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Methane (, ) is a chemical compound with the
chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, commas and ...
(one
carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three occur naturally, ...

carbon
atom bonded to four
hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

hydrogen
atoms). It is a
group-14 hydride Group 14 hydrides are chemical compounds composed of hydrogen atoms and carbon group, group 14 atoms (the elements of group 14 are carbon, silicon, germanium, tin, lead and flerovium). Tetrahydrides The tetrahydride series has the chemical formula ...
, the simplest
alkane In , an alkane, or paraffin (a historical that also has ), is an . In other words, an alkane consists of and atoms arranged in a structure in which all the s are . Alkanes have the general chemical formula C''n''H2''n''+2. The alkanes rang ...
, and the main constituent of
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon In , a hydrocarbon is an consisting entirely of and . Hydrocarbons are examples of s. Hydrocarbons are generally colourless and hydrophobic wit ...

natural gas
. The relative abundance of methane on
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wit ...

Earth
makes it an economically attractive
fuel A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical conc ...

fuel
, although capturing and storing it poses technical challenges due to its
gaseous Gas is one of the four fundamental states of matter (the others being solid Solid is one of the four fundamental states of matter (the others being liquid A liquid is a nearly incompressible fluid In physics, a fluid is a su ...
state under normal conditions for temperature and pressure. Naturally occurring methane is found both below ground and under the
seafloor The seabed (also known as the seafloor, sea floor, or ocean floor) is the bottom of the ocean, no matter how deep. All floors of the ocean are known as 'seabeds'. Structure Most of the oceans have a common structure, created by common ph ...

seafloor
and is formed by both geological and biological processes. The largest reservoir of methane is under the seafloor in the form of
methane clathrate . Methane clathrate (CH4·5.75H2O) or (4CH4·23H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large am ...

methane clathrate
s. When methane reaches the surface and the
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...
, it is known as
atmospheric methane Atmospheric methane is the methane present in Earth's atmosphere. Atmospheric methane concentrations are of interest because it is one of the most potent greenhouse gases in Earth's atmosphere. Atmospheric methane is rising. The 20-year global w ...
. The Earth's atmospheric methane concentration has increased by about 150% since 1750, and it accounts for 20% of the total
radiative forcing Radiative forcing is the change in energy flux in the atmosphere caused by Climate variability and change, natural or Human impact on the environment#Impacts on climate, anthropogenic factors of climate change as measured by watts / metre2. It is ...
from all of the long-lived and globally mixed
greenhouse gas A greenhouse gas (GHG or GhG) is a gas Gas is one of the four fundamental states of matter (the others being solid, liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformat ...
es. It has also been detected on other planets, including
Mars Mars is the fourth planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to ...

Mars
, which has implications for
astrobiology Astrobiology, formerly known as exobiology, is an interdisciplinary scientific field concerned with the origins, early evolution, distribution, and future of life Life is a characteristic that distinguishes physical entities that have ...
research.


Properties and bonding

Methane is a
tetrahedral In , a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular , is a composed of four , six straight , and four . The tetrahedron is the simplest of all the ordinary and the only one that has fewer than 5 faces. The t ...
molecule with four equivalent C–H bonds. Its electronic structure is described by four bonding molecular orbitals (MOs) resulting from the overlap of the valence orbitals on
C
C
and
H
H
. The lowest-energy MO is the result of the overlap of the 2s 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. At
room temperature Colloquially, room temperature is the range of air temperature Temperature is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy, present in all matter, which is the source of the occurrence of heat ...
and
standard pressure Standard temperature and pressure are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure ...
, methane is a colorless, odorless gas. The familiar smell of natural gas as used in homes is achieved by the addition of an
odorant An aroma compound, also known as an odorant, aroma, fragrance or flavor, is a chemical compound that has a smell or odor. For an individual chemical or class of chemical compounds to impart a smell or fragrance, it must be sufficiently Volatili ...
, usually blends containing
tert-butylthiol ''tert''-Butylthiol, also known as 2-methylpropane-2-thiol, 2-methyl-2-propanethiol, ''tert''-butyl mercaptan (TBM), and ''t''-BuSH, is an organosulfur compound with the formula (CH3)3CSH. This thiol A thiol () or thiol derivative is any organos ...

