Naturally occurring methane is found both below ground and under theseafloor and is formed by both geological and biological processes. The largestreservoir of methane is under the seafloor in the form ofmethane clathrates. When methane reaches the surface and theatmosphere, it is known asatmospheric methane.[10]
The Earth's atmospheric methane concentrationhas increased by about 160% since 1750, with the overwhelming percentage caused by human activity.[11] It accounted for 20% of the totalradiative forcing from all of the long-lived and globally mixedgreenhouse gases, according to the 2021Intergovernmental Panel on Climate Change report.[12] Strong, rapid and sustained reductions in methane emissions could limit near-term warming and improve air quality by reducing global surface ozone.[13]
Methane has also been detected on other planets, includingMars, which has implications forastrobiology research.[14]
Methane is atetrahedral molecule with four equivalentC–H bonds. Itselectronic structure is described by four bonding molecular orbitals (MOs) resulting from the overlap of the valence orbitals onC andH. The lowest-energy MO is the result of the overlap of the2s orbital on carbon with the in-phase combination of the 1s orbitals on the four hydrogen atoms. Above this energy level is a triply degenerate set of MOs that involve overlap of the 2p orbitals on carbon with various linear combinations of the 1s orbitals on hydrogen. The resulting "three-over-one" bonding scheme is consistent with photoelectron spectroscopic measurements.
Methane is an odorless, colourless and transparent gas atstandard temperature and pressure.[15] It does absorb visible light, especially at the red end of the spectrum, due toovertone bands, but the effect is only noticeable if the light path is very long. This is what givesUranus andNeptune their blue or bluish-green colors, as light passes through their atmospheres containing methane and is then scattered back out.[16]
The familiar smell of natural gas as used in homes is achieved by the addition of anodorant, usually blends containingtert-butylthiol, as a safety measure. Methane has a boiling point of −161.5 °C at a pressure of oneatmosphere.[3] As a gas, it isflammable over a range of concentrations (5.4%–17%) in air atstandard pressure.
Solid methane exists in severalmodifications, of which nine are known.[17] Cooling methane at normal pressure results in the formation of methane I. This substance crystallizes in the cubic system (space group Fm3m). The positions of the hydrogen atoms are not fixed in methane I, i.e. methane molecules may rotate freely. Therefore, it is aplastic crystal.[18]
Partialoxidation of methane tomethanol (CH3OH), a more convenient, liquid fuel, is challenging because the reaction typically progresses all the way tocarbon dioxide andwater even with an insufficient supply ofoxygen. Theenzymemethane monooxygenase produces methanol from methane, but cannot be used for industrial-scale reactions.[19] Some homogeneouslycatalyzed systems and heterogeneous systems have been developed, but all have significant drawbacks. These generally operate by generating protected products which are shielded from overoxidation. Examples include theCatalytica system, copperzeolites, and iron zeolites stabilizing thealpha-oxygen active site.[20]
A variety ofpositive ions derived from methane have been observed, mostly as unstable species in low-pressure gas mixtures. These includemethenium or methyl cationCH+3, methane cationCH+4, andmethanium or protonated methaneCH+5. Some of these have beendetected in outer space. Methanium can also be produced as diluted solutions from methane withsuperacids.Cations with higher charge, such asCH2+6 andCH3+7, have been studied theoretically and conjectured to be stable.[23]
Given appropriate conditions, methane reacts withhalogenradicals as follows:
•X + CH4 → HX + •CH3
•CH3 + X2 → CH3X + •X
where X is ahalogen:fluorine (F),chlorine (Cl),bromine (Br), oriodine (I). This mechanism for this process is calledfree radical halogenation. It is initiated whenUV light or some otherradical initiator (likeperoxides) produces a halogenatom. A two-stepchain reaction ensues in which the halogen atom abstracts a hydrogen atom from a methane molecule, resulting in the formation of ahydrogen halide molecule and amethyl radical (•CH3). The methyl radical then reacts with a molecule of the halogen to form a molecule of the halomethane, with a new halogen atom as byproduct.[26] Similar reactions can occur on the halogenated product, leading to replacement of additional hydrogen atoms by halogen atoms withdihalomethane,trihalomethane, and ultimately,tetrahalomethane structures, depending upon reaction conditions and the halogen-to-methane ratio.
