Methane
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 odorants, 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 over 100 years of 23 (IPCC Third Assessment Report) i.e. when averaged over 100 years each kg of CH4 warms the earth 23 times as much as the same mass of CO2.
The Earth's mantle contains huge amounts of methane and is the main reservoir. Large amounts of methane are emitted to the atmosphere through Mud volcanoes which are connected with deep geological faults or as the main consituent of biogas formed naturally by anaerobic digestion.
Methane has a wide range of thermodynamic stability.
Properties
At room temperature and pressure, methane is a colorless, odorless gas. It has a boiling point of −162°C at 1 atmosphere pressure and is flammable.
Potential health effects
Methane is not toxic by any route. The immediate health hazard is that it may cause thermal burns. It is flammable and may form mixtures with air that are flammable or explosive. Methane is violently reactive with oxidizers, halogens, and some halogen compounds. Methane is an asphyxiant and may displace oxygen in a workplace atmosphere. Asphyxia may result if the oxygen concentration is reduced to below 18% by displacement. The concentrations at which flammable or explosive mixtures form are much lower than the concentration at which asphyxiation risk is significant. When structures are built on or near landfills, methane off-gas can penetrate the building interior and expose occupants to significant levels of methane. Some buildings have specially engineered recovery systems below their basements, to actively capture such fugitive off-gas and vent it away from the building. An example of this type of system is in the Dakin building, Brisbane, California.
Reactions of methane
The reactions with methane are: combustion, hydrogen activation, and halogen reaction.
Combustion
In the combustion of methane several steps are involved:
Methane forms a methyl radical (CH3), which reacts with oxygen forming formaldehyde (HCHO or H2CO). The formaldehyde gives a formyl 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 + ½O2 → 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 + ½O2 → 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. Despite the high activation barrier for breaking the C-H bond, CH4 is still the principal starting material for manufacture of hydrogen. 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.
Reactions with halogens
Methane undergoes reactions with all the halogens given correct conditions. The reactions occur as follows:
- CH4 + X2 → CH3X + HX
Where X is either fluorine (F), chlorine (Cl), bromine (Br) or sometimes iodine (I).
This mechanism for this process is called free radical substitution and it occurs as follows:
Initiation:
- X2 → 2Xˑ
Propagation:
- CH4 + Xˑ → ˑCH3 + HX
- ˑCH3 + ˑX2 → CH3X + Xˑ
Termination:
- 2Xˑ → X2
- ˑCH3 + X → CH3X
- ˑCH3 + ˑCH3 → CH3CH3
This reaction can occur as far as CX4 where the halide ions will have completely displaced the hydrogens
Uses
Fuel
For more on the use of methane as a fuel, see: natural gas
Methane is important for electrical generation by burning it as a fuel in a gas turbine or steam boiler. Compared to other hydrocarbon fuels, burning methane produces less carbon dioxide for each unit of heat released. Also, methane's heat of combustion is about 902 kJ/mol, which is lower than any other hydrocarbon, but if a ratio is made with the atomic weight (16.0 g/mol) divided by the heat of combustion (902 kJ/mol) it is found that methane, being the simplest hydrocarbon, actually produces the most heat per gram than other complex hydrocarbons. In many cities, methane is piped into homes for domestic heating and cooking purposes. In this context it is usually known as natural gas. At sea level, 1 cubic foot of methane will produce roughly 1,000 BTU of energy.
Industrial uses
Methane is used in industrial chemical processes and may be transported in liquid or refrigerated liquid form. While leaks from a liquid container are initially heavier than air, the gas is lighter than air. Gas pipelines distribute large amounts of natural gas, of which methane is a significant component.
In the chemical industry, methane is the feedstock of choice for the production of hydrogen, methanol, acetic acid, and acetic anhydride. When used to produce any of these chemicals, methane is first converted to synthesis gas, a mixture of carbon monoxide and hydrogen, by steam reforming. In this process, methane and steam react on a nickel catalyst at high temperatures (700–1100 °C).
- CH4 + H2O → CO + 3H2
The ratio of carbon monoxide to hydrogen in synthesis gas can then be adjusted via the water gas shift reaction to the appropriate value for the intended purpose.
- CO + H2O ⇌ CO2 + H2
Less significant methane-derived chemicals include acetylene, prepared by passing methane through an electric arc, and the chloromethanes (chloromethane, dichloromethane, chloroform, and carbon tetrachloride), produced by reacting methane with chlorine gas. However, the use of these chemicals is declining, acetylene as it is replaced by less costly substitutes, and the chloromethanes due to health and environmental concerns.
