The use of renewable energy is becoming more important due to an increasing energy demand and international goals. Renewable energy plays an important role in substituting fossil fuels, reducing greenhouse gas emissions and protecting the climate. Biomass can be considered as organic matter, in which the energy of sunlight is stored in chemical bonds. It can be used as a solid, liquid or gaseous source of energy for the production of heat, electricity and fuels. Therefore biomass offers a good opportunity for decentralized energy generation. Especially technologies based on fluidized bed combustion and gasification play an important role for biogenic solid feedstocks. An inert bed material is normally used for fluidized bed combustion. On the other hand, to promote the primarily desired gasification reactions in fluidized bed gasification, a catalytic active bed material is used. Interactions between biomass ash and bed material leading to layer formation on the surface of bed particles used both in fluidized bed combustion and gasification can have different consequences on the process. Layer formation can lead to defluidization due to agglomeration but also to a high catalytic activity towards tar reduction. The aim of this thesis is to analyse and compare the influence of different parameters, such as feedstock and bed material composition, in thermochemical conversion processes on agglomeration and layer formation mechanisms. Therefore bed material samples from a bench-scale fluidized bed combustion reactor and from a dual fluidized bed steam gasification reactor after thermochemical conversion processes are analysed. The investigated feedstocks are bark, straw, chicken manure and mixtures of those. These feedstocks are examples of residues from paper industry and agricultural production and animal biomass waste. The bed material types studied are the commercially used inert quartz, the catalytic active limestone and the alternative potassium feldspar. The bed material samples were analysed via scanning electron microscopy and energy dispersive X-ray spectroscopy. The results of the analyses serve as a basis for identifying the mechanisms of agglomeration and layer formation and for confirming theoretical concepts. The observed tendency for layer formation is highest in quartz, followed by potassium feldspar and lime. Layer formation on quartz and potassium feldspar can be promoted through the use of lime as additional bed material. The layers are partly distinguishable into an inner and outer layer with dominance of silicon, calcium, potassium and aluminium. The initial layer formation on quartz is taking place due to molten ash of potassium silicates, where calcium is diffusing into the grain surface. This can be confirmed with existing literature. However, identified layer formation on potassium feldspar using phosphorus-rich feedstocks can not be explained by existing wood and woody biomass literature. Layer formation on lime is dominated by the interaction of calcium and magnesium. Slight modifications in feedstock composition, theoretical ash content and process time had no effects on the elemental composition of the layers. Detected agglomeration phenomena, when using potassium feldspar bed material, indicate a coating induced agglomeration. Layers and agglomeration regions have the same elemental composition with dominance of silicon, calcium, phosphorus, potassium and aluminium. Agglomeration for lime could not be detected. The phosphorus stemming from phosphorus-rich feedstocks, such as chicken manure, was found in the layers. This phosphorus was mainly detected in the outer layer if the layers are distinguishable into an inner and outer layer. This observation was confirmed with existing literature for quartz.