Mechanical Modification of Naturally Occurring Limestone for High

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Mechanical Modification of Naturally Occurring Limestone for HighTemperature CO2 Capture Jian Sun, Wenqiang Liu,* Mingkui Li, Xinwei Yang, Wenyu Wang, Yingchao Hu, Hongqiang Chen, Xian Li, and Minghou Xu* State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, People’s Republic of China S Supporting Information *

ABSTRACT: The rapid decrease of CO2 capture capacity is one of the most challenging problems hindering the use of naturally occurring limestone in the calcium looping process. In this work, the mechanical modification method (dry planetary ball milling) was used to improve the cyclic CO2 capture performance of naturally occurring limestone. Low-cost Bayer aluminum hydroxide sourced from the industrial-scale production of alumina from bauxite ore was used as the precursor of the inert support to enhance the CO2 sorption stability of the ball-milled sorbents. It was found that the CO2 uptake of the milled sorbents could be further improved by increasing the ball-milling time because this generated more amounts of fine particles. Moreover, the pellets produced from ball-milled limestone powder possessed a relatively high CO2 capture capacity of 0.252 g/g in the 25th cycle, which is nearly 1.3 times the capture capacity of naturally occurring limestone powder. This indicates that the combination of mechanical modification and pelletization is an effective approach to produce highly efficient CO2 capture pellets from naturally occurring limestone.

1. INTRODUCTION Nowadays, the combustion of fossil fuels results in the emission of CO2, which is the primary contributor for global warming.1,2 The calcium looping process (CLP) for high-temperature CO2 capture is a promising technology to mitigate global warming.3−5 The CLP consists of a carbonation reactor and a calcination reactor, which remove CO2 by the reversible reaction CaO + CO2 ↔ CaCO3. CO2 in the flue gas is stripped by the CaO-based sorbent in the carbonation reactor. The carbonated CaO-based sorbent is regenerated in the calcination reactor in an oxy-fuel combustion process, and the produced CO2 gas stream can be further used and sequestrated.6−8 To avoid the high CO2 removal cost, naturally occurring limestone is generally used to produce CO2 capture sorbents because of its low price.9,10 Unfortunately, the CaO-based sorbent derived from naturally occurring limestone has a poor CO2 sorption durability, and the CO2 capture capacity rapidly decays during carbonation/calcination cycles.11,12 This is mainly because the sintering of CaCO3 at high temperatures causes particle aggregation and reduces the porous surface area for CO2 sorption.13−15 Hence, a number of methods (i.e., doping,16,17 acid modification,18−20 thermal activation,21 hydration,22,23 and simultaneous hydration−impregnation24) have been investigated to improve the cyclic CO2 sorption performance of naturally occurring limestone, as summarized in Table 1. The modification methods listed mainly aim to increase the surface area or form a stabilized skeletal structure of the CaO-based sorbent, which have been proven to effectively enhance the CO2 capture capacity and stability of naturally occurring limestone. In addition to the aforementioned common modification methods, a mechanical modification method via high-energy ball-milling treatment has also been reported.25,26 High-energy © XXXX American Chemical Society

ball milling is a simple and highly effective powder processing technique that has been widely used to make alloys, nanocrystalline metals, and structural materials.27,28 However, the number of studies regarding mechanical modification of CaO-based sorbents to improve high-temperature CO 2 sorption performance is still limited. Sayyah et al.25 reported that wet planetary ball milling could substantially improve the CO2 sorption capacity of CaO-based sorbents. Furthermore, MgO used as the inert support was found to effectively mitigate the sintering of CaCO3 and to help maintain the stable CO2 uptake of the ball-milled sorbents. However, there are many other inert support materials to be studied during the ballmilling treatment processes. For example, Al-based,29−33 Labased,34 Ti-based,35,36 Mn-based,37 Ce-based,38 Y-based,39,40 Li-based,41 Nd-based,42 and Zr-based43,44 materials could also be selected as the inert supports for CaO sorbents. Particularly, Al-based inert supports have been investigated the most in the existing literature because of their splendidly enhanced CO2 sorption capacity and low cost. Hence, it would be interesting to discover the effect of Al-based inert supports on the CO2 sorption of CaO-based sorbents during ball-milling treatment processes. Moreover, a method to use the finely ball-milled sorbent powders to prepare highly efficient CO2 capture pellets is also needed. Therefore, this work aims to modify naturally occurring limestone to improve its high-temperature CO2 capture performance via two routes: (1) mechanical modification and (2) integrating mechanical modification with Al-based inert solid support additions. Dry planetary ball milling was used as Received: May 11, 2016 Revised: July 15, 2016

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DOI: 10.1021/acs.energyfuels.6b01131 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Table 1. Summary of the Typical Modification Methods for the CO2 Capture Performance Enhancements of Naturally Occurring Limestone and the Testing Conditions for the Obtained Sorbents (Only Representative Results) reference

modified method

González et al.16

reactor

doping (KCl)

TGA

doping (NaCl)

TGA fixed bed

Ridha et al.20

acid modification (tartaric acid) acid modification (pyroligneous acid) acid modification (formic acid)

Manovic et al.21

thermal activation

TGA

Wu et al.22

hydration/pelletization (water) hydration (ethanol/water solutions) hydration−impregnation (sea salt)

TGA

Salvador et al.

17

Hu et al.18 Li et al.19

Li et al.23 Xu et al.24

carbonation condition 700 °C, 15% CO2, and 10 min 700 °C, 15% CO2, and 20 min 650 °C, 15% CO2, and 30 min 700 °C, 15% CO2, and 20 min 650 °C, 15% CO2, and 20 min 700 °C, 15% CO2, and 30 min 800 °C, 25% CO2, and 30 min 700 °C, 15% CO2, and 20 min 850 °C, 100% CO2, and 15 min

fixed bed TGA

fixed bed fixed bed

the method for mechanical modification of naturally occurring limestone. Bayer aluminum hydroxide (BAh), a low-cost intermediate product of the Bayer process used for industrialscale production of alumina from bauxite ore, was selected as the Al-based inert solid support. The ratios of the BAh additions (5, 10, 20, and 40 wt %) and the ball-milling times (1, 2, and 4 h) were both compared to identify the optimum parameters for the production of mechanically modified CaObased sorbents. Moreover, sorbent pellets were prepared by combining the ball-milling treatment and extrusion−spheronization from naturally occurring limestone for the first time, in consideration of the sorbent powders easily being elutriated from the fluidized-based CLP. The cyclic CO2 uptake and mechanical properties of the pellets were investigated to evaluate their practicability.

calcination condition

cycle number

last cycle capacity (g/g)

900 °C, 15% CO2, and 10 min

13

∼0.26

850 °C and 100% N2

14

∼0.39

900 °C, 100% N2, and 10 min

26

∼0.28

850 °C, 100% N2, and 15 min

103

∼0.23

850 °C, 100% N2, and 5 min

20

∼0.22

850 °C, 100% N2, and 10 min

30

∼0.38

800 °C, 100% N2, and 15 min

27

∼0.36

920 °C, 80% CO2/20% O2, and 15 min 850 °C, 100% N2, and 15 min

15

∼0.39

40

∼0.31

Figure 1. XRD pattern of BAh.

2. EXPERIMENTAL SECTION in Figure 2. A total of 10 g of raw powders (naturally occurring limestone or BAh) were placed in an alumina ceramic jar (250 mL) operated in a planetary ball mill (QM-3SP2, Nanjing University Instrument Corporation, China) at 250 rpm. A range of mechanically modified sorbents were prepared with different amounts of BAh (5, 10, 20, and 40 wt %) and different milling times (1, 2, and 4 h). The pellets with particle size fractions of 0.9−1.25 mm were prepared from the milled limestone powder using an extrusion−spheronization method. More details on the pelletization method for the CaObased pellets are presented in our previous works.45 The naturally occurring limestone is denoted as CC, and the ball-milled limestone without BAh doping is named as CC-B. The mechanically modified sorbents with BAh are named CAx, and x refers to the ratio of BAh. For instance, a sample with 10 wt % BAh is designated as CA10, and the other sorbents are named following the same rule. 2.3. CaO-Based Sorbent Testing. The cyclic CO2 capture performance of the sorbents was tested using thermogravimetric analysis (TGA, Pyris 1). The initial mass of all of the tested sorbents was approximately 30 mg, and the total reaction gas flow was maintained at 100 mL/min. Before the capture cycles commenced, the sample was heated to 850 °C at a rate of 20 °C/min in an atmosphere of 100 vol % N2. The sample was calcined for 10 min at the calcination temperature to attempt to achieve complete calcination of the fresh limestone. During the carbonation/calcination testing process, carbonation reactions were allowed to occur in 15 vol % CO2 (N2 used as the balance gas) for 30 min, and the calcination conditions were maintained at 850 °C for 2 min in pure N2 (or 900 °C for 5 min in 40 vol % CO2). It is worth noting that, although the TGA apparatus

2.1. Raw Materials. The naturally occurring limestone used in this study came from the Fude chemical plant located in Tongling of Anhui province in China. After the limestone was ground and sieved, limestone particles with diameters less than 170 μm were collected. BAh was purchased from a chemical plant in Baise of Guangxi province in China. The crushed BAh was sieved, and particles with a size fraction of 75−150 μm were used as the inert solid support. The results from X-ray fluorescence (XRF) analysis using a focused-beam XRF spectrometer (model EAGLE III) for limestone and BAh are shown in Table 2. The X-ray diffraction (XRD) of BAh is depicted in Figure 1, which was analyzed using an X-ray diffractometer (model X’Pert PRO); the major constituent of BAh was gibbsite [Al(OH)3]. 2.2. Preparation of the CaO-Based Sorbent. The preparation process of the mechanically modified CaO-based sorbents is presented

Table 2. XRF Analysis of Raw Limestone and BAh component

limestone (wt %)

BAh (wt %)

CaO Al2O3 SiO2 SO3 Cl2O TiO2 Fe2O3 loss on ignition (LOI)

55.27

0.09 62.12 2.56

0.58 0.13

0.1 43.92

0.84 0.14 0.02 34.24 B


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Figure 2. Schematic sketch of the preparation process of the mechanically modified sorbents. is inferior to the real fluid-bed CO2 capture systems used to test new sorbents because of its confined laboratory test conditions, it is still the most reasonable option considering the cost of large-scale tests.46 The CaO carbonation conversion (Xn, %), the CO2 capture capacity (Cn, grams of CO2 per grams of calcined sorbent, g/g), and the CO2 uptake loss rate (Ln, %) were defined to evaluate the CO2 capture performance of the sorbent. The specific formulas for Xn, Cn, and Ln can be found in our previous works.7,45 A laser particle size analyzer (Malvern, model MASTERSIZER 3000) was used to analyze the particle size distributions of freshly ballmilled sorbents, using water as the dispersant. The crystal phases of the initial calcined, mechanically modified sorbents were analyzed using an X-ray diffractometer (model X’Pert PRO). Field emission scanning electron microscopy (FSEM, model Nova NanoSEM 450) was used to observe the surface morphologies of the initial calcined and cycled CaO-based sorbents. The samples were dispersed on a conductive adhesive carbon tab and coated with carbon prior to image acquisition to obtain better electronic signals. The specific surface area, pore volume, and average pore width of the initial calcined sorbents were measured via a Micromeritics ASAP 2020 accelerated surface area and porosimetry system. The Brunauer−Emmett−Teller (BET) surface area and average pore width were analyzed using the N2 adsorption and desorption isotherms.

Figure 3. (a) CO2 capture capacity of the CC and CC-B and (b) 1st and (c) 25th CO2 capture rates and CO2 capture capacity of the CC and CC-B. Calcination, 2 min at 850 °C in 100 vol % N2; carbonation, 30 min at 650 °C in 15 vol % CO2.

diffusion-controlled stage) of the CaO-based sorbent. The schematic for the identification of the three stages from the curve of the CO2 capture rate is depicted in Figure S1 of the Supporting Information. As illustrated in panels b and c of Figure 3, in comparison to the CO2 capture rate in the 1st and 25th cycles, the chemical-reaction-controlled stage of the CC and CC-B during the 30 min carbonation process both shortened with subsequent cycles. This can mainly be attributed to the sorbent sintering, leading to the compact inner structure, which limits the diffusion of CO2 to further react with interior, free CaO.47 Moreover, the CC displayed a faster CO2 sorption than the CC-B during the beginning of the chemical-reaction-controlled stage of sorbent carbonation. The higher specific surface area of the CC (Table 3) is responsible Table 3. Specific Surface Area, Pore Volume, and Average Pore Width of the Initial Calcined Sorbents

3. RESULTS AND DISCUSSION 3.1. Mechanically Modified Sorbents Prepared via Ball Milling. To investigate the effect of ball-milling treatments on the cyclic CO2 capture performance, the ball-milled limestone (CC-B) was tested using TGA. The naturally occurring limestone (CC) was also tested for comparison purposes. As depicted in Figure 3a, it is clear that ball-milling treatments effectively improve the cyclic CO2 capture performance by comparing the CO2 capture capacity (Cn) of the CC and CC-B. For instance, the Cn of the CC-B is 0.322 g/g in the 25th cycle, which is over 1.6 times that of CC. This finding is consistent with previous results,25 in which wet planetary ball milling was applied to modify the CaCO3 reagent for CO2 capture. The CO2 capture rate (Cn derivative with respect to carbonation time) could help to improve the understanding of the carbonation process (including the chemical-reaction-controlled stage, the transition stage, and the product layer

sorbent

sorbent surface area, BET (m2/g)

pore volume, BJH (cm3/g)

average pore width (nm)

CC CC-B CA5 CA10 CA40

12.19 10.13 16.31 19.91 36.03

0.103 0.067 0.081 0.105 0.141

33.75 26.45 19.84 21.17 15.60

for the faster CO2 sorption because the overall reaction rate in the chemical-reaction-controlled stage is given by the intrinsic reactivity multiplied by the surface area. After 25 cycles, the CC-B maintained an apparent higher CO2 capture rate than that of the CC, demonstrating the outstanding reactivity. Although the specific surface area and pore volume of the CC-B are slightly lower than those of the CC, the CC-B displayed a better cyclic CO2 capture performance. This could C


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Energy & Fuels probably be explained from the surface morphologies of the calcined CC and CC-B. It is clear that more nano-scaled pores existed on the surface of the calcined CC (Figure 4a), which

Figure 5. (a) CO2 capture capacity of the CC-B pellet during 50 cycles in comparison to CC and CC-B and (b) weight loss of the CC-B pellet with the increasing rotation number. The conditions are the same as for the samples shown in Figure 2.

of the porous structure of the original powder.45,51 However, the Cn of the CC-B pellets is still apparently superior to that of the CC (raw naturally occurring limestone powder). The CC-B pellets displayed a high Cn of 0.188 g/g in the 50th cycle, which is nearly comparable to the 25th Cn (0.196 g/g) of the CC. As shown in Figure 4e, the CC-B pellet has a good capability to resist the thermal stress during carbonation/calcination cycles because no apparent cracking was observed on the surface of the cycled CC-B pellets. Moreover, the cycled pellets still maintained a relatively porous microstructure that was responsible for its high CO2 sorption capacity during the carbonation/calcination cycles (Figure 4f). The attrition propensity and mechanical strength of the sorbent are two very important factors to evaluate its practical application. A friability tester (CS-2) was used to test the resistance to attrition of the CC-B pellets; the detailed testing parameters and the calculation methods are presented in our previous work.45 As shown in Figure 5b, the CC-B pellet possesses a relatively good anti-attrition ability (a weight loss of only 6.76% after 4000 rations). The ability to resist attrition of the CC-B pellet is even comparable to that of cement-bound extruded particles reported in previous work.49 The compression strength was used to represent the mechanical strength of the CC-B pellet, which is defined as the ratio of the crushing force to the cross-sectional area of the pellet.45 The peak force to crush a single pellet was measured using a mini-compression tester (LYYS-1000N), and 15 pellets were measured to ensure the reliability of data. It was found that the average compression strength of the CC-B pellet was 4.98 ± 0.53 MPa, which is superior to cement-bound pellets with a size range of 2−2.36 mm.45 Comprehensively considering the CO2 uptake and mechanical property of the CC-B pellet, the combination of the ball-milling treatment and extrusion−spheronization is a promising method to modify naturally occurring limestone for CO2 capture in the CLP. 3.2. Mechanically Modified Sorbents Prepared via Simultaneous Ball Milling and BAh Additions. Al-based supports have been proven to be effective for the enhancement of the anti-sintering ability of the CaO-based sorbent during high-temperature CO2 capture processes.32 BAh is a low-cost intermediate product of the Bayer process used for industrial-

Figure 4. Surface morphology images of sorbents: (a and b) calcined CC and CC-B, (c) CC after 25 cycles, (d) CC-B after 25 cycles, and (e and f) CC-B pellet after 50 cycles.

formed as a result of CO2 escaping from the interior of the sorbent. In contrast, a large number of small particles and microsized cavities were observed on the surface of the calcined CC-B (Figure 4b). The existence of more nano-scaled pores (Figure S2 of the Supporting Information) contributed to the higher specific surface area and pore volume for the CC. During the cyclic CO2 capture process, the nanosized pores easily disappeared because of the deterioration of the sorbent sintering, as shown in Figure 4c. However, the microsized cavities generated by ball milling were not sensitive to sorbent sintering, and hence, a porous structure could be maintained (Figure 4d). This stabilized porous structure of the cycled CCB is beneficial to increase its reactive surface area, and therefore, a slower degradation of the CO2 sorption performance could be expected. Although finely milled limestone powders have good cyclic CO2 capture performance, they are easy to be elutriated from the fluidized-bed reactors.48−50 Therefore, for practical use of the milled fines in the CLP, pelletization processing is very necessary. In this work, the pellets were produced from the ballmilled limestone powders and the cyclic CO2 sorption performance was investigated. As illustrated in Figure 5a, the CC-B pellet showed a lower Cn (CO2 capture capacity) in comparison to the limestone powder after the ball-milling treatment (CC-B). The deterioration of the CO2 uptake of sorbent after pelletization processing is due to the destruction D


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Energy & Fuels scale production of alumina from bauxite ore.52 In this work, to further improve the CO2 capture performance of ball-milled limestone, different ratios (5, 10, 20, and 40 wt %) of BAh were added to naturally occurring limestone prior to the ball-milling treatment. Freshly ball-milled sorbents (CA5, CA10, CA20, and CA40) were analyzed by FSEM/energy-dispersive X-ray spectroscopy (EDX), and the EDX mapping images of the calcium and aluminum elements are depicted in Figure S3 of the Supporting Information. The aluminum element completely derived from BAh can reflect the distribution of BAh. It was found that the more BAh added to the sorbent, the more highly dispersed aluminum was observed, which indicates that there was a more homogeneous distribution of BAh within the sorbents. When the BAh-added sorbents wer calcined using TGA, two weight loss peaks could be observed starting at ∼150 and ∼530 °C, respectively, owing to the dehydration of BAh and the decomposition of CaCO3 (Figure 6). By comparison, the sorbents without BAh only had one weight loss peak as a result of the decomposition of CaCO3.

Figure 7. (a) CO2 capture capacity and (b) CO2 uptake loss of ballmilled sorbents with different BAh/limestone ratios. The conditions are the same as for the samples shown in Figure 2.

Figure 6. TGA results of the CaO-based sorbent with different ratios of BAh in N2 at 100 mL/min. The temperature increases to 850 °C at a heating rate of 20 °C/min.

The CO2 capture capacities (Cn) and CO2 uptake loss (Ln) of the ball-milled sorbents with different ratios of BAh are illustrated in Figure 7. Apparently, the initial Cn gradually decreased as the ratio of BAh increased (Figure 7a) as a result of the reduction of active CaO. However, the addition of BAh can retard the CO2 uptake loss of the CaO-based sorbents during multiple carbonation/calcination cycles. As shown in Figure 7b, the Ln of the sorbents gradually reduced as the BAh ratio increased, indicating its stable CO2 sorption capability. For instance, the Ln dropped from 47.13 to 32.39% when the added BAh increased from 0 to 40 wt % in the ball-milled sorbents. Comprehensively considering the trade-off between the CO2 capture capacity and sorbent durability, the optimum addition ratio of BAh was found to be 5 wt % in this study. The effectiveness of BAh in enhancing the CO2 sorption stability could be explained through XRD of the original and modified CaO-based sorbents after calcination (Figure 8). Diffraction peaks appearing at 2θ = 32.26°, 37.38°, 53.92°, 64.23°, and 67.42° for (111), (200), (220), (311), and (222) could be attributed to the typical crystalline phase of CaO [Joint Committee on Powder Diffraction Standards (JCPDS), 04-0777].53 The crystalline phase of tricalcium aluminate (Ca3Al3O6) was found for the BAh-added sorbents, and its

Figure 8. XRD patterns of the initial calcined sorbents.

intensity increased with increasing ratios of BAh. It was reported that Ca3Al3O6 has a high thermal stability and good anti-sintering ability.29 Evenly dispersed Ca3Al3O6 is able to alleviate the aggregation of CaO grains and, thereby, improve the cyclic CO2 capture performance. Generally, Ca3Al3O6 is more stable among Ca−Al−O phases, which is produced from deeper reaction between Ca12Al14O33 and CaO at 1100 °C.35,54 However, Ca12Al14O33 was not found in the ball-milled sorbent after the calcination at 850 °C. This is probably because the ball-milling process promotes the formation of Ca3Al3O6. A similar phenomenon also occurred on the synthetic sorbent from carbide slag and aluminum nitrate hydrate by the method of combustion synthesis at 800 °C.32 The specific surface area, pore volume, and average pore width of the ball-milled sorbents with different addition ratios E


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Energy & Fuels of BAh are listed in Table 3. Apparently, the addition of BAh is beneficial in that it increases the specific area and pore volume of the ball-milled sorbents, and larger amounts of BAh result in higher specific areas and pore volumes. For instance, CA40 (containing 40 wt % BAh) showed the highest specific area of 36.03 m2/g in comparison to 10.13 m2/g for CC-B (without BAh addition). The Barrett−Joyner−Halenda (BJH) pore size distributions of the ball-milled sorbents after calcination are illustrated in Figure 9. It was found that the pore volumes in the

Figure 10. CO2 capture capacity of (a) CA5 and (b) CA10 milled for different times. The conditions are the same as for the samples shown in Figure 2. Figure 9. Pore size distributions of the calcined sorbents with different addition ratios of BAh.

range of 2−15 nm dramatically increase with increasing amounts of BAh. The BAh-added sorbents also maintained more pore volume in the larger pore range of 20−50 nm, which is responsible for rapid diffusion of CO2 to react with interior, free CaO.55 3.3. Effect of Ball-Milling Time on the Cyclic CO2 Uptake of Modified Sorbents. It was found that the milling time was closely related to the properties of the ball-milled samples.25,56 Therefore, we further investigated the effect of different ball-milling times (1, 2, and 4 h) on the CO2 uptake of CA5 (5 wt % BAh addition) and CA10 (10 wt % BAh addition). As shown in Figure 10, the CO2 capture capacity (Cn) of all of the sorbents treated by ball milling apparently improved when the milling time was increased from 1 to 2 h, with an increased percentage of 25.2% for CA5 and 14.3% for CA10. Increasing the milling time to 4 h caused the 25th Cn of CA10 to increase by 35.7% compared to the samples milled for 1 h. However, there was no obvious improvement in the CO2 uptake for the sorbent with a low amount (5 wt %) of BAh addition when the milling time was increased from 2 to 4 h. This indicates that a milling time of 2 h is sufficient to make 5 wt % BAh effectively disperse throughout the ball-milled samples, and longer milling times are needed for higher ratios (10 wt %) of BAh to ensure effective dispersion of BAh. However, a very long ball-milling time may be not suitable for practical preparations of the highly efficient CO2 capture sorbent during a realistic operation, because it would reduce the preparation efficiency and increase energy consumption. The changes of CO2 capture capacities and CO2 capture rates versus the carbonation times of the sorbents milled for different times in the 25th cycle are depicted in Figure 11. The chemical-reaction-controlled stage and product layer diffusioncontrolled stage can be clearly identified during the carbonation

Figure 11. CO2 capture capacity curve and CO2 capture rates in the 25th cycle of (a) CA5 and (b) CA10 milled for different times.

reaction process, which has been widely reported in previous literature.45,57 Interior, free CaO wrapped by the formed CaCO3 product layer with the deepening of the carbonation reaction leads to the reduction of the CO2 capture rate because of the increasing diffusion resistance of CO2. The milling time mainly affects the transition of CaO carbonation from the chemical-reaction-controlled stage to the product layer diffusion-controlled stage. For instance, apparently higher F


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Figure 12. FSEM images of the CA10 sample after 25 carbonation/calcination cycles with different ball-milling treatment times of (a) 1 h, (b) 2 h, and (c) 4 h.

sintering.58,59 However, in comparison to the CC, the CO2 sorption observed during 25 carbonation/calcination cycles was obviously superior for CA10-4h (Figure 13). The results demonstrate that the ball-milling treatment can effectively improve the cyclic CO2 sorption performance of naturally occurring limestone when calcined under high CO2 partial pressure.

CO2 capture rates are observed for the sorbents with the addition of 10 wt % BAh milled for 2 or 4 h in comparison to that milled for 1 h during the transition stage. Generally, the generation of more fine particles is considered to be the reason for the improved CO2 capture performance of the ball-milled sorbents.25 The increased ball-milling time can effectively reduce the particle size of the CaO-based sorbents; approximately 90 vol % of the freshly milled CA10-1h (ball milled for 1 h) sorbent particles were less than 18.3 μm, whereas 90 vol % of the freshly milled CA10-4h (ball milled for 4 h) sorbent particles were less than 12.9 μm (Figure S4 of the Supporting Information). Moreover, it was found that more nano-sized particles were contained in the freshly milled CA104h. After 25 carbonation/calcination cycles, the cycled CA104h sorbent still possessed more nano-sized particles (∼125− 200 nm) than the samples milled for 1 h, as shown in Figure 12. It has been reported that nanosized particles can be sufficiently carbonated during the carbonation process, which leads to the superior CO2 capture performance for CA10 milled for 4 h. Moreover, it is worth recognizing that more realistic testing conditions would be helpful to evaluate the performance of sorbents during practical applications. Tests to investigate the cyclic CO2 uptake of CA10-4h (ball-milled for 4 h) and CC at 900 °C in 40 vol % CO2 (the maximal CO2 concentration limited using TGA) were conducted. Their cyclic CO2 capture capacity change profiles are presented in Figure 13. After 25 cycles, CA10-4h calcined under 40 vol % CO2 displayed a relatively low CO2 capture capacity of 0.296 g/g compared to that calcined in pure N2 (0.371 g/g; Figure 10). This indicates that the existence of CO2 at the calcination stage causes inferior CO2 uptake for CA10-4h because of the accelerated sorbent

4. CONCLUSION The combination of planetary ball milling and BAh additions was carried out to modify naturally occurring limestone for high-temperature CO2 capture. It was found that the planetary ball-milling treatment is able to effectively improve the cyclic CO2 capture performance of naturally occurring limestone. The CO2 sorption durability of the ball-milled sorbents could be further enhanced with the addition of BAh. Moreover, better CO2 sorption performance could be obtained by increasing the BAh ratios or prolonging the milling time. The ball-milled limestone powder could be pelletized by the extrusion− spheronization method to prepare CaO-based pellets with relatively low attrition propensities. Although the pelletization of ball-milled limestone powders would lead to the reduction of the CO2 uptake, the pellets still possess a superior CO2 sorption performance to naturally occurring limestone powders. Therefore, the combination of planetary ball milling and BAh additions is an effective, simple, and environmentally friendly pre-modification method to enhance the CO2 sorption performance of naturally occurring limestone.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01131. Schematic diagram of the identification of the carbonation stage from the CO2 capture rate curve (Figure S1), pore size distributions of the calcined CC and CC-B (Figure S2), FSEM/EDX mapping images of mechanically modified sorbents (Figure S3), and laser particle size analysis of the CC and freshly ball-milled CA10 samples (Figure S4) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +86-027-87542417-8301. Fax: +86-27-87545526. E-mail: [email protected]. *Telephone: +86-027-87542417-8301. Fax: +86-27-87545526. E-mail: [email protected].

Figure 13. Cyclic CO2 capture capacity change profiles of CC and CA10-4h calcined under more realistic conditions. G


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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research is supported by the National Natural Science Foundation of China (51306063 and 21306059). The authors appreciate the Foundation of State Key Laboratory of Coal Combustion (FSKLCCB1602) and the Analytical and Testing Center at the Huazhong University of Science and Technology.

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