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Article

A Carbide Slag-Based, Ca12Al14O33-Stabilized Sorbent Prepared by the Hydrothermal Template Method Enabling Efficient CO2 Capture

by
Xiaotong Ma
1,
Yingjie Li
1,*,
Yi Qian
1 and
Zeyan Wang
2
1
School of Energy and Power Engineering, Shandong University, Jinan 250061, China
2
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China
*
Author to whom correspondence should be addressed.
Energies 2019, 12(13), 2617; https://doi.org/10.3390/en12132617
Submission received: 25 May 2019 / Revised: 27 June 2019 / Accepted: 4 July 2019 / Published: 8 July 2019
(This article belongs to the Special Issue Carbon Capture, Storage and Utilization)
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Abstract

:
Calcium looping is a promising technology to capture CO2 from the process of coal-fired power generation and gasification of coal/biomass for hydrogen production. The decay of CO2 capture activities of calcium-based sorbents is one of the main problems holding back the development of the technology. Taking carbide slag as a main raw material and Ca12Al14O33 as a support, highly active CO2 sorbents were prepared using the hydrothermal template method in this work. The effects of support ratio, cycle number, and reaction conditions were evaluated. The results show that Ca12Al14O33 generated effectively improves the cyclic stability of CO2 capture by synthetic sorbents. When the Al2O3 addition is 5%, or the Ca12Al14O33 content is 10%, the synthetic sorbent possesses the highest cyclic CO2 capture performance. Under harsh calcination conditions, the CO2 capture capacity of the synthetic sorbent after 30 cycles is 0.29 g/g, which is 80% higher than that of carbide slag. The superiority of the synthetic sorbent on the CO2 capture kinetics mainly reflects at the diffusion-controlled stage. The cumulative pore volume of the synthetic sorbent within the range of 10–100 nm is 2.4 times as high as that of calcined carbide slag. The structure of the synthetic sorbent reduces the CO2 diffusion resistance, and thus leads to better CO2 capture performance and reaction rate.

Graphical Abstract

1. Introduction

With the frequent occurrence of various extreme climates around the world and the increasingly intensified global climate change caused by greenhouse gases, a global consensus has been reached to reduce greenhouse gas emissions [1,2,3]. The Special Report on Global Warming of 1.5 °C adopted by the 48th plenary session of the Intergovernmental Panel on Climate Change (IPCC) [4] illustrates the point that human factors have caused global average temperature to increase by 1 °C compared with those before industrialization. If this trend continues, the global temperature could reach 1.5 °C or even 2 °C above the baseline, that is, pre-industrial level, sometime between 2030 and 2052. The main reason is that humans have been using fossil fuels (such as coal, oil, and so on) in large quantities, and the combustion process accompanied by heavy emissions of CO2 and other greenhouse gases. Calcium looping (CaL) process is one of the feasible large-scale CO2 capture technologies at present, which is applicable to the process of coal-fired power generation and gasification of coal/biomass for hydrogen production [5,6,7,8,9]. However, one of the problems restricting the development of this technology is the attenuation of CO2 capture activity of CaO-based sorbents due to high-temperature sintering [10,11,12].
So far, how to improve the CO2 capture performance of CaO-based sorbents has become a research hotspot. Researchers have put forward many methods to solve this problem, aiming at optimizing CaO-based sorbents to maintain good CO2 absorption activity and anti-sintering performance in the cyclic process, and finally obtaining a high CO2 capture efficiency. These methods include hydration [13,14], thermal pre-treatment [15,16], use of porous CaO-based precursors [17,18], and doping [19,20,21,22]. Unfortunately, the modified sorbents still suffer from a pore blocking in the microstructure. Therefore, some inert materials serving as microstructure stabilizers have been added in the CaO-based sorbents [5,23,24,25,26].
Studies have shown that calcium aluminates can effectively stabilize the pore structure of CaO and retard the sintering of CaO particles [27,28,29]. CaO-based, calcium aluminates-stabilized sorbents have gained increasing interest owing to their advantages such as wide material sources, good sintering resistance, and high cyclic CO2 capture capacity. The formation of calcium aluminates depended on different synthesis conditions and precursors. Zhou et al. [30] found that by means of mixing calcium nitrate and aluminum nitrate, CaO was uniformly mixed with Ca9Al6O18, an inert support. Li et al. [31] proved that the uniform distribution of Ca, Al, and O atoms in the CaO/Ca3Al2O6 composite sorbent was the main reason for the high CO2 capture capacity of the sorbent. Vanga et al. [5] assessed a CaO/Ca12Al14O33 sorbent over 200 multiple cycles and the stabilizing role of spacer Ca12Al14O33 ceramic against sorbent decay was confirmed. Its CO2 capture capacity drops by 13% up to 200 cycles. Rui et al. [24] reported that Ca3Al2O6 was formed in the synthetic sorbent with the addition of 10 mol% Al2O3. The CO2 capture capacity of the synthetic sorbent exceeded that of the benchmark CaO by more than 300% after 20 cycles. In addition, some low-cost industrial products or natural minerals, such as fly ash [32], attapulgite [33], kaolin [34,35], and cement [36,37,38], were also used to prepare CaO-based, calcium aluminates-stabilized sorbents. Chen et al. [32] found that doping fly ash into limestone could significantly improve the CO2 capture performance of limestone, and Ca12Al14O33 was generated as the support. After 20 cycles, the CO2 absorption amount of the sorbent doped with fly ash was twice that of undoped limestone. Chen et al. [33] modified limestone by attapulgite using the wet mixing method, and the obtained Ca3Al10O18 promoted the anti-sintering performance of the sorbents at a high temperature. The sorbent doped with 15 wt.% attapulgite displayed better CO2 capture performance than limestone by an increase of 128% after 20 cycles under the same condition (calcination at 950 °C in 100% CO2 and carbonation at 700 °C in 15% CO2/85% N2). Ridha et al. [34] used kaolin as both an aluminum source and a binder to granulate limestone. After 10 cycles under the calcination condition of 920 °C and pure CO2, the carbonation conversion was 17%. Luo et al. [36] obtained a CaO/Ca12Al14O33 sorbent by mixing sol-gel CaO powder with cement, which proved that the presence of Ca12Al14O33 improved the CO2 capture performance and cycling stability of the synthetic sorbent. At the carbonation and calcination temperatures of 650 °C and 850 °C, respectively, the CO2 capture capacity after 50 cycles reached 0.43 g/g. Sun et al. [37] prepared pellets from limestone, cement, and rice husks via blending and pelletizing, which had a high CO2 capture capacity and mechanical strength.
A large amount of carbide slag, mainly composed of Ca(OH)2, is discharged from the polyvinyl chloride (PVC) and chlor-alkali industries every year [39,40]. In China, the annual output of PVC in a chlor-alkali plant is about 600,000 tons, accompanied by about 1.1 million tons of carbide slag generated per year [41]. In 2016, PVC production capacity reached 23 million tons in China, accounting for 40% of the global production capacity. This means that carbide slag output is more than 40 million tons a year. The piled-up carbide slag causes damage to the surrounding ecological environment and threatens human health. In addition, it is difficult to recycle carbide slag on a large scale, which results in the waste of calcium resources. Therefore, the utilization of carbide slag as CO2 sorbents is supposed to solve the practical problem of waste accumulation and lower the cost of the CaL process.
It has been proven that carbon templating is a feasible way to synthesize ordered mesoporous CO2 sorbents. Also, past work involved testing the sorbents prepared using carbide slag and alumina cement with the carbon template obtained from the hydrothermal carbonation of glucose. Although the synthetic sorbent showed stable CO2 capture performance, its initial CO2 uptake was low. It is necessary to study the method to further improve the CO2 uptake. Li et al. [42] found that the limestone treated with acetic acid solution had high CO2 capture activity, and the carbonation conversion after 20 cycles was three times higher than that of untreated limestone. With the treatment by acetic acid, carbide slag with poor solubility in water can be converted into calcium acetate as a soluble material. It is generally known that soluble precursors are favorable for the homogenous distribution of CaO and inert supports, which is supposed to enhance the sintering resistance of CaO-based sorbents [30,31]. In addition, the pore structure of sorbents can be further improved to help increase the initial CO2 uptake.
In this work, the synthetic sorbent with acid-washed carbide slag and aluminum nitrate as precursors by hydrothermal template method was reported. The CO2 capture characteristics of synthetic sorbents were studied to validate the practical ability. Morphological characterizations were also used to explain the relationship between the micro pore structure and the performance.

