Natural Gas: What is it?
Natural gas, a domestically produced gaseous fuel, is readily available through the utility infrastructure. Whether produced via conventional or renewable methods, this clean-burning alternative fuel must be compressed or liquefied for use in vehicles.
Types of Natural Gas as Alternative Fuel Transportation
Compressed Natural Gas (CNG)
CNG is produced by compressing natural gas to less than 1% of its volume at standard atmospheric pressure. To provide adequate driving range, CNG is stored onboard a vehicle in a compressed gaseous state at a pressure of up to 3,600 pounds per square inch.
CNG is used in light-, medium-, and heavy-duty applications. A CNG-powered vehicle gets about the same fuel economy as a conventional gasoline vehicle on a GGE basis. One GGE equals about 5.66 pounds of CNG.
Liquified Natural Gas (LNG)
LNG is natural gas in its liquid form. LNG is produced by purifying natural gas and super-cooling it to -260°F to turn it into a liquid. During the process known as liquefaction, natural gas is cooled below its boiling point, removing most of the extraneous compounds found in the fuel. The remaining natural gas is primarily methane with small amounts of other hydrocarbons.
Because of LNG’s relatively high production cost, as well as the need to store it in expensive cryogenic tanks, the fuel’s widespread use in commercial applications has been limited. LNG must be kept at cold temperatures and is stored in double-walled, vacuum-insulated pressure vessels. LNG is suitable for trucks that require longer ranges because liquid is denser than gas and, therefore, more energy can be stored by volume. LNG is typically used in medium- and heavy-duty vehicles. One GGE equals about 1.5 gallons of LNG.
Renewable Natural Gas (RNG)
Renewable natural gas (RNG), also known as biomethane, is produced from organic materials—such as waste from landfills and livestock—through anaerobic digestion. RNG qualifies as an advanced biofuel under the Renewable Fuel Standard. Because RNG is chemically identical to fossil-derived conventional natural gas, it can use the existing natural gas distribution system and must be compressed or liquefied for use in vehicles.
Benefits of Natural Gas as an Alternative Fuel
- Reduces carbon dioxide emissions 10-15%; for RNG that numbers shoots to over 80%
- Emits little or no particulate matter (e.g., PM-10, PM-2.5); the reductions are massive as compared to pre-2008 diesel vehicles
- Reduces carbon monoxide emissions 50%-97%
- Reduces nitrogen oxide emissions 35%-60%
- Emits fewer toxic and carcinogenic pollutants
- Reduces non-methane hydrocarbon emissions 50%-75%
- Emits little or no particulate matter
- Virtually eliminates evaporative emissions