Effect of Si/Al additives on Cl fate during MSWI fly ash thermal treating process

SiO2 and Al2O3 (Si/Al) are proper additives for municipal solid waste incineration (MSWI) fly ash melting. However, Si/Al additives impact the emission of chlorine (Cl), which is harmful to the ecology and environment. In this study, co-thermal treating experiments of fly ash and SiO2 or Al2O3 were carried out to determine chlorine migration and transformation. Results show that the release of HCl is significantly promoted by the addition of SiO2 or Al2O3. Thermodynamic equilibrium calculation shows that the reaction rate between additives (SiO2 or Al2O3) and CaCl2 is the main factor limiting the release of HCl. Besides, SiO2 has an obvious effect on reducing Cl percentage in residues, as the addition of 30% SiO2 raises the Cl removal rate to 99.72% at 1250 °C. Main component in the residuals is Ca-silicate after addition of SiO2. Compared to SiO2, the addition of Al2O3 is more likely to retain Cl in the residues. Al2O3 reacts with fly ash to generate Cl-containing minerals, such as Ca10(SiO4)3(SO4)3Cl2 and Ca12Al10.6Si3.4O32Cl5.4, leading to a lower removal rate of Cl. Moreover, Cl combined with Na and K is transformed to secondary fly ash by volatilization and condensation. Our findings may assist in pollutant discharge control in developing a new melting furnace. •Both SiO2 and Al2O3 can significantly increase HCl emission concentrations, and decrease initial emission temperature.•Cl is mainly enriched in secondary fly ash by volatilization and condensation in the form of NaCl and KCl.•SiO2 has significant promotion effect on Cl removal,•Adding Al2O3 into fly ash is advantageous for high melting point mineral (Ca12Al10.6Si3.4O32Cl5.4) generation.•In the melting process, additive Al2O3 blending ratio need to be control.