Uniformity of delignification during kraft pulping of hardwood chips: impact of wood structure and the importance of impregnation

Abstract

This study examines the homogeneity of lignin and xylan removal in model sapwood chips of alder (Alnus glutinosa), birch (Betula pubescens), beech (Fagus sylvatica), and aspen (Populus tremula) during batch kraft pulping. The first experiments focused on investigating the effect of hardwood morphology on the evolution of pulping. By comparing the gradients of lignin and xylan inside the chips at different stages of delignification, it was established that the process is more uniform in hardwoods of lower density (high porosity), like aspen. Hence, characteristics such as large and frequent vessels, thin fiber walls, and large lumens may help to achieve pulps with more homogeneous composition and to avoid shives. The influence of other morphological features was not clear. The results also suggested that the impact of morphology was mostly connected to the impregnation step, as the porosity of the sapwood dictated the initial concentration of hydroxide ions within the chips. Thus, a second set of experiments was conducted to evaluate the possibility to overcome this impact by utilizing liquors with high alkali content during pulping. Among the evaluated conditions, the use of liquor containing 2 mol HO⁻/kg during impregnation followed by cooking with 0.55 mol HO⁻/kg was shown to be a reasonable approach to increase the uniformity of delignification.