Power Plant Engineering

226 POWER PLANT ENGINEERING

tity of fines is limited because the crushing force, due to gravity is not large. Bradford breakers easily
reject foreign matter and produce relatively uniform, size coals. They are usually used at the mine but
may also be used at the plant.
Other simple devices called roll crushers, which have single or double rolls or rotors equipped
with teeth, have been used but have not proven very satisfactory. because of their inability to produce
coal of uniform size.

Casting

Perforated

Plate

Litter

Fig. 7.5. Bradford Breaker.
Pulverizers. The pulverizing process is composed of several stages. The first is the feeding
system, which must automatically control the fuel-feed rate according to the boiler demand and the air
rates required for drying (below) and transporting pulverized fuel to the burner (primary air). The next
stage is drying. One important property of coal being prepared for pulverization! is that it be dry and
dusty. Because coals have varying quantities of moisture and in order that lower-rank coals can be used,
dryers are an integral part of pulverizing equipment. Part of the air from the steam-generator air preheater,
the primary air, is forced into the pulverizer at 650°F or more by the primary air fan. There it is mixed
with the coal as it is being circulated and ground.
The heart of the equipment is the pulverizer, also called grinding mill. Grinding is accomplished
by impact, attrition, crushing, or combinations of these. There are several commonly used pulverizers,
classified by speed:
(1) low-speed: the ball-tube mill
(2) medium-speed: the ball-and-race and roll-and-race mill
(3) high-speed: the impact or hammer mill, and the attrition mill.
The low-speed ball-tube mill, one of the oldest on the market, is basically a hollow cylinder with
conical ends and heavy-cast wear-resistant liners, less than half-filled with forged steel balls of mixed
size. Pulverization is accomplished by attrition and impact as the balls and coal ascend and fall with
cylinder rotation. Primary air is circulated over the charge to carry the pulverized coal to classifiers
(below). The ball tube mill is dependable and requires low maintenance, but it is larger and heavier n
construction, consumes more power than others, and because of poor air circulation, works less effi-
ciently with wet coals. It has now been replaced with more efficient types
The medium-speed ball-and-race and roll-and race pulverizers are the type in most use nowa-
days. They operate on the principles of crushing and attrition. Pulverzation takes place between two
surfaces, one rolling on top of the other. The rolling elements may be balls or ring-shaped rolls that rolls