TITTLE: AN ANALYSIS OF THE EFFECT OF CARBONATION CONDITIONS ON CAO
DEACTIVATION CURVES
Authors: B. Arias*1, J.C. Abanades1, G.S. Grasa2
1
Instituto Nacional del Carbón, CSIC, C/ Francisco Pintado Fe, No. 26, 33011, Oviedo, Spain.
2
Instituto de Carboquímica, CSIC, C/Miguel Luesma Castán, No. 4, 50015 Zaragoza, Spain.
*Corresponding author: Dr. Borja Arias
Instituto Nacional del Carbón, C.S.I.C.
C/Francisco Pintado Fe, Nº26
33011 Oviedo (Spain)
Telephone: +34 985 11 90 57
Fax: +34 985 29 76 62
E-mail: borja@incar.csic.es
1
AN ANALYSIS OF THE EFFECT OF CARBONATION CONDITIONS ON CAO
DEACTIVATION CURVES
B. Arias1, J.C. Abanades1, G.S. Grasa2
1
Instituto Nacional del Carbón, CSIC, C/ Francisco Pintado Fe, No. 26, 33011, Oviedo, Spain.
2
Instituto de Carboquímica, CSIC, C/Miguel Luesma Castán, No. 4, 50015 Zaragoza, Spain.
borja@incar.csic.es, abanades@incar.csic.es, gga@incar.csic.es
Abstract
There is a growing interest in developing high temperature CO2 capture looping systems using
CaO as a regenerable sorbent. The evolution of the sorbent properties with the number of cycles
plays an important role in the design of these systems. One of the key variables to be determined
is the sorbent’s CO2 carrying capacity at the end of the so called fast carbonation stage. This is
the only useful conversion for practical purposes and it is know to decrease with the number of
cycles. It is obviously important to obtain experimental CaO deactivation curves from sorbent
laboratory tests in conditions as close as possible those to an industrial system. This paper
reviews previously reported results and investigates the effect of carbonation conditions on the
CO2 carrying capacities of CaO. An upgraded version of an existing deactivation model is used
to provide a better interpretation of the sorbent deactivation trends observed. The inclusion in the
deactivation model of an additional diffusion controlled carbonation stage may help to explain
some important discrepancies and observations reported in the literature. This review highlights
the need for an improved methodology to avoid the distortional diffusional effects in the
determination of CaO deactivation curves.
Keywords: CO2 capture, carbonation, deactivation, modeling
*Corresponding author e-mail address: borja@incar.csic.es; Tel.:+34 985119057; Fax.: +34 985 297662
2
1. Introduction
There is a need for low cost and more energy efficient CO2 capture technologies. A range
of CaO-based solid looping systems are being proposed for this objective [1,2,3,4]. These
processes are based on the carbonation/calcination equilibrium of CO2 on CaO and they consist
of at least two steps. In the fast carbonation step CO2 is absorbed by CaO up to a certain limit
(which is known to decay with the number of cycles). This is followed by a regeneration step,
the aim of which is to produce, by calcination, a concentrated stream of CO2 ready for
purification, compression and safe CO2 storage.
For CO2 capture reactors to operate efficiently, it is essential that both reactions proceed
rapidly. In postcombustion applications, the need to treat the large flow of flue gases at
atmospheric pressure and temperatures of around 650ºC entails very short contact times between
the gases and solids. In addition, the large solid circulation rates of active CaO required to
capture CO2 lead solid residence times of only a few minutes in the carbonator reactor. The
average concentration of CO2 in a carbonator reactor operating with a high capture efficiency of
CO2 from flue gases at atmospheric pressure will be very low. Therefore, in principle any test
that aims to measure the maximum carrying capacity of a CaO sorbent for postcombustion CO2
capture should be designed for carbonation times of no more that a few minutes and for CO2
volume fractions of below 0.1. However, it is generally accepted that the transition between the
fast carbonation regime and the slow carbonation regime is very sharp [5,6,7,8] and relatively
unaffected by carbonation conditions [9]. Some experimental works have reported on the effect
of the carbonation conditions on the sorbent carrying capacity during CO2 capture tests. For
example, Lysikov et al. [10] performed a parametric study on the effect of the carbonation
conditions on sorbent conversion using carbonation times between 7.5 and 30 min, and
3
carbonation temperatures ranging between 750 and 850 ºC. They found that the cycling
conditions influence the value of the residual CO2 carrying capacity of the sorbent. Similar
findings were reported by Manovic et al. [11], who studied the effect of carbonation/calcination
conditions on sorbent activity in a TGA.
Despite the influence of carbonation conditions on sorbent performance and the narrow
operation window of CFB carbonators, several publications report deactivation curves for CaO
under a wide variety of experimental conditions. Typically, deactivation curves are obtained in
TGA under isothermal conditions. In these cases the transition between calcination and
carbonation is achieved by modifying the composition of the atmosphere. The disadvantage of
this approach is that carbonation of the sorbent is usually carried out at high temperatures in
