Page 137 - Wastewater Solids Incineration Systems

104 Wastewater Solids Incineration Systems

precludes operation with high combustion air temperatures. In some variations of
the multiple-hearth incineration process, wet scrubber exhaust is reheated to 760 to
870°C (1400 to 1600°F) in a secondary combustion unit. Here the energy in the fur-
nace exhaust could be used to preheat the scrubber exhaust flue gas, thus reducing
auxiliary fuel requirements for secondary combustion. In this case, such recovery
allows high-temperature treatment with lower energy costs.
No matter what form heat recovery takes, the goal should be economical plant
operation, both in fuel efficiency and operating and maintenance costs. Energy
recovery should first address the primary objective: efficient and effective combus-
tion of waste residue. Remaining energy can then be used for secondary purposes.
This chapter covers the key concepts and considerations in heat recovery and
reuse applications. Finally, design and operational details of major types of equip-
ment used for heat recovery are described.

2.0 CONSIDERATIONS IN HEAT RECOVERY
AND REUSE APPLICATIONS

The source of recoverable energy is the hot furnace exhaust flue gas. As a resource,
these flue gases contain most of the heat energy input to the system, including the
heating value of the feed cake and auxiliary fuel and the sensible heat of the combus-
tion air. A small portion of this energy is lost to the surrounding environment
through radiation and convection, and some is lost because of the unburned com-
bustibles. These losses, however, are relatively minor when compared with the total
heat energy input to the furnace.
The inherent nature of fluid bed systems provides several primary heat recovery
opportunities to reduce auxiliary fuel requirements. Fluid bed systems are able to
accommodate relatively high levels of combustion air preheat, typically as high as
650°C (1200°F) and sometimes greater. Furnace exhaust temperatures may be as high
as 870°C (1600°F) under normal operating conditions. There is always sufficient
energy available in the exhaust gases to preheat the typical amount of combustion air
(stoichiometric amount plus 40% excess air) to as much as 650°C. In fact, there is typ-
ically a meaningful surplus of recoverable energy in the exhaust flue gases that can
be applied to other secondary uses.
One unique attribute of exhaust flue gas is its relatively high moisture content.
Essentially, all moisture in the furnace feed cake is contained in the exhaust gases as
water vapor. In most cases, water vapor accounts for 25 to 45% of the furnace exhaust