Lignin oxidation mechanisms under oxygen delignification conditions. Part 3: Reaction pathways and modeling

A detailed kinetic model for the lignin oxidation chemistry is presented. It is based mainly on the mechanisms and kinetics presented in the literature. Parameters that could not be found in the literature were regressed against the experimental data obtained from oxidation experiments with softwood kraft lignin. In addition to the detailed model for the chemistry, acid-base equilibrium reactions and gas-liquid mass transfer were modeled. Most of the experimental observations could be reproduced with the developed model. The reasons behind the behavior of guaiacyl and condensed phenols are discussed. The reaction routes affecting lignin solubilization and chemical consumption are presented. Modeling of acid-base equilibria proved to be important because acid-base pairs of reactants react differently. Carbon dioxide buffers the pH and in this way affects the chemical reactions through the pH. A similar model could also be developed for other lignin treatments; for example, waste water purification or chemicals production in new biorefinery concepts. The developed reaction scheme will be used as a part of oxygen delignification model.