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Author(s): Juha Fiskari & Petri Kilpeläinen, 
Title: Acid sulfite pulping of Acacia mangium and Eucalyptus pellita as a pretreatment 
method for multiproduct biorefineries 
Year: 2021 
Version: Published version 
Copyright:   The Author(s) 2021 
Rights: CC BY-NC-ND 4.0 
Rights url: http://creativecommons.org/licenses/by-nc-nd/4.0/ 
 
Please cite the original version: 
Fiskari J., Kilpeläinen P. (2021). Acid sulfite pulping of Acacia mangium and Eucalyptus pellita as a 
pretreatment method for multiproduct biorefineries. Asia-Pacific Journal of Chemical Engineering. 
e2707. https://doi.org/10.1002/apj.2707. 
R E S E A R CH AR T I C L E
Acid sulfite pulping of Acacia mangium and Eucalyptus
pellita as a pretreatment method for multiproduct
biorefineries
Juha Fiskari1 | Petri Kilpeläinen2
1Fibre Science and Communication
Network (FSCN), Mid Sweden University,
Sundsvall, Sweden
2Natural Resources Institute Finland
(LUKE), Espoo, Finland
Correspondence
Juha Fiskari, Mid Sweden University,
Fibre Science and Communication
Network (FSCN), Sundsvall, Sweden.
Email: juha.fiskari@miun.se
Abstract
Conversion of biomass into saleable biochemicals and fuels requires the use of
a pretreatment to enable subsequent processing. Acid sulfite pulping is one of
the most cost-effective strategies, because the chemicals are inexpensive and
the technology is available on an industrial scale. It also allows the simulta-
neous production of cellulosic fibers and lignosulfonate. However, too little is
known about the feasibility of acid sulfite pulping of tropical hardwoods. The
objective of this research was to gain a better understanding of the response of
Acacia mangium and Eucalyptus pellita in acid sulfite pulping. The plantation-
grown hardwood chip samples were obtained from Sabah, Malaysia. The
sulfite cooking experiments were carried out in autoclaves with temperatures
of 130
C and 140
C and varied chemical charges. The results revealed that a
cooking temperature of 140
C was needed to reach kappa numbers below
30, but this also resulted in much reduced fiber length and higher fines content
than 130
C, probably due to the intensified acid hydrolysis. To reach kappa
numbers below 20, more severe cooking conditions are needed. These results
demonstrate that using A. mangium and E. pellita as feedstocks allows feasible
production of chemical pulp and sulfonated lignin, which are intermediate
products for biorefineries.
KEYWORD S
chemical pulp, fermentable sugars, lignocellulosic biomass, spent sulfite liquor, sulfite
process
1 | INTRODUCTION
The importance of many acacia and eucalyptus species as
feedstock for pulping has been growing in the past
decades. Both Acacia mangium and Eucalyptus pellita
have been planted for pulpwood and timber in many
areas of the subtropics and tropics, including parts of
South East Asia.1 These feedstocks are sometimes under-
utilized and thus available as relatively inexpensive raw
material for nonconventional industrial processes, as
well. On the other hand, the technologies for converting
biomass into saleable products are in constant need of
Received: 20 June 2021 Revised: 30 August 2021 Accepted: 2 September 2021
DOI: 10.1002/apj.2707
This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any
medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
© 2021 The Authors. Asia-Pacific Journal of Chemical Engineering published by Curtin University and John Wiley & Sons Ltd.
Asia-Pac J Chem Eng. 2021;e2707. wileyonlinelibrary.com/journal/apj 1 of 8
https://doi.org/10.1002/apj.2707
development to meet techno-economic criteria required
for investment.
Sulfite pulping starts with the feeding of wood chips
and cooking liquor to a digester. The sulfite cooking
liquor is a reaction product of sulfur dioxide, a base
cation, and water; it also contains an excess of SO2. Base
cations employed in industrial sulfite pulping processes
are ammonium (NH4+), calcium (Ca2+), magnesium
(Mg2+), or sodium (Na+). There are several modifications
of sulfite pulping, which are designated according to the
pH of the cooking liquor.2 This makes sulfite pulping
very flexible and allows the production of many pulp
grades which can be used for a broad range of applica-
tions. Currently, acid sulfite pulping process (pH range
1–2) is almost exclusively employed for dissolving pulp
production.2,3 Dissolving pulp has typically high cellulose
content while lignin, hemicellulose, and extractives con-
tents are very low; the desired contents depend on the
dissolving pulp grade. Industrial experience on sulfite
pulping with mainly softwood as the feedstock has
proved that sulfite process can be successfully incorpo-
rated with hydrolysis of sugars from hemicellulose and
cellulose.4 The latter creates opportunities for various
biorefinery concepts, which are based on sulfite pulping
technology, where the main products are biochemicals or
bio-based transportation fuels instead of papermaking
fibers or dissolving pulp. Moreover, this type of bio-
refinery can also be designed to “swing” between differ-
ent products or their combinations. It is apparent that
product yield distribution and the overall productivity of
the plant vary significantly5,6—as the feedstock availabil-
ity and product prices fluctuate with market conditions.
