Difference between revisions of "Methane" - New World Encyclopedia

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== Occurrence ==
 
== Occurrence ==
===Natural gas fields===
 
  
Methane is found in geological deposits known as ''natural gas fields''. It is associated with other hydrocarbons and sometimes also [[helium]] and [[nitrogen]]. The gas is formed by the decay of [[organic matter]] under anaerobic conditions (that is, in the absence of oxygen) beneath the Earth's surface. In general, natural gas is found in sediments buried deeper and at higher temperatures than those that contain [[petroleum]]. Natural gas fields are the main source from which methane is extracted for human use.
+
Large quantities of methane are present in geological deposits known as ''natural gas fields''. It is associated with other hydrocarbons and sometimes also [[helium]] and [[nitrogen]]. In general, natural gas is found in sediments buried deeper and at higher temperatures than those that contain [[petroleum]]. It is formed by the decay of [[organic matter]] under anaerobic conditions (that is, in the absence of oxygen) beneath the Earth's surface. Natural gas fields are the main source from which methane is extracted for human use.
  
 
'''Biogas''' is another source of methane. It is a mixture of methane, carbon dioxide, and small amounts of other gases. It is generated by the [[fermentation]] of organic matter—including manure, wastewater sludge, municipal solid waste, or other biodegradable feedstock—under anaerobic conditions. Biogas is also called '''swamp gas''', '''landfill gas''', or '''marsh gas''', depending on where it is produced.
 
'''Biogas''' is another source of methane. It is a mixture of methane, carbon dioxide, and small amounts of other gases. It is generated by the [[fermentation]] of organic matter—including manure, wastewater sludge, municipal solid waste, or other biodegradable feedstock—under anaerobic conditions. Biogas is also called '''swamp gas''', '''landfill gas''', or '''marsh gas''', depending on where it is produced.
  
 
Methane can also be extracted from [[coal]] deposits. In industrial settings, it can be produced by chemical reactions between hydrogen and common atmospheric gases. In addition, methane is a component of cattle flatulence.
 
Methane can also be extracted from [[coal]] deposits. In industrial settings, it can be produced by chemical reactions between hydrogen and common atmospheric gases. In addition, methane is a component of cattle flatulence.
 
*The process of producing biogas is popular for treating many types of organic waste. It provides a convenient way of turning waste into [[electricity]], decreasing the amount of waste to be disposed of, and destroying disease-causing [[microbe]]s present in the waste stream.
 
  
 
[[Image:Burning hydrate inlay US Office Naval Research.jpg|right|frame|"Burning ice." Methane clathrate consists of methane trapped in ice crystals. Upon heating, this material releases methane, which can burn. Inset: Molecular structure of a methane clathrate.<br>Source: [[USGS]]]]
 
[[Image:Burning hydrate inlay US Office Naval Research.jpg|right|frame|"Burning ice." Methane clathrate consists of methane trapped in ice crystals. Upon heating, this material releases methane, which can burn. Inset: Molecular structure of a methane clathrate.<br>Source: [[USGS]]]]
  
'''Methane clathrate''', also called '''methane hydrate''' or '''methane ice''', is a form of [[water (molecule)|water]] [[ice]] that contains a large amount of [[methane]] within its [[crystal]] structure (a [[clathrate hydrate]]). Originally thought to occur only in the outer regions of the [[solar system]] where temperatures are low and water ice is common, extremely
+
Large deposits of methane have been found in a form known as '''methane clathrate''', under sediments on the [[ocean]] floors. Also known as '''methane hydrate''' or '''methane ice''', it consists of methane molecules held in cagelike, crystalline structures of frozen water. These structures remain stable at temperatures up to 18°C. Methane clathrates are thought to be formed when methane gas streams rising from geological faults come in contact with cold seawater. One liter of methane clathrate solid would contain, on average, 168 liters of methane gas (at 25°C and ).
  
Large deposits of methane have been found in a form known as '''methane clathrate''' under sediments on the [[ocean]] floors. Also known as '''methane hydrate''' or '''methane ice''', it consists of methane molecules held in cagelike structures of frozen water.
+
*The process of producing biogas is popular for treating many types of organic waste. It provides a convenient way of turning waste into [[electricity]], decreasing the amount of waste to be disposed of, and destroying disease-causing [[microbe]]s present in the waste stream.
 
