Coaxial Dielectric Barrier Discharge Reactor Research Articles

Physicochemical Characteristics and Antimicrobial Efficacy of Plasma-Activated Water Produced by an Air-Operated Coaxial Dielectric Barrier Discharge Plasma

In this study, Plasma-Activated Water (PAW) was synthesized using a coaxial Dielectric Barrier Discharge (DBD) reactor, benefiting from the elevated capacity of air-flow-assisted DBD discharges to enhance nitrogen-based species concentration. By manipulating operational parameters, including gas flow rate, activation time, and DI water volume, we achieved significant concentrations of reactive oxygen and nitrogen species (RONS). As a result, the PAW obtained displayed pronounced physicochemical attributes: a pH of 2.06, an ORP of 275 mV, conductivity of 3 mS/cm, and TDS of 1200 mg/L. A pivotal aspect of this research was the evaluation of the reactor’s efficiency, as indicated by metrics like the specific input energy and ozone efficiency yield. The antimicrobial potential of the PAW was also assessed against pathogenic microbes, with remarkable reductions in viability for both Staphylococcus aureus and Escherichia coli (99.99%) and a more moderate decrease for Candida albicans (37%). These findings underscore the capability of coaxial DBD reactors in crafting high-quality PAW with significant antimicrobial properties, necessitating further studies to validate its broad-spectrum and safe applications.

Sterilization of Water-Based Cutting Fluids Using Compact Air-Cooled Coaxial Dielectric Barrier Discharge Reactor with Bubbler

Odor and discoloration in water-based cutting fluids are caused by the growth of microorganisms and putrefying bacteria. This significantly reduces cutting performance, prevents rust, and deteriorates the working environment. To overcome these drawbacks, we developed a compact air-cooled coaxial dielectric barrier discharge (DBD) with a bubbler. Bacteria and microorganisms living in waste cutting fluids were sterilized by the high concentration of ozone produced under the optimized conditions of the compact air-cooled coaxial DBD. Moreover, it was confirmed that 99.99% of bacteria and microorganisms were completely removed. Ozone was found to not affect property changes such as the composition, concentration, and pH of the water-based cutting fluids. The chromaticity and complex odor of waste cutting liquids were thus found to have been improved by the effects of microorganism and bacterial sterilization. We conclude that the proposed a compact air-cooled coaxial DBD with a bubbler is an efficient method for sterilizing water-based cutting fluids.

Chain scission modification mode in plasma catalytic n-undecane decomposition: In situ probing of intermediates and reaction pathways

To provide insight into the mechanism of plasma-catalytic long-chain alkane oxidation from the aspect of chain scission, n-undecane (C11H24) decomposition over γ-Al2O3 and Ag/γ-Al2O3 were investigated in a coaxial dielectric barrier discharge (DBD) reactor. The highest n-undecane conversion (89.7 %) and CO2 selectivity (SCO2, 40.0 %) were achieved in Plasma + Ag/γ-Al2O3 system. It was found that the simultaneous activation of O3 and C11H24 played a very important role in chain scission, which could be promoted by the formation of Ag-O-Al entity introduced by the strong interaction between Ag and γ-Al2O3. Different chain scission behaviors and reaction pathways were proposed by a combined analysis of GC-MS and in-situ DRIFTS. In Plasma + Ag/γ-Al2O3 system, the reaction of chain scission and oxidation preferred to take place in the middle of the chain, producing the ketone and aldehyde species with low carbon number, which were easy to be converted to CO2 and H2O. Instead, without the highly efficient activation of O3 and C11H24, the oxidation of the C11H24 over the surface of γ-Al2O3 was more likely to take place on the edge of the chain, leading to the accumulation of long-chain aldehyde, which lower values of C11H24 conversion and SCO2 (45.6 % and 21.7 %, respectively) in Plasma + γ-Al2O3 system.

