Molten Chloride Salt Technology for Next-Generation CSP Plants: Compatibility of cost-effective Fe-based steels with Mg-purified molten MgCl2-KCl-NaCl at 700°C

Abstract

The next-generation concentrating solar power (CSP) plant can be equipped with a high-temperature thermal energy storage (TES) system and supercritical CO2 (sCO2) Brayton power cycle, whose operating temperatures are higher than 700 °C, for a higher energy conversion efficiency and lower levelized cost of electricity (LCOE). MgCl2-KCl-NaCl is a promising candidate of such high-temperature TES material and heat transfer fluid (HTF) due to its low cost and excellent thermophysical properties. Using Fe-based (Fe: ≥50 wt.%) alloys as the main structural material for the chloride-based TES system is the key to ensuring its cost competitiveness. However, it is universally believed that Fe-based alloys have unacceptably high corrosion rates in unpurified molten MgCl2-KCl-NaCl. Theoretically, purification with Mg metal can reduce the corrosion rates of Fe-based alloys to acceptable low levels (<15 µm/year). In this work, to experimentally verify this theory and prove feasibility of cost-effective Fe-based alloys as the high-temperature structural material for the chloride-based TES, Stainless Steel (SS) 310 and Incoloy (In) 800H were immersed in the Mg-purified molten MgCl2-KCl-NaCl at 700 °C for 2000 hours. The SEM and EDX results show that after immersion, the typical Cr-depleted corrosion layers on the samples are negligibly thin (< 3 µm). Based on mass loss and microstructural analysis results, the corrosion rates of SS 310 and In 800H are only 12.4 and 5.3 µm/year, respectively. Therefore, from the perspective of corrosion, the cost-effective Fe-based alloys possess good compatibility with the Mg-purified molten MgCl2-KCl-NaCl. According to preliminary calculation, the cost of TES using chlorides at >700°C could be potentially reduced close to that using commercial nitrates/nitrites at ≤565°C, leading to a significant reduction of the LCOE of CSP with higher operating temperatures.