Investigation of Sulfation of CaO and CaCO3 in Eutectic CaF2–CaCl2

Abstract

To achieve deep cuts in greenhouse gas emissions from stationary point sources such as carbon intensive industry and fossil power generation, carbon capture and storage (CCS) will be a necessity. Carbon capture in molten salts (CCMS) is an emerging technology with high potential. In CCMS, the reversible reaction between calcium oxide (CaO) and carbon dioxide (CO2) to form calcium carbonate (CaCO3) is utilized, with the active substances being dissolved or partly dissolved in molten salts. CCMS has previously demonstrated promising properties for carbon capture with simulated flue gases on the laboratory scale. The next step in the technology development is to test the performance under more realistic gas compositions. In the present work, the focus is on sulfur dioxide (SO2) and oxygen (O2), a combination often coexisting with CO2 in flue gases from power production or industry. Experiments have been performed to investigate indirect sulfation of CaO and direct sulfation of CaCO3 in a eutectic mixture of calcium fluoride (CaF2) and calcium chloride (CaCl2) because it is not known whether these are thermodynamically favored in the molten state. Simulated flue gases containing 0.5 vol % SO2 and 3 vol % O2 have been used. In addition, combined capture experiments with 14 vol % CO2 and levels of SO2 up to 0.5 vol % have been performed. The composition of the outlet gas from the reactor was analyzed by Fourier transform infrared spectroscopy. The results indicate that both direct and indirect sulfation occur and that carbon capture is only slightly, but not significantly, inhibited in accordance with the level of SO2 in the flue gas. Furthermore, it was shown that CO2 capture in molten salts is possible at temperatures above the decomposition temperature for CaCO3 in solid-state calcium looping. This is attributed to higher activities when the active substances are dissolved in molten salts. Based on experimental data and thermodynamic modeling, it was estimated that the ratio of activity coefficients for CaO and CaCO3 is between 5 and 6 when dissolved in eutectic CaF2–CaCl2.