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Abstract

A gliding arc plasmatron (GAP) is very promising for CO2 conversion into value-added chemicals, but to further improve this important application, a better understanding of the arc behavior is indispensable. Therefore, we study here for the first time the dynamic arc behavior of the GAP by means of a high-speed camera, for different reactor configurations and in a wide range of operating conditions. This allows us to provide a complete image of the behavior of the gliding arc. More specifically, the arc body shape, diameter, movement and rotation speed are analyzed and discussed. Clearly, the arc movement and shape relies on a number of factors, such as gas turbulence, outlet diameter, electrode surface, gas contraction and buoyance force. Furthermore, we also compare the experimentally measured arc movement to a state-of-the-art 3D-plasma model, which predicts the plasma movement and rotation speed with very good accuracy, to gain further insight in the underlying mechanisms. Finally, we correlate the arc dynamics with the CO2 conversion and energy efficiency, at exactly the same conditions, to explain the effect of these parameters on the CO2 conversion process. This work is important for understanding and optimizing the GAP for CO2 conversion.

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... The operation conditions of the GAP can be easily controlled within a broad area using a variation of plasmatron geometry, gas flow rate, current or voltage and can provide extensive information concerning the gliding arc physics [7]. Experimental results recently obtained by different research groups concerning the gliding arc characterization, show the formation of plasma objects, which might have an axial magnetic field caused by the interaction of micro-vortices with a gliding arc plasma [8,9,10]. The formation of the so called "arc lump", was established by the interaction of micro-vortices with an AC gliding arc [8]. ...
... These plasma objects possess a rotational axis, which angle in respect to the plasma channel axis strongly influences the total resistance of the Gliding Arc (GA) channel. The production of similar arc lumps was also observed in [9], by the optimization of a GAP applied for CO2 conversion, and in a GAP operated with nitrogen flow [10]. ...
... To test this assumption, an influence of the plasma-diamagnetism must be excluded, so the formation of a plasma core with an axial magnetic field inside of the plasma spot near the electrode can be studied. Formation of a cathode spot inside of a gliding arc plasmatron, a contraction of the plasma channel, a drastic increase of current density, overheating and erosion of the cathode material, which are correlated with an increase of the gas flow and thereby possible turbulence, was shown in [9,23]. ...
Article
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In this work a gliding arc plasmatron consisting of a filamentary discharge rotating in a nitrogen vortex flow at low DC current (I = 100 mA) is investigated. The gas flow swirl of the plasmatron is produced by six tangential gas inlets. The Reynolds number of the nitrogen flow through these tubes at the flow rate of Q = 10 slm amounts to about 2400, which is in the intermediate range. Under these conditions, the formation of micro-vortices can be caused by small gas flow disturbances like e.g. a tube edge. The operation of the GA plasmatron at these conditions is accompanied by the production of plasma spots at the anode surface, namely near the gas inlets. Melted and solidified metal is found in erosion traces left by plasma spots at the anode surface. It is established that melting of stainless steel cannot be caused by an axial current of I = 100 mA of plasma spots and an helical current is supposed. This assumption is confirmed by microscope images of eroded traces with toroidal melting areas. These experimental results corroborate a hypothesis of previous studies, concerning the gliding arc physics, about the formation of plasma objects with an axial magnetic field by the interaction of micro-vortices with the plasma channel.
... In this GAP, the swirl generator is on the same side as the gas outlet. The plasma channel is confined within the inner flow, where heat transfer to the wall is strongly reduced, but the energy efficiency and ionization degree are increased [18,23]. Moreover, gas mixing and thus, gas conversion is improved [17,21,22]. ...
... Plasma plumes are plasma objects with larger diameter and higher emission intensity inside the GAP plasma channel. In principle, the position of the plasma plume in the GAP depends on the gas mixture, gas flow and electric current [23] and can reach the surface of the metal electrodes. In this case, the electrode material is overheated and vaporized. ...
... The GAP is operated in a nitrogen flow of 10 slm with a purity of 99.999% and a total electric current of 230 mA DC. Based on CO 2 flow simulations for this GA plasmatron, presented in [23], we assume that the rotational speed in the anode tube amounts to 700 revolutions per second under our experimental conditions. It was also supposed in [23] that local oscillations in flow speed cause the formation of micro-vortices by turbulent gas flow through the plasma reactor. ...
Article
A gliding arc plasmatron (GAP), which is very promising for purification and gas conversion, is characterized in nitrogen using optical emission spectroscopy and high-speed photography, because the cross sections of electron impact excitation of N2 are well known. The gas temperature (of about 5500 K), the electron density (up to 1.5 × 10¹⁵ cm⁻³) and the reduced electric field (of about 37 Td) are determined using an absolutely calibrated intensified charge-coupled device (ICCD) camera, equipped with an in-house made optical arrangement for simultaneous two-wavelength diagnostics, adapted to the transient behavior of a GA channel in turbulent gas flow. The intensities of nitrogen molecular emission bands, N2(C-B,0-0) as well as (B-X,0-0), are measured simultaneously. The electron density and the reduced electric field are determined at a spatial resolution of 30 μm, using numerical simulation and measured emission intensities, applying the Abel inversion of the ICCD images. The temporal behavior of the GA plasma channel and the formation of plasma plumes are studied using a high-speed camera. Based on the determined plasma parameters, we suggest that the plasma plume formation is due to the magnetization of electrons in the plasma channel of the GAP by an axial magnetic field in the plasma vortex.
... The discharge zone can either be a plasma-disc or three-dimensional rotating arc [18]. Typically, the arc rotation is achieved using multiple tangential gas entry holes [11,15,[19][20][21][22][23][24][25][26] and the configuration is known as a rotating gliding arc (RGA). The RGA system is also used along with the external magnets [12,[27][28][29] designated as a magnetic rotating gliding arc (MRGA). ...
... Reactor wall was used as both the electrodes with insulation separating them. [25] of kHz frequency was studied. The electrode configurations (type 1 to 6) of RGA are briefly discussed in the subsequent paragraphs. ...
... Type-6: Gliding Arc Plasmatron (GAP) of type-6 [25,30] was designed with the anode (positive electrode) and the cathode (negative electrode) being the reactor wall itself as shown in table 1. ...
... Among them are thermochemical processes, electrolysis [1] and plasma catalysis; the latter has the smallest technology readiness level (TRL) but also offers a large potential for future improvements [2][3][4]. Attention is focused mostly on four types of plasma reactors: dielectric barrier discharges (DBDs) [5], gliding arc (GA) [6,7], atmospheric pressure glow discharges (APGD) [8,9] and microwave (MW) plasmas [10,11]. Increasing their energy efficiency and conversion at ambient pressure is the main point of concern. ...
... They were selected based on performance from a broader range of systems prev ously reviewed [17], considering more recent work. Gliding arcs provide high efficien at ambient pressure; a vortex is often used to increase the discharge volume [6,7]. Glidin arcs also obtain good results without vortex flow [18]. ...
... They were selected based on performance from a broader range of systems previously reviewed [17], considering more recent work. Gliding arcs provide high efficiency at ambient pressure; a vortex is often used to increase the discharge volume [6,7]. Gliding arcs also obtain good results without vortex flow [18]. ...
Article
Full-text available
Plasma technology reaches rapidly increasing efficiency in catalytic applications. One such application is the splitting reaction of CO2 to oxygen and carbon monoxide. This reaction could be a cornerstone of power-to-X processes that utilize electricity to produce value-added compounds such as chemicals and fuels. However, it poses problems in practice due to its highly endothermal nature and challenging selectivity. In this communication a glow discharge plasma reactor is presented that achieves high energy efficiency in the CO2 splitting reaction. To achieve this, a magnetic field is used to increase the discharge volume. Combined with laminar gas flow, this leads to even energy distribution in the working gas. Thus, the reactor achieves very high energy efficiency of up to 45% while also reaching high CO2 conversion efficiency. These results are briefly explained and then compared to other plasma technologies. Lastly, cutting edge energy efficiencies of competing technologies such as CO2 electrolysis are discussed in comparison.
... The GA has been known for more than one hundred years [10], but it has attracted increasing attention only in the last 30 years for its promising applications in energy and environmental fields [7][8][9][10][11][12][13]. The traditional GA was composed of two diverging electrodes. ...
... The GA breakdowns at the shortest gap and cyclically goes through 'breakdownthermal-nonthermal phases' [1]. This GA reactor can supply favorable conditions for chemical reactions, but it exhibits a low gas conversion as the traditional GA reactor configuration employs a high gas flow rate to push the arc, and thus the gas residence time and gas-treated fraction are limited [11]. Therefore, several new 3D types of GA reactors were developed to enhance the arc movement, gas residence time and interaction between arc channel and surrounding gas [12,13]. ...
