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

The use of atmospheric rotating gliding arc (RGA) plasma is proposed as a facile, scalable and catalyst-free approach to synthesizing hydrogen (H-2) and graphene sheets from coalbed methane (CBM). CH4 is used as a CBM surrogate. Based on a 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, the selectivity of H-2 and the properties of solid generated carbon. Nevertheless, the basic properties of RGA plasma and its role in CH4 conversion are scarcely mentioned. In the present work, a 3D RGA model, with a detailed nonequilibrium CH4/Ar plasma chemistry, is developed to validate the previous experiments on CBM conversion, aiming in particular at the distribution of H-2 and other gas products. Our results demonstrate that the dynamics of RGA is derived from the joint effects of electron convection, electron migration and electron diffusion, and is prominently determined by the variation of the gas flow rate and applied voltage. Subsequently, a combined experimental and chemical kinetical simulation is performed to analyze the selectivity of gas products in an RGA reaction, taking into consideration the formation and loss pathways of crucial targeted substances (such as CH4, C2H2, H-2 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 its initial spherical structure into folded multilayer graphene sheets, while the size of the graphene sheets is slightly affected by the change in gas flow rate.