Presented by V.G.Shivaprasad

Air or gas is passed through an inert bed of solid particles called bed material. sand or crushed refractory on grid (Bed)

Purpose of bed material:- good mixing of fuel and air maintaining uniform temperature bouncy and gravitational forces are balanced & ideal environment for excellent combustion is created

Ignition energy:- For providing ignition energy pilot flame gas igniters or oil

guns or charcoal dipped in diesel are used.

Remove pilot flame gas igniter and oil guns.

Supply the full load air by FD fan.

Maintain Furnace pressure between -3mmwc to

-5mmwc.

Start filling the bed material in all compartments .

Wind box pressure readings in all compartments are

to be uniform.

1. By external combustor2. By oil burner provision

inside furnace3. spreading LDO dipped

charcoal

Close - Primary Air Line bottom gates. Close - PA line individual dampers and

compartments. Close - secondary dampers. Close - oil burner air supply dampers. Start - ID fan . Open - first compartment damper and allow the

air per compartment. Other compartments are to be checked one by

one. Wind box differential pressure to be measured. Finally allow the total air and open all

compartment valves.

(1) When the bed material is above 600°C:-

Oil burners are not required. The coal can be supplied directly into

the bed gradually and raise the temperature around 800°-850°C.

Bed temperature should not drop below 750°C.

The other compartments can also be made active by proper coal supply and air flow.

Hot start-up contd (2) If the bed temperature is less

than 600°C:- Start ID fan, FD fan and PA fan as

per startup procedure. If bed level is too high drain the

bed material. Start the oil burners and raise the

bed temperature above 600°C.

During no load condition & shut down, bed slumping operation required.

Combustion control has to be changed to manual mode.

Reduce fuel flow and cut off fuel feeding to the slumping compartments.

Slump the extreme bed instead of doing first or middle bed

Reduce air flow and close the wind box damper of the slumping compartment.

Finally completely put off the coal feeders and primary air dampers of the slumped bed.

Further reduction in boiler loads, additional compartments has to be slumped.

Turn down - Compartmental Operation

500

600

700

800

900

1000

0 10 20 30 40 50 60 70 80 90 100 110 120

Load, % MCR

Bed

Tem

per

atu

re, D

eg-C

12 - 37 % 28 - 65 %

46 - 90 %60 - 110 %

When the limestone is injected into the furnace, the following reactions occur:-

Oxidation of sulphur S+O2--> SO2 Limestone is calcined to form calcium oxide CaCO3 --> CaO + CO2 –425 kcal/kg Sulphur dioxide gas reacts with solid CaO SO2 + 1/2 O2 + CaO --> CaSO4+3740 kcal/kg The resulting calcium - sulphate based ashes are

chemically stable and are easily disposed. This ash can be used as a raw material for cement

manufacturing, concrete blocks, road base, structural fills, etc.

Residence time is greater than conventional firing.

Ignition temperature is constantly supplied by a hot

bed material which act as “Thermal flywheel”.

Turbulence is promoted by fluidization. Combustion takes place at 800°-900°C

resulting in reduced NOₓ formation.

SO₂ emissions can be reduced by the injection of sorbent into the bed.

Combustion conditions are relatively uniform through out the combustor.

The fluidizing velocity (4-6m/s) is high.

The solids are separated from the flue gases in cyclone separator.

Individual particles may recycle anything from 10 to 50 times.

Combustion contd

The loop seal is a U-shaped refractive line device which prevents gas flow short circuiting from lower furnace to cyclone.

The fuel is fed back to the boiler through the loop seal return legs.

Ash and unburned carbon are re-circulated, probably many times.

Because of recirculation of the bed material, particle residence times are relatively longer.

Combustion contd

Fuel preparation Fuel handling Velocity of air Furnace pressure SOx and NOx emission

Combustion controls:- • Support fuel flow • Furnace pressure • Bed temperature • Bed level

• Air flow • Limestone feed Furnace safety controls:-

Different from PF boiler To safely start-up, shut-down and monitor the boiler

during operation and equipment malfunction. There is no distinct flame and the reliance on bed

temperature to ensure safety controls unique.

Turbine bypass system:-•Not common for all CFB.•Main system used for dissipating the large thermal inertia of a CFB, when there is a turbine full load trip.•The system is complex and uses of large valves which have to open quickly.

Controls contd

FBC advantage:- Suitable for firing any kind of fuel. No limitation on fuel quality. Economy on construction. Reduced maintenance. No slagging & fouling on tubes. Low Nox / Sox emission. Low operating cost. The ash quality is better & non-toxic, which

can be used for mixing with cement.

FBC disadvantage:- In FBC boilers we can not turn off the boiler

instantly as we can do it in pulverized fuel boiler.