Studies on CO₂ uptake by CaO/Ca₃Al₂O₆ sorbent in calcium looping cycles

Abstract

In this work, limestone, aluminum nitrate hydrate, and glycerol water solution by combustion synthesis method were proposed to prepare a synthetic CaO/Ca₃Al₂O₆ sorbent for CO₂ capture in calcium looping cycles. The effects of the mass ratio of CaO to Al₂O₃, cycle number, carbonation conditions, and calcination conditions on the CO₂ uptake by the obtained synthetic sorbent in the repeated carbonation/calcination cycles were studied in a dual fixed-bed reactor and a thermogravimetric analyzer. The optimum mass ratio of CaO to Al₂O₃ was 90:10 in the preparation process of the synthetic sorbent, which exhibited a 0.43 g g⁻¹ of CO₂ uptake after 50 cycles. The main compositions of the synthetic sorbent contained the mass ratio of CaO:Al₂O₃ = 90:10 were CaO and Ca₃Al₂O₆, and the mass ratio of CaO to Ca₃Al₂O₆ was 74:26. The CO₂ uptake by CaO/Ca₃Al₂O₆ increases rapidly with the carbonation time in previous 5 min and then rises slowly after 5 min. The carbonation time to reach the maximum CO₂ uptake rate of CaO/Ca₃Al₂O₆ was much sooner than that of CaO derived from limestone in each cycle. The optimum carbonation temperature window of CaO/Ca₃Al₂O₆ was 650–700 °C. CaO/Ca₃Al₂O₆ sorbent possessed obviously higher sintering resistance than CaO under the more severe calcination conditions in the cycles. The high CO₂ uptake capacity of CaO/Ca₃Al₂O₆ was attributed to its stable porous structure in the multiple carbonation/calcination cycles.

Introduction

Recently, CO₂ capture technologies have been significantly developed. Adsorption methods using solid materials are an alternative to the absorption technology in the processes of purification gases from CO₂ [1]. The development of solid materials with potential application for CO₂ capture is a topic of great scientific interest [2–4]. Calcium-based sorbents are widely used for the control of SO₂ emissions [5–10]. The calcium-based sorbents can be also used to capture CO₂. Calcium looping, i.e., repetitive carbonation/calcination cycles of CaO, is a promising CO₂ capture technology for fossil fuel-fired power plants [11, 12]. The calcium looping involves a carbonation process, where CaO absorbs CO₂ to generate CaCO₃ at 650–750 °C in a carbonator, and a calcination process, where the CaCO₃ is calcined under oxy-combustion or steam to produce a pure CO₂ stream (for reclaim and storage) and CaO in a calciner at temperature above 850 °C. After the carbonaiton/calcination cycle, the regenerated CaO returns to the carbonator for the next CO₂ adsorption process [13].

