CO2 Capture Performance of Mesoporous Synthetic Sorbent

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CO2 Capture Performance of Mesoporous Synthetic Sorbent Fabricated Using Carbide Slag under Realistic Calcium Looping Conditions Xiaotong Ma,† Yingjie Li,*,† Changyun Chi,† Wan Zhang,† Jiewen Shi,† and Lunbo Duan‡ †

School of Energy and Power Engineering, Shandong University, Jinan 250061, China Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, Nanjing 210096, China

‡

ABSTRACT: Calcium looping is techno-economically feasible for industrial CO2 reduction. Carbide slag as a waste from the PVC (polyvinyl chloride) industry is a good candidate for low-cost calcium-based CO2 sorbents. A novel synthetic sorbent with rich mesopores was fabricated from high alumina cement, carbide slag, and byproduct of biodiesel, in order to overcome the loss in CO2 capture capacity of calcium-based sorbents with the number of carbonation/calcination cycles. The CO2 capture capacities of synthetic sorbents were examined under the severe calcination condition of high CO2 concentration and high temperature, which is close to the actual atmosphere for industrial applications. The effects of high alumina cement addition, byproduct of biodiesel addition, calcination condition, and steam addition in carbonation atmosphere on CO2 capture by the synthetic sorbents were also discussed. Results show that the synthetic sorbent with 90 wt % CaO achieves the highest CO2 capture capacity of about 0.27 g/g after 30 cycles under the realistic calcination condition, which is 1.7 times higher than that of carbide slag. N2 physisorption measurement reveals typical mesoporous structure in the synthetic sorbent with 90 wt % CaO and large amounts of pores in the range of 10−100 nm are maintained over 10 cycles. C12A7 (Ca12Al14O33) and C2AS (Ca2Al2SiO7) are found in the synthetic sorbent, which are uniformly distributed as the pore skeleton between CaO grains. The combining effect of the mesoporous structure and stabilization of pores over the repeated cycles is responsible for the high CO2 capture capacity of the synthetic sorbent. The synthesized mesoporous CO2 sorbent appears promising for the implementation of the cost-effective CO2 capture technique.

1. INTRODUCTION The increasing CO2 emission from industries has become the major concern about greenhouse gas emission worldwide.1−3 The continued growth of energy demand increases the impact of greenhouse gases (in particular, CO2) on environmental degradation.4 Appropriation measures should be taken quickly to reduce the risk of environmental damage, e.g., climate change.5,6 Calcium looping (CaL) is deemed as one of the most promising technologies for large-scale CO2 capture due to its low operational cost, high CO2 capture efficiency, and potential use in H2 production.7 Natural materials such as limestone and dolomite have attracted attention for CO2 capture because of their abundant resources and low cost.8,9 However, the calciumbased sorbents are faced with a decrease in CO2 uptake during repeated carbonation/calcination cycles. Besides, the energy consumption of CaO regeneration from CaCO3 is provided by the oxy-fuel combustion. Therefore, the CO2 concentration in the calciner is up to 95%, which implies that the temperature must be maintained over 930 °C to achieve full decomposition and fast kinetics within the short residence time in the calcination process.10 High temperature leads to a dramatic decay in CO2 uptake of the calcium-based sorbents mainly because of the sintering of CaO particles.11 Large amounts of fresh materials need to be supplemented accordingly to compensate for this partial loss in CO2 uptake, which is definitely uneconomical. The long-term CO2 capture performance becomes one of the key concerns for industrial applications of CaL.12 © XXXX American Chemical Society

Considerable research has prepared high-active calciumbased sorbents for CO2 capture by various methods, such as hydration,13 modification with acid or ethanol−water solution,14,15 sol−gel combustion synthesis (SGCS), and wet mixing combustion,16,17 and the use of biomass-based poreforming templates and organometallic precursors.18−20 The sorbents prepared by these methods possessed porous structure that was beneficial for the gas−solid reaction between CaO and CO2. The pores in the range of 10−100 nm in diameter greatly contributed to higher CO2 uptakes of the calcium-based sorbents.13,21 Lu et al.22 thought that micropores (50 nm) were contributive to the gas diffusion and mechanical stability of sorbents with the decreasing surface area. Wang et al.15 pointed out that more mesopores (2−50 nm) were supposed to make for carbonation of CaO at high temperature, as it could keep the balance of gas diffusion and enough surface area for carbonation. The mesoporous structure is also preferred during the pellet process of the calcium-based sorbents.23 Sun et al.24 fabricated mesoporous core−shell-structured CaO-based spheriform CO2 sorbents by a repeated impregnation coating process combined with the mesoscopic surfactant-templating method and found that the sorbents with a 1 μm mesoporous zirconia shell exhibited the lowest activity loss of only 30.8% Received: March 6, 2017 Revised: April 29, 2017 Published: May 25, 2017 A

