Carbonation Behavior and the Reaction Kinetic of a New Dry

Article pubs.acs.org/IECR

Carbonation Behavior and the Reaction Kinetic of a New Dry Potassium-Based Sorbent for CO2 Capture Chuanwen Zhao, Xiaoping Chen,* and Changsui Zhao Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of Energy & Environment, Southeast University, Nanjing 210096, China S Supporting Information *

ABSTRACT: The carbonation behaviors of K2CO3 generated by calcination of KHCO3 were investigated with a pressurized thermo gravimetric apparatus, and the shrinking-core model in the noncatalytic heterogeneous reaction systems was used to explain the kinetics of the reaction between K2CO3, CO2, and H2O using analysis of the experimental breakthrough data. The carbonation reaction process can be divided into two stage-controlled regions, one is the surface chemical reaction-controlled region at the initial stage and another is the internal diffusion-controlled region at the last stage. The total amount of carbonation conversion is mainly dependent on the first stage. The reaction rate of this stage decreases as the reaction temperature increases. It increases in the same temperature when the CO2 and H2O concentrations increase. The total carbonation conversion decreases as the pressure increases. On the basis of the Arrhenius equation, the apparent activation energy and pre-exponential factor for these two stages are calculated, when the temperature is in the range of 55−80 °C and the pressure is 0.1 MPa. They are 33.4 kJ/mol and 3.56 cm/min for the surface chemical reaction-controlled region and 99.1 kJ/mol and 4.01 × 10−22 cm2/min for the internal diffusion-controlled region. This paper provides theoretical basis for the further study on the capture of CO2 from flue gas using dry potassium-based sorbents.

1. INTRODUCTION Global warming has attracted significant attention in recent years. CO2 capture from fossil-fuel fired power plants is of critical importance.1 The various CO2 capture options include precombustion decarbonization,2 O2 combustion with CO2 recycle,3 chemical looping combustion,4 and postcombustion capture.5 Each option is currently being explored, but to date an effective capturing process which is inexpensive and has a low energy demand does not exist. Recently, the approach of using dry alkali metal-based sorbent for CO2 capture in low temperature flue gas has received increasing attention as an innovative concept which might meet the needs for a costeffective and energy efficient option.6−23 For this technology, 1 mol of alkali metal carbonate can potentially absorb 1 mol of CO2 at low temperature by the reaction:

transport reactors. It is found that K2CO3/AC and K2CO3/ Al2O3 have the potential to be employed as the excellent sorbents for CO2 capture due to their high CO2 capture capacities and rapid carbonation reaction rates.8−10 The multicycle behavior of these sorbent is also particularly good for CO2 capture capacity and attrition resistance. The process development includes effects of operation conditions such as reaction temperature, gas composition, operation pressure and gaseous impurities on CO2 capture behavior,11 continuous operation of the CO2 capture process,12 and economic evaluation of this process.13 It is found that this process is significantly lower in cost and more energy efficient than conventional MEA technology. There are still many problems that need to be solved. First, the global carbonation reaction rates for both pure Na2CO3 and pure K2CO3 are rather slow. The carbonation conversion rate of sodium-based sorbent reached 65% in 100 min,7 and of pure K2CO3 reached 45% in 2 h.14 It is necessary to research a proper pure alkali metal carbonate used as the active component of the sorbents. In order to couple the regeneration reactor with the carbonation reactor in continuous operation of the CO2 capture process, it is important to know the reaction rates of these two processes well. However, the studies on the carbonation and regeneration reaction kinetics of the supported alkali metal-based sorbents are currently very limited. The kinetic of CO2 sorption of pure K2CO3 or pure Na2CO3 under moist conditions is studied in a fixed-bed reactor.15,16 The shrinking-core model, the homogeneous model, and the

60 − 80 ° C

M 2CO3(s) + CO2 (g) + H 2O(g) XoooooooooooY 2MHCO3(s) 120 − 200 ° C

(M = Na, K)

This process can be readily used for retrofitting existing facilities and easily integrated with new power generation facilities. It is ideally suited for coal-fired power plants incorporating wet flue gas desulfurization, due to the associated cooling and saturation of the flue gas. It is expected to be both cost-effective and energy-efficient. Current research is so far focused on two main areas: sorbent development and process development. In the sorbent development area, pure sodium carbonate and potassium carbonate were tested as the active component of the sorbents. Supported sorbents have also been examined in order to provide needed attrition resistance in the fluidized bed or © 2012 American Chemical Society

Received: June 14, 2012 Accepted: October 19, 2012 Published: October 19, 2012 14361

dx.doi.org/10.1021/ie302497r | Ind. Eng. Chem. Res. 2012, 51, 14361−14366

Industrial & Engineering Chemistry Research

Article

As the temperature is constant in the reactor, the heat transfer in this system is neglected. The external diffusion is eliminated in TGA by a series test of changing gas flow. Analyzed with the TGA test results in a previous study,19 it is found that the diffusion rate of H2O in the particle of K2CO3-1 and the reaction rate of H2O with the solid particle are much lower than that of CO2. The unreacted shrinking reaction core model for the carbonation kinetic of K2CO3-1 can be developed based on the diffusion and reaction process of H2O with the solid particle. The CO2 concentration is in large excess for the samples in TGA, so we assume that it is kept constant in the whole process. On the basis of this shrinking-core model, the concentration of K2CO3 at the reaction interface is kept the same as the initial concentration of K2CO3. It is assumed that K2CO3 reacts with CO2 and H2O to form KHCO3 by the following eq 1:

deactivation model are all explored to explain the kinetics, and these values are compared with the experimental data. Research Triangle Institute (RTI) derives kinetic rate constants for the carbonation reaction of Na2CO3 and compares these values with thermogravimetric analysis (TGA) results from Louisiana State University (LSU).17 The primary kinetic parameters are obtained, and RTI develops a shrinking-core type model to simulate the carbonization of Na2CO3 in a roughly conical TGA cup. All the investigations are aimed at the whole bed layer. As the relatively large concentration of carbon dioxide in utility flue gas suggests fluidized reactors are an appropriate contacting scheme for sorbent capture, the reaction kinetics of a single particle are more useful. As a result, a detailed investigation on reaction kinetic behaviors of alkali-metal based sorbents is required. K2CO3 generated by calcination of KHCO3 (defined as K2CO3-1), was found to have excellent carbonation capacity.14 The problem of low carbonation reaction rate can be solved by using this sorbent. The carbonation reaction mechanism of this sorbent was studied by being compared with pure K2CO3 and K2CO3 dehydrated from K2CO3·1.5H2O,18 and the CO2 capture behaviors were investigated through an orthogonal test method.19 (Orthogonal experiment design was used for selecting the optimum condition of experimental factors and nine groups of experiment plans with three levels and four factors, L9 (34) were adopted.) Focused on the carbonation reaction rate of this sorbent, the carbonation reaction kinetics characteristics should be investigated. The objective of this paper is to obtain the carbonation reaction kinetic of the particle of K2CO3-1. The reaction mechanism is investigated more clearly by comparing the theoretical model with the experimental data.

K 2CO3(s) + CO2 (g) + H 2O(g) → 2KHCO3(s)

(1)

For this model, the internal diffusion rate of H2O is expressed as −

dn H 2 O dt

⎛ dC H 2 O ⎞ = 4πrC 2De⎜ ⎟ ⎝ dr ⎠ r = r

C

(2)

The rate of chemical reaction, which occurs at the interface of the solid, is given as follows: −

dn H 2 O dt

0 CC = 4πrC 2k SC K0 2CO3CCO 2 H 2O

(3)

H2O cannot be accumulated in the product layer, because only the diffusion process occurs, while no chemical reaction occurs in the layer of KHCO3. As a result, the relationship between inflow and outflow quantity of the diffusion process is expressed as

2. THEORETICAL MODELING AND ANALYSIS As reported,18 the total surface area and pore volume for K2CO3-1 are less than 5 m2/g and 0.02 cm3/g, respectively, and 90% of the pores are less than 20 nm. The internal diffusion is the main influencing factor for the carbonation of the particle because the microstructure of the particle of K2CO3-1 is poor. The chemical reaction rate is deduced to be much higher than the diffusion rate. As a result, the unreacted shrinking reaction core model is chosen for the carbonation process of K2CO3-1. On the basis of this model,24−28 the particle of K2CO3-1 is defined as a spherical particle, K2CO3 reacts with CO2 and H2O to form KHCO3, and the carbonation reaction process is shown in Figure 1.

⎛ dC H 2 O ⎞ 4πrC 2De⎜ ⎟ ⎝ dr ⎠ r = r

C

⎛ dC H 2 O ⎞ = 4πrC 2De⎜ ⎟ ⎝ d t ⎠ r = r − dr (4)

= constant

The relationship between the chemical reaction rate of H2O and K2CO3 is expressed as −

dn H 2 O dt

=−

dnK 2CO3 (5)

dt

In this model, the chemical reaction rate of K2CO3 is expressed as −

dnK 2CO3 dt

= 4πrC 2C K0 2CO3

drC dt

(6)

where nH2O (mol) and CH2O (mol/cm3) are the amount and the concentration of H2O, respectively. CHC 2O (mol/cm3) is the concentration for H2O at the reaction interface. C0CO2 (mol/ cm3) is the initial concentrations of CO2. rc (cm) is the distance of the reaction interface with the particle center, and t (min) is the reaction time. nK2CO3 (mol) is the amount of K2CO3, and C0K2CO3 (mol/cm3) is the initial concentration of K2CO3. kS is

Figure 1. Idealized reaction process of the particle of K2CO3 generated by calcination of KHCO3. 14362

dx.doi.org/10.1021/ie302497r | Ind. Eng. Chem. Res. 2012, 51, 14361−14366

Industrial & Engineering Chemistry Research

Article

K2CO3 calcined from KHCO3 are 3.89 m2/g and 13.4 × 10−3 cm3/g, respectively. 3.2. Apparatus and Procedure. The carbonation tests of K2CO3-1 were processed with a TherMax 500 TGA in the gas composition of CO2, H2O, and a balance of N2. CO2 and N2 were obtained from high-purity cylinders with mass flow controllers used to control flow. H2O was fed with a piston pump and was heated to ensure complete vaporization before mixing with a stream of either CO2/N2 or N2. To eliminate the effect of external diffusion on carbonation, the gas flow rate was chosen as 500 mL min−1. Consulting refs 7, 11, and 19, the carbonation temperature was chosen between 55 and 80 °C. The CO2 concentration was chosen to approximate the flue gas composition between 5% and 20%. On the basis of the saturated vapor pressure in the carbonation temperature, the H2O concentration was chosen between 0 and 21%. The operation pressure was chosen between 0.1 and 0.5 MPa. On the basis of the theoretical value increment corresponding to the complete conversion of K2CO3 to KHCO3, the carbonation conversion η for K2CO3-1 is calculated from eq 18.

the reaction rate constant per unit reaction interface, and De is the diffusion coefficient for H2O in the product layer. The initial condition is r = R,

C H2O = C H0 2O

(7)

r = rC ,

C H2O = C HC2O

(8)

CH0 2O (mol/cm3) is the initial concentrations of H2O, and R (cm) is the radius of the particle. The mathematical expression of the reaction time is calculated as follows: t=

0 2 rC ⎞ C K 2CO3R ⎛ ⎛ ⎛ rC ⎞2 ⎜1 − ⎟ + ⎜ ⎟ ⎜ 1 3 + ⎜ 0 ⎝R⎠ R ⎠ 6DeC H0 2O ⎝ k SCCO C0 ⎝ 2 H 2O

R

⎛ ⎛ r ⎞ 3 ⎞⎞ − 2⎜1 − ⎜ C ⎟ ⎟⎟⎟ ⎝ R ⎠ ⎠⎠ ⎝

(9)

The carbonation conversion (η) (the weight change of K2CO3) is defined as ⎛ η = ⎜⎜1 − ⎝

4 ⎞ πr 3C 0 3 C K 2CO3 ⎟ 4 πR3C K0 2CO3 ⎟⎠ 3

⎛ ⎛ r ⎞3 ⎞ × 100% = ⎜1 − ⎜ C ⎟ ⎟ × 100% ⎝R⎠ ⎠ ⎝

η=

The relationship between the carbonation conversion (η) and the reaction time is expressed as R 0 k SCCO C0 2 H 2O

(1 − (1 − η)1/3 ) +

C K0 2CO3R2 (11)

On the basis of the different controlled regions for the noncatalytic heterogeneous reaction systems, eq 11 can be simplified as t = A1g(η) (in surface chemical reaction‐controlled region)

t = A 2 P(η)

(12)

(in internal diffusion‐controlled region) (13)

where g(η) = 1 − (1 − η)1/3

(14)

P(η) = 1 − 3(1 − η)2/3 + 2(1 − η)

(15)

A1 =

A2 =

R 0 k SCCO C0 2 H 2O

(16)

C K0 2CO3R2 6DeC H0 2O

× 100% (18)

4. RESULTS AND DISCUSSION 4.1. Typical Test Results Analysis. A typical carbonation test was carried out, and the carbonation conversion (η) changes with reaction time are shown in Figure 2a, which is calculated from eq 18. The experimental data is processed with eqs 12 and 13, and the results are shown in parts b and c of Figure 2, respectively. As shown in Figure 2a, the carbonation conversion (η) increases to 80.4% in 20 min with a high reaction rate and then increases about 3.0% in the next 20 min. Processed with eqs 12 and 13, the results show that the process is a surface chemical reaction-controlled region in the first 20 min, and it is an internal diffusion-controlled region after this time. 1/A1 and 1/ A2 are 2.45 and 0.17 × 10−2 min−1, respectively. As 1/A1 is much higher than 1/A2, the total amount of carbonation conversion is mainly dependent on the first stage. In order to investigate the carbonation reaction kinetics deeply, the tests were carried out at different conditions and the results were analyzed as follows. 4.2. Effect of Temperature. Keeping the pressure at 0.1 MPa, the carbonation reactions were carried out at different temperatures in the same gas composition of 15% CO2, 15% H2O, and balanced N2. The results are shown in Figure 3. Figure 3 shows that η increases with reaction time and reaches the plateau in 30 min for all conditions. η is about 80% in 30 min in the temperature of 55−70 °C. Above 75 °C reaction temperature, η is only 42.3% in 20 min. This observation is consistent with the results for Na2CO3 generated by calcination of NaHCO3 and a potassium-based sorbent sorbKX35.7,11 Because the carbonation reaction is reversible and highly exothermic, the low carbonation conversion at a higher temperature is attributed to the backward reaction of eq 1.

6DeC H0 2O

(1 − 3(1 − η)2/3 + 2(1 − η))

w(0)(2MKHCO3 − M K 2CO3)

where t is the reaction time, w(t) is the weight of sorbent at time t, w(0) is the sorbent weight at the beginning of carbonation, and MK2CO3 and MKHCO3 are the molecular weight of K2CO3 and KHCO3, respectively.

(10)

t=

M K 2CO3(w(t ) − w(0))

(17)

3. EXPERIMENT AND MATERIALS 3.1. Sample Preparation. K2CO3 used in this study was obtained from calcined of KHCO3. Analytical reagent KHCO3 were provided by Shanghai Fine Chemical Co., Ltd. KHCO3 were 99.5% pure, the average particle sizes were chosen as 20 μm, and R is 10−3 cm. The surface area and pore volume for 14363

dx.doi.org/10.1021/ie302497r | Ind. Eng. Chem. Res. 2012, 51, 14361−14366

Industrial & Engineering Chemistry Research

Article

decreases from 8.06 × 105 to 3.41 × 105 with the temperature increasing from 55 to 80 °C, while De decreases from 19.3 × 10−7 to 1.75 × 10−7. This result shows that the carbonation process of K2CO3-1 is consistent with the Arrhenius formula. It is necessary for this reaction be carried out at a low temperature. 4.3. Effect of CO2 Concentration. Figure 4 shows the effect of CO2 concentration on the carbonation reactions in the same conditions of 65 °C and 15% H2O at 0.1 MPa.

Figure 4. Effect of CO2 concentration on carbonation conversion for K2CO3 calcined from KHCO3.

As shown in Figure 4, η is about 75% in 25 min for all the conditions. The effect of CO2 concentration on carbonation of K2CO3-1 is not significant when it is in the range of 5−20%. This test result confirms the assumption that CO2 concentration at the reaction interface is kept constant in the whole process. The calculated results are listed in Table S2 in the Supporting Information. Although 1/A1 increases as the CO2 concentration increases, the time decreases for the surface chemical reaction-controlled region. With the integration of the effect of reaction rate and the reaction time, the carbonation conversion is not changed when the CO2 concentration increases from 5% to 20%. De is kept constant around 1.50 × 10−6, while kS decreases from 8.83 × 105 to 3.53 × 105. It is deduced that KHCO3 is produced at a high reaction rate surrounding the exterior of the sorbent particles at the beginning as the reaction rate increases. The diffusion process of H2O is affected by accumulating of the product at the exterior of the sorbent, so the H2O concentration at the reaction interface decreases. Because kSis obtained as the average value of the whole reaction process, it is affected by the above process. It leads the time to be decreased for the chemical reaction-controlled region. 4.4. Effect of H2O Concentration. The effect of H2O concentration on the carbonation reactions in the same condition of 65 °C and 15% CO2 at 0.1 MPa is shown in Figure 5. There was no carbonation reaction occurring for K2CO3-1 without H2O provided, implying the significance of H2O in the carbonation process. η is only 24.5% in the first 21.6 min when the H2O concentration is 3% and the total conversion is only 68.2% in 43 min. η is only 14.8% in 12.2 min when the H2O concentration increases to 6%, and the total conversion is 81.7% in 30 min. η increases from 68.2% to 83.4% as the H2O concentration increases from 3% to 21% in 50 min. The curves of η are different when the H2O concentration is lower than 15%, and they are similar when the H2O concentration is higher than 15%, while η reaches more than 80% in 20 min. This test result confirms the theoretical basis of the diffusion

Figure 2. Typical carbonation test result: (a) carbonation conversion, (b) processed with eq 12, and (c) processed with eq 13.

Figure 3. Effect of temperature on carbonation conversion for K2CO3 calcined from KHCO3.

The experimental datas are processed with eqs 12 and 13, then kS and De are calculated from eqs 16 and 17. On the basis of the density of K2CO3 (2.428 g/cm3), C0K2CO3 is calculated to be 1.76 × 10−2 mol/cm3. C0CO2 and CH0 2O are both 6.7 × 10−6 mol/cm3, respectively. The results are listed in Table S1 in the Supporting Information. As shown in Table S1 in the Supporting Information, 1/A1 and 1/A2 decrease as the temperature increases from 55 to 80 °C. 1/A1 is only 1.5 × 10−2 min−1, and the time is only 12 min for the surface chemical reaction-controlled region when the temperature is higher than 75 °C. This is the reason that η is only 42.3% for the test results. All these results are attributed to that kS and De decrease with temperature increasing. kS 14364

dx.doi.org/10.1021/ie302497r | Ind. Eng. Chem. Res. 2012, 51, 14361−14366

Industrial & Engineering Chemistry Research

Article

Figure 5. Effect of H2O concentration on carbonation conversion of K2CO3 calcined from KHCO3.

