The fate of chlorine during MSW incineration: Vaporization, transformation, deposition, corrosion and remedies

Municipal solid waste (MSW) incineration plays an important role in waste treatment systems throughout the world, due to the advantages of fast volume reduction by 80-90%, heat recovery, and power generation. However, waste-to-energy (WtE) plants have low electrical efficiency of 15-25%, due to the low steam temperature and pressure used in order to minimize boiler deposition and corrosion problems. Undoubtedly, the high Cl-content in MSW is the reason for the severe corrosion problem. Chlorine also forms volatile compounds with trace metals (e.g., Zn, Pb), and, influences the fate of other key elements, e.g., Na, K, and S. Different from alkali metals in biomass, which have been thoroughly investigated, the behavior of chlorine during MSW incineration has not been systematically and comprehensively studied. Up until now, there are few in-depth studies that have been conducted on the thermal behavior of chlorine or on the remedial measures against Cl-induced problems. An up-to-date review on the behavior of chlorine from incineration via freeboard chemistry to corrosive attack is therefore needed, in order to provide knowledge on process optimization and reactor design, thereby enabling high-efficient energy utilization and safe operation of large-scale WtE units. This review provides a critical summary of the progress of research on chlorine in MSW (origins, species, and analytical methods): the thermal behavior of chlorine, including chlorine vaporization, aerosol formation and transformation (freeboard chemistry), deposit formation, and Cl-initiated corrosion mechanisms. In addition, the interrelationship of chlorine with other key elements (S, Na, K, Zn, Pb), and, the chlorine roadmap in the incineration process is presented, along with the influence of feedstock composition and the temperature of both the flue gas and boiler tube metal on chlorine-induced deposition and corrosion. Mitigation measures against Cl-initiated problems such as Segher boiler prisms, mixed secondary air injection, and eco-tube systems, are also thoroughly discussed. Finally, challenges and further research questions, are identified. (C) 2019 Elsevier Ltd. All rights reserved.