Page 211 - Wastewater Solids Incineration Systems

Ash Handling and Recycling 177

All ash from a fluidized bed incinerator is carried off in the exhaust gas as fly ash.
In addition to the ash, fluid bed sand breaks down into smaller particles, which also
will be carried off in this gas stream. This ash is subsequently captured in the waste
heat boiler, economizer, baghouse, wet scrubber, or electrostatic precipitator, as
applicable for the given installation.

3.0 ASH HANDLING
In the wet or hydraulic ash handling system, ash is removed as a slurry. Wet systems
tend to be more prevalent on fluid bed systems coupled with wet scrubbers because
the majority of ash is removed from the process train in a wet form. Wet systems may
also be found in MHFs, where sluicing water is used to convey bottom ash to dis-
posal. In a fluid bed incinerator, ash slurry is drained from the bottom of the wet
scrubber. In either case, the ash slurry is conveyed to a lagoon or to mechanical thick-
ening and dewatering equipment. In the MHF system, dust can be released at
grinders, access doors, and discharge chutes before sluicing. Steam can be generated
from sluicing water evaporation as the hot dry ash mixes with the water. Seals and
gaskets must be inspected regularly and replaced as necessary. In any wet ash han-
dling application, the temperature of the slurry must be considered from a safety as
well as a material properties viewpoint.

3.1 Wet Systems

3.1.1 Conveyance

3.1.1.1 Ash Sluiceways
Sluiceways transport ash by gravity. The sluiceway will discharge ash to a lagoon.
The sluiceway is typically a rectangular concrete channel that may be lined with an
abrasion-resistant material. The sluiceway should be designed using the principles of
open-channel hydraulics. When a wet scrubber is used on the incinerator exhaust
gases, scrubber water can be used to sluice the incinerator bottom ash.

3.1.1.2 Ash Slurry Well
When ash slurry is pumped to a disposal point, a well is typically required. The well
provides some storage capacity and the proper hydraulic conditions for suction of
the ash slurry pumps. A mixer may be installed in the well to provide proper mixing
of water and ash. Alternatively, a self-cleaning wet well approach may be used.