2. Experimental

2.1. Sorbent Preparation

The carbide slag was sampled from a chlor-alkali plant in Shandong Province, China. The chemical components of the raw carbide slag were analyzed by X-ray fluorescence (XRF), as shown in Table 1. Aluminum nitrate, acetic acid, and glucose used in this work were analytical reagents.
The template synthesis procedure was the same as in the previous work [43]. Aluminum nitrate was used as the aluminum precursor, acetic acid-washed carbide slag was used as the Ca precursor, and glucose was used as the carbon template precursor for template synthesis in this work. The acid-washing process of carbide slag was described below. Predetermined amounts of carbide slag were placed into a beaker. Deionized water was added at a carbide slag to deionized water ratio of 1 g/8 mL and the suspension was stirred evenly. Acetic acid was poured slowly into the suspension at a molar ratio of Ca to acetic acid of 2:1, and then it was stirred in a water bath at 50 °C for 20 min. The mixture was filtered and the filtrate was dried at 120 °C until all solids were separated. After grinding, the acetic acid-washed carbide slag was obtained. Synthetic sorbents were prepared by the hydrothermal template method. Glucose was dissolved in 60 mL deionized water to obtain a solution with a concentration of 1 mol/L. Six grams of acetic-washed carbide slag and aluminum nitrate with different CaO/Al2O3 weight ratios (97.5:2.5, 95:5, 92.5:7.5, and 90:10) were added to the solution and stirred in a water bath at 60 °C for 30 min. The mixture was poured into a high-pressure kettle for a hydrothermal synthesis at 180 °C for 6 h. The precipitate was collected after separation, washed with water and ethanol, dried, and pyrolyzed under a nitrogen flow at 450 °C for 2 h with a heating rate of 3 °C/min. Finally, the sample obtained was calcined under air at 600 °C for 1 h in a muffle furnace. It is worth noting that the errors of the CaO/Al2O3 ratios are less than 0.2%, considering that the complexity exists in carbide slag and the insoluble matter is removed during the acid-washing process. The synthetic sorbents were symbolized by HTx, where x is the weight percentage of Al2O3. As a comparison, the acetic acid-washed carbide slag and aluminum nitrate were directly mixed in a solution phase, dried, and calcined in air atmosphere at 600 °C for 1 h. The synthetic sorbent with the Al2O3 content of 5% was prepared by the wet mixing method, which is denoted as WM5. Weighed amounts of acetic acid-washed carbide slag and aluminum nitrate were mixed in a glass beaker. Water was added with stirring and the solution obtained was then pre-calcined under air at 600 °C for 1 h.

2.2. Cyclic CO2 Capture Test

The cyclic calcination/carbonation reaction was performed in a dual fixed-bed reactor (DFBR) with a steam generator, as shown in Figure 1. The two reactors were a calciner and a carbonator, respectively. About 200 mg of the sample was placed into the calciner and maintained for 10 min for the full decomposition of the sample. Subsequently, the sample was cooled for 2.5 min in N2 and weighed by a precise Mettler Toledo-XS105DU electronic balance (±0.1 mg). The sample was placed in the carbonator and maintained at 700 °C for 20 min. The weight of the sample after the carbonation was also measured, after which the calcination/carbonation cycles began. The preliminary experiments on the discontinuous 30 cycles of calcination/carbonation with cooling steps and continuous 10, 20, and 30 cycles without cooling steps were performed using carbide slag, respectively. The results showed that the cooling steps had a small impact on the CO2 capture performance obtained, as shown in Figure 2. Therefore, masses of the sample were measured after each step in the following experiments to evaluate the CO2 capture performances of the sample after all cycles and avoid the random error. The gas flow rates were controlled by mass flow meters to meet the designed reaction atmosphere and steam was generated from a steam generator. All gases were premixed evenly before they were sent into reactors together. The total flow rate was 2 L/min. The samples were calcined in the conditions of 850 °C—100% H2O, 850 °C—85% H2O/15% CO2, or 920 °C—70% CO2/N2, and carbonated at 700 °C under 15% CO2/N2 or 10% H2O/15% CO2/N2. The cyclic CO2 capture capacities and the carbonation conversions of the samples were calculated from the recorded weight changes during repeated calcination and carbonation reactions, as shown in Equations (1) and (2).
C N = m N − m cal , N m 0 ,
X N = m N − m cal , N m 0 ⋅ M CaCO 3 M CO 2 ,
where N is the number of calcination/carbonation cycles; CN and XN denote the CO2 capture capacity and the carbonation conversion of the sorbent after the Nth cycle, respectively; m0 is the initial mass of the sorbent after calcination, g; mN and mcal,N are the masses of the sorbent after the Nth carbonation and calcination, respectively, g; and MCaCO3 and MCO2 are the molar masses of CaCO3 and CO2, respectively, g/mol.
The CO2 capture kinetics of the synthetic sorbent was evaluated by a thermo-gravimetric analyzer (TGA, Mettler Toledo TGA/SDTA851e). Around 5 mg of the pristine sample or the sample that had experienced 10 cycles in DFBR was placed in an alumina pan and put in the furnace together. Temperature was raised from ambient temperature to the carbonation temperature of 700 °C at a rate of 30 °C/min under a N2 flow of 120 mL/min. Once the temperature was reached, the N2 flow was switched to a flow of 120 mL/min CO2. Carbonation was performed for 30 min. The mass change of the sample was continuously monitored and the CO2 capture capacity (CN) and the CO2 capture rate (μt) were calculated, respectively, as shown in Equations (1) and (3).
μ t = d C N d t

2.3. Characterization

Selected samples were characterized with regards to crystalline phase using X-ray diffraction instrument (XRD, D/Max-III, Rigaku Co., Ltd, Japan), pore characteristics using N2 adsorption equipment (Micromeritics ASAP 2020-M, Micrometer Co., Ltd, Shanghai, China), and micro-morphologies using field emission scanning electron microscope (FE-SEM, SUPRATM 55, Zeiss Co., Ltd, Germany).

3. Results and Discussion

3.1. Composition of the Synthetic Sorbent

The XRD patterns of the synthetic sorbents are shown in Figure 3. It can be found that the phase compositions of the synthetic sorbent are unchanged when the CaO/Al2O3 ratios are different. The two synthetic sorbents are mainly composed of CaO and Ca12Al14O33. The peak values of HT10 are higher than those of HT5, which indicates that the Ca12Al14O33 content is higher with a lower CaO/Al2O3 ratio. Apparently, CaO is derived from the decomposition of the acetic acid-washed carbide slag, i.e., calcium acetate. In addition, Al2O3 is generated by the decomposition of aluminum nitrate. A portion of CaO reacts with Al2O3 at a high temperature during the preparation process and Ca12Al14O33 is formed, as shown in Equation (4) [44,45]. The computed mass ratio of CaO/Ca12Al14O33 in HT5 and HT10 is about 90:10 and 80:20, respectively. Ca12Al14O33 is one of the effective support materials for CaO-based sorbents, because of its high stability and separation effect on CaO particles under a high temperature [32,46].
12 CaO   +   7 Al 2 O 3 → Ca 12 Al 14 O 33

