Heavy Metals Migration and In-Situ Solidification in the Co-Combustion of Solid Waste Fuels: Rules and Differences

Abstract

The discharge of heavy metals (HMs) significantly limits the application of sewage sludge (SS) and coal slime (CS) in synergistic heat treatment as two types of low-calorific-value solid waste. The migration and transformation of HMs (Pb, Cd, Zn, Cu, Cr and Ni) during the co-combustion at 700-1000°C were quantitatively studied by combining DFT (density functional theory), thermodynamic analysis and combustion experiments. The results showed that the interaction between SS and CS inhibited the volatilization of HMs, with the most pronounced solidification effect occurring at 900℃ . The enrichment rates of Pb, Cd, Zn, and Cu were 5.3%, 6.3%, 6.4%, and 4.2% higher than the theoretical values, respectively. In-situ solidification of HMs occurred through physical and chemical pathways. The physical pathway involved melting silicon and aluminum in mixed fuels, reducing volatility of HMs via covering effects. Chemically, the minerals in SS and CS interacted with HMs to solidify them in the bottom ash. KH2PO4 in SS inhibited Zn and Pb volatilization by promoting ZnPO4(s) and PbPO2(s) formation, while sulfides in CS preferentially reacted with Zn and Pb, preventing volatile chlorides like ZnCl2(g) and PbCl2(g) from forming. Furthermore, the kaolinite contained in SS and CS deformed at high temperature to form metakaolin, aiding the in-situ solidification of Pb and Cd through adsorption and heterogeneous reactions. Electron transfer induction occurred between LUMO orbital of the metakaolin and HOMO orbital of the PbCl2 and PbO. This study is vital for improving the treatment of low-calorific-value solid waste and solidifying HMs in situ.

Keywords: Heavy metal, Migration rule, Sewage sludge, Coal slime, In-situ solidification