Combustion

COMBUSTIONThis is a set of lectures on Combustion Basics (there is little here on practical systems), organised in the following topics: • Combustion characteristics. A descriptive presentation of what is combustion, what it is for (applications), how it is done (practical combustors), and what was historically known. • Fuels, Fuel properties, Fuel consumption, and Pyrotechnics. An extensive descriptive presentation, with an historical development review. • Combustor characteristics. A short description, following a block diagram approach, of what to study in a generic combustor: intake, internals, heat and work flows and exhaust. • Environmental effects and hazards in combustion. A descriptive presentation of potential source of damege in combustion applications, with a review of fire safety, pollutant emissions, and generic safety management. • Combustion thermodynamics. A mathematical formulation of equilibrium conditions, based on the extent of reaction and the affinity of reaction, and with emphasis on the enthalpy of reaction (the maximum heat, or heating value), the exergy of reaction (the maximum work) and the exhaust equilibrium composition. • Combustion kinetics. A descriptive presentation of the detailed mechanism of reaction rates, activation energy and its modification by catalysts. • Combustion models . A mathematical formulation of some key combustion problems: combustion at rest, premixed combustion and non-premixed combustion, with emphasis on flame geometry. • Combustion instrumentation. A descriptive presentation of devices and procedures for the setting, control and diagnosis of combustion processes. Back to index COMBUSTION CHARACTERISTICS Combustion characteristics (Introduction) .................................................................................................... 1 Combustion fundamentals (What it is) ..................................................................................................... 1 What it is not ......................................................................................................................................... 3 Thermal free-flame gaseous combustion .............................................................................................. 4 Thermal trapped-flame combustion in porous media ........................................................................... 4 Smouldering (Thermal non-flame combustion of porous media) ......................................................... 5 Catalytic combustion............................................................................................................................. 5 Detonating combustion ......................................................................................................................... 6 Combustion applications (What it is for) .................................................................................................. 6 Heating .................................................................................................................................................. 7 Propulsion and electricity...................................................................................................................... 7 Absorption refrigeration ........................................................................................................................ 7 Chemical transformations ..................................................................................................................... 8 Combustion systems types (How it is done) ............................................................................................. 8 Steady combustion chambers ................................................................................................................ 8 Unsteady combustion chambers............................................................................................................ 9 Catalytic combustors ........................................................................................................................... 10 Porous burners..................................................................................................................................... 10 Fluidised bed combustion ................................................................................................................... 11 Open fires ............................................................................................................................................ 11 Combustion history (What was known).................................................................................................. 11 Fuel history ......................................................................................................................................... 11 History of Combustion theories .......................................................................................................... 12 COMBUSTION CHARACTERISTICS (INTRODUCTION) There is much more to combustion than a fuel in air and an ignition source. To better appreciate the wide range of involved phenomena, a description of combustion basics (combustion types and processes), and combustion applications (combustor types and systems), is presented here, before a more rigorous treatment of the thermodynamics, kinetics and metrology of combustion. COMBUSTION FUNDAMENTALS (WHAT IT IS) Everyone knows from infancy what a fire is; humans have always felt a mixture of fear and magical appeal for fire. Combustion is burning, a self-propagating oxidative chemical reaction producing light, heat, smoke and gases in a flame front. Combustion is a process and fire is the actual outcome. What does it mean in more detail?  Combustion means burning (lat. cum urere-ustus = burn), e.g. burning wood in air, natural gas in air (or CH4/O2/N2 mixtures in general), hydrogen with oxygen (H2/O2 in gaseous or liquid forms, and not only H2 in O2, but O2 in H2), and more bizarre burnings, such as sodium with chlorine (Na(s)/Cl2(g)), aluminium powder with water, magnesium powder with carbon dioxide, nitrocellulose (cellulose is -(C6H10O5)n- with n=300..2000) within any medium, etc. But the meaning of combustion is usually restricted to easily flammable substances (typical fuels) in ambient air. Fuels and oxidisers are presented aside.  Self-propagating, means that, once it ignites, it goes on, sustained by the high temperatures and radicals (active species) produced, until either the fuel or the oxidiser practically runs out, or an extinguishing agent is applied that prevents fuel-and-air mixing, or cools the system well below autoignition, or scavenges active species. Notice the two sequential steps in combustion: first there is an endothermic process of ignition, followed by a much more powerful exothermic process of runaway oxidation that propagates the process. Any exothermic adiabatic system will show a thermal runaway at some high enough temperature (autoignition), but for real non-adiabatic systems, ignition criteria are governed by an interplay between heat-release rate and heat-loss rate. Materials are termed non-combustible if they cannot be ignited below 1000 K. Combustion propagation is usually a slow process; very slow indeed for solid and liquid fuels: <0.1 mm/s for a candle flame (that were used as clocks to measure hours), 1 cm/s for flames spreading on solid fuels, usually <0.5 m/s for premixed fuel-and-air gases burning at rest (although it may be raised to 100 m/s in high-turbulent flows, and may reach >1000 m/s in detonations). However, a common feeling and fear is that combustion is explosive, because the pressure-rise in confined spaces may give rise to violent mechanical explosions (brick walls are not pressure vessels). According to the mechanism for propagation, several types of combustion processes may be distinguished:  Thermal free-flame gaseous combustion: the usual case for combustion, e.g. in a candle, a lighter, a bunsen, an internal combustion engine (reciprocating or turbine), a furnace, a boiler, etc.  Thermal trapped-flame combustion in porous refractory media: a new combustion procedure derived from the common one above-mentioned, to substantially increase combustion intensity and stability, but problems of refractory materials exist.  Smouldering (or Thermal non-flame combustion of porous media): the slow burning without flame of porous combustible matter, e.g. cigarettes, wood or coal embers, etc. It has little engineering use, but great safety interest because uncontrolled fires usually start by smouldering.  Catalytic combustion: active species, instead of temperature, may propagate the combustion process, usually without flaming: e.g. room-temperature reaction of hydrogen and oxygen in a platinum surface (it may flame if a large contact area exists, as in a porous catalyst that releases so much heat as to ignite the rest).  Detonating combustion: when the combustion process is coupled to a high-pressure shock wave, travelling at supersonic speed.   Oxidative chemical reaction. Combustion is an electron-exchange reaction (a ‘redox’ one), not a simple electron-cloud distortion as in proton-exchange (acid-base) reaction. The fuel atoms supply electrons and get oxidised, whereas the oxidiser atoms get the electrons (get reduced). Light, heat, smoke and gases in a flame front. Combustion results in a large temperature increase in the products, typically from 300 K to 2500 K, causing them to be in the gas state (except for soot and more rare refractory particles) and establishing a radiation imbalance in the infrared and visible ranges. Light emission in flames only approaches blackbody radiation if there are solid particles, such as soot found in non-premixed flames (e.g. yellow bunsen). For premixed flames, light is by chemiluminescence in special spectral bands, and very dim in intensity (e.g. blue bunsen). It is the visible light of non-premixed flames that has been traditionally identified with combustion (it is the standard symbol for fire). In fact, it might help to think of the flame as an invisible, very hot, burning interface, made visible by non-burning incandescent substances passing by or being created, for instance soot particles in non-premixed flames (their sublimation temperature is around Tsubl=3900 K), sodium ions in salt-seeded flames (above the salt boiling point Tb=1690 K), or calcium oxide in limelight (Tb=3100 K). The latter was used in the 19th c. in theatres as the brightest, most natural-colour artificial light available, being produced by placing a block of lime against a hydrogen/oxygen jet flame (practically invisible in spite of its 4000 K temperature; lime melts at 2850 K). The Sun also gives light and heat (at a temperature of 5800 K in its surface) but by nuclear fusion reactions in the interior (where the temperature may reach 107 K) and not by chemical combustion. What it is not Combustion vs. explosions Combustion means burning and explosion means bursting, i.e. combustion is a relatively slow chemical process yielding light and heat, whereas explosion is a sudden mechanical process causing rupture and noise, due to great pressure forces that may be originated chemically (e.g. from a confined combustion), thermally (as in boilers, even electrically heated), mechanically (as in a balloon or any other gaspressurised vessel), nuclearly, etc. Detonation, the supersonic combustion taking place under some circumstances in premixed fuel/oxidiser gaseous mixtures and many explosives, is studied aside. Combustion vs. fuel cells Fuel cells are electrochemical generators, like batteries but with continuous fuel-and-oxidiser supply. Reactions inside a fuel cell, although globally equivalent to combustion, are not properly combustion because they do not self-propagate (reaction in a fuel cell stops as soon as the electrical load is switched off, it shows no thermal-runaway). A non-premixed burner (e.g. a lighter) may be thought of as controllable as a fuel cell (as soon as the fuel injection stops, combustion ceases), but it does not simply starts over if reopened. A controllable-area catalytic combustor, however, more closely resembles a fuel cell: no need of igniter, simple reaction control, and for small active areas there is no runaway (a big difference is that fuel cell directly generates electricity and the catalytic combustor just heat). The igniter in a combustor (a spark or a hot wire) and the electrical connector in a fuel cell, act as catalysts that provide a gateway for the reaction; in both cases there is an electron-transfer reaction (redox reaction), the main difference being that the transfer of electrons from fuel to oxidiser is restricted in a fuel cell by electrode-interface-area and electrolyte-ion-diffusion, with the external electrical connector required all the time, whereas in normal combustion the electrons transfer is only limited by diffusion in the bulk, and the igniter is only needed to start the process. Entropy generation, positive in both cases, tends to zero in a fuel cell at very low intensities, but it is always above a certain finite value in combustion. Combustion vs. oxidation Combustion is a self-propagating oxidative chemical reaction characterised by a thermal runaway; i.e. it is a quick exothermal oxidation. The same system may undergo slow oxidation or combustion, with the same initial and final states, but with different paths (e.g. paper turns yellow (and brittle) with the years because of slow oxidation, but may burn in seconds). Notice also that oxidation may be exothermic or endothermic, whereas combustion is always very exothermic. g. Steady free flames demand an astonishing fine balance for heat and mass flows (e. thermal free-flame combustion cannot propagate inside a metal tube of less than 2 mm. this kind of 'thermal' combustion cannot propagate (safety lamps and quenching grids are based on this fact). If adiabaticity of the initial ignition region is prevented by nearby heat sinks. e. burn at the surface of a molten mixture of the metal and its oxide. tantalum or zirconium. but also fluidised). that self-propagates as a result of the high temperature (1500. Only refractory fuels like coal to some extent. flames may be sustained inside liquids in a suitable gas envelop. iron and titanium.g. and this fact is used in fire-fighting. what can be achieved by holding a lit free flame close to the solid for some time. and the burning or flame-propagation speed is only limited by the chemical kinetics of the reactions involved and heat diffusion forward.3500 K) developed after initial ignition (e. In order to maintain a steady underwater flame (e. it is necessary to form a stable bubble. which have low boiling points. slowly decreasing gas injection speed allows the flame to go backwards and penetrate the porous media. Thermal flames are almost always established in a gas phase: in a fuel/air gas mixture or in the fuel vapours diffusing in air. According to the initial state of mixing of fuel and air. . 5% and 15% of fuel by volume of mixture for methane/air flames. from liquid or solid fuels. as a cold solid wall. In premixed combustion the unburnt gas is already a perfect mixture of fuel and air. respectively. However. having intermediate melting points for both the metals and the oxides.g. although increasing temperature widens this range. whereas aluminium and magnesium. The presence of condense matter is always a handicap (all condensed fuels burn worse than their vapours). the speed being fixed by the flow-rates of fuel and oxidiser that must approach the flame by diffusion from each side. If the injected gas speed matches the deflagration speed.THERMAL FREE-FLAME GASEOUS COMBUSTION This is the usual case for combustion. unless the solid is hot enough (say >1000 K). vaporize and then burn in the gas phase. combustion process can be classified in the limit as premixed and non-premixed (real processes are in between). Thermal free-flame combustion cannot propagate if the air/fuel ratio lies outside of the lower and upper flammability limits at ambient conditions: e. if a premixed methane/air stream is forced through a finite porous medium (usually solid. the flame sits steadily at the mouth and. which is being heated by the slowly moving flame front. think why a candle flame sits at a precise distance up the wick). The process is also known as filtration combustion. since the exhaust gases cannot maintain it by themselves (a great deal of skill is required of divers who perform this kind of work). due to a more-or-less adiabatic conversion of chemical-bond energy to internal-thermal energy within a reacting gas mixture (for condensed fuels. by a spark). and ignited at the exit. after the porous end gets hot.g. the free-flame formed may travel upstream or downstream according to the flow speed. Neither can flames propagate also at very low pressures (a fire safety rescue in spacecraft). whereas in non-premixed combustion there is not a characteristic burning or flame-propagation speed. with a corresponding premixed and nonpremixed flame. burn at the solid surface.. THERMAL TRAPPED-FLAME COMBUSTION IN POROUS MEDIA Flames cannot propagate through small holes in a solid (this is how Davy's safety lamp work).g. usually achieved with an additional compressed-air jet-stream introduced around the tip of the torch. For instance. a hydrogen or acetylene welding torch). for premixed methane/air stoichiometric mixtures. the latent heats for vaporisation and possibly decomposition has to be subtracted). They may work with premixed flows and with a non-premixed fuel-flow and ambient air. e. taking the oxidiser from the ambient through its pores. but with change of external texture (which chars) and smoke emission (sometimes very toxic).e. In spite of this basic materials difficulty.Notice that the flame-front temperature is lower than the adiabatic value when the front moves backwards. an heterogeneous reaction. with little or no visible flame. CATALYTIC COMBUSTION Even in the close proximity of cold solids.5 m/s to 4 m/s (they reach 3000 kW/m2 instead of the 300 kW/m2 of normal burners). if the gas injection speed is increased to force the flame to travel downstream. however. now combustion 'of' a porous-media-fuel is analysed. A big fire-safety problem is that. i.g. smouldering may undergo a sudden transition to flaming. During drawing. SiC and FeCrAl-alloys being used below 2000 K with the advantage that they have larger thermal conductivities and mechanical resistance.1 to 1 times full load. a dim burning happens with maximum temperatures of some 850±50 K at the centre and some 650±50 K at the periphery. to help an early detection. smouldering can progress undetected for long periods of time (smoke detectors are used. SMOULDERING (THERMAL NON-FLAME COMBUSTION OF POROUS MEDIA) Before. Presently only Al2O3 and ZrO2 can work above 2000 K.  Wider range of ignitable compositions (lower limit decreases from 5% to 4% in methane/air flames. but. The porous fuel is inside a porous cylindrical envelop (of paper in a cigarette. Once the cigarette lighted.  More compact because the deflagration speed increases from 0. widening of ignition limits and less emissions. yielding lower emissions). The most developed catalytic combustor uses C4H10/air over Pt-coated ceramic matrix at 500 ºC. combustion 'in' a porous-media was considered.2. but diffusion is very inefficient. Low-temperature catalytic combustors work in the 300.9 to =2. combustion may be sustained if there is some catalytic substance that lowers the activation energy and sustains the combustion process at low temperature at the porous surface. allowing for leaner mixtures to be burnt. increasing the air ratio from =1. then its temperature is above the adiabatic value (it is moving into an already hot solid).1400 K. porous-medium burners have several advantages:  Less NOx emissions because of lower temperatures. Some porous (solid) fuels may sustain a self-propagating combustion inside their matrix. under certain circumstances. Porous media combustion was developed aiming to stabilise premixed flames near their lower stability limit. at a very low rate and low temperature. and high-temperature catalytic combustors work in the 700. or a full tobacco leaf in a cigar) containing crashed dried tobacco leaves (they were smoked or chewed by American Indians since ancient times because of the euphoric action of nicotine). against 0. A typical smouldering process takes place in the tip of a lighted cigarette. respectively. the burning taking place in the insides. but not the higher burning rate (that was due to thermal conduction along the hot solid matrix. and thus only 600 kW/m 2 are . if left in still air without drawing. due to very low temperatures involved)..5 to 1 for normal burners.  Wider power-modulation range (0. are maintained or increases here (<1 ppm for NOx and CO. where buoyancy forces are absent. those values go up to 1000±50 K and 850±50 K. CH4/air over Pt requires 350 ºC. so that start/stop cycles and accumulators are avoided).600 K range. Thus. The advantage of power modulation. that is why some forced convection of cabin-air and avionics-air is always procured in spacecrafts.. H2/air react over Pt at room temperature. transportation propulsion. where both heterogeneous and homogeneous reactions take place. Combustion (controlled and uncontrolled) releases an average of 101012 W in the globe. To advance the efficient use of fossil fuels (quickly exhausted) to satisfy our needs (e.g. most of the times light. Combustion is key to humankind today as in the past and the foreseeable future. but today only electrical lamps are used. part of the fuel slips to the exhaust (increasing pollution and expense). Combustion always gives off heat and gases. the general interest to broad our knowledge of the world). Is it just then a subject of historical interest? For instance. it is the major environmental-pollution process. and the major personal danger in fires).achieved instead of the 3000 kW/m2 of thermal porous-medium burners. as compared to 0. and sometimes smoke (a nuisance in non-premixed flames. part of which was transformed to UVshield O3 by solar radiation and thus allowed the start and survival of primitive plants. 31012 W due to geothermal heat flux and 120 0001012 W due to the solar absorption. small flames detach from the outer surface and at about 1000 kW/m2 a blue flame at some 1900 K forms immediately close to the outer surface. 3. Roman Emperor Augustus established a corps of fire-fighting watchmen already in 24 b. the only lighting method was combustion. if a thin porous solid is doped with a catalyst and a premixed methane/air stream is forced through. but mostly in compounds) and Earth ecosphere is plenty of fuel (all living matter plus fossil fuels).C. or even the whole fuel if the catalyst cools down and deactivates. and fire is one of the most feared natural disasters. from pre-historic times (some 500 000 years ago. 4. but if more gases are fed. To understand it (i. Combustion is the major energy release mechanism in the Earth (it is gravitational dissipation in jovian moons). To alleviate the problems associated to combustion emissions. DETONATING COMBUSTION Supersonic combustion. Plant photosynthesis has generated all O2 in the atmosphere and all living (and fossil) fuels on Earth. an interesting trade-off solution when the catalyst is too expensive and very little can be used (in full catalytic mode. for instance. 4. releasing much of the lower heating value. for its own sake. there was no O2 but some H2O that generated a small amount of O2 by solar-radiation hydrolysis. Why? Because the Earth atmosphere is plenty of free-molecular oxygen (50%wt of the whole ecosphere is oxygen. At intermediate temperatures. it is worldwide the major energy-release process (nearly 90% of the world primary energy is to be burnt).e. and under Supersonic combustion in /Combustion kinetics. so. what causes the matrix to reach some 1500 K at the outer surface (the hotter) in a flameless regime with a power of up to 500 kW/m2. To minimise the risks and damages of controlled combustion and uncontrolled fires (safe fire handling). electricity generation). Why? It is nearly a miracle because at the start of the Earth ages. called detonation. their fire-fighting techniques . that only reaches now some 600 K. Detonating combustion is considered under Explosives in Pyrotechnics.. bone and stone. The influence of the flow rates is important.5109 yr ago. at least) to the end of XIX c. 2.51012 W due to nuclear disintegration (controlled and uncontrolled). an hybrid regime of catalytic-assisted thermal combustion may be developed. an exothermic oxidation of the fuel takes place if the temperature is >600 K. COMBUSTION APPLICATIONS (WHAT IT IS FOR) Combustion is an old technology to humankind. is realised when combustion gets coupled to a shock wave travelling at supersonic speed. why spending effort on candescent lighting? We want to devote an effort to understand combustion processes for several reasons: 1. if there is insufficient catalyst. only second to simple mechanical tools made of wood. at a much later times. Energy efficiency in heating is usually measured as the fraction of the lower heating value of the fuel that is fed to the system to be heated (the rest is lost by the flue). roughly).2 to 0. in which. may burn for one minute consuming some 5010-3 kg of oxygen. and it is close to 1 (that is not a limit: heat pumps can give more).5 m long. For cutting or piercing thick ceramic objects. as for carbon).5). or by the specific fuel consumption (of the order of 60 g/MJ or 200 g/kWh). Internal chemical energy is first released to internal thermal energy (high temperature). gas generation for fuel-tank inertisation. For electricity generation. 0. ABSORPTION REFRIGERATION Absorption-refrigeration machines use heat (usually from steam or from a fuel burning) to separate the working vapour from a high-pressure liquid mixture. under water. by means of heat engines (vapour and gas cycles). Explosives applications are considered aside. steam).e.. with additional oxygen to burn the metal (that is a fuel itself. the change is x [mph]=235/x [L/100 km]). fireworks. and opening doors and windows in enclosures to get rid of smoke and let fresh-air in for people sake (unfortunately feeding the combustion process. aromatherapy. By an absorption refrigerator driven directly by the combustion products or through an intermediate fluid (e. reforming. PROPULSION AND ELECTRICITY Motion and electricity generation. rescue signals. and it could be similarly done for mechanical power. excluding photosynthesis and uncontrolled fires) is by combustion (plus a 5% hydraulic and a 7% nuclear fission. A special heating application is the cutting of materials. oxidation. removing the fuel by cutting it aside with an axe (or. too). but nowadays it is a rarity: candles.  Propulsion and electricity generation. energy efficiency is measured by the ratio of electrical energy delivered divided by the lower heating value of the fuel (it typically ranges from 0. There are other unusual applications of combustion. This combustion-released energy (as any energy) is used today for:  Heating.  Refrigeration. etc. or within mud and slurries). producing a >5000 K flame that would pierce a 150 mm-thick steel slab in less than 10 s (a 0. Even nowadays 88% of world commercial energy consumption (i. and then heat transfer to the load takes place. constitutes the foundations of Thermodynamics.g. HEATING Heating for space conditioning or for materials processing (domestic and industrial) has always been the basic combustion application.  Light emission was a major application of combustion up to the XIX c. etc. incineration. or directly the combustion products.5 m deep hole can be pierced in stone or concrete in a few minutes. as smoke generation for visualization.are still applied: removing heat by throwing water with a bucket passed from hand to hand. anywhere: in the air. Mainly by means of a heat engine with a working fluid that may be an independent one (e. The working vapour condenses in a heat exchanger with the ambient. although for cars it is usually done in litres of fuel consumed per 100 km travelled (or in miles per gallon in the USA.  Chemical transformations. Mainly as a heating application. and is flashed through a restriction to a low-pressure heat exchanger where the cooling . steam). an iron tube act as the fuel that burns (once ignited) with oxygen being supplied through it (sometimes in the cryogenic liquid state). propane or oxy-acetylene torch. hint: think of M+O2=MO2. A small 5 mm-diameter tube.g. but sometimes directly for reduction. a thermal lance is used. Thick metal plates can easily be cut with a normal gasoline. by providing firebreaks). Recently. no surveillance. producing a bright and long yellow flame with maximum temperature T≈2500 K with maximum spatial deviations of T≈1000 K. e. whereas. 1a) for emission reduction. as combustion). and to how the propagation is maintained. and domestic appliances: from the cooking range to the gas lighter. 20 years life). 1b) is a friendly. reliable. but it does not mean that the process is locally steady. The combustion process may approach the non-premixed flame model or the premixed flame model. efficient and environmentally-clean combustion appliance. most practical systems operate in a highly turbulent regime. is fed by natural gas that burns with 3% excess air. and only produces 80 mg/m3 of NOx as major pollution (besides the CO2 inherent to the fuel carbon content). that is nearly stoichiometric (see Fig. provides some 20 kW of space heating or hot water for sanitary use. Walls must be cooled to avoid materials problems at the high temperatures involved (>2000 K). is small and silent. due to exhaust condensation). exiting into a liquid absorber that is pumped and closes the loop. energy efficiency is usually measured as the amount of material processed divided by the amount of fuel consumed. heat recovery. Energy efficiency is usually measured as the amount of cooling divided by the amount of fuel or steam consumed (in vapourcompression refrigeration. require an exergy supply (other materials processes are used as an exergy source. depending on the materials. Exhaust gas recirculation (EGR) helps to keep a high uniform temperature inside the combustor. but lower than the hottest region in normal flames. no servicing. gas turbines. steam turbine boilers. it is permanently ready (safe. partial recirculation of exhaust gases is being applied (instead of the secondary air in Fig. one may distinguish several type of combustors. cooking. COMBUSTION SYSTEM TYPES (HOW IT IS DONE) According to the steadiness of the control frontier (real or imaginary). boilers. excess air is used to cool the main burnt flow.g. autoignition and flame stabilisation. in normal combustion. Steady refers to the average process. extracting 94% of the maximum heating value (105% in terms of its LHV. . and. mineral reduction. 1a). CHEMICAL TRANSFORMATIONS Some materials processing. incineration may have positive or negative heat balance.g. according to how much fuel-and-air mixing exists before burning. traditionally grouped in steady combustion chambers and unsteady combustors. most of the times. In the typical case of energy consumption. air enters at 300 K with 21% O2. furnaces. a maximum T≈1800 K with maximum spatial deviations of T≈100 K. STEADY COMBUSTION CHAMBERS Steady state combustion takes place in industrial heaters. distillation. entering the air/EGR mixture at some 1500 K with as low as 2% O2 and producing a wide dim green flame. energy efficiency is measured by the ratio of cooling energy to electrical or mechanical energy input). e.effect is produced. A modern home boiler (Fig. and =1. pulverised (fine dust) or fluidised (coarse granulate made to levitate by air entrainment). extracting some 50% of the maximum work-value of the fuel (some 30% energy efficiency in terms of its LHV).Fig. b) domestic water heater. 20 years life). both of Diesel and of Otto type. whereas USA cars are typically 4-litre. Unsteady combustion chamber in an internal combustion engine. The combustion process may approach the non-premixed flame model (for the major part of the fuel burning in a diesel engine). 80-kW engines. SI-fuels must have high volatilities (or be gaseous) for quick mixing. but gasified (pyrolysed).1 g/km of CO. 1. . and is able to power a 5-seat car at 170 km/h consuming 3 litres every 100 km (at 90 km/h on good roads).1.05 for Otto engines. or the premixed flame model (for the spark-ignition engine and the initial stage for compressionignition engines). or Diesel).. Presently. The best ICE are direct injection (DI) exhaust gas recirculation (EGR) engines. reliable (2 years typical maintenance-free.02 g/km particulate matter. UNSTEADY COMBUSTION CHAMBERS Unsteady combustion chambers are used in reciprocating internal combustion engines (ICE). it is small and not too noisy and shaky. Typical air/fuel relative ratios are =0. A modern direct-injection turbo-charging diesel-engine for a small family-car (like in Audi A2 1.3 g/km of NO x as major pollution (and <0. Fig. however. contrary to steady combustors. although it is expensive and only worth for >3 MW. >100 kW-engines. Solid fuels are no longer burnt in chunks over a grate. =1.9.2. The normal working for a SI-engine is with a premixture of fuel and air at the stoichiometric ratio. and <0. that the thermal inertia of the wall materials makes higher peak temperatures allowable in the gas. whereas for a CI-engine non-premixed fuel is added to highly-compressed air. Steady combustion chamber layouts: a) industrial burner.1. The two basic ICE realisations are the spark ignition engine (SI. not further mentioned here because of lack of use.. EU and Japan cars are typically 2-litre. easy to start. to avoid knocking. diesel oil. the latter with stratified mixture (by injection control) and exhaust catalytic converter. or Otto) and the compression ignition engine (CI.1 for large (>500 kW) slow (<200 rpm) marine Diesel engines.2 TDI.3. or VW Lupo 3L TDI) is a friendly commodity: it is safe. and have a high resistance to autoignition. yielding some 100 gCO2/km). it injects the fuel directly to its three cylinders in line at more than 100 MPa when the compressed air is at some 6 MPa and 700 ºC.. tuning the output power by essentially chocking the entrance duct. producing less than 0. The period of burning and fluids renovation is so short.8 for normal Diesel engines. 4-cylinder. CI-fuels must have a low autoignition temperature and short delay. also in the (non-reciprocating) Wankel engine. Dry (<15% water) coal powder (1 mm size) can be burnt in cyclonic or screw burners. besides the CO2 inherent to the fuel. 2. 10 years typical mean-time between failure.6. providing some 50 kW of mechanical power with its 1200 cm3 total piston displacement and 18:1 compression ratio. 6-cylinder. gasoline. to dry wood and other biomass).g. with a non-premixed fuel flow that meets a back-diffusing air flow at the exit.  Catalytic converters. They may work with a premixed flow (fuel and air been forced through one end). because of increase heat transfer and flame stability. where alveolar flames stabilise themselves for a wide range of flow rates (e. where very lean mixture may be burnt at moderate temperatures. for safety). achieved electrically (from the mains.2 for CH4/air). where heat is generated by burning a main fuel (C4H10.g.2. but most usually they require some initial heating (as in Pt for CH4/air). producing a better combustion for a steady regime (it is difficult to control the time of ignition if unsteady): less soot and lower NOx emissions (lower temperatures) than a diesel engine. CH4.  Catalytic-assisted burners. where emissions are remediated without interest in heat generation (e. Depending on the application. . but they are expensive and bulky. with a compact heat exchanger. most modern cars have a catalytic converter in the exhaust pipe (see Three-way catalytic converter). showing promise of very high combustion intensities (3000 kW/m2 of cross-section) and low emissions. They are not so much developed as the simple thermal combustors above-mentioned (steady and unsteady). They are being introduced in gas-turbine engines and plants. Catalysts may work at room temperature (as in Pt for H2/O2). because of the surface effect on heterogeneous reactions. by a conventional pilot flame.. more rarely. practical systems consist of monolithic honeycombs or granular material (porous media of 1 mm typical porous size). They have the advantage of very complete oxidation (low emissions) and low-temperature work.It seems that both Otto and Diesel engines are converging towards an hybrid. to cure the epoxy used to join pipes in gasoducts and pipelines. the temperature at which the catalytic pad will sustain a chemical reaction with the fuel and ambient air. where it is heated until it enters the second larger-pore hottest solid matrix. The combustion catalyst per excellence is a platinum doped refractory matrix. or even by a room-temperature catalytic combustor (as in hydrogen-assisted natural-gas catalytic combustor). POROUS BURNERS Porous burners are porous non-catalytic solids inside which a high-temperature combustion takes place with small flames trapped in the pores. or with a battery). the new Homogeneous Charge Compression Ignition engine (HCCI). where fuel injection during the compression stroke creates a very lean mixture that auto-ignites by premixed compression near the top dead centre.g. and the fuel supply is regulated to operate steadily at some 700 K. CO and VOC in car exhausts are completely oxidised). or. They are in the development stage. from =1. from v=0.. avoiding the formation of NOx. This is the most developed application.4 m/s for CH4/air) and air/fuel relative ratios (e.g. and higher efficiency that an Otto engine (but unburnt emissions in between). Although simple stagnation flows over a catalytic plate are studied in the lab. They are mainly used as radiant catalytic heaters for stoves and open-air industrial heating (e. Premixed porous burners consist of two sequential stages: the premix fuel/air stream first enters a finepore hot solid matrix (below the flame quenching size. H2) in a catalytic porous media. a third stage may be directly added to the burner. CATALYTIC COMBUSTORS Catalytic combustors are porous catalytic solids inside which a low-temperature combustion takes place. The element preheats the catalyst bed to some 400 K. They may be used for:  Catalytic burners. but when the drag on the particles overcome its weight. A smoulder temperature may be identified to describe the flammability behaviour of a flat dust layer on a hot surface (e. but can be shorten to less than 0. may sustain a low-temperature combustion inside with air penetrating by diffusion. porous burners could be installed in wall niches instead of protruding significantly.. being more compact. FUEL HISTORY This theme can be found in Fuels. the lowest temperature of a heated surface capable of igniting a 5 mm thick dust) COMBUSTION HISTORY (WHAT WAS KNOWN) Although the history of combustion is related to the history of fuels. noise. Smouldering is not intentionally used in engineering. for small upward speeds the particulates act just as a filter. that must be 1 m to 3 m long if conventional (to protect from the flame). The disadvantages are the difficult start-up. Mineral fuels (coal. from the traditional fireplace to the wild uncontrolled forest fire. and erosion of the heat-exchanger walls (that is partially or totally submerged to limit the maximum temperature within the bed). The main advantage of this modern type of combustors is that additives can be easily added and the porous material easily refurbished. This compactness is also an advantage for industrial heaters and driers. held over a porous surface. in Fuels. By 'open fires' we here just mean unenclosed combustion setups. passing by the humble candle flame. either solid like wood or particulate like dust. it can be found under “Torches. Briefly. . difficult ignition (it is ignited above the bed). Steady burning is achieved at 1100 K to 1300 K (uniform red-hot bed). Fluidised bed combustion is currently used in some coal-fired power plants.The wide power modulation capability of porous burners make them ideal for applications such as home water heaters. where the power needed to heat sanitary water cannot be less than say 15 kW and is increasing. and Biological fuels in the XXI c. but below 1100 K the burning is noisy with violent small bursts. but it is very important in uncontrolled fires. It is also been used to burn high-moisture fuels like agriculture and industrial residues and sludge. or sand or other granular refractory). Besides. instead of a rigid solid matrix. FLUIDISED BED COMBUSTION Fluidised bed combustion is conceptually similar to porous burner combustion. empty or filled with porous media). with more or less ‘bubbling’ or ‘boiling’ when speed is increased until at a certain value is reached (the terminal velocity) beyond which the particles are ejected from the bed and carried away with the stream. where coal-desulfuration is achieved during combustion by adding lime to the bed. OPEN FIRES Most practical combustion systems are enclosed inside easily-identifiable walls (combustion chambers. oil lamps and candles”. whereas the power for space heating is decreasing from typically 20 kW to 5 kW for betterinsulated apartments.5 m using a porous burner. and above 1300 K the particulate may start fusing and clogging. Because the information here compiled is basically historical. crude oil and natural gas). we treated them here separately because under ‘fuels’ we focused on empirical findings and under ‘combustion’ we centre the attention on theories. Porous materials. Fuel history has the subheadings there: Biological fuels before the XX c. a high-temperature combustion takes place within a particulate system (the fuel particles themselves. but. a fluidised regime is establish (at the fluidising speed).g. When an upward air-stream (or a premixed fuel/air stream) is established. as usually achieved. wood. However. water. besides the fuel. It had a magical appeal: Prometheus stole it from the gods. Phlogiston principles: . A summary of the chronological development of combustion theory is presented in Table 2. sparking stones or compressing air. 2. Becher and G.C. Around 500 b.. The flame. it was only learnt in the 17th c. Proposed by the German physician and chemist Georg Ernst Stahl-1697 and discarded by Antoine Lavoisier-1783-Réflexions sur le phlogistique. electrical). Count Rumford rejected the caloric theory in his essay 'An experimental enquiry concerning the source of the heat which is excited by friction'. (starting in 1826) at the Royal Institution. Human respiration and its analogy to a candle. Around 1660. thermal. The components of water are oxygen and hydrogen. Components of air: oxygen and nitrogen. aside of the caloric substance (the heating component). Heraclitus of Ephesus held that fire is the primordial substance of the universe and that all things are in perpetual change (like a vital flame). whereas metals did it slowly (rust). by Mayer and Joule. 4. was only generally accepted after mid 19th century. Afterwards. to avoid air convection and heat loss.HISTORY OF COMBUSTION THEORIES Fire has always been a cause of panic. Lavoisier.C. Fuels released phlogiston quickly. The concept of energy (chemical. Although already in 1798. imaginary fluids were thought to explain the facts. it was not until the 1840s that it was finally discarded and replaced by the principle of energy conservation. that air was also needed. Joseph Priestley-1796 tried to come back to phlogiston. but not just hitting the wood).. The most important (actually six of them) was published as "The Chemical History of a Candle" in 1860: 1. rejected the phlogiston theory.C. it was common practice to preserve fire overnight by putting a fire cover (curfew) over it.Stahl introduced the theory of phlogiston as a fluid that flowed away in combustion processes. The need of air Phlogiston Table 2. Empedocles of Sicily set the theory of the four universal elements: fire. (he added aether as the quintessence. but it was Lavoisier in 1777 the first to realise that oxygen was the key. both to combustion and to animal respiration. as an extension of the primitive mechanical idea of combination of 'head' (potential energy) and 'vis viva' (kinetic energy). A good appreciation of the level acquired in combustion theory can be grasped from the titles of Michael Faraday's famous series of Christmas Lectures for Children. but also of immense power. in his paper "Réflexions sur le phlogistique" of 1783. (lat. sulfur could not be ignited by concentrated light. and earth. water from combustion. Coming down to our physics. Around 450 b.J. delivered in the mid 19th c. J. Construction of a candle. Product from a candle: carbon dioxide. phlegma=fire spirit): a substance (with mass) stored in fuels and released in chemical reactions (the flammability component).E. 6. further developed and spread by Aristotle around 350 b. Before that. Much earlier than the need of air for combustion was realised. under vacuum. Robert Boyle-1663 found that combustion requires air. air. a fluid that flowed away with heat (some authors even advocated for another fluid for cooling processes. it was appreciated from the beginning that combustion required a fuel (e. around 1663. Chronology of combustion theories. Boyle realised that. 5. Products of combustion. but set forth the caloric theory. in his Metaphysics). according to Greek mythology.g. 3. Necessity of air for combustion. but not stone) and an igniter (by rubbing woods. the frigoric). What was phlogiston: H2 (released by metals when pouring an acid over) CO2 (exhaled by animals). making the metal heavier when rusted.g. and may suddenly release it if burned. Bunsen-1860 repeated Newton's experiment of light dispersion by a prism. Water dissolves phlogiston. and water gas. that escapes from them when bodies are heated to burn.e. At Heidelberg. but does not destroy it. later changed to subscript.  Phlogiston can be weighted by difference before and after burning the fuel. Thermal model Mallard and Le Châtelier. but with the light of flame. e. Fixed Proposed by Proust-1799: chemical reactions take a fixed amount of substances proportions relative to another. NO. particularly with premixed flames. or to air if plants are burnt. Caloric Proposed by Lavoisier-1777 and discarded by Mayer-1842. coal. Around 1815 he discovered that flames cannot go through wire meshes and developed the miner’s safety lamp. Emissions CO+H2. at École de Mines de Paris. and returns to animals when they eat plants. fuels cannot yield all their phlogiston. developed his premixed burner in 1855. Mass Proposed by Lavoisier-1777: the overall mass is preserved in any confined chemical conservation reaction. Premixed flame Robert Bunsen. oxygen affinity. i. e. Flame Sir Humphry Davy published in 1800 his personal trials on the physiological effects quenching of some gases of importance to combustion (laughing gas. a substance (with mass) released in thermal processes. electricity was a substance (with mass) released in electrical processes. Bohr-1913 proposed the orbital theory of electrons in the atom. He also established the taxonomy of chemical compounds. Similarly. and explained the emission spectra of simple atoms in absence of magnetic fields (quantum mechanics explains everything up to now). which nearly poisoned him). Phlogiston is a material fluid present in fuels (wood. Multiple Proposed by Dalton-1803: different chemical reactions between same reactives take proportions a fixed amounts of substances relative to another. Flame spectra measuring flame temperatures and flame speeds. and identified oxygen-in-the-air as the main oxidiser. Plants slowly take phlogiston from the air on sunshine. and the excess appears un-reacted (quite different to a mixture). is stored in the air and plants. The phlogiston released by metals when rusting may be fed-back from phlogiston-containing substances (metals are obtained from their ores by calcination with wood or coal). and discovered that it was not a continuum but separated bands. the quantum numbers. the ratio between which are small whole numbers. If air is trapped inside a bell. and carbon and hydrogen as the main fuelconstituents.g.  Air is a phlogiston absorber. H2O). Chapman and Jouguet in 1900 distinguished deflagrations from detonations and computed detonation speeds.e. and the bands changed with addition of different substances to the flame (the spectrum was characteristic of the substance).  Phlogiston is conserved. metals and living beings). Profs. animals exhalate phlogiston by the mouth. . Question. studied flame propagation in 1868 and proposed the first flame structure theory in 1883. or O2? Answer: the closest idea is to -O2. Berzelius-1826 accurately measured the relative masses in chemical reactions and established chemical-formula notation (the first letter of the Latin name with the relative factor as a superscript. But metals release so little phlogiston that it floats in air. i. Prof. needed to let it escape from fuels. Diffusion model Burke and Shumann in 1928 made the first theoretical computation of non-premixed flame height and shape.Thermal Ignition: Semenov's theory of 1928 (uniform temperature). 173 (1953)). including turbulence. Steady-state and partial-equilibrium approximations. the founder of the U. von Kármán and G. (Back to Combustion) . FrankKamenetskii's theory of 1945 (with thermal gradients). acoustic-coupled instabilities. Finite rate Asymptotic methods for different chemical and flow time scales: activation energy chemistry and reaction-rate ratio asymptotics. Proc. producing his famous Aerothermochemistry notes and lectures (T. unsteadiness. Zeldovich theory of 1943. multiphase flow. ThermalZeldovich in the 1940s and Sivashiusky in the 1970s analysed thermal-diffusive diffusive model instabilities in two-dimensional flames. to compile and spread multidisciplinary knowledge on combustion science. CFD codes Simulation of multidimensional reacting fluid flow. Millán. Thermo-fluidTheodore von Kármaán. Thermal explosions. and a co-founder of the Combustion theory Institute. organised and led (under NATO and UN auspices) an international team. around 1950. of the Jet Propulsion Laboratory (1944). He also proposed in 1943 his thermal model for the formation mechanism of NO. Instit. 4.S. Combust. Institute of Aeronautical Sciences chemistry (1933). etc. ...................................................................................................................................................................................10 For fuel cells...........................................................................................................................................................................................................16 FUELS WHAT IS A FUEL A fuel (from Old French feuaile..................................................................................................................................................................................5 Gas .......................................................................................................................................................3 What is the problem with fuels? ........................................................................................ .........................................................................................................................................6 By marketing ......... i........................................... ..................................................................................................................................................................................................14 Biological fuels in the XXI c..........7 Petroleum fuels ......................6 Natural or primary fuels ............................................................................3 Hydrocarbon nomenclature ...11 The fireplace and the chimney ..................................................................................................................................................................................................................... ultimately from Latin focus fireplace.................................5 Fossil fuels ........... from feu fire.......................................................................................................................................................................................................................1 The oxidiser.................................7 Non-commercial...............................................................................5 Renewable fuels .............4 Fuel types .........................5 Liquid ....................................................................1 What is a fuel ............................................................................................................................. propellants and explosives .............................................................................................................5 By period of natural renovation ....................5 Solid .....................................................FUELS Fuels .................................................................. hearth) is a substance that may be burned in air (or any other oxidant-containing substance)............................................................................7 By application ........................ oil lamps and candles ............................................... ...................................................................................................................................................................................................................................................e..................................8 For compression ignition engines .......................................7 Commercial ......................12 Engines on biological fuels .........................................................................................................................................................................................................................9 For boilers .........................................................................5 By physical state ................................................................................................6 Artificial or secondary fuels ................................................................8 For spark ignition engines ...........................................................................................................................................................10 Fuel history ..........................................................................................6 By production stage (resources type) ..........................................................................10 For pyrotechnics....................................................................................11 Biomass fuels before the XX c............................................................................................................... that so quickly reacts with oxygen that heat and light is emitted in the form of a sustained flame..........................................................................................................14 Mineral fuels ...............................................................................................................................................................................................................................................................................................2 What is a fuel used for .............................................................11 Torches...............................................................9 For small portable applications ...............................................9 For gas turbine engines .................................................................................... and dephlogisted air (O2. Oxygen-enriched air (up to pure oxygen) accelerates combustion processes and give way to higher temperatures. only after Fe. nitrocellulose (-(C6H10O5)nwith n=300. which constitutes the conventional fuels. natural gas in air (approximated by CH4/O2/N2). Air is a constant-composition gas mixture. i. Besides normal air (21% O2). usually from the exhaust stream (exhaust gas recirculation. 40%wt). to form CaCO3. combustion processes). Before Lavoisier in 1776 gave name to most chemical compounds. but again the main Al-compounds are already fully oxidised. zirconium powder in carbon dioxide (Zr/CO2). CaO. both in the air as CO2. Lean-oxygen air is sometimes used as oxidiser (to diminish emissions). from which it was obtained with sulfuric acid) was discovered by Priestley in 1771 and found to be unable to support life or combustion. and it is found basically oxidised. which may range from nearly 0% up to 4%.9%wt in the whole Earth crust.Although the chemical reactions of wood in air. and only 0.e. the most abundant element of the Universe. all gases were named as air variants (the word gas was introduced by Paracelsus around 1520 to name intangible substances). with 20% to 21% oxygen. the ninth most abundant element). hydrogen. mephitic air (N2. and in the whole planet Earth it has 30%wt. although it combines endothermically with oxygen at high temperatures to get the unwanted NOx pollutants. hydrogen in oxygen (H2/O2). are all of the same chemical type (self-propagating highlyexothermic re-dox reactions. usually 'fuels' and 'combustion' only refer to easily flammable substances in air (the air is the oxidiser needed by a fuel to burn. It is also the most abundant in the hydrosphere. sodium in chlorine (Na(s)/Cl2(g)). IGR may be just 4% but in 2-stroke engines may reach 60%. In the atmosphere. Carbon has a poor abundance: <0. coal in air. In the whole ecosphere it is the first with 50%wt. Silicon follows as most abundant in the crust (28 %wt). the residue of combustion inside a bell jar) was discovered by Rutherford in 1772 and found to be 4/5 parts of normal air. in a 4-stroke engine. as bauxite (Al2O3) and silicates like Al2O3Si. a first glance on it seems appropriate). it fixed to quicklime. except for great variations in a minority component. but also by letting a sizeable amount of burnt gases within the cylinder in piston engines (internal gas recirculation. . where it comprises only a 3%wt. is scarcely available in the ecosphere. mainly fully oxidised as water in the hydrosphere. Oxygen is then readily available from Earth's atmosphere. water vapour. i. obtained by adding to ambient air a part of the burnt gases. as sand (SiO2) and silicate rocks (like CaO3Si). animal-fat in air. with 23%wt. there is no silane gas (SiH4) free in Nature. that is why it is the main oxidiser. Most combustion data for fuels refer to its burning with normal air. 'poisonous' to respiration.e. obtained by dripping an acid over a metal) was discovered by Priestley in 1766. fixed air (CO2. and in the rocks as CaCO3. 86%wt. flammable air (H2. The most abundant element on Earth's crust is oxygen. THE OXIDISER Oxygen in the air is the basic oxidant for fuels: nitrogen is basically inert. EGR). with typically 2% in volume at the Earth surface.. obtained by heating HgO2 or KClO3) was discovered by Priestley in 1774.1%wt in Earth crust. forming CnHmcompounds (hydrocarbons). but most natural Si-compounds are already fully oxidised. and it is needed in larger quantities than fuels. IGR). As for the fuels. with 47%wt.. so. Aluminium follows (8 %wt).2000) in any medium. but there is some part un-oxidised in living and fossil matter. etc. silicon in oxygen (Si/O2). it is second to nitrogen. whereas oxygen-depleted air may prevent combustion (usual fuels cannot be ignited if xO2<10%). ports and urban areas. • Pollution by fuels can be negligible if clean renewable fuels are used. and sources must be found (for energy. but these uses are minority. WHAT IS THE PROBLEM WITH FUELS? Several: • Fuels are dangerous. and secondary fuels (artificial fuels) may be difficult to manufacture. are used (see Fuel consumption. minerals). Primary fuels (natural fuels) may be difficult to find. to remove the ground for aeration. water. particularly meat and fish). from low river courses up to their dwellings) and waste waters out. once at hand. to say the less. paints (who has never used a coal chunk to draw).WHAT IS A FUEL USED FOR Fuels are mostly used as convenient energy stores because of their high specific energy release when burnt with omnipresent ambient air (or other specific oxidiser). most liquid fuels are cancerous). as just discussed. or even two different engines). • Fuels are pollutant when burnt (and even before.g. Fuels. and use.e.g. both locally and at a global scale. because they accumulate a lot of chemical energy that may be accidentally released. that their associated problems are but to be solved. causing economic and political instabilities). i. as well as storage and end-use details. Elaborating on the above problems: • Danger in fuel handling can be controlled and reduced to a relatively very low risk (in comparison with other accepted risks. to turn the potter-mill. e. with the only nuisance of safety (uncontrolled combustion) and pollution (toxic emissions during storage and when burnt. to throw weapons. natural gas has very low density. A common problem to all human needs (except air. to mill the grain. Besides metabolic energy needed by any living being and supplied by the catalytic reaction of food and oxygen (some 100 W for an adult person). or for chemical transformations. polymers from petroleum).). basically a fuel is a C-Hcompound that combines with oxygen to yield CO2 (a natural compound of no harm if it does not accumulate. transportation to a better place must be arranged. etc. A short-term palliative to vehicle-engine pollution is to force a dual-fuel system (with dual fuel-reservoirs and engine controls. for transportation of goods and people. and leaving the present more-convenient but more-pollutant fuels to highway and cruising. The atoms . to cook their meals. Energy is a basic need to humans. for telecommunications. for food (all food is perishable. • Fuels are difficult to handle: coal is very dirty. fuels are very easy to store. food. But. dirtiness.g.. for more details) for heat generation. but. and for electrical energy. lubricants. Storage is sometimes the most cumbersome stage. to bring potable water in (e. fuels are so convenient energy storage systems. in comparison with other energy store systems. Specific energy storage values for fuels are presented aside. as for biofuels) and H2O (the most life-compatible compound). using a non-pollutant fuel inside cities. for work generation. that we have to devote a sizeable effort within our limited capabilities. • Fuels are scarce (fossil sources are being depleted) and the sources are unevenly spread (most petroleum reserves are in the Middle East.. transport. the same fuel substance may be also used as a feedstock in chemical synthesis (e. as in transportation or sports). • Scarcity of fuels is like scarcity of water: what we mean is scarcity of cheap good-quality sources. descendants of homo habilis need energy to change Nature to better suit their needs: to heat their home. for industry. causing deathly thermal and chemical effects. crude-oil is too viscous. they are presently the major contribution to environmental pollution. and energy in general. in most cases) is that energy is not available at the location and time we desired. and so on. crude oil). Hydrogenated compounds may be: • Aliphatic (oils and fats. Water-like fuels seem the best to handle. etc. toluene (-CH3).e. Methanol comes from wood distillation or natural gas reforming. like sugars. miscible with water. graphite.e. and cycloparaffins or naphtenes). Alcohol derivatives (obtained from alcohols and isobutene. mixing alcohol and sulfuric acid). glycerols if three OH)). hydrocarbons may be: hydrogenated. • Aldehydes: R-CHO.. oxygenated. • Alcohols: R-OH (phenols if R is aromatic. reforming by synthesis with vapour or air. what is presently done by first gasifying it).• that make the fuel and oxidiser are preserved after combustion. cyclic (homo. and oil derivative. nitrobenzene (-NO2). • Esters: R-COO-R'. the problem is an ancient one: it is hard to become a farmer if you can find suitable wild plants. etc. water. or after hydrolysis from starch). Besides the ubiquitous carbon. Ethanol comes from biomass fermentation (directly from sugars. • From chemical reforming (e. . and with the addition of some external energy (freely available from the Sun) those atoms can be arranged to form the initial fuel and oxidiser molecules. glycols if double OH-group. organic chemical compounds of carbon and hydrogen atoms. used as solvents. little odour): alkanes (saturated hydrocarbons: paraffins (linear chain). and pure hydrogen (H2). converted to liquid.. isoparaffins (branched chain). so coal should be liquefied (i.. Volatile fragrant fats (also found on fruits and flowers).g. Used in the manufacture of chemicals. From acid+alcohol→ester+water: R-COOH+R'-OH→R-COO-R'+H2O. 2-rings: naphtalenes. or triple bond). HYDROCARBON NOMENCLATURE Most practical fuels are hydrocarbon mixtures (i. starch and cellulose. aniline (-NH2). and natural gas should be liquefied (as done with coal gases by Fischer-Tropsch process. Volatile flammable pungent liquids. the main exception are pure carbon (C. Oxygenated compounds may be: • Carbohydrates: Cn(H2O)m. Liquid fuels are a must. cracking of heavy molecules. Dehydrogenated alcohols. used in the manufacture of chemicals and as solvents. Some chemistry refreshing is appropriate here to better understand fuels: • Hydrocarbon nomenclature: • By type of additional atoms. air and space). in the lab. • Ethers: R-O-R'. alkynes (acetylenes). and maybe some additional ones). • By mixture fraction specification (all natural fuels are mixtures): • From fractional distillation of raw materials (wood. crude-oil is distilled. They have the divalent carbonyl group =CO. double. pyrolysis. nitrogenated. particularly for vehicle propulsion (in land. • Aromatic (strong smell): 1-ring: benzene.or heterocyclic). • By type of molecular shape. styrene (--CH=CH2)). but think on a fuel-cell car that runs on H2+(1/2)O2=H2O and uses off-line-produced solar electricity (in the garage or at the station) to recharge the tank by electrolysis H2O=H2+(1/2)O2. chain (linear or branched). alkenes (ethylenes or olefins). 3-rings: anthracenes. • Cetones (or ketones): R-CO-R'. It may be difficult to think of such a new gadget added to our cars that would regenerate the gasoline and air from the exhaust gases. animals and fuels. Thus. • By type of carbon bonding (single. 4-rings: tetracenes. since the 1920s). coal. like syngas). since coal is already a mixture). phenol (-OH). . crude-oil and natural gas) were formed slowly (during millions of years. ethers. Gas As natural gas. mainly at certain remote epochs.. alcohols. acetylene. gas. typically from 3% down to 2% U-235 in nuclear fuel rods). imines (R.). Two differences. diesel.g.Nitrogenated compounds may be amines (primary amines R-NH2. manufacture gas (from coal or oil residue) and biogas (from manure or sewage). if only 0. azides (RN3). Notice that the usual U. amides (RCO-NH2). nitro-compounds (R-NO2). Notice. nitriles (R-C≡N). by the way. up to 10 km depth. although large pressure might stabilise them also at higher depths and temperatures (at 300 km it might be 10 GPa and 1000 ºC). Table 2. however. They are then non-renewable at humankind periods. that nuclear fuel reserves are also short-term: the 2⋅109 kg present commercial reserves would last some 50 years. not uniformly.S. and that the usual British word is ‘petrol’. secondary R-NH-R'. Liquid As crude-oil derivatives (gasoline. sulfur ignites easily. whereas “reserves” refers to that portion of resources that can be economically recovered at today's selling prices.5% of natural uranium is profitable used as in present nuclear plants (0. FUEL TYPES BY PHYSICAL STATE Solid As coal (mineral). charcoal (from wood) and biomass (wood.