Bioresource Technology 155 (2014) 307–313

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Bioresource Technology

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Kinetics of SO2–ethanol–water (SEW) fractionation of hardwoodand softwood biomass

0960-8524/$ - see front matter � 2014 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.biortech.2013.12.100

⇑ Corresponding author. Tel.: +358 503440934.E-mail address: minna.yamamoto@aalto.fi (M. Yamamoto).

Minna Yamamoto a,⇑, Mikhail Iakovlev a, Adriaan van Heiningen a,b

a Department of Forest Products Technology, Aalto University, FI-00076 Aalto, Finlandb Department of Chemical and Biological Engineering, University of Maine, 5737 Jenness Hall, Orono, ME 04469-5737, USA

h i g h l i g h t s

� Softwood and hardwood biomass is fractionated by SO2–ethanol–water technology.� Hemicellulose dissolution rate is similar for softwood and hardwood.� Biomass (branches etc.) fractionation is compared with stem wood fractionation.� Softwood biomass delignification is inferior likely due to high amount of bark.

a r t i c l e i n f o

Article history:Received 7 November 2013Received in revised form 18 December 2013Accepted 22 December 2013Available online 2 January 2014

Keywords:BiomassBiorefineryMass balancePretreatmentSO2–ethanol–water fractionation

a b s t r a c t

SO2–ethanol–water (SEW) fractionation of forest residues (tree tops, stumps, branches) was investigatedto demonstrate the potential of this method for forest biorefineries. The effect of fractionation time ondissolution of wood components was studied. Total mass balances of fractionation show that ligninand hemicelluloses are rapidly dissolved in the spent fractionation liquor whereas cellulose is fully pre-served in the solid residue throughout the fractionation treatment. Within 20 min treatment at 150 �C(SO2:EtOH:H2O = 12:43.5:44.5, by weight, L:W ratio 6 L kg�1), 89% of hardwood lignin and 74% of hemi-celluloses are dissolved. The corresponding values for softwood biomass are 64% and 74%, respectively,indicating slower delignification but equal hemicellulose removal. Additionally, sulfur content of thefeedstocks, solid fractionation residues and spent liquors were analyzed to determine the degree of ligninsulfonation. The obtained results are compared with the stem wood fractionation results.

� 2014 Elsevier Ltd. All rights reserved.

Introduction

Due to depletion of easily available oil resources and concernsabout energy security and climate change, a large research effortis ongoing within the area of renewable fuel production. It is essen-tial to find techno-economically feasible and environmentallysound solutions for alternative fuel production. Biomass has beenidentified as the only practical source for liquid fuels to replace fos-sil feedstocks (Huber et al., 2006).

Numerous pathways for the conversion of biomass into renew-able fuel are under development. These vary from chemical andbiochemical (e.g. anaerobic digestion, acid or enzymatic hydrolysiscombined with fermentation) to thermochemical (e.g. pyrolysis,gasification) conversion. The conversion technique affects the endproduct properties, so that different conversion methods have dif-ferent potential applications. Presently, there is a strong preferencetowards utilizing sustainable non-food raw materials, such as

lignocellulosics, instead of starch and vegetable oil. In lignocellu-losic biomass cellulose is the dominant component, and serves asfeedstock for glucose production by hydrolytic cleavage. Amongvarious hydrolysis techniques, enzymatic depolymerization ispresently considered the most economically attractive approach(Hamelinck et al., 2005). However, enzymes are unable to signifi-cantly hydrolyze cellulose in untreated biomass. This is explainedby the fact that cellulose is a high molecular weight polymer whichexists to a large extent in a highly ordered, crystalline form, andthat the cellulose entities called microfibrils are embedded into amorphologically complex and inaccessible matrix of polymers,mainly hemicelluloses and lignin (Himmel et al., 2007).

Therefore, a large number of so-called pretreatment studieshave been performed to overcome the resistance of cellulose toenzymatic hydrolysis by removing the interfering lignin and hemi-celluloses and opening the biomass structure through the creationof a network of pores. These pretreatment methods include steamexplosion, dilute acid and several alkaline treatments. Unfortu-nately, most of the pretreatments are not effective for softwoodas its structure is especially resistant to enzymatic hydrolysis. Gen-

SO2 / EtOH / H2O Screened feedstock:SW / HW / Mixture

FractionationSO2:EtOH:H2O = 12:43.5:44.5 (by weight), L:W ratio 6 L kg-1, 150°C, 20/30/60/90 min

Cooling, squeezing the solids

Solid residues Spent liquors

Washing twice with 40% EtOH, 60°C and twice with H2O, 23°C

Spent liquor analyses:Dry solids

CarbohydratesUronic acids

LigninAsh and sulfur

Furfural and HMFpH

Washed solids

Screening

Pulp analyses: Yield

Kappa numberViscosity and DPCarbohydratesUronic acids

LigninExtractives

Ash and sulfur Acetyl groups

Rejects

Feedstock analyses:

Bark contentCarbohydratesUronic acids

LigninExtractives

Ash and sulfurAcetyl groups

Fig. 1. Experimental stages.

