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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. Model-ing 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.

Advanced analysis methods have been developed to follow the reactions of lignin during alkaline oxygen delignification conditions more comprehensively than before. This aim was attained by monitoring both the lignin macromolecule and the dissolved reaction products. Softwood (SW) and hardwood (HW) kraft spent liquor lignins were studied as substrates under various reaction conditions. The decrease in the contents of different types of free phenolic hydroxyl groups and the concurrent formation of carboxylic acids was followed by 31P NMR of the phosphitylated products. In addition, the formation of acidic degradation products with low molecular weight was determined by capillary zone electrophoresis (CE). This way, it was possible to distinguish the carboxylic acids bound to the lignin macromolecule from the cleaved reaction products, even if they partly co-precipitated during sample preparation. Peak deconvolution was applied to get information on syringyl type phenolic structures and on C(5) condensed guaiacyl structures in hardwood lignin. Pyrolysis-GC/MS was applied to provide additional information about the distribution of guaiacyl/syringyl/p-hydroxyphenyl (G/S/H) type lignin subunits, as well as changes in the phenylpropane side chain.