MIL-HDBK-1003/6
Coal Firing. The selection of the most appropriate burning
5.2.2
equipment for a specific application is dependent on factors which include
economics, operating characteristics, type of coal, and environmental
standards. Currently, the most common coal fired boilers fall into two main
categories: stokers and pulverized coal units.
Stokers. There are four main types of coal stokers which are
5.2.2.1
classified according to the way the fuel is fed and by the type of grate used.
These types are the underfeed stoker, the vibrating-grate stoker, the
traveling-grate stoker, and the spreader stoker. Refer to Table 14.
a) Underfeed Stoker. The underfeed stoker is generally applied to
the small capacity boilers in a range 3,000 to 25,000 lb of steam per hour
(0.378 to 3.15 kg/s). See Figure 12. An agitator on the hopper assures a
continuous supply of coal to the stoker feed ram. The coal ram provides a
positive feed from the hopper to the single retort. A heavy pusher bar with
auxiliary blocks at the bottom of the retort distributes the coal to produce a
uniform fuel bed from front to back of the unit. The underfeed stoker will
handle a wide range of coals having ash fusion temperatures above 2,400
degrees F (1315.6 degrees C). Coal with ash-softening temperature less than
2,400 degrees F (1315.6 degrees C) will normally form clinkers when applied to
the underfeed type stoker. Free-burning type coals are not normally used with
the underfeed type stoker. The modern underfeed stoker is equipped with
overfire air jets to provide turbulence to mix volatile hydrocarbons with the
air for complete combustion.
b) Vibrating-Grate Stoker. The vibrating-grate stoker usually has
a range of 10,000 to 25,000 lbs of steam per hour (1.26 to 3.15 kg/s) and is
used extensively in mass burning applications. See Figure 13. It is unique
in that it combines water cooling protection of the grate surface with
intermittent grate vibration for movement of the fuel bed down the inclined
grate. The frequency of the vibration of the grates is used to regulate the
combustion rate. The ash is automatically discharged at the rear. It
requires high quality boiler feedwater due to localized high temperature spots
along the cooling tubes when grate sections are exposed to furnace
temperatures; these hot spots increase the possibility of scaling. As with
traveling-grate stokers, the vibrating-grate stoker has individually
controlled air zones for better air flow control.
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