Chemical Transformation of Inorganic Species in Thermochemical Conversion of Waste-Derived Fuels - The Role of Oxygen Carriers

Doktorsavhandling, 2023

Waste-derived fuels are used increasingly in heat and power production systems in Sweden. Thermal conversion of waste-derived and biomass fuels offers the possibility of achieving carbon dioxide-neutral or even negative emissions. To limit global warming, it is essential to integrate these systems with carbon capture and storage. However, using alternative fuels raises challenges due to their complex compositions and their contents of heavy metals and other inorganic species. Chemical looping technologies have great potential for lowering costs for CO2 capture and reducing emissions of pollutants, such as NOx. These processes utilize metal oxides, or oxygen carriers (OCs), to transfer oxygen from air to fuel. However, the fates of the inorganic ash species in the presence of OCs are not well understood. The aim of this thesis is to provide a better understanding of the chemical transformations that occur in chemical looping applications, focusing on the heavy metals Zn, Cu, and Pb.

In this thesis, the reaction pathways of Zn, Cu and Pb are studied using combined theoretical and experimental approaches. Samples derived from combustion and gasification processes that utilize OCs are studied in detail by XRD, SEM-EDX and XPS. Zn and Cu are observed to interact with the OC and form ferrites under both combustion and gasification conditions. The formation of ferrites is shown to play an important role in the pathways for these elements. For the Fe-Ti-based OC ilmenite, Zn is incorporated into the ash layer while Cu is found to accumulate inside the ilmenite particles. The interaction between Zn and ilmenite is studied in greater detail in laboratory-scale experiments. It is observed that reaction with Zn is promoted after ilmenite has undergone consecutive reduction and oxidation cycles, owing to the formation of an Fe-rich layer on the external surface. Pb is concentrated in the fly ash regardless of the chemical looping technology and OC types investigated in this thesis.

The chemical speciation of Zn, Cu, and Pb in chemical looping processes is further considered with respect to the oxygen carrier type, temperature, reduction potential, and other ash components. The correlation of theoretical and experimental observations enables the identification of systems that were not well-described by thermodynamic equilibrium calculations (TECs). To improve the predictive potentials of TECs, thermodynamic databases are expanded by incorporating data i) available in the literature, and ii) from first principle calculations. For the latter, thermodynamic data is obtained for experimentally identified crystalline phases that are not available in the literature. This expansion has contributed to the updated and most comprehensive thermodynamic database for combined OC and ash systems. The database was implemented to study the phase stability during chemical looping combustion (CLC) of waste-derived fuels, providing the first insights into the chemical speciation of inorganic ash species. The results indicate that a major fraction of the problematic compounds exits the fuel reactor with the gas, preventing corrosion of the heat transfer surfaces in the air reactor.