Article
pubs.acs.org/EF
K2CO3/Al2O3 for Capturing CO2 in Flue Gas from Power Plants. Part 1:
Carbonation Behaviors of K2CO3/Al2O3
Chuanwen Zhao, Xiaoping Chen,* and Changsui Zhao
School of Energy and Environment, Southeast University, Nanjing 210096, People’s Republic of China
ABSTRACT: The present paper is the first part of a series of papers about a systematical investigation on the application of the
K2CO3/Al2O3 sorbent for capturing CO2 in flue gas. It was focused on the carbonation behaviors of K2CO3/Al2O3 in a
thermogravimetric analyzer. The effects of the temperature, gas composition, and pressure on the reactions were studied by
analyzing the experimental breakthrough data. It was found that K2CO3/Al2O3 shows a high CO2 capture capacity and its
carbonation conversion reaches 68.3−91.8% in 20 min when the reaction temperature is in the range of 55−75 °C, the CO2
concentration is in the range of 5−20%, the H2O concentration is in the range of 12−21%, and the pressure is 0.1 MPa. The total
carbonation conversion mainly depends upon the reaction in the first 5 min, and the reaction rate reaches the maximum in about
2 min. The total carbonation conversion increases with the increase of CO2 and H2O concentrations but decreases with the
increase of the temperature and pressure. Among the factors studied, the H2O concentration and pressure were found to have a
significant impact on the carbonation. Moreover, water pretreatment of the sorbent plays an important role in the carbonation
reaction.
1. INTRODUCTION
Applying dry alkali-metal-based sorbents for capturing CO2 of
flue gas has been recently investigated as an innovative
concept.1−3 Various sorbents have been proposed and
developed for the application. Previous studies found that
CO2 capture capacity was greatly dependent upon the sorbents
used and the operation conditions.4−12 Some common
problems were confronted with most of the sorbents proposed
in the studies. For example, the global carbonation reaction
rates of the alkali-metal-based sorbents were generally slow,
resulting in slow carbonation conversion of the sorbents. The
appropriate operation conditions for carbonation and regeneration reactions were not confirmed.
To develop an efficient sorbent and also to solve the
problems above, extensive research works were performed in
our group. Our previous studies13,14 found that K2CO3
generated from calcining KHCO3 had excellent carbonation
capacity. Furthermore, when loading this sorbent as the active
component on γ-Al2O3, the resulting K2CO3/Al2O3 sorbent
had a fast carbonation reaction rate and achieved high
carbonation conversion.15,16 A preliminary study showed that
K2CO3/Al2O3 had excellent CO2 capture capacity and
regeneration performance in a bubbling fluidized-bed reactor
during 10 cycles,17 indicating that K2CO3/Al2O3 has potential
to be used as a sorbent for a large-scale CO2-capture process.
For the sake of developing this sorbent for industrial
application, several key issues should be solved thoroughly.
First, understanding the effects of operation conditions,
including the temperature, gas composition, heating rate, and
pressure, on the carbonation and regeneration reactions are
essential for describing the carbonation behaviors and
determining appropriate reaction conditions of this sorbent.
Second, the loading amount of the active component is a
crucial parameter for the sorbent, and the attrition resistance
performance of the sorbent is also substantial for its application
© 2012 American Chemical Society
in the fluidized-bed reactor. Therefore, an embedded study on
them is highly needed. Third, concerning the application of
K2CO3/Al2O3 in practice, the CO2 capture capacity and
regeneration property in the fluidized-bed reactor should be
investigated by long-time and multi-cycle operation. Lastly, the
effect of impurity gases, including SO2, NO, and HCl, existing
in the flue gas on the carbonation reaction needs to be under
consideration.
Aiming at clarifying the above issues, a systematical
investigation was carried out to investigate CO2-capture
behaviors of K2CO3/Al2O3. The carbonation and regeneration
behaviors of K2CO3/Al2O3 were investigated with thermogravimetric analysis (TGA). A bubbling fluidized-bed reactor was
used to study the effect of the loading amount, SO2, and NO in
the flue gas on the CO2-capture behavior, 80 carbonation/
regeneration cycle behaviors, and abrasion characteristics of
K2CO3/Al2O3 particles. The results will be reported in a series
of papers. The present paper is the first part, which is addressed
to the carbonation behaviors of K2CO3/Al2O3, investigated
using a pressurized TGA approach.
