Methane Conversion to C2 Hydrocarbons by Abnormal Glow Discharge at Atmospheric Pressure

Methane conversion to C2 hydrocarbons has been investigated with the addition of hydrogen in a plasma reactor of abnormal glow discharge at atmospheric pressure. The aim of this experiment is to minimize coke formation and improve discharge stability. The typical conditions in the experiment are 300 ml of total feed flux and 400 W of discharge power. The experimental results show that methane conversion is from 91.6% to 35.2% in mol, acetylene selectivity is from 90.2% to 57.6%, and ethylene selectivity is approximately from 7.8% to 3.6%, where the coke increases gradually along with the increase of CH4/H2 from 2 ： 8 to 9 ： 1. A stable discharge for a considerable running time can be obtained only at a lower ratio of CH4/H2 = 2：8 or 3： 7. These phenomena indicate that the coke deposition during methane conversion is obviously reduced by adding a large amount of hydrogen during an abnormal glow discharge. A qualitative interpretation is presented, namely, with abundant hydrogen, the possibility that hydrogen molecules are activated to hydrogen radicals is increased with the help of the abnormal glow discharge. These hydrogen radicals react with carbon radicals to form C2 hydrocarbon products. Therefore, the deposition of coke is restrained.     

Glow Discharge Induced Hydroxyl Radical Degradation of 2-Naphthylamine

In an aqueous solution, normal electrolysis at high voltages switches over spontaneously to glow discharge electrolysis and gives rise to hydroxyl radical, hydrogen peroxide, and aqueous electron, as well as several other active species. Hydroxyl radical directly attacks organic contaminants to make them oxidized. In the present paper, 2-naphthylamine is eventually degraded into hydrogen carbonate and carbon dioxide. The degradation process is analyzed by using an Ultraviolet (UV) absorption spectrum, high-performance liquid chromatography (HPLC) and chemical oxygen demand (COD). It is demonstrated that 2-naphthylamine (co =30 mg·1-1) is completely converted within 2h at 30℃and 600 V by glow discharge electrolysis, and the degradation is strongly dependent upon the presence of ferrous ions. COD is ascended in the absence of ferrous ions and descended in the presence of them.     

Three modes of a direct-current plasma jet operated underwater to degrade methylene blue

A direct-current air plasma jet operated underwater presents three stable modes including an intermittently-pulsed discharge, a periodically-pulsed discharge and a continuous discharge with increasing the power voltage. The three discharge modes have different appearances for the plasma plumes. Moreover, gap voltage-current characteristics indicate that the continuous discharge is in a normal glow regime. Spectral lines from reactive species(OH, N_2, N_2~+, H_α,and O) have been revealed in the emission spectrum of the plasma jet operated underwater.Spectral intensities emitted from OH radical and oxygen atom increase with increasing the power voltage or the gas flow rate, indicating that reactive species are abundant. These reactive species cause the degradation of the methylene blue dye in solution. Effects of the experimental parameters such as the power voltage, the gas flow rate and the treatment time are investigated on the degradation efficiency. Results indicate that the degradation efficiency increases with increasing the power voltage, the gas flow rate or the treatment time. Compared with degradation in the intermittently-pulsed mode or the periodically-pulsed one, it is more efficient in the continuous mode, reaching 98% after 21 min treatment.     

Review on reactive species in water treatment using electrical discharge plasma: formation,measurement, mechanisms and mass transfer

In the electrical discharge plasma process, various chemical and physical processes can participate in the removal of contaminants. In this paper, the chemical and physical processes that occur as a result of the electrical discharge plasma are reviewed, and their possible roles in the degradation of contaminants are discussed. Measurement methods for the quantification of important reactive species and their advantages and shortcomings are presented. Approaches on how to enhance the diffusion of the reactive species in solution are examined. In addition, the formation of typical reactive species in different electrical discharge plasma is compared.     

The mechanism study of low-pressure air plasma cleaning on large-aperture optical surface unraveled by experiment and reactive molecular dynamics simulation

Low-pressure air plasma cleaning is an effective method for removing organic contaminants on large-aperture optical components in situ in the inertial confinement fusion facility. Chemical reactions play a significant role in plasma cleaning, which is a complex process involving abundant bond cleavage and species generation. In this work, experiments and reactive molecular dynamics simulations were carried out to unravel the reaction mechanism between the benchmark organic contaminants of dibutyl phthalate and air plasma. The optical emission spectroscopy was used to study the overall evolution behaviors of excited molecular species and radical signals from air plasma as a reference to simulations. Detailed reaction pathways were revealed and characterized, and specific intermediate radicals and products were analyzed during experiments and simulation. The reactive species in the air plasma, such as O, HO₂ and O₃ radicals, played a crucial role in cleaving organic molecular structures. Together, our findings provide an atomic-level understanding of complex reaction processes of low-pressure air plasma cleaning mechanisms and are essential for its application in industrial plasma cleaning.     

