Mechanism of CO2 Capture Technology Based on the

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Mechanism of CO2 capture technology based on phosphogypsum reduction thermal decomposition process Siqi Zhao, Liping Ma, Jie Yang, Dalong Zheng, Hongpan Liu, and Jing Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01673 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017

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Mechanism of CO2 capture technology based on phosphogypsum reduction thermal decomposition process Siqi Zhao, Liping Ma*, Jie Yang, Dalong Zheng, Hongpan Liu, Jing Yang

Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming, Yunnan 650093, China HIGHLIGHTS:

Phosphogypsum was used as a calcium-based absorbent to capture CO2.

Carbonation period is via gas-liquid-solid reaction in fluidized bed.

The lower the L/S ratio is, the higher the CO2 capture capacity is.

The capture product CaCO3 has better mineralize conversion.

ABSTRACT: CO2 emission all over the world is constantly on the rise. In recent years, CO2 capture technology has been improved and innovated. Phosphogypsum (PG) is a solid product of industrial solid waste with the increasing annual accumulation. This paper has carried out thermal decomposition of phosphogypsum under the reduction atmosphere to produce by-product CaS, and to carry out mineralization capture of CO2 in a gas-liquid-solid three-phase fluidized bed reactor. Results show that the fluidized bed is more effective and can effectually shorten the reaction time. Meanwhile, different liquid-solid (L/S) ratios, different temperature and pressure have been researched to study the theory of CO2 capture by phosphogysum. It has been found that the lower the L/S rate is, the higher the CO2 capture capacity is. At the same time, theoretical calculations show that high temperature promote the decomposition of CaCO3, thereby inhibiting the capture of CO2, so the whole *Corresponding author. E-mail address: [email protected]

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carbonation process is operated under ambient temperature and pressure. Meanwhile, this paper has been studied and discussed the calcium migration path and migration mode from the decomposition of PG to the mineralization of CO2, proposes a new idea of CO2 capture and makes a certain contribution of PG resource utilization. Keywords: Phosphogypsum decomposition, CO2 capture, Liquid-solid ratio, Calcium migration, Fluid bed carbonation. 1. Introduction Human activity was considered to be the main reason of the greenhouse gas CO2 emission [1]. The acceleration of industrialization process, the excess combustion of fossil fuel, the destruction of forest vegetation and the rapid development of modern agriculture, lead to a sharp increase of CO2 concentration which further cause a rise in global sea levels, a reduction in fresh water resources, extreme climate, human health, and severe damage to the global ecosystem. It had been pointed out that: By the year of 2050, if we decrease the global average temperature for 2.0-2.4 ºC compared with the year of 2000, the global needs to reduce nearly 50%-80% [2] carbon emission. Thus, a scientific and reasonable technique to solve CO2 emissions is imminent. CO2 capture refers to the process of separating and enriching CO2 which is produced in the combustion process of fossil fuel by metallurgy, cement, power and other industries. According to the differences of capture methods, CO2 capture mainly consists of Pre-combustion, Post-combustion and Oxy-fuel combustion. Carbon dioxide capture and storage (CCS) is one of the most important measures for scientific reduction of greenhouse gases during periods of extreme energy demand growth [3-5].

