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Methane ( or /ˈmiːθeɪn/) is a
chemical compound with the
chemical formula . 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. It is generally transported in bulk by
pipeline in its natural gas form, or
LNG carriers in its liquefied form; few countries transport it by truck.
Methane was discovered and isolated by Alessandro Volta between 1776 and 1778 when studying marsh gas from Lake Maggiore.
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.
Methane also affects the degradation of the ozone layer.
The mole fraction of methane in the Earth's atmosphere in 1998 was 1745 nmol/mol (parts per billion, ppb), up from 700 nmol/mol in 1750. By 2008, however, global methane levels, which had stayed mostly flat since 1998, had risen to 1,800 nmol/mol. By 2010, methane levels, at least in the Arctic, were measured at 1850 nmol/mol, a level scientists described as being higher than at any time in the previous 400,000 years. Historically, methane concentrations in the world's atmosphere have ranged between 300 and 400 nmol/mol during glacial periods commonly known as ice ages, and between 600 to 700 nmol/mol during the warm interglacial periods.
In addition, there is a large (but unknown) amount of methane in methane clathrates in the ocean floors. The Earth's crust contains huge amounts of methane. Large amounts of methane are produced by methanogenesis. Other sources include mud volcanoes, which are connected with deep geological faults; landfill; and livestock (primarily ruminants) from enteric fermentation.
Properties
Methane is the major component of natural gas, about 87% by volume. At
room temperature and
standard pressure, methane is a colorless, odorless gas; the smell characteristic of natural gas as used in homes is an artificial safety measure caused by the addition of an
odorant, often
methanethiol or
ethanethiol. Methane has a boiling point of −161 °
C (−257.8 °
F) at a pressure of one
atmosphere. As a gas it is
flammable only over a narrow range of concentrations (5–15%) in air. Liquid methane does not burn unless subjected to high pressure (normally 4–5 atmospheres).
Potential health effects
Methane is not toxic; however, it is extremely flammable and may form
explosive mixtures with air. Methane is violently reactive with
oxidizers,
halogens, and some halogen-containing compounds. Methane is also an
asphyxiant and may displace
oxygen in an enclosed space.
Asphyxia may result if the oxygen concentration is reduced to below 19.5% by displacement. The concentration of methane where asphyxiation risk becomes significant is much higher than the 5–15% concentration that forms flammable or explosive mixtures. When structures are built on or near
landfills, methane off-gas can penetrate the buildings' interiors 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
Main reactions with methane are:
combustion,
steam reforming to
syngas, and
halogenation. In general, methane reactions are hard to control. Partial oxidation to
methanol, for example, is difficult to achieve; the reaction typically progresses all the way to
carbon dioxide and
water.
Combustion
In the
combustion of methane, several steps are involved:
Methane is thought to form a formaldehyde (HCHO or ). 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 oxidizes, forming , releasing heat. This occurs very quickly, usually in significantly less than a millisecond.
:2 H2 + O2 → 2 H2O
Finally, the CO oxidizes, forming and releasing more heat. This process is generally slower than the other chemical steps, and typically requires a few to several milliseconds to occur.
:2 CO + O2 → 2 CO2
The result of the above is the following total equation:
:CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l) (ΔH = −891 kJ/mol (at standard conditions))
where bracketed "g" stands for gaseous form and bracketed "l" stands for liquid form.
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, is still the principal starting material for manufacture of hydrogen in
steam reforming. The search for
catalysts that facilitate C–H bond activation in methane and other low
alkanes has considerable industrial importance.
In
oxidative coupling of methane the synthetic target is ethylene. In
catalytic methane decomposition the syntheic targets are ultra-pure hydrogen and high-value carbon composite materials
Reactions with halogens
Methane reacts with all halogens given appropriate conditions, as follows:
:CH4 + X2 → CH3X + HX
where X is a halogen: fluorine (F), chlorine (Cl), bromine (Br), or iodine (I). This mechanism for this process is called free radical halogenation.
