Populus tomentosa was torrefled under carbon dioxide in a tubular reactor to investigate the effects of temperature (240-320 degrees 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 torrefled 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 torrefled sample showed good thermal stability as determined by the thermal analysis. In addition, the combustion reactivity increased from 4.13%/(min degrees C) in the raw material to 4.98%/(min degrees C) for the sample subjected to torrefaction at 280 degrees C for 75 min. The data provides meaningful insight on the upgradation of poplar through torrefaction. (C) 2017 Elsevier Ltd. All rights reserved.