Modelling of fluid flows and chanelling in fixed charbed

In order to examine how an uneven flow distribution in the char bed of a biomass gasifier affects the gasification process numerical models have been developed. The flow distribution in the char bed is modelled using Erguns pressure drop equation. One type of models developed is thin film models. Application of these models showed that a relatively small fraction of the gas was flowing through a part of the bed that was fuelled with smaller particles compared to a part of the bed that was fuelled with larger particles. The calculations also show that if the height of the two different parts of the bed were not evened out the bed would not be stable. However, if transport of particles from one part of the bed to the other occurred the bed would be stable. 2-D flow models of the char gasification reactor were developed. With these models calculations have been performed for different configurations of a gasification reactor. It was found that the production rate of the gasifier is only reduced with 12% when the outlet area of the reactor is reduced with 85%. A CFD model of the flow in a char coal gasifier was developed. Sub-models of the gasification reactions, particle shrinkage etc. was included in the model. However, it was troublesome. Therefore CFD models should only be chosen if the freeboard above the char bed is included in the model