Superheated steam drying and torrefaction of wet biomass: The effect on products characteristics

Drying process is inevitable for torrefaction of nature biomass which contains considerable moisture. Drying process would retard the temperature building of biomass during superheated steam (SHS) torrefaction and hence affect the torrefaction process as well as change the properties of products. Feedstocks with different moisture contents were torrefied to develop a drying model to describe the SHS drying process and predicate the temperature of feedstock. Torrefied biomass was analyzed to get its physicochemical properties, pyrolysis behaviors, and combustion performance. It was found that torrefaction would occur during the drying process. The drying process can be roughly divided into three stages: the rapid cooling stage, the rapid heating stage, and the stable heating stage. The rapid cooling stage and rapid heating stage are dominated by drying processes, while the stable heating stage is usually accompanied by a severe torrefaction process. Wet feedstock need longer residence time to achieve equivalent physicochemical properties and pyrolysis behaviors as that of dry feedstock. The HHV of dry sample could be increased by 40.8 %, and that of wet sample also could be increased by 34.77%. However, the combustion behaviors of the products have changed after the thermal treatment. It is found that ignition and burnout temperatures of SHS torrefied biomass were greatly affected by the initial moisture content in feedstock.