However, CO₂ uptake capacities of the calcium-based sorbents decrease with the number of carbonation/calcination cycles [14–16]. Various methods have been used to enhance CO₂ uptake capacities of the calcium-based sorbents [17–19]. The CO₂ uptake by the synthetic calcium-based sorbents prepared by dispersing CaO precursors across the various support materials such as Al₂O₃ [20], MgO [21], CaTiO₃ [22], SiO₂ [23], Y₂O₃ [24], and cement [25] has been reported. The supporters can stabilize effectively the pore structure and improve CO₂ uptake capacity of the synthetic sorbent. Among those supports, Al₂O₃ is more prospective because of the low cost and the high improvement in CO₂ uptake capacity of CaO. Al₂O₃ can react with CaO to generate the various calcium aluminates such as Ca₁₂Al₁₄O₃₃ [20, 26–29], Ca₉Al₆O₁₈ [30], and Ca₃Al₂O₆ [31] under the different conditions. The calcium aluminates are good supporter to improve CO₂ uptake capacity of CaO. The types of the calcium aluminates are determined by the synthesis methods and the raw materials including CaO precursor (such as limestone [20], nano-CaCO₃ [26], calcium acetate [27], Ca(NO₃)₂ [28], calcium naphthenate [29] and Ca(C₆H₅O₇)₂ [30]), Al₂O₃ precursor (such as Al(NO₃)₃ [20], aluminum sol [26], aluminum acetylacetonate [29]), and dispersant/solvent (2-propanol [20, 27], citric acid [28, 31], xylene [29])). Li et al. [20] used Al(NO₃)₃·9H₂O and CaO into 2-propanol water solution to prepare CaO/Ca₁₂Al₁₄O₃₃ sorbent (the mass ratio of CaO to Ca₁₂Al₁₄O₃₃ = 75:25) by drying and subsequent calcination, which achieved 0.41 g g⁻¹ of CO₂ uptake after 50 cycles. Zhou et al. [30] used Ca(C₆H₅O₇)₂ and Al(NO₃)₃ to produce CaO/Ca₉Al₆O₁₈ sorbent (the mass ratio of CaO to Al₂O₃ = 90:10) and found that its CO₂ capture capacity was 0.51 g g⁻¹ after 28 cycles. Zhang et al. [31] used CaCO₃, citric acid, and aluminum nitrate by citrate preparation route and four step heating mode to fabricate CaO/Ca₃Al₂O₆ (the mass ratio of CaO to Al₂O₃ = 91:9), which retained 0.34 g g⁻¹ of CO₂ uptake after 100 cycles.

It is necessary to prepare high active and cheap CO₂ sorbent in the calcium looping cycles. In this work, we used the limestone, the aluminum nitrate hydrate, and the glycerol water solution by the combustion synthesis method to fabricate the synthetic CO₂ sorbent. The precursor of CaO is the limestone, and it is cheaper than nano-CaCO₃, calcium acetate, Ca(NO₃)₂, calcium naphthenate, and Ca(C₆H₅O₇)₂. The cost of the glycerol used in the preparation of the synthetic sorbent is also lower than that of 2-propanol, citric acid, and xylene. The glycerol as a byproduct can be also obtained from the preparation of the biodiesel fuel [32]. CaO derived from the limestone and the aluminum nitrate hydrate were dissolved in the glycerol water solution. The glycerol was inflammable. The synthetic sorbent was synthesized by Ca²⁺ and Al³⁺ in the glycerol water solution by the combustion. Thus, the combustion process of the solution is also the synthesis process of Ca²⁺ and Al³⁺. The effects of the mass ratio of CaO to Al₂O₃, the cycle number, the carbonation conditions, and the calcination conditions on the CO₂ uptake by the obtained synthetic sorbent were studied. The microstructure of the synthetic sorbent in the calcium looping cycles was also examined.

Experimental

A typical limestone in Shandong Province, China, was the precursor of CaO. The chemical compositions of the limestone were analyzed by X-ray fluorescence (XRF, AXIOS PW4400) as shown in Table 1. The limestone was completely decomposed into CaO at 850 °C for 20 min. CaO derived from the limestone was ground and sieved below 0.125 mm. The aluminum nitrate hydrate (analytical grade Al(NO₃)₃·9H₂O, Shanghai Qingxi Chemical Technology Co., Ltd, China) and the glycerol (analytical grade C₃H₈O₃, Tianjin Kemiou Chemical Reagent Co., Ltd, China) were also used in the preparation of the synthetic calcium-based sorbent. Firstly, 60 mL glycerol was dissolved in 50 mL deionized water and stirred at 25 °C for 20 min and then, the solution was heated from 25 to 80 °C. Secondly, 10 g CaO and some Al(NO₃)₃·9H₂O were added in the solution and stirred at 80 °C. In order to obtain different the synthetic sorbents, the mass ratios of CaO derived from the limestone to Al₂O₃ derived from Al(NO₃)₃·9H₂O were specified as 85:15, 90:10, and 95:5, respectively. CaO reacted with water to generate Ca(OH)₂, which could be dissolved in the glycerol. After Ca(OH)₂ and Al(NO₃)₃·9H₂O were completely dissolved in the glycerol water solution, the solution was sent to a muffle furnace for the combustion synthesis process. Later, the solution was combusted in the muffle furnace (800 °C) under air atmosphere for 1 h. The rapid burning of the glycerol was observed in the combustion process. Simultaneously, the calcium aluminate was synthesized by Ca²⁺ and Al³⁺ in the solution in the combustion process of the glycerol. Therefore, the combustion process was also synthesis process. And the synthetic sorbent was obtained after the combustion synthesis. The synthetic sorbent was ground and sieved to size <0.125 mm.