2. EXPERIMENTAL SECTION
2.1. Samples. K2CO3/Al2O3 used in this study was prepared by
impregnating K2CO3 on γ-Al2O3. K2CO3 was provided as an analytical
reagent, and a special γ-Al2O3 was supplied by the Research Institute of
Nanjing Chemical Industry Group. The preparation process of the
sorbent consisted of three steps: mixed and impregnation, dried at 105
°C for dehydration, and calcined at 300 °C. The loading amount of
K2CO3 was determined to be 28.5 wt %. The generated K2CO3/Al2O3
sorbent is in the form of particles in the size range of 10−30 μm, with
a mean size of 20 μm. Its surface area and pore volume are 71.4 m2/g
and 0.27 cm3/g, respectively, and the mean pore size is 13 nm. More
Received: March 1, 2011
Revised: August 16, 2011
Published: January 12, 2012
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information about the method of the sorbent preparation and its
microscopic structure characterization were reported in detail in a
previous paper.15
The amount of alkali metal impregnated was determined by an
Advant’XP X-ray fluorescence (XRF). An accelerated surface area and
porosimetry (ASAP) 2020 system with N2 adsorption−desorption was
used for surface area and pore structure determinations.
2.2. Apparatus and Procedure. Experimental studies for
carbonation of K2CO3/Al2O3 were performed with TherMax 500
TGA in simulating flue gas composed of CO2, H2O, and a balance of
N2. The gas flow rate was set to be 500 mL/min. The mass of the
sorbent loaded in TGA was about 100 mg. Considering the high water
adsorption capacity of K2CO3/Al2O3, the fresh sorbent was first heated
to 200 °C for dehydration and then the temperature decreased to a
certain value for the carbonation reaction. The details about this
experimental system were reported in our previous paper.17
To determine the optimum reaction condition for carbonation, the
carbonation of K2CO3/Al2O3 was carried out under a wide range of
reaction conditions by varying the carbonation temperature between
55 and 80 °C, the CO2 concentration between 5 and 20%, the H2O
concentration between 0 and 21%, and the operation pressure between
0.1 and 0.5 MPa.
As confirmed by X-ray diffraction (XRD) analysis in the previous
study,15 the main carbonation product of K2CO3/Al2O3 was KHCO3.
On the basis of the theoretical value increment corresponding to the
complete conversion of K2CO3 to KHCO3, the carbonation
conversion (η) and reaction rate (rc) are calculated as
η=
rc =
MK2CO3(w(t ) − w(0))
αw(0)(2MKHCO3 − MK2CO3)
MK2CO3
dw
dt
αw(0)(2MKHCO3 − MK2CO3)
Figure 1. Effect of the temperature on (a) carbonation conversion and
(b) reaction rate.
when the reaction was carried out in 60−80 °C. In contrast,
Na2CO3 calcined from NaHCO3 was found to have better
reactivity than other sodium-based sorbents.21 Its total
conversion was 76% in 200 min with NaHCO3 as the product
when the reaction temperature was 60 °C. The conversion was
95% in 200 min with Wegscheider’s salt as the product when
the reaction temperature was 70 °C. No carbonation occurred
at 80 °C when the CO2 concentration was 5%.3 While the
carbonation temperature range of the sodium-based sorbents
was generally narrow, that of the potassium-based sorbents was
found relatively wider. With 2 h of reaction, the weight of
K2CO3 increased 26, 12, and 6% when the carbonation
temperatures were 60, 80, and 100 °C, respectively.22 The
carbonation reactivity of K2CO3 calcined from KHCO3 was
better than K2CO3, and the total conversion was 74−83% in 40
min when the temperature was in the range of 60−70 °C.23 In
comparison to the carbonation behaviors of all of the abovementioned sorbents reported in the literature, the carbonation
conversion and reaction rate of K2CO3/Al2O3 shown in Figure
1 are higher at the same reaction condition. What is especially
significant is that the reaction rate in the first 5 min was much
higher. As reported previously,16 after K2CO3 had been loaded
on Al2O3, the total surface area and pore volume of the sorbent
were greatly increased and the active components were
uniformly distributed on the surface of Al2O3 in the form of
many small aggregates. As a result, the CO2 capture capacity
increased significantly.
3.2. Effect of the CO2 Concentration on Carbonation.
Figure 2 shows the effect of the CO2 concentration on the
carbonation conversion and reaction rate of K2CO3/Al2O3 in
the same condition of 65 °C and 15% H2O at 0.1 MPa.
As shown in panels a and b of Figure 2, when the CO2
concentration increases from 5 to 20%, η increases from 67.9 to
76.9% in 25 min and the maximum reaction rate increases from
6.9 to 28.7%/min. It means that the effect of the CO2
concentration is not significant on the total carbonation
conversion but is significant on the reaction rate. These are
attributed to the change of the concentration driving force.
× 100%
(1)
× 100%
(2)
where t is the reaction time, w(t) and w(0) are the weights of the
sorbent at time t and at the beginning of carbonation, respectively, α is
the K2CO3 loading amount of the sorbents, dw/dt is the change of the
weight of the sorbent with time, and MK2CO3 and MKHCO3 are the
molecular weights of K2CO3 and KHCO3, respectively.
