Torrefaction pretreatment for regulating the co-production of gaseous and biochar during lignin pyrolysis

Torrefaction pretreatment is considered an effective strategy to regulate the physicochemical properties of lignin and its subsequent pyrolysis behavior. In this study, lignin was subjected to torrefaction at different temperatures and residence times prior to pyrolysis in order to investigate the evolution of lignin structure and the distribution of biochar and gaseous pyrolysis products. The results showed that increasing torrefaction severity affected both lignin properties and product distribution. The biochar yield increased from 65.01% to 75.99%, while the gas yield rose from 10.02% to 14.34%. Structural characterization indicated that torrefaction promoted the carbonization and deoxygenation of lignin, leading to a higher fixed carbon content, lower O/C ratio and improved thermal stability. The resulting biochar exhibited enhanced structural ordering, with the Raman I D /I G ratio decreasing from 4.44 to 3.14, indicating a higher degree of graphitization. In terms of gaseous products, the yields of CO, H₂ and CH₄ increased, with CO rising from 27.87 to 56.16 mL/g, while CO₂ decreased slightly. These results demonstrate that torrefaction pretreatment can effectively regulate lignin structure and pyrolysis pathways, providing insights for the efficient and high-value utilization of lignin through thermochemical conversion.