Dry torrefaction of biomass - Torrefied products and torrefaction kinetics using the distributed activation energy model

Torrefaction is a thermal treatment applied to improve biomass properties for long-term storage and thermochemical conversion of wood. The torrefaction kinetic behavior of two woody biomass samples (aspen and birch) and their individual constituents (cellulose, xylan -hemicellulose proxy- and lignin) was studied in detail. The materials were torrefied in a fixed-bed reactor up to 280 °C and were characterized after 15 min and 60 min treatments in terms of their energy density, energy yield, H, O and C contents, humidity uptake and FTIR vibrational spectra. Regardless of mildness (240 °C) or severity (280 °C) of torrefaction, hemicellulose was the most reactive. Compared to torrefied cellulose, lignin exhibited considerable activity and increased heating value under severe torrefaction (280 °C). Likewise, the increase in heating value of torrefied cellulose was less significant than that of lignin due to cellulose energy loss because of production of tar compounds. The release of volatiles versus time for cellulose, xylan and lignin torrefaction reactions measured using a thermogravimetric method was also modeled for each lump by means of the distributed activation energy model and a three-parameter log-normal distribution function. The model was found to fit the experimental data for xylan and lignin torrefaction while it showed slight under-prediction in the case of cellulose torrefaction. The distributed activation energy models developed for the individual lumps were combined into a meta-model to apprehend the torrefaction kinetics of actual (aspen and birch) woody biomass by assuming parallel and independent involvement of their three constituents present under various compositions. The model was found to represent with very good agreement the experimental data of aspen and birch torrefaction.