CO2 capture performance of CaO modified with by-product of biodiesel at calcium looping conditions

Accepted Manuscript CO2 capture performance of CaO modified with by-product of biodiesel at calcium looping conditions Changyun Chi, Yingjie Li, Xiaotong Ma, Lunbo Duan PII: DOI: Reference:

S1385-8947(17)30932-4 http://dx.doi.org/10.1016/j.cej.2017.05.163 CEJ 17062

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

6 April 2017 26 May 2017 27 May 2017

Please cite this article as: C. Chi, Y. Li, X. Ma, L. Duan, CO2 capture performance of CaO modified with by-product of biodiesel at calcium looping conditions, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/ j.cej.2017.05.163

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CO2 capture performance of CaO modified with by-product of biodiesel at calcium looping conditions Changyun Chia, Yingjie Li∗a, Xiaotong Maa, Lunbo Duanb a

School of Energy and Power Engineering, Shandong University, Jinan, 250061, China

b

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, Nanjing, 210096, China

Abstract: A novel method that CaO was modified with the by-product of biodiesel by the combustion was proposed to improve its CO2 capture capacity at calcium looping conditions. The CO2 capture performance of CaO modified with the by-product of biodiesel during the calcium looping cycles was investigated in a twin fixed-bed reactor and a thermogravimetric analyzer. The effects of ratio of by-product of biodiesel to CaO, combustion duration and temperature on the CO2 capture performance of CaO modified with the by-product of biodiesel were studied. When the ratio of by-product of biodiesel/CaO is 25mL/g, the modified CaO achieves the highest CO2 capture capacity during the cycles. The feasible combustion temperature and duration are 800 oC and 60 min, respectively. CO2 capture capacity of the modified CaO can retain 0.5 g CO2/g sorbent after 20 cycles (carbonation at 700 oC for 20 min in 20% CO2/80% N2, calcination at 850 o

C for 10 min in N2), which is higher than that of the modified CaO with the various organic

solutions. The modified CaO still shows much higher CO2 capture capacity than original CaO under the severe calcination condition. The by-product of biodiesel modification greatly improves CO2 capture rate of CaO in the chemical-controlled stage, but shows a little effect on rate in the diffusion-controlled stage. The cyclic CO2 capture capacity of the deactivated CaO is significantly

∗

Corresponding author: Tel.: +86-531-88392414; E-mail: [email protected] (Y. Li). 1

reactivated after the by-product of biodiesel modification. The modified CaO exhibits more porous structure and higher sintering resistance during the CO2 capture cycles. Keywords: CaO; modification; combustion; calcium looping; CO2 capture

1 Introduction Nowadays, the increasing CO2 emissions have become a huge problem for mankind. The vast majority of CO2 results from human activities, especially from the combustion of fossil fuels [1]. The emissions have increased rapidly in recent years and caused the greenhouse gas effect. Carbon capture and storage (CCS) has been proposed as a potential option for reducing CO2 emissions released by stationary sources such as power plants and other industrial processes [2, 3]. Calcium looping cycles (i.e. carbonation/calcination cycles), containing the repetitive reaction between CaO and CO2, which is considered as emerging technology for CO2 capture from high temperature post-combustion capture and the hydrogen production process because of its advantages in economy [4, 5]. In carbonation/calcination cycles, the flue gas containing CO2 from fossil-fired power plant is sent into a carbonator, where CO2 reacts with CaO to form CaCO3 at 650-700 oC. Then CaCO3 is transported to a calciner, where CaCO3 decomposes into CaO and CO2 at high temperature. CO2 from the calciner can be concentrated for storage, and the regenerated CaO is again sent into the carbonator for the next cycle. The discharged spent CaO can be used as the material for the cement manufacture [5]. Calcium-based sorbents have advantages in wide resource and low price. But CO2 capture capacities

of

the

calcium-based

sorbents

decrease

gradually with the

number

of

carbonation/calcination cycles due to the sintering of the sorbents [6, 7]. The sintering results in

