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

Direct-fired supercritical CO2 (sCO(2)) power cycles are a pathway to low-CO2 fossil energy but contain O-2 and H2O in the sCO(2) from combustion. The effect of impurities on structural steels was investigated at 450 degrees-650 degrees C in 30 MPa sCO(2). 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%O-2 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%O-2. 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 degrees C but not at 450 degrees or 550 degrees C.