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

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

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

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

Combined manganese oxides as oxygen carriers for biomass combustion — Ash interactions

Carbon capture and storage (CCS) has been acknowledged as an important strategy for mitigation of climate change. Although highly applicable for fossil fuels, CCS with biomass could have the added advantage of resulting in negative emissions of carbon dioxide. One promising carbon capture technology is chemical-looping combustion (CLC). In CLC the reactors are filled with metal oxide bed material called oxygen carriers. Before CLC can be implemented for biomass combustion at a large scale, biomass ash components interaction with oxygen carriers needs to be further understood. Four combined manganese oxides Mn3O4-SiO2, Mn3O4-SiO2-TiO2, Mn3O4-Fe2O3 and Mn3O4-Fe2O3-Al2O3 were exposed to common biomass ash components K, Ca and P. The ash components can exist in many forms, but here the compounds CaCO3, K2CO3 and CaHPO4 were used. Exposures were performed at 900 \ub0C for six hours in oxidising, reducing and inert conditions. Crystalline phases were analysed by XRD and morphology examined with SEM-EDX. Results show that oxygen carrier particles containing silicon were more likely to form agglomerates, especially in combination with potassium, whereas the particles including iron were more stable. MnFeAl was the oxygen carrier that showed least agglomerating behaviour while simultaneously showing a propensity to absorb some ash components. Some inconsistencies between thermodynamic predictions and experimental results is observed. This may be explained by lack of relevant data in the used databases, were only a few of the oxygen carrier-ash systems and subsystems have been optimised. Further optimisation related to manganese rich systems should be performed to obtain reliable results

Fate of lead, copper, zinc and antimony during chemical looping gasification of automotive shredder residue

Gasification experiments in this study were performed in a 2–4 MW indirect gasifier coupled to a semi-commercial CFB combustor at Chalmers University of Technology. Experiments were carried out during 13 days with automotive shredder residue (ASR), giving a unique opportunity to investigate the bed material under realistic conditions and with long residence times. The metal rich ash was accumulated in the bed, gaining some oxygen carrying capabilities, creating a chemical looping gasification (CLG) process. This study aims to expand the knowledge about the chemistry of zinc, copper, lead and antimony during CLG of ASR. Several experimental methods have been utilized, such as XRD, SEM-EDX and XPS along with detailed thermodynamic calculations to study chemical transformations that can occur in the system. Thermodynamic calculations showed that the reduction potential affect the phase distribution of these elements, where highly reduction conditions result in heavy metals dissolving in the slag phase. Copper and zinc ferrites, lead silicates and antimony oxides were identified at the particle surfaces in the bottom ash. The formation of an iron rich ash layer plays an important role, especially for copper and zinc speciation. The main pathways in the complex CLG system have been discussed in detail

Performance of iron sand as an oxygen carrier at high reduction degrees and its potential use for chemical looping gasification

Iron sand as an industrial by-product has a reasonable iron content (35 wt% Fe) and low economical cost. The reactivity of iron sand as an oxygen carrier was examined in a bubbling fluidized bed reactor using both gaseous and solid fuels at 850–975 \ub0C. Pre-reductions of iron sand were performed prior to fuel conversion to adapt the less-oxygen-requiring environment in chemical looping gasification (CLG). Based on the investigations using CO and CH4, iron sand has an oxygen transfer capacity of around 1 wt%, which is lower than that of ilmenite. The conversion of pine forest residue char to CO and H2 was higher when using iron sand compared to ilmenite. Depending on the mass conversion degree of iron sand, the activation energy of pine forest residue char conversion using iron sand was between 187 and 234 kJ/mol, which is slightly lower than that of ilmenite. Neither agglomeration nor defluidization of an iron sand bed occurred even at high reduction degrees. These suggests that iron sand can be utilized as an oxygen carrier in CLG. Furthermore, this study presents novel findings in the crystalline phase transformation of iron sand at various degrees of oxidation, altogether with relevant thermodynamic stable phases

Investigating the Interaction between Ilmenite and Zinc for Chemical Looping

The iron and titanium oxide ilmenite is a benchmark oxygen carrier for chemical looping combustion (CLC) and oxygen carrier-aided combustion (OCAC). Both of them are combustion technologies for biomass and waste fuels with lower emissions and low costs for carbon capture. Here, the interaction between the ash component zinc and oxygen carrier ilmenite is studied in a two-staged vertical tube reactor. Three types of ilmenites─Norwegian rock ilmenite, synthesized ilmenite, and ilmenite extracted after 200 h of OCAC in a full-scale fluidized bed unit─were exposed to gas-phase Zn and ZnCl2. Following the exposure, samples were analyzed concerning morphology, chemical distribution, composition, and crystalline phases. The observations were complemented with thermodynamic equilibrium calculations. It is observed that the iron-rich layer formed on the external surface of rock ilmenite after activation promotes the reaction with both gaseous zinc compounds, with zinc ferrite formed in the external Fe-rich layer. In contrast, ilmenite with no segregation of Fe and Ti showed to interact less with zinc species. Metallic Zn penetrated the particles, while the interaction depth was shallow with ZnCl2\ua0for all investigated ilmenite oxygen carriers. The gaseous conditions, particle ash layer composition, and iron availability are shown to play an important role in the interaction between zinc compounds and ilmenite particles. Based on these results, interaction mechanisms for Zn and ZnCl2\ua0are proposed. This interaction could have environmental implications for the toxicity of ash streams from waste combustion in addition to possibilities for Zn recycling

Interaction of oxygen carriers with common biomass ash components

Carbon capture and storage (CCS) has been proposed as a bridging technology between the current energy production and a future renewable energy system. One promising carbon capture technology is chemical-looping combustion (CLC). In CLC the reactors are filled with metal oxide bed material called oxygen carriers. The interaction between oxygen carriers and biomass ashes is a poorly explored field. To make CLC a viable process, and thereby creating carbon emission reductions, more knowledge about the interactions between biomass ashes and oxygen carriers is needed. This study investigated solid-state reactions of three promising oxygen carriers, hematite, hausmannite and synthesised ilmenite with different biomass ash components. Oxygen carriers were exposed with the ash components: calcium carbonate, silica and potassium carbonate at 900 \ub0C and at different reducing potentials. Crystalline phases of the exposed samples were determined using powder x-ray diffraction (XRD). Results showed that the oxygen carriers hausmannite and hematite interact to a higher extent compared to synthesised ilmenite regarding both physical characteristics and detectable phases. Synthesised ilmenite formed new phases only in systems including potassium. Thermodynamic calculations were performed on the multicomponent system and compared with experimental results. The results suggest that optimisation of systems involving manganese and potassium should be performed