Sources of methane
Natural gas fields
The major source of methane is extraction from geological deposits known as natural gas fields. It is associated with other hydrocarbon fuels and sometimes accompanied by helium and nitrogen. The gas at shallow levels (low pressure) is formed by anaerobic decay of organic matter deep under the Earth's surface. In general, sediments buried deeper and at higher temperatures than those which give oil generate natural gas.
Alternative sources
Apart from gas fields an alternative method of obtaining methane is via biogas generated by the fermentation of organic matter including manure, wastewater sludge, municipal solid waste, or any other biodegradable feedstock, under anaerobic conditions. Industrially, methane can be created from common atmospheric gases and hydrogen (produced, perhaps, by electrolysis) through chemical reactions such as the Sabatier process, Fischer-Tropsch process. Coal bed methane extraction is a method for extracting methane from a coal deposit. It is also caused by cows' natural gas.
Methane in Earth's atmosphere
Methane in the earth's atmosphere is an important greenhouse gas with a Global warming potential of 23 over a 100 year period. Its concentation has increased by about 150% since 1750 and it accounts for 20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases [1].
The average concentration of methane at the Earth's surface in 1998 was 1,745 ppb [2]. Its concentration is higher in the northern hemisphere as most sources (both natural and human) are larger. The concentrations vary seasonally with a minimum in the late summer.
Methane is created near the surface, and it is carried into the stratosphere by rising air in the tropics. Uncontrolled build-up of methane in Earth's atmosphere is naturally checked—although human influence can upset this natural regulation—by methane's reaction with a molecule known as the hydroxyl radical, a hydrogen-oxygen molecule formed when single oxygen atoms react with water vapor.
Early in the Earth's history—about 3.5 billion years ago—there was 1,000 times as much methane in the atmosphere as there is now. The earliest methane was released into the atmosphere by volcanic activity. During this time, Earth's earliest life appeared. These first, ancient bacteria added to the methane concentration by converting hydrogen and carbon dioxide into methane and water. Oxygen did not become a major part of the atmosphere until photosynthetic organisms evolved later in Earth's history. With no oxygen, methane stayed in the atmosphere longer and at higher concentrations than it does today.
Emissions of methane
Houweling et al. (1999) give the following values for methane emissions [3]:
| Origin | CH4 emission (Tg/yr) |
| Natural emissions | |
|---|---|
| Wetlands (incl rice production) | 225 |
| Ocean | 20 |
| Termites | 15 |
| Hydrates | 10 |
| Natural total | 290 |
| Anthropogenic emissions | |
| Energy | 110 |
| Landfills | 40 |
| Ruminants | 115 |
| Waste treatment | 25 |
| Biomass burning | 40 |
| Anthropogenic total | 330 |
Slightly over half of the total emission is due to human activity [4].
Living plants (e.g. forests) have recently been identified as a potentially important source of methane. The recent paper calculated emissions of 62–236 Tg yr-1, and "this newly identified source may have important implications". [5], [6]. However the authors stress "our findings are preliminary with regard to the methane emission strength".[7]
See also Flatulence tax.
Removal processes
The major removal mechanism of methane from the atmosphere is by reaction with the hydroxyl radical (·OH), which may be produced when a cosmic ray strikes a molecule of water vapor:
This reaction in the troposphere gives a methane lifetime of 9.6 years. Two more minor sinks are soil sinks (160 year lifetime) and stratospheric loss by reaction with , and in the stratosphere (120 year lifetime), giving a net lifetime of 8.4 years. [8]
Sudden release from methane clathrates
At high pressures, such as are found on the bottom of the ocean, methane forms a solid clathrate with water, known as methane hydrate. 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.
One source estimates the size of the methane hydrate deposits of the oceans at ten million million tons (10 exagrams). Theories suggest that should global warming cause them to heat up sufficiently, all of this methane could again be suddenly released into the atmosphere. Since methane is twenty-three times stronger (for a given weight, averaged over 100 years) than CO2 as a greenhouse gas; this would immensely magnify the greenhouse effect, heating Earth to unprecedented levels.
Extraterrestrial methane
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.
See also
- Alkane, a type of hydrocarbon of which methane is simplest member.
- Anaerobic digestion
- Anaerobic respiration
- Biogas
- 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.
- Organic gas
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