The goal of the present work is to develop the methods and tools to achieve reliable information on the local conditions in a CFB combustor. As shown before, such information are crucial for a further scale-up of CFB boilers. The lateral mixing of fresh fuels into the combustion chamber is only one example. The concept of simulation-supported measurements presented above will be applied to fulfill the task. Measurements in a large-scale CFB combustor have to be performed and a model for large-scale circulating fluidized bed combustors has to be developed.The simulation model shall describe all major mechanisms during fuel feeding, devolatilization and mixing of fresh fuel particles, subsequent combustion of char and volatiles and formation of emissions. These processes have to be described sufficiently detailed so that the relevant phenomena, e.g. at the fuel or secondary air injection points, can be investigated and compared to the measurements. At the same time reasonable calculation times shall be maintained.