Coal was formed from plant life under the action of immense pressures and temperatures prevailing within the earth's crust over a period encompassing millions of years. The major elements present in the "organic portion" of coal are carbon, hydrogen, oxygen, nitrogen and sulfur. Sulfur (mostly as iron pyrite) is also present as part of the "inorganic portion" or ash in the coal along with oxides of alumina, silica, iron, alkaline earths and alkalis. Coal also contains some chlorine. Coal is classified into the following four types according to the degree of metamorphism:

• anthracite which is low in volatile matter (which forms tars, oils and gasses when coal is heated) and consists of mostly carbon (fixed carbon)
• bituminous which contains significant amounts of the volatile matter and typically exhibit swelling or caking properties when heated
• sub-bituminous is a younger coal and contains in addition to the volatile matter, significant amounts of moisture
• lignite is the youngest form of coal (when peat is not included in the broader definition of coal types) and is very high in moisture content resulting in a much lower heating value than the other types of coal.

Almost all of the coal consumed in the world is for electric power generation by combusting the coal in boilers and generating steam to power a turbine. Coal is being used to a limited extent in gasification based plants to produce gas to fuel gas turbine based combined cycles (IGCCs) and in some countries such as China for chemicals synthesis. With more advanced gas turbines under development, coal based IGCC will have a strong economic and environmental basis to compete with boiler based power plants.

Coal Consumption1 and Recoverable Reserves2,3

The following table shows the coal consumption and the estimated reserves in 1995 in some countries (multiply ST by 0.9072 to obtain MT or Mega Gram):