Modelling and feedback measurements give new understanding about recovery boiler behaviour

Modern recovery boilers are bigger and bigger, they have to generate more power from the bio fuel, with high efficiency and low emissions. The availability has to be close to 100 %, and the interval between the shut downs is increasing from one year to 11/2 years. Much of the factors, which influence the demands originate from the phenomena, which happen in furnace. Also the flow fields over the heat transfer surfaces have effect on fouling, availability and corrosion. The presentation describes some of the simulations in different areas of the boiler, and various feedback measurements, and their results. A new tool, which is under preparation in co-operation between VTT and Andritz, is demonstrated in the presentation. A model of deforming char bed has been developed, which is a sub model for Fluent 6 to predict the shape of the char bed during stationary furnace simulation. In this model the size of the bed remains fixed, but the shape of the bed can change and it can be predicted during the solution process. During the solution process the update of bed shape is coupled with the Lagrangian particle tracking of the CFD code which is used to describe the in-flight behaviour of black liquor droplets and their interaction with the gas phase. The new shape of the bed is predicted using the cellular automata based model, and it depends on the current shape of the bed and on the material flows of the bed, which include the droplets landing on the bed and bed consumption due to chemical reactions on the bed surface and the outflow of the smelt via smelt spouts. The char bed is described in Fluent by introducing suitable source terms so that the gas mixture over the bed surface recognises the surface of the bed as a wall surface. Feedback for the furnace simulations is not available from the fields inside the furnace; the shape of the char bed can be recorded, as well some other parameters on the frame of the furnace. Such data is shown in the presentation.