Reciprocating Engines

Reciprocating internal combustion engines are a widespread and well-known technology. North American production exceeds 35 million units per year for automobiles, trucks, construction and mining equipment, marine propulsion, lawn care, and a diverse set of power generation applications. A variety of stationary engine products are available for a range of power generation market applications and duty cycles including standby and emergency power, peaking service, intermediate and baseload power, and combined heat and power (CHP). Reciprocating engines are available for power generation applications in sizes ranging from a few kilowatts to over 5 MW. A schematic of a reciprocating engine based CHP system is shown in Figure 4-1.

There are two basic types of reciprocating engines - spark ignition (SI) and compression ignition (CI). Spark ignition engines for power generation use natural gas as the preferred fuel, although they can be set up to run on propane, gasoline, or landfill gas. Compression ignition engines (often called diesel engines) operate on diesel fuel or heavy oil, or they can be set up to run in a dual-fuel configuration that burns primarily natural gas with a small amount of diesel pilot fuel.

Diesel engines have historically been the most popular type of reciprocating engine for both small and large power generation applications. However, in the United States and other industrialized nations, diesel engines are increasingly restricted to emergency standby or limited duty-cycle service because of air emission concerns. As a result, the natural gas-fueled SI engine is now a popular choice for the higher-duty-cycle stationary power market (over 500 hr/yr).

Current generation natural gas engines offer low first cost, fast start-up, proven reliability when properly maintained, excellent load-following characteristics, and significant heat recovery potential. Electric efficiencies of natural gas engines range from 28% LHV for small stoichiometric engines (<100 kW) to over 40% LHV for very large lean burn engines (> 3 MW) (1). Waste heat can be recovered from the hot engine exhaust and from the engine cooling systems to produce either hot water or low pressure steam for CHP applications. Overall CHP system efficiencies (electricity and useful thermal energy) of 70 to 80% are routinely achieved with natural gas engine systems.

Reciprocating engine technology has improved dramatically over the past three decades, driven by economic and environmental pressures for power density improvements (more output per unit of engine displacement), increased fuel efficiency and reduced emissions. Computer systems have greatly advanced reciprocating engine design and control, accelerating advanced engine designs and making possible more precise control and diagnostic monitoring of the engine process. Stationary engine manufacturers and worldwide engine R&D firms continue to drive advanced engine technology, including accelerating the diffusion of technology and concepts from the automotive market to the stationary market.

The emissions signature of natural gas SI engines in particular has improved significantly in the last decade through better design and control of the combustion process and through the use of exhaust catalysts. Advanced lean burn natural gas engines are available that produce NO x levels as low as 50 ppmv @ 15% O 2 (dry basis).

4.1.1 Technology Description

There are two primary reciprocating engine designs relevant to stationary power generation applications - the spark ignition Otto-cycle engine and the compression ignition Diesel-cycle engine. The essential mechanical components of the Otto-cycle and Diesel-cycle are the same. Both use a cylindrical combustion chamber in which a close fitting piston travels the length of the cylinder. The piston is connected to a crankshaft that transforms the linear motion of the piston into the rotary motion of the crankshaft. Most engines have multiple cylinders that power a single crankshaft.

The primary difference between the Otto and Diesel cycles is the method of igniting the fuel. Spark ignition engines (Otto-cycle) use a spark plug to ignite a pre-mixed air fuel mixture introduced into the cylinder. Compression ignition engines (Diesel-cycle) compress the air introduced into the cylinder to a high pressure, raising its temperature to the auto-ignition temperature of the fuel which is injected at high pressure.

