Coke formation during the pyrolysis of bio-oil: Further understanding on the evolution of radicals

The radical reactions dominate the thermochemical conversion reaction network of organics. Bio-oil, as the liquid organic fuel, has many potential applications via thermochemical conversion, while the coke formation is a particularly serious problem in these processes. To clarify the effects of radical reactions on the coke formation during the pyrolysis of bio-oil at 200–500 °C, the stable radicals in the pyrolytic products were measured by the Electron Paramagnetic Resonance (EPR) spectrometry, and the active radicals during pyrolysis were quantified by amounts of hydrogen donated by 9,10-dihydrophenanthrene (DHP). The results indicated that the amount of stable radicals generated after bio-oil pyrolysis was proportional to temperature, and these stable radicals were trapped in solid coke due to the steric hindrance effect of coke structure. The content of oxygen-containing radicals and heteroatoms decreased with the increase of temperature. However, the increase of temperature and the extension of reaction time were more favorable to the formation of the σ and π type oxygen-containing radicals and aromatic radicals. The amount of stable radicals was about 0.25–1.5% of the amount of active radicals during bio-oil pyrolysis. When DHP effectively blocked the reaction of the active radical, the polycondensation reaction of bio-oil was inhibited, and the content of coke precursor was significantly reduced, thus inhibiting the formation of coke during the bio-oil pyrolysis.