Comparison of superheated steam and conventional torrefaction of pine wood: Solid product properties, productivity, and process evaluation

Superheated steam (SHS) is utilized as the sole heat source and carrier gas for the torrefaction process to improve feedstock properties and productivity. This study compares the performance of SHS torrefaction with conventional torrefaction that usually uses an external heat source and nitrogen as the carrier gas. Sample properties, chemical functional groups, and combustion and pyrolysis behaviors of torrefied pine wood from the two torrefaction methods are presented. Torrefaction was conducted at temperatures of 200, 250 or 300 °C. Two sample mass loadings (i.e., small at 30 g and large at 160 g) were adopted for productivity comparison between the two torrefaction methods. Results indicated that pine wood torrefied at 300 °C with 30 g by SHS had the highest carbon content (70.88 wt%) and HHV (28.50 MJ·kg −1 ), and the lowest oxygen (18.91 wt%) and volatile matter (39.99 wt%) contents among all treatment conditions. The Van Krevelen diagram showed that the pine wood torrefied by SHS with 160 g produced comparable O/C and H/C ratios to those of conventional torrefaction with 30 g at the same torrefaction temperatures, indicating the improved productivity of SHS torrefaction. Furthermore, the combustion and pyrolysis behaviors of torrefied pine wood clearly demonstrated that SHS torrefaction enhanced the thermal degradation of holocellulose. The evaluation of SHS and conventional torrefaction suggests that SHS torrefaction is practical for industrial application owing to its superior productivity and energy efficiency.