Salt purification for reducing corrosivity of molten chloride salts for thermal energy storage in next-generation concentrated solar power plants

Abstract

Molten MgCl2/NaCl/KCl chloride salts are promising high-temperature (up to 800°C) thermal energy storage (TES) materials in next-generation concentrated solar power (CSP) plants. They offer a potential for higher energy conversion efficiency and lower levelized cost of electricity (LCOE) due to low material costs and high thermal stability, but can exacerbate corrosion of structural materials at high temperatures. Corrosion rates of metallic structural materials in contact with the molten MgCl2/NaCl/KCl salts are closely related to the concen-tration of the main corrosive impurity MgOH+ in the molten chlorides, which is one of the hy-drolysis products of the hydrated MgCl2. In this work, an improved titration method is used to accurately measure the MgOH+ concentration. The electrochemical method based on cyclic voltammetry (CV) for in-situ monitoring MgOHCl concentration is calibrated using this im-proved titration process. Subsequently, a 1000 h corrosion test is performed to study the cor-rosion behavior of selected alloys in the presence of excess Mg corrosion inhibitor (2.8 wt.%) in the molten salt. The results of CV and titration show that the peak current densities obtained from CV are proportional to the concentrations of MgOH+ obtained from titration. The slopes of the peak current density vs. the concentration of MgOH+ at 500, 600 and 700 °C deter-mined in this work are 5.5, 7.0 and 2.1, respectively. These slopes and the Mg corrosion inhibitor are expected to be used for impurity monitoring and reduction in a corrosion control sys-tem (CCS) for the molten MgCl2/NaCl/KCl salts TES system.