Plasma-assisted ammonia synthesis in a packed-bed dielectric barrier discharge reactor: roles of dielectric constant and thermal conductivity of packing materials

In this article, plasma-assisted NH₃ synthesis directly from N₂ and H₂ over packing materials with different dielectric constants (BaTiO₃, TiO₂ and SiO₂) and thermal conductivities (BeO, AlN and Al₂O₃) at room temperature and atmospheric pressure is reported. The higher dielectric constant and thermal conductivity of packing material are found to be the key parameters in enhancing the NH₃ synthesis performance. The NH₃ concentration of 1344 ppm is achieved in the presence of BaTiO₃, which is 106% higher than that of SiO₂, at the specific input energy (SIE) of 5.4 kJ·l⁻¹. The presence of materials with higher dielectric constant, i.e. BaTiO3 and TiO₂ in this work, would contribute to the increase of electron energy and energy injected to plasma, which is conductive to the generation of chemically active species by electron-impact reactions. Therefore, the employment of packing materials with higher dielectric constant has proved to be beneficial for NH₃ synthesis. Compared to that of Al₂O₃, the presence of BeO and AlN yields 31.0% and 16.9% improvement in NH₃ concentration, respectively, at the SIE of 5.4 kJ·l⁻¹. The results of IR imaging show that the addition of BeO decreases the surface temperature of the packed region by 20.5% to 70.3°C and results in an extension of entropy increment compared to that of Al₂O₃, at the SIE of 5.4 kJ·l⁻¹. The results indicate that the presence of materials with higher thermal conductivity is beneficial for NH₃ synthesis, which has been confirmed by the lower surface temperature and higher entropy increment of the packed region. In addition, when SIE is higher than the optimal value, further increasing SIE would lead to the decrease of energy efficiency, which would be related to the exacerbation in reverse reaction of NH₃ formation reactions.     

Effects of nitrogen on ozone synthesis in packed-bed dielectric barrier discharge

The effects of nitrogen on ozone synthesis are studied in a coaxial cylinder generator with dielectric barrier discharge (DBD) and pack-bed dielectric barrier discharge (PB-DBD). A series of 10 h discharge experiments are conducted adopting a bare stainless electrode and bare copper electrode. Results show that the material of the electrode can affect the ozone synthesis. It is inferred that the ozone zero phenomenon (OZP) may be induced from ozone decomposing by metallic oxide catalysis. Packing dielectric particles can reduce the OZP. Adding a certain amount of nitrogen into the oxygen feed gas can further eliminate the OZP, and increase the ozone concentration significantly, but decreases the maximum energy efficiency of ozone generators. Initial analysis indicates that the optimal proportion of nitrogen addition is inversely related to the average reduced electric field strength in the discharge region.     

Experimental study on the parameter optimization and application of a packed-bed dielectric barrier discharge reactor in diesel particulate filter regeneration

To compensate for the shortcomings of the thermal and catalytic regeneration of the diesel particulate filter(DPF),a self-designed packed-bed dielectric barrier discharge(DBD)reactor for DPF regeneration was developed.The DBD reactor with the main active substance of nonthermal plasma(NTP)as the target parameter was optimized by adjusting the feed gas,packing particles(material or size),and cooling water temperature.Moreover,a set of optimal working parameters(gas source,O2;packing particles,1.2-1.4 mm ZrO2;and cooling water temperature,20℃)was selected to evaluate the effect of different O3 concentrations on DPF regeneration.The research results showed that selecting packing particles with high dielectric constant and large particles,as well as reducing the cooling water temperature,with oxygen as the feed gas,contributed to an increase in O3 concentration.During DPF regeneration,the following changes were observed:the power of the NTP reactor decreased to lower than 100 W,the O3 concentration increased from 15 g m-3 to 45 g m-3,the CO and CO2 volume fractions of the particulate matter decomposition products increased,and the peak regeneration temperature increased to 173.4℃.The peak temperature arrival time was 60 min earlier,indicating that the regeneration rate of DPF increased with the increase in O3 concentration.However,the O3 utilization rate(the amount of carbon deposit removed per unit volume O3)initially increased and then decreased;when the O3 concentration was set to 25 g m-3,the highest O3 utilization rate was reached.The packed-bed DBD technology contributed to the increase in the concentration of NTP active substances and the regeneration efficiency of DPF.It provides a theoretical and experimental basis for high-efficiency regeneration of DPF at low temperatures(＜200℃).     

