Coal - Types

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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