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, also referred to as 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 are used primarily as fuel for steam-electric power generation. Additionally, it is an important source of light aromatic hydrocarbons for the chemical synthesis industry.
● Bituminous coal, 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 also 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; a harder, glossy, black coal used primarily for residential and commercial space heating. It may be divided further into metamorphicallyaltered bituminous coal and petrified oil, as from the deposits in Pennsylvania
● Graphite, technically the highest rank, but difficult to ignite and is not so 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. According to the German classification, coal is classified as follows:
| NAME | VOLATILES % | C CARBON % | H HYDROGEN % | O OXYGEN % | S SULFUR % | HEAT CONTENT KJ/KG |
| Braunkohle(Lignite) | 45-65 | 60-75 | 6.0-5.8 | 34-17 | 0.5-3 | <28470 |
| Flammkohle(Flame coal) | 40-45 | 75-82 | 6.0-5.8 | >9.8 | ~1 | <32870 |
| Gasflammkohle(Gas flame coal) | 35-40 | 82-85 | 5.8-5.6 | 9.8-7.3 | ~1 | <33910 |
| Gaskohle (Gas coal) | 28-35 | 85-87.5 | 5.6-5.0 | 7.3-4.5 | ~1 | <34960 |
| Fettkohle (Fat coal) | 19-28 | 87.5-89.5 | 5.0-4.5 | 4.5-3.2 | ~1 | <35380 |
| Esskohle(Forge coal) | 14-19 | 89.5-90.5 | 4.5-4.0 | 3.2-2.8 | ~1 | <35380 |
| Magerkohle(Non baking coal) | 10-14 | 90.5-91.5 | 4.0-3.75 | 2.8-3.5 | ~1 | 35380 |
| Anthrazit(Anthracite) | 7-12 | >91.5 | <3.75 | <2.5 | ~1 | <35300 |
| Percent by weight |
The middle six grades in the table represent a progressive transition from the English-language sub-bituminous to bituminous coal, while the last class is an approximate equivalent to anthracite, but more inclusive (the U.S. anthracite has < 6% volatiles).
Cannel coal (sometimes called “candle coal”), is a variety of fine-grained, high-rank coal with significant hydrogen content. It consists primarily of “exinite” macerals, now termed “liptinite”.
