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 CO2and 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.
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Machala et al. (2000) studied this question.