(R'-) C=NH). Canadian. some 20 years latter in both cases). Notice that 'resources' refers to the total amount in Nature. and New Zealand word for gasoline is simply ‘gas’. whereas the rest dates from 150 million years) by high-pressure-decomposition of trapped vegetable matter during extreme global warming. BY PERIOD OF NATURAL RENOVATION Fossil fuels Fossil fuels (coal. Estimated reserves and availability of fossil fuels (oil-discovery peaked in 1960s and oil-production is expected to peak around 2007. producing a pungent blue flame.g. using today's technologies and under today's legislation. cyanates (ROCN). Fossil fuels are found trapped in Earth’s crust. oil derivatives (LPG). but also waxes. but also LPG at low temperatures. e. aluminium particles are used in the rocket boosters for heavy-lift launchers such as the Space Shuttle and Ariane 5). that new breeder reactors can burn not only the U-235 . metals and nonmetals (e. American oil was formed some 90 million years ago. times some 70% of burning depth of the enriched stuff. Commercial reserve-2000 Reserve/Consumption-2000 Coal 250 yr 1000⋅1012 kg 12 Crude oil 40 yr 100⋅10 kg 12 Natural gas 70 yr 150⋅10 kg. and will eventually be commercially depleted. imides ((R-CO)2-N-R').71% U-235 in natural uranium. dung). esters. etc. apply to commercial reserves of nuclear fuel: first. fueloil).. most of South Africa's diesel fuel is currently produced this way (they also use a 50/50 Jet A-1 substitute. industrial residues. • Solid: wood. acetylene. • Reformed from natural fuels (fossil or biomass. However. may be obtained by distilling the raw material. particularly with oil and gas. has been met in the past and in the present by primary fuels (biofuels in the past and fossil fuels during the last two centuries). In comparison with fossil fuels. crude oil is never used as a primary fuel because there is no economy (residual crude-oil products are cheaper) and because it is difficult to handle (being a mixture of very light and very heavy substances. Renewable fuels Renewable fuels (biomass) are formed in a year or a few years basis (synthetic fuels may come from fossil or from renewable sources): • Gaseous: biogas from anaerobic fermentation or gasogen gas from pyrolysis of biomass. They are also called synthetic fuels. BY PRODUCTION STAGE (RESOURCES TYPE) Natural or primary fuels Any commodity can be artificially produced.g. animal residues. its handling causes cavitation. to make machines work. Tb=98 ºC) was obtained by electrical discharge on a CO/H2 mixture (syngas). renewable fuels are more disperse. removal of sulfur. natural gas and biomass. by chemical reactions with heat. more moisture and ash content. or domestic waste). was made to react in presence of Fe. urban waste. • Fossil fuels: coal. Artificial or secondary fuels • Distillates from natural fuels (fossil or biomass. wood and fuel crops). Some pumping is usually needed. Synthetic liquid fuels are most promising because of their high energy density. but it may cost a lot. and so on. vapour traps and sticky clogs). synthetic gasoline. crude oil (not used unprocessed). Actually. to yield a liquid hydrocarbon blend (containing from methane to heavy waxes) from which gasoline-like and diesel-like fuels were obtained (e.g. first results date from 1897 when formaldehyde (HCHO. esters (biodiesel). plus alcohols used as additives and mixtures. liquid. fuel pellets (from wood or vegetable residues). half from oil. or from human activity waste (agriculture residues.fraction but the main U-238 fraction and other fissionable ores. desulfurated syngas (generated by passing water vapour over hot coal). . synthetic oils. The need for energy. nitrogen. and may be gaseous. They are obtained by mining (coal) or welling (oil and gas). and second. or solid: hydrogen. cattle manure. that nuclear ores have been only marginally prospected and price increments of several orders of magnitude are tolerable due to the small share of fuel price in nuclear power. half from coal). charcoal. humankind progress has always been based on finding raw materials that with no cost or little cost could satisfy their needs. and coke. where. • Biofuels (from biomass). have less energy content. and ash. but the main milestone is the Fischer-Tropsch process of 1923. but without chemical reaction): all petroleum derivatives. steam or partial air). • Liquid: alcohols. modern oil-refinery products really come from a combination of physical methods (distillation) and chemical methods (reforming and cracking). ethers (biopetrol). Ru and Co catalysts. 24H2+12CO=C12H24+12H2O). agriculture residues. Syngas preparation is the most expensive stage in the process due to materials handling: purification of input coal. charcoal. to transport people and goods. They can be directly taken from nature (e. and require more handling effort (but they are renewable). synthetic gases (syngas). and non-hydrogen non-carbon fuels. They may be directly used as fuels (burnt with air). Fe).g. Table 3. 2Al+(3/2)O2=Al2O3 is used for aluminothermy. are considered non-commercial or special fuels. • Special commercial fuels.. Old town gas. dung)... obtained by fractional distillation and reforming. used as intermediate energy stores. Mg. such as West Texas Intermediate. all vehicle fuels. and small and medium stationary applications fuels are petroleum derivatives. like metals (Si.• Methanol synthesis is another important process. metals (Fe. It was the main fuel for the Industrial Revolution in the XIX c. hydrazine: N2H4). with ρ=610 kg/m3. Most crude-oil is now traded in relation to the spot price of certain market crudes.12 730. as monopropellants (e. but also lignite. Si). black powder. since they are first produced from their oxides (with cheap natural energy) and afterwards burnt to form their oxides (providing valuable artificial energy).4 580 0. BY MARKETING Non-commercial Some biomass materials (municipal solid waste (MSW). The spot price is the price of an individual cargo of crude traded at a particular location. through dehydration to dimethylether 2CH3OH=CH3OCH3+H2O).g. They were developed in parallel with the corresponding internal combustion engine during the XX c. i. as said before (it only burns in uncontrolled fires). cars gases (LPG) Gasoline 300. and still traded for domestic use during the first half of the XX c. Mg. Exotic fuels (not obtained from fossil or biomass fuels): hydrogen from water electrolysis. Crude oil is not used directly as a fuel. • Crude-oil derivatives (see Table 3). manufactured from coal or crude-oil. is no longer on the market. or as intermediate products. North Sea Brent. Ammonia is not used as a fuel because of its high auto-ignition temperature (925 K) and the difficulty of complete burning (there is NH3 in the exhaust). Dubai.5 −100 domestic petroleum heating.760 0. Petroleum fuels More than 50% of world's primary energy comes nowadays from petroleum.5 −30 cars . Boiling Boiling Carbon Density Viscosity Flash Main use range range chain (liquid at 15 ºC) at 40 ºC point Tb [K] Tb [ºC] range ρ [kg/m3] ν⋅106 [m2/s] Tflash [ºC] Liquefied <300 <30 1. 2N2H4=2NH3+N2+H2). • Natural gas. might yield 109 kg/m3 of H2 whereas the most advanced hydrogen storage systems (metal hydrides) only store 25 kg/m3 of H2. Commercial • Coal... as a final fuel (CO+2H2=CH3OH).g.g.200 4. Main commercial fuels derivatives from crude-oil. or as an intermediate step to gasoline and diesel (e. Al.e. like acetylene (used for cutting and welding). It seems to be the main fuel for the immediate future.500 30. whereas Si+2H2O=SiO2+2H2 is used to produce hydrogen fuel (Al+3HCl=AlCl3+(3/2)H2 is only used in the lab).. but it might have a future as a hydrogen carrier since liquid NH3 at 20 ºC (p>854 kPa). Main commercial fuels and their physical data (that change according to the market requirements) are presented in Table 3. etc. NH3. e. or Alaskan North Slope.. Al. but today it is only traded to power stations and heavy industries. local industrial wastes. and their main averaged properties. mainly bituminous coal.. rocket propellants (e. with composition varying widely from nearly 100% propane in cold countries. MTBE (methanol tertiary butyl ether. whereas for use-alone up to 10% water can be accepted.14 500. 20% in Greece)..g. but some other alternative fuels exist. 100% in UK.. and less avid for water. • Methanol is rarely used. Most propane-fuelled-engines can work indistinctly with propane or gasoline (dual-fuel engines). or gaseous for carburated engines. and yields less emissions than gasoline (less than a half.3 kg/m3 at 20 ºC)... propylene. is used in Brazil) but added up to 20% to gasoline (E20-fuel or gasohol) to avoid engine changes.650 150. power plants and cogeneration plants because it is clean at entrance and exhaust. They are better additives than alcohols because they are not so volatile. It has good octane number (of order 100. heaters industry. C6H14O).20 780. The reference fuel is gasoline (also named petrol. . ETBE has lower volatility. • Ethers. and higher octane number (RON=116). CNG. Bioethanol is preferentially made from cellulosic biomass materials instead of from more expensive traditional feedstock (starch crops)..800 300. • Natural gas (mostly as a compressed gas.. regularly added in a 10% to gasoline in EU (but there are concerns about its cancerous properties).40 aircrafts cars. increasing the efficiency up to 40% (the engine must be specially tuned). Usually not pure but added up to 20% to gasoline.. RON=107.850 820. which is then fed into a reactor vessel in the presence of a catalyst to produce methanol and water vapour. obtained by catalytic reaction of isobutene with bioethanol (with only 55% isobutene. at its vapour pressure (around 1 MPa).. Anhydrous ethanol (<0. what allows for a compression rate of 12 instead of 9. 35% methanol).4%vol for water-in-fuel.350 10. (CH3)3-CO-CH3 or C5H12O).. LNG.Kerosene Diesel 450. 50% in the Netherlands.3%vol for fuel-in-water and 1. It is fed liquid to direct injection engines. is a petroleum derivate (65% isobutene. not so corrosive. particularly at small loads).. butane and butylenes. Little used in cars because of the storage. lower water-solubility (2. to only 20. mainly as a denaturaliser).e. 35% in France. at some 500 kPa).600 200. ships industry. A better alternative is ETBE (ethanol tertiary butyl ether.880 3 3 40 40 Fuel oil distillate Fuel oil residue 600.6% water) is required for gasoline mixtures. Must be heated BY APPLICATION For spark ignition engines Spark ignition (SI) internal combustion engine (ICE) fuels (Otto fuels) require knock retardation (oxygenated compounds increase the octane number (RON=research-octane-number) and decrease CO and HC emissions).30 840. shortened to 'gas' in the USA).300 10. increasing with propane content up to RON=112). i. boats. at 30 MPa. ETBE forms an azeotrope with ethanol (63% mol ETBE).. Methanol is produced by steam reforming of natural gas to create a synthesis gas. LPG is a mixture of mainly propane.1010 500 100 >800 >500 20.. that comes from petroleum): CH3CH2OH+(CH3CH)2=(CH3)3-CO-CH2CH3. but more and more used in slave-fleet buses. or nearly pure for new 'versatile fuel vehicles' (E80-fuel only has 20% gasoline. 30% in Spain.. but sometimes in as a cryogenic liquid. • Gasoline (Eurograde-95 in UE.500 15.. its solubility is only 4.930 10 60 930. Usually not pure (E100-fuel. ships. Premium-95 in USA).30% propane in hot countries (e. • Propane (or better LPG). lorries. • Ethanol and bioethanol. pure ethanol. RON=108. and it has RON=130. The gas is usually added by direct injection in the cylinders before ignition (to avoid high injection pressures). • Natural gas is used for stationary applications. • Natural gas can be used as main fuel in a diesel engine if a small amount of diesel fuel is used for compression ignition (dual fuel engine). C16H34. basically 99% kerosene with 1% additives to enhance cold operations and thermal stability. significantly reducing particulate-matter emissions (at the expense of some reduction in cetane-number. • Kerosene is used for mobile applications. • Pulverised coal • Fluidised-bed coal • Fuel oil • For small heaters (like domestic water heaters for space heating or for hot water) . For gas turbine engines As they are internal combustion engines (ICE). The same approach may be used to burn other high-autoignition-temperature fuels (e. however. It has high heating value and low flash point. the best is to discard the mixture. • Fuel oil (heavy fuel or residual oil) is only used in large marine engines.• Hydrogen is only used in research prototypes as a possible intermediate to future integrated hydrogen systems. colza. Aircraft jet engines currently use Jet A-1 fuel. • Diesel oil (gasoil) from crude-oil distillation. they cannot work with solid or heavy fuels. The first jet engine. Notice that a mixture of gasoline and kerosene makes no good for either the SI-engine (less octanenumber) or the CI-engine (less lubrication). In case of accidental mixing. although gas turbine engines are in principle more tolerant on fuel type than gasoline and diesel engines. • Natural vegetable oils (sunflower. RON=130. high-performance gas turbine demand high-performance fuels. provide the high temperatures needed for subsequent autoignition of the main natural gas loading. but some other alternative fuels exist. obtained by transesterification of natural oils) can directly substitute diesel oil in CI engines (a mixture of 30% biodiesel and 70% fossil diesel is on the market). up to 10% in volume of anhydrous ethanol (E10-diesel) may be burnt on unmodified CI-engines. decreasing pollutant emissions. non-toxic and biodegradable. hydrogen) in autoignition mode (diesel mode). defined as a cetane / methyl-napthalene mixture which has the same ignition delay-time as the test fuel). Hybrid solutions in which a small amount of natural gas injected in a small pre-chamber and ignited in a spark plug. Biodiesel is renewable. The use of diesel/bioethanol mixtures in CI-engines is being investigated..g. The reference fuel is named diesel (formerly gasoil). are being developed.20 times that of diesel) and glycerine waste. that is why it is measured as 'cetane number'. that flew in the Heinkel He 178 on 27 August 1939. n-hexadecane. used gasoline as fuel. For boilers • For external combustion engines (vapour turbines) and very large heaters. ignitionspeed and lubrication). • Biodiesel (a mono-alkyl-ester mixture. For compression ignition engines Compression ignition (CI) internal combustion engine (ICE) fuels (Diesel fuels) require very high injection pressure and low autoignition temperature and delay (as for cetane. besides. soybean) are usually not directly used because of their high viscosity (10. natural gas. etc. or get rid of by soaking with water. For pyrotechnics. • Acetylene (see Illumination. smokeless and non-hazardous are required: • For lighters (portable fire sources) • Matches (see Pyrotechnics) • GLP (see SI-engines). Keep gas lighters below 50 ºC. the maximum of any fuel. Since 2004. • Gasoline (see SI-engines) • Waxed-wick fire-lighters • For illumination • GLP (see SI-engines) • Kerosene (see SI-engines) • Waxes (candles) • Acetylene (C2H2. with two 75 kW PEM fuel cell stacks each. A typical workshop bottle of 40 litres. costing some 1 200 000 € each. and then connecting it to an upside-down GLP reservoir (enhanced by heating and shaking the reservoir and cooling the lighter).• • • Natural gas (if a network is available) LPG Diesel oil For small portable applications Fuel. but the storage problem is not yet solved satisfactorily. After cooking. or by hydrocarbon cracking. at 1. roughly). in liquid form. cutting) • GLP (see SI-engines. • For heating (cooking. Most used at movable cooking ranges and stoves.5 MPa. propellants and explosives (See Pyrotechnics. light it and left it for half-an-hour until the coal is ready to cook (it gets to embers covered by a fine grey ash). methanol. • Reformed hydrogen from other (fossil or natural) fuels (e. above). but it is usually traded in high-pressure bottles.pdf) . which are odourless. by passing an electric arc through calcinated limestone (CaO) and coal tar. the fire may be extinguished by air sophocation. 3500 K. pressed sawdust). and it is produced from CaC2 and water.). instead of the 200 000 € for a normal bus. Acetylene can be produced in situ. there are city buses powered by fuel cells operating in several cities (three. above). The oxy-acetylene torch is the common tool for manual cutting and welding. CaC2 was first obtained in 1892 while searching for aluminium synthesis.g. in Madrid. dissolved in acetone (and stabilised within a solid porous material) since pure acetylene may decompose explosively if p>205 kPa. and the unburnt fuel left in place ready for a new fire. Gas lighters are refilled by first letting the remaining gas escape (enhanced by warming up). It is used in chemical synthesis (80%) and welding (20%). The butane gas lighter dates back to 1933 and it is the most used today. BBQ-hints: (BBQ=bar-byqueue): place the charcoal in the BBQ-pan and some firelighters in the middle cavities. For fuel cells • Hydrogen is the nominal fuel for low temperature fuel cells. yields some 6 m3 of acetylene at room conditions (the flow rate should not be higher than 1 m3/h to avoid acetone carry-over). plumbing. • Barbecue pellets or briquettes (charcoal. or by partial oxidation of natural gas. ethyne). because of its high heating power and high combustion temperature. etc. wild fires must date from the beginning of terrestrial vegetation (evidence of fire has been found in coal deposits formed 350 million years ago). they are still appreciated for their aesthetic and natural flavour. Elementary carbon is not abundant on Earth (<0. It must be realised. and the same in Anatolia 200 000 years ago. H-compounds or Si-compounds will take over? The first biofuels used were: firewood. and as a strong fuel in metallurgy. The process of smoking meats and fish might have been naturally discovered that way. animal fat. where Bunsen mastered the premixed combustion with gaseous fuels in his famous burner.FUEL HISTORY Humans must have mastered fire some 500 000 years ago (from the time of Homo Heidelbergensis). the importance of the wall surrounding the fireplace was appreciated to the point of making a dedicated smoke tunnel: the chimney. The term biomass (and biological fuels) is usually restricted to living matter (not fossilised). i. but when the roof was added to protect from the rain. but it is the basic constituent of life and it is found unoxidised on all living and fossil matter. was used as a paint pigment in the Palaeolithic. The fireplace and the chimney Hearths were used for lighting. religion. For instance. and perpetual fires were maintained in front of principal temples. but the first fuels used by humans were from biomass (living matter used as a source of energy. excavations at Torralba (Spain) suggest fire-hunting for elephants.e. Since 500 000 years ago. however. animal dung. may be the best solution to heat with wood today. Chimney fireplaces have been in use at every home for heating and cooking until the middle of the XX c. Would it continue like that in future?. Perhaps the next step was to have fireplaces in the open but surrounded by large stones that protect against gusts (and radiate heat). communication. later a large flat stone and since the XIX c. An opening in the roof was left to get rid of smoke and. Most fuels used nowadays are fossil (remnant of plants that existed in the distant past). but soon the benefit of having the fireplace protected by a shelter wall was discovered and the hearth enter the cave.1%wt in the crust). Charcoal.. charcoal. They used to light the flame by the sun’s rays captured at the centre of a recipient called a skaphia (the ancestor of the parabolic mirror used today for lighting the Olympic flame). notably firewood). and vegetable oils. cooking. horses. how embarrassing a fireplace in a modern house may be. already in recent centuries. or some new kind of fuels like artificial C-compounds. The first human-controlled fires might have been done out in the open where smoke presented no problem. up to Classical Greece (myth of Prometheus). Of course.C. The simple fire place was an earthy hearth surrounded by stones.. But only non-premixed flames with condensed fuels were used up to 1850. to its take-over by coke in late XVIII c. heating and cooking. fighting. humankind used natural carbon-compounds as their main fuel. BIOMASS FUELS BEFORE THE XX C. heating. an enclosed wood stove with glass doors on the fireplace. although no longer needed because of oil and gas heaters and cooking ranges. with the smoke burden. perhaps the first severe anthropogenic environment pollutant (metabolic waste is not so asphyxiant).. wild cattle. deer and woolly rhinoceroses 400 000 years ago. the problem of smoke pollution and fire safety was aggravated. from its earliest developments in 5000 B. partially burnt wood to yield a more energetic and biological-inert fuel. and remember that burning wet or green wood generates . a flat metal plate. Primitive humans must have used fire for lighting. and. Fire was a sacred element to most ancient cultures. dating from around 20 000 B. suggesting to narrow the air entrance by covering most of the front with a hanging cloth (or better a plaster wall). .C. which is covered with heavy refractory masonry. so they would radiate better. electric light marked the decline of fuel lamps. later enhanced to a reed or tow soaked in molten fat or oil.C. the carbon arc lamp. 1). with widely angled side-walls. The next light source. and enclosed chambers (furnaces) are required to reach higher temperatures.. a burning branch plucked from a fire. looked like saucers (later on with a groove) and burned olive oil in a pottery or bronze container (a modern oil lamp made of brass is shown in Fig. a kind of torch with liquid fuel pumped up by capillarity. on a column of hot air of height L: ∆p=∆ρgL. the first one dated comes from an earthquake in Venice in 1347. Thompson (Count of Rumford) in 1796 suggested to make the fireplace shallower. when the incandescent carbonfilament lamp was also developed (by Thomas Edison in the 1880s). only were in widespread since 400 B. Franklin in 1745 wrote some guidelines to avoid smoky chimneys. taking over first the traditional tungsten-filament incandescent bulb (developed in the 1920s.C. A chimney cap is always used to keep out the rain. Torches date back at least to 50 000 B.C. and later by the fluorescent gas-discharge lamp (in widespread use since the 1940s. B. and being retired in the 2010s because of its low efficiency). A chimney works on the Archimedes' principle: generating a pressure-imbalance draught. Much later on. Rosin in 1932 that the sudden expansion is counterproductive). painted wood or glossy paper that generate toxic gases). the first remains were found at Lascaux famous painting cave. to keep smoke from coming out into the room. and not just the front side roasted and a frozen back). Although the carbon arc lamp is assisted by the burning of graphite rods. oil lamps and candles Special devices were invented to transport fire (portable burning appliances). The first portable fire was the torch. was not developed until the early 1800s (by Sir Humphry Davy). and with a streamlined flue throat with a sudden expansion to avoid smoke rebuff (it was shown by P. which practically disappear from 1900 onwards. Oil lamps were common in Egypt in 3000 B. must have been developed. say. animal fat in a bowl (sea-shell or skull) with a grass wick. or naturally impregnated with resin or pitch. 900 K. it is difficult to approach. Chimneys get coated with soot and. if not cleaned regularly. B. made of clay and burning seed oil with a cotton wick.C. although introduced around 1900). Greek lamps from 500 B. Torches. The chimney is a rather recent invention. being able to remove more heat from already warm walls by air convection than the chemical heat supplied by the fuel (that is why walls extended around fireplaces to have comfort within. may catch fire. although known before. For the same reason. and bird droppings. mainly for lighting.O. and to widen the flue. A chimney is a very effective ventilation set-up (much more than a whole in a wall). to inhibit downdraughts and as spark arrestor. ∆p. leaves. Modern fireplaces have a pre-cast iron box with a pre-cast streamlined chimney throat. Candles.creosote vapours that may inflamate in the chimney (and never burn plastics. although it was not in widespread use until late in that century. are formed at some 1500 K (the hottest zone. it would suck too much fuel. by weaving the cotton fibres flat and treating them with a mordant (e. combined with glycerine. Bee-wax candles were used by wealthy Romans as a rarity: they produce a bright flame. or casting. blue at the well-ventilated bottom. Candle usage widespread during the Middle Ages to light monasteries and churches. Spermaceti candles were made of oil present in the head cavities of sperm whales. Since the early XIX c.C. is found off the centre on the edge of the brighter yellow portion). Improvements on the wick were also achieved in mid XIX c. brighter and longer lasting wax. living standards improved as evidenced by the increasing availability of candlesticks and candleholders and their appearance in households. by radiation to melt the solid fuel. usually from sheep or cattle. what causes the wick tip to stand. the candle flame flickers a lot and may be extinct. The candle was known in Egypt (clay candle holders are found dating 3000 B. by removing glycerine. dimmer light and higher pollution than oil lamps. and that. form . Chevreul discovered that tallow was a mixture of two fatty acids. stearic acid and oleic acid. or extrusion.). or wax.g. generating gases that diffuse through the inner cold bulb and reach the thin combustion zone. had a dark yellowish colour and gave off a nasty smell. he achieved a harder. enhancing safety and handling (a no-spill lamp). or rolling. where it is pyrolysed at some 900 K (dark zone in the flame). what causes the fluffy wick tip to bend within the cold zone of the flame and fall into the molten wax. or by moulding. Candle making may be by a process of sequential dipping. The solid fuel melted at some 55 ºC to 65 ºC by radiated heat. Japanese and preColombian Americans also used candles. or better of stearin. migrating up the wick by capillary action. yellow at the richer upper part. instead of just twisting the cotton fibres. up to 1700 K. onwards. tuned to feedback heat.Fig. The candle is as a kind of solid-oil lamp. including soot particles. by reaction with outside air. ordinary candles were made of animal fat (tallow). in absence of a force field. the candle slowly burns with a very precarious spherical blue flame). From the XVI c. as under microgravity. candles are made of crude-oil bleached paraffins. causing a mesh if not snuffing (cutting the charred part of the wick). after the chemist M. boric acid). they burned with so a bright light that a spermaceti candle flame was used as a standard light measure for photometry. but candles were not used because of the higher cost. A smoky kerosene oil-lamp. whose flow is smoothly coupled to burning rate through buoyancy convection (notice that if air is artificially supplied. curl and burn. do not smoke.E. 1. and produce a fragrant odour while burning. made of frozen oil. If the wick would not curl and lean out of the flame and burn. Chinese. where products.. semisolid fat. Candles were made by coating a wick (the fuel pump) with pitch or animal fat. the fluorescent lamp was invented in 1938). and smoke. its second consumer. that allowed air to reach the centre of the flame. Around year 1880. driven by the Industrial Revolution hunger to feed steam boilers. cars were designed to burn the coarse petrol distillate of the time. what means that fossil-fuel emissions must be artificially counterbalanced (e. Up to the modelT Ford. but only used when available on site. pushing combustion illumination to the corner. alcohol in Otto’s engine. and as a home heating fuel in England since Roman times and during the Middle Ages. the tonne-coal-equivalent (1 tce=30 GJ) unit was used. with two direct consequences: • They are short-term commercially available (they will be exhausted at current trends in one or two generations). introduced the hollow-cylinder wick. but wood has a small heating value and was been exhausted. less ash content. the Otto engine and Diesel’s engine. also in England. History of coal Coal was known since ancient times. Their major drawback is being not renewable. and up to mid 20th c. is the condensation of vaporised hydrocarbons not yet pyrolysed (it can be easily lighted). which increases the greenhouse effect. Whale oil and colza oil were most used at this time. History of natural condensed hydrocarbons . primary world energy came from traditional biomass and coal 50%/50%. and natural gas. But during the XX c. and burying it in a non-pollutant way). In the mid XIX c. A major development in the oil lamp took place in the 1780's when Argand. except for coal. lower moisture content. and liquid biofuels were too much expensive. They are basically coal. later substituted by the tonne-oilequivalent unit (1 toe=42 GJ). The white smoke seen when a candle is blown out. or any mixture of them. the petroleum industry swept all the fuel markets. and. • The increase pollution. They have several advantages over biomass: fossil fuels are more concentrated. but mining really started in the XVIII c. all started their development burning biomass fuel: wood in the boilers. without short-term natural recycling. by getting back excess atmospheric CO2. coal also replaced charcoal in making black-powder and explosives. more constant composition. yielding a brighter flame. until kerosene came into scene in mid XIX c. or alcohol (distilled from fermented sugars). MINERAL FUELS Mineral fuels were later discovered and only in the last three centuries massively used (up to depletion!).too much soot. his assistant Quinquet added the glass chimney that bears his name (Da Vinci also sketched it). and vegetable oil in Diesel’s. a Swiss chemist. Olive oil lamps made in brass have been in use in rural Spain up to mid 20th century. petroleum. and particularly global warming. in Wales during the Bronze Age. Engines on biological fuels Steam engines. A great amount of coal is also used in the iron industry. have higher energy density. otherwise charcoal from wood was used.g.. Coal was used in China in 1100 BC. but not until Coolidge's ductile wolfram replaced Edison's wretched carbon filaments (incandescent lamps have been improved by filling with gas and most recently by using a halogen gas to recycle vaporised wolfram. and finally Thomas Edison invented the incandescent light in 1879. Latin petroleum stone oil). Tpour>0 ºC). History of natural gas Humans must have found many natural-gas sources seeping from the ground (by their hissing or bubbling. roof and road paving). and hydrogen-sulphide content. In the XIX c. or the 32 gallon London ale barrel. but in USA. releasing a 50/50 methane/air mix. Crude oil has always aroused interest as a substitute of vegetal and animal oils. where E. seven years after Drake drilled his well. the 31. the most famous might be one on Mount Parnassus around 1000 B. It was in 1866. that Pennsylvania producers confirmed the 42-gallon barrel as their standard volume unit (not a real container). Drake found oil in 1859 in a 23-m deep well. Ohio) to produce kerosene. barges. odorants must be artificially added for safety alarm). New York. John D. hydrocarbon dewpoint. that flows. it may be crude oil (petroleum. adhesive and waterproof (boat.C. and used for lighting. Today only fluid crude-oil is put in the market. for lighting purposes. Tflash<65 ºC. sealing boats (Noah's Ark) and paving streets. Pennsylvania. because they usually have no smell. generically called bitumen (Latin bitumen sticky) were found in Mesopotamia 5 000 years ago. Tpour<0 ºC) or asphalt (Tflash>65 ºC. as opposed to. It is only after 1950 that big gas pipelines (gasoducts) were built (in USA and Russia). Crude oil is always transported in bulk by tankers. Crude-oil today is usually measured in 'toe' (metric ton oil equivalent). pipelines.). and all of it is distilled. by William A. and asphalt (Greek ασφαλτοσ binder) that does not flow. oil well drilling started. or the 36 gallon London beer barrel. natural gas was used locally in many USA states. Two kind of bitumen were found: crude oil (or petroleum. Drake refined the crude oil into kerosene for lighting. Greece. As demand for kerosene for illumination grew. just after a few years). there are many other barrel and gallon sizes). unsuitable to gasoducts. the basic unit remains the 'barrel' (of 42 gallons of 231 cubic inches. and gasoline and other products made during refining were simply thrown away because people had no use for them. heating. Natural gas for transmission companies must generally meet certain pipeline quality specifications with respect to water content. heating value. and time to fill them. The generic name for natural condensed hydrocarbons is bitumen. gluing bricks together. and. Persia. The first success was in Titusville. This first well intentionally drilled to obtain natural gas was drilled in 1821 in Fredonia. handled at >100 ºC to overcome yield stress. with >80%wt C and >15%wt H. according to the fluidity. Rockefeller enters the oil refining business in 1870 forming the Standard Oil Company (Cleveland. sticky or liquid). Hart: a 9 m deep well to enhance a surface seepage of natural gas. say. Egyptians coated mummies and sealed pyramids with pitch. and trucks. but get mixed with air in mining. They are non-crystalline highly viscous. Coal seams store CH4 inside micropores. though there are no more old-fashioned wooden crude-oil-barrel since Drake's time except in museums (they run out of barrels. . In 1771 George Washington (the 1st USA President) acquired a piece of ground in what is now West Virginia because it contained an oil-and-gas spring. asphalt being the residue (more than 80% of asphalt production is used for road paving.Natural condensed hydrocarbons (solid. black or dark brown pastes. or lately as a water emulsion with 30%w asphalt particles about 1 µm size). but the ones they could not miss were the burning springs (ignited by a lightning) known as 'eternal fires' referred to in most ancient traditions (India.5 gallon wine barrel. specially for transport applications. to improve sanitation). sewage. soya). animal waste. Besides. municipal solid waste (MSW)). but their importance during the Industrial Revolution fade away. fossil fuel depletion indicators. • Biomass fuels are also contaminant. dictated that he demonstrated his engine at the World Exhibition in Paris in 1900 using peanut oil as fuel (biomass). and fossil fuels were slowly 'cooked' over the aeons to separate water and most of the oxygen. because living matter is roughly a water suspension of oxygenated hydrocarbons. Some biofuel production methods considered are: • Ethanol by fermentation of biomass sugars. biofuels are at the stage again. although other times distinctions are introduced and then biofuels may refer to biomass derivatives directly substituting fossil fuels for the same combustor. Biomass fuels have always presented several problems that might have been forgotten during the two centuries when we have profited from massive fossil fuel sources: • Biomass-fuel sources are not found concentrated in Nature (contrary to fossil-fuel fields). and. domestic waste). straw.g. living matter and their residues are a handy alternative to dying fossil fuels. not contributing to global warming (because the CO2 produced compensates with that synthesised from the air during the biomass growth). and local and global pollution associated to fossil fuels. dioxins) if not properly treated. some biomass fuels have non-fuel components that must be separated (e. he intended to burn coal powder (fossil). but producing much more particulates and new chemical emissions (e. Late in the XX c. soil in forest-waste.g. metals in industrial waste).g. the kind of fuel best fitted to both engines and stationary combustors. forest wood waste. as fuels continue to be the best solution for energy storage. biomass may be restricted to unprocessed biomass (forest waste. Use of biomass fuels has never been abandoned. • Biomass fuels are mostly solid.g. biomass fuels and renewable fuels. starch or cellulose by yeast or bacteria. may be used indistinctly if they refer to natural or artificial fuels obtained from renewable sources.BIOLOGICAL FUELS IN THE XXI C.g. colza. urban solid waste). In Japan. • Biomass fuels are less energetic than fossil fuels. sunflower. The terms: biofuels. but problems with that choice (later solved with oil fuels). although sometimes it is compensated by the need to get rid of that matter (e. a bacteria has been bred which produces ethanol from paper or rice-straw without any pretreatment.. and renewable may include fuels like hydrogen obtained by electrolysis and not from biomass. • Oils (biodiesel) by reforming oleaginoseous plant seeds (e. and some pre-processing is needed (gasification. A good example of this biomass back up in those days is that when Rudolph Diesel developed in 1893 his compression-ignition engine. crops and agriculture waste. liquefaction) to produce fluid fuels. to prevent forest fires. and there is an inherent inefficiency in collecting them (e. have being pressing to come back from 'black fossil power' to 'green renewable power'. • Methane (actually a biogas mixture) by anaerobic digestion of biomass waste (manure. And for the time being. But the move to biofuels is not based on their short-term advantage over fossil fuels but on the longterm need to have fuels of any kind. The marine microscopic algae Botryococcus Braunii has been shown to accumulate a quantity of hydrocarbons amounting to 75% of their dry weight . but what is life: a self-organised system based on water photolysis? For mobile applications. or from water electrolysis by solar or wind energy (of course.g.• • Methanol from wood-waste distillation. this sometimes called 'solar fuel' is not bio in the sense that it has nothing to do with living matter. Another possible biofuel in the future may be hydrogen produced biologically. however. by a photosynthesis of hydrogen instead of carbohydrates (there are some algae that do that). e. Producing hydrogen by other sustainable means (direct solar energy or wind energy). As an aid in transition from fossil fuels to biofuels. Hydrogen by reforming other biofuels (e. producing methanol or hydrogen from natural gas). seems closer in the future.g. because of higher energy density and simpler infrastructure.g. mixtures of both are being progressively put on the market for old engines and combustors. and new engines and combustors are progressively developed to run on 'biofuel prototypes' derived from current fossil fuels (e. gaseous biofuels being restricted to stationary applications. ethanol or methane). liquid biofuels are preferred (ethanol and biodiesel). (Back to Combustion) . ............12 Purity .................................................................................................................................................................................................... etc.........................................13 Uses .......................................................................................... most of the times................g..........FUEL PROPERTIES Fuel properties......... the explosion limits depend on the boundary conditions for a given fuel/oxidiser pair)............................. on particular fuels.........................................................................................................................................................................pdf..............16 Biomass .... LPG and methane hydrates ....6 Heavy fueloil ..... However.......................................................................................................................................................................13 Coal ..................................................................................15 Heating values .......... can be found apart in Fuels..........................................................16 Composition ................................14 Air requirement for theoretical combustion ....................................................................................................................................................g................................................................... Kerosene............... ..................................................................................13 Types .................................................................................................................... air..........................13 Origin ..........................................18 Fuel pyrolysis ......................................................................................................... chemical formula.......................................... and Jet fuel ................ mainly physicochemical data..................................................................................................................................................................................................................................................................................... density............................ and so on.......................................................................................16 Wood .......................12 Price comparison of hydrogen energy ........................................................................... can be assigned some physical and chemical properties (e.... vapour pressure............................. biogas... as for any other type of substance.........2 Bioethanol and ETBE .................. Fuel price..................... in spite of the fact that these properties depend on the oxidiser (e.............................................................................................................................14 Composition: proximate analysis and ultimate analysis ..................g.................................................................. What follows is just a collection of additional notes................. A summary table of fuel properties for normal combustion in air can also be found there............ including some relevant properties..............................................................................................1 Crude oil ..........9 Storage & transport ........................... risk............................................9 Production ....................6 Fueloil ..........................................5 Biodiesel.................................... Fuel consumption and Pyrotechnics are also covered separately.............................................................. thermal capacity....................................................................................... combustion properties are also assigned to fuels....................................................................................................................................................2 Gasoline .....................................7 Natural gas..........................................................................................................................11 Safety .......................................................................4 Jet fuels ... An introduction on fuels and fuel types...............3 Diesel oil...............13 Hint: Hydrogen balloon combustion & explosion ........ availability. could also be considered fuel properties (attributes).......7 Hydrogen ........ pure oxygen) and the actual process (e............................................................................................................................................................................................................................................................19 FUEL PROPERTIES Fuels................................................................. Eurosuper-98 (both lead-free). Vapour pressure. some 10% in weight of gasoline is in the vapour state at 300 K.1%. Exponential temperature variation fit.. . Vapours are heavier than air (2 to 3 times).2010-6 m2/s at 20 ºC. as the more volatile (and less dense) components are lost. Price: for a 100 $/barrel (very variable). brown or dark-green colour. the density of some crude oils may increase enough for the oils to sink below the water surface. Boiling and solidification points. and liquid-vapour above 280 K. vapours start to decompose at about 900 K. which could not then be economically transported long distances.5. after considerable evaporation.80 kPa at 38 ºC). Ignition limits: lower 0.7 MJ/kg.. floats on water). Crude-oil vapours are mainly short-chain hydrocarbons (only about 10% in volume have more than 4 carbons).CRUDE OIL Crude oil is not used directly as a fuel but as a feedstuff for the petrochemical factories to produce commercial fuels. but since 1950. when heating a previously subcooled sample at constant standard pressure. The density of spilled oil will also increase with time. resins 5%wt. Viscosity=510-6. and some 90% when at 440 K). so that.. Flash-point and autoignition temperature: some 230 K and 700 K approximately. LHV=42. fluorescence-spectroscopy and infrared-spectroscopy techniques. Surface tension: 0. Typically 900 kg/m3 (from 700 kg/m3 to 1000 kg/m3 at 20 ºC. Pour point= 5. that can be established by a combination of gas-chromatography..029 N/m with its vapours. Density. and additional chemicals.. Oil refineries were originally placed near the oil fields.g. Viscosity=0.15 ºC. Linear temperature variation fit. When heating at 100 kPa a frozen crude-oil sample (from below 210 K). Theoretical air/fuel ratio: A=14. Average Eurosuper values are: HHV=45. plastics.9 MJ/kg. 0. Sulfur content is 0. solid-liquid equilibrium may exist in the range 210 K to 280 K. for instance.90 kPa at 20 ºC. in part because natural gas.20 kPa at 20 ºC (40. Vapour pressure. 5. (e. Heating value.15%. Thermal expansion coefficient=90010-6 K-1 (automatic temperature compensation for volume metered fuels is mandatory in some countries). typically 70 kPa at 20 ºC.6 $/GJ.. and that may be used. Each crude-oil field has a different composition. Heavy metals <100 ppm. aromatic or sulphide odour. <0. Saturated hydrocarbons content is around 60%wt.5 kg air by kg fuel. for strategic reasons crude oil was transported by tankers and oleoducts to local refineries. Composition. Most data are highly variable with crude-oil field.5. upper 7.510-6 m2/s at 20 ºC. Not well defined because they are mixtures. In the USA: Regular (97 RON) and Premium (95 RON). The characteristic time for evaporation of crude-oil spills at sea is 1 day (25% in volume evaporated). Organoleptic: black.4%wt.. typical ranges are given. 50. Freezing and boiling points. synthetic rubbers.63 $/L.023 N/m with water. with 159 L/barrel it is 0. aromatics 30%wt. Density=750 kg/m3 (from 720 kg/m3 to 760 kg/m3 at 20 ºC). GASOLINE Types. Solubility.2%wt.. due mainly to volatile compounds. In EU: Eurosuper-95. was available to fuel the highly energy-intensive refining process. it is 16. and with 900 kg/m3 and 42 GJ/toe. crude-oil derivatives may be associated to their source field). in forensic analysis of oil spills at sea (even after refining. Anhydrous ethanol (<0. . in some 0.2H12. value added tax 13.. yield soot. to prevent knocking. 16% iso-pentane. Table 2 presents some data.9 -C-) 5% olefins (alkenes) bad smell (is decreasing) 35% aromatics (benzenes) toxic. In Europe.20. This is a measure of autoignition resistance in a spark-ignition engine. Its typical hydrocarbon composition is presented in Table 1. i. station benefit 6. Gasoline composition*. Solubility in water depends on the actual compounds: hydrocarbons are very insoluble in water. low RON (is increasing) 30% branched (iso-) high RON 15% cycle 40% unsaturated (5.006%wt Price. Pb(C2H5)4.099 kg/mol. high RON <500 ppm Sulfur in 2000 <100 ppm Sulfur for 2005 *A sample showed 21% cycle-hexane. 15% toluene. Pure bioethanol (E100-fuel) is by far the most produced biofuel. a colourless oily insoluble liquid. 3% naphthalene. 16% ethyl-bencene. Gasoline composition has changed in parallel with SI-engine development. More widespread practice has been to add up to 20% to gasoline by volume (E20-fuel or gasohol) to avoid the need of engine modifications. mainly as a denaturaliser). Solubility data at 25 ºC of some gasoline compounds. 17% iso-octane.8 -C-) 15% lineal (n-) best combustion.06%wt cyclohexane 0.e.6% water) is required for gasoline mixtures. meaning that gasoline has a relative large time-lag between injection in hot air and autoignition. Nearly pure bioethanol is used for new 'versatile fuel vehicles' (E80fuel only has 20% gasoline.6). sulfur was removed at that time because it inhibited the octane-enhancing effect of the tetraethyl lead. Average molar mass is M=0. the price structure is roughly: refinery output 20.. special fuel tax 60.01%wt iso-octane 0. although this is irrelevant in typical gasoline applications (spark ignition). in % of retail price. was used as an additive from 1950 to 1995. with variations of 30% amongst countries (in USA some 1 €/L). and ultimate analysis (by weight.0003%wt 0.Octane number (RON)=92. Substance Solubility of substance in water Water solubility in substance ethanol (& methanol) 100%wt 100%wt benzene 0. and used as a gasoline blend.98.5 €/L. and all other <1%.006%wt 0. about 1. being the volume percentage of iso-octane in a iso-octane / n-heptane mixture having the same anti-knocking characteristic when tested in a variable-compression-ratio engine.1 grams of lead per litre.18%wt 0. 12% n-decane.. but alcohols readily mix. Lead tetraethyl. 60% saturated (4. see coal analysis below for more details): 87%C and 13%H (corresponds roughly to C7. mainly in Brazil and USA. Composition. Table 2. Cetane number=5. In Europe in 2013.. whereas for use-alone up to 10% water can be accepted. Bioethanol and ETBE Bioethanol is bio-fuel substitute of gasoline. it is ethanol obtained from biomass (not from fossil fuels). Table 1. transport 1. In a batch fermentation process. a bacteria has been bred which produces ethanol from paper or rice-straw without any pre-treatment. The alcohol leaves the top of the final column at 96% strength. LHV=36 MJ/kg). free of sulfur and with shorter and branched carbon-chains more resistant to thermal breakdown. Steps processes in ethanol production are:  Milling (the feedstock passes through hammer mills. The meal is mixed with water and an enzyme (alpha-amylase) and keept to 95 ºC to reduce bacteria levels and get a pulpy state. it is used in rocketry usually with liquid oxygen as the oxidiser (RP1/LOX bipropellant). The tendency to change to biofuels or GTL fuels is also applicable here. Kerosene is a crude-oil distillate similar to petrodiesel but with a wider-fraction distillation (see Petroleum fuels). multicolumn distillation system where the alcohol is removed from the solids and the water. a byproduct sold to the carbonate-beverage industry). and the residue from the base of the column is further processes into a high protein-content nutrient used for livestock feed. In all cases. The mash is cooled and a secondary enzyme (gluco-amylase) added to convert the liquefied starch to fermentable sugars (dextrose). because kerosene has no cetane-number specification and thus it may have large ignition delays (producing lots of unburnt emissions and engine rough-running by high-pressure peaks). . MTBE (methanol tertiary butyl ether. KEROSENE. DIESEL. Bioethanol is preferentially made from cellulosic biomass materials instead of from more expensive traditional feedstock such that starch crops (obtaining it from sugar-feedstocks is even more expensive). which grind it into a fine meal). but alternatives are increasingly being developed for partial or total substitution of petrodiesel.ETBE (ethanol tertiary butyl ether. to make it unfit for human consumption. the fermenting mash is allowed to flow. ETBE-15 fuel is a blend of gasoline with 15% in volume of ETBE. or cascade. and diesel-fuel less cold-start ability. C6H14O. Contrary to its etymology. also named gas to liquid fuels. The mash is pumped to the continuous flow. The other gasoline-substitute ether. and synthetic diesel (usually from a gas fuel coming from coal reforming or biomass. not so corrosive. through several fermenters until the mash is fully fermented and then leaves the final tank. AND JET FUEL Diesel fuel is any liquid fuel used in diesel engines. diesel nowadays must be free of sulfur. Diesel and kerosene should not be taken as fully interchangeable fuels at present.  Distillation: The fermented mash contains about 10% ethanol. ETBE is obtained by catalytic reaction of bioethanol with isobutene (45%/55% in weight): CH3CH2OH+(CH3CH)2=(CH3)3-CO-CH2CH3. originally obtained from crude-oil distillation (petrodiesel).  Fermentation. as well as all the nonfermentable solids from the feedstock and the yeast cells. Jet fuel is kerosene-based. To note that isobutene comes from petroleum. In Japan. most ethanol plants use a molecular sieve to capture the remaining water and get anhydrous ethanol (>99. and less avid for water. with special additives (<1%). 35% methanol). kerosene has less lubricity. the mash stays in one fermenter for about 48 hours before the distillation process is started. such as biodiesel (from vegetal oils).e.  Saccharification. GTL). is a better ingredient than bioethanol because it is not so volatile. Using a continuous process. besides.8%wt pure). =760 kg/m3. (CH3)3-CO-CH3).  Denaturing: Fuel ethanol is denatured with a small amount (2%-5%) of some product such as gasoline. less lubricant). present-day kerosene and derivatives are less waxy than diesel (i.  Dehydration: To get rid of the water in the azeotrope. is a full petroleum derivate (65% isobutene. Rocket propellant RP-1 (also named Refined Petroleum) is a refined jet fuel. Yeast is added to the mash to ferment the sugars to ethanol and carbon dioxide (CO2. 860 kg/m3 at 40 ºC).5..2% S)....2%): corrosion inhibitor. 38 ºC for Jet A-1 and JP-8 (typical value for Jet A-1 is Tflash=50 ºC.. same properties as type A.e. B for industries (agriculture. and No. In EU: type A for road vehicles.. Composition. plus special additives (1. some 13. with whole number of atoms and typical carbon-chain-length.6 MJ/kg. is used in very cold weather. anti-icing.. special fuel tax 60.010-6 m2/s for biodiesel.. calculated from the fuel density and viscosity). with a freezing temperature of Tf=50 ºC (47 ºC as a limit). Jet A. as C11H21. by weight. LHV=43 MJ/kg (HHV=40 MJ/kg for biodiesel). value added tax 13.6. 4.5% kerosene for JP-5 and JP-8. with composition distribution from 5 to 15 carbon chains).2 (e. may yield some 84. 100% kerosene for Jet A-1). or C12H23.43. 880 kg/m3 for biodiesel (860. In Europe. From the ultimate analysis one may establish a reduced molecular formula (per unit carbon atom) of CH n with n=1.15% H. some 66% of saturated hydrocarbons (linear and cycle chains). aviation gasoline (avgas) is used to power piston-engine aircraft. In Europe in 2013. in % of retail price. C for heating (not for engines. or structural (mass fraction of identified molecules). 0. and Tflash=20 ºC for JP-4. with larger cetane numbers having lower ignition delays. 2 Distillate (Diesel). JP-5.8. 99.900 kg/m3 at 40 ºC).g. for 86% C and 14% H. Heating value. but red-coloured for different taxation). station benefit 6. and anti-static compounds.86% C. Minimum flash point is 60 ºC for JP-5. Boiling and freezing points. with a vapour pressure at this point of 1.. and Jet B) and military (JP-4.. 1 Distillate (Kerosene). 1 kPa at 38 ºC).. If the structural analysis is also considered. by weight.. is only used in very cold climates. and 760 kg/m3 for Jet B. the price structure is roughly: refinery output 20. Not well defined because they are mixtures.65 for biodiesel. mass fraction of chemical elements). Jet B (also named JP-4. The ultimate analysis of desulfurized kerosenes (<0. diesel costs about 1. JP-8. a different burning-quality index is used. and in military aircraft.010-6. Composition of biodiesel. Vapour pressure=1.430 K for biodiesel). Flash-point=50 ºC typical (40 ºC minimum). HHV=47 MJ/kg.. or C13H26.. This is only of interest in compressionignition engines. or C12H26.01% S. In general. and Jet B (Tf<50 ºC). dodecene and tridecene are the most usual surrogates). and 4% olefins (unsaturated hydrocarbons).. This is a measure of a fuel's ignition delay. may be: 77% C. The structural analysis shows. In the range 310. 30% aromatics (benzene derivatives).010-6. the commercial name of JP-4..5 kPa at 38 ºC for kerosene. n=(14/1)/(86/12=1.10 kPa at 38 ºC for diesel and JP-4. Jet fuels Jet fuel is used for commercial (Jet A-1. 60. Jet A-1 comprises hydrocarbon chains with 9 to 15 carbon atoms.4 €/L. for heavy fueloil. 11% O. by volume. Jet A-1 is the international standard jet fuel. the time period between the start of injection and start of combustion (ignition) of the fuel. The analysis can be ultimate (i. Price. Jet A (with Tf=40 ºC) is a low-grade Jet A-1 only and mostly used in USA.55). blue-coloured). Thermal expansion coefficient=80010-6 K-1. They are basically mixtures of kerosene and gasoline (half-&-half for JP-4. anti-fouling. In USA: No. Typical density at 15 ºC is 810 kg/m3 for Jet A-1.. a mean molecular formula can be found (i.340 K (370. All natural fuels are mixtures (and most synthetic fuels too).Diesel types.4. and 1% impurities and additives.010-6 m2/s at 40 ºC) for diesel.) jet propulsion. and only valid for light distillate fuels (because of the test engine. these fuels remain liquid down to 30 ºC (some antifreeze additives may be added to guarantee that). or C14H30. Density=830 kg/m3 (780.e. fishing. Jet fuel must withstand 150 ºC without fouling (dissolved oxygen in fuel exposed to air reacts with the .. They all have a lower heating value of 42. with variations of 20% amongst countries (in USA some 1 €/L). 12% H. Viscosity=310-6 m2/s (2.95). transport 1. 0.5 kPa.8. Cetane number=45 (between 40. 1. Biodiesel surrogates are longer-chain hydrocarbons than petrodiesel: C13H28. Jet A-1 specification is Tflash=494 ºC at 100 kPa (but it might decrease to Tflash=15 ºC at cruise altitude with 25 kPa). which are then separated from each other and purified. .1010 kg/m3.30)10-6 m2/s at 100 ºC). but more expensive. or C15H32. cleaner. and some 10% glycerol forms. safer. further heating leads to thermal cracking. Density. There are two basic types of fueloil: Distillate fueloil (lighter. although fueloil is cheaper than diesel oil. Pour point in the range 5. thinner.hydrocarbons to form peroxides and eventually deposits after few hours). Vapour pressure. and REE=rape ethyl ester). soybean. (10. Fuel tank ullage can be inertized with nitrogen-enriched air with xO2<12%. it is more difficult to handle (must be settled.. 1. Rocket propellant RP-1 fuel properties may be assumed to be the same as jet fuel properties. 0. the term 'fuel oil' also includes diesel and kerosene. then mixed with an alcohol (usually methanol) and a catalyst (usually sodium or potassium hydroxide). Some 900.. Varies a lot with composition and temperature. Widely variable with composition. C14H30. Fig. and leave a sludge at the bottom of the tanks).010-6 m2/s. 1. Jet A-1 surrogate is 1-dodecene (C12H24). better for cold-start) and Residual fueloil (heavier. They are only used for industrial and marine applications because. some 100010-6 m2/s at 20 ºC (400010-6 m2/s at 10 ºC. sometimes. or even animal fat) is first filtered. some distillate is added to residual fueloil to get a desired viscosity.8 $/L (about 20 €/GJ in terms of LHV). Price: Jet A-1 sells at some 0. Jet A-1 viscosity at 20 ºC is about 8. renewable. Composition.10 ºC. then pre-processed with alkali to remove free fatty acids. non-toxic and biodegradable direct substitute of petroleum diesel in compression-ignition engines. Colza is also known as rape (RME=rape methyl ester. better lubrication). Transesterification of vegetable oil to biodiesel (R is typically a 16 to 18 C-atoms hydrocarbon with 1 to 3 double bounds. Fueloils are usually graded by their viscosity at 50 ºC (ISO-8217). where a new or used oil (sunflower.1 kPa at 20 ºC. Biodiesel Biodiesel is a biomass-derived fuel. Varies with composition and temperature. Distillate fueloils are similar to diesel oil. Often. FUELOIL Types. Viscosity.. Biodiesel is a mono-alkyl-ester mixture obtained from natural oils. pre-heated and filtered.. more powerful. JP-4 has Tflash=20 ºC. colza. whereas Jet B (also named JP-4) surrogate is ndecane C10H22. Notice that. Fig. the oil's triglycerides react to form esters and glycerol. Must be heated for handling (it is usually required to have <50010-6 m2/s for pumping and <1510-6 m2/s for injectors). currently produced by a process called transesterification. particularly in the USA. thicker. Usually 10% methanol (non-renewable) is added. Nigeria. 0. with sensors and control valves every 25 km and pumping stations every 100 km) and LNG-ships (Liquefied Natural Gas carriers typically of 140 000 m3 in capacity. and it is then called 'wet gas'. =990 kg/m3 at 15 ºC. NATURAL GAS. dehydrated (by glycol absorption. waste oil from other industries are often added. 0. Saudi Arabia. LPG AND METHANE HYDRATES Natural gas is a flammable gaseous mixture.g. 0.5 m in diameter welded steel pipes (tested at 16 MPa) with a concrete overcoat to protect it from anchors.Price. Mind also that HHV of LNG is some 2 MJ/kg lower than natural gas. metallic particles from the refinery equipment. to avoid water freezing and hydrate formation). CO. transport costs are significant. composed mainly of methane: 70. 39 MJ/m3 for NG from UK. It is found on many underground cavities. carbon dioxide and hydrogen sulfide to prevent solid plugs and corrosion. plus some 0. Price of natural gas varies not only with time but among world regions (because. 42 MJ/m3 for NG from Algeria. but underground cavities (natural or artificial) seem a better solution. Biogas is a flammable gaseous mixture. The production may range from .. 33 MJ/m3 for NG from Netherlands.g. Alaska). 70% in Libya. it is sweetened (H 2S and CO2 are removed by amine absorption). 50 m apart. It is the fuel used in large marine vessels because of price (about half the price of distillates). HFO (also named Bunker-C. e. New Zealand) or linked to petroleum fields (e. The liquefaction of natural gas requires the removal of the non-hydrocarbon components of natural gas such as water. Typically half of crude-oil price. Since the mid-20th century it is traded by large continental gasoducts (up to 2 m in diameter. under the Gibraltar Strait. composed mainly of methane and carbon dioxide. consists of two 0. and may contain dispersed solid or semi-solid particles (asphaltenes. HHV=36 MJ/m3 for NG from Russia. Large LNG tanks of up to 50 000 m 3 and gasholders of up to 100 000 m3 are used as accumulators. by isentropic expansion). either as free deposits (e. Indonesia.2% C3H8. Liquefied natural gas tankers (LNG ships) were developed in 1960s.. 38 MJ/m3 for NG from USA. particularly on volume basis. or Residual fuel) may have a composition of 88%wt C.5% by 2020).5% water. Differences in natural gas composition have a sizeable impact on heating value.. which has a viscosity of 30010-6 m2/s at 50 ºC (300 cSt). A typical HFO is IF-300 (Intermediate Fuel). and minor concentrations of H2O. due to its low temperature. and the Algeria-Morocco-Spain one in 1997. natural gas associated to oil fields may contain appreciable fractions of butane and heavier hydrocarbons. and some dumped chemical wastes). 1%wt S. CO2. and the flash-point at 60. HFO leaves a carbonaceous residue in the tanks. Heavy fueloil Heavy fueloil (HFO) is the residue of crude oil distillation that still flows (the quasi-solid residue is asphalt). Algeria.. 99% in Alaska).g.80 ºC.g. etc. price is around 12 $/GJ in Europe and 4 $/GJ in USA. contrary to crude oil. BIOGAS. 2510-6 m2/s at 100 ºC. 1. 250 000 m3 for new ones). and may have up to 5% of sulfur (MARPOL directive is to limit it to 3.5% by 2012 and to 0. and particularly if cryogenic transport is involved). 10%wt H. and some liquefying fractions are extracted (to produce ‘LPG’. He. minerals and other leftovers from the oil source. Before putting the dry natural gas on the market. with 45 km undersea length up to 400 m deep (the one under Sicily Strait reaches 600 m depth). etc. The Algeria-Italy submarinegasoduct started operation in 1983. obtained by anaerobic fermentation of condensed biomass (manure or sewage). The latter gasoduct.13% C2H6. in 2013. based on LHV.99% CH4 (e. HHV=43 MJ/kg. N2.5%wt H2O.1%wt ash. the later step being the controlling stage. Table 3. 100% in UK. and portable 'camping gas' bottles (containing some 2.70 m3 of biogas per cubic metre of manure. Pure methane. lasting 10. with composition varying widely from nearly 100% propane in cold countries..2% H2 and <1% H2S and H2SO4 before desulfuration.. ethanethiol. commercial propane and commercial butane) are not good for some laboratory work.74 0.4 kg of LPG. iso-butane.. 290 mm diameter and 376 mm height).g. propylene.. Data for some gaseous fuels. 35% in France. that is 20.74 0. 2% pentane.45% CO2.95 Nm3).30% propane in hot countries (e.1 g/kg H2O and 1 mg/kg mercaptans.2 c 520 liquid 560 liquid HHV (LHV) [MJ/kg] 54. whereas normal conditions are usually defined as 0 ºC and 101 kPa. b Standard conditions are usually defined as 15 ºC and 100 kPa.. LPG is also marketed in small expandable containers for laboratory use (containing some 50. propane and butane can be easily found from local chemicals suppliers.g.. For higher rates or cold ambient. 200. NG NG NG Propane Butane Biogas a (Alaska) (Algeria ) (North Sea) (commercial) commercial) (typical)  at 15 ºC [kg/m3]b 0.74 2.4% N2. 1. 50% in the Netherlands...5 ºC and L=580 kg/m3 at 20 ºC). n-butane. 1.30 days within a digestor (depending on the temperature. e. yielding monomers that are made to ferment by other bacteria. 1. LPG (liquefied petroleum gas) are petroleum derivative mixtures (gaseous at ambient temperature. 25% propane. 190 g is the commonest).40 ºC). 2. but handled as liquids at their vapour pressure.g. 0. The new aluminium bottle holds 6 kg (13 kg total.. which follows a different treatment than the one pumped through gasoducts (e. the traditional bottle for domestic use (UD125) holds 12.65% CH4. if the commercial mixtures traded (natural gas.4 gas.300 g of LPG. yielding alcohol that later turns to acetic acid and finally decomposes to methane plus carbon dioxide. For small lab demonstrations they may also be obtained in situ. CH3CH2SH) are introduced for safety because its detection threshold for human smell is 0. In Spain. propane bottles works better. where biomass is first hydrolysed by some bacteria in absence of oxygen. and butylenes.900 kPa). methane content may be as low as 83% in the latter case)..20. 30% in Spain..1. to only 20. e.3 (49) 53 (48) 50 (46) 49 (45) 33 (30) CH4 %vol 99 89 82 60 C2H6 %vol 8 9 C3H8 %vol 2 5 >80 <30 C4H10 %vol 1 2 <20 >70 olefins %vol <20 <20 N2 %vol 1 1 2d CO2 %vol 1 40 a For natural gas delivered through LNG carriers. 20% in Greece. c Thermal expansion of liquid propane =1.g. and odour markers (sulfurcontaining chemicals.3 (49) 54..4 ppm in volume). All gaseous fuels are odourless (except those containing traces of H2S).95 normal cubic metres (sometimes written 1 Sm3=0. d Typical biogas composition: 55. as thiols or mercaptans.5 kg of commercial butane (56% n-butane.1. with a rough molar composition of 40% propane and 60% butanes (n-butane and isobutane). 35. with Tb=0.8 kg is commonest). 17% iso-butane. mainly constituted by propane. 2. For vehicles EN-589-1993 applies. thus one standard cubic metre equals 0.0 gas.510-3 K-1. . in 1899. characterized by H-bonds and regular open cavities. Present use is mainly for chemical synthesis (e.. CH4·6H2O) fizzle and evaporate quickly when depressurised. but for the using of hydrogen as an intermediate energy carrier (like electricity). as did his discoverer.methane may be easily produced by means of Al4C3(s)+6H2O(l)=3CH4(g)+2Al2O3(s). or by heating a 50/50 mix of anhydrous sodium acetate and sodium hydroxide: NaOH(s)+NaC2H3O2(s)=CH4(g)+Na2CO3(s). Since this methane comes from very large-time biomass decomposition. but clathrates. as a cryogenic fuel in rockets.e. can stabilize the water lattices and form “hydrates". and as a fuel-cell fuel. stabilised to a solid state by incorporating small guest non-polar molecules of appropriate size (to which they are not bonded. Methane hydrates (approx. and cleanly converted back to water. the American Mathews. hydrogen-energy appears as the final solution to face the energy-environment dilemma of scarcity and pollution.5 ºC) in plant-covered moist places like the continental sediments on the sea floor and permafrost soil on high-latitude lands. HYDROGEN In the long term. Strictly speaking. CH4+aH2O=(2+a)H2+bCO2+cCO is carried out at 1150 K with Co-Ni catalysts. oxygen or hydrogen are much more difficult to stabilise in water. Hydrates soils are prone to accidental landslides. living matter and fossil matter. yielding some 150 times its volume of methane. to drive fuel cells engines and clean combustors. not only for the much-pursued nuclear-fusion power stations (using hydrogen isotopes). On Earth. they are not hydrates (chemical compounds of a definite formula). an unstable network (they tend to the liquid state) of host polar molecules like water. but availability .  Production at large (world production in 2010 was 40·109 kg). cleanly produced from water and solar energy. i.g. and released CH4 losses are worse: 20 times more relative greenhouse effect that CO2. Discovered in 1766 by Cavendish (used in 1520 by Paracelsus as inflammable air). is based on fossil feedstock:  Some 50% of world H2 production is from natural gas reforming. in cryogenic research. Methane hydrates are solid icy-balls (of some centimetres in size) found trapped under high pressure (>30 MPa) and chilling temperatures (0. particularly during exploitation. but >2000 ppm-CO is left and PEFC-type fuel cells required <20 ppm-CO. They might be the major source of natural gas in the future. it is found combined in water. Production  Pure hydrogen (H2) is an artificial product on Earth (1 ppm in the atmosphere). Natural gas reforming is presently the best method to produce hydrogen while renewable sources are being developed. it is easier to purify until <1000 ppm-CO and add oxygen to get rid of the CO at the catalyst (but if more O2 is used. The process. for the hydrogenation of fats. In MCFC & SOFC the CO is an additional fuel. Instead of fully purifying the H2. ammonia). the problem of global warming remains: it yields CO2 on burning. where they may block valves. carbon dioxide. metallurgy. only van-der-Waals forces act to stabilise the network). ceramics. Besides methane. it reacts with H2 at the catalyst producing just heat). what constitutes a high risk to extraction platforms. presently they are a nuisance in high-pressure gasoducts. but nearly 100% of Jupiter atmosphere and 90% of all atoms in the Universe (nearly 3/4 of its total mass). and larger hydrocarbons such as ethane and propane. hydrogen sulphide. smaller molecules like nitrogen. and named by Lavoisier in 1781 First massive production in 1782 by Jacques Charles (Fe(s)+2HCl(aq)=FCl2(aq)+H2(g)) to inflate a balloon (he flew 25 km from Paris on the same year of Montgolfier's brothers fly with a hot-air balloon). The more aromatics in gasoline. In practice there is an intermediate steps (if not. sometimes catalytic (PCO). NH3. proton-exchange-membrane. syngas=synthesis gas.  Si(s)+2H2O(l)=SiO2(s)+2H2(g)+339 kJ/mol. and Na2SiO3(aq)+H2O(l)=2NaOH(aq)+SiO2(s)-85 kJ/mol at 220 ºC.  NG (natural gas. afterwards shift-converted (reduces CO to 1%).  