High-Temperature Corrosion in Biomass- and Waste-fired Boilers: Current Challenges and the Impact of Integrating Carbon Capture Technology

Doctoral thesis, 2025

Substituting fossil fuels with biomass and waste-derived fuels is a recognized strategy for promoting more-sustainable heat and electricity production. However, compared to fossil fuels, biomass and waste have higher levels of corrosive species, which upon combustion can interact with critical boiler components and cause significant corrosion. This results in reduced electricity output and makes the transition to more-sustainable fuels less-appealing to key stakeholders. To enable increased electricity production, it is essential to enhance our understanding of corrosion mechanisms in this environment.

This thesis investigates corrosion phenomena related to the combustion of biomass and waste, through a combination of laboratory experiments, field exposures and kinetic modeling. In particular, it looks at the following topics: corrosion of superheaters positioned in the loop seal of a circulating fluidized bed (CFB) waste-fired boiler; PbCl2-induced corrosion of low-alloyed steels; and kinetic modeling of corrosion in superheater tubes. The latter aspect is approached through a novel methodology that combines oxide microstructural analyses from laboratory and field exposures with thermodynamic kinetic simulations. The corrosion products and microstructures were characterized using a combination of x-ray diffraction (XRD) and electron backscattered diffraction (EBSD) for phase identification and scanning electron microscopy (SEM) coupled with energy-dispersive x-ray spectroscopy (EDX) for microstructural evaluation. Broad ion beam (BIB) milling was used to prepare high-quality cross-sectional samples.

The results reveal that corrosion plays an important role in the degradation rates of various alloy types in the loop seal of CFB waste-fired boilers. FeCrAl alloys exhibit material loss rates comparable to those of conventional nickel-based alloys, positioning them as cost-effective alternatives. However, significant nitridation was observed in these alloys, and further studies are required to understand its impact on their protective and mechanical properties. Furthermore, PbCl2-induced corrosion on low-alloyed steels was found to be severe, with oxide scale delamination identified as a key degradation mechanism. This phenomenon is attributed to the formation of metal chlorides at the metal/oxide interface, which weakens scale adhesion and accelerates corrosion. Microstructural analyses of superheater tubes exposed for up to 4 years in the convective pass of a CFB waste-fired boiler revealed the presence of dense and adherent oxide scales on top of the alloy substrate, supporting the use of parabolic oxide growth modeling. Discrepancies between the simulated material losses and measured material losses in superheaters that were exposed in a commercial boiler were observed, likely attributable to cyclic corrosion influenced by chlorine load and boiler operating conditions.

Lastly, this thesis explores high-temperature corrosion in chemical looping combustion (CLC), a promising technology for achieving negative CO2 emissions and efficient electricity production with sustainable fuels. A novel laboratory setup was developed to simulate continuous alkali release in the air reactor. The results show that conventional chromia-forming alloys experience accelerated corrosion, whereas an FeCrAl alloy exhibits strong corrosion resistance under the same conditions.