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What is Coal Ash?

Production of Coal Ash

Coal-fired power stations burn pulverized coal in a boiler, and that energy is converted into electricity. The coal ash, which is produced in this combustion, is broadly classified into two types according to the equipment where it is collected. The first type is bottom ash, which is lumps of porous that is collected from the "bottom" of boilers. The second type is fly ash, which is spherical particles of ash floating in the combustion gas. These particles are captured by electric precipitators. The ratio of bottom ash to fly ash is approximately 1:9.

Fly Ash

Particles of ash produced in the combustion of coal float in high temperature combustion gas and are captured by electric precipitators or bag filters. These captured spherical particles are known as fly ash.


When used as a concrete admixture, fly ash demonstrates the following characteristics.

  • Improved fluidity
  • Reduced heat of hydration
  • Suppressed alkali-aggregate reaction
  • Improved long-term strength
  • Improved watertightness

Bottom Ash

Particles of ash produced during combustion inside the boiler melt and fuse, and the pulverized clumps of porous ash that fall to the bottom of the boiler. These are called Bottom Ash.


They have a porous structure, so the CBR (California Bearing Ratio) value, which indicates the capacity to bear compacting, is stable. Bottom Ash possess good drainage and permeability as well as outstanding capacity to retain water.

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