Bottom ash

Bottom ash

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Bottom ash refers to part of the non-combustible residues of combustion. In an industrial context, it usually refers to coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

 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
Fly ash
Fly ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal...

. The clinker
Clinker (waste)
Clinker is a general name given to waste from industrial processes — particularly those that involve smelting metals, burning fossil fuels and using a blacksmith's forge which will usually result in a large buildup of clinker around the tuyère...

s fall by themselves into the water or sometimes by poking manually, and get cooled.

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 to dumping yard far away. 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 (SSC) submerged in a water trough below the furnace which quenches hot ash as it falls from the combustion chamber and removes the wet ash continuously up a de-watering slope before onward discharge into mechanical conveyors or directly to storage silos.

Alternatively bottom ash can be conveyed using the dry technology, the MAC (Magaldi Ash Cooler) System, originally introduced since mid '80s by the Italian company Magaldi Power S.p.A. and in recent years proposed also by Clyde Bergemann's DRYCON system. These systems eliminate water usage in the cooling and conveying of bottom ash. The system cools ash using only a small controlled amount of ambient air.

Bottom ash may be used as an aggregate in road construction and concrete, where it is known as furnace bottom ash (FBA), to distinguish it from incinerator bottom ash
Incinerator bottom ash
Incinerator bottom ash is a form of ash produced in incineration facilities. This material is discharged from the moving grate of municipal solid waste incinerators. Following combustion the ash typically has a small amount of ferrous metals contained within it. This ash can be processed to...

 (IBA), the non-combustible elements remaining after incineration
Incineration is a waste treatment process that involves the combustion of organic substances contained in waste materials. Incineration and other high temperature waste treatment systems are described as "thermal treatment". Incineration of waste materials converts the waste into ash, flue gas, and...

. It was also used in the making of the concrete blocks used to construct many high-rise flats in London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

in the 1960s.

Clyde Bergemann DRYCON Brochure

CASE STUDY: Efficient dry bottom ash handling with Clyde Bergemann DRYCON at Heyuan Power Plant, China

External links

  • EcoSmart Concrete : A site dedicated to the use of Fly ash and other supplementary cementing materials in concrete.
  • LondonWaste: LondonWaste website where you can see how bottom ash is processed to make aggregate.