Removal of pollutants by plasma catalytic processes

Abstract

Removal of molecular atmospheric pollutants by non-thermal plasmas is under study since the beginning of the eighties. It has been shown that pulsed electrical discharges, such as dielectric barrier or corona discharges, are powerful means to eliminate Volatile Organic Compounds (VOCs) from the ambient air, or to treat flue gases which contain nitrogen oxide. However it is now recognised that, for several pollutants, the use of the plasma alone does not allow a complete elimination of the undesirable molecule. For example NO is oxidised in the air plasma to form other oxides like NO 2 and N 2 O 5 , and reactions of oxygen atoms or hydroxyl radicals produced by the discharge with VOCs can lead not only to H 2 O and CO 2 but to a number of by-products following the partial oxidation of the molecule, which can be as undesirable than the compound to be initially removed from effluents. This is particularly the case when the electrical energy deposited in the gas flow must be kept as low as possible in order to design a low energy cost equipment. As a result addition of a catalyst together with the pulsed discharge is now investigated in various laboratories in order to achieve a complete oxidation of VOCs, i.e. the so-called de-COV process, or a complete reduction of NO X (NO and NO to produce N 2 and O 2 , i.e. the so-called de-NO X process, at low energy consumption. This paper is a short review of works which have been done these last years in that domain, specifically on NO X and some selected VOC molecules.