Disposal of waste tyres for energy recovery and safe environment—Review

PergamonEnergy Convers. Mgmt Vol. 39, No. 5/6, pp, 511-528, 1998 © 1997 ElsevierScienceLtd. All rights reserved Printed in Great Britain PII: S0196-8904(97)00044-7 0196-8904/98 $19.00 + 0.00 DISPOSAL OF WASTE TYRES FOR ENERGY RECOVERY AND SAFE ENVIRONMENT--REVIEW V. K. SHARMA, 1'* M. MINCARINI, I F. FORTUNA, 1 F. COGNINI 2 and G. CORNACCHIA ~ ~ENEA C.R. Trisaia, Environment Department, AMB-TEIN-RIF Unit, 75025 Policoro (MT), Italy and 2ENEA C.R. Trisaia, Innovation Department, INN-NUMA-TECMA, 75025 Policoro (MT), Italy (Received 3 March 1997) Abstract--This article presents the essential features of different practical means applied for the disposal of waste tyres. Considerable attention has been devoted to new, more efficient and environmentally attractive methods, such as incinerators with energy recovery systems, pyrolysis using microwave energy, etc. © 1997 Elsevier Science Ltd. Waste tyre Air pollution Energy and environment Incineration Pyrolysis INTRODUCTION The energy crisis and environmental degradation are the main problems mankind is facing today. These problems owe their origin to a growing population, rapid industrialisation and huge quantities of solid refuse which are generated daily. To alleviate part of our energy crisis and environmental degradation, it has become imperative to make use of appropriate technologies for the possible recovery of resources from non-conventional sources, like organic wastes, plastic wastes, used rubber tyres, etc. The problems related to the disposal of organic wastes available from urban, agro-industries or agricultural farms, etc. using both aerobic and anaerobic biotechnology have already been addressed by several researchers and more recently by Sharma et al. [1, 2]. The problems related to the disposal of other solid wastes such as plastic and waste rubber tyres, have also been addressed. A variety of global and national policies are also being developed and proposed worldwide. It is, however, to be noted that, no doubt, considerable research and development efforts to recover energy and useful by-products, combat relevant disposal problems, minimise pollution effects, etc. have been made in many industrialised countries, but the fact remains that, even at present (at least in most of the developing countries), a considerable portion of waste tyres is still being disposed of by using the most common techniques, such as incineration (without the recovery of value) and landfill. Both these methods lead to a waste of potentially valuable raw materials together with many other problems. Stockpiling, the other current disposal option, is even less attractive--the waste tyres are unsightly and pose a considerable fire hazard. In view of the problems caused by the combustibility of waste tyres, it is not surprising that moves are afoot to harness this energy. It has been observed that, by using novel processes for recycling of wastes, incinerators with energy recovery systems, the pyrolysis process using electric or microwave heating, etc., it is possible to conserve available resources, recover useful products (fuel, by products such as char and steel, etc.) and combat disposal problems and minimise pollution effects [3-8]. Available results indicate that the pyrolysis process has a promising future. A number of designs and their performances have already been described by various researchers [9-15], but there still seems to be many operational problems associated with the models available on the market. *Visiting scientist from New Delhi, India; to whom all correspondence should be addressed. 511 For registered vehicles Waste rubber produced from each car = 5 kg/yr Waste rubber produced from each heavy vehicle (e. stabiliser).g. truck) = 100 kg/yr Ratio between cars and heavy vehicles = 10:1 For old damaged (off the road) vehicles Waste rubber produced from each car = 20 kg/yr Waste rubber produced from each heavy vehicle (e. of old damaged (off the road) vehicles. such as incinerators and pyrolysis. corresponding to each registered and old damaged vehicle. etc. truck) = 200 kg/yr Ratio between cars and heavy vehicles = 20:1 Using the expression mentioned above. i. reinforcing agents (carbon black).: DISPOSALOF WASTE TYRES--REVIEW Given this context. experiences had so far with recent developments. COMPOSITION AND CHARACTERISTICS OF USED TYRES While considering the disposal of used tyres. to be noted that. the facts mentioned below were also considered while evaluating the contribution from both registered and old damaged (off the road) vehicles. QUANTITY OF WASTE TYRES PRODUCED IN ITALY: AN ESTIMATION Knowledge about the waste rubber tyres available in the country is essential to decide the size as well as type of disposal method. The quantity of waste rubber produced annually (in tons) can be estimated using the expression: Waste rubber tyres produced annually (t/yr)= (No. this number will certainly increase. • • • • • • knowledge about the registered and old damaged (off the road) vehicles average miles covered by each tyre average tyre life unitary weight of tyre weight reduction due to wearing number of tyres used contemporaneously. anti-oxonant. represent the average values for the waste tyres produced in a year (kg/yr). It is.512 SHARMA et al. The production of tyres uses several materials and substances. i. it is essential to be aware of the different materials and substances used in the production of tyres.g. plastificants (hydrocarbon oils). it has been estimated that nearly 380. 