Types
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. It is also used as a conditioner for soil to make it more able to retain and slowly release water.
- Lignite, or brown coal, is the lowest rank of coal and used almost exclusively as fuel for electric power generation. Jet, a compact form of lignite, 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 a dense sedimentary rock, usually black, but sometimes dark brown, often with well-defined bands of bright and dull material; it is 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 US. 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. According to the German classification, coal is classified as follows:
| German Classification | English Designation | 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 | <28,470 |
| Flammkohle | Flame coal | 40-45 | 75-82 | 6.0-5.8 | >9.8 | ~1 | <32,870 |
| Gasflammkohle | Gas flame coal | 35-40 | 82-85 | 5.8-5.6 | 9.8-7.3 | ~1 | <33,910 |
| Gaskohle | Gas coal | 28-35 | 85-87.5 | 5.6-5.0 | 7.3-4.5 | ~1 | <34,960 |
| Fettkohle | Fat coal | 19-28 | 87.5-89.5 | 5.0-4.5 | 4.5-3.2 | ~1 | <35,380 |
| Esskohle | Forge coal | 14-19 | 89.5-90.5 | 4.5-4.0 | 3.2-2.8 | ~1 | <35,380 |
| Magerkohle | Nonbaking coal | 10-14 | 90.5-91.5 | 4.0-3.75 | 2.8-3.5 | ~1 | 35,380 |
| Anthrazit | Anthracite | 7-12 | >91.5 | <3.75 | <2.5 | ~1 | <35,300 |
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 (US 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".
Read more about this topic: Coal
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