308 M. Yamamoto et al. / Bioresource Technology 155 (2014) 307–313

erally, efforts to further increase the accessibility towards celluloseby increasing the severity of the pretreatment simultaneously leadto unwanted degradation of carbohydrates, primarily hemicellu-loses (Mosier et al., 2005; Galbe and Zacchi, 2002). In addition,the lignin subjected to acidic pretreatments is changed to renderit unacceptable for applications other than energy production(Carvalheiro et al., 2008). Therefore, the pretreatment approachgenerally fails to meet the requirements of the biorefinery conceptwhere each component of the feedstock is used at a value signifi-cantly above its energy content (Van Heiningen, 2006). Moreover,the recovery of chemicals used in many of these pretreatment pro-cesses would be unrealistically expensive.

The present paper describes a process which aims at addressingthe aforementioned deficiencies. In this process, both softwoodand hardwood forest biomass are separated into cellulosic fibers,hemicellulose sugars and lignin by treatment in hot, pressurizedSO2–ethanol–water (SEW) solutions. The released cellulosic fibersare then enzymatically hydrolyzed, and the combined celluloseand hemicellulose sugar streams are subjected to Acetone–Buta-nol–Ethanol (ABE) fermentation to produce a mixture of fuel com-patible chemicals. The SO2–ethanol–water (SEW) fractionationtechnology has been recently studied by our group (Iakovlevet al., 2009, 2011, 2013; Sixta et al., 2013; Iakovlev and van Hein-ingen, 2012a,b). It has been suggested as a technology suitable fornovel biorefineries where wood components are utilized morecomprehensively than in traditional commercial pulping pro-cesses, such as kraft or acid sulfite pulping. Good quality paper-making and dissolving pulp can be obtained from the cellulosefraction or alternatively, cellulosic biomass residue can be hydro-lyzed to glucose by enzymes. In addition, the relatively simplerecovery of chemicals by evaporation of SO2 and ethanol is a clearadvantage. The conversion of dissolved wood in the SEW liquor tomonomer sugars is part of a patented process termed AVAP™ byAmerican Process Inc., a member of the industrial consortium sup-porting the present research (Retsina and Pylkkanen, 2011).

Presently, SO2–ethanol–water treatment is used by our group aspart of a project where the target is to produce biofuel from cheapand low quality forest residues, such as stumps, branches, twigsand thinnings. Fractionation is the first processing step, wherehemicelluloses and lignin dissolve in the fractionation liquor andcellulose is preserved in the solid residue. The released spent frac-tionation liquor is then conditioned in order to recover most of theethanol and SO2 by simple evaporation, and make the concentratedsolution suitable for subsequent fermentation. Simultaneously theshare of monomeric hemicellulose sugars is increased due to acidhydrolysis reactions ongoing during the spent liquor recovery (Skl-avounos et al., 2011, 2013b). The cellulosic fibers released by frac-tionation are treated by enzymes to produce a glucose solutionwhich is combined with the conditioned hemicellulose monosugarstream so that all sugars present in wood are available for subse-quent fermentation.

Benefits of this biorefinery concept include efficient dissolutionof lignin and hemicelluloses within relatively short time, low sugarlosses, hydrolysis of hemicelluloses without enzymes or additionalacids, and low concentrations of inhibitive compounds formed (Skl-avounos et al., 2011). Even mixtures of lignocellulosics from differ-ent species including softwoods can be treated simultaneously.Additionally, cellulosic residues are relatively easily hydrolyzed bycommercial enzyme mixtures (Yamamoto et al., 2012). The ABE fer-mentation technology demonstrated within this project (Survaseet al., 2011, 2012) allows production of biochemicals highly suitableto be blended in gasoline. Jurgens et al. (2012) have recently re-viewed the advantages and challenges in biobutanol production.

This study investigates the optimal SEW fractionation condi-tions when softwood and hardwood biomass are used as feed-stocks, and the aim is to release all monosaccharides present in

wood without significant mass losses. In a previous publication(Yamamoto et al., 2011), it was found that mixtures of softwood(SW) and hardwood (HW) biomass and recycled fibers can be trea-ted simultaneously in one batch. This omnivorous character is adistinctive advantage for biorefineries, and therefore this featurewas studied further. In this paper, the fractionation kinetics ofSW and HW biomass separately and as mixtures are presented.Observations are based on the analyses of the chemical composi-tion of the process streams.

Methods

The raw materials studied for this biorefinery concept includesoftwood (SW) and hardwood (HW) forest residues, which consistof tree tops, branches and twigs. Green biomass chips werescreened (SCAN-CM 40:01) and air-dried before fractionation todecrease the heterogeneity of the feedstocks. Iakovlev et al.(2013) have shown that SEW treatment efficiency is not affectedby the dry matter content of the feedstock and this finding was alsoconfirmed for SW biomass. Bark content of the screened and air-dried chips was determined by manually removing bark from thechips. The measurement was done according to SCAN-CM 42:06with the exception of using only 100–120 g chips per analysis.

SW and HW biomass were treated by SEW fractionation and theresulting solid residues (pulps) and spent fractionation liquorswere analyzed to determine their chemical composition. Fig. 1shows the conditions during the fractionation procedure and anal-yses carried out along the experimental stages. Fractionation wasdone in silicon oil bath using bombs of 220 mL each filled with25 g (o.d. basis) biomass. The composition and charge of the cook-ing liquor (SO2:EtOH:H2O = 12:43.5:44.5, by weight; liquor-to-wood (L:W) ratio 6 L kg�1) as well as the temperature (150 �C)

M. Yamamoto et al. / Bioresource Technology 155 (2014) 307–313 309

were kept constant in the experiments, whereas the cooking dura-tion varied from 20 to 60 min. The reported durations include 8–9 min of equivalent heat-up time. After fractionation, the bombswere cooled in cold water. Spent liquors were collected by squeez-ing the pulp suspension contained in washing bags. Pulps werewashed twice with 40% v/v ethanol–water at 60 �C (L:W 2 L kg�1)and twice with deionized water at room temperature (L:W20 L kg�1). Optimization of SEW treatment and sufficient pulpwashing of stem wood chips have been determined by Iakovlevet al. (2009, 2011) and Iakovlev and van Heiningen (2012a,b).

The solid yield of pulping and the solids in spent liquor weredetermined immediately after pulping by evaporation to drynessat 105 �C (SCAN-C3:78 and SCAN-N1:61, respectively). The pulpswere screened with a 0.35 mm slotted sieve plate and the amountof rejects was determined gravimetrically. Kappa number was mea-sured according to SCAN-C 1:00. Intrinsic viscosity in cupriethylen-ediamine (CED) solution was analyzed according to SCAN-CM 15:99,and the pulps with kappa number higher than 35 were exposed tochlorite delignification (T230 om-66) prior to viscosity measure-ment. Degree of polymerization of cellulose was calculated fromthe viscosity according to da Silva Perez and van Heiningen (2002).

Extractives content was analyzed gravimetrically through ace-tone extraction (SCAN 49:03). Carbohydrates and lignin weredetermined according to NREL/TP-510–42618 which includes dou-ble stage sulfuric acid hydrolysis followed by HPAEC-PAD (DionexICS-3000, CarboPac PA20 column) analysis. Before the acid hydro-lysis, liquor samples were evaporated to dryness (Iakovlev and vanHeiningen, 2012a). Acid insoluble lignin was analyzed gravimetri-cally while acid soluble lignin was determined by measuring theabsorbance at 205 nm (Shimadzu UV-2550 spectrophotometer).The absorptivity values used were 128 and 110 L g�1 cm�1 forSW and HW, respectively. Acetyl groups were measured as aceticacid according to the same standard by HPLC (Dionex UltiMate3000 with diode array detector, Acclaim OA column). Cellulosecontent of feedstocks and pulps was calculated based on the man-nose-to-glucose ratio of 1.6 and 4.15 reported for hardwood andsoftwood glucomannan, respectively (Janson, 1974). The ash con-tent was measured at 575 �C according to NREL/TP-510–42622.Analysis of uronic acids was based on acid methanolysis withGC-FID detection (Shimadzu GC-17A, NB-30 capillary column)(Iakovlev and van Heiningen, 2012a; Sundberg et al., 1996). Furfu-ral and HMF in spent liquor were analyzed by HPLC. The determi-nation of sulfur content was done as reported by Iakovlev and vanHeiningen (2012a). The results are expressed as weight% on ovendry feedstock (% o.d.f.), if not otherwise indicated.

Results and discussion

Overall mass balances

Forest residues are a very heterogeneous feedstock. Earlier frac-tionation experiments performed on green chips with a wide par-ticle size distribution (accepted from screens Ø7, Ø13 and //8 mm)were characterized by low reproducibility (Yamamoto et al., 2011;Rakkolainen et al., 2010). In order to achieve a higher reproducibil-ity a more homogenous feedstock in terms of moisture content (i.e.air-dried) and particle size (only accepted from screens Ø7 andØ13 mm) is needed. The smaller size biomass particles had a high-er lignin and extractives content and thus, smaller carbohydratecontent, likely due to the increased amount of bark and small twigspresent, with softwood bark having a lignin content of about 40%and extractives content of about 20% (Fengel and Wegener,1989). Bark content of SW and HW biomass was 28.0 ± 2.3% and7.2 ± 1.1%, respectively.

Mass balances of fractionation are presented in Table 1. Overallmass balances are close to 100% indicating no significant losses

during the course of fractionation experiments. However, as re-ported earlier (Yamamoto et al., 2011), HW biomass mass balancesdetermined by the yield of pulp and spent liquor were constantlyabout 97% despite the fact that results were corrected for aceticacid evaporated during the dry solids analysis. The reason for thissmall mass balance loss is likely due to volatilization of low-boilingdissolved components (for example methanol) during the dry sol-ids analysis.

Fig. 2 shows the distribution of wood components in the solidresidue and spent liquor during the SEW fractionation. It is clearthat cellulose is mostly retained in the solid residue whereas hemi-celluloses and lignin are rapidly dissolved in the spent liquor. Somedifferences, for example in the delignification rate of HW and SWbiomass, are observed and these will be discussed in more detailin the following sections.

Viscosity and degree of polymerization of cellulose

As expected, the intrinsic viscosity and degree of polymeriza-tion of cellulose decreased systematically with increasing fraction-ation time (Table 1 and Fig. 3a). However, the viscosity andcellulose DP of HW pulps dropped faster than that of the SW pulpduring the course of SEW fractionation. A similar behavior hasbeen observed earlier in the experiments conducted on spruceand beech stem wood by Iakovlev et al. (2011). Since the amountof sulfonic acids and thereby the acidity (pH values indicated in Ta-ble 1) in both spent liquors are about the same, there must be an-other reason for the difference in the cellulose cleavage rate. It hasbeen speculated that this is related to the better accessibility ofamorphous cellulose regions in HW (Iakovlev et al., 2011). Also,the fact that enzymatic hydrolysis on HW lignocellulosics is knownto be easier than that of SW (Galbe and Zacchi, 2002) supports thatHW cellulose is easier to access. The faster delignification of HWlikely contributes to the better accessibility of cellulose and there-by faster decrease in viscosity.

Another observation reveals that in the beginning of fraction-ation, the viscosity and cellulose DP of SW and HW biomass pulpsare notably lower than those for stem wood pulps (Fig. 3a). How-ever, after 30 min fractionation the differences observed in viscos-ity values even out. It is known that the fiber dimensions inbranches are smaller and chemical composition different fromthose of stem wood (Hakkila, 1989). The DP of cellulose is some-what lower in bark than that of stem wood (Fengel and Wegener,1989) but data on the DP of cellulose in branches has not been re-ported. However, cellulose in compression wood is typically lesscrystalline than in normal wood (Alen, 2011), and this might havecontributed to the initially lower values of the cellulose DP inbiomass.

Delignification and sulfonation of lignin

Delignification is efficient during SEW treatment. However,there are clear differences in the delignification of HW and SW spe-cies, as can be seen in the Fig. 3b showing the lignin content in thesolid fractions. For the reported conditions, 89% of HW lignin is dis-solved in the spent liquor within 20 min treatment whereas thedegree of delignification for SW is only 64% at 20 min. Higher resis-tance of SW towards delignification is generally known and it re-sults mainly from the notable differences in lignin chemistry(Achyuthan et al., 2010). Also, here the notably higher bark contentin SW biomass must have contributed to inferior delignification.Both residual lignin contents and kappa numbers of HW biomasspulps are comparable to the results obtained for beech stem wood(Tables 1 and 2). However, the SW biomass kappa numbers andresidual lignin contents are much higher than those observed forstem softwood. This is likely due to the high amount of bark in

Table 1Complete mass balances and pulp properties of SW and HW biomass during SEW fractionation at conditions: SO2:EtOH:H2O = 12:43.5:44.5 (by weight), L:W ratio 6 L kg�1 and150 �C.

Feedstock SW biomass HW biomass Mixture of SW and HW

Fractionation time (min) 0 20 30 60 90 0 20 30 60 0 c 30 60

Kappa number 115.9 96.8 67.8 84.3 27.1 23.3 18.9 44.2 34.9Intrinsic viscosity (mL/g) 857 783 556 410 966 801 462 849 516Cellulose DP 3740 3240 2120 1470 4580 3600 1730 3560 1940pH of the spent liquor 1.06 1.01 0.99 0.99 0.96 0.98 0.95 0.94 0.92Mass balance (% o.d.f.)Solid yield of fractionation 53.5 45.7 41.3 38.2 49.0 44.3 39.9 45.2 39.9Rejects 4.7 2.9 1.1 0.4 1.0 1.2 0.5 1.1 0.4Neutral carbohydratesa 52.7 38.5 34.0 33.0 30.8 63.1 46.8 41.6 39.3 57.9 38.7 36.0Cellulose 31.5 32.8 30.1 30.4 29.2 36.2 39.2 36.5 36.6 33.8 34.1 33.6Non-cellulosic glucan 1.4 0.4 0.3 0.2 0.1 1.2 0.6 0.4 0.3 1.3 0.3 0.2Xylan 8.5 3.4 2.1 1.5 0.8 21.7 6.1 4.0 2.0 15.1 3.3 1.7Mannan 5.9 1.8 1.3 0.9 0.6 1.9 0.9 0.7 0.4 3.9 1.0 0.6Galactan 3.2 0.1 0.1 0.0 0.0 0.8 0.0 0.0 0.0 2.0 0.0 0.0Arabinan 1.7 0.0 0.0 0.0 0.0 0.7 0.0 0.0 0.0 1.2 0.0 0.0Rhamnan 0.4 0.0 0.0 0.0 0.0 0.5 0.0 0.0 0.0 0.5 0.0 0.0Galacturonic acid 2.6 n.m. 0.2 0.0 0.0 2.8 n.m. 0.0 0.0 2.7 n.m. n.m.4-O-Me-glucuronic acid 0.8 n.m. 0.2 0.1 0.0 1.5 n.m. 0.1 0.1 1.1 n.m. n.m.Acetyl groups 1.3 0.3 0.1 0.1 0.0 3.8 0.6 0.4 0.1 2.6 0.3 0.1Lignin 33.9 12.1 8.7 6.1 5.9 25.8 2.8 2.0 1.4 29.8 4.3 2.9Acid insoluble 32.7 11.8 8.5 5.9 5.7 21.9 2.2 1.6 1.2 27.3 4.0 2.7Acid soluble 1.2 0.3 0.2 0.2 0.2 3.9 0.6 0.3 0.2 2.5 0.3 0.2Ash 2.6 0.8 0.8 0.6 0.4 0.5 0.1 0.1 0.1 1.5 0.3 0.2Extractives 4.1 1.0 0.8 0.9 0.8 2.8 0.3 0.4 0.5 3.5 0.5 0.4Total analyzed 97.9 52.8 44.7 40.9 38.0 100.3 50.6 44.6 41.4 99.1 44.1 39.6Solids in spent liquor 47.3 56.1 60.4 63.1 47.5 52.7 56.2 53.4 58.8Neutral carbohydratesa 16.8 19.0 20.0 20.3 19.3 21.3 23.2 20.9 22.4Glucose 2.0 2.5 2.9 3.4 0.8 1.2 2.0 1.8 2.5Xylose 6.5 6.8 7.8 6.2 15.6 16.7 17.6 12.8 12.9Mannose 3.4 4.2 4.6 5.5 0.9 1.1 1.5 2.9 3.5Galactose 2.8 3.4 2.5 3.4 0.8 0.9 0.9 1.6 1.8Arabinose 1.8 1.7 1.7 1.4 0.6 0.8 0.6 1.2 1.1Rhamnose 0.4 0.4 0.4 0.3 0.5 0.5 0.5 0.5 0.5Galacturonic acid n.m. 2.4 2.3 1.4 n.m. 2.2 1.7 2.3 1.94-O-Me-glucuronic acid n.m. 1.0 1.1 0.9 n.m. 1.7 1.9 1.5 1.5Acetyl groups b 1.0 1.2 1.2 1.3 3.2 3.4 3.7 2.2 2.5Lignin 22.2 28.0 31.5 33.7 22.0 25.6 28.1 26.4 28.9Acid insoluble 18.9 23.0 23.8 25.8 16.0 17.3 17.5 19.6 20.2Acid soluble 3.3 5.0 7.6 7.9 5.9 8.3 10.6 6.8 8.7Lignin, feedstock-pulp 21.7 25.1 27.7 28.0 23.1 23.9 24.4 25.6 27.0Ash 1.2 1.1 1.4 1.5 0.4 0.6 0.6 0.9 1.3Extractivesb 3.2 3.3 3.2 3.3 2.5 2.4 2.3 3.0 3.0Furfural 0.0 0.1 0.3 0.4 0.1 0.4 0.6 0.1 0.5HMF 0.1 0.0 0.0 0.1 0.0 0.0 0.1 0.0 0.1Total analyzed 44.6 56.2 61.0 62.9 47.4 57.7 62.1 57.3 62.0Total (by dry solids) 97.9 100.9 101.9 101.7 101.2 100.3 96.5 97.0 96.2 99.1 98.6 98.7

a All carbohydrates are given as anhydrosugars.b Acetic acid as acetyl groups, values calculated by the difference of feedstock and pulp.c Values calculated as an average of SW and HW biomass.

0

20

40

60

80

100

0 20 30 60 90

Com

pone

nt (%

o.d

.f.)

