A coupled transport and reaction model is formulated to investigate the effects of various parameters on biomass pyrolysis. The model takes into account formation of chars, tars and gases through mechanisms including both primary reactions of the virgin biomass degradation and secondary reactions of the primary tar. All main transport phenomena, unsteadiness of the gas/vapor-phase processes, variation of the reacting medium properties and particle shrinkage are also described. Numerical simulation of the problem of a wooden particle, subjected to an assigned external radiation, is used to analyze time and space evolution of the main variables and product distribution as the shrinkage parameters and the intensity of the heat flux are varied. The effects of the orientation of the anisotropic wood grain relative to the one-dimensional heat flux are also investigated.