14 and 28. forms the tyre.e. accelerating agents (to facilitate the sulphur action). the description of the most appropriate models. mainly because of the consumption of the tread.000 tons of waste tyres are produced each year in Italy [17]. the composition . of registered vehicles. however. Emphasis is. as the population and associated automotive industries grow. protective agents (anti-oxidising. are discussed in the present review article. originates the mixture that. In addition.14 + No. given to the disposal of waste tyres using thermo-chemical processes. It is. vulcanising agents (sulphur and sulphur compounds). however. mixed with several ingredients. It is to be noted that the following parameters were duly considered while evaluating the contribution due from both registered and old damaged vehicles. A mixture is generally composed of: • • • • • • elastomer (natural or synthetic rubber). Natural or synthetic rubber.000--400. however.e. upon vulcanisation and coupling with the wire gauze.28)/1000 The numerical values used in the above equation. respectively. to be noted that. p o t e n t carcinogens. if not shredded. etc. It is true that landfilling of waste tyres for disposal purposes has a reasonably low investment and is easy to design. to be noted that devulcanisation processes. an attempt has been made to address the problem in question and to give readers (who. are involved in this technology) a broad and detailed overview of various aspects of the recovery of value from this waste. but in reality.5 16. In fact. In view of the problems mentioned above. one way or the other. the other current disposal option. they can cause instability in the landfill. Table 1. mainly because the tyres are unsightly and pose a serious fire hazard causing atmospheric pollution. as they are already collected and sorted by retreading companies. Over the past decade or so.2 1 6 Used (%) 43 21 27 -2 1 6 .SHARMA et al. DISPOSAL OF U S E D R U B B E R T Y R E S W I T H M A T E R I A L R E C O V E R Y Because of the vulcanised nature of the rubber. dioxins (toxic hydrocarbon) and poly-nuclear aromatic h y d r o c a r b o n s . it is not surprising that moves are afoot to harness this energy for electricity generation. are used to reproduce products similar to the initial ones. Using appropriate technologies.r e u s e of rubber material: using this methodology. Composition of new and used rubber tyres [16] Car Truck Tyre composition Elastomer Carbon black Steel Textile fibres Zinc oxide Sulphur Other oil New (%) 48 22 15 5 1. It is. several schemes to tackle the waste tyres problem have been proposed. This is especially true in the case of the rubber tyre for both car and truck as demonstrated in Table 1. There has been a spate of interest in tyres as people realise their value as a resource and become aware of the problems arising from their inappropriate disposal.: DISPOSAL OF WASTE TYRES--REVIEW 513 of a used tyre is slightly different from the new one. this is a waste of potentially valuable raw materials and leads to m a n y other problems. used tyres are not directly reusable in the production cycle. however. The number of landfill sites willing to accept tyres is diminishing. Stockpiling. are rarely used.. even today. including zinc oxide. devulcanisation processes.5 1 1 7. . Keeping in view the facts mentioned above. Mechanical milling. Description of the most appropriate models is discussed in the text following. The concepts under investigation can be categorised as: . waste tyres present a significant recycling opportunity.5 New (%) 45 22 25 -2.r e c o v e r y of materials and energy. to be noted that.2 1 8 Used (%) 47 21. For these reasons.5 5. the used tyre is milled to obtain a powder or a granular material having a specific granulometry. it is necessary to treat the rubber with a devulcanisation process for regeneration and reuse as virgin materials. cryogenic milling.. because of their high operating cost. the vulcanisation transforms the elastomer into a non-fusible and insoluble substance. the waste tyres. DISPOSAL OF W A S T E T Y R E S The scale of the disposal problem is considerable. upon being pre-treated. develop and operate. the vast majority of waste tyres are simply dumped or buried in landfill sites. Moreover. On the other hand. is even less attractive. as the tyres do not degrade.. are the different technologies used for this purpose. however. It is. but in later years. Primary granulator 4. Description of a mechanicaltrituration plant. ... to be noted that further research and development efforts are still needed to meet the ever increasing demand of good product quality. 1. As a result of this operation... No doubt.514 SHARMA et al.... It is. 2). Vertical depression column Fig.. Densimeter separator 7.6 0 and -100°C. shown in Fig.. mainly in many industrialised countries. .. It is true that much of the early work on tyre remoulding or retreading has been done in Italy (with more than 30% of the waste tyres retreaded). it is possible to obtain very pure materials.