CONCLUSIONS Both the TGA and fixed bed reactor torrefaction test results showed that the torrefaction rate was affected by the particle size, especially at high temperatures. Although the temperature gradient inside particles of smaller than 1 mm during torrefaction is very small, the internal diffusion of generated vapors inside particles affects the global torrefaction reaction rate. The nonshrinking core particle model with a first order torrefaction reaction can be applied to give a reasonable prediction of the reaction rate data, accounting for the particle size effect. By fitting the reaction model to the TGA weight loss data, the effective diffusivity of torrefaction volatiles was determined, which was orders of magnitude lower than the Knudsen diffusion coefficient estimated based on the microfibril pores of the solid wood but within the range reported for gas diffusion through solids materials. The densification tests showed that the energy consumption for making torrefied pellets increased with increasing the particle size and the degree of torrefaction, and the quality of torrefied pellets (hydrophobicity and hardness) could be improved with decreasing the particle size and increasing the severity of torrefaction treatment.