3. RESULTS AND DISCUSSION
3.1. Effect of the Reaction Temperature on Carbonation. The carbonation of K2CO3/Al2O3 in the simulating
flue gas with the composition of 15% CO2, 15% H2O, and N2
balanced at 0.1 MPa was carried out at different temperatures.
The carbonation conversion (η) and the reaction rate (rc)
changing with the reaction time are presented in Figure 1.
Figure 1a shows that the total conversion η increases with the
reaction time for all reaction temperatures. However, η
decreases with the temperature increasing. It can be seen that
η reaches 68.3−91.2% in 20 min when the reaction temperature
is in the range of 55−75 °C, while η is only 57.9% in 18 min
when the reaction temperature is 80 °C. The low carbonation
conversion at a higher temperature is attributed to the
reduction in the concentration driving force because the
carbonation reaction is reversible and highly exothermic. Figure
1b shows that the total carbonation conversion is mainly
dependent upon the reaction in the first 5 min, and the reaction
rate reaches a maximum at about 2 min and then decreases at
all temperatures.
For an alkali-metal-based sorbent, the temperature is an
important factor for the carbonation reaction. The studies on
sodium-based sorbents reported in the literature indicated that
the carbonation reactivity of Na2CO3, Na2CO3·H2O, and
Na2CO3 calcined from sodium sesquicarbonate was weak18−20
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Figure 2. Effect of the CO2 concentration on (a) carbonation
conversion and (b) reaction rate.
Figure 3. Effect of the H2O concentration on (a) carbonation
conversion and (b) reaction rate.
Because the carbonation reaction is fast in the first 5−10 min,
the total carbonation conversion is not significantly affected in
25 min. It was reported that24 the carbonation conversion and
the reaction rate of Na2CO3 calcined from NaHCO3 increased
in 250 min, when the CO2 concentration increased from 5 to
8%. This is consistent with the observation for K2CO3/Al2O3
here. These results emphasize the importance of choosing the
CO2 concentration. In previous papers,4−6 the CO2 capture
capacities of many potassium-based sorbents were studied in a
fixed-bed reactor with the CO2 concentration of only 1% in
reaction gas. Because the CO2 concentration is between 10 and
20% in the flue gas from power plants, the difference in the
CO2-capture behavior may be caused by the CO2 concentration.
3.3. Effect of the H2O Concentration on Carbonation.
The effect of the H2O concentration on the carbonation
conversion and reaction rate of K2CO3/Al2O3 in the same
condition of 65 °C and 15% CO2 at 0.1 MPa is shown in Figure
3.
It can be seen that there was actually no reaction occurring
for K2CO3/Al2O3 if no H2O was provided, implying that CO2
could not be adsorbed without the presence of H2O. Figure 3a
shows that η increases significantly from 26.1 to 85.1% in 25
min when the H2O concentration in the gas mixture was
increased from 3 to 21%. The total carbonation conversion only
increases for 9.0% when the CO2 concentration increases from
5 to 20% in Figure 2a, while it increases for 59.0% when the
H2O concentration increases from 3 to 21% in the same
reaction time of 25 min. It indicates that the effect of the H2O
concentration on the total carbonation conversion is more
significant than that of the CO2 concentration. Figure 3b shows
that the maximum reaction rate increases from 3.4 to 28.6%/
min when the H2O concentration increases from 3 to 15%, and
then it remains at about 30%/min when the H2O concentration
increases from 15 to 21%. The reason is that, for the
carbonation of K2CO3/Al2O3, H2O is first adsorbed on the
surface of the sorbent and then CO2 reacts with the adsorbed
H2O and K2CO3 to produce KHCO3.16 In these processes,
H2O adsorption is considered as the rate-controlling step. As
the H2O concentration increases, the diffusion and adsorption
capacities of H2O in the sorbent are improved; therefore, the
total carbonation conversion increases as the H2O concentration increases. When the H2O concentration is high enough,
the effect of the H2O concentration change on carbonation will
become not significant.
3.4. Effect of the Water Pretreatment on Carbonation.
It was found that the carbonation capacity decreased after water
pretreatment for both Na2CO3 calcined from NaHCO3 and
K2CO3 calcined from KHCO3,13,14,19 while the carbonation
capacity increased after water pretreatment for the alkali-metalbased sorbents, such as K2CO3/activated carbon (AC),
sorbNX35, and sorbKX35.7,9,11 To confirm whether the CO2
capture capacity was excellent after water pretreatment for
K2CO3/Al2O3, the effect of the water pretreatment on
carbonation is shown in Figure 4.
Figure 4. Effect of the water pretreatment on the carbonation reaction.
