Fragmentation mechanism of CaO-based CO2 adsorbent pellets stored in moist air and its anti-fragmentation strategy via hydrothermal Shell-Core structuring

Calcium-based adsorbents hold great promise for large-scale CO 2 capture. However, CaO pellets suffer fragmentation when stored in moist air, limiting their application. Numerous studies demonstrate that the presence of water significantly affects the structural integrity of adsorbent pellets. However, most of these studies focus on high-temperature conditions, while limited attention to moist air conditions. This study investigated the effect of different hydration pathways on the structural integrity of CaO pellets and revealed the fragmentation mechanism under moist air. Multiscale characterizations showed that CaO pellets exposed to moist air or liquid water underwent severe fragmentation, primarily caused by liquid-water hydration reactions. FT-IR and TGA results confirmed the presence of residual water in the fragmented pellets, while Rietveld refinement showed a significant drop in crystallinity and rise in microstrain. These changes compromised the structural integrity of pellets. In contrast, the pellets undergoing high-temperature steam hydration retained structural integrity, showing high crystallinity and low microstrain. Based on this, a hydrothermal shell-core strategy was proposed to enhance the anti-fragmentation performance of CaO pellets. A dense Ca(OH) 2 shell formed via high-temperature steam hydration isolated the CaO core from moist air, significantly enhancing structural integrity. Pellets with over 60 % hydration conversion retained 74.07 % structural integrity after 48 h of exposure to moist air (30 °C and 85 % RH), three times higher than unhydrated pellets, while CO 2 capture capacity of the revised pellets surpassed 0.6 g CO 2 /g adsorbent. These findings provide both mechanistic basis and design strategy for the production, transportation, and storage of calcium-based adsorbent pellets in moist environments.