Methane

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The simplest hydrocarbon, methane, is a gas with a chemical formula of CH₄. Pure methane is odorless, but when used commercially is usually mixed with small quantities of strongly-smelling sulfur compounds such as ethyl mercaptan to enable the detection of leaks.

A principal component of natural gas, methane is a significant fuel. Burning one molecule of methane in the presence of oxygen releases one molecule of CO₂ (carbon dioxide) and two molecules of H₂O (water):

CH₄ + 2O₂ → CO₂ + 2H₂O

Methane is a greenhouse gas with a global warming potential of 21.

Sources of methane

Principal methane sources are

• decomposition of organic wastes
• natural sources (marshes): 23 %
• mineral fuel extraction: 20 %, see Coal bed methane extraction

Methane is extracted from geological deposits as a mineral fuel which is associated with other hydrocarbon fuels.

• the processes of digestion in animals (cattle): 17 %
• bacteria found in rice plantations: 12 %
• biomass anaerobic heating or combustion

60% of the world emissions are from sources affected by humans. They come primarily from agricultural and other human activities. During the past 200 years, the concentration of this gas in the atmosphere doubled, passing from 0.8 to 1.7 ppm.

Methane is also classified as a biogas because it can be created by the (anaerobic) decomposition of certain organic matters.

• Industrial sources

Methane can be created and used industrially, and perhaps in nature, by chemical reactions such as the Sabatier process, Fischer-Tropsch process, and steam reforming. Similar gases and materials are often present in geologic and volcanic processes.

• At high pressures, such as are found on the bottom of the ocean, methane forms a solid clathrate with water. An unknown but possibly very large quantity of methane is trapped in this form in ocean sediments. The sudden release of large volumes of methane from such sediments into the atmosphere has been suggested as a possible cause for rapid global warming events in the earth's distant past, such as the Paleocene-Eocene thermal maximum of 55 million years ago.

Reactions of methane

• Combustion

In the combustion of methane several steps are involved:

Methane forms to a methyl radical (CH₃), which reacts to formaldehyde (HCHO or H₂CO). The formaldehyde reacts to a formal radical (HCO), which then forms carbon monoxide (CO). The process is called oxidative pyrolysis:

CH₄ + O₂ → CO + H₂ + H₂O

Following oxidative pyrolysis, the H₂ oxidizes, forming H₂O, replenishing the active species, and releasing heat. This occurs very quickly, usually in less than a millisecond.

H₂ + ½ O₂ → H₂O

Finally, the CO oxidizes, forming CO₂ and releasing more heat. This process is generally slower than the other chemical steps, and typically requires a few to several milliseconds to occur.

CO + ½ O₂ → CO₂

• Hydrogen activation

The strength of the carbon-hydrogen covalent bond in methane is among the strongest in all hydrocarbons, and thus its use as a chemical feedstock is limited. The search for catalysts which can facilitate C-H bond activation in methane and other low alkanes is an area of research with considerable industrial significance.

• Halogenation

Methane not on Earth

Methane has been detected or is believed to exist in several locations of the solar system. It is believed to have been created by abiotic processes, with the possible exception of Mars.

• Jupiter
• Mars
• Saturn
  • Iapetus
  • Titan
• Neptune
  • Triton
• Uranus
  • Ariel
  • Miranda
  • Oberon
  • Titania
  • Umbriel
• Comet Halley
• Comet Hyakutake

Traces of methane gas are present in the thin atmosphere of the Earth's Moon.

Methane has also been detected in interstellar clouds.

Units of measure

• Methane is a greenhouse gas with a global warming potential of 21.
• One cubic meter (m³) at normal pressure has a mass of 717 grams.

See also

• alkane, a type of hydrocarbon of which methane is simplest member.
• methane clathrate, form of water ice which contains methane.
• methanogen, archaea that produce methane as a metabolic by-product.
• methanogenesis, the formation of methane by microbes.
• methanotroph, bacteria that are able to grow using methane as their only source of carbon and energy.
• methyl group, a functional group similar to methane

External links

• Methane Hydrates (http://marine.usgs.gov/fact-sheets/gas-hydrates/title.html)

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