Table 1 summarizes some bench-scale test results of torrefaction of wood with different operating conditions reported in the open literature. Most previous torrefaction studies were conducted in fixed bed reactors. Besides the reaction temperature, reaction residence time, and wood species, particle size is also an important design parameter for torrefaction and densification, which was not reported in some studies. Also, Table 1 shows that there are inconsistent results from different research groups when one compares the data obtained at same operating conditions, such as 523 K for 30 and 60 min, respectively. This can be partly attributed to the difference in the reactor configurations and heating rate, but
particle size may also play important role. As Prins et al. (2006) postulated, for pyrolysis of wood below 573 K, the reactions were the rate-limiting step for particles smaller than 2 mm where the impact of intraparticle heat and mass transfer becomes insignificant.10 In this study, both a TG and a fixed bed reactor have been used to study particle size effect on torrefaction. A press machine was used to make torrefied pellets for the evaluation of the quality of the torrefied pellet based on density, energy yield, durability, and equilibrium water uptake rate.