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Coal classification
There are many compositional differences between the coals mined from the different coal deposits worldwide. The different types of coal are most usually classified by rank which depends upon the degree of transformation from the original source (i.e., decayed plants) and is therefore a measure of a coal's age. As the process of progressive transformation took place, the heating value and the fixed carbon content of the coal increased and the amount of volatile matter in the coal decreased. The method of ranking coals used in the United States and Canada was developed by the American Society for Testing and Materials (ASTM) and is based on a number of parameters obtained by various prescribed tests:
- Heating value: The energy released as heat when coal (or any other substance) undergoes complete combustion with oxygen.
- Volatile matter: The portion of a coal sample which, when heated in the absence of air at prescribed conditions, is released as gases. It includes carbon dioxide, volatile organics and inorganic gases containing sulfur and nitrogen.
- Moisture: The water inherently contained within the coal and existing in the coal in its natural state of deposition. It as measured as the amount of water released when a coal sample is heated at prescribed conditions. It does not include any free water on the surface of the coal. Such free water is removed by by air-drying the coal sample being tested.
- Ash: The inorganic residue remaining after a coal sample is completely burned and is largely composed of compounds of silica, aluminum, iron, calcium, magnesium and others. The ash may vary considerably from the mineral matter present in the coal (such as clay, quartz, pyrites and gypsum) before being burned.
- Fixed carbon: The remaining organic matter after the volatile matter and moisture have been released. It is typically calculated by subtracting from 100 the percentages of volatile matter, moisture and ash. It is composed primarily of carbon with lesser amounts of hydrogen, nitrogen and sulfur.
The ASTM ranking system is presented in the table below:
Class or Rank |
Group |
Fixed Carbon (b) (wt % dry mmf) |
Volatile Matter (b) (wt % dry mmf) |
Gross Heating Value (c) (MJ/kg moist mmf) | |||
---|---|---|---|---|---|---|---|
Equal or greater than |
Less than |
Greater than |
Equal or less than |
Equal or greater than |
Less than | ||
Anthracitic |
Metaanthracite (d) Anthracite (d) Semianthracite (d) |
98 92 86 |
98 92 |
2 8 |
2 8 14 |
||
Bituminous |
Low-volatile bituminous (d) Medium-volatile bituminous (d) High-volatile A bituminous High-volatile B bituminous High-volatile C bituminous (e) High-volatile C bituminous (f) |
78 69 |
86 78 69 |
14 22 31 |
22 31 |
32.55 30.23 26.74 24.41 |
|
Subbituminous |
Subbituminous A Subbituminous B Subbituminous C |
24.41 22.09 19.30 |
26.74 24.41 22.09 | ||||
Lignite |
Lignite A Lignite B |
14.65 |
19.30 14.65 | ||||
(a) This classification does not include a few coals (referred to as unbanded coals) having unusual physical and chemical properties falling within the fixed carbon and heating value ranges of the high-volatile bituminous and subbituminous ranks. (b) Percentage by weight on a dry and mineral matter free basis (mmf). |
The anthracitic coals, with the highest contents of fixed carbon and lowest contents of volatile material, have the highest rank. The lignite coals,with the lowest contents of fixed carbon and highest contents of volatile matter, have the lowest rank. The bituminous and subbituminous coals (in that order) are ranked between the anthracitic and lignite coal.
As a broad generality, the anthracitic coals have the highest heating value and the lignite coals have the lowest heating values.
Coal Rank |
Proximate Analysis (wt % ar) |
Ultimate Analysis (wt % maf) |
Net Heating Value (maf) (MJ/kg) | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Fixed carbon |
Volatile matter |
Moisture |
Ash |
C |
H |
O |
N |
S | |||
Anthracite | 81.8 | 7.7 | 4.5 | 6.0 | 91.8 | 3.6 | 2.5 | 1.4 | 0.7 | 36.2 | |
Bituminous | 54.9 | 35.6 | 5.3 | 4.2 | 82.8 | 5.1 | 10.1 | 1.4 | 0.6 | 36.1 | |
Subbituminous | 43.6 | 34.7 | 110.5 | 11.2 | 76.4 | 5.6 | 14.9 | 1.7 | 1.4 | 31.8 | |
Lignite | 27.8 | 24.9 | 36.9 | 10.4 | 71.0 | 4.3 | 23.2 | 1.1 | 0.4 | 26.7 | |
Notes: • wt % = percent by weight ar = as received maf = moisture and ash free |
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- ↑ Chris Higman and Maarten van der Burgt (2008). Coal Gasification, 2nd Edition. Gulf Professional Publishers. ISBN 0-7506-8528-X.