The Master’s thesis presents the methodology for simplified modeling of the fluidized bed in the boiler, which uses wood biomass as a primary fuel for combustion. Methodology is based on energy balance of the gasification process that can be described within the limiting frame of reference of the commercial code for the 3D combustion analysis. The developed methodology is then used on the model of 9MW boiler combusting wood biomass and validated using temperature measurements preformed on the actual boiler of our industrial partner. Produced model shows satisfactory functionality and regarding to the used simplification and needed computing times it was able to give us reasonably accurate results that can be used for further directions in optimization process of the flow and temperature field in the boiler. On the basis of validated model we further looked into the problem of secondary air inlets effect. We studied secondary air intake effect on the boiler combusting process phenomena regarding to their number, location and air distribution in terms of ratio between fresh gas and recirculated flue gasses. A key contribution from the study is confirmed possibility of simplified modeling of fluidized bed combustion and findings about the reorganization of air inlets. We reorganized air inlets in a way that led to the establishment of more suitable flow and temperature regime, which will presumably enable us to reduce the possibilities of formation of deposits on the wall of the combustion chamber.