... The arc behaviors in a forward or reverse vortex gas flow have been investigated by experiment and modeling [16,17]. Ramaker et al and Trenchev et al have studied the arc dynamic in reverse vortex Ar and CO 2 flows by combining experiment and fluid modeling to reveal the correlation between arc behaviors and gas conversion [11,18]. In our previous works, the temporal evolution of annular-mode GA in a vortex air flow was studied to understand the breakdown and dynamic of GA [19]. ...
... In this work, we designed and tested a novel GA reactor, based on our insights obtained from reverse-vortex flow GA reactors [11,18,20]. More specifically, we developed a dual-vortex electrode configuration, supported by fluid dynamics and plasma simulations. ...
... The effect of heat insulation of the GA in a reverse-vortex flow has been shown through modelling [18,23]. With no doubt, the arc is indeed forced in the reactor centre [18,20]. This configuration, however, results in a quasi-static hot cathode spot on the cathode cap, as can be verified by the photograph in Fig. 1(b). ...
... Some difference in light emission can be spotted in Fig. 1(b), and variations in plasma density and temperature have been predicted through modelling [18,23]. Due to the intense arc contraction, the discharge is rather thin (see Fig. 1(b)), which was indeed verified in simulations [18] and experiments [20]. The discharge radius is typically no more than 2 mm. ...
Article
Atmospheric pressure gliding arc (GA) discharges are gaining increasing interest for CO2 conversion and other gas conversion applications, due to their simplicity and high energy efficiency. However, they are characterized by some drawbacks, such as non-uniform gas treatment, limiting the conversion, as well as the development of a hot cathode spot, resulting in severe electrode degradation. In this work, we built a dual-vortex plasmatron, which is a GA plasma reactor with innovative electrode configuration, to solve the above problems. The design aims to improve the CO2 conversion capability of the GA reactor by elongating the arc in two directions, to increase the residence time of the gas inside the arc, and to actively cool the cathode spot by rotation of the arc and gas convection. The measured CO2 conversion and corresponding energy efficiency indeed look very promising. In addition, we developed a fluid dynamics non-thermal plasma model with argon chemistry, to study the arc behavior in the reactor and to explain the experimental results. Keywords: Plasma; Modeling; Reactor; CO2 conversion, CFD, Vortex, Argon, Atmospheric pressure
... This feature of GAD greatly facilitates the production of efficiently active species especially for vibrationally excited states, and thus GAD exhibits high energy efficiency and fast reaction rate. GAD has been widely used in many fields, such as pollution control [8][9][10], energy conversion [11][12][13][14], surface treatment [15][16][17][18] and material synthesis [19,20]. ...
... Furthermore, the residence time of reaction gas inside plasma is rather short [31]. Therefore, in an attempt to overcome the disadvantages of conventional GAD, 3D GAD configurations have recently been developed with spiral-ring or truncated cone electrodes to enhance the interaction between arc and reactant gas [12,[31][32][33][34][35][36][37][38][39][40]. Amongst those GAD designs, the GAD in the reverse vortex flow is recognized as the most efficient one. ...
... The similar restrike process with the classical 2D GAD was reported by Zhao et al [38] and Zhang et al [46] in vortex flow 3D GAD reactor. The simulation for 3D GAD in a reverse vortex flow was also developed with a hypothesis as the GA ignites at narrowest gap and then glides until the longest length [12]. However, besides the restrike mode, a continuous gliding of arc in GAD was also reported [47,48] as turn-over mode in the thermal arc discharge [49,50]. ...
... Besides a conventional rotating arc reactor with straight cylindrical grounded tube [49,51,54], a nozzle-type rotating arc reactor with a nozzle as grounded electrode has been developed as well [22,25,50,52,57]. In addition, there exist other types of 3D arc reactors, e.g., referred to as vortex stabilized gliding arc discharge or gliding arc plasmatron [21,[27][28][29]31,33,[58][59][60][61]. In all of these 3D arc reactors, the arcs rotate in 3D space by a swirling flow inlet or by a magnetic field [54,62]. ...
... We realize that the model is for argon, so it might be different from the experiments, but developing this model in the CO 2 /CH 4 mixture with all the chemistry would lead to unacceptable calculation times. Moreover, the arc behaviour can be very similar between different gases, as proven before [59] to be accurate enough for estimating the arc rotation. The calculated plasma density is around 10 21 m − 3 , which is typical for atmospheric pressure arc discharges [33,60]. ...
Article
Full-text available
Arc plasma technology is gaining increasing interest for a variety of chemical reaction applications. In this study, we demonstrate how modifying the reactor geometry can significantly enhance the chemical reaction performance. Using dry reforming of methane as a model reaction, we studied different rotating arc reactors (conventional rotating arc reactor and nozzle-type rotating arc reactor) to evaluate the effect of attaching a downstream nozzle. The nozzle structure focuses the heat to a confined reaction volume, resulting in enhanced heat transfer from the arc into gas activation and reduced heat losses to the reactor walls. Compared to the conventional rotating arc reactor, this yields much higher CH4 and CO2 conversion (i.e., 74% and 49%, respectively, versus 40% and 28% in the conventional reactor, at 5 kJ/L) as well as energy efficiency (i.e., 53% versus 36%). The different performance in both reactors was explained by both experiments (measurements of temperature and oscillogram of current and voltage) and numerical modelling of the gas flow dynamics, heat transfer and fluid plasma of the reactor chambers. The results provide important insights for design optimization of arc plasma reactors for various chemical reactions.
... GA is a promising plasma source for the synthesis of a range of fuels and chemicals [36,[76][77][78][79][80][81][82][83][84][85][86][87][88] that combines the advantages of both non-thermal plasma and thermal plasma [89][90][91][92]. It is often referred to as a 'warm' plasma as the gas can attain a temperature of >1000 K, which is in between the gas temperatures of thermal and non-thermal plasmas. ...
... Extensive efforts have been dedicated to the investigation of the physical and chemical characteristics of GA using both experimental methods and modeling approaches [76,[93][94][95][96][97][98][99]. GA has been demonstrated to be effective for vibrational excitation of chemical molecules, which is viewed as the most energy-efficient means for the dissociation of CO 2 [80,95,96]. Indeed, the average electron temperature of GA is around 1-2 eV, which is conducive for populating low energy vibrational levels of CO 2 . ...
Article
Increasing attention has been drawn to carbon dioxide (CO2) conversion into higher-value platform chemicals and synthetic fuels due to global warming. These reactions require a large amount of thermal energy in order to proceed, which is ascribable to the high stability of the bonds in CO2. Non-thermal plasma (NTP)-catalytic CO2 conversion has emerged as a promising method to significantly reduce the reaction temperature as plasma can activate CO2 at as low as room temperature and atmosphere pressure. However, this technology requires a paradigm shift in process design to enhance plasma-catalytic performance. CO2 conversion using plasma-catalysis has great potential to increase reaction efficiencies due to the synergetic effects between the plasma and catalysts. It is crucial to present the recent progress in CO2 conversion and utilization whilst providing a research prospects framework and direction for future research in both industries and laboratories. Herein, a comprehensive review of recent, encouraging research achievements in CO2 conversion using NTP is provided. The topics reviewed in this work are: i) the recent progress in different NTP sources in relation to product selectivity, conversion, and energy efficiency; ii) plasma-based CO2 reactions and applications; iii) CO2 conversion integrated with CO2 capture; and iv) current challenges and future perspectives. The high market value of the possible products from this process, including chemicals and fuels, make commercialization of the process feasible. Furthermore, the selectivities of these products can be further improved by developing suitable catalysts with effective sensitivities and performances under the intricate conditions needed to make these products. There is an urgent need for further studies to be performed in this emerging field.
... However the nozzle used to generate the gas stream could represent a gas flow bottleneck, which limits the amount of gas to be treated and requires a high pressure of the gas on the nozzle entry. The research on the utilization of gliding arc for gas treatment lead to invention of several discharge designs, where the discharge distribution could be further shaped by e.g. the rotation of conical electrode [8] or creation of gas vortex in discharge chamber [9,10]. ...
Article
The large-scale plasma treatment of waste gas in industrial or municipal conditions requires high efficiency of plasma conversion process at high processing speed, i.e., large volumetric flow. The integration of the plasma unit into existing systems puts demands on the pipe-system compatibility and minimal pressure drop due to adoption of plasma processing step. These conditions are met at the innovative rotating electrode gliding arc plasma unit described in this article. The system consists of propeller-shaped high voltage electrode inside grounded metallic tube. The design of HV electrode eliminates the pressure drop inside the air system, contrary the plasma unit itself is capable of driving the waste gas at volumetric flow up to 300 m<sup>3</sup>/hr for 20 cm pipe diameter. In the article the first results on pilot study of waste air treatment will be given for selected volatile organic compounds together with basic characteristic of the plasma unit used.
