'Natural gas' is a
gaseous
fossil fuel consisting primarily of
methane but including significant quantities of
ethane,
butane,
propane,
carbon dioxide,
nitrogen,
helium and
hydrogen sulfide.
[1] It is found in
oil fields and
natural gas fields, and in
coal beds (as
coalbed methane). 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,
marshes, and
landfills (see
landfill gas), as well as
sewage sludge and
manure by way of
anaerobic digesters, in addition to
enteric fermentation particularly in
cattle. Natural gas is often informally referred to as simply 'gas', especially when compared to other energy sources such as electricity. Before natural gas can be used as a fuel, it must undergo extensive
processing to remove almost all materials other than methane. The by-products of that processing include ethane, propane, butanes,
pentanes and higher molecular weight hydrocarbons, elemental sulfur, and sometimes
helium and nitrogen.
Chemical composition
The primary component of natural gas is
methane (
CH4), the shortest and lightest
hydrocarbon molecule. It also contains heavier gaseous hydrocarbons such as
ethane (
C2H6),
propane (
C3H8) and
butane (
C4H10), as well as other
sulfur containing gases, in varying amounts, see also
natural gas condensate. Natural gas also contains and is the primary market source of
helium.
| 'Component' | 'wt. %' |
|---|
| Methane (CH4) | 70-90 |
| Ethane (C2H6) | 5-15 |
| Propane (C3H8) and Butane (C4H10) | < 5 |
| CO2, N2, H2S, etc. | balance |
Nitrogen, helium, carbon dioxide and trace amounts of hydrogen sulfide, water and
odorants can also be present
[2].
Mercury is also present in small amounts in natural gas extracted from some fields
[3]. The exact composition of natural gas varies between gas fields.
Organosulfur compounds and
hydrogen sulfide are common contaminants which must be removed prior to most uses. Gas with a significant amount of sulfur impurities, such as
hydrogen sulfide, is termed
sour gas; gas with sulfur or carbon dioxide impurities is
acid gas. Processed natural gas that is available to end-users is tasteless and odorless, however, before gas is distributed to end-users, it is odorized by adding small amounts of odorants (mixtures of
t-butyl mercaptan,
isopropyl mercaptan,
tetrahydrothiophene,
dimethyl sulfide and other sulfur compounds), to assist in
leak detection. Processed natural gas is, in itself, harmless to the human body, 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.
Natural gas can also be hazardous to life and property through an
explosion. Natural gas is lighter than air, and so tends to escape 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 buildings. 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 non-existent, due to the escaping nature of the gas, and the need to maintain concentrations between 5% and 15% to trigger explosions.
Energy content, statistics and pricing
Quantities of natural gas are measured in
normal cubic meters (corresponding to 0°C at 1
atm) or in
standard cubic feet (corresponding to 60°F and 30
inHg).
The
gross heat of combustion of one
normal cubic meter of commercial quality natural gas is around 39
megajoules (≈10.8
kWh), but this can vary by several percent.
In
US units, one
standard cubic foot of natural gas produces around 1,000
British Thermal Units (BTUs). The actual heating value when the water formed does not condense is the
net heat of combustion and can be as much as 10% less.
The price of natural gas varies greatly depending on location and type of consumer, but
as of 2006 a price of $10 per 1000 cubic feet is typical in the United States. This corresponds to around $10 per million BTU's, or around $10 per
gigajoule. Natural gas in the United States is traded as a
futures contract on the
New York Mercantile Exchange. Each contract is for 10,000 MMBTU (
gigajoules), or 10 billion BTU's. Thus, if the price of gas is $10 per million BTU's on the NYMEX, the contract is worth $100,000. In the United States, at retail, natural gas is often sold in units of
therms (th); 1 therm = 100,000 BTU.
Gas meters measure the volume of gas used, and this is converted to therms by multiplying the volume by the energy content of the gas used during that period, which varies slightly over time. Wholesale transactions are generally done in
decatherms (Dth), or in thousand decatherms (MDth), or in million decatherms (MMDth). A million decatherms is roughly a billion cubic feet of natural gas.
Natural gas is also traded as a commodity in Europe, principally at the United Kingdom
NBP and related European hubs, such as the
TTF in the Netherlands.
In the rest of the world, LNG and LPG is traded in metric tons or mmBTU as spot deliveries. Long term contracts are signed in metric tons - and to convert from one system to the other requires should better be described here, than a very isolated market. A cubic foot is a volumetric measure, MT is weight. The LNG and LPG is transported by special ships/containers, as the gas is liquified - LPG cryonic. The specification of each LNG/LPG cargo will usually contain the energy content, but this information is in general not available to the public.
Natural gas processing
A natural gas processing plant
The image below is a schematic
block flow diagram of a typical natural gas processing plant. It shows the various unit processes used to convert raw natural gas into sales gas pipelined to the end user markets.
The block flow diagram also shows how processing of the raw natural gas yields byproduct sulfur, byproduct ethane, and natural gas liquids (NGL) propane, butanes and natural gasoline (denoted as
pentanes +).
[4][5][6][7][8]
Schematic flow diagram of a typical natural gas processing plant
Storage and transport
The major difficulty in the use of natural gas is
transportation and
storage because of its low density. 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 representing colder areas to speak publicly of potential shortages.
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 facilities or into a pipeline.
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 illegal in many countries. Additionally, companies now recognize that value for the gas may be achieved with LNG, CNG, or other transportation methods to end-users in the future. The gas is now re-ed 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
desalinization. Similarly, some landfills 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 practicable.
With 15 nations accounting for 84% of the world-wide production, access to natural gas has become a significant factor in international economics and politics. In this respect, control over the pipelines is a major strategic factor.
[9]
Use
Power generation
Natural gas is a major source of
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. Natural gas burns cleaner than other
fossil fuels, such as oil and coal, and produces less carbon dioxide per unit energy released. 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.
[10] 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, the natural gas supply is expected 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 could be exhausted around the year 2085.
Hydrogen
Natural gas can be used to produce
hydrogen, with one common method being the
hydrogen reformer. Hydrogen has various applications: it is a primary feedstock for the chemical industry, a hydrogenating agent, an important commodity for oil refineries, and a fuel source in
hydrogen vehicles.
Natural gas vehicles
Compressed natural gas (
methane) is used as a clean alternative to other
automobile fuels such as
gasoline (petrol) and
diesel.
As of 2005, the countries with the largest number of
natural gas vehicles were
Argentina,
Brazil,
Pakistan,
Italy,
India and
Bangladesh.
[11] The energy efficiency is generally equal to that of gasoline engines, but lower compared with modern diesel engines. Benzine vehicles converted to run on gas suffer of the low-compression ratio their engines have, resulting in a cropping of delivered power while running on natural gas (10%-15%). CNG factory-made engines, however, use a higher compression ratio, due to the high number of
octane (120-130) of this fuel.
Liquified petroleum gas (a
propane and
butane blend) is also used to fuel vehicles. LPG and CNG vehicle fuel systems are not compatible. CNG also requires higher pressure tanks which are typically much heavier than those used for LPG.
Residential domestic use
Natural gas is supplied to homes, where it is used for such purposes as
cooking in natural gas-powered ranges and/or ovens, natural gas-heated
clothes dryers,
heating/
cooling and
central heating. Home or other building heating may include boilers,
furnaces, and
water heaters. CNG is used in
rural homes without connections to
piped-in
public utility services, or with portable
grills. However, due to CNG being less economical than LPG, LPG (Propane) is the dominant source of rural gas.
Fertilizer
Natural gas is a major feedstock for the production of
ammonia, via the
Haber process, for use in
fertilizer production.
Aviation
Russian aircraft manufacturer
Tupolev is currently running a development program to produce LNG- and
hydrogen-powered aircraft.
[12] The program has been running since the mid-
1970s, and seeks to develop LNG and hydrogen variants of the
Tu-204 and
Tu-334 passenger aircraft, and also the
Tu-330 cargo aircraft. It claims that at current market prices, an LNG-powered aircraft would cost 5,000
roubles less to operate per ton, roughly equivalent to 60%, with considerable reductions to
carbon monoxide,
hydrocarbon and
nitrogen oxide emissions.
Other
Natural gas is also used in the manufacture of
fabrics,
glass,
steel,
plastics,
paint, and other products.
Sources
Natural gas production by country (countries in brown and then red have the largest production)
Natural gas
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 natural gas industry is producing gas from increasingly more challenging
resource types: sour gas, tight gas,
shale gas and
coalbed methane.
The world's largest gas field by far is
Qatar's offshore
North Field, estimated to have 25 trillion cubic metres
[13] (900 trillion cubic feet) of gas in place - enough to last more than 200 years at optimum production levels. The second largest natural gas field is the
South Pars Gas Field in
Iranian waters in the
Persian Gulf. Connected to Qatar's North Field, it has estimated reserves of 8 to 14 trillion cubic metres
[14] (280-500 trillion cubic feet) of gas; see
List of natural gas fields.
Town gas
Town gas is a mixture of methane and other gases that can be used in a similar way to natural gas and can be produced by treating
coal chemically. This is a historic technology, still used as 'best solution' in some local circumstances, although coal gasification is not usually economic at current gas prices. However, depending upon infrastructure considerations, it remains a future possibility.
Biogas
Methanogenic archaea are responsible for all biological sources of methane, some in symbiotic relationships with other life forms, including
termites,
ruminants, and cultivated crops. Methane released directly into the atmosphere would be considered a
pollutant, however, methane in the atmosphere is oxidised, producing carbon dioxide and water. Methane in the atmosphere has a half life of seven years, meaning that every seven years, half of the methane present is converted to carbon dioxide and water.
U.S. Natural Gas Production 1900 - 2005 Source: EIA
Future sources of
methane, the principal component of natural gas, include landfill gas, biogas and methane hydrate. Biogas, and especially landfill gas, are already used in some areas, but their use could be greatly expanded. Landfill gas is a type of biogas, but biogas usually refers to gas produced from organic material that has not been mixed with other waste.
Landfill gas is created from the decomposition of waste in
landfills. If the gas is not removed, the pressure may get so high that it works its way to the surface, causing damage to the landfill structure, unpleasant odor, vegetation die-off and an
explosion hazard. The gas can be vented to the atmosphere,
flared or burned to produce
electricity or
heat. Experimental systems were being proposed for use in parts
Hertfordshire, UK and
Lyon in France.
Once
water vapor is removed, about half of landfill gas is methane. Almost all of the rest is
carbon dioxide, but there are also small amounts of
nitrogen,
oxygen and
hydrogen. There are usually trace amounts of
hydrogen sulfide and
siloxanes, but their concentration varies widely. Landfill gas cannot be distributed through natural gas pipelines unless it is cleaned up to the same quality. It is usually more economical to combust the gas on site or within a short distance of the landfill using a dedicated pipeline. Water vapor is often removed, even if combusting the gas on site. If low temperatures condense out the water from the gas,
siloxanes can be lowered as well because they tend to condense out with the water vapour. Other non-methane components may also be removed in order to meet
emission standards, to prevent fouling of the equipment or for environmental considerations. Co-firing landfill gas with natural gas improves combustion, which lowers emissions.
Biogas is usually produced using agricultural waste materials, such as otherwise unusable parts of plants and
manure. Biogas can also be produced by separating
organic materials from waste that otherwise goes to landfills. This is more efficient than just capturing the landfill gas it produces. Using materials that would otherwise generate no income, or even cost money to get rid of, improves the profitability and energy balance of biogas production.
Anaerobic lagoons produce biogas from manure, while biogas reactors can be used for manure or plant parts. Like landfill gas, biogas is mostly methane and carbon dioxide, with small amounts of nitrogen, oxygen and hydrogen. However, with the exception of pesticides, there are usually lower levels of contaminants.
Hydrates
A speculative source of enormous quantities of methane is from
methane hydrate, found under sediments in the oceans. However,
as of 2006 no technology has been developed to recover it economically.
Safety
In any form, a minute amount of
odorant such as t-butyl
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, thiophane 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. Odorants are considered non-toxic in the extremely low concentrations occurring in natural gas delivered to the end user.
In
mines, where methane seeping from rock formations has no odor,
sensors are used, and mining apparatuses have 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. However, 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 (H
2S). This untreated gas is
toxic.
Amine gas treating, an industrial scale process which removes
acidic gaseous components, is often used to remove hydrogen sulfide from natural gas.
[15]
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.
Natural Gas heating systems are the leading cause of carbon monoxide deaths in the United States, according to the U.S. Consumer Product Safety Commission. When a natural gas heating system malfunctions, it produces odorless carbon monoxide. With no fumes or smoke to give warning, poisoning victims are easily asphyxiated by the carbon monoxide.
See also
★
abiogenic petroleum origin
★
Carbon dioxide (CO
2)
★
Compressed natural gas (CNG)
★
Fuel station
★
Future energy development
★
Hydrogen vehicle
★
Liquefied natural gas (LNG)
★
Natural gas storage
★
List of North American natural gas pipelines
★
Natural gas prices
★
Natural gas processing
★
North American natural gas crisis
★
Steam reforming
★
World energy resources and consumption
External links
★
American Gas Association - distributor trade group
★
CERA - Energy research group's Natural Gas Research
★
DOE/EIA Natural Gas Data Page
★
Economic History - Manufactured and Natural Gas Industry
★
Interstate Natural Gas Association of America - pipeline trade group
★
Natural Gas Supply Association - producer trade group
★
Oil and Gas Directory
★
Oil and Gas Eurasia - Oil and Gas News, Technology and Analysis
★
Platt's - Natural Gas Industry News
★
Top 20 Greatest Natural Gas Reserves by Country
Natural gas vehicles
★
CNG Forum
★
International Association for Natural Gas Vehicles
★
National Alternative Fuels Training Consortium - Alternative Fuel Vehicle Training
★
Northeast Sustainable Energy Association - Natural Gas Factsheet (PDF)
References
1. Natural gas overview
2. WVU's AFVTP - Propane Review, West Virginia University
3. Using Gas Geochemistry to Assess Mercury Risk, OilTracers, 2006
4. ''Natural Gas Processing: The Crucial Link Between Natural Gas Production and Its Transportation to Market''
5. ''Example Gas Plant''
6. ''From Purification to Liquefaction Gas Processing''
7. ''Feed-Gas Treatment Design for the Pearl GTL Project''
8. ''Benefits of integrating NGL extraction and LNG liquefaction''
9. The Contours of the New Cold War
10. Natural Gas and the Environment
11. International Statistics (IANG website page)
12. PSC Tupolev - Development of Cryogenic Fuel Aircraft
13. Background note: Qatar
14. Pars Special Economic Energy Zone
15. NaturalGas.org - Processing Natural Gas
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