Effect of Impurities on Supercritical Carbon Dioxide Compatibility

This project will study the effect of impurities (e.g., O2, H2O) on compatibility of structural materials in supercritical carbon dioxide (sCO2) Brayton cycle systems, particularly for direct-fired systems. For economically scaling up to commercial power production, the increased use of Fe-based alloys is needed and impurity studies at (450-650 °C) will determine operating limitations for 9-12% Cr and austenitic steels. In addition to measuring reaction rates and characterizing reaction products, post-exposure room-temperature tensile properties will be used to quantify compatibility as a function of temperature, time, and impurity level. After establishing baseline behavior, coatings and shot peening will be evaluated to increase the maximum temperature capability of Fe-based structural alloys. This information will be used to continue the development of a lifetime model for various classes of structural alloys with and without surface modifications. Previously, high O2 impurity levels (0.25-1%) have been found to increase reaction rates of both Fe- and Ni-based alloys at 750 °C/300 bar (30 MPa). Further work is needed to isolate O2 and H2O effects including the use of isotopic tracers. An additional project goal is to understand creep behavior of thin-walled sections for Fe- and Ni-based alloy heat exchangers.