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Bottom ash is part of the non-combustible residue of combustion in apower plant,boiler,furnace, orincinerator. In an industrial context, it has traditionally referred tocoal combustion and comprises traces of combustibles embedded in formingclinkers and sticking to hot side walls of a coal-burning furnace during its operation. The portion of theash that escapes up thechimney or stack is referred to asfly ash. The clinkers fall by themselves into the bottom hopper of a coal-burning furnace and are cooled. The above portion of the ash is also referred to as bottom ash.
Most bottom ash generated at U.S. power plants is stored inash ponds, which can cause serious environmental damage if they experience structural failures.
In a conventionalwater-impounded hopper (WIH) system, the clinker lumps get crushed to small sizes by clinker grinders mounted under water and fall down into a trough, where a waterejector takes them out to a sump. From there, it is pumped out by suitablerotary pumps. In another arrangement, a continuouslink chain scrapes out the clinkers from under water and feeds them to clinker grinders outside the bottom ash hopper.
Modern municipal waste incinerators reduce the production ofdioxins by incinerating at 850 to 950 degrees Celsius for at least two seconds, formingincinerator bottom ash as byproduct.
In United States facilities, the ash waste is typically pumped toash ponds, the most common disposal method.[1] Some power plants operate a dry disposal system withlandfills.
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In the United States, coal ash is a major component of the nation's industrial waste stream.[2] In 2017, 38.2 million short tons (34.7×10^6 t) of fly ash, and 9.7 million short tons (8.8×10^6 t) of bottom ash, were generated.[3] Coal contains trace levels ofarsenic,barium,beryllium,boron,cadmium,chromium,thallium,selenium,molybdenum, andmercury, many of which are highly toxic to humans and other life. Coal ash, a product of combustion, concentrates these elements and can contaminate groundwater or surface waters if there are leaks from an ash pond.[4]
Most U.S. power plants do not usegeomembranes, leachate collection systems, or other flow controls often found inmunicipal solid waste landfills.[5] Following a 2008 failure that caused the Tennessee Valley Authority'sKingston Fossil Plant coal fly ash slurry spill, theU.S. Environmental Protection Agency (EPA) began developing regulations that would apply to all ash ponds in the U.S. The EPA published its "Part A" final rule for Coal Combustion Residuals (CCR) on August 28, 2020, requiring all unlined ash ponds to retrofit with liners or close by April 11, 2021. Some facilities may apply to obtain additional time—up to 2028—to find alternatives for managing ash wastes before closing their surface impoundments.[6][7][8] EPA published its "CCR Part B" rule on November 12, 2020, which allows certain facilities to use an alternative liner, based on a demonstration that human health and the environment will not be affected.[9] Further litigation on the CCR regulation is pending as of 2021.[10]
Bottom ash can be extracted, cooled, and conveyed using dry ash handling technology. When left dry, the ash can be used to make concrete, bricks, and other useful materials. There are also several environmental benefits.[11]
Bottom ash may be used as raw alternative material, replacing earth or sand or aggregates, for example inroad construction and incement kilns (clinker production). A noticeable other use is asgrowing medium inhorticulture (usually after sieving). In the United Kingdom it is known as furnace bottom ash (FBA), to distinguish it fromincinerator bottom ash (IBA), the non-combustible elements remaining afterincineration. A pioneer use of bottom ash was in the production of concrete blocks used to construct many high-rise flats inLondon in the 1960s.[citation needed]