MIL-HDBK-1003/6
particulate loading of the burning gases within the furnace which, without fly
ash reinjection, would result in a high carbon loss in the fly ash. For units
with capacities of 25,000 lbs of steam per hour (3.15 kg/s) and over, front
discharge traveling-grates are commonly used with spreader stokers. Dump,
vibrating, reciprocating and oscillating grates are also available. With a
high particulate loading, the spreader stoker requires the use of
electrostatic precipitator or baghouse collectors to prevent particulate
Pulverized Coal (PC) Firing. Pulverized-coal-fired boilers are
5.2.2.2
typically used in applications of 100,000 PPH of steam and above. In PC
boilers, coal is finely ground in a pulverizer or mill, blown into the
furnace, and burned in suspension. The rate of fuel flow is controlled by a
variable speed coal feeder which supplies coal to the pulverizer. Although
some of the coal ash forms deposits on tubes and some falls to the bottom of
the boiler, most of the ash is swept out with the flue gas stream. The PC
boiler must therefore have a bottom ash hopper, soot blowers, and a
particulate collection system. Although bituminous coals are the most common
fuel in PC boilers, almost any coal can be used. The most important
properties in selecting a fuel for PC firing are; ignition and flame
stability, ash characteristics, and grindability. Flame stability can be
insured by firing fuels with a minimum of about 20 percent volatile matter.
The amount of ash should be minimized and the ash fusion temperature should be
sufficiently high to avoid slagging problems. The ease of pulverizing a coal
is a measure of its grindability. This is usually indicated on the Hardgrove
Grindability Index where those with a low index are harder to grind. The
principal types of pulverizers can be classified under high, medium, and slow
speeds. Proper air flow is essential to the operation of any type of
pulverizer. The preheated air performs three functions; moving and separating
the coal within the mill, drying the coal, and transporting the coal to the
furnace.
a) High Speed Pulverizers. The simplest pulverizer is the high
speed impact mill. In this machine, a series of swinging hammers mounted on a
shaft impact coal fed into the grinding chamber which is lined with abrasion
resistant material. A disadvantage to these mills is their susceptibility to
damage by foreign materials. The main advantage is the low initial cost of
these units.
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