VALAMPURI ENERGY
As geological processes apply pressure to dead biotic material over time, under suitable conditions it is transformed successively into:
- Peat, considered to be a precursor of coal, has industrial importance as a fuel in some regions, for example, Ireland and Finland. In its dehydrated form, peat is a highly effective absorbent for fuel and oil spills on land and water
- Lignite or brown coal, is the lowest rank of coal and used almost exclusively as fuel for electric power generation. Jet is a compact form of lignite that is sometimes polished and has been used as an ornamental stone since the Upper Palaeolithic
- Sub-bituminous coal, whose properties range from those of lignite to those of bituminous coal, is used primarily as fuel for steam-electric power generation and is an important source of light aromatic hydrocarbons for the chemical synthesis industry.
- Bituminous coal is dense sedimentary rock, black but sometimes dark brown often with well-defined bands of bright and dull material, used primarily as fuel in steam-electric power generation, with substantial quantities used for heat and power applications in manufacturing and to make coke
- Steam coal is a grade between bituminous coal and anthracite, once widely used as a fuel for steam locomotives. In this specialized use it is sometimes known as sea-coal in the U.S. Small steam coal (dry small steam nuts or DSSN) was used as a fuel for domestic water heating
- Anthracite, the highest rank of coal is a harder, glossy, black coal used primarily for residential and commercial space heating. It may be divided further into metamorphically altered bituminous coal and petrified oil, as from the deposits in Pennsylvania
- Graphite, technically the highest rank is difficult to ignite and is not commonly used as fuel: it is mostly used in pencils and, when powdered, as a lubricant.
The classification of coal is generally based on the content of volatiles. However, the exact classification varies between countries.