-. TYRE REMOULDING Tyre remoulding or retreading is nothing but the most direct form of tyre recycling. Crushing chamber 2.: DISPOSALOF WASTE TYRES--REVIEW ® ¢9 I. Elevator 3. The principal aim of using a rotating blade is to separate the rubber from the metallic part. -... a considerable variation in terms of the technology employed has been obtained. In this process. . "-'' .-. ' ]' Q Planimetric Scheme 1... the milling is generally done by rotating blades. CRYOGENIC MILLING With this process (Fig.. the low cost of the plant and the absence of air emissions are the major advantages. the rubber is cooled using liquid nitrogen at a temperature ranging between . Flow Scheme (9 ! 11 1 I I-. 1. but high consumption of electric power and limited markets for the products obtained are the main drawbacks that need further research efforts. thus... and a new tread rubber strip is fused to the old carcass by vulcanisation. Magnetic separator 6. . The concept in question is used to replace the worn rubber by a new tread section. MECHANICAL MILLING Using this technology. both due to the economic potential and significant R&D efforts. the rubber becomes fragile and. by using the latest generation plants. however.-. reliability and many technical conditions set by the world market.. the remaining tread is ground away from a tyre to be remoulded. The literature survey very clearly demonstrates the economic potential of this process. Secondary granulator 5. It has been observed that. 19 Stockingbin Floatingtable / x 211 2.13 Stockingbin 2.7 Scroll Elevator Magneticseparator 2.5 2. new waste tyres disposal options are being developed and implemented.2.6 2. pyrolysis.) seem to be the most advanced and significant disposal methods.17 2. but there still seems to be many technical.9 kg to treat just 1 kg of rubber) makes the process very expensive.1 Stocking bin 34 1 Jl 3~! []I] 2~ 21Q Fig.21 Vibratingsieve 3.2 Hopper 2. cost effective and efficient designs will be discussed in the following paragraphs.14 2. The main advantage of this process consists in the possibility obtaining very fine powder (up to one hundred microns). 1.: DISPOSAL OF WASTE TYRES--REVIEW 515 is milled very easily into very fine powder either by a disk or hammer mill.8 3.8 Belt conveyer Crusher Vibrating conveyer Elevator Cryogenicrotary chillier 3. 1. is subdivided into the following categories.10 Cylindrical sieve Vibratingfloors Magneticseparator Granulatorstorage bin 2. gasification.15 Pneumatic conveyer 2.5 3. Resource recovery and thermal treatment (incineration.1 4. economical and some social problems related to different models available in the market. Description of cryogenic trituration for the waste tyre. 2.12 Gravimetricseparator 2. 2. High consumption of both energy and liquid nitrogen (approximately 0. THERMAL TREATMENT The thermal treatment process. No doubt. a number of designs and their performance have already been described by various researchers. etc. 1.3 3. in general. It is in this context that the description of the most appropriate.2 3.9 3.4 2.9 Vibratingconveyer 2. capable of alleviating some of the more pressing problems observed in this field.7 2. .1.6 3.4 Cooling unit Rotary machine Rubberdust metering station Pneumaticcarder Hammer milling Vibratingconveyer 3. DISPOSAL OF USED TYRES WITH ENERGY AND MATERIAL RECOVERY In light of the overall environmental impact along with the drive toward energy and material conservation.11 Mechanicalsieve 2.e.SHARMA et al.10 Pne.msfic conveyer 3.18 Pneumaticconveyer 2.3.16 Stockingbin 2. i.20 Granulatormilling Floatingtable 4. • Combustion (incineration). The walls and furnace beds must be able to withstand the high temperatures (approx. corrosive action of the gases. etc. • Pyrolysis. etc. such materials have much higher heating values than municipal refuse. dimension and structural characteristics (density. a furnace and pollution control devices. It is. mineral content. plant design. In general. such as (1) maximum heat recovery. 2. it is necessary to design suitable incinerators using appropriate materials.: DISPOSAL OF WASTE TYRES---REVIEW • Gasification. The incinerator has to be designed for good burning and the prevention of soot delivery. to be noted that the following problems are associated with thermal treatment of waste tyres: • Disposal of ash: lead and cadmium salts used as stabilisers in tyre production remain as ashes. Here. (3) environmentally acceptable process. . some toxic gases. The volume of waste tyres can be reduced by more than 90%. be kept in mind that solid fuels being rich in volatile matter develop long flames and thus require the firebox to be larger than in a traditional boiler. the waste tyres. H2S. Furthermore. thus requiring additional systems for their proper treatment. becomes self-supporting. insufficient oxygen supply. heat exchange surface. such as higher temperature. resin content. charging mechanisms. manufacture and operating conditions. • Toxic gases: when tyres are burnt.. COs content. Non-polluting and capable of destroying most of the organic substances harmful to human health. are used as fuel in the incinerators. defined