Hardwood delignification

Sammanfattning: For the necessary transition into a sustainable and circular society, lignocellulosic resources are set to play a crucial role in replacing fossil-based materials. In the valorisation of lignocellulosic materials, kraft pulping is one of the key processes. However, challenges regarding increasing the resource efficiency and expanding the scope of products mean that an improved understanding of the mechanisms behind the process is needed. The work presented in this thesis summarises two studies aimed at investigating the interplay between transport and reaction mechanisms in the kraft delignification of hardwood. In the first study, the impact of mass transport during the pulping of wood chips has been investigated by comparing the liquor fraction within the chip, with that of the bulk. The study observed considerable differences in the concentration and structure of dissolved wood components between the liquor inside the wood chip and the surrounding bulk liquor, implying that the transport between these two fractions is slower than the reactions involved. In addition, even under mild impregnation conditions, the transport of alkali has been found to have a minor, yet persistent, effect on the subsequent delignification. The second study compared the delignification behaviour of milled wood from four hardwood species: birch, beech, aspen, and alder. Small, yet significant, differences in the delignification rate were found among the species, although these differences could not be attributed to variations in the molecular weight or structure of the dissolved wood components. However, the molecular weight of the dissolved lignin continuously increased throughout pulping, indicating that transport mechanisms within the cell wall influences the overall rate of delignification. Lastly, parts of the xylan have been found to extract rapidly in polymeric form at the start of pulping, while other parts remained as residuals in the pulp.