A relatively low-grade fossil fuel formed from partial decomposition of decaying animal and plant matter. Coal is created over millions of years under intense pressure from the layers of rock and other sediments that settle over the material in the intervening years.

Coal is one of the world's primary sources of fuel for electrical energy generation. It is not nearly as efficient or concentrated a fuel as petroleum, but it is much more widely available and usually far cheaper to extract from the Earth. Its low relative fuel rating translates to a high relative impurity rating, which means that extra care must be taken when coal is burned to insure that the release of waste products such as fly ash and sulfur dioxide are kept to a minimum. Efforts to reduce these emissions have only been undertaken on a global level within the last two or three decades.

There are four common grades of coal, and the quality of these grades is expressed as their heating value. Anthracite is a hard, jet-black coal with high heating value which is currently mined commercially only in a few selected areas. Most North American anthracite is mined in Northeast Pennsylvania. Bituminous coal is a softer black coal with a somewhat lower fuel value mined in Nova Scotia, Kentucky, Pennsylvania and West Virginia. It is the most common commercially-mined grade of coal. Subbituminous coal has a lower heating value and is either dull black or slightly brownish in color. It is mined chiefly in areas where superior grades of coal are either unavailable or too expensive to import. Lignite is a brownish-black coal, the lowest of the four commercial grades, and is usually only mined where no other low-cost fuels are available.

A ton of coal (2,000 lb.) measures approximately 29 cubic feet. A tonne (metric ton or 1,000kg) of coal is approximately 10% heavier and bulkier than a ton.

Energy content varies widely even among the same grades of coal. A typical ton of bituminous coal will yield 22-26 million BTU. Depending on the efficiency of the production facility, this can yield anywhere from 2,000 to 4,000 kilowatt-hours of electricity.

See also:

fossil fuel, Clean Air Act, British Thermal Unit, fly ash, sulfur dioxide, electrostatic precipitator, scrubber, flue gas particulate collector