Methane

Methane (US: /ˈmɛθeɪn/or UK: /ˈmiːθeɪn/) is a chemical compound with the chemical formula CH4 (one atom of carbon and four atoms of hydrogen). It is a group-14 hydride and the simplest alkane, and is the main constituent of natural gas. The relative abundance of methane on Earth makes it an attractive fuel, although capturing and storing it poses challenges due to its gaseous state under normal conditions for temperature and pressure.

Natural occurring methane is found both below ground and under the sea floor, and is formed by both geological and biological processes. The largest reservoir of methane is under the seafloor in the form of methane clathrates. When methane reaches the surface and the atmosphere, it is known as atmospheric methane.[6] 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.[7] Methane has also been detected on other planets, including Mars, which has implications for astrobiology research.[8]

Methane
Stereo, skeletal formula of methane with some measurements added
Ball and stick model of methane
Spacefill model of methane
Names
Preferred IUPAC name
Methane[1]
Systematic IUPAC name
Carbane (never recommended[1])
Other names
  • Marsh gas
  • Natural gas
  • Carbon tetrahydride
  • Hydrogen carbide
Identifiers
3D model (JSmol)
3DMet B01453
1718732
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.000.739
EC Number 200-812-7
59
KEGG
MeSH Methane
RTECS number PA1490000
UN number 1971
Properties
CH4
Molar mass 16.043 g·mol−1
Appearance Colorless gas
Odor Odorless
Density
  • 0.657 g·L−1 (gas, 25 °C, 1 atm)
  • 0.717 g·L−1 (gas, 0 °C, 1 atm)
  • 422.62 g·L−1 (liquid, −162 °C)[2]
Melting point −182.5 °C; −296.4 °F; 90.7 K
Boiling point −161.50 °C; −258.70 °F; 111.65 K[3]
22.7 mg·L−1
Solubility Soluble in ethanol, diethyl ether, benzene, toluene, methanol, acetone and insoluble in water
log P 1.09
14 nmol·Pa−1·kg−1
Conjugate acid Methanium
Conjugate base Methyl anion
−12.2×10−6 cm3·mol−1
Structure
Td
Tetrahedron
0 D
Thermochemistry
35.69 J·(K·mol)−1
186.25 J·(K·mol)−1
−74.87 kJ·mol−1
−891.1 to −890.3 kJ·mol−1
Hazards[4]
Safety data sheet See: data page
GHS pictograms The flame pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)
GHS signal word DANGER
H220
P210
NFPA 704
Flash point −188 °C (−306.4 °F; 85.1 K)
537 °C (999 °F; 810 K)
Explosive limits 4.4–17%
Related compounds
Related alkanes
Supplementary data page
Refractive index (n),
Dielectric constantr), etc.
Thermodynamic
data
Phase behaviour
solid–liquid–gas
UV, IR, NMR, MS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Properties and bonding

Methane is a tetrahedral 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 and 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 and standard pressure, methane is a colorless, odorless gas.[9] The familiar smell of natural gas as used in homes is achieved by the addition of an odorant, usually blends containing tert-butylthiol, as a safety measure. Methane has a boiling point of −164 °C (−257.8 °F) at a pressure of one atmosphere.[10] As a gas it is flammable over a range of concentrations (5.4–17%) in air at standard pressure.

Solid methane exists in several modifications. Presently nine are known.[11] 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 a plastic crystal.[12]

Chemical reactions

The primary chemical reactions of methane are combustion, steam reforming to syngas, and halogenation. In general, methane reactions are difficult to control.

Selective oxidation

Partial oxidation of methane to methanol is challenging because the reaction typically progresses all the way to carbon dioxide and water even with an insufficient supply of oxygen. The enzymes methane monooxygenase produces methanol from methane, but cannot be used for industrial-scale reactions.[13] Some homogeneously 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 zeolites, and iron zeolites stabilizing the alpha-oxygen active site.[14]

One group of bacteria drive methane oxidation with nitrite as the oxidant in the absence of oxygen, giving rise to the so-called anaerobic oxidation of methane.[15]

Acid-base reactions

Like other hydrocarbons, methane is a very weak acid. Its pKa in DMSO is estimated to be 56.[16] It cannot be deprotonated in solution, but the conjugate base is known in forms such as methyllithium.

A variety of positive ions derived from methane have been observed, mostly as unstable species in low-pressure gas mixtures. These include methenium or methyl cation CH+
3
, methane cation CH+
4
, and methanium or protonated methane CH+
5
. Some of these have been detected in outer space. Methanium can also be produced as diluted solutions from methane with superacids. Cations with higher charge, such as CH2+
6
and CH3+
7
, have been studied theoretically and conjectured to be stable.[17]

Despite the strength of its C–H bonds, there is intense interest in catalysts that facilitate C–H bond activation in methane (and other lower numbered alkanes).[18]

Combustion

The fire within her
Methane bubbles can be burned on a wet hand without injury.

