Novel optimized process for utilization of CaO-based sorbent for capturing CO ₂ and SO ₂ sequentially

Calcium oxide (CaO) based sorbents, used for CO ₂ capture in calcination/carbonation cycles, reduce in activity with increasing numbers of cycles, thus reducing the economic benefits of this capture process, generating a significant amount of spent sorbent and raising the potential for pollution from its disposal. This paper presents research into the use of this spent sorbent for the prior capture of sulfur oxides (SO _x), as required when the cycle is used with coal-derived flue gases. Using a thermogravimetric analyzer (TGA), the sulfation behaviors of two limestones from Hebei Province (named LA and LB) were examined after different numbers of calcination/ carbonation cycles. The results show that the sulfation conversions for spent sorbent of the LA and LB after 40 cycles were, respectively, about 4% and 14% less than that of the original sorbent, indicating that spent sorbents from CO ₂ capture cycles remain active for SO ₂ retention. As the number of cycles increased, there was a difference in the sulfation conversions between LA and LB, probably due to the difference in pore structure development for the two limestones as the sulfation reaction progressed. The surface morphology and pore structure of sorbents after different numbers of cycles were examined by scanning electron microscopy (SEM) and nitrogen adsorption/desorption spectroscopy. It was found that the pore size distributions show two peaks in all cases, and as the number of cycles increased, the larger pores fuse together while the number of small pores was not markedly diminished, which is not consistent with the existing sintering model. The observed sintered pore structure provides greater product growing space for reactions to occur in the pore inner surface, which benefits sulfation more than carbonation due to their different product molar volumes, helping to explain why cycled sorbent samples remain active in sulfation.