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Nowadays, CO2 sequestration technologies include geologic sequestration, ocean carbon sequestration and industry carbon sequestration. It has been mentioned that possible advanced technology over the follow years, which are expected to arise from an eventual adoption of CCS as standard practice for all large stationary fossil fuel installations, are also identified [6]. According to the International Energy Agency, by 2050, global energy conservation by CCS could reduce greenhouse gas emissions by around 19%. Calcium-based absorbent is one of the most sufficient industry carbon sequestration methods that been widely used in recent years. Calcium based absorbent includes of natural Ca-based absorbent and modified Ca-based sorbent. In addition, the natural Ca-based absorbents mainly refer to the limestone and dolomite, which could be found in abundance at low price. The purpose of modification of the absorbent is achieved by adding a high melting point inert substance into the absorbent or by subjecting the absorbent to pretreatment. Salvador C et al. [7] demonstrated that calcium titanate can effectively improve the regenerative stability of the absorbent by adding Na salt into it. Aihara et al. illustrated that calcium titanate can effectively improve the regenerative stability of the absorbent. Furthermore, the modified absorbent has a significant increase in the ability to capture CO2, but with the addition of additives, the relative economic costs also increase [8]. Considerable efforts have been devoted to the research of calcium based absorbent [9-14]. The basic theory is to use Ca-based compounds and CO2 carbonation cycle reaction to achieve the purpose of capture. Today, the most widely used Ca-based

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absorbents are CaO, Ca(OH)2 and so on. The use of CaO to capture CO2 is reversible. The entire capture process consists of roughly two parts, namely carbonation and high temperature calcination. CaO or Ca(OH)2 and other Ca-based absorbents react with CO2 to generate CaCO3, then CaCO3 was decarburized at high temperature to CaO and CO2, followed by cyclic capture. Phosphogypsum(PG) is a byproduct of industrial hydrometallurgical phosphoric acid, and its main component is calcium sulfate dihydrate (CaSO4·2H2O). It is reported that about 3.75 tons of phosphogypsum will produce about 1t of phosphate fertilizer [15]. At present, phosphogypsum could be used in: production of sulfuric acid co-production cement, production of chemical raw materials (calcium chloride, ammonium sulfate, potassium sulfate, sodium sulfate, thiourea, sodium hydrogen phosphate, hydroxyapatite nanoparticles, etc.) [16-18], and production of building materials, purification of water polluted by heavy metal and improvement of soil condition. The proportion of the composition of phosphogypsum is closely related to the origin, grade and phosphoric acid production process of phosphate rock, but the composition is basically unchanged. Much work so far has focused on Ca-looping and CaO which decomposed from PG to mineralize CO2 [19-21]. In our study, CaS as the main components of phosphogypsum decomposition slag, has good CO2 capture efficiency. In this paper, phosphorus gypsum decomposition slag is used as calcium base absorbent. The mineralize product CaCO3 also displays a desirable conversion rate which has a certain significance of further utilization. According to the idea of "waste and waste",

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the waste residue and exhaust gas are treated at the same time, which provides a new solution for CO2 capture and resource utilization of phosphogypsum. 2. Experiment 2.1 Material and preparation The PG sample used in this research was obtained from Yunnan, China, which phase analyze measured by X-ray fluorescence (ZSX100e) are listed in Table 1. And, lignite sample was acquired from Yunnan, China, which chemical compositions are shown in Table 2. The PG and lignite samples were firstly screened at 100 mesh and then drying in a constant temperature oven at 65ºC for 24 hours. As for the water used in experiments, the ultrapure water was collected at a resistivity of 18.25 ohms. Table 1. Main components of PG (wt%) (dry basis) Component Content (wt%)

Tol SO3

CaO

SiO2

Tol P2O5

P2O5 (aq)

Tol F

F (aq)

33.85

28.52

17.06

5.06

0.47

0.52

0.12

Component

Fe2O3

Al2O3

MgO

Na2O

MnO

Acid-insol uble

PH

Content (wt%)

0.212

0.236

0.030

0.043

0.002

19.23

3.11

Table 2. Element content and parameter analysis of lignite (wt%) Element Content (wt%)

C 62.47

H 4.36

O 30.73

N 1.35

Component

Ash

Volatile matter

Moisture

Fix carbon

Content (wt%)

0.212

0.236

0.030

0.043

S 1.09 Low heating value (kJ/kg) 0.002

2.2 Experiment facilities TG-DTA experiments were performed on XXWRT-2C Thermogravimetric analyzer (Beijing Optician Plant). PG decomposing process was investigated in KTL1600 (Nanjing University Experiment Factory). And the carbonation process was carried 5


