Sub-bituminous coals are the second division of low-rank coals. They are transitional between lignite and bituminous coal. In the U.S. rank classification, sub-bituminous coals and their subdivisions (A, B, C) are supposed to be based on calorific value. By ASTM standards, sub-bituminous coals have calorific (heating) values of 8,300 to 11,500 Btu/lb (ASTM, 2014). In practice, however, there is considerable overlap between what is called a higher rank sub-bituminous coal (subbituminous A) and a lower rank bituminous (high volatile C) coal. For example, coal beds of the Illinois Basin (and western Kentucky) are considered high volatile C bituminous coals although they have calorific values of less than 11,500 Btu/lb. Vitrinite reflectance (%Ro) in subbituminous coals is generally Ro between 0.4 to 0.5%Ro. As with calorific values, reflectance values for what are considered bituminous C and subbituminous A coals in the United States overlap.
Sub-bituminous C (lower rank sub-bituminous) coals in the U.S. rank system are brown and earthy like lignites, whereas, sub-bituminous A (higher rank sub-bituminous) coals are gray to black and shiny like bituminous coals. Sub-bituminous coals are sometimes referred to as black lignite (Jackson, 1997).
Physical and Chemical Changes (Sub-bituminous Rank)
During coalification of sub-bituminous coals, moisture loss continues, but biochemical gelification is replaced by geochemical gelification, also termed vitrinization. Vitrinization is the process by which low-rank components of coals called huminite macerals are transformed into vitreous (shiny) vitrinite macerals, which are characteristic of higher rank coals. Vitrinization is followed by bituminization, in which biogenic breakdown of the coal matrix is replaced by thermal breakdown (Mukhopadhya and Hatcher, 1993; Levine, 1993; Teichmüller, 1989; Stach et al., 1982). Above the rank of sub-bituminous B coals, most of the changes to coal chemistry and structure are driven by increased thermal (heating) rather than biogenic processess. The phase of increased thermally-driven changes is termed catagenesis in maturation studies (e.g., Tissot and Welte, 1984).