Metallic Alloys for Primary Surface Recuperators

Abstract

The efficiency of small and intermediate-size gas turbine engines can be significantly increased by the use of a primary surface recuperator, which uses waste heat from the exhaust gas to preheat the compressor discharge air before it enters the combustor. The result is lower fuel consumption to reach a particular firing temperature. The construction and operation of a primary surface recuperator present numerous challenges in the area of materials selection. Experiences with stainless steels and nickel-base alloys in construction and application in Solar Turbines’ Mercury 50 gas turbine will be presented, along with the results of extensive laboratory creep and oxidation testing and post-test evaluation. Oxidation testing in humidified air has been carried out on a variety of commercially available stainless steel and nickel-base alloy thin foils considered as materials of construction for primary surface recuperators. Two predominant degradation modes have been identified. The active mode generally depends on the exposure conditions and the alloy composition. Alloys which are rich in iron tend to suffer from accelerated oxidation, while alloys with higher chromium and nickel contents tend to exhibit oxide scale evaporation via the formation of volatile chromium-bearing species. The active mechanism is evident in the oxidation kinetics, the oxide scale morphology and composition, and in observations of compositional changes in the metal alloy substrate.