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  • 1.
    Carlborg, Markus
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Refractory corrosion in biomass gasification2018Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    To stop the net emission of CO2 to the atmosphere, we need to reduce our dependency of fossil fuels. Although a switch to a bio-based feedstock hardly can replace the total amount of fossils used today, utilization of biomass does still have a role in a future in combination with other techniques. Valuable chemicals today derived from fossils can also be produced from biomass with similar or new technology. One such technique is the entrained flow gasification where biomass is converted into synthesis gas. This gas can then be used as a building stone to produce a wide range of chemicals.

    Slagging and corrosion problems are challenges presented by the ash forming elements in biomass during thermochemical energy conversion. The high temperature in the entrained flow process together with ash forming elements is creating a harsh environment for construction materials in the reactor. Severe corrosion and high wear rates of the lining material is a hurdle that has to be overcome to make the process more efficient.

    The objective of this work is to investigate the nature of the destructive interaction between ash forming elements and refractory materials to provide new knowledge necessary for optimal refractory choice in entrained flow gasification of woody biomass. This has been done by studying materials exposed to slags in both controlled laboratory environments and pilot scale trials. Morphology, elemental composition and distribution of refractories and slag were investigated with scanning electron microscopy and energy dispersive X-ray spectroscopy. Crystalline phases were investigated with X-ray diffraction, and thermodynamic equilibrium calculations were done in efforts to explain and make predictions of the interaction between slag and refractory.

    Observations of slag infiltration and formation of new phases in porous materials indicate severe deterioration. The presence of Si in the materials is limiting intrusion by increasing the viscosity of infiltrated slag. This is however only a temporary delay of severe wear considering the large amount of slag that is expected to pass the refractory surface. Zircon (or zirconium) (element or mineral?) based material show promising properties when modeled with thermodynamic equilibrium, but disassembling of sintered material and dissociation of individual grains was seen after exposure to a Si- and Ca-rich slag. Fused cast materials have a minimal slag contact where the only interaction is on the immediate hot face. Dissolution was however observed when exposed to a silicate-based slag, as was the formation of NaAlO2 after contact with black liquor.

  • 2.
    Carlborg, Markus
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Backman, Rainer
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Landälv, Ingvar
    Characterization of spent spinel-based refractory lining from a 3 MW black liquor gasifierManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    Black liquor gasification is dependent on minimizing heat loss to the surroundings and thus needs to be well insulated. In combination with the high temperature and basic black liquor, a very corrosive environment is created on the hot face of such a reactor. Therefore the wall system is required to be chemically and thermally stable at the same time as it has insulating properties. These cannot easily be combined in the same material and therefore layers with different properties can be used in combination. Penetration of species through the lining can lead to further reactions with other construction materials, less suited for chemical resistance, corrosion of the pressure shell is an example with catastrophic consequences. This paper investigates two castable and one fused cast spinel (MgAl2O4) refractory after about 1 600 hours, and one fused cast material used for 15 000 hours of operation in a 3 MWth black liquor gasifier. Infiltration of Na, followed by destruction of microstructure, and extensive formation of NaAlO2 was observed throughout the whole castable materials, while it was mainly restricted to the hot face of the fused cast materials. Formation of NaAlO2 leads to a volumetric expansion which eventually lead to an increased pressure on the steel shell. In addition, the expansion of the bricks can cause stress and by that spallation and material loss.

  • 3.
    Carlborg, Markus
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Boström, Dan
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Kannabiran, Sankar
    Höganäs Bjuf AB.
    Backman, Rainer
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Interaction between ash forming elements in woody biomass and two high alumina refractories part 1: effects on morphology and elemental distributionManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    To gain more knowledge about possibly destructive effects of ash-forming elements in woody biomass on refractory materials in entrained flow gasification, an exposure study was performed on two high alumina refractories. The materials, a pre-fired castable consisting of about 63 weight-% Al2O3, and a phosphate bonded brick with 83 weight-% Al2O3 was exposed to synthetic ash mixtures at 1050°C and 1 atm CO2 for 7 days. This paper presents distribution of ash-forming elements and morphology of the samples microstructure, while identification and distribution of crystalline compounds is presented in a separate paper. In the samples, potassium (K) had infiltrated the materials and reacted with different components, while calcium (Ca) did not seem to have any direct effect during these conditions. The matrix of the castable absorbed much K, became clogged and produced a distinct border between reacted and unaffected matrix. The coarser matrix of the phosphate bonded brick retained much of its porosity and had ash transported further into the material without a clear distinction between reacted and unaffected matrix. Grains with >30 atomic-% Si, formed a layer enriched in K, with a thickness up to 40 µm and cracks propagating through it. Grains mainly consisting of Al2O3 seemed unaffected by the exposure. When the ash was rich in SiO2, a melt was produced that restricted the attack on the refractories to the surface and coarser pores.

  • 4.
    Carlborg, Markus
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Boström, Dan
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Kannabiran, Sankar
    Höganäs Bjuf AB.
    Backman, Rainer
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Interaction between ash forming elements in woody biomass and two high alumina refractories part 2: transformation of crystalline compoundsManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    Two high alumina refractories, one brick and one pre fired castable was exposed to pure K2CO3, K2CO3 + CaCO3, and K2CO3 + CaCO3 + SiO2 at 1050°C and a CO2 atmosphere. A stratified investigation of crystalline phases was made with polycrystalline x-ray diffraction, and thermodynamic equilibrium calculations were performed to explore possible formation paths. A monoclinic polymorph of KAlSiO4 was formed to a large extent in both materials exposed to pure K2CO3. Throughout the affected part of the castable and a small layer close to the surface of the brick, a solid solution between KAlO2 and KAlSiO4 formed, K1-xAl1-xSixO2, x = 0.19. The affected area of the castable had 30-50 %wt new phases and made a sharp transition to unaffected material. The concentration of new phases in the brick was decreasing at an even rate from about 40 to 15%wt throughout the whole material thickness of 14 mm. Exposure to K2CO3 and CaCO3 showed the same phases and behavior, but no Ca-bearing phases could be detected. The mixture containing K2CO3, CaCO3 and SiO2 did not penetrate far into the material but formed the same phases in the affected areas. Wollastonite (CaSiO3) formed in the slag on top of these materials. The major mechanism for formation of new phases is suggested to be the formation of an initial melt composed of K2O and SiO2. This liquid is then dissolving refractory components and forms a liquid in equilibrium with KAlSiO4 and K1-xAl1-xSixO2.

  • 5.
    Carlborg, Markus
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Weiland, Fredrik
    Ma, Charlie
    Luleå University of Technology, Luleå, Sweden.
    Backman, Rainer
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Landälv, Ingvar
    Wiinikka, Henrik
    Exposure of refractory materials during high-temperature gasification of a woody biomass and peat mixture2018Ingår i: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 38, nr 2, s. 777-787Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Finding resilient refractory materials for slagging gasification systems have the potential to reduce costs and improve the overall plant availability by extending the service life. In this study, different refractory materials were evaluated under slagging gasification conditions. Refractory probes were continuously exposed for up to 27 h in an atmospheric, oxygen blown, entrained flow gasifier fired with a mixture of bark and peat powder. Slag infiltration depth and microstructure were studied using SEM EDS. Crystalline phases were identified with powder XRD. Increased levels of Al, originating from refractory materials, were seen in all slags. The fused cast materials were least affected, even though dissolution and slag penetration could still be observed. Thermodynamic equilibrium calculations were done for mixtures of refractory and slag, from which phase assemblages were predicted and viscosities for the liquid parts were estimated.

  • 6.
    Eriksson, Matias
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik. Nordkalk AB, Köping, Sverige.
    Carlborg, Markus
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Broström, Markus
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Characterization of ring deposits inside a quicklime producing long rotary kiln2019Ingår i: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 33, nr 11, s. 11731-11740Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Ring deposits are common problems in rotary kiln operations. The ring is constantly subjected to thermal and mechanical wear counteracting the growth of the ring. If the ring hardens or if the growth of the ring is too rapid the kiln needs to be shut down and the ring removed, reducing the operational time and profitability of the process. In the present study, ring deposits from a limestone fed long rotary kiln producing quicklime was sampled and characterized in detail by SEM-EDS, dynamic rate TG and XRD. This work identifies three hardening mechanisms active in the kiln, an increased densification of the ring deposits near the refractory surface, the formation of calcite and spurrite through carbonation of the ring deposits, and the intrusion of molten fuel ash and product into the refractory, resulting in a strong attachment of the deposit to the refractory surface. The work also concludes that a significant part of the ring deposit has its origin in the fuel ash, contributing to deposit mass and increasing ring growth rate.

  • 7.
    Holmgren, Per
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik. Umeå Universitet.
    Carlborg, Markus
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik. Umeå Universitet.
    Broström, Markus
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik. Umeå Universitet.
    Slag Formation During Entrained Flow Gasification: Calcium Rich Bark Fuel with KHCO3 AdditiveManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    Managing slag properties is of utmost importance for successful operation of entrained flow gasifiers. The present study details some aspects of slag formed from a softwood bark fuel, and especially the situation with only small amounts of mineral contaminants, meaning composition is shifted from Si- towards P-dominated ash. Wood bark with and without KHCO3 additive was gasified between 850 °C and 1300 °C at O2 stoichiometric ratio (λ) 0.6 to study the resulting ash properties and the influence of the additive. The ash particles collided with a flat impact probe inside the hot reactor, with particle impact angles varied between 90° to 30°. The reactor and probe were constructed to allow for long-distance microscope data collection close to the surface of the probe. In situ PIV and SEM-EDS of deposit samples from lab scale entrained flow gasification experiments were used for evaluation, while XRD was used to characterize carbonates. High potassium release was found but numerous spherical ash particles indicated lower ash melting temperatures than expected from the bulk ash composition. These new findings propose a mechanism for melt formations involving carbonates rich in potassium and phosphorous, followed by K-release and calcination leading to solidification.

