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 to subdivide bituminous coals. In this system, bituminous coals have Ro between 0.5 and 1.9.
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, gradually transforming the molecular phase of the coal from a water-based system to a bitumen-based system (Teichmüller, 1974; 1989; Levine, 1993; Mukhopadhyay and Hatcher, 1993). Bitumen is petroleum tar or natural asphalt.
Coalification from 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.
The formation of thermogenic methane begins in approximately the medium-volatile bituminous rank. 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.