The aim of this work was to investigate the microstructure and the mechanical properties of laser-welded joints combined of Dual Phase DP800 and DP1000 high strength thin steel sheets. Microstructural and hardness measurements as well as tensile and fatigue tests have been carried out. The welded joints (WJ) comprised of similar/dissimilar steels with similar/dissimilar thickness were consisted of different zones and exhibited similar microstructural characteristics. The trend of microhardness for all WJs was consistent, characterized by the highest value at hardening zone (HZ) and lowest at softening zone (SZ). The degree of softening was 20 and 8% for the DP1000 and DP800 WJ, respectively, and the size of SZ was wider in the WJ combinations of DP1000 than DP800. The tensile test fractures were located at the base material (BM) for all DP800 weldments, while the fractures occurred at the fusion zone (FZ) for the weldments with DP1000 and those with dissimilar sheet thicknesses. The DP800-DP1000 weldment presented similar yield strength (YS, 747 MPa) and ultimate tensile strength (UTS, 858 MPa) values but lower elongation (EI, 5.1%) in comparison with the DP800-DP800 weldment (YS 701 MPa, UTS 868 MPa, EI 7.9%), which showed similar strength properties as the BM of DP800. However, the EI of DP1000-DP1000 weldment was 1.9%, much lower in comparison with the BM of DP1000. The DP800-DP1000 weldment with dissimilar thicknesses showed the highest YS (955 MPa) and UTS (1075 MPa) values compared with the other weldments, but with the lowest EI (1.2%). The fatigue fractures occurred at the WJ for all types of weldments. The DP800-DP800 weldment had the highest fatigue limit (348 MPa) and DP800-DP1000 with dissimilar thicknesses had the lowest fatigue limit (<200 MPa). The fatigue crack initiated from the weld surface.

Although more than a hundred papers dealing with the agglomeration problem in combustion and gasification of biomass can be found in the literature, very few studies focusing on the bed particle layer formation process in fluidized bed combustion (FBC) and fluidized bed gasification (FBG) can be found. With increased knowledge of the bed particle layer formation process — i.e. the main route behind bed agglomeration and bed material deposition in wood combustion/gasification — suitable combinations of fuel/bed material and/or bed material management measures can be suggested. This would not only aim to reduce the risk of ash related operational problems but also to enhance the catalytic activity of the bed material (e.g. for tar removal in gasification). The present investigation was therefore undertaken to determine the layer formation process on and within typical bed materials (i.e. quartz and olivine) and for a potentially interesting new bed material, K-feldspar.

Bed material samples were collected from four different combustion and two different gasification appliances: two bubbling fluidized beds (BFB) (5 kW_th/30 MW_th), two full-scale circulating fluidized beds (CFB) (90/122 MW_th), and two dual fluidized bed gasifiers (DFB) (8/15 MW_th). Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD) were used to explore layer morphology and elemental composition and to gain information about crystalline phases of the layers. Phase diagrams and thermodynamic equilibrium calculations (TECs) were used to interpret the melting behavior of the layers and the melt fragments in deposits. In addition, a diffusion model was used to interpret the layer growth process.