atmospheres with high partial pressures of CO2. An example of this experimental procedure can
be found in an early work by Barker [5] in which he studied the evolution of the CO2 carrying
capacity of a sorbent produced from calcium carbonate in a TGA in isothermal conditions at 866
ºC, using pure CO2 for the carbonation step and pure nitrogen for calcination. A similar
experimental procedure for long-term calcination/carbonation cycling was employed by Sun et
al. [12] and Chen et al. [13] to study the performance of limestone-derived CaO and pretreated
limestones and dolomites, respectively.
The objectives of the present paper are to analyze the effect of different carbonation
conditions on the sorbent carrying capacity of CaO in Ca-looping systems and to propose a
quantitative model that accounts for the observed experimental effects. One of the outcomes of
this study is a recommendation that a more adequate methodology should be used in laboratory
tests to determine the parameters that define the CaO deactivation curves necessary to design
continuous Ca-based capture system systems.
4
2. Analysis of CaO deactivation curves
It is well known that carbonation is only fast for a limited interval of Ca conversion and
that this conversion decreases with the number of cycles. Much effort has been devoted to
understanding the decay in activity of both natural and synthetically derived CaO sorbents. We
still consider that the basic carbonation model outlined by Barker [5] is useful for describing the
main features of the process. Unlike other gas solid reactions (like the sulfation of CaO), pore
blockage mechanisms are not usually responsible for the limited conversion of CaO after several
cycles of carbonation and calcination. In this case, the diffusion of CO2 through the built up layer
of calcium carbonate limits the conversion capacity of the sorbent.
A schematic representation of the progress of the carbonation reaction is shown in Figure
1. During the fast reaction stage, reaction takes place on the free surface of CaO via the
nucleation and growth of CaCO3. The carbonation process during this stage is limited by the
kinetics of the reaction between CaO and CO2. Under typical reaction carbonation conditions
(temperature: 600-700 ºC, CO2 concentration: 10-15 %vol), the conclusion of the fast reaction
stage can be achieved in the order of few minutes. The end of the fast stage takes place when the
growing CaCO3 collapses to form a continuous layer over the unreacted sorbent. Many textural
studies have focused on this phenomenon and have concluded that the thickness of the layer
produced is fairly constant during the cycles and has a value of around 49 nm at the end of the
fast reaction stage [14]. Therefore, the carbonation conversion under the kinetic regime (XKN)
should be proportional to the free surface (SN) and the thickness of the newly formed layer (h).
X K N  SN
h M CaO
 CaO VM Ca CO3
(1)
5
This assumption is even more valid for highly cycled particles, since the diameter of the
pores is greater than the thickness of the layer of CaCO3. This ensures that there is enough room
in the pore network to accommodate the layer of product despite the different molar volumes of
CaO and CaCO3 (16.9 and 36.9 cm3/mol respectively).
Due to the sintering of the sorbent, the free surface of the sorbent is reduced during the
carbonation/calcination cycles and thus the carbonation conversion under the fast reaction
regime decays. Many semi-empirical models have been proposed to describe the decay of
carrying capacity of the sorbent with the number of cycles [10, 12, 15, 16]. In this work we have
used a semi-empirical model proposed by Grasa et al [16]. This equation allows the sorbent
conversion (XKN) to be calculated as a function of the number of cycles (N), as the sorbent
reaches the point of maximum conversion under the fast reaction regime:
XKN 
1
1
k N
(1  X r )
 Xr
(2)
where k is the deactivation constant and Xr is the residual conversion after an infinite number of
cycles. These parameters are intrinsic to each sorbent, and depend on the rate of sintering of the
particles.
Once the free surface has been covered by CaCO3, the carbonation reaction can still
continue as the CO2 diffuses through the product layer (gaining a certain extra conversion XDN).
Under this regime, the chemical reaction and the diffusion of CO2 are controlling the overall
process. The kinetics of the diffusion and mechanisms of reaction between CaO and CO2 under
the diffusion controlled regime have been studied in the literature by several authors and models
are available for calculating the evolution of the sorbent conversion [6, 17, 18]. One of the most
6
frequently used models for analyzing the heterogeneous reactions between solids and gases is the
random pore model (RPM) proposed by Bathia et al. [19, 20]. We adapted this model in a
previous work to the carbonation of multicycled CaO particles [18]. By means of this model, the
conversion of the solids can be calculated using the following equation as long as the chemical
kinetics and the diffusion through the product layer are controlling the overall reaction rate:
2



