The present work addresses the study of the pyrolysis of biomass particle, with the aim to improve the comprehensive mathematical model of the thermochemical processes involving solids decomposition. A new CFD model for the biomass pyrolysis was developed at the particle scale in order to properly describe the relative role of reaction kinetics and transport phenomena. The model is able to solve the Navier-Stokes equations for both the gas and solid porous phase. The code employs the open-source OpenFOAM® framework to effectively manage the computational meshes and the discretization of fundamental governing equations. The mathematical algorithm is based on the PIMPLE method for transient solver and exploit the operator-splitting technique that allows the separation of the transport and the reactive term in order to handle complex computational geometries minimizing the computational effort. The model was tested with experimental data for both reactive and non-reactive conditions. The code is able to provide correct information about temperature distribution within the particle, gas, tar and char formation rates.