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Bioresource Technology 102 (2011) 2916–2924
Contents lists available at ScienceDirect
Bioresource Technology
journal homepage: www.elsevier.com/locate/biortech
Pretreatment efficiency and structural characterization of rice straw
by an integrated process of dilute-acid and steam explosion for bioethanol
production
Wen-Hua Chen ⇑, Ben-Li Pen, Ching-Tsung Yu, Wen-Song Hwang
Cellulosic Ethanol Program, Institute of Nuclear Energy Research, Taoyuan, Taiwan, ROC
a r t i c l e
i n f o
Article history:
Received 17 June 2010
Received in revised form 9 November 2010
Accepted 11 November 2010
Available online 4 December 2010
Keywords:
Rice straw
Dilute-acid pretreatment
Acid-catalyzed steam explosion
Integrated pretreatment process
Enzymatic hydrolysis
a b s t r a c t
The combined pretreatment of rice straw using dilute-acid and steam explosion followed by enzymatic
hydrolysis was investigated and compared with acid-catalyzed steam explosion pretreatment. In addition
to measuring the chemical composition, including glucan, xylan and lignin content, changes in rice straw
features after pretreatment were investigated in terms of the straw’s physical properties. These properties
included crystallinity, surface area, mean particle size and scanning electron microscopy imagery. The
effect of acid concentration on the acid-catalyzed steam explosion was studied in a range between 1%
and 15% acid at 180 °C for 2 min. We also investigated the influence of the residence time of the steam
explosion in the combined pretreatment and the optimum conditions for the dilute-acid hydrolysis step
in order to develop an integrated process for the dilute-acid and steam explosion. The optimum operational conditions for the first dilute-acid hydrolysis step were determined to be 165 °C for 2 min with
2% H2SO4 and for the second steam explosion step was to be carried out at 180 °C for 20 min; this gave
the most favorable combination in terms of an integrated process. We found that rice straw pretreated
by the dilute-acid/steam explosions had a higher xylose yield, a lower level of inhibitor in the hydrolysate
and a greater degree of enzymatic hydrolysis; this resulted in a 1.5-fold increase in the overall sugar yield
when compared to the acid-catalyzed steam explosion.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Lignocellulosic materials such as agricultural, hardwood and
softwood residues are potential sources of sugars for ethanol
production. Currently, a large number of studies regarding the
utilization of lignocellulosic biomass as a feedstock for producing
bioethanol are being carried out. A number of processes based on
the application of enzymes for ethanol production from lignocellulosic biomass have been proposed (Himmel et al., 2007). In all cases,
pretreatment of the biomass is required to hydrolyze the hemicellulose, and make the cellulose more accessible to the enzymes. An
efficient pretreatment system is crucial to the enzymatic hydrolysis
and thus the fermentation process, which are the essential steps in
the cellulose conversion process to produce bioethanol (Mosier
et al., 2005).
Dilute-acid hydrolysis, steam explosion, liquid hot water extraction, alkaline hydrolysis, ammonia treatment, and biological
⇑ Corresponding author. Address: Cellulosic Ethanol Program, Institute of Nuclear
Energy Research, Atomic Energy Committee, Executive Yuan, No. 1000, Wenhua Rd,
Jiaan Village, Longtan Township, Taoyuan County 32546, Taiwan, ROC. Tel.: +886 3
4711400x5115; fax: +886 3 4711410.
E-mail address: wenhua@iner.gov.tw (W.-H. Chen).
0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biortech.2010.11.052
processes have been used for pretreatment. Among these various
types of pretreatment, dilute-acid hydrolysis and the steam explosion technique are commonly used; these are more effective than
others because they have already been successfully developed for
pretreatment, are known to improve sugar recovery (particularly
xylose in the aqueous phase) and enhance opening up of the pretreated fibers to enzymatic digestion (Wyman et al., 2005; Yang
and Wyman, 2008; Alvira et al., 2010).
Rice straw is the most abundant agricultural waste around the
world, and the amount available in Taiwan has been estimated to
be 700–800 million tons every year. With over 90% of the worldwide production, most rice straw is produced in Asia. Therefore, rice
straw is considered to be an attractive alternative for fuel ethanol
production in Asia (Binod et al., 2010). The Institute of Nuclear
Energy Research (INER), a governmental organization in Taiwan,
has been developing lignocellulosic fuel ethanol technology using
rice straw in order to build a test cellulosic ethanol plant. At
present, although many researchers have studied steam explosion
(Moniruzzaman, 1996a,b; Nakamura et al., 2001; Jin and Chen,
2006, 2007) and dilute-acid pretreatment (Roberto et al., 2003;
Karimi et al., 2006; Guo et al., 2009; Hsu et al., 2010) using rice
straw, only a few investigations of the pretreatment effects on
overall total sugar recovery when dilute acid and steam explosion
W.-H. Chen et al. / Bioresource Technology 102 (2011) 2916–2924
the glucose yield from the subsequent enzymatic hydrolysis. In
addition, the structural properties of the pretreated rice straw solid
residues after the different pretreatment processes were also
investigated by measuring surface area, mean particle size and
crystallinity as well as SEM microscopy imagery. The aim was to
estimate the pretreatment efficiencies of the different processes
and to clarify the effects of acid pretreatment and explosion on
the enzymatic hydrolysis.
pretreatment of rice straw are combined have been described (Vlasenko et al., 1997). In general in this context, overall sugar yield has
been reported to be the most import parameter when evaluating
the production cost of bioethanol. Therefore, studying how the pretreatment process affects overall sugar yield in combination with
the enzymatic hydrolysis process is very important.
It has been shown that highly severe conditions are required
during steam pretreatment to improve the enzymatic digestibility of
cellulose, but these conditions will also cause greater degradation
of xylose. In other words, obtaining both hemicellulose (xylose)
recovery and high cellulose utility at the same time is difficult
(Thomsen et al., 2006, 2008). Therefore, a two step dilute-acid
pretreatment has been proposed. The first step is performed under
low severity to give a high recovery of hemicellulose-derived
fermentable sugars in the liquid; the second step is carried out
under greater severity in order to make the cellulose more accessible
to enzymatic attack (Nguyen et al., 2000; Boussaid et al., 2000; Kim
et al., 2001; Söderström et al., 2003; Thomsen et al., 2006; Sørensen
et al., 2008; Monavari et al., 2009).
Based on the above viewpoint, we propose a two-step pretreatment process (an integrated process of dilute-acid and steam
explosion) to improve the overall saccharification efficiency with
rice straw. The first step is a dilute-acid hydrolysis to maximize
the xylose conversion and the second step is steam explosion to
destroy the rice straw in order to enhance enzymatic hydrolysis.
