Page 145 - Wastewater Solids Incineration Systems

112 Wastewater Solids Incineration Systems

to within satisfactory limits and to protect the air preheater equipment. De-rating of
the system capacity is rarely an acceptable approach to address these “off-design”
conditions. Fortunately, several creative approaches have been developed to address
such conditions without sacrificing system capacity under abnormal feed cake char-
acteristics or system operating efficiency when conditions are normal. One approach
is to partially bypass a controlled amount of cold air from the inlet to the outlet of the
air preheater when less air preheat is required. Another approach is to control the
bleed of hot air from the discharge of the air preheater to the atmosphere (or stack).
Several incineration systems have waste heat recovery boilers or economizers to
capture available energy in flue gases for use elsewhere in the plant, or both. Rarely
is there a perfect match between available energy and independent energy demand;
therefore, these systems are often equipped with bypass ducts around the heat
recovery equipment. The bypasses can be throttled to match energy demands of the
plant or to isolate the heat recovery unit from the flue gas stream when energy
demand is not present or the energy recovery equipment is out of service. Despite
good intentions, these bypass ducts do not typically provide a complete barrier and
seal against flowing hot flue gases; maintenance of the heat recovery equipment typi-
cally requires a complete system shutdown. Bypasses can be used to isolate the
equipment from the flue gases to allow the incinerator to operate until shutdown is
convenient for heat recovery system maintenance and repair. Care should be taken in
design of the system to protect the off-line heat recovery equipment from damage
from the ever-present leakage around the bypass dampers. Protection may require
some form of supplemental pressurized air purge for the isolated equipment or evap-
orative cooling sprays in the ductwork upstream of the unit to limit flue gas temper-
atures to acceptable levels to protect off-line equipment.

2.3.3 Safety Considerations
Safety is a significant concern in the operation of complex, high-temperature equip-
ment such as those found in incinerators and energy recovery systems. Expansion
joints are required at key points in the system because of thermal expansion and con-
traction of equipment. Several good expansion joint designs exist, such as joints with
multiple barriers and pressurized air purge features. In the long run, though, poten-
tial for rupture and leakage through these expansion joints is a significant concern.
Many earlier simple fluid bed systems were equipped only with a combustion air
preheater and not with secondary energy recovery devices. Flue gases passed directly
from the outlet of the air preheater through appropriate ductwork to the inlet of the wet