Development of Coatings for Gas Turbines Burning Biomass and Waste-Fuels

Abstract

Worldwide, carbon dioxide emission reductions are in progress following the Kyoto Protocol implementation programme to mitigate climate change. More stringent reductions are expected to follow the present programme which ends in 2012. In addition to reducing carbon dioxide emissions, the major climate change mitigation policy is the elimination of waste. This project addresses both aspects, by facilitating the use of biomass and waste fuels in the gas turbines of highly efficient, integrated gasification combined cycle electricity generating units. Gases from the gasification of these fuels contain potentially damaging contaminants which, when combusted in gas turbines, will initiate hot corrosion. To resist hot corrosion, but still maximise gas turbine efficiency, the hot components of gas turbines require protective coatings. Five activities in this project required original research to meet the objectives. Firstly, to identify potentially damaging species in gasifier gases, which could remain after hot gas cleaning and, following combustion, initiate hot corrosion along the gas path of the gas turbine. Thermodynamic assessments, using MTDATA software, identified cadmium and lead species that could initiate hot corrosion in the gas turbine. The second research activity, involved Type II hot corrosion tests of the identified species on superalloys and typical commercial coatings. These tests simulated the same corrosion environment as in industrial high temperature gas turbine operation. Test results confirmed the thermodynamic assessments, with hot corrosion being initiated on all items tested, and was worse with lead and/or cadmium additions. The third research activity was to develop novel hot corrosion protective coatings. The approach was to develop the most economic coatings, which would provide comparable, or superior, hot corrosion performance to that provided by well proven commercial coatings already used with fossil fuel firing. From previous research at Cranfield, published literature, and after aluminising and silicon modified aluminising CVD trials, single-step silicon modified aluminising was adopted as the basis for novel coating development. The fourth research activity consisted of cyclic oxidation tests and, type II and type I hot corrosion tests, to assess the oxidation and hot corrosion protection provided by the novel coatings on IN738LC and CMSX-4 substrates. Cyclic oxidation tests at 950C and 1050C showed the novel coatings produced by CVD, at a soak temperature of 1050C and soak period of one hour, were superior for both substrates. Microstructurally, TCP phases were formed in CMSX-4 samples which could reduce mechanical strength in service. The TCP phases were observed in the high silicon containing coatings through a reaction with refractory metals diffusing outward from the CMSX-4. This was most noticeable in samples cyclically oxidised at 1050C for long times. Results of hot corrosion tests undertaken at 700C (type II) and 900C (type I) showed novel coatings on IN738LC samples to be more resistant than commercial coatings. Those on CMSX-4 samples had similar hot corrosion resistance to commercial coatings. The novel coatings provided high levels of hot corrosion resistance, which could be enhanced by improvements in deposition. The fifth research activity was to carry out EB-PVD TBC trials on an IN738LC turbine blade, which demonstrated that the novel coating provided an effective bond for the TBC. It is concluded that the novel, single-step silicon-aluminide coatings developed in this project, with identified improvements in quality, will provide effective hot corrosion resistance for gas turbines burning gasified biomass and waste fuels.