Coal reforming. Pb 40106 kg/year at 0. water only 11%. Order of magnitude world production and cost for metals is: Fe 0. Si is not good for direct fuel (in a SOFC) due to the very small anode voltage. CuHv+uH2O = (u+v/2)H2+uCO. are not made available (and in that case with CO2 sequestration). and (only for PEFC) finally selectively oxidised (to <10 ppm CO). compared to methanol).7 kgcoal/kgSi). gaseous fuels are no good for storage and transportation. No good because of pollution and high temperature work. Direct electrolysis of molten SiO2 is not developed. Several reforming processes exist:  Partial oxidation (PO). the worst reforming (that is why diesel is bad). Gasoline may be thermally decomposed at >800 ºC (or best at 300 ºC with Nicatalyst). It is the exothermic reaction with deficient oxygen. it is first desulfurised (from previously added safety odorants!). dripping a strong acid over a metal.2 €/kg.65.  Another 20% of world production was based on coal reforming (declining rapidly): C+H2O= H2+CO at 1300 K and CO+H2O = H2 + CO2 with Fe0-CrO2-ThO2 catalyst. PEM. but gives rise to very high temperatures (2000 K) and pollutants (NOx.of natural gas is in question if new major sources. however. producing 109 kg/year at 1 €/kg (consuming 29 MJ/kgSi plus 2. This may be the best H2 production method in the long term.  Na(s)+H2O(l)= (1/2)H2(g)+NaOH(aq).  Ethanol reforming (ethanol has 13%wt of hydrogen). as seabed clathrates.5%wt of hydrogen). For methanol reforming. but contaminates a lot (and desulfurisation is required). Reforming a fuel is producing another fuel from it. Presently Si is produced by SiO2-reduction with charcoal in an electric-arc furnace.4 €/kg. Methane has 25%wt of hydrogen. Al 25109 kg/year at 0.e. Natural gas is most used today in low-temperature fuel cells (PEFC and PAFC). As fossil fuels are being exhausted. HCN) without appropriate catalysts. Pt 15000 €/kg. endothermic but good energy rate (78%). N2H4 (flammable liquid).  From other intermediate storage compounds (too expensive at present): NH3 (liquid at 1 MPa & 298 K.  Some 30% of world production is based on naphtha reforming in crude-oil refineries. CH3OH(vap)+½O2=2H2+CO2+667 kJ/mol (in reality yields 40% H2(g) instead of 67%).4 €/kg. it is far too slow): Si(s)+2NaOH(aq)+H2O(l)=Na2SiO3(aq)+2H2(g)+424 kJ/mol at room temperature. and not electrolysis with renewable-energy.  Production at intermediate locations (for transportation or for stationary applications) by reforming (see 'Reforming' details below)  Methanol reforming (methanol has 12. This technique may become commercial (Si is non-toxic non-CO2 and non-CO producing.g. but it is a poison).  A small percentile of world production (<4%) is based on water electrolysis from cheap hydroelectric energy in Canada and Scandinavia: H2O = H2+(1/2)O2. with e=0. electrolysers).  Gasoline reforming (gasoline has some 16%wt of hydrogen. diesel has a little less and is not used). i. a Pd . it is the simplest reforming process.  Production in the lab  Zn(s)+2HCl(aq)= H2(g)+ZnCl2(aq). then steamreformed (yields 10% CO). It is the purest H2. water electrolysers seem to be the most popular hydrogen sources in the future. Electrolysers with liquid potash lye produce hydrogen cheaper than other kinds of electrolysers (e. methane) or LPG (propane+butane) reforming. With more H2O(g) may yield H2+CO2 (40%/60%).  Chemically stabilised in clathrates (i. SR).18 m3 and 25 kg tank). For methane. it is not used in practice.g.e. although low pressure favours the reaction. CH4(g)+CO2(g)=2H2+2CO-248 kJ/mol. cobalt) at room temperature. CH4(g)+H2O=3H2+CO-206 kJ/mol is carried out at 900 K to 1200 K in a gas furnace. A non-flammable non-volatile alkaline aqueous solution of sodium borohydride (NaBH4) at 20%wt. For methanol. using a Ni-catalyst on silica.  Chemically absorbed in metal hydrides (up to 2%. CH3OH(vap)+H2O(vap)=3H2+CO2-49 kJ/mol (in reality yields 70% H2(g) instead of 75%. notice that no CO is involved. It is the endothermic cracking at high temperature. at 250 ºC. this is presently the most economic method of H2 production. 0. 0. Although this requires a refrigerated storage at 250 K. stored by moderate overpressure and/or cooling. 3 kg. after SR (or PO or AR) the gas flow is exposed to a selective membrane that yields 90% H2. but a very complex one because of the required external heating (only used for production >500 kg/day). a Pt or Ni catalysts on alumina are used. 3 kg for 500 km in a car. 0.e.g. 3 kg.10 m3 and 45 kg tank).  Chemically combined with alkaline metals. 1. that clogs the catalyst). 2CO=C+CO2. it is best suited to PAFC because of the CO2. they are realised by slow compression of water and H2(g) at room temperature up to 200 MPa followed by cooling to some 250 K. and it presently costs 35 MJ/kg just to liquefy H2(g) from ambient conditions (30% of its lower heating value). Power is controlled by insertion/removal of the catalyst. i. For natural-gas reforming. hardly dissolves in liquids (e. Requires container inspection every few years. CH4(g)+½O2=2H2+CO+36 kJ/mol (in reality yielding 1. It is not reforming itself but a post-processing stage to reforming.3 molH2/molCH4 instead of 2). Methanol reforming. or with Cu/ZnO).3 MPa really.g. Storage & transport History of hydrogen storage and transport is always associated to the Hindenburg-1937 and Challenger-1986 catastrophes (as nuclear energy to the Hiroshima bomb).6 ppm by weight in water at normal conditions). working at 1200 K (a big problem is the formation of soot. Hydrogen. CH4(g)=2H2(g)+C(s)-75 kJ/mol. and given-off by depression and/or heating. and PAFC have the advantage that the vapour is produced with the by-product heat. Autothermic reforming (AR). It is just a combined PO+SR process that it is adiabatic overall. For methane. Hydrogen clathrates are more difficult to get than methane clathrates. Thermal decomposition (TD).g. gives off pure H2(g) by contact with a catalyst (ruthenium.  Cryogenic liquid at 20 K and pamb (up to 0. Several hydrogen storage systems may be used:  Compressed gas at 30 MPa and Tamb (e.     catalyst is used. With present dewar-tanks it evaporates in two weeks. by exothermic hydrolysis: NaBH4(aq)+2H2O=4H2+NaBO2(aq). It is the endothermic reaction with water vapour. Membrane reactor. that must be regenerated with oxygen from time to time to get rid of the carbon deposited. metastable crystal networks of host polar molecules like water). Carbon-dioxide reforming (not much used). .05 m3 of LaNi5H6). this could be easily managed for instance with a small venting of innocuous liquid nitrogen vapours. although it works for PEFC. Natural-gas reforming. as most other gases. e. e. CH3OH(vap)=2H2+CO-95 kJ/mol (it is not used alone but adding water. it is the most widely used reforming process and the one that yields more hydrogen. with Ni-catalyst on alumina plus a final pass through Pt to further oxidise CO to CO2. Steam reforming (SR). with Ni plus a final pass through Pt to further oxidise CO to CO2. CO+H2O=CO2+H2+41 kJ/mol. It is the most used storage method for small applications. and consumes up to 20% of HHV. smaller energy for ignition (15 times less).5% Oxygen (O2) <500 ppm . with the same or even less damage to people and goods. which. Purity Required hydrogen purity depends on intended use (it is easy to understand that requirements for the ammonia industry are different than those for food hydrogenation). while mooring to a 50 m high metallic docking tower in stormy weather. hydrogen is not so much dangerous (some claim it is safer). As further reassurance on H2 safety. and burns with not toxic fumes (most deaths caused by fire are actually due to deadly fumes and gases). Hydrogen is most dangerous in poorly-ventilated and closed spaces. the other victims fall or jumped to the void in despair. But. in ventilated spaces. hydrogen inside the bags. but to its badly-designed canvass coating. it must be acknowledged that.59% instead of 6. but afterwards the pressure can be lowered without leakage (safe to transport and store). and released by heating. the one offered at the few hydrogen-supply-stations) is compiled in Table 4. Compressed hydrogen storage posses the additional problem of pressure-vessel explosions. The often-used stereotypical example of the Hindenburg catastrophe was not due to its lifting hydrogen filling. but much wider flammability limits (in air. Safety Hydrogen is a dangerous flammable gas. Component Tolerance Inert gases (He+Ar+N2. all of them by diesel flames in the cabin. For fuel cell applications. Gas trapped in glass micro-spheres or nano-fibres. it is said). 4. a helium-filled air-ship would have had the same type accident and caused the same casualties. due to risk of explosion by deflagration or even detonation. nearly invisible non-premixed flame. Hydrogen is not toxic itself. Very high pressure is needed to put the gas inside. minimising possible horizontal spreads (most ignition sources and valuables accumulate horizontally).. in relative terms to other fuels. It is curious that 'hydrogen is roughly as light in air as in water': 7% (of course meaning that the gas has 7% of the density of air. as done with all other odourless flammable gases.14%).6 mm instead of 2 mm). an electrostatic spark ignited the coating and set fire to everything combustible: the canvas. made of a flammable combination of dark iron-oxide and reflective aluminium paints to avoid solar heating. with the same self-ignition temperature as methane (850 K).. but great advances have been recently made on safe-breakage of very high pressure containers made of laminated fibrereinforced polymers.75% instead of 5. its main advantage being its extreme lightness. Admissible impurities in hydrogen supply for fuel cell vehicles. Table 4.…) <1% Agua líquida (H2O(l)) <0. smaller quenching distance (0. detonability limits are 18. standard purity (e. town gas (50% hydrogen) was pipelined to most dwellings in most large cities for nearly a century. and the liquid has 7% of the density of water. makes H2 leaks and flames to vertically escape quickly.15%). and more prone to detonation (in air. entirely similar to any other compressed gas vessel.. although the last couple can never be in thermal equilibrium).. only 8 people by burning.g. before natural gas took over. and engine diesel fuel (35 out of the 97 passengers lost their lives. A safety problem related to purity is that presently hydrogen cannot be marked with any suitable odorant to ease early leak-detection by humans. It is concluded now (at the time it was attributed to a H2-leakage) that. but yields less heat also). with brilliant lustre and very low moisture. Estimations in 2000 gave a reserve/consumption ratio of some 250 years. the gases inside the balloon quickly react to produce yellow flames and a large bang. the remaining texture of the original vegetation can be discerned. It has some 2.Carbon oxides (CO+CO2) Hydrocarbons (total. hydrogen costs four times the price of natural gas for the same energy content. lignite. is the most abundant form of coal.006 €/MJ. oxygen gas from the air begins to effuse through the tiny pores in the balloon material. However. . or brown coal. 20% in Russia. Coal was known since ancient times. more brittle and scarce coal. by MJ-produced (before taxes). Types Coals are classified in a maturity rank according to age and fixed carbon content:  Anthracite is the hardest.020 €/MJ.. non-renewable and very-polluting source. methanol 0. 15% in China.10% of trapped water.  Lignite. not a mineral) formed some 300 million years ago (Carboniferous Period. (<5% of trapped water). a similar stuff obtained from wood. and the typical heating value is 30 MJ/kg. Origin Coal is a compact black or dark-brown sedimentary rock (a mixture. and peat. It has some 40. When ignited with a lighted candle.012 €/MJ. a charcoal. internal combustion engines 30 €/kW (20-yr life). After a while. by high pressure and temperature anaerobic decomposition of dead plants (mainly ferns). Sometimes. most often.: from more to less ‘cooking’: anthracite. electrical utility stations cost 500 €/kW (20-yr life). dinosaurs era.  As for year 2000. was used (charcoal leaves less ashes. bituminous coal. but the degree of metamorphosis varies a lot and several types of coal can be found today. proven commercial reserve in 2000 were >10001012 kg (some 30% in USA. purest. and 100 Myr ago). but only used when available on site. but it is a finite.01 ppm <0.025 €/MJ (notice that fuel energy refers to its higher heating value). Hint: Hydrogen balloon combustion & explosion A balloon filled with hydrogen gas burns when ignited but does not produce a large bang. although smaller deposits exists from 200 Myr. gasoline 0. if a balloon filled with hydrogen is allowed to remain untouched for a while. O2 and some other non-reactive gases. COAL Coal is the most plentiful fuel available on Earth..  Price of energy.60% of trapped water. It is a dense black solid. too precious for a fuel (it is used for chemicals). and the typical heating value is 15 MJ/kg. including lubricants) Sulfur Ammonia Inorganics <2 ppm <1 ppm <1 ppm <0. As for year 2000. by kW-installed.30%.  Bituminous coal is a dense black solid that frequently contains bright bands with a brilliant lustre. 15% in EU-15). electricity 0.. fuel cells 3500 €/kW (<10-yr life). Volatiles may reach up to 30% in some lignite. Volatile matter range is 10. the balloon contains a mixture of H2. solar cells 5000 €/kW (<10-yr life). hydrogen 0. As for year 2000.01% Price comparison of hydrogen energy  Price of an electricity generator. or on a dry basis. and a solid remains (coke: nearly pure carbon. Composition: proximate analysis and ultimate analysis Coal is a natural composite material. Coal tar is further distilled: a first fraction yields benzene and toluene. a liquid pours out (coal tar). when oil took over. mainly restricted to electricity generation in large combustion power plants (38% of world electricity generation). also in England. A great amount of coal is used to make iron (reducing iron ore). but mining really took off in the 18th c. and it is not used as commercial fuel (it must be dried to at least 30% in water to be burned. lit his home with gas lamps that burned coal gas. specifying: water + percentage of C. that coal started to be used in the metallurgical industry. O. and it is possible sometimes to identify the remains of individual leaves in peat. H. and as a home heating fuel in England since Roman times and during the Middle Ages (Newcastle coast was plentiful of so-called sea-coal). and its composition. specifying: fixed coal + volatile coal + ash + water.  Ultimate analysis.. even today. H2. HCN). is measured by weight. Since coal consumption in its raw solid form is cumbersome. and referred to either to as-received coal (wet coal). In the mid 19th c. CO2. Murdock. being a solid substance. Uses Coal was used in China in 1100 BC. and moisture contents is computed by differential weighting after heating the sample to 103 ºC for several hours. gas lamps had come into use as street lamps in London. coal also replaced charcoal in making black-powder and explosives. several fluidification processes have been developed:  Pulverisation by mechanical grinding. Sulfur content is computed either by chemically converting all S-ions to sulfate-ions and precipitation to BaSO4. Two types of coal analysis are commonly used:  Proximate analysis. although mines some 500 m deep have been worked (following a surface coal layer deeper and deeper). used in blast furnaces). a third fraction creosotes. Plant decomposition has progressed only partially. a Scottish engineer. a second fraction naphthalene's. by heating coal. used in blast furnaces to yield clean concentrated heat. Platt. Coal had the leading share in world energy production from 1800 to 1950. is almost pure carbon but amorphous. The Fischer–Tropsch process (1920s) convert a mixture of carbon monoxide and hydrogen into liquid hydrocarbons (as done by Germany in II World War. to separate volatile matter. In 1792. with the coke discovery in 1603 by H.. synthetic gasoline was transparent). and S. or on a moisture-free-ash-free basis. driven by the Industrial Revolution hunger to feed steam boilers.90% of trapped water. more than half of the world coal is from surface mining (open cut or strip mining). Heating coal with air (and preferably also water vapour) produces coal gas (town gas) and leaves a solid residue (coke). that is distilled aside. In any case. Coke. cellulose being still the main component. It was after a wood-shortage in West Europe in late 16th c. N. Early in the XIX c. a fourth anthracenes. coal is first drained. Coal is not so valued as to worth it deep underground mining. Peat has some 70. in Wales during the Bronze Age. and was used in open fires). and the residue is called coal-tar pitch.  Distillation by pyrolysis.  Gasification by pyrolysis. and still has a large share: 24% of primary energy consumption. or by chemically fixing the sulfur . not a single chemical compound. not as graphite) obtained by anaerobic heating of coal. produces a mixture of volatiles (CH4. Heating coal in absence of air at >1000 K. for the mass-percentages composition).. water mass divided by dry mass (what is left at 103 ºC).2% by weight of oxygen in the air. Notice. ash. 5% O and 5% ash by weight).. H-.43∙%H0. minus the contribution of its oxygen content: .90% is dry ash-free coal.. coal composition.e. than coal from the mine has less %C by weight (some 70%) than oil (85%) or natural gas (75%).  Hydrogen content in dry ash-free coal is in the range 5. Overall dry ash-free coal has a composition of:  Carbon content in dry ash-free coal is in the range 80.  Nitrogen is typically 0. i. but all modern power plants have flue gas desulfuration units (scrubbers) to get rid of it by absorption in a lime spray. O2. and S. Wood moisture is 20. the percentage in carbon is measured from CO2 content of complete burning the sample.oxides formed on combustion of the sample. Fixed coal is computed by subtracting from sample weight moisture. In the proximate analysis. because it is associated to iron in pyrite chunks that settle. A typical dry coal composition is 85% C. once water is eliminated and sulfur and ash measured aside.2% in dry ash-free coal....  The remaining 70. HHV=0. the air demand is the sum of the C-.80%. what gives for the higher heating value. since it is chemically combined with hydrogen and carbon atoms (the HHV reduction per kilogramme of oxygen would be 17. In terms of the dry ultimate analysis of a coal. sulfur and volatile coal. volatile coal is computed by differential weighting after heating the dry sample to 950 ºC for several minutes (volatiles consist mainly of H 2. although there is a difference in composition and weight between initial mineral matter and final ash. CO. C2H6.5 kg of air per kg of fuel. however. H2O and maybe other volatile organic compounds).15∙%O in MJ/kg. CH4. further divided in volatile matter (20. Ash is measured as the solid residue on combustion.  Sulfur is typically 1. Wood ash is 1.  Oxygen content in dry ash-free coal is in the range 1. and the amount of oxygen is computed by subtracting from the total..e.10% by weight (it may range from 1% in dry ash-free anthracite to some 45% in wood just cut).9 MJ/kg if combined with H.. 5% H.airdemand.60% moisture. Air requirement for theoretical combustion Pure carbon demands 11. as received. the amount of hydrogen is computed as 1/9 of the weight of water produced in combustion. a empirical value of 15 MJ/kg is often used.5% in dry ash-free coal.20% of ash..40% by weight in dry ash-free basis) and fixed coal (the solid residue). corresponding to 1 mol of oxygen per mol of carbon (C+O2=CO2) and 23.3 MJ/kg if combined with C. the latter is most often used in analysis. In ultimate analysis. This oxygen decreases both the theoretical air required for complete combustion of the fuel and its heating value. Lignites can have 30. i. or 12.15% of moisture. The main impurity in coal combustion has been sulfur: 2.90% by weight..6% by weight. once water is eliminated.. u%C+v%H+w%O+x%N+y%S+z%Ash.3%.33∙%C+1. the amount of nitrogen (usually less than 1% in weight) is measured aside or neglected. the solid residue after burning. In summary. Some sulfur can be removed by just grinding and washing.. is as follows:  5.10% in weight.  10.. WOOD Wood is the hard..9 MJ/kg if combined with H (286 kJ per half-a-mole of oxygen. and S. to 5%wt in artificially-dried furniture-wood). an empirical value of 15 MJ/kg is often used.5 kJ/mol). u%C+v%H+w%O+x%N+y%S+z%Ash: HHV  0. either solid (coal. e. or 12. Composition Wood is a natural composite material consisting of hollow polymer fibres in a polymer matrix. Hard wood (resistant to sawing) comes from deciduous broad-leafed trees: oak. since it is already bonded to hydrogen and carbon atoms.67  %C   8  %H   1 %O   1 %S (e.10000 and M=500.10000 kg/mol). particularly those with oxygen. H-. and HHV=8. fir. what gives for the higher heating value of a coal in terms of its dry ultimate analysis..328  %C   1. The HHV reduction per kilogramme of oxygen would be 17. Soft woods come from pine. diesel.15  %O   0. from 60%wt in freshly cut trees.5 kg/kg for %C=100) 23. elm and fruit trees. The hollow fibres are tubular cells (most of them dead).20 MJ/kg for brown coal (lignite). Chemically.45%% cellulose (a long homopolysaccharide. H2+(1/2)O2=H2O+286 kJ/mol).oxidation. Wood is used for timber (construction).5 kJ/mol per mole of oxygen.43  %H   0. for paper making. which could not be further oxidised.. A0=11.8 MJ/kg. Price of coal for thermal power plants in 2013 is around 60 $/t (at 30 MJ/kg it means 2 $/GJ. assumed to be bond-free. natural gas may be 5 times more expensive). C+O2=CO2+393.g. porous. dry wood is an aggregate of 40.30 MJ/kg for soft coal (bituminous coal)..e.3 MJ/kg if combined with C (393. Heating values Pure carbon has a higher heating value of HHV=32. Equations (1-2) can be used for any kind of complex fuel of unknown molecular formula (for which the ultimate analysis per weight is available).13 MJ/kg for peat and schist (but peat briquettes may reach 17 MJ/kg).g. corresponding to the enthalpy of reaction C+O2=CO2+393. the matrix is made of hemicellulose and lignin. minus the contribution of the existing bonds.30% hemicellulose (a short-chain polymer with . HHV=10. Actual coals would have a heating value made up of the contribution of C-.A0  2. mostly the debris.5 kJ/mol.. wood).. (C 6H10O5)n with n=5000.2 (1) with typical coal values of A0=10 kg of air per kg of coal. Trees are cut in winter to minimise initial water content.09  %S [MJ/kg] (2) with typical values in the range HHV=20. cedar. heavy fuel or crude-oil). with cellulosic walls (70% cellulose and 30% lignin) holding aqueous solutions in the inside space (water content varies a lot. and as a fuel (up to 30% of wood production in industrialised countries. Paper is made from chemically and mechanically processed wood fibres (typically 30 mm in diameter and 2 mm long) which are self-binding when dried from a wet state.. fibrous substance found beneath the bark of trees and shrubs. 20. oxygen decreases the heating value for complete combustion. but up to 90% in developing countries). or liquid (gasoline. i. much lower than any other fossil fuel. 7 O 43..37 0.72 2.85 17 N 0. Cellulose.. Cambium divides every spring.g. the elementary chemical composition of wood is presented in Table 5. Table 5. Thermal conductivity.00 0. such as tannins. 2. but it forms very porous structure that makes it very hygroscopic (e.30 40. Fig. It depends on water and air content (porosity).30 kg/mol) and 20. it does not dissolve water.18 MJ/kg.24 30.250 and M=20..1 W/(mK) transversally.16 MJ/kg. or cotton.02 0.03 0. Old xylem vessels (the core or duramen) die and become clogged with dry metabolic wastes. and resins. Dry wood is a very good insulator due to the air spaces. with k=0. wood pellets may have HHV=17. and wood chips HHV=13. It depends on water and air content (porosity). It depends on water content (moisture). 2) is the most abundant naturally occurring organic substance.10000).. pure cellulose is a white crystalline powder). dyes.00 0. Fig. apical tissue divides axially. the outermost are alive and allow for water and nutrients supply to the leaves from the roots).96 3. There is also a tissue that transports water transversally from xylem to cambium (wood rays) Density. wood. Molecular structure of cellulose ((C6H10O5)n with n=5000.04 0. just providing structural support to the plant. and on direction (anisotropy).00 0.15 MJ/kg. Wood composition (%wt) and lower heating values. increasing the length).08 34. increasing the diameter some 2 mm (conifers) up to 7 mm soft woods). paper. whereas the outer part is mainly cambium and phloem (living conduits that return elaborated sap from the leaves to everywhere). Maximum is HHV=20 MJ/kg for dry wood.46 25..20 4. .. that is practically pure cellulose.01 S 0. with some 7% ash (the main components being SiO2 and CaO).23 0.18 20 H 6.. Heating value.3 W/(mK) along the fibres and k=0. yielding xylem to the interior and phloem to the outside (cambium divides only transversally. The inner part is mainly xylem (mostly dead conduits of dxylem=25·10-6 m.31 0.n=150. Maximum is cellulose=1550 kg/m3. but very soft wood (balsa wood) only has 50 kg/m3.30% lignin (a reticular polymer with a 3-D structure that glues all together). %wt water 0 20 40 60 C 50. log wood HHV=15.15 Total 100 100 100 100 LHV kJ/kg 19900 15400 10950 6500 The biological structure of wood is basically a hard hollowed tissue in a cylindrical arrangement. say between 10 m and 100 m.00 ash 0. instead of producing the ethanol from cellulose in the straw. as when cereals are grown for either food or bioethanol. and to minimise pollution. Nobel Laureate Paul Crutzen published findings that the release of Nitrous Oxide (N2O) from rapeseed oil... or natural gas (fossil fuels). but it is creative. aside of domestic waste. pellets…).12 MJ/kg. biofuels from nonfood crops and from algae. is by transforming raw biomass into gas (known as biogas or syngas). palm oil… In the future. depending on market price. which may be used as a commercial fuel instead of just burning it to get rid of. since. to a first approximation. but dioxin emission is a problem. but the preferred way to easy handling and transportation. Examples of biomass composition (%wt). should be used. carbon neutral. An additional problem is the impact on water resources. with HHV=7. there is the case of developing countries where wood is the only fuel. sugar cane. or recovered from other industries waste (forestry. although in October 2007. petroleum. biomass is also used as a fertiliser (compost). contribute more to global warming than the fossil fuels they replace. effort and thought to burn firewood. etc. building (e. bringing you closer to Nature.g. but its importance is marginal. It takes time. Biomass is a renewable fuel.Why using wood as a fuel. Table 6 presents some examples of biomass composition: Table 6. most common energy crops are: corn. Presently. farming. food industry…). urban and animal waste might be included too. some crops and some fields are of double use. It excludes organic material which has been transformed by geological processes into coal. Biomass can be directly burned in furnaces and boilers. sorghum. biomass is synonymous of vegetable matter used as fuel (biofuel). timber). liquid (which may range from alcohols to tars). paper industry and other chemical stuff. Besides the biofuel production here discussed. barley. straw in adobe and roofs. either grown for that purpose. if a gas or oil burner works smoothly and effortless?.20% biofuel fraction. At present. and solid (char. and. Sugar cane bagasse Pine sawdust Almond shells Grape stalks moisture 7 9 12 8 C 46 45 41 41 H 5 5 5 6 O 40 39 39 40 N 0 0 1 0 S 0 0 0 0 ash 1 1 3 5 PCI kJ/kg 16 200 16 400 16 000 16 700 A problem facing energetic crops is the impact on food crop price (the food vs. soybeans. The traditional biomass through the ages has been wood. there is the case of wood residues from forest cleaning or industry. inspiring and really warm. besides any practical justification. . and corn (maize). at present. fuel debate). First. rapeseed. Second. in the sense that the CO 2 released in biofuel combustion was previously captured from the environment during biomass growth. But. there is the aesthetic reason of the intrinsic beauty of wood fire and its cultural heritage (our ancestors developed around the fireplace heart). Municipal solid waste (MSW) has great organic content and can be used as a fuel in incineration power plants. liquid biofuels (bioethanol and biodiesel) are mixed with oil derivatives (gasoline and diesel) in a 5%. Biomass Here. Wood and wood residues are basically cellulose.Minimum Autoignition Combustion  [kg/m3] Tg sition Td ignition Tflas Tself-ign HHV [MJ/kg] . toluene and xylene. carbon dioxide and other trapped gases. the process is best known as reforming. coal. PMMA). Some plastics decompose to their monomers (e. crude oil. produce very toxic gases. slowly decreasing wood strength.. with a sudden 80% weight loss from 300 ºC to 400 ºC (600 K to 700 K). paper and sugar industries) generates three different energy products in different quantities: coke (char). Material Density Softening Decompo. up to 400 K. In catalytic high-pressure pyrolysis the biomass is heated at a high pressure with a reducing gas and a catalyst. Between 100 ºC to 200 ºC (400. Table 7. anthracene and phenanthrine. Pyrolysis of plastics and rubbers. it is not a partial oxidation or combustion process). as water vapour. and the autoignition is around 400 ºC (700 K). and a synthetic gasoline is produced. Above 400 ºC (>700 K) all volatile material is gone. usually imply just a moisture loss (although de-polymerisation may occur after some time).g. Pyrolysis is the major step for crude-oil cracking in the petroleum industry to get lighter fuels (and to get ethylene and hydrogen for synthetic chemicals). In the presence of air. non-combustible products are released. in order to eliminate any polymerisation. Thermal softening. as a first batch up to 500 K. leaving a solid residue called coal-tar pitch. with more than 80%wt carbon. but it is really partially pyrolysed cellulose best approximated by the empirical formula C7H4O). and heating value of some materials. biomass) to get more suitable fuels. from 550 K to 600 K mainly creosotes.e. taking place in most uncontrolled fires. or organic wastes from the cellulose. a mixture of benzene. When heated. Recently. giving a slow-burning glow or a prominent flame according to the heat losses. We focus here on pyrolysis of some raw fuels (wood. and from 600 K to 650 K a mixture of quinoline. and a carbonaceous char (charcoal) remains (charcoal is often approximated as pure carbon. The vapours can be cooled to get a liquid condensate (known as coal tar) and then distilled to yield. When water is added during the pyrolysis of a fuel. and some internal depolymerisation starts. catalytic pyrolysis is being developed to drive the process to the most demanding yields. as when PVC decomposes to HCl). decomposition. whereas others generate new substances. i. Pyrolysis implies chemical reactions. Most of the times. High-temperature resisting plastics decompose to almost-exclusive carbonaceous residue. In low-pressure fast pyrolysis the biomass is fast heated and allowed to decompose into vaporised oils that are fast cooled and condensed. wood tends to dry. Above 200 ºC (>500 K) the cellulosic polymers decompose yielding volatile organic compounds. by heating coal to more than 1200 K.FUEL PYROLYSIS Pyrolysis is the chemical decomposition of compounds caused by high temperatures with no access to air or oxygen (i. from 500 K to 550 K mainly naphthalene. up to 100 ºC (<400 K). used in metallurgy and as a fuel). the flash point is around 250 ºC (500 K). A now-obsolete method for making methyl alcohol (wood alcohol) was to pyrolyse wood chips and separate the volatile alcohol by fractional distillation. with a weight loss of some 10% from 100 ºC to 300 ºC (400 K to 600 K). Pyrolysis of biomass (straw and other agricultural feedstock. oils (tar) and gases (syngas). like paper. the decomposition products are separated by distillation (the whole process is called dry or destructive distillation). coal or crude oil. the first stages in heating a fuel. (PE. used as a heavy fuel or as a binder in making coal or coke briquettes.500 K). Similar organic products are obtained from the pyrolysis of any natural organic material: wood. to loose moisture by evaporation. The earliest use of pyrolysis was to produce coke (a smokeless form of coal without volatiles.e. 300 kg/mol. saccharification and fermentation were mastered (like in ruminants and some insects like termites). masking the H-bonds and making cellulose highly insoluble in water. glass-transition temperature is not applicable to thermosetting polymers) Cellulose is the most abundant polymer on earth.. paper). Not Applicable (e. with size of order 10-7 m and M50.5 Polypropylene (PP) 910 170 ºC 350 ºC 350 ºC 370 ºC 46 Polystyrene (PS) 1040 100 ºC 300 ºC 350 ºC 500 ºC 42 Polyurethane (PU) 1100 NA 300 ºC 400 ºC PVC (polyvinylchloride) 1400 75 ºC 200 ºC 350 ºC 450 ºC 20 Teflon (PTFE) 2250 330 ºC 500 ºC 550 ºC 600 ºC Wood 500 NA 200 ºC 300 ºC 400 ºC 25 NA.2000. -(C6H10O5)n.with n=300..Bakelite (Phenol1300 NA formaldehyde. but not digestible by humans. PA) 1140 220 ºC 300 ºC 450 ºC 500 ºC 32 Polyacrylonitrile (PAN) 250 ºC 450 ºC 550 ºC Polyester 1380 260 ºC 18 Polyethylene (PE) 930 100 ºC 350 ºC 350 ºC 370 ºC 46. The linear chain is very coiled. if its hydrolysis. but it forms very porous structure that make it very hygroscopic (e. it would be a less expensive source of glucose than starch. (Back to Combustion) .g. Cellulose is the main constituent of plant cell walls. consisting of long unbranched chains of H-bond-linked glucose units.g. Like most polymers it is not a well-defined molecule but a family of closely related chemical structures known as polysaccharides. PF) Cellulose (90% in cotton) 1600 NA 280 ºC 300 ºC 400 ºC 17 Coal 1400 NA 200 ºC 28 Methacrylate (PMMA) 1180 85 ºC 180 ºC 300 ºC 450 ºC 26 Nylon (Polyamide. .... 1 Fuel consumption .. but who knows the answer..)................. cooling.......................................... Year 2000 Year 2020 prediction .. every conceivable non-inert system (from biological organisms to just mechanical clockwork mechanisms) generates entropy......... mostly from fossil fuels)...... • Energy used for transportation is now sixty times larger than 500 years ago (the start of ocean travels)............................................................. 8 References ........ At present we only use two final-user commercial-energy carriers: fuels (piped or batch-delivered) and electricity (wired through the grid............. seems no so energy-eager (compare a videoconference-meeting with a presence-meeting arranged via individual car transportation)........ This is the consequence of most people living in large service cities... Short summary of fuel share in world energy utilization................................................... 4 Fuels in end-use energy consumption ................................................................................ coming from 2400 W/cap of primary energy: some 300 W of electricity (produced from some 900 W of primary energy............................................ and must be compensated by an exergy input to keep the process steady............. and in the foreseeable future........................ humankind needs energy services: lighting............................. Is that surge in transportation-energy consumption really needed? Human mobility a basic human need..................... humankind energy expenditure has grown differently on different energy services: • Energy used to procure food and water is now (say year 2000.............................. pulling and pushing goods (actuators)..................................... which must be evacuated as heat....................... Table 1........ presently....................... trying to extrapolate into the future..... If we look into the past....... radio and TV transponders......... and powering information devices (semaphores....... wasting nowadays several hours a day to go to work and back home? • Telecommunication technology. but to what extent? Is it not really a burden sometime............ plus some 1500 W of end fuels (refined from raw fossil fuels.... as summarised in Table 1...................................1 FUEL CONSUMPTION Fuels and energy utilisation ....... Most of the energy trade involves fuels.......................... 2 Fuels in world energy production ........................ Human metabolism needs some 100 W/cap (100 watts per capita)................... transporting goods and people......... 8 Energy future..... heating............................................................ 3 Fuels in electricity production.............................. heating.......... or stored in batteries)..... in the past................. 9 FUELS AND ENERGY UTILISATION Besides air........................ computing machines..................... However..................................... and food................... and only few people devoted to provide food and water to all...................... We might question how much energy we need to satisfy our wanted services. per capita) five times larger than 106 years ago................................................................................... and used for transportation.. 6 Energy management ... 5 Fuel-to-energy conversion factors ...... water....... on the other hand.................... and so on)........................... and humankind consumes some additional 1800 W/cap of final energy............. 6. solar.. This work may be used to produce propulsion. perfumes. nuclear fusion might take over). The analyses of the utilization of: energy as a commodity (sources. b Gross values: 3200 W/cap (→620⋅1018 J/yr=14600 Mtoe/yr). population. more heat. or cold.6·109 cap. both as primary energy (i.g. water-mills and wind-mills were large contributors. directly extracted from natural sources and put on the market). e. a most fortunate situation. are used for heat generation. . treated aside. The burning process. To produce heat in a burner (thermo-chemical converter). in the far future. Fuel consumption. a 34-fold for hydrogen) mass of air is required to burn a given mass of fuel. polymer synthesis. GDP. animal power. 2. pharmaceuticals.e. or for materials processing. or for materials processing. and be jointly dealt with physico-chemical properties of fuels. Fuels may be considered as primary energy (i. Fuels. for candescent lighting. for work generation.). as energy carriers or secondary-energy source (i. may be considered as market fuel-properties. When dealing with world-wide-average energy usage. refrigerator.1·109 cap.. Fuel consumption Fuel consumption.. because of their high specific energy-release when burnt with ambient air. population. mineral oils. both at source and at destination (up to the Middle Ages. gas. biofuels and synthetic fuels) are mainly used as convenient energy stores.. cold. as fuel price and fuel availability. not considered here furthermore. or heat pump) to produce power. mainly polymers (fibres. as energy source. and air is freely available everywhere anytime (has not to be carried on). This electricity may be used to produce propulsion. or electricity. To produce materials (and heat) in a reactor (chemo-chemical converter). oil. for cold generation. for feeding a thermal machine (heat engine. a 18 Gross values: 2400 W/cap (→460⋅10 J/yr=10900 Mtoe/yr). is today the major contributor (near 90%) to energy use. but we have preferred to deal with separately here. Fuels are also used for non-burning purposes. as found in Nature) and final energy source (i. fuels may be used (chronological or difficulty ordering): 1. or for chemical transformations (see details in What fuels are used for). we must recall how uneven (unfair) the distribution can be. 7. The substances collectively known as fuels (basically coal. To produce work (and heat) in a heat engine (mechano-chemical converter). oils. as for the chemical synthesis of materials. cosmetics. or more heat. cold. This heat may be used for direct heating. 10000 €/cap. GDP. or as final energy (bought by the end-user for final consumption).e.2 Primary energya Energy carriers (end use) 90% Fuels 84% Fuels 7% Nuclear 16% Electricity 3% Hydro Electricity production: • 66% Fuels • 17% Nuclear • 17% Hydro Primary energyb Energy carriers (end use) 90% Fuels 82% Fuels 5% Nuclear 18% Electricity 3% Hydro Electricity production: 2% Wind • 65% Fuels • 10% Nuclear • 15% Hydro • 10% Wind. In a summary. storage and consumption) is sometimes called Energetics. To produce electricity (and heat) in a fuel cell (electro-chemical converter). plastics. however. etc.e. is not essential for the release of fuel-and-oxidiser energy. with a third of mankind presently lacking electricity. 6700 €/cap. or more heat.e. 4. as input to the end user). 3. the same global process takes place in fuel cells without combustion. because a 15-fold (for hydrocarbons. manufactured fuels such as crude-oil distillates and synthetic fuels). transportation. indirect heating (heat exchangers). fuels and energy. wind-mill fields.g. From IEA http://www. can be used indistinctly both for primary and for final consumption. but the average per unit time (e. etc. resource consumption) is computed from the budgets and estimates of industrial producers: • Coal mines and coal importers • Crude-oil extractors and importers • Natural gas extractors and importers • Nuclear energy generators • Renewable energies: hydroelectric plants.3 Fuels major share in world energy market (80% to 90%) means that the two terms. We agree that a year period is a more natural unit of time than a second. Primary energy consumption in % (year 2000). since the data may have not higher accuracy (1 yr=π·107 s gives less than 0. for human activities (who measures salaries in €/s?).g. without such a rational as above. and one may use in most cases the simple approximation 1 yr=30·106 s. Some energy unit conversions are presented in Table 3 (e. it is worth considering that all terrestrial energy (except the minor contribution of gravitational tidal energy) is ultimately of nuclear origin: nuclear fission inside the Earth generates geothermal energy (also a minor share of the overall Earth energy budget). b) in TW. Time evolution of world annual primary-energy consumption: a) in Mtoe. in the mid term (a year) to biomass energy. or 2. FUELS IN WORLD ENERGY PRODUCTION Primary energy production (i. which is less than 5% below the exact figure. Fig. that is the dominant commercial source nowadays. and nuclear fusion at the Sun providing the major energy input.e.e. Most of them are accounted basically by the subsidies they ask for. biomass industries.4 kW/cap).iea. 11 000 Mtoe/yr. The total primary energy consumption in the world (year 2000) was 460⋅1018 J/yr (i. solar energy fields. or an average of 15 TW in the world. some places have daylight and others night. that some people used indistinctly 'electricity' and 'energy'. however. Table 2 presents the distribution by type of energy source and its time evolution. that divided by 6. with a typical nuclear power station of 1 GW).4 kW average per person). 460⋅1018 J/yr=15⋅1012 W=130⋅1012 kWh/yr=11 000⋅106 toe/yr.1⋅109 people corresponds to 2. Besides. but conversion errors and encumbrance are minimised if only SI-units are used.org/. . 1. some have summertime and others winter). Beware. that is partially converted in the short term (weeks) to hydraulic energy and wind energy. world averages have less dispersion than local ones (at a given instant.g. On the other hand. Traditional energy balances are presented in toe-units (tonne-oil-equivalent) per year) or other odd units. Table 2.5% error). and in the very long term (million years) to fossil fuels. in 1012 W=1 TW) seems a more rational rate measure and allows easier comparison with single power devices (e. 16⋅10-18 J Erg 1 erg = 0.6⋅106 J Thermie (106 cal) 1 thermie = 4.1⋅1018 J/yr=120⋅106 toe/yr=4 kW/cap. only amount to a 10% of world energy coverage (6% in the UE) and all the rest come from exhaustible sources. 500 kg of crude-oil and 300 kg of natural gas (i. Per capita consumption of energy is oddly distributed (more than food.1⋅1018 J Exajoule 1 EJ = 1. some 30% of all primary energy was used in the production of electricity. but less than water): the 2.2⋅103 J Quad (1015 BTU) 1 Quad = 1. basically electricity production (but also town gas and coque production) from primary energy sources.1⋅109 J British Thermal Unit 1 BTU = 1. there is a firm will however. Spain primary energy consumption is 5.10⋅10-6 J Notice that renewable energy sources (RES) in 2000.1⋅1018 J/yr) 81 85 18 18 43 53 19 13 1 <1 15 13 4 2 Table 3. 300 kg of oxygen from the air.1 kW/cap and the averaged Sun input on Earth of 30 000 kW/cap. and less than 1 kW/cap in the Third World).0⋅1018 J Electronvolt 1 eV = 0. It may be interesting to compare fuel consumption (basically energy) to other basic human needs: world annual per-capita consumption is some 1000 kg of drinking water.159 m ) 1 barrel = 6. to come back to a more sustainable exploitation of energy resources. more if biomass from developing countries were added).2 kW/cap average comes from 4 kW/cap in EU. 2 and Table 4. split by type of primary energy in Fig. and the objective is of covering by renewable sources up to 30% of the world energy production in 2020 and up to 60% in 2100 (UE target to 2010 is 12% of RES). In 2000.1⋅103 J Kilocalorie (Calorie) 1 kcal = 4. and 600 kg of coal.11⋅109 J 3 Barrel of crude (0. Unit Equivalence Tonne oil equivalent (toe) 1 toe = 42⋅109 J Tonne coal equivalent 1 tce = 30⋅109 J 3 Cubic metre of natural gas (STP) 1 m NG = 40⋅106 J Kilowatthour 1 kWh = 3. basically hydroelectric and biomass.4 Fuels coal crude oil natural gas biomass (not traded) Nuclear Hydroelectric World First world Third world 18 18 (460⋅10 J/yr) (250⋅10 J/yr) (190⋅1018 J/yr) 90 86 95 24 20 29 36 41 26 21 22 20 9 3 20 7 11 2 3 3 3 EU-15 Spain 18 (65⋅10 J/yr) (5. FUELS IN ELECTRICITY PRODUCTION An intermediate step in energy utilisation (between primary consumption and final consumption) is energy transformation into a more useful form of energy.e. Some energy unit conversions. Notice that the . using today's technologies and under today's legislation. whereas “reserves” refers to that portion of resources that can be economically recovered at today's selling prices. 1400 kg of traded energy-products.2⋅106 J Therm (105 BTU) 1 therm = 0. Notice that 'resources' refers to the total amount in Nature. 200 kg of solid food. It might be compared with the metabolic consumption of 0. 8 kW/cap in USA. public and household sectors (it excludes deliveries to the energy transformation sector and to the energy industries themselves).org/. 3 and Table 5) or by type of end use (Table 6). commercial. agricultural.org/.7 TW= (440 GW= (300 GW= (20 GW= 15 000 TWh/yr= 3900 TWh= 2500 TWh= 170 TWh= 54⋅1018 J/yr) 14⋅1018 J/yr) 9⋅1018 J/yr) 0. in spite that many first-grade books say that electricity comes from water).5 major source for electricity is the combustion of fuels. Electricity production: total and percentage by source type (year 2000). It can be measured by type of final energy (Fig.5⋅1018 J/yr) Fuels 65 73 51 46 coal 37 52 27 39 crude oil 9 3 6 3 natural gas 17 16 18 4 biomass (not traded) 2 2 <1 Nuclear 20 20 34 35 Hydroelectric 15 7 15 19 FUELS IN END-USE ENERGY CONSUMPTION Final energy consumption is the energy finally consumed in the transport. Final energy use by energy type (year 2000). % in the world % in USA % in EU-15 % in Spain (1.iea. From IEA http://www. Fig. Time evolution of world annual final-energy consumption (in Mtoe). . industrial. From IEA http://www. 2. 3.iea. Time evolution of world annual electricity production (in TWh. Table 4. Table 5. Fig. • Per tonne of freight: 3. Fossil fuels will continue to provide the largest share in vehicle power consumption. hospitals) 15 11 10 Residential (home) 20 23 15 Non-energy consumption 10 3 About 20% of world primary energy (30% of the final-energy consumption) is used to power transportation (1% coal. In equivalent fuel litres per 100 km. Table 6. extractive) 30 32 35 Transportation 25 31 40 Commerce and services (offices. 40% in EU). The largest share in electricity generation is by coal (50% world-wide. Nearly 40% of final-energy consumption in UE takes place inside buildings (heating. plus 3% electricity).8⋅1018 J/yr) Industry (construction. gas. 6 L by car. FUEL-TO-ENERGY CONVERSION FACTORS Notice that the conversion from material budgets (coal. supplemented by some 8% renewable biofuels and some 5% hydrogen (from fossil and renewable sources).0 MJ/km by plane.0 MJ/km by truck.5 MJ/km by train. the figures are: 10 L by plane.6 Fuels coal crude oil natural gas biomass (not traded) Electricity % in the world (315⋅1018 J/yr) 84 15 41 17 13 16 % in EU-15 % in Spain 18 (44⋅10 J/yr) (3. uranium) to energy budgets is an agreed standard and not the thermodynamic exergy (the difference is not very large however.8⋅1018 J/yr=86⋅106 toe/yr.9 MJ/km by bus or train.7 MJ/km by ship and 0. EU forecast for year 2020 stills base >80% of that power from fossil fuels. Rough average energy consumption in transportation is: • Per passenger: 3. . oil. being also responsible for some 20% of the projected increase in both global energy demand and greenhouse gas emissions until 2030. 0. 6% gas. except for nuclear fuels).8⋅1018 J/yr) 79 81 3 3 56 64 26 14 4 21 19 The total final energy consumption in the world is 315⋅1018 J/yr=7000⋅106 toe/yr. lighting.8 MJ/km by car. Some 17% of anthropogenic CO2 emissions also come from transport. with a rising on natural gas to 10%. Final energy consumption in Spain is 3. cooling and other appliances). 3L by bus. the lower heating value is chosen for coal and oil. the higher heating valued for natural gas. Final energy use by sector (year 2000). manufacturing. % in the world % in EU-15 % in Spain (315⋅1018 J/yr) (44⋅1018 J/yr) (3. some 200 g/km for motorcycles. some 200 g/km for cars (down to 130 g/km for new cars in EU from 2012). and 2 L by train. and some 60 g/(km·pax) for trains. 1. and 0. some 80 g/(km·pax) for buses. It is appropriate here to quote the CO2 emissions of different transportation means: some 250 g/(km·pax) for plains (down to 100 g/(km·pax) for the most efficient). 90% oil-derivatives. in an isolated system. a given uranium-ore. The actual amount of nuclear raw-material used depends a lot on the technology used. like in a common heat pump.086 toe/MWh Nuclear energy 0. taking no account of the amount of uranium used. per person and year. i.2606 toe/MWh Geothermal energy 0. 0. it is worth mentioning that the main mass-percentage in fossil fuels is carbon. from the 11 000 Mtoe/yr of primary energy).2 tonnes of hydrogen. a standard value of 10% in energy efficiency is assumed for geothermal plants.96 toe/tonne Natural gas 0.090 toe/thermie Hydraulic energy 0. From that. Similarly. from stoichiometry C+O2=CO2). a raw energy of 3Ee is accounted for (1 MWh→0. would yield some 50 times more electricity if processed in a breeder reactor (where most of the fertile U-238 atoms transform in fissile Pu239 atoms) than if processed in a normal reactor.02 toe/tonne LPG 1.06 toe/tonne Diesel 1. world CO2 emissions in 2005 were 24·1012 kg (6500 MtC/yr. To measure energy-consumption efficiency. because of the 'unavoidable energy losses' in the production and transportation processes (e. Recommended conversion factors. we are shovelling 1 tonne of carbon from below ground to the troposphere above us (and that tonne was bonded to some 0.03 toe/tonne Fuel-oil 0.g. world production 440 EJ/yr and world consumption 315 EJ/yr). energy available for work. • Exergy.. just says that: • Energy is conserved. e. Talking about mass-to-energy conversion factors.58 toe/tonne Coal black lignite 0.e.7 tonnes of oxygen). if the only energy need of humans were comfort heat.7 For nuclear power plants. Thermodynamics. i.2606 toe).e. and is released bonded to 3. however. cooking power and even sanitary hot water. two ratios are used: • Per capita energy consumption . that produces three or four times the energy it consumes. can only decrease with time in every process.13 toe/tonne Gasoline 1.07 toe/tonne Kerosene 1. in an otherwise isolated system in a given environment. so that in crude words. Table 7. no matter the processes taking place.g.32 toe/tonne Coal brown lignite 0. this final energy consumption could be met by extracting a much smaller primary energy from fossil fuels and forcing the rest of the energy to come from the ambient. to release the average energy we trade in the world.086/0.33=0. Notice that final-energy consumption must be less than the primary-energy production. a standard value of 33% in energy efficiency is assumed. which burns with oxygen in the air to yield nearly four-times more mass of carbon dioxide (44 g every 12 g.86 toe/MWh From IEA (International Energy Agency) data.18 toe/tonne Crude oil 1. for an amount Ee of electricity generated. Fuel Ascribed energy Coal bit. subsidies on renewable energies) to procure the following social guaranties (safety. security. The trend. with two associated consequences: • Environmental impact. for instance. etc. Besides. then cheap-coal and storable-hydroelectric power stations. The key problem is that energy consumption is growing not proportionally to population growth (as food may be). and so on. but at a much higher rate (because of the 'developed' way-of-life. Some environment impact always exists. strategy to fulfil demand variations. and other energy sources do not show a clear alternative: nuclear fission has the unsolved problem of waste fuel and proliferation. There are other aspects related to fuel consumption that have not been considered here: strategic reserves. and particularly to the changing-input conditions (low hydraulic year. taxes on fossil energy) and incentives (e. which generates global-warming gases and chemical pollution (global and local).g. weekend second-residences) pay energy (and water.g. • Reliable energy supply. windmill and nuclear plants being always enabled. Some unreliability always exists. the design capacity of available power plants must be larger than the expected average production. and its supply has been traditionally managed by public administrations. and society and individuals must establish the level of acceptable risks (should you carry a gas-lighter in your pocket?.) at the same price as central city dwellers? Should large energy consumers pay more or less per unit energy consumed? • Reasonable energy impact on the environment. etc. and renewable energy sources are not so powerful neither free of environmental impact (e. how wide should the margin in supply voltage or frequency be acceptable?). in an airplane?.8 • Per gross-national-internal-product energy consumption. to adapt electricity generation to demand. unexpected shut-downs. due to this changing-load effect. and it is increasing. and new energy demands from a crowed world. like massive water desalination). transportation and end use) may cause. knowing that the costs grow exponentially (should a one-minute electricity-dropout in a commercial store be considered an admissible minor nuisance. On another side. in the case of Spain this external dependence is >70%). that are more expensive to run. Third-world countries consume three times more energy per GNP than developed countries. however. Should disperse occasional users (e. and society and individuals must establish the level of acceptable impact that energy utilisation (production. • Reasonable energy pricing.g. Energy management Energy is a first-need good. Europe imports more than 50% of the primary energy it consumes (in 2000. Some risks always exist. and society and individuals must establish appropriate levels of reliability. low winds. and then combined-cycle natural-gas plants. as food and water. in view that electricity can hardly be accumulated. telephone. with non-storable hydroelectric. On the strategic dependence side. an order of power-plant activation priority is established. or a great costly disturbance to be protected from?. waste management. . affordability and sustainability): • Reasonable energy safety.g. marketing policies. pre-programmed maintenance. even darker than the future of clean water and food. ENERGY FUTURE The future of energy (as a human commodity) looks dark nowadays. is towards a free-market management with political restrictions (e. because the largest share in energy production comes from fuel combustion. should the domestic grid voltage be low or high?). effects of wind mills on fauna and landscape). 9 • Scarcity of cheap resources. their problem with nuclear waste perhaps being solved in the future. International Energy Agency (IEA). World Energy Council (WEC). looking forward to solving some of the inconveniences (being alert for new possibilities). capturing CO2 emissions from traditional exhaust gases (e. or too rigid to allow for sensible choices (e. the best energy diet may be. intrinsically non-proliferating. Nuclear fusion research must be further encouraged. Perhaps the best summing up is: • Make people aware of this gigantic energy-problem by fostering scientific and social education. Public acceptance is a pre-requisite in developed societies. fears and utopias). but their remote risk of massive life destruction renders them too risky for wide-world proliferation (energy consumption in the future will increase the most amongst presently underdeveloped societies). at present. . for consumers to minimize the real cost/benefit ratio.org. and weighting more on those showing better promise at the time being. without which.g. Power plants intrinsically safe to runaway. and making best use of fissionable material. diversify the effort according to actual achievements (facts) and reasonable expectations (expert prospective.worldenergy. using the carbonatationcalcination process). are being exhausted at a quicker pace than new reserves are found. e.g. gas. References • • www. should be developed. economic criteria and technology availability are powerless.g. • New nuclear fission plants can alleviate in the short term the energy problem. etc. Energy seems to be presently too cheap for people to care about (e. There is no proportion between the ratio of R&D expenses and consumption expenses. Cleaner and more energy-efficient combustion processes must be develop for the traditional fuels. variety and temperance (with green matter being preferable to meat. but not as a present panacea: nowadays. • Invest in basic and applied research on energy management and related environmental impact. • Make energy economy more explicit (including waste management and health-care costs).iea. and second. without preconceptions. at least. because readily-available oil. first. using natural-gas combined-cycle plants with a thermal efficiency nearly double than old coal-fired plants. a 26-member-states policy-advice cooperative agency. or directly from the fuel by reformation of the fossil fuel to less-contaminant fuels before combustion (what drives towards the hydrogen economy). as for a food diet.g. In particular: • New fossil fuel plants seem to be unavoidable for decades to come. thermal solar energy plants. As a clear solution to this energy problem is presently not at hand. or helped by the oxy-combustion process. as being the only panacea in the horizon. Avoid being too enthusiast on a single goal. although wind energy is developing faster. when buying powered appliances. and coal deposits. biomass cultures for biofuels (from non-alimentary plants). and paying attention to 'oysters and lobsters').org. • New renewable plants must be promoted. nor in decades to come. Among renewables. a UN-accredited non-government. • Meanwhile. www. biofuels are not yet alternatives vehicle fossil-fuels). between energy technologies and other technologies. the two approaches with wider future are. or when using vehicles). they cannot provide a complete substitute to fossil-fuel plants. even subsidized if one takes account of the social costs implied in traditional power plants (from human health to world politics). the most rational approach might be to push along several fronts. nonprofit organisation. European Union's Directorate-General for Energy and Transport.html.10 • • Back http://europa. . BP is a global energy company.bp.eu.com/statisticalreview. http://www.int/comm/energy/index_en. ... all providing oxygen... because liquid mixtures are too unstable..................7 Propellants ............... with some gluing agent to keep them bounded...................................................... fire) refers to making fire by chemical reaction.... they should be part of the same molecule (single-base pyrotechnics) with zero or slightly positive oxygen balance.................. or metal powders.............................9 References ........... or gases................................... It is always done by combustion of a fuel and an oxidiser (a red-ox reaction)...................... The majority of cases.........8 Ariane fuel.......................................................................2 Explosives .... PROPELLANTS AND EXPLOSIVES Pyrotechnics... Nitrogen atoms are found in most explosives......................... but distinguishes from normal combustion in the speed: combustion refers to slow processes whereas pyrotechnics is associated to almost instantaneous combustion (solid-rocket propellants being in between)........... propellants and explosives................................ fireworks...... and the associate projection of entrained solid ......................................... chlorates..................................................................... The reducing agents may be charcoal (carbon).... even better....... peroxides....... and premixed explosive gases are considered under normal combustion............................... they should be highly exothermic........ are treated as combustion processes... oxides........................................8 Shuttle fuels............................................... propellants................... noise. By physical state pyrotechnics may be grouped as:  Solids.. There are no single-base pyrotechnic gases (they would decompose)............................................................... because they are more stable and easier to handle................... and they should be in condense form and generate a lot of gas.. Separate liquids......... this shock wave (the blast caused by rapidly expanding gases)............................... ............................... heat..........................6 Trinitrotoluene ... Very unstable even if single-base......................................4 The match ................ sulfur.......... Notice that all practical double-base pyrotechnics are powder solids mixed-up.7 Ammonium nitrate .....................1 Applications: blasters.................... An explosion is a mechanical process generating a destructive high-pressure wave in a fluid..............................................................................................................................  Liquids...................PYROTECHNICS..............1 Classification .3 Greek fire .... sparklers..................... fuel and oxidiser must be premixed (double-base pyrotechnics) or..........................................................................................................................  Gases... matches.......................................................................................................................................... with the goal to produce light............................. In double-base pyrotechnics..... For pyrotechnics to be effective.. chromates and perchlorates (the best)..........................................................................................................................4 Nitroglycerine and dynamite .............................................................. like LH2 and LOX used in cryogenic rockets... because they yield nitrogen molecules that release great energy and expanding gases (the bond energy of N≡N is 941 kJ/mol)........ the oxidising agents may be nitrates.......... airbags.................................................................................................................... PROPELLANTS AND EXPLOSIVES CLASSIFICATION Pyrotechnics (from Greek .9 PYROTECHNICS......................... as nitrocellulose and nitroglycerine..4 Black powder ........................ pyrotechnics are grouped as:  Explosives.g. tetrazole (CH2N4) and derivatives. a strong alkali that cause eyes irritation). propagating at a few cm/s or m/s within the material. Less dangerous pyrotechnics are taking over NaN3. lead azide). Airbags are car-safety-devices that use a pyrotechnic inflator. quick-release devices. a high-power electrical discharge in a solid. By use. the device is called a firecracker (see below). e. APPLICATIONS: FIREWORKS. for airbag inflators. with the consequent change in burning rate (recession speed vr is modelled by Vielle's law. for rockets and weaponry. BLASTERS. but too short to break eardrums) and white smoke of talcum powder (used to lubricate the deployment). and the process is known as detonation. If NaN3 is exposed to water in a landfill.7). SPARKLERS. If the blast is just to cause an abrupt noise. as fires. toxic fumes. which in rockets is around 10 MPa. They are also called high-explosives. all these systems may explode. generate a supersonic reaction wave. generating a lot of hot expanding gases..g. a mist of combustible particles in air. beware of un-deployed airbags.g. and widespread to all new cars in late 1990s.. cause mechanical damage by impact (besides other possible associated risks.4<n<0. by chemical decomposition. PROPELLANTS. rapidly producing a great quantity of nitrogen gas that inflates a nylon or polyester bag in some 50 milliseconds (it inflates at 100 m/s). They are also called low-explosives. Airbag deployment is harsh: it generates some toxic substances (NaOH. concerns have been raised regarding landfill pollution. The main difference between rocket propellants and gun propellants is the working pressure reached. Substances that. They commercially started in the late 1970s in USA and Germany. A low combustion temperature (2000.. it generates a bang (some 170 dB.. ANFO). by chemical decomposition. a nuclear reaction.. TNT. . AIRBAGS. and more stable materials secondary explosives (e. like triazole (C2H3N3).. They are high-explosives that undergo supersonic combustion when detonated by a low-explosive or shock-wave (they slowly burn if just approached by a flame). as in combustion. Sensitive materials that can be exploded by a relatively small amount of heat or pressure are called primary explosives (e. black powder. MATCHES. radioactive waste. dynamite. Since most airbags will never be deployed and since each airbag contains between 50 and 150 grams of NaN3. with 0. propagating at several km/s within the material. nitroglycerine. the bag is micro-perforated to allow progressive cushioning by deflation when the passenger-body hits it.debris.g. e. In case of accident. According to their purpose. pyrotechnics may be classified as:  Blasters. A container with pressurised gas. Substances that. chemical attack or high temperature. demolition. and the process is known as deflagration. with insignificant blasting. They generate a large gas stream (like all other pyrotechnics) that is channelled with one free end to give propulsive thrust to a projectile or to the combustor body. and weaponry (warhead).  Propellants. volatile liquid.  Propellants. which is an extremely toxic. vrpn.2300 K) is preferable to avoid massive formation of toxic CO and NO. and in guns more than 100 MPa..). and it usually causes burns to passengers. a high-explosive. generate a subsonic reaction wave. They were based on the combustion of sodium azide (NaN 3) with an oxidiser. a confined mixture of premixed flammable gases. generating a lot of hot expanding gases. it is converted into hydrazoic acid. tunnelling. either by friction.  Gas generators. for mining. titanium. and at p=400 kPa more than 90% people die. nuclearly. above p=200 kPa there may be some casualties. An explosion is a travelling wave with a sizeable pressure jump across. orange CaCl-band. Mg. Black-powder slowly burns if in the open. etc.     