To mitigate global warming and achieve a sustainable society, innovative technologies for efficient CO2 utilization are required. Integrated CO2 capture and reduction (CCR) using dual-function materials (DFMs) is favorable owing to its potentially low energy consumption, capital investment, and processing costs. Although numerous studies have focused on catalytic science, continuous and steady-state CCR operations have not been sufficiently addressed from an engineering perspective. In this study, a circulating fluidized bed (CFB) system is investigated for continuous CCR to syngas (CO + H2). In the CFB system, transition-metal-free DFM (Na/Al2O3) particles are circulated between two bubbling fluidized-bed reactors. The DFM captures CO2 in one reactor (CO2 capture reactor) and reduces the captured CO2 to CO by the reaction with H2 in the other reactor (H2 reactor). The effluent gas concentrations from both reactors reach steady state and are maintained for over 8 h. For the product gas from the H2 reactor, the CO2 conversion and CO selectivity exceed 80 % and 99 %, respectively. However, the H2 conversion is <20 %, indicating a potential challenge for the CFB system for integrated CCR. Furthermore, this study confirms that the H2/CO ratio for syngas can be controlled by adjusting the experimental conditions (particularly, the H2 flow rate). Consequently, the CFB system can be modified to facilitate the interaction between H2 gas and the DFM particles.@en

Integrated CO2 capture and conversion (ICCC) using dual-function materials (DFMs) is one of the key technologies for addressing critical global environmental and energy issues. DFMs generally consist of alkali or alkaline earth metals for CO2 capture and transition metal catalysts for CO2 conversion. In this study, we studied the ICCC to CO using transition-metal-free DFMs to demonstrate their potential to directly produce syngas from atmospheric-level CO2. Among the DFMs prepared herein, Na/Al2O3 exhibited excellent performance and achieved a CO2 conversion exceeding 90% and CO selectivity exceeding 95% at a reaction temperature of 450-500 °C. Na/Al2O3 maintained its capture and conversion capacity throughout a 50-cycle stability test without significant deactivation. Furthermore, in the scale-up experiments using Na/Al2O3 DFM, a syngas-like mixture an H2/CO molar ratio of 3.3 (48.1 vol% H2 and 14.5 vol% CO) was directly obtained from 400 ppm CO2. These results suggest that ICCC using the transition-metal-free Na/Al2O3 DFM may be practicable provided the CO2 capture capacity of the DFM is further improved while maintaining the aforementioned advantages.

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Carbon capture and utilization (CCU) technologies, such as CO₂ methanation, generally require energy-intensive CO₂ capture and separation processes prior to catalytic CO₂ conversion. In contrast, integrated CO₂ capture and reduction (CCR) technologies that use dual function materials (DFM) can directly convert low-concentration CO₂ in flue gas or atmosphere into high-concentration CH₄ or CO. In this study, we demonstrate a circulating fluidized bed (CFB) approach to enable continuous operation of CCR. In the CFB approach, the DFM (Na/Ni/Al₂O₃) circulates between two bubbling fluidized beds to enable steady-state cyclic operation of (1) selective capture of CO₂ in flue gas/air and (2) hydrogenation of the captured CO₂. We succeeded in the continuous synthesis of CH₄ with high CO₂ capture efficiency (>88 %) and high H₂ conversion (>85 %) yielding mainly CH₄ (selectivity > 99 %) as the product at high concentration (>20 % CH₄) using 2 % CO₂/N₂ as the model flue gas.

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