Micro- and Macrokinetics of the nitrite formation in ‘Solar Salt’

Abstract

A nitrate salt mixture with the commercial name ‘Solar Salt’ is the state-of-the-art storage material for sensible heat storage in concentrating solar power (CSP) plants. The storage units operate up to a temperature of about 565 °C. A higher maximum temperature improves the efficiency of the conversion heat-to-power, but also drives decomposition reactions. The decomposition products include harmful gases and corrosive ionic species. We investigated the intrinsic reaction kinetics of the reversible reaction of nitrate ions to nitrite ions and oxygen, which is fundamental in the reaction sequence. The applied rate law is expected to comprise both forward and reverse reaction, the chemical equilibrium, and the effect of the oxygen partial pressure. Consistent with the Arrhenius equation, the rate constants and consequently also the macroscopic reaction velocity increase with temperature. Kinetics description of the reaction in terms of a mathematical equation is included in a predictive model of the ‘Solar Salt’ composition. Thermogravimetric analysis (TGA) experiments revealed that mass transport limitations can be neglected in salt films with small thicknesses. Measurements with samples of 1 mm and smaller can be applied for investigation of reaction kinetics. A methodology was applied that allows separation of salt evaporation effects, and formation of reaction gases during the data analysis procedure. The reaction kinetics are analyzed with a series of TGA experiments with different temperature profiles. The results are used to validate a mechanistic reaction model that is based on a kinetic rate law. The obtained deviations of the simulation curves from experimental results are discussed with regard to the applicability of the microkinetics description. Deviations are higher for temperatures up to 450 °C, and decrease considerably above 500 °C, which is ascribed to smaller relative fluctuations of the TGA signal at high temperatures. It should be emphasized that this study is the first to validate intrinsic reaction kinetics of the nitrite and nitrate formation in ‘Solar Salt’. Finally, the reaction kinetics from the TGA experiments are compared to appropriate larger scale experiments with sample thicknesses of around 3 and 46 cm. The reaction is significantly decelerated leading to the conclusion that, besides intrinsic reaction kinetics, mass transport plays a major role in those experiments. Gas bubble formation and diffusion of oxygen gas in the molten salt seem to contribute to the macrokinetics.