A new three-stage biomass gasifier is proposed to generate product gas with low tar content. The first stage is dedicated for biomass pyrolysis. While the main roles of the second and third stages, which are superposed and separated by a gas distributor plate, are product pyrolysis gases combustion and gasification, respectively; noting that gasification is realized in a fluidized bed. In order to study and improve the process fundamentals, a cold flow model is developed to understand system dynamics and parameters. A computational fluid dynamics (CFD) study has been conducted using the Eulerian-multiphase model approach applied within the combustion and gasification zones. A complete CFD simulation of flow through the gas distributor is realized. Three values of the distributor perforated area: 1%, 3% and 5% and three bed materials were investigated to study the effect of the solid density and particle size on the fluidized bed. The aim of this work is to study the effect different parameters such as the distributor design, gas flow rate, fluidization velocity, solid density and solid particle size. In this work, gas and solid velocities above the distributor, pressure drop across the distributor, distributor to bed pressure drop ratio, solid volume fraction, bed pressure drop and the bed expansion ratio have been examined. Results show that the perforated area of a distributor has a great influence on the uniformity of the fluidization. The hydrodynamics of solid-gas fluidized beds are shown strongly affected by the gas flow rate and the properties of the bed material.