Fuel Processing Technology 91 (2010) 903–909
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Fuel Processing Technology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Effect of cellulose, lignin, alkali and alkaline earth metallic species on biomass
pyrolysis and gasification
Dangzhen Lv, Minghou Xu ⁎, Xiaowei Liu, Zhonghua Zhan, Zhiyuan Li, Hong Yao ⁎
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history:
Received 13 April 2009
Received in revised form 2 June 2009
Accepted 27 September 2009
Keywords:
Biomass
Cellulose
Lignin
AAEM species
Pyrolysis and gasification
a b s t r a c t
Fundamental pyrolysis/gasification characteristics of natural biomass and acid-washed biomass without
alkali and alkaline earth metals (AAEM) were investigated by a thermogravimetric analyzer (TGA) and a
fixed-bed reactor. In these experiments, six types of biomass were used and the contents of cellulose, lignin
and AAEM species in the biomass were measured. It was observed that the characteristic of biomass
pyrolysis and gasification was dependent on its components and AAEM species on the basis of TGA
experiments. During biomass pyrolysis, the tar and gas yields increased with the growth of cellulose content,
but the char yield decreased. There were two reactions indicating two major decomposition mechanisms.
The first stage of decomposition showed rapid mass decrease due to the volatilization of cellulose, while the
second stage became slow attributed to the lignin decomposition. The higher the cellulose content, the faster
the pyrolysis rate. In contrast, the pyrolysis rate of biomass with higher lignin content became slower. In
addition, the rises of cellulose content elevated the peak temperature of gasification and prolonged the
gasification time. Meanwhile, the effect of AAEM species on gasification behavior was studied by comparing
unwashed and acid-washed biomass. AAEM species increased the peak gasification value, whereas decreased
initial gasification temperature. It revealed that the activity of biomass gasification was attributed to the
interaction between AAEM–cellulose/lignin.
Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction
Due to the ever increasing world population and the continually
improving living standards, the demand for energy resources has risen
dramatically, and this condition will continue. However, dramatic
increases in the use of fossil fuel will certainly have adverse health
effects and environmental pollution [1]. Currently, biomass is recognized
as the third largest and renewable energy resource with zero CO2 net
emission in the world [2–6]. Meanwhile, biomass energy accounts for
14–15% of total world energy consumption [7,8]. Hence, the exploitation
and utilization of biomass energy are effective and necessary for
relieving the pressures caused by environmental pollution and fossil
fuel shortage.
Recently, extensive researches aiming to develop the renewable
energy resources from biomass have been carried out [9–18], especially
pyrolysis and gasification, which are particularly suitable for the
utilization of biomass. An early work by Pereira [11] concluded that
the main components of biomass were total cellulose and lignin, in
addition to extractives, water and mineral matter (mainly as K). Fout
⁎ Corresponding authors. Tel./fax: +86 27 87545526.
E-mail addresses: [email protected] (M. Xu), [email protected] (H. Yao).
and Reina [13,14] reported that the thermal decomposition of the main
components took place via complex mechanisms. Raveendran [15]
studied on isolated biomass components as well as synthetic biomass.
Both the pyrolysis characteristics and product distribution of biomass
showed a direct summative correlation based on biomass component.
Gani [16] investigated that for the biomass with higher cellulose
content, the pyrolysis rate became faster, while higher lignin content
gave a slower pyrolysis rate. Furthermore, among the factors influencing
pyrolysis and gasification behavior, more and more interests were
focused on the distributed AAEM species in biomass. Raveendran [15]
investigated that ash (mainly as AAEM species) present in biomass
seemed to have a strong influence on both the pyrolysis characteristics
and the product distribution. Hsisheng and Raveendran [19,20]
suggested that deashing increased the volatile yield, initial decomposition temperature and rate of pyrolysis. Fahmi [21] carried out TGA
analysis on the untreated and treated biomass samples. They found that
the AAEM species had a strong catalytic effect, particularly potassium
during pyrolysis. Reviews done by Wu and Li [22–24] showed that the
interactions between the volatiles and the char particles enhanced the
volatilization of AAEM species. Meanwhile, the catalytic activity of
AAEM species was deeply dependent on the interaction between AAEM
species and char structure.
