HIGH YIELDS OF KRAFT PULP FROM RAPID- GROWTH HYBRID POPLAR TREES

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1 U.S.D.A., FOREST SERVICE RESEARCH PAPER FPL HIGH YIELDS OF KRAFT PULP FROM RAPID- GROWTH HYBRID POPLAR TREES U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY MADISON, WISCONSIN

2 ABSTRACT Kraft pulps were made from whole hybrid poplar trees and components including bark using growth rotations of 1, 3, 5, 11, and 24 years. The best production resulted from a rotation of 11 years on the basis of per acre per year. Satisfactory pulps were made from all of the materials and the presence of bark posed no special problems in either pulping or bleaching. The 1-year-old material was unique in that it was stronger than pulps made from the 3- and 5 year-old trees and that little or no refining energy was required to develop the strength of pulp.

3 HIGH YIELDS OF KRAFT PULP FROM RAPID- GROWTH HYBRID POPLAR TREES By J. F. LAUNDRIE, Chemical Engineer Forest Products Laboratory, 1 Forest Service U.S. Department of Agriculture, Madison, Wis. and J. G. BERBEE, Professor of Plant Pathology and Forestry, University of Wisconsin INTRODUCTION The conversion of trees into products in the United States traditionally has been inefficient primarily because of an abundant supply of highquality trees. Now, however, there is predicted a timber supply shortage as a result of rapidly in 2 creasing demands for wood and wood fiber (3). Improved forest products utilization efficiency offers perhaps one of the greatest potentials for extending our timber resource at minimum expense. One possible way of obtaining improved utilization efficiency in the pulp and paper industry could be to use more of the tree, as strongly advocated by Young (8) and others. However, the widespread belief that not more than 1 percent bark in the chips can be tolerated appears to be one of the major obstacles preventing the fuller utilization of the whole tree for kraft pulp production. For this reason, effort has been made to develop effective methods for bark-chip separation and segregation (1, 2, 7). The alternative of leaving the bark with the chips and removing it during pulping or subsequent operations has received little attention. One purpose of this study, therefore, was to obtain and provide information concerning the chemical requirement, yield, and quality of kraft pulps made from whole trees including bark. Another way of obtaining improved utilization efficiency in the pulp and paper industry could be to use fast-growing species which reproduce readily and which could be handled as an agricultural crop with intensive growth culture and mechanized harvesting. Previous work with sycamore by the U.S. Forest Service, the University of Georgia, and the Georgia Forest Research Council has shown that this practice does have some merit in the South (4). The poplars (populus spp.) also fall into this category. The Plant Pathology Department of the University of Wisconsin has de v o t e d considerable research effort to the development, selection, and production of rapidly growing disease- and insectresistant poplars, primarily of the Aigeiros (black poplar) type. A natural hybrid poplar clone, resembling eastern cottonwood (Populus deltoides Marsh.) and tentatively designated P. x euramericena (Dode) Guiner cv Wisconsin 5 was chosen for these pulping studies. Details relating to the establishment, maintenance, and productivity of hybrid poplar plantations will be considered in a forthcoming paper. 1 Maintained in cooperation with the University of Wisconsin, Madison, Wis. 2 UnderIined numbers in parentheses refer to Literature Cited.

