Chemical equilibria and intrinsic kinetics of reactions in molten nitrate salt

Kurzfassung

The nitrate salt mixture Solar Salt is applied as heat storage material in large-scale energy storage systems that are essential components of concentrating solar power plants to enable dispatchable electricity production. The maximum storage temperature does not exceed 565 °C in existing systems, but is intended to increase in the future to improve the efficiency and the capacity of the storage technology. However, chemical reactions in the molten Solar Salt intensify and accelerate with increasing temperature, leading to thermal instability of the storage material, and to decomposition products including corrosive ions and toxic gases. The decomposition process must be understood to maintain material stability at increased temperatures. In this thesis, the reactions that form the relevant decomposition products are experimentally investigated and mathematically described. The reaction of nitrate ions to nitrite ions constitutes the first step of the decomposition process, and the chemical equilibrium is described in two temperature regimes (450-550 and 560-630 °C). The intrinsic kinetics of the nitrite formation are investigated up to 550 °C by thermogravimetric analysis, and a differential rate law for both forward and reverse reaction kinetics is derived. In a second step, the nitrite ions are considered to decompose to oxide ions. Uniquely, this thesis presents experiments that prove chemical equilibrium including oxide ions in Solar Salt. Adding nitrous gases to a synthetic air purge stabilizes the oxide content, which is demonstrated at 600 and 620 °C. These results are particularly valuable, because oxide ions have shown to aggravate steel corrosion in Solar Salt, and thereby reduce the lifetime of the storage system. The intrinsic kinetics of the oxide formation are investigated in air atmosphere up to 630 °C. A mathematical kinetics description is developed, and the parameters are obtained by fitting the experimental results. Overall, the decomposition reactions form a consistent network, characterized by chemical equilibria and intrinsic kinetics of two reactions. The results can improve molten salt heat storage technology by providing Solar Salt stability predictions at existing and higher operation temperatures.