Bottom ash

From Global Energy Monitor
This article is part of the Global Energy Monitor coverage of the Tennessee sludge spill
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Bottom ash is one kind of waste produced by coal-fired power plants in the coal combustion process. Along with fly ash, which is captured by pollution control equipment from coal plant chimneys, it is often referred to simply as coal ash.

Coal bottom ash is a coarse, granular byproduct of coal combustion that is collected from the bottom of coal furnaces in power plants. When pulverized coal is burned in a dry bottom boiler, approximately 80 percent of the unburned material is captured from the chimney as fly ash. The remaining 20 percent is bottom ash, which is collected in a water-filled hopper at the bottom of the furnace. In wet bottom boilers, bottom ash is kept in a molten state and collected when it flows into the ash hopper below. The water in the ash hopper immediately fractures the molten material into crystallized pellets. In these cases the bottom ash is referred to as boiler slag (also known as "black beauty"), a hard, black, glassy material.[1]

Coal ash

The 1.05 billion tons of coal burned each year in the United States contain 109 tons of mercury, 7884 tons of arsenic, 1167 tons of beryllium, 750 tons of cadmium, 8810 tons of chromium, 9339 tons of nickel, and 2587 tons of selenium. On top of emitting 1.9 billion tons of carbon dioxide each year, coal-fired power plants in the United States also create 120 million tons of toxic waste. That means each of the nation's 500 coal-fired power plants produces an average 240,000 tons of toxic waste each year. A power plant that operates for 40 years will leave behind 9.6 million tons of toxic waste.[2] This coal combustion waste (CCW) constitutes the nation's second largest waste stream after municipal solid waste.[3]

When coal is burned, toxins in the coal are released into the furnace and smokestack. With modern air pollution controls, airborne toxins are captured through filtration systems before they can become airborne, and contained in a fine ash called coal ash, fly ash, or coal combustion waste. As a result, heavy metals such as mercury are concentrated in what the EPA considers "recycled air pollution control residue."[4]

Coal ash contains large quantities of toxic metals, including 44 tons of mercury, 4601 tons of arsenic, 970 tons of beryllium, 496 tons of cadmium, 6275 tons of chromium, 6533 tons of nickel, and 1305 tons of selenium.[2] In 2006, coal plants in the United States produced almost 72 million tons of fly ash, up 50 percent since 1993.[4]

Other uses for bottom ash

The utilities industry has promoted the reuse of coal combustion waste because of the growing amount produced each year — 131 million tons in 2007, up from less than 90 million tons in 1990. Uses for coal ash include construction fill, dry wall, cement, and mine reclamation. In 2007, 50 million tons of fly ash was used for agriculture purposes, such as improving the soil’s ability to hold water, in spite of a 1999 EPA warning about high levels of arsenic.[5] Boiler slag is used as blasting grit and in roofing shingles. Both materials have also been used in snow and ice control and as components of asphalt paving.[1]



  1. 1.0 1.1 Coal Bottom Ash/Boiler Slag, Turner-Fairbank Highway Research Center, U.S. Department of Transportation, accessed August 2009.
  2. 2.0 2.1 "Green Coal?," Rachel's Environment & Health News, November 6, 2008.
  3. Sue Sturgis, "Coal's ticking timebomb: Could disaster strike a coal ash dump near you?," Institute for Southern Studies, January 2009
  4. 4.0 4.1 "Fly ash: Culprit at Lafarge? Residue of coal-burning is being examined as possible source of mercury pollution," Times Union, October 26, 2008.
  5. Shaila Dewan, "Hundreds of Coal Ash Dumps Lack Regulation," New York Times, January 7, 2009.

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