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TA 4.R: Supercritical Carbon Dioxide Brayton Cycle Commercial experience with sCO2 shaft seals to date is largely based upon CO2 transport and injection operations in support of enhanced oil recovery. This application has been successfully met in a range of low temperature supercritical fluid conditions at low to moderate shaft speeds (3,600 to 8,000 rpm) by non- contacting CO2-film dry-gas seals. Off-the-shelf solutions for high pressure sCO2 sealing do not presently meet the requirements for the high temperatures that would be required at the turbine end of a sCO2 power cycle. Bearings: Proper design and selection of bearings is frequently one of the most important factors to turbomachinery system performance and reliability. The technical challenge is to determine the approach that is suitable for the conversion system and develop a specific configuration that results in acceptably low frictional loss and also survives the corrosion and erosion environment. A number of different technologies may be considered, but, for high-speed, high-load applications such as power production turbomachinery, fluid film (hydrodynamic) bearings and rolling element bearings have historically been the industry workhorses. Materials Each application will have specific material design and code requirements. For example, ASME Code Section III stipulates that only five materials may be used for construction of nuclear reactor components for high temperature service. The use of the high-temperature nuclear construction materials will be required for all the components of the primary and secondary systems of advanced nuclear plants. However, it is not clear where the boundary of material use for the balance-of-plant will be drawn and where other materials may be used. Corrosion considerations for the use of the Section III high-temperature nuclear construction materials and for the use of materials in Sections I and VIII power boilers and unfired pressure vessels are not addressed by the ASME Code, other than by the general requirement stating that such corrosion shall not compromise the required section thickness or strength of the components. Materials reliability uncertainties include: carburization and sensitization, high-temperature corrosion, erosion, creep, and fatigue. The effects of material interactions can impact the design, reliability, and lifetime of essentially all system components. These uncertainties and R&D needs are discussed below. Carburization and Sensitization: Internal carburization and sensitization is a long-term concern for conventional austenitic stainless steels such as types 321 and 347, but is less of a concern for higher alloyed materials like alloy 709 and Ni-base alloys where the solubility of C is much lower. Similar carburization of ferritic-martensitic steels also has been observed at 550°-650°C. If these less expensive ferritic-martensitic and austenitic steels are to be used, R&D is needed on the long-term carburization behavior and maximum use temperature of these alloys to identify degradation mechanisms and for prediction of useful life. High Temperature Corrosion: Based on relatively short-term oxidation tests, the high-temperature oxidation behavior of candidate advanced ultra-supercritical steam cycle (A-USC) alloys was found to be as good as, or better, in sCO2 than in sH2O, making them candidate alloys for indirect sCO2 power cycle components also. These leading alloy candidates need to be tested for longer time periods (e.g. 1,000-5,000 hours) at 20-35 MPa at target temperatures (650-750°C) in sCO2 to establish oxidation reaction kinetics and quantify the rate of internal carburization. Furthermore, the long-term effect of various joining techniques (e.g. diffusion bonding, brazing, etc.) on reaction rates needs to be determined. For the direct-fired concept, only information at 1 bar is available on how impurities such as O2, H2O, and others introduced in the CO2 stream from the combustion of fuel may affect corrosion rates. Thus, oxidation/corrosion data results in supercritical conditions are needed. Erosion: Erosion is a significant issue for the closed Brayton Cycle systems at two sCO2 test facilities: SNL and Bettis. Substantial erosion in the turbine blade and inlet nozzle has been observed. It is believed that this is caused by residual debris in the loop and/or small particulates that originate from the spallation of corrosion products of different materials and at different locations within the loop. These particles are entrained through the nozzle vane and turbine, thus causing erosion. The problem in sCO2 cycles is exacerbated by the fact that 16 QuadrennialTechnologyReview2015PDF Image | Advancing Clean Electric Power Technologies
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