Light generators (sparklers). a powder mixture of iron oxide and aluminium or magnesium dust. thermally (as in boilers. Underwater torches use a mixture of high-gassing solid reactives that. One may find precedents in the incendiary substances developed in the Middle Ages. which is a mix of ferrous oxide (Fe2O3. a 3 kPa jump may break window panes. fuel pellets for field stoves) or military purpose (incendiary grenades. it is better to educate on dangers. mechanically (as in a balloon or any other gas pressurised vessel). essentially rust) and aluminium. Aluminium has already been mentioned as an incendiary metal. Na) are added to the black powder in order to create bright light (yellow-white by hot emission at >1500 K) and coloured shimmering sparks (by actual particle burning and gas line-emission: yellow Na-line. magnesium. and depleted uranium. or the condensation of water-vapour forced over dry ice or liquid nitrogen.g. and it is known that little explosions may occur in the fireplace that cause a loud noise and throw sparks away.g. for rescue signals or military purpose (smoke grenades). was used to spread fire by the splash of liquid iron generated: 2Al(s)+Fe2O3(s)=Al2O3(s)+2Fe(l))). but not supersonic combustion). The author reminds the reader that the information collected below is intended to satisfy human curiosity. in air. green BaCl-band. Heat generators. it explodes (yields a high pressure pulse. Smoke generators. than to let them explore on their own. and advises other people to be so prudent (simple things may kill if put to bad use. a few meters). Notice that an explosion is a sudden mechanical process causing rupture and noise. like the donkey bone in Cain and Abel story.1 kPa. EXPLOSIVES CAUTION. Some metal powders (Al. Thermite is often used in demolition grenades to burn or melt down military gear that has to be abandoned to an enemy. They all burn at very high temperatures. The author is wise enough to keep away of foreseeable dangers. Other incendiary metals include zirconium. blue CuCl-band). Fe. when ignited. Fire was used by humans since 500 000 years ago. as for children. beyond p=200 kPa the wave becomes supersonic in ambient air. a 10 kPa jump may throw down people. even electrically heated). a very loud noise (e. from a confined combustion). Noise generators (firecrackers). Fire generators. for fireworks or for rescue signals. nearly hurting the ear. The thermite reaction is Fe2O3+2Al=Al2O3+2Fe. but control of 'sudden fires' is a very recent happening. due to great pressure forces that may be originated chemically (e. a close-up turbine) produce an acoustic wave with p<0. Theatre 'smoke' is just a mist formed by condensation in the ambient of boiling glycol entrained by an air jet. . for domestic use (matches. The reaction burns very hot and releases a tremendous amount of energy. a 20 kPa jump may throw down thin walls and a 50 kPa jump thick walls. but confined within the paper wrapping of a firecracker. red SrCl-band. A particularly useful metallic incendiary is "thermite". but explosives are risky even if put to good use). Zn. creates enough pressure gases to keep water away (they only work at small depths. for fireworks or for rescue signals. thermite. Smoke from combustion is an aerosol formed by a suspension of microscopic solid particles from the poor combustion of carbonaceous fuels. the piezoelectric spark is substituted by a low-temperature combustion process initiated by rub-heating of phosphorus or one of its compounds (pure white phosphorus catches fire spontaneously in air). on burning. abundant. Black powder is an excellent pyrotechnic in many respects. an aluminium soap. it gives off a dirty smoke with a characteristic smell.. In the 14th century. he wrote (in code because of the lethal nature of the material) that when heating a finely ground mixture of 6 weights of saltpetre with 5 weights of charcoal and 5 weights of sulphur. flash cotton. less dangerous is lycopodium powder. Magicians commonly use different forms of nitrocellulose (flash paper. the usual way to start a fire was by striking a flint-stone with an iron to get a spark. they manage to destroy the Arab's wooden ships during the siege of Constantinople in 670 a. and sulfur. in concentrated sulfuric acid to get rid of water) was developed by F.Greek fire Greek fire is a water-resistant fuel-mix used by the Byzantines. Its present-time successor. but. giving way to smokeless powders (of which guncotton was the first. Another incendiary is FAE (acronym for fuel-air explosives) that sprays out an aerosol cloud of a hydrocarbon liquid. and it can be easily ignited with a spark. in the proportions 75:15:10 by weight. Sulfur. demanded a thorough cleaning of old fire arms for maintenance. though it may explodes under confinement). or polystyrene plastic beads). a shiny white crystalline material that could be found on the walls of caves or in well-aged manure piles. A binder (e. The English monk Roger Bacon described a formula for it in 1242. also known as gunpowder. Abel in 1865. decrease the ignition temperature (and provided additional fuel). The large amount of solids formed (>50 % in mass) makes powder combustion very sooty. or from sodium palmitrate). and the compound slurry can easily coat a support wire or fill a tube. when cellulose nitrate (nitrocellulose. close to an easily burning material (tinder). and reasonably safe: non-toxic ingredients easily shaped.D. . Napalm (an acronym from naphtha and palmic acid. Saltpetre is potassium nitrate. C6H7O2(OH)3+3HNO3=C6H7O2(ONO2)3+3H2O. non-detonating (it burns readily.A. is a highly incendiary jelly. With it. followed by cordite) that are used almost exclusively since then. this fact. A simple approximate stoichiometry for its reaction is 4KNO3+7C+S=K2CO3(s)+K2S(s)+3CO2(g)+3CO(g)+2N2(g). petroleum and quicklime. Black powder Black powder may be considered the first pyrotechnic. custard powder and even powdered milk. charcoal. usually consisting of a naphtha liquid made viscous and sticky with a thickener: a sodium soap. but by deflagration. was used to propel ammunition until late 19th century. found in volcanic deposits. a moistened starch or sugar slurry) is used to give shape to the mixture. it can be stored indefinitely if kept very dry. Black powder knowledge spreaded to the West in the Middle Ages. black powder led to the development of fire weapons. Greek fire was a mixture of pitch. flash string) to produce bright flashes of fire. a vigorous flame suddenly appears. The match Up to the beginning of the XIX c. used by fire-breathing magicians. but it is really partially pyrolysed cellulose best approximated by the empirical formula C7H4O. Black powder.g. sulfur. It was known in China more than 1000 years ago. often approximated as carbon. the formula for black powder was refined to a mix of saltpetre. that burned vigorously and could not be extinguished with water. Eventually. and the fact that moisture turned some of the soot into a caustic corroding solution (with KOH). and used to make firecrackers and rockets for public entertainment and to frighten enemies in combat. and then ignites it to create a flaming explosion over a wide area. Charcoal is pyrolysed wood. In the friction match (from Old French meiche). Its raw materials are cheap. Transition from white to red phosphorous may occur by boiling or by exposure to sunlight. do not have the igniter (the phosphorus) in the same place at the booster (the head). Black phosphorus is produced by heating white phosphorus in the presence of a mercury catalyst. decomposing the potassium chlorate. Matches were produced on a commercial scale first in 1833 at Darmstadt (D).. rubbed against another wood. The equilibrium phase at standard conditions is a transparent (whitish-yellow by impurities) soft crystalline solid named white phosphorus (M=0. but the common friction match was invented by the English chemist John Walker in 1826 (it is said that he accidentally scraped the stick he was using to mix phosphorus with antimony sulphide and potassium chlorate over a rough surface and caught fire). that in 1680 tried to enhance the old friction method of making fire. tray retention of the box. The basic design of the match may be split in three parts: igniter (phosphorus in air). tipped with a mixture of potassium chlorate and sugar. but 85 percent of the phosphorus is found in the bones and teeth). violet . friction resistance. booster (potassium chlorate oxidiser mixed with sulfur fuel) and sustainer.. the slow-burning splint to which the head is glued with a binder (such as gum arabic or wax. but it was not reliable. by using small wood-sticks impregnated with sulfur (used in black powder) and phosphorus (just discovered in 1669 by Hennig Brand. When the match is scratched over it. Red phosphorus is relatively stable and easy to handle (it is an amorphous solid wit =2340 kg/m3. 2) Take only one match. first patented by Pash in 1844 in Sweden. Phosphorus is commercially obtained from phosphate ores. which burns spontaneously in air. ancient name for the planet Venus when appearing before sunrise) can have several allotropic forms: white. arranged as tetrahedral units of four atoms). the heat from friction causes some red phosphorus to become white phosphorus. =1820 kg/m3. There were other chemically-ignited matches developed. the match-head must be rubbed against a special surface (glued on the package of matches) to get ignited. i. present in every cell. A few basic safety-rules on how to light a match should be pursued (at least for child education): 1) Verify there is no flammable substances nearby. red. and phoros. as the one in Paris. Burning a match releases around 1 kJ of heat. 3) Close the cover of the .031 kg/mol.e. The striking surface is coated with powdered glass and red phosphorus mixed in a binder. The match box widespread with smoker fashion in XX c. in1805. Tm=44 ºC. who extracted it by evaporating urine to dryness and distilling the residue with sand. producing necrosis of the jaw-bone and mental disorder. in USA.The first trial to make matches is due to R. spontaneously ignited when brought into contact with sulfuric acid (soaked in asbestos inside a bottle). Phosphorus (Gr. Tb=277 ºC. The ordinary yellow phosphorous initially used was highly poisonous to match-makers. Phosphorescence is the emission of light (the glow can only be seen in darkness) by slow oxidation of white phosphorus in air (red phosphorus does not phosphoresce). and only second (after calcium) in animals (0.7%. which has been dipped in a fireproofing agent to keep it from burning too easily. liberating oxygen that quickly reacts with sulfur and lights the wood of the match. etc. as well as non-combustion-related problems as splint strength.7%wt). but white phosphorus is a deadly poison that spontaneously ignites at room temperature (it is kept under water).. light. and the ease-of-use of ‘strike-anywhere’ matches. and the match box developed in late XIX c. etc. and it is the least reactive and of least commercial value. Phosphorus is the most abundant mineral element in plants (0. splint afterglow. phos. that sublimates at 417 ºC). Safety matches were slowly introduced into the market because technical difficulties in producing red phosphorus. Boyle. Safety matches. Matches and their boxes are tested to avoid burning problems as exploding heads... also preventing moisture). where a thin strip of wood or cardboard. looking for the philosopher's stone). spitting heads. bearer. heat resistance. until development of the disposable gas-lighter in 1969. holding the match an arm's length away. 4) Strike the match away from the direction of the body. but he failed: a Swedish chemical manufacturer. Nitroglycerine tablets and sprays are used as heart medication (diluted by inert matter and completely non-explosive). and in 1887 ballistite. In low or deflagrating explosives the explosion propagates through the material at subsonic speed through a sustained combustion process. handled.  He found a detonator: dynamite could not be set off by a spark or a flame (it was even safer than black powder). 5) Throw a match away only after the flame is extinguished and cool to the touch (a waste basket is not an ash tray). or lead azide.227 kg/mol (Fig. M=0. not toys (do not allow children to play with match-boxes or lighters. Nobel developed in 1875 blasting gelatine (gelatinised nitroglycerine and nitrocellulose). and have the respective formulas of Hg(N3)2. Sobrero decided that the liquid he called "nitroglycerine" was dangerous and tried to keep it a secret. setting off an explosion that nearly killed him. . A tiny amount of nitroglycerine (<1mg) placed under the tongue causes blood vessels to dilate. began to produce nitroglycerine for rock blasting in 1863. Besides dynamite in 1867. and detonated. 6) Matches are fire-making tools. where the quick.5 MJ. which is a dangerous and unstable liquid explosive. Liquid nitroglycerine is very unsafe to handle (Nobel's brother Emile was killed while working with it). Nitroglycerine. a blasting cap consisting of a small black power charge with a cord fuse. releasing a lot of gas and energy: C3H5(NO3)3(s) = 3CO2(g)+(5/2)H2O(g)+(3/2)N2(g)+(1/4)O2(g) + 1. to set it off. or glycerine. whereas in high explosives the explosion propagates by a supersonic detonation. The first commercial high explosive was found in 1846 by an Italian chemist named Ascanio Sobrero adding glycerol to a mixture of nitric and sulfuric acids. instantly lowering blood pressure. C3H8O3 (Fig. the nitric acid triester of glycerol. CHONO2(CH2ONO2)2. Nobel called it dynamite. 1b). Hg(CNO)2. Pb(N3)2. Alfred Nobel.match box before striking the match. Nitroglycerine and dynamite Explosives may be grouped in low explosives (like black powder) and high explosives (like dynamite). a smokeless powder (a mixture of nitrocellulose and nitroglycerine). In 1865 Nobel devised the first detonator. cool flame was not likely to ignite the explosive methane-coal dust mixture in the tunnel. These are salts of hydrazoic acid (HN3). This material could be packed into cardboard tubes and reliably transported. even when empty). that can be detonated underwater. 1a) is a syrupy liquid. is a dense oily liquid with a density of 1600 kg/m3 that melts at 80 ºC and detonates if heated to 218 °C or if subjected to mechanical shock. Detonators have traditionally been made from fulminate of mercury. Alfred Nobel. Glycerol. But he made two major findings:  He found a stabiliser: nitroglycerine absorbed in diatomaceous earth (a porous clay that consisted of the deposits of the skeletons of tiny sea creatures laid down aeons before) is stable to chocks. and Pb(N3)2. Nobel's detonator was a significant step forward in the development of modern explosives technology. began to produce nitroglycerine for rock blasting in 1863. a by-product of soap manufacture. Dynamite was particularly useful in coal-mine blasting. Trinitrotoluene (TNT) The first military high explosive to be put into service was picric acid, C6H2OH(NO2)3, a toxic yellow crystalline substance used by the French in 1885. However, picric acid has a high melting point, making the process of filling shells with it difficult; reacts with heavy metals to form very toxic compounds; and tends to be sensitive. Another explosive, trinitrotoluene (TNT, trilite, Fig. 1d), was first discovered in the 1860s, but was not used until the German military adopted it in 1902. It was widely used in World War I. It is relatively insensitive to chock and can be melted at low temperature to allow it to be poured into bombs and shells. Fig. 1. a) glycerol (glycerine), b) trinitroglycerine, c) toluene, d) trinitrotoluene (trilite, TNT). Ammonium nitrate Ammonium nitrate (NH4NO3,=1750 kg/m3, M=0.080 kg/mol, Tf=445 K, Tdecomp=470 K) is a colourless odourless highly soluble crystalline solid, obtained by the reaction between gaseous ammonia and aqueous nitric acid, and used mainly as a fertiliser and in explosives. In the 1950's ammonium nitrate derivative explosives were commercialised. Ammonium nitrate is stable if pure, even in the molten state or in aqueous solutions, yielding NH4NO3=2H2O+N2O when slowly heated in the open, but violently exploding as NH4NO3=2H2O+N2+½O2 when subjected to hot confinement or a strong shock. Contaminants increase the explosion hazard, particularly if acid. If pure (<0.2%wt combustible matter within) it is catalogued as a strong oxidiser by the U.S. Department of Transportation, but for >0.2% of combustible substances as an explosive. One of the worst blasts ever happened took place on 16 April 1947 on a French freighter docked at Texas City and loaded with ammonium nitrate fertiliser. The ship caught on fire in the morning and attracted a crowd of spectators along the shoreline, who believed they were a safe distance away. In mid-morning the ship exploded, killing hundreds with the tremendous blast, sending a tidal wave surging over the shoreline, and setting refineries on the waterfront on fire. A second ship loaded with fertiliser exploded after midnight, but emergency workers were given warning, they evacuated the vicinity of the vessel, and only two people were killed. Fires burned for six days after the disaster. Official casualty estimates came to a total of 567. An explosion in Toulouse on 21-09-2001 of some 250 tons of ammonium nitrate in a large fertiliser factory caused 31 deaths and more than 2000 injuries. A truck with 25 tonnes exploded in Spain on 09-03-2004 in a car accident, leaving 2 deaths and a 3 m diameter crater. ANFO, ammonium nitrate and fuel oil, is the most common explosive nowadays for surface-mine blasting, having practically replaced dynamite, due to lower cost and safer handling In the early 1980s, US Army researchers experimented with "cubane" a cubical C-network hydrocarbon synthesised in 1960s of double the density of gasoline; its high density means faster propagation of breakdown reaction, leading to a more powerful and compact explosive. Octaninitrocubane consists of a cubic core of eight carbon atoms, with an N2O group attached to each corner of the cube; it seems to be very stable, twice as powerful as TNT and its breakdown products are non-toxic carbon and nitrogen compounds. PROPELLANTS Propellants are used to force an object to move forwards, from a firework-rocket, to a rocket to put people on the Moon. As propellants must be carried aboard, condense phase propellants (solid or liquid) are much preferable. Liquid propellants may be grouped as:  Single base liquids, such as hydrazine (N2H4(l)) and derivatives (as mono-methyl-hydrazine MMH N2H3CH3(l)) and unsymmetrical-dimethyl-hydrazine UDMH N2H2(CH3)2(l))), or hydrogen peroxide (H2O2 more than 70%wt concentrated), that are stable at ordinary temperatures, but decompose into hot gases when exposed to suitable catalysts or high temperatures (e.g. 2N2H4=2NH3+N2+H2).  Double base liquids, stored apart. As oxidiser, liquid oxygen (LOX) or nitrogen tetroxide (NO4) may be used. As fuels, liquid hydrogen or kerosene may be used. Solid propellants (they do not allow intermittent operation) are categorised as "single-base", "doublebase", and "multi-base" or "composite" powders or pastes:  Single-base powders, such as cordite, are mostly nitrocellulose (guncotton with some nitroglycerine). Black powder was used for solid-propellant rockets in the first third of the 20th c. They burn cool and cause little barrel wear in firearms. Double-base composite powders are now generally used as a propellant for solid-fuel rockets.  Double-base powders are mixtures of nitroglycerine and nitrocellulose such as ballistite, and burn hotter.  Composite powders are modern formulations that do not contain nitrocellulose or nitroglycerine, instead using more modern propellants that burn cool but are as powerful as double-base powder. Ammonium perchlorate composite propellant (APCP), one of the best solid rocket fuels, can be cast into shape (instead of pressed as a powder), making its setup more simple and reliable. Hypergolic propellant are substance pairs that ignite upon contact with each other without a spark, heat or other external aid, such as N2H4/N2O4, N2H4/NO4, aniline and red fuming nitric acid (concentrated nitric acid in which nitrogen dioxide is dissolved), etc. Shuttle fuels The Shuttle (the Space Transportation System) has, for take-off, two solid rocket boosters (2 minutes burn) and three liquid-fuelled main engines, providing in total up to 30106 N thrust (80% solid rockets, 20% liquid rockets) to lift the 2106 kg total mass. Besides, the Orbiter (what most people call the Space Shuttle) has two propulsion systems: OMS (Orbital Manoeuvring System) used to change orbit and to return to earth, and the RCS (Reaction Control System) used for station-keeping and attitude control; both systems burn liquid hydrazine with cryogenic oxygen. Each solid rocket is 45 m high and 3.7 m in diameter, has 500 000 kg of fuel (590 000 kg total mass) and yields 13106 N thrust during the first few seconds, slightly decreasing during the 2 minutes burn. Their solid propellant consists of a mixture of aluminium powder (fuel, 16%wt), ammonium perchlorate powder (NH4ClO4, oxidant, 70%wt), and a dash of iron oxide powder (catalyst, 0.4%wt), held together with a polymer binder (12%wt) and epoxy resin (curing, 2%wt). The burning reaction is basically NH4ClO4=2H2O+0.5Cl2+0.975O2+0.25N2+0.25N2O+167.4 kJ (lower heating value). The white cloud formed at launch is due to alumina particles. Russia uses liquid-fuel rockets also as boosters. The liquid rocket engines use liquid hydrogen and liquid oxygen contained in the external tank, the largest element of the Shuttle (8.4 m in diameter, 2000 m3 of cryogenic propellants with a multilayer thermal protection 25 mm thick). Hydrogen is supplied from a 1450 m3 bottom tank (106 t of LH2 at 20.6 K and 300 kPa, =73 kg/m3), at a rate of 210 kg/s (3 m3/s), while oxygen is supplied from a 540 m3 top tank (630 t of LOX at 90.6 K and 250 kPa, =1150 kg/m3), at a rate of 1300 kg/s (1.1 m3/s), both through 0.43 m diameter feed lines, during the 8.5 minutes burning time, positioning the Shuttle at 115 km height. Notice that for 1300 kg/s of LOX, the stoichiometric LH2 is 1300/8=163 kg/s, but 210 kg/s is supplied, this excess hydrogen being used as coolant to protect the engine. Each of the three rockets yield up to 2106 N thrust. Ariane fuel Ariane 5 has two solid boosters and two liquid-propellant stages, with a total thrust of 11106 N and a total take-off mass of 750103 kg. Each booster is 31 m long and 3 m in diameter, uses 240103 kg of a PCA + HTPB + aluminium solid mixture, yielding 5106 N thrust during 130 s (up to 55 km). Stage 1 is 31 m long and 5.4 m in diameter, using 160103 kg of LH2/LOX and yielding 1106 N thrust during 580 s; the combustion chamber is at 10 MPa. Stage 2 is 3.3 m long and 4 m in diameter, using 10103 kg of monomethyl hydrazine / nitrogen tetroxide and yielding 27103 N thrust during 1100 s. REFERENCES http://en.wikipedia.org/wiki/Pyrotechnics. R.E. Reish, 1996, "Airbags", Scientific American, 116. J.A. Conkling, 1996, "Pyrotechnics", Scientific American, 102. (Back to Combustion) 1 COMBUSTOR CHARACTERISTICS Combustor characteristics ............................................................................................................................. 1 Inlet flow (intake)...................................................................................................................................... 2 Dual fuel combustors ............................................................................................................................ 3 Internal flow .............................................................................................................................................. 4 Outlet flow (exhaust) ................................................................................................................................ 4 Heat and work flow ................................................................................................................................... 4 Power intensity...................................................................................................................................... 5 Efficiency .............................................................................................................................................. 5 Power modulation ................................................................................................................................. 5 Combustor internals .................................................................................................................................. 6 Type of flames ...................................................................................................................................... 6 Type of geometry .................................................................................................................................. 6 COMBUSTOR CHARACTERISTICS A combustor is the equipment where combustion takes place, usually a combustion chamber, and it can be a complex system that is best studied by considering its main parts and processes separately. The different types of combustors were reviewed in Combustion characteristics; we here intend to give a general idea of the basic parts of all combustors. The analysis of a combustor may be, in order of increasing generality:  Specific combustor types: steady and unsteady combustion chambers, and practical burners.  Generic combustor subsystems and processes: intake, injectors, igniters, energy flows, exhaust, safety and controls.  Basic combustion processes: combustion at rest, non-premixed flame, premixed flame (deflagration wave, detonation), pollutant generation.  Fundamentals: Chemistry (in particular Chemical Kinetics), Thermodynamics (in particular Chemical Thermodynamics), Heat and Mass Transfer, Fluid Mechanics (in particular gas flow and turbulence)...  Ancillary sciences (needed for the modelling and validation of theories and practical design): Mathematics (analytical and numerical modelling) and experimentation (test rig preparation, instrumentation, data acquisition and analysis). The practical goal in the analysis of combustion devices is the prediction of its performance (for present and future combustors), in terms of the multiple physico-chemical phenomena involved; it is thence a prerequisite to analysis the latter. We try to focus here on combustion basics, i.e. consider a generic combustion system as used in applications (Fig. 1), but analyse mainly its fundamentals. The objective is to understand the influence of the main parameters in combustion, in terms of predictive models (not just descriptions of particular cases). Q Inlet Combustor Environment Outlet F O P W 200 MPa for direct injection compression ignition Diesel-engines (DICI). =A/A0. yF.2 Fig.. O: oxidiser.  Mixture ratio specification. convenience (better if fluids). or at the intake manifold as in most current Otto engines. For coal chunks and other solid fuels burning over a grate. either far upstream in the carburettor of a classical premixed-combustion engines. xF. etc. octane index promoters.  Mixture preparation. Most modern injection systems are electronically controlled. e=-1. Most industrial burners and SI-engines use near stoichiometric mixtures. Types: ambient air is the most used because it is free. price. whereas CI-engines and gas turbines use very lean mixtures. etc. and other oxidisers (as for rocket propulsion) may be used. Lean pre-vaporised premixed combustion (LPP) is under study to reduce the temperature and NOx emissions in gas turbines. or vaporised into the oxidiser stream. one for under-fire air. it is the same in molar and mass basis). or non-premixed. instead of air ratio). a range that is extended in stratified-charge SI-engines (direct gasoline injection may work up to <3. the best in mixture preparation is to split the air intake in two streams. excess air. a) Generic steady-state combustion chamber and its interactions.6. =1/. It depends on the type of combustion wanted: premixed. Inlet flow (intake) What to study:  Fuels. b) unsteady chamber in reciprocating engines (F: fuel. There are several ways to specify the mixture ratio: fuel molar-fraction in the mixture. mixture (massflow-rate) fraction. Liquid fuels must be quickly vaporised within the oxidiser (only vapours burn). lubricants. A detailed description of fuels can be found in Fuels. the outlet may (theoretically) be considered split in as many ducts as chemical species are (to provide a common thermochemical reference to each species). A. indirect fuel injection at the manifold may be at 300. is named excess air ratio or even excess air.400 kPa. but preheated air. additives (water. air/fuel ratio (molar or mass). Q: heat. availability. f. Types. there may be neither inlet not outlet (in reciprocating engines with valves closed). The liquid fuel may be supplied at high pressure into the combustion chamber in order to quickly form a burning spray.pdf.10 MPa for direct injection spark ignition Otto-engines (DISI) and 30. W: work). it the same in molar and mass basis). air ratio (to stoichiometric air for constant fuel flow. the inlet duct may be actually split in two (one for the fuel and another one for the oxidiser). . SI-engines using three-way afterburning catalysers must use stoichiometric mixtures (=1) for proper operation.. and the other for over-fire air. also known as aeration or simply 'lambda'. In Otto engines. or within the cylinders (direct injection). depleted air (mixed with exhaust recirculation). fuel/air ratio (molar or mass).  Oxidisers. but homogeneouscharge SI-engines may work up to <1. and direct natural-gas injection up to <10 of overall mixture ratio). 1. P: products. whereas direct injection in the cylinder at the end of the compression phase may be at 3. stabilisers). and consider other cases as variations of this simple scheme: the enclosure may be imaginary as in a simple candle flame. equivalence ratio (the fuel/air ratio relative to stoichiometry. fuel mass-fraction. It is good to always keep in mind the steady-state combustor with one inlet and one outlet (a simple control volume approach). (Note that sometimes . enriched air (up to pure oxygen).. Fuel and oxidiser must be pumped for normal pressurised combustion (they are aspirated in depression combustors). and nowadays to limit pollutant emissions (e. as when a fleet of city buses is transformed from diesel-oil to natural gas (to reduce pollution) and the modified engines cannot work with diesel-oil any more. and (in most cases) to complement the required fuel load (up to 20% diesel oil plus more than 80% natural gas is the common approach). a gasoline spark-ignition engine cannot be fed with any other fuel. Dual fuel combustors can switch to a different fuel by just manually or automatically turning a control valve. since the combustion process is similar or even better (e. where a gasoline engine system is modified to run also on propane (LPG really) or methane (really compressed natural gas CNG typically at 20 MPa. or very little. and cheap dirty fuel-oil for deep-sea operation.g. etc. by non-premixed feeding.). intake-port injection or directcylinder injection). but this is of little interest. C+aO2=bCO+cCO2+dO2 or aCuHvOwNxSy+aA(c21O2+c79N2)+cH2O+dO2=(xiMi). And similarly for a kerosene-fuelled gas turbine or any other internal combustion application. etc. depending on the choice of unit-fuel-amount or unit-exhaust-amount of substance. convection. later to cope with fuel restrictions (gasogens during the wars). for spark engines: carburetion. like a boiler and some type of furnaces. evaporation and so on.g. within small variations. of course.  Diesel dual-fuel engines. For instance. LPG and NG have larger octanenumber than gasolines). except when an existing combustor is to be definitively transformed. what might be reduced by compressing LNG if available. really. external combustion applications. Notice that sometimes the term dual-fuel diesel engine refers to the capability to run on light and heavy fuel .3 Chemical notation (particularly hydrocarbon chemistry) and stoichiometry (the proportions in which atoms are combined in molecules) must be considered. DUAL FUEL COMBUSTORS Combustors have been traditionally classified according to the type of fuel they were designed for. for most practical systems. city fleets of taxis and buses on LPG and NG. as when up to 20% bio-ethanol is added to normal gasoline. Dual fuel diesels presently run only in the 4-stroke mode. clean diesel oil for port and coastal operations of ships. initially to be able to burn new fuels in old systems (coal in wood grates. and for their size. may run with heat produced by the combustion of any kind of fuel.g. where a diesel engine system is modified to run also on CNG or LNG. We do not consider under the dual-fuel heading the capability to use a fuel and electricity (as a second energy source). The common dual-fuel systems in use are:  Otto dual-fuel engines. but the hearth where the combustion takes place is also fuel-specific. C+O2=CO2 or C+(1/2)O2=CO) and actual mixture equations (e. The problem here is that the high-pressure gas-fuel-injection would consume a lot of power (some 30% of the heating value of the gas). Actual mixing. some amount of a compatible fuel may be added. a liquid-to-vapour converter (if any). of course. and distinction made between stoichiometric equations (e. or. any combustor prepared for any fuel might be fully modified to run on any other fuel. for a given oxidiser/fuel ratio.).g.g. But there has always been a push to provide a dual-fuel capability. Of course. natural gas in oil burners. The changes required are: the additional fuel tank (to be refilled in situ or exchanged and refilled off-line). Otto dual-fuel engines switch from one fuel to the other by a simple feeding switch. oil in coal hearths. may be considered also here (e. or liquefied natural gas LNG from a cryogenic reservoir). a in the first instance or A in the second. a gas mixer or a gas injector device. the fuel lock valve. manually or automatically operated. and the switching valve for the driver (most times an additional fuel storage meter is added). by feeding the gas at the modified intake manifold and using some diesel oil in the normal way to ignite the mixture (always). The engine itself is not modified. 5 kJ/mol (-gr). A detailed description of pollutant emissions and fire safety can be found in Environmental effects and hazards in combustion. Most dual-fuel systems work in the normal non-premixed mode. exhaust gas composition (complete-oxidation model. gas turbines running on kerosene or natural gas.. besides the normal heavy-fuel oil. Brayton dual-fuel engines.e. Outlet flow (exhaust) What to study here: types of exhausts conditions according to heat transfer (from isothermal to adiabatic). heat exchangers. it is sometimes wrongly assumed that a given chemical reaction always gives more heat than work. emission of contaminants) and exhaust temperature. Notice also that most diesel-oil engines would run on biodiesel with none or minimal change. i. for the same fuel/oxidiser mixture.pdf. and many times with forced swirling at the injectors. Closely related to the exhaust ducting is the chimney. and even on straight vegetable oil with some modifications.p0)). The use of exhaust scrubbers and exhaust catalysts are crucial for pollutant control. Combustor walls usually have to withstand high temperatures.50 m/s for aeronautical turbines. In modern combustors. and thence an inner lining of refractory material (fire bricks or monolithic) is applied. it might yield even more work than the maximum heating value. Internal flow Internal flow field depends a lot on the type of combustor. amount of heat transfer (from zero to the calorific value). Heat and work flow What to study: energy balance for the combustor: dE=dQ+dW+Hi*dni. A typical dual-fuel external-combustion applications is the steam turbine in a classical LNG carrier ship. and a drastic reduction around 1992 (another 40% from 1990 to 2000). A control electronic unit is required to adjust the dual-fuel supply. amount of work transfer (from zero to the maximum value: the exergy of reaction).. where the natural-gas boil-off is burnt in the boilers. the main one being convection but radiation effects sometimes crucial. the major pollution source in cities. Heat sinks in combustion: walls. Notice that heat and work are path variables. as for instance with C+O2=CO2. chemical-equilibrium model with (xip/p0)i=exp(K(T. heating of a furnace load. recirculation of exhaust gases (EGR) is a common practice. possible condensation. the internal flow is highly turbulent.25 m/s for industrial gas turbines and 25. that has a maximum heat potential at standard conditions of 393. In all practical cases. and some liquid fuel is always required to quickly ignite the mixture at each cycle (engine start-up is done with 100% diesel oil). Environmental effects are usually identified with pollutant emission through the tail-pipe because this is the major source. Heat transfer mechanisms in combustion. . Emissions from cars. the vertical part of the exhaust piping intended to create a natural draught and a far disposal of pollutant emissions.6 kJ/mol (-hr) and a maximum work potential of 394. Typical mean flow velocity inside combustion chambers of gas turbines are in the range 15. like within furnaces. used in stationary and large mobile combustors.4  oils. have realised a unitary reduction (per car) of 50% from 1970 to 1990. Furthermore. but there are others (see below). External combustion applications running on dual fuel are basically a dual burner within the same combustor. being different for constant-volume combustion than for constant-pressure combustion. but some lean premixed combustion systems (vaporising the liquid fuel before combustion) are also been developed. Combustion intensity is the product of mean reaction rate times fuel heating value. but the more-powerful normal premixed flame gets unstable below 50% of full load (flash-back extinction occurs. Sometimes. without a secondary on-purpose heat-extracting fluid).  Combustion efficiency. Outside of this flame-stability range. EFFICIENCY Different kinds of efficiencies can be defined:  System efficiency.. In any case. and CO and NOx increase). is some 30% for a boiler and 40% for an engine). or per crosssectional area perpendicular to the main flow in the combustor (in kW/m2). Non-premixed flames are stable in a very wide range of flow rates. per unit of fuel exergy input. It is usually defined as the ratio of useful energy extracted from the system (a boiler. the usual assumption in most combustors is that the heat lost through the external combustor walls is negligible against the enthalpy flows involved. since hydrogen oxidises more readily.. POWER MODULATION Combustors are usually optimised to work at a nominal load.. and thus of power. volatile organic compounds) and condensables. and blow-off extinction in the upper limit). Typical values are 90% for a boiler and 40% for an engine. but nitrogen oxides may be present too. inside a porous matrix burner). Hot water (in line) requires a minimum of 15 kW for small periods (less than 15 minutes). Real combustion processes do not yield their maximum heating value because of unburnt emissions (soot. i. and the heat loses through the wall.e. or new porous combustors developed.e. either a nonpremixed flame is allowed (with size increase. the temperature must be increased (e. an exergy ratio being sounder (the exergy extracted. an engine) to the available heating value of the fuel supplied (care must be paid to find if the HHV or the LHV has been chosen). Notice that this definition corresponds to a system efficiency if one considers the combustion gases themselves as the energy-extracting medium (like the heated water in a boiler).150 . consider the case of home boilers. or a catalyst must be used (catalytic combustion may be used to stabilise low temperature flames instead of increasing their intensity). but are usually required to work at different loads.g. They usually provide space heating and hot water. with the 'lost fractions' being the residual chemical energy of unburned components in the exhaust.. but the numerical values usually clarify the distinction. Combustion efficiency in hydrocarbons practically coincides with the carbon conversion fraction from fuel to CO2. the area being burnt must be increased (e. In a fix wall (i. CO. on combustion intensity.g.5 POWER INTENSITY As for other engineering processes.  Combustion efficiency in 'adiabatic combustors'. the rest of the heating potential goes as enthalpy of exhaust gases. combustion efficiency is defined as the fraction of the LHV that exits as thermal enthalpy (sensible and latent) of the exhaust stream. to increase the former. i. to the maximum heating value of the fuel (roughly the HHV). the main interest is not on integral heat and work. whereas a 100. but on heat and work flow-rates.e. For instance. by turbulence). These are just energy ratios that do not take into account thermodynamic limits. a boiler efficiency is called combustion efficiency. or as heat loses to the environment through the walls. without work exchange) combustion chamber intended to be adiabatic (like in a gas-turbine combustor. either per volume of combustor (in kW/m 3). or some burners must be switched off (causing uneven temperatures and start-stop increased emissions). It is usually defined as the ratio of actual heat released with intake at standard conditions and exhaust transported to standard conditions at constant composition. typical vales in well-operated combustors is >98%. partial-.g. etc. basically generic CFD codes. but instrumentation is also applied to the intake to regulate the flow and to the exhaust to monitor emissions.15 kW of heating. The normal premixed flow.. the best is to provide a hot-water storage tank of some 50. the one used in traditional water heaters. Usually identified with low-temperature catalytic combustion. second. operation and maintenance details. yields an unstable flame below 50% of full load.  Non-premixed combustion. .. extinction (effect of air/fuel ratio. turbine. Chemical kinetics is perhaps the hardest theme to study.  Two-dimensional combustion models. first-. quenching).  Homogeneous combustion of condensed fuels. as for planar and axial jets. if more than 15 kW were needed (e..6 m2. depending on wall insulation and external temperature. i.g.100 litres. flame detectors). TYPE OF GEOMETRY Concerning the geometry of the combustion process. branching. usually divided in:  Unsteady process: ignition (local or global light-off). combustion processes may be grouped as:  Non-flame combustion. to evaluate the combustion rates and required sizes and times. initiation-..15 kW (heating is temporary stopped when hot water is needed). the traditional division in combustor modelling is:  Uniform combustion model. anchored and travelling flames. Usually identified with porous media combustion.and termination-reactions.  Three-dimensional combustion models. matching the demand by an on/off control that increases pollutant emissions at every start. and analytical models for internal processes and kinetics (flow and chemistry). as for plug-flow reactors and spherical combustion. TYPE OF FLAMES According to the type of flame. boiler. which depend a lot on the particular application envisaged: furnace.  Premixed combustion. We have focused here only on the characterisation of combustors from the combustion point-of-view. thus current home boilers may be modulated between 7.e. mechanisms for ignition. Sometimes measurement and control systems are here considered (e. etc. if intermittent user-demands (e. and if startup-times of the boiler are to be skipped. as for still-reactors and well-stirred reactors.g. types of flames. usually divided in:  Homogeneous combustion of gaseous fuels.  Steady process: deflagration speed. as a butane jet in air. for better comfort on hot water demands..and frozenequilibrium reactions. Arrhenius law.  Non-visible-flame combustion. propagation-. On the other hand. as premixed hydrogen/air or methanol/air. law of mass action. Topics to be studied include: speed of chemical reactions.  Heterogeneous combustion of condensed fuels. as for coal burning and solid charring. although there are also nearly-invisible flames.and third-order reactions. 4-person family home in mid-latitudes may require from 2. Chemical kinetics and physical transport processes may become too complex if the flow field is not drastically simplified. as gasoline vapours in air. stabilization and extinction. steady. not considering other important aspects of combustors as their construction. two showers at a time). Combustor internals What to study: description of types of burners (see Combustor types). at the expense of its continuous heat loss. at the sink tap) are to be smoothed.  One-dimensional combustion models. elementary reaction mechanism. due to the intricate mathematical models (very stiff problems because of the widely different orders of magnitude involved) and the scarcity of reliable experimental data. the higher the pressure loss in the combustor). and secondary air to dilute the burnt gases and lower the temperature some 500 K before entry to the turbine. in spite of being cooled by secondary air on both sides. Back to Combustion .7 For instance. an outer case holds the pressurised gases against the low external ambient pressure. 2. special thermalresistance alloys are still required for the liner. Fuel is injected behind the leeward dome of the liner through a swirler to accelerate mixing (liquid fuel gets atomised. the higher the turbulence. however. in jet-engine combustor. Diagrams of components and flow paths in a gas turbine combustor (from Wikipedia). and air flow gets more turbulent. and an inner perforated liner holds the hot burning gases while providing primary air sustain the flame. Fig. ....................................... 14 Risk: a combination of hazard and damage ..................................... As usual... b) loss of health (morbidity). 5 Inert atmospheres .............................. Damage may be caused to individuals or to the environment in general...... ...................................................................... damage to property (deformations............................................................................ Coal handling produces respiratory hazards..................................................................................................................................................... 14 Safety management .......................................... 2 Fire safety education ...... here........................ 6 Pollutant emissions .............. and damage to people (injury and loss of life)................ fear............................................ Protection and remediation ......................................... Damage may be ranked in top-down severity order as: a) loss of life (mortality)....................................... Liquefied petroleum gases....... there is also damage associated to the management of fuels (and oxidisers......................................................................... 9 Vibrations and noise................... smoke).................................. and particularly cryogenic fluids like LNG.. ................................................................. 15 Glossary of terms: accident............................................................................ 14 Electromagnetic interferences ............ 8 Fuel tank and crank-case ventilation ................................ if special)............................................................................................................................................ reduce uncontrolled ignition-sources (sparks and hot spots).......... 9 Exhaust emissions and pollution ....... diesel instead of gasoline....................................................................... 14 Emergency response ............................... 2 Fuel detection ............................... 5 Smoke detectors .................. 15 Prevention of accidents .................................................... and.. besides the many services it provides to humankind............................ LPG.................................................g.......... one has to be aware of it............. burnings and explosions)................................. 1 Fire safety........ decrease the impact of the controlled combustion processes (emissions).......................................... it may cause nuisance (e................................................................................................................. A general summary on safety management is included below................................. rely on safer fuels (e............................................... 7 Emission quantification.. and............................................................................................ reduce unnecessary fuel stores..................................................................... Besides damage caused by the combustion process itself........................................................................................................................................................................................................... loss of strength.......g......... 5 Water sprinklers ....... pyrotechnics hazards (from amusement firecrackers to weaponry) are not considered under the combustion heading.............................................................. and plan for the best rescue actions in case of uncontrolled combustion (fire detection and fighting)........... some techniques to minimise risks associated to combustion are studied........ and crude-oil derivatives are carcinogenic............... Combustion is a physico-chemical hazard.. noise................ 16 ENVIRONMENTAL EFFECTS AND HAZARDS IN COMBUSTION Combustion is a hazard...ENVIRONMENTAL EFFECTS AND HAZARDS IN COMBUSTION Environmental effects and hazards in combustion ....... may cause severe frostbite and structural damage (carbon.................. 15 Analysis of accidents .. to minimise its impact......... 6 Fire-breaks and fire extinguishers ...................................................................................................................................... avoid fuel leakages and provide fuel detectors........................................... exposure............and low-alloy steels show a marked ductile to brittle transition at freezing temperatures).... natural gas instead of butane)............................................... c) loss of property and d) loss of activity................................... 4 Fire detection.....  Chemical hazards: oxygen depletion. As a rule. porous media) and post-combustion treatment (catalytic conversion).g. from deadly explosions to inconvenient electromagnetic interferences.  Minimising risk: fire safety education. i.  Continuous degradation. The most effective protection is based on proper fire safety education. fire-fighting brigades). due to pollutant emissions during normal combustion. and not only instructions about particular items. thermal (excessive heat. it teaches people and safety personnel a lot). xCO>1% for 1 minute is mortal (maximum permissible 50 ppm). i. out-range temperatures). gas poison. People are the primary cause of all accidents. or by active means (fireextinguishers). their consequences foreseen (minimising damage). Only a descriptive view of the subject is here presented (some theoretical insight can be found on Combustion Kinetics). hydrocarbon vapours in concentration >0. oily rags. Fire safety. fire sprinklers and fire extinguishing. This impact is fight by developing better fuels (sulfur-free diesel. to arson. The effects of combustion on the environmental may be grouped in two categories:  Sudden uncontrolled events (combustion accidents). FIRE SAFETY EDUCATION Education is the best prevention. SO2. This impact is fought with proper prevention (minimising risk and educating people) and proper fire fighting (smoke detectors. NO2. HCl. automatic firesprinklers. plan a fire safety procedure (e. which may be originated from a combustion process. Protection and remediation Fear of the fire has always frightened humans. We have tried to follow a top-down approach in the analysis of the environmental effects of combustion. rubbish) and even dust may be a fire hazard (old stores must be kept tidy. even at a pre-established time. CNH. Smoking is reported to be a major cause of fire accidents. inert atmospheres. Clean habits already help to prevent fires. radiation (blinding flare). evacuate multi-store buildings once a year. education on fundamental principles. unleaded gasoline). from children left playing with matches. are presented aside. smoke detectors and fire-doors. liquid poisons. solid poisons. too!). Risks must be prevented first (minimising risk). since accumulation of waste (packing material. fighting).e. is the best preventive measure.1% for 1 minute is mortal). whereas >1% ppm produces . and rehearse it (e. Hazard types to humans (for goods there are only two concerns: either destruction by fire.A quick-summary of hazard types associated to combustion may be:  Physical hazards: mechanical (explosion). there are many fires that are not fortuitous accidents but directly caused by irresponsible people. Contrary to common sense. aerosols.g. Thermal effects on materials in general. portable fire-extinguishers. For every identified risk. Again. better combustion processes (fluidised bed. gateways. get a hose or a water bucket nearby when making an outdoor fire).  Minimising damage: fire-breaks. The main toxic gas in combustion is CO. their occurrence appropriately handled (early warning. and the consequent damage allocated in justice. other toxic gases may be SH2 (xSH2>0. NH3. Fire protection can be achieved by passive means (using fire-resistant materials).e. NO. poisoning.1% in volume (1000 ppm) cause eye irritation after some minutes. or fire promotion):  Toxicity of emissions. second-degree burns (large blisters on the skin). Burning. i. i. a small blister). the maximum heating taking place 3010 minutes after ignition  Tbuildup=1100100 K. if <19. dehydration and ablation of tissues. particles and droplets aerosols. i. even in the driest.g. a circular oil-pool fire of a 1 m2.   unconsciousness after a few minutes. Open fires show larger burning rates because of better air supply. a very hot environment. insufficient oxygen (asphyxia means lack of pulse). i. Smoke. but the temperature at the flame tips is 65050 K in open fires and 80050 K in enclosed fires. may range from mild skin burns to incineration. In both cases there is a turbulent flame that completely fills the space above the fuel to some height (nearly independent of pool size for >1 m2). suddenly. at 90 ºC and less than 10% RH for some minutes may be healthy). At 40 ºC and 100% RH death follows after a few hours (but a steamsauna. pressure jumps. Suffocation and hyperthermia. Anoxia.5%. what may be explained in terms of the fuel/oxidiser characteristics. significant changes take place. after one and a half or two hours after ignition. with A=345 K and B=8 min-1. i. and.01 m/s over solid fuels. dry and painless).07. There are some basic differences between open and enclosed fires: flame tip temperature.e. or indoors in the middle of say a 3∙3∙3 m3 room.22%. In both cases the bulk region is at a nearly uniform temperature of 1100100 K. oxygen is locally depleted and CO and HCN abound..5% work with oxy-mask.. Breathable oxygen concentration range is 19.e. the values for a 1 m 2 kerosene pool being 0. a build-up period of some 10 minutes after ignition. Consider the same fuel in air geometry. if >10%&<20% be on alert. Nordic sauna. but below xO2<15% muscle fatigue is noticeable. i. Each fire is unique. and calcination. T=Tamb+Alog(1+Bt). CO2 is not toxic. resulting in heat exhaustion and heat stroke. Burning may be due to hot objects.05 kg/(m2∙s) for enclosed fires and some 0. The ISO standard fire-event is just an exponential heating.e. at 40 ºC for a few minutes may be healthy. due to the different heat radiation loss in each case. Monitored by electrochemistry (ZrO2). burning rate. oil spillage) at room temperature is at some 0.g. if >22% risk of fire. The spreading of fire over a quiescent liquid-fuel pool (e. cause eyes and nose irritation and orientation lost (panic). i. the ceiling layer. death follows after a few minutes (but a dry sauna. at xO2<5% consciousness stops. while sufficient heat is released for a thermal runaway. and some 0.0. at xO2<10% reasoning stops. usually graded as first-degree burns (skin surface painful and red.e. i. but most domestic and industrial fires (explosions not included) have the following common characteristics:  tbuildup=105 minutes. if the enclosed fire is close to a wall.e.e. Normal air has xO2=21%.02. most important.  Tresidual=700100 K.08 kg/(m2∙s) for open fires.. a maximum heating (maximum temperature relative to the ambient) of 1100100 K. above xO2=18% normal breathing can be sustained. Turkish bath. inability to evacuate metabolic heat to the environment.  tmax heat=3010 minutes.. if >20% leave the room. a residual slowly-decreasing heating of Tamb+700100 K. at 50 ºC and 100% RH death follows after a few minutes). or direct flame contact: superficial skin burn (1st and 2nd degree) takes 6 hours at 45 ºC but 1 second at 70 ºC.e. either outdoors. third-degree burns (destruction of both epidermis and fatty dermis. . At 120 ºC.e.0.1 m/s. in calm air.3 m in total). e. plus some flame tongues or flame tips going further up (some 2. Monitored by explosimeters (oxidation in a hot Pt-wire): ok if <10% LEL (). just deprives of oxygen: adding >10% CO2 to air is mortal after 1 minute. in closed spaces). Depending on openings and ventilation. Accidental ignition sources are so common (sharp knocks. Fuel-gas leakage detectors are also called explosimetres. an oxidiser (that is always available. since they will not be ignited by accidental sparks or distant flames. that must be specially treated. and any uncontrolled treated as an immediate fire hazard with ample ventilation and evacuation. The usual fuel-leakage detection method for combustible substances is visual inspection of liquid-fuel tanks. fire safety starts by identifying and detecting fuel sources. of course. Non-flammable combustible substances are of secondary concern to fire safety. until the flashover temperature of 90050 K (typical autoignition temperature) is reached in the ceiling layer with the sudden burning of all combustible gases generated by pyrolysis and vaporisation of other combustible matter existing in the room. Besides pyrotechnics and explosives.1 kPa for a few minutes (depending on ventilation). Flammable substances must be so labelled.2. electrostatic sparks). They are usually based on the electrical resistance variation of a platinum wire.Enclosed fires give way to significant pressure changes in the interior. If the fire is extinguished. cigarettes.. electrical contacts. If the fire goes on and a flashover in the ceiling layer takes place. FUEL DETECTION A fire needs a fuel. . after a few minutes from ignition. inside pressure suddenly rises some 0. a sudden pressure increase occurs that may break windows or walls. A two-zone thermal model. due to the initial expansion of hot gases. stoves and other hot points). Flammable liquids (gasoline. however. butane and propane. and the vertical speed of gases also decrease due to air entrainment (from a maximum of up to 10 m/s). Because ignition sources are commonly present (cigarettes. and the emergency measures should be proportional to spillage and inversely proportional to flash-point temperature. either along pipes and combustor (gas leak). is often used as a first stage in the analysis of enclosed fire development. due to contraction on cooling of the hot gases inside. a uniform ceiling layer of hot gases floating over a floor layer of fresh gases and a narrow hot plume. due to the temperature increase caused by rapid catalytic oxidation of the fuel-air mixture at room temperature in the surface of the catalyst. they form ignitable mixtures with air during the process of mixing (and perhaps after full mixing. enclosed fires build a hot ceiling layer that slowly increase temperature with time. that the risk of ignition is very high.0. Portable system with field replaceable measuring cells are in the market capable of sensing minute concentrations of natural gas. or in the ambient (explosimetres). fuels may be classified as flammable substances (gaseous fuels and high-vapour-pressure condensed fuels) and combustible substances (lowvapour-pressure condensed fuels). and an ignition source. amongst them. there is a small underpressure of order 0. then the pressure fluctuates around the original ambient pressure. due to the hindered heat losses. only liquid fuels (particularly gas-oil and kerosene) are of special risk. liquefied petroleum gases) and gaseous fuels are an obvious hazard since on a leakage occasion. before incurring on the expense of CFD codes. alcohols.8 kPa for a few minutes after fire builds up. exchanging mass and energy. The most important difference. ambient air). is that. heaters. although in both cases the temperature slightly decreases with height further up the flame tips. it is not enough to have a quantity less than the lower ignition limit in a closed room. more generally. Water sprinklers are very reliable and safe (the chances of a sprinkler accidentally going off are remote) and minimise the water damage associated to fire fighting. americium. triggering the alarm. The detector can be used to ring an alarm or directly fight the fire by active means. There are two basic type of smoke detectors:  Ionization detectors.  Photo-electric detectors. They contain a very small amount of radioactive material (e. fire alarms usually require a temperature above 60 ºC plus a heating rate above 1 ºC/min. WATER SPRINKLERS Automatic water sprinklers. making an electrical path. to avoid explosions. and water is discharged only through sprinklers that have opened due to exposure to heat. SMOKE DETECTORS Smoke detectors are used to provide a quick alarm (acoustic at least) once a fire breaks out. Some flame detectors and thermal detectors can be found in Combustion instrumentation or in Thermal effects on materials. They were introduced as back as the XIX c. a low-melting-point or thermoelastic device. since fire extinguishers always cause damage. the transmissivity changes. an aluminium rod that expands and breaks a glass stopper). connected to a wet-pipe system (only suited for areas not subject to freezing). For thermal detection. all the others remaining intact. a radioactive metallic element produced by bombardment of plutonium with high energy neutrons) that ionizes the air. where an electric current is established. the flow of water out of the sprinkler system trips a monitoring device. They contain a light source (usually a small bulb) and a photocell. When smoke enters within the light path. In large systems. it can be detected by different methods:  Smoke detectors  Flame detectors (infrared or visible)  Heat detectors or. Smoke detectors should be located in exposed areas of the room. typically a water flow detector. for that purpose. and they are based on an operating element. triggering the alarm. and the smoke detector trigger might be due to a false alarm). and not in dead-air corners (a forced air convection is applied in spacecraft racks. .. using low-melting-point or thermoelastic switchers (e. They are usually combined with automatic water sprinklers (after a warning period of alert.Sometimes. Mercaptan odorants are always added to gaseous fuels so that they can be detected by scent before reaching explosive levels. care should be taken not to confuse fire with other inoffensive heating. FIRE DETECTION Once a fire breaks out.g. thermal detectors.g. since very light fuels like H2 or very heavy gas fuels as diethyl ether (C4H10O) will stratify a lot. When smoke enters within the path. aside. or any other fire-fighting fluid. which activates a central fire alarm. and to enhance heat removal). the smoke molecules attach themselves to the ions and the electric current changes. usually by water sprinklers. water pressure pushes the sprinkler cap aside. provide both fire detection and suppression. that clears the water exit. producing a fine and high-efficiency water-vapour mist that maximize cooling. etc. an early alarm to trained personnel is most of the times better than a personal attempt to extinguish a fire. relative to the surrounding air.e. where the oxygen molar fraction in air xO2=0. liquid. foam. and dry-powder (NH4H2PO4. e. Besides fire prevention.08 by adding exhaust gases from a combustor (the air/fuel relative ratio cannot be too large. A fire-break is anything that stops a fire from spreading: a strip of cleared land in a forest. it must always have some overpressure. and very difficult to get rid of ignition sources. CO2. is good enough). HFC-227ea. B and C):  Type-A fires involve ordinary solid combustible substances (coal. mono-ammonium phosphate. Inert atmospheres are routinely used onboard crude-oil tankers. vehicles. and can be extinguished with a cooling agent (water. are all good for types A. and fuel tanks. When a sprinkler head is activated by a fire.08 by adding nitrogen (nitrogen with 5% impurities.Dry-pipe systems are essentially the same as wet-pipe systems except that the pipes in the protected area contain pressurized air. i. depleting oxygen from the air) may be the best fireprevention action. etc. Procuring a local inert atmosphere (i. a hydraulic seal. or ammonium dihydrogen phosphate. low flash-point chemical cargo ships. non-freezing. earth) to arrest the generation . Extinguishers must be located near escape routes. rubber and many plastics).. the pressurized air is released and water flow occurs. as obtained with selective permeation membranes. nitrogen systems are used as super dry (Tdew<200 K). INERT ATMOSPHERES Many industrial fires take place when handling flammable materials or when welding near combustible materials. cloth.21 is reduced below xO2=0. In most circumstances. HFC-236fa. ADP). electrical or metal. industrial installations. a fire-retardant door. since it is impossible to get rid of combustible materials. halons. but for large fires the first action must be to isolate the damage with fire-breaks. non-oxidative.  Burnt atmosphere. allowing for freezing temperatures. wood. up to =2 is good enough). an inert atmosphere is one having xO2<5% (not to be confused with an under-lean atmosphere.e.g. But there are several other characteristics that make a material fire-resistant to different degrees. extinguishing fires with minimal amounts of water (a few litres for a typical room). For a tank to be really protected with an inert atmosphere. A new fire-sprinkler technology is based on delivering water at very high pressure (requires special pumping). Fires and extinguisher are classified according to the material that is on fire: solid. Small fires may be fought with fire extinguishers. they neither selfinflame. where the oxygen molar fraction in air xO2=0.21 is reduced below xO2=0. nor can be ignited by a near spark (ISO 1182). FIRE-BREAKS AND FIRE EXTINGUISHERS Once a fire is noticed. although many extinguishers are good for several types of fire (and so are labelled. a masonry wall. their substitutes (e.g. wires.7 m3/s from air by membrane technology. non-corrosive atmospheres. which is one with less flammable vapours than the lower ignition limit for the fuel). if when heated to 1000 K in a furnace for 3 hours. FM-200 and Inertgen). The two basic inertisation procedures are:  Nitrogen atmosphere. Materials are usually termed non-combustible if they cannot combine with oxygen at all or if their flashpoint is higher than 1000 K. Fire-resistance tests are performed on building construction elements (ISO-384). the largest nitrogen-generator producing 1. which have been substituted by heptafluoropropane (R227. both locally and globally. but the latter usually requires straightforward measures as firebreaks (removing further fuel ingest). a liquefied gas). which has substituted CO2. and may inflame many hours later by a blow of air. portable extinguishers. etc. liquids are called flammable only if their flash-point is below 48 ºC (that of diesel). Inertgen. a compressed gas). natural gas. solvents). the resulting x O2<10% is already low to sustain fire propagation (but able to keep human conscience during several minutes). GLP. hoses. Typical hazards at home are cloth fires by cigarettes. iron. axles. Type-B fires involve flammable liquids and gases (oil. C3HF7. . and Halon 1301 (CF3Br. do not carry the pan outside or throw water over). and must be extinguished with an electrically insulating air-barrier (e. spreading chemicals that decrease the flammability of wood. The most used were Halon 1211 (CF2ClBr. Powder fire extinguishers are loaded with NH4H2PO4 or NaHCO3 powders and a pressurised CO 2 cartridge that. Typical hazards at home are fires at the kitchen by overheated oil or grease when cooking (covering with a lid is usually enough to stop the fire. the standard fire-extinguisher fluids during the last half of the 20th century were halogenated hydrocarbons (halons). the rest being called combustible liquids. Type D fires involve metals such as magnesium. with the advantage that they do not leave a mesh of foam. Inergen (IG-541) is a mixture of 52 % N2. because life must live at the expense of the environment. i. CO2 fire extinguishers are loaded with liquid CO2 that.   of fuel vapours. Pollutant emissions Life is a polluting process. Type-C fires involve electrical potentials (wiring. to avoid the problem of blocking blood decarboxylation. water tanks. pushes down and expels the powder as a fine mist that dissociates to CO2 and H2O in contact with the fire. bromochlorodifluoromethane. 40% Ar and 8% CO2. when mixed with air more or less half and half (they are almost isodense). breathing apparatuses. The problem is that the amount and concentration of pollutants emitted by human activities has gone too far and seriously menace life. They are used to fight domestic. whereas the resulting x CO2<4% is low enough to not blocking blood release of CO2 at the lungs. ladders. their embers remain hot inside for long. aluminium. FM-200) and never with water (the fire would spread). energized electrical equipment). they are very dangerous and should not be handled by untrained personnel. titanium or sodium. water may cause electrocution).e. These fires are smoky and cause disorientation and toxic fumes (remember: the air in a fire is cleaner at the floor). lacquers. Fire engine trucks carry an extensive assortment of tools and equipment: pumps. paints. grease. also named FM-200). gasoline. and which penetrates everywhere (the mesh they create may outbalance their use in small fires). flooding with water. bromotrifluoromethane.g. As for refrigerant fluids. communication equipment. which had to be substituted in the 1990s by non-ozone-depletion fluids. Beware of arrested solid fires. CO2. Sometimes. when the valve is triggered. when triggered. industrial and forest fires. fuse boxes. where the liquid flashes to a mist of dry-ice particles and CO2-gas. electric lights and generators. or spreading smothering materials such as dirt or sand. focused along a long funnel (that may become very cold and should not be touched). stoves and unsafe playing with lighters and matches. is expelled from the bottom through a riser pipe and finally expanded at the end of the high-pressure hose. and are extinguished with an air-barrier dry-chemical (CO2.  