Power Plant Engineering

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

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FUELS AND COMBUSTION
231
of coal that contain a high ash content with a minimum of 6 percent to as high as 25 percent, and a high
volatile matter, more than 15 percent, to obtain the necessary high rates of combustion. A wide range of
moisture is allowable with pre-drying. One limitation is that ash should not contain a high sulfur content
or a high Fe
2
O
3
; (CaO + MgO) ratio. Such a coal has a tendency to form high ash-fusion temperature
materials such as iron and iron sulfide in the slag, which negates the main advantage of cyclone firing.
The main advantage is the removal of much of the ash, about 60 percent, ao molten slag that is
collected on the cyclone walls by centrifugal action and drained off the bottom to a slag-disintegrating
tank below. Thus only 40 percent ash leave, with the flue gases, compared with about 80 percent for
pulverized-coal firing. this materially reduces erosion and fouling of steam-generator surfaces as well as
the size of dust-removal precipitators or bag houses at steam-generator exit. Other advantages are that
only crushed coal is used and no pulverization equipment is needed and that the boiler size is reduced.
Cyclone-furnace firing uses a range of coal sizes averaging 95 percent passing a 4-mesh screen.
The disadvantages are higher forced-draft fan pressures and therefore higher power requirements,
the inability to use the coals mentioned above, and the formation of relatively more oxides of nitrogen,
NO
2
which are air pollutants, in the combustion process.
The cyclone is essentially a water-cooled horizontal cylinder (Fig. 7.10) located outside the main
boiler furnace, in which the crushed coal is fed and fired with very high rates of heat release. Combustion
of the coal is completed before the resulting hot gases enter the boiler furnace. The crushed coal is fed
into the cyclone burner at left along with primary air, which is about 20 percent of combustion or
secondary air. The primary air enters the burner tangentially, thus imparting a centrifugal motion to the
coal. The secondary air is also admitted tangentially at the top of the cyclone at high speed, imparting
further centrifugal motion. A small quantity of air, called tertiary air, is admitted at the center.
The whirling motion of air and coal results in large heat-release-rate volumetric densities, between
450,000 and 800,000 Btu/(h.ft) (about 4700 to 8300 kW/m
3
), and high combustion temperatures, more
than 3000°F (1650°C). These high temperatures melt the ash into a liquid slag that covers the surface of
the cyclone and eventually drains through the slag-tap opening to a slag tank at the bottom of the boiler
Replaceable
Rear lines
Oil Burner
Radial Burner
Primary Air
Tertiary Air
Crushed Coal Inlet
Emergency Standby
Oil Burner
Secondary Air
Gas Burners
Re-entrant
Throat
Slag Tap Opening