TM 5-815-1/AFR 19-6
of the furnace. Conversely, when air flow is
continuous stopping of sections is required to
reduced below the minimum fluidizing
control load for extended periods, the
velocity, the bed slumps and fluidization
fluidized bed boiler may become a big user of
stops.
auxiliary fuel to maintain bed temperature.
(2) The fuel feed systems available are either
(4) Major limitations of the bubbling bed design
under-bed feed system or over-the-bed feed
are high calcium/sulfur ratios, low
system. The under-bed feed system is quite
combustion efficiency, limited turndown
complex. It requires coal at less than 8
without sectionalization of the furnace bottom
percent surface moisture and crushed to
and complexity of the under bed feed system
about 6 MM top size to minimize plugging
required to minimize elutriation of unburned
the coal pipes. Operating and maintenance
fines. Typical fluidized bed combustors of
costs are usually high for the under-bed feed
this type are shown in figures 13-4 and 13-5.
h. In the circulating fluidized bed boiler, the fuel is
system. The major advantage of the under-
bed feed system is that with use of recycle
fed into the lower combustion chamber and primary air
combustion efficiency approaches 99 percent.
is introduced under the bed.
The over-bed feed system is an adaptation of
(1) Because of the turbulence and velocity in the
the spreader stoker system for conventional
circulating bed, the fuel mixes with the bed
boilers. This system has a potential problem
material quickly and uniformly. Since there is
of effective carbon utilization. Carbon
not a definite bed depth when operating, the
elutriation can be as high as 10 percent.
density of the bed varies throughout the sys-
(3) Some bubbling bed units have sectionalized
tem, with the highest density at the level
or modular design for turndown or load
where the fuel is introduced. Secondary air is
response. This allows a section to be cut in or
introduced at various levels to ensure solids
out as required. Some are actually divided
circulation, provide stage combustion for NOx
with water cooled or refractory walls. This
reduction, and supply air for continuous fines
type unit should be matched to the facility
combustion in the upper part of the combus-
demand pro-file to avoid continual bed
tion chamber.
slumping and operator attention. When
(2) Combustion takes place at about 1600
13-3
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