Frac�ona�on �me (min)

Cellulose

Hemis, acetyl

Lignin Dissolvedsugars

Lignin

Extrac�ves

SOLID RESIDUE

SPENT LIQUOR Extrac�vesAsh

AshFurfural and HMF

0

20

40

60

80

100

0 20 30 60

Com

pone

nt (%

o.d

.f.)

Frac�ona�on �me (min)

Cellulose

Hemis, acetyl

Lignin

Dissolvedsugars

Lignin

Extrac�ves

SOLID RESIDUE

SPENT LIQUOR Extrac�vesAsh

AshFurfural and HMFa b

Fig. 2. The distribution of wood components determined in feedstock, solid residue and spent liquor during HW (a) and SW (b) biomass SEW fractionation at conditions:SO2:EtOH:H2O = 12:43.5:44.5 (by weight), L:W ratio 6 L kg�1 and 150 �C.

310 M. Yamamoto et al. / Bioresource Technology 155 (2014) 307–313

0

2

4

6

8

10

12

0 20 40 60 80 100Deg

ree

of p

olym

eriz

a�on

of c

ellu

lose

(DPx

10-3

)

Frac�ona�on �me (min)

SW biomass

HW biomass

Spruce

Beech

0

5

10

15

20

25

30

35

40

0 20 40 60 80 100

Lign

in in

sol

id fr

ac�

on (%

o.d

.f.)

Frac�ona�on �me (min)

Spruce

SW biomass

Beech

HW biomass

0

5

10

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20

25

0 20 40 60 80 100

Xyla

n in

sol

id fr

ac�

on (%

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Frac�ona�on �me (min)

SW biomass

HW biomass

Spruce

0

2

4

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12

14

0 20 40 60 80 100

Man

nan

in s

olid

frac

�on

(% o

.d.f

.)

Frac�ona�on �me (min)

SW biomass

HW biomass

Spruce

c d

b a

Fig. 3. SO2–ethanol–water fractionation kinetics of different feedstocks produced at conditions: SO2:EtOH:H2O = 12:43.5:44.5 (by weight), L:W ratio 6 L kg�1 and 150 �C. (a)Viscosity-average cellulose DP; (b) delignification; (c) residual xylan; (d) residual mannan in the solid fractions. The curves for beech and spruce are calculated according tothe earlier published fractionation kinetics (Iakovlev et al., 2013, 2011, 2009).

Table 2Spruce and beech pulp properties calculated according to earlier published fractionation kinetics on air-dried chips at conditions: SO2:EtOH:H2O = 12:43.5:44.5 (by weight), L:Wratio 6 L kg�1 and 150 �C (Iakovlev et al., 2009, 2011, 2013).

Feedstock Spruce stem wood Beech stem wood

Fractionation time (min) 0 20 30 60 90 0 20 30 60

Kappa number 90.0 38.0 <10 <10 55–59 19–20 n.p.Intrinsic viscosity in CED (mL/g) 1260 930 540 380 1400–1500 900–1000 n.p.Cellulose DP 6960 4320 2030 1320 8100 4100 1600Mass balance (% o.d.f.)Solid yield of fractionation 58.2 51.2 44.9 43.2 54–58 45–48 n.p.Xylose 5.3 2.4 1.9 1.1 0.6 19.5 n.m. n.m. n.m.Mannose 12.8 3.7 2.9 1.4 0.7 1.1 n.m. n.m. n.m.Lignin 27.7 9.1 3.6 <1a <1a 26.1 6.3 1.9 <1.5a

a Residual delignification phase, n.m. – not measured, n.p. – not-predictable.

M. Yamamoto et al. / Bioresource Technology 155 (2014) 307–313 311

SW biomass and thus, higher amount of lignin and extractives.Condensation reactions may occur between lignin and extractivesduring fractionation causing impaired delignification. Also theinorganics content is higher in biomass than in stem wood (Sklavo-unos et al., 2013a; Yamamoto et al., 2011), and it may impair thefractionation by reducing the acidity of the spent fractionation li-quor. Furthermore, it is known that the lignin structures presentin twigs and branches are different from those in stem wood dueto the presence of compression wood which is known to containmore highly condensed lignin (Alen, 2011). Also, the higher densityof branch wood (Mäkelä, 1977) may retard the penetration ofcooking chemicals into wood and thus, impair the delignification.Nevertheless, the reasons behind impaired delignification of SWbiomass will be studied in more detail.

If delignification curves (Fig. 3b) of spruce stem wood and SWbiomass are compared, it can be seen that after the 8 min heat-

up period the delignification of both proceeds rapidly until20 min. However, after that it is clear that SW biomass delignifica-tion slows down significantly leading to a much higher residual lig-nin content compared to SW stem wood. This is likely due to thepresence of bark resulting in higher amount of lignin and extrac-tives and possibly condensation reactions. When the decrease inlignin content of pulps is compared to the increase in lignin con-tent in the liquor (Table 1) it is clear that the latter is too high,probably due to overestimation of the acid-soluble lignin (due toa higher actual extinction coefficient). Also the extractives presentin spent liquor must be responsible for the overestimation of theamount of dissolved lignin.

