Methane

The simplest hydrocarbon, methane is a gas with a chemical formula of C H₄.

  H-C-H

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

Due to the heat and attack by the active species, the methane reacts 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

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.

Pure methane is odorless, but when used as a fuel is usually mixed with small quantities of strongly-smelling sulfur compounds such as ethyl mercaptan to enable the detection of leaks.

Methane is a greenhouse gas with a global warming potential of 22 (meaning that it has 22 times the warming ability of carbon dioxide).

Methane results from the decomposition of certain organic matters in the absence of oxygen. It is therefore also classified as a biogas.

Principal sources are

decomposition of organic wastes
natural sources (marshes) : 23 %
fossil fuel extraction : 20 % See Coal bed methane extraction
the processes of digestion of animals (cattle) : 17 %
bacteria found in rice plantations : 12 %
biomass anaerobic heating or combustion

80% of the world emissions are of human source. 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

See also