Improving Physical Properties of Kraft Hardwood Pulps by Copulping

Feb 21, 2013 - Wheat straw and corn stover were substituted for hardwood chips in the amounts of 10, 15, and 20 wt % in kraft pulping experiments whil...
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Improving Physical Properties of Kraft Hardwood Pulps by Copulping with Agricultural Residues Mikhail V. Levit,† Lenong Allison,‡ Jim Bradbury,§ and Arthur J. Ragauskas*,†,‡ †

School of Chemistry and Biochemistry and ‡Institute of Paper Science and Technology, Georgia Institute of Technology, Atlanta, Georgia, 30032, United States § NewPage Corporation, Wisconsin Rapids, Wisconsin, 54494, United States S Supporting Information *

ABSTRACT: Wheat straw and corn stover are promising biorenewable resources that can be utilized in the pulp and paper industry. This study examines changes in the physical strength of pulps made from blends of agricultural residue and hardwoods. Wheat straw and corn stover were substituted for hardwood chips in the amounts of 10, 15, and 20 wt % in kraft pulping experiments while keeping the H-factor constant. This substitution allows a maximum 29% increase in tensile index and 12% increase in tear index for unrefined samples containing wheat straw. Pulp yields and kappa numbers changed slightly with increasing woodchip replacement levels. Three substituted pulps were bleached using a relatively mild OD(E + P + O)D sequence, and the viscosities and sugar profiles were traced throughout the process. The strength improvement can be attributed to the increased xylan contents of pulps made with agricultural residue. Fully bleached pulps preserved the improved mechanical properties, and an attempt was made to correlate the xylan content with the degree of strength development.

1. INTRODUCTION Agricultural residues, derived from corn stover, wheat straw, sugar cane bagasse, and other crop waste products, present a valuable source of plentiful lignocellulosic biomass and have been investigated as feedstocks to optimize the convenience and energy efficiency of chemical and mechanical pulping.1−5 The results of these studies suggest that a number of viable routes can take advantage of pulping nonwoody biomass and that the technological process should then be adjusted to accommodate a specific feedstock. Moreover, alternative fiber sources should be chosen based on their availability in the geographic area where tailored pulp production will take place to reduce transportation costs. Improvements have also been achieved in the quality of the end products and the optimization of the pulping conditions.6,7 North America has a substantial variety of promising feedstock substitution options for alternative fiber pulping, despite the wide availability of woody biomass, which has historically made alternative fiber sources less attractive for industry. In recent years, environmental concerns8,9 and new competing demands for wood resources and land10,11 have revived interest in the utilization of nonwoody resources for pulp and paper manufacturing. In contrast, a principally different situation has evolved in the Far East, particularly in China, where the pulp and paper industry has traditionally utilized various nonwoody residues because of the scarcity of virgin wood fiber but has experienced an even more prominent increase in paper demand, which drove a number of recent research efforts.12−14 Another interesting technological development has been the discovery of xylan retention during kraft pulping, which imparts beneficial physical properties on kraft pulps. A recent study15 correlated the improvement of the mechanical properties of kraft pulps with the amount and molecular weight of xylan © 2013 American Chemical Society

added, which was accomplished by a comparison of the tensile index and tensile stiffness parameters of pulp samples made with different contents of xylan. Such findings suggest that incorporating xylan-rich agricultural residues (ag residues) should be beneficial for the development of strength properties in such composite pulps. Wheat straw (WS) and corn stover (CS) are two types of ag residues that are particularly abundant in the United States and especially in the Midwest region. There is already a fairly established technology for growing, fertilizing, harvesting, and processing wheat and corn, as well as equipment for baling, packaging, loading, and transporting the material. These two aspects, namely, positive influence of xylan on the mechanical properties of pulp and the availability of low-cost raw materials, were the basis for investigating the effects of pulping agricultural fiber rich in xylan blended with hardwood chips. Ag residue can principally be utilized in two different ways in papermaking, whereby it is pulped as a pure alternative fiber feedstock (such as wheat straw) and treated as a complete replacement of woodchips16 or pulped and bleached as batches of nonwood material and blended as the resulting ag pulps with conventional brownstock acquired from woodchips as furnish for the paper machine.4,17,18 No reported cases of chemical pulping blends of wood and nonwood feedstock together exist aside from one semichemical process studied in 2002.19 Specifically, this study indicated an increase in the strength properties of refined pulps made from blends of wheat straw and red oak chips with maxima at approximately the 30% Received: Revised: Accepted: Published: 3300

