Fluidized Bed Combustion (FBC)
“Fluidized bed combustion (FBC) is a combustion technology used to burn solid fuels.”
A bed of solid particles is said to be fluidized when the pressurized fluid (liquid or gas) is passed through the medium and causes the solid particles to behave like a fluid under certain conditions.
Fluidization causes the transformation of the state of solid particles from static to dynamic.
- Bubbling Fluidized Bed Combustion (BFBC)
- Circulating Fluidized Bed Combustion (CFBC).
Bubbling FBC is used for Fuels with lower heating values such as Rice Husk.
Under such sort, the main factors leading to fluidization are as follows:
- Solid Fuel Particle Size
- Air Fuel Mixture
Fluidized Bed Combustion takes place when the forced draught fan supplies air to the Furnace of the Boiler. In the furnace, and is (used for Bubbling phenomenon) placed on the Bed and is heated before fluidization, the air enters the bed from the nozzles fitted on the Furnace Bed. And above the nozzles; the sand opposes the upward motion of the air.
But at sufficient velocities, when the pressure applied by the air becomes equal to the weight of the sand, fluidization of the sand occurs.
Now the fuel supplied by fuel conveyor is fed to the preheated bubbling sand and gets combusted away. This phenomenon also ensures complete combustion of the Fuel.
The heat released during combustion heats up the surrounding boiler tubes and generates steam. The major advantages of Bubbling Fluidized Bed Combustion are the enhanced thermal efficiency, easy ash removal.
Another type is the Circulating Fluidized Bed Combustion; it is applied to fuels with higher heating values such as Petcoke.
In this, the unburned fuel is fed again to the furnace with the help of a Forced Draught fan and ducts, ensuring enhanced combustion and higher heating and provides excellent fuel flexibility.
Also, the fluidizing velocity of Air in CFBC is comparatively higher than that of BFBC. One of the major drawbacks is the power consumption.
The motors installed in the Forced Draught Fan consume more power than the one installed in the same capacity Boiler’s ( wood/coal fired) Forced Draught Fan, because of elevated levels of draught requirement to create fluidization.