Hydrolysis of molten CaCl<sub>2</sub>-CaF<sub>2</sub> with additions of CaO

Espen Olsen; Maria Hansen; Heidi S. Nygård; Espen Olsen; Maria Hansen; Heidi S. Nygård

doi:10.3934/energy.2017.6.873

AIMS Energy

2017, Volume 5, Issue 6: 873-886. doi: 10.3934/energy.2017.6.873

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Hydrolysis of molten CaCl₂-CaF₂ with additions of CaO

Faculty of Science and Technology, Norwegian University of Life Sciences (NMBU), Drøbakveien 31, N-1432 Ås, Norway

Received: 19 July 2017 Accepted: 19 October 2017 Published: 30 October 2017

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.
- carbon capture,
- molten salts,
- hydrolysis,
- flue gases,
- Ca-looping
Citation: Espen Olsen, Maria Hansen, Heidi S. Nygård. Hydrolysis of molten CaCl2-CaF2 with additions of CaO[J]. AIMS Energy, 2017, 5(6): 873-886. doi: 10.3934/energy.2017.6.873

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Abstract

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.

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