Effects of HCl, SO2 and H2O in flue gas on the condensation behavior of Pb and Cd vapors in the cooling section of municipal solid waste incineration

This study aims to clarify the temperature-dependence of the condensation behavior of heavy metal vapors upon flue gas cooling during municipal solid waste (MSW) incineration. Two heavy metals, lead (Pb) and cadmium (Cd), were investigated in a lab-scale rotary kiln reactor coupled with a multi-stage cooling zone. Three gas components, HCl, SO2 and H2O, were doped into flue gas to clarify their effects on the deposition behavior of inorganic vapors in cooling zone. The results indicate that, once vaporized, the gaseous PbCl2 and CdCl2 commenced to condense with flue gas temperature dropping to 853K. A rapid quenching of flue gas, i.e. supercooling, decreased the high temperature (853-623K) deposition fractions of the gaseous species, which were far lower than that predicted by thermodynamic equilibrium modeling. The presence of SO2 and H2O in flue gas facilitated the conversion of gaseous chlorides into sulfates, which in turn enhanced the deposition of Pd and Cd vapors at the temperatures above 853K as the dew point of sulfate is higher than the corresponding chloride. Co-existence of Pb and Cd vapors in flue gas also influenced the deposition behavior of single metals. It has been confirmed that, the deposition of CdSO4 was remarkably shifted to higher temperature range due to its heterogeneous nucleation with pre-existing PbSO4 nuclei which were preferentially condensed as a result of its high partial vapor pressure at higher temperatures. The chemical reaction for inorganic vapors and their heterogeneous coagulation are influential in terms of the condensation of heavy metal vapors on flue gas cooling, which greatly compensated the negative effect of supercooling, and thus, shifted the composition of metallic deposits from toxic chlorides to less harmful sulfates which preferentially condense into solid particulates in flue gas.