Basic Arrangement of FBC System and its Advantages and Disadvantages

1. The basic arrangement of FBC system is represented in Figure.
2. It consists of a shell with a distributor plate at its bottom with holes in it.
3. The shell carries the boiler evaporator tubes which remain immersed in the fluidized bed as shown in figure.
4. Coal of 6 to 13 mm size with dolamite or lime stone are fed on the distributor plate.
5. Air is supplied from the bottom of the distributor plate at a velocity more than the minimum velocity of fluidization, therefore, the solid coal will remain freely suspended in the gas stream on burning.
6. The heat energy released due to combustion is rapidly transferred at high rates to water in the evaporator tubes and the generated steam is taken out.
7. The sulphur content of coal is converted into SO₂ on burning.
8. It is mostly absorbed by dolamite, hence, the flue gases leaving the bed are almost free from SO₂ gas.
9. The molten ash formed is taken out from the top as shown.
10. Such a system reduces the size of the boiler considerably due to high rates of heat transfer.

Advantages of Fluidized Bed Combustion Systems :

1. Coal used is 2 to 3% of bed volume. Remainder space is occupied by the inert material like sintered ash, fused alumina, sand etc. These materials help in controlling the bed temperatures in the range of 800°C to 1000°C.
2. The low temperatures of combustion in FBC allows ash to remain soft and less abrasive.
3. SO₂ emissions are much less due to addition of dolamite.
4. NO_X, emissions are low since the amount of excess air used is less and temperatures maintained are low in FBC in the range of 800-1000°C compared to pulverized coal combustion system. It is so, since the fixation of free nitrogen at low temperatures does not occur.
5. Its initial and operational costs are low.
6. The rates of heat transfer are much higher since the system behaves like a violently burning liquid.
7. Combustion efficiencies obtained are as high as 99.5%.
8. Coals with high ash contents can be used.
9. Volatilization of alkali compounds does not occur, therefore, their deposits on boiler tubes is minimised.
10. Size of the plant and its cost is considerably reduced.

1. The coal feeding and ash handling systems are complex.
2. It requires crushed coal of proper size.
3. It needs auxiliary starting system like an oil or gas burner.
4. Large variations in load cannot be met with.
5. Difficult to control carbon carryover with the flue gases.