Kinetics and fusion characteristics of municipal solid waste incineration fly ash during thermal treatment

During the thermal treatment of municipal solid waste incinerators (MSWI) fly ashes (FA), the fusion characteristics, kinetics, and phase transition are the theoretical basis on heavy metals solidification improvement and energy consumption reduction. In this study, the thermal behavior of MSWI FA from two typical furnaces in China, i.e., grate furnaces (GF) and circulating fluidized bed (CFB), are systematically investigated. The results show that FA melting can be divided into five stages but with different positions and areas of weightlessness peaks between two types of FA. We identify four kinetic models from which to obtain the apparent activation energy and pre-exponential factor during the melting transition. The apparent activation energy of GF FA ranges from 466 to 1016.2 kJ/mol, which is generally higher than that of CFB FA (380.1–636.6 kJ/mol). The weightlessness rate of GF FA is approximately three times that of CFB FA because of its high chloride load. Silicon dioxide (SiO2) in FA is prone to react with other compounds to form low-melting-temperature minerals at 1100 °C, such as Ca2MgSi2O7. Therefore, the fusion temperatures of CFB FA with a higher content of SiO2 are about 150 °C lower than that of GF FA, which is consistent with the theory that the fusion temperature of FA is at its lowest when the ratio of basic to acid oxides approaches 1. Finally, SiO2–CaO–Al2O3 ternary phase diagrams, ash fusion temperatures, and several fouling and slagging indices are verified to predict the ash fusion characteristics of MSWI FA.