High-temperature CO₂ capture cycles for CaO-based pellets with kaolin-based binders

Pellets from natural and acetified Havelock limestone were synthesized and tested for in situ post-combustion CO₂ capture in Ca-looping cycles. Natural kaolin and Al(OH)₃ obtained from acid leaching of kaolin were used as binders. The results showed that after 30 cycles, pellets prepared from acetified limestone with 10vol.% acetic acid and CaO/Al(OH)₃ mass ratio of 5.5 had a somewhat higher CO₂ uptake of 0.13g/g (g CO₂/g sorbent), compared to 0.12g/g for pellets prepared from natural limestone and Al(OH)₃. These uptakes corresponded to 56% and 36.7% of their initial capture capacities, respectively. By contrast, natural sorbent and pellets prepared with kaolin demonstrated a significant decay in their CO₂ uptake down to 21% and 21.7% of their initial capacities, respectively. The superiority of acetified sorbents with Al(OH)₃ binders appears to be due to the creation of a highly developed porous structure. In particular, α-Al₂O₃ generated from Al(OH)₃ appears to create a stable framework dominated by mesopores, which also inhibits sintering over multiple cycles. However, acetification itself widens pores and alters pore surface area and volume, providing more space for CaCO₃ growth. Unfortunately, water addition during pellet synthesis appears to compromise this porous structure, thus reducing the original benefits of acetification.