The sulfite pulping does not degrade lignin to the
same extent as the kraft cooking process. The generated
lignosulfonates are useful by-products that have a
steadily growing demand and a relatively stable market.
In 2010, the total global production of lignosulfonates
was approximately 1.2 million tons which accounted for
about 10% of the lignin in the biomass that was pulped.7
Major applications include concrete additives (plasti-
cizers), dispersing agents, and industrial binders.8,9 These
modified natural polymers are nontoxic and non-
hazardous, which is why they are increasingly finding
new uses, such as water treatment applications, or in the
food industry.10 Moreover, significant research efforts are
allocated to the continuous development of new lignosul-
fonate products, such as those made of hardwood with
well-engineered properties. By employing posttreatments,
hardwood lignosulfonate properties may be tailored to
specific applications.7 The latter allows a major improve-
ment in the product profitability of hardwood-based lig-
nosulfonates, which are often considered of less value
than those produced from softwoods.
In relation to the dominant kraft pulping process,
sulfite pulping has a rather comparable environmental
impact today.11 For example, the sulfur dioxide
remaining in the combustion gases is recycled, and the
elimination of all chlorine bleaching chemicals is easier
to achieve with sulfite pulps, which are known to have
better bleachability than kraft pulp. Moreover, as the
spent sulfite liquor (SSL) contains a significant amount of
lignocellulosic material, and it is currently being
converted to various value-added products instead of
being discharged with effluents.
This paper summarizes a laboratory research on
A. mangium and E. pellita and their feasibility as feed-
stocks for biorefinery. Our research included chemical
analyses of the hardwood samples, as well as laboratory
cooking experiments and the characterization of the
obtained pulp samples for kappa number and fiber qual-
ity. Fiber length and fines content may not appear to be
important attributes in this research since the resulting
pulp is not intended for papermaking. However, too short
fibers, fiber fragments, and fines are difficult to retain in
the pulp in the subsequent washing and screening opera-
tions, which results in decreased yield. It is, therefore, the
processability of pulp that makes the fiber length and the
fines content important parameters in a feasibility assess-
ment for a biorefinery process. In this investigation, we
also analyzed lignosulfonate samples isolated from SSL
for their molecular weight (MW), which is among the
most important characteristics that contribute to ligno-
sulfonate functionality.
2 | EXPERIMENTAL
2.1 | Materials
The plantation-grown A. mangium and E. pellita chips
were obtained from a company that operates man-made
plantations and a chip mill, all of which are located in
Sabah, Malaysia (the island of Borneo). Samples of the
chips are shown in Figure 1. The sapwood of A. mangium
is white and clearly defined against the darker brown
heartwood. E. pellita chips have more uniform reddish-
brown color, making it more difficult to distinguish sap-
wood from heartwood. The chips had been dried before
shipping, and both samples had a moisture content of
approximately 6%. The low moisture content had to be
taken into account by ensuring sufficient impregnation
of the cooking liquor. According to previous experience
published elsewhere, sulfate (kraft) cooking of eucalyptus
chips with a very low moisture content appeared
to be manageable with only minor process-related
challenges.12
2 of 8 FISKARI AND KILPELÄINEN
The hardwood chips were screened with a laboratory-
scale classifier according to SCAN-CM 40:94 test method,
and only the accept fraction was used for acid sulfite
cooking experiments.
The Natural Resources Institute Finland (LUKE)
analyzed screened hardwood chip samples (the accept
fraction) for carbohydrates, lignin, and extractives. Hemi-
celluloses were determined using method based on acid
methanolysis13 and cellulose with acid hydrolysis.14 The
total lignin content of the samples was determined
according to KCL method N:o 115b:82. The KCL method
included also a correction due to the ultraviolet
(UV) absorption of carbohydrate degradation products.15
More information about this analytical procedure and
how it relates to some alternative lignin analysis methods
has been published elsewhere.16 Prior to the Klason lig-
nin analyses, the lipophilic extractives were first extracted
with hexane and thereafter the hydrophilic extractives
with an acetone:water (95:5 v/v) mixture with accelerated
solvent extractor ASE-350 (Dionex, Sunnyvale, CA). A
more detailed description of the extractives analysis has
been published previously.17 The chemical constituents
of the wood samples are summarized in Table 1.
