Chemical Engineering Science 273 (2023) 118646
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Chemical Engineering Science
journal homepage: www.elsevier.com/locate/ces
Kinetics of CaCO3 decomposition at low CO2 partial pressure in a vacuum
fixed bed
Wenfei Yue a,b, Wenli Song a,c,⇑, Chuigang Fan a,c,⇑, Songgeng Li a,b
a
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
School of Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
c
Sino-Danish College, University of Chinese Academy of Sciences, Beijing 100049, China
b
h i g h l i g h t s
Intrinsic reaction kinetic parameters of CaCO3 decomposition in vacuum are obtained.
Criterion eliminating CO2 partial pressure on CaCO3 decomposition rate is proposed.
The activation energy in vacuum is lower than that in atmospheric pressure.
a r t i c l e
i n f o
Article history:
Received 11 January 2023
Received in revised form 22 February 2023
Accepted 13 March 2023
Available online 17 March 2023
Keywords:
Kinetics
CaCO3 decomposition
Isothermal
Vacuum
Fixed bed
a b s t r a c t
Calcium looping often suffers from sintering during CaCO3 decomposition process, and CaCO3 decomposition in vacuum is a potential solution. Kinetic analysis of CaCO3 decomposition in vacuum is seldom,
and most of the kinetics in the literature is obtained from thermogravimetric analyzer in atmospheric
pressure. In addition, the reported kinetic data seems scattered, and varies depending on the analyzer
employed. This study is focused on the intrinsic kinetics of isothermal CaCO3 decomposition in a vacuum
fixed bed from 700 °C to 850 °C. Besides conventional internal/external mass transfer elimination via
reducing size/increasing gas velocity, the effect of CO2 partial pressure is specially discussed under vacuum condition. It is found that, this influence could be eliminated only when the relative difference
between equilibrium pressure and maximum CO2 partial pressure (Peq -P*CO2)/Peq was higher than 0.98.
Tests at atmospheric pressure were also conducted with the same particle size and gas volumetric feeding rate for comparison. The intrinsic reaction rate constants of CaCO3 decomposition in vacuum are
1.05 10-3 s1– 4.76 10-3 s1 between 700 °C and 850 °C, and the activation energy of this reaction
is 92.58 kJ/mol.
Ó 2023 Elsevier Ltd. All rights reserved.
1. Introduction
Calcium looping process is a promising technology for carbon
dioxide capture and energy storage (Blamey and Anthony, 2010;
Tregambi and Montagnaro, 2015). The process is based on the
reversible reaction between calcium oxide and calcium carbonate,
and two corresponding reactions need to be investigated: (a) carbon dioxide capture by calcium oxide (carbonation); and (b) thermal decomposition of calcium carbonate (calcination) in order to
regenerate calcium oxide for a new cycle. Many investigators have
focused on the kinetics of CaO carbonation and the modification of
CaO sorbents to achieve higher CO2 capture capacity and cyclic sta⇑ Corresponding authors at: State Key Laboratory of Multiphase Complex
Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing
100190, China.
E-mail addresses: wlsong@ipe.ac.cn (W. Song), cgfan@ipe.ac.cn (C. Fan).
https://doi.org/10.1016/j.ces.2023.118646
0009-2509/Ó 2023 Elsevier Ltd. All rights reserved.
bility (Erans and Manovic, 2016; Chen and Duan, 2020). Reactivity
decay is the main problem of calcium looping, and some methods
have been proposed to improve the CO2 capture capacity of CaO
sorbents, including hydration, thermal pretreatment, doping and
chemical pretreatment, incorporation of support with high Taman
temperature, or synthesis of CaO sorbents (Erans and Manovic,
2016; Chen and Duan, 2020; Raganati and Ammendola, 2023). In
addition, CaO regeneration at high temperature leads to sintering
and crystallization, resulting in a decrease in the reaction surface
available for carbonation and a significant loss of CO2 capture
capacity. The researchers found that under vacuum conditions,
CaCO3 could decompose at a relative low pressure and temperature, allowing it to complete decomposition in a shorter residence
time. Furthermore, CaCO3 decomposition at low pressure mitigates
CaO sintering and decay of CaO activity over multiple cycles
(Sarrión and Perejón, 2021).