2

the pores blockage, which is not beneficial to CO2 capture by the sorbent [8-11]. Thus, it is necessary and significant to improve cyclic CO2 capture performance of the calcium-based sorbents during the calcium looping cycles [12, 13]. Increasing the active surface area and pore volume of the calcium-based sorbent is an effective method to improve its CO2 capture capacity and cyclic stability, which can be implemented by hydration [14-18], doping [19-29], thermal pre-treatment [30, 31], chemical treatments [33-43], etc. Among the chemical treatment methods, the solution modification (e.g. organic solution) is usually used to modify calcium-based sorbents to improve CO2 capture capacity [32]. Li et al. [33] found that the CaO modified with ethanol-water solution exhibited higher CO2 capture capacity than hydrated CaO. Wang et al. [34] also used ethanol solution to modify CaO and they found ethanol could enhance the hydration reaction of CaO and the calcines derived from the modified CaO possessed higher porosity. The various organic acid solutions such as formic acid, oxalic acid, vinegar, acetic acid, propionic acid, citric acid, gluconic acid, pyroligneous acid can react with calcium-based sorbents to produce the organic calcium salts. After the calcination of the organic calcium salts, the porous CaO sorbents are formed due to the release of a certain amount of gases during the calcination. Li et al. [38, 39] found the carbonation conversions of limestone and dolomite were both significantly improved by acetic acid solution modification. And the surface areas and pore volumes of the calcines derived from the two sorbents were also increased after the modification. They pointed out that organic acid modification improved the sintering resistance of the sorbents. Ridha et al. [35] used four organic acid solutions including acetic acid, formic acid, vinegar and oxalic acid to treat the limestone and the corresponding modified limestone achieved 0.26, 0.24, 0.17 and 0.28 g CO2/g sorbent after 20 cycles, respectively. Sun et al. [40] fabricated

3

the modified limestone with propionic acid and its CO2 capture capacity was 0.24 g CO2/ g sorbents after 100 cycles, which was about 4 times than that of the untreated limestone due to a significant increase in the porosity. Ridha et al. [36] used formic acid solution to modify the calcium-based sorbent and they found that the microstructure of the modified sorbent was related to acid concentration in the solution. Radfarnia and Iliuta [41] obtained the citric acid modified limestone and its CO2 capture capacity was 0.415g CO2/g sorbent after 18 cycles. They found the majority of small pores (less than 300 nm) of the sorbent apparently vanished, leading to the formation of larger ones (less than 400 µm), which played a positive role in promoting the CO2 capture performance. In addition, they pointed out that limestone treatment using acetic acid derivatives was introduced as a powerful technique to improve the durability and adsorption capacity of natural limestone. Wang et al. [44] thought that the production process for solution modification of calcium-based sorbents was simple and convenient for use in practice. However, the cost of the organic solution modification is not low. Therefore, the more inexpensive and efficient organic solution modification to improve CO2 capture performance of calcium-based sorbent is welcome. The biodiesel is a kind of promising alternative fuel, which is generally produced by the transesterification reaction of triglycerides and methanol [45]. Glycerol is the main composition of the by-product of biodiesel (BPB), which accounts for up to 90%. BPB is an important industrial resource, and has the advantages of low price and availability. In our previous research, Ma et al. [20] synthesized CaO/MgO sorbent by using magnesium and BPB, which achieved higher CO2 capture capacity than natural sorbents. And its CO2 capture capacity retained 0.42 g CO2/g sorbent after 20 cycles. The effect of MgO on cyclic CO2 capture by the sorbent was revealed. However,

4

the individual effect of BPB on CO2 capture performance and pore structure of the sorbent was not discussed. In this work, CaO was the modified with BPB by the combustion to enhance its CO2 capture capacity at calcium looping conditions. The effects of ratio of BPB/CaO, combustion duration and temperature on the CO2 capture performance of the modified CaO with BPB were studied on a twin fixed-bed reactor and a thermogravimetric analyzer. The CO2 capture capacities and rates of the modified CaO and the original CaO during the cycles were also compared. In addition, the microstructure of the modified CaO was detected, in order to reveal the mechanism of BPB modification. 2 Experimental 2.1 Materials and sorbents preparation The analytically pure CaO (CaO≥99.0 wt.%, Tianjin Kermel Chemical Reagent Co., Ltd., China) was used in this work. BPB containing high concentration of glycerol (> 90%) was sampled from the transesterification process of the peanut oil reacted with methanol (reaction conditions: alkali catalyst, reaction temperature of 64 °C, molar ratio of methanol to peanut oil of 12:1 and reaction time of 2h) [45]. There is not an ash content in BPB. The procedures of the different modified CaO are shown in Fig. 1. A certain amount of CaO was added into the distilled water by fully mixing at the ratio of 1g CaO:10mL distilled water in a beaker. After the complete hydration reaction, the different volumes of BPB addition were added in the beaks with the ratios of BPB/CaO of 5mL/1g, 10mL/1g, 25mL/1g, 50mL/1g and 100mL/1g, respectively. And then the solutions were placed in an electric-heated thermostat water bath for stirring for 60 min at 60 oC. The solutions from mentioned above steps were combusted at 800-900oC in a muffle furnace