tert-butylthiol
, as a safety measure. Methane has a boiling point of −161.5  °C at a pressure of one
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...
. As a gas, it is
flammable , Germany ) , image_map = , map_caption = , map_width = 250px , capital = Berlin , coordinates = , largest_city = capital , languages_type = Official language , languages = German language, German , demonym = Germans, German , g ...
over a range of concentrations (5.4–17%) in air at
standard pressure Standard temperature and pressure are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure ...
. Solid methane exists in several
modifications Modification may refer to: * Special education#Modifications, Modifications of school work for students with special educational needs * Modifications (genetics), changes in appearance arising from changes in the environment * Posttranslational mo ...
. Presently nine are known. Cooling methane at normal pressure results in the formation of methane I. This substance crystallizes in the cubic system (
space group In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it unchan ...
Fmm). The positions of the hydrogen atoms are not fixed in methane I, i.e. methane molecules may rotate freely. Therefore, it is a
plastic crystalA plastic crystal is a crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all dir ...
.


Chemical reactions

The primary chemical reactions of methane are
combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...
,
steam reforming Steam reforming or steam methane reforming is a method for producing syngas (hydrogen and carbon monoxide) by reaction of hydrocarbons with water. Commonly natural gas is the feedstock. The main purpose of this technology is hydrogen production. The ...
to
syngas Syngas, or synthesis gas, is a fuel gas Fuel gas is any one of a number of fuels that under ordinary conditions are gaseous. Many fuel gases are composed of hydrocarbons (such as methane or propane), hydrogen Hydrogen is the chemica ...
, and
halogenation In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, ...

halogenation
. In general, methane reactions are difficult to control.


Selective oxidation

Partial
oxidation Redox (reduction–oxidation, pronunciation: or ) is a type of chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter ...

oxidation
of methane to
methanol Methanol, also known as methyl alcohol, amongst other names, is a chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be se ...

methanol
, a more convenient, liquid fuel, is challenging because the reaction typically progresses all the way to
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
and
water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...

water
even with an insufficient supply of
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
. The
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
methane monooxygenase Methane monooxygenase (MMO) is an enzyme capable of oxidizing the C-H bond in methane as well as other alkanes. Methane monooxygenase belongs to the class of oxidoreductase enzymes (). There are two forms of MMO: the well-studied soluble form (sMM ...
produces methanol from methane, but cannot be used for industrial-scale reactions. Some homogeneously
catalyzed that utilizes a low-temperature oxidation catalyst to convert carbon monoxide to less toxic carbon dioxide at room temperature. It can also remove formaldehyde from the air. Catalysis () is the process of increasing the reaction rate, rate of a ...

catalyzed
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 the Catalytica system, copper
zeolite Zeolites are microporous, aluminosilicate Aluminosilicate mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Eart ...

zeolite
s, and iron zeolites stabilizing the
alpha-oxygen Alpha-oxygen (α-O) is a reactive oxygen species formed from an oxygen-atom abstraction (OAT) from nitrous oxide (N2O) by alpha-iron (α-Fe) Catalysis, catalysts. The latter is defined as a high spin (S=2) divalent iron(II) ion in a constrained squa ...
active site. One group of
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
catalyze methane oxidation with
nitrite The nitrite polyatomic ion, ion has the chemical formula . Nitrite (mostly sodium nitrite) is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name ...

nitrite
as the
oxidant 125px, Dangerous goods label for oxidizing agents An oxidizing agent, also known as an oxidant or oxidizer, is a substance that has the ability to oxidize (mild reducing agent) are added to powdered potassium permanganate Potassium perma ...

oxidant
in the absence of
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
, giving rise to the so-called
anaerobic oxidation of methane Anaerobic oxidation of methane (AOM) is a microbial process occurring in anoxic marine and freshwater sediments. During AOM methane Methane ( or ) is a chemical compound with the chemical formula (one atom of carbon and four atoms of hydrog ...
.