DOI: 10.1021/acs.energyfuels.7b00676 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Chemical Components of Carbide Slag (wt %) CaO

MgO

SiO2

Al2O3

Fe2O3

SrO

Ti2O

Others

Loss on ignition

69.52

0.02

2.34

1.52

0.17

0.03

0.03

0.57

25.80

Table 2. Chemical Components of High Alumina Cement (wt %) CaO

MgO

SiO2

Al2O3

Fe2O3

SrO

Ti2O

K2O

Others

39.02

0.57

8.41

45.76

2.46

0.05

2.15

1.03

0.55

PVC from a chlor-alkali plant in China is about 0.6 million tons per year, and the output of the dry carbide slag is about 0.9−1.1 million tons a year, which means about several hundred thousand tons of CaO derived from carbide slag is produced every year. Zhang et al.39 reported that carbide slag possesses better cyclic CO2 capture performance and higher attrition resistance than limestone in a fluidized bed reactor. Sun et al.19 prepared carbide slag pellets by the extrusion−spheronization method, and pellets with the addition of 20 wt % microcrystalline cellulose displayed a carbonation conversion of 53% in the 25th cycle. However, these works focused on mild calcination conditions (N2 and relatively low temperature). Many researchers have proved that the high concentration of CO2 during calcination has an adverse effect on the carbonation reactivity of the regenerated CaO, and the CO2 concentration plays a crucial role on the behavior of calcium-based sorbents.40,41 Thus, the development of synthetic sorbents with high CO2 uptakes under realistic operation conditions (calcination under high CO2 concentration at above 900 °C) is significantly important and offers valuable reference to practical applications. This study presented here was conducted to produce a synthetic CO2 sorbent with rich mesopores using carbide slag as the calcium precursor, high alumina cement as the support material, and byproduct of biodiesel (BPB) as the pore-forming agent. The synthetic sorbent was further tested under realistic operation conditions during cyclic carbonation/calcination cycles. The CO2 capture behaviors of the synthetic sorbents under the severe calcination condition including high CO2 concentration and high temperature above 900 °C were investigated during the repetitive cycles. The effects of high alumina cement addition, BPB addition, calcination condition, and steam addition in carbonation atmosphere on CO2 capture by the synthetic sorbents were discussed. In addition, the CO2 capture performances of the synthetic sorbents were interpreted according to detailed morphology and chemical characterizations. This work aimed at investigating the cyclic reaction mechanism, and thus, only the CO2 uptakes of the synthetic sorbents were considered. The collision and attrition behaviors would be the future work if the viable CO2 capture performance of the synthetic sorbents could be obtained.

after 20 cycles. The combustion process using organic solvents as pore-forming agents, e.g., citric acid and glycerol, dispersed between CaO particles is an interesting way to improve the structural properties of the calcium-based sorbents. Luo et al.16 found that the violent combustion of citric acid contributed to the formation of a porous CaO structure, which allowed the sorbent to absorb CO2 in the succeeding cycles with high efficiency. Li et al.25 prepared CaO/Ca3Al2O6 by combustion synthesis and pointed out that the addition of glycerol during the preparation process exhibited an improvement on the cyclic CO2 capture capacity of the synthetic sorbent, possibly due to the quick release of CO2 and water vapor in the combustion synthesis step. Apart from the improvement for calcium-based sorbents, the cost should be considered when it comes to choose the practical agents. In our recent work,26 the byproduct of the transesterification process for biodiesel production was found to be a good alternative to glycerol that could lead to an effective improvement in CO2 uptakes because of surface cracking and an increase of the reactive surface area during the combustion process, and it is significantly cheap. Biodiesel is one of the promising alternative fuels because it is renewable and environmentally friendly.27 Its global output stands at about 30 million tons per year. About 1 ton of glycerol accompanies each ton of biodiesel. Therefore, the cost for synthesizing sorbents can be further reduced if large amounts of byproduct of biodiesel can be reutilized during the calcium looping. However, the improved pore structure with the addition of byproduct of biodiesel was not maintained for a long period of cycles due to the severe sintering of sorbents especially under the realistic calcination condition.26 The use of supports offers a promising way to increase the sintering resistance of calcium-based sorbents. A great portion of works have succeeded in the effective dispersion of a stable and inert matrix between active CaO, which avoids the aggregation of CaO grains under high temperature. Refractory materials, such as MgO,16,28 Al2O3,18 Y2O3,29,30 Ca2SiO4,31 MnO2,32 and some cheaper support materials such as fly ash,33 rice husk ash,34 bentonite,35 attapulgite,36 and kaolin,37 have been reported so far. The properties of synthetic sorbents with supports, together with encouraging preliminary results, led us to study them in more detail. In this regard, the two general thoughts of overcoming the loss-in-capacity problem of the calcium-based sorbents are to create mesoporous sorbents and stabilize these mesopores during long carbonation/calcination cycles. Some industries generate large quantities of high calcium content wastes every year which are always land-filled and cause the waste of calcium sources. The utilization of wastes, such as the steel slag from the iron and steel industry,1 white mud from the paper industry,38 and carbide slag from the PVC (polyvinyl chloride) and chlor-alkali industries,19 into potential candidates for CO2 capture is promising from the two perspectives of environmental protection and economics. The typical output of