5

under air atmosphere for 10-120 min, and then the various modified CaO with BPB were obtained. The obtained sorbent was crushed and screened to a particle size below 0.125 mm. In order to eliminate the interference of hydration process on the BPB, the hydrated CaO was produced according to the procedure of the modified CaO with BPB except the addition of BPB, as shown in Fig. 1. In addition, in order to study the reactivation of the deactivated CaO by BPB modification, CaO which had experienced 10 carbonation/calcination cycles was modified with BPB (the ratio of BPB/CaO of 25mL/g), as presented in Fig. 1. The three modified sorbents including BPB-CaO, H-CaO and BPB-D-CaO were obtained, which denote the modified CaO with BPB, hydrated CaO and the modified deactivated CaO with BPB, respectively. O-CaO denotes the original CaO. 2.2 CO2 capture tests CO2 capture tests were performed in a twin fixed-bed reactor (TFBR), as shown in Fig. 2. TFBR includes a calciner and a carbonator designed at the atmospheric pressure. The actual temperatures of the reactors are controlled by silicon carbide rods and measured by thermocouples placed in the center of the reactors. Samples (about 500 mg) were sent to the carbonator for 20 min at 700oC in 20% CO2/80% N2, then put into the calciner for 10 min complete calcination (under three conditions: at 850 oC in pure N2, at 920 oC in 70% CO2/30% N2 and at 950 o C in pure CO2) and repeatedly shifted between the two reactors. The thermocouples were close to the samples, and the carbonation atmosphere was shifted when the temperature of the thermocouples corresponded with the furnace. In order to guarantee the appropriate relative velocity of gas-solid, the gas flow rates in the calciner and the carbonator were both 2L/min. The samples after the carbonation and calcination reactions were taken out from TFBR and put into a nitrogen-filled

6

dryer to prevent the atmospheric moisture and CO2 until they were close to room temperature. And then the mass changes of the samples were measured by an electronic balance (Mettler Toledo-XS105DU) with the accuracy of 0.1 mg. The CO2 capture capacity of the sample during the cycles, which means CO2 adsorption amount per unit mass of the sample, was calculated by CN =

m N (t ) − mcal, N m0

(1)

A thermogravimetric analyzer (TGA, Mettler-Toledo TGA/SDTA85le) was adopted to examine the CO2 capture rates of O-CaO, H-CaO, BPB-CaO during the 1st and the 11th cycles. Besides, the CO2 capture capacities and rates of O-CaO in the 10th cycle and BPB-D-CaO in the 11th cycle were tested in TGA. The initial sorbents and the cycled sorbents sampled from the TFBR which had experienced 10 cycles were respectively put into the furnace of TGA and the temperature was raised to 700 oC at a rate of 25 oC/min under pure N2. When the temperature of the furnace reached 700 oC, the atmosphere was changed into 20% CO2/80% N2 and the sorbent were then carbonated for 20 min under this atmosphere. The reaction gas flow speed was 120 mL/min and the protection gas (N2) flow speed was 30 mL/min. The CO2 capture capacity of the sorbent in TGA was calculated by Eq. (1), and the CO2 capture rate was defined as

rN =

dCN dt

(2)

where rN is CO2 capture rate of the sorbent at t during the Nth cycle, g/(g·s). 2.3 Characterization The phase constitutions of the samples were characterized by a D/Max-IIIA X-ray diffraction (XRD). The microstructures of O-CaO, H-CaO and BPB-CaO during the cycles were analyzed by a SUPRATM 55 field emission scan electron microscope (SEM). A TECNAI 20 U-TWIN high

7

resolution

transmission

election

microscopy

(HTEM)

and

selected

area electron diffraction (SAED) were used to observe the crystal morphology of the samples. The surface areas and the pore volumes of the samples were analyzed by a Micromeritics ASAP 2020-M nitrogen absorption analyzer, which were calculated by Brunauer-Emmet-Teller (BET) equations and Barrett-Joyner-Halenda (BJH) model, respectively. 3 Results and Discussion 3.1 Effect of ratio of BPB to CaO during preparation of BPB-CaO on CO2 capture The XRD pattern of BPB-CaO is shown in Fig. 3. It is found that the main composition of BPB-CaO is CaO, which is the same as that of the original CaO. This indicates that the BPB modification does not change the main composition of the sorbent. However, the BPB modification has a significant effect on the pore structure of the sorbent, which will be discussed in the section 3.6.

