Evolution of Stable Free Radicals during Bio-Oil Pyrolysis and Its Relation to Coke Formation: An in Situ EPR Study

Thermal–chemical conversion is needed to improve the quality of bio-oil. When bio-oil is heated, it polymerizes to form coke, causing severe problems to its upgrading processes and thus hindering its further utilization. As stable free radicals are generated and attached to coke during the active radical polymerization of bio-oil, figuring out the evolution of stable free radicals is a key to revealing the coke formation mechanisms. In this study, bio-oil pyrolysis was conducted between 300 and 600 °C, coupled with in situ electron paramagnetic resonance (EPR) detection. The results indicate that the stable free radicals generated during pyrolysis are related to bio-oil polymerization and coke structure change. In the heating stage, the stable free radicals are generated from the polymerization of bio-oil and the condensation of coke, and the process can be expressed by two first-order kinetics with activation energies of 14.57 and 10.51 kJ/mol, respectively. The aromatic structure stable radicals are generated mainly by the polymerization of bio-oil with an activation energy of 12.66 kJ/mol. In the cooling stage, the free radical reaction would be caused by coke shrinkage, leading to further changes in coke structure.