The flow chart for the acid-catalyzed steam explosion and the integrated process of dilute-acid and steam explosion is shown in
Fig. 1. The goal of this study was to define the optimal conditions
of the different pretreatment processes of rice straw, including
hydrothermal, steam explosion, acid-catalyzed steam explosion,
dilute-acid pretreatment and the integrated process. These were
evaluated by estimating the xylose yield after pretreatment and
(a)
2. Methods
2.1. Biomass materials
Air-dried rice straw with a moisture content of 10% was reduced to about 2 cm in length by chopping and used as the raw
material. The rice straw was mainly obtained from a single private
farm in Taiwan. The obtained composition of the raw rice straw
was glucan 31.1 ± 0.8%, xylan 18.7 ± 0.2%, arabinan 3.6 ± 0.1%,
extractive 9.0 ± 1.2%, ash 14.5 ± 0.1% and lignin 13.3 ± 0.6% on a
dry weight basis.
2.2. Pretreatment
The dilute-acid and steam explosion pretreatment of the rice
straw was carried out in a steam explosion apparatus. The steam
explosion apparatus consisted of a steam generator, a reactor and
a receiver. The capacity of the reactor was 6 L, while the maximum
working pressure and temperature was 10 kg/cm2 and 180 °C,
respectively. Approximately 100 g of chopped rice straw with or
without soaking in an appropriate amount of sulfuric acid solution
at 15% (w/w) solid loading for 4 h was introduced into the reactor,
Rice straw
Presoaking (impregnation)
Acid-catalyzed
steam explosion
Dilute H2SO4
Solid
Solid/Liquid
Separation
Enzymatic hydrolysis
Liquid
Saturation steam
Xylose-rich hydrolysate
Xylose yield (P)
Glucose yield (P)
Xylose yield (E)
Glucose yield (E)
Total saccharification yield
(b)
Rice straw
Presoaking (impregnation)
Dilute-acid pretreatment
(Step 1)
2917
Dilute H2SO4
Saturation steam
Solid/Liquid Solid
Separation
Steam explosion
(Step 2)
Solid/Liquid
Separation
Liquid
Solid
Saturation steam
Xylose-rich hydrolysate
Enzymatic hydrolysis
Xylose yield (P)
Xylose yield (E)
Glucose yield (P)
Glucose yield (E)
Total saccharification yield
Fig. 1. The flow charts of (a) the acid-catalyzed steam explosion and (b) the proposed integrated process of dilute-acid and steam explosion in this study.
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W.-H. Chen et al. / Bioresource Technology 102 (2011) 2916–2924
which was then steam heated. The reactor heat-up time varied
from 10 to 30 s, depending on the pretreatment temperature. After
the desired steaming time (2–20 min), a 200 ball valve at the bottom
of the reactor was used to rapidly reduce the vessel to atmospheric
pressure. This resulted in the solid and liquid (slurry) products
being explosively released into the flash receiver with cooling. In
the case of the dilute-acid pretreatment experiment, no such
explosive release occurred and the pretreated mixture was transferred from the reactor after the selected retention time and
collected. The resulting mixtures were filtered and the total
volume of liquid in the filtrate and the weight of solid residue were
recorded.
In the study, the rice straw was pretreated by an acid-catalyzed
steam explosion using different sulfuric acid concentrations in a
range between 1% and 15% (w/w) at 180 °C (10 kg/cm2) for
2 min. With regard to the integrated process of dilute-acid and
steam explosion pretreatment. As shown in Fig. 1(b), the first step
is dilute-acid hydrolysis to maximize the xylose conversion and
the second step is steam explosion. The xylose-rich hydrolysate
after dilute-acid pretreatment was obtained by filter press separation and the cellulose-rich cake was sequentially pretreated by the
steam explosion process. The dilute acid (the first step) pretreatment were performed at 125–165 °C for 2–5 min using 1–8%
(w/w) acid concentration. The liquid fraction was the xylose-rich
hydrolysate and the solid fraction was sequentially treated by
the second steam explosion at 180 °C (10 kg/cm2) for 20 min.
2.3. Enzymatic hydrolysis
The wet pretreated residue (about 1 g dry weight) was transferred to a 250 ml shake flask and then the pH was adjusted to
4.8 by adding 50 mM acetate buffer at 2% (w/v) solid loading.
The flask was put in a shaking water bath (FIRSTEK; Model
B603D) at 50 °C and 100 rpm for 30 min. An appropriate amount
of cellulase with a filter paper activity of 60 FPU/ml (Genencor,
Spezyme CP) was added at this point. The digestion of the cellulose
was conducted at a cellulase activity level of 20 FPU/g cellulose.
The enzymatic digestion was carried out at 50 °C and 100 rpm
for 72 h. All the experiments were performed in triplicate, with
the average value being reported.
Win2 V. 2.1). A scanning electron microscope (SEM) (Hitachi
S-4800 SEM instrument operated at 10–15 kV accelerated voltage)
was used to observe surface morphology and size of the rice straw
before and after pretreatment (Yu et al., 2009). The structure of the
pretreated residues was characterized by powder X-ray diffraction
(XRD) (Bruker D8 Advance). The particle size distribution of
pretreated residues was measured by a Beckmann Coulter LS 230
Laser Diffraction Particle Size Analyzer. Detail description of the
related experiments can be found in our previous report (Hsu
et al. 2010).
2.5. The definition of conversion and saccharification yield
The conversions of pretreatment and enzymatic hydrolysis are
calculated by the following equations:
Xylose conversion (P) (%) = [(xylose (g) released in pretreatment)/(total xylose (g) in the feedstock)] 100
Glucose conversion (P) (%) = [(glucose (g) released in pretreatment)/(total glucose (g) in the feedstock)] 100
Xylan conversion (E) (%) = [(xylose (g) released in enzymatic
hydrolysis)/(total xylan (g) in the feedstock)] 100
Cellulose conversion (E) (%) = [(glucose (g) released in enzymatic hydrolysis)/(total cellulose (g) in the feedstock) ] 100
The definition of total saccharification yield (overall sugar yield)
is the summation of xylose and glucose monomers saccharification
yields. The xylose/glucose saccharification yields are calculated by
the following equations:
Xylose saccharification yield (%) = [(xylose (g) released in pretreatment (P) and enzymatic hydrolysis (E))/(total xylose and
glucose (g) in the feedstock)] 100
Glucose saccharification yield (%) = [(glucose (g) released in pretreatment (P) and enzymatic hydrolysis (E))/(total xylose and
glucose (g) in the feedstock)] 100
Total saccharification yield (%) = [(xylose and glucose (g)
released in pretreatment (P) and enzymatic hydrolysis (E))/
(total xylose and glucose (g) in the feedstock)] 100
3. Results and discussion
2.4. Analysis methods
The identified components are extractive, ash, cellulose, hemicellulose and lignin of the untreated and treated rice straw were
determined according to Laboratory Analytical Procedures from
the National Renewable Energy Laboratory (NREL) (Sluiter et al.,
2005). The carbohydrate contents were determined by measuring
the hemicellulose (xylan, arabinan) and cellulose (glucan) of derived sugars.
The composition of the hydrolysates after pretreatment and
enzymatic hydrolysis was found to include cellobiose, glucose,
xylose, arabinose, acetic acid and furfural, which were filtered
through a 0.45 lm-pore-size filter and measured by high performance liquid chromatography (HPLC) using a packed column
(Agilent 1200 series HPLC equipped with Coregrl-87H3 Column
Transgenomic Technologies at 65 °C with 4 mM H2SO4 as the eluent at a flow rate of 1.0 mL/min).