Furthermore, the degree of sulfonation of the residual solidsand dissolved lignin was determined to estimate the maximumpossible SO2 recovery in the process, as well as to evaluate the lig-nin properties. The sulfur balance of SEW treatment on stem wood

312 M. Yamamoto et al. / Bioresource Technology 155 (2014) 307–313

has been previously studied by Iakovlev and van Heiningen(2012a). In the present study the target was to confirm whetherthese observations are also valid for biomass feedstocks. It wasfound that the sulfur content of the residual solids and dissolvedlignin is slightly higher than that of spruce stem wood (Table 3).The higher sulfonation degree of biomass is thought to be due tothe higher content of inorganics present in biomass (Yamamotoet al., 2011). This likely leads to a higher content of hydrosulfiteions in the spent liquor and thus, to increased sulfonation (Iakovlevand van Heiningen, 2012b). Another factor responsible for thehigher sulfonation degree may be the higher temperature (150 �Cvs. 135 �C) used in biomass fractionation.

Additionally, it can be seen in Table 3 that SW and HW biomasshad the same degree of sulfonation (0.1 mol S/mol C9 in pulp and0.36 mol S/mol C9 in liquor) despite the fact that HW is delignifiedsignificantly faster. Also, similar to stem wood SEW treatment, thesulfonation was clearly lower than observed for acid sulfite pulping(0.35 and 0.6, respectively) (Rydholm, 1965). Thus, despite the highinitial charge of SO2 in the fresh fractionation liquor, the degree oflignin sulfonation is very low. This is advantageous from a sulfurrecovery point of view, since the rest of the sulfur applied (95–97% of total SO2 charged) is recoverable by distillation (Iakovlevand van Heiningen, 2012a).

One of the main hurdles for lignocellulosic biorefineries is theinhibition caused by phenolic compounds, weak acids and furanderivatives in both the enzymatic hydrolysis and the fermentation(Kim et al., 2013). In this process, the target is to sufficiently delig-nify and wash the solid residue/pulp in order to secure efficientenzymatic hydrolysis. The SEW pulps and spent liquors werecooled down in cold water bucket before washing and then re-heated to 60 �C. If washing would be performed on warm pulpwithout excess cooling, the residual lignin content is expected tobe slightly lower due to less lignin re-deposition on cellulose. Thiscould somewhat improve the enzymatic hydrolysis of pulp. How-ever, the harmful phenolic compounds are concentrated in thespent liquor and washing liquor, resulting in inferior fermentation.Especially hot water washing is stated to release more inhibitivecompounds from pretreated lignocellulose (Kim et al., 2013). Thus,the content of inhibitors has to be reduced through suitable condi-tioning stages prior to fermentation (Sklavounos et al., 2013b).

Dissolution of carbohydrates

Sugar balances (Table 1 and Fig. 2) show that both HW and SWhemicelluloses dissolve in the spent fractionation liquor at aboutthe same rate, while the amount of residual hemicelluloses inHW pulp is higher because its initial hemicellulose content is con-siderably higher. Cellulose was fully preserved in the solid residuefor both the biomass and stem wood pulps. After 20 and 30 minfractionation, about 74% and 84% of hemicelluloses, respectively,are dissolved in the spent liquor. Efficient dissolution of hemicell-

Table 3Sulfur content of feedstocks, residual lignin in pulps and dissolved lignin in spent liquors. F150 �C, 30 min (biomass) and 135 �C, 80 min (spruce).

Feedstock Sulfur content

In biomass In residual lignin% o.d.f. % o.d.f. S/C9

SW biomass 0.03 0.16 0.11HW biomass 0.01 0.04 0.11Mixed biomass 0.09 0.12Sprucea 0.004 0.05 0.08Spruce, sulfiteb 0.2 0.35

Assumed lignin molar mass is 190 g mol�1 (Rydholm, 1965, p. 186).a Iakovlev and van Heiningen, 2012a.b Rydholm, 1965, p. 490.

ulosesis important, since subsequent enzymatic hydrolysis isknown to be hindered by residual hemicelluloses and lignin in pulp(Mansfield et al., 1999). However, utilizing longer fractionationtime would not only increase the amount of dissolved hemicellu-loses but also increase degradation of the dissolved sugars into fur-anic compounds. This is seen clearly for HW fractionation results(Table 1), where the furfural increases multiple times when timeof fractionation increases from 20 to 60 min while the amount ofxylose recovered in solid and liquid phase gradually decreases(21.7%, 20.7% and 19.6% after 20, 30 and 60 min). Thus it is benefi-cial to avoid long fractionation times.