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out by a gas-liquid-solid three-phase fluidized bed. KM9106 complex fuel gas analyzer (KANE Company, British) and D/max-3BPEX-P96 XRD (Japan) were used to analyze the gas components and powder phase. UPT-I -5/10/20T was used to provide ultrapure water. 2.3 Experiment method Experiment method includes two steps, (1) Tube furnace calcination. The PG and lignite powders were mixed with a mole ratio being 0.25, then calcined in tube furnace under the temperature more than 900 °C with nitrogen (99.999%) atmosphere [22]. (2) Fluidized bed carbonation. The decomposition slag and ultrapure water with different liquid-solid ratio were injected into three-phase fluid bed. And the CO2 (20%) atmosphere was inlet in the equipment at the same time. The characterization of the products would be represented.

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Figure 1. Process flow diagram of CO2 capture based on PG decomposition. 3

Results and discussion

3.1 Theoretical analysis Table 2 shows the element content of lignite which used in this study. The main reactions through this system may occur as follows, which R1-R4 represent for the decomposition process and R5-R8 stand for carbonation period: CaSO4 + 4C → CaS + 4CO

(R1)

CaSO4 + 4CO → CaS + 4CO2

(R2)

CaSO4 + CO → CaO + CO2 + SO2

(R3)

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CaSO4 + 3CaS → 4CaO + 4S

(R4)

CaS + 2H2O → Ca(OH)2 + H2S

(R5)

Ca(OH)2 + CO2 → CaCO3 + H2O

(R6)

CaS + H2O + CO2 → CaCO3 + H2S

(R7)

CaCO3 → CaO + CO2

(R8)

Different mole ratio of PG and lignite were tested in this study in order to get the most pure CaS decomposition. Throughout the decompose process, it was found that with the increase of mole ratio, the conversion of CaS presents a gradual increase in trend which almost arrive at 98.47% [22]. The mole ratio chosen in this work was 0.25, and the main reactions were R1 and R2, which the amount of calcium sulfide produced by the most. During the process of R5 and R7, H2S was produced and then cyclic into the decomposition process to further reduce the PG sample according to it reducing characteristic. Thus, both realize the CO2 capture and the cyclic reuse of H2S. During the carbonation period (R5, R6), different liquid-solid ratio, temperature and pressure show dissimilar carbonation reaction processes. Reaction time, purity of carbonation products and other conditions may be influenced by the changes. However, temperature and pressure parameters did not show a palpable change during the reaction. Change curves under different temperature in Fig.2 reflect the variation of Gibbs free energy at R1 to R7. The delta G values in Fig. 2 are calculated by Factsage 7.1 under ideally status, atmospheric pressure range and the temperature increases by every 100 °C from 0 °C to 1000 °C. Fig.2 (a) and Fig.2 (b) on behalf of PG

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decomposition and carbonation periods, respectively. In Fig.2 (a), when temperature is less than 425 °C, ∆G value in R1 is positive which illustrates R1 is not a spontaneous reaction. However, when the temperature is more than 425 °C to 1000 °C, R1 present to be a spontaneous process. What’s more, the Gibbs free energy of R2 has not changed significantly from 0 °C to 1000 °C and displays a spontaneous process always. Thus, which demonstrate that the variety of temperature has little effect on R2. While R3 and R4 reflect a similar variation trend of Gibbs free energy under 0 °C to 1000 °C. ∆G values in these two reactions are still positive which explain that R3 and R4 are not spontaneous reactions during PG decomposition process. It should be pointed out that R5 and R6 in Fig.1 (b) together compose of carbonation reaction (R7). As shown in Fig.2 (b), with the increase of temperature, ∆G value in R7 still present to rise. When the temperature is less than 500 °C, ∆G value in R7 is negative displays a spontaneous process, while ∆G value is positive when the temperature is from 500 °C to 1000 °C. Therefore, it can be concluded that relative high temperature is not a promote factor for carbonation reaction and ambient temperature under normal pressure will be chosen in carbonation process.