BET surface area information of the pretreated residues was
measured by N2 adsorption/desorption measurements (Quantachrome instrument; model: NOVA 4200e) done at 196 °C (77 K).
Prior to measurement, all biomass materials were dried at 40 °C
for 48 h and then a sample (0.5–1 g) was put into the sample tube
of the Quantachrome instrument and degassed using a vacuum
for 16 h. The BET surface area and pore volume were obtained from
the N2 adsorption/desorption curves using NOVA software (NOVA
3.1. Effect of the method used for residues recovery on enzymatic
digestibility
Two different processes for explosion pretreatment can be
used; one is to explosively relieve the pressure, such as an ammonia fiber explosion (AFEX) (Teymouri et al., 2005), and the other is
steam explosion, which results in pretreated products draining
away (Moniruzzaman, 1996a, 1996b; Vlasenko et al., 1997; Cara
et al., 2006; Ewanick et al., 2007; Ruiza et al., 2008). To explore
the effect of the explosion processes on structure and enzymatic
digestibility of pretreated residues, samples pretreated by the
different steam explosion processes, namely explosively relieving
the pressure (denoted as HR-P) and steam explosion accompanied
by drainage of the pretreated products (denoted as HR-SE) were
prepared at 180 °C for 20 min and compared to the direct cooling
process under the same steam pretreatment condition (hydrothermal process; denoted as HR). In this context, the crystallinity and
BET surface area of the lignocellulosic biomass are thought to play
important roles in susceptibility to enzyme digestion; therefore,
we measured the crystallinity and BET surface area of the
pretreated residue in order to use these parameters to reflect
pretreatment efficiency.
Table 1 summarizes the total solid recovery, sugar composition,
physical properties, enzymatic hydrolysis conversion and overall
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W.-H. Chen et al. / Bioresource Technology 102 (2011) 2916–2924
Table 1
The total solid recovery, sugar composition, physical properties, enzymatic hydrolysis conversions and overall sugar yield of untreated and treated rice straw.
Sample
a
Untreated
HR
HR-P
HR-SE
Total solid recovery (%)
–
80.6
77.8
85.4
Sugar composition in residuesb
Physical properties
c
2
Enzymatic hydrolysis conversion (%)d
Glucose (%)
Xylose (%)
CrI (%)
BET (m /g)
Cellulose
Xylan
34.5 ± 0.8
49.3 ± 0.7
48.7 ± 0.4
46.9 ± 0.9
21.3 ± 0.2
16.6 ± 0.4
18.3 ± 0.3
18.7 ± 0.5
51.3
54.1
55.8
58.4
1.32
4.32
3.77
5.51
5.2 ± 0.5
34.6 ± 3.5
38.7 ± 0.5
54.8 ± 0.9
–
29.7 ± 3.3
32.0 ± 1.4
33.0 ± 0.8
Overall sugar yield (%)e
–
32.5 ± 2.5
34.2 ± 4.8
49.2 ± 1.2
a
HR represents hydrothermal at 180 °C for 20 min; HR-P represents explosively relieving the pressure after hydrothermal process; HR-SE represents steam explosion
accompanied by drainage of the pretreated products after hydrothermal.
b
Each value is the average of three replicates and calculated on the basis of dry weight.
c
CrI (%) = (I002 – Iam)/I002 100. Where I002 is intensity of the 002 peak at 2h = 22.4° and Iam is intensity of the background scatter at 2h = 18.7° (Kim and Holtzapple, 2006).
d
Cellulose to glucose and xylan to xylose conversion after 72 h of enzymatic hydrolysis. Each value is the average of three replicates.
e
Overall sugar yield is based on the recovered monosugers (glucose and xylose) obtained after 72 h of enzymatic hydrolysis and in the liquid fraction after pretreatment
per dry weight of the raw rice straw. (Total sugar content in the raw rice straw is 55.8 g/100 g.).
sugar yield of the untreated and treated rice straw. The xylose contents of the three treated samples are similar and were reduced
from an untreated value of 21.3% to 16.6% and the glucose contents
were increased from 34.5% when untreated up to 49.3%. Furthermore, the RCI (relative crystalline index), specific BET surface area
and enzymatic hydrolysis conversion are all found to have the following order: HR-SE > HR-P > HR >> untreated rice straw. Although
an enzymatic hydrolysis enhancement of rice straw pretreated by
both steam explosion processes was observed, it is clear that HRSE is better at conversion than HR-P. This suggests that the shear
stress through thermal explosion decomposition and the flow of
treated slurry create tensile forces that break the rice straws into
more fragmental small pieces of tissue; this results in further substantial breakdown of the lignocellulosic structure. Therefore, the
BET surface area of the rice straw and the accessibility of the cellulose components to degradation by enzymes are greatly increased
from 1.32% to 5.51% and 5.2% to 54.8%, respectively.
Based on the results above, it can be seen that the method used
for product recovery obviously affects the surface area of the rice
straw and the enzymatic hydrolysis efficiency. Thus, the steam
explosion accompanied by drainage of the pretreated products
process was selected for the pretreatment efficiency studies below
using rice straw.
3.2. Acid-catalyzed steam explosion pretreatment
In general, the hemicellulose fraction of the lignocellulose complex is mainly solubilized, while cellulose and lignin remain in the
fiber fraction during the acid-catalyzed steam explosion pretreatment. Xylan is the major component present in the hemicellulose
structure of rice straw, so the presence of xylose in the filtrate
can be used as an indicator of the solubilization level of hemicellulose, which is easily utilized by the fermentation process. In this
work, the rice straw was pretreated by an acid-catalyzed steam
explosion using different sulfuric acid concentrations in a range between 1% and 15% at 180 °C (10 kg/cm2) for 2 min. The flow chart
for the acid-catalyzed steam explosion is shown in Fig. 1(a). The
sugar compositions, physical properties and enzymatic hydrolysis
of the pretreated residues obtained by the acid-catalyzed steam
explosion using different acid concentration is presented in Table
2. The xylan, glucan and lignin contents obtained in the pretreated
residues were 0.5–2.7%, 45.2–53.4% and 20.8–28.9%, respectively.
Normally, xylan content has been found to decrease with increasing acid concentration and eventually plateaus at approximately
3% sulfuric acid; by contrast, glucan content increases, mainly
due to the removal of most of the hemicelluloses by acid-catalyzed
hydrolysis. It has been shown that there is a relative increase in the
amount of lignin present after treatment due to the removal of
hemicelluloses. With regard to fermentation inhibitors, the con-
centrations of furfural and acetic acid were lower than 1 g/L when
the acid concentration was lower than 3%. On the other hand, the
concentrations of furfural and acetic acid increased to 1.5–3.5
and 2.9–3.5 g/L, respectively, at acid concentration higher than 3%.
