Evolution of oxygen-containing functional groups during torrefaction and its effect on pyrolysis behaviors and hydrogen production from different lignocellulosic biomasses

• Torrefaction facilitated the conversion of aliphatic to aromatic rings in biomass; • O-containing functional groups in hemicellulose and cellulose visibly reduced; • Torrefaction enhanced H₂ and CH₄ percentage in pyrolysis gas; • Phenolic content in tar increased significantly, particularly for CR (49.17%); • Torrefaction altered the pyrolysis reaction mechanisms. Three representative lignocellulosic biomasses, including cedar (CR), wheat straw (WSt), and walnut shell (WSh), were firstly torrefied in a tube furnace at 250°C to investigate the evolution of oxygen-containing functional groups (OFGs) during torrefaction and then pyrolysized at 900°C to explore the effect of torrefaction on pyrolysis behaviors and hydrogen production. The OFGs of raw and torrefied biomass were monitored by 13 C-NMR and FTIR. Pyrolysis gas and tar collected in fixed bed were analyzed by GC and GC-MS, respectively. Moreover, pyrolysis kinetics of three kinds of biomass were studied by the thermogravimetric analyzer. Results demonstrated that torrefaction of biomass effectively reduced OFGs in hemicellulose and cellulose, while facilitating the conversion of aliphatic carbon to aromatic rings. Torrefaction at 250°C remarkable improved both the low heating value of pyrolysis gas and the quality of tar. Specifically, CO₂ and CO yields were obviously decreased, and the selectivity of H-containing gases (H₂ and CH₄) increased after torrefaction. Notably, the phenolic content in pyrolysis tar increased significantly after torrefaction, particularly for CR (49.17%). Kinetic analysis revealed that the activation energy of torrefied CR was substantially lower than that of raw biomass within a conversion rate range of 0.2–0.7. This reduction minimized energy input requirements during the main pyrolysis stage and enhanced reaction system homogeneity. Furthermore, Z(α) master plots indicated that torrefaction altered the pyrolysis reaction mechanisms, with the extent of variation being biomass-dependent.