Page 118 - Wastewater Solids Incineration Systems

Combustion Technology 87

In many cases, the auxiliary combustion air was supplied by an air system that included
fans or blowers. This type of system has been retrofitted on some older MHFs.

9.3.6 Access Doors
Hinged access doors are constructed of ductile or cast iron. The door is lined with
castable refractory. Each door has a sliding or hinged plate that acts as an observation
port. The hinged door is mounted on a cast iron frame that is bolted to the inciner-
ator shell. The door is kept closed by bolted clamps. There are typically two doors per
hearth located 180° apart. The door sizes vary but are large enough to permit entry of
workers to a cold furnace.

9.3.7 Emergency Bypass
An emergency bypass damper is provided to vent furnace gases in the event of a
power failure or loss of induced draft fan. The purpose is to prevent unburnt hot
gases from being blown into the building through furnace doors because of pressure
buildup that would occur when the induced draft fan stopped. It also prevents the
internal furnace from overheating because of cake in the furnace continuing to burn.
The emergency bypass system includes an emergency bypass damper and stack.
The emergency bypass damper is connected to the stack by a duct that is either con-
nected to the top of the furnace or to the exhaust gas duct. The damper is arranged to
fail open, which means that upon loss of power or signal, the damper opens. The
damper is typically of the butterfly type and is refractory lined. Guillotine-type,
refractory-lined dampers (single and double) also have been used. Ducting between
the furnace and the damper is typically refractory lined. The stack is typically con-
structed of stainless steel and does not have refractory lining but may be insulated
externally for personnel protection.
9.3.8 Instrumentation
The MHF is equipped with pressure, temperature, and oxygen monitoring instru-
mentation to allow the operator to supervise operations. Instrumentation also pro-
vides process measurements that are incorporated into automatic control systems,
such as hearth temperature control through burners and auxiliary combustion air
dampers and furnace draft control through induced draft fan damper modulation or
speed adjustment.
Temperature is typically measured by Type K thermocouples mounted in ther-
mowells in the furnace wall. Thermocouples typically extend up to 300 mm (1 ft) into