Conversion of coalbed methane surrogate into hydrogen and graphene sheets using rotating gliding arc plasma

Atmospheric rotating gliding arc (RGA) plasma is proposed as a facile, scalable and catalyst-free approach to synthesize hydrogen (H2) and graphene sheets from coalbed methane (CBM). CH4 is used as a CBM surrogate. Based on previous investigation of discharge properties, product distribution and energy efficiency, the operating parameters such as CH4 concentration, applied voltage and gas flow rate can effectively affect the CH4 conversion rate, selectivity of H2 and properties of generated carbon solid. Nevertheless, the basic properties of RGA plasma and its role in CH4 conversion are scarcely touched. In present work, a 3D RGA model, with a detailed non-equilibrium CH4/Ar plasma chemistry, is developed to validate with the previous experiments of CBM conversion, especially aiming at distribution of H2 and other gas products. Our results demonstrate that dynamics of RGA is derived from the joint effect of electron convection, electron migration and electron diffusion, and is prominently determined by the variation of gas flow rate and applied voltage. Subsequently, a combined experimental and chemical kinetical simulation is performed to analyze the selectivity of gas products in RGA reaction, with the consideration of the formation and loss pathways of crucial targeted substances (such as CH4, C2H2, H2 and H radicals) and corresponding contribution rates. Additionally, the effects of operating conditions on the properties of solid products are investigated by scanning electron microscopy (SEM) and Raman spectroscopy. The results show that increasing the applied voltage and decreasing CH4 concentration will change the solid carbon from initial spherical structure into a folded multi-layer graphene sheets, while the size of graphene sheets is slightly affected by the change of gas flow rate.