Ca2Al2SiO7

Article pubs.acs.org/IECR

Enhanced Reaction Performance with Hematite/Ca2Al2SiO7 Oxygen Carrier in Chemical Looping Combustion of Coal Tao Song, Laihong Shen,* Wanjun Guo, Dingqian Chen, and Jun Xiao Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, Nanjing, 210096, China ABSTRACT: Chemical looping combustion (CLC) has been as a viable and efficient alternative for coal combustion with CO2 inherent separation. Iron ore is a promising candidate for CLC of coal due to its low cost. Due to a limitation of particle size of the iron ore used in the fluidized bed, the fines for the iron ore after sieving with a particle size of Fe60C40 > Fe80C20. Further, it can be found that the oxygen transferred rates for all the five samples were higher compared to the coal conversion rates, indicating that the char gasification is the limiting step in the coal CLC process. If the coal was rapidly gasified, the gasification products were quickly oxidized by the present oxygen carrier samples, which had the potential oxygen transfer ability. 3.3. Temperature Effect. The positive effect of Fe70C30, Ni-OC, and K-OC on enhancing coal conversion was observed and confirmed in the above sections. This test was to investigate the temperature effect on the reaction performance of the three samples of K-OC, Ni-OC, and Fe70C30 in the CLC process of coal. A series of temperature of 800 to 950 °C was evaluated. Figure 7 shows the effect of reducing temperature on the carbon conversion rate (a, c, e) and coal gasification rate (b, d, f) with the oxygen carrier samples of Fe70C30, Ni-OC, and KOC. At 800 °C, excluding the char elutriation from the reactor, the incompletely remaining carbon conversion with the sample of Fe70C30 and Ni-OC was observed. This suggested a slow gasification rate at this temperature. However, for K-OC, the complete remaining carbon conversion reached at 23 min. At 850 °C, the carbon conversion with the sample of Fe70C30 was still not incomplete. As for Ni-OC and K-OC, the complete carbon conversion reached at 35 and 19 min, respectively. At the reducing temperature higher than 900 °C, the carbon conversion with all the three oxygen carrier samples reached completely. Further, it can be observed from these figures, the maximum carbon conversion rates and gasification rates with all the three samples progressively increased with the temperature. The reaction time to reach a complete carbon conversion was progressively shortened. Table 3 gives a summary of data evaluations including the reaction time needed to reach a 95% of carbon conversion, average coal gasification rate, average carbon conversion rate, and average oxygen transferred rate with the oxygen carrier samples of Fe70C30, Ni-OC, and K-OC at different reducing

Table 2. Maximum Gasification Rate, Reaction Time, Average Coal Gasification Rate, Average Carbon Conversion Rate, and Average Oxygen Transferred Rate with the Oxygen Carrier Samples of Fe60C40, Fe70C30, Fe80C20, Ni-OC, and K-OCa maximum gasification rate (%/min) b reaction time (min) c average gasification rate (%/min) c average carbon conversion rate (%/min) c average oxygen transferred rate (%/min)

Fe60C40

Fe70C30

Fe80C20

Ni-OC

K-OC

4.45

6.09

3.61

8.17

81.99

31 2.70

27 3.42

33 2.17

23 4.45

6.50 22.52

5.36

6.53

4.58

8.24

32.72

10.18

12.49

8.76

15.90

56.10

Reaction temperature: 900 °C. Steam concentration: 50%. Coal fed weight: 0.5 g. bTime needed to reach 95% of the final carbon conversion efficiency. cAverage rates to reach 95% of the final carbon conversion efficiency. a

sample of Ni-OC is 1.65%, which is higher than those observed in Figure 2 of section 3.1. As seen in Figure 4b, this value for the sample of K-OC dramatically increases to 8.77%. It indicated that, when the sample of K-OC was used as an oxygen carrier, the char after devolatilization was rapidly gasified and its gasification products were rapidly oxidized by the oxygen carriers to CO2 and steam. However, due to the fast char gasification, there were some reducing gases released without being oxidized by the oxygen carrier sample of K-OC, as shown in Figure 4b. After excluding the inner N2, the CO2 concentrations were equal to 96.1% and 78.2% with the samples of Ni-OC and K-OC, respectively. Figure 5 presents the carbon conversion rate, coal gasification rate, and oxygen transfer rate as a function of reducing time with the oxygen carrier samples of Fe70C30, Ni-OC, and KOC, respectively. It can be seen that the carbon conversion rate, coal gasification rate, and oxygen transfer rate were significantly improved when the K-OC oxygen carrier sample was used. As for the sample of Ni-OC used, it had a positive effect on improving the carbon conversion rate and gasification rate compared to use the sample of Fe70C30. However, this effect was weak compared to the K-OC sample. As summarized in Table 2, compared to Fe70C30 sample, the reaction time needed was remarkably shortened to 6.5 min when K-OC was used. Meanwhile, the average carbon conversion rate (32.72%/min) was significantly improved by

