Cold-end corrosion caused by hygroscopic ammonium chloride in thermal conversion of biomass and waste

Optimization of energy recovery and efficient use of the energy in flue gases is of high importance for the economy of power plants. The fuel quality and additives to mitigate corrosion may dramatically affect cold-end deposits and corrosion. Ammonium chloride may form in the cold-end of the boiler if the flue gases contain both HCl(g) and NH ₃(g). Ammonia may be present in the cold-end when using ammonium-based additives or operating the boiler at low load leading to an NH ₃(g) slip. Ammonium chloride is a hygroscopic salt, which can cause corrosion of pre-heaters and flue gas cleaning equipment NH ₃(g) and HCl(g) can also pass through filters and form NH ₄Cl(s) after the baghouse filters. In this work, the hygroscopic properties of NH ₄Cl were determined by measuring the conductivity of the salt during decrease of temperature at various water vapor concentrations (10–35 vol%). The corrosiveness of NH ₄Cl salt on carbon steel was studied at 25 vol% water vapor in the temperature range of 70–160 °C. The cross-sections were analyzed using SEM-EDX, and corrosion rates were determined from panorama SEM images by analyzing the material loss in about 50 000 points. The material loss was also determined gravimetrically after removing the corrosion products with citric acid. The work revealed that hygroscopic NH ₄Cl is highly corrosive on carbon steel when water is absorbed. Water uptake and corrosion occurred at relative humidities well below the deliquescence relative humidity and depended on the temperature. An empirical equation for water uptake by NH ₄Cl was determined for water vapor concentrations between 10 and 35 vol% H ₂O. As corrosion proceeded, chlorine was enriched close to the carbon steel surface due to the formation of iron chlorides. The results can be used to avoid conditions typical for NH ₄Cl deposit build-up and create strategies to prevent corrosion.