Kinetics of CO₂ Capture with Calcium Oxide during Direct Air Capture in a Fluidized Bed

The bulk of research in carbon capture involves high CO₂ concentrations. This work instead describes the kinetics of CaO carbonation in mixtures with <i>p</i>CO₂ between 0.38 and 2.70 vol % and at temperatures between 400 and 650 °C. The reaction was studied in a bed of SiO₂ fluidized at flow rates corresponding to <i>U/U</i> _mf of ∼4. Lower concentrations of CO₂ connected to the rates of mass transfer of CO₂ of the same order of magnitude as the rates of carbonation, thus, were eliminated from the kinetic analysis. The introduction of steam to the gas mixture (2 vol %) increased the rates of carbonation, demonstrating a pseudocatalytic effect, yet diminishing at higher temperatures. A rate expression generally accepted in the literature for high concentration CO₂ in CaO carbonation was assessed for its applicability at near-equilibrium conditions, demonstrating that the order of the rate expression changes between 0 and 1, increasing at higher temperatures. Using nonlinear regression, the experimental values were fitted to the Langmuir-Hinshelwood rate expression. The obtained parameters indicate CO₂ sorption and desorption being equilibrated, with the overall capture dominated mainly by slow kinetics of chemical reactions.