3.2. Comparison of Preparation Methods

The cyclic CO2 capture capacities of synthetic sorbents prepared by different methods are analyzed, as shown in Figure 4. The initial CO2 capture capacities, C1, of HT5 and WM5 are 0.63 g/g and 0.61 g/g, which are 8.9% and 4.3% higher, respectively, than that of carbide slag. This phenomenon is mainly because of the fact that Ca(OH)2 in carbide slag reacts with acetic acid to form Ca(CH3COO)2. Ca(CH3COO)2 decomposes at a high temperature and thus gas is released, leaving a large number of pores in the synthetic sorbents. As a result, the diffusion resistance of CO2 during carbonation process is weakened and the CO2 capture performance of HT5 is improved. During the following 10 cycles, the CO2 capture capacity of WM5 decreases by 13.2%, while the value for HT5 is only about 8%. Given that the combustion of carbon template contributes to the formation of a porous microstructure, the well-distributed CaO and Ca12Al14O33 results in a stable and porous surface of the synthetic sorbent when the hydrothermal template method is applied. Therefore, HT5 shows a higher CO2 capture capacity than WM5. In addition, C1 of HT5 is 38% higher than the synthetic sorbent with the addition of 5% cement. After 10 cycles, C10 of HT5 is 1.5 times as high as that of the one with 5% cement. This indicates that the synthetic sorbent prepared using acetic acid-washed carbide slag and aluminum nitrate as soluble precursors possesses better CO2 capture performance than that prepared using carbide slag and cement as insoluble precursors. However, from an economic point of view, the use of carbide slag and cement is more cost-effective than the use of acetic acid-washed carbide slag and aluminum nitrate in preparation. With the increase in cycle numbers, the CO2 capture capacities of the three synthetic sorbents are relatively stable, and attenuations of the CO2 capture capacities are slower than that of carbide slag. After the 10 cycles, C10 of HT5 is 0.58 g/g, which is 1.1 times and 2 times those of WM5 and carbide slag, respectively. To sum up, HT5 achieves the best CO2 capture performance during the cyclic CO2 capture process.
The comparisons between CO2 capture capacities of HT5 and diverse CaO-based, calcium aluminates-stabilized sorbents reported elsewhere are summarized in Table 2. Li et al. [31] fabricated a CaO/Ca3Al2O6 sorbent using the same calcium and aluminum precursors as this study by the combustion method. C10 of the synthetic sorbent is 0.45 g/g, which is about 20% lower than the value of HT5. Luo et al. [47] reported the synthesis of sorbents supported by different materials, such as Ca2MnO4, La2O3, Ca12Al14O33, and MgO, using the sol-gel technique, and conformed better performance of the CaO/Ca12Al14O33 sorbent than others. However, a large amount of calcium nitrate decomposes under the high-temperature preparation process and toxic nitrogen oxides, which are adverse to the environment and human health, are released with it. This is the disadvantage of this method. Broda et al. [48] used carbon gel as a template to fabricate synthetic CaO-based sorbents containing Ca12Al14O33, which achieved C10 of 0.56 g/g when the calcination atmosphere was 100% N2. In comparison, it can be found that HT5 shows superior CO2 capture performance. Therefore, the synthetic sorbent prepared by hydrothermal template method is a kind of sorbent with good CO2 capture performance and cyclic stability.

3.3. Effect of Support Ratios

The influence of the Al2O3 ratio (2.5–10%) in the preparation process on the cyclic CO2 capture performance of synthetic sorbents is studied, as shown in Figure 5. It can be seen from Figure 5a that all of synthetic sorbents prepared by the hydrothermal template method show more stable CO2 capture performance than carbide slag under the calcination condition of 100% N2. After 10 cycles, C10 of HT2.5, HT5, HT7.5, and HT10 is 0.62 g/g, 0.58 g/g, 0.57 g/g, and 0.52 g/g, respectively, which is only 4.7%, 8%, 5.5%, and 3% lower than C1 of those sorbents. The results show that the sintering resistance of synthetic sorbents can be improved with the addition of Al2O3 in the range of 2.5–10%. This is because of Ca12Al14O33 generated by the solid-phase reaction between Al2O3 and CaO, which effectively improves the anti-sintering performance of the synthetic sorbents and the cyclic stability of CO2 capture. As the amount of Al2O3 increases, more CaO reacts with Al2O3 to generate Ca12Al14O33, resulting in the decrease of CaO content and the theoretical maximum CO2 capture capacity of the synthetic sorbent. Therefore, there is an appropriate amount of Al2O3 to enable the synthetic sorbent to obtain higher CO2 capture performance and cycling stability.
In the industrial application, the regeneration of CaO-based sorbents is carried out under a high concentration of CO2, which corresponds to a high calcination temperature. Valverde et al. [50] and Manovic et al. [51] both came to the conclusion that CaO particles suffered from severe sintering under the calcination condition of 920 °C, 70% CO2/N2. Therefore, the high concentration of N2 is a mild calcination condition and the high concentration of CO2 is a relatively harsh calcination condition. As shown in Figure 5b, the CO2 capture capacities of synthetic sorbents decrease slowly with the increase of cycle numbers under the harsh calcination condition, while that of carbide slag decreases rapidly. For example, the calculated attenuation ratios of HT2.5, HT5, HT7.5, and HT10 are 45.3%, 38.8%, 36.6%, and 34.4%, respectively, during the following 10 cycles, while the value of carbide slag is as high as 65.9%. This suggests that the synthetic sorbents show relatively higher sintering resistance than carbide slag. After 10 cycles, C10 of HT2.5, HT5, HT7.5, and HT10 is 68.6%, 80.9%, 78.2%, and 64.1% higher, respectively, than that of carbide slag. Although C1 of HT2.5 is higher than that of HT5, its CO2 capture capacity decreases more noticeably with the number of cycles. At the 10th cycle, HT5 is the best performing synthetic sorbent in this set of experiments, with a CO2 capture capacity of 0.38 g/g. The average CO2 capture capacity of HT5 is 0.48 g/g in the following 10 cycles, which is 7% higher than that of HT2.5. By comprehensive consideration of CO2 capture capacity and stability, the following experiments are focused on HT5.

