Steam coal (sometimes called thermal coal) is a grade of coal used in electric power plants to generate steam to create electricity. Most of the coals mined in the United States are steam coals. Steam coals for power plants must meet quality and heating characteristics of the boiler design and for the design of pollution-control equipment at a power plant.
Grades of steam coal are generally related to sulfur content and ash yield. Typically, the term low-sulfur coal is used for coals with less than 1 percent sulfur. In the United States, the Clean Air Act Amendments (1990) set a limit for sulfur emissions at power plants of 1.2 pounds of sulfur per 10,000 Btu, which amounts to approximately 1 to 1.2 percent sulfur in a bituminous coal (Cobb and Eble, 1992; U.S. Energy Information Administration, 1997). Hence, sulfur is used to define grades of coals that meet emissions standards. Low-ash coals are grades of coal with ash yields below 10 percent. High-ash coals are generally coals above 10 percent ash yield.