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Fuel Processing Technology 92 (2011) 1849
–
1854
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Fuel Processing Technology
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Change of pyrolysis characteristics and structure of woody biomass due to steam
explosion pretreatment
, Kentaro Umeki
1
, Weihong Yang, Wlodzimierz Blasiak
Division of Energy and Furnace Technology, Department of Materials Science and Engineering, Royal Institute of Technology (KTH), Brinellvagen 23, SE-100 44 Stockholm, Sweden
⁎
Amit Kumar Biswas
article info
abstract
Article history:
Received 15 March 2011
Received in revised form 17 April 2011
Accepted 25 April 2011
Available online 2 June 2011
Steam explosion (SE) pretreatment has been implemented for the production of wood pellet. This paper
investigated changes in biomass structure due to implication of steam explosion process by its pyrolysis
behavior/characteristics. Salix wood chip was treated by SE at different pretreatment conditions, and then
pyrolysis characteristic was examined by thermogravimetric analyzer (TGA) at heating rate of 10 K/min. Both
pyrolysis characteristics and structure of biomass were altered due to SE pretreatment. Hemicellulose
decomposition region shifted to low temperature range due to the depolymerization caused by SE
pretreatment. The peak intensities of cellulose decreased at mild pretreatment condition while they increased
at severe conditions. Lignin reactivity also increased due to SE pretreatment. However, severe pretreatment
condition resulted in reduction of lignin reactivity due to condensation and re-polymerization reaction. In
summary, higher pretreatment temperature provided more active biomass compared with milder
pretreatment conditions.
Keywords:
Steam explosion
Woody biomass
Pyrolysis
Reactivity
Thermogravimetry
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
celluloses into solution
[11]
. Additionally, both cellulose and lignin are
also altered depending on the severity of the process
[12,13]
.
Pyrolysis is one of the major conversion steps during thermo-
chemical conversion of solid fuels. Therefore, it is important to focus
on the pyrolysis characteristics of SE treated biomass since variation
in main components of biomass has signi
In recent time, wood pellet industry has experienced dramatic
expansion in energy market at annual production of 13 million ton in
2009
[1]
. The main barometers of the physical quality of wood pellets
are hardness, speci
c weight, sensitivity to moisture and heating
cant effect on its reaction
behavior. Despite a number of studies on thermochemical behavior of
steam pretreated biomass has been conducted, most of them were
performed on the residue which had gone through simultaneous
sacchari
value
[2
4]
. Different approaches have been considered to improve
the quality of the wood pellet, for example, torrefaction and fast
pyrolysis. Those technologies are mostly in research and development
phase and require further efforts on development and commercial-
ization
[5,6]
. Recently steam explosion (SE) pretreatment, previously
devoted to ethanol production and binderless panel production
[7
–
cation and fermentation (SSF)
[14
–
16]
.
In such studies,
lignocelluloses structure was further modi
ed due to SSF. A limited
number of studies have been previously reported considering
pyrolysis of SE residue
[11,17,18]
. Xu et al.
[17]
observed increase in
char yield after pyrolysis for steam pretreated wool
9]
,
has been brought into attention for improvement of wood pellet. SE
pellet has proved to provide improved physical properties of pellet
[5]
. Woody biomass consists of cell wall mainly with polysaccharides
(cellulose and hemicelluloses) and aromatic polymers named lignin.
In steam explosion (SE) process, biomass is exposed to saturated
steam at the temperatures range from 453 K to 513 K with a wide
range of residence time, which results in both morphological and
chemical changes in wood
[10]
. SE pretreatment is known to bring
adequate disruption of carbohydrate linkage by releasing hemi-
–
ber residue. They
associated this observation with removal of loose substances of
biomass during steam explosion. Deepa et al.
[18]
observed slight
change in degradation temperature of hemicellulose in SE pretreated
banana
ber residue which was attributed to the presence of trace
quantity of hemicellulose. Negro et al.
[11]
observed shift in lignin
peak towards lower temperature for severe pretreated residue in
comparison with the mildest condition. They suggested development
of thermolabile chemical bonds in lignin when severity of pretreat-
ment was high. Although those studies provide pyrolysis behavior of
SE residue, no study was found which investigated detailed effect of
process parameters of steam explosion (i.e. pretreatment tempera-
ture and time) on the pyrolysis characteristics of SE residue.
