Evaluation of the co-product pulp from Salix viminalis energy crops

b i o m a s s a n d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 3 4 2 e1 3 4 7

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Evaluation of the co-product pulp from Salix viminalis energy crops Jean-Michel Lavoie*, Eva Capek-Menard, Esteban Chornet Industrial Research Chair on Cellulosic Ethanol and 2nd Generation Biofuels, Department of Chemical Engineering, Universite´ de Sherbrooke, Sherbrooke, Que´bec, Canada J1K 2R1

article info

abstract

Article history:

Salix viminalis is among the species that are considered as promising for future energy

Received 12 January 2009

crops in marginal lands throughout Canada. Whilst research conducted on Salix species is

Received in revised form

oriented, to a large extent, towards the hydrolytic production of sugars for fermentation,

14 March 2010

we have considered co-product fibre since it adds potential value to an integrated use of

Accepted 16 April 2010

the biomass. In this study, extractives, lignin, hemicellulose, holocellulose and a-cellulose have been quantified using ASTM or TAPPI standard methods. Carbohydrates found in the

Keywords:

hemicellulose were also quantified using HPLC. Hemicellulose comparison for one and

Biomass

three years old samples showed that the xylose content increases with age during this

Biorefinery

growth period. Kraft pulp has been produced at a bench scale (few kg range) using estab-

Cellulose fibre

lished pulping conditions. The pulp was tested following ATTPC standard methods. Pulp

Energy crops

yields were of 29% and 34% for one and three years old samples respectively. The average

Kraft pulping

fibre lengths were shorter for the younger samples, 0.35 mm, as compared with the older

Salix viminalis

samples, 0.41 mm. Short fibres are not necessarily related to the species as much as to the fact that the samples used for the pulping were from young wood. When compared to industrial pulp, the basket willow pulp showed mechanical properties comparable to hardwood pulp which implies that this pulp could eventually be used in similar applications. Comparison with other energy crops shows advantages from the perspective of using this energy crop in Canada to co-produce pulp and, following suitable hydrolysis, sugars for fermentation to ethanol. ª 2010 Elsevier Ltd. All rights reserved.

1.

Introduction

The concept of lignocellulosic biorefineries is gaining renewed attention for the production of biofuel but mostly ethanol and biodiesel. Even if cellulosic ethanol does not compete with food resources, it might, on a longer period, be a concern for the forest conservation. Foreseeing such a problem, research has been oriented toward the so-called “energy crops” which would produce lignocellulosic material on low quality soil while contributing to CO2 fixation. Among the species

considered for this purpose, Salix viminalis, a fast growing woody shrub has shown significant growth potential in Canadian marginal lands [1]. Combining biofuels with value added fibre could be an approach that easily links with existing industry. In the last few years a few species have been investigated for their potential as energy-crops. Specific studies have been reported on Arundo donax, Cynara cardunculus, Miscanthus sinensis, Panicum virgatum, Sorghum bicolor [2], Eucalyptus gunnii, Populus trichocarpa and S. viminalis [3]. The latter is of particular

* Corresponding author. Tel.: þ1 819 821 8000/65505; fax: þ1 819 821 7955. E-mail address: [email protected] (J.-M. Lavoie). 0961-9534/$ e see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2010.04.023