... In order to overcome these drawbacks, a rotating gliding arc (RGAD) without using magnetic field has been developed in our lab. Novel and simple electrode configuration is designed compared to the existing ones in literature [9][10][11][12][13][14][15][16][17]. To our knowledge, no studies have been carried out yet on the effect of number of tangential entry holes (NH) and flow rate (Q) on arc dynamics and electrical characteristics. ...
Conference Paper
Full-text available
A new rotating gliding arc (RGAD) type reactor is developed without using magnets, with novel approach to electrode configuration suitable for gas cleaning applications. In this work, it is attempted to understand and verify the influence of gas flow dynamics on voltage characteristics and dynamics of the rotating gliding arc. Experiment is conducted using Argon gas by varying (a) 'number of tangential entry holes (NH)' of gas vortex and (b) 'flow rate (Q)'. Influence of gas flow dynamics is observed and verified by the (a) existence of linear functionality between arc rotational frequency (farc) and Reynolds number (Re) and (b) effect of gas flow nature (laminar or turbulent) and its disturbance on the nature of voltage waveforms (regular or irregular periodicity). This work is important for understanding and optimising the reactor for specific applications.
... 29 This novel type of GA plasma is promising for gas conversion at atmospheric pressure, as demonstrated already for CO 2 conversion in pure CO 2 30 and in a CO 2 −N 2 gas mixture, 31 as well as for dry reforming of methane, 32 but it has not yet been applied for NO x formation in a N 2 −O 2 gas mixture. To better understand and improve the GAP for gas conversion, the underlying mechanisms have been studied, both computationally 33−35 and experimentally, 36,37 but only in argon, pure CO 2 , and pure N 2 , while the chemistry has also been modeled in CO 2 −CH 4 32 and CO 2 −N 2 31 gas mixtures. In order to elucidate the underlying mechanisms in a N 2 −O 2 gas mixture in the GAP reactor, we combine our experiments with a zero-dimensional (0D) chemical kinetics model. ...
Article
Plasma technology provides a sustainable, fossil-free method for N2 fixation, i.e., the conversion of inert atmospheric N2 into valuable substances, such as NOx or ammonia. In this work, we present a novel gliding arc plasmatron at atmospheric pressure for NOx production at different N2/O2 gas feed ratios, offering a promising NOx yield of 1.5 % with an energy cost of 3.6 MJ/mol NOx produced. To explain the underlying mechanisms, we present chemical kinetics model, validated by experiments, which provides insight into the NOx formation pathways and into the ambivalent role of the vibrational kinetics. This allows us to pinpoint the factors limiting the yield and energy cost, which can help to further improve the process.
... The revolution period is approximately 0.7 ms. We have also performed high-speed photography experiments of the arc rotation in our experimental reactor, showing a similar behaviour and rotational speed 28 While the argon model provides valuable information of the discharge formation, the main purpose of this work is to model the GAP operating in CO2. A complex CO2 chemistry would yield excessive calculation times if using a 3D model. ...
Article
The gliding arc plasmatron (GAP) is a highly efficient atmospheric plasma source, which is very promising for CO2 conversion applications. To understand its operation principles and to improve its application, we present here comprehensive modelling results, obtained by means of computational fluid dynamics simulations and plasma modelling. Due to the complexity of the CO2 plasma, a full 3D plasma model would be computationally impractical. Therefore, we combine a 3D turbulent gas flow model with a 2D plasma and gas heating model in order to calculate the plasma parameters and CO2 conversion characteristics. In addition, a complete 3D gas flow and plasma model with simplified argon chemistry is used to evaluate the gliding arc evolution in space and time. The calculated values are compared with experimental data from literature as much as possible, in order to validate the model. The insights obtained in this study are very helpful for improving the application of CO2 conversion, as they allow us to identify the limiting factors in the performance, based on which solutions can be provided on how to further improve the capabilities of CO2 conversion in the GAP.
... However, only very few diagnostic studies are available on the estimation of basic plasma parameters such as electron density (n e ) and electron temperature (T e ). [21][22][23] Determining T e , n e , and electron energy distribution function (EEDF) becomes essential to optimize/control plasma systems. 24,25 As electrons are responsible for initiation of various reactions in plasma systems, their energies can be estimated by a macroscopic scaling parameter defined as reduced electric field ( E No ). ...
Article
This work reports average electron temperature ( T e) and electron density ( n e) of an atmospheric argon rotating gliding arc ( R G A), operated in glow-type mode, under transitional and turbulent flows. Both T e and n e were calculated near the shortest ( δ) and longest ( Δ) gap between the electrodes, by two different methods using two separate measurements: (1) optical emission spectroscopy ( O E S) and (2) physical–electrical. T e calculated from (a) collisional radiative model ( C R M) ( O E S) and (b) BOLSIG+ [physical–electrical, reduced electric field ( E N o ) as input], differed each other by 16%–26% at δ and 6% at Δ. T e was maximum at δ ( > 2 eV) and minimum near Δ (1.6–1.7 eV). Similarly, the E N o was maximum near the δ (5–8 Td) and minimum near Δ, reaching an asymptotic value (1 Td). By benchmarking T e from C R M, the expected E N o near δ was corrected to 3 Td. The calculated C R M intensity agreed well with that of the measured for most of the emission lines indicating a well optimized model. The average n e near δ and Δ from Stark broadening ( O E S) was 4.8– 8.0 × 10 21 m − 3, which is an order higher than the n e calculated through current density (physical–electrical). T e and n e were not affected by gas flow, attributed to the glow-type mode operation. To the best of authors’ knowledge, this work reports for the first time (a) an optimized C R M for R G As (fine-structure resolved), (b) the poly-diagnostic approach to estimate plasma parameters, and (c) the validation of E N o calculated using physical–electrical measurements.
... The dynamics of RGAs and MRGAs is still being explored [8]. The reported works focussed mostly in understanding the general behaviour of the discharge such as the discharge's generation and motion dynamics [8,[17][18][19][20][21][22][23][24][25], electrical characteristics [8,[17][18][19][20][22][23][24][25], and emission/spectral characteristics [22,[25][26][27]; only a few works among them reported the effect ofQ [8,19,[24][25][26]28] on plasma characteristics, and a very few characterized the Re [8,19]. Guofeng et al [19] studied theQs having the flow regime between laminar and transitional (1200 < Re < 2600), and observed a linear relation between the Re and the 'root-mean-square' of V (V RMS ). ...
Article
Full-text available
The work reports the effect of flow regime on plasma characteristics of an atmospheric N 2 rotating gliding arc ( RGA ). When changed from transitional (5 SLPM) to turbulent (50 SLPM) flow, operation mode transitioned from glow to spark discharge due to frequent reignition events; the average reduced electric field ( E/N ) and electron temperature raised (38→92 Td, 0.84→2.2 eV); and gas temperature ( T g ) slightly cooled (2973→2807 K). Molecules generated for 100 eV of energy input (G–factor) increased by a factor of 20 and 65, for the chemically active singlet and triplet metastable states of N 2 , respectively—a promising feature for chemical applications. A sudden three fold increase in the energy efficiency, achieving a destruction of 3.0±0.2 g·kWh ⁻¹ of dilute toluene (112±10 ppmV) at highly turbulent flow corroborated the enhancement of the G–factor, E/N and T g ; and indicated the sensitivity of plasma properties to the flow regime. Interestingly, for flows having Reynolds number ≥3×10 ⁴ , the bandhead of N 2 ⁺ shifted from 0–0 at 391.4 nm to 3–3 at 383.3 nm attributed to higher-level perturbations, showing again the sensitivity. The smallest eddies (η≈6 μm) is less than the discharge diameter (d d ≈220±90 μm), and thermal/mass Péclet number>1. The eddies of size < d d advected the plasma species, wrinkled/distorted the discharge, and increased the reignition events, eventually affected the plasma properties including the chemical performance (energy efficiency), which is observed in this work.
Article
Numerous studies have shown that dielectric barrier discharge (DBD) and DBD-like plasma jets interact with a treated surface in a complex manner. Eroded traces after treatment cannot be explained by conventional plasma-surface interaction theory. The mechanisms of a controlled formation of these plasma objects is still unclear. In this work, the authors show that the formation rate and characteristics of eroded traces, treating a titanium surface, can be controlled by process design and the combination of materials used. A thin (0.45 μm) layer of titanium film is deposited onto a glass substrate and is then treated in the effluent of a non-equilibrium atmospheric pressure plasma jet (N-APPJ) operated with argon or krypton flow. Plasma spots with diameters ranging from 100-700 μm are observed using an intensified digital camera on the titanium film surface. These plasma objects are strongly inhomogeneous, forming a core with a very high current density and leave erosion holes with diameters of about 1 μm. By using krypton as a working gas, effective erosion of the titanium substrate can be shown, whereas by using argon no traces are detected. For the latter case, traces can be provoked by deposition of a thin aluminum layer on top of the titanium substrate, by creation of artificial scratches or by an additional swirling flow around the discharge. Based on the experimental results presented in this and previous papers, it is assumed that plasma spots with dense cores are produced by an interaction of micro-vortices within the plasma channel and by the formation of an extremely high axial magnetic field. This assumption is confirmed by destruction of the treated surface material, extraction of paramagnetic atoms and toroidal substrate heating, which is most likely caused by a helical current of the plasma spot.
