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Natural gas

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LPG-Gas stove
Many stoves use natural gas.
Natural gas, commonly referred to as gas, is a gaseous fossil fuel consisting primarily of methane. It is found in oil fields and natural gas fields, and in smaller quantities, coal beds. Particular interest to engineering is its use in large quantities for electrical power generation.

Other sourcesEdit

When methane-rich gases are produced by the anaerobic decay of non-fossil organic material, these are referred to as biogas. Sources of biogas include swamps, which produce swamp gas; marshes, which produce marsh gas; landfills, which produce landfill gas, as well as sewage sludge and manure, by way of anaerobic digesters, in addition to Enteric fermentation particularly in cattle.

PropertiesEdit

Natural gas is tasteless and odorless.

Methane is an extremely efficient greenhouse gas which may contribute to enhanced global warming when free in the atmosphere, and such free methane, would then be considered a pollutant rather than a useful energy resource. However, methane in the atmosphere is oxidised, producing carbon dioxide and water, so that the greenhouse effect of released methane is relatively short-lived. Also, natural gas, when burned, produces much less carbon dioxide than more carbonaceous fuel sources, such as coal. Significant biological sources of methane are termites, ruminants and cultivation. Estimated emissions are 15, 75 and 100 million tons per year respectively.

Chemical composition and energy contentEdit

Chemical compositionEdit

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 sulphur containing gases, in varying amounts, see also natural gas condensate.

Organosulfur compounds and hydrogen sulfide (H2S see acid gas) are common contaminants, which must be removed prior to most uses. Gas with a significant amount of sulfur impurities is termed "sour".

Energy contentEdit

Combustion of one cubic metre of commercial quality natural gas yields 38 megajoules (10.6 kWh). Equivalently, one cubic foot of natural gas produces 1031 British Thermal Units (BTUs).

Supply to end usersEdit

Since natural gas is tasteless and odorless, before gas is distributed to end-users, it is odorized by adding thiols, to assist in leak detection. Natural gas is, in itself, harmless to the human body -- unlike carbon monoxide, for instance, it is not a poison. However, natural gas is a simple asphyxiant and can kill if it displaces air to the point where the oxygen content will not support life.

HazardousEdit

Natural gas can also be hazardous to life and property through an explosion. Natural gas is lighter than air, and so tends to dissipate into the atmosphere. But when natural gas is confined, such as within a house, gas concentrations can reach explosive mixtures and, if ignited, result in blasts that could destroy buldings. Methane has a Lower Explosive Limit of 5% in air, and an Upper Explosive Limit of 15%.

Explosive concerns with compressed natural gas used in vehicles are almost nonexistent, due to the escaping nature of the gas, and the need to maintain concentrations between 5% and 15% to trigger explosions.

Storage and transportEdit

180px-Polyethylene gas main
Polyethylene gas main being laid in a trench.

The major difficulty in the use of natural gas is transportation and storage. Natural gas pipelines are economical, but are impractical across oceans.

Even the onland pipe lines of steel have to be provided with galvanic protection against corrossion and suitable electrical insulation covering increasing the cost to some extent.

LNG carriers can be used to transport liquefied natural gas (LNG) across oceans, while tank trucks can carry liquefied or compressed natural gas (CNG) over shorter distances. They may transport natural gas directly to end-users or to distribution points, such as pipelines for further transport. These may have a higher cost requiring additional facilities for liquefaction or compression at the production point, and then gasification or decompression at end-use facitilies or into a pipeline.

For electric power generating stations who consume enormous quantities the gas is supplied direct from gas wells with on line processing but without any storage at sending end. As an on line processing is involved gas is expected to be used non stop at generating stations without any break.

When gas is drawn from gas wells, the sending through pipe line involves reduction in gas pressure with consequent seperation of condensate (that is liquid petroleum). This is however stored in limited quantities in suitable tanks.

FlaringEdit

In the past, the natural gas which was recovered in the course of recovering petroleum could not be profitably sold, and was simply burned at the oil field (known as flaring). This wasteful practice is now almost extinct, the same being reused. One method is to re-inject back into the formation for later recovery. This also assists oil pumping by keeping underground pressures higher. In Saudi Arabia, in the late 1970s, a "Master Gas System" was created, ending the need for flaring. The natural gas is used to generate electricity and heat for desalinisation. Similarly, some land-fills that also discharge methane gases have been set-up to capture the methane and generate electricity.

Natural gas is often stored in underground caverns formed inside depleted gas reservoirs from previous gas wells, salt domes, or in tanks as liquefied natural gas. The gas is injected during periods of low demand and extracted during periods of higher demand. Storage near the ultimate end-users helps to best meet volatile demands, but this may not always be practical.

UsesEdit

Power generationEdit

Natural gas is important as a major source for electricity generation through the use of gas turbines and steam turbines. 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 fewer 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. [1] 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. Also, natural gas is said to peak around the year 2030, 20 years after the peak of oil. It is also projected that the world's supply of natural gas should finish around the year 2085.

Hydrogen manufactureEdit

Natural gas can be used to produce hydrogen that can be used in hydrogen vehicles.

Fertiliser manufactureEdit

Natural gas is a major feedstock for the production of ammonia, via the Haber process, for use in fertilizer production.

Vehicle fuelEdit

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.

The energy efficiency is generally equal to that of gasoline engines, but lower compared with modern diesel engines, partially due to the fact that natural gas engines function using the Otto Cycle, but research is on its way to improve the process (Westport Cycle).

ResidentialEdit

Natural gas is supplied to homes where it is used for such purposes as cooking and heating/cooling. CNG is used in rural homes without connections to piped-in public utility services, or with portable grills.

OtherEdit

Natural gas is also used in the manufacture of fabrics, glass, steel, plastics, paint, and other products.

Future sourcesEdit

300px-Natural gas production world
Natural gas production by country (countries in brown and then red have the largest production)

Natural gas is commercially produced from oil fields and natural gas fields. Gas produced from oil wells is called casinghead gas or associated gas. The biggest natural gas field is located in Urengoy gas field, Russia, with a reserve of 1013 m³. See also List of natural gas fields.

Town gas is a mixture of methane and other gases which can be used in a similar way to natural gas and can be produced by treating coal chemically. This a historic technology still used as 'best solution' in some local circumstances, although coal gasification is not usually economic at current gas prices, depending upon infrastructure considerations.

Possible future sourcesEdit

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 biogas, methane gases 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 the source separation of organic materials from the waste stream that by using an anaerobic digester, the methane can be used to produce usable energy. This can be improved by adding other organic material (plants as well as slaughter house waste) to the digester.

A speculative source of enormous quantities of methane is from methane hydrate, found under sediments in the oceans. At present (2006), no technology has been developed to recover this source of energy economically.

SafetyEdit

In any form, a minute amount of odorant such as methyl mercaptan, with a rotting-cabbage-like smell, is added to the otherwise colorless and odorless gas, so that leaks can be detected before a fire or explosion occurs. Sometimes a related compound, ethyl mercaptan is used, with a rotten-egg smell. Adding odorant to natural gas began in the United States after the 1937 New London School explosion. The buildup of gas in the school went unnoticed, killing three hundred students and faculty when it ignited.

Although concentrated thiols are extremely toxic, it is considered non-toxic in the extremely low concentrations in which it occurs in natural gas delivered to the end user. For example, a safe exposure level to ethyl mercaptan at 5 parts per million over an eight-hour period has been established by the American Congress of Government and Industrial Hygienists (ACGIH). Actual concentrations used by gas companies are on the order of 5 parts per billion (5 parts in 109), one-thousandth the maximum safe limit.

In mines, where methane seeping from rock formations has no odor, sensors are used, and mining apparatus has been specifically developed to avoid ignition sources, e.g., the Davy lamp.

Explosions caused by natural gas leaks occur a few times each year. Individual homes, small businesses and boats 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. Also, considering the tens of millions of structures that use the fuel, the individual risk of using natural gas is very low.

Some gas fields yield 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.

ReferencesEdit

See alsoEdit

External linksEdit

Natural Gas DirectoriesEdit

Natural gas vehiclesEdit

North AmericaEdit

South AsiaEdit

Pollution and allergyEdit

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