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Figure 2. Change curves of Gibbs free energy under different temperatures. Three-phase diagram of CaS-CO2-H2O The there-phase diagram of CaS-CO2-H2O were calculated by Factsage 7.1 at ideally status, 298K (25 °C) and 1 atm (see Fig. 3). In the phase diagram, the three fixations represent three different substances, and the different regions from 1 to 10 represent the products of the different reactants, respectively. The shadow part is the reflection product with the liquid-solid ratio of 6 in this study. In this section, CaS transfers into CaCO3 and the main form of calcium element is CaCO3. Three-phase diagram can provide reliable theoretical guidance and the theoretical basis for us to change the operating conditions reasonably during the experiments.

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Figure 3. Three-phase diagram of CaS-CO2-H2O. 3.2 TG-DTA analysis The samples were analyzed by TG-DTA analyzer under the N2 atmosphere with N2 flow rate being 50ml/min. The heating temperature ranged from ambient temperature to 1100 °C. As shown in Fig. 4, (a), (b), (c) represent PG sample, lignite sample and compound sample, respectively. It can be concluded from Fig. 4 (a) that CaSO4·2H2O dehydrated into CaSO4 at a temperature of 125-220 °C with a 16.47% weight loss which was accompanied with an endothermic phenomenon according to the DTA curve. PG weight loss then did not show a definite decrease at 220-1000 °C. When the temperature was more than 1000 °C, TG curve showed a decrease during that period, which demonstrated that PG itself began to decompose under that temperature. Fig. 4 (b) exhibits a weightlessness during TG experiment. The weight loss can be divided into three sections according to the temperature: 0-150 °C, 150-800 °C and 800-1100 °C. When the temperature is 0-150 °C, the weightlessness of lignite and an endothermic peak can be observed. During the temperature of 150-1100 °C, lignite has a weight loss with an exothermic peak. When the temperature is at 150-800 °C, the weight loss in TG curve mainly contributed to the gasification of lignite. Furthermore, lignite continues to lose weight at 800-1100 °C which has a difference with the research by Yang J [22]. This difference may attribute to the flow rate of N2, while the flow rate is 10 ml/min (Yang J), the air in TG-DTA analyzer system may not 11


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be exhausted and reducing environment has not completely formed which hinder the further reaction of lignite. 50 ml/min of N2 flow rate in our research can easily exhaust the air in TG-DTA analyzer system, and lignite then reaction in the reducing atmosphere at 800-1100 °C which causes the weight loss during that temperature. A similar evident endothermic peak in DTA curve apparently formed with a 23.77% weight loss at the temperature of 120-200 °C which shown in Fig. 4 (c). The weight loss (see Fig. 4 (c)) in TG attributes to the mass loss of PG and lignite. During 200-1000 °C, the weight loss in TG accompanies with an exothermic peak. The reactions (R1-R4) between PG and lignite cause 19.22% weight loss. When the temperature is at 1000-1100 °C, 2.99% weight loss in TG curve is caused by the decomposition of PG and gasification of lignite.

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Figure 4. TG-DTA curves of (a) PG, (b) lignite, (c) compound sample. 3.3 CO2 capture capacity 3.3.1

Effect of different temperature

Theoretical analysis has been proved that high temperature will not promote the result of CO2 carbonation reaction as well as capture rate. R8 is a side reaction during 13


