Page 143 - Wastewater Solids Incineration Systems

110 Wastewater Solids Incineration Systems

TABLE 6.1 Typical composition of wet flue gas (volume basis).

Parameter Fluid bed systems Multiple-hearth systems
Oxygen (O ) 3.5 to 6% 6 to 10%
2
Nitrogen content (N ) 45 to 55% 50 to 65%
2
Carbon dioxide (CO ) 7 to 10% 5 to 8%
2
Water vapor (H O) 35 to 45% 25 to 35%
2
Sulfur dioxide (SO ) 0 to 1500 ppm 0 to 1000 ppm
2
Hydrogen chloride (HCl) 0 to 1000 ppm 0 to 700 ppm
Acid dewpoint* 50 to 90°C (125 to 200°F) 50 to 90°C (125 to 200°F)
*Add 56°C (100°F) design safety factor to account for potential presence of SO .
3

Values are typical ranges of parameters of interest from a heat transfer and corro-
sion perspective only and are for illustration purposes; no individual incinerator is
“typical.” Each application must be evaluated using the specific thermodynamic
characteristics of the feed cake and the design and operating parameters of the case
being analyzed.
2.3.2 Process Considerations
One of the key process considerations when designing and operating a heat recovery
and reuse system is the real-world variability in operating conditions that the system
must be able to accommodate. Incineration systems often must operate with a wide
range of feed cake characteristics. Sometimes key characteristics, such as cake solids
and volatile solids content, are subject to sudden fluctuations on a daily or hourly
basis. Further, most WWTPs experience seasonal variations in the amount and
quality of the feed cake. Some systems are operated on campaigns of hours, days, or
weeks and are then either placed in a “hot” standby condition or are temporarily
shutdown. Other systems operate more or less continuously; however, the feed rate
to the system reflects the actual solids load at the plant, which may be significantly
less than the design point.
Most incineration systems are designed to be able to operate over some range of
turndown and to accommodate variations in characteristics of the feed cake. It is
important that equal attention be paid to designing the heat recovery and reuse sys-
tems with appropriate features and providing ways to handle these variations, which
can sometimes be a challenging endeavor.