The fluid-particle interaction and the impact of shrinkage on pyrolysis of biomass inside a 150 g/h fluidised bed reactor is modelled. Two 500 mu m in diameter biomass particles are injected into the fluidised bed with different shrinkage conditions. The two different conditions consist of (1) shrinkage equal to the volume left by the solid devolatilization. and (2) shrinkage parameters equal to approximately half of particle volume. The effect of shrinkage is analysed in terms of heat and momentum transfer as well as product yields. pyrolysis time and particle size considering spherical geometries. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. Heat transfer from the bubbling bed to the discrete biomass particle, as well as biomass reaction kinetics are modelled according to the literature. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase. FLUENT 6.2 has been used as the modelling framework of the simulations with the whole pyrolysis model incorporated in the form of user defined function (UDF). (C) 2009 Elsevier B.V. All rights reserved.