Mass-to-energy emission ratio. besides the unavoidable CO2 in carbon-containing fuels that contribute to the greenhouse warming (the legacy left by these emissions would be felt mainly by our offspring). even splitting between methane and NMHC (non-methane hydrocarbons). of course. the term HC (hydrocarbons) is used. 40 g/m3 of PM10. It is intended to weight the burden against the real benefit of the combustion process. units of g/kg are common. 1000 g/m3 of CO. EERi. defined as the mass of i-contaminant emitted divided by the energy supplied by the fuel burnt. e.5. . instead of VOC. and product losses from the combustor shell. 30 g/m3 of O3 and 20 g/m3 of SO2. DERi. fuel losses at the inlet. defined as the mass of i-contaminant emitted divided by the mass of fuel burnt. whereas Otto engines do just in the reverse order. Diesel engines produce more pollutants in the stated order (more PM and less VOC). NOx. the goodness of transportation is not the power of the engine but the distance the payload travels. etc. It is intended to weight the burden against the benefit of the combustion process. SOx. Some 40% of man-produced VOCs come from transport (not only through the tail-pipe but from reservoirs and at the stations). To give an idea of the pollution in a major city.g. but it is the best method from the combustion point-of-view to quantify emissions. etc. mass-to-distance travelled emission ratio. EIi. This ratio serves to compare pollution from other energy sources different to combustion. to make a chimney for venting the fireplace. but handling of fuels. 70 g/m3 of NO2. the average figures for measured values in Madrid are: 1000 g/m3 of CO. or PM10 to explicitly restrict to sizes <10 m). city cycle or highway. Main contaminants. respiratory and visual irritation by smoke and noxious gases. hyperthermia by heat. animals.g. e. to build power stations aside. Mechanical pollution (noise) and electromagnetic pollution (interferences. This measure is independent of the application at hand and the dilution applied. or water heated energy for boilers). irrespective of the number of actual occupants). plants and goods: explosion danger (in confined places). and the payload is not fixed beforehand (e. Although a dimensionless ratio. thus it does not enter into considerations of the actual effects of the emissions or the real need of the process. open flame danger. The effects on the environment are usually identified with pollutant emission through the tail-pipe of combustors.g. but great care must be paid to ascertain if the energy supplied is just the heating value of the fuel burnt. are dealt apart. toxicity from CO (and other toxic gases in chemical fires). Soot is formed in non-premixed flames and on premixed flames for equivalence ratios >1. distanceemissions ratios are given for a car in a given trajectory. defined as the mass of icontaminant emitted divided by the distance travelled. EMI).  Mass-to-purpose.The first and traditional approach to fight pollution is to go away or throw it away. and particulate matter (PM. suffocation or anoxia from CO2. or the final energy delivered to the payload (shaft work for combustion engines.g. but with little relevance since the emissions are so different (e. are: VOCs (Volatile Organic Compounds). but time is also important. to have out-side-air sealed combustion boilers. are other sources (up to 20% of the hydrocarbon emissions in a car do not go out along the tail-pipe). but great care must be paid to measure the benefit. EMISSION QUANTIFICATION Quantifying emissions is not a trivial issue and several methods can be followed:  Mass-to-mass emission indices. CO. fossil power stations). nuclear vs. Mass losses and energy losses may be a danger to humans. Sometimes. Exposure to air pollution can cause adverse health effects. oxides of nitrogen (NOx). most acute in children. Environmental pollution locally depends on the amount and type of industrial activities. A major milestone in world pollution control was achieved in the Kiotto-1997 protocol. The crank-case (oil box) is also ventilated to the admission pipe. ultrasonic. volatile organic compounds (VOC. and vice versa. organometallic and others. Fuel tanks are pressurised only for gaseous fuels. and the rest to industrial and other activities). wild fires. climatology. About 1 % of the exhaust gas stream is harmful. rocket propulsion) give off many more hazardous contaminants not considered here: halogenated (dioxines. In premixed combustion. have a great impact on the environment. but CO2 is already considered a pollutant due to the overall green-house-effect contribution if non-renewable fuels are used. the transport sector is a major source of air pollution (amongst it. 30% due to transport engines. 15109 kgNOx.FUEL TANK AND CRANK-CASE VENTILATION Up to 20% of the hydrocarbon emissions in a car may not go out along the tail-pipe but directly from the fuel tank. and the dominant source in urban areas.. ICE are responsible for nearly half of the world contamination load. Just in EU. For condensed fuels the key leakage diagnostic is spillage tracking. Nowadays. We intend here to analyse pollutants going out the exhaust pipe of mobile and stationary combustors that. the annual emission of CO2 is some 31012 kgCO2 (40% due to electricity generation. road transport takes more than 80%). Continuous emissions monitoring (CEM) is mandatory in most major combustors. and is stabilised (other emissions are slowly declining: 15109 kgVOC. waste incineration. the cultural heritage). liquid-fuel tanks are no longer directly ventilated to the ambient but through an active-carbon filter (canister) that retains the fuel vapours. and catalytic methods are used. also named hydrocarbons: HC) and particulate matter (PM). UV-fluorescence. Non-premixed combustion may be much more pollutant: the flame sits near the stoichiometric diffusion-rates and thus . although in concentration much lower than CO2 and H2O (typically less than 1%). etc. liquid-fuel tanks are not pressurised but ventilated to prevent build-up of pressure and facilitate refuelling. the crank-case and other fuel leakages.). because there is an important fuel loss from the combustion chamber to the oil box through the slipping rings. EXHAUST EMISSIONS AND POLLUTION The complete combustion of CuHvOwNx fuels would only yield CO2 and H2O as new compounds. furanes. Combustion processes where non-commercial fuels are burn (e. particularly during cold operation (at starts). weather conditions. emissions depend upon the air-fuel ratio. Water is thought to be processed globally by the hydrological cycle (although local condensations may be corrosive and be the best culture for moulds and bacteria). density and age of vehicle fleet. and consists of carbon monoxide (CO). whereas for gases. asthmatics. efficiency in combustion. and can damage vegetation and materials (notably. during normal engine operation. but unfortunately when the concentration of CO and HC decreases the concentration of NOx increases.g. and the elderly. hydrogen chloride). Fuel leakage can be detected by several methods. etc. 10109 kgSO2. bubbling (usually by applying a soap solution). where a target on anthropogenic greenhouse gases emissions of less than 5% the 1990 world figures was established for 2008. In most developed countries. some fresh air is passed through the canister to the admission pipe to burn the vapours and regenerate the filter. and to use low-C fuels (shift from coal with C/H=1. and a 5% already produces troubles after 1 hour. better.g. Use of CO2 as by-product to other industries is too small: as a chemical stuff for methanol and urea synthesis.04% molar). Some other large scale ‘enhanced natural ways’ to get rid of CO2 have been suggested. To get pure oxygen from air. environmentally). floods. There is little concern with local contamination by CO2 emissions. hurricanes. and change to a hydrogen economy. CO2 is used to help recover residual crude-oil at some wells. get rid of the solid C. as an inert gas in welding and inertization of large fuel installations (dry stoichiometric exhaust gas from a combustor is the cheapest choice). However. the foreseeable reduction in CO2 emissions seem not enough. which is some 85% by weight C. This has no sense for coal. a 10%CO2 is fatal to a person after a few minutes. CO2 capture may be based on different approaches:  Capture the emitted CO2 from conventional sources. although new approaches are been tried. to natural gas with C/H=0.its temperature is very high. in the preparation of carbonated beverages. etc.  Capture the emitted CO2 in a better way by modifying the combustion process. it may cause local suffocation. and the pure fuel approaching it gets pyrolysed with large production of soot (that give the characteristic yellow colour to non-premixed flames) and other volatiles. i. This is impractical once the CO2 is diluted in the atmosphere (x=0. with a small global warming but larger regional changes (desertification. as a solvent in the cleaning industry. or sorption) is presently too expensive. a reforming stage can be used to decompose CH4=C+2H2.  An indirect way out to CO2 capture is to reforest.15. by anoxia. and eventually to H2 with C/H=0). and technologies to capture the CO2 before it goes to the atmosphere must be applied in the near future. engine and associated transmissions. yielding a pure CO2 exhaust. or CO).20% molar). The main concern of CO2 emissions is at global scale. but a possible solution for natural gas (e.e. as promoting carbonatation of marine biota . causing the formation of NOx. or a CO2 / H2O gas mixture easily separated by water condensation. CO2 Carbon dioxide (CO2) is an unavoidable emission in the combustion of carbon-containing fuels. CO. being an inert gas like nitrogen. and then reduced to yield pore O2 and the regenerated metal  Fuel decarbonisation before combustion. with the positive correlation found between anthropogenic CO2 generation and the increase of CO2 fraction in air (e. but chemical separation has made good progress in the carbonatation-calcination process (CO(gas mix)+CaO=CaCO. but is being tried in large coal-fuelled power stations (with x=0. separated. and the foreseeable consequences on climate change due to the associated increase in the global greenhouse effect. as using an intermediate metal that gets oxidised.25. There are two ways to decrease CO2 emissions: to decrease overall emissions by increasing the energy efficiency of the combustor. and seems impractical too in transport. Physical separation (by membrane technology. and so on.5. traditional cryogenic distillation or membrane separation may be used. capture the carbon (in the form of C. cryogenic distillation. followed by CaCO+heat=CaO+CO). to 380 ppm molar in 2007). Like nitrogen.3 and liquid fuels with C/H=0. which is feared to be highly non-linear. CO 2 is always partially recirculated.. to GLP and. To avoid too high temperatures. The only way out is then CO2 capture (or sequestration).g. as a refrigerant (including dry ice applications). and permanent disposal in deep soil reservoirs (deep-sea disposal seems too risky. from 310 ppm molar in 1950. for the trees to fix the atmospheric CO2 into biomass (which can be later used as fuel source or not). to be performed with pure oxygen what is known as oxy-combustion. Measured in the blood. mainly NO and NO2. starting by proper food preparation. some dinitrogen tetroxide N2O4 (2NO2=N2O4(g)+57 kJ/mol) is exothermically formed. and two approaches are followed to avoid their emission: avoidance of high temperature formation (by using very lean mixtures. When heating an ampoule containing NO2 from above. but also N2O. N2O4 and N2O5. NO concentration can be measured by chemiluminescence with ozone: NO+O3=NO2+O2+h. N2O4 is a colourless heavier gas that appears at the bottom (because of buoyancy). like in a hotel room. chewing and so on).g. CO Carbon monoxide (CO) is found in exhaust emissions due to a poor combustion process (i. and so on (it is a fact that only a third of the CO 2 emissions accumulate in the atmosphere. i.g. exhaust gas recirculation.by seeding the sea-surface with iron particles.e. usually by a catalytic oxidiser (e. NO (N2O4=2NO+O2. N2O3. contrary to unburnt emissions. too rich a mixture. rapidly converts to yellow NO2. nitrogen oxide.e. the rest being captured by the oceans). unburnt fuel pyrolises at crevices. lowers the blood oxygen content by producing carboxyhemoglobin. CO and CO2 are usually measured by infrared absorption. is a colourless gas that in the presence of atmospheric oxygen. idle. for clean air standard EU sets a limit of 40 gNO2/m3. the tree-way catalyser in Otto engines). Two types of approaches may be followed to fight CO emissions (besides minimising fuel use): to avoid CO formation inside (e.1). but their decomposition may be very slow. In uncontrolled fires. a blue liquid condensate first develops (a strong mixture of N2O4 in NO2). For clean air standard EU sets a limit of 10 mgCO/m3 in a 8 hours average. as a result. a brown gas at normal conditions (but readily condensable. catalytic burners. They are formed at very high temperatures in the presence of air (there is a high peak in the range =1. or not enough residence time for equilibrium). although the reaction would be more displaced to the left in equilibrium at high temperature (but kinetics dominates). and full power conditions. Even as low a proportion as 0. it takes time for a good digestion. a blue solid appears. Nitrogen oxide. so that it might be said the NO x emission is a sign of good combustion. A third gas appears when heating at 600 ºC. CO is a deadly poison that reduces the ability of the blood to absorb oxygen and.5 percent by volume of CO in the air can prove fatal within 1 hour (>50% carboxyhemoglobin in blood). Tb=11 ºC). and to eliminate the CO inside the tailpipe.. When NO2 or N2O4 are cooled. NO2 smells pungently and causes pronounced irritation of the respiratory system if > 10 ppm. 0. as in large combustion chambers and large marine engines. porous burners. due to the fact that it destroys the lung tissue. mainly consisting of N2O3. water injection). Notice that the peak in NOx production practically coincides with the range of maximum combustion efficiency (minimum entropy production). NOx NOx stands for all nitrogen oxides. Atmospheric . they dissociate. where the residence time is near one second (combustion is similar to eating. and all appear from atmospheric nitrogen during combustion with air (coals and heavy fuel-oils have some intrinsic nitrogen also. also transparent. NO. N2O is a powerful greenhouse gas. particularly in the Otto engine at cold starts. All nitrogen oxides are unstable at ambient conditions when pure. The most polluting of NOx-components is nitrogen dioxide. and is fatal if >100 ppm after minutes. using stratified charging). and catalytic reduction at the exhaust. N2O2.05% produces headache after 10 hours (10% carboxyhemoglobin in blood). typical concentrations of up to 5% are achieved after the fire runs away of control. up to a few percent by weight). NO2. Unburnt pyrolysed fuel would be in negligible amounts if sufficient time for equilibrium at the low exit temperatures were allowed.1. and after further cooling. deadly if >0. the tree-way catalyser in Otto engines. as the more dangerous to health.ozone. and phosgene from PVC burning (COCl2.g.g. it is formed by reaction with air of NOx emissions. polycyclic aromatic hydrocarbons (PAH). separate analyses have been already applied to natural gas combustion. deadly if >0.g. but Otto engines are the major source of them because of the small residence time (some milliseconds for combustion. VOC is a cul de sac. against near one second in premixed industrial burners). and to eliminate the VOC inside the tailpipe. ammonia (NH3. O3. NO x concentration may be measured by chemiluminescence or by infrared absorption. are a group of chemicals with Tb<250 ºC that includes important air pollutants like benzene.g. very toxic substances are released. For clean air standard EU sets a limit of 5 gC6H6/m3. one of the key reactions being NO(g) + (3/4)O2(g) +(1/2)H2O = H+(aq) + NO3-(aq). but the worst are sizes <2 m. VOC (and CO) emissions should be very low for premixed combustion with excess air (even with stoichiometric air). VOC may be measured by flame ionisation or by infrared absorption. coming from unburnt fuel and pyrolysed fuel.3 butadiene. wool. plastics and flesh.20% (but up to 50% for small loads) is currently applied in all kind of new engines (Diesel and Otto). 1. classifying its VOC (or gaseous HC) as methane and non-methane (MHC and NMHC). usually by a catalytic reductor (e. comprising all chemical emissions except the singled-out H2O. and fly ash from coal and waste combustion . VOC are not only due to the fuel but to the lubrication oil that seep through the segments and gets burnt (in the small two-stroke engines oil is add directly to the fuel). and found up to 0. the tree-way catalyser in Otto engines). Two types of approaches may be followed to fight VOC emissions (besides minimising fuel use): to avoid VOC formation inside (e. lakes and rivers ecosystems. as hydrogen cyanide (HCN. is another pollutant (contrary to stratospheric ozone). and.1%). since there have widely different effects on the environment.. BTEX (benzene.6 at 288 K) that give way to acid rain. although not directly emitted in combustors. ethylbenzene and xylene).1% in typical home fires). because the fresh mixture is directly thrown to the exhaust to sweep the burnt gases in the cylinder. using exhaust recirculation.1 ppm). that is the cause of the bad smell from tail-pipes. and the expected trend is to go on with the singularisation of emitted substances. Two types of approaches may be followed to fight NOx emissions (besides minimising fuel use): to avoid NOx formation inside by avoiding high temperatures (e. and the associated small size of the combustion chamber (limited to say half a litre per cylinder for this fact). the urea catalyser in Diesel engines). Nitrogen oxides also combine with water vapour to form acid mists (pH<5. In that move. using water-emulsified fuel).3%. toluene. using stratified charging). and others. or PM10 to explicitly restrict to sizes <10 m) is harmful to the respiratory system for sizes smaller than say 10 m (larger particles do not follow the air stream and get stuck at the nose and trachea). damaging forest. deadly if >0. and to eliminate the NOx inside the tailpipe. using lean mixtures.g. Other approaches split further the bunch of substances identifying e. VOC Volatile organic compounds. CO2. Particulate matter consists of soot from all kind of hydrocarbon combustion (mainly in non-premixed flames). CO and NOx. Some amount of exhaust gas recirculation (EGR). By the way. deadly if >0. and acrylic aldehyde (CH2CHCHO.3% in half an hour). The most pollutant are the small two-stroke Otto engines used in motorcycles and gardening. for reciprocating engines. with typical solid substances as wood. PM Particulate matter (PM. typically in the range 10. From uncontrolled fires. deadly if >10 ppm). hydrogen sulphide in rubber and flesh burning (H2S. usually by a catalytic oxidiser (e. Emission regulations In the European Union. Table 1 presents the typical emission rates of new (2000) cars.5% in sulfur (down to 1.5). adding lime for CaO+SO2+2H2=CaSO3·2H2O.and incineration. and the trend has been to get off the market sulfur-containing fuels (by desulfurising those that need it). SO2 has been shown to have detrimental effects on the selective reduction of NOx with ammonia (SCR and SNCR). exhaust emission limits and their testing are regulated: 1992 Euro I (adoption of catalytic converters). 1997 Euro II. Premixed combustion starts producing soot for air-to-fuel relative ratios <0. but also form by atmospheric reactions of NOx and SO2 emissions (forming nitrates and sulphates). but emphasis here is on engineering applications. by formation of the highly corrosive ammonium bisulphate. Two types of approaches may be followed to fight PM emissions (besides minimising fuel use): to avoid PM formation inside (e.g. or in the exhaust (deSOx dry or wet scrubbers).g. as done in Diesel engines). Remedies have been the elevation in fuel-injection pressure (up to 200 MPa). Finer particles. that should be periodically regenerated (e.5 m (named PM2. are even worse than PM10. Power plants alone contribute to 2/3 of SO2 global generation in 2000 (and 1/4 of NOx and 1/3 of mercury. PM and smoke are distinguished by the measuring method: PM is measured by weighting a filter.5% in special areas like in EU seas).g. of sizes smaller than 2. are well-known sources of particulate matter associated to combustion (as well as some related processes as deep frying). Table 1. or implementing desulfurising agents in fluidised-bed combustion (e. depending on the fuel. preheating of air.5 or 0. For clean air standard EU sets a limit of 20 g/m3 for sizes <10 m. respectively). due to very inefficient burning of fuel drops on cold surfaces. or limestone). Nowadays only very large marine engines and large power stations still burn sulfur-containing fuels (residual fuel oil and coal. although the dirty exhaust associated to the combustion of sulfur-containing fuels usually prevents the use of catalysers. SO2 Sulfur dioxide (SO2) emissions mainly depends on type of fuel and not on combustion details. and smoke by light absorption. by passing hot gases rich in oxygen. In the USA. a non-visible-smoke exhaust is mandatory in practically all types of engines (even for ships). Nowadays. from the fuel). and exhaust filtering. even in 2005 the IMOMARPOL limit on marine fuel is 4. using homogeneous mixture compression and suitable injection rates). 2000 Euro III and 2005 Euro IV. in a per-shaft-energy basis.g. and thus responsible for emission control. consuming thence 1 kWh of work every 5 km or so. they are issued from combustor exhaust. the Environmental Protection Agency (EPA) is the organism in charge of protecting human health and safeguarding the natural environment.) Diesel Exhaust manifold Emissions Exhaust manifold Diesel Emissions . as per-distance travelled. incense burning. Table 2 compares these internal combustion engines with other power-producing devices. Particulate matter was also characteristic of diesel engines at low loads (e. causing severe local and global pollution (acid rain). during acceleration). with a dense dark smoke at the exhaust. charcoal grills and the like. Otto Otto (tree-way cat. Tobacco smoke. Typical emission rates of new (2000) cars a.6. and to eliminate the PM at the exhaust by an appropriate filter. Notice that a typical car running at 100 km/h demands a propulsive power of some 20 kW. 05 0.4 0. but restricting the use of powerful machinery. or flammable materials and explosives).30 ppm <0.5% NO: 100. freely using glassware. or NOx<400 ppm for biogas engines). NO x<250 ppm. plenty of possible sources of injury (hazard). below 200 rpm).8 2 NOx 1 5 0.03 g/km >0. b VIBRATIONS AND NOISE Combustion generates acoustic waves due to shear flows and turbulent fluctuations. the electromagnetic contamination due to operation of electrical and electronic devices in reciprocating engines keeps increasing since the old days (1897) when the magneto substituted the open-flame hot-tube ignition system in Otto engines. Typical emission rates and fuel consumption of different power engines..2 g/km >0. Coal Steam Diesel ICE Diesel ICE Gas Turbine Gas Turbine Phosphoric Acid Fuel Cell b Power Plant Euro III with catalyst Oil-fired NG-firedc NG-fuelled CO2 800 500 600 800 600 450 CO 5 10 VOC 0.1.1% 0. That is why muffler silencers are installed in all exhaust pipes for sound attenuation.g..03 g/km >0.01 g/km Stationary engines have tighter emission regulations (e. c Gas Turbine NOx 2000-regulation <5 ppm in the exhaust. is a source of environmental pollution.1000 ppm NO2: 10.01 g/km <0.1% NO: 100. This aerodynamic noise. would create an unbearable loud noise.08 0 Fuel-in >400 >170 >170 >200 >200 >120 a Notice that specific fuel consumption is the inverse of specific energy output (e.g.001 g/km (wet basis)b 4% 0. b Table 2. but we must be aware of risks and try its optimum management..g. and in Diesel engines some 7%.CO2 CO VOC NOx PM a (wet basis)* 8.1 0 SO2 3 0 0..10% 0..01 g/m3 >100 g/km >0. cutlery.001 g/m3 >120 g/km >0. and different levels of possible impact (damage). exhaust gases leave the cylinder under high pressure that.5. NOx. VOC and PM).. Water molar fraction in Otto engine exhaust is some 15%. in g/kWh a.1% 0. We cannot envisage life without risks (it might turn that risk is a native ingredient in life evolution). ELECTROMAGNETIC INTERFERENCES In spite of the undeniable fact that combustion pollution is mainly due to tail-pipe chemical emission (CO.30 ppm >0. if allowed to escape to the atmosphere directly.1000 ppm NO2: 10..1 0.004 PM 0. Marine diesel (>150 kW engines) NOx 2000-regulation <10 g/kWh (up to 17 g/kWh for very low engines..05 g/km >0. matches. . In reciprocating engines in particular. SAFETY MANAGEMENT Risk: a combination of hazard and damage Humankind has always evolved in a risky environment.. CO<700 ppm. 200 g/kWh = 18 MJ/kg)..7 1 0. accepting minor risks and injuries as natural events (e. besides the possible mechanical noise associated to oscillating or vibrating solid parts in combustors.2 g/km >0. Ask for help? Do not panic but hurry up. transport and use. Isolate damage! Has it ended? Avoid escalation in damage. Immediate cause of the accident (first explanation). the second part of risk management). usually delegated on social authorities. Why it happened. Hazard identification and labelling (the first part of risk management). 5. Many times. 2.The basic rule in risk management is that "the higher the possible damage. isolate causes and damage extension: close valves. Immediate responsibility: first explanation of why it happened (had proper prevention measures been taken?). How it happened. when travelling in sophisticated vehicles. Identification of the accident: what happened. as when settling by river banks that might flood.g. alert the closest knowledgeable people.e. Known risks are less feared of (fear might increase risk). Galen. firemen). evaluating risk by comparison with other better-known risks). to control that risks have been evaluated. Both. and rehearse emergency procedures (e. 2. to best identify the accident for proper acting and for reporting (identify the type of damage and its extent). What happened? Keep calm. or when using powerful tools. efficient and respectful development of human activities at reasonable risk. 6. however. 5. Organise actions to minimise overall cost. 4. most minor accidents and unsafe exposures are recoverable. and bald remedies requiring a century are useless). put further risks away (including children). Who pays. How it was fought. Analysis of accidents 1. There is. Individuals cannot be on permanent watch. What happened. Rules updating.g. and procedures have been adopted in developed societies to guarantee a safe. in the 2nd century. Hazards must be categorised and regulated during production. 6. under roofs that might fall over. Injuries? Keep priorities: is life threatening? Do not think about property until life and health is safe. 3. evacuation rules).g. already said that doses alone make the difference between medicine and poison. Risk acceptance is a subjective measure. Keep priorities (benefit/cost) on what can be rescue. but society must keep watch teams alert all the time (e. accident and countermeasures. 3. Risk management can be split in several stages: 1. possible damage prediction. always realizing that we cannot devote our life to risk prevention (e.g. Remediation. 7. 2. and people may change priorities with experience. a demand on risk management. Rescue. sometimes fighting accidents cause increased damage. when and where. .g. Be better prepared for emergences. Risk assessment (e. Prevention of accidents Individuals and society accept risks in accordance to the benefit the risky activity brings or may bring. 4. first aid box). Damage control. so that quick temporal repairs are often applied (they should not contribute to enlarge risks). 3. policemen. An early alert to professional teams is key to control large emergencies. Responsibilities. Consequences identification (i. the lower the acceptable hazards". Emergency response A quick-list of actions to do after an accident occurs may be (intervention protocol: 1. rescue and remediation. society and individuals must keep proportional emergency stores (e. damage costs increase with time due to lack of benefit of use. exposure. by viruses. Perceived risk may be more frightening than real danger (a USA poll amongst risk experts rated the three most-risky activities from a given set of 30 as: driving. Prepare intervention protocols to repair possible damage. changing interfaces (e. aggression or attack. multitasking. Prepare intervention protocols to fight in case of accidents (if they can. events per time (e. early alert. conditions. Exposure Cumulative amount of hazardous substance or electromagnetic radiation. smoking and alcohol drinking. it is side-effects or after-effects. and minimising acceptable risks).4. 1 per million). It is very important to have an early detection. IR. out-range). thermal (fire. Hazard Potential source of damage (threat). it is said spoil. Danger Imminent or probable risk. Hazard types:  Physical hazards: mechanical (chock.  Chemical hazards: oxygen depletion. particulates).g.  Ergonomic and psychological hazards: bad posture. it is an incident (minor offence). received by living or non-living systems. Injury Physical damage or hurt. Defence and protection against accidents is usually split in prevention (before) and response (after). Explosion Sudden conversion of potential energy (mechanical or chemical) into kinetic energy of a high-pressure gas released (pre-existing or being produced). 10 deaths per year). with the production of a bang noise.. 5. Also moral harm (from injustice). liquid poisons.g. nuclear or biological. Severe accidents (that affects a lot of people and cause great damage) are termed disasters or catastrophes. handguns and smoking). Damage usually refers to a sudden event. aerosols. If damage is not sudden. chemical. UV. but indirect. Distribution of risk according to profiteers. but intentional. by prions.e. threatening circumstances. i. Spoil. bad timing. Distribute damage. they will). etc. but above all to avoid escalation by improper action (quick-response systems may become unstable). Insurance companies aim at reducing the financial risk of individuals by spreading financial loss to a community. bad planning. property or the environment. Accident Unexpected undesirable sudden event that cause direct damage. If damage is not direct. fractions of event cases (e. processes and activities involved. it is wear or illness. solid poisons. If damage is not undesirable. 7. MW. radiation (ionising. but progressive.. explosion. flare. vibration and noise. Risk Product of probability by consequences of a hazardous event. gas poison. 6. if more progressive. chock. Fear Subjective feeling of insecurity or risk (real or imaginary). Hazards may be physical.g. fear.g. Risk likelihood may be imminent or remote. Acceptability identification (avoiding risks disproportionate to benefits. Glossary of terms: accident. early containment and early fight. dealing with the public). RW). . and some mechanical work (forcing to move or shattering nearby objects). the same poll amongst college students yielded: nuclear power. and they are inherent to the materials. weather) or people (from thieves to sabotage and military attack). Damage Actual harm caused to people. Defence Protection against threat or attack from enemy circumstances (e. electrical (chock).  Biological hazards: infection by bacteria. Sudden damage to the human body is specifically termed injury. evacuation). Sudden spoil and accumulated spoil are named damage. Environmental effects may be physical (wear. exposure). spillage). (Back to Combustion) . weather). chemical (reactions). or the environment (pollution. bacteria) or radiological (ultraviolet. temperature.Spoil Risk possible consequences may be on health (deaths. Degradation of usefulness by exposure to harmful environments. nuclear). property (ruin. injuries. biological (viruses. .... 11 Equilibrium composition............................... mostly in gas phase............................ a steady-state combustor burning natural gas (here idealised as pure methane) in air............... so that the same overall results apply in all three cases....................... maximum work obtainable.................... The study of combustion is based on the more general subject of Thermodynamics of Chemical Reactions........................... It may be helpful to have in mind a concrete instance of a combustion process................... and assumes the combustor is large enough for the exhaust to be in equilibrium (no longer reacting.............................................e................................... 6 Thermochemistry .............................. etc........ in spite of their details being so different...... no gradients).................................................................................... flame geometry....... The overall process in combustion is analogous to those taking place in fuel cells and living-matter respiration...................... with some heat output (as in a domestic water heater)......................................................................... 5 Water vapour condensation .. or any other gradient or discontinuity within)............... the geometry of the flame........... efficient and clean design and operation of fire-making devices...... This model already emphasises the black-box approach typical of thermodynamic analysis (it does not look into the internal details of how the fuel and air mix............. extinction..................................... propagation............................ 8 Heating value ....................... extent and affinity .......... etc......................................................................................................................................................................................................................................... 4 Equilibrium ........................ 4 Chemical potential expressions .. producing light........................................................ Combustion Thermodynamics focuses on the former physico-chemical phenomena: fuel/air ratios............................................. combustion is characterised by the very high temperatures reached...... 1........... 2 Thermodynamics of mixtures ...... in terms of the multiple physico-chemical phenomena involved............. heat and smoke in a nearly-adiabatic flame front.. and kinetics (how we get it..... exhaust composition...................................................................................................1 COMBUSTION THERMODYNAMICS Combustion Thermodynamics .......................................... 7 Enthalpy of formation and absolute entropy . whereas Combustion kinetics focuses on mixing process......................................... and we proposed the idealised burner sketched in Fig.................................................................................................................................. 4 Thermal capacity averaging . for a safe........................ at what rate).................................... We shall deal here only with the peculiarities of the combustion reaction.. it is thence a prerequisite to analysis the latter.................. 1 Thermodynamics fundamentals .................................................... usually called Thermochemistry....................... ignition................... stability...................... Those phenomena may be split in two groups: equilibrium behaviour (what we need and what we get)........... 6 Stoichiometry... 14 COMBUSTION THERMODYNAMICS COMBUSTION Combustion is a self-propagating exothermic oxidative chemical reaction.................................................... with focus on the thermodynamics of a fuel-and-air gas-phase reaction.......................................................................... ........ 10 Adiabatic combustion temperature ................................... heating values... 1 Combustion ...................................................... i...... 8 Maximum work .................. The practical goal in combustion study is the prediction of its performance.......................... These basic general principles and other particular assumptions on the behaviour of some type of substances. A succinct description of the process in Fig. The exhaust composition for stoichiometric mixture consists of 71% by volume of N2.ni). give way to a formulation that is actually applied when solving problems. their mixing. Q. with a maximum heat output of 55 MJ/kgCH4 (or 37 MJ/m3CH4. the traditional formulation of Thermodynamics is based on these principles:  Zero Law: there exists an state-function named temperature.  Second Law: there exists a state-function named entropy.e. T(E.  First Law: there exists a path-function named heat. THERMODYNAMICS FUNDAMENTALS Perhaps a rough summary of Thermodynamics Fundamentals seems appropriate at the beginning of a new application of the general theory. 19% H2O.5). aromatic-hydrocarbon vapours and maybe soot. that are noxious to the health: CO. although the massive use of combustion renders their effects very obvious. the higher heating value of methane) that would decrease as the exhaust temperature increase until a maximum when no heat is exchanged. As it will be shown below. such that all entropy variations tend to die for any processes at that limit T=0 K. For such a system in such states. Those figures already show that thermodynamic properties of exhaust gases can be approximated by those of air for a crude analysis (as air properties can be approximated by those of nitrogen). the heat flow at the frontier is just the change in stored energy minus the work received by the system. volume V and amount of each chemical species ni. measuring the distribution of thermal energy within the system. even the inert gas CO2 is considered an undesirable emission. with the equality holding only for the limit case of a nondissipating process. S(E. such that when two control-mass systems having different temperatures exchange energy.e. We should only consider systems such that their equilibrium states are just characterised by its energy E. ST0 K0. oxidisers. since it contributes to the menacing global warming and its associated climate changes. some 2200 K (the adiabatic combustion temperature for the stoichiometric mixture). NO. A detailed description of fuels. . i. they show the importance of water as the only condensable gas in the exhaust.5 m3 of air are required for the complete combustion of 1 m3 of methane at same p-T conditions (the molar stoichiometric air/fuel ratio is A0=9. and the small proportion of contaminant emissions. can be found elsewhere. and that is here briefly refreshed. such that for any process in a control-mass system. such that if two systems having different temperatures are put in contact. 1. similarly.ni). indicating the thermal level of the system. at least 9.2 CH4 air combustor products heat Fig. 1 may be as follows. i. Combustion emissions. Nowadays. A simplified model of a natural-gas combustor. flowing from the hotter to the colder one until both reached the same value (equilibrium). its variation is lower-bounded to SdQ/T.V. the ignition process and the kinetics of its propagation. 9% CO2. Q=EW. NO2. measuring the thermal energy exchanged by the system. following our Thermodynamics Lectures.  Third Law: there exists a singular value for the state-function entropy. and much less than 1% of undesirable gases called ‘emissions’. Exercise 1. instead of the recourse to ‘as you may know’.V. their energy varies. 314 J/(mol∙K). In particular. and the perfect substance model for stored thermal energy. should be kept in mind: E=Q+W (energy balance of a control mass) (1) E=mcvT (stored thermal energy model for a perfect substance) (2) Details on the concept of entropy. particularly the omni-present steady-state mass and energy balances: 0 openings    vA me with m 0  Q W  (8) openings  me hte (9) . for a gas of molar mass M.3 J/(mol·K). should be retained: S   dQ  dEmdf dU  pdV  T T (general expressions for entropy change) (3) T2 p  mR ln 2 T1 p1 (entropy change in a perfect gas model) (4) PGM S  mc p ln The combination of energy and entropy called exergy. can be found aside. measuring the maximum work obtainable from a system. Ru=8. the ideal gas equation of state is omnipresent in all combustion studies: pV=nRT with R=8. and thus the equations developed in Control Volume analysis must be known. or in the form   p RT or v  RT p (7) where the same symbol is used for the universal gas constant. or the minimum work required to reach a global non-equilibrium state. for pure substances. dilation and compressibility can be distinguished: H≡U+pV.  T p V T p V p T v T v (6) T But it must be remembered that. is. an additive and conservative function of kinetic and potential terms. there are only two independent state variables. but its basic expression as a function of other variables. others intensive) are used to simplify the analysis of systems. and its particularisation for a perfect gas. an additive non-conservative function measuring the internal distribution of energy and other conservative properties. amongst which enthalpy.3 Details on the concept of energy. for a control mass:   Wumin  Wu Suniv 0  E  p0 V  T0 S  (5) Many thermodynamic variables (some extensive. and the particular gas constant. cv  T . Most combustion processes take place within a control volume. and all others can be obtained by a combination of this two. c p  T s T  p h 1 V s u 1 V  .  . thermal capacities. but the energy balance of a control mass system (one that cannot exchange mass with the surroundings). can be found aside. R≡ Ru/M. the combustor. the former comes from the equality of the crossed second derivatives 2G/(Tni)=si=2G/(niT)=i/T and from gi=hiTsi.: C G  U  pV  TS  H  TS   ni i (11) i 1 The differential form of the Gibbs function is used a lot in thermochemistry: dG=SdT+Vdp+idni (12) Several useful relations can be derived from it. subtracting (12) to the total differential of (11). in a similar way as temperature measures the escaping tendency of thermal energy. i.ni) being a maximum.e. that G=ini. and the exhaust is always a mixture of burnt gases (except in the two ideal cases C+O 2=CO2 and H2+(1/2)O2=H2O). pressure p=TS/V. i. at least of a fuel and an oxidiser (usually air. the escaping tendency of chemical energy. i. what implies.e. instead of with the entropy of an isolated system. the Gibbs-Duhem equation. that a more detailed summary is here included. and hte the total specific enthalpy for each entrance (or exit) with mass flow rate me . Multi-phasic mixtures. First.e. more usually used in the form of Antoine’s fitting: dp dT  sat hlv Tvlv → ln p hlv  1 1  p   A    → ln T p0 RT  T T0  p0 B Tunit C (10) The Thermodynamics of mixtures is so important to combustion. from Gibbs equation and Euler theorem for homogeneous functions. For a mixture in contact with an infinite environment at T=constant and p=constant. that the temperature T=U/S.p. EQUILIBRIUM An isolated mixture with conservative amounts of substance ni tends to reach an equilibrium state defined by its entropy S(U. It can be deduced. i.e. another mixture itself!). The general theory of Thermodynamics of Mixtures is developed aside. CHEMICAL POTENTIAL EXPRESSIONS The chemical potential. what yields: . and pressure the escaping tendency of compression energy.V. and it may be enough to recall the equation for the vapour pressure curve. it is better to work with the Gibbs function for the system G(T. as the ambient atmosphere. Q the heat received. are uniform at equilibrium (in absence of external forces). can be found aside. measures the tendency for an species to migrate. THERMODYNAMICS OF MIXTURES Combustion always involve a mixture. Phase changes in pure substances are needed in combustion only to deal with pure liquid fuels. and mixture segregation due to external force fields. the general dependence of i(T) and i(p). but the important points to combustion are summarised here. and chemical potential for each conservative species i=TS/ni. Clapeyron’s equation. i.: 0=SdTVdp+nidi (13) Second. in absence of external fields.4 W being the shaft work input.ni)U+pVTS=HTS. There are few combustion problems were equations (7-9) are not involved. g. OH… 34 Water vapour: H2O 47 Carbon dioxide: CO2 54 Monoatomic molecules: Ar.p. but at 3000 K cp=3100 J/(kgK)=56 J/(molK). THERMAL CAPACITY AVERAGING The perfect gas model.e. for high-precision computations. temperature correlations for thermal capacities must be used. on the other hand. Gas cp [J/(molK)] Diatomic molecules: N2. Fig. but a good averaged value of cp must be taken in combustion studies. i. On the other hand. i(p) comes directly from the equality of the crossed second derivatives 2G/(pni)=vi=2G/(nip)=i/p. Figure 2 gives a plot of cp(T) for the most important combustion gases. the chemical potential for an species i takes the form: i(T. besides the ideal gas equation of state (7). N … 21 For a more crude manual analysis. and varies with pressure logarithmically according to (12) and vi=RT/p. O.1)+RTln(p/p)+RTlnxi (16) indicating that the chemical potential varies with temperature according to the enthalpy function (13). NO. Table 1. the drastic simplification of assuming an averaged thermal capacity for the exhaust mixture of say cp=36 J/(molK) may be cost effective. as the traditional JANAF tables. IGM. assumes constant thermal capacities cp.p. H. for air at 300 K cp=1000 J/(kgK)=29 J/(molK). or other polynomial fitting. For preliminary computations. averaged values that may be used for preliminary computations are presented in Table 1. . the usual averaged values are presented in Table 1. ni For an ideal gaseous mixture. H2. since cp varies considerably: e. Variation of thermal capacity with temperature for gases of interest in combustion. ni known as van’t Hoff equation.: i p  vi (15) T . but at 3000 K cp=1240 J/(kgK)=36 J/(molK). He.xi)IGM=i(T.5  i T 1  T  hi (14) p . O2. and varies with the molar fraction xi also logarithmically. simplifying the computations a lot. Mean values used for constant-thermal-capacity models in combustion. from that. and for water-vapour at 300 K cp=1900 J/(kgK)=34 J/(molK). CO. 2. CO+(1/2)O2=CO2. For the case of one condensing species (water).vap. Leaving aside the important problem of acid rain formation in the combustion of sulfur-containing fuels. the molar fraction of species i in the vapour phase.6 WATER VAPOUR CONDENSATION Except in a few theoretical cases (C+O2=CO2. is equal to the vapour pressure for the pure i-component at that temperature. the molar mass changes even less because the increase due to the formation of carbon dioxide practically compensates with the decrease due to water formation.. xvap (notice that ‘vap’ now refers to the substance and not only to the phase) dissolved in a given flue gas at given pressure p. one arrives at Raoult's law (see Mixtures): xi. water is a genuine combustion product. water is the only condensable component in the products. pi* (obtained from Clapeyron’s equation or Antoine’s correlation). divided by the pressure of the mixture. condensing temperature. being produced in such sizeable amounts (typically xH2O≈10%) that its detailed account on mass and energy balances is of paramount importance). the important equations are:  Maximum water-vapour fraction. see Fuel properties).vap/xi. It often helps to think of the pressure of the mixture as the summation of partial pressures attributed to each i-component. that really take place also with some water vapour). that should be changed from cp=1000 J/(kgK)=29 J/(molK) for air at about 15 ºC to cp=1100 J/(kgK)=32 J/(molK) for a typical exhaust mixture at about 50 ºC.liqp*(T). etc. divided by the molar fraction in the liquid phase. and pure hydrocarbons (commercial fuels are complex mixtures of hydrocarbons. for the two-phase system at equilibrium (humid exhaust plus condensate) at temperature T: p=pi. Gaseous mixtures with a condensable component are dealt with in detail for the case of humid air aside. Tdew. and water molar fraction xi. pixi. and temperature T: xvap=pi*(T)/p  Dew point. p.liq=pi*(T)/p (17) i. i. the only modification is in the thermal capacity of non-condensable gases. but the approximation of non-condensable products by air may be good enough. Here we are concerned mostly with humid flue gases. implies uniform chemical potential for each species I.vap=pi*(Tdew)/p  (18) (19) Total pressure. Recalling that liquid-vapour equilibrium.non-cond+pwater*(T) (20) THERMOCHEMISTRY A detailed thermodynamic treatment of generic reacting systems can be found aside. for ideal two-phase mixtures.e. so that at liquid-vapour equilibrium pi=xi. . p. carbon. S+O2=SO2.e.vapp. for a given pressure p. if not. We will focus here just in combustion reactions of simple fuels: hydrogen. it is solved from: xi. identified by their molecular formula Mi (Mi is also used for their molar mass).5 mola/molf=17 kga/kgf).  Rich mixtures (more fuel than needed. According to the stoichiometric ratio for full oxidation of a fuel. The relative air-tofuel ratio  is with respect to the stoichiometric air-to-fuel ratio A0. Notice that the stoichiometric coefficients change if one writes 0=2H2+O2-2H2O. and sometimes an arrow is used instead of the equal sign). The ratio air-to-fuel in molar base is An≡nair/nfuel. (21) serves as the mass conservation equation if Mi is the molar mass of species i.e. The i are called stoichiometric coefficients for that reaction. EXTENT AND AFFINITY A combustion process involves a set {Mi. or 0=H2O-H2-(1/2)O2. namely the fuel-to-air ratio. and serves to build the set of elementary conservation equations when the molecular form of Mi is considered. the inverse of these functions are often used. but excess fuel will pyrolise to small-molecule fuels. i. the chemical affinity. In any case. is defined: .g. e.5·0. that. though non-dimensional. As for the products of combustion. to distinguish between n-octane and iso-octane).76·N2)=CO2+2·H2O+7. e. and Am=AnMa/Mf=9. is An=2·(1+3.R}.016=17.R (e. Most commercial fuels are hydrocarbons (chemical notation should be briefly refreshed. i=1. Notice. In the USA. to measure the tendency to progress.029/0.  Stoichiometric mixtures (with the precise or theoretical amount of fuel as established in a given reaction as (21)). CH4+2·(O2+3. or 0=H2+(1/2)O2-H2O. as above). undergoing a simultaneous set of chemical reactions {Ri. i=1. although it is often stated simply as A (but notice the values are different. since most water is condensed and left behind).  (Greek letter xi.5.g. H2  21 O2  H2O ) (21) where the first form is preferred for kinetic studies when a direction in the process is implicit (it is said to occur from reactants (left) converting into products (right). and in massic base An≡nair/nfuel. A. ≡A/A0 (now independent of the molar or massic base). each one specified by its so-called stoichiometric equation: C C C i 1 i 1 i 1  ir' M i   ir'' M i or 0   ir M i for r=1.52·N2. to avoid condensation of water inside the instruments.76)/1=9.g.. When a control-mass mixture with initial composition ni0 reacts. measurements of exhaust gases are taken on a dry mixture that is obtained by passing the exhaust gases through desiccants (an ice bath is often good enough.C} of chemical species i. air/fuel mixtures fed to a combustor are classified as:  Lean mixtures (little fuel content. sometimes written as A=9.. and the equivalence ratio =1/. however. excess of air). the molar fractions are always used (also said 'volume fractions' since they are the same with the ideal-gas mixture model predominant in combustion exhaust). whereas the last form is more simple for equilibrium studies where no direction is privileged. a convenient normalised mole-account system) is defined as:  ni  ni 0 i for a given 0 =   i Mi (22) The extent of a reaction marks the state of progress. the extent of the reaction at any later time. f=1/A (either in molar or massic base. theoretical air ratio for methane. and only small molecules appear at the exhaust).. also called progress or degree of advancement of a reaction.7 STOICHIOMETRY. The best references are:  Enthalpy reference: zero enthalpy is assigned to the most stable natural form of each chemical elements at standard temperature and pressure (T=298 K and p=100 kPa). It is customary to include in the thermochemical tabulation not only hf and s. energy and entropy reference states for each atom must be agreed in reacting systems. and cooling water) and output approaching standard conditions (298 K and 100 kPa). the reaction rate d/dt is not. but atoms are.g. according to ds=(cp/T)dTvdp. a flow calorimeter with inputs (fuel and air mixture. also called practical calorific value. Although both. .e. Thus. hPHV (or its molar value). and tabulated. i. for perfect gases h=hf+cp(TT)).e. References are free to choose by the observer. known as absolute standard entropies. gf. but they must necessarily be consistently related. since it is an experimental fact explained by information theory that entropy changes in the limit T0 also tend to zero. although it is redundant since: C g f  h f  T    i si for the reaction of formation of the compound: 0=   i Mi . while the cooling flow-rate is traded off for best resolution (least uncertainty in Q  mw cw Tw  mf hPHV ). are experimentally measured (usually by calorimetry. is dG=Ad0 for the whole set of reactions. (25) i 1 HEATING VALUE The standard practical heating value (PHV. that have no influence on reaction equilibrium. most of the times indirectly) and tabulated. i. Enthalpies for nonelementary species at standard temperature and pressure. Entropy values at the standard state (T=298 K and p=100 kPa). called standard enthalpies of formation. since the heating value is independent of excess air. the specific combustion enthalpy. The fuel is burnt with excess air for complete combustion. are thermodynamic state functions. dG=Ad0 → progress. ENTHALPY OF FORMATION AND ABSOLUTE ENTROPY Compounds are not conserved-entities in combustion. since the real limit. only if A>0. PCV) of a fuel is defined as the heat transfer to cooling water in a Junkers-type calorimeter. Enthalpy of an species at other temperature and pressure are computed from the general relation dh=cpdT+(1T)vdp. i.  Entropy reference: zero entropy is assigned to each species (not just to each elementary species but compounds too) at 0 K and any pressure. It is important to notice that reactions with negative affinities can naturally progress only at the expense of other reactions with positive and larger affinities. are computed by integration of experimentally measured (usually by spectrometry) thermal capacity data.e. s. hf. i. but also the standard Gibbs function of formation of compounds from their elements.8 C A    i  i (23) i 1 such that Gibbs function variations are: C dG   SdT  Vdp   i dni   SdT  Vdp  Ad  (24) i 1 the minus sign being introduced in the definition of A to ensure that natural evolution (entropy increase in an isolated system. similarly to heat transfer.e. d/dt>0. according to the model used (e. or Gibbs-function decrease in a system at T and p constant) corresponds to a positive affinity. instead of the energy and entropy reference state for each compound used in non-reacting systems. at T and p constant. affinity and extent. Reaction rates depend a lot on the presence of catalysts. the HHV for methane is (from the Combustion Data Table) hHHV=890 kJ/mol (hHHV=55 MJ/kg). However. either initially or at the end. For combustion at constant volume of a control mass (e. and depends very little with temperature. For instance. and only CO2 and H2O were produced and all the water in the exhaust were condensed.5)8. there will be just one tail-pipe and at 25 ºC the water will be some 95% condensed (forming a mist of micrometric droplets) and some 5% dissolved in the gas stream. the heat release is hHHV=hLHV=283 kJ/mol. For instance. 890 kJ by mole of methane will be released to the ambient (55 MJ by kg of methane). . both inlets (methane and air) being at 298 K. 100 kPa and species-separated). the negative of the reaction enthalpy (hLHV=hr. to know if they are HHV or LHV. i. since energy is only defined between two states of a closed system. and some traces of unburnt hydrocarbons and possibly soot will also show up in the exhaust.g. a more involved analysis is required. for instance.9 However. Thus. be it in a flow system (the most usual case) or in a control mass (e. and the measured heating value would be slightly smaller. whereas if the initial state is the same but the burning is inside a rigid vessel at constant volume. sometimes known as high calorific value HCV) is defined as the heat release to the ambient when fuel. heat is a path variable. the heat release is practically independent on pressure. pV=nRT and Q=HVp=H(nfinalninitial)RT. by EW|Q=0. For combustion at constant pressure.3298=282 kJ/mol for an stoichiometric CO/O2 mixture. a so called standard higher heating value (HHV or hHHV.g.e. Q=(HpV)=HVp. called lower heating value (LHV or hLHV). for methane. oxidiser and products flow at 298 K. If there is no condensed matter. the same equation as for gaseous mixtures may be applied just considering the non-condensed species. the energy balance is: Q=H=nfuelhHHV (26) assuming all water is in condensed form. not a state function. that assumes all water exits at 100 kPa and 25 ºC but in the gaseous state (a virtual state not possible in practice for pure water). by QEW. even the difference between burning at constant pressure or at constant volume is not significant. a theoretical standard is defined by assuming that inlet and outlet flows run through separated pipes for each pure chemical species. heat is only defined as energy transfer through an impermeable boundary. its combustion being CH4+2O2=CO2+2H2O. recall that it has no sense to talk about the 'energy content' of a fuel or of any other system. and computed by adding the latent heat of the water-vapour (exiting the calorimeter above-mentioned). for initial and final gaseous states. in an ideal Diesel cycle). it is advantageous for the calculations to establish a different standard. in an ideal Otto cycle). to the PHV. the heat release may range from 283 kJ/mol for a very diluted mixture to 283103+(11. for instance. and the outlet is cooled to 298 K. if 1 mol of CO is burnt at constant pressure in air at 298 K and 100 kPa. but since the available data are enthalpies. being for a given constant-pressure process. the energy balance is Q=E=U. similarly. Care is needed when using tabulated heating values. the LHV is hLHV=890244=802 kJ/mol. thus. if the saturation pressure of the condense matter at the standard temperature is small. Fortunately for combustion reactions. in order to provide a common reference to allow direct combination of heating values for several reactions. If there is some condensed matter. meaning that if a steady flow of methane is made to burn with air (or oxygen) at 100 kPa. Finally.comb). although. Notice that the heating value of a fuel depends on input and output conditions and the path followed. HHV and LHV are defined as state functions. Notice also that the heating value of a fuel must be understood as the heat released in its combustion with air (or any inert mixture with oxygen). for applications where the exhaust gases are above their dew point (a very common situation since the exhaust dew point is typically below 60 ºC). In practice.44 MJ/kg or 44 kJ/mol of water produced). the LHV is equal to the HHV minus the vaporisation enthalpy of the exhaust water at 25 ºC (2. =htT0s. no work will be produced (at the combustion chamber).e. The energy balance of the combustor relates the heat and work flows with the entrance and exit temperatures.in and hLHV  hHHV  vH2O hlv (27) i. for H2+(1/2)O2=H2O a maximum of =286103298(701310. some work will be produced. or with the internal energy of reaction at constant pressure for the case: U/|V. for the theoretical combustion of methane with oxygen.3298=284 kJ/mol for an stoichiometric H2/O2 mixture.83 for hydrogen (but mind that it would be 1. If an energy efficiency is defined as the quotient between work produced and heating value.T. hHHV  hr   H  C C i 1 i 1   i hi   p .e. as much as 237 kJ/mol of work might be generated.9% for C8H18. for the liquid-to-vapour phase change at T=298 K is hlv=44. 817 kJ/mol of work might be produced (the remainder to the heating value being evacuated as heat. The work obtainable from the combustion of a fuel depends on the actual process (as does the heat release). the larger variation being with the physical state of water at the end: for long-chain hydrocarbons the LHV is 6. or nothing of the two. or a value close to the HHV in the practical case.5205)=237103 J/mol. to 286103+(01. the heat release may range from 242 kJ/mol for a very diluted gaseous mixture. if the combustion is performed in a rigid open chamber (as in a gas-turbine). =gr=hrTisi=890103298(214+2701862205)=817103 J/mol. Similarly. i. For instance.0 kJ/mol=2442 kJ/kg. .i are standard enthalpies of formation of the participating compounds i=1. and si their absolute standard entropies. All the above-defined heating values are global values for a non-equilibrium combustion process between the initial and the final states quoted.5% lower than the HHV. only work.T+U/p|. but no difference is found if the instantaneous equilibrium changes are introduced and the heating value is defined as the enthalpy of reaction (changed of sign because the assumed direction for the heat release)..C.iTisi for a given 0=   i Mi (28) where hf. but if it is performed in a chamber with moving parts (as in a reciprocating engine). i. In summary. on account of the energy balance q+w=ht). MAXIMUM WORK A combustion process may exchange both heat and work with the surroundings.5)8.e. The value for the molar enthalpy of water. hLHV=242 kJ/mol if all the exhaust water were in its vapour state. whereas if the initial state is the same but the burning is inside a rigid vessel at constant volume. for a steady process in the presence of an environment at temperature T0 is the change in flow-exergy from inlet to outlet. that for standard conditions is: wmin==gr=hrTsr=ihf. The maximum obtainable work. only heat.T=U/|p.92 for methane and 0. CH4+2O2=CO2+2H2O. the difference increasing with decreasing carbon content (6. Similarly. 9.T ni hi nfuel. the ratio of the change of the enthalpy of a reacting system at equilibrium to the change in the extent of reaction when an infinitesimal advance in the extent of the combustion reaction (scaled per unit of amount of fuel) takes place.Tp/|V.002 for carbon). the heat release at constant volume can be identified with the internal energy of reaction at constant volume.1% for CH4 and 15. the heat release would be hHHV=286 kJ/mol if the exhaust water were fully condensed. that would produce near-zero gaseous moles.4% for H2).e. the maximum efficiency would be 0. or the minimum required work to reverse the process. the heat release slightly varies from constant pressure to constant volume combustion.10 if 1 mol of H2 is burnt at constant pressure in air at 298 K and 100 kPa. i. because of the very poor thermal conductivity of gases and the low speed (<0.g. of the hot exhaust stream relative to the atmosphere. their initial thermodynamic state (e. stoichiometric or rich mixtures). premixed gases at 298 K and 100 kPa. since the chemical exergy of dilution into the atmosphere is much smaller. lean. Burning with pure oxygen increases the temperature of the adiabatic exhaust and thus its exergy. |r|=237 J/mol. for instance.3 kJ/mol. all the exergy of the reaction would be lost by entropy generation inside. H2O=1. to the exergy of the CH4+2O2=CO2+2H2O reaction. CO2=20 kJ/mol. to be compared with the exergy from H2+(1/2)O2=H2O. it may be assumed adiabatic to a first approximation. one must computed the flow exergy. In spite of this advantage of higher temperatures (already discovered by Carnot in the most general case). some 190 kJ/mol of exergy per mole of hydrogen may be obtained from the very hot adiabatic exhaust at some 4000 K and 100 kPa (difficult to compute because of dissociations). overestimates the temperature in the case of high radiation emission in non-premixed flames. in the case of H2/O2.11 Recall here that there is a small difference between the maximum work obtainable from combustion and the maximum work obtainable from the fuel (in spite of ambient-air having no exergy). however. Although an indirect realisation of the same global reaction can approach that maximum work. and subtracting the exergy of two moles of oxygen. if a constant-pressure combustion process (the difference with a constant-volume process being small) were very slow. For instance. To begin with. practical combustion is most of the times with common air. the exergy of hydrogen at the standard state is H2=236 kJ/mol.5 m/s laminar burning speed for ordinary hydrocarbon/air mixtures). The adiabatic temperature is a thermodynamic state variable that results from the conversion of internal chemical energy to internal thermal energy after the combustion process takes place. O2=3. since common materials cannot withstand such high temperatures. =htT0s. since the thermochemical values refer to separate streams) have some chemical exergy relative to the atmosphere. and for high speeds flows relative to solids (as in afterburner heat-exchangers). and depends on the actual fuel and oxidiser (e. and two moles of water. but some work may be obtained afterwards. even in some excess to further decrease the temperature achieved. and the type and proportions of the compounds formed. If the process is rapid.e. ADIABATIC COMBUSTION TEMPERATURE The adiabatic model very accurately predicts homogeneous combustion temperatures in gas phase. no work is actually produced. CH4/air). because the streams (to be considered separate components. as most combustions really are. To evaluate the maximum work obtainable from the adiabatically burnt gases. or just after a flame. due to the unavoidable entropy generation inherent to the combustion process (entropy increase without entropy flow). the heating value being spent in heating the exhaust gases themselves instead of in heat transfer to another medium. or to some 180 kJ/mol in the adiabatic products at 870 kPa and 2950 K after isochoric combustion with stoichiometric air. but the practical difficulties in ensuring . For instance. its exergy. direct combustion processes cannot approach those values. the heat release would just dissipate to the environment without any possible work. i. |r|=817 J/mol. The adiabatic model. Similarly. since the exhaust is hotter than the environment. even when at T and p. For hydrogen at standard conditions. computed by adding the exergy of one mole of carbon dioxide.g. the maximum work obtainable from methane is CH4=831 kJ/mol (against the 817 kJ/mol above). It can be measured at the exhaust of an adiabatic combustor.g. their ratio (e. the maximum obtainable work (236 kJ/mol in an ideal fuel cell) is reduced to some 170 kJ/mol in the adiabatic exhaust at 100 kPa and 2480 K after isobaric combustion of hydrogen with stoichiometric air. although energies associated to mixing and pressure are negligible).9 kJ/mol. It is good enough to retain just the thermomechanical exergy. as in fuel cells. and assuming there is no phase change from the standard state to the actual input or output states (see below). The main choices to write the so called mixture equation are: per unit amount-of-substance of input stream. and per unit amount-of-fuel in the input stream. what is partially solved assuming a set of compounds dictated by experience. but their molar fractions may be functions of the temperature and pressure (e.g.i   xij (hi  hf . But. per unit amount-of-substance of output stream. particularly from the thermometer probe. and a thermal part (from the standard state to their actual state at the opening) of the participating compounds i=1. according to the scale of amount of substance chosen. With this complete-oxidation model. the energy balance for a steady state combustor is C   C  0  Q  nentry  xi hi  xi c pi (T  T  )  nexit  xi hi  xi c pi (T  T  ) i 1 i 1  (30) Notice that water must be treated as an ideal gas to apply (30). This approach is useful for combustion of lean mixtures in air. but not for combustion in pure oxygen where temperatures are higher. is computed to be 235050 K. in good agreement with a measure of 2300100 K.. otherwise. for instance.i )  dt j 1 j 1 i 1  i 1  (29) where the enthalpy contributions at each opening j is split in a chemical part (proportional to the molar fraction i of each chemical component of standard enthalpies of formation hf. better.C. Notice that the components are assumed known at entry and exit openings. A drastic simplification of the energy balance occurs if one assumes complete combustion. the enthalpy of phase change must be added. With this simplification the adiabatic temperature for a stoichiometric H2/air mixture. in clear disagreement with a measure of 3000100 K. because then one can resolve the exhaust composition independently of temperature. molar balance for a combustor may be established in different ways. The actual mass balance or. The problem with the computation is that the type and proportions of the compounds formed must be known.12 minimal heat losses. assuming W  0 (no work input/output through the impermeable walls). separating the entry flows and the exit flows.i. The last two usually take the form: C C i 1 i 1 aM fuel  bM air  cM other   xi M i and M fuel  AM air  BM other   i M i (31) . and assuming that they are in thermodynamic equilibrium at that pressure and temperature. when there is chemical equilibrium at an exit). render a theoretical computation more precise that the actual measurement. The energy balance for a generic reactor is: openings openings C dE C   W  Q   n j h j  W  Q   n j  xij hf . the temperature is the unknown result and thus the equilibrium composition cannot be directly computed. even for the simplest case of a constant-pressure combustion process (very good for open combustors since the pressure only rises some 2% after a deflagration). whereas the adiabatic temperature for a stoichiometric H2/O2 mixture is computed to be 5400200 K. yielding an implicit and stiff algebraic mathematical problem. hLHV.6/4).76N2)=nCO2+(m/2)H2O+3. in spite of the widely different heating values (e. CnHm. cp=47 J/(molK) for water vapour. lowering adiabatic temperature. and it happens that the amount of substance of the products varies almost proportional to the molar heating value. plus the thermal energy in the intake stream. and cp=54 J/(molK) for carbon dioxide (Table 2). since water is taken as a gas). A and B are numeric factors (A=b/a is the air/fuel ratio). One may wonder why all vales of the maximum adiabatic combustion temperature in air are so close (2300200 K. .e. CnHm+(n+m/4)·(O2+3. The adiabatic jump is TadT=hLHV/nicp. Of course. is thought to be the best compromise on precision/effort for non-programmed computations. hLHV=10 MJ/kg for CO and hLHV=(1422. which makes TadT=(n·394+m·121)/(n·0. A simple explanation can be found for hydrocarbons. and taking a mean value cp.e. or due to lack of oxygen in rich flames. per unit amount-of-substance exiting.g. whereas reducing the pressure increases dissociation in accordance with Le Châtelier principle. which is (n·394+m·242/2) kJ/mol.058) K. e. from combustion data Table).13 where a.air=40 J/(mol·K) for the exhaust gas. cp=1000 J/(kgK)=29 J/(molK) for cold air.76·(n+m/4)·N2. i. i. For the complete stoichiometric combustion in air. the total mole number in the products is n·(1+3. computation of adiabatic temperatures is coupled to computation of equilibrium composition. and i are the amounts of exhaust species per unit amount of fuel (not the stoichiometric coefficients). c. yields:  0qa h  fuel     air     other  c p fuel (T  T )  b h  c pair (T  T )  c h  q  ahLHV  (ac p fuel  bc pair  cc pother )(Tin  T  )     x h  c pother (T  T )  Cexhaust  i 1 xi c pi (Tout  T  ) Cexhaust i 1 i  i  c pi (T  T  )  (32) which can be read as follows: the thermal energy in the exhaust stream (the last term) is the contribution of the external heat added through the walls per unit of exhaust q (zero for an adiabatic process).76)+m·(1/2+3.4)=140 MJ/kg for H2). their LHV may be approximated by the LHV of nC+(m/2)H2. a biparametric function almost constant. or better cp=34 J/(molK) to account for the growth of thermal capacity with temperature. of value TadT2000 K. but the first form of (33) with cp=34 J/(molK) for any diatomic molecule.air. The adiabatic combustion temperature is obtained from (32) with q=0:  Tad  T  ahLHV  (ac p fuel  bc pair  cc pother )(Tin  T  ) Cexhaust  i 1 xi c pi T  ahLHV c pair (33) where the last simplification (a most crude approach) neglects the thermal enthalpy input. and approximates the thermal properties of the exhaust gas mixture by those of air. plus the chemical heating value of the fuel input (the lower heating value. xi are the molar fractions in the exhaust (xi=1).190+m·0. the energy balance (30) with the definition of LHV in (27). the best precision is obtained with the integration cp/T)dT instead of the approximation cpT in (30).g. The effect of pressure on adiabatic temperature is through its effect on equilibrium composition: at high pressure dissociation is negligible and the completecombustion adiabatic temperature gets its highest value. b. Mi the molar masses of each component (associated to the molecular formula stated. aC+bAir=xCOCO+xCO2CO2+xO2O2+xN2N2). Choosing the first option in (31). When dissociation is important due to the high temperatures associated to near-stoichiometric combustion. for each reaction r=1. on top of that. if the combustion process has had time enough to develop completely. that for a reacting ideal gas mixture is (see Chemical reactions):  p  xi      i 1 p  C i   i  p  K (T .. The Q-values of a closed system have a tendency to reach a limiting value over time. The exhaust from a combustor may be at equilibrium (not with the surroundings but within itself. for a given reaction 0=iMi. is an acceptable approximation to many combustion processes. there are always some partial reactions so slow that they do not reach equilibrium in the times considered. Back to Combustion . as for some trace contaminants. i=0. serves to explain why a further simplification. although it may be considered at a metastable equilibrium (it would not react if not ignited). for each component i=1. and the constants gr (standard Gibbs function of reaction) and gr (standard enthalpy of reaction) are computed from the standard enthalpies and Gibbs functions of formation by: C C i 1 i 1 hr   i h fi . the complete combustion model. and. the equilibrium constant K.C. but the assumption of perfect chemical equilibrium is a very good approximation to the overall real combustion process. i.e. p )     p     i  g h  T    exp  r  r  1   RT  T   RT (34) where the approximation lnK=A+B/T for the reaction constant K has been applied.14 EQUILIBRIUM COMPOSITION The intake to a combustor is not at equilibrium (it would not react if it were). The latter establishes a relation between molar fractions in the equilibrium composition xi.. Ar=0. In practice. g r   i g fi (35) For systems that are not at equilibrium yet. the ratio calculated from the mass-action law is called a reaction quotient Q. in chemical equilibrium at a given T and p). or even at separate fuel and air inlets at their own equilibrium conditions before mixing. Chemical equilibrium in absence of external force fields implies no gradient of chemical potential.R. and T and p. and no affinity to go on. ........................................................................................................................................................................................................................................ 