Page 100 - Wastewater Solids Incineration Systems

Combustion Technology 69

7.0 FLUID BED INCINERATION SUBSYSTEMS

As shown in the process flow diagrams in Figure 5.8 and Figure 5.9, the incineration
system can be divided into subsystems, which are described in the next several sections.

7.1 Air System
Air supplied to the incineration system could be divided in three categories: flu-
idizing air, purge air, and atomizing air.

7.1.1 Fluidizing Air
Combustion air (same as fluidizing air) is typically supplied by a multistage cen-
trifugal blower. The system shown in Figure 5.8 operates under positive pressure and
is referred to as a push-type system. Complete gas tightness of the system is essential
because the pressure in the system is greater than atmospheric and the temperature
of the air and gases is high. This type of system is common in North America, where
heat recovery is aimed at autogenous operation and plume suppression and wet-ash
types of air pollution are typical. In contrast, the system shown in Figure 5.9 operates
at both positive and negative pressure, with the zero point typically at the furnace
outlet. This type of system is referred to as a push-pull type system. In this case, an
additional induced draft fan is required to ensure that negative pressure is main-
tained in the exhaust gas system. Push-pull systems are typically used where waste-
heat boilers and dry-ash type systems—which are more difficult to make gastight—
help avoid any risk of leakage of hot gases and dry ash.
In a hot or warm wind box incinerator, the air is preheated in a shell and tube
heat exchanger, which uses the reactor offgases as the hot medium. Heat recovery by
combustion air preheating reduces supplemental fuel and increases capacity of the
plant. In a cold wind box, combustion air from the centrifugal air blower is supplied
directly to the metal wind box.
The fluidizing air blower is designed with several stages to provide the required
outlet pressure. The main control of the incineration process capacity is regulated by
the quantity of fluidizing air entering the wind box. The airflow is controlled by a
damper on the blower inlet side, and measured by a flow meter.

7.1.2 Purge Air
Purge air is used in all ports to the incinerator and also to all expansion joints and
pressure taps in the ductwork to keep them cool and free of sand and ash. Purge air
can be either high pressure or low pressure. For example, high-pressure air is sup-