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Effects of Drying Methods on Wet Chemistry Synthesis of AlStabilized CaO Sorbents for Cyclic CO2 Capture Yinqiang Song,†,§ Guozhao Ji,†,§ Xiao Zhao,† Xu He,† Xiaomin Cui,† and Ming Zhao*,†,‡ †

School of Environment, Tsinghua University, Beijing 100084, China Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education, Beijing, 100084, China

‡

S Supporting Information *

ABSTRACT: Al-stabilized CaO sorbents synthesized by wet chemistry methods have demonstrated effectiveness to mitigate CaO sintering during Ca-looping cycles (CaO + CO2 ⇌ CaCO3) for CO2 capture. To further screen the synthesis techniques and recipes, a series of Al-stabilized CaO sorbents, namely, CaO−Ca9Al6O18 hybrid materials, derived from cosolutions of calcium acetate and aluminum nitrate were prepared using three different drying methods, i.e., freeze drying, spray drying, and evaporation drying. These sorbents were then characterized by X-ray diffraction, N2 physisorption, scanning electron microscopy, and energy dispersive spectrometry. The effects of drying methods on the CO2 capture performance of the sorbents were analyzed comprehensively. Out of the three drying methods, spray drying enabled the optimal textural property and the hard skeleton with sufficient mechanical strength, resulting in the supreme CO2 capture capacity. Furthermore, it was found that, by spray drying, the inert spacer Ca9Al6O18 could play the most significant role in stabilizing the cyclic sorption reactivity of CaO. For spray dried samples, the SD70 sample with 70 wt % CaO and 30 wt % Ca9Al6O18 could well balance the capacity and stability under mild conditions. Its advantage was much more pronounced under severe conditions, where SD70 overtook other samples in CO2 uptake capacity from the fourth cycle and maintained the highest CaO conversion all through the 30 cycles.

1. INTRODUCTION As a promising strategy to mitigate global warming, carbon capture and storage (CCS) technologies have attracted increasing attention for both laboratory and industrial scales. Currently, CCS technologies can be realized by three pathways: precombustion, postcombustion, and oxy-fuel combustion, where postcombustion capture (PCC) technologies are the easiest to retrofit the extant power-generation infrastructure.1,2 To date, a wide range of technologies have been applied for implementation of CCS such as amine scrubbing, chilled ammonia, alkali-metal carbonates, membranes, and calcium looping. Despite that amine scrubbing is likely the most mature technology for industrial applications, calcium looping has attracted more attention for bench- or pilot-scale investigations for its greater potential to lower the CCS cost toward the market acceptance level.3 Natural CaO-based sorbents derived from limestone or dolomite have captured CO2 effectively even with quite low CO2 partial pressures.4−6 However, these sorbents decay rapidly over multiple cycles due to sintering of sorbent particles. To address this major challenge, researchers are highly motivated to develop an effective strategy for sintering resistance, including but not limited to hydration treatment of CaO,7−12 thermal pretreatment of sorbents,13−15 modification of materials,16−18 and incorporation of inert support materials.19−23 Compared to other approaches, using inert refractory particles as the stabilizer seems more popular in recent studies due to the significant sintering mitigation capacity. Such “spacer” materials include Al 2 O 3 , 21,22,24−26 MgO, 20,27 SiO2,23,28,29 ZrO2,20,30 TiO2,23,31 Y2O3,20 etc. From the economic point of view, the addition of new components increases the synthesis cost of sorbents. This increment of cost, © XXXX American Chemical Society

