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Compatibility Of Steels At 450°-650°C In Supercritical CO2 With O2 And H2O Additions

Using supercritical CO2 (sCO2) as a working fluid is being explored for a number of power generation technologies including fossil, nuclear, geothermal, concentrating solar power (CSP) and waste heat recovery1-7.  The various sCO2 cycles are attractive because of the low critical point (31°C/73.8 bar) and the reduced work of compression compared to an ideal gas.  While CO2 is sometimes described as inert, there is a long history of component degradation in subcritical and supercritical CO2 and a particular concern about internal carburization8-16

Product Number: 51322-18018-SG
Author: B. A. Pint, M. J. Lance, R. Pillai, J. R. Keiser
Publication Date: 2022
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Direct-fired supercritical CO2 (sCO2) power cycles are being commercialized to revolutionize the use of fossil fuels as a low-emission power source.  However, the cycle will increase O2 and H2O in the sCO2 and the implications of these additions have not been fully studied, particularly for lower cost steels that are needed in the lower temperature segments of the plant.  Representative 9 and 12%Cr ferriticmartensitic (FM) steels and conventional and advanced austenitic steels were evaluated at 450-650°C in sCO2 with 1%O2 and 0.1%H2O at 300 bar to determine their maximum use temperatures. Compared to research grade (low impurity) sCO2 in indirect-fired cycles, the mass gains and scale thickness were not significantly changed for FM steels: both formed thick duplex Fe-rich scales with and without impurities.  For austenitic steels, higher mass gains were observed at all temperatures with increased Fe-rich oxide nodule formation.  After 1000 h at 650°C, the measured bulk C content was high for all of the steels with the addition of impurities suggesting a lower maximum operating temperature for steels. The impact of the environment on the post-exposure room temperature tensile properties was also evaluated and compared to 1000 h Ar anneals.

Direct-fired supercritical CO2 (sCO2) power cycles are being commercialized to revolutionize the use of fossil fuels as a low-emission power source.  However, the cycle will increase O2 and H2O in the sCO2 and the implications of these additions have not been fully studied, particularly for lower cost steels that are needed in the lower temperature segments of the plant.  Representative 9 and 12%Cr ferriticmartensitic (FM) steels and conventional and advanced austenitic steels were evaluated at 450-650°C in sCO2 with 1%O2 and 0.1%H2O at 300 bar to determine their maximum use temperatures. Compared to research grade (low impurity) sCO2 in indirect-fired cycles, the mass gains and scale thickness were not significantly changed for FM steels: both formed thick duplex Fe-rich scales with and without impurities.  For austenitic steels, higher mass gains were observed at all temperatures with increased Fe-rich oxide nodule formation.  After 1000 h at 650°C, the measured bulk C content was high for all of the steels with the addition of impurities suggesting a lower maximum operating temperature for steels. The impact of the environment on the post-exposure room temperature tensile properties was also evaluated and compared to 1000 h Ar anneals.

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