The effect of the ratio of BPB/CaO on CO2 capture capacity of BPB-CaO during the cycles in TFBR is demonstrated in Fig. 4. Compared to the H-CaO, BPB-CaO prepared by the various ratios of BPB/CaO remains higher and more stable C N. It is found that CN of BPB-CaO increases with increasing the ratio of BPB/CaO from 5mL/g to 25mL/g, because more Ca(OH)2 is dissolved in BPB during the preparation of BPB-CaO with increasing the ratio of BPB/CaO, which means more porous structure is possibly formed after the combustion. However, as the ratio of BPB/CaO increases from 25 mL/g to 100mL/g, CN of BPB-CaO decreases. That is maybe because the too high ratio of BPB/CaO leads to high temperature of the sorbent during the combustion process, which aggravates the sintering of BPB-CaO. Therefore, when the ratio of BPB/CaO is 25mL/g, BPB-CaO achieves the highest CO2 capture capacity. In this case, C10 of BPB-CaO retains 0.65 8

g/g, which is 1.6 times than that of the H-CaO. Thus, the ratio of BPB/CaO of 25mL/g is used during the preparation of BPB-CaO in the following sections.

3.2 Effects of combustion temperature and duration during preparation of BPB-CaO on CO2 capture The effects of the combustion duration and the temperature on CO2 capture capacity of BPB-CaO in TFBR are illustrated in Fig. 5. H-CaO shows better CO2 capture performance than O-CaO, but the CO2 capture capacity of H-CaO rapidly decays with the cycle number. The calcination duration has little effect on CO2 capture by H-CaO. The obtained BPB-CaO at the various preparation conditions shows higher CO2 capture capacity than O-CaO and H-CaO. The combustion temperature and duration have an important effect on CO2 capture capacity of BPB-CaO. When the combustion temperature is 800 oC, BPB-CaO shows higher and more stable CO2 capture capacity during the cycles. C10 of BPB-CaO retains 0.65 g/g at the combustion temperature of 800 oC for 60 min, which is only 6.1% lower than C1. As the combustion temperature increases, the obtained BPB-CaO shows a reduction in CN. When the combustion temperature increases from 800 to 900 oC for 60 min, C 1 and C10 of BPB-CaO decay by 19.8% and 28.2%, respectively. This is because the high combustion temperature (over 850 oC) intensifies the sintering of BPB-CaO, which impedes CO2 adsorption during the cycles. The combustion duration also shows an effect on CO2 capture by BPB-CaO. CN of BPB-CaO increases with the combustion duration rising from 10 to 60 min. However, as the duration increases from 60 to 120 min, C N has almost no change. This is because the prolonged combustion duration is beneficial for the burnout of BPB, which is beneficial for the formation of the porous structure of BPB-CaO, but too long duration does not improve the pore structure further. Therefore, the

9

feasible combustion temperature and duration are 800 oC and 60 min, respectively, which are used during the preparation of BPB-CaO in the following sections. The comparison of CO2 capture capacities of BPB-CaO in TFBR and the modified CaO with the various organic solutions such as ethanol, acetic acid, formic acid, propionic acid, etc. reported in the references is presented in Fig. 6. The summary of the reaction conditions for the modified CaO with the various organic solutions in Fig. 6 is shown in Table 1. BPB-CaO exhibits higher CN (N > 3) than the modified CaO with the various organic solutions. C20 of BPB-CaO retains 0.5 g/g, which is 1.3 and 2.3 times as high as those of propionic acid-CaO [40] and formic acid-CaO [35]. It indicates that BPB-CaO possesses higher CO2 capture capacity during the calcium looping cycles, compared to the modified CaO with the various organic solutions. 3.3 Effect of calcination condition on CO2 capture Oxy-fuel combustion is used to offer the heat for the calcination of CaCO3 in the calciner, which produces the flue gas containing high concentration of CO2. Thus, it is necessary to research CO2 capture by BPB-CaO calcined under high concentration of CO2 at high temperature. The high temperature and the high CO2 concentration in the calcination aggravate the sintering of the sorbent, which is not beneficial for CO2 capture [46, 47]. The CO2 capture capacities of BPB-CaO and O-CaO under the mild calcination condition (850 oC and N2) and the severe calcination condition (920 oC and 70% CO2/30% N2) in TFBR are depicted in Fig. 7. It is found that when the mild calcination condition is switched to the severe calcination condition, C1 and C10 of BPB-CaO decrease by 7% and 41%, respectively. It indicates that the severe calcination condition leads to the reduction in CN of BPB-CaO. However, BPB-CaO still shows much higher CN than O-CaO under the severe calcination condition. C1 and C10 of BPB-CaO are 1.5 and 1.7