however, might be offset or even surpassed by the gains from the enhanced stability. Therefore, a relatively cheap spacer and facile recipe are pursued. Al-based spacer materials (Al2O3, Ca12Al14O33, Ca9Al6O18, etc.) have been regarded as promising candidates. Feng et al.32 prepared a CaO-based sorbent stabilized by Al2O3 with a wet impregnation method, and because of low active CaO content in the sorbent, the CO2 capacity was sacrificed too much. In a similar manner, Zhang et al.24 prepared a CaO−Al2O3 sorbent with a four-step heating mode instead of the traditional one-step calcination procedure, which was believed to favor the formation of porous structure and thus improvement of long-term cyclic stability of this sorbent. Li et al.26 used a wet mixing method to synthesize the CaO−Ca12Al14O33 sorbent and investigated the effect of calcination temperature on the phase of inert support as well as CO2 capture performance. By using a sol−gel method, Radfarnia et al.25 developed a Ca9Al6O18-stabilized CaO sorbent, and ascribed the enhanced sorption to the superior structure and high dispersion of Ca9Al6O18 binder throughout the CaO matrix. Zhou et al.33 prepared various CaO-based sorbents with different calcium and aluminum precursors in order to investigate the correlation between the structural properties and CO2 capture performance of the sorbents. It has been well-known that the textural properties of Cabased sorbents such as surface area, pore size, and pore volume remarkably affect the CO2 sorption performance. By using advanced drying techniques such as spray drying and freeze drying, more favorable textural properties could be expected. Received: August 8, 2017 Revised: September 19, 2017 Published: September 20, 2017 A

DOI: 10.1021/acs.energyfuels.7b02330 Energy Fuels XXXX, XXX, XXX−XXX

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Since the component of a sorbent is known, the theoretical fraction of CaO in that sorbent could be calculated according to the amount of the calcium and aluminum precursors used in the preparation of the sorbent. The textural properties, including Brunauer−Emmett−Teller (BET) surface area, Barrett−Joyner−Halenda (BJH) pore size distribution, pore volume, and average pore diameter, were measured by N2 absorption−desorption isotherms at −196 °C. All of the samples were degassed at 300 °C before analysis. To obtain an insight of micromorphology of the sorbents and clarify the possible relationship between morphology and the CO2 capture performance, a field emission scanning electron microscope (FESEM) with an energy dispersive spectrometer (EDS) for elemental analysis was used for pertinent measurements. 2.3. Tests of the Performance. To measure the performance of the sorbents for CO2 capture over multiple carbonation−calcination cycles, a series of tests were conducted in a thermogravimetric analyzer (TGA). A general condition was set up as 30 min of carbonation at 650 °C in a flow of mixture gas (15 vol % CO2/85 vol % N2); after carbonation, the sample was heated to 850 °C with a temperature ramping rate of 20 °C min−1 in pure N2. On arriving at 850 °C, the sorbents were then instantly cooled to 650 °C with the same temperature ramping rate in pure N2, and thus a cycle ended. In the test above, the total gas flow rate was always maintained at 100 mL min−1. All spray dried sorbents were further tested under severe conditions, where a shorter carbonation time (5 min), a higher CO2 partial pressure in both carbonation and calcination atmosphere (90 vol % CO2/10 vol % N2), and a higher calcination temperature (950 °C) were adopted to enable a sintering-favored scenario. Carbonation temperature, ramping rate, and overall purging rate were kept unchanged. For the Al-stabilized CaO sorbents, three indices, (1) the net CO2 capture capacity (the mass of CO2 absorbed by unit mass of a sorbent), (2) the CaO conversion (the percentage of active CaO converted to carbonate; inert component neglected), and (3) the decay rate, were calculated according to the weight change of samples measured by TGA in order to evaluate the performance of the sorbents. The mathematical definitions of the CO2 capture capacity and the corresponding CaO conversion are as follows

However, these drying techniques have not been extensively studied, especially in the context of Ca-based sorbents.29,34,35 In the present study, a series of CaO-based sorbents derived from cosolutions of calcium acetate and aluminum nitrate were prepared. The conventional wet chemical method was used as the benchmark, i.e., evaporating solvent by heating up the solution. Freeze drying and spray drying were applied to dry the solutions as well, and the obtained products were compared to the benchmark sorbent in terms of CO2 capture performance over multiple cycles. This work has two major objectives: for one thing, to illustrate the influence of the three different drying techniques on the structural properties of the sorbents as well as the corresponding CO2 capture performance; for another, to screen the sorbent with supreme CO2 capture performance, which may be used in the sorption-enhanced steam reforming (SESR) process for H2 production later.36−41

2. EXPERIMENTAL SECTION 2.1. Preparation of the Sorbents. The Al-stabilized CaO sorbents mentioned in this work refer to a hybrid material which contains active component (CaO) and inert stabilizers (or “spacers”, i.e., Ca−Al composite without reactivity for CO2 capture). Such a hybrid material was synthesized by a wet chemistry method, which includes three steps: (1) dissolving precursors of Ca and Al, namely, calcium acetate and aluminum nitrate, respectively, into a cosolution; (2) drying the cosolutions to obtain well mixed fine particles of the precursors; (3) calcining the mixed powders to get the CaO−stabilizer hybrid.

Ca(CH3COO)2(s) → CaO(s) + 2CO2(g) + 3H 2O(g)

(1)

4Al(NO3)3(s) → 2Al 2O3(s) + 12NO2(g) + 3O2(g)

(2)

9CaO(s) + 3Al 2O3(s) → Ca 9Al 6O18(s)

(3)

During the calcination step, the precursors decomposed first (eqs 1 and 2) followed by a solid−solid reaction between the oxides of Ca and Al with the formation of the stabilizer, Ca9Al6O18. The remaining CaO particles were evenly spaced by the stabilizer so that the cyclic stability was remarkably enhanced. It should be noted that the formula of the stabilizer was determined by the following experiments (see section 3.1). By stoichiometrical calculation, a range of cosolutions were prepared corresponding to different active CaO fractions from the final calcined sorbents such as 60, 70, 80, and 90 wt %. Each cosolution was equally divided into three pieces that were subject to evaporation drying, freeze drying, and spray drying, respectively. For simplicity, evaporation drying, freeze drying, and spray drying were denoted as ED, FD, and SD, respectively. Taking SD90 as an example, SD indicates that the sorbent was dried by spray drying, and 90 denotes the active CaO fraction of this sorbent to be 90 wt %. Particularly, the pure CaO samples prepared by evaporation drying, freeze drying, and spray drying were denoted as ED100, FD100, and SD100, respectively. The spray drying process was conducted with a laboratory spray drier maintained at 300 °C for the air inlet temperature. For the freeze drying process, the liquid was precooled to about −78 °C for about 6 h in a refrigerator, and was then placed in a vacuum chamber at approximately −60 °C and 0.1 mbar for about 24 h. As for the evaporation drying process, the liquid was placed in an electric oven maintained at 60 °C for ∼24 h. The dried precursors were then ground into fine powders, which were calcined in a muffle oven at 900 °C for 90 min with a temperature ramping rate of 10 °C min−1 later. Finally, the synthesized sorbents were stored in a vacuum desiccator for use in the following tests. 2.2. Characterization of the Sorbents. In order to find out the structure of the sorbents, the as-prepared sorbents were analyzed by Xray diffraction (XRD, Cu Kα radiation) with a 2θ range of 10−90°.

C=

mmax − mmin mmin

(4)

X=

(mmax − mmin) × MCaO mmin × η × MCO2

(5)

where C and X represent the CO2 capture capacity and the CaO conversion, respectively; mmax is the weight of the sample at the end of carbonation in a given cycle and mmin is the corresponding initial sample weight in the same cycle; η stands for the CaO mass fraction in a sorbent; and MCaO, MCO2 are the molar weights of CaO and CO2, respectively. Besides the cyclic tests mentioned above, several full carbonation tests were conducted in the same TGA to estimate the actual fraction of active CaO in the sorbents. The test condition was that, with a temperature ramping rate of 20 °C min−1, ∼5 mg of sorbents were heated up to 850 °C in N2 to remove impurities like moisture and absorbed CO2, and then maintained at 650 °C in a CO2 gas flow (50 vol % CO2/50 vol % N2) for 300 min which can ensure nearly complete carbonation. For the full carbonation tests, the total gas flow rate was maintained at 30 mL min−1. The weight change measured by the TGA was attributed to the carbonation of active CaO component only, and thus, the actual CaO mass fraction could be estimated.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Sorbents. Figure 1 shows the XRD patterns of SD60−100, FD60, and ED60 sorbents, where only the diffraction peaks assigned to CaO (2θ = 32.2, B

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Figure 1. XRD patterns of samples with varied active CaO fractions and different drying methods. Figure 2. SEM images of calcined samples from different drying methods: (a) ED100, (b) ED70, (c) FD100, (d) FD70, (e) SD100, (f) SD70.

