Fly ash generation and collection

The fly ash produced from the burning of pulverized coal in a coal-fired boiler. Typically, coal is pulverized and blown with air into the boiler's combustion chamber. Where it ignites immediately generating heat and producing a molten mineral residue. Boiler tubes extract heat from the boiler, cooling the flue gas which causes the molten mineral residue to harden and form ash (Figure 1).

It is a fine material with spherical particles. It very much resembles volcanic ashes and also named as ‘Pozzolans’. Fly ash is generated from various organic and inorganic constituents present in feed coals and is produced at a temperature of 1200-17000C. Indian coal has high ash contents that vary from 30 to 50%.
The lighter fine ash particles are termed as flyash which remain suspended in the flue gas. While coarse ash particles, referred to as bottom ash or slag, fall to the bottom of the combustion chamber. Flyash is removed by particulate emission control devices such as electrostic precipitators or filter fabric baghouses.
The component of flyash vary considerably depending upon the source of the coal being burned, but all flyash includes substantial amounts of silicon dioxide and calcium oxide , both being endemic ingredients in many coal bearing rock strata.

About 80% of all the ash leaves the furnace as flyash. When pulverized coal is combusted in
a wet-bottom (or slag-tap) furnace, 50% of the ash is retained in the furnace and the other 50
percent being entrained in the flue gas. In a cyclone furnace, where crushed coal is used 70
to 80 % of the ash is retained as boiler slag and only 20 to 30 % leaves the furnace as dry-ash
in the flue gas.

More see: Some properties of flyash for mine backfilling

Sundeep choudhary, Full text