Plasma driven catalysis is a promising technology for waste gas treatment characterized by higher energy efficiencies, high mineralization rates and low by-product formation. The combination of heterogeneous catalysts with non-thermal plasma can be operated in two configurations: positioning the catalyst in the discharge zone (in-plasma catalysis) or downflow the discharge zone (post plasma catalysis). In a first part of the review, changes of plasma properties resulting from the introduction of catalyst material are discussed. It has been reported that discharge types can even change. Accordingly, it was reported that microdischarges are formed within the catalyst pores. Changing plasma characteristics can eventually result in enhanced production of new active species, increasing the oxidizing power of the plasma discharge. In a second part, it is discussed that plasma discharges also affect catalyst properties such as a change in chemical composition, enhancement in surface area or change of catalytic structure. These phenomena partially explain why catalyst adsorption kinetics of airborne pollutants are affected when exposed to plasma discharges. It is also reviewed that the synergy of combining plasma with catalysts can not only be attributed to the production of new reactive species. Also plasma photon emission or thermal hot-spots can initiate catalytic pollutant oxidation reactions. To conclude, an overview of recently published manuscripts concerning plasma catalysis for volatile organic compounds abatement is given. It is also discussed why heterogeneous plasma catalysis has high potential for the simultaneous abatement of NO x and hydrocarbons.

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Combining non-thermal plasma with heterogeneous catalysis in waste gas treatment: A review

In this paper, we give a detailed review of recent work carried out on the numerical characterization of non-thermal gas discharge plasmas in air at atmospheric pressure. First, we briefly describe the theory of discharge development for dielectric barrier discharges, which is central to the production of non-equilibrium plasma, and we present a hydrodynamic model to approximate the evolution of charge densities. The model consists of the continuity equations for electrons, positive and negative ions coupled to Poisson's equation for the electric field. We then describe features of the finite element flux corrected transport algorithm, which has been developed to specifically aim for accuracy (no spurious diffusion or oscillations), efficiency (through the use of unstructured grids) and ease of extension to complex 3D geometries in the framework of the hydrodynamic model in gas discharges. We summarize the numerical work done by other authors who have applied different methods to various models and then we present highlights of our own work, which includes code validation, comparisons with existing results and modelling of radio frequency systems, dc discharges, secondary effects such as photoionization and plasma production in the presence of dielectrics. The extension of the code to 3D for more realistic simulations is demonstrated together with the adaptive meshing technique, which serves to achieve higher efficiency. Finally, we illustrate the versatility of our scheme by using it to simulate the transition from non-thermal to thermal discharges.We conclude that numerical modelling and, in particular, the extension to 3D can be used to shed new light on the processes involved with the production and control of atmospheric plasma, which plays an important role in a host of emerging technologies.

https://iopscience.iop.org/article/10.1088/0022-3727/38/20/R01/pdf

Numerical modelling of atmospheric pressure gas discharges leading to plasma production

In this paper, a generalized leader inception model is proposed. It is based on an iterative geometrical analysis of the background potential distribution of an earthed structure to simulate the first meters of propagation of an upward connecting leader. By assuming a static field approach, the leader stabilization fields and the striking distances were computed for a lightning rod and for a building. The obtained results were compared with the existing leader inception criteria. Furthermore, in order to validate the model, the leader inception condition was computed for a triggered lightning experiment. Excellent agreement with the experimental results was obtained. The present model has several advantages in comparison with the existing leader inception criteria. One of them is related to the fact that the proposed model can be used to analyze the effect of the space charge on the upward leader inception.

A simplified physical model to determine the lightning upward connecting leader inception

A critical analysis of photoionization as the major process for seed electron production ahead of the cathode-directed streamer has been made. The accumulation of ions between pulses and fast electron detachment in an electric field could be an effective source of seed electrons for repetitively pulsed discharges in electronegative gases. Measurements and two-dimensional calculations in the hydrodynamic approximation of streamer parameters in air (anode current, propagation velocity and radiative channel diameter) can only agree if one assumes uniform background pre-ionization and not by taking into account only the photoionization process generally adopted for pulsed discharges in air. The results of analytical estimates and full three-dimensional simulations confirm that the seed charge distribution in the undisturbed gap has an effect on the streamer head formation and leads to the streamer branching phenomena.

