Effect of pressure and thermal cycling on compatibility in CO₂ for concentrated solar power applications

A lifetime model is being developed for supercritical CO₂ (sCO₂) compatibility for the 30 year duty life for concentrated solar power (CSP) applications at <700°C to achieve higher efficiencies than other power cycles. Alloys 740H, 282, 625 and Fe-base alloy 25 are being evaluated in 500-h cycles at 1 bar and 300 bar, and 10-h cycles in 1 bar industrial grade CO₂ at 700°- 800°C. For comparison, companion experiments are being conducted in 1 bar air or O₂. Using mass change, all of the alloys showed low mass gains with parabolic rate constants below the performance metric after 1000 h. However, alloy 25 showed a higher rate at 700°C in 300 bar sCO₂ and did not follow an Arrhenius relationship. After 1500 h in 1 bar CO₂, a much faster rate was observed for alloy 25 due to the formation of Fe₂O₃, but a similar increase was not observed in 300 bar CO₂. Oxide thickness measurements have been completed after 1000 h in each condition. Only minor differences were noted between the 1 and 300 bar exposures. Up to 4,000 h exposures in 10-h cycles has not resulted in evidence of scale spallation but very small mass losses for alloy 625 were consistently observed. As longer exposures times are completed, quantification of the reaction products as a function time will be used to better model the degradation rate and additional characterization techniques will be included to further develop the model.