Effect of biofuel-derived contaminants on coated industrial gas turbines blade materials

Combined cycle power systems offer increased efficiency of electricity generation and lower environmental emissions of CO₂, SO_x and NO₂, as well as being adaptable to most fossil/biofuels. Industrial gas turbines are at the heart of such power stations and are being developed to perform at higher firing temperatures and pressures to achieve even greater efficiencies, with lower emissions. Fuel gases derived from renewable fuels, such as biogases from digestors or syngases from solid fuel gasification, may contain contaminants that are extremely corrosive to the gas turbine components (e.g. blades and vanes) located in the hot combusted gas path. Such damage can result in a gradual loss of turbine efficiency and reliability. Therefore, it is of paramount importance that the materials usedfor gas turbine components that operate in these environments provide acceptable and predictable in-service life times. Single crystal superalloys (e.g. CMSX-4) were developed to have improved mechanical properties (creep and fatigue) at increasing component operating temperatures, especially in relatively clean aero-engine operating environments. This paper describes work carried out to investigate the development of hot corrosion processes on CMSX-4 (uncoated and Pt-Al coated) in a range of potential environments for blade materials in industrial gas turbines fired on biomass derived fuel gases. A series of laboratory tests has been carried out using the 'deposit recoat' technique, with exposure conditions covering: deposits of 80/20 and 50/50 (Na/K)²SO⁴, with additions oflead, a gas composition of 100 vpm SO_x, 100 vpm HCl in simulated combustion gases, deposition flux of 15mg/cm²/h, temperature of 700 8C, for periods up to 1000 h. During their exposure the materials were monitored using traditional mass changemethods. However, quantitative damage data in terms of metal loss was obtained using dimensional metrology, pre-exposure contact measurements combined with post-exposure measurements of damage observed by optical microscopy on polished cross-sections. These measurement methods allowed the distribution of damage to be determined and the material sensitivity to such hot corrosion processes to be quantified.