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Methane (Rev #16, changes)

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This page is about methane, a fuel and a greenhouse gas.

Basic chemistry and use as a fuel

From Wikipedia:

Methane is a chemical compound with the chemical formula CH 4CH_4. It is the simplest alkane, and the principal component of natural gas. Methane’s bond angles are 109.5 degrees. Burning methane in the presence of oxygen produces carbon dioxide and water.

The relative abundance of methane makes it an attractive fuel. However, because it is a gas at normal temperature and pressure, methane is difficult to transport from its source. In its natural gas form, it is generally transported in bulk by pipeline or LNG carriers; few countries transport it by truck.

Lifetime of methane

Chemists are sometimes surprised to hear that in the atmosphere the average lifetime of a molecule of methane is about a decade. After all, methane is the most reduced form of carbon. Outside the atmosphere it can be stable and accumulate over millions of years e.g. in methane hydrate ocean sediments. In the atmosphere

Methane is primarily oxidized by reaction with hydroxyl radical, OH, in a stew of chemical reactions in the troposphere, similar to the chemistry in a candle flame. Additional sinks, accounting for about 10% of the methane, include destruction by ultraviolet (UV) light in the stratosphere and oxidation by bacteria in soils (see table 1).

OH is a water molecule missing a hydrogen atom and hungry to get that hydrogen back. (…) The lifetime of methane in the atmosphere depends on the concentration of OH. At night, or trapped in a dark bubble in an ice core, methane can sit in oxygen forever without reacting much, (…) The concentration of OH determines the rate at which the atmosphere can cleanse itself of reduced gases such as methane, CO, and hydrocarbons

  • David Archer, The Global Carbon Cycle, Chapter 5

Methane as a greenhouse gas

From Wikipedia:

Methane is a relatively potent greenhouse gas. Compared with carbon dioxide, it has a high global warming potential of 72 (calculated over a period of 20 years) or 25 (for a time period of 100 years). It has a net lifetime of about 10 years and is primarily removed by reaction with hydroxyl radicals in the atmosphere, producing carbon dioxide and water.

(Note that carbon dioxide is also a greenhouse gas. It has a lower global warming potential than methane, but it effectively stays in the atmosphere much longer.)

In addition, there is a large volume of methane deposited in the Arctic whose stability is poorly understood.

The effect of rising CO2 on plant life is a controversial and important topic. A new meta-analysis in Nature claims that more CO2 boosts soil emissions of nitrous oxide in all ecosystems, and more methane in in rice paddies and wetlands:

The claimed explanation is that higher CO2 concentrations reduce plant water use, making soils wetter, in turn reducing the availability of oxygen in soil, favoring microorganisms that make methane and N2O. Also: increasing CO2 makes plants grow faster, which supplies these microorganisms with extra energy.

The methane cycle

For more information about the Methane cycle.


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category: carbon