Particulate and SO2 Emissions
A. Cyclone Separators
Typically used to remove particulate from a gas stream, the gas enters tangentially at the top of a cylinder and is forced downward into a spiral motion. The particles exit the bottom while the gas turns upward into the vortex and leaves through the top of the unit. Pressure drops through cyclones are usually from 13 to 17 mm water gauge. Although seldom adequate by themselves, cyclone separators are often an effective first step in pollution control.
Separating solids or liquids from a gas is one use of scrubbers. However, separating a soluble gas from other gases is the application where scrubbers see the most action. Typically, a gas enters the bottom of a scrubber and moves upward while a liquid is sprayed from the top. The soluble gas is carried away by the liquid exiting out the bottom of the unit. The most common application is flue gas desulfurization using ammonia as the solvent or spray liquid. Pressure drops through scrubbers are usually low if they're sized properly, and scrubbers are generally about 50% efficient so multiple units are sometimes required or packing may be used to increase efficiency. The contaminated liquid exiting the scrubber represents it's own disposal problem.
C. Semidry Scrubbers
The advantage of semidry scrubbers is in that they remove contaminates by way of a solid waste that is easier to dispose of (less expensive). Initially, the scrubbing medium is wet (such as a lime or soda ash slurry) then a spray dryer is used to atomize the slurry into the gas which evaporates the water in the droplets. As this takes place, the acid in the gas neutralizes the alkali material and forms a fine white solid. Most of the white solids are removed at the bottom of the scrubber while some are carried into the gas stream and have to be removed by a filter or electrostatic precipitator (discussed later). Although semidry systems cost 5-15% more than wet systems, when combined with a fabric filter, they can achieve 90-95% efficiencies.
Dry scrubbers are sometimes used in a very similar fashion, but without the help of gas-liquid-solid mass transfer, these systems use much higher amounts of the solid alkali materials.
D. Electrostatic Precipitators
Boasting an efficiency in excess of 99%, electrostatic precipitators are very effective at removing tiny particles from gas streams. Gas flows through a rectangular duct containing rows of metallic strips. The strips are negatively charged by way of a small voltage that is applied (about 200 W for every 1000 ft3/min of gas). The efficiency is a result of the precipitators applying the collecting force to the particles only and not the gas. Periodically, the precipitators have to be taken offline and cleaned.