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Methane ( or ) is a chemical compound with the chemical formula CH 4 (one atom of carbon and four atoms of hydrogen). It is the simplest alkane and the main component of natural gas. The relative abundance of methane makes it an attractive fuel, though capturing and storing it poses challenges due to its gaseous state found at normal conditions. In its natural state, methane is found both below ground, and under the sea floor, where it often finds its way to the surface and in the earth's atmosphere where it is known as atmospheric methane.[4]
In November 1776, methane was first scientifically identified by Italian physicist Alessandro Volta in the marshes of Lake Maggiore straddling Italy and Switzerland, having been inspired to search for the substance after reading a paper written by Benjamin Franklin about "flammable air".[5] Volta captured the gas rising from the marsh, and by 1778 had isolated the pure gas.[6] He also demonstrated means to ignite the gas with an electric spark.[6]
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 level in energy 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.[7] The familiar smell of natural gas as used in homes is a safety measure achieved by the addition of an odorant, usually blends containing tert-butylthiol. Methane has a boiling point of −161 °C (−257.8 °F) at a pressure of one atmosphere.[8] As a gas it is flammable over a range of concentrations (4.4–17%) in air at standard pressure.
Main reactions with methane are: combustion, steam reforming to syngas, and halogenation. In general, methane reactions are difficult to control. Partial oxidation to methanol, for example, is challenging because the reaction typically progresses all the way to carbon dioxide and water even with incomplete amounts of oxygen. The enzymes methane monooxygenase can produce methanol from methane, but they cannot be used for industrial scale reactions.[9]
Like other hydrocarbons, methane is a very weak acid. Its pKa in DMSO is estimated to be 56.[10] It cannot be deprotonated in solution, but the conjugate base with methyllithium is known.
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 super acids. Cations with higher charge, such as CH2+ 6 and CH3+ 7, have been studied theoretically and conjectured to be stable.[11]
Despite the strength of its C-H bonds, there is intense interest in catalysts that facilitate C–H bond activation in methane (and other low alkanes).[12]
Methane's heat of combustion is 55.5 MJ/kg.[13] Combustion of methane is a multiple step reaction. The following equations are part of the process, with the net result being:
CH4 + 2 O2 → CO2 + 2 H2O (ΔH = −891 kJ/mol (at standard conditions))
The species M* signifies an energetic third body, from which energy is transferred during a molecular collision. Formaldehyde (HCHO or H 2CO) is an early intermediate (reaction 7). Oxidation of formaldehyde gives the formyl radical (HCO; reactions 8–10), which then give carbon monoxide (CO) (reactions 11, 12 & 13). Any resulting H2 oxidizes to H2O or other intermediates (reaction 14, 15). Finally, the CO oxidizes, forming CO2 (reaction 16). In the final stages (reactions 17–19), energy is transferred back to other third bodies. The overall speed of reaction is a function of the concentration of the various entities during the combustion process. The higher the temperature, the greater the concentration of radical species and the more rapid the combustion process.[14]
Methane reacts with halogens given appropriate conditions as follows:
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 with UV light or some other radical initiator. A chlorine atom is generated from elemental chlorine, which abstracts a hydrogen atom from methane, resulting in the formation of hydrogen chloride. The resulting methyl radical, CH3•, can combine with another chlorine molecule to give methyl chloride (CH3Cl) and a chlorine atom. This chlorine atom can then react with another methane (or methyl chloride) molecule, repeating the chlorination cycle.[15] Similar reactions can produce dichloromethane (CH2Cl2), chloroform (CHCl3), and, ultimately, carbon tetrachloride (CCl4), depending upon reaction conditions and the chlorine to methane ratio.
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.
Methane is important for electrical generation by burning it as a fuel in a gas turbine or steam generator. Compared to other hydrocarbon fuels, burning methane produces less carbon dioxide for each unit of heat released. At about 891 kJ/mol, methane's heat of combustion is lower than any other hydrocarbon but the ratio of the heat of combustion (891 kJ/mol) to the molecular mass (16.0 g/mol, of which 12.0 g/mol is carbon) shows that methane, being the simplest hydrocarbon, produces more heat per mass unit (55.7 kJ/g) than other complex hydrocarbons. In many cities, methane is piped into homes for domestic heating and cooking purposes. 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.
