Fluidized Bed Combustion Boilers (FBCB)

1. Minimum fluidization velocity of air was defined the velocity of air needed at which the mixture of fuel and air starts behaving like a fluid and the burning of the fuel in such state was called fluidized bed combustion boilers.
2. We shall now discuss the classification and working of some of the fluidized bed combustion boilers.
3. Fluidized bed combustion boilers (FBCB) are more suitable in India, since we have large quantities of coal of inferior quality with high ash and sulphur content.
4. With the industrial development , we are required to reduce emissions which are reduced by use of FCBC . It also reduces the size of plant.

Classification of Fluidized Bed Combustion Boilers :

1. The combustion chamber of a circulating fluidized bed combustion boiler operates under atmospheric pressure due which these are also called as atmospheric fluidized bed combustion boilers.
2. These types of boiler are presently in developed state and these are being used in many countries.
3. These boilers are further classified as underfeed and overfeed boilers depending upon whether the loading of fuel and supply of air is from the bottom of the bed or on the top of the bed respectively.
4. The underfeed atmospheric fluidized bed combustion boilers (FBCB) are compact in design, however, they have high operational costs.
5. Whereas, the overfeed FBCB are simple in operation with less operational cost but these give lower heat transfer rates per m of area of the bed A circulating fluidized bed combustion boiler (CFBCB) is shown in Figure A.
6. These boilers are called as circulating type FBCB sinice a good portion of the particles exit the combustion chamber with the flue gases.
7. The particles are then separated from the exhaust and re-injected into the fluidized bed system.
8. The fluidized air flows through the grid plate from the air plenum chamber into the bed where the combustion of coal occurs.
9. The coal is crushed between 6 mm to 22 mm size before supplied to the combustion chamber with lime stone by the feed hooper.
10. The mixture of fuel and air becomes a swirling mixture in the combustion chamber and the rapid combustion takes place at a temperature of (800°C-900°C).
11. Heat is transferred to the submerged tube bank.
12. Lime stone allows the sulphur to remain in the bed and formation of H₂SO₄, is prevented.
13. The lower combustion temperatures prevent the formation of NO_X, thus the inferior quality of cont can be used and no pulveriser is needed.
14. The products of combustion leaving the bed contain large portion of unburned carbon particles that are collected in the cyclone separator.
15. It separates the carbon, particles from dust particles by centrifugal action.
16. The carbon particles are returned back to the combustion chamber to complete their combustion.
17. The boiler water tubes are located in the furnace and the steam is generated at a rapid rate.

1. Pulverized coal is not needed.
2. NO_X formation does not take place.
3. Desulphurization of coal take place at the bed due to addition of lime stone, therefore the exhaust gases can be cooled to lower temperature before discharged through the chimney.
4. Slugging problem is reduced due to combustion at low temperature.
5. Low grade coal can be used.
6. Cheaper alloys can be used for its construction due to low combustion temperatures of fuel.
7. Heat release rates are 10 to 15 times higher compared to conventional boilers.
8. Surface heat transfer rates are 2 to 3 times compared to conventional boiler.
9. Size of the plant is considerably reduced.

2. Pressurized Fluidized Bed Combustion Boiler (PFBCB) :

1. Pressurized FBC boiler is the latest system used for generation of power in which air is supplied under pressure at 10 bar approximately with the help of an air compressor driven by a gas turbine as shown in Figure B.
2. The supply of air under pressure results into better rates of heat transfer compared to circulating FBC boiler.
3. The mixture of fuel and lime stone is supplied under pressure to the fluidized bed having a part of evaporator tubes immersed in the bed and remainder tubes as free board tubes above the bed.
4. The ash is collected at the bottom.
5. The flue gases along with unburnt carbon leaving the combustion chamber are passed through a cyclone separator where the unburned carbon particles are separated from the gas due to centrifugal action from the mixture.
6. Separated carbon particles are returned to fluidized bed to complete their combustion.
7. The flue gases from separator are now passed through a dust separator.
8. The cleaned flue gas is then supplied to the gas turbine where it expands upto the atmospheric pressure.
9. The power of the gas turbine is used to run an air compressor for supplying the compressed air to the combustion chamber.
10. FBC systems have various designs e.g. FBC with gas turbine, FBC with steam turbine, FBC with SO, recovery system etc.

1. High rate of coal loading with high rate of combustion of fuel is achieved.
2. Load variation on plant can be met easily by varying the coal loading on plant.
3. Considerable reduction in volume of furnace, hence size of plant is reduced drastically.
4. High steam generation rates are achieved.
5. Low NO_X emissions.
6. Reduced formation of SO₂.
7. The cost of plant is reduced.
8. Requires reduced air velocity compared to CFBCB.

1. Combustion rates cannot be controlled.
2. Life of plant is low compared to conventional boilers.
3. Erosion of walls of the furnace.
4. Complexity of operation.