The cooking tests were carried out in 2L autoclaves.
Acid sulfite cooking liquor contains higher free SO2 con-
tent compared with bisulfite cooking liquors. In this
research, acid sulfite chemical charges were free SO2 of
21%, 24%, and 28% while the corresponding combined
SO2 charges were 4.2%, 4.8%, and 5.6%, respectively. The
base cation was sodium. In addition to the varied chemi-
cal charges, varied temperatures at 130
C and 140
C
were also included in the tests.
Finally, the resulting pulp samples were screened, and
the reject content of each pulp sample was determined
gravimetrically. Kappa numbers of the pulp samples were
measured in accordance to ISO 302:2004 test method.
Length-weighted average fiber length and fines content
of the pulp samples were measured with a Kajaani
FS-200 fiber length analyzer. It measures fiber length via
an indirect optical method. One of its strongest points
is the large number of fibers sized in a short time.18
2.2 | Chemical characterization of the
acacia and eucalyptus samples
According to the lignin analyses, the wood of
A. mangium contained 32.8% lignin while the lignin
content of E. pellita was 37.3%.
Cellulose contents of both hardwood species,
measured as the concentration of galactose, were found
to be quite similar. A. mangium and E. pellita had cellu-
lose contents of 43.2% and 42.5%, respectively.
The hemicellulose analysis results revealed that
A. mangium had a lower total concentration of hemicel-
lulose than E. pellita, 207 and 268 mg/g, respectively.
These values can also be expressed as 20.7% and 26.8%,
respectively. The breakdown of total hemicellulose
contents to individual hemicellulose components is
presented in Figure 2.
All individual hemicellulose components analyzed in
this research exhibited a higher content for E. pellita than
A. mangium. Xylose was the most dominant hemicellu-
lose component for both wood species. The xylose con-
tent of E. pellita was 137 mg/g while that of A. mangium
was 107 mg/g. The most prominent difference between
these two wood samples was the content of galactose:
24.7 mg/g for E. pellita while A. mangium only had
9.5 mg/g.
In addition, A. mangium and E. pellita contained sig-
nificant amounts of extractives, 2.6% and 1.4%, respec-
tively. The extractive content for A. mangium wood in
this research was much higher than extractive analysis
results published elsewhere.19 Moreover, virtually all
remaining knots or “knotwood” in the original chip lot
had been separated in screening from the accept fraction;
they mostly ended up in the overthick fraction. Hard-
wood knots typically have a higher content of extractives
than the corresponding heartwood.20 However, according
to the results of a study on A. mangium grown in
FIGURE 1 Eucalyptus pellita (left) and Acacia mangium chips
(right) from East Malaysia
TABLE 1 Chemical composition of A. mangium and E. pellita
wood samples
Chemical constituent A. mangium E. pellita
Extractives, % 2.6 1.4
Lignin, % 32.9 37.3
Cellulose, % 43.2 42.5
Hemicellulose, % 20.7 26.8
Sum of components, % 99.4 108.0
FISKARI AND KILPELÄINEN 3 of 8
Indonesia, knots contained smaller amounts of lipophilic
extractives than heartwood or sapwood.19
The extractive content of E. pellita wood in this
research was relatively low when compared to the
information published elsewhere.21
Table 1 summarizes all analyzed chemical constitu-
ents and their concentrations (%).
The acetyl groups of hemicellulose were not quanti-
fied in this research. They would probably have added “a
couple of percentage points” to the sum of the chemical
components. In addition, it is obvious that wood contains
small amounts of inorganics, typically some 1–2% for
debarked wood.22 The inorganics are usually measured
as residual constituents or the ash content of wood.
2.3 | Characterization of lignosulfonates
in spent liquor samples
Among other important characteristics, lignosultonate
functionality is defined by its MW. In order to perform
an accurate determination of the weight-averaged MW of
lignosulfonates, Mw, we employed size-exclusion chroma-
tography (SEC) in combination with light-scattering
techniques. More detailed descriptions of the method are
available elsewhere.23,24
3 | RESULTS AND DISCUSSION
3.1 | Acid sulfite cooking experiments
For both acacia and eucalyptus, kappa number decreased
substantially with increased cooking temperature. That
also indicated that at 130
C, the pulp was not properly
digested. All kappa numbers from the cooking at 130
C
were very high, 60 or higher. Eucalyptus has somewhat
higher kappa numbers than acacia with the same
cooking conditions, but the difference is not consistent.