 
Methane clathrates are common constituents of the shallow marine geosphere, and they occur both in deep sedimentary structures, and as outcrops on the ocean floor. Methane hydrates are believed to form by migration of gas from depth along [[geological fault]]s, followed by precipitation, or crystallization, on contact of the rising gas stream with cold sea water.
 
 
 
Methane clathrates remain stable at temperatures up to 18 °C. The average methane clathrate hydrate composition is 1 [[mole (unit)|mole]] of methane for every 5.75 moles of water, though this is dependent on how many methane molecules "fit" into the various cage structures of the water lattice. The observed density is around 0.9 g/cm<sup>3</sup>. One liter of methane clathrate solid would therefore contain, on average, 168 liters of methane gas (at [[Standard temperature and pressure|STP]]).
 
 
 
Methane forms a '''structure I hydrate''' with two [[dodecahedral]] (20 water molecules) and six [[polyhedron|tetrakaidecahedral]] (24 water molecules) water cages per unit cell. The hydratation value of 20 can be determined experimentally by [[Magic angle spinning|MAS NMR]] (Dec, 2005). A methane clathrate spectrum recorded at 275 [[kelvin|K]] and 3.1 [[MPa]] shows a peak for each cage type and a separate peak for gas phase methane.
 
  
 
==Properties==
 
==Properties==

Revision as of 16:57, 4 July 2006

Methane
Methane Methane
General
Other names Marsh gas
Molecular formula CH4
SMILES C
Molar mass 16.04 g/mol
Appearance colourless gas
CAS number [74-82-8]
Properties
Density and phase 0.717 kg/m3, gas
Solubility in water 3.5 mL g/100 ml (17°C)
Melting point −182.5°C (90.6 K)
Boiling point −161.6°C (111.55 K)
Triple point 90.7 K, 0.117 bar
Critical temperature 190.5°K (−82.6°C) at 4.6 MPa (45 atm)
Structure
Molecular shape tetrahedral
Symmetry group Td
Dipole moment Zero
Hazards
MSDS External MSDS
EU classification Highly flammable (F+)
NFPA 704

NFPA 704.svg

4
1
0
 
R-phrases R12
S-phrases S2, S9, S16, S33
Flash point −188°C
Autoignition temperature 537°C
Maximum burning
temperature:
2148°C
Explosive limits 5–15%
Supplementary data page
Structure and
properties
Thermodynamic
data
Spectral data UV, IR, NMR, MS
Related compounds
Related alkanes Ethane
Propane
Related compounds Methanol
Chloromethane
Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)
Infobox disclaimer and references

The simplest hydrocarbon, methane, is a gas with a chemical formula of CH4. In chemical terms it is classified as an alkane.

Pure methane is odorless, but when used commercially is usually mixed with small quantities of odorants, strongly-smelling sulfur compounds such as ethyl mercaptan to enable the detection of leaks.

Methane is the main component of natural gas, which is widely used as a fuel.

Methane is a greenhouse gas with a global warming potential over 100 years of 23 (IPCC Third Assessment Report) i.e. when averaged over 100 years each kg of CH4 warms the earth 23 times as much as the same mass of CO2.

The Earth's mantle contains huge amounts of methane and is the main reservoir. Large amounts of methane are emitted to the atmosphere through mud volcanoes that are connected to deep geological faults.


Occurrence

Large quantities of methane are present in geological deposits known as natural gas fields. It is associated with other hydrocarbons and sometimes also helium and nitrogen. In general, natural gas is found in sediments buried deeper and at higher temperatures than those that contain petroleum. It is formed by the decay of organic matter under anaerobic conditions (that is, in the absence of oxygen) beneath the Earth's surface. Natural gas fields are the main source from which methane is extracted for human use.

Biogas is another source of methane. It is a mixture of methane, carbon dioxide, and small amounts of other gases. It is generated by the fermentation of organic matter—including manure, wastewater sludge, municipal solid waste, or other biodegradable feedstock—under anaerobic conditions. Biogas is also called swamp gas, landfill gas, or marsh gas, depending on where it is produced.

Methane can also be extracted from coal deposits. In industrial settings, it can be produced by chemical reactions between hydrogen and common atmospheric gases. In addition, methane is a component of cattle flatulence.

"Burning ice." Methane clathrate consists of methane trapped in ice crystals. Upon heating, this material releases methane, which can burn. Inset: Molecular structure of a methane clathrate.
Source: USGS

Large deposits of methane have been found in a form known as methane clathrate, under sediments on the ocean floors. Also known as methane hydrate or methane ice, it consists of methane molecules held in cagelike, crystalline structures of frozen water. These structures remain stable at temperatures up to 18°C. Methane clathrates are thought to be formed when methane gas streams rising from geological faults come in contact with cold seawater. One liter of methane clathrate solid would contain, on average, 168 liters of methane gas (at 25°C and ).

