Characteristics and performance of CaO-based high temperature CO₂ sorbents derived from a sol–gel process with different supports

Abstract

The calcium looping process with CaO-based sorbents is one of the most important technologies for high-temperature scrubbing of CO₂. However, the CO₂ uptake capacities of natural CaO-based sorbents decayed rapidly during cyclic reactions. Thus, sorbents featuring cyclic stability should be developed for repeated utilisation. In this work, different inert solid materials were used as effective metal skeletons to improve the CO₂ uptake behaviour of the CaO-based sorbents. A dual fixed-bed reactor was used to investigate the CO₂ uptake performance of the sorbents during long-term cycles. The microstructures of the calcined sorbents were analysed by XRD and SEM tests. Results showed that the melting point of the inert supports was a pivotal factor affecting the CO₂ uptake behaviour. Zr-, Mg-, Y-, La- and Al-based supports were found to be outstanding supporting materials, whereas Co-, Zn-, Ba- and Fe-based supports were counteractive. Moreover, the calcination conditions significantly influenced the cyclic stability of the synthesised sorbents. After calcination at higher temperature in a pure CO₂ atmosphere, most of them became more vulnerable to sintering. However, the 90% CaO–10% ZrO₂ sorbent maintained a CO₂ uptake capacity of 0.5311 g CO₂ per g sorbent after 20 cycles. After 10 h of severe sintering stage, this material still demonstrated a capacity of 0.6493 g CO₂ per g sorbent. The metal skeleton of the sorbent possibly prevented crystal boundary migration and the recrystallisation process.

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1 Introduction

Global warming caused by anthropogenic emission of CO₂ into the atmosphere has become a public concern and could be potentially resolved by implementing a CO₂ capture and storage (CCS) strategy.^1,2 Within this strategy, various CO₂ uptake technologies have been put forward;^2–12 among them, calcium looping is one of the most important methods for high-temperature scrubbing of CO₂ from fuel gas, syngas, etc. Current economic projections suggest that this process may uptake CO₂ at a cost of approximately $20 per ton of avoided CO₂.¹³ This mechanism is mainly based on absorption/regeneration reactions of CaO through the following reactions: CaO + CO₂ → CaCO₃; CaCO₃ → CaO + CO₂. However, the cyclic CO₂ uptake behaviour of CaO derived from natural sources generally underwent rapid deterioration during repeated calcination/carbonation processes, and this phenomenon has been generally attributed to the sintering of the sorbents at high temperatures.

To improve the CO₂ capture performance of the calcium-based sorbents, three primary approaches have been extensively employed: (1) modification of the stability and structure of CaO sorbents, such as citric acid,¹⁴ propionic,¹⁵ KCl or K₂CO₃,¹⁶ HBr,¹⁷ sea water,¹⁸ biodiesel¹⁹ and ethanol/water;²⁰ (2) incorporation of inert materials, such as CaZrO₃,^21–24 MgO,^25,26 La₂O₃,^27,28 Ca₁₂Al₁₄O₃₃,^28–30 TiO₂,³¹ Ca₂MnO₄ (ref. 32) and Y₂O₃;^33–35 and (3) advanced preparation technologies, such as precipitation,³⁶ precipitated calcium carbonate (PCC),^37,38 dry planetary ball-milling method,³⁹ combustion synthesis,⁴⁰ simultaneous hydration–impregnation (SHI) method,¹⁸ self-assembly template synthesis (SATS),³¹ sol–gel^24,25,41 and CTAB-assisted sol–gel method,⁴² flame spray pyrolysis (FSP) technology,⁴³ biomass-based pore-forming method⁴⁴ and so on.^45,46

However, few studies have compared the effects of different support materials on the CO₂ capture performance of the synthetic sorbents prepared by the same method. Moreover, the reaction conditions differed in various studies. Thus, the results of different supports are difficult to compare in a parallel manner. Furthermore, the effects of low-melting-point oxides (such as CoO, BaO, ZnO and Fe₂O₃) on the calcium-based sorbents have not been reported yet. Therefore, different synthetic sorbents should be prepared using a promising method, and the calcium-looping cycle should be tested under the same condition.

