| 1. |
- Rebbling, Anders, 1980-
(författare)
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Application of fuel design to mitigate ash-related problems during combustion of biomass
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The energy supply of today is, through the use of fossil energy carriers,contributing to increased net emissions of greenhouse gases. This hasseveral negative effects on our environment and our climate. In order toreduce the impact of this, and possibly to reverse some of the effects, allrenewable energy sources must be used. Biomass is the renewable energycarrier that has the greatest potential to reduce net greenhouse gasemissions, but the transition from fossil fuels to biofuels is challenging.The combustion of biomass is associated with various technical andenvironmental problems such as slagging, corrosion, and emissions ofparticles, soot, or harmful chemical compounds. Most of these problemsare linked to ash chemical reactions involving alkali metals. Therefore, toreduce the risk of operational and environmental problems, it is importantto understand and control the ash transformation reactions involvingalkali metals.The research presented in this thesis has focused on the development oftools, such as models and indices, for predicting the behaviour of variousbiofuels during combustion, and on the development of the concept of fueldesign and implementation of the same during industrial combustion ofbiomass. The development of easy-to-use tools for predicting problematicash behaviour is crucial in order to make it possible to increase the use ofbiomass as an alternative to fossil fuels. The tools presented here are basedon theoretical and empirical knowledge and can be used to predictchallenges concerning the fuel ash composition and to propose relevantfuel design measures.The purpose of fuel design, as used here, is to broaden the fuel feedstockand to increase the usability of biomass in the global energy system. Thisis achieved through measures to change the ash chemical composition inorder to enhance beneficial properties, or reduce problematic properties,via the use of additives or blending of two or more different fuels.The present thesis extends the foundation of knowledge regarding fuel ashtransformation reactions and their implications for operational problemsthrough in-depth laboratory studies and analyses. Furthermore, thefeasibility of applying this extended knowledge in the medium and largescaleindustrial combustion of biomass is demonstrated and validated. More specifically, a slagging index has been developed using the results ofseveral years of combustion experiments. Fuel designs based on the indexwas demonstrated during normal operation in local and district heatingplants. Furthermore, a model was developed for predicting slaggingproblems that take into account both the chemical composition of the fueland the burner technology.Several studies have also been performed on different fuel designs basedon the same foundation as the index and the model. Additives to supply forexample calcium and sulphur, as well as the clay kaolin, have been used toreduce both technical and environmental problems.The conclusion is that fuel design, based on ash chemistry, is a possiblepath for increased fuel flexibility and a broader feedstock for bioenergy.
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| 2. |
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| 3. |
- Hannl, Thomas Karl, M.Sc. 1993-
(författare)
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Fluidized bed combustion and gasification for phosphorus recovery by co-conversion of sewage sludge with biomass
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In recent years, the thermal conversion of sewage sludge has proven its applicability for managing this inevitably generated waste. The viability arises from the concomitant features of recovering energy or valuable compounds, the breakdown of potentially harmful organic compounds, the separation or immobilization of heavy metals, and the formation of volume-reduced, sanitized residues. The inorganic residue after thermal conversion of municipal sewage sludge, i.e., the ash, is generally rich in phosphorus (P). However, P in sewage sludge ash is mostly present in a chemical association that is poorly plant-available, e.g., apatite and whitlockite. Since sewage sludge ashes represent a P-rich resource, a number of different post-processing methods have been fathomed to extract P or alter its association in the ash. While extraction methods often focus on eluting P with acids, methods to alter the P-association in the ash rely mostly on thermochemical post-processing with additives. A way of enhancing the plant-availability of P in the ash is the thermochemical treatment with alkali additives, e.g., (Na,K)2SO4 and (Na,K)2CO3, leading to the formation of alkali-bearing phosphates of improved plant-availability. Providing the necessary physiochemical conditions for this phosphate alteration process, there is a potential to achieve the formation of alkali-bearing phosphates already