| dc.description.abstract | An investigation was made into the rates of moisture content, extractives and
basic wood density losses that occur when trembling aspen (Populus tremuloides Michx.) wood
chips are stored during different seasons of the year, in a chip pile. Seasoning is required for
the pulping of trembling aspen in order to reduce the extractive content to a level that will
minimize the occurrences of pitch build-up and problems in the kraft pulping process. The
hypothesis for this study was that there are significant differences in losses of extractives,
moisture content and basic density within a chip pile·and that the extractives losses in the pile
centre would match extractives losses in roundwood in a much shorter time. The change of
extractives can be achieved in normal outside chip storage periods by using chips from the
interior of a chip pile or in shorter periods through steaming chips under controlled conditions.
Time was a very significant factor during the storage. The study found that the closer to
the middle of the pile, the hotter the temperatures became. The largest rate of heat build-up
was found in the first 12 days. The middle of the pile showed the lowest average moisture content
after 4 months (i.e., 38.2%), whereas the top of the pile showed the highest moisture content
after 4 months (i.e., 45.2%). The absolute rate of extractives loss was, on average, 0.3%/month for
the 4 month study period. However, for the first month, the absolute rate was 1.0%/month
(i.e., 31% of the original) and for the second month it dropped down to 0.6%/month (i.e.,
39 % of the original). There were no significant changes in the extractive content from month
2 to month 4. The bottom middle of the chip pile resulted in the largest loss of
extractives (i.e., final extractive content of 1.8% after 4 months). No comparison was made
between the aspen chip pile built in the summer with the one built in the winter since the winter
pile remained frozen for the duration of the study. It was calculated that the extractive content
after 1.5 weeks in the summer pile would equal the extractive content (i.e., 2.5%) of the
aged roundwood (i.e., for 1 year). A minimum of emphasis should be placed on this since the
sample size used to calculate the extractive content of the aged roundwood was small. The
rate of basic density loss in the summer chip pile was 2.4%/month for the 4 month study
period. The
average moisture content of the fresh arriving chips was 47.9% as compared with the moisture
content of the digester chips (i.e., 40.2%). The extractive content of the fresh arriving
chips was 3. 6 %, as compared with 2.9% for the digester chips. There were no significant
differences between the average basic densities for the fresh
arriving chips and the digester chips (i.e., 0.41 g/cm3) . The study showed that
steaming does not accelerate the loss of extractives in aspen chips. A method of chip reclaim is
presented that collects, with the aid of an auger travelling underneath the chip pile, only the
bottom middle chips. As these chips are collected, chips from the other regions of the pile fill in
the gaps. A continuous chip seasoning period of 2 weeks is recommended to allow for the maximum
amount of seasoning in the shortest time period. A chip pile inventory of 42 308 m3 would be
sufficient to allow for a 2 week inventory, however, a chip inventory of 3 weeks is recommended
(i.e., 63 462 m3 to allow for an extra weeks buffer). It is also recommended that Avenor consider
continuous monitoring of the digester extractive contents to know that they are sufficiently low,
as well as to try and correlate any operating problems in the mill
with the extractive content of the chips that had entered. | |