Sol–gel process^{30,42,47–49} has been extensively reported as a good candidate method to prepare calcium-based sorbents, in which calcium and support materials can be mixed at molecular level; and the sorbents can be prepared with small grains, which were favourable to solid–gas reaction. Therefore, in this work, 10 CaO-based sorbents were synthesised by a sol–gel process supported with various materials, and their cyclic behaviour was investigated using a fixed-dual bed system. This work aims to study the effects of support materials on the CO₂ uptake performance of CaO-based sorbents, as well as to screen optimal oxide for calcium-based sorbents. Notably, the melting points of the 10 oxides are different, which varied from 1565 °C of Fe₂O₃ to 2709 °C of ZrO₂. Additionally, considering that calcination condition can significantly influence the cyclic stability of the sorbents, the cyclic CO₂ uptake behaviour of the sorbents was examined under different calcination conditions. At last, field emission scanning electron microscopy (FSEM) was used to investigate the microstructure changes of the sorbents during sintering stages.

2 Experimental

2.1 Materials and preparation

All of the reagents used in this paper were analytical grade from Sinopharm Chemical Reagent Co., Ltd. The preparation method is based on a standard sol–gel process. And the different sorbents were produced as follows:

Pre-calculated amounts of citric acid monohydrate, calcium nitrate tetrahydrate and other metal nitrates (precursor of the supporting materials) were added into deionised water at a citric acid/metal ion molar ratio of 1 : 1 and a metal ion/water molar ratio of 1 : 40 at room temperature. Afterwards, the mixture was continuously stirred at 70 °C in an electrically heated magnetic stirring apparatus for 5 h. The formed translucent wet sol was then aged for 12 h at room temperature. Subsequently, the aged gel was placed in a dryer at 80 °C for 5 h and then dried at 120 °C for another 12 h until a dry gelatine was formed. The dried gelatine was ignited at 400 °C and then burned off in a muffle furnace at 400 °C. When the combustion process was completed, the loose grey powder was then calcined at 850 °C for another 3 h. Finally, the novel CaO-based sorbents were obtained. Notably, the obtained sorbents were all transformed into oxides as discussed later in XRD patterns.

The amounts of Ca precursor and support precursors were pre-determined to ensure that the weight ratio of CaO and inert support in the sorbents would be as designed (assuming that the support materials in the sorbent were only stabile oxides; in this study, Ca₁₂Al₁₄O₃₃ can be seen as 12CaO + 7Al₂O₃, and the same as Ca₂Fe₂O₅). To investigate the effect of the different supporting oxides on the CO₂ capture performance of the calcium-based sorbents, we adopted a uniform mass ratio of CaO to metal oxides. An optimum weight fraction of CaO exists in the sorbent, that is, lower than which the sorbent contained insufficient active CaO and beyond which the sorbent lost its sintering-resistant property; study⁵⁰ showed that 90% was the optimal weight fraction for the supported Ca-based sorbents.

In this work, various sorbents were prepared by citric sol–gel method as aforesaid, and CaO weight fractions in the sorbent were all 90%. The final sorbents obtained are named 90CaO–10M_xO_y, which represents the mass ratios of CaO to support oxides at 90/10 ratio (M_xO_y represents the support oxides).

2.2 Sorbent testing

The cyclic carbonation/calcination performance of the sorbents was tested in a fixed-dual bed reaction system and a thermogravimetric analyzer (TGA, Pyris 1), as described previously.^18,44 And the reaction condition was shown in Table 1 below. The variation in the mass of the sample during the calcination/carbonation cycles was weighted by a high precision electric balance, and the CO₂ uptake capacity of the sorbent and the conversion of CaO were calculated as follows:

C_N = (m_N − m₀)/m₀ (1)

X_N = (m_N − m₀)M_CaO/bm₀M_CO2 (2)

where, C_N and X_N are the maximum CO₂ uptake capacity of the sorbent and CaO conversion during the N_th cycle, m_N is the mass of the sorbent after the N_th carbonation, m₀ is the mass of the sorbent after initial calcination, b is the content of CaO in the initial sorbents (here are all 90%), and M_CaO and M_CO2 are molar mass of CaO and CO₂.

Table 1 Reaction conditions in the fixed-dual bed system

	T/°C	t/min	Reaction atmosphere
Carbonation	700	30	20% CO2, 80% N2
Calcination	850	10	100% N2

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The crystal structure parameters for calcined samples were examined by an X-ray diffraction (XRD, PANalytical B.V) analyzer with Cu Kα radiation at a scanning speed of 0.05° per s. The microstructures of the calcined samples were investigated by field-emission scanning electron microscopy (FSEM, Nova NanoSEM 450) with 10 kV of accelerating voltage.