during the thermal conversion of sewage sludge.This work investigates the potential of forming K-bearing phosphates in fluidized bed co-combustion and co-gasification processes of P-rich sewage sludge and K-rich agricultural residues. The focus was set on the fate and role of P in the interaction of the main ash-forming elements based on thermodynamic equilibrium studies, lab-scale investigations, and bench-scale fluidized bed experiments. Additionally, the benefits, e.g., fuel flexibility and high conversion rate, and ash-related risks due to interaction of ash and bed material when using fluidized bed systems are elaborated with a focus on bed material selection and investigating the operational modes of combustion and gasification.The applicability of K- and Na-feldspar bed material in a pilot-scale indirect gasification system was investigated to provide a potential substitute for commonly used bed materials such as olivine and quartz. Olivine often contains heavy metals that could contaminate recovered ashes. Quartz may react with fuel-derived K, which could hamper the targeted formation of K-bearing phosphates and lead to bed material fragmentation and bed agglomeration. The bed material analysis of feldspar used in indirect wood gasification showed significant differences in the interaction phenomena between K- and Na-feldspar with the fuel ash. While both feldspar types displayed Ca-reaction and ash deposition layers on the particle surface, the interaction of Na-feldspar with K additionally led to the formation of K-reaction layers, possibly decreasing the bed particle integrity. The results suggest that K-feldspar is the preferred bed material option in terms of process stability and limiting the potential for side reactions of K when aiming for phosphate alteration toward K-bearing phosphates.Thermodynamic equilibrium calculations (TEC) were conducted with a focus on the fate of P and melting tendencies for a wide range of chemical compositions in biomass ashes and fuel mixtures between sewage sludge and the agricultural residues wheat straw (rich in Si and K) and sunflower husks (rich in K and Ca). The results for the K-Ca-Mg-P-Si-O system were validated with literature references, and an outline of practical implications was given. The results for sewage sludge and mixtures with agricultural residues functioned as a seminal tool for fuel design in experimental investigations. The thermodynamic preference for forming alkali-bearing phosphates in competition with pure Ca-phosphates and incorporating K in silicates could be shown. The analysis of the K-Ca-Mg-P-Si-O system highlighted the influence of elemental ratios between and within the Lewis acid formers (Mg, K, Ca) and the Lewis base formers (Si, P) on the fate of P and the ash melting tendency. The TEC for sewage sludge and mixtures with agricultural residues predicted that these elemental ratios are also the determining factors in the presence of large quantities of Al and Fe.Experimental research regarding the underlying ash chemistry with a focus on the fate of P was conducted in a single-pellet reactor and bench-scale fluidized bed combustion and gasification processes. The approach used sewage sludge pellets and co-pelletized mixtures of sewage sludge with wheat straw and sunflower husks to determine the P-recovery potentials and ash-related operational risks. The parameters were chosen with relevance to practical applications of fluidized bed technologies. The experimental findings supported the TEC results in several aspects, such as the preference for Ca-phosphate formation in sewage sludge ash and the exclusion of Fe from the bulk ash matrix. However, the results also showed practical limitations for the formation of K-bearing phosphates in fuel mixture ashes. The identified limitations were the reaction of K with Si, the high stability of Ca-rich orthophosphates, and the limited interaction of ash-forming elements in char residues from gasification processes. Furthermore, the results from the fluidized bed experiments highlighted the necessity for amendments in terms of fuel selection and fuel mixing to avoid operational risks such as bed agglomeration. The results of the conducted experimental investigations suggest that using K-feldspar as bed material in sewage sludge co-conversion setups with agricultural residues might benefit the incorporation of K in the P-rich ash fractions.The results and discussions presented in this work allowed for the assessment of crucial process and fuel parameters for fluidized bed conversion systems using sewage sludge fuels and biomass fuel mixtures focusing on the formation of K-bearing phosphates. The importance of the ash transformation chemistry and its impact on selecting a suitable bed material could be outlined based on experimental and modeling data. The outcome shall assist the design of future large-scale applications in terms of a viable process and fuel design for energy and resource recovery from sewage sludge and agricultural residues.
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| 4. |
- Nordin, Andreas, et al.
