Energy consumption in refining of Scots pine and Norway spruce TMP is governed by fibre morphology and ultrastructure

Studies on the different behaviour of Scots pine and Norway spruce during thermomechanical pulping were conducted to find reasons for explaining the greater energy consumption of pine. Well characterized wood samples from clear-cut trees and thinnings were used in pilot scale trials to produce highly characterized pulps. Structural and topochemical differences found between pine and spruce likely to have a strong negative impact on energy included: i) presence of a greater proportion of latewood fibres in pine annual rings with thick secondary cell walls and concomitant reduced fibre collapsibility; ii) ultrastructural differences in the location of fractures at the fibre secondary wall level during primary refining with pine showing greater resistance to refining although it defibrated easier than spruce; iii) that fibre separation and fibre development were sequential processes in pine and not concurrent as in spruce; and iv) the greater amounts of extractives in pine and their redistribution onto the surfaces of fibres and fines during processing