Non-Oxidative Ethane Dehydrogenation in a Packed-Bed DBD Plasma Reactor

Plasma-assisted conversion of ethane (C2H6) can produce value-added chemical building blocks using green electricity. Here we employ a simple packed-bed coaxial dielectric barrier discharge (DBD) reactor to convert C2H6 at mild operating conditions unattainable by conventional thermocatalysis. Ethylene (C2H4), acetylene (C2H2), and methane (CH4) are the main products along with small fractions of C3 and C4 hydrocarbons. Interestingly, the C2H4 selectivity is primarily correlated to C2H6 conversion, dominated by electron dissociation and recombination reactions irrespective of the dielectric properties of the packed bed material (SiO2, Al2O3, ZrO2, TiO2, and BaTiO3), packing material size, supplied power, and C2H6 concentration. While a distortion of the electric field and discharge propagation results in varying dissipated power as materials change, the C2H4 energy yield remains constant. The particle size appears to affect conversion mainly due to pressure alterations. Pd/SiO2 catalyst can change the selectivity, favoring saturated species by expending hydrogen.

Basic metal oxide integrated DBD packed bed reactor for the decomposition of CO2

A coaxial dielectric barrier discharge (DBD) reactor was used to perform catalytic non-thermal plasma for CO2 conversion under ambient conditions. A systematic study was made to understand the role of basic oxides in activating weakly acidic CO2. For this purpose, MO/γ-Al2O3 (M = Mg, Ca, Sr, and Ba) catalysts were synthesized and integrated with the DBD reactor. The applied voltage was varied from 16 kV to 22 kV to determine the effect of applied voltage on CO2 conversion. Plasma discharge generates high-energy electrons that activate the basic oxides. Integration of basic metal oxides with the non-thermal plasma reactor resulted in a higher CO2 conversion. The adsorption of weakly acidic CO2 on the basic sites is responsible for the higher conversion of catalytic plasma reactor over plasma reactor alone. Among the basic metal oxides studied, SrO loaded γ-Al2O3 resulted in the best conversion, where CO2 conversion of ∼ 12% and energy efficiency of ∼ 1.46 mmol kJ−1 was attained at a power of 1.8 W. The concentration of O2 and O3 was measured during the reaction. The hybridized system's superior performance may be due to increased charge deposition and altered gas-phase chemistry because of catalyst integration. BOLSIG + software was used to compute the mean electron energies and electron energy distribution function for various packing conditions.

Non-Thermal Plasma Incorporated with Cu-Mn/γ-Al2O3 for Mixed Benzene Series VOCs’ Degradation

In this work, a coaxial dielectric barrier discharge (DBD) reactor was constructed to degrade the mixture of toluene and o-xylene, two typical benzene series. The Cu-MnO2/γ-Al2O3 series catalysts prepared by redox and impregnation methods were filled into the plasma device to degrade VOCs synergistically and explore the degradation effect. The experimental results showed that the introduction of a Cu-doped MnO2 catalyst significantly improved the pollutants’ removal efficiency and CO2 selectivity, and greatly inhibited the formation of by-products. Among them, Cu0.15Mn/γ-Al2O3 showed the highest removal efficiency (toluene was 100% and o-xylene was 100%), and the best CO2 selectivity (92.73%). The XRD, BET, XPS and SEM results confirmed that the synergistic effect between Cu and Mn in the Cu-Mn solid solution could promote the amount and reducibility of the surface active oxygen species, which improved the catalytic performance. Finally, the toluene and o-xylene decomposition pathways in the NTP catalytic system were speculated according to the detected organic matter. This work provides a theoretical and experimental basis for the application of DBD-catalyzed hybrid benzene series VOCs.

Insights into the Role of Nanorod-Shaped MnO2 and CeO2 in a Plasma Catalysis System for Methanol Oxidation.

Published papers highlight the roles of the catalysts in plasma catalysis systems, and it is essential to provide deep insight into the mechanism of the reaction. In this work, a coaxial dielectric barrier discharge (DBD) reactor packed with γ-MnO2 and CeO2 with similar nanorod morphologies and particle sizes was used for methanol oxidation at atmospheric pressure and room temperature. The experimental results showed that both γ-MnO2 and CeO2 exhibited good performance in methanol conversion (up to 100%), but the CO2 selectivity of CeO2 (up to 59.3%) was much higher than that of γ-MnO2 (up to 28.6%). Catalyst characterization results indicated that CeO2 contained more surface-active oxygen species, adsorbed more methanol and utilized more plasma-induced active species than γ-MnO2. In addition, in situ Raman spectroscopy and Fourier transform infrared spectroscopy (FT-IR) were applied with a novel in situ cell to reveal the major factors affecting the catalytic performance in methanol oxidation. More reactive oxygen species (O22-, O2-) from ozone decomposition were produced on CeO2 compared with γ-MnO2, and less of the intermediate product formate accumulated on the CeO2. The combined results showed that CeO2 was a more effective catalyst than γ-MnO2 for methanol oxidation in the plasma catalysis system.

Improvement in discharge characteristics and energy yield of ozone generation via configuration optimization of a coaxial dielectric barrier discharge reactor

Dielectric barrier discharge (DBD) has been widely employed in ozone generation. However, the technology still exhibits relatively low energy yield (EY) referring to its theoretical value. In this work, EY of ozone generation was improved by optimizing the mesh number, electrode length, and dielectric material in a coaxial DBD reactor with two wire mesh electrodes. Meanwhile, the discharge characteristics were investigated to elucidate the effect of reactor configuration on EY. Results showed that the discharge characteristics were improved by increasing the mesh number, electrode length, and relative permittivity. When the mesh number was increased from 40 to 100, an improvement of approximately 48% in EY was obtained. Additionally, higher EY values were obtained using corundum as the dielectric material relative to polytetrafluoroethylene and quartz. Ultimately, EY in the optimal DBD reactor could reach 326.77 g·(kW·h)−1. Compared with the reported DBD reactor, the coaxial DBD reactor with the mesh electrode and the dielectric material of corundum could effectively improve EY, which lays a foundation for the design of high-efficiency coaxial DBD reactor.

Sustainability Assessment of the Utilization of CO2 in a Dielectric Barrier Discharge Reactor Powered by Photovoltaic Energy

The direct activation of diluted CO2 in argon was studied in a co-axial dielectric barrier discharge (DBD) reactor powered by photovoltaic energy. The influence of the initial CO2 and argon concentration on the CO2 decomposition to form CO was investigated using a copper-based catalyst in the discharge zone. It was observed that the CO2 conversion was higher at lower CO2 concentrations. The presence of the diluent gas (argon) was also studied and it was observed how it has a high influence on the decomposition of CO2, improving the conversion at high argon concentrations. At the highest observed energy efficiency (1.7%), the CO2 conversion obtained was 40.2%. It was observed that a way to enhance the sustainability of the process was to use photovoltaic energy. Taking into account a life cycle assessment approach (LCA), it was estimated that within the best-case scenario, it would be feasible to counterbalance 97% of the CO2 emissions related to the process.

Plasma Chemical Oxygen Conversion in an Industry‐Oriented Reactor Design

AbstractThe use of non‐thermal plasma enables chemical conversions as well as the removal of trace gas components. This application purpose has already been demonstrated for volume flow rates on a laboratory scale using a coaxial dielectric barrier discharge reactor. The next milestone is now to scale up this technology for application purposes with higher volume flow rates. In the present paper, the first successful conversion results from a steel reactor developed for the purpose of upscaling are presented.

A novel double dielectric barrier discharge reactor for toluene abatement: Role of different discharge zones and reactive species

A coaxial cylindrical double dielectric barrier discharge (DDBD) reactor with two separate discharge zone was designed and the role of its different discharge zones and reactive species were explored in toluene degradation. The inner tube (the space between the high voltage electrode and the inner barrier) had a higher gas temperature and reactive species concentration due to its stronger discharge intensity compared with the outer tube (the space between the inner and outer barriers). The two discharge zones exhibited similar toluene removal efficiency, whereas the mineralization rate of the inner tube exceeded that of the outer tube under 18–22 kV, and the result was opposite under 24–28 kV. The gas-flow direction had no significant effect on the toluene removal efficiency, but the mineralization rate of the down-flow reactor (23.7%–54.4%) exceeded that of the reverse-flow reactor (20.7%–45.5%). The inner and outer tubes mainly responsible for the initial degradation and further oxidation of toluene, respectively. The role of long-lived (N2(A3∑u+) and metastable O2 (a1Δg and b1Σg+)) and short-lived (N2(C3∏u), N2(B3∏g), and atomic O (1D and 3P)) reactive species on toluene degradation and oxidation was analyzed. The plausible toluene degradation mechanism in the DDBD reactor was proposed based on the role of different discharge zones and reactive species. The finding herein is significant in the design, optimization and application of DBD reactors.

Real‐time measurement of axial temperature in a coaxial dielectric barrier discharge reactor and synergistic effect evaluation for in‐plasma catalytic CO2 reduction

AbstractDue to serious difficulty in the measurement of catalyst‐bed temperature (Tb) for in‐plasma catalysis (IPC), the evaluation of the IPC synergistic effect is still disputed. Herein, we reported a real‐time measurement of axial temperature (Tax), which is much closer to Tb than wall temperature (Tw), using a thermocouple in a catalyst‐packed coaxial dielectric barrier discharge reactor for the first time. In plasma catalytic CO2 reduction, the effects of discharge power and flow rate on Tax, Tw, and CO2 conversion were examined. On the basis of Tax, compared with the thermal catalytic case, the plasma catalytic case only shows a weak synergistic effect. It was confirmed that the synergistic effect is overestimated if based on Tw (except for the heat‐insulated or quasi‐adiabatic reactors).

Mathematical modeling of the electric field in the planar and coaxial dielectric barrier discharge reactor in water treatment application

PurposeTo design a pulse power water treatment system, it is necessary to design a reactor optimally. One of the most essential types of reactors used in water treatment is the dielectric barrier discharge (DBD) reactor. The purpose of this paper is to model the electric field in the two types of planar and coaxial reactors to have an accurate analytical formula for using in the optimal design according to the required electric field of the treatment.Design/methodology/approachThe method proposed in this paper focuses on the voltage of different areas in the reactor and different boundary conditions to obtain the surface charge density. In this regard, parameters of the dielectric and treated material, as well as the reactor dimension, have been affected in the equations. To confirm the analytical results, the finite element method simulation has been performed, and it shows the accuracy of this method.FindingsThe exact analytical equation of the electric field is found within the discharge zone of the planar and coaxial DBD reactors. These equations can predict the values of different parameters of the reactor required to purify the material before each design and it does not even require simulation.Originality/valueThe electric field formula presented in this paper can allow the manufacturers of pulse power water treatment systems to optimize their design easily, cost-effectively and in less time. Also, the formulas provided are completely general and remain effective for all materials.

Effect of megapore particles packing on dielectric barrier discharge, O3 generation and benzene degradation

Recently, packed-bed discharge plasma technologies have been widely studied for treatment of volatile organic compounds (VOCs), due to the good performance in improving the degradation and mineralization of VOCs. In this paper, a coaxial cylindrical dielectric barrier discharge reactor packed with porous material of micron-sized pores was used for degradation of benzene, and the discharge characteristics and ozone generation characteristics were studied. When the discharge length was 12 cm and the filling length was 5 cm, the packed particles in the discharge area significantly increased the number of micro-discharges, and the current amplitude and density increased with the pore size of packed particles, but the discharge power and ozone concentration showed a trend of first increasing and then decreasing. The discharge power and ozone production reached the maximum when the size of pore former was 75 μm, correspondingly, the degradation efficiency of benzene was the highest.

Plasma-assisted catalysis for ammonia synthesis in a dielectric barrier discharge reactor: key surface reaction steps and potential causes of low energy yield

Ammonia synthesis experiments were carried out in a coaxial dielectric barrier discharge (DBD) reactor packed with several different supports and metal catalysts. There was a marked increase in the reaction rate, over that obtained in an empty DBD plasma reactor, upon introduction of a packed bed of γ-Al2O3, Ru/γ-Al2O3, or SiO2 particles. The difference in the reaction rates over γ-Al2O3 and Ru/γ-Al2O3 was minimal. Complementary zero-dimensional plasma kinetic model analysis was also performed using inputs from experimental data. This kinetic analysis allowed for gas phase reactions, Eley–Rideal (E–R) reactions, and direct adsorption of radical species on the γ-Al2O3 surface. On the metal surface, dissociative adsorption of N2 and H2, and Langmuir–Hinshelwood reactions were also included. This analysis revealed that, under the conditions of our experiments, ammonia synthesis proceeds principally by the formation of reactive radicals in the gas phase, which then adsorb and participate in E–R reactions on both the metal and support material surfaces. This finding illustrates a challenge for substantially increasing the energy yield for plasma-assisted ammonia synthesis in typical DBD reactors containing packed catalyst beads.

Revealing the active sites of the structured Ni-based catalysts for one-step CO2/CH4 conversion into oxygenates by plasma-catalysis

The direct conversion of CO2/CH4 into liquid chemicals is an attractive way for using carbon resources, but it is still unclear which types of catalysts are suitable for plasma-involved catalysis. Herein, a serious of nickel foam (NF) supported Ni-based catalysts derived from Layered double hydroxides (LDH) were newly designed by a green hydrothermal strategy. The crystal structures, morphologies, and the redox behaviors of these catalysts (NiAl-LDH/NF, NiO/NF, Ni/NF and NiGa/NF) were systemically characterized. Plasma-enabled direct conversion of CO2/CH4 was performed in a co-axial dielectric barrier discharge reactor using a nanosecond repetitive-pulse power, and the results showed that the selectivities of the liquid oxygenates were strongly depended on their microstructures, surface compositions (valence states) and reducibilities. The zero-valent Ni/NF and NiGa/NF showed high total liquid selectivity (> 30 %) and dominant CH3COOH (> 15 %), while the Ni2+ in NiO/NF gave the comparable activities for CH3OH (8.9 %) and CH3COOH (9.6 %). Unexpectedly, NiAl-LDH/NF exhibited the highest CH3OH selectivity (12.3 %) and negligible carbon deposition. According to systemic characterizations, an “−OH reservoir” mechanism was proposed that numerous −OH groups over the ultrathin nanosheets (∼60 nm) of NiAl-LDH may react with the interfacial CHx* radicals and, on the other hand, possess inferior electron affinity for electronegative C atom, thus endowing NiAl-LDH the prominent CH3OH selectivity and high resistance to carbon deposition.

Influence of Water Cooling for Outer Electrode on the Discharge Characteristics of an Atmospheric Coaxial DBD Reactor

Dielectric barrier discharge (DBD) has attracted increasing attention for the applications in the gas phase plasma chemical reactions. The utilization of electrode cooling has been proved to enhance the performance of plasma chemical reactions in the coaxial DBD reactor. However, the universal understanding on how the electrode cooling improves the plasma reaction is very limited. Here, we investigated the discharge characteristics in the N2 DBD with and without water cooling for outer electrode. The influence of water cooling on the discharge characteristics of the N2 DBD was examined by the diagnostics of electrical, optical, and temperature characters. The results showed that at the same initial applied voltage, the N2 DBD with water cooling exhibited a more stable discharge with higher intensity in comparison with that without water cooling, which was reflected by the smaller decrease and higher level in applied voltage, number of current pulses, transferred charge, relative intensity of the main reactive species, and energy efficiency as a function of discharge time. From the temperature measurement, the temperature of the DBD reactor was maintained in the lower and more stable level in the presence of water cooling, indicating that more energy was injected into the plasma reactions rather than wasted as heat dissipation to the environment, which led to the higher energy efficiency in the N2 DBD with water cooling. Overall, this work provides essential references for the development and optimization of the DBD reactors in practical applications (e.g., chemical synthesis using plasma processes).

Dry Reforming of Methane over Ni/γ‐MgO Catalysts in a Coaxial Dielectric Barrier Discharge Reactor

AbstractDry reforming of methane (DRM) over Ni/γ‐MgO catalysts in a coaxial dielectric barrier discharge (DBD) reactor was investigated and compared with DRM without catalysts. Firstly, the catalysts Ni/MgO, Ni/5%CeO2/MgO, and Ni/7%CeO2/MgO were prepared, their characteristics were specified by X‐ray diffraction, and then, they were partially packed into the DBD reactor. Various parameters such as the discharge power, flow rate, and CH4/CO2 molar ratio were explored. The presence of catalysts in the plasma reactor increased the yield and selectivity of H2 and CO. Furthermore, the effect of a reduced catalyst (Ni/7%CeO2/MgO reduced in argon‐hydrogen plasma) was evaluated, showing that CH4 can be converted with activated catalyst without extra power. Activated Ni/7%CeO2/MgO alone was able to convert CH4 to over 99 % for 30 min after reduction.

Ni and Cu oxide supported γ-Al2O3 packed DBD plasma reactor for CO2 activation

The direct activation of undiluted CO2 is carried out in a co-axial dielectric barrier discharge (DBD) reactor. The variation of the electrical discharge parameters and their influence on CO2 decomposition is investigated with the integration of 15 % MO/γ-Al2O3 (M = Ni, Cu) catalyst in the discharge zone. The electrical discharge is found to shift from the filamentary to a combination of surface and micro filamentary discharge on catalyst integration to NTP and also leads to the higher conversion of CO2 than DBD alone. The highest conversion of CO2 (15.7 %) with the energy efficiency of 1.597 mmol/kJ is achieved under CuO/γ-Al2O3 integrated NTP system, whereas the maximum of carbon balance (94.4 %) reaches with 4% CeO2 addition to CuO/Al2O3 catalyst. The oxygen vacancy of the catalyst plays a vital role in improving the performance, especially, the oxygen buffer property of CeO2 facilitates the recombination reaction and contributes to obtaining the highest carbon balance.

A promising plasma-catalytic approach towards single-step methane conversion to oxygenates at room temperature

Direct conversion of methane into chemicals and fuels under mild conditions has been considered as a ‘holy grail’ of chemistry and catalysis in the 21st century. Plasma-catalytic partial oxidation of methane (POM) to higher-value liquid fuels and chemicals over supported transition metal catalysts (Ni/γ-Al2O3, Cu/γ-Al2O3 and Fe/γ-Al2O3) has been investigated in a co-axial dielectric barrier discharge (DBD) reactor at room temperature and atmospheric pressure. The selectivity of oxygenates was 58.3% in the plasma POM reaction without a catalyst, while the combination of DBD with the catalysts enhanced the selectivity of oxygenates up to 71.5%. Of the three catalysts, Fe/γ-Al2O3 showed the highest methanol selectivity of 36.0% and a significant methanol yield of 4.7%, while the use of Cu/γ-Al2O3 improved the selectivity of C2 oxygenates to 9.4%, which can be attributed to the presence of more acid sites on the surfaces of the Cu catalyst. The possible reaction pathways in the plasma-catalytic POM reaction have been explored by combined means of plasma electrical and optical diagnostics, analysis of gas and liquid products, as well as comprehensive catalyst characterization. The plausible reaction routes for the production of major oxygenate (methanol) on the Fe/γ-Al2O3 surfaces have been proposed. The surface CHx species are found to be critical for methanol synthesis; they can be formed through the direct adsorption of CHx radicals generated in the plasma gas-phase reactions or through the dissociation of adsorbed CH4 on the catalyst surface.