Dry scrubber

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Figure 1. An electrostatic precipitator is one type of dry scrubber.[1]

A dry scrubber or dry scrubber system is one type of scrubber that is used to remove harmful materials from industrial exhaust gases before they are released into the environment. Dry scrubbers are the type most commonly used in plants today, and they utilize a collection of dry substances to remove acidic gases that contribute to acid rain.[2]

Dry scrubbers work similarly to other scrubbers. The system sprays a collection of dry reagents into an exhaust stream. These chemicals can react differently depending on which material they are specifically targeting for removal. Some of these materials neutralize harmful pollutants in the stream through a chemical reaction, while others cause a material to react and turn into a different substance. That substance then falls out of the gas stream or is caught in a particle screen.

Use

Dry scrubbers are used mainly to remove acid gases from combustion sources. Generally, this is done by introducing a series of dry reactants to exhaust gas at high speeds. This neutralizes the pollutants in the gas. This task is done in three steps: gas cooling, reagent injection, and filtering.[3] First, gas cooling must be done to prepare exhaust gases. In the gas cooling system, emission gases are cooled to make it easier to remove pollutants and other toxins from the gas. The exhaust gas is diluted using an evaporative cooler.[3] Once the gas has been significantly cooled the reagent injection can begin. It is in this step that the harmful components are actually removed from the gas. Components of the dry reagent are generally chosen because of their neutralizing properties, thus sodium bicarbonate is frequently included. A variety of powders are mixed together and fired at high pressures into the exhaust gas. Chemical reactions occur that reduce the acidity of the gas and remove harmful pollutants. The final step is using a fabric filter to capture the used scrubbing powder as the cleaned gas exits the scrubbing chamber. This spent powder can sometimes be cleansed and reused for dry scrubbing, but frequently it must be disposed since it cannot be properly washed.

Advantages and Disadvantages

Dry scrubbing has been embraced widely, and one of the biggest advantages to using it is the positive environmental impact. The removal of hazardous substances from exhaust gas is important as it prevents a large number of pollutants from escaping into the air. Dry scrubbers are also more commonly used than wet scrubbers mainly because they produce comparatively little waste material. Most of this material that is sprayed into the exhaust is burned off in the heat of the stream or is caught in a filter. The use of dry scrubbers is less expensive as there is no associated cost with removing, transporting, and storing waste water from wet scrubbers.[3]

One disadvantage of dry scrubbing is the residual waste powder that must be disposed of because it is a hazardous material. This waste must be handled by specialists because of its chemical makeup. Another disadvantage is that the process of scrubbing is expensive. The final disadvantage to using a dry scrubber is that they cannot remove all contaminants from the gas.[3]

For Further Reading

References

  1. Wikimedia Commons. (July 15, 2015). Electrostatic Precipitator [Online]. Available: https://upload.wikimedia.org/wikipedia/commons/c/c4/Elektrofilter_Maria_Gugging.jpg
  2. A. Atkins and M. Escudier, A dictionary of mechanical engineering. Oxford: Oxford University Press, 2013.
  3. 3.0 3.1 3.2 3.3 Institute.unido.org, 2018. [Online]. Available: https://institute.unido.org/wp-content/uploads/2014/11/25.-Air-Pollution-Control-Technologies-Compendium.pdf. [Accessed: 26- Jul- 2018].


Authors and Editors

Bethel Afework, Jordan Hanania, Kailyn Stenhouse, Jason Donev
Last updated: September 3, 2018
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