Methane's heat of combustion is 55.5 MJ/kg.[19] Combustion of methane is a multiple step reaction summarized as follows:

CH4 + 2 O2 → CO2 + 2 H2O (ΔH = −891 kJ/mol, at standard conditions)

Peters four-step chemistry is a systematically reduced four-step chemistry which explains the burning of methane.

Methane radical reactions

Given appropriate conditions, methane reacts with halogen radicals as follows:

X• + CH4 → HX + CH3
CH3• + X2 → CH3X + X•

where X is a halogen: fluorine (F), chlorine (Cl), bromine (Br), or iodine (I). This mechanism for this process is called free radical halogenation. It is initiated when UV light or some other radical initiator (like peroxides) produces a halogen atom. A two-step chain reaction ensues in which the halogen atom abstracts a hydrogen atom from a methane molecule, resulting in the formation of a hydrogen halide molecule and a methyl 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. [20] Similar reactions can occur on the halogenated product, leading to replacement of additional hydrogen atoms by halogen atoms with dihalomethane, trihalomethane, and ultimately, tetrahalomethane 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). 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 for ovens, homes, water heaters, kilns, automobiles,[21][22] turbines, and other things. Activated carbon is used to store methane. Gaseous[23] methane is also used as a rocket fuel when combined with liquid oxygen, as in the BE-4 and Raptor engines.

As the major constituent of natural gas, methane is important for electricity generation by burning it as a fuel in a gas turbine or steam generator. Compared to other hydrocarbon fuels, methane produces less carbon dioxide for each unit of heat released. At about 891 kJ/mol, methane's heat of combustion 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[24] 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, which is considered to have an energy content of 39 megajoules per cubic meter, or 1,000 BTU per standard cubic foot. Liquefied natural gas (LNG) is predominantly methane (CH4) converted into liquid form for ease of storage or transport.

Refined liquid methane is used as a rocket fuel.[25] Methane is reported to offer the advantage over kerosene of depositing less carbon on the internal parts of rocket motors, reducing the difficulty of re-use of boosters.

Chemical feedstock

Natural gas, which is mostly composed of methane, is used to produce hydrogen gas on an industrial scale. Steam Methane Reforming (SMR), is the most common method of producing commercial bulk hydrogen gas. More than 50 million metric tons are produced annually worldwide (2013), principally from SMR of natural gas. [26] Much of this hydrogen is used in petroleum refineries, in the production of chemicals and in food processing. Very large quantities of hydrogen are used in the 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 CO and H2, known as "Water gas" and "Syn-gas":

CH4 + H2OCO + 3 H2

This reaction is strongly endothermic (consumes heat, ΔHr= 206 kJ/mol). Additional hydrogen is obtained by the reaction of CO with water via the water-gas shift reaction.

CO + H2O ⇌ CO2 + H2

This reaction is mildly exothermic (produces heat, ΔHr= -41 kJ/mol).

Methane is also subjected to free-radical chlorination in the production of chloromethanes, although methanol is a more typical precursor.[27]

Generation

Diagram of sustainable methane fuel production

Geological routes

The two main routes for geological methane generation are (i) organic (thermally generated, or thermogenic) and (ii) inorganic (abiotic)[28]. Thermogenic methane occurs due to the breakup of organic matter at elevated temperatures and pressures in deep sedimentary 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 serpentinization.[29][30]

Biological routes

Most of Earth's methane is biogenic and is produced by methanogenesis[31][32], a form of anaerobic respiration only known to be conducted by some members of the domain, Archaea. Methanogens occupy landfills and other soils[33], ruminants (for example cows or cattle)[34], the guts of termites, and the anoxic sediments below the seafloor and the bottom of lakes. Rice fields also generate large amounts of methane during plant growth.[35] 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 methyl coenzyme M reductase (MCR).[36]

CSIRO ScienceImage 1898 Testing Sheep for Methane Production
Testing Australian sheep for exhaled methane production (2001), CSIRO

Ruminants

Ruminants, such as cattle, belch methane, accounting for ~22% of the U.S. annual methane emissions to the atmosphere.[37] One study reported that the livestock sector in general (primarily cattle, chickens, and pigs) produces 37% of all human-induced methane.[38] A 2009 study found that at a conservative estimate, at least 51% of global greenhouse gas emissions were attributable to the life cycle and supply chain of livestock products, meaning all meat, dairy, and by-products, and their transportation.[39] A 2013 study estimated that livestock accounted for 44% of human-induced methane and ~15% of human-induced greenhouse gas emissions.[40] Many efforts are underway to reduce livestock methane production, such as medical treatments and dietary adjustments,[41][42] and to trap the gas to use as energy.[43] The state of California has been particularly active in this area.[44]

Seafloor Sediments

Most of the subseafloor is anoxic because oxygen is removed by aerobic microorganisms within the first few centimeters of the sediment. Below the oxygen replete seafloor, methanogens produce methane that is either used by other organisms or becomes trapped in gas hydrates. Other organisms utilize methane for energy and are known as methanotrophs (methane-eating). Consortia of Archaea and Bacteria have been found to oxidize methane via Anaerobic Oxidation of Methane (AOM); the organisms responsible for this are Anaerobic Methanotrophic Archaea (ANME) and Sulfate-Reducing Bacteria (SRB).[45]

Industrial routes

There is little incentive to produce methane industrially. Methane is produced by 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.

