Solâgel-Derived Synthetic CaO-Based CO2 Sorbents Incorporated

Article pubs.acs.org/IECR

Sol−gel-Derived Synthetic CaO-Based CO2 Sorbents Incorporated with Different Inert Materials Changjun Zhao, Zhiming Zhou,* and Zhenmin Cheng State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: A series of CaO-based sorbents incorporated with different inert materials (Ca2SiO4, Ca3Al2O6, CaTiO3, CaZrO3, and MgO) were prepared by a simple citrate sol−gel method. The structure−property relationship of the sorbents was explored, and the cyclic CO2 capture capacity and stability of the sorbents were evaluated. It was found that the complexation time, dispersion agent, calcium precursor, and inert material had significant effects on the sorbent structure and performance. The best CaO-based sorbent stabilized by CaZrO3 (29.1 wt %) had a stable CO2 capture capacity of 0.45 gCO2/gsorbent over 30 consecutive cycles, which was mainly ascribed to the small grains, accessible pores, and homogeneous distribution of CaO and CaZrO3 in the sorbent. Under severe but more realistic conditions (10 min of carbonation in 15% CO2/N2 at 650 °C, 5 min of calcination in 80% CO2/N2 at 1000 °C), this sorbent exhibited a constant capacity of 0.16 gCO2/gsorbent (carbonation conversion of 29%) after 50 cycles.

1. INTRODUCTION It is well-known that fossil fuel-derived CO2 emission contributes greatly to global warming and climate change,1 and development of high-efficiency and inexpensive CO2 capture and separation technologies has attracted much attention from both academia and industry.2,3 Among various technologies, the Ca-looping technology, based on the reversible reaction between CaO and CO2 to form CaCO3, has shown a considerable potential for CO2 postcombustion capture.4−6 Natural minerals such as limestone and dolomite are presumed to be promising sorbents for the Ca-looping process because of their low cost, wide availability, high CO2 capture capacity, and fast sorption−regeneration kinetics.7,8 However, these sorbents suffer from poor stability during cyclic CO2 capture, i.e., their sorption capacities toward CO2 decrease rapidly with cycling, especially in the first several carbonation− calcination cycles.9−11 Improvement of the cyclic stability of CaO-based sorbents by different approaches is therefore the subject of a number of investigations, including reactivation by hydration,12−14 thermal pretreatment,15−17 and doped and supported sorbents.18−32 Incorporation of inert materials, such as Al2O3,18−22 MgO,23,24 TiO2,25,26 ZrO2,27−29 SiO2,30−32 etc., into active CaO to form doped and supported sorbents is regarded as one of the most effective ways for preparation of high-performance CaO-based sorbents, owing to the inert material that acts as a support matrix for CaO and thus effectively delays sintering of CaO and CaCO3 at high temperature.33,34 Several techniques have been proposed to prepare these sorbents, including hydration, precipitation, dry or wet mixing, and sol−gel routes.35,36 In particular, the sol−gel method can ensure to a large extent the homogeneous distribution of CaO and inert materials, which in turn stabilizes the micromorphology of the sorbent and inhibits sintering during multiple cycles.21,37 Several researchers have reported the sol−gel-derived synthetic CaO sorbents without addition of other materials and their CO2 capture performance.38−41 Although the © 2014 American Chemical Society

synthetic unsupported CaO exhibited higher CO2 capture capacity and better stability compared to natural sorbents such as limestone, their cyclic stability and CaO utilization were still inferior to those of many supported sorbents. For instance, as reported by Luo et al.,39 after 20 carbonation−calcination cycles, a sol−gel-derived CaO had carbonation conversion of about 48%, while the conversion of a CaO-La2O3 sorbent prepared by the same method was around 71%. To date, only a limited number of studies have been carried out on the sol−gel-made supported CaO-based sorbents. Luo et al.42,43 developed a sol−gel combustion synthesis (SGCS) method to prepare CaO-La2O3 and CaO-Al2O3. The CaOLa2O3 sorbent with a molar ratio of Ca to La of 10:1 exhibited the best performance of a CO2 capture capacity of 0.44 gCO2/ gsorbent under mild calcination condition (at 850 °C in pure N2 for 10 min) after 20 cycles,42 while the corresponding capacity for CaO-Al2O3 with 80 wt % CaO was 0.43 gCO2/gsorbent.43 Broda et al.29,37,44,45 synthesized CaO-Al2O3 and CaO-ZrO2 sorbents through the hydrolysis of metal alkoxides. The best CaO-Al2O3 sorbent containing 91 wt % CaO had a CO2 capture capacity of 0.51 gCO2/gsorbent after 30 cycles under mild conditions (carbonation at 750 °C in 40% CO2/N2 for 20 min, calcination at 750 °C in pure N2 for 20 min),45 whereas the best CaO-ZrO2 containing about 70 wt % CaO showed a capture capacity of 0.34 gCO2/gsorbent after 90 cycles under other conditions (carbonation at 800 °C in 50% CO2/N2 for 5 min, calcination at 800 °C in pure N2 for 15 min).29 Recently, we prepared CaO-Ca9Al6O18 (or Ca3Al2O6) sorbents via the hydrolysis of aluminum isopropoxide followed by addition of calcium precursors.21 The best CaO-Ca9Al6O18 with 90 wt % CaO displayed a capacity of 0.59 gCO2/gsorbent after 35 cycles Received: Revised: Accepted: Published: 14065

