Coal is a readily combustible rock containing more than 50 percent by weight of carbonaceous material formed from compaction and indurations of variously altered plant remains similar to those in peat.
After a considerable amount of time, heat, and burial pressure, it is metamorphosed from peat to lignite. Lignite is considered to be "immature" coal at this stage of development because it is still somewhat light in color and it remains soft.
|102||3.1 - 9.0|
|201||9.1 - 13.5||Dry steam coals||Low volatile steam coals|
|202||13.6 - 15.0|
|203||15.1 - 17.0||Cooking steams coals|
|204||17.1 - 19.5|
|206||19.1 - 19.5||Heat altered low volatile steam coals|
|301||19.6 - 32.0||Prime cooking coals||Medium volatile coals|
|305||19.6 - 32.0||Mainly heat altered coals|
|306||19.6 - 32.0|
|401||32.1 - 36.0||Very strongly coking coals||High volatile coals|
|501||32.1 - 36.0||Strongly coking coals|
|601||32.1 - 36.0||Medium coking coals|
|701||32.1||Weakly coking coals|
|801||32.1 - 36.0||Very weakly coking coals|
|901||32.1 - 36.0||Non-coking coals|
1) Volatile matter - dry mineral matter free basis. In coal, those products, exclusive of moisture, given off as gas and vapor determined analytically.
Anthracite coal creates a steady and clean flame and is preferred for domestic heating. Furthermore it burn longer with more heat than the other types.