Table 1 Chemical compositions of the limestone by XRF/mass %

A dual fixed-bed reactor (DFR) [4] operated under atmospheric pressure was used to determine the CO₂ uptake by the synthetic sorbents in the carbonation/calcination cycles. The reactor mainly comprises a carbonator operated in the range of 650–725 °C and a calciner operated in the range of 850–950 °C. The sample boat containing about 1 g sample was repeatedly shifted between the carbonator and the calciner. The sample was firstly carbonated for 20 min under a gas mixer including 15 % CO₂ and 85 % N₂ in the carbonator and then was calcined for 10 min under the pure N₂ or CO₂ in the calciner. And then, the first cycle for CO₂ uptake was finished. At each cycle, the sample mass was weighed after the carbonation and the calcination by an electronic balance (precision accuracy is 0.1 mg). CO₂ uptake capacity was used to describe the CO₂ uptake performance of the synthetic sorbent during the multiple carbonation/calcination cycles as follows:

$$ C_{\text{N}} = \frac{{m_{{{\text{car,N}}}} (t) - m_{{{\text{cal,N}}}} }}{{m_{0} }} $$

(1)

where t is carbonation time, min. C _N means CO₂ uptake capacity of sample in the Nth cycle which indicates CO₂ adsorption amount per unit mass of sample, g g⁻¹. m ₀ represents mass of initial sample, g. m _cal,N is sample mass after complete calcination in the Nth cycle, g. m _car,N (t) denotes sample mass after carbonation at t in the Nth cycle, g.

The CO₂ uptake performance of the synthetic sorbent with the reaction time was studied in an atmospheric thermogravimetric analyzer (TG, Mettler Toledo TGA/SDTA851^e). The uncycled and the cycled sorbents which had experienced various cycles from DFR were selected as the samples in TG. The sample mass was 5 mg. The temperature of TG increased from the room temperature to the carbonation temperature (650–700 °C) with a heating rate of 30 °C min⁻¹ and was kept 15 min at this temperature in pure N₂. Then, the atmosphere was switch to the carbonation atmosphere (15 % CO₂ and 85 % N₂). The CO₂ uptake capacity of the sample was computed according to Eq. 1. The CO₂ uptake rate of the sample was defined as follows:

$$ v_{\text{N}} = \frac{{{\text{d}}C_{\text{N}} }}{{{\text{d}}t}} $$

(2)

where v _N denotes CO₂ uptake rate of sample in the Nth cycle, g g⁻¹ min⁻¹.

The phase compositions of the sample from DFR were examined by an X-ray diffraction (XRD, D/Max-IIIA). The apparent morphologies of the samples after different carbonation/calcination cycles sampled from DFR were detected by a scanning electron microscope (SEM, JEOL JSM-7600F). The element distributions in the surface of the sample from DFR were examined by an energy-dispersive X-ray (EDX, Oxford INCA sight X). The microstructure parameters of the sample from DFR were detected by an accelerated surface area and porosimetry system (Micromeritics, ASAP 2020-M) abased on N₂ adsorption method. It should be mentioned that the surface area and the pore volume of the sample were computed by BET method and BJH model, respectively.

