Torrefaction subsequent to pelletization: Characterization and analysis of furfural residue and sawdust pellets

Torrefaction integrated with pelletization has gained increasingly interest as it enhances the characteristics of fuel pellets (e.g. hydrophobicity and energy density). In current study, torrefaction of furfural residue pellets (FRPs) and sawdust pellets (SPs) was performed by employing tubular reactor furnace, and quality of pellets was compared. The characteristics of both types of pellets were significantly improved with increasing torrefaction temperature from 200 °C to 300 °C and residence time from 15 min to 30 min. The highest lower heating value of 23.78 MJ/kg and energy density ratio (1.27) for torrefied furfural residue pellets (TFRPs) and 26.76 MJ/kg and 1.46 for torrefied sawdust pellets (TSPs) were achieved at 300 °C and 120 min. Increasing torrefaction temperature and residence time, the volumetric energy densities of TFRPs increased from 25.69 (at 200 °C and 15 min) to 27.59 kJ/m³ (at 300 °C and 120 min), while those of TSPs correspondingly decreased from 20.81 to 16.69 kJ/m³. The highest true densities (i.e. 2.40 and 1.85 g/cm³) and porosities (i.e. 52 and 65 v %) of TFRPs and TSPs were achieved at 300 °C and 120 min, much higher than those of un-torrefied pellets. Moisture uptake of TFRPs and TSPs at 300 °C were only 1.4 wt% and 2.0-2.8 wt%, respectively, showing strong water-resistant ability. The crystallinity of cellulose in FRPs was found higher than that of SPs, while the crystallinity of cellulose in TFRPs was found lower than that of TSPs at same process conditions. FTIR showed that O-H bond was destroyed after torrefaction for both FRP and SP.