Sub-bituminous coal

Sub-bituminous coal is a type of coal whose properties range from those of lignite to those of bituminous coal and are used primarily as fuel for steam-electric power generation.

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Sub-bituminous coal 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. It contains 20-30% inherent moisture by weight. The heat content of sub-bituminous coal ranges from 17 to 24 million British thermal units (Btu) per short ton (20 to 28 megajoules per kilogram) on a moist, mineral-matter-free basis. The heat content of sub-bituminous coal consumed in the United States averages 17 to 18 million Btu per short ton (20 to 21 MJ/kg), on the as-received basis (i.e., containing both inherent moisture and mineral matter). A major source of sub-bituminous coal in the United States is the Powder River Basin in Wyoming.

Its relatively low density and high water content renders some types of sub-bituminous coal susceptible to spontaneous combustion if not packed densely during storage in order to exclude free air flow.