Sub-bituminous coal is a lower grade of coal that contains 35–45% carbon. The properties of this type are between those of lignite, the lowest grade of coal, and those of bituminous coal, the second-highest grade of coal.[1] Sub-bituminous coal is primarily used as a fuel for steam-electric power generation.
Sub-bituminous coals may be dull, dark brown to black, soft and crumbly at the lower end of the range, to bright jet-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 11,500 BTu/lb or 19.3 to 26.7 MJ/kg. Their relatively low density and high water content renders some types of sub-bituminous coals susceptible to spontaneous 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 the Powder River Basin in Wyoming.
Sub-bituminous coals, in the United States, typically have a sulfur content less than 1% by weight, which makes them an attractive choice for power plants to reduce SO2 emissions under the Acid Rain Program.
Sub-bituminous coals release large quantities of greenhouse gases when burned, compared to higher grades of coal.[3]
| Coal types by grade (lowest to highest) |  | |
|---|---|---|
| Coal combustion | ||
| Coal mining |
| |
Notes: [1] Peat is considered a precursor to coal. [2] Graphite is only technically considered a coal type. | ||
