Oxygen carrier aided combustion (OCAC) of two waste fuels - Experimental and theoretical study of the interaction between ilmenite and zinc, copper and lead

Highlights: • Characterization of industrial samples derived from two OCAC plants utilizing waste fuels. • Multicomponent thermodynamic calculations for investigated systems. • Fate of Zn, Cu and Pb using thermodynamics and detailed characterization. • Different reduction potentials affected phase formation. • Mixed oxides including Zn, Cu and Pb were identified on the particle surface. Zinc, copper and lead are amongst the more abundant trace metals in waste fuels such as municipal solid waste and recovered waste wood. The ashes from waste fuels could contain high contents of these metals, which could be valuable but also toxic in certain environments. Oxygen carrier aided combustion, OCAC, is a novel technology for combustion of biomass and waste. Utilizing oxygen carriers could affect the fate of these metals and have implications for stability and recycling. The aim of this work is to study the fate of zinc, copper and lead during oxygen carrier aided combustion of two waste fuels utilizing ilmenite as an oxygen carrier. In total, four samples have been obtained from two different industrial fluidized bed boilers using ilmenite as bed material. Due to low concentrations, bulk analysis methods are not suitable for speciation, i.e. SEM/EDX and XRD. Hence, this investigation utilizes high resolution x-ray photoelectron spectroscopy (XPS), coupled to detailed thermodynamic modelling, with the aim of understanding trace metal speciation, distribution and phase composition. Characterization of the four samples show that iron at the surface of ilmenite particles interact with both copper and zinc to form ferrites, CuFe₂O₄ and ZnFe₂O₄. Lead, on the other hand, is more prone to end up in the fly ash as condensed PbCl₂, but the mixed oxide PbTiO₃ could be identified at the oxygen carrier surface. Thermodynamic calculations were shown to be in line with the identified compounds.