Impact of torrefaction pretreatment on the pyrolytic behavior of hemicellulose: A systematic analysis of biochar, bio-oil, and gaseous products

To address the issue of high oxygen content and poor product quality resulting from the inherent instability of hemicellulose, torrefaction was applied as a pretreatment. Herein, xylan was chosen as a model compound representing hemicellulose and subjected to torrefaction at 200–300 °C for 15–45 min to investigate its physicochemical evolution. The torrefied solids were subsequently pyrolyzed, and the resulting distribution of biochar, bio-oil, and gaseous products was characterized. Analyses by Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Raman spectroscopy confirmed that torrefaction effectively removed oxygen-containing functional groups and facilitated the formation of aromatic structures, thereby enhancing the structural ordering of the carbon matrix. As the severity of the torrefaction pretreatment increased, the higher heating value (HHV) of the treated hemicellulose improved markedly. With respect to the pyrolysis products, torrefaction was found to enhance the yield of biochar and accelerate its graphitization. For the gaseous products, although the total yield decreased, the sample torrefied at 300 °C for 45 min exhibited elevated yields of CO (68.85 mL/g) and H₂ (29.03 mL/g), compared to those from the raw material (59.27 mL/g and 24.12 mL/g, respectively). Concurrently, the volume fraction of CO₂ decreased from 58.38% to 35.82%, which consequently elevated the HHV of the gas product from 7.51 to 12.18 MJ/m³ . In the case of liquid products, the HHV of bio-oil increased from 19.87 to 25.05 MJ/kg. Meanwhile, the relative content of acidic compounds decreased from 29.77% to 21.67%, whereas the contents of furans and ketones increased notably. This study elucidates the correlation between the microstructural evolution of hemicellulose during torrefaction and the distribution of tri-phase pyrolysis products, providing a basis for the high-value utilization of biomass. • Torrefaction reduced oxygenated groups and promoted aromatic structures. • Biochar yield and carbon structural ordering increased with torrefaction severity. • Torrefaction enriched CO and H₂ while suppressing CO₂ in pyrolysis gas. • Gas heating value increased despite a reduction in total gas yield. • Bio-oil quality improved with higher HHV and lower acidic compound content.