Results and discussion

Effect of mass ratio of CaO to Al₂O₃ on cyclic CO₂ uptake by synthetic sorbent

Figure 1 shows the cyclic CO₂ uptake capacity of the synthetic calcium-based sorbents contained the various mass ratios of CaO to Al₂O₃ in the carbonation/calcination cycles in DFR. The CO₂ uptake capacity of the synthetic sorbent contained the mass ratio of CaO:Al₂O₃ = 95:5 is higher than that of the sorbent possessed the mass ratio of CaO:Al₂O₃ = 90:10 in the previous eight cycles, but the CO₂ uptake capacity of the former is lower than that of the latter after nine cycles. The synthetic sorbent possessed the mass ratio of CaO:Al₂O₃ = 85:15 exhibits lower CO₂ uptake capacity than the sorbent possessed the mass ratio of CaO:Al₂O₃ = 90:10. Thus, the optimum mass ratio of CaO to Al₂O₃ is 90:10. The CaO content of the synthetic sorbent is less than that of the limestone. At the same time, the CO₂ capture activity of CaO derived from the limestone is high during the first cycle. Therefore, the CO₂ capture capacity of CaO is higher that that of the synthetic sorbent (the mass ratio of CaO to Al₂O₃ = 90:10) after one cycle. The CO₂ uptake capacity of the synthetic sorbent (the mass ratio of CaO:Al₂O₃ = 90:10) drops slowly with the number of cycles. As the cycle number increases from 1 to 50, CO₂ uptake capacity of the synthetic sorbent (the mass ratio of CaO:Al₂O₃ = 90:10) decreases by 17 %, while that of CaO drops 88 %. The CO₂ uptake capacity of the synthetic sorbent (the mass ratio of CaO:Al₂O₃ = 90:10) after 50 cycles is 0.43 g g⁻¹, which is about six times as high as that of CaO after 50 cycles for the same reaction conditions.

Fig. 1

Cyclic CO₂ uptake by synthetic sorbent contained different mass ratios of CaO:Al₂O₃ in DFR (carbonation: 700 °C, 15 % CO₂, 20 min; calcination: 850 °C, 10 min, pure N₂)

The XRD spectrum of the obtained synthetic sorbent contained the mass ratio of CaO:Al₂O₃ = 90:10 is presented in Fig. 2. The XRD quantitative analysis shows that the main compositions of the synthetic sorbent contained the mass ratio of CaO:Al₂O₃ = 90:10 are CaO and Ca₃Al₂O₆, and the mass ratio of CaO to Ca₃Al₂O₆ is 74:26. Thus, the following researches focus on the CO₂ uptake by CaO/Ca₃Al₂O₆ (mass ratio of CaO:Ca₃Al₂O₆ = 74:26). The SEM–EDS mapping of the surface of CaO/Ca₃Al₂O₆ sorbent is shown in Fig. 3. Ca, Al, and O elements disperse uniformly in the surface of the initial synthetic sorbent. It suggests that CaO as the CO₂ sorbent and Ca₃Al₂O₆ as the inert support are evenly mixed in the synthetic sorbent. The glycerol is soluble in the water. At the same time, Ca(OH)₂ generated by the hydration of CaO and Al(NO₃)₃·9H₂O are dissolved in the glycerol and the water, respectively. Thus, the obtained solution is the homogeneous solution. Ca²⁺ and Al³⁺ are distributed uniformly in the glycerol water solution. Therefore, CaO and Ca₃Al₂O₆ uniformly distribute in the obtained synthetic sorbent after the combustion synthesis. The high CO₂ uptake capacity of the synthetic sorbent in the cycles is attributed to the good dispersion of CaO and Ca₃Al₂O₆.

Fig. 2

XRD spectrum of initial synthetic sorbent contained mass ratio of CaO:Al₂O₃ = 90:10

Fig. 3

SEM-EDS mapping of initial CaO/Ca₃Al₂O₆ sorbent (mass ratio of CaO:Ca₃Al₂O₆ = 74:26)

Effect of cycle number on cyclic CO₂ uptake by CaO/Ca₃Al₂O₆

The CO₂ uptake capacities and rates of CaO/Ca₃Al₂O₆ sorbent (the mass ratio of CaO:Ca₃Al₂O₆ = 74:26) and CaO derived from the limestone with the number of cycles in TG are plotted in Fig. 4. The CO₂ uptake capacities of CaO/Ca₃Al₂O₆ and CaO increase rapidly with the carbonation time in the previous 5 min and then rise slowly after 5 min, as shown in Fig. 4a. Although the CO₂ uptake capacity of CaO/Ca₃Al₂O₆ is lower than that of CaO in the first cycle at the same reaction time, the CO₂ uptake capacity of the former is much higher than that of CaO in the other cycles. For example, the CO₂ uptake capacities of CaO/Ca₃Al₂O₆ at 5 min in the 10th and the 50th cycles are 2.1 and 6.2 times as high as those of CaO for the same reaction conditions, respectively. For the same reaction time in the previous 2.5 min, the CO₂ uptake capacity of CaO/Ca₃Al₂O₆ remains almost the same with the number of cycles, whereas that of CaO decays rapidly, as illustrated in Fig. 4a. The CO₂ uptake rate is obtained according to Eq. 2, as presented in Fig. 4b. It is found that the carbonation time to reach the maximum CO₂ uptake rate of CaO/Ca₃Al₂O₆ is much sooner than that of CaO in each cycle. The CO₂ uptake rate of CaO/Ca₃Al₂O₆ is higher than that of CaO in previous 5 min except in the first cycle. After 5 min, the CO₂ uptake rate of CaO becomes almost zero, whereas CaO/Ca₃Al₂O₆ still remains the high reaction rate.

Fig. 4

CO₂ uptake capacities and rates of CaO/Ca₃Al₂O₆ sorbent contained mass ratio of CaO:Ca₃Al₂O₆ = 74:26 and CaO in various cycles with reaction time in TG (carbonation: 700 °C, 15 % CO₂, 20 min; calcination: 850 °C, 10 min, pure N₂). a CO₂ uptake capacity, b CO₂ uptake rate

Effect of carbonation conditions on cyclic CO₂ uptake by CaO/Ca₃Al₂O₆

The optimum carbonation temperature window of the general calcium-based sorbent is 650–700 °C in the carbonation/calcination cycles [33, 34]. It is necessary to examine the specific carbonation temperature which is favorable to the CO₂ uptake by CaO/Ca₃Al₂O₆. The effect of the carbonation temperature on the cyclic CO₂ uptake by CaO/Ca₃Al₂O₆ sorbent contained the mass ratio of CaO:Ca₃Al₂O₆ = 74:26 and CaO derived from the limestone is depicted in Fig. 5. In the range of 650–720 °C, CaO/Ca₃Al₂O₆ exhibits slightly higher CO₂ uptake capacity at 700 °C. It is found that the optimum carbonation temperature window of CaO/Ca₃Al₂O₆ is still 650–700 °C. CO₂ uptake capacities of CaO/Ca₃Al₂O₆ at 650, 680, and 700 °C for 20-min carbonation are 4.8, 3.1, and 4.1 times as high as those of CaO after 20 cycles, respectively.

Fig. 5

Effect of carbonation temperature on CO₂ uptake capacities of CaO/Ca₃Al₂O₆ sorbent contained mass ratio of CaO:Ca₃Al₂O₆ = 74:26 and CaO in DFR (carbonation: 15 % CO₂, 20 min; calcination: 10 min, 850 °C, pure N₂)