as the ratio of thermal energy output to global energy input. thus causing disposal problems. are generated. a storage pit and tipping area. Volatile matter content. it is the design of the furnace and its effective efficiency that play an important role concerning general combustion performance. etc. • Appropriate incinerators: to tackle the problems. are the principal functional parameters of a combustion facility firebox. structure. INCINERATION In this process. The combustion process is spontaneous above 400°C. The heat generated during incineration produces steam which may be used to heat and air condition buildings or for industrial processing or the production of electricity. require more combustion area and higher flame temperature. highly exothermic and.. Temperature. it should be stressed that high combustion efficiency. incinerator cranes. HCN. HC1. (2) low air pollution emissions. such as SO2. 1150°C) generated by the combustion process. The above mentioned thermal treatment processes have advantages. such as 1. the waste tyres are burnt under controlled high temperature combustion. therefore. The processes are net energy producers with possible material recovery. Burning refuse in steam-generating incinerators and its use as a supplemental fuel are the most advanced and proven waste energy utilisation technologies.516 SHARMA et al. A typical incinerator is comprised of a set of scales. however. once initiated. etc. usually depend upon interdependent factors. The use of refuse as a supplemental fuel in power producing plants offers many advantages. At the design stage. area/volume ratio.). excess air. are the main physical characteristics that affect boiler efficiency. • Soot: imperfect burning of the waste material will produce soot. In thermal technologies. etc. having a calorific value of 7500-8000 kcal/kg. such as the fuel's physical characteristics. In general. moisture. 3. the incineration of waste tyres may be defined as the reduction of combustible wastes to inert residue by controlled high-temperature combustion. it should. Post-combustion chamber 6 .. the waste material is fed through the feed door and deposited on the fire grate. but experimental results.Chimney Fig. The need for a post-combustion chamber and problems relevant to grate packing (in the case of melting of metallic fraction). --Rotary kiln. and thus. no doubt. However.Ash removal system 3 . 3. having a pre-combustion chamber large enough to reduce the problems relevant to thermal shock of the refractory.S H A R M A et al. the disadvantages of incinerators are: (1) (2) (3) (4) large capital investment.Rubber feed box 2 . Use of the combustion on grate kiln technology is justified economically. Continuous multistage combustion takes place in the plant. . General layout of the plant process for tyres burning using a mobile-grate kiln. caused by the high volume of gases produced. Here. (4) reduced power production cost. it is. etc. where an auxiliary burner may be employed if the temperature is low. skilled labour/operator is required to operate the system. when compared to other treatment technologies. reduces significantly the radiant effects of the flame temperature.Heat recovery boiler 7 . are much lower. especially for large-sized plants. relatively high operating cost. Concerning the size of the furnace. Combustion on grate kiln (Figs 3 and 4) Combustion on a fire grate seems to be the most appropriate technology for waste tyres treatment. It is also to be noted that the stresses on the refractory. to be noted that a large furnace generally causes high inertia. The gases produced during combustion contain particulate matter as well as incompletely oxidised compounds.R E V I E W 517 Combustible gas to the burner Tyres feeder I 3 4 _ _ ('~ 6 z Cooling air Combustion air Cooling water - l 1 1 . The thermal technologies. etc. Final oxidation of valorised products takes place in the secondary burning chamber. The gas ignited on the furnace floor is mixed with primary air. --Fluidised bed kiln.Flue-gas treatment system 4 . are the disadvantages of such a plant. in general. need of flue gas depuration.: DISPOSAL OF WASTE T Y R E S . are discussed under the headings: --Grate kiln. very clearly demonstrate the validity and acceptability of the technology. however.Mobile-grate kiln 5 . made available by different research groups working in this field. Flue g a s h e a t e x c h a n g e r 7 = Generator Fig. proven technology. The kiln is a large metal cylinder lined with refractory material with its axis slightly inclined.Ash conveyer 5 . multistage combustion. It is mounted on rollers and slowly rotated by an electric motor. whereas the need of a post-combustion chamber and particulate filtration to control the gases. such as SO2 and NOx. 4. 2. it can be stated that the rotary kiln combustion plant. The waste tyres. use of refractory having good thermal shock resistance. presents the following characteristics: 1. .Textile Pdter 6 . i. generating steam. are a few disadvantages that need further efforts to improve the system. in different sizes. Mobilegrate kiln rubber burning plant.Fan b l o w e r 7 2 . 6 ! . the need of a minimal plant size to justify the capital investment. are the main advantages. 