MIL-HDBK-1003/6
c) Traveling-Grate Stoker - Front Gravity Feed. For plant
capacities in the range of 20,000 lb of steam per hour to 160,000 lb of steam
per hour (2.52 to 20.16 kg/s), the traveling-grate stoker method of firing can
be used for moderately changing wide load swings. See Figure 14. These
boilers will handle fuels that have widely varying characteristics, from low
volatile anthracite, coke breeze to high and medium volatile bituminous.
Refer to Table 14. They are particularly efficient with free-burning Mid-West
coals and can handle lignite and sub-bituminous coals. These boilers are
characterized by the coal being fed onto a grate at one end of the furnace,
the coal burning on the grate as it travels through the furnace, and the ash
being dumped at the opposite end of the furnace. A coal gate is used to
control the thickness of the coal bed entering the furnace. The typical
furnace configuration includes a front refractory arch and a long rear arch,
which are important when using the traveling-grate stoker to burn very low
volatile fuels, such as anthracite or coke breeze. The hot front arch is used
to maintain combustion and heat coal entering the furnace. The rear arch
directs any remaining volatile matter from the burnout zone to the flame zone
where it can be burned. The feature of the traveling-grate stoker that
provides for the utilization of such a wide variety of fuel types is the
under-grate air zoning. These units normally have from five to nine
individual air zones which can control the amount of air admitted to the fuel
bed as it travels from the free end of the stoker to the discharge. Although
this provides the stoker operation with tremendous flexibility to obtain
complete combustion with the various sizes and types of fuel, strongly caking
coals have a tendency to mat and prevent proper air distribution. Since the
fuel bed on the traveling-grate stoker is not agitated by vibration as the bed
(usually 4 inches to 6 inches (101.6 mm to 152.4 mm) depth) is moving from the
feed end toward the discharge end, the amount of particulate fluidization is
very low. This means that the revolving grate stoker has a low particulate
pollution characteristic as compared to other types of fuel burning strokers.
Chain grate stokers are not recommended except to burn low fusion coal with
high clinkering tendencies.
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