2. EXPERIMENTAL SECTION 2.1. Sorbent Preparation. Carbide slag was sampled from a chlor-alkali plant in Shandong Province, China. Carbide slag was dried overnight at 105 °C, passed through a 0.125 mm sieve, and calcined at 800 °C for 10 min prior to the experiments. The chemical components (expressed in the form of oxides) of the carbide slag were determined by X-ray fluorescence (XRF), as shown in Table 1. The high alumina cement was supplied by a cement plant in Jinan, Shandong Province, China, and its components by the XRF were shown in Table 2. The BPB (>90 wt % of glycerol content) was obtained from the transesterification process of peanut oil (transesterification conditions: molar ratio of methanol to peanut oil of 12:1 and reaction time of 2 h, B


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Energy & Fuels reaction temperature of 64 °C, CaO as catalyst and CaO addition percentage of 6%).42 The novel synthetic CO2 sorbents were fabricated from carbide slag, high alumina cement, and BPB as follows: the calcined carbide slag (2 g in total) was put in a glass beaker. 10 mL of deionized water, the weighted amounts of high alumina cement, and 10−200 mL of BPB were then added in order under stirring at room temperature. Subsequently, the suspension was stirred at 60 °C for 60 min and then placed in a muffle furnace at 700 °C under air for 60 min. The combustion process includes the combustion of BPB and the solidstate reaction of carbide slag and high alumina cement. Then the synthetic sorbents were obtained. During the preparation process, the various synthetic sorbents containing 90, 80, and 70 wt % CaO derived from carbide slag were prepared by adjusting the addition of high alumina cement to 60.9, 254, and 502.3 mg, respectively. Besides, the modification of carbide slag only with the addition of BPB was also performed according to the same procedure mentioned above. C-H90, C-H-80, and C-H-70 denote the contents of CaO derived from the carbide slag in synthetic sorbents are 90, 80, and 70 wt %, respectively. 2.2. Cyclic CO2 Capture Tests. The cyclic CO2 capture capacities of the synthetic sorbents were evaluated in a dual fixed-bed reactor (DFBR) including a carbonator and a calciner, as shown in Figure 1.

after 20 min of carbonation was weighed again. To eliminate the experimental error arising in the mass difference of the pan at different temperatures, the pan was weighed after cooling down in a drying vessel for 2.5 min. The carbonation/calcination cycles were operated with the repeating procedures mentioned above. The CO2 capture capacity is defined as the CO2 adsorption amount per unit mass of the sorbent, as follows:

CN =

mN − mcal , N m0

(1)

where N is the number of carbonation/calcination cycles; CN represents the CO2 capture capacity after N cycles, g/g; m0 is the initial mass of the sorbent, g; mN is the mass of the sorbent after the Nth carbonation, g; and mcal,N is the mass of the sorbent after the Nth calcination, g. The calcined carbide slag and synthetic sorbents experiencing different carbonation/calcination cycles in the DFBR (carbonation: 15% CO2/N2, 700 °C and 20 min; calcination: 70% CO2/N2, 920 °C and 10 min) were collected, and their carbonation behaviors were tested in a Mettler Toledo TGA/SDTA851e thermal gravimetric analyzer (TGA). In the stream of 100% N2, the sample (about 5 mg) was sent into the TGA and the temperature increased from room temperature to 700 °C with a heating rate of 30 °C/min. Then the atmosphere was switched to 15% CO2/N2 with a total flow rate of 120 mL/min and maintained for 30 min. The CO2 capture capacity was calculated with the mass change as well according to eq 1. 2.3. Characterization. The phase compositions of the high alumina cement and synthetic sorbents were characterized by X-ray diffraction (XRD, D/Max-III). The pore structure parameters of synthetic sorbents and carbide slag, such as surface area, pore volume, and pore size distribution, were evaluated with a nitrogen adsorption analyzer (Micromeritics TriStar II 3020). A field emission scanning electron microscope (SEM, SUPRATM 55) was used to detect the apparent morphologies of synthetic sorbents and carbide slag before and after cycles. A transmission election microscope (TEM, JEOL2100F) was used to observe the crystal morphology of synthetic sorbents. The element distribution and content of the synthetic sorbent were scanned with an energy dispersive X-ray (EDX, Oxford INCA sight X). Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) spectra were performed on a Vertex 70 FTIR analyzer to detect functional groups in synthetic sorbents.

