High Temperature Corrosion of Superheaters in Biomass - and Waste-Fired Boilers: Combat on two fronts
Carbon dioxide is a greenhouse gas contributing to the greenhouse effect and a significant contribution 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 that deteriorate superheaters and other 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 material solutions.
The aim of this thesis is to increase knowledge on accelerated corrosion in biomass- and waste-fired boilers and to develop solutions that mitigate the corrosion.
In order to increase knowledge on accelerated corrosion in these environments, the corrosiveness of alkali salts (e.g NaCl or KCl) towards stainless steels has been investigated in several laboratory studies. However, a large discrepancy in corrosion attack has been observed between laboratory and field investigations. Therefore, a new setup was developed in laboratory in order to better mimic the boiler environment in a well-controlled manner. The present study involves two commercial stainless steels: 304L (18% Cr – 8% Ni type of alloy) and Sanicro 28 (higher chromium and nickel contents). The exposures were performed under continuous KCl deposition in an environment containing O2 and H2O at 600 °C for 24 hours. The results showed:
- Continuous KCl deposition caused a corrosion attack similar to the attack observed in boilers.
- Continuous KCl deposition accelerated the corrosion attack compared to pre-deposited KCl.
- Regions with large amounts of deposited KCl experienced faster corrosion.
- The alloy with higher chromium and nickel content exhibited an increased corrosion resistance in this environment.
Increasing knowledge on corrosion mechanisms is important for the development of efficient ways of mitigating corrosion. It is possible to mitigate a corrosion attack in two ways: improving the materials or changing the surrounding environment of the materials.
A solution to improving the materials is the utilization of coatings. Three different nickel-based coatings (NiCr, NiAl and NiCrAlY) HVAF (High Velocity Air Fuel)-sprayed onto a low-alloyed steel (16Mo3) were investigated. Their protectiveness was tested in two different environments for boiler purposes: A mildly corrosive environment (O2 + H2O) and a highly corrosive environment (O2 + H2O + KCl). The results showed that the NiCr coating did not remain protective since chlorides were detected within the coating and at the coating/substrate interface. In contrast, NiAl and NiCrAlY coatings performed well in both environments with minor oxidation.
The other approach to mitigating corrosion is changing the surrounding environment of the materials. A field study investigated the potential of a new superheater position in a boiler, predicted with CFD (Computational Fluid Dynamics) calculations. The impact of different operational parameters of the boiler on the deposit composition and amount were tested. The results showed that it is possible to decrease the amount of corrosive species (chlorides) in the deposits by changing the settings of the boiler. Moreover, the corrosion attack of a fixed installation of several superheater materials was investigated after 8000 hours. The analysis focused on 347H (18% Cr – 8% Ni type of alloy) material and revealed a corrosion attack similar to the new setup with continuous KCl deposition in laboratory.