This paper deals with the two dimensional modelling of transient pyrolysis of biomass in a fixed bed. The model that is developed mainly relies on the theory of volume averaging. This model includes heat and mass transfer and pyrolysis reaction. It allows the prediction of temperature, moisture content organic material content, and char content, as well as the prediction of internal evolution of the composition of the gas held and released from the sample under investigation. This model is used in order to describe pine wood pyrolysis taking place in a macro thermobalance which is also presented in the paper. Comparison of the numerical results with the experimental ones, in terms of mass loss of the sample, shows good agreement and validates the model. A deeper analysis of the results allows a better understanding of the processes involved in the thermal degradation.