Prolonged wood storage causes mainly brightness problems for TMP

During summer-time storing wood loses moisture and brightness. Storing induced wood dryness may cause problems in mechanical pulp production. Norway spruce logs were stored in piles over summer and compared to fresh wood as a raw material for TMP. The reduced wood moisture content does not require changes in thermomechanical pulping as long as the moisture content remains above the fiber saturation point. The amount of colored substances in wood increases already during the storing, and further in the refining, resulting reduced pulp brightness. This brightness loss can be mostly recovered by peroxide bleaching. However, at high brightness levels of 75-80%, the use of stored wood can increase the required peroxide charge by 8-12 kg/BDt. Combined washing and pressing treatment of the chips provides further brightness improvement.