   


1





1




 1 

 

X D  1  exp  

2
2
 






(3)
Following the methodology outlined in Figure 1, we calculated the carbonation
conversion (XN) for each cycle as the contribution of two terms, one corresponding to the
fraction that reacts under the fast reaction regime (XKN) and the other corresponding to the
diffusion controlled stage (XDN) given by equation 3:
XN=XKN+XDN
(4)
CO2
3
CaO
1
XD
2
CaCO 3
2
XN
CaO
XK
1
CO2
CaCO3
CaO
3
Time
Figure 1. Schematic representation of the sorbent conversion during carbonation.
7
The term XKN can be calculated from Eq. 2, but the application of this equation is not
straightforward for N>1 because all of the previous cycles have experienced a certain gain in
additional conversion XDN and therefore XN does not necessarily adjust to Eq. 2 with the intrinsic
k and Xr parameters. When the carbonation times and conditions allow a certain diffusion
controlled period as in Figure 1, the carbonation reaction extends beyond XKN. This means that a
fraction of the deactivated CaO (1-XKN) is transformed into CaCO3. After this material has been
calcined in the next cycle, the subsequent decomposition of the CaCO3 formed during the
diffusion controlled period yields CaO that is as active as any other form of CaCO3. In other
words, if we assume that CaCO3 does not have “memory”, the carrying capacity of the sorbent
after each calcination does not depend on N but on the carbonate conversion in the previous
cycle. Under these conditions it has been proven to be useful in previous works [21, 22] to use
the concept of Nage. This can be defined as the “actual age” of a particle with the carrying
capacity XN, or alternatively as the number of equivalent carbonation/calcination cycles that a
natural sorbent would require to achieve a conversion equal to XN (reaching in each cycle the
exact point of maximum carbonation conversion at which the fast reaction stage is completed, as
given by Eq. 2). Nage can be calculated using Eq. 2 as:
1
1
 1 
N age    

