Power Plant Engineering

228 POWER PLANT ENGINEERING

The bin system is essentially a batch system by which the pulverized coal is prepared away from
the furnace and the resulting pulverized-coal-primary-air mixture goes to a cyclone separator and fabric
bag filter that separate and exhaust the moisture laden air to the atmosphere and discharge the pulverized
coal to storage bins (Fig. 7.7). From there, the coal is pneumatically conveyed through pipelines to
utilization bins near the furnace for use as required. The bin system was widely used before pulverizing
equipment became reliable enough for continuous steady operation. Because of the many stages of
drying, storing, transporting, etc., the bin system is subject to fire hazards. Nevertheless, it is still in use
in many older plants. It has, however, given way to the direct-firing system, which is used exclusively in
modern plants.

Pulverized

Coal Bin

To feed

Pulverizer Hopper

Cyclone

Separator

Bag Filter

Exhaust

Coal Tripper

and Conveyor

Pulverizer

Fan

Air Heater

Coal Feeder

Raw Coal

Bunker

Fig. 7.7

Compared with the bin system the direct-firing system has greater simplicity and hence greater
safety, lower space requirements, lower capital and operating costs, and greater plant cleanliness. As its
name implies, it continuously processes the coal from the storage receiving bunker through a feeder,
pulverizer, and primary-air fan, to the furnace burners (Fig. 7.7(a)). (Another version of this system, less
used, places the fan on the outlet side of the pulverizer. Fuel flow is suited to load demand by a combi-
nation of controls on the feeder and on the primary-air fan in order to give air-fuel ratios suitable for the
various steam-generator loads. The control operating range on any one direct firing pulverizer system is
only about 3 to 1. Large steam generators are provided with more than one pulverizer system, each
feeding a number of burners, so that a wide control range is possible by varying the number of pulverizers
and the load on each

Burners A pulverized-coal burner is not too dissimilar to an oil burner. The latter must atomize
the liquid fuel to give a large surface-to-volume ratio of fuel for proper interaction with the combustion
air. A pulverized-coal burner already receives dried pulverized coal in suspension in the primary air and
mixes it with the main combustion air from the steam-generator air preheater. The surface-to-volume ratio