The sugar conversions of pretreatment (P) and the subsequent
enzymatic digestibility (E) of pretreated residues obtained after
72 h are shown in Fig. 2. From Fig. 2, it can be seen that the xylose
conversion in the pretreatment step (P) decreases with acid
concentration, while the glucose conversion (P) seems independent of acid concentration. A maximum glucose conversion (E) of
72.5% was attained at a 3% acid concentration for subsequent enzymatic hydrolysis after 72 h. Moreover, when the acid concentration
was increased to 8%, a significant increase in cellobiose conversion
with a corresponding decrease in the amount of glucose was
observed. This phenomenon can be ascribed to the presence of a
b-glucosidase inhibitor such as lignin, which is produced by high
sulfuric acid concentration pretreatment and results in an altered
ratio of glucose and cellobiose. In other words, increasingly higher
sulfuric acid concentrations during pretreatment lead to the
pretreated biomass having greater hydrophobicity; this results in
an increased adsorption of cellulose enzyme onto the lignin droplets by means of hydrogen bonds (Selig et al., 2007; Guo et al.,
2008). Merino and Cherry (2007) also reported that unwashed
acid-pretreated corn stover was found to have a lower cellulose
conversion during enzymatic hydrolysis than washed acid-pretreated corn stover. Therefore, washing pretreated biomass or an
inhibitor removal step could be essential to preventing a lower
conversion yield from pretreated biomass. In our results, the enzymatic hydrolysis result of washing rice straw residues treated by a
high acid content (11%) provides further support for the above
argument. No cellobiose was observed and the glucose conversion
was increased from 25.0% to 85.2% after hydrolysis of the washed
steam-exploded rice straw for 72 h. The results show that the optimum acid concentration is 3% at 180 °C (10 kg/cm2) for 2 min by
acid-catalyzed steam explosion in the reaction system, and a high
acid concentration is disadvantageous to both the xylose yield after
pretreatment and the glucose yield from the subsequent enzymatic
hydrolysis.
The purpose of pretreatment is not only to remove xylose from
hemicellulose but also to increase the rate of enzymatic hydrolysis
of cellulose to glucose. Thus, it is necessary to estimate pretreatment efficiency by combining total xylose and glucose conversion
yield after both the pretreatment and enzymatic hydrolysis
processes; this is because the purpose of pretreatment is to
produce sugars for conversion to bioethanol. Our results show that
only 49.6% of the total saccharification yield was obtained using
the acid-catalyzed steam explosion pretreatment at a combined
severity factor (CSF = log Ro pH; where Ro = texp [(Tr 100)/
14.75]) of 1.6 and enzymatic hydrolysis with 20 FPU/g of cellulose
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W.-H. Chen et al. / Bioresource Technology 102 (2011) 2916–2924
Table 2
Effect of acid concentration of the acid-catalyzed steam explosion pretreatment on the compositions, physical properties and enzymatic hydrolysis of the pretreated residues.
Samplea
Total solid recovery (%)
Composition content (%)c
Glucan
AC-SE-1b
AC-SE-2
AC-SE-3
AC-SE-8
AC-SE-11
AC-SE-15
a
b
c
d
e
59.8
53.0
44.3
54.7
34.5
38.2
45.2 ± 0.9
46.7 ± 1.1
53.4 ± 1.4
48.0 ± 1.0
46.7 ± 1.2
49.0 ± 0.8
Xylan
Physical properties
d
Lignin
2.7 ± 0.6
1.4 ± 0.2
1.2 ± 0.2
0.9 ± 0.1
0.7 ± 0.1
0.5 ± 0.1
ASL
AIL
Total
4.1 ± 0.2
3.6 ± 0.3
3.7 ± 0.3
3.5 ± 0.4
3.5 ± 0.2
3.2 ± 0.4
16.7 ± 1.4
20.9 ± 1.7
22.6 ± 1.2
24.4 ± 1.5
25.3 ± 1.2
25.0 ± 2.3
20.8 ± 1.4
24.4 ± 1.6
26.3 ± 1.4
28.1 ± 1.5
28.9 ± 1.5
28.2 ± 2.5
2
Enzymatic hydrolysis conversion (%)e
CrI (%)
BET (m /g)
Cellulose
Xylan
55.1
56.9
57.1
58.7
56.9
59.6
5.59
9.73
4.82
5.87
5.93
6.63
58.7 ± 1.0
68.0 ± 0.8
72.5 ± 1.1
57.8 ± 0.9
25.0 ± 1.7
10.8 ± 0.3
6.8 ± 0.8
4.8 ± 0.8
4.7 ± 0.5
1.1 ± 0.2
2.4 ± 0.4
9.2 ± 1.1
All the acid-catalyzed steam explosion experiments were performed at 180 °C for 2 min.
AC-SE-1 where 1 represents acid concentration (w/w)%.
Each value is the average of three replicates and calculated on the basis of dry weight.
CrI (%) = (I002 Iam)/I002 100 where I002 is intensity of the 002 peak at 2h = 22.4° and Iam is intensity of the background scatter at 2h = 18.7° (Kim and Holtzapple, 2006).
Cellulose to glucose and xylan to xylose conversion after 72 h of enzymatic hydrolysis. Each value is the average of three replicates.
Conversion
(g/g xylan or glucan)
0.8
Xylose(P)
Glucose(P)
Xylose(E)
Glucose(E)
Cellobiose(E)
0.6
0.4
0.2
0.0
0
4
8
12
16
H2SO4(%)
Fig. 2. Effect of sulfuric acid concentration on the obtained sugar conversions of
pretreatment (P) and the sequential enzymatic digestibility (E) of pretreated
residues using acid-catalyzed steam explosion at 180 °C (10 kg/cm2) for 2 min.
loading after 72 h (Chum et al., 1990). Again, this implies that
reaching both high hemicellulose (xylose) recovery and high cellulose utility at the same time is impossible (Thomsen et al., 2008).
3.3. Two-step pretreatment process (integrated process of dilute-acid
and steam explosion)
As mentioned above, high glucose conversion during enzymatic
hydrolysis can be obtained under severe pretreatment operational
conditions, but these higher severity conditions result in a lower
xylose recovery. Thus, we propose a two-step pretreatment to improve the overall saccharification efficiency. The first step is diluteacid hydrolysis to maximize the xylose conversion and the second
step is steam explosion to break the protective sheath around the
cellulose such that it becomes more vulnerable to enzymatic
hydrolysis. A wide range of conditions were used to determine
the most favorable combination.
3.3.1. The residence time of the steam explosion process
The steam explosion pretreatment step was performed using the
rice straw residue obtained from the first dilute-acid pretreatment
at 145 °C and 3% H2SO4 for 10 min (CSF = 1.6). The xylose conversion was 71.6% and the cellulose-rich residue contained mainly
48.4% glucan, 3.3% xylan and 23.6% lignin. This was then steam treated in the second step for various residence times (1, 3, 5, 10, 15 and
20 min) at 180 °C (10 kg/cm2) and then the cake underwent a sudden explosion. The change in composition, physical properties and
enzymatic hydrolysis of the steam explosion pretreated residues
are shown in Table 3.
