Structural and thermal properties of Populus tomentosa during carbon dioxide torrefaction

Populus tomentosa was torrefied under carbon dioxide in a tubular reactor to investigate the effects of temperature (240-320 °C) and reaction time (30-120 min) on the solid char. The influences of the reaction conditions on mass yield, energy yield and higher heating value (HHV) were investigated by using response surface methodology. The structural and thermal properties of the torrefied poplar were comprehensively characterized with multiple techniques. Results indicated that torrefaction temperature showed a greater influence on mass yield, energy yield and HHV than time. Torrefaction led to substantial changes of the structural properties of poplar, as evidenced by the variation of microstructure, elements, and CrI value. The torrefied sample showed good thermal stability as determined by the thermal analysis. In addition, the combustion reactivity increased from 4.13%/(min °C) in the raw material to 4.98%/(min °C) for the sample subjected to torrefaction at 280 °C for 75 min. The data provides meaningful insight on the upgradation of poplar through torrefaction.