Volatilization Characteristics And Emissions Control Of Lead And Cadmium During Waste Incineration

Because of the advantages of small space occupation, large disposal capacity and high volume reduction rate, waste incineration has been widely used in the word. In recent China, the increasing phenomenon of garbage-surrounded city makes the waste incineration enter a high speed period of expansion. Municipal solid waste (MSW) contains semi-volatile toxic heavy metals like lead (Pb) and cadmium (Cd), which can vaporize in furnace, then condense to submicron particles in flue gas and finally pass through bag filter into atmosphere causing the damage to human health. This paper chose Pb and Cd as research objects to investigate their volatilization characteristics and emissions control during waste incineration.Firstly, the thermal stability and the volatilization kinetics of many pure substances of Pb and Cd were investigated by equilibrium calculation and thermogravimetric analysis. In air atmosphere and the incineration temperature range, oxdies and chlorides were more stable and other substances would convert to them. The sequence of initial evaporization temperature followed:CdO (～1040℃)> PbO (～860℃)> CdCl2 (～500℃)≈PbCl2 (～500℃) and the sequence of evaporization rate followed: CdCl2>PbCl2>>PbO>CdO. The volatility of chlorides were much stronger than that of oxides.Then, the thermodynamic equilibrium of Pb and Cd during the waste incineration was calculated on the basis of the minimization of the total Gibbs energy. The effect of incineration condition and elements input and the related mechanisms were investigated. In oxygen-rich condition, the increasing temperature could cause the conversion of Pb and Cd chlorides to oxides by the reaction with H2O. In fuel-rich condition, the increasing temperature could cause the conversion of Pb and Cd from sulfides to elementary substances. CaO could be used as desulfurizer to prevent NaCl from reacting with SO2 to release Cl, or antichlor, but they were invalid at high temperature. In the fixed-bed incineration experiments, the initial volatilization temperatures of Pb and Cd were lowered for the reducing atmosphere on the surface or inside of solid fuel which converted oxides to elementary substances. The effects of PVC and NaCl on Pb and Cd volatilization were close but different in process and mechanism. The Cl release from PVC at 300℃ caused the chloridization of heavy metals at low temperature. NaCl chloridized heavy metals at ～800℃ directly by the reaction with Si and Al. CaO could inhibit the low temperature chloridization from PVC below 700℃, but was invalid to the chloridization from NaCl.Next, according to the volatilization characteristics of heavy metals during incineration, the theoretical investigations were carried out. On the basis of first-principles, the Pb vapor and Cd vapor adsorption on kaolinite surfaces were investigated by density functional theory (DFT) calculation. The adsorption occurred for the unsaturated Al atoms and the O atoms losing H atoms on Al surface. The adsorption products followed the sequence of oxide> chloride> single atom. The stability of oxide adsorption was due to the O-Al. In the previous experiments, the lower capture efficiency of CdO than that of PbO had nothing to do with chemical adsorption but diffusion process. The unstable chloride adsorption could transform to the stable oxide adsorption through de-HCI process with high energy barrier. In the previous experiments, the lower capture efficiency of CdCl2 than that of PbCl2 was due to the different desorption abilities of middle products.At last, the process and performance of kaolinite to capture submicron Pb and Cd were investigated by using the lab-scale fluidized bed incinerator. The capture process was (1) chemical adsorption of metal vapor, (2) clusters formation of kaolinite particles, (3) conglutination of nano scale particles on kaolinite surface and (4) capture by dedusting equipments. The chemical adsorption and the effect on conglutination of Pb was stronger than those of Cd. When it was evaluated by capture efficiency, increasing temperature was in favor of performance enhancement and increasing addition amount caused the performance limitation. When it was evaluated by total toxicity emission control efficiency, increasing temperature was invalid to enhance performance and no performance limitation was found with increasing addition amount. The acid soak modification derived from DFT calculation results was tested to be effective to enhance the performance of kaolinite.