The effect of the carbonation temperature on the CO₂ uptake capacities and rates of CaO/Ca₃Al₂O₆ (the mass ratio of CaO:Ca₃Al₂O₆ = 74:26) and CaO in the 10th cycle in TG are displayed in Fig. 6. The influence of the carbonation temperature on the CO₂ uptake capacity and rate of CaO is greater than that of CaO/Ca₃Al₂O₆, especially in previous 5 min. For example, with the carbonation temperature increasing from 650 to 700 °C, the CO₂ uptake capacities of CaO/Ca₃Al₂O₆ and CaO for 3-min carbonation in the 10th cycle increase by 6 and 58 %, respectively, as shown in Fig. 6a. The time to reach the maximum CO₂ uptake rate of CaO/Ca₃Al₂O₆ is shortened with the carbonation temperature increasing from 650 to 700 °C in the 10th cycle, as presented in Fig. 6b. The maximum CO₂ uptake rates of CaO/Ca₃Al₂O₆ in the range of 650–700 °C are almost the same. However, the maximum CO₂ uptake rates of CaO in the range of 650–700 °C exhibit great differences.

Fig. 6

CO₂ uptake by CaO/Ca₃Al₂O₆ contained mass ratio of CaO:Ca₃Al₂O₆ = 74:26 and CaO at various carbonation temperatures in the 10th cycle with carbonation time in TG (carbonation: 15 % CO₂, 20 min; calcination: 10 min, 850 °C, pure N₂). a CO₂ uptake capacity, b CO₂ uptake rate

Effect of calcination conditions on cyclic CO₂ uptake by CaO/Ca₃Al₂O₆

CO₂ is captured by CaO in the carbonator to turn into CaCO₃. In order to provide the heat of regeneration of CaCO₃ and produce a pure CO₂ stream (>95 %, dry) for sequestration and recycling in the industrial application, the oxy-combustion of fuel is employed in the calciner. Therefore, the calcination of the carbonated sorbent was carried out in the atmosphere of the almost pure CO₂. It is necessary to investigate the CO₂ uptake by CaO/Ca₃Al₂O₆ for calcination under pure CO₂. Figure 7 shows the temperature and the atmosphere in the calcination on the cyclic CO₂ uptake by CaO/Ca₃Al₂O₆ sorbent contained the mass ratio of CaO:Ca₃Al₂O₆ = 74:26 and CaO derived from the limestone. The previous research reported that improving the calcination temperature intensified the sintering of the calcium-based sorbents and it decreased the CO₂ uptake capacity [15, 35, 36]. As the calcination temperature rises, CaO/Ca₃Al₂O₆ and CaO both exhibit a drop in the CO₂ uptake capacity due to the sintering, but the sintering resistances of the two sorbents are obviously different. With increasing the calcination temperature from 850 to 950 °C under pure N₂, CO₂ uptake capacities of CaO/Ca₃Al₂O₆ and CaO after 20 cycles decay by 14 and 37 %, respectively. Moreover, the high CO₂ concentration in the calcination atmosphere aggravates the sintering [37]. When calcination temperature is 950 °C, CO₂ uptake capacities of CaO/Ca₃Al₂O₆ and CaO calcined under pure CO₂ after 20 cycles are 12 and 40 % lower than those of the two sorbents calcined under pure N₂, respectively. Accordingly, CO₂ uptake capacities of CaO/Ca₃Al₂O₆ and CaO after 20 cycles under the severe calcination conditions (950 °C, pure CO₂) are about 25 and 62 % lower, respectively, compared with those of the two sorbents under the moderate calcination conditions (850 °C, pure N₂).

Fig. 7

Effect of calcination temperature and atmosphere on CO₂ uptake by CaO/Ca₃Al₂O₆ sorbent contained mass ratio of CaO:Ca₃Al₂O₆ = 74:26 and CaO in DFR (carbonation: 700 °C, 15 % CO₂, 20 min; calcination: 10 min)

