Formation Of Microdischarges Research Articles

Effect of Ni NP morphology on catalyst performance in non-thermal plasma-assisted dry reforming of methane

Supported nickel nanoparticle catalysts, with differing nickel morphologies, were studied in the non-thermal-plasma-promoted dry reforming of methane. Catalysts with nickel particles containing a greater ratio of (111) lattice planes and surface structures in the nanoscale, i.e. dendritic and flake-like particles, showed higher conversions and yields (CH4 conversions of 65 % and 22 % and CO2 conversions of 43 % and 33 % respectively with H2 yields of 42 % and 34 % and CO yields of 31 % and 24 % respectively) than spherical analogues which had conversions of 26 % (CH4) and 16 % (CO2) and yields of 21 % (H2) and 11 % (CO). This is thought to be related to the favourability of (111) faces for CH4 dissociation and the potential for the formation of micro-discharges around their small surface features. All materials tested showed higher CH4 conversion and H2 production when compared with reactions carried out in the absence of a catalyst. The latter reactions showed a greater extent of competing side reactions, resulting in lower conversions and yields.

Application of dielectric barrier discharge (DBD) plasma packed with glass and ceramic pellets for SO2 removal at ambient temperature: optimization and modeling using response surface methodology

Air pollution is a major health problem in developing countries and has adverse effects on human health and the environment. Non-thermal plasma is an effective air pollution treatment technology. In this research, the performance of a dielectric barrier discharge (DBD) plasma reactor packed with glass and ceramic pellets was evaluated in the removal of SO2 as a major air pollutant from air in ambient temperature. The response surface methodology was used to evaluate the effect of three key parameters (concentration of gas, gas flow rate, and voltage) as well as their simultaneous effects and interactions on the SO2 removal process. Reduced cubic models were derived to predict the SO2 removal efficiency (RE) and energy yield (EY). Analysis of variance results showed that the packed-bed reactors (PBRs) studied were more energy efficient and had a high SO2 RE which was at least four times more than that of the non-packed reactor. Moreover, the results showed that the performance of ceramic pellets was better than that of glass pellets in PBRs. This may be due to the porous surface of ceramic pellets which allows the formation of microdischarges in the fine cavities of a porous surface when placed in a plasma discharge zone. The maximum SO2 RE and EY were obtained at 94% and 0.81 g kWh−1, respectively under the optimal conditions of a concentration of gas of 750 ppm, a gas flow rate of 2 l min−1, and a voltage of 18 kV, which were achieved by the DBD plasma packed with ceramic pellets. Finally, the results of the model’s predictions and the experiments showed good agreement.

Self-Organization Kinetics of Microarc Oxidation: Nonequilibrium-State Electrode Reaction Kinetics

Here, we present a new method for studying and gaining new insights into microarc oxidation (MAO) and its corresponding nonequilibrium-state electrode reaction kinetics. We observed that the fundamental condition of microdischarge formation was the resistance nonequilibrium in the oxide film caused by the existence of a defective region. Specifically, the current always first passed through a path with the lowest resistance. After the first discharge, a ceramic phase (e.g., α-Al2O3) was formed, which changed the conductive properties of the defective region, and the resistance in the defective region was subsequently increased, which sent feedback to the next current distribution. This fundamental and inherent law rendered controlling and predicting the microarc system kinetics by the external environment difficult. Therefore, we developed a microarc algorithm and proposed a feedback coefficient k, which determines whether the anode current distribution is inclined to or deviates from the equilibrium state. Finally, we analyzed the k-frequency relationship and used frequency to experimentally describe various kinetic behaviors of MAO.

Can plasma be formed in catalyst pores? A modeling investigation

We investigate microdischarge formation inside catalyst pores by a two-dimensional fluid model for various pore sizes in the μm-range and for various applied voltages. Indeed, this is a poorly understood phenomenon in plasma catalysis. The calculations are performed for a dielectric barrier discharge in helium, at atmospheric pressure. The electron and ion densities, electron temperature, electric field and potential, as well as the electron impact ionization and excitation rate and the densities of excited plasma species, are examined for a better understanding of the characteristics of the plasma inside a pore. The results indicate that the pore size and the applied voltage are critical parameters for the formation of a microdischarge inside a pore. At an applied voltage of 20kV, our calculations reveal that the ionization mainly takes place inside the pore, and the electron density shows a significant increase near and in the pore for pore sizes larger than 200μm, whereas the effect of the pore on the total ion density is evident even for 10μm pores. When the pore size is fixed at 30μm, the presence of the pore has no significant influence on the plasma properties at an applied voltage of 2kV. Upon increasing the voltage, the ionization process is enhanced due to the strong electric field and high electron temperature, and the ion density shows a remarkable increase near and in the pore for voltages above 10kV. These results indicate that the plasma species can be formed inside pores of structured catalysts (in the μm range), and they may interact with the catalyst surface, and affect the plasma catalytic process.