As shown in Table 3, glucan content increased from 34.5% to
48.4% and xylan content obvious decreased from 21.3% to 3.3%
after the first dilute-acid pretreatment. Moreover, glucan content
increased from 48.4% to 55.3% and xylan content decreased from
3.3% to 1.3% after steam explosion. It can be seen that glucan and
xylan content showed in the range of 51.6–55.3% and 1.2–1.9%
for various steaming residence times. In comparison, the treated
residue using the dilute-acid pretreatment shows apparent change
in glucan and xylan content, whereas, only slightly decrease in
glucan and xylan content in the steam explosion step after 1 min
steaming time. On the other hand, glucan and xylan contents
almost unchanged with increasing residence time (3–20 min).
Therefore, it is noted that the main component of the contents
remained practically unchanged with increasing residence time,
while variations in glucan and xylan content for the samples before
and after steam treated explosion were observed. The results
reveal that the steam cooking time seemed to have no apparent
influence on glucan, xylan and lignin contents in the treated
samples, whereas the explosion process did result in some alterations in those components. In general, an increase in the cellulose
crystallinity index with residence time was observed in the steam
explosion treated samples and reached a plateau after 5 min.
Furthermore, the mean particle size of the pretreated solid suspensions was reduced to ca. 237.7 lm within 10 min and slightly
decreased with steaming time, whereas the BET surface area was
observed to apparently increase after 10 min. In terms of enzymatic hydrolysis, it was shown that the maximum glucose conversion (90.6%) and BET surface area (19.5 m2/g) were obtained after
20 min steaming time.
Based on the above experimental results, the influence of steam
explosion is greater than steam cooking on the glucan and xylan
contents remaining in the pretreated residues, while the physical
characteristics, such as crystallinity, BET surface area and particle
size, as well as enzymatic hydrolysis, depend not only on the steam
explosion but also on the residence time in the two-step pretreatment process. In other words, it is suggested that the effect of the
steam explosion is higher than that of steam cooking in terms of
action on the glucan and xylan contents, while the physical characteristics and enzymatic hydrolysis are influenced by both steam
cooking and the explosion process during the two-step pretreatment process.
A number of researchers have reported that the best model for
xylose formation and degradation is a pair of parallel first-order
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W.-H. Chen et al. / Bioresource Technology 102 (2011) 2916–2924
Table 3
Effect of the steaming time of the steam explosion step of the integrated process on the compositions, physical properties and enzymatic hydrolysis of the pretreated residues.
Residence time min)
Untreated
0a
1
3
5
10
15
20
a
b
c
d
Composition content (%)b
Physical properties
Glucan
CrI (%)c
BET (m2/g)
Mean Particle Size (lm)
Cellulose
Xylan
51.3
52.2
53.1
56.9
57.6
57.7
56.9
59.0
1.32
4.43
4.53
5.34
5.99
14.0
16.4
19.5
–
–
429.8
385.9
301.7
237.7
235.5
233.9
5.2 ± 0.5
66.2 ± 1.1
64.2 ± 0.2
68.7 ± 1.3
75.0 ± 0.8
76.2 ± 0.5
82.3 ± 2.2
90.6 ± 1.8
–
75.1 ± 3.4
66.0 ± 0.8
64.4 ± 0.4
87.0 ± 0.8
92.9 ± 5.1
84.3 ± 3.9
82.8 ± 4.5
34.5 ± 0.8
48.4 ± 1.4
52.9 ± 0.9
52.8 ± 1.5
53.3 ± 0.4
51.6 ± 1.5
52.5 ± 0.3
55.3 ± 0.5
Xylan
Lignin (%)
21.3 ± 0.2
3.3 ± 0.3
1.9 ± 0.0
1.8 ± 0.1
1.4 ± 0.0
1.2 ± 0.0
1.4 ± 0.1
1.3 ± 0.0
ASL
AIL
Total
4.7 ± 0.3
4.0 ± 0.5
4.4 ± 0.6
4.1 ± 0.1
4.2 ± 0.4
4.6 ± 0.5
4.2 ± 0.1
3.9 ± 0.6
9.4 ± 0.5
19.6 ± 1.6
22.6 ± 2.2
19.3 ± 0.7
21.7 ± 1.6
18.2 ± 1.4
22.1 ± 1.9
17.9 ± 0.9
13.3 ± 0.6
23.6 ± 1.9
27.1 ± 2.2
23.4 ± 0.8
25.9 ± 1.4
22.8 ± 1.7
26.3 ± 1.8
21.8 ± 1.5
Enzymatic hydrolysis
conversion (%)d
0 represents the dilute-acid pretreatment of the integrated process.
Each value is the average of three replicates and calculated on the basis of dry weight.
CrI (%) = (I002 – Iam)/I002 100 where I002 is intensity of the 002 peak at 2h = 22.4° and Iam is intensity of the background scatter at 2h = 18.7° (Kim and Holtzapple, 2006).
Cellulose to glucose and xylan to xylose conversion after 72 h of enzymatic hydrolysis. Each value is the average of three replicates.
Table 4
The compositions, physical properties and enzymatic hydrolysis of the pretreated residues after the integrated pretreatment process.
Sample
The first step (dilute-acid pretreatment)
The second step (steam explosion)b
Condition
Composition content (%)d
T (°C)
AP-SE-0.15a
AP-SE-0.72
AP-SE-0.97
AP-SE-1.02
AP-SE-1.19
AP-SE-1.38
AP-SE-1.91
a
b
c
d
e
165
140
125
145
150
165
145
t (min)
2
5
5
5
5
2
5
Acid (%)
0.9
1.1
5.7
2.7
1.6
2.1
8.0
EH
conversion (%)c
37.4 ± 4.0
45.6 ± 1.6
42.8 ± 1.6
48.9 ± 3.7
38.4 ± 3.7
11.5 ± 1.0
0.9 ± 0.1
Glucan
50.2 ± 2.1
54.7 ± 0.6
53.8 ± 0.6
56.1 ± 1.7
48.1 ± 0.3
54.5 ± 1.0
47.2 ± 0.6
Xylan
6.2 ± 0.7
2.1 ± 0.0
1.0 ± 0.2
1.1 ± 0.1
1.2 ± 0.2
4.2 ± 0.2
0.9 ± 0.4
Physical properties
e
Lignin
ASL
AIL
Total
3.5 ± 0.1
3.2 ± 0.2
2.9 ± 0.1
2.8 ± 0.1
3.0 ± 0.1
3.1 ± 0.2
2.6 ± 0.1
25.5 ± 1.7
21.9 ± 0.9
20.3 ± 1.4
25.3 ± 0.6
22.2 ± 1.5
23.1 ± 0.8
32.0 ± 2.4
29.0 ± 1.8
25.1 ± 1.1
23.2 ± 1.3
28.1 ± 0.6
25.2 ± 1.6
26.2 ± 0.5
34.6 ± 2.6
EH conversion (%)c
CrI (%)
BET
(m2/g)
Cellulose
Xylan
60.5
69.5
59.4
60.7
61.1
61.0
65.4
11.01
9.68
12.88
14.33
10.56
9.75
10.56
60.7 ± 1.0
81.4 ± 2.4
71.4 ± 0.3
93.1 ± 2.0
77.4 ± 1.6
84.2 ± 2.0
73.5 ± 1.7
57.4 ± 1.7
69.0 ± 4.5
46.2 ± 2.0
72.1 ± 2.4
90.0 ± 5.0
68.8 ± 2.5
35.2 ± 1.1
AP-SE-0.15 where 0.15 represents the combined severity factor (CSF) of the first step (dilute-acid pretreatment) where CSF = log Ro – pH; Ro = texp [(Tr 100)/14.75]).
