Thermal Regulation of CO₂ Methanation Using Al-Cu-Si Alloy-Based Microencapsulated Phase Change Materials

CO₂ methanation is an effective carbon utilization technology. It involves synthesizing CH₄ by reacting CO₂, recovered from industrial processes, with H₂ produced from renewable energy sources. However, this exothermic reaction causes significant increase in catalyst temperature, leading to thermal runaway, which can reduce CH₄ selectivity, activity, and catalyst lifespan. Therefore, effective thermal regulation of the reactor is crucial for stable CO₂ methanation.<br/>This study focuses on latent heat storage (LHS) for thermal regulation. LHS uses latent heat from the solid-liquid phase transition of phase change materials (PCM), offering high heat storage density, constant temperature heat input/output, and excellent cycling performance. LHS was expected to maintain a constant reactor temperature at the PCM’s melting point during the reaction. An Al-Cu-Si alloy PCM, with a melting point of 520°C, is suitable for thermal regulation within the reaction’s operating temperature range of 300−600°C. However, the leakage of PCM in its liquid phase and its high corrosiveness are major challenges and necessitate encapsulation.<br/>Our research group developed a microencapsulated PCM (MEPCM) consisting of a PCM core and an α-Al₂O₃ shell. The α-Al₂O₃ shell enhances thermal and chemical stability, enabling MEPCM use as composite molding and catalyst support. By loading the catalyst onto MEPCM pellets, we fabricated catalyst pellets with integrated heat storage function. The close proximity of the catalyst and PCM at the microscale allows in-situ heat recovery, facilitating passive thermal regulation and suppressing sudden thermal runaway.<br/>In this study, catalysts consisting of Al-Cu-Si alloy-based MEPCM pellets loaded with Ni were prepared and evaluated for thermal regulation and catalytic reaction characteristics in a fixed-bed reactor.<br/>Catalyst pellets were prepared by loading Ni onto 1 mm diameter pellets composed of Al-Cu-Si alloy-based MEPCM (70 vol%), α-Al₂O₃ (20 vol%), and yttria-stabilized-zirconia (ZrO₂-3 mol% Y₂O₃) (10 vol%). A quartz tube with a 20 mm inner diameter was filled with the catalyst pellets to a height of 100 mm. The catalyst bed was heated to 500°C under Ar flow, and Ni was activated under Ar/H₂ flow for 30 min. The catalyst test was conducted for 30 min under an Ar/H₂/CO₂ flow as the feed gas. Temperatures at 0, 25, 50, 75, and 100 mm (denoted as <i>T</i>₀, <i>T</i>₂₅, <i>T</i>₅₀, <i>T</i>₇₅, and <i>T</i>₁₀₀, respectively) from the catalyst bed inlet were measured using multi-point thermocouples installed in the catalyst bed to assess the thermal regulation. The product gas was analyzed using Q-mass spectrometry to evaluate the catalysts' reactivity.<br/>The results showed temporary temperature stagnation near the PCM melting point at <i>T</i>₂₅, <i>T</i>₅₀, <i>T</i>₇₅, and <i>T</i>₁₀₀, with the duration of stagnation increasing towards the catalyst bed outlet. This temperature stagnation demonstrated the thermal regulatory effect of the MEPCM. Near the inlet, most of the CO₂ and H₂ reacted with the catalyst, generating substantial heat. As most of the CO₂ and H₂ were consumed near the inlet, the generated heat decreased towards the latter half of the bed, thereby increasing the stagnation duration towards the exit. These results suggest that MEPCM can effectively regulate the thermal environment of CO₂ methanation.