Kinetic analysis of the carbonation reactions for the capture of CO2 from air via the Ca(OH)2–CaCO3–CaO solar thermochemical cycle

A thermogravimetric analysis of the carbonation of CaO and Ca(OH)2 with 500 ppm CO2 in air at 200–450 °C is performed as part of a three-step thermochemical cycle to capture CO2 from air using concentrating solar energy. The rate of CaO-carbonation is fitted to an unreacted core kinetic model that encompasses intrinsic chemical reaction followed by intra-particle diffusion. In contrast, the Ca(OH)2-carbonation is less hindered by diffusion while catalyzed by water formation, and its rate is fitted to a chemically-controlled kinetic model at the solid interface not covered by CaCO3. The rates of both carbonation reactions increase with temperature, peak at 400–450 °C, and decrease above 450 °C as a result of the thermodynamically favored reverse CaCO3-decomposition. Avrami's empirical rate law is applied to describe the CO2 uptake from the continuous air flow by CaO and Ca(OH)2, with and without added water. The addition of water vapor significantly enhances the reaction kinetics to the extent that, in the first 20 min, the reaction proceeds at a rate that is 22 and nine times faster than that observed for the dry carbonation of CaO and Ca(OH)2, respectively.