Power Plant Engineering

Fig. 4.26. Bin or Central System.
Advantages
l. The pulverising mill grinds the coal at a steady rate irrespective of boiler feed.
2. There is always some coal in reserve. Thus any occasional breakdown in the coal supply will
not effect the coal feed to the burner.
3. For a given boiler capacity pulverising mill of small capacity will be required as compared to
unit system.
Disadvantages
1. The initial cost of the system is high.
2. Coal transportation system is quite complicated.
3. The system requires more space.
To a large extent the performance of pulverised fuel system depends upon the mill performance.
The pulverised mill should satisfy the following requirements:
1. It should deliver the rated tonnage of coal.

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STEAM POWER PLANT
163
2. Pulverised coal produced by it should be of satisfactory fineness over a wide range of capaci-
ties.
3. It should be quiet in operation.
4. Its power consumption should be low.
5. Maintenance cost of the mill should be low.
Fig. 4.27 shows the equipments for unit and central system of pulverised coal handling plant.
Raw Coal
Primary Crusher
Magnetic Separator
Coal Drier
Coal Bunkers
Unit System
Central System
Scale
Scale
Pulveriser
Furnace
Burners
Furnace
Pulveriser
Central Bin
Feeder
Burners
Fig. 4.27. Equipments for Central and Unit System.
4.9 PULVERISED COAL BURNERS
Burners are used to burn the pulverised coal. The main difference between the various burners
lies in the rapidity of air-coal mixing i.e., turbulence. For bituminous coals the turbulent type of burner
is used whereas for low volatile coals the burners with long flame should be used. A pulverised coal
burner should satisfy the following requirements:
(i) It should mix the coal and primary air thoroughly and should bring this mixture before it
enters the furnace in contact with additional air known as secondary air to create sufficient turbulence.
(ii) It should deliver and air to the furnace in right proportions and should maintain stable igni-
tion of coal air mixture and control flame shape and travel in the furnace. The flame shape is controlled
by the secondary air vanes and other control adjustments incorporated into the burner. Secondary air if
supplied in too much quantity may cool the mixture and prevent its heating to ignition temperature.

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POWER PLANT ENGINEERING
(iii) Coal air mixture should move away from the burner at a rate equal to flame front travel in
order to avoid flash back into the burner.
Raw Coal
Bunker
Pulverizer
Feeder
Control
Damper
Primary
Air Fan
Pulverized Fuel
and Air Piping
Pulverized Fuel
Burners
Cold (Tempering) Air
From Forced Draft Fan
Tempering Air
Damper
Hot Air From
Boiler Air Heater
Furnace
Burner Windbox