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