Over the past few decades, a growing interest in the thermal conversion of alternative fuels such as biomass and waste-derived fuels has been observed among the energy-producing companies. Apart from meeting the increasing demand for sustainable heat and power production, other advantages such as reducing global warming and ameliorating landfilling issues have been identified. Among the available thermal conversion technologies, combustion in grate-fired furnaces is by far the most common mode of fuel conversion. In recent years, Fluidized-Bed (FB) technologies have grown to become one of the most suitable technologies for combustion and gasification of biomass and waste-derived fuels.In spite of the benefits, however, some difficulties are attributed to the thermal conversion of the alternative fuels. Ash-related issues could be a potential problem, as low-grade fuels may include considerable concentrations of ash-forming elements such as K, Na, S, Ca, Mg, P, Si and Cl. These elements undergo many undesirable chemical and physical transformations during the thermal conversion, and often cause operational problems such as deposition-related issues, slag formation in furnaces, corrosion of the heat transfer surfaces, and bed agglomeration of the fluidized-beds. Ash-related problems in the utility boilers are a major concern that may result in decreased efficiency, unscheduled outages, equipment failures, increased cleaning and high maintenance costs.This thesis investigated the ash behavior and ash-related problems in two different FB conversion systems: a Bubbling Fluidized-Bed (BFB) boiler combusting solid waste, and a Dual Fluidized-Bed (DFB) gasifier using biomass as feedstock. Full-scale measurements, chemical analysis of fuel and ash, as well as thermodynamic equilibrium modeling have been carried out for the BFB boiler (Papers I-IV), to investigate the impact of reduced-bed temperature (RBT) and also co-combustion of animal waste (AW) on the ash transformation behavior and the extent of ash-related issues in the boiler. For the DFB gasifier (Paper V), a thermodynamic equilibrium model was developed to assess the risk of bed agglomeration when forest residues are used as feedstock.The experimental results showed that the RBT and AW co-combustion could decrease or even resolve the ash-related issues in the BFB boiler, resulting in a lower deposit-growth rate in the superheater region, eliminating agglomerates, and a less corrosive deposit (in RBT case). Thermodynamic equilibrium modeling of the BFB boiler gave a better understanding of the ash transformation behavior, and also proved to be a reliable tool for predicting the risk of bed agglomeration and fouling. The modeling of the DFB gasifier indicated a low risk of bed agglomeration using the forest residues as feedstock and olivine as bed material, which was in good agreement following the observations in a full-scale DFB gasifier.

One of the most sustainable pathways of sewage sludge treatment in recent years has been thermal conversion. The benefits of thermal treatment of sewage sludge are the recovery of energy or valuable chemical products, the destruction of harmful organic compounds, the separation of heavy metals from the P-rich coarse ash fraction, and the decreased and sanitized ash volume. The ashes created by these thermal conversion processes of sewage sludge are often rich in P that is mostly present in minerals with low plant-availability such as apatite. Due to the enrichment of P in the created ashes, a variety of post-processing steps have been developed to recover P from sewage sludge ashes. One proven way for the sus-tainable recovery of P from such ashes is thermal post-processing with alkaline salts, e.g., Na2SO4 or K2CO3, which was able to transform less plant-available phosphates in the sewage sludge into more plant-available alkali-bearing phos-phates. Based on these results, one could facilitate creating these phosphates with enhanced plant-availability by providing the chemical potential to form them already during the thermal conversion process of sewage sludge.   This thesis aims to increase the current knowledge about the ash transformation processes of P and to suggest suitable process parameters for the alteration of the phosphate speciation in sewage sludge ashes by co-conversion with alkaline-rich agricultural residues. More specifically, the possibility of incorporating K derived from agricultural residues in phosphate structures derived from sewage sludge was evaluated with respect to the influence of the process temperature, the conver-sion atmosphere, and the fuel mixture. The studied parameters were chosen to generate knowledge relevant for fluidized bed gasification processes, with a spe-cial focus on dual fluidized bed (DFB) gasification systems. The applicability of feldspar bed materials in fluidized bed gasification systems was investigated to enable the substitution of the commonly used olivine, which often contains heavy metals (potentially contaminating recovered ashes), and quartz, which is very reactive towards fuel-derived K and potentially leads to bed material fragmentation and bed agglomeration (Paper I & II). Subsequently, the thermodynamic potential for the alteration of the P-species in sewage sludge ash during co-combustion and co-gasification processes with agricultural residues was investigated (Paper III). Thereafter, an experimental evaluation of the ash transformation chemistry in thermal conversion processes of sewage sludge with different types of alkali-rich agricultural residues in temperatures relevant for flu-idized bed technology was conducted (Paper IV & V). The methodology employed was chosen with respect to the state of technology of the specific investigated process. Paper I & II applied SEM, EDS, XRD, and thermodynamic equilibrium modeling for bed material samples derived from an industrial indirect gasifier. Paper III applied thermodynamic equilibrium calcula-tions to theoretically evaluate ash compositions resulting from co-conversion of sewage sludge and agricultural residues. Paper IV & V employed SEM, EDS, ICP-AES/MS, XRD, and thermochemical modeling on ash samples derived from single pellet lab-scale experiments. The results obtained by analysis of bed material from indirect wood gasification showed the difference in interaction mechanism for K-feldspar and Na-feldspar, most notably the enhanced disintegration of Na-feldspar by K originating from the fuel (Paper I & II). Thermodynamic models employed for fuel mixtures of sewage sludge and agricultural residues showed the thermodynamic preference for the formation of the desired alkali-bearing phosphates (Paper III). Experi-ments conducted with these fuel mixtures (Paper IV & V) supported the theo-retical findings, and the influence of temperature and process conditions could be obtained. However, practical investigations also showed that attainment of the desired ash composition is subject to significant restrictions. Derived from the elaborated results and discussions, it was possible to assess the critical process and fuel parameters for the development of up-scaled gasification processes focusing on the conversion of sewage sludge with the aim of creating improved phosphate formation in the ash. The selection of a suitable bed material in fluidized bed conversion and the transformation mechanisms defining the ash chemistry were found to be of vital importance for future applications. The pur-suit of the predefined aims in reference to P-recovery from sewage sludge has led to a multitude of suggestions for suitable process parameters that must be ad-dressed in future bench- and pilot-scale experimental runs.