Natural Gas Engine Emissions

2.5 Emissions

Emissions of criteria pollutants – oxides of nitrogen (NOx), carbon monoxide (CO), and volatile organic compounds (VOCs – unburned, non-methane hydrocarbons) – are the primary environmental concern with reciprocating engines operating on natural gas. Emissions of sulfur compounds (SOx) depend only on the sulfur content of the fuel. SOx emissions are an issue only in large, slow speed diesels firing heavy oils. SOx emissions from natural gas engines are assumed to be less than 0.0006 lb/MMBtu.28 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. Particulate emissions from 4-stroke lean burn natural gas engines are 4,000 times lower than for an uncontrolled diesel engine.

2.5.1 Emissions Characteristics

2.5.1.1 Nitrogen Oxides (NOx)

NOx emissions are usually the primary concern with natural gas engines and are a mixture of (mostly) NO and NO2 in variable composition. In measurement, NOx is reported as parts per million by volume in which both species count equally (e.g., ppmv at 15 percent O2, dry). Other common units for reporting NOx in reciprocating engines are g/hp-hr and g/kWh, or as an output rate such as lb/hr. Among natural gas engine options, lean burn natural gas engines produce the lowest NOx emissions directly from the engine. However, rich burn engines can more effectively make use of three way catalysts (TWC) to produce very low emissions. If lean burn engines must meet extremely low emissions levels, as in California CARB 2007 standards of .07 lb/MWh then selective catalytic reduction must be added. Rich burn engines would qualify for this standard by taking a CHP credit for avoided boiler emissions. Lean burn engines can meet the standard using selective catalytic reduction (SCR). Both rich burn and lean burn engines have been certified for operation in Southern California meeting the stringent California Air Resources Board (CARB) 2007 standards.

The control of peak flame temperature through lean burn conditions has been the primary combustion approach to limiting NOx formation in gas engines. Diesel engines produce higher combustion temperatures and more NOx 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.29

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 NOx. In contrast, lean-premixed homogeneous combustion used in lean burn gas engines results in lower combustion temperatures and lower NOx production.

For any engine there are generally trade-offs between low NOx emissions and high efficiency. There are also trade-offs between low NOx 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 NOx 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.

2.5.1.2 Carbon Monoxide (CO)

CO and VOCs both result from incomplete combustion. CO emissions result when there is inadequate oxygen or insufficient residence time at high temperature. Cooling at the combustion chamber walls and reaction quenching in the exhaust process also contribute to incomplete combustion and increased CO emissions. Excessively lean conditions can lead to incomplete and unstable combustion and high CO levels. Therefore, control of NOx through lean combustion can increase CO and VOC emissions out of the engine.

2.5.1.3 Unburned Hydrocarbons

Volatile hydrocarbons also called volatile organic compounds (VOCs) can encompass a wide range of compounds, some of which are hazardous air pollutants. These compounds are discharged into the atmosphere when some portion of the fuel remains unburned or just partially burned. Some organics are carried over as unreacted trace constituents of the fuel, while others may be pyrolysis products of the heavier hydrocarbons in the gas. Volatile hydrocarbon emissions from reciprocating engines are normally reported as non-methane hydrocarbons (NMHCs).

2.5.1.4 Carbon Dioxide (CO2)

While not considered a pollutant in the ordinary sense of directly affecting health, emissions of carbon dioxide (CO2) are of concern due to its contribution to climate change. The amount of CO2 emitted is a function of both fuel carbon content and system efficiency. The fuel carbon content of natural gas is 34 lbs carbon/MMBtu; oil is 48 lbs carbon/MMBtu; and (ash-free) coal is 66 lbs carbon/MMBtu. As converted to CO2 in the exhaust, these values are 117 lb/MMBtu for natural gas, 160 lb/MMBtu for diesel oil, and 205-226 lb/MMBtu for coal.

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