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, ash content, low calorific value and propensity to spontaneous combustion during storage and transport.

Power to methane is a technology that uses electrical power to produce hydrogen from water by electrolysis and uses the Sabatier reaction to combine hydrogen with carbon dioxide to produce methane. As of 2016, 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 generators and solar arrays. However, as currently very large amounts of natural gas are used in power plants (e.g. CCGT) to produce electric energy, the losses in efficiency are not acceptable.

Laboratory synthesis

Methane can be produced by protonation of methyl lithium and methylmagnesium iodide. In practice, a requirement for pure methane will be filled with a steel gas bottle from standard 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 fuels, and sometimes accompanied by helium and nitrogen. Methane is produced at shallow levels (low pressure) by anaerobic 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 oil.

Methane 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.

Atmospheric methane

Mlo ch4 ts obs 03437
Methane concentration evolution from 1987 to December 2018 at Mauna Loa (Hawaii).

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. 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.[46]

Methane is an important greenhouse gas 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.[47][48]

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 vapor which is by far the largest component of the greenhouse effect).[49]

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 ocean floor within the 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).[50] 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.[51][52] The global mass of carbon stored in gas clathrates is still uncertain and has been estimated as high as 12,500 Gt carbon and as low as 500 Gt carbon.[53] The estimate has declined over time with a most recent estimate of ~1800 Gt carbon.[54] 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 relatively newly discovered source of methane was discovered in an ultraslow spreading ridge in the Arctic.[55] 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.[54] 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.[56][57][58][59]

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 methane production from local materials found on Mars[60] or Titan), providing fuel for a return journey.[25][61]

Mars

Methane has been detected on all planets of the solar system and most of the larger moons. With the possible exception of Mars, it is believed to have come from abiotic processes.[62][63]

PIA19088-MarsCuriosityRover-MethaneSource-20141216
Methane (CH4) on Mars – potential sources and sinks.

The Curiosity rover has documented seasonal fluctuations of atmospheric methane levels on Mars. These fluctuations peaked at the end of the Martian summer at 0.6 parts per billion.[64][65][66][67][68][69][70][71]

Methane has been proposed as a possible rocket propellant on future Mars missions due in part to the possibility of synthesizing it on the planet by in situ resource utilization.[72] An adaptation of the Sabatier methanation reaction may be used with a mixed catalyst bed and a reverse water-gas shift in a single reactor to produce methane from the raw materials available on Mars, utilizing water from the Martian subsoil and carbon dioxide in the Martian atmosphere.[60]

Methane could be produced by a non-biological process called ’'serpentinization[a] involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.[73]

History

In November 1776, methane was first scientifically identified by 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".[74] Volta collected the gas rising from the marsh, and by 1778 had isolated the pure gas.[75] He also demonstrated that the gas could be ignited with an electric spark.[75]

The name "methane" was coined in 1866 by the German chemist August Wilhelm von Hofmann.[76] The name was derived from 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" (A.D.1840) or directly from the French "méthyle" which is a back-formation from the French "méthylène" (corresponding to English "methylene"), the root of which is coined from the Greek "methy" (related to English "mead") and "hyle" (meaning "wood"). The radical is named after this because it was first detected in wood alcohol. 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).

Safety

Methane is nontoxic, yet it is extremely flammable and may form explosive mixtures with air. Methane is also an asphyxiant if the oxygen concentration is reduced to below about 16% by displacement, as most people can 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. Landfills are the single largest source of U.S. man-made methane emissions.[77]

Methane gas explosions are responsible for many deadly mining disasters.[78] A methane gas explosion was the cause of the Upper Big Branch coal mine disaster in West Virginia on April 5, 2010, killing 29.[79]

Industrial processes which produce methane are often required to control or abate their methane emissions, along with other VOCs. A thermal oxidizer is the most common type of air pollution control equipment used to reduce methane emissions.[80]

See also

Notes

  1. ^ There are many serpentinization reactions. Olivine is a solid solution between forsterite and fayalite 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): 18 Mg2SiO4 + 6 Fe2SiO4 + 26 H2O + CO2 → 12 Mg3Si2O5(OH)4 + 4 Fe3O4 + CH4

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External links

Alkane

In organic chemistry, an alkane, or paraffin (a historical name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which all the carbon–carbon bonds are single. Alkanes have the general chemical formula CnH2n+2. The alkanes range in complexity from the simplest case of methane (CH4), where n = 1 (sometimes called the parent molecule), to arbitrarily large and complex molecules, like pentacontane (C50H102) or 6-ethyl-2-methyl-5-(1-methylethyl) octane, an isomer of tetradecane (C14H30).

