Page 105 - Wastewater Solids Incineration Systems

74 Wastewater Solids Incineration Systems

Expansion joints are constructed of high-temperature metal. The expansion joints
are refractory lined and of metallic bellows construction to allow for duct and equip-
ment movement caused by temperature fluctuations.
Purge air is provided to the bellows to prevent sand and fly ash from passing
through and restricting movement of the joint.

7.7 Process Control System
The control system is designed with alarms and interlocks to ensure safe operation. In
general, interlocking is based on a few fundamental philosophies. None of the combus-
tion operations can be started until the various safety checks are cleared. Safety checks
include airflow rates per design conditions, water flow rates to the Venturi scrubber,
and so forth. Typically, the control system consists of programmable logic controllers
(PLC) and programmable controllers with a screen control monitor as interface. All
process information recorded by the instrument and control equipment is displayed on
the operator’s graphics computer screen for plant monitoring.
For the safety of the operation, the plant is fitted with temperature elements
called thermocouples. The thermocouples are used to give control signals to the var-
ious combustion control loops associated with the incinerator operation and to deter-
mine the bed temperature span, which is an indication of fluidization quality. The
incinerator is also fitted with pressure taps. The differential pressures in the bed indi-
cate bed height and are also used to monitor the quality of fluidization. A wide span
of bed pressure differential generally indicates a well-fluidized bed. Thermocouples
and pressure taps are also used in the heat recovery and air pollution control sys-
tems. Water flow and airflow are measured by a mass flow meter.
The fluidized bed exhaust is supplied with an oxygen sampling and monitoring
system to assist the operating personnel in monitoring combustion and function as a
source of interlocks and alarms.
The operation of the fluid bed and its performance depend on the feed rates of the
three major flows to the incinerator—air, cake, and supplementary fuel. Airflow can be
set constant, although the composition of the dewatered cake, especially the solids con-
tent and, therefore, the cake feed rate, varies with time. The variation of cake feed rate is
the primary reason for the need to observe and control the process continuously. The con-
trol is simple and is based on only two parameters of control: temperature and excess air
(or oxygen). These parameters are continuously monitored during the process.
7.7.1 Bed Temperature
Thermocouples placed in the sidewall of the furnace bed measure the bed tempera-
ture. The thermocouples are averaged to get the bed temperature for control. The