James R. Keiser

In turbomachinery, internal leakage flow accounts for up to 3% of the total thermodynamic cycle energy loss. Tradeoff must be made between the sealing efficiency (smaller clearance) and the friction and wear issues for interfering with the shaft (larger clearance). This ORNL-Danfoss joint effort developed a novel hybrid seal composed of carbon nanotube (CNT)-coated metal meshes. The CNT growth process was based on a self-catalyzing chemical vapor deposition and these multiwall CNTs were well aligned with high crystallinity. This hybrid material structure takes advantage of the CNT’s low-friction nature and uses the metal mesh as an extendable backbone. Full-scale experimental seals were designed, fabricated, and optimized for sealing performance and durability. The CNT-coated metal mesh seal demonstrated superior gas sealing efficiency to the baseline labyrinth seal and significantly improved shaft surface protection compared with the state-of-the-art superalloy brush seal on the static rig and full-scale compressor dynamometer tests. The CNT-metal mesh seal is low-cost and scalable and can potentially benefit wide applications, including CSP and other power generation, marine, automotive, and HVAC.

Direct-fired supercritical CO₂ (sCO₂) power cycles are a pathway to low-CO₂ fossil energy but contain high levels of O₂ and H₂O in the sCO₂. The effect of impurities on structural steels was investigated at 450°-650°C in 300 bar sCO₂. The test matrix included 9 and 12%Cr ferritic-martensitic (FM) steels and conventional and advanced austenitic steels exposed for 1000-2000 h with and without additions of 1%O₂ and 0.1%H₂O to simulate the cycle after water removal. For FM steels, the mass gains and scale thicknesses were similar with and without impurities with the formation of thick, duplex Fe-rich scales in all cases including the observation that Fe₂O₃ only formed with 1%O₂. For the austenitic steels, higher mass gains were observed at all temperatures with increased formation of Fe-rich oxides when impurities were added. Carbon ingress was assessed by bulk combustion analysis, glow discharge optical emission spectroscopy and measuring post-exposure room temperature tensile properties. Bulk C content was strongly increased at 650°C but not at 450° or 550°C.

The carburizing supercritical CO₂ (sCO₂) environment limits the use of lower cost steels in the lower temperature (450–650°C) portions of the sCO₂ Brayton cycle because of concerns about internal carburization and embrittlement. Results on a ferritic–martensitic steel and conventional and advanced austenitic steels at 450–650°C in 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. Pack Al and Cr coatings were evaluated at 650°C on T91 and 316H substrates and showed some benefit for up to 2000 h at 650°C, especially without impurities. However, characterization indicated Al₂O₃ was not formed and Cr‐rich carbides formed in the Cr coatings. With the addition of impurities in the sCO₂, the coatings were less protective at 650°C. Subsequent exposures at 600°C in sCO₂ showed similar behavior. Postexposure evaluations included measuring the bulk C content and room temperature tensile properties. Improvements were indicated but the tensile results were complicated by the high temperature pack coating process affecting the substrate properties.

The U.S. Department of Energy's Office of Scientific and Technical Information

The U.S. Department of Energy's Office of Scientific and Technical Information