The aim of this research is to observe the effect of SE conditions on
the reactivity of woody biomass during pyrolysis process. Structural
Corresponding author. Tel.: +46 8790 8459; fax: +46 8207 681.
E-mail addresses:
(A.K. Biswas),
(K. Umeki),
(W. Yang),
(W. Blasiak).
1
Present address: Division of Energy Science, Department of Engineering Science
and Mathematics, Luleå University of Technology, SE-971 87 Luleå, Sweden.
⁎
–
0378-3820/$
see front matter © 2011 Elsevier B.V. All rights reserved.
doi:
1850
A.K. Biswas et al. / Fuel Processing Technology 92 (2011) 1849
1854
–
changes of biomass were examined to address the reason why
reactivity changed due to SE. Salix chips were used as samples for SE
pretreatment. Thereafter, thermogravimetric analysis with raw
sample and pretreated residues was performed under pure nitrogen
atmosphere.
rm the repeatability in experimental measurement,
experiments of both TGA and XRD were performed repeatedly.
To con
3. Results and discussions
3.1. Effect of SE condition on the reactivity of residue
2. Experimental
To have comprehensive view of change in reactivity by pretreat-
ment of biomass, pyrolysis temperatures at different conversion ratios
were examined. In previous studies
[19]
, temperature at 50% (T
50
)of
conversion ratio was used to describe pyrolysis reactivity where lower
T
50
temperature indicates faster decomposition of the biomass. In this
study, pyrolysis temperatures at three different conversion ratios,
10%, 50%, and 90%, were used as listed in
Table 2
with their
corresponding weight loss rate.
Pretreatment lowered initial decomposition temperature (T
10
)
that is attributed to modi
10 mm was used for
pretreatment experiment. The ultimate and proximate analysis of
untreated biomass sample is shown in
Table 1
. The moisture content
of fresh biomass was 46%. Wood was collected and chopped and
stored in a plastic bag at 277 K prior to experiment. Eight SE
experiments were performed on Salix wood chips using a laboratory
scale reactor by varying two process parameters, temperature (Tp)
and time (t). Three pretreatment temperatures were chosen: 478, 493
and 501 K. For pretreatment temperature of 478 K and 493 K,
pretreatment time was chosen as 6 min, 9 min and 12 min. For
501 K, pretreatment time was set as 6 min and 12 min. The detailed
description of test facility and experimental procedure is explained
elsewhere
[5,7]
. The steam used was in saturated condition. After
pretreatment, biomass was separated from liquid and dried in air to
reduce moisture content.
Pyrolysis of SE residue was performed in a thermogravimetric
analyzer (TG, PerkinElmer) under nitrogen atmosphere. The nitrogen
ow rate was kept at around 300 ml min
−
1
at standard state. Prior to
the experiments, samples were ground and size less than 0.125 mm
was used to minimize intra-particle heat and mass transfer effect on
the pyrolysis behavior. A sample weight of around 5 mg was used in
every occasion and placed in a crucible. Initially, the biomass was
heated to 373 K and kept for at least half an hour under the nitrogen
atmosphere to remove all the moisture content from biomass.
Afterwards, biomass sample was heated from 373 K to 1023 K at a
heating rate of 10 K/min. The residual mass and sample temperature
were recorded every 4 s. Residual mass ratio, derivation of thermo-
gravimetry (DTG) and conversion ratio are represented by following
equation:
Short rotation willow (Salix) of chip size 2
–
ed structure and breakdown of hemi-
celluloses from biomass as discussed later. However, almost no
change in reaction intensity (wt.%/K) was observed at 10% of
conversion ratio for pretreated materials. It indicates the increased
reactivity of pretreated biomass since the equivalent reaction
intensity was observed at lower temperature. No signi
cant alteration
in T
50
was observed in pretreated residue although pretreatment
decreased the corresponding reaction intensity (wt.%/K) when
pretreatment temperature was 478 K. When pretreatment tempera-
ture was further increased to 493 K and 501 K, reaction intensity was
observed to increase signi
cantly. Temperature at 90% of conversion
ratio (T
90
), which is an indication of overall conversion of pyrolysis
process, showed that pretreatment at 478 K made the biomass more
resistance to thermal decomposition. However, further increase in
pretreatment temperature to 493 K and 501 K showed the equivalent
temperature (T
90
) of pretreated biomass compared to untreated
biomass.
