1 October 1997
dampers are maintained in a fixed position once they have been set for the waste. In such cases,
the feed rate of waste is adjusted to match the flow of air rather than vice versa.
b. Furnace Temperature Control. To insure complete combustion, the furnace outlet
temperature must be maintained at its set point. In packaged and small modular units, temperature
control is the primary parameter for adjusting the waste feed rate. However, temperature is used by
all technologies to influence the firing rate to some degree. Temperature limit controls may be used
to initiate a water spray or to activate the under-fire air flow controller to abruptly reduce the
chamber temperature. In excess-air units and in the secondary zone of starved-air units, excessive
temperature is corrected by increasing the airflow. If the temperatures become too low, the
auxiliary gas/oil-fired burners will ignite. In all cases, the temperature is sensed by a thermocouple
mounted in the side wall or in the crown of the combustion chamber. The controller will use the
temperature signal to initiate alarms or make automatic adjustments.
c. Furnace Pressure Control.. Various actions will cause pressure fluctuations in the
incinerator and systems that follow; such as, variations in air flow to the incinerator, operation of
internal rams, and feed door opening and closing; therefore, it is necessary to have a draft control
system. Normally, a forced draft (FD) fan is used on large units to supply air to the combustion air
ducting system. An induced draft (ID) fan is used to draw combustion gases from the incinerator
through the heat recovery boiler and air pollution control system. (Note: It is preferable to pass
clean air through the fan.) The draft control system balances the operation of these two fans and
maintains a nominal primary chamber pressure of (-) 0.1 in. of water column (vacuum).
Irrespective of how air flow is controlled, a negative pressure is maintained throughout the
combustion gas flow path to preclude toxic gas leaks from the system.
d. Cooling Control System. Heat recovery is used on all but the smallest incinerators.
Typically, packaged units and modular unit secondary combustion chambers are followed by a
convection heat exchanger that heats hot water or produces steam. The design of the heat-
recovery system provides for the cooling of flue gases from a nominal 1,500-1700F down to 350-
450F. Temperature control of the flue gases is absolutely necessary before the gases go through
the pollution control system and ID fan. If an over-temperature upset condition occurs, cooling is
accomplished by a water spray in the ducting, dilution of the gas stream with tempering air, and/or
"dumping" the gas directly to the atmosphere by means of a "dump valve" located between the
secondary combustion chamber and the heat recovery device.
e. Combustion Monitors. The quality of the combustion process is monitored by devices
sensing CO, O2, CO2, and total hydrocarbon (THC) levels in the flue gas leaving the incinerator.
The sensors provide input to the computer control system to automatically adjusts feed rates and
air flows to maintain optimal combustion process conditions. Output of these sensors may also go
to a recorder for environmental emissions monitoring. As federal and state regulations become
more stringent, the use of more sophisticated monitors and controls become commonplace.