The objective of this research was to characterise the deposit build-up and its effect on heat transfer in biomass (especially wood) combustion. With the knowledge gained in this project it is possible to develop the methodologies and mathematical models for better understanding of changes in heat transfer caused by wood combustion. With the models and simulations interrelation between fuel constituents and deposit formation tendency can be described and the causalities between heat transfer, in-furnace conditions and fuel & fuel-ash characteristics established. The results can be used in estimating and optimising the factors that have effect on plant's availability. Furthermore, we can estimate the effects caused by some problematic fuel blends and additives on heat transfer, which are not otherwise possible to study. In general, the results can be used in enhancing boiler availability and the use of wood fuels. Project started in March 2003 and it was ended in March 2004. Experts of the Energy Production research field at VTT Processes carried out the majority of the research activities. System Technology Laboratory of Oulu University was responsible for the power plant automation and for the research on boiler control technology. Following companies participated in the project: Etelä-Savon Energia Oy, Foster Wheeler Energia Oy, Vapo Oy, Simpele pasteboard factory of M-Real Oyj and Coresto Oy.