3 Results and discussion

3.1 Comparison of different pure CaO sorbents

The cyclic CO₂ uptakes behaviour of micro-CaCO₃ (common CaCO₃, AR), nano-CaCO₃ and citric sol–gel CaO was analysed, and the results are shown in Fig. 1. The CO₂ uptake capacities of the sorbents all experienced a rapid decay, and this phenomenon was generally ascribed to sintering, which occurred during high-temperature reactions. However, compared with micro-CaCO₃ and nano-CaCO₃, the cyclic CO₂ uptake behaviour of citric sol–gel CaO was much better. The sol–gel CaO sorbent gained the highest CO₂ uptake capacity during the initial carbonation and also behaved more stability during long-term cycles. Its CO₂ uptake capacity remains as high as 0.44 g CO₂ per g sorbent after 10 cycles, which is 60% higher than that of nano-CaO and 131.6% higher than that of micro-CaO.

Fig. 1 CO₂ uptake volume versus number of cycles: sol–gel CaO, nano-CaO, micro-CaO.

In order to clarify the mechanism of higher performance of the sol–gel sorbent, further investigation via FSEM analysis showed micromorphology of these three CaCO₃ precursors. As shown in Fig. 2, the grains of the micro-CaCO₃ were much larger than those of nano-CaCO₃ and sol–gel CaCO₃, and the crystal structure of micro-CaCO₃ was compact; in addition, the clearance between particles was much smaller. However, the micromorphology of the sol–gel CaCO₃ was fluffy, and porous. The developed porous structure may be in favor of the diffusion of CO₂ into the sorbent interior, and then contribute to higher CO₂ uptake volume. Besides, the well-interlinked macro-pores maybe good scaffolds that inhibited agglomeration of the grains of sol–gel sorbents at high temperatures.

Fig. 2 FSEM images of the Ca-precursors: (a) micro-CaCO₃; (b) nano-CaCO₃; (c) sol–gel CaCO₃.

The fluffy structure was formed from the preparation processes, including the homogeneous mixing of Ca²⁺, nitrate, citrate, and other ions. Metal ions combined with coordinate bonds of citric acid during the hydrolytic process and formed catenulate colloidal sol. Though the ageing process, the sol dried and poly-condensed continually, and formed net structures gel. Subsequently, the aged gel dried in high temperatures to remove the water and formed a xerogel.

When the xerogel was ignited in the muffle at 400 °C, a severe redox reaction occurred as the followings:

Ca(NO₃)₂·4H₂O + 5/6C₆H₈O₇·H₂O + 5/4O₂ → CaO + 5CO₂ + 49/6H₂O + N₂ (3)

The drastic combustion process of xerogel released a large amount of heat and gases and produced an improved fluffy structure. The fluffy microstructure was favoured for diffusion of CO₂ through the sorbents, and the fine grains distributed in the fluffy space slowed down the sintering progress during the repeated calcium-looping cycles.

3.2 Comparison of different kinds of metallic oxides

Owing to the high performance of the sol–gel sorbents, we adopted the above method to produce a variety of sol–gel sorbents supported by different metal ions. All the sorbents were prepared at a mass ratio of CaO to supporting oxide of 90/10.

The cyclic CO₂ uptake performances of these sol–gel CaOs were investigated, and the results are shown in Fig. 3. The sol–gel CaOs supported by ZrO₂, Y₂O₃, MgO, La₂O₃ and Al₂O₃ all demonstrated excellent CO₂ uptake performance during long-term cycles; in particular, 90CaO–10ZrO₂ showed almost no decrease during the 20 cycles. While, the sorbents supported by CeO and Fe₂O₃ behaved a marginally better cyclic stability during the initial 10 cycles and then decayed as the sol–gel CaO (without support). However, the CO₂ uptake capacities of 90CaO–10BaO, 90CaO–10ZnO and 90CaO–10CoO showed similar trends, and these sorbents all behaved much worse than the sol–gel CaO. Their cyclic CO₂ uptake volumes were all plummeted rapidly, especially the CO₂ uptake volume of 90CaO–10CoO decreased to around 0.1 g CO₂ per g sorbent after 4 cycles.

Fig. 3 Cyclic CO₂ uptake behaviour of sorbents supported by different metallic oxides.

