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A new nano CaO-based CO2 adsorbent preparedusing an adsorption phase technique
ARTICLE in CHEMICAL ENGINEERING JOURNAL · FEBRUARY 2013
Impact Factor: 4.32 · DOI: 10.1016/j.cej.2012.11.095
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A new nano CaO-based CO2 adsorbent prepared using an adsorption
phase technique
Yan Wang a, Yanqing Zhu a,b, Sufang Wu a,c,⇑
a Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, PR Chinab Institute of Environmental and Municipal Engineering, North China University of Water Resources and Electric Power, Zhengzhou, Henan 450011, PR Chinac Zhejiang Provincial Engineering Research Center of Industrial Boiler and Furnace Flue Gas Pollution Control, Hangzhou 311202, PR China
h i g h l i g h t s
" Using ‘‘adsorption phase technique’’,
a coating layer of 4.5 nm–11.6 nm
was formed with the TiO2 content
increasing.
" TiO2 content played an important
role of CO2 adsorption durability.
" Compact factor between 0.8 and 1.3
was tested to relate the adsorption
stability.
g r a p h i c a l a b s t r a c t
a r t i c l e i n f o
Article history:Received 13 August 2012
Received in revised form19 November 2012
Accepted 21 November 2012
Available online 29 November 2012
Keywords:
CO2 adsorbent
Nano CaO
Adsorption phase technique
a b s t r a c t
This study describes for the first time micro-scale hydrolysis has been used in the adsorption phase toprepare a nano CaO-based CO2 adsorbent with a highly durable sorption capacity. The hydrolysis of
Ti(OC4H9)4 to form TiO2 was used to prepare TiO2-coated nano CaCO3, which was then calcinated to pre-
pare a nano CaO-based CO2 adsorbent with a controlled coating layer. The coating compactness was
defined for the first time in this study to describe the mole ratio of Ti to Ca on the surface of the nano
CaCO3. The coating compactness and the durability of the sorption capacity of samples with varying
TiO2 content, hydrolysis temperature, and ester concentration were studied in detail. The properties of
the reactive adsorption of the prepared nano CaO-based CO 2 adsorbents were tested using a thermo-
gravimetric analyzer. The results showed that, of the conditions tested, the TiO 2 content exerts the most
influence on the durability of the sorption capacity. The nano CaCO 3 that was coated with 10 wt.% TiO2
and prepared under 20 C, which has a corresponding coating compactness of 1.0, exhibited a much more
durable CO2 sorption capacity than the other prepared samples.
Crown Copyright 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction
The capture of CO2 through the use of a CaO-based adsorbent
plays an important role in the efficient separation of CO2 from
combustion/gasification gases [1–3] and sorption-enhanced hydro-
gen production processes [4–6]. This capture is based on the
reversible carbonation reaction of CaO [7]. CaO is a potential adsor-
bent because of its high reactive sorption capacity and the
abundance of its natural precursors, such as limestone (CaCO3)
[8] and dolomites (Ca, Mg(CO3)2) [9,10]. However, the CaO-based
adsorbents exhibit a rapid decay in their absorption capacity dur-
ing multiple carbonation–calcination reaction cycles [11]. It is
widely believed that the capacity decay is mainly due to the sinter-
ing of CaO and CaCO3 in the regeneration process [12], the physical
aggregation of the crystals, which leads to an increased particle
size, or the loss of porosity that is caused by the volume reduction
of the small pores [13].
Compared with natural adsorbents, nano CaCO3 has drawn
increasing attention [14–17] because of its higher reactive sorption
capacity, fast reaction rate and its significant improvement in the
durability of the adsorbent. However, because nano CaCO3 has a
high ratio surface area and a high surface energy, it aggregates
1385-8947/$ - see front matter Crown Copyright 2012 Published by Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2012.11.095
⇑ Corresponding author at: Department of Chemical and Biological Engineering,
Zhejiang University, Hangzhou 310027, PR China. Tel.: +86 571 87953138.
E-mail address: [email protected] (S. Wu).