Figure 6. Effect of pressure on carbonation conversion for K2CO3 calcined from KHCO3.

and reaction process of H2O with the solid particle for this unreacted shrinking reaction core model. The calculated results are listed in Table S3 in the Supporting Information. As shown in Table S3 in the Supporting Information, 1/A1 increases from 0.33 × 10−2 to 2.45 × 10−2 min−1 as the H2O concentration increases from 3% to 21%. When the H2O concentration is in the range of 3−6%, kS and De are kept constant around 3.7 × 105 and 2.4 × 10−5, respectively, and the carbonation conversion is low in the chemical reactioncontrolled region and the total conversion mainly depends on the diffusion-controlled region. When the H2O concentration is in the range of 9−21%, kS increases to 5.2 × 105 while De decreases to 5.01 × 10−7. 1/A1 is much higher than 1/A2, and the total conversion mainly depends on the chemical reactioncontrolled region. Our previous paper14 reported that K4H2(CO3)3·1.5H2O is produced at first for K2CO3-1 and then reacts with CO2 to form KHCO3 in the carbonation conditions. As shown in Table S2 in the Supporting Information, the effect of CO2 concentration on the reaction rate is insignificant. It implied that the reaction is fast for K4H2(CO3)3·1.5H2O with CO2. On the contrary, the effect of H2O concentration on the reaction rate is significant. The production process of K4H2(CO3)3·1.5H2O is considered as the rate-determining step. When the H2O concentration is low, the production of K4H2(CO3)3·1.5H2O is low, so the reaction rate for the rate-determining step is low. After the amount of K4H2(CO3)3·1.5H2O is produced, it can react with CO2 in a high rate. When the H2O concentration is high enough, this effect on the carbonation reaction disappears. That is the reason that the curves of η are different for different H2O concentrations. 4.4. Effect of Operation Pressure. The carbonations of K2CO3-1 were carried out at various pressures in the same reaction conditions of 55 °C, 15% CO2, 15% H2O, and N2 balanced. Results are shown in Figure 6. Figure 6 shows that η of K2CO3-1 decreases from 80.4% to 40.9% in 45 min as the pressure increases from 0.1 to 0.5 MPa. The calculated results are listed in Table S4 in the Supporting Information. As shown in Table S4 in the Supporting Information, the total amount of carbonation conversion is mainly dependent on the chemical reaction-controlled region, when the pressure increases from 0.1 to 0.3 MPa. 1/A1 decreases as the pressure increases, while1/A2 remains constant. The time decreases to 5 min for the chemical reaction-controlled region, while it increases to 40 min for the diffusion-controlled region when the pressure is higher than 0.3 MPa. Although 1/A1 increases as the pressure increases to 0.5 MPa, 1/A2 decreases to a low value. As a result, the total carbonation conversion decreases as the

pressure increases. The main reason is deduced that the diffusion coefficient of water vapor in the sorbent decreases as the pressure increases. 4.5. Chemical Kinetic Parameters Calculation. The values of kS and De with different temperature are shown in Table S1 in the Supporting Information. The Arrhenius plots are shown in Figure 7.

Figure 7. Arrhenius plot of the carbonation reaction for K2CO3 calcined from KHCO3 (a) for the reaction rate constant and (b) for the diffusion coefficient.

Using the values of the slopes and intercepts of these straight lines, the apparent activation energy and pre-exponential factor for these two stages are calculated, when the temperature is in the range of 55−80 °C and the pressure is 0.1 MPa. They are 33.4 kJ mol−1 and 3.56 cm/min for the surface chemical reaction-controlled region and 98.98 kJ mol−1 and 2.07 × 10−22 cm2/min for the internal diffusion-controlled region.

