Page 101 - Wastewater Solids Incineration Systems

70 Wastewater Solids Incineration Systems

plied from a compressed system for the roof spray nozzles and pressure ports to keep
them cool and free of deposits. Purge air from the fluidizing air blower can be used
in annular sleeves of all ports, including site ports, bed oil guns, sand inlet/outlet,
sludge inlet, and roof sprays.

7.1.3 Atomizing Air
Atomizing air at approximately 48 to 55 kPa (7 to 8 psig) is required when fuel oil
is selected as auxiliary fuel. Atomizing air can be provided by an injection-purge
air blower.

7.2 Feed System
Solids from the wastewater process are dewatered first. Dewatering can be achieved
in a plate filter press, a belt filter press, or a centrifuge decanter. A belt press typically
can dewater undigested sludge to approximately 25 to 30% dry solids. High-perfor-
mance centrifuges typically can dewater to approximately 30 to 35% dry solids.
Polymer is used in both the belt press and centrifuge decanter as a dewatering aid.
Continuous, even transport and distribution of the dewatered cake to the furnace
is an important adjunct to stable and economical operation. Past practice was to use
screw extrusion feeders for dry cake and progressing cavity pumps for wet cake.
Today, hydraulic piston pumps are mostly used to convey the cake from dewatering
equipment to the furnace. The piston pump is preferable to others because of its flex-
ibility and insensitivity to feed quality. Recent developments have been made in the
design of progressing cavity pumps for pumping cake.
Two types of feed operations can be found in the literature: overbed feeding and
in-bed feeding. Overbed feeding consists of dropping the feed cake either by gravity
or by air spraying on to the bed from the freeboard sidewall or from the roof of the
furnace. In-bed feeding consists of conveying the feed cake at high pressure directly
to the bed of sand at a height of approximately 1.2 m (4 ft) under the bed surface area.
Overbed feeding is simple but is prone to bypassing of uncombusted feed cake par-
ticles into the exhaust. Overbed feeding is more commonly used in other applications
such as fluid bed boilers burning coal or other solid waste-fuel, in which a cyclone is
installed at the exhaust of the furnace to return the unburned carbon back to the bed.
In-bed feeding is mostly used for incineration of wastewater solids because the
combustion process is slower and in two stages (evaporation and combustion). The
feeding location, at 1.2 m (4 ft) under the bed surface or 30 cm (1 ft) above the distrib-
utor, ensures that maximum possible retention time of the feed cake particles is