37.3, 53.9, 64.2, and 67.4°) and Ca9Al6O18 (2θ = 33.2, 47.6, and 59.3°) were observed, suggesting that the sorbents were comprised of only CaO and Ca9Al6O18 regardless of the drying methods, which excluded the possibility of forming other compounds such as Al2O3 or Ca12Al14O33,20,21,26,42,43 at least at the XRD detection level. As expected, the intensity of Ca9Al6O18 peaks gradually increased with decreasing CaO fraction in the sorbent. The FWHM (full width at halfmaximum) of SD samples decreased from 0.265 (SD60) to 0.183 (SD100), implying the growing crystallite size with increasing CaO fraction.44,45 The FWHM of FD60 and ED60 were 0.227 and 0.233, respectively, indicating the crystallite size order as FD60 > ED60 > SD60. As shown in Figure 1, except for CaO, only Ca9Al6O18 was detected; therefore, it was postulated that all of the Al from aluminum nitrate precursor was converted to form Ca9Al6O18. The molar quantity of Ca joining the formation of Ca9Al6O18 would be 1.5 times of Al; meanwhile, the rest of the Ca was all present in the form of CaO. On the basis of this assumption, CaO fractions of the sorbents could be calculated theoretically from the precursor recipe. Figure S1 shows the full carbonation curves of several sorbents, by which the actual active CaO fractions of the sorbents could be calculated. It was found that the measured CaO fractions were consistent with the theoretical value calculated by assuming that only two phases, i.e., CaO and Ca9Al6O18, were present in the sorbents. This result further confirmed the accuracy of the formula identification of the stabilizer. The microstructures of both pure CaO and Al-stabilized sorbents (with 70 wt % CaO, 30 wt % Ca9Al6O18) observed by FESEM are shown in Figure 2. The pure CaO sorbents from different drying methods did not exhibit much difference in the morphology. However, for Al-stabilized ones with 70 wt % fraction of CaO, drying methods resulted in distinctive changes. Particles of ED70 tended to agglomerate with each other and get densified, whereas FD70 and SD70 displayed much more gaps and channels than ED70. Some very tiny pores, which look like spots, were observed in FD70. The particles in SD70 were slightly more scattered than ED70 and FD70. Moreover, it

should be noted that, in all of the sorbents, calcium and aluminum are uniformly distributed according to the EDS figures (Figure S2 in the Supporting Information), thus making it possible that the well distributed calcium aluminates can mitigate the sintering of the sorbents particles effectively. SD70, FD70, and ED70 were selected for the N 2 physisorption test. The isotherms in Figure 3 compared the textural properties of the three samples. The spray dried SD70 showed a hysteresis between P/P0 = 0.4 and P/P0 = 0.9, indicating the existence of a mesoporous structure of the SD70 sample. The freeze dried FD70 did not present an evident hysteresis in the isotherm, demonstrating the insufficiency of mesopores. However, the hysteresis was not observed at all in ED70, which implied the absence of mesopores. Figure 3d shows the pore size distributions derived from the desorption isotherms by the BJH method. More pores fell between 2 and 3 nm for SD70 than FD and ED samples. Specially, SD70 displayed a sharp peak at d = 3.8 nm, demonstrating the majority of the pores are around this size. FD70 sample had less mesopores than SD70, but it was rich in macrospores around d = 62 nm. ED70 showed the lowest pore volume throughout the entire pore size range, suggesting that evaporation drying resulted in the densest materials. Table 1 summarized the surface area and pore volume of the three samples. SD70 showed the largest surface area, followed by FD70 and ED70, which is consistent with the observation in SEM images (Figure 2). The largest pore volume was seen for FD70, likely owing to more large pores around d = 62 nm. The N2 physisorption test was also undertaken for pure CaO from three drying methods. The isotherms, pore size distribution, summarized surface area and pore volume information were provided in Figure S3 and Table S1. Each sample showed the same trend as corresponding 70 wt % CaO samples but with lower adsorption quantity and pore volume. 3.2. Tests of the Performance. Figure 4 compares the effect of different drying methods and CaO fractions on the C

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Figure 3. N2 physisorption isotherms and pore size distributions of calcined samples from different drying methods: (a) isotherm of SD70, (b) isotherm of FD70, (c) isotherm of ED70, (d) pore size distributions of SD70, FD70, and ED70. STP: standard temperature and pressure (T = 273.15 K, P = 1 × 105 Pa).

spray drying also underwent condensation when liquid was evaporating. However, the condensation time acting on the material is much shorter than that during evaporation drying. The piled-up particles with the sufficient gaps between each other constructed the porous structure of SD samples. Particles with uniform size would result in uniform pore sizes, as shown in Figure 3d. In freeze drying, under vacuum conditions, ice was sublimated directly to vapor from the frozen precursor without undergoing a liquid phase; thus, the space previously occupied by water would be left vacant as pores and the dry material finally formed a porous framework. This explained why FD sample exhibited the largest pore volume in Table 1. The TEM images of ED70, FD70, and SD70 samples were displayed in Figure S4. The mapping of Al element demonstrated that the spray dried sample (SD70) performed the most uniform distribution of Ca9Al6O18. The evaporation dried (ED70) and freeze dried sample (FD70) showed a relatively poorer uniformity of the Al element, suggesting the Ca9Al6O18 spacer was less uniform in the materials. Manovic and Anthony46 proposed a pore-skeleton model to correlate the sorption phenomena and microstructure of the sorbent. It is postulated that the sorbent consists of two parts (Figure 6): the hard skeleton which stabilizes and protects the particle morphology and the soft skeleton which is exposed to the pores and changes the morphology easily during carbonation and calcination. It is more likely to form the hard skeleton after spray drying and evaporation drying due to the formation of particles. However, for freeze dried material, a hard skeleton was unlikely to form; this is because the ice was removed in situ, and the remaining material was very dispersive and too thin (or too fluffy) to contain both a hard skeleton and a soft skeleton; instead, it is highly possible to form only a soft skeleton. A soft skeleton with weak mechanical strength was not able to stabilize the morphology during carbonation− calcination cycles; thus, the porous structure could not be

Table 1. Surface Area and Pore Volume of Calcined Samples from Different Drying Methods sample

surface area (m2 g−1)

pore volume (cm3 g−1)

SD70 FD70 ED70

15.5 9.4 7.5

0.056 0.099 0.048

CO2 capture capacity of the samples. For the Al-stabilized sorbents, the SD ones displayed the supreme performance for CO2 capture over 30 cycles, followed by the ED and FD sorbents. This order is consistent with that of the FWHM, which indicates that better sorption could be achieved for smaller crystallite sizes. The data in Figure 4 together with textural properties in Table 1 implied that large surface area could lead to high CO2 sorption capacity, but large pore volume is not strictly correlated to the sorption performance. This is because the CO2 capture by CaO is a chemisorption process which takes place from the gas−solid interface to the bulk solid. Large pore volume could only favor physisorption during which gas molecules are adsorbed onto the solid surface and then fill in the vacant pores. For pure CaO sorbents, FD 100 showed slightly better performance than ED100, and still, SD100 captured the most CO2. To further illustrate the difference of the three drying methods, the schematic of these drying processes was depicted in Figure 5. Initially, the precursor was well dissolved in water to achieve homogeneity. Evaporation drying removed the water from the liquid−gas interface. During the drying process, surface tension in the liquid pulls the precursor and densifies the material; consequently, a dense chunk was formed after drying. Spray drying scattered the solution into tiny droplets (or clouds). The water evacuation from the tiny droplets can be completed in a very short time, and then, uniform solid particles formed. It should be noted that the material from D

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Figure 4. Cyclic CO2 sorption capacity of samples from different drying methods: (a) with 60 wt % CaO, (b) with 70 wt % CaO, (c) with 80 wt % CaO, (d) with 90 wt % CaO, (e) with 100 wt % CaO.

Figure 5. Schematic of pore formation from different drying methods. Figure 6. Pore-skeleton model. Reproduced from Manovic and Anthony.46

maintained. This is possibly the reason that FD samples exhibited a lower performance than ED samples. The effect of active CaO fraction on the cyclic sorption capacity and conversion is presented in Figure 7. Theoretically, the higher the CaO fraction of a sorbent is, the larger its CO2 capture capacity should be, regardless of the sintering problem. However, due to sintering of the CaCO3 particles, a certain amount of the inert spacer must be incorporated into the CaO matrix to resist sintering. Since the long-term performance of a sorbent is influenced by these two opposing factors (fractions of active CaO and inert support), monotonically increasing the CaO fraction could not result in a higher CO2 capacity in the cyclic test. The CO2 capacity of ED90 overtook that of ED100 throughout the 30 cycles (Figure 7b). For freeze dried samples, FD90 and FD100 almost converged to the same capacity, namely, 0.41 g of CO2/g of sorbent. In spray dried samples,

SD70 could reach the same CO2 capacity of SD80, SD90, and SD100 after 30 cycles in spite of its lower CaO fraction, namely, 0.44 g of CO2/g of sorbent. This value is much higher than the reported sorption capacity at the 30th cycle in other open literature.16,47−51 Even though SD70 contains less CaO than SD80, SD90, and SD100, the capacity of SD70 after cycles could benefit from the improved stability owing to more Ca9Al6O18 in the material. The insets of Figure 7a−c depict the sum of CO2 captured in 30 cycles per gram of sorbents. For those sorbents with the same CaO:spacer ratio, the spray dried ones captured remarkably more CO2 than both the freeze dried and evaporation dried ones. For FD and ED samples, those with E

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Figure 7. Cyclic sorption capacity and CaO conversion of samples with varied active CaO fractions: (a) net sorption capacity of freeze dried samples, (b) net sorption capacity of evaporation dried samples, (c) net sorption capacity of spray dried samples, (d) CaO conversion of freeze dried samples, (e) CaO conversion of evaporation dried samples, (f) CaO conversion of spray dried samples. The insets of parts a, b, and c are the sum of capacities over 30 cycles.

Figure 8. Cyclic sorption capacity and conversion of spray dried samples under severe conditions: (a) cyclic capacity (the inset is the sum of capacities over 30 cycles); (b) cyclic CaO conversion.

Nevertheless, the spacers worked differently with varied drying methods. For spray dried samples, increasing spacer fractions could enhance the CaO conversion more greatly than either the FD or ED ones, especially when the spacer fraction increased from 20 (SD80) to 30 (SD70) wt %. The best conversion after 30 cycles was observed for SD60 (83%), which is 16 and 14% higher than that of FD60 and ED60, respectively. It should be noted that SD70 was slightly behind SD60 in terms of CaO conversion (Figure 7f) but succeeded completely in net CO2 capacity (Figure 7c). In view of the superior performance of the spray dried sorbents, further sorption tests using the SD samples were conducted under a severe condition which is more realistic to industrial application. In postcombustion CO2 capture by

90 wt % active CaO captured the most CO2 in 30 cycles, while, for spray dried samples, SD80, SD90, and SD100 captured a similar amount of CO2 and the overall CO2 uptake dropped down a bit when the spacer fraction increased to 30 wt % (SD70). However, SD70 even approached FD90 and ED90 in terms of total capacity in 30 cycles (∼14 g of CO2/g of sorbent) and remarkably overtook both the freeze dried and evaporation dried ones with 30 wt % spacer (FD70 and ED70). The CaO conversions during the cyclic carbonation− calcination tests plotted in Figure 7d−f indicated that the addition of inert Ca9Al6O18 into active CaO matrix enhanced the conversion of CaO. It showed a positive correlation between the fraction of inert Ca9Al6O18 and the conversion value. This is a general rule for all three types of sorbents. F

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form a hard skeleton which maintains the original porosity, thus leading to lower cyclic stability during multiple carbonation− calcination cycles. By contrast, spray drying was a superior option to drying the precursor, as the produced sorbent showed the greatest surface area; furthermore, the spray drying method tended to form both a hard and soft skeleton to maintain a stable structure. Particularly, the SD70 sample, namely, the spray dried sorbent with 70 wt % CaO and 30 wt % stabilizer (Ca9Al6O18), presented the supreme cyclic performance that maintained the net CO2 capture capacity at 0.44 g of CO2/g of sorbent even after 30 carbonation−calcination cycles under mild conditions; the corresponding CaO conversion at the same time was kept at 77%. Under the severe conditions, the demand for stability was stricter than that in mild tests, as the capacity decayed more rapidly with cycles. SD70 captured the most CO2 in 30 cycles among all of the spray dried samples due to the outstanding stability and decent capacity. This indicated that 70 wt % CaO:30 wt % Ca9Al6O18 optimally balanced the reduction of active CaO fraction and the benefit of stabilization by using the inert spacers, and thus, SD70 was identified as the most promising candidate for high-temperature CO2 capture, compared to the rest of the ones in this work.