The effects of the calcination conditions on the CO₂ uptake capacities and rates of CaO/Ca₃Al₂O₆ (the mass ratio of CaO to Ca₃Al₂O₆ = 74:26) and CaO in the 10th cycle in TG are depicted in Fig. 8. The higher temperature and CO₂ concentration in the calcination process lead to drop in the CO₂ uptake capacity of CaO/Ca₃Al₂O₆ and CaO at the same carbonation time in the 10th cycle, but CaO/Ca₃Al₂O₆ retains the higher CO₂ uptake capacity under the severe calcination conditions. For example, when the moderate conditions (850 °C, pure N₂) are changed into the severe conditions (950 °C, pure CO₂), the CO₂ uptake capacity of CaO/Ca₃Al₂O₆ and CaO at 3 min in the 10th cycle drops by approximately 24 and 61 %, as plotted in Fig. 8a. When the calcination conditions become more severe, the CO₂ uptake rates of CaO/Ca₃Al₂O₆ and CaO in the 10th cycle decrease and the time to reach the maximum CO₂ uptake rates is also prolonged, as illustrated in Fig. 8b. It is found that the CO₂ uptake rate of CaO/Ca₃Al₂O₆ is much higher than that of CaO under the more severe calcination conditions at the same carbonation time in the 10th cycle. The results from DFR and TG indicate that CaO/Ca₃Al₂O₆ sorbent possesses obviously higher sintering resistance than CaO under the more severe calcination conditions.

Fig. 8

CO₂ uptake by CaO/Ca₃Al₂O₆ (mass ratio of CaO:Ca₃Al₂O₆ = 74:26) and CaO in the 10th cycle under various calcination conditions in TG (carbonation: 700 °C, 15 % CO₂, 20 min; calcination: 10 min). a CO₂ uptake capacity, b CO₂ uptake rate

The precursors, the preparation method, and CO₂ uptake capacity of CaO/Ca₃Al₂O₆ (mass ratio of CaO:Ca₃Al₂O₆ = 74:26) are compared with those of CaO/calcium aluminates reported in the literature, as presented in Table 2. It can be found that CaO/Ca₃Al₂O₆ sorbent prepared from the limestone, the aluminum nitrate hydrate, and the glycerol water solution by the combustion synthesis exhibits higher CO₂ uptake capacity than some synthetic sorbents such as CaO/Ca₁₂Al₁₄O₃₃ [20, 26, 27] and CaO/Ca₃Al₂O₆ [31]. The materials such as the limestone and the glycerol in the preparation of CaO/Ca₃Al₂O₆ sorbent are also not costly, compared with those in the preparation of the synthetic sorbent reported in the references. Thus, the obtained CaO/Ca₃Al₂O₆ sorbent by the combustion synthesis is a promising CO₂ sorbent in the calcium looping technology.

Table 2 Comparison of CO₂ uptake between CaO/Ca₃Al₂O₆ obtained in this work and other CaO/calcium aluminates presented in literature

Microstructure analysis

The apparent morphologies of CaO/Ca₃Al₂O₆ (the mass ratio of CaO:Ca₃Al₂O₆ = 74:26) and CaO after 1 and 10 cycles detected by SEM analysis are presented in Fig. 9. Compared with that of CaO, the surface of CaO/Ca₃Al₂O₆ appears more porous and loose than that of CaO after the same cycles. As the number of cycles rises from 1 to 10, lots of pores in the surface of CaO are blocked due to the sintering, but the surface of CaO/Ca₃Al₂O₆ still retains the porous structure. The combustion products (CO₂ and water vapor) are quickly released from the synthetic sorbent in the combustion synthesis step, which maybe leads to the formation of the porous structure. The stable porous structure of CaO/Ca₃Al₂O₆ is favorable for maintaining the high cyclic CO₂ uptake capacity.

Fig. 9

SEM images of CaO/Ca₃Al₂O₆ (mass ratio of CaO:Ca₃Al₂O₆ = 74:26) and CaO with number of cycles (carbonation: 700 °C, 15 % CO₂, 20 min; calcination: 10 min, 850 °C, pure N₂). a CaO after 1 cycle, b CaO/Ca₃Al₂O₆ after 1 cycle, c CaO after 10 cycle, d CaO/Ca₃Al₂O₆ after 10 cycle