10

times as high as those of O-CaO under the severe calcination condition, respectively. Especially, CN of BPB-CaO calcined under the severe calcination condition is higher than that of O-CaO calcined under the mild calcination condition. It suggests that BPB-CaO also seems promising as the CO2 sorbent under the severe calcination condition. 3.4 The effect of BPB modification carbide slag on CO2 capture performance Carbide slag is a kind of industrial solid waste from the production of acetylene by hydration of calcium carbide. The same method shown in section 3.1 is used to prepare carbide slag modified with BPB, and the CO2 uptake capacity of carbide slag modified with BPB in TFBR is shown in Fig. 8. C1 and C10 of carbide slag modified with BPB are 20.3% and 49.6% higher than those of carbide slag calcined at 850 o C in 99.999% N2, respectively. It suggests that the BPB modification has a positive effect on CO2 capture capacities of the various calcium-based sorbents. 3.5 CO2 capture rates of BPB-CaO The CO2 capture capacities and rates of O-CaO, H-CaO and BPB-CaO in TGA are illustrated in Fig. 9. The TGA curves indicate that the carbonation reactions of the various sorbents are characterized by two evident stages: an initial chemical-controlled stage (within about 150 s) and followed by a diffusion-controlled stage (above 150 s), as shown in Fig. 8(a). It is found that although the initial chemical-controlled stage is very short, the most of CaO in the sorbent converts into CaCO3 in this stage. The carbonation of the calcium-based sorbent predominantly takes place during the chemical-controlled stage. As shown in Fig. 9(a), C1 of BPB-CaO is 35.7% higher than that of O-CaO at 150 s. C11 of BPB-CaO at 150 s is 0.43 g/g, which is 95.5% and 126.3% higher than those of H-CaO and O-CaO, respectively. rN of BPB-CaO is significantly higher than that of O-CaO and H-CaO during the 11th cycle, as shown in Fig. 9(b). r11 of

11

BPB-CaO attains its maximum value at 64 s, which is about 1.3 times than that of O-CaO. The BPB modification greatly improves CO2 capture rate of CaO in the chemical-controlled stage, but shows a little effect on rN in the diffusion-controlled stage. 3.6 Effect of BPB modification on CO2 capture by deactivated CaO Fig. 10 shows the effect of the BPB modification on the CO2 capture performance of O-CaO in TFBR which had experienced 10 cycles under the severe calcination condition (950 oC and 100% CO2). O-CaO which had experienced 10 cycles after the BPB modification is called BPB-D-CaO. It is found that the CO2 capture capacity of O-CaO which had experienced 10 cycles is very low, about 0.19 g/g. However, the deactivated O-CaO is reactivated by BPB modification. C11 of BPB-D-CaO calcined at 850 oC under N2 and at 920 oC under 70% CO2 /30% N2 are about 3.6 and 3.0 times as high as C10 of O-CaO, respectively. As the cycle number increases from 10 to 20, CN of BPB-D-CaO decays under the various calcination conditions, but it is significantly higher than CN (N=1-10) of O-CaO. C20 of BPB-D-CaO calcined at 850 oC under N2 and at 920 oC 70% CO2/30% N2 are 3.1 and 1.8 times as high as C10 of O-CaO, respectively. The BPB modification possibly improves the pore structure of the deactivated CaO and increases its porosity. Therefore, the CO2 capture capacity of the deactivated CaO is significantly reactivated by the BPB modification. The CO2 capture capacities and rates of O-CaO during the 10th cycle and BPB-D-CaO during the 11th cycle with the reaction time in TGA are illustrated in Fig. 11. For the same reaction time, C11 of BPB-D-CaO is much higher than C10 of O-CaO. At 200 s, C11 of BPB-D-CaO is 3 times as high as C10 of O-CaO. The maximum r11 of BPB-D-CaO is about 2.1 times as high as maximum r10 of O-CaO. It shows that the CO2 capture rate of the deactivated CaO is greatly enhanced by the

