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February 24, 1997


Air Pollution Control


Purpose:

To show how air pollution is controlled.

Grade level:

6th, 7th & 8th grades

Essential Elements:

(Science) 1 (A) Properly demonstrate the use of laboratory equipment; 2 (A) Observe physical and chemical properties of matter; 5 (A) Measure physical and chemical properties of matter.

Objectives:

At the end of the lesson the student will be able to distinguish between an electrostatic precipitator and a wet scrubber and the principles behind the operation of these control techniques.

Focus:

When any product is made by industry, waste may be produced that can pollute the air. Wet scrubbers and electrostatic precipitators are two devices used to clean up the air waste stream before it enters the atmosphere.

Background:

Air contaminants are emitted into the atmosphere as particulates, aerosols, vapors, or gases. The most common methods of eliminating or reducing pollutants to an acceptable level are destroying the pollutant by thermal or catalytic combustion, changing the pollutant to a less toxic form, or collecting the pollution by use of equipment to prevent its escape into the atmosphere. Pollutant recovery may be accomplished by the use of one or more of the following:

Baghouses -

Dry particulates are trapped on filters made of cloth, paper or similar materials. Particles are shaken or blown from the filters down into a collection hopper. Baghouses are used to control air pollutants from steel mills, foundries, and other industrial furnaces and can collect more than 98 percent of the particulates.

Cyclones -

Dust-laden gas is whirled very rapidly inside a collector shaped like a cylinder. The swirling motion creates centrifugal forces causing the particles to be thrown against the walls of the cylinder and to drop into a hopper. Cyclones are used for controlling pollutants from cotton gins, rock crushers, and many other industrial processes and can remove up to 95 percent of solid pollutants.

Electrostatic precipitators -

By use of static electricity, they attract particles in much the same way that static electricity in clothing picks up small bits of dust and lint. Electrostatic precipitators, 98 to 99 percent effective, are used instead of baghouses when the particles are suspended in very hot gases, such as in emissions from power plants, steel and paper mills, smelters, and cement plants.

Wet scrubbers -

Particulates, vapors, and gases are controlled by passing the gas stream through a liquid solution. Scrubbers are used on coal burning power plants, asphalt/concrete plants, and a variety of other facilities that emit sulfur dioxides, hydrogen sulfides, and other gases with a high water solubility. Wet scrubbers are often used for corrosive, acidic, or basic gas streams. ( Note that recovery control devices include adsorption and condenser techniques as well.)

Activities:

Here are two activities to help demonstrate how a wet scubber and an electostatic precipitator work.

Activity 1 demonstrates how to build a wet scrubber.

Activity 2 demonstrates how to build an electrostatic precipitator.

Closure:

Restate objectives, begin questioning.

  1. Which type of air cleaner would be the best for removing particles?
  2. Which type of air cleaner would be the best for removing waste gases?
  3. Does a wet scrubber clean up all of the pollutants?
  4. What problems are produced by having too many pollutants in the air we breathe?
  5. If industry is just part of the problem, what can we do to control the amount of air pollution that we cause?

Reference:

Activities from from Holt, Winston's Environmental Science.


Acknowledgment:

Lyn Mock, Stephen F. Austin University Nacogdoches TES Course, 1994


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