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The degree of 'metamorphisrn' or coalification undergone by a coal, as it matures from peat to anthracite, has an important bearing on its physical and chemical properties, and is referred to as the 'rank' of the coal. Low rank coals, such as lignite and sub-bituminous coals, are typically softer, friable materials with a dull, earthy appearance; they are characterised by high moisture levels and a low carbon content, and hence a low energy content
Lignite (soft). This type of coal contains a lot of moisture and breaks apart easily. Of the four types, lignite contains the least amount of carbon. Also called brown coal, lignite is used mainly at electricity-generating plants. These lower ranked coals are browner and softer. Llignites have a high oxygen content (up to 30 percent), a relatively low carbon content (60-75 percent on a dry basis), and a high moisture content (30-70 percent)
Subbituminous (medium-soft). usually appear dull black and waxy. They have a carbon content between 71 and 77 percent and a moisture content of up to 10 percent and are used for electricity generation or can be converted to liquid and gaseous fuels
Bituminous (medium-hard). are dense black solids, frequently containing bands with a brilliant lustre. The carbon content of these coals ranges from 78 to 91 percent and the water content from 1.5 to 7 percent. It is used to generate electricity and to produce coke, a coal residue used in the steel industry.
Anthracite (hard). This type of coal has the highest carbon content. Anthracite burns slowly and makes a good heating fuel for homes.
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