Fate of Trace Elements in Thermochemical Conversion of Waste Fuels Using Oxygen Carriers

Licentiate thesis, 2021

The metals zinc, copper and lead are amongst the more abundant trace elements in waste fuels. The fate of these elements is important to study because they can affect the thermochemical conversion process and end up in ashes. With respect to the latter, this could have environmental implications when the ashes are used or landfilled but may also open up for the possibility of recycling. Utilizing metal oxides, so called oxygen carriers, as bed material in fluidized bed combustion could affect the fate of these metals. The interaction between heavy metals and oxygen carriers is an unexplored field of research. In this thesis a combined theoretical and experimental approach is used to study the fate of Zn, Cu and Pb in presence of oxygen carriers. Analysis methods such as scanning electron microscopy and x-ray diffraction were utilized to study morphology and main crystalline phases. Due to low concentrations x-ray photoelectron spectroscopy (XPS) was also used to study the trace elements on the surface and cross section of oxygen carrier particles. Thermodynamic calculations and a user defined database were applied to study phase formation for a range of parameters.
Solid samples were obtained from industrial fluidized bed applications using oxygen carriers. The availability of samples from commercial units burning wastes provided a unique opportunity to study the trace element chemistry, as the long residence times of solids will allow for sufficient trace element interaction to be able to characterize appropriately. Analyzing ilmenite particles revealed incorporation of Zn the ash layer and accumulation of Cu inside the particles. During chemical looping gasification of a metal rich fuel and olivine, one major observation related to the surface enrichment of Cu and Zn, also in the form of ferrites. Thus, Fe is shown to play an important role for the interaction between the bed material and Cu and Zn. Pb is mainly concentrated in the fly ashes, during both olivine and ilmenite operation, although some lead chlorides, silicates and/or titanates were identified on the particles. Experimental findings and thermodynamic calculations indicate that the trace element chemistry is not only dependent on the oxygen carrier but also other ash components, for example K, Si and Cl. The proposed methodology in this thesis and the knowledge gained, can be applicable for other technologies using oxygen carriers, for example chemical looping combustion.