0378-3820/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.fuproc.2009.09.014
904
D. Lv et al. / Fuel Processing Technology 91 (2010) 903–909
From these points, the cellulose and lignin contents in biomass are
some of the important parameters to evaluate the pyrolysis and
gasification characteristics. Meanwhile, AAEM species, either as carboxylates forming part of its organic matter or as inorganic matter
(particularly as potassium), affect pyrolysis and gasification behaviors.
However, fundamental pyrolysis and gasification characteristics for
biomass based on the main components and AAEM species have not
been elucidated clearly, especially the AAEM species interacting with
cellulose and lignin. Hence, this paper aims to concentrate on the effects
of AAEM species, cellulose/lignin and AAEM–cellulose/lignin interaction
on pyrolysis and gasification characteristics of biomass.
2. Experimental
2.1. Feedstocks and preparation
Pine sawdust categorized as a woody biomass, and sugar cane
bagasse, rice-straw, rice-husk, cotton-stalk and corncob as agricultural
biomass, were used as the test samples. Biomass samples were milled to
produce the particles in two sizes: below 200 µm for TGA and about
1 mm for fast pyrolysis test. In order to illustrate the effect of cellulose
and lignin on the fundamental pyrolysis and gasification characteristics,
pure cellulose and lignin chemicals were also tested in TG experiments.
The properties of the biomass samples, cellulose and lignin chemicals
are shown in Table 1. The results show that all types of biomass contain
high volatile matter and a small amount of ash, except rice-straw and
rice-husk. The cellulose and lignin contents analyses in biomass were
conducted in this study. The analytical method of cellulose and lignin
contents was according to TAPPI [25–27].
The acid-washed biomass (AW biomass) without AAEM species
was prepared by washing biomass with a dilute vitriolic acid (H2SO4).
2 g of biomass was dispersed in 100 ml 1 M H2SO4. The mixture was
heated to 353 K while being stirred for 4 h and then cooled to room
temperature, repeatedly filtered and washed with double-distilled
water until no AAEM species were detectable. The remaining solid
was dried in the vacuum drier at 328 K again for 24 h.
The AAEM species content of biomass was determined by the
induced coupled plasma emission spectrometer (ICP-AES). A total of
0.1 g of the oven dried material was placed in a 100 mL Teflon digestion
tube, and 10 ml HNO3 and 2 ml H2O2 were added. The sample was
mixed by swirling and kept at room temperature for 2 h prior to heating
overnight. When the biomass was completely digested and then
filtered, the solution was subjected to ICP-AES, repeated twice with an
average standard deviation of 5–7% for each element.
2.2. Thermogravimetric analysis (TGA) and fast pyrolysis test in a
fixed-bed reactor
2.2.1. TGA for pyrolysis and gasification
Fundamental tests on pyrolysis and gasification for several kinds of
biomass were conducted using a thermogravimetric (TG) analyzer. The
Table 1
Proximate and ultimate analysis of the biomass, cellulose and lignin.
Samples
Cellulose
Bagasse
Corncob
Cotton-stalk
Rice-straw
Pine sawdust
Rice-husk
Lignin
experimental conditions were shown in Table 2. The biomass and
biomass char of approximately 5 mg were weighed accurately. The N2
and CO2 gases were supplied for the pyrolysis and gasification experiment, respectively. The sample was heated from 298 K to 1173 K for
pyrolysis at the heating rate of 20 K/min, and from 298 to 1473 K at the
heating rate of 15 K/min for gasification.
2.2.2. Fast pyrolysis test in a fixed-bed reactor
Fast pyrolysis of biomass was conducted in a fixed-bed reactor
(Fig. 1). A quartz tube, 700 mm in length and 50 mm in inner
diameter, was used to prevent catalytic reaction at high temperatures.
The carrier gas N2 entered the reactor at the top of the quartz tube and
was heated as it moved downward. The temperature in the reactor
was measured by a thermocouple. For each experiment run, the
reactor tube was heated to the designed temperatures of 773, 873,
973 and 1073 K, respectively. Then the biomass sample was added
into the high temperature zone so that fast pyrolysis took place in the
constant temperature zone of the reactor. The volatile matter passed
through the porous quartz filter and the char remained. Then the
volatile products were cooled in an ice-cooled condenser, where the
tar was separated by a silica filter. The gaseous products were washed
with water (saturated with CaCl2) and dried before being analyzed by
a gas chromatograph (Agilent GC-3000). Besides, after biomass char
holding at the temperature for 30 min, the reactor was removed from
the furnace and cooled to room temperature with continuing N2 flow.
