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  • 1.
    Falk, Joel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Effect of fuel composition and combustion conditions on phosphorus behavior during combustion of biomass2018Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Due to concerns for climate change and future supply of phosphorus fertilizer within agriculture, there has been an increased interest in the combustion of phosphorus containing waste residues and opportunity biomass fuels. Previous research has shown that during combustion, phosphorus has large impact on ash transformation reactions and may decrease or increase ash-related problems such as slag formation and bed agglomeration. This is a serious concern if new types of biomass are to be added for heat and power production. Additionally, plant studies and leaching tests of P-rich biomass ash indicate that the plant availability of phosphorus varies greatly with its association in the ash. As such, the ash transformation behavior of phosphorus is of great importance for the success of such ventures. While several studies have been made on the behavior of phosphorus during combustion, no comprehensive study has been made evaluating the effect of fuel composition and combustion conditions.

    In this work, the behavior of phosphorus was determined for a wide range of fuels and combustion conditions. More specifically, the objective was to determine (i) the effect of fuel ash composition and combustion technologies on the fate of phosphorus during combustion, (ii) investigate potential difference in the behavior of phosphorus during combustion of sewage sludge and plant based biomassand (iii)the effect of phosphorus on slag formation and bed agglomeration for the co-combustion of a wide range of plant based biomasses.

    The investigation was carried out by comparing experimental data gathered from the combustion of 26different biomass fuelsor fuel blends in a bench scale bubbling fluidized bed (5 kW, 18 experiments), an underfed pellet burner (20kW, 10 experiments) and a swirling powder burner (150 kW, 7 experiments). This included chemical characterization of bed ash, bottom ash and fly ash fractions by X-ray diffraction (XRD), scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) in addition to qualitative measures of slagging-and bed agglomeration tendencies.

    It was found that phosphorus, irrespective of combustion technology and fuel composition, was mainly found in bed-, and bottom ash fractions and/or coarse fly ash fractions (>1μm). Based on the crystalline phase composition of the phosphates found in bed-, bottom-and coarse fly ash samples, phosphate speciation was correlated to the molar ratio between P, Ca and Mg for all three combustion technologies. Based on these results, it would be possible to control the behavior of phosphorus during combustion and the plant availability of phosphates in biomassash by designing fuel blends based on their fuel ash composition.

    In fluidized bed combustion, it was found that for similar combustion conditions and fuel ash compositions (with respect to K, Ca and P), the speciation of phosphorus in coarse ash fractions was significantly different from experiments with plant based biomass compared to sewage sludge. Unlike ash from plant based biomass, the crystalline phase composition of ash from sewage sludge did not change with the relative concentration of K, Ca andP in the fuel. The results suggest that the reaction pathway of phosphorus during combustion of sewage sludge is different to plant based biomass due to difference in the association of phosphorus in the fuel.

    The effect of phosphorus on slag formation and bed agglomeration in biomass combustion was mainly related to the relative fuel ash concentration of K, Ca, Mg, Si and P. In fluidized bed combustion, P contributes to the formation of agglomerates through the melt induced mechanism, through complex interaction with K, Ca, Mg and Si. Similarly, in fixed bed combustion the composition of slag indicatedthatslag formation involves the formation of P and Si rich ash melt with a varying content of K, Ca and Mg. In both cases, the severity of problems was related to the melting behavior of the (CaO,MgO)-K2O-(SiO2,P2O5) multicomponent system.

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  • 2.
    Falk, Joel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Fate of phosphorus in combustion of biomass Manuscript (preprint) (Other academic)
    Abstract [en]

    Integrating phosphorus recovery with a sustainable bioenergy system using biogenic residues in thermochemical conversion processes simultaneously addresses global challenges in phosphorus management and climate change. Studies on phosphorus recovery from ash fractions by direct use of ash as fertilizers rarely report what fuels and process produced the ash, whether the chosen ash fractions are suitable, or what phosphates are actually present in the ash. Here, we discuss the fate of phosphorus in combustion of various biomasses as a function of fuel composition and combustion technology by identifying trends in crystalline phosphate formation. The main deciding factor for which phosphate structures will form is relative concentrations of ash-forming elements in initial fuel or fuel blends, where calcium and magnesium are the most important counter-ions. Phosphate structures including potassium improves the recovery potential and may be produced in thermochemical conversion processes by adjusting total fuel feedstock composition through fuel blending or additives.

