Coal was formed from remains of plants from several hundred millions years ago partially decomposed. These remains were settled in regions where waterlogged or swampy regions prevailed. These conditions avoid complete decomposition making possible the gradual peat formation. Peat is not considered coal actually, but is an essential step to coal formation. This formation process is called "coalification" and it is essentially a progressive change from peat to anthracite passing through different types of coals. This process not only depends on the time, but also on temperature and burial pressure (Speight 1983).
Schematic representation of the coalification process (Speight 1983)
2. Coal Classification
Coals can be classified by its "rank". The rank of the coal can be defined as the degree of alteration that coal experiments during the coalification. On the one hand there are Lignite and sub-bituminous coals which are Low rank coals. These have a lower content in Carbon, are lighter and have higher moisture levels. On the other hand there is Bituminous coals and Anthracite which are high rank coals. They have a higher content in carbon which means they have more energy content. They have lower moisture levels and a more vitreous appearance (University of Kentucky, 2012).
3. Coal Composition
Coal is an organic sediment which can be described in several ways. The most common way is in terms of the elemental composition. J. G. Speight, (1991) suggested that coal can classified on the basis of the general formula:
Cn Hm Nx Oy Sz (where n, m, x, y and z are number of atoms of each element)
Coal is not a homogeneous material. It is heterogeneous and is contaminated by different types of impurities w...
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Classification and Rank of Coal. University of Kentucky. (2012). [ONLINE] Available at: http://www.uky.edu/KGS/coal/coalkinds.htm. [Accessed 12 November 13].