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Methane is the most abundant organic compound in the Earth's atmosphere, where it acts as a greenhouse gas and thus has implications for global climate change.
The total annual source of methane to the atmosphere was estimated in 1988 at about 540 Tg (range of 400-640 Tg [1 teragram = 1012 g]) of methane per year by Ron Oremland (USGS, Water Resources Discipline) and Ralph Cicerone (University of California, Irvine). The sources include (1) enteric fermentation (formed in the intestines of livestock); (2) natural wetlands; (3) rice paddies; (4) biomass burning; (5) termites; (6) landfills; (7) oceans; (8) freshwater; (9) hydrates; (10) coal mining; and (11) gas drilling, venting, and transmission. Sources 1 through 8 produce methane containing mainly modern carbon (with 14C), whereas sources 9 through 11 produce methane containing mainly ancient carbon (without 14C).
Notably absent from these identified sources, except for hydrates, is the contribution of geologically sourced methane from naturally occurring gas seeps, which is now estimated to be about 20 Tg of methane per year, including about 5 Tg of methane per year from mud volcanoes. Sources of atmospheric methane are nearly balanced by a methane sink, whereby methane reacts with hydroxyl in the atmosphere to form carbon dioxide. Large additions of methane to the atmosphere for example, from the decomposition of gas hydratecould upset this balance and lead to global climate change.
Methane is purposely recovered from the Earth for the energy that it releases during combustion. Natural gas, which is composed mainly of methane, is currently a primary fossil-fuel commodity along with oil and coal. Because methane burns more cleanly to carbon dioxide than do oil and coal, methane is now a preferred energy resource.
The total world endowment of conventional natural gas is about 16 quadrillion cubic feet, or about 320,000 Tg of methane, which is being used at a rate of almost 2,000 Tg of methane per year. This conventional endowment of methane may be supplemented in the future with methane from gas hydrate.
The hydrogen content of methane (4 hydrogen atoms to 1 carbon atom) is the highest of any organic compound. Thus, methane is a potential source of hydrogen to provide energy for motive power, if cost-effective ways can ever be devised to remove hydrogen from carbon in the methane molecule. Prototype vehicles using hydrogen fuel cells are already being tested as a prelude to a possible future hydrogen economy.
in this issue:
Future of Natural Gas
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