Experimental and numerical investigation of the morphological changes of a natural limestone-based CO2 sorbent over repeated carbonation-calcination cycles

Morphological changes of natural limestone-based CO₂ sorbents during the cyclic transition between CaO and CaCO₃ affect their carbonation rate and cyclic CO₂ uptake. We examine the evolution of the pore structure of Havelock limestone during carbonation in the ranges (I) 2–100 nm, (II) 200–3000 nm and (III) > 3000 nm with unprecedented detail, and correlate morphological changes with the observed carbonation rate. Pores of region (I) are fully filled with CaCO₃ at a CaO conversion > 60 % (1st cycle), leading to a loss of ∼ 90 % of the total surface area of the sorbent, whereas pores of region (II) are only partially filled, and pores of region (III) remain largely unaffected. Throughout the carbonation reaction in the 1st and 10th cycle, the observed carbonation rate decreases linearly with the decreasing total surface area of the sorbent. Supported by kinetic and morphological modelling, our findings challenge the widely used concept of a CaCO₃ product layer of critical thickness limiting CO₂ diffusion to CaO, implying that the reaction is limited by diffusion as soon as the surface of CaO is fully covered with CaCO₃ crystallites. Our results further provide a perspective on the design of efficient CaO-based sorbents by tuning their pore diameter to be larger than > 100 nm, such that the pore volume (and the respective surface area) can be largely regenerated over cycling, in turn yielding a high cyclic CO₂ uptake.