Fate of heavy metals during fluidized-bed thermal treatment of municipal solid waste

Municipal solid waste pyrolysis, gasification and incineration experiments were conducted in a fluidized bed reactor to investigate the effect of redox atmosphere and temperature on the fate of heavy metals (Cd, Pb, Zn and Cu). The results show that higher temperature and reductive atmosphere are effective for the evaporation of Cd, Pb and Zn; while the oxidative condition favors the migration of Cu. Thermodynamic equilibrium calculation confirms that the metal species differ a lot under redox conditions. Metals are mainly existed as elemental forms or sulfide under reductive atmosphere; and their volatility depends on the metal boiling point dominantly. On the contrary, the formation of metal chlorides such as CuCl₃ increases the volatilization of Cu in an oxidative atmosphere. Besides, the evaporation of Cd, Pb and Zn is postponed under oxidative condition due to the metal-matrix reactions. The distribution of heavy metals in different fractions was further examined. The majority of the vaporized metals are condensed and enriched in the fly ash upon cooling. Entrainment also poses a significant influence on the migration of metals, which is determined by the characteristics of high gas velocity used in fluidized bed.