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sulfide (Na2S), and sodium carbonate (Na2CO3) were determined by titration with hydrochloric acid in so-called “ABC titrations”.22 The procedure is outlined in TAPPI Standard Test Method T 624 cm-85 for kraft and soda white and green liquors and in TAPPI Standard Test Method T 625 cm-85 for soda and sulfate black liquor. A modified version of this test was employed for these studies using an automatic titrator. A Mettler DL21 automatic titrator was used to perform a modified version of the ABC test for analysis of white and black liquor samples. This modified ABC test procedure was conducted using a series of acid titrations (hydrochloric acid) to determine the concentrations of HO−, HS−, and CO32− ions. The hydrochloric acid concentration was 0.500 N. Analysis of the metal contents of pulping liquors was performed using an OPTIMA 7300 DV inductively coupled plasma-optical emission spectrometer from Perkin-Elmer (Wellesley, MA). 2.4. Handsheet Making Procedures. Test handsheets were made from pulp by dewatering on a screen with 0.104mm openings. These sheets were pressed at 345 kPa for 300 s and then dried for 600 s on a plate dryer that was set at 105 °C. Eight handsheet samples were formed for each set conditions. The basis weight was 60.00 g/m2. All handsheets were conditioned according to TAPPI standard conditions before testing. 2.5. Bleaching Sequence. Three ag-residue-substituted pulps were bleached using a relatively mild OD(E + P + O)D sequence. Oxygen delignification (O) was performed for 1 h at 10% consistency and 90 °C at 690 kPa and a 1.5% charge of sodium hydroxide. The chlorine dioxide (D) stage was performed in a stirred batch reactor at 3.5% consistency and 50 °C for 0.75 h, at 0.20 kappa factor based on the washed kappa number. The alkaline extraction with hydrogen peroxide and oxygen (E + P + O) stage was carried out at 10% consistency and 75 °C for a total of 1.25 h in a Parr reactor with stirring. Sodium hydroxide (1.00%) and hydrogen peroxide (0.40%) were added to the preheated pulp in the mixer, which was then pressurized with oxygen to a gauge pressure of 240 kPa. The oxygen pressure was decreased to 0 kPa over a period of 0.25 h in 20 kPa increments. The pulp was held in the reactor for an additional 1 h. The chlorine dioxide brightening stage was conducted at 10.0% consistency in polyester bags using 0.50−0.80% ClO2 based on the oven dry weight of the pulp. The pH of the pulp was adjusted to provide a terminal pH of 3.0, so as to provide the best delignification results by promoting the rate of reaction between pulp and chlorite ion and decreasing the amount of residual chlorite ion. The bag was then sealed and transferred to a water bath maintained at 75.0 °C for 3 h. 2.6. Carbohydrate, Lignin, and Ash Contents. The carbohydrate profiles and lignin contents of the raw materials and pulps were measured as previously described in the literature.23,24 In brief, the material (0.175 g dry weight) was hydrolyzed with 72% sulfuric acid for 1 h, diluted to 3% sulfuric acid with water, and then autoclaved at 121.0 °C for 1 h. After this series of steps, the samples were filtered, and the residue was dried and weighed to give the Klason lignin content. The carbohydrates were quantified using high-performance anionexchange chromatography with pulsed amperometric detection (HPAEC-PAD). The system used was a Dionex ICS-3000 ion chromatograph with a CarboPacTM PA-1 column. The column was set at 23.0 °C, eluent A was 100% DDI water (18 MΩ·cm), and eluent B was 200 mM NaOH. The flow rate was 0.30 mL/

replacement level; however, the fate of the carbohydrates was not investigated, nor were any bleaching experiments reported. The main drawback of pulping ag residue has been the widely explored theme of transfer of inorganic components to black liquor and finally into the recovery boiler, which tends to be very case-specific at each particular mill.20,21 It depends on the source of the feedstock and might require the adjustment of the pulping parameters and recovery operations. Reported herein is a comprehensive study of relatively low substitution levels of ag residue for wood furnish. Pulping ag residue with mixed hardwoods resembles a conventional kraft cook and is shown not to affect the overall performance of the digester.

