Definição e significado de Methane

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Methane is a chemical compound with the chemical formula CH₄. It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees. Burning methane in the presence of oxygen produces carbon dioxide and water. The relative abundance of methane makes it an attractive fuel. However, because it is a gas at normal temperature and pressure, methane is difficult to transport from its source. In its natural gas form, it is generally transported in bulk by pipeline or LNG carriers; few countries transport it by truck.

Methane was discovered and isolated by Alessandro Volta between 1776 and 1778 when studying marsh gas from Lake Maggiore.

Methane is a relatively potent greenhouse gas with a high global warming potential of 72 (averaged over 20 years) or 25 (averaged over 100 years).^[2] Methane in the atmosphere is eventually oxidized, producing carbon dioxide and water. As a result, methane in the atmosphere has a half life of seven years.

The abundance of methane in the Earth's atmosphere in 1998 was 1745 parts per billion, up from 700 ppb in 1750. Methane can trap about 20 times the heat of CO₂. In the same time period, CO₂ increased from 278 to 365 parts per million. The radiative forcing effect due to this increase in methane abundance is about one-third of that of the CO₂ increase.^[3] In addition, there is a large, but unknown, amount of methane in methane clathrates in the ocean floors. The Earth's crust contains huge amounts of methane. Large amounts of methane are produced anaerobically by methanogenesis. Other sources include mud volcanoes, which are connected with deep geological faults, landfill and livestock (primarily ruminants) from enteric fermentation.

Global warming has the potential to trigger the release of large amounts of methane, currently stored as methane clathrates and further cause Arctic methane release from permafrost.

Properties

Methane is the major component of natural gas, about 87% by volume. At room temperature and standard pressure, methane is a colorless, odorless gas; the smell characteristic of natural gas as used in homes is an artificial safety measure caused by the addition of an odorant, often methanethiol or ethanethiol. Methane has a boiling point of −161 °C at a pressure of one atmosphere. As a gas it is flammable only over a narrow range of concentrations (5–15%) in air. Liquid methane does not burn unless subjected to high pressure (normally 4–5 atmospheres).

Potential health effects

Methane is not toxic; however, 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 19.5% by displacement^{[citation needed]}. 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.^{[citation needed]} An example of this type of system is in the Dakin Building, Brisbane, California.

Reactions of methane

Main reactions with methane are: combustion, steam reforming to syngas, and halogenation. 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.

Combustion

In the combustion of methane, several steps are involved:

Methane is believed to form a formaldehyde (HCHO or H₂CO). The formaldehyde gives a formyl radical (HCO), which then forms carbon monoxide (CO). The process is called oxidative pyrolysis:

CH₄ + O₂ → CO + H₂ + H₂O

Following oxidative pyrolysis, the H₂ oxidizes, forming H₂O, releasing heat. This occurs very quickly, usually in significantly less than a millisecond.

2 H₂ + O₂ → 2 H₂O

Finally, the CO oxidizes, forming CO₂ and releasing more heat. This process is generally slower than the other chemical steps, and typically requires a few to several milliseconds to occur.

2 CO + O₂ → 2 CO₂

The result of the above is the following total equation:

CH₄(g) + 2 O₂(g) → CO₂(g) + 2 H₂O(l) -890 kJ/mol

where bracketed "g" stands for gaseous form and bracketed "l" stands for liquid form.

Hydrogen activation

The strength of the carbon-hydrogen covalent bond in methane is among the strongest in all hydrocarbons, and thus its use as a chemical feedstock is limited. Despite the high activation barrier for breaking the C–H bond, CH₄ is still the principal starting material for manufacture of hydrogen in steam reforming. The search for catalysts which can facilitate C–H bond activation in methane and other low alkanes is an area of research with considerable industrial significance.

Reactions with halogens

Methane reacts with all halogens given appropriate conditions, as follows:

CH₄ + X₂ → CH₃X + HX

where X is a halogen: fluorine (F), chlorine (Cl), bromine (Br), or iodine (I). This mechanism for this process is called free radical halogenation.When X is Cl, this mechanism has the following form:

1. Radical generation:

The needed energy comes from UV radiation or heating,

2. Radical exchange:

CH₄ + Cl· → CH₃· + HCl + 14 kJ

CH₃· + Cl₂ → CH₃Cl + Cl· + 100 kJ

3. Radical extermination:

2 Cl· → Cl₂ + 239 kJ

CH₃· + Cl· → CH₃Cl + 339 kJ

2 CH₃· → CH₃CH₃ + 347 kJ

If methane and X₂ are used in equimolar quantities, CH₂X₂, CHX₃, and even CX₄ are formed. Using a large excess of CH₄ reduces the production of CH₂X₂, CHX₃, CX₄, and thus more CH₃X is formed.