Figure 1. Diagram of DFBR. The steam generator that was equipped on the DFBR can provide a steady stream of water vapor. 500 mg of sample was placed in an alumina pan and sent into the calciner at 920 °C under a mixture flow of 70% CO2/N2 or at 850 °C under one of three options: 100% N2, 100% H2O, and 85% H2O/15% CO2. Before being introduced into the reactor, each gas (N2 and CO2) from the gas cylinder was controlled by a needle valve, and the mass flow rate of the each gas was regulated by a mass flowmeter. The total amount of gas flow was 2 L/min. The deionized water was heated up to 200 °C, and the stream of water vapor was dosed into reactors by means of a pump in the streams of 1.7 and 2 mL/min (with the pump speed of 21 and 24.8 r/min). All the gases were mixed in a blending tub, and the gas mixture was then sent to DFBR. The pan was taken out from the calciner after 10 min of calcination, weighed, and sent into the carbonator at 700 °C under a mixture flow of 15% CO2/N2 or 10% H2O/15% CO2/N2. The sorbent

3. RESULTS AND DISCUSSION 3.1. Characterization of Synthetic Sorbents. XRD patterns of the high alumina cement and the various synthetic CO2 sorbents (C-H-90, C-H-80, and C-H-70) are presented in Figure 2. The high alumina cement is mainly composed of C2AS (Ca2Al2SiO7) and CA (CaAl2O4), with a small amount of CA2 (CaAl4O7) as shown in Figure 2(a). It can be noticed that the diffraction peak distributions are similar among C-H-90, C-

Figure 2. XRD patterns of (a) high alumina cement and (b) synthetic sorbents. C


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Al atoms. The existence of SiO4− is probably due to a minor constituent of SiO2 in carbide slag. The double peaks at 1423 and 1478 cm−1 are the characteristic peaks belonging to vaterite.52 They are observed in spectra of both carbide slag and the synthetic sorbents, which is attributed to the exposure to air. The reason that this main reflection could not be detected by the XRD is the higher sensitivity of the FTIR apparatus.47 The absorption band at 3644 cm−1 that appears in carbide slag is assigned to the O−H stretching vibrations associated with hydration products when exposed to ambient humidity, as shown in Figure 3(d). The TEM image of C-H-90 as a representative of the synthetic sorbents is shown in Figure 4. The honeycomb-like

H-80, and C-H-70, as illustrated in Figure 2(b). The diffraction peaks of three synthetic sorbents belong to CaO, C12A7 (Ca 12 Al 14O 33), and C 2AS, which shows that the new composition, C12A7, is formed, while C2AS in the high alumina cement remains stable without chemical changes during the preparation process. This indicates that most of CA and CA2 react with CaO, as shown in eqs 2 and 3, respectively. There’s large amounts of evidence suggesting that the support material, C12A7, acts as a skeleton and decreases the aggregation and sintering of adjacent CaO crystallites during the calcination at high temperature.12,43−45 Besides, Yan et al.46 confirmed the formation of C2AS through mechanical mixing of CaO and coal fly ash and found that the dispersed C2AS was responsible for the high cyclic CO2 uptake and strong sintering resistance of synthetic sorbents. Some weak diffraction peaks belonging to Ca(OH)2 are observed in the synthetic sorbents because a small part of exposed CaO absorbs moisture from the air before the test. Therefore, in fact, the synthetic sorbents are only made up of CaO and two inert minerals as supports, C12A7 and C2AS. 5CaO + 7CaAl 2O4 → Ca12Al14O33

(2)

17CaO + 7CaAl4O7 → 2Ca12Al14O33

(3)

Figure 3 shows the FT-IR spectra of the synthetic sorbents and calcined carbide slag, which supplies more useful

Figure 4. TEM images of C-H-90 with the corresponding electron diffraction pattern.