June 25, 2014 August 24, 2014 August 26, 2014 August 26, 2014 dx.doi.org/10.1021/ie502559t | Ind. Eng. Chem. Res. 2014, 53, 14065−14074

Industrial & Engineering Chemistry Research

Article

Table 1. Nomenclature for the Synthetic CaO-Based Sorbents

a

sorbent

calcium precursor

precursor of inert material

mass ratio of CaO/MxOya

complexation time (h)

dispersion agent

CN-Al-6 CN-Al-6-0-P300 CN-Al-6-2-P300 CN-Al-6-8-P300 CN-Al-6-2-EG CN-Al-6-2-P10000 CA-Al-6-2-P300 CL-Al-6-2-P300 CG-Al-6-2-P300 CH-Al-6-2-P300 CN-Zr-6-2-P300 CN-Si-6-2-P300 CN-Ti-6-2-P300 CN-Mg-6-2-P300 CN-Zr-4-2-P300 CN-Zr-8-2-P300 CN-Al-4-2-P300 CN-Al-8-2-P300 CaO

CN CN CN CN CN CN CA CL CG CH CN CN CN CN CN CN CN CN CN

Al(NO3)3 Al(NO3)3 Al(NO3)3 Al(NO3)3 Al(NO3)3 Al(NO3)3 Al(NO3)3 Al(NO3)3 Al(NO3)3 Al(NO3)3 Zr(NO3)4 Si(OC2H5)4 Ti(OC4H9)4 Mg(NO3)2 Zr(NO3)4 Zr(NO3)4 Al(NO3)3 Al(NO3)3 −

6 6 6 6 6 6 6 6 6 6 6 6 6 6 4 8 4 8 −

0 0 2 8 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

− PEG300 PEG300 PEG300 EG PEG10000 PEG300 PEG300 PEG300 PEG300 PEG300 PEG300 PEG300 PEG300 PEG300 PEG300 PEG300 PEG300 PEG300

MxOy represents Al2O3, ZrO2, SiO2, TiO2, or MgO depending on the precursor of inert material.

(carbonation at 650 °C in 15% CO2/N2 for 30 min, calcination at 800 °C in pure N2 for 10 min). Liu et al.46 synthesized mesoscopic CaO-Ca12Al14O33 hollow sphere sorbents with tunable cavity size and shell thickness by using sulfonated polystyrene as hard template. The best sorbent with 90 wt % CaO had a capacity of 0.68 gCO2/gsorbent(carbonation at 650 °C in 15% CO2/N2 for 30 min, calcination at 900 °C in pure N2 for 10 min), which was close to the theoretical maximum possible capacity (0.70 gCO2/gsorbent). Very recently, core−shell sorbents with mesoporous SiO2 shells were prepared by a wet coating process combined with evaporation-induced sol−gel technique,47 and the sorbent with 53 wt % CaO possessed 0.13 gCO2/gsorbent after 31 cycles (carbonation at 675 °C in pure CO2 for 20 min, calcination at 850 °C in pure N2 for 10 min), corresponding to an activity loss of 46%. The above investigations indicate that various types of inert materials can be incorporated into the CaO-based sorbents through sol−gel techniques, but which one would be better in improving the CO2 capture performance of the sorbent? It is difficult to determine from the literature available because either the sorbents were prepared by different sol−gel methods (the citrate route, hydrolysis of metal alkoxide, with or without template) or their CO2 capture performance was evaluated under different carbonation−calcination conditions.37 In fact, very little research has been performed to compare the CO2 capture performance of different supported CaO-based sorbents,48,49 and these sorbents were prepared by flame spray pyrolysis48 or acidification of limestone coupled with wet mixing and two-step calcination,49 rather than by the sol−gel process. In this work, a series of CaO-based sorbents incorporated with different inert materials (Al-, Mg-, Ti-, Zr-, and Sicontaining compounds) were prepared by a citrate sol−gel method. First, the effects of preparation procedures and calcium precursors on the sorbent performance were investigated; then, the supported sorbents doped with various materials were compared for their capacity and stability in capturing CO2; finally, the screened sorbents with high CO2 capture perform-

ance were evaluated under severe but more realistic conditions. The main objective of the present work is to explore the structure−property relationship of the sol−gel-derived sorbents and develop CaO-based sorbents with high CO2 capture capacity and stability.