IUPAC defines alkanes as "acyclic branched or unbranched hydrocarbons having the general formula CnH2n+2, and therefore consisting entirely of hydrogen atoms and saturated carbon atoms". However, some sources use the term to denote any saturated hydrocarbon, including those that are either monocyclic (i.e. the cycloalkanes) or polycyclic, despite their having a different general formula (i.e. cycloalkanes are CnH2n).

In an alkane, each carbon atom is sp3-hybridized with 4 sigma bonds (either C–C or C–H), and each hydrogen atom is joined to one of the carbon atoms (in a C–H bond). The longest series of linked carbon atoms in a molecule is known as its carbon skeleton or carbon backbone. The number of carbon atoms may be considered as the size of the alkane.

One group of the higher alkanes are waxes, solids at standard ambient temperature and pressure (SATP), for which the number of carbon atoms in the carbon backbone is greater than about 17.

With their repeated –CH2 units, the alkanes constitute a homologous series of organic compounds in which the members differ in molecular mass by multiples of 14.03 u (the total mass of each such methylene-bridge unit, which comprises a single carbon atom of mass 12.01 u and two hydrogen atoms of mass ~1.01 u each).

Alkanes are not very reactive and have little biological activity. They can be viewed as molecular trees upon which can be hung the more active/reactive functional groups of biological molecules.

The alkanes have two main commercial sources: petroleum (crude oil) and natural gas.

An alkyl group, generally abbreviated with the symbol R, is a functional group that, like an alkane, consists solely of single-bonded carbon and hydrogen atoms connected acyclically—for example, a methyl or ethyl group.

Anaerobic digestion

Anaerobic digestion is a collection of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to manage waste or to produce fuels. Much of the fermentation used industrially to produce food and drink products, as well as home fermentation, uses anaerobic digestion.

Anaerobic digestion occurs naturally in some soils and in lake and oceanic basin sediments, where it is usually referred to as "anaerobic activity". This is the source of marsh gas methane as discovered by Alessandro Volta in 1776.The digestion process begins with bacterial hydrolysis of the input materials. Insoluble organic polymers, such as carbohydrates, are broken down to soluble derivatives that become available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. These bacteria convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens convert these products to methane and carbon dioxide. The methanogenic archaea populations play an indispensable role in anaerobic wastewater treatments.Anaerobic digestion is used as part of the process to treat biodegradable waste and sewage sludge. As part of an integrated waste management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobic digesters can also be fed with purpose-grown energy crops, such as maize.Anaerobic digestion is widely used as a source of renewable energy. The process produces a biogas, consisting of methane, carbon dioxide, and traces of other ‘contaminant’ gases. This biogas can be used directly as fuel, in combined heat and power gas engines or upgraded to natural gas-quality biomethane. The nutrient-rich digestate also produced can be used as fertilizer.

With the re-use of waste as a resource and new technological approaches that have lowered capital costs, anaerobic digestion has in recent years received increased attention among governments in a number of countries, among these the United Kingdom (2011), Germany and Denmark (2011).

Arctic methane emissions

Arctic methane release is the release of methane from seas and soils in permafrost regions of the Arctic. While it is a long-term natural process, methane release is exacerbated by global warming. This results in negative effects, as methane is itself a powerful greenhouse gas.

The Arctic region is one of the many natural sources of the greenhouse gas methane. Global warming accelerates its release, due to both release of methane from existing stores, and from methanogenesis in rotting biomass. Large quantities of methane are stored in the Arctic in natural gas deposits, permafrost, and as undersea clathrates. Permafrost and clathrates degrade on warming, thus large releases of methane from these sources may arise as a result of global warming. Other sources of methane include submarine taliks, river transport, ice complex retreat, submarine permafrost and decaying gas hydrate deposits.Concentrations in the Arctic atmosphere are higher by 8–10% than that in the Antarctic atmosphere. During cold glacier epochs, this gradient decreases to practically insignificant levels. Land ecosystems are considered the main sources of this asymmetry, although it has been suggested that "the role of the Arctic Ocean is significantly underestimated." Soil temperature and moisture levels have been found to be significant variables in soil methane fluxes in tundra environments.

