Purification strategy and effect of impurities on corrosivity of dehydrated carnallite for thermal solar applications

Abstract

Thermal and chemical purifications empower molten chlorides for next generation concentrating solar power (CSP) applications.

Molten Chloride Thermophysical Properties, Chemical Optimization, and Purification

“…All experiments with Mg addition (0.25-0.75 wt.%) showed signs of excess Mg droplets at the bottom of the salt sample by visual post-purification inspection. Similar behavior had been observed in [28]. Therefore, 0.25 wt.% was the least amount of Mg that provided the best balance between effective reduction of MgOHCl and minimum excess of Mg after purification.…”

“…This range is believed to be an estimation only because (1) the calculation is very sensitive to pH changes because pH is defined as the logarithmic of H + ions and the specified resolution of pH probe is only 0.01 26 , and (2) some HCl molecules may not be captured by the buffer if they react with the furnace vessel hardware and outlet gas tubing 27 and/or escape the vessel hardware through leaks. 28 In addition, the total weight loss during the purification step is 0.578 ± 0.098 wt.%, 29 higher than the estimated weight fraction of HCl. Such discrepancy in mass balance again suggests that not all mass loss is from HCl release and moisture can be responsible for the rest of the mass loss, which corroborates the assumption used by the calculation in the previous section that not all released H2O participates in hydrolysis to form HCl.…”

“…27 Inconel vessel can indeed react with HCl at high temperatures. 28 The purification is always performed with a slight positive pressure (up to a few psig) in the furnace vessel. 29 Based on multiple purifications with different Mg addition and gas sparging rate.…”

“…Molten chloride salts have been proposed as an alternative HTF chemistry for the next generation (Gen3) CSP technology operating at higher temperatures in order to maximize the benefit of using the sCO2 power cycle [1,2]. A significant number of research efforts [3,4,[13][14][15][16][17][18][19][20][21][22]5,[23][24][25][26][27][28][6][7][8][9][10][11][12] have been devoted to investigating molten chlorides' chemistry, thermodynamics, corrosion, and properties, especially the ternary system of NaCl-KCl-MgCl2 due to its availability and low cost.…”

“…The primary corrosion mechanism is believed to be correlated to the presence of magnesium hydroxychloride, MgOHCl, impurity at the operation temperature [2,19,[28][29][30]. This corrosive impurity is fundamentally caused by the hygroscopic nature of the MgCl2 component in the ternary salt (i.e., MgCl2 takes various forms of hydrates, such as MgCl2•H2O, MgCl2•2H2O, MgCl2•4H2O, and MgCl2•6H2O).…”