Study On The Partitioning And Speciation Of Cd And Pb During Municipal Solid Waste Incineration

This thesis combines experimental study and thermodynamic equilibrium calculations to explore the characteristics of cadmium (Cd) and lead (Pb) partitioning and speciation during municipal solid waste (MSW) incineration. In the tubular furnace designed and commissioned by the author, experiments were conducted to investigate the impact of chlorine, sulfur, moisture, sorbents and operating conditions (including incineration temperature and residence time) on the partitioning and speciation behaviors of Cd and Pb. After comparing the expetimenatal data with the calculated equilibrium form distributions based on Gibbs Free Energy Minimization, hereby are the findings of this study:Chlorine can effectively increase the volatility of Cd and Pb. Both the organic chlorine (PVC) and inorganic chlorine (NaCl) retain more than 97% Cd and 90% Pb partition in fly ash. However, the mechanisms that influenced the partitioning and speciation behaviors of Cd and Pb with these two forms of Chlorine are different from each other. Inorganic chlorine tends to react with Pb to form PbCl₂, With Cd, CdCl₂ could be easily fromed. NaCl could even react directly with CdCl₂ to form CdCl₂(NaCl)₆ or Na₂CdCl₄. For orgnic chlorine, however, apart from its tendancy to react with Cd and Pb to form chloride as inorganic chlorine does, it also encourages the formation of various chlorates such as Na[Cd(ClO₄)₃], Pb(ClO₄)₂ and PbCl2O4.Different types of sulfur have different impacts on the partitioning and speciation of Cd and Pb. With Cd, S and Na₂S tends to form CdS which is hardly volatile, and increased the retention of Cd in bottom ash by 4.2% and 20.1% respectively. In contrast, Na₂SO₄ (Na₂SO₃) tends to form CdSO₄, which also increases the chance for Cd to remain in fly ash. All forms of sulfur tend to decrease the Pb retention in bottom ash by more than 25%. PbSO4 is the main form of Pb compounds in fly ashThe moisture in flue gas mainly affects the chemical equilibrium shift of heavy metal reactions, whilst the moisture in MSW affects the MSW incineration characteristic. The experimental results indicate that adsorptive capacity order of three sorbents towards Cd is as follows: Al₂O₃>SiO₂>CaO. Besides chemical adsorption, sorbents also process certain physics adsorption capacity. Sulfur and chlorine can narrow the effectiveness temperature range of adsorption. The effective adsorptive capacity is not affected by the presence of sulfur and chlorine, although chlorine might deactivate Al₂O₃ in Cd adsorption.Temperature affects both the vapor pressure of Cd and Pb compounds and MSW incineration characteristic. Temperature increase can significantly enhance the volatility of heavy metals. Cd and Pb do not exist in flue gas if temperature below 850°C. 8.49% Cd partitioned in flue gas when temperature was 950°C. Pb did not transfer into flue gas during the whole experimental temperature range. Cd and Pb were more likely to evaporate at the initial stage. During the initial 3min–6min, residence time had a significantly impact on the migration of Cd and Pb, however, this impact almost completely diminished after 6min.The findings of this study should enhance the understanding of heavy metals behaviors during MSW incineration in China and provide valuable information to the improvement of incineration furnace design and the development of heavy metals control technologyies.