 k  X N  Xr 1 Xr



(5)
It is possible to illustrate the practical use of Nage by using data already published by other
authors who have reported series of calcination-carbonation cycles after a long cycle of
carbonation [5, 12, 13, 23, 24]. After carrying out several calcination carbonation cycles to allow
the carrying capacity of the sorbent to fall in accordance with Eq. 2, they performed one further
carbonation step that extended well beyond XKN. After calcination in the next cycle, the
8
decomposition of the “extra” CaCO3 formed during the slow reaction period in the previous long
carbonation cycle follows the typical deactivation curve. Figure 2 shows as an example some of
the experimental results obtained by Barker [5] and Sun et al. [12] (see references for more
details of the experimental apparatus and procedure). The data from Chen [13] are not included
in Figure 2, because the original sample was a deactivated sorbent from thermal pre-treatment,
and the behaviour of the sorbent reactivated by carbonation cannot be compared. The data from
Manovic [23] were obtained using a synthetic sorbent and so have not been presented in Figure
2.
0.8
Original sorbent (1)
Reactivated sorbent (1)
XN
0.6
Original sorbent (2)
Reactivated sorbent (2)
0.4
0.2
0.0
0
10
20
30
Number of cycles
Figure 2. Experimental results of sorbent reactivation by carbonation (sorbent 1 [5], sorbent 2
[12]) using the concept of Nage.
In the case of the results obtained by Barker [5] (see Figure 2), the sorbent undergoes a
25 calcination/carbonation cycles up to its point of maximum conversion under the fast reaction
regime. After that it is reactivated by extending the carbonation time for 24 hours, and then
tested again during 10 cycles. As can be seen, after being reactivated, the sorbent achieves in
9
cycle N=26, a carbonation conversion typical of a sorbent that has been cycled 6 times
(XN=0.37). In Figure 2, the Nage is used on the x-axis instead of the number of cycles and all the
points fall on the same curve irrespective of their experimental cycle number. Similarly, the
results of Sun et al. [12] correspond to a sorbent that has been cycled up to its residual
conversion (N>500), at which point it is carbonated for 24 h to reach a conversion of 0.50. This
is equivalent to a Nage of 2.6. In this figure, also the quality of the fit of all these data with the
general equation for a fresh sorbent (Eq. 2) is remarkable.
These results evidence the importance of using the concept Nage on XN, instead of the
number of cycles (N). This concept has also been used in previous papers [21, 25]. The
introduction of Nage in this work allows us to calculate the sorbent conversion at the end of the
fast carbonation stage after each calcination step (XKN), as long as we know the value XN from
the previous cycle (resulting from the addition of both the fast and slow reaction regime of
Figure 1, as indicated by Eq. 2). This is achieved by simply increasing the cycle number from
Nage to Nage+1 in Eq. 2. However, to calculate the final conversion with Eq. 5 and to estimate the
conversion term associated with the diffusional regime (XDN), we use Eq. 3 taking into account
that only a fraction of the sorbent (1-XKN) reacts under the diffusion controlled regime. In
summary by adding the two terms of conversion, the conversion of the sorbent at the end of each
cycle (XN) can be obtained as follows:
2




  Nage  



 
 


 1   Nage  Nage  1 
 Nage 

 
 



