Increasingly stringent environmental regulation imposed on both the military and civilian sectors has created a growing demand for alternative abatement methods for a variety of hazardous compounds. One alternative, the nonthermal plasma, shows promise of providing an efficient means for the destruction of dilute concentrations of hazardous air pollutants. The Dahlgren Laboratory of the Naval Surface Warfare Center has extensively investigated one type of nonthermal plasma discharge, the pulsed corona reactor, for the destruction of volatile organic compounds and chemical warfare agents. In this reactor, a fast rise time (∼10 ns), short duration (<100 ns), high-voltage pulse is repetitively delivered to a wire-cylinder electrode geometry, thereby producing a multitude of streamer discharges along its length. The resulting nonthermal plasma contains highly reactive chemical radicals which can interact with and destroy the hazardous molecules entrained in the ambient atmosphere flowing through the reactor volume. Increased electrical efficiency was obtained using a combination of high efficiency constant-current capacitor-charging, high repetition-rate spark gap switching, and resonant energy transfer to the reactor. Promising results have been obtained for toluene, methylene chloride, and dichlorodifluoromethane in air at concentrations of a few hundred parts per million. The device has been operated at voltages up to 30 kV, pulse repetition rates up to 1.4 kHz, and flow rates up to 60 ℓ/min. Detailed electrical measurements have been made to properly characterize the electrical properties of the pulsed corona reactor and to validate subsequent improvements in the reactor energy efficiency.
One new non-chemical method for removal of biofouling utilizes pulsed acoustic waves above the cavitation threshold to remove accumulated scale and/or biofouling from the inside walls of piping and other enclosed structures. The pulsed acoustic wave successively removes accumulated deposits as the arc-discharge source is moved down the tube by an operator. We describe a program developing a compact, portable tube-cleaning system for use in utility and U.S. Navy scheduled plant maintenance. Results from a laboratory demonstration with typical heat-exchanger tubing taken from a Tennessee Valley Authority power plant are presented. In addition results from a field experiment conducted at the U.S. Navy Marine Corrosion Test Facility, Ft. Lauderdale, Florida, are presented that demonstrated significant (order of magnitude) reduction in biofouling associated with pulsed acoustic shock wave treatment at intensities below the cavitation threshold.
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