Power Plant Engineering

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

Fig. 7.10

furnace, where it is solidified and broken for removal. The slag layer that forms on the walls of the

cyclone provides insulation against too much heat loss through the walls and contributes to the effi-