Nage 
1
1

  1 
  1  exp  1  
 (6)
XN 
 1
 

 Nage

1
 Nage 2  Nage 2
 k N age  
 k N age  




 1 X r
  1 X r
 





where,
10
 Nage 
 Nage 
 Nage 
4  LNage (1   )
(7)
2
S Nage
2 k s a  1   
M CaO b D p S Nage
(8)
ks (Cb  Ce ) S Nage tcarb
(9)
1   
The terms Nage, Nage and Nage in each cycle can be calculated by determining the chemical and
diffusion parameters under the carbonation conditions used (ks, D), and the evolution of the
structural parameters (SNage, LNage) during the cycles. As in the case of Nage, this may be related
to the conversion attained in the previous cycle XN-1 (see reference [18] for more details).
Therefore, in order to calculate XN for the entire multi-cycle testing, the value for Nage of the
sorbent at the beginning of the first cycle must be known. When the sorbent is produced by fresh
limestone, Nage has a value of 1 and XKN1 would be equal 0.70 (from Eq. 2 and the values of
Xr=0.075 and k=0.52). Now, the conversion at the end of the first carbonation cycle X1 can be
calculated by adding the diffusion part as in Eq. 6. Since this conversion will be higher than
XKN1, the value of Nage will be below 1 for the first cycle of extended carbonation. As mentioned
before, the kinetic part of the sorbent conversion for the next cycle (XK2) can be determined by
calculating Nage in the second cycle (Nage2 = Nage1 +1) and by using Eq. 6 to obtain X2. This
methodology repeated over many cycles allows the final sorbent carbonation conversion to be
calculated at different carbonation conditions (reaction time, temperature and CO2 partial
pressure) in each cycle.
In order to illustrate the effect of the carbonation conditions, by allowing for an extra gain
of conversion (XDN) in each cycle during a multi-cyclic carbonation-calcination test, we applied
11
Eq. 6 using the parameters for the RPM derived by Grasa et al. in a previous work for a natural
limestone (Imeco, see Table 1) [18]. We also fixed the carbonation temperature at 650 ºC and a
CO2 volumetric fraction of 10% at atmospheric pressure. The values used for k and Xr were 0.52
and 0.075, respectively.
The calculated results are presented in Figure 3, which includes as a parameter the
additional carbonation reaction time under the diffusion controlled regime from 0 minutes to
several hours (note that the carbonation reaction time associated with the end of the fast reaction
regime is only about 30 seconds under these conditions). As can be seen in the figure, long
extended carbonation times have a great influence on “apparent sorbent carrying capacity (XN)”
along cycling, even for moderate carbonation times. When the sorbent reacts only under the fast
reaction stage (tcarb=0), the conversion with cycles (XN) can be calculated directly with Eq. 2.
Only in this case, does the value of Nage correspond with the cycle number (N). However, when
the sample reacts under the diffusion controlled regime, the value of Nage will be lower than the
cycle number (N) due to the regeneration of the sorbent.
Under these experimental conditions, even when the carbonation time under the diffusion
controlled stage is short, the reactivation of the sorbent accumulates in each cycle, and after a
high number of cycles it exercises an increasing influence. This effect can be misleading when
determining the intrinsic parameters (k and Xr) and Eq. 2 is used directly if the experimental data
are not obtained exclusively under the fast reaction stage. In addition, these results demonstrate
that extended carbonation conditions in each cycle can yield unrealistically high values for the
residual activity (Xr) and the deactivation constant (k).
12
0.8
0 min
5 min
20 min
60 min
300 min
1440 min
XN
0.6
0.4
0.2
0
0
10
20
30
40
50
Number of cycles
Figure 3. Effect of carbonation time under the diffusion controlled regime
To show the importance of the testing conditions and the magnitude of error that may
affect the determination of the intrinsic sorbent parameters (k y Xr) when using Eq. 2, we fitted
the data calculated from the previous model to Eq. 2 and the constants Xr and k were then
calculated. Figure 4 represents the apparent residual conversion in the sorbent when carbonation
is extended beyond the fast reaction regime. According to the RPM, the increase in sorbent
conversion under the diffusion controlled regime is higher for low carbonation times, while the
reaction rate decreases, as the reaction time increases. A similar dependence is observed on
carbonation time in the case of apparent residual conversion. It can be seen in Figure 4 that
apparent Xr increases notably with carbonation time at low values, while it tends to stabilize for
long reaction times. For example, when the carbonation time is extended to 15 min, the value of
apparent Xr is twice the intrinsic value.
13
0.6
Xr apparent
0.5
0.4
0.20
0.3
0.15
0.2
0.10
0.1
0.05
0
0.0
0
250
500
750
10
1000
20
30
1250
tcarb (min)
Figure 4. Variation of the apparent residual conversion (Xr) with carbonation time (tcarb).
These results show that the constants Xr and k calculated by fitting the experimental
parameters to Eq. 2 depend on the testing conditions when the sorbent is allowed to react under
the diffusion controlled regime. This simple exercise highlights the importance of the testing
conditions during multi-cycling testing and an adequate fitting of the experimental data obtained,
as the residual conversion of the sorbent is one of the critical parameters employed in the design
of reactors.
In order to determine the intrinsic values of Xr and k for a sorbent, the carbonation
conditions in the experimental set up must reproduce as closely as a possible the operation
conditions at industrial scale (short carbonation times (no more than 5 minutes), carbonation
temperature in the range of 650 ºC, and average CO2 volume fraction below 10% at atmospheric
pressure). Under these conditions, the sorbent reacts mainly under the fast reaction regime. The
diffusion controlled stage may then be avoided and the data can be fitted directly to Eq. 2.
However, if the experimental set up allows reaction to take place under the diffusion controlled
14
regime, Eq. 2 cannot be used directly and instead the calculation procedure explained above and
Eq. 6 must be used to derive Xr and k. However, this approach requires the knowledge of other
parameters in order to calculate XDN (ks, D, S0, L0). The drawback of using Eq. 6 is the large
number of parameters involved, which have to be determined experimentally or be calculated by
fitting the parameters. This alternative also requires more experimental work to be carried out to
obtain data at different conditions of carbonation (temperature, reaction time,…).
In order to reduce the number of parameters involved in the calculation of XN when the
sorbent reacts under the diffusion controlled regime, we have attempted to derive a new equation
for reducing the experimental work needed to determine the intrinsic values of Xr and k. Our
approach for simplifying the calculation of XN assumes that the gain in activity under the
diffusion-controlled regime in each carbonation cycle is proportional to that achieved under the
kinetic regime (XKN). Assuming this to be so, the carbonation conversion in each cycle (XN) can
be calculated by means of the following equation:




