Coal is a solid, brittle, combustible, carbonaceous rock formed by the decomposition and alteration of vegetation by compaction, temperature, and pressure. It varies in color from brown to black and is usually stratified. The source of the vegetation is often moss and other low plant forms, but some coals contain significant amounts of wood. Coal deposits are usually called beds or seams and can range from fractions of an inch to hundreds of feet in thickness. Coals are found in all geologic periods from Silurian through Quaternary, but the earliest commercially important coals are found in rocks of Mississippian age (Carboniferous in Europe). Coals generally form in either intercontinental basins in fluvial environments or in paralic basins open to marine incursions.
Coal is found on every continent, and world coal reserves exceed 1 trillion tons. However, the largest reserves are found in the U.S., former Soviet Union, and China. The U.S. and former Soviet Union each have about 23% of the world's reserves, and China has about 11%. China, however, is the world's lead producer followed by the U.S.
About 13% of the U.S. is underlain by coal which occurs in 37 of 50 states. Over 2,000 active surface and underground mines are producing approximately 1 billion tons of coal a year in the U.S. At current consumption rates, the U.S. has a 240 year supply of coal reserves. Coals are generally termed steam (for use by power plants for electric generation) or coking (for conversion to coke for iron and steel making). About 56% of the power generated in the U.S. comes from power plants fueled by coal, the country's most abundant fossil fuel. A lesser amount of coal is also used for residential, commercial, and industrial applications.
Coal is generally classified according to rank. Rank classifications are based on a coal's content of fixed carbon, volatile carbon compounds, water, and ash, its heating value, and its coking properties. In the coalification process, coal first takes the form of peat, then progresses through lignite (brown coal), bituminous (soft coal), and finally to anthracite (hard coal) and graphite. Lignite has a low heating value and a high moisture content of 30 to 40 %. Since it weathers easily, lignite is not transported far but used to generate electricity in power plants located close to mines as in California, Louisiana, Montana, North Dakota, and Texas. Bituminous coal is the most common form of coal in the U.S.; it is black and contains bands of both bright and dull material. The moisture content of bituminous coal is usually under 20%, and it is mined in the Appalachian, Interior and Rocky Mountain coal fields. Anthracite is jet black with a high luster. It is the highest rank of economically usable coal with a moisture content less than 15%. In the U.S., anthracite is mined principally in Pennsylvania and used for space heating and electric generation
Coal is the principal fuel used for electric generation in the United States, and annual production has been relatively steady the last few years at 1.1 to 1.2 billion short tons. Increasing demand for energy is improving coal markets domestically and abroad, and rising energy prices are improving market conditions for coal significantly. Companies that were just trying survive a year ago are beginning to explore for new reserves, and emerging clean-coal technologies are paving the way for increased coal utilization without environmental consequence. Coal research remains dynamic, and the U.S. Department of Energy is planning to expand the clean-coal technology program. Carbon sequestration is also spurring on new avenues of coal research because carbon dioxide and other waste gases from power plants can be injected into coal and can be used to enhance coalbed methane production.