Experimental study on the synergy between lignin and cellulose during co-torrefaction and its impact on the properties and structural characteristics of pyrolysis biochar

Abstract

This study aims to investigate the synergy between lignin and cellulose during co-torrefaction, as well as the influence of torrefaction temperature and mixing ratio on the basic properties and structural characteristics of the feedstocks and biochar. To this end, both single-component and co-torrefaction pretreatments with lignin and cellulose feedstocks were conducted at 220 ℃, 260 ℃, and 300 ℃, followed by pyrolysis at 550 ℃. The results showed that the torrefaction process led to increased fixed carbon and C content, lower volatiles and O content, and enhanced higher heating value (HHV). In addition, torrefaction decreased the crystallinity of cellulose while increasing that of lignin. For the co-torrefaction samples, crystallinity generally decreased after torrefaction, and this decrease was positively correlated with the cellulose ratio. Torrefaction also enhanced the degree of graphitization of the feedstocks through the removal of oxygen-containing functional groups. However, the elemental content and HHV of the biochar were largely unaffected. It was also found that lignin and cellulose exhibited synergy during co-torrefaction, which promoted the transition of feedstocks and biochar to higher carbon content and HHV while accelerating the removal of oxygen-containing functional groups. XPS analysis showed that under medium and high temperatures (260 ℃ and 300 ℃), co-torrefaction inhibited the decomposition of functional groups such as C-C, CC, C-H, -OH, and C-O-C in the biochar, while promoting the decomposition of oxygen-containing functional groups such as CO, -COOH, and -COO. This study elucidates the notable role of co-torrefaction in improving the properties and pyrolysis performance of biomass components.