A critical process phenomenon in fluidized bed combustion and gasification is the bed particle layer formation which might be problematic in cases where it instigates bed agglomeration and bed material deposition or could improve the process performance in cases where it has a positive effect on tar reduction in biomass gasification. The interactions between ash forming matter for a wide variety of wood-derived fuels with different types of bed materials in fluidized bed combustion and gasification have been profusely studied, and the underlying mechanism of bed particle layer formation has been suggested. Howbeit, the influence of bed material’s surface properties on bed layer characteristics has not been elaborated in the literature. In this thesis, the effect of surface morphology on the formation of bed particle layers and crack layers has been investigated for different bed material types in combustion and gasification of woody biomass. Samples were selected from different full-scale and bench-scale conversion units at different ages from the start-up to investigate the development of the bed layer at different surface morphologies over the bed particle. Natural sand samples (consisting mainly of quartz and K-feldspar) were taken from a 30 MWth bubbling fluidized bed (BFB30) combustion unit and a 90 MWth circulating fluidized bed (CFB90) combustion unit, K-feldspar and olivine bed samples were taken from a dual fluidized bed (DFB) gasification unit (consisting of a 2-4 MWth bubbling fluidized bed gasifier and a 12 MWth circulating fluidized bed combustor), and ilmenite bed samples were taken from a 5 kWth bench scale bubbling fluidized bed (BFB5)combustion unit. X-ray microtomography analysis (XMT) was utilized to inspect the bed layer distribution and determine surface morphology in 3D, including measurements of the bed particle layer thickness over the particle surface. Distribution of the crack layers inside the bed core was also observed through the XMT images. Moreover, scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) along with transmission electron microscopy (TEM) were employed to investigate the elemental compositional distribution and to support interpretation of XMT results. The results revealed that for all types of bed materials, regardless of the conversion process, both inner and outer layers could be observed over the convex-shaped regions on the bed particle surfaces after some days, while the concave-shaped regions are mainly covered with a thin inner layer. Consequently, a thicker overall layer could be observed over the convex-shaped regions compared to the concave regions. Results from the combustion processes units show that K-feldspar bed particles retain a thinner overall layer compared to quartz when both are exposed to the same process condition and fuel ash composition. Moreover, almost no tendency for crack layer formation was observed in K-feldspar particles while up to 19% and 32% of the quartz bed particle could be engaged by the crack layers in bed samples taken from the BFB30 and CFB90, respectively. It was also observed that the crack layers are initiated from the concave-shaped regions on the bed surface where gaseous alkali can penetrate into the bed core through the inward cracks in the thin inner layer. In the BFB5 combustor, it was observed for the samples taken 6 and 12 hours after the start-up that a thin Fe-rich layer ascends over the convex areas over the ilmenite surface due to the outward migration of Fe from the bed particle core. Over time, this iron-rich layer is covered with a Ca-rich outer layer that obstructs further migration of the Fe through the convex areas. For the older particles (i.e., 30 and 42 hours from the start-up) high concentration of Fe was observed in the concaves and more porous areas inside the bed particle core. Measurement of the average overall layer thickness for typical K-feldspar and olivine bed particles from the DFB gasifier showed that the latter exhibits thinner overall bed layer thickness. The elemental composition of the bed layer over the concave and convex regions was observed to be different for both bed materials. A slightly higher Fe concentration was observed over the convex areas on olivine. A similar trend was likewise noticed for Mn on both bed types.
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:ltu-93678 |
| Date | January 2022 |
| Creators | Valizadeh, Ali |
| Publisher | Luleå tekniska universitet, Energivetenskap, Luleå |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Licentiate thesis, comprehensive summary, info:eu-repo/semantics/masterThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | Licentiate thesis / Luleå University of Technology, 1402-1757 |
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