Direct CO₂ Uptake Performance of Mn‐Modified Ca(OH)₂ in Repeated Carbonation‐Calcination‐Hydration Cycles: Experiment and Density Functional Theory Study

Abstract Ca(OH)₂ can be used in CO₂ uptake from the flue gas in fossil‐fired power plants. Compared with indirect method where Ca(OH)₂ is first converted into CaO and then uptakes CO₂, direct CO₂ uptake by Ca(OH)₂ occurs at lower temperature, leading to slighter sintering. However, the direct CO₂ uptake capacity of Ca(OH)₂ should be further enhanced. In this study, Ca(OH)₂ is doped with Mn to improve its direct CO₂ uptake capacity in carbonation‐calcination‐hydration cycles. The cyclic tests are conducted in a triple fixed‐bed reactor. The mechanism of CO₂ adsorbed on Mn‐modified Ca(OH)₂ is determined by density functional theory calculations. In the first cycle, Mn‐modified sorbents acquire highest carbonation conversion at 400 °C, which is 40 °C lower than Ca(OH)₂. The modified Ca(OH)₂ with Ca/Mn molar ratio of 100:0.75 exhibits the best performance, and its carbonation conversions reach 0.76 and 0.59 in the 1st and 20th cycles, respectively, which are 28% and 18% higher than those of Ca(OH)₂. Furthermore, carbonation conversion of this Mn‐modified Ca(OH)₂ decreases by within 1.00% from cycle 5 to 20. Mn addition boots oxygen vacancy, increases the surface area and pore volume, and reduces the energy barrier for carbonation of Ca(OH)₂. Mn‐modified Ca(OH)₂ as CO₂ sorbent appears promising.