CO2 capture performance of calcium-based sorbent doped with manganese salts during calcium looping cycle

Applied Energy 89 (2012) 368–373

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CO2 capture performance of calcium-based sorbent doped with manganese salts during calcium looping cycle Rongyue Sun, Yingjie Li ⇑, Hongling Liu, Shuimu Wu, Chunmei Lu ⇑ School of Energy and Power Engineering, Shandong University, Jinan 250061, China

a r t i c l e

i n f o

Article history: Received 13 March 2011 Received in revised form 26 July 2011 Accepted 28 July 2011 Available online 3 September 2011 Keywords: Calcium-based sorbent Manganese salts Additive Calcination/carbonation CO2 capture

a b s t r a c t The effects of manganese salts including Mn(NO3)2 and MnCO3 on CO2 capture performance of calciumbased sorbent during cyclic calcination/carbonation reactions were investigated. Mn(NO3)2 and MnCO3 were added by wet impregnation method. The cyclic CO2 capture capacities of Mn(NO3)2-doped CaCO3, MnCO3-doped CaCO3 and original CaCO3 were studied in a twin fixed-bed reactor and a thermo-gravimetric analyzer (TGA), respectively. The results show that the addition of manganese salts improves the cyclic carbonation conversions of CaCO3 except the previous cycles. When the Mn/Ca molar ratios are 1/100 for Mn(NO3)2-doped CaCO3 and 1.5/100 for MnCO3-doped CaCO3, the highest carbonation conversions are achieved respectively. The carbonation temperature of 700–720 °C is beneficial to CO2 capture of Mn-doped CaCO3. The residual carbonation conversions of Mn(NO3)2-doped and MnCO3-doped CaCO3 are 0.27 and 0.24 respectively after 100 cycles, compared with the conversion of 0.16 for original one after the same number of cycles. Compared with calcined original CaCO3, better pore structure is kept for calcined Mn-doped CaCO3 during calcium looping cycle. The pore volume of calcined MnCO3-doped CaCO3 is 2.4 times as high as that of calcined original CaCO3 after 20 cycles. The pores of calcined MnCO3-doped CaCO3 in the pore size range of 27–142 nm are more abundant relative to clacined original one. That is why modification by manganese salts can improve cyclic CO2 capture capacity of CaCO3. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction

X N ¼ fmN ð1  fw Þ þ fw

Pollutants from coal burning have caused serious environmental problems, and many technologies have been proposed to control the pollutants emissions [1–4]. Global warming, mainly due to CO2 emissions from combustion of fossil fuels, has been a public concern. CO2 capture and storage is believed to be viable to achieve deep cuts in CO2 emissions. Various available options such as O2/ CO2 combustion, membranes, chemical-looping combustion and calcium looping cycle, have been seriously investigated [5–8]. Calcium looping cycle, proposed by Shimizu et al. [9], is considered as one of the most potential methods to control CO2 emissions by using very cheap and widely available sorbents such as limestone and dolomite. Previous studies showed that calcination/carbonation reactions of calcium-based sorbents were far from reversible in practice [10,11]. The maximum carbonation conversions of the sorbents decrease dramatically with increasing calcination/carbonation cycles. Abanades and Alvarez [12] developed a semi-empirical correlation between carbonation conversion and cycle number by analyzing data on carbonation conversions of CaO during multiple cycles:

where fm = 0.77 and fw = 0.17 are fit for many different series of limestones. Sintering of CaO during calcination at high temperature is believed to be the major cause of the deactivation [11,13]. Recently, there have been a number of investigations that focused on modifications of natural calcium-based sorbents to enhance the CO2 capture capacity, attempting to reduce the cost for the separation of CO2 from flue gas. Manovic and Anthony [14] claimed that after steam reactivation, the spent calcium-based sorbent had better characteristics for CO2 capture than the natural one. Li et al. [15] found that natural calcium-based sorbents modified with acetic acid exhibited better reversibility than the original sorbents during multiple cycles. Manovic and Anthony [16] reported that calcium-based sorbents showed higher carbonation conversions over a longer series of cycles than original sorbents after thermal pretreatment at high temperature. However, the modifications of natural sorbents result in an increase in the cost. Pretreatment of natural sorbent with a small quantity of additive is one of the simple and attractive methods, as a means of delaying the degradation of carbonation conversions. Some researchers [17,18] believed that the use of sodium salts to enhance CO2 capture capacity produced improvements to some degree in fixed bed. It was also proved by Aihara et al. [19] that the addition of calcium titanate could prevent sintering of reactive par-