Oxygen glow discharge cleaning in nuclear fusion devices

Laboratory experiments were carried out as part of the preparations of an oxygen glow discharge cleaning experiment in ASDEX Upgrade. They aimed at evaluating the effect of mixing the oxygen with helium, the collateral damage caused by the glow discharge, as well as the influence of impurities in the films being eroded. Oxygen concentrations below 20% in helium are sufficient to achieve high erosion rates. The discharge can lead to the formation of oxide layers on surfaces which - as demonstrated for tungsten - can be rapidly reduced by post-treatment in a hydrogen discharge. For carbon, aluminum and iron the physical sputtering yield may become similar to the erosion yield of redeposited layers, but it is by more than one order of magnitude smaller for tungsten. Using a-B:C:H films with varying boron content, it was found that impurities can cause the erosion rate to drop by orders of magnitude.     

Activation of peroxydisulfate by gas-liquid pulsed discharge plasma to enhance the degradation of p-nitrophenol

Pulsed discharge in water and over water surfaces generates ultraviolet radiation,local high temperature,shock waves,and chemical reactive species,including hydroxyl radicals,hydrogen peroxide,and ozone.Pulsed discharge plasma (PDP) can oxidize and mineralize pollutants very efficiently,but high energy consumption restricts its application for industrial wastewater treatment.A novel method for improving the energy efficiency of wastewater treatment by PDP was proposed,in which peroxydisulfate (PDS) was added to wastewater and PDS was activated by PDP to produce more strong oxidizing radicals,including sulfate radicals and hydroxyl radicals,leading to a higher oxidation capacity for the PDP system.The experimental results show that the increase in solution conductivity slightly decreased the discharge power of the pulse discharge over the water surface.An increase in the discharge intensity improved the activation of PDS and therefore the degradation efficiency and energy efficiency of p-nitrophenol (PNP).An increase in the addition dosage of PDS greatly facilitated the degradation of PNP at a molar ratio of PDS to PNP of lower than 80∶1,but the performance enhancement was no longer obvious at a dosage of more than 80∶1.Under an applied voltage of 20 kV and a gas discharge gap of 2 rmm,the degradation efficiency and energy efficiency of the PNP reached 90.7％ and 45.0 mg kWh-1 for the plasma/PDS system,respectively,which was 34％ and 18.0 mg kWh-1 higher than for the discharge plasma treatment alone.Analysis of the physical and chemical effects indicated that ozone and hydrogen peroxide were important for PNP degradation and UV irradiation and heat from the discharge plasma might be the main physical effects for the activation of PDS.     

Fuel hydrogen retention of tungsten and the reduction by inert gas glow discharges

Polycrystalline tungsten was exposed to deuterium glow discharge followed by He, Ne or Ar glow discharge. The amount of retained deuterium in the tungsten was measured using residual gas analysis. The amount of desorbed deuterium during the inert gas glow discharge was also measured. The amount of retained deuterium was 2–3 times larger compared with a case of stainless steel. The ratios of desorbed amount of deuterium by He, Ne and Ar glow discharges were 4.6, 3.1 and 2.9%, respectively. These values were one order of magnitude smaller compared with the case of stainless steel. The inert gas glow discharge is not suitable to reduce the fuel hydrogen retention for tungsten walls. However, the wall baking with a temperature higher than 700 K is suitable to reduce the fuel hydrogen retention. It is also shown that the use of deuterium glow discharge is effective to reduce the in-vessel tritium inventory in fusion reactors through the hydrogen isotope exchange.     

Investigation of flame structure in plasma-assisted turbulent premixed methane-air flame

The mechanism of plasma-assisted combustion at increasing discharge voltage is investigated in detail at two distinctive system schemes (pretreatment of reactants and direct in situ discharge).OH-planar laser-induced fluorescence (PLIF) technique is used to diagnose the turbulent structure methane-air flame,and the experimental apparatus consists of dump burner,plasma-generating system,gas supply system and OH-PLIF system.Results have shown that the effect of pretreatment of reactants on flame can be categorized into three regimes:regime Ⅰ for voltage lower than 6.6 kV;regime Ⅱ for voltage between 6.6 and 11.1 kV;and regime Ⅲ for voltage between 11.1 and 12.5 kV.In regime Ⅰ,aerodynamic effect and slower oxidation of higher hydrocarbons generated around the inner electrode tip plays a dominate role,while in regime Ⅲ,the temperature rising effect will probably superimpose on the chemical effect and amplify it.For wire-cylinder dielectric barrier discharge reactor with spatially uneven electric field,the amount of radicals and hydrocarbons are decreased monotonically in radial direction which affects the flame shape.With regard to in situ plasma discharge in flames,the discharge pattern changes from streamer type to glow type.Compared with the case of reactants pretreatment,the flame propagates further in the upstream direction.In the discharge region,the OH intensity is highest for in situ plasma assisted combustion,indicating that the plasma energy is coupled into flame reaction zone.     

