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19:12, 13 March 2017
is a [[
product consisting of
that can be
[[petroleum]] extraction and production (known as associated natural gas), extracted from separate natural gas fields (non-associated natural gas), or extracted from other areas such as coal-beds or [[shale]] (continuous or unconventional gas).<ref>U.S. Geological Survey. ''Natural Gas Production in the United States''. 2002. http://pubs.usgs.gov/fs/fs-0113-01/fs-0113-01.pdf</ref> It consists of about 75%
and the rest is made up of [[ethane]] (15%) and
,<ref>http://scifun.chem.wisc.edu/chemweek/methane/methane.html</ref> but this varies by region and degree of refinement. It is naturally odorless, so odor is added to make a leak detectable.
When burned for [[electricity]] production (its most common use), natural gas produces significantly less [[pollution]] than [[coal]], the most popular source of energy in the United States for electricity production. Natural gas produces no [[sulfur dioxide]] (the main contributor to [[acid rain]], and which coal produces, though in much smaller quantities after the 1990 amendment to the [[Clean Air Act]]). Natural gas also produces over 77% less [[nitrogen oxide]] air pollutants (important contributors to smog and respiratory health problems) compared to coal, on average.<ref>Proops, J.L.R., et. al. "The lifetime pollution implications of various types of electricity generation: an input-output analysis." 1996. ''Energy Policy'' Vol. 24, No. 3, pp. 229-237.</ref> As such, it is especially useful in urban environments (e.g., in fueling [[bus]]es) where local concentrations of airborne pollutants can cause certain health issues. If considering [[carbon dioxide]]
a pollutant, natural gas produces over 36% less CO<sub>2</sub> emissions from electricity generation compared to coal, on average. In the U.S. in 2008, natural gas made up about 20% of electricity production<ref>U.S. Energy Information Administration. ''Annual Energy Outlook 2010'' - Electricity Demand. http://www.eia.doe.gov/oiaf/archive/aeo10/electricity.html</ref> and 16% of CO<sub>2</sub> emissions from electricity production, compared to 48% and 82%, respectively, for coal. CO<sub>2</sub> emissions from electricity generation make up 41% of total U.S. CO<sub>2</sub> emissions.<ref>U.S. Energy Information Administration. ''Annual Energy Outlook 2010'' - Emissions Projections. http://www.eia.doe.gov/oiaf/archive/aeo10/emission.html</ref>
Canadian National is testing two railroad [[locomotive]]s which have been converted to use natural gas.<ref>http://www.railwaygazette.com/news/freight/single-view/view/natural-gas-fuelled-locomotives-on-test
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