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). The heat content of sub-bituminous coals range from 8300 to 11,500 BTu/lb or 19,306 to 26,749 kJ/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.
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.
- The Babcock & Wilcox Company. Steam: its generation and use. The Babcock & Wilcox Company. pp. 36–5. ISBN 0-9634570-1-2.