Scrubber
From Wikipedia, the free encyclopedia
- This article is about the pollution control device, for other uses, see scrubber (disambiguation).
A scrubber is an industrial pollution control device, usually installed on the exhaust flue gas stacks of large furnaces, but may also be used on any number of other air exhaust systems.
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[edit] 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.
[edit] Wet scrubbing
A wet scrubber is used to clean air or other gases of various pollutants 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 sprays, packed towers or a venturi. Wet scrubbers will often significantly increase the proportion of water in waste gases of industrial processes which can be seen in a stack plume.
[edit] Acid neutralization
One common form of scrubbing is to neutralize corrosive vaporized acids in the exhaust steam, such as sulfur dioxide that is emitted when coal containing sulfur is burned. These vaporized acids create smog, and can cause acid rain that eats away at sculptures and stonework.
The scrubbing of acids involves powdered lime (calcium carbonate), which may be mixed with water to form a paste or can also be used dry. The lime is sprayed into the exhaust gas stream inside a reaction chamber. The lime reacts with the acid, neutralizes it, and absorbs it, turning into gypsum which is removed from the exhaust gas stream and can then be used for making other products.
Another method is the use of the dry sorbent scrubber. Limestone pellets (~5 mm)are introduced to the top of a tank where they are distributed via gravity and evenly spaced baffles. Acid gas flows countercurrently to the falling limestone and reacts with the surface of the pellet. The pellets fall into a hopper which is collected by a screw and fed into a device which scrapes the outside layer of the reacted limestone off, at which point it is ready to be pushed back up to the top of the tower. This method is not as efficient as lime injection, and also produces limestone dust as a pollutant in the stack exhaust.
[edit] Mercury removal
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 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.
[edit] 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.
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.
[edit] 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.
