Effect of pressure and impurities on oxidation in supercritical CO2

Both indirect‐ and direct‐fired supercritical CO₂ cycles for high‐efficiency power generation are expected to have impurities that may greatly alter the compatibility of Fe‐ and Ni‐based structural alloys in these environments. Recent work has attempted to quantify reaction rates at 750°C in simulated laboratory environments with controlled impurity levels at ambient pressure, as well as under supercritical conditions (30 MPa). With low impurity levels in research and industrial‐grade CO₂, pressure appeared to have only a limited effect on oxide thickness and internal oxidation and reaction products were similar to those formed in laboratory air. However, a direct‐fired simulation at 750°C/30 MPa in CO₂ + 1%O₂ + 0.25%H₂O has found an increased mass gain and characterization after 2,500‐hr exposures have found thicker reaction products, especially for Fe‐based alloys. At these impurity levels, pressure may have a significant effect on the role of impurities.