2. EXPERIMENTAL SECTION 2.1. Sorbent Preparation. The sol−gel-derived CaObased sorbents were prepared via the citrate route. The calcium precursors included calcium hydroxide (CH), calcium nitrate tetrahydrate (CN), calcium acetate monohydrate (CA), calcium lactate pentahydrate (CL), and calcium gluconate monohydrate (CG). The precursors of inert materials were aluminum nitrate nonahydrate (Al), magnesium nitrate hexahydrate (Mg), zirconium nitrate pentahydrate (Zr), tetrabutyl titanate (Ti), and tetraethyl orthosilicate (Si). The dispersion agents used were ethylene glycol (EG) and polyethylene glycol (PEG with a molecular weight of 300 or 10 000). In a typical synthesis, predetermined amounts of calcium precursor, precursor of inert materials, citric acid (the molar ratio of citric acid to cation was 1.2), and deionized water (the molar ratio of water to citric acid was 50) were first added into a three-neck round-bottom flask placed in an oil bath, and the pH value of the solution was adjusted to 1−2 by nitric acid. The mixture was stirred continuously by a magnetic stir bar under reflux at 80 °C for a certain time period (complexation time, defined as the time interval between addition of precursors and addition of dispersion agent); then, an amount of EG or PEG (the mass ratio of EG or PEG to citric acid was 0.5) was added, and the solution was allowed to stir under reflux for 6 h at 105 °C. Finally, the formed gel was dried at 110 °C for 12 h and ground into a fine powder, followed by calcination in a muffle furnace at 850 °C for 2 h with a heating rate of 5 °C/min. For simplicity, the synthetic sorbents are named as XX-YY-nt-D (Table 1): XX stands for the calcium precursor, and YY refers to the precursor of inert material; n is the mass ratio of CaO to the oxide derived from YY, t the complexation time, and D the dispersion agent. For example, CN-Al-6-2-P300 was 14066

dx.doi.org/10.1021/ie502559t | Ind. Eng. Chem. Res. 2014, 53, 14065−14074


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obtained from calcium nitrate and aluminum nitrate with a CaO/Al2O3 mass ratio of 6, and the complexation time before addition of PEG300 was 2 h. In a special case where no dispersion agent is used, the sorbent is simply denoted by XXYY-n. 2.2. Sorbent Characterization. The crystalline structure of the sorbent was determined by X-ray diffraction (XRD, Rigaku D/Max 2550) using Cu Kα radiation. The data were measured over a 2θ range of 10−80° with a step size of 0.02°. The textural properties of the sorbent, such as Brunauer− Emmett−Teller (BET) surface area, pore volume, and average pore diameter, were obtained by N2 adsorption−desorption analysis at −196 °C using a Micromeritics ASAP 2010 instrument. Prior to N2 adsorption measurements the sample was degassed at 150 °C and 133.3 Pa for 6 h. The microstructure of the sorbent was observed by field emission scanning electron microscopy (FESEM, Nova NanoSEM 450). The elemental analysis of the sorbent was carried out using an energy dispersive spectrometry instrument (EDS, EDAX Genesis XM2 system) attached to a high-resolution transmission electron microscopy instrument (HRTEM, JEOL JEM2100). The samples were prepared by suspending the ground sorbents in ethanol and sonicating for 30 min. A drop of the suspension was deposited on a carbon-coated copper grid and evaporated in air at room temperature. 2.3. Performance Tests. The CO2 capture performance of the synthetic sorbents over multiple consecutive carbonation− calcination cycles was evaluated by thermogravimetric analysis instrument (TGA; WRT-3P, Shanghai Precision & Scientific Instrument Co., Ltd.). For each run, about 10 mg of fresh sorbent (50−75 μm) was loaded in a platinum basket which was placed in the furnace of the TGA instrument. The temperature and the sample weight were continuously recorded in a computer, and the gas (N2, CO2) flow rates were controlled by mass flow meters. Except where specified otherwise, one complete cycle consisted of 20 min of carbonation at 650 °C in 15% CO2/N2 and 5 min of calcination at 900 °C in pure N2. After carbonation or calcination, the TGA instrument was heated or cooled to 900 or 650 °C, respectively, at a rate of 25 °C/min in a flow of pure N2. The total gas flow rate during the experiment was kept at 50 mL/min. Detailed information was reported elsewhere.20 The performances of different sorbents were compared in terms of the CO2 capture capacity (mass of CO2 absorbed per unit mass of sorbent) and the cyclic stability.

Figure 1. Effect of the complexation time on the cyclic CO2 capture capacity of the synthetic CaO-based sorbents.

or PEG. If the complexation time is too short, the complexation reaction will compete with the polyesterification process, which will exert a negative effect on the formation of stable cation− citric acid complexes. If the complexation time is too long, the excessive complexation between cations and citric acid will result in few carboxylic acid groups left in citric acid to react with EG or PEG, which will cause particle aggregation. In accordance with this explanation, the order of the average grain size of the three sorbents is CN-Al-6-8-P300 > CN-Al-6-0-P300 > CN-Al-6-2-P300, as shown in Figure 2, and correspondingly, the CO2 capture capacity of the sorbents increases in the opposite order (Figure 1) because the CaO-based sorbent with smaller grains usually shows good CO2 capture performance.21 The effect of the dispersion agent on sorbent activity is given in Figure 3. As expected, CN-Al-6 without the addition of dispersion agents displays larger grains (Figure 2) and thus lower activity compared to that of the sorbents prepared with the aid of EG or PEG. This is because EG or PEG can provide a polymeric network to prevent mobility of cations through polyesterification with citric acid on the one hand,50,51 and combustion of EG or PEG during calcination of the sorbent precursor can generate a porous structure for CO2 diffusion inside the sorbent on the other hand. It can be clearly seen from Figure 2 that CN-Al-6-2-EG, CN-Al-6-2-P300, and CNAl-6-2-P10000 have structures that are more fluffy and porous than that of CN-Al-6. In addition, the sorbents with the aid of PEG show CO2 capture capacities higher than the one with EG, and the length of the molecular chains of PEG (MW = 300 or 10 000) has almost no effect on the sorbent activity. In the following study, the sorbents are manufactured using a complexation time of 2 h and PEG300 as the dispersion agent. 3.2. Effect of Calcium Precursors. The effect of calcium precursors on the CO2 capture performance of the synthetic sorbents is investigated using five different precursors. Figure 4 shows the XRD patterns of the five developed sorbents. All sorbents consist of active CaO (JCPDS entry 77-2010) and inert Ca3Al2O6 (JCPDS entry 38-1429), except for CH-Al-6-2P300 composed of CaO and Ca12Al14O33 (JCPDS entry 702144). This is consistent with our previous results showing that the inert materials could be Al2O3, Ca3Al2O6, or Ca12Al14O33, depending on the Ca and Al precursors used.20 The micromorphologies of the five sorbents together with a CaO sorbent serving as a reference (Table 1) are presented in Figure 5. All these sorbents except CH-Al-6-2-P300 possess small