⇑ Corresponding authors. Tel.: +86 531 88392414 (Y. Li), +86 531 88392264 (C. Lu). E-mail addresses: [email protected] (Y. Li), [email protected] (C. Lu). 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.07.051

ð1Þ

R. Sun et al. / Applied Energy 89 (2012) 368–373

369

Nomenclature Ar fm fw MCaO MCO2 m0

content of CaO in initial sample when Mn/Ca molar ratio is r,% model parameter model parameter molar mass of CaO, g/mol molar mass of CO2, g/mol initial mass of sample, g

ticles and stabilize the reversibility of the cyclic reactions. Roesch et al. [20] investigated the carbonation behavior of cesium-doped sorbents and found that the calcium-based sorbent doped with 20 wt.% cesium reached the highest carbonation conversion, but the cesium is so expensive that it is not suitable as the additive. Li et al. [21] demonstrated that KMnO4 could enhance the CO2 capture performance of CaCO3 during multiple cycles. KMnO4 decomposed into potassium oxide and manganese oxides during calcination at high temperature, and potassium oxide was proved to be beneficial to CO2 capture of CaCO3 [18]. However, the effect of manganese oxides on CO2 capture capacity of CaCO3 is not that explicit. Manganese salts are abundantly distributed in the natural environment. The modification of CaCO3 with a small amount of manganese salts will not cause the cost of the sorbent to increase a lot. If a small amount of addition of manganese salts can improve the CO2 capture capacity, it will be an effective and economical method for the modification of calcium-based sorbents. This paper looks at pretreatment of CaCO3 by manganese salts as a means of delaying the deactivation of the sorbent over multiple cycles. The effects of Mn(NO3)2 and MnCO3, as the precursors of manganese oxides, on CO2 capture performance of CaCO3 were investigated. 2. Experimental work 2.1. Sorbent preparation Manganese salts doped CaCO3, i.e., Mn-doped CaCO3, were synthesized by wet impregnation method using analytically pure CaCO3 (>99.0%). The manganese salts were analytically pure Mn(NO3)2 (50% manganese nitrate water solution) and MnCO3 (>99.0%). All the sorbents were prepared by mixing the manganese salts with CaCO3. The molar ratio of Mn/Ca ranged from 0% to 4%. 10 g CaCO3 and appropriate amount of manganese salts were mixed in 200 mL distilled water. The beaker containing the slurry was put into a constant temperature bath to keep the modification temperature constant at 60 °C and stirred for 3 h. Then the sample was dried in an oven at 120 °C, the particle size of the sorbent was below 0.125 mm. 2.2. Cyclic calcination/carbonation The cyclic calcination/carbonation reactions were studied in a twin fixed-bed reactor system, including a carbonation reactor and a calcination reactor, as shown in Fig. 1. The dimensions of the furnaces for the carbonation reactor and the calcination reactor are both ¢60  400 mm. The length of the constant temperature zone of the furnaces for two reactors is 150 mm. The size of the sample boat is 60  30  15 mm. The sample boat containing 500 mg sorbents can be shifted between the two reactors to complete the multiple cycles. The effects of different variables (molar ratio of Mn/Ca, carbonation temperature, calcination temperature) on the carbonation process were studied in multiple calcination/ carbonation cycles. The sample was calcined at 850–1000 °C in

mCal mN N XN

mass of sample after calcination, g mass of sample after Nth carbonation, g number of cycles carbonation conversion after N cycles

pure N2 and carbonated at 650–750 °C in a 15vol.-%CO2/85vol.%N2 gas mixture at atmospheric pressure. The total gas flow rates for carbonation and calcination are both 1 L/min which are controlled by rotameters. CO2 at a flow rate of 0.15 L/min and N2 at a flow rate of 0.85 L/min are mixed enough before entering the carbonation reactor. The calcination time and carbonation time were 10 min and 20 min, respectively, confirmed by preliminary experiments. The sample mass was weighed by an electronic balance (Shimadzu, AW120) after calcination and carbonation. The carbonation conversion was calculated according to the mass change as follows:

XN ¼

mN  mcal M CaO  m0 Ar M CO2

ð2Þ

A thermo-gravimetric analyzer (Mettler–Toledo TGA/SDTA 851e) was used to investigate the carbonation kinetics during the cycles. The samples used for TGA experiment were selected from the carbonation reactor after 9 and 99 cycles. Then, the carbonation experiments of the three sorbents with the reaction time after 1, 10 and 100 cycles were done in TGA. The reacting gas mixture in TGA was the same as that in the twin fixed-bed reactor. The sample weight used in this experiment was 10 ± 0.1 mg. After the reaction temperature increased to 850 °C at a heating rate of 30 °C/min, the sample was calcined for 15 min in pure N2. Then, the temperature decreased to 700 °C at 30 °C/min. The gas mixture was switched to carbonation atmosphere, and the carbonation time was 30 min. The total flow rate was 0.12 L/min during the experiment. 2.3. Microstructure analysis To help understand how the modification using manganese salts affect the CO2 capture capacity of CaCO3, samples after 10th and 100th calcination were collected for a scanning electron microscope (SEM, JEOL JSM-7600F) analysis. The surface areas and pore size distributions of the calcined MnCO3-doped CaCO3 and the calcined original CaCO3 after 20 cycles were also analyzed by Micromeritics ASAP 2020 Accelerated Surface Area and Porosimetry analyzer (N2 adsorption method). The surface area and pore size distribution were calculated by BET method and BJH model, respectively. 3. Results and discussion 3.1. Effect of Mn/Ca molar ratio on cyclic CO2 capture capacity The cyclic carbonation conversions of CaCO3 doped with different molar ratios of Mn/Ca are plotted in Figs. 2 and 3. After the first cycle, the carbonation conversions of Mn(NO3)2-doped CaCO3 are lower than original one. However, Mn(NO3)2-doped CaCO3 exhibits a slight decay in carbonation conversion and ultimately reflects better CO2 capture capacity than original one after 2 or 3 cycles.

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valve Computer Rotameter

Temperature controller

Electronic balance

Sample boat CO2

N2 Carbonator reactor

Calcination reactor

Fig. 1. Diagram of twin fixed-bed calcination/carbonation reactor system at atmospheric pressure.

versions during cyclic reactions decrease if continuing to increase the addition quantity of manganese salts over the optimum composition. Mn(NO3)2-doped CaCO3 achieves the highest cyclic carbonation conversions when the Mn/Ca molar ratio is 1/100. Similarly, the optimum molar ratio of Mn/Ca for MnCO3-doped CaCO3 is 1.5/100.

0.8 Original Mn/Ca=0.5/100 Mn/Ca=1.0/100 Mn/Ca=2.0/100 Mn/Ca=4.0/100

0.7

XN

0.6 0.5

3.2. Effect of reaction temperature on cyclic CO2 capture capacity

0.4

Grasa et al. [22] reported that the calcium-based sorbent could achieve higher carbonation conversions when the carbonation temperature was between 650 °C and 720 °C. The effect of carbonation temperature on the cyclic carbonation conversions of Mndoped CaCO3 was investigated. Figs. 4 and 5 indicate that the effects of carbonation temperature on carbonation conversions of Mn(NO3)2-doped CaCO3 and MnCO3-doped CaCO3 are similar. Mn-doped sorbent presents the maximum cyclic carbonation conversion carbonated between 700 °C and 720 °C. When carbonated at other temperatures, Mn-doped CaCO3 reflects faster decay in carbonation conversion. The carbonation conversion of Mn(NO3)2-doped CaCO3 carbonated at 700 °C after 10 cycles is 0.42, which is 31.3% higher than that carbonated at 650 °C. The carbonation conversion of MnCO3-doped CaCO3 for carbonation at 700 °C after 10 cycles is 0.38, which is 35.7% greater than that carbonated at 650 °C. Higher calcination temperature is beneficial to the decomposition of CaCO3 [23]. However, too high calcination temperature

0.3 0.2

0

4

8

12

16

20

N Fig. 2. Cyclic carbonation conversions of Mn(NO3)2-doped CaCO3 at different Mn/Ca molar ratios for calcination at 850 °C and carbonation at 700 °C.

