Power Plant Engineering

FUELS AND COMBUSTION 227

that roll between two races, in the manner of a ball bearing. Fig. 7.6 shows an example of the former.

The balls are between a top stationary race or ring and a rotating bottom ring, which is driven by the

vertical, aft of the pulverizer. Primary air causes coal feed to circulate between the grinding elements,

and when it becomes fine enough, it becomes suspended in the air and is : carried to the classifier.

Grinding pressure is varied for the most efficient grinding of various coals by externally adjustable

springs on top of the stationary ring. The ball- and race pulverizer has ball circle diameters varying

between 17 and 76 in and capacities between 1J to 20 tons/h. The roll-and-race pulverizer is operated at

lower speeds and larger sizes. A typical one has an 89-in ball circle diameter, a 12 ft diameter, and a 22.5

ft height overall, weighs 150 tons, and is driven by a 700 hp motor. 3 oth types are suitable for direct-

firing systems (see below).

High-speed pulverizers use hammer beaters that revolve in a chamber equipped with high-wear-

resistant liners. They are mostly used with low-rank coals with high-moisture content and use flue gas

for drying. They are not widely used for pulverized coal systems.

Burner Pipe

Shutoff

Valve

Coal-and-Air

Discharge Openings

(individual Burner Lines)

Classifier

Raw

Coal

Feeder

Air Seal

Driving

Ring

Pyrites

Tr a p

Windbox

Fig. 7.6
The classifier referred to above is located at the pulverizer exit. It is usually a cyclone with
adjustable inlet vanes. The classifier separates oversized coal and returns it to the grinders to maintain
the proper fineness for the particular application and coal used. Adjustment is obtained by varying the
gas-suspension velocity in the classifier by adjusting the inlet vanes.
The Pulverized-Coal System. A total pulverized-coal system comprises pulverizing, delivery,
and burning equipment. It must be capable of both continuous operation and rapid change as required by
load demands. There are two main systems: the bin or storage system and the direct-firing system.