structure can be found, in which piled pores exist between crystallites in C-H-90. Moreover, the domains in C-H-90 are well-crystallized and produce characteristic diffraction rings, as shown in the embedded image of Figure 4. Thus, C-H-90 is composed of polycrystals. The corresponding EDX analysis is given in Figure 5. It shows that C-H-90 mainly consists of Ca, O, Al, and Si elements, which agrees with the XRD analysis, as shown in Figure 5(a) and (b). The elemental distribution confirms a good and homogeneous distribution of all elements throughout C-H-90. The uniform distribution of supports provides an interconnected skeleton, which may retard the agglomeration of surrounded CaO grains and thus enhances the sintering resistance of the synthetic sorbents under high temperature. Figure 6 presents the magnified TEM images of C-H-90. It is interesting to notice that large amounts of mesopores are found in the interior of grains, as shown in Figure 6(a). This phenomenon indicates that the rapid combustion of the BPB as the pore-forming agent during the preparation of the sorbent possible creates a mesoporous structure of C-H-90. Some dissociated regions (one marked in green) are found in the crystallite, as shown in Figure 6(b). The partial dislocations create a stacking defect, which can influence the mass transfer of gas.53,54 Sun et al.55 found that the solid-phase ionic diffusion is dominated by the counter-current diffusion of inward CO32− groups and outward O2− anions during the carbonation process of the calcium-based sorbents. Namely, the O2− diffuses outward from the CaCO3/CaO interface to solid/gas interface to form CO32− groups, which then diffuses inward through the CaCO3 layer to form new CaCO3. Here, the defects may accelerate the motions of O2− and CO3−, which is beneficial for the carbonation rate of the synthetic sorbents.

Figure 3. FT-IR spectra of synthetic sorbents and calcined carbide slag.

information to further define the phase compositions. The broad band located in the region 750−900 cm−1 is associated with stretching of the Al−O bond in AlO4− tetrahedra from C12A7,47 which further demonstrates the XRD results. Moreover, with increasing the high alumina cement content, the shift of the bands from 871 cm−1 to lower wavenumber, 848 cm−1, is observed. The phenomenon indicates that the binding energy and stability of the functional group, Al−O, become greater, which is consistent with the result reported by Madroñero et al.48 Besides the fact that the bands around 690 cm−1 of synthetic sorbents are assigned to the Al−O bond,49 it can be seen that the depth of the bands becomes more pronounced as the high alumina cement content increases, which indicates that C12A7 content increases with the increase of the high alumina cement. The band located at 1120 cm−1 is speculated to be caused by two factors: one is attributed to O−Si−O bending of the SiO4− group; the other indicates the formation of the Al− O−Si bond.50,51 These vibrational modes appear at frequencies relatively close to each other due to the similar mass of Si and D


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Figure 5. SEM-EDX results of C-H-90: (a) SEM image; (b) EDX analysis.

Figure 6. Magnified TEM images (magnification factor: ×50,000 for (a) and ×300,000 for (b)).

3.2. CO2 Capture Performance of Synthetic Sorbent under Realistic Calcium Looping Conditions. The cyclic CO2 capture capacities of three synthetic sorbents (C-H-90, CH-80, and C-H-70) are plotted in Figure 8. The CO2 capture performances of the synthetic sorbents and the carbide slag are also compared. C-H-90, C-H-80, and C-H-70 all exhibit better CO2 capture performance than the carbide slag under the realistic calcination condition. The C10 values of C-H-90, C-H-

N2 physisorption measurement reveals type IV isotherms for C-H-90, as shown in Figure 7. The hysteresis loop is observed in the range of 0.7−1.0 P/P0, suggesting the typical mesoporous structure in C-H-90. An extremely sharp increase in adsorption during the high-pressure stage (P/P0 = 0.8−1.0) can be observed, which is caused by intergranular gaps between the particles.56 E


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Figure 7. N2 adsorption−desorption isotherms of C-H-90.

Figure 9. CO2 capture performances of C-H-90 in multicycle process (carbonation: 15% CO2/N2, 700 °C and 20 min; calcination: 70% CO2/N2, 920 °C and 10 min).

experiment data of the general limestones when k = 0.52 and Xr = 0.075. The CO2 capture capacities of the synthetic sorbents in the literature as a function of cement content are presented in Figure 10, and the various reaction conditions are summarized

Figure 8. Effect of cement addition on the CO2 capture capacities of synthetic sorbents during 10 cycles (carbonation: 15% CO2/N2, 700 °C and 20 min; calcination: 70% CO2/N2, 920 °C and 10 min).