Characteristics of an Underwater Plasma Source with a Porous Glass Membrane

SUMMARYThe advanced oxidation process (AOP) with hydroxyl radicals (OH) is considered to be useful for water purification through oxidation; therefore, we proposed an underwater plasma source with a porous glass membrane which performed the functions of bubble supply and the formation of microdischarges. We observed bubble formation and light emission from discharges on the surface of the membrane using a high‐speed camera and a spectroscope, respectively. In addition, the dependence of gas pressure on the generation of OH radical and the decomposition of recalcitrant organic matter were examined. The obtained results showed that AOP may be more effective when the operating gas pressure is set higher than a pressure which could form bubbles.

Dielectric Barrier Discharge Ozonizer Using the Transformerless Single-Switch Resonant Converter for Portable Applications

This paper describes the analysis, design, and construction of an ozonizer based on a single-switch resonant converter (SSRC). The main contribution of the work is the removal of the transformer from the SSRC, resulting in much smaller footprint and higher efficiency. High voltage (up to 4.0 $\hbox{kV}_{{\rm p} - {\rm p}}$ ) is achieved using a resonant tank, which comprises of an LC circuit combined with the chamber's equivalent circuit components. Despite the absence of the transformer, the topology is able to generate sufficiently high voltage to initiate and sustain the formation of micro-discharges. Geometrically, the chamber is constructed as a planar type, using muscovite mica as dielectric. The main advantage of using mica is its availability in thin sheet (less than 1 mm), which then allows for the lowering of the initiation voltage. To validate the analysis and design, an experimental prototype of the ozonizer is build. It is found that the ozonizer draws very little power, i.e., less than 8 W with very high ozone yield (over 120 g/kWh at 1.0 L/min). The maximum measured efficiency is 75%. The proposed ozonizer is fed from a low voltage power source (up to 40 V dc) and hence its suitability for a wide range of portable applications.

Characteristics of an Underwater Plasma Source with a Porous Glass Membrane

Advanced oxidation process (AOP) with hydroxyl radicals (OH) is considered to be useful for water purification through oxidation; therefore, we proposed an underwater plasma source with a porous glass membrane which functioned in bubble supply and the formation of micro-discharges. We observed bubble formation and light emission from discharge on the surface of the membrane using a high-speed camera and a spectroscope, respectively. Additionally, dependence of gas pressure on generation of OH radical and decomposition of recalcitrant organic matter were examined. The obtained results showed that AOP might be more effective when the operating gas pressure was set higher than a pressure which could form bubbles.

Pulsed Discharge Regeneration of Diesel Particulate Filters

A novel method for the removal of soot from a diesel particulate filter using pulsed electric discharges is presented. High voltage pulses of between 18 and 25 kV of nano to microsecond duration and with pulse energies of typically 100–200 mJ were applied to the filter via a series spark gap. Initial slow erosion of the soot layer proceeds via the formation of microdischarges. Subsequent spark discharges removed the accumulated soot more effectively from a larger filter volume. Average soot removal rates of ∼0.1–0.2 g/min were achieved at 50 Hz breakdown frequency by optimizing both electrode geometry and breakdown voltage. On-engine long term testing of the technology showed soot removal by pulsed discharge to be reliable, efficient and uniform; a total of 100 g of soot was deposited and removed over 18 filter regeneration cycles.

Phenomena of Microdischarges in Microplasma

Microscope photos of the microdischarges in dielectric barrier discharge microplasma were taken for the discharge gaps ranging from 30 to 100 μm. The microdischarge formation phenomena in all the stages were observed for microplasma generated in N2/Ar mixture by a pulse power supply. A diffused glowlike microplasma was observed with the increase in Ar concentration in N2/Ar mixture.