For quartz bed particles taken from BFB, the younger particles (< around 1 day) had only one thin layer, but for particles older than 3 days, the layer consisted of inner and outer layers. In addition to the inner and outer layers, a K-rich inner-inner layer was found for bed particles taken from CFB and DFB. No outer layers were found for quartz bed particles taken from DFB. The thin/absence of an outer layer could have resulted from the more significant attrition between particles in CFB and DFB. Reduced availability of Ca and a risk of layer breakage from the particle lead to the formation of the inner-inner layer. Similar elemental compositions of the layers upon the quartz bed particles taken from different fluidized bed techniques were found. The inner-inner layers are dominated by Si, K and Ca (excluding O), and the outer layers are rich in Ca, Si and Mg, which seem to resemble more closely the fuel ash composition. The inner layers, mainly consisted of Si and Ca, were found to have higher concentrations of Ca for older particles. The layer thickness increases with particle age, but the growth rate decreases. Melt was estimated to exist in the inner layer for younger particles (< around 1 day) and in the inner-inner layer. The existence of partially melted inner-inner layers, in particles from CFB and DFB, points towards higher risk of bed agglomeration in these techniques compared to BFB. Based on the experimental results, thermodynamic equilibrium calculations, and diffusion model analyses, a layer formation process on quartz bed particle was suggested: the layer formation is initiated by reaction of gaseous K compounds with quartz to form K-rich silicate melt, which prompts the diffusion of Ca²⁺. The gradual incorporation of Ca into the melt followed by the precipitation of Ca-silicates, e.g. Ca₂SiO₄, will result in the continuous inner layer growth. However, because of increasing concentration of Ca and release of K from the inner layer, the melt disappears in the inner layer and the layer formation process gradually becomes Ca diffusion controlled. The diffusion resistance increases with increasing thickness of a more Ca-rich layer, resulting in a decreasing layer growth rate.

Crack layers with similar compositions dominated by Si, K and Ca were observed in relatively old quartz bed particles. A melt was predicted to exist in the crack layer according to thermodynamic equilibrium calculations. The crack layers found in quartz particles from BFB and CFB connect with the cracks in the inner layer, whereas for bed samples collected from DFB, the crack layers were found along existing cracks in the quartz particle. The different morphologies may indicate different routes of formation for crack layers in bed particles from different fluidized bed technologies. For quartz particles from BFB and CFB, crack formation through the inner layer down to the interface between the inner layer and the core of quartz bed particle initiates the cracks in the quartz bed particle. This allows for diffusion of gaseous alkali compounds to react with quartz in the bed particle core, thereby forming crack layers. The reaction is accelerated with bridge formation between crack layers. This may later lead to the breakdown of the bed particle into smaller alkali-silicate-rich fragments.

For K-feldspar bed particles from BFB and CFB, only one layer was found for particles with an age of 1 day. For bed particles with ages older than 3 days, two layers including a homogenous inner layer containing cracks and a more particle-rich outer layer can be distinguished. Compared to bed particles from BFB with similar ages, the outer layer is thinner for bed particles from CFB. The inner layer is dominated by Ca, Si and Al (excluding O), whereas the outer layer is dominated by Ca, Si and Mg. The average concentration of Ca in the inner layer increases with bed particle age. Increasing layer thickness with decreasing growth rate was found, similar to that on quartz particles. For particles from DFB, the inner layer is also mainly consisted of Ca and Si, but cracks in the inner layer were not found. For all the particles, the Ca/Si molar ratio in the layer decreases towards the bed particle core and the change of concentration is more significant at the bed particle core/layer interface. The overall inner layer growth is resultant from the gradual incorporation of Ca into the layer.

For olivine bed particles from DFB, the younger bed particles (< around 24 h) have only one layer, but after 24 h, an inner layer and an outer layer appear. Furthermore, for bed particles older than 180 h, the inner layer is separated into a distinguishable Ca-rich and Mg-rich zone. Two kinds of cracks in the inner layer either perpendicular or parallel to the particle surface were observed. Compared to the younger bed particles, the Ca concentration in the layer of older particles is much higher. A detailed mechanism for layer formation on olivine particles in fluidized bed gasification (most likely also applicable to combustion) based on the interaction between woody biomass ash and olivine has been proposed. The proposed mechanism is based on a solid-solid substitution reaction. However, a possible enabling step in the form of a Ca²⁺ transport via melts may occur. Ca²⁺ is incorporated into the crystal structure of olivine by replacing either Fe²⁺ or Mg²⁺. This substitution occurs via intermediate states where Ca-Mg silicates, such as CaMgSiO₄, are formed. Mg²⁺ released from the crystal structure most likely forms MgO, which can be found in a distinguishable zone between the main particle layers. Due to a difference in the bond lengths between Mg/Fe and incorporated Ca²⁺ with their respective neighboring oxygen atoms, the crystal structure shifts, resulting in formation of cracks.

