Effect of Impurities on the Compatibility of Steels in Supercritical CO₂ at 450∘-650∘C

Direct-fired supercritical CO₂ (sCO₂) power cycles are a pathway to low-CO₂ fossil energy but contain O₂ and H₂O in the sCO₂ from combustion. The effect of impurities on structural steels was investigated at 450^∘-650 ^∘C in 30 MPa sCO₂. The test matrix included 9 and 12%Cr ferritic-martensitic (FM) steels and conventional and advanced austenitic steels exposed for 1000-2000 h with and without additions of 1%O₂ and 0.1%H₂O to simulate the cycle after water removal. For FM steels, the mass gains and scale thicknesses were similar with and without impurities with the formation of thick, duplex Fe-rich scales in all cases including the observation that Fe₂O₃ only formed with 1%O₂. For the austenitic steels, higher mass gains were observed at all temperatures with increased formation of Fe-rich oxides when impurities were added. Carbon ingress was assessed by bulk combustion analysis, glow discharge optical emission spectroscopy (GDOES) and measuring postexposure room temperature tensile properties. Bulk C content was strongly increased at 650 ^∘C but not at 450^∘ or 550 ^∘C.