3. RESULTS AND DISCUSSION 3.1. Effect of Preparation Procedure. The effect of the complexation time on the sorbent activity is shown in Figure 1. Among CN-Al-6-0-P300, CN-Al-6-2-P300, and CN-Al-6-8P300 prepared under different complexation time (0, 2, and 8 h, respectively) but using the same precursor and dispersion agent, CN-Al-6-2-P300 exhibits the highest activity for CO2 capture, whereas CN-Al-6-8-P300 has the lowest activity. It appears that a complexation time that is too short (0 h) or too long (8 h) is unfavorable, which can be explained in terms of the reaction chemistry of the polymerizable-complex route used for the synthesis of CaO-based sorbents. This method makes use of citric acid to form stable cation complexes, which are then polyesterified with EG or PEG to form a polymeric resin.50,51 The compositional homogeneity of the final sorbent is undoubtedly affected by the complexation of cations with citric acid and the polyesterification between citric acid and EG 14067



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Figure 2. Schematic showing the effect of different preparation procedures on the micromorphology of the CaO-based sorbents.

Figure 3. Effect of different dispersion agents on the cyclic CO2 capture capacity of the synthetic CaO-based sorbents.

Figure 5. FESEM images of the synthetic CaO-based sorbents derived from different calcium precursors.

leaf-like grains has a relatively smooth surface and few pores, implying a low CO2 capture capacity. As shown in Figure 6, the CO2 capture capacity of the pure CaO drops significantly from 0.60 gCO2/gsorbent at the 1st cycle to 0.50 gCO2/gsorbent at the 5th cycle and then decreases to 0.35 gCO2/gsorbent at the 15th cycle. In contrast, the capacity of CNAl-6-2-P300 increases from 0.47 (1st cycle) to 0.55 gCO2/gsorbent (5th cycle) and then slightly decreases to 0.50 gCO2/gsorbent (15th cycle). Although the pure CaO has an initial capacity higher than that of CN-Al-6-2-P300 owing to its high CaO content, its stability over multiple cycles is much poorer. This is closely related to the structure of the sorbent. Despite the fact

Figure 4. XRD patterns of the synthetic CaO-based sorbents derived from different calcium precursors.

grains, rough surfaces, and many accessible pores, which can facilitate the diffusion of CO2 through the CaCO3 product layer formed on the surface of CaO34 and help to increase the CO2 capture capacity of the sorbents. CH-Al-6-2-P300 consisting of 14068



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improved stability of the supported CaO-based sorbents is mainly ascribed to the inert support material (Ca3Al2O6 or Ca12Al14O33) that effectively inhibits the sintering of CaO particles during calcination.33 For the five supported sorbents, the initial capacity increases in the order CL-Al-6-2-P300 > CN-Al-6-2-P300 > CG-Al-6-2-P300 > CA-Al-6-2-P300 > CHAl-6-2-P300. This is also the order of the BET surface area and the opposite order of the average pore diameter (Table 2). Table 2. Textural Properties of Various CaO-Based Sorbents

Figure 6. Comparison of the CO2 capture performance of CaO and CN-Al-6-2-P300.

that the fresh CaO has a rough surface and small grains, its surface after 15 cycles becomes much smoother together with larger grains, which leads to the decreased capacity.52 On the contrary, CN-Al-6-2-P300 after 15 cycles retains a fluffy surface, small grains, and interconnected pores, and in fact, the grains of CN-Al-6-2-P300 after 15 cycles become smaller than the fresh grains. As a result of the evolution of the microstructure of the sorbent, the CO2 capture capacity of CN-Al-6-2-P300 after 15 cycles is higher than that of the fresh sorbent, which is normally called the “self-activation” phenomenon.53 This phenomenon is also observed for other supported sorbents (Figure 7). Figure 7 further compares the CO2 capture performance of the sorbents derived from various calcium precursors. All the supported sorbents have stability higher than that of CaO. The CO2 capture capacity of each supported sorbent after 15 cycles is larger than 0.4 gCO2/gsorbent, exceeding that of CaO. The

sorbent

BET surface area (m2/g)

pore volume (cm3/g)

average pore diameter (nm)

