ACTA PHYSICO-CHIMICA SINICA
Volume 22, Issue 7, July 2006
Online English edition of the Chinese language journal
Cite this article as: Acta Phys. -Chim. Sin., 2006, 22(7), 786-790.
RESEARCH PAPER
Two-step Consecutive Reaction Model of Biomass Thermal
Decomposition by DSC
Haixiang Chen,
Nai’an Liu*,
Weicheng Fan
State Key Laboratory of Fire Science, University of Science and Technology of China, Anhui 230026, P. R. China
Abstract:
The thermal decomposition of one kind of biomass in air has been investigated by differential scanning calorimetry
(DSC) analyzer. The results indicate that the heating process of samples from ambient temperature to 923 K at low heating rates
shows two obvious exothermic peaks. According to the decomposition mechanism, the first exothermic step is attributed to oxidative
degradation of hemicellulose and cellulose, and the second exothermic step is attributed to lignin degradation and char oxidation.
The reaction model of the studied biomass thermal decomposition has been studied by iso-conversional methods and optimization
computation. The results suggest that the two-step consecutive reaction model is suitable to describe the exotherm of biomass
thermal decomposition in air.
Key Words: Biomass; Thermal decomposition; Kinetic model; DSC
Thermal decomposition of biomass is an important research
topic in the field of fire safety science. According to statistics,
the mean forest area per person is less than 1.1×103 m2 and the
mean burnt area every year is 8.2×109 m2, from 1950 to 1990,
in China[1]. The theoretical modeling of forest fire spread and
the engineering of fire barrier bands require the basic understanding of biomass decomposition processes. The knowledge
of thermal decomposition kinetics of biomass provides the
fundamental clues to understand the mechanisms of biomass
ignition and combustion. Studies on biomass decomposition
kinetics may be of enormous help for improving the physical
fire spread models and developing the methods for selection
of fire-resistant tree species. In addition, biomass decomposition is also an important topic in the field of chemical engineering. The production of bio-oil from biomass through pyrolysis has a market with a wide prospect and is an effective
way to meet the increasing petroleum demands of China.
In the past decades, thermal decomposition of biomass has
been extensively investigated, whereby, quite a number of
apparent mass loss kinetic models have been established by
thermogravimetric (TG) analysis. Inspection of the published
reports reveals that the kinetic models fall into two types: (1)
regarding biomass as consisting of a single pseudo component,
using its solid-gas reaction kinetics during the overall temperature range, to describe the mass loss kinetics[2, 3]; (2)
Viewing the biomass as consisting of multiple pseudo components, and modeling biomass decomposition by the superposition of the overall decompositions of these
components[4–6]. Liu et al.[7, 8] analyzed the shortcomings of
these models and proposed a new kinetic model, called the
“First Order Pseudo Bi-component Separate-stage Model
(PBSM-O1)”, according to the two-peak features of derivative
TG curves.
Application of the TG technique to study biomass thermal
decomposition is still in vigorous progress. At the same time,
another thermal analysis technique named differential scanning calorimetry (DSC) may be regarded as a supplement of
TG analysis. However, so far the DSC thermogram has been
only qualitatively explained in the research of biomass decomposition kinetics. In this article, the thermal decomposition of one Chinese tree species in air has been examined, by
quantitative kinetic analysis on the DSC thermogram, to pro-
Received: November 21, 2005; Revised: February 21, 2006.
Corresponding author. Email: [email protected]; Tel: +86551-3601668.
The project was supported by the National Natural Science Foundation of China (50323005, 50576090), the China NKBRSF Project (2001CB409600), the Program for
New Century Excellent Talents in Chinese Universities, and the Anhui Excellent Youth Scientist Foundation (2004-2005).
*
Copyright © 2006, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved.
Chinese edition available online at www.whxb.pku.edu.cn
CHEN Haixiang et al. / Acta Physico-Chimica Sinica, 2006, 22(7): 786-790
pose a kinetic model of biomass decomposition.
1
Experimental
A NETZSCH STA 409C Thermobalance was used to record
TG and DSC thermograms of the biomass decomposition simultaneously. The tree species used was oil-tea wood, which
was from Shucheng County, Anhui Province, China. Oil-tea
trees were extensively planted in the forest zone because of its
high economic value. Research on the decomposition behavior
of these trees might help assess their fire-resistance ability in
the kinetic sense.