2. METHODS 2.1. Raw Materials and Feedstock. All chemical reagents used in this study were purchased from VWR International (Radnor, PA) and Sigma-Aldrich (St. Louis, MO) and used as received. The mixed hardwood chips were received from a pulp mill in the Midwest region of the United States. The wheat straw (Triticum aestivum L.) and corn stover (Zea mays L.) were collected at the Agricultural Research Station of the University of Wisconsin-Madison in Arlington, WI. The ag residue was air-dry and contained leaves, stems, and a small amount (less than 1.0 wt %) of seeds for WS and kernels or chopped fractions of cobs for CS. Both WS and CS were ground using a Wiley mill to pass through a 1.00-mm screen prior to being cooked to provide evidence for the fundamental aspect of dissolving the hemicelluloses. Distilled deionized (DDI) water was used in all instances when water was required. The woodchips were air-dried and screened through a 6.00-mm bar screen. The wood chips and ag residue were stored in a cold room at 4 °C before use to prevent the deterioration of components. 2.2. Kraft Pulping Conditions. Kraft pulping experiments were performed in an electrically heated, rotating, multivessel digester. The digester system was manufactured by Aurora Technical Products Ltd., Savona, BC, Canada. The ratio of ag residue to wood chips loaded into the digester was varied and is noted in Table 1. The pulping conditions were as follows: Table 1. Kraft Pulps Studied and Sample Abbreviations

a

pulp ID

feedstock compositiona

ag residue

HW100 WS10 WS15 WS20 CS10 CS15 CS20

100:0 90:10 85:15 80:20 90:10 85:15 80:20

− wheat straw wheat straw wheat straw corn stover corn stover corn stover

Ratio of mixed hardwoods to ag residue (% oven dry weight).

liquor/biomass ratio, 4:1; effective alkalinity (EA), 16.0%; sulfidity, 22.0%; maximum temperature, 165 °C; target Hfactor, 800. The kraft cook was interrupted at the appropriate H-factor, and each vessel was cooled immediately in a cold water bath. The cooked mixture of wood chips and ag residue was disintegrated in an industrial-type blender for 300 s and screened and washed in a valley-type screen. The screen had slots varying from 0.15 to 0.20 mm. The rejected fibers were those that did not pass through the screen. 2.3. Pulping Liquor Titrations and Analysis. The relative proportions of sodium hydroxide (NaOH), sodium 3301

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hardwoods, the lignin content of CS was considerably lower. This difference suggested the possibility for better interchangeability of wheat straw with woodchips in our study. 3.2. Kraft Pulping Studies. A series of initial kraft cooks were conducted employing three different replacement levels. A comparison of the essential pulp characteristics from the pulping experiments is presented in Table 3. The information

min. The ash contents of the feedstocks were determined according to TAPPI Standard Test Method T 211 om-07. 2.7. Viscosity Measurements. Pulp viscosity values were determined in accordance with TAPPI Standard Test Method T 230 om-94 “Viscosity of Pulp (capillary viscometer method)”. The moisture content was determined for air-dried pulp and was used to weigh 0.2500 g (oven dry) of pulp. The weighed pulp was solvated with cupriethylenediamine and passed through two Cannon Fenske size-150 viscometers at 25.0 °C. The viscometers were carefully cleaned with nitric acid, water, and acetone and dried between measurements. Two separate viscometer readings were obtained for each sample, and each sample was run twice (total of four viscometer readings). The standard deviation for the brownstock pulps was 0.38 cP for four replicates of one pulp, and the standard deviation for different pulps obtained under the same pulping conditions was 0.45 cP. 2.8. Physical Property Testing. The tensile, tear, and burst properties of the handsheets were tested according to TAPPI standards. The samples were kept and analyzed under a controlled environment in accordance with TAPPI standard conditions. Tensile index measurements were automatically performed using a QC 1000 tensile tester from Thwing-Albert Instrument Co. (Philadelphia, PA) for all samples except the fully bleached pulps, for which a Lorentzen & Wettre tensile tester with fracture toughness was used. For tear parameter measurements, an Elmendorf tearing tester from Thwing-Albert Instrument Co. was used. The errors did not exceed 4.0% between samples and are included in the tables of results.

