The cost of creating an inert atmosphere is one of the main expenses associated with torrefaction. For reducing the operating cost, torrefaction of poplar sawdust is conducted in a semi-closed system without an inert atmosphere. The effect of self-generated volatiles from torrefaction in a semi-closed system on mass and energy yields, HHV, elemental analysis, thermal properties, chemical structures, grindability and hydrophobicity of torrefied biomass was investigated, and then compared with torrefaction in nitrogen medium. Results show that properties of torrified biomass in a semi-closed system are almost the same as those in nitrogen medium. However, the thermal performance of torrefied biomass obtained from a semi-closed system is superior to that from nitrogen medium because of the relatively easy accessed thermal reactions, as indicated by low peak temperature in DTG. The decreased peak temperature is related to oxidized cellulose and lignin with increased contents of carboxyl, carbonyl, and ether groups in torrefied biomass obtained in the presence of generated volatiles. (C) 2018 Elsevier Ltd. All rights reserved.