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interest in many areas of Canada since it has demonstrated efficient growth potential and soil adaptability. Over the years, multiple studies have been made on this species and in particular in its potential for CO2 fixation and its use for decontamination of heavy-metal-rich soils. On this subject, recent work by Adler et al. showed the effects of different types of bio-fertilizer on the metal content found in S. viminalis [4]. While the latter article was oriented mostly toward the cropping conditions, other researchers have oriented their efforts toward the conversion of such a biomass. Sassner et al. worked on the conversion of S. viminalis first using SO2 [5] and more recently using H2SO4 [6]. Whilst both their goals and process steps used are different from those presented in this manuscript, several points are relevant. Adler et al. looked for the defibration of the lignocellulosic material to make it suitable for enzymatic treatment to eventually produce ethanol. Even though they used steam explosion, they used cooking times varying from 4 to 12 min and an acid catalyst which produces a significant hydrolysis of the cellulose fibre [4]. Production of glucose is a major target for the willow crops since it leads to ethanol production via fermentation processes that can handle both the hexoses and the pentoses. But the cellulose itself, as fibre, is also valuable and could be, in a biorefinery process, an added value co-product target of the integrated conversion process. The present research is oriented toward the conversion of S. viminalis into added value products, among which its pulp potential will be investigated. Even though the utilization of willow species have been considered before in literature [7,8], the potential of young crops ranging from 1 to 3 years old has not been, to our knowledge, investigated so far.

2.

Materials and methods

2.1.

Sampling

The 1 and 3 years old samples of S. viminalis were obtained from Agro E´nergie in St-Roch de l’Achigan (Que´bec, Canada). Same species have been reported to produce close to 4528 tonne per km2 [1] although, in the actual case, samples of S. viminalis produced an average of 1200 dry tonne per km2. The one year old samples of Salix viminalis were harvested in   the municipality of St-Roch de l’Achigan (45 040 44.4900 N, 74 110 11.5400 O with 55 m of elevation). The one year old sample had an average diameter of 11.22 mm, average height of 1.82 m and the average weight of each plant was of 0.31 kg. The bark/wood/foliage ratio was not evaluated during this research although previous reports showed that the bark/ wood ratio for one year old shot was close to 28%wt. [4]. The three years old samples of S. viminalis were harvested in the  municipality of Huntingdon (Que´bec, Canada) (45 490 59.6100 N,  73 380 12.1700 O with 52 m of elevation). The three years old sample had an average diameter of 33.71 mm, average height of 4.90 m and the average weight of each plant was of 3.66 kg. The bark/wood/foliage ratio was not evaluated during this research although previous reports showed that the bark/ wood ratio for one year old shot was close to 25%wt. [4]. Samples were chipped to 3  3  1 cm right after harvesting (October 2007), placed in separated containers and then

Table 1 e Identification of the test and the standard techniques used for characterization of Salix viminalis pulp. Test

Standard technique identification

Pulp disintegration Freeness Determination Laboratory Screening of Pulp (Pulmac-Type Instrument) Fibre classification (Bauer-McNett) Fibre Length by Automated Optical Analyzer Using Polarized Light Forming Handsheets for Physical Tests with Pulp Forming Handsheets for Optical Tests with Pulp (British Sheet Machine Method) Grammage Brightness Colour Measurement with a Diffuse/Zero Geometry Tristimulus Reflectometer Opacity Thickness and Apparent Density Internal Tear Resistance Bursting Strength Length of rupture Module of elasticity TEA

ATPPC C.10 ATPPC C.1 ATPPC C.12 ATPPC C.5U ATPPC B.4P ATPPC C.4 ATPPC C.5

ATPPC D.3 ATPPC E.1 ATPPC E.5

ATPPC ATPPC ATPPC ATPPC ATPPC ATPPC ATPPC

E.2 D.4 D.9 D.8 D.34 D.34 D.34

transported to our facilities at the University de Sherbrooke. Once at destination, the samples were homogenized manually then transferred in 1 kg air tight plastic containers which were stored at 15  C until utilization. Since the overall potential of the plant was considered during this experiment, the tissues from the trees were not separated one from the other and were evaluated as a whole although the 1 kg samples were homogenized manually before any transformation.

2.2.

Extract content

Extractions were performed following the ASTM D1107-56 protocol. Biomass was grinded again then sieved at

Table 2 e Identification of the extractibles, ash, holocellulose, a-cellulose and lignin for the 1 and 3 years old sample of Salix viminalis. Analysis

1 YEAR Dry wood, wt.%

3 YEAR Dry wood, wt.%

3.40 5.40 1.71 63.73 29.16 30.23

2.82 5.05 1.34 64.50 27.50 30.99

Extractibles EtOH/Tol Extractibles H2O Ash Holocellulose Lignin a-cellulosea a Comprised in holocellulose.