43 Relation between rate coefficients and equilibrium constants ..................................................................................... 15 Fuel sprays ............................. propagation and extinction ... 14 Flash point (ignition) ..................... 40 Types of elementary reactions ................................................................................................................................................... 24 Propagation of premixed flames ........................... 4 Evaporation ............................................ 38 Turbulent premixed flames .......................................................................... 36 Axisymmetric turbulent jet flames .......................................................................................................................................................... 41 Arrhenius law................................................................................................................................................................................................................................................................ 42 Collision theory ....................................................................................................................................................... 46 Selective catalytic reduction (SCR) ............................................................................................................................................................................................................................................................................................................................................ 39 Mass action law ............................................. 7 Ignition and extinction .......................................................................................................................... 46 The three-way catalytic converter .......................... 25 Laminar combustion .................. 38 Chemical kinetics................................................................................................................................................................................... 30 Flame quenching....................................................................................................... 13 Gas fuel jet ................................ 45 Catalysis .................................................................................................................................................................................................................................................................. 5 Droplet evaporation ................................................................................................................................................................................................................... 12 Propagation of non-premixed flames.......................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................... 29 Flame stabilisation ............................................................ 31 Autoignition temperature .................................................................... 3 Diffusion .. 3 Mixture ratio specification .................................................................... 16 Droplet combustion .... 33 Supersonic combustion ............................................................................................................................................................................... 5 Evaporation from a planar surface ...... 13 Condensed fuel ......................................................................................................................................................................................................................................................... 34 Turbulent combustion ......................................................................... 3 Convection ..................................................................................................................... 44 Kinetics of NOx formation .............................................................................................................................................................................................................................................................................................................................................................................................................. 14 Flame stabilisation by porous feeding: oil lamps and the candle flame ............................... 31 Flammability limits ................................. 26 Deflagration speed ....................................................................................................................................................................... 47 Catalytic combustion ................................................................ 47 ................... 2 Physical mixing and its effects on ignition.................................. 39 Reaction mechanism and reaction rate ........ 13 Flame length ................................................. 2 Mixing................................. 18 Particle combustion ..............COMBUSTION KINETICS Combustion kinetics . 27 Flame thickness . 10 Igniter types ........................................................................................................................................................................................................................................................................................................................................................................................ A vivid example of the controlling effect of mixing is presented in Fig. evolving at a sizeable pace (i. establish limits to natural and artificial processes.g. and determines that end state (which might be reached also by secularly-slow oxidation). and does not deal with the burning rate. 1. 1. but the path actually followed. and hydrocarbon fuels cannot burn in CO2/air mixtures if xO2<15% (<6% for H2 fuel). the piece of paper in air will oxidize very slowly (unnoticeable to the eye). without ignition. generates a flame-front that sustains itself. blue. PHYSICAL MIXING AND ITS EFFECTS ON IGNITION. Fig. Hydrocarbon fuels cannot burn in N2/O2 mixtures if xO2<12% (<5% for H2 fuel). due to the lack of air draught by buoyant convection. one on the ground and the other on a space platform (no buoyancy effects). it just says that the system paper/air might reach a more stable equilibrium state (more entropy) by burning. candle flames on earth are long. or at a negligible rate (i. propagation and extinction. as in slow oxidation). Thermodynamics says that a fuel and air may naturally react. not even after being ignited. and rate of chemical reaction once mixed (including a review of reaction mechanisms). in the monitoring time-span. For instance.e. the fuel will not burn completely if there is not enough room inside for a good air-convection to the flame.e. Some basic models of global combustors and of flame structure are dealt with apart. whereas under weightlessness they are nearly spherical. Thermodynamics does not say that a piece of paper will burn in air. because. as in combustion). i. Candle flames on earth (left) and on a space station under weightlessness (right). One of the basic features of combustion is its self-spreading power: under usual conditions.e. At room temperature. but if the kinetics is too slow. that transmits the activation to the . requires some exergy input from outside). may proceed in an isolated system) or artificial (i.e.e. depends on other circumstances. more quickly than monitored. showing two similar candle flames. PROPAGATION AND EXTINCTION It cannot be stressed enough that kinetics is what finally controls combustion (or any other reaction). as in explosions). once ignited. convection and diffusion might not supply enough oxygen to maintain the minimum heat release needed for propagation. even if with more than stoichiometric air is enclosed. may not burn completely if ignited (e. more slowly than monitored. as oxygen concentration gets reduced.COMBUSTION KINETICS Thermodynamic laws. Topics covered below are: rate of physical mixing and its effects on ignition. bounds to the possible paths. Thermodynamics indicates if the reaction is natural (i. Some illustrative examples are: the burning of a piece of paper or a candle inside a closed container. slender and yellow. and burn much slower (sometimes get extinguished). i. For instance. and the pace (the process rate).e.e. a piece of paper enclosed in a transparent container with more air than the theoretical one. by a concentrated light). a fuel/air mixture. It is Kinetics science which deals with how fast things happen: instantly (i. an observer concludes that there is no reaction. Two extreme cases of mixing are considered in combustion: combustion in a premixed system (prepared well-before-hand. mass fraction yF. . But mixing is a slow physical process if not forced by convection (large-scale transport) and turbulence (large-scale to small-scale transport). (1). to the wakes behind vehicles of any sort.e. it does not require an energy expenditure). Two limit cases can be considered for this propagation according to the state of the mixture: combustion propagation when fuel and air are at each other side of the flame (i. and not only inside the combustor itself. the basic kinetic law for mass diffusion is Fick’s law:   ji =− Di ∇ρi (Fick’s law.g. selective force fields may yield diffusion (e. similar to Fourier’s law for heat transfer q =−k ∇T ) (1)  ji ≡ m i A being the diffusion-mass-flow-rate of species i per unit area. mass-diffusion due to a speciesconcentration gradient. alternatively. known as Soret effect. with the same value in molar and mass basis). massdiffusion due to a temperature gradient. with the same value in molar and mass basis). equivalence ratio φ (the actual fuel/air ratio relative to the stoichiometric one.g. fuel reformers. from the piping of water. is a natural process (i. decreasing bulk differences). relative speeds and chemical composition (with the natural stratification in the presence of gravity or another force field).e. gases. mixture fraction (mass-flow rate ratio of injected fuel to mass flow rate of products). Diffusion Actual mixing of chemical species is governed by mass transfer laws. but most of the times those cross-coupling fluxes are negligible. fuels. if fuel and oxidizer gases are brought to contact and enough time allowed.fresh mixture through heat and mass transfer. there are also secondary fluxes associated to other possible gradients (e. etc. Besides. for a non-premixed mixture).e. there may be heat-diffusion due to a species-concentration gradient.. Di the mass-diffusivity for species i in the given mixture. Mixture ratio specification Mixture ratio specification may use different units: molar fraction of fuel in the mixture xF. and ρi=yiρ the mass-density of species i in the given mixture.. In a homogeneous media. the mixture ratio specification is established before-hand. whereas for non-premixed combustion. Mass transfer is essential to combustion. and heat-diffusion due to a pressure gradient). Notice that only the flux associated to the main driving force is considered in Eq. which is a special case of combined heat and mass transfer reacting system. etc. and mass-diffusion due to a pressure gradient.e. known as Dufour effect. but for other combustion-related reactors as in the after-burning catalysts. Mixing (i. as the laminar diffusing contrails left by jet aircraft). i. fuel-to-air ratio f (molar or mass). For premixed combustion..e. very similar to heat transfer laws for conduction (diffusion) and convection. MIXING Mixing is a pre-requisite for combustion. so that. Turbulent mixing is the rule in all practical fluid flows at scales larger than the millimetre. for a premixed flame). without phase changes or chemical reactions. to all atmosphere. a perfect mixing would take place in their energy level (temperature). air-to-fuel ratio A (molar or mass). ions in an electric field). the mixture ratio specification depends on the actual feeding flow-rates of fuel and air. and combustion propagation when both fuel and air are at the same side of the flame (i. ocean and stellar motions (there are some exceptions. air relative ratio λ (the air/fuel ratio relative to stoichiometry. and combustion in the common-interface layer where non-premixed fuel and air come into contact.. or well-stirred). empirical correlations applicable just to the boundary values are often used as: hm L  = Sh ( ReL . But. the balance equations for mass-transfer and heat-transfer. Because of the nearly-equal values of Di and a for gases (Di≈a≈10-5 m2/s). the solutal and thermal convection correlations are sketched (Sc≡ν/Di. are studied with Brownian-motion mechanics. diffusing within a background mixture of averaged properties. are here presented jointly: Magnitude Chemical species i Thermal energy Accumulation Production wi ∂yi = ρ ∂t ∂T ∂t = φ ρc p Diffusive flux Convective flux  + Di∇ 2 yi − ∇ ⋅ ( yi v ) (2) + a∇ 2T  − ∇ ⋅ (Tv ) (3) with wi being the mass-production rate by chemical reaction. as in heat transfer.Notice that only molecular diffusion is considered here.). As in the study of convective heat transfer.g. To better grasp the similarity between species diffusion and heat diffusion. T. smoke). mass diffusion relaxation times are much smaller than their thermal counterparts. a jet of hot air emerging to ambient air). instead of having to migrate by its own random fluctuations. and particles >10-6 m with Newton mechanics. that the boundary conditions in practical solutal-convection problems can be very different to the classical heat-convection problems where a single fluid sweeps a hot or cold rigid boundary. Notice. yi (one for each species). Reynolds number of the imposed flow Re. i. the thermal and solutal relaxation times in absence of convection. i.g.g. one usually resorts to empirical correlations to compute mass-transfer nondimensional parameters (Sherwood Sh. Typical values for Di and a are given in Mass diffusivity data. instead of solving the whole fluid-dynamic problem with (2-3) and momentum equation. Rayleigh number of the imposed thermal gradient Ra. are nearly equal (trel≈L2/Di≈L2/a). applied to a unit-volume system. Pr ) for heat convection  =  k Sh ≡ (4) where both. particles in the range 10-8. the evaporation of liquid fuels studied below) can be separated from the more complex gas-phase combustion. although most problems are modelled as a binary system of one species of interest. but. the fluid flow should be solved in conjunction to diffusion. mist. or mass-Nusselt number) in terms of non-dimensional stimuli (e. and a=k/(ρcp) the thermal diffusivity..e. a jet of fuel gas emerging to ambient air) and thermal convection (e. but many mass-transfer functions. Convection A quicker mixing process than diffusion is convection. etc. The case of a submerged jet is a good case of similarity between solutal convection (e. Notice that there is only one driving heat-transfer-function. In liquids and solids. where bulk fluid-flow transports species as if encapsulated. however. for particle sizes <10-8 m. Phase changing systems are more conspicuous. Thus. and ρi is bound to ρi<<1 kg/m3 for diffusion in air under normal conditions. Sc ) for mass convection  Di   hL Nu ≡ Nu ( ReL . in the burning of solid fuels the process are entangled because there may be . Pr≡ν/a). T is not bound. although most of the times the process of phase change (e. in spite of the fact that the coefficients in (1) are widely different (Di≈10-5 m2/s and k≈10-2 W/(m·K)). 10-6 m (soot.g. drying. When considering the vaporisation of practical fuel droplets (like diesel oil). and may take place in pure substances. but the air outside is stirred enough as to maintain constant conditions at the mouth (T0. It is often simpler and more efficient to transport fuels in condensed form. due to a normal concentration-gradient of that species in the gas close to the interface. desalination by reverse osmosis and other membrane processes. oxidation. in the real test-tube case. Perhaps the most clarifying difference is that boiling is a bulk process (bubbles form at hot points. as well as a more general mass-transfer topic in mechanical engineering (humidification. the variation with time of the composition and vaporisation temperature may be very important due to multi-component equilibrium. cooling towers) and chemical engineering (reactors. (2) with its initial and boundary conditions would solve the problem.). We only consider here evaporation of a pure liquid. usually the walls of a heated container). Evaporation should not be confused with boiling (which may also be properly called vaporisation). Evaporation is a basic topic in combustion of condensed fuels. controlled by diffusion of both. we follow on here just with the evaporation process). Consider a test-tube with water in open air. Evaporation from a planar surface Let us start by the simplest one-dimensional planar diffusion-controlled evaporation problem. Assuming that the air in the tube is quiescent. etc. whereas evaporation is a freesurface process (no bubbles form. the initial and final phases take place with a detached vapour flame (at some 2600 K). and the interface region cools) that only happens in mixtures. in air (a mixture). the evaporation of naphthalene in air (sometimes called sublimation). scrubbing. usually n-octane for gasolines. controlled by a carbon layer (perhaps Al4C3) formed by heterogeneous reaction of carbon monoxide there (and not by an alumina layer as thought. Tm(Al2O3)=2320 K). typically water. it is common practice in theoretical analysis to assimilate commercial fuels to pure-component reference-fuels. at the interface between a condensed phase and a gas mixture. materials processing. whereas an intermediate stage takes place at the surface. although the first integration can be skipped directly establishing the series of mass conservation relations: . and safer to burn them with nonpremixed flames. the evaporation of ammonia from an open bottle of water-ammonia solution in air (water evaporates too). Examples: the evaporation of water from a glass of water in air (the level decreases some 1 mm/day).decomposition reactions within the solid (as in wood burning). or heterogeneous combustion at the interface (as in coal burning and metal burning). species and heat. electrochemistry. however. in which case. consuming two thirds of the mass. EVAPORATION Evaporation (sometimes called vaporisation) is the net flux of some species. and assuming a steady state (the water level is thought to be kept steady by some slow liquid supply from the bottom. fuel droplet evaporation from the injector spray constitutes a first stage to the combustion of the generated vapours within the oxidiser stream (droplet burning is dealt with below. the evaporation of ethanol from a glass of wine in air (water evaporates too). which is the change of phase within the liquid phase due to an increase in temperature or a decrease in pressure. Eq. are just heat transfer problems). For instance. Volatiles liquids and volatiles solids are smelly. p0. Evaporation and condensation in a mixture are always combined mass-and-heat transfer problems (boiling and condensation in a pure substance and the Stefan problem of melting or solidification. and ndodecane or n-tetradecane for diesel oils. φ0). the liquid level would slowly decrease).. when aluminium particles burn in a carbondioxide atmosphere (2Al+3CO2→Al2O3+3CO). g. Notice that. assumed to be that of two-phase equilibrium. and z1−z0 being the depth (the diffusion length). with the constant-density approximation in the gas phase. Dividing by ρ and integrating.13 mm/day for water down 10 cm in a test-tube). 0. Raoult’s law (see Mixtures).3 kPa  − 0.1 mm/day). and φ is the relative humidity of the ambient.g. for a 1 cm diffusion layer. water) and the mixture (practically that of ambient air.029 kg/mol 100 kPa 100 kPa  where Di was found from Mass diffusivity data. e.∆yi. ethanol evaporates some 10 times faster than water at 20 ºC vliq=12 mm/day).2 ⋅10−5 m 2 /s 0. 100 kPa and 60% humidity ratio.018 kg/mol  2. respectively.2 kg/m3 2. i. just after he introduced the partial-pressure concept. at the ambient and at liquidlevel equilibrium are related to the pure vapour pressure at that temperature by Raoults' law:  φ pi* (Tamb ) M i = y at the ambient  i1 p Mm xi M i Mi  = yi = xi (7)  ∑ xi M i M m  y = pi* (Tliq ) M i at the liquid surface  i0 p Mm  where Mi and Mm are the molar mass of species i (the volatile liquid.∆z). who first said that the evaporation rate is proportional to the difference in partial pressure of water vapour from the saturated boundary layer. and that it increases with free air velocity. to that of the air far aside.e. This result might have been anticipated by a simple dimensional analysis of the function v=v(Di.g. or Antoine fitting). Dalton in 1801. For water evaporation in ambient air. and vliq the speed of liquid injection (really. the evaporation rate for water at 20 ºC in ambient air at 20 ºC.5·10-8 m/s (1. The first analytical . yi. Notice that the evaporation rate increases with vapour pressure (e. ρi=yiρ.m A  mass-flow-rate per unit area dρ = =ρi v + ji = ρv = ρi v + vdiffi + ρi v − Di i =ρliq vliq (5) 0    dz  mass-flow-rate mass of i mass of i ) ( mass-flow-rate of i per unit area of other species per unit area convected diffused mass-flow-rate of liquid to keep the level steady where ρ is the density of the mixture (assumed constant in a first approximation). for small x’s). with the relation ρv=ρliqvliq from (5). and the linearization in (6) yields: pi* (Tamb )  ρv ρ Di M i  pi (Tliq ) vliq = = −φ   p  ρliq ρliq ∆z M m  p * (8) e. is: vliq 1.3 kPa 2. and the vapour-pressure value obtained from steam tables (or from Clapeyron equation. the global velocity is also constant along the tube.3 mm/day)   3 −2 1000 kg/m 1 ⋅10 m 0. and it falls with deepness (e. (6). the liquid-level descent with time). close to the liquid surface). yi0 the massfraction at the bottom of the gas column (i. The evaporation speed. the value of the global velocity is obtained: 1 − yi1 ln 1 − yi0 yi <<1 yi − yi0 d ln (1 − yi ) dy D dyi → v =Di → v =Di =− Di 1 v =yi v − Di i → v =− i (6) 1 − yi dz dz dz z1 − z0 z1 − z0 yi1 being the mass-fraction of species i at the mouth (assumed to be that of the ambient).6 =1. whereas for a less volatile fuel like n-decane it is <0.g. It was J.e. the mass-fraction of water-vapour. in a droplet evaporation process. The expected radial profiles for the mass fraction and temperature fields are presented in Fig. 2. we have assumed known. The analysis of evaporation in very hot environments (e. Fig. Convection would increase the evaporation rate. The effect of wind speed is to decrease the boundary layer thickness. Stefan in 1872. and the large temperature and concentration variation makes the constant-property assumption less accurate. the droplet radius will decrease with time since there is not such an imaginary supply). due to buoyancy of the lighter humid mixture nearby. for a small amount of liquid the liquid quasi-steady temperature would be the wetbulb temperature (see Humid air). but in reality. The mass and energy balance for this strictly-steady system (with a liquid source at r=0 and a vapour sink at r=∞). ∆z. besides. or at least p*(T∞)>p). because the two temperatures were assumed known. making the approximation above acceptable. a little below ambient temperature. at the steady state. natural convection effects. Radial profiles of temperature and mass fraction of the volatile component. consequently increasing the evaporation rate. from sun rays. Evaporation not only implies a mass transfer but also a heat transfer. T∞>Tcr. although they are still applicable at the liquid surface because the droplet tends to reach a quasi-steady-state temperature close to its boiling point.e.10 µm). quickly accommodate their speed to that of the surrounding gas flow. e. is neglected in this analysis. Besides. since the vaporization enthalpy at the liquid surface must come from either the liquid side or the air side.g. This heattransfer implication was not apparent in the numerical application of (8) just done. the temperature at the liquid interface will depend on the heat transfer and energy balance (the effect of a possible radiation trapping there. as found in mists and spray combustion.. suppressing forced-convection effects afterwards.model of evaporation is due to J. Consider the system defined by the control volume enclosed by a generic sphere of radius r>r0. kept steady by an imaginary supply of liquid at the origin (in reality. of course. To start with. directly related to the diffusion depth. Droplet evaporation Evaporation of a droplet in a quiescent atmosphere is a more practical application of this diffusioncontrolled model. can be set as follows: Magnitude Accumulation Mass balance (global) 0 Production Diffusive flux = m L +0 Convective flux − m ( r ) (9) . since small drops (say of 1.g. gets more involved. Raoult's law is no longer applicable to the far field (r→∞) because the liquid could not be in twophase equilibrium with such a hot gas (i. will enter here also). r0 being the radius of the droplet. inside a furnace). 2. the linearisation in (6) may no longer be valid. i.Mass balance (volatile species) 0 = m L + ADi dρi dr − m ( r ) yi (10) Energy balance 0 = −m L hLV0 + Ak dT dr − m ( r ) c pi (T − T0 ) (11) where the enthalpy reference has been chosen h=0 for the vapour.∞ ⇒ tevap = 2 r0. Notice that the thermal capacity in the convective flux must be the vapour one. stating that the droplet life-time. the diameter.0 (T0 ) 2k ln ln 1 + i  1 − yi . and one concludes that the global mass-flow-rate of gases across the spherical surface.∞ ⇒ ⇒ Di dyi 1 = d ⇒ 2 v0 r0 1 − yi r dr02 2 ρ Di 1 − yi . or simply the r2-law.ini =  c p (T∞ − T0 )  2 ρ Di 1 − yi .0.e. and ρiviA=ρvA= m . d 2 (t= ) d 02 − 4 Kt ). because the sum ΣρivicpiA reduces to just the vapour term ρivicpiA (because dry air cannot move in the steady state). There is. often. finally yields: = m = ρ vA constant  →= v (r ) v0 r02 r2 (12) Dividing (10) by m L = ρ vA and upon integration. is independent of r.ini K (13) i. yi. yields: Di dyi r 2 Di dyi = − yi  →= − yi 0 1+ 0 1+ v dr v0 r02 dr ρliq dr0 D 1 − yi . at the liquid temperature T0 (that is why a −hLV.0 =− ≡ −K ln ρliq dt 1 − yi . tevap. m ( r ) . the bulk radial velocity v.ini r0. In (9). an internal unknown in (13). however. the well-known Langmuir’s law (1918). and. Dividing now the energy balance (11) by m L = ρ vA and upon integration yields: 0= −hLV0 + ln k dT − c pi (T − T0 ) ⇒ ρ v dr hLV0 + c pi (T∞ − T0 ) hLV0 = ⇒ tevap hLV0 ρv r 2 ρv dT dr = = − − 0 2 0 dr ⇒ k kr + c pi (T − T0 ) dr c p ρliq 0 r0 c pi ρ v0 r0 dt = = − k k ⇒ hLV + c pi (T∞ − T0 ) dr02 2k = − ln 0 ρliq c pi hLV0 dt 2 2 r0. from m = ρ vA . d. evaporation always implies a coupled heat and mass transfer. even if injected hotter). m L stands for the mass-flow-rate of liquid consumed (the parameter sought).0 =− v0 = i ln ρ dt r0 1 − yi . assuming constant gas density ρ (pressure is constant and temperature does not vary too much).0 input at the origin is accounted for in (11). and with A=4πr2. which is controlled by the energy balance (11): the heat for the liquid-to-vapour transition must come from the gas phase (droplets always attain a quasisteady temperature below that of the environment.∞ ρliq hLV0 ρliq c pi   (14) . is used instead of radius (with our K. is proportional to the square of the initial radius. if the gas is not too hot (let say for T∞<100 ºC). if a 0. or analytically by an enthalpy matching. is some 1. For instance. and tevap. evaporating in some 0. instead of the spherical volume of radius r>r0. The vapour-mass-fraction close to the droplet is not 'near 1' as one might guess from such a 'burning' environment. i.6 ºC. and.where the latter equality comes from (13). used above to establish the mass and energy balances (9-11). sometimes. yi. It can be shown that. and evaporates in some 16 s.1 mm in diameter in ambient air at 25 ºC. T0 is the wet-bulb temperature. the surrounding atmosphere may already containing some fuel vapours from the evaporation of surrounding droplets in a spray.∞ ) ρliq hLV0 ρliq (15) which. together with yi .e. a water droplet 0. Fuel droplets are usually assumed to evaporate in a zero-fuel environment. and typical Fluid Mechanics formulation applied: D ( ∑ ρi ei ) Dt  k d  2 dT  dT  + ∇ ⋅ ( ρi hi vi ) = −∇ ⋅ q  → 0 + ρv c pv vv = 2 r  r dr  dr  dr (16) which.2∙10-3 s. For instance.0 (T0 ) = ( pi* (T0 ) p ) ( M i M m ) . Notice that the choice of system for the analysis can be changed. relating mass diffusion and heat diffusion. but a mere 16% (notice that the air surrounding the drop is just about 63 ºC and not 1000 ºC). and having only one unknown T0 after imposing Raoult's law (7): yi .1 mm in diameter water-droplet is injected in air at 1000 ºC. but not in the overall mass balance (9). what is equivalent to linearising the logarithms in (14). into 'dry air' but. the droplet-temperature quickly (in less than 1 ms) adapts to a temperature of 63 ºC. As in the planar-geometry evaporation case treated above. attains a temperature of Twet bulb=17. would yield a similar equation to (11): r2 r dT dr cp ρ v0 02= v k d  2 dT r r 2 dr  dr dT  → ρ v0 r02 c pv = (T − T0 ) kr 2 − ρ v0 r02 hlv0   dr  (17)  lv0 = 4π r02 k dT dr r = r .ini ⇒= tevap = 2k (T∞ − T0 ) 2 ρ Di ( yi .0. practically coinciding with the adiabatic-saturation temperature analysed in Humid air and easily found with the psychrometric chart. The initial relative motion decays also quickly (in some 10 ms). for room-temperature water droplets. 100 kPa and 50%RH. reducing to: 2 2 r0. and is the closure for the evaporation problem. A second where the last term comes from the energy balance at the surface. convection effects in droplet evaporation can be neglected in the species balance (10) and in the energy balance (11). a unit area in a spherical surface of radius r>r0 could have been used.17 s. after performing a first integration from r0 to r. the evaporation time for a 10 µm n-decane droplet issuing at 400 K (50 ºC below its boiling point) into air preheated to 800 K in a furnace.ini r0.0 (T0 ) − yi . mh 0 integration would yield: . knowing from the mass balances (9) that ρivi=ρv=ρv0r02/r2. As another example.0 (T0 ) = ( pi* (T0 ) p ) ( M i M m ) . allow the finding of T0. if an activation energy is not externally supplied. a flame. H2(g)/F2(g).e. i. to cause a selfacceleration of the reaction. the mixture H2(g)/Cl2(g) does not burn by contact in the dark. develops). but to provide a practical control of fire. an ignition source. burns without an ignition source. as for most engineering problems. One may consider different ways for the fuel/oxidiser reaction to run away: • Contact ignition. by just coming into contact some special fuels with special oxidisers (ignition by contact with a hot object is covered under autoignition).e. The latter is a very convenient ignition method. by striking a piece of flint and steel together) as in prehistoric times. besides analysing the steady running. below). or the presence of Pt. gas turbines. Although not properly a combustion process.e. Pd or Ni ignites it. Pure iron in minute amounts also burn by contact with air (this is how sparks are produced when hitting a piece of iron on a flint). P4(s)+3NaOH(aq)+3H2O(l)= PH3(g)+3NaH2PO2(aq). All combustion reactions have positive affinities and thus could proceed naturally in an isolated system. but requires a premixture at least locally near the spark plug (see Flash point. Na(s)+H2O(l)=Na(OH(aq)+(1/2)H2(g). like an electric spark or a hot object. obtained by immersion of phosphorous in a sodium hydroxide solution. After ignition. not only to better understand combustion. the reaction self-propagates because a flame is an ignition source. There are substances (called pyrophoric substances) which have autoignition temperatures below that of the environment (say <15 ºC). so that the combustion takes place as soon as they are put in contact (they cannot exist premixed). Also. Ignition of a reactive system may be homogeneous (only for perfectly premixed systems uniformly heated. but sooner or later. the mixture of hydrogen and fluorine. when a solution of white phosphorus in carbon disulfide is used to write on paper. i. based on the production of a tiny burning particle (e. At room conditions. • Spark ignition. IGNITION AND EXTINCTION We shall deal basically with steady-state kinetics and propagation. i. or on the electric arc produced between two closed electrodes when the dielectric strength of the medium is surpassed (3 kV/mm for air). one has to face the more complex phenomena of starting up and stopping down the process (ignition and extinction). furnaces and boilers. by locally supplying the activation energy required to triggers the reaction. the rapid exothermic oxidation of metallic sodium in contact with water. the carbon disulfide evaporates within a few minutes and the paper chars. Dimethyl-hydrazyne (N2H2(CH3)2(l)) and dinitrogen tetroxide (N2O4(g)) also burn just by contact. escapes at the free surface and react with air. as any other hot object. or heterogeneous.g. hlv + c pv (T∞ − T0 )  ρ v0 r0 c pv ρliq vliq r0 c pv ρ v0 r02 dr dT (18) =  → ln  0 =  = 2   hlv0 + c pv (T − T0 ) k r h k k lv 0   and so on as from (14). i. and the mixture H2(g)/O2(g) also burns at room conditions by contact in the presence of platinum catalyst. There have been accidents where oilimpregnated rags have got fire by themselves because of the rapid oxidation of oil in air with such a large contact area and low heat transfer. may release so much energy as to ignite the hydrogen produced.e. used in Otto engines. but most common fuels and oxidisers combine too slowly at ambient conditions. as in compression ignition). An order-of-magnitude estimation of the energy required for ignition is the energy required to heat a cubic volume of size . Even in normal air there may be spontaneous combustion of some fuels. PH3(g)+2O2(g)= (1/4)P4O10(s)+(3/2)H2O(g). but intense light. as when locally initiated by a spark or a catalyst in either a premixed or a non-premixed system (a subsequent deflagration front. as when phosfine bubbles. For instance. The traditional fire-fighting method of massively adding water. Tesla in 1890 (also attributed to R. but it may be more difficult to stop.g. but also by hot-surface ignition. It is estimated that the spark produces a plasma filament at some 20 000 K in the discharge. This value corresponds to the quenching distance for stoichiometric mixtures. releasing negligible heat). The spark plug was invented by N. it goes from 1. for lean and rich mixtures the quenching distance grows parabolically towards infinite at the ignition limits (e. or. for methane(air. to prevent flame propagation. . to 4 mm at λ=0.8 mm at λ=1. although sometimes a pilot flame is maintained if operation is discontinuous (e. from which the normal deflagration theory applies. and the many radicals needed for flame propagation). quickly turning into a hot gas-ball of one or two millimetres in diameter at some 5000 K. Cold solid walls.8 and λ=1. i. with their massive thermal sinks. to ignite a cold liquid jet being injected in hot surrounding air). works mainly as a cooling agent to lower the temperature below the autoignition value. more commonly. Combustion may be difficult to start. but in steady combustors like gas turbines. in some domestic hot-water heaters). and avoiding any more fuel to be near the flame. the effect of a sizeable spark (>1 mJ) in a sizeable container (larger than a few millimetres by all sides). or to a dual-fuel Diesel engines (as when a lean natural gas / air mixture is premixed and made to ignite by compression-ignition of a low-autoignition secondary fuel pilot injection). Autoignition. namely. Benz). also act as cooling agents that prevent flame propagation near them. by raising the temperature of the gases by rapid compression or by heat transfer. as well as the process of ignition and extinction themselves. ∆Eign~L3thρcp∆Tign~(10−3)3∙1∙103∙103~10−3 J. only used purposely in minute model engines. Once ignited. a H2(g)/O2(g) mixture within a glass or a steel bottle at room temperature and pressure is not seen to react (hardly any molecule yield fertile chocks. by cooling and/or by dilution of its reactive species (fuel.e. furnaces and boilers. usually by compression ignition. Autoignition by hot spot is usually an undesirable event. safety lamps and quenching grids are based on that fact. a certain balance between the heat released (depending on chemical kinetics) and heat transfer towards the fresh mixture.8). for a flame to propagate. Besides the obvious extinction procedure of letting all the fuel to burn (the amount of oxidiser is assumed infinite in terrestrial applications). or the presence of a sizeable amount of platinum. is required to ignite the system at ambient conditions (it may also be ignited by heating the mixture to 850 K at 100 kPa whatever the container). By using a catalyst to lower the activation energy. This is a very convenient ignition method.e.e.. either previously mixed. or to a fully premixed mixture (as in some new stationary Diesel engines where all the fuel is injected before ignition takes place by compression). the system only needs to be ignited at start of the duty (usually by spark plugs). by raising the temperature of the oxidiser in a sudden compression and then injecting the fuel when desired and at the desired rate. a hard knock. whereas the typical CO2 fireextinguisher works mainly as a dilutant to prevent air coming into the flame (see Fire safety). Combustion ignition in reciprocating engines must be stimulated at each cycle (either by a spark or by quick compression). which may be applied to the non-premixed mixture as in the usual Diesel engine (i. Catalytic ignition. what explains the existence of flammability limits and quenching distance (explained below). For instance. a combustion process may be arrested by providing unfavourable conditions to its propagation.g. premixed methane/air combustion cannot propagate inside a metal tube of less than 2 mm bore. i. oxidiser. must be warranted. as mentioned above.• • Lth to the autoignition temperature. Bosh and to K. • Catalytic igniter. sustaining combustion even of other combustible mixtures. Created when some natural crystalline substances (quartz. Igniter types • Thermal • Rapid-compression heating • Air-compression igniter. some primitive people knocked in a crude piston-cylinder device to ignite some dry wood-shavings inside. For propane and butane flames. producing a steady blow-torch type of flame nearly-invisible in the bright (sometimes a ceramic flame-window that glows when hot is used to indicate the lighter is on). nearly invisible and much more powerful. • Two-electrode sparks.e. wind-proof lighters. A summary of ignition devices. is to get ready dry flammable tinder (fine wood shavings. classified by the ignition method. Sometimes the wire is a catalyst (see below). the combination of pulse-mode semi-conductor lasers and optical fibres seem most promising. Platinum wire gets red-hot. >3 kV/mm). follows.Many types of igniters have been developed. Non-sensitive to moisture. Non-sensitive to moisture. a specially prepared chemical substance quickly reacts and forms a flame. are produced in 'jet lighters' (miniature bunsens). Sensitive to moisture. . since prehistoric times). as important as the lighter. as in a diesel engine. the catalytic wire is usually preheated electrically. induction-type premixed flames. i. Non-sensitive to moisture. Usually. All lighters until 1990 where of non-premixed type. but since then. etc. hydrogen with fluoride).). causing opposite electrical charges to concentrate on each side of the crystal. Electric discharge through a ∼1 mm air-gap (requires a coil autotransformer to overcome rupture voltage in gases. • Dissipative heating • Mechanical energy dissipation. With a little friction. Little sensitive to moisture. • Electronic sparks. • Pure chemical (without heating) • Simple contact of special chemicals (e. • Spark heating • The flint-and steel spark. • Piezoelectric sparks. on use since 1850. Since 1980. • Pyrotechnic heating • The match. Very sensitive to moisture. gasoline vapour (since 1932) or waxy wool (tinder. • Radiation heating • Solar radiation and a glass magnifier (water-filled bottles. when in contact with an hydrogen/air mixture. straw. It is the most handy igniter since XIX c. • Pulse laser can be focused on the most convenient spot. or even a clear ice ball) focussing light onto tinder. a gripping steel wheel is force to rotate a little against a small flint stone. Sensitive to moisture. reaching >1000 K. • Electrical energy dissipation. down from corn or from birds or seeds. The bow and drill is the most-efficient simple-friction firemaking procedure since prehistoric times. barium titanate) are hammered (a mechanical trigger is used). dried grass. to be successful in using an igniter to start a camp fire. In spite of all the above possibilities.g. This device is used in modern. and iron sparks are projected over some easily flammable substance: LPG (since 1933). A battery and a wire or a steel-wool load that gets red-hot by Joule heating. what implies very fast kinetics and very thin flame thickness. and they are quite luminous (yellow.e. because they naturally hold attached to the injector rim over a very wide regime range. both yF=0 and yO=0 at the flame front. shows that ignition takes place (after some delay time) in the lean and hotter zone sited some 10 droplet-radius afar (with relative air/fuel ratios λ=2. but sometimes not so easy with other combustible material). as everybody lighting a bunsen realises.Ignition is a requisite for combustion. that are nearly invisible (alcohol flames are difficult to see. and an oxidiser-region with no fuel (yF=0. some ignition agent must be applied at the mixing layer (outside this fluid interface. This Burke and Schumann's model of fast chemical kinetics is the first step in most analysis of non-premixed combustion. too). concentration fields. The usual way to ignite this local premixture is by an electrical spark or a pilot flame.e. The main characteristic of a non-premixed flame is its size and shape. not having a defined propagation speed and flammability limits as premixed flames. The fluid field is split by the flame in a fuel-region with no oxidiser (yO=0). with typical values of K heavily dependent on temperature (K=10-7.3). besides. and the averaging principle for fluctuating flames. They are easier to establish than premixed flames. what is an advantage for old lighting-methods and long-range heating. in spite of being less power-intensive and more pollutant than premixed combustion. Non-premixed flames are most used in practical system (coal burning. diesel engines. Burke and Schumann established in 1928 the first model of non-premixed flames (coaxial flows with the same speed). i. propane/air jet flames issuing from a 0. open fires). gas-fired or oil-fired industrial burners. Detailed studies in simple configurations. but auto-ignition at high temperature is used in diesel engines. candle flame.8 mm tube.5. and solid-fuel rockets). and they are more radiant. d2(t)=d02−Kt. yFyO=0 in the whole field). if chemical kinetics is so fast that the global process is diffusioncontrolled. the flame sitting just where the two reactive flows meet with stoichiometric rates (i. Unsteady non-premixed flames occur in diesel engines. because of blackbody emission by soot) except for hydrogen flames. Flame length depends on the measuring principle: visible light. but all flame requires diffusion) are established by some ignition process (usually by a spark or by high temperature of the oxidiser). the fluid is not flammable because only fuel or oxidiser exist)..10-6 m2/s in droplet combustion in cold or hot air). temperature field. the most common example may be the flame of a cigarette lighter. droplet burning) or turbulent (e.g. near the boundary between fuel and oxidiser. The square-law for droplet-diameter evolution. where   ρO vO M O +ν O ρ FvF M F = 0 ). because of their higher safety and ease of control.. applies very well to both the non-reacting evaporation and to the combustion processes. based on the assumption that the characteristic chemical time for reaction is much shorter than the time required for the reactives to reach the flame (mixing time). other radiations.g. To ignite a non-premixed fuel/oxidiser mixture. PROPAGATION OF NON-PREMIXED FLAMES Non-premixed flames (also diffusion flames. linearly increase their height with exit velocity up to 12 m/s (Re≈1000). e. It also depends on buoyancy. with . for a reaction F+νOO→νPP. GAS FUEL JET Flame length Flame length is the most characteristic parameter of a non-premixed flame established at the mouth of an injector.g. gas turbines. but fire-keeping demands also self-sustained conditions for its propagation (very easily obtained with flammable material. Non-premixed flames may be laminar (e. like the autoignition of a small fuel droplet in hot air (well above the autoignition temperature). Lfl=tdifw0. may be given by the distance travelled by the jet at the exit speed w0 during the period of fuel diffusion tdif.e. at w0=12 m/s. . at high speeds. (19) For instance. for a steady methane jet of 10 mm diameter issuing in air at 80 mm/s. LH2/LCH4≈0. from yF=1 at the jet centre. buoyancy should not be as relevant as it appears in practice (perhaps the behaviour would tend to match. when they shorten to some 0.06. A quick guess for the laminar flame length.g.18 m for buoyant flames. concentrates in the mixing shell where the flame locates at small heights. decreases with size in the form f n ≈ f 0 / D D0 . all this depends on temperature. combustion is not possible. from 10% (for methane/air) to 50% (for propane/air) of their heat release is lost by radiation. with tdif=(1−yF. Notice that.39 m. carbonmonoxide flame are much shorter than hydrocarbon flames. but with a more pronounced peak between 4 µm and 5 µm due to molecular band emission by H2O and CO2. until they blow out at 33 m/s. if blow-out did not prevent it). Re<1000) the flame length is proportional to the volumetric flow-rate. Di=10−5 m2/s and yF. or above the upper limit. greatly reducing flame length.25 m for buoyant flames (on the ground) and 0. but the main first natural frequency. special swirlinducing nozzles are used to quickly mix fuel and air.2. air is coaxially fed with the fuel. Notice that with this model (only applicable to laminar jet flames. below). against the 0. from the in-flame soot (with a smooth maximum at CWien/T=2900/2500=1. as soon as evaporation proceeds.2 µm. with D0=0. for the propane/air jet-flame above-mentioned. in between the lower and upper flammability limits. and increases with stoichiometric air/fuel ratio (e. soot volume fractions profiles have a two-bell shape up to some 50 mm height. we get Lfl=0. Further increasing of exit velocity causes a transition to turbulent flames with heights receding to 0. until some 35 m/s. and wide-spreads to the centre downstream (e. with f0=1. For gas-jet flames of a few kilowatts issuing from a few-millimetre injector.18 m. outside the flame). Turbulent non-premixed flames flicker with a wide frequency spectra. with a fuel-vapour mole fraction that may be below the lower flammability limit (see Flammability limits. Non-premixed flames may radiate a lot of energy.stq at the flame front.25 m measured. whereas for non-buoyant flames heights further increase to 0.stq)w0D02/(4Di). CONDENSED FUEL Flash point (ignition) Another aspect related to the combustion of non-premixed mixtures is the so called ‘flash point’ of a condensed fuel in the presence of ambient air. and one central bell-shape further along the axis).stq=1/(1+16)=0.33 m for non-buoyant flames (under microgravity). to the stoichiometric value yF=yF. Lfl. fn.8 mm.2.g. but.heights of 0.e. a gaseous fuel/air mixture develops near the liquid surface. i. what may increase the flame length and pollutant emissions if the co-flow pattern reduces the mixing.5 Hz and D0=1 m. In the limit case of the initial liquid fuel before evaporation (i. LC4H10/LCH4≈3). Similarly to the temperature field. since vapour pressure exponentially grows with temperature. mainly by blackbody emission at some 2500 K. soot production in a gas-fuel jet flame.29 m before blow out at 40 m/s. To prevent this. In most practical non-premixed burners.stq)(D0/2)2/Di and thence: Lfl=(1−yF. LCO/LCH4≈0. no fuel-vapours in the air). D being the diameter of the fuel-injection tube or the mean horizontal size for open fires (it seems that this is due to the Kelvin-Helmholtz vortex-ring instability in the shear layer of the hot plume. Of course. but a local molten pool is maintained by heat radiation from the flame). All these problems are solved when the flame can get stabilised around the tip of a porous solid that pumps liquid-fuel by capillary suction towards a well-ventilated region. perhaps just holding close a match for a while. the non-premixed flame will travel along the fuel surface. Some oil lamps developed in the 19th century used gravity-pumping instead of capillary pumping. can significantly change the flash point. . A detailed description of the historical developments of oil lamps (where the fuel is already in the liquid state). providing the heating and the igniter at the same time.g. Torches. and above that are labelled ‘combustible’. tallow or pitch. Fuel Tflash Methane −188 ºC 85 K n-Butane −63 ºC 210 K Iso-Octane −12 ºC 261 K n-Heptane −4 ºC 269 K Methanol 12 ºC 285 K Ethanol 12 ºC 285 K n-Octane 13 ºC 286 K n-Decane 44 ºC 317 K Diesel 50 ºC 323 K Kerosene 57 ºC 330 K n-Dodecane 71 ºC 344 K Lubrication oil 200 ºC 500 K Liquids (and solids) with flash points below 40 ºC (or 50 ºC. requiring extensive safety measures for its handling and transport. the flash point for diesel. alcohol/water mixtures) show higher flash points. Sprays of pure fuel show lower flash points. Table 1. its humidity. the composition of the oxidising atmosphere. the choice is arbitrary and depends on legislation) are labelled ‘flammable’. A pool of light fuel oil with a flash point above 60 °C cannot be ignited with a match. and of candles (where the fuel is in the solid state. but sprays of diluted fuels (e. Once ignited. can be found aside. usually becoming unstable. Crude-oil is easy to ignite because of its many volatiles. acetone (−18 ºC). Many common organic-liquid solvents have a flash point below room temperature. The state of the fuel sample (particularly for solid fuels). and even extinguishing by poor ventilation (lack of air).The flash point is the lowest temperature at which a liquid has a sufficient vapour pressure to ignite when a small pilot flame is brought close to the surface of the liquid. Flame stabilisation by porous feeding: oil lamps and the candle flame Condensed fuels can be ignited by a spark if they are above their flash-point temperature (at least locally). in History of Fuels. benzene (−11 ºC). posing a great fire hazard: e. oil lamps and candles). isopropanol and ethanol have flash points below the threshold of 38 °C at concentrations as low as 30% by weight in water. if not. Table 1 shows some flash point data (more can be found aside). what is the foundation of oil lamps and candles. e. diethyl ether (−45 °C).g. Wood shaving ignites more readily than a massive piece of wood. Flash-point temperature for liquid fuels in air at 100 kPa.g. Some of them will maintain their flammability risk even at concentrations as low as 10% by weight in water. they have to be heated before ignition starts. but crude-oil spills at sea cannot be ignited because volatiles swiftly go away and heavies cannot be heated enough because of the large thermal sink of underlying water (oil spills have to be cleaned mechanically and with chemical solvents). and set alight by an igniter). or finely dividing the fuel (sprays and powders). a torch being a hybrid device between a candle and an oil lamp (a torch is a wooden or tow shaft dipped in wax. liquid fuels are used in the majority of vehicle-engines (cars. trucks. The structure of a typical candle flame is depicted in Fig.45 mm in still air.120 W (LHVwax=44 MJ/kg). until the development of electric light at the end of the 19th century.. 600 K). when flickering occurs in turbulent flames. .. so that. and.. soot production quickly rise. These flames radiate a lot of energy. Fig. ships. yielding some 60. 3. the exception is when the injected liquid can be left to fully evaporate and a subsequent premixed flame produced. as in gasoline engines.. and it is enhanced by increasing the residence time within the flame and by the presence of polycyclic aromatic hydrocarbons (PAH). the small visible part more than justified their prominent use for artificial lighting since prehistoric times.10 mm. Fuel sprays Because of the advantage of easy storage and high energy intensity. although most of this low-temperature blackbody-radiation is in the infrared spectrum. and it is of utmost importance for a good burning to promote the interface area between liquid and gas. what is responsible for the typical yellow colour of non-premixed flames. 3. nearly a perfect blackbody at 5800 K). then the vapours heat up to the flash point (around 230 ºC. The wax can be of animal origin (esters of fatty acids. Soot formation is a relatively slow chemical process of nucleation and aggregation. with a flame height of 40. i.e. and aircraft). the wick holds some solid wax in its pores (from previous use or from manufacturing). When a match is brought nearby. Tm=36.4 ºC). in most cases. ignition starts.10 g/h of fuel. a 20 mm in diameter candle burns some 5. A volume fraction of soot less than 1 ppm already gives luminous blackbody emission.The length of this type of flames depends to some extent on the length of the wick. like stearin and beeswax) or of mineral origin (long-chain paraffins. beyond eicosane. and a quasi-steady-state regime is reached. where. and a maximum flame diameter of some 8. what can be controlled in oil lamps (too long wicks will burn themselves if unable to pump enough liquid). first the wax melts (between 50 ºC and 60 ºC). C20H42. trains. Mixing processes and structure of a candle flame. A typical candle flame has a colour temperature of some 1700 K. The combustion is performed by injection of the liquid fuel in an oxidiser gas stream (usually air). a non-premixed flame will be established. Initially. and in a sizeable part of stationary engines and power plants. This is similar to the incandescent radiation from a filament electrically heated to near 3000 K as in the traditional incandescent bulb-lamp (white light corresponds to the solar spectrum. Wall impingement is undesirable in most circumstances. 4. In the first wind-induced regime. but still being much larger than the . what requires very high pressure jumps. forms a spray that breaks down to a myriad of droplets.The scattering of the injected liquid into fine droplets (spray. are less used in combustion. is usually achieved mechanically by creating a high-speed jet (>100 m/s. The break-up length. drops larger than the jet diameter are cast by capillary instability. 5). and given liquid (of density ρ. wetted surfaces introduce an undesirable time-lag in the fuel control loop. and the break-up length grows proportionally to the speed (A to D). air-entrained or water-entrained jets. Think about how fast the processes follow each other: in a few milliseconds. For a given injector diameter D. a thin liquid stream in turbulent motion (which may cavitate and flash-boil when approaching the tip of the fuel injector). 5. Liquid-jet break-up regimes in terms of Reynolds number Re≡ρuD/µ. see Fig. and generate unwanted emissions. viscosity µ. or atomisation. the length where the first droplets appear (at the jet surface). ignite. Other methods of spraying. which disintegrate into droplets (much smaller than the jet diameter). droplet formation and evaporation in a diesel injector. and surface tension σ). and the break-up length decreases too /E to F). tends to zero in atomised jets (Fig. 4). In the Rayleigh regime. the only variable is injection speed u. burn. increasing emissions by poor combustion near cold walls. even in gasoline injection at the manifoild. Jet break-up. i. Fig. and Ohnesorge number Oh≡µ/(σρD)1/2. droplet size decreases to the range of jet diameter. Fig. currently up to 200 MPa in some diesel injectors). by shear forces. which evaporate.e. as impinging jets. droplet size further decreases. a cloud of fuel vapour. which supplies fuel vapours . and ignition delay time.diameter. heat-up period. is ignited by a injection of a small amount of diesel oil at the usual high pressure near the top dead centre. Due to the fine atomisation achieved by the high pressure injector.e. F to H). evaporation rate. we assume it to be spherically-symmetric (a good approximation under microgravity. with a very effective atomisation. and break-up length tends to zero (F to G. what governs the path length before burnout (and consequently the minimum size of the combustion chamber required). In the second wind-induced regime. a lean mixture of air / natural gas is usually formed in the admission manifold. Typically. a vertical plume develops. the droplet temperature keeps a constant value T0 (a heat-and-mass diffusion trade-off value to be found. by the way. but it is of paramount importance for the understanding of spray combustion. and soot formation continues until there is liquid remaining. Additionally. Pressure. The key parameter to be found is the droplet burning time. sketched in Fig. To further simplify the single-droplet problem. including flame dynamics. but up to 800 K are measured before proper ignition). combustion chemistry. Presently. it is only of interest in gasoline injectors in the intake manifold. pyrolyses and burns. if the injection enthalpy is higher than the saturated-liquid enthalpy at the discharge pressure). develops around the liquid jet already after a few millimetres from the injector. 6. Steady non-premixed flames occur in liquid-fired gas turbines. species diffusion. on Earth. can give rise to flash-boiling (i. because. Droplet combustion A single droplet of fuel burning in air. or by autoignition in hot air (above autoignition temperature). and reaction activation) had been settled. but later decreases in size. soot starts to form at the tip of the jet due to thermal pyrolysis of air-depleted vapours. boilers and furnaces. following the shrinking of the droplet (but not linearly proportional as deduced below with a simple model). Notice that LPG injection. lower than its boiling point). the flame increases in size initially by thermal expansion and internal convection. has an influence on several aspects of the droplet-burning process. and kerosene gas-turbines. helped also by the larger autoignition delay of methane than that of diesel oil. In the dual-fuel diesel engine. a jet of liquid fuel is injected (at some 350 K once warmed by the hot engine). whereas unsteady non-premixed flames occur in diesel engines. As for droplet evaporation. with heat from the flame diffusing inwards and forcing evaporation. the temperature increase during compression of the mixture is high enough to auto-ignite the oil (480 K for diesel/air) but not enough to auto-ignite the gaseous mixture (850 K for methane/air). assumed constant during droplet combustion. and the required residence time for unsteady processes (and consequently the minimum period in reciprocating engines). droplet combustion experiments show that. inside hot air (heated to some 950 K by adiabatic compression). and. we can imagine a liquid source at the centre of the drop to keep a strict steady state. either by a spark in cold air (above the flash-point temperature). at the surface. and heated gasoline and diesel injection. attaining more than 2500 K and showing chemiluminiscent radiation. at a distance of a few centimetres. this rich vapour mixture already attains its autoignition temperature (the minimum for diesel/air is 480 K. after ignition. beyond that. after the initial processes of ignition (heating. a very practical arrangement used in all liquid-fuel burners. there is a longer liquid spike at the core that grows with speed. well-mixed with air. once compressed to the normal compression ratio of the diesel engine. is not of any practical use (except for research). The atomization regime stands beyond this zero-break-up-length point (beyond G). We here assume that the combustion has already been started. In this steady-rate regime. we focus just on the quasisteady rate of burning. For instance. microgravity also allows easy droplet positioning by levitation). all diesel engines. from the start. and part of the heat released. to the sink at the flame. with both fuel and oxidiser concentrations approaching zero at the flame (with slopes proportional to the stoichiometry). of algebraic amount −m O related to the fuel flow-rate by the stoichiometry at the flame: m O M O =ν O m F M F . diffusing outwards and feeding a non-premixed flame. Fig. a) Temperature and species-concentration profiles (combustion in air). b) Evolution of drop radius. fed also from the outside by a diffusive flow of oxygen from the air. We start the analysis of droplet combustion by rewriting the equations for droplet evaporation (9-11). for a spherical volume of radius r (now it must be r0<r<r1). we must choose a sphere with r>r1 (there is no oxidiser in r<r1). there is no sink for yP or yN at r0): Magnitude Accumulation Production Diffusive flux Mass balance (global) 0 = m F +0 Mass balance of fuel (volatile species) 0 = m F + ADF ρ Energy balance 0 = −m F hLV0 + Ak Convective flux dy F dr dT dr − m ( r ) (20) − m ( r ) yF (21) − m ( r ) c pF (T − T0 ) (22) These equations must be now supplemented with the mass diffusion balances for the other species (all minus one. For the oxidiser. with time.with a mass fraction yF0 close to the liquid. Droplet combustion. To simplify nomenclature. The choice of system for analysis is always a control volume of radius r in strict steady state (with appropriate sources and sinks at discontinuities). and volume. with yF+yP+yN=1 and only the fuel flowing (from the source at the origin. the global mass balance. located at r1/r0≈10. we make use of the stoichiometric relation. F+νOO→νPP. notice that the stoichiometric coefficients νi are in molar basis (that is why the molar masses appear in the mass-equations). A possible inert gas. 6. The oxidiser has a source at r→∞ and a sink at the flame (at r=r1). i. area. if we make use of the global balance).. and energy balance.e. diffuse outwards. notably nitrogen foe combustion in air. Products being generated at the flame (we assume the single-step reaction model F+νOO→νPP). species mass balance. Magnitude Accumulation Mass balance of oxidiser (r>r1) 0 Mass balance of products (r>r1) 0 Mass balance of products (r<r1) 0 Production Diffusive flux Convective flux M dy = −m Fν O O + ADO ρ O − m ( r ) yO (23) MF dr M dy = m Fν P P + ADP ρ P − m ( r ) yP (24) MF dr = 0 + ADP ρ dyP dr − m ( r ) yP (25) . completes the set of species playing (one must have at every point yF+yO+yP+yN=1). The overall mass balance (20) is still valid for any r. v0. but total mass is conserved. equation (22). yields: DF dyF r 2 DF dyF 1+ − yF ⇒ 0 = − yF v dr v0 r02 dr DF 1 1 ⇒ ln (1 − yF. the global mass-flow-rate of gases across any spherical surface. Thus. equation (21). since there are sources and sinks of species at the flame front.