The cooking temperature of 140
C resulted in kappa
numbers between 23 and 31 for eucalyptus and between
22 and 26 for acacia. This confirms that in terms of kappa
number, the response of A. mangium to acid sulfite
cooking was a little better than that of E. pellita. Kappa
number results of the acid sulfite cooking experiments
are presented in Figure 3.
The limited extent of delignification with A. mangium
and E. pellita wood may be attributed to their dense wood
structures. The exact densities of our hardwood chip sam-
ples were not known, but it is apparent that an inten-
sively managed industrial tree plantation aims at
maximizing the performance of trees, and thus, the
highest possible wood density is also an important objec-
tive. For example, Ismaili et al. reported a basic density of
540 kg/m3 for A. mangium grown in Malaysian Borneo.25
Other studies have reported basic densities of 464 and
494 kg/m3 for A. mangium grown in Malaysia.26,27 The
density of E. pellita grown at a plantation in Indonesian
Borneo has been reported to vary from 556 to 652 kg/
m3.28 For comparison, another two genera, namely,
Macaranga spp. and Endospermum spp., also fast-
growing timber trees grown in Malaysia, only have basic
densities of 371 and 389 kg/m3, respectively.27 Moreover,
in the case of A. mangium, the relatively high content of
extractives may further retard the delignification by gen-
erating cross-links with lignin during acid sulfite pulping.
The visual appearance of the pulp samples after the
acid sulfite pretreatment shows no apparent difference
FIGURE 2 Hemicellulose analysis
results of A. mangium and E. pellita. The
list of hemicellulosic components (top to
bottom) 4-O-methyl glucuronic acid,
galacturonic acid, glucuronic acid,
rhamnose, arabinose, xylose, galactose,
glucose, and mannose
4 of 8 FISKARI AND KILPELÄINEN
between the two species. However, both soft and hard
particles of undercooked wood material can be observed.
It seems that the cooking temperature of 130
C resulted
in higher reject content than the cooking temperature of
140
C. Pulp reject contents, calculated as % on oven dry
pulp, are presented in Table 2.
As can be seen in Table 2, the lower cooking tempera-
ture of 130
C generated more reject than 140
C. Cooking
with acacia resulted in much higher reject than with
eucalyptus. Especially with the lower cooking tempera-
ture (130
C), the reject content of A. mangium was
unusually high, up to 21% on oven dry pulp.
All pulp samples were also characterized for fiber
length and fines using Kajaani FS-200 fiber length
analyzer after sulfite cooking and the removal of reject.
Kajaani FS-200 is specifically designed to evaluate fiber-
length distributions of cellulosic fibers.18 Pulp and paper
industries frequently discuss fiber length in terms of
length-weighted average. Fiber quality results are
presented in Figure 4.
According to the results, the longest fibers (measured
as length-weighted average length, Lw) were obtained
with the lower cooking temperature (130
C), between
0.65 and 0.70 mm for both acacia and eucalyptus pulp
samples. With the higher cooking temperature, 140
C,
the average fiber length was reduced significantly. The
impact of cooking chemical charge was a much less pro-
nounced. A. mangium pulp samples had slightly longer
fibers than those of E. pellita, when compared with the
same cooking conditions. The highest fiber length values
presented here are lower than those published in the lit-
erature, for example, 0.93 mm for A. mangium 26 and
0.904 mm for E. pellita,28 which reflect their full potential
as papermaking fibers. With acid sulfite cooking, it is
hardly possible to reach the full potential of fiber length
due to acid hydrolysis of cellulose polymers. A. mangium
pulp samples had clearly higher fines contents than those
of E. pellita at both cooking temperatures.
We also characterized lignosulfonates isolated from
the SSL samples. By fermentation of the hexoses and by
chemical degradation of the pentoses, purified lignosulfo-
nate products are obtained.8 Lignosulfonate functionality
in various industrial uses is greatly defined by its
MW. For various applications such as dispersants, a
greater MW is beneficial.29 Figure 5 summarizes weight-
averaged MW (Mw) results of all six lignosulfonate
samples.
