Unimolecular Decomposition Pathway for the Vapor-Phase Cracking of Eugenol, A Biomass Tar Compound

To design optimal thermochemical processes for the conversion of biomass into chemicals, fuels, and electrical power, an understanding of the mechanisms for the secondary vapor-phase cracking of tar compounds is crucial. Despite the many studies examining the homogeneous secondary cracking of biomass tar existing in the literature, its thermal decomposition reaction pathways are not completely understood. Much of this lack of understanding is due to the complex, heterogeneous nature of biomass tar. A useful approach is to examine the pyrolysis of model-fuel compounds that are actual components or are representative of compounds found in biomass tar. In this study, we focus on eugenol, a model-fuel compound representative of the lignin-derived components found in biomass tar. We conduct pyrolysis experiments at temperatures of 300-900 degrees C and one second residence time using a non-isothermal laminar-flow reactor system. We report the variation in the experimental yield of light product gases as functions of the reactor temperature. We examine a reaction pathway for the unimolecular decomposition of eugenol with consideration of the experimental product distributions and analogous reactions based on established decomposition mechanisms of similar compounds. We examine the detailed energetics of the unimolecular decomposition route using computational chemistry calculations at the B3LYP/6-311G+(d,p) level of theory. The results presented in this study would be of relevance to the pyrolysis, gasification, and combustion of biomass.