The dominating elements in the inner layers are similar for each kind of bed material from BFB, CFB, and DFB, indicating limited effects of atmosphere on the inner layer formation. The initiation of layer formation differs depending on the bed material, but increasing Ca concentration in the inner layer with time for all bed materials indicates that the layer growth resulted from the incorporation of Ca into the layer. Compared to quartz, K-feldspar and olivine are more promising bed materials in wood combustion/gasification, especially in CFB and DFB techniques, from the perspective of mitigating bed agglomeration and bed material deposit build-up.  

Bed agglomeration during combustion and gasification of woody biomass fuels in quartz beds has been frequently studied, and chemical mechanisms responsible for bed agglomeration have been suggested. However, few studies have focused on the bed material deposition on walls, in cyclones, and return legs in fluidized bed combustion. Part of these bed material depositions originates from sticky fragments of alkali-rich silicates formed after crack formation in older quartz bed particles. The crack layer formation in quartz bed particles in fluidized bed combustion of woody biomass was therefore investigated by collecting bed material samples of different ages from full-scale bubbling and circulating fluidized bed facilities. Scanning electron microscopy/energy-dispersive spectroscopy was used to analyze the crack morphology and composition of the layer surrounding the cracks. For quartz bed particles with an age of some days, a crack in the quartz bed particle was observed in connection to the irregular interface between the inner layer and the core of the bed particle. The crack layer composition is similar for quartz particles with different ages and for samples taken from different fluidized bed techniques. Their composition is dominated by Si, K, Ca, and Na (except O). These crack layers become deeper, wider, and more common as bed particle age increases. The crack layers eventually connect with each other, and the whole quartz particle is transformed into smaller quartz cores surrounded by crack layers, which were observed in particles older than 1 week. From the characterization work, a crack formation process including three phases is proposed on the basis of the presumption that the initial crack layer formation resulted from the presence of induced cracks in the inner quartz bed particle layer. Fragmentation after the third phase is likely responsible for the formation of sticky alkali silicate deposit formation, and a weekly complete exchange of the bed is therefore recommended to avoid problematic deposits in combustion of woody-type biomass in fluidized bed combustion

Despite frequent reports on layer characteristics on quartz bed particles, few studies have been found focusing on the layer characteristics on K-feldspar bed particles. The layer characteristics of K-feldspar bed particles in fluidized bed combustion of woody biomass was therefore investigated by collecting bed material samples of different ages from large-scale bubbling and circulating fluidized bed facilities. Scanning electron microscopy/energy-dispersive spectroscopy was used to analyze the layer morphology and elemental composition. For particles with an age of 1 day, a thin layer rich in Si, Ca and Al was found. For particles older than some days, an inner more homogenous layer containing cracks and an outer more particle-rich layer were observed. The outer layer was thinner for K-feldspar bed particles sampled from circulating fluidized bed, compared to particles from bubbling fluidized bed. The concentration of Ca in the inner layer increases towards bed particle surface, the molar ratio of Si/Al is maintained, and the molar ratio of K/Al decreases compared to the K-feldspar. The inner layer thickness for quartz and K-feldspar bed particles collected at the same operation conditions was found to be similar. No crack layers, as have been observed in quartz particles, were found in the core of the K-feldspar bed particles. The results suggest that the diffusion and reaction of Ca2+ into/with the feldspar particle play an important role on the inner layer formation process.

