Page 142 - Wastewater Solids Incineration Systems

Heat Recovery and Reuse 109

These and other applications will be discussed in”Heat Reuse Applications” sec-
tion of this chapter. As fluidized bed systems become more integrated with other
solids processing systems—such as anaerobic digestion, dewatering, or thermal
drying systems—more direct markets for this energy will develop.
The focus up to this point has been on fluid bed systems because they offer the
greatest potential for primary heat recovery. Multiple-hearth systems, on the other
hand, cannot typically make effective use of combustion air preheat temperatures
higher than approximately 200°C (400°F). Moreover, furnace exhaust temperatures
from multiple-hearth systems are typically no higher than 760°C (1400°F) when high-
temperature afterburning is provided and are more often in the range of 480 to 650°C
(900 to 1200°F). Further, multiple-hearth systems typically operate at 75 to 150%
excess air, so there is a higher mass flow of flue gases per unit of feed cake compared
with fluid bed systems. In general, exhaust gases from a multiple-hearth system are
nearly equivalent in resource value to exhaust gases from a high-temperature air pre-
heater on a fluid bed system. If afterburning is provided on the multiple-hearth
system, energy recovery potential is even higher. Therefore, regardless of the type of
furnace used, the potential for secondary heat recovery and reuse is approximately
the same when exhaust flue gases are cooled to 180 to 200°C (350 to 400°F).

2.3 Application Considerations
Several issues must be considered when designing or operating primary and sec-
ondary energy recovery equipment and systems. These issues include addressing the
unique needs and composition of flue gases from the particular type of incinerator
(fluid bed or multiple hearth) and accommodating the myriad process and feed vari-
ations. Many systems have been designed to operate well for a continuous feed at a
maximum feed rate and may be severely challenged under conditions such as turn-
down or variations in the thermodynamic properties of the feed cake.

2.3.1 Gas Composition
Exhaust flue gases have many undesirable traits including high moisture content and
fairly high acid gas content upstream of the wet scrubber. In fluid bed systems, essen-
tially all of the ash in the feed cake exits the furnace with the flue gas. In multiple-
hearth systems, this discharge is usually lower. Flue gases, however, still carry 5 to 15%
of the ash from the feed. The potential fouling characteristics and particulate loading
should be addressed during the system design phase. Table 6.1 lists typical composi-
tion values for exhaust flue gases from fluid bed and multiple-hearth incinerators.