The FB-combustion of several biomass fuels (bark, wood in different forms, biosludge, chicken litter, REF and forest residue) with different bed materials (quartz, natural sand, and recently introduced GR Granule) has been studied in this work. The bed samples have been collected during the laboratory, pilot, and full scale tests. The chemical composition of the coating layers and agglomerates were characterized by SEM-EDX. Line scans, point analyses, and X-ray maps were used to characterize the coating layers further. It was observed that the chemical composition of the coating of the bed particles depends both on the fuel type and on the bed material. The thickness of coating layers was also dependent on fuel and combustion history. The coatings were often seen to contain several superimposed layers. The innermost layer mainly contained alkali silicates, whereas the outermost layer was calcium-rich or magnesium-rich. In some cases the coating layers seem to protect the bed particles from agglomeration.

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