CN-Al-6-2-P300 CA-Al-6-2-P300 CH-Al-6-2-P300 CG-Al-6-2-P300 CL-Al-6-2-P300 CN-Zr-6-2-P300 CN-Si-6-2-P300 CN-Ti-6-2-P300 CN-Mg-6-2-P300 CN-Al-4-2-P300 CN-Al-8-2-P300 CN-Zr-4-2-P300 CN-Zr-8-2-P300

12.0 8.8 4.1 9.3 14.4 13.4 23.8 20.5 16.4 9.0 8.2 20.3 18.6

0.014 0.024 0.027 0.021 0.032 0.037 0.19 0.118 0.079 0.019 0.023 0.085 0.053

13.0 18.9 32.9 16.8 12.6 13.9 33.0 27.8 25.4 13.3 11.3 18.2 14.5

However, there is no direct correlation between the initial capacity and the pore volume. CH-Al-6-2-P300 has the lowest initial capacity, which is in accordance with its microstructure (Figure 5). The order of capacity of these sorbents is almost unchanged after 15 cycles, and CL-Al-6-2-P300 and CN-Al-6-2P300 have the largest capacity. In view of the low cost and high availability, calcium nitrate is selected as the calcium precursor in the following study. 3.3. Effect of Inert Materials. The effect of inert materials on the CO2 capture performance of the CaO-based sorbents is studied using five different materials, i.e., Al-, Mg-, Ti-, Zr-, and Si-containing compounds. The crystalline structures of these sorbents are presented in Figure 8. For CN-Si-6-2-P300, CNAl-6-2-P300, CN-Ti-6-2-P300, CN-Zr-6-2-P300, and CN-Mg6-2-P300, the inert materials acting as support matrices for CaO are Ca2SiO4 (JCPDS entry 33-0302), Ca3Al2O6 (JCPDS entry 38-1429), CaTiO3 (JCPDS entry 22-0153), CaZrO3 (JCPDS entry 35-0790), and MgO (JCPDS entry 45-0946), respectively. It is clear that except for MgO, all other oxides are reacted with CaO at high calcination temperature to form different Ca-containing salts. Figure 9 shows the CO2 capture performance of the above five sorbents during 30 carbonation−calcination cycles. The initial capacities of the sorbents follow the order CN-Mg-6-2P300 > CN-Al-6-2-P300, CN-Ti-6-2-P300 > CN-Zr-6-2-P300 > CN-Si-6-2-P300, which is not in line with the order of the BET surface area (Table 2). This disagreement results mainly from the different inert materials in the sorbents. On the one hand, both CaO and the inert material contribute to the BET surface area of the sorbent, but keep in mind that only the surface area occupied by CaO determines the amount of CO2 absorbed. On the other hand, different properties of these inert materials and different CaO content in the sorbents result in the inconsistency between the order of the BET surface area and that of the CaO surface area. Thus, the order of the initial capacity of the sorbents (incorporated with different inert

Figure 7. Effect of different calcium precursors on the cyclic CO2 capture capacity of the synthetic CaO-based sorbents. 14069



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features. This result is probably due to the small content of MgO in the sorbent (14.3 wt %), which is lower than the theoretical minimum value (17.3 wt % predicted by Liu et al.23 based on a 3-dimentional percolation theory) to effectively separate CaO. Indeed, CN-Mg-6-2-P300 displays more sintering after 30 cycles compared to other sorbents. Among the five synthetic sorbents, the CaZrO3-stabilized CN-Zr-6-2P300 appears to have the best CO2 capture performance, as evidenced not only from its excellent cyclic stability (Figure 9) but also from the well-preserved morphology after 30 cycles (Figure 10). The good performance of Zr-containing CaObased sorbents in CO2 capture has also been reported by other investigators.29,54 3.4. Effect of CaO Content. The effect of the CaO content on the sorbent capacity and stability is investigated using the Al- and Zr-containing CaO-based sorbents with three different CaO/Al2O3 or CaO/ZrO2 mass ratios (4, 6, and 8). The XRD analyses (not presented here) show that the inert support materials for Al- and Zr-containing sorbents are still Ca3Al2O6 and CaZrO3, respectively, independent of the CaO/Al2O3 or CaO/ZrO2 ratio. Figure 11 shows the FESEM images of these sorbents, which indicate that the CaO content has less effect on the sorbent morphology. On the other hand, the CaZrO3stabilized sorbents show smaller grains than the Ca3Al2O6stabilized sorbents, and accordingly, the former have larger surface area and pore volume than the latter, as listed in Table 2. Figure 12 compares the CO2 capture performance of the Ca3Al2O6- and CaZrO3-stabilized CaO sorbents. With an increase in the CaO/Al2O3 ratio, the sorbent capacity is expected to increase, but its cyclic stability becomes worse. The CO2 capture capacity of CN-Al-4-2-P300 decreases from the maximum value of 0.44 gCO2/gsorbent at the 6th cycle to 0.36 gCO2/gsorbent at the 30th cycle, with an average decay rate of 0.7%/cycle, while for CN-Al-8-2-P300, the average decay rate is 1.0%/cycle (from 0.57 gCO2/gsorbent at the 4th cycle to 0.41 gCO2/gsorbent at the 30th cycle). In addition, CN-Al-6-2-P300 and CN-Al-8-2-P300 have similar capacities, indicating the lower CaO utilization in CN-Al-8-2-P300. On the other hand, for the CaZrO3-stabilized sorbents, CN-Zr-4-2-P300 and CNZr-6-2-P300 exhibit the same capacities, but CN-Zr-8-2-P300 shows increased capacity and decreased stability. In general, the CaZrO3-stabilized sorbents have better cyclic stability than the Ca3Al2O6-stabilized sorbents, agreeing well with the morphological and textural characteristics of these sorbents. Figure 13 shows the carbonation conversion profiles of various CaZrO3stabilized sorbents. The carbonation conversion is calculated by

Figure 8. XRD patterns of the synthetic CaO-based sorbents incorporated with different inert materials.

