Hydrolysis of molten CaCl₂-CaF₂ with additions of CaO

• Main Authors: Espen Olsen, Maria Hansen, Heidi S. Nygård
• Format: Article
• Language: English
• Published: AIMS Press 2017-10-01
• Series: AIMS Energy
• Subjects: carbon capture; molten salts; hydrolysis; flue gases; Ca-looping
• Online Access: http://www.aimspress.com/energy/article/1674/fulltext.html

Calcium halide based molten salts have recently attracted interest for a number of applications such as direct reduction of oxides for metal production and as liquefying agent in cyclic sorption processes for CO₂ by CaO from dilute flue gases (Ca-looping). A fundamental aspect of these melts is the possible hydrolysis reaction upon exposure to gaseous H₂O forming corrosive and poisonous hydrogen halides. In this work experiments have been performed investigating the formation of HCl and HF from a molten salt consisting of a 13.8 wt% CaF₂ in CaCl₂ eutectic exposed to a flowing gas consisting of 10 vol% H₂O in N₂. Hydrolysis has been investigated as function of content of CaO and temperature. HCl and HF are shown to be formed at elevated temperatures; HCl forms to a substantially larger extent than HF. Addition of CaO has a marked, limiting effect on the hydrolysis. Thermodynamic modeling of the reaction indicates activity coefficients for CaO above unity in the system. For cyclic CO₂-capture based on thermal swing, it is advisable to keep the temperature in the carbonation (absorption) reactor well below 850 ℃ while maintaining a high CaO content if molten CaCl₂ is employed. Similar conclusions can be drawn with regards to CaF₂.