Sub-bituminous coal is considered a black coal, although its appearance varies from bright black to dull dark brown. Its consistency ranges from hard and strong to soft and crumbly, because it is an intermediate stage of coal between bituminous and brown coal (lignite).
Sub-bituminous coal is widely used for generating steam power and industrial purposes. Sometimes called "black lignite," sub-bituminous coal is not stable when exposed to air. It tends to disintegrate.
Sub-bituminous coal contains more moisture and volatile matter than bituminous coals, but lower sulfur levels.
Heating value: Sub-bituminous coal has a heat value of approximately 8,500 to 13,000 Btu per pound, as mined.
Characteristics: Sub-Bituminous coal is non-coking and has less sulfur but more moisture (approximately 10 to 45 percent) and volatile matter (up to 45 percent) than bituminous coals. Carbon content is 35-45 percent and ash ranges up to 10 percent. Sulfur content is generally under 2 percent by weight. Approximately 0.5 to 2 percent of sub-bituminous coal's weight is nitrogen.
Combustion of sub-bituminous coal can lead to hazardous emissions that include particulate matter (PM), sulfur oxides (SOx), nitrogen oxides (NOx), and mercury (Hg).
Sub-bituminous coals produces ash that is more alkaline than other coal ash. This characteristic can help reduce acid rain caused by coal-fired power plant emissions. Adding sub-bituminous coal to bituminous coal introduces alkaline byproducts that are able to bind sulfur compounds released by bituminous coal and therefore reduce acid mist formation.
When sub-bituminous coal is burned at higher temperatures, its carbon monoxide emissions are reduced. As a result, small combustion units and poorly maintained ones are likely to increase pollution output. People who use sub-bituminous coal in a home furnace or firebox say that bigger lumps produce less smoke and no clinkers. High ash content can be a drawback, however.
Availability: Moderate. Approximately 30% of available coal resources in the U.S. are Sub-Bituminous. The U.S. far surpasses other countries in its quantity of sub-bituminous coal resources, with estimated reserves of approximately 300,000 million tonnes. Other countries with notable resources include Brazil, Indonesia, and the Ukraine.
Location: Wyoming, Illinois, Montana, and other locations west of the Mississippi river.
Ranking: Sub-bituminous ranks 3rd in heat and carbon content compared with other types of coal, according to ASTM D388 - 05 Standard Classification of Coals by Rank.
Learn about other types of coal
#3 Ranked Coal - Sub-bituminous