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material and, as such, will incur additional costs. Use of a fabric filter as a collection step
after the dry scrubber is limited by possible clogging problems and/or temperature limita-
tions. If either concern exists, an ESP must be used for the final collection step.
3.3.3. Thermal Desorption
A thermal desorber is typically a semidry scrubber followed by a baghouse. The
scrubber removes acid gases, and the baghouse removes particulate matter. Such a con-
trol scenario will normally provide needed removal efficiencies for acid gases as well
as heavy metals and trace organics.

3.3.4. General Remediation Applications
Two distinct limiting features of semidry absorption technology should be considered:
1. Very high removal efficiencies, as per wet scrubbing, are not possible.
2. As the absorbent is slurried, the dry waste produced in dry scrubbing will add to solid-waste
disposal problems and costs.
The advantages of dry absorption are as follows:
1. Heavy metal contaminants can be removed in dry scrubbing. This is not possible in wet
scrubbing.
2. Acid gases and trace organics are also removed in dry scrubbing, as well as some chlorinated
dioxin and furan compounds.
3. Addition of activated carbon will further boost heavy metal removal efficiency.

3.4. Dry Scrubber Design
3.4.1. General Design Considerations
When properly designed and operated, a SDA system followed by a baghouse will
achieve a removal efficiency as high as 99%+ of pollutants commonly present in an incin-
erator exhaust stream (acid gases and heavy metals). In older systems, removal rates of
70–80% of the pollutants present are common. A spray dryer with an ESP is capable of
dioxin and furan removals of 98%. Additionally, SDA systems are used to treat hot pol-
luted gas streams. A maximum temperature of 1000°C is possible for a gas stream being
treated. Designs exist for a wide range of gas flow rates. SDA technology, however, is not
useful for the control of gas streams with low concentrations of contaminants (28).
3.4.2. Semidry Scrubber Sizing
When designing a SDA system, the following concerns will govern the design process:
1. Polluted residence time, which is a function of the following:

(a) Vessel volume/size
(b) Polluted gas flow rate
(c) Polluted gas inlet and outlet temperatures

2. Reagent slurry flow, which will determine the following:
(a) Size of slaking equipment
(b) Pump sizes
(c) Atomization equipment requirements