Adhesive coating layers on bed particles have been thought to be the cause of the agglomeration in fluidized-bed combustion. The formation of coating layers on particles of different bed materials (quartz, natural sand, and GR Granule) has been studied when the fluidized-bed combustion of various biomass fuels (bark, different types of wood fuel, peat, biosludge, chicken litter, REF, and forest residue) occurs. The bed samples have been collected during the laboratory-scale, pilot-scale, and full-scale boiler tests. The chemical composition of the coating layers were characterized by scanning electron microscopy, coupled with energy-dispersive spectroscopy (SEM-EDS), through the utilization of line scans, as well as point and domain analyses. X-ray maps were also recorded to characterize the coating layers. The coatings on the bed particles often consisted of several superimposed layers. The innermost layer mainly contained alkali silicates, whereas the outermost layer was rich in calcium or magnesium, depending on the composition of both the bed and the fuel. Coating layers that contain potassium or sodium may be adhesive and cause the formation of agglomerates, which would explain why quartz and natural sand bed materials are problematic when fuels with high alkali-metal content are combusted. The magnesium-containing coating layers on the GR Granule particles may protect the bed particles from agglomeration, and, consequently, this bed material can be used in the combustion of otherwise problematic fuels.