Materials for Supercritical CO2 Power Cycles

The deployment of supercritical CO2 power systems will require cost-effective materials that can withstand a combination of mechanical stress, corrosive and erosive environments for upwards of 100,000 hours service life. Advanced materials that have been developed for various power generation systems over the years may also be suitable for use in the supercritical CO2 power cycle. For example, several nickel-based superalloys (Inconel 740H and Haynes 282) have been developed specifically for the advanced ultrasupercritical steam cycles. However, supercritical CO2 cycle environments pose some unique challenges for materials, including the following:

  • Supercritical CO2 can cause chemical instabilities on the surface of materials i.e. oxidation potential, carburization potential. Impurities in the working fluid can increase these potentials.
  • Oxidation and carburization can take place in the growing cracks on the components under cyclic loading causing mechanical instabilities which can lead to premature failure.
  • Creep life of thin wall sections in compact heat exchangers may be lower compared to bulk property of the same material.
  • High velocity turbulent flow of dense supercritical CO2 can cause erosion problems in the components.
    Materials joined together by welding, diffusion bonding, or brazing can be especially vulnerable in the sCO2 environments since the resulting joint materials are not necessarily optimized for these aggressive environments.
  • Direct sCO2 cycles pose greater challenges for construction materials due to more corrosive chemistry of the working fluid (CO2, O2, H2O, and impurities) and higher operating temperatures.

The effects of materials-environment interactions can impact the design, reliability, and lifetime of most system components. These uncertainties and R&D needs are discussed below.

High Temperature Corrosion
Based on relatively short-term oxidation tests, the high-temperature corrosion (degradation of material surface through chemical reactions) behavior of candidate advanced ultra-supercritical steam cycle (A-USC) alloys was found to be as good as, or better, in supercritical CO2 than in supercritical H2O, making them candidate alloys for indirect sCO2 power cycle components also. These leading alloy candidates need to be tested for long terms at target temperatures and pressures in sCO2 to establish oxidation reaction kinetics and quantify the rate of internal carburization (carbon ingress into material resulting in formation of subsurface metal carbides). Furthermore, the long-term effect of various joining techniques (e.g. diffusion bonding, brazing, etc.) on reaction rates needs to be determined in the sCO2 cycle conditions. For the direct-fired concept, how impurities such as O2, H2O and others introduced in the CO2 stream from the combustion of fuel may affect corrosion rates need to be determined in supercritical conditions.

Creep and Fatigue
Creep, the tendency of a solid material to deform slowly and permanently as a result of mechanical stresses below its yield strength at elevated temperatures, and fatigue, a failure mechanism that occurs when the component experiences cyclic stresses or strains that produce permanent damage, are primary potential limitations that must be accommodated in the design of sCO2 systems. If the system design constraints drive designs to more-corrosion resistant materials (e.g., due to CO2 interactions), there may be a need to obtain creep rate data for those materials if sufficient data are not available from the manufacturer. Furthermore, creep and fatigue behavior of joints (diffusion bonded or brazed) may need to be investigated as a part of the design methodology of compact heat exchangers. In addition to creep and fatigue as purely mechanical consideration, effect of environment on these mechanical properties may need to be evaluated.

Erosion
Erosion (removal of material from the surface of a component) can be a significant issue for sCO2 cycle systems. Erosion in the turbine blade and inlet nozzle can be 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 entrained in a flow of a high density fluid at very high velocities through the nozzle vane and turbine can cause erosion.

NETL supports several projects in Materials R&D.