Changes in Aspen Bark Stored in Outdoor Piles

Increasing use of bark for fuel has led to questions as to the losses and other changes that occur during outdoor pile storage. The possibility of spontaneous ignition in bark piles is of special concern. This study examined the storage characteristics of aspen bark. Two aspen bark piles, 40 feet by 40 feet by 20 feet high, were built in October of 1974 and 1975 at a northern Wisconsin mill site. The 1974 pile contained bark which came directly from a ring debarker; the 1975 pile was built using bark that had been put through a hammermill after it came from the ring debarker. Temperatures were observed at the centers of the piles using thermistors placed at 5, 10, and 15 feet above the base. Bark substance losses (ovendry material) were determined by placing bark samples contained in nylon mesh bags at each of these locations in each pile and retrieving them after 1 year of storage. Maximum pile temperatures were attained in about 3 weeks and were about 160?F for both piles. Bark substance losses varied with height above the base of the pile, being greatest at the 15-foot level (about 25 percent) and least at the 5-foot level (about 5 percent). Moisture content also varied with the height and was greatest at the 15-foot level. The pH of the bark decreased during storage from an initial value of 4.6 to final values ranging between 2.6 and 3.3. Bark particle size did not significantly affect pile temperatures, weight losses, or changes in pH and moisture content.

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