Acid–base reactions

Like other
hydrocarbon In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, prop ...
s, methane is an extremely
weak acid Acid strength is the tendency of an acid An acid is a molecule or ion capable of donating a proton (hydrogen ion H+) (a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid), or, alternatively, capable of forming a covalent bond wi ...

weak acid
. Its pKa in
DMSO Dimethyl sulfoxide (DMSO) is an organosulfur compound Organosulfur compounds are organic compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements ...

DMSO
is estimated to be 56. It cannot be in solution, but the
conjugate base A conjugate acid, within the Brønsted–Lowry acid–base theory The Brønsted–Lowry theory (also called proton theory of acids and bases) is an acid–base reaction An acid–base reaction is a chemical reaction A chemical reactio ...
is known in forms such as
methyllithium Methyllithium is the simplest organolithium reagentOrganolithium reagents are organometallic compounds that contain carbon – lithium bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group ...
. A variety of positive ions derived from methane have been observed, mostly as unstable species in low-pressure gas mixtures. These include
methenium In organic chemistry, methenium (also called methylium, carbenium, methyl cation, or protonated methylene) is a cation An ion () is a particle, atom or molecule with a net electric charge, electrical charge. The charge of the electron is ...
or methyl cation , methane cation , and
methanium In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they unde ...
or protonated methane . Some of these have been detected in outer space. Methanium can also be produced as diluted solutions from methane with
superacid According to the classical definition, a superacid is an acid An acid is a molecule or ion capable of donating a proton (hydrogen ion H+) (a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid), or, alternatively, capable of forming ...
s.
Cation An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are u ...
s with higher charge, such as and , have been studied theoretically and conjectured to be stable. Despite the
strength Physical strength *Physical strength, as in people or animals *Hysterical strength, extreme strength occurring when people are in life-and-death situations *Superhuman strength, great physical strength far above human capability *A common attrib ...
of its C–H bonds, there is intense interest in
catalysts that utilizes a low-temperature oxidation catalyst to convert carbon monoxide to less toxic carbon dioxide Carbon dioxide (chemical formula ) is a colorless gas with a density about 53% higher than that of dry air. Carbon dioxide molecules ...
that facilitate C–H bond activation in methane (and other lower numbered
alkanes In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, pr ...

alkanes
).


Combustion

Methane's
heat of combustion The heating value (or energy value or calorific value) of a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, a material with a definite chemical composi ...
is 55.5 MJ/kg.
Combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...
of methane is a multiple step reaction summarized as follows: :CH4 + 2 O2 → CO2 + 2 H2O (, at standard conditions)
Peters four-step chemistryPeters four-step chemistry is a systematically reduced mechanism for methane combustion, named after Norbert Peters (engineer), Norbert Peters, who derived it in 1985. The mechanism reads as :CH4 + 2H + H2O -> CO + 4H2 :CO + H2O CO2 + H2 :H ...
is a systematically reduced four-step chemistry that explains the burning of methane.


Methane radical reactions

Given appropriate conditions, methane reacts with
halogen The halogens () are a group A group is a number A number is a mathematical object used to counting, count, measurement, measure, and nominal number, label. The original examples are the natural numbers 1, 2, 3, 4, and so forth. Numbers can b ...

halogen
radicals as follows: :X• + CH4 → HX + CH3• :CH3• + X2 → CH3X + X• where X is a
halogen The halogens () are a group A group is a number A number is a mathematical object used to counting, count, measurement, measure, and nominal number, label. The original examples are the natural numbers 1, 2, 3, 4, and so forth. Numbers can b ...

halogen
:
fluorine Fluorine is a chemical element with the Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard conditions for temperature and pressure, standard conditions as a highly toxic, pale yellow Diatomic molecule ...

fluorine
(F),
chlorine Chlorine is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemica ...

