Page 246 - Air Pollution Control Engineering

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Wet and Dry Scrubbing 225

plants and solid-waste incinerators. SDA is a widely accepted and well-documented air
pollution control technology. Alkali slurry is often injected in SDA by atomizers (single
or multiple) or with dual-fluid spray nozzles. The optimum slurry droplet size in SDA
is 50–90 µm. All types of flow schemes are found in SDA equipment: downflow,
upflow, upflow using cyclonic precollection, as well as with single or multiple gas
inlets. Typical removal efficiencies achieved in a SDA system are 95% for SO and HF
2
and 99%+ for HCl and SO . Polluted gas streams having temperatures as high as
3
1000°C may be treated with SDAs. This is possible as the result of the rapid cooling of
gas that will take place as the result of evaporative cooling. Evaporation of water will
bring the temperature of the hot gas below 200°C. Needed residence time for the pol-
luted gas stream being treated is 10–18 s. Approximately 25% of the reaction products
are recovered in the SDA unit as ash (15).

3.3. Dry Scrubbing Applications
3.3.1. General Downstream and Upstream Dry Scrubbing Applications
After dry scrubbing (downstream), a fabric filter or an ESP will be used to collect
particulates. The efficiency of acid gas removal is enhanced by the presence of the par-
ticulate collection step. Generally, fabric filters improve such efficiency better than an
ESP. Collected solids are also often suitable for recycling, which may recover some of
the operation cost of the pollution control system.
Power plant air emissions are commonly treated and controlled with dry scrubber
technology. Standard designs and operational data are therefore available for such appli-
cations. Less well understood is the use of dry scrubbers to control air emissions from
waste incinerators. Numerous control schemes are possible, but no industry standard for
treating air emissions from waste incinerators has emerged to date. Combinations of
various types of scrubber is possible, as well as fabric filter, ESP, and so forth, to solve
a particular air pollution problem. Example solutions are discussed below.

3.3.2. Incineration Pollution Control
The pollutants in an incinerator exhaust gas flow may be controlled using semidry
scrubbing, with varying removal efficiencies. Particles that are sticky, gummy, and/or
corrosive are processed easily in dry scrubbers. Much of the control process occurs in
the dryer section, including chemical and moisture addition and, subsequently, most of the
absorption as well. Some additional absorption will also occur in the dust collection stage.
Dry absorption is best suited for control of trace metals, acid gas, and trace organic pol-
lutants. Dry scrubbers often have high power costs because fluid nozzles operate at high
pressure. Power consumption is lower for rotary atomizers than fluid nozzles. Wet
absorbers generate lower chemical and disposal costs compared with dry scrubbers. A
major advantage of dry scrubbers is lower initial capital expenditures compared to a wet
scrubber system.
The final product of semidry scrubbing (see Fig. 6B) will be hygroscopic and contain a
large soluble fraction. This end product also tends to cling more (or stick) than fly ash, so
it is more difficult to handle than the latter. In addition to some fly ash, the final product of
dry scrubbing will contain some trace heavy metals along with trace amounts of organic
compounds. Consequently, the final product will always be classified as a hazardous waste