Plasma Chemistry and Plasma Processing (v.38, #3)

The Production of Ketene and C5O2 from CO2, N2 and CH4 in a Non-thermal Plasma Catalysed by Earth-Abundant Elements: An In-Situ FTIR Study by P. A. Christensen; Abd Halim Bin Md Ali; Z. T. A. W. Mashhadani; M. A. Carroll; P. A. Martin (461-484).
In situ Fourier Transform Infra Red spectroscopy was employed to study the plasma glow region of a non-thermal plasma between two Macor dielectrics and fed with CO2, CH4 and N2. CO, HCN and formaldehyde were produced and rapidly attained steady-state conditions. In addition, the chain oxides C5O2 and ketene were observed (the first time such species have been seen in the dry reforming of methane) and a liquid film was produced comprising multiple components, one of which was acetamide. The data were interpreted in terms of catalysis by the Macor dielectric and a wholly novel, multizone model in which the liquid film plays a direct and important role. The data obtained in the plasma experiments were compared to an analogous, thermal experiment. Importantly, the results from this work could have major implications across the fields of catalysis, synthesis and origin-of-life chemistry.
Keywords: Non-thermal plasma; Ketene; CO2 ; Conversion; Catalysis; Infrared spectroscopy; Acetamide; C5O2

Nitrogen Fixation and NO Conversion using Dielectric Barrier Discharge Reactor: Identification and Evolution of Products by Xiaolong Tang; Jiangen Wang; Honghong Yi; Shunzheng Zhao; Fengyu Gao; Chao chu (485-501).
The recombination (synthesis and conversion) of nitric oxide was investigated using dielectric barrier discharge reactor at atmospheric pressure. In this work, products identification and its evolution of different gas components have been studied. In the NO/O2/N2 systems, nitric oxide (NO) can be removed via chemical oxidation and chemical reduction, and corresponding products are NO2 and N2, respectively. In the O2/N2 systems, N2O5 producing from the interaction of NO3 with NO2 was also observed. There is an optimum SED at which the highest NOx yield and best NO conversion efficiency will be achieved. In the H2O/O2/N2 systems, the formation of NO2, HNO2 and HNO3 were observed in both NF and NO conversion. The N2O molecule, as a byproduct of plasma chemical reaction, was observed in all the experiments when the H2O or O2 is presence in the simulated gas. The lowest energy cost of NO conversion is achieved at the SED of 1250 J/L.
Keywords: Interconversion; Non-thermal plasma; Specific energy density; HNO2 ; N2O

Methane decomposition in plasma reactor is a green process and can be considered as an economical route to produce COx-free hydrogen. The present study aimed to design and construct a plasma reactor with a unique feature of stable operation to provide an opportunity for direct decomposition of methane at almost ambient temperature. The reactor performance was evaluated in terms of hydrogen selectivity and methane conversion under various feed flow rate, plasma power, and electrode velocity. The main product was hydrogen with a small amount of C2 hydrocarbons where C2 refers to ethane, ethylene, and acetylene. In addition, the role of the degree of non-equilibrium state in plasma reactor performance was studied to provide a better understanding of the complex behavior of the cold plasma reactor. Better performance was observed through the rotation of high voltage electrode, compared to fixed electrode in terms of methane conversion attributed to the uniform dispersion of plasma power and effective distribution of active species.
Keywords: Methane; Decomposition; Hydrogen; Rotating electrodes; Plasma

The aim of this work is to study the role of argon during plasma reforming of methane and carbon dioxide in order to convert Biogas into liquid fuels. Mixtures of synthetic CH4 and CO2, representing typical biogas compositions, were processed in a surface dielectric barrier discharge reactor in the presence of argon, which is considered to improve the discharge conditions. Our measurements showed that at constant feed flow rate and constant applied power, increasing the argon percentage from 0 to 66% in the feed, leads to increase the electron density up to 60% and the electron mean energy up to 50%. In these conditions, the absolute conversions of CH4 and CO2 are improved respectively from 19 to 43% and from 11 to 25%, the H2/CO ratio enhances up to 0.9. However, despite these improvements, the addition of argon beyond 33% decreases the carbon balance by deposition of black carbon and soot, decreases the selectivity of liquid products and finally lowers the energy efficiency of CH4 + CO2 mixture conversion. Meanwhile the selectivity of 10 liquid fuels principally alcohols, ketones and light organic acids, obtained in a yield of 3 wt%, depends also on the flow rate of argon in the feed mixture.
Keywords: Biogas; Dry reforming; DBD; Liquid fuels; Argon

