Ash components of biomass fuels can cause fouling, slagging, and bed material agglomeration during thermal utilization in fluidized bed combustion and gasification plants. The influence of ash components on these problems in dual fluidized bed biomass gasification plants is investigated in an industrial scale plant in Gussing, Austria. Samples of fouling are analyzed, and the results are evaluated. The samples were analyzed by X-ray fluorescence analysis and thermal analyses such as thermogravimetric analysis, differential thermal analysis, and differential scanning calorimetry. Mass balances of inorganic matter are presented, evaluating different loop configurations. The analyses showed high potassium contents compared to the fuel ash composition in fouling of up to 23% by weight. The potassium content of fly ash with a particle size smaller than 200 mu m is half that of coarse fly ash with a particle size larger than 200 mu m. The thermal analyses showed a large difference between samples of inorganic streams such as fly ash or fly char and fouling. Different fractions of fly ash samples (particle fraction smaller than 200 mu m and particle fraction larger than 200 mu m) showed similar thermal behavior: endothermic weight losses at around 400 degrees C and around 720-820 degrees C caused by decomposition of carbonates. The composition of inorganic matters of fly ash and fly char is similar. The elemental composition of deposits at the cyclone wall and the first heat exchanger in the flue gas path showed high potassium contents up to 23.6%. While samples of fly ash and fly char did not show significant melting in their thermal behavior, melting could be detected with fouling at temperatures higher than 1000 degrees C. Mass balances of inorganic matter showed a flow of potassium oxide from the combustion reactor to the gasification reactor, which leads to unexpected high potassium concentrations in the fly char. A reduction of ash loops reduces the amount of potassium that is transferred from the combustion reactor to the gasification reactor. Recommendations are made for the operation of dual fluidized bed gasification plants in terms of ash handling to reduce tendencies for fouling, slagging, and bed material agglomeration.