Bituminous coals are black, shiny, and generally hard. They are a medium-rank coal. Bituminous coals generally have calorific values above 11,500 Btu/lb andvolatile matter below 14% (ASTM, Jackson, 1997). In the Illinois Basin (and western Kentucky), however, the lower rank end of what are termed bituminous coals in the United States have calorific values of 11,100 to 11,300 Btu/lb. The international coal classification system usesvitrinite reflectance (%Ro) to subdivide bituminous coals. In this system, bituminous coals have %Ro between 0.5 and 1.9. As for calorific value, reflectance values for what are considered bituminous C and subbituminous A coals in the United States overlap.
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 orfixed 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).
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.