Methane may be transported as a refrigerated liquid (liquefied natural gas, orLNG). 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.
Methane is used as afuel for ovens, homes, water heaters, kilns, automobiles,[27][28] rockets, turbines, etc.
As the major constituent ofnatural gas, methane is important forelectricity generation by burning it as a fuel in agas turbine orsteam generator. Compared to otherhydrocarbon fuels, methane produces lesscarbon dioxide for each unit of heat released. At about 891 kJ/mol, methane'sheat of combustion is lower than that of any other hydrocarbon, but the ratio of the heat of combustion (891 kJ/mol) to the molecular mass (16.0 g/mol, of which 12.0 g/mol is carbon) shows that methane, being the simplest hydrocarbon, produces more heat per mass unit (55.7 kJ/g) than other complex hydrocarbons. In many areas with a dense enough population, methane is piped into homes and businesses forheating, cooking, and industrial uses. In this context it is usually known asnatural gas, which is considered to have an energy content of 39megajoules per cubic meter, or 1,000BTU perstandard cubic foot.Liquefied natural gas (LNG) is predominantly methane converted into liquid form for ease of storage or transport.
Refinedliquid methane as well as LNG isused as arocket fuel,[29] when combined withliquid oxygen, as in theTQ-12,BE-4,Raptor,YF-215, andAeon engines.[30] Due to the similarities between methane and LNG such engines are commonly grouped together under the termmethalox.
As aliquid rocket propellant, a methane/liquid oxygen combination offers the advantage overkerosene/liquid oxygen combination, or kerolox, of producing small exhaust molecules, reducing coking or deposition ofsoot on engine components. Methane is easier to store than hydrogen due to its higher boiling point and density, as well as its lack ofhydrogen embrittlement.[31][32] The lowermolecular weight of the exhaust also increases the fraction of the heat energy which is in the form of kinetic energy available for propulsion, increasing thespecific impulse of the rocket. Compared toliquid hydrogen, thespecific energy of methane is lower but this disadvantage is offset by methane's greater density and temperature range, allowing for smaller and lighter tankage for a given fuel mass. Liquid methane has a temperature range (91–112 K) nearly compatible with liquid oxygen (54–90 K). The fuel currently sees use in operational launch vehicles such asZhuque-2,Vulcan andNew Glenn as well as in-development launchers such asStarship,Neutron,Terran R,Nova, andLong March 9.[33]
Natural gas, which is mostly composed of methane, is used to produce hydrogen gas on an industrial scale.Steam methane reforming (SMR), or simply known as steam reforming, is the standard industrial method of producing commercial bulk hydrogen gas. More than 50 million metric tons are produced annually worldwide (2013), principally from the SMR of natural gas.[34] Much of this hydrogen is used inpetroleumrefineries, in the production of chemicals and in food processing. Very large quantities of hydrogen are used in theindustrial synthesis of ammonia.
At high temperatures (700–1100 °C) and in the presence of ametal-basedcatalyst (nickel), steam reacts with methane to yield a mixture ofCO andH2, known as "water gas" or "syngas":
CH4 + H2O ⇌ CO + 3 H2
This reaction is stronglyendothermic (consumes heat, ΔHr = 206 kJ/mol).Additional hydrogen is obtained by the reaction ofCO with water via thewater-gas shift reaction:
CO + H2O ⇌ CO2 + H2
This reaction is mildlyexothermic (produces heat, ΔHr = −41 kJ/mol).
Methane is also subjected to free-radicalchlorination in the production of chloromethanes, althoughmethanol is a more typical precursor.[35]
Hydrogen can also be produced via the direct decomposition of methane, also known as methanepyrolysis, which, unlike steam reforming, produces nogreenhouse gases (GHG). The heat needed for the reaction can also be GHG emission free, e.g. from concentrated sunlight, renewable electricity, or burning some of the produced hydrogen. If the methane is frombiogas then the process can be acarbon sink. Temperatures in excess of 1200 °C are required to break the bonds of methane to produce hydrogen gas and solid carbon.[36] Through the use of a suitable catalyst the reaction temperature can be reduced to between 550 and 900 °C depending on the chosen catalyst. Dozens of catalysts have been tested, including unsupported and supported metal catalysts, carbonaceous and metal-carbon catalysts.[37]
The reaction is moderately endothermic as shown in the reaction equation below.[38]
Global methane budget (2017). Shows natural sources and sinks (green), anthropogenic sources (orange), and mixed natural and anthropogenic sources (hatched orange-green for 'biomass and biofuel burning').
Methane can be generated through geological, biological or industrial routes.
Abiotic sources of methane[example needed] have been found in more than 20 countries and in several deep ocean regions so far.
The two main routes for geological methane generation are (i) organic (thermally generated, or thermogenic) and (ii) inorganic (abiotic).[14] Thermogenic methane occurs due to the breakup of organic matter at elevated temperatures and pressures in deep sedimentarystrata. Most methane in sedimentary basins is thermogenic; therefore, thermogenic methane is the most important source of natural gas. Thermogenic methane components are typically considered to be relic (from an earlier time). Generally, formation of thermogenic methane (at depth) can occur through organic matter breakup, or organic synthesis. Both ways can involve microorganisms (methanogenesis), but may also occur inorganically. The processes involved can also consume methane, with and without microorganisms.
The more important source of methane at depth (crystalline bedrock) is abiotic. Abiotic means that methane is created from inorganic compounds, without biological activity, either through magmatic processes[example needed] or via water-rock reactions that occur at low temperatures and pressures, likeserpentinization.[39][40]
Most of Earth's methane isbiogenic and is produced bymethanogenesis,[41][42] a form of anaerobic respiration only known to be conducted by some members of the domainArchaea.[43] Methanogens occur inlandfills andsoils,[44]ruminants (for example,cattle),[45] the guts of termites, and theanoxic sediments below the seafloor and the bottom of lakes.
This multistep process is used by these microorganisms for energy. The net reaction of methanogenesis is:
Testing Australian sheep for exhaled methane production (2001),CSIROThis image represents a ruminant, specifically a sheep, producing methane in the four stages of hydrolysis, acidogenesis, acetogenesis, and methanogenesis.
Wetlands are the largest natural sources of methane to the atmosphere,[47] accounting for approximately 20–30% of atmospheric methane.[48] Climate change is increasing the amount of methane released from wetlands due to increased temperatures and altered rainfall patterns. This phenomenon is calledwetland methane feedback.[49]
Rice cultivation generates as much as 12% of total global methane emissions due to the long-term flooding of rice fields.[50]
Ruminants such as cattle belch out methane, accounting for about 22% of the U.S. annual methane emissions to the atmosphere.[51] One study reported that the livestock sector in general (primarily cattle, chickens, and pigs) produces 37% of all human-induced methane.[52] A 2013 study estimated that livestock accounted for 44% of human-induced methane and about 15% of human-induced greenhouse gas emissions.[53] Many efforts are underway to reduce livestock methane production, such as medical treatments and dietary adjustments,[54][55] and to trap the gas to use its combustion energy.[56]
Most of the subseafloor isanoxic because oxygen is removed byaerobic microorganisms within the first few centimeters of thesediment. Below the oxygen-replete seafloor,methanogens produce methane that is either used by other organisms or becomes trapped ingas hydrates.[43] These other organisms that utilize methane for energy are known asmethanotrophs ('methane-eating'), and are the main reason why little methane generated at depth reaches the sea surface.[43] Consortia of Archaea and Bacteria have been found to oxidize methane viaanaerobic oxidation of methane (AOM); the organisms responsible for this are anaerobicmethanotrophic Archaea (ANME) andsulfate-reducing bacteria (SRB).[57]
Given its cheap abundance in natural gas, there is little incentive to produce methane industrially. Methane can be produced byhydrogenating carbon dioxide through theSabatier process. Methane is also a side product of the hydrogenation of carbon monoxide in theFischer–Tropsch process, which is practiced on a large scale to produce longer-chain molecules than methane.
