CO2 Capture Performance of CaO in Molten NaCl-CaCl2: Effect of Operating Parameters and KCl Modification of Molten Salt

Dissolving CaO into molten salts can overcome its structural collapse and thermal sintering issues during CO2 capture by constructing a homogeneous liquid reaction environment. Herein, a molten CaO/NaCl-CaCl2 system was developed, and the effects of operating parameters and KCl addition on its CO2 capture performance were systematically investigated. Compared with solid CaO, molten NaCl-CaCl2 significantly boosted the CO2 capture capacity of CaO. CaO was fully dissolved in molten NaCl-CaCl2 at 600°C–700°C and dissociated into Ca2+ and mobile alkaline O2−, forming a uniform liquid ionic environment for CO2 capture, where O2− anions served as active sites for CO2 conversion to CO32−. The marked enhancement of CO2 capture of CaO in molten NaCl-CaCl2 was attributed to eliminated sorbent sintering, accelerated interfacial O2− transport, and bypassed CaCO3 layer barrier, with the molten salt exhibiting high stability and uniform dispersion of CaO/CaCO3 during the process. 650°C, 200 mL·min−1 10% CO2/Ar gas, and 10 wt.% CaO relative to molten salt were identified as the favorable operating parameters under the testing conditions, achieving a CO2 uptake of 14.42 mmol·g−1 and CaO conversion rate of 80.73%. This condition balanced CO2 capture kinetics, interfacial O2−/CO2 transfer, and CO32− stability in Molten salt; excess temperature aggravated carbonate decomposition, while improper gas flow rate or CaO loading reduced efficiency. The addition of 10 wt.% KCl further improved the capture performance by lowering molten salt viscosity and promoting O2− migration, especially relieving O2− depletion in the mid-late capture stage to sustain interfacial activity, lifting CO2 uptake to 15.56 mmol·g−1 and CaO conversion rate to 86.56%. This study demonstrates that molten NaCl-CaCl2 integrated with compositional tuning is an effective strategy for enhancing CaO-based high-temperature CO2 capture.