3.2. Pyrolysis characteristics of untreated biomass
Biomass consists of three major components which are cellulose,
hemicellulose, and lignin. It has been recognized that those
components can be characterized by means of derivative thermo-
gravimetry (DTG)
[19,20]
. In other terms, due to inherent difference
in structure of those components, it is possible to qualitatively identify
characteristics of those components from their intensity and location
in DTG. In general, hemicellulose decomposition occurs within the
range of 423 to 623 K, cellulose decomposes within the range of 623 to
773 K, and lignin decomposition ranges from 623 K to beyond 773 K
[20]
.
Residual mass ratio and DTG of untreated biomass against
temperature are shown in
Fig. 1
. DTG distribution showed different
=
m
i
m
o
Residual mass ratio
;
α
1
ð
Þ
d
dT
Derivation of thermogravimetry DTG =
−
2
ð
Þ
X =
m
0
−
m
i
Conversion ratio
;
ð
3
Þ
m
0
−
m
∞
X-Ray diffractometry (XRD) of both untreated and pretreated
biomass was carried out using a diffractometer (Siemens, D 5000),
with monochromatic Cu K
=0.154180 nm), generated
at 35 kV and 40 mA. The diffracted intensity was measured in a 2
α
radiation (
λ
θ
Table 2
Pyrolysis temperature and weight loss at three different conversion ratios, 10%, 50%,
and 90%.
range between 10° and 30° for every 0.02°.
Pretreatment conditions
Conversion ratio (%)
Table 1
Proximate and ultimate analyses of untreated biomass, dry basis.
Temperature
[K]
Time
[min]
10
50
90
[% db]
Temp
[K]
DTG
[wt.%/K]
Temp
[K]
DTG
[wt.%/K]
Temp
[K]
DTG
[wt.%/K]
Proximate analysis
Fixed carbon
16.4
Untreated Biomass
–
568
0.10
641
0.67
762
0.05
Volatile
81.20
478
6
530
0.12
639
0.53
807
0.03
Ash
2.40
9
520
0.12
637
0.56
856
0.03
Ultimate analysis
Carbon (C)
12
514
0.10
639
0.52
866
0.03
49.40
493
6
552
0.10
643
1.00
758
0.06
Hydrogen (H)
6.10
9
541
0.10
641
0.89
774
0.05
Oxygen (O)
41.80
12
545
0.13
634
0.96
767
0.05
Nitrogen (N)
0.29
501
6
533
0.11
630
0.86
766
0.05
Sulfur (S)
0.043
12
534
0.11
633
0.87
759
0.07
A.K. Biswas et al. / Fuel Processing Technology 92 (2011) 1849
1854
1851
–
100
1
1.2
Untreated
6 min
9 min
12 min
1
0.8
0.5
50
0.6
0.4
0.2
0
0
0
400
500
600
700
800
900
400
500
600
700
800
900
Temperature [K]
Temperature [K]
Fig. 1.
Residual mass ratio and DTG of raw biomass as a function of reaction
temperature.
Fig. 3.
Change of DTG distributions due to steam explosion at (493 K; 6, 9 and 12 min).
times were 6, 9 and 12 min. In every occasion, the highest peak was
identi
peaks at different temperatures. The main peak appeared at around
657 K that corresponds to decomposition of cellulose. Before cellulose
peak (657 K), no de
ed at around 643 K. This peak stands for decomposition of
celluloses. When pretreatment time was 6 min, a relatively broaden
region with some small shoulders was observed before the cellulose
peak (645 K) in comparison with untreated biomass. That broadened
region represents decomposition of transformed hemicelluloses.
When pretreated residue produced at 478 K and 9 min was tested,
several peaks were observed before cellulose peak with a notable
peak at 425 K. Further increase of pretreatment time to 12 min
showed only one peak at 426 K.
