Biomass is increasingly used as a feedstock in global energy production. This may present operational challenges in energy conversion processes which are related to the inorganic content of these biomasses. As a larger variety of biomass is used the need for a basic understanding of ash transformation reactions becomes increasingly important. This is not only to reduce operational problems but also to facilitate the use of ash as a nutrient source for new biomass production. Ash transformation reactions were examined in the present work using the Lewis acid-base concept. The model presented in Paper I was further extended and discussed, including the definition of tertiary ash transformation reactions as reaction steps where negatively charged molecular ions, Lewis bases, other than hydroxides are present in the reactants. The effect of such reactions for bonding of various metal ions, Lewis acids, were discussed. It was found that the formation of various phosphates through secondary and tertiary ash transformation reactions is important for the behaviour of biomass ash in combustion. The suggested model was supported by findings in Papers II-VIII. The experimental findings in Papers II-VIII were discussed in terms of ash transformation reactions. The fuel design choices made to investigate the effect of phosphorus in particular on ash transformation reactions were high-lighted. Addition of phosphoric acid to woody-type and agricultural biomasses showed that phosphate formation has a large influence on the speciation of Si, S, and Cl. Co-combustion of a problematic agricultural residue with other biomasses showed that the relation between phosphorus, alkali and alkaline earth metal content is important. Co-combustion of biosolids with wheat straw was shown to greatly improve the combustion properties of wheat straw. It was suggested that fuel analyses should be presented using molar concentration (mole/kg) in diagrams based on ash transformation reactions and elements forming Lewis acids or bases. This may facilitate the assessment of the combustion behaviour of a fuel. Some comments were made on fuel design and additives, specifically pointing out that phosphorus content should always be carefully considered in relation to alkali and alkaline earth metals in fuels and fuel blends.

Pressurised entrained-flow gasification (PEFG) of woody biomass has the potential to produce high purity syngas for the production of vital chemicals, e.g., biofuels. However, ash-related issues such as reactor blockages and refractory corrosion need to be addressed before this potential can be realised from a technical perspective. These undesirable consequences can be brought about by slag formation involving inorganic ash-forming elements and the chemical transformations that they undergo during fuel conversion. The objective of this study was to elucidate the ash transformations of the major ash-forming elements and the slag formation process. A pilot-scale PEFG reactor was used as the basis of the study, gasifying different woody biomass-based fuels including wood, bark, and a bark/peat mixture. Different ash fractions were collected and chemically analysed. Reactor slags had elemental distributions differing from that of the fuel ash, indicating the occurrence of fractionation of ash material during fuel conversion. Fly ash particles from a bark campaign were also heterogeneous with particles exhibiting differing compositions and physical properties; e.g., molten and crystalline formations. Si was consistently enriched in the reactor slags compared to other major ash-forming elements, while analyses of other ash fractions indicated that K was likely volatilised to a significant extent. In terms of slag behaviour, near-wall temperatures of approximately 1050-1200 °C inside the reactor were insufficient to form flowing ash slag for continuous extraction of ash material during firing the woody biomass fuels alone. However, fuel blending of a bark fuel with a silica-rich peat changed the chemical composition of the reactor slags and bulk slag flow behaviour was evident. Thermochemical equilibrium calculations supported the importance of Si in melt formation and in lowering solidus and liquidus temperatures of Ca-rich slag compositions that are typical from clean wood and bark. Viscosity estimations also showed the impact that solids have upon slag flow behaviour and corresponded qualitatively to the experimental observations. Corrosion of reactor refractory was observed. The mullite-based refractory of the reactor formed a slag with the fuel ash slag, which caused the former to flux away. Reactor blockages were also resultant because of the high viscosity of this slag near the outlet.  A preliminary study into the corrosion of different refractories was also carried out, based on firing a bark/peat mixture.  Alumina-rich refractories consisting of corundum, hibonite, mullite, and andalusite tended to form anorthite and exhibited varying degrees of degradation. Infiltration of slag was evident for all the samples and was a severe mode of degradation for some refractories. For fused-cast periclase and spinel-based refractories, slag infiltration was limited to voids and no extensive signs of refractory dissolution were found. This is also supported by a thermochemical equilibrium calculations mimicking slag infiltration that incorporated viscosity estimations. The findings from this thesis contribute towards the development of woody biomass PEFG by highlighting issues concerning ash fractionation, slag behaviours and ash\slash refractory interaction that should be investigated further.