chlorine
(Cl),
bromine Bromine is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elemen ...

bromine
(Br), or
iodine Iodine is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical ele ...

iodine
(I). This mechanism for this process is called
free radical halogenationIn organic chemistry, free-radical halogenation is a type of halogenation In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: t ...
. It is initiated when
UV light Ultraviolet (UV) is a form of with from 10 (with a corresponding frequency around 30 ) to 400 nm (750 ), shorter than that of , but longer than s. UV radiation is present in , and constitutes about 10% of the total electromagnetic radi ...
or some other
radical initiator In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...
(like
peroxides Peroxides are a group of compounds with the structure R−O−O−R, where R = any element. The O−O group in a peroxide is called the peroxide group or peroxo group. The nomenclature is somewhat variable. The most common peroxide is hydrogen ...
) produces a halogen
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

atom
. A two-step
chain reaction A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback Positive feedback (exacerbating feedback, self-reinforcing feedback) is a pro ...
ensues in which the halogen atom abstracts a hydrogen atom from a methane molecule, resulting in the formation of a
hydrogen halideHydrogen halides are diatomic Diatomic molecules are molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically ...
molecule and a
methyl radical Methyl (also systematically named trihydridocarbon) is an organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Du ...
(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. Similar reactions can occur on the halogenated product, leading to replacement of additional hydrogen atoms by halogen atoms with
dihalomethane The dihalomethanes are organic compounds in which two hydrogen atoms in methane are replaced by a halogen. They belong to the haloalkanes or to the subgroup of halomethanes and contains ten members. There are four members with only one sort of halo ...
,
trihalomethane Trihalomethanes (THMs) are chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together ...
, and ultimately,
tetrahalomethaneTetrahalomethanes are fully Halogenation, halogenated methane derivatives of general formula CBrkCllFmInAtp, where:k+l+m+n+p=4Tetrahalomethanes are on the border of Inorganic chemistry, inorganic and Organic chemistry, organic chemistry, thus they ca ...
structures, depending upon reaction conditions and the halogen-to-methane ratio.


Uses

Methane is used in industrial chemical processes and may be transported as a refrigerated liquid (liquefied natural gas, or
LNG Liquefied natural gas (LNG) is natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon gas mixture consisting of methane and commonly including varying amounts of other higher alkanes, and s ...

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.


Fuel

Methane is used as a
fuel A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical conc ...

fuel
for ovens, homes, water heaters, kilns, automobiles, turbines, etc.
Activated carbon Activated carbon, also called activated charcoal, is a form of carbon Carbon (from la, carbo "coal") is a chemical element with the Symbol (chemistry), symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—making f ...

Activated carbon
is used to store methane. Refined liquid methane is used as a
rocket fuel Rocket propellant is the reaction mass Working mass, also referred to as reaction mass, is a mass Mass is the physical quantity, quantity of ''matter'' in a physical body. It is also a measure (mathematics), measure of the body's ''inerti ...
, when combined with
liquid oxygen Liquid oxygen—abbreviated LOx, LOX or Lox in the aerospace Aerospace is a term used to collectively refer to the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)' ...
, as in the
BE-4 The Blue Engine 4 or BE-4 is an oxygen-rich liquefied-natural-gas-fueled Staged combustion cycle, staged-combustion rocket engine under development by Blue Origin. The BE-4 is being developed with private and public funding. The engine has been ...
and
Raptor Raptor or RAPTOR may refer to: Animals * Raptor (bird) Birds of prey, also known as raptors, include species of bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves , characterised by feathers ...
engines. As the major constituent of
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon In , a hydrocarbon is an consisting entirely of and . Hydrocarbons are examples of s. Hydrocarbons are generally colourless and hydrophobic wit ...