The maximal conversions of different CaO are list in Table 2. The CaOs supported by ZrO₂, Y₂O₃, MgO, La₂O₃ and Al₂O₃ all shown outstanding conversion during the repeated cycles; they achieved not only the highest conversion during the first cycle but also maintained a significantly high value over 20 cycles. The optimal sorbent was 90CaO–10ZrO₂, and its CaO conversion was 95.51%, which was even higher than that of the sol–gel sorbent without support; in addition, the value maintained 85.95% after 20 cycles. The 90CaO–10CeO showed a similar trend, with good conversion during the first cycle, but retained unfavourable performances during the 20th cycle. However, the CaOs supported by BaO, ZnO, Fe₂O₃ and CoO all demonstrated inferior performances during long-term cycles, and they behaved slightly lower initial conversion, as well as terrible conversion after 20 cycles; in particular, the CaO conversion of 90CaO–10CoO decreased rapidly from 66.2% at the first cycle to 6.57% after the 20th cycle.

Table 2 Carbonation conversion of the different sorbents during the first and 20th cycle

XN	During the first cycle	During the 20th cycle
Sol gel-CaO	94.16%	42.1%
CaO–ZrO2	95.51%	85.95%
CaO–Y2O3	97.3%	73.08%
CaO–MgO	94.66%	75.85%
CaO–La2O3	96.68%	69.89%
CaO–Al2O3	86.15%	65.59%
CaO–CeO	84.65%	48.26%
CaO–BaO	94.68%	35.93%
CaO–ZnO	82.89%	30.91%
CaO–Fe2O3	73.05%	29.72%
CaO–CoO	66.2%	6.57%

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Considering that the temperature exerts more obvious effect on the transmission of ions or atoms, the melting-point temperature is very important in studying the sintering of oxides. The diffusion of cavities will be apparent when the ionic compounds are above about 3/4 of the melting-point. Generally, sintering occurred far below the melting-point temperature of solid matter. When a solid matter reaches the Tammann temperature, which is generally 50% to 75% of its melting-point temperature, the diffusion of ions and cavities will be activated; consequently, grain boundary integration will occur, and agglomeration will become evident. The melting-point temperature and Tammann temperature of the relative matter are shown in Table 3. The Tammann temperature of CaCO₃ is 533 °C, which is lower than the reaction temperatures, and then CaCO₃ grains will sinter and lead to porous degree reduction and activity decline. However, inertia oxides in uniform support will improve the sintering resistance and stabilize the CO₂ uptake activity of the CaO-based sorbents. The Tammann temperature of CaO is highest among the materials, followed by ZrO₂; however the Tammann temperatures of ZnO, Fe₂O₃ and CoO are all below the calcination temperature. This may explain the optimum cyclic CO₂ uptake behaviour of CaO–ZrO₂ sorbents and the improved performance of CaO–Y₂O₃, CaO–MgO, CaO–La₂O₃ and CaO–Al₂O₃ sorbents. These findings may also justify the worst performance of CaO–ZnO, CaO–Fe₂O₃ and CaO–CoO sorbents.

Table 3 Melting-point temperature and Tammann temperature of the relative objects

Compound	Melting-point temperature (°C)	Tammann temperature (°C)
CaO^51,52	2572	1313
CaCO3 (ref. 52 and 53)	1339	533
ZrO2 (ref. 52 and 54)	2709	1218
Y2O3 (ref. 55)	2410	1200
MgO^51,52,54	2852	1188
La2O3 (ref. 55)	2217	1013
Al2O3 (ref. 51 and 52)	2000	990
CeO^53	2400	1064
BaO^55	1923	982
ZnO^56	1975	710
Fe2O3 (ref. 51 and 54)	1565	698
CoO^52,53	1830	779

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3.3 Characteristics of the different synthetic sorbents

The initial CaO conversion of 90CaO–10Al₂O₃ was 86.15%, which was lower than that of the sol–gel CaO without support, but this material maintained 65.59% of conversion after 20 cycles, which was higher than that of CaO without support. This finding may be ascribed to the reaction that occurred between CaO and Al₂O₃ at high temperatures and formed inert materials (Ca₁₂Al₁₄O₃₃) as shown in Fig. 4. Through the reaction, the active CaO contents decreased, which can maintain a more stable conversion during long-term cycles because of the formed inert Ca₁₂Al₁₄O₃₃ suppressing the sintering of the sorbents. Ca₁₂Al₁₄O₃₃ in the sorbent cannot react with CO₂ but can provide stable metal skeletons to prevent the CaO grains from agglomerating during the repeated cyclic calcination and carbonation processes, as well as to retard the sintering of the sorbents. In conclusion, 90CaO–10Al₂O₃ remains a promising sorbent for calcium-looping technology.