Experimental analysis on the nonlinear behavior of DC barrier discharge plasmas

Nonlinear behavior of glow discharge plasmas is experimentally investigated.The glow is generated between a barrier semiconductor electrode,Chromium doped namely Gallium Arsenide(Ga As:Cr),as a cathode and an Indium–Tin Oxide(ITO) coated glass electrode as an anode,in reverse bias.The planar nature of electrodes provides symmetry in spatial geometry.The discharge behaves oscillatory in the time domain,with single and sometimes multiperiodicities in plasma current and voltage characteristics.In this paper,harmonic frequency generation and transition to chaotic behavior is investigated.The observed current–voltage characteristics of the discharge are discussed in detail.     

Optimization Study of Pulsed DC Nitrogen-Hydrogen Plasma in the Presence of an Active Screen Cage

A glow discharge plasma nitriding reactor in the presence of an active screen cage is optimized in terms of current density,filling pressure and hydrogen concentrations using optical emission spectroscopy（OES）.The samples of AISI 304 are nitrided for different treatment times under optimum conditions.The treated samples were analyzed by X-ray diffraction（XRD） to explore the changes induced in the crystallographic structure.The XRD pattern confirmed the formation of iron and chromium nitrides arising from incorporation of nitrogen as an interstitial solid solution in the iron lattice.A Vickers microhardness tester was used to evaluate the surface hardness as a function of treatment time（h）.The results showed clear evidence of improved surface hardness and a substantial amount of decrease in the treatment time compared with the previous work.     

Experimental investigation on n-decane plasma cracking in an atmospheric-pressure argon environment

In order to solve the problem of the difficulty of igniting and steadily propagating a continuous rotating detonation engine when using liquid hydrocarbon fuel, an experiment was carried out using a dielectric barrier discharge excited by a nanosecond power supply to crack n-decane, the single alternative fuel to aviation kerosene, in a pre-heated argon environment. By changing the voltages and the discharge frequencies, the concentrations of different components as well as a number of different species were acquired. The generating mechanism of olefins and alkanes together with their competition mechanism were acquired. The influence of the voltage on isomer products was also analyzed. The results demonstrate that the bond energy distribution and the species generating condition are the main factors affecting the formation of the products. With the increasing of voltage and discharge frequency, small molecule olefins, large molecular olefins, large molecular alkanes, small molecular alkanes, and hydrogen were detected, and in turn, their concentrations were also increased except for ethylene; what is more, when the voltage was increased over 8.5 kV, the n-butene converted to trans-butene, and the n-pentene converted to isoamylene.     

Numerical simulation of atmospheric pulse-modulated radio-frequency glow discharge ignition characteristics assisted by a pulsed discharge

A one-dimensional self-consistent fluid numerical model was developed to study the ignition characteristics of a pulse-modulated(PM) radio-frequency(RF) glow discharge in atmospheric helium assisted by a sub-microsecond voltage excited pulsed discharge. The temporal evolution of discharge current density and electron density during PM RF discharge burst was investigated to demonstrate the discharge ignition characteristics with or without the pulsed discharge. Under the assistance of pulsed discharge, the electron density in RF discharge burst reaches the magnitude of 1.87 × 10~(17) m~(-3) within 10 RF cycles, accompanied by the formation of sheath structure. It proposes that the pulsed discharge plays an important role in the ignition of PM RF discharge burst. Furthermore, the dynamics of PM RF glow discharge are demonstrated by the spatiotemporal evolution of the electron density with and without pulsed discharge. The spatial profiles of electron density, electron energy and electric field at specific time instants are given to explain the assistive role of the pulsed discharge on PM RF discharge ignition.     

Novel Plasma Reactor with Rotary Helix Electrode Used in Coupling of CH4 at Atmospheric Pressure

At the ambient temperature and pressure a glow discharge plasma was used as a new approach for the coupling of methane with the newly-developed rotary multidentate helix electrode. In the presence of hydrogen, the effects of the input peak voltages and gas flow rates on methane conversion, C2 single pass yield and selectivity were investigated, and then the results were compared with those from the three-disc multidentate electrode. This demonstrated, on an experimental scale, that the rotary multidentate helix electrode was better than the multidentate three-disc electrode as there was little accumulation of coke, and the C2 yield per pass was 69.85% and C2 selectivity over 99.14% with 70.46% methane conversion at an input peak voltage of 2300 V and 60 ml/min gas flow rate.     