Article
CO2 conversion by plasma technology is gaining increasing interest. We present a carbon (charcoal) bed placed after a Gliding Arc Plasmatron (GAP) reactor, to enhance the CO2 conversion, promote O/O2 removal and increase the CO fraction in the exhaust mixture. By means of an innovative (silo) system, the carbon is constantly supplied, to avoid carbon depletion upon reaction with O/O2. Using this carbon bed, the CO2 conversion is enhanced by almost a factor of two (from 7.6 to 12.6 %), while the CO concentration even increases by a factor of three (from 7.2 to 21.9 %), and O2 is completely removed from the exhaust mixture. Moreover, the energy efficiency of the conversion process drastically increases from 27.9 to 45.4 %, and the energy cost significantly drops from 41.9 to 25.4 kJ.L⁻¹. We also present the temperature as a function of distance from the reactor outlet, as well as the CO2, CO and O2 concentrations and the temperature in the carbon bed as a function of time, which is important for understanding the underlying mechanisms. Indeed, these time-resolved measurements reveal that the initial enhancements in CO2 conversion and in CO concentration are not maintained in our current setup. Therefore, we present a model to study the gasification of carbon with different feed gases (i.e., O2, CO and CO2 separately), from which we can conclude that the oxygen coverage at the surface plays a key role in determining the product composition and the rate of carbon consumption. Indeed, our model insights indicate that the drop in CO2 conversion and in CO concentration after a few minutes is attributed to deactivation of the carbon bed, due to rapid formation of oxygen complexes at the surface.
Article
Full-text available
Conversion of CO2 into CO with plasma processing is a potential method to transform intermittent sustainable electricity into storable chemical energy. The main challenges for developing this technology are how to get efficient CO2 conversion with high energy efficiency and how to prove its feasibility on an industrial scale. In this paper we review the mechanisms and performance of different plasma methodologies used in CO2 conversion. With the goals of obtaining efficient conversion and high energy efficiency, as well as industrial feasibility in mind, we emphasize a promising new approach of CO2 conversion by using a thermal plasma in combination with a Carbon co-reactant.
Article
This article investigates the effect of gas flow rate on arc discharge mode and plasma chemistry in a nonmagnetic rotating gliding arc reactor. This article is conducted using oxygen as a plasma forming gas under transient (5 LPM) , turbulent (25 LPM) , and highly turbulent (50 LPM) flow conditions. The voltageendash current ( V-I ) characteristics reveal discharge modes, such as glow mode under transient flow (I < 1 A) , glowendash spark transition mode under turbulent flow (I < 1 A & I - 1 A) , and spark mode under highly turbulent flow (I - 1 A) . Arc completes full rotation under transient flow, whereas it is blown off before completing full rotation under turbulent flows. The captured optical emission lines of the discharge indicate domination of excitation reactions under glow and glowendash spark transition modes and domination of both the excitation and electron impact ionization reactions under spark mode. These observations reveal that the gas dynamics changes the discharge mode of the rotating arc that in turn alters the plasma chemistry, which is a positive feature to promote specific reaction pathways.
Article
CO2 conversion with renewable power is gaining increasing interest to solve the current energy and environmental issues. Plasma is a very promising technology because it supplies a non-thermal condition for CO2 activation and can easily be connected to the intermittent renewable electricity. Gliding arc (GA) plasma in all types of plasmas has the most potential for practical application of plasma based CO2 conversion because it operates at atmospheric pressure and reaches a high energy efficiency. However, GA technology is not mature yet, the CO2 conversion in the GA plasmas are still too low. So far, the existing GA reactors still need to be developed and improved. The physical and chemical mechanism and the most efficient conversion route of CO2 in GA are still not clear enough. To further improve the GA based CO2 conversion, the state-of-the-art technology is reviewed to know the technology development has been made and limitations need to be overcome. This paper overviews the arc behaviors and GA plasma parameters, and addresses standard definitions and reliable analysis method for evaluating plasma based CO2 conversion. In addition, the GA reactor development, the gas conversion performance of GA, effect of conditional parameters and the reaction mechanism for CO2 conversion in GA plasma were discussed.
Article
Dry reforming (DR) of n-heptane and coupling of DR with partial oxidation reforming (POR) were investigated through experiments and simulations in a gliding arc discharge (GAD) plasma reactor. The effects of the input power, the molar ratio of oxygen to carbon, and the residence time on the DR process were evaluated according to the conversion rate, product selectivity and the energy yield of hydrogen. The highest CO2 conversion was 42.6%, which was higher than other CO2 splitting technologies in the gliding arc discharge plasma. The coupling of DR and POR achieved better results by adding air to the DR of n-heptane to form a thermal coupling process. Optical emission spectra of n-heptane DR and coupling DR with POR of n-heptane were investigated, respectively. Different reactive species including C2(A³Πg-X³Πu), CO²⁺(A²Πu-X²Πg), CO(a’³Σ⁺-a³Π), CO(d³Δ-a³Π) and O⁺(⁴P-⁴S⁰) were detected. A zero-dimension (0-D) model of heptane DR containing 163 reactions was established in Chemkin to describe the effect of O/C. The 0-D model of coupling DR with POR with 193 reactions was also established.
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Plasma-based gas conversion has great potential for enabling carbon-free fertilizer production powered by renewable electricity. Sustaining an energy-efficient plasma process without eroding the containment vessel is currently a significant challenge, limiting scaling to higher powers and throughputs. Isolation of the plasma from contact with any solid surfaces is an advantage, which both limits energy loss to the walls and prevents material erosion that could lead to disastrous soil contamination. This paper presents highly energy-efficient nitrogen fixation from air into NOx by microwave plasma, with the plasma filament isolated at the center of a quartz tube using a vortex gas flow. NOx production is found to scale very efficiently when increasing both gas flow rate and absorbed power. The lowest energy cost recorded of ∼2 MJ/mol, for a total NOx production of ∼3.8%, is the lowest reported up to now for atmospheric pressure plasmas.
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Direct current atmospheric pressure discharges offer a unique set of properties, that make them ideal for the splitting of CO2. In this study, a reactor with a maximum plasma power of 220 W is presented that operates in the glow-to-arc transition region. It facilitates the splitting of CO2 with up to 43% energy efficiency in the plasma at a high CO2 conversion of 27%. The goal of this study was to improve the splitting efficiency of CO2 and gain an understanding for the scaling effects involved in the process. To achieve this, a tubular plasma reactor with adequate driver was constructed, in which the discharge is formed between a pin and ring electrode. The reactor represents an improved version of our previous design, utilizing optimized gas flow. The plasma is forced into a disc-like volume by an axial magnetic field. The relationship of electrical characteristics of the plasma and the applied magnetic field were studied successfully. It was revealed that the magnetic field can be used to tune the burn voltage and stabilize the plasma. The shape and rotation rate of the plasma in the magnetic field were investigated. Splitting of CO2 was performed under a wide range of parameters, the ideal operation conditions could be determined.
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In this work, we consider the rational design of a plasma catalysis system for the splitting of carbon dioxide (CO2) via one-dimensional PIC/MCC simulations. We show that field emission-driven microdischarges (also called microplasmas) can selectively excite the asymmetric stretch mode, which is known to be more favorable to the dissociation of CO2, relative to the symmetric stretch mode. These results suggest that field emission-driven microdischarges could form the basis for intentionally coupling the plasma state with a catalyst for improving the performance of CO2 reforming.
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The dynamics of a low-power (<40 W) rotating gliding arc in atmospheric pressure argon sustained by a homemade dual DC power supply between a conical cathode and a grounded sleeve anode and forced into motion by the combined action of a vortex gas flow (<26.3 SLPM) and axial magnetic field (∼0.043-0.122 T) was studied. High-speed imaging and electrical diagnostics were used to gain an understanding of the projected arc length, position of the attachment point on the central cathode, and frequency of arc rotation. The electrical signals revealed a glow-type mode of operation, which characteristically low current and high voltage. The square root dependency of the rotating arc frequency on the applied current predicted by a simplified model considering the gas vortex drag and Lorentz forces was confirmed with measurements. Rotation frequencies in the ∼100-300 Hz range with average projected arc lengths of 4-8 mm creating a large stabilized reactive volume were obtained. We further demonstrated that the Lorentz force acting on the rotating gliding arc is equivalent to the hydrodynamic drag caused by an argon flow rate of ∼8 SLPM for this device configuration.