Chemical Engineering Journal 218 (2013) 39–45
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c e j
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more easily, which makes sintering a serious problem. Several
methods with varying degrees of success have been proposed to in-
crease the life cycle performance of CaO-based sorbents by pre-
venting the sintering of CaO. These methods include the
incorporation of inert materials, such as Al2O3 [18], SiO2 [19,20],
MgO [21,22], and CaTiO3 [14] through several chemical synthesis
and physical methods, the utilization of potentially sintering-resis-
tant calcium precursors [23,24], the hydration treatment of CaO to
regain its reactivity [25,26], and the development of high surface
area CaCO3 [27].
The addition of an inert material is a general way to modify the
nano adsorbent to obtain better durability. In previous studies, the
combination of an inert material and the adsorbent was usually
achieved through immediate wet mixing [28], precipitation
[29,30] or surface coating [20]. The coating of a CaO-based adsor-
bent is an effective way to prevent sintering. The surface coating
material behaves as a partition wall that prevents the agglomera-
tion of CaO particles. Therefore, controlling the coating layer is
important for improving durability. However, previous research
on coating-modified CaO-based adsorbents has not reported the
proper preparation of a controlled coating layer [14,20].
The adsorption phase technique [31,32] uses the adsorption
phase as a microreactor to coat the surface of the adsorbent. It is
a remarkable technique because the particle size is efficiently
and easily controlled. In this paper, the adsorption phase technique
was first used to prepare TiO2-coated nano calcium carbonate. A
controlled coating layer was obtained through the micro-scale
hydrolysis of Ti(OC4H9)4 in the adsorption phase. The effects of
the TiO2 content, hydrolysis temperature, and ester concentration
on the CO2 adsorption performance and the relationship between
the durability and the nano calcium carbonate coating compact-
ness were studied.
2. Experimental
2.1. Reagents and instruments
Nano CaCO3 (>95% purity) with a particle size of 70 nm with
water (Huzhou Ling Hua Ltd., China) was used as the CaO-based
sorbent precursor. Tetrabutyl titanate (Ti(OC4H9)4; Shanpu Shang-
hai Chemical Co. Ltd.) was used as the source of TiO2, and ethanol
(Wuxi Jingke Chemical Co. Ltd.) was used as the solvent. An A-type
zeolite (Sinopharm Chemical Reagent Co. Ltd.) was used to remove
the water from the nano CaCO3.
A thermo-gravimetric analyzer (TGA, Pyris1, Perkin-Elmer, USA)
was used for the reactive sorption capacity measurements. X-ray
photoelectron spectroscopy (XPS, VG ESCALAB MK II, UK) was used
to detect the type and relative content of the surface elements. The
morphology of the sorbent was investigated using a transmission
electron microscope (TEM, JEM-1200EX, USA).
2.2. Preparation of TiO 2-coated nano CaCO 3 using an adsorption phasetechnique
An 18-g mass of aqueous nano CaCO3 was dispersed in dehy-
drated ethanol using an ultrasonic dispersion method to form a
suspension and then placed into heat-treated zeolite to dehydrate.
A clear dehydrated ethanol solution with a specific concentration
of Ti(OC4H9)4 was added dropwise into the dehydrated suspension
while stirring. After all of the solution was added, the solution was
stirred for an additional 1–2 h. The solids were collected by vac-
uum filtration, dried and then heat-treated at 500 C for 2 h until
the solid sample was obtained.
TiO2-coated nano CaCO3 samples with different TiO2 contents
can be obtained by changing the amount of n-butyl titanate. In
addition to the TiO2
content, the hydrolysis temperature and the
concentration of the n-butyl titanate ethanol solution were also
varied to prepare the different samples. All of the sample prepara-
tion conditions are listed in Table 1.
The preparation method is based on the adsorption phase tech-
nique. The mechanism by which TiO2 is formed on the nano cal-
cium carbonate is shown in Fig. 1. The nano CaCO3 with water
was dehydrated using zeolite, and only a thin layer of water on
the round surface of the nano CaCO3 particle was maintained.
