Whole Tree Pulping Experiment by Bill Fuller
July 11, 2011
To: Kevin Tudor – Domtar/Mt. Sterling
From: Bill Fuller
Subject: Utilization of Wood with Bark
Before I get into the discussion of the attached technical papers, a review of the history of utilizing un-debarked wood in the form of whole tree chips, whole stemwood chips, unprocessed tops, “puckerbrush”, stumps and
a variety of other tree parts chipped without removal of bark, foliage, dirt and slow squirrels. While some work was done prior to the 1970’s, the rapid expansion of the pulp and paper industry in that period caused a short-fall of conventional roundwood and forest products residual chips with a consequent rapid increase in price. Previous academic work in the biomass estimation and whole tree utilization had demonstrated the ability to remove 25-100% more wood from a forest by utilizing more than the merchantable size trees and tree parts (both above and below ground). With the dramatic fiber shortage, an industry cooperative was formed under the sponsorship of TAPPI and the American Pulpwood Association. This group shared information on woods chipping field trials, quality of chips from types of harvest methods, methods to analyze the contaminants resulting from woods chipping and encouraged equipment suppliers to develop methods to both chip and upgrade chips in the woods. Much of this information was not shared outside the group and only some of it was published. I am providing the work that was published, some of the better presentations made by group members, the best public research work (mostly from Canada) and several translations of work in Sweden and Finland where wood supplies were also, and continue to be, critical.
Analysis of Bark
The Institute of Paper Science and Technology thoroughly studied the composition of all North American commercial species beginning in the 1960’s. Their work is summarized in the attached Excel file. Two things are most important. Bark, particularly softwood, usually contains more resinous materials than wood and therefore, has greater potential to cause appearance problems in pulp and paper products. Second, there is fiber in bark, but not a lot. The pulp yield of softwood bark ranges from 20-30%, but there are no fibers long enough to be useful in papermaking. In hardwoods, the pulp yield is 30-40% and 5-20% of the fibers are comparable to wood fibers in terms of paper quality. A “forest legend” that bark completely pulps away is not true. The issues are whether the yield is high enough; the fiber is of sufficient quality and whether the pitch and gums will cause pulp appearance problems. Lab and pilot studies can provide reliable information on yield and fiber quality that can be extrapolated to mill production. However, actual mill trials are needed to determine whether a mill bleaching and pulp cleaning system can prevent product appearance problems.
Since the time whole tree utilization was investigated and full-scale mill trials were run, significant improvements in pulp bleaching have been implemented. Most important are the substitution of chlorine dioxide for chlorine bleaching and the use of oxygen and hydrogen peroxide. While environmental issues were the primary reason for using these bleaching agents, their ability to remove the colored extractives contained in bark was also an important driver.
The success of oxidative bleaching has been significant in enabling mills in northern regions of Canada to tolerate bark levels over 5% that occur in winter. I can provide specific data on a former Weyerhaeuser mill if it would be useful.
Biomass studies and field trials showed that 25-50% more above-ground softwood biomass could be harvested with whole tree utilization methods. This increase was from tops, branches and small stems.
From the outset, it was recognized that woods chipping of softwood whole trees was least likely to succeed due to:
- difficulty feeding tops through the woods chipper and resulting poor chip size distribution, especially long twigs and slivers that plug conveyors and screens
- persistent foliage and tendency of needles to plug screens
- low fiber quality of compression wood that comprises more than half of branchwood
- resinous nature of bark
- high bark/wood ratio in small diameter trees
- high amount of dirt accumulated on bark and foliage when the trees are skidded to the chipper at roadside.
During this period, companies that had been actively establishing Southern pine plantations, we looking forward to the need to thin stands at about 15-20 years of age. Whole tree chipping was considered the best alternative for removing this material since conversion of 4-6” stems to solid wood products was not at all economical and it had to be removed to remove fire and beetle infestation hazards.
As an aside, Interstate Paper in Riceboro, GA, cooperated with Hercules since the early 1960’s to extract old, resinous field stumps that remained in the ground from old-growth Southern pine stands that had been converted to farmland. They were removed between crops, cleaned in a drum debarker, washed, shredded and steam-extracted to recover pure pine rosin. The extracted wood was screened to remove fines and oversize and blended with normal chips for pulping in the Interstate mill. The fiber quality was found to be equivalent to stemwood. As a result, whole tree utilization studies in the 1970’s also considered stump wood, but with little success due to contamination and the cost of extraction.
Few mills attempted to utilize softwood with bark for bleached products. It was successfully used to manufacture linerboard at substitution levels up to 20-25%. At that time, increased bark particles in this packaging product were not as significant as they are now. Appearance and print quality are now important factors for boxes, particularly in export markets.
Biomass studies and field trials showed that 50-100% more above-ground hardwood biomass could be harvested with whole tree utilization methods. This large increase was from large branches above the main stem and small trees.
Hardwood whole tree utilization has always been regarded as the most feasible and economically attractive. Some of the drawbacks cited above still were of concern, but the factors making it attractive were:
- doubling the amount of wood harvested from an acre of land resulted in a very attractive wood cost that could offset the increased pulp mill operating disadvantages
- the usable fiber yield from pulped bark further helped offset costs
- hardwood bark does not contain the high amounts of pitch and resin that softwoods contain
- hardwood whole trees chip more uniformly
How long did the use of whole tree chips last?
By 1980, the use of whole tree chips declined.
- Supply of clean chips caught up with demand. The source of these chips was:
- The increased costs in pulp mills from softwood bark, foliage and knotwood were not offset by the lower wood cost.
- Hardwood whole tree chips continued to be used for corrugating medium and in a few bleached mills where the wood costs were almost 50% lower than conventional chips. Corrugating medium abrasiveness was a significant disadvantage to box plants where corrugating roll wear was severe. Weyerhaeuser continued to use whole tree hardwood for medium until the early 1990’s.
- Several technologies were developed to remove bark, foliage and abrasive grit from whole tree chips. The most promising were advanced to the pilot plant level, but none were fully implemented. The bark removal efficiency was a maximum of 75% but at that level, good wood loss was as high as 20%. Abrasive material removal was 80-90%. As satellite chip mills and chain flail debarking improved, the economics of whole tree chip upgrading were not favorable.
- Some linerboard mills discontinued debarking pulpwood in conventional wood yards. The economics of this practice depend on mill specific process and product limitations.
The first publication you should look at is the presentation by Steffes. He was a researcher at Crown Zellerbach. His summary of work done on Western U.S. species, it is completely applicable to Southern wood supplies. His conclusions are an excellent overview of the details in the other publications.
It is tempting to go into lots of details on the technology and details and I will be glad to do that if needed. There are more reports and data available.