High Temperature Corrosion Behavior in Biomass- and Waste-Fired Boilers - Insights into catastrophic corrosion and corrosion mitigation techniques

Abstract: Carbon dioxide is contributing to the greenhouse effect and a significant part comes from the use of fossil fuels. Utilizing more renewable fuels is therefore a solution to decrease the net release of CO2 emissions to the atmosphere. This can be achieved by substituting fossil fuels, such as coal and oil, with renewable fuels, such as biomass and waste. However, the combustion of these renewable fuels releases corrosive species creating corrosion challenges for critical parts of the plants, resulting in lower electrical efficiency and higher maintenance costs. It is therefore necessary to understand and investigate the corrosion attack that occurs in this type of environment, in order to find sustainable solutions. High-temperature corrosion research can be divided into two steps: laboratory investigations focus on determining the role or influence of different parameters such as water vapor content or specific corrosive species, while field investigations are more oriented towards in-situ studies to test ideas developed within laboratory studies for mitigation of the corrosion attack. Both approaches are included within the scope of this thesis to broaden the knowledge about the accelerated corrosion of steels, as well as investigate different techniques of mitigation of high-temperature corrosion occurring in biomass- and waste-fired boilers. The laboratory investigations in this thesis focused on KCl-induced corrosion of steels at elevated temperatures (600 °C). A setup allowing a continuous supply of a corrosive species (KCl) during the whole exposure was developed. This generated a corrosive environment mimicking a boiler environment and a corrosion attack better resembling the attack observed in commercial boilers compared to previous methods. Within this setup, four commercial/steels/alloys were investigated and selected in such a way that they represent a broad range of material classes, but also to contain an increasing amount of nickel (from 0 wt.% to 62 wt.%). The results showed that the corrosion attack could be divided into two corrosion morphologies: general corrosion and steel grain boundary attack. Increasing the nickel content resulted in a decrease of the thickness of the general oxide scales. The corrosion attack of the steel grain boundary exhibited a different trend. The attack became more severe when nickel is part of the alloy’s composition. In order to improve the understanding of the corrosion attack in this type of environment, an in-depth study of intergranular corrosion (steel grain boundary attack) in a stainless steel (304L) using the state-of-the-art combination of 3D tomography and TEM was performed. The grain boundary corrosion attack was characterized in detail and revealed new insights of how this complex attack progresses. Very small amounts of Cl were observed in the corrosion front together with very large depletion zones. The design of a new material exhibiting outstanding corrosion resistance properties, excellent mechanical strength, and reasonable costs for application in biomass- and waste-fired boilers is both difficult and time-demanding. Another approach is to separate these properties using a base material (with good mechanical properties) with a corrosion resistant coating. In this work, a set of nickel-based coatings (NiCr, NiAl and NiCrAlY) were investigated in a KCl-rich environment for up to 168 hours. The results showed that the NiAl and NiCrAlY coatings performed well. However, it is necessary to perform a more advanced investigation, i.e. longer durations of exposure, to ascertain their reliability. Another way to mitigate the high-temperature corrosion experienced in boilers would be to alter the environment. Two studies of this alternative solution were investigated in this thesis. A potential new position for superheaters predicted via CFD (Computational Fluid Dynamics) calculations was studied and showed that it is possible to decrease the amount of corrosive species (chlorides) in the deposits. Within the similar scope of making the environment milder for the materials, another technical solution was investigated, the Sulfur Recirculation technique. In this setup, the corrosion history caused by a variation in fuel (corrosion memory effect) was studied and the results showed that the corrosion memory effect can influence positively (when exposed first in a mild environment) and negatively (when exposed first in a corrosive environment) the future corrosion behavior of a material.