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Emission Control and Monitoring 159

The additional particulate capture will contain some volatile hydrocarbons,
which would be detected in the “back-half” of the method 5 particulate test. The tray
scrubber shown in Figure 7.6 contains an additional gas-liquid separation device and
two additional trays. With this design, a caustic solution is circulated to just the top
two trays to achieve a high degree of acid gas control. Typically with this arrange-
ment, SO and HCl removals of 95 and 98%, respectively, can be achieved.
2
2.5.2 Advantages and Disadvantages
The advantages of the tray scrubber are that pressure losses in the system range from
0.5 to 0.7 kPa (2 to 3 in w.c.) per tray. Because three trays are typically used, the
overall pressure drop is approximately 2.2 to 3 kPa (9 to 12 in w.c.). To achieve sub-
cooling of the flue gas to 38 to 43°C (100 to 110°F), liquid flow rates of 3.4 to 6.7 L/m 3
of saturated flue gas (25 to 50 gpm per 1000 acfm) (i.e., following the Venturi
scrubber) are required.
Because this type of scrubber is inefficient in removing small diameter parti-
cles, it cannot be used as the sole particulate control device. It also requires large
quantities of water to operate, which then must be treated. This excess water is typ-
ically not a problem at a WWTP. However, this can be a significant concern at
WWTPs that are required to produce a high-quality effluent and want to minimize
recycle flows to the WWTP.
2.5.3 Operation and Maintenance
Because the performance of a tray scrubber is a function of the pressure drop across
the trays, a differential pressure indicator is typically used to continuously record
these figures. Removal efficiency of an impingement tray scrubber is relatively con-
stant from 40 to 100% of the maximum design flue gas flow rate.
Even without moving parts, tray scrubbers can require significant maintenance.
Water cooling of the gas stream can form condensed organics on the interior surfaces
of the chamber and on the trays if combustion in the incinerator is incomplete, as is
the case with an MHF. On some MHFs, this buildup of greasy slime can reach thick-
nesses of 6 to 13 mm (0.25 to 0.50 in) and can also occur in ductwork and in the
induced draft fan housing. This type of fouling has not been a significant problem
with fluid bed incinerators.
The warm, moist atmosphere in the scrubber can also promote the growth of a
biological mass on the trays, requiring high-pressure sprays for removal. The use
of a pressure differential indicator across the trays can identify this problem during
operation. The use of chlorinated plant effluent can lessen the growth of biomass