1
XN  
 X r   (1  k D )


1
 (1  X )  k N

r


(10)
where kD is the proportional constant. The calculation procedure for this equation is similar that
described above, but in this case the sorbent conversion during the carbonation step is calculated
using Eq. 10 instead of Eq. 6. To compare the results of both equations, we fitted previously
obtained results using the expression derived from the RPM, into equation Eq. 10 for different
carbonation times, using the values of k=0.52 and Xr=0.075. Figure 5 compares the results
obtained by both methods.
15
0.8
0 min
20 min
60 min
300 min
XN
0.6
0.4
0.2
0.0
0
10
20
30
Number of cycles
40
50
Figure 5. Comparison of the results obtained with Eq.6 (continuous lines) and Eq. 10 (filled
symbols).
As can be seen in this figure, despite the simplicity of Eq. 10, this expression can predict
reasonably well the results obtained by the more complex equation derived from the RPM for
different carbonation times by using only one fitting parameter (kD). The parameter kD was
calculated for different carbonation times and the values obtained are represented in Figure 6.
0.30
KD
0.20
0.10
0.00
0
250
500
750
1000
1250
tcarb (min)
Figure 6. Variation of KD with carbonation time
16
The simplified equation proposed can be used to determine experimentally the intrinsic
constants of one sorbent (k and Xr) by performing a minimum of two multi-cyclic tests using
different carbonation times. To validate the previous model and illustrate the determination
procedure, we have attempted to re-interpret previously published data obtained from multicyclic carbonation-calcination tests for long carbonation times. Figure 7a shows two series of
data obtained using two carbonation times, the first one for 24 h of carbonation, and the second
one allowing the sample to react up its maximum conversion under the fast reaction regime (see
[5] for more details). The second series of data (see Figure 7b) were obtained using carbonation
times of 7.5 and 30 min respectively (see [10] for more details).
1.0
a
Barker-1
0.8
Barker-2
XN
0.6
0.4
0.2
0.0
0
5
10
15
20
25
Number of cycles
1.0
Lysikov-1
0.8
Lysikov-2
XN
0.6
0.4
0.2
b
0.0
0
10
20
30
40
50
60
Number of cycles
Figure 7. Comparison of the experimental data obtained by Barker [5] and Lysikov et al. [10]
with the results obtained from Eq. 10.
17
These experimental data were fitted to Eq. 10, and the parameters Xr, k and kD were
calculated by the least square method. In this fitting exercise, the values of Xr and k must the
same for each sorbent during both experimental runs, while the parameter kD will have a
different value for each run as the carbonation time is modified. The comparison between
experimental data and the calculated values is shown in Figure 7. As can be seen, Eq. 10 is able
to predict the experimental results reasonably well, indicating that the different behaviors
observed during the experimental runs are due mainly to the different carbonation conditions
used.
The intrinsic values of Xr and k of both sorbents are presented in Table 2. As can be seen,
similar Xr and k values were obtained for both sorbents. If we compare the values of kD obtained
using these experimental results, it can be observed that they are in agreement with those
determined theoretically for the Imeco limestone (see Figure 6). The value of kD for the first
experimental run of Barker has a value of 0. This result is not surprising as, during this
experimental test, the sample was allowed to react up to its maximum conversion in the fast
reaction regime. However, when it was allowed to react for 24 h during the carbonation period,
the performance of the sorbent was completely different. Under these experimental conditions,
kD reached a value of 0.26, indicating that the conversion achieved in each cycle (XN) was 26%
higher than that of the fast reaction stage (XKN).
From the results presented by Lysikov, it can be seen that the differences in carbonation
time are smaller, 7.5 and 30 min respectively, whereas the values of kD are closer. This increase
