Working Principle of Natural Gas Engine for Power Generation
Current natural gas engines for power generation 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 30 percent LHV (27 percent HHV) for small stoichiometric engines (<100 kW) to over 46 percent LHV (42 percent HHV) for large lean burn engines (> 3 MW).9,10 Waste heat recovered from the hot engine exhaust and from the engine cooling systems produces either hot water or low pressure steam for CHP applications. Overall CHP system efficiencies (electricity and useful thermal energy) of up to 80 percent (HHV) can be achieved.
Spark ignition engines use spark plugs, with a high-intensity spark of timed duration, to ignite a compressed fuel-air mixture within the cylinder. Natural gas is the predominant spark ignition engine fuel used in electric generation and CHP applications. Other gaseous and volatile liquid fuels, ranging from landfill gas to propane to gasoline, can be used with the proper fuel system, engine compression ratio, and tuning. American manufacturers began to develop large natural gas engines for the burgeoning gas transmission industry after World War II. Smaller engines were developed (or converted from diesel blocks) for gas gathering and other stationary applications as the natural gas infrastructure developed. Natural gas engines for power generation applications are primarily 4-stroke engines, available in sizes up to about 18 MW.
Depending on the engine size, one of two ignition techniques ignites the natural gas:
• Open chamber – the spark plug tip is exposed in the combustion chamber of the cylinder, directly igniting the compressed fuel-air mixture. Open chamber ignition is applicable to any engine operating near the stoichiometric air/fuel ratio for up to moderately lean mixtures.
• Precombustion chamber – a staged combustion process where the spark plug is housed in a small chamber mounted on the cylinder head. This cylinder is charged with a rich mixture of fuel and air, which upon ignition shoots into the main combustion chamber in the cylinder as a high energy torch. This technique provides sufficient ignition energy to light off very lean fuel-air mixtures used in large bore engines.
The simplest natural gas engines operate with a 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 9 The exact ratio of air to fuel that is required for complete combustion is called the stoichiometric ratio. If there is less or more air than needed for complete combustion the engine is called rich burn or lean burn respectively. 10 Most efficiencies quoted in this report are based on higher heating value (HHV), which includes the heat of condensation of the water vapor in the combustion products. In engineering and scientific literature the lower heating value (LHV – which does not include the heat of condensation of the water vapor in the combustion products) is often used. The HHV is greater than the LHV by approximately 10% with natural gas as the fuel (i.e., 50% LHV is equivalent to 45% HHV). Higher Heating Values are about 6% greater for oil (liquid petroleum products) and 5% for coal. 11 Stoichiometric ratio is the chemically correct ratio of fuel to air for complete combustion, i.e., there is no unused fuel or oxygen after combustion.
12 Lean mixture is a mixture of fuel and air in which an excess of air is supplied in relation to the amount needed for complete combustion; similarly, a rich mixture is a mixture of fuel and air in which an excess of fuel is supplied in relation to the amount needed for complete combustion. 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.
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 NOx emissions. The lean burn approach in reciprocating engines is analogous to dry low-NOx 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 can exceed those of diesel engines of the same size. Natural gas engine efficiencies range from about 28 percent (LHV) for small engines (<50 kW) to 46 percent (LHV) for the largest high performance, lean burn engines. Lean burn engines tuned for maximum efficiency may produce twice the NOx emissions as the same engine tuned for minimum NOx. Tuning for low NOx 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 use 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.13 Due to the derating effects from lower BMEP, the spark ignition versions of smaller diesel engines may produce only 60 to 80 percent of the power output of the parent diesel.
Manufacturers often enlarge cylinder bore by about 5 to 10 percent to increase the power to levels which meet or exceed their diesel counterparts. The $/kW capital costs of large, high performance natural gas spark ignition engines are typically on a similar level to 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.