4. So. A fluidised bed plant foresees the following principal components. possibility of using different wastes. Finally. etc. The final combustion takes place in the kiln. possibility maintaining the temperature around 800°C. thus driving off the volatile constituents. The solid residue is discharged from the kiln into the refuse collecting device.e.Cleaning of flue gas 3 . 7) is principally a combustion chamber.518 SHARMA et al. continuous removal of ashes. Hot gases generated in the process pass through the heat exchangers.V a p o u r turbine 4 .: DISPOSALOF WASTE TYRES--REVIEW Rotary kiln combustion (Figs 5 and 6) The rotary kiln furnace consists of the rectangular drying and ignition zone and rotating combustion kiln. low operating cost. are passed through the drying and ignition zone. The volatiles driven off in the ignition zone go through a passage above the kiln and join the hot gases effluent from the kiln in a secondary combustion chamber where the combustion is completed. Fluidised bed combustion The fluidised bed kiln (Fig. generally. etc. 3. The material is continuously injected into the fluidised bed furnace where it burns while being kept in suspensionby air blowing from the underside. valorisation of volatile substances. to conclude. SHARMA et al. 5.: DISPOSAL OF WASTE TYRES--REVIEW 519 . etc. G e n e r a l layout of the plant process for waste tyres burning --Fuel system. induced draft fan and baghouse. The system can be operated using the elastomer together with other industrial wastes (sludge. i.Burner 53 6 8 - I 4 79 A - box Rotary-kiln Post-combustion chamber Scrubbing tower E Calmness chamber Post-combustion burner Flue-gas scrubbing system and ash separator Sedimentation system for sludge treatment Solution for scrubbing B - Waste tyres feed C - Combustible oil to the burners D . where fine particle loading is collected before discharge of the gas to the atmosphere. . etc. uniform temperature inside the combustion chamber results in reduced NOx emission (<400 ppm). --high operating flexibility.A J i: c D Rubber feed 2 . --Control and measurement unit. thus.). gypsum. steel scraps.A i r to the burners E . producing electricity. such as --feeding of different wastes. --Fluidised bed heater. a greater differential in pressure from the inlet to the outlet of the turbine. The fluidised bed reactor has many advantages. F i g . --System for the removal of solid combustion residue (ash. The gas then passes through the mechanical dust collector. --System for sand and limestone supply for the abatement of SO2 and HC1 produced during the combustion.Water F - Recycling water for scrubbing using horizontal rotary kiln. the efficiency of the unit is improved because the cooling water produces a vacuum and. Ash and sand material will be removed from the bottom of the unit and taken to the town landfill. The superheated steam generated in the boiler is directed to the steam turbine connected to the generator. --low levels of harmful emission. By directing the steam exhaust into a condenser. wood. --System for gas cleaning and its removal. the evaporator and economiser section of the boiler.) and coal. Hot gases and light particulates from the process are raised in the furnace and channelled to superheaters. --high combustion efficiency (> 98%).e. '" "i . Chimney 2. . 4 5L] t" • . the waste material is heated further. but it still has a few disadvantages. Post combustion 9. elimination of the post-combustion chamber makes the system more economical. and they can be converted into energy to either sustain the process or produce excess power. 8. an endothermic process (thermal degradation of materials caused by heating in an oxygen-free or oxygen deficient atmosphere). --possibility for the over packing of low melting ashes. General layout of Eneal's horizontal rotary kiln combustion plant installed at Marangoni (Rovereto. --considerable feedstock preparation (necessity of trituration). chemically. can be used as fuels within the pyrolysis system or for an adjacent plant such as a combined heat and power system. Boiler 7. up to the required temperature. are sent back to the retort for volatilisation). As mentioned above. Textile filter 3. producing lower emissions of nitrogen oxide and sulphur oxide than incinerators--the conventional technology. Italy). PYROLYSIS Pyrolysis. Product to be recycled Fig. pyrolysis also allows valuable materials to be recovered. generally begins with preheating of the shredded materials fed to a retort.: DISPOSAL OF WASTE TYRES--REVIEW In addition. the gas is cleaned by liquid separators. Supply line 4. as shown in Fig. 6. The oil and gas. for gas production. The products of pyrolysis represent about 50% of the initial volume of the organic matter. Primary reactor 5. the technology seems to be the most appropriate for the treatment of waste tyres. offers an environmentally attractive method of reducing the world's waste tyre backlog. depending upon the process. --lack of experimental data at the commercial level. No doubt. Vapour 8. in turn.