Phase Equilibria Evaluation for CO2 Capture: CaO-CaF2-NaF, CaCO3-NaF-CaF2, and Na2CO3-CaF2-NaF

This paper presents partial phase relations for the systems NaF-CaF2, NaF-CaF2-CaO, NaF-CaF2-CaCO3, NaF-CaF2-NaCO3, and NaF-CaF2-Na2CO3-CaCO3. The data were obtained by thermal analysis (TA), thermodynamic calculations (Fact Sage), and X-ray diffraction (XRD) of quenched samples. There was particular emphasis on revealing the phase formations of CaO, CaCO3, and Na2CO3 in the eutectic composition of NaF-CaF2 for the development of a novel carbon capture technology. Therefore, a thermal analysis of the binary CaF2-NaF system was performed first. A eutectic point was found at 31.9 mol % CaF2 and 814.8 degrees C. The liquidus isotherms and phase regions of the Na-Ca//F-O and Na-Ca//F-CO3 systems were mapped using FactSage. In addition, some compositions in the NaF-CaF2-CaO and NaF-CaF2-Na2CO3-CaCO3 systems were studied by TA. XRD analysis of the quenched samples was applied for phase identification. Based on the FactSage simulation and the experimental data, the solubility of CaO increases with increasing CaF2 concentration in NaF melt. It was derived that carbonates (Na2CO3 and CaCO3) in NaF-CaF2 are present as intermediate compounds. The reactions of NaF and CaCO3 and the formation of complex carbonates are discussed. In general, there is good agreement between experimental data and simulations.