4 Three-, 5-, and 11-year-old trees of this clone were available in plantations at the University of Wisconsin Arlington Farms. Twenty-four-yearold trees were available from the University of Wisconsin Gugel Farm in Madison. Because of the large number of 1-year-old trees required, it was necessary to use a mixture of 20 similar clones, including Wisconsin 5 for pulping trials involving this age group. All of the 1-year-old trees were grown at Boscobel State Nursery, Boscobel, Wis. The 1-year-old trees were of particular interest because they not only had a high yield potential on a per acre per year basis, but also because they would be handled as an agricultural crop, Another purpose of this study, therefore, was to obtain and provide information concerning the chemical requirement, yield, and quality of kraft pulps made from 1-year-old and older trees, and to estimate kraft pulp yields with various rotation lengths on a per acre per year basis. Having this information, the pulp and paper industry would be in a position to determine whether or not specific percentages of these pulps could be used in the manufacture of their products and to judge the value of obtaining fiber from trees grown and harvested as an agricultural crop. EXPERIMENTAL Wood Harvesting and Preparation for Pulping Kraft Pulping Each of the trees harvested was cut as close to Bomb scale digestions were made using the the groundline as practicable. The entire tree in separated wood and bark samples from the smaller all cases was transported to the Laboratory and diameter trees and branches. A few of the other separated into the following categories: Bole, samples were also pulped in the bombs to first branches less than 2 inches diameter outside establish cooking conditions before making the bark (DOB), and branches larger than 2 inches larger digester cooks. DOB. The branchless 1-year-old trees, the 3- Standard procedures were used for both bomb year-old boles, and branchwood less than 2 inches and digester scale cooks (5). For bomb cooks, DOB from all of the trees were converted into the pulps made from the wood chips were de 1/2-inch chips without first removing the bark. watered on coarse fritted glass funnels and To determine the amounts of bark on this small washed with hot water until the effluent was diameter wood and to obtain both bark and bark- clear. The pulps made from the bark samples free wood for subsequent bomb scale pulping would not dewater in the fritted glass funnels, so studies, representative samples of at least 200 the procedure was changed to centrifugation with grams (moisture-free) were taken and separated repeated washings. The washed pulps were dried by hand with a jackknife. and yield was determined. After drying, samples The bark was removed from half of the 5-, 11-, were taken or Kappa number determination. and 24-year-old boles and branches larger than For the digester cooks, the screened and par 2 inches DOB, and l/2-inch chips were made from tially dewatered pulps were transferred to a both the unbarked and debarked materials. The canvas bag, and the consistency was raised to bark removed from this larger diameter material about 30 percent in a hydraulic press. The pulp was also converted into 1/2-inch chips. Most of cakes were shredded, weighed, and sampled for the chips were made in a commercial-sized, yield and Kappa number determination, four-knifed chipper. The materials less than 1/2-inch DOB and bark samples were cut into 1/2-inch lengths with a pruning shears. FPL 186 2

5 Cleaning A pulp made from the 1-year-old whole trees was centrifugally cleaned before bleaching. After diluting the pulp to a consistency of 0.5 percent, it was pumped through a 3-inch-diameter centrifugal cleaner having a 1/8-inch-diameter tip opening. The inlet pressure was controlled at 40 pounds per square inch, while the backpressure was held at 5 pounds per square inch. After cleaning, the pulp was partially dewatered in a 100 mesh stainless steel screenbox Bleaching Selected pulps were bleached to about 88 percent brightness using chlorination, caustic soda extraction, chlorine dioxide, and in some instances an additional stage of caustic soda extraction and another application of chlorine dioxide. Five hundred grams (moisture-free) of each pulp was bleached to provide enough pulp for the determination of physical and optical properties. DISCUSSION OF RESULTS Although all components of the whole trees, both with and without bark, were pulped and tested, this discussion will be limited to only those results concerning bark-free boles, boles with bark, and whole trees. All other data are included in the appendix Active Alkali Requirement The amount of active alkali consumed to obtain pulps with the same Kappa number, increased with the amount of bark contained with the chips (fig. 1). The 24-year-old whole tree chip mixture containing 21.6 percent bark consumed 10.8 percent active alkali, while at the other extreme, the 1-year-old whole tree chip mixture containing 43.8 percent bark, consumed 15.0 percent active alkali. While no attempt was made to optimize the cooking conditions to obtain the same residual concentration of active alkali in the black liquor, it is evident that the amount of active alkali charged also increased with increasing amounts of bark. The amount of bark, however, was not the only factor affecting the amount of active alkali consumed. While the bark-free 11- and 24-year-old boles both consumed 10.9 percent active alkali, increasing amounts of active alkali were consumed by the bark-free boles as tree age decreased. The cause for this increase is not known, although it can be postulated that an increase in the relative amount of pith with decreasing age could be the cause, Unfortunately, the amount of pith was not measured in this study. Using the black liquors from the 1- and 24-yearold whole tree digestions, the total solids measured 17.4 and 12.7 percent and the heating value of these solids measured 6,660 and 7,200 B.t.u per pound, respectively. The amount of total solids in the black liquor from 1-year-old whole tree digestions is considerably higher than that of the liquor from the 24-year-old whole tree digestion, not only because of the increased amounts of organic material dissolved, but also because of the higher amounts of active alkali required. The increased amount of inorganic chemicals in the black liquor from the 1-year-old whole tree digestion is, of course, the reasonthat the heating value of these solids is lower than the solids in the black liquor from the 24-year-old whole tree digestion. These data are given here only for the reader to obtain an estimate in the differences between these two liquors, and should not be used per se for the calculation of chemical or energy balances since the effect of black liquor recycle is not included. Digester Pulp Yield The effect of tree age on digester pulp yield is given in figure 2. As expected, the yields of pulps from the boles with bark were in all cases lower than those of the bark-free boles. This, of course, was mainly due to the lower yields obtained from the bark portion of the chip mixture. (Bark yields are given in appendix table 2.) While the barkfree boles from the 11- and 24-year-old trees 3

