Page 282 - Wastewater Solids Incineration Systems

244 Wastewater Solids Incineration Systems

(1600 to 1800°F). This temperature range can vary depending on the ash fusion tem-
perature of the feed cake solids and design of the incinerator and refractory specifica-
tions. The bed temperature system generates low and high abnormal bed tempera-
ture alarms when the temperature drops below the cake solids enable temperature or
rises above the cake solids and auxiliary fuel shutdown temperature.
The freeboard temperature system controls the temperature of the incinerator
offgases to maintain a safe temperature. This temperature depends on the ash fusion
temperature. If the temperature of offgases rises to the ash fusion temperature, ash
builds up on the exhaust duct and eventually plugs it, thereby preventing gases from
leaving the incinerator and creating a safety hazard.
The freeboard temperature control system typically consists of multiple water
sprays mounted in the incinerator roof. These sprays come on in sequence as the free-
board temperature rises. The water supply to these sprays is critical; if water pres-
sure varies, a booster pump is required. If freeboard temperature rises beyond
acceptable levels, cake and auxiliary feeds will be stopped.
A secondary freeboard temperature control system that has been successfully
used in some facilities consists of a separate blower that injects air to the freeboard
through multiple nozzles. The air valve can be modulated automatically or manually.
The freeboard air injection system maintains the freeboard temperature below the
temperature that initiates the water sprays. The freeboard air injection system results
in a more uniform temperature distribution to the incinerator. Some of the air that
would typically be injected as fluidizing air is injected as freeboard air. This compen-
sates for the usual temperature difference between the bed and freeboard.
Although many of the fluid bed incinerator components are automated, control of
the incinerator temperature is not entirely automatic. The operator must monitor and
adjust the feed cake rate and the fluidizing airflow. The feed cake rate varies according
to the production of cake by the dewatering operation and the auxiliary fuel system
automatically compensates for small changes in feed cake rate. However, the feed
cake equipment may require adjustments in speed to compensate for changes in the
cake production rate. Adjustments to the fluidizing airflow may be required if the
oxygen in the offgases moves outside of the acceptable operating range. If the inciner-
ator is a hot wind box type, the operator may also be required to periodically adjust
the quantity of offgases passing through the gas-to-air heat exchanger. The bed and
freeboard temperature are recorded on the incinerator temperature recorder to assist
the operator in monitoring and controlling the incinerator temperatures.