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Investigating the kinetics of anisole: a simple lignin model compound
Koirala, Yogesh
Koirala, Yogesh
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2015
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Abstract
Thermochemical conversion of biomass is a potentially important process for providing a re-newable source of fuels and chemicals. One hurdle to an economically viable process is better control of the pyrolysis products of the biomass component lignin. Development of a kinetic model that could predict the impact of various operating conditions on the conversion and prod-uct yield could substantially accelerate commercial development. It is essential that the kinetic model describe the reactions at the molecular level with theoretically consistent rates, to confi-dently explore the various regions of potential operating parameters. In this thesis, anisole (C6H5OCH3) is used as a model compound since it has the weak phenolic-carbon linkage so prevalent in lignin. Anisole pyrolysis experiments were performed under conditions similar to those proposed for fast pyrolysis of biomass: T= 525-650 °C, t~0.4 s, P ~0.82 atm. For one set of experiments the initial anisole mole fraction was 0.75% with the balance inerts. In a second set, 5% H2 was added to shift the mole fractions of some of the important radicals in the system. A detailed kinetic mechanism was developed by combining an anisole model from the literature with an extensively validated CSM hydrocarbon pyrolysis model. A combination of sensitivity analysis and rates of production analysis demonstrated that the recombination of me-thyl radicals and phenoxy (the products of the initial dissociation of anisole) was a critical reac-tion. This recombination is complicated since the phenoxy radical assumes three different reso-nant structures, resulting in three different recombination products (anisole, o-methylcyclohexadienone , and p- methylcyclohexadienone ). This reaction network (the C7H8O potential energy surface), was characterized using CBS-QB3 electronic structure calculations. The resulting surface was significantly different from earlier estimates and led to improved pre-dictions. Similar electronic structure calculations were performed for many other reactions in-volving oxygenated species. All of these reactions were added to the mechanism and a sensitivity analysis was performed to refine the model. The only change that could easily be justified was an increase to the barrier for o-cresol formation on the C7H8O potential energy surface by 2 kcal/mol to improve the cresol predictions. No other adjustments were made and the predictions of this model were then used to compare to the collected data in this work as well as several earlier studies reported in the literature. These comparisons of the model predictions to these multiple data sets suggest that this fundamentally-based mechanism has led to an improved description of anisole pyrolysis and has identified specific issues that need to be addressed to further improve the kinetic description. The biggest remaining issue is to identify theoretically consistent reaction pathways that will divert some of the cresol product to phenol.
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