Thermal Regulation of CO<sub>2</sub> Methanation Reaction in a Fixed Bed Reactor Filled with Ni Catalyst with Latent Heat Storage Function: Effect of Axial Gradient Arrangement

This study investigates a passive type of thermal regulation for CO2 methanation using catalyst pellets, comprising Ni catalysts supported on pellets composed of an Al-Cu-Si based microencapsulated phase change material (MEPCM, heat storage temp.: 519 °C), with latent heat storage functions. The effects of catalyst bed configurations; a uniform type and multi-stage types on temperature profiles in the catalyst beds and CO2 conversion were mainly investigated. Here, uniform type is only Ni/Al-Cu-Si MEPCM pellets were packed. The multi-stage types are with different mass fractions of Ni/MEPCM pellets and MEPCM pellets (no catalyst) in each stage. Compared to the uniform configuration, the multi-stage gradient configuration demonstrated an extended duration of temperature stagnation at around the heat storage temperature of the MEPCM and a reduction in the steady-state temperature at the inlet-side location. This effect can be attributed to the dilution of the catalyst bed with Al-Cu-Si MEPCM pellets, which suppressed the heat generated by the reaction and prevented excessive temperature increases. On the other hand, at the downstream temperature, the two-stage gradient and three-stage gradient configurations exhibited shorter durations of temperature stagnation and higher steady-state temperatures. In addition, the CO2 conversion observed for the multi-stage gradient configurations were lower than that of the uniform configuration.