Vast specifications from locations all over the world.
There are several different types of coal, that have different properties usually dependent on their age and the depth to which they have been buried under other rocks.
In some parts of the world (for example New Zealand), coal development is accelerated by volcanic heat or crustal stresses. The degree of coal development is referred to as a coal’s “rank,” with peat being the lowest rank coal and anthracite the highest.
Peat is the layer of vegetable material directly underlying the growing zone of a coal forming environment. The vegetable material shows very little alteration and contains the roots of living plants. Peat is widely used as a domestic fuel in rural parts of Scotland and Ireland.
Lignite is geologically very young (upwards of around 40,000 years). It is brown and can be soft and fibrous, containing discernible plant material. It also contains large amounts of moisture (around 70%) and so has a low energy content: around 8 to 10 megajoules per kilogram. This coal is mined extensively in the Latrobe Valley south east of Melbourne, Australia. As the coal develops it loses its fibrous character and darkens in colour.
In Australia, black coal ranges from Cretaceous age (65 to 105 million years ago) to mid Permian age (up to 260 million years ago). They are all black; some are sooty and still quite high in moisture (sub-bituminous coal), including the coal mined at Collie, which is sometimes termed a “black lignite.”
Coals that get more deeply buried by other rocks lose more moisture and start to lose their oxygen and hydrogen. These coals are harder and shinier (bituminous coal). These are typical of most of the coals mined in NSW and Queensland, which have energy contents around 24 to 28 megajoules per kilogram. These coals generally have less than 3% moisture, but some power stations in NSW and Queensland burn coal at up to 30% ash.
Anthracite is a hard, black, shiny form of coal that contains virtually no moisture and very low volatile content. Because of this, it burns with little or no smoke and is sold as a “smokeless fuel”.
In Australia, coals only approach anthracite composition where bituminous coal seams have been compressed further by local crustal movements (at Yarrabee and Baralaba, both in Queensland). Anthracites can have energy contents up to about 32 megajoules per kilogram, depending on the ash content.
Coal rank has little to do with quality; as a coal matures its ash content actually increases as a proportion because of the loss of moisture and volatiles. Lower rank coals may have lower energy contents, but they tend to be more reactive (they burn faster) because of their porosity and resultant higher surface area.