  • 8.
    Kou, Wen
    et al.
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi.
    Gabrielsson, Klas
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi.
    Borhani, Adrian
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi.
    Carlborg, Markus
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Molin Thorén, Margareta
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi.
    The effect of artificial aging on high translucent zirconia2019Ingår i: Biomaterial Investigations in Dentistry, ISSN 2641-5275, Vol. 6, nr 1, s. 54-60Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Zirconia is known for its high strength but lacking translucency. Recently, a new type of high translucent zirconia, 5 mol% yttria partially stabilized zirconia (5Y-PSZ), with a larger fraction of cubic zirconia phase has become commercially available. However, the resistance to aging of these commercially available zirconia materials is not yet fully established.

    Purpose: The aim of the present study was to analyze the effects of artificial aging on surface roughness, transparency, phase transformation and biaxial flexural strength of two 5Y-PSZ products, DD cubeX2 and Prettau Anterior.

    Materials and methods: The artificial aging was performed in an autoclave under 2 bars of pressure at 134 °C for 10 hours, which is estimated to correspond to 30–40 years in vivo. Artificial aging for 10 hours had no significant effect on surface roughness, transparency, or phase transformation for either of the tested materials.

    Results: DD cubeX2 had higher mean flexural strength than Prettau Anterior both before and after artificial aging for 10 hours (p < .05). DD cubeX2 showed, however, a significant reduction in flexural strength after artificial aging (p < .05), whereas Prettau Anterior showed a slight increase in flexural strength after artificial aging but not at a significant level.

    Conclusion: Within the limitation of the present study, both DD cubeX2 and Prettau Anterior seems to be relatively resistant to aging. However, a wider range of measured flexural strength indicated that Prettau Anterior probably is a less stable material than DD cubeX2, which also means that the flexural strength of DD cubeX2 could be more predictable.

  • 9.
    Ma, Charlie
    et al.
    Energy Engineering, Division of Energy Science, Luleå University of Technology, Luleå, Sweden.
    Carlborg, Markus
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Hedman, Henry
    Wennebro, Jonas
    Weiland, Fredrik
    Wiinikka, Henrik
    Backman, Rainer
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Öhman, Marcus
    Ash Formation in Pilot-Scale Pressurized Entrained-Flow Gasification of Bark and a Bark/Peat Mixture2016Ingår i: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 30, nr 12, s. 10543-10554Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Pressurized entrained-flow gasification (PEFG) of bark and a bark/peat mixture (BPM) was carried out in a pilot-scale reactor (600 kWth, 7 bar(a)) with the objective of studying ash transformations and behaviors. The bark fuel produced a sintered but nonflowing reactor slag, while the BPM fuel produced a flowing reactor slag. Si was enriched within these slags compared to their original fuel ash compositions, especially in the bark campaign, which indicated extensive ash matter fractionation. Thermodynamically, the Si contents largely accounted for the differences in the predicted solidus/liquidus temperatures and melt formations of the reactor slags. Suspension flow viscosity estimations were in qualitative agreement with observations and highlighted potential difficulties in controlling slag flow. Quench solids from the bark campaign were mainly composed of heterogeneous particles resembling reactor fly ash particles, while those from the BPM campaign were flowing slags with likely chemical interactions with the wall refractory. Quench effluents and raw syngas particles were dominated by elevated levels of K that, along with other chemical aspects, indicated KOH(g) and/or K(g) were likely formed during PEFG. Overall, the results provide information toward development of woody biomass PEFG and indicate that detailed understanding of the ash matter fractionation behavior is essential.