Pressure and Discharge-Voltage Dependence of Self-Sustaining Pulses in Air-Glow Discharge

The details of formation of micro discharges in air-glow discharge plasma are experimentally studied. The number of micro discharges formed per second is strongly related to both pressure and discharge voltage. This number tends to show reflections of the Patchen curve as far as its pressure dependence is concerned. The discharge-voltage dependence indicates that the transition from normal to abnormal glow discharge is not a sudden one, but has its roots during the normal glow stage and is initiated by the micro discharges which can be regarded as the early stage of abnormal glow.

Role of the volume and surface breakdown in a formation of microdischarges in a steady-state DBD

The results of an experimental study on a spatial-time behavior of microdischarges (MDs) in steady-state dielectric barrier discharge (DBD) are presented. MDs of DBD have a spatial “memory”, i.e. every subsequent MD appears exactly at the same place that was occupied by the preceding MD. In most cases each MD appears at its fixed place only once by every half-period (HP). Spatial “memory” is derived from slow recombination of plasma in the MDs channels for a period between two neighbor HPs. In steady-state DBD each plasma column was formed only one-time due to local avalanche-streamer breakdown in the very first (initial) gas gap breakdown under inception voltage $U^*$ . After that DBD is sustained under voltage lower than $U^*$ . For the plane-to-plane DBD having the restricted electrode area there is a critical voltage U 1: DBD is in a steady-state if U > U 1 but the DBD decays slowly at voltages below U 1. The decay takes many HPs and occurs due to decreasing the number of MDs inside the gap because of their Brownian motion from central region to the outside of the discharge area. In steady-state DBD there is no correlation between an appearance of alone MD and phase of the applied voltage – each MD has a great scatter in its appearance at the HP. This scatter is attributed to the dispersion in a threshold voltage for local surface breakdowns around the MD base. So, in steady-state DBD the MD volume plasma is responsible for an existence of spatial “memory” (i.e. where the MD appears) but the surface charge distribution around MD is responsible for MD time dispersion (i.e. when the MD appears).

Optical emission spectroscopy studies of discharge mechanism and plasma characteristics during plasma electrolytic oxidation of magnesium in different electrolytes

The discharge mechanism of the plasma electrolytic oxidation (PEO) process in different electrolytes was investigated by examining the variation of the optical emission spectra (OES). The spectrum of active species existed in the bubble layer. The bubble layer was initially broken down, followed by the breakdown of the dielectric barrier layer. Breakdown is the initial stage of discharge. A micro-discharge formation model, which assumes that the discharge ignition in the bubble layer developed at the oxide/electrolyte interface, was proposed. The active plasma species that appeared in different electrolytes during the PEO process were also studied. The appearance order of the excited active plasma species depended on the energy that the orbit transition of the species needed, but was not related to the anion concentration in the electrolyte. The anions in the electrolyte, except the OH −, also had little influence on the composition of the active plasma species during the PEO process. The active plasma species were mainly composed of metal atoms, metal cations, and gases produced by water decomposition. The electron temperature of the excited hydrogen was between 6 × 10 3 and 3 × 10 4 K. The high temperature provided the possibility of ceramic film melting and sintering. The source and transition of the active plasma species were also studied. They were found to undergo dissociation, ionization, and excitation processes.

Numerical Simulation of NO Oxidation in Dielectric Barrier Discharge with Microdischarge Formation

In this work, we developed a simulation model for plasma-enhanced chemical reaction in dielectric barrier discharge (DBD) in consideration of filamentary microdischarge formation. The first stage is the calculation of single filamentary microdischarge developed in DBD with parallel plate geometry using the fluid model. The second stage is the calculation of chemical processes with a set of rate equations using the radical production yield obtained in the filamentary microdischarge calculation. This model was applied to the NO oxidation process, which is often important in NOx removal using gas discharges, in atmospheric pressure N2/O2/NO mixtures under various discharge conditions. The calculated NO oxidation property has a quantitatively good agreement with the experimental one under wide discharge conditions. Therefore, the method used in this work is valid for the prediction of chemical processes in DBD. We also discuss the effect of gas temperature and gas residence time on NO oxidation.

Influence of the Surface Conductivity on the Stability of a Glow Dielectric-Barrier Discharge

The aim of this paper is to illustrate how a noninsulating surface in contact with a glow dielectric-barrier discharge induces the formation of microdischarges. Short-exposure-time pictures synchronized with the discharge show that microdischarges only occur when the conducting surface is on the cathode side and their duration is on the order of 5 mus. This long duration is confirmed by surface scanning-electron-microscopy imaging.