80, and C-H-70 are 53%, 21%, and 18% higher than those of carbide slag, respectively. The lower CO2 capture capacities of C-H-70 during the previous cycles compared with the carbide slag may be attributed to the higher alumina cement content, which decreases the active CaO for CO2 capture in synthetic sorbents. However, the CN value of C-H-70 drops by 50% as the cycle number increases from 1 to 10. This reveals that the addition of the high alumina cement enhances the cyclic stabilities of synthetic sorbents but it may restrain the theoretical maximum CO2 capture capacity because of the reduction of CaO content. C-H-90 exhibits the highest cyclic CO2 capture capacity among three synthetic sorbents. Therefore, the CO2 capture performance of C-H-90 during the multiple cycles is illustrated in Figure 9. It is found that the C30 value of C-H-90 is approximately 0.27 g/g, which is 1.7 times as high as that of carbide slag and 2.6 times higher than that of the general limestone fitted by the empirical formula (eq 4) from Grasa and Abanades.57 The high and stable CO2 capture capacity of C-H-90 may be attributed to the mesoporous structure and the presence of the inert components, C12A7 and C2AS, which are sufficient to maintain the pore structure and hinder the agglomeration of CaO particles under the severe calcination condition. XN =

1 1 1 − Xr

+ kN

Figure 10. Comparison of CO2 capture capacities of synthetic sorbents prepared from various calcium precursors and cements.

Table 3. Reaction Conditions Reported in the References Involved in Figure 10* ref 58 59 45

60 44

+ Xr (4)

This study

where XN represents the carbonation conversion after N cycles, mol/mol; k is the deactivation constant; and Xr is the residual carbonation conversion. The equation is adopted to fit the

Calcium precursor Limestonea Limestoneb CaO Commercial hydrated lime Ca(OH)2 Limestone CaO Ca(OH)2 Carbide slag

Carbonation condition

Calcination condition

Cycle No.

850 °C, 100% CO2, 10 min

850 °C, 100% N2, 10 min

30

650 °C, 15% CO2/ N2, 15 min 650 °C, 15% CO2/ N2, 30 min

850 °C, 100% N2, 10 min 900 °C, N2, 30 min

50

650 °C, 15% CO2/ N2, 30 min 800 °C, 25% CO2/ N2, 30 min

900 °C, 100% N2, 10 min 800 °C, 100% N2, 15 min

16

800 °C, 25% CO2/ N2, 30 min

900 °C, 100% N2, 10 min

18

90 120 30

* Note: Different species of cement were used in aCA-14 aluminate cement and bSecar 51 aluminate cement.

F


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Energy & Fuels accordingly in Table 3. It should be noted that all of the synthetic sorbents reported elsewhere were calcined under mild atmosphere (100% N2). C-H-90 in this study shows better CO2 capture performance than those in the literature, which means the novel synthetic CO2 sorbents fabricated from carbide slag, high alumina cement, and BPB are a kind of highly effective CO2 sorbents. To validate the effects of the addition of BPB, a comparison was made between the conditions with and without BPB of both C-H-90 and carbide slag, as shown in Figure 11. The

Figure 12. Carbonation behaviors of C-H-90 and the carbide slag in TGA (carbonation: 15% CO2/N2, 700 °C and 20 min; calcination: 70% CO2/N2, 920 °C and 10 min).

more mesopores. In this situation, carbonation cannot be easily hindered by the CaCO3 product layer. Thus, there is a gradual and retarded transition from the kinetically controlled carbonation stage to the diffusion controlled one. Besides, CN of C-H-90 increases faster than that of the carbide slag, with the reaction time, during the diffusion controlled stage, indicating a higher carbonation rate. This phenomenon is in good consistence with the hypothesis about the stacking defects influencing the carbonation kinetics by accelerating the motions of O2− and CO3−. The higher CN of C-H-90 during the carbonation process, compared with the carbide slag, confirms the optimal performance of C-H-90. The calcination temperature is one of the key factors affecting the CO2 capture behavior of the calcium-based sorbent. It must be higher than the equilibrium temperature of the reaction between CaO and CO2, which is related to the CO2 concentration in the calcination atmosphere.41 From this perspective, reducing the CO2 partial pressure under the realistic calcium condition is supposed to reduce the decay rate of CO 2 capture by a calcium-based sorbent. A high concentration of N2 in the calcination atmosphere only has a research value and cannot be applied in reality because CO2 cannot be separated and concentrated easily in this situation. Diluted CO2 concentration by H2O is feasible and promising in theory, considering that CO2 can be separated after a simple condensation process, as reported in the earlier work.26 Researches showed that the presence of steam in the calcination atmosphere can lower the partial pressure of CO2, which can decrease the decomposition temperature and thus hinder the sintering. Wang et al.61 proposed that steam can weaken the bond between CaO and CO2, which was beneficial for the rapid decomposition of CaCO3. Champagne et al.62 reported that steam changed the morphology of the sorbent while calcination was occurring, resulting in a structure which increases carrying capacity. Oxy-steam combustion can replace the traditional oxy-fuel combustion that generates a high concentration of H2O.63 Figure 13 shows the CO2 capture performances of C-H-90 under high concentration of H2O. Except for the calcination under high CO2 concentration (70%), 850 °C is applied, since it is high enough for the complete and rapid decomposition of CaCO3 under high concentration of N2 and H2O. As expected, C-H-90 achieves the highest CO2 capture capacities and stabilities under pure N2, the mildest calcination condition. Under this condition, C10 of C-H-90 is 0.57 g/g, which is about twice as high as that of carbide slag. Moreover, for the calcination under pure H2O, the C10 values of C-H-90 and the

