Natural Gas as Fuel
Table showing number of CNG vehicles and refueling stations in top 10 countries. Source: International Association of Natural Gas Vehicles 2007; United Nations Statistics Division 2007.
Natural gas is extracted from oil wells, coal beds, natural gas fields, and landfills. After processing, natural gas contains a mixture of hydrocarbons: between 70% and 90% methane (CH4), between 5% and 15% ethane (C2H6), and smaller amounts of propane (C3H8) and butane (C4H10). Combustion of these hydrocarbons releases carbon dioxide and water and produces chemical energy.
Natural gas has a very low energy content at normal atmospheric pressure. Carrying a sufficient amount of natural gas to power a vehicle any distance would take a huge storage tank; therefore, natural gas for transportation is pressurized to several hundred atmospheres (2000 psi to 3600 psi or 13.8 MPa to 24.8 MPa). This compressed natural gas (CNG) still has a quarter or less of the energy content in gasoline and thus requires larger storage tanks at refueling stations and on the vehicles themselves. Refueling of CNG vehicles is also slower than refueling with liquid fuels, requiring about double the time to fill the tank.
Locations of public compressed natural gas refueling stations in California. Consumer Energy Center 2006.
On the plus side, natural gas costs less on the world market than gasoline or diesel fuel. Some countries, such as Argentina and Pakistan, have greater reserves of natural gas than oil [1] and have promoted its use.
Conversion of engines from gasoline to CNG is straightforward, and natural gas deserves its reputation as a “clean” fuel because it produces fewer particulates, non-methyl hydrocarbons, and NOX (a contributor to photochemical smog) during combustion than gasoline or petrodiesel. Buses in areas suffering from air pollution are often powered by CNG and may be distinguished by the large storage tank on their roofs. Most urban areas in the United States have CNG refueling petrodiesel. [2] Buses in areas suffering from air pollution are often powered by CNG and may be distinguished by the large storage tank on their roofs. Most urban areas in the United States have CNG refueling stations; for example, California now has more than 200 CNG refueling stations.
More importantly, combustion of natural gas emits smaller amounts of greenhouse gases than any fuel except for hydrogen. Thus in an energy transition to wind and solar power, use of natural gas to provide base load to the electric grid could be vital. Leakage of natural gas during extraction, refining, distribution, and combustion is an issue because its main constituent, CH4, is a greenhouse gas with 23 times the global warming potential of CO2. Such leakage is small, however, and CNG vehicles typically emit about 12% less greenhouse gases than do gasoline-powered vehicles. [3]
Natural gas will condense into a liquid when cooled to between -120° and -170°C, depending on the relative proportions of the various constituent hydrocarbons. Liquefied natural gas (LNG) has an energy content similar to other liquid fuels and thus can be stored in smaller tanks and transferred more quickly than CNG. Such storage tanks, however, require heavy duty thermal isolation to keep the LNG from boiling. Because of the difficulties of working with a cryogenic liquid (i.e., liquid that must be stored at a very low temperature), use of LNG has been generally limited to long-distance transport of natural gas when CNG pipelines are unavailable. For example, cargo ships carry LNG for transport across ocean.
[1] BP (2007) BP Statistical Review of World Energy, BP p.l.c., London, http://www.bp.com/multipleimagesection.do?categoryId=9017892&contentId=7033503.
[2] U.S. Environmental Protection Agency (1999) Modeling Emission Factors for Compressed Natural Gas Vehicles, Washington, D.C., http://www.epa.gov/otaq/models/mobile6/m6ful004.pdf.
[3] Science Applications International Corporation (2002) Greenhouse Gas Emission Reductions and Natural Gas Vehicles, National Energy Technology Laboratory, Pittsburgh, PA, http://www.osti.gov/bridge/servlets/purl/816573-V0Mlif/native/816573.PDF.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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