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. 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) 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, and 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.^[4]

Research is being conducted by NASA on methane's potential as a rocket fuel.^[5] One advantage of methane is that it is abundant in many parts of the solar system and it could potentially be harvested in situ (i.e. on the surface of another solar-system body), providing fuel for a return journey.^[6]

Current methane engines in development produce a thrust of 7,500 pounds , which is far from the seven million pounds needed to launch the space shuttle. Instead, such engines will most likely propel voyages from our moon or send robotic expeditions to other planets in the solar system.^[7]

Recently methane emitted from coal mines has been successfully converted to electricity.^[8]

Industrial uses

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.

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).

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 is replaced by less costly substitutes, and the use of chloromethanes is diminishing due to health and environmental concerns.

Sources of methane for human use

Natural gas fields

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 and reworked methane from deep under the Earth's surface. In general, sediments buried deeper and at higher temperatures than those which give oil generate natural gas. Methane is also produced in considerable quantities from the decaying organic wastes of solid waste landfills.

Alternative sources

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 (including landfills), or any other biodegradable feedstock, under anaerobic conditions. 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.^[9] The livestock sector in general (primarily cattle, chickens, and pigs) produces 37% of all human-induced methane".^[10] However animals "that put their energies into making gas are less efficient at producing milk and meat". Early research has found a number of medical treatments and dietary adjustments that help limit the production of methane in ruminants.^[11][12][13]

Industrially, methane can be created from common atmospheric gases and hydrogen (produced, for example, 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, while enhanced coal bed methane recovery is a method of recovering methane from an non-minable coal seam.

Scientific experiments have given variable results in determining whether plants are a source of methane emissions.^[14][15][16]

Atmospheric methane

Methane is created near the Earth's surface, primarily in soils, rivers/seas and in animal guts. 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.

Methane in the Earth's atmosphere is an important greenhouse gas with a global warming potential of 25 compared to CO₂ over a 100-year period. This means that a methane emission will have 25 times the impact on temperature of a carbon dioxide emission of the same mass over the following 100 years. 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 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.^[17] 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 such a more effective greenhouse gas. Recently methane emitted from coal mines has been successfully converted to electricity.

Arctic methane release from permafrost and clathrates is an expected consequence of global warming.

In prehistoric times, large methane excursions have been linked with dramatic shifts in the Earth's climate, notably during the Paleocene-Eocene thermal maximum and during the Permian-Triassic extinction event, which was the worst ever mass extinction.

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.

• Moon - traces are outgased from the surface^[18]
• Mars - the atmosphere contains 10 ppb methane. In January 2009, NASA scientists announced that they had discovered that the planet often vents methane into the atmosphere in specific areas, leading some to speculate this may be a sign of biological activity going on below the surface.^[19]
• Jupiter - the atmosphere contains about 0.3% methane
• Saturn - the atmosphere contains about 0.4% methane
  • Iapetus
  • Titan — the atmosphere contains 1.6% methane and thousands of methane lakes have been detected on the surface^[20]
  • Enceladus - the atmosphere contains 1.7% methane^[21]
• Uranus - the atmosphere contains 2.3% methane
  • Ariel - methane is believed to be a constituent of Ariel's surface ice
  • Miranda
  • Oberon - about 20% of Oberon's surface ice is composed of methane-related carbon/nitrogen compounds
  • Titania - about 20% of Titania's surface ice is composed of methane-related organic compounds
  • Umbriel - methane is a constituent of Umbriel's surface ice
• Neptune - the atmosphere contains 1.6% methane
  • Triton - Triton has a tenuous nitrogen atmosphere with small amounts of methane near the surface.^[22][23]
• Pluto - spectroscopic analysis of Pluto's surface reveals it to contain traces of methane^[24][25]
  • Charon - methane is believed to be present on Charon, but it is not completely confirmed^[26]
• Eris - infrared light from the object revealed the presence of methane ice
• Comet Halley
• Comet Hyakutake - terrestrial observations found ethane and methane in the comet^[27]
• Extrasolar planet HD 189733b - This is the first detection of an organic compound on a planet outside the solar system. Its origin is unknown, since the planet's high temperature (700°C) would normally favor the formation of carbon monoxide instead.^[28]
• Interstellar clouds^[29]

See also

References

External links

• Gavin Schmidt, Methane: A Scientific Journey from Obscurity to Climate Super-Stardom, NASA Goddard, September 2004
• Methane thermodynamics
• International Chemical Safety Card 0291
• Methane Hydrates
• Computational Chemistry Wiki
• Safety data for methane
• Dynamic Viscosity of Methane
• Thermal Conductivity of Methane
• METHANE-EATING BUG HOLDS PROMISE FOR CUTTING GREENHOUSE GAS. Media Release, GNS Science, New Zealand]
• Catalytic conversion of methane to more useful chemicals and fuels
• Methane as a Savior of the Dairy Industry