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Natural gas, the second most abundant fossil fuel after coal, contains methane, a mix of light non-methane hydrocarbons, hydrogen sulfide, carbon dioxide, water vapor, nitrogen, helium, and other trace gases. In most cases the raw natural gas has to be upgraded to pipeline specification in a gas processing plant before it is injected in the distribution system. The processing involves removal of water and H2S to prevent corrosion, and the removal of heavier hydrocarbons to prevent condensation in the pipeline. The removed hydrocarbons are used as a valuable feedstock for producing LPG and petrochemicals.
The main constituent of pipeline-quality natural gas is methane, which makes up about 80-99% of the total. The remainder is primarily ethane, inert gases (N2, CO2), and smaller amounts of propane and higher hydrocarbons. Typical composition of natural gas for vehicular use is illustrated in Table 1, which summarizes selected specifications by the U.S. EPA and California ARB .
|Constituent||EPA Cert Fuel||ARB Cert Fuel||ARB In-Use Fuel|
|Methane||89.0 (min)||90.0±1||88.0 (min)|
|Ethane||4.5 (max)||4.0±0.5||6.0 (max)|
|C3 and higher||2.3 (max)||2.0±0.3||3.0 (max)|
|C6 and higher||0.2 (max)||0.2 (max)||0.2 (max)|
|Hydrogen||-||0.1 (max)||0.1 (max)|
|Carbon monoxide||-||0.1 (max)||0.1 (max)|
|Oxygen||0.6 (max)||0.6 (max)||1.0 (max)|
|Inert gases (CO2 + N2)||4.0 (max)||3.5±0.5||1.5-4.5|
The properties of methane and selected other hydrocarbon components of natural gas are listed in Table 2 .
|Lower heating value, MJ/kg||50.01||47.48||46.35||45.78|
|Liquid density, kg/m3||466||572||501||519|
|Liquid energy density, MJ/dm3||23.30||27.16||23.22||23.76|
|Gas energy density, MJ/m3||32.6||58.4||84.4||79.4|
|Gas specific gravity*||0.55||1.05||1.55||1.47|
|Boiling point, °C||-164||-89||-42||-47|
|Research octane No.||>127||-||109||-|
|Motor octane No.||122||101||96||84|
|* relative to air, 25°C|
Natural gas, biogas and pure methane could be all treated as relatively similar fuels from combustion viewpoint, although it should be noted that the composition is different. Notably, poor quality biogas produced from some landfill sources may contain silicon compounds, which can poison catalytic devices if used for fueling of catalyst-equipped engines.
Utilization of natural gas could be either in compressed form as CNG or in liquefied form, as LNG. The difference regarding engine performance and emissions between these options is relatively small. In the LNG case, injection of liquid fuel is possible.