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
Unit 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.
Advantages:
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
Advantages
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.
Disadvantages
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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