Page 148 - Wastewater Solids Incineration Systems

Heat Recovery and Reuse 115

Steam turbine equipment includes backpressure and condensing turbine
designs. With either style of turbine, it is advisable to provide some amount of steam
superheat upstream of the turbine to reduce or eliminate moisture formation at the
low-pressure end of the turbine. When power generation is used, the maximum
amount of power that can be generated through secondary energy recovery is much
greater than the total power demands for the incineration system itself. Thus it is pos-
sible to support part of the electrical load of the balance of the WWTP in this manner.
It is unlikely that the amount of power generated will exceed the balance of plant
demands, so the potential for sale of power to the utility is minimal.
In general, most applications to date use exhaust gas reheat for aesthetic reasons,
such as for plume suppression. Although it does accomplish this goal, it also pro-
vides enhanced dispersion of stack exhaust gases and can often help to meet
emerging strict regulations on air toxics found in many jurisdictions. It is expected
that more incineration systems will incorporate this feature in the future.
There are several ways to reheat exhaust gases. One method involves providing
a second flue gas-to-flue gas (or flue gas-to-air) heat exchanger downstream of the
main combustion air preheater on fluid bed systems or at a convenient point down-
stream of the furnace in multiple-hearth systems. If an induced draft fan is available,
then exhaust can simply be routed through the heat exchanger to pick up heat before
discharge through the stack. For purely “push” systems, with no induced draft fan, a
smaller fan may be provided that discharges fresh air through the heat exchanger
and blends it with the flue gas upstream of or within the stack itself.
Another technique for reheating exhaust gas in fluid bed systems involves
making the fluidizing air blower and the combustion air preheater slightly larger
than required for the combustion air alone. A relatively small side stream of hot
(540°C [1000°F] or greater) pressurized air is then blended with exhaust flue gas
upstream of or within the stack itself. With this method, the operator has the advan-
tage of greater control and can suspend operations in an emergency to provide addi-
tional combustion air. In addition, extra air passing through the combustion air pre-
heater can reduce tube metal temperatures, providing some additional protection for
the preheater. At turndown, if combustion air temperatures rise too far, some excess
air can be bled to the stack to serve as “extra” reheat air while providing added air
preheater protection.

3.3 Typical Energy Recovery Flow Sheets

Figure 6.4 shows a generic flow sheet illustrating the typical arrangement of energy
recovery equipment in an integrated fluid bed system. Both primary and secondary