A two-dimensional theoretical and numerical model of biomass pyrolysis has been developed to determine the kinetic of wood particles pyrolysis in a fixed bed reactor. The adopted chemical model is coupled with a global transport bi-dimensional, and two temperatures model. The three modes of heat transfer (conduction, convection and/or radiation) between the solid and gas phases, and the wall are considered. The system of all unsteady and reactive equations is resolved by a finite volume method with a hybrid spatial interpolation and an implicit interpolation for time. The resulting algebraic equations are then resolved by using the bi-conjugate gradient stabilized and normalized method. The obtained model is validated in several experimental cases for data of one biomass particle. This model is applied to predict the pyrolysis phenomenon in a fixed bed of particles. Using the proposed numerical model the time space evolutions, of temperature and char fraction, are presented and discussed. In this configuration, the obtained numerical results agree with the experimental data found in the literature.