1d3v PIC/MCC simulation of dielectric barrier discharge dynamics in hydrogen sulfide

In this study, we computationally examined the dynamics of dielectric barrier discharge in hydrogen sulfide. The simulations were performed with a 1d3v particle-in-cell/Monte Carlo collision model in which a parallel-plate electrode geometry with dielectrics was used. Particle recombination process is represented in the model. The discharge mode was found to be initially Townsend discharge developing from the cathode to the anode, and at the peak of the current, a more stable glow discharge develops from the anode to the cathode. A higher applied voltage results in sufficient secondary electrons to trigger a second current peak, and then the current amplitude increases. As the frequency is increased, it leads to the advance of the phase and an increase in the amplitude of the current peak. A higher dielectric permittivity also makes the discharge occur earlier and more violently in the gap.     

Effect of Ne Glow Discharge on Ion Density Control in LHD

Neon glow discharge cleaning was firstly attempted in Large Helical Device (LHD)instead of He glow discharge to remove hydrogen neutrals and to control the ion density n_i. TheNe glow discharge continued for 8 hours overnight after a three-day experiment. At the secondnight H_α emission became weaker than the emission usually observed in the He glow discharge. Aclear reduction of the hydrogen influx was also observed in neutral beam injection (NBI) dischargeswith Ne puff, whereas the neon recycling was strongly enhanced with appearance of a flat densityprofile. As a result, the lowest density limit was further reduced down to 0.2×10～(13) cm～(-3). Theuse of Ar puff formed a peaked density profile with a high T_i of 7 keV.     

Excited Electron Temperature and Molecular Vibration Temperature During Transition from Filamentary Discharge to Glow Discharge in Surface Discharge in Air

A dielectric barrier surface discharge device was used to investigate the transition from a filamentary discharge to a glow discharge in air at different gas pressures. Discharge images and waveforms of the applied voltage and discharge current were recorded simultaneously, and it was found that the discharge could transit from filamentary to glow with the decrease in pressure. Optical emission spectra during the transition from a filamentary discharge to a glow one were recorded. Excited electron temperature can be determined from the ratios of the relative intensities of spectral lines while molecular vibration temperature can be measured by analysing spectral lines of the N2 second positive band system. The results show that both the excited temperature and molecular vibration temperature increase with the decrease in the gas pressure. Qualitative explanations are given.     

Various patterns in dielectric barrier glow discharges simulated by a dynamic model

In this paper, various patterns of dielectric barrier glow discharge simulated by a phenomenological dynamic model are reported. The model is constructed based on the basic dynamic process of dielectric barrier glow discharge and involves the voltage-transfer characteristic as well as the lateral inhibition effect. In simulations, different driving voltage profiles are applied to achieve one or two pulsed discharges in each half-period and the corresponding pattern evolution is investigated. The final stable patterns driven by a rectangular wave voltage organize simply as stationary striations or hexagonal lattices. The patterns driven by a multi-step wave appear to be much more complicated, with complementary striations, concentric rings and square superlattices observed. The evolutions of these patterns from the initial uniform state are described and it is found that the spreading of the inhibition effect plays a key role in these evolutions. The numerical simulations in the present work are in excellent accordance with previous experiments and fluid modeling. This dynamic model proves to be a convenient and promising approach to reproducing different pattern structures and pattern evolutions in dielectric barrier glow discharge systems.     

Degradation of Microcystin-LR by Gas-Liquid Interfacial Discharge Plasma

In this study, we report on the degradation of microcystin-LR （MC-LR） by gas- liquid interracial discharge plasma. The influences of operation parameters such as average input voltage, electrode distance and gas flow rate are investigated. Experimental results indicate that the input voltage and gas flow rate have positive influences on MC-LR degradation, while the electrode distance has a negative one. After 6 min discharge with 25 kV average input voltage and 60 L/h air aerati by discharge both in on, the degradation rate of MC-LR achieves 75.3%. distilled water and MC-LR solution are measured H202 and 03 generated Moreover, an emission spectroscopy is used as an indicator of the processes that take place on the gas-liquid boundary and inside plasma. Varied types of radicals （O, .OH, CO, 03, etc.） are proved to be present in the gas phase during gas-liquid interfacial discharge.     

Oxidative Degradation of 4-chlorophenol in Aqueous Induced by Plasma with Submersed Glow Discharge Electrolysis

The oxidative degradation of 4-chlorophenol （4-CP） in aqueous solution induced by plasma with submersed glow discharge has been investigated. The concentration of 4-CP and the reaction intermediates were determined by high performance liquid chromatography （HPLC）. Various influencing factors such as the initial pH, the concentration of 4-CP and the catalytic action of Fe^2＋ were examined. The results indicate that 4-CP is eventually degraded into inorganic ion, dioxide carbon and water. The attack of hydroxyl radicals on the benzene rings of 4-CP in the initial stage of oxidative reactions is presumed to be a key step. They also suggest that the reaction is of a pseudo-first order kinetic reaction and the proposed method is an efficient way for the 4-CP degradation,