A mathematical model for intra-particle transport phenomena and chemical reactions is coupled with an external heal transfer model, taking into account fluid-bed hydrodynamics, to predict the fast pyrolysis characteristics of cellulosic fuels. Good agreement is obtained between predicted and measured product yields as functions of the reactor temperature. For practical applications aimed at liquid fuel production, particle size and external temperatures greatly affect the average particle heating rate (values roughly comprised between 300 and I K/s), whereas the actual degradation temperature vary in a narrow range (600-725 K). Consequently, variations in the conversion time are significantly larger than in product distribution and yields. Finally, comparisons are made with the Ranz-Marshall correlation and the limit case of infinitely fast external heat transfer rates.