Eighty-six percent of the coal used in the United States is burned by electric power plants [1] to produce electricity. When burned, coal generates energy in the form of heat. In a power plant that uses coal as fuel, this heat converts water into steam, which is pressurized to spin the shaft of a turbine. This spinning shaft [2] drives a generator that converts the mechanical energy of the rotation into electric power.

Coal is also used in the steel industry. The steel industry uses coal by first heating it and converting it into coke [3], a hard substance consisting of nearly pure carbon. The coke is combined with iron ore [4] and limestone [5]. Then the mixture is heated to produce iron. Other industries use different coal gases given off during the coke-forming process [6] to make fertilizers, solvents [7], medicine, pesticides, and other products.

Fuel companies convert coal into easily transportable gas [8] or liquid fuels [9]. Coal-based vapor fuels [10] are produced through the process of gasification [11]. Gasification may be accomplished either at the site of the coalmine [12] or in processing plants [13]. In processing plants, the coal is heated in the presence of steam and oxygen to produce synthesis gas [14], a mixture of carbon monoxide [15], hydrogen, and methane [16] used directly as fuel or refined into cleaner-burning gas [17].

On-site gasification [18] is accomplished by controlled, incomplete burning of an underground coal bed while adding air and steam. To do this, workers ignite the coal bed, pump air and steam underground into the burning coal, and then pump the resulting gases from the ground. Once the gases are withdrawn, they may be burned to produce heat or generate electricity. Or they may be used in synthetic gases to produce chemicals or to help create liquid fuels .

Liquefaction [19] processes convert coal into a liquid fuel that has a composition similar to that of crude petroleum [20] Liquefaction. Coal can be liquefied either by direct or indirect processes. However, because coal is a hydrogen-deficient hydrocarbon [21], any process used to convert coal to liquid or other alternative fuels [22] must add hydrogen. Four general methods are used for liquefaction: (1) pyrolysis [23] and hydrocarbonization [24], in which coal is heated in the absence of air or in a stream of hydrogen; (2) solvent extraction [25], in which coal hydrocarbons are selectively dissolved and hydrogen is added to produce the desired liquids; (3) catalytic liquefaction [26], in which hydrogenation [27] takes place in the presence of a catalyst; and (4) indirect liquefaction, in which carbon monoxide and hydrogen are combined in the presence of a catalyst.

NOTES TO THE TEXT

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[2] spinning shaft����ת��

[3] coke����̿

[4] iron ore������ʯ

[5] limestone��ʯ����

[6] coke-forming process����̿�γɹ���

[7] solvents���ܼ�

[8] easily tr

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