primary and secondary cyclone type air and ash separators. Ash is dumped from the
separators into the storage silo through air lock type automatic gates. Air from the
secondary separator is passed through a bag filter for residual ash removal prior to the
air being admitted to the vacuum producing exhausters.
10.1.1.2 Pressure System. Compressed air for conveying fly ash is produced by motor
driven positive displacement rotary blowers. An air lock tank type feeder is located at
each fly ash hopper outlet or discharge. The feeder discharge is attached to the
conveyer pipeline. The feeder is closed to the conveyor pipe and opened to the ash
hopper during the feeder loading cycle. When full, the feeder is closed to the ash
hopper, pressurized with compressed air, and then opened to the conveyor pipe allowing
the fly ash to discharge into the conveyor pipe air stream. The air-entrained fly ash
is then conveyed by the pipeline and discharged into the storage silo. The silo air
vent shall have an exhaust fan which discharges the vented air to the boiler flue gas
dust collector, or a separate bag filter may be used to clean the silo vented air. A
pressure system should be used when the fly ash conveying distance is greater than can
be served by a vacuum system. Pressurized conveyors can be used to convey fly ash
several thousand feet.
10.1.1.3 Vacuum-Pressure System. A combination of vacuum and pressure conveying of fly
ash should be considered where conveying distances rule out the use of vacuum alone.
This type of system permits the use of vacuum to remove fly ash from hoppers at a high
rate to a collecting point nearby where the fly ash is continuously transferred to a
pressurized conveyor for discharge at a remote terminal point.
Hydraulic. Hydraulic ash removal systems, using high pressure water as the
conveying medium for bottom ash, are used only for utility type boilers and will not be
considered for most Navy installations. In this system, bottom ash hoppers are fitted
with ash discharge gates each followed by a crusher which in turn discharges to a
centrifugal materials handling pump or a hydraulic ejector utilizing high pressure water
as the motive force and ash conveying medium. The ash is conveyed in a slurry
(approximately 20 percent ash by weight) to a disposal area or pond, or to dewatering
bins. Dewatering bins allow storage of bottom ash up to several days. After the water
is drained from one of the dewatering bins, ash is discharged into trucks, rail cars or
barges for final disposal.
Dense Phase. In the dense phase system, material is pushed through a pipeline
as a fluidized slug at higher ash to air ratios than conventional dilute phase
conveying. The dense phase system requires fine, dry, fluidizable material such as fly
ash. Coarse or damp material cannot be conveyed by this method. Because of the
associated history of line plugging and other mechanical difficulties, there have been
only a limited number of successful installations of dense phase systems in the United
States. Need for further developmental work is indicated and is ongoing. Careful
investigation should be made before considering the use of a dense phase system.
Bottom Ash Hoppers
Stoker Firing. The bottom ash hopper should be sized to provide not less than
8 hours, preferably 12 hours of storage at the maximum bottom ash production rate. For
bulk density of bottom ash, use 45 lb/ft3 for eastern bituminous coal and 35 lbs/ft3 for
western sub-bituminous coal. Sizing of the bottom ash hopper volume should also
consider that the bottom ash will contain unburned coal in the amount of 5 to 12 percent
of the bottom ash. Design structural loads shall be based on 70 pounds per cubic foot.
Discharge gates shall be air-cylinder operated with intermediate positioning capability.
Pulverized Coal Firing. For a water impounded bottom ash hopper, the hopper
volume should be sized to provide 8 hours of storage at the maximum production rate, or
12 hours of storage based on the maximum expected amount of ash to be produced for any
12-hour period. The bulk densities of bottom ash as given above under stoker coal
firing may be used also for pulverized coal firing.