Thermodegradation characterization of hardwoods and softwoods in torrefaction and transition zone between torrefaction and pyrolysis

•Woody biomass is thermally treated in the torrefaction and transition zones.•Two hardwood and two softwood species are investigated.•Hardwoods and softwoods exhibit drastic weight loss at 240–330 °C.•Atomic H/C and O/C ratios decrease vigorously as the temperature rises to 300 °C.•Wood treatment in the transition zone is not a suitable operation due to low solid yield. Torrefaction and pyrolysis are two common thermochemical conversion processes to convert biomass into biofuel. This study aims to treat woody biomass within the temperature range of torrefaction and transition zone at 210–390 °C. The weight loss of the hardwoods (up to 64 wt%) and softwoods (up to 60 wt%) exhibit a drastic change at temperatures of 240–300 °C and 240–330 °C, respectively. The biomass shrinkage ratio (about 15–22%) along the tangential and radial directions is more pronounced than that along the longitudinal direction. The H/C and O/C ratios decrease as the temperature increases to 300 °C. Thermogravimetric analysis shows that the treated hardwoods and softwoods exhibit drastic changes at temperatures of 270–300 °C and 300–330 °C, respectively. Fourier-transform infrared spectroscopy analysis shows that cellulose constituent in hardwoods is easier to degrade than that of softwoods. The transition zone is not a suitable operation for treating wood due to its low solid yield.