Pyrolysis characteristics of torrefied kraft lignin prepared under oxidative and non-oxidative atmospheres

Waste lignin is a promising feedstock for the generation of energy and chemicals. Nevertheless, the thermochemical conversion of waste lignin is limited by several obstacles, such as the low yield of the overall process and low production of final products. In this study, lignin was subjected to oxidative torrefaction, and the chemical structures and pyrolysis products of torrefied lignin were investigated. Oxidative torrefaction increased the calorific value of lignin by removing some functional groups and generating more solid products. Low temperature (230℃) and oxygen concentration (8–10%) were conducive to the production of phenolic (up to 82.16%) and hydrocarbon substances from lignin, and oxidative torrefaction also facilitated the conversion of waste lignin into unconsolidated products after pyrolysis. This finding indicates that oxidative torrefaction is a mild and economically feasible process for the pretreatment of biomass, which favors the thermochemical conversion of lignin into energy and high-value chemicals. • Lignin can be efficiently transformed into biochar by aerobic torrefaction. • Torrefaction atmosphere directly affects the output of biochar. • Oxidative torrefaction elevated the calorific value of lignin. • Lower temperatures in an oxygenated atmosphere favors the pyrolysis of lignin.