When an ethanol solution of tetrabutyl titanate was added drop-
wise into the dehydrated suspension, the Ti(OC4H9)4 spread to
the water layer and quickly hydrolyzed. Different operating condi-
tions resulted in different distributions of TiO2 on the surface of the
nano CaCO3 and different compact factors.
2.3. Compactness definition and characterization test
The compact factor C is defined in Eq. (1) to quantify the coating
compactness. This factor is the relative molar content ratio of the
coating material (titanium) and the material to be coated (calcium)
on the surface of the nano CaCO3. It describes the coating status
and, in this paper, represents the dispersion of TiO2 on the surface
of the nano CaCO3.
Table 1
Preparation conditions of samples.
Sample Nos. 1 2 3 4 5 6 7 8 9 10 11 12 13 14
TiO2
content/wt.% 0 5 6.5 8 10 12 15 20 10 10 10 10 10 10
T /C 11.5 15 15 12.5 14.5 12 12.5 0 20 40 70 22 22
Ester concentration/wt.% 9.6 9.6 9.6 9.6 9.6 9.6 9.6 9.6 9.6 9.6 9.6 5 17.5
Fig. 1. Mechanism image of ester hydrolysis of adsorption phase reaction.
40 Y. Wang et al. / Chemical Engineering Journal 218 (2013) 39–45
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C ¼ nTi
nCa
; ð1Þ
In Eq. (1), n refers to the molar number of elements.
Based on the locations of the characteristic lines that appear in
the XPS energy spectrum, the types of elements that are found on
the surface can be identified. The XPS energy spectrum of sample
No. 5 is shown in Fig. 2. The elements Ti, Ca, C, and O were ob-
served on the surface. Once a specific energy region of Ti and Ca
are chosen for further scanning, a single element spectrum is ob-
tained, as shown in Fig. 3a and b. The photoelectron line intensity
(photoelectron peak area) reflects the number of atomic levels;
thus, we can obtain the relative molar content of the surface ele-
ments. The compact factor can be calculated from the following
equation based on the Atomic Sensitivity Factor (ASF) method:
C ¼ nTi
nCa
¼ I Ti=S Ti
I Ca=S Ca
; ð2Þ
where I is the peak area of the XPS test data fitting and S is the sen-
sitivity factor, which is related to the element and equipment used
(in this instrument, S Ti = 1.1 and S Ca = 0.71).
2.4. Sorption capacity test method
The cyclic CO2 sorption capacity was measured with a thermo-
gravimetric analyzer. The tests were conducted under carbonation,
with a CO2 partial pressure of 0.02 MPa in N2 at 600 C for 10 min,
and calcination in N2 at atmospheric pressure and 725 C for
10 min. The temperature was increased at a rate of 15 C/min
and decreased at a rate of 40 C/min. The cyclic CO2 sorption capac-
ity tests were also conducted in a fixed bed reactor. We used the
reactive sorption capacity to analyze the reaction of CaO with
CO2. The reactive sorption capacity and decay ratio were calculated
according to the following equations:
Reactive sorption capacity ¼ CO2 sorption amount
The mass of CaO in adsorbent
ð g CO2= g CaOÞ; ð3Þ
Decay ratio ¼ðSC Þ1 ðSC Þn
ðSC Þ1
100%; ð4Þ
where (SC )1 is the reactive sorption capacity of the first run and(SC )n is the reactive sorption capacity of the nth run.
3. Results and discussion
3.1. Effects of TiO 2 content on the durability of the CO 2 adsorption
cycles
To investigate the effect of the TiO2 content on the cyclic perfor-
mance, seven samples (from No. 2 to No. 8) with different TiO2
mass fractions of 5%, 6.5%, 8%, 10%, 12%, 15%, and 20% were pre-pared. The TEM images of several typical samples are shown in
Fig. 4.
As shown in Fig. 4, the more dispersed nano calcium carbonate
particles were lighter in color.1 The formation of the TiO2 layer
darkened the colors of the edges. As the TiO2 content increases,
the thickness and distribution of the dark parts become more obvi-
ous. The samples with 5% and 10% TiO2 showed a thin layer and a
partial coating around the edges of the particles. The sample with
20% TiO2 showed a thick and almost complete coating around the
edges of the particles. The thickness of the coating layer ranged
from 4.5 nm to 11.6 nm as the TiO2 content increased from 5% to
20%.
