Cyclic CO2 Capture Behavior of Limestone Modified with

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Cyclic CO2 Capture Behavior of Limestone Modified with Pyroligneous Acid (PA) during Calcium Looping Cycles Yingjie Li,* Rongyue Sun, Hongling Liu, and Chunmei Lu School of Energy and Power Engineering, Shandong University, Jinan 250061, China ABSTRACT: In this work, pyroligneous acid (PA) was used to modify limestone, and the CO2 capture capacity in calcination/ carbonation cycles was consequently strengthened. The cyclic CO2 capture behavior of PA-modified limestone was investigated in a thermogravimetric analyzer and a dual fixed-bed reactor. The main component of the PA-modified limestone was found to be calcium acetate hydrate. The optimum ratio of PA to limestone was found to be 20 mL/g. The PA-modified limestone was found to retain a higher CO2 capture capacity for carbonation at 700 °C. The carbonation conversion of the PA-modified limestone achieved 0.33 after 103 cycles, whereas, for the original sorbent, the conversion was just 0.078 under the same reaction conditions. The modification of limestone increased the surface area and pore volume of the calcined material during cycles. Furthermore, the calcined form of the modified limestone had more pores in the ranges of 1.8
4.6 and 18
155 nm, and as a result, its surface area and pore volume were both expanded.

1. INTRODUCTION Recently, it has been widely realized that CO2 capture, storage, and utilization are of extreme importance in that the accumulation of CO2 in the atmosphere can cause global warming. In the past, various technologies and processes to capture CO2 were proposed, and solid CO2 sorbents were explored comprehensively.1 Additionally, the calcium looping cycle, that is, using a calcium-based sorbent for repeated calcination/carbonation cycles involving the reversible reaction between CaO and CO2, was regarded to be encouraging for CO2 removal,2,3 and its applications in both precombustion CO2 capture [e.g., sorptionenhanced hydrogen production,4
6 HyPr-RING (hydrogen production by reaction-integrated novel gasification),7 and zeroemission carbon (ZEC) hydrogasification8] and postcombustion CO2 capture9,10 were reported in the literature. Although low-cost natural calcium-based sorbents such as limestone and dolomite can be used as CO2 sorbents, their CO2 capture capacities can attenuate greatly with increasing numbers of calcination/carbonation cycles.9,11 Therefore, to capture CO2 efficiently in calcium looping cycles, more fresh sorbent is needed, and consequently, more spent sorbent is generated. Hence, in process design, it is highly required to abate the deactivation of natural sorbents and minimize the sorbent makeup flow during long-term cycles. More recently, a number of researchers have paid more attention to improving the CO2 capture capacity of natural limestone during long-term calcination/carbonation cycles. In particular, different methods to reduce the decay of the sorbent in reactivity have been investigated.12
19 It has been reported that the organic calcium-based sorbent produced by the acetification reaction between calcium-based sorbent and acetic acid exhibits good behavior.20
27 However, the inherent problem with this method is that the acetic acid is very expensive and, as a result, organic calcium-based sorbent is costly as well. Therefore, in this work, pyroligneous acid (PA) as a substitute for acetic acid solution was employed to modify limestone. Because PA is the r 2011 American Chemical Society

product of the flash (fast) pyrolysis of biomass,28 the cost of PA is significantly lower than that of acetic acid solution. In general, PA consists of water, formic and acetic acids (less than 10%), a mixture of carboxylic acids, several aldehydes and alcohols, pyrolytic lignin, and some organic and/or inorganic impurities.28,29 Although PA is much less expensive than acetic acid solution, it contains too many impurities other than acetic acid. These impurities have an uncertain effect on CO2 capture by PAmodified limestone. Hence, in this article, experimental results on the CO2 capture behavior and microstructure of limestone modified by PA under different reaction conditions during multiple cycles are discussed. Additionally, the CO2 capture behaviors of natural limestone and PA-modified limestone are compared.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. In the current experiments, natural limestone was modified with pyroligneous acid (PA) in an electromagnetic agitator at ambient temperature and pressure. The chemical components of natural limestone were analyzed by X-ray fluorescence (XRF), and the organic components in PA were analyzed by gas chromatography/mass spectrometry (Finnigan Trace DSQ instrument), as reported in Tables 1 and 2 (components less than 1% not included), respectively. In the experiments, the ratio of PA to limestone ranged from 5 to 30 mL/g. The reaction time was 2 h, and then the limestone modified by PA was dehydrated at 110 °C in a drying cabinet. The particle size of the samples was below 0.125 mm. 2.2. Cyclic Carbonation in TGA. The carbonation behavior was investigated by thermogravimetric analysis (TGA). To investigate the cyclic carbonation behavior of the sorbents, samples of Received: April 7, 2011 Accepted: August 2, 2011 Revised: July 25, 2011 Published: August 02, 2011 10222

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Table 1. Chemical Components of the Natural Limestone component

content (wt %)

component

content (wt %)

CaO

52.08

Al2O3

0.53

MgO

1.32

Na2O

0.02

SiO2

3.32

others

Fe2O3

0.03

LOI

0.47 42.23

Table 2. Main Organic Components in PA (wt %) component

content (wt %)

water

83.32

component acetol

content (wt %) 3.32

acetic acid

6.87

acetone

1.21

methyl alcohol

2.56

others

2.72

the original sorbents (10 ( 0.1 mg) and the carbonated sorbents (10 ( 0.1 mg) from sorbents that had been subjected to 4, 9, and 99 calcination/carbonation cycles in a dual fixed-bed reactor were chosen as the samples for TGA. Therefore, the carbonation behaviors of sorbents were obtained as a function of reaction time during the 1st, 5th, 10th, and 100th cycles. The furnace temperature of the TGA apparatus was increased to a calcination temperature of 850
1000 °C at a heating rate of 30 °C min
1, and the sample was held for 15 min at the ultimate calcination temperature under pure N2. After the calcination was completed, the temperature was decreased for carbonation at 600
700 °C under pure N2. Then, the reaction atmosphere was switched to a 15% CO2/85% N2 gas mixture, and the calcined sample was carbonated for 30 min. The cyclic carbonation conversion of the sorbent during the carbonation process was calculated as mN
mNcal WCaO XN ¼ m0 b WCO2

dXN dt

3. RESULTS AND DISCUSSION 3.1. XRD Analysis of PA-Modified Limestone. The XRD spectrum of PA-modified limestone, shown in Figure 2, indicates that the main component of the modified limestone was calcium acetate hydrate, Ca(CH3COO)2 3 H2O, when the ratio of PA to limestone was 20 mL/g. According to the calcination of calcium acetate reported by Adanez et al.,30 the modified limestone can be decomposed as CaðCH3 COOÞ2 3 H2 O f CaCO3 þ CH3 COCH3 þ H2 O

ð3Þ

ð1Þ

where XN denotes the carbonation conversion of sorbent at time t (carbonation time) during the Nth cycle; b is the content of CaO in the initial sorbent (%); mN represents the mass of carbonated sorbent at time t during the Nth cycle (mg); mN cal is the mass of sorbent after the completion of calcination during the Nth cycle (mg); and WCaO and WCO2 are the molar masses of CaO and CO2, respectively (g/mol). The carbonation rate of the sorbent was computed by rcarb, N ¼

been purged off, and then the sample boat was moved to the carbonator. The actual temperature of the sample was measured with a thermocouple mounted in the center of the sample boat. The carbonation conversion of the sample after N cycles was calculated by eq 1. 2.4. Phase and Microstructure Analysis. The phase analysis of the PA-modified limestone was perfomed on a Rigaku X-ray diffraction (XRD) analyzer, and the surface morphologies of the calcined samples after different numbers of cycles in the DFR were analyzed by field-emission scanning electron microscopy (SEM). A Micromeritics ASAP 2020-M nitrogen adsorption analyzer was used to measure the microstructure parameters, including surface area, pore area, pore volume, and pore size distribution of the calcined samples after different numbers of cycles in the DFR. It should be mentioned that the surface area and pore volume of each sample were calculated by the Brunauer
Emmett
Teller (BET) method and the Barrett
Joyner
Halenda (BJH) model, respectively, and the pore area was obtained by the BJH model.

ð2Þ

where rcarb,N is the carbonation rate of sorbent at time t during the Nth carbonation (1/s). 2.3. Cyclic CO2 Capture in DFR. A schematic of a dual fixedbed reactor (DFR) including a carbonator and a calciner operated at atmospheric pressure is shown in Figure 1. The sample boat containing the sorbent (about 2 g) can be shifted between two reactors. The reacting gas was measured by flow meter and introduced into the reactor. The variation in the sample mass was measured with a delicate electronic balance. The sample was calcined at 850 and 960 °C in pure N2 and carbonated at 600
740 °C in a 15% CO2/85% N2 gas mixture at atmospheric pressure. Based on preliminary experiments, the carbonation time and the calcination reaction time were specified to be 20 and 15 min, respectively. To prevent calcined sample from recarbonation in the calciner, the gas flow was switched to N2 immediately until the CO2 had

CaCO3 f CaO þ CO2

ð4Þ

3.2. Cyclic Carbonation Kinetics of PA-Modified Limestone. Parts a and b of Figure 3 show the carbonation rates

and conversions, respectively, of natural and PA-modified limestones as functions of reaction time during calcination/carbonation cycles as determined by TGA. It was found that the carbonations of the two sorbents obviously occurred in two stages after each cycle. In the first stage, the cyclic carbonation rates of the two sorbents increased rapidly with the reaction time, as shown in Figure 3a. The reaction times corresponding to the maximum carbonation rates of the PA-modified limestone and the original limestone at each cycle were about 48 and 100 s, respectively. However, in the second stage (above the reaction time corresponding to the maximum carbonation rate), the carbonation rates of the sorbents decreased as a result of the formation of a certain thickness of CaCO3 layer that prevented the further carbonation of unreacted CaO, so that the carbonation conversions increased slowly, as seen in Figure 3b. Compared with the original limestone at the same cycle number, the maximum carbonation rate of the PA-modified limestone was higher. After 300 s, the carbonation rate of the original limestone tended toward 0, whereas that of the PA-modified limestone retained a certain value, as shown in Figure 3b. This reveals that the diffusion capacity of CO2 through the product layer was strengthened on the PA-modified limestone. Figure 3b also indicates that the PA-modified limestone did not provide a better carbonation conversion than the original sorbent during the first cycle. 10223

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Figure 1. Schematic diagram of the DFR at atmospheric pressure.

Figure 2. XRD spectrum of PA-modified limestone (PA/limestone ratio = 20 mL/g).

However, PA had a gradually increasing impact on the carbonation conversion of the limestone with the number of cycles. For example, the carbonation conversion of the PA-modified limestone during the 5th cycle at 800 s was 1.2 times that of the original limestone under the same reaction conditions, whereas the conversion of the former during the 100th cycle at 800 s was 3.1 times that of the latter under the same reaction conditions. 3.3. Effect of the PA-to-Limestone Ratio on the Cyclic Carbonation Conversion of PA-Modified Limestone. Figure 4 shows the carbonation conversions obtained in the DFR for the PA-modified limestone at different ratios of PA to limestone. A ratio of 0 actually cooresponds to the original limestone. It was observed that the cyclic carbonation conversion increased with increasing ratio of PA to limestone. When the ratio was 20 mL/g, the cyclic carbonation conversion of the modified limestone was significantly enhanced. For the ratio of 20 mL/g, the carbonation conversion of the modified limestone was 0.46 after 17 cycles, whereas the conversion of the original sorbent was just 0.18 under the same reaction conditions. However, for a certain ratio, the impact of the modified limestone on carbonation conversion ultimately tends to be leveled off. For instance, when the ratio is raised from 20 mL/g to 30 mL/g, the modified limestone may cause an increase of just 6% in the carbonation conversion after 17 cycles, in that the main component of the limestone after the treatment becomes the calcium acetate hydrate, when the ratio reaches 20 mL/g. In addition, the cost increases with the increase of the ratio, so the optimal ratio of PA to limestone should be 20 mL/g.

Figure 3. Cyclic carbonation kinetics of original limestone and PAmodified limestone (PA/limestone ratio = 20 mL/g, carbonation temperature = 700 °C, calcination temperature = 850 °C).

3.4. Effect of Reaction Temperature on the Cyclic Carbonation Conversion of PA-Modified Limestone. The effect of

carbonation temperature on cyclic carbonation conversion of the PAmodified limestone at the mentioned optimal ratio of PA to limestone in the DFR is depicted in Figure 5. CaO derived from natural limestone and calcium acetate may exhibit better CO2 capture capacity at about 600
700 °C.9 Therefore, in the current experiments, the carbonation temperature was specified in the range from 600 to 740 °C. It is revealed by the experimental results that the modified limestone can reach the maximum carbonation conversion at 700 °C. 10224

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Figure 7. Long-term carbonation behavior of PA-modified limestone (PA/limestone ratio = 20 mL/g, carbonation temperature = 700 °C, calcination temperature = 850 °C). Figure 4. Effect of ratio of PA to limestone on carbonation conversions of PA-modified limestone (carbonation temperature = 700 °C, calcination temperature = 850 °C).

Figure 8. SEM images of calcined materials derived from the (a) original and (b) PA-modified limestone sorbents after 20 cycles (PA/limestone ratio = 20 mL/g, carbonation temperature = 700 °C, calcination temperature = 850 °C).

Figure 5. Cyclic carbonation conversions of PA-modified limestone at different carbonation temperatures (PA/limestone ratio = 20 mL/g, calcination temperature = 850 °C).

Figure 6. Cyclic carbonation conversions of PA-modified limestone at different calcination temperatures (PA/limestone ratio = 20 mL/g, carbonation temperature = 700 °C).

The high calcination temperature is beneficial to decomposition of limestone. However, if the temperature is too high, sintering of calcium-based sorbents can occur and the decay of carbonation conversion for calcium-based sorbents may be caused owing to the induced blockage of many pores in sorbents.31 Grasa and Abanades32 found that the calcination temperature above 950 °C accelerated the decay in CO2 capture capacity. Figure 6 shows the effect of calcination temperature on the cyclic carbonation conversion of the PA-modified limestone at the optimal ratio of PA to limestone in the DFR. It reveals that the both two sorbents exhibit decrease in the cyclic carbonation conversions with increasing the calcination temperature. The carbonation conversion of the PA-modified limestone after 17 cycles for calcination at 960 °C is four times as high as that of the original limestone under the same reaction conditions. In addition, the PA-modified limestone for calcination at 960 °C exhibited an even greater carbonation conversion compared with the natural limestone at 850 °C. Moreover, the PA-modified limestone was able to maintain a higher CO2 capture capacity at calcination temperatures above 950 °C. This shows that the favorable calcination temperature range for natural limestone is below 950 °C, whereas, for the modified limestone, it is above 950 °C. Figure 7 presents the long-term carbonation behaviors of the original and PA-modified limestones in the DFR. The original limestone showed a sharp decay in the carbonation conversion during the first 20 cycles, and then the decay became slow after these 20 cycles. The original limestone had a carbonation conversion of 0.078 after 103 cycles, as reported by Lisbona et al.33 10225

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Figure 9. SEM images of calcined materials derived from the (a) original and (b) PA-modified limestone sorbents after 100 cycles (PA/limestone ratio = 20 mL/g, carbonation temperature = 700 °C, calcination temperature = 850 °C). Figure 11. Pore size distributions of the calcined sorbents during calcination/carbonation cycles (PA/limestone ratio = 20 mL/g, carbonation temperature = 700 °C, calcination temperature = 850 °C).

Figure 10. Surface areas, pore areas, and pore volumes of calcined sorbents during calcination/carbonation cycles (PA/limestone ratio = 20 mL/g, carbonation temperature = 700 °C, calcination temperature = 850 °C).

and Grasa et al.34 The carbonation conversion of the modified limestone reached approximately 0.33 after 103 cycles, which is 4.2 times higher than that of the original limestone under the same reaction condidtions. 3.5. Microstructure Analysis of PA-Modified Limestone over Multiple Cycles. SEM micrographs of the calcined materials derived from the original limestone and the PA-modified limestone after 20 cycles are presented in Figure 8. The calcined material derived from the original limestone after 20 cycles appeared compact, with no visible pores in the surface of the particle, because the series of calcination/carbonation cycles

aggravated the sintering, as shown in Figure 8a. The blockage and collapse of pores of the original limestone can hinder the passage of CO2 as the carbonation proceeds. As a result, the CO2 capture capacity of the original limestone dropped sharply with the number of cycles. However, a number of loose, expansive, and penetrable pores appeared in the surface of the calcined material derived from the PA-modified limestone after 20 cycles, as shown in Figure 8b. This structure is beneficial to CO2 diffusion in the sorbents. Figure 9 shows SEM images of the calcined materials derived from the original limestone and the PA-modified limestone after 100 cycles. It can be observed that less sintering occurred in the surface of the calcined material derived from the modified limestone, which resulted in a higher CO2 capture capacity after 100 cycles. Figure 10 shows the surface areas, pore areas, and pore volumes of the calcined materials derived from the original and PA-modified limestones after 3, 10, and 20 cycles. It was found that the differences in the surface areas and pore areas after the same number of cycles were slight. The surface areas of the calcined materials derived from the PA-modified limestone after 3, 10, and 20 cycles were 1.7, 2.1, and 2.1 times larger than those derived from the original limestone after the same number of cycles. Similarly, the pore volume of the calcined material derived from the PA-modified limestone was greater than that derived from original limestone after the same number of cycles, as presented in Figure 10b. These results reveal that the modification of limestone with PA can increase the surface area and the pore volume of the calcined material during multiple cycles, because great amounts of volatile compounds can be release from the modified limestone during the first calcination and, consequently, a number of pores can be generated. The calcined calciumbased sorbents with larger surface areas and pore volumes can behave more effectively for CO2 capture, because the rate of gas
solid reactions is primarily determined by the total area accessible to the gas and by the space available for the reaction products.35 Figure 11 shows the pore size distributions of the calcined materials derived from the original limestone and the PA-modified limestone after 3, 10, and 20 cycles, which were calculated using ASAP 2020 software. The two-peak behavior associated with the pore size distribution curve was properly captured. One peak occurred in the range of 1.8
4.6 nm, and the other appeared at 18
155 nm. It was found that, compared with the calcined material derived from the original limestone at the same cycle 10226

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Industrial & Engineering Chemistry Research number, the calcined material derived from the PA-modified limestone had more pores in the 1.8
4.6- and 18
155-nm ranges after multiple cycles. The surface areas of the two sorbents mainly depend on the pores in the 1.8
4.6-nm size range, and their pore volumes are dominantly presented by the pores in the range of 18
155 nm. Therefore, it is easy to understand why the calcined material derived from the PA-modified limestone had a larger surface area and pore volume than that derived from the original limestone after the same number of cycles.

4. CONCLUSIONS Pyroligneous acid (PA) was primarily used to modify limestone to enhance the cyclic CO2 capture capacity during calcium looping cycles. It was found that the main component of the modified limestone was calcium acetate hydrate. Compared with the experimental data for the original limestone after the same numbers of cycles, the maximum carbonation rate of the PAmodified limestone was higher. PA had a gradually increasing impact on the carbonation kinetics of the sorbents with increasing cycle number. In addition, the CO2 capture capacity of the modified limestone was significantly influenced by the ratio of PA to limestone. It was observed that the optimal ratio was 20 mL/g. Furthermore, the modified limestone reached the maximum carbonation conversion at 700 °C. For the natural limestone, the favorable calcination temperature range is below 950 °C, whereas it is above 950 °C for the modified limestone. Moreover, the carbonation conversion of the modified limestone was approximately 0.33 after 103 cycles, which is 4.2 times higher than that of the original limestone under the same reaction conditions. The calcined material derived from the modified limestone exhibited better pore structure parameters including surface area, pore volume, and pore size distribution. Therefore, according to the results of this work, PA-modified limestone can be employed as a CO2 sorbent. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: +86-531-88392414. E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support from the Independent Innovation Foundation of Shandong University (2009GN042), China Postdoctoral Science Foundation Funded Project (20090461205), and Special Funds for Postdoctoral Innovative Projects of Shandong Province (200902019) is greatly appreciated. ’ REFERENCES (1) Wang, Q.; Luo, J.; Zhong, Z.; Borgna, A. CO2 capture by solid adsorbents and their applications: Current status and new trends. Energy Environ. Sci. 2011, 4, 42. (2) Anthony, E. J. Solid looping cycles: A new technology for coal conversion. Ind. Eng. Chem. Res. 2008, 47, 1747. (3) Abanades, J. C.; Grasa, G.; Alonso, M.; Rodriguez, N.; Anthony, E. J.; Romeo, L. M. Cost structure of a postcombustion CO2 capture system using CaO. Environ. Sci. Tehnol. 2007, 41, 5523. (4) Harrison, D. P. Sorption-enhanced hydrogen production: A review. Ind. Eng. Chem. Res. 2008, 47, 6486. (5) Florin, N. H.; Harris, A. T. Enhance hydrogen production from biomass with in situ carbon dioxide capture using calcium oxide sorbents. Chem. Eng. Sci. 2008, 63, 287.

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