(författare)
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Co-Combustion of Municipal Sewage Sludge and Biomass in a Grate Fired Boiler for Phosphorus Recovery in Bottom Ash
- 2020
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Ingår i: Energies. - : MDPI. - 1996-1073. ; 13:7
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Tidskriftsartikel (refereegranskat)abstract
- Phosphorus has been identified as a critical element by the European Union and recycling efforts are increasingly common. An important phosphorus-containing waste stream for recycling is municipal sewage sludge (MSS), which is used directly as fertilizer to farmland. However, it contains pollutants such as heavy metals, pharmaceutical residues, polychlorinated bi-phenyls (PCBs) and nano-plastics. The interest in combustion of MSS is continuously growing, as it both reduces the volume as well as destroys the organic materials and could separate certain heavy metals from the produced ashes. This results in ashes with a potential for either direct use as fertilizer or as a suitable feedstock for upgrading processes. The aim of this study was to investigate co-combustion of MSS and biomass to create a phosphorus-rich bottom ash with a low heavy metal content. A laboratory-scale fixed-bed reactor in addition to an 8 MWth grate-boiler was used for the experimental work. The concentration of phosphorus and selected heavy metals in the bottom ashes were compared to European Union regulation on fertilizers, ash application to Swedish forests and Swedish regulations on sewage sludge application to farmland. Element concentrations were determined by ICP-AES complemented by analysis of spatial distribution with SEM-EDS and XRD analysis to determine crystalline compounds. The results show that most of the phosphorus was retained in the bottom ash, corresponding to 9-16 wt.% P2O5, while the concentration of cadmium, mercury, lead and zinc was below the limits of the regulations. However, copper, chromium and nickel concentrations exceeded these standards.
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| 5. |
- Skoglund, Nils, et al.
(författare)
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Characterisation of ash particles from co-combustion of bark and sludges from pulp and paper industry
- 2023
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Ingår i: Fuel. - : Elsevier. - 0016-2361 .- 1873-7153. ; 340
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Tidskriftsartikel (refereegranskat)abstract
- Recycling phosphorus from waste streams for fertilization purposes could contribute to a sustainable society. The production in the pulp and paper industry results in several waste streams, among others nutrient-rich sludges in different forms. This study presents a detailed chemical and 3D characterization of ash from co-combustion of bark and two types of sludges from a paper mill; mixed sludge and biosludge. The combustion performance was investigated for these experiments and advanced analysis methods were used to characterise the ashes to correlate chemical and physical properties relevant for nutrient recycling. The elemental composition was determined by energy-dispersive X-ray spectroscopy; dominating crystalline phases by X-ray diffraction; and morphology, porosity, pore size distribution and active surface area of the slag were analysed with synchrotron-based X-ray micro-tomography and image analysis. Slag was formed in all combustion experiments to a large extent with increasing amounts with a higher proportion of sludge. Nutrient amounts indicate that slag particles from co-combustion of both biosludge and mixed sludge can be useful either as a soil improvement directly or for recovery processes. Slag from combustion of 30 wt% biosludge and 70 wt% bark contained the highest amount of phosphorus, 9 at% on a C and O free basis. Evaluation of tomography data showed that discrete and open pores could be distinguished on a micrometre scale. The porosity of the slag varied between the replicates and fuel mixtures, on average between 17 and 23 vol% for the bark and sludge mixtures. Open pore volume displayed large variations, on average 39–56 vol% of the pores were open pores connected to the surrounding volume. For all samples, 90 % of the pores were small, with an equivalent diameter under 30 μm, but the largest pore volume (80–90 %) consists of pores with an equivalent diameter over 75 μm. In soils, pores with a minimum equivalent diameter over 30 μm generally transmit water and the smaller pores store water. The slag particles have relatively thick walls, with few pore openings to the surroundings, indicating that the slag needs to be pre-treated by milling or crushing before application in the soil.
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| 6. |
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| 7. |
- Strandberg, Anna, Dr, 1984-, et al.
(författare)
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AI-assisted deep learning segmentation and quantitative analysis of X-ray microtomography data from biomass ashes
- 2024
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Ingår i: MethodsX. - : Elsevier. - 1258-780X .- 2215-0161. ; 13
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Tidskriftsartikel (refereegranskat)abstract
- X-ray microtomography is a non-destructive method that allows for detailed three-dimensional visualisation of the internal microstructure of materials. In the context of using phosphorus-rich residual streams in combustion for further ash recycling, physical properties of ash particles can play a crucial role in ensuring effective nutrient return and sustainable practices. In previous work, parameters such as surface area, porosity, and pore size distribution, were determined for ash particles. However, the image analysis involved binary segmentation followed by time-consuming manual corrections. The current work presents a method to implement deep learning segmentation and an approach for quantitative analysis of morphology, porosity, and internal microstructure. Deep learning segmentation was applied to microtomography data. The model, with U-Net architecture, was trained using manual input and algorithm prediction. The trained and validated deep learning model could accurately segment material (ash) and air (pores and background) for these heterogeneous particles.Quantitative analysis was performed for the segmented data on porosity, open pore volume, pore size distribution, sphericity, particle wall thickness and specific surface area.Material features with similar intensities but different patterns, intensity variations in the background and artefacts could not be separated by manual segmentation – this challenge was resolved using the deep learning approach.