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Table 3 e Identification of the carbohydrates found following the hydrolysis of the hemicellulose of 1 and 3 years old of Salix viminalis.

Table 5 e Colour measurement with a diffuse/zero geometry tristimulus reflectometer of the 1 and 3 years old samples of Kraft pulp produced from Salix viminalis.

Carbohydrates

Sample

1 YEAR Dry Hydrolysate, wt.%

3 YEAR Dry Hydrolysate, wt.%

36.62 15.84 9.62 37.83

20.59 7.50 9.54 62.35

Arabinose Galactose Glucose Xylose

0.2e0.5 mm after what it was placed in extraction thimbles in a Soxhlet and extracted with a mixture of ethanol and toluene (50/50) for a minimum of 4 h. The extracts were weighted after removal of the solvent and complete drying. The extracted biomass is rinsed then extracted again by Soxhlet using water. As for the previous extraction, the water extraction must be performed for a minimum of 4 h.

2.3.

Ash content

Ash content was obtained using the TAPPI T-15 standard method. Biomass, previously sieved at 0.5 mm was heated at 575  C for a minimum of 3 h. Samples were then dried in a desiccator filled with drierite until they reached room temperature.

2.4.

Hemicellulose

a*

b*

63.88 71.45

2.61 2.18

12.97 12.42

at a temperature varying from 95 to 100  C the solution was filtered and then neutralized with sulphuric acid instead of acetic acid. The solution resulting from the first part of this analysis should have a high content of dimeric, trimeric and/or oligomeric carbohydrates structure; therefore it is hydrolyzed again with a mixture of 2 wt.% H2SO4 for 60 min at 120  C. Quantification of the sugars was made by a Dionex HPLC system model DX-500 equipped with an AS50 autosampler, a GP50 pump and an ED40 electrochemical detector. Column used to this purpose was a Dionex PA-10 (4  250 mm) with a guard column model Dionex PA-10 (4  50 mm). Detection was made with a pulsed amperometry detector (PAD). Elution was made with a 4 mM of an aqueous mixture of sodium hydroxyde with a 1.0 mL per minute flow. Qualification of carbohydrates was made by comparison to pure compounds. Arabinose, glucose, galactose, xylose and mannose were all bought from Sigma. Quantification was made using fucose (also bought at Sigma) as an internal standard.

2.6.

Holocellulose

Quantification of the holocellulose was made using the TAPPI standard method T-9 (similar to ASTM D-1104). To obtain this value, the lignin part of the biomass was depolymerized and solubilised using sodium chlorite and acetic acid. The remaining fraction contained both cellulose and hemicellulose which was then dried and weighted.

2.5.

1 Y.O. 3 Y.O.

L*

a-Cellulose

Quantification of a-cellulose was made using holocellulose as starting material. The technique used for this purposed is the ASTM method D1103-60 (similar to TAPPI T203 05-1974) which implies the solubilisation of shorter chains of cellulose and degraded cellulose using a sodium hydroxide mixture.

2.7.

Lignin

Lignin was quantified using the ASTM D1106-56 method. The technique requires the solubilisation of carbohydrate in an

Isolation of the hemicellulose was made using the TAPPI standard method T-212. The wood samples were treated with a solution of diluted alkali (1 wt.% NaOH) and after 1 h cooking

Table 4 e Mechanical and optical properties of the Kraft pulp produced from 1 and 3 year old samples of Salix viminalis. Standard test Pulp yield ATPPC C.1 ATPPC C.12 ATPPC D.3 ATPPC E.1 ATPPC E.2 ATPPC D.4 ATPPC D.9 ATPPC D.8 ATPPC D.34 ATPPC D.34 ATPPC D.34

1 YEAR

3 YEAR

28.59% 412 mL 0.72% 60.2 gm2 24.4% 99.8% 1.85 cm3 g1 3.41 mN*m2 g1 1.86 kPa*m2 g1 4.61 km 1.44 MPa 26.6 Jm2

33.64% 454 mL 0.16% 60.1 gm2 33.3% 99.5% 1.79 cm3 g1 3.14 mN*m2 g1 1.56 kPa*m2 g1 4.24 km 1.19 MPa 19.6 Jm2

Fig. 1 e Fibre length of Kraft pulp obtained from Salix viminalis by automated optical analyzer using polarized light.