Figure 11. Effects of BPB addition on the CO2 capture performance of C-H-90 and carbide slag (carbonation: 15% CO2/N2, 700 °C and 20 min; calcination: 70% CO2/N2, 920 °C and 10 min).

obvious increase in the CO2 capture capacities of carbide slag is observed when BPB is added, e.g., the CO2 capture capacities of modified carbide slag only with the addition of BPB are 0.27 g/ g after 10 cycles, which is 26% higher than that of carbide slag without BPB. This may be attributed to the better pore structure formed due to the release of CO2 and H2O during the combustion of BPB. As the ratio of BPB/CaO increases from 20 mL/g to 100 mL/g, the CO2 capture capacities of C-H-90 increase correspondingly. It can be noted that the additional amount of BPB has a small effect on the CO2 capture capacity of C-H-90 when the ratio of BPB/CaO is higher than 25 mL/g. Therefore, there’s no need to add so much pore-forming agent because the excess BPB may just burn itself and has no influence on the CO2 capture performance of the synthetic sorbents. When the CO2 capture performance and cost of the synthetic sorbents are considered together, the optimum ratio of BPB/CaO is 25 mL/g. Biodiesel and BPB are conventionally produced via the transesterification, where the various biomasses, such as oil-bearing crops, alga, etc., react with short-chain alcohol. From the total life cycle point of view, the net CO2 emission is zero during the combustion of BPB. Figure 12 shows the carbonation behaviors of C-H-90 and carbide slag in TGA during the first and 10th cycles. It shows that the carbonation behavior of C-H-90 makes little difference compared to that of carbide slag in the first cycle. The slightly higher reaction rate in the previous 100 s of C-H-90 may be attributed to the larger interface between CO2 molecular and CaO constructed by large amounts of mesopores displayed in Figure 6. During the 10th cycle, it can be observed that the kinetically controlled carbonation stage for the carbide slag is quickly changed to the diffusion controlled stage, while carbonation of C-H-90 proceeds slowly and lasts longer at the kinetically controlled one. This may be due to the highsintering-resistance materials (C12A7 and C2AS) in the sorbents as frameworks, which maintains good microstructure, especially G


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the carbide slag during CO2 capture cycles. The surface of the calcined carbide slag presents partial adhesions of CaO particles, while that of initial C-H-90 shows separated particles with well-fined boundaries, as shown in Figure 15(a) and (c). This is supposed to be related to the release of CO2 and H2O during the rapid combustion of BPB as the pore-forming agent that results in these slit pores and gaps between grains in C-H90, which has been intuitively observed in Figure 6(a). Subsequently, the particles in the calcined carbide slag enlarge after 10 cycles, leaving a less porous surface, as shown in Figure 15(b). The morphology of C-H-90 after 10 cycles shows a slight grain growth and the formation of some macropores, but it is relatively more stable than that of calcined carbide slag, as shown in Figure 15(d), indicating just a slight sintering of C-H90 under the realistic calcination condition. This proves the supporting effect of C12A7 and C2AS on the pore structure of C-H-90 and also leads to the better CO2 capture performance of the synthetic sorbent under realistic calcium looping conditions. The BET surface areas and pore volumes of calcined C-H-90 and carbide slag show a significant difference, as summarized in Table 4. C-H-90 achieves better and stable pore characteristics compared with carbide slag. After 10 carbonation/calcination cycles, the surface area and pore volume of C-H-90 are 12.21 m2·g−1 and 0.055 cm3·g−1, which are 3.31 and 3.44 times greater than those of calcined carbide slag, respectively. Larger surface area and pore volume are likely to result in the higher CO2 capture capacity of C-H-90. Additionally, the pore size distributions before cycles (N = 0) and after 10 cycles (N = 10) of C-H-90 and carbide slag are plotted in Figure 16. Carbide slag suffers a great loss of pores over the cycles, which leads to high diffusion resistance of CO2 and poor CO2 capture capacity. As for C-H-90, although the pores in the diameter less than 10 nm dramatically decrease, large amounts of pores in the range of 10−100 nm in the diameter are maintained over 10 cycles, which has been proven to be the main contributors for CO2 capture. This indicates that C-H-90 has better sinteringresistance nature, which is believed to be owing to the inert materials, C12A7 and C2AS, effectively hindering the sintering.

