Page 338 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 323

plant of coal dust and equipment that accumulates the and coal fed to the burners. The coal cannot be simply
fines is an ongoing task. All those activities require more ground down. It has to be dried as well because it does
personnel. The lower cost of coal justifies the added cost contain water and the grind would become muddy
of personnel to handle it. without drying it. To dry the coal the pulverizers are
Coal can tend to “cake” before entering the stoker. supplied with preheated combustion air from an air pre-
The large pieces of compressed, usually wet coal will not heater or, in the case of some small plants, steam heated
burn completely in the furnace unless it is broken up. air.
Preventing caking is accomplished in the handling and One type of equipment that pulverizes the coal
preparation of the coal. Keeping the coal dry by unload- is a ball mill. It consists of a large drum mounted with
ing cars or trucks before it rains or snows and limiting its axis on the horizontal and filled with cast iron balls.
exposure of the fuel to water will reduce caking. The trunions (extensions at the center of the heads of
Clinkers is the name we give to chunks of un- the drum which serve as a shaft) are hollow so air and
burned coal and ash that form in the furnace. Those coal can be fed into one end and the pulverized mixture
large particles can jam stokers and ash handling equip- leaves the other. As the drum rotates the balls are lifted
ment. They’re usually formed when you get low ash fu- and dropped on the coal to crush it. The finely ground
sion temperature coal or coal with a lot of dirt and other particles are carried out with the heated air.
materials in it that melt at the normal furnace tempera- Bowl mills consist of a bowl spinning on a vertical
tures. They can also form when you get a hot spot in the shaft with rollers inside that roll around on the inside of
furnace that is higher than the ash fusion temperature the bowl crushing the coal that’s dumped into the bowl.
(see fuels). When they form, clinkers have to be broken Some use balls instead of rollers. The crushed coal is car-
up to prevent them forming a blockage in the fire that re- ried away by heated air directed up around the bowl.
duces output and increases temperatures in other areas Hammer mills use something comparable to sev-
of the grate. The operator has to watch the coal bed and eral metal hammers that swing freely on a shaft connec-
use special tools with one end inserted into the furnace tion. The metal hammers pound on an accumulation of
to break up the clinkers. coal to break it into fines that are carried away by the air.
Another operation that operators perform with Attrition mills are something like a combination of
coal stokers is “dressing” the fire. Despite all provisions fan and grinder with pins on the circumference of the
the coal never distributes perfectly evenly over the fan wheel that strike the coal particles to crush them.
grate. Dressing the bed (the layer of coal on the grate) is The attrition mills have stricter sizing requirements for
accomplished with tools like those used for clinkers to feed than the others and mill capabilities vary with con-
move the coal around until the bed depth is uniform and struction and manufacturer.
burning evenly. The fans or blowers that transport the coal and
Breaking clinkers and dressing a coal fire are ac- air mixture to the burners are called primary air fans or
tivities that require on the job training and experience to exhausters with the latter term reserved for those that
do it well. I’ll have to admit I could never do it well but move the coal laden air. Most installations use exhaust-
I have observed several operators that, in my opinion, ers to limit potential leakage of powdered coal into the
were artists when it came to dressing a fire. plant. The fuel and air mixture exits the mill into the
exhauster inlet which discharges the mix under pressure
Coal burners to the burners. In smaller equipment the pulverizer and
Coal burners are principally designed to burn the exhauster are all in the same housing.
fuel in suspension so it has to be pulverized before it’s What’s probably the most important part of a
delivered to the burner. The bottom of a furnace fitted pulverizer—burner combination is the classifier. It’s
with pulverized coal burners will have means to re- normally a static device (no moving parts) that sepa-
move the ash that drops out of the fire but much of the rates large particles from the stream of coal dust and air
ash is transported through the boiler to be removed by heading to the burners and returns those particles to the
dust collectors on the boiler outlet. Pulverizers form an mill for further grinding. The normal requirements for
integral part of most coal burners. There are (or were, pulverized coal leaving a classifier are at least 85% of the
I’m not sure there are any) plants that burned pulver- coal through a 200 mesh sieve and no more than 5% over
ized coal from storage but most plants have an integral a 5 mesh sieve. Finally, the mixture of coal and primary
pulverizer that grinds the coal to fine powder and mixes air has to be fuel rich to provide a stable point for igni-
it with primary air to produce a fuel rich stream of air tion of the fuel at the exit of the coal nozzle.