~i 1 q 1. such as --high operating costs. Char and steel are also recovered from the process. for example. the machines use high temperatures and an oxygen free environment to decompose tyres. Steel scrap 6. To remove oils or tars (which.520 SHARMA et al. The process. In the retort. By recycling rather than burning. IMPORTANT INSTALLATIONS In order to exploit waste tyres as a potential resource for electricity generation as well as raw materials. Chimney Fig. CO. can be used to heat the pyrolysis reactor. Elevator 5. Limestone storage 8. Burner starter 2. The percentage of both oil and gaseous fractions depends upon the operating conditions. Tyre grinder 4. H2S. whereas the gaseous fraction is composed of non-condensable organics. . The oil. CH4. gas. H2.S H A R M A et al. Convective section 3. oil residue. several small to medium scale projects have been undertaken in different industrialised countries. It has been observed from experiments conducted by researchers in different countries that an increase in reactor temperature results not only in a corresponding increase in the heating value of the product but also the percentage increase in H2. on the other hand.: DISPOSAL O F W A S T E T Y R E S . can be used as a fuel in a conventional furnace. therefore. the pyrolysis of waste tyres has had limited application world-wide but has been used at both pilot scale and full scale with a considerable degree of success. It is further to be noted that the oil fraction (mainly composed of olephinic and aromatic hydrocarbon) can condense at normal pressure and temperature. Textile filter 10.R E V I E W 521 Used tyres 2 Effluent gaseous output 8 6 9 lo ® ~---_ Secondary air Primary I. Fluidized -bed furnace 9. The much larger volumes of air and vapours and grit and dust emissions associated with the incinerator are. still operating are given below. Description of a fluidised bed tyres burning plant. To date. The non-condensable gas. Pyrolysis is not a combustion process and is performed in closed vessels. is mainly composed of light hydrocarbons and. such as temperature and pressure in the pyrolysis reactor. hence. CH4. having a heating value of the order of 10. such products are available as: --33% --35% --12% --20% char fraction. CO and other hydrocarbons. etc.000 kcal/Nm 3. Tyre's storage bin 6. NH3.. Sand storage 7. metallic fraction. not present. is used as a fuel or for the production of activated carbons or as an inert additive in the rubber. The other important by-product. 7. Brief descriptions of a few such successful plants. char. residence time and type of catalyst employed. Generally. : DISPOSAL OF WASTE TYRES--REVIEW ?/ t i o v 1 e~ e~ "E e~ I .e t:h 06 t .522 SHARMA et al. 110 dm 3 in volume and 0. has been designed and constructed by Elm Energy and Recycling (U. whole or shredded tyres are fed continuously via two purge locks to the reaction zone where they meet hot recycled gas. The vapour is passed through a condenser to give fuel oil. These are followed by a radiant chamber which heats up to 30-40% of the steam. (D) Pyrolysis pilot-plant for the treatment of plastic/rubber wastes A pilot scale pyrolysis plant has been installed at the ENEA research centre. Trisaia. requirements.C. The emissions are considerably lower than those found in coal fired plants. The operation of the plant is based upon the technology that utilises waste rubber tyres. The residue left in the crucible consists of scrap steel. in Italy. it would also be possible to destroy other organic materials. and the process gives dry products. with a weight fraction of about 17%.: DISPOSAL OF WASTE TYRES--REVIEW 523 (A) 30 megawatt Elm's electricity generation plant The 30 megawatt electricity generation plant.6 F . The microwaves break down the tyres. In contrast to Beven's thermal pyrolyser. by using microwave techniques.000 tons per annum. plastics and hazardous chemicals. the main product here in the system under investigation is the high value product called activated carbon. 1992. readers are requested to see Energy Digest. Vol. The airtight lid is lowered into the kiln. for the very first time. it is felt that. nearly 100. and coke (roughly 40%). Each of these consists of three pulse hearth furnaces designed to give a long combustion time at 800°C. Waste material is introduced at one end of the kiln and rolls slowly down the length of the kiln after successive rotations. It is heated externally by using electric elements surrounding the reactor chamber. can pyrolyse up to 1000 tpy. A greater throughput of the batch process can obviously be achieved by running several units in parallel.SHARMA et al.) Limited.K. BRC claims a carbon recovery of 36%. The pyrolysis unit. It is felt that. first of its kind in Europe as well as the largest individual generation of electricity from renewable energy sources. Use of advanced flue gas clean up systems ensures the emission level is well under control as well as within E. a one tonne batch of whole or shredded tyres is charged into a crucible which is lowered into the reaction vessel.4 m in diameter. and the gas fractions can be fed back to the boiler to help heat the kiln. 9.c.). with microwaves. The plant is still under thorough experimentation. On a mass balance. The pyrolyser operates under a positive pressure of nitrogen (300 mm w. The concept of an incinerator with an energy recovery system has been used while constructing the plant based on five incinerators. Any sulphur in the