The estimated annual generation in India are:
|Byproduct||Industrial Source||Million TPA|
|Fly Ash||Thermal plants/Steam boilers||120 *|
|Lime Sludge||Paper, gas, sugar||5|
|Gypsum||Fertilizer / Chemical||2|
|Slag||Steel / Iron||10|
|Rice Husk Ash||Rice husk fired boilers||4|
|Miscellaneous||Aluminum and others||9|
The ash content varies from 5 to 45% in different coals. Indian coals are known for their high ash content in the range of 30-45%. Out of this ash, about 20% is over burnt and settled as bottom ash and 80% of ash flies along with flue gas and trapped in electrostatic precipitators (ESP) or bag filters.
Lime comes as byproduct in two forms:
- In paper and certain chemical industries, lime gets into reaction and comes out as calcium carbonate. Each ton of CaO gives 1.78 tons of CaCO3.
- In the production of acetylene gas, calcium carbide is treated with water by which calcium hydroxide comes as byproduct at 1.32 times to that of CaO.
When rock phosphate is digested with sulphuric acid, for every ton of phos acid 2.6 tons of gypsum (dihydrate) is generated. Gypsum (anhydrite) also emerges in the desulphurisation of flue gases when high sulphur coal is burnt in boilers.
Each ton of steel production contributes for the generation of 0.4 to 0.6 tons of slag. Lime, silica, alumina, associated with iron ore and added fluxes, get into molten state along with metal. If this molten slag is allowed to cool gradually, it gets into crystalline dusty material without any binding property. The molten slag, when quenched with water, attains glassy structure and, upon grinding, becomes cementitious.
Rice Husk Ash
Each ton of rice paddy gives raise to about 0.2 tons of rice husk which, in turn, contains 20% of rice husk ash (RHA).
RHA is high pure silica of not less than 90% and emerges in amorphous state when rice husk is combusted in controlled conditions.