The reactive sorption capacity test results are shown in Fig. 5.
The sample marked 0% was not coated with TiO2. The initial reac-
tive sorption capacity of each sample was high, decayed rapidly in
the initial five to six cycles, and then decayed slowly. After 30 cy-
cles of carbonation–calcination, the reactive sorption capacity of
most samples remained at a relatively stable value; this was true
for all samples except the samples with 5% and 6.5% TiO2, which
continued to decay. The non-TiO2 coated sample continued to de-
cay with a lower reactive sorption capacity than the other samples,
which indicates that the coating operation improved the durability
of the sorption capacity. A few samples, such as those with 10% and
12% TiO2, showed an increase in the CO2 reactive sorption capacity
after 20 cycles. We conjecture that this phenomenon is caused by
self-reactivation [14,33]. An additional 30 cycles of carbonation–
calcination of the 10% TiO2 sample were run after the initial 30 cy-
cles. The sorption capacity was almost stable after the 40th run, as
shown in Fig. 6.The results of the BET surface area measurements on samples 1
and 5 before and after several runs are shown in Table 2. As shown
in the results, after 12 runs, the surface area and the average pore
size of sample No. 5, which was coated with TiO2, were maintained
at almost constant levels, whereas those of sample No. 1 decreased.
This finding indicates that the TiO2 coating may provide a partition
wall that prevents the agglomeration of the CaO particles and thus
improves the durability of the reaction sorption capacity of the
coated samples.
Fig. 7 shows the effect of the TiO2 content on the sorption
capacity in the 1st and the 30th runs. With increasing TiO2 content,
the sorption capacity of the initial sorbent decreased and that of
the sample in the 30th run first increased and then decreased. In
the first run, the increasing content of the coating material led toa reduction in the relative content of CaO, which reduces the num-
ber of CO2 adsorption sites and decreases the CO2 sorption capac-
ity. As the cycles of adsorption and desorption progressed, the
activity of CaO decreased, and thus, the sorption capacity de-
creased. When the TiO2 content was greater than 8%, the decay ra-
tio was smaller, which meant that the durability was improved.
Taking into account the sorption capacity, a TiO2 content in the
range of 8–10% was considered to be optimal. Among those values,
after 30 cycles, the sample with 10% TiO2 had the highest sorption
capacity, with a value of 0.404 g of CO2/g of CaO, and the lowest
decay ratio at 28.3%, which results in the best adsorption
0 100 200 300 400 500 600 700 800 900 1000
I n t e n s
i t y ( a . u . )
Binding Energy (eV)
C1s
O1sO KVV
Ca2p3/2&1/2
Ca2s
Ti2p3/2&1/2
Ca LMM
Ti2s
Ti LMM
Fig. 2. XPS full spectrum scan of TiO2 coated CaO-based adsorbent.
1 For interpretation of color in Fig. 4, the reader is referred to the web version of this article.
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performance. In S.F. Wu’s research, the reactive sorption capacityof TiO2/CaCO3 was 4.2 mol/kg after 40 runs, which was 14.3% lower
than that of sample No. 5.
3.2. Effects of hydrolysis conditions on the durability of the CO 2
adsorption cycles
Because the n-butyl titanate hydrolysis reaction is a fast reac-
tion, the hydrolysis conditions have a great impact on the reaction
rate, which in turn affects the formation and growth of the coating
layer. Hydrolysis temperatures varying from 0 C to 70 C and ester
concentrations in the range of 5–17.5% were studied in this section.
Fig. 8 shows the CO2 reactive sorption capacities of the adsor-
bent samples No. 5 and 9–12, which were prepared under different
temperatures. As shown in the figure, the different hydrolysis tem-peratures had little influence on the reactive sorption capacity of
the initial cycles. As the cycles progressed, the impact of the tem-perature became apparent. The sorption capacities of the samples
prepared under 40 C and70 C temperature conditions were grad-
ually reduced; these were not as stable as the other three samples,
which were prepared under temperatures in the range of 0–20 C.