Methane in the form of compressed natural gas is used as a vehicle fuel and is claimed to be more environmentally friendly than other fossil fuels such as gasoline/petrol and diesel.[16] Research into adsorption methods of methane storage for use as an automotive fuel has been conducted.[17]
Liquefied natural gas or LNG is natural gas (predominantly methane, CH4) that has been converted to liquid form for ease of storage or transport.
Liquefied natural gas takes up about 1/600th the volume of natural gas in the gaseous state. It is odorless, colorless, non-toxic and non-corrosive. Hazards include flammability after vaporization into a gaseous state, freezing and asphyxia.
The liquefaction process involves removal of certain components, such as dust, acid gases, helium, water, and heavy hydrocarbons, which could cause difficulty downstream. The natural gas is then condensed into a liquid at close to atmospheric pressure (maximum transport pressure set at around 25 kPa or 3.6 psi) by cooling it to approximately −162 °C (−260 °F).
LNG achieves a higher reduction in volume than compressed natural gas (CNG) so that the energy density of LNG is 2.4 times greater than that of CNG or 60% of that of diesel fuel.[18] This makes LNG cost efficient to transport over long distances where pipelines do not exist. Specially designed cryogenic sea vessels (LNG carriers) or cryogenic road tankers are used for its transport.
LNG, when it is not highly refined for special uses, is principally used for transporting natural gas to markets, where it is regasified and distributed as pipeline natural gas. It is also used in LNG-fueled road vehicles as is beginning to be seen with some trucks in commercial operation, which have been achieving payback periods of approximately four years on the higher initial investment required in LNG equipment on the trucks and LNG infrastructure to support fueling.[19] However, it remains more common to design vehicles to use compressed natural gas. As of 2002, the relatively higher cost of LNG production and the need to store LNG in more expensive cryogenic tanks had slowed widespread commercial use.[20]
Power to gas is a technology which converts electrical power to a gas fuel. The method is used to convert carbon dioxide and water to methane, (see natural gas) using electrolysis and the Sabatier reaction. The excess power or off peak power generated by wind generators or solar arrays could theoretically be used for load balancing in the energy grid.
In a highly refined form, liquid methane is used as a rocket fuel.[21]
While investigations of methane use have existed for decades, no production methane engines have yet been used on orbital spaceflights.[22] This is changing, and liquid methane has recently been selected for the active development of a variety of bipropellant rocket engines.
Since the 1990s, a number of Russian rockets have been proposed to use liquid methane.[23][24] One 1990s Russian engine proposal was the RD-192, a methane/LOX variant of the RD-191.[24]
In 2005, US companies, Orbitech and XCOR Aerospace, developed a demonstration liquid oxygen/liquid methane rocket engine and a larger 7,500 pounds-force (33 kN)-thrust engine in 2007 for potential use as the CEV lunar return engine, before the CEV program was later cancelled.[25][26][27]
More recently the American private space company SpaceX announced in 2012 an initiative to develop liquid methane rocket engines,[28] including, initially, the very large Raptor rocket engine.[29]
Raptor is being designed to produce 4.4 meganewtons (1,000,000 lbf) of thrust with a vacuum specific impulse (Isp) of 363 seconds and a sea-level Isp of 321 seconds,[30] and is expected to begin component-level testing in 2014.[31] In February 2014, the Raptor engine design was revealed to be of the highly efficient and theoretically more reliable full-flow staged combustion cycle type, where both propellant streams—oxidizer and fuel—will be completely in the gas phase before they enter the combustion chamber. Prior to 2014, only two full-flow rocket engines have ever progressed sufficiently to be tested on test stands, but neither engine completed development or flew on a flight vehicle.[30]
In October 2013, the China Aerospace Science and Technology Corporation, a state-owned contractor for the Chinese space program, announced that it had completed a first ignition test on a new LOX methane rocket engine. No engine size was provided.[32]
In September 2014, another American private space company—Blue Origin announced work on a large methane rocket engine. The new engine, the Blue Engine 4, or BE-4, has been designed to produce 2,400 kilonewtons (550,000 lbf) of thrust. While initially planned to be used exclusively on a Blue Origin proprietary launch vehicle, it will now be used on a new United Launch Alliance (ULA) engine on an new launch vehicle that is a successor to the Atlas V. ULA expects the first flight of the new launch vehicle no earlier than 2019.[33]