6 Figure 1.--Active alkali requirement to obtain 20 Kappa number kraft pulps. M both gave yields of 52.8 percent, the bark-free boles from the younger trees all had lower yields with the 1-year-old trees having the lowest at 44.1 percent. The digester pulp yields from the whole trees, similarly, decreased as tree age decreased and as the proportion of bark in the chip mixture increased. A yield of only 32.6 percent was obtained from the 1-year-old whole trees. While most of these lower yields are due to the presence of bark in the chip mixture, part of the overall loss in yield from the younger trees is caused by the high active alkali requirement and overcooking of the wood portion of the chip mixture as shown in figure 3. Yields from separate wood and bark cooking were calculated from weighted yields of the individual components of the whole tree (appendix tables 1 and 2) and compared to the yields actually obtained. For the 1-year-old trees, the calculated yield was 36.1 percent compared to the 32.6 percent actually obtained. Similar results were obtained with the 3- and 5 year-old whole trees, although the differences were not as large. FPL 186 4

7 Figure 2.--Kraft pulp yield of whole hybrid poplar trees. M Annual Per Acre Pulp Yields To determine potential kraft pulp yields on a per acre per year basis, it was necessary to calculate total dry weights produced with rotation lengths of 1, 3, 5, 11, and 24 years. The dry weights of the component parts of individual trees or groups of trees of the different ages were measured. Data thus were obtained on the amounts of bole wood and branchwood over and under 2 inches in diameter both with and without bark (table 1). Yields from trees of different ages are given in table 2. The dry weights of the 3- and particularly of the 5-year-old trees are relatively low because of comparatively slow diameter growth in the plantations sampled. Conversely, the 11-year-old tree was larger than the average individual in the plot. The 24-year-old tree was irreversibly suppressed by crowded growing conditions and consequently does not reflect the growth potential of that age. It was included only to obtain comparative data on the relationships of pulp yields and characteristics to tree age. 5

8 Figure 3.--Loss of yield from combined cooking of wood and bark. M Increases in overall yield as a consequence of pulping the whole tree rather than bark-free boles only also are given in table 2. For 1-, 3-, 5-, 11-, and 24-year-old trees, inclusion of bark and branches increased the amount of dry matter pulped by 66, 92, 56, 54, and 58 percent, respectively, Through utilization of the entire treetops, pulp yields per tree were increased by 25 percent or more, although pulp strength was somewhat reduced. The greatest yield increase (59 pct.) was obtained in the pulping of 3-year-old material because at that age, bark and branches accounted for almost one-half of the total dry weight. The smallest yield increase (25 pct.) was obtained with 1-year-old trees because of the absence of branches and a much reduced digester yield. For 5- and 11-year-old trees, pulp yields per tree were increased by approximately 35 percent by utilizing all of the above-ground parts of the trees. Estimates of per acre per year yields from trees of different ages are presented in table 3. For the 1-year-old and 11-year-old materials, the tree populations given are based on actual spacings within the plots sampled. The 3-yearold and 5-year-old trees were grown at wider spacings than indicated in table 3, but repeated trials have indicated that growth rates of the hybrid poplar used in these trials is not adversely affected until after the basal area exceeds 160 square feet per acre. This figure was used to calculate the populations given for the 3- and 5-year-old trees. The yields given in table 3 are expressed as dry weight of wood or trees, as dry weight of FPL 186 6

