Direct and indirect plasma oxidation of NOx was tested in a medium-scale test-bench at gas flows of 50 slm (3 m(3)/h). For direct plasma oxidation the synthetic flue gas was directed through a stacked DBD reactor. For indirect plasma oxidation, a DBD reactor was used to generate ozone from pure O-2 and the plasma treated gas including ozone was mixed with flue gas at the entrance of a 6 m long serpentine-like reaction chamber which allowed reaction times longer than 10 s. At relatively low NOx concentrations of 200 ppm, both oxidation methods gave similar results. However, the temperature increase of the DBD reactor decreased the long-term efficiency of direct plasma oxidation. At the same time, the efficiency of indirect oxidation increased at elevated reactor temperatures. Additional experiments were carried out to investigate the improvement of indirect oxidation by the introduction of catalyst to the reaction zone. Small-scale experiments with TiO2 powder demonstrated considerable efficiency gain for NOx oxidation while in medium-scale experiments, the efficiency improvement remained negligible. (c) 2014 Elsevier Ltd. All rights reserved.