One advantage of methane is that it is abundant in many parts of the solar system and it could potentially be harvested on the surface of another solar-system body (in particular, using In Situ Resource Utilization on Mars[34] and Titan), providing fuel for a return journey.[21][35]
NASA's Project Morpheus has developed a restartable LOX methane rocket engine with 5,000 pounds-force (22 kN) thrust and a specific impulse of 321 seconds suitable for inspace applications including landers. Small LOX methane thrusters 5–15 pounds-force (22–67 N) were also developed suitable for use in a Reaction Control System (RCS).[36][37]
Although there is great interest in converting methane into useful or more easily liquefied compounds, the only practical processes are relatively unselective. In the chemical industry, methane is converted to synthesis gas, a mixture of carbon monoxide and hydrogen, by steam reforming. This endergonic process (requiring energy) utilizes nickel catalysts and requires high temperatures, around 700–1100 °C:
Related chemistries are exploited in the Haber-Bosch Synthesis of ammonia from air, which is reduced with natural gas to a mixture of carbon dioxide, water, and ammonia.
Methane is also subjected to free-radical chlorination in the production of chloromethanes, although methanol is a more typical precursor.[38]
Naturally occurring methane is mainly produced by the process of methanogenesis. This multistep process is used by microorganisms as an energy source. The net reaction is:
The final step in the process is catalyzed by the enzyme landfill, ruminants (e.g., cattle), and the guts of termites.
It is uncertain if plants are a source of methane emissions.[39][40][41]
Methane could also be produced by a non-biological process called serpentinization[1] involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.[42]
Methane can be 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. This technology is practiced on a large scale to produce longer chain molecules than methane.
Natural gas is so abundant that the intentional production of methane is relatively rare. The only large scale facility of this kind 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 which is otherwise very hard to transport for its weight, ash content, low calorific value and propensity to spontaneous combustion during storage and transport.
An adaptation of the Sabatier methanation reaction may be used via 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.[34]
Methane can also be produced by the destructive distillation of acetic acid in the presence of soda lime or similar. Acetic acid is decarboxylated in this process. Methane can also be prepared by reaction of aluminium carbide with water or strong acids.
Methane was discovered and isolated by oil generate natural gas.
It is generally transported in bulk by pipeline in its natural gas form, or LNG carriers in its liquefied form; few countries transport it by truck.
Apart from gas fields, an alternative method of obtaining methane is via manure, wastewater sludge, municipal solid waste (including landfills), or any other biodegradable feedstock, under anaerobic conditions. Rice fields also generate large amounts of methane during plant growth. Methane hydrates/clathrates (ice-like combinations of methane and water on the sea floor, found in vast quantities) are a potential future source of methane. Cattle belch methane accounts for 16% of the world's annual methane emissions to the atmosphere.[43] One study reported that the livestock sector in general (primarily cattle, chickens, and pigs) produces 37% of all human-induced methane.[44] Early research has found a number of medical treatments and dietary adjustments that help slightly limit the production of methane in ruminants.[45][46] A more recent study, in 2009, 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.[47] Many efforts are underway to reduce livestock methane production and trap the gas to use as energy.[48]
Paleoclimatology research published in Current Biology suggests that flatulence from dinosaurs may have warmed the Earth.[49]
Methane is created near the Earth's surface, primarily by microorganisms by the process of methanogenesis. It is carried into the stratosphere by rising air in the tropics. Uncontrolled build-up of methane in the atmosphere is naturally checked – although human influence can upset this natural regulation – by methane's reaction with hydroxyl radicals formed from singlet oxygen atoms and with water vapor. It has a net lifetime of about 10 years,[51] and is primarily removed by conversion to carbon dioxide and water.