A computational model of a barrier discharge in air at atmospheric pressure: the role of residual surface charges in microdischarge formation

A two-dimensional (axially symmetric) computational model of the microdischarge formation in a barrier discharge for short (1–2 mm) gaps in air at atmospheric pressure is proposed. A non-homogeneous electric field, caused by a residual non-uniform charging of the dielectric barriers, is considered as an important reason for the filament formation by a Townsend mechanism. The ion–electron emission from the dielectric covering the cathode is treated as the principal secondary process; the secondary emission coefficient in the model is selected to be consistent with the Paschen voltage. For an applied sinusoidal voltage, this follows the microdischarge development on a microsecond scale. It is shown that even a slight inhomogeneity of the initial electric field leads to the formation of a narrow microdischarge channel.The two-dimensional dynamics of the radiation from a microdischarge for the case of the second positive and the first negative systems of nitrogen is simulated and compared with recent experimental data. The effects of the secondary emission coefficient and of a distribution of residual surface charges are investigated. It is shown that the level of inhomogeneity of the residual surface charge distribution does not affect the radius of the microdischarge channel, but affects its two-dimensional structure.The proposed model explains satisfactorily the experimental results for the velocity of the cathode-directed ionizing wave and the emission of the N2 second positive system from a microdischarge.

Cross‐Correlation Spectroscopy Applied to the Investigation of Barrier Discharges in N2/O2 Mixtures at Atmospheric Pressure

AbstractThe techniques of spatially resolved cross‐correlation spectroscopy (CCS) and current pulse oscillography were used to carry out systematic investigations of the barrier discharge (BD) in the binary gas mixtures N2/O2 at atmospheric pressure. In the case of the BD filamentary mode, which is characterized by the formation of microdischarges (MDs) of short duration, the spatio‐temporal distributions of the BD radiation intensities were recorded for the spectral bands of the 0‐0 transitions of the 2nd positive (λ = 337 nm) and 1st negative system of molecular nitrogen (λ = 391 nm). The velocities of the cathode‐directed ionising waves as well as the effective lifetimes of the excited states N 2(C 3Πu )υ ′=0 and N +2 (B 2Σ+u )υ ′=0 were evaluated from the CCS data. The two‐dimensional optical scanning of the MD channel (in axial and radial directions of the MD) was carried out for the BD operated in the gas mixture consisting of 6 vol.% of O2 and 94 vol.% of N2. In the middle of gap, the MD channel diameter was found to be about 0.3 mm and to expand towards both electrodes. On the dielectrics, outward propagating discharges were observed. In pure nitrogen the diffuse mode of barrier discharge was investigated. Surprisingly, under the conditions studied, the transition to the filamentary mode already starts for O2 admixtures to nitrogen of ≥400 ppm. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Generation of microdischarges in porous materials

Formation of microdischarges in porous dielectric materials generated by de high voltage has been investigated. Two geometrical configurations (sandwich type and corona type) and several materials with different pore size and thickness have been tested. The I-V characteristics and current pulse characteristics have been recorded and supplemented with the photographs of the discharge. Stable microdischarges have been successfully generated only using materials with a certain pore size and in the limited range of the I-V characteristics.

Discharge characterization in plasma electrolytic oxidation of aluminium

Digital video imaging of the plasma electrolytic oxidation (PEO) of aluminium has been performed, which allowed evaluation of both dimensional characteristics of individual microdischarges appearing at the oxide–electrolyte interface and their collective behaviour throughout the oxidation process. It has been shown that the microdischarge cross-sectional dimensions vary within the range 0.01–1.35 mm2. In the course of PEO processing, small localized events (<0.03 mm3) always dominate in the microdischarge spatial distribution and the relative proportion of medium-sized to very large microdischarges is gradually redistributed in favour of the latter. Temporal dependences have been found for the fraction of surface area instantaneously experiencing the discharge, as well as for the spatial and current densities of the microdischarge. Discharge mechanisms occurring during PEO are discussed and a model of microdischarge formation is suggested, assuming the possibility of free-electron generation and glow discharge ignition in the gaseous media developed at the oxide–electrolyte interface. First approximation evaluations of thermal processes in the oxide layer under the discharge conditions have been considered. The estimated ranges of the microdischarge current density (50–18 kA m−2) and duration (0.25–3.5 ms) sufficient for initiating phase transitions (e.g. γ–α transformation and melting) in the surface oxide layer are shown to be in good agreement with experimental data.

Modelling of dielectric barrier discharge chemistry

Abstract