Scrubber
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224809714
2008-07-10T14:47:44Z
74.13.31.40
:''This article is about the pollution control device, for other uses, see [[scrubber (disambiguation)]].
'''Scrubber''' systems are a diverse group of [[air pollution]] control devices that can be used to remove [[particulate]]s 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 [[slurry]] into 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 [[combustion]] may 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 scrubbing ===
A [[wet scrubber]] is used to clean [[air]] or other [[gas]]es of various [[pollutant]]s and [[dust]] particles. 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 bed|packed]] towers or an [[aspirator]]. [[Wet scrubber]]s will often significantly increase the proportion of water in waste gases of industrial processes which can be seen in a stack plume.
<br />
[http://en.wikipedia.org/wiki/Image:Wet_scrubber1.jpg Typical wet scrubber]<br />Compliance agencies typically place minimum DP thresholds on wet scrubber.
===Dry scrubbing===<!-- This section is linked from [[Air pollution]] -->
A [[dry scrubber|dry]] or [[dry scrubber|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 [[wastewater]] handling/disposal requirements. Dry scrubbing systems are used to remove [[acid gas]]es (such as SO<sub>2</sub> and HCl) primarily from [[combustion]] sources.
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]] [[sorbent]] material 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 injection|dry sorbent injectors (DSIs)]] or as [[spray dryer|spray dryer absorbers (SDAs)]]. Spray dryer absorbers are also called semi-dry scrubbers or spray dryers.
'''Dry sorbent injection''' involves the addition of an [[alkaline]] material (usually [[hydrated lime]] or [[soda ash]]) into the gas stream to react with the [[acid gas]]es. The sorbent can be injected directly into several different locations: the combustion process, the [[flue gas]] duct (ahead of the particulate control device), or an open reaction chamber (if one exists). The acid gases react with the alkaline [[sorbent]]s to form solid [[salt]]s which are removed in the particulate control device. These simple systems can achieve only limited acid gas (SO<sub>2</sub> 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 waste]] incinerators and a few [[municipal waste]] combustors.
In '''spray dryer absorbers''', the [[flue gas]]es 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 [[salt]]s 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 [[absorption|absorber]] tower. Spray dryers are capable of achieving high (80+%) acid gas removal efficiencies. These devices have been used on industrial and utility [[boiler]]s and [[municipal waste]] combustors.
=== Mercury removal ===
[[mercury (element)|Mercury]] has no known beneficial uses in nature, but it is a common substance found in coal that must also be removed. [[Wet scrubber]]s are only effective for mercury removal under certain conditions. Mercury vapor in its elemental form, Hg<sup>0</sup>, is insoluble in the scrubber slurry and not removed. Oxidized mercury, Hg<sup>2+</sup>, 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 reduction]] unit 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 [[gypsum]] of sufficient quality that can be used to manufacture [[drywall]] and 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 [[landfill]] that prevents the mercury from seeping out into the environment.
== Bacteria spread ==
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' disease]] in [[Norway]] was 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 weeks. This particular system had undergone regular cleaning routines every three weeks. This case is believed to be the first documented case of a scrubber being the source of such bacteria spread.
== See also ==
* [[Wet scrubber]]
** [[Baffle spray scrubber]]
** [[Ejector venturi scrubber]]
** [[Liquid-to-gas ratio]]
** [[Mechanically aided scrubber]]
** [[Spray tower]]
** [[Spray Nozzle]]
** [[Venturi scrubber]]
** [[Air Turbo-Impactor]]
* [[Flue gas desulfurization]]
* [[Oil desulfurization]]
* [[Electrostatic precipitator]]
== External links ==
*[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 ''[[Encarta|Microsoft Encarta]]''
*[http://antoine.frostburg.edu/chem/senese/101/inorganic/faq/waste-so2-to-wallboard.shtml How Sulfuric (SO<sub>2</sub>) waste is chemically removed from emissions]
[[Category:Air pollution]]
[[Category:Pollution control technologies]]
[[Category:Air pollution control systems]]
[[Category:Scrubbers]]
[[Category:Acid gas control]]
[[Category:Chemical engineering]]
[[Category:Industrial processes]]
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