Each sample was steam heated at 180 °C (10 kg/cm2) for 20 min and then explosion.
Cellulose to glucose conversion after 72 h of enzymatic hydrolysis. Each value is the average of three replicates.
Each value is the average of three replicates and calculated on the basis of dry weight.
CrI (%) = (I002 – Iam)/I002 100 where I002 is intensity of the 002 peak at 2h = 22.4° and Iam is intensity of the background scatter at 2h = 18.7° (Kim and Holtzapple, 2006).
reactions: one for easy hydrolysis and the other for difficult hydrolysis (Himmel et al., 2007). The easily hydrolysable fraction has
been reported to be 60–80% of the hemicelluloses in various lignocellulosic materials. Based on this information, we are able to
explain why most xylose (72%), which is easy to hydrolyze, was
extracted from the xylan of the hemicellulose after the first
dilute-acid treated under the low severity (145 °C, 3% H2SO4, and
10 min). The xylan content then did not change, even with an
extended steam cooking time as the residual xylan is part of the
difficult hydrolysable fraction. However, the explosion is sufficient
to affect the stubborn xylan molecules. The removal of xylan
increases the porosity of the rice straw and therefore increases
water diffusion inside the channels of the substrate. As a result,
the physical characteristics of the treated rice straw were changed
due to autohydrolysis and adiabatic explosion of water in the pores
of the acid-pretreated residue.
3.3.2. The optimum conditions for the dilute-acid process
Using the above experimental results, it was found that the maximum glucose conversion and BET surface area were obtained after
20 min of steaming time at 180 °C. Thus, the second steam explosion was selected to operate at 180 °C (10 kg/cm2) for 20 min. The
first step of dilute-acid hydrolysis was then performed at different
operational conditions (1–8% acid concentration, 125–165 °C and
2–5 min). The liquid fraction was the xylose-rich hydrolysate and
the solid fraction was sequentially treated by the second steam
explosion at 180 °C (10 kg/cm2) for 20 min. Table 4 summarized
the compositions, physical properties and enzymatic hydrolysis of
the pretreated solid residues produced by the integrated pretreatment process under different severity conditions of the dilute-acid
pretreatment.
The xylose and glucose conversions obtained from pretreatment
versus combined severity factor (CSF) of the dilute-acid hydrolysis
(the first step) and the acid-catalyzed steam explosion are shown
in Fig. 3. The xylose conversion reached a plateau (75%) at
CSF = 1.0–2.0 and we expect that the xylose conversion would
decrease at even higher severities (Kabel et al., 2007). In the case
of the subsequent enzymatic digestibility of pretreated residues
after the second steam explosion step, glucose conversions of
73–93% were obtained after 72 h at 20 FPU/g of cellulose without
b-glucosidase supplementation.
The total saccharification yield made by combining the total
xylose and glucose conversion yield for both processes is an index
of pretreatment efficiency for ethanol bioconversion. Fig. 4 shows a
comparison between acid-catalyzed steam explosion and the integrated pretreatment process, using the total saccharification yield
versus the combined severity factor (CSF). Xylose was mainly
obtained from the pretreatment process, while glucose was mainly
obtained from enzymatic hydrolysis. Both of the pretreatment
processes have higher glucose yields for the enzymatic hydrolysis
(glucose yield (E)) than that expected for acid-catalyzed steam
explosion under the higher severity condition (CSF P 1.7) due to
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W.-H. Chen et al. / Bioresource Technology 102 (2011) 2916–2924
1.0
Conversion
(g/g xylan or glucan)
Xylose Glucose
Acid-catalyzed SE
Two-step process
0.8
0.6
0.4
0.2
0.0
0.0
0.5
1.0
1.5
2.0
CSF
Fig. 3. The obtained xylose and glucose conversions for pretreatment versus
combined severity factor (CSF) of the dilute-acid hydrolysis (the first step) and the
acid-catalyzed steam explosion.
ification yield shows an approximately 1.5-fold increase in that
achieved with an experiment performed at CSF = 1.6 over the
acid-catalyzed steam explosion pretreatment. The result shows
that the optimum operational conditions for the first dilute-acid
hydrolysis step were determined to be 165 °C for 2 min with 2%
H2SO4 and for the second steam explosion step was to be carried
out at 180 °C for 20 min; this gave the most favorable combination
in terms of an integrated process. Under the condition, a total
75.9% of xylan and 77.1% of glucan were converted to xylose and
glucose, respectively. Moreover, Karimi et al. studied the hydrolysis of rice straw using the two step dilute-acid pretreatment and
reported that the ability of the two-stage hydrolysis to depolymerize 78.9% xylan to xylose and 46.6% of glucan to glucose (Karimi
et al., 2006). In our study, 73.7% of xylan and 7.7% of glucan were
converted to xylose and glucose by the integrated pretreatment
process. In comparison, the glucose yield from the pretreatment
step obtained in our study is lower than the obtained by Karimi
et al. using two-step steam pretreatment. However, the goal of pretreatment is to remain a large part of glucan in the treated residues
to proceed the subsequent enzymatic hydrolysis for bioethanol
production.
Saccharification yield
(g/g glucan and xylan in raw material)
3.4. Characterization of pretreated rice straw
1.0
0.8
Acid-catalyzed SE
Two-step process
Xyloe(P)
Glucose(P)
Xylose(E)
Glucose(E)
0.6
0.4
0.2
0.0
1.3 1.5 1.6 1.7 1.9 2.1
0.2 0.7 0.8 0.9 1.0 1.2 1.4 1.9
CSF
Fig. 4. Total saccharification yield versus combined severity factor for the acidcatalyzed steam explosion and the combined severity factor (CSF) of the dilute-acid
hydrolysis (the first step) of the integrated pretreatment process. Total saccharification yield (g sugar/100 g xylose and glucose in raw rice straw) expressed the
sum of glucose (E) and xylose (E) yields obtained after 72 h of the sequential
enzymatic hydrolysis step and xylose (P) and glucose (P) yields obtained in the
pretreatment step.