First the wood sample was dried and then ground and
sieved so that the particle size was specified to be less than
300 μm. The initial amounts of the sample were all kept at 10
mg or so for the experiments. In the tests, the temperature was
increased to 923 K or so until the samples had a constant mass,
at heating rates of 5, 10, 15, and 25 K•min–1. An air stream
was continuously passed into the furnace at a flow rate of 42
mL·min–1 (at normal temperature and atmospheric pressure).
Two runs were performed with the same experimental conditions, and the experiment reproducibility was verified.
2
2.1
Results and discussion
Qualitative analysis of DSC curves
DSC curves of the oil-tea wood sample, decomposed in the
thermal analyzer, are shown in Fig.1, which shows two obvious exothermic peaks. Corresponding to the heating rates of 5,
10, 15, and 25 K·min–1, the temperatures for the first exothermic peak are 590.6, 600.3, 615.6, and 618.2 K, and those for
the second are 693.3, 700.7, 714.5, and 732.6 K, respectively.
The two-peak features of the DSC curves may be explained by
comparing the corresponding TG curves recorded simultaneously. In the TG experiments, the two major mass loss processes occurred in the temperature ranges of 563–613 K and
713–773 K, leading to two mass loss peaks in the DTG
curves[7, 8]. The first mass loss step is mainly contributed to the
oxidative degradation of hemicellulose and cellulose, and the
second to lignin degradation and char oxidation.
The heat flow rate during the mass loss processes is recorded by the DSC thermogram, which shows two exothermic
peaks. Fig.1 indicates that with increase of heating rate, the
heat flow rate of the decomposition rises and the second peak
becomes gradually sharper than the first, which suggests that
the increase of heating rate favors char oxidation. When the
heating rate increases, the biomass resident time in the low
temperature range will be shorter, this does not favor the volatile release, but activates char oxidation, so the oxidative reaction at higher temperatures will be more violent and will release more heat.
2.2
Analysis method of decomposition kinetics
Generally, a one-step thermal decomposition process of a
solid can be expressed as
S(solid) ® R(solid) + V(gas)
which can be formulated with the following rate equation
dα
A
= exp( - Eα / RT )dT
(1)
f (α ) b
where a is the thermal decomposition fraction of the solid, β
(K·min–1) is the heating rate, Ea (kJ·mol–1) is the activation
energy, A (s–1) is the preexponential factor, and R (8.314
J·mol–1·K–1) is the gas constant. T (K) is the absolute temperature. The specific form of f(a) represents the hypothetical
reaction function. The past decades witnessed many kinetic
analysis methods developed on the basis of the above rate
equation. During the past years, the iso-conversional methods
have been used widely, as they can evaluate the activation
energy without any prior knowledge of the reaction model
function. The two most used iso-conversional methods are
(I) Friedman method[9]
E
æ bda ö
lnç
÷ = ln[Af (a )] - a
RT
è dT ø
(2)
By using data at different heating rates under a constant
fraction α, the plot of ln(βdα/dT) versus 1/T leads to a straight
line. Therefore, the apparent activation energy Ea can be determined from the slope term of the regression line.
(II) OFW method[10, 11]
é R a 1
ù
E
ln b @ -5.333 - ln ê
da ú - 1.052 a
(3)
AE
f
(
a
)
RT
0
a
ë
û
ò
Fig.1
DSC curves of the decomposition process of oil-tea
wood samples in air at different heating rates
Using the same data sampling method as in the Friedman
method, the plot of lnβ versus 1/T results in a straight line, and
Ea can be evaluated from the slope term.
The two methods are used to calculate the activation energies of the DSC curves. First, the data are sampled from the
raw DSC curves in the temperature range of 453–853 K, with
a data step of 0.5 K. The start temperature of 453 K denotes
the temperature when the decomposition reaction has just
started. Under this temperature the moisture should fully
evaporate. The residue mass of the sample at above the end
CHEN Haixiang et al. / Acta Physico-Chimica Sinica, 2006, 22(7): 786-790
temperature of 853 K remains nearly constant, as showed in
Fig.1. The conversion fraction of biomass decomposition can
be calculated as
t
ò
a=
ò
ti
tf
ti
[DSC(t ) - Baseling(t )] dt
[DSC(t ) - Baseline(t )] dt
(4)
where ti is the start time of the decomposition and tf is the end
time. DSC signal is influenced by the baseline excursion of
the apparatus, so it should be corrected in advance[12]. The
conversion fraction and its derivative of sample decomposition in air are shown in Fig.2.