  • 3.
    Falk, Joel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    The fate and ash transformations of phosphorus in combustion of biomass and sewage sludge2022Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The combustion of phosphorus (P)-rich biomass has a significant potential to increase the stock of biomass resources available for renewable heat and power production. In addition, the P-rich ashes have the potential as a fertilizer and could reduce the need for non-renewable P resources in agriculture. However, several technical challenges must be resolved to realize this potential.

    During combustion, the ash-forming matter in the fuel undergoes numerous chemical transformations, which can result in the formation of ash melts. Excessive melt formation can lead to durable ash deposits in and around the combustion zone and on heat exchangers, which can lower combustion performance and, in severe cases, lead to a complete shutdown of the process. Further, mono-combustion of P-rich residues such as sewage sludge results in the formation of phosphates with poor plant availability, which can significantly limit the value of the ash as a fertilizer. By co-combustion the sludge with K-rich biomass, it may be possible to alter the chemical speciation of P towards more plant-available phases while simultaneously managing the risk of ash-related operational issues. This work investigates the effect of combustion technology, fuel ash composition, and chemical association of P in the fuel on the fate, i.e., distribution and speciation, and ash transformations of P in combustion and co-combustion of biomass and sewage sludge.

    The basis of the study was experiments performed in three different combustion technologies, including a fluidized bed (5 kW, 730-800 °C), a fixed bed (20 kW, 950-1250 °C), and a powder burner (150 kW, ~1100°C). The fuels and fuel mixtures included P-rich and P-poor woody biomass, agricultural residues, and sewage sludge, which constitute a wide range of ash compositions in terms of K, Ca, Mg, Fe, Al, Si, and P. The residual ashes from the experiments were collected and chemically characterized with the original fuels and fuel mixtures to determine the ash transformation reactions of P. The experiments were complemented by thermodynamic equilibrium calculations (TECs), which aided the interpretation of experimental data and predicted the risk for operational issues related to the melting of coarse ash fractions.

    The major share of fuel P was found in coarse ash fractions such as bed ash particles, bottom ash, slag, cyclone ash, wind side deposits, and coarse fly ash. A low share of fuel P was found in fine ash fractions such as leeside deposits and PM1. This generally matched the predictions by TEC, which indicated that P was stable in condensed phases at the relevant compositions and conditions during the combustion experiments. The powder burner experiments produced the highest share of fuel P in PM1 (4-14 wt.%), followed by fixed bed combustion (<4 wt.%), with fluidized bed combustion having the lowest share (<0.6 wt.%). In addition, the experiments with sewage sludge indicated a significantly lower P share in PM1 for a given combustion technology than the other biomass fuels, ranging from <0.2 wt.% in the fluidized bed and <1.2 wt.% in the fixed bed.

    Combustion and co-combustion of woody biomass and agricultural residues resulted in the formation of a wide range of ortho-, pyro-, and metaphosphates associated with K, Ca, and Mg. Combustion of woody biomass generally resulted in a high share of Ca-orthophosphates, whereas agricultural residues had a higher share of K-rich ortho- and pyrophosphates. Irrespective of biomass assortment, the speciation of P in the ash from combustion and co-combustion followed general trends with respect to the fuel ash composition of the biomass mixture. The frequency and share of pyro- and metaphosphates identified in the coarse ash fractions tended to increase with the relative concentration of P to K, Ca, and Mg in the fuel mixture. A similar correlation was found between the share of K-rich phosphates and the relative concentration of K to Ca and Mg.

    The crystalline phosphate phases identified in the coarse ash fractions from sewage sludge and K-rich biomass experiments were mainly Fe-rich and Ca-rich orthophosphate. The frequency and share of Fe-rich orthophosphates decreased with the relative P to K, Ca, and Mg concentration in the fuel mixture. However, the sewage sludge mixtures were less prone to form K-rich orthophosphates than the biomass mixtures for a given composition in terms of P to K, Ca, and Mg.

    Based on TECs, it was possible to qualitatively predict ash-related issues related to the melting behavior of coarse ash fractions, such as slag formation, for woody biomass and agricultural residues by considering the K, Ca, Mg, Si, and P content in the fuel. The share of network formers (SiO2, PO2.5) to total ash oxides had the largest overall influence on the melting tendency of the ash mixture, followed by the ratio of K2O to total network modifiers (K2O, CaO, MgO), which had a high impact on ash mixtures with high relative shares of SiO2. The slagging tendency of fuel mixtures with a high share of sewage sludge could not be predicted based on the melting behavior of the K-Ca-Mg-Si-P-O system due to the high relative share of Fe and Al. The experimental results indicated that the slagging tendency of the sewage sludge was significantly improved by co-combustion with moderate amounts of wheat straw or sunflower husk.

