Identification of free radicals in pyrolysis oil and their impact on bio-oil stability
The existence of radicals in pyrolysis oil generated from loblolly pine in three different reactor systems was verified with electron paramagnetic resonance (EPR) spectroscopy. Characterization of the bio-oil via its sub-fractions revealed the radicals were preferentially located in the bio-oil lignin fraction, especially in the higher molecular weight lignin. Based on the observed EPR spectra (which lacked hyperfine structure) and low g-factors, the radicals are proposed to be stable, carbon-centered and delocalized in a highly conjugated lignin π system. Furthermore, this study also examined the impact of radicals on bio-oil aging severity using an accelerated aging method and the addition of radical scavengers. Preliminary results support the hypothesis that bio-oil radicals are present in a stable state because radical scavengers showed negligible effects on controlling pyrolytic lignin condensation. Only a mild radical concentration reduction was observed after bio-oil accelerated aging.