Fly ash is a fine, powdery material that is a by-product of burning pulverized coal in thermal power plants. When coal is combusted, non-combustible mineral impurities are expelled with flue gases. As these gases cool, the ash particles solidify into spherical glassy particles and are collected through electrostatic precipitators or bag filters.
The primary chemical constituents of fly ash include oxides of silicon (SiO2), aluminum (Al2O3), iron (Fe2O3), and calcium (CaO), accompanied by lesser amounts of oxides of magnesium, potassium, sodium, titanium, and sulphur.
The process of fly ash generation begins with the combustion of pulverized coal in a boiler furnace at high temperatures. As the coal burns, the inorganic mineral matter within the coal melts. These molten particles are carried upward by the exhaust gases (flue gases).
As the gases cool, the molten particles solidify into tiny, spherical glassy particles — this is fly ash. The flue gases carrying fly ash are then passed through electrostatic precipitators or fabric bag filters, which capture the majority of the fly ash particles. The collected fly ash is then stored in large silos or ash ponds for further handling and utilization.