Kinetic roles of vibrational excitation in RF plasma assisted methane pyrolysis

A combined experimental and simulational work was carried out in this paper to investigate the kinetic effects of non-equilibrium excitation by direct electron impact on low temperature pyrolysis of CH4 in a RF dielectric barrier discharge. Special attention was placed on the vibrational chemistry of CH4 and some other important products including H-2, C2H2, C2H4, C2H6 and C3H8 largely produced in CH4/He discharge under an intermediate reduced electric field ranging 51-80 Td. A detailed kinetic mechanism incorporating a set of electron impact reactions, electron-ion recombination reactions, negative ions attachment reactions, charge exchange reactions, reactions involving vibrationally excited molecules and the relaxation process of vibrationally excited species was assembled and experimentally validated. The modeling results showed a reasonable agreement with the experimentally measured results in terms of CH4 conversion and products production including C-2 hydrocarbons and hydrogen. A linear increasing trend of methane conversion with increasing plasma power input was discovered, which suggested a strong dependence of molecular excitation on energy input. Both the CH4/He mole ratio and the reactor temperature play significant roles in CH4 conversion and major products production. The experimental results showed that the selectivity of value-added products C2H4 and H-2 keeps essentially unchanged with increasing energy input, mostly because the contribution CH4 ionization and He excitation effectively compete with vibrational excitation and dissociation of CH4 molecule with the E/N value increasing. The calculated results showed that the typical relaxation time of vibrational states is comparable to the gas-kinetics time in a CH4/He discharge mixture, thus the vibrationally excited molecules can significantly accelerate chemical reactions through an effective decrease of activation energy. The path flux analysis revealed that the vibrationally excited molecules CH4(v) and H-2(v) enhanced chain propagation reactions, such as CH(4()v)+HCH3+H-2, CH4(V)+CHC2H4+H, and H-2(v)+CCH+H, further stimulating the production of active radicals and final products. Specifically, H-2(V)+CCH+H was responsible for 7.9% of CH radical formation and CH4(v)+CHC2H4+H accounted for 31.4% of total C2H4 production. This kinetic study provides new sights in demonstrating the contribution of vibrationally excited molecules in RF plasma assisted methane pyrolysis. (C) 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.