Page 172 - Wastewater Solids Incineration Systems

Emission Control and Monitoring 139

the higher alkalinity of plant effluent in comparison with potable water. If a greater
degree of SO or HCl control is required, a caustic scrubbing section can be added to
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the tray scrubber. Typically, this is done during design by including two additional
trays on the tray scrubber and circulating a caustic solution to just these two trays.
With caustic scrubbing, 95% control of SO can be achieved and even greater level of
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HCl control can be obtained. In general HCl is easier to control than SO . Hydrogen
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fluoride (HF) is also present in small quantities in the flue gases. This acid gas is
readily removed by wet and dry scrubbing systems, as shown in Table 7.2.
Although wet scrubbing typically is used for acid gas control at WWTPs, dry
emission control systems are possible. Such systems are extensively used in the
power utility industry to control SO emissions from coal-fired power plants. Table
2
7.2 lists alternate acid gas control technologies, their operating temperatures, and
their achievable acid gas control efficiencies. With a dry injection system, a dry adsor-
bent such as lime is injected to the ductwork upstream of a dry particulate collector
(fabric filter or ESP). The lime reacts with the acids in the flue gas forming insoluble
salts (such as CaCl and CaSO ), which are collected in the dry collector. With a spray
4
dry absorber (SDA), a lime slurry is sprayed into an absorber vessel upstream of the
fabric filter or ESP. The moisture from the slurry cools the flue gas and enhances the
lime neutralization reaction. A spray dryer is significantly more effective in control-
ling SO (90%) than a dry injection system (50%). Also the use of a fabric filter instead
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of an ESP enhances SO removal. Wet scrubbing systems are also highly effective.
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However, the disadvantage of wet scrubbers is that they are only marginally effec-
tive in removing sulfur trioxide (SO ) (25 to 40%), whereas an SDA with fabric filter
3

TABLE 7.2 Effectiveness of acid gas control systems.
Pollutant removal (%)
Control system Temperature* HCl HF SO
2
Dry injection fabric filter 160–180°C (320–356°F) 80 98 50
Spray dry absorber ESP 140–160°C (284–320°F) 95 99 50–70
with recycle 140–160°C (284–320°F) 95 99 70–90
Spray dry absorber fabric filter 140–160°C (284–320°F) 95 99 70–90
with recycle 140–160°C (284–320°F) 95 99 80–95
Wet scrubber 40–50°C (104–122°F) 95 99 90
Wet/dry scrubber 40–50°C (104–122°F) 95 99 90

*Temperature at the exit of the control device.