Sub-bituminous coal is a lower grade ofcoal that contains 35–45%carbon. The properties of this type are between those oflignite, the lowest grade of coal, and those ofbituminous coal, the second-highest grade of coal.[1] Sub-bituminous coal is primarily used as a fuel forsteam-electric power generation.
Sub-bituminous coals may be dull, dark brown to black, soft and crumbly at the lower end of the range, to brightjet-black,hard, and relatively strong at the upper end. They contain 15-30% inherent moisture by weight and are non-coking (undergo little swelling upon heating).[2] The heat content of sub-bituminous coals range from 8300 to 11,500BTu/lb or 19.3 to 26.7MJ/kg. Their relatively low density and high water content renders some types of sub-bituminous coals susceptible tospontaneous combustion if not packed densely during storage in order to exclude free air flow.
A major source of sub-bituminous coal in the United States is thePowder River Basin inWyoming.
Sub-bituminous coals, in theUnited States, typically have asulfur content less than 1% byweight, which makes them an attractive choice forpower plants to reduceSO2 emissions under theAcid Rain Program.
Sub-bituminous coals release large quantities ofgreenhouse gases when burned, compared to higher grades of coal.[3]
| Coal types by grade (lowest to highest) |  | |
|---|---|---|
| Coal combustion | ||
| Coal mining |
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Note: [1] Peat is considered a precursor to coal. Graphite is only technically considered a coal type. | ||