Types of pulverised coal firing system
(i) Unit system (or) Direct System
(ii) Bin (or) Central system
(iii)Semi direct firing system.
Pulverised Coal Firing System:
Pulverised coal firing is done by two systems:
i) Unit system or Direct System.
ii) Bin or Central system
In this system, the raw coal from the coal bunker drops on to the feeder.
Hot air is passed through coal in the factor to dry the coal. The coal is then transferred to the pulverising mill where it is pulverised. Primary air is supplied to the mill, by the fan. The mixture of pulverised coal and primary air then flows to burner where secondary air is added. The unit system is so called from the fact that each burner or a burner group and pulverizer constitute a unit.
1. The system is simple and cheaper than the central system
2. There is direct control of combustion from the pulverising mill.
3. Coal transportation system is simple.
Central or Bin System
It is shown in figure. Crushed coal from the raw coal bunker is fed by gravity to a dryer where hot air is passed through the coal to dry it. The dryer may use waste flue gasses, preheated air or bleeder steam as drying agent. The dry coal is then transferred to the pulverising mill. The pulverised coal obtained is transferred to the pulverised coal bunker (bin). The transporting air is separated from the coal in the cyclone separator. The primary air is mixed with the coal at the feeder and the mixture is supplied to the burner.
Figure: Central or Bin system
1. 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 affect 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.
1. The initial cost of the system is high
2. Coal transport system is quite complicated
3. The system requires more space.
Semidirect Firing System:
A cyclone separator between the pulverizer and furnace separates the conveying medium from the coal. The hot primary air separated in the cyclone is used by the exhauster or primary air fan to push the coal particles, falling by gravity from the cyclone, through the burners into the furnace.
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