Microkinetics of the reaction [formula omitted] in molten sodium nitrate and potassium nitrate salt

Abstract

Latest development in the concentrating solar power (CSP) tower technology for energy production and storage intends to find a heat transfer and storage material that provides stability at elevated temperatures. The binary nitrate salt mixture Solar Salt (sodium nitrate, 60 wt.% and potassium nitrate, 40 wt.%) is utilized up to its state-of-the-art temperature limit of 560 °C. Knowledge of the ongoing reactions is demanded to assess the potential of Solar Salt for higher temperature applications. The reversible reaction NO3-⇌NO2-+0.5 O2 is fundamental in a sequence of decomposition reactions. It is investigated by thermogravimetric experiments and ion chromatography post-analysis with regard to the chemical equilibrium, and to reaction kinetics. The reaction enthalpy and entropy of 97 +/- 9 kJ mol−1 and 85 +/- 12 J·mol−1 K−1, respectively, coincide with literature values. A detailed reaction mechanism based on literature findings is drafted and discussed. The corresponding kinetic rate law is simplified by means of rate law approximations and by focusing on the reduction reaction path. The kinetic rate constants of this study exceed previous findings, probably due to successful exclusion of mass transport limitations in the <1 mm thin salt samples. The activation energies and pre-exponential factors amount to 212 +/- 11 kJ·mol−1 and 694546727 +/- 6 s−1, and 115 +/- 11 kJ·mol−1 and 25770 +/- 6 s−1 for the reduction and oxidation reaction, respectively.