12

BPB modification. The BPB modification increases the surface area and pore volume of BPB-CaO (as shown in section 3.6), which promotes the CO2 diffusion and improves the CO2 capture rate. Thus, the BPB modification is a good method to improve the CO2 capture performance of the deactivated CaO. 3.7 Microstructure analysis The apparent morphology of BPB-CaO observed by SEM is illustrated in Fig. 12. The BPB-CaO presents the various micro-structures such as globular, spiral, lamellar, scaphoid and cellular structures, as presented in Fig. 12(a). At higher magnification as shown in Fig. 12(b), the surface of BPB-CaO seems highly porous, like popcorn. This is maybe because the combustion product rapidly releases during the preparation process of BPB-CaO. The well-developed pore structure of BPB-CaO is beneficial to CO2 diffusion and absorption. The pore structures of calcium-based sorbents during carbonation/calcination cycles have the important effect on their CO2 capture performances [48-51]. SEM images of O-CaO, H-CaO and BPB-CaO are presented in Fig. 13. The few pores can be observed in the surface of O-CaO and fusion phenomenon of CaO grains in O-CaO after 10 cycles are obviously observed, as illustrated in Fig. 13(a) and (b). H-CaO possesses more fragmentation than O-CaO as shown in Fig. 13(c). After 10 cycles, H-CaO shows evidently larger porosity than O-CaO, as illustrated in Fig. 13(d), so C10 of the former one is higher than that of latter one. BPB-CaO exhibits more porous and loose structure than O-CaO and H-CaO for the same cycles, as shown in Fig. 13(e) and (f). It is observed that the size of CaO grains in BPB-CaO is much smaller than those of O-CaO and H-CaO for the same number of cycles. In addition, CaO grains in BPB-CaO grow more slowly

13

with the number of cycles, compared to those of the other two sorbents. This suggests that BPB-CaO possesses greater sintering resistance than O-CaO and H-CaO. TEM analysis is used to examine the ultrastructures of O-CaO and BPB-CaO, as demonstrated in Fig. 14(a) and (b), respectively. The selected area electron diffraction patterns of O-CaO and BPB-CaO are separately given out, and the two sorbents both give appearances like polycrystals. The diffraction rings are in good consistent with XRD results. However, the diffraction rings of BPB-CaO are less dramatic than those of O-CaO. This is related to the growth of CaO grains, and doesn’t represent the bad crystallinity. As shown in Fig. 14(b), it is clearly seen that BPB-CaO possesses lots of pores in the range of 20-50 nm in diameter and these mesoporous are beneficial for CO2 adsorption by the calcium-based sorbent [50]. Compared to BPB-CaO, there are few pores in the range of 20-50 nm in diameter in O-CaO, as shown in Fig. 14(a). Therefore, BPB-CaO exhibits better CO2 capture performance than O-CaO. The surface areas and the pore volumes of O-CaO, H-CaO and BPB-CaO after 10 carbonation/calcination cycles in TFBR are presented in Table 2. BPB-CaO has larger surface area and pore volume than the other two sorbents. The surface area of BPB-CaO is about 2.9 and 1.8 times as large as those of O-CaO and H-CaO, respectively. The pore volume of BPB-CaO is about twice as large as those of O-CaO and H-CaO, respectively. The larger surface area and pore volume of BPB are prone to facilitate its CO2 capture capacity. Fig. 15 shows the pore size distributions of O-CaO, H-CaO and BPB-CaO after 10 cycles in TFBR. The volume of pores in the entire pore size range measured for BPB-CaO is obviously higher than those for O-CaO and H-CaO after 10 cycles, especially pores distributed in 20-100 nm in diameter. As shown in Table 3, the volume of pore distributed in 20-100 nm in diameter for