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Atmospheric pressure plasmas (APPs) are at the core of diverse technological applications in materials processing, chemical synthesis, resource recovery, water treatment, and medicine, among others. APPs span a wide range of power density, from low-power non-thermal to high-power thermal discharges, and typically involve interactions with a stream of working gas, processing material, or gas environment. The article provides an overview of computational fluid dynamics (CFD) approaches, from mathematical models to software strategies, for the analysis of APP flows. Its focus is flows with large variations in ionization degree and significant fluid dynamic-thermal-electromagnetic coupling, as particularly found in mid- to high-power discharges. The advances achieved and challenges faced by CFD of APP flows have been driven by established and emerging applications, and can be broadly characterized in terms of model fidelity and numerical accuracy. Fidelity refers to the degree of underlying phenomena captured by the model, whereas accuracy to the precision of the numerical solution of the model. Two distinct numerical accuracy challenges are addressed: the capture of instability and pattern formation phenomena, and of plasma-gas interaction and turbulence; as well as two representative fidelity challenges: radiative transport under nonequilibrium conditions, and nonequilibrium electron and particle kinetics. The article aims to provide guidance to researchers, from modelers and code developers to open-source and commercial software users, working on CFD analyses of APP flows within technological contexts.
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Mechanical energy‐induced CO2 reduction is a promising strategy for reducing greenhouse gas emissions and simultaneously harvesting mechanical energy. Unfortunately, the low energy conversion efficiency is still an open challenge. Here, multiple‐pulse, flow‐type triboelectric plasma with dual functions of harvesting mechanical energy and driving chemical reactions is introduced to efficiently reduce CO2. CO selectivity of 92.4% is achieved under normal temperature and pressure, and the CO and O2 evolution rates reach 12.4 and 6.7 µmol h−1, respectively. The maximum energy conversion efficiencies of 2.3% from mechanical to chemical energy and 31.9% from electrical to chemical energy are reached. The low average electron energy in triboelectric plasma and vibrational excitation dissociation of CO2 with low barrier is revealed by optical emission spectra and plasma simulations, which enable the high energy conversion efficiency. The approach of triboelectric plasma reduction reported here provides a promising strategy for efficient utilization of renewable and dispersed mechanical energy. Triboelectric plasma creates simultaneously harvesting mechanical energy and driving CO2 reduction to CO, efficiently increasing the overall energy efficiency from mechanical to chemical energy. Moreover, this system shows well‐suited to the irregularity and volatility of mechanical energy and the flow of CO2 gas. This strategy exhibits potential in mechanical energy storage and mechanical energy‐driven CO2 reduction.
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CO2 conversion into value-added chemicals and fuels is considered as one of the great challenges of the 21st century. Due to the limitations of the traditional thermal approaches, several novel technologies are being developed. One promising approach in this field, which has received little attention to date, is plasma technology. Its advantages include mild operating conditions, easy upscaling, and gas activation by energetic electrons instead of heat. This allows thermodynamically difficult reactions, such as CO2 splitting and the dry reformation of methane, to occur with reasonable energy cost. In this review, after exploring the traditional thermal approaches, we have provided a brief overview of the fierce competition between various novel approaches in a quest to find the most effective and efficient CO2 conversion technology. This is needed to critically assess whether plasma technology can be successful in an already crowded arena. The following questions need to be answered in this regard: are there key advantages to using plasma technology over other novel approaches, and if so, what is the flip side to the use of this technology? Can plasma technology be successful on its own, or can synergies be achieved by combining it with other technologies? To answer these specific questions and to evaluate the potentials and limitations of plasma technology in general, this review presents the current state-of-the-art and a critical assessment of plasma-based CO2 conversion, as well as the future challenges for its practical implementation.
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An AC-pulsed tornado gliding arc plasma was employed for CO2 conversion via CO2 decomposition and dry reforming reactions. A stable and high-efficient constant arc length discharge mode was obtained in this plasma reactor. And then, CO2 conversion was studied under this discharge mode. In the case of CH4/CO2 = 0, CO2 was converted to CO and O2 via the CO2 decomposition reaction. Energy efficiency of 29 % was attained at CO2 conversion of 6 %. With strong reducing agent CH4 added into CO2, the main contributor of CO2 conversion changed from CO2 decomposition to dry reforming of CH4. Conversions of CH4 and CO2, energy efficiency and energy cost changed sharply at CO2/CH4 ratios lower than 1/4, while they changed slowly at CH4/CO2 ratios above 1/4. In the case of CH4/CO2 = 2/3, energy efficiency of 68 % and syngas energy cost of 1.6 eV/mole were achieved at CH4 conversion of 29 % and CO2 conversion of 22 %.
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A chemical kinetics model is developed for a CO 2 /N 2 microwave plasma, focusing especially on the vibrational levels of both CO 2 and N 2. The model is used to calculate the CO 2 and N 2 conversion as well as the energy efficiency of CO 2 conversion for different power densities and for N 2 fractions in the CO 2 /N 2 gas mixture ranging from 0 to 90%. The calculation results are compared with measurements, and agreements within 23% and 33% are generally found for the CO 2 conversion and N 2 conversion, respectively. To explain the observed trends, the destruction and formation processes of both CO 2 and N 2 are analyzed, as well as the vibrational distribution functions of both CO 2 and N 2. The results indicate that N 2 contributes in populating the lower asymmetric levels of CO 2 , leading to a higher absolute CO 2 conversion upon increasing N 2 fraction. However, the effective CO 2 conversion drops because there is less CO 2 initially present in the gas mixture; thus, the energy efficiency also drops with rising N 2 fraction.
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A non-thermal gliding arc discharge was generated at atmospheric pressure in an air flow. The dynamics of the plasma column and tracer particles were recorded using two synchronized high-speed cameras. Whereas the data analysis for such systems has previously been performed in 2D (analyzing the single camera image), we provide here a 3D data analysis that includes 3D reconstructions of the plasma column and 3D particle tracking velocimetry based on discrete tomography methods. The 3D analysis, in particular, the determination of the 3D slip velocity between the plasma column and the gas flow, gives more realistic insight into the convection cooling process. Additionally, with the determination of the 3D slip velocity and the 3D length of the plasma column, we give more accurate estimates for the drag force, the electric field strength, the power per unit length, and the radius of the conducting zone of the plasma column.
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In this study we report on a 2D fluid model of a gliding arc discharge in argon. Despite the 3D nature of the discharge, 2D models are found to be capable of providing very useful information about the operation of the discharge. We employ two models—an axisymmetric and a Cartesian one. We show that for the considered experiment and the conditions of a low current arc (around 30 mA) in argon, there is no significant heating of the cathode surface and the discharge is sustained by field electron emission from the cathode accompanied by the formation of a cathode spot. The obtained discharge power and voltage are relatively sensitive to the surface properties and particularly to the surface roughness, causing effectively an amplification of the normal electric field. The arc body and anode region are not influenced by this and depend mainly on the current value. The gliding of the arc is modelled by means of a 2D Cartesian model. The arc–electrode contact points are analysed and the gliding mechanism along the electrode surface is discussed. Following experimental observations, the cathode spot is simulated as jumping from one point to another. A complete arc cycle is modelled from initial ignition to arc decay. The results show that there is no interaction between the successive gliding arcs.
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The dissociation of CO2 and the formation of CO, O3, and O2 were studied in a dielectric barrier discharge (DBD) at atmospheric pressure by means of ex-situ infrared absorption spectroscopy. CO mixing ratios of 0.1%–4.4% were determined for specific injected energies between 0.1 and 20 eV per molecule (0.3–70 kJ/l). A lower limit of the gas temperature of 320–480 K was estimated from the wall temperature of the quartz reactor as measured with an infrared camera. The formation of CO in the DBD could be described as function of the total number of transferred charges during the residence time of the gas in the active plasma zone. An almost stoichiometric CO:O2 ratio of 2:1 was observed along with a strongly temperature dependent O3 production up to 0.075%. Although the ideal range for an efficient CO2 dissociation in plasmas of 1 eV per molecule for the specific injected energy was covered, the energy efficiency remained below 5% for all conditions. The present results indicate a reaction mechanism which is initiated by electron impact processes followed by charge transfer reactions and non-negligible surface enhanced O and CO recombination. While electron-driven CO2 dissociation is relatively energy inefficient by itself, fast O recombination and the low gas temperatures inhibit the synergistic reuse of atomic oxygen in a secondary CO2 + O dissociation step.
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The dynamic behavior of an argon gliding arc dis- charge has been investigated by means of electrical diagnostics and high-speed photography. Two different arc breakdown regimes are identified: initial breakdown in the narrowest electrode gap and restrike breakdown on the electrode surface. It is also found that the anode and cathode arc roots exhibit different motion behaviors.