calcium looping, the sorbent regeneration is generally implemented in the presence of high fractions of CO2. In this work, 90 vol % CO2 was adopted for both carbonation and calcination. The former was done at 650 °C for 5 min, and the latter, by ramping to 950 °C at 40 °C min−1 without dwell time. The capacity results of spray dried samples during 30 cycles under the severe conditions were shown in Figure 8a, and the corresponding CaO conversions were plotted in Figure 8b. Although carbonation time was shortened greatly, the initial CO2 uptake in the first cycle was greatly increased compared to the mild tests. All five samples approached 100% conversion for cycle 1 (Figure 8b), presumably due to higher CO2 partial pressure during carbonation. However, unlike the results under mild conditions, the CO2 capacity decayed rapidly, especially for SD80, SD90, and SD100. The serious decay issue amplified the demand for stabilization as well as the optimization of spacer fraction. It seemed that for the long-term test SD70 held the optimum balancing point between stabilizer and CaO. Even though SD70 started at a lower capacity level than other CaOricher sorbents, it surpassed them from the fourth cycle. This overtaking point happened much earlier than the mildcondition tests, indicating a much more significant stabilizing function of the spacer. More importantly, the fast decay period of SD70 was much shorter than all of the other SD samples even including SD60. The cyclic curve of SD70 was maintained rather flat after four cycles, leading to 31% more CO2 capture than the second best sample SD90 in the total 30 cycles (see the inset bar graph in Figure 8a). The severe condition changed the rule in mild tests that CaO conversions positively correlated with stabilizer fractions. SD100, SD90, and SD80 showed similar CaO conversions which were much lower than that of SD70 all through the 30 cycles. These CaO-rich sorbents could only convert 20−30% CaO into CaCO3 after 30 cycles, while SD70 was stabilized at more than 60% conversion (Figure 8b). SD60 did not further increase the conversion as expected, and instead showed a considerable drop compared to SD70. This turning point clearly suggested the optimum spacer fraction. For spray dried samples, mild tests did not show such a turning point, although SD70 and SD60 did not differ much in terms of CaO conversion (Figure 7f). It was hard to obtain direct evidence to explain this. A possible reason could be that, under higher CO2 partial pressure during calcination, CaCO3 existed at much higher temperature, which greatly enhanced the sintering effect due to the low Tammann temperature of CaCO3.49,52 Under such a circumstance, too much spacer matter together with longer-lasting CaCO3 hindered mass transfer and prolonged the diffusion-controlled stage, whose reaction rate was much lower than that of the kinetic-controlled stage in carbonation of CaO.33,53

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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.7b02330. Figures showing the detection of actual CaO fraction by full sorption tests, EDS mapping images of the surface of selected samples, N2 physisorption isotherms and pore size distributions of calcined samples from different drying methods, and TEM mapping images of the ED70, FD70, and SD70 samples and table showing the surface area and pore volume of calcined samples from different drying methods (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 10 62784701. E-mail: [email protected]. cn. ORCID

Guozhao Ji: 0000-0003-4556-2675 Ming Zhao: 0000-0002-5801-5593 Author Contributions §

Y.S., G.J.: These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China for the support (Grant No. 51506112). G.J. is grateful for China Postdoctoral Science Foundation (Grant No. 2017M610910).

4. CONCLUSIONS The wet chemistry technique was applied to synthesize various Al-stabilized CaO sorbents using freeze drying, spray drying, and evaporation drying. The role of drying methods for such synthesis was investigated via a series of characterizations and TGA tests, and distinct morphology and cyclic performance of these sorbents were examined. The simple evaporation drying caused material contraction due to the surface tension of precursor solutions. The resulting low surface area and pore volume were not favorable for CO2 capture. Freeze drying circumvented the contraction and improved surface area and pore volume. However, the fluffy structure was insufficient to

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REFERENCES

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DOI: 10.1021/acs.energyfuels.7b02330 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.7b02330 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.7b02330 Energy Fuels XXXX, XXX, XXX−XXX