Experimental study and process evaluation of coproduction graphene, acetylene and hydrogen from methane pyrolysis by thermal plasma

Abstract

This paper developed a process for the coproduction of graphene, acetylene and hydrogen from methane pyrolysis by thermal plasma. Bench-scale tests under three specific energy input (SEI) were carried out to investigate the morphology of carbon nanomaterials and the product selectivity. The process was evaluated by exergy and exergoeconomic analyses. Results indicated that a higher SEI improved the selectivity of graphene nanoflakes (GNFs) and reduced their defects. The product selectivity of GNFs played a critical role in the decrease of total fuel exergy per unit of GNFs as the SEI increased. The plasma reactor exhibited the highest exergy loss ratio in the entire process due to heat dissipation. The quenching device exhibited the second-highest exergy loss ratio in the process, stemming from irreversible entropy increase causing exergy loss. Improving the design of the plasma reactor to enhance thermal efficiency and implementing a feasible waste heat recovery system were identified as effective strategies to enhance exergy efficiency. Excluding consideration of co-products, GNFs with high SEI had the lowest cost of 199.41 CNY/kg, when co-products were considered, the lowest cost decreases by 30%, and the low SEI had the lowest GNFs cost at 140.71 CNY/kg. This indicated that there was enough investment potential to tailor the properties of GNFs produced by such processes for application scenarios. The unit exergy cost of GNFs decreased from 0.89 CNY/MJ to 0.80 CNY/MJ as the SEI of the plasma reactor increased. Subsequently, it rose to 1.16 CNY/MJ. Prioritizing the increase in investment costs for the plasma reactor to reduce exergy loss costs should be considered first, and optimizing the waste heat recovery process was also a key direction to minimize exergy loss costs.