Power Plant Engineering

166 POWER PLANT ENGINEERING

Raw Coal

Bunker

Conveyor

Breaker

Cyclone

Furnace

Valve

Crusher

Rotary

Seal

Feeder

Primary

Air

Slag

Coal Pipe Conveyor Ta n k

Fig. 4.31

4.9.1 Cyclone Fired Boilers

In cyclone fired boilers the fur-nace is arranged as a horizontal cylinder. The pulverised fuel is
bed along the periphery of the cylinder. The hot gases travel axially into the water tube section having

a tight helix path. The temperature generated in the combustion zone is quite high and because of this

the tubes are coated with fused ash which goes on collecting the ash particles going in the flue gases.

The out going gases contain particles less than 20 microns.

The cyclone furnaces can successfully burn coals having low ash fusion temperature. The
cyclone furnace is operated under combustion air pressure of 700 to 1000 mm of water gauge.

Cyclone fired boilers have the following advantages : (i) Quick load variations can be easily

handled. (ii) Nearly 55% of ash in coal is burnt in the form of liquid slag. The ash can be removed in the

molten form. (iii) The slag can be used as a building material. (iv) Fly-ash problem is reduced to much

lower limits.

4.10 Water Walls

Larger central station type boilers have water cooled furnaces. The combustion space of a fur-

nace is shielded wholly or partially by small diameter tubes placed side by side. Water from the boiler

is made to circulate through these tubes which connect lower and upper headers of boiler.

The provision of water walls is advantageous due to following reasons: (1) These walls provide
a protection to the furnace against high temperatures. (2) They avoid the erosion of the refractory

material and insulation. (3) The evaporation capacity of the boiler is increased.