Natural gas is a gas produced by the anaerobic decay of organic material. It is usually found in oil fields and natural gas fields, but is also generated in swamps and marshes (where it is called swamp gas or marsh gas), in landfill sites, and during digestion in animals (see flatulence).
The primary component of natural gas is methane (CH4), the shortest and lightest hydrocarbon molecule. It may also contain heavier gaseous hydrocarbons such as ethane (C2H6), propane (C3H8) and butane (C4H10), as well as other gases, in varying amounts, see also natural gas condensate.
Hydrogen sulfide (H2S see acid gas) and mercury (Hg) are common contaminants, which must be removed prior to most uses.
Combustion of one hundred cubic feet (1 ccf) of commercial quality natural gas typically yields approximately 1 therm (100,000 British thermal units, 30 kWh). One cubic meter yields 38 MJ (10.6 kWh).
The major difficulty in the use of natural gas is transportation. Natural gas pipelines are economical, but are impractical across oceans. Many existing pipelines in North America are close to reaching their capacity prompting some politicians in colder climates to speak publicly of potential shortages. Liquefied natural gas (LNG) tankers are also used, but have higher cost and safety problems. In many cases, as with oil fields in Saudi Arabia, the natural gas which is recovered in the course of recovering petroleum cannot be profitably sold, and is simply burned at the oil field (known as flaring). This wasteful practice is now illegal in many countries, especially since it adds greenhouse gas pollution to the atmosphere, and since a profitable method may be found in the future. The gas is instead re-injected back into the ground for possible later recovery, and to assist oil pumping by keeping underground pressures higher.
Natural gas is often stored as Compressed Natural Gas or CNG.
Many politicians and prominent figures in North America have spoken publicly about a possible natural gas crisis. This list includes former Secretary of Energy Spencer Abraham, Chairman of the Federal Reserve Alan Greenspan, Ontario Minister of Energy Dwight Duncan.
The natural gas crisis is typically described by the increasing price of natural gas in the U.S. over the last few years due to the decline in indigenous supply and the increase in demand for electricity generation. The price has become so high that many industrial users, mainly in the petrochemical industry, have closed their plants causing loss of jobs. Alan Greenspan has suggested that a solution to the natural gas crisis is the importation of liquified natural gas, or LNG.
Natural gas is important as a major source for electricity generation through the use of gas turbines and steamturbines. Particularly high efficiencies can be achieved through combining gas turbines with a steam turbine in combined cycle mode. Environmentally, natural gas burns cleaner than other fossil fuels, such as oil and coal, and produces less greenhouse gases. For an equivalent amount of heat, burning natural gas produces about 30% less carbon dioxide than burning petroleum and about 45% less than burning coal.  (http://www.naturalgas.org/environment/naturalgas.asp#greenhouse) Combined cycle power generation using natural gas is thus the cleanest source of power available using fossil fuels, and this technology is widely used wherever gas can be obtained at a reasonable cost. Fuel cell technology may eventually provide cleaner options for converting natural gas into electricity, but as yet it is not price-competitive.
Natural gas vehicles
Compressed natural gas (and LPG) is used as a clean alternative to other automobile fuels. As of 2003, the countries with the largest number of natural gas vehicles were Argentina, Brazil, Pakistan, Italy, and India.
Natural gas is supplied to Homes where it is used for such purposes as cooking and heating. CNG is used in rural Homes without connections to piped-in public utility services, or with portable grills.
Natural gas is a major feedstock for the production of ammonia, via the Haber process, for use in fertilizer production.
Natural gas is commercially produced from oil fields and natural gas fields.
Possible future sources
One experimental idea is to use the methane gas that is naturally produced from landfills to supply power to cities. Tests have shown that methane gas could be a financially sustainable power source.
There are plans in Ontario to capture the methane gasses rising from the manure of cattle caged in a factory farm and to use that gas to provide power to a small town.
There is also the possibility that with source separation of organic materials from the waste stream that by using an anerobic digester, the methane can be used to produce useable energy. This can be improved by adding other organic material (plants as well as slaughter house waste) to the digester.
In any form, a strong bad scent (such as ethanethiol) is deliberately added to the otherwise colorless and odorless gas, so that leaks can be detected by the smell before an explosion occurs. In mines, sensors are used and mining apparatus has been specifically developed to avoid ignition sources (e.g. the Davy lamp). Adding scent to natural gas began after the 1937 New London School explosion. The buildup of gas in the school went unnoticed, and killed three hundred students and faculty when it ignited.
Explosions caused by natural gas leaks occur a few times each year. Individual Homes and small businesses are most frequently affected when an internal leak builds up gas inside the structure. Frequently, the blast will be enough to significantly damage a building but leave it standing. In these cases, the people inside tend to have minor to moderate injuries. Occasionally, the gas can collect in high enough quantities to cause a deadly explosion, disintegrating one or more buildings in the process. The gas usually dissipates readily outdoors, but can sometimes collect in dangerous quantities if weather conditions are right. Considering the tens of millions of structures that use the fuel, the risks of using natural gas are very low.
Natural gas is non-toxic, though some gas fields yield 'acid gas' or 'sour gas' containing hydrogen sulfide. This untreated gas is toxic.
Extraction of natural gas (or oil) leads to decrease in pressure in the reservoir. This in turn may lead to subsidence at ground level. Subsidence may affect ecosystems, waterways, sewer and water supply systems, foundations etc.
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