Bed particle layer formation is a crucial aspect of fluidized bed conversion of woody biomass, which can positively and negatively impact the process performance. The layer formation may lead to issues such as bed agglomeration and bed material deposition or reduced oxygen transport capacity of oxygen carriers. However, bed layer formation may positively affect the product gas composition in gasification. While extensive research has been conducted on the interactions between the ash-forming constituents from various wood-derived fuels and different bed materials, the influence of the bed particle surface properties on the bed layer characteristics remains largely unexplored in the existing literature.

This thesis thoroughly studied the role of bed particle surface morphology on the bed layer formation and its characteristics across different bed particle types within various fluidized bed conversion processes of woody biomass. The bed particle samples were collected from different bench-scale, semi-industrialscale, and industrial-scale conversion units at different time intervals from the start-up to assess the bed particle layer formation process throughout the bed particle age in different conversion processes of woody biomass. Natural sand bed 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 were obtained from a dual fluidized bed (DFB12-4) indirect gasification unit comprising a 12 MWth circulating fluidized bed combustor and a 2-4 MWth bubbling fluidized bed gasifier. Different types of ilmenite samples were taken from a 5 kWth bench-scale bubbling fluidized bed (BFB5) and a 12 MWth circulating fluidized bed (CFB12) combustion unit. Additionally, quartz and natural sand samples (comprising mainly quartz, K-feldspar, and Na-feldspar) were gathered from three different industrial fast pyrolysis plants. Bed particles found in the samples were subjected to different analysis methods. Scanning electron microscopy (SEM), coupled with energy dispersive spectroscopy (EDS), served as a primary tool for obtaining information regarding the morphology and elemental composition of different layers present on the surface of bed particles and inside the bed particle core. In instances where layers were too thin to be adequately examined via SEM/EDS, transmission electron microscopy (TEM) was employed as a complementary analysis method. X-ray microtomography (XMT) was also utilized to explore the distribution of bed particle layers on the particle surfaces. This method facilitated both qualitative and quantitative assessments, including observation of the surface morphology of the bed particles, analysis of the bed layer distribution on the bed particle surface, along with measurements of bed particle layer thickness and the volume fraction of various features throughout the bed particles.

Comprehensive analysis of the results from different characterization techniques showed that for all studied bed particles, regardless of their chemical composition, the inner layer (i.e., the Ca-reaction layer) was thicker on convex areas and thinner or entirely missing in the concave regions. The outer layer, mainly consisting of Ca compounds (i.e., the ash-deposition layer), was more likely to be found on the convex areas of the bed particle surface. Overall, the total bed particle layer thickness (inner and outer layer) was larger on the convex areas compared to the concaves. Consequently, the concave regions can facilitate mass transfer to and from the bed particle core even after the full development of the Ca-rich inner layer. Therefore, in the case of using quartz and Na-feldspar particles where there was a high chemical potential to react with fuel-derived gaseous alkali, the inner-inner layer and the crack layers (together referred to as the K-reaction layers) were connected to concaves on the bed particle surface. For the studied oxygen carrier (i.e., ilmenite), where most of the convex regions at the bed particle surface were covered with the Ca-rich layer, Fe could still migrate to the bed particle surface through concaves.

Bed particle layer characteristics observed in the fast pyrolysis plants were, to some extent, different compared to those in the combustion and gasification of woody biomass. In general, the layers were considerably thinner in the fast pyrolysis process, with a similar exposure time compared to combustion and gasification. Crack layers were not detected in quartz and Na-feldspar bed particles in any of the studied fast pyrolysis plants, and the inner layer had a lower Ca concentration than that in the combustion or gasification. Further, only the Ca-reaction layer was identified on feldspar bed particles. However, the distribution pattern of the bed particle layers at different morphologies on the bed particle surface resembled that in combustion and gasification.

The findings indicated that apart from the chemical composition, the surface morphology of the bed particles plays a vital role in determining their performance throughout the fuel conversion process in the fluidized bed. Specifically, as the crack layer formation in quartz bed particles is linked to the concave-shaped areas on the bed particle surface, quartz bed particles characterized by fewer concaves experience less fragmentation during the conversion process. Conversely, when employing ilmenite as an oxygen carrier, the presence of concaves on the particle surface facilitates the outward migration of Fe over prolonged exposures, albeit potentially compromising the structural integrity of the bed materials. Thus, a trade-off exists between achieving a desired oxygen-carrying capacity and maintaining structural integrity over extended durations. Previous studies have suggested that the formation of a Ca-rich layer on bed particles can positively influence the composition of the product gas in the gasification process. Results from this work showed that the Ca-rich layer on bed particles intended for gasification, such as K-feldspar and olivine, is thicker and more evenly distributed on particles possessing a surface morphology featuring more frequent convex shapes.