Figure 9. Effect of different inert materials on the cyclic CO2 capture capacity of the synthetic CaO-based sorbents.

materials) does not follow the sequence of the BET surface area. Up to now, to the best of our knowledge, no effective methods have been found to measure the CaO surface area of the sorbents. Different from the order of the initial capacity, after 30 cycles CN-Al-6-2-P300 and CN-Zr-6-2-P300 have the largest CO2 capture capacity of about 0.45 gCO2/gsorbent. When it comes to the cyclic stability of the synthetic sorbents, except for CN-Mg-6-2-P300 whose capacity decreases rapidly with cycling, all other sorbents show good stability after 20 cycles. The good stability of the synthetic sorbents is mainly attributed to, as mentioned above, the micromorphologies of the sorbents and the inert support materials. As shown in Figure 10, the fresh sorbents have small grains, rough surfaces, and accessible pores. Moreover, active CaO and inert materials are well distributed in the sorbents according to the EDS maps. All these features are prerequisites for achieving high-performance CaO-based sorbents. The spent sorbents after 30 cycles preserve to some extent their morphologies, although sintering of some grains takes place. However, CN-Mg-6-2-P300 is an exception in spite of the fact that it possesses the advantageous

conversion =

m − m0 MCaO · · 100% m0 x MCO2

(1)

where m0 and m are the initial and final sorbent weights, respectively; x is the mass content of CaO in the sorbent; MCaO and MCO2 are the molar weights of CaO and CO2, respectively. The CaO (or CaZrO3) content in CN-Zr-4-2-P300, CN-Zr-62-P300, and CN-Zr-8-2-P300 are calculated to be 70.9 (29.1), 79.2 (20.8), and 83.8 (16.2) wt %, respectively, by assuming that the sorbents are composed only of CaO and CaZrO3. Obviously, CN-Zr-4-2-P300 has the best CO2 capture performance, with a stable conversion of about 82% after 20 cycles. Moreover, its micro-morphology is well-preserved after 30 cycles, as illustrated by the inset images in Figure 13. 14070



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Figure 10. FESEM images of fresh and spent (after 30 cycles) sorbents incorporated with different inert materials as well as the EDS maps of fresh sorbents.

Figure 12. Effect of the CaO content on the cyclic CO2 capture capacity of Ca3Al2O6- and CaZrO3-stabilized sorbents.

Figure 11. FESEM images of Ca3Al2O6- and CaZrO3-stabilized CaObased sorbents with different CaO content.

3.5. Effect of Carbonation−Calcination Conditions. The best sorbent developed in this work, CN-Zr-4-2-P300, is further evaluated under other carbonation−calcination conditions (Table 3) in order to study the effects of shorter carbonation time (10 min), higher calcination temperature (1000 °C), and higher CO2 concentration of the calcination atmosphere (80% CO2). In particular, condition 4 listed in

Figure 13. Effect of the CaO content on the cyclic carbonation conversion of CaZrO3-stabilized sorbents.

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the self-activation phenomenon observed under condition 1 disappears under other conditions, indicating that 10 min of carbonation is not enough to induce the self-activation of CNZr-4-2-P300. Compared to condition 2, a higher calcination temperature (condition 3) leads to a lower CO2 uptake, likely because of more sintering of CaO occurring, as confirmed by SEM analysis. Furthermore, the sorbent capacity drops dramatically in the first 10 cycles in condition 3, similar to that in condition 2, and then stabilizes at 0.22 gCO2/gsorbent. Unlike those observed in condition 2 and condition 3, when the calcination step is conducted under high CO2 pressure (condition 4), a relatively stable CO2 uptake is noted for the initial 11 cycles, followed by a rapid decay from the 12th to the 45th cycle and finally a leveling off (0.16 gCO2/gsorbent, corresponding to the carbonation conversion of 29%). The initial stable period in condition 4 is probably due to the further carbonation at high CO2 pressure occurring when the 15% CO2/N2 atmosphere for carbonation is switched to 80% CO2/ N2 for calcination. This process is continued until equilibrium is reached (at about 870 °C), allowing more CaO to react with CO2, which is considered to increase the CO2 uptake.37,57 On the other hand, both high temperature and high CO 2 concentration result in much more sintering of CaO (Figure 14), which is unfavorable for CO2 capture. Therefore, the initial stability of the sorbent in condition 4 can be ascribed to the balance of the above positive and negative effects. It is worth noting that, although CN-Zr-4-2-P300 experiences severe calcination conditions and sintering of CaO inevitably takes place, this sorbent still preserves many pores and cracks after 50 cycles, demonstrating its good CO2 capture performance. As listed in Table 4, natural limestones usually exhibit performance poorer than that of CN-Zr-4-2-P300 under similar carbonation−calcinations conditions. Nevertheless, at the present stage, it is too early to conclude that the CaZrO3stabilized CaO-based sorbent developed here is superior to limestones in postcombustion CO2 capture at the industrial scale because the preparation cost of the synthetic sorbent is higher than that of natural limestones. The savings obtained by cycling the more stable and reactive synthetic sorbent must at least balance the increased preparation cost,4 and detailed technoeconomic analyses are needed to assess the economic feasibility of the sorbent.8,35 Additionally, the synthetic sorbent is not limited to postcombustion CO2 capture. It can be applied

