Power Plant Engineering

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

---

FUELS AND COMBUSTION
229
Raw Coal
Bunker
Bin
Gate
Feeder
Pulverizer
Control
Damper
Primary
Air fan
Pulverized Fuel
and Air Piping
Pulverized Fuel
Burners
Hot Air
Damper
Tempering Air
Damper
Cold (Tempering) Air
from Forced Draft Fan
Hot Air from
Boiler Air Heater
Boiler
Front Wall
Burner
Windbox
Basement Floor