Molten salt chemistry in nitrate salt storage systems: Linking experiments and modeling

Solar Salt above 600 °C: Impact of Experimental Design on Thermodynamic Stability Results

“…The entire dataset presented herewith (red symbols), which was performed in an experimental setup with a reduced corrosion impact, now, indeed, shows a significant shift towards lower values of ln(K 1 ) compared to the published data with steel components (black symbols). Consequently, the present paper confirms that solar salt is more stable than previous studies suggested, and the reported reaction enthalpy ( ∆H = 95 kJ/mol) and entropy (∆S = 85 J/molK) published by Sötz [12] are also accurate for 600 and 620 • C. Another verification of this concept is given with the experiment performed at 620 • C and 20% oxygen (without NO). Here, the ln(K 1 ) value is the second highest (−2.8030) of the study because oxide ion formation occurred unopposed.…”

“…In a recent study [28], it was shown that above 600 • C, also at a 100 g scale, oxide ion formation leads to a shift of the equilibrium position to the nitrite side and higher values of ln(K 1 ) (Equation ( 7)). The dotted line is extrapolated from low-temperature data from another study by Sötz et al 2019 [12] without measurable amounts of oxide ions. This line is interpreted as the ideal nitrate-nitrite equilibrium composition, without the influence of oxide ion formation and without mass transport effects (at a mg sample scale).…”

“…So far, the first decomposition step of NO 3 − -anions (Equation ( 1)) has been proven valid [11,12]. For the second decomposition step, several reaction mechanisms were suggested, and, for solar salt above 600 • C, the majority of them can be described by a combination of Equations ( 2) and (3) [13][14][15][16].…”

“…In consequence, the interpretation of those results leads to false thermodynamic predictions, especially on the high-temperature properties of solar salt. However, the authors suggested that the data of Sötz et al [12] generated at temperatures below 550 • C are in fact applicable for elevated temperatures if side reactions are reduced to a minimum. With this study, by more careful experimental design improvements, the requirements for generating data without side reactions were met.…”

“…Reprinted/adapted with permission from Ref [27]. 2021, MDPI,[12] 2019, Elsevier B.V.,[32] 2020 Elsevier B.V.,[26], 2020, International Solar Energy Society,[28] 2023, Elsevier B.V.)…”