Abstract:

Molten salt energy storage is important for applications such as concentrated solar power generation and enhancing the flexibility of coal-fired power plants. However, various applications require these salts to operate at significantly higher temperatures. This study systematically investigates the high-temperature stability and decomposition products of two typical nitrate salts, Solar Salt and Hitec under air conditions. The salts were prepared by melting pure salts at low temperatures, then dried and stored in glove boxes to protect them from water and oxygen. The oxidation and decomposition processes were characterized by thermogravimetric analysis. Ion chromatography was used to measure the proportion of nitrite ions, and potentiometric titration was used to determine oxide content. Prior to the formal experiments, all three testing methods underwent error calibration to ensure the accuracy of the test results. The thermogravimetry test conducted in a synthetic air atmosphere explains the reason why Hitec salt has a higher decomposition temperature (with 3% weight loss) than Solar salt. At high temperatures, Hitec salt first reacts with oxygen and gains weight at 450 ℃ before decomposition and subsequent weight loss because of its high initial concentration of nitrite ions. In contrast, solar salts begin continuous decomposition and weight loss at 500 ℃, generating oxygen and nitrogen oxides. This analysis indicates that the components of Hitec salts are more prone to change at high temperatures, which increases their melting point. Therefore, it cannot be used at higher temperatures. Long-term stability tests conducted in a muffle furnace revealed that, at high temperatures, Solar and Hitec salts produce approximately the same nitrite ratio after equilibrium. This is attributed to their similar Na⁺ to K⁺ ratios, which lead to similar chemical equilibria. Under an air atmosphere, the proportion of nitrite ions increased with temperature, rising from 4.36% (molar fraction) at 550 ℃ to 7.34% at 600 ℃. The test results on the formation of oxides at high temperatures showed that, in the open system, the Hitec salt continuously decomposes and produces oxides at 580 ℃. This result can be attributed to the direct emission of gaseous products. At 200 h, the oxide mass fraction of Hitec salt reached more than 9%, with obvious oxide precipitation. These findings demonstrate that the use of nitrate molten salts at higher temperatures may require the consideration of a closed system to inhibit oxide formation.

Key words: thermal energy storage, molten salt, high-temperature stability, solar salt, Hitec salt