Fig. 4 XRD patterns of calcined sample of 90CaO–10Al₂O₃: 1 – CaO; 2 – Ca₁₂Al₁₄O₃₃.

The modification mechanism of the CaO–ZrO₂, CaO–Y₂O₃, CaO–MgO, CaO–La₂O₃ may be similar with the CaO–Ca₁₂Al₁₄O₃₃. The higher sintering temperature of these inert metal oxide supports, as shown in Fig. 5, provided well developed frameworks to stabilize the CaO sorbents, and lead to stable conversions during 20 cycles.

Fig. 5 XRD patterns of calcined sorbents (90CaO–10ZrO₂; 90CaO–10Y₂O₃; 90CaO–10MgO; 90CaO–10La₂O₅): 1 – CaO; 2 – ZrO₂; 3 – Y₂O₃; 4 – MgO; 5 – La₂O₃.

However, oxides such as BaO, ZnO, Fe₂O₃ and CoO have much lower melting temperatures; and CO₂ uptake performance of sorbents supported by these oxides behaved much worse, accordingly more severe sintering occurred (as discussed latter). It is worth noting that, the Fe₂O₃ had the lowest melting point among these oxides, but behaved much better than the 90CaO–10CoO. It may be caused by the formation of Ca₂Fe₂O₅, as shown in Fig. 6, which is a kind of much higher melting point inert material, and it can act as an inert scaffold inhibit the sorbents from further sintering. In addition, the intensity of BaO, ZnO and CoO were all weak in the XRD patterns, it may be the result that eutectic formed during cycles at high temperatures. The eutectic spoiled the porous solid surface, and then prevented the diffuse of CO₂ molecular into the sorbent interior.

Fig. 6 XRD patterns of calcined sorbents (90CaO–10CoO; 90CaO–10Fe₂O₃; 90CaO–10ZnO; 90CaO–10BaO): 1 – CaO; 2 – CoO; 3 – Fe₂O₃; 4 – Ca₂Fe₂O₅; 5 – ZnO; 6 – BaO.

Table 4 presents the crystallite size and lattice distortion changes of the different CaO-based sorbents after 0 cycle and 20 cycles. Interestingly, the fresh CaO grain sizes of the better behaved supported sorbents (e.g., 90CaO–10ZrO₂, 90CaO–Y₂O₃, 90CaO–10MgO, 90CaO–10La₂O₃, and 90CaO–10Al₂O₃) are all smaller due to the supporting method. On the other hand, the fresh CaO grain sizes of the worse behaved supported sorbents (e.g., 90CaO–10BaO, 90CaO–10Fe₂O₃ and 90CaO–10CoO) are all bigger

Table 4 Crystallite size and lattice distortion of the different CaO by XRD analysis

Sample	Dhkl/nm			D̄/nm	Lattice distortion%
	111	200	220
CaO 0 cycle	64.3	64.2	57.1	61.9	0.130
CaO–ZrO2 0 cycle	42.9	43.5	33.9	40.1	0.265
CaO–Y2O3 0 cycle	42.8	39.9	57.5	46.7	0.241
CaO–MgO 0 cycle	47.5	43.3	66.3	52.4	0.181
CaO–La2O3 0 cycle	43.0	53.3	50.0	48.8	0.188
CaO–Al2O3 0 cycle	52.0	47.7	56.0	51.9	0.210
CaO–CeO 0 cycle	52.0	39.9	46.2	46.0	0.236
CaO–BaO 0 cycle	65.6	77.0	96.9	79.8	0.146
CaO–ZnO 0 cycle	52.0	52.7	50.6	51.8	0.208
CaO–Fe2O3 0 cycle	75.9	66.6	81.8	74.7	0.149
CaO–CoO 0 cycle	75.9	52.8	96.9	75.2	0.159
CaO 20 cycles	72.8	48.7	84.8	68.8	0.137
CaO–ZrO2 20 cycles	47.1	42.8	50.7	46.9	0.231
CaO–Y2O3 20 cycles	89.9	58.7	70.8	73.1	0.153
CaO–MgO 20 cycles	47.0	58.6	70.7	58.9	0.196
CaO–La2O3 20 cycles	65.6	77.0	66.0	69.5	0.158
CaO–Al2O3 20 cycles	89.9	43.5	81.8	71.7	0.17
CaO–CeO 20 cycles	65.6	66.6	81.7	71.3	0.158
CaO–BaO 20 cycles	75.9	91.2	96.9	88.0	0.130
CaO–ZnO 20 cycles	89.9	43.5	96.8	76.7	0.165
CaO–Fe2O3 20 cycles	89.9	91.2	81.8	87.6	0.126
CaO–CoO 20 cycles	92.0	62.8	96.9	83.9	0.123

