Fly Ash: Definitions and Divisions of Fly Ash, Composition of Coal

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Non-aqueous, non-gaseous remains of burnt particles are known as ash.

Ash is Categorised into Three Types

  • Fly ash: It is very fine material and arises from the flue gases which are captured by Electrostatic Precipitators.
  • Bottom Ash: It is coarse in nature but do not fly with flue gas. These are collected from the bottom of the boiler.
  • Economizer/APH ash: It has similar characteristics to that of bottom ash and are recovered from the Economizer hopper and APH hopper. These are formed in very small quantity.

Definitions of Fly Ash

  • Siliceous or siliceous and aluminous material which in itself possesses little or no cementitious value, chemically react with Calcium Hydroxide (lime) in presence of water at ordinary temperature and form soluble compound comprises cementitious property similar to cement.
  • Includes substantial amount of Silicon Dioxide, Aluminium oxide and iron oxide and small fraction of other constituents like MgO, Na2O, K2O, chloride, unburnt carbon and heavy metals in traces.

Divisions of Fly Ash

Siliceous Fly Ash

  • These are normally produced from anthracite or bituminous coal
  • It has pozzolanic properties
  • It consists of reactive calcium oxide less than 10 percent

Calcareous Fly Ash

  • These are produced from lignite or sub bituminous coal
  • It has both pozzolanic and hydraulic properties
  • It consists of reactive calcium not less than 10 percent by mass.

Composition of Coal

Composition of Coal

Utilization of Fly Ash

Utilization of Fly Ash
Utilization of Fly Ash

Multiple Choice Questions

1. The ash which is coarser in nature and is found in bottom of boiler is known as a) Fly ash b) Bottom ash c) Economizer d) All of the above Answer: b

2. Which among the following is not found in fly ash? a) Silicon dioxide b) Aluminium oxide c) Iron oxide d) Calcium carbonate Answer: d

3. Which among the following is a high value utilization of fly ash? a) Pozzolan cement b) Highway base c) Light weight refractory d) All of the above Answer: c 4. Which ash is produced from lignite or sub bituminous coal? a) Calcareous fly ash b) Siliceous fly ash c) Both of the above d) None of the above Answer: a

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