1、附录一：英文原文 原文 ： Natural gas From Wikipedia, the free encyclopedia Jump to: navigation, search For other uses, see Natural gas (disambiguation). Natural gas is a naturally occurring hydrocarbon gas mixture consisting primarily of methane, with up to 20 percent1 concentration of other hydrocarbons (usua
2、lly ethane) as well as small amounts of impurities such as carbon dioxide. Natural gas is widely used and is an important energy source in many applications including heating buildings, generating electricity, providing heat and power to industry and vehicles and is also a feedstock in the manufactu
3、re of products such as fertilizers. Natural gas is found in deep underground natural rock formations or associated with other hydrocarbon reservoirs, in coal beds, and as methane clathrates. Most natural gas was created over time by two mechanisms: biogenic and thermogenic. Biogenic gas is created b
4、y methanogenic organisms in marshes, bogs, landfills, and shallow sediments. Deeper in the earth, at greater temperature and pressure, thermogenic gas is created from buried organic material.2 Before natural gas can be used as a fuel, it must undergo processing to clean the gas and remove impurities
5、 including water in order to meet the specifications of marketable natural gas. The by-products of processing include ethane, propane, butanes, pentanes, and higher molecular weight hydrocarbons, hydrogen sulphide (which may be converted into pure sulfur), carbon dioxide, water vapor, and sometimes
6、helium and nitrogen. In the 19th century, natural gas was usually obtained as a byproduct of producing oil, since the small, light gas carbon chains came out of solution as the extracted fluids underwent pressure reduction from the reservoir to the surface, similar to uncapping a bottle of soda pop
7、where the carbon dioxide effervesces. Unwanted natural gas was a disposal problem in the active oil fields. If there was not a market for natural gas near the wellhead it was virtually valueless since it had to be piped to the end user. In the 19th century and early 20th century, such unwanted gas w
8、as usually burned off in the oil fields. Today, unwanted gas (or stranded gas without a market) associated with oil extraction often is returned to the reservoir with injection wells while awaiting a possible future market or to repressurize the formation, which can enhance extraction rates from oth
9、er wells. In regions with a high natural gas demand (such as the US), pipelines are constructed when economically feasible to move the gas from the wellsite to the end consumer. Another possibility is to export the natural gas as a liquid. Gas-to-liquids (GTL) is a developing technology that convert
10、s stranded natural gas into synthetic gasoline, diesel, or jet fuel through the Fischer-Tropsch process developed during World War II by Germany. Such fuel can be transported to users through conventional pipelines and tankers. Proponents claim GTL burns cleaner than comparable petroleum fuels. Most
11、 major international oil companies are in an advanced stage of GTL production, with a world-scale (140,000 barrels (22,000 m3) a day) GTL plant in Qatar scheduled to be in production before 2010.dated info Natural gas can be associated (found in oil fields) or non-associated (isolated in natural gas
12、 fields), and is also found in coal beds (as coalbed methane). It sometimes contains significant amounts of ethane, propane, butane, and pentane heavier hydrocarbons removed for commercial use prior to the methane being sold as a consumer fuel or chemical plant feedstock. Non-hydrocarbons such as ca
13、rbon dioxide, nitrogen, helium (rarely), and hydrogen sulfide must also be removed before the natural gas can be transported.3 Natural gas is commercially extracted from oil fields and natural gas fields. Gas extracted from oil wells is called casinghead gas or associated gas. The natural gas indust
14、ry is extracting gas from increasingly more challenging resource types: sour gas, tight gas, shale gas, and coalbed methane. The worlds largest proven gas reserves are located in Russia, with 4.7571013 m (1.681015 cubic feet). With the Gazprom company, Russia is frequently the worlds largest natural
15、 gas extractor. Major proven resources (in billion cubic meters) are world 175,400 (2006), Russia 47,570 (2006), Iran 26,370 (2006), Qatar 25,790 (2007), Saudi Arabia 6,568 (2006) and United Arab Emirates 5,823 (2006). It is estimated that there are about 900 trillion cubic meters of unconventional
16、gas such as shale gas, of which 180 trillion may be recoverable.4 In turn, many studies from MIT, Black & Veatch and the DOE - see natural gas - will account for a larger portion of electricity generation and heat in the future.5 The worlds largest gas field is Qatars offshore North Field, estimated
17、 to have 25 trillion cubic meters6 (9.01014cubic feet) of gas in place enough to last more than 420 yearscitation needed at optimum extraction levels. The second largest natural gas field is the South Pars Gas Field in Iranian waters in the Persian Gulf. Located next to Qatars North Field, it has an
18、 estimated reserve of 8 to 14 trillion cubic meters7 (2.81014 to 5.01014 cubic feet) of gas. Because natural gas is not a pure product, as the reservoir pressure drops when non-associated gas is extracted from a field under supercritical (pressure/temperature) conditions, the higher molecular weight
19、 components may partially condense upon isothermic depressurizing an effect called retrograde condensation. The liquid thus formed may get trapped as the pores of the gas reservoir get deposited. One method to deal with this problem is to re-inject dried gas free of condensate to maintain the underg
20、round pressure and to allow re-evaporation and extraction of condensates. More frequently, the liquid condenses at the surface, and one of the tasks of the gas plant to collect this condensate. The resulting liquid is called natural gas liquid (NGL) and has commercial value. Town gas Town gas, a syn
21、thetically produced mixture of methane and other gases, mainly the highly toxic carbon monoxide, is used in a similar way to natural gas and can be produced by treating coal chemically. This is a historical technology, not usually economically competitive with other sources of fuel gas today. But there are still some specific cases where it is the best option and it may be so into the future. Most town gashouses located in the eastern US in the late 19th and early 20th