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CO2 capture. Furthermore, CaCO3 begin to decompose at almost 600 °C and high temperature makes R7 (R5, R6) and R8 compete with each other. Fig.5 shows the concentration of reactants and resultants at different temperature conditions in the carbonation period which the data in fig. 5 are obtained by a theoretical calculation by Factsage 7.1. It displays a variable trend that with the increase of temperature, the conversion of CaCO3 begins to decrease, CO2 and CaS present to be increase. Meanwhile, when the temperature is more than 700 °C, the conversion of CaCO3 even decreases into 0. This phenomenon demonstrates that this process should be carried out in the process of maintaining a reasonable temperature range. CaCO3 begin to decompose at almost 600 °C in our research is coherent with the relative decompose experiments of CaCO3.While the CaCO3 concentration begin to decrease at almost 300 °C and finishes at 600 °C (see fig. 5) can be explained that with the increase of temperature, the reaction of positive reaction direction was suppressed, which inhibit the formation of CaCO3. And under 600 °C, the positive reaction almost not participate into reaction and CaCO3 begin to decompose which showing the trend in figure 5. The lower the temperature, the better the CO2 capture capacity. The illustration of phenomenon coincides with that of Fig. 2. With the increase of temperature, the ∆G value of R6 and R7 increase which demonstrates that high temperature has an inhibitory effect on the reaction. Thus, we recommend ambient temperature to reaction in this process.

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Figure 5. Effect of different temperature on the carbonation process. 3.3.2

Effect of different pressure

Different pressure has also been studied according to R8 (see Fig. 6 ) which has been calculated by Factsage 7.1 under a room temperature (298K). It’s clearly that the concentration of CaCO3 and H2S has not changed significantly and stable at nearly 0.9995mol. Meanwhile, the concentration of CaS and CO2 expose a similar status and has not changed obviously. Thus, this phenomenon has verified that the exchange of pressure has little effect on R8. So, increasing the pressure has little noteworthy on CO2 capture.

Figure 6. Effect of different pressure on the carbonation process. 15


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One thing has to be mentioned that, the experiments of different temperature and pressure may better reveal the capture characteristics. Therefore, we used software to simulate an assuming react environment to reflect in this research. However, as for the influence of pressure and temperature in experiments, we mainly based on the pre-study of our research group [38, 39] indicating that the carbonation process of the best or optimal temperature and pressure conditions is of normal temperature and pressure, with the best pre-study conditions, we are now mainly focusing on the study of PG decomposition, different L/S ratio, Ca migration and so on. 3.3.3

Effect of different L/S ratio

For the capture of carbon dioxide, a large number of studies have been done to compare the capture efficiency of physical absorbers, zeolite-based absorbers and chemical absorbers [23-27]. Arenillas A et al. [28] put forward the idea to use fly ash derived carbon with amine PEI to capture CO2 and the capture capacity were from 4 to 6 wt% at 75 ºC. Radfarnia H R et al. [29] prepared a highly efficient Al-stabilized CaO (CaO 92.5 wt%, / Al2O3 7.5, wt%) sorbent via sol-gel method where the absorption is less than 60%. CO2 concentration curves in Fig. 6 show the capture capacity of different liquid-solid ratio. See from Fig. 7, our capture capacity is more than 70% when the reaction time is more than 24 min averagely. As the reaction time increase, different liquid-solid ratio exhibit different capture capacity. Different L/S ratio curves (see fig. 7) exhibit different trend of CO2 concentration which is a parameter to measure CO2 capture capacity. While under every same reaction time, the CO2 concentration of small L/S ratio is lower than the big one. Since the CO2

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concentration is the outlet concentration of CO2, and the lower the concentration, the better the capture capacity. As shown above, it is safe to say that small L/S ratio has a good capture capacity than the large one and with the extension of L/S ratio the capture capacity present a trend of gradually weakened. The capture capacity shows a sharply decrease at the reaction time of nearly 26-32 min which indicate the decomposition slag almost conversed into CaCO3 [30]. The whole reaction may contain for nearly 48 min in an ambient temperature.

