Power Plant Engineering

FUELS AND COMBUSTION 229

Raw Coal

Bunker

Bin

Gate

Feeder

Pulverizer DamperControl

Primary

Air fan

Pulverized Fuel

and Air Piping

Pulverized Fuel

Burners

Hot Air

Damper

Tempering Air

Damper

Cold (Tempering) Air

from Forced Draft Fan

Boiler Air HeaterHot Air from

Boiler

Front Wall

Burner

Windbox

Basement Floor
Fig. 7.7(a)
of pulverized coal or fineness requirements vary, though not too greatly, from coal to coal (the higher the
fixed carbon, the finer the coal). For example, pulverized coal with 80 percent passing a 200 mesh
screen and 99.5 percent passing a 50-mesh screen possesses a surface area of approximately 1500 cm^2 /g
with more than 97 percent of that surface area passing the 200-mesh screen.

The fuel burners may be arranged in one of two configurations. In the first. individual burners,
usually arranged horizontally from one or opposite walls, are independent of each other and provide
separate flame envelopes. In the second, the burners are arranged so that the fuel and air injected by
them interact and produce a single flame envelope. In this configuration the burners are such that fuel
and air are injected from the four corners of the furnace along lines that are tangents to an imaginary
horizontal circle within the furnace, thus causing a rotative motion and intensive mixing and a flame
envelope that fills the furnace area. Vertical firing is also used but is more complex and used only for
hard-to-ignite fuels.

The burners themselves can be used to burn pulverized coal only (Fig. 7.8) or all three primary
fuels, i.e. pulverized coal, oil, or gas (Fig. 7.9). In Fig. 7.8, the coal impeller promotes the mixing of fuel
with the primary air and the tangential door built into the wind box provide turbulence of the main
combustion, or secondary, air to help mix it with the fuel-primary-air mixture leaving the impeller.

Regulating

Rod

Coal Nozzle

Air Register Door

(Secondary Air)(Oil) LighterWindbox

Water CooledFurnace Wall Coal Impelier With studded TubesRefractory Throat

Fig. 7.8