Page 191 - Wastewater Solids Incineration Systems

158 Wastewater Solids Incineration Systems

repair of the Venturi damper are necessary to ensure that dampers are in proper
working order. Also silicon carbide linings are subject to high velocities and stresses
and can break away from the scrubber housing. If possible, operation at pressure
drops of 6.2 kPa (25 in w.c.) or less will prolong the life of these linings.

2.5 Tray Scrubbers
As shown in Figure 7.6, a tray scrubber is commonly used following a Venturi
scrubber to act as a complete wet scrubbing system. The tray scrubber consists of a
vertical cylinder in which the exhaust gases enter at the bottom and exit through the
top. The gas stream first passes through a gas-liquid separation device that is used to
separate the Venturi water (heavily laden with particulate matter) from the tray
water. The water contains a relatively small amount of particulate matter and typi-
cally is returned to the WWTP headworks. The above gas stream then proceeds
through three or more scrubber trays that allow contact between the exhaust gas and
the scrubbing water. The scrubber water is introduced on the top tray and flows
countercurrent to the gas stream. Large quantities of water are used to cool the gas to
temperatures of 38 to 49°C (100 to 120°F).
Two types of trays are commonly used: perforated sheet metal plates or impinge-
ment trays. With perforated sheet metal plates, each tray can have one or two layers of
perforated plates. With two perforated sheets, the metal plates are separated by a small
gap and positioned so that the holes are not aligned and the gas flows through a con-
toured pathway. An impingement tray consists of a perforated metal sheet with metal
strips or “targets” located above the holes in the perforated plate. As the gas flows
through the holes in the plate, it is then forced to contact the metal targets, which
increases turbulence and scrubbing effectiveness. Packed scrubbers have also been
used on some systems but they are more prone to fouling and plugging, particularly if
they are not preceded by a dry collector such as fabric filter or ESP. A demister section
is located above the trays to remove large water droplets from the gas stream. Typically
a zigzag-type baffle demister is used because it is less prone to fouling.

2.5.1 Performance
A tray scrubber has three beneficial effects:

1. Condenses aerosols in the flue gas, hence enhances particulate capture.
2. Removes water vapor from the gas stream, hence minimizes plume forma-
tion from the stack.
3. Removes the bulk of the acid gases in the flue gas.