A 1-DIMENSIONAL MODEL OF INDIRECT BIOMASS GASIFICATION IN A DUAL FLUIDISED BED SYSTEM

Abstract –A 1-dimensional model is used to analyse how solids material circulation, biomass reactivity and gas mixing affect the char conversion in the Chalmers 2-4 MW indirect biomass gasifier. For the geometry and operational conditions particular to this unit, the model predicts a peak in char conversion for a solids circulation rate of around 3 kg/s. Char conversion is found to increase substantially with biomass reactivity and the level of gas mixing. At the experimental rate of solids circulation (6 kg/s), modelled char conversion values lie between 9% and 39 % and are thus quite far from the experimental value of 2 % (although potentially ranging between 0 and 10 % due to experimental uncertainty). An explanation of the higher char conversion provided by the model could be the uncertainty in the reactivity of the biomass used. A further possible explanation, which has been studied by means of modelling in this work, is the gas mixing. The model uses expressions for the gas mixing which have been derived from measurements in smaller lab units with a high-pressure drop gas distributor, i.e. which induce a better gas mixing than the limited one existing in the large-scale unit studied here, caused by the presence of large bubbles and regions with weak fluidisation. 1