The United States produces 100 to 130 million tons of combustion wastes annually at coal-fired power plants. Regulations control the handling of solid wastes from the boiler and pollution-control equipment at coal-fired power plants. In most cases, these materials are disposed of in landfills. However, reuse of coal combustion wastes as byproducts is permitted and 40 to 60 million tons (10 to 15%) are recycled annually according to the American Coal Ash Association.

Common pollution control equipment on coal-fired power plants and the byproduct wastes produced by each

Boiler slag is produced in coal-fired power plants that use wet-bottom boilers. It forms from melted minerals left over from coal combustion. This molten ash is quenched with water to form a solid, black, glassy waste product, called slag. Boiler slag is commonly re-purposed as a useful combustion byproduct. The slag material can be ground into particles which are used as roofing granules and blasting grit, and for snow and ice control (like sand) (Kalyoncu and Olson, 2001; American Coal Ash Association, 2016).

Bottom ash is produced in dry-bottom boilers, which are the most common types of boilers in coal-fired power plants. Bottom ash consists of the melted minerals left over from coal combustion. The ash settles on the walls and at the base of the boiler as a gray, sandy material. The ash is periodically removed and disposed of in landfills or re-purposed as a useful coal-combustion byproduct. Approximately 15 million tons of bottom ash are produced annually in the United States, and 10 to 15% is recycled as useful byproducts. Bottom ash is used in many ways but mostly (1) as road base; (2) for snow and ice control (like sand); (3) in structural fills; (4) in cement and grout; and (5) in mining applications such as mine fills, where permitted (Kalyoncu and Olson, 2001; American Coal Ash Association, 2016).

Fly ash is the fine particulate (including ash, dust, soot, and cinders) material produced from coal combustion. It consists of the mineral matter in the coal which did not combust in the furnace, and did not settle to the bottom of the boiler as bottom slag or bottom ash. It is carried upward with gaseous by-products from the furnace into the flue gas. Fly ash can be captured in an electrostatic precipitator or filter baghouses. Between 50 and 70 million tons of fly ash are produced in the United States annually. A little more than half of the fly ash is disposed of in landfills, but 40 to 45% (more than 20 million tons) is re-cycled as useful byproducts. Fly ash is used in many ways but mostly (1) to make concrete and grout; (2) for use in structural fills; (3) for waste stabilization (thickening, solidifying); (4) in cement production; and (5) in mining applications such as mine fills, where permitted (Kalyoncu and Olson, 2001; American Coal Ash Association, 2016).

Flue-gas desulfurization (FGD, scrubber) wastes are “wet” or “dry” depending on the type of scrubber used. Where wet scrubbers are used, the waste slurry is transported by pipes to a holding pond, where it settles, slowly filling the pond. Dry scrubbers produce a dry waste product. Both types of ash are disposed of according to federal and state regulations, or can be used as useful byproducts. The annual amount of FGD wastes produced is increasing as more coal-fired power plants have added scrubbers to meet Federal emissions regulations. In 2014, four million tons of synthetic gypsum was produced from scrubbers; half was disposed of in landfills, and half was recycled as useful byproducts; mostly to make wallboard. Wallboard (plasterboard, drywall) is essentially a gypsum paste sandwiched between two pieces of paper. The synthetic gypsum produced by scrubbers can be used the same as natural gypsum in wallboard. Some of the largest wallboard plants in the Nation use scrubber wastes. FGD wastes are also used (1) as structural fills; (2) in concrete and grout; and (3) in mining applications as mine fills, etc., where permitted (Kalyoncu and Olson, 2001; American Coal Ash Association, 2016).


References for Power Plant and Coal-Combustion Byproducts

  • Adriano, D.C., Page, A.L., Elseewi, A.A., Chang, A.C. and Straughan, I., 1980, Utilization and disposal of fly ash and other coal residues in terrestrial ecosystems: A review: Journal of Environmental Quality, v. 9, no. 3, p.333-344.
  • Energy Information Administration, 2016, Annual coal report: EIA website,, accessed 2016.
    Greb, S.F., Eble, C.F., Peters, D.C., and Papp, A.R., 2006, Coal and the Environment: American Geological Institute, Environmental Education Series, 64 p.
  • Kalyoncu, R.S. and Olson, D.W., 2001, Coal combustion products: U.S. Geological Survey, Coal Combustion Products, 12 p.
  • National Energy Technology Laboratory (NETL), 2016, Coal: U.S. Department of Energy, NETL website,, accessed 2016
  • National Research Council, 1975, Underground disposal of coal mine wastes: a report to the National Science Foundation Study Committee to assess the feasibility of returning underground coal mine wastes to the mined-out areas: National Academy Press, Washington, D.C., 172  p.
  • U.S. Department of Transportation, 2008, User guidelines for waste and byproduct materials in pavement construction: Federal Highway Administration Research and Technology, On-line Publication no. FHWA-RD-97-148, website i, accessed 2016.
  • Beechler, D.S. and Jahnke, J.A., 1981, APTI Course 413–Control of particulate emissions–Student manual: U.S. Environmental Protection Agency, Air Pollution Training Institute, No. EPA-450/2-80-066, varied pagination.
  • U.S. Environmental Protection Agency (EPA), 1998, Bituminous and subbituminous coal combustion: Technology and Transfer Network Clearinghouse for Inventories and Emission Factors, Washington, D.C., AP-42, Fifth Edition, v. 1, chapter 1: External Combustion Sources, 52 p.
  • U.S. Environmental Protection Agency (EPA), 1999, Report to Congress–Wastes from the combustion of fossil fuels: Office of Solid Waste and Emergency Response, Washington, D.C., EPA 530-S-99-010, Executive Summary, v. 1, 40 p., Methods, Findings, and Recommendations, v. 2, 232 p.



Last Modified on 2023-01-05
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