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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