Natural Gas

Hannu Jääskeläinen, Peter Ahlvik

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Abstract: Natural gas (NG) can be used as fuel in spark ignited engines (Otto cycle) or else it can be utilized in direct injection or dual fuel engines operated in the Diesel cycle. While NG engines can be designed to produce lower tailpipe GHG emissions than diesel, the greenhouse gas effect of natural gas energy critically depends on methane losses in natural gas production, distribution and utilization. Traditionally, NG engines had lower NOx and PM emissions compared to diesel. With the introduction of clean fuels and advanced emission control technologies in diesel engines, similar low emission levels can be achieved using both natural gas and diesel fuel.

Natural Gas Properties

Natural gas, the second most abundant fossil fuel after coal, contains methane (CH4), 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 US EPA and California ARB [176].

Table 1
Natural gas specifications, USA and California (mole %)
Constituent EPA Light-Duty Certification Fuel CARB Certification Fuel CARB 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 [176].

Table 2
Properties of natural gas components
  Methane Ethane Propane Propene
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 a 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 (NG) could be either in the form of compressed natural gas (CNG) or liquefied natural gas (LNG). The difference regarding engine performance and emissions between these options is relatively small. In the LNG case, injection of liquid fuel is possible.

LNG can be further categorized based on its temperature. “Cold” LNG is dispensed at less than -143°C and 0.34 MPa while “warm” LNG at -125 to -131°C and 0.69 to 0.93 MPa [4373]. While warm LNG is sometimes referred to as “saturated” LNG and cold LNG as “unsaturated”, this terminology is confusing. The vapor phase of both cold and warm LNG can be saturated because the liquid and vapor phases will equilibrate under storage conditions.