The char sample was then recovered from the reactor for subsequent
analysis.
3. Results and discussion
3.1. Cellulose and lignin contents and AAEM species analyzed in biomass
Cellulose and lignin are generally recognized as the main components in biomass [28]. Holocellulose (cellulose and hemicellulose) and
lignin were determined by chemical analysis for the six types of biomass
selected. Fig. 2 shows the cellulose and lignin contents in biomass. The
term holocellulose (total cellulose) is composed of cellulose and
hemicellulose. The weight fraction except for the cellulose and lignin
fraction corresponds to the fraction of acid-soluble hydrocarbons in
biomass. As shown in Fig. 2, all of the biomass samples contain more
cellulose than lignin. The cellulose and lignin contents vary from 55% to
85% and from 10% to 35%, respectively. The bagasse contains the highest
amount of cellulose, up to about 82% and the rice-husk has a maximum
of lignin, up to about 34%. Besides, Fig. 3 shows a clear positive
correlation predicting the AAEM species distribution in biomass. The
total AAEM species (by ICP-AES) increase as the cellulose content
increases.
3.2. Pyrolysis behavior of several types of biomass, cellulose and lignin
Typical TG curves of pyrolysis in N2 are shown in Fig. 4 for bagasse,
rice-straw, rice-husk and cotton-stalk, comparing with cellulose and
Table 2
Experimental conditions in TG analysis.
Proximate analysis (wt.%, ad.)
Ultimate analysis (wt.%, ad.)
M
A
V
FC
C
H
O
N
S
Samples
Pine sawdust, bagasse, rice-straw, rice-husk, cotton-stalk and corncob
5.50
5.48
7.75
8.87
8.54
7.90
7.67
13.10
–
2.84
8.13
5.46
10.50
2.68
14.96
12.35
89.20
77.83
66.81
68.70
65.77
71.39
60.69
35.74
5.30
13.85
17.31
16.97
15.19
18.03
16.68
38.81
42.06
46.28
41.62
42.70
39.28
45.73
38.45
47.65
6.06
6.32
6.07
6.25
6.17
6.50
5.68
4.30
46.38
38.87
35.64
35.52
33.44
36.76
32.37
19.60
–
0.21
0.63
0.94
1.41
0.23
0.56
2.92
–
–
0.16
0.26
0.66
0.20
0.31
0.08
Sample weight
Gas flow rate
Pyrolysis
[mg]
[ml/min]
Heating rate [K/min]
Atmosphere
Holding temperature [K]
Heating rate [K/min]
Atmosphere
Holding temperature [K]
Gasification
5.0
100
20
N2
1173
15
CO2
1573
D. Lv et al. / Fuel Processing Technology 91 (2010) 903–909
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Fig. 3. A correction between total AAEM species and cellulose content as measured from
ICP-AES.
Fig. 1. Schematic diagram of a free fall reactor.
lignin chemicals. The vertical axis represents fraction of mass decline.
In the full temperature range, Fig. 4 shows evident disparity in weight
loss behavior for the six types of biomass. It can be seen that biomass
pyrolysis can be divided into the following two ranges: rapid mass
decrease due to cellulose volatilization and slow mass decrease
attributed to lignin decomposition. Focusing on the pattern of the
curves of mass decrease (Fig. 4), the cellulose chemical decomposes at
rapid pyrolysis rate at low temperatures and within a narrow
temperature range. The lignin begins to react at higher temperatures
and the decomposition rate becomes slower than that of the cellulose
chemical. As shown in Fig. 4, the pyrolysis reaction for cellulose is
finished completely between 473 K and 673 K, whereas the lignin
begins to degrade at high temperatures above 573 K and still
continues to decrease even above 1073 K. This is explained by the
fact that the cellulose compounds have the structure of branching chain
Fig. 2. Cellulose and lignin contents in six types of biomass.
of polysaccharides and no aromatic compounds, which are easily
volatilized. However, the lignin consists of various –O- and C–Ccontaining functional groups and aromatic structural units [29].
