Kraft pulping of birch: Insights into delignification and xylan removal

Licentiate thesis, 2024

Kraft pulping is a well-established process in the production of many wood-based materials, holding a strategic position in the transition towards a bioeconomy. Nevertheless, further process development is needed to increase its resource-efficiency, assimilate new feedstocks and meet new specifications. Thus, the two studies presented in this thesis focused on investigating the mechanisms governing the removal of lignin and carbohydrates during kraft pulping of birch, aiming to identify the conditions leading to evenly delignified pulps. 

The first study assessed the influence of mass transport on pulping by using impregnation liquors with different compositions and by comparing black liquor fractions collected from the bulk (surrounding the wood chips) and from the pore system of wood. The results showed that the transport of OH- and SH- during impregnation led to more extensive delignification throughout cooking. Moreover, when analyzing the black liquor fractions, higher concentrations and an initial accumulation of lignin and xylan were observed in the pore system. Also, the structure of the dissolved wood components in the pore and bulk fractions differed, highlighting the influence of mass transport resistance in the cell walls and pore system of wood.  

The second study evaluated the local evolution of pulping within model wood chips under different cooking conditions. The extent of lignin and xylan removal strongly depended on the available hydroxide content within the chips. In addition, lignin was removed more uniformly from wood when using low cooking temperatures (145°C). Lastly, pulping was shown to progress faster in the longitudinal direction of the chips.