3.4. Effect of Reaction Conditions

Studies have shown that O2/H2O combustion is a promising technology with many advantages, such as easy operation and energy saving, compared with O2/CO2 combustion. Therefore, the calcination atmosphere with a high concentration of steam and a high concentration of CO2 belong to actual calcination conditions. The cyclic CO2 capture capacities of HT5 under three calcination conditions (850 °C—100% H2O, 850 °C—85% H2O/15% CO2, and 920 °C—70% CO2/N2) are evaluated, as shown in Figure 6. Both HT5 and carbide slag show the best CO2 capture performance under the calcination atmosphere of 100% H2O. C10 of HT5 and carbide slag is 0.46 g/g and 0.28 g/g, which is 21.9% and 34.6% higher, respectively, than those calcined under the high concentration of CO2. Moreover, C10 of carbide slag under the calcination atmosphere of 85% H2O/15% CO2 is 10.6% higher than that when the calcination under the high concentration of CO2 is applied, although this difference between C10 of HT5 under the two conditions is not significant. Studies have shown that the presence of CO2 in the calcination atmosphere accelerates the process of sintering and reduces the specific surface area of CaO-based sorbents [52,53]. The undiminished CO2 capture capacity of HT5 under the high concentration of CO2 means that HT5 has better sintering resistance than carbide slag. Calcination under the high concentration of H2O is more conducive to the cyclic stability of HT5 during the CO2 capture process. HT5 shows better CO2 capture performance than carbide slag under all of three calcination conditions. The average CO2 capture capacities of HT5 in the 10 cycles are 1.4 times, 1.4 times, and 1.5 times as high as those of carbide slag, respectively.
The effect of H2O existing in the carbonation atmosphere on the CO2 capture by HT5 is studied under the harsh calcination condition, as shown in Figure 7. It is apparent that HT5 and carbide slag achieve higher CO2 capture capacities in different degrees with the presence of 10% H2O in the carbonation atmosphere. C10 of HT5 and carbide slag is increased by 9.2% and 56.2%, respectively, in the presence of H2O. The result shows that, by contrast, the presence of H2O has a greater effect on promoting the CO2 capture performance of carbide slag. This is because the presence of H2O enhances the proliferation of O2- ions in CaO [54,55]. Carbide slag suffers from serious sintering in the cyclic process, which leads to less porous structure. On this occasion, O2- ions must penetrate the CaCO3 product layer in carbide slag to contact with external CO2. Therefore, the CO2 diffusion resistance for carbide slag increases. The good sintering resistance of HT5 is conducive to the relative stability of pore structure, which results in a lower diffusion resistance of CO2. This is the explanation for the more pronounced influence of H2O on the CO2 capture performance of carbide slag than that of HT5. After 10 cycles, C10 of HT5 carbonated with the presence of H2O exceeds that of carbide slag by 26.5%.

3.5. CO2 Capture during Extended Cycles

Figure 8 shows the CO2 capture performance of HT5 during 100 cycles under mild and harsh calcination conditions. It is interesting to notice that the capture performance degradation mainly occurs in the previous 30 cycles. After 100 cycles, the CO2 capture capacities of HT5 under mild and harsh calcination conditions decrease by 51.2% and 61.1%, respectively, while those of carbide slag decrease by 69.7% and 78%, respectively. Therefore, HT5 possesses better stability during the long-term cycles. Under the harsh calcination condition, the average CO2 capture capacity of HT5 in the 100 cycles is 0.29 g/g, which is 72.1% higher than the value of carbide slag. C100 of HT5 is 0.31 g/g and 0.24 g/g under the mild and harsh calcination conditions, respectively, which are both 1.8 times as high as those of carbide slag under the same conditions. In conclusion, HT5 has better long-term CO2 capture performance under both mild and harsh calcination conditions. To maintain the overall CO2 capture efficiency at a high level in the industrial application, deactivated sorbents are continually discharged and a great quantity of fresh sorbents are added during the CaL process [8]. Studies have shown that energy consumption apparently decreases with the increased cyclic CO2 capture capacity of the calcium-based sorbents [56,57]. Therefore, the utilization of HT5 is supposed to result in the decrease in make-up sorbents and energy saving compared with the situation when carbide slag is applied.

