REACTION CHEMISTRY AND OPTIMIZATION OF PLASMA REMEDIATION OF NXOY FROM GAS STREAMS

Increasing environmental awareness and regulatory pressure have motivated investigations into energy efficient methods to remove oxides of nitrogen (NxOy) from gas streams resulting from the combustion of fossil fuels. Plasma remediation of NxOy is potentially an efficient removal technique due to the relative ease of generating reactants by electron-impact processes. Previous works have investigated the use of electron-beam, corona, and dielectric barrier discharge (DBD) generated plasmas for this purpose. In those works, reduction (N+NO-->N-2+O) and oxidation (NO2+OH-->HNO3) reactions were identified as major removal channels. A computational study of the plasma remediation of NxOy from humid air using repetitively pulsed DBDs is reported. The dominant reaction pathways are discussed and scaling laws are proposed to optimize the energy efficiency of removal. Three reaction periods are identified: the current pulse (during which electron-impact processes generate radicals), the postpulse remediative period (during which NxOy is removed), and the interpulse period (during which the densities of various nitrogen oxides are reapportioned with little net removal). The lifetimes of reactants (OH and O-3 in particular) determine the length of these periods and hence the optimum repetition frequency. Optimum repetition rates are typically less than hundreds of Hz. It is also found that a larger number of current pulses producing less energy deposition per pulse results in a higher removal efficiency due to reduced competition from radical-radical reactions which deplete the reactants. The production of unwanted species (e.g., O-3 and N2O) can be minimized by reducing or terminating power deposition when the densities NO and NO2 have been reduced to ppm levels. The energy efficiency of remediation generally increases with increasing water content by removing NOx through the oxidation channel, although at the price of producing an acidic end product. (C) 1995 American Institute of Physics.