Figure 7. CO2 concentration curves in different liquid-solid ratio. CO2 capture rate (Xc (%)): Under the gas-liquid three-phase reaction system, the capacity of decomposition slag mineralized CO2 gas in a three-phase fluidized bed. It can be expressed as Eq. (E1). Xc 1

out

100% E1 in

In which Xout (%) and Xin (%) are the export concentration and initial concentration of CO2, respectively. As shown in Fig. 8, CO2 capture rate in different liquid-solid ratios show a gradually diminution which coincide with the CO2 concentration curves. From 0-8min 17


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the capture rate of L/S=6 stays at more than 90% which is better than other ratios. From 10-26 min the capture rate of different L/S arrive at a relative stable stage. What’s more, from 26-36 min the capture rate decrease harshly which indicate an upcoming exhaustion of the reactants. As for the nearly last 10 minutes, the capture rate almost maintain at 0.2%. Thus, the carbonation process in three-phase fluid bed presents a favorable CO2 capture capacity and sufficient reaction ability.

Figure 8. CO2 capture rate in different liquid-solid ratio. 18


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3.4 XRD analysis The samples were carried on X-ray diffraction studies. XRD patterns in Figure. 9 show the bulk structural information on different kinds of slag. The crystalline phases are all substituted with chemical compound identification using MDI Jade 6.0. As shown from the depiction (a), PG sample mainly consists of CaSO4 and SiO2, while CaSO4 was mostly identified at 2θ=26.639 , 31.367°, 38.640°, respectively. CaSO4 was the main reactant during thermal decomposition. In the process of PG decomposition, CaSO4·2H2O gradually dehydrate to generate CaSO4. Then the compound of CaSO4 and lignite with mixing proportion to generate CaS and other products at a reasonable temperature under N2 atmosphere. As shown in Figure 9 (b), CaS was the target product and wide-angle X-ray diffraction showed broad peaks at 31.406°, 44.996°, 55.883°, 65.504°, 74.423°, 82.980°, respectively. SiO2 component found in decomposition slag derives from PG sample and did not participate in reaction because of the relatively low decomposition temperature. The diffraction angle of SiO2 compared with that in Figure 9 (a) was not change. The XRD pattern of PG decomposition reflected a sufficient reaction of PG and lignite at the proper temperature. As can be seen from part (c) of Figure 9, in stage of carbonate mineralization, decomposition slag CaS was mineralized into CaCO3 with a definite ratio of CO2 and ultrapure water. The XRD pattern in Figure 9 (c) displayed a sufficient carbonation process, and the main carbonated product was CaCO3, which was identified mostly at 2θ=21.004°, 24.900°, 27.047°, 32.779°, 43.848°, 49.098°, 50.077°, 59.853°, respectively. It must also be mentioned that SiO2 in raw material

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was not involved in the reaction of PG decomposition and carbonation, and it could be detected at 2θ=26.639° all the time. The whole transitive process of Ca element can be expressed as follow: High temperature and reducing atmosphere prompt the decomposition of PG and lignite which make CaSO4 transformed into CaS. What’s more, under the action of CO2 and H2O, CaS was further mineralized into CaCO3. In addition to SiO2 which consisted in raw materials, phosphogypsum converted into CaCO3 mostly (see Fig. 9). This phenomenon elucidated that SiO2 demonstrates a better thermal stability and the influence on the whole reaction process made by SiO2 remains to be further explored. It must be mentioned that, compared with fig. 9, Figure 3 was calculated by the software Factsage 7.1 which can provide certain theoretical basis but cannot fully represent for the actual experimental process. Because of the complexity of the solid waste composition, the carbonation products also contain impurities. The XRD patterns in fig. 9, in fact, C and S elementary substance do exist but in very trace. In order to focus on the main resultant and conversion of Ca-based products: CaSO4, CaS and CaCO3, we have not marked in Figure 9.