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The microstructure evolution with cyclic numbers of the different sol–gel CaO was investigated by field emission scanning electron microscopy (FSEM). As shown in the SEM image, the phenomena of grain enlargement and porosity decrease have not been found in sol–gel CaO [Fig. 7(a) and (b)] and sol–gel 90CaO–10MgO [Fig. 7(b) and (c)] when compare with the micro-granular façades between the fresh sorbents and the sorbents after 20 cycles. The fresh sol–gel 90CaO–10ZnO [Fig. 7(e)] was fluffy (slightly less than the two sorbents above), while the sorbent after 20 cycles [Fig. 7(f)] was grain-closed and void-less. It may be affected by the lower melting point of ZnO, and the ZnO inside the grains should formed eutectics and promoted the migration of ionic, which consequently closed the pore, along with the grain growth. Interestingly, a compact and hardened micro-granular was formed in the 90CaO–10ZnO sorbent after 20 cycles, the CO₂ molecules were difficult to react with the CaO inner the granular by diffusion. On the other hand, the 90CaO–10ZnO remained porous microstructure during 20 cycles, which in favor of CO₂ molecular diffusion into the product layer; thus it will behaved much better CO₂ uptake capacity during long-term calcium looping process.

Fig. 7 SEM images of different calcined sorbents derived from sol–gel method: (a) fresh CaO, (b) CaO after 20 cycles; (c) fresh 90CaO–10MgO, (d) 90CaO–10MgO after 20 cycles; (e) fresh 90CaO–10ZnO, (f) 90CaO–10ZnO after 20 cycles.

3.4 Kinetic analysis of CO₂ capture performance of the calcium-based sorbents

After 20 cycles, the cyclic performance of the synthetic sorbents followed the order CaO–ZrO₂ > CaO–MgO > CaO–Y₂O₃ > CaO–La₂O₃ > CaO–Al₂O₃ > CaO–CeO > CaO > CaO–BaO > CaO–ZnO > CaO–Fe₂O₃ > CaO–CoO. CaO–MgO was slightly inferior to CaO–ZrO₂, however, it still hold a relatively high CO₂ capture capacity during long-term cycles, and the cost of magnesium precursor was much cheaper. Hence, we believe that CaO–MgO should be further investigated. The industrial fluidised bed reactor has a typically short contact time. Thus, the fast reaction stage will be very important. Kinetic analyses of the micro-CaCO₃ and the 90CaO–10MgO were tested in a thermogravimetric analyzer (TGA, Pyris 1). Results are shown in Fig. 8. The carbonation conversion of 90CaO–10MgO was 40% higher than that of micro-CaCO₃ during the first cycle. However, after 20 cycles, the conversion of 90CaO–10MgO was about twice that of the micro-CaCO₃. The reason may be the faster reaction rate of 90CaO–10MgO than that of the micro-CaCO₃ sorbent during long-term cycles; and 90CaO–10MgO should retained preferable carbonation rates after repeated cycles, whereas micro-CaCO₃ underwent a rapid decline. The carbonation curves at the first and the 20th cycles of the micro-CaCO₃ and 90CaO–10MgO are shown in Fig. 9. Evidently, the 90CaO–10MgO sorbent showed faster reaction rate than that of the micro-CaCO₃ during both the first cycle and the 20th cycle. The fast reaction rate should be ascribed to the fluffy and porous microstructure of sorbents derived from the sol–gel method.

Fig. 8 Cyclic performance of micro-CaCO₃ and 90CaO–10MgO sorbent (carbonation: 20% CO₂, 700 °C; calcination: 100% N₂, 850 °C).

Fig. 9 CO₂ adsorption behaviour of micro-CaCO₃ and 90CaO–10MgO sorbent as a function of time at the first and the 20th cycle.