  • 10.
    Rebbling, Anders
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Fagerström, Jonathan
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Steinvall, Erik
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Carlborg, Markus
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Öhman, Marcus
    Energy Engineering, Division of Energy Science, Department of Engineering Sciences & Mathematics, Luleå University of Technology.
    Boman, Christoffer
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Reduction of Alkali Release by Two Fuel Additives at Different Bed Temperatures during Grate Combustion of Woody Biomass2019Ingår i: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 33, nr 11, s. 11041-11048Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The use of small- and medium-scale combustion of biomass for energy utilization is expected to grow in the coming decades. To meet standards and legislation regarding particle emissions and to reduce corrosion and deposit formation, it is crucial to reduce the release of alkali species from the fuel. This can be achieved by capturing the volatile alkali in the residual bottom ash as more thermally stable compounds. In this work, we investigate the combination of primary measures, i.e., process parameters and fuel additives, for reduction of the release of K and Na from the fuel bed during fixed bed combustion. In addition, the influence of these combined measures on fine particle emissions was explored. The results showed a clear influence of the process parameters, herein bed temperature, and that a significant reduction of the alkali release and PM1 emissions can be achieved by correct settings. Furthermore, the application of additives (kaolin and diammonium sulfate) reduced both K and Na release even further. The observed effects on the release behavior was mainly explained by the formation of KAlSiO4 and K2SO4 during addition of kaolin and diammonium sulfate, respectively. This work therefore emphasizes the importance of good control over the fuel bed conditions, especially temperature, when these additives are applied. To reduce the potential deactivation (for kaolinite) and melting (for K2SO4), the control of bed temperature is vital. Thus, it was concluded that the release of volatile alkali species and related fine particle emissions in small- and medium-scale biomass heat and power plants using wood fuels could be significantly reduced by a correct combination of controlling the combustion parameters and the use of fuel additives.

  • 11.
    Rebbling, Anders
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Sundberg, Peter
    Energy Engineering, Division of Energy Science, Luleå University of Technology.
    Fagerström, Jonathan
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Carlborg, Markus
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Tullin, Claes
    Energy Engineering, Division of Energy Science, Luleå University of Technology.
    Boström, Dan
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Öhman, Marcus
    Energy Engineering, Division of Energy Science, Luleå University of Technology.
    Boman, Christoffer
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Skoglund, Nils
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Kemiska institutionen.
    Demonstrating fuel design to reduce particulate emissions and control slagging in industrial scale grate combustion of woody biomassManuskript (preprint) (Övrigt vetenskapligt)
  • 12.
    Rebbling, Anders
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Sundberg, Peter
    Energy Engineering, Division of Energy Science, Dept. of Engineering Sciences & Mathematics, Luleå University of Technology.
    Falk, Joel
    Energy Engineering, Division of Energy Science, Dept. of Engineering Sciences & Mathematics, Luleå University of Technology.
    Fagerström, Jonathan
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Carlborg, Markus
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Torshage, Erik
    Umeå Energi AB.
    Anundi, Per
    Umeå Energi AB.
    Benckert, Åsa
    Umeå Energi AB.
    Öhman, Marcus
    Energy Engineering, Division of Energy Science, Dept. of Engineering Sciences & Mathematics, Luleå University of Technology.
    Boman, Christoffer
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Application of kaolin additive for reduction of fine particle emissions during medium-scale fixed bed combustion of woody biomassManuskript (preprint) (Övrigt vetenskapligt)
  • 13.
    Strandberg, Anna
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Carlborg, Markus
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Boman, Christoffer
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Broström, Markus
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Ash formation and transformation during combustion of poplar wood pelletsManuskript (preprint) (Övrigt vetenskapligt)
  • 14.
    Strandberg, Anna
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Carlborg, Markus
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Boman, Christoffer
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Broström, Markus
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Ash Transformation During Single-Pellet Combustion of a Silicon-Poor Woody Biomass2019Ingår i: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 33, nr 8, s. 7770-7777Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Biomass fuels with calcium and potassium as the main ash-forming elements are expected to form ash consisting mainly of carbonates and oxides. These carbonates are stable in a rather narrow temperature range, which in turn depends on the Ca/K ratio, as well as on the surrounding atmosphere. The objective of the present study was to perform a detailed characterization of ash formation and transformation at a single-pellet level during combustion of silicon-poor woody biomass fuel. Combustion tests were performed with poplar in a single-pellet isothermal thermogravimetric analyzer operated at different temperatures and atmospheres and quenched at different stages of fuel conversion. The char and residual ashes were characterized for morphology and chemical composition. The focus of the experimental work in this study was on the time (conversion) resolved ash formation and transformations at the late part of the char combustion phase. Thermodynamic equilibrium calculations were used both to design the experiments and to support the interpretation of experimental results. It was concluded that carbonates were, in general, stable at low temperatures (here, 600–800 °C), identified as CaCO3, K2Ca2(CO3)3, and K2Ca(CO3)2, and decomposed at higher temperatures. In addition, a combined carbonate and phosphate phase in the form of carbonate apatite, Ca9.9(PO4)6(CO3)0.9, was also found, mainly at lower temperatures. However, for char/ash samples quenched before full conversion, CaCO3 was still found at temperatures higher than expected, possibly explained by the stabilizing effect of locally higher CO2 partial pressure within the burning fuel particles. Thus, the results of the present study provide new insights into conversion-based ash formation and transformation in a burning fuel particle with relevance for combustion of Si-poor woody biomass fuels.

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