Flue-gas desulfurization (FGD) began in England in 1935. The
technology remained dormant until the mid-1960s when it became active primarily
in the United States and Japan. Since then, over 50 FGD processes have been
developed, differing in the chemical reagents and the resultant end products.
The most common FGD system is a lime/limestone wet scrubber.
After the flue gas has been treated in the precipitation (or baghouse), it
passes through the induced fans and enters the SO2 scrubber. If the required
SO2 removal efficiency is less than 85%, a fraction of the flue gas can be
treated while bypassing the rest to mix with and reheat the saturated flue gas
leaving the scrubber.
For higher-sulfur fuels requiring SO2 removal efficiencies
of 90% or greater, the entire flue-gas stream must be treated. Upon leaving the
SO2 absorption section, the flue gas is passed through entrainment separators
to remove any slurry droplets mixed with the gas.
The saturated flue gas is then reheated approximately 25 to
50°F above the water dewpoint before it is vented to the stack. For low- to
medium-sulfur fuels, an alternate scrubbing technology is dry scrubbing.
This process minimizes water consumption and eliminates the
requirement for flue-gas reheating but requires more expensive additives than
the wet limestone systems. The typical dry SO2 absorber is a cocurrent
classifying spray dryer.
Flue gas enters the top of the absorber through inlet
assemblies containing swirl vanes. The absorbent is injected pneumatically into
the center of each swirler assembly by ultrasonic atomizing nozzles that
require an air pressure of about 60 lb/in2 (gage).
Slurry feed pressures are 10 to 15 lb/in2 (gage). The
compressed air induces primary dispersion of the absorbent slurry by mechanical
shear forces produced by the two fluid streams.
Final dispersion is accomplished by shattering the droplets
with ultrasonic energy produced by the compressed air used with a proprietary
nozzle design. Then ultrasonic nozzles generate extremely fine droplets, which
have diameters that range from 10 to 50 #m, as shown by photographic studies.
The flue-gas outlet design requires that effluent gases make
a 180° turn before leaving the absorber. Besides eliminating product
accumulation in the outlet duct, the abrupt directional change also allows the
larger particles to drop out in the absorber product hopper.
This design curtails the particulate loading to the fabric
filter. Consequently, the number of cleaning cycles as well as abrasion of the
filter medium are reduced.
As compared with ordinary fly-ash collection applications,
fabric filters together with dry scrubbing offer a broader choice of design
options. In conventional fly-ash collection applications, the fabric filter
experiences flue-gas temperatures about 100 to 150°F higher than encountered in
dry scrubbing.
Filter media unsuitable at the higher temperatures can be
used when the fabric filter follows a dry absorber. In particular, acrylic
fibers become attractive because of their strength and flex characteristics, as
well as their ability to support more vigorous cleaning methods like mechanical
shaking.
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