Sub-Bituminous Coal Characteristics and Uses
More Carbon Dense Than Lignite, but Softer Than Bituminous Coal
Sub-bituminous coal is considered black coal, even though its appearance varies from bright black to dull dark brown. Its consistency ranges from hard and strong to soft and brittle due to its intermediate stage—between bituminous and brown coal (lignite). The 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. This type of coal contains more moisture and volatile matter than other bituminous coal types, but it has lower sulfur levels. Sub-bituminous coal has a heat value of approximately 8,500 to 13,000 British thermal units per pound, as mined.
Sub-bituminous coal is noncoking and contains less sulfur but more moisture (approximately 10 to 45 percent) and volatile matter (up to 45 percent) than other bituminous coal types. It has 35 to 45 percent carbon content, and its ash content ranges up to 10 percent. The coal's sulfur content is generally less than 2 percent by weight. Nitrogen makes up approximately 0.5 to 2 percent of the coal's weight. Sub-bituminous coal typically is found near the surface, which results in lower mining costs, making it a relatively inexpensive coal.
Sub-bituminous coal combustion can lead to hazardous emissions that include particulate matter (PM), sulfur oxides (SOx), nitrogen oxides (NOx), and mercury (Hg). It also produces ash that has higher alkaline content than other coal ash. This characteristic can help reduce acid rain typically caused by coal-fired power plant emissions. Adding sub-bituminous coal to bituminous coal introduces alkaline byproducts that bind to 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 home furnaces or fireboxes say that bigger lumps produce less smoke and no clinkers. High ash content can be a drawback, however.
Environmental concerns have prompted electricity power plants to use sub-bituminous coal and lignite in place of bituminous coal. Typically, coal mined from freshwater basins in the western United States contains lower sulfur levels, which makes it preferable for industrial uses, according to the Environmental Protection Agency. The EPA further notes that approximately 95 percent of sulfur in bituminous coal is emitted into the atmosphere as a gas, whereas sub-bituminous coal emits less when burned.
Other Facts About Sub-Bituminous Coal
Availability: Moderate. Approximately 30 percent of available coal resources in the United States are sub-bituminous. The United States far surpasses other countries in its quantity of sub-bituminous coal resources, with estimated reserves of approximately 300,000 million tons. Other countries with notable resources include Brazil, Indonesia, and the Ukraine.
Mining Locations: 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. The complete rankings: