Carbonation Condition and Modeling Studies of Calcium-Based

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Carbonation Condition and Modeling Studies of Calcium-Based Sorbent in the Fixed-Bed Reactor Shengping Wang,* Shasha Fan, Yujun Zhao, Lijing Fan, Shuyao Liu, and Xinbin Ma Key Laboratory for Green Chemical Technology, Department of Chemical Technology, School of Chemical Engineering and Technology, Tianjin University; Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China ABSTRACT: Carbonation process of calcium-based sorbent, as a promising technique for CO2 capture, was examined in a fixed-bed reactor. A method was established to calculate the exact adsorption capacity using CO2 Non-Dispersive Infrared Radiation (NDIR) analyzer as the analytical unit. The factors affecting the carbonation process of calcium-based sorbent were investigated thoroughly. The conversion and adsorption capacity of sorbent reached 65% and 41%, respectively, when CO2 partial pressure was 0.01 MPa at 600 °C, exhibiting an obvious advantage in terms of a relatively low carbonation temperature. The results showed a first-order reaction with respect to CO2 partial pressure. Increasing carbonation pressure was an effective method to improve adsorption capacity, and conversion and adsorption capacity of sorbent achieved 85% and 53% at 1.3 MPa, respectively. The existence of water was beneficial to the carbonation process and the influence can be explained by the process of water hydration of CaO. The Random Pore Model (RPM) was applied to describe the carbonation process in the reactioncontrolled stage and the predicted data obtained from the model showed good accordance with the experimental results. shrinking core model,28 random pore model (RPM),4,29 and iron reaction mechanism.30 However, most research have been carried out through TG instead of the fixed-bed reactor. The random pore model which correlates the carbonation behavior with the internal pore structure by three structural parameters was applied to the carbonation process through TG analysis by Grasa29 and Zhou.4 However, it is still unknown whether this model fits well with the carbonation process occurred in the fixed-bed reactor. In this paper, we will investigate the carbonation process of the calcium-based sorbent and obtain the most optimal operating conditions in the fixed-bed reactor. A method would be established to calculate the exact carbonation conversion and adsorption capacity of the sorbent by using the CO2 non-dispersive infrared radiation (NDIR) analyzer (XLZ-1090). Furthermore, how the factors affect the carbonation conversion and adsorption capacity will also be investigated. The random pore model was used to describe the carbonation behavior of the calcium-based sorbent and build the relationship between adsorption capacity and time in the fixed-bed reactor.

1. INTRODUCTION Carbon dioxide (CO2) is widely recognized as the major anthropogenic contributor to the greenhouse effect and climate change, and the CO2 emissions will correspondingly increase with improvement in the consumption of fossil fuels.1−3 Thus, CO2 capture techniques are receiving increasing attention over the past decade.4 Among multiple techniques, the method using calcium-based sorbents via the carbonation/calcination reaction(CCR) for CO2 capture, which was proposed by Silaban and Harrison,5 has attracted much attention due to its low cost, wide availability, and high efficiency.6−8 It is well-known that the use of calcium-based sorbents, including natural material such as limestone and dolomite,9,10 and synthetic material11,12 for transferring CO2, has been studied extensively. However, there still exist two main problems. One is the decay in capacity after multiple carbonation/calcination cycles. Many ways have been tried to reactivate the calcium-based sorbents, such as thermal pretreatment,10 hydration with stream,13,14 dealing with ethanol solutions,15,16 synthesizing the precipitated calcium carbonate,17,18 incorporating into inert support materials,19−21 and use of nanomaterial.22−24 The other problem is that the studies of the carbonation process of calcium-based sorbents in the fixed-bed reactor are not so comprehensive and extensive until now. On one hand, few investigations of the factors affecting the carbonation process have been carried out in a fixed-bed reactor. Li et al.25 ascertained the effect of carbonation and calcination temperature on the carbonation conversion using the modified calcium oxide sorbent in a twin fixed-bed reactor. Pacciani et al.26 only investigated the influence of CO2 concentration on the carbonation process. So, it is still necessary to study the influence of different factors on the carbonation process in the fixed-bed reactor thoroughly. On the other hand, different types of models have been reported to describe the carbonation process, such as the apparent model,27 © 2014 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Preparation of Sorbent. Sorbent precursors were prepared by the coprecipitation method reported previously.31,32 Solution A was prepared by dissolving Ca(NO3)2· 4H2O (Tianjin Kelmel, 99.9%) and Al(NO3)3·9H2O (Tianjin Kelmel, 99.0%) at room temperature, with desirable Ca2+/Al3+ molar ratios (Ca2+/Al3+=8). Solution B was prepared by dissolving NaOH (Tianjin Kelmel, 96.0%) and Na2CO3 Received: Revised: Accepted: Published: 10457

February 24, 2014 May 10, 2014 June 4, 2014 June 4, 2014 dx.doi.org/10.1021/ie500789g | Ind. Eng. Chem. Res. 2014, 53, 10457−10464

Industrial & Engineering Chemistry Research

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(Tianjin Kelmel, 99.8%) in deionized water. Then, solution B was quickly added to solution A with vigorous stirring. The resultant slurry was filtered and washed with deionized water, followed by drying at 80 °C in an oven to obtain raw sorbent powders. 2.2. Characterization. Crystalline structure of samples were performed by X-ray diffraction (XRD) on a Rigaku D/ max-2500 diffractometer operating in a 2θ range of 5−90° at a scanning rate of 0.02°/s. Scanning electron microscope (SEM) was used to attain surface morphology of the calcium-based sorbents employing a Hitachi S4800 field-emission microscope at 10.0 kV. Textual properties of the sorbent were determined by the mercury intrusion porosimetry (MIP) test using a Micromeritics Autopore IV. The MIP test is mainly used to characterize the macroporous structure of the material. The pore size distribution can be obtained according to the different amount of mercury pressed into the pore of the material at different pressure, ranging from 3.378 KPa to 206 850 KPa. 2.3. Apparatus and Procedure. The cyclic carbonation/ calcination reaction for calcium-based sorbent was experimentally studied in a fixed-bed reactor made of quartz with an inner diameter of 8 mm. The experimental system for the fixed-bed reactor consists of a mixing chamber, a fixed-bed reactor with temperature control, and a CO2 analysis unit. CO2 in the outlet was monitored online by a NDIR analyzer, the precision of which was within 2%. Water was introduced into the reaction zone by bubbling N2 through a glass saturator filled with deionized water at a certain temperature when the influence of water content on the carbonation process was applied. Different factors (particle diameter, partial pressure of CO2, carbonation temperature, carbonation pressure, and water partial pressure) affecting the carbonation process were inspected in a series of carbonation tests. The raw sorbent powder was first compressed to particles with different mesh number (10−80) through a tablet press. Subsequently, the sorbent particles were pretreated in flowing N2 at 700 °C for 40 min and then the pretreated sorbent (about 1g) was put into the fixed-bed reactor. The carbonation temperature varied from 400−600 °C in an atmosphere of CO2 and N2 and calcination was carried out at 700 °C in pure N2. The exact flow rate was set by mass controllers. The carbonation time and calcination time were 30 and 50 min, respectively. A relatively exact adsorption capacity was obtained by using the CO2 NDIR analyzer. In the calculating process, N2 was chosen as the internal reference and the flow rate of CO2 was obtained according to the CO2 concentration monitored by the analyzer. Then, the adsorption capacity can be obtained according to the following equation: Capacity (%) =

(∫ VCO2ref dt − ∫ VCO2adsdt )44 (60)(1000)(22.4)m

× 100

×

The conversion of CaO can be obtained through the following equation: Conversion = Capacity ×

(3)

where MCaO and MCO2 represent molar mass of CaO and CO2 respectively, and a is the mass content of CaO in the sorbent. The sorbent after pretreatment is composed of CaO and Ca12Al14O33, as proved by X-ray diffraction (Figure 1a). So, the mass content of CaO in the sorbent is calculated to be about 79.7 wt %.

Figure 1. XRD patterns of the sorbents (a, fresh; b, after one cycle; c, after ten cycles).

■

RESULTS AND DISCUSSION 3.1. Effects of Particle Size. To investigate the influence of particle size of the calcium-based sorbent on the carbonation reaction rate, conversion and adsorption capacity, four particle size fractions (mesh numbers 10−20, 20−40, 40−60, 60−80) were tested in the apparatus. Experiment results were plotted in Figure 2. It can be seen that the differences in the slopes of the curves were inconspicuous, indicating that the particle size had little influence on the carbonation rate. The effect of particle size on the conversion and adsorption capacity was not obvious either, which was consistent with the results of others.33 In general, both the carbonation rate and adsorption capacity showed a little superiority when the mesh number was 20−40. So, we choose this particle size for further investigation. 3.2. Effects of CO2 Partial Pressure. The driving force for CO2 adsorption is the difference between the CO2 partial pressure and the corresponding equilibrium partial pressure in the carbonation process.34 It is noted that the equilibrium partial pressure of CO2 is about 0.0001 MPa at 600 °C according to the thermodynamic equilibrium correlation of Baker.35 The range of CO2 partial pressure investigated in this study is from 0.005 to 0.012 MPa, which is far above the equilibrium pressure. Thus, the carbonation reaction is not restricted by thermodynamic equilibrium and the driving force can be considered equal to the CO2 partial pressure existing in the system. The influence of the CO2 partial pressure on the carbonation process at 600 °C is shown in Figure 3. It can be seen that the CO2 partial pressure had an important effect on

273 273 + T (1)

where VCO2ref is the flow rate of CO2 in the blank test; VCO2ads is the flow rate of CO2 in the carbonation process; m is the mass of the calcium-based sorbent put into the reactor; T is the room temperature. The CO2 adsorption capacity was expressed in the following form: Adsorption capacity (%) mass of the adsorbed CO2 = × 100 mass of the sorbent

MCaO 1 × MCO2 a

(2) 10458

dx.doi.org/10.1021/ie500789g | Ind. Eng. Chem. Res. 2014, 53, 10457−10464


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Figure 2. Capacity and conversion of the sorbents with different particle size.

Figure 3. Capacity and conversion of the sorbents at different CO2 partial pressure.

respect to CO2 in Figure 4. As can be seen in Figure 4, the linearity manifested was quite good, showing an apparent firstorder reaction, in accordance with the results of Mess et al. obtained at the chemically controlled stage.36,37 3.3. Effects of Carbonation Temperature. The carbonation reaction was carried out in a range of temperature from 400 to 600 °C, close to the operation conditions in the methanation process. The carbonation curves at different temperatures are shown in Figure 5. The figure illustrated that the carbonation temperature had a stronger effect on the conversion and adsorption capacity compared with CO2 partial pressure. With the increasing temperature from 400 to 600 °C the conversion and adsorption capacity increased sharply from 19% to 65% and 11.6% to 41%, respectively. As reported by Pacciani et al.,26 the adsorption capacity was about 20% when the CO2 molar fraction is 0.14 using the synthetic calcium oxide sorbent at 750 °C, far above 600 °C. Meanwhile, this high carbonation temperature would be accompanied by a higher calcination temperature, meaning a great deal of energy consumption. This CO2 adsorption capacity of 41% offered a great opportunity for the industrial application of calcium-based sorbent at 600 °C. As for the carbonation rates, there were some improvements in the temperature range of 400 and 550 °C, but then, the rate remained almost the same with further increasing temperatures. Some researchers38 considered the inflection point as the demarcation point between the reaction controlled stage and the diffusion controlled stage. It is noted that the inflection points of the carbonation curves showed obvious distinction, as marked in Figure 5. Though the difference of carbonation rates at these temperatures was inconspicuous, the time needed to achieve the demarcation points of the curves gradually became longer with increasing

Figure 4. Mean carbonation rate vs CO2 partial pressure.

the conversion and adsorption capacity. When the partial pressure increased from 0.005 to 0.01 MPa, the conversion and adsorption capacity increased obviously. However, the adsorption performance showed little change with further improvement of CO2 partial pressure. Hence 0.01 MPa was chosen as the optimal CO2 partial pressure during the carbonation process. Apart from adsorption capacity, the results also indicated that CO2 partial pressure as a kinetic factor may affect the carbonation rate. It is obvious that the slopes of the curves were strongly affected by the CO2 partial pressure in the reaction controlled period. The mean carbonation rate of this period was plotted against the CO2 partial pressure in order to investigate the reaction order with 10459



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Figure 5. Capacity and conversion of the sorbents at different carbonation temperatures.

Figure 6. Capacity and conversion of the sorbents at different carbonation pressure.

Figure 7. Capacity and conversion of the sorbents at different water partial pressure.

Figure 8. Morphology of the sorbents after one cycle at different water partial pressure (a) without water, (b) 0.0013 MPa, (c) 0.0017 MPa.

temperature, indicating a gradually longer period of time that the carbonation process continued at the reaction controlled stage. It is well-known that the adsorption capacity obtained at the reaction controlled stage dominated the whole capacity, so the time that the reaction maintained at this stage was also an important factor to the ultimate capacity compared with the reaction rate. This was the reason that the capacity obtained at 600 °C is higher than that at 550 °C though the reaction rates were almost the same. 3.4. Effects of Carbonation Pressure. The effect of carbonation pressure on the CO2 capture was investigated by ranging the pressure from 0 to 1.5 MPa. As shown in Figure 6,

it can be seen that the total pressure played a positive role in the carbonation process. The conversion and adsorption capacity improved greatly from 66% to 85% and 41% to 53% by increasing the pressure from 0 to 1.3 MPa, respectively, because high pressure is favorable for improving the reaction rate. However, the further increased reaction rate could lead to a quick formation of the product layer, which hindered the contact between CO2 and CaO. So, the capacity remained almost the same when the pressure was increased further. Therefore, 1.3 MPa was an appropriate pressure for the carbonation process. 10460



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carbonation process, the surface was smooth, firm, and looked whole without obvious small particles. When water partial pressure reached 0.0013 MPa, distinct changes could be seen from the surface of the sorbent. The surface was rough, loose, and porous, and consisted of countless small particles. With further increase of the partial pressure, the small particles caused by the hydration process reduced and the surface became slightly compact again. This changing process can be explained by the influence of the water hydration of CaO.39 Once solid CaO contacts with water, the chemical conversion from CaO to Ca(OH)2 occurs instantly. The chemical reaction induces solid volume expansion from CaO (16.7 cm3/mol) to Ca(OH)2 (33.5 cm3/mol) and a consequent increase in internal stress. In the previous studies,40 it has been reported that Ca(OH)2 has extremely low tensile strength and weak crack resistance. Hence, the formed Ca(OH)2 product layer cannot sustain the increased internal stress and disintegrates into small fragments, resulting in the formation of loose and porous structure. This change was beneficial to the reaction between CaO and CO2, so the conversion and adsorption capacity improved compared with the occasion of no water. After this reaction process between CaO with H2O, the formed Ca(OH)2 further interacts with water physically,39,41 resulting in the stronger agglomeration of small particles, as shown in the SEM images. This physical interaction process finally led to a decrease in adsorption capacity. Therefore, the good effect of water on the carbonation process was probably due to the formation of Ca(OH)2 as an intermediate by the hydration process. 3.6. Stability of the Sorbent. The stability of the calciumbased sorbent was investigated in the presence and absence of water at the most optimal operating conditions, respectively, and the results are shown in Figure 9. From Figure 9, it can be seen that the adsorption capacity and conversion declined obviously with the increasing cycle numbers when water was not introduced into the system. The conversion and adsorption capacity decreased from 67% to 27% and 41% to 18% after ten cycles, respectively. However, the existence of water improved the stability of the sorbent greatly. The conversion and adsorption capacity could still maintain 54% and 34% after ten cycles, respectively. As shown in Figure 1, the crystalline structures of the sorbents with the different cycle number were characterized by XRD, and the crystalline size of CaO is listed in Table 1. XRD patterns in Figure 1 displayed the existence of both CaO and Ca12Al14O33 in the three sorbents and no other crystalline phases were detected. From Table 1, it can be seen that the crystallite size increased obviously with the increasing cycle number, which resulted from the sintering of the sorbent. Further, connecting the adsorption capacities and conversions of the sorbents with the different carbonation/calcination cycle, it could be concluded that the increase of CaO crystallite size would lead to the decay of the conversion and adsorption capacity of sorbent. 3.7. Model. The Random Pore Model developed by Bhatia and Perlmutter42,43 was applied to the carbonation system. This model is a typical gas−solid reaction model and developed in terms of the pore size distribution of the reacting solid. The pore structure was considered as a network of randomly interconnected pores in the RPM model, according to what the particle structural parameters were defined. The RPM model considered the solid phase as the continuous phase and correlated the adsorption behavior with the pore structure.4

Figure 9. Stability of the sorbent (Square: without water. Circle: water partial pressure 0.0013 MPa. Adsorption at 600 °C, desorption at 700 °C, CO2 partial pressure 0.01 MPa).

Table 1. Crystallite Size of the Sorbents sorbent

crystallite size (nm)

a (fresh) b (after one cycle) c (after 10 cycles)

37 42 86

Figure 10. Pore size distribution of the sorbents.

Table 2. Structural Parameters S0 (m2/m3)

L0 (m/m3)

ε

Ψ

1.219 × 10

6.222 × 1013

0.499

2.633

7

3.5. Effects of Water Partial Pressure. Sorbent reactivation by steam hydration has been proposed as an appropriate method to improve the adsorption capacity in TG measurements.13 However, few studies were investigated on the influence of water partial pressure on the carbonation performance of sorbents in the fixed-bed reactor. Figure 7 displayed the effect of water partial pressure on the adsorption performance in the fixed-bed reactor at 600 °C. It was noted that the conversion and adsorption capacity increased first, then decreased with the increase of water partial pressure and achieved the highest conversion and capacity when the partial pressure was 0.0013 MPa. Figure 8 showed the morphology of the sorbents after one carbonation cycle at different water partial pressures. When there was no water existing in the 10461



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Figure 11. Comparison of experimental capacity curves and those predicted by the RPM model under different carbonation temperatures. Symbols: predicted results. Lines: experimental data.

Table 3. Values of K′ and t0

4πL0(1 − ε)

Ψ=

T (°C)

400

500

550

600

K′ t0 (min)

0.0195 1.8

0.0345 3.2

0.0705 4

0.0715 5

(5)

where S0 and L0 represent the initial surface area and total pore length per unit of volume in the porous system, respectively, and ε is the initial porosity. S0, L0, and ε can be obtained from the results of mercury intrusion porosimetry, as follows:29

This model does not have special requirements for the morphology or particle size of the material. The aim of applying this model is to obtain a specific expression to describe the adsorption capacity against time in the fixed-bed reactor and further predict the adsorption capacity achieved in the reaction-controlled stage through the expression. A general expression for the instantaneous solid− gas reaction rate applicable to porous systems in the presence of product layer diffusion resistance according to the model was listed as follows: ksS0C(1 − x) 1 − ψ ln(1 − x) dx = βZ ⎡ ⎤ dt (1 − ε)⎣1 + ψ ( 1 − ψ ln(1 − x) − 1)⎦

S0 2

S0 = 2

L0 =

ε=

∫0

∫0

∫0

∞

∞

v0(r ) dr r

v0(r ) πr 2

dr

(6)

(7)

∞

v0(r )dr

(8)

As shown in Figure 10, the sorbent presented a bimodal pore size distribution, which was obtained by mercury intrusion porosimetry. The structural parameters of S0, L0, and ε can be acquired through the pore size distribution and the results are listed in Table 2. It is well-known that the carbonation period can be divided into two parts, the reaction-controlled stage and the diffusion-controlled stage. It was commonly believed that the adsorption capacity was mainly obtained in the reactioncontrolled stage.38 So, we focused on this stage and its expression can be derived from eq 3 as follows:

(4)

The parameter C is the CO2 concentration in the carbonation process. The parameter β and Z represent the modified Biot modulus and the molar volume ratio of carbonation product to reactant, respectively. The expression correlates the adsorption capacity, x, with the structural parameter Ψ, which accounts for the textural property of the sorbent in terms of 10462


Industrial & Engineering Chemistry Research k S (C − Ce)t 1 [ 1 − Ψ ln(1 − x) − 1] = RPM 0 b Ψ 2(1 − ε)

f (Ψ) = K′ =

1 [ 1 − Ψ ln(1 − x) − 1] Ψ

kRPMS0(C b − Ce) 2(1 − ε)

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The RPM model has been applied to describe the carbonation behaviors of the sorbent in the reaction-controlled stage. The results showed that the predicted data obtained from the RPM was in good accordance with the experimental data. We can also give a rough value of the adsorption capacity acquired in the reaction-controlled stage under some certain carbonation conditions using this model.

(9)

(10)

■

(11)

The experimental data showed that the relationship between f(Ψ) and t is linear and through the linear relationship the theoretical data can be predicted. As shown in Figure 11, the predicted data was in good agreement with the experimental results over the range of temperatures tested, indicating that the RPM was suitable for the carbonation process in the fixed-bed reactor. According to the structure parameters mentioned above, when CO2 partial pressure was 0.01 MPa, eq 9 can be expressed as follows: ⎡ 1 − (2.633K ′t + 1)2 ⎤ 0 ⎥ x = 1 − exp⎢ 2.633 ⎦ ⎣

Corresponding Author

*Fax: +86-22-87401818. Email: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support by Natural Science Foundation of China (NSFC) (Grant No. 21176179), the Program for New Century Excellent Talents in University (NCET-13-0411), and the Program of Introducing Talents of Discipline to Universities (B06006) is gratefully acknowledged.

■

(12)

where t0 represents the time maintained in the reaction controlled stage. The values of K′ and t0 at different carbonation temperatures are listed in Table 3. From Table 3, it can be seen that the values of K′ and t0 improved with increasing temperature. However, the value of K′ remained almost the same when the temperature reached 550 °C. Thus, temperature was used to estimate K′ and t0, as shown empirically in eqs 13 and 14: K ′ = − 0.11 + 3.128 × 10−4T

(400 °C ≤ T ≤ 550 °C)

(400 °C ≤ T ≤ 600 °C)

REFERENCES

(1) Kierzkowska, A. M.; Pacciani, R.; Müller, C. R. CaO-based CO2 sorbents: From fundamentals to the development of new, highly effective materials. ChemSusChem 2013, 6 (7), 1130−1148. (2) Chen, H.; Zhao, C.; Li, Y.; Chen, X. CO2 capture performance of calcium-based sorbents in a pressurized carbonation/calcination loop. Energy Fuels 2010, 24 (10), 5751−5756. (3) Wang, S.; Yan, S.; Ma, X.; Gong, J. Recent advances in capture of carbon dioxide using alkali-metal-based oxides. Energy Environ. Sci. 2011, 4 (10), 3805−3819. (4) Zhou, Z.; Xu, P.; Xie, M.; Cheng, Z.; Yuan, W. Modeling of the carbonation kinetics of a synthetic CaO-based sorbent. Chem. Eng. Sci. 2013, 95 (0), 283−290. (5) Silaban, A.; Harrison, D. P. High temperature capture of carbon dioxide: Characteristics of the reversible reaction between CaO(s) and CO2(g). Chem. Eng. Com 1995, 137 (1), 177−190. (6) Anthony, E. J. Ca looping technology: Current status, developments, and future directions. Greenhouse Gases: Sci. Technol. 2011, 1 (1), 36−47. (7) Blamey, J.; Anthony, E. J.; Wang, J.; Fennell, P. S. The calcium looping cycle for large-scale CO2 capture. Prog. Energy Combust. Sci. 2010, 36 (2), 260−279. (8) Yu, F.-C.; Phalak, N.; Sun, Z.; Fan, L.-S. Activation strategies for calcium-based sorbents for CO2 capture: A perspective. Ind. Eng. Chem. Res. 2011, 51 (4), 2133−2142. (9) Sun, P.; Lim, C. J.; Grace, J. R. Cyclic CO2 capture by limestonederived sorbent during prolonged calcination/carbonation cycling. AIChE J. 2008, 54 (6), 1668−1677. (10) Chen, Z.; Song, H. S.; Portillo, M.; Lim, C. J.; Grace, J. R.; Anthony, E. J. Long-term calcination/carbonation cycling and thermal pretreatment for CO2 capture by limestone and dolomite. Energy Fuels 2009, 23 (3), 1437−1444. (11) Kierzkowska, A. M.; Poulikakos, L. V.; Broda, M.; Müller, C. R. Synthesis of calcium-based, Al2O3-stabilized sorbents for CO2 capture using a co-precipitation technique. Int. J. Greenhouse Gas Control 2013, 15 (0), 48−54. (12) Broda, M.; Müller, C. R. Synthesis of highly efficient, Ca-Based, Al2O3-stabilized, carbon gel-templated CO2 sorbents. Adv. Mater. 2012, 24 (22), 3059−3064. (13) Manovic, V.; Anthony, E. J. Steam reactivation of spent CaObased sorbent for multiple CO2 capture cycles. Environ. Sci. Technol. 2007, 41 (4), 1420−1425. (14) Martínez, I.; Grasa, G.; Murillo, R.; Arias, B.; Abanades, J. Evaluation of CO2 carrying capacity of reactivated CaO by hydration. Energy Fuels 2011, 25 (3), 1294−1301.

(13)

t0 = −4.583 + 0.0158T

AUTHOR INFORMATION

(14)

Correspondingly, adsorption capacity achieved in the reactioncontrolled stage at the temperature range from 400 to 600 °C can be predicted through the expressions above.

4. CONCLUSIONS A calcium-based sorbent was prepared by coprecipitation method, and it exhibited good conversion and adsorption capacity in the fixed-bed reactor with the obvious advantage of requiring a relatively low carbonation temperature. The analytical unit is a CO2 NDIR analyzer, which is different from other methods, thus establishing a mature method to obtain the exact adsorption capacity using this equipment. The factors affecting the carbonation process including particle size, CO2 partial pressure, carbonation temperature, carbonation pressure, and water partial pressure were generally investigated and the optimum conditions were confirmed during the process. With the increasing of carbonation temperatures and CO2 partial pressure the conversion and adsorption capacity improved and reached 65% and 41%, respectively, when the partial pressure was 0.01 MPa at 600 °C and the results showed a first-order reaction with respect to CO2 partial pressure. The carbonation pressure was also an important factor to the process and the conversion and adsorption capacity achieved 85% and 53% at 1.3 MPa, respectively. When water was introduced into the system, the conversion and adsorption capacity improved further. The most appropriate water partial pressure was 0.0013 MPa because of the formation of a loose and porous structure originating from water hydration of CaO. 10463



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Carbonation Condition and Modeling Studies of Calcium-Based

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