W. Yue, W. Song, C. Fan et al.
Chemical Engineering Science 273 (2023) 118646
CaCO3 decomposition reaction, CaCO3 ðsÞ CaOðsÞ þ CO2 ðgÞ
Dr Hhm ð298KÞ ¼ 178kJ=mol, proceeds only when CO2 partial pressure
in gas phase above solid surface is less than the equilibrium pressure. According to temperature dependence of the equilibrium
pressure, low CO2 concentration or CO2 pressure can reduce
decomposition temperature of CaCO3 (Sarrión and Perejón, 2021;
Sakadjian and Iyer, 2007).
The reaction temperature and CO2 partial pressure during
CaCO3 decomposition directly affect structural and morphological
properties (such as crystallite size, surface area, and porosity) of
derived CaO, which in turn determine its subsequent CO2 capture
performance (Sakadjian and Iyer, 2007; Dunstan and Donat,
2021; Beruto and Searcy, 2004; Zhong and Bjerle, 1993). The influence of CO2 on CaCO3 decomposition mainly embodies in its limitation effect on decomposition rate and conversion (Escardino and
García-Ten, 2013; Qin and Feng, 2015). Stanmore et al. (Stanmore
and Gilot, 2005) observed that CO2 partial pressure showed no
influence on CaCO3 decomposition when CO2 partial pressure
was well below the equilibrium pressure. It is also reported that
when CO2 partial pressure is very small compared with equilibrium pressure, structural transformation and product desorption
will occur extremely rapidly and rate limiting step of CaCO3
decomposition will be chemical reaction control (Valverde, 2015;
Valverde and Sanchez-Jimenez, 2015; Valverde and Medina, 2015).
Since the kinetic parameters are essential to the practical
implementation of calcium looping process (Fedunik-Hofman and
Bayon, 2019c,b), researchers performed CaCO3 decomposition
experiments using different experimental equipment, calcium
sources, and atmosphere to determine these parameters
(Sakadjian and Iyer, 2007; Escardino and García-Ten, 2013;
Sanders and Gallagher, 2002; Fernandez and Turrado, 2019;
Tsuboi and Koga, 2018). Table 1 summarizes the studies of CaCO3
decomposition kinetics, including not only experimental conditions: experimental setup, particle size, atmosphere and temperature range, but also activation energy and reaction rate constant.
Most of the experimental data of CaCO3 decomposition have been
obtained from thermal gravimetric analyzer (TGA). High gas flow
rate can minimize external diffusion, and the choice of the gas flow
rate is usually limited by the type of TGA (Fedunik-Hofman and
Bayon, 2019b). CaCO3 samples come from different sources, and
particle sizes are not explicitly stated in some literature. Different
reaction kinetic models are used to determine the kinetic parameters. So, the CaCO3 decomposition data are scattered, and the
obtained activation energy mainly falls within the range of 110–
210 kJ/mol. In addition, researches have shown that CaCO3 decomposition in CO2 atmosphere slows down the decomposition rate
and increases the activation energy of this reaction (Escardino
and García-Ten, 2013; Fedunik-Hofman and Bayon, 2019a,b;
Martínez and Grasa, 2012).
It has been shown that vacuum has effect on CaCO3 decomposition, CaO structure and multicycle stability. Tian et al. (Tian and
Hou, 2021) found in a research of dolomite calcinations that vacuum could indeed accelerate the decomposition. Cao et al. (Cao
and Wang, 2014) also testified that vacuum lime product had more
developed pore structure, better sphericity, slighter sintering, and
better crystallization habits than products from conventional
ambient pressure kilns. Sarrión et al. (Sarrión and Perejón, 2021)
observed that CaCO3 decomposition under CO2 at low pressure
could reduce the temperature and slow down the deactivation of
CaO sorbent. Moreover, CaCO3 decomposition in vacuum should
be carried out in fixed bed. Fixed bed reactors are usually the ones
with the easiest and most economic design, construction and operation (Alvarez Rivero and Rodrigues, 2022).