Investigation on discharge characteristics of a coaxial dielectric barrier discharge reactor driven by AC and ns power sources

A coaxial dielectric barrier discharge(DBD) reactor with double layer dielectric barriers has been developed for exhaust gas treatment and excited either by AC power or nanosecond(ns)pulse to generate atmospheric pressure plasma. The comparative study on the discharge characteristics of the discharge uniformity, power deposition, energy efficiency, and operation temperature between AC and ns pulsed coaxial DBD is carried out in terms of optical and electrical characteristics and operation temperature for optimizing the coaxial DBD reactor performance. The voltages across the air gap and dielectric layer and the conduction and displacement currents are extracted from the applied voltages and measured currents of AC and ns pulsed coaxial DBDs for the calculation of the power depositions and energy efficiencies through an equivalent electrical model. The discharge uniformity and operating temperature of the coaxial DBD reactor are monitored and analyzed by optical images and infrared camera. A heat conduction model is used to calculate the temperature of the internal quartz tube. It is found that the ns pulsed coaxial DBD has a much higher instantaneous power deposition in plasma, a lower total power consumption, and a higher energy efficiency compared with that excited by AC power and is more homogeneous and stable. The temperature of the outside wall of the AC and ns pulse excited coaxial DBD reaches 158 ℃ and 64.3 ℃ after 900 s operation, respectively.The experimental results on the comparison of the discharge characteristics of coaxial DBDs excited by different powers are significant for understanding of the mechanism of DBDs,reducing energy loss, and optimizing the performance of coaxial DBD in industrial applications.     

Electrical model and experimental analysis of a double spiral structure surface dielectric barrier discharge

A surface dielectric barrier discharge (SDBD) can discharge at atmospheric pressure and produce a large area of low-temperature plasma. An SDBD plasma reactor based on the double spiral structure is introduced in this paper. To study the discharge mechanism of SDBD, an equivalent circuit model was proposed based on the analysis of the micro-discharge process of SDBD. Matlab/Simulink is used to simulate and compare the voltage–current waves, Lissajous and discharge power with the experimental results. The consistency of the results verifies the validity of the SDBD equivalent circuit model. Maxwell software based on the finite elements method is used to analyze the electrostatic field distribution of the device, which can better explain the relationship between the discharge image and the electrostatic field distribution. The combination of equivalent circuit simulation and electrostatic field simulation can provide better guidance for optimizing a plasma generator. Finally, the device is used to treat PM2.5 and formaldehyde. The test results show that the degradation rate of PM2.5 can reach 78% after 24 min, and formaldehyde is about 31.5% after 10 min of plasma treatment.     

Methane conversion using a dielectric barrier discharge reactor at atmospheric pressure for hydrogen production

In the present paper,we carried out a theoretical study of dielectric barrier discharge (DBD) filled with pure methane gas.The homogeneous discharge model used in this work includes a plasma chemistry unit,an electrical circuit,and the Boltzmann equation.The model was applied to the case of a sinusoidal voltage at a period frequency of 50 kHz and under a gas pressure of 600 Torr.We investigated the temporal variation of electrical and kinetic discharge parameters such as plasma and dielectric voltages,the discharge current density,electric field,deposited power density,and the species concentration.We also checked the physical model validity by comparing its results with experimental work.According to the results discussed herein,the dielectric capacitance is the parameter that has the greatest effect on the methane conversion and H2/CH4 ratio.This work enriches the knowledge for the improvement of DBD for CH4 conversion and hydrogen production.     

Generation of reactive species in atmospheric pressure dielectric barrier discharge with liquid water

Atmospheric pressure helium/water dielectric barrier discharge(DBD) plasma is used to investigate the generation of reactive species in a gas–liquid interface and in a liquid. The emission intensity of the reactive species is measured by optical emission spectroscopy(OES)with different discharge powers at the gas–liquid interface. Spectrophotometry is used to analyze the reactive species induced by the plasma in the liquid. The concentration of OH radicals reaches 2.2 μm after 3 min of discharge treatment. In addition, the concentration of primary longlived reactive species such as H_2O_2, NO_3~- and O_3 are measured based on plasma treatment time.After 5 min of discharge treatment, the concentration of H_2O_2, NO_3~-, and O_3 increased from 0 mg?·?L~(-1) to 96 mg?·?L~(-1), 19.5 mg?·?L~(-1), and 3.5 mg?·?L~(-1), respectively. The water treated by plasma still contained a considerable concentration of reactive species after 6 h of storage. The results will contribute to optimizing the DBD plasma system for biological decontamination.     