0.8 Original Mn/Ca=0.5/100 Mn/Ca=1.0/100 Mn/Ca=1.5/100 Mn/Ca=2.0/100 Mn/Ca=3.0/100

0.7

XN

0.6 0.5

0.8 o

650 C o 680 C o 700 C o 720 C o 750 C

0.4 0.7

0.3 0.2

0

4

8

12

16

20

N Fig. 3. Cyclic carbonation conversions of MnCO3-doped CaCO3 at different Mn/Ca molar ratios for calcination at 850 °C and carbonation at 700 °C.

XN

0.6 0.5 0.4 0.3 The performance of MnCO3-doped CaCO3 is similar to that of Mn(NO3)2-doped CaCO3. The addition of manganese salts to CaCO3 has an optimum content. At the beginning, the sorbent achieves higher carbonation conversion with increasing the addition quantity of manganese salts. However, more addition quantity does not always mean higher carbonation conversion. The carbonation con-

0.2

0

2

4

6

8

10

N Fig. 4. Effect of carbonation temperature on carbonation conversion of Mn(NO3)2doped CaCO3 with Mn/Ca molar ratio of 1/100 for calcination at 850 °C.

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0.8 o

650 C o 680 C o 700 C o 720 C o 750 C

0.7

XN

0.6 0.5 0.4 0.3 0.2

0

2

4

6

8

10

N

(a) Carbonation conversion

Fig. 5. Effect of carbonation temperature on carbonation conversion of MnCO3doped CaCO3 with Mn/Ca molar ratio of 1.5/100 for calcination at 850 °C.

Orginal, 1 cycle Mn(NO3)2-doped, 1 cycle MnCO3 -doped, 1 cycle Orginal, 10 cycles Mn(NO3)2-doped, 10 cycles MnCO3-doped, 10 cycle Orginal, 100 cycles Mn(NO3)2-doped, 100 cycles MnCO3 -doped, 100 cycle

0.005

0.8

o

carbonation rate/ 1/s

Orginal, 850 C o MnCO3-doped, 850 C o Mn(NO3)2-doped, 850 C o Orginal, 900 C o MnCO3-doped, 900 C o Mn(NO3)2-doped, 900 C o Orginal, 1000 C

0.6

XN

0.006

0.4

0.004 0.003 0.002 0.001

o

MnCO3-doped, 1000 C o Mn(NO3)2-doped, 1000 C

0.2

0

3

6

9

12

15

18

21

N Fig. 6. Effect of calcination temperature on carbonation conversions of original and Mn-doped CaCO3 with Mn/Ca molar ratio of 1/100 and 1.5/100 for Mn(NO3)2-doped and MnCO3-doped CaCO3 for carbonation at 700 °C.

0

200

400

600

800

1000

1200

carbonation time/ s

(b) Carbonation rate Fig. 8. Cyclic carbonation kinetics of Mn-doped and original CaCO3 for calcination at 850 °C and carbonation at 700 °C.

0.4

conversions of original and Mn-doped CaCO3. All the sorbents show sharper decrease in carbonation conversion with number of cycles at higher calcination temperature. The carbonation conversions of original CaCO3, Mn(NO3)2-doped CaCO3 and MnCO3-doped CaCO3 reduce by 30.1%, 23.8%, and 22.1% for calcination at 900 °C after 10 cycles, respectively, compared with that calcined at 850 °C after the same number of cycles. MnCO3-doped CaCO3 and Mn(NO3)2-doped CaCO3 present almost the same CO2 capture capacity as the original CaCO3 for calcination at 1000 °C. Too high calcination temperature is unfavorable for CO2 capture of Mndoped CaCO3.

0.3

3.3. Long-term CO2 capture capacity of Mn-doped CaCO3

0.2

Fig. 7 shows the carbonation conversions of original and Mndoped CaCO3 for calcination at 850 °C and carbonation at 700 °C during 100 cycles. The Mn/Ca molar ratio is 1/100 and 1.5/100 for Mn(NO3)2-doped and MnCO3-doped CaCO3, respectively. The decay of the carbonation conversions occurs mostly in the previous 30 cycles. The carbonation conversions decay in a very slow speed from the 40th cycle to the 100th cycle. For original CaCO3, the carbonation conversion after 100 cycles is 0.16, which is accordant with Eq. (1). The Mn-doped CaCO3 reflects a greater long-term CO2 capture capacity. The carbonation conversion of Mn(NO3)2doped CaCO3 is 0.27 after 100 cycles, which is 69% higher than that

0.8 Orginal MnCO3-doped with Mn/Ca ratio of 1.5/100 Mn(NO3)2-doped with Mn/Ca ratio of 1/100 Eq. (1)

0.7 0.6 0.5

XN

0.000

0.1

0

20

40

60

80

100

N Fig. 7. Comparison of carbonation conversions between Mn-doped and original CaCO3 during long-term cycles for calcination at 850 °C and carbonation at 700 °C.

leads to aggravation of sintering, and that is responsible for the decay in carbonation conversions with the number of cycles [24]. Fig. 6 exhibits the effect of calcination temperature on carbonation

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(a) Calcined original CaCO3

(b) Calcined Mn(NO3)2-doped CaCO3

(c) Calcined MnCO3-doped CaCO3

Fig. 9. SEM images for calcined sorbents after 10 cycles for calcination at 850 °C and carbonation at 700 °C.

(a) Calcined original CaCO3

(b) Calcined Mn(NO3)2-doped CaCO3

(c) Calcined MnCO3-doped CaCO3

Fig. 10. SEM images for calcined sorbents after 100 cycles for calcination at 850 °C and carbonation at 700 °C.

of the original one. The conversion is 0.24 for MnCO3-doped CaCO3. The CO2 capture capacities of Mn(NO3)2-doped CaCO3 and MnCO3doped CaCO3 are almost the same during long-term cycles. 3.4. Carbonation kinetics analysis Fig. 8a and b show the cyclic carbonation conversions and carbonation rates of original CaCO3 and Mn-doped CaCO3 with carbonation time, respectively. The carbonation rate integrates the effect of the chemical reaction rate and the CO2 diffusion. CO2 diffusion through CaO grains increases the quantity of sorbent which can react with CO2 at the beginning of the reaction, and the carbonation rate increases. The carbonation rate reaches a maximum when a balance between chemical reaction and CO2 diffusion is achieved. Then, the resistance of CO2 diffusion increases due to the formation of CaCO3 product layer out CaO core, and the carbonation rate decreases quickly. It can be observed that Mn-doped CaCO3 exhibits a little slower carbonation rate than original one after 1 cycle, and the 30-min carbonation conversion is also lower. However, with cycle number increasing, Mn-doped CaCO3 achieves higher carbonation rates and 30-min carbonation conversions after 10 and 100 cycles. 3.5. Microstructure analysis The pore structure of the calcined sorbents during cyclic reactions has significant effect on the carbonation conversion [25,26]. The SEM observations on calcined original CaCO3 and calcined Mn-doped CaCO3 with optimum additions of manganese salts after 10 cycles and 100 cycles are shown in Figs. 9 and 10, respectively. It can be observed that Mn-doped CaCO3 appears porous, and the

pores are interconnected with each other after 10 cycles, as shown in Fig. 9b and c. For calcined original CaCO3, the particles are a little denser, and the pores are mutually uncorrelated with each other as seen in Fig. 9a. As a result, the pores have more possibility to be blocked compared with Mn-doped CaCO3 during carbonation period, and CO2 diffusion inside the particles is restricted. Fig. 10 illustrates that the blockage and shrinking of the pores become serious after 100 cycles. No visibly clear pores could be seen in the surface of calcined original CaCO3, as shown in Fig. 10a. It can be observed from Fig. 10b and c that much better pore structure is kept after 100 cycles for Mn-doped CaCO3. BET surface area, pore volume and average pore diameter of calcined MnCO3-doped CaCO3 and calcined original CaCO3 after 20 cycles for calcination at 850 °C and carbonation at 700 °C are presented in Table 1. BET surface area of calcined MnCO3-doped CaCO3 is almost equal to that of calcined original CaCO3 after 20 cycles. However, it can be seen that the pore volume of calcined MnCO3-doped CaCO3 is 2.4 times of that of calcined original CaCO3 after 20 cycles. The modification by manganese salts remarkably increases the pore volume of the sorbent compared with original CaCO3. The average pore diameter of calcined MnCO3-doped CaCO3 is 114% higher than calcined original one. Table 1 BET surface area, pore volume and average pore diameter of calcined MnCO3-doped CaCO3 and calcined original CaCO3 after 20 cycles. Sample