Consequently, the lignin is more thermally stable than cellulose during
pyrolysis. Comparing these curves, bagasse approaches to that for the
cellulose, since it contains the highest cellulose content as shown in
Fig. 2. On the other hand, the curve for rice-husk with a maximum
amount of lignin content relatively comes near to that for the lignin. The
pyrolysis tests were also conducted in the micro fluidized bed reaction
analyzer (MFBRA) developed by Institute of Process Engineering of CAS
to clarify the gas evolution characteristics during pyrolysis. The results
identify that the gas components of H2, CH4, CO and CO2 have different
time to start and end the gas evolution. The lower the temperature, the
more obvious the time difference was. Meanwhile, the cellulose has
higher CO yield, and the lignin owns higher H2 and CH4 yield. These
results clarify that the pyrolysis characteristic of biomass is strongly
dependent on its own components.
3.3. Gasification behavior of several types of biomass, cellulose and lignin
char
To understand the effect of cellulose and lignin contents on
gasification characteristics of biomass, the DTG curves for gasification
Fig. 4. TG curves of biomass, cellulose and lignin during pyrolysis in N2 atmosphere at
the heating rate 20 K/min.
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D. Lv et al. / Fuel Processing Technology 91 (2010) 903–909
Fig. 5. DTG curves of biomass and cellulose char during gasification in CO2 atmosphere
at the heating rate 15 K/min.
in the natural biomass char with different components and pure
cellulose char were compared in Fig. 5. Firstly, their char were
obtained from fast pyrolysis at 1073 K under nitrogen atmosphere in
the fixed-bed reactor. After the volatile completely evolved, the char
samples were used for gasification by TG analyzer. Fig. 5 shows that
higher cellulose content in biomass elevates the peak temperature of
gasification and prolongs the gasification time. As can be seen from
Fig. 5, the rice-husk char with the highest lignin content shows the
peak of the DTG curve firstly, whereas the peak for the pure cellulose
appears lastly. This is due to the probable diversities of the char
microstructure caused by great differences in cellulose and lignin
contents in biomass.
Morphological structure changes before and after pyrolysis were
observed by a scanning electron microscope (SEM). Fig. 6 shows the
surface morphologies of bagasse, rice-straw, rice-husk and cellulose
chemical, respectively. From Fig. 6a and d, it can be observed that
bagasse and cellulose chemical have a fibrous structure. However, as
shown in Fig. 6b and c, morphologies of rice-straw and rice-husk are
observed to be lumpy solid. Meanwhile, Fig. 7 shows the biomass char
morphological structures after pyrolysis, respectively. It can be seen
from Fig. 7a and d that the structures of bagasse and cellulose
chemical are still fibrous. However, for rice-straw and rice-husk
shown in Fig. 7b and c, it can be observed that the structures of both
change evidently. During pyrolysis, rice-straw and rice-husk chars
become porous structures, and the BET surfaces increase obviously.
The BET surface areas of rice-straw are 0.74 m2/g for natural sample
and 23.98 m2/g for char sample, respectively. Meanwhile, the BET
surface area of rice-husk increases from 0.93 to 84.39 m2/g after
pyrolysis. Hence CO2 can easily diffuse inside the particle during
gasification. This is one reason why the gasification characteristics for
biomass significantly differ from each other. From these observation
results, the gasification reactivity of biomass is greatly dependent on
the biomass char structure.
Fig. 6. Biomass morphology.
D. Lv et al. / Fuel Processing Technology 91 (2010) 903–909
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Fig. 7. Biomass char morphology during fast pyrolysis in the fixed-bed reactor.
3.4. Effect of cellulose and lignin contents on the yields of product during
pyrolysis
Fig. 8 presents the yields of product distribution from fast pyrolysis
of bagasse, cotton-stalk, rice-straw and rice-husk from 773 to 1073 K.
The results show that the gas yields smoothly increase with the
increase of the temperature, while the tar and char yields decrease. At
the whole temperature range, the main evolved product of cellulose is
tar, which contains levoglucosan as the major component with
aldehydes, ketones and organic acid in addition to water and char. But
comparing with cellulose, the product of lignin is dominated by char,
and also involves the release of water and tar. Consequently, the
cellulose components of biomass are mainly responsible for the
volatile matter, while lignin is the main contributor to the char.
