Decomposition of volatile organic compounds using non-thermal plasmas

Abstract

Volatile organic compounds (VOCs) are among the most common anthropogenic air pollutants. They have been linked to various human diseases, including cancer, cardiovascular disease, lung, and respiratory diseases. As a result, reducing VOC emissions has become a significant concern and a significant research area worldwide. Non-thermal plasmas (NTPs) are an attractive technique for removing VOC emissions from ambient air. This research concerned the use of a non-thermal plasma dielectric barrier discharge (DBD) method to remove various volatile organic compound emissions from ambient air. The model VOC compounds chosen were hexane, cyclohexane, benzene, and methanol. The principal aim of this research was to improve the performance of a non-thermal plasma DBD reactor on the removal efficiency of VOCs at ambient temperature and atmospheric pressure. The effects of key process parameters such as carrier gases (nitrogen, dry and humidified air), plasma power, oxygen concentrations, residence time and inlet concentration on the removal efficiency, product selectivity and elimination of unwanted by-products were investigated. These investigations showed that the removal efficiencies of the VOCs generally increased with increasing plasma power and residence time, regardless of the carrier gas used. The maximum removal efficiencies of the three 6-carbon hydrocarbons used, hexane (94.4%), cyclohexane (98.2%) and benzene (93.7%), were achieved in humidified air plasma. In contrast, the 96.7% maximum methanol removal efficiency was obtained in dry air plasma. However, the removal efficiency of VOCs decreased with increasing inlet concentration due to the increased number of VOC molecules flowing into the plasma reactor at constant discharge length, plasma power and residence time. It was found that increasing O2 concentration from 0 to 21% increased the removal efficiency and selectivity to CO2 due to the increase in the generation of oxygen radicals. The decomposition products were CO2, CO, H2, and lower hydrocarbons (C1-C5), depending on the model VOC and the carrier gas used. O3 concentrations were below 10 ppm in dry air plasma for all the studied VOCs, and NOX was not detected in any carrier gas. To some extent, the hexane, cyclohexane, and benzene decomposed into solid residue due to the oligomerisation of hydrocarbon radicals produced in the DBD plasma system in pure nitrogen and dry air plasma, which would cause arcing and blockage problems after prolonged operation. The effect of water vapour was investigated to determine whether it would reduce the formation of these solid residues in the DBD reactor. Water at RH= 25% (hexane and cyclohexane) and 35% (benzene) significantly increased the removal efficiency and the CO2 selectivity while eliminating the solid residue and NOX in the NTP-decomposition of VOCs, probably via the formation of potent OH radicals. Humidification generally improved the removal efficiency and increased the yield of H2 and ii CO2 selectivity. These results imply that the decomposition of VOCs by non-thermal plasma DBDs is dominated by the effect of OH and O radicals. The addition of water to the decomposition process suppressed O3 formation and reduced selectivity to CO in all tested conditions. Therefore, the performance of DBD plasmas in removing hexane, cyclohexane, benzene, and perhaps other VOCs would improve when operating in humid conditions or when the inlet gas stream is humidified. In addition, the technique of eliminating solid residue by the addition of water vapour provides a simple solution to a problem that is limiting the application of DBD systems of VOC removal and similar applications