Bottom ash

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.

Modern municipal waste incinerators reduce the production of dioxins by incinerating at 850 to 950 degrees Celsius for at least two seconds, forming incinerator bottom ash as byproduct.

Waste handling

In United States facilities, the ash waste is typically pumped to ash ponds, the most common disposal method.[1] Some power plants operate a dry disposal system with landfills.

Environmental impacts

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 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.[4]

Most U.S. power plants do not use geomembranes, leachate collection systems, or other flow controls often found in municipal solid waste landfills.[5] 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 2019.[6] 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.[7]

Ash recycling

These days bottom ash can be extracted, cooled and conveyed using dry ash technology from various companies. Dry ash handling has many benefits. When left dry the ash can be used to make concrete 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.

See also

References

  1. McLaren, R.J. & DiGioia, A.M. (1987). "The Typical Engineering Properties of Fly Ash". In Woods, R. D. (ed.). Geotechnical Practice for Waste Disposal '87; Proceedings of a Specialty Conference. Reston, VA: American Society of Civil Engineers. ISBN 9780872626041.
  2. "Coal Ash Basics". EPA. 2019-02-05.
  3. 2017 Coal Combustion Product Production & Use Survey Report (PDF) (Report). Farmington Hills, MI: American Coal Ash Association. 2018.
  4. Schlossberg, Tatiana (2017-04-15). "2 Tennessee Cases Bring Coal's Hidden Hazard to Light". The New York Times.
  5. Kessler, K. A. (1981). "Wet Disposal of Fossil Plant Waste Case History". Journal of the Energy Division. American Society of Civil Engineers. 107 (2).
  6. Green, Douglas H.; Houlihan, Michael (2019-04-24). "D.C. Circuit Court Remands CCR Deadline Extension to EPA". Environment, Energy, and Resources Section. Washington, DC: American Bar Association.
  7. EPA. "Hazardous and Solid Waste Management System: Disposal of Coal Combustion Residuals From Electric Utilities; A Holistic Approach to Closure Part A: Deadline To Initiate Closure." Proposed rule. Federal Register, 84 FR 65941. 2019-12-02.
  • EcoSmart Concrete - Use of Fly ash and other supplementary cementing materials in concrete
  • LondonWaste - How bottom ash is processed to make aggregate
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