Furthermore, as can be seen from Fig. 8, the yields of gas increase as
the cellulose content grows, but the yields of char and tar decrease. At
773 K, the gas, char and tar yields of bagasse with the highest cellulose
content are 66.7%, 15.5%, 17.8% and 85.6%, 7.9%, 6.5% at 1073 K,
respectively. Meanwhile, the yields for rice-husk with the highest
lignin content are 42.9%, 47.9%, 9.2% at 773 K and 66.6%, 29.9%, 3.5% at
1073 K, respectively.
3.5. Effect of AAEM species on gasification of biomass
To further discern the effect of AAEM species on gasification
behaviors, a comparison between unwashed and acid-washed biomass
samples during pyrolysis was made and presented in Fig. 9. Firstly,
pyrolysis of acid-washed samples was under 1073 K tested in the fixed-
bed reactor, and the nitrogen gas was supplied. After the volatile
completely evolved, the char samples were used for gasification by TG
analyzer. It can be seen that the acid-washed samples have lower peak
values than the unwashed and the initial gasification temperature
increases simultaneously. This might be attributed to the effect of AAEM
species on catalytic gasification reactions. Fig. 10 shows the DTG curves of
natural and acid-washed biomass during gasification. It is interesting to
see that the gasification behavior of the acid-washed biomass has a
similar phenomenon with that of the unwashed, and the increase of
cellulose content also elevates the peak temperature and prolongs the
gasification time. The results indicate that the characteristics of biomass
gasification, are also strongly correlated with biomass components.
Therefore, the gasification activity of biomass is a result of the interaction
between AAEM species and cellulose/lignin, and is further greatly
dependent on the char structure.
4. Conclusions
Pyrolysis and gasification of six types of biomass in this study were
investigated by a thermogravimetric analyzer and a fixed-bed reactor.
Each kind of biomass had a pyrolysis and gasification characteristic
based on its own individual components, especially cellulose/lignin
and AAEM species content. The following conclusions can be drawn
from the experimental results:
1) The main components in biomass consisted of cellulose and lignin. The
cellulose content was more than lignin for the biomass samples
selected in this study, which varied from 55% to 85% and from 10% to
908
D. Lv et al. / Fuel Processing Technology 91 (2010) 903–909
Fig. 9. DTG curves of biomass char during gasification in CO2 atmosphere at the heating
rate 15 K/min.
Fig. 8. Yields of product during fast pyrolysis in the fixed-bed reactor.
35%, respectively. Meanwhile, a correlation predicting the distribution
of AAEM species was that the total AAEM species content increased
with the increase of the cellulose content.
2) During pyrolysis, the reactions for biomass demonstrated two
stages. The first stage showed rapid mass decrease due to the
cellulose volatilization, and the second stage decreased slowly due
to the lignin decomposition. The higher the cellulose content, the
faster the pyrolysis rate. In contrast, the pyrolysis rate of biomass
became slower with the lignin content increasing. Meanwhile,
during fast pyrolysis under the fixed-bed reactor, the tar and gas
yields increased with the increase of the cellulose content, but char
yield decreased.
3) Furthermore, gasification activity was greatly dependent on the
char structure and considerably influenced by the interaction
between cellulose/lignin and AAEM species in biomass. The
increasing cellulose content elevated the peak temperature of
gasification and prolonged the gasification time. Meanwhile, the
AAEM species existing in biomass increased the char reactivity,
resulting in the decrease of initial gasification temperature and the
increase of the peak gasification value. Therefore cellulose/lignin,
AAEM species and AAEM–cellulose/lignin interaction were the
essential parameters on pyrolysis and gasification characteristics
for biomass.
D. Lv et al. / Fuel Processing Technology 91 (2010) 903–909
Fig. 10. DTG curves during gasification of unwashed and acid-washed biomass char in
CO2 atmosphere at the heating rate of 15 K/min.
Acknowledgements
This work was partly supported by the National Natural Science
Foundation of China (50721140649, 50721005, 50811120107), the
National Key Basic Research and Development Program of China
(2008CB417201), Program for New Century Excellent Talents in
University, and the Graduates' Innovation Fund of Huazhong University
of Sci. and Tech. (HF-06-001-08-121).
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