percent of U.S. coal production by tonnage, but because of their lower heating values, only about 40 percent by heating value. The distinction between tonnage and energy content is particularly important when considering possible shifts in coal supply and demand by producing and consuming regions. Subbituminous coals are produced almost exclusively in the PRB of Wyoming and Montana. EIA’s forecasts of increased coal production over the next three decades (see
Coal Rank and Quality
Although the term coal refers to any readily combustible rock containing more than 50 percent by weight of organic matter, coals differ considerably in their physical and chemical characteristics (Table 4.2) and these differences have pronounced impacts on their value and use. Coals in the United States are classified by “rank,” a method of distinguishing coals on the basis of their fixed carbon content, volatile matter content, heating value, and agglomerating characteristics.1 Coal rank is defined as “the degree of metamorphism, or progressive alteration, in the natural series from lignite to anthracite. Higher-rank coal is classified according to the fixed carbon on a dry basis, lower-rank coal, according to Btu [heating value] on a moist basis” (AGI, 1997).2 Differences in sedimentary depositional environments and differences in the geological history of the coal strata result in differences in mineral matter content and composition, as well as differing concentrations of some of the important minor elements (e.g., sulfur, chlorine). These differing characteristics impact coal utilization in both the electricity generation and metallurgical markets.
The term coal quality is used to distinguish the range of different commercial steam coals that are produced directly by mining or are produced by coal cleaning.3 Generally, coal quality for steam coals (i.e., coal used for electricity generation) refers to differences in heating value and sulfur content (Table 4.2), although other characteristics such as grindability or ash fusion characteristics are also specified in coal sale agreements. While not as obvious as the impact of sulfur content on environmental emissions, differences in the moisture content and heating values among different coal types affect CO2 emissions upon combustion, with higher-rank bituminous coals producing 7 to 14 percent lower emissions than subbituminous coals on a net calorific value basis (Winschel, 1990).