:"This article is about the pollution control device, for other uses, see
Scrubber systems are a diverse group of
air pollutioncontrol devices that can be used to remove some particulates and/or gases from industrial exhaust streams. Traditionally, the term "scrubber" has referred to pollution control devices that used liquid to "scrub" unwanted pollutants from a gas stream. Recently, the term is also used to describe systems that inject a dry reagent or slurryinto a dirty exhaust stream to "scrub out" acid gases. Scrubbers are one of the primary devices that control gaseous emissions, especially acid gases.
Removal and neutralization
The exhaust gases of
combustionmay at times contain substances considered harmful to the environment, and it is the job of the scrubber to either remove those substances from the exhaust gas stream, or to neutralize those substances so that they cannot do any harm once emitted into the environment as part of the exhaust gas stream...
wet scrubberis used to clean airor other gases of various pollutants and dustparticles. Wet scrubbing works via the contact of target compounds or particulate matter with the scrubbing solution. Solutions may simply be water(for dust) or complex solutions of reagents that specifically target certain compounds.
Removal efficiency of pollutants is improved by increasing residence time in the scrubber or by the increase of surface area of the scrubber solution by the use of a
spray nozzle, packed towers or an aspirator. Wet scrubbers will often significantly increase the proportion of water in waste gases of industrial processes which can be seen in a stack plume.
Compliance agencies typically place minimum DP thresholds on wet scrubber.
A dry or semi-dry scrubbing system, unlike the
wet scrubber, does not saturate the flue gas stream that is being treated with moisture. In some cases no moisture is added; while in other designs only the amount of moisture that can be evaporated in the flue gas without condensing is added. Therefore, dry scrubbers do not have a stack steam plume or wastewaterhandling/disposal requirements. Dry scrubbing systems are used to remove acid gases (such as SO2 and HCl) primarily from combustionsources.
There are a number of dry type scrubbing system designs. However, all consist of two main sections or devices: a device to introduce the
acid gas sorbentmaterial into the gas stream and a particulate matter control device to remove reaction products, excess sorbent material as well as any particulate matter already in the flue gas.
Dry scrubbing systems can be categorized as dry sorbent injectors (DSIs) or as spray dryer absorbers (SDAs). Spray dryer absorbers are also called semi-dry scrubbers or spray dryers.
Dry sorbent injection involves the addition of an
alkalinematerial (usually hydrated limeor soda ash) into the gas stream to react with the acid gases. The sorbent can be injected directly into several different locations: the combustion process, the flue gasduct (ahead of the particulate control device), or an open reaction chamber (if one exists). The acid gases react with the alkaline sorbents to form solid salts which are removed in the particulate control device. These simple systems can achieve only limited acid gas (SO2 and HCl) removal efficiencies. Higher collection efficiencies can be achieved by increasing the flue gas humidity(i.e., cooling using water spray). These devices have been used on medical wasteincinerators and a few municipal wastecombustors.
In spray dryer absorbers, the
flue gases are introduced into an absorbing tower (dryer) where the gases are contacted with a finely atomized alkaline slurry. Acid gases are absorbed by the slurry mixture and react to form solid salts which are removed by the particulate control device. The heat of the flue gas is used to evaporate all the water droplets, leaving a non-saturated flue gas to exit the absorber tower. Spray dryers are capable of achieving high (80+%) acid gas removal efficiencies. These devices have been used on industrial and utility boilers and municipal wastecombustors.
Mercury has no known beneficial uses in nature, but it is a common substance found in coal that must also be removed.
Wet scrubbers are only effective for mercury removal under certain conditions. Mercury vapor in its elemental form, Hg0, is insoluble in the scrubber slurry and not removed. Oxidized mercury, Hg2+, compounds are more soluble in the scrubber slurry and can be captured. The type of coal burned as well as the presence of a selective catalytic reductionunit both affect the ratio of elemental to oxidized mercury in the flue gas and thus the degree to which the mercury is removed.
Scrubber waste products
One side effect of scrubbing is that the process only moves the unwanted substance from the exhaust gases into a solid paste or powder form. If there is no useful purpose for this solid waste, it must be either contained or buried to prevent environmental contamination. Limestone-based scrubbers can produce a synthetic
gypsumof sufficient quality that can be used to manufacture drywalland other industrial products.
Mercury removal results in a waste product that either needs further processing to extract the raw mercury, or must be buried in a special hazardous wastes
landfillthat prevents the mercury from seeping out into the environment.
Until recently, scrubbers have not been associated with health risks involving bacteria spread as a result of inadequate cleaning, unlike other devices such as
cooling towers. However, a 2005 outbreak of Legionnaires' diseasein Norwaywas proven to emanate from a scrubber, causing ten deaths and more than fifty cases of infection as it spread the bacteria through the air during a period of only two scrubbers being the source of such bacteria
Baffle spray scrubber
Ejector venturi scrubber
Mechanically aided scrubber
Flue gas desulfurization
* [http://macrotek.net Macrotek Inc. (Supplier)]
* [http://encarta.msn.com/media_461547679/Anatomy_of_an_Air_Scrubber.html Cross section of an air scrubber system] – From "Microsoft Encarta"
* [http://antoine.frostburg.edu/chem/senese/101/inorganic/faq/waste-so2-to-wallboard.shtml How Sulfuric (SO2) waste is chemically removed from emissions]
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