Torrefaction of wood: Part 1. Weight loss kinetics

Torrefaction is a thermal treatment step in the relatively low temperature range of 225–300 °C, which aims to produce a fuel with increased energy density by decomposing the reactive hemicellulose fraction. The weight loss kinetics for torrefaction of willow, a deciduous wood type, was studied by isothermal thermogravimetry. A two-step reaction in series model was found to give an accurate description. For the two steps, activation energies of 76.0 and 151.7 kJ/mol, respectively, and pre-exponential factors of 2.48 × 104 and 1.10 × 1010 kg kg−1 s−1, respectively, were found. The first reaction step has a high solid yield (70–88 wt%, decreasing with temperature), whereas less mass is conserved in the second step (41 wt%). The fast initial step may be representative for hemicellulose decomposition, whereas the slower subsequent reaction represents cellulose decomposition and secondary charring of hemicellulose fragments. The kinetic model is applied to give recommendations for industrial torrefaction process conditions, notably operating temperature, residence time and particle size.