Kinetics of CO<inf>2</inf> Capture with Calcium Oxide during Direct Air Capture in a Fluidized Bed

Abstract

The bulk of research in carbon capture involves high CO2 concentrations. This work instead describes the kinetics of CaO carbonation in mixtures with pCO2 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 U/Umf 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.