Imrich Morva

Removal of cyclohexanone in transition electric discharges at atmospheric pressure

Two new types of streamer-induced electric discharges operating in a non-uniform electric field in air at atmospheric pressure were applied to the removal of volatile organic compounds (VOCs). The first type is a pulseless dc discharge with physical properties corresponding to the glow discharge. The second, also supplied by a dc high voltage of both polarities, is a spontaneously pulsing discharge operating in the regime of the streamer-to-spark transition, the spark phase being too short to reach local thermodynamic equilibrium conditions. Both discharges are able to generate a non-thermal plasma, as resolved from their rotational and vibrational temperatures. The influences of these discharges on the removal of cyclohexanone at various gas flow rates and concentrations (600-6000 ppm) were compared. The removal efficiencies achieved were about 50-60%, and the energy costs were 16-100 eV/molecule at various energy densities. Special conditions where CO2 and other gaseous products are minor and dominant products appear in the condensed phase can be obtained, especially in the spontaneously pulsing transition discharge. We explain some plasmochemical processes induced by the discharges by considering heterogeneous effects of the copper electrode surface. The role of active nitrogen and the formation of the NCO radical are probably key factors leading to the formation of the condensation product based on amino acids, here produced for the first time from VOCs, as well as in the overall energy cycle resulting in low energy costs of the process. The small pilot-scale reactor based on the spontaneously pulsing transition discharge has been successfully applied to the removal of cyclohexanone in the mixture with other VOCs with no noxious gas output. This validates the possibility of the application of such a type of reactor for larger scales.

Cathode and anode falls of arcs with fusible electrodes

Anode and cathode potentials for electrodes of mild steel in stagnant air or flowing argon and CO2 have been obtained both of the voltage distribution during arc quenching and of a measured melting velocity. The potentials are independent of arc current in the observed region 40-200 A. The energy for melting a unit length of a wire electrode has been established as the energy input for breaking a short-circuit bridge of the observed wire. The important role of neutral particle flux to melted electrodes has been revealed. A model of rapid arc quenching due to temperature and field emission of electrons has been suggested.

Effect of cathode surface properties on glow-to-arc transition in a short positive corona gap in ambient air

Measurements were made of current growth waveforms corresponding to the glow-to-arc transition in a 1 cm positive point-to-plane gap in conditions similar to those in pulsed corona devices used for flue gas cleaning. The glow-to-arc transition was significantly accelerated using a freshly polished unconditioned cathode surface. A duration of glow-to-arc transition as short as 25 ns was observed using a graphite-coated cathode. A possible explanation in terms of the cathode sheath break-ups is presented.

Plasma Technologies for Reducing CO2 Emissions From Combustion Exhaust with Toxic Admixtures to Utilisable Products

The method reported here provides a contribution to CO2 and combustion exhaust utilisation. A multifunctional system for gas removal was tested on various sources of exhaust (internal combustion engine, brown coal boiler, bituminous pulverised coal boiler, gas boiler, glass oven, VOC sources) in full-scale or by-pass gas flow volumes.A spontaneously-pulsing, direct-current electric discharge operating in a corona geometry was used. The discharge has strongly shining channels migrating quickly along the stressed electrode. The synergetic effect of electric discharge and heterogeneous catalysis on the organometallic part of the product formed on the non-stressed electrode was responsible for the specific character of the products. The final product of the process is a powder with a fractal structure on the microscopic level with low specific mass and insoluble in water. The main component (95%) of the solid product is an amorphous condensate of amino acids with about 5% of organometallic compound with catalytic properties. The product was analysed using IR absorption spectrometry, microscopic photography, HPLC and thermogravimetry. The following amino acids were observed in the final product: alanine, serine, glycine, aspartic acid, lysine, arginine, methionine, histidine.

Thermochemical aspects of the conversion of the gaseous system CO2-N2-H2O into a solid mixture of amino acids

Conversion of the gaseous mixture CO2(g)+N2(g)+H2O(g) to a solid amino acid condensate in an electric discharge plasma has high efficiency of the energy transfer from the different plasma components into chemical processes. The basic activation process is activation of the N2 metastable electronic state, followed by formation of NCO* and ON-NCO free-radicals and generation of many reactive radicals. These radicals help to overcome the high activation energy of thermal dissociation of N2 to N (950 kJ=9.846 eV).The major product is a statistical polycondensate containing the amino acids: arginine, lysine, histidine, methionine, glycine, alanine, serine and aspartic acid. This information was obtained by comparing the IR spectra of the products with reference IR absorption spectra of pure components. Identification of the individual amino acids in the solid product was performed by HPLC, when samples were dissolved using 6 M HCl applied at 100°C for 24 h. Properties of the condensate were estimated using thermogravimetric analysis. Small amounts of oxamidato complexes and oligo pyrrole structures are formed on the electrode surface giving the surface catalytic properties. The gas cleaning process has practical applicability (production of useful fertilizers, reduction of the CO2 concentration in the atmosphere) and may also contribute to explanation of the origin of life on Earth.

Ignition of a welding arc during a short-circuit of melted electrodes

It is shown that an arc discharge in a welding process with fusible electrodes (gas-metal arc welding) starts before a short-circuit bridge, made by a metal drop between the electrodes, is broken. The possibility of this premature ignition is proved by voltage and current measurements, by analysis of the electrical field near the neck of the drop and also by simulation of the increase in current in an electrical discharge.

Determination of the temperature of electrons heated by microwaves from the spectral line intensities emitted by the neon afterglow plasma

The electron temperature dependence of the dissociative recombination coefficient of ions with electrons was investigated in the afterglow of a pulsed high frequency discharge using both microwave cavity and optical emission spectrometry methods. The present experimental results show the close correlation between the relative intensities of the emitted lines, the electron temperature and the incident microwave power. It is proposed to calibrate the electron temperature as a function of absorbed microwave power by measuring the spectral line intensities from Ne(3p) to Ne(3s) transitions. It is confirmed that the electron temperature dependence of the dissociative recombination coefficient of molecular ions follows the power law .

Reply to comment on `Ignition of a welding arc during a short-circuit of melted electrodes' (1999 J. Phys. D: Appl. Phys. vol 32, p 1058)

It is shown that estimations made in the premature arc scenario are in good agreement with the observed results in spite of some simplifications and uncertainties pointed out by Blumschein. The temperature dependence of electrical resistivity does not influence the geometry of the drops neck and the magnetic pressure does not suppress the evaporation dramatically. The reached current density casts doubt upon the crucial role of isochoric heating in the arc ignition. The shunting arc, used in plasma-based ion implantation, gives experimental evidence of the premature arc scenario. The relative field strength (600 Td, used also in the premature arc scenario) is sufficient for the shunting arc ignition.