5. CONCLUSIONS The carbonation reaction process can be divided into two stage-controlled regions, one is a surface chemical reactioncontrolled region at the initial stage and another is an internal diffusion-controlled region at the last stage. The total amount 14365

dx.doi.org/10.1021/ie302497r | Ind. Eng. Chem. Res. 2012, 51, 14361−14366

Industrial & Engineering Chemistry Research

Article

(10) Zhao, C. W.; Chen, X. P.; Zhao, C. S. Carbonation and activecomponent-distribution behaviors of several potassium-based sorbents. Ind. Eng. Chem. Res. 2011, 50 (8), 4464−4470. (11) Seo, Y. W.; Jo, S. H.; Ryu, C. K.; Yi, C. K. Effects of water vapor pretreatment time and reaction temperature on CO2 capture characteristics of a sodium-based solid sorbent in a bubbling fluidized-bed reactor. Chemosphere 2007, 69, 712−718. (12) Yi, C. K.; Jo, S. H.; Seo, Y.; Lee, J. B.; Ryu, C. K. Continuous operation of the potassium-based dry sorbent CO2 capture process with two fluidized-bed reactors. Int. J. Greenhouse Gas Control. 2007, 1, 31−36. (13) Shigemoto, N.; Yanagihara, T.; Sugiyama, S.; Hayashi, H. Material balance and energy consumption for CO2 recovery from moist flue gas employing K2CO3-on-activated carbon and its evaluation for practical adaptation. Energy Fuels 2006, 20, 721−726. (14) Zhao, C.; Chen, X.; Zhao, C.; Liu, Y. Carbonation and hydration characteristics of dry potassium-based Sorbents for CO2 capture. Energy Fuels 2009, 23, 1766−1769. (15) Park, S. W.; Sung, D. H.; Choi, B. S.; Lee, J. W.; Kumazawa, H. Carbonation kinetics of potassium carbonate by carbon dioxide. J. Ind. Eng. Chem. 2006, 12, 522−530. (16) Park, S. W.; Sung, D. H.; Choi, B. S.; Oh, K. J.; Moon, K. H. Sorption of carbon dioxide onto sodium carbonate. Sep. Sci. Technol. 2006, 41, 2665−2684. (17) Green, D. A.; Turk, B. S.; Gupta, R. P.; McMichael, W. J.; Harrison, D. P.; Liang, Y. Carbon dioxide capture from flue gas using dry regenerable sorbents. Quarterly Technical Progress Report; Research Triangle Institute: Research Triangle Park, NC, April 2002. (18) Zhao, C.; Chen, X.; Zhao, C. Carbonation behavior of K2CO3 with different microstructure used as an active component of dry sorbents for CO2 capture. Ind. Eng. Chem. Res. 2010, 49, 12212− 12216. (19) Zhao, C.; Chen, X.; Zhao, C. Study on CO2 Capture Using dry potassium-based sorbents through orthogonal test method. Int. J. Greenhouse Gas Control 2010, 4, 655−658. (20) Xiao, G. K.; Singh, R.; Chaffee, A.; Webley, P. Advanced adsorbents based on MgO and K2CO3 for capture of CO2 at elevated temperatures. Int. J. Greenhouse Gas Control 2011, 5 (4), 634−639. (21) Li, L.; Li, Y.; Wen, X.; Wang, F.; Zhao, N.; Xiao, F. K.; Wei, W.; Sun, Y. H. CO2 capture over K2CO3/MgO/Al2O3 dry sorbent in a fluidized bed. Energy Fuels 2011, 25 (8), 3835−3842. (22) Zhang, B. T.; Fan, M. H.; Bland, A. E. CO2 separation by a new solid K-Fe sorbent. Energy Fuels 2011, 25, 1919−1925. (23) Okunev, A. G.; Sharonov, V. E.; Aristov, Y. I.; Parmon, V. N. Sorption of carbon dioxide from wet gases by K2CO3-in-porous matrix: Influence of the matrix nature. React. Kinet. Catal. Lett. 2000, 71 (2), 355−362. (24) Sohn, H. Y.; Szekely, J. A structural model for gas-solid reactions with a moving boundary-III: A general dimensionless representation of the irreversible reaction between a porous solid and a reactant gas. Chem. Eng. Sci. 1972, 27, 763−778. (25) Sohn, H. Y.; Szekely, J. A structural model for gas-solid reactions with a moving boundary-IV: Langmuir-Hinshelwood kinetics. Chem. Eng. Sci. 1973, 28, 1169−1177. (26) Szekely, J.; Evans, J. W. A structural model for gas-solid reactions with a moving boundary. Chem. Eng. Sci. 1970, 25, 1091− 1107. (27) Szekely, J.; Evans, J. W. A structural model for gas-solid reactions with a moving boundary-II: The effect of grain size, porosity and temperature on the reaction of porous pellets. Chem. Eng. Sci. 1971, 26, 1901−1913. (28) Weisz, P. B.; Goodwin, R. D. Combustion of carbonaceous deposits within porous catalyst particles I: Diffusion-controlled kinetics. J. Catal. 1963, 2, 397−404.

