环境

Natural gas is a gaseous fossil fuel consisting primarily of methane but including significant quantities of ethane, propane, butane, and pentane—heavier hydrocarbons removed prior to use as a consumer fuel —as well as carbon dioxide, nitrogen, helium and hydrogen sulfide. It is found in oil fields (associated) either dissolved or isolated in natural gas fields (non associated), 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.

Since natural gas is not a pure product, when non associated gas is extracted from a field under supercritical (pressure/temperature) conditions, it may partially condense upon isothermic depressurizing--an effect called retrograde condensation. The liquids thus formed may get trapped by depositing in the pores of the gas reservoir. One method to deal with this problem is to reinject dried gas free of condensate to maintain the underground pressure and to allow reevaporation and extraction of condensates.

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 often also contains heavier gaseous hydrocarbons such as ethane (C2H6), propane (C3H8) and butane (H10), as well as other sulfur containing gases, in varying amounts, see also natural gas condensate. Natural gas that contains hydrocarbons other than methane is called wet natural gas. Natural gas consisting only of methane is called dry natural gas.

Nitrogen, helium, carbon dioxide and trace amounts of hydrogen sulfide, water and odorants can also be present [2]. Natural gas also contains and is the primary market source of helium. Mercury is also present in small amounts in natural gas extracted from some fields[3]. The exact composition of natural gas varies ween 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 mercaptanthiol, 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 ween 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 101.325 kPaA) or in standard cubic feet (corresponding to 60 °F (16 °C) and 14.73 PSIA). 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 1,030 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. In 2007, a price of $7 per 1,000 cubic feet (28 m³) was typical in the United States. The typical caloric value of natural gas is roughly 1,000 BTU per cubic foot, depending on gas composition. This corresponds to around $7 per million BTU's, or around $7 per gigajoule. In April 2008, the wholesale price was $10 per 1,000 cubic feet (28 m³) ($10/MBTU) [5]. The residential price varies from 50% to 300% more than the wholesale price. At the end of 2007, this was $12-$16 per 1000 ft3 (or MBTU) [6]. 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, retail sales are often 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 (liquified natural gas) and LPG (liquified petroleum gas) is traded in metric tons or mmBTU as spot deliveries. Long term contracts are signed in metric tons. The LNG and LPG is transported by specialized transport ships, as the gas is liquified at cryogenic temperatures. 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
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 +).

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-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. Satellite observation unfortunately shows that some large gas-producing countries still use flaring[12] and venting[13] routinely. The natural gas is used to generate electricity and heat for desalination. 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.....

天然气（Natural Gas）是一种主要由甲烷组成的气态化石。它主要存在于油田和天然气田，也有少量出于煤层。

当非化石的有机物质经过厌氧腐烂时，会产生富含甲烷的气体，这种气体就被称作生物气（沼气）。生物气的来源地包括森林和草地间的沼泽、填埋场、下水道中的淤泥、粪肥，由细菌的厌氧分解而产生。生物气还包括胃肠涨气（例如：），胃肠气最通常来自于牛羊等家畜。

当甲烷散逸到大气层中时，它将是一种直接促使全球变暖愈演愈烈的温室气体。这种飘散的甲烷，就会被视作一种污染物，而不是一种有用的能源。然而，在大气中的甲烷一旦与臭氧发生氧化反应，就会变成二氧化碳和水，因此排放甲烷所导致的温室效应相对短暂。而且就而言，天然气要比煤这类石炭纪产生的二氧化碳要少得多。甲烷的重要生物形式来源是白蚁、反刍动物（如牛羊）和人类对土地的耕种。据估计，这三者的散发量分别是每年15、75和100百万吨（年散发总量约为1亿吨）。