rubber from the vulcanisation process can be removed from the carbon by acid washing. In the process. side by side. The operating design capacity of the plant is to generate nearly 30 megawatts of electricity. air leakage through the system is negligible. (C) BRC Environmental's 5000tpy pyrolysis plant A pyrolysis plant has been designed by BRC Environmental to handle 5000 tpy of waste tyres. For more detailed technical and performance information. At the beginning of the experiment. developed at AEA Technology's Harwell Laboratory for Hebert Beven & Company. and the tyres are heated to 450-600°C for about 10 h. Here. the waste material is fed into a hopper with a maximum ECM 3 9 / 5 . This separates the vapour into fuel gas (hydrogen and methane) and light and heavy oils. shown in Fig. The idea of using microwave energy has been proposed. 21(2). A reburn tunnel at approximately 1000°C burns the remaining gases and then passes to a boiler plant. and the vapour passes to a quench cooler and distillation column. (B) AEA's lO00tpy tyre pyrolysis plant A pyrolyser. there are no problems of heat transfer. In addition. consists of a reactor of the rotary kiln type. for electricity generation without polluting the environment while producing valuable reusable by-products. notably clinical wastes. Transport of the granular material can be assisted by tilting the axis of rotation of the kiln a few degrees so that the charge end is higher than the discharge end. 9 t~ u~ .o < ro Z m ~o .: DISPOSAL OF WASTE TYRES--REVIEW |< °o °° oo .o o 0~ .rb" 8 ..2"°T " ~ .524 SHARMA et al. hermetically sealed and equipped with a screw feeding device. 12.for treatment of hazardous waste This system (Figs 10 and 11). etc. To remove the fuel oils and cool the gaseous stream further. both located before the wet scrubbing system. The non-condensable gases from the jacketed pipe condenser are filtered (using a demister filter) and are passed through a wet scrubbing system designed to remove acid components by NaOH (4%) injection. shown in Fig. is treated in a jacketed pipe condenser.: DISPOSAL OF WASTE T Y R E S .3 kPa. catalytic cracking and reduction and oxidation reactions..R E V I E W 525 holding capacity of 400 dm 3. 10. (E) Gas([ication/pyrolysis plant . could be mixed homogeneously.acuum pyrolysis pilot plant [19] The objective of the pilot plant built and tested during the fall of 1987 in Saint-Amable near Montreal was to minimise secondary reactions such as thermal cracking. the output from the scrubber. i. the problems mentioned above could be solved by combining both systems together.e. The reactor was externally fired with gas and a small portion of the Sewagesludge Fuel gas Gas Cleaning [ I Oxygen I Plastic waste Shredder fluff Tyres Householdwaste Gascleaning IM. (F) SahTt-Amable t. repolymerisation and recondensation reactions. Such gases are monitored on line. using both the concepts of pyrolysis and gasification. Feeding of the material across the reactor is continuous until the hopper becomes empty. mentioned above.p_oonc°" Metals I Slag st!° Wastewater 1 Sulphur Fig..s. .S H A R M A et al. oil and coke. encountered in gasification. Because of the fact that. The principal objective of designing such a hybrid system was to overcome problems such as fineness of grinding and supply of homogeneous material. material of size up to 200 mm could be fed and the products obtained from the pyrolysis. gas phase collision. Subsequently. the cleaned fuel gases along with propane gas (as a supporting gas) are combusted in a flare to supply the thermal energy required to heat the process nitrogen used for oxygen removal. the maximum feeding rate is 48 kg/h.++. by process gas chromatography and with sampling equipment for laboratory analysis. The system. The solid residue of pyrolysis (char) is continuously discharged in a water cooled tank. to about 30C. in pyrolysis. has been designed. The gaseous products formed during pyrolysis pass through a scrubber that removes tar and cools the gases to about 250'~C. developed and experimented by researchers from VEBA OEL Technologies in Germany [18]. using the concept of vacuum pyrolysis. Block diagram of combined pyrolysis and gasification plant for waste treatment [18]. was designed to decompose continuously 200 kg/h of steel belt tyres at a pressure of 1. 526 S H A R M A et al.: DISPOSAL OF WASTE T Y R E S .9 ..R E V I E W ~ ~o~ ~ ) 2 o~ b 2Z]~_ o r~ 0) =b (D (D 5---- ~m ~o t:h ~ I~ ~ u A~ O b c~ ~ E~ .o 5~ u .I3 8 r© © < G ~b b r~3 GA © © u~ . for stadia and for new rubber articles. however. is to be used in the elastomeric mixture. Cooling screw 11. Crusher 13. till today.S H A R M A et al. In Great Britain. In Germany. The end products would be used in the preparation of asphalt mixtures to be used for road covering. The carbon black was recovered at the bottom of another water head which connected the bottom of the reactor and the ground. Vacuum reactor 10. is planned. at Durham and Grantham. 3. with an overall capacity of 170. Heavy oil quencher 16. Two other incinerators. an incinerator for the treatment of waste tyres.r ~9 ) 1.000 tpy of waste rubber tyres. Similarly. pyrolysis oil. The vapours were quenched by two scrubbers set in series.. the project has been working continuously. 