Fig.3 shows the curves of the activation energy versus conversion obtained by the iso-conversional methods. It is obvious that the activation energies by the Friedman method
nearly keep at a value of 120 kJ·mol–1 when α < 0.2, and then
decrease to 94.5 kJ·mol–1 (α»0.35), and later increase quickly.
Comparatively, the activation energies evaluated by the OFW
method decrease to nearly 110 kJ·mol–1 (α»0.41) at first, and
then increase. Obviously, the results obtained from the two
methods show some difference, but all imply the biomass decomposition is complex, and is consisted of a number of reactions. Although there is some difference between the two activation energy curves, they both can be divided into two parts
at nearly α»0.4. This reflects the two-peak features of the DSC
curves in the kinetic sense, and 0.4 is the approximate ratio of
the heat release from the first peak to the total heat release.
Fig.3
Activation energy versus conversion factor computed
by iso-conversional methods
This fact implies that a two-step kinetic model may be suitable
to describe the thermal decomposition of biomass.
2.3
Kinetic model of decomposition
In introduction, it is mentioned that the mass loss behavior
of biomass decomposition has been extensively investigated,
whereas the variation process of heat flux has been rarely
modeled. The two types of kinetic models mentioned in introduction do not consider the decomposition process includes
two continuous steps and they ignore the intermediate product
(char). The shortcoming of the separate-stage model is that the
separation point of the two stages cannot be determined, as the
two peaks of the DSC signals significantly overlap each other.
As indicated above, the two-step consecutive reaction model
is suitable to describe the heat flux signal recorded by DSC.
The two-step consecutive reaction model was adopted in
many researches regarding polymer decomposition[13, 14],
though it encounters the difficulty in optimization computation.
The two-step consecutive reaction model describes the decomposition process as follows:
k1
S ¾¾®
R 1 + V1
(5)
k 2 (O 2 )
R1 ¾¾
¾
¾® R 2 + V2
(6)
where S represents the sample, R the residue (char or ash), V
the volatiles. wS, wR are the mass fractions of the sample and
residue. If nth-order reaction functions are assumed for the
two processes and the reaction orders are n1 and n2, respectively, the following rate equations can be developed.
dwS / dt = - A1e
- Ea1 / RT
n
wS1
dwR / dt = A1e - Ea1 / RT wS1 - A2 e - Ea2 / RT wR2
n
1
Fig.2
Conversion curves and its derivative curves of sample
decomposition process
n
(7)
1
After baseline calibration, the recorded DSC signal, DSC(t),
can be expressed as[14]
dw
(8)
DSC(t)-Baseline(t)=Q1 dwS +Q2 R 1
dt
dt
where Qi is the heat release for each step.
The total heat release Q of decomposition is determined by
CHEN Haixiang et al. / Acta Physico-Chimica Sinica, 2006, 22(7): 786-790
Q = Q1+Q2=
ò
tf
ti
[DSC(t ) -Baseline(t)]dt
Table 1
By combining Eq. (8) and the differential form of Eq. (4),
the following equation can be obtained
dw
da 1
= (Q1 dwS +Q2 R 1 )
dt
Q
dt
dt
(9)
Eq. (7) and Eq. (9) are the coupled nonlinear differential
equations, for which optimization computation is generally
used to extract the kinetic parameters of the two steps. The
multivariable nonlinear regression method embedded in the
“NETZSCH Thermokinetics” software[15] was used to evaluate
the activation energy, reexponential factor, and reaction order
for each step, and the rate equations were finally determined
to be
dwS / dt = -2.10´105 e-88000 / RT wS1.98
dwR1 / dt = 2.10´105e-88000 / RT w1S.98 1.33
2.52´109 e-157100 / RT wR 1
(9)
Fig.4 shows the comparison of the experimental DSC
curves with the simulated curves by optimization. The corresponding curves of the two groups agree well with each other,
indicating that the two-step consecutive reaction model can
well describe biomass decomposition in air. Table 1 gives out
the kinetic parameters. Comparing with the results of Fig.3,
the activation energy of the first step in the Table is lower to
some extent and close to the minimum of the curves in Fig.3,
and the activation energy of the second step is nearly equal to
the average of the last part of the curves.
2.4
Comparison and discussion of kinetic parameters
On the basis of the DSC thermograms, a two-step consecutive reaction model of biomass decomposition has been proposed in the previous part. In this paragraph the existing kinetic models and kinetic parameters in published reports have
been compared.