Role of electronegative gas admixtures in streamer start, propagation and branching phenomena

In this review we describe a transient type of gas discharge which is commonly called a streamer discharge, as well as a few related phenomena in pulsed discharges. Streamers are propagating ionization fronts with self-organized field enhancement at their tips that can appear in gases at (or close to) atmospheric pressure. They are the precursors of other discharges like sparks and lightning, but they also occur in for example corona reactors or plasma jets which are used for a variety of plasma chemical purposes. When enough space is available, streamers can also form at much lower pressures, like in the case of sprite discharges high up in the atmosphere.
We explain the structure and basic underlying physics of streamer discharges, and how they scale with gas density. We discuss the chemistry and applications of streamers, and describe their two main stages in detail: inception and propagation. We also look at some other topics, like interaction with flow and heat, related pulsed discharges, and electron runaway and high energy radiation. Finally, we discuss streamer simulations and diagnostics in quite some detail.
This review is written with two purposes in mind: First, we describe recent results on the physics of streamer discharges, with a focus on the work performed in our groups. We also describe recent developments in diagnostics and simulations of streamers. Second, we provide background information on the above-mentioned aspects of streamers. This review can therefore be used as a tutorial by researchers starting to work in the field of streamer physics.

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The physics of streamer discharge phenomena

By measuring the currents associated with the streamer discharge along silicone rubber surfaces, parameters of streamer propagation such as the minimum field of streamer crossing, the field of stable streamer propagation, the mean velocity and the streamer charge distribution have been analyzed and compared to the streamer discharge in air alone. Clear differences were observed in the measured currents for the individual surfaces at low background fields (285 kV/m). For higher fields the streamer crosses the gap almost independently of the surface type. The minimum streamer field was found to be slightly increased compared to air. The field of stable streamer propagation also was higher than in air. It is /spl sim/ 570 kV/m, larger than that of the streamer discharge in air (/spl sim/ 500), under the same conditions. The streamer speed was found slightly increased in the presence of the silicone rubber surface and the distinction between the individual surfaces was modest. A discussion on possible mechanisms for the observed differences in the streamer speed and currents with and without the insulator surfaces is presented. The net positive charge of the streamer along an insulating surface seems to be distributed along the streamer channel rather than localized in the front part of the channel as the case for the streamer in air.

Positive streamer discharges along insulating surfaces

3D Simulations of Streamer Branching in Air

The results of numerical simulations of a positive streamer development in air in a weak and uniform electric field are presented. Streamer dynamics is considered in an electrode gap of 33 mm in length and the configuration was 'protrusion on a plate-plate'. This particular configuration was chosen in order to perform direct comparison between simulated results and experimental data. The electrostatic field in such a system decreases rapidly with increase of the distance from the protrusion (anode) and the region with a weak and uniform background field covers similar to 30 mm of the gap. The parameters of the propagating streamer are studied at six different values of the background field strength: 0.24; 0.30; 0.345; 0.37; 0.43 and 0.50 MV m(-1). Stable streamer development (with constant velocity) takes place in a field of 0.5 MV m(-1) (the stability field) but the streamer is able to cross the gap in a background field of 0.3 MV m(-1). These values are in excellent agreement with experimental data. During the stable streamer propagation, the electron density and plasma conductivity in the discharge channel and the electric field at its front remain constant. In a background field lower than 0.5 MV m(-1), the discharge front velocity and the electric field at the front decrease linearly with an increase of streamer length. The discharge propagation in the stability field is associated with an increase of electrostatic energy at the streamer front but it decreases if the streamer develops in a weaker electric field. This behaviour is accompanied by a constant Joule dissipation at 0.5 MV m(-1) and decreasing energy losses at the streamer front in a weaker background electric field. (Less)

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The propagation of positive streamers in a weak and uniform background electric field

The Franklin lightning conductor: conditions necessary for the initiation of a connecting leader

To study the performance of compact systems for microwave generation, a series of experiments have been performed with a microwave source powered directly by a Marx generator. The system consists of a 20-stage 400-kV/400-J Marx generator, a powerful 40-kV charger, a reflex triode, and a vircator-type microwave source. Different parametric studies were performed such as variation of the anode-cathode distances and the emitting area of the cathode. The results have been analyzed and compared to an equivalent electric circuit model of the system. The experiments, generating microwave frequencies between 3 and 5 GHz, can be fairly accurately reproduced by the model both in terms of discharge currents and microwave frequencies

Mose Akyuz

Papers

Positive streamer discharges along insulating surfaces

3D Simulations of Streamer Branching in Air

The propagation of positive streamers in a weak and uniform background electric field

The Franklin lightning conductor: conditions necessary for the initiation of a connecting leader

Study of a Compact HPM System With a Reflex Triode and a Marx Generator