Utilization of biomass as feedstock in dual fluidized bed steam gasification is a promising technology for the substitution of fossil energy carriers. Experience from industrial-scale power plants showed an alteration of the olivine bed material due to interaction with biomass ash components. This change results mainly in the formation of Ca-rich layers on the bed particles. In this paper, a mechanism for layer formation is proposed and compared to the better understood mechanism for layer formation on quartz bed particles. Olivine bed material was sampled at an industrial-scale power plant before the start of operation and at predefined times after the operation had commenced. Therefore, time-dependent layer formation under industrial-scale conditions could be investigated. The proposed mechanism suggests that the interaction between wood biomass ash and olivine bed particles is based on a solid-solid substitution reaction, where Ca²⁺ is incorporated into the crystal structure. As a consequence, Fe^2+/3+ and Mg²⁺ ions are expelled as oxides. This substitution results in the formation of cracks in the particle layer due to a volume expansion in the crystal structure once Ca²⁺ is incorporated. The results of this work are compared to relevant published results, including those related to quartz bed particles

Agglomeration is among one of the major problems in the operation of fluidized bed boilers. The formation of bed particle layers is thought to play an important role on the occurrence of agglomeration in wood-fired fluidized (quartz) beds. In spite of frequent experimental reports on the quartz bed particle layer characteristics, the underlying bed layer formation process has not yet been presented. By combining our previously experimental results on layer characteristics for samples with durations from 4 h to 23 days, with phase diagrams, thermochemical equilibrium calculations, and a diffusion model, a mechanism of quartz bed particle layer formation was proposed. For younger bed particles (

The use of biomass as feedstock for gasification is a promising way of producing not only electricity and heat but also fuels for transportation and synthetic chemicals. Dual fluid bed steam gasification has proven to be suitable for this purpose. Olivine is currently the most commonly used bed material in this process due to its good agglomeration performance and its catalytic effectiveness in the reduction of biomass tars. However, as olivine contains heavy metals such as nickel and chromium, no further usage of the nutrient-rich ash is possible, and additional operational costs arise due to necessary disposal of the ash fractions. This paper investigates possible alternative bed materials and their suitability for dual fluid bed gasification systems focusing on the behavior of the naturally occurring minerals olivine, quartz, and K-feldspar in terms of agglomeration and fracturing at typical temperatures. To this end, samples of bed materials with layer formation on their particles were collected at the industrial biomass combined heat and power (CHP) plant in Senden, Germany, which uses olivine as the bed material and woody biomass as feedstock. The low cost logging residue feedstock contains mineral impurities such as quartz and K-feldspar, which become mixed into the fluidized bed during operation. Using experimental and thermochemical analysis, it was found that the layers on olivine and K-feldspar showed a significantly lower agglomeration tendency than quartz. Significant fracturing of particles or their layers could be detected for olivine and quartz, whereas K-feldspar layers were characterized by a higher stability. High catalytic activity is predicted for all three minerals once Ca-rich particle layers are fully developed. However, quartz may be less active during the buildup of the layers due to lower amounts of Ca in the initial layer formation

A promising way to substitute fossil fuels for production of electricity, heat, fuels for transportation and synthetic chemicals is biomass steam gasification in a dual fluidized bed (DFB). Using lower-cost feedstock, such as logging residues, instead of stemwood, improves the economic operation. In Senden, near Ulm in Germany, the first plant using logging residues is successfully operated by Stadtwerke Ulm. The major difficulties are slagging and deposit build-up. This paper characterizes inorganic components of ash forming matter and draws conclusions regarding mechanisms of deposit build-up. Olivine is used as bed material. Impurities, e.g., quartz, brought into the fluidized bed with the feedstock play a critical role. Interaction with biomass ash leads to formation of potassium silicates, decreasing the melting temperature. Recirculation of coarse ash back into combustion leads to enrichment of critical fragments. Improving the management of inorganic streams and controlling temperature levels is essential for operation with logging residues.

