Page 266 - Wastewater Solids Incineration Systems

228 Wastewater Solids Incineration Systems

within the incinerator. Of particular concern, however, is the increased volume of wet
feed cake on the top hearth. If too great a volume of feed cake builds up on the top
hearth, the center shaft motor can overload and trip out.

2.6.4 Draft Control
The induced draft fan must be used during normal operation. Induced draft serves
three functions:
• It provides a negative pressure within the incinerator preventing gases from
escaping through incinerator openings,
• It draws oxygen through the air ports into the incinerator to support and con-
trol the combustion process, and
• It provides a vehicle for exhausting the combustion gases.

Draft is measured by a pressure sensor located within the incinerator. Chapter 9,
Instrumentation and Control, contains additional details. The draft control damper
maintains a specific negative pressure in response to the sensor setpoint.
As the incinerator dampers function, the pressure within the incinerator changes.
Under normal conditions, the negative pressure set point should be set between
0.07 and 0 kPa ( 0.30 and 0 in water). The negative pressure setpoint should be set
as high as possible while maintaining a negative pressure throughout the incinerator.
If the pressure setpoint is too close to zero the pressure within the incinerator can
increase to positive pressure conditions when the inlet air dampers open. This causes
the emergency bypass stack damper to open, creating natural draft. If the pressure
set point is too low, excessive fly ash can be carried out of the incinerator with the
exhaust gases.
If the induced draft fan fails, interlocks must immediately stop the feed cake
from entering the incinerator and open the emergency bypass stack damper. The
incinerator emergency bypass stack vents incinerator gases to the atmosphere during
an emergency, allowing the material in the incinerator to properly burnout. In addi-
tion, it provides draft when the incinerator is in hot standby.
The emergency bypass stack must be adequate in size and cannot be used during
normal operation as it allows the incinerator exhaust gases to bypass the air pollu-
tion control system. Opening of the emergency bypass stack damper should be tied
into the shutdown of the feed cake system to the incinerator. Operators have had to
open their emergency bypass stack on a regular basis to relieve pressure buildup in