When X is Cl, this mechanism has the following form:
1. Radical generation:
:
The needed energy comes from UV radiation or heating,
2. Radical exchange:
:CH4 + Cl· → CH3· + HCl (ΔH = −14 kJ)
:CH3· + Cl2 → CH3Cl + Cl· (ΔH = −100 kJ)
3. Radical extermination:
:2 Cl· → Cl2 (ΔH = −239 kJ)
:CH3· + Cl· → CH3Cl (ΔH = −339 kJ)
:2 CH3· → CH3CH3 (ΔH = −347 kJ)
If methane and X2 are used in equimolar quantities, CH2X2, CHX3, and even CX4 are formed. Using a large excess of CH4 reduces the production of CH2X2, CHX3, CX4, and thus more CH3X is formed.
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. At about 891 kJ/mol, methane's heat of combustion is lower than any other hydrocarbon but the ratio of the heat of combustion (891 kJ/mol) to the molecular mass (16.0 g/mol) shows that methane, being the simplest hydrocarbon, produces more heat per mass unit (55.7 kJ/g) 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, and is considered to have an energy content of 39 megajoules per cubic meter, or 1,000 BTU per standard cubic foot.
Methane in the form of compressed natural gas is used as a vehicle fuel, and is claimed more environmentally friendly than other fossil fuels such as gasoline/petrol and diesel. Research into adsorption methods of methane storage for this purpose has been conducted.
Research is being conducted by NASA on methane's potential as a rocket fuel. One advantage of methane is that it is abundant in many parts of the solar system and it could potentially be harvested in situ (i.e. on the surface of another solar-system body), providing fuel for a return journey.
Current methane engines in development produce a thrust of , which is far from the needed to launch the Space Shuttle. Instead, such engines will most likely propel voyages from the Moon or send robotic expeditions to other planets in the solar system.
Recently methane emitted from coal mines has been successfully converted to electricity.
Industrial uses
Methane is used in industrial chemical processes and may be transported as a refrigerated liquid (liquefied natural gas, or
LNG). While leaks from a refrigerated liquid container are initially heavier than air due to the increased density of the cold gas, the gas at ambient temperature is lighter than air.
Gas pipelines distribute large amounts of natural gas, of which methane is the principal 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).
:
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 is replaced by less costly substitutes, and the use of chloromethanes is diminishing due to health and environmental concerns.
Sources of methane for human use
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) forms by
anaerobic decay of
organic matter and reworked methane from deep under the Earth's surface. In general, sediments buried deeper and at higher temperatures than those that contain
oil generate natural gas. Methane is also produced in considerable quantities from the decaying organic wastes of
solid waste landfills.
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 (including landfills), or any other biodegradable feedstock, under anaerobic conditions.
Rice fields also generate large amounts of methane during plant growth.
Methane hydrates/clathrates (ice-like combinations of methane and water on the sea floor, found in vast quantities) are a potential future source of methane. Cattle belch methane accounts for 16% of the world's annual methane emissions to the atmosphere. The livestock sector in general (primarily cattle, chickens, and pigs) produces 37% of all human-induced methane. Early research has found a number of medical treatments and dietary adjustments that help slightly limit the production of methane in
ruminants.
Industrially, methane can be created from carbon dioxide and hydrogen or carbon monoxide and hydrogen through chemical reactions such as the Sabatier process or the Fischer-Tropsch process (although Fischer-Tropsch is usually used to produce longer chain molecules than methane). Coal bed methane extraction is a method for extracting methane from a coal deposit, while enhanced coal bed methane recovery is a method of recovering methane from an non-minable coal seam.
Scientific experiments have given variable results in determining whether plants are a source of methane emissions.
Atmospheric methane
Methane is created near the Earth's surface, primarily in soils, rivers/seas and in animal innards. It is carried into the
stratosphere by rising air in the
tropics. Uncontrolled build-up of methane in the atmosphere is naturally checked — although human influence can upset this natural regulation — by methane's reaction with
hydroxyl radicals formed from
singlet oxygen atoms and with water vapor.