Table 3. Results Obtained for Brownstocks from Cooks to Target H-Factor of 800

Table 2. Chemical Compositions of Raw Materials Utilized in the Study hardwood

wheat straw

corn stover

sugar profile (% of carbohydrate fraction) arabinose galactose glucose xylose mannose klason lignin (% content of feedstock) ash content (%) material density (kg/m3)

0.9 1.4 68.4 26.3 3.1 22.85 0.50 176.4

3.3 1.0 63.7 32.0 − 22.41 5.17 267.3

4.6 2.2 60.9 32.3 − 14.96 7.38 249.8

kappa number

viscosity (cP)

screened yield (%)

total yield (%)

HW100 WS10 WS15 WS20 CS10 CS15 CS20

18.1 19.0 21.7 22.8 21.9 23.1 22.5

18.0 17.2 19.1 20.2 16.3 17.4 19.9

41.97 42.03 43.04 43.64 41.78 43.54 44.39

42.19 42.89 43.26 43.87 41.96 44.03 44.67

indicates that replacement levels of woodchips with ag residue even as low as 10% led to an increase in kappa number relative to that of the control hardwood chips sample. This phenomenon continued to develop further with increasing ratio of WS or CS to wood in the mixture. The unexpected decrease in kappa number value from sample CS15 (23.1) to sample CS20 (22.5) might be due to the initial relatively low lignin content in corn stover, so that, after the kraft pulping process, the difference in residual lignin in samples CS15 and CS20 can be considered to be negligibly small given the potentially different responses of the other biomass components to pulping. This hypothesis must be further verified. Beginning with 15% ag-residue substitution, the viscosities of the pulped mixtures of material began to rise, and an increase of almost 26% was reached for sample WS20 relative to the pure hardwood control. This observation needs to be taken into consideration when the scaled-up process is carried out, because of the changes that it introduces into the fluid flow in papermaking. It can be speculated that such changes occur primarily as a result of the redistribution of xylan and the increase in hemicellulose/cellulose ratio in pulps containing ag residue. The screened yields of ag-residue-substituted pulps not only were preserved but also showed a slight increase compared to the control sample containing 100% hardwoods. The amount of rejects after screening in each case was 1.0% or less, which confirms that the choice of H-factor was reasonable and that virtually no undercooking took place. Although it could be argued that decreasing the H-factor would lead to extended efficiency in terms of time and energy savings, our experiments were optimized for comparisons between the control sample and ag-residue-substituted ones. Thus, it is evident that such conditions are suitable for copulping and lead to higher overall yields, which means even a potential pulp gain. As previously mentioned, the xylan content is a crucial parameter responsible for interfiber bonding and, consequently, could increase physical strength properties for ag-containing samples. The carbohydrate (sugar) profiles of brownstock collected after cooking are presented in Table S1 (Supporting Information). The xylose levels in the hydrolysates increased gradually with the addition of ag residue and assumed values somewhat intermediate between those of the raw materials

3. RESULTS AND DISCUSSION 3.1. Feedstock Analysis. The initial phase of this work consisted of detailed characterization of the acquired feedstock, as reported in Table 2. These results generally correlated with

component

pulp ID

the compositional analyses of WS and CS reported in previous works by various authors.4,13,25,26 Nevertheless, because of its high dependency on the geographic region of growth, weather conditions, soil peculiarities, and harvesting and storage procedures, the composition of a feedstock varies to a certain extent and needs to be experimentally determined.25,26 For both WS and CS, only trace amounts of mannose were present in the hydrolysates. However, significant differences with respect to chemical composition were observed between the different feedstocks such that both ag residues had xylan contents about 23% higher than that of hardwood furnish and, whereas WS had a lignin content very close to that of 3302