Figure 13. CO2 capture performances of C-H-90 and the carbide slag under different calcination conditions (carbonation: 15% CO2/N2, 700 °C and 20 min; calcination: 10 min).

carbide slag are 0.43 and 0.25 g/g, respectively, which are 39% and 6% higher than those calcined under high concentration of CO2. This is attributed to the lower calcination temperature.61 With the diluted CO2 concentration by H2O, i.e., 85% H2O/ 15% CO2, C10 of C-H-90 is 0.34 g/g, which is 10.44% higher than that calcined under 70% CO2/30% N2. It is also 1.47 times as high as that of carbide slag calcined under 85% H2O/15% CO2. Therefore, the high concentration of H2O is another realistic calcination condition except high concentration of CO2, and it is more conducive to CO2 capture by the synthetic sorbent and more energy-saving. Normally, flue gas from a coal-fired power plant contains 5%−10% steam, bringing the necessity of understanding how the steam presented in the carbonation condition affects the carbonation performance of the synthetic sorbent. Figure 14

4. CONCLUSIONS A high-active and low-cost synthetic calcium-based sorbent with stable mesoporous structure for CO2 capture was fabricated using carbide slag, high alumina cement, and BPB. In such a process, a bunch of mesopores confirmed by both TEM and N2 adsorption analysis that are beneficial for CO2 capture are easily obtained and inert materials, C12A7 and C2AS, are uniformly distributed in the synthetic sorbent. The synthetic sorbent with 90 wt % CaO (C-H-90) exhibits the best CO2 capture performance, achieving the CO2 capture capacity of around 0.27 g/g under the realistic calcination condition after 30 cycles, which is 1.7 times as high as that of carbide slag. During the multiple carbonation/calcination cycles, the incorporation of C12A7 and C2AS inhibits the agglomeration and sintering of CaO particles, thereby maintaining large amounts of pores in the range of 10−100 nm in the diameter over 10 cycles of C-H-90 that contributes to the better CO2 capture performance. Diluted CO2 concentration by steam in the calcination atmosphere reduces the decay rate of the synthetic sorbent. Besides, the presence of 10% H2O in the carbonation significantly improves the CO2 capture capacity of C-H-90 by 34%. The superior CO2 capture

Figure 14. CO2 capture performances of C-H-90 and the carbide slag with the presence of steam in carbonation atmosphere (carbonation: 700 °C and 20 min; calcination: 70% CO2/N2, 920 °C and 10 min).

shows the effect of steam addition in carbonation atmosphere on the CO2 capture capacity of C-H-90. It is found that the presence of 10% H2O enhances the CO2 capture by C-H-90 and the carbide slag under the realistic calcination condition. For example, the C10 values of C-H-90 and the carbide slag in the presence of 10% H2O are 0.43 and 0.27 g/g, respectively, which are 34% and 31% higher than those in the absence of H2O in carbonation atmosphere. Yang and Xiao64 proposed that the enhancement of CO2 capture performance by adding H2O could be attributed to the formation of surface hydroxyl groups and bicarbonates as intermediates. Another hypothesis on steam enhancing product layer diffusion has been recently confirmed and supported.65,66 3.3. Microstructure Analysis of the Synthetic Sorbent during Cycles. Figure 15 shows SEM images of C-H-90 and H


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Energy & Fuels

Figure 15. SEM images of C-H-90 and the calcined carbide slag (carbonation: 15% CO2/N2, 700 °C and 20 min; calcination: 70% CO2/N2, 920 °C and 10 min): (a) calcined raw carbide slag, (b) calcined carbide slag after 10 cycles, (c) initial C-H-90, and (d) calcined C-H-90 after 10 cycles.

Notes

Table 4. BET Surface Areas and BJH Pore Volumes of Calcined C-H-90 and Carbide Slag after Different Cycles Sample

Cycle No.

BET surface area, m2/g

BJH pore volume, cm3/g

Carbide slag Carbide slag C-H-90 C-H-90

0 10 0 10

10.31 3.69 15.35 12.21

0.048 0.016 0.078 0.055

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51376003), Primary Research & Development Plan of Shandong Province, China (2016GSF117001), and the Fundamental Research Funds of Shandong University, China (2014JC049). We thank Hui Li for providing the BPB for this work.

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Figure 16. Pore size distributions of calcined C-H-90 and carbide slag after different cycles (carbonation: 15% CO2/N2, 700 °C and 20 min; calcination: 70% CO2/N2, 920 °C and 10 min).

performance of the synthetic sorbent makes it possible to fully utilize the industrial wastes into the cost-effective CO2 capture.

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Xiaotong Ma: 0000-0001-9008-4268 I


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CO2 Capture Performance of Mesoporous Synthetic Sorbent

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