B. M. Penetrante

Comparison of electrical discharge techniques for nonthermal plasma processing of NO in N/sub 2/

This paper presents a comparative assessment of three types of electrical discharge reactors: 1) pulsed corona, 2) dielectric-barrier discharge, and 3) dielectric-pellet bed reactor. The emphasis is on the efficiency for electron-impact dissociation of N/sub 2/(e+N/sub 2//spl rarr/e+N+N) and the subsequent chemical reduction of NO by nitrogen atoms (N+NO/spl rarr/N/sub 2/+O). By measuring the concentration of NO as a function of input energy density in dilute mixtures of NO in N/sub 2/, it is possible to determine the specific energy cost for the dissociation of N/sub 2/. Our experimental results show that the specific energy consumption (eV per NO molecule reduced) of different types of electrical discharge reactors are all similar. These results imply that, during radical production in electrical discharge reactors, the electric field experienced by the plasma is space-charge shielded to approximately the same value. The specific energy consumption for the dissociation of N/sub 2/ using electrical discharge processing is measured to be around 240 eV per nitrogen atom produced. In the NO-N/sub 2/ mixture, this corresponds to a specific energy consumption of around 240 eV per NO molecule reduced. >

Kinetic Analysis of Non-Thermal Plasmas Used for Pollution Control

Non-thermal plasma techniques are being developed for the treatment of many gas-phase pollutants. In these methods electrical energy from electron beams or electrical discharges is directed selectively into the production of electrons, ions and radicals, or into molecular excitations that will result in the efficient destruction of the unwanted species. The processes by which this can be achieved are described. Kinetic analysis of the deposition of energy into contaminated air is illustrated by studies of the decomposition of nitrogen oxides, methylene chloride, carbon tetrachloride and methanol.

Identification of mechanisms for decomposition of air pollutants by non-thermal plasma processing

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.

Pulsed corona and dielectric‐barrier discharge processing of NO in N2

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 N2dischargeplasma 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.

Environmental applications of low-temperature plasmas

Treatment of NOx 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 NOx conversion chemistry in a plasma. It is shown that the plasma by itself cannot chemically reduce NOx to N2 in the highly oxidizing environment of a diesel engine exhaust. To implement the reduction of NOx to N2, it is necessary to combine the plasma with a heterogeneous process that can chemically reduce NO2 to N2. Data is presented that demonstrates how the selective partial oxidation of NO to NO2 in a plasma can be utilized to enhance the selective reduction of NOx to N2 by a catalyst.

Electron beam and pulsed corona processing of volatile organic compounds in gas streams

This paper presents experimental results on non-thermal plasma processing of atmospheric-pressure gas streams containing dilute concentrations of various volatile organic compounds (VOCs). This investigation used a compact electron beam reactor and a pulsed corona reactor to study the effects of background gas composition and gas temperature on the decomposition chemistry and elecmcal energy efficiency. The elecmcal energy consumption is characterized for the decomposition of a variety of VOCs, including carbon tetrachloride, mchloroethylene, methylene chloride, benzene, acetone and methanol. For most of the VOCs investigated, electron beam processing is more energy efficient than pulsed corona processing. For VOCs (such as carbon tetrachloride) that require copious amounts of electrons for its decomposition, electron beam processing is remarkably more energy efficient. For some VOCs the decomposition process is limited by their reaction rate with the plasma-produced radicals and/or by the occurrence of back reactions. In these cases, the energy consumption can be minimized by operating at high (but non- combusting) temperatures.

Electron beam and pulsed corona processing of carbon tetrachloride in atmospheric pressure gas streams

Experimental results are presented on electron beam and pulsed corona processing of atmospheric-pressure gas streams containing dilute concentrations of carbon tetrachloride (CCl4). Electron beam processing is remarkably more energy efficient than pulsed corona processing in decomposing CCl4. The specific energy consumption in each reactor is consistent with dissociative electron attachment as the dominant decomposition pathway. The energy efficiency of the plasma process is insensitive to the gas temperature, at least up to 300°C. By doing the experiments using both dry air and N2, the contribution of O radicals in the decomposition of CCl4 is assessed. A discussion of the chemical kinetics starting from the initial decomposition of CCl4 to the formation of products is presented.

PLASMA-ASSISTED DECOMPOSITION OF METHANOL AND TRICHLOROETHYLENE IN ATMOSPHERIC PRESSURE AIR STREAMS BY ELECTRICAL DISCHARGE PROCESSING

Experiments are presented on the plasma‐assisted decomposition of dilute concentrations of methanol and trichloroethylene in atmospheric pressure air streams by electrical discharge processing. This investigation used two types of discharge reactors, a dielectric‐barrier and a pulsed corona discharge reactor, to study the effects of gas temperature and electrical energy input on the decomposition chemistry and byproduct formation. Our experimental data on both methanol and trichloroethylene show that, under identical gas conditions, the type of electrical discharge reactor does not affect the energy requirements for decomposition or byproduct formation. Our experiments on methanol show that discharge processing converts methanol to COx with an energy yield that increases with temperature. In contrast to the results from methanol, COx is only a minor product in the decomposition of trichloroethylene. In addition, higher temperatures decrease the energy yield for trichloroethylene. This effect may be due to...

Plasma-assisted catalytic reduction of NOx

Many studies suggest that lean-NOx SCR proceeds via oxidation of NO to NO¬ by oxygen, followed by the reaction of the NO¬ with hydrocarbons. On catalysts that are not very effective in catalyzing the equilibration of NO+O¬ and NO¬, the rate of N¬ formation is substantially higher when the input NOx is NO¬ 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 NO¬, and another component catalyzes the reaction between NO¬ 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 NO¬ 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¬. 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. efficiency for reduction of NOx

Feasibility of plasma aftertreatment for simultaneous control of NOx and particulates

Plasma reactors can be operated as a particulate trap or as a NO{sub x} converter. The soluble organic fraction (SOF) of the trapped particulates can be utilized for the oxidation of NO to NO{sub 2}. The NO{sub 2} can then be used to non-thermally oxidize the carbon fraction of the particulates. This paper examines the energy density required for oxidation of the SOF hydrocarbons and the fate of NO{sub 2} during the oxidation of the particulate carbon. The energy density required for complete oxidation of the SOF hydrocarbons is shown to be unacceptably large. The reaction of NO{sub 2} with carbon is shown to lead mainly to backconversion of NO{sub 2} to NO. These results suggest that the use of a catalyst in combination with the plasma will be required to efficiently reduce the NO{sub x} and oxidize the SOF hydrocarbons.