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aqueous mixture which implies a preliminary hydrolysis with a strong inorganic acid. The unaltered lignin is then dried and weighted.

2.8.

Kraft pulping

Kraft pulping was made using an adapted version of a laboratory scale pulping process described by Smook [9]. In a reactor, biomass was combined with a solution of NaOH (60 gL1), Na2CO3 (11 gL1) and Na2S·9H2O (71 gL1) and cooked at a rising temperature until it stabilizes at 160  C, temperature at which the system was held for 3 h or until the H factor reaches 1500. The biomass was processed directly as produced in the field without any other size reduction process. Since the biomass was not debarked and that it was estimated that its lignin content was higher than normal hardwoods, cooking procedure used in this situation was comparable to that of softwood as described by Smook [9].

2.9.

Pulp and paper testing

Pulp potential of S. viminalis was made according to standard ATPPC test. Name of the tests and the identification of the related standard test are mentioned in Table 1.

3.

Results and discussion

The properties of pulp for an energy crop as S. viminalis are closely related to the macromolecular composition of its tissues. Whilst cellulose has a direct impact on the pulp yield, hemicelluloses and lignin will influence both the pulp and the chemical content of the black liquor. Table 2 shows the extract, ash, holocellulose, lignin, and cellulose content of both 1 and 3 years old samples of S. viminalis. Hemicelluloses content for the two samples is reported in Table 3. As shown in Table 2, extractives content is overall higher in younger samples accounting for a higher quantity of primary and secondary metabolites found in the sap. The primary metabolites are used for the production of tissues while secondary metabolites should be used mainly for defence since the plant is vulnerable in its early growth stage. Qualification and quantification of extractives will be done in a forthcoming research project. The differences that were observed in the macromolecular compounds (lignin, holocellulose and a-cellulose) can be linked to the occurrence of bark and foliage in the feedstock. Lignin is more concentrated in bark and the latter represent a higher volume in one year old samples than in three years old samples. Comparable investigations have been made in literature (a heterogeneous feedstock composed of Salix schwerinii and S. viminalis of unmentioned ages). The lignin content was reported as 26.4 wt.% of the anhydrous mass while ash content was reported to be less than 1 wt.% [6]. The proportion of xylose tends to increase in the total hemicelluloses fraction in relation with maturation. Whilst the glucose concentration tends to stay relatively unchanged, the contents of arabinose and galactose tend to decrease. Comparable rates of carbohydrates in hemicelluloses have been reported recently by Kovacs [10].

Fig. 2 e Fibre length of Kraft pulp obtained from Salix viminalis by the Bauer-McNett classification.

As for the production of kraft pulp from S. viminalis, rates as well as mechanical test are mentioned in Table 4 and Table 5 while fibre lengths are depicted in Fig. 1 and Fig. 2. Production of kraft pulp was higher with the 3 years old samples which can, once again, be directly linked to the occurrence of a higher proportional concentration of bark in the 1 year old samples. But even though the production of pulp was more efficient with 3 years old samples, the overall mechanical properties of the pulp appear superior for the 1 year-old sample as shown by the burst strength, the internal tear resistance, the length of rupture, the module of elasticity and

Table 6 e Mechanical properties of Salix viminalis kraft pulp from 1 to 3 years old samples in comparison to common industrial gradea kraft pulp. Standard test

