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Introduction to Methane Gas
One of the most important methane gas facts is that it has very beneficial uses as an energy source when burned as a fuel. However, it must also be noted that methane gas is an air pollutant when released into the atmosphere. Read on in this article for more details about methane properties, methane gas as a fuel (both in natural gas and in biogas), production of biogas, methane gas as an air pollutant, and sources of methane in the atmosphere.
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Methane Properties and Methane Gas Hazards
Methane is a colorless, odorless gas. Its chemical formula is CH4. The schematic diagram of a methane molecule shows that each molecule has one atom of carbon and four atoms of hydrogen. Methane is a gas, lighter than air, at normal temperatures and pressures. It will condense to a liquid at -164oC and will become solid at -183oC.
Methane is in a chemically reduced state and will readily oxidize (burn as a fuel), giving off heat and producing carbon dioxide and water by the following chemical reaction:
CH4(g) + 2 O2(g) → 2 CO2(g) + H2O(l)
The primary methane gas hazards are its flammability, its explosive potential, and the possibility of asphyxiation. Mixtures of about 5% to 15% methane in air make an explosive mixture. Asphyxiation can be caused by breathing air with a high concentration of methane, because the high concentration of methane can reduce the oxygen level below that which is needed for life.
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Methane in Natural Gas as a Fuel
Natural gas is composed primarily of methane, and it is widely used around the world as a fuel. The largest uses of natural gas as a fuel are for space heat, water heating, and the generation of electricity. Natural gas typically contains about 87 - 96% methane by volume, with most of the remainder being ethane. Methane has a heating value of about 1000 Btu/ft3.
The methane in natural gas was formed from anaerobic decomposition of organic (plant and/or animal) matter that was trapped underground long in the past. It remains there until tapped and extracted by natural gas wells.
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Biogas - a Fuel Produced by a Methane Digester
Methane gas will be produced by anaerobic decomposition if waste organic matter is simply kept in a closed space without oxygen present. The process will be very slow, however, if the organic matter isn't "seeded" with the appropriate anaerobic bacteria. This process of decomposing waste organic matter and producing methane is called anaerobic digestion, and the system for carrying out the process is sometimes called a methane digester.
The methane containing gas produced by this process is usually referred to as biogas. Biogas is a less concentrated fuel than natural gas. It will typically contain about 50 to 75% methane, with most of the remainder being carbon dioxide, along with small to trace amounts of nitrogen, oxygen, water vapor, hydrogen sulfide, and various organic compounds.
As a way of using biogas, there is interest in how to convert methane gas to electricity.
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Methane Gas as an Air Pollutant
Methane gas is a hydrocarbon and thus will contribute to photochemical smog in locations where that type of air pollution is a problem. Also, as more widely publicized, methane gas is a greenhouse gas. As described in the article What are the Five Major Greenhouse Gases?, greenhouse gases trap incoming solar radiation in the earth's atmosphere and contribute to global warming and climate change.
Methane gas is produced and released into the atmosphere by natural processes in which organic matter is maintained in an anaerobic condition, such as from bogs and marshes. It is also produced where concentrated livestock waste is present and in sanitary landfills. Many wastewater treatment plants use anaerobic digestion to stabilize wastewater sludge, and thus produce methane gas as a byproduct. Some plants use the methane gas they produce as a fuel for heat or to generate electricity. Otherwise it is typically burned in a flare, so that the methane gas isn't released into the atmosphere.
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References and Image Credits
References for Further Information:
1. Methane Molecule - NASA website
2. Methane Digester - Gopal Aggarwal