9 Table 1.--Sizes and physical compositions of 1-, 3-, 5-, 11-, and 24-year-old hybrid poplar trees Table 2.--Increases in kraft pulp yields by using boles with bark and whole trees kraft pulp, and as equivalent cords. The equiv- substantially higher with the 11-year rotation. The alent cords were obtained by multiplying the yields obtained with the very young trees were kraft pulp yields by a conversion factor of 2.325, approximately twice as high as the normal maxithe number of rough cords of poplar normally mum per acre per year yields obtained from required to produce 1 ton of kraft pulp (6). natural poplar (Populus tremuloides Michx.) Yields obtained with 1-, 3-, and 5-year rota- grown in a 40-year rotation in the Lake States. tions were approximately the same, but were Yields from the 11-year rotation amounted to 7

10 Table 3.--Estimated kraft pulp yields per acre per year from bark- free boles, boles with bark, and whole trees 1 Except for the 1-year-old material, space requirements were calculated to produce a basal area of approximately 160 square feet at the end of the rotation with maximum anticipated growth rates. 2 Equivalent cords of unpeeled bolts required to produce the kraft pulp yields given in the preceding column based on cords per ton. 3 The 1-year-old trees, consisting of a mixture of 20 similar clones, had 2-year-old roots. Space requirements were based on the 3-year-survival of trees cut back to the ground each spring (original spacing 0.5 square feet per tree). 4 The 11-year-old tree actually pulped was larger than the average individual in the 30-tree plot. Yields given are based on growth data from a tree of average size (moisture-free weights of the bark-free bole, the bole with bark, and the whole tree were 159, 184, and 245 pounds, respectively). Projections based on the pulped tree gave yields of 5.2, 5.5, and 7.2 equivalent cords with utilization of the bark-free bole, the bole with bark, and the whole tree, respectively. more than a threefold increase in productivity. The utilization of whole trees rather than barkfree boles increased yields by 25 to 36 percent. Pulp and Handsheet Properties Original freeness.--the effect of tree age on the original freeness is given in figure 4. The original freeness of the pulps made from the boles with bark and the whole trees of the 11 and 24-year-old trees are only slightly lower than those of the bark-free bole pulps. As tree age decreased and the proportion of bark in the chip mixture increased, however, the original freeness also decreased. The 1-year-old whole tree pulp had an original freeness of only 395 milliliters (Canadian Standard) and in forming a standard TAPPI handsheet had a drainage time of 8 seconds which was twice the time needed to form a handsheet from the whole tree pulp made from the 24-year-old tree. Although removal of the fines passing through a 200-mesh screen increased the freeness of the 1-year-old whole tree pulp from 395 to 510 milliliters and decreased the drainage time to 5 seconds, this procedure is not recommended because 17.7 percent of the pulp passed through a 200-mesh screen and this, in effect, would reduce the digester yield from 32.6 percent to 26.8 percent, Increased washer capacity would be a better solution to the problem of slow drainage because, as will be discussed later, this low freeness pulp does have some unique advantages. FPL 186 8

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12 Figure 5.--Average length of 50 whole kraft pulp fibers. M Strength Properties trees. Surprisingly, however, the pulp made from the 1-year-old whole trees was stronger than All of the pulps made in the digester were those made from either the 3- or the 5-year-old evaluated for strength development in a labora- whole trees. For example, at a burst factor of 55, tory beater and handsheets were made and tested the pulp made from the 1-year-old whole tree had for physical properties according to standard a tear factor of 64 while the pulps made from the TAPPI methods. These data are given in appendix 3- and 5-year-old whole tree had tear factors of table 3. about 58. The pulp made from the 1-year-old Summarized in figure 6 are burst and tear fac- whole tree was also unique in that very little tors of the unbleached pulps made from the whole refining energy was required to develop its trees. Original and final freeness, with beating bursting strength and in doing so there was only times, are also shown. As expected, the pulps a slight loss in tearing resistance. Comparable made from the 11- and 24-year-old whole trees results were also obtained from the bleached were superior to those made from the younger pulps made from the whole trees as shown in FPL