1 3 Birch Poplar Maple Loblolly Jack YEAR YEAR Pine Pine

ATTP C.1 412 454 449 409 317 (mL) ATPPC B.4P 0.35 0.41 0.39 0.39 0.38 (mm) ATPPC E.1 24.4 33.3 78.9 24.7 60.2 (%) 1.85 1.79 1.69 2.17 1.80 ATPPC D.4 (cm3g1) ATPPC D.9 3.41 3.14 4.25 4.09 3.69 (mN*m2 g1) 1.86 1.56 1.98 1.35 2.23 ATPPC D.8 (kPa*m2 g1) ATPPC D.34 4.61 4.24 2.11 2.67 1.84 (km) a Source: Domtar [11].

636

484

1.0e1.2 90

0.79 90

1.84

1.68

5.99

4.47

0.95

1.09

e

3.65

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Table 7 e Mechanical properties of Salix viminalis kraft pulp from 1 to 3 years old samples in comparison to ‘energy crop’ pulps made from Populus (13) and Miscantus (14) species. Species P. alba x P. grandidentata P.deltoides x P. maximowiczii P. deltoides Miscanthus x giganteus Salix viminalis 1 Y.O. S. viminalis 3 Y.O.

ATPPC C.1 ATPPC B.4P mL mm

ATPPC D.34 Jm2

ATPPC D.9 mN*m2g1

ATPPC D.34 %

ATPPC D.8 kPa*m2g1

ATPPC E.1 %

ATPPC E.2 %

515

0.78

168

8.1

e

e

e

e

432

0.65

148

6.3

e

e

e

e

505 e

0.76 e

196 e

7.7 5.7

e 2.7

e 2.7

e 45.7

e 86

412 454

0.35 0.41

3.41 3.14

1.44 1.19

1.86 1.56

24.4 33.3

99.8 99.5

26.6 19.6

the TEA (Table 4). The 3 year-old pulp has a higher level of freeness and a lesser amount of residues upon pulping. The colour of the pulp seems to be different for the older samples which are also brighter (Tables 4 and 5) than for the younger samples since brightness is at 33.3% for the 3 years old sample compared to 24.4% for the younger samples. The L*a*b* colour test also shows comparable values for the a* and the b* indexes while the L*, related to brightness, is higher for the 3 years old sample. (Figs. 1 and 2). Usually, in terms of pulp and paper, the major problem encountered when using young shoots as for S. viminalis, is that the length of the fibres is generally short since the tree is in his early stages of development and therefore the cellulose fibres had not had the chance to be as developed as in older specimens. Figs. 1 and 2 provide data on the fibre length according to the automated optical analyzer and the BauerMcNett classification. Both tests corroborate and show that the older specimen has a significant higher amount of long fibres thus, to a certain extent; it improves its pulp potential. The ponderated average lengths of the fibres are of 0.35 for the younger and 0.41 mm for the older specimens. Table 6 shows

a comparison of the S. viminalis kraft pulps in comparison with commercial softwood and hardwood species. Even if the length of the fibres are lower than the common softwood species, the values observed for S. viminalis are still comparable to hardwoods such as birch, poplar or maple even though the willow samples were likely much younger than the other species. Brightness seems to be much lower for the willow but no mention is made in the references as to the level of the lignin still present in the fibre which might be a cause for lower levels of brightness in willow. The tearing resistance of willow pulp was generally lower than the other commercial species while the bursting strength was shown to be higher. The variation of both these factors could be related to the shorter fibre length. The results for the length of rupture are quite peculiar since there are significantly higher for S. viminalis not only when compared to hardwood but also softwood. As for other ‘energy-crops’, few data are available in literature. Nevertheless, it was still possible to compare S. viminalis in this case with two other genus; Miscanthus [13] and Populus [12]. The poplar species that were used in the latter study were at least 7.5 years old [12] which has a direct influence on fibre length and as a consequence, on the mechanical properties of the pulp. The maturity of the samples might be the major reason why the results obtained from poplar and willows are far apart in Table 7. The Miscanthus giganteus samples used for pulping by Cappelletto et al. [13] were composed of significatively longer fibres (see Fig. 3) which might be one of the reasons explaining why the mechanical properties were higher for this pulp. As for the optical difference it is simply related to the fact that the Miscanthus pulp considered in Table 7 was bleached. The reason explaining the higher content of fibre is only partially related to the species since these researchers had developed and applied a sieving technique allowing the recuperation of a higher number of long fibres.

