Thermodynamic model of flue gas cooling path and implications on heavy metal recovery from MSWI fly ash

Solid residuals of municipal solid waste incineration (MSWI), i.e. bottom ash and fly ash contain significant quantities of heavy metals. Nearly 80’000 tons of MSWI fly ash is produced annually in Switzerland. From the combustion of waste wood, another 6’000t of waste wood fly ash arise annually, which show heavy metal concentrations in the range of MSWI fly ash. In Switzerland, the current practice is to stabilize and dispose of the residues from waste incineration on landfills or underground storage. For MSWI fly ash, heavy metal recovery through acid leaching will be mandatory from 2026 onwards. For waste wood fly ash, heavy metals will have to be recovered from 2024 onwards and it is a matter of discussion, whether a co-treatment with MSWI fly ash could be expedient. The heavy metals are recovered from MSWI fly ash through acid fly ash leaching (FLUWA process). The MSWI fly ash here represents a mix of the different ash fractions (boiler- and electrostatic precipitator ashes) that arise along the flue gas cooling path. With regard to the obligation of heavy metal recovery from MSWI fly ash, it has not yet been conclusively defined whether the heavy metal recovery from boiler ash is expedient from a technical and ecological point of view, as little data exists on its geochemical characteristics and heavy metal binding forms. The recovery of metals from the waste makes a valuable contribution to a sustainable closure of material cycles, as metals from primary raw material mining can partly be returned to the material cycle and the environmental pollution though land- filling is minimized. This thesis focusses on inventory of volatile heavy metals (e.g. Zn, Pb, Cu, Cd) in MSWI fly ash. Detailed characterization in combination with leaching experiments allow new insights on their chemical and mineralogical composition, heavy metal binding forms and leachability. Thermodynamic modeling was used as complemen- tary approach to investigate ash-forming processes and transport mechanisms occurring during cooling of the flue gas. The influence of flue gas composition on phase assemblages and the formation conditions of PCDD/-F thereby represent further investigated aspects. It has been shown that the MSWI boiler ashes are significantly different materials than the electrostatic precipitator ash and that their share on the bulk fly ash is approximately 25 - 30%. The boiler ash consists mainly of acid-buffering and inert refractory minerals, as well as amorphous glass phase and shows considerably lower heavy metal concentrations than the electrostatic precipitator ash. The thermodynamic simulations predict, that Zn and Cu in the boiler ash fractions are preferably stable as silicates (Zn2SiO4) and oxides (e.g. ZnO, Fe2ZnO4, CuO), phases that have also been analytically verified. The elec- trostatic precipitator ash is dominated by a chloride and sulfate matrix which shows a low acid-buffering capacity and shows much higher concentrations in the heavy metals of interest (Zn, Cd, Cu, Pb), predominantly in the form of easily soluble salts. These obser- vations suggest that the electrostatic precipitator ash shows a higher potential for heavy metal recovery. The thermodynamic simulations confirm that the S/Cl ratio in the flue gas primarily controls the predicted equilibrium phase assemblages, while variations in O2 concentration affects the composition of the ash to a minor extent. The thermodynamic modelling further indicate that the formation of gaseous PCDD/-F should take place at extremely reducing conditions which are not expected for typical bulk flue gas conditions. It is suggested that necessary reducing conditions may establish, e.g. in close vicinity to the combustion of plastic or other solid particles. The results of this thesis give new in- sights into the ash-forming processes during the cooling of the flue gas and contribute to a better understanding of speciation of heavy metals in the ash, as well as of the leachability of individual fractions of MSWI fly ash and waste wood fly ash. The data may serve as a basis for life cycle assessment and to concretize the enforcement aid for heavy metal recovery. In particular, further development of thermodynamic modelling approach opens opportunity for theoretical investigation of the combustion condition in the incineration plant, optimization of the combustion process and potential minimization of toxic organic pollutants