Until now, most of the studies on CaCO3 decomposition are generally carried out in atmospheric pressure. While compared with
CaCO3 decomposition in atmospheric pressure, the kinetics of
CaCO3 decomposition in vacuum can be regarded as being rare.
Some researchers (Darroudi and Searcy, 1981; Garcı´a-Labiano
and Abad, 2002; Hyatt and Cutler, 1958) observed different kinetics in different CO2 partial pressures, and CO2 partial pressure
could be reduced significantly under vacuum condition. The aim
of this paper focuses on CaCO3 decomposition kinetics in vacuum.
Both the internal/external diffusion and CO2 partial pressure
effects are studied and eliminated in fixed bed reactor, and the
effect of CO2 partial pressure on the CaCO3 decomposition under
vacuum condition is specially discussed. Finally, the reaction rate
and intrinsic kinetic parameters of CaCO3 decomposition in vacuum are obtained. In addition, a comparative analysis of the kinetics obtained from vacuum and atmospheric pressure is performed.
2. Experimental
2.1. Materials
Analytical reagent calcium carbonate (Sinopharm Chemical
Reagent Co., Ltd., purity 99%) was used in the CaCO3 decomposition kinetic experiment. The size fractions used in experiments
were 2.7–3.0 mm, 1.25–1.60 mm, 0.9–1.25 mm, 75–96 lm and
48–75 lm, respectively.
2.2. Experimental setup
A lab-scale fixed bed reactor has been set up for CaCO3 decomposition experiments. Fig. 1 illustrates a detailed schematic flow
diagram of the reaction system. The inner diameter of the reactor
is 22 mm and the length is 800 mm. The reactor is heated by a
three-stage electric furnace. The temperature of the reactor is controlled by a thermocouple attached to the wall of the furnace, and
temperature of the sample is monitored by a thermocouple
inserted into the reactor bed. Flow rate of gas is controlled by mass
flow controller (MFC) with accuracy of 1%. A metered CO2 or N2 as
purge stream is introduced into the reactor inlet. The necessary
vacuum condition in the reactor required for the experiments is
provided by a vacuum pump, and the absolute pressure of 1 kPa
can be achieved. A nondispersive infrared (NDIR) analyzer downstream of the vacuum pump is used to monitor CO2 concentration
(range 0–20%) with the resolution of 1%. And another metered dilution stream of N2 is introduced before the NDIR analyzer to ensure
that CO2 concentration doesn’t exceed the full-scale value of the
analyzer. Sampling interval of the analyzer is set to 1 second.
2.3. Determination of kinetic parameters
CaCO3 decomposition experiments are carried out as follows. To
avoid the decomposition of CaCO3 during temperature-rise period,
the reactor is continuously purged with CO2 before the target temperature is attained. Fig. 2 shows CaCO3 decomposition in TGA in
CO2 atmospheric pressure, and it can be found that the initial
decomposition temperature of CaCO3 in pure CO2 is about
905 °C. Hence, CaCO3 does not decompose at the specified target
temperature (700 °C, 750 °C, 800 °C and 850 °C) during heating
in CO2 atmospheric pressure.
CaCO3 decomposition experiments are carried out under
isothermal conditions. Before each run, around 1.5 g CaCO3 sample
is loaded into the hanging basket of the reactor. The CO2 purge
stream is kept at 50 mL/min in atmospheric pressure during heating, and van out of the reactor via valve 1. The vacuum pump is
switched on and a dilution stream of N2 with 1000 mL/min is introduced before the vacuum pump.
