Effect of pressure and thermal cycling on long-term oxidation in supercritical CO₂

Concentrating solar power (CSP) plant designers are interested in supercritical CO₂ (sCO₂) for the power block to achieve >50% electrical efficiency at >700°C. The goal of this project was to develop a long-term (>100 kh) lifetime model for sCO₂ compatibility using 10-15 kh laboratory exposures. Alloys N06625, N07740 and N07208 were evaluated in long-term exposures at 750°C using 500-h cycles in laboratory air, 1 bar industrial grade CO₂ and 300 bar supercritical CO₂, and using 10-h cycles in 1 bar CO₂ and O₂. Mass change data and quantification of the oxide scale thickness and depth of internal attack after 1,000-10,000 h exposures indicate that these materials are compatible with the sCO₂ environment at 750°C. Thermal cycling to simulate the solar duty cycle did not result in scale spallation after 15 kh and comparison of the 1 and 300 bar results did not show a significant effect of pressure on the reaction and no significant internal carburization was observed under these conditions.