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Cleaning up with a smokestack's siren song.

Cleaning up with a smokestack's siren song

A siren's painfully loud wail may somedaysignal the cleansing of minute particles from gases escaping a coal-fired power plant's smokestack. The idea is to use high levels of acoustic energy to force micron-sized particles to collide and stick together. The resulting increase in particle size means that conventional cleanup techniques, normally inefficient for removing particles less than five microns across, can be used more effectively to reduce pollutant emissions from power plants.

Although the concept of acoustic agglomerationis almost a century old, only in the last decade or so have researchers, funded by the Department of Energy, taken a close look at the process and come up with an approach that appears to be technically and economically feasible. Gerhard Reethof of Pennsylvania State University in University Park heads one group involved in this research, and David T. Shaw of the State University of New York at Buffalo in Amherst, N.Y., leads another. They discussed the status of their work in Indianapolis at a recent Acoustical Society of America meeting.

One of the biggest difficulties has beenobtaining a reliable, high-efficiency sound source. Reethof has developed a 2,000-watt siren that generates a 160-decibel sound at a frequency between 1,000 and 2,000 hertz. In contrast, a typical air-raid siren, which operates at a lower frequency, has a power of only a few hundred watts. Reethof is now designing a 4,000-watt siren. Other types of sound sources are also being developed.

The siren's loudness isn't likely tocreate a noise problem, because the siren would be encased within a large, thick-walled structure through which gases and entrained fly ash from the combustion of coal would pass on their way to cleanup devices such as electrostatic precipitators, baghouses and cyclones. The level of noise escaping from this chamber would be lower than the noise level in the rest of the power plant.

"The research results . . . show conclusivelythat acoustic agglomeration of fly ash can be accomplished,' says Reethof. However, before a large-scale demonstration can be attempted, he says, further research on several important aspects of the coagulation process is needed. One scientific question concerns whether agglomerated particles can, without falling apart, withstand the rigors of cleansing devices such as cyclones. Another concerns whether the particles are truly spherical, as theoretical models of the process assume (but which may or may not be necessary for trapping).

Nevertheless, says Reethof, "I think weare ready to try it on a small scale.' He proposes testing the effectiveness of the process by diverting and treating a portion of the gas flow at an existing power plant.

Shaw is interested in using acousticagglomeration to control dust particles in high-temperature, high-pressure, highly corrosive environments where other control methods are ineffective. Acoustic agglomeration, he suggests, may serve as a "preconditioner' for removing dust from hot gases used to run a gas turbine or for decontaminating an airstream after an accidental radioactivity release. The chemical industry may also find the process useful for quickly separating chemical products from a gas stream.

Osman K. Mawardi of Case WesternReserve University in Cleveland is investigating the possibility of using much lower levels of sound energy to separate solid particles suspended in a gas. He suggests combining the sound field with an electric or magnetic field to achieve either particle agglomeration or removal. This method eliminates the need for generating high-intensity sound fields and considerably lowers the potential cost of the process. "On paper,' he says, "it looks very encouraging.'
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Title Annotation:research on using high levels of acoustic energy to clean minute particles escaping from plant smokestacks
Author:Peterson, Ivars
Publication:Science News
Date:May 30, 1987
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