For all of the cases shown in Figure 4, the sorbent was
pretreated in 15% H2O for 30 min and then the gas
composition was changed to 15% CO2 concentration and
various H2O concentrations with N2 balance. It can be seen
that the dimensionless weight only increases to 1.02 after water
pretreatment. Additionally, no K2CO3·1.5H2O was detected in
the product of K2CO3/Al2O3 by the XRD analysis. Therefore,
the increase in the weight is believed to result from the physical
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3.4%/min. It can be seen that, when the pressure is higher than
0.2 MPa, η is less than 50% in 35 min and the maximum
reaction rate is less than 10%/min, implying that increasing the
pressure significantly depresses the carbonation reaction. The
main reason may be the decrease of the diffusion coefficient of
water vapor in the sorbent with increasing the pressure.
Because H2O adsorption is the rate-controlling step of the
carbonation process of K2CO3/Al2O3, the slow diffusion rate of
H2O certainly has a significant effect on the total carbonation
conversion. Therefore, it is necessary to keep the pressure at
atmospheric pressure for achieving a good carbonation
performance.
Traditional combustion-based systems for power generation
are typically operated at near ambient pressures, while the
advanced integrated gasification combined cycle systems are
operated at elevated pressures.25,26 This implies that the
K2CO3/Al2O3 sorbent may be more suitable for CO2 capture in
traditional combustion-based systems.
adsorption of H2O. Although no H2O was continued to supply
after water pretreatment, the dimensionless weight increased to
1.089 in 47 min, corresponding to 70.4% conversion of K2CO3
to KHCO3 in CO2 and N2. When the H2O concentration was
increased, the dimensionless weight increased from 1.089 to
1.096 in 47 min, corresponding to the increase of η from 70.4
to 75.8%. The change in carbonation curves is less than the
result in Figure 4 at the same reaction condition, suggesting
that the water pretreatment plays an important role in the
carbonation process.
The study on the carbonation of K2CO3/AC found that,16
with water pretreatment, the hydration reaction occurred to
form K2CO3·1.5H2O, which was rapidly transformed into
KHCO3 once CO2 was provided. For K2CO3/Al2O3, H2O is
physically adsorbed during water pretreatment and then the
carbonation reaction is rapid when CO2 is given. However, after
water pretreatment, the carbonation conversions decreased
significantly for all analytical reagent samples of alkali-metalbased sorbents.13,14,19 The reason is deduced to be that, after
loaded on Al2O3, the microstructure and the distribution
behavior of K2CO3 are significantly changed.
In the flue gas of power plants, the CO2 concentration is
usually in the range of 10−20% and the H2O concentration can
be as high as 8−17%. Because the concentrations of CO2 and
H2O generally vary with time, it is hard to keep them with a
constant stoichiometric ratio. It can be found from Figure 4
that the effect of the H2O concentration in the flue gas on the
carbonation becomes insignificant for K2CO3/Al2O3 with water
pretreatment. As a result, the sorbent can be regenerated in a
H2O atmosphere in real operations. In this way, the CO2
capture capacity of K2CO3/Al2O3 will be improved.
3.5. Effect of the Pressure on Carbonation. The
carbonation of K2CO3/Al2O3 was carried out at various
pressures in the same reaction condition of 55 °C, 15% CO2,
15% H2O, and N2 balanced. Results are shown in Figure 5.
4. CONCLUSION
The carbonation behaviors of K2CO3/Al2O3 were systematically investigated using TGA. K2CO3/Al2O3 shows high CO2
capture capacity, and the carbonation conversion reaches 68.3−
91.8% in 20 min under reaction conditions of simulated flue
gas. The total carbonation conversion is mainly dependent
upon the reaction in the first 5 min, and the reaction rate
reaches the maximum at about 2 min. The total carbonation
conversion increases with the increase of CO2 and H2O
concentrations but decreases with the increase of the
temperature and pressure. Water pretreatment of the sorbent
plays an important role in the carbonation reaction. After water
pretreatment, the effect of the H2O concentration in the flue
gas on carbonation becomes not significant for K2CO3/Al2O3.
As a result, the sorbent can be regenerated in a H2O
atmosphere in real operations. In this way, the CO2 capture
capacity of K2CO3/Al2O3 will be improved. It is necessary to
keep the pressure at atmospheric pressure. This sorbent may be
more suitable for capture of carbon dioxide in traditional
combustion-based systems.
■
AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-25-83793453. Fax: +86-25-83793453. E-mail:
[email protected].
■
ACKNOWLEDGMENTS
Financial support from the National High Technology
Research and Development Program of China
(2009AA05Z311), the National Natural Science Foundation
(50876021), the National Basic Research Program of China
(2011CB707301), and the Scientific Research Foundation of
Graduate School of Southeast University (YBJJ1001) is
sincerely acknowledged.
■
Figure 5. Effect of the pressure on (a) carbonation conversion and (b)
reaction rate.
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