Methane in the Earth's atmosphere is an important greenhouse gas with a global warming potential of 25 compared to CO2 over a 100-year period (although accepted figures probably represents an underestimate). This means that a methane emission will have 25 times the impact on temperature of a carbon dioxide emission of the same mass over the following 100 years. Methane has a large effect for a brief period (a net lifetime of 8.4 years in the atmosphere), whereas carbon dioxide has a small effect for a long period (over 100 years). Because of this difference in effect and time period, the global warming potential of methane over a 20 year time period is 72. The Earth's atmospheric methane concentration 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 (these gases don't include water vapour which is by far the largest component of the greenhouse effect). Usually, excess methane from landfills and other natural producers of methane is burned so CO2 is released into the atmosphere instead of methane, because methane is a more effective greenhouse gas. Recently, methane emitted from coal mines has been successfully utilized to generate electricity.
Arctic methane release from permafrost and clathrates is an expected consequence of global warming.
In prehistoric times, large methane excursions have been linked with dramatic shifts in the Earth's climate, notably during the Paleocene-Eocene thermal maximum and during the Permian-Triassic extinction event, which was the worst ever mass extinction.
Extraterrestrial methane
Methane has been detected or is believed to exist in several locations of the
solar system. In most cases, it is believed to have been created by
abiotic processes. Possible exceptions are
Mars and
Titan.
Moon – traces are outgassed from the surface
Mars – the atmosphere contains 10 nmol/mol methane. In January 2009, NASA scientists announced that they had discovered that the planet often vents methane into the atmosphere in specific areas, leading some to speculate this may be a sign of biological activity going on below the surface.
Jupiter – the atmosphere contains about 0.3% methane
Saturn – the atmosphere contains about 0.4% methane
*Iapetus
Titan — the atmosphere contains 1.6% methane and thousands of methane lakes have been detected on the surface In the upper atmosphere the methane is converted into more complex molecules including acetylene, a process that also produces molecular hydrogen. There is evidence that acetylene and hydrogen are recycled into methane near the surface. This suggests the presence either of an exotic catalyst, or an unfamiliar form of methanogenic life.
Enceladus – the atmosphere contains 1.7% methane
Uranus – the atmosphere contains 2.3% methane
*Ariel – methane is believed to be a constituent of Ariel's surface ice
*Miranda
*Oberon – about 20% of Oberon's surface ice is composed of methane-related carbon/nitrogen compounds
*Titania – about 20% of Titania's surface ice is composed of methane-related organic compounds
*Umbriel – methane is a constituent of Umbriel's surface ice
Neptune – the atmosphere contains 1.6% methane
Triton – Triton has a tenuous nitrogen atmosphere with small amounts of methane near the surface.
Pluto –
spectroscopic analysis of Pluto's surface reveals it to contain traces of methane
Charon – methane is believed present on Charon, but it is not completely confirmed
Eris – infrared light from the object revealed the presence of methane ice
Comet Halley
Comet Hyakutake – terrestrial observations found
ethane and methane in the comet
Extrasolar planet HD 189733b – This is the first detection of an organic compound on a planet outside the solar system. Its origin is unknown, since the planet's high temperature (700 °C) would normally favor the formation of
carbon monoxide instead.
Interstellar clouds
See also
2007 Zasyadko mine disaster
Abiogenic petroleum origin
Aerobic methane production
Anaerobic digestion
Anaerobic respiration
Arctic methane release
Biogas
Coal Oil Point seep field
Energy density
Greenhouse gas
Halomethane, halogenated methane derivatives
List of alkanes
Methanation
Methane clathrate, form of water ice that 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
Thomas Gold
References
External links
Gavin Schmidt, Methane: A Scientific Journey from Obscurity to Climate Super-Stardom, NASA Goddard, September 2004
Methane thermodynamics
International Chemical Safety Card 0291
Methane Hydrates
Safety data for methane
Methane-eating bug holds promise for cutting greenhouse gas. Media Release, GNS Science, New Zealand]
Catalytic conversion of methane to more useful chemicals and fuels
Methane as a Savior of the Dairy Industry
Category:Anaerobic digestion
Category:Greenhouse gases
Category:Fuels
Category:Fuel gas