    Based on the combined results, it was possible to establish four fuel ash molar ratios correlated with the speciation of P in the produced coarse ash fractions and the risk of slag formation in fixed-bed combustion. These ratios were used to recommend practical fuel mixing strategies that could enable the production of combustion ashes with high P-plant availability while simultaneously managing the risk of severe slag formation.

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  • 4.
    Falk, Joel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Hannl, Thomas Karl
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Skoglund, Nils
    Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics, Umeå University, SE-90187 Umeå, Sweden; BEST-Bioenergy and Sustainable Technologies GmbH, Inffeldgasse 21b, AT-8010 Graz, Austria; Institute of Chemical, Environmental & Bioscience Engineering, TU Vienna, AT-1060 Vienna, Austria.
    Backman, Rainer
    Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics, Umeå University, SE-90187 Umeå, Sweden.
    Öhman, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Thermodynamic Equilibrium Study on the Melting Tendency of the K-Ca-Mg-P-Si-O System with Relevance to Woody and Agricultural Biomass Ash Compositions2022In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 36, no 13, p. 7035-7051Article in journal (Refereed)
    Abstract [en]

    A major challenge in the combustion of biomass fuels is the heterogeneity of ash-forming elements, which may cause a wide range of ash-related problems. Understanding the melting tendency of the coarse ash fractions is necessary to mitigate agglomeration and slagging. This work aims to evaluate the melting tendency of the K-Ca-Mg-Si-P-O system by use of thermodynamic equilibrium calculations. The formation of condensed phases were systematically assessed in a combustion atmosphere, varying temperatures, and composition. Compositional ranges were based on fuel ash data extracted from the Phyllis 2 database. The speciation and degree of polymerization of phosphates, silicates, and melts were evaluated and indicated a systematic variation in composition. The melt fraction was predicted as a function of temperature and composition. The melting tendency was modeled for three systems, i.e., a P-dominated, a Si-dominated, and a mixed Si-P system. Four ratios between K2O, CaO, MgO, SiO2, and P2O5 were found to have a large effect on the melting tendency of the ash mixtures: the ratio between network formers (SiO2, P2O5), K2O to total network modifiers, CaO to CaO + MgO, and the ratio of network formers to total ash oxides. This modeling approach showed qualitative agreement with ash-related issues seen in previous lab-scale experiments in bubbling fluidized bed and fixed bed combustion. Practical implications of the results are discussed from the perspective of fuel design with the aim of preventing ash-related problems. This study presents a novel method of applying thermodynamic equilibrium calculations for a broad range of compositions and shows potential for predicting ash-related issues related to the melting of coarse ash fractions.

  • 5.
    Falk, Joel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Hannl, Thomas Karl
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Öhman, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Hedayati, Ali
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Skoglund, Nils
    Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics, Umeå University, SE-90187 Umeå, Sweden; BEST−Bioenergy and Sustainable Technologies GmbH, Inffeldgasse 21b, AT-8010 Graz, Austria; Institute of Chemical, Environmental & Bioscience Engineering, TU Vienna, AT-1060 Vienna, Austria.
    Ash Transformation during Fixed-Bed Co-combustion of Sewage Sludge and Agricultural Residues with a Focus on Phosphorus2023In: ACS Omega, E-ISSN 2470-1343, Vol. 8, no 14, p. 13162-13176Article in journal (Refereed)
    Abstract [en]

    This work investigates the ash transformation during fixed-bed co-combustion of sewage sludge mixtures with the agricultural residues wheat straw and sunflower husks, focusing on the fate of phosphorus (P) in the resulting ash fractions. The study aims to determine suitable process parameters for fixed-bed combustion of fuels previously investigated in single-pellet experiments. The pure fuels and fuel mixtures were combusted in a 20 kWth residential pellet burner while monitoring the flue gas composition, temperature, and particulate matter formation. Subsequently, the different ash fractions were collected and characterized by CHN, SEM/EDS, and XRD analysis. The results showed that co-combustion of sewage sludge and agricultural residues reduced the formation of particulate matter as well as the formation of slag. Co-combustion of sewage sludge with either agricultural residue resulted in a change in phosphate speciation, displaying higher shares of Ca and lower shares of Fe and Al in the formed orthophosphates as well as amorphous phases containing higher shares of K. The formation of K-bearing phosphates was hindered by the spatial association of P with Ca and Fe in the sewage sludge, the incorporation of available K in K-Al silicates, and the depletion of K in the P-rich melt phase. Compared to mono-combustion, co-combustion experiments showed the potential for improving the combustion performance and reducing the risk of slag formation. The outcome suggests that co-combustion is a feasible path to integrate waste streams in fixed-bed energy conversion with simultaneous formation of phosphates enabling P recovery.

