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Bottom ash is part of the non-combustible residue of combustion in a power plant, boiler, furnace or incinerator. In an industrial context, it has traditionally referred to coal combustion and comprises traces of combustibles embedded in forming clinkers and sticking to hot side walls of a coal-burning furnace during its operation. The portion of the ash that escapes up the chimney or stack is, however, referred to as fly 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 in ash ponds, which can cause serious environmental damage if there are structural failures of the ponds.
Ash handling processes
In a conventional water impounded hopper (WIH) system, the clinker lumps get crushed to small sizes by clinker grinders mounted under water and fall down into a trough from where a water ejector takes them out to a sump. From there it is pumped out by suitable rotary pumps. In another arrangement a continuous link chain scrapes out the clinkers from under water and feeds them to clinker grinders outside the bottom ash hopper.
More modern systems adopt a continuous removal philosophy. Essentially, a heavy duty chain conveyor submerged in a water bath below the furnace which quenches hot ash as it falls from the combustion chamber and removes the wet ash continuously up to a de-watering slope before onward discharge into mechanical conveyors or directly to storage silos.
In the United States, coal ash is a major component of the nation's industrial waste stream. In 2017, 38.2 million short tons (34.7×106 t) of fly ash, and 9.7 million short tons (8.8×106 t) of bottom ash were generated. Coal contains trace levels of arsenic, barium, beryllium, boron, cadmium, chromium, thallium, selenium, molybdenum and mercury, 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.
Most U.S. power plants do not use geomembranes, leachate collection systems, or other flow controls often found in municipal solid waste landfills. Following a 2008 failure that caused the Tennessee Valley Authority’s Kingston Fossil Plant coal fly ash slurry spill, the U.S. Environmental Protection Agency (EPA) began developing regulations that would apply to all ash ponds in the U.S. EPA published a Coal Combustion Residuals (CCR) regulation in 2015, which remains in litigation as of 2020. On December 2, 2019 EPA published a proposed rule that would require power plants to stop placing ash in unlined ponds by August 31, 2020, although some facilities could obtain additional time—up to eight years—to find alternatives for managing ash wastes before closing their surface impoundments.
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.
Bottom ash may be used as raw alternative material, replacing earth or sand or aggregates, for example in road construction and in cement kilns (clinker production). A noticeable other use is as growing medium in horticulture (usually after sieving). In the United Kingdom it is known as furnace bottom ash (FBA), to distinguish it from incinerator bottom ash (IBA), the non-combustible elements remaining after incineration. A pioneer use of bottom ash was in the production of concrete blocks used to construct many high-rise flats in London in the 1960s.
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