The surface areas and the pore volumes of CaO/Ca₃Al₂O₆ (mass ratio of CaO:Ca₃Al₂O₆ = 74:26) and CaO in 20 cycles are shown in Fig. 10. The surface area and the pore volume of CaO/Ca₃Al₂O₆ keep more stable than those of CaO with the cycle number. As the cycle number increases from 1 to 20, the surface areas of CaO/Ca₃Al₂O₆ and CaO drop about 18 and 75 %, respectively. After 20 cycles, the surface area and the pore volume of CaO/Ca₃Al₂O₆ are approximately 4.8 and 2.2 times as high as those of CaO, respectively. The high and stable surface area and pore volume of CaO/Ca₃Al₂O₆ sorbent in the cycles facilitate the high cyclic CO₂ uptake capacity.

Fig. 10

Surface area and pore volume of CaO/Ca₃Al₂O₆ (mass ratio of CaO:Ca₃Al₂O₆ = 74:26) and CaO with number of cycles (carbonation: 700 °C, 15 % CO₂, 20 min; calcination: 10 min, 850 °C, pure N₂). a Surface area, b Pore volume

The pore volume distributions of CaO/Ca₃Al₂O₆ (the mass ratio of CaO:Ca₃Al₂O₆ = 74:26) and CaO in 20 cycles are depicted in Fig. 11. There are two peaks in the pore volume distribution curves of the two sorbents. One peak is in 2–5 nm, and another peak is in 20–100 nm. It is found that volume of pores in the entire measured range for CaO/Ca₃Al₂O₆ decreases slightly, whereas that for CaO drops rapidly. The small pores are prone to be blocked in the CO₂ uptake by the sorbent, whereas the pores in 20–100 nm in diameter are important felids for CO₂ uptake [38, 39]. As the cycle number increases from 1 to 20, the volumes of pores in 20–100 nm in diameter for CaO/Ca₃Al₂O₆ and CaO decrease by 9 and 57 %, respectively. It reveals that Ca₃Al₂O₆ is good supporter and maintains stable in the pore structure of CaO/Ca₃Al₂O₆ during the multiple cycles. Therefore, CaO/Ca₃Al₂O₆ (the mass ratio of CaO:Ca₃Al₂O₆ = 74:26) can retain high CO₂ uptake activity in the cycles.

Fig. 11

Pore volume distributions of CaO/Ca₃Al₂O₆ (mass ratio of CaO:Ca₃Al₂O₆ = 74:26) and CaO with number of cycles (carbonation: 700 °C, 15 % CO₂, 20 min; calcination: 10 min, 850 °C, pure N₂)

Conclusions

The limestone, the aluminum nitrate hydrate, and the glycerol water solution as the materials were used to prepare a new CaO/Ca₃Al₂O₆ sorbent by the combustion synthesis. CaO/Ca₃Al₂O₆ sorbent contained the mass ratio of CaO to Al₂O₃ = 90:10 exhibits higher CO₂ uptake capacity, and its main compositions are CaO and Ca₃Al₂O₆ (the mass ratio of CaO to Ca₃Al₂O₆ is 74:26). The CO₂ uptake capacity of CaO/Ca₃Al₂O₆ sorbent after 50 cycles can retain 0.43 g g⁻¹, which is about six times as high as that of CaO derived from the limestone for the same reaction conditions. The CO₂ uptake rate of CaO/Ca₃Al₂O₆ is much higher than that of CaO during previous 5 min after two cycles. The optimum carbonation temperature window of CaO/Ca₃Al₂O₆ is 650–700 °C. CaO/Ca₃Al₂O₆ shows higher sintering resistance than CaO under the more severe calcination conditions. After 20 cycles, the surface area and the pore volume of CaO/Ca₃Al₂O₆ are about 4.8 and 2.2 times as high as those of CaO, respectively. CaO/Ca₃Al₂O₆ possesses more pores in the range of 20–100 nm in diameter during the cycles, which is helpful to maintain high CO₂ uptake capacity. CaO/Ca₃Al₂O₆ obtained by the combustion synthesis is promising as a high active CO₂ sorbent in the calcium looping cycles.