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| 8. |
- Strandberg, Anna, Dr, 1984-, et al.
(författare)
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Char conversion characterized by synchrotron based X-ray micro-tomography and SEM-EDS analysis
- 2017
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Ingår i: European Biomass Conference and Exhibition Proceedings. - : ETA-Florence Renewable Energies. - 2282-5819. ; , s. 485-491
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Konferensbidrag (refereegranskat)abstract
- Fuel and ash properties were studied during fuel conversion by careful examination of char samples at different degrees of char oxidation. Two types of lignocellulosic pellets with different compositions of ash forming elements were used: poplar and straw from wheat. The charred pellets were investigated by synchrotron-based X-ray micro-tomography to create 3D images of the development of cracks, internal cavities, and ash layers during conversion. Furthermore, SEM-EDS was used to for detailed chemical and morphological information of the ash layers formed. The pore development during pellet conversion was found to deviate from what has previously been described for the structure of solid wood particles. Large cracks and internal cavities were formed extensively already during devolatilization. For poplar, no mobility of the ash forming elements were observed as the burnout proceeded. Ash layer properties varied between the two fuels: poplar formed a porous, permeable, low density and Ca rich ash, whereas wheat straw ash accumulated on the surface in the form of high density melt that develop into bubbles on the surface. As the conversion proceeded, the ash layer covered more of the active char surface area, but without totally blocking the gas transport.
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| 9. |
- Strandberg, Anna, Dr, 1984-, et al.
(författare)
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Characterization of porosity and microstructure of phosphorus-rich ash particles with X-ray micro-tomography
- 2019
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Konferensbidrag (refereegranskat)abstract
- A large proportion of the nutrients supplied from forest and agricultural sector are lost today, both through leaching and removing of produced biomass. Sustainable and efficient recycling of macro- and micro nutrients can be done by combustion / co-combustion of residual streams, which makes it possible to utilize for example the phosphorus-bound fraction in the produced ash for further processing and recycling. The porosity of the ash is important for, among other things, leaching and water-retaining ability when returning to the soil. The purpose of the project is to provide detailed knowledge of porosity and internal microstructure of ash particles from combustion of residual biomass streams, by using X-ray based micro-tomography and image analysis. The results provide new insights into how ash porosity and micro structure differs between different ashes, depending on fuel and the choice of conversion process.
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| 10. |
- Strandberg, Anna, Dr, 1984-, et al.
(författare)
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Morphological characterisation of ash particles from co-combustion of sewage sludge and wheat straw with X-ray microtomography
- 2021
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Ingår i: Waste Management. - : Elsevier. - 0956-053X .- 1879-2456. ; 135, s. 30-39
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Tidskriftsartikel (refereegranskat)abstract
- Combustion of phosphorus-rich residual streams can produce nutrient-rich ashes and these can be used either in further processing or as materials for direct nutrient recycling. The latter requires knowledge on morphological parameters of such ash particles that may impact plant growth, nutrient availability, and soil physical properties. The present work aims to determine the porosity, pore size, and specific surface area of ash particles, and discuss these properties in light of literature concerning interaction with soil water and plant roots. Bottom ash particles from combustion of sewage sludge and wheat straw and their co-combustion were analysed with X-ray microtomography. Image analysis provided information on morphology, specific surface area, porosity, and pore structure on a micrometre scale resolution. Co-combusting sewage sludge with wheat straw resulted in differences in ash particles' porosity and pore structure compared to combustion of pure fuels. Pure wheat straw ash displayed 62 vol% porosity while there was no apparent difference between 10 wt% or 30 wt% mixtures of sewage sludge, with a porosity of 29–31 vol%. Open pore volume comprise the largest part of the porosity (72–99 vol%) enabling interaction between surrounding pore water and nutrients.Overall, the ash particles display large open volume fractions and thin particle walls which may lead to rapid weathering and extensive interaction with soil water. The particles generally contained pore openings over 200 µm towards the surroundings, which provide opportunities for interaction with microbes and roots from a variety of plant species in addition to nutrient transport by soil water.
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