3.6. Apparent CO2 Capture Kinetics

Figure 9a,b show the CO2 capture capacities and the apparent CO2 capture rates of HT5 with time during the 1st and 11th carbonation processes, respectively. The CO2 capture capacities of HT5 and carbide slag increase rapidly with time in the previous 50 s and the increase becomes slower after 50 s, as shown in Figure 9a. In the 1st carbonation, the CO2 capture capacity of HT5 is almost same as that of carbide slag within 100 s, gradually higher than that of carbide slag after 100 s, and eventually 4.3% higher than that of carbide slag after 30 min. In the 11th carbonation, The CO2 capture capacities of HT5 and carbide slag are at lower levels compared with those in the 1st carbonation as a result of high-temperature sintering. The difference is a significant reduction in the CO2 capture capacity of carbide slag in the previous 100 s. The final CO2 capture capacity of HT5 is obviously higher and the cycle stability is better than that of carbide slag.
The apparent CO2 capture rates of HT5 and carbide slag are shown in Figure 9b. During the 1st and 11th carbonation processes, HT5 and carbide slag have relatively high CO2 capture rates in the previous 75 s, and the CO2 capture rates decreases gradually after 75 s. In the 1st carbonation, the CO2 capture rates of HT5 and carbide slag reach their maximum values at around 45 s. The CO2 capture rate of carbide slag is slightly higher than that of HT5 in the previous 45 s, while it falls rapidly and remains lower than that of HT5 after 45 s. This phenomenon suggests that the more CO2 diffusion degree in HT5 contributes to the process of carbonation. The CO2 capture rates of HT5 and carbide slag in the 11th carbonation are relatively lower than those in the 1st carbonation. However, the increased cycle number has less impact on carbonation performance of HT5 than that of carbide slag. The maximum CO2 capture rate of carbide slag moves forward to 35 s and the duration of chemical reaction-controlled stage is shortened. After 10 cycles, HT5 possesses higher CO2 capture rate than carbide slag.
The shrinking core model is used to describe the CO2 capture kinetics because of its high accuracy [58,59]. At the chemical reaction-controlled and diffusion-controlled stages, the carbonation conversions of CaO-based sorbents with time are described as Equations (5) and (6) [60,61].
1 − ( 1 − X N ) 1 / 3 = k C ( P CO 2 − P eq ) t ,
1 − 3 ( 1 − X N ) 2 / 3 + 2 ( 1 − X N ) = k D ( P CO 2 − P eq ) t ,
where kC and kD represent the apparent reaction rate constants at the two stages, respectively, MPa−1∙s−1. Peq and PCO2 represent the equilibrium concentration and partial pressure of CO2, respectively, MPa. By means of the linear fitting, the relations between carbonation conversion and time for HT5 and carbide slag at the two stages are obtained, as shown in Figure 10a,b. The value of kC and kD are calculated accordingly and the results are given in Table 3. According to the correlation coefficients, that is, R2, the conclusion can be drawn that the shrinking core model matches better with the results at the chemical reaction-controlled stage than those at the diffusion-controlled stage. The apparent reaction rate constants of HT5 and carbide slag are reduced with cycles. kC is much higher than kD, which suggests that the CO2 capture rate of the CaO-based sorbent at the chemical reaction-controlled stage is far above that at the diffusion-controlled stage. In the 1st carbonation, kC of carbide slag is higher than that of HT5, while kD of carbide slag is relatively lower. This phenomenon indicates that the structure of HT5 is more favorable to the diffusion of CO2 inside the sorbents, and thus the process of carbonation reaction. After 10 cycles, kC and kD of HT5 are 26.3% and 58.9% higher than those of carbide slag, respectively. Therefore, the superiority of the synthetic sorbent prepared using the hydrothermal template method on the CO2 capture kinetics mainly reflects at the diffusion-controlled stage.

3.7. Microstructure Analysis

The apparent morphologies of HT5 and carbide slag after 10 cycles are observed by SEM apparatus, as shown in Figure 10. Carbide slag has smooth and dense surface morphology, as shown in Figure 11a. This phenomenon is attributed to sintering. Low porosity hinders CO2 diffusion, which results in poor CO2 capture performance of carbide slag under the harsh calcination condition. It can be seen from Figure 11b that HT5 undergoing 10 cycles under the harsh calcination condition is sphere-shaped. With a magnification factor of 50,000, it can be found that the average grain size of HT5 is smaller than that of carbide slag, as illustrated in Figure 11c,d. In addition, HT5 has larger porosity than carbide slag. The result proves the superiority of the pore structure of HT5 again, and this is the reason for the high CO2 capture capacity of HT5 under harsh calcination conditions.
N2 adsorption analysis is conducted on the pore characteristics of HT5 and carbide slag after 10 cycles calcined under the harsh calcination condition, as shown in Figure 12. The N2 adsorption–desorption isotherms, Figure 12a, reveal the type IV curves and the presence of a hysteresis loop for both HT5 and carbide slag. Thus, they have mesoporous structures and the maximum quantity adsorbed of the synthetic sorbent is higher than that of carbide slag. In order to further explore the mechanism of the influence of the pore properties on the CO2 capture characteristics of the synthetic sorbent, the surface areas and pore volumes of HT5 and carbide slag are analyzed, as shown in Table 4. After 10 cycles, the surface area and the pore volume of HT5 are 7 m2/g and 0.038 cm3/g, respectively, which are more than double those of calcined carbide slag. Figure 12b shows the pore volume distribution of HT5. It can be clearly seen that the pore volume of the synthetic sorbent in the range of 2–100 nm is significantly improved after the preparation process by the hydrothermal template method. Studies have shown that the CO2 capture rate that CaO-based sorbents can achieve is mainly determined by the pores within the range of 10–100 nm [62]. The cumulative pore volume of calcined carbide slag within the range of 10–100 nm is 0.0009 cm3/g, while the cumulative pore volume of HT5 within the same range is 0.0022 cm3/g, 2.4 times as high as that of calcined carbide slag. The pore structure of HT5 is more conducive than that of carbide slag to the diffusion of CO2 and the carbonation reaction of inner CaO. This result is consistent with the above calculated reaction kinetics, which indicates that the good pore structure of HT5 is the main reason for the high reaction rate constant.