8. (G) Other installations In California (U. Fig. 12.: DISPOSAL OF WASTE T Y R E S . to be installed at Wolverhampton. Vibratory screen 14. 5. Schematic of the Saint-Amable pilot plant.000 tons per year is under thorough experimentation.000 tons per year. to be noted that. Ethos Rubber Inc. having a high content of zinc oxides.e. The plant has been constructed jointly by Continental and Lurgi GmbH.R E V I E W 527 . The flue gases are to be treated with a fabric filter and the powder. Discharge screw 12.000 and 9000 tons of waste tyres per year. research and development efforts are in progress concerning the use of waste tyres not only to solve our environmental problems but also to produce electricity. low rate of the heat transferred to the rubber per unit area of reactor surface. The plant has a capacity of 100. i..t---- 12 8 II ( . has decided to realise a new trituration plant for tyres available from trucks and cars. in other European countries. respectively. Light oil quencher Decanter Vacuum pump Flare stack Heavy oil storage Light oil storage Magnetic separator Steel recovery I I C Q.).. Feed conveyer 9. The purpose of this plant is to produce 20 megawatts of electric power.. .i.ii:: 0 000 / . 4.A. 7..:~.S. Carbon black handling system 15. 6. with overall treatment capacity of 24. 2. It is. an incinerator for the treatment of waste tyres with an overall treatment capacity of nearly 50. besides the main limitation of the system. are also in the construction phase. R. Elsevier Applied Science. p r o d u c t i o n o f corrosive combustion products. 248.. F. and Notestein. K. A t present. Fluidized bed combustion of scrap tires. S. 1990. both technical and operational. Analytical and Applied Pyrolysis. Shang. P. 1989.. in short-to-medium term perspective.: DISPOSAL OF WASTE TYRES---REVIEW CONCLUSIONS F r o m the discussion made so far concerning the use o f different treatment technologies to tackle the waste tyre problem with energy and material recovery options. Walter. and Taylor. 1474. 13.. Istituto per l'Ambiente. 125. and Pollesel. 15. V. and Scialdoni. 12.. 1994. and Sinn. M. P. Energy Convers. K. F. Eng.. 1996. E. M. 40--41. Cornacchia. 3. Besler. Department of Energy.. Canditelli. Private communication. M. in general: 1. 1978... 4. 6. T. D. and Unsworth. Renewable Energy. etc. E. Sharma. 35. P.528 SHARMA et al. 203. J. 94074. 30. Buekens and A. 1997. U. 1981. and Taylor. . however. to be noted that m a n y problems.. July. The pyrolysis process (though in its initial R & D stages). C. Rausa. L. 72. London. having higher energy recovery values than incineration (70% vs 41%) and also being attractive environmentally. from ICTP. 11. Crosio. R. G. DOE/METC-8614068. L. Coronidi. need to be solved before the said process could satisfy the reliability and economic conditions set by the world market. J. D.. 29. Mincarini.. 1995. V. CR Casaccia.B-TEIN-G-004. 48. ENEA. G.A. Corodini. Pederzani. Working Group on Recycling. G. 1. Marani. B. C. 19. K. Pyrolysis and Gasification. J. 5. Carugati. 38(5). Analytical and Applied Pyrolysis. Roy. Journal. Furlan. Milano. 107.. 180. and Pollesel. 69. R. 2. ed. Conservation and Recycling. Risparmio Energetico. 1994. Kaminsky. A. and Roseveare. and Ranaldi. V... As far as pulverisation o f tyres is concerned. H. Proc. Mgmt. seem to be the most advantageous both technically and economically. 1997. Chem. Berti. O. 423. Maniatis. Castelluccio. Canditelli. FISIA/A.. no doubt. V.. 1991. T. Fortuna.p.. Fortuna. Process Engg Environmental Protection. W. Trieste for his constant encouragement throughout the course of this work. and Cornacchia. Chang. Technical notes from Annual Report No. 138. Fuel. C... Acknowledgements-43ne of the authors (VKS) would like to express his sincere thanks to Professor G. 1997. Allott. T.S. F. M. incinerators with energy recovery systems. Fortuna. Y.. M. 1992. 1997. Bridgwater. and Scialdoni. 17. M. Vol. 1990. 453. 18. Bass. the following conclusions could be drawn.. ACS Symposium Series No. RT/AMB/90. A. Rapporto 92/03.. Rome. R.. Milano. 16. it is to be noted that. ISSN/1120-5555. Fuel. 23.. 3. Labrecque. Relazione Finale. T. 10. 1990. REFERENCES 1. excessive heat generated during combustion. K.. 130. Sharma. are a few problems that need futher attention. Mincarini. and Senneca. the cryogenic pulverisation process appears to be the m o s t appropriate one. but more air requirement. Capetti. Resources. 8. and Sharma. 1469.. seems to be another potential solution for possible energy as well as useful by-products recovery. Relazione tecnica. P. 2. J. 1993. William. M.. however. no. Eniricerche S. and De Caumia... and Cornacchia.. 40--41. 14.. higher flame temperature. Published work cited in the present document is also gratefully acknowledged. J. Resources Conservation and Recycling. Williams. 1980. 9. G. Ferrero. 403. E. Mei. M. it can. C. K. 45. J. G. W. p. K... 7. 1991. An Int. It is... be stated that incineration with energy recovery is o f greater manufacturing and operating complexity and is better suited for large power plants.. G. In reference to the thermal destruction technologies. B. 4. 17. Roy.A. lnd. notwithstanding the problems encountered which show further development w o r k to be undertaken.