Fig. 3
provides DTG distribution against temperature of pretreated
residue when pretreatment temperature was 493 K for residence time
6, 9 and 12 min with that of untreated material. Similar to previous
case (
Fig. 2
), a peak at around 423 K was observed for pretreatment
time 6 and 9 min before cellulose peak (643 K). However, further
increment of pretreatment time to 12 min did not show any de
nite peak was observed. Moreover cellulose peak
was appeared unsymmetrical. According to Bridgeman
[21]
, hemicel-
lulose content in willow is around 14%. Therefore, it can be
interpreted that low amount of hemicelluloses in Salix makes
hemicelluloses decomposition to merge with cellulose decomposi-
tion, hence, attributed to the unsymmetrical shape at that region.
Beyond 657 K, several broaden shoulders appeared at different
temperatures. Those shoulders can be attributed to deformation of
lignin components. This re
ects that lignin of Salix decomposed in
different steps during pyrolysis rather than having uniform decom-
position over temperature. Upon observation, it should be pointed out
that DTG of untreated Salix revealed a different zone of decomposition
for different biomass components. Therefore, it is possible to
qualitatively justify changes in lignocelluloses' structure and their
corresponding pyrolysis characteristics.
nite
peak before cellulose peak. When pretreatment temperature was
further increased to 501 K (
Fig. 4
), peak at around 423 K was observed
for the case where pretreatment time was 6 min. However, no such
peak in that region was observed when pretreatment time was
12 min.
The region before cellulose peak in every occasion showed overall
higher intensity of decomposition comparing to untreated biomass. In
addition, this region for pretreated biomass shifted to lower
3.3. Change of biomass structure and pyrolysis characteristics
by pretreatment
DTG distribution against temperature for pretreated biomass at
478 K is shown in
Fig. 2
with that of untreated biomass. Pretreatment
1.2
1.2
Untreated
Untreated
6 min
9 min
12 min
6 min
12 min
1
1
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0
0
400
500
600
700
800
900
400
500
600
700
800
900
Temperature [K]
Temperature [K]
Fig. 2.
Change of DTG distributions due to steam explosion (478 K; 6, 9 and 12 min).
Fig. 4.
Change of DTG distributions due to steam explosion at (501 K; 6 and 12 min).
1852
A.K. Biswas et al. / Fuel Processing Technology 92 (2011) 1849
1854
–
temperature zone than that of untreated biomass. In general,
hardwood hemicelluloses are mostly comprised of xylan (4-O-
methylglucuronoxylans)
[12]
. This component goes through depoly-
merization reaction and reduces hemicelluloses to smaller molecular
weight components which in turn exhibit sensitivity to low
temperature of pyrolysis. The observed peak at around 423 K
(
Figs. 2, 3 and 4
) can be attributed to the cross-linking reactions of
lique
cellulose, and recrystallined the Paracrystalline by releasing free
water molecule of wood cell. However, increased crytallinity does not
show any dependency on cellulose decomposition. Ye and coworkers
[27]
observed increase in crystallinity in steam exploded biomass
while they also observed reduction in mean hydrogen bond strength
and degree of polymerization in steam pretreated biomass. Hence,
nature of cellulose peak intensity in DTG might be also related to the
strength of hydrogen bond and degree of polymerization. However,
further detailed research is required to explain this nature of cellulose
decomposition of pretreated biomass. Although both pretreatment
parameters, temperature and time, played signi
ed
D
-xylose.
D
-xylose is the major monosaccharide of xylan and
its melting point varies from 417 K to 424 K
[22,23]
. With the
increment of pretreatment conditions, pretreatment temperature and
time, hemicellulose can be hydrolyzed to monosaccharide. It can be
thought that
D
-xylose molecules that had been distributed to the
wood structure came into contact with other molecules after melting,
and the cross-linking reactions occurred. Further increment of both
pretreatment temperature and time (i.e. 493 K and 12 min) resulted
in the disappearance of peak around 423 K, which indicates the
destruction of monomerized
D
-xylose to smaller molecules.
Cellulose maximum peak intensity varied incoherently with
increase of pretreatment temperature. The peak intensities were
found to be around 0.9 wt.%/K for the raw biomass, 0.5
cant roles in
alteration of hemicelluloses, the effect of pretreatment temperature
was more transparent on the thermal decomposition of cellulose
under the examined conditions.
