New sight on the lignin torrefaction pretreatment: Relevance between the evolution of chemical structure and the properties of torrefied gaseous, liquid, and solid products.

In order to reveal the deoxygenation mechanism of lignin torrefaction, the relevance between evolution of chemical structure of torrefied lignin and the properties of torrefied gaseous, liquid, and solid products was established in this study. Results showed that the contents of oxygen element, βO4 linkages, oxygen-containing functional groups (aliphatic OH, aliphatic COOH, aromatic OCH3) in lignin decreased with the increase of the torrefaction temperature from 210 to 300 °C. The oxygen removal efficiency of lignin torrefaction reached the maximum value of 25.53% at 300 °C. The removed oxygen in the torrefied lignin was transferred into the torrefied gaseous product (e.g. CO2, H2O, and CO) and torrefied liquid product (e.g. G-type and P-type phenols, acids). Among the torrefied gaseous products, CO2 was the dominant oxygen carrier, followed by CO and H2O. Among the torrefied liquid products, G-type phenols were the dominant oxygen carrier, followed by P-type phenols and acids.