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Evaluation of Coatings to Improve Steel Compatibility in Supercritical CO2

Supercritical CO2 (sCO2) has many attractive features as a working fluid including its low critical point (31°C/73.8 bar) and the reduced work of compression compared to an ideal gas. Thus, it is being explored for many different applications including fossil, nuclear, geothermal, concentrating solar power (CSP) and waste heat recovery. However, CO2 environments are known to carburize steels6-20 which limits their usage to lower temperatures (450°C21 for 9%Cr steels) than in steam boilers.

Product Number: 51323-19207-SG
Author: B. A. Pint, R. Pillai, J. R. Keiser
Publication Date: 2023
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The use of a direct-fired supercritical CO2 (sCO2) power Allam cycle could revolutionize fossil energy as a low-emission power source. However, the carburizing sCO2 environment may limit the use of lower cost steels in the lower temperature portions of the plant because of concerns about embrittlement. Initial studies on representative ferritic-martensitic (FM) steels and conventional and advanced austenitic steels at 450-650°C in 300 bar (30 MPa) sCO2 with and without 1%O2 and 0.1%H2O additions have indicated that sCO2 environments will have lower maximum operating temperatures compared to steam plants. In this study, pack coated steels were evaluated including chromizing and aluminizing. Initial 500-1000 h results showed some benefit of coating especially for the Cr coatings at 600° and 650°C. Characterization included measuring the post-exposure room temperature tensile properties to assess the coating effect on embrittlement typically associated with carbon ingress.

The use of a direct-fired supercritical CO2 (sCO2) power Allam cycle could revolutionize fossil energy as a low-emission power source. However, the carburizing sCO2 environment may limit the use of lower cost steels in the lower temperature portions of the plant because of concerns about embrittlement. Initial studies on representative ferritic-martensitic (FM) steels and conventional and advanced austenitic steels at 450-650°C in 300 bar (30 MPa) sCO2 with and without 1%O2 and 0.1%H2O additions have indicated that sCO2 environments will have lower maximum operating temperatures compared to steam plants. In this study, pack coated steels were evaluated including chromizing and aluminizing. Initial 500-1000 h results showed some benefit of coating especially for the Cr coatings at 600° and 650°C. Characterization included measuring the post-exposure room temperature tensile properties to assess the coating effect on embrittlement typically associated with carbon ingress.

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