Atmosphere of Mars

The atmosphere of the planet Mars is composed mostly of carbon dioxide (95.3%). The atmospheric pressure on the Martian surface averages 600 pascals (0.087 psi; 6.0 mbar), about 0.6% of Earth's mean sea level pressure of 101.3 kilopascals (14.69 psi; 1.013 bar). It ranges from a low of 30 pascals (0.0044 psi; 0.30 mbar) on Olympus Mons's peak to over 1,155 pascals (0.1675 psi; 11.55 mbar) in the depths of Hellas Planitia. This pressure is well below the Armstrong limit for the unprotected human body. Mars's atmospheric mass of 25 teratonnes compares to Earth's 5148 teratonnes; Mars has a scale height of 11.1 kilometres (6.9 mi) versus Earth's 8.5 kilometres (5.3 mi).The Martian atmosphere consists of approximately 96% carbon dioxide, 1.9% argon, 1.9% nitrogen, and traces of free oxygen, carbon monoxide, water and methane, among other gases, for a mean molar mass of 43.34 g/mol. There has been renewed interest in its composition since the detection of traces of methane in 2003 that may indicate life but may also be produced by a geochemical process, volcanic or hydrothermal activity.The atmosphere is quite dusty, giving the Martian sky a light brown or orange-red color when seen from the surface; data from the Mars Exploration Rovers indicate suspended particles of roughly 1.5 micrometres in diameter.On 16 December 2014, NASA reported detecting an unusual increase, then decrease, in the amounts of methane in the atmosphere of the planet Mars. Organic chemicals have been detected in powder drilled from a rock by the Curiosity rover. Based on deuterium to hydrogen ratio studies, much of the water at Gale Crater on Mars was found to have been lost during ancient times, before the lakebed in the crater was formed; afterwards, large amounts of water continued to be lost.On 18 March 2015, NASA reported the detection of an aurora that is not fully understood and an unexplained dust cloud in the atmosphere of Mars.On 4 April 2015, NASA reported studies, based on measurements by the Sample Analysis at Mars (SAM) instrument on the Curiosity rover, of the Martian atmosphere using xenon and argon isotopes. Results provided support for a "vigorous" loss of atmosphere early in the history of Mars and were consistent with an atmospheric signature found in bits of atmosphere captured in some Martian meteorites found on Earth. This was further supported by results from the MAVEN orbiter circling Mars, that the solar wind is responsible for stripping away the atmosphere of Mars over the years.In September 2017, NASA reported radiation levels on the surface of the planet Mars were temporarily doubled, and were associated with an aurora 25 times brighter than any observed earlier due to a massive, and unexpected, solar storm in the middle of the month.On 1 June 2018, NASA scientists detected signs of a dust storm (see image) on the planet Mars which resulted in the end of the solar-powered Opportunity rover's mission since the dust blocked the sunlight (see image) needed to operate; as of 12 June, the storm is the worst ever recorded at the surface of the planet, and spanned an area about the size of North America and Russia combined (about a quarter of the planet); as of 13 June, Opportunity was reported to be experiencing serious communication problems due to the dust storm; a NASA teleconference about the dust storm was presented on 13 June 2018 at 01:30 pm/et/usa and is available for replay. In July 2018, researchers reported that the largest single source of dust on the planet Mars comes from the Medusae Fossae Formation.On 7 June 2018, NASA announced a cyclical seasonal variation in atmospheric methane.

Biogas

Biogas is the mixture of gases produced by the breakdown of organic matter in the absence of oxygen. Biogas can be produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste or food waste. Biogas is a renewable energy source.

Biogas is produced by anaerobic digestion with methanogen or anaerobic organisms, which digest material inside a closed system, or fermentation of biodegradable materials. This closed system is called an anaerobic digester, biodigester or a bioreactor.Biogas is primarily methane (CH4) and carbon dioxide (CO2) and may have small amounts of hydrogen sulfide (H2S), moisture and siloxanes. The gases methane, hydrogen, and carbon monoxide (CO) can be combusted or oxidized with oxygen. This energy release allows biogas to be used as a fuel; it can be used for any heating purpose, such as cooking. It can also be used in a gas engine to convert the energy in the gas into electricity and heat.Biogas can be compressed, the same way as natural gas is compressed to CNG, and used to power motor vehicles. In the United Kingdom, for example, biogas is estimated to have the potential to replace around 17% of vehicle fuel. It qualifies for renewable energy subsidies in some parts of the world. Biogas can be cleaned and upgraded to natural gas standards, when it becomes bio-methane. Biogas is considered to be a renewable resource because its production-and-use cycle is continuous, and it generates no net carbon dioxide. As the organic material grows, it is converted and used. It then regrows in a continually repeating cycle. From a carbon perspective, as much carbon dioxide is absorbed from the atmosphere in the growth of the primary bio-resource as is released, when the material is ultimately converted to energy.

Carbon tetrachloride

Carbon tetrachloride, also known by many other names (the most notable being tetrachloromethane, also recognized by the IUPAC, carbon tet in the cleaning industry, Halon-104 in firefighting, and Refrigerant-10 in HVACR) is an organic compound with the chemical formula CCl4. It is a colourless liquid with a "sweet" smell that can be detected at low levels. It has practically no flammability at lower temperatures. It was formerly widely used in fire extinguishers, as a precursor to refrigerants and as a cleaning agent, but has since been phased out because of toxicity and safety concerns. Exposure to high concentrations of carbon tetrachloride (including vapor) can affect the central nervous system, degenerate the liver and kidneys. Prolonged exposure can be fatal.

