Kinetics\nof CO<sub>2</sub> Capture with Calcium Oxide\nduring Direct Air Capture in a Fluidized Bed

The bulk of research\nin carbon capture involves high\nCO<sub>2</sub> concentrations. This work instead describes the kinetics\nof CaO\ncarbonation in mixtures with <i>p</i>CO<sub>2</sub> between\n0.38 and 2.70 vol % and at temperatures between 400 and 650 °C.\nThe reaction was studied in a bed of SiO<sub>2</sub> fluidized at\nflow rates corresponding to <i>U/U</i><sub>mf</sub> of ∼4.\nLower concentrations of CO<sub>2</sub> connected to the rates of mass\ntransfer of CO<sub>2</sub> of the same order of magnitude as the rates\nof carbonation, thus, were eliminated from the kinetic analysis. The\nintroduction of steam to the gas mixture (2 vol %) increased the rates\nof carbonation, demonstrating a pseudocatalytic effect, yet diminishing\nat higher temperatures. A rate expression generally accepted in the\nliterature for high concentration CO<sub>2</sub> in CaO carbonation\nwas assessed for its applicability at near-equilibrium conditions,\ndemonstrating that the order of the rate expression changes between\n0 and 1, increasing at higher temperatures. Using nonlinear regression,\nthe experimental values were fitted to the Langmuir–Hinshelwood\nrate expression. The obtained parameters indicate CO<sub>2</sub> sorption\nand desorption being equilibrated, with the overall capture dominated\nmainly by slow kinetics of chemical reactions.