High-Temperature compatibility of structural alloys with supercritical and subcritical CO2

Abstract

In searching for new working fluids for power generation,supercritical CO2 (sCO2) offers some attractive features forefficient cycles. However, compatibility with structural alloysis a concern. NiCr-based alloys have excellent compatibilityat 600°-800°C at 20-30 MPa sCO2. However, conventionalsteels have restrictions in temperature because of carburization andaccelerated oxidation in sCO2, similar to observations in CO2. Toassess the impact of carburization on steel mechanical properties,small (25mm long) dogbone tensile bars are being exposed and testedafter exposure at 450°-650°C. Only highly alloyed advanced austeniticsteels are resistant to carburization at 650°-750°C, suggesting that Crrichoxide scales are good barriers to C ingress. Above 800°C, it isonly possible to conduct subcritical evaluations at this time, but initialresults suggest most conventional high-temperature Fe- and Ni-basedalloys are more rapidly degraded by CO2 at higher temperatures.Coatings are a potential solution that require more study.