The aim of the present study is to develop a 1D modelling tool representing biomass gasification in a dual fluidized bed. This tool should be used with the objective to control and scale-up the process for different pilot scales and operating conditions. This implies that the modelling tool should be based on a phenomenological description but also that the calculations are not too much time-consuming. A one-dimensional steady state model was thus developed for the gasification reactor, combining a description of the biomass thermochemical conversion phenomena and of the hydrodynamic behaviour of the fluidized bed. The reactor is divided into two zones, namely, dense zone and freeboard zone. For the dense zone, the model assumes the existence of two phases -bubble and emulsion- with chemical reactions occurring in both phases (biomass devolatilization, char gasification, water-gas shift). In the freeboard, a plug flow is assumed and the only reaction considered is water-gas shift. Mass balance is globally solved across the entire gasifier. The model allows calculating axial concentrations of the gas species in the reactor as well as the global yields. The parameters of the calculations are temperature of the gasifier, biomass composition, biomass and steam feeding rates.