used as the primary pressure sealing device. The fluidizing velocity in
systems ranges from 4.3 to 9.1 m/second 14 fps to 30 fps. The design shall
permit maintenance of full load with loss of a single feed point. Screw
conveyors shall be designed to operate continuously. Sorbent feed for CFB
systems shall be capable of providing the required proportion of sorbent
over the load range. These systems shall consist of a sorbent metering
device weigh belt feeder and be fed by gravity, pneumatic injection, or air
swept feeder. The sorbent shall be fed in-bed, under-bed, or over-bed, and
shall be fed separately from the fuel.
2.5.6
Fluidized Bed Combustion Area
Parts subject to severe wear or deterioration shall be replaceable with a
minimum of dismantling. The fluid bed combustor shall be designed to
insure that no combustion air entering the bed is allowed to bypass contact
with bed solids. Air distribution to the fluid bed will be through a
number of nozzles located in the bed support plate. Average superficial
velocity above the bed at the point of fuel and limestone feed shall be
between 1.2 to 3.7 m/second 4 to 12 fps at bed temperature and mix zone
pressure. The pressure drop through the air distributor plate shall be 3.7
kPa 15 inches water gauge minimum. The walls of the fluid bed combustor
shall be constructed of water-cooled tubes which form a part of the steam
water circuit of the boiler. These walls shall be of membrane wall type of
construction. The distributor shall be designated to assist with removal
of large tramp material, where expected.
2.5.7
Bed Material Letdown System
The bed material letdown system shall be capable of removing and cooling
solids from the bed including spent sorbent, unfired fuel, bottom ash,
rocks, and debris. The system shall be designed to operate continuously
and to cool all the solids going to disposal to 135 degrees C 275 degrees F
or less. The system shall be sized to continuously remove at least 80
percent of the total solids generated from the boiler, unless otherwise
approved. The design of the bed letdown system shall include personnel
safety considerations such as prevention of spillage of hot bed material
from the unit.
2.5.8
Burners and Fuel Piping
The burners and burner equipment shall be suitable for continuous service.
Parts subject to severe wear or deterioration shall be replaceable with a
minimum of dismantling. All valves or controls shall be mounted outside
the boiler front and air housing. The burner design shall provide positive
and uniform mixing of the air and fuel at all loads, and shall produce
sufficient turbulence to preclude stratification. The burner design shall
permit the firing of No. 2 fuel oil. In-duct/over-bed burners shall be of
the air atomizing type. The burners and/or ignitors shall have an
aggregate capacity capable of bringing a cold unit up to ignition
temperature using either precalcined limestone or spent bed material in 4
hours for AFBs, and 8 to 12 hours for CFBs, depending upon refractory
limitations. Equipment for remote positioning shall be furnished, if
required, of each burner gun at either the fully retracted or fully
inserted position. The positioning equipment shall be air operated; shall
include limit switches to indicate the position of the gun and all
necessary drives, linkages, and mechanisms; and shall automatically purge
the fuel from the guns before retracting.
SECTION 15559A
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