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Non-thermal plasma processing methods have been shown to be effective for treating dilute concentrations of pollutants in large-volume atmospheric-pressure air streams. This paper presents results from basic experimental and theoretical studies aimed at identifying the main reactions responsible for the decomposition of four representative compounds: carbon tetrachloride, methylene chloride, trichloroethylene and methanol. Each of these compounds is shown to be decomposed by a different plasma species: electrons, nitrogen atoms, oxygen radicals and positive ions, respectively. By understanding what plasma species is responsible for the decomposition of a pollutant molecule, it is possible to establish the electrical power requirements of the plasma reactor and help identify the initial reactions that lead to the subsequent process chemistry. These studies are essential for predicting the scaling of the process to commercial size units.

Experimental results on pulsed corona and dielectric-barrier discharge processing of very dilute concentrations of NO in N2 are presented. These NO reduction experiments measure the G value for electron-impact dissociation of N2 and are used to infer the effective electron mean energy in an N2 discharge plasma at atmospheric pressure. The data have been obtained from three different laboratories using widely differing electrode structures, voltage wave forms, power measurements, and chemical analyses. The NO reduction yields from the discharge reactors tested are all similar, corresponding to an electron mean energy of 4.0±0.5 eV.

August 24, 1998This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author. PREPRINTThis paper was prepared for submittal to the DISCLAIMER This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes. ABSTRACTMany studies suggest that lean-NO x SCR proceeds via oxidation of NO to NO 2 by oxygen, followed by the reaction of the NO 2 with hydrocarbons. On catalysts that are not very effective in catalyzing the equilibration of NO+O 2 and NO 2 , the rate of N 2 formation is substantially higher when the input NO x is NO 2 instead of NO. The apparent bifunctional mechanism in the SCR of NO x has prompted the use of mechanically mixed catalyst components, in which one component is used to accelerate the oxidation of NO to NO 2 , and another component catalyzes the reaction between NO 2 and the hydrocarbon. Catalysts that previously were regarded as inactive for NO x reduction could therefore become efficient when mixed with an oxidation catalyst. Preconverting NO to NO 2 opens the opportunity for a wider range of SCR catalysts and perhaps improves the durability of these catalysts. This paper describes the use of a non-thermal plasma as an efficient means for selective partial oxidation of NO to NO 2 . When combined with some types of SCR catalyst, the plasma can greatly enhance the NO x reduction and eliminate some of the deficiencies encountered in an entirely catalyst-based approach.

Treatment of NO x in diesel engine exhaust represents a big opportunity for the environmental application of low-temperature plasmas. This paper discusses the effect of gas composition on the NO x conversion chemistry in a plasma. It is shown that the plasma by itself cannot chemically reduce NO x to N 2 in the highly oxidizing environment of a diesel engine exhaust. To implement the reduction of NO x to N 2 , it is necessary to combine the plasma with a heterogeneous process that can chemically reduce NO 2 to N 2 . Data is presented that demonstrates how the selective partial oxidation of NO to NO 2 in a plasma can be utilized to enhance the selective reduction of NO x to N 2 by a catalyst.

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Abstract

This letter presents measurements of the specific energy consumption (eV per molecule) for electron-impact dissociation of N2 (e+N2→e+N+N) in a pulsed corona and an electron beam reactor. Measurements were done using 100 pm of NO in N2. In this mixture the removal of NO is dominated by the reduction reaction N+NO→N2+O. By measuring the specific energy consumption for reduction of NO, these experiments provide a good measure of the specific energy consumption for electron-impact dissociation of N2. The specific energy consumption using pulsed corona processing is 480 eV per dissociated N2 molecule. For electron beam processing, the specific energy consumption is 80 eV per dissociated N2 molecule.