Evaluation of Coatings to Improve Steel Compatibility in Supercritical CO₂

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.

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^1-5. However, CO₂ environments are known to carburize steels^6-20 which limits their usage to lower temperatures (450°C²¹ for 9%Cr steels) than in steam boilers²². Of particular concern are the O₂ and H₂O impurities associated with burning natural gas in the Allam cycle⁵, which may be the first economical fossil energy with CO₂ capture. Initial results suggested that these impurities increase the reaction rate^18,20,23,24.

Oxidation-resistant coatings are an obvious potential solution for sCO₂ environments and similar Cr- and Al- rich coatings have been investigated in CO₂ that were beneficial in steam^25-32. Beginning with a baseline of uncoated steel exposures in sCO₂ with and without impurities at 450°-650°C^19,20, pack coated steels were exposed for 500-1000 h and then characterized. In addition to examining reaction rates and characterizing reaction products, room temperature tensile properties were measured before and after exposure to confirm that the coatings were preventing ductility loss due to carburization.