Competition between homogeneous and plasma chemistry in methane conversion at high temperatures: A global kinetic and graph theory analysis

Non-thermal plasmas are a promising technology for the conversion of methane (CH 4 ) into valuable chemistries that could be the foundation for the robust production of value-added products from sources such as natural gas. Previous work has shown a sensitivity of CH 4 conversion to the bulk reaction temperature in a dielectric barrier discharge (DBD) reactor, resulting in inhibited conversion at reaction temperatures above 600 °C. Understanding the interplay between plasma chemistry and thermal homogeneous reactions at these elevated temperatures is imperative for optimizing the conversion process and reactor design. In this work, we utilized the chemical kinetic modeling software ZDPlasKin to investigate the changes to the decomposition products of the gas-phase effluent in pure CH 4 and CH 4 /argon (Ar) DBD plasmas at various bulk reaction temperatures observed in prior experiments. Reaction networks were constructed from kinetic simulations, where species were represented by nodes and the network edges were weighted according to reaction rates. To simplify the complex reaction network, edge betweenness centrality (EBC) was applied to quantify the relative importance of reactions in the network and used to provide insight into changes in the reaction pathways at high temperatures. Results show a temperature-dependent change to the species product distribution and that higher temperatures affect the chain propagation and termination of reactions, resulting in reactions that promote the formation of CH 4 , thereby reducing conversion. The effect of temperature on homogeneous reactions, rather than temperature-dependent changes to the plasma, is the main cause of the observed inhibition in DBD plasmas at high temperatures.