The formation of sticky layers on bed particles has been considered as a prerequisite for bed agglomeration in fluidized bed combustion and gasification of woody fuels. In addition, the layer formed makes the bed particle less resistant against fragmentation. The fragments from quartz particles can result in deposition. The present investigation was undertaken to determine the layer formation process on quartz bed particles during combustion of wood-derived fuels and to determine the effect of quartz particle layers on deposit build-up in full-scale dual fluidized bed (olivine) steam gasification of logging residues. Bed material samples were collected from three different combustion appliances: bench-scale bubbling fluidized bed, full-scale bubbling fluidized bed and full-scale circulating fluidized bed. These were collected at different sampling times from start-up with fresh bed material. In addition, samples of deposits, bed material, coarse ash, fine ash, and fly coke from a dual fluidized bed gasification process were also collected. Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD) were used to explore layer morphology, chemical composition and to gain information about crystalline phases of the layers. Significant differences in layer morphology and composition were found for quartz bed particles with different ages. For bed samples with operational durations of less than 1 day, only one thin Ca-, Si-, O- and K-rich homogeneous quartz bed particle layer with a relatively high K/Ca molar ratio was found. For quartz bed particles with an age of around 1 day to 2 weeks, an outer more particlerich coating layer was also found. During the initial days of this period, the layer growth rate was high but decreased over time, with decreasing K/Ca and increasing Ca/Si molar ratios in the inner bed particle layer. For bed particles with ages between 2 to 3 weeks, a much lower layer growth rate was observed. At the same time, the Ca/Si molar ratio reached high values and the K concentration remained at a very low level. In addition to these layer formation processes mentioned, an innerinner-/ crack layer was also formed simultaneously in the circulating fluidized bed (CFB) quartz bed particles along with the inner bed particle layer. By combining the experimental results on layer characteristics for samples with durations from 4 h to 23 days, with phase diagrams, thermochemical equilibrium calculations and a diffusion model, a mechanism of quartz bed particle layer formation was proposed. For younger bed particles (< around 1 day), the layer growth process is accelerated due to a high diffusion of calcium in a K-rich silicate melt. But with continuous addition of calcium into the layer, the amount of melt decreases and crystalline Ca-silicates starts to form. Ca2SiO4 is the dominating crystalline phase in the inner layer, while the formation of CaSiO3 and Ca3SiO5 are favored for younger and older bed particles, respectively. The decreasing amount of melt and formation of crystalline phases resulted in low diffusion rates of calcium in the inner layer and the layer growth process becomes diffusioncontrolled after around 1 day. The observation of formation of Ca3SiO5 in a thick bed layer after around two weeks may indicate substantially higher diffusion resistance and lower layer growth rate. The practical implication of the results from this work is that a low bed material renewal rate during wood-derived fired bubbling fluidized bed boilers using natural sand is recommended. The formation of high-melting Ca-silicates for older quartz bed particles protects the bed particle layer surface from further attack by potassium, leading to reduced agglomeration tendency. In dual fluidized (olivine) bed steam gasification, impurities, mainly composed of quartz particles brought into the fluidized bed with the feedstock, play a critical role for deposit formation in the post-combustion zone. Interaction between biomass ash and the quartz particles leads to formation of sticky potassium-rich silicate layers. Recirculation of coarse ash back into the combustion zone therefore leads to the enrichment of critical fragments. Improving the management of inorganic streams and controlling of temperature levels are therefore essential in operating with woody biomass fuels containing impurities (i.e. sand minerals).

The formation mechanism of K2Ti2O5 was investigated with TiO2 microparticles and nanoparticles as precursors by thermogravimetric (TG) technique. A method of direct multivariate non-linear regression was applied for simultaneous calculation of solid-state reaction kinetic parameters from TG curves. TG results show more regular decrease from initial reaction temperature with TiO2 nanoparticles as raw material compared with TiO2 microparticles, while mass losses finish at similar temperatures under the experimental conditions. From the mechanism and kinetic parameters, the reactions with the two materials are complex consecutive processes, and reaction rate constants increase with temperature and decrease with conversion. The reaction proceedings could be significantly hindered when the diffusion process of reactant species becomes rate-limiting in the later stage of reaction process. The reaction active sites on initial TiO2 particles and formation of product layers may be responsible to the changes of reaction rate constant. The calculated results are in good agreement with experimental ones.