Power Plant Engineering

230 POWER PLANT ENGINEERING

Multi Purpose

Oil Lighter

Spin Vanes

Water

Cooled

Throat

Windbox

Inner AirRegisterOuter Air

Register

Fig. 7.9

Table 7.2 : Excess air required by some fuel systems

Fuel System Excess air, %

Coal Pulverized, completely water-cooled furnace 15—20

Pulverized, partially water-cooled furnace 15—40

Spreader stoker 30—60

Chain grate and travelling stoker 15—50

Crushed, cyclone furnace 10—15

Fuel oil Oil burners 5—10

Multifuel burners 10—20

Gas : Gas burners 5—10

Multifuel burners 7—12

The total air-fuel ratio is greater than stoichrometric (chemically correct) but just enough to
ensure complete combustion without wasting energy by adding too much sensible heat to the air. Table
7.2 gives the range of excess air, percent of theoretical, necessary for good combustion of some fuels.
Initial ignition of the burners is accomplished in a variety of ways including a light-fuel oil jet, itself
spark-ignited. This igniter is usually energized long enough to ensure a self-sustaining flame. The con-
trol equipment ranges from manual to a remotely operated programmed sequence. The igniters may be
kept only for seconds in the case of fuel oil or gas. In the case of pulverized coal, however, they are
usually kept much longer, sometimes for hours, until the combustion-zone temperature is high enough
to ensure a self-sustaining flame. It may also be necessary to activate the igniter at very light loads,
especially for coals of low volatility. The impeller is the part of the burner that is subject to severe
maintenance problems and is usually replaced once a year or so.

7.7 Cyclone Furnaces

Cyclone-furnace firing, developed in the 1940s, represents the most significant step in coal firing
since the introduction of pulverized-coal firing in the 1920s. It is now widely used to burn poorer grades