14

BPB-CaO after 10 cycles is 1.8 and 1.5 times than that for O-CaO and H-CaO, respectively.. The pores distributed in 20-100 nm in diameter are important for the carbonation reaction of CaO [51]. Thus, it’s the reason why BPB-CaO achieves higher CO2 capture capacity than O-CaO and H-CaO especially after 10 cycles. 4 Conclusions The CO2 capture performance of the modified CaO with BPB at the calcium looping conditions was investigated. The BPB-CaO not only achieves better CO2 capture capacity than O-CaO and H-CaO, but shows faster reaction rate in the chemical-controlled stage. The main composition of BPB-CaO is still CaO. When the ratio of BPB/CaO, the combustion temperature and the combustion duration are 25mL/g, 800 oC and 60 min, respectively, BPB-CaO can retain high cyclic CO2 capture capacity. BPB-CaO exhibits higher CN (N > 3) than the modified CaO with the various organic solutions such as acetic acid, citric acid and ethanol in the published works. C20 of BPB-CaO can retain 0.5 g/g. In addition, the deactivated CaO can also been reactivated by BPB modification. C11 of BPB-D-CaO calcined under pure N2 and 70%CO2/30%N2 are 0.69 g/g and 0.57 g/g, which are 3.6 and 3.0 times as high as C10 of O-CaO, respectively. BPB-CaO exhibits porous and loose pore structure than O-CaO and H-CaO during the cycles. In addition, BPB-CaO has greater sintering resistance. The pore volumes and surface areas of BPB-CaO are obvious higher than those of O-CaO and H-CaO after 10 cycles. BPB-CaO possesses more abundant pores in the range of 20-100 nm in diameter, which is beneficial to CO2 capture. CaO modified with BPB by the combustion shows superior CO2 capture performance and appears promising. Acknowledgment

15

Financial support from the National Natural Science Foundation of China (51376003) and Primary Research & Development Plan of Shandong Province (2016GSF117001) is gratefully appreciated. Nomenclature N

number of the carbonation/calcination cycles;

t

carbonation time, s;

CN

CO2 capture capacity of sample after N cycles, g (CO2 )/g (sorbent);

m0

initial mass of the sample, g;

mN (t)

mass of the carbonated sample at t after N cycles, g;

mcal, N

mass of the calcined sample after N cycles, g;

rN

CO2 capture rate of samble at t during the Nth cycle, g/(g·s);

O-CaO

original CaO;

H-CaO

hydrated CaO;

BPB-CaO

modified CaO with BPB;

BPB-D-CaO

modified deactivated CaO with BPB.

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Table Captions Table 1 Summary of reaction conditions in Fig. 6. Table 2 Surface areas and pore volumes of O-CaO, H-CaO and BPB-CaO after 10 cycles in TFBR.

Table 3 Volumes of pores in 20-100 nm in diameter for O-CaO, H-CaO and BPB-CaO after 10 cycles

23

Figure Captions Fig. 1 Preparation procedures of various modified sorbents. Fig. 2 Schematic diagram of TFBR. Fig. 3 XRD pattern of BPB-CaO (preparation process: BPB/CaO ratio of 25mL/g, combustion at 800 o

C for 60 min).

Fig. 4 Effect of ratio of BPB/CaO on CO2 capture capacity of BPB-CaO prepared at 800 oC for 60 min in TFBR (calcination at 850 oC for 10 min in pure N2 and carbonation at 700 o C for 20 min in 20% CO2/80% N2). Fig. 5 Effects of different combustion durations and temperatures on CO2 capture capacity of BPB-CaO in TFBR (calcination at 850 oC for 10 min in N2 and carbonation at 700 oC for 20 min in 20% CO2/80% N2). Fig. 6 Comparison of BPB-CaO in TFBR and modified CaO with various organic solutions in CO2 capture capacity. Fig. 7 Effect of calcination condition on CO2 capture capacity of BPB-CaO in TFBR (carbonation at 700 oC for 20 min in 20% CO2/80% N2, calcination at various conditions for 10 min). Fig. 8 CO2 capture capacity of carbide slag modified with BPB in TFBR (carbonation at 700 oC for 20 min in 20% CO2/80% N2, calcination at 850 oC for 10 min in N2). Fig. 9 Cyclic CO2 capture capacity and carbonation rates of O-CaO, H-CaO and BPB-CaO in TGA: (a) CO2 capture capacity, (b) CO2 capture rate (calcination at 850 oC for 10 min in N2 and carbonation at 700 oC for 20 min in 20% CO2/80% N2). Fig. 10 Effect of BPB modification on CO2 capture by deactivated CaO under different calcination conditions in TGA (calcination for 10 min and carbonation at 700 oC for 20 min in 20% CO2/80% N2).