Signi
cant alteration in the region beyond cellulose peak was also
observed in pretreated residue. For pretreatment temperature of
478 K and 6 min (
Fig. 2
), lignin decomposed gradually after cellulose
peak. In every occasion, peak intensity was observed to be higher than
untreated biomass. In addition, a de
nite shoulder was observed at
around 943 K for each pretreated samples. When pretreatment
temperature was further increased (i.e. 493 K and 501 K), in
Figs. 3
and 4
, the region beyond cellulose peak showed a slight shift towards
lower temperature. This shift in peak can be suggested to the
–
0.56 wt.%/K
for pretreated biomass at 478 K, 0.95
–
1.03 wt.%/K for pretreated
biomass at 493 K, and 0.94
0.96 wt.%/K for pretreated biomass at
501 K. Previous studies showed that cellulose decomposition of
biomass was related to alkali metal content and crystallinity of
biomass
[20,24]
. Higher alkali metal content tends to reduce cellulose
decomposition temperature and rate during pyrolysis
[24]
. In our
previous study, it was found that SE pretreatment reduced alkali
metal content in biomass substantially
[5]
. Especially potassium (K)
content was reduced with increase of pretreatment temperature.
Therefore, no certain dependency of alkali metal on cellulose
decomposition rate and temperature can be correlated.
To examine the effect of crystallinity on cellulose decomposition,
X-ray diffraction (XRD) analysis was performed on three samples
including untreated biomass.
Fig. 5
represents diffraction pattern with
2
–
a
1.2
1
0.8
θ
that varies from 10° to 30°. Two broad peaks were observed at the
0.6
2
values at around 15° and 22° for untreated biomass, which
represents 101 and 002 lattice spacing in cellulose of wood
[25]
. Those
peaks became narrow when biomass was pretreated at 478 K for
12 min. XRD pattern of pretreated residue produced at 501 K and
12 min exhibited similar narrow and intense peak at those positions.
These observations suggest increase of crystallinity in pretreated
biomass. Similar observation was made by Yamashiki and coworkers
[26]
for steam exploded residue. They explained that high temper-
ature water penetrated to the Paracrystalline and amorphous part of
θ
0.4
0.2
0
400
500
600
700
800
900
Temperature [K]
b
1.2
1
501 K
0.8
478 K
0.6
0.4
Untreated
0.2
10
15
20
25
30
0
400
500
600
700
800
900
2
θ
[deg]
Temperature [K]
Fig. 5.
XRD patterns of raw biomass and steam exploded biomass (478 K and 501 K;
12 min).
Fig. 6.
Repeatability of DTG curves: (a) 478 K; 12 min and (b) 501 K; 12 min.
A.K. Biswas et al. / Fuel Processing Technology 92 (2011) 1849
1854
1853
–
Table 3
Repeatability of crystallinity investigation by XRD.
Nomenclature
m
i
mass [mg]
m
o
initial mass [mg]
Pretreatment conditions
Crystallinity index [
−
]
m
∞
nal mass [mg]
Temperature [K]
Time [min]
1st
2nd
T
temperature [K]
Untreated biomass
–
0.252
–
X
conversion ratio [
−
]
478
12
0.352
0.363
α
Residual mass [
−
]
501
12
0.430
0.396
formation of thermolabile chemical bond due to increase of the
severity of the process. However, intensity of those peaks reduced
with the increase of pretreatment temperature from 478 K to 493 K
and 501 K.
Reduction in intensity of the lignin peak during increase of
pretreatment temperature can be attributed to the increase of Klason
lignin in the biomass due to condensation and repolymerization
reaction between decomposition product of hemicellulose and lignin.
According to Chau and Wayman
[28]
,atdrasticpretreatment
condition, some reactive components from hemicellulose such as
furfural may react with lignin and increase fraction of acid insoluble
lignin in biomass. Ramons et al.
[12]
mentioned that at severe
condition,
Acknowledgements
Authors would like to thank Prof. K.V. Rao and Prof. Gudio Zacchi
for letting us use their test facilities. The authors also acknowledge
nancial support from EU and KIC Innoenergy.
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