Detailed kinetics of substituted phenolic species in pyrolysis bio-oils

Fast biomass pyrolysis is an effective and promising process to obtain high yields of bio-oils, whose upgrading provides valuable fuels for energy application or chemicals for industry. The growing interest in the use of bio-oils in combustion devices to produce energy motivates this study, in which we present the first comprehensive kinetic model to describe systematically the pyrolysis and combustion of substituted phenolic species, considered as reference components in bio-oil surrogate mixtures. In fact, bio-oils are complex liquid mixtures, containing a large variety of oxygenated organic species. Within these species, substituted phenolic compounds are one of the most significant fractions (∼20-30 wt%). A reliable characterization of the combustion properties and pollution potential of bio-oils strongly depends on the accurate knowledge of their combustion chemistry. While some experimental and kinetic modeling studies on pyrolysis and combustion of phenol, anisole, and catechol are available in the literature, only limited efforts have been devoted to the understanding of the decomposition and oxidation kinetics of guaiacol (2-methoxyphenol) and vanillin (4-hydroxy-3-methoxybenzaldehyde). Accurate theoretical calculations of bond dissociation energies have been performed to assess proximity effects originating from multiple substitutions on the aromatic ring. Based on these evaluations and on previous studies, rate rules and reference kinetic parameters are proposed for major pyrolysis and combustion reaction classes. Satisfactory comparisons of model predictions with experimental data of pyrolysis and combustion of anisole, catechol, guaiacol, and vanillin hierarchically support the development and the reliability of the proposed kinetic model. This work provides a valuable basis for further developments and strongly motivates additional experimental, theoretical, and kinetic modelling efforts in the area of reference components for bio-oil surrogates.