In our previous study, we showed that features of vascular
bundles, epidermis and silica cells of air-dried untreated rice straw
could be observed by SEM microscopy and reported that the
exterior area of the epidermis revealed multiple areas of damaged
after dilute-acid pretreatment because of the hydrophilic nature
of silica (Yu et al., 2009). In this report, the microscopic observations of the change in the different pretreatment processes were
also studied using scanning electron microscopy (SEM). The exterior epidermis of the residue fibers after dilute-acid pretreatment
(AP) reveals multiple structures. Moreover, the vasculofibrous bundles of HR-SE (the residue fibers after steam explosion) show no
obvious peeling phenomenon with the steam explosion, whereas
comparatively large amounts of microfibers and destructive perforations of the lumen can be observed in SEM images of the residue
treated by acid-catalyzed steam explosion (AC-SE). The rice straw
treated by the integrated pretreatment process (AP-SE) also shows
significant destruction derived structures. This indicates that the
proton ions of H2SO4 should be able to penetrate into the rice straw
surface and further catalyze the de-acetylation and hydrolysis; this
will break the b(1 ? 4)-glycosidic bonds of the hemicelluloses and
promote the defibrillation of rice straw by sequential steam
explosion.
inhibition at high acid concentrations. This suggests the steam
explosion process actually contributes to enzymatic hydrolysis
efficiency. On the other hand, the xylose yield (P) is variable in
terms of the dilute-acid pretreatment for the integrated pretreatment process or acid-catalyzed steam explosion. This suggests
the xylose yield from pretreatment (xylose yield (P)) has more
weight in terms of total saccharification yield than the glucose
yield from enzymatic hydrolysis (glucose yield (E)). As shown in
Fig. 4, 75% of the maximum total saccharification yield was obtained using the integrated pretreatment process of dilute-acid
and steam explosion (two-step pretreatment process) at a combined severity factor of 1.4 in the first step (dilute-acid pretreatment) and enzymatic hydrolysis with 20 FPU/g of cellulose
loading after 72 h. The maximum total sugar yield obtained in
our study is about the same as the obtained from sawdust by
Söderström et al. (77%) using two-step steam pretreatment
(Söderström et al., 2003). Specifically, the maximum total sacchar-
Glucose yield (g/g glucan)
1.0
0.8
0.6
0.4
Untreat
Acid-catalyzed SE (AC-SE)
Two-step process (AP-SE)
Dilute-acid pretreatment (AP)
0.2
0.0
2
4
6
8
10
12
14
16
18
BET(m2/g)
Fig. 5. The relationship between the BET surface area of the rice straw and the
glucose yield in terms of enzyme digestibility by cellulase.
W.-H. Chen et al. / Bioresource Technology 102 (2011) 2916–2924
The SEM microscopic image of the residue after acid-catalyzed
steam explosion (AC-SE) shows some particles attached to the
external surface of fragments of AC-SE. These particles might be released lignin that have been re-deposited and are called lignin
droplets, or other surface degradation products (Selig et al.,
2007). No similar particles can be observed on the external surface
of fragments from the residue treated by the integrated pretreatment process (AP-SE). This might be explained by these particles
being removed by the steam washing during the second steam
explosion step. In our results, acid-soluble lignin (ASL) contents
were observed decreased from 5.3 ± 0.2% to 3.5 ± 0.1% (AP-SE0.15), from 5.2 ± 0.3% to 3.2 ± 0.2% (AP-SE-0.72) and from
3.6 ± 0.1% to 2.6 ± 0.1% (AP-SE-1.91) after the second steam explosion step. In other words, acid-soluble lignin decreased after the
second steam explosion step regardless of the combined severity
factor (CSF) of the first dilute-acid pretreatment step. This change
of acid-soluble lignin provides further support for the above
argument.
The relationship between the BET surface area of rice straw and
enzyme digestibility by cellulase is shown in Fig. 5. The BET surface
areas of the pretreated rice straw using AP (dilute-acid pretreatment), AC-SE (acid-catalyzed steam explosion) and AP-SE (integrated pretreatment process) were found to range from 3.5 to 6,
2.5 to 10 and 10.5 to 16.5 m2/g, respectively. The glucose yield in
terms of enzyme digestibility of the AC-SE (57.8–72.5%) shows a
higher glucose yield (10–30%) than that of the AP (37.4–48.9%)
with a similar BET surface area. On the other hand, AP-SE pretreated rice straw shows a higher glucose yield (71.8–93.1%) and
a larger BET surface area (10.5–16.5 m2/g), indicating that most
of the hemicellulose in AP-SE has been solubilized and removed
after the integrated pretreatment process, which increases the surface area and makes enzymatic hydrolysis easier.
Overall, in order to convert hemicellulose and cellulose for use
in a cellulosic ethanol production process, the two-step pretreatment process has attractive advantages, but further evaluation is
required due to disadvantages in terms of increased machinery
cost, such as the reactor and filter that are used in the two-step
process. In this study, our experiments were carried out in a 6 L
reaction system with a maximum steam pressure of 10 kg/cm2.
We expect that a higher saccharification yield might be obtained
if the acid-catalyzed steam explosion pretreatment was carried
out at a higher steam pressure. Therefore, a batch operation reactor
system and a bench-scale continuous operation reactor system
equipped with a higher base steam pressure for the acid-catalyzed
steam explosion pretreatment and the two-step pretreatment process are under construction and investigation at our institute.
4. Conclusions
We have proposed a two-step process to improve the overall
saccharification efficiency. The first step is dilute-acid hydrolysis
to maximize the xylose conversion and the second step is steam
explosion to destroy the rice straw structure to enhance enzymatic
hydrolysis. The effects of product recovery, dilute-acid and steam
explosion on the structural feature properties of the residue were
also explored. The integrated process shows attractive advantages
for the utility of holocellulose in the feedstock for cellulosic ethanol production.
Acknowledgements
The authors thank Mr. Ming-Chia Yiu, Wei-Fan Huang and ShihYi Lee for assistance with the pretreatment test and also appreciate
the technical assistance of Dr. Wei-His Chen, Mr. Chen-Chien Lin,
Jing-Feng Wang and Miss. Ling-Chuan Cheng.
2923
References
Alvira, P., Tomás-Pejó, E., Ballesteros, M., Negro, M.J., 2010. Pretreatment
technologies for an efficient bioethanol production process based on
enzymatic hydrolysis: a review. Bioresour. Technol. 101, 4851–4861.
Binod, P., Sindhu, R., Singhania, R.R., Vikram, S., Devi, L., Nagalakshmi, S., Kurien, N.,
Sukumaran, R.K., Pandey, A., 2010. Bioethanol production from rice straw: an
overview. Bioresour. Technol. 101, 4767–4774.
Boussaid, A.L., Esteghlalian, A.R., Gregg, D.J., Lee, K.H., Saddler, J.N., 2000. Steam
pretreatment of Douglas–Fir wood chips. Appl. Biochem. Biotechnol. 84–86,
693–705.
Cara, C., Ruiz, E., Oliva, J.M., Sáez, F., Castro, E., 2006. Enhanced enzymatic hydrolysis
of olive tree wood by steam explosion and alkaline peroxide delignification.
Process Biochem. 41, 423–429.
Chum, H.L., Johnson, D.K., Block, S.K., Overend, R.P., 1990. Pretreatment-catalyst
effects of combined severity parameter. Appl. Biochem. Biotechnol. 24 (25), 1–
14.
Ewanick, S.M., Bura, R., Saddler, J.N., 2007. Acid-catalyzed steam pretreatment of
Lodgepole Pine and subsequent enzymatic hydrolysis and fermentation to
ethanol. Biotechnol. Bioeng. 98, 737–746.
Guo, G.L., Chen, W.H., Chen, W.H., Men, L.C., Hwang, W.S., 2008. Characterization of
dilute acid pretreatment of silvergrass for ethanol production. Bioresour.