∞ MF 1 = M r1 νO O MF DO dyO 1 = −d ⇒ 2 M v0 r0 ν r O + yO O MF (30) The energy balance in r0<r<r1. and the bulk radial velocity v. assuming constant gas density ρ (no as good an approximation as for the droplet evaporation case studied before. m ( r ) . relates to the radius recession rate in the real process through the mass conservation at the liquid surface: ρliq dr0  dr  ρ vA= ρ v0 A0= ρliq  − 0  A0  → v0= − ρ dt  dt  (28) The species balance of fuel in r0<r<r1. is independent of r. divided by m F = ρ vA and upon integration. equation (23). yields: M D dy 0= −ν O O + O O − yO MF v dr ⇒ DO ln v0 r02 νO M O r 2 DO dyO ⇒ 0= −ν O + − yO M F v0 r02 dr ⇒ MO + yO. temperature varies a lot in droplet combustion). on either side of the flame. divided by m F = ρ vA and upon integration yields: −hLV0 0= ln k dT + − c p (T − T0 ) ⇒ ρ v dr hLV0 + c p (T1 − T0 ) hLV0 hLV0 ρ v0 r02 ρ v0 r02 1 ρv dT = − − dr = dr = d ⇒ + c p (T − T0 ) k kr 2 k r dr c p ρliq 0 r0 c p ρ v0 r02  1 1  c p ρ v0 r0 dt = = −  −  k r r k k  0 1 ⇒ . although pressure is also assumed constant. yields: v0 r02 = m ρ= vA constant = m F  → v= (r ) 2 r (27) The convective speed in the gas phase close to the drop.Mass balance of nitrogen (any r) 0 = 0 + ADO ρ dyO dr − m ( r ) yO (26) the latter being redundant since at any stage we have yF+yO+yP+yN=1. divided by m F = ρ vA and upon integration. since.0 ) = − 2 v0 r0 r0 r1 0= 1+ ⇒ DF dyF 1 d = ⇒ 2 v0 r0 1 − yF r (29) The species balance of oxidiser in r1<r<∞. ini K (31) i. however. T0. cp. including inerts. p=100 kPa. pressure and oxygen concentration (e.575/0. Notice that this constant K for the burning problem will differ in value from that of the simple evaporation problem.i their molar thermal capacities. tburn. ambient air temperature. the species balances can be used to get the actual droplet temperature and other internal unknowns. νP=12+13=25.6N2 + 7575 kJ/mol and consequently: . is justified by practical results (r1/r0≥10).5O2 + 69. the values cp=34 J/(mol·K)=1200 J/(kg·K) may be used for combustion in air (care is needed. T⊕=298 K is the standard reference in Thermochemistry. Tb=489 K. getting the well-known r2-law (similar to Langmuir’s law (1918) for droplet evaporation). and Tad=2600 K). and MO=0. which is the adiabatic flame temperature.170=44. ni are the molar coefficients of the products (including any possible inert species). for n-dodecane. hLVb=257 kJ/kg. because the influence on (31) is not large). usually that of air at some appropriate conditions intermediate between the extreme temperatures.g. One can make the approximation of considering just one thermal-capacity value for the products mixture.6N2= 12CO2 +13H2O + 69. d 2 (t= ) d 02 − 4 Kt ). a pure-substance model for diesel oil (a commercial distillate). λ=0. but the droplet life-time is directly given by the integrated energy equation (28) because we know the flame temperature T1. relative to the fuel. is used instead of radius (with our K. and products-mass-fractions.⇒  c (T − T )  dr02 2k ln 1 + p 1 0  ≡ − K =−  hLV0 dt ρliq c p   ⇒ tburn = 2 r0. data are: initial droplet radius (e. The actual mixture used in the case of burning in air is: Mixture used: C12H26 + 18. The simplification introduced along (31). As a rule of thumb.6 MJ/kg.23). The first derivation of this burning time was done by Spalding and Godsave in 1953. to account for the mol-ratio or the mass-ratio of products. afterwards. νO=37/2=18.170 kg/mol.e. or any other suitable value. and cp. is proportional to the square of the initial radius. r0.5. cp=1000 J/(kg·K). the latter obtained from the thermochemical analysis: Stoichiometry: C12H26 + (37/2)O2 = 12CO2 +13H2O + 7575 kJ/mol the latter obtained from LHVmolar=−Σνihf.52)−13(−241. for a given fuel droplet in a given environment. d. Tad. T∞=300 K. Later. stating that the droplet life-time. as fuel-mass-fraction near the liquid surface. hLV0.ini=10-5 m).82)+(−291)]=7575 kJ/mol and thence LHVmass=7. properties of fuel (e. ρliq=780 kg/m3. the energy balance gives directly the burning rate. In summary.g. the flame position r1 will be found from (27). and its corresponding vaporisation enthalpy. an order-of-magnitude value for the adiabatic temperature of Tad=2500 K may be used for a first approximation. MF=0.i=−[−12(−393.029 kg/mol). are approximated by the boiling-point values (Tb and hLVb. If the steady-state temperature of the drop. the diameter. 1/r1<<1/r0.024 W/(m·K). thermal properties of ambient air (for the given state. LHVmass=45 MJ/kg. (we neglect radiation losses) given by: = T1 T= T⊕ + ad LHVmolar prod ∑ν c (32) i pi where LHVmolar stands for the lower heating value of the fuel in molar basis.g. often. yO∞=0. 3 10−9 kg/s = dt The flame radius.ini = 2π r0.170   M F   i. Associated to the burning rate in terms of radius (31) is the burning rate in terms of fuel mass: dr0 2π r0 K = 2π K r0.032   ν MO    O   2 0.024  1200 ( 2650 − 489 )  −6 m ln 1 + =  =0.∞ DO MF 1 = ln 2 M v0 r0 r1 νO O MF  c (T − T )   1200 ( 2650 − 489 )  ln 1 + p 1 0  ln 1 +    hLV0 257 ⋅103 ln (1 + 10 ) r1     = 37 = = = r0 ln (1 + 0. The mass fraction of fuel vapours close to the drop.12 ⋅10−6 =⋅ 2.6) ⋅ 34 ∑ν icp i or.028 m prod ⋅1200 cp 0.e.12 ⋅10−6 −5 2 ⇒ tburn i. we get: . r1.6 ⋅103 298 + 2630 K = = 0.5 ⋅ 0.2ini − Kt ⇒ m F . r0) can be obtained from the oxidiser mass balance (23): m F = −4π r02  c p (T1 − T0 )  2 k r d 1 ln 1 +  0 ≈ D ρliq dr0 ρcp O v0 = − h r1 ρliq LV 0   ρ dt 2 dt  → = = ρ r0         y y DO ln 1 + O.23  y ln 1 + ln 1 + O. the flame stabilises itself at a distance of some 37 radii from the drop (of course.032 + 69.∞  15 0. with an uncertainty proportional to the simplifications assumed).∞   ν MO   ν MO  O O    MF  M F    the latter obtained by substituting the burning-rate result (31) and introducing the approximation of equal thermal and solutal diffusivities in the gas phase (Schmidt number unity).∞  ln 1 + O. can be computed from the fuel mass balance (21).170 m fuel From the energy balance (31) we get the burning time: r2 tburn =0. Proceeding as before.ini . in mass terms: T1 = Tad = T⊕ + LHVmass 44.ini (10 ) =0. (relative to the drop radius.e.170 + 18.12 ⋅10 780 ⋅1200  257 ⋅103 s  2 r0. a r0.066 )        0..83 ⋅10−3 s = = K 0. With our example data: M ν O O + yO.003 ⋅ 0. with K  c (T − T )   c (T − T )  2k 2k ln 1 + p 1 0   ln 1 + p 1 b  =  ρliq c p ρliq c p  hLV0 hLVb    K= 2 2 ⋅ 0.LHV 7575 ⋅103 T1 = Tad = T ⊕ + prod molar = 298 + = 2650 K (12 + 13 + 69.8 ms (typical uncertainties are ±20%).6 ⋅ 0.ini K = 2π 0.ini=10 µm n-dodecane droplet burns in 0. 0.0  yP. since there is neither fuel nor oxidiser at the flame radius (see BurkeSchuman's model.77}.03. As for the composition profiles along r.1/yP.0 ) = − liq 2 dt ρ DF ρliq dr0 v0 = − ρ dt c p (T1 − T0 )  1 ⇒ =  hLV0  yF.1=0.1=0.0.∞→0.65. what yields exponential profiles for all the variations.yP. and.170  ln   0.1 =0.0. yN. at the flame. but up to a generic r. what yields: MP M D dy D MF 1 = 0 ν P P + P P − yP  → P2 ln = M F v dr v0 r0 ν M P − y r1 P P.0   c (T − T )  ln 1 + p 1 0    hLV0 r1   ⇒  → = r0 y  ln  P.06}.0.35. The mass fraction of products.e.0}={0.91.65}.0 r1  1200 ( 2650 − 489 )  ln 1 +  257 ⋅103   =37 ⇒  0. the rest must be inert component.0. but now with yP.1 MF νP ⇒  1200 ( 2650 − 489 )  ln 1 +  257 ⋅103   =37 ⇒ 0. what yields the relation yN.1   PM  F  yP. above)..yO. equations (29-31) should not be integrated between the limits. the composition is {yF.35  ln    yP.1 = v dr v0 r0 yP.DF 1 ln (1 − yF. In the inner region the equation is (25).0. and at r→∞ {yF.1/yN.23.0 ) = 2 v0 r0 r0  (1 − y ) 1 + F.e. as sketched in Fig.0. The last development could have been applied to the inert component.35.0.yO.030    25 0.0.0.0.0. r=r0.0. in the outer region can be computed from the products mass balance for r>r1 (24).35 i. at the flame.yP.yP.e.0. there is some 91% in mass of fuel vapours close to the drop (the uncertainty may be large).0 = 1+ 1 hLV0 c p (T1 − T0 ) = 1+ k ρcp ≈ DO =  c (T1 − T0 )  − ln 1 + p  ⇒   h LV 0   1 1 = = 0.170   c (T − T )  ln 1 + p 1 0    hLV0 r1   ⇒  →= M r0   P  νP M  F  ln  ν M P − y  P..0}={0.yN.03 i. r=r1.0. what yields: 0 0+ = ⇒ y DP dyO D 1 − yP  → P2 ln P.91 3 1 257 ⋅10 1+ 10 1200 ( 2650 − 489 ) i.1   0. yP.0.yO. 6.030  25 − yP.0  1 dr02 ρ  → ln (1 − yF. the composition is {yF. similarly to that of oxidiser.yN. close to the drop.0=yP. .0. yP.0 =0.1   yP.0.0.0}={0.yN. etc. catalysts.e. with yF=1 and yO=0 in the solid phase. with a radial profiles as sketched in Fig. • Inert solid material (ash) may be a sizeable part of solid fuel (e. • For 1500 K<T0<2500 K. Fine solid particles. The surface temperature in carbon combustion depends on how quick heterogeneous reactions proceed relative to the species-diffusion rates. trace contaminants.g. according to C+(1/2)O2→CO. • Contrary to liquids. driven by a gas-phase homogeneous combustion with a blue free flame at some distance from the surface. a heterogeneous endothermic reaction sets at the surface. and a homogeneous detached flame as in droplet burning. Leaving aside coal composition (see Coal analysis. evaporation is so effective (carbon sublimates at 3910 K at 100 kPa) that a detached flame develops as in droplet combustion. but now there are discontinuities in the species concentration at the front. a heterogeneous combustion process is established at the surface according to C+O2→CO2. the combustion products are usually solid oxides (even at the high-temperatures involved). because there are no surface tension effects as in droplets. fuel and oxidiser flow-rates still must verify ρO vO M O +ν O ρ F vF M F = 0 at the reaction front. • For T0>3500 K. and yF=0 and yO>0 in the gas phase. The nature of coal combustion depends a lot on composition and temperature. • Under some circumstances.Particle combustion We consider here the combustion of solid particles. 1500 K<T0<2500 K. when the surface temperatures (after ignition) does not go over 1500 K. two combustion fronts may develop: the heterogeneous one at the particle surface mentioned above. once the heterogeneous reaction C+O2→CO2 is established. with a radial profiles as sketched in Fig. whereas for higher temperatures. however. are usually the result of a milling process that tends to produce not too-elongated particles. • For 2500 K<T0<3500 K. . the combustion of solids can take place at the interface (heterogeneous combustion). i. The main differences to the latter are: • Particles shape can depart a lot from the spherical shape. a surface temperature is in the range 1000 K<T0<1500 K is obtained. according to C+CO2→2CO. they vaporise before. • Solid particles are far more difficult to ignite than droplets (they have higher flash points). This is because liquids cannot get hot enough to sustain a rapid surface-oxidation process with the adsorbed oxidiser. the external free-flame disappears. 7a. Ignition and combustion of pulverized coal particles is of major interest to modern coal-fuelled power plants. leaving a porous solid layer around the burning fuel core. the oxidiser concentration decreasing towards zero as temperature increase. what depends on geometry. 7c. although oxygen dissociation becomes important too. But solids can indeed get very hot and yield a sizeable rate of chemical recombination at their surface layer. a detached flame appears. the effect of temperature on pure-carbon combustion is as follows (temperature range is very sensitive to humidity): • For T0<1500 K. For a reaction scheme   F+νOO→νPP. in Fuel properties). leaving just the heterogeneous combustion at the surface according to C+(1/2)O2→CO. Combustion of metal powders and polymeric dust in air is a major safety concern in many powder industries (e. saw-dust in wood processing). In the simplest case of diffusion-limited combustion at low ambient temperatures. flour in food processing. according to CO+(1/2)O2→CO2. significantly distorting mass and heat transfer. liquid particles being considered above under Droplet combustion. in some coals).g. • In the case of metallic particles. 0 MF 0 − ln  = −K = −  ρ sol ρ sol ν M O ν M P − y   dt P P. Exercise 1. c) Radial temperature and species concentration profiles when a detached flame appears (1500 K<T0<2500 K). what is plotted in Fig. not only in most gas-fired burners (from cooking ranges to industrial furnaces) but also in spark-ignition engines and some gas turbines. with surface temperature. is CO).g. one gets (yO. Bunsen conceived his famous burner in 1855. b) Correlation of species concentration close to the surface. with HV=33 MJ/kg for C+O2→CO2. but it was only very recently. The species and energy balances for the case without free flame.∞   dr 2  2 ρ DO 2 ρ DO yO. Partial oxidation of carbon particles at high temperature PROPAGATION OF PREMIXED FLAMES People learnt to make non-premixed flames on torches some 500 000 years ago. after R.∞ − yO. 7.g.0 MF  0 = − ln  −K = − ρ sol ρ sol ν M O  dt ν MO + y  O.W.0/(44/12)=1200(T0−T∞)/(33·106). with T in kelvin.0  O  MF   MF    HV + c p (T∞ − T ⊕ )   dr02 2k 2k c p (T∞ − T0 ) − ln  = −K = −  ⊕ c p ρ sol  HV + c p (T0 − T )  c p ρ sol HV  dt     (33) what relates all other variables with the surface temperature attained.0)/(32/12)=yP.0  O  OM  MF F    M    νP P   dr 2  2 ρ DP 2 ρ DP yP. because premixed combustion is more energy-intense and less pollutant than nonpremixed flames. . e. assuming DO=DP=k/(ρcp). Combustion of a carbon particle: a) Radial temperature and species concentration profiles when 1000 K<T0<1500 K (the only product. closely following the analysis detailed above for droplet burning. for 1000 K<T0<1500 K. 7b. that premixed flames are used.Fig. Premixed flames may be laminar (e. P. and cp=1200 J/(kg/K).∞−yO. yield:   MO  νO + yO. lab bunsen burner) or turbulent (industrial burners).. g. DISI engines (first developed by Mitsubishi in 1995). a 10% in volume of methane in air. even when the fuel is already a gas. because there is a lot of time from mixture preparation outside the cylinder to ignition after compression. The quickest way is by letting a fine dispersion of liquid fuel (obtained by high-pressure injection) to evaporate and mix with air (as in gasoline engines). rich premixed methane/air flames (e. is perfect. easily flammable). otherwise. The main feature of premixed flames is their characteristic propagation speed. being ignited by a spark). a huge volume of gas is required to make it optically thick (several metres). although the gases are at similar temperature in both types of flames. whereas lean premixed methane/air flames are green due to C2 radicals. equivalent to the steady-state in the adiabatic motion of a planar heatsource. quiescent flames are the easiest to analyse. This light emission is not due to blackbody radiation because. just a thermal model. and alcohol and hydrogen flames are purple and tenuous. one of the weakness of Otto engines). also named GDI for gasoline direct injection). For mixing to occur in any reasonable engineering-time-span. For instance. The strong temperature gradient is one of the main characteristic of a flame. but there are other attributes that distinguish them from non-premixed flames. none the least their colour: contrary to the yellow colour of non-premixed flames (due to blackbody emission by soot particles). for the new port-fuel-injection models and the old carburettor models). However. the most instructive to study. assuming as a first approximation a planar one-dimensional propagation. only admit air to the cylinders (without a throttling valve). Although the predominant colour in premixed flames is blue. it is easier to consider a flame propagating inside a long tube filled with a premixture (e.Full mixing of a fuel in air takes a lot of time. LAMINAR COMBUSTION Perfectly premixed. as in a cooking range. 8). Mallard and Le Châtelier proposed in 1883 a model for the propagation of a flame in quiescent premixed gases (laminar deflagration). only some characteristic spectral bands show up over the tenuous bulk emission. i. gaseous jets are more expensive to produce (require more power for pressurisation) and are less ready to disperse (recall the long bunsen tube). only rich near the spark-plug (i. The additional non-premixed flame surrounding the rich premixed methane/air flame is also blue (and not yellow. and a prerequisite to understand any combustion phenomena. assuming that the heat-release zone was much smaller than the temperature-change zone (Fig. at ambient temperature and pressure. as in normal non-premixed flames) because now there is little fuel remaining and consequently soot formation is hindered. propane and LPG flames are greener and brighter. those corresponding to the species present. it must always be made by turbulent convection. when some primary air is used) are blue/green due to CH and C2 radicals. the mixing is not so good in the new direct injection spark ignition engines (DISI. premixed flames show a green-bluish coloration and much lower luminous intensities.g. as for methane and natural gas. which causes the burning of the overall lean mixture (increasing a lot the efficiency on partial loads. Although the most common laminar premixed flame is obtained at the tip of the bunsen burner. The degree of mixing in traditional Otto engines (both. with the injector (placed near the spar-plug) controlled in such a way that as to have a stratified mixture. which work with a stratified mixture. and inject fuel within the cylinder during the compression stroke. .e.e. since there may be no time enough from injection to ignition. 5 m/s. for the laminar deflagration speed VL=0. is related to the deflagration speed by just Lth=a/VL. Besides.e. for burning velocity. for deflagration velocity. what agrees very well with typical values of Lth=10-3 m and VL=0. for laminar speed. premixed. fresh gases (i. but at 1500 K a=0. etc. A more refined analysis yields LthVL=2a. A further analysis of the laminar deflagration speed VL follows. ρVL dx dx dx ∂x Tb − T f dt  a  (34) where for the fresh mixture entering from the left at Tf and the burnt products exiting to the right at Tb. ρ=1 kg/m3. for the thermal capacity cp=1000 J(kg∙K). Deflagration speed The laminar deflagration velocity VL is the flame-front speed relative to the quiescent. but VL seems preferable because it reminds of the laminar character (practical combustion is . and its equivalent thermal model. for flame velocity. some 100 W per square centimetre of flame). or VD. from the exponential argument in (34). this is valid for −∞<x≤0 (it is T=Tb for x≥0). for a mean temperature of 1500 K (a=2⋅10-5 m2/s at room temperature. having substituted typical values for density of fresh gases. = k 2 . VL. The most important feature of this thermal conduction model is obtained by a mere order of magnitude analysis that shows that the thermal thickness. or Vf. VL = exp  L  ρ = −∇ ⋅ q .e. Lth.35⋅10-3 m2/s.4 m/s. i. and for the temperature increase Tb−Tf=2000 K.. The heat release per unit area is = Q / A ρ f VL c p (Tb − T f ) ≈ 1 MW/m 2 (i.Fig. or Vb. per unit area).35⋅10-3 m2/s) one gets LthVL=0. In axes moving with the front.e. the energy balance (heat equation) for a generic unit volume yields: T − Tf dh ∂ 2T dT d 2T dh   xV  =a 2 . it is sometimes named SL. the volumetric rate of fresh mixture being burnt. Internal structure of a premixed laminar flame. 8.. 3 m/s for C4H10/O2. 9.e.65 and at λ=1. VL=0. It depends on oxidiser composition (air. must be proportional to the overall concentration of fresh gases. For air. For premixed methane/air mixtures at ambient conditions.5 m/s for most hydrocarbons: VL=0.g. upon substitution of LthVb=a in ξ /V =cf/tth and elimination yields: VL = a ξ = cf V a M m  −dcF  ρ f  dt  (35) . from VL=0.10% of its maximum value. pf. not to zero). • Effect of fuel. The laminar deflagration velocity VL varies with type of fuel. and pressure. but VL=0. λ=A/A0. CO. Fig. for some fuels.08 has been found under microgravity conditions).7∙10-6 (m/s)/K2 L and p0=100 kPa. VL=7. C6H6. O2.9 m/s for CH4/O2. c2=3. Tf (e.5% butane.. VL decreases from VL=0. VL=3. VL increases one order of magnitude from xO2=0. Effect of pressure.21 to xO2=1 (e. VL increases exponentially with fresh gases temperature. VL varies with the dilutant: with Ar and He it is double than with N2. ξ /V .15 m/s for H2/air flames when adding 3.15 m/s at λ=0. • Effect of oxidiser. relative air ratio in the mixture.2 m/s at 600 K). with c1=0. and inversely-proportional to the time taken to burn the gases inside the flame thickness. φ=1/λ. Other striking case is the reduction of VL from 2. to tth=Lth/VL.5 m/s for C2H2/O2 and VL=14 m/s for H2/O2). Also.2% H2O causes an eight-fold increase of VL. VL=1. • Effect of mixture ratio. and decreases parabolically towards the ignition limits (to some 30%.7. to VL=0. Laminar deflagration speed vs. type of oxidiser.6 m/s for C3H8/O2. although adding an inert gas lowers the deflagration speed. as in the striking case of CO/O2 combustion.5 m/s for H2. at room conditions.75 m/s for C2H4. equivalence ratio. the observable speed of a flame in a tube only coincides with VL for the open-exit / closed-intake tube).45 m/s at 300 K to VL=1.g. Tf. i. so that. for a given xO2. in some special cases it may act as a positive catalyst and increase it.036 m/s at λ=2. VL has a maximum value of near 0.g. temperature. VL=3. where a 0. H2O2). C3H8. Experiments show that VL has a maximum value for λ slightly over λ=1. Fig. Some prediction of all the above-described VL-dependence may be obtained following the thermal model above.45 m/s for CH4.1 m/s. 9 presents additional data.7 m/s to 0. cf=p/RuTf=ρfMm. An empirical correlation from Andrews and Bradley for stoichiometric CH4/air flames is V= ( c1 + c2T 2 ) p p0 .45 m/s at λ=1. VL=3.05. However.85 m/s for CH3-CH2OH. VL decreases with increasing pressure to a half from 100 kPa to 1 MPa (it increases 50% from 100 kPa to 10 kPa). below). a minimum value of VL=0.6 m/s for C2H2 and VL=3. Notice that pure oxygen may give rise to a detonation (see Supersonic combustion.turbulent) and the apparent flame speed may vary with the configuration (e. The burning rate density. • • Effect of temperature. g. usually grouped in stationary and moving flames. a flame stabilised over a porous burner is better than the travelling flame along a tube.g.e. ρfVL=ρbvb. in either premixed or non-premixed flames.45 m/s in free-flames to 4 m/s in porous-media flames. It is important to keep in mind that the products leave the flame at a quicker pace than the deflagration speed. vb/VL=(pf/pb)(Mf/Mb)(Tb/Tf). For planar geometries.4 m/s flame speed towards the fresh gases. where the last factor in the right-hand-side product is the dominant (of order 7 or 8. and a non-symmetric parabolic flame in horizontal propagation. But the effect of the chemical kinetics is difficult to guess beyond Arrhenius’s law and the law of mass action.6 for methane in air). since the effect of gravity on the later. or some chemical thickness.5 m/s to the other side. and the threshold levels). apply also here: the uncertainty in measuring the thickness of a solid sheet is well understood. gives a ±20% approximation to the deflagration speed). νO=15. . what about for the thickness of a gaseous layer as in a flame? One possible answer may be its thermal thickness (once we agree on a certain norm. ρf the unburnt mixture density. or a given monochromatic band. The visible thickness is always smaller than the thermal thickness (say one half). due to the effect of temperature on mass conservation. 8). central ignition inside a soap bubble filled with a premixed fuel/air mixture may be used. propagation speed greatly increases (parabolically) with Reynolds number in the turbulent regime.or with a more refined analysis: = VL a (1 +ν O ) M F  −dcF  ρ f  dt  (36) where Mm and MF are the molar masses of the mixture and the fuel. and the visible layer stays at the downstream-end of the thermal layer (see Fig. usually less than a millimetre.g. i. what is profitably used in practical systems (typical turbulent speeds are some 5 times larger than the laminar speed).g. maximum deflagration speed increases from 0. Thus. e. the other two being of order 1). e. but. divided by the flame area. the products are ejected at some 2. for methane/air. in the planar geometry. Some comments made when analysing non-premixed flame lengths. the mean distance between points with temperature say 1% greater than the entering fresh mixture and the points with temperature say 1% lower than the exiting products. from a temperature profile as in Fig. times the cross-section area. Flames inside porous media have higher values. the typical bunsen flame may be used. For cylindrical geometries. The laminar deflagration speeds here consider refer to open flames. and better if a planar velocity profile for the issuing jet is achieved by a contraction (in any case. using a certain photographic plate. there where radicals recombine. νO is the stoichiometric mass coefficient for the oxidiser (O2) relative to unit mass of fuel (e. For spherical geometries. the overall injection speed. It must be reminded that all this refers to the laminar speed. Flame thickness The flame front is very thin. There are many procedures to measure laminar deflagration speeds. and dcF/dt the speed of reaction for the fuel. 8). but 10 times in ρc). even for an elastic material. e. gives way to different speeds for upward and downward propagation. etc. But another possibility may be its light-intensity thickness (once we agree also on a certain norm. for a typical 0. explained because of the 10-fold increases in thermal diffusivity a (100 times in k. increasing the burning area and the speed. For instance. is related to the deflagration speed by just LthVL=a. given way to a vast range of speeds at their wakes. i. • Local hot spots (e. Fig. fuel and air are supplied the stability diagram is presented in terms of the equivalence ratio of the mixture supplied. except in a very short range of matching the propagation speed. some primary air is added to the fuel (more than the lower flammability limit and less than the stoichiometric ratio). This applies to nearstoichiometric deflagration speeds. that may grow to 1 cm at very low pressures (10 kPa). whereas CO. a premixed flame is established. causes a neat production and consequent emission. 10. if there is not enough time to equilibrium. the mixture (within its ignition limits) must be fed between twice and five times the laminar deflagration speed. similar to the main products (H2O and CO2). if a stable CH4/air premixed-flame is to be realised at the mouth of a bunsen burner. These partially-premixed combined flames are the usual feature in bunsen burners and domestic gas-stoves and gas-heaters. The trade-off between flame stability (greater for non-premixed flames) and flame energy intensity (greater for premixed flames) is usually solved by a sequential combination of a premixed flame and a non-premixed flame. followed by a less pronounced consumption rate profile. . Stability of bunsen flames in terms of primary air. Lth grows from 1 mm at λ=1. Lth. the thermal thickness. for premixed methane/air mixtures at ambient conditions. The main reactives (fuel and oxygen) show a simple consumption rate profile (say a Gauss bell in the dci/dt profile). ceramics with large thermal inertia) that permanently ignite the mixture. that naturally stabilise at the shear layer between fuel and oxidiser because they cannot propagate to one side or the other. that.As reasoned before. that show a simple production rate profile. giving rise to detached flows with a vast range of speeds. • Swirling by jet induction or bypass flows. To help develop appropriate kinetic models. premixed flames have their own propagation speed and. they tend to flashback or blowoff. 10. flame thickness grows parabolically (e. Several methods are employed to stabilise a partially or totally premixed flame a desired location in a combustor. Their stability diagram. against natural uncontrolled fluctuations and the intended operation range. When both. all methods trying to procure a region where natural flame speeds are matched: • Sudden flow expansion. to Lth=10 mm at λ=0. NOx and radicals show first a neat production rate profile.6 and at λ=2. and the remaining fuel is burnt in a second non-premixed flame with another supply of air (e.e. giving rise to a recirculation with a vast range of speeds • Bluff-bodies. Flame stabilisation Contrary to non-premixed flames.g. when fed with the fresh mixture.g. from the ambient). with typical values of 1 mm. it is very important to diagnose the species production rate profiles within the flame. outside that range. in terms of the primary air (that used in the premixed flame) is presented in Fig.g. For instance. for stoichiometric fuel/air mixtures at 100 kPa and 20 ºC. UFL (or rich flammability limit or upper explosion limit. are called flammability limits (also ignition or explosive limits). dQ=1.The easiest way to light a bunsen is with the primary-air openings closed (0% primary air. so that if the premixed jet is ignited at one side of the mesh. with corrugation (non-circular ducts have slightly lower values). the flame cannot traverse to the other side. fuel molar fraction in the gaseous mixture. excited by a spark of some 10 J. Lean mixtures are more unstable. infinite equivalence ratio of the feeding mixture) because this flame is most stable (it would detach only at an order of magnitude larger exit speeds).g. xstq=9. they tend to stay anchored to the burner under most variable circumstances. i. because a certain energy balance must be accomplished between the chemical energy release and heat transfer. 11). LFL (also known as lower explosive limit LEL. but if it is greater than xUFL. %vol.g.8 mm for CH4O but dQ=0. dQ=2. Some typical flammability limits are presented in Table 2. Those values can be converted to other units: e. Common units for LFL and UFL are molar units. and a better combustion can be achieved by increasing the primary air to 60% or 70% of the stoichiometric (i. as in combustion within porous media). what is the same. a subsequent escape of mixture will yield flammable mixtures with omnipresent air.5%. xUFL=14%} from Mass . or decreasing the equivalence ratio φ to 1. and decrease with pressure. Quenching distance is typically of 2 mm for normal hydrocarbon flames (e. dQ=2.9).g. with passage sizes below 3 mm to prevent flame ingestion. by a spark stimulation) if there is too-much air or too-much fuel. A fine wire mesh prevents flame propagation through.e. dQ=1. Flammability limits Non-premixed flames are very stable in the sense that. and.7.e. or upper ignition limit). Flame quenching Quenching is the extinction of a flame by an excessive heat-loss towards relatively-cold solid boundaries nearby. if more air than that corresponding to the lower flammability limit is supplied. if the fuel molar fraction is lower than xLFL there is no further risk of accidental combustion. defining a flammability range.9 mm for CH4. or. for methane. These values increase with the air/fuel ratio outside the minimum value (corresponding to λ≈0. since. The amounts of gaseous fuel (usually in volume terms in the mixture) between which a flame propagates in a quiescent mixture at ambient conditions. or lower ignition limit LIL).0 mm for n-C8H18. and with the presence of catalysts. increasing the relative air/fuel ratio λ to 0. in percent volume. Domestic gas-fired appliances usually have multi-hole or multi-blade burners (slightly corrugated slots to increase mechanical stiffness).1 mm for C3H8. 11. but this flame is very long and yellow. once ignited. Quenching prevents the propagation of combustion along narrow tubes or orifices and through fine wire-meshes (see Fig. The lower (or lean) flammability limit. Fig. but a premixed fuel/air mixture may not propagate a local combustion (e.5). with temperature (the quenching effect is lost at high temperature.6 mm for H2). the mixture cannot be ignited at room temperature. {xLFL=5%.8 or 1.6 or 0. is of greater interest than the upper flammability limit. up to a point where the mixture is no longer flammable at room conditions.61 850 CH4.g.. Αstq=9. where only the xi of the actual fuels. close it.g. ethanol). Another way to verify this tank-flammability behaviour is to put some liquid fuel in an Erlenmeyer flask (e.. φstq=1.0 5. all at 100 kPa. Flammability limits above refer to fuel and air at room temperature and pressure conditions. Fuel Lower Stoichiometric Upper Minimum Autoignition Flammability mixture Flammability quenching temperature Limit. The effects of changes in these parameters are: • With pure oxygen. for C2H6/O2 the change is to xFL=3.g. Le Châtelier proposed in 1898 a rule to compute the lower flammability limit for a mixture of gaseous fuels of molar fractions xi: xLFLm=1/Σ(xi/xLFLi). LFL Limit.7 12. gasoline. ΑLFL=34}.8 750 C4H10.5. for C4H10/O2 to xFL=1.14%vol to xFL=5.95%vol.2. .34 5. the flammable range gets smaller. for CH4/air mixtures.1 480 C12H26. methane 4.75%vol to xFL=4.g. n-octane 1. this point is reached with 21% of CO2 in volume.5 3. in the event of an electrostatic spark inside the tank).64 13 1. or 35% N2.95%vol). for C3H8/O2 to xFL=2.0 1. minimum quenching diameter at 25 ºC. propane 2.65 6 500 C8H18. Flammability limits (in %vol) for premixed fuel/air gaseous mixtures at 25 ºC. ether 1. acetylene 2.75 80 2. not so the LFL (e. three cases can be considered for the flammability risk (e.5 7. Notice that for CH4/O2 mixtures up to 80% N2 may be needed to have a non-ignitable mixture (Fig.0 4.45%vol..5. ethanol 3. and autoignition temperature. molar air/fuel ratios {ΑUFL=5. n-dodecane 0. 5 cm3 in a 100 cm3 beaker). n-butane 1.60%vol.80 1. diesel oil.3 6. for H2/O2 the change is from xFL=4.7 560 C8H18.. carbon monoxide 14 29.3 600 CH4O. Table. not of the possible inert components.1 1. diesel oil will not burn. ΑLFL=19}.9 9. according to its equilibrium vapour pressure.g.8 800 C3H8.5 1. or 48% He. e. • With the addition of an inert gas.65 6 690 C10H22.9 29. gasoline burns with a non-premixed flame outside the beaker.60 1. methanol. φUFL=2}. for CH4/O2 the change is from xFL=5. ether.5 680 C2H6O.87 6.5 75 0. UFL distance [mm] [K] CO. λLFL=2}.54 19 630 C4H10O.g. ethane 3.95 1.13 37 440 Notice that for a liquid fuel tank at room conditions.. and fuels with vapour pressure pF*(T)>xUFLp pose no spark-risk inside the tank (e. hydrogen 3.5 700 C7H16. 2. agitate. or 26% of water vapour. Αstq=17.4 2. 12). etc.3 36 1. n-decane 0.12 480 C2H2. λstq=1. and approach a lighter flame.5 70 900 H2.5. open it. may be expressed also in terms of equivalent ratios {φLFL=0.8 3. fuel oil).. n-heptane 1. fuels with vapour pressure xLFLp<pF*(T)<xUFLp pose a great spark-risk (e.diffusivity data. methanol 6. must be considered. iso-octane 0. although in the latter case the venting gases will be flammable when in contact with more air. the upper flammability limit increases a lot. mass air/fuel ratios {ΑUFL=10. pF*(T): fuels with vapour pressure lower than xLFLp pose no spark-risk (e. relative air/fuel ratios {λUFL=0.13 8.5..0 850 C2H6.47 14 2. acetone). and methanol burns violently inside the flask (without explosion because of the wide-enough opening).g..66%vol.52.40%vol and for C2H2/O2 xFL=2.7.02 9. Moreover.. sulfur has Tautoign=240 °C. Practical use of that is made in diesel engines..g.9% and xUFL=14% at 0. outside the flammability range). some values of which can be found in Table 2). xLFL=4. it must be recalled that the quoted autoignition temperatures refer to sudden inflammation. Additionally.15. xLFL=3. and in some steady combustors where high exhaust-gas-recirculation rates yields high-enough intake temperatures.g. Influence of an inert gas on the flammability limits of CH4/air and CH4/O2 mixtures at room conditions. and xLFL=3.4. In reality.1 MPa. size. composition.• • With temperature.g.. in the new dual-fuel diesel engines. >480 K for diesel oil).. or for CH4 at 300 ºC xFL=3.8. Autoignition may be a hazard in some special cases at low temperature: phosphorus has an autoignition temperature of only Tautoign=34 °C. 850 K for methane/air and 825 K for methane/oxygen). autoignition may occur at much lower temperatures after some self-heating period (e. since some fuel gets vaporised and mixed with air before autoignition actually occurs (that is why a two-function Weibe model best fit actual pressure plots). The minimum temperature for this to occur at 100 kPa in air is called autoignition temperature (also known as spontaneous ignition temperature. A non-premixed fuel/oxidiser mixture will also start burning at high temperature.45. the flammable range widens: e. Autoignition temperature A premixed fuel/oxidiser mixture will spontaneously burn at elevated temperature irrespective of pressure. but. a H2/air mixture .g.0. 12. carbon disulphide has Tautoign=90 °C. Combustion in this case is not by a flame spatially propagating but by homogeneous combustion of the whole volume at once. some premixed combustion takes place in diesel engines too.g. where intake air is quickly compressed to get it hot enough for the fuel to burn by contact with the hot air. implying the process is governed more by the Arrhenius expression than by concentrations. the flammable range gets slightly wider (particularly along the rich side) up to an autoignition temperature (depending on pressure and composition) where any mixture ratio will burn. SIT. Tautoign is just the temperature at which the rate of generation of heat becomes greater than its rate of dissipation.1% and xUFL=60% at 20 MPa. but here the propagation is as for normal non-premixed flames: at the rate of supply. Substitution of oxygen for air has little effect on autoignition temperature (e. <850 K for natural gas) and made to burn by compression ignition of a minute injection at high pressure of low-autoignition-temperature fuel (e. etc. etc.8% and xUFL=55% at 10 MPa. Fig.83%vol. for methane/air. compressed below its high autoignition temperature (e. For instance. a lean premixture (prepared with a high-octane fuel at intake pressures) is fed to the cylinders (with relative air/fuel ratio in the range λ=0. Autoignition is related to heat release and dissipation.17%vol. if the system is well insulated. diethyl ether has Tautoign=160 °C (it can be ignited by a hot plate). for H2 at 300 ºC xFL=2. With pressure. according to van't Hoff. will suddenly burn by itself at some 850 K, but it has been reported that in well insulated envelops it selfignites after a couple of hours at 750 K). SUPERSONIC COMBUSTION Combustion is a slow process (think of the maximum premixed laminar flame speed of 0.45 m/s for methane/air, or the slow burning of wood in the fireplace). High burning rates required in engineering applications, due to short residence time dictated by kinematics in reciprocating engines or size in other combustors, are achieved by promoting turbulence (e.g. in a gas turbine combustor the average speed is some 50 m/s, two orders of magnitude larger than typical laminar flame speeds, without the flame being blow-off (aided by some flame-holder devices). But, could the flame speed be raised to say 2000 m/s? The answer is yes, supersonic combustion sometimes takes place accidentally in coal mines and other closed places, as well as within condensed explosives, but no controlled application has been developed yet. H2/O2 and C2H2/O2 mixtures are particularly prone to detonation. Notice that the ignition limits for supersonic combustion are usually narrower that for deflagration; e.g. for H2/air premixed combustion, the flammability limits change from xFL=4..75%vol to detonability limits of xDL=18..59%vol. Supersonic combustion, called detonation, is realised coupled to a shock wave travelling at supersonic speed. Deflagration to detonation transition (DDT) is difficult to understand. If premixed gases inside a tube closed at both ends are ignited at one end, a laminar flame first develops, travelling fast being pushed by the expanding hot products behind. The inverted small pressure-jump across the flame generates local pressure pulses that wrinkles the flame, creates turbulence, extends the burning area and increases the burning rate, with a positive feedback that, if positively combined with pressure pulses reflected from the other end, might compressed the fresh mixture to the autoignition temperature. Although it is very difficult to have a planar geometry in practice (both in subsonic and supersonic combustion), we analyse just the planar case because it yields the maximum benefit/cost for learning. For a planar fluid front, be it a subsonic deflagration, a sonic acoustic wave, a supersonic shock wave, or a supersonic combustion behind a shock wave, the mass, momentum and energy balance equations through it at a steady state are: ρ= ρ= m A 1v1 2 v2 (37) p1 + ρ1v12 =p2 + ρ 2 v22 (38) h1 + v12 v2 + q = h2 + 2 2 2 (39) where m A is the mass-flow-rate per unit area traversing the front, and q=yFPCI is the heat release equivalent to the chemical energy of combustion (if there is such), that is taken apart from the thermomechanical enthalpy h. The intention is to analyse the possible exiting conditions for a given entry conditions, what is traditionally visualised in the pressure-volume diagram (p-v, or better p-1/ρ, in terms of density, since we keep v for velocities). From (37) and (38) one gets the so called Rankine line (a straight line in the p-1/ρ diagram):  1 1  dp2 p2 − p1 = − ρ 2 v22 − ρ1v12 = −  −  m A2 → = −m A2 1  ρ 2 ρ1  d ( ) ρ2 (40) showing that, for a given entry conditions (point A in Fig. 13) the only possible exiting conditions lay in the second and fourth quadrant from A, since m A is always positive. Fig. 13. Possible exiting conditions (Rankine straight line) after a planar fluid wave, in the pressurevolume diagram, for a given entry conditions at point A. After a normal flame, point F is reached (the Rankine line would join A and F). Acoustic waves take place as oscillations around point A along q=0 (the Rankine line would be the local tangent). Normal chock waves correspond to points like N (the Rankine line would join A and N). A detonation corresponds to point C-J (Chapman-Jouguet), the tangent point for the Rankine line from A to the Hugoniot curve q>0 (positive heat release). From (37), (38) and (39) one gets the so called Hugoniot curve (or Rankine-Hugoniot curve):  v2 v2  v +v v + v p − p1 p − p1  1 1  h2 − h1 =q −  2 − 1  =q − 2 1 ( v2 − v1 ) =q + 2 1 2 =q + 2  +  (41) 2 2 2  ρ 2 ρ1  ρ1v1  2 2 that, with the perfect gas model (∆h=cp∆T, T=p/(ρR), cp/R=γ/(γ -1) yields: p2 − p1  1 1  γ  p2 p1  q  −  =+  +  2  ρ 2 ρ1  γ − 1  ρ 2 ρ1  (42) that, although not evident, corresponds to a kind of hyperbolic curve in the p-1/ρ diagram (Fig. 13), displaced to the right of point A because of the q term (always positive). In the case of a normal flame, the jump through the front is from A to F, with a small pressure drop (some 2%) and a large specific-volume increase (some 7 times). The case of acoustic waves correspond to small departures to the left of A along the curve for q=0. Assuming negligible energy dissipation, the process is isentropic, and from the differential forms of equations (37) and (38), d(ρv)=0 and dp+d(ρv2)=0, one gets the general equation for the speed of sound:  ρ 2v 2  1 dp = v2 → = vsound dp + d  dp + ρ 2 v 2 d   = 0 → = dρ ρ  ρ  ∂p PGM = γ RT (43) ∂ρ S where the particularisation for the perfect gas model (PGM) can simply be obtained from differentiating the logarithm of (p/ρ)γ=constant. The case of a normal shock wave correspond to large departures to the left of A along the curve for q=0. Substituting ρ=p/(RT) and v = M γ RT in equations (37), (38) and (39) for the perfect gas model, M being the Mach number M≡v/vsound), one gets: pM p = constant M γ RT → RT T p p + ρv2 = p+ M 2γ RT = p 1+ γ M 2 = constant RT v2 M 2γ RT  γ −1 2  h + = c pT + → T 1 + M = constant 2 2 2   ρv = ( ) (44) (45) (46) from which one may get an explicit expressions for the exiting Mach number: M2 = 2 + ( γ − 1) M 12 2γ M 12 − ( γ − 1) (47) and, upon substitution, the rest of the variables. Notice, by the way, that the Hugoniot curve must lay to the right of the isentropic curve (p/ρ)γ=constant passing by A, since there can only be entropy increases in real adiabatic processes. Finally, for the detonation case, the exiting conditions correspond precisely to the Chapman-Jouguet point (C-J in Fig. 13); the rationale is that, if it were above C-J, the exiting velocity would be subsonic and the combustion would not be able to sustain that strong shock wave, whereas if it were below C-J, the exiting velocity would be supersonic and the combustion would tend to accelerate the shock wave; that is why it is found in practice that detonation waves always approach the C-J point. The internal structure of the detonation can be separated in a very thin leading region (a few molecular free-paths) of a strong shock with a pressure jump of say 40-fold (and temperature and density jumps of say 7-fold and 5-fold, respectively) leaving the stream subsonic with a local M≈0.4, followed by a much thicker combustion region where the fluid accelerates towards the local sonic speed M=1 while increasing further the temperature and decreasing both pressure and density. For a stoichiometric methane/air mixture at ambient conditions, the detonation velocity is 1800 m/s (advancing speed relative to quiescent fresh gases), varying little with pressure and composition and more with temperature, the corresponding Mach number is M1=5.1, and the exiting temperature and pressure 2780 K and 1.72 MPa.. TURBULENT COMBUSTION Combustion usually takes place in a turbulent flow, from combustion chambers in gas turbines, boilers and furnaces, to reciprocating engines and uncontrolled fires. But, as in Fluid Mechanics, a larger share of the learning effort is devoted to study laminar flows because they are more amenable, and more advanced turbulent models are presently built upon them. The basic characteristic of turbulence is that all local variables (velocities, pressure, temperature, concentrations) show a random fluctuation superimposed to a running average changing more slowly (or being constant if the flow is steady in the average; turbulence is always unsteady in the detail). This randomness does not mean the flow is not deterministic; it means that the flow is too sensitive to uncontrolled initial and boundary perturbations. In pipe flow, for instance, turbulent slugs may appear at low Reynolds number (Re~200), decaying more slowly the higher this value, until at Re~2300 they no longer decay in most setups; but in very smooth walls, smooth entry and quiet environment, the laminar regime has been maintained up to Re~100 000. Turbulence introduces a very effective exchange of mass, momentum and energy in cross-stream directions, increasing physical transport by orders of magnitude, what is a blessing in engineering problems limited by the mixing rate, as in combustion (obviously for non-premixed flames, but also for extending the flame front in premixed flames, and for the dispersion of pollutant emissions). For instance, for a velocity component u, along a given direction, x, a very quick scanning (e.g. laserdoppler, hot-wire or ultrasonic velocimetry) would yield a fluctuating variable u(t,x,y,z) at a given point (x,y,z). But u(t) is aperiodic and its average would depend on the time-span considered, ∆t. In practice, one can find suitable ∆t values that depend on the problem at hand; for typical engineering problems it may be ∆t<10-2 s, i.e. fluctuations below 102 Hz are retained, and above that are averaged), whereas for meteorological problems it may be ∆t≈103 s, i.e. fluctuations below 10-3 Hz are retained, and above that are averaged). The theoretical value for ∆t would be determined by the autocorrelation of the function being averaged. Assuming ∆t is well defined, the fluctuating variable could be decomposed in a running mean, u , and a random fluctuation u’: = u  u + u'  mean inst. with u = fluct. 1 t +∆t ∆t ∫t udt (48) Turbulence theories try to model this statistic problem by reducing the number of representative parameters. Unfortunately, the non-linear convective term in Navier-Stokes equations make the statistic analysis non-linear, introducing higher-order coupling terms at any stage in the decomposition problem, giving rise to a closure problem. For instance, the momentum equation along x for incompressible flow, when averaged, reduces to: ∂ ( ρ ( u + u ') ) ∂ ( p + p ')   + ∇ ⋅ ( ρ ( u + u ')( v + v ') ) = − + ρ g x + µ∇ 2 ( u + u ') ∂t ∂x ∂ ( ρu ) ∂ ( p)   + ∇ ⋅ ( ρ uv ) + ∇ ⋅ ρ u ' v ' = − + ρ g x + µ∇ 2u → ∂t ∂x ( ) (49) since the mean of single random variables is zero. The new term appearing in turbulent flows (the third in the left hand side) may be transposed to the right-hand-side of (49) and thence it is called turbulent Reynolds stress, defining an empirical turbulent viscosity, µt, usually much larger than the viscous one):  −∇ ⋅ ρ u ' v ' ≡ µt ∇ 2u ( ) (50) Turbulence is always a three-dimensional spatial-temporal problem, and thus spatial and temporal correlations are sought. The parameters introduced to quantify turbulence are of two types: • Turbulent intensities of the variables, i.e. root-square-means in the temporal fluctuations of velocities, temperature, concentrations…), that may be of the same order as the mean values. • Turbulent scales in the coordinates, defining a region of fluid with correlated properties (an eddy or vortex, since turbulent motion is highly rotational), that may be of the same order as the geometrical size of the problem. Several length scales may be defined in turbulent flows; from the largest to the smallest: the main length of the geometry, L (e.g. the diameter in a pipe flow), the large-eddy scale, LLE= ∞ 0 )u 'rms ( r )) , the Taylor mesoscale, LTY= u 'rms /(∂u '/ ∂y ) rms , and the Kolmogorof ∫r0 u '(r0 )u '(r )dr /(u 'rms3 (r1/4 microscale, LKG=(ν /ε) =[ν3(LLE/u’rm)/((3/2)u’2rm )]1/4 (where ν is the kinematic viscosity and ε the energy dissipation rate, ε=ν(Σ∂ui'/∂xi)rms), beyond which there is no further convective-transport but viscous-dissipation. Associated Reynolds numbers are defined as ReL=uL/ν, ReLE=u’LLE/ν, ReTY=u’LTY/ν ). Larger diameter flames do not blow off.and ReKG=u’LKG/ν. • Homogeneous turbulence. the incoming stream detaches at the exit lip. More elaborated models are being tried. D0. This conical solution is only valid from z/D0>8 to z/D0<100 with the origin at z/D0=0. • Isotropic turbulence. the flame appears wrinkled in many connected flamelets along the main flame front. The radius where the axial speed is half of the central speed. and the lateral one (assumed isotropic). i. iu≡u’rm/ u . • For turbulent flows.6. when du’rm/dθ=0 for all fluctuating components in all angular directions. when it blows-off and extinguishes. is dominated by turbulence intensity. when du’rm/dt=0 for all fluctuating components. with a flame height increasing linearly with exit velocity (see Flame length). or liquid in liquid. etc. r99. Non-premixed combustion of an axisymmetric gaseous fuel jet in air show a laminar behaviour up to Re=w0D0/ν<1000. i. • For laminar flows (Re=w0D0/ν<1000 or even Re<10 for non-parabolic jets) de semi-angle is small and given by r1/2/z=6/Re (e. there is a conical similarity solution in r/z. the specific turbulent kinetic energy. explaining why natural-gas and oil-well fires cannot be extinguished by blowing out. Sometimes only local lateral isotropy is imposed. r1/2. For instance. 3. with the central speed decreasing hyperbolically as w=(3/32)(w0D0/z)Re. the large-eddy simulation (where only smaller scales are averaged) and the direct numerical simulation of the whole statistical problem (only affordable for academic configurations). the relative turbulent intensity. the turbulent kinetic energy dissipation rate. when du’rm/dx=0 for all fluctuating components in all directions. For larger turbulence-intensity . is used to characterise the width of the jet. ε. ℓm (defined by µt = ρ  2m ∂ ( u ) / ∂y ). Axisymmetric turbulent jet flames Perhaps the more representative case of turbulent flow in combustion is the turbulent jet. and turbulent flames up to 40 m/s. for a methane/air jet issuing from an orifice 1 mm in diameter. Turbulent premixed flames Laminar premixed flames were characterised by a propagation speed that only depended on fluid properties (fuel and oxidiser type. as the Reynolds stresses model in terms of ternary correlations.5º for Re=100). however. µt. all due to the non-linear character of turbulence. Several turbulent intensities may be defined in turbulent flows: the eddy viscosity just introduced in (50). beyond which the flame detaches from the rim of the injector but remains stabilised in a lifted position (increasing with flow-rate to more than 10 diameters) up to 80 m/s. concentrations. with the central speed decreasing hyperbolically as w=6. For low turbulence-intensity levels. there is a continuum in length scales (the turbulent cascade) and in time scales (the frequency spectrum of energy distribution is continuum).g. or either where it is 1%. Re>2000.e. This conical solution is only valid for z/D0>9. temperature and pressure).e. When one stream issues into an otherwise quiescent fluid (gas in gas. laminar flames are obtained for injection speeds up to 20 m/s. the semi-angle is larger (typically 9º). with a fully-turbulent flame-length independent of speed for higher Reynolds numbers. the traditional k-ε model is widely used.6w0D0/z. In reality. Trying to render the formulation of real turbulent flows more amenable. some of the following simplifications may be imposed on the model: • Steady turbulence. two turbulent intensities are retained: the axial one along the local mean velocity. Turbulent premixed flame speed. the case of a liquid in a gas being very different). k ≡ u '2rms / 2 . becoming a predominantly axial flow (jet) that decays along the axial length while entraining external fluid and broadening in a more-or-less conical flow. the turbulent mixing length. with the ratios LLE/LTY=ReLE3/4 and LLE/LKG=ReLE1/2. for large distances compared to the exit diameter. A transition to turbulent jet combustion takes place at higher Re (up to Re>2000 for H2 but Re>9000 for C3H8). both independent of the Reynolds number. or when an already excited molecule hits another and decompose. a more realistic elementary mechanism must be thought of in terms of uni-molecular. First. until a generalised bulk reaction zone develops. Uni-molecular reactions are very rare at low temperature (the fraction of molecules with very large speeds according to the Maxwell-Boltzmann distribution law is not enough to flame the system and propagate the reaction. i. and let us think at the molecular level of detail. the flamelets get disconnected. respectively. oxygen. work and temperature can be obtained from a combustion reaction. Example: the combustion of hydrogen must start by dissociation of a hydrogen molecule (it requires less energy than oxygen dissociation): H2+M→H+H+M. . except when locally excited (e. but says nothing about the reaction rate. but they are more fertile (a larger fraction of chocks yield chemical change) because there are more possibilities for the redistribution of momentum and energy. .e.e. An example in the combustion of hydrogen may be H+OH+M→H2O+M. ξeq. for a given reaction 0 = ∑ν i M i dt V Vν i dt ν i dt (51) where ci is the molar concentration of species i in the mixture (the common units used being mol per litre. but not half-a-molecule. spark ignition) or globally heated (autoignition). and the SI unit mol/m3). we should write 2H2+O2→2H2O because we focus the attention on the forward reaction and not on the backward one (i. where M stands for an unspecified molecule (hydrogen. bimolecular and ter-molecular steps. • Bimolecular reactions are those where two molecules of reactives recombine by collision because of their very high relative speed or because they are already excited.levels. The interest now is in the reaction rate. • Uni-molecular reactions are those where one molecule of reactive breaks down by collision with an unspecified molecule at a very high relative speed. being transported by the eddies. REACTION MECHANISM AND REACTION RATE Consider the reaction of hydrogen with oxygen to yield water. how quick the process takes place. that we usually write as H2+(1/2)O2=H2O. But before specific kinetic models can be developed to study basic combustion processes. Instead of the above global reaction. as in Thermochemistry. ξ . The unspecified molecule is required to balance the momentum and energy of the uni-molecular chemical change. we should write 2H2+O2=2H2O because you can have half-a-mole. a review of general chemical kinetics is due. for a given reaction (or better its density ξ /V . Second. example in the combustion of hydrogen may be H2+O2→HO2+H and H2+OH→H2O+H. Recalling: ξ≡ ni − ni 0 νi dni dci d ξ ξ = . to have an intensive variable). • Ter-molecular reactions are those where three molecules happen to collide together at the same time. it is clearly unrealistic to think that in a single step two H-H molecules interact with one O=O molecule to yield two molecules. what is key to define the size of combustors. ξ = = . mol/L. Ter-molecular collisions are very much improbable than bimolecular collisions. CHEMICAL KINETICS Thermochemistry considers how much heat. and how is the exhaust composition. But third.g. the possible decomposition of water to hydrogen and oxygen). instead of the reaction advance at equilibrium. a hot wire). current successful models use nearly 300 elementary reactions involving some 50 species. a reaction mechanism is dependent on the range of temperatures and pressures involved. hydroxyl radical. H2O. p )[H 2 ]1. the rate of chemical reaction is directly proportional to the concentration of the reacting substances raised to particular exponents”. (53) for the global combustion reaction of some hydrocarbons (units for A.2 (1. it is found: ξ d [H 2 O] −d [H 2 ] −d [O 2 ] = = = = k (T . first presented in 1864. A and TA are empirical coefficients whose values are given in Table 3. for porous burning kinetics. for very-low-pressure kinetics (wall effects become predominant). For the combustion of methane in air. for ignition kinetics. hydroperoxide radical. For instance.7) in the example). and in a second step CO further oxidises to CO2 (for lean mixtures).5 [O2 ]0. The sum of the exponents is called ‘reaction order’ (order 2. OH. Empirical coefficients in Eq. HO2. O. H. and a. For hydrocarbons. But for either the global reaction or any elementary reaction in a reaction mechanism. and even for the global complete combustion to CO2 and H2O: Fuel+aO2 → bCO2+cH2O with d [ Fuel] dt a b  −T  = − [ Fuel] [ O 2 ] A exp  A   T  (53) Where a.g. but the mechanism is very complex. for the influence of pressure there was little knowledge and little interest. b. b and c are the stoichiometric coefficients. for the combustion of hydrogen in oxygen at some temperature and pressure.The many intermediate species appearing (e. in their “Studies Concerning Affinity”. Table 3. Most of the times. etc. and each exponent is called ‘reaction order in that species’ (1.…).g. the two controlling steps seem to be the formation of molecular hydrogen from hydrogen radicals (e.5 order in H2 and 0. OH. a reaction mechanism with some 40 elementary reactions involving 8 species (H2. H2O and H2O2) has been found appropriate to model the real reaction in an ample temperature and pressure range. and different mechanisms have to be developed for free-flame kinetics. now called the law of mass action: “at a constant temperature and pressure. O2. For instance. what they called ‘the action of masses in chemical reactions’. but how about the influence of the reactives themselves? The answer is the law of mass action.7 order in O2).7 for 2H2+O2→2H2O V 2dt 2dt dt (52) where the common notation for concentrations [M]≡cM has been used. H.p) is called ‘rate coefficient’.5+0. atomic hydrogen radical.g. in the combustion of hydrogen in oxygen. Theoretical kinetic models have been developed for practical combustion processes.…) are called radicals (e. . Empirical reaction rates for hydrocarbons have been developed based on the simplification that the fuel (with the oxidiser) first oxidises to CO and H2O. the question remains: what are the parameters that influence the rate of advance? It was well-known that increasing temperature had a tremendous influence on the reaction rate. a and b compatible with concentrations in mol/m3). they are electrically neutral but with unpaired electrons. Mass action law The Norwegian chemistry professors Cato Guldberg and Peter Waage (brothers in law). CH4+H=CH3+H2) and the consumption of molecular oxygen by combination with hydrogen radicals (H+O2=HO2). The coefficient k(T. basically for combustion of hydrogen and of methane. with tens of elementary species and hundreds of elementary reactions. H+H+M→H2+M (without it. e.6 15 000 K 0. • Chain propagation reactions are the majority in a reaction mechanism. For instance. and the rate coefficient can be computed from two concentration measurements (at two instants). the main type of reactions are first. It was Lindemann in 1922 who first explained the reaction mechanism for uni-molecular decompositions. although not as probable as bimolecular collisions. only limited by the total amount of initial reactives. In order to render amenable the solution of the mathematical problem for the reaction mechanism. Initiation steps generate radicals from stable molecules (but they are very rare).25 1.65 15 000 K 0. Example: H2+OH→H2O+H. • Chain branching occurs when one radical yields more than one radical upon collision with a stable molecule.Fuel Methane Propane Butane n-Octane Methanol A 83·109 27·106 23·106 15·106 10·106 a b TA -0. several simplifications are introduced. isomerisation. the main types of elementary reactions are: • Chain initiation and chain termination reactions. For a generic elementary reaction aA+bB=Products. The concentration of the active species varies exponentially with time. double decomposition) since bimolecular reactions are the most common. According to their effect. the law of mass action is then: ξ −dc A −dcB = = = k (T .3 1.15 1.5 15 000 K But the real importance of the law of mass actions comes when applied to elementary reactions. d[A]/dt=k[A]. Ter-molecular collisions. the frozen equilibrium. the partial equilibrium. simple collisions between two active radicals are not productive). the most common being the steady state. are much more fertile and thus many third-order reactions appear in reaction mechanisms. and the high activation energy. • Third order reactions serve to model ter-molecular reactions.g. CH4+O2= CH3+HO2). and OH+H+M→H2O+M is a termination reaction in water formation. • First order reactions serve to model decomposition reactions A+M→B+M when the concentration of the unspecified molecule M is much larger than the reactive species A. Examples: H2+M→H+H+M is an initiation reaction in hydrogen decomposition. p ) c Aa cBb for aA+bB→P V adt bdt (54) Types of elementary reactions According to their reaction order. second and third order reactions.25 1. .3 15 000 K 0. Combustion is the traditional example of chain reaction (also nuclear fission). Combustion reactions generally start with the generic initiation step RH+O2=R+HO2. Propagation steps produce new radicals by consuming other radicals (in the same amount. for H+O2→OH+O the reaction speed density is d[OH]/dt=k[H][O2]. It is this branching effect that characterises what are known shortly as ‘chain reactions’. since then the reaction order is just the number of molecules participating. The common instance is the recombination of two active radicals after collision with a third body that absorbs energy. • Second order reactions serve to model the majority of elementary reactions (formation of activated complex. reactions that become more and more productive because more and more active radicals are generated.1 1.g. causing and exponential growth rate termed ‘explosive’. Example: O+H2→OH+H.5 15 000 K 0. and termination steps consume radicals yielding the end products (they are also very rare). where R is a hydrocarbon radical (e. see below). with enzymes). in the Zeldovich mechanism of NO formation (O+N2↔NO+N and N+O2↔NO+O) this approximation implies d[N]/dt=k1[O][N2]-k2[N][O2] =0. Thermodynamics of Chemical Reactions): d ln K −hr⊕ = 1 R d T (55) where hr⊕ is the standard reaction enthalpy and R the gas constant. where first the reaction rate increases but then decreases with further temperature rise. with values of order 104 K for combustion reactions (Table 3). This reduces a differential equation to an algebraic one. Arrhenius proposed: − Ea k (T ) = Ae RT (56) and gave to EA the interpretation of an energy barrier that the molecules have to jump over. Well. as the presence of catalysts. as shown below.• • • The steady state approximation is the assumption that very active radicals form and decompose at the same (high) rate. The partial equilibrium approximation is applied when there are two species linked by a single fast reaction in both senses (forward and backward). K. and for gaseous reactions is already taken into account through the concentration of species. most chemical reactions accelerate when increasing temperature. The effect of pressure is negligible in condense-phase reactions.e. then. to actually react. The frozen equilibrium approximation is the assumption that slow rate reactions can be solved after faster reactions are solved. Arrhenius law Once the effect of the concentration of reactives has been singled out with the law of mass action. named activation temperature. but it can be deduced also from collision theory assuming that the probability of a fertile chock is proportional to the number of molecules with kinetic energy above a threshold (the activation energy). This eliminates a differential equation.g. Maxwell-Boltzmann distribution of molecular kinetic energies shows that the fraction of the total number of molecules. the former being sensitive only to the integral effect of the quicker processes. based on the integration of the theoretical deduction by van’t Hoff in 1884 of his famous equation for the variation of the equilibrium reaction constant. 14): . But the effect of temperature is very great. Coefficient A in (42) is known as the pre-exponential factor or the collision factor. with negligible accumulation in the system. with normalised kinetic energy between ε and ε+dε (ε is the kinetic energy divided by the mean kinetic energy). thence EA is termed ‘activation energy’. and its integral from ε to ∞ are (Fig. This reduces a differential equation to an algebraic one. pressure and other side effects. TA. The Arrhenius exponential temperature dependence. notably in catalytically-controlled biological systems (i. Svante Arrhenius proposed in 1889 a dependence of rate coefficients with temperature. For instance. f. with reaction rates doubling just by increasing temperature a few tens degrees. at least in the ideal gas model. but there are some exceptions. was deduced from van’t Hoff equation for equilibrium. since it can be calculated from collision theory. Another variable. and it is much larger than the denominator in (56) for combustion reactions. with temperature (see e. is introduced instead of the activation energy as TA≡EA/R. the rate constant in (32) will depend on temperature. or the type of solvent for liquid-phase reactions. the concentration of one of them can be simply put as proportional to the other through the equilibrium constant for that equation (forward and backward). In effect. according to the MaxellBoltzmann distribution. and substituting the characteristic speed. i. ∆T. changes in electronic configuration). and an empirical exponent. without chemical change. The reaction rate density should be proportional to the number of collisions per unit volume and unit time. the Arrhenius exponential factor tends to 1 for large temperatures. multiplied by the probability that a collision be fertile (i. i. the final result is: . One may recover the reasoning in terms of temperature variations. contrary to what might be thought by saying that reaction rates grow exponentially with T (in fact. T0.e. i. That is. v the modulus of its velocity. and k Boltzmann’s constant (the gas constant divided by Avogadro’s number. that produces new species. fd ε = 2 π ∞ ( ) ε e −ε d ε → F (ε ) = 1 − erf ε + ∫ fd ε = ε 2 π ε e −ε ε >>1 2 =ε e −ε π with 1 2 mv ε≡2 3 kT 2 (57) where m is the mass of a molecule. 