Reactivity of CaO with CO2 in molten CaF2–NaF: formation and decomposition of carbonates

• Published in: Energy science & engineering 2016-05, Vol.4 (3), p.205-216
• Main Authors: Tomkute, Viktorija, Solheim, Asbjørn, Sakirzanovas, Simas, Olsen, Espen
• Format: Article
• Language: English
• Subjects: Calcium fluoride; calcium oxide; carbon dioxide; carbonate looping; molten salt; sodium fluoride
• ISSN: 2050-0505; 2050-0505
• DOI: 10.1002/ese3.120

The characteristics of CO2 reacting with CaO in a molten eutectic mixture of CaF2 and NaF has been investigated. Calculations of the Gibbs free energy, temperature analysis of the decomposition of the formed carbonates, and XRD analyses of quenched samples taken during CO2 absorption or desorption were employed to identify the phases present in the melt. Efficient CO2 absorption from a simulated flue gas was observed, due to a combined reaction where CaO initially reacts with CO2 and forms CaCO3. Subsequently, Na2CO3 is formed by an ion exchange reaction between CaCO3 and NaF. It was found that the CaO activity is highest in the temperature range 826–834°C. Increasing the CaO concentration from 5 to 20 wt% in the molten salt resulted in reduced CO2 reactivity efficiency, probably because of precipitation and agglomeration of the sorbent. The total carbonation conversion was independent of the CO2 concentration in the inlet gas, and the sorbent carrying capacity was in the range 0.722–0.743 g CO2/g CaO corresponding to 0.037–0.144 g CO2/g total liquid. Decarbonation was conducted by raising the temperature. 40% conversion back to CaO was recorded at 1160°C. The recorded curves for the CO2 concentration in the outlet gas exhibited a rapid desorption step followed by a slow step. Calcium looping by CaO dispersed in a molten salt containing NaF has been studied.