Page 146 - Wastewater Solids Incineration Systems

Heat Recovery and Reuse 113

scrubbing system. In many cases, the air preheater was “hard-connected” to the fur-
nace and the only expansion joint was at the inlet to the wet scrubber. Many of these
systems operated under pressure developed by the fluidizing air blower all the way to
the stack discharge. These configurations are referred to as “push” systems. The disad-
vantage of this system is that if a leakage develops then hot flue gases may be released
into the operations building, resulting in a potentially severe safety hazard.
Systems with more extensive heat recovery equipment, particularly those having
waste heat boilers or economizers, often are designed to operate under slight nega-
tive pressure downstream of the furnace. In this design, an induced draft fan down-
stream of the wet scrubber pulls flue gases from the furnace through the system and
discharges them to the stack. The fluidizing air blower is only designed to develop
the pressure necessary to deliver the combustion (fluidizing) air into the furnace and
through the bed zone. These designs are referred to as “push-pull” systems. The dis-
advantage of this approach is that two fans must be operated and maintained. Air
from any leakage that develops, however, will be drawn into the hot flue gases from
the operations building area, which is safer.

3.0 HEAT REUSE APPLICATIONS
Although a variety of equipment is available to recover energy from furnace exhaust
flue gases, extensive recovery has rarely been practiced. The only exception is that most
fluid bed systems use combustion air preheaters for primary energy recovery. Multiple-
hearth systems do not have a meaningful demand for primary energy recovery. In the
past, the potential benefits of secondary energy recovery were largely ignored in favor
of a simpler system with no linkages to other plant processes and demands.
Newer incineration systems generally are based on fluid bed technology. Many
of these systems contain more extensive primary and secondary heat recovery and
reuse systems. Although many older incinerators use multiple-hearth technology,
these too provide opportunities for retrofitting with heat recovery systems, most
often involving secondary recovery only.

3.1 Primary Energy Recovery Systems
Primary recovery systems are almost exclusively related to preheating of combustion
air, using flue gas to air heat exchangers, before its introduction to the furnace.
Although combustion air preheating is rarely used with multiple-hearth systems,
there are some installations that are designed for a modest preheat level of 200°C