Influence of non-uniform electric field distribution on the atmospheric pressure air dielectric barrier discharge

The dielectric barrier discharge(DBD) in air at atmospheric pressure is not suitable for industrial applications due to its randomly distributed discharge filaments. In this paper, the influence of the electric field distribution on the uniformity of DBD is theoretically analyzed and experimentally verified. It is found that a certain degree of uneven electric field distributions can control the development of electron avalanches and regulate their transition to streamers in the gap. The discharge phenomena and electrical characteristics prove that an enhanced Townsend discharge can be formed in atmospheric-pressure air with a curved-plate electrode. The spectral analysis further confirms that the gas temperature of the plasma produced by the curved-plate electrode is close to room temperature, which is beneficial for industrial applications. This paper presents the relationship between the electron avalanche transition and the formation of a uniform DBD, which can provide some references for the development and applications of the DBD in the future.     

A novel double dielectric barrier discharge reactor with high field emission and secondary electron emission for toluene abatement

Dielectric barrier discharge (DBD) has been extensively investigated in the fields of environment and energy, whereas its practical implementation is still limited due to its unsatisfactory energy efficiency. In order to improve the energy efficiency of DBD, a novel double dielectric barrier discharge (NDDBD) reactor with high field emission and secondary electron emission was developed and compared with traditional DDBD (TDDBD) configuration. Firstly, the discharge characteristics of the two DDBD reactors were analyzed. Compared to TDDBD, the NDDBD reactor exhibited much stronger discharge intensity, higher transferred charge, dissipated power and gas temperature due to the effective utilization of cathode field emission and secondary electron emission. Subsequently, toluene abatement performance of the two reactors was evaluated. The toluene decomposition efficiency and mineralization rate of NDDBD were much higher than that of TDDBD, which were 86.44%–100% versus 28.17%–80.48% and 17.16%–43.42% versus 7.17%–16.44% at 2.17–15.12 W and 1.24–4.90 W respectively. NDDBD also exhibited higher energy yield than TDDBD, whereas the overall energy constant ${k}_{{\rm{overall}}}$ of the two reactors were similar. Finally, plausible toluene decomposition pathway in TDDBD and NDDBD was suggested based on organic intermediates that generated from toluene degradation. The finding of this study is expected to provide reference for the design and optimization of DBD reactor for volatile organic compounds control and other applications.     

Experimental and simulated investigation of microdischarge characteristics in a pin-to-pin dielectric barrier discharge (DBD) reactor

Both experimental and simulated studies of microdischarge (MD) are carried out in a dielectric barrier discharge with a pin-to-pin gap of 3.5 mm, ignited by a sinusoidal voltage with a peak voltage of 10 kV and a driving frequency of 5 kHz. Statistical results have shown that the probability of the single current pulse in the positive half-period (HP) reaches 73.6% under these conditions. Experimental results show that great luminous intensity is concentrated on the dielectric surface and the tip of the metal electrode. A 1D plasma fluid model is implemented by coupling the species continuity equations, electron energy density equations, Poisson equation, and Helmholtz equations to analyze the MD dynamics on the microscale. The simulated results are in good qualitative agreement with the experimental results. The simulated results show that the MD dynamics can be divided into three phases: the Townsend phase, the streamer propagation phase, and the discharge decay phase. During the streamer propagation phase, the electric field and electron density increase with the streamer propagation from the anode to the cathode, and their maximal values reach 625.48 Td and 2.31 × 1019 m−3, as well as 790.13 Td and 3.58 × 1019 m−3 in the positive and negative HP, respectively. Furthermore, a transient glow-like discharge is detected around the anode during the same period of streamer propagation. The formation of transient glow-like discharge is attributed to electrons drifting back to the anode, which is driven by the residual voltage in the air gap.     

