A one-dimensional unsteady-state heat transfer model coupled with time dependent mass loss equation of wood particles exposed to drying and pyrolysis conditions was developed. The kinetic parameters were determined experimentally and the regime and characteristics of the conversion were evaluated in terms of the particle size and reactor temperature. The order of mass loss exponent varied from n =1 at temperatures lower than 350 degrees C to n = 0.5 at temperatures higher that 350 degrees C. The analysis of conversion time, the time when particles lost 90% of their mass, showed that thermal treatment of particles larger than 0.5 mm were controlled by internal thermal resistances. The valid range of particle size to use the simplified lumped model depends on the fluid temperature around the particles. The critical particle size was 0.6-0.7 mm for the fluid temperature of 500 degrees C and 0.9-1.0 mm for the fluid temperature of 100 degrees C. (C) 2016 Elsevier Ltd. All rights reserved.