Page 224 - Air Pollution Control Engineering

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just after an incineration process, the quencher cools the exhaust gas formed in the pro-
cess to saturation, or near saturation, temperature. In so doing, the volume of the gas to
be treated in the next step is greatly reduced. The approach to saturation temperature is
a function of liquid rate, droplet size and gas residence time. Also, as the humidity of the
gas is increased in the quencher, improved absorption (higher efficiency) is supported
in the next step of the treatment process.
A quencher is also a scrubber, to a limited degree. With alkali liquor, the quencher
can approach 50% removal efficiency of acid gases. This reduces the size of the high-
efficiency packed tower or Venturi unit required after the quencher.

2.3. Wet Scrubber Applications

2.3.1. General Downstream and Upstream Applications
As previously described, a scrubber system moves a pollutant(s) from the gas phase
into the liquid phase. Therefore, after scrubbing, a liquid separator is often required.
Typical liquid separators are mist eliminators, cyclones (or sometimes called hydrocy-
clones to specify liquid vs air cyclones), and swirl vanes. All of these separators use
impaction or centrifugal force to remove liquid droplets (coalesce) in the process
stream. Mist eliminators, as previously discussed, are either mesh-pad or chevron-
blade type.
A typical application example of a wet scrubber is the treatment of an acid gas stream
with a quencher (cool, condense, some removal), then a Venturi for PM removal, followed
by a scrubber (packed or spray tower type) to complete the removal of acid gas from
the polluted airstream. Wet scrubbers also often follow an ESP or fabric-filter unit oper-
ation. This scenario is common when high PM removal is required and such removal
cannot be accomplished with a single Venturi step. An example of this is a polluted
airstream containing acid gas, heavy metals, and, possibly, organic residues. Several air
pollution control process steps, each targeted for one of the above pollutants, are needed
to fully cleanse the air.

2.3.2. Incineration Pollution Control
Incineration or combustion processes produce pollutants that must be removed from
an airstream prior to atmospheric release. Possible pollutants formed are acid gases,
carbon dioxide, carbon monoxide, nitric oxides (NO ), heavy metals, and particulates.
x
If fine PMs (<10 µm) are not a concern and/or if total PM removal required is not
needed, a wet scrubber will probably be used to treat the polluted air. All types of wet
scrubber described here are a possible solution for air pollution control. Often, the ability
of a wet scrubber to remove all of the pollutants mentioned earlier makes this option the
easiest to use for a given air pollution control problem.

2.3.3. Thermal Desorption
Awet scrubber is sometimes useful for thermal desorptional, though its PM removal
capacity is less than that of a fabric filter (or baghouse) or ESP. As flue gas from an
incineration process is cooled to near-saturation temperature in a wet scrubber, the dis-
persion properties of the released flue gas may cause a plume to form. If this occurs, the
gas will need reheating to eliminate the plume.