Technol. 99, 6046–6053.
Guo, G.L., Hsu, T.C., Chen, W.H., Chen, W.H., Hwang, W.S., 2009. Characterization of
enzymatic saccharification for acid-pretreated lignocellulosic materials with
different lignin composition. Enzyme Microb. Technol. 45, 80–87.
Himmel, M.E., Ding, S.Y., Johnson, D.K., Adney, W.S., Nimlos, M.R., Brady, J.W., Foust,
T.D., 2007. Biomass recalcitrance: engineering plants and enzymes for biofuels
production. Science 315, 804–807.
Hsu, T.C., Guo, G.L., Chen, W.H., Hwang, W.S., 2010. Effect of dilute acid
pretreatment of rice straw on structural properties and enzymatic hydrolysis.
Bioresour. Technol. 101, 4907–4913.
Jin, S., Chen, H., 2006. Superfine grinding of steam-exploded rice straw and its
enzymatic hydrolysis. Biochem. Eng. J. 30, 225–230.
Jin, S., Chen, H., 2007. Fractionation of fibrous fraction from steam exploded rice
straw. Process Biochemistry 42, 188–192.
Kabel, M.A., Bos, G., Zeevalking, J., Voragen, A.G.J., Schols, H.A., 2007.
Effect of pretreatment severity on xylan solubility and enzymatic breakdown
of the remaining cellulose from wheat straw. Bioresour. Technol. 98, 2034–
2042.
Karimi, K., Kheradmandinia, S., Taherzadeh, M.J., 2006. Conversion of rice straw to
sugars by dilute-acid hydrolysis. Biomass Bioenergy 30, 247–253.
Kim, S., Holtzapple, M.T., 2006. Effect of structures on enzyme digestibility of corn
stover. Bioresour. Technol. 97, 583–591.
Kim, K.H., Tucker, M.P., Keller, F.A., Aden, A., Nguyen, Q.A., 2001. Continuous
countercurrent extraction of hemicelluloses from pretreated wood residues.
Appl. Biochem. Biotechnol. 91–93, 253–267.
Merino, S., Cherry, J., 2007. Progress and challenges in enzyme development for
biomass utilization. Adv. Biochem. Eng. Biotechnol. 108, 95–120.
Monavari, S., Galbe, M., Zacchi, G., 2009. The influence of solid/liquid separation
techniques on the sugar yield in two-step dilute acid hydrolysis of softwood
followed by enzymatic hydrolysis. Biotechnol. Biofuels 2 (6).
Moniruzzaman, M., 1996a. Saccharification and alcohol fermentation of steamexploded rice straw. Bioresour. Technol. 55, 111–117.
Moniruzzaman, M., 1996b. Effect of steam explosion on the physicochemical
properties and enzymatic saccharification of rice straw. Appl. Biochem.
Biotechnol. 59, 283–297.
Mosier, N., Wyman, C., Dale, B., Elander, R., Lee, Y.Y., Holtzapple, M., Ladisch, M.,
2005. Features of promising technologies for pretreatment of lignocellulosic
biomass. Bioresour. Technol. 96, 673–686.
Nakamura, Y., Sawada, T., Inoue, E., 2001. Enhanced ethanol production from
enzymatically treated steam-exploded rice straw using extractive fermentation.
J. Chem. Technol. Biotechnol. 76, 879–884.
Nguyen, Q.A., Tucker, M.P., Keller, F.A., Eddy, F.P., 2000. Two-stage dilute-acid
pretreatment of softwoods. Appl. Biochem. Biotechnol. 84–86, 561–576.
Roberto, I.C., Mussatto, S.I., Rodrigues, C.L.B., 2003. Dilute-acid hydrolysis for
optimization of xylose recovery from rice straw in a semi-pilot reactor. Ind.
Crop. Prod. 17, 171–176.
Ruiza, E., Cara, C., Manzanaresb, P., Ballesterosb, M., Castroa, E., 2008. Evaluation of
steam explosion pre-treatment for enzymatic hydrolysis of sunflower stalks.
Enzyme Micro. Technol. 42, 160–166.
Selig, M.J., Viamajala, S.Decker, S.R., Tucker, M.P., Himmel, M.E., Vinzant, T.B., 2007.
Deposition of lignin droplets produced during dilute acid pretreatment of
maize stems retards enzymatic hydrolysis of cellulose. Biotechnol. Prog. 23,
1333–1339.
Söderström, J., Pilcher, L., Galbe, M., Zacchi, G., 2003. Two-step steam pretreatment
of softwood by dilute H2SO4 impregnation for ethanol production. Biomass
Bioenergy 24, 475–486.
Sørensen, A.Teller, P.J., Hilstrøn, T., Ahring, B.K., 2008. Hydrolysis of Miscanthus for
bioethanol production using dilute acid presoaking combined with wet explosion
pre-treatment and enzymatic treatment. Bioresour. Technol. 99, 6602–6607.
Sluiter, A., Hames, B., Ruiz, R., Scarlata, C., Sluiter, J., Templeton, D., 2005.
Determination of structural carbohydrates and lignin in biomass, Laboratory
Analytical Procedure from the National Renewable Energy Laboratory Biomass
Analysis Technology (NREL BAT) Team.
2924
W.-H. Chen et al. / Bioresource Technology 102 (2011) 2916–2924
Teymouri, F., Laureano-Perez, L., Alizadeh, H., Dale, B.E., 2005. Optimization of the
ammonia fiber explosion (AFEX) treatment parameters for enzymatic
hydrolysis of corn stover. Bioresour. Technol. 96, 2014–2018.
Thomsen, M.H., Thygesen, A., Christensen, B.H., Larsen, J., Jørgensen, H., Thomsen,
A.B., 2006. Preliminary results on optimising hydrothermal pre-treatment used
in co-production of biofuels. Appl. Biochem. Biotechnol. 129–132, 448–460.
Thomsen, M.H., Thygesen, A., Thomsen, A.B., 2008. Hydrothermal treatment of
wheat straw at pilot plant scale using a three-step reactor system aiming at
high hemicellulose recovery, high cellulose digestibility and low lignin
hydrolysis. Bioresour. Technol. 99 (10), 4221–4228.
Vlasenko, E.Y., Ding, H., Labavitch, J.M., Shoemaker, S.P., 1997. Enzymatic hydrolysis
of pretreated rice straw. Bioresour. Technol. 59, 109–119.
Wyman, C.E., Dale, B.E., Elander, R.T., Holtzapple, M., Ladisch, M.R., Lee, Y.Y., 2005.
Coordinated development of leading biomass pretreatment technologies.
Bioresour. Technol. 96, 1959–1966.
Yang, B., Wyman, C.E., 2008. Pretreatment: the key to unlocking low-cost cellulosic
ethanl. Biofuels Bioprod. Bior. 2, 26–40.
Yu, C.T.Chen, W.H., Men, L.C., Hwang, W.S., 2009. Microscopic structure features
changes of rice straw treated by boiled acid solution. Ind. Crop. Prod. 29, 308–315.
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