14. and its integral from ε to ∞. and σ is the averaged diameter of the molecules. Ta.e. The argument is as follows. The molecular kinetic-energy distribution also shows a T-1/2 pre-exponential temperature-dependence (the ε term). Notice that there is −1/T and not T in the exponential argument in (56).Fig. Maxwell-Boltzmann distribution. and expanding exp(−TA/(T0+∆T)) ∝ exp(∆T). most collisions are sterile in the sense that only yield thermal redistribution of energy. k=R/NA). cB. v . not to infinity). F. for a molecule A with molecules B (assume there are type A and type B molecules only). f. Collision theory Collision theory for bimolecular reactions gives a simple explanation to both the mass action law and Arrhenius law. changes in translational. can be estimated roughly as the number of intersecting points (molecules centres) in a cylindrical volume of length vdt and diameter double than the average molecule diameter (double because only their centre points were considered before).e.e. When all molecules are considered for the number of bimolecular collisions. but it does not corresponds well to experimental data.:  number of collisions for one molecule  N B π ( 2σ ) v  = unit volume  unit time 4   V 2 (58) where NB/V is the number of B-molecules per unit volume (its molar concentration. the fraction of molecules with kinetic energies larger than a given value is proportional to an exponential factor in temperature of the form exp(−1/T). relative to a given value. times Avogadro’s number). of molecular kinetic energies (relative to the mean kinetic energy). is sometimes used instead. The number of collisions per unit volume. rotational or vibrational energies. b and EA are experimentally determined because the collision theory do not provide any values for them. i. (61) for some elementary reactions in the combustion of hydrogen with oxygen (from Warnatz-1984).6 MJ/mol 300. the quotient of rate coefficients (forward/backward) must equal the equilibrium constant. and K stands for the temperature unit.2·10 (cm /gmol)/s -0. In effect. Finally.e. (52) to both senses of a generic reaction aA+bB ↔ cC+dD.8k B T 2  number of total collisions  πσ   = c A cB πµ AB  unit volume  unit time  (59) On the other hand. instead of the SI-units (m3/mol)n-1/s.e. one gets: .0 0 1000.2500 K 7 3 O+H2→OH+H 1.1 MJ/mol 300.5000 K Relation between rate coefficients and equilibrium constants Global reactions indicate overall transformations. For real work. although a more refined activated-complex theory (with bonding energies and steric factors) could be developed to supply estimations of those parameters in terms of more basic data.0 31. in spite of the fact that most of the times T b is written in (61) instead of (T/K)b. the kelvin. can be assumed to have no privileged direction of advance. they should be at equilibrium (generating products at the same rate as they react back). TA=EA/R in kelvin units.. i..5·10 (cm /gmol)/s 2. the fertility of the OH+H collision to yield H2O must depend on the orientation of the linear OH-radical relative to the centre of the H radical (chocks where the O-atom lies in the middle will be more productive). i.. 14). there is an assumed direction of advance. or as activation temperatures. Reaction A b EA Temperature range 17 3 H+O2→OH+O 1.4·1023 (cm3/gmol)2/s -2. for the same high speed. applying Eq. B is commonly given in CGS-units of (cm3/gmol)n-1/s.2500 K H+OH+H2O→2H2O 1. indicate detailed interactions that. not just to species A). however.6·1017 (cm3/gmol)/s 0 478 MJ/mol 2000. For a reaction at equilibrium. ceteris paribus.g.:  probability that a   − EA    ∝ exp    RT   collision is fertile  (60) where EA is the activation energy for the reaction (for the reaction A+B→P. Empirical values in Eq. where n is the reaction order. Considering just the tail of the Maxell-Boltzmann distribution. the three parameters A. Elementary reactions. the probability that a collision be fertile must be proportional to the fraction of molecules with a high-enough speed (the tail of the Maxell-Boltzmann distribution. to have a non-dimensional exponent b (the K is omitted in most writings).91 69. with a clear distinction between starting reactives and end products.3000 K H2O+H2O→H+OH+H2O 1. Fig. and to some steric factor that accounts for the fact that. it is proportional to the exponential of the chosen lower energy bound (43). as said above. Table 4 gives a sample of experimental values. in principle. the geometry of the multi-atom molecules on the collision must be important to fertility (e.. Activation energy data are given in MJ/mol units (usually written as kJ/gmol).e. Exponent b is tabulated in non-dimensional form. Table 4. In summary: ξ −dc A −dcB  −E  T  = = = c AcB A   exp  A  V dt dt K  RT  b (61) where A is the pre-exponential factor as in Arrhenius law. but typical combustion processes are really entangled. Normal combustion in air produces some 1000. Several kinetic mechanisms have been worked out of combustion interest. particularly the formation of emissions.4000 ppm of NO and some 10.8 ⋅103 .. TA = 21000 K (64) = −cNO cO A exp  A  with A = mol ⋅ s dt  T  dcNO m3  −T  NO+H → N+OH 1. HCHO and CHO. p ) c Aa cBb RT  ξb c d   = kb (T . O. To simplify the kinetic mechanism. the relation between rate coefficients at equilibrium and the equilibrium constant is used to evaluate either the backward or the foreword rate coefficient. the oxidation pattern seems to be CH4→CH3→HCHO→HCO→CO→CO2. CO2. The best known example is the Zeldovich-kinetics mechanism for NOx formation. that is dominant. H2O.. other are relatively simple. p )  ξ f =  kb (T . p )cC cD  V Measurement of rate coefficient is so difficult that it is usual to get just one significant figure in accuracy.40 ppm of NO2. CH3. two basic assumptions are often introduced. p ) = K (T . The extended Zeldovich mechanism is: dcNO m3  −TA  8 O+N 2 → NO+N 1. Although NOx formation may come from a N-containing fuel. OH. a short-mechanism for CH4/air combustion already involves 14 species: CH4. as the one step kinetics for ozone formation (O2/O3/O kinetics). CO . the thermal formation after the flame. p )c Aa cBb  ⊕ c + d − a −b k f (T . based on the measurement of the other. as the burning of hydrogen with halogens or the formation of nitrogen oxides.H2 . In the case of a lean combustion. the steady state and the partial equilibrium hypothesis: • Some radicals are assumed to be produced and consumed at a steady rate (not valid at the start and the end of the process) • Some elementary reactions are assumed to be so quick that they are at equilibrium (forward rate equal to backward rate) at the time scale considered. H. O2. p ) cCc cDd  V ⊕  p  (62) ξb ⇒ == K c (T . TA = 38000 K (63) = cO cN2 A exp   with A = mol ⋅ s dt  T  dcNO m3  −T  NO+O → N+O 2 3. The rate of NO formation is computed from: dcNO k k k = k1cO cN2 − 1 cNO cN − k2 cNO cO + 2 cN cN2 − k3cNO cH + 3 cN cOH dt K1 K2 K3 (66) . or from reaction in the hot-products region. KINETICS OF NOX FORMATION Chemical kinetics has not yet been able to provide accurate models for practical combustion reactions. for lean adiabatic combustion it is the latter.7 ⋅108 . Some of them are very simple. TA = 25000 K (65) = −cNO cH A exp  A  with A = mol ⋅ s dt  T  the last reaction being added to the basic Zeldovich mechanism to better model near-stoichiometric combustion cases. OH2. but has proved invaluable to understand some basic associated processes. so that even far from equilibrium. from reaction of N2 in air within the flame front.ξf  = k f (T . N2.8 ⋅10 . Metal catalysts are usually transition metals such as iron. complex organic compounds that contain a protein entity. and are the most active in Nature. • Heterogeneous catalysts. The converter shell contains a ceramic "honeycomb" which is coated with a noble-metal combination (platinum. • Enzymes. They are biological catalysts. Gasoline is processed by a catalytic reaction carried out over alumina silicates (zeolites). as perovskite (a greyish-black mineral form of calcium titanate. as shown for instance by their effect on ethyl alcohol. a 'hygienic lamp and smoke absorber'. etc. They are often coordination compounds of transition metals soluble in the liquid medium in which the reaction takes place. the decomposition of water into hydrogen and oxygen at room temperature. in the presence of an alumina catalyst. Of little relevance in combustion. In 1887 the French pharmacist M. metal oxides. which catalyzes removal of sulfur from crude petroleum. the catalytic after-treatment of the exhaust gas is considerably more effective than the purely thermal after-burning of the exhaust gases in a thermal reactor. which catalyzes the oxidation of hydrogen to water and of ammonia to nitric acid. and the reduction of the nitrogen oxides. although some other catalysts are being tried. molybdenum sulphide. that. The term "three-way" means that all three toxic substances CO. Only lead-free gasoline may be used with noble-metal converters because the lead otherwise destroys the catalytic properties of the metals. but no catalyst will facilitate the reverse reaction. The .W. It should be emphasized that a catalyst only speeds up the rate of a reaction that is thermodynamically allowed. K1 and K1 being their equilibrium constants. Using a modern catalytic converter. Dobereiner in 1820 in the hydrogen/oxygen reaction to form water at room temperature. A platinum catalyst will initiate and accelerate the reaction of hydrogen and oxygen to form water. if the alumina is replaced by copper. with some rare-earth elements. with a wick connected to a burner made up of ceramic and black platinum. more than 90% of the amount of toxic substances can be converted to harmless substances. k2 and k3 being the rate coefficients for the three reactions above. They are finely divided solids (metals.with k1. which catalyzes the synthesis of ammonia. metal sulphides. The latter reaction is not allowed by thermodynamics to run alone. that consists on an alcohol lamp (with secret essential-oil odorants). HC and NOx are eliminated at the same time. The so called three-way catalytic converter (TWC. K1. ethyl alcohol will react to form diethyl ether or ethylene. which is stable at ambient temperature. when heated up for a couple of minutes. The combustion catalyst per excellence is platinum (Pt). the catalytic property of Pt were discovered by the German chemist J. has come into widespread use for gasoline engines (on all new cars since 1993). the products become acetaldehyde and hydrogen. Catalysts are usually classified as: • Homogeneous catalysts. has the ability to retain a smokeless incandescence at some 500 ºC (it was much used in mortuaries and hospitals. cN2. and cO. even if its rate is so small as to be negligible. which is used in certain high-temperature ceramic superconductors). and may be highly selective in their activity. and. see Combustion Instrumentation). Berger patented the Berger lamp. which catalyzes the reforming of petroleum to high-octane gasoline. or acidic oxides) that catalyze reactions in a fluid media. platinum on alumina. and nowadays in aromatherapy). CATALYSIS A catalyst is a substance that changes the velocity of a chemical reaction while not being changed itself overall. cO2 and cOH corresponding to the equilibrium exhaust composition in absence of NO. although there are really only two different processes: the oxidation of the unburnt emissions (CO and HC). cN. The three-way catalytic converter For unburnt emissions. platinum. palladium and rhodium). and so an extremely accurate closed-loop electronic control (featuring almost zero lag). Moreover. or even better (<1 ppm for NOx and CO. specifically designed to get rid of the NOx since there is little CO and HC in lean-combustion engines. Ethanol and other hydrocarbons have also been proved efficient for SCR. and thus it cannot propagate through normal cold porous media. but not for sewage and landfill biogases because sulfur and heavy metals content rapidly deactivate the catalyst. with steady temperatures in the catalytic matrix from 700 K to 1400 K.catalytic converter also works on natural-gas burning engines. The urea aqueous solution (40%wt) is carried in a reservoir and fed by a dosing pump (the amount of urea depends on engine load) to a compressed-air spraying system. widening of ignition limits and less emissions. If the fuel contains sulfur (as some heavy fuel oils). The advantages of power modulation. the best open-loop enginecontrol would be unable to maintain the air-fuel mixture within the close tolerances required for optimum work of the three-way catalytic converter. But flames can propagate through hot porous solids and through cold porous solidcatalysts. and only if running stoichiometric. and it is found that near-equimolar NO/NO2 mixtures proceed much faster than pure NO or NO2. to generate ammonia by thermo-hydrolysis (NH2)2CO+heat=HNCO+NH3 followed by hydrolysis of the isocyanoic acid HNCO+H2O+heat=NH3+CO2. In the first case the reaction is directly NO+NO2+2NH3=2N2+3H2O. CO and HC are oxidised with a platinum catalysts in a first-order reaction mechanism since only the CO or HC concentration is controlling. other catalysts are being tried for lean-mixture gasoline and all diesel engines (always lean). as well as other selective non-catalytic reducers (SNCR).g. For lean-mixture gasoline engines. ξ / V = k (T ) xCO ). ceramics or polymers). there is the problem of condensation of (NH4)2SO4 downstream (that is why the SCR is placed after the exhaust turbine). for CO+(1/2)O2→CO2. The three-way catalytic converter only reduce NOx in reach mixtures and only oxidise CO and HC in lean mixtures. are similar to high-temperature porous combustion. best results (>90% NOx reduction) are presently obtained by spraying urea in the flue gases. must be added to the air-fuel-mixture management system. oxygen concentration being much higher in lean combustion products (e. due to very low temperatures involved). The catalytic substance helps to maintain the combustion process at low temperature at the surface. The urea SCR only works at T>250 ºC. Catalytic combustion The usual thermal free-flame combustion process cannot be sustained in the vicinity (say within 1 mm) of normal cold solids (metals. as when a mixture of H2/air reacts over Pt at room temperature (CH4/air over Pt requires 350 ºC). with a very thin overlapping region near the stoichiometric air-fuel ratio (a deviation of only 1 % has considerable adverse effects on either oxidation or reduction efficiency). That is why they are only useful for gasoline engines. that in its absence would require much higher temperatures: the minimum autoignition temperature for homogeneous H2/air combustion is 850 K Catalytic combustion works for premixed flows and for a non-premixed fuel flow in ambient air. in a selective catalytic reduction (SCR) process through a platinum-coated titanium-oxide matrix. with an oxygen sensor at the exhaust (the lambda probe). Selective catalytic reduction (SCR) As the successful three-way catalytic converter only works close to stoichiometric mixing. but not the higher burning . For diesel engines. whereas in the second case additional steps involving ammonium nitrate take place. a blue flame with some 1900 K forms immediately close to the outer surface. and thus only 600 kW/m2 are achieved instead of the 3000 kW/m2 of thermal porous-medium burners. small flames detach from the outer surface and. or even the whole fuel if the catalyst cools down and deactivates. The influence of the flowrates is important.rate (that was due to thermal conduction along the hot solid matrix. an hybrid regime of catalytic-assisted thermal combustion may be developed. if a thin porous solid is doped with a catalyst and a premixed methane/air stream is forced through. where both heterogeneous and homogeneous reactions take place. that only reaches now some 600 K. but if more gases are fed. releasing much of the lower heating value. if there is insufficient catalyst. part of the fuel slips to the exhaust (increasing pollution and expense). an interesting trade-off solution when the catalyst is too expensive and very little can be used (in full catalytic mode. what causes the matrix to reach some 1500 K at the outer surface (the hotter) in a flameless regime with a power of up to 500 kW/m2. an exothermic oxidation of the fuel takes place if the temperature is >600 K. At intermediate temperatures. at about 1000 kW/m2. Back to Combustion . for instance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................................ 15 Tomography......................................................................................................................................................................................................................................................................... 8 Exhaust control ............................... 13 Radiation sources .......................................................................................................................................................................................................................................... 10 Temperature measurement..... 4 Experimental techniques .................................................... 14 Holography ...................................................................................................... 3 Experiment diagnostics ........................... 16 Raman scattering (RS) .................... 3 Experiment stimulation and control........................................................................... 16 Laser induced fluorescence (LIF) ....................................... 13 Object types .................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................... 16 Coherent anti-Stokes Raman Scattering (CARS) ..................................................................................................................... 2 Experiment conditioning ........................... 11 (Radiation) Spectrometry.......................................................... 11 Fundamentals .......................................................... 15 Speckle interferometry (SI) .............. 15 Interferometry ........................................ 9 Particle image velocimetry (PIV) .............................................................................................................................................................................................. 13 Detector types ................................................................................... 9 Speckle velocimetry (SV) ...................................................................................................................................................................................... 11 Optical diagnostic techniques ............................................... 11 Applications ...................................................... 10 Analytical techniques............. 16 ................................................................................................................................................................................................................................................................................................................. 15 Moire deflectometry (MD) ................................................ 10 Gas chromatography ................................................................................................................ 6 Fuel and air supply.................................................................................................................................................................................................................................................................................................................... 16 Laser induced incandescence (LII) .............................................................. 13 Radiation emission and radiation absorption ...................................................... 12 Classification .................................................................................................................................................................................................................................................................................................... 9 Hot-wire anemometry (HWA) ................................................................................... 2 Experiment planning................................................. 3 Experiment analysis ............................................................................................................................. 13 Radiation scattering ......................................................... 7 Flame diagnostics .............................................................................................................................................................................................. 7 Emission detectors .................................................................................................................................................................................................................................. 9 Laser doppler velocimetry (LDV) .........................................................................................................................................................................................................COMBUSTION INSTRUMENTATION Experimentation and instrumentation .............................................................. 10 Mass spectrometry ................................................. 4 Controls and safety .................................................................... 6 Flame detectors ................... 8 Flow measurement ............................................... air supply and control. specific controls. for investigation of special details. Instrumentation for research on combustion is much more sophisticated than instrumentation for proper operation of a combustor. i. Experiments are purposely tried events. but the key rule may be that. a mathematical model to use for making predictions (checkable expectations).e. control and diagnose all the variables of interest may become rather complex. on the other hand. Depending on the application of the combustion process.e. heating. the better (and try it at least twice!). . emissions detection. instrumentation may be aimed at: • Normal operation of a working system (reliability and economy are the drivers) • Experimental research in a test-rig or prototype system (accuracy is the driver).g. According to its main objective. o Stimulation (or actuators). ignition. on one hand. eventful and active (requiring power or a trigger) to force some stimuli or avoid it. Instrumentation equipment serves the following purposes: • Controls. cooling. test section. piping. all experimental research make use of common operation instruments (e. e. laser diagnostics). Experimental techniques for combustion (and in general) may be grouped according to its function of conditioning. We envelop these techniques. and planning for eventual malfunctions (experimental robustness). fuel supply and control. with a planning insensible to uncontrolled environmental perturbations (Taguchi’s experiment-quality approach). to check some preconceptions.e. Combustion always involves complex thermo-chemical and fluid-mechanical interactions. Instrumentation heavily depends on the objective of the test. i. that may be split in: o Conditioning. stimulation and diagnosing the event. as the internal structure of a flame (including radicals evolution). instruments for routine operation were in the past research instruments. reservoirs. flame detection. and. redundancy in measurement. EXPERIMENT PLANNING Design of experiments (DOE) is a difficult creative endeavour.g. a feasibility study to find the weak points and choose the most reliable tools for actual implementation. flow controls). with a previous planning phase and a final analysis phase. both aspects are covered. for proper operation or. usually permanent and passive (not requiring power) to establish a known configuration: structure. development of an ordered set of nominal trials to perform (known procedures).. engine tachometers and dynamometers). of the initial state and of the boundary conditions (sometimes depending on measurements). power supply. overall diagnosis. the instrumentation to condition. exhaust. Instrumentation is the actual set of devices used and their way of implementation to carry out the experiment (experimentation). Clearly defined experiment objectives (known target). validation of the results is greatly enhanced by the accumulation of data that demonstrate with a high degree of confidence that the process is repetitive (without the need of lots of trials). i. additional variables and instruments may be of interest (e.e.g. usually involving sophisticated tools for their implementation. i. are a guarantee to success. in the more demanding case. the simpler the experiment. besides special sophisticated instruments (e. exposed in chronological order of execution.g. etc. to heat engines. Aiming at research instrumentation. Experiments must be designed robust. but they are not unconnected. and other interfaces. and thus. generic controls.EXPERIMENTATION AND INSTRUMENTATION The aim here is to analyse the devices (and their implementation) used to measure and control combustion process. One aspect of robustness is redundancy. regulate or control its action. Advanced diagnostics rely on spectrometric methods using laser instrumentation and advanced computer modelling codes. Besides the transducer itself. thermal (temperatures) and chemical (species). nowadays interfacing to a digital computer for automated operation (data acquisition and control. make intrusive techniques only applicable to intake and exhaust control. there are other controls for the fluid supply (e. but. very sophisticated techniques are usually applied. access doors and viewing windows.e. but fluid physics has always used flow visualization for immediate diagnosis). and large gradients (106 K/m). coolers). most of the time optical (or better radiometric) in nature. and there is a big jump from academic setups to industrial prototypes. EXPERIMENT DIAGNOSTICS Diagnostics means the taking of measurements and samples with the aim at identifying how the system behaves in space and time. sensors and actuators. on measurement techniques. remote operation. air blower). etc. digital signal processing.). Experiment conditioning is so application-dependent. DAQ. and not merely combustion control (with its inherent diagnostics). since the high temperatures (>1000 K). as for other aerodynamic and hydrodynamic problems.• Diagnostics. DSP. etc. EXPERIMENT CONDITIONING Besides instrumentation for diagnostics and control. feeding ducts and exhaust. thermal state (heaters. The most demanding task is measuring active species within a flame (radicals and ions). but it is most desirable for analysis too (sending samples to a distant laboratory may still be necessary for some special biological or chemical analysis. Our main aim in this survey of combustion instrumentation and experimental techniques is really here. combustion equipment always includes the basic infrastructure to set up the process. i.e. to condition and control the configuration: combustor body (its structure and supports). both. thermal insulation. as for typical thermofluid-mechanical experiments. EXPERIMENT STIMULATION AND CONTROL The key stimuli in combustion is the ignition event (usually an electric spark). Diagnostics instrumentation in combustion is based on three main types of variables: flow (velocities). safety devices. fluid flow-rate (valves). We do not extend here on combustion stimuli but on diagnostics. . When research on combustion is implied. but non-contact coupling (electromagnetic) is increasingly being used for sensing (and sometimes for actuators). that no further details are presented here. i. small scales (<1 mm). Most sensors (and actuators) act by direct contact with the object. Real time diagnosis is required for real-time control of the experiment. not without recalling that any sensing involves some stimuli on the test-subject (there is no measuring without perturbation). usually require some power and signal conditioning (digital or analogue). including the initial state and boundary conditions. etc. Combustion experiments depend on so many intermingled parameters that it has not been possible to carry out scale-model experiments.g. fuel pump. sensors to measure the evolution of selected variables. Every stimulation requires some sensors to timeline. so that sensors should be studied prior to actuators (and the number and variety of sensors in a test-rig is much greater than that of actuators). Timing in combustion is not as critical as in other fields. i. EXPERIMENTAL TECHNIQUES Experimental techniques may be classified as above in configuration setups. ultrasounds. NTC) and metallic resistance thermometer devices (RTD). or the multiplicity of particle sizes (fuel sprays. etc. in that order. Wolfhard flat-flame premixed burner in 1939. safety-related. concentration). Time measurement Many experiments on combustion deal basically with steady states. etc. concentration and temperature. stimuli (actuators: flow inducers. etc. However. it is very different to check that a certain magnitude is within allowable bounds (e. For combustion instrumentation. Thermometry The contact thermometers most used in combustion are thermocouples. standard test-rig. as when Ràmàn scattering is used to measure composition. but timing is important in any case (all steady states start and end). thermistors (negative temperature coefficient resistors. and diagnostics (sensors: flow. Low varying geometries as liquid level. data fitting to models and not just to generic curves and surfaces. soot) usually require optical techniques. that a big problem in data analysis is the reduction of the overwhelming amount of data taken. smoke plumes. and so on.e. statistical analysis helps a lot. For combustion experimentation. special burners have been devised. as we said before. it should be recalled that besides the sensor or actuator itself (the transducer). temperature. actuator position (for stimuli or for sensor location). spark plugs). Pandya-Weimberg opposite-jets diffusion burner in 1963. capacitive sensors. but the most important is to have a parametric model to fit the data to. and particularly in combustion research. The traditional classification of experimental techniques is. some power supply and conditioning circuit is usually involved. Here we follow the traditional approach. emissions-related. air-related. that to investigate the formation of soot. according to the type of variable being measured or controlled: temperature measurement. valves. sometimes grouped in physical and chemical techniques. but a separate presentation of general non-intrusive diagnostics is included below.g. Wolfhard-Parker twin-slot diffusion burner in 1949. as flame fronts. More . tracers. starting from the now-standard axisymmetric Bunsen premixed burner in 1855. More subtle edges. are measured with potentiometers. But nowadays. fuel/air-related.. to check the concentration of CO in a vehicle exhaust). pressure. solidfuel borders. composition. we deal here only with instrumentation. however. level. except when analysing the initial phase of the ignition process. experimental techniques could also be classified according to function as: fuel-related. flow rate or velocity. and leave actual data acquisition and analysis aside. the same experimental technique is used to measure several magnitudes at once. Redundancy is so important to guarantee data validation. edges and particle measurement Fixed geometry is measured before or after the experiment. Geometry. specially built research set-up.EXPERIMENT ANALYSIS The analysis of an experiment depends a lot on its purpose. or according to the setup: operating installation. and flame-structure-related (only for combustion research). H. but most advanced analytical techniques are non-intrusive and on-line. for CO-toxicity the maximum human exposure is 50 ppm. or gas chromatography. and are used to follow the highfrequency pulsation at the intake and exhaust ports in reciprocating engines.. e-. The main components in combustion processes are: N2. where rapid changes and large pressures are involved. oxygen was removed by a NH4Cl/CuCl solution. NO2. CHO+. selective absorption. . Measuring the velocity field is much more cumbersome at least because of the amount of data implied. Many times. O2. or selective solid-electrolyte conductometry). turbine-meters and other more sophisticated methods as thermal capacity and Coriolis effect devices. the amount of CO2 was first measured by volume subtraction after passing the gas through a NaOH solution. CHO. Chemical sampling may be intrusive. followed by chemical analysis (using Orsat selective absorbers. is singled out. O. as explained below. or local to know the velocity field. Chemical analysis Different stages in a combustion processes may demand chemical analysis. may be used. Velocimetry and flow rating Flow meters may be global. fuel. to know the overall mass flow rate or volume flow rate. but for different function. thermal or optical. through a quartz tube of some 1 mm or less. a sample of the mixture is analysed off-line and discarded. tank weighting or level change in condense fuels. and CO by H2SO2. often through a separation process of chromatography. since it is widely used to directly measure concentrations of many different gases (CO. or mass spectrometry. to the most sophisticated spectroscopic techniques. or general flow meters as rotameters. analytical techniques may be grouped as: • Chemical methods of analysis: characteristic reactions.5% (the instruments are different). and later radiometry and mass spectrometry. calibrated nozzles and diaphragms. qualitative and quantitative finding of the composition in a mixture: intake (fuel and air analysis). e. main or trace. and sophisticated methods are used. Note that composition ranges may be different not only for different species. H2S. afterwards. inside (radicals formed for kinetic studies). i. but only in the ppm-range). Before gas chromatography took over in mid XX c. CO2 and H2O. etc. and there piezoelectric sensors are used (quartz-crystal transducers develop an electrical charge when compressed. ranging from electrical. After filtering solid particles and dehumidifying the exhaust sample. For small pressure differences the best are silicon-chip capacitance sensors. classical or quantum. or non-intrusive sampling (radiometric) that may be passive or active. In the first case. • Physical methods of chemical analysis. Piezoresistive transducers are semiconductors (doped silicon) that change their electrical resistence upon compression. etc. OH-. OH. Piezometry Pressure measurement (piezometry) is not a difficult problem in combustion except in reciprocating engines. NOx. Sometimes a special chemical type.g.advanced non-contact thermometers (sometimes named pyrometers) are dealt with below under Optical techniques. the electrochemical one.e. SO2. trace components are: CO. and exhaust (emissions). the standard method in exhaust analysis was developed by Orsat in late XIX c. In general. non-saturated hydrocarbons were removed by a KOH and pyrogalic-acid solution. whereas for COcombustion the minimum concentration for ignition is 12. HCl. they are bored into the head of the cylinder or adapted within a modified spark-plug). OH2. electrochemical techniques. A simple fix regulation may produce unwanted air/fuel ratios due to fluctuations in fuel and/or air supply (pressure or composition). whereas for instance xCO2 decreases parabolically with A-A0. through the electrolyte (a ceramic sheet of ZrO2). the engine suction itself. pneumatic and hydraulic actuators. an O2-detector in the exhaust is used to control the air/fuel ratio (an oxygen sensor is chosen because xO2 monotonically grows. The usual fuel flowrate control is a solenoid valve (a needle electrovalve).01 (domestic water-heaters work with 10. with A-A0. the frequency or the wave profile). solid fuels usually require more handling and preparation.. that is why they were placed at the exhaust manifold). To that purpose. or changes in load condition (e. are used to control the process. if any. Air flow metering with integrated temperature sensor is fed to the electronic control unit (ECU). automatically or manually operated.0. . besides. and on both sides! of λ=1.g. piezoelectric) may be part of the diagnostics. Commercial liquid fuels require pumping and filtration. and has become critical for operation of exhaust catalysts. Heating power control is based on fuel-supply control. as seen in Fig. but for purging purposes to bring the system to known safe conditions.. either on/off or modular. for that reason.50% excess air). as in vehicle engines and coal-fires burners.1 from λ=1. They are electrochemical cells yielding a voltage depending on the difference in oxygen concentration (O2. and later on the more sophisticated diagnostic techniques. 1). Air must be supplied not only for combustion. Several O2-detectors have been developed since the old λ-probe (Saab/Bosh-1977) that revolutionised electronic ignition and injection control in Otto engines. usually based on the rapid oxidation of the fuel at room temperature in the surface of a catalyst (a Pt-wire that gets hot and changes its electrical resistance in the presence of a reactive atmosphere). secondary air in a burner does not follows fuel flow rate). FUEL AND AIR SUPPLY Fuel supply Most fluid fuels are already available at a supply pressure (either bottled or piped).2). electrical. Related to fuel presence and the possibility of uncontrolled combustion is the 'explosimeter'. a fuel-gas sensor. and sometimes also heating systems. Air/fuel ratio Good control of air/fuel ratio is important in premixed flames for efficiency and polution-avoidance. neither of those additional systems are dealt with here. They only work when hot (>300 ºC.really) between the exhaust and the ambient air (highly non-linear emf..Experimental techniques are described below. Other resistive semiconductor probes have been tried without too much success (TiO2. first those related to the overall control and safety of the process. quasi-linearly (xO2=0. Manometers (diaphragm. Since 1990 all λ-probes (in the front and at the rear of the catalyser) are heated to work also when idle and at part throttle. CONTROLS AND SAFETY The general goal of combustion instrumentation is to procure a safe. SnO2).00±0. or a natural draught induced by the fuel supply. Air supply The air supplier may be a variable speed fan (the speed is varied by changing the voltage. The electrodes are gas-permeable platinum layers. besides the sensor being more expensive). combustion process for the intended use: operation or research. If not. the output depends on the operating temperature. energy-efficient and emission-free. where stoichiometry is now maintained to λ=1. some pumping should be implemented. 4. and sometimes a counter electrode too) and a small volume of an acid or base solution (the . since very light fuels like H2 or very heavy gas fuels as diethyl ether (C4H10O) will stratify a lot. and the most used method is the zirconium-oxide cell explained above. Portable system with field replaceable measuring cells are in the market capable of sensing minute concentrations of natural gas. • Flue gas emission analysers. the main and the pilot fuel supply (a manual start is needed. bends a burdom-type vapour-pressure phial. A combination of UV/IR sensors is better. Lightning and arc-welding may cause false alarms. being the method presently used in home appliances. Usually based on selective radiation emission or absorption.Fig. EMISSION DETECTORS • Gas leak detectors and explosimeters. • Ultra-violet emission (UV with λ=0. some are good for close-proximity detection and others for overall surveillance (indoors or outdoors): • Thermal. or on electrochemical cells.0. Usually based on the electrical resistance variation of a platinum wire. it is not enough to have a quantity less than the LEL in a closed room. Oxygen in the exhaust is measured to know the air/fuel ratio used. A probe that changes with temperature (bends a bimetallic strip. butane and propane.1. or in the ambient (explosimeters). Functional details of a lambda probe sensor. • Ionisation. a photomultiplier tube with an spectral filter may sense characteristic radiation emissions. It is very quick and can be automated. consisting of coated electrodes (sensing. either along pipes and combustor (gas leak). and fail-safe. due to the temperature increase caused by catalytic oxidation of the fuel-air mixture. so they are only used in large equipment. For known flame types.7 µm or better λCO2=4. to avoid explosions. reference. a wire that melts may be used to break a contact.. Other O2 and NOx sensors are based on wet electrochemical cells. It is better to use several wavelengths to discard solar radiation reflections. • Chemiluminescence.6 µm). usually the user holding a push-button while the thermocouple gets hot). simple. An emf is generated that may power a solenoid to keep both. It was the standard for small appliances as home water heaters.2. Just for safety. • Thermoelectric. Solar and lamp radiation may cause false alarms. FLAME DETECTORS Different types of fire alarms and flame detectors exist. 1. Measures the change in electrical conductivity of air through a flame (a flame is a plasma with some 1 ppm charged-particles).)..3 µm). Sometimes. • Infra-red emission (IR with λCO2=2. They are pasive devices (no need of power). All radiometric methods are expensive. in spite of the fact that it is not passive (it requires power). etc. • electrolyte); gases diffuse through orifices on the sensing face to the porous sensing electrode, reaching the electrolyte, and generating a very small electrical current proportional to gas concentration; their response time is low, and they have a consumable counter electrode. Portable system with field replaceable measuring cells are in the market capable of measuring at once flue velocity (0..50 m/s), H2O (0..30%), O2 (0..25%), CO (0..10000 ppm), NO (0..1000 ppm), NO2 (0..1000 ppm), SO2 (0..1000 ppm), differential temperature (0..1000 K) and CO2 (0..25%). Excess air and energy efficiency can be easily computed from those measurements. Smoke and particulate analysers. Usually based on light transmission, scattering or reflection, or by β-radiation absorption, or by the tribo-electric measuring principle (the tribological probe measures the charge on the particles that strike a metallic rod, which depends on the flow velocity and the concentration of the dust in the flue gas). Detector must be calibrated from time to time (according to required standards), using certified concentrations of test gases. FLAME DIAGNOSTICS The most conspicuous feature of combustion to analyse is the visible flame. Most flames flicker, and it is difficult to have a steady flame to look at: it must be protected from minute air-drafts and fuel-supply perturbations. Flames tend to get anchored to solid edges because there are ample ranges in temperature and velocities nearby, where stabilisation may take place. When flames cannot stabilise on the rim of the burner, flame holders (flow gutters in I-, V- or H-shape) are placed downstream of the fuel injectors, as in gas turbine combustion chambers where flames are stabilised in streams up to 100 m/s). Although contact heat devices may be used for flame detection and analysis as said above, optical (nonintrusive) techniques are preferred, either based on classical optical effects (ray geometry and photometry, as intensity absorption, particles, edges, shadowgraphy, interferometry, moiré, speckle, polarisation, etc.), or based on quantum effects (spectral intensity analysis: the frequency identifies the species, and the intensity gives their concentration). A generic overview of optical diagnostic techniques is given below, after flow and chemical instrumentation is presented. Laminar premixed flame experiments are the standard way to get validation data for combustion chemistry models (e.g. for pollutants and soot formation). EXHAUST CONTROL Flow rates are controlled at the intake by virtue of fuel and oxidiser supply systems; the aim here is how to provide a safe exhaust for the operation of combustors. The usual procedure is to get rid of the exhaust gases through a chimney to the atmosphere, far enough to minimise nuisance and danger (overheating, asphyxia, intoxication, deflagration). Sometimes the exhaust is cleaned to minimise emissions, but we focus here on fuel releases. Normal practice to cope with unavoidable fuel releases through the exhaust, due to lack of appropriate ignition or unexpected extinction, is to force fresh air for a while to dilute the mixture below its ignition limit, before any other trial is performed. A better procedure in experimental setups is to pass the exhaust through a pilot flame to guarantee that explosive mixtures do not build up. FLOW MEASUREMENT Flow measurement is used for accounting and for control. Basic fluid-mechanical instrumentation, as used in other applications to measure liquid tank level, flow velocity fields, turbulence level, pressure, particle size and distribution, etc., are also used in combustion instrumentation: positive-displacement counters (as the domestic gas meter), differential-pressure meters (diaphragm and venturi meters), turbine flow-meters, hot-wire anemometers (HWA), laser doppler velocimetry (LDV), particle image velocimetry (PIV), ultrasonic velocimetry, etc. The most precise flow-rate sensors are the heater-based for gases (thermal capacity) and the Coriolis-based for liquids. Advanced optical techniques are covered below. As said before, there is an emphasis on particle characterisation in combustion diagnostics, both because of the importance of soot formation and particulate emissions, and because of the importance of spray combustion, what is related to flow measurement (e.g. PIV may be used to measure single-fluid flow by adding tracer particles, or to measure particle velocities in proper two-phase flows: fuel sprays and exhaust soot). Modern high-speed digital cameras are used nowadays to better analyse two-phase flows. HOT-WIRE ANEMOMETRY (HWA) In this technique, a very small metal wire is heated and the power dissipated and temperature reached are measured (the electrical resistance depends on temperature); as the amount of cooling depends on the convective velocity of the fluid in which it is immersed, proper calibration provides a measure of fluid velocity in terms of power supply for a constant temperature difference, or in terms of temperature difference for a constant power supply. The wire is made of platinum or wolfram, and is very delicate, with diameter in the range 0.5..5 µm and lengths from 0.1..2 mm, making HWA only usable for relatively clean gases like ambient air (that is why they are usually named anemometers). Because of the geometry, a single hot wire only yields one component of the velocity field, so that multi-sensor probes are used for three-dimensional fields. Notice that by just measuring the electrical resistance, hot-wires are also thermometers. PARTICLE IMAGE VELOCIMETRY (PIV) In this technique the fluid is seeded with small particles (of order of 10 µm or larger), of the same density as the fluid, in an amount such that there are as many as possible without overlapping in the image. In two-dimensional PIV, a thin powerful sheet of light is shed on the object (preferably by a pulsed laser, to avoid heating by absorption), and the light scattered by the individual particles is focused in an image plane, where their positions are tracked to compute velocity vectors from consecutive images. Stereographic and photogrametric and holographic techniques are being used to provide direct threedimensional measurements. This technique best applies to quasi-steady, quasi-planar flows. Refractive index gradients could distort the image. Collimated light may be used to avoid parallax distortions. SPECKLE VELOCIMETRY (SV) Similar to PIV but with many more and smaller particles (of order of 1 µm) whose individual images overlap, forming speckles (a small granulation) without any meaning to direct observation; but if the speckle pattern is illuminated with a coherent light beam, Young interference fringes appear, proportional to the average particle separation, that, if subtracted from a reference speckle image, gives the apparent velocity field. LASER DOPPLER VELOCIMETRY (LDV) In this technique, the time tracers take to cross along consecutive fringes formed at the intersection of two nearly-parallel beams from a laser, is measured in a scattered-light detector. As this simple setup only yields the speed modulus in one direction (along the in-plane counter-bisect), timing shifts and multiple laser beam-pairs are used to measure the whole velocity vector at a point. In spite of its fast and precise response and non-intrusive character (most of the times there is no need to add the tracers, as common fluids as air and water always carry fine particles in suspension), LDV has the strong handicap of being a one-point sampling technique (i.e. zero-dimensional, when most velocity fields are three-dimensional). TEMPERATURE MEASUREMENT Combustion thermometry usually focuses onto the gas phase; measurement of surface temperatures in conducts and combustor walls is also important, but not so demanding (except perhaps at moving surfaces as gas-turbine rotor blades). Measuring high temperatures in a gas is a difficult subject since thermal conductance from gas to probe is poor, heat conduction through the lead wires and the metallic sheath is important, and radiative coupling between probe and walls is high. Moreover, in thin, fluctuating, reacting, hot regions (as within a flame), there is response-time problem, probe micro-vibrations problems, materials-resistance problems, and basic non-equilibrium ill-defined-temperature problems. For high spatial and temporal resolutions, fine-gauge thermocouples are the best; platinum-resistance probes (Pt-100) are too large, and thermistors (NTC) have a small temperature range. Accuracy and response time are key issues. As for any contact thermometer, one assumes that thermal equilibrium of the sensor and the object is established, but with low-conducting gases at high temperatures, losses through the probe support and to the walls are large and only a steady state is reached. The response time, of the order of 1 s for 1 mm size thermocouples, may be lowered down to 10-2 s for the smallest 10 µm wide thermocouples (at the expense of durability and accuracy). For high temperature measurement in combustion, thermocouples must be sheath with SiO2 to avoid metal oxidation and catalytic effects. But intrusive thermometers as the thermocouple cannot provide whole field temperatures in fluctuating flows, as in reciprocating internal combustion engines; knowledge of the spatial distribution of gas temperature prior to ignition is needed for modelling engine combustion with stratified load and/or exhaust gas recirculation, since large temperature inhomogeneities are involved. Non-intrusive radiometric temperature measurement is preferred for advanced multidimensional analysis, the physical probes been used for calibration. Thermometry based on gas density measured by the refractive index is easy, but resolution is low at high temperatures. Infrared thermometry based on CO2 and H2O bands, an ultraviolet radiometry at the 0.309 µm band due to OH are also used. ANALYTICAL TECHNIQUES Analytical techniques refer here to chemical-composition measurement. A short description of some of the traditional analytical techniques used in combustion follows, with the more advanced optical diagnostics being covered under the Optical Diagnostic Techniques heading, below. Sometimes the main products in the combustion of organic matter are separated before further exhaust analysis; water vapour is traditionally absorbed and weighted in phosphorus-oxide, P4O10(s)+6H2O(v,l)=4H3PO4(s), and carbon dioxide in sodium hydroxide, NaOH(s)+CO2(g)=NaHCO3(s). GAS CHROMATOGRAPHY Chromatographic techniques (gas or liquid) are based, as for other means of mixture separation, in the natural segregation of species between two immiscible phases. In gas chromatography, a small sample is diluted in an inert gas carrier (He, H2, Ar, N2...) and forced to flow through a porous media or a stationary liquid, what introduces a selective speed-lag (dependent on size and affinity of species to plug material) that allows selective elution for collection or discarding, i.e. a fractioning, as when a drop of mixed dyes spread over a tissue (the origin of chromatography). After separation, traditional chemical analysis may be performed, but, with proper calibration with known samples at standard temperature and pressure, a catalogue of retention index can be prepared, and qualitative (just by a look-at table) and quantitative (e.g. by light absorption) measures can be obtained. Taking physical samples in combustion processes is not simple because the probe may induce catalytic reactions at those high temperatures. The probe is typically a narrow SiO2 tube (down to 0.1 mm in diameter have been achieved) MASS SPECTROMETRY In mass spectrometry, a small sample is put into vacuum and bombarded with an electron beam to produce a stream of charged fragments in different proportions and mass-to-charge ratios (ion source generation), that, when subjected to a magnetic or electric field, produces a proportional deviation, i.e. a fractioning of different mass-to-charge ratios, with the intensity at the target being proportional to concentration. (RADIATION) SPECTROMETRY Note. Spectrometry is understood to refer only to electromagnetic spectrometry, i.e. spectro-photometry or spectro-radiometry; no mass spectrometry. Sometimes, spectrometry is also used as a generic name for all quantum-optical techniques. Three types of radiation effects are used in spectrometry: emission, absorption, and scattering. Either the wavelength, λ, or the wavenumber k=1/λ, or the frequency, ν=c/λ (with c the speed of light), is measured (only the frequency is independent of the refractive index). Details are covered below. Although high-resolution microwave and infrared spectroscopy are applied to optical diagnostics in combustion, visual laser spectroscopy is the most common advanced technique for flame analysis: Raman spectroscopy, Coherent Anti Raman Spectrometry (CARS), Laser Induced Fluorescence (LIF). OPTICAL DIAGNOSTIC TECHNIQUES APPLICATIONS Optical diagnostic techniques are commonly used in all kinds of fluid diagnostics, not only in combustion, but they are specially critical to flame diagnostics because of the hardship of the environment and small size. Optical diagnostic techniques may serve to study: • Geometry: fluid boundaries, moving objects, moving fluid-interface fronts, etc. • Particle size and distribution: nephelometry, Rayleigh scattering. • Velocimetry: particle image velocimetry (PIV), laser doppler velocimetry. • Thermometry: disappearing-filament pyrometry, sodium-line reversal. • Concentrations: refractometry (schlieren, moiré). The fact that most novel diagnostic techniques, both for physical analysis and for chemical analysis, are based on electromagnetic radiations, is because of the following advantages of optical systems: • They are non-intrusive • • • Whole field sensing possibility Immediate qualitative diagnosis with possibility of quantitative analysis Digital video processing automation possibility Optical diagnostic techniques naturally started by photographic recording of what the observer’s eye was watching; it first developed along classical optics, but most applications nowadays are based on quantum optical effects (spectrometry). Optical techniques have some disadvantages, however. First of all, the radiation coming from the object may go through a complex path (windows, intermediate fluids, environmental air, and so on), with optical properties not well known or controllable. Second, the optical setups used to be most delicate and expensive, but introduction of fibre-optics and afocal optics has alleviated some of these problems. Third, optical techniques need to be calibrated in a similar configuration (a laminar premixed flame is usually used as a standard). FUNDAMENTALS Electromagnetic radiation can best be understood at the microscopic level in the particle-sense (from the wave/particle dualism), as a photon gas; a photon has zero mass at rest and an energy proportional to its frequency, E=hν. The interaction of electromagnetic radiation with a material substance may be small, giving rise to thermal interactions (energy redistribution without molecular breakings; only translational, rotational, vibrational and electronic energy level rearrangement), or the interaction may be stronger (giving rise to loss of electrons in the molecule, i.e. ionisation and chemical reactions, or even causing nuclear reactions). Energy jumps in the interaction matter-radiation are widely separated in the spectra and are studied independently (the experimental techniques and equipment are different): nuclear transitions yield γ-rays independently of atomic and molecular structure, atomic (electronic) transitions yield X-ray and UV-ray (fine spectral lines) independently of nuclear and molecular structure, and molecular transitions (vibration and rotation) yield visible and IR bands (thick lines) independently of nuclear and atomic structure (e.g. ions radiate the same as neutral atoms in the visible and IR). Radiation is constantly emitted by matter at equilibrium due to the continuous energy transitions associated to the prevailing temperature. A system out of equilibrium may emit additional radiation depending on the reactions taking place inside. Most optical techniques, however, rely on the analysis of stimulated emission due to an apply coherent radiation source (laser), although the stimuli may also be a diffuse light, an electron beam, an electric or magnetic field, a spark, an arc, a flame, etc.). Stimulated radiation due to a coherent radiation source may cause: • Optical resonance when the source frequency coincides with some natural frequency of the system, and the molecules radiate at the same frequency and in all directions, without lag. • Chemiluminescence emission, when the source frequency coincides with some natural frequency of the system, and the molecules radiate at a lower frequency and in all directions, with a lag that may be >10-4 s (and then it is called phosphorescence), or <10-4 s (and it is called fluorescence). • Elastic scattering (Rayleigh scattering), when the system is excited with any frequency and emits at the same frequency with a small amplitude (some 10-3 the intensity of the source, larger at larger frequencies), and non-isotropic but lobular and polarised. • Inelastic scattering (Raman scattering, 1928), when the system is excited with any frequency and emits at the several different frequencies with a very small amplitude (some 10-6 the intensity of the source) and non-isotropic but lobular and polarised. The frequency differences with the source depends on the type of molecules, and they may be below the exciting one (called Stokes lines) or above it (called anti-Stokes lines). CLASSIFICATION Three main items must be considered in optical diagnostics: the object, the radiation source, and the radiation sensor. OBJECT TYPES According to the object, optical diagnostic techniques may be classified in two main groups: • Heterogeneous systems, basically particles (isolated or forming a mist) in a fluid matrix, and fluid boundaries. • Homogeneous systems, basically smoothly changing fluid volumes. RADIATION SOURCES According to the radiation source, optical diagnostic techniques may be classified as follows: • Own radiation emission by the object (thermal radiation and chemiluminescent radiation) • External radiation source o Visible white light, usually from incandescent lamps, but sometimes from special luminaries (Nernst, Globar, Hg, Na). o Monochromatic light, mostly from lasers (monochromators with white light yield much lower spectral power). Although it depends on temperature, typical He-Ne lasers have 10 mW and λ=0.633 µm (the iodine stabilized Helium-Neon laser has λ=0.6329914 µm), cheap diode lasers may have 10 mW and λ~0.8 µm (AlGaAs) or λ=0.65 µm (AlGaInP), powerful NdYAG lasers have 100 W and λ=1.064 µm (invisible near infrared), and the most powerful, CO2 lasers have >1 kW and λ=10.600 µm (invisible far infrared). Excimer lasers are pulsed gas discharge lasers which produce optical output in the ultraviolet (the actual wavelength can be changed by changing the gas mixture). DETECTOR TYPES According to what is detected (image), optical diagnostic techniques may be classified as follows: • Radiation origin: object own emission (by its temperature or internal processes), source own emission outlined by the object, source reflection on the object and surroundings (typical eyesight), source refraction along the object, object emission stimulated by the external source (dispersion, luminescence). • Image information: ray intensity (bright, contrast, colour and saturation of typical eye-sight imaging), ray deflection refractometry (moiré, schlieren, shadowgraphy), interferometry (radiation phase measurement), and holography (intensity and phase measurement). • Spatial dimensionality: point sampling, line of sight, bidimensional, tridimensional. • Spectral dimensionality: white light, monochromatic. • Spectral frequency band: visible, infrared, ultraviolet, microwave, etc. • Image sensor: photochemical or photoelectric. The traditional chemical photogram (Gr. photo, light, gram, message) was much used since its development by Daguerre in 1839 until the end of the XX c., that was substituted by digital photography. Photoelectric sensors may be onedimensional (photodiodes, photomultipliers, field-effect transistors) or twodimensional (vidicon tubes, charge coupled devices, CCD, are the most used). Image sensors (eye, photographic plate, CCD) are two-dimensional. RADIATION EMISSION AND RADIATION ABSORPTION The own emitted radiation from a sample can be used to diagnose hot or excited objects as flames. The radiation absorbed by an object from a known wide-band light, can be used to diagnose cold and hot objects. Molecules absorb light only at certain characteristic wavelengths called its spectrum. At infrared wavelengths, the spectrum results from vibrations of the atoms in the molecule, while at visible and ultraviolet wavelengths the spectrum is caused by the electrons orbiting the molecule. The spectrum can be calculated from quantum mechanics or measured in the laboratory Condense matter yields a continuous spectrum dependent on its temperature (it approaches blackbody radiation for dielectric materials like soot), but gases yield clearly separated spectral bands (they would require optical sizes of many metres to approach blackbody radiation). Radiation emission may be used to measure temperature or to analyse species and concentrations. If the object is cold, it can be raised to high temperatures by contact with a hot wire or by exposure to a flame (e.g. by dipping an inert platinum wire in the solution or powder sample and putting it on a Bunsen flame). Light is often said to have a colour temperature (not the real temperature of the emitting body, but the one of a blackbody that would give the same colour sensation); the colour temperature of some common light sources are: 2000 K for a candle flame, 3000 for an incandescent lamp, 4000 K for a carbon-arc or a magnesium-flash light, 6000 for direct sunlight and 10 000 K for open sky. The colloquial usage of "red hot," "white hot," and so on, is part of the colour sequence black, red, orange, yellow, white, and bluish white, seen as an object is heated to successively higher temperatures. RADIATION SCATTERING When electromagnetic radiation impinges on a material particle, some part is re-radiated at different angles from incidence (what is known as scatter). Scattering radiation depends on the size of the particle and radiation-source wavelength. The interaction may be described as follows: • For large particles (say >1 mm that is the typical minimum width of a collimated beam, very much larger than the source wavelength, λ), some part of it is absorbed, some part is reflected (both specularly and diffusively), and the rest is refracted according to the transmissivity of the particle. The geometrical optics approximation applies (i.e. ray tracing), without any influence of λ. • For medium-size particles (say from 1 µm to 1 mm) the bending of outlining light-rays, i.e. edge diffraction, has to be considered. Diffraction is the name given to any deviation from the laws of geometrical optics; it was modelled by Fresnel in 1818. Diffraction in the far zone, the Fraunhofer approximation, is good enough in this size range, instead of the full Mie scattering theory. • For small particles (say 1 µm, comparable to the wavelength of the light used, λ), some part of the incident radiation is scattered in a non-isotropic, non-symmetric lobular pattern with intensity independent of frequency, larger to the foreword, what is known as Mie scattering. Tyndall effect is due to Mie scattering, as well as other whitish effects are: clouds, dispersion opalescence, foams, mists, and dust haze (they scatter all wavelengths equally). • For very small particles (<1/10 µm, or better λ/10), some part of the incident radiation is scattered in a non-isotropic, symmetric lobular pattern with intensity proportional to the fourth power of the frequency, what is known as Rayleigh scattering. The blue colour of sky and the reddish colours at sunset and sunrise result from Rayleigh scattering. Scattered light intensity depends on species concentration and on species cross-section, so that if one is known, the other may be found (this simple technique is widely used in combustion, and named Rayleigh line scattering, RLS). Most scattered radiation has the same frequency as the incoming radiation (Rayleigh scattering), but some parts have different frequencies and are given the specific name of Raman scattering. Radiation dispersed from a monochromatic source is the best mean to diagnose flames. Scattered radiation is fractioned in a tuneable monochromator (a prism or better a grating) and focused onto a light scattered by the object in the scene is focused by a lens onto a 2D-plate.detector (visual or infrared). and later the three-dimensional structure is built with computer aid. for viewing. for viewing. Usually a reference speckle is shot ob the object before the flow starts. so that continuous subtraction to the subsequent speckles yields a sequence of refractive maps. split in two. For reconstruction. Holography can be used to visualise objects and particles. and store the interference of the reference beam with the beam scattered by the object. When two such speckle images are subtracted (optically or digitally). then through a scattering solid. developed by Gabor in 1947. and the combination is focused on an image recorder. isolated from external noise) to work. then it is combined with the other beam. is used. TOMOGRAPHY Tomography is an image synthesis technique based on building a three-dimensional visualisation by juxtaposition of many two-dimensional adjacent images (sectional views). HOLOGRAPHY Holography. irrespective of the eye position. INTERFEROMETRY Interferometry is the measuring of the refractive index field of an object phase by means of the interference pattern formed when a reference light beam is combined with the beam going through the object. flame sheets are diagnosed at a time. SPECKLE INTERFEROMETRY (SI) A coherent light is split in two collimated beams. for measuring the actual species (based on wavelength) and its concentration (based on intensity detection). the procedure is to split a coherent light beam. shine one part on the scene. what sheds a virtual replica of the initial scene ‘as if the objects were there’.e. one beam passes through the object. or to produce interferograms. the recording plate acts like a window through which the observer can peep through. . In traditional imaging. To build a hologram. Holography requires a large temporal coherence of the source light (a very monochromatic laser) to produce sharp interferences. we just look at the image with any light and we see a 2D-frozen image of the original scene. i. i. is the method for recording and reconstruction the whole information of an optical scene (intensity and phase of the light waves). Tomography is used in flame structure research to avoid the accumulated contribution from all the scene.e. a clear image of the refractive index field appears. Many different types of interferometers have been developed. and that is why a single laser beam. The refractive index field is directly related to the density of the fluid object. allowing to resolve one of them if the other is known. For reconstruction. building a planar projection of the scene proportional to light intensity without any phase information. Both beams have to be spatial and time coherent. the recorded hologram is places in a beam of the same coherent light used for recording. generating a speckle pattern (a blurred image to the eye). and large spatial coherence of the source light (a very planar beam) to enable reconstruction by any similar laser and not necessarily the recording one. what depends on temperature and composition. most of them requiring an optical bench (a very rigid setup. LASER INDUCED INCANDESCENCE (LII) This technique is used for detailed analysis of soot formation.MOIRE DEFLECTOMETRY (MD) A collimated light beam (preferably coherent) goes through an object phase. a line or sheet is shed onto the flame and only soot incandescence in that part is sampled. since the natural emission is much dimmer. RAMAN SCATTERING (RS) Raman scattering is the most used spectrometric technique of analysis. The superposition of the two fine linear gratings yields an X-pattern of intensity fringes (moiré pattern) that is distorted by the ray deflection within the object path. whereas the relative intensities amongst the lines for a single species is related to the local temperature (in reality. It measures the inelastic radiation scattered by molecules when a strong laser light is shone on them (the signal is weak). Coherent anti-Stokes Raman scattering is being used for accurate temperature measurements in research on turbine combustors. (back to Combustion) . giving only an overall picture of a three-dimensional and often unsteady phenomena. using excimer lasers at 248 nm and 308 nm tracer molecules (3-pentanone. The shifting in wavelength from the source depends on the type of molecules. The sensitivity of the instrument can be tuned by adjusting the distance between gratings and their relative rotation. that may be different to the translational temperature if there is no local equilibrium). LIF has been successfully used. whose concentration depends on the line intensities. a laser of appropriate wavelength is focused on the sample so as to excite the electronic energy levels in some type of molecules. two lasers are focused on the sample (a point. with their frequencies tuned to enhance the response of one type of molecule by the process of stimulated emission. then through two separated parallel gratings and is finally focused on an image plane. notably OH-radicals and polycyclic aromatic hydrocarbons (PAH). LASER INDUCED FLUORESCENCE (LIF) In this technique. that upon subsequent relaxation yield a characteristic emission. Visible radiation from a sooting flame comes from all regions of the flame. sheet or volume) at the same time. 10%wt) added to the fuel (iso-octane). to the vibrational-rotational temperature of the molecules. on optically accessible engines to measure two-dimensional temperature fields and fuel concentration fields. for instance. however. before and after ignition. with a powerful laser. Coherent anti-Stokes Raman Scattering (CARS) In this technique.