Low-carbon hydrogen production by molten metal–catalyzed methane pyrolysis: Catalysts, reactor design, and process development

This review focuses on low-carbon H2 production via the non-oxidative decomposition of CH4. The plasma-based methane decomposition, water splitting, nuclear thermochemical cycles, and steam methane reforming were compared with those of molten metal (MM)-based CH4 pyrolysis based on thermodynamic, techno-economic, and environmental aspects. The selection of MM catalysts and reactor materials was described for CH4 pyrolysis, followed by sustainable heat sources and reactor configurations. An electromagnetic levitation method was presented to elucidate the intrinsic reaction rates based on the bubble surface area, regardless of the reactor type and residence time. Models including the physical properties of the gas and liquid phases, reaction kinetics, and mass transfer of carbon were then discussed for the effective design of MM-based bubble column reactors (MMBCRs). Moreover, a process flow diagram integrating natural gas pre-treatment, CH4 pyrolysis reaction, H2 and carbon separations, and H2 storage was introduced for commercial-scale H2 production. As carbon byproduct is three times the H2 weight, the applications of carbon products were investigated to improve the economic feasibility of MM-based CH4 pyrolysis. Metal impurities in the carbon byproduct should be removed to increase the purity and convert carbon into a high-value-added material. This review culminates with conclusions and future perspectives on low-carbon H2 production using MMBCRs.