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The long history of plasma application for fuel conversion shows that reasonably low specific energy requirement has been achieved in most cases using non-equilibrium systems with relatively high local temperature ('warm' plasmas). Analysis of reasons for this trend presented in this paper indicates that transitional warm plasma discharge systems are optimal for large-scale fuel processing. This analysis also reveals one specific feature of warm discharges that was not discussed earlier: warm discharge-based plasma-chemical systems are very sensitive to gas temperature and chemical reactions. When temperature reaches the level that is high enough to support chemical reactions in a particular system (ignition temperature), chemical reactions produce high concentration of excited molecules, and these molecules form a basis for stepwise ionization. This results in a significant drop in the energy necessary to support electric discharge in the system for two reasons. First, stepwise ionization that requires relatively low electron energy overcomes direct ionization that is typical for low-temperature non-equilibrium plasmas and requires much higher ionization energy. Second, high temperature of surrounding gas reduces heat losses from the discharge channel, while a significant portion of the discharge energy in warm plasma systems should be spent to compensate these losses. Thus, an intensive chemical reaction, e.g. combustion, supports the existence of a warm electric discharge.
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The process of CO2 dissociation was studied in a non-equilibrium gliding arc plasmatron (GAP). The GAP was designed not only for efficient reforming but also to ensure significant variability of reactor parameters. The effect of vortex flow configuration on efficiency was also studied in the reactor by comparing forward vortex flow and reverse vortex flow. The maximum thermodynamic efficiency of the dissociation process was determined to be approximately 43%. The high level of efficiency may be attributed to non-equilibrium vibrational excitation of CO2 and a high-temperature gradient between gliding arc and the surrounding gas that results in fast quenching.
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This document describes the current formulation of the SST turbulence models, as well as a number of model enhancements. The model enhancements cover a modified near wall treatment of the equations, which allows for a more flexible grid generation process and a zonal DES formulation, which reduces the problem of grid induced separation for industrial flow simulations. Results for a complete aircraft configuration with and without engine nacelles will be shown.
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The aim of this work consists of the evaluation of atmospheric pressure dielectric barrier discharges for the conversion of greenhouse gases into useful compounds. Therefore, pure CO 2 feed flows are administered to the discharge zone at varying discharge frequency, power input, gas temperature and feed flow rates, aiming at the formation of CO and O 2 . The discharge obtained in CO 2 is characterized as a filamentary mode with a microdischarge zone in each half cycle of the applied voltage. It is shown that the most important parameter affecting the CO 2 -conversion levels is the gas flow rate. At low flow rates, both the conversion and the CO-yield are significantly higher. In addition, also an increase in the gas temperature and the power input give rise to higher conversion levels, although the effect on the CO-yield is limited. The optimum discharge frequency depends on the power input level and it cannot be unambiguously stated that higher frequencies give rise to increased conversion levels. A maximum CO 2 conversion of 30% is achieved at a flow rate of 0.05 L min −1 , a power density of 14.75 W cm −3 and a frequency of 60 kHz. The most energy efficient conversions are achieved at a flow rate of 0.2 L min −1 , a power density of 11 W cm −3 and a discharge frequency of 30 kHz.
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CH4–CO2 reforming is of rapid growing interest for reasons of the continuous decrease of petroleum resources and the emphasis on the environmental situation for greenhouse gas mitigation. Plasma technology is considered to be one of potential ways for CH4–CO2 reforming. This paper presents an overview of CH4–CO2 reforming by cold plasmas and thermal plasma. The evaluations for their performances and the key factors in different plasmas are given. In particular, the attention is focused on how to achieve higher conversions at high feed-gas flow rate, so as to lessen the energy consumption in the process by plasma to meet the requirements of industrial application. To obtain the aim, three key factors, electron density, plasma temperature and reactor configuration related to the process are emphasized. Considering the current status of CH4–CO2 reforming by plasma, there is an opportunity to improve the energy conversion efficiency and the treatment capacity of the process by optimizing both plasma form and reactor design in future work.
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Low temperature plasmas are gaining a lot of interest for environmental and energy applications. A large research field in these applications is the conversion of CO2 into chemicals and fuels. Since CO2 is a very stable molecule, a key performance indicator for the research on plasma-based CO2 conversion is the energy efficiency. Until now, the energy efficiency in atmospheric plasma reactors is quite low, and therefore we employ here a novel type of plasma reactor, the Gliding Arc Plasmatron (GAP). This paper provides a detailed experimental and computational study of the CO2 conversion, as well as the energy cost and efficiency in a GAP. A comparison with thermal conversion, other plasma types and other novel CO2 conversion technologies is made to find out whether this novel plasma reactor can provide a significant contribution to the much-needed efficient conversion of CO2. From these comparisons it becomes evident that our results currently obtained are less than a factor two away from being cost competitive and already outperform several other novel technologies. Furthermore, we indicate how the performance of the GAP can still be improved by further exploiting its non-equilibrium character. Hence, it is clear that the GAP is very promising for CO2 conversion.
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CO2 conversion into value-added chemicals is gaining increasing interest in recent years, and a gliding arc plasma has great potential for this purpose, because of its high energy efficiency. In this study, a chemical reaction kinetics model is presented to study the CO2 splitting in a gliding arc discharge. The calculated conversion and energy efficiency are in good agreement with experimental data in a range of different operating conditions. Therefore, this reaction kinetics model can be used to elucidate the dominant chemical reactions contributing to CO2 destruction and formation. Based on this reaction pathway analysis, the restricting factors for CO2 conversion are figured out, i.e., the reverse reactions and the small treated gas fraction. This allows us to propose some solutions in order to improve the CO2 conversion, such as decreasing the gas temperature, by using a high frequency discharge, or increasing the power density, by using a micro-scale gliding arc reactor, or by removing the reverse reactions, which could be realized in practice by adding possible scavengers for O atoms, such as CH4. Finally, we compare our results with other types of plasmas in terms of conversion and energy efficiency, and the results illustrate that gliding arc discharges are indeed quite promising for CO2 conversion, certainly when keeping in mind the possible solutions for further performance improvement.
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The effect of introducing a photocatalytically active coating inside a plasma unit is investigated. This technique combines the advantages of high product selectivity from catalysis and the fast start-up from plasma technology. In this study, a preselected TiO2 coating is applied on the collector electrode of a DC corona discharge unit as NTP reactor, in order to study the oxidation of ethylene. For both positive and negative polarities an enhanced mineralisation is observed while the formation of by-products drastically decreases. The plasma catalytic unit gave the best results when using negative polarity at a voltage of 15 kV. This shows the potential of plasma catalysis as indoor air purification technology.
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Power-to-gas is a storage technology aiming to convert surplus electricity from renewable energy sources like wind and solar power into gaseous fuels compatible with the current network infrastructure. Results of CO2 dissociation in a vortex-stabilized microwave plasma reactor are presented. The microwave field, residence time, quenching, and vortex configuration were varied to investigate their influence on energy- and conversion efficiency of CO2 dissociation. Significant deterioration of the energy efficiency is observed at forward vortex plasmas upon increasing pressure in the range of 100 mbar towards atmospheric pressure, which is mitigated by using a reverse vortex flow configuration of the plasma reactor. Data from optical emission shows that under all conditions covered by the experiments the gas temperature is in excess of 4000 K, suggesting a predominant thermal dissociation. Different strategies are proposed to enhance energy and conversion efficiencies of plasma-driven dissociation of CO2.
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In this computational study, a gliding arc plasma reactor with a reverse-vortex flow stabilization is modelled for the first time by a fluid plasma description. The plasma reactor operates with argon gas at atmospheric pressure. The gas flow is simulated using the k-ϵ Reynolds-averaged Navier-Stokes turbulent model. A quasi-neutral fluid plasma model is used for computing the plasma properties. The plasma arc movement in the reactor is observed, and the results for the gas flow, electrical characteristics, plasma density, electron temperature, and gas temperature are analyzed.
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The use of plasma technology for CO2 splitting is gaining increasing interest, but one of the major obstacles to date for industrial implementation is the considerable energy cost. We demonstrate that the introduction of a packing of dielectric zirconia (ZrO2) beads into a dielectric barrier discharge (DBD) plasma reactor can enhance the CO2 conversion and energy efficiency up to a factor 1.9 and 2.2, respectively, compared to that in a normal (unpacked) DBD reactor. We obtained a maximum conversion of 42 % and a maximum energy efficiency of 9.6 %. However, it is the ability of the packing to almost double both the conversion and the energy efficiency simultaneously at certain input parameters that makes it very promising. The improved conversion and energy efficiency can be explained by the higher values of the local electric field and electron energy near the contact points of the beads and the lower breakdown voltage, demonstrated by 2 D fluid modeling.
