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

Ca(OH) 2 can be used in CO 2 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 CO 2 , direct CO 2 uptake by Ca(OH) 2 occurs at lower temperature, leading to slighter sintering. However, the direct CO 2 uptake capacity of Ca(OH) 2 should be further enhanced. In this study, Ca(OH) 2 is doped with Mn to improve its direct CO 2 uptake capacity in carbonation-calcination-hydration cycles. The cyclic tests are conducted in a triple fixed-bed reactor. The mechanism of CO 2 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 CO 2 sorbent appears promising.