Page 99 - Wastewater Solids Incineration Systems

68 Wastewater Solids Incineration Systems

upper boundary. Its height depends on the amount of sand in the bed. For
incineration of wastewater solids, an expanded bed of approximately 1.5 m
(5 ft) is typically used. The sidewalls slope outwardly from bottom to top of
the bed to ensure water vapor expansion and to keep the gas velocity within
acceptable limits. The walls are also equipped with nozzles and ports for
sludge and auxiliary fuel injection and for various instruments.
• The freeboard section—the space above the bed is called the freeboard or dis-
engagement zone. It acts as a retention chamber for the combustion gases to
ensure a sufficient residence time at the temperature required and as a separa-
tion chamber for the bed material particles to disengage from the gas. To
ensure complete combustion of any escaping volatile hydrocarbons (from the
bed), the freeboard must be sized to provide approximately 6.5 seconds of gas
residence-time minimum. It is typically 4.6 m (15 ft) high. The shape of the
freeboard could be a cylindrical straight shape or a conical teardrop shape.
The cylindrical straight shape is generally designed based on a gas velocity of
0.76 m/s (2.5 ft/sec). In the conical teardrop shape, the freeboard is expanded
laterally along its height to maximize residence time and to reduce the gas
velocity further. The gas velocity at the top of the teardrop shaped freeboard is
0.64 m/s (2.1 ft/sec). The exhaust gas duct is installed in the center of the roof
dome to minimize gas bypassing and dead zone and to maximize residence
time in the freeboard. The gradient of decreasing gas velocity in the freeboard
and a lower gas velocity at the exhaust minimizes sand losses.

6.2 Cold or Warm Wind Box Fluid Bed
A cold wind box (or warm wind box) incinerator is used for feed material that can be
incinerated without heat recovery (or with moderate heat recovery). Wind box air
temperature is typically limited to less than 400°C (750°F). As the hot wind box, the
cold (or warm) is also composed of four sections, and the designs in general are quite
similar, except as follows:

• The wind box is not refractory-lined and the bed support and air distributor
can be a metal alloy plate. To sustain the high temperature of the bed, the top
of the plate is usually refractory lined.

• The preheat burner is installed in the freeboard, angled downward to heat the
top of the fluidized sand bed during startup.