An example of large-scale coal-to-methane gasification is theGreat Plains Synfuels plant, started in 1984 inBeulah, North Dakota as a way to develop abundant local resources of low-gradelignite, a resource that is otherwise difficult to transport for its weight,ash content, low calorific value and propensity tospontaneous combustion during storage and transport. A number of similar plants exist around the world, although mostly these plants are targeted towards the production of long chain alkanes for use asgasoline,diesel, or feedstock to other processes.
Methane is the major component of natural gas, about 87% by volume. The major source of methane is extraction from geological deposits known asnatural gas fields, withcoal seam gas extraction becoming a major source (seecoal bed methane extraction, a method for extracting methane from acoal deposit, whileenhanced coal bed methane recovery is a method of recovering methane from non-mineable coal seams). It is associated with otherhydrocarbon fuels, and sometimes accompanied byhelium andnitrogen. Methane is produced at shallow levels (low pressure) byanaerobicdecay oforganic matter and reworked methane from deep under the Earth's surface. In general, thesediments that generate natural gas are buried deeper and at higher temperatures than those that containoil.
Methane is generally transported in bulk bypipeline in its natural gas form, or by LNG carriers in its liquefied form; few countries transport it by truck.
Methane (CH4) measured by the Advanced Global Atmospheric Gases Experiment (AGAGE) in the lower atmosphere (troposphere) at stations around the world. Abundances are given as pollution free monthly mean mole fractions inparts-per-billion.
Methane is an importantgreenhouse gas, responsible for around 30% of the rise in global temperatures since the industrial revolution.[58]
Methane has aglobal warming potential (GWP) of 29.8 ± 11 compared toCO2 (potential of 1) over a 100-year period, and 82.5 ± 25.8 over a 20-year period.[59] This means that, for example, aleak of one tonne of methane is equivalent to emitting 82.5 tonnes of carbon dioxide. Burning methane and producing carbon dioxide also reduces the greenhouse gas impact compared to simply venting methane to the atmosphere.
Sources of global methane emissions
As methane is gradually converted into carbon dioxide (and water) in the atmosphere, these values include the climate forcing from the carbon dioxide produced from methane over these timescales.
Annual global methane emissions are currently approximately 580 Mt,[60] 40% of which is from natural sources and the remaining 60% originating from human activity, known as anthropogenic emissions. The largest anthropogenic source isagriculture, responsible for around one quarter of emissions, closely followed by theenergy sector, which includes emissions from coal, oil, natural gas and biofuels.[61]
Historic methane concentrations in the world's atmosphere have ranged between 300 and 400 nmol/mol during glacial periods commonly known asice ages, and between 600 and 700 nmol/mol during the warminterglacial periods. A 2012 NASA website said the oceans were a potential important source of Arctic methane,[62] but more recent studies associate increasing methane levels as caused by human activity.[11]
Global monitoring of atmospheric methane concentrations began in the 1980s.[11] The Earth's atmospheric methane concentration has increased 160% since preindustrial levels in the mid-18th century.[11] In 2013, atmospheric methane accounted for 20% of the totalradiative forcing from all of the long-lived and globally mixed greenhouse gases.[63] Between 2011 and 2019 the annual average increase of methane in the atmosphere was 1866 ppb.[12] From 2015 to 2019 sharp rises in levels of atmospheric methane were recorded.[64][65]
In 2019, the atmospheric methane concentration was higher than at any time in the last 800,000 years. As stated in theAR6 of theIPCC, "Since 1750, increases inCO2 (47%) andCH4 (156%) concentrations far exceed, and increases inN2O (23%) are similar to, the natural multi-millennial changes between glacial and interglacial periods over at least the past 800,000 years (very high confidence)".[12][a][66]
In February 2020, it was reported thatfugitive emissions andgas venting from thefossil fuel industry may have been significantly underestimated.[67][68] The largest annual increase occurred in 2021 with the overwhelming percentage caused by human activity.[11]
Climate change can increase atmospheric methane levels by increasing methane production in natural ecosystems, forming aclimate change feedback.[43][69] Another explanation for the rise in methane emissions could be a slowdown of the chemical reaction that removes methane from the atmosphere.[70]
Over 100 countries have signed theGlobal Methane Pledge, launched in 2021, promising to cut their methane emissions by 30% by 2030.[71] This could avoid 0.2 °C of warming globally by 2050, although there have been calls for higher commitments in order to reach this target.[72] TheInternational Energy Agency's 2022 report states "the most cost-effective opportunities for methane abatement are in the energy sector, especially in oil and gas operations".[73]
Methane clathrates (also known as methane hydrates) are solid cages of water molecules that trap single molecules of methane. Significant reservoirs of methane clathrates have been found in arctic permafrost and alongcontinental margins beneath theocean floor within thegas clathrate stability zone, located at high pressures (1 to 100 MPa; lower end requires lower temperature) and low temperatures (< 15 °C; upper end requires higher pressure).[74] Methane clathrates can form from biogenic methane, thermogenic methane, or a mix of the two. These deposits are both a potential source of methane fuel as well as a potential contributor to global warming.[75][76] The global mass of carbon stored in gas clathrates is still uncertain and has been estimated as high as 12,500Gt carbon and as low as 500 Gt carbon.[49] The estimate has declined over time with a most recent estimate of ≈1800 Gt carbon.[77] A large part of this uncertainty is due to our knowledge gap in sources and sinks of methane and the distribution of methane clathrates at the global scale. For example, a source of methane was discovered relatively recently in anultraslow spreading ridge in the Arctic.[48] Some climate models suggest that today's methane emission regime from the ocean floor is potentially similar to that during the period of thePaleocene–Eocene Thermal Maximum (PETM) around 55.5 million years ago, although there are no data indicating that methane from clathrate dissociation currently reaches the atmosphere.[77]Arctic methane release frompermafrost and seafloor methane clathrates is a potential consequence and further cause ofglobal warming; this is known as theclathrate gun hypothesis.[78][79][80][81] Data from 2016 indicate that Arctic permafrost thaws faster than predicted.[82]
In May 2023The Guardian published a report blamingTurkmenistan as the worst in the world for methanesuper emitting. The data collected by Kayrros researchers indicate that two large Turkmen fossil fuel fieldsleaked 2.6 million and 1.8 millionmetric tonnes of methane in 2022 alone, pumping theCO2 equivalent of 366 million tonnes into the atmosphere, surpassing the annual CO2 emissions of theUnited Kingdom.[88]
This sectionis missing information about where extraterrestrial abiotic methane comes from (Big Bang? supernova? mineral deposits reacting?). Please expand the section to include this information. Further details may exist on thetalk page.(June 2024)
Methane is abundant in many parts of the Solar System and potentially could be harvested on the surface of another Solar System body (in particular, usingmethane production from local materials found onMars[89] orTitan), providing fuel for a return journey.[29][90]
Methane has been detected on all planets of theSolar System and most of the larger moons.[citation needed] With the possible exception ofMars, it is believed to have come fromabiotic processes.[92][93]
Methane (CH4) on Mars – potential sources and sinks
Methane could be produced by a non-biological process calledserpentinization[b] involving water, carbon dioxide, and the mineralolivine, which is known to be common on Mars.[103]
Methane has been detected in vast abundance onTitan, the largest moon ofSaturn. It comprises a significant portion ofits atmosphere and also exists in a liquid form on its surface, where it comprises the majority of the liquid in Titan'svast lakes of hydrocarbons, thesecond largest of which is believed to be almost pure methane in composition.[104]
The presence of stable lakes of liquid methane on Titan, as well as the surface of Titan being highly chemically active and rich in organic compounds, has led scientists to consider thepossibility of life existing within Titan's lakes, using methane as a solvent in the place of water for Earth-based life[105] and using hydrogen in the atmosphere to derive energy withacetylene.[106]
