The simplest hydrocarbon, methane, is a gas (at
standard temperature and pressure, STP) with a chemical formula of
CH4. Pure methane is odorless, but when used commercially is usually
mixed with small quantities of odorants, strongly-smelling sulfur
compounds such as ethanethiol (also called ethyl mercaptan), to
enable the detection of leaks.
The principal component of natural gas, methane is a
significant and plentiful fuel. Burning one molecule of methane in
the presence of oxygen releases one molecule of CO2 (carbon dioxide)
and two molecules of H2O (water):
CH4 + 2O2 ? CO2 + 2H2O
Methane's relative abundance and clean burning process
makes it a very attractive fuel. However, because it is a gas and
not a liquid or solid, methane is difficult to transport from the
areas that produce it to the areas that consume it. Converting
methane to forms that are more easily transported, such as LNG
(Liquified Natural Gas) and methanol, is an active area of
Methane is a greenhouse gas with a global warming
potential over 100 years of 23. 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 crust contains huge amounts of methane.
Large amounts of methane are emitted to the atmosphere through mud
volcanoes which are connected with deep geological faults or as the
main constituent of biogas formed naturally by anaerobic
At room temperature and standard pressure, methane is
a colorless, odorless gas. It has a boiling point of -162 °C at 1.00
atmosphere and is extremely flammable.
Methane is not toxic. The immediate health hazard is
that it may cause burns if it ignites. It is highly flammable and
may form explosive mixtures with air. Methane is violently reactive
with oxidizers, halogens, 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 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 buildings'
interiors 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. An example of this type of system is in
the Dakin Building, Brisbane, California.
Main reactions with methane are: combustion, hydrogen
activation, and halogen reaction. In general, methane reactions are
hard to control; partial oxidation to methanol, for example, is
difficult to achieve; the reaction typically progresses all the way
to carbon dioxide and water.
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 (16.0
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 unit mass 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. One
standard cubic foot of methane will produce roughly 1,000 BTU (1.06
MJ = 293 W-hr) of energy.
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 a significant component.
In the chemical industry, methane is the feedstock of
choice for the production of hydrogen, methanol, acetic acid, and
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.
The major source of methane is extraction from
geological deposits known as natural gas fields. It is associated
with other hydrocarbon fuels and sometimes accompanied by helium and
nitrogen. The gas at shallow levels (low pressure) is formed by
anaerobic decay of organic matter deep under the Earth's surface. In
general, sediments buried deeper and at higher temperatures than
those which give oil generate natural gas.
Apart from gas fields an alternative method of
obtaining methane is via biogas generated by the fermentation of
organic matter including manure, wastewater sludge, municipal solid
waste, or any other biodegradable feedstock, under anaerobic
conditions. Industrially, methane can be created from common
atmospheric gases and hydrogen (produced, perhaps, by electrolysis)
through chemical reactions such as the Sabatier process,
Fischer-Tropsch process. Coal bed methane extraction is a method for
extracting methane from a coal deposit. It is also caused by cows'
Methane in Earth's
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.
The average concentration of methane at the Earth's
surface in 1998 was 1,745 ppb. 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
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.
|Wetlands (incl rice
Slightly over half of the total emission is due to human