The carbonation process consists of a chemical reaction controlled stage and a product layer diffusion controlled stage. It can be seen that the reaction between CaO and CO₂ is characterised as an initial fast reaction controlled regime and then switches to a slower product layer diffusion-controlled regime. At the first cycle, both the sorbents were fluffy and porous, and the reaction-controlled regimes were followed by reaction and diffusion joint controlled regimes, as shown in Fig. 9(a). After multiple cycles, the chemical reaction regimes and diffusion regimes can be clearly identified during the carbonation process, as shown in Fig. 9(b). However, after multiple cycles, the reaction controlled regimes were shorter and the conversions were decayed. Moreover, it is obvious that the 90CaO–10MgO sorbent showed much faster reaction rate than the micro-CaCO₃ during the fast reaction controlled regimes, and the maximum reaction rates of the 90CaO–10MgO were 97% and 160% higher than the micro-CaCO₃ in the first and the 20th cycle, respectively. Consequently, the 90CaO–10MgO is a promising sorbent in calcium-looping process for large-scale.

3.5 Effect of reaction conditions on calcium looping process

In practical application of calcium looping process, in order to achieve high concentration of CO₂, the calcination stage usually conducted in pure CO₂. Accordingly, the calcination temperature should exceedingly high, and this severe condition would render the calcium-based sorbents to lower CO₂ uptake capacity as a result of sintering. Hence, it was increasingly meaningful to investigate the effect of calcination condition on CO₂ uptake behaviour during long-term cycles. In this work, we studied the effects of different calcination conditions on some calcium-based sorbents during repeated 20 cycles in a fixed-bed react system. And the reaction condition is outlined in Table 5, and here we adopted three kinds of calcination conditions.

Table 5 Reaction conditions in the fixed-dual bed system

	T/°C	t/min	Reaction atmosphere
Carbonation	700	30	20% CO2, 80% N2
Calcination 1	850	10	100% N2
Calcination 2	950	10	100% N2
Calcination 3	950	10	100% CO2

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The sol–gel 90CaO–10ZrO₂ has been proven to be the optimum sorbent during long-term cycles; we took advantage of this sorbent, the sol–gel CaO without supporting and micro-CaCO₃ to study the different calcination conditions affected by the CO₂ uptake behaviour during the repeated cycles. And the result is shown in Fig. 10; obviously, both the higher temperature and the pure CO₂ atmosphere go against the CO₂ uptake behaviour. Regardless whether running on mild calcination condition (850 °C, 100% N₂), or severe ones (850 °C, 100% CO₂; 950 °C, 100% CO₂), the decay of CO₂ uptake capacity of 90CaO–10ZrO₂ sorbent was relatively small; more precisely, running on the extremely harshest condition, this sorbents retained 0.531 g CO₂ per g sorbent uptake capacity (Fig. 11). Both the micro-CaO and the sol–gel CaO demonstrated inferior cycling stability (Fig. 12 and 13); even in the mildest calcination condition, the CO₂ uptake capacities have decreased by more than half during 20 cycles; these materials are also more vulnerable to severe calcination conditions, and CaOs showed the optimal cycling stability at 850 °C under pure N₂ calcination condition, and worst activity at 950 °C under pure CO₂.

Fig. 10 Cyclic CO₂ uptake behaviour of three sorbents in different calcination conditions.

Fig. 11 CO₂ uptake capacity of sol–gel 90CaO–10ZrO₂ in different calcination conditions.

Fig. 12 CO₂ uptake capacity of sol–gel CaO in different calcination conditions.

Fig. 13 CO₂ uptake capacity of micro-CaO in different calcination conditions.

At the same calcination temperatures, the CO₂ uptake capacity decreased more serious in pure CO₂ atmosphere probably as a result of slower decomposition rate. Consequently, sorbents exist as CaCO₃ in prolonged periods. In addition, the melting point of CaO is 2572 °C, whereas that of CaCO₃ is 1339 °C and the Tammann point is considered as about 533 °C, which is much lower than the calcination temperature; thus, further serious sintering may occur in this severe condition.

To verify this conjecture, we calcined the sorbents at pure N₂ and pure CO₂ at a long period and then tested the CO₂ uptake performance; the result is shown in Fig. 14. These sorbents exhibited worse CO₂ uptake performances after 10 h of calcination in pure CO₂ than that in pure N₂, especially the sol–gel CaO and micro-CaO. Additionally, after calcination at a longer time, the capacity became severe.