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  • 6.
    Falk, Joel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Skoglund, Nils
    Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics, Umeå University, Umeå, Sweden.
    Grimm, Alejandro
    Department of Forest Biomaterials and Technology, Swedish University of Agricultural Sciences, Umeå, Sweden.
    Öhman, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Fate of Phosphorus in Fixed Bed Combustion of Biomass and Sewage Sludge2020In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 34, no 4, p. 4587-4594Article in journal (Refereed)
    Abstract [en]

    The recovery of phosphorus (P) from societal waste streams, such as sewage sludge, could make a significant contribution to alleviating the global dependency upon non-renewable phosphate sources, such as phosphate rock. This study aims to determine the effect of fuel ash composition, chemical association, and combustion technology on the fate of P in ashes from the combustion of sewage sludge and biomass blends to enable more efficient P recovery from combustion ashes. Experiments were performed in a fixed bed pellet burner (20 kW), combusting two sewage sludge blends and three biomass blends of similar fuel ash composition but with different P source (sewage sludge, dried distiller’s grain with solubles, or phosphoric acid). Slag, bottom ash, and particulate matter samples were collected and analyzed by scanning electron microscopy–energy-dispersive X-ray spectroscopy and X-ray diffraction for morphology and elemental and crystalline phase composition and compared to results from experiments in fluidized bed combustion using the same fuel blends reported separately. The distribution and elemental composition of ash fractions indicated that sub-micrometer particles contained a minor share of fuel P, with the significant share of fuel P found in the slag and bottom ash fractions. No apparent difference in phosphate speciation could be observed between the slag and bottom ash from sewage sludge blends and biomass blends, with a range of crystalline Ca, Mg, and K phosphates detected in the ash. By comparison, only Ca-rich phosphates were detected in the ashes from the combustion of the sewage sludge blends in the bench-scale fluidized bed. The difference in P speciation between the technologies was attributed to a difference in the process temperature between the two technologies. In comparison to fluidized bed combustion, fixed bed combustion favored the formation of (Ca, Mg)–K phosphates rather than Ca phosphates for similar fuel blends.

  • 7.
    Falk, Joel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Skoglund, Nils
    Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics, Umeå University, Umeå, Sweden.
    Grimm, Alejandro
    Department of Forest Biomaterials and Technology, Swedish University of Agricultural Sciences, Umeå, Sweden.
    Öhman, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Systematic Evaluation of the Fate of Phosphorus in Fluidized Bed Combustion of Biomass and Sewage Sludge2020In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 34, no 4, p. 3984-3995Article, review/survey (Refereed)
    Abstract [en]

    Comprehensive knowledge concerning the behavior of phosphorus (P) during combustion is necessary to enable more efficient recovery of P from combustion ashes for agricultural purposes. To this end, parameters that influence the distribution and speciation of P in combustion ashes are important because they may influence which ash fractions are suitable for P recovery. This study aims to determine the fate of P as a result of fuel ash composition and chemical association in the fuel during fluidized bed combustion by a systemic review of previous work. The synthesis was performed by comparing scanning electron microscopy–energy-dispersive X-ray spectroscopy and X-ray diffraction chemical analyses of bed ash, fly ash particles, and deposits from fluidized bed combustion of different blends of P-poor (logging residues or wheat straw) and P-rich (sewage sludge, dried distiller’s grain with solubles, or phosphoric acid) fuels and additives. The blends were produced to have a similar ash composition but with a different P source. The distribution of P among ash fractions indicated that P is mainly found in the coarse ash fractions (bed and cyclone ash), irrespective of fuel ash composition or chemical association in the fuel. The chemical speciation of P in coarse ash fractions differed between biomass blends containing sewage sludge compared to blends with phosphoric acid or dried distiller’s grain with solubles. Phosphates in the ash from the two sewage sludge blends included predominantly Ca with minor inclusion of other cations. In contrast, ashes from the blends with phosphoric acid or dried distiller’s grain with solubles contained phosphates with a significant amount of K, Ca, and Mg. The difference in phosphate speciation could not solely be explained by the combustion conditions and the elemental composition of the ash fractions. These results show that it is necessary to consider the chemical association of P in the fuel to predict the type of phosphates that will form in fluidized bed combustion ashes.