natural gas
, methane is important for electricity generation by burning it as a fuel in a gas turbine or Boiler (power generation), steam generator. Compared to other fossil fuel, hydrocarbon fuels, methane produces less
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
for each unit of heat released. At about 891 kJ/mol, methane's
heat of combustion The heating value (or energy value or calorific value) of a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, a material with a definite chemical composi ...
is lower than that of any other hydrocarbon. However, it produces more heat per mass (55.7 kJ/g) than any other organic molecule due to its relatively large content of hydrogen, which accounts for 55% of the heat of combustion but contributes only 25% of the molecular mass of methane. In many cities, methane is piped into homes for domestic heating and cooking. In this context it is usually known as
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon In , a hydrocarbon is an consisting entirely of and . Hydrocarbons are examples of s. Hydrocarbons are generally colourless and hydrophobic wit ...

natural gas
, which is considered to have an energy content of 39 megajoules per cubic meter, or 1,000 British thermal unit, BTU per standard cubic foot. Liquefied natural gas (LNG) is predominantly methane (CH4) converted into liquid form for ease of storage or transport. As a Liquid-propellant rocket, liquid rocket fuel, methane offers the advantage over kerosene of producing small exhaust molecules. This deposits less soot on the internal parts of rocket motors, reducing the difficulty of booster re-use. The lower molecular weight of the exhaust also increases the fraction of the heat energy which is in the form of kinetic energy available for propulsion, increasing the specific impulse of the rocket. Liquid methane also has a temperature range (91–112 K) nearly compatible with
liquid oxygen Liquid oxygen—abbreviated LOx, LOX or Lox in the aerospace Aerospace is a term used to collectively refer to the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)' ...
(54–90 K).


Chemical feedstock

Natural gas, which is mostly composed of methane, is used to produce hydrogen gas on an industrial scale. Steam reforming, 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. Much of this hydrogen is used in petroleum Refinery, refineries, in the production of chemicals and in food processing. Very large quantities of hydrogen are used in the Ammonia production, industrial synthesis of ammonia. At high temperatures (700–1100 °C) and in the presence of a metal-based catalyst (nickel), steam reacts with methane to yield a mixture of Carbon monoxide, CO and Dihydrogen, H2, known as "water gas" or "
syngas Syngas, or synthesis gas, is a fuel gas Fuel gas is any one of a number of fuels that under ordinary conditions are gaseous. Many fuel gases are composed of hydrocarbons (such as methane or propane), hydrogen Hydrogen is the chemica ...
": : CH4 + water, H2O ⇌ carbon monoxide, CO + 3 hydrogen, H2 This reaction is strongly endothermic (consumes heat, Δ''H''r = 206 kJ/mol). Additional hydrogen is obtained by the reaction of carbon monoxide, CO with water via the water-gas shift reaction: : CO + H2O ⇌ carbon dioxide, CO2 + H2 This reaction is mildly exothermic (produces heat, Δ''H''r = −41 kJ/mol). Methane is also subjected to free-radical chlorination reaction, chlorination in the production of chloromethanes, although
methanol Methanol, also known as methyl alcohol, amongst other names, is a chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be se ...

methanol
is a more typical precursor.Rossberg, M. ''et al.'' (2006) "Chlorinated Hydrocarbons" in ''Ullmann's Encyclopedia of Industrial Chemistry'', Wiley-VCH, Weinheim. .


Generation


Geological routes

The two main routes for geological methane generation are (i) organic (thermally generated, or thermogenic) and (ii) inorganic (Abiotic component, abiotic). Thermogenic methane occurs due to the breakup of organic matter at elevated temperatures and pressures in deep sedimentary Stratum, strata. 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 or via water-rock reactions that occur at low temperatures and pressures, like Serpentinite, serpentinization.


Biological routes

Most of Earth's methane is Biogenic substance, biogenic and is produced by methanogenesis,Lessner, Daniel J. (Dec 2009) Methanogenesis Biochemistry. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000573.pub2] a form of anaerobic respiration only known to be conducted by some members of the domain Archaea. Methanogens occupy landfills and other Soil gas, soils, ruminants (for example, cattle), the guts of termites, and the Anoxic waters, anoxic sediments below the seafloor and the bottom of lakes. Rice fields also generate large amounts of methane during plant growth. This multistep process is used by these microorganisms for energy. The net reaction of methanogenesis is: :CO2 + 4 H2→ CH4 + 2 H2O The final step in the process is catalyzed by the enzyme Coenzyme-B sulfoethylthiotransferase, methyl coenzyme M reductase (MCR).