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The strong non-equilibrium conditions provided by the plasma phase offer the opportunity to beat traditional thermal process energy efficiencies via preferential excitation of molecular vibrations. Simple molecular physics considerations are presented to explain potential dissociation pathways in a plasma and their effect on energy efficiency. A common microwave reactor approach is evaluated experimentally with Rayleigh scattering and Fourier transform infrared spectroscopy to assess gas temperatures (exceeding 104 K) and conversion degrees (up to 30%), respectively. The results are interpreted on basis of estimates of the plasma dynamics obtained with electron energy distribution functions calculated with a Boltzmann solver. It indicates that the intrinsic electron energies are higher than is favorable for preferential vibrational excitation due to dissociative excitation, which causes thermodynamic equilibrium chemistry to dominate. The highest observed energy efficiencies of 45% do indicate that still non-equilibrium dynamics had been at play. A novel approach involving additives of low ionization potential to tailor the electron energies to the vibrational excitation regime is proposed.
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Decomposition of carbon dioxide (CO2) by gliding arc plasma was examined. The plasma device consisted of two or four triangular stainless steel plates. The gas enters through a nozzle tube from the upper part of the cylindrical reactor and exits at the bottom part of the reactor. Some additional gases, such as N2, O2, air, and H2O, have been used to study the dilution effects on CO2 conversion. A simple kinetic reaction model was developed to investigate the pathways of plasma reaction. Experimental results indicate that the conversion of pure CO 2 reaches 18% at the total gas flow rate of 0.85 L/min and produces CO and O2 as the final products. The effects of additional gases injection are described in detail in this paper.
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This unique book provides a fundamental introduction to all aspects of modern plasma chemistry. The book describes mechanisms and kinetics of chemical processes in plasma, plasma statistics, thermodynamics, fluid mechanics, and electrodynamics, as well as all major electric discharges applied in plasma chemistry. The book considers most of the major applications of plasma chemistry from electronics to thermal coatings, from treatment of polymers to fuel conversion and hydrogen production, and from plasma metallurgy to plasma medicine. The book can be helpful to engineers, scientists, and students interested in plasma physics, plasma chemistry, plasma engineering, and combustion, as well as in chemical physics, lasers, energy systems, and environmental control. The book contains an extensive database on plasma kinetics and thermodynamics as well as a lot of convenient numerical formulas for practical calculations related to specific plasma-chemical processes and applications. The book contains a large number of problems and concept questions that are helpful in university courses related to plasma, lasers, combustion, chemical kinetics, statistics and thermodynamics, and high-temperature and high-energy fluid mechanics.
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This paper reports on the study of repetitive nanosecond-pulsed discharge splitting of carbon dioxide (CO2) for the production of CO. Gas chromatography is used to analyze the composition of the reformed gas when CO2 is exposed to high-voltage (15 kV) very short (10 ns) electrical discharges that deposit as much as 0.4 mJ of energy at a rate of 30 kHz. Conversion rate and energy efficiency are obtained while the discharge pressure is varied between 2.4 and 5.1 atm. At the tested conditions, the maximum conversion rate and energy efficiency are found to be 7.3% and 11.5%, respectively. The energy efficiency drops slightly with increased pressure because of the decreased electric field and electron energy per molecule. An energy balance analysis of a set of CO2 plasma reactions reveals that the dominant dissociation pathway under these conditions passes through the excitation of CO2 (10.5 eV) followed by autodissociation into CO and O, which are often in excited states.
Article
This paper demonstrates that the CO2 conversion in a dielectric barrier discharge rises drastically upon addition of Ar or He, and the effect is more pronounced for Ar than for He. The effective CO2 conversion, on the other hand, drops upon addition of Ar or He, which is logical due to the lower CO2 content in the gas mixture, and the same is true for the energy efficiency, because a considerable fraction of the energy is then consumed into ionization/excitation of Ar or He atoms. The higher absolute CO2 conversion upon addition of Ar or He can be explained by studying in detail the Lissajous plots and the current profiles. The breakdown voltage is lower in the CO2/Ar and CO2/He mixtures, and the discharge gap is more filled with plasma, which enhances the possibility for CO2 conversion. The rates of electron impact excitation–dissociation of CO2, estimated from the electron densities and mean electron energies, are indeed higher in the CO2/Ar and (to a lower extent) in the CO2/He mixtures, compared to the pure CO2 plasma. Moreover, charge transfer between Ar+ or Ar2+ ions and CO2, followed by electron-ion dissociative recombination of the CO2+ ions, might also contribute to, or even be dominant for the CO2 dissociation. All these effects can explain the higher CO2 conversion, especially upon addition of Ar, but also upon addition of He.
Article
Plasma technology is gaining increasing interest for the splitting of CO2 into CO and O2. We have performed experiments to study this process in a dielectric barrier discharge (DBD) plasma with a wide range of parameters. The frequency and dielectric material did not affect the CO2 conversion and energy efficiency, but the discharge gap can have a considerable effect. The specific energy input has the most important effect on the CO2 conversion and energy efficiency. We have also presented a plasma chemistry model for CO2 splitting, which shows reasonable agreement with the experimental conversion and energy efficiency. This model is used to elucidate the critical reactions that are mostly responsible for the CO2 conversion. Finally, we have compared our results with other CO2 splitting techniques and we identified the limitations as well as the benefits and future possibilities in terms of modifications of DBD plasmas for greenhouse gas conversion in general.
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A crucial step in the plasma splitting of carbon dioxide is the separation of the conversion products, and this is not straightforward, especially for separating O2 from CO2 and CO. In this work the trapping of atomic oxygen by adding a hydrogen source, which enhances the chemical conversion into water, is demonstrated. The experimental and modelling results show that by adding 3% of H2 and 2% of CH4 most of the oxygen can be trapped at a CO2 conversion of ±2.5%. The identified products formed by the addition of CH4 or H2 are mainly H2O and in the case of CH4 also H2. Adding a hydrogen source thus leads to the removal of O2, leaving behind a gas mixture that can be more easily separated.
Article
We present a zero-dimensional kinetic model of CO2 splitting in non-equilibrium plasmas. The model includes a description of the CO2 vibrational kinetics (25 vibrational levels up to the dissociation limit of the molecule), taking into account state-specific VT and VV relaxation reactions and the effect of vibrational excitation on other chemical reactions. The model is applied to study the reaction kinetics of CO2 splitting in an atmospheric-pressure dielectric barrier discharge (DBD) and in a moderate-pressure microwave discharge. The model results are in qualitative agreement with published experimental works. We show that the CO2 conversion and its energy efficiency are very different in these two types of discharges, which reflects the important dissociation mechanisms involved. In the microwave discharge, excitation of the vibrational levels promotes efficient dissociation when the specific energy input is higher than a critical value (2.0 eV/molecule under the conditions examined). The calculated energy efficiency of the process has a maximum of 23%. In the DBD, vibrationally excited levels do not contribute significantly to the dissociation of CO2 and the calculated energy efficiency of the process is much lower (5%).
Article
Conversion of CO2 into CO and O is studied in a flowing gas surfaguide pulsed microwave discharge operating with CO2 and CO2 + N2 gas mixtures under different conditions. Optical emission spectroscopy, including actinometry (using N2), vibrational (N2 molecule) and rotational (CO and N2 molecules) analysis are utilized. Both time- and space-resolved measurements are performed. The results show the essential changes of the CO2 conversion rate, its energetic efficiency, and the gas and vibrational temperatures along the gas flow direction in the discharge. The spatial distribution of the power absorbed in the plasma is analyzed. It is also confirmed that the vibrational excitation is a key factor in the CO2 dissociation process in this type of plasma. It is suggested that the obtained dissociation rates can be further optimized by varying the gas composition, as well as the power applied to the discharge.
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An alternating-current (AC) gliding arc reactor has been developed offering a new route for the co-generation of syngas and value-added carbon nanomaterials by plasma dry reforming of methane. Different carbon nanostructures including spherical carbon nanoparticles, multi-wall carbon nanotubes and amorphous carbon have been obtained as by-products of syngas generation in the plasma system. Optical emission spectra of the discharge demonstrate the formation of different reactive species (Al, CO, CH, C-2, H-alpha, H-beta and O) in the plasma dry reforming reaction. The effect of different operating parameters (feed flow rate, input power and CH4/CO2 molar ratio) on the performance of the plasma process has been evaluated in terms of the conversion of feed gas, product selectivity and energy conversion efficiency. It is interesting to note that gliding arc plasma can be used to generate much cleaner gas products of which syngas is the main one. The results also show that the energy efficiency of dry reforming using gliding arc plasma is an order of magnitude higher than that for processing using dielectric barrier or corona discharges. Both of these can be attributed to the higher electron density in the order of 10(23) m(-3) generated in the gliding arc plasma. Copyright
Article
The continual and increasing use of fossil fuels throughout the world has advanced concerns of atmospheric carbon dioxide (CO2) concentrations, causing a swell of scientific interest to ease the predicted effects of global warming. This work experimentally investigates the conversion of CO2 to carbon monoxide (CO) and oxygen in an atmospheric pressure microwave plasma/catalyst system. Diagnostics such as mass spectrometry and optical emission spectroscopy are used to identify the gas species present after plasma treatment and to measure plasma temperatures. The CO2 gas is first treated with plasma alone, and is then treated with a combination of plasma and rhodium (Rh) catalyst material. While the plasma system alone is able to achieve a 20% energy efficiency, the Rh catalyst actually causes a drop in efficiency due to reverse reactions occurring on the surface. The plasma temperature measurements indicate thermal equilibrium between Tr and Tv around 6000–7000 K.