  • 8.
    Hedayati, Ali
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Falk, Joel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Borén, Eleonora
    Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden; Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics, Umeå University, SE-901 87 Umeå, Sweden.
    Lindgren, Robert
    Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics, Umeå University, SE-901 87 Umeå, Sweden.
    Skoglund, Nils
    Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics, Umeå University, SE-901 87 Umeå, Sweden.
    Boman, Christoffer
    Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics, Umeå University, SE-901 87 Umeå, Sweden.
    Öhman, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Ash Transformation during Fixed-Bed Combustion of Agricultural Biomass with a Focus on Potassium and Phosphorus2022In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 36, no 7, p. 3640-3653Article in journal (Refereed)
    Abstract [en]

    In this study, ash transformation during fixed-bed combustion of different agricultural opportunity fuels was investigated with a special focus on potassium (K) and phosphorus (P). The fuel pellets were combusted in an underfed fixed-bed pellet burner. Residual ashes (bottom ash and slag) and particulate matter were collected and characterized by scanning electron microscopy–energy-dispersive X-ray spectroscopy, X-ray diffraction, inductively coupled plasma, and ion chromatography. The interpretation of the results was supported by thermodynamic equilibrium calculations. For all fuels, almost all P (>97%) was found in residual-/coarse ash fractions, while K showed different degrees of volatilization, depending on fuel composition. During combustion of poplar, which represents Ca–K-rich fuels, a carbonate melt rich in K and Ca decomposed into CaO, CO2, and gaseous K species at sufficiently high temperatures. Ca5(PO4)3OH was the main P-containing crystalline phase in the bottom ash. For wheat straw and grass, representing Si–K-rich fuels, a lower degree of K volatilization was observed than for poplar. P was found here in amorphous phosphosilicates and CaKPO4. For wheat grain residues, representing P–K-rich fuels, a high degree of both K and P retention was observed due to the interaction of K and P with the fuel-bed constituents, i.e., char, ash, and slag. The residual ash was almost completely melted and rich in P, K, and Mg. P was found in amorphous phosphates and different crystalline phases such as KMgPO4, K2CaP2O7, K2MgP2O7, and K4Mg4(P2O7)3. In general, the results therefore imply that an interaction between ash-forming elements in a single burning fuel particle and the surrounding bed ash or slag is important for the overall retention of P and K during fuel conversion in fixed-bed combustion of agricultural biomass fuels.

  • 9.
    Strandberg, Anna
    et al.
    Umeå University, Department of Applied Physics and Electronics, Thermochemical Energy Conversion Laboratory, SE 901 87 Umeå, Sweden; Swedish University of Agricultural Sciences, Department of Forest Biomaterials and Technology, SE 901 83 Umeå, Sweden.
    Thyrel, Mikael
    Swedish University of Agricultural Sciences, Department of Forest Biomaterials and Technology, SE 901 83 Umeå, Sweden.
    Falk, Joel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Öhman, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Skoglund, Nils
    Umeå University, Department of Applied Physics and Electronics, Thermochemical Energy Conversion Laboratory, SE 901 87 Umeå, Sweden.
    Morphology and phosphate distribution in bottom ash particles from fixed-bed co-combustion of sewage sludge and two agricultural residues2024In: Waste Management, ISSN 0956-053X, E-ISSN 1879-2456, Vol. 177, p. 56-65Article in journal (Refereed)
    Abstract [en]

    The purpose of this study was to provide detailed knowledge of the morphological properties of ash particles, including the volumetric fractions and 3D distributions of phosphates that lay within them. The ash particles came from digested sewage sludge co-combusted with K- and Si-rich wheat straw or K-rich sunflower husks. X-ray micro-tomography were combined with elemental composition and crystalline phase information to analyse the ash particles in 3D.

    Analyses of differences in the X-ray attenuation enabled calculation of 3D phosphate distributions that showed high heterogeneity in the slag particles. This is underscored by a distinct absence of phosphates in iron-rich and silicon-rich parts. The slag from silicate-based wheat straw mixtures had lower average attenuation than that from sunflower husks mixtures, which contained more calcium. Calculated shares of phosphates between 7 and 17 vol% were obtained, where the highest value for a single assigned phosphate was observed in hard slag from wheat straw with 10 % sewage sludge. The porosity was notably higher for particles from pure wheat straw combustion (62 vol%), compared to the other samples (15–35 vol%). A high open pore volume fraction (60–97 vol%) indicates that a large part of the pores can be accessed by the surroundings. For all samples, more than 60 % of the discrete (closed) pores had an equivalent diameter < 30 μm, while the largest volume fraction consisted of pores with an equivalent diameter > 75 μm. Slag from sunflower husk mixtures had larger pore volumes and a greater relative number of discrete pores >75 µm compared to wheat straw mixtures.

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