13 Figure 6.--Freeness, beating time, and strength of unbleached kraft pulps of whole hybrid poplar trees. M figure 7. The data for the 1-year-old whole tree pulp were obtained using an uncleaned pulp (appendix table 4, digestion 5041X). Centrifugal cleaning of this type of pulp prior to bleaching did not significantly affect the strength properties of the handsheets (appendix table 4, digestion 5051X). The burst and tear factors of the unbleached pulps made from the 11-year-old boles, both with and without bark, are compared to values obtained from the 11-year-old whole tree pulp in figure 8. Similarly, the strength values of the pulps made from the 24-year-old tree boles are compared to the values obtained from the 24-year-old whole tree pulp in figure 9. At both ages, similar results were obtained, although of different magnitude. The strongest pulps were those made from the bark-free boles and the weakest pulps were those made from the whole trees. For example, at a burst factor of 65 the 11-year-old bark-free bole pulp had a tear factor of 86, while the bole with bark pulp had a tear factor of 81 and the 11-yearold whole tree pulp had a tear factor of 63. Similarly, the 24-year-old bark-free bole pulp at a burst factor of 65 had a tear factor of 91 while the bole with bark pulp had a tear factor of 84, and the 24-year-old whole tree pulp had a tear factor of

14 Figure 7.--Freeness, beating time, and strength of bleached kraft pulps of whole hybrid poplar trees. M Optical Properties of Bleached Pulps pulp made from the bark-free boles of the 24 year-old tree. The opacity and specific scattering The optical properties of the bleached pulps coefficient values of all of the whole tree pulps made from the whole trees are given in table 4. are slightly higher than those of the pulp made Also included for comparison is the bleached from the 24-year-old bark-free bole. FPL

15 Figure 8.--Freeness, beating time, and strength of unbleached kraft pulps of 11-year-old hybrid poplar trees. M The most striking differences between pulps were in dirt count. Surprisingly, the bleached pulp made from the 24-year-old bark-free bole was only slightly cleaner than the pulp made from the 24-year-old whole tree. The cleanest pulp was made from the 11-year-old whole tree. The amount of dirt in the bleached pulps made from the 3- and 5-year-old whole trees was almost twice that found in the 24-year-old bark-free bole pulp. The uncleaned bleached pulp from the 1-year-old whole trees contained almost three times as much dirt as that found in the 24-yearold bark-free bole pulp. However, centrifugal cleaning of this pulp prior to bleaching reduced the amount of dirt to almost the same level as the 24-year-old, bark-free bole pulp. 13

16 Figure 9.--Freeness, beating time, and strength of unbleached kraft pulps of 24-year-old hybrid poplar trees. M FPL

17 Table 4.--Optical properties of bleached whole tree pulp handsheets 1 1 = 0.02 through 0.10 sq. m., 2 = 0.15 through 0.40 sq. m., 3 = 0.60 through 2.50 sq. m., 4 = larger than 2.50 sq. m. 2 Centrifugally cleaned before bleaching. 3 Boles without bark. CONCLUSIONS (1) A relatively high anual yield per acre was produced with 1-year-old hybrid poplar trees that could be harvested and chipped in the field with presently available farm machinery. A satisfactory pulp was produced from this material and inclusion of the bark posed no special problems in pulping. Centrifugal cleaning of this pulp prior to bleaching is an effective means of satisfactorily reducing the amount of dirt in the bleached pulp without significantly changing the strength properties of handsheets. One of the major advantages of this pulp was that little or no refining energy was required to develop the strength of the pulp. Disadvantages included the relatively low digester yield and possibly the low pulp freeness which would require increased washer capacity. (2) There was no apparent advantage either from the standpoint of productivity or pulp quality to 3- and 5-year rotations over 1-year rotations. (3) The highest overall yields per acre per year were obtained with an 11-year rotation and the pulp produced from bark-free boles was generally comparable to, commercial aspen kraft pulps. Inclusion of the bark, resulted in small increases in overall per acre per year yields and in slight reductions in pulp strength. Pulping the whole tree increased the per acre per year yield markedly but decreased the pulp strength significantly. (4) Although pulp from the 24-year-old tree was slightly stronger than that from the 11-yearold tree, there appeared to be no major advantage to rotations longer than 10 to 12 years. 15

