Z. Zakrzewski

CFC-11 destruction by microwave torch generated atmospheric-pressure nitrogen discharge

A novel plasma method and its application for destruction of Freons using a moderate-power (several hundred watts) microwave torch discharge (MTD) in atmospheric-pressure flowing nitrogen are presented. The capability of the MTD to decompose Freons is demonstrated using a chlorofluorocarbon CCl3F (Freon CFC-11) as an example. The gas flow rate and microwave power (2.45 GHz) delivered to the MTD were 1–3 litre min−1 and 200–400 W, respectively. Concentration of the CFC-11 in the nitrogen was up to 50%. The results show that the decomposition efficiency of CFC-11 is up to 100% with the removal rate of several hundred g h−1 and energy efficiency of about 1 kg kWh−1. This impressive performance, superior to that of other methods, is achieved without generating any significant unwanted by-products. As a result of this investigation, a relatively low-cost prototype system for Freon destruction based on a moderate-power MTD and a scrubber is proposed.

Propagation characteristics of surface waves sustaining atmospheric pressure discharges : the influence of the discharge processes

The propagation characteristics of microwave frequency surface waves along atmospheric pressure plasma columns sustained in capillary tubes by these same waves are studied analytically and numerically. In contrast to what has been done to date in such a pressure range, our analysis includes the modelling of the discharge processes in order to account for their interdependence with the waves propagation. To describe these processes and to determine the plasma parameters, we turn to a two-temperature model for non-equilibrium plasmas. The analysis of the wave dispersion is performed using a cold plasma, linear dielectric description; it assumes the local uniformity approximation, which is scrutinized. Also, we assume that the phase and attenuation coefficients of the wave, under given discharge conditions, depend only on the amount of power dissipated in the plasma per unit length. The obtained results are compared with known experimental data.

Hazardous gas treatment using atmospheric pressure microwave discharges

Atmospheric pressure microwave discharge methods and devices used for producing non-thermal plasmas for control of gaseous pollutants are described in this paper. The main part of the paper is concerned with microwave torch discharges (MTDs). Results of laboratory experiments on plasma abatement of several volatile organic compounds (VOCs) in their mixtures with either synthetic air or nitrogen in low (∼100 W) and moderate (200–400 W) microwave torch plasmas at atmospheric pressure are presented. Three types of MTD generators, i.e. low-power coaxial-line-based MTDs, moderate-power waveguide-based coaxial-line MTDs and moderate-power waveguide-based MTDs were used. The gas flow rate and microwave (2.45 GHz) power delivered to the discharge were in the range of 1–3 litre min −1 and 100–400 W, respectively. The concentrations of the processed gaseous pollutants were from several to several tens of per cent. The results showed that the MTD plasmas fully decomposed the VOCs at a relatively low energy cost. The energy efficiency of decomposition of several gaseous pollutants reached 1000 g (kW-h) −1 . This suggests that MTD plasmas can be useful tools for decomposition of highly concentrated VOCs. (Some figures in this article are in colour only in the electronic version)

Designing an efficient microwave-plasma source, independent of operating conditions, at atmospheric pressure

The surfaguide is a waveguide-based electromagnetic-surface-wave launcher that allows sustaining long plasma columns using microwaves. Its electrodynamic characteristics are examined experimentally and theoretically in the perspective of achieving an efficient plasma source without any need for impedance matching retuning as operating conditions are varied over a broad range. The plasma source design and its modelling using equivalent-circuit theory are described and a simple procedure is provided to determine the optimum dimensions of the surfaguide that maximize the transfer of microwave power to plasma. As an example, with an optimized surfaguide, the reflected power in an N2 discharge at atmospheric pressure stays below 3% for powers in the 2?6?kW range and gas flow rates in the 30?150?l?min?1 domain under varying concentrations (< 2%) of admixed gases such as SF6, O2 and argon.

Propagation characteristics of electromagnetic waves along a dense plasma filament

The characteristics of electromagnetic waves propagating along dense plasma filaments, as encountered in atmospheric pressure discharges, are examined in the microwave frequency range; they turn out to be surface waves. Results of numerical calculations of the dependence of the phase and attenuation coefficients on the plasma parameters are presented. In the limit of large electron densities, this guided wave is akin to a Sommerfeld wave and the propagation can be described in an analytical form.

