Heterodox Torrefaction of the Forest Residue with a Twin-Screw Extrusion Reactor

Torrefaction of bark, a common forestry and wood processing residue, was carried out by using a twin-screw extrusion reactor to produce solid biofuels and collect condensable compounds. Unlike conventional torrefaction, this work was carried out under heterodox conditions with temperatures between 275 and 375 °C and retention times of less than 7 min. The effect of key variables, such as temperature, screw rotation speed, and screw design, on the properties and distribution of solid and liquid products was analyzed. Similar to conventional torrefaction, both weight loss and heating values of products are positively correlated to the treatment severity when the torrefaction is conducted with the same screw design. Applying higher temperatures (350 °C and above) drastically increased the yields of biochemicals. For 500 g of feedstock, 280 mg of furfural was produced under 350 °C for ∼5 min, and the yields of phenols and guaiacol reached 400 and 100 mg, respectively, at 375 °C. Applying shear damage by changing the design of the screws was effective in further improving the torrefaction performance, as indicated by the distribution of products and energy content of the obtained biochar, but the yields of biochemicals decreased, presumably because the adsorption of chemicals was enhanced. This study recommends applying high temperature and a short reaction time as an approach to increase the treatment capacity of torrefaction equipment and the yields of biochemicals.