Ruminants

Ruminants, such as cattle, belch methane, accounting for about 22% of the U.S. annual methane emissions to the atmosphere. One study reported that the livestock sector in general (primarily cattle, chickens, and pigs) produces 37% of all human-induced methane. A 2013 study estimated that livestock accounted for 44% of human-induced methane and about 15% of human-induced greenhouse gas emissions. Many efforts are underway to reduce livestock methane production, such as medical treatments and dietary adjustments, and to trap the gas to use its combustion energy.


Seafloor sediments

Most of the subseafloor is Anoxic waters, anoxic because oxygen is removed by Aerobic respiration, aerobic microorganisms within the first few centimeters of the seafloor sediment, sediment. Below the oxygen-replete seafloor, methanogens produce methane that is either used by other organisms or becomes trapped in Clathrate hydrate, gas hydrates. These other organisms that utilize methane for energy are known as methanotrophs ('methane-eating'), and are the main reason why little methane generated at depth reaches the sea surface. Consortia of Archaea and Bacteria have been found to oxidize methane via
anaerobic oxidation of methane Anaerobic oxidation of methane (AOM) is a microbial process occurring in anoxic marine and freshwater sediments. During AOM methane Methane ( or ) is a chemical compound with the chemical formula (one atom of carbon and four atoms of hydrog ...
(AOM); the organisms responsible for this are anaerobic Methanotroph, methanotrophic Archaea (ANME) and Sulfate-reducing microorganism, sulfate-reducing bacteria (SRB).


Industrial routes

Given its cheap abundance in natural gas, there is little incentive to produce methane industrially. Methane can be produced by hydrogenation, hydrogenating carbon dioxide through the Sabatier process. Methane is also a side product of the hydrogenation of carbon monoxide in the Fischer–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 the Great Plains Synfuels plant, started in 1984 in Beulah, North Dakota as a way to develop abundant local resources of low-grade lignite, a resource that is otherwise difficult to transport for its weight, Coal assay#Ash, ash content, low calorific value and propensity to spontaneous 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 as gasoline, Diesel fuel, diesel, or feedstock to other processes. Power to gas#Power to methane, Power to methane is a technology that uses electricity, electrical power to produce hydrogen from water by electrolysis and uses the Sabatier reaction to combine hydrogen with
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
to produce methane. As of 2021, this is mostly under development and not in large-scale use. Theoretically, the process could be used as a buffer for excess and off-peak power generated by highly fluctuating wind turbines and solar arrays. However, as currently very large amounts of natural gas are used in power plants (e.g. Combined cycle, CCGT) to produce electric energy, the losses in efficiency are not acceptable.


Laboratory synthesis

Methane can be produced by protonation of methyl lithium or a methyl Grignard reagent such as methylmagnesium chloride. It can also be made from anhydrous sodium acetate and dry sodium hydroxide, mixed and heated above 300 °C (with sodium carbonate as byproduct). In practice, a requirement for pure methane can easily be fulfilled by steel gas bottle from standard gas suppliers.


Occurrence

Methane was discovered and isolated by Alessandro Volta between 1776 and 1778 when studying marsh gas from Lake Maggiore. It is the major component of natural gas, about 87% by volume. The major source of methane is extraction from geological deposits known as natural gas fields, with coal seam gas extraction becoming a major source (see coal bed methane extraction, a method for extracting methane from a coal deposit, while enhanced coal bed methane recovery is a method of recovering methane from non-mineable coal seams). It is associated with other
hydrocarbon In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, prop ...
fuels, and sometimes accompanied by helium and nitrogen. Methane is produced at shallow levels (low pressure) by anaerobic organism, anaerobic Decomposition, decay of organic matter and reworked methane from deep under the Earth's surface. In general, the sediments that generate natural gas are buried deeper and at higher temperatures than those that contain Petroleum, oil. Methane is generally transported in bulk by Pipeline transport, pipeline in its natural gas form, or by LNG carriers in its liquefied form; few countries transport it by truck.


