Methane

From New World Encyclopedia
Revision as of 01:42, 12 July 2006 by Dinshaw Dadachanji (talk | contribs) (→‎See also: condensing)
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 found in association with other hydrocarbons and sometimes also helium and nitrogen. In general, natural gas is present 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 currently 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.

"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

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

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 100 kPa pressure).

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. It is violently reactive with oxidizers, halogens, and some halogen compounds.

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

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, but its immediate health hazard is that it may cause thermal burns. Given that it is flammable, it may form mixtures with air that are explosive. If it displaces oxygen within an enclosed space, and the oxygen concentration drops below 18%, it can lead to asphyxiation. The concentrations at which flammable or explosive mixtures form are much lower than the concentration at which the risk of asphyxiation 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 an important fuel. Being the simplest hydrocarbon, the combustion of methane produces more heat per gram than other hydrocarbons. In addition, compared to other hydrocarbon fuels, burning methane produces less carbon dioxide for each unit of heat released.

In the form of natural gas, methane is regularly distributed via gas pipelines. In many cities, it is piped into homes for domestic heating and cooking purposes. Moreover, large quantities are burned to drive gas or steam turbines, to generate electricity.

When biogas is used as a fuel, it provides a convenient way to turn waste into electricity. The process of producing biogas decreases the amount of waste to be disposed of and destroys disease-causing microbes that are present in the waste stream.

Industrial uses

In the chemical industry, methane is the feedstock of choice for the production of hydrogen, methanol, acetic acid, and acetic anhydride. To produce any of these chemicals, methane is first made to react with steam in the presence of a nickel catalyst at high temperatures (700–1,100°C). The product, known as "synthesis gas," is a mixture of carbon monoxide and hydrogen.

CH4 + H2O → CO + 3H2

The synthesis gas is then manipulated to produce the intended products.

Moreover, acetylene is prepared by passing methane through an electric arc. When methane is made to react with chlorine gas, various chloromethanes are produced: chloromethane, dichloromethane, chloroform, and carbon tetrachloride. The use of these chemicals, however, is declining. Acetylene may be replaced by less costly substitutes, and the chloromethanes are used less often because of 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 beyond our planet, as listed below.

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.
  • Methanogen, archaea that produce methane as a metabolic by-product.
  • Methanotroph, bacteria that are able to grow using methane as their only source of carbon and energy.

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

 

Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

The history of this article since it was imported to New World Encyclopedia:

Note: Some restrictions may apply to use of individual images which are separately licensed.