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Liquid Transportation Fuels from Coal and Biomass: Technological Status, Costs, and Environmental Impacts
COMPRESSED NATURAL GAS
In 2007, the main U.S. uses for natural gas were apportioned as follows: electric-power generation, 30 percent; industrial use, 29 percent; residential use, 20 percent; and commercial use, 13 percent (EIA, 2008). Only 0.11 percent was used as fuel in transportation vehicles. It is the primary feedstock for fertilizers and petrochemicals. The cleanest and most efficient hydrocarbon fuel, natural gas is environmentally superior to coal for electric-power generation, and for similar reasons it could be a sound choice for transportation fuels.
The chapter on fossil fuel of the report America’s Energy Future: Technologyand Transformation (NAS-NAE-NRC, 2009) provides estimates of U.S. natural gas resources. Current natural-gas consumption needs are met mainly by domestic production. A switch to natural gas for a large segment of U.S. transportation use would probably trigger increased importation of natural gas or fuels produced from natural gas.
Technologies for producing transportation fuels from natural gas are ready for deployment by 2020. If natural gas were used for transportation instead of for electricity, there would be a potential to supply roughly one-fifth to one-fourth of transportation needs from North American natural-gas reserves, but only with investment in distribution infrastructure. Supplying more would require importing natural gas.
Compressed natural gas fuels natural-gas vehicles (NGVs). Natural gas is not a liquid fuel and it must be compressed to supply sufficient fuel for a vehicle. In 2008, there were more than 150,000 NGVs and 1,500 NGV fueling stations in the United States. Natural gas is sold in gallons of gasoline equivalent; a gallon of gasoline equivalent has the same energy content (124,800 Btu) as a gallon of gasoline. NGVs are more expensive than hybrid or gasoline vehicles. The Civic GX NGV has a manufacturer’s suggested retail price of $24,590 compared with $22,600 for the hybrid sedan and $15,010 for the regular sedan (Rock, 2008).
Of all the fossil fuels, natural gas produces the least carbon dioxide (CO2) when burned because it contains the lowest carbon:hydrogen ratio. It also releases smaller amounts of criteria air pollutants. NGVs emit unburned methane (which has a higher greenhouse forcing potential than CO2), but this may be compensated for by the substantial reduction in CO2 emission. Dedicated NGVs emit less carbon monoxide (CO), nonmethane organic gas, nitrogen oxides (NOx), and CO2 than do gasoline vehicles.