In general, the sorption capacity of the sample prepared under
40 C was higher than that of the sample prepared under 70 C.
Thus, a lower temperature is relatively favorable, and a tempera-
ture range of 0–20 C is optimal.
The reactant concentration of butyl titanate dissolved in the
ethanol solution also had some impact on the adsorption perfor-
mance of the adsorbent. Fig. 9 shows the CO2 reactive sorption
capacities of the adsorbent samples Nos. 10, 13, and 14, which
were prepared under the different ester concentrations of 5%,
9.6%, and 17.5%, respectively. As the cycles progressed, when theester concentration was 17.5%, the sorption capacity gradually
Fig. 4. TEM images of samples with different TiO2 content. (a) Non-TiO2 coated nano CaCO3 (sample No. 1), (b) 5% TiO2 (sample No. 2), (c) 10% TiO2 (sample No. 5), (d) 20%
TiO2 (sample No. 8).
340 342 344 346 348 350 352 354 356 358
0
100
200
300
400
500
600 scanning curve of Ca2p
FittedCurves
FittedCurves
I n t e
n s i t y ( a . u . )
Binding Energy (eV)
454 456 458 460 462 464 466 468 470
0
100
200
300
400
500
600 scanning curve of Ti2p
FittedCurves
FittedCurves
I n t e
n s i t y ( a . u . )
Binding Energy (eV)
a b
Fig. 3. XPS spectra of element (a) Ca, (b) Ti.
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decayed and did not reach a steady state even after 30 cycles. In
contrast, when the ester concentration was relatively low, the
sorption capacity was basically stable after only 10 cycles.
3.3. Compactness and durability
The adsorption performance of the novel adsorbent was corre-
lated with the compactness of the coating layer. The compact fac-
tors of the TiO2 coating layers of samples with different adsorptionperformances are shown in Table 3.
0 5 10 15 20
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
1st run
30th run
r e a c t i v e s o r p t i o n c a p a c i t y ( g C O
2 / g C a O )
TiO2 content/%
Fig. 7. Effect of TiO2 content on sorption capacity of 1st and 30th run.
0 5 10 15 20 25 30
0.1
0.2
0.3
0.4
0.5
0.6
r e a c t i v e
s o r p t i o n c a p a c i t y ( g C O
2 / g C a O )
number of cycles
Fig. 8. CO2 reactive sorption capacity of sample Nos. 5, and 9–12 prepared under
different temperatures varying from 0 C to 70 C.
0 5 10 15 20 25 30
0.1
0.2
0.3
0.4
0.5
0.6
r e a c t i v e s o r p t i o n c a p a c i t y ( g C O
2
/ g C a O )
number of cycles
5%
9.6%
17.5%
Fig. 9. CO2 reactive sorption capacity of sample Nos. 10, and 13,14 prepared underdifferent ester concentrations of 5–17.5%.
0 10 20 30 40 50 60
0.0
0.1
0.2
0.3
0.4
0.5
0.6
r e a c t i v e s
o r p t i o n c a p a c i t y ( g C O
2 / g C a O )
number of cycles
10%TiO2
0%TiO2
Fig. 6. CO2 reactive sorption capacity of sample No. 5.
Table 2
BET analytic results of samples 1 and 5 before and after several runs.
Sample BET surface area (m2/g) Average pore size (nm)
No. 1 No. 5 No. 1 No. 5
Fresh 5.90 20.76 20.80 19.14
After 12 runs 4.35 10.92 18.42 12.42
After 20 runs 3.33 10.33 16.60 13.18
0 5 1 0 1 5 2 0 2 5 3 0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
r e a c t i v e s o r p t i o n c a p a
c i t y ( g C O
2 / g C a O )
number of cycles
0% 5%
6.5% 8%
10% 12%
15% 20%
Fig. 5. CO2 reactive sorption capacity of sample Nos. 1–8 with different TiO2
content (carbonation: CO2 partial pressure of 0.02 MPa at 600 C for 10 min, and
calcination with an atmospheric pressure in N2 at 725 C for 10 min).
