Power Plant Engineering

STEAM POWER PLANT 161

4.7.3 Shaft Mill

Fig. 4.24 shows fuel pulverization with a shaft mill. The fuel from bunker is moved to feeder via
automatic balance. Then from duct fuel goes to mill where it is crushed by beaters secured on the
spindle of the mill rotor.

Bunker

Automatic

balance

Feeder

Duct

Shaft

Mill

Air duct

Hole

Fig. 4.24. Pulverization with a Shaft Mill.

The pulverised fuel is dried up and then blown into shaft by hot air. Secondary air is delivered

into the furnace through holes to burn the fuel completely.

4.8 PULVERISED COAL FIRING

Pulverised coal firing is done by two system :

(i) Unit System or Direct System.

(ii) Bin or Central System.

Unit System. In this system (Fig. 4.25) the raw coal from the coal bunker drops on to the feeder.

Raw Coal

Banker

Feeder

Hot Air

Fan

Pulverising

Mill

Burner

Secondary Air

Primary Air

Coal

Furnace

Fig. 4.25. Unit or Direct System.