4.

Fig. 3 e Fibre length of Kraft pulp obtained from Salix viminalis and Miscanthus giganteus by the Bauer-McNett classification.

Conclusion

It has been shown in this study that S. viminalis, can provide cellulosic fibre derived from kraft pulping. Although the results from the mechanical test made on the pulp were comparable to those of mature hardwoods, a net advantage of the willow would be its fast growth as well as a high biomass

b i o m a s s a n d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 3 4 2 e1 3 4 7

production. Pulp yields ranged between 29 and 35% and the biomass was not sieved before the pulping process. Other energy crops mentioned in the literature have often shown higher mechanical properties using older specimens and/or sieved biomass. Even if the kraft pulping-only approach for such feedstock will not be sufficient to justify an industrial pulping process, the production of cellulose fibre as an added value co-product in a biorefinery process aimed at biofuels and/or green chemicals has merit and this study proves to be feasible.

Acknowledgement We would like to acknowledge our industrial partners; Enerkem, Greenfield Ethanol, CRB Innovations and the Quebec government for their financial support of the Industrial Chair. We would also like to thank Francis Allard of Agroenergy for providing the biomass used in this research.

references

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[4] Adler A, Dimitriou I, Aronsson P, Verwijst T, Weih M. Wood fuel quality of two Salix viminalis stands fertilised with sludge, ash and sludge-ash mixtures. Biomass and Bioenergy 2008;32(10):914e25. [5] Sassner P, Galbe M, Zacchi G. Steam pretreatment of Salix with and without SO2 impregnation for production of bioethanol. Applied Biochemistry and Biotechnology 2005; 121e124:1101e17. [6] Sassner P, Maartensson C-G, Galbe M, Zacchi G. Steam pretreatment of H2SO4-impregnated Salix for the production of bioethanol. Bioresource Technology 2008;99 (1):137e45. [7] Janin G, Durand A. Can the willow be a raw material for the paper industry. Papeterie 1973;95(5):332e4. 337-40. [8] Jayme G, Harders-Steinhauser M, Mohrberg W. Pulp from willow. Holz als Roh- und Werkstoff 1953;11:276e83. [9] Smook GA. Handbook fir pulp and paper technologists. Vancouver: Angus Wilde Publication; 2002. 79e103. [10] Kovacs K, Megyeri L, Szakacs G, Kubicek CP, Galbe M, Zacchi G. Trichoderma atroviride mutants with enhanced production of cellulase and b-glucosidase on pretreated willow. Enzyme and Microbial Technology 2008;43(1):48e55. [11] Domtar website, http://www.domtar.com/en/pulp/index. asp. Birch data, http://www.domtar.com/en/pulp/products/ 426.asp. poplar data, http://www.domtar.com/en/pulp/ products/425.asp. maple data, http://www.domtar.com/en/ pulp/products/428.asp. loblolly pine data, http://www. domtar.com/en/pulp/products/424.asp. and jack pine data, http://www.domtar.com/en/pulp/products/430.asp [accessed 04.12.08]. [12] Francis RC, Hanna RB, Shin S-J, Brown AF, Riemenschneider DE. Papermaking characteristics of three Populus clones grown in the north-central United States. Biomass and Bioenergy 2006;30(8e9):803e8. [13] Cappelletto P, Mongardini F, Barberi B, Sannibale M, Brizzi M, Pignatelli V. Papermaking pulps from the fibrous fraction of Miscanthus x Giganteus. Industrial Crops and Products 2000;11 (2e3):205e10.