4. Conclusions

In this work, a carbide slag-based, Ca12Al14O33-stabilized sorbent was prepared by the hydrothermal template method with soluble calcium and aluminum sources. The main conclusions are as follows. The synthetic sorbent prepared using acetic acid-washed carbide slag and aluminum nitrate as soluble precursors has higher initial CO2 capture capacity than that prepared using carbide slag and cement as insoluble precursors. When the addition amount of Al2O3 is 5%, the CO2 capture capacities of the synthetic sorbent after 30 cycles decrease by 37.7% and 53.9% under mild and harsh calcination conditions, respectively, while those of carbide slag decrease by 62.6% and 74.7%, respectively. The synthetic sorbent possesses higher CO2 capture performance and sintering resistance. The calcination condition of high concentration of steam is more favorable for the synthetic sorbent to maintain the cyclic stability in the CO2 capture process. After 10 cycles, the calculated reaction rate constants of the synthetic sorbent are 26.3% and 58.9% higher, respectively, than those of carbide slag. Specifically, the good pore structure is the main reason for the high CO2 capture capacity of the synthetic sorbent.

Author Contributions

Conceptualization, X.M.; methodology, X.M.; project administration—Y.L.; validation, Y.Q. and Z.W.; writing—original draft preparation, X.M.; writing—review and editing, Y.L.