Once the temperature is reached, the purge stream with the
specified flow rate is switched from CO2 to N2, and out of the reac2
W. Yue, W. Song, C. Fan et al.
Table 1
Summary of studies on kinetics of CaCO3 decomposition.
researchers
CaCO3 source
particle size
reactor
atmosphere
temperature
(°C)
activation
energy
(kJ/mol)
reaction rate constant
method
Zhong and Bjerle (1993)
limestone
—
TGA
vacuum
600–1000
110.63
2.85 10-7 ms1 (800 °C)
Criado and Gonzalez (1995)
CaCO3(AR)
—
TGA
707–900
191 ± 5
—
Sanders and Gallagher (2002)
Sakadjian and Iyer (2007)
CaCO3(AR)
limestone
—
45–75 lm
TGA-DSC
rotary calciner
550–1000
700–750
139–223
163.79
—
0.00123 s1 (750 °C)
model-free
—
Chen and Liu (2010)
CaCO3
<74 lm
TGA
1.3–20.0%
CO2
Ar
vacuum,
sweep gas
N2
N2
shrinking
core model
model-fitting
580–870
0.00256 min1 (800 °C)
CaCO3(AR)
limestone A
limestone B
limestone
nano CaCO3
CaCO3(AR)
limestone
eggshell
carbonated CaO
synthesized CaCO3
45–75 lm
<300 lm
fluidized bed
TGA
Ar
0–50% CO2
700–900
820–910
model-free and modelfitting
shrinking core model
grain model
<0.225 mm
36.2–88.2 nm
0.1 mm
75–150 lm
—
250–300 lm
—
tubular kiln
TGA
TGA
TGA
TGA-DTA
fixed bed
TGA
Pechini-synthesized
CaCO3
limestone
—
TGA
<50 lm
850–950
500–800
775–900
600–800
550–780
590–650
650–850
1000
550–850
910–1000
790–990
Scaltsoyiannes and Lemonidou (2020)
limestone
45 < dp < 75 lm
drop tube
reactor
fixed bed
air
Ar
60%CO2
N2
80% CO2
—
N2
CO2
N2
CO2
0–85% CO2
203.6
194.1
142.73
112.4
91.7
139
124.7–176.9
157.5
103.6
200–210
201
204
1220
164
307
195
Arcenegui-Troya and Sanchez-Jimenez
(2021)
Silakhori and Jafarian (2021)
limestone
20–130 lm
pure CaCO3
Liu and Hao (2021)
pure CaCO3
Tian and Hou (2021)
Li and Wang (2022)
Savuto and Stendardo (2022)
Da and Zhou (2023)
Yu and Yue (2010)
Martínez and Grasa (2012)
3
Escardino and García-Ten (2013)
Cui and Xue (2013)
Qin and Feng (2015)
Ramezani and Tremain (2017)
Tsuboi and Koga (2018)
Giammaria and Lefferts (2019)
Fedunik-Hofman and Bayon (2019c)
Fedunik-Hofman and Bayon (2019a)
Fernandez and Turrado (2019)
1
0.075 s (800 °C)
0.0584 s1 (870 °C, PCO2 = 0 kPa)
—
7.7 106 kmol m-2min1 (850 °C)
—
0.00104 s1 (800 °C)
65.69 min1 (800 °C)
—
3.63 10-8 mols1 (650 °C)
—
0.012 min1 (max)
0.023 min1 (max)
—
-6
grainy pellet model
model-free
shrinking core model
model-fitting
Friedman
—
Coats-Redfern
Friedman
—
2
1
2% CO2
825–885
210 ± 17
TGA
N2
570–640
175
0.72 10 kmol m
PCO2 = 0 kPa)
0.174 s1 (640 °C)
—
TGA
830–920
161.7–194.1
—
model-fitting
MFBRA
700–1100
model-fitting
600–950
128
272
313
—
dolomite
50 lm and
80 lm
0.15 mm
10–90%
steam
N2
100% CO2
vacuum
—
model-fitting
limestone
dolomite
pure CaCO3
150 lm
—
—
0/30% CO2
CO2
N2
750–920
650–850
527–850
182
641
192.0 ± 5
0.0256 kgm2s1 (750 °C)
