Gliding arc gas discharge

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Abstract

The sliding arc discharge starts at the shortest distance between the electrodes, then moves with the gas flow at a velocity of about 10 m/s and the length l of the arc column increases together with the voltage. When the length of the gliding arc exceeds its critical value lcrit, heat losses from the plasma column begin to exceed the energy supplied by the source, and it is not possible to sustain the plasma in a state of thermodynamic equilibrium. As a result, a fast transition into a non-equilibrium phase occurs. The discharge plasma cools rapidly to a gas temperature of about T0=1000 K and the plasma conductivity is maintained by a high value of the electron temperature Te=1 eV (about 11 000 K). After this fast transition, the gliding arc continues its evolution, but under non-equilibrium conditions (TeT0). The specific heat losses Wcrit in this regime are much smaller than in the equilibrium regime (numerically about three times less). The discharge length increases up to a new critical value of l≅3lcrit. The main part of the gliding arc power (up to 75–80%) can be dissipated in the non-equilibrium zone. After the decay of the non-equilibrium discharge, the evolution repeats from the initial break-down. This permits the stimulation of chemical reactions in regimes quite different from conventional combustion and environmental situations. It provides an alternative approach to addressing energy conservation and environmental control. In the first part of this paper, the gas discharge physics are described. The second part reviews the chemical reaction in the gliding arc plasma and some possible applications.

Introduction

The number of industrial applications of plasma technologies is extensive and involves many industries [1], [2], [3], [4], [5], [6], [7]. High energy efficiency, specific productivity, and selectivity may be achieved in plasmas for a wide range of chemical processes. As an example, for CO2 dissociation in non-equilibrium plasmas under supersonic flow conditions, it is possible to selectively introduce up to 90% of total discharge power in CO production when the vibrational temperature is about 4000 K and the translational temperature is only 0(100) K [7], [9], [10], [11]. The specific productivity of a such a supersonic reactor achieves 1 000 000 l/h, with power levels up to 1 MW [7], [8]. The key point for practical use of any chemical process in a particular plasma system is to find the proper regime and optimal plasma parameters among the numerous possibilities intrinsic to systems far from equilibrium. In particular it is desired to provide high operating power for the plasma chemical reactor together with high selectivity of the energy input while simultaneously maintaining non-equilibrium plasma conditions [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15].

Generally, two very different kinds of plasmas are used for chemical applications. Thermal plasma generators have been designed for many diverse industrial applications covering a wide range of operating power levels from less than 1 kW to over 50 MW. However, in spite of providing sufficient power levels, these are not well adapted to the purposes of plasma chemistry, where selective treatment of reactants (through the excitation of molecular vibrations or electron excitation) and high efficiency are required. The main drawbacks of using thermal plasmas for plasma chemical applications are overheating of the reaction media when energy is uniformly consumed by the reagents into all degrees of freedom, and hence high energy consumption required to provide special quenching of the reagents, etc. Because of these drawbacks, the energy efficiency and selectivity of such systems are rather small (only one of many plasma chemical processes developed in the first decades of the century remains, i.e. the production of acetylene from light hydrocarbons in Germany, Hülls).

An alternative approach for plasma chemical gas processing is the non-thermal one. Silent, glow, corona, short pulse, microwave or radio frequency (RF) electrical discharges are directly produced in the processed gas, mostly under low pressure [7], [10], [13], [14], [15]. The glow discharge in a low pressure gas is a simple and inexpensive way to achieve a non-thermal plasma [13]. Here, the ionization processes induced by the electric field dominate the thermal ones and give relatively high energy electrons as well as excited ions, atoms and molecules which promote selective chemical transitions [7], [11]. However, the power of glow discharges is limited by the glow to arc transition [13], [15]. Gas, initially below 1000 K, becomes hot (>6000 K), and the electron temperature, initially high enough (>12 000 K), cools close to the gas temperature. The discharge voltage decreases during such a transition making it necessary to increase the current in order to keep the power at the same level which in turn leads to thermalization of the gas. Thus, cold non-equilibrium plasmas created by conventional glow discharges offer good selectivity and efficiency, but at limited pressure and power levels.

In these two general types of plasma discharges, it is impossible to simultaneously keep a high level of non-equilibrium, high electron temperature and high electron density, whereas most prospective plasma chemical applications simultaneously require high power for high reactor productivity and a high degree of non-equilibrium to support selective chemical processes.

These parameters are somewhat achievable in microwave discharges [7], [10]. The skin effect here permits simultaneous achievement of a high level of electron density and a high electric field (and hence a high electronic temperature as well) in the relatively cold gas. Existing super high frequency discharge technology can be used to generate dense (log ne (electrons/cm3)=13) non-equilibrium plasmas (Te=1–2 eV, Tv=3000–5000 K, T0=800–1500 K, for supersonic flow T0≤150 K and less) at pressures up to 200–300 torr and at power levels reaching 1 MW [7].

Recently, a simpler technique offering similar advantages has been proposed [16], the gliding arc. Such a gliding arc occurs when the plasma is generated between two or more diverging electrodes placed in a fast gas flow. It operates at atmospheric pressure or higher and the dissipated power at non-equilibrium conditions reaches 40 kW per electrode pair. The incontestable advantage of the gliding arc compared with microwave systems is its cost; it is much less expensive compared to microwave plasma devices.

The description of this new plasma discharge and its possible applications is the main objective of the present paper. In the first part of this paper, we provide the physical description of the gliding arc plasma. Attention is paid to the evolution of the electrical characteristics of the gliding arc. The models of the equilibrium zone, non-equilibrium zone and of the phenomenon of fast transition into non-equilibrium are discussed. The mechanisms of that transition are explained on the basis of typical instabilities occurring due to the slow increase in the electric field during the gliding arc evolution. The plasma chemical applications of the gliding arc such as natural gas processing, waste and exhaust treatment, hydrogen sulfide dissociation, etc., will be considered in the second part.

Section snippets

Gliding arc physical phenomenon

Let us consider a simple case of a direct current gliding arc in air [16], driven by two generators. A typical electrical scheme of the circuit is shown in Fig. 1. The gliding arc consists of a high voltage generator (up to 5000 V) used to ignite the discharge and a second power generator (with a voltage up to 1 kV, and a total current J up to 60 A). A variable resistor (R=0–25 Ω) is in series with a self-inductance having a typical value L=25 mH. In principle, more advanced schemes, such as an AC

Chemical reactions in gliding arc plasma

Gliding arc plasma reactors can process directly, at negligible pressure drop, different gases (Ar, air, water vapor, O2, H2, N2, H2S, CO, CO2, hydrocarbons, and their mixtures), preheated and cold, in the pressure range 0.5–5 atm.

As was shown, the electrical energy is directly introduced into the reaction volume to create a non-equilibrium and very reactive environment for promoting the chemical transformations of interest. Up to 80% of the electrical energy may be directly absorbed by

Conclusion

It was shown that the atmospheric pressure, high power gliding arc discharge has a complicated time–space structure including quasi-equilibrium and non-equilibrium periods, as well as a fast transition FENETRe (window) between them, where the specific ionization instability creates the conditions for supporting a relatively cold plasma.

The initial quasi-equilibrium plasma of the gliding arc is stable, takes a relatively small part of the total power, and provides the preliminary plasma

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