Identification of free radicals in pyrolysis oil and their impact on bio-oil stability

Abstract

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.