Characterization of an Air-Based Coaxial Dielectric Barrier Discharge Plasma Source for Biofilm Eradication by Juliana Soler-Arango; Graciela Brelles-Mariño; Antonio Rodero; Maria C. Garcia (535-556).
Air-based atmospheric-pressure cold plasmas are a source of charged particles, excited species, radicals, and UV rays, known to induce degradation of biomaterials. In this work we characterize an air-based Dielectric Barrier Discharge plasma source designed for biofilm eradication, and study plasmas generated under different conditions by Optical Emission Spectroscopy. The main excited species in air-based plasmas are N2 (C3Πu) molecules and the gas temperatures never exceed 335 K, decreasing as air amounts increase in the feeding gas. Excited oxygen atoms and OH species are only detected in discharges generated in argon-containing gases. The temperature of the effluent remains below 308 K. Air-based plasmas are useful for biofilm eradication as they produce high amounts of ozone at a low gas temperature.
Keywords: Atmospheric plasmas; Biofilm eradication; Cold plasmas; Gas temperature; Optical emission spectroscopy

DC Discharge Electronic Non-equilibrium Effects Investigations on a M = 2 Rarefied Supersonic Flow Over a Flat Plate by Sabrina Hamdoun; Bachir Liani; Amina Ait Oumeziane; Jean-Denis Parisse (557-571).
This work aims at studying the effects of a low-pressure Argon discharge (P = 0.5 Torr) on a supersonic Argon flow (M = 2) around a flat plate. The observed phenomena during high speed-flow control with a plasma discharge are exhaustively described. The present investigation is of great interest not only to aviation but also to numerous other areas like the wind power industry. The computations have been carried out using the DC discharge and the High Mach Number Flow Comsol Multiphysics modules. To simulate the DC discharge, chemical reactions near the cathode region along with their corresponding Townsend coefficients need to be defined. The latter are calculated using the Bolsig + computer code. The other reactions cross sections are imported from the LXCAT data base. The imported data are used to calculate the reactions rates. The plasma discharge effects on the rarefied supersonic flow are described using a 2D hydrodynamical model under the Drift–Diffusion approximation. The hydrodynamical model was validated by comparing its results for a supersonic air flow with experiments. The main results on an Argon supersonic flow coupled to an Argon discharge show an increase in the pitot pressure and the shock angle.
Keywords: Plasma actuators; Glow discharge; Supersonic flow; COMSOL multiphysics

Study on Elemental Mercury Oxidation by Non-thermal Plasma with Calcium Chloride Enhancement by Jun Zhang; Yufeng Duan; Weixin Zhao; Chun Zhu; Qiang Zhou; Weike Ding (573-586).
Converting of elemental mercury (Hg0) into oxidized mercury compounds is considered to be an effective method for prompting mercury removal of conventional flue gas treatment systems. The technology of non-thermal plasma (NTP) with CaCl2 treatment was utilized to oxidize Hg0 in this paper. The effects of input energy and different flue gas components like O2, NO and SO2 on Hg0 oxidation were investigated. The results showed that the Hg0 oxidation efficiency with CaCl2 treatment was increased obviously compare with single NTP treatment, and would be more than 90% when the input energy was 120 J/L. The O2 in simulated flue gas played an important role for mercury oxidation. The Hg0 oxidation efficiency and O3 outlet concentration were increased with the increase of O2 content. However, both NO and SO2 inhibited Hg0 oxidation due to their competitive consumption of O3 and O. Compare with single NTP technology, the effects of NO and SO2 on Hg0 oxidation under NTP with CaCl2 treatment were decreased obviously. The technology of Temperature-Programmed Decomposition and Desorption was utilized to analyze the mercury speciation adsorbed or deposited on AC and CaCl2 surface. The results concluded that the main Hg0 oxidation products by NTP with CaCl2 treatment were HgO, Hg2O and HgCl2, which indicated that the chlorine of CaCl2 surface was the main reason for enhancement of Hg0 oxidation.
Keywords: Non-thermal plasma; Calcium chloride; Elemental mercury; Mercury chloride

Behavior of Carbon-Containing Impurities in the Process of Plasma-Chemical Distillation of Sulfur by Leonid Mochalov; Roman Kornev; Alexander Logunov; Mikhail Kudryashov; Aleksandr Mashin; Andrey Vorotyntsev; Vladimir Vorotyntsev (587-598).
Sulfur was purified by plasma-chemical distillation at low pressure (0.1 Torr) under dynamic vacuum conditions. RF (40 MHz) non-equilibrium plasma discharge was used for initiation of chemical interactions. The carbon-containing impurities behavior was studied. The possible mechanism of their conversion has been discussed. The effectiveness of the suggested method has been compared with the ‘traditional’ distillation in terms of removal of heterophase impurities.
Keywords: Plasma-chemical distillation; Carbon impurities behavior; Optical emission spectroscopy; Sulfur purification

