Cyclic Performance of Waste-Derived SiO2 Stabilized, CaO-Based

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Cyclic performance of waste-derived SiO2 stabilized, CaO-based sorbents for fast CO2 capture Feng Yan, Jianguo Jiang, Kaimin Li, Sicong Tian, Zongwen Liu, Jeffrey Shi, Xuejing Chen, Jingyuan Fei, and Yuxiang Lu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01903 • Publication Date (Web): 30 Sep 2016 Downloaded from http://pubs.acs.org on October 3, 2016

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ACS Sustainable Chemistry & Engineering

Cyclic performance of waste-derived SiO2 stabilized, CaO-based sorbents for fast CO2 capture Feng Yan, a,d Jianguo Jiang,

a,b,c *

Kaimin Li, a Sicong Tian, a Zongwen Liu, d Jeffrey Shi, d Xuejing

Chen, a Jingyuan Fei, d and Yuxiang Lu d a

School of Environment, Tsinghua University, Beijing, China.

b

Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education, Beijing, China.

c

Collaborative Innovation Center for Regional Environmental Quality, Tsinghua University, Beijing, China.

d

School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006, Australia

*

Corresponding author:

Prof. Dr. Jianguo Jiang (School of Environment, Tsinghua University, Beijing 100084, China.) Tel./Fax.: +86 01062783548; E-mail address: [email protected]

ABSTRACT: Calcium-looping technology has been identified as one of the most favorable CO2 capture techniques for the implementation of carbon capture, utilization, and storage (CCUS); however, the rapid deactivation of CaO sorbents due to sintering is currently a major barrier of this technology. We report for the first time an environmentally benign and cost-effective strategy to reduce sintering by adding waste-derived nanosilica, synthesized from photovoltaic waste (SiCl4), into Cao-based sorbents through a simple dry mixing procedure. The as-synthesized sorbent (90%CaCO3-W) resulted in final CO2 uptake of 0.32 g(CO2) g(CaO)−1 within 5-min carbonation. Even under the most severe calcination conditions (at 920ºC in pure CO2), it still maintained a stable capture capacity, with CO2 uptake of 0.23 g(CO2) g(CaO)−1 after 30 cycles. Additionally, the CO2 uptake percentage reached ~90% in the fast carbonation stage (~20 s), which is of great significance for real applications. The most likely stabilization mechanism was considered on the basis of N2 physisorption isotherms and X-ray diffraction patterns. It was concluded that stable and refractory larnite (Ca2SiO4) particles were formed during 2-h thermal pretreatment at 900ºC, leading to sintering resistance. This strategy significantly enhanced the cyclic stability and carbonation rate of CaO-based sorbents through the reuse of SiCl4, and is thus a green technology for scaled-up fast CO2 capture. KEYWORDS: CO2 capture; CaO-based sorbent; Nanosilica; Photovoltaic waste; Refractory stabilizer

1

ACS Paragon Plus Environment


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INTRODUCTION The concentration of carbon dioxide (CO2) in the Earth’s atmosphere has increased by 40% since pre-industrial times and had reached 391 ppm by 2011. And atmospheric CO2 plays a major role in climate change and ocean acidification.1 A most promising strategy of CO2 capture, utilization, and storage (CCUS) has been proposed to mitigate the long-term impact of continuous CO2 emissions from fossil fuels.2 Generally, the process of CO2 capture accounts for 50–80% of the total cost of CCUS,3 thus, it is crucial to develop applicable CO2 capture methods that are both technically and economically feasible. Calcium-looping technology, based on the reversible carbonation-calcination reaction of CaO-based sorbents,4 has been identified as one of the most favorable CO2 capture techniques, due to its high theoretical capacity and low costs of CO2 avoidance.5 Additionally, this process has recently been investigated to assess its implementation in a diverse range of industrial applications, such as the enhancement of steam methane reforming via in-situ CO2 sorption,6 the transport of concentrated solar power,7 and the inherent energy storage for the decarbonization of coal-fired power plants.8 Although CaO sorbent naturally derived from limestone or dolomite is inexpensive and widely available,9 its sorption capacity declines rapidly to only 10% of the theoretical capacity after several cycles, due to serious sintering and attrition.10 Hence, different pre-treatments, such as hydration,11 thermal treatment,12 or acid modification,13 have been investigated to resist the decay of the sorption capacity. Even so, the modest CO2 uptake improvements that have been achieved imply that severe sintering still occurs on the unstable CaO structure,14 leading to a similar decay of limestone under realistic calcination conditions. 15 Currently, the most effective strategy is to incorporate CaO into inert materials, such as Al2O3, 16 - 18 MgO, 19 SiO2, 20 ZrO2, 21 Y2O3, 22 cement,23,24 and coal fly ash,25 which could achieve more stable composite- structures and higher cyclic CO2 uptake. Further efforts have been made to disperse the CaO particles uniformly through various wet-process methods, including carbon templating,16 co-precipitation,19 sol-gel,26 and wet mixing.23 Efficient sorbent materials, synthesized through facile and cost-effective processes, still need to be identified to ensure the economic viability of CO2 capture. Silica oxide is widely distributed in the earth’s surface and many different types of solid waste, and is thus a cheap and widely available inert material. Li et al.27 and Chen et al.28 showed that rice 2


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husk ash (containing 93 wt. % SiO2) could effectively enhance the cyclic CO2 uptake of synthetic sorbents, due to the formation of dicalcium silicate (Ca2SiO4), with a high Tammann temperature (TT). Valverde et al.29 and Sanchez-Jimenez et al.30 suggested that nanostructured silica could rapidly accelerate the carbonation rate and improve the mechanical strength of synthetic sorbents, resulting in almost a doubling of the residual capture capacity compared to the original sorbent in a short carbonation time (5 min). Recently, advanced synthesis methods, such as co-precipitation31 and sol-gel, 32 were also reported to synthesize Si/Ca-composite sorbents, and both achieved superior cyclic CO2 uptake (Table 2). Nevertheless, considering the limited residence time33 and the large-scale production demand in real project, the cost-effective synthesis of SiO2 stabilized, CaO-based sorbents with high cyclic sorption capacity and fast sorption/desorption kinetics remains a research priority. In our previous study, 34 nanosilica was synthesized from photovoltaic waste (silicon tetrachloride, SiCl4) using a low-temperature vapor-phase hydrolysis method, and a superior performance was achieved compared with commercial fumed silica. Considering the large specific surface area, the high thermal stability and the scalable production process, this waste-derived nanosilica might be an ideal source of inert material and could simultaneously enhance the carbonation rate. In this study, waste-derived SiO2 was first introduced into CaO-based sorbents via dry mixing for fast CO2 capture, and commercial precipitated SiO2, fumed SiO2, and mesoporous SiO2 were also considered for comparison. Cyclic carbonation-calcination tests with a short carbonation time (5 min) and a series of characterizations were performed, to investigate the effect of waste-derived SiO2 and particle size on cyclic CO2 uptake of the synthetic CaO-based sorbents. Additionally, the best synthetic sorbent was chosen to study the influence of carbonation and calcination conditions on cyclic CO2 uptake, which enabled comparison of the results with previous study. This facile and scalable synthesis process could both enhance the reuse of photovoltaic waste and mitigate the CO2 emission via fast CO2 capture.

EXPERIMENTAL SECTION Sorbent Preparation The waste-derived SiO2 was synthesized through the hydrolysis of SiCl4 (99 wt. %; LDK Solar Ltd., Xinyu, China) at 150ºC, and was collected and dried at 105ºC for 2 h. Three kinds of 3



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commercial nanosilica were considered for comparison, namely, precipitated SiO2 (Sipernat-306; Evonik, Essen, Germany), fumed SiO2 (S5130; Sigma-Aldrich, USA), and mesoporous SiO2 (MCM-41; JCNANO, Nanjing, China). Although CaO decomposed from organic acid calcium exhibited relatively high pore volumes and was a better calcium precursor,35 a common and inexpensive calcium carbonate (CaCO3; 99 wt. %; Sinopharm, Shanghai, China) was used for its higher carbonation rate.36 The CaCO3 and SiO2 powders were physically dry-mixed in an agate mortar for 10 min, and the mixture was then calcined in a muffle furnace at 900ºC for 2 h to obtain the synthetic sorbents. All sorbent samples were designated by their synthesis conditions, e.g., “90%CaCO3-W” for 90 wt. % CaO content in the sorbent, and the silica source of waste-derived SiO2. To investigate the effect of particle size, the CaCO3 powder and the waste-derived SiO2 powder were separately sieved to obtain particle sizes of 0–74 µm and 0–37 µm, and then “90%CaCO3-W (0–74 µm)” and “90%CaCO3-W (0–37 µm)” were synthesized through the same process as for “90%CaCO3-W”. Characterization The X-ray diffraction (XRD) patterns of the synthetic sorbents were recorded using a diffractometer (D/max 2500 PC; Rigaku, Tokyo, Japan) with Cu-Kα radiation, The diffraction data were recorded in the 2θ range of 10–90º with a scanning rate of 2º min−1. The particle size distributions of the raw materials and the synthetic sorbents were examined using a dry powder laser particle sizer (LS 13320; Beckman Coulter, USA). N2 adsorption-desorption isotherms of particles at 77 K were collected on a gas adsorption analyzer (ASAP2020; Micrometrics Instrument, USA). All samples were degassed at 90ºC for 1 h and at 350ºC for 4 h in a vacuum atmosphere. Specific

surface

area

was

determined

by

the

N2

adsorption

branch

using

the

Brunauer-Emmett-Teller (BET) equation over the P/P0 range of 0.05–0.25. Total pore volume was calculated from the amount of gas adsorbed at P/P0 = 0.99, and micropore volume was calculated by the t-plot method. Average pore size and pore distribution were derived from the N2 desorption branch using the Barrett-Joyner-Halenda (BJH) method. Cyclic CO2 Capture Cyclic CO2 capture tests were performed on thermal gravimetric analyzer (TGA; TGA/DSC 2; Mettler-Toledo, Switzerland) with a high-sensitivity balance (< 0.1 µg). The furnace temperature, 4


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the gas flow and the sample weight were automatically controlled and recorded using Mettler STARe software (version 13.0). First, the synthetic sorbent (8–10 mg) was preheated (30°C min−1) to the carbonation temperature (700ºC) under an N2 flow of 60 mL min−1. Once the carbonation temperature was reached, the sorbent was subjected to a 100 vol. % CO2 flow of 60 mL min−1 for 5 min. Next, the calcination was conducted by heating (heating rate 100°C min−1) and keeping the sorbent at 900°C for 3 min under an N2 flow of 60 mL min−1. After calcination, a new cycle was started when the temperature was decreased (cooling rate 100°C min−1) to 700ºC and the atmosphere was changed back to a 100 vol. % CO2 flow of 60 mL min−1. Additionally, a constant N2 flow of 20 mL min−1 was maintained as a protective flow over the microbalance. The “90%CaCO3-W” sample was used as an example to investigate the effect of cyclic sorption conditions. The carbonation temperature was changed to either 700 or 750ºC and the carbonation atmosphere was changed to either a 15 or 100 vol. % CO2 flow of 60 mL min-1, to determine the effect of carbonation temperature and CO2 partial pressure. The calcination was performed either at 700°C under an N2 flow of 60 mL min-1 or at 920°C under a 100 vol. % CO2 flow of 60 mL min-1. The carbonation-calcination cycle was repeated 30 times for each sorbent, and the cyclic CO2 uptake of the sorbents [X, g(CO2) g(sorbent)−1] was calculated on the basis of continuous weight changes: X N = ( m1N − m2 N ) m0 , where m0, m1N and m2N (mg) refer to the initial sorbent mass and the sorbent weight after N cycles of carbonation and calcination, respectively.

RESULTS AND DISCUSSION Effect of Waste-derived SiO2 Addition Figure 1-(a) plotted the CO2 uptake in 30 cycles for the synthetic sorbents prepared with different SiO2/CaO ratios. Although Chen et al. noted that a longer carbonation time resulted in higher CO2 uptake for CaO-based sorbent,28 lengthening the carbonation time is not feasible for practical applications. Thus, we limited the carbonation time to 5 min in this study, considering the short residence time in a fluidized bed reactor.33 The conventional sorbent (100%CaCO3) possessed a high initial CO2 uptake of 0.55 g(CO2) g(sorbent)−1, but rapidly lost 82% of its capture capacity after 30 cycles, which was in agreement with the previous study.37 Sanchez-Jimenez et al also tested the cyclic performance of limestone under carbonation atmosphere of 15 vol.% CO2,30 which possessed an initial CO2 uptake of 0.58 g(CO2) g(sorbent)−1 and lost 75% of its capture capacity 5



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after 30 cycles. The results revealed that reducing the CO2 partial pressure would mitigate the decay of CO2 uptake. With regard to the synthetic sorbents, “90%CaCO3-W” exhibited significantly slower decay (1.15% per cycle) in CO2 uptake with increasing cycle number, and possessed a CO2 uptake of 0.30 g(CO2) g(sorbent)−1 after 30 cycles. This represented a marked increase of 205% compared with that of “100%CaCO3”. The stability enhancement of synthetic sorbents could be explained by the 2-h thermal pretreatment and effective separation of CaO particles by the inert material. Manovic et al. reported that thermal pretreatment (800–1300°C) would decrease the initial CO2 uptake of CaO-based sorbent but increase the CO2 uptake at longer cycles.12 This favorable influence could be explained using the pore-skeleton model:12 The pore-skeleton would change and form a new inward (hard) skeleton during the pretreatment, which could protect the morphology of particles and enhance the long-term CO2 uptake. Another reason for the stability could be a new crystalline phase, larnite (Ca2SiO4), which was detected from the XRD pattern of “90%CaCO3-W” (Figure 2), via the diffraction peaks at 2θ = 32.05, 32.14, and 32.59° for (-3 0 1), (-1 2 1), and (0 0 2) reflections, respectively (JCPDS 33-0302). Larnite, formed by the reaction of CaCO3 and SiO2, generally exhibits good chemical durability as well as a high Tamman temperature of 929°C (Table S1),33 and may act as an ideal inert support to prevent the sintering of CaO-based sorbent. Therefore, the synthetic sorbents could maintain relatively high CO2 uptake over multiple carbonation-calcination cycles. Additionally, it was necessary to study the optimal SiO2/CaO ratio for the synthetic sorbents, because the sorbent will sacrifice its active CaO component above the ratio,38 but will lose its sintering resistance below the ratio. 39 When the SiO2/CaO ratio was increased to 15:85, “85%CaCO3-W” exhibited lower cyclic CO2 uptake and also a slower decay (1.09% per cycle), namely, 0.39 and 0.27 g(CO2) g(sorbent)−1 for the 1st and 30th cycles, respectively. This was due to (i) lower proportion of active CaO in the initial mixture and (ii) higher consumption of active CaO in larnite formation. When the SiO2/CaO ratio was decreased to 5:95, “95%CaCO3-W” exhibited higher initial CO2 uptake of 0.49 g(CO2) g(sorbent)−1, but rapider decay of 38.63% after 30 cycles, indicating a limited stabilization effect of larnite. The XRD results also confirmed that the reflections of larnite were rarely observed in the pattern of “95%CaCO3-W”. It was notable that the diffraction peaks of cristobalite (SiO2, JCPDS 76-0935) existed in all synthetic sorbents, implying 6


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that the reaction between CaCO3 and SiO2 did not fully complete after thermal pretreatment. Despite this, the formation of larnite continued throughout the cyclic carbonation-calcination tests, as Li et al. certified.31 Considering the cyclic CO2 uptake and the long-term stability, the optimal SiO2/CaO ratio (around 10:90) coincided well with the results of previous studies31,32 and the theoretical value predicted by the three-dimensional percolation theory.40 Three kinds of commercial nanosilica were compared, and the detailed N2 physisorption isotherms and pore size distributions of the nanosilica and synthetic sorbents were presented in Figures S1 and S2. Waste-derived SiO2, precipitated SiO2 and fumed SiO2 all possessed type II N2 physisorption isotherms with an H3 hysteresis loop, while mesoporous SiO2 exhibited typical type IV N2 physisorption isotherms and had a narrow pore size distribution centered at 3.0–3.5 nm. Figure 1-(b) showed the cyclic performance of synthetic sorbents prepared from different silica sources. Because the synthetic sorbents have been thermally pretreated for 2 h and physically doped with the inert material (larnite), they all possessed a better cyclic stability than “100%CaCO3”. The cyclic stability and the final CO2 uptake of synthetic sorbents were both ranked in the same order as the average pore size (Table S2): “90%CaCO3-W” > “90%CaCO3-P” > “90%CaCO3-M” > “90%CaCO3-F”. Although “90%CaCO3-W” (7.46 m2 g−1) had a slightly smaller surface area than “90%CaCO3-M” (8.36 m2 g−1), the larger pore size could provide an easier diffusion channel for CO2.41 Lu et al. also suggested that the dominant factor causing the deterioration of carbonation could sometimes be the blockage and collapse of pore structure rather than the sintering effect.42

Figure 1. CO2 uptake as a function of cycle for (a) synthetic sorbents prepared with the indicated SiO2/CaO ratios, and (b) synthetic sorbents prepared from different silica sources. Conditions: carbonation in 100 vol. % CO2 at 700°C for 5 min; calcination in 100 vol. % N2 at 900°C for 3 min. 7



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Figure 2. X-ray diffraction patterns of synthetic sorbents prepared with the indicated SiO2/CaO ratios.

Effect of Particle Size The particle size of CaCO3 powder was mainly distributed in the range of 12–48 µm with an average size of 29 µm, while the waste-derived SiO2 powder exhibited a larger average size of 127 µm (Figure S3). This difference in particle size may result in insufficient contact between CaCO3 and SiO2, which would further reduce the stabilizing effect of larnite. The particle size distribution curve of “90%CaCO3-W” (Figure 3) showed two separate peaks in the range of 10–70 µm and 70– 180 µm, which were associated with CaCO3 particles and SiO2 particles, respectively. A smaller particle size offers more potential to incorporate inert material into the CaCO3 powder matrix via the dry mixing method, enabling the sintering to be avoided at a more microscopic scale. Thus, the raw materials were sieved separately to study the effect of particle size on the cyclic CO2 uptake. The particle size distributions of both “90%CaCO3-W (0-74 µm)” and “90%CaCO3-W (0-37 µm)” showed only one peak concentrated in the ranges of 13–73 µm and 5–35 µm, respectively. Figure 4 plotted the CO2 uptake of synthetic sorbents of different particle sizes over 30 cycles of carbonation-calcination. The initial CO2 uptakes of synthetic sorbents were 0.46, 0.43 and 0.41 g(CO2) g(sorbent)-1 for “90%CaCO3-W”, “90%CaCO3-W (0-74 µm)” and “90%CaCO3-W (0-37 µm)”, respectively, with the CO2 uptake declining as the particle size decreased. The decrease in the initial CO2 uptake may be due to the greater consumption of CaO to form the inert larnite, because the smaller particle size leads to an easier reaction. However, the sorption stability of synthetic sorbents was enhanced as the particle size decreased, leading to slower decay of 0.70% per cycle 8


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and higher final CO2 uptake of 0.32 g(CO2) g(sorbent)−1 for “90%CaCO3-W (0-37 µm)”. As we expected, this result confirmed that more microscopic scale mixing would achieve a more uniform mix between particles and higher stability of SiO2 stabilized, CaO-based sorbents. On the other hand, an appropriate particle size is also important to avoid the attrition in fluidized bed reactors, namely, particle with average size > 90 µm will be required for practical applications of calcium looping.43 Nevertheless, Sanchez-Jimenez et al. reported that the formation of calcium silicate and a nanostructured silica matrix would enhance the mechanical strength of sorbents.30 Li et al. suggested that palletization by gelatin would not affect the cyclic CO2 sorption performance of CaCO3/mesoporous silica composite sorbents (CaCO3@mSiO2).31 Thus, the waste-derived SiO2 stabilized, CaO-based sorbents may likely have potential for scalable applications in fluidized bed reactors.

Figure 3. Particle size distribution curves of synthetic sorbents of different particle sizes.

9



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Figure 4. CO2 uptake as a function of cycle for synthetic sorbents of different particle sizes. Operational conditions were the same as in Figure 1.

As well discussed in the literature,44 the carbonation of CaO-based sorbents proceeds along two well-differentiated stages: (i) an initial fast carbonation stage controlled by surface reaction, associated with the filling of pores; and (ii) a subsequent significantly slower carbonation stage controlled by diffusion, in which a layer of CaCO3 is deposited on the outer surface of the particles (Figure S4). However, CO2 capture is limited in practice by the short residence time, which mainly constrains capture to the fast carbonation stage.33 Thus, CO2 uptake in the fast carbonation stage is a more important indicator for CaO-based sorbents, which is critically affected by the surface area available for fast carbonation.45 To better evaluate this key indicator, the duration (∆tN, s) and the CO2 uptake percentage (ηN, %) in the fast carbonation stage were calculated based on Figure S4 and were summarized in Table 1. The fast carbonation stage was generally shortened from ~50 s to ~20 s after the first cycle, since the pore size of sorbents was enlarged during the carbonation-calcination process,25 which made surface reactions easier. For the CO2 uptake percentage, the ηN of “100%CaCO3” initially reached 73.08%, but rapidly decreased to 46.36–52.13% since the 2nd cycle, because the CaO particles would sharply lose their available surface area under serious sintering.10 In contrast, the ηN of “90%CaCO3-W” gradually increased over the multiple cycles and reached 87.04% in the 30th cycle. When the particle size was decreased, “90%CaCO3-W (0-37 µm)” possessed an even higher η1 of 84.29% and could eventually achieve 90.97% of the total CO2 uptake within the fast carbonation stage (∆t30 ≈ 20 s). Therefore, owing to the nanostructure effect and the sintering resistance, the presence of nano-SiO2 could substantially enhance the CO2 uptake percentage during the fast 10


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carbonation, which would be of great significance for the development of calcium looping technology. Table 1. The duration and the CO2 uptake percentage of the fast carbonation stage for sorbents of different particle sizes during the 1st, 2nd, 15th, and 30th cycles 2nd cycle

1st cycle

30th cycle

∆t1 a

η1 b

∆t2

η2

∆t15

η15

∆t30

η30

(s)

(%)

(s)

(%)

(s)

(%)

(s)

(%)

100%CaCO3

50

73.08

20

52.13

20

48.41

20

46.36

90%CaCO3-W

50

66.17

20

83.13

20

86.79

20

87.04

90%CaCO3-W (0-74 µm)

50

74.06

20

87.46

20

88.19

20

88.52

90%CaCO3-W (0-37 µm)

50

84.29

20

88.87

20

90.61

20

90.97

Samples

a

15th cycle

The duration of the fast carbonation stage during the Nth cycle, ∆tN (s). b The CO2 uptake percentage of the fast

carbonation stage during the Nth cycle (ηN, %), η N = X FN X N × 100% , where XFN and XN refer to the CO2 uptake of the fast carbonation stage and the whole carbonation process, respectively.

Effect of Carbonation-calcination Conditions On the basis of the equilibrium partial pressure of CO2 over CaO (Figure S5),46 the carbonation temperature and CO2 partial pressure play important roles on sorption capacity of CaO-based sorbents; e.g., a theoretical highest temperature of 778°C should not be exceeded for a 15 vol. % CO2 flow. Figure 5-(a) showed the cyclic performance of “90%CaCO3-W” under different carbonation conditions. In a pure CO2 atmosphere, a higher carbonation temperature of 750°C accelerated the reaction rate and enhanced the CO2 uptake in the initial cycles, but resulted in a lower CO2 uptake of 0.28 g(CO2) g(sorbent)−1 after 30 cycles, indicating that increasing the carbonation temperature would intensify the sintering of sorbents. When the CO2 partial pressure was lowered to a realistic concentration in flue gas (~15 vol. %),47 the cyclic CO2 uptake at a carbonation temperature of 700°C was significantly higher than that at 750°C, which was in accordance with the previous conclusion that the optimal carbonation temperature was 660–710°C for cyclic CO2 capture in 15 vol. % CO2.27 Besides, the cyclic stability was slightly enhanced in 15 vol. % CO2, namely, “90%CaCO3-W” possessed an initial CO2 uptake of 0.40 g(CO2) g(sorbent)−1 and a decay of 1.05% per cycle at 700°C. Manovic et al. also argued that reducing the CO2 partial 11



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pressure could mitigate the sintering, and thus would result in better long-term performance of sorbents.23 In a real calcium looping process, higher CO2 concentration (> 90 vol. %) is required to produce concentrated CO2 streams; thus, the minimum temperature for a calcination reaction should be at least 900°C.46 In this study, a temperature of 920°C was selected for a calcination atmosphere composed of pure CO2, since Ridha et al. suggested that it was more adequate for calcination than 900°C.48 As expected, the initial CO2 uptake was independent of calcination conditions; however, the CO2 uptake decreased more rapidly (decay rate of 1.60% per cycle) in subsequent cycles, when the calcination atmosphere was changed from pure N2 to pure CO2 (Figure 5). Nevertheless, “90%CaCO3-W” still possessed a higher CO2 uptake of 0.23 g(CO2) g(sorbent)−1 after 30 cycles, compared with the coal fly ash stabilized, CaO-based sorbent [0.21 g(CO2) g(sorbent)−1] under such realistic calcination conditions.36 When “90%CaCO3-W” was tested under both realistic carbonation and calcination conditions, it exhibited a little faster decay of CO2 uptakes under such severe conditions, i.e., a CO2 uptake of 0.40 and 0.19 g(CO2) g(sorbent)−1 for the first and 30th cycles, respectively. Although the “recarbonation” process in a calcination atmosphere of pure CO2 increased the cyclic capacity as is reported,49 it is far from offsetting the rapider decay of CO2 uptake caused by more serious sintering.50 This serious sintering is mainly caused by the CaCO3 phase (Tammann temperature = 524°C) exposed at ~920°C, which is more readily sintered than CaO (Tammann temperature = 1170°C). In contrast, the sorbents decarbonate as soon as the atmosphere is changed to pure N2, and already exist in the CaO phase when the temperature reaches ~900°C; thus, the sintering of sorbents would be significantly mitigated under mild calcination conditions. Because sorbents would decarbonate to the CaO phase as soon as the atmosphere was changed, a similar trend in cyclic CO2 uptake was observed between the calcination temperatures of 700 and 900°C, leading to final CO2 uptakes of 0.32 and 0.30 g(CO2) g(sorbent)−1, respectively, indicating that the net effect of calcination temperature (in the range of 700–900°C) was negligible. The CO2 uptake of CaO-based sorbents is significantly influenced by the calcium precursor, namely, CaO derived from calcium organic acid usually had a higher surface area and pore volume, and thus higher CO2 uptake.35 However, the CO2 uptake in the fast carbonation stage is the most crucial phase as discussed above; thus, we insisted to use the cheap and abundant CaCO3 as the 12


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calcium precursor in this study. It was found that a new phase (larnite) formed during pretreatment and mitigated CaO crystal grains from sintering. The as-synthesized sorbent, “90%CaCO3-W”, possessed a CO2 uptake of of 0.32 g(CO2) g(CaO)−1 within 5-min carbonation after 30 cycles and a low average deactivation rate of 1.03% per cycle. Even under the realistic carbonation and calcination conditions, it still maintained a stable capture capacity, with a final CO2 uptake of 0.19 g(CO2) g(CaO)−1. Considering the limited carbonation time and the superior performance under realistic conditions, the as-synthesized sorbents had comparatively high CO2 uptake and good cyclic stability compared with the other SiO2 stabilized, CaO-based sorbents (Table 2). Although it is necessary to further study the pelletizing of sorbents and the cyclic performance in fluidized bed reactors in the future, these results provide a facile and cost-effective approach to scaled-up high temperature CO2 capture, with the simultaneous reuse of photovoltaic waste.

Figure 5. (a) CO2 uptake as a function of cycle for “90%CaCO3-W” under different carbonation temperatures and CO2 partial pressures (carbonation for 5 min and calcination in 100 vol. % N2 at 900°C for 3 min); (b) under different calcination conditions (carbonation in 100 vol. % CO2 at 700°C for 5 min and calcination for 3 min) and realistic conditions (carbonation in 15 vol. % CO2 at 700°C for 5 min and calcination in 100 vol. % CO2 at 920°C for 3 min).

13



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Table 2. Synthetic SiO2 stabilized, CaO-based sorbents: Effectiveness and stability over cyclic CO2 capture Carbonation

CaO Silica source

content (wt. %)

90 SiCl4

g, k,

t

CO2

T

t

CO2

(ºC) (min) (%)

(ºC) (min) (%)

700

700

5

100

CO2 uptake

N, Cycles

X1

Xresidue

a

g(CO2) g(sorbent)-1

Loss b (%)

3

0c

30

0.46

0.32

1.03

c

30

0.46

0.30

1.15

30

0.46

0.23

1.63

30

0.40

0.27

1.05

90

700

5

100

900

3

0

90

700

5

100

920

3

100

5

15

c

900

3

0

c

920

3

100

30

0.40

0.19

1.71

90

700

c

90

700

5

15

Quartz h, l, 42

90

700

300

30 d

700

30

0d

4

0.69

0.67

0.97

Quartz g, k,

65

800

25

15 c

800

15

0c

13

0.46

0.21

4.19

47

700

15

15 c

850

20

0c

50

0.25

0.15

0.78

76

700

25

15 c, f 950

10

0c

10

0.52

0.44

1.40

c

10

0.42

0.39

0.70

51

Husk ash i, l, 27 Husk ash i, l, 28

a

T

Calcination

76

700

10

15

SBA-15 h, l, 20

68

700

60

Aerosil-R974 i, k, o, 29

79

650

Aerosil-300 j, l, o, 30

79

Na2SiO3 g, m, 31 TEOS h, n, 32

c, f

950

5

0

100

910

30

0c

40

0.50

0.40

0.51

5

15 e

850

5

0e

100

0.51

0.18

0.65

650

5

15 e

850

5

0e

25

0.093

0.087

0.27

90

650

40

100

850

35

0c

50

0.57

0.26

1.08

89

650

30

15 c

850

10

0c

15

0.53

0.52

0.10

X1 and Xresidue refer to the CO2 uptake in the first and last cycles, respectively.

b

The average change in CO2

c

uptake capacity per cycle: (1-Xresidue/X1)/N*100%. The concentration of CO2 in the atmosphere was adjusted by N2.

d

The concentration of CO2 in the atmosphere was adjusted by He.

atmosphere was adjusted by air.

f

e

The concentration of CO2 in the

The pressure of the carbonator was enhanced to 0.5 MPa.

h

i

g

The calcium j

precursor was CaCO3. The calcium precursor was Ca(Ac)2. The calcium precursor was Ca(OH)2. The calcium precursor was Ca(NO3)2. k Synthesis by dry mixing. l Synthesis by wet mixing. m Synthesis by co-precipitation. n

Synthesis by sol-gel.

o

Aerosil-R974 is hydrophobic fumed silica and Aerosil-300 is hydrophilic fumed silica,

which were supplied by Evonik Industries (Essen, Germany).

ASSOCIATED CONTENT Supporting Information Characterization of raw materials and synthetic sorbents, including particle size distributions, carbonation kinetics, N2 physisorption isotherms, pore size distributions and structural parameters; Tammann temperature of typical minerals. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION 14


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Corresponding Author * Prof. Dr. Jianguo Jiang. Tel. / Fax: +8610-62783548. E-mail: [email protected]. Present Addresses * School of Environment, Tsinghua University, Beijing 100084, China. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (grant no. 21576156) and the Tsinghua University Initiative Scientific Research Program (grant no. 2014z22075).

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For Table of Contents Use Only Title: Cyclic performance of waste-derived SiO2 stabilized, CaO-based sorbents for fast CO2 capture Feng Yan, Jianguo Jiang, Kaimin Li, Sicong Tian, Zongwen Liu, Jeffrey Shi, Xuejing Chen, Jingyuan Fei, and Yuxiang Lu Brief synopsis: This strategy significantly enhances the cyclic CO2 uptake and sorption rate of CaO-based sorbents through reuse of photovoltaic waste, and thus provides a facile and cost-effective approach to scaled-up fast CO2 capture.

Table of Contents (TOC)

19



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TOC 47x26mm (300 x 300 DPI)


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Figure 1. CO2 uptake as a function of cycle for (a) synthetic sorbents prepared with the indicated SiO2/CaO ratios, and (b) synthetic sorbents prepared from different silica sources. Conditions: carbonation in 100 vol. % CO2 at 700°C for 5 min; calcination in 100 vol. % N2 at 900°C for 3 min. 199x70mm (300 x 300 DPI)



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Figure 2. X-ray diffraction patterns of synthetic sorbents prepared with the indicated SiO2/CaO ratios. 99x70mm (300 x 300 DPI)


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Figure 3. Particle size distribution curves of synthetic sorbents of different particle sizes. 99x112mm (300 x 300 DPI)



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Figure 4. CO2 uptake as a function of cycle for synthetic sorbents of different particle sizes. Operational conditions were the same as in Figure 1. 99x70mm (300 x 300 DPI)


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Figure 5. (a) CO2 uptake as a function of cycle for “90%CaCO3-W” under different carbonation temperatures and CO2 partial pressures (carbonation for 5 min and calcination in 100 vol. % N2 at 900°C for 3 min); (b) under different calcination conditions (carbonation in 100 vol. % CO2 at 700°C for 5 min and calcination for 3 min) and realistic conditions (carbonation in 15 vol. % CO2 at 700°C for 5 min and calcination in 100 vol. % CO2 at 920°C for 3 min). 199x70mm (300 x 300 DPI)


Cyclic Performance of Waste-Derived SiO2 Stabilized, CaO-Based

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