Page 139 - Wastewater Solids Incineration Systems

106 Wastewater Solids Incineration Systems

factor greater than the acid dewpoint, the practical lower limit for heat recovery sys-
tems is approximately 175 to 200°C (350 to 400°F). As illustrated in Figure 6.1, typical
recoverable energy above this limiting temperature range is approximately 50% of the
total energy in the flue gas at a starting temperature of 870°C. This proportion may
change slightly depending on site-specific factors such as furnace exhaust tempera-
ture, actual flue gas composition, and moisture content, among others.

2.1 Potential Uses for Primary Energy Recovery

The most important use for recovered energy is primary recovery to reduce or elimi-
nate auxiliary fuel requirements for combustion. The most common form of primary
recovery is for preheating of the combustion air to the system. As noted earlier, this is
more appropriate for fluid bed incinerators because these systems can take advan-
tage of preheat temperatures as high as 650°C (1200°F) or more. Another form of pri-
mary recovery is using excess energy to thermally dewater incinerator feed cake. The
feed cake is dried to the point that little or no auxiliary fuel is required, which typi-
cally requires two steps. First, the system either generates steam or heats a thermal
fluid. Second, an indirect dryer uses that steam or fluid in the drying process.
Nearly all fluid bed systems have some form of heat recovery for combustion-air
preheat purposes. To illustrate the value of preheating combustion air, Figure 6.2
shows a graph of auxiliary fuel requirements as a function of preheat temperature for
a 25% solids feed cake to a fluid bed operating at 40% excess air and an exhaust tem-
perature of 870°C (1600°F). Volatile solids levels of 50, 60, and 70% are shown for
illustration purposes.
Figure 6.2 shows the wide range of potential auxiliary fuel requirements and the
dramatic reduction that can be effected by preheating combustion air to the system. In
this example, a feed cake having 70% volatile solids is nearly autogenous (requires zero
fuel) at an exhaust temperature of 870°C and a combustion air preheat level of 650°C
(1200°F). As a practical matter, most fluid bed systems operate at a slightly lower
exhaust temperature. So this air preheat temperature would typically reflect autoge-
nous combustion in the range of 840 to 870°C (1550 to 1600°F), which would be accept-
able for design purposes. In this example, preheating combustion air to 650°C reduces
auxiliary fuel requirements to levels from 5 to 35% of those without preheat.
To put this into perspective, it is useful to consider the practicality of preheating
combustion air to 650°C using the available energy in the furnace exhaust flue gases.
The fraction of total flue gas energy required to achieve various combustion air preheat
levels is presented graphically in Figure 6.3, using the same example as in Figure 6.2.