Fuel Gas Efficiency BMP - Efficient Use of Fuel Gas in Engines (Module 7).pdf

May 2008Disclaimer This publication was prepared for the Canadian Association of Petroleum Producers, the Gas Processing Association Canada, the Alberta Department of Energy, the Alberta Energy Resources and Conservation Board, Small Explorers and Producers Association of Canada and Natural Resources Canada by CETAC-West. While it is believed that the information contained herein is reliable under the conditions and subject to the limitations set out, CETAC-West and the funding organizations do not guarantee its accuracy. The use of this report or any information contained will be at the user’s sole risk, regardless of any fault or negligence of CETAC-West or the sponsors. Acknowledgements This Fuel Gas Efficiency Best Management Practice Series was developed by CETAC WEST with contributions from: • • • • • • • • • Accurata Inc. Clearstone Engineering Ltd. RCL Environmental REM Technology Inc. Sensor Environmental Services Ltd. Sirius Products Inc. Sulphur Experts Inc. Amine Experts Inc. Tartan Engineering CETAC-WEST is a private sector, not-for-profit corporation with a mandate to encourage advancements in environmental and economic performance in Western Canada. The corporation has formed linkages between technology producers, industry experts, and industry associates to facilitate this process. Since 2000, CETAC-WEST has sponsored a highly successful ecoefficiency program aimed at reducing energy consumption in the Upstream Oil and Gas Industry. Head Office # 420, 715 - 5th Ave SW Calgary, Alberta Canada T2P2X6 Tel: (403) 777-9595 Fax: (403) 777-9599 [email protected] Table of Contents 1. Applicability and Objectives ............................................ 1 2. Basic Improvement Strategies......................................... 2 2.1 2.2 2.3 2.3 Technology and Equipment Types of Fuel Improving Efficiency Training and Expertise 3. Inspection, Monitoring and Record Keeping................ 10 4. Rapid Feasibility Assessment of Reciprocating Engine Components ....................................................... 11 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 Number of Operating Hours Ignition Type Spark Plugs Fuel Composition and Quality Engine Governor Control System Type Dry Paper Element Air Filtration System Oil Bath Air Filtration System Exhaust System Backpressure Inlet Air Variances Catalytic Converter Lean Air-Fuel Ratio Engine Utilization Engine Management Systems 5. Operational Checks, Testing and Ajustments.............. 16 5.1 5.2 5.3 Operational Checks Determining BSFC monitoring Fuel Efficient Operation 6. Appendices...................................................................... 22 Appendix A Appendix B Appendix C Appendix D Appendix E Appendix F Appendix G Brake Specific Fuel Consumption Emission and Greenhouse Gas Aspects Engine Considerations Fuel Gas Meter Requirements Case Studies Glossary of Terms References Figures Figure 2.1 Engine Heat Source Figure 5.1 Logic Diagram: Engine Fuel Consumption Optimization Figure A1 Reciprocating Engine Part Load Efficiency Figure A2 Reciprocating Engine Part Load BSFC Figure A3 Small Stoichiometric Naturally Aspirated Reciprocating Engine BSFC Figure A4 Large Stiochiometric Naturally Aspirated Reciprocating Engine BSFC Figure A5 Medium Stiochiometric Naturally Aspirated Reciprocating Engine BSFC Figure A6 Large Turbocharged Stiochiometric Reciprocating Engine BSFC Figure A7 Medium Turbocharged Stoichiometric Reciprocating Engine BSFC Figure A8 Very Large Turbocharged Lean Reciprocating Engine BSFC Figure A9 Large Turbocharged Lean Reciprocating Engine BSFC Figure A10 Medium Turbocharged Lean Reciprocating Engine BCFC Figure B1 Typical Emissions vs Air-Fuel Ratio Tables Table 2.1 Target BSFC in Natural Gas Fuelled Reciprocating Engines Table 5.1 Engine Condition Checklist Table 5.2 BSFC Calculation Table A1 Engine Fuel Comparison Table A2 Engine Fuel Delivery Methods Table A3 Heat Content of Fuel Table A4 Reciprocating Engine BSFC at Various Speeds and Loads Table B1 Emission Components Table B2 Nitrogen Oxide Emission Standards (BC) Table B3 Emission Component Importance Factors Background The issue of fuel gas consumption is increasingly important to the oil and gas industry. The development of this Best Management Practice (BMP) Module is sponsored by the Canadian Association of Petroleum Producers (CAPP), the Gas Processing Association Canada (GPAC), the Alberta Department of Energy, Small Explorers and Producers Association of Canada (SEPAC) Natural Resources Canada (NRC) and the Energy Resources and Conservation Board (ERCB) to promote the efficient use of fuel gas in engines used in the upstream oil and gas sector. It is part of a series of 17 modules addressing fuel gas efficiency in a range of devices. This BMP Module: • • identifies the typical impediments to achieving high levels of operating efficiency with respect to fuel gas consumption; presents strategies for achieving cost effective improvements through inspection, maintenance, operating practices and the replacement of underperforming components; and identifies technical considerations and limitations. • The aim is to provide practical guidance to operators for achieving fuel gas efficient operation while recognizing the specific requirements of individual engines and their service requirements. 3. ne 4. um m en ts 5. Ch em ica lI Ch em ica lI Pn a Fl a r t ic I ns in g t ru m en ts tru e 4. um a Fl a r t ic I ing ns 5. tru C nj ec tio 3. P 6. F 6. F 6. F 7. 7. he m ica lI 7. 1. Ga 2. ther Pu ing Sy 3. mpj s Pn ac ks tem eu s 4. m at Fl i c ar ing Ins 5. m nje ire ct d io H En ea n Pu m gin ter ps 8. s Co es m p 9. Gl ress io yc 10 ol n .D De es hy ic 11 dr . F can t D ato ue rs eh lG y as M dr at o ea su rs re m en t MODULE 7 of 17: Engines nje ire c d He tion En P gin ater ump 8. s s Co es m p 9. Gl ress io yc 10 ol n .D De es hy ic 11 dr . F can t D a to ue 12 l G ehy rs dr . F as at M ra e 13 cti as ors u o .R na rem ef 16 rig tion ent .T er at ai lG ion a en ts s EFFICIENT USE OF FUEL GAS IN THE UPSTREAM OIL AND GAS INDUSTRY ire d H En ea n Pu m gin ter ps 8. s e Co s m p 9. Gl ress io yc 10 ol n .D De es hy ic 11 dr . F can t D a to ue rs 12 l G eh yd . F as r a M r 13 act eas tors . R ion ure a m e 14 frig tion ent . A er at m 15 ine ion .S 16 ulp . T hu r a 1 7 il G R e c a .A o s cid Inc ver G iner y as a In tion je cti on In c in er at io n 1. Applicability and Objectives This module provides guidance for operating staff to recognize when fuel consumption is higher than the minimum achievable for a specific application. The determination of fuel gas efficiency is made by prescribed calculations that yield the efficiency of the engine based upon the fuel input and power required for the application. The majority of engines use sweet natural gas or sales gas for fuel but may also use propane or diesel. This module is applicable to all types of fuel but differences in heating values must be considered for efficiency calculations. The information provided in this module serves to outline opportunities for optimization of engines used in the upstream oil and gas industry. Tools are provided for supervisory and operations personnel to evaluate engines and not only identify, but quantify opportunities for optimization. Engines are devices used to convert fuel energy into mechanical energy. The mechanical power is for loads such as gas compressors, pumps, electrical generators and other devices. The mechanical power from the engine equals the power required by the load. This module outlines and quantifies opportunities for operations and supervisory personnel to optimize engines based on their load requirements. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 1 of 70 2. Basic Improvement Strategy The most significant elements of long-term operating efficiency are the application of best available technology, implementation of operating and maintenance systems and management commitment. Efficient operation of rotating equipment requires: • • • • • knowing what each equipment element was designed to do and what it is currently required to do, the conduct of periodic checks and adjustments, routine testing and correction of abnormalities, assessment of opportunities to install upgrades and replacement of inefficient equipment, and retention of records. 2.1 Technology and Equipment The first step in moving toward higher levels of fuel gas efficiency should always be to understand what the engine was designed to do and what modifications have been made since it was placed in service. This should provide an early indication of the suitability of the installed equipment for the service and if the equipment is likely to be able to meet the prescribed performance standards. Knowledge of the equipment will also help to identify what changes may be required to achieve higher levels of fuel gas efficiency. Following this, efforts should be made to bring the installation in line with manufacture‘s specifications for the installation, use and maintenance of the equipment. Section 5 of this Module provides guidance for the assessment of engines. Guidance on assessing the driven equipment is contained in Module 8: Compressors. The two main types of engines available are reciprocating and turbine. Each has characteristics that allow a designer to make a best choice according to the application requirement. In the upstream oil and gas industry the most common fuel is natural gas. The following discusses the variation in engine types according to: • • • • • fuel type, natural gas variations, separable engine types, integral engine types, turbines. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 2 of 70 Proven processes and technology are available to both optimize and sustain equipment operation. The important issue is to recognize an opportunity for application to the technology in this regard. More importantly is to consult with industry expertise to understand existing technologies. Engine Uses Engines are employed in upstream oil and gas applications to drive a variety of equipment. The selection of the driver is dictated by the lowest cost engine that fits the duty of the application. Considerations governing engine choice are speed (rpm), fuel quality, altitude, ambient temperature and the characteristics of the load. Speed can be adjusted by gears to suit the requirements of the driven equipment (increase or decrease). The complexities of the gear box and torsion aspects add cost and maintenance requirements. Engine rated speed is also related to the size of the engine. Reciprocating engine speeds vary between 1,800 RPM for the smaller engines (up to 1,000 HP) and 900 RPM for the larger models (up to 4,500 HP). The typical maximum operating speeds for the most popular reciprocating engine models in upstream applications is 1,800 RPM (80 to 600 HP), 1,200 RPM (700 to 1,800 HP) and 1,000 RPM (1,800 to 3,000 HP). Gas turbines naturally run at much higher speeds. Smaller models also run faster than larger models. The rotor tip speed needs to be higher on the smaller models to maintain efficiency. Turbine operating speeds in excess of 10,000 RPM are common. Fuel quality will also dictate the choice of engine. Upstream oil and gas applications often take fuel from the process stream and it may not be sufficiently conditioned to meet the engine manufacturer’s standard. Reciprocating engines are quite sensitive to fuel quality. The manufacturer will specify a power de-rate to accommodate hot fuels (higher in propane and butane). Limits on the H2S content of the fuel are also imposed. They specify sweet, clean, dry fuel gas with a methane content of over 90% (typically at 900 Btu) for most engines to develop full nameplate power. Gas turbines are more accommodating to different qualities of fuel. The manufacturer will configure the engine to burn a wider range of Btu value fuels as well as gas with much higher levels of H2S content than a reciprocating engine. Elevation and ambient temperature play a significant role in developing all the rated power for an engine. Increasing the ambient temperature of the combustion air will decrease the density of the air. Thus less combustion air enters the engine to mix with the fuel than at the rated conditions (factory test cell). The engine manufacturers de-rate the nameplate power for warmer combustion air. Similarly, higher elevation has the same affect by reducing the air density. Turbochargers and superchargers will offset the affect of higher Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 3 of 70 temperature or elevation on power de-rates by increasing the benchmark at which the manufacturer will apply the power reduction. The magnitude of power de-rate is specific to engine model and manufacturer. Power de-rate must be considered for the speed and demand of the load (driven equipment). Engines can be used to drive portable or stationary equipment. Upstream applications are primarily stationary applications, albeit the equipment may be mounted on a mobile base. The reciprocating engine design allows for the conversion of a reciprocating motion as noted by the up and down travel of the piston to a rotary motion using a lever action against a shaft that is placed at right angles to the input force. This conversion to rotary motion provides a constant force that is capable of spinning a driven member for the purpose of doing work. Appendix C provides a detailed discussion of engine design and fuel implications. The natural gas powered reciprocating engines used within the industry range in power from 5 to 10,000 brake horsepower. The operating speeds vary from 500 rpm to 1800 rpm. Engine displacement ranges up to *285 L / 17,400 cubic inches of displacement (*example: Waukesha 16V-AT27GL). The first reciprocating engines incorporated an integral design with the compressor frame. These engines incorporated a large diameter cylinder bore and long piston stroke. The volume of the air-fuel mixture contained within the cylinder for combustion places limits on the speed at which the engine can operate. The greater the volume of the air-fuel mixture within the cylinder the more time required to complete the combustion process. The rate at which the flame front spreads through the air-fuel mixture to initiate and complete the combustion process limits the engine speed. The greater the volume of air-fuel within the cylinder consequently more time required to complete the burn process. High speed engines are designed with small bores and short strokes. The burn process is completed very quickly and can be repeated more often allowing for higher operating speeds. The industry requirements expanded from large plant settings where gas was drawn from the fields for processing and compression to meet sales line requirements. The shift towards the installation of compression closer to the gas fields and in some cases wellhead locations drove the development of equipment that is more readily installed, easily moved and provides a better match up to the driven equipment. The combined engine and compressor approach did not offer the versatility required to meet the oil and gas industry needs. Common integral engine manufacturers include Cooper Bessemer, Clarke, Dresser Rand, and Ajax (currently produced). The upstream oil and gas sector contains a limited population of this equipment. Since the typical worker will not encounter integral compressors we will not discuss detailed fuel efficiency aspects. General engine principals will, however, apply to the engine side. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 4 of 70 The modern high speed natural gas fuelled engine design is capable of providing the same horsepower as the much larger integral design. The rotating components of the new high speed engine design are lighter in weight which reduces the parasitic engine horsepower loss related to maintaining inertia forces. The new high speed designed engine provide smaller contact surfaces where the piston rings meet the liner bore, offer reduced areas of the crankshaft bearing areas, a smaller camshaft drive train and lighter cylinder head valve springing. The result is an engine design that delivers the same horsepower but in a much more efficient manner that provides the benefits of reduced fuel consumption rates. Today the use of separable equipment is prevalent throughout the industry with a few exceptions that are designed to accommodate site specific applications. The focus of this study is therefore dedicated to the separable engine drivers. Turbine Engines Turbine engines are constructed with three basic processes. The first process is an air compressor that takes in air and compresses it in a centrifugal wheel compressor. The second process is combustion. Fuel is added to the compressed air and blended to a stoichiometric mixture and then ignited. The third process is expansion. The ignited fuel enters another centrifugal style wheel (power turbine). Gas expansion drives the wheel and produces engine power. Gas turbine efficiency range is typically 25% to 35%. Efficiency tends to increase as the size of the turbine increases. They are renowned for low efficiency and complex controls and drive systems. Running speeds are high and gear boxes may be required to reduce the speed for the driven equipment. The affects of torsion is an important consideration when evaluating loads for turbine engines. The attractive aspects of the gas turbine are high reliability, long service intervals and flexible fuel use. The gas turbine can burn a variety of fuels with minor changes to the engine. They are capable of burning gasoline, diesel, propane and natural gas. It is also capable of burning sour natural gas fuel without damage to the engine (environmental considerations notwithstanding). This can make it an attractive engine option for remote sites without utility infrastructure that are producing sour natural gas. Many models and sizes of industrial gas turbines are available. The market is dominated by six major manufacturers. Between them, they produce about 40 different models. The power range for traditional machines (excluding microturbines) starts at about 1000 HP and extend to the tens of thousands. They are often specifically designed for the driven application and sold as a modular assembly. Typical applications include generators, centrifugal and other Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 5 of 70 rotary compressors and sometimes reciprocating compressors. They are most commonly found in mainline transmission booster service or large process applications. Their use in upstream applications is limited. As such, they comprise a low population in upstream oil and gas installations. We will discuss gas turbines here for general information and to acquaint the reader with their characteristics. We will not discuss detailed fuel efficiency for these engines since they are typically not encountered. The engine manufacturer should be consulted for specific training for those workers who need to operate gas turbines. 2.2 Types of Fuel The most commonly available fuels in the upstream oil and gas industry are • • • • propane, diesel, gasoline, natural gas. Normally reciprocating engines are designed to operate on one type of fuel, although some are able to operate on two fuels. Natural gas is the most common fuel source in upstream oil and gas operations. It will be considered alone for the focus of this study. A comparison of fuels is provided in Appendix A. The fuels are delivered in different methods according to the fuel type. With a carburetor the air and fuel are mixed before going into the engine cylinder, whereas with fuel injection the fuel is added either to the air when the intake valve is open or directly to the engine cylinder. To convert the fuel type for an engine, consult specialists in this field. Conversions An engine designed for natural gas can use propane, with adjustments and viceversa. A gasoline engine can be converted to natural gas or propane with a change to the fuel delivery device. Dual Fuel A diesel engine can be converted to dual fuel – natural gas and diesel (typically >90% natural gas) with a conversion kit, but cannot be easily converted to only natural gas. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 6 of 70 Similarly turbines are designed for a specific fuel type. Conversion from one fuel type to another may require extensive modification. 2.3 Improving Efficiency Decisions to carry out adjustments or replace components should be made on an individual basis with consideration for health, safety, environmental and economic considerations. Where adjustments to existing systems are practical, these should be carried out at the time of testing. At that time, minor component replacements should also be undertaken. When equipment shut down is required to undertake improvements, the repair/replacement may be delayed until the next planned shutdown provided this does not pose any safety concern. The purpose of an engine or motor is to provide mechanical power to the load. Efficiency is the fraction of the rate at which mechanical energy is produced (kW or HP) compared to the rate at which energy is used by the engine or motor. For an engine or turbine the efficiency is the output in kW divided by the heat input in kW (1 kW = 1 kJ-s) of the fuel. The losses are the exhaust heat and friction. For engines and turbines a more common rating is the brake specific fuel consumption described below. In a natural gas engine or turbine, only some 30 to 40% of the fuel energy is converted to mechanical power. The remainder of the energy is given off as heat as shown schematically below. Exhaust gases at 500°C+ 30 to 35% input energy Coolants at 80°C+ 30 to 35% of input energy Figure 2.1 Engine Heat Sources Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 7 of 70 Brake specific fuel consumption, BSFC, is the ratio of the rate of heat energy going into an engine to the mechanical power produced by an engine. For a turbine or engine the rate of energy comes from combustion heat content of the fuel, which is commonly measured per hour (h) in units of Btu/h or kJ/h. The ratio of this to the power output is (Btu/h)/HP = Btu/HP-h or (kJ/h)/kW = kJ/kW-h, also known as the Brake Specific Fuel Consumption (BSFC). For engines the energy in is the lower heating value (LHV) of the fuel rather than the gross (or higher) heating value (GHV)i. The power out is that delivered at the engine crank, commonly known as the Brake power. For a detailed analysis of BSFC calculation refer to Appendix A. Brake Specific Fuel Consumption varies dramatically for each size of engine and the nature of its combustion system. BSFC in units of Btu/BHP-h typically ranges from 6,500 to 11,000 depending on speed and load. These values are the lowest achievable fuel consumption based on lab test results by the engine manufacturers in ideal conditions. Fuel consumption for engines in the field can easily be up to 30% higher than the ideal conditions. Tables and graphs are included to illustrate the expected performance of each engine type and a discussion of sources of inefficiency. Key factors in optimization are the controls to deal with changes to conditions that affect the engine. Requirements and opportunities with engine management systems are reviewed. Variable operating ranges and a variety of engine combustion systems do not offer a single target measure for fuel efficiency. Table 2.1 shows target BSFC values for various sizes of engines. The table represents natural gas fuel in reciprocating engines under full load and at full speed. We have imposed a 20% premium on the OEM (Original Equipment Manufacturer) values to determine the best possible fuel consumption in field conditions. This premium is based on field test results and our experience with field engines over a variety of applications. Table 2.1 Target BSFC in Natural Gas Fuelled Reciprocating Engines Size and Combustion System Large Naturally Aspirated Stoichiometric Medium Naturally Aspirated Stoichiometric Small Naturally Aspirated Stoichiometric Large Turbocharged Stoichiometric Medium Turbocharged Stoichiometric Very Large Turbocharged Lean Large Turbocharged Lean Medium Large Turbocharged Lean BSFC (Btu/BHP-h) 9,240 9,480 9,840 9,360 9,120 8,040 8,760 8,760 Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 8 of 70 Appendix A offers detailed performance characteristics over a range of speeds and power outputs. Appendices B, C and D offer information on regulatory aspects with respect to emissions regulations, greenhouse gases, fuel meter requirements, and safety. Section 5 of this module provides guidance for assessing performance deficiencies and possible corrective actions. The Appendices provide information on the factors that can have significant impact on engine efficiency. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 9 of 70 2.4 Training and Expertise It is important to include training and education in any efficiency enhancing program. When an operating system is properly understood, correctly operated and adequately maintained the operations group will be in a better position to provide feedback. An understanding of the equipment and operating scenarios that may impact the operating efficiency is critical to identifying additional opportunities for improvement. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 10 of 70 3. Inspection, Monitoring and Record Keeping Operators should have a record program to support the company’s rotating equipment testing and improvement system. Proper record keeping should assist in ensuring that sub-optimal equipment is identified and that appropriate follow-up actions are implemented. This information will also assist in establishing the assessment frequency for each piece of equipment to achieve cost-effective fuel gas efficiency improvements. Although each company will define its record keeping system, an effective program will include the following information: • • • • • data sheets for each package, expected fuel gas consumption for each engine, records of changes for each unit that have been performed efficiency testing results and economic analysis performed on underutilized engines that have not been adjusted or modified. Record keeping in support of a company’s fuel gas estimates, where measurement is not provided, may be audited by the ERCB to assess compliance. In addition, records need to be maintained to demonstrate compliance with ERCB Directives related to NOx and SOx emissions. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 11 of 70 4. Rapid Feasibility Assessment Reciprocating engine operation requires the management of many variables. This section provides a discussion of the variables that affect fuel consumption in common engine types. Good equipment condition is a critical element in reducing fuel consumption; however, this document is not intended to provide a detailed engine maintenance schedule. That task is considerable for each model of engine and is best left to the equipment manufacturers. Metrics for evaluating fuel efficiency and how fuel efficient operation can be maintained are the focus of this discussion. Operations personnel who understand the variables are better equipped to enhance equipment performance and maximize component service life. The engine condition and fuel consumption will vary with the following aspects. 4.1 Number of Operating Hours Maintain records of when the equipment was installed, last overhauled or nearing a major maintenance milestone. Maintain records as to what items were checked, adjusted, repaired or replaced. Improperly adjusted and worn equipment uses more fuel than new. 4.2 Ignition Type The first generation ignition systems are far less efficient than modern systems. A modern, adaptive ignition system will reduce fuel consumption. 4.3 Spark Plugs Engines may be fitted with spark plugs originating from different suppliers. The service life of spark plugs will vary due to materials used in their manufacture, fuel composition, engine loading and engine speeds. Another factor to consider is the condition of the ignition system firing the plug. An often overlooked performance related problem is the spark plug installation procedure itself. The spark plug body will deform if over tightened during installation. Failure to use a torque wrench is the leading cause of this failure. An over-tightened sparkplug will allow combustion gases to escape up through the plug interior and cause overheating and voltage leakage as carbon trails develop. An improperly tightened spark plug is also a safety concern as it can separate from its casing and leaves the combustion chamber open to atmosphere. Plugs that are not seated properly will not transfer combustion process heat through the plug to the water cooled engine casting. Failure to achieve this heat transfer will result in Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 12 of 70 poor engine performance. The use of precious metal spark plugs will increase the plug life. Using high quality spark plugs suitable for the application will reduce fuel consumption. 4.4 Fuel Composition and Quality Changes in the fuel gas supply may occur during operation as wellhead gas supply streams co-mingle. Well flow adjustments or the introduction of new well flow will impact the fuel stream and fuel gas composition. Engine ignition timing may be affected by fuels with a higher energy value which may reduce the available power. Conditioning the fuel and installing an adaptive engine management system can help to restore full power to the engine and reduce fuel consumption. 4.5 Engine Governor Control System Type Mechanical governor systems are still in wide use as they are inexpensive, easy to maintain and simple to adjust. Mechanical governor systems are also prone to speed instability. The amplitude of instability will increase as the linkage wears and therefore a means for improved and sustained speed stability is needed. An electronic governor system will provide that type of control. A number of electronic governor control systems are available for replacement of the existing equipment or on newer engines as factory installed equipment. The purpose of an engine governor is to adjust engine speed to match the load. Over time, as the governor linkage wears, a wider range of speed variance will develop. The speed instability allowed by the worn linkage permits the engine to surge. The maximum speed attained during a surge event may exceed the high engine speed set point and shut the unit down. Typically the operator will reduce the operating speed to avoid these nuisance shut down resulting in sub-optimal engine utilization and inefficiency. A narrow speed control band will allow the engine to operate closer to the maximum or desired speed. Engine performance and the driven equipment production can also be optimized by having a tight range of speed control. Improved utilization of the existing equipment that avoids adding more power, will result in increased operational efficiency and ultimately less fuel consumption. 4.6 Dry Paper Element Air Filtration System Research by manufacturers has caused a shift away from the oil bath filtration system towards the dry paper elements. The easily replaced or reusable chemically treated paper pleated air filter elements are proven by engine Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 13 of 70 manufactures to provide a 99.9% filtration capability. The high efficiency offers the best protection for the engine and is not impacted by ambient air temperatures as compared to oil bath filtration systems which call for seasonal oil changes. An optional feature is the air filter differential pressure sensor that will shut the engine down should the filter element restriction increase signalling that the filters require attention. A differential pressure increase across the paper element filters will result in poor engine performance, power loss and increased fuel usage. 4.7 Oil Bath Air Filtration System Oil bath air filtration systems have a long service history and remain in widespread use. Ensuring that these filters perform properly requires regular monitoring and maintenance. The oil reservoir oil level must be maintained within specifications and debris accumulations in the reservoir sump must be removed. An over-filled reservoir or an incorrect oil viscosity will result in oil being drawn into the intake manifold and being consumed within the combustion chamber. This will impact exhaust emissions. Failure to properly maintain the filter will allow airborne impurities to enter into the engine. Poorly maintained oil bath filters do not restrict the air flow rate but render the design characteristics of the oil bath system ineffective. The entrance of airborne particulates and other debris such as insects has been shown to increase liner and piston wear rates. This increases the rate of oil contamination which leads to increases in engine wear and results in higher fuel consumption. 4.8 Exhaust System Backpressure The amount of exhaust backpressure will vary among engine types and the individual installation design. Elevated backpressure in the engine crankcase will affect the engine performance. Engine manufacturers provide specifications to ensure that system design is within guidelines. Engines that begin to exhibit higher than normal backpressure need to be inspected for exhaust system restrictions or internal failures. Increased exhaust system pressure indicates inefficiencies which increase fuel consumption. 4.9 Inlet Air Variances The temperature of the intake air, moisture content, and barometric pressure constantly change. Very warm inlet air temperature reduces the efficiency of the engine and increases the fuel consumed to produce the same power due to a decrease in air density. Cooler air, being denser, allows more air to enter the combustion chamber so the available power will increase; however, extremely cold air may result in turbocharger surge on engines with more than one Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 14 of 70 turbocharger (a warm air draw system is normally recommended for cold climates). 