Coalbed methane

Coalbed methane (CBM or coal-bed methane), coalbed gas, coal seam gas (CSG), or coal-mine methane (CMM) is a form of natural gas extracted from coal beds. In recent decades it has become an important source of energy in United States, Canada, Australia, and other countries.

The term refers to methane adsorbed into the solid matrix of the coal. It is called 'sweet gas' because of its lack of hydrogen sulfide. The presence of this gas is well known from its occurrence in underground coal mining, where it presents a serious safety risk. Coalbed methane is distinct from a typical sandstone or other conventional gas reservoir, as the methane is stored within the coal by a process called adsorption. The methane is in a near-liquid state, lining the inside of pores within the coal (called the matrix). The open fractures in the coal (called the cleats) can also contain free gas or can be saturated with water.Unlike much natural gas from conventional reservoirs, coalbed methane contains very little heavier hydrocarbons such as propane or butane, and no natural-gas condensate. It often contains up to a few percent carbon dioxide.

Eocene

The Eocene ( ) Epoch, lasting from 56 to 33.9 million years ago, is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Paleocene Epoch to the beginning of the Oligocene Epoch. The start of the Eocene is marked by a brief period in which the concentration of the carbon isotope 13C in the atmosphere was exceptionally low in comparison with the more common isotope 12C. The end is set at a major extinction event called the Grande Coupure (the "Great Break" in continuity) or the Eocene–Oligocene extinction event, which may be related to the impact of one or more large bolides in Siberia and in what is now Chesapeake Bay. As with other geologic periods, the strata that define the start and end of the epoch are well identified, though their exact dates are slightly uncertain.

The name Eocene comes from the Ancient Greek ἠώς (ēṓs, "dawn") and καινός (kainós, "new") and refers to the "dawn" of modern ('new') fauna that appeared during the epoch.

Extraterrestrial atmosphere

The study of extraterrestrial atmospheres is an active field of research, both as an aspect of astronomy and to gain insight into Earth's atmosphere. In addition to Earth, many of the other astronomical objects in the Solar System have atmospheres. These include all the gas giants, as well as Mars, Venus, and Pluto. Several moons and other bodies also have atmospheres, as do comets and the Sun. There is evidence that extrasolar planets can have an atmosphere. Comparisons of these atmospheres to one another and to Earth's atmosphere broaden our basic understanding of atmospheric processes such as the greenhouse effect, aerosol and cloud physics, and atmospheric chemistry and dynamics.

Flatulence

Flatulence is defined in the medical literature as "flatus expelled through the anus" or the "quality or state of being flatulent", which is defined in turn as "marked by or affected with gases generated in the intestine or stomach; likely to cause digestive flatulence". The root of these words is from the Latin flatus – "a blowing, a breaking wind". Flatus is also the medical word for gas generated in the stomach or bowels. Despite these standard definitions, a proportion of intestinal gas may be swallowed environmental air, and hence flatus is not totally generated in the stomach or bowels. The scientific study of this area of medicine is termed flatology.It is normal for humans to pass flatus through the rectum, although the volume and frequency may vary greatly between individuals. It is also normal for intestinal gas passed through the rectum to have a characteristic feculent smell, although this too may vary in concentration. Flatus is brought to the rectum by specialised contractions of the muscles in the intestines and colon. The noises commonly associated with flatulence ("blowing a raspberry") are caused by the vibration of anal sphincters, and occasionally by the closed buttocks. Both the noise and smell associated with flatus leaving the anus can be sources of embarrassment or comedy in many cultures.

There are five general symptoms related to intestinal gas: pain, bloating and abdominal distension, excessive flatus volume, excessive flatus smell and gas incontinence. Furthermore, eructation ("an act or instance of belching", colloquially known as "burping") is sometimes included under the topic of flatulence. When excessive or malodorous, flatus can be a sign of a health disorder, such as irritable bowel syndrome, celiac disease or lactose intolerance.

Greenhouse gas

A greenhouse gas is a gas that absorbs and emits radiant energy within the thermal infrared range. Greenhouse gases cause the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide and ozone. Without greenhouse gases, the average temperature of Earth's surface would be about −18 °C (0 °F), rather than the present average of 15 °C (59 °F). The atmospheres of Venus, Mars and Titan also contain greenhouse gases.

Human activities since the beginning of the Industrial Revolution (around 1750) have produced a 45% increase in the atmospheric concentration of carbon dioxide (CO2), from 280 ppm in 1750 to 406 ppm in early 2017. This increase has occurred despite the uptake of more than half of the emissions by various natural "sinks" involved in the carbon cycle. The vast majority of anthropogenic carbon dioxide emissions (i.e., emissions produced by human activities) come from combustion of fossil fuels, principally coal, oil, and natural gas, with additional contributions coming from deforestation, changes in land use, soil erosion and agriculture (including livestock).Should greenhouse gas emissions continue at their rate in 2017, global warming could cause Earth's surface temperature to exceed historical values as early as 2047, with potentially harmful effects on ecosystems, biodiversity and human livelihoods. At current emission rates temperatures could increase by 2 °C, which the United Nations' IPCC designated as the upper limit to avoid "dangerous" levels, by 2036.

