Coal is mixed with sand and limestone/dolomite (to absorb SOx pollutant gases). Coal is combusted with the help of hot primary air from air pre-heater. However, here, coal doesn’t combust violently resulting into flames, but rather the heat resulting from combustion is instantaneously exchanged with the water tubes running through the coal-bed-air suspension bed. Thus, the combustion of coal takes place at a relatively lower temperature of around 850-900 deg C, necessitating lower thermal insulation of the boiler compared to PC boiler.
Advantages of CFB boiler
1. Lower combustion temperature translates into lower thermal insulation needs of the boiler, directly resulting in savings.
2. As most of the SOx gases are absorbed by the bed, flue gases can be cooled to lower temperatures than PC boiler, thereby increasing the amount of potential heat exchange.
3. Unburnt coal particles are recycled back with the help of cyclone separators. These separators separate heavy unburnt coal particles from the lighter ash particles by centrifugal action. The unburnt coal is then fed back into the bed for combustion. This results in direct savings on fuel.
PC (Pulverised Coal) Boiler
The fuel burnt in PC boiler is coal, which is pulverised (grinded into small powder form) in a coal grinding mill. Pulverised coal is often conveyed to the burners in boiler through pneumatic means (by forcing coal particles by using moving air) from the coal pulveriser mill.
Some issues with the PC boiler:
1. Coal is combusted at temperatures of about 1600-1800 deg C. At such high temperatures, NOx, which is a notorious pollutant, is formed.
2. SOx pollutants are carried away in flue gases, which condense to form highly corrosive sulphuric acid at temperatures around 140-160 deg C. Thus, the exhaust temperature has to be maintained above this temperature to avoid large-scale corrosion of boiler water tubes. This limits the boiler energy extraction efficiency and constitutes the largest heat loss from the boiler.
3. Unburnt coal particles can’t be recycled, because there’s no economical way to separate unburnt coal particles from the ash and flue gases.