Fig. 14 CO₂ uptake capacities of three sorbents after various long-term calcination conditions.

Further investigation via SEM analysis illustrated visual changes of the three sorbents during 20 cycles. After being supported by ZrO₂ [Fig. 15(a)–(c)], the sorbent became smaller grain size, complete crystal shape and porous. This porous microstructure was in favour of diffusion of CO₂ molecular into the CaO particle, consequently, improving the cyclic reactivity. After 20 cycles [Fig. 15(b) and (c)], the well-developed porous structure remained, and the sintering state was improved, which accordingly explained well cycling stability. However, the grain size of sol–gel CaO without support was obviously enlarged, and the inter-grain was compact, as shown in Fig. 15(d); after 20 cycles, the grains fused into a bulk and became less porous, as shown in Fig. 15(e). While the micro-CaCO₃ grains were larger than the sol–gel CaO and 90CaO–10ZrO₂ CaO, and the former was very compact and less porous, as shown in Fig. 15(f). After 20 cycles, the micro-CaCO₃ grains suffered severest sintering, as shown in Fig. 15(g) and (h), which explained the least CO₂ uptake capacity after 20 cycles. The grains in the sol–gel CaO and micro-CaO grew abnormally after multi-cycles, and this phenomenon may be considered recrystallisation of the grains, which accelerated the sintering. While uniformly dispersed ZrO₂ among 90CaO–10ZrO₂ sorbents prevented crystal boundary migration and suppressed the recrystallisation process, thereby significantly improving the cycling stability. In addition, sintering condition was severe during calcination in pure CO₂ than that in pure N₂ atmosphere; which explained the decreased CO₂ uptake capacity.

Fig. 15 SEM images of different sorbents: (a) fresh 90CaO–10ZrO₂ after carbonation; (b) 90CaO–10ZrO₂ after 20 cycles, 950 °C, pure N₂; (c) 90CaO–10ZrO₂ after 20 cycles, 950 °C, pure CO₂; (d) fresh sol–gel CaO after carbonation; (e) sol–gel CaO after 20 cycles, 950 °C, pure N₂; (f) micro-CaCO₃; (g) micro-CaO after 20 cycles, 950 °C, pure N₂; (h) micro-CaO after 20 cycles, 950 °C, pure CO₂.

4 Conclusions

In this work, various CaO based sorbents were prepared by sol–gel process and tested in a fixed-dual bed system. After 20 cycles, the cyclic performance of the synthetic sorbents followed the order CaO–ZrO₂ > CaO–MgO > CaO–Y₂O₃ > CaO–La₂O₃ > CaO–Al₂O₃ > CaO–CeO > CaO > CaO–BaO > CaO–ZnO > CaO–Fe₂O₃ > CaO–CoO.

The former five supports, i.e., ZrO₂, Y₂O₃, MgO, La₂O₃ and Al₂O₃ exhibited high melting temperatures and can act as stable inert metal skeletons to prevent CaO from sintering, thereby improving the CO₂ uptake performance. Unfortunately, the latter five supports, especially ZnO, CoO and Fe₂O₃, accelerated the movement of grain boundary and blocked the porous structure, thus demonstrating the worst CO₂ uptake performance. Notably, among the 10 kinds of the supports, Al₂O₃ and Fe₂O₃ supports reacted with CaO and formed more stable inert Ca₁₂Al₁₄O₃₃, and Ca₂Fe₂O₅, respectively. And the Ca₁₂Al₁₄O₃₃ was more effective than the Ca₂Fe₂O₅.

Calcination condition exerts an important effect on the cyclic stability of the sorbents. When calcined at higher temperature in pure CO₂ atmosphere, the sorbents became vulnerable to sintering. The optimum sorbent is 90CaO–10ZrO₂. The morphology tested by SEM showed that ZrO₂ can inhibit grain growth by suppressing the movement of grain boundary, thereby maintaining a porous stable structure; however, the sol–gel CaO (without support) and micro-CaCO₃ crystal agglomeration was obvious, leading to unfavourable CO₂ uptake performance.

Acknowledgements

The authors are grateful for the financial support from the National Natural Science Foundation of China (51606076), National Key Research Program of China (2016YFB0600801) and the Analytical and Testing Center of Huazhong University of Science and Technology (HUST) for XRD and SEM measurements.

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