M.C. Hsiao

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. >

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.

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...

Electron‐impact dissociation of molecular nitrogen in atmospheric‐pressure nonthermal plasma reactors

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.

DECOMPOSITION OF METHYLENE CHLORIDE BY ELECTRON BEAM AND PULSED CORONA PROCESSING

Experiments on the plasma-assisted decomposition of dilute concentrations of methylene chloride in atmospheric-pressure N2 and dry air streams by electron beam and pulsed corona processing are presented. This paper presents the first experimental evidence showing that the decomposition of methylene chloride in a non-thermal plasma at ambient gas temperature proceeds via reaction with nitrogen atoms. The decompositions is more efficient with energy deposition in electron beam generated plasmas because of the higher rate for electron-impact dissociation of N2. In dry air mixture, the decomposition of methylene chloride is degraded substantially because the nitrogen atoms are consumed in the production of nitrogen oxides. At higher gas temperatures (300°C), the decomposition of methylene chloride in dry air is shown to proceed via reaction with oxygen atoms. The main products of methylene chloride decomposition in dry air mixtures are CO, CO2, HCl, and probably Cl2.

FUNDAMENTAL LIMITS ON NOX REDUCTION BY PLASMA

This paper discusses the gas-phase reaction mechanisms for removal of NO{sub x} in a plasma. The effect of 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 NO{sub x} reduction using the plasma alone. The fundamental limit on the minimum electrical energy consumption that will be required to implement NO{sub x} reduction in any type of plasma reactor is established.

Electron-impact ionization of air molecules and its application to the abatement of volatile organic compounds

In this paper the authors present data on the non-thermal plasma processing of two representative VOCs: carbon tetrachloride and methanol. The investigation used a compact electron beam reactor, and two types of discharge reactors: a pulsed corona and a dielectric-barrier discharge. To the knowledge of the authors, this is the first comparison of the energy efficiency of electron beam, pulsed corona and dielectric-barrier discharge processing of these VOCs under identical gas conditions. For most electrical discharge reactors the analysis suggests that the attainable electron mean energy is rather limited and cannot be significantly enhanced by changing the electrode configuration or voltage waveform. The experimental data confirms that there is no significant difference in the performance of the pulsed corona and dielectric-barrier discharge reactors. The authors observe that electron beam processing is remarkably more energy efficiency than electrical discharge processing in decomposing either of these VOC molecules. During electron beam processing, the specific energy consumption is consistent with the energy required for the ionization of the background air molecules. For carbon tetrachloride, the dominant decomposition pathway is dissociative electron attachment. For methanol, the dominant decomposition pathway is dissociative charge exchange.

Decomposition of methylene chloride in non-thermal plasmas

Summary form only given, as follows. Identification of the mechanism and plasma species responsible for methylene chloride decomposition is important for choosing the most energy efficient type of non-thermal plasma reactor. The paper presents the first experimental evidence showing that the decomposition of methylene chloride in a non-thermal plasma at ambient gas temperature proceeds via reaction with nitrogen atoms. The data is also the first comparison of the energy efficiency of electron beam and pulsed corona processing of methylene chloride under identical gas conditions. We observe that electron beam processing is more energy efficient because of its higher rate for electron-impact dissociation of N/sub 2/. In dry air mixtures, the decomposition of methylene chloride is degraded substantially because the nitrogen atoms are consumed in the production of nitrogen oxides. At higher gas temperatures (300/spl deg/C), the decomposition of methylene chloride in dry air is shown to proceed via reaction with oxygen atoms. The main products of methylene chloride decomposition in dry air mixtures are CO, CO/sub 2/, HCl, and Cl/sub 2/.