Table 3. Carbonation−Calcination Conditions Used for CNZr-4-2-P300 condition 1 2 3 4

temperature (°C) 650 650 650 650

(900)a (900) (1000) (1000)

time (min) 20 10 10 10

(5) (5) (5) (5)

atmosphere 15% CO2/85% N2 (100% N2) 15% CO2/85% N2 (100% N2) 15% CO2/85% N2 (100% N2) 15% CO2/85% N2 (80% CO2/20% N2)

a

The data outside and inside the parentheses correspond to carbonation and calcination conditions, respectively.

Table 3 is used to mimic realistic CO2 capture conditions in fluidized-bed combustion systems.55 Figure 14 shows the influence of different carbonation− calcination conditions on the cyclic CO2 capture capacity of

Figure 14. Effect of the carbonation−calcination conditions on the cyclic CO2 capture capacity of the synthetic CN-Zr-4-2-P300 sorbent as well as the FESEM images of spent sorbents.

CN-Zr-4-2-P300, together with the FESEM images of spent sorbents. Compared to the reference condition (condition 1), a shortened carbonation period (condition 2) undoubtedly decreases the amount of CO2 absorbed,55,56 and meanwhile,

Table 4. CO2 Capture Capacity of CN-Zr-4-2-P300 and Natural Limestones over Multiple Cycles under Realistic Conditions sorbent CN-Zr-4-2-P300

La Blanca limestone

Cadomin limestone

limestone

limestone

carbonation

calcination

15% CO2 650 °C 10 min 100% CO2 650 °C 20 Min 20% CO2 700 °C 10 min 15% CO2 650 °C 20 min 15% CO2 700 °C 20 min

80% CO2 1000 °C 5 min 100% CO2 950 °C 0 min 100% CO2 950 °C 0 min 80% CO2 920 °C 15 min 100% CO2 950 °C 15 min

number of cycles

capacity ((g of CO2)/(g of sorbent))

conversion (%)

reference

50

0.16

29

this work

14

0.06

8

9

30

0.11

19

55

50/100

0.08/0.05

10/7

58

40

0.09

12

59

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for the sorption-enhanced steam methane reforming process,60 which can improve the energy efficiency and reduce the environmental impact of the hydrogen production process.61