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The TiO2 content and the compact factor both increased in sam-
ples Nos. 2, 4, 7, and 8. It is easy to understand that an increased
TiO2 content indicates that more TiO2 is gathered on the surface
of the nano calcium carbonate. Thus, the relative content of tita-
nium to calcium on the surface was greater. The hydrolysis tem-
perature is critical for the control of the reaction kinetics that
determines the formation rate and the growth pattern of TiO2,
which results in the dispersion of TiO2 on the surface of the nano
calcium carbonate. Higher temperatures were conducive to thegrowth of the Ti(OH)4 precipitate crystal, which led to an inferior
dispersion of TiO2 and thus a smaller compact factor.
As shown in Fig. 10, the sorption capacity in the 30th run first
increases and then decreases with increasing compact factor. A
compact factor in the range of 0.8–1.3 resulted in a small decay
ratio and a high sorption capacity in the 30th run; this range was
thus regarded as an optimal range of C . When C was less than
0.85, the sorption capacity decayed the most throughout the 30-
cycles because the coating layer was not dense and the distribution
of the coating material on the surface was not uniform. Thus, this
coating was not able to prevent the agglomeration of the CaCO3
or the sintering of the nano particles. When C was greater than
1.3, the coating layer was uniform and sufficiently dense. For
example, the compact factor of sample 8 was 1.89, and the sorptioncapacity of this sample gradually stabilized after the first few ini-
tial cycles of decay. However, the increase in the coating material
content led to a reduction in the relative content of CaO, which
greatly reduced the overall sorption capacity of the sorbent.
There were two main factors that affected the compact factor C :
the coating content and the operating conditions, which include
the temperature and the concentration used in the preparation of
the absorbent. The coating content mainly determined the coating
thickness, whereas the operating conditions regulated the coating
uniformity. When the coating material content was constant, both
the sorption capacity and the durability of the sorbent prepared at
a low temperature and a low reactant concentration were im-
proved probably because these conditions reduce the reaction rate,
thus making the coating material on the surface of the nanocalcium carbonate distribute more evenly. In addition, when the
compact factor was close to 1.0, the compactness was considered
to be improved.
4. Conclusion
The application of an adsorption phase technique for the prep-
aration of a TiO2-coated nano-CaO based CO2 adsorbent with high
durability and a controlled coating layer was introduced in thisstudy. A coating layer of 4.5–11.6 nm was formed with increasing
TiO2 content. The TiO2 coating can prevent the sintering of nano
CaCO3 during multiple carbonation/calcination cycles. Among the
TiO2 content, the hydrolysis temperature, and the ester concentra-
tion, the TiO2 content has the most effect on the durability of resul-
tant absorbent for CO2 adsorption. The optimal content of TiO2
ranges from 8% to 12%. The optimal hydrolysis conditions included
a relatively low temperature in the range of 0–20 C and a low
reactant concentration of 5–10 wt.%. The compact factor is a
quantitative description of the status of the TiO2 coating layer
and differs when the TiO2-coated nano-CaCO3 is prepared under
different conditions. When the compact factor is between 0.9 and
1.3, the compactness is considered to be optimal because the
adsorption stability of these samples is the best of the samplestested.
Acknowledgments
The National Natural Science Foundation of China is thanked for
its financial support (20876142). Kimberly Braches from McMaster
University is also acknowledged for her editing assistance.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cej.2012.11.095.
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Table 3
The compact factors of TiO2 coating layer under different experimental conditions.
Sample Nos. 2 4 7 8 10 11 12
C 0.81 1.31 1.52 1.89 1.03 1.09 0.77
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.00.1
0.2
0.3
0.4
0.5
0.6
0.7
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r e a c t i v e s o r p t i o n c a p a c i t y ( g C O
2 / g C a O )
compact factor
0
20
40
60
80
decay ratio of 30th run
d e c a y r a t i o ( % )
Fig. 10. The relationship between compact factor and the 30th sorption capacity
and decay ratio.
44 Y. Wang et al. / Chemical Engineering Journal 218 (2013) 39–45
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