4.10 Catalytic Converter A catalytic element is required to treat the exhaust stream on stoichometric engines when their emissions are higher than allowed by regulatory limits. Catalytic converters and the associated control systems require ongoing maintenance and condition monitoring to ensure optimal performance for emission reduction and ultimately fuel gas savings. Emissions treated by the catalyst are typically lower than those from a pre-chamber lean burn engine design at first but then increase over time. If a catalytic converter is used, the exhaust emissions will be reduced but without a savings in fuel gas consumption. The air-fuel ratio is maintained quite rich to maintain hot operating temperatures and a supply of unburned hydrocarbons for efficient catalyst performance. This achieves the required emission reduction but has a cost of higher fuel consumption. It should be noted that turbocharged rich burn engines can be converted to operate with leaner air-fuel mixtures using certain engine management systems. This “rich to lean” conversion offers the benefits of lower fuel consumption (typically 10% to 18%) and emissions without the penalty of full lean burn problems. It also avoids the use of a catalytic converter and extends the service life of the engine. Details may be discovered in the PTAC report on the REMVue system qualification listed in the references 4.11 Lean Air-Fuel Ratio Lean burn engines use a very lean air-fuel mixture to reduce emissions. The airfuel mixture is typically maintained at approximately 30:1 and this is so lean that a spark plug will not ignite the mixture. To overcome this problem, a prechamber attached to the main combustion chamber is filled with a stoichometric mixture of air and fuel. The mixture in the pre-chamber is ignited by the spark plug and a flame front travels into the main combustion chamber to ignite the lean air-fuel mixture. Since their introduction, the lean burn engines have a reputation in the industry as being difficult to start and costly to maintain. Fuel consumption is reduced on lean burn engines compared with rich burn engines. 4.12 Engine Utilization Low engine power utilization can affect engine reliability. Turbocharged engines in particular are susceptible to oil leaks and coking when loaded less than 60% of the power rating. Carbon deposits and head damage can also result when any Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 15 of 70 engine is run at low speed and load. However, increasing the speed will consume more fuel than required for the application. Options should be considered for modification or replacement of the engine for circumstances where low loads affect engine reliability. 4.13 Engine Management Systems Any engine will benefit from installing an engine management system. It provides adaptive control and, as a result a savings in fuel consumption. The engine management system allows the operator to change the engine tuning from “best power” to “best fuel” and maintain stable engine operation. Engines tend to be more sensitive to load changes (and shut down) when run at leaner air-fuel ratios (“best fuel”). Factory installed engine controls are less refined and control best at one set point. Engines, however, are required to operate over a wide range of conditions, loads and speeds which results in considerable efficiency loss. Typically a 3% to 5% fuel consumption reduction is available due to more stable and adaptive engine control alone. An appropriate engine management system is required to maintain a leaner air-fuel ratio and reduce fuel consumption as much as possible. Automatic engine control capability eliminates the periodic adjustments required with other systems (i.e. on going carburetor adjustments to attain emission levels). Optimized operation is assured at all times with an adaptive control system. Another benefit of adaptive control is that the engine speed and driven load are not set back to accommodate transient upset conditions. This can produce incremental production gains and fuel gas savings. When a compressor performance measurement system and process control valves are added to an engine, more sophisticated automatic compensation routines may be developed. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 16 of 70 5. Operational Checks, Testing and Adjustments The impact of performance and service requirements on efficiency necessitates the identification of what constitutes good performance for each piece of rotating equipment over the range of process conditions that are normally encountered. This may require a compromise to maintain the desired operation of one piece of equipment over another. Attaining these levels of performance may not be possible in all cases especially when a unit is grossly oversized in its current application. Units that cannot meet the objective are candidates for upgrade or replacement. The variety of variables and components make fuel efficiency testing a complex effort. An engine checklist and logic diagram is presented in Table 5.1 and Figure 5.1 to help operations personnel sort through the various details to optimize an engine. The order of tasks is designed to make the process as efficient as possible and lead the worker through the controls and capacity issues described in Section 4. It also identifies which conditions dictate a change in control or when physical parameters must be changed to accommodate a repair or replacement option. The goal is to use only the engine fuel required to suit an optimized application. 5.1 Operational Checks The first part of any fuel consumption efficiency effort is to understand how the equipment is operating. Indicators of efficient operation can be gathered from engine power utilization, intake manifold pressure, cylinder temperatures, fuel quality and other aspects. However, these all require further analysis to allow the operator to understand how efficient an engine is running. A more accurate and intuitive measure of efficiency can be derived from monitoring the brake specific fuel consumption (BSFC) of the engine. Any change in engine efficiency or fuel quality will trigger an increase in the BSFC. The purpose of the checklist is primarily to ensure that the engine is operating properly. These checks can be performed during a site “walk around”. They require instrumentation installed on the equipment or portable measurement devices. They should be recorded on daily basis depending on the availability of operating personnel. The time to carry out the visual checks is in the range of 30 to 60 minutes. Efficiency tests are a formal survey of equipment performance. Such testing will establish the performance of the rotating equipment and identify where action needs to be taken to improve performance. The driven equipment must also be optimized as discussed in Module 8: Compression in order to optimize the engine. An efficiency test may require several hours or days to perform. An operator must assess the results with adjustments and retesting to determine the Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 17 of 70 affect of each change. The task can be overwhelming for large fleets in changing field conditions. An automated dynamic software tracking system is recommended to assist the operator with assessing equipment life and ranking the candidates deserving attention. The logic diagram will also help to work through the efficiency test. Operations personnel may wish to add the parameters identified in Table 5.1 to their daily log data sheets. 5.2 Determining Brake Specific Fuel Consumption BSFC is defined as Btu/ (BHP*Hr). It is a convenient efficiency measure because it provides a uniform basis for comparison that includes the power and the energy value of the fuel. In order to determine BSFC, the engine must be equipped with an individual fuel gas meter, gas property data, power and a means to perform the calculation. We have provided a table that will assist the worker with calculating the BSFC. Appendix A details fuel property calculation and various conversion factors. A smaller BSFC number means lower fuel consumption. The calculation requires fuel flow measurement that can be converted to the total energy consumed (Btu value of the gas must be known). The power used in the denominator is calculated based on power used at the time of the measurement. Several tools may be used to predict power but these must be calibrated against an actual measurement. A “recip trap” or similar diagnostic tool can determine the power used by a reciprocating compressor. The OEM (original equipment manufacturer) compressor software may then be tuned to produce the same results as the measured values. Screw compressors are not as easy to measure the actual power. A field torsional measurement may be used or the manufacturer can sometimes advise the accuracy of their software power predictions. In any event, the engine manufactures’ power predictions using manifold pressure are not sufficiently accurate. They advise that it should be used as a guide only. It is important to measure the fuel consumption of each individual engine. Using one meter for a variety of engines will mask inefficiencies in individual engines. OEM fuel predictions from their test data is also virtually impossible to reproduce in the field. Actual fuel consumption can be as much as 30% higher than the factory predicted values. The information required to determine the OEM fuel predicted for comparison purposes is also included in Appendix A. Knowledge of the typical deviation will allow use of the OEM prediction as a guide for comparison of unknown engines (i.e. new to the fleet, moved or replaced). Field fuel consumption results may also vary between seemingly identical models due to subtle mechanical design differences and load variables. Engine fuel gas streams may be derived from a single common source, blended at the header or from another source. The compressed gas stream is also subject to variables such as header inlet design, outlet pipe design and plant routing. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 18 of 70 Table 5.1 Engine Condition Checklist Site Location: Unit Number: Engine Model Driven Equipment Model: Unit In Service: (Y/N): Item Hours from last major overhaul Hours from last top end overhaul Hours from last service Fuel pressure Fuel constituents Ignition timing Engine speed Speed control swings Engine misfiring Air filter elements Oil bath air filter Exhaust back pressure Catalytic converter Air-fuel ratio Engine management system Greenhouse gas emissions Driven equipment optimized Engine power utilization Control valve position Adaptive controls active Date: Time: Serial #: Serial #: Outside Temp Activity Record hours Record hours Record hours Check pressure Take sample & cf last sample Record set point Record RPM Record ΔRPM (swing range) Yes, no? Check condition and ΔP Check condition and bypass Check engine crankcase P Check condition and ΔP Record mixture Active? Record settings Attach exhaust analysis Yes, no (last check date)? Record power used Record position Yes, no? Records General Observations and Conditions: Operator: Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 19 of 70 Figure 5.1 Engine Fuel Consumption Optimization Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 20 of 70 Site Location: Unit Number: Engine Model Driven Equipment Model: Unit In Service: (Y/N): Item Fuel gas density Date: Time: Serial #: Serial #: Outside Temp Activity Values lb/scf from gas analysis OR (if gas density not stated) Fuel gas specific gravity No units (SG; relative density) Density of air lb/scf at standard conditions 0.0871 Fuel gas density lb/scf = SG * 0.0871 Fuel mass flow lb/hr Fuel flow rate from mass flow scf/h = mass flow÷gas density OR (if flow rate is stated on a volume basis) Fuel flow rate as volume flow scf/h = volume flow rate Fuel gas heating value BTU/scf from gas analysis Btu/h Volume flow * heating value Power Horsepower (calibrated calc) BSFC (calculated) Btu/BHP*h = Btu/h÷Power Engine speed Record RPM Power Horsepower (calibrated calc) BSFC (theoretical) Obtain from OEM data Actual vs OEM BSFC Actual BSFC – OEM BSFC Table 5.2 BSFC Calculation General Observations and Conditions: Operator: Conversions: 1 Btu/HP-h = 1.414 kJ/kW-h 1 HP = 0.7457 kW 1 Btu = 1.05435 kJ Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 21 of 70 5.3 Monitoring Fuel Efficient Operation Trending BSFC can show changes in engine performance and give indications of overall engine health. In order to trend BSFC, the engine must be equipped with an individual fuel gas meter and a means to perform the calculation. Refer to section 5.2 for a detailed method of manual calculation. A trend with decreasing BSFC means that fuel consumption is decreasing. The converse is true for increasing BSFC. Workers should review the BSFC and efficiency of their engines each month. If time and resources do no permit a monthly review then a quarterly review should be undertaken as a minimum. If conditions for the driven equipment or the engine have changed then the BSFC should be reviewed again to determine a new baseline. It takes a concerted effort to monitor engine performance and the influences that affect changes in engine operation. Continuous vigilance is needed to maintain optimized operations. Operators of large fleets will recognize that this is a daunting task. An automated software system approach to monitoring engine performance with an integrated operator interface is recommended. The calculation can be incorporated in the control panel software. Gathering the fuel gas flow rates is made easier if the panel can log the data on a real time basis. Estimating load using predetermined algorithms is also easier when performed in the panel. It is imperative that engine manifold pressure is NOT used as the power prediction. Tests have proven that power can be substantially overestimated using that approach. REMVue panels produced by REM Technology Inc (A division of Spartan Controls) contain a proven algorithm that is calibrated to the actual measured power. It is accurate, reliable and scalable. Data from the panel can be incorporated into the log data collection system for review by operations personnel. Another approach is to install a centralized software program that will calculate the parameters based on electronically communicated log data. Continuous monitoring is then possible with warnings when the BSFC drifts from the desired value. Fleet management software is available that present ranked operating and efficiency data for review. This is by far the most practical approach to trending unit and engine performance. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 22 of 70 Appendix A Brake Specific Fuel Consumption The most commonly available fuels in the upstream gas and oil industry are: • • • • propane, diesel, gasoline and, natural gas. Normally reciprocating engines are designed to operate on one type of fuel, although some are able to operate on two fuels. A comparison of these fuels is provided in Appendix A. Table A.1 Fuel Comparison Fuel Type Propane Diesel Gasoline Natural gas Electricity Costii $0.90 per litre $1.00 per litre $1.20 per litre $6.00 per GJ $0.08 per kW-h Density kg/l 0.51 0.85 0.73 Cost per GJ $38.48 $27.73 $37.12 $6.00 Cost per 1000 HP-h* TBD $276 TBD $44.26 $65.00 * Cost of producing 1000 HP of mechanical power for 1 hour; a brake specific fuel consumption of 7000 Btu/HP-h is used for propane, gas, and natural gas engines, while a brake specific fuel consumption of 0.152 kg/HP-h is used for diesel enginesiii. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 23 of 70 The fuels are delivered in different methods according to the fuel type. Table A 2 Fuel Delivery Methods Fuel Type Propane Diesel Gasoline Natural gas Fuel state Gas Liquid Liquid Gas Fuel delivery Carburetor or Fuel injection Fuel injector Carburetor or Fuel injection Carburetor or Fuel injection Typical compression ratio 8 to 1 14 to 1 8 to 1 8 to 1 With a carburetor the air and fuel are mixed before going into the engine cylinder, whereas with fuel injection the fuel is added either to the air when the intake valve is open or directly to the engine cylinder. To convert the fuel type for an engine, consult specialists in this field. Brake specific fuel consumption, BSFC, is the ratio of the rate of heat energy going into an engine to the mechanical power produced by an engine. For a turbine or engine the rate of energy comes from combustion heat content of the fuel, which is commonly measured per hour (h) in units of Btu/h or kJ/h. The ratio of this to the power output is (Btu/h)/HP = Btu/HP-h or (kJ/h)/kW = kJ/kW-h, also known as the Brake Specific Fuel Consumption (BSFC). For engines the energy in is the lower heating value (LHV) of the fuel rather than the gross (or higher) heating value (GHV)iv. The power out is that delivered at the engine crank, commonly known as the Brake power. Useful conversion factors 1 kW = 1.341 HP 1 kJ = 0.9485 Btu 1 kJ/kW-h = 0.7073 Btu/HP-h 1 HP = 0.7457 kW 1 Btu = 1.05435 kJ 1 Btu/HP-h = 1.414 kJ/kW-h The percent efficiency is given by: % efficiency = 100*2546/BSFC where BSFC is in Btu/HP-h or % efficiency = 100*3414.4/BSFC where BSFC is in kJ/kW For example the rating 7200 Btu/HP-h = 100*2546/7200 = 35.4 % efficiency. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 24 of 70 The BSFC allows the performance of an engine to be expressed in a way that is independent of the engine type and size. The smaller BSFC indicates better engine efficiency. The BSFC of an engine or turbine in service may be determined from the following measurements: • • • fuel composition (gas analysis), engine fuel consumption rate – volumetric or mass flow, engine power output (HP). The table below allows a user to calculate the heating content of the fuel either on a mass or volumetric basis. Table A3 Heat Content of Fuel Volume (molar) Fr 0.9441 0.0023 0.0002 0.0003 0.0001 0.0001 0.0002 0.0003 0.0028 LHV 3 MJ/m 33.85 60.25 55.79 86.16 81.21 111.67 112.06 137.67 137.85 163.91 189.83 11.93 10.19 21.84 0.046 0.001 0.0026 Density at 15 C kg/m3 Gas 0.678 1.271 1.271 1.865 1.865 2.458 2.458 3.051 3.051 3.645 4.231 1.188 0.085 1.441 1.185 1.355 0.169 1.691 1.860 0.760 1.000 LHV Vol Amt 3 MJ/m Vol(fr)*33.85 Vol(fr)*60.25 Vol(fr)*55.79 Vol(fr)*86.16 Vol(fr)*81.21 Vol(fr)*111.67 Vol(fr)*112.06 Vol(fr)*137.67 Vol(fr)*137.85 Vol(fr)*163.91 Vol(fr)*189.83 Vol(fr)*11.93 Vol(fr)*10.19 Vol(fr)*21.84 0.000 0.000 0.000 0.000 0.000 0.000 Column Total MJ/m3 LHV Mass Amt MJ/kg Vol(fr)*49.89 Vol(fr)*47.41 Vol(fr)*43.90 Vol(fr)*46.20 Vol(fr)*43.55 Vol(fr)*45.43 Vol(fr)*45.59 Vol(fr)*45.12 Vol(fr)*46.18 Vol(fr)*44.97 Vol(fr)*44.87 Vol(fr)*10.04 Vol(fr)*119.57 Vol(fr)*15.15 0.000 0.000 0.000 0.000 0.000 0.000 Column Total MJ/kg Component Methane Ethane Ethene Propane Propene Iso-Butane N-Butane Iso-Pentane N-Pentane N-Hexane N-Heptane Carbon Monoxide Hydrogen Hydrogen sulphide Nitrogen Oxygen Helium Argon Carbon dioxide Water vapor Sums Formula CH4 C2 H6 C2 H4 C3 H8 C3 H6 C4H10 C4H10 C5H12 C5H12 C6H14 C7H16 CO H2 H2 S N2 O2 He Ar CO2 H2 O Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 25 of 70 Then the heat content of the fuel going into the engine is either: Fuel in heat rate = Mass flow rate (kg/h) * LHV (MJ/kg) = MJ/h OR Fuel in heat rate = Volume flow rate (m3/h) * LHV (MJ/m3) = MJ/h To convert to Btu/h multiply by 1054.35 To convert to kJ/h multiply by 1000 The engine or turbine output power can be determined from the load. The total engine load consists of three parts: • • • the load – generator, compressor or pump, the cooling fan, if driven by the engine, auxiliary equipment such as an auxiliary water pump and electrical generator if driven by the engine. The engine output is the sum of the load power + fan power + auxiliary equipment power. Then: • • BSFC (Btu/HP-h) = Fuel in heat rate (Btu/h) / Engine output (HP) BSFC (kJ/kW-h) = Fuel in heat rate (kJ/h) / Engine output (kW) For a generator, the generator output may be measured in kW and multiplied by a factor 100/ (100 – Effg) where Effg is the generator loss percentage to account for friction and heating. For a reciprocating gas compressor, the load may be measured with a compressor analyzer with an allowance for friction losses, or calculated from the measured process parameters and cylinder specificationsv. For a screw compressor, each manufacturer has load calculation software. As a note of caution, such software is normally valid for the slide valve position at 100%, and may be in error for slide valve positions less than 100%. Each pump manufacturer has a proprietary load calculation method. The power consumed by the cooling fan operating at the rated speed is normally available from the manufacturer. If the fan pitch is unchanged, the fan load varies as (RPM)3, so, as an example, a fan with a load rating of 45 HP at 1200 RPM will, at 1000 RPM, consume (1000/1200)3*45 = 26 HP. In the absence on details on the cooling fan a reasonable value is 3 to 4% of the rated engine load for an enginecompressor combination and 2% of rated engine load for a generator load. The Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 26 of 70 power consumed by auxiliary equipment should also be estimated and included. Frequently these are relatively small (< 10 HP) and not included. In special cases, items normally included in the engine package, such as the oil pump and main coolant pump are powered by line powered electric motors. In such cases the engine rated power is larger than the manufacturer’s specification. Engine Ratings A manufacturer normally supplies an engine rating for continuous use. Often there are temperature (coolant and ambient) and altitude de-rating factors. Check the manufacturer’s manual to determine if these de-ratings apply for a particular site. Part Load Efficiency An engine has to power various engine components such as the water pump, oil pump, cam drives and overcome internal friction, no matter what the external load. At part loads, a larger fraction of the fuel input energy is used to power these engine components compared to full load operation. The graph below is an example of the reduction in engine efficiency, as defined by crankshaft output, at part loads. BSFC - Turbocharged Stoichoimetric 10000 9500 BTU/HP-h 9000 8500 8000 7500 7000 40% 60% 80% % Load 100% 120% 1200 RPM 1000 RPM 800 RPM Figure A.1 Part Load Reciprocating Engine Efficiency The efficiency loss occurs for all engines at part load. The amount depends on the engine type and the BSFC curve for the engine. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 27 of 70 W aukesha 7042 Efficiency relative to full load 1.05 Relative Efficiency 1.00 0.95 0.90 0.85 0.80 0.75 40% 50% 60% 70% 80% Turbocharged Naturally Aspirated 90% 100% 110% Percent Load Figure A.2 Load Reciprocating Engine BSFC The amount of power developed by an engine tends to be proportional to the RPM. Hence the rated output load also is proportional to the RPM. The power used by the engine components and friction also become smaller as the RPM becomes smaller. Hence, if the full engine output at the rated RPM is not required, it is more advantageous with respect to efficiency to operate an engine at reduced RPM but at a higher percentage load. This is shown below by BSFC as a function of percent load (at that RPM) for a range of engine RPM values. For example, if the required output power required for the engine is 900 HP, the table below shows the expected BSFC for operating at 3 different speeds. Table A.4 Reciprocating Engine BSFC at Various Speeds and Loads Engine Speed 800 1000 1200 % load 91% 73% 61% BSFC 7550 8000 8500 Efficiency Comparison Reference 6% worse 13% worse If, for example, the clearances on the compressor cylinders can be adjusted to achieve the desired flow at reduced RPM, an engine efficiency improvement can be expected. BSFC and Engine Type The brake specific fuel consumption rating is valid for both 2 and 4 stroke cycle engines. It is a valuable way to compare the performance of different engines. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 28 of 70 Generally the BSFC becomes smaller (engine is more efficient) as the displacement per cylinder becomes larger. Also, the air-fuel ratio control can affect BSFC and efficiency from differences in combustion completeness and heat loss during a combustion cycle. An engine with lean combustion, excess air, will operate with a lower BSFC (more efficiently) than an engine with a stoichiometric air-fuel ratio. This occurs because the excess air with lean combustion ensures the fuel is fully combusted in the engine cylinder, whereas with a stoichiometric engine there is both unburned fuel and carbon monoxide (CO) in the exhaust gases. These components are more fully combusted in the exhaust catalyst, producing heat in the catalyst. In other words, some of the fuel combustion occurs in the catalyst where it cannot contribute to engine power. The BSFC of such engines are generally higher than for lean combustion engines. If an engine uses a mechanically driven blower or super-charger for intake air compression, this reduces the crankshaft power available for the load. Such engines have a BSFC some 1500 to 2000 Btu/HP-h larger than the equivalent turbo-charged engine. An exhaust driven turbocharger does not increase the BSFC as an exhaust driven turbocharger gets its energy from the hot exhaust gases produced by the engine. Pre-combustion chambers (PCC) are small volumes adjacent to the main combustion volume, where the spark plug ignites a richer mixture than is in the main chamber. The burning gases ejected from the PCC into the main chamber readily ignite the air-fuel mixture in the main chamber. PCCs are used on larger engines to overcome the difficulty in igniting a lean air-fuel mixture and the reduced flame speeds of lean mixtures. Since the relative volume of the PCCs is small relative to the main chamber, there is little effect on BSFC compared to open chamber designs with no PCCs. BSFC for Different Reciprocating Engine Types and Sizes The types of reciprocating engines are • • • naturally aspirated, turbocharged stoichiometric, turbocharged lean. The sizes for purposes of this discussion are • • • • small (bore < 4.0 inches), medium (bore 4.5 to 6.5 inches), large (bore 6.5 to 10 inches), very large (bore > 10.1 inches). Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 29 of 70 Naturally Aspirated Engines A naturally aspirated engine may be operated at stoichiometric, rich, or lean. Stoichiometric operation is required if the engine has a 3-way catalyst for NOx and CO reduction. Rich operation is sometimes used to reduce NOx without a catalyst, but with higher fuel consumption and high carbon monoxide (CO) emissions. Slightly lean operation (2% exhaust O2) results in improved fuel efficiency but with increased NOx and higher exhaust temperatures. Leaner operation (> 4% exhaust O2) can result in improved fuel efficiency and reduced NOx, but results in a maximum load de-rating. General Note for the BSFC Curves The BSFC curves are normally taken with the engine oil and water pump included but without a fan, alternator, and other optional equipment. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 30 of 70 Applicable engines: Cummins G5.9, Arrow VRG220, Ford LRG-425 BTU/HP-h 10000 9500 9000 8500 8000 7500 BSFC NA Stoichiometric - Small Not including fan, alternator, optional equipment 7000 1000 1200 1400 1600 1800 2000 2200 2400 RPM BSFC NA Stochiometric - Small 12000 11000 BTU/HP-h 10000 9000 8000 7000 30% 1800 RPM Not including fan, alternator, optional equipment 50% 70% % Load 90% 110% Figure A.3 Small Stoichiometric Naturally Aspirated Reciprocating Engine BSF (Natural Gas) Generally these small engines (< 100 HP) have an RPM at which the best full load efficiency is achieved. Above and below that RPM, the fuel efficiency becomes worse as shown here. The optimum RPM with regard to fuel efficiency depends on the engine design. Hence, operation at speeds different to the rated RPM may result in lower fuel efficiency. Information should be requested from the engine manufacturer. No curves are shown for small turbocharged engines as these are not common in the field. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 31 of 70 Applicable engines BSFC NA Stoichiom etric Large BTU/HP-h Waukesha VHP series F2895G, F3521G, L5108G, L5790G, L7042G, P9390G White Superior G825-6; G825-8; G825-12 Cat 3500 NA series 3512NA; 3516NA 10500 10000 9500 9000 8500 8000 7500 7000 40% 50% 60% 70% 80% 90% 100% 110% 1000 RPM 1200 RPM 800 RPM Load% Figure A.4 Large Stoichiometric Naturally Aspirated Reciprocating Engine BSFC (Natural Gas) Applicable Engines: Waukesha VGF Series F18G, H24G, L36G, P48G Cat 3400 NA Series G3406NA, G3408NA, G3412NA Cat 3300 NA Series G3304NA, G3306NA BSFC NA Stoichiometric Medium 10500 10000 BTU/HP-h 9500 9000 8500 8000 7500 7000 40% 50% 60% 70% 80% 90% 100% 110% 1800 RPM 1400 RPM Percent Load Figure A.5 Medium Stoichiometric Naturally Aspirated Reciprocating Engine BSFC (Natural Gas Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 32 of 70 Turbocharged Engines A turbocharged engine uses an exhaust driven turbo-charger to increase the manifold air pressure and therefore the rated load. These engines may be operated at stoichiometric, rich, or lean. Stoichiometric operation is required if the engine has a 3-way catalyst for NOx and CO reduction. Rich operation is sometimes used to reduce NOx without a catalyst, but with higher fuel consumption and high carbon monoxide (CO) emissions. Slightly lean operation (2% exhaust O2) results in improved fuel efficiency but with increased NOx and higher exhaust temperatures. Leaner operation (> 4% exhaust O2) can result in improved fuel efficiency and reduced NOx, but results in a maximum load derating. Applicable Engines Waukesha VHP Series F2895GSI, F3521/3524 GSI, L5108GSI, L5790/5794GSI, L7042/7044GSI, P9390GSI 10000 9500 BSFC - Turbocharged Stoichoimetric Large Engines 13900 13400 12900 11900 11400 10900 10400 9900 50% 60% 70% 80% 90% 100% 110% 1200 RPM 1000 RPM 800 RPM BTU/HP-h 8500 8000 7500 7000 40% % Load Figure A.6 Large Turbocharged Stoichiometric Reciprocating Engine BSFC (Natural Gas) Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 33 of 70 kJ/kW-h 9000 12400 BTU/HP-h 8500 8000 7500 7000 40% 50% 60% 70% 80% 90% 100% 12400 11900 11400 10900 10400 9900 Cat 3400 Series G3406TA, G3408TA G3412TA 110% Percent Load Figure A.7 Medium Turbocharged Stoichiometric Reciprocating Engine BSFC (Natural Gas) B TU /H P-h Applicable Engines: Waukesha AT27 Series 8V-AT27GL, 12VAT27GL, 16VAT27GL Cat 3606, 3612, 3616 BSFC Lean - Turbocharged (PCC) Very Large Engine 9000 8500 8000 7500 7000 6500 40% 1000 RPM 900 RPM 13200 12700 12200 11700 11200 10700 10200 9700 50% 60% 70% 80% 90% 100% 110% 9200 120% Percent Load Figure A.8 Very Large Turbocharged Lean Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 34 of 70 kJ/kW-h Applicable engines: Waukesha VGF Series F18GSID H24GSID L36GSID P48GSID BSFC - Turbocharged Stoichiometric Medium Engines 10000 9500 9000 13900 13400 12900 1800 RPM 1400 RPM Applicable Engines Waukesha F2895GL, H3521GL, L5790LE, L7042GL, P9390GL, REM Rich to lean GSI conversion BSFC Lean - Turbocharged (PCC) Large Engine 9500 9000 1200 RPM 1000 RPM 800 RPM 13200 12700 12200 11700 11200 10700 10200 9700 9200 120% BTU/HP-h 8500 8000 7500 7000 6500 40% 50% 60% 70% 80% 90% 100% 110% Percent Load Figure A.9 Large Turbocharged Lean Reciprocating Engine BSFC (Natural Gas) BTU/HP-h Applicable Engines Waukesha F18GL H24GL L36GL P48GL Cat 3400 Series G3412LE BSFC Lean - Turbocharged Medium Engine 9500 9000 8500 8000 7500 7000 6500 40% 1800 RPM 1400 RPM 13200 12700 12200 11700 11200 10700 10200 9700 50% 60% 70% 80% 90% 100% 110% 9200 120% Percent Load Figure A.10 Medium Turbocharged Lean Reciprocating Engine BSFC (Natural Gas) Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 35 of 70 Appendix B Emissions and Greenhouse Gas Aspects Emissions Regulations Natural gas engines and turbines emit a range of gases resulting from combustion. The main gases emitted are listed below with a note on current and possible future regulations in Western Canada. Table B.1 Emission Components Emission Water Carbon Dioxide Nitric oxide Nitrogen dioxide Nitrous oxide Carbon monoxide Volatile organics Non-methane hydrocarbons Methane Formula Notes H2O CO2 Greenhouse gas NO NO2 N2O CO VOC NMHC CH4 Part of NOx Part of NOx Greenhouse gas Toxic gas Organic gases excluding methane and ethane; All hydrocarbons except methane A greenhouse gas from unburned fuel Intermediate combustion product; lifetime 2 to 6 hours; Carcinogenic Maximum fuel gas concentrations specified by engine manufacturer. Almost all is combusted in the engine From combustion of H2S Very low from NG engines Regulation None No current regulation Provincial NOx regulations Provincial NOx regulations No current regulation Normally not regulated except for plant specific limits No current regulation No current regulation No current regulation No current regulation Well below regulatory limits Formaldehyde CH2O Hydrogen Sulphide H2S Sulphur dioxide Particulates SO2 Regulated; normally site specific No current regulation Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 36 of 70 Alberta Regulation – Alberta - INFORMATIONAL LETTER IL 88-5 June 1988 A few key items are listed below. For more complete information consult http://www.eub.gov.ab.ca/bbs/ils/ils/pdf/il88-05.pdf “Alberta Environment requires "low NOx" emission technology to be used at all new compression facilities and at all expansions of existing compression facilities when the unit size is greater than 600 kW” • • • 600 kW=804 HP, For greater than 600 kW, NOx < 6 g/kW-h (< 4.5 g/HP-h), No regulation for engines less than 600 kW. BC regulation – Environmental Management Act OIL AND GAS WASTE REGULATION July 2005 A few key items are listed below. For more complete information consult http://www.qp.gov.bc.ca/statreg/reg/E/EnvMgmt/254_2005.htm Registration and Authorization of Operations “The air contaminants discharged from each driver with a rated power of greater than 600 kilowatts installed after February 26, 1997 and all drivers with a rated power greater than 100 kilowatts installed after January 1, 2006 comply with the requirements set out in Schedule 1” From Schedule 1: Table B.2 Nitrogen Oxide Emission Standards (BC) Maximum Nitrogen Oxide Emitted (NOx as NO2, grams per kilowatt hour) 2.7 6.7 10.7 Fuel Used to Power Driver Natural Gas Natural gas/liquid fuel combinations Liquid fuel Does not apply for operation < 200 hours per year. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 37 of 70 Other: Fees are charged for emissions • • • Fees will be charged for discharges of sulphur and sulphur oxides and NOx, All sulphur compounds will be assumed to be completely combusted to sulphur dioxide, Fees for sulphur compounds will be assessed on the basis of the fee per tonne of sulphur dioxide discharged as set in Schedule B of B.C. Reg. 299/92, and Fees for NOx will be assessed on the basis of the fee per tonne of nitrogen dioxide discharged as set in Schedule B of B.C. Reg. 299/92, using a calculation methodology for converting nitrogen oxides to nitrogen dioxide as specified by a director. • Saskatchewan Regulations - Air Monitoring Directive for Saskatchewan ~ DRAFT ~ EPB 377 April 2007 A few key items are listed below. For more complete information consult http://www.se.gov.sk.ca/environment/protection/air/Air%20Monitoring%20Directiv e%20for%20Saskatchewan.pdf Ambient Monitoring • Submit yearly estimates of SO2, NOx and CO2 in tonnes per year in the annual environmental report. Report emergency release of H2S in estimated tonnes per year. Shall not exhibit opacity greater than 40% averaged over a period of 6 consecutive minutes. Operate a flare stack system with a minimum of 12.2 meter height to ensure proper ground level dispersion of SO2 emissions. Passive monitoring for H2S and total sulphation (SO3) on a quarterly basis at a minimum of 2 locations or join the regional air shed association. Routinely monitor and minimize fugitive air emissions from the plant. Follow CCME’s Best Management Practices for the control of benzene emissions. • • • • • No regulations specific to Natural gas engines or turbines. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 38 of 70 Greenhouse Gas Emissions Engine and turbines emit greenhouse gases, GHG, as shown in the table below. Carbon dioxide is the reference greenhouse gas. Other gases have a greenhouse effect which is shown by a mass factor. The sum of the greenhouse gases, multiplied by the appropriate factor give a carbon dioxide equivalent amount, CO2(e). Table B.3 Emission Component Importance Factors Gas Carbon Dioxide Methane Sources Fuel combustion Unburned fuel Engine misfire Instrument gas venting Natural gas leaks Compressor packing vents Engine and compressor crankcase venting Inefficient flares Engine starting gas Equipment blow-downs Pig pressuring and venting Dehydrators 3-way catalysts Factor 1 23vi Nitrous Oxide 296vii Carbon Dioxide – The amount of carbon dioxide emitted is proportional to the amount of fuel consumed. Hence an increase in system efficiency will result in a reduction in CO2. Examples of efficiency improvements are engine rich-to-lean conversion, RPM optimization et.al. Methane - Methane is a potent greenhouse gas. A reduction in the escape of methane to the atmosphere can dramatically reduce the CO2(e) for a facility. Obvious improvements result from leak reduction and minimization of equipment blow-downs. Improvements are possible with the use of SlipStreamTM technology where low pressure vented gases can be combusted in an engine. Nitrous Oxide – Nitrous oxide, N2O, is distinct from NO and NO2, which, while produced by engines, have short lifetimes in the atmosphere. Nitrous oxide has a residence time in the atmosphere of over 100 years. While natural gas engines and turbines do not produce N2O, some is produced by 3-way catalysts, particularly those that are poorly maintained. Two methods are normally employed to estimate the greenhouse gas emissions, namely CAPP and IPCC. The CAPP method utilizes the measured fuel gas consumption alone and assumes typical fuel gas composition and thus also assumes typical values of CH4 and N2O. The CAPP value will thus produce the most stable results for trending purposes. The IPCC method considers the actual Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 39 of 70 fuel gas composition as well as CH4 and N2O values. It places an importance factor on the relative contribution to greenhouse gases of the components of emissions as they react in the atmosphere. The IPCC method accounts for the dual role of carbon as it oxidizes in the atmosphere via CH4, and then CO2 after 11.5 years. The formulae are shown below: CAPP Method: CO 2e = 2.012 tonnes / e3m3 of fuel combusted IPCC Method: CO 2e = 310 * N 2O + 21 * CH 4 + ∑ Fuel Gas Carbon Elements It is normally assumed that the unburned hydrocarbons are all methane in these estimates. This is a good assumption for a first approximation but other components are actually present as unburned hydrocarbons and these should also be quantified. The N2O is an element that is typically not measured in the equipment manufacturer’s lab or in the field. The high importance factor assigned makes it significant even if the amount is small. Indeed it is important to note that tests on gasoline engines fitted with catalytic converters produced significantly higher amounts of N2O after the exhaust gases reacted with the catalyst. It warrants further exploration to determine if this is the case for natural gas engines using catalysts. Clearly, including any amount of N2O contribution will be significant. Typical field emissions measurements do not include CH4 and neither laboratory or field measurements include N2O components. The quality of field data is typically insufficient to support a conclusive analysis using the IPCC method. The CAPP method ignores the fuel gas composition and does a poor job of accounting for CH4 and N2O elements. Site Regulations Normally the regulatory authorities require both a site and equipment application. The environmental authorities may or may not establish emissions regulations specific to a particular site in addition to any engine/turbine regulations. Engine Control The combustion process and the resulting emissions have a strong dependence on the air to fuel ratio and the ambient conditions, most notably temperature. The effect of air fuel control is shown by the graph below that shows NOx, CO and unburned fuel exhaust emissions as a function of the excess air amount. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 40 of 70 Figure B.1 Typical Emissions vs Air-Fuel Ratio 10000 9000 8000 7000 6000 5000 4000 3000 2000 1000 0 0.8 CO 1 2 HC NOx 3 4 Exhaust PPM 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Lambda - e xce ss air 1. 2. 3. 4. Rich Stoichiometric Lean – best fuel (meets Alta NOx limits) Lean – low NOx (can achieve < 1 g/HP-h NOx) If the engine is equipped with a 3-way catalyst to reduce the emissions of NOx and CO, the air-fuel ratio control must be quite precise, as the limits for reduction of both NOx and CO are quite narrow. In stoichiometric or rich-burn engines much of the energy goes “up the stack” or to the catalytic converter in the form of CO and unburned methane. The control system must maintain the desired air-fuel ratio over the expected ranges of • • • • fuel quality (heating value), engine load, engine speed, ambient temperature. In general mechanical control systems (e.g. with regulators, levers etc.) are incapable of achieving the desired control over the expected operational ranges above. Both theory and practice show that electronic control is much superior. Nevertheless, not all electronic control systems are capable of achieving good control for all the above operational ranges, so due diligence is required. A good control system leads to improved reliability, as the engine/turbine is operated within the manufacturer’s design expectations. This has been proven time and time again in practice. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 41 of 70 Appendix C Engine Considerations Engine Design The oil and gas industry utilized modified diesel engines to drive gas compression equipment, refrigeration compressors and to power electrical generators. The term gasification describes the process of converting an engine from diesel or gasoline fuels to natural gas. The ruggedness of the diesel engine design provided a benefit to the industry in the form of reliability and exceptional major component life. The move towards the development of what is termed a pure natural gas engine design was a combination of the need for a wider range of engine drivers, diesel engine conversion costs and an opportunity to reduce overall costs as the diesel engine components were over designed for natural gas service. The engine manufacturers responded to the increasing and diverse needs of the industry. They expanded the range of products to ensure that horsepower output could be closely matched to needs. This enabled the user to select the best possible combination of operating speeds, operating ranges, horsepower ratings, fuel compositions, engine ignition systems and engine control and monitoring systems to meet specific site requirements. The natural gas engines used today are fully capable of 24/7 continuous operation, deliver the full rated horsepower and are controlled and monitored by fully automated systems. Technological advances in the areas of ignition systems, speed control, loading, and exhaust emission reduction have improved starting and run time reliability. The natural gas engine provides components are physically lighter in weight and dimensionally smaller than what the diesel engine conversions offered. The lightening of materials is due to the lower Btu energy forces that are created with the combustion of natural gas fuels within the combustion chamber. The reduction in the overall mass of the engine components provides reduced mechanical forces, lower overall casting weight and physical dimensions. The reduction in mechanical forces and casting weight also allowed for a reduction in the amount of structural support and foundation support required. Engine design and fuel delivery systems vary substantially. The following description provides a brief orientation of engine design aspects. Two Stroke Design The two stroke engine design has seen limited use in the oil and gas industry. The design is suited to scenarios where the fuel gas supply contains traces of hydrogen sulphide gas that exceed levels acceptable to the manufacturers of separable four stroke engine designs. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 42 of 70 The sequence of the two stroke engine operation begins with the piston moving downwards in the cylinder bore during its power stroke. On its travel downwards the top of the piston will pass an exhaust port and the pressurized exhaust gases will exit. The piston continues downwards in its travel. During this phase it compresses the air-fuel-oil mixture within in the crankcase. The top of the piston then passes a transfer port and the compressed crankcase charge is allowed into the cylinder and the remaining exhaust is forced out. The compression stroke begins with the air-fuel-oil mixture charge in the cylinder. As the piston begins to move upwards the compression charge in the cylinder draws a vacuum in the crankcase, pulling in more air, fuel, and oil from the carburetor. The compressed charge is then ignited by a spark plug and the resultant explosion of the air-fuel-oil mixture drives the piston downwards in the cylinder and the process is repeated. The two stroke design uses the space below the piston for air intake and compression. This allows the area above the piston to be used for the power and exhaust strokes. The design offers an advantage in that there is a power stroke for every revolution of the crank, instead of every second revolution as in a fourstroke engine. Four Stroke Design The four stroke engine design is the most prevalent within the industry. The sequence of operation starts with the piston being positioned at the top dead center (TDC). This means that the piston position is now furthest away from the crankshaft. The travel of the piston in the cylinder bore downwards from the TDC position is referred to as the intake of the piston (first stroke). As the piston travels down the cylinder bore a vacuum is created and the air-fuel mixture is drawn into the cylinder aided by the atmospheric pressure or forced into the cylinder by a turbocharger. The intake valve closes as the piston begins it travel upwards compressing the air-fuel mixture within the cylinder bore. As the piston nears the top of its compression travel (second stroke) the air-fuel mixture is ignited by the spark plug. The combustion process that then takes place results in a rapid expansion of the ignited gases and the piston is driven down into the bore delivering its power (third stroke). At the bottom of its stroke the piston begins its return journey and expels the burnt combustion gases out of the cylinder past the exhaust valve (fourth stroke). Naturally Aspirated Engine with Stoichiometric Air-Fuel Mixture The naturally aspirated engine design relies on the atmospheric pressure differential created when the piston moves downwards in the cylinder bore to draw in the air-fuel mixture. The term stoichiometric refers to the perfect ratio of air to fuel (16.8:1) that matches the oxygen and hydrocarbon molecules so as to create a complete combustion process; the exhaust stream analysis would show that there are no unburned hydrocarbons or free oxygen exiting the combustion chamber. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 43 of 70 Turbocharged Engine with Stoichiometric Air-Fuel Mixture The turbocharged engine design utilizes the exhaust stream thermal flow energy to drive an impeller fitted to a shaft that is fitted with a turbine wheel that forces air into the intake manifold of the engine building a reserve that is of sufficient quality so as to force the air-fuel mixture under pressure into the cylinder bore. This process is capable of increasing engine horsepower by approximately 30% over the same cubic inch displacement that a naturally aspirated version of the same engine would be capable of producing. As is the case with the naturally aspirated engine design the term stoichiometric refers to the perfect ratio of air to fuel (16.8:1) that matches the oxygen and hydrocarbon molecules so as to create a complete combustion process; the exhaust stream analysis would show that there are no unburned hydrocarbons or free oxygen exiting the combustion chamber. Turbo-charged Engine with Lean Burn Technology The turbocharged lean burn engine design utilizes the exhaust stream thermal flow energy to drive an impeller fitted to a shaft that is fitted with an over sized turbine wheel capable of delivering approximately 80% more air than a standard turbocharger equipped engine. This forces more air into the intake manifold of the engine building a much higher reserve of air. The net result is that the ratio of air-fuel mixture entering under pressure into the cylinder bore may range from 25:1 to 32:1 dependant upon engine design. As is the case with the standard turbocharger design this process is capable of increasing engine horsepower by approximately 30% over the same cubic inch displacement that a naturally aspirated version of the same engine would be capable of producing. The exhaust gas stream is measured for the percentage of free oxygen as a means of setting the air fuel ratio of the engine. The excess oxygen forced into the cylinder ensures that the combustion process is fully completed prior to exiting into the exhaust manifold. Supercharged Engine with Mechanical Forced Air Induction Design The supercharged engine design utilizes a mechanically driven rotary meshing impeller design to force air into the intake manifold. The mechanical drive approach provides an even response to changes in speed and loads as it not reliant upon the thermal energy generated by the exhaust stream to drive an impeller. Engine Operating Speed Ranges The speed ranges of the natural gas fuelled spark ignited reciprocating engines typically range from 600 to 1800 rpm. The lean burn designed engines are the exception as their oversized turbocharger capacity requires a specific amount of thermal heat energy to drive the turbine wheel shaft. The speed range of these Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 44 of 70 engines is limited. If the engine is to be operated at a lower rpm a low speed turbocharger cartridge is installed to match the thermal energy level that is being generated by the engine. Separable Engine and Compressor Design The term separable defines the engine driver and the driven components are individual rotating elements connected by couplings. This allows mixing and matching of a wide range of drivers, driven equipment, gearboxes and accessories. One engine model could be used to drive a compressor, a water pump, an electrical power generator or other rotating elements. Integral Reciprocating Engine and Compressor Design The integral design incorporated a low speed, long piston stroke approach to generate horsepower and to compress natural gas. The combination of the engine and compressor into a single unit eliminated the need for drive belts, gear box drives or clutch assemblies that would manage the transfer of engine power to the driven component. At the time, industrial equipment was evolving and the designers working in that period of time offered a modern approach to moving large volumes of natural gas. The design utilized a single casting combining the engine power cylinders with the compression cylinders. The industry has steadily phased out the use of the integral design over the years due to manufactures obsolescence issues, resultant difficultly in sourcing of replacement parts and the emergence of the separable concept that offers features and benefits that greatly improved the economics of plant and field development and ongoing operations. The integral engine/compressor designers created a number of models and variations. The power cylinders were typically arranged in the vertical plain or in a “V” configuration. The number of power cylinders would vary from 1 to 16, dependant upon design philosophy. The compression portion of the design incorporated varying numbers of cylinders and methods of attachment. The dimensions of the units would vary widely based upon the design of the crankcase and the oil sumps. It was not unusual for a plant to provide a basement located below floor level that would accommodate a secondary filtration set up so as to facilitate engine filter and oil changes without having to shut down the engine. The fuel selection process, fuel management systems, ignition strategy and monitoring and control systems are comparable to the separable engine design. Surviving integral installations are represented by a number of different manufacturers and utilize a variety of monitoring and control systems. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 45 of 70 Fuel Selection A fuel that is most readily deliverable or available to the site is usually the basis upon which an engine is selected. The type of fuel and its composition will determine the engine design selected. The fuel type will determine the combustion cylinder compression ratio, ignition timing, fuel delivery system and control system. Fuel supply management will address storage, conditioning, filtration, regulating supply and delivery pressure rates. The following aspects must be considered in fuel selection. Heating Value The internal combustion engine design is based upon the Btu; British thermal heating value of the fuel that will be consumed. The physical state in which the fuel is supplied for combustion purposes, i.e. gaseous or liquid and the stability of the fuel as determined by an octane or cetane rating, will determine the delivery system design. Ignition Strategy Dependant upon the fuel selected the engine designer will incorporate an ignition strategy. The natural gas or gasoline engine will employ a controlled electrical spark source of ignition. Diesel fuel oils will rely on auto-ignition. This is based upon the heat of compression generated within the combustion chamber as the piston travels towards top dead center compressing the air trapped above the piston. Gaseous State Fuel Natural gas is a methane based fuel in a gaseous state. Propane is stored in a liquid state and must be converted to a gaseous state prior to being mixed with the atmosphere. These fuels enter into a mixing chamber referred to as a carburetor to create an air-fuel ratio specific homogenous mixture that can be readily ignited within the combustion chamber by a timed ignition spark event. Liquid State Fuel Fuels that are in liquid state (gasoline or diesel) require a mechanical process to assist in the atomization of the liquid to enable the commingling process with the incoming air prior to entering into the combustion chamber. Once the mixture has entered into the combustion chamber, a timed spark event (gasoline engines) provides the source of ignition. In the case of diesel fuelled engines, auto-ignition ignites the air-fuel mixture. Gasoline Fuel Atomization Methodology The venturi effect is the traditional and low cost approach used to atomize gasoline. Engine fuel efficiencies and power outputs have been improved Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 46 of 70 through the introduction of pressurized injection systems that enhance the atomization process. This process has greatly improved cold starting capabilities and enhanced engine responsiveness. Diesel Fuel Atomization Methodology The oil based diesel fuel requires a high pressure injection system to facilitate the atomization of the fuel to enhance the commingling process with the atmosphere within the engine combustion chamber. The injection is timed to coincide with the compression cycle and the heat that is being generated as a result. The heat build up due to the high compression ratios is sufficient to auto-ignite the mixture of diesel fuel and air. Engine Compression Ratios The Btu and composition of a fuel determines the combustion cylinder compression ratio options. The ratio of compression is determined by measuring the volume of the cylinder when the piston is at the bottom of it stroke, as compared to the volume of the cylinder when the piston is at the top of its stroke. Higher compression ratio engines produce more power per cylinder than lower compression ratio engines. The increased power output is related to the air fuel mixture within the cylinder being placed under an increased pressure during the compression stroke compounding the expansion pressures within the cylinder combustion chamber. A higher explosive expansion force is exerted upon the piston top which translates into an increase in overall horsepower. Fuel Heating Value The heating value is the amount of energy that a fuel produces for a unit of volume. The following discussion compares the common fuels used for engines. Natural Gas The Btu heating value of natural gas may range from 400 – 1250 per cubic foot of gas. The Btu heating value of the hydrocarbon based methane gas is dependant upon the source. The engine designer determines the horsepower rating by analyzing a sample of the fuel supply to determining Btu value and establish the amount of energy that will be is created within the combustion chamber when the gas is combusted. The percentage content of methane gas is taken into consideration. A scenario in which the collected methane gas is emitted during the processing of organic waste products may be dealing with Btu value as low as 400 per cubic foot of gas. Conversely methane gas contained within a well reservoir may have a Btu that exceeds 1250 Btu per cubic foot of gas. The enhanced heat value of the reservoir sourced may be attributed constituents contained within the stream such as butane and hydrogen. A precise analysis of the fuel gas stream is required to ensure that the engine designs compatibility. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 47 of 70 Propane The Btu heating value of liquefied petroleum gas is approximately 2,500 Btu per cubic foot of gas. This is a fuel stored in a liquid state that must be converted to a gaseous state prior to entering the engine combustion chamber. When compared to natural gas propane is 1.5 times denser and contains a mixture of butane, butylenes and propylene. Gasoline The Btu heating value of gasoline is approximately 150,000 Btu/Imp gallons. This is a fuel stored in a liquid state and must undergo a mechanical atomization process to ensure it is evening distributed during its commingling phase with the incoming air stream. Gasoline may be enhanced with benzene to increase octane ratings and may include toluene, naphthalene and triethylbenzene. An octane rating system is used by industry to establish fuel grade standards to identify the fuel grade’s auto ignition resistance characteristic referred to as fuel stability. Diesel Fuel The Btu heating value of diesel fuel is approximately 166,600 Btu/Imp gallons. This fuel is stored in a liquid state and must undergo a mechanical atomization pressure injection process to ensure that it will mix in with the air being compressed within the cylinder combustion chamber. The use of a cetane rating system establishes the fuel grade by measuring its tendency to auto ignite, which is a desirable characteristic as the diesel engine design relies on the heat generated during the compression stroke to ignite the air fuel mixture. Engine Service Life The internal combustion engine design must be rugged enough to ensure that its rotating forces can be controlled. Personnel safety and acceptable service life are the two major design considerations. The combustion process is an explosive event that generates useable energy and emits or radiates waste heat by-products. The engine cooling system design may utilize air to air or liquid to air heat transfer methods. Lubricating oils are selected to reduce frictional contact with moving parts, provide internal component cooling and to carry away any contaminates that form during operation. It is not unusual for an engine to consume some lubricating oils during normal operation. The rate of consumption is dependant upon the engine design, the severity of service and the lubricant selected. The oil supplier provides a specification and the operator of the engine is duty bound to select a lubrication product that has been approved for use by the engine designer. Over the service life of the engine assembly wear will occur that may lead to oil consumption. There is a range of formulated lubrication products within the industry that can be matched to the service duty of the equipment. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 48 of 70 Engine designers and component manufactures utilize their knowledge base in selecting materials and development of a design to be compatible with the fuel(s) that will be consumed. The characteristics of the fuel during handling, metering and combustion are taken into consideration when designing monitoring and control systems. The primary objective by suppliers is to provide an engine that can be safely operated and will develop the horsepower that is required. The ingestion of fuels other than what an engine has been designed for will result in poor engine performance, a reduced service life of components and contribute to the failure of the engine assembly. Hot Fuel Management The term hot fuels refer to the increased Btu values contained within a natural gas fuel source. Higher Btu values the greater the force that will be exerted against the top of the piston during the combustion process. If a hot mixture is ignited too early the resultant explosive expansion force is compounded and will affect engine performance and damage components. The ignition of the air fuel mixture within the combustion cylinder is a controlled and precisely timed event. The point at which the ignition source is introduced and the duration of the ignition input is determined by the fuel type. Engine designers determined that the hotter the fuel the greater the increase in the rate of expansion and forces generated during the air fuel mixture combustion process. Advanced ignition timing may either impede the pistons travel upwards during the compression cycle or increase the internal pressures within the cylinder bore. The result affects the engine’s design performance. Advanced ignition timing may result in a compounding of the expansion forces within the cylinder. As the piston is traveling upwards compressing the air fuel mixture, the forces of inertia created by the engine flywheel and the other rotating components combine with the increased rate of expansion with the potential of over stressing components. The physical evidence of this damaging event is melted or fractured pistons, broken piston and oil control rings, bending of the connecting rods, excessively worn connecting rod and main bearings. If undetected the progressive engine damage may result in a catastrophic failure of the assembly. Engine designers often retard timing and de-rate the engine to compensate for hot fuels. The octane rating of the fuel is a measure of the fuels resistance to auto-ignite during this compression process. This is also referred to as fuel stability. As the piston travels upwards during its compression stroke the air fuel mixture is compressed into a smaller area and the internal temperature begins to rise. Engine control and monitoring systems capable of detecting and reacting to detonation or pre-ignition events have been developed and are available for installation on engines that may be at risk. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 49 of 70 Natural Gas Natural gas is a gaseous fossil fuel primarily consisting of methane and may include varying quantities of ethane, butane, propane, carbon dioxide, nitrogen, helium, hydrogen sulphide and water vapour. The approximant heating value of natural gas is 1000 Btu per standard cubic foot. The use of natural gas as a fuel source within the oil and gas industry is based on its ready availability in a wide range of settings. These factors eliminate the need to source and manage the cost and risks associated with the transportation of other fuel types such as diesel or gasoline. The development of remote locations in restricted access locals has been made possible through the use of natural gas as a direct fuel source. Alternate Fuels Propane or diesel fuel is often employed for back-up or stand-by applications where the primary fuel supplies might be interrupted by a plant outage. Electrical power generation is the typical application for standby equipment. The preferred fuel choice may be natural gas however, should a situation arise where the fuel gas source is interrupted a back up unit would ensure that essential services are not interrupted. The use of alternate fuels requires the incorporation of specialized equipment within the engine design. The typical conversion used in the oil and gas industry is natural gas to propane. In situations where methane gas is being collected from an organic process and subject to supply fluctuations an engine can be equipped with controls that can accommodate a make up fuel supply and offset the primary fuel source. Whenever the commingling of different fuel streams is undertaken the engine management systems must also be capable of automatically adjusting engine timing and fuel metering. The oil and gas industry, along with engine designers have recognized that there can be substantial differences in Btu values and quality of the primary fuels used within the oil and gas industry. The engine designer’s test cells are typically located in populated areas and draw upon municipal grade natural gas supplies. Therefore generally accepted standards upon which natural gas engine designs are based upon is 900 Btu per standard cubic foot. Propane is rated at approximately 2500 Btu per standard cubic foot and bio gas can range as low as 400 Btu per standard cubic foot. When designing for bio gas application the engines are rated on the lowest anticipated Btu value of fuel. During engine operation changes in fuel values are accommodated by automated fuel delivery and loading systems. The quality of natural gas may be affected by the source and the process used to condition the fuel prior to use. The Btu values are dependant upon the amount of other hydrocarbon sources present in the stream. Wet gas (hot gas) will exhibit a Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 50 of 70 higher Btu rating than gas that has been processed and stripped of excess hydrocarbons. The incoming natural gas stream may also contain varying percentages corrosive elements such as carbon dioxide and hydrogen sulphide. These constituents contribute to the formation of acids when combined with the water. On occasion particulate matter may be transported within the gas stream and must be filtered out. The particulates act as an abrasive that will contribute to combustion chamber wear and contamination of lubricating oils. Liquefied propane is stored in pressurized containers that have been filled from a refined source. This eliminates the possibility of contamination and provides a consistent Btu value. Diesel fuels require conditioning to accommodate seasonal temperature variances. Diesel fuel storage tanks may be contaminated by water moisture that forms on the tank walls during hot and cold ambient temperature cycles. The storage tanks are equipped with open to atmosphere breathers to manage temperature related fuel expansion and contraction cycles. Metal containers typically used to store diesel fuels are prone to rusting over time. This oxidation process releases fine particulates into the fuel that over time will contribute to blocking filtration systems. Fuel delivery system maintenance is critical as the pressurized fuel delivery systems incorporate extremely tight tolerances that can be easily damaged by the entrance foreign materials. Methane bio gas is sourced from the decomposition of organic matter. The methane gas is used to fuel engines that drive electrical generators and, on occasion, co-generation is employed. The decomposition process emitting the methane source may be a landfill site or from waste water treatment faculties. Some experimentation is being conducted regarding the collection of methane emissions from animal waste processing plants. No matter what the stream source, specialized equipment is required. In addition to the design considerations, handling procedures to control bio hazards for personnel safety reasons and contaminates to protect the integrity of the equipment are also required. Natural Gas Fuel Supply Sources Utility Gas Pipeline A utility gas pipeline provides access to a source of fuel gas that has been treated and can typically be counted on to deliver a consistent Btu value. This gas will have been processed. During processing the gas would have been passed through several phases of separation to remove hydrocarbon liquids and water vapour. A sample will be analyzed to confirm what constitutes may be present in the stream and to establish a Btu value that is used to determine the engine timing. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 51 of 70 On Site Plant Process Source Plant discharge gas is often selected as the primary source for fuel simply because it is readily available. The gas is usually moved through several phases of mechanical separation and or filtration medium to remove hydrocarbon liquids and water vapour. In addition to this the gas stream may have also been processed through a refrigeration process to further remove liquid hydrocarbons and water vapour. This gas would have passed through one or more stages of compression. It should be noted that during the compression stages the gas stream will pick up traces of compressor piston ring and piston rod seal packing box lubricating oils. The compressor lubrication rates are closely monitored to prevent excessive amounts from entering into the gas stream. The Btu value of the plant discharge gas is dependant upon the incoming gas stream constituents and level of processing that is applied within the plant prior to the gas been used as a source of fuel. Wellhead Gas Wellhead natural gas Btu values may vary widely and can be in a constant state of flux depending upon flow rates and contributing zones. Each of the reservoir zones will supply varying quantities of water, liquid hydrocarbons and in some fine particulate matter. The use of in water knock outs, separators and line heaters help to capture liquids and solids. While an effort is made to manage the incoming stream and render it as a usable fuel source an analysis of the fuel is required in order to identify the percentage of constituents, Btu value and ensure that any potential corrosive elements have been identified. The quality of well head gas varies by site and the relocation of equipment demands that a review of the equipment be undertaken to ensure compatibility with the new fuel source. Unpredictable Fuel Quality Variances To ensure that the performance and mechanical integrity of an engine is maintained, regularly scheduled gas analysis is performed. This approach will help to provide a record of the fuel source quality. Any shifts in the percentage composition of the fuel will be noted during a review of the gas analysis report and corrective action can be taken. The gas field gathering systems are constantly in flux and subtle changes can occur in Btu values. Automated fuel management control systems have been developed that can monitor and respond accordingly. These systems ensure that the air-fuel mixtures are efficient and that levels of exhaust emissions are maintained within specifications. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 52 of 70 Appendix D Fuel Gas Meter Requirements To determine the efficiency of an engine or turbine, the fuel gas flow should be measured for each operating engine or turbine. It is not sufficient to use the gas meter for the entire site. A combination of individual engine and turbine fuel flow together with a site flow will enable the fuel gas usage by other components such as building heaters, instrument gas supplies, and leaks to be more easily assessed. A periodic fuel gas analysis is needed to determine the heat content of the fuel gas (see section on BSFC). The sampling and analysis frequency depends on the possibility of composition changes. Note that gas composition can change from well to well and also, if the fuel gas is from a gas treatment plant, the current status of the treatment plant (e.g. refrigeration unit may not be operating to specifications from time to time).Engines are often in vibration service and the implications to the meter must be considered. In lean natural gas engines, the amount of excess air for good efficiency, exhaust emissions, and reliability depends on fuel flow. The heating value of the fuel can vary significantly depending on the hydrocarbon mix. The table to the right shows the lower heating value (LHV) per standard cubic foot (scf) for four components often found in engine fuel gas. The table below compares three gas flow meter types: • • • orifice plate, coriolis, thermal. GAS Methane Ethane Propane nButane LHV – BTU/SCF 912 1622 2376 3020 PROPERTY 1.1.1 Measuring principle VOLUMETRIC (ORIFICE) Volume THERMAL MASS Specific heat CORIOLIS Mass Temperature Fuel changes Vibration effect Reliability “Dirty” gas influence Needs temperature measurement Poor Low Good Some; increases apparent Temperature compensated Good Low Good, see below Some; decreases apparent Temperature compensated Best High Good None Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 53 of 70 Pressure drop Turndown ratio Piping requirements flow Low, depends on orifice size Typically 10 to 1 Straight pipe typically 38” before and 25” after Rosemount Manufacturers 1 to 20 psi depending on flow Typically 100 to 1 Typically 10 or 20 to 1 for gases Typically no obstructions 10 No piping limits; pipes must pipe diameters upstream transmit minimal strain to the and 5 downstream meter. Fox Thermal Instruments MicroMotion flow Very low; < 0.2 psi Measuring Principle The orifice meter uses the pressure drop across an orifice plate in a straight pipe run to calculate volumetric flow. The pressure difference, the upstream or downstream pressure and the gas temperature must be measured. A device using a similar principle is the V-cone meter. For accuracy, there are generally significant piping requirements discussed in more detail below. Other designs may have different requirements. The orifice meter measures volume flow, which is sometimes converted to mass flow by multiplication by a pre-determined, userentered density value. The Coriolis meter is based on the Coriolis Effect and uses the transverse force created by a mass traveling in a curved path. It is a direct measurement of the mass flow. The thermal mass meter measures the thermal conductivity of the flowing gas. The thermal conductivity is very nearly proportional to mass flow. Thermistors measure the heat loss to a flowing gas. Temperature The orifice meter requires a temperature measurement of the gas to provide the flow in standard units (i.e.: scf/h). The thermal meter has internal temperature compensation while the Coriolis meter measures true mass flow and does not require a temperature adjustment. Fuel Changes For many applications the fuel heating content may change. A volumetric meter cannot sense such a change. If the fuel consists mainly of hydrocarbons, then the thermal mass flow meter and the Coriolis meter provide a measurement that correlates well with heating value changes. The graph here, where other hydrocarbons are normalized to methane, shows the relative differences. A fuel meter based on a volumetric flow principle is not suitable for installations where significant changes in the fuel heating content are expected. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 54 of 70 Fuel Heating Value Error 50% 0% Percent error -50% -100% -150% -200% -250% Methane Ethane Propane nButane relative to methane Coriolis Thermal Volumetric If the fuel contains significant and changeable amounts (typically more than 5%) of inert gases such as Argon, carbon dioxide, or nitrogen, a direct measurement of the fuel heating value may be required for any of these meters. Vibration Effects Both the orifice type and the thermal mass flow meters have low errors due to vibration while the Coriolis type is affected to a greater degree. Selection of location and vibration isolation should be used to minimize vibration induced flow signal noise with a Coriolis meter for engine fuel measurement. For smaller engines where the flow is low (poor signal to noise ratio), the other meters should be used. The table here shows some comparative measurements for two meters in the same application where vibration was a problem. A longer averaging time for the thermal meter would further reduce the noise. AVERAGING TIME(S) 12.8 0.25 PEAK TO PEAK SIGNAL NOISE % ±5 ± 2.5 METER TYPE Coriolis (MicroMotion) Thermal (Fox) Reliability Subject to proper installation, the reliability of each of the meter types is high and suitable for engine applications. There have been some reports of Coriolis meter failures where the meters were mounted with significant strain placed on the meter — for example, misaligned flanges. In all cases, the reliability of fuel meters to determine engine air requirements far exceeds the reliability of exhaust oxygen sensors (an alternate method for engine air control). Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 55 of 70 Dirty” Gas Influence Sometimes the engine fuel gas may contain particulate matter. Build-up may affect the readings of any of the meters. The Coriolis meter can be re-zeroed when the flow is zero. For the orifice plate meters, the orifice plate and probe of the thermal meter may require cleaning. RTI has had no reports of meter errors due to dirty gas. Pressure Drop For applications where the fuel gas pressure is low, the pressure drop of the Coriolis meter may be too large. This R50 Meter - Pressure drop Natural Gas graph shows the pressure drop versus Flow - kg/h flow for the Micromotion R50 Coriolis 0 100 200 300 400 meter. 30 The pressure drop for the orifice plate meter is relatively low, depending on the orifice diameter relative to the pipe diameter and should be calculated where pressure loss is a concern. The pressure drop for the thermal mass flow meter is the lowest of all the meter types. Turndown Ratio The turndown ratio is defined by the signal to noise ratio for the application. For most large engine fuel flow measurements the turndown ratios for the orifice and Coriolis meters are sufficient. For smaller engines or for accuracy for low flow applications, the thermal mass flow meter is recommended. Piping Requirements For best accuracy, the piping upstream and downstream of an orifice plate meter must be straight and uniform in diameter (about 20 diameters upstream and 10 to 15 downstream). The requirements are less stringent for the thermal mass flow meter (10 diameters upstream and 5 downstream). The Coriolis meter has no such requirements except for those mentioned above in the Reliability section. 25 Pr drop - psi 20 15 10 5 0 0 200 400 600 800 Flow lb/h 2.5 2 1.5 1 0.5 0 1000 Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 56 of 70 Accuracy - % of Flow 3 Meter Examples Mass Flow Meter Installed: Thermal Mass Flow Meter: Orifice meter run and Rosemount 3095 flow transmitter: Summary If the gas flow is measured by a device based on a volumetric flow principle, large errors arise in the manifold air pressure set point determined by the control system as the fuel gas composition changes. All fuel meter measurement sensors are more reliable than the exhaust oxygen sensors for air-fuel control. Flow measurements that rely on gas properties such as mass or specific heat properties scale better to the heating value of the fuel components in mainly hydrocarbon fuel. Such devices are the Coriolis mass flow meter and the thermal mass flow meter. The graph shows the percentage error in measuring the heating value of the gas flowing through the meter according to meter type. The poorest fuel meter is the volumetric type and this type should not be used where there is a possibility of a change to the gas composition. The best meter type is the Coriolis type due to the smallest errors with changes of gas mixture. The thermal mass flow meter is the best choice for applications where the fuel flow is small (engines below about 500 HP) or where vibration is significant. A REMVue® engine control system is the only system that uses a mass flow meter to operate their controls and, as such, it is the only system that is dynamically adaptive to changes in fuel quality. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 57 of 70 Appendix E Case Studies Case Study I: Fuel Gas Meters for Each Engine Objective: Show why individual fuel gas meters are required for each engine. Site Configuration:Three units in parallel driving reciprocating compressors. Equipment Description: Unit 1 Engine: Waukesha 7042GSI (Turbocharged 1232 horsepower at 1000 RPM) Compressor: Ingersoll Rand RDS, four cylinder, single stage Ignition: MPI set at 24 degrees BTDC Engine Management System: REMVue 500 with AFR (rich to lean conversion) Unit 2 Engine: Waukesha 7042GSI (Turbocharged 1232 horsepower at 1000 RPM) Compressor: Ingersoll Rand RDS, four cylinder, single stage Ignition: MPI set at 24 degrees BTDC Engine Management System: REMVue 500 with AFR (rich to lean conversion) Unit 3 Engine: Waukesha 7042GSI (Turbocharged 1232 horsepower at 1000 RPM) Compressor: Ingersoll Rand RDS, four cylinder, single stage Ignition: IQ 500 set at 24 degrees BTDC Engine Management System: Waukesha standard stoichiometric carbureted Data: (actual field tests for a Southern Alberta site: see PTAC report reference) Unit Engine Speed Lambda Fuel consumption Fuel consumption Compressor horsepower Percent of engine load BSFC (1000 Btu/scf gas) Utility gas station flow RPM Number KG/H E3M3/D Kw Percent Btu/BHP-h E3M3/D 700 1.514 91 3.04 379 59 8806 20.1 Unit 1 900 1.375 153 5.11 623 75 9007 22.4 1000 1.403 211 7.05 898 98 8618 21.3 700 1.462 95 3.18 379 59 9193 19.4 Unit 2 900 1.393 156 5.21 621 75 9213 22.0 1000 1.348 214 7.15 876 97 8960 14.2 700 1.010 112 3.74 388 60 10587 0.0 Unit 3 900 1.015 172.5 5.77 625 76 10123 0.0 1000 1.030 230 7.69 920 100 9169 24.2 Discussion: Compare data at the same load and speed operating points for each engine. The consumption and BSFC is different for each engine at each point. The engine management systems affect the fuel consumption as well as the load and equipment condition. Furthermore, the station flow rate including other site utilities will not equal the total flow through all engines. Estimating fuel Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 58 of 70 consumption for each engine based on combined metering is not accurate enough to determine individual engine performance. Case Study II: OEM vs Actual Fuel Consumption Objective: Show how OEM fuel consumption predictions are less than the actual. Site Configuration: Three units in parallel driving reciprocating compressors. Equipment Description: (use one factory equipped engine for this comparison) Unit 3 Engine: Waukesha 7042GSI (Turbocharged 1232 horsepower at 1000 RPM) Compressor: Ingersoll Rand RDS, four cylinder, single stage Ignition: IQ 500 set at 24 degrees BTDC Engine Management System: Waukesha standard stoichiometric carbureted OEM values Waukesha L7042GSI OEM Predicted Values (Initial 0.3 O2 % @ 920 kW, 1000 RPM, 24 BTDC) Data: (actual field tests for a Southern Alberta site: see PTAC report reference) Unit Engine Speed Lambda Fuel consumption Fuel consumption Compressor horsepower Percent of engine load BSFC (1000 Btu/scf gas) RPM Number KG/H E3M3/D Kw Percent Btu/BHP-h OEM Prediction 700 1.015 95.5 3.19 432 70 8108 900 1.015 143.6 4.80 665 80 7920 1000 1.015 189.0 6.32 893 97 7762 700 1.010 112 3.74 388 60 10587 Unit 3 900 1.015 172.5 5.77 625 76 10123 1000 1.030 230 7.69 920 100 9169 