The purpose of the present research is to develop a numerical model for the biomass gasification processes suitable for the dimensioning of industrial installation. The end use of the producer gas being often internal combustion engines systems, the model concentrates on the quantitative and qualitative aspects of the process, with a particular interest in the problem of tar formation and destruction. This study focuses on the first two steps of the process: drying and pyrolysis. The pyrolysis model presented in this paper consists in a coupling between a heat transfer model based on a nodal approach and a chemical model for the thermal decomposition of the pyrolysed material. Drying is considered as a sub process included in pyrolysis. The shown results are in good agreement with experimental data.