Coal is a rock made of almost pure carbon. The coal in different deposits have different compositions, therefore, coal is classified in various categories. Sub-bituminous coal or black lignite is a category of coal which appears as grey-black or dark brown. It ranges from hard to soft as it represents an intermediate stage between low quality lignite and higher quality bituminous coal. The carbon content of sub-bituminous coal varies from 70-76%. Sub-bituminous coals are among the younger coals geologically and are approximately 251 million years old, with an energy density that ranges from 18-23 MJ/kg. This type of coal is among the most common, with 30% of coal resources being sub-bituminous.
Sub-bituminous coal is used in generating steam for the production of electricity, and thus frequently used in power plants. Moreover, sub-bituminous coal can be liquefied and converted into petroleum and gas.
The use of this coal can lead to hazardous emissions, particularly of harmful smoke, soot, sulfur oxides (SOx), nitrogen oxides (NOx), and mercury. However, burning the coal at higher temperatures reduces the amount of carbon monoxide that is released.