of carbonation conversion is mainly dependent on the chemical reaction-controlled region. The reaction rate at this reactioncontrolled region decreases as the reaction temperature increases. It increases in the same temperature when the CO2 and H2O concentrations increase. The total carbonation conversion decreases as the pressure increases. On the basis of the Arrhenius equation, the apparent activation energy and pre-exponential factor for these two stages are calculated, when the temperature is in the range of 55−80 °C and the pressure is 0.1 MPa. They are 33.4 kJ mol−1 and 3.56 cm/min for the surface chemical reaction-controlled region and 98.98 kJ mol−1 and 2.07 × 10−22 cm2/min for the internal diffusion-controlled region.

■

ASSOCIATED CONTENT

* Supporting Information S

Four tables. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +86 25 83793453. Fax: +86 25 83793453. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation (Grant No. 50876021), the National High Technology Research and Development Program of China (Grant No. 2009AA05Z311), the Foundation of Graduate Creative Program of Jiangsu Province (Grant No. CX08B_141Z), and the Scientific Research Foundation of Graduate School of Southeast University (Grant YBJJ1001) are sincerely acknowledged.

■

REFERENCES

(1) Wall, T. F. Combustion processes for carbon capture. Proc. Combust. Inst. 2007, 31 (1), 31−47. (2) Brown, T. D. Coal gasificationcombined cycles for electricity production. Prog. Energy Combust. Sci. 1982, 8 (4), 277−301. (3) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Oxy-fuel combustion technology for coal-fired power generation. Prog. Energy Combust. Sci. 2005, 31 (4), 283−307. (4) Hossain, M. M.; de Lasa, H. I. Chemical-looping combustion (CLC) for inherent CO2 separations- A review. Chem. Eng. Sci. 2008, 63, 4433−4451. (5) Oexmann, J.; Hensel, C.; Kather, A. Post-combustion CO2 capture from coal-fired power plants: Preliminary evaluation of an integrated chemical absorption process with piperazine-promoted potassium carbonate. Int. J. Greenhouse Gas Control. 2008, 2, 539−552. (6) Hoffman, J. S.; Pennline, H. W. Study of regenerable sorbents for CO2 capture. J. Energy Environ. Res. 2001, 1, 90−100. (7) Liang, Y.; Harrison, D. P.; Gupta, R. P.; Green, D. A.; McMichael, W. J. Carbon dioxide capture using dry sodium-based sorbents. Energy Fuels 2004, 18, 569−575. (8) Lee, S. C.; Choi, B. Y.; Lee, T. J.; Ryu, C. K.; Ahn, Y. S.; Kim, J. C. CO2 absorption and regeneration of alkali metal-based solid sorbents. Catal. Today 2006, 111, 385−390. (9) Hayashi, H.; Taniuchi, J.; Furuyashiki, N.; Sugiyama, S.; Hirano, S.; Shigemoto, N.; Nonaka, T. Efficient recovery of carbon dioxide from flue gases of coal-fired power plants by cyclic fixed-bed operations over K2CO3-on-carbon. Ind. Eng. Chem. Res. 1998, 37, 185−191. 14366

dx.doi.org/10.1021/ie302497r | Ind. Eng. Chem. Res. 2012, 51, 14361−14366