WHAT IS COAL?
Coal is a black or dark brown combustible rock made primarily of carbon. It was formed millions of years ago when ferns, plants and trees died and fell into swamps. The swamp conditions prevented the organisms from decaying completely and after millions of years of intense heat and pressure coal was formed1
Coal is classified into four main types, or ranks, based on carbon and heat content. The general rule is that the higher the grade of coal, the cleaner it burns and the more versatile its uses.
- Lignite(25%-35% carbon): Also referred to as brown coal, is the lowest rank of coal and used almost exclusively as fuel for electricity power generation.
- Sub-bituminous coal (35%-45% carbon): Properties range from those of lignite to those of bituminous coal. It is used primarily as fuel for electricity power generation. This coal generally has a lower sulfur content than other types, which makes it attractive for use because it burns cleaner.
- Bituminous coal (45%-86% carbon):Black and sometimes dark brown, often with well-defined bands of bright and dull material. It is used primarily as fuel in electricity power generation, with substantial quantities also used for heat and power applications in manufacturing and to make coke for steel making.
- Anthracite coal (86%-97% carbon):A hard, glossy black coal that is used for home heating and steel making 2
Coal is extracted from the earth through underground mining or surface mining. The choice of mining method is largely determined by the geology of the coal deposit and its distance to the surface. Underground mining currently accounts for a larger share of world coal production than surface mining.
Coal can be burned for heating or to produce electricity. To convert thermal coal to electricity, it is first milled to a fine powder, which increases the surface area and allows it to burn more quickly. The hot gases and heat energy produced from combustion converts water into steam to run a turbine and generator3
High quality coal is also a useful raw material; for example, it can be converted to coke for steel-making. Coal can also be converted to liquid or synthetic gas by advanced chemical processes, making it a possible, but costly, replacement for natural gas or liquid fuels for transportation.