Materials and Coatings for Superheater Tubes in Biomass- and Waste-fired Boilers

Licentiate thesis, 2023

Combustion of biomass and waste fuels has strong potential as an alternative renewable energy source for meeting the increasing global energy demand, while reducing the net release of CO2 into the atmosphere. The combustion of biomass and waste releases flue gases that contain high amount of alkali salts and water vapour, which drive the corrosion of metallic boiler components. Corrosion issues can lead to reduced material life-time, costly boiler system failures, and eventually reduced electricity efficiency. The focus of corrosion research in these environments has primarily been on the initiation of corrosion and short-term exposures. However, to combat corrosion, it is important to gain an in-depth understanding of the corrosion behaviour of the material during long-term exposure.
For this work, a novel experimental set-up that mimics the corrosive nature of the boiler environment was developed, to investigate the long-term corrosion behaviours of materials and coatings. Several relevant chromia- and alumina-forming alloys were exposed as bulk materials and as overlay weld coatings in a KCl(s)/KCl(g)-rich environment at 600°C. The corrosion behaviours were investigated using advanced ion and electron microscopy, together with thermodynamic equilibrium calculations and kinetic simulations.
The results show that all alloys experience breakaway oxidation and transition into a secondary corrosion regime, i.e., forming double-layered oxide scales (outward- and inward-growing scales). Within the secondary corrosion regime, alloys may form either fast-growing and less-protective scales (poor secondary protection) or slow-growing and more-protective scales (good secondary protection), depending on the oxide microstructure formed after the breakaway process. The oxide microstructure is influenced by the content of alloying elements, as well as by the microstructure of the alloy. Bulk materials exhibit better corrosion resistance than the overlay weld coatings of similar composition, due to faster formation of the protective secondary scale. The results also indicate that the high-alloy steels experience phase transformation in the inward-growing scales from a less-protective spinel oxide to a more-protective corundum-type oxide, which leads to better corrosion resistance during long exposures