Ecological biomass and waste offer alternative to fossil fuels in power generation industry. Operating boilers with biomass and waste, however, accelerate the corrosion of the boilers calling for the development of new highly corrosion and wear resistant coatings.
Several deposition techniques for corrosion protection, such as high velocity oxygen fuel (HVOF), high velocity air fuel (HVAF), twin wire arc and cold spraying, are commercially available. These commercial solutions result in different microstructures and levels of corrosion protection.
At the current state of the work, the coatings can be ranked based on their resistance as follows: Twin wire arc spray > HVOF > Cold spray (He) > Cold spray (N 2) > HVAF where twin wire arc spray coating is the most resistant. However, intense work is applied in the optimisation of the newest cold spray and HVAF processes and significant improvements are soon expected.
In this study several metallic thermally sprayed coatings have been studied to obtain a relevant comparison of their corrosion resistance in high temperature and chlorine containing environment. The correlation between microstructure and corrosion behaviour permitted to identify the weakest features of the coatings and so giving directions for further developments.
Chlorine could penetrate through the particle boundaries of the coatings forming metal chlorides and accelerating corrosion for all the coatings samples indicating that such regions are microstructural weak points. These finding enable the development of new protective coatings with high corrosion and wear resistance extending the service life of metal components for power generation.
In the last years, the industry of power generation has been gradually reducing the use of fossil fuels thus, converting toward the use of ecological biomass and waste, advancing to meet the EU climate and energy targets. Biomass and waste are alternative ecological fuels largely available in Europe. The former is considered a carbon neutral fuel while the use of waste is considered beneficial in terms landfill managing and lifecycle impact of products. However, operating boilers with such fuels originates harsh environments entailing severe corrosion of the fireside metal components.
Waste and biomass are rich in chlorides of sodium, potassium and include minor amounts of zinc and heavy metals chlorides such as lead. These compounds may react directly with alloying elements of the metal components and thus triggering and accelerating corrosion. Besides, maintenance downs of implants, such corrosive fuels force boilers to operate at lower temperatures than fossil fuels, vastly reducing the plant efficiency . Corrosion protection is generally provided by boiler design, environment control and protective coatings. This project focuses on protective coatings and aims at developing highly corrosion and wear resistant coatings able to extend service life of metal components for power generation.
At the current stage of the work, the results are mainly used for coating development at laboratory scale. A successful outcome will result in the application of the coatings in actual commercial boilers. This means that all the companies involved in the production chain will benefit from superior quality boiler with extended life and increased efficiency. On a wider point of view, enhanced efficiency of renewable fuelled boilers will make them more appealing and spread their installation. Benefits are accountable in reduced dependence on imported fossil fuels and greenhouse gas emissions.
Davide Fantozzi, Tampere University of Technology
TTY, VTT, Valmet Technologies, Metso Flow Control, Outotec, Abloy, Virtasen Koneistamo, Kuopion Konepaja, Oseir, Kokkola LCC, Telatek Service, TKM TTT Finland Oy.