Article
In this paper, the splitting of CO2 in a pulsed plasma system, such as a dielectric barrier discharge (DBD), is evaluated from a chemical point of view by means of numerical modeling. For this purpose, a chemical reaction set of CO2 in an atmospheric pressure plasma is developed, including the vibrational states of CO2, O-2, and CO. The simulated pulses are matched to the conditions of a filament (or microdischarge) and repeated with intervals of 1 mu s. The influence of vibrationally excited CO2 as well as other neutral species, ions, and electrons on the CO2 splitting is discussed. Our calculations predict that the electrons have the largest contribution to the CO2 splitting at the conditions under study, by electron impact dissociation. The contribution of vibrationally excited CO2 levels in the splitting of CO2 is found be 6.4%, when only considering one microdischarge pulse and its afterglow, but it can be much higher for consecutive discharge pulses, as is typical for a filamentary DBD, when the interpulse time is short enough and accumulation effects in the vibrationally excited CO2 densities can occur.
Article
The decomposition of CO2 in a dielectric packed-bed plasma reactor has been studied. It was found that the dielectric properties and morphology of packing dielectric pellets play important roles in the reaction due to their influence on the electron energy distribution in the plasma. The acid–base properties of the packing materials also affect the reaction through the chemisorption of CO2 on basic sites of the materials. Heterogeneous reactions on the solid surfaces of the dielectric materials also play a role in the reaction, which was also confirmed through the investigation of the influence of the discharge length on the reaction. The reverse reaction of CO2 decomposition, the oxidation of CO, was also investigated to further understand the role of dielectric materials in the plasma and their effect on plasma reactions. Both the decomposition of CO2 and the oxidation of CO in non-packed or dielectric packed reactors are first-ordered.
Article
One possible solution to mitigating the effects of high atmospheric concentrations of carbon dioxide (CO2) is the use of a plasma source to break apart the molecule into carbon monoxide (CO) and oxygen. This work experimentally investigates the efficiency of dissociation of CO2 in a 1-kW radio-frequency (rf) plasma source operating at 13.56-MHz in a low-pressure discharge. Mass spectrometry diagnostics are used to determine the species present in the discharge, and these measurements are used to calculate the energy efficiency and conversion efficiency of CO2 dissociation in the rf plasma source. Experimental results have found that the conversion efficiency of CO2 to CO can reach values near 90%, however energy efficiency reaches a maximum of 3%. A theoretical energy cost analysis is also given as a method to evaluate the effectiveness of any plasma system designed for CO2 emissions reduction.
Article
The results of experimental and theoretical studies of chemical reactions in nonequilibrium plasmas are reviewed. Special attention is given to processes stimulated by vibrational excitation of the ground electronic state of the reacting molecules in the plasma. General patterns in the kinetics of these reactions are discussed, and the optimum discharge parameters for maximum energy efficiency are noted. Specific plasma-chemical processes—the dissociation of CO2 and H2O and the synthesis of nitrogen oxides—are described. Experimental results are presented for hf, microwave, glow, plasma-beam, and non-self-sustained discharges, for plasma radiolysis, etc.
Article
On board hydrogen production out of hydrocarbons (reforming) for fuel cells feed is subject to problems when used with traditional catalysts. High device weight, a relatively long transient time and poisoning problems make the integration onboard a vehicle complex. In response to these challenges, hydrocarbons reforming processes assisted by non-thermal plasmas for hydrogen production have been implemented over recent years. This paper aims to provide an overview of the setting up, feasibility and efficiency of the existing technologies here investigated. This state-of-the-art technology review explains the key characteristics of plasma reforming through various original approaches. The performances of some of the systems are then compared against each other and discussed.
Article
Methane conversion using gliding arc plasma has been studied. The process was conducted at atmospheric pressure. Four kinds of additive gases—helium, argon, nitrogen, and CO2—were used to investigate their effects on methane conversion, as well as product selectivity, and discharged power. Methane conversion was increased with the increasing concentration of helium, argon, and nitrogen in the feed gas but decreased when CO2 concentration increased. Qualitatively, hydrogen and acetylene were the major gas products. No liquid product was produced.
Article
This paper attempts to give an overview of gas discharge plasmas in a broad perspective. It is meant for plasma spectroscopists who are familiar with analytical plasmas (glow discharges, ICPs and microwave discharges), but who are not so well aware of other applications of these and related plasmas. In the first part, an overview will be given of the various types of existing gas discharge plasmas, and their working principles will be briefly explained. In the second part, the most important applications will be outlined.
Article
Numerous indoor sources emit volatile organic compounds, NOx and O3 which all have a negative effect on human health. Previous work proved that plasma technology seems promising. However, some disadvantages occur such as high energy cost and the formation of by-products.In this work in-plasma (IPC) and post-plasma (PPC) catalysis was investigated for indoor air purification. Introducing a TiO2 catalyst in-plasma was not effective in ozone reduction. Adding only 10 g of CuOMnO2/TiO2 post-plasma position, resulted in a reduction of the ozone outlet concentration by a factor 7. Humidity proved to have a limiting effect on ozone removal rates.In dry air toluene (Cin = 0.5 ppmv) was removed three times more efficient by introducing Aerolyst®7706 TiO2 (IPC). In humid conditions (RH = 27%) performance of this IPC decreased: toluene abatement was only 1.5 times more efficient compared with the non-catalytic plasma oxidation. For dry air inserting 10 g CuOMnO2/TiO2 down flow the plasma reactor modules (PPC), resulted in toluene removal efficiencies up to 40 times higher. In humid gas streams, PPC toluene removal efficiency decreased by competitive adsorption.NOx production by the corona discharge was monitored. In dry air and for an energy density of 10 J L−1, the NO2 outlet concentration was 1500 ppbv, while this is three times lower at 50% RH. Both heterogeneous catalysts (TiO2 and CuOMnO2/TiO2) proved capable to reduce NOx levels in the outlet gas stream by up to 90%.Deactivation of the catalyst material may be explained by the formation of HNO3 in the plasma discharge. These molecules adsorb on the catalyst, surface nitrate ion concentrations were 0.184 mg/m2 and 0.143 mg/m2 for TiO2 and MnO2–CuO/TiO2 catalysts, respectively.
Article
Conversion of carbon dioxide (CO2) using gliding arc plasma was performed. The research was done to investigate the effect of variation of total gas flow rates and addition of auxiliary gases--N2, O2, air, water--to the CO2 conversion process. This system shows higher power efficiency than other nonthermal plasma methods. Experiment results indicate the conversion of CO2 reaches 18% at total gas flow rate of 0.8 L/min and produces CO and O2 as the main gaseous products. Among auxiliary gases, only N2 gives positive effect on CO2 conversion and the power efficiency at N2 concentration of 95% and total gas flow rate of 2 L/min increases about three times compared to pure CO2 process.
CO 2 conversion in a microwave plasma reactor in the presence of N 2 : modelling and experimental validation
  • S Heijkers
  • R Snoeckx
  • T Kozák
  • T Silva
  • T Godfroid
  • N Britun
  • R Snyders
  • A Bogaerts
Heijkers S, Snoeckx R, Kozák T, Silva T, Godfroid T, Britun N, Snyders R and Bogaerts A 2015 CO 2 conversion in a microwave plasma reactor in the presence of N 2 : modelling and experimental validation O-15-6 O-15-6
  • A Indarto
  • J-W Choi
  • H Lee
  • H Song
Indarto A, Choi J-W, Lee H and Song H K 2006 Conversion of CO 2 by gliding arc plasma Environ. Eng. Sci. 23 1033-43
Non-equilibrium plasma-chemical process of CO 2 decomposition in a supersonic microwave discharge
  • R I Asisov
  • A K Vakar
  • V K Jivotov
  • M F Krotov
  • O A Zinoviev
  • B V Potapkin
  • A A Rusanov
  • V Rusanov
  • A A Fridman
Asisov R I, Vakar A K, Jivotov V K, Krotov M F, Zinoviev O A, Potapkin B V, Rusanov A A, Rusanov V D and Fridman A A 1983 Non-equilibrium plasma-chemical process of CO 2 decomposition in a supersonic microwave discharge Proc. USSR Acad. Sci. 271 94-7