Page 193 - Wastewater Solids Incineration Systems

160 Wastewater Solids Incineration Systems

on the scrubber trays. The demister also is subject to fouling and plugging from
soot or biological growths. The scrubber should be designed with an access door
for each of the scrubber trays and the demister. Furthermore, the trays and
demister should be constructed in sections such that they can be removed from the
scrubber for periodic cleaning.
Another concern with this type of scrubber is maintaining a uniform flow of
water across the trays. The flow of water across the trays is controlled by weirs at
one end (and sometimes both ends) of the trays. The weir depth should be at least
80 mm (3 in) to maintain a pressure drop of 0.5 to 0.7 kPa (2 to 3 in w.c.) across each
tray. Weirs are sometimes not installed perfectly level. Also slime buildup or metal
warping from temperature excursions (resulting from temporary loss of scrubber
water) can alter weirs and result in a non-uniform depth of water across each tray.
Thus, it is important to periodically inspect the inside of a tray scrubber particu-
larly after process upsets. A typical scrubber tray and weir combination is shown in
Figure 7.11.
Another design consideration is that the scrubber drain pipe must be atmospher-
ically separated from the scrubber. Otherwise, ambient air will be drawn into the
scrubber from the drain pipe, or, if it is under positive pressure, flue gas will be
blown out the drain pipe. To prevent this from happening, an S or P trap must be pro-
vided at the scrubber drain as shown in Figure 7.11. If the scrubber is under negative
pressure, the height of the drain trap, dimension A, must be greater than the max-
imum negative pressure experienced by the scrubber. If the scrubber is under posi-
tive pressure (as is often the case with a fluid bed incinerator), the depth of the
U-shaped section of the trap must be greater than the positive pressure in the system.
Formation of foam within the scrubber can also clog the unit. Some plants have
experienced excessive foaming of scrubber water, which can prevent exhaust gases
from moving through the trays. In such circumstances, a complete chemical analysis
of the scrubber water should be performed to identify the foaming agent so that it
can be removed from the wastewater stream. If this is not possible, antifoaming
agents can be used.

2.6 Dry Electrostatic Precipitators
Dry ESPs have been used for years on utility power boilers that combust coal. They
have not been used on MHF incinerators in the United States because of the high
moisture and volatile organic content of exhaust gases. However, dry ESPs have been
used on fluid bed incinerator installations in Europe, the United Kingdom, and, on at