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depending on the replaced percentage of wood chips. These results indicate that our pulping conditions did not have a detrimental effect on overall xylan preservation. 3.3. Kraft Cooking Liquor Analysis. Black free liquors were collected at the end of the cook and titrated to determine the final alkalinity and sulfidity. In general, the active alkali (AA) and effective alkali (EA) values were very uniform regardless of the feedstock composition (Table S2, Supporting Information). This is a favorable characteristic of the proposed copulping process because it suggests suitability for the typical kraft mill recovery system to a first approximation. It also emphasizes that applying the same white pulping liquor to slightly different mixtures of woodchips and ag residue produces pulps with different properties. Therefore, the composition of pulping chemicals remains fixed, which is beneficial from the perspective of mill operation. Along with these advantages, the similar sulfidity and alkalinity of the used pulping liquors can potentially make it possible to meet environmental requirements without changing the existing pulping infrastructure. Another important characteristic of black liquors is the higher heating value (HHV). The ag-residue-substituted cooks produced liquors with HHVs that were somewhat lower than that of the control sample, although the values were relatively close, which would imply the same order of magnitude should the liquors from the ag-residue-substituted materials be streamed to the regular recovery system, as summarized in Table S3 in the Supporting Information. The concentrations of metals in the black liquors were also analyzed, as this parameter provides valuable information for evaluating possible chemical recovery options. In particular, the silicon content of black liquor significantly affects the feasibility of using ag residues in the kraft pulping process. The experimental results summarized in Table S3 (Supporting Information) demonstrate that even a 10% substitution of woodchips for wheat straw led to a nearly 2-fold increase in silicon content of black liquor and the trend continued, with nearly 4 times as much silicon in sample WS20 compared to the 100% hardwood control. Such an evident transfer of silicon from the ag residue into the pulping solution can be explained by the high silicon content initially present in wheat straw. 3.4. Physical Properties of Unbleached Pulps. Differences in tensile strength between the control sample and the samples with ag-residue substitution were also evaluated. This comparison served as evidence for the feasibility of pulping mixed hardwoods and ag residue together in one digester and further established the basis for a quantitative determination of the positive effects in strength improvement that copulping provides. All brown pulps were tested unrefined, and a significant increase in tensile strength was found for both types of ag residue, as summarized in Table 4. Substitution with wheat straw seemed to have a more pronounced effect than substitution with corn stover, as the improvements in tensile index were 11.7% and 9.2%, respectively. Although the maximum effect was achieved at higher replacement levels, 10%- and 15%-substituted pulps would still be an attractive alternative to industry given that the costs of ag residue are much lower than those of virgin wood. 3.5. Bleaching Studies: Sugar Profiles and Viscosity. Bleaching conditions were chosen according to the recommendations found in the literature to accommodate the relatively mild treatment of pulps made of ag residue and

Table 4. Mechanical Testing of Handsheets Made of Unbleached Unrefined Pulps pulp ID HW100 WS10 WS15 WS20 CS10 CS15 CS20

tensile index (N·m/g)

breaking length (km)

± ± ± ± ± ± ±

6.65 6.73 6.84 7.42 6.71 6.81 7.27

65.2 66.0 67.1 72.8 65.8 66.8 71.2

1.2 1.1 1.2 1.4 1.3 1.1 1.5

hardwood chips.27 A control sample and pulps with the highest xylan contents (WS20, WS15, and CS20) were selected for bleaching investigation to determine the extent to which the positive effect of strength improvement would be preserved. Brightness, as the target parameter for fine paper grades, was traced throughout the bleaching sequence (Figure 1) with measurements after oxygen delignification, after alkaline extraction with hydrogen peroxide and oxygen, and after the entire process. These results represent the relative effectiveness of the bleaching stages in terms of the ISO brightness gains for different pulps. Carbohydrate analysis was performed for two intermediate bleached pulps and for the fully bleached samples, and these results are presented in Table S4 (Supporting Information). It is obvious that the amount of xylose in the pulps increased with the increase in the initial ag-residue contribution to the starting mixture. Oxygen delignification had the strongest effect on the sugar compositions of the pulps in general compared to brownstock, even though the conditions for this stage were relatively mild. Compared with the control sample of mixed hardwoods, the samples with ag residue exhibited an average xylan increase of about 2.0%, which is believed to be responsible for the strength improvement of handsheets. Viscosity measurements exhibited a drop for bleached pulps compared to brownstock. The measured values of viscosity after each stage are presented in Table S5 (Supporting Information). It can be seen that the ag-residue-substituted pulps all had lower viscosities, and a trend was observed in which the ag residue was responsible for the lower viscosities of the bleached pulps, with the most pronounced effect in oxygen delignification stage. Although the initial viscosities of brownstock containing ag residues were higher than those of hardwoods alone, the bleaching treatment caused ag-residue fibers to respond more readily. Overall, the viscosities of the agresidue-substituted pulps were 20% lower than that of the control sample. 3.6. Physical Properties of Bleached Pulps. The fully bleached pulps in this study represent the end product; therefore, the tensile, tear, and burst properties were measured for each sample (Table 5). As expected from the brownstock data and carbohydrate analysis, ag-residue-containing pulps all exhibited improved mechanical properties. An unexpected observation was the higher tear strength of sample WS15 compared to sample WS20. This trend could not be tested for the CS series because it was not selected for bleaching experiments, but it might be related to a favorable interaction of hardwood fibers with ag residue resulting in an actual maximum in tear strength as a function of substitution level. 3303

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Figure 1. ISO brightness values of pulps collected after several different bleaching stages.

