CNG and LNG as Alternative Fuels
Two forms of natural gas are used in vehicles: CNG and LNG. Both are clean-burning, domestically produced, relatively low priced, and widely available. Because of the gaseous nature of this fuel, when stored onboard a vehicle, it must be in either a compressed gaseous (CNG) or liquefied (LNG) state. CNG and LNG are considered alternative fuels under the Energy Policy Act of 1992.
Natural gas is sold in units of diesel or gasoline gallon equivalents (DGEs or GGEs) based on the energy content of a gallon of gasoline or diesel fuel.
Compressed Natural Gas
To provide adequate driving range for a vehicle, CNG is stored in cylinders at a pressure of 3,000 to 3,600 pounds per square inch. A CNG-powered vehicle gets about the same fuel economy as a conventional gasoline vehicle on a GGE basis. A GGE equals about 5.66 pounds of CNG. CNG is used in light-, medium-, and heavy duty applications.
Liquefied Natural Gas
LNG is produced by purifying natural gas and super-cooling it to -260°F to turn it into a liquid. Because it must be kept at cold temperatures, LNG is stored in double-walled, vacuum-insulated pressure vessels. LNG is good for trucks needing a longer range because liquid is more dense than gas (CNG) and, therefore, more energy can be stored by volume in a given tank. LNG is typically used in medium- and heavy duty vehicles; a GGE equals about 1.5 gallons of LNG.
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Source: Alternative Fuels Data Center