in carbonation time has a great impact on sorbent conversion during the experimental run. If the
experimental results presented Lysikov are fitted to Eq. 2, the apparent residual conversions
18
calculated are 0.20 and 0.11 for 30 and 7.5 min respectively, which differs greatly from the
intrinsic value of 0.051.
The analysis presented in this work was carried out by studying the effect of carbonation
time. However, it should be noted that the effect of other variables could be analyzed by the
same method. Thus, the partial pressure of CO2 could also be used as a variable and could be
modified during the experimental runs, since it has a similar influence on sorbent conversion
(XN) as carbonation time (see Eq. 6). Carbonation temperature is another variable susceptible to
modification. However, the dependence of XN on temperature is more complex, as it modifies
the diffusion (D) and kinetic (ks) parameters, as well as the equilibrium partial pressure of CO2
(Ce).
Analysis of the data available in the literature has shown that some of the differences
observed in the sorbent behavior may be due to the different carbonation conditions used in the
experimental runs. Direct fitting of the data, without taking into account the effect of the
carbonation conditions on sorbent conversion, may lead to inaccurate determination of the values
of Xr and k and may cause us to attribute to the sorbent carrying capacity that would not hold up
under the conditions of a real system.
3. Conclusions
In this work the effect of carbonation conditions on sorbent activity has been analyzed. It
has been shown that the carrying capacity of a sorbent during cyclic testing depends on the
experimental conditions if the sorbent is allowed to react under the slow diffusion controlled
regime. This is due to the partial reactivation of the sorbent when carbonation is extended
beyond the fast reaction stage. In order to obtain useful deactivation parameters, the sorbent must
19
be tested under experimental conditions that do not allow the reaction to proceed under the slow
diffusion controlled stage.
The carrying capacity of a sorbent during each cycle can be calculated by adding two
terms, one corresponding to the fraction that reacts under the fast reaction regime and the other
corresponding to the diffusion controlled stage. The intrinsic deactivation parameters (k and Xr)
of a sorbent can be calculated using the methodology proposed in this work and using the
experimental data obtained under the diffusion controlled regime. For this purpose a minimum of
two runs under different carbonation conditions must be carried out.
Application of the proposed methodology to previously published data shows that great
differences can be encountered between the intrinsic values and those calculated by fitting the
experimental results to models that do not take into account the regeneration effect of extending
carbonation beyond the fast reaction regime.
20
NOTATION
a, b
C
D0
Dp
EaD
Eak
h
k
kD
ks
ks0
LNage
MCaO
N
SN
tcarb
VMCaCO3
XDN
XKN
XN
Xr
Z
ρCaO




stoichiometric coefficients for carbonation reaction
concentration of CO2, kmol/m3; b, bulk concentration; e, equilibrium
Pre-exponential factor for diffusivity, m2/s
apparent product layer diffusion, m2/s
Activation energy for the combined diffusion and kinetic regime, kJ/mol
Activation energy for the kinetic regime, kJ/mol
product layer thickness, m
sorbent deactivation constant
proportional constant in Eq. 10
rate constant for surface reaction, m4/kmols
pre-exponential factor for chemical reaction, m4/kmols
Total length of pore system, m/m3
molecular weight of CaO, kg/kmol
number of calcination/carbonation cycles
reaction surface, m2/m3
carbonation time under the diffusion controlled regime, s
molar volume of CaCO3, m3/kmol
CaO molar conversion under the diffusion controlled regime
CaO molar conversion under the fast reaction regime
CaO molar conversion in each cycle
residual CaO conversion
ratio volume fraction after and before reaction
CaO density, kg/m3
4πLN(1- ε)/S2N
2ksaρ(1- ε)/MCaObDpSN
ks(Cb-Ce)SNtcarb/(1- ε)
porosity
21
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