Page 283 - Wastewater Solids Incineration Systems

Incinerator Operations 245

3.3.2 Draft Control
Two types of draft systems are used for fluid bed incinerators: pressurized and bal-
anced. In a pressurized system, the fluidizing air blowers provide the motive force
for the fluidizing air and the incinerator exhaust gases. No draft control is required
for this system, although a minimum quantity of fluidizing air must be injected to the
bed to keep it fluidized. If the fluidizing airflow rate falls below the required level,
cake and auxiliary fuel feeds are stopped.
In the balanced draft, or push-pull system, an induced draft fan is used by the
freeboard controller to maintain a slightly negative pressure in the freeboard. The
freeboard pressure signal is transmitted to the freeboard pressure controller which
either modulates the induced draft fan inlet dampers or varies the fan speed. The fan
must be running before cake and auxiliary fuel can be fed to the bed. The cakes and
auxiliary fuel feeds are stopped if the fan trips or the freeboard pressure is outside
the acceptable operating range. The acceptable freeboard pressure operating range is
typically between 2.5 to 2.5 kPa ( 10 to 10 in water).
If the incinerator is equipped with a Venturi scrubber, the pressure drop must
remain constant to provide optimum particulate control. Pressure drops may be
changed by adjusting the throat opening or the water flow rate, although adjust-
ments to the water flow rate will result in only minor alterations to the pressure drop.
Throat adjustment can be accomplished either automatically or manually. If the
throat adjustment is performed automatically, the actuator on the throat damper will
modulate to maintain a constant pressure drop. If performed manually, the operator
must make periodic adjustments to the throat damper opening.

3.4 Emergency Operations
3.4.1 Power Failure
A power failure to a fluid bed incinerator will shut down the incinerator system,
including all blowers and controls, but cake solids will continue to burn as long as
oxygen is available. Although cake solids will continue to burn for only a short time,
the cake’s volatile solids may continue to boil off for longer. When power is restored,
the operator must purge the incinerator of all combustible gases before attempting to
light a burner. A cold or warm startup procedure, depending on the incinerator tem-
perature, must be followed. The operator must perform a complete system check
prior to restarting the fluid bed incinerator.