A Kinetic Model of CO₂ Absorption in Molten CaO-CaF₂-CaCl₂

Abstract

Post-combustion CO₂ capture is a promising method for removing CO₂ from processes where emissions cannot be mitigated by renewable energy input and where the chemical reactions required for production emit CO₂, e.g. calcination of calcium carbonate (CaCO₃) for cement production. One promising capture method is carbon capture in molten salts (CCMS). CCMS is a thermal swing gas-liquid process that utilizes CaO carbonation to absorb CO₂. The molten salt used in this work is 15 wt% CaO in eutectic CaCl₂-CaF₂ (86.2 : 13.8 wt%). The CaCl₂-CaF₂-CaO system has been found to have high cyclic absorption capacity (0.6 g CO₂/g CaO), though reaction kinetics has yet to be studied. By utilizing a novel experimental setup, data is collected, and a kinetic model is developed, which can be used in a techno-economic evaluation. The model proposes a simplified description of the CaCl₂-CaF₂-CaO system, with the assumption that the reaction is a first order elementary reaction where CaO and CO₂ react to form CaCO₃ without any solubility of CO₂ in the molten salt. CO₂ concentration, temperature, wt% CaO and surface area of molten salt are parameters in the proposed kinetic model. The result is a kinetic model that accurately fits the experimental data with an R² value above 0.98. It has been found that increasing the CO₂ concentration and decreasing the temperature yield a higher CaO to CaCO₃ equilibrium conversion.