Hardwood delignification

Licentiate thesis, 2024

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.