(3) Markewitz, P.; Kuckshinrichs, W.; Leitner, W.; Linssen, J.; Zapp, P.; Bongartz, R.; Schreiber, A.; Müller, T. E. Worldwide Innovations in the Development of Carbon Capture Technologies and the Utilization of CO2. Energy Environ. Sci. 2012, 5, 7281−7305. (4) Blamey, J.; Anthony, E. J.; Wang, J.; Fennell, P. S. The Calcium Looping Cycle for Large-Scale CO2 Capture. Prog. Energy Combust. Sci. 2010, 36, 260−279. (5) Anthony, E. J. Ca Looping Technology: Current Status, Developments and Future Directions. Greenhouse Gas Sci. Technol. 2011, 1, 36−47. (6) Fan, L. S.; Zeng, L.; Wang, W.; Luo, S. W. Chemical Looping Process for CO2 Capture and Carbonaceous Fuel Conversion − Prospect and Opportunity. Energy Environ. Sci. 2012, 5, 7254−7280. (7) Feng, B.; An, H.; Tan, E. Screening of CO2 Adsorbing Materials for Zero Emission Powder Generation Systems. Energy Fuels 2007, 21, 426−434. (8) Valverde, J. M. Ca-based Synthetic Materials with Enhanced CO2 Capture Efficiency. J. Mater. Chem. A 2013, 1, 447−468. (9) Abanades, J. C.; Alvarez, D. Conversion Limits in the Reaction of CO2 with Lime. Energy Fuels 2003, 17, 308−315. (10) Grasa, G. S.; Abanades, J. C. CO2 Capture Capacity of CaO in Long Series of Carbonation/ Calcination Cycles. Ind. Eng. Chem. Res. 2006, 45, 8846−8851. (11) Sun, P.; Lim, C. J.; Grace, J. R. Cyclic CO2 Capture by Limestone-Derived Sorbent During Prolonged Calcination/Carbonation Cycling. AIChE J. 2008, 54, 1668−1677. (12) Arias, B.; Grasa, G. S.; Abanades, J. C. Effect of Sorbent Hydration on the Average Activity of CaO in a Ca-Looping System. Chem. Eng. J. 2010, 163, 324−330. (13) Yin, J. J.; Zhang, C.; Qin, C. L.; Liu, W. Q.; An, H.; Chen, G.; Feng, B. Reactivation of Calcium-Based Sorbent by Water Hydration for CO2 Capture. Chem. Eng. J. 2012, 198−199, 38−44. (14) Wang, S. P.; Shen, H.; Fan, S. S.; Zhao, Y. J.; Ma, X. B.; Gong, J. L. Enhanced CO2 Adsorption Capacity and Stability Using CaO-Based Adsorbents Treated by Hydration. AIChE J. 2013, 59, 3586−3593. (15) Manovic, V.; Anthony, E. J. Thermal Activation of CaO-Based Sorbent and Self-Reactivation during CO2 Capture Looping Cycles. Environ. Sci. Technol. 2008, 42, 4170−4174. (16) Chen, Z. X.; Song, H. S.; Portillo, M.; Lim, C. J.; Grace, J. R.; Anthony, E. J. Long-Term Calcination/Carbonation Cycling and Thermal Pretreatment for CO2 Capture by Limestone and Dolomite. Energy Fuels 2009, 23, 1437−1444. (17) Ozcan, D. C.; Shanks, B. H.; Wheelock, T. D. Improving the Stability of a CaO-Based Sorbents for CO2 by Thermal Pretreatment. Ind. Eng. Chem. Res. 2011, 50, 6933−6942. (18) Li, Z. S.; Cai, N. S.; Huang, Y. Y.; Han, H. J. Synthesis, Experimental Studies and Analysis of a New Calcium-Based Carbon Dioxide Absorbent. Energy Fuels 2005, 19, 1447−1452. (19) Martavaltzi, C. S.; Lemonidou, A. A. Development of New CaO Based Sorbent Materials for CO2 Removal at High Temperature. Microporous Mesoporous Mater. 2008, 110, 119−127. (20) Zhou, Z. M.; Qi, Y.; Xie, M. M.; Cheng, Z. M.; Yuan, W. K. Synthesis of CaO-Based Sorbents through Incorporation of Alumina/ Aluminate and Their CO2 Capture Performance. Chem. Eng. Sci. 2012, 74, 172−180. (21) Xu, P.; Xie, M. M.; Cheng, Z. M.; Zhou, Z. M. CO2 Capture Performance of CaO-Based Sorbents Prepared by a Sol-Gel Method. Ind. Eng. Chem. Res. 2013, 52, 12161−12169. (22) Angeli, S. D.; Martavaltzi, C. S.; Lemonidou, A. A. Development of a Novel-Synthesized Ca-Based CO2 Sorbent for Multicycle Operation: Parametric Study of Sorption. Fuel 2014, 127, 62−69. (23) Liu, W. Q.; Feng, B.; Wu, Y. Q.; Wang, G. X.; Barry, J.; Diniz Da Costa, J. C. Synthesis of Sintering-Resistant Sorbents for CO2 Capture. Environ. Sci. Technol. 2010, 44, 3093−3097. (24) Park, J. H.; Yi, K. B. Effects of Preparation Method on Cyclic Stability and CO2 Absorption Capacity of Synthetic CaO-MgO Absorbent for Sorption-Enhanced Hydrogen Production. Int. J. Hydrogen Energy 2012, 37, 95−102.

4. CONCLUSIONS In this work we made use of a citrate sol−gel method to prepare a series of CaO-based sorbents incorporated with different inert stabilizers (Ca2SiO4, Ca3Al2O6, CaTiO3, CaZrO3, and MgO). The structures of the developed sorbents were studied by XRD, N2 physisorption, FESEM, and EDS on the one hand, and their cyclic CO2 capture performances were evaluated by TGA on the other hand, through which the structure−property relationship of the sorbents was explored. Some synthesis parameters including complexation time, type of dispersion agent, type of calcium precursor, and type of stabilizer were found to have a significant impact on the structure and performance of the sorbents. The best sorbent developed in this study was a CaZrO3-stabilized sorbent with a CaO/ZrO2 mass ratio of 4 (70.9 wt % CaO and 29.1 wt % CaZrO3), which was prepared under the optimal condition of 2 h of complexation, PEG300 as the dispersion agent, and calcium nitrate as the calcium precursor. This sorbent exhibited a stable CO2 capture capacity of 0.45 gCO2/gsorbent during 30 consecutive carbonation−calcination cycles (carbonation at 650 °C in 15% CO2/N2 for 20 min and calcination at 900 °C in pure N2 for 5 min), corresponding to a carbonation conversion of 82%. Some important features of the sorbent, including small grains, accessible pores, and homogeneous distribution of CaO and stabilizer in the sorbent, contributes to the improved CO2 capture performance. Under severe but more realistic conditions (shorter carbonation time and higher calcination temperature in the presence of CO2), the best sorbent showed decreased CO2 capture capacity associated with sintering of CaO, but many pores and cracks were still present in the sorbent, giving rise to a stable capacity of 0.16 gCO2/gsorbent (carbonation conversion of 29%) after 50 cycles (carbonation at 650 °C in 15% CO2/N2 for 10 min and calcination at 1000 °C in 80% CO2/N2 for 5 min).

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +86-21-64252230. Fax: +86-21-64253528. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Natural Science Foundation of China (21276076), the Program for New Century Excellent Talents in University (NCET-130801), the Fundamental Research Funds for the Central Universities (222201313011), and the “111” project (B08021).

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