Reaction mechanism explorations on non-thermal plasma reforming of CO2-CH4 by combining kinetics modeling and emission spectroscopy measurements

Abstract

Plasma-based dry reforming in an atmospheric dielectric barrier discharge (DBD) reactor has been thoroughly studied by combination of experiments and detailed kinetics simulation. With the assistances of in-situ optical emission spectroscopy (OES) measurements and reaction pathway analysis based on plasma kinetic simulation considering non-constant electric field and thermal effects, this study provided deep insights into the reaction mechanism of plasma-based dry reforming under varied feeding gas composition, gas residence time and operation temperature. A CH4 conversion as high as 44.76 % could be achieved without catalyst at 300 °C, with an SEI of 78 J/cm3 and feeding CH4 ratio of 50 %. Increase of feeding CH4 proportions were found to enhance the generation of C2 and C3 hydrocarbons. Consistently, the OES analysis showed that the relative intensities of spectra bands induced by deexcitation of CH, CO, and C2 also increased monotonically. With the same increment in SEI, increasing gas residence time led to larger promotions in CH4 and CO2 conversion, as being compared to increase of input power. The reason could be explained as that CH4 and CO2 molecules experienced more successive micro-discharges with a longer gas residence time, so more energy would be devoted into CH4/CO2 conversion, whereas more energy was dissipated into heat as rudely increased the discharge power due to the limit of reaction time. CO2 conversion continuously decreased with increasing gas temperature, owing to the recombination of CO and O enhanced at higher gas temperatures. Reaction pathway analysis showed that CH4 conversion was mainly induced by electronic dissociation and ionization at the discharge stage, while CO2 was mainly converted by recombination reaction CO2 + CH2 → CH2O + CO and also electron collision vibrational reaction e + CO2 → e + CO2(vn). The formation of C2 hydrocarbon products was supposed to follow the path of CH4 ↔ CH3/CH → C2H6 ↔ C2H5 ↔ C2H4 → C2H2.