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High voltage insulation is one of the main areas of pulsed power research and development since the surface of an insulator exposed to vacuum can fail electrically at an applied field more than an order or magnitude below the bulk dielectric strength of the insulator. This is troublesome for applications where high voltage conditioning of the insulator and electrodes is not practical and where relatively long pulses, on the order of several microseconds, are required. Here we give a summary of our approach to modeling and simulation efforts and experimental investigations for understanding flashover mechanism. The computational work is comprised of both filed and particle-in-cell modeling with state-of-the-art commercial codes. Experiments were performed in using an available 100- kV, 10- s pulse generator and vacuum chamber. The initial experiments were done with polyethylene insulator material in the shape of a truncated cone cut at +45 angle between flat electrodes with a gap of 1.0 cm. The insulator was sized so there were no flashovers or breakdowns under nominal operating conditions. Insulator flashover or gap closure was induced by introducing a plasma source, a tuft of velvet, in proximity to the insulator or electrode., See also ADM002371. 2013 IEEE Pulsed Power Conference, Digest of Technical Papers 1976-2013, and Abstracts of the 2013 IEEE International Conference on Plasma Science. IEEE International Pulsed Power Conference (19th). Held in San Francisco, CA on 16-21 June 2013., The original document contains color images.

Plasma reactors can be operated as a particulate trap or as a NOx converter. Particulate trapping in a plasma reactor can be accomplished by electrostatic precipitation. The soluble organic fraction of the trapped particulates can be utilized for the hydrocarbon- enhanced oxidation of NO to NO2. The NO2 can then be used to non-thermally oxidize the carbon fraction of the particulates. The oxidation of the carbon fraction by NO2 can lead to reduction of NOx or backconversion of NO2 to NO. This paper examines the hydrocarbon and electrical energy density requirements in a plasma for maximum NOx conversion in both heavy-duty and light-duty diesel engine exhaust. The energy density required for complete oxidation of hydrocarbons is also examined and shown to be much greater than that required for maximum NOx conversion. The reaction of NO2 with carbon is shown to lead mainly to backconversion of NO2 to NO. These results suggest that the combination of the plasma with a catalyst will be required to reduce the NOx and oxidize the hydrocarbons. The plasma reactor can be operated occasionally in the arc mode to thermally oxidize the carbon fraction of the particulates.

Several catalytic aftertreatment technologies rely on the conversion of NO to NO2 to achieve efficient reduction of NO(x) and particulates in diesel exhaust. These technologies include the use of selective catalytic reduction of NO(x) with hydrocarbons, NO(x) adsorption, and continuously regenerated particulate trapping. These technologies require low sulfur fuel because the catalyst component that is active in converting NO to NO2 is also active in converting SO2 to SO3. The SO3 leads to increase in particulates and/or poison active sites on the catalyst. A non-thermal plasma can be used for the selective partial oxidation of NO to NO2 in the gas phase under diesel engine exhaust conditions. This paper discusses how a non-thermal plasma can efficiently oxidize NO to NO2 without oxidizing SO2 to SO3.

Many studies suggest that lean-NOx SCR proceeds via oxidation of NO to NO2 by oxygen, followed by the reaction of the NO2 with hydrocarbons. On catalysts that are not very effective in catalyzing the equilibration of NO+O2 and NO2, the rate of N2 formation is substantially higher when the input NOx is NO2 instead of NO. The apparent bifunctional mechanism in the SCR of NOx has prompted the use of mechanically mixed catalyst components, in which one component is used to accelerate the oxidation of NO to NO2, and another component catalyzes the reaction between NO2 and the hydrocarbon. Catalysts that previously were regarded as inactive for NOx reduction could therefore become efficient when mixed with an oxidation catalyst. Preconverting NO to NO2 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 NO2. When combined with some types of SCR catalyst, the plasma can greatly enhance the NOx reduction and eliminate some of the deficiencies encountered in an entirely catalyst-based approach., Prepared in collaboration with Lawrence Livermore National Lab., Lawrence, CA, Northwestern Univ., Evanston, IL, and Engelhard Corp., East Newark, NJ.

This paper discusses the gas-phase reaction mechanisms for removal of NOx in a plasma. The effect oxygen content on the competition between the reduction and oxidation processes is discussed. The effect of the electron kinetic energy distribution on the radical production and subsequent chemistry is then discussed in order to predict the best performance that can be achieved for NOx reduction using the plasma alone. The fundamental limit on the minimum electrical energy consumption that will be required to implement NOx reduction in any type of plasma reactor is established., International Spring Fuels and Lubricants Meeting held in Dearborn, Michigan on May 5-8, 1997.

This paper examines how hydrocarbons affect the non-thermal plasma treatment of NOx in lean-burn engine exhausts. We have found that the NO is mainly oxidized to NO2 by NO + HO2 --> NO2 + OH, NO + RO2 --> NO2 + RO where R is a hydrocarbon radical. The O and OH radicals produced by electron-impact dissociation are consumed mainly by reactions with the hydrocarbons rather than with NO. The hydrocarbons lower the energy cost for the oxidation of NO by converting 0 and OH to HO2; the OH radical is then reproduced when NO is oxidized by HO2. This cyclic process leads to a very efficient utilization of the plasma-produced radicals for the selective partial oxidation of NO to NO2. This result suggests that gas-phase reactions in the plasma alone cannot lead to the chemical reduction of NOx. Any reduction of NOx to N2 can only be accomplished through heterogeneous reactions of NO2 with surfaces or particulates, Presented at 1997 Diesel Engine Emissions Reduction Workshop.

This paper examines the gas-phase chemical reduction of NOx. It is shown that under the best conditions, the plasma can chemically reduce 1.6 grams of NOx per brake-horsepower-hour when 5% of the engine output energy is delivered to the plasma. This NOx reduction efficiency is a fundamental limit that cannot be exceeded in the absence of heterogeneous reactions or chemical additives. Higher NOx removal efficiencies would suggest reactions of plasma oxidation products (such as NO2 or nitric acid) on surfaces or particulates.

Recent developments in catalytic control of NOx are revealing the significance of NO2 as an intermediary for achieving higher NOx removal efficiencies. This paper discusses the combination of the plasma with a catalyst to improve the selective reduction of NOx under lean-burn conditions. It is shown that the main effect of the plasma is to enhance the gas-phase oxidation of NO to NO2. The reduction of NOx to N2 is then accomplished by the heterogeneous reaction of NO2 with activated hydrocarbons on the catalyst surface. By using a plasma, one can take advantage of a new class of catalysts that are potentially more durable, more active, more selective and more sulfur-tolerant compared to conventional lean-NOx catalysts. The plasma-assisted catalytic reduction process can be implemented with any type of plasma reactor and does not require a specific type of electrical power supply. It can also easily accommodate any type of catalyst support structure., Prepared in cooperation with Engelhard Corporation.

Recent Studies suggest that the conversion of NO to NO2 is an important intermediate step in the selective catalytic reduction (SCR) of NOx to N2. These studies have prompted the development of schemes that use an oxidation catalyst to convert NO to NO2, followed by a reduction catalyst to convert NO2 to N2. Multi-stage SCR offers high NOx reduction efficiency from catalysts that, separately, are not very active for reduction of NO, and alleviates the problem of selectivity between NO reduction and hydrocarbon oxidation. A plasma can also be used to oxidize NO to NO2. This paper compares the multi-stage catalytic scheme with the plasma-assisted catalytic scheme for reduction of NOx in lean-burn engine exhausts. The advantages of plasma oxidation over catalytic oxidation are presented., Proceedings of the 1997 Diesel Engine Emissions Reduction Workshop.