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 °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.

The vapor-phase cracking of 4-vinylguaiacol has been investigated in a nonisothermal, laminar-flow reactor at temperatures between 300 and 900 °C and a residence time of one second. Products identified by gas chromatography were oxygenated compounds such as phenols, cresols, furans, ketones, and aldehydes, single-ring and polycyclic aromatic hydrocarbons, C1–C6 hydrocarbon gases, and carbon monoxide. Temperature had a marked effect in governing the overall product composition. Conversion of 4-vinylguaiacol to products increased above 400 °C and was completed at 550 °C. Reaction rate parameters derived from the conversion data were A = 1013 s–1 and E a = 45.3 kcal mol–1. The oxygenated compounds were observed as products in the range 450–800 °C, peaking in yields below 700 °C. The aromatic hydrocarbons and light gases dominated the product composition above 600 °C, especially at 900 °C, the highest temperature investigated. On the basis of the experimental data showing the effect of temperature on product composition, reaction pathways leading to products formation are proposed.

Lumped kinetics for the vapor-phase cracking of 4-propylguaiacol, a model compound representative of components found in primary tar derived from lignin, has been investigated. Analysis of the products from pyrolysis experiments in a laminar-flow reactor at temperatures between 300 and 900 °C and a residence time of 1 s revealed that the products can be lumped into three compound classes: oxygen-containing compounds, single- and multiring aromatic hydrocarbons, and permanent gases. Temperature was found to have a marked effect in governing the overall product composition. The oxygen-containing compounds peaked in yield between 500 and 700 °C. The aromatic hydrocarbons and permanent gases dominated the product composition above 600 °C, especially at 900 °C, the highest temperature investigated. A lumped kinetic model with three irreversible first-order reactions was developed to model the experimental data. This model was extended to one with eight first-order irreversible reactions. Optimized reaction-rate parameters for each reaction in both models were determined by fitting the experimental data using a plug-flow reactor model.

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