Surface area (m2/g)

Pore volume (cm3/g)

Average pore diameter (nm)

MnCO3-doped CaCO3 Original CaCO3

5.04

0.0400

31.7

4.80

0.0166

14.8

R. Sun et al. / Applied Energy 89 (2012) 368–373

142 nm. The pore structure of calcined Mn-doped CaCO3 is beneficial to the diffusion of CO2 to the surface of the sorbent, and that is why Mn-doped CaCO3 reflects higher carbonation conversions than original one.

MnCO3-doped Original

3

pore volume distribution cm /(g.nm)

0.0018 0.0015

373

0.0012

Acknowledgments

0.0009

This research was supported by National Natural Science Foundation of China (51006064), Independent Innovation Foundation of Shandong University (2009GN042U), and Graduate Innovation Foundation of Shandong University (yyx10040).

0.0006 0.0003 0.0000

References

1

10

100

pore size/ nm Fig. 11. Pore size distributions of MnCO3-doped and original CaCO3 after 20 calcinations for calcination at 850 °C and carbonation at 700 °C.

Pore volume distributions were calculated to understand the effect of modification of CaCO3 by manganese salts on the carbonation reaction, as shown in Fig. 11. The pore volume of calcined MnCO3-doped CaCO3 mainly distributes in the pore size range of 27–142 nm. The pore volume in the pore size range of 2–20 nm is 0.0021 cm3/g for MnCO3-doped CaCO3 after 20 cycles, appreciably lower than that of the original one, for which the value is 0.0031 cm3/g. However, the pore of calcined MnCO3-doped CaCO3 in the pore size range of 27–142 nm is much more abundant relative to clacined original one. The pore volume of calcined original CaCO3 in the pore size range of 27–142 nm decreases significantly, due to sintering of the sorbent at high calcination temperature. For MnCO3-doped CaCO3, the pore volume in the pore size range of 27– 142 nm is 0.0342 cm3/g after 20 calcinations, 4.5 times of that of original CaCO3 under the same reaction conditions. The pore size distribution of calcined Mn-doped CaCO3 makes it easier for the diffusion of CO2 to the surface of the sorbent to react with CaO. Therefore, the CO2 capture capacity of CaCO3 is enhanced after modification by manganese salts. 4. Conclusion CaCO3 doped by manganese salts exhibits better CO2 capture capacity than the original one during cyclic calcination/carbonation reactions. Although the carbonation conversions are lower than the original one during the previous cycles, Mn-doped CaCO3 shows slight decay with increasing the cycle number and achieves higher carbonation conversions over a longer series of cycles. The optimum Mn/Ca molar ratios for Mn(NO3)2-doped CaCO3 and MnCO3-doped CaCO3 are 1/100 and 1.5/100, respectively. Mndoped CaCO3 achieves higher CO2 carbonation conversions at 700–720 °C than at other carbonation temperatures. Too high calcination temperature is unfavorable for CO2 capture using Mndoped CaCO3. The long-term CO2 capture capacity is enhanced by the addition of manganese salts. The carbonation conversion of Mn(NO3)2-doped CaCO3 keeps steady at 0.27 after 100 cycles, which is 69% higher than the original one at the same reaction conditions. The conversion for MnCO3-doped CaCO3 is 0.24. The effect of Mn(NO3)2 and MnCO3 on CO2 capture capacity of CaCO3 is basically the same. The modification by manganese salts increases the carbonation rate of CaCO3. The SEM and N2 adsorption analysis show that better pore structure is kept for Mn-doped CaCO3 during the multiple calcination/carbonation cycles. The pore volume of calcined MnCO3-doped CaCO3 is much more abundant relative to calcined original one, especially in the pore size range of 27–

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