Conversion of CO2 in a cylindrical dielectric barrier discharge reactor: Effects of plasma processing parameters and reactor design

Direct conversion of CO2 into CO and O-2 was carried out in a cylindrical dielectric barrier discharge (DBD) reactor at atmospheric pressure and low temperatures. The influence of plasma processing parameters (e.g. discharge frequency, CO2 flow rate, discharge power, discharge length, discharge gap, and dielectric material's thickness) on the plasma CO2 conversion and energy efficiency of the process was investigated. The major products of this reaction were CO and O-2, which suggests the stoichiometric conversion of CO2 into CO and O-2 was achieved. The results indicate that discharge frequency has a negligible influence on the conversion of CO2 at a constant discharge power. Increasing discharge power or decreasing feed flow rate enhanced CO2 conversion but lowered the energy efficiency when all other parameters were kept constant. In addition, decreasing the discharge gap and the dielectric material's thickness, or enlarging the discharge length, increased both conversion of CO2 and process efficiency. Two regression models for the plasma process were developed to elucidate the relative significance of these plasma processing parameters on the plasma conversion of CO2. It was found that the flow rate of CO2 and discharge power play the most important role in CO2 conversion, while the energy efficiency is most significantly influenced by the discharge power. Modification of the plasma design was explored by using different inner and outer electrodes in the DBD system. The combination of the Al foil outer electrode with the stainless steel (SS) screw-type inner electrode showed an enhanced CO2 conversion and energy efficiency compared to other electrode forms, leading to the maximum CO2 conversion of 27.2% and maximum energy efficiency of 10.4% in this work. The use of the SS screw-type inner electrode could lead to an enhanced local electric field near the sharp edge of the electrode, while the use of Al foil as an outer electrode could enlarge the effectiveness of discharge area compared to the mesh electrode. Both effects contributed to the enhanced efficiency of the plasma CO2 conversion. (C) 2017 Elsevier Ltd. All rights reserved.