CALCIUM CHLORIDE AND ITS ROLE ON SUPERHEATER CORROSION IN BIOMASS COMBUSTION

Deposit build-up and superheater corrosion have been detected in a boiler firing calcium-rich biomass and sludge. The calcium can indirectly impact superheater corrosion by capturing sulfur, thus leading to insufficient sulfation of alkali chlorides, which are considered very corrosive. However, deposit analyses from the boiler also indicated the presence of CaCl2 and calcium-bearing corrosion products. While it is well known that CaCl2 can be formed in the presence of CaO and HCl, its corrosive impact on superheaters and reactivity is not. A measurement campaign was conducted at the full-scale boiler to clarify the presence of CaCl2. Additional deposit and superheater samples from an industrial fluidized bed boiler were analyzed using SEM-EDX and XRD with a focus on the effects of CaCl2. Furthermore, the corrosivity and stability of CaCl2 are further studied in the laboratory and by thermodynamic calculations. One-week high-temperature (450-550 °C) corrosion tests were done with two steels (10CrMo and AISI 347). To keep CaCl2 stable at these temperatures, HCl was added to the feed gases, which also consisted of 20 vol% H2O, 5 vol% O2, and the rest N2. After exposure, the samples were cut to reveal the cross-section, and the corrosion products were analyzed with SEM/EDX. The reactivity and stability of CaCl2 were studied with a thermogravimetric analyzer, and the stability was calculated with Factsage 8.1. Sodium and potassium chlorides may also be present in deposits when utilizing various biomass fuels. These can form relatively low-melting eutectics with CaCl2 and may increase corrosion significantly. Accordingly, additional corrosion experiments with a salt mixture of CaCl2, KCl, and NaCl were performed to study the combined effect of the chlorides. This work will reveal the role of CaCl2 on superheater corrosion when utilizing Ca-rich biomass fuels.