Engines are further categorized by crankshaft speed (rpm), operating cycle (2- or 4-stroke), and whether turbocharging is used. Reciprocating engines are also categorized by their original design purpose - automotive, truck, industrial, locomotive and marine. Many automotive engine models are used in hundreds of small-scale stationary power, CHP, irrigation, and chiller applications. These are generally low-priced engines due to large production volumes. However, unless conservatively rated, these engines have limited durability. Truck engines have the cost benefit of production volume and are designed for reasonably long life (e.g., one million miles) and offer longer durability than automotive engines in stationary applications. A number of truck engines are available as stationary engines. Engines intended for industrial use are designed for durability and for a wide range of mechanical drive and electric power applications. Their sizes range from 20 kW up to 6 MW, including industrialized truck engines in the 200 to 600 kW range and industrially applied marine and locomotive engines above 1 MW.

Both the spark ignition and the diesel 4-stroke engines most relevant to stationary power generation applications complete a power cycle in four strokes of the piston within the cylinder:

The simplest natural gas engines operate with natural aspiration of air and fuel into the cylinder (via a carburetor or other mixer) by the suction of the intake stroke. High performance natural gas engines are turbocharged to force more air into the cylinders. Natural gas spark ignition engines operate at modest compression ratios (compared with diesel engines) in the range of 9:1 to 12:1 depending on engine design and turbocharging. Modest compression is required to prevent auto-ignition of the fuel and engine knock, which can cause serious engine damage.(2)

Using high energy ignition technology, very lean fuel-air mixtures can be burned in natural gas engines, lowering peak temperatures within the cylinders and resulting in reduced NO x emissions. The lean burn approach in reciprocating engines is analogous to dry low-NO x combustors in gas turbines. All major natural gas engine manufacturers offer lean burn, low emission models and are engaged in R&D to further improve their performance.

Natural gas spark ignition engine efficiencies are typically lower than diesel engines because of their lower compression ratios. However, large, high performance lean burn engine efficiencies approach those of diesel engines of the same size. Natural gas engine efficiencies range from about 28% (LHV) for small engines (<50 kW) to over 40% (LHV) for the largest high performance, lean burn engines. Lean burn engines tuned for maximum efficiency may produce twice the NO x emissions as the same engine tuned for minimum NO x . Tuning for low NO x typically results in a sacrifice of 1 to 1.5 percentage points in electric generating efficiency from the highest level achievable.

Many natural gas spark ignition engines are derived from diesel engines, i.e., they are built using the same block, crankshaft, main bearings, camshaft, and connecting rods as the diesel engine. However, natural gas spark ignition engines operate at lower brake mean effective pressure (BMEP) and peak pressure levels to prevent knock. (3) Due to the derating effects from lower BMEP, the spark ignition versions of diesel engines often produce only 60 to 80% of the power output of the parent diesel. Manufacturers often enlarge cylinder bore about 5 to 10% to increase the power, but this is only partial compensation for the derated output. Consequently, the $/kW capital costs of natural gas spark ignition engines are generally higher than the diesel engines from which they were derived. However, by operating at lower cylinder pressure and bearing loads as well as in the cleaner combustion environment of natural gas, spark ignition engines generally offer the benefits of extended component life compared to their diesel parents.

Conventional, low pressure gas injection engines typically require about 5 to 10% pilot fuel and may be derated to about 80 to 95% of the rated diesel capacity to avoid detonation. The minimum pilot fuel requirement is generally set by the turndown ratio of the diesel fuel injection system. Conventional diesel injectors cannot reliably turn down to less than 5 to 6% of the full load injection rate. Natural gas input is controlled at each cylinder by injecting gas before the air intake valves open. NO x emissions of conventional dual fuel engines are generally in the 5 to 8 gm/kWh range (compared to lean burn natural gas engines with NO x emissions in the 0.7 to 2.5 gm/kWh range).

High pressure gas injection engines attempt to reduce derating by injecting natural gas at very high pressures (3,600 to 5,100 psig) directly into the main combustion chamber as the pilot fuel is injected. However, the parasitic power for gas compression can be as high as 4 to 7% of the rated power output - partly offsetting the benefit of reduced derating. This technology has not proved particularly popular because of this issue and the additional equipment costs required for gas injection. Pilot fuel consumption is typically 3 to 8% and NO x emissions are generally in the 5 to 8 gm/kWh range.

