در شرایط استاندارد دما و فشار این گاز بی بو و بیرنگ و نافذتر و سبکتر از هوا است و اولین ترکیب سلسله هیدروکربنهای اشباع شده به شمار می رود. این گاز در طبیعت از تجزیه و پوسیده شدن مواد آلی به ویژه فساد گیاهان در مردابها حاصل میشود، به همین جهت آن را «گاز مرداب» نیز مینامند.
Naturally occurring methane is found both below ground and under the seafloor, 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. 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. Methane has also been detected on other planets, including Mars, which has implications for astrobiology research.
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.
Solid methane exists in several modifications. 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 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.
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 enzymemethane monooxygenase produces methanol from methane, but cannot be used for industrial-scale reactions. 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.
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.
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.
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 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.
As a 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 (54–90 K).
Natural gas, which is mostly composed of methane, is used to produce hydrogen gas on an industrial scale. Steam methane reforming (SMR), or simply known as steam reforming, is the most common 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 petroleumrefineries, 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" or "syngas":
The two main routes for geological methane generation are (i) organic (thermally generated, or thermogenic) and (ii) inorganic (abiotic). 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.
Most of Earth's methane is biogenic and is produced by methanogenesis, a form of anaerobic respiration only known to be conducted by some members of the domain Archaea. Methanogens occupy landfills and other soils,ruminants (for example cows or cattle), 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. This multistep process is used by these microorganisms for energy. The net reaction of methanogenesis is:
Testing Australian sheep for exhaled methane production (2001), CSIRO
This image represents a ruminant, more specifically a sheep producing methane within the four stages of hydrolysis, acidogenesis, acetogenesis, and methanogenesis.
Ruminants, such as cattle, belch methane, accounting for ~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 ~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 as energy.
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 turbines 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.
Methane (CH4) measured by the Advanced Global Atmospheric Gases Experiment (AGAGE) in the lower atmosphere (troposphere) at stations around the world. Abundances are given as pollution free monthly mean mole fractions in parts-per-trillion.
Methane concentration evolution from 1987 to September 2020 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.
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).
From 2015 to 2019 sharp rises in levels of atmospheric methane have been recorded. In February 2020, it was reported methane emissions from the fossil fuel industry may have been significantly underestimated.
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). 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. 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. 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 relatively newly discovered source of methane was discovered in an 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.
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". Volta collected the gas rising from the marsh, and by 1778 had isolated the pure gas. He also demonstrated that the gas could be ignited with an electric spark.
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, 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).
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 (mol), kilogram, and gram.
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.
^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
^ ab"Front Matter". Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. pp. 3–4. doi:10.1039/9781849733069-FP001. ISBN978-0-85404-182-4. Methane is a retained name (see P-12.3) that is preferred to the systematic name ‘carbane’, a name never recommended to replace methane, but used to derive the names ‘carbene’ and ‘carbyne’ for the radicals H2C2• and HC3•, respectively.
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^"Blue Origin BE-4 Engine". Retrieved June 14, 2019. We chose LNG because it is highly efficient, low cost and widely available. Unlike kerosene, LNG can be used to self-pressurize its tank. Known as autogenous repressurization, this eliminates the need for costly and complex systems that draw on Earth’s scarce helium reserves. LNG also possesses clean combustion characteristics even at low throttle, simplifying engine reuse compared to kerosene fuels.
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