Many countries around the world, including our own, depend on coal and other fossil fuels to produce electricity. A natural result from the burning of fossil fuels, particularly coal, is the emission of flyash. Ash is mineral matter present in the fuel. For a pulverized coal unit, 60-80% of ash leaves with the flue gas. Historically, flyash emissions have received the greatest attention since they are easily seen leaving smokestacks.
Two emission control devices for flyash are the traditional fabric filters and the more recent electrostatic precipitators. The fabric filters are large baghouse filters having a high maintenance cost (the cloth bags have a life of 18 to 36 months, butcan be temporarily cleaned by shaking or backflushing with air). These fabric filters are inherently large structures resulting in a large pressure drop, which reduces the plant efficiency. Electrostatic precipitators have collection efficiency of 99%, but do not work well for flyash with a high electrical resistivity (as commonly results from combustion of low-sulfur coal). In addition, the designer must avoid allowing unburned gas to enter the electrostatic precipitator since the gas could be ignited.
The salt & pepper collector/selector, and repelling balloon experiments serve to illustrate the basis of an electrostatic precipitator. In these experiments a type of electrostatic collector and electrostatic selector are created. This same principle is used to keep the environment clean today. A description of a more elaborate demonstration of how an electrostatic precipitator works using a Van de Graaff generator may be found at http://www.physics.umd.edu/lecdem/services/demos/demosj2/j2-15.htm.
The fluegas laden with flyash is sent through pipes having negatively charged plates which give the particles a negative charge. The particles are then routed past positively charged plates, or grounded plates, which attract the now negatively-charged ash particles. The particles stick to the positive plates until they are collected. The air that leaves the plates is then clean from harmful pollutants. Just as the spoon picked the salt and pepper up from the surface they were on, the electrostatic precipitator extracts the pollutants out of the air. For a more detailed overview of an electrostatic precipitator, see Powerspan Corp. (formerly Zero Emmisions Technology) homepage at www.powerspancorp.com/news/precipitator.shtml which also includes a couple of nice schematics. Photographs of electrostatic precipitators at coal-fired power plants can be found at http://www4.ncsu.edu/~frey/apcespph.html.
Electrostatic precipitators are not only used in utility applications but also other industries (for other exhaust gas particles) such as cement (dust), pulp & paper (salt cake & lime dust), petrochemicals (sulfuric acid mist), and steel (dust & fumes).
As we can see Electrical Engineers can play an important part in the fight against pollution. Through devices such as the electrostatic precipitator, electrical engineers can protect the environment from harm. Such a design also appeals to the general public as the electricity can be produced cheaply. The electrostatic precipitator is just one example of a device designed by electrical engineers to help the environment. Engineers are responsible for considering environmental impact as part of their original design work.