In the article by Di Blasi et al.[16], the mass loss process of
wood in air is divided into three steps: the very rapid degradation of all the main components (the activation energy is about
75–100 kJ·mol–1); the slow degradation of lignin and hemicellulose (86–92 kJ·mol–1); and the combustion of the solid
Fig.4
Comparison of experimental DSC curves with
simulated curves by optimization
Kinetic parameters of two-step consecutive reaction
model for the decomposition process of oil-tea wood
sample in air
Reaction step
Ea (kJ·mol–1)
A /s–1
n
Step 1: Eq. (5)
88.0
2.10 × 105
1.98
Step 2: Eq. (6)
157.1
2.52 × 109
1.33
Ea: activation energy; A: pre-exponential factor; n: reaction order
residue (71–90 kJ·mol–1). Momoh et al.[17] observed two
well-defined pyrolysis stages, which occurred over the temperature ranges of 474–699 K and 670–730 K in the decomposition process of some tropical timbers in air, and attributed
them to biomass devolatilization and subsequent char oxidation. By assuming first-order reaction function, the activation
energies are about 101–136 and 35–65 kJ·mol–1, respectively.
Cordero et al.[18] found the mass loss process of four wood
species also contained devolatilization and oxidation stages,
and the activation energies were about 89–93 kJ·mol–1 and
65–83 kJ·mol–1. Senneca et al.[19] observed that the decomposition process of the lignocellulosic material (Robinia Pseudoacacia) in air consisted of two mass loss steps with activation energies of 130 and 110 kJ·mol–1 and reaction order of 2.8
and 1.2. Liu et al.[8] used first-order reaction function and obtained activation energies of 52–99 and 87–202 kJ•mol–1 for
biomass samples. Chen et al.[20] used the same model and obtained activation energies of 80 and 82 kJ·mol–1 for oil-tea
wood decomposed in air. Calvo et al.[21] observed three stages
in the rice straw heating process: drying, devolatilization, and
burning. The activation energy of the second step is 78 or 121
kJ·mol–1 (using two different computation methods) and the
reaction order is 2.39, and those of the third step is 100 or 128
kJ·mol–1 and 1.3. Similar research is abundant, but not listed
here. All these articles reveal that the process of dry biomass
samples decomposed in air in TGA experiments can be divided into two steps: devolatilization and subsequent combustion, which support our explanation of DSC thermograms in
the previous part. Table 2 lists the mentioned parameters. It is
obvious that the activation energy (88.0 kJ·mol–1) of the first
step, obtained from the DSC curves, is in the activation-energy range of the devolatilization process reported in
the published reports, especially it agrees well with those (80
kJ·mol–1) of the same sample[20]. However, the activation energy (157.1 kJ·mol–1) of the second step, obtained from the
DSC curves is generally higher than the reported values. From
Table 2, it is observed that the reported activation value with
the reaction order of 1 is relatively lower, whereas those with
a higher reaction order are higher. If first-order reaction function is assumed to simulate the DSC curves, we can obtain the
activation energies of the two steps as 78.7 and 109.3 kJ·mol–1,
respectively. These values are in the range of Table 2, but they
do not agree with the values calculated by the iso-conversional
methods, and the accordance of simulated and experimental
CHEN Haixiang et al. / Acta Physico-Chimica Sinica, 2006, 22(7): 786-790
Table 2
Kinetic parameters of biomass decomposition reported in the literature
Step 1
–1
Step 2
Ref.
Ea / (kJ·mol )
n
Ea / (kJ·mol–1)
n
75-100; 86-92
1?
71-90
1?
[16]
101-136
1
35-65
1
[17]
89-93
1?
65-83
1?
[18]
130
2.8
110
1.2
[19]
52-99
1
87-202
1
[8]
80
1
82
1
[20]
78; 121
2.39
100; 128
1.3
88.0
1.98
157.1
1.33
[21]
This research
tion process and evaluate the kinetic parameters. The kinetic
parameters agree well with those of the TGA curves in the
published reports. The reaction model based on the basis of
DSC thermograms is expected to be applied in the field of
biomass thermal conversion technology and fire safety science.
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3
Conclusions
The DSC analyzer has been used to investigate the thermal
decomposition of one kind of biomass in air. The results indicate that the heating process of samples from ambient temperature to 923 K, at low heating rates, shows two obvious
exothermic peaks. According to the decomposition mechanism
and the comparison of TGA experiments in the published reports, the first exothermic step is attributed to the oxidative
degradation of hemicellulose and cellulose, and the second to
lignin degradation and char oxidation.
The results of the iso-conversional methods reveal the
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