Figure 4. Gas concentration distributions with the oxygen carrier samples of Ni-OC (a) and K-OC (b). Reaction temperature: 900 °C. Steam concentration: 50%. Coal fed weight: 0.5 g. 9577

dx.doi.org/10.1021/ie4012613 | Ind. Eng. Chem. Res. 2013, 52, 9573−9585

Industrial & Engineering Chemistry Research

Article

Figure 5. Carbon conversion rate (a), coal gasification rate (b), and oxygen transfer rate (c) with the oxygen carrier samples of Fe70C30, Ni-OC, and K-OC. Reaction temperature: 900 °C. Steam concentration: 50%. Coal fed weight: 0.5 g.

reactivity was also progressively increased with the temperature. Its reactivity at the temperature range of 850−900 °C showed no significant increase. However, when the temperature increased from 900 to 950 °C, its reactivity remarkably increased. The average carbon conversion rate remarkably increased from 4.45%/min at 900 °C to 22.80%/min at 950 °C. 3.4. Cyclical Performance. The cyclically reducing/ oxidizing performance of the three samples of Fe70C30, NiOC, and K-OC was investigated. In this test, 20 cycles were performed. Although the gas mixture of 5% O2 balanced with N2 was used instead of air to prevent the temperature increase in the oxidation process, a temperature increase was also found during the experiments. Therefore, during the cyclically reducing/oxidizing experiments, the oxidation temperature was about 900 ± 5−9 °C. The reducing temperature was designated as 900 °C for each cycle with all the three samples tested in this section. Figure 8 shows the carbon conversion rate in the CLC process of coal with the oxygen carrier samples of Fe70C30 (a), Ni-OC (b), and K-OC (c) during 20 reducing/oxidizing cycles. For the Fe70C30 oxygen carrier, there was a stable reactivity with the cycles, as shown in Figure 8a. For the Ni-OC, as shown in Figure 8b, the carbon conversion rate first increases during the first reduction process as marked with a dotted line and then decreases slowly with the reaction time. The carbon conversion rates reach the initial peak after around 4.5 (0.15), 5.5 (0.13), and 7.5 min (0.11), which correspond to the first reduction, tenth reduction, and twentieth reduction, respectively. This suggested that there was a slight decrease of its reactivity with the cycles. As shown in Figure 8c, the carbon conversion rate for the K-OC is much higher than that for the samples of Fe70C30 and Ni-OC during the 20 cycles, but it keeps on decreasing during the initial cycles and is finally

Figure 6. Average carbon conversion rate, coal gasification rate, and oxygen transfer rate with all the five oxygen carrier samples.

temperatures of 800−950 °C. The highest reactivity for the oxygen carrier sample of K-OC was observed in the temperature range of 800−950 °C. The average carbon conversion rate for K-OC sample remarkably increased by nearly 4.6 times from 12.14%/min at 800 °C to 55.87%/min at 950 °C. The average gasification rate for K-OC sample remarkably increased by nearly 5.1 times from 6.76%/min at 800 °C to 34.64%/min at 950 °C. It should be stated that at this reducing temperature of 950 °C, the oxygen transferred rate was larger than 1. It meant that the oxygen transferred from the oxygen carrier to fuel was not sufficient in the oxygen transfer step of Fe2O3 to Fe3O4. Therefore, the reducing temperature for K-OC sample in the CLC process of coal was optimized as 900 °C. As for Ni-OC oxygen carrier sample, the 9578



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Figure 7. Effect of reducing temperature on carbon conversion rate (a, c, e) and coal gasification rate (b, d, f) with the oxygen carrier samples of Fe70C30, Ni-OC, and K-OC. Reduction temperature: 800, 850, and 950 °C. Steam concentration: 50%. Coal fed weight: 0.5 g.

