Representation of Coal Rank in the NEMS Coal Market Module

The thermal grades represented in the NEMS Coal Market Module (CMM) primarily correspond to three ranks of coal: bituminous, subbituminous and lignite. In the United States, coals are grouped into specific rank categories based on fixed carbon content, volatile matter, heating value, and agglomerating (or caking) properties. The classification of coals according to rank is based on their degree of progressive alteration from lignite to anthracite.

In the CMM, bituminous coal is represented by two thermal grades: (1) a premium bituminous coal that is supplied to the domestic and foreign coking coal sectors and used to make coke for the steelmaking process; and (2) a bituminous steam coal consumed in the electricity, industrial, and residential/commercial sectors. Like bituminous steam coal, subbituminous coal and lignite also are consumed in the electricity, industrial, and residential/commercial sectors. Anthracite coal from Pennsylvania is not uniquely modeled in the CMM but is grouped with bituminous coal in Northern Appalachia (Pennsylvania, Ohio, northern West Virginia, and Maryland). An additional supply curve representing supplies of waste bituminous and anthracite coals in Northern Appalachia is also represented in the CMM. Currently, waste coals are consumed primarily by independent power producers.

There is some indication coal rank is correlated with the capability of different technologies to remove Hg from the stack gases of electric power plants (see Table 5), but it is not entirely clear why Hg removal rates vary by coal rank. A number of factors are known to affect Hg removal, such as chlorine content of the coal, the chemical state of the Hg in the coal (elemental or in compound), boiler temperature and firing type, and flue gas temperature. Others are not yet well understood, such as the ability of fly ash itself (generated during combustion) to absorb Hg. Chlorine reacts with elemental Hg during combustion to form oxidized Hg, which is more effectively removed from the flue gas of coal-fired units equipped with wet SO₂ scrubbers.^a

Data on chlorine content, from the U.S. Environmental Protection Agency’s 1999 Information Collection Request, typically indicate a substantial difference in chlorine content between bituminous and subbituminous coals. For example, the average chlorine content associated with the CMM coal supply curves for bituminous coals from the Northern Appalachian and Central Appalachian (southern West Virginia, Virginia and eastern Kentucky) regions ranges from approximately 800 to 1,200 parts per million (ppm), whereas the average chlorine content of low-sulfur subbituminous coal from the Powder River Basin (Wyoming and Montana) region is 120 ppm.

^aN. Shick, “Mercury’s Pathways to Fish,” EPRI Journal, Vol. 8 (December 22, 2000).