Investigation of nanosecond-pulsed dielectric barrier discharge actuators with powered electrodes of different exposures

Nanosecond-pulsed dielectric barrier discharge actuators with powered electrodes of different exposures were investigated numerically by using a newly proposed plasma kinetic model. The governing equations include the coupled continuity plasma discharge equation, drift-diffusion equation, electron energy equation, Poisson's equation, and the Navier–Stokes equations.Powered electrodes of three different exposures were simulated to understand the effect of surface exposure on plasma discharge and surrounding flow field. Our study showed that the fully exposed powered electrode resulted in earlier reduced electric field breakdown and more intensive discharge characteristics than partially exposed and rounded-exposed ones. Our study also showed that the reduced electric field and heat release concentrated near the right upper tip of the powered electrode. The fully exposed electrode also led to stronger shock wave, higher heating temperature, and larger heated area.     

Surface treatment of titanium dioxide nanopowder using rotary electrode dielectric barrier discharge reactor

Titanium dioxide (TiO2) nanopowder (P-25;Degussa AG) was treated using dielectric barrier discharge (DBD) in a rotary electrode DBD (RE-DBD) reactor.Its electrical and optical characteristics were investigated during RE-DBD generation.The treated TiO2 nanopowder properties and structures were analyzed using x-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR).After RE-DBD treatment,XRD measurements indicated that the anatase peak theta positions shifted from 25.3° to 25.1°,which can be attributed to the substitution of new functional groups in the TiO2 lattice.The FTIR results show that hydroxyl groups (OH) at 3400 cm-1 increased considerably.The mechanism used to modify the TiO2 nanopowder surface by air DBD treatment was confirmed from optical emission spectrum measurements.Reactive species,such as OH radical,ozone and atomic oxygen can play key roles in hydroxyl formation on the TiO2 nanopowder surface.     

Enhanced removal of ultrafine particles from kerosene combustion using a dielectric barrier discharge reactor packed with porous alumina balls

Ultrafine particles(UFPs) are harmful to human beings, and their effective removal from the environment is an urgent necessity. In this study, a dielectric barrier discharge(DBD) reactor packed with porous alumina(PA) balls driven by a pulse power supply was developed to remove the UFPs(ranging from 20 to 100 nm) from the exhaust gases of kerosene combustion. Five types of DBD reactors were established to evaluate the effect of plasma catalysis on the removal efficiency of UFPs. The influences of gas flow rate, peak voltage and pulse frequency of different reactors on UFPs removal were investigated. It was found that a high total UFP removal of91.4% can be achieved in the DBD reactor entirely packed with PA balls. The results can be attributed to the enhanced charge effect of the UFPs with PA balls in the discharge space. The UFP removals by diffusion deposition and electrostatic attraction were further calculated,indicating that particle charging is vital to achieve high removal efficiency for UFPs.     

Simulated and experimental studies on the array dielectric barrier discharge of water electrodes

A kind of dielectric barrier discharge(DBD) device composed of water electrodes with 3×3forms can produce large-area low-temperature plasmas at atmospheric pressure.To reflect the discharge characteristics of DBD better,a dynamic simulation model,which is based on the voltage controlled current source(CCS),is established,then the established model in Matlab/Simulink is used to simulate the DBD in air.The voltage-current waves and Lissajous at a voltage of 10 kV,11 kV and 12 kV peak value with a frequency of 15 kHz are studied.The change of the discharge power of DBD with a different amplitude and frequency of applied voltage is also analyzed.The result shows the voltage-current waves,Lissajous and discharge power of DBD under different conditions from the simulation agree well with those of the experiment.In addition,we propose a method to calculate the dielectric barrier capacitance C_d and the gap capacitance C_g,which is valid through analyzing the variation of capacitance at different voltage amplitudes.     

Hollow hexagonal pattern with surface discharges in a dielectric barrier discharge

The hollow hexagonal pattern involved in surface discharges is firstly investigated in a?dielectric barrier discharge system. The spatiotemporal structures of the pattern are studied using an intensified charge-coupled device and photomultiplier. Instantaneous images taken by an intensified charge-coupled device and optical correlation measurements show that the surface discharges are induced by volume discharges. The optical signals indicate that the discharge filaments constituting the hexagonal frame discharge randomly at the first current pulse or the second pulse, once?or twice. There is no?interleaving of several sub-lattices, which indicates that the ‘memory' effect is no longer in force due to surface discharges. By using the emission spectrum method, both the molecule vibration temperature?and electron density of the surface discharges are larger than that of the volume discharges.     

