Stabilization of Solar Salt at 650 °C – Thermodynamics and practical implications for thermal energy storage systems

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

“…Additionally, oxide formation rates with the same NO partial pressure are quite similar (Figure 5, bottom right: 6 × 10 −6 mol%/h) and are not affected by the nitrite ion concentration. This finding agrees with the result of the previously mentioned study [28], where the oxide formation rate did not correlate with the nitrite concentration but with the temperature. In addition, in a similar way, the oxide ion equilibrium levels show a weak impact on the nitrite concentration (Figure 4).…”

“…It is concluded that all NO x gas species (200 ppm) reduce the extent of the nitrite decomposition (Equation ( 2)), which allows for equilibration of the nitrate-nitrite reactions without the impact of oxide ion formation. This is the first evidence for the suggestion in the literature [26,28] that above 600 • C, nitrite decomposition needs to be suppressed to achieve a proper thermodynamic nitrate-nitrite equilibrium (see Section 3.5, Equation ( 7)). When comparing 200 ppm of NO and NO 2 , both lead to stable oxide ion concentrations at 600 • C and 620 • C, whereas NO 2 gives lower oxide ion values than NO.…”

“…In their kinetic study, Nissen and Meeker [11] have already revealed that it is necessary to eliminate oxygen (and nitrite) mass transport effects in order to achieve reproducible data, but they assumed that there were no errors in the thermodynamic equilibrium data. However, in a more recent study [28], it was found that above 600 • C, all of the present thermodynamic datasets for solar salt are most likely afflicted by decomposition reactions or mass transport limitations and do not reflect a thermodynamic equilibrium. In consequence, the interpretation of those results leads to false thermodynamic predictions, especially on the high-temperature properties of solar salt.…”

“…As a result, the content of nitrate and nitrite is given in absolute values based on a standardized calibration of the IC method. A detailed description of the experimental procedure and calibration is given elsewhere [28]. The oxide ion contents were determined by automated acid base titration (Metrohm Titrando 800, Herisau, Switzerland) under an inert purge gas of N 2 (5.0-grade Linde gas).…”

“…It has been shown in a different study [27] that nitrous gases stabilize the molten salt, and an established Equilibrium levels were calculated from anion content after the reaction quotient (ratio between educt and product) obtained a stable value. For NO 3 − and NO 2 − (Equation (1)), this is the case after approximately 300 h. Oxide formation rates are often afflicted with a strong error, especially if a corrosion reaction cannot be excluded [28]. It has been shown in a different study [27] that nitrous gases stabilize the molten salt, and an established oxide equilibrium allows comparison between different experiments.…”