Atmospheric methane

In 2010, methane levels in the Arctic were measured at 1850 nmol/mol. This level is over twice as high as at any time in the last 400,000 years. Keeling Curve, Historic 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 and 700 nmol/mol during the warm interglacial periods. The Earth's oceans are a potential important source of Arctic methane. Methane is an important
greenhouse gas A greenhouse gas (GHG or GhG) is a gas Gas is one of the four fundamental states of matter (the others being solid, liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformat ...
with a global warming potential of 34 compared to CO2 (potential of 1) over a 100-year period, and 72 over a 20-year period.Intergovernmental Panel on Climate Change, IPCC Fifth Assessment Report
Table 8.7, Chap. 8, p. 8–58
(PDF; 8,0 MB)
The Earth's atmospheric methane concentration has increased by about 150% since 1750, and it accounts for 20% of the total
radiative forcing Radiative forcing is the change in energy flux in the atmosphere caused by Climate variability and change, natural or Human impact on the environment#Impacts on climate, anthropogenic factors of climate change as measured by watts / metre2. It is ...
from all of the long-lived and globally mixed greenhouse gases. The IPCC Sixth Assessment Report, AR6 of the Intergovernmental Panel on Climate Change, IPCC states: "Observed increases in well-mixed greenhouse gas (GHG) concentrations since around 1750 are unequivocally caused by human activities. Since 2011 (measurements reported in AR5), concentrations have continued to increase in the atmosphere, reaching annual averages of 410 ppm for carbon dioxide (CO2), 1866 ppb for methane (CH4), and 332 ppb for nitrous oxide (N2O) in 2019. (…) In 2019, atmospheric CO2 concentrations were higher than at any time in at least 2 million years (high confidence), and concentrations of CH4 and N2O were higher than at any time in at least 800,000 years (very high confidence). Since 1750, increases in CO2 (47%) and CH4 (156%) concentrations far exceed, and increases in N2O (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)". From 2015 to 2019 sharp rises in levels of atmospheric methane have been recorded. In February 2020, it was reported that fugitive emissions and gas venting from the fossil fuel industry may have been significantly underestimated. Climate change can increase atmospheric methane levels by increasing methane production in natural ecosystems, forming a Climate change feedback.


Clathrates

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 along continental margins beneath the Seabed, ocean floor within the Gas hydrate stability zone, gas 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). 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.Miller, G. Tyler (2007). ''Sustaining the Earth: An Integrated Approach''. U.S.A.: Thomson Advantage Books, p. 160. The global mass of carbon stored in gas clathrates is still uncertain and has been estimated as high as 12,500 Gigaton, Gt carbon and as low as 500 Gt carbon. The estimate has declined over time with a most recent estimate of ~1800 Gt carbon. 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 an Mid-ocean ridge, ultraslow spreading ridge in the Arctic. Some climate models suggest that today's methane emission regime from the ocean floor is potentially similar to that during the period of the Paleocene–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. Arctic methane release from permafrost and seafloor methane clathrates is a potential consequence and further cause of global warming; this is known as the clathrate gun hypothesis. Data from 2016 indicate that Arctic permafrost thaws faster than predicted.


Extraterrestrial methane


Interstellar medium

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, using In situ resource utilization, methane production from local materials found on
Mars Mars is the fourth planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to ...

Mars
or Titan (moon), Titan), providing fuel for a return journey.