Formation of honeycomb-Kagome hexagonal superlattice pattern with dark discharges in dielectric barrier discharge

A honeycomb-Kagome hexagonal superlattice pattern with dark discharges is observed in a dielectric barrier discharge system for the first time. The spatiotemporal structure of the honeycomb-Kagome hexagonal superlattice pattern with dark discharges is investigated by an intensified charge-coupled device and the photomultipliers show that it is an interleaving of three different sub-lattices, which are bright-spot, invisible honeycomb lattice, and Kagome lattice with invisible frameworks and dim-spots, respectively. The invisible honeycomb lattices and Kagome lattices are actually composed of dark discharges. By using the optical emission spectra method, it is found that the plasma parameters of the three different sub-lattices are different. The influence of the dark discharges on pattern formation is discussed. The results may have significance for the investigation of the dark discharges and will accelerate the development of self-organized pattern dynamics.     

Influences of frequency on nitrogen fixation of dielectric barrier discharge in air

The influences of frequency on nitrogen fixation of dielectric barrier discharge in air were studied by electrical diagnostics, gas detection and infrared detection methods. The system power, nitrogen oxide concentration, voltage–current waveform, dielectric surface temperature distribution and filamentous discharge pictures were measured, and then the energy yield was calculated; paper studied their changing tendencies in the presence of frequency. Results show that frequency has strong influences on nitrogen fixation. When the parameters of reaction chamber and amplitude of applied voltage is fixed, with the increasing of frequency, the system power increases; in 5-10 kHz, nitrogen oxide gas concentration up to 1113.7 mg m~(-3), and 7 k Hz is the optimal nitrogen fixation frequency whose energy yield is 20.5 mg(m~3 W)~(-1).     

Development of a battery-operated floating-electrode dielectric barrier discharge plasma device and its characteristics

In this work, a portable floating-electrode dielectric barrier discharge(FE-DBD) device is designed with a rechargeable battery as the power supply. The characteristics of the FE-DBD with a metal electrode and human hand are studied and compared. The human contact safety is verified by calculating the current through the human body based on the equivalent circuit model. Escherichia coli inactivation experiments confirm the efficacy of the FE-DBD device in the envisaged applications.     

High-efficiency removal of NO_x using dielectric barrier discharge nonthermal plasma with water as an outer electrode

With the rapid increase in the number of cars and the development of industry, nitrogen oxide(NO_x)emissions have become a serious and pressing problem. This work reports on the development of a water-cooled dielectric barrier discharge reactor for gaseous NOxremoval at low temperature. The characteristics of the reactor are evaluated with and without packing of the reaction tube with 2 mm diameter dielectric beads composed of glass, ZnO, MnO_2, ZrO_2, or Fe_2O_3. It is found that the use of a water-cooled tube reduces the temperature, which stabilizes the reaction, and provides a much greater NO conversion efficiency(28.8%) than that obtained using quartz tube(14.1%) at a frequency of 8 k Hz with an input voltage of 6.8 k V. Furthermore,under equivalent conditions, packing the reactor tube with glass beads greatly increases the NO conversion efficiency to 95.85%. This is because the dielectric beads alter the distribution of the electric field due to the influence of polarization at the glass bead surfaces, which ultimately enhances the plasma discharge intensity. The presence of the dielectric beads increases the gas residence time within the reactor. Experimental verification and a theoretical basis are provided for the industrial application of the proposed plasma NO removal process employing dielectric bead packing.     

Measurement of the first Townsend's ionization coefficient for atmospheric pressure neon by a dielectric barrier discharge

Fast photography and optical emission spectroscopy are implemented in a 5 mm neon gap dielectric barrier discharge (DBD) at atmospheric pressure with quartz glass used as the dielectric layer. Results show that it starts with a Townsend discharge and ends at a sub-normal glow discharge in neon DBD. Based on the Townsend discharge, the first ionization coefficient of neon is measured. The measurements are consistent with those at low pressure. Optical emission spectroscopy indicates that the spectra are mainly composed of atomic lines of neon, molecular bands and molecular ion bands originating from inevitable gas impurities (mainly nitrogen). Moreover, spectral lines emitted from atomic neon corresponding to the transitions (2p⁵ 3p → 2p⁵ 3s) are predominant. Although the second positive system of N₂(C³Π_u → B³Π_g) is observed, their intensities are too weak compared with neon's spectrum. The molecular nitrogen ion line of 391.4 nm is observed. It reveals that Penning ionization between high energy neon excited states and the inevitable gas impurities plays an important role in the value of the α coefficient.