Modelling of atmospheric pressure, RF and microwave discharges sustained by travelling waves

A channel-type model for RF and microwave discharges, operating under a local thermodynamic equilibrium (LTE) regime and sustained by travelling waves, is proposed. Such discharges are, as a rule, axially non-uniform. The channel model presented includes this feature. From the analysis that follows, two functions of the value of the heat flux potential at the axis (SA) turn out to be necessary to determine the spatial structure of the discharge. These are L(SA), the power dissipated per unit length of the plasma column, and alpha (SA), the wave attenuation coefficient. From these, the spatial distribution S(r,z) of the heat flux potential can be obtained. Under LTE conditions, this is sufficient to determine the distributions of all plasma parameters of interest. As an example, the spatial structure of a nitrogen discharge sustained by a surface wave is calculated.

Decomposition of fluorohydrocarbons in atmospheric-pressure flowing air using coaxial-line-based microwave torch plasma

Results of the investigation of decomposition of fluorohydrocarbons C2H2F4 (HFC-134a) and CHClF2 (CFC-22) in atmospheric-pressure flowing air using a coaxial-line-based microwave torch plasma are presented. Concentrations of the fluorohydrocarbons in the flowing air were up to 10 %. The decomposition efficiency of both C2H2F4 and CHClF2 was almost 100 %. This suggests the coaxial-line-based microwave torch plasma can be a useful tool for decomposition of highly-concentrated fluorohydrocarbons in air at atmospheric pressure.

Surface‐wave sustained plasmas: Toward a better understanding of RF and microwave discharges

An approach is presented that unifies the description of the various existing RF and microwave discharges. It is based on two essential facts: (i) it is not the spatial distribution of the high frequency (HF) electric field intensity but its spatial average that plays in the power transfer to the plasma; (ii) the power θ required to maintain an electron in the discharge is governed by charged particle losses, which are independent of the HF E‐field. This enables one to model separately the maintenance processes of HF discharges and the electrodynamic properties of HF circuits sustaining the plasma, although the discharge and the HF field are actually coupled self‐consistently. The influence of the field frequency on the properties of these plasmas is also summarized.

Microwave Torch Plasmas for Decomposition of Gaseous Pollutants

Abstract Results of the study of decomposition of volatile organic compounds (VOCs including Freons) in their mixtures with either synthetic air or nitrogen, and nitrogen oxides NOx in their mixtures with N2 or Ar in low (~ 100 W) and moderate-power (200-400 W) microwave torch plasmas at atmospheric pressure are presented. Three types of microwave torch discharge (MTD) generators, i.e. the low-power coaxial-line-based MID, the moderate-power waveguide-based coaxial-line MTD and the moderate-power waveguide-based MTD generators were used. The gas flow rate and microwave power (2.45 GHz) delivered to the discharge were in the range of 1÷3 l/min and 100÷ 400 W, respectively. Concentrations of the processed gaseous pollutants usually were from several up to several tens percent. The energy efficiency of decomposition of several gaseous pollutants reached 1000 g/kWh. It was found that the microwave torch plasmas fully decomposed the pollutants at relatively low energy cost. This suggests that the MTD plasma can be a useful tool for decomposition of highly-concentrated gaseous pollutants.

Microwave Plasma Sources for Gas Processing

In this paper atmospheric pressure microwave discharge methods and devices used for producing the non- thermal plasmas for processing of gases are presented. The main part of the paper concerns the microwave plasma sources (MPSs) for environmental protection applications. A few types of the MPSs, i.e. waveguide-based surface wa ve sustained MPS, coaxial-line-based and waveguide-based no zzle-type MPSs, waveguide-based nozzleless cylinder-typ e MPS and MPS for microdischarges are presented. Also, results of the laboratory experiments on the plasma processing of several highly-concentrated (up to several tens percent) vorganie compounds (VOCs), including Freon-type refrigerants, in the moderate (200-400 W) waveguide-bas ed nozzle-type MPS (2.45 GHz) are presented. The results showed that the microwave discharge plasma fully decomposed the VOCs at relatively Iow energy cost. The energy efficiency of VOCs decomposition reached 1000 g/kWh. This suggests that the microwave discharge plasma can be a useful tool for environmerital protection applications. In this paper also results of the use of the waveguide-ba sed nozzleless cylinder-type MPS to methane reforming into hy drogen are presented.