Loop Type of Inductively Coupled Thermal Plasmas System for Rapid Two-Dimensional Oxidation of Si Substrate Surface by Takumi Tsuchiya; Yasunori Tanaka; Y. Maruyama; A. Fujita; M. K. S. Tial; Y. Uesugi; T. Ishijima; T. Yukimoto; H. Kawaura (599-620).
This paper describes the use of loop-type inductively coupled thermal plasmas (loop-ICTP) for two-dimensional (2D) rapid oxidation processing. The unique and original loop-ICTP torch has been developed for large-area rapid materials processing. We applied the loop-ICTP to surface oxidation of a Si substrate as an example of materials processing. A part of the $$hbox {Ar/O}_2$$ Ar/O2 loop-ICTP is formed, lying linearly on the surface of the substrate. In addition, scanning the Si substrate enabled 2D oxidation for the whole Si substrate surface. The uniformity of the oxide layer thickness and the oxidation rate were estimated by measuring the thickness of the oxide layer fabricated on the Si substrate. As a result, controlling the pressure offered more uniform oxide layer thickness, and gas injection onto the linear plasma on the substrate improved the uniformity of the oxide thickness. It should be noted that only three minutes of exposure of $$hbox {Ar/O}_2$$ Ar/O2 loop-ICTP is sufficient to create an oxide layer with approximately 100 nm thickness for a 2-inch Si substrate surface.
Keywords: Thermal plasma; Inductively coupled plasma; Oxidation; Silicon; Surface modification

Formation of Copper–Nickel Alloy from Their Oxide Mixtures Through Reduction by Low-Temperature Hydrogen Plasma by Kali Charan Sabat; Raja Kishore Paramguru; Barada Kanta Mishra (621-635).
The paper presents experimental results of a new method of production of nanoparticles of copper–nickel alloys from the reduction of the mixture of cupric oxide (CuO) and nickel oxide (NiO) by low-temperature hydrogen plasma in a microwave assisted hydrogen plasma set-up. The microwave power and hydrogen flow-rate used for the current investigation are 750 W and 2.5 × 10−6 m3 s−1 respectively. The addition of NiO–CuO, in proportion to result in alloys of 90Cu:10Ni and 70Cu:30Ni, not only removed the induction period from the kinetic plot of CuO reduction but also, improved the reduction rate of CuO. The XRD analysis of the product exhibited a single-phase peak with a d-spacing lying between Cu and Ni, which satisfies the Vergard’s law, indicating the Cu–Ni alloy formation. The lattice parameter decreases from 3.6221 (90Cu:10Ni) to 3.595 Ǻ (70Cu:30Ni), due to the smaller atomic radius of Ni (0.1246 nm) than that of Cu (0.1278 nm). The crystallite size, calculated by applying Scherrer’s formula, in both cases is found to be 31.7 nm.
Keywords: Cu–Ni alloy; Low-temperature hydrogen plasma; Reduction of metal oxides; Vergard’s law; Nanoparticles

Measurement of Anode Arc Attachment Movement in DC Arc Plasma Torch at Atmospheric Pressure by P. Ondac; A. Maslani; M. Hrabovsky; J. Jenista (637-654).
Direct current (DC) arc plasma torches are widely used in various industrial applications. Studying processes in their anode area helps to extend their lifetime, and stabilize the plasma flow for plasma applications. This paper reports detailed observations of the fast movement (above 100 m/s) of the anode arc attachment in a hybrid water-argon DC arc plasma torch with an external anode. We measured a mean electrical conductivity of a plasma volume above the anode and found a relation between the attachment movement and the anode erosion. Further, we measured average attachment speed, the average period of the restrike process and the average distances travelled by the attachment under different experimental conditions such as different values of the arc electric current, argon flow rate and different anode configurations. For our measurements, we used a high-speed camera and a high-voltage probe. Our results are in agreement with a model of plasma generated by a hybrid plasma torch and with spectroscopy measurements. The results describe the movement of the anode arc attachment in detail and provide experimental data on average plasma electrical conductivity in hot anode areas. Both the measurements of the mean electrical conductivity and the procedure for quantitative comparisons of anode erosion can be used also in water plasma torches and theoretically also in gas plasma torches.
Keywords: DC arc plasma torch; Arc attachment movement; Anode area; Anode erosion; High-speed camera