Bottom Ash
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Bottom ash is part of the non-
combustible A combustible material is something that can burn (i.e., ''combust'') in air. A combustible material is flammable if it ignites easily at ambient temperatures. In other words, a combustible material ignites with some effort and a flammable mat ...
residue of combustion in a
power plant A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the electricity generation, generation of electric power. Power stations are generally connected to an el ...
,
boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, centr ...
,
furnace A furnace is a structure in which heat is produced with the help of combustion. Furnace may also refer to: Appliances Buildings * Furnace (central heating): a furnace , or a heater or boiler , used to generate heat for buildings * Boiler, used t ...
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 Ash or ashes are the solid remnants of fires. Specifically, ''ash'' refers to all non-aqueous, non- gaseous residues that remain after something burns. In analytical chemistry, to analyse the mineral and metal content of chemical samples, 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 pond An ash pond, also called a coal ash basin or surface impoundment,U.S. Environmental Protection Agency (EPA), Washington, D.C. "Hazardous and Solid Waste Management System; Identification and Listing of Special Wastes; Disposal of Coal Combustion ...
s, which can cause serious environmental damage if they experience structural failures.


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 pump A rotary vane pump is a positive-displacement pump that consists of vanes mounted to a rotor that rotates inside a cavity. In some cases these vanes can have variable length and/or be tensioned to maintain contact with the walls as the pump rotates ...
s. In another arrangement a continuous
link chain A chain is a serial assembly of connected pieces, called links, typically made of metal, with an overall character similar to that of a rope in that it is flexible and curved in compression but linear, rigid, and load-bearing in tension. A ...
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 Incinerator bottom ash (IBA) is a form of ash produced in incineration facilities. This material is discharged from the moving grate of municipal solid waste incinerators. Once IBA is processed by removing contaminants, it can be used as an aggreg ...
as byproduct.


Waste handling

In United States facilities, the ash waste is typically pumped to
ash pond An ash pond, also called a coal ash basin or surface impoundment,U.S. Environmental Protection Agency (EPA), Washington, D.C. "Hazardous and Solid Waste Management System; Identification and Listing of Special Wastes; Disposal of Coal Combustion ...
s, the most common disposal method. 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. In 2017, of fly ash, and of bottom ash were generated. Coal contains trace levels of
arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, ...
, barium,
beryllium Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to form m ...
, boron,
cadmium Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...
, chromium,
thallium Thallium is a chemical element with the symbol Tl and atomic number 81. It is a gray post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Chemists William Crookes an ...
,
selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...
, 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
geomembrane A geomembrane is very low permeability synthetic membrane liner or barrier used with any geotechnical engineering related material so as to control fluid (liquid or gas) migration in a human-made project, structure, or system. Geomembranes are ...
s, leachate collection systems, or other flow controls often found in
municipal solid waste Municipal solid waste (MSW), commonly known as trash or garbage in the United States and rubbish in Britain, is a waste type consisting of everyday items that are discarded by the public. "Garbage" can also refer specifically to food waste ...
landfills. Following a 2008 failure that caused the Tennessee Valley Authority’s
Kingston Fossil Plant coal fly ash slurry spill The Kingston Fossil Plant coal fly ash slurry spill was an environmental and industrial disaster that occurred on Monday December 22, 2008, when a dike ruptured at a coal ash pond at the Tennessee Valley Authority's Kingston Fossil Plant in ...
, the
U.S. Environmental Protection Agency The Environmental Protection Agency (EPA) is an Independent agencies of the United States government, independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon pro ...
(EPA) began developing regulations that would apply to all ash ponds in the U.S. 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. 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. Further litigation on the CCR regulation is pending as of 2021.


Ash recycling

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 A road is a linear way for the conveyance of traffic that mostly has an improved surface for use by vehicles (motorized and non-motorized) and pedestrians. Unlike streets, the main function of roads is transportation. There are many types of ...
and in
cement kiln Cement kilns are used for the pyroprocessing stage of manufacture of portland and other types of hydraulic cement, in which calcium carbonate reacts with silica-bearing minerals to form a mixture of calcium silicates. Over a billion tonnes of ...
s ( clinker production). A noticeable other use is as
growing medium A growth medium or culture medium is a solid, liquid, or semi-solid designed to support the growth of a population of microorganisms or cells via the process of cell proliferation or small plants like the moss ''Physcomitrella patens''. Different ...
in
horticulture Horticulture is the branch of agriculture that deals with the art, science, technology, and business of plant cultivation. It includes the cultivation of fruits, vegetables, nuts, seeds, herbs, sprouts, mushrooms, algae, flowers, seaweeds and no ...
(usually after sieving). In the United Kingdom it is known as furnace bottom ash (FBA), to distinguish it from
incinerator bottom ash Incinerator bottom ash (IBA) is a form of ash produced in incineration facilities. This material is discharged from the moving grate of municipal solid waste incinerators. Once IBA is processed by removing contaminants, it can be used as an aggreg ...
(IBA), the non-combustible elements remaining after
incineration Incineration is a waste treatment process that involves the combustion of substances contained in waste materials. Industrial plants for waste incineration are commonly referred to as waste-to-energy facilities. Incineration and other high ...
. A pioneer use of bottom ash was in the production of concrete blocks used to construct many high-rise flats in
London London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary dow ...
in the 1960s.


See also

* Coal combustion products * Fly ash *
Health effects of coal ash Coal ash, also known as coal combustion residuals (CCRs), is the mineral residue that remains from burning coal. Exposure to coal ash and to the toxic substances it contains may pose a health risk to workers in coal-fired power plants and resident ...
* Industrial wastewater treatment


References

{{Reflist


External links


EcoSmart Concrete
– Use of fly ash and other supplementary cementing materials in concrete
LondonWaste
– How bottom ash is processed to make aggregate Environmental impact of the coal industry Waste Water pollution