Vapor-Phase Cracking of Eugenol: Distribution of Tar Products as Functions of Temperature and Residence Time

Abstract

Knowledge of the product distribution and kinetics of the secondary vapor-phase cracking of biomass tar is crucial for the design and optimization of biomass thermochemical conversion processes, such as fast pyrolysis, gasification, and combustion. In this study, we have examined the distribution of condensable tar products from the vapor-phase cracking of eugenol, a model-fuel compound representative of the lignin structural entities found in biomass. Pyrolysis experiments were investigated at different temperatures (300–900 °C) and residence times (τ = 1 and 3 s) in a non-isothermal laminar-flow reactor operated at atmospheric pressure. Tar products were analyzed using gas chromatography with mass spectrometric detection. Eugenol conversion commenced rapidly above 350 °C at τ = 3 s and above 400 °C at τ = 1 s. Complete conversion was attained at 550 and 600 °C at τ = 3 and 1 s, respectively. A numerical model incorporating laminar-flow, non-isothermal temperature profile and pseudo-unimolecular kinetics was developed to model the eugenol conversion data. The model agreed very well with the experimental data, and the derived global kinetic parameters for overall eugenol decomposition were A = 1014 s–1 and Ea = 50.7 kcal mol–1. Tar products were comprised of oxygenates and aromatic hydrocarbons. Identified oxygenates include phenols, furans, ethers, and acids. The aromatics consisted of single-ring aromatic compounds and polycyclic aromatic hydrocarbons. Oxygenates were observed as tar products in the temperature range of 350–850 °C at both τ = 1 and 3 s. Aromatic hydrocarbons were observed as tar products at temperatures above 650 °C. Most of the identified aromatic hydrocarbons showed an increase in the yield above 650 °C at both τ = 1 and 3 s. At all temperatures, yields of oxygenates and aromatics were generally higher at τ = 1 s than at τ = 3 s. Mass balance closure was not achieved above 400 and 500 °C at τ = 3 and 1 s, respectively, with discrepancies in the mass balances increasing with an increase in the temperature. The discrepancies in the mass balances can be attributed to the formation of noncondensable gas products.