Synthesis of stabilized calcium looping absorbents for high-tem

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Environmental and Carbon Dioxide Issues

Demonstration of polymorphic spacing strategy against sintering: Synthesis of stabilized calcium looping absorbents for high-temperature CO sorption 2

Ming Zhao, Yinqiang Song, Guozhao Ji, and Xiao Zhao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00648 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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Demonstration of polymorphic spacing strategy against sintering: Synthesis of stabilized calcium looping absorbents for high-temperature CO2 sorption Ming Zhaoa,b, Yinqiang Songa,b, Guozhao Jia,b, Xiao Zhaoa,c,*

a

School of Environment, Tsinghua University, Beijing 100084, China.

b

Key Laboratory for Solid Waste Management and Environment Safety, Ministry of

Education, Beijing, 100084, China c

College of Water Resources & Civil Engineering, China Agricultural University,

Beijing 100083, China. * Corresponding author. Email: [email protected]

ABSTRACT. To decrease the sintering deterioration of CaO sorbents in multiple CO2-capture-release cycles, we synthesized a series of stabilized CaO sorbents incorporated with silica through freeze-drying and heat-drying, the latter of which was referred to as benchmark. The ratio of Ca and Si precursors was varied to control the reactive loadings of CaO (from 70 to 100 wt. %) and the fraction of spacers in the sorbents. The characterization results show that the freeze-drying method produces sorbents with higher specific area and larger pore volume than the heat-drying method. Moreover, the stability test of over 30 cycles demonstrated that the freeze-dried samples exhibited better performance with higher stability and total CO2 uptake. The HRTEM image shows that Ca2SiO4 crystallites as spacers are distributed within the matrix of CaO crystallites. The optimal spacer loading was determined to be ~10

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wt. % and the optimal reaction temperature was found to be 700 ºC. Finally, the best sorbent was tested under harsh conditions and it maintained a stable capture capacity with a CO2 uptake of 0.21 g CO2·g-1 sorbent even at 30th cycle. The performance of the sorbent in this work was then systematically compared with the ones reported in literatures. The use of Si-based spacer and freeze-drying have significant potential to enhance the stability of CaO sorbents.

1. Introduction The increasing concentration of atmospheric CO2 plays a key role in global warming, ocean acidification and sea level rise.1, 2 Carbon capture and storage (CCS) technologies have been proposed to stabilize the atmospheric CO2 level and the process of carbon capture is considered to account for 50-80 % of the total cost of CCS.3,

4

So far, post-combustion amine scrubbing using

amine-based solvents (usually monoethanolamine, MEA) is believed to be the maturest carbon capture technology.5, 6 However, it is still challenged on the account of its high energy and capital costs, solvent degradation and environmental issues.7-9 Recently, there has been an increasing number of studies investigating new solid sorbents, such as MgO,10 Na2ZrO311 and Li4SiO4.12 MgO sorbent is specially suitable for moderate-temperature CO2 capture but it suffers from low capture capacity due to rigid MgCO3 coating during carbonation.10 For Na2ZrO3 sorbent, it is vital to optimize synthesis parameters and thus promise better sodium diffusion, which would offset the deactivation caused by sintering problem.11 Despite benign sorption kinetics, Li4SiO4 is far from industrial application due to its low CO2 capture capacity.12 2


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As a potential alternative, calcium-looping (CaL) technology has attracted wide investigations with a number of demonstration projects throughout the world. It is considered to be a “second-generation” CCS technology,13 and potentially a cheaper option compared to the benchmark of MEA according to the prediction of a few techno-economic models.3, 14 CaL simply adopts reversible carbonation-calcination reactions of CaO-based sorbents. The limestone derived CaO sorbents bear several merits, including low cost, high theoretical capacity and sufficient mechanical strength.15 However, these sorbents are vulnerable to deactivation caused by sintering problem and pore blockage.16 The Tammann temperature (TTammann) of a material indicates the empirical temperature at which rapid sintering occurs. Although it is difficult for CaO (TTammann=1170 °C) to sinter during CaL processes (600-900 °C), the densification of CaCO3 occurs owing to its low Tammann temperature (TTammann=533 °C). To prevent the decay of sorbent capacity, inert metal oxides with high Tammann temperature were used as supports. Typical supports includes MgO,16 Al2O3,17 ZrO2,18 TiO2,19 and Y2O320 and enhanced stability was reported for most of the cases. Efficient sorbents over repeated cycles are still in urgent demand to ensure the economic viability of CaL technology. Moreover, CaL-plant has been proposed to be synchronized to the cement-plant, in which the spent sorbents can be used to make cement.21 The product in a typical cement clinker usually consists of 67% of CaO, 22% SiO2, 5% Al2O3, 3% Fe2O3 and 3 % others.22 To this end, the supports like MgO, ZrO2, TiO2, and Y2O3 are unwanted impurities to produce cement, while the use of SiO2 or small amount of Al2O3 exhibits prominent advantages as pointed out in our previous communication article.23 3


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Silicon is the second most abundant element in the earth surface and silica oxide is widely available.24 The application of SiO2 as support for CaO has been rarely reported, mainly due to its low Tammann temperature (TTammann=664 °C).23, 25, 26 However, after calcining the mixture of CaO and SiO2, a cement compatible material -Ca2SiO4 (namely CaO·SiO2, C2S) -can be formed, which holds a high Tammann temperature (TTammann=929 °C). More importantly, the polymorphic transition of Ca2SiO4 and its corresponding volumetric effect can refresh the pore structure of CaO and thus enhance the stability of sorbent dramatically. Belite (Ca2SiO4) itself is inert to carbonation/decarbonation.23 In our previous work,23 we have demonstrated the superiority of this polymorphic spacer over the “conventional” spacers. Yet, the previous work simply adopted costly TEOS as silicon source to synthesize sorbents with a fixed spacer fraction, and tested them merely under mild conditions. To validate the strategy of Ca2SiO4 as polymorphic spacer, more detailed work is still in need. As an extended research of that work, we committed to make a thorough investigation of Ca2SiO4 as a polymorphic spacer in this work from at least three perspectives: 1) exploring the feasibility of using a cheaper silicon source, i.e., Na2SiO3; 2) systematically investigating the effects of more parameters on the performance of sorbents, including the effects of spacer fractions and drying methods, and more detailed kinetics study was conducted; 3) testing the sorbents under much severer conditions to simulate realistic industrial scenarios.

2. Materials and methods

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2.1 Chemicals All chemical reagents used in this study are of analytical grade or higher. Sodium silicate (Na2SiO3·9H2O, 99%), calcium acetate (Ca(CH3COO)2·H2O, 99.5%) were obtained from Xilong Co. (Guangdong, China). All water utilized in the experiments was deionized water (18 MΩ·cm). Pure CO2 and N2 (Qianxi Co., China; 99.999%) were mixed to prepare simulated flue gas.

2.2 Sorbent synthesis To synthesize hydrated silica gel, 5 g of sodium silicate (Na2SiO3·9H2O) was dissolved into 100 ml of deionized water at 80 °C, and the solution was then subjected to CO2 bubbling (15 vol. % CO2 balanced with 85 vol. % N2 at a total flow rate of 80 ml min-1) for 3 h under constant stirring. The clear solution of Na2SiO3 turned to turbid due to the formation of hydrated silica. After cooling to room temperature, the hydrated silica was recovered by filtration and washing. The fresh hydrated silica gel mainly consists of SiO2·nH2O. To find out the silicon fraction, 1 g of the gel was heated at a rate of 10 °C·min-1 to 500 °C to remove the crystal water completely. The residual after calcination was regarded as pure SiO2 and then the silicon fraction can be obtained by calculation. Two g of fresh hydrated silica (SiO2·nH2O) as silicon precursor was added into 100 ml of deionized water under continuous stirring. Based on the obtained Si fraction, designated amount of calcium acetate (Ca(CH3COO)2·H2O) was added into mixture to control the ratio of Si and Ca precursors to achieve desired ratio of Ca2SiO4 and CaO. To make freeze-dried sorbents, half of mixture was pre-frozen to -78 °C, followed by an overnight drying process in a freeze dryer maintained at -50 °C and 0.1 mbar; to make heat-dried samples, the other half of the mixture was dried at 60 °C 5


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under constant stirring. Both types of dried powders were heated to 950 °C for 2 h (with a heating ramp of 10 °C·min-1) and then ground to get the sorbents. During the calcination, the crystalline belite (Ca2SiO4) can be formed through a solid-solid reaction which is more associated with the manufacture of cement clinker:27

H4SiO4 +2Ca(CH3COO)2 +8O2 → Ca 2SiO4 +8CO2 +8H2O

(1)

The synthetic sorbents were designated according to their synthesis process and active CaO fraction, e.g., FD90 for the freeze-dried sorbent comprising 90 wt. % of active CaO; HD100 for a heat-dried pure CaO.

2.3 Characterization Powder X-ray diffraction (XRD) was conducted on a Dmax/2500 X-ray diffractometer (Rigaku, Japan) with a 2θ range of 10- 90º (Cu Kα radiation, λ= 0.1542 nm). N2 physisorption was carried out on a Quantachrome autosorb iQ-C instrument. Before analysis, the sorbent was degassed at 300 °C until the pressure difference in the sample chamber was lower than 25 µmHg min-1. The Brunauer-Emmett-Teller (BET) surface area was estimated based on the N2 adsorption-desorption isotherms with a P/P0 range of 0.05-0.3. Total pore volume and pore size distribution were calculated from the desorption results according to the Barrett-Joyner-Halenda (BJH) method. Morphological studies of samples were carried out using scanning electron microscopy (SEM, Zeiss MERLIN VP Compact). All the sorbents for SEM were sputter-coated with a layer of platinum (~5 nm). To gain an insight of detailed microstructure of the sorbents, atomic lattice imaging and elemental mapping were carried out on a transmission electron microscope (TEM, JEM-2010F, Japan) with a 200 kV electron beam. All the samples for TEM were dispersed in ethanol first and then casted onto holey carbon films. Elemental mappings for Ca, O and Si were 6


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conducted using JEM-2010F fitted with an electron energy loss spectroscopy detector equipped with an Oxford energy-dispersive spectrometer (EDS).

2.4 Carbon capture test Cyclic carbonation/decarbonation tests were carried out to evaluate CO2 capture performance of the sorbents using a thermogravimetric analyzer (TGA, TA Instruments, TGA-Q50). In each test, ~ 20 mg of sample was first subjected to a precalcination at 850 °C in pure N2 (100 ml·min-1) for 5 min to remove the presorbed impurities. Then the chamber temperature was controlled to 700 °C under a stream of 15 vol. % CO2 balanced by N2 at 100 ml·min-1 for 20 min and then decarbonated at 850 °C in of a pure stream of N2 (100 ml·min-1) for 5 min. Next cycle was instantly started after the decarbonation process. The ramp between carbonation and decarbonation was maintained at 40 °C·min-1. To investigate the effect of operating conditions on performance of the sorbents, FD90 was selected as a representative and subjected to a series of tests. For the effect of carbonation temperature, FD90 was carbonated at 600 °C, 650°C, 700 °C, and 750 °C under otherwise identical conditions, respectively. For the effect of harsh or high temperature decarbonation conditions, FD90 was decarbonated under 100 ml min-1 pure CO2 or N2 at 920 °C for 5 min. Five key parameters of the sorbents, i.e., CO2 uptake (Ci, g-CO2·g-sorbent-1), total CO2 uptake in 30 cycles (CT, g-CO2·g-sorbent-1), CaO conversion (Xi, %), average capacity loss of each cycle (A, g-CO2·g-sorbent-1, cycle), and performance number (N), were calculated based on the continuous weight change during multiple carbonation/decarbonation cycles to evaluate their CO2 capture performance. These parameters were defined as follows: 7


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Ci =

mmax,i − mmin,i mmin,i

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(2)

30

CT = ∑ Ci

(3)

i =1

Xi =

(mmax, i − mmin, i ) × MCaO ×100% mmin, i ×η × MCO 2

A=

Cnmax − Cn ' n − n'

N=

Cnmax ' A

(4)

(5)

(6)

where mmax,i is the sample weight at the end of carbonation in cycle i, and mmin,i is the corresponding initial sample weight in the same cycle; η is the CaO weight fraction in a sorbent, and MCaO, MCO2 are the molecular weight of CaO and CO2, respectively; Cnmax is the maximum CO2 uptake over tested cycles and the ' corresponding cycle number is n’, Cn is the CO2 uptake of the nth cycle, in this case n equals to 30. The parameters A and N were proposed by Liu’s group for the first time28 to evaluate the CO2 capture performance of CaO-based sorbents over long cycles. The sorbents with lower A values and higher N values usually exhibit better sintering-resistant ability.

3. Results and discussion 3.1 Characterization of the sorbents Sorbents dried by two methods (HD as heat-drying; FD as freeze-drying) with various CaO fractions (from 70 to 100 wt. %) were examined by XRD and the patterns are 8


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given in Fig. 1. The presence of CaO (JCPDS No. 48-1467) can be confirmed in both types of sorbents (Fig. 1a and 1c). Some typical peaks referring to belite, Ca2SiO4 (JCPDS No. 33-0302), though they are weak peaks, can be identified in detailed XRD patterns as shown in Fig. 1b and 1d. Both types of sorbents contained a mixture of orthorhombic α and monoclinic β phase of Ca2SiO4. The results agreed well with our previous investigation and more detailed discussion on the transformation between two phases can be found in it.23 Evidently, no Ca2SiO4 detected in both HD100 and FD100, while sharp peaks for HD70 to HD90 and FD70 to FD90 confirm the formation of Ca2SiO4. The crystallite sizes for CaO estimated using Scherrer Equation with respect to the (hkl) Miller indices (111), (200) and (220) are tabulated in Table S1 (Supporting Information). For both type of pure CaO, namely HD100 and FD100, the crystallite sizes were larger than 100 nm, which is too high value to apply Scherrer Equation. With the introduction of Ca2SiO4 as support, the crystallite sizes decreased. In addition, higher amount of support induced smaller sizes. It is noteworthy that the freeze-dried samples hold smaller crystallite sizes compared to heat-dried samples under otherwise identical conditions. For example, the estimated CaO crystallite sizes of FD90 at phase (111), (200) and (220) are 60.9, 79.6 and 72.2 nm respectively, while the sizes of HD90 are 62.7, 80.6 and 77.6 nm, suggesting the freeze-drying may produce smaller sized CaO crystallite. Analysis by TEM of FD90 as a representative is shown in Fig. 2. The aggregates of particles in size ~200-400 nm can be seen in Fig. 2a and the elemental mapping implies homogeneous distribution of Ca, O and Si. The uniform distribution of Ca2SiO4 as spacer can mitigate the sintering of CaO in the calcination process effectively. Fig. 2b presents the high-resolution TEM (HRTEM) and the planes of CaO with d-spacing value of 0.3118, 0.2479, and 0.2537 nm can be assigned as (110), 9


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(002) and (002) planes, respectively. From the atomic lattice image, the planes of Ca2SiO4 (210) are identified with an interplanar distance of 0.2473 nm, again, confirming the presence of Ca2SiO4. Fig. 2c depicts a closer view of the atomic lattice, and it is evident that the Ca2SiO4 crystals are distributed within the matrix of CaO crystals.

Fig. 1 Powder XRD patterns of (a) and (b) heat-dried sorbents with CaO fraction varied from 70 to 100 wt. % (HD70 to HD100); (c) and (d) freeze-dried sorbents with CaO fraction varied from 70 to 100 wt. % (FD70 to FD100).

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Fig. 2 TEM images of FD90: (a) TEM image and associated elemental X-ray maps, (b) HRTEM image of atomic lattice indexed to CaO and Ca2SiO4 phases, and (c) a closer view of atomic lattice.

Next, we used SEM to observe the morphologies of heat-dried sorbents (as shown in Fig. 3) and freeze-dried sorbents (Fig. 4). The material morphology greatly depends on the synthesis method, i.e. drying processes in this work. In the heat-drying process, the solution evaporated from the liquid-gas interface, leaving a densified chunk formed due to the surface tension of the solution. However, in the course of freeze-drying, firstly, the dendritic ice crystals are formed during the pre-freezing, and then the ice can be sublimated directly into vapor at -50 °C under vacuuming condition. During the freeze-drying, the mobility of solution was limited so that agglomeration and phase segregation were lessened.29 Needle-like pores can be found for freeze-dried samples,23 and similar structure was observed in Fig. 4a-4f. For pure CaO, freeze-dried samples (FD100 as shown in Fig. 4d) exhibited more pores than the heat-dried ones (HD100 in Fig. 3d). For the stabilized samples, FD series samples seemed looser than the HD series. However, after 30 cycles, sintering effect induced enlarged particle size and closed pores were found for both types of sorbents.

Fig. 3 SEM images of heat-dried sorbents with CaO fraction varied from 70 to 100 11


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wt. % (HD70 to HD100). Magnification factor: ×20000 for (a)-(h) and ×5000 for the insets.

Fig. 4 SEM images of freeze-dried sorbents with CaO fraction varied from 70 to 100 wt. % (FD70 to FD100). Magnification factor: ×20000 for (a)-(h) and ×5000 for the insets.

Fig. 5 The specific BET surface area and pore volume of fresh synthetic sorbents.

Textural properties including the specific BET surface area and pore volume of fresh synthetic sorbents are shown in Fig. 5. For pure CaO, the specific surface area of freeze dried samples (FD100, 10.6 m2·g-1) was higher than that of the heat-dried ones 12


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(HD100, 7.1 m2·g-1). Freeze-drying also was found to produce CaO sorbents with larger pore volumes, i.e. the pore volume of FD100 and HD100 were 0.043 and 0.031 cm3·g-1, respectively. According to Fig. 5, the introduction of spacer (HD70-90 and FD70-90) can enlarge specific surface area and pore volume for both types of sorbents. Interestingly, the overall trend is that higher surface area and larger pore volume were found for sorbents with higher fraction of spacer. Moreover, higher surface area and larger pore volumes were recorded for freeze-dried samples than the heat-dried samples with identical spacer loadings. Detailed N2 physisorption isotherms and pore size distribution can be found in Fig. S1 for heat-dried samples and Fig. S2 for freeze-dried samples. As shown in Fig. S1d, the heat-dried CaO (HD100) did not present any hysteresis suggesting the insufficiency of mesopores of this sample. The results of pore size distribution also demonstrated that almost negligible pores with diameter of ~ 3 nm was detected for HD100 (Fig. S1e). However, for freeze-dried CaO (FD100), hysteresis was found between P/P0=0.5 and P/P0=0.8 as shown in Fig. S2d, and mesopores with diameter of ~ 3 nm was observed (Fig. S2e). No pores between 30-150 nm were observed for both HD100 and FD100. For both heat-dried and freeze-dried samples, spacer stabilized samples present typical irreversible Type IVa isotherms according to the 2015 IUPAC classification30 (as shown in Fig. S1a-d and Fig. S2a-d). The recorded hysteresis loops were Type H3, which are usually given by non-rigid aggregates of particles.30 For both types of sorbents, the addition of spacer induced the formation of some mesopores (with diameter ~ 3 nm and 30-50 nm) and more importantly, some more macropores (with diameter ~ 50-150 nm), which can provide reactive sites for carbonation.23

3.2 Effects of drying method and CaO fraction on CO2 capture behavior 13


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The effects of drying method and active CaO fraction on the cyclic sorption capacity and CaO conversion is presented in Fig. 6. For heat-dried samples as shown in Fig. 6b, it is evident that the CaO conversion rate of the sorbents followed the order of HD70 > HD80 > HD90 > HD100, suggesting that higher spacer fraction can improve the CaO conversion of the sorbents. However, high CaO conversion cannot guarantee high CO2 capture capacity over cycles. The overall long-term stability of a sorbent is not only dependent on the active CaO fraction, but also relying on the fraction of inert spacer to resist sintering. Therefore, the performance of a sorbent is the balance of these two factors and an optimal dosage of spacer is expected. As depicted in Fig. 6a, the CO2 uptake of these sorbents increased in the first ~5 cycles followed by gradual deactivation in the 5-30th cycles. A fast decay of capacity over cycles was found for pure heat-dried CaO (HD100), while the use of spacer could inhibit the capacity loss (HD70, HD80 and HD90) resulting in more stable performance in the 10+ cycles. The CO2 uptake of HD90 throughout 30 cycles overtook that of HD70 and HD80, suggesting the optimal loading of spacer is around 10 wt. %. The total CO2 uptake in 30 cycles (CT) is compared in the inset of Fig. 6a, and apparently, HD90 is the best sorbent among all the HD samples with a CT of 13.4 g CO2·g-1 sorbent.

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Fig. 6 The cyclic sorption capacity and CaO conversion of sorbents with varied CaO fractions: (a) net sorption capacity of heat-dried samples, (b) CaO conversion of heat-dried samples, (c) net sorption capacity of freeze-dried samples, and (d) CaO conversion of freeze-dried samples. The insets of (a) and (c) describes the total CO2 uptake in 30 cycles for all the samples. Experimental conditions: CO2 concentration of 15% at 700 ºC for 20 min for carbonation, and pure N2 at 850 ºC for 5 min for calcination.

For the freeze-dried samples, higher spacer loading could also enhance the CaO conversion as shown in Fig. 6d. Again, these sorbents experienced an improvement of reactivity in the initial 5 cycles at “self-activation” stage. HD90 was found to be the best sorbent with a CT of 14.0 g CO2·g-1 sorbent and selected for further study on the effects of other conditions. 15


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The results with regard to correlation between average capacity loss of each cycle (A, g CO2·g-1 sorbent, cycle) and performance number (N) were plotted in Fig. 7. Previous study suggests that the CaO sorbents with lower A values and higher N values usually exhibit better sintering-resistant ability.28 The values of A and N for HD100 were comparable to those of FD100, suggesting their similar CO2 capture performance over cycles. The introduction of spacer lowered the A values and increased the N values for both types of samples. Notably, lower A values and higher N values were found for freeze- dried samples than those of heat-dried samples with same spacer loadings, indicating that freeze-drying can produce more stable sorbents. Fig.7 displays evidence that synthesis method did influence the trend of CO2 capture capacity decay, even though the performance of HD70, HD80 and HD100 seemed similar in Fig. 6. Freeze-drying can produce dendritic ice crystals which can leave enlarged pores when solution sublimates away.23 As such, an interlinked pore structure and larger pore volume were produced which may have benefit with the diffusion of CO2 during the carbonation process.

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Fig. 7 Comparison of A (average capacity loss) and N (performance number) for different sorbents.

During the carbonation process, the CO2 transfers through grain boundary diffusion and surface diffusion. With increasing cycles, grain size can be enlarged and specific surface area can be reduced with less reactive sites available. Therefore, the interactions between sorbents and CO2 are changed from fast, surface-controlled reaction to slow, solid state diffusion-controlled process within the sorbent lattice.31 To mitigate this, we adopted the idea of phase transition of belite during the temperature swings of CaL as reported in our previous work.23 That work demonstrated that α’-C2S begins to form at temperature > 690 ºC whereas it can transform to β-C2S at temperature < 680 ºC, experiencing a ~ 2% of volumetric expansion. This volumetric expansion was larger than conventional thermal expansion of many other monomorphic spacers for several orders of magnitude, and thus the corresponding volumetric expansion force proved highly effective in resistance of sintering stresses.23, 32 17


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3.3 Effects of carbonation temperature on CO2 capture behavior Fig. 8 depicts the cyclic sorption behavior and CaO conversion of FD90 at different carbonation temperatures. As shown in Fig. 8a, low capacity and stable performance were observed at 600 and 650 ºC, while higher capacity was recorded at 700 ºC. Rapid deactivation and suppressed capacity were found at 750 ºC. The CaO conversion also exhibited the same trend (Fig. 8b).

Fig. 8 The (a) cyclic sorption capacity and (b) CaO conversion of FD90 under varied carbonation temperatures. Carbonation conditions: CO2 concentration of 15% at 600 to 750 ºC for 20 min; calcination condition: pure N2 at 850 ºC for 5 min.

The equilibrium of reaction between CaO and CO2 highly depends on temperature, and theoretically, decarbonation of CaCO3 would occur at temperature of 778 ºC under CO2 concentration of 15 %.33 Higher temperature can pose several effects on the interactions between CO2 and CaO-based sorbents. Positively, faster and more intensive carbonation reaction may occur at elevated temperature owing to more intensive contact between CO2 and CaO molecules at higher temperature.25 Hence, better performance was recorded at higher temperature in the range of 600 to 700 ºC. However, negative effects of high temperature include: 1) decarbonation 18


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becomes more predominant at higher temperature since the carbonation reaction is reversible; 2) intensified the sintering effect.27 Therefore, temperature of 750 ºC, which is the highest temperature tested in this work, might have caused the reaction equilibrium move to decarbonation direction to some extent, resulting in an inhibited capacity. In addition, the exaggerated sintering effect at 750 ºC induced a rapid capacity loss over cycles.

Fig. 9 The kinetic profile (a) and sorption reaction rate (b) of CO2 uptake of the 30th cycle at different temperatures. Carbonation conditions: CO2 concentration of 15% at 600 to 750 ºC for 20 min; calcination condition: pure N2 at 850 ºC for 5 min.

In order to interpret the CO2 sorption performance at different temperature, the kinetic profiles of CO2 uptake of the 30th cycle at different temperatures are plotted in Fig. 9. The reaction rate was generally a product of reaction coefficient and reaction driving force: n dα m  −E  = k0 exp   (1 − α ) ( C − Ceq ) dt  RT 

(7)

Where α is the sorbent conversion, k0 exp(-E/RT) represents the reaction coefficient and (1-α)m(C-Ceq)n stands for the driving force.34 As shown in Fig. 9, the CO2 sorption rate continuously increased with temperature increasing from 600 to 700 19


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ºC mainly due to the improved reaction coefficient at elevated temperatures. Whereas the initial sorption rate decreased significantly when further increasing the temperature from 700 to 750 ºC, suggesting that the reaction rate also depends on the driving force. The increment of equilibrium CO2 concentration Ceq decreased the driving force,35 thus leading to slower reaction. These findings indicate that 700 ºC is the optimum temperature for CO2 capture by FD90. 3.4 Effects of calcination conditions on CO2 capture behavior Fig. 10 presents the cyclic performance and CaO conversion of FD90 under different calcination conditions. In a pure N2 atmosphere, a higher carbonation temperature (920 ºC) suppressed the CO2 capture capacity of FD90 to 0.35 g CO2·g-1 sorbent after 30 cycles, which is much lower than the value obtained under mild conditions (0.41 g CO2·g-1 sorbent). In a pure CO2 stream at temperature of 920 ºC, the capacity was decreased further to only 0.21 g CO2·g-1 sorbent at cycle 30, implying the atmosphere during calcination plays an important role on capture capacity of sorbents.

Fig. 10 The (a) cyclic sorption capacity and (b) CaO conversion of FD90 under varied calcination conditions. Carbonation condition: CO2 concentration of 15% at 700 ºC for 20 min; calcination conditions: pure N2 at 850 ºC for 5 min for mild conditions, pure N2 or CO2 at 920 ºC for 5 min.

20


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In a real calciner of CaL, CO2 concentration > 90 % is required in order to obtain a high-purity stream of CO2 from the effluent, therefore temperature > 900 ºC is needed.36 Thus, we tested the temperature of 920 ºC with pure stream of N2 or CO2. The sintering effect is strongly enhanced at higher temperature and higher CO2 partial pressure. Although “over-carbonation” occurs as the gas switched to pure CO2,27 it is far from enough to offset the rapid deactivation of the sorbents. The high calcination temperature of 920 ºC is much higher than the Tammann temperature of CaCO3 (TTammann=524 °C), thereby CaCO3 in the calciner can be greatly influenced by sintering effect. The Tammann temperature of CaO is as high as 1170 ºC, yet the Tammann temperature of belite is 929 ºC, indicating partial or slow sintering is also possible in a calciner at 920 ºC.

3.5 Comparison with other Si-stabilized CaO sorbents Based on the CO2 capture capacity over cycles, the belite stabilized CaO in this work was compared with recently reported Si-stabilized CaO sorbents as tabulated in Table 1. We have been aware that it is a huge challenge to make direct comparisons, therefore we summarized the key information including synthesis conditions, carbonation/calcination conditions, CaO fraction, etc. The use of lab-scale conditions for developed new materials is quite common. Although highly stable capture performance of CaO-based sorbents over cycles were reported, their testing conditions have been criticized to be “unrealistic” or “too mild”.27 Thus, the use of harsh conditions, especially during the calcination stage is highly suggested. As presented in Table 1, most of the studies were conducted in TGA under mild calcination conditions,23, 37-42 and only three articles investigated the performance of sorbents under harsh conditions.25-27 Under mild conditions, the belite stabilized CaO 21


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in this work showed better stability and higher capacity than those reported in literature, under both temperature and pressure swings. A CaO sorbent was stabilized by quartz derived spacer40 and excellent performance over cycles were reported. Yet the tests were conducted under pressure swings, which is unrealistic for post-combustion capture. Additionally, even under harsh calcination conditions, the overall capacity over cycles of the sorbents in this work surpasses the other recently reported sorbents.

22


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Table 1. Summary of the CO2 capture performance of different SiO2 stabilized CaO sorbents over cycles.

Silica source/Ref. 37

Synthesis method

Aerosil-R974/ Husk ash/38 Na2SiO3/39 Nano SiO2/26

Dry mixing Wet mixing Wet mixing Dry mixing

Quartz/40 Quartz/41 SBA-15/42 SiCl4/25

Wet mixing Dry mixing Wet mixing Dry mixing

TEOS/23 TEOS/27

Freeze-drying Freeze-drying

Hydrated silica gel/ this work

Freeze-drying

CO2 uptake (g CO2·g-1 sorbent) C1 Cfinal 0.51 0.18 0.25 0.15 0.57 0.26 0.40 0.20 0.44 0.21 0.69 0.68 0.46 0.21 0.50 0.40 0.46 0.30 0.40 0.27 0.40 0.19 0.53 0.52 0.41 0.12 0.41 0.08

CaO fraction (wt. %) 79 47 90 90 90 90 65 68 90 90 90 70 70 70

T (°C) 650 700 650 700 700 700 800 700 700 700 700 650 650 650

t (min) 5 15 40 5 25 300 25 60 5 5 5 30 5 5

CO2 (%) 15 15 100 15 15 30 15 100 100 15 15 15 15 15

T (°C) 850 850 850 920 920 700 800 910 700 900 920 850 850 950

t (min) 5 20 35 3 5 30 15 30 3 3 3 0 1 1

CO2 (%) 0 0 0 100 100 0 0 0 0 0 100 0 100 100

100 50 50 30 30 4 13 40 30 30 30 15 15 15

90

700

20

15

850

5

0

30

0.49

0.41

90

700

20

15

920

5

100

30

0.48

0.21

Carbonation

Calcination

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Cycles

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4. Conclusions This study provides a systematic investigation on the effects of synthesis conditions on the performance of belite (Ca2SiO4) stabilized CaO sorbents. The major findings are summarized as follows: 1)

Sorbents with higher specific area and larger pore volume can be synthesized

through freeze-drying method than the ones obtained by heat-dried method. Additionally, the freeze-dried samples showed better performance over cycles; 2)

The optimal loading of belite as spacer was found to be 10 wt. % and the

sample FD90 exhibited a total CO2 capture capacity of 14.0 g CO2·g-1 sorbent in 30 cycles; 3)

The spacers were heterogeneously distributed in the sorbents and according

to the HRTEM image, the Ca2SiO4 crystallites are distributed within the matrix of CaO crystallites; 4)

The cyclic performance of FD90 is highly dependent on the carbonation

temperature and 700 ºC is the optimal carbonation temperature under 15 vol.% CO2 / 85 vol.% N2; 5)

Even under harsh calcination conditions, the sorbents in this work still

surpass the other recently reported sorbents, with a capacity of 0.21 g CO2·g-1 sorbent after 30 cycles.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (grant number: 51506112). X. Zhao is grateful for the support by General Financial Grant from the China Postdoctoral Science Foundation (grant number: 2016M601056). G. Ji 24


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is grateful for the financial support from China Postdoctoral Science Foundation (grant number: 2017M610910) and International Postdoctoral Exchange Fellowship Program. The helpful assistance from Abdul Raheem for proofreading is gratefully acknowledged.

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Synthesis of stabilized calcium looping absorbents for high-tem

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