—
—
shrinking core model
Kissinger method
KAS method
(780 °C,
uniform conversion
model
non-parametric kinetics
Chemical Engineering Science 273 (2023) 118646
tubular
furnace
MFB-TGA
TG-DSC
TGA
s
W. Yue, W. Song, C. Fan et al.
Chemical Engineering Science 273 (2023) 118646
order n can be determined by the slope. Then the reaction rate constants at different temperatures can also be derived.
Since CaCO3 decomposition reaction is generally considered to
conform to Arrhenius Law, the reaction rate constant can be also
expressed by Eq. (2.4):
tor via valve 2, then fed into the vacuum pump. Absolute pressure
in the reactor drops from 101 kPa to 1 kPa while the specified temperature is held constant during the decomposition reaction.
Fig. 3 illustrates typical curves of CO2 concentration, absolute
pressure and CO2 partial pressure in the reactor over time. The
experiment was carried out in vacuum at 800 °C with 300 mL/
min purge N2. The absolute pressure in the reactor can be reduced
rapidly from atmospheric pressure to 1 kPa. CaCO3 decomposition
reaction is considered to complete when CO2 concentration at outlet of the reactor is lower than 0.05%. As shown in Fig. 3 b), CO2 partial pressure experiences a rapid rising followed by a gradual
decreasing with time. This experiment is repeated three times,
and the relative standard deviation of CaCO3 decomposition conversion is 1.50%.
CaCO3 decomposition conversion is determined by measured
CO2 concentration, which is defined as the ratio of the molar of
the released CO2 to CaCO3 sample in the decomposition according
to Eq. (2.1):
Rt
n
CO2
a ¼ mCaCO
¼
3
ð
cCO
M CaCO
3
2
1c0 2 Þ V N2 dt
CO2
Þ
V m ð298
273
MCaCO3
mCaCO3
E
ln kðTÞ ¼ ln A þ RT
ð2:1Þ
3. Results and discussion
3.1. Influence of internal and external diffusion
In order to determine the intrinsic kinetics of CaCO3 decomposition reaction, internal and external diffusion should be eliminated. The effect of internal diffusion depends strongly on
particle size. Therefore, a set of experiments were carried out with
different particle sizes of CaCO3, and other conditions were kept as
the same. Five size fractions were used (2.7–3.0 mm, 1.25–
1.60 mm, 0.9–1.25 mm, 75–96 lm, and 48–75 lm) to explore
the effect of particle size.
Data collected from CaCO3 decomposition with different particle sizes in vacuum at 800 °C are shown in Fig. 4. CaCO3 decomposition rate constant (k) increases with the decreased particle size,
and the CaCO3 decomposition rate does not change when particle
size is small enough. The size fraction of CaCO3 in subsequent
experiments was set to 48–75 lm.
Elimination of external mass transfer is accomplished by varying superficial gas velocity of purge gas N2. Fig. 5 shows the change
of decomposition rate at different N2 flow rates. Increasing the gas
velocity of N2 from 2.60 to 9.05 m/s, the decomposition rate is in
the range of 3.63 10-3 s1– 3.74 10-3 s1, and the effect of N2
ð2:2Þ
where a is the conversion, t is the reaction time, kðTÞ is reaction rate
constant depending on temperature, and n is the reaction order.
