Fluidized Bed Combustion (FBC) has emerged as a viable alternative and has significant advantages compared to conventional combustion system and provides many advantages such as flexible fuel higher combustion efficiency and reduces emission of harmful pollutant such as SOx and NOx.
Fuels can be burned in fluidized bed combustion are coal, reject washing clothes, rice husks, bagasse and other agricultural wastes. Fluidized bed combustion boiler has wide capacity range between 0.5 ton/hour to more than 100 ton/hour. When air or gas is distributed evenly passed upward through a bed of solid particles such as sand which is supported on a fine mesh, the particles will be undisturbed at low velocity. As air velocity is gradually increased, it becomes a state where particles are suspended in the air stream-bed which is called as “fluidized”.
With the increase in air velocity, there is bubble formation, strong turbulence, rapid mixing and formation of the dense bed surface. Solid bed particles show properties of boiling liquid and looks like a fluid “bubbling fluidized bed”. If sand particles in fluidized sate are heated to the temperature of flame coal and coal is injected continuously into the bed, the coal will burn rapidly and the bed reaches uniform temperature.
Fluidized Bed Combustion (FBC) takes place at temperature about 840 C to 950 C. Because the temperature is much below the ash fusion temperature, the melting of ash and the problem related therein can be avoided. The lower combustion temperature is reached due to high heat transfer coefficient as a result of rapid mixing in the fluidized bed and effective extraction from bed through heat transfer between tubes and wall bed.
The gas velocity is reached between minimum fluidization velocity and particle entrainment velocity. This ensures stable operation of the bed and avoids particle entrainment in the flue gas.