had a perfect reactivity to enhance coal gasification and oxidize the gasification products. Also, this oxygen carrier sample showed a stable reactivity during the multiple reducing/ oxidizing cycles. Further, for the pure iron ore used for a comparison, as shown in Figure 9d, the CO2 concentration was stabilized as about 70%, which was much lower compared to all the samples produced in this work. It suggested that the reactivity of the oxygen carrier samples produced was improved compared to the pure iron ore. 3.5. Attrition Behavior. The working conditions for the oxygen carrier are severe during the CLC process of coal. For Fe-based oxygen carrier, the particles were tended to be progressively cracked and crumbling after multiple cycles.11 The attrition behavior of the three samples of Fe70C30, NiOC, and K-OC was investigated, as shown in Figures 10 and 11. The fluidizing gas velocities during the reducing process and oxidizing process were calculated as about 0.17 and 0.21 m/s, respectively.

constant. This indicated that after 10 cycles, the reactivity of the K-OC oxygen carrier was stable. Figure 9 shows the gas yields of CO2, CO, and H2 for the oxygen carrier samples of Fe70C30 (a), Ni-OC (b), K-OC (c), and pure iron ore (d) during 20 reducing/oxidizing cycles. The pure iron ore was used for a comparison. This iron ore has been used in our previous work.25 When Fe70C30 was used, there was a stable CO2 evolution about 90% due to its stable reactivity. The slightly decreasing reactivity for the samples of Ni-OC with the cycles, as discussed in the above, resulted in a decrease of CO2 evolution and an increase of CO and H2 evolution, as shown in Figure 9b. When the K-OC was used, during the 20 cycles, the yield of CO decreased from 4.7% to 1.4% during the initial five cycles and stabilized. Correspondingly, the yield of CO2 first increased from 78% to 97% during the initial five cycles and stabilized. It was clear that the yields of CO and H2 for the K-OC were much lower than that for the Ni-OC and Fe70C30 while the yield of CO2 was much higher during all the 20 cycles. All the results supported that the K-OC 9579



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crushing by the machine. These particles with irregular shape were easy to be attrited. During the following cycles, the attrition rates decreased to lower values. Compared to the attrition rates for the three samples, the Fe70C30 showed a good resistance to attrition in the CLC process of coal. 3.6. Mechanism Exploration. 3.6.1. Catalytic Mechanism in CLC Process of Coal. From the summary of the experiments results obtained and illustrated in the above, K-OC and Ni-OC showed an enhanced reactivity to oxidize the coal gasification products in the CLC process of coal. Meanwhile, the coal gasification rate and conversion rate significantly increased. In the reducing process, the coal and oxygen carriers were mixed in the reactor, where coal was gasified by means of gasification agent of steam (R1−R3). The coal pyrolysis/ gasification products subsequently reacted with the oxygen carrier particles to yield CO2 and water, R4.

Table 3. Effect of Reducing Reaction Temperature on the Reaction Time, Average Coal Gasification Rate, Average Carbon Conversion Rate, and Average Oxygen Transferred Rate with the Oxygen Carrier Samples of Fe70C30, Ni-OC, and K-OCa reaction temperature (°C) oxygen carrier samples and data evaluation Fe70C30 reaction time (min) c average gasification rate (%/min) c average carbon conversion rate (%/min) c average oxygen transferred rate (%/min) Ni-OC b reaction time (min) c average gasification rate (%/min) c average carbon conversion rate (%/min) c average oxygen transferred rate (%/min) K-OC b reaction time (min) c average gasification rate (%/min) c average carbon conversion rate (%/min) c average oxygen transferred rate (%/min) b

800

850

900

950

− 0.27 0.83

− 1.43 3.33

27 3.42 6.53

17 6.76 12.14

1.47

6.47

12.49

23.8

− 0.95 2.47

25 4.10 7.50

23 4.45 8.24

5.50 22.80 38.12

coal → volatile matter + char(C)

(R1)

C + H 2O → CO + H 2

(R2)

4.54

14.34

15.90

74.16

CO + H 2O → CO2 + H 2

(R3)

