Purification strategy and effect of impurities on corrosivity of dehydrated carnallite for thermal solar applications.

Youyang Zhao,Judith Vidal,Noah Klammer

doi:10.1039/c9ra09352d

Youyang Zhao, Judith Vidal + Show 1 more

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https://doi.org/10.1039/c9ra09352d

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Abstract

This paper presents a purification method for dehydrated carnallite (DC)—a commercial ternary MgCl2–KCl–NaCl salt—for concentrating solar power (CSP) applications based on a thermal and chemical treatment using the reduction power of Mg. The purification is effective at reducing MgOH+ by an order of magnitude—from around 5 wt% in non-treated salt to less than 0.5 wt% in post-purification salt. The corresponding decrease in the measured corrosion rate of Haynes 230 at 800 °C from >3200 μm per year to around 40 μm per year indicates that soluble MgOH+ is indeed correlated to corrosion. The addition of elemental Mg serves as both a scavenger of impurities and corrosion potential control, which are considered the primary mechanisms for corrosion mitigation.

Highlights

To enable the use of a supercritical carbon dioxide Brayton power cycle, next-generation (Gen3) concentrating solar power (CSP) requires a heat-transfer uid (HTF) and thermal energy storage (TES) medium that can operate in the temperature range of 500–750 C
The MgOHCl content further drops during the chemical puri cation
It should be noted that multiple measurements were performed on non-treated Dehydrated carnallite (DC) salt, DC salt a er heat treatment at 117 C for 8 hours, and thermally + chemically puri ed salt (i.e., 1.7 wt% of Mg)

Summary

Introduction

To enable the use of a supercritical carbon dioxide (sCO2) Brayton power cycle, next-generation (Gen3) concentrating solar power (CSP) requires a heat-transfer uid (HTF) and thermal energy storage (TES) medium that can operate in the temperature range of 500–750 C. This operational temperature differs from the current Gen2's molten nitrate HTF and TES medium, which is stable up to $565 C. The hygroscopic nature of MgCl2 has been known for a long time in the literature.[1,2,3,4,5,6,7,8,9] Stepwise dehydration of MgCl2$6H2O to form lower hydrates can be achieved by heating MgCl2$6H2O to different temperatures as shown by eqn (1a)–(1d).[4]

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