Acknowledgments

Financial support from National Natural Science Foundation of China (51876105) and the Fundamental Research Funds of Shandong University, China (2018JC039) is gratefully appreciated.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Diagram of dual fixed-bed reactor (DFBR) with steam generator.
Figure 1. Diagram of dual fixed-bed reactor (DFBR) with steam generator.
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Figure 2. Preliminary experiments on calcination/carbonation processes with/without cooling steps (carbide slag as the sample, carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination in 100% N2 at 850 °C).
Figure 2. Preliminary experiments on calcination/carbonation processes with/without cooling steps (carbide slag as the sample, carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination in 100% N2 at 850 °C).
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Figure 3. X-ray diffraction (XRD) pattern of synthetic sorbents HT5 and HT10.
Figure 3. X-ray diffraction (XRD) pattern of synthetic sorbents HT5 and HT10.
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Figure 4. CO2 capture capacities of sorbents prepared using different methods (glucose addition of 1 mol/L for the hydrothermal template method, pre-calcination at 600 °C for 1 h; carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination in 100% N2 at 850 °C for 10 min).
Figure 4. CO2 capture capacities of sorbents prepared using different methods (glucose addition of 1 mol/L for the hydrothermal template method, pre-calcination at 600 °C for 1 h; carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination in 100% N2 at 850 °C for 10 min).
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Figure 5. CO2 capture capacities of synthetic sorbents prepared using hydrothermal template method under the (a) mild and (b) harsh calcination conditions (carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination (mild) in 100% N2 at 850 °C or (harsh) in 70% CO2/N2 at 920 °C for 10 min).
Figure 5. CO2 capture capacities of synthetic sorbents prepared using hydrothermal template method under the (a) mild and (b) harsh calcination conditions (carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination (mild) in 100% N2 at 850 °C or (harsh) in 70% CO2/N2 at 920 °C for 10 min).
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Figure 6. Effects of calcination conditions on the CO2 capture capacities of HT5 (carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination for 10 min).
Figure 6. Effects of calcination conditions on the CO2 capture capacities of HT5 (carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination for 10 min).
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Figure 7. Effects of H2O existing in the carbonation atmosphere on the CO2 capture capacities of HT5 (carbonation at 700 °C for 20 min, calcination in 70% CO2/N2 at 920 °C for 10 min).
Figure 7. Effects of H2O existing in the carbonation atmosphere on the CO2 capture capacities of HT5 (carbonation at 700 °C for 20 min, calcination in 70% CO2/N2 at 920 °C for 10 min).
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Figure 8. Long-term CO2 capture capacities of HT5 (carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination for 10 min).
Figure 8. Long-term CO2 capture capacities of HT5 (carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination for 10 min).
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Figure 9. Apparent CO2 capture kinetics of HT5 tested on thermo-gravimetric analyzer (TGA): (a) CO2 capture capacities and (b) apparent CO2 capture rates (carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination in 100% N2 at 850 °C).
Figure 9. Apparent CO2 capture kinetics of HT5 tested on thermo-gravimetric analyzer (TGA): (a) CO2 capture capacities and (b) apparent CO2 capture rates (carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination in 100% N2 at 850 °C).
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Figure 10. Carbonation kinetics curves of HT5 at (a) the chemical reaction-controlled stage and (b) the diffusion-controlled stage fitted by the shrinking core model (carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination in 100% N2 at 850 °C).
Figure 10. Carbonation kinetics curves of HT5 at (a) the chemical reaction-controlled stage and (b) the diffusion-controlled stage fitted by the shrinking core model (carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination in 100% N2 at 850 °C).
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Figure 11. Scanning electron microscope (SEM) images of (a), (c) carbide slag and (b), (d) HT5 after 10 cycles with different magnification factors, ×20,000 and ×50,000 (carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination in 70% CO2/N2 at 920 °C for 10 min).
Figure 11. Scanning electron microscope (SEM) images of (a), (c) carbide slag and (b), (d) HT5 after 10 cycles with different magnification factors, ×20,000 and ×50,000 (carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination in 70% CO2/N2 at 920 °C for 10 min).
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Figure 12. Pore characteristics of HT5 and carbide slag: (a) N2 adsorption–desorption isotherms, (b) pore volume distributions after 10 cycles (carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination in 70% CO2/N2 at 920 °C for 10 min).
Figure 12. Pore characteristics of HT5 and carbide slag: (a) N2 adsorption–desorption isotherms, (b) pore volume distributions after 10 cycles (carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination in 70% CO2/N2 at 920 °C for 10 min).
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Table
Table 1. Chemical components of raw carbide slag (wt.%).
Table 1. Chemical components of raw carbide slag (wt.%).
CaOMgOSiO2Al2O3Fe2O3SrO Ti2OOthersLoss on Ignition
69.520.022.341.520.170.030.030.5725.80
Table
Table 2. Comparison of CO2 capture performance for different CaO-based, calcium aluminates-stabilized sorbents.
Table 2. Comparison of CO2 capture performance for different CaO-based, calcium aluminates-stabilized sorbents.
Calcium AluminatesCaO Ratios (wt.%)PrecursorsMethodsReaction ConditionsC10
Ca3Al2O6 [41]90Carbide slag;
Al(NO3)3		CombustionCarbonation: T1, A1, t3;
Calcination: T3, A3, t1		0.45
Ca9Al6O18 [49]90Ca(C6H5O7)2;
Al(NO3)3		MixingCarbonation: T1, A1, t3;
Calcination: 800 °C, A3, t1		0.55
Ca12Al14O33 [47]80Ca(NO3)2;
Al(NO3)3		Sol-gelCarbonation: T2, A1, 2.5 min;
Calcination: 950 °C, CO2, 2.5 min		0.35
Ca12Al14O33 [36]75Sol-gel CaO;
Cement		Dry mixingCarbonation: T2, A1, 15 min;
Calcination: T3, A3, t1		0.48
Ca12Al14O33 [37]90Limestone;
Cement		Wet mixingCarbonation: T2, A1, t3;
Calcination: 900 °C, A3, t1		0.28
Ca12Al14O33 [48]90Ca(NO3)2;
Al(NO3)3		Carbon gel TemplatingCarbonation:750 °C, 60%CO2, t2;
Calcination: 750 °C, A3, t2		0.56
Ca12Al14O33 (this work)90Carbide slag;
Al(NO3) 3		Hydrothermal TemplatingCarbonation: T1, A1, t2;
Calcination: T3, A3, t1		0.58
Note: T1 denotes 650 °C; T2 denotes 700 °C; T3 denotes 850 °C; A1 denotes 15% CO2/N2; A2 denotes 20% CO2/N2; A3 denotes 100% N2; t1 denotes 10 min; t2 denotes 20 min; t3 denotes 30 min.
Table
Table 3. Apparent reaction rate constants of HT5 and carbide slag after fitting.
Table 3. Apparent reaction rate constants of HT5 and carbide slag after fitting.
Samples.NkCR2kD × 103R2
Carbide slag10.1190.9832.9780.968
HT510.1030.9903.9640.942
Carbide slag110.05711.2920.974
HT5110.0720.9942.0540.959
Table
Table 4. Surface areas and pore volumes of HT5 and carbide slag (carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination in 70% CO2/N2 at 920 °C for 10 min).
Table 4. Surface areas and pore volumes of HT5 and carbide slag (carbonation in 15% CO2/N2 at 700 °C for 20 min, calcination in 70% CO2/N2 at 920 °C for 10 min).
SamplesNBET Surface Area m2/gBJH Pore Volume cm3/g
Carbide slag0100.048
1030.017
HT50120.053
1070.038
Note: BET denotes Brunner−Emmet−Teller; BJH denotes Barrett-Joyner-Halenda.

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Ma, X.; Li, Y.; Qian, Y.; Wang, Z. A Carbide Slag-Based, Ca12Al14O33-Stabilized Sorbent Prepared by the Hydrothermal Template Method Enabling Efficient CO2 Capture. Energies 2019, 12, 2617. https://doi.org/10.3390/en12132617

AMA Style

Ma X, Li Y, Qian Y, Wang Z. A Carbide Slag-Based, Ca12Al14O33-Stabilized Sorbent Prepared by the Hydrothermal Template Method Enabling Efficient CO2 Capture. Energies. 2019; 12(13):2617. https://doi.org/10.3390/en12132617

Chicago/Turabian Style

Ma, Xiaotong, Yingjie Li, Yi Qian, and Zeyan Wang. 2019. "A Carbide Slag-Based, Ca12Al14O33-Stabilized Sorbent Prepared by the Hydrothermal Template Method Enabling Efficient CO2 Capture" Energies 12, no. 13: 2617. https://doi.org/10.3390/en12132617

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