Page 147 - Wastewater Solids Incineration Systems

114 Wastewater Solids Incineration Systems

(400°F), which can be achieved using a high-pressure (3110 kPa [450 psig]) steam coil.
These applications typically have a high-pressure waste heat boiler to recover energy
from hot flue gases in the form of steam. Part of the steam generated is used to pre-
heat combustion air; the balance is available for other purposes.
A second form of primary recovery involves the use of energy from furnace
exhaust flue gases—either as steam or a heated thermal fluid—to indirectly heat
the thermal dewatering system. This type of system can be modulated to remove
additional moisture from mechanically dewatered feed cake, up to the point where
the cake is autogenous (or nearly so) in the combustion system. Typically, this
process involves thermally dewatering the cake to 27 to 33% solids, depending on
the particular characteristics of the cake being processed. Integration of a thermal
dewatering system to the incineration process provides enhanced flexibility to
accommodate changes in feed cake characteristics and turndown operations in an
efficient manner.

3.2 Secondary Energy Recovery Systems

Most secondary recovery systems are an add-on to the primary recovery systems.
Occasionally, the primary and secondary recovery systems are integrated into a
single unit that can serve both primary and secondary energy demands. In general,
energy in flue gases can be converted to heated air or flue gas using gas-to-gas heat
exchangers, to high- or low-pressure steam using firetube or watertube boilers, or to
heated water or thermal fluids using gas-to-liquid heat exchangers or economizers.
The choice of what equipment to use is based on a review of available energy and the
nature and magnitude of energy demands to be served.
There are several uses for common secondary energy recovery and reuse appli-
cations:

• Reheating of exhaust flue gases downstream of the scrubber before discharge
for plume suppression.
• Generation of high-pressure steam to operate turbine-driven equipment or
electrical power generators.
• Generation of steam or hot water for building or structure heating, potable
water heating, or hot flushing operations.
• Heating water or a thermal fluid to meet process demands in the plant, such as
heating of digesters or preheating the feed to centrifuge dewatering equipment.