Working Principle of Natural Gas Engines for Power Generation
by Starlight Generator dieselgeneratortechCurrent
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.11
•
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.12
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.
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Created on Apr 19th 2019 21:00. Viewed 236 times.