Taking the logarithm of both sides of Eq. (2.2), it can be
described as:
ln
da
¼ ln kðTÞ þ n lnð1 aÞ
dt
ð2:5Þ
Based on Eq. (2.5), fitting ln kðTÞ against 1=T gives a regression
straight-line with ðE=RTÞ as slope and ln A as intercept. Thus,
the activation energy and pre-exponential factor can be obtained.
a– CaCO3 decomposition conversion; nCO2 – molar of CO2 produced
by CaCO3 decomposition, mol; mCaCO3 – mass of CaCO3 sample, g;
MCaCO3 – molar mass of CaCO3, g/mol; cCO2 – CO2 concentration, %;
c0CO2 – CO2 concentration in the blank experiment without CaCO3
sample, %; V N2 – total N2 flow rate in the reaction system, L/s; t–
CaCO3 decomposition time, s; V m – gas molar volume, 22.4 L/mol.
According to the fundamentals of chemical kinetics, the intrinsic kinetics of CaCO3 decomposition can be expressed in the form
of power-law, as shown in Eq. (2.2):
da
¼ kðTÞð1 aÞn
dt
ð2:4Þ
where A (s1) and E (kJ/mol) are the pre-exponential factor and activation energy, respectively, and R (J/molK) is the universal gas
constant.
The Arrhenius parameters can be obtained conveniently by
Arrhenius equation in its natural logarithmic form:
c0CO
2
0 1cCO
E
kðTÞ ¼ A exp RT
ð2:3Þ
For an isothermal condition, based on Eq. (2.3), ln (da/dt) can be
fitted against ln (1-a) to obtain a straight line, and the reaction
Fig. 1. Schematic diagram of fixed bed reactor system.
4
Chemical Engineering Science 273 (2023) 118646
W. Yue, W. Song, C. Fan et al.
3.2. Influence of CO2 partial pressure
CaCO3 decomposition is a reversible reaction and CO2 partial
pressure in the reaction system has a significant effect on the
decomposition rate. To determine the intrinsic kinetics of CaCO3
decomposition reaction, it is necessary to ensure that CO2 partial
pressure is well below the CO2 equilibrium pressure during CaCO3
decomposition process. CO2 equilibrium pressure Peq depends on
temperature, and it can be determined by Eq. (3.1) (Silakhori and
Jafarian, 2021).
lgPeq ðkPaÞ ¼ 9:079 8307:83
T
ð3:1Þ
Since CO2 concentration varies with the reaction time during
CaCO3 decomposition process, the CO2 partial pressure at highest
CO2 concentration in the reactor is defined as maximum CO2 partial pressure and denoted as P*CO2. Fig. 6 shows the CaCO3 decomposition experiment results with different P*CO2 at 700 °C and
800 °C. As shown in Fig. 6 a), the purge N2 gas flow rate is increased
from 0 to 500 mL/min, and the corresponding maximum partial
pressure decreases from 1 kPa to 0.034 kPa. The decomposition
rate increases as P*CO2 decreases, but no longer significant as
P*CO2 is below 0.056 kPa. In contrast, when P*CO2 is lower than
0.385 kPa at 800 °C, the decomposition rate of CaCO3 is no longer
Fig. 2. CaCO3 decomposition temperature obtained from TGA in CO2 atmospheric
pressure.
gas velocity on CaCO3 decomposition rate is negligible. Hence, the
gas velocity of N2 used for CaCO3 decomposition test is higher than
2.60 m/s.
Fig. 3. Parameters of the reaction system in the experiment (a) CO2 concentration, absolute pressure and (b) CO2 partial pressure in the reactor versus time.
Fig. 4. Effect of particle size on CaCO3 decomposition in vacuum at 800 °C (a) CaCO3 decomposition conversion versus time (b) decomposition rate at different particle sizes.
5
W. Yue, W. Song, C. Fan et al.
Chemical Engineering Science 273 (2023) 118646
Fig. 5. Effect of N2 flow velocity on CaCO3 decomposition in vacuum at 800 °C (a) CaCO3 decomposition conversion versus time (b) decomposition rate at different gas
velocities.
Fig. 6. Effect of P*CO2 on CaCO3 decomposition in vacuum at (a)700 °C and (b)800 °C.