11 13.68 20.88

8.50 16.79 25.88

6.50 22.52 32.72

3.50 34.64 55.87

37.67

50.51

56.10

Fe2O3 + CO + H 2 + volatile matter → Fe3O4 /FeO/Fe + CO2 + H 2O

Generally, the coal gasification rate in the CLC process is much related to the reactivity of the oxygen carriers, since the more reactive oxygen carrier can rapidly oxidize the gasification products, such as CO or H2, which were believed to be the gasification inhibitors. Without the inhibitory effect, the coal gasification rate would increase. However, if the reactivity of the oxygen carriers were designated as a constant value, the presence of some additives also can improve the coal conversion in the CLC process.24,25 Yu et al.24 found that the high reduction rate of coal char with Fe2O3 oxygen carrier can be achieved with the addition of alkali carbonates, which were impregnated on the Fe2O3 oxygen carrier. Gu et al.25 investigated the reaction performance of K2CO3 impregnated iron ore oxygen carrier in the CLC process of coal. The results indicated that compared to the original iron ore, the K2CO3-decorated iron ore promoted the reaction rate and shorten the time to obtain reaction balance. The similar results were obtained in the present work. In the coal CLC process, the coal gasification rate and conversion rate significantly increased, although the additives of K or Ni were not impregnated on the coal. That is, in the case of enhancing coal conversion during CLC process catalyzed by some additives, the mechanism was significantly different compared to the typical coal catalytic gasification process. All the information supported that in the CLC process of coal, the additives with catalytic effect can improve the coal gasification rate, even though the catalysts were not intimate with the coal. The reason can be ascribed to the contact, which was inevitable between the catalysts and coal particles in the fluidized bed. This resulted in a lower gasification temperature of coal needed depending on the effect of catalysts.27 After coal was catalytically gasified, the gasification products were further oxidized by the oxygen carriers, such as the K-OC sample used in this work. 3.6.2. Enhancing Mechanism for the K-OC and Ni-OC Samples. In order to detect the mechanism, the phase characterizations of the particles were analyzed. Figure 12 presents XRD analyzing results for the fresh sample of K-OC

d

109.38

Reaction temperature: 800, 850, 900, and 950 °C. Steam concentration: 50%. Coal fed weight: 0.5 g. bTime needed to reach 95% of the final carbon conversion efficiency. “−” indicates that carbon conversion was not complete. cAverage rates to reach 95% of the final carbon conversion efficiency. dMeans that the oxygen transferred from the oxygen carrier to coal was not sufficient. a

Figure 10 shows the mass loss during the 20 multiple cycles of the three samples. In this test, the agglomeration due to the increasing particle size was not found in all the three oxygen carrier samples. Particles larger than 0.45 mm were not seen in all the samples. This indicated that the addition of cement can prevent the occurrence of the potential agglomeration. In terms of the sum of the mass loss to divide by the mass inventory of the oxygen carriers, the total of about 1.8, 2.3, and 2.7 wt % corresponding to the oxygen carrier samples of Fe70C30, KOC, and Ni-OC, respectively, was elutriated from the reactor due to attrition. During the 20 cycles, a total of 23.3 h experiments was used to calculate the attrition rates with the three oxygen carrier samples. The attrition rates (Lf, %/h) can be determined as Lf =

Δmfines 1 Δt m

(R4)

(12)

where Δmfines and m are the weight loss during the 20 cycles and the primary weight of the oxygen carrier samples, g. Δt is the reaction time, h. Results showed that the attrition rates for Fe70C30, K-OC, and Ni-OC samples were 0.077, 0.098, and 0.11%/h, respectively. These attrition rates are likely to be slightly higher and an improved calcination temperature during oxygen carrier preparation step may be beneficial to decrease the attrition rates.21 The attrition rates during the cycles for the three samples are presented in Figure 11. Results indicated that in the initial first cycle the attrition rates were unexpected to be higher. It was due to the irregular shape of the samples after 9580



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Figure 8. Effect of cycles on the carbon conversion rate for the oxygen carrier samples of Fe70C30 (a), Ni-OC (b), and K-OC (c). Reduction temperature: 900 °C. Oxidation temperature: 900 ± 5−9 °C. Steam concentration: 50%. Coal fed weight for each cycle: 0.5 g. Cycles: 20 reducing/ oxidizing cycles.

Figure 9. Gas yields of CO2, CO, and H2 for the oxygen carrier samples of Fe70C30 (a), Ni-OC (b), K-OC (c), and pure iron ore (d) during 20 reducing/oxidizing cycles. Reduction temperature: 900 °C. Oxidation temperature: 900 ± 5−9 °C. Steam concentration: 50%. Coal fed weight for each cycle: 0.5 g. Cycles: 20 reducing/oxidizing cycles.

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constant of Fe2O4/Fe3O4 for the reduction reaction with H2 and CO at the present reducing temperature is much higher than the one for NiO/Ni. A higher equilibrium constant represents a higher conversion of the reducing gas. That is, in the presence of Fe2O3, the coal gasification gases are oxidized by Fe2O3, faster than those by NiO. Therefore, it can be deduced that NiO in this sample was not as an active phase to transfer lattice oxygen to fuel. The unique oxygen transferred carrier was Fe2O3, which supplied the lattice oxygen to fuel gasification products in the reduction step of Fe2O3 to Fe3O4. Further, it is known that Ni is