Interactions of cellulose- and lignin-derived radicals during pyrolysis: An in-situ Electron Paramagnetic Resonance (EPR) study

Lignocellulosic biomass can be converted to biochar, bio-oil and gas through reactions of non-volatile stabilised free radicals (SFRs) and volatile radicals during thermal conversion. In the process, the interactions between radicals from different biomass components play an essential role. Figuring out the interactions of radicals during pyrolysis holds the key to revealing the interaction mechanism of biomass components. In this study, the evolution of SFRs during the co-pyrolysis of cellulose and lignin was explored using in-situ electron paramagnetic resonance (EPR) detection. The interactions of cellulose- and lignin-derived radicals were analysed based on the variation of SFRs, thermogravimetric behavior and Raman spectrum measurement. The results indicate that the interactions of radicals influence char yield, char structure and the SFRs evolution during pyrolysis. The interaction of volatile radicals and SFRs increases the char yield and promotes the condensation of aromatic structure in biochar. At temperatures above 400 °C, the interaction between cellulose- and lignin-derived SFRs leads to the coupling of radicals, which forms an unstable structure in the hot nascent biochar. In the subsequent cooling process, the weak bonds from interactions may break due to the structure shrinkage, which promotes radical reactions and further changes the char structure.