Page 185 - Wastewater Solids Incineration Systems

152 Wastewater Solids Incineration Systems

liquid. The high velocity gas stream atomizes the liquid into fine droplets that col-
lect the fine particulate. The primary collection mechanism is inertial impaction in
which the micrometer and submicrometer size particles are driven into the larger
size water droplets. Condensation on the submicrometer particles increases their
mass and contributes to their removal by the impaction mechanism (Perry and
Chilton, 1973). Downstream of the Venturi throat, the particulate-laden droplets
collide and agglomerate into larger size, heavier droplets which are driven into the
flooded elbow at the bottom of the Venturi. Finer size droplets are collected in the
downstream tray scrubber. The pressure drop through the Venturi determines the
particulate removal efficiency. The greater the pressure drop, the greater the
removal of submicron size particles.
Figure 7.6 shows a vertical plume bob type Venturi scrubber which is commonly
used on a fluid bed incinerator. An automatic actuator can move the plume bob up
and down adjusting the size of the opening in the throat and thereby adjusting the
pressure drop through the throat. Another commonly used Venturi throat, shown in
Figure 7.7, has a rectangular throat and is equipped with one or two bomb-bay type
dampers that allow for varying the throat opening. Other types of Venturi scrubbers
operate with a fixed throat and maintain a constant pressure drop across the throat
by varying the amount of water added as the gas flow changes. These types are not
common on municipal incinerators.
Relatively recently, multiple Venturi scrubbers have been used. This type of
scrubber is different in that condensation of the gas stream is performed before the
Venturi section. A typical multiple Venturi scrubber is shown in Figure 7.8. In the first
stage, the flue gas flows through a low pressure drop quenching section and then
enters a vertical scrubber tower. The scrubber tower contains sub-cooling trays that
cool flue gas to 38 to 49°C (100 to 120°F). The flue gas then enters the Venturi stage
which consists of several Venturi throats. At the inlet of each Venturi throat, high-
pressure water at 2100 kPa (300 psig) (or alternatively water and compressed air) is
atomized through fine nozzles to create fine water droplets necessary to remove
micrometer and submicrometer size particles.

2.4.1 Performance
Particulate removal efficiency of a Venturi scrubber is dependent on the pressure
drop across the throat of the Venturi. In general, the greater the pressure drop, the
greater the particulate removal. Particulate removal efficiencies as a function of par-
ticle size and Venturi pressure drop are shown in Figure 7.9 (Schiffner and Hesketh,