Mars

Methane has been detected on all planets of the Solar System and most of the larger moons. With the possible exception of Life on Mars, Mars, it is believed to have come from Abiogenic petroleum origin, abiotic processes. The Curiosity (rover), ''Curiosity'' rover has documented seasonal fluctuations of Atmosphere of Mars, atmospheric methane levels on Mars. These fluctuations peaked at the end of the Martian summer at 0.6 parts per billion. Methane has been proposed as a possible rocket propellant on future Human mission to Mars, Mars missions due in part to the possibility of synthesizing it on the planet by In situ resource utilization#Mars, in situ resource utilization. An adaptation of the Sabatier reaction, Sabatier methanation reaction may be used with a mixed catalyst bed and a Water-gas shift reaction#Reverse water-gas shift, reverse water-gas shift in a single reactor to produce methane from the raw materials available on Mars, utilizing water from the Martian soil, Martian subsoil and
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
in the Atmosphere of Mars, Martian atmosphere. Methane could be produced by a non-biological process called serpentinite, serpentinization involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.


History

In November 1776, methane was first scientifically identified by Italian people, Italian physicist Alessandro Volta in the marshes of Lake Maggiore straddling Italy and Switzerland. Volta was inspired to search for the substance after reading a paper written by Benjamin Franklin about "flammable air".Volta, Alessandro (1777
''Lettere del Signor Don Alessandro Volta ... Sull' Aria Inflammable Nativa Delle Paludi''
[Letters of Signor Don Alessandro Volta ... on the flammable native air of the marshes], Milan, Italy: Giuseppe Marelli.
Volta collected the gas rising from the marsh, and by 1778 had isolated pure methane. He also demonstrated that the gas could be ignited with an electric spark. Following the Felling mine disasters#1812 disaster, Felling mine disaster of 1812 in which 92 men perished, Sir Humphry Davy established that the feared firedamp was in fact largely methane. The name "methane" was coined in 1866 by the German chemist August Wilhelm von Hofmann. The name was derived from Methanol#History, methanol.


Etymology

Etymologically, the word "''methane''" is coined from the chemical suffix "''-ane''", which denotes substances belonging to the alkane family; and the word "''methyl''", which is derived from the German "''methyl''" (1840) or directly from the French "''méthyle''", which is a back-formation from the French "''méthylène''" (corresponding to English "methylene"), the root of which was coined by Jean-Baptiste Dumas and Eugène Péligot in 1834 from the Greek "''methy''" (wine) (related to English "mead") and "''hyle''" (meaning "wood"). The radical is named after this because it was first detected in
methanol Methanol, also known as methyl alcohol, amongst other names, is a chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be se ...

methanol
, 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 chemist August Wilhelm von Hofmann (1818–1892).


Abbreviations

The abbreviation CH4-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 of CH4-C. CH4-C can also mean the methane-carbon ratio, which is 1.33 by mass. Methane at scales of the atmosphere is commonly measured in teragrams (Tg CH4) or millions of metric tons (MMT CH4), which mean the same thing. Other standard units are also used, such as nanomole (nmol, one billionth of a mole), Mole (unit), mole (mol), kilogram, and gram.


Safety

Methane is nontoxic, yet it is extremely flammable and may form explosive mixtures with air. Methane is also an asphyxiant gas, asphyxiant if the oxygen concentration is reduced to below about 16% by displacement, as most people can Cabin pressurization#Need for cabin pressurization, tolerate a reduction from 21% to 16% without ill effects. The concentration of methane at which asphyxiation risk becomes significant is much higher than the 5–15% concentration in a flammable or explosive mixture. Methane off-gas can penetrate the interiors of buildings near landfills and expose occupants to significant levels of methane. Some buildings have specially engineered recovery systems below their basements to actively capture this gas and vent it away from the building. Methane gas explosions are responsible for many deadly mining disasters. A methane gas explosion was the cause of the Upper Big Branch Mine disaster, Upper Big Branch coal mine disaster in West Virginia on April 5, 2010, killing 29.


See also


Notes


References


Cited sources

*


External links


Methane
at ''The Periodic Table of Videos'' (University of Nottingham)
International Chemical Safety Card 0291


– United States Geological Survey *
CDC – Handbook for Methane Control in Mining
{{Authority control Methane, Anaerobic digestion Fuel gas Fuels Greenhouse gases Industrial gases Gaseous signaling molecules