Cold-end corrosion in biomass combustion – Role of calcium chloride in the deposit

Abstract

Using sustainable fuels in power production may lead to an increased risk of corrosion of various boiler parts. In biomass and waste combustion, hygroscopic and deliquescent deposits can cause corrosion of, e.g., preheaters and flue gas cleaning equipment. In the colder part of the boiler, calcium chloride (CaCl2) may form when utilizing fuels containing chlorine. Calcium chloride is a deliquescent salt which can absorb moisture from the flue gas leading to the dissolution of the salt itself. When calcium chloride deliquesces, it forms a corrosive electrolyte that can cause severe corrosion. In this work, full-scale corrosion probe measurements in a biomass boiler and laboratory experiments were carried out to study whether calcium compounds and especially CaCl2 might cause corrosion at different temperatures and humidity conditions. The impact of CaCl2 on the corrosion of carbon and stainless steel was studied in more details at various temperatures (80, 100 and 120 °C) and flue gas moisture levels (15–25 vol% H2O) in a laboratory furnace. Different exposure times were used (24, 72 and 168 h), and the effect of mixtures of CaCl2 and calcium carbonate (CaCO3) was also studied. The corrosion rate was gravimetrically measured, and the sample cross-sections were analysed with SEM-EDX to verify the impact of different elements on the corrosion. Full-scale measurements showed chlorine-induced low-temperature corrosion. The main species in the deposit were calcium and chlorine, and corrosion was observed at conditions at which CaCl2 deliquesces. The laboratory work showed that although the deposit contains mainly calcium carbonate, which is not deliquescent, the highest corrosion rate (>1 mm/year) was found with 5 wt% CaCl2 in the salt deposit mixture. The tests showed that the corrosion rate is linear with time, and severe corrosion of carbon steel occurs when CaCl2 deliquesces. Stainless steel did not show any measurable corrosion at the conditions tested.