18 LITERATURE CITED 1. Blackford, J. M Separating bark from wood chips. Forest Prod. J. 11(11): Erickson, J. R Bark removal after chipping--a progress report. Pulp and Pap. Mag. of Can. 71(3): Hair, D Use of regression equations for projecting trends in demand for paper and board. U.S. Forest Serv. Forest Resource Rep, No. 18, USDA, Washington, D.C. 4. McAlpine, R. G., Brown, C. L., Herrich, A. M., and Ruark, H. E Silage sycamore. Forest Farmer 26(1): Sanyer, Necmi, and Laundrie, J. F Factors affecting yield increase and fiber quality in polysulfide pulping of loblolly pine, other softwoods, and red oak. Tappi 47(10): U.S. Department of Agriculture Pulp yields for various processes and wood species. Forest Serv. Res. Note FPL-031, U.S. Forest Prod. Lab., Madison, Wis. 7. Wesner, A. L Vac-sink for recovery of pulpwood chips from wood-bark waste. Pulp and Pap. 36(17): Young, H. E The complete tree concept--a challenge and an opportunity. Soc. Amer. For. Annu. Meeting Proc., pp ACKNOWLEDGMENT Research was supported by the Forest Products Resources, Prof. J. E. Kuntz of the Department Laboratory, Forest Service, U.S. Department of of Plant Pathology, University of Wisconsin, par- Agriculture, and by the College of Agricultural ticipated in the initiation of the project and and Life Sciences, University of Wisconsin established three of the five plantations sampled Madison, with financial assistance and coopera- in this study, tion from the Wisconsin Department of Natural FPL

19 Appendix Table 1.--Kraft pulping 1 of 1-, 3-, 5-, 11-, and 24-year-old hybrid poplar trees 17

20 Appendix Table 1.--Kraft pulping 1 of 1-, 3-, 5-, 11-, and 24-year-old hybrid poplar trees--cont. FPL

21 Appendix Table 2. --Kraft pulping 1 of bark from 1-, 3-, 5-, 11-, and 24- year- old hybrid poplar trees 19

22 Appendix Table 3.--Pulp and handsheet properties of unbleached kraft pulps from , 11-, and 24-year-old hybrid poplar trees FPL

23 Appendix Table 3.--Pulp and handsheet properties of unbleached kraft pulps from , , and 24-year-old hybrid poplar trees--cont. 21

24 Appendix Table 4.--Handsheet properties of bleached kraft pulps from 1-, 3-, 5-, 11-, and 24-year-old hybrid poplar trees FPL

25 ABOUT THE FOREST SERVICE.... As our Nation grows, people expect and need more from their forests--more wood; more water, fish and wildlife: more recreation and natural beauty; more special forest products and forage. The Forest Service of the U.S. Department of Agriculture helps to fulfill these expectations and needs through three major activities: * Conducting forest and range research at over 75 locations ranging from Puerto Rico to Alaska to Hawaii, * Participating with all State forestry agencies in cooperative programs to protect, improve, and wisely use our Country s 395 million acres of State, local, and private forest lands. * Managing and p rot e c t i n g the 187-million acre National Forest System. The Forest Service does this by encouraging use of the new knowledge that research scientists develop; by setting an example in managing, under sustained yield, the National Forests and Grasslands for multiple use purposes: and by cooperating with all States and with private citizens in their efforts to achieve better management, protection, and use of forest resources, Traditionally, Forest Service people have been active members of the communities and towns in which they live and work. They strive to secure for all, continuous benefits from the Country s forest resources. For more than 60 years, the Forest Service has been serving the Nation as a leading natural resource conservation agency, 23

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