A comparative study of the cell wall level delignification behaviour of four Nordic hardwoods during kraft pulping

Wood is a heterogeneous material with significant variation among species. This inherent complexity poses a challenge to the continuous expansion of our understanding of the kraft process; yet previous pulping research has mainly been limited to a few species. This study investigates variations among some less studied species and their cell wall level delignification behaviour during kraft pulping. Ground wood of birch, beech, aspen, and alder were pulped at near-constant composition and temperature conditions. Minor, yet significant, differences in the rates of their delignification were observed: aspen had a pronounced fast rate during the initial stage, whereas alder delignified more slowly relative to its high initial lignin content. The dissolution of xylan was substantially faster for birch. In contrast, no substantial differences were detected between the species in the molecular weight and structure of the dissolved wood components, suggesting that the different delignification behaviours stem from variations in the residual phase. The molecular weight distribution of dissolved lignin was uniform during the initial stage of pulping, which is indicative of rapid and extensive fragmentation. Subsequently, the weight increased continuously for the remainder of the process, suggesting that the mass transfer within the cell wall influenced the overall delignification kinetics.