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Figure 9. XRD patterns of PG, PG Decomposition and Carbon products, respectively. 3.5 Analysis of Ca migration mechanism Ca-looping (CaL) has been extensively studied in the CO2 capture process. CaL has a low cost, a wide range of usefulness [31, 32]. In the process of cycling, the vast majority of studies are based on CaO as the absorbent [33, 34], the introduction of CO2 atmosphere at about 650 ºC combustion adsorption. Then, the carbonated solid recycle furnace is calcined to regenerate CaO. Li et al. used calcium magnesium acetate for CO2 capture [35]. Miranda-Pizarro J et al. [36] proposed a Ca-Mg mixed acetates adsorbent for multicycle CO2 capture. In this study, calcium sulfide was used as calcium-based absorbent in the cycle of calcium element from PG decomposition to calcium sulfide carbonation process. XPS patterns of PG samples and decomposition slag are shown in fig. 10. Fig. 10(a)-(b) show the XPS spectra of Ca 2p. As shown in Fig. 10(a), the peaks in Ca 2p appeared at the binding energy of 346.97 eV is the characteristic value for sulphate which can be defined as CaSO4 [37]. As shown in Fig. 10(b), the binding energy located at 347.47 eV is attributed to Ca 2p as CaS [37]. The different binding energy 21


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for CaSO4 and CaS between our research and reference may attribute to the reaction of impurities in phosphogypsum.

Figure 10. XPS patterns of (a) PG sample, (b) decomposition slag, respectively. Ca migration route in the study can be expressed as Fig. 11 It can be seen from the schematic diagram that one molecule of CaSO4·2H2O initially dehydrates two molecules of water into CaSO4. Furthermore, in the reducing decomposition stage, one molecule of CaSO4 is restored into equal CaS by C and CO according to R1 and R2, which accompanied by the breaking and formation of chemical bonds. And then, with the addition of water, Ca-S double bond ruptured into two single bonds and each single bond then combined with a hydroxyl into Ca(OH)2 which derived from water

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molecule. Through the injected of CO2, the hydroxyl in Ca(OH)2 dehydrogenized and one of C-O double bond from CO2 then connect with O bond, finally obtain the target product CaCO3. Meanwhile, the valence of Ca (+2) element has not been changed during this whole reactions. Thus, the two-step reaction for CO2 capture including calcination and carbonation may be summarized as gas-liquid-solid three phases reaction and chemical bonds break up.

Figure 11. Ca migration route in molecular scale.

4

Conclusion In this work, the CO2 capture capacity of CaS derived from PG decomposition and

Ca migration route have been studied under certain conditions. Different liquid-solid ratios were experimented at ambient temperature in a three-phase gas-liquid-solid fluid bed. The carbonation process was carried out in a three-phase fluid bed which to get more sufficient reaction. Considering the economic costs and experiment effects, CO2 (20%) atmosphere and ultrapure water were used during that period. According to the idea of "waste and waste", it provides a new possibility for the greenhouse gas CO2 capture and makes a reasonable solution for global deposition of phosphogypsum 23


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at the same time. What’s more, the carbonation product CaCO3 also can be used as chemical industry products which show an environmental protection and sustainable development. (1) High temperature will promote the decomposition of CaCO3, refrain the carbonation reaction balance and further restrain the CO2 capture capacity. (2) It was demonstrated that better CO2 capture capacity could be obtained with relative low L/S ratios rather than high ratios. (3) Carbonation process was under ambient temperature and atmospheric pressure which conserve the thermal energy to some extent. Notes The authors declare no competing financial interest. Acknowledgments Financial support for this project was provided by National Natural Science Foundation of China (No.21666016), which is greatly acknowledged. References [1] Boubaker K, Colantoni A, Marucci A, et al. Renewable Energy, 2016, 90, 248-256. [2] International Energy Agency, Carbon dioxide capture and storage. China Environmental Science Press, 2010. [3] Rahman F A, Aziz M M A, Saidur R, et al. Renewable & Sustainable Energy

Reviews, 2017, 71, 112-126. [4] Wennersten R, Sun Q, Li H, et al. Journal of Cleaner Production, 2015, 103,

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Mechanism of CO2 Capture Technology Based on the

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