Methane also affects the degradation of the ozone layer.[52][53]
In addition, there is a large (but unknown) amount of methane in methane clathrates in the ocean floors as well as the Earth's crust. Most methane is the result of biological process called methanogenesis.
In 2010, methane levels in the Arctic were measured at 1850 nmol/mol, a level over twice as high as at any time in the 400,000 years prior to the industrial revolution. Historically, methane concentrations in the world's atmosphere have ranged between 300 and 400 nmol/mol during glacial periods commonly known as ice ages, and between 600 to 700 nmol/mol during the warm interglacial periods. Recent research suggests that the Earth's oceans are a potentially important new source of Arctic methane.[54]
A
Compressed natural gas is touted as the 'cleanest burning' alternative fuel available, since the simplicity of the methane molecule reduces tailpipe emissions of different pollutants by 35 to 97%. Not quite as dramatic is the reduction in net greenhouse-gas emissions, which is about the same as corn-grain ethanol at about a 20% reduction over gasoline
"We are going to do methane." Musk announced as he described his future plans for reusable launch vehicles including those designed to take astronauts to Mars within 15 years, "The energy cost of methane is the lowest and it has a slight Isp (Specific Impulse) advantage over Kerosene" said Musk adding, "And it does not have the pain in the ass factor that hydrogen has".
"SpaceX's initial plan will be to build a lox/methane rocket for a future upper stage codenamed Raptor....The new Raptor upper stage engine is likely to be only the first engine in a series of lox/methane engines...".
Methane has been detected or is believed to exist on all planets of the solar system, as well as on most of the larger moons. In most cases, it is believed to have been created by abiotic processes. Possible exceptions are Mars and Titan.
Methane gas explosions are responsible for many deadly mining disasters.[64] A methane gas explosion was the cause of the Upper Big Branch coal mine disaster in West Virginia on April 5, 2010, killing 25.[65]
Methane is not toxic, yet it is extremely flammable and may form explosive mixtures with air. Methane is violently reactive with oxidizers, halogen, and some halogen-containing compounds. Methane is also an asphyxiant and may displace oxygen in an enclosed space. Asphyxia may result if the oxygen concentration is reduced to below 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.
Arctic methane release from permafrost and methane clathrates is an expected consequence and further cause of global warming.[61][62][63]
Methane is essentially insoluble in water, but it can be trapped in ice forming a similar solid. Significant deposits of methane clathrate have been found under sediments on the ocean floors of Earth at large depths.
Methane has a large effect for a brief period (a net lifetime of 8.4 years in the atmosphere), whereas carbon dioxide has a small effect for a long period (over 100 years). Because of this difference in effect and time period, the global warming potential of methane over a 20-year time period is 72. The Earth's atmospheric methane concentration has increased by about 150% since 1750, and it accounts for 20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases (these gases don't include water vapor which is by far the largest component of the greenhouse effect).[60] Usually, excess methane from landfills and other natural producers of methane is burned so CO2 is released into the atmosphere instead of methane, because methane is a more effective greenhouse gas. Recently, methane emitted from coal mines has been successfully utilized to generate electricity.
The IPCC Fifth Assessment Report determined that methane in the Earth's atmosphere is an important greenhouse gas with a global warming potential of 34 compared to CO2 over a 100-year period (although accepted figures probably represent an underestimate[57][58]). This means that a methane emission will have 34 times the effect on temperature of a carbon dioxide emission of the same mass over the following 100 years. And methane has 33 times the effect when accounted for aerosol interactions.[59]
[56] Possible adverse effects projected as the gas escapes into the atmosphere are estimated to have the potential of a sixty trillion dollar impact on the world economy.[55]
Solar System, Mars Reconnaissance Orbiter, Oxygen, Mercury (planet), Moon
Ice, Oxygen, Hydrogen, Energy, Drinking water
Diamond, Graphite, Solar system, Coal, /anolanthanide Chemistry
Methane, Ethane, Oxygen, Pentane, Jupiter
Jupiter, Saturn, Hydrogen, Carbon, Methane
Mars, Argon, Methane, Carbon dioxide, Oxygen
Carbon dioxide, Global warming, Methane, Coal, Natural gas
Moon, Solar System, European Space Agency, Saturn, Earth