  • The process of producing biogas is popular for treating many types of organic waste. It provides a convenient way of turning waste into electricity, decreasing the amount of waste to be disposed of, and destroying disease-causing microbes present in the waste stream.

Properties

At room temperature and pressure, methane is a colorless, odorless gas. It has a boiling point of −162°C at 1 atmosphere pressure and is highly flammable.

Each molecule of methane consists of four atoms of hydrogen attached to a single atom of carbon through covalent bonds. The molecule is shaped like a tetrahedron, with the carbon atom at the center and the four hydrogen atoms occupying the four corners of the tetrahedron.

Reactions of methane

Combustion

When methane is burned in the presence of oxygen, the reaction—called a combustion reaction—produces carbon dioxide, water, and a large amount of heat. In molecular terms, the chemical reaction can be represented as follows:

CH4 + 2O2 → CO2 + 2H2O + heat

Methane burns relatively cleanly compared to coal.

Hydrogen activation

In methane, the carbon-hydrogen covalent bond is among the strongest in all hydrocarbons. In chemical terms, there is a high "activation barrier" to break this C-H bond—in other words, considerable energy is required to break it. Nonetheless, methane is still the principal starting material for the manufacture of hydrogen. The search for catalysts that can lower the activation barrier and other small-molecule alkanes is an area of research with considerable industrial significance.

Reactions with halogens

Under the proper conditions, methane reacts with all the halogens. The general reaction can be represented as follows:

CH4 + X2 → CH3X + HX

Here, X is either fluorine (F), chlorine (Cl), bromine (Br), or sometimes iodine (I).

This reaction may continue, so that CH3X reacts with X2 to produce CH2X2; CH2X2 in turn can react with X2 to produce CHX3; and CHX3 can react further with X2 to produce CX4.

Potential health effects

Methane is not toxic by any route. The immediate health hazard is that it may cause thermal burns. It is flammable and may form mixtures with air that are flammable or explosive. Methane is violently reactive with oxidizers, halogens, and some halogen compounds. Methane is an asphyxiant and may displace oxygen in a workplace atmosphere. Asphyxia may result if the oxygen concentration is reduced to below 18% by displacement. The concentrations at which flammable or explosive mixtures form are much lower than the concentration at which asphyxiation risk is significant. When structures are built on or near landfills, methane off-gas can penetrate the building interior and expose occupants to significant levels of methane. Some buildings have specially engineered recovery systems below their basements, to actively capture such fugitive off-gas and vent it away from the building.

Uses

Fuel

For more on the use of methane as a fuel, see: natural gas

Methane is important for electrical generation by burning it as a fuel in a gas turbine or steam boiler. Compared to other hydrocarbon fuels, burning methane produces less carbon dioxide for each unit of heat released. Also, methane's heat of combustion is about 902 kJ/mol, which is lower than any other hydrocarbon, but if a ratio is made with the atomic weight (16 g/mol) divided by the heat of combustion (902 kJ/mol) it is found that methane, being the simplest hydrocarbon, actually produces the most heat per gram 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.

Industrial uses

Methane is used in industrial chemical processes and may be transported in liquid or refrigerated liquid form. While leaks from a liquid container are initially heavier than air, the gas is lighter than air. Gas pipelines distribute large amounts of natural gas, of which methane is a significant component.

In the chemical industry, methane is the feedstock of choice for the production of hydrogen, methanol, acetic acid, and acetic anhydride. When used to produce any of these chemicals, methane is first converted to synthesis gas, a mixture of carbon monoxide and hydrogen, by steam reforming. In this process, methane and steam react on a nickel catalyst at high temperatures (700–1100 °C).

CH4 + H2O → CO + 3H2

The ratio of carbon monoxide to hydrogen in synthesis gas can then be adjusted via the water gas shift reaction to the appropriate value for the intended purpose.

CO + H2O ⇌ CO2 + H2

Less significant methane-derived chemicals include acetylene, prepared by passing methane through an electric arc, and the chloromethanes (chloromethane, dichloromethane, chloroform, and carbon tetrachloride), produced by reacting methane with chlorine gas. However, the use of these chemicals is declining, acetylene as it is replaced by less costly substitutes, and the chloromethanes due to health and environmental concerns.