Discussion: Compare data at the same load and speed operating points. The consumption and BSFC is different at each point. The load, site conditions and equipment condition affect the fuel consumption. Note that the OEM predicted fuel consumption is about 20% lower than what is actually consumed. Case Study III: Lean Burn Conversion Objective: Show how a REMVue system can reduce fuel consumption. Site Configuration: Three units in parallel driving reciprocating compressors. Equipment Description: Unit 1 Engine: Waukesha 7042GSI (Turbocharged 1232 horsepower at 1000 RPM) Compressor: Ingersoll Rand RDS, four cylinder, single stage Ignition: MPI set at 24 degrees BTDC Engine Management System: REMVue 500 with AFR (rich to lean conversion) Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 59 of 70 Unit 2 Engine: Waukesha 7042GSI (Turbocharged 1232 horsepower at 1000 RPM) Compressor: Ingersoll Rand RDS, four cylinder, single stage Ignition: MPI set at 24 degrees BTDC Engine Management System: REMVue 500 with AFR (rich to lean conversion) Unit 3 Engine: Waukesha 7042GSI (Turbocharged 1232 horsepower at 1000 RPM) Compressor: Ingersoll Rand RDS, four cylinder, single stage Ignition: IQ 500 set at 24 degrees BTDC Engine Management System: Waukesha standard stoichiometric carbureted Data: (actual field tests for a Southern Alberta site: see PTAC report reference) Unit Engine Speed O2 CO CO CO2 CO2 NO NOx NOx NOx CH4 (or CxHy) Lambda Fuel consumption Fuel consumption Compressor horsepower Percent of engine load BSFC (1000 Btu/scf gas) Average Exhaust Temperature RPM Percent PPM kg/hr Percent kg/hr PPM PPM kg/hr g/bhp-hr % Number KG/H E3M3/D Kw Percent Btu/BHP-h Degrees C Unit 3 (factory) 700 0.2 15330 26.02 11.6 309 2077 2079 5.78 11.12 0.686 1.010 112 3.74 388 60 10587 567 900 0.3 1282 3.4 11.5 478 3521 3528 15.31 18.27 0.560 1.015 172.5 5.77 625 76 10123 639 1000 0.6 2100 7.46 11.4 390 4344 4345 25.3 20.5 0.046 1.030 230 7.69 920 100 9169 690 Unit 1 (REMVue) 700 7.1 220 0.46 7.7 250 1292 1297 4.4 0 0.449 1.514 91 3.04 379 59 8806 455 900 5.7 160 0.51 8.5 421 3008 3029 15.67 18.76 0.096 1.375 153 5.11 623 75 9007 532 1000 6 165 0.73 8.4 585 2649 2664 19.39 16.1 0.153 1.403 211 7.05 898 98 8618 574 Unit 2 (REMVue) 700 6.6 250 0.52 8 261 2377 2385 8.14 16.03 0.120 1.462 95 3.18 379 59 9193 475 900 5.9 225 0.73 8.4 429 3025 3042 16.26 19.52 0.088 1.393 156 5.21 621 75 9213 548 1000 5.4 220 0.95 8.7 590 3934 3961 28.1 23.92 0.121 1.348 214 7.15 876 97 8960 597 Discussion Compare data at the same load and speed operating points for each engine. The consumption and BSFC is different for each engine at each point. The engine management systems affect the fuel consumption as well as the load and equipment condition. The REMVue engine management system allows operation at a leaner air fuel ratio. The engine runs cooler (note exhaust temperatures) and uses less fuel at the same load and speed. Lower BSFC shows efficiency improvement. Reduction of greenhouse gas emissions is also achieved by reducing CO and CO2. Case Study IV: Low Engine Load Adaptation Objective: Show how an underutilized engine service life can be extended. Site Configuration: One reciprocating compressor using 50% power utilization. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 60 of 70 Equipment Description Engine: Waukesha 7042GSI (Turbocharged 1478 horsepower at 1200 RPM) Engine Management System: Waukesha standard stoichiometric carbureted Data: (Waukesha published data) Waukesha L7042 GSI (turbocharged, 8:1 compression ratio, 130 F AC) Waukesha L7042 G (naturally aspirated, 8:1 compression ratio) Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 61 of 70 Discussion: Turbocharged engines require positive intake manifold pressure to provide acceptable service intervals. Loads that require less than about 60% rated power utilization will not maintain a positive intake manifold pressure. This will allow oil coking on the turbocharger bearings and increase carbon deposits on cylinder heads and valves. More frequent service intervals will be required for cleaning to avoid expensive repairs. One alternative to replacing the engine may be to remove the turbocharger. The system is then converted to a naturally aspirated engine. This will avoid the deposits and increase the service interval. Compare the charts above for the two fuel delivery systems using the same engine speed at 750 HP. It suggests that fuel consumption will also be reduced after the conversion. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 62 of 70 Appendix F Glossary of Terms A AIR-FUEL RATIO - The ratio of the weight, or volume, of air to fuel. AMBIENT AIR - The air that surrounds the equipment. The standard ambient air for performance calculations is air at 80 °F, 60% relative humidity, and a barometric pressure of 29.921 in. Hg, giving a specific humidity of 0.013 lb of water vapour per lb of dry air. AMBIENT TEMPERATURE - The temperature of the air surrounding the equipment. ATMOSPHERIC AIR - Air under the prevailing atmospheric conditions. ATMOSPHERIC PRESSURE - The barometric reading of pressure exerted by the atmosphere. At sea level 14.7 lb per sq in. or 29.92 in. of mercury. B BAROMETRIC PRESSURE - Atmospheric pressure as determined by a barometer usually expressed in inches of mercury. BRAKE SPECIFIC FUEL CONSUMPTION (BSFC) – A measure of fuel efficiency for gas engines. It is normalized with load and power to establish a uniform means of comparison. Imperial units are most common: Btu/(BHP-h). BRITISH THERMAL UNIT (Btu) - The mean British Thermal Unit is 1/180 of the heat required to raise the temperature of 1 lb of water from 32 °F to 212 °F at a constant atmospheric pressure. A Btu is essentially 252 calories. C C - Carbon element, CO - Carbon monoxide. CO2 - Carbon dioxide. CLEARANCE – The amount of volume not used in compression for a reciprocating compressor cylinder. Clearance adjustment devices can variable head end volume pockets, fixed volume bottles, valve chairs or cylinder end unloading devices. COMBUSTIBLE LOSS - The loss representing the unliberated thermal energy occasioned by failure to oxidize completely some of the combustible matter in the fuel. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 63 of 70 COMBUSTION - The rapid chemical combination of oxygen with the combustible elements of a fuel resulting in the release of heat. COMBUSTION AIR - Air used in the combustion process. Air contains oxygen which is required to combust fuel. COMBUSTION EFFICIENCY - The effectiveness of the engine to completely burn the fuel. The air content will vary depending on the combustion process used for the engine while converting all combustibles in the fuel to useful energy. COMPLETE COMBUSTION - The complete oxidation of all the combustible constituents of a fuel. COMPPRESSION RATIO - The ratio of discharge pressure to suction pressure. The use of absolute pressure units is recommended when calculating compression ratio. COMPRESSOR FRAME – The compressor frame for a screw compressor is the main pressure retaining housing that contains the lubrication, rotors and running gear. In the case of a reciprocating compressor, the frame does not retain pressure but houses the lubrication and running gear of the rotating elements. COMPRESSOR CYLINDER - The pressure retaining components where the pistons travel to compress the gas in a reciprocating compressor. Cylinders may be referred to as single acting or double acting depending if the gas is compressed on one or both sides of the piston. Cylinders are available in many sizes and pressure rating to allow flexible compression strategies and multiple stages of compression. Normally one cylinder is attached one throw of the compressor frame but tandem cylinders are also available where two bore sizes are installed on one throw. CONDUCTION - The transmission of heat through and by means of matter unaccompanied by any obvious motion of the matter. D DESIGN LOAD - The load for which equipment is designed, is considered the maximum load to be carried. DEW POINT - The temperature at which condensation starts. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 64 of 70 DISTANCE PIECE – The housing assembly containing the piston rod and crosshead guide between the frame and cylinders for a reciprocating compressor is known as the distance piece. It can contain none, one or two compartments separated by oil wipers with optional purge and vent assemblies. Distance piece compartment lengths should be long enough that one point on the compressor rod can not pass through more than one packing or oil wiper assembly. The purpose of the compartments is to isolate the frame from corrosive gases leaking from the cylinder. DRY GAS - Gas containing no water vapour. E EFFICIENCY - The ratio of output to input. See also Combustion and Thermal Efficiency. ENGINE SPEED – The number of revolutions an engine turns in a unit of time. Normally expressed in RPM or sometimes Hz. EXCESS AIR - Air supplied for combustion in excess of that theoretically required for complete oxidation. F FLUE GAS - The gaseous product of combustion in the flue to the stack. FUEL-AIR MIXTURE - Mixture of fuel and air. FUEL-AIR RATIO - The ratio of the weight, or volume, of fuel to air. G GAS ANALYSIS - The determination of the constituents of a gaseous mixture. GAS PRESSURE REGULATOR - A spring loaded, dead weighted or pressure balanced device which will maintain the gas pressure to the burner supply line. GAUGE PRESSURE - The pressure above atmospheric pressure. H HEAT BALANCE - An accounting of the distribution of the heat input, output and losses. HEATING SURFACE - Those surfaces which are exposed to products of combustion on one side and water on the other. This surface is measured on the side receiving the heat. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 65 of 70 HEATING VALUE - The quantity of heat released by a fuel through complete combustion. It is commonly expressed in Btu per lb, per gallon, or cu-ft. HERTZ (Hz) – A unit of frequency that is defined as one cycle per second. Hertz are sometimes used as an alternate means to express engine speed. I INCOMPLETE COMBUSTION - The partial oxidation of the combustible constituents of a fuel. INTEGRAL ROTATING EUQIPMENT – Two or more rotating equipment elements that share the same crank case and crankshaft. A compressor and engine combined into one frame forms and integral compressor. Screw or other compressors may combine a gear set in one common housing with the compressor. M MMBtu - Millions of Btus (British Thermal Units). MOISTURE - Water in the liquid or vapour phase. N NATURALLY ASPIRATED – Atmospheric pressure engine combustion air that is drawn into the engine using the piston downward stroke as the motivation for the air flow (think of the engine as an air pump). NOx - Abbreviation for the sum of nitrogen mon-oxide, NO, and nitrogen dioxide, NO2. O ORIFICE - A calibrated opening or nozzle used to deliver fuel gas. P ppm - Abbreviation for parts per million. Used in chemical determinations as one part per million parts by weight. PACKING – The packing assemblies or packing cases seal around the piston rod to retain gas in the reciprocating compressor cylinder. The assemblies are lubricated, sometime cooled and may be purged depending on the application. PRODUCTS OF COMBUSTION - The gases, vapours, and solids resulting form the combustion of fuel. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 66 of 70 R RATED CAPACITY - The manufacturers stated capacity rating for mechanical equipment; for instance, the maximum continuous power for which and engine is designed. ROTATIONS PER MINUTE (RPM) – The number of rotations of the crankshaft per minute. It is common units of operating speed for rotating equipment. S SEPARABLE ROTATING EQUIPMENT – Rotating equipment elements that do not share a common crank shaft or crankcase. Rotating element shafts are connected by coupling(s). SHELL - The cylindrical portion of a pressure vessel. SLIDE VALVE – A movable device on a screw compressor that exposes a variable length of the rotors to afford compression. A slide valve allows some inefficiency due to internal gas recirculation. SPECIFIC HEAT - The quantity of heat, expressed in Btu, required to raise the temperature of 1 lb of a substance 1 deg F. STACK - A vertical conduit, which due to the difference in density between internal and external gases, creates a draft at its base. SUPERCHARGER – An engine combustion air compressor driven from the crankshaft using a mechanical drive. T THEORETICAL AIR - The quantity of air required for perfect combustion. THERMAL EFFICIENCY - The efficiency of a heater, based on the ratio of heat absorbed to total heat input. This does not include heat loss from the boiler shell. TOTAL AIR - The total quantity of air supplied to the fuel and products of combustion. Percent total air is the ratio of total air to theoretical air, expressed as percent. TURBOCHARGER – An engine combustion air compressor driven from the exhaust gases using a turbine wheel and gas expansion. TURNDOWN RATIO - Ratio of maximum to minimum operating speed or throughput. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 67 of 70 U UNBURNED COMBUSTIBLE - The combustible portion of the fuel that is not completely oxidized. V VE – The internal volumetric ratio of a reciprocating compressor cylinder. It is normally adjustable and affects drive train loading, power consumption and efficiency. Vi – Volume index; the internal volumetric ratio of suction volume to discharge volume for a screw compressor. It is normally adjustable and affects bearing life, power consumption and efficiency. Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 68 of 70 Appendix G References 1 “An Investigation into REMVue Technology and the Effects on Compressor Reliability” Prepared by Venessa Veres (BP Canada employee) May 2005 2 REM Technology Inc information from their website; http://www.remtechnology.com/news/magazine2.htm “A VALIDATION STUDY FOR A REMVue 500/A ENGINE MANAGEMENT SYSTEM”, Prepared for PetroCanada Oil and Gas by Accurata Inc; Frank Zahner, Ken Terrell, Howard Malm, Philip Croteau, Jody Hood, June 2005 REMVue Field Test Results for BP 2004 Installations, Prepared for BP Canada Energy by Accurata Inc; Bill Gibb, Ken Terrell, Frank Zahner, September 30 2005 5 4 3 Natural Gas Engines – Reducing Greenhouse Gases, Prepared for Combustion Canada Conference by REM Technology, Howard Malm, September 22-24 2003 6 Fuel System Management Alternatives For Rich Burn Engines, Prepared for Devon Canada by Accurata Inc; Frank Zahner, Ken Terrell, December 2003 7 Personal correspondence with Waukesha and White Superior representatives as well as REM Technology Inc personnel (Howard Malm, Cam Dowler, Lorne Tuck, Greg Brown, Wade Mowat), 2003 to present. 8 Nitrous Oxide Emissions from Light Duty Vehicles by Vera F. Ballantyne, Peter Howes, and Lief Stephanson – Environment Canada, SAE paper 940304 9 Gas processing graphic – Producers Technology Transfer Workshop by Robinson – Occidental Oil and Gas and EPA’s Natural Gas STAR Program Midland, TX June 8, 2006 (Permission to use graphic granted June 14 2007 by Roger Fernandez of the US Natural Gas STAR Program) 10 Emissions and Efficiency Enhancements with REM AFR Systems, Prepared by Accurata Inc; by Bill Gibb, Ken Terrell, Frank Zahner – Petroleum Technology Alliance Canada, March 2006 11 Natural Gas Engines – Reducing Greenhouse Gases, Prepared for Combustion Technology Conference by REM Technology Inc; by Howard Malm, September 22-24 2003 12 13 14 Waukesha technical data and literature Caterpillar technical data and literature Cummins technical data and literature Rev Date 27/05/2008 Page 69 of 70 Efficient Use of Fuel Gas in Engines Module 7 of 17 15 16 Arrow technical data and literature REM Technology Inc generously provided the discussion on flow meter selection and installation for engines (Appendix C). Howard Malm is the author (July 2005). Endnotes 1 The Gross heating value includes the heat from the condensation of the combustion water vapour to liquid water at the standard temperature (15.5 C / 60 F), whereas the Lower heating value excludes this heat. 2 Prices used were typical at the time this report was made. To adjust for different prices, multiply by the ratio of the new price to the price in the table. Note that brake specific fuel consumption depends on many factors; the values used for this table are test cell values for full load operation. The Gross heating value includes the heat from the condensation of the combustion water vapour to liquid water at the standard temperature (15.5 C / 60 F), whereas the Lower heating value excludes this heat. 5 6 7 4 3 Programs are available from Ariel Compressor or PIC Division of Spartan Controls. 100 year equivalent International Panel on Climate Change - 2001 100 year equivalent International Panel on Climate Change – 2001 Efficient Use of Fuel Gas in Engines Module 7 of 17 Rev Date 27/05/2008 Page 70 of 70