Table 5. Summary of Mechanical Testing of Fully Bleached (ISO Brightness 89) Pulps tensile fully bleached pulp ID

tensile strength (kN/m)

HW100 WS20 WS15 CS20

1.253 1.763 1.553 1.645

tear

tensile index (N·m/g)

breaking length (km)

tear strength (mN)

± ± ± ±

1968 2728 2421 2522

60.52 67.74 75.79 78.45

19.3 26.8 23.7 24.7

0.8 1.0 0.9 0.8

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4. CONCLUSIONS A comprehensive original study on employing ag residue for copulping with mixed hardwoods has been conducted. The feasibility of pulping under such conditions was demonstrated, and a trend was observed for the selected pulps indicating improvements in physical properties with increasing ag-residue substitution levels up to 20%. This phenomenon can be attributed to the increased xylan contents of the resulting pulps. The improvement in tensile strength carried through the oxygen delignification stage and a three-stage bleaching sequence to achieve a target brightness of 89 ISO; therefore, such pulps can be used for the manufacture of fine papers. However, the viscosities and glucose contents of ag-reinforced pulps were found to be lower than those of the control sample, which might be indicative of a decrease in the degree of polymerization. Further investigation will be aimed at explaining the origin of this phenomenon.

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tear index (mN·m2/g)

burst strength (kPa)

± ± ± ±

75.0 88.3 86.5 85.6

0.932 1.028 1.139 1.179

0.06 0.04 0.05 0.05

burst index (kPa·m2/g) 1.15 1.34 1.32 1.29

± ± ± ±

0.04 0.05 0.04 0.06

ACKNOWLEDGMENTS

The authors acknowledge financial support from the PSE Fellowship program at IPST@GT and NewPage Corporation. This work is part of M.V.L.’s requirements for the degree of Ph.D. at Georgia Institute of Technology.

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REFERENCES

(1) Leponiemi, A. Non-wood pulping possibilities - a challenge for the chemical pulping industry. Appita J. 2008, 61 (3), 234−243. (2) Diaz, M. J.; Eugenio, M. E.; Lopez, F.; Alaejos, J. Paper from olive tree residues. Ind. Crops Prod. 2005, 21, 211−221. (3) Rodriguez, A.; Moral, A.; Serrano, L.; Labidi, J.; Jimenez, L. Rice straw pulp obtained by using various methods. Bioresour. Technol. 2008, 99, 2881−2886. (4) Jimenez, L.; Rodriguez, A.; Perez, A.; Moral, A.; Serrano, L. Alternative raw materials and pulping process using clean technologies. Ind. Crops Prod. 2008, 28, 11−16. (5) Lopez, F.; Alfaro, A.; Garcia, M. M.; Diaz, M. J.; Calero, A. M.; Ariza, J. Pulp and paper from tagasaste. Chem. Eng. Res. Des. 2004, 82, 1029−1036. (6) Jimenez, L.; Maestre, F.; DeLaTorre, M.; Perez, I. Organosolv pulping of wheat straw by use of methanol−water mixtures. Tappi J. 1997, 80 (12), 148−154. (7) Jimenez, L.; Ramos, E.; Rodriguez, A.; De la Torre, M. J.; Ferrer, J. L. Optimization of pulping conditions of abaca. An alternative raw material for producing cellulose pulp. Bioresour. Technol. 2005, 96 (9), 977−983. (8) McKinley, D. C.; Ryan, M. G.; Birdsey, R. A.; Giardina, C. P.; Harmon, M. E.; Heath, L. S.; Houghton, R. A.; Jackson, R. B.; Morrison, J. F.; Murray, B. C.; Pataki, D. E.; Skog, K. E. A synthesis of current knowledge on forests and carbon storage in the United States. Ecol. Appl. 2011, 21 (6), 1902−1924. (9) Hurtt, G. C.; Chini, L. P.; Frolking, S.; Betts, R. A.; Feddema, J.; Fischer, G.; Fisk, J. P.; Hibbard, K.; Houghton, R. A.; Janetos, A.;

ASSOCIATED CONTENT

S Supporting Information *

Detailed carbohydrate analysis data for brownstock and partially and fully bleached pulps, results of titration of pulping liquors, higher heating values and metal contents of liquors, and viscosities of pulps. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 3304

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Industrial & Engineering Chemistry Research

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dx.doi.org/10.1021/ie303095s | Ind. Eng. Chem. Res. 2013, 52, 3300−3305