Fig. 7. Effect of relative pressure difference (Peq-P*CO2)/Peq on CaCO3 decomposition
rate.
Fig. 8. Influence of CO2 partial pressure on CaCO3 decomposition at different
temperatures.
6
Chemical Engineering Science 273 (2023) 118646
W. Yue, W. Song, C. Fan et al.
Table 2
Experimental conditions for intrinsic kinetics of CaCO3 decomposition in vacuum.
temperature (°C)
size fraction (lm)
N2 purge stream (mL/min)
ug (m/s)
P*CO2 (kPa)
700
750
800
850
48–75
300
4.62
5.08
5.83
6.62
0.056
0.098
0.176
0.241
Fig. 9. CaCO3 decomposition conversion versus time at different temperatures in (a) vacuum and (b) atmospheric pressure.
CO2 partial pressure and CO2 equilibrium pressure for the CaCO3
decomposition reaction. The effect of CO2 partial pressure on
decomposition can be safely ignored when (Peq-P*CO2)/Peq is above
0.98.
According to this criterion, combined with experimental data
obtained from tests conducted at different P*CO2, the shaded area
in Fig. 8 gives the experimental operation region where CaCO3
decomposition reaction is not affected by CO2 partial pressure.
Based on all results and discussion in Sections 3.1 and 3.2, the
experimental parameters used to investigate the intrinsic kinetics
of CaCO3 decomposition in vacuum are tabulated in Table 2. The
gas velocity is higher than 2.60 m/s and P*CO2 is lower than that calculated by (Peq-P*CO2)/Peq 0.98.
changed. Therefore, with the increase of decomposition temperature, the effect of P*CO2 on CaCO3 decomposition reaction is
weakened.
The pressure difference between CO2 equilibrium pressure and
CO2 partial pressure Peq-P*CO2 is generally considered as the driving
force for CaCO3 decomposition (Wang and Lin, 2007; Salaudeen
and Acharya, 2018). The relationship between reaction temperature and relative pressure difference; i.e., the ratio of Peq-P*CO2 to
Peq, is exhibited in Fig. 7. CaCO3 decomposition rate increases with
increasing relative pressure difference. When the relative pressure
difference exceeds 0.98, the decomposition reaction rate will not
increase. Thus, this study provides a clear relationship between
3.3. Kinetic analysis of CaCO3 decomposition
Fig. 9 shows the effect of temperature on CaCO3 decomposition
rate under different decomposition conditions. For comparison,
CaCO3 decomposition experiments in atmospheric pressure are
carried out from 750 °C to 850 °C. Size fraction of CaCO3 particles
and N2 gas flow rate employed are the same as those used in vacuum tests. It should be noted that atmospheric pressure test is not
performed at 700 °C, because CaCO3 decomposition rate is too slow
in such circumstances. As expected, the decomposition rate of
CaCO3 increases with the increasing temperature under both
decomposition conditions. For vacuum tests, when reaction temperature is increased from 700 °C to 750 °C, time for a complete
decomposition is decreased significantly, but when the temperature is higher than 750 °C, the tendency slows down. Similar trend
is also observed in atmospheric tests in Fig. 9 b).
For both vacuum and atmospheric pressure cases, the reaction
order and reaction rate constant were calculated according to Section 2.3. The temperature dependence of reaction rate constant is
plotted in Fig. 10, while the fitted reaction orders are given in
Table 3. The measured reaction rate constant for vacuum decom-
Fig. 10. Reaction rate constants of CaCO3 decomposition at different temperatures
in vacuum and atmospheric pressure.
7
W. Yue, W. Song, C. Fan et al.
Chemical Engineering Science 273 (2023) 118646
Table 3
Kinetic parameters of CaCO3 decomposition in different absolute pressures.
temperature (°C)
absolute pressure (kPa)
reaction order
R2
activation energy (kJ/mol)
lnA (s1)
700–850
750–850
1
101
0.66
0.58
0.9699
0.9329
92.58
173.13
4.68
12.15
Table 4
Relative pressure difference at different temperatures in vacuum and atmospheric
pressure.