Methane in Earth's atmosphere

Methane concentrations graph
Computer models showing the amount of methane (parts per million by volume) at the surface (top) and in the stratosphere (bottom).

Methane in the earth's atmosphere is an important greenhouse gas with a Global warming potential of 23 over a 100 year period. Its concentation 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 [1].

The average concentration of methane at the Earth's surface in 1998 was 1,745 ppb [2]. Its concentration is higher in the northern hemisphere as most sources (both natural and human) are larger. The concentrations vary seasonally with a minimum in the late summer.

Methane is created near the surface, and it is carried into the stratosphere by rising air in the tropics. Uncontrolled build-up of methane in Earth's atmosphere is naturally checked—although human influence can upset this natural regulation—by methane's reaction with a molecule known as the hydroxyl radical, a hydrogen-oxygen molecule formed when single oxygen atoms react with water vapor.

Early in the Earth's history—about 3.5 billion years ago—there was 1,000 times as much methane in the atmosphere as there is now. The earliest methane was released into the atmosphere by volcanic activity. During this time, Earth's earliest life appeared. These first, ancient bacteria added to the methane concentration by converting hydrogen and carbon dioxide into methane and water. Oxygen did not become a major part of the atmosphere until photosynthetic organisms evolved later in Earth's history. With no oxygen, methane stayed in the atmosphere longer and at higher concentrations than it does today.

Emissions of methane

Houweling et al. (1999) give the following values for methane emissions [3]:

Origin CH4 emission (Tg/yr)
Natural emissions
Wetlands (incl rice production) 225
Ocean 20
Termites 15
Hydrates 10
Natural total 290
Anthropogenic emissions
Energy 110
Landfills 40
Ruminants 115
Waste treatment 25
Biomass burning 40
Anthropogenic total 330

Slightly over half of the total emission is due to human activity [4].

Living plants (e.g. forests) have recently been identified as a potentially important source of methane. The recent paper calculated emissions of 62–236 Tg yr-1, and "this newly identified source may have important implications". [5], [6]. However the authors stress "our findings are preliminary with regard to the methane emission strength".[7]

See also Flatulence tax.

Removal processes

The major removal mechanism of methane from the atmosphere is by reaction with the hydroxyl radical (·OH), which may be produced when a cosmic ray strikes a molecule of water vapor:

This reaction in the troposphere gives a methane lifetime of 9.6 years. Two more minor sinks are soil sinks (160 year lifetime) and stratospheric loss by reaction with , and in the stratosphere (120 year lifetime), giving a net lifetime of 8.4 years. [8]

Sudden release from methane clathrates

At high pressures, such as are found on the bottom of the ocean, methane forms a solid clathrate with water, known as methane hydrate. An unknown, but possibly very large quantity of methane is trapped in this form in ocean sediments. The sudden release of large volumes of methane from such sediments into the atmosphere has been suggested as a possible cause for rapid global warming events in the earth's distant past, such as the Paleocene-Eocene thermal maximum of 55 million years ago.

One source estimates the size of the methane hydrate deposits of the oceans at ten million million tons (10 exagrams). Theories suggest that should global warming cause them to heat up sufficiently, all of this methane could again be suddenly released into the atmosphere. Since methane is twenty-three times stronger (for a given weight, averaged over 100 years) than CO2 as a greenhouse gas; this would immensely magnify the greenhouse effect, heating Earth to unprecedented levels.

Extraterrestrial methane

Methane has been detected or is believed to exist in several locations of the solar system. It is believed to have been created by abiotic processes, with the possible exception of Mars.

Traces of methane gas are present in the thin atmosphere of the Earth's Moon.

Methane has also been detected in interstellar clouds.

See also

  • Alkane, a type of hydrocarbon of which methane is simplest member.
  • Methane clathrate, form of water ice which contains methane.
  • Methanogen, archaea that produce methane as a metabolic by-product.
  • Methanogenesis, the formation of methane by microbes.
  • Methanotroph, bacteria that are able to grow using methane as their only source of carbon and energy.
  • Methyl group, a functional group similar to methane.

External links


 
Alkanes

methane
CH4

|
 

ethane
C2H6

|
 

propane
C3H8

|
 

butane
C4H10

|
 

pentane
C5H12

|
 

hexane
C6H14

heptane
C7H16

|
 

octane
C8H18

|
 

nonane
C9H20

|
 

decane
C10H22

|
 

undecane
C11H24

|
 

dodecane
C12H26

 

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