24

Fig. 11 Effect of BPB modification on CO2 capture by deactivated CaO with reaction time in TGA (Calcination at 920 oC for 10 min in 70% CO2/30% N2 and carbonation at 700 oC for 20 min in 20% CO2/80% N2). Fig. 12 SEM images of BPB-CaO: (a) low magnification, (b) high magnification (preparation process: BPB/CaO ratio of 25mL/g, combustion at 800 oC for 60 min). Fig. 13 SEM images of various sorbents: (a) O-CaO, (b) O-CaO after 10 cycles, (c) H-CaO, (d) H-CaO after 10 cycles, (e) BPB-CaO, (f) BPB-CaO after 10 cycles (calcination at 850 oC for 10min in N2 and carbonation at 700 oC for 20min in 20% CO2/80% N2). Fig. 14 TEM images of O-CaO (a) and BPB-CaO (b) (calcination at 850 oC for 10min in N2 and carbonation at 700 oC for 20min in 20% CO2/80% N2). Fig. 15 Pore size distributions of O-CaO, H-CaO and BPB-CaO after 10 cycles in TFBR (calcination at 850 oC for 10min in N2 and carbonation at 700 oC for 20min in 20% CO2/ 80% N2).

25

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9(a)

Figure 9(b)

Figure 10

Figure 11

Figure 12(a)

Figure 12(b)

Figure 13(a)

Figure 13(b)

Figure 13(c)

Figure 13(d)

Figure 13(e)

Figure 13(f)

Figure 14(a)

Figure 14(b)

Figure 15

Table 1 Summary of reaction conditions in Fig. 6. Solution

Material

Calcination

Carbonation

Calcined

920 oC, 80% CO2/20% O2,

[33] 700 oC, 15% CO 2/85% N2, 20 min

Ethanol limestone

15 min 920 oC, 80 % CO2/20%

Acetic acid

Ref.

[38] o

Limestone

650 C, 15% CO 2/85% N2, 20 min O2, 15 min 920oC, 80% CO2/20% O2,

Acetic acid

[39] o

Dolomite

650 C, 15% CO 2/85% N2, 20 min 15 min

Propionic

Calcined

[40] 850 oC, 100% N2, 10 min

acid

700 oC, 15% CO2/85% N 2, 20min

limestone

Pyroligneous

[43] o

o

Limestone

850 C, 100% N2, 15 min

700 C, 15% CO 2/85% N2, 20 min

Vinegar

Limestone

850 oC, 100% N2, 5 min

650 oC, 15% CO 2/85% N2, 20 min

[35]

Oxalic acid

Limestone

850 oC, 100% N2, 5 min

650oC, 15%CO2/85% N 2, 20 min

[35]

Formic acid

Limestone

850 oC, 100% N2, 5 min

650 oC, 15%CO2/85% N2, 20 min

[35]

Citric acid

Limestone

750 oC, 100% Ar, 10 min

700 oC,15% CO2/ 85% N2, 30 min

[41]

850 oC, 99.999% N2 , 10

700 oC, 20% CO2/ 80% N2, 20

min

min

acid

BPB

CaO

26

Table 2 Surface areas and pore volumes of O-CaO, H-CaO and BPB-CaO after 10 cycles in TFBR. Sample

Surface area (m2/g)

Pore volume (cm3/g)

O-CaO

3.90

0.031

H-CaO

6.13

0.030

BPB-CaO

11.21

0.061

27

Table 3 Volumes of pores in 20-100 nm in diameter for O-CaO, H-CaO and BPB-CaO after 10 cycles Sample

Volume of pores in 20-100nm (cm3/g)

O-CaO

0.021

H-CaO

0.015

BPB-CaO

0.038

28

Highlights

By-product of biodiesel (BPB) is used to modify CaO by combustion. Modified CaO possesses much higher cyclic CO2 capture capacity than original CaO. Modified CaO seems promising under severe calcination condition. CO2 capture capacity of deactivated CaO is reactivated by BPB modification. Pore structure of CaO is significanly improved by BPB modification.

29

TEM image of BPB-CaO

SEM image of BPB-CaO