Besides decreasing the fermentable sugar yield, furfural andhydroxymethylfurfural (HMF) are known to inhibit bacterial fer-mentation and thereby their amounts in the conditioned liquorhave to be minimized. Furfural is partially removed during the li-quor evaporation stage prior to fermentation. However, HMF is en-riched in the conditioned liquor since it is not volatile and can alsobe formed during spent liquor conditioning process (Sklavounoset al., 2011).

Arabinose and galactose located in hemicellulose side chainsare fully dissolved within 20 min, whereas glucose, mannose andxylose units mainly present in linear backbone of hemicellulosesare not so easily released. Also 4-O-Me-glucuronic acid being a sidechain of xylan is not completely removed since the glucuronidebond is known to be relatively stable towards acid hydrolysis.The fact that hemicellulose removal notably slows down towardsthe end of fractionation is likely to be related to their locationwithin lignocellulosic matrix of lignin and cellulose. Nevertheless,the hemicelluloses dissolution pattern of SW and HW biomass(Fig. 3c and d) is similar to that of stem wood spruce (Iakovlevet al., 2009, 2011). Interestingly, the mannan content in SW bio-mass was only about half of that in spruce stem wood, likely dueto the high content of compression wood in softwood branches.Compression wood is known to have only about half the amountof glucomannan compared to that of regular wood (Alen, 2011).

Galacturonic acid and rhamnose, which constitute pectins, wereanalyzed in the feedstocks, pulps and spent liquors to observe theirdissolution behavior. The data in Table 1 shows that these pectincomponents are almost fully dissolved in the spent liquor within30 min treatment. Moreover, it can also be seen that galacturonicacid starts to degrade in the spent liquor at longer times similarlyto that observed for monomeric hemicelluloses. Thus longer frac-tionation times not only lead to less fermentable sugars but alsoproduce CO2 by decarboxylation of the galacturonic acid. In addi-tion, unwanted furfural and insoluble humins are reported to beformed from uronic acids (Feather and Harris, 1966).

In all of the analyzed properties, simultaneous treatment ofmixed SW and HW biomass consistently led to results in betweenthe values obtained when treating SW and HW separately. Basedon the results obtained in the present study, there is no reason pre-venting simultaneous processing of these and other lignocellulo-

ractionation conditions: SO2:EtOH:H2O = 12:43.5:44.5 (by weight), L/W ratio 6 L kg�1;

In dissolved lignin Total% o.d.f. S/C9 % o.d.f. S/C9

1.53 0.36 1.69 0.301.46 0.36 1.49 0.341.32 0.31 1.41 0.281.01 0.25 1.06 0.232–3 0.5–0.7 2–3

M. Yamamoto et al. / Bioresource Technology 155 (2014) 307–313 313

sics, both wood and non-wood. Previous SEW kinetics studies ofdifferent type of biomasses (Iakovlev et al., 2011) has revealed this‘‘omnivorous’’ nature of SEW fractionation.

Conversion of the biomass sugars into a biofuel

Sklavounos et al. (2013b, 2011) have been developing a condi-tioning process to allow ABE fermentation of SEW spent liquors.The conditioning steps include evaporation, steam stripping, lim-ing and catalytic oxidation. Conditioned spent fractionation liquorsproduced from spruce (Sklavounos et al., 2013b; Survase et al.,2011) and SW biomass have been successfully fermented into amixture of butanol, acetone and ethanol. A productivity of about4.86 g/L/h and a total ABE solvent yield of 27% has been achievedfor conditioned spruce SEW spent liquors.

Conclusions

SO2–ethanol–water technology efficiently dissolves hemicellu-lose sugars and lignin from forest biomass, such as branches andtree tops. Cellulose is preserved in the solid residue which is highlysuitable for enzymatic hydrolysis. Dissolved sugars can be con-verted into biofuel for example through ABE fermentation technol-ogy. Hemicellulose removal rates of softwood and hardwoodbiomass are comparable and similar to those of stem wood. How-ever, softwood biomass delignification is slower and residual lignincontent higher, likely due to bark and thereby higher amount oflignin and extractives. Also, sulfonation of biomass is somewhathigher than that of stem wood.

Acknowledgements

Authors want to thank Myrtel Kåll, Maarit Niemi and HeikkiTulokas for their help with the HPAEC and IC analyses. We alsothank Prof. Raimo Alén for providing Schöniger combustion appa-ratus. Financial support of TEKES (Finnish Funding Agency forTechnology and Innovation) and industrial partners (ABB, Ameri-can Process Inc., Andritz, Kemira, Neste Oil, Ruukki Group, St1 Bio-fuels, Stora Enso, UPM) through the BioRefine program is greatlyacknowledged.

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