Temperature (°C)
750
800
850
(Peq - P*CO2)/Peq
vacuum test
atmospheric pressure test
0.9892
0.9919
0.9950
0.8492
0.7421
0.8201
Fig. 12 shows CO2 partial pressure in the reactor during CaCO3
decomposition process in vacuum and atmospheric pressure tests
at different temperatures. The CO2 partial pressure in vacuum is
one order of magnitude lower than that in atmospheric pressure.
The values of corresponding maximum CO2 partial pressure P*CO2
are also marked in the figure. For vacuum tests, P*CO2 varies from
0.10 kPa to 0.24 kPa; while for atmospheric pressure tests, the values of P*CO2 are between 1.37 kPa and 8.63 kPa.
As shown in Table 4, the specific values of the relative pressure
difference (Peq -P*CO2)/Peq in atmospheric experiments are also calculated and compared with vacuum data. It is clear that relative
pressure difference in atmospheric pressure tests is 0.74–0.85,
much lower than 0.98. This may be the main reason for the lower
decomposition rate, and the scattered experimental results
obtained in atmospheric pressure.
Fig. 11. Arrhenius plot for CaCO3 decomposition.
position varies from 0.001 s1 to 0.005 s1. Compared with atmospheric pressure cases, the reaction rate constant and reaction
order of CaCO3 decomposition in vacuum are significantly higher.
Arrhenius plots are shown in Fig. 11, according to which the
kinetic parameters of both vacuum and atmospheric pressure tests
are evaluated and tabulated in Table 3. The activation energy for
CaCO3 decomposition in vacuum is 92.58 kJ/mol.
From Figs. 10–11 and Table 3, it can be found that energy barrier in vacuum is much lower than that in atmospheric pressure.
Compared with the data in Table 1, the activation energy obtained
in vacuum is well below most published data obtained in atmospheric pressure (e.g. activation energy of 110–210 kJ/mol).
4. Conclusions
Most of kinetic experimental data for CaCO3 thermal decomposition in literature are obtained from TGA tests, so it is difficult to
exclude the influence of external diffusion and CO2 partial pressure. In this paper, intrinsic kinetics is studied in a vacuum fixed
bed from 700 °C to 850 °C. The influence of CO2 partial pressure
on CaCO3 decomposition rate is particularly discussed. It is found
that CO2 partial pressure in vacuum decomposition is much lower
than that in atmospheric pressure. Meanwhile, the condition for
Fig. 12. CO2 partial pressure in vacuum and atmospheric pressure tests.
8
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W. Yue, W. Song, C. Fan et al.
eliminating the influence of CO2 partial pressure on the measurement of CaCO3 decomposition rate is determined. Conclusions
include:
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(1) The effect of CO2 partial pressure on CaCO3 decomposition in
vacuum can be eliminated when relative pressure difference
(Peq -P*CO2)/Peq is higher than 0.98.
(2) Intrinsic reaction rate constant of CaCO3 decomposition
measured in vacuum varies within 1.05 10-3 s1–
4.76 10-3 s1 between 700 °C and 850 °C. The activation
energy of this reaction is 92.58 kJ/mol.
(3) Compared with kinetic data from atmospheric pressure
decomposition in current and other published studies, the
too high CO2 partial pressure may be the main reason for
the lower decomposition rate, and the scattered experimental results.
CRediT authorship contribution statement
Wenfei Yue: Investigation, Formal analysis, Visualization, Validation, Writing – original draft. Wenli Song: Conceptualization,
Methodology, Formal analysis, Supervision. Chuigang Fan: Writing
– original draft, Visualization, Supervision. Songgeng Li: Resources,
Writing – review & editing.
Data availability
Data will be made available on request.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
The authors are grateful for the financial support by the International Cooperative Program (2018YFE111100-01) of Ministry of
Science & Technology of China.
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