The discovery of methane is credited toItalian physicistAlessandro Volta, who characterized numerous properties including itsflammability limit and origin from decaying organic matter.[107]
Volta was initially motivated by reports of inflammable air present in marshes by his friend Father Carlo Giuseppe Campi. While on a fishing trip toLake Maggiore straddlingItaly andSwitzerland in November 1776, he noticed the presence of bubbles in the nearby marshes and decided to investigate. Volta collected the gas rising from the marsh and demonstrated that the gas was inflammable.[107][108]
Volta notes similar observations of inflammable air were present previously in scientific literature, including a letter written byBenjamin Franklin.[109]
Etymologically, the wordmethane is coined from the chemical suffix "-ane", which denotes substances belonging to the alkane family; and the wordmethyl, which is derived from the GermanMethyl (1840) or directly from the Frenchméthyle, which is a back-formation from the Frenchméthylène (corresponding to English "methylene"), the root of which was coined byJean-Baptiste Dumas andEugène Péligot in 1834 from the Greekμέθυméthy (wine) (related to English "mead") andὕληhýlē (meaning "wood"). The radical is named after this because it was first detected inmethanol, an alcohol first isolated by distillation of wood. The chemical suffix-ane is from the coordinating chemical suffix-ine which is from Latin feminine suffix-ina which is applied to represent abstracts. The coordination of "-ane", "-ene", "-one", etc. was proposed in 1866 by German chemistAugust Wilhelm von Hofmann.[113]
The abbreviationCH4-C can mean the mass of carbon contained in a mass of methane, and the mass of methane is always 1.33 times the mass ofCH4-C.[114][115]CH4-C can also mean the methane-carbon ratio, which is 1.33 by mass.[116] Methane at scales of the atmosphere is commonly measured in teragrams (TgCH4) or millions of metric tons (MMTCH4), which mean the same thing.[117] Other standard units are also used, such as nanomole (nmol, one billionth of a mole),mole (mol),kilogram, andgram.
Methane is anasphyxiant gas, meaning that it is non-toxic and the primary health hazard is displacement of oxygen in high enough concentrations, potentially causing death byasphyxiation. No systemic toxicity has been detected at 5% concentration in air.
^In 2013Intergovernmental Panel on Climate Change (IPCC) scientists warned atmospheric concentrations of methane had "exceeded the pre-industrial levels by about 150% which represented "levels unprecedented in at least the last 800,000 years."
^There are manyserpentinization reactions.Olivine is asolid solution betweenforsterite andfayalite whose general formula is(Fe,Mg)2SiO4. The reaction producing methane from olivine can be written as:Forsterite + Fayalite + Water + Carbonic acid → Serpentine + Magnetite + Methane, or (in balanced form):
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^Cornell, Clayton B. (April 29, 2008)."Natural Gas Cars: CNG Fuel Almost Free in Some Parts of the Country". Archived fromthe original on January 20, 2019. RetrievedJuly 25, 2009.Compressed natural gas is touted as the 'cleanest burning' alternative fuel available, since the simplicity of the methane molecule reduces tailpipe emissions of different pollutants by 35 to 97%. Not quite as dramatic is the reduction in net greenhouse-gas emissions, which is about the same as corn-grain ethanol at about a 20% reduction over gasoline
^"Blue Origin BE-4 Engine".Archived from the original on October 1, 2021. RetrievedJune 14, 2019.We chose LNG because it is highly efficient, low cost and widely available. Unlike kerosene, LNG can be used to self-pressurize its tank. Known as autogenous repressurization, this eliminates the need for costly and complex systems that draw on Earth's scarce helium reserves. LNG also possesses clean combustion characteristics even at low throttle, simplifying engine reuse compared to kerosene fuels.
^Thiel, Volker (2018), "Methane Carbon Cycling in the Past: Insights from Hydrocarbon and Lipid Biomarkers", in Wilkes, Heinz (ed.),Hydrocarbons, Oils and Lipids: Diversity, Origin, Chemistry and Fate, Handbook of Hydrocarbon and Lipid Microbiology, Springer International Publishing, pp. 1–30,doi:10.1007/978-3-319-54529-5_6-1,ISBN978-3-319-54529-5,S2CID105761461
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