Why People are Using Natural Gas Generators


Internal combustion engine-powered generator sets fueled by natural gas are commanding more attention these days as interest grows in on-site power production equipment that is both efficient and environmentally friendly. In response to this interest, manufacturers have introduced natural gas engine-powered generator sets that feature new “lean-burn” technology. The combustion is considered “lean” when excess air is introduced into the engine along with the fuel. This produces two positive effects. First, the excess air reduces the temperature of the combustion process and this reduces the amount of oxides of nitrogen (NOx) produced by nearly half, compared to a conventional natural gas engine. Second, since there is also excess oxygen available, the combustion process is more efficient and more power is produced from the same amount of fuel.


Any air/fuel reaction requires an energy source to initiate combustion. In natural gas engines, the spark plug performs this function. In the new lean-burn engines, the combustion process is enhanced by pre-mixing the air and fuel upstream of the turbocharger before introduction into the cylinder. This creates a more concentrated mixture in the combustion chamber and reduces the occurrence of “knocking” or detonation. To prevent either knocking or misfiring, the combustion process must be controlled within a narrow operating window. Charge air temperatures and volume, together with air to fuel ratio and compression ratio, are constantly monitored. The microprocessor-based engine controller regulates the fuel flow and air/gas mixture and ignition timing. New lean-burn engines are designed to operate at a lean air/gas ratio of Lambda = 1.7.(Traditional stoichiometric natural gas engines have an air/gas ratio of Lambda = 1.0).In the chart that plots Break Mean Effective Pressure (BMEP) against Air Excess (Lambda), the operating window is a very narrow band where efficiency peaks and where Nox is near its minimum. A richer mixture (stoichiometric) can potentially produce knocking and higher Nox emissions; a leaner mixture than Lambda 1.7 may not combust reliably and cause misfiring, which raises HC emissions. Full-authority electronic engines, sensors and microprocessors in the new lean-burn engines are critical for maintaining combustion within these boundaries. The design of the lean-burn engine incorporates a simple open combustion chamber housed in the piston crown. The shape of the piston crown introduces turbulence in the incoming air/fuel mixture that promotes more complete combustion by thoroughly exposing it to the advancing flame front. The flame plate of the cylinder head is regular (flat) and the spark plug is centrally located. The air and gas are mixed using a governor controlled gas nozzle operating in a venturi.


One of the results of this technology is significantly reduced emissions in the exhaust. lean-burn gas engine generators can have NOx emissions as low as .85 grams/BHP-hr, and produce low amounts of hydrocarbons (HC), carbon monoxide(CO) and particulate matter (PM).This allows the generator sets to meet the most stringent air quality regulations without after treatment devices in the exhaust stream. For even lower emissions, lean-burn gas engine generator sets are also available with factory-integrated after treatment options such as Selective Catalytic Reduction(SCR) and Oxidation Catalysts, resulting in Nox levels at or below 0.15 grams/BHP-hr. With these after treatment options, the gas engine generators have been shown to meet the most stringent prime power emissions regulations anywhere in the world.


Another advantage of the lean-burn technology with full-authority electronic engine controls is the ability to operate on gas with a wide range of quality. A measurement called the Methane Number (MN) is used to determine fuel gas suitability as an engine fuel. Most natural gas has an MN from 70 to 97,and pipeline quality gas typically has an MN of about 75. Resource recovery gas from landfills or sewage treatment facilities is typically of lower quality, but is often suitable for use in lean-burn engines. Some lean-burn gas engine generators will operate on gas with an MN of 50 or greater, providing excellent fuel flexibility. However, gas with a MN below 70 may require derating of the generator output.
Lean-burn gas engine generator sets are setting a new standard for fuel efficiency, high power output for their size, and for low emissions. In regions with supplies of natural gas, these generator sets are providing highly reliable electric power for utility peaking, distributed generation, prime power and for combined heat and power systems.


Below 100kw, Natural Gas gensets generally cost less than an equivalent diesel counterpart. Also, there are fewer moving parts in a natural gas genset, therefore maintenance costs tend to be lower. Installed cost of a natural gas unit is even lower if fuel storage is not required.


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