Unlocking synergistic benefits of the calcium looping-calcium hydroxide integration for energy storage: A perspective on sorbent performance

• Steam enhances CO 2 diffusion, improving conversion in the diffusion phase. • Multicycle tests show steam slows reactivity loss compared to tests without steam. • Steam injection boosts residual conversion, improving multicycle performance. • Interleaving hydration/dehydration cycles increases residual conversion to 0.98. • Hydration cycles are more stable and resistant to deactivation than carbonation. Calcium Looping has recently attracted attention as a high temperature thermochemical energy storage system. However, significant sintering due to the high temperatures hampers the recyclability of CaO. Hydration and hydroxylation has been explored as a method to regenerate the spent CaO. This study investigates a novel synergistic integration of carbonation (CaCO 3 /CaO) and hydroxylation (CaO/Ca(OH) 2 ) reactions. Calcination was conducted in N 2 and N 2 /H 2 O mixtures with 29 % steam content. Carbonation was conducted in CO 2 /H 2 O mixture with similar steam concentrations. Results show that steam plays a dual role: during calcination, it promotes the formation of large pores on the CaO surface, and during carbonation, it enhances mineralization, resulting in larger CaCO 3 grains. Also, steam promotes CO 2 diffusion through the CaCO 3 layer and, at the same time, significantly mitigates the deactivation of CaO along the cycles. Specifically, sequential calcination/carbonation cycles without steam yield a residual conversion value of 0.14. Steam injection improved residual conversion to 0.27. Alternatively, the interleaving of hydroxylation/dehydroxylation cycles in the sequence further increased this value to 0.64 without steam and up to 0.76 with steam injection. Hydroxylation/dehydroxylation cycles alone demonstrated high stability, with a residual conversion of 0.98 when interleaved with calcination/carbonation cycles under 29 % steam conditions. Additionally, frequent hydroxylation/dehydroxylation cycles improve overall conversion stability, highlighting their synergistic benefits within the integrated process. This work underscores the potential of integrating Calcium Looping with Calcium Hydroxide for improved multicycle performance and opens pathways for scaling experiments to pilot systems, alongside assessing the efficiency and economic viability of this integrated approach.