Formation of NH4Cl and its role on cold-end corrosion in CFB combustion

There is an increased demand to operate CFB boilers at various loads to cope with changes in energy demand. The combination of load changes and the use of additives to mitigate emissions has been shown to have dramatic effects on coldend deposits and corrosion. Ammonium chloride (NH4Cl) may form in the coldend of the flue gas channel if NH3 and HCl are present in the flue gases. During low load operation of a CFB boiler, some NH3 released from the fuel nitrogen in the furnace may remain unreacted in the cooled flue gases. Furthermore, an NH3 slip may form when using ammonia injection for NOX mitigation. Ammonium chloride can cause corrosion and deposit build-up in the cold-end, e.g. corrosion of economizers, air-preheaters, and flue gas cleaning equipment. In this work, the conditions where NH4Cl is stable were determined by thermodynamic calculations. The hygroscopic properties of NH4Cl was determined in controlled conditions with
a chronoamperometric setup for flue gas conditions to evaluate the corrosiveness of the salt at lower temperatures. Additionally, corrosion experiments with NH4Cl on carbon steel and austenitic stainless steel were performed at 80-160 °C. SEM-EDX analyses of the corrosion products were performed to understand the corrosion mechanism. The results of this work can be used to optimize material temperatures and avoid corrosion caused by NH4Cl.