technical NOTES

11. Coal Mill Booster Fan

Preheater inert hot gases are used for drying in the coal mill. Normally coal mill
will be kept close to the preheater fan to take care of this requirement. If there is
an advantage in layout for other benefits sometime coal mill is kept away than
normal. In this case there are two options of taking the hot gases to the coal mill.
Option – 1
Coal mill fan is sized larger to take care of the extra pressure drop and pulls the hot
gases from the preheater fan outlet including the extra distances and bends.
Option - 2
Booster fan is added before the mill which pulls the required hot gases to the
coal mill circuit. Coal mill fan takes gases from only the mill inlet after the booster
fan in a normal way.
Though option-1 is simple and required minimum changes, option -2 is advantages in
long run due to the following reasons.
Safety
In option 1, coal mill fan has to take gases from the preheater fan outlet. The extra
pressure drop created by the ducting including the bends has to be taken care by
the coal mill fan. This creates higher suction in the mill inlet and also at bag filter
inlet. This high suction creates additional air leakage during operation in the
system which increases O2 content in the circuit. This is not good for coal mill
operation because of it diluting the inert atmosphere. Hence option 2 is advantage
from safety point of view.
Power Consumption
In option 1, the extra pressure drop created by the ducting and bends before the
mill will be added to the coal mill fan which handles much higher volume including
recirculation and leakage air than the booster fan in the option 2. Hence system
specific power consumption of option 1 will be higher than option 2. Booster fan
pulls only the required hot gases and hence consumes less power. Leakage air in
option 1 due to high suction takes additional power consumption.
Operational Control
Booster fan in option 2 has better operational control than option 1.
Case Study: Calculations

Operation of Coal Mill with and without Booster Fan

S.N. Operating parameters UOM

Without

booster fan

With booster

fan

1 Coal mill required flow m3/sec 53.2 53.2

2 coal mill inlet draft mmwg -80 -80

3 Coal mill inlet temperature degC 150 150