Explain the FBC (Fluidized Bed combustion) and neat diagram

• Principal of FBC :

1. A fluidized bed may be defined as the bed of solid particles behaving as a fluid.
2.  When a gas is passed through a packet bed of finely divided solid particles, it experiences a pressure drop across the bed.
3. At low gas velocities, this pressure drop is small and does not disturb the particles. But if the gas velocity is increased further, a stage is reached, when particles are suspended in the gas stream and the packet bed becomes a 'fluidized bed'.
4. With further increase in gas velocity, the bed becomes turbulent and rapid mixing of particles occurs.
5. In general, the behavior of this mixture of solid particles and gas is like a fluid. Burning of a fuel in such a state is known as fluidized bed combustion (FBC).

• Working :

1. Fig. given below shows the arrangement of the FBC system.
2. The fuel and inert material dolomite are fed on the distributor plate and from its bottom air is supplied.
3. The high velocity of air keeps the solid feed material in suspending condition during burning.
4. The generated heat is rapidly transferred to the water passing through the tubes immersed in the bed and generated steam is taken out.
5.  During the burning sulphur dioxide formed is absorbed by the dolomite and prevents its escape with the exhaust gases. The molten slag is tapped from the top surface of the bed.
6. The primary object of using the inert material is to control the bed temperature, it accounts for 90% of the bed volume.

• Advantages :

1. As a result of better heat transfer, the unit size and hence the capital costs are reduced.
2. Since combustion temperatures are low, the fouling and corrosion of tubes is reduced considerably.
3. Pollution is controlled and combustion of high-sulphur coal is possible.

• Disadvantages :

1. The major drawback is that the air has to be supplied at a high pressure so as to support the bed, the fan power is increased sufficiently so controls are difficult and plant life is low. 
2. 
   Fluidized bed systems are generally smaller for a given thermal output than conventional equipment.