Bituminous coals are black, shiny, and generally hard. They are a medium-rank coal. Bituminous coals have calorific values above 11,500 Btu/lb and volatile matter below 14% (ASTM, Jackson, 1997). The international coal classification system uses vitrinite reflectance (%Ro) to subdivide bituminous coals. In this system, bituminous coals have %Ro between 0.5 and 1.9. There is overlap between reflectance values for bituminous C and subbituminous A coals.
In the U.S. classification system, bituminous coals are divided into five subdivisions. High-volatile bituminous coals (high volatile B and C) are classified using gross calorific (heating) value. High volatile bituminous C coals have calorific values of 11,500 to 13,000 Btu/lb. High volatile bituminous B coals have calorific values of 13,000 to 14,000 Btu/lb. Above the high volatile bituminous rank, however, calorific values are less distinctive for increasing rank. Medium and low volatile bituminous ranks in the U.S. system, are defined based on volatile matter or fixed carbon rather than calorific value. Medium volatile bituminous coals have 22 to 31% volatile matter on a dry mineral-matter free basis. This corresponds to a fixed carbon content of 69 to 78%. Low volatile bituminous coals have 14 to 22% volatile matter on a dry mineral matter free basis. This corresponds to a fixed carbon content of 78 to 86% (ASTM, 2014).
Physical and Chemical Changes (Bituminous Rank)
Bituminous coals begin with abundant volatile matter (high volatile bituminous coals) but with increasing rank, volatile matter is lost forming medium and then low volatile bituminous coals (e.g., Stach et al., 1982). Chemically, the process of bituminization continues through the bituminous rank. The term “bituminous” comes from the increase in bitumen at this rank. Bitumen is petroleum tar or natural asphalt. Heating coal to the rank of high bituminous coal is approximately equivalent to the maturation level in organic-rich source rocks at which oil is generated (Dow, 1977; Teichmüller, 1974; 1989; Tissot and Welte, 1984; Levine, 1993; Mukhopadhyay and Hatcher, 1993). Bitumen is petroleum tar or natural asphalt.
Coalification from the upper high volatile A bituminous through low volatile bituminous ranks is dominated by thermal degradation and “cracking” of the molecular components of the coal and expulsion of thermogenic methane (methane evolved from heating). The process of cracking, also called “repolymerization” essentially breaks down complex carbon compounds into simpler carbon compounds. This process is part of the catagenesis phase of maturation. Thermogenic methane generation begins in the early catagenesis stage, but peak methane expulsion from the coal matrix occurs in mid-catagenesis, in medium and low volatile bituminous ranks (Dow, 1977; Hunt, 1979; Teichmüller, 1974; 1989; Tissot and Welte, 1984). This is why some medium volatile bituminous and higher rank coals are gassy underground. Methane is flammable, so cautions are taken in underground mines to monitor and prevent the buildup of methane leaking from higher rank coal beds to prevent underground mine explosions.