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See also Directory: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 coal beds. 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.

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 sulphur containing gases, in varying amounts, see also natural gas condensate. Natural gas also contains and is the primary market source of helium.

Nitrogen, helium, carbon dioxide and trace amounts of hydrogen sulfide, water and odorants can also be present. Mercury is also present in small amounts in natural gas extracted from some fields. 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 and often referred to as "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 thiols, 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 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 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 and statistics

Combustion of one cubic metre of commercial quality natural gas yields 39 megajoules (10.6 kWh). Equivalently, one cubic foot of natural gas produces 1031 British Thermal Units (BTUs).

In the USA, at retail, natural gas is often sold in units of therms (th); 1 therm = 100,000 BTU. 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.

Storage and transport

The major difficulty in the use of natural gas is transportation and storage. 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.

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, especially since it adds greenhouse gas pollution to the earth's atmosphere. 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-injected 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 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 practicable.

Liquefied Natural Gas (LNG)

When natural gas is cooled to a temperature of approximately -260°F at atmospheric pressure it condenses to a liquid called liquefied natural gas (LNG). One volume of this liquid takes up about 1/600th the volume of natural gas at a stove burner tip. LNG is only about 45% the density of water. LNG is odorless, colorless, non-corrosive, and non-toxic. When vaporized it burns only in concentrations of 5% to 15% when mixed with air. Neither LNG, nor its vapor, can explode in an unconfined environment. Natural gas is composed primarily of methane (typically, at least 90%), but may also contain ethane, propane and heavier hydrocarbons. Small quantities of nitrogen, oxygen, carbon dioxide, sulfur compounds, and water may also be found in "pipeline" natural gas. The liquefaction process removes the oxygen, carbon dioxide, sulfur compounds, and water. The process can also be designed to purify the LNG to almost 100% methane.

Liquefied natural gas or LNG is natural gas that has been processed to remove impurities and heavy hydrocarbons and then condensed into a liquid at almost atmospheric pressure (Maximum Transport Pressure set around 25 KPa) by cooling it to approximately -163 degrees Celsius. LNG is transported by specially designed cryogenic sea vessels and cryogenic road tankers; and stored in specially designed tanks. LNG is about 1/614th the volume of natural gas at standard temperature and pressure (STP), making it much more cost-efficient to transport over long distances where pipelines do not exist. Where moving natural gas by pipelines is not possible or economical, it can be transported by LNG vessels, where the most common tank types are membrane(prismatic), Moss Rosenberg (spheres) or Self-Supporting Prismathic Type.

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.

Basic Facts on LNG

LNG offers an energy density comparable to petrol and diesel fuels and produces less pollution, but its relatively high cost of production and the need to store it in expensive cryogenic tanks have prevented its widespread use in commercial applications.

Conditions required to condense natural gas depend on its precise composition, the market that it will be sold to and the process being used, but typically involve temperatures between -120 and -170 degrees Celsius (pure methane liquefies at -161.6 C) and pressures of between 101 and 6000 kPa (14.7 and 870 lbf/in²). High pressure natural gas that is condensed is then reduced in pressure for storage and shipping.

The density of LNG is roughly 0.41 to 0.5 kg/L, depending on temperature, pressure and composition. In comparison water has a density of 1.0 kg/L.

LNG does not have a specific heat value as it is made from natural gas, which is a mixture of different gases. The heat value depends on the source of gas that is used and the process that is used to liquefy the gas. The higher heating value of LNG is estimated to be 24MJ/L at -164 degrees Celsius. This corresponds to a lower heating value of 21MJ/L.

The natural gas fed into the LNG plant will be treated to remove water, hydrogen sulfide, carbon dioxide and other components that will freeze (e.g., benzene) under the low temperatures needed for storage or be destructive to the liquefaction facility. Purified LNG typically contains more than 90% methane. It also contains small amounts of ethane, propane, butane and some heavier alkanes. The purification process can be designed to give almost 100% methane.

The most important infrastructure needed for LNG production and transportation is an LNG plant consisting of one or more LNG trains, each of which is an independent unit for gas liquefaction. The largest LNG train is the SEGAS Plant in Egypt with a capacity of 5mtpa. Exxon Mobil operating Qatargas stage 2, of which one train has a production ability of 5 million ton per annum (mtpa). Other facilities needed are load-out terminals for loading the LNG onto vehicles, LNG vessels for transportation, and a receiving terminal at the destination for discharge and regasification, where the LNG is reheated and turned into gas. Regasification terminals are usually connected to a storage and pipeline distribution network to distribute natural gas to local distribution companies (LDCs) or Independent Power Plants (IPPs).

In 1964 the UK and France were the LNG buyers under the world’s first LNG trade from Algeria, witnessing a new era of energy. As most LNG plants are located in "stranded" areas not served by pipelines, the costs of LNG treatment and transportation were so huge that development has been slow during the past half century. The construction of an LNG plant costs USD 1-3 billion, a receiving terminal costs USD 0.5-1 billion, and LNG vessels cost USD 0.2-0.3 billion. Compared with the crude oil, the natural gas market is small but mature. The commercial development of LNG is a style called value chain, which means LNG suppliers first confirm the downstream buyers and then sign 20-25 year contracts with strict terms and structures for gas pricing. Only when the customers were confirmed and the development of a greenfield project deemed economically feasible could the sponsors of an LNG project invest in their development and operation. Thus, the LNG business has been regarded as a game of the rich, where only players with strong financial and political resources could get involved. Major international oil companies (IOCs) such as BP, ExxonMobil, Royal Dutch Shell; and national oil companies (NOCs) such as Pertamina, Petronas were active players. Japan, South Korea and Taiwan imported large sums of LNG due to their shortage of energy. In 2002 Japan imported 54 million tons of LNG, representing 48% of the LNG trade around the world that year. Also in 2002, South Korea imported 17.7 million tons and Taiwan 5.33 million tons. These three major buyers purchase approximately 70% of the world's LNG demand.

In recent years, as more players take part in investment, both in downstream and upstream, and new technologies are adopted, the prices for construction of LNG plants, receiving terminals and vessels have fallen, making LNG a more competitive means of energy distribution. The standard price for a 125,000-cubic-meter LNG vessel built in European and Japanese shipyards used to be USD 250 million. When Korean and Chinese shipyards entered the race, increased competition reduced profit margins and improved efficiency, reducing costs 60%. The per-ton construction cost of a LNG liquefaction plant fell steadily from the 1970s through the 1990s, with the cost reduced to approximately 35%.

Due to energy shortage concerns, many new LNG terminals are being contemplated in the United States. Concerns over the safety of such facilities has created extensive controversy in the regions where plans have been created to build such facilities. One such location is in the Long Island Sound between Connecticut and Long Island. Broadwater Energy, an effort between TransCanada Corp. and Shell (A British-Dutch Corporation) wishes to build a LNG terminal in the sound on the New York side. Local politicians including the Suffolk County Executive have raised questions about the terminal. New York Senators Chuck Schumer and Hillary Clinton have both announced their opposition to the project. Several terminal proposals along the coast of Maine have also been met with high levels of resistence and questions.

Trade in LNG

LNG is shipped around the world in specially constructed seagoing vessels. The trade of LNG is completed by signing a sale and purchase agreement (SPA) between a supplier and receiving terminal, and by signing a gas sale agreement (GSA) between a receiving terminal and end-users. Most of the contract terms used to be DEX or Ex Ship, which meant the seller was responsible for the transportation. But with low shipbuilding costs, and the buyer preferring to ensure reliable and stable supply, there are more and more contract terms of FOB, under which the buyer is responsible for the transportation, which is realized by the buyer owning the vessel or signing a long-term charter agreement with independent carriers. The agreements for LNG trade used to be long-term portfolios that were relatively inflexible both in price and volume. If the annual contract quantity is confirmed, the buyer is obliged to take and pay for the product, or pay for it even if not taken, which is called the obligation of take or pay (TOP). In contrast to LNG imported to North America, where the price is pegged to Henry Hub, most of the LNG imported to Asia is pegged to crude oil prices by a formula consisting of indexation called the Japan Crude Cocktail (JCC).

The pricing structure that has been widely used in Asian LNG SPAs is as follows: PLNG = A+B×Pcrude oil, where A refers to a term that represents various non-oil factors, but usually a constant determined by negotiation at a level that can prevent LNG prices from falling below a certain level. It thus varies regardless of oil price fluctuation. Typical figures of ex-ship contracts range from USD 0.7 to 0.9. B is a degree of indexation to oil prices; typical figures are 0.1485 or 0.1558, and Pcrude oil usually denominated in JCC. PLNG and Pcrude oil stand for price of oil in USD per million British Thermal Unit (MMBTU (in the fuel industry, M stands for 1000 and MM for 1 000 000)). With the demand of LNG moving up and down, the price of LNG moves in a "S" curve. With new demand from China, India and US increasing dramatically, and crude oil price skyrocketing, the LNG price is on the rise too.

In the mid 1990s LNG was a buyer's market. At the request of buyers, the SPAs began to adopt some flexibilities on volume and price. The buyers had more upward and downward flexibilities in TOP, and short-term SPAs less than 15 years came into effect. At the same time, alternative destinations for cargo and arbitrage were also allowed. By the turn of the 21st century, the market was again in favor of sellers. Sellers now propose rigid SPAs and would like an association similar to OPEC to be established to protect their interests. It is certain that the competition between sellers and buyers will go on. Receiving terminals exist in several countries (see the list of importing countries in table below; China is expected to move onto the list by 2006), allowing gas imports from other areas (see list of exporting countries in table below).

The United States Department of Energy's Energy Information Administration provides estimates of LNG trade in 2002 as follows:

In 2005, Egyptian NG production outpaced consumption and it joined the LNG exporting countries.

LNG safety and accidents

In its liquid state, LNG is not explosive. For an explosion to occur with LNG, it must first vaporize and then mix with air in the proper proportions (the explosive range is 5% to 15%), and then be ignited afterwards. Serious accidents involving LNG to date are listed below:

• 1944, 20 October. The East Ohio Natural Gas Company experienced a failure of an LNG tank in Cleveland, Ohio. 128 people perished in the explosion and fire. The tank did not have a dike retaining wall, and it was made during World War II, when metal rationing was very strict. The steel of the tank was made with an extremely low amount of nickel, which made the tank brittle when exposed to the extreme cold of LNG, and the tank ruptured, spilling LNG into the city sewer system.
• 1973, February,Staten Island, New York. While repairing the interior of an empty storage tank, a fire started. The pressure increased inside the tank so fast the concrete dome on the tank lifted and then collapsed falling inside the tank and killing the 37 construction workers below.
• 1979, Lusby, Maryland, at the Cove Point LNG facility a pump seal failed, releasing gas vapors, which entered and settled in an electrical conduit. A worker switched off a circuit breaker, igniting the gas vapors, killing a worker and causing heavy damage to the building. National fire codes were changed as a result of the accident.
• 2004, 19 January, Skikda, Algeria. Explosion at Sonatrach LNG liquefaction facility. 27 killed, 80 injured, three LNG trains destroyed, 2004 production was down 76% for the year. A cold hydrocarbon leak occurred introducing the high-pressure steam boiler with gases via a combustion air fan. The explosion inside the boiler fire box precipitated a larger explosion of vapors outside the box.

Seaborne LNG transport tankers (including their loading terminals) have not had a major accident in over 33,000 voyages since maritime inception in 1959. There have, however, been several significant incidents with LNG ships, but with no spills. In addition to accidents, terrorism experts are concerned that intentional sabotage could lead to unprecedented releases, resulting in massive fires and other damaging effects. The latter may include detonations (producing large blast waves) and deflagration-to-detonation transition phenomena. As the Department of Energy notes in its December 2004 report (Sandia National Labs, SAND2004-6258), the available testing data on LNG spills are based on releases of very small size in comparison to releases expected from intentional attacks. Despite intense local opposition, the Federal Energy Regulatory Commission has approved a site permit for an LNG terminal in Fall River, Massachusetts in a densely populated harbor area.

LNG storage

LNG tanks are always of double-wall construction with extremely efficient insulation between the walls. Large tanks are low aspect ratio (height to width) and cylindrical in design with a domed roof. Storage pressures in these tanks are very low, less than 5 psig. Smaller quantities, 70,000 gallons and less, are stored in horizontal or vertical, vacuum-jacketed, pressure vessels. These tanks may be at pressures any where from less than 5 psig to over 250 psig. LNG must be maintained cold (at least below -117°F) to remain a liquid, independent of pressure.

LNG refrigeration

The insulation, as efficient as it is, will not keep the temperature of LNG cold by itself. LNG is stored as a "boiling cryogen," that is, it is a very cold liquid at its boiling point for the pressure it is being stored. Stored LNG is analogous to boiling water, only 470°F colder. The temperature of boiling water (212°F) does not change, even with increased heat, as it is cooled by evaporation (steam generation). In much the same way, LNG will stay at near constant temperature if kept at constant pressure. This phenomenon is called "autorefrigeration". As long as the steam (LNG vapor boil off) is allowed to leave the tea kettle (tank), the temperature will remain constant. If the vapor is not drawn off, then the pressure and temperature inside the vessel will rise. However, even at 100 psig, the LNG temperature will still be only about -200°F.

Compressed natural gas (CNG)

Compressed natural gas (CNG) is natural gas pressurized and stored in welding bottle-like tanks at pressures up to 3,600 psig. Typically, it is same composition of the local "pipeline" gas, with some of the water removed. CNG and LNG are both delivered to the engines as low pressure vapor (ounces to 300 psig). CNG is often misrepresented as the only form natural gas can be used as vehicle fuel. LNG can be used to make CNG. This process requires much less capital intensive equipment and about 15% of the operating and maintenance costs.

Liquid Petroleum Gas (LPG)

Liquid petroleum gas (LPG, and sometimes called propane) is often confused with LNG and vice versa. They are not the same and the differences are significant. Varieties of LPG bought and sold include mixes that are primarily propane, mixes that are primarily butane, and mixes including both propane and butane, depending on the season—in winter more propane, in summer more butane. In some countries LPG is composed primarily of propane (upwards to 95%) and smaller quantities of butane. LPG compared to natural gas has significantly higher heating value and requires a different gas-to-air mixture for good combustion (see: Wobble index).

LPG can be stored as a liquid in tanks by applying pressure alone. While the distribution of LNG requires heavy infrastructure investments (pipelines, etc.), LPG is portable. This fact makes LPG very interesting for developing countries and rural areas. LPG (sometimes called autogas) has also been used as fuel in light duty vehicles for many years. An increasing number of petrol stations around the world offers LPG pumps as well. A final example that should not be forgotten is that the "bottled gas" can often be found under BBQ grills.

Natural gas crisis

Many politicians and prominent figures in North America have spoken publicly about a possible natural gas crisis. This includes former Secretary of Energy Spencer Abraham, former Chairman of the Federal Reserve Alan Greenspan, and 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. Indigenous supply has fallen from 20,570,295 MMcf in 2001 to 19,144,768 MMcf in 2005. Because of the drop in production (exacerbated by the dramatic hit to production that came from Hurricanes Katrina and Rita)and the continuing growth in demand, the price has become so high that many industrial users, mainly in the petrochemical industry, have closed their plants causing loss of jobs. Greenspan has suggested that a solution to the natural gas crisis is the import of LNG.

This solution is both capital intensive and politically charged due to the public perception that LNG terminals are explosive risks, especially in the wake of the 9/11 terrorist attacks in the United States. The U.S. Department of Homeland Security is responsible for maintaining their security.

New or expanded LNG terminals create tough infrastructure problems and require high capital spending. LNG terminals require a very spacious—at least 40 feet (12.2 m) deep — harbor, as well as being sheltered from wind and waves. These "suitable" sites are thus deep in well-populated seaports, which are also burdened with right-of-way concerns for LNG pipelines, or conversely, required to also host the LNG expansion plant facilities and end use (petrochemical) plants amidst the high population densities of major cities, with the associated fumes, multiple serious risks to safety.

Typically, to attain "well-sheltered" waters, suitable harbor sites are well up rivers or estuaries, which are unlikely to be dredged deep enough. Since these very large vessels must move slowly and ponderously in restricted waters, the transit times to and from the terminal become costly, as multiple tugboats and security boats shelter and safeguard the large vessels. Operationally, LNG tankers are (for example, in Boston) effectively given sole use of the harbor, forced to arrive and depart during non-peak hours, and precluded from occupying the same harbor until the first is well-departed. These factors increase operating costs and make capital investment less attractive.

To substantially increase the amount of LNG used to supply natural gas to North America, not only must "re-gasification" plants be built on North American shores -- difficult for the reasons stated above -- someone also must put substantial, new liquefaction stations in Indonesia, the Middle East, and Africa, in order to concentrate the gas generally associated with oil production in those areas. A substantial expansion of the fleet of LNG carriers also must occur, to move the huge amount of fuel needed to make up for the coming shortfall in Northeast America.

Uses

Power generation

Natural gas is 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. Natural gas burns cleaner than other fossil fuels, such as oil and coal, and produces less greenhouse gas 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 [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, the natural gas supply 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 be exhausted around the year 2085.

Hydrogen

Natural gas can be used to produce hydrogen that can be used 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, and India. [2] 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).

Liquified petroleum gas (a propane and butane blend) is also used to fuel vehicles, but it is only suitable for gasoline engines. 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, and heating/cooling. 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.

Fertilizer

Natural gas is a major feedstock for the production of ammonia, via the Haber process, for use in fertilizer production.1,2,and..3

Other

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

Sources

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 largest two natural gas fields are probably South Pars Gas Field in Iran and Urengoy gas field in Russia, with reserves on the order of 10¹³ m³. See also List of natural gas fields. Qatar also has 25 trillion cubic meters of natural gas (5% of the world's proven supply), enough to last 250 years at current production levels.

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 is 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.

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.

Possible future sources

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.

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. Other non-methane components may also be removed in order to meet emissions 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 unmerchantable parts of plants and manure. Biogas can also be produced by separating organic materials from waste that otherwise goes to landfills, which 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 are used to 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.

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 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 (H₂S). This untreated gas is toxic. Scrubbers which remove acidic gaseous components can be used to remove hydrogen sulfide from natural gas. 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.

Related concepts

• LPG: Liquified petroleum gas

External articles and references

General

• Oil and Gas Directory
• Natural Gas industry news
• Natural Gas Generators
• India: How New Delhi used CNG to ease pollution
• Pollutant chemical pollutant chemical that can worsen both classical allergy and chemical sensitivity.
• Wikipedia contributors, Wikipedia: The Free Encyclopedia. Wikimedia Foundation. <http://en.wikipedia.org>.
• WVU's AFVTP - Propane Review, West Virginia University
• Using Gas Geochemistry to Assess Mercury Risk, OilTracers, 2006
• U.S. Natural Gas Marketed Production 1900 - 2005 (MMcf) (in English). U.S. Department of Energy].
• California Energy Commission Frequently Asked Questions About LNG
• The Global Liquefied Natural Gas Market: Status and Outlook @ the EIA - DOE
• Liquefied Natural Gas in the US: Federal Energy Regulatory Commission (FERC)
• Alternative Fuel Vehicle Training From the National Alternative Fuels Training Consortium.
• Ventura LNG Task Force A local community group in Ventura CA working on LNG issues.
• LNG Law Blog Your Daily Online Source for Liquefied Natural Gas
• [3] Poten & Partners: A Collection of Articles Relating to LNG
• LNG transportation
  • Transportation of LNG in Small LNG ships

Natural gas vehicles

• International Natural Gas Vehicle Statistics
• Alternative Fuel Vehicle Training From the National Alternative Fuels Training Consortium.
• IANGV - International Association for Natural Gas Vehicles
• Northeast Sustainable Energy Association (PDF)

North America

• American Gas Association
• Appalachian Basin Natural Gas Production Information
• What is Compressed Natural Gas?
• Could CNG work in America?
• Natural Gas Supply Association
• Institute of Gas Technology
• Manufactured and Natural Gas Industry (EH.Net Encyclopedia of Economic History)

See also

• Directory:Natural Gas

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- PESWiki home page