The Mw values of E. pellita lignosulfonates were sub-
stantially higher than those of A. mangium under the
same pulping conditions. There appears to be a positive
correlation between lignosulfonate Mw and lignin con-
tent in wood. The avergage Mw ranged from 11 600 to
18 900 g/mol for E. pellita and 7600 to 10 700 g/mol for
A. mangium. The lower cooking temperature of 130
C
resulted in higher average Mw than 140
C. This suggests
that the higher Mw values were not resulting from exten-
sive lignin condensation under severe cooking condi-
tions.30 In all, both A. mangium and E. pellita yielded
FIGURE 3 Kappa numbers after
sulfite cooking. The chemical charges of
combined and total SO2 are presented
between the x-axis and wood species
TABLE 2 Pulp reject contents (% on oven dry pulp) after acid
sulfite pretreatment
Sample 5.6%/28% 4.8%/24% 4.2%/21%
E. pellita, 130
C 0.7 1.1 1.9
A. mangium, 130
C 15.8 13.8 21.0
E. pellita, 140
C 1.2 0.5 0.9
A. mangium, 140
C 3.5 5.9 7.6
Note: The chemical charges of combined and total SO2 are shown in the top
row.
FISKARI AND KILPELÄINEN 5 of 8
quite high Mw values, especially with the cooking
temperature of 130
C. For comparison, Mw values of
E. globulus and E. grandis lignosulfonates have been
reported to be much lower, 6300 and 5700 g/mol, respec-
tively.24 On the other hand, lignin obtained from labora-
tory kraft pulping of E. pellita was found to have a very
large polydispersity, but the Mw values of different frac-
tions averaged 2500–2600 g/mol only.31,32
Dispersants and concrete admixtures probably
account for the broadest use of lignosulfonates and
sulfonated lignin. This is due to their appropriate MW
(10 000–50 000 g/mol).9 In this range of Mw, commercial
lignosulfonate qualities are typically produced of SSL
from softwood pulping. However, according to Ghorbani
et al., several commercial spruce lignosulfonate products
had Mw values ranging from 5780 to 11 390 g/mol.
33 In
our research, the high Mw of E. pellita lignosulfonate pro-
vides therefore very interesting opportunities for
product engineering, although it is obvious that further
research on this subject is necessary. Other important
lignosulfonate parameters, such as the degree of sulfona-
tion, the content of methoxyl groups, or polydispersity,
must also be investigated.
4 | CONCLUSIONS
According to the results, E. pellita chips showed more
positive response in sulfite pretreatment than
A. mangium. After acid sulfite pretreatment of 130
C,
kappa numbers and average fiber lengths of the both spe-
cies were similar, but reject amounts of acacia were much
FIGURE 4 Fiber analysis results
(Kajaani FS-200). The chemical charges
of combined and total SO2 are shown
between the x-axis and wood species.
Fines content results (%) are shown
numerically on top of each bar
FIGURE 5 Weight-averaged
molecular weight (Mw) results of
lignosulfonates from SSL samples
6 of 8 FISKARI AND KILPELÄINEN
higher than those of eucalyptus; 140
C cooking tempera-
ture was needed to reach kappa numbers below
30, which, in turn, resulted in much reduced fiber length
and high fines content, apparently due to the acid hydro-
lysis of cellulose polymers. Moreover, to reach kappa
numbers below 20, even more severe cooking conditions
would be needed. Nevertheless, our results demonstrate
that it is possible to delignify both A. mangium and
E. pellita by acid sulfite pretreatment to reasonably low
kappa number, which allows the further processing of
the separated fibers as well as the carbohydrates
dissolved in the SSL. The hydrolysis of hemicelluloses
into monomer sugars allows their utilization as feedstock
for fermentation into biofuels and biochemical products.
In addition, our results show that the resulting lignosul-
fonates had unexpectedly high MW, especially those of
E. pellita. The production of high MW lignosulfonate
may open up very interesting opportunities for lignin
product engineering. In all, the pulp, the dissolved
carbohydrates, and the resulting lignin may well find
their uses as raw materials for multiproduct biorefineries.
The data obtained from this research can serve as funda-
mental information in the selection of suitable wood
species to be applied as feedstock in an acid sulfite-based
biorefinery process.
CONFLICT OF INTEREST
The authors declare that they have no conflicts of
interest.
ORCID
Juha Fiskari https://orcid.org/0000-0002-8097-6040
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How to cite this article: Fiskari J, Kilpeläinen P.
Acid sulfite pulping of Acacia mangium and
Eucalyptus pellita as a pretreatment method for
multiproduct biorefineries. Asia-Pac J Chem Eng.
2021;e2707. doi:10.1002/apj.2707
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