Hydrocarbon

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons from which one hydrogen atom has been removed are functional groups called hydrocarbyls. Because carbon has 4 electrons in its outermost shell (and because each covalent bond requires a donation of 1 electron, per atom, to the bond) carbon has exactly four bonds to make, and is only stable if all 4 of these bonds are used.

Aromatic hydrocarbons (arenes), alkanes, cycloalkanes and alkyne-based compounds are different types of hydrocarbons.

Most hydrocarbons found on Earth naturally occur in crude oil, where decomposed organic matter provides an abundance of carbon and hydrogen which, when bonded, can catenate to form seemingly limitless chains.

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 amount of methane is trapped within a crystal structure of water, forming a solid similar to ice. Originally thought to occur only in the outer regions of the Solar System, where temperatures are low and water ice is common, significant deposits of methane clathrate have been found under sediments on the ocean floors of the Earth.Methane clathrates are common constituents of the shallow marine geosphere and they occur in deep sedimentary structures and form outcrops on the ocean floor. Methane hydrates are believed to form by the precipitation or crystallisation of methane migrating from deep along geological faults. Precipitation occurs when the methane comes in contact with water within the sea bed subject to temperature and pressure. In 2008, research on Antarctic Vostok and EPICA Dome C ice cores revealed that methane clathrates were also present in deep Antarctic ice cores and record a history of atmospheric methane concentrations, dating to 800,000 years ago. The ice-core methane clathrate record is a primary source of data for global warming research, along with oxygen and carbon dioxide.

Methane monooxygenase

Methane monooxygenase, or 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 (EC 1.14.13.25).

There are two well-studied forms of MMO: the soluble form (sMMO) and the particulate form (pMMO). The active site in sMMO contains a di-iron center bridged by an oxygen atom (Fe-O-Fe), whereas the active site in pMMO utilizes copper, although some propose that pMMO also uses iron. Structures of both proteins have been determined by X-ray crystallography; however, the location and mechanism of the active site in pMMO is still poorly understood and is an area of active research.

The particulate methane monooxygenase and related ammonia monooxygenase are integral membrane proteins, occurring in methanotrophs and ammonia oxidisers,respectively, which are thought to be related. These enzymes have a relatively wide substrate specificity and can catalyse the oxidation of a range of substrates including ammonia, methane, halogenated hydrocarbons, and aromatic molecules. These enzymes are composed of 3 subunits - A (InterPro: IPR003393), B (InterPro: IPR006833) and C (InterPro: IPR006980) - and contain various metal centers, including copper. Particulate methane monooxygenase from Methylococcus capsulatus is an ABC homotrimer, which contains mononuclear and dinuclear copper metal centers, and a third metal center containing a metal ion whose identity in vivo is not certain.The A subunit from Methylococcus capsulatus (Bath) resides primarily within the membrane and consists of 7 transmembrane helices and a beta-hairpin, which interacts with the soluble region of the B subunit. A conserved glutamate residue is thought to contribute to a metal center.Methane monooxygenases are found in methanotrophic bacteria, a class of bacteria that exist at the interface of aerobic (oxygen-containing) and anaerobic (oxygen-devoid) environments. One of the more widely studied bacteria of this type is Methylococcus capsulatus (Bath). This bacterium was discovered in the hot springs of Bath, England.

Methanogen

Methanogens are microorganisms that produce methane as a metabolic byproduct in hypoxic conditions. They are prokaryotic and belong to the domain of archaea. They are common in wetlands, where they are responsible for marsh gas, and in the digestive tracts of animals such as ruminants and humans, where they are responsible for the methane content of belching in ruminants and flatulence in humans. In marine sediments the biological production of methane, also termed methanogenesis, is generally confined to where sulfates are depleted, below the top layers. Moreover, methanogenic archaea populations play an indispensable role in anaerobic wastewater treatments. Others are extremophiles, found in environments such as hot springs and submarine hydrothermal vents as well as in the "solid" rock of Earth's crust, kilometers below the surface.

Natural gas

Natural gas (also called fossil gas) is a naturally occurring hydrocarbon gas mixture consisting primarily of methane, but commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carbon dioxide, nitrogen, hydrogen sulfide, or helium. It is formed when layers of decomposing plant and animal matter are exposed to intense heat and pressure under the surface of the Earth over millions of years. The energy that the plants originally obtained from the sun is stored in the form of chemical bonds in the gas.Natural gas is a non-renewable hydrocarbon used as a source of energy for heating, cooking, and electricity generation. It is also used as a fuel for vehicles and as a chemical feedstock in the manufacture of plastics and other commercially important organic chemicals.

Natural gas is a major cause of global warming, both in itself when leaked and also due to the carbon dioxide it produces when burnt.Natural gas is found in deep underground rock formations or associated with other hydrocarbon reservoirs in coal beds and as methane clathrates. Petroleum is another resource and fossil fuel found in close proximity to and with natural gas. Most natural gas was created over time by two mechanisms: biogenic and thermogenic. Biogenic gas is created by methanogenic organisms in marshes, bogs, landfills, and shallow sediments. Deeper in the earth, at greater temperature and pressure, thermogenic gas is created from buried organic material.In petroleum production gas is sometimes burnt as flare gas. Before natural gas can be used as a fuel, most, but not all, must be processed to remove impurities, including water, to meet the specifications of marketable natural gas. The by-products of this processing include: ethane, propane, butanes, pentanes, and higher molecular weight hydrocarbons, hydrogen sulfide (which may be converted into pure sulfur), carbon dioxide, water vapor, and sometimes helium and nitrogen.

Natural gas is often informally referred to simply as "gas", especially when compared to other energy sources such as oil or coal. However, it is not to be confused with gasoline, especially in North America, where the term gasoline is often shortened in colloquial usage to gas.

Steam reforming

Steam reforming or steam methane reforming is a chemical synthesis for producing syngas, hydrogen, carbon monoxide from hydrocarbon fuels such as natural gas. This is achieved in a processing device called a reformer which reacts steam at high temperature and pressure with methane in the presence of a nickel catalyst. The steam methane reformer is widely used in industry to make hydrogen, also called "grey hydrogen", from natural gas. With the use of CCUS technology it is possible to capture the carbon dioxide, after which the product will be called "blue hydrogen".

There is also interest in the development of much smaller units based on similar technology to produce hydrogen as a feedstock for fuel cells. Small-scale steam reforming units to supply fuel cells are currently the subject of research and development, typically involving the reforming of methanol, but other fuels are also being considered such as propane, gasoline, autogas, diesel fuel, and ethanol.

Titan (moon)

Titan is the largest moon of Saturn and the second-largest natural satellite in the Solar System. It is the only moon known to have a dense atmosphere.

Titan is the sixth gravitationally rounded moon from Saturn. Frequently described as a planet-like moon, Titan is 50% larger than Earth's moon and 80% more massive. It is the second-largest moon in the Solar System after Jupiter's moon Ganymede, and is larger than the planet Mercury, but only 40% as massive. Discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan was the first known moon of Saturn, and the sixth known planetary satellite (after Earth's moon and the four Galilean moons of Jupiter). Titan orbits Saturn at 20 Saturn radii. From Titan's surface, Saturn subtends an arc of 5.09 degrees and would appear 11.4 times larger in the sky than the Moon from Earth.

Titan is primarily composed of ice and rocky material. Much as with Venus before the Space Age, the dense opaque atmosphere prevented understanding of Titan's surface until the Cassini–Huygens mission in 2004 provided new information, including the discovery of liquid hydrocarbon lakes in Titan's polar regions. The geologically young surface is generally smooth, with few impact craters, although mountains and several possible cryovolcanoes have been found.

The atmosphere of Titan is largely nitrogen; minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog. The climate—including wind and rain—creates surface features similar to those of Earth, such as dunes, rivers, lakes, seas (probably of liquid methane and ethane), and deltas, and is dominated by seasonal weather patterns as on Earth. With its liquids (both surface and subsurface) and robust nitrogen atmosphere, Titan's methane cycle is analogous to Earth's water cycle, at the much lower temperature of about 94 K (−179.2 °C; −290.5 °F).

Volatile organic compound

Volatile organic compounds (VOCs) are organic chemicals that have a high vapor pressure at ordinary room temperature. Their high vapor pressure results from a low boiling point, which causes large numbers of molecules to evaporate or sublimate from the liquid or solid form of the compound and enter the surrounding air, a trait known as volatility. For example, formaldehyde, which evaporates from paint and releases from materials like resin, has a boiling point of only –19 °C (–2 °F).

VOCs are numerous, varied, and ubiquitous. They include both human-made and naturally occurring chemical compounds. Most scents or odors are of VOCs. VOCs play an important role in communication between plants, and messages from plants to animals. Some VOCs are dangerous to human health or cause harm to the environment. Anthropogenic VOCs are regulated by law, especially indoors, where concentrations are the highest. Harmful VOCs typically are not acutely toxic, but have compounding long-term health effects. Because the concentrations are usually low and the symptoms slow to develop, research into VOCs and their effects is difficult.

Types
Infrastructure
Uses
Molecules
Deuterated
molecules
Unconfirmed
Related
Alkali metal hydrides
Alkaline earth hydrides
Group 13 hydrides
Group 14 hydrides
Pnictogen hydrides
Hydrogen chalcogenides
Hydrogen halides
Transition metal hydrides
Lanthanide hydrides
Actinide hydrides

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