Identified corrosion and erosion mechanisms in SCO2 Brayton Cycles.

Abstract

Executive SummarySupercritical Carbon Dioxide (S-CO2) is an efficient and flexible working fluid for power production. Research to interface S-CO2 systems with nuclear, thermal solar, and fossil energy sources is currently underway. To proceed, we must address concerns regarding compatibility of materials, at high temperature, and compatibility between significantly different heat transfer fluids.Dry, pure S-CO2 is thought to be relatively inert [1], while the addition of ppm levels of water and oxygen result in formation of a protective chromia layer and iron oxide [2]. Thin oxides are favorable as diffusion barriers, and for their minimal impact on heat transfer. While S-CO2 is typically understood to be the secondary fluid, many varieties of primary fluids exist for nuclear applications. Molten salts, for use in the Molten Salt Reactor concept, are given as an example to contrast the materials requirements of primary and secondary fluids. Thin chromia layers are soluble in molten salt systems (nitrate, chloride, and fluoride based salts) [3-8], making materials selection for heat exchangers a precarious balancing act between high temperature oxidation (S-CO2) and metal dissolution (salt side of heat exchanger).Because concerns have been raised regarding component lifetimes, S-CO2 work has begun to characterize starting materials and to establish a baseline by analysis of 1) as-received stainless steel piping, and 2) piping exposed to S-CO2 under typical operating conditions with Sandia National Laboratories' Brayton systems.A second issue discovered by SNL involves substantial erosion in the turbine blade and inlet nozzle. It is believed that this is caused by small particulates that originate from different materials around the loop that are entrained by the S-CO2 to the nozzle, where they impact the inlet nozzle vanes, causing erosion. We believe that, in some way, this is linked to the purity of the S-CO2, the corrosion contaminants, and the metal particulates that are present in the loop and its components.

Review of supercritical CO2power cycles integrated with CSP

“…Dry and pure sCO 2 is inert, but adulteration of a little water or oxygen, even at the ppm level, can be heavily corrosive, leading to the formation of oxide layer on the metal surface. 71,72 Typically, ppm level concentration of water and oxygen exists in the industrial grade carbon dioxide. 73 In addition, abrasion between moving part and sCO 2 may result in erosion, as have been observed in a RC Brayton cycle testing platform.…”

“…Dry and pure sCO 2 is inert, but adulteration of a little water or oxygen, even at the ppm level, can be heavily corrosive, leading to the formation of oxide layer on the metal surface . Typically, ppm level concentration of water and oxygen exists in the industrial grade carbon dioxide .…”

The application of supercritical CO2 in nuclear engineering: A review

“…43,44 The same issues have also been discussed by Pham et al 45 Sandia National Laboratories provided an overview of corrosion mechanisms in SCO 2 , as well as in molten salts for nuclear applications. 46 They found that the uncertainty in the longterm reliability of various components has become more visible, when more hours of operation applied to the Recompression Closed Loop Brayton Cycle power loop. Besides, the reasonable materials are also issues for the application of SCO 2 in Nuclear Engineering.…”

Testing and Evaluating of Structural Materials for CSP Applications

“…In addition, the flow of the compressed hot gas through the system could intensify corrosion while mechanically leading to erosion damage on the material surface. Sandia National Laboratories have discovered erosion issues in the inlet nozzle and turbine blades of their Recompression Brayton Cycle (RCBC) testing platform (43). The exact erosion mechanism is currently unknown but particulate matter high in Fe is thought to be the cause.…”