High-Temperature Corrosion in Waste-Fired Boilers: Insights into material selection for fluidized bed heat exchangers and the corrosivity of PbCl2

Licentiate thesis, 2023

The replacement of fossil fuels with waste-derived fuels for heat and electricity production has gained significant momentum in the European Union (EU), especially in the wake of the implementation of stricter waste management directives and the pursuit of ambitious climate goals. Combustion of waste, however, leads to the formation of a complex and highly corrosive flue gas that cause severe corrosion on important boiler equipment. Consequently, corrosion has significant impact on the operational cost and electrical efficiency of the power plant. As an approach to improve boiler efficiency, modern designed circulating fluidized bed (CFB) waste-fired boilers conduct the final heating of the superheated steam within the loop seal region. However, there is limited published research on the performance of different alloy types in this region of the boiler, and the influence of corrosive and erosive events on the material degradation mechanisms remains relatively elusive. Furthermore, in the water wall region of waste-fired boilers, elevated concentrations of Pb-containing compounds such as PbCl2 have been identified and linked to the accelerated corrosion rate of the tube material. Nevertheless, there is a limited comprehension of the underlying corrosion mechanism in PbCl2-induced corrosion of low-alloyed steel, which is often used as tube material in this area of the boiler.

The aim of this thesis is to address these aforementioned concerns through a combination of field exposures and laboratory studies. A field exposure study was conducted on a commercial CFB boiler to assess the performance of relevant alloy types for superheater application in the loop seal region of the boiler and to improve the understanding of the synergetic effect of corrosion and erosion attacks on the material degradation mechanism. The results from the study revealed that novel FeCrAl alloys exhibited comparable material loss to a conventional nickel-based alloy, positioning them as viable candidates for this application with the potential to reduce material costs. However, significant internal Al-nitridation was observed for this material, and further studies are required in order to understand its impact on the materials' corrosive protective properties. Lastly, the results observed in this study underscore that corrosion rather than erosion is the principal driving force for the observed material losses, highlighting the importance of considering corrosion mitigation strategies when choosing suitable materials for this application.

Additionally, a time-resolved laboratory study was carried out to investigate the corrosive nature of PbCl2(s) on low-alloyed steel at 400°C in a humid environment. Based on the findings presented in this work, it was shown that the presence of PbCl2(s) significantly accelerated the corrosion rate of the steel substrate. The corrosion attack is argued to be driven by the extreme reactivity of PbCl2(s) in the studied environment which leads to a local release of HCl(g) and the introduction of metal chlorides to the metal/oxide interface that promotes severe delamination, void formation, and development of cracks within the oxide scale. The results suggests that Cl plays a pivotal role in both the initiating and propagating corrosion mechanisms of PbCl2-induced corrosion on low-alloyed steels, whereas the Pb compound in PbCl2 do not demonstrate any corrosion-accelerating properties.