Page 197 - Wastewater Solids Incineration Systems

164 Wastewater Solids Incineration Systems

this problem. Also, the ash hoppers should have heaters to keep the ash warm and in
a free-flowing state. If the ash is allowed to cool and absorb moisture, it will set up
and cause plugging problems. A related problem is plugging or malfunction of the
rotary valves that control the discharge of ash from the ash hoppers.
Although a host of clogging and particulate build-up problems can occur when
an incompletely burnt out exhaust gas is treated in a dry ESP, these problems have
not occurred on such installations on fluid bed incinerators. In fact, European experi-
ence with dry ESPs on fluid bed incinerators indicates that they operate well and
require minimal maintenance.

2.7 Wet Electrostatic Precipitators

A wet ESP operates similarly to a dry ESP but contains a washing mechanism to
counteract the buildup of volatile or particulate matter on collector surfaces. A typ-
ical wet ESP, as shown in Figure 7.13, operates like a dry ESP in that there are elec-
trodes which charge the incoming particles and collection plates or tubes to attract
the particles. With a wet ESP, however, a thin film of water is continuously flowing
down the surfaces of the collection tubes. This water film helps to remove the col-
lected particles. The exhaust gas entering the wet ESP must be cooled and fully satu-
rated before entering the unit. In addition, a fine mist generating system is typically
used at the inlet of the wet ESP to create very fine water droplets which are collected
on the surface of the collection tubes and are important in maintaining a wetted sur-
face and continuous flow of water down the collection tubes. Also periodically,
flushing sprays above the collection tubes are used to wash the particles off the tube
collecting surfaces.
2.7.1 Performance
Performance of the wet ESP is equal to or better than the dry ESP. Particulate collec-
tion efficiencies of 99% or more can be expected.

2.7.2 Advantages and Disadvantages
The advantages of the wet ESP are as follows:
• The unit does not require a heat recovery device such as a waste heat boiler for
cooling the incoming gases.
• The unit can be used after a conventional wet scrubbing system (Venturi and
impingement tray scrubbers) to achieve additional removal of particulate
matter and metals.