The efficient use of resources and sustainable recovery of various materials are important to minimize the anthropogenic impact on the climate and environment. One such resource is the phosphorus present in manure and sewage sludge. Various technologies are currently being developed to recover the phosphorus for the use of fertilizers in agricultural applications. Thermochemical conversion presents an opportunity to recover energy from these materials. At the same time, elements can be recovered in ash fractions, potentially harmful organic substances can be destroyed and heavy metals fractionated from the P. Mono-combustion of sewage sludge mainly produce apatite, which is not plant available and useful for fertilization. Co-combustion/-gasification with other fuels enables modification of ash transformation pathways and also remedy potential problems, such as bed agglomeration, associated with e.g. agricultural residues used as fuels. This thesis aims to increase the current knowledge in ash transformation of phosphorus-rich materials in cocombustion/-gasification with woody and agricultural fuels in process temperatures relevant for fluidized bed systems. The work focuses on i) possibility for formation of plant-available K-bearing phosphates ii) the effect of fuel ash composition and chemical association of P in the fuel on the distribution and speciation of P and iii) interaction of P-rich ash with bed material in fluidized beds. Experiments were carried out in bench-scale bubbling fluidized bed (BFB), macro-TGA (thermogravimetric analysis) combustion reactors and a dual fluidized bed (DFB) gasification reactor. Fuels studied were mixtures of chicken litter together with wheat straw and bark, and mixtures of digested sewage sludge combined with wheat straw and sunflower husk. Ash fraction and bed materials were collected and analyzed using ICP-OES/MS, SEM-EDS and XRD techniques. For the mixture of chicken litter and K- and Si-rich wheat straw, combusted in BFB, P and Si together with K and Ca formed homogeneous ash particles with large amounts of potentially amorphous iv content. A similar behavior was observed in sewage sludge and wheat straw mixtures where P and Si were likely present in a melt that is amorphous after extraction. In addition to these particles, P was also found in crystalline compounds such as hydroxyapatite, whitlockite and CaKPO4. For mixtures with Ca-rich bark, most of the phosphate formed was in the form of hydroxyapatite. In the interaction of ash with bed material, P captures Ca and K in phosphates, decreasing the interactions of these elements with the bed material, and thus can decrease the risk for bed agglomeration. The findings show that it is possible to modify the ash transformation of P towards K-bearing phosphates by co-combustion. Furthermore, they suggest that it is possible to recover most of the phosphorus in coarse ash fractions through co-combustion of P-rich materials with agricultural fuels. This means that P and volatile heavy metals can be separated into different ash fractions. This also increases the possibility of utilizing existing boilers for recovery of P as well as increased their flexibility to different fuels. To further validate the agricultural value of the produced ashes, plant growth studies have to be performed.

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.

The direct variable costs for heat and electricity production based on solid biomass fuel combustion is approximately 3-5 times lower than the costs in a fossil fuel-oil based boiler in Sweden. In addition waste derived biomass fuels are typically much cheaper than biomass not classified as waste. The introduction of the waste derived fuels; wastewater treatment sludge, demolition wood, and animal waste in a 50MWth circulating fluidized bed (CFB) biomass boiler located in Perstorp, Sweden, led to rapid deposit buildup in superheaters, heavy ash accumulation in economizers and failing boiler tubes and vortex finders that forced frequent boiler shutdowns. This in turn increased the use of expensive oil (fossil fuel) in backup boilers and the CO2 footprint of the on-site energy conversion system. This work aims to increase the general mechanistic understanding of combustion systems using complex fuels, and includes: A mapping of the boiler failure and preventive maintenance statistics; elemental composition analysis of ash, deposits and fuel fractions; flue-gas composition measurements; chemical speciation analysis; an attempt to describe the overall ash transformation reactions and mass balance throughout the combustion process. Scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) was used to analyze the elemental composition of ash and deposits. The SEM-EDS results were used together with data from X-ray powder diffraction (XRD) analysis, thermodynamic phase data, and equilibrium calculations in an attempt to quantify the crystalline phases and the overall ash transformation of the process. Based on the findings concerning ash transformation and the failure statistics, it has been possible to identify generic key parameters regarding boiler design and process parameters, enabling major improvements of the CFB boiler availability, a lower overall energy conversion cost and a reduced CO2 footprint. / Den direkta rörliga kostnaden för värme-och elproduktion baserad på fast biobränsle är ungefär 3-5 gånger lägre än kostnaden för fossiloljebaserad produktion. Avfallsklassade fasta biobränslen är vidare oftast betydligt billigare än fasta biobränslen som inte är klassade som avfall. Införandet av de avfallsklassade bränslena; reningsslam, rivningsvirke, och animaliskt avfall i en 50MWth cirkulerande fluidiserad bädd (CFB) -panna, ledde till kraftig beläggningstillväxt i överhettare och ackumulering av aska i ekonomisers, samt haveri av panntuber och centrumrör i cyklonerna, som tvingade fram frekventa pannstopp. Detta ökade i sin tur användningen aveldningsolja (fossilt bränsle) i reservkrafts-pannor vilket resulterade i ett större CO2 utsläpp och en högre kostnad för energiomvandlingen på siten. Detta arbete syftar till att öka den allmänna mekanistiska förståelsen av förbränningssystem som använder komplexa bränslen, och omfattar; haveri- och underhållsstatistik, elementarsammansättningsanalys av aska, beläggningar och bränslefraktioner, rökgasens sammansättning, kemisk specificering av askor och beläggningar, ett försök att beskriva de övergripande askomvandlingsreaktionerna, samt en massbalans för förbränningsprocessen. Svepelektronmikroskop (SEM) utrustat med energidispersiv röntgenspektroskopi (EDS) användes för att analysera den elementära sammansättningen av aska och beläggningar. SEM-EDS-resultaten användes tillsammans med pulverröntgendiffraktionsanalys (XRD), termodynamiska fasdata, och jämviktsberäkningar i ett försök att kvantifiera de kristallina faserna och de övergripande askomvandlingsreaktionerna i processen. Baserat på resultaten rörande askomvandling och haveristatistik, har det varit möjligt att identifiera generiska nyckelparametrar gällande panndesign och processparametrar, som möjliggjort stora förbättringar av CFB pannans tillgänglighet, en lägre totalkostnad för energiomvandlingen på siten samt ett minskat CO2-utsläpp.