Formation of Coal edit
The generally accepted theory of coal formation is that coal is the result of geologic processes occurring over long periods of time to dead plant matter. Most coal formation began in the coal-bearing period when large tracts of swampland formed across much of the world. Although the majority of the world's coal reserves come from Carboniferous coal, coal formation continues to the present day, as it is a process that occurs progressively as coal is upgraded over time to higher ranks. Coal formation is a process that occurs over geologic time, and as such is not a renewable process within the timeframe of human history.
Coal formation begins as dead plant matter accumulates over time, and in the absence of decomposition (whether due to anoxic conditions or suitable bacteria and animals had not yet evolved) is buried under sediment. Over time, the accumulated plant matter becomes buried deeper underground and by the application of heat and pressure begins to undergo metamorphic processes that reduces the water content of the material, and the chemical composition changes to increase the carbon content of the coal. Over time the coal passes through successive ranks as it is upgraded:
As geological processes apply pressure to dead biotic matter over time, under suitable conditions it is transformed successively into
- Peat, considered to be a precursor of coal. It has industrial importance as a fuel in some countries, for example, Ireland and Finland.
- 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 Iron Age.
- Sub-bituminous coal, whose properties range from those of lignite to those of bituminous coal and 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, a dense mineral, 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.
- Anthracite, the highest rank; a harder, glossy, black coal used primarily for residential and commercial space heating. It may be divided further in to metamorphically altered 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:[1]
| 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 |
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 < 8% volatiles).
- ^ Eberhard Lindner; Chemie für Ingenieure; Lindner Verlag Karlsruhe, S. 258