Micropilot prechamber engines are similar to spark ignition prechamber engines in that the pilot fuel injected into a prechamber provides a high energy torch that ignites the very lean, compressed fuel air mixture in the cylinder. Leaner mixtures than spark ignition engines are achievable since the energy provided by the diesel-fueled micropilot chamber is higher than that obtained with a spark ignition prechamber. Micropilot dual fuel engines with 1% pilot fuel can operate at or close to the diesel engine's compression ratio and BMEP, so little, if any, derating occurs. In this case the high power density and low $/kW cost advantage of the original diesel engine are retained and engine efficiency at 75 to 100% load is close to that of the 100% diesel engine. NO x and other emissions are comparable to those of lean burn spark ignition prechamber engines (NO x emissions in the 0.7 to 2.5 gm/kWh range). These engines must be equipped with conventional diesel fuel injectors in order to operate on 100% diesel.

Several independent developers and engine manufacturers are testing and commercializing dual fuel retrofit kits for converting existing diesel engines to dual fuel operation. The level of sophistication of these kits varies widely and some require major engine modifications. Derating, efficiencies, and emissions also vary widely and have yet to be fully tested or certified. However, dual fuel conversions are not expected to be as low in emissions as dedicated natural gas engines. In addition, manufacturers may not honor warrantees on an engine that has been retrofitted by an independent third party.

4.1.2 Performance

Table 4-1 summarizes performance characteristics for typical commercially available natural gas spark ignition engine CHP systems over a 100 kW to 5 MW size range. This size range covers the majority of the market applications for engine-driven CHP. Heat rates and efficiencies shown were taken from manufacturers' specifications and industry publications. Available thermal energy was calculated from published engine data on engine exhaust temperatures and engine jacket and lube system coolant flows. CHP thermal recovery estimates are based on producing hot water for process or space heating needs. As shown in the table, 50 to 60% of the waste heat from engine systems is recovered from jacket cooling water and lube oil cooling systems at a temperature too low to produce steam. This feature is generally less critical in commercial/institutional applications where it is more common to have hot water thermal loads. Steam can be produced from the exhaust heat if required (maximum pressure of 150 psig), but if no hot water is needed, the amount of heat recovered from the engine is reduced and total CHP system efficiency drops accordingly.

The data in the table show that electrical efficiency increases as engine size becomes larger. As electrical efficiency increases, the absolute quantity of thermal energy available to produce useful thermal energy decreases per unit of power output, and the ratio of power to heat for the CHP system generally increases. A changing ratio of power to heat impacts project economics and may affect the decisions that customers make in terms of CHP acceptance, sizing, and the desirability of selling power.

4.1.3 Emissions

Exhaust emissions are the primary environmental concern with reciprocating engines. The primary pollutants are oxides of nitrogen (NO x ), carbon monoxide (CO), and volatile organic compounds (VOCs - unburned, non-methane hydrocarbons). Other pollutants such as oxides of sulfur (SO x ) and particulate matter (PM) are primarily dependent on the fuel used. Emissions of sulfur compounds, primarily SO 2 , are related to the sulfur content of the fuel. Engines operating on natural gas or distillate oil, which has been desulfurized in the refinery, emit insignificant levels of SO x. In general, SO x emissions are an issue only in large, slow speed diesels firing heavy oils. Particulate matter (PM) can be an important pollutant for engines using liquid fuels. Ash and metallic additives in the fuel contribute to PM in the exhaust.

NO x emissions are usually the primary concern with natural gas engines and are a mixture of (mostly) NO and NO 2 in variable composition. In measurement, NO x is reported as parts per million by volume in which both species count equally (e.g., ppmv at 15% O 2 , dry). Other common units for reporting NO x in reciprocating engines are gm/hp-hr and gm/kWh, or as an output rate such as lbs/hr. Among engine options, lean burn natural gas engines produce the lowest NO x emissions and diesel engines produce the highest (without further exhaust treatment).

The control of peak flame temperature through lean burn conditions has been the primary combustion approach to limiting NO x formation in gas engines. Diesel engines produce higher combustion temperatures and more NO x than lean burn gas engines, even though the overall diesel engine air/fuel ratio may be very lean. There are three reasons for this: (1) heterogeneous near-stoichiometric combustion; (2) the higher adiabatic flame temperature of distillate fuel; and (3) fuel-bound nitrogen. The diesel fuel is atomized as it is injected and dispersed in the combustion chamber. Combustion largely occurs at near-stoichiometric conditions at the air-droplet and air-fuel vapor interfaces, resulting in maximum temperatures and higher NO x . In contrast, lean-premixed homogeneous combustion used in lean burn gas engines results in lower combustion temperatures and lower NO x production.

For any engine there are generally trade-offs between low NO x emissions and high efficiency. There are also trade-offs between low NO x emissions and emissions of the products of incomplete combustion (CO and unburned hydrocarbons). There are three main approaches to these trade-offs that come into play depending on regulations and economics. One approach is to control for lowest NO x accepting a fuel efficiency penalty and possibly higher CO and hydrocarbon emissions. A second option is finding an optimal balance between emissions and efficiency. A third option is to design for highest efficiency and use post-combustion exhaust treatment.

There are several types of catalytic exhaust gas treatment processes that are applicable to various types of reciprocating engines - three-way catalyst, selective catalytic reduction, oxidation catalysts, and lean NO x catalysts.

The catalytic three-way conversion process (TWC) is the basic automotive catalytic converter process that reduces concentrations of all three major criteria pollutants - NO x , CO and VOCs. The TWC is also called non-selective catalytic reduction (NSCR). NO x and CO reductions are generally greater than 90%, and VOCs are reduced approximately 80% in a properly controlled TWC system. Because the conversions of NO x to N 2 and CO and hydrocarbons to CO 2 and H 2 O will not take place in an atmosphere with excess oxygen (exhaust gas must contain less than 0.5% O 2 ), TWCs are only effective with stoichiometric or rich-burning engines. Typical "engine out" NO x emission rates for a rich burn engine are 10 to 15 gm/bhp-hr. NO x emissions with TWC control are as low as 0.15 gm/bhp-hr.

Stoichiometric and rich burn engines generally have lower efficiencies than lean burn engines. The TWC system also increases maintenance costs by as much as 25%. TWCs are based on noble metal catalysts that are vulnerable to poisoning and masking, limiting their use to engines operated with clean fuels - e.g., natural gas and unleaded gasoline. Also, the engines must use lubricants that do not generate catalyst poisoning compounds and have low concentrations of heavy and base metal additives. Unburned fuel, unburned lube oil, and particulate matter can also foul the catalyst. TWC technology is not applicable to lean burn gas engines or diesels.

Lean burn engines equipped with selective catalytic reduction (SCR) technology selectively reduces NO x to N 2 in the presence of a reducing agent. NO x reductions of 80 to 90% are achievable with SCR. Higher reductions are possible with the use of more catalyst or more reducing agent, or both. The two agents used commercially are ammonia (NH 3 in anhydrous liquid form or aqueous solution) and aqueous urea. Urea decomposes in the hot exhaust gas and SCR reactor, releasing ammonia. Approximately 0.9 to 1.0 moles of ammonia is required per mole of NO x at the SCR reactor inlet in order to achieve an 80 to 90% NO x reduction.

SCR systems add a significant cost burden to the installation cost and maintenance cost of an engine system, and can severely impact the economic feasibility of smaller engine projects. SCR requires on-site storage of ammonia, a hazardous chemical. In addition ammonia can "slip" through the process unreacted, contributing to environmental health concerns.

Oxidation catalysts generally are precious metal compounds that promote oxidation of CO and hydrocarbons to CO 2 and H 2 O in the presence of excess O 2 . CO and NMHC conversion levels of 98 to 99% are achievable. Methane conversion may approach 60 to 70%. Oxidation catalysts are now widely used with all types of engines, including diesel engines. They are being used increasingly with lean burn gas engines to reduce their relatively high CO and hydrocarbon emissions.

Lean-NO x catalysts utilize a hydrocarbon reductant (usually the engine fuel) injected upstream of the catalyst to reduce NO x . While still under development, it appears that NO x reduction of 80% and both CO and NMHC emissions reductions of 60% may be possible. Long-term testing, however, has raised issues about sustained performance of the catalysts. Current lean-NO x catalysts are prone to poisoning by both lube oil and fuel sulfur. Both precious metal and base metal catalysts are highly intolerant of sulfur. Fuel use can be significant with this technology - the high NO x output of diesel engines would require approximately 3% of the engine fuel consumption for the catalyst system.

4.1.4 CHP Applications

Potential distributed generation applications for reciprocating engines include standby, peak shaving, grid support, and CHP applications in which hot water, low pressure steam, or waste-heat-fired absorption chillers are required. Reciprocating engines are also used extensively as direct mechanical drives in applications such as water pumping, air and gas compression and chilling/refrigeration. While the use of reciprocating engines is expected to grow in various distributed generation applications, the most prevalent on-site generation application for natural gas SI engines has traditionally been CHP, and this trend is likely to continue. The economics of natural gas engines in on-site generation applications is enhanced by effective use of the thermal energy contained in the exhaust gas and cooling systems, which generally represents 60 to 70% of the inlet fuel energy.

4.1.5 Thermal Energy Generation

There are four sources of usable waste heat from a reciprocating engine: exhaust gas, engine jacket cooling water, lube oil cooling water, and turbocharger cooling. Heat can generally be recovered in the form of hot water or low pressure steam (<30 psig). Medium pressure steam (up to about 150 psig) can be generated from the engine's high temperature exhaust gas, but the hot exhaust gas contains only about one half of the available thermal energy from a reciprocating engine. Some industrial CHP applications use the engine exhaust gas directly for process drying. Generally, the hot water and low pressure steam produced by reciprocating engine CHP systems is appropriate for low temperature process needs, space heating, potable water heating, and to drive absorption chillers providing cold water, air conditioning or refrigeration.

The most common method of recovering engine heat is the closed-loop cooling system as shown in Figure 4-2. These systems are designed to cool the engine by forced circulation of a coolant through engine passages and an external heat exchanger. An excess heat exchanger transfers engine heat to a cooling tower or radiator when there is excess heat generated. Closed-loop water cooling systems can operate at coolant temperatures from 190 to 250°F. Depending on the engine and CHP system's requirements, the lube oil cooling and turbocharger aftercooling may be either separate or part of the jacket cooling system.

Ebullient cooling systems cool the engine by natural circulation of a boiling coolant through the engine. This type of cooling system is typically used in conjunction with exhaust heat recovery for production of low-pressure steam. Cooling water is introduced at the bottom of the engine where the transferred heat begins to boil the coolant generating two-phase flow. The formation of bubbles lowers the density of the coolant, causing a natural circulation to the top of the engine.

The coolant at the engine outlet is maintained at saturated steam conditions and is usually limited to 250°F and a maximum of 15 psig. Inlet cooling water is also near saturation conditions and is generally 2 to 3°F below the outlet temperature. The uniform temperature throughout the coolant circuit extends engine life and contributes to improved combustion efficiencies.

Exhaust heat is typically used to generate hot water to about 230°F or low-pressure steam (up to 150 psig). Only a portion of the exhaust heat can be recovered since exhaust gas temperatures are generally kept above temperature thresholds to prevent the corrosive effects of condensation in the exhaust piping. For this reason, most heat recovery units are designed for a 250 to 350°F exhaust outlet temperature.

Exhaust heat recovery can be independent of the engine cooling system or coupled with it. For example, hot water from the engine cooling can be used as feedwater or feedwater preheat to the exhaust recovery unit. In a typical district heating system, jacket cooling, lube oil cooling, single stage aftercooling and exhaust gas heat recovery are all integrated for steam production.

4.1.6 Current Market Applications

There were an estimated 1,055 engine-based CHP systems operating in the United States in 2000 representing over 800 MW of electric capacity. Facility capacities range from 30 kW to 30 MW, with many larger facilities comprised of multiple units. Reciprocating engine CHP systems are installed in a variety of applications as shown in Figure 4-3. Spark ignited engines fueled by natural gas or other gaseous fuels represent 84% of the installed reciprocating engine CHP capacity.

Thermal loads most amenable to engine-driven CHP systems in commercial/institutional buildings are space heating and hot water requirements. The simplest thermal load to supply is hot water. Primary applications for CHP in the commercial/institutional sectors are those building types with relatively high and coincident electric and hot water demand such as colleges and universities, hospitals and nursing homes, and lodging. Office buildings, and certain warehousing and mercantile/service applications can be economic applications for CHP if space heating needs can be incorporated. Technology development efforts targeted at heat activated cooling/refrigeration and thermally regenerated desiccants are designed to expand the application of engine-driven CHP by increasing the thermal energy loads in certain building types. Use of CHP thermal output for absorption cooling and/or desiccant dehumidification could increase the size and improve the economics of CHP systems in existing CHP markets such as schools, lodging, nursing homes and hospitals. Use of these advanced technologies in applications such as restaurants, supermarkets and refrigerated warehouses provides a base thermal load that opens these applications to CHP.

A typical commercial application for reciprocating engine CHP is a hospital or health care facility with a 1 MW CHP system comprised of multiple 200 to 300 kW natural gas engine gensets. The system is designed to satisfy the baseload electric needs of the facility. Approximately 1.6 MW thermal (MW th ) of hot water is recovered from engine exhaust and engine cooling systems to provide space heating and domestic hot water to the facility, and to drive absorption chillers for space conditioning during summer months. Overall efficiency of this type of CHP system can exceed 70%.

Engine-driven CHP can be used in a variety of industrial applications where hot water or low pressure steam is required for process needs or space heating. A typical industrial application for engine CHP would be a food processing plant with a 2 MW natural gas engine-driven CHP system comprised of multiple 500 to 800 kW engine gensets. The system provides baseload power to the facility and approximately 2.2 MW th low pressure steam for process heating and washdown. Overall efficiency for a CHP system of this type approaches 75%.

4.1.7 CHP Potential

The economics of engines in on-site power generation applications often depend on effective use of the thermal energy contained in the exhaust gas and cooling systems, which generally represents 60 to 70% of the inlet fuel energy. Most of the waste heat is available in the engine exhaust and jacket coolant, while smaller amounts can be recovered from the lube oil cooler and the turbocharger's intercooler and aftercooler (if so equipped). The most common use of this heat is to generate hot water or low pressure steam for process use or for space heating, process needs, domestic hot water or absorption cooling. However, the engine exhaust gases can also be used as a source of direct energy for drying or other direct heat processes.

Heat in the engine jacket coolant accounts for up to 30% of the energy input and is capable of producing 200 to 210°F hot water. Some engines, such as those with high pressure or ebullient cooling systems, can operate with water jacket temperatures up to 265°F. Engine exhaust heat represents from 30 to 50% of the available waste heat. Exhaust temperatures of 850 to 1200°F are typical. By recovering heat in the cooling systems and exhaust, approximately 60 to 70% of the fuel's energy can be effectively utilized to produce both power and useful thermal energy.