Scrubber design is directed toward air pollution control through the removal of harmful contaminants from exhaust streams created by power generation, manufacturing, and other industrial applications. Types of scrubber design include wet scrubbers using liquids or slurries, dry scrubbers using powdered chemicals, and combustion equipment that burns off toxic constituents. Units may be regenerative types that recycle the accumulated contaminants or non-regenerative equipment that collects the material for disposal. Pollutants removed may include particulates, fumes, gases, and vapors.
Units using liquids, i.e., wet scrubbers, can be configured with a variety of technologies. Venturi scrubbers are designed to accelerate the exhaust stream through a narrowed cone-shaped duct where the scrubbing liquid, often water, is sprayed to maximize contact with the gas stream. Particulate residue sticks to the droplets of liquid, and the wet particulate passes through a centrifugal separator to segregate it from the balance of the exhaust stream.
Spray scrubbers, such as the type often used in petroleum-burning power plants, wet the gas stream with a curtain of liquid or slurry inside a tower or chamber. The wetting materials are chosen for their specific chemical reactions with the exhaust to recover the maximum possible quantity of pollutants. Factors such as density, viscosity, and temperature affect the efficiency of these units.
A more recent scrubber design is the condensation scrubber. Steam is injected into a saturated gas stream and condensation forms as droplets around the fine particulates. The droplets are separated using mechanisms such as a mist eliminator.
Dry scrubbers are frequently used to neutralize acidic exhausts. Dry chemical powder is injected at high pressure into the gas stream to react with the acidic elements. To maximize the chemical reaction, the exhaust temperature may be modified before treatment. The neutralized chemicals are captured through a filtration system, thus leaving a cleansed exhaust stream to be discharged.
Combustion units are another stage in scrubber design. These units typically work in tandem with other types of scrubbers. The gas stream is heated and oxidized to destroy insoluble toxic components. High temperature burning reduces pollutants in the exhaust stream to a level where wet or dry scrubbers can be used to complete the cleaning process.
Regenerative scrubber design allows for reuse of the chemicals collected in the scrubber for secondary uses. Generally the cost of installation of a regenerative scrubber is higher, but the resale or reuse of the recycled chemicals can help to offset the operating expense. A non-regenerative scrubber is most often less costly to install, but the byproducts of the scrubbing process may incur disposal fees when they are removed, thus increasing operating costs.