Coal formed from the remains of plants that probably grew in swamplike conditions. There are four main classes of coal based on the ratio of fixed carbon, volatile matter, and moisture content. Of the estimated reserves of 550 billion tons, more than 400 billion tons can be mined by known methods. At the present rate of consumption these reserves would last 300 years. The most important use of coal in the United States is for electric-power generation. There are three major types of pollutants caused from the burning of coal. Approaches to the problem of burning coal cleanly to help eliminate pollutants include removing offending contaminants before the coal is burned by gasification, liquefaction, or solvent-refining; burn it more cleanly; or clean the coal smoke after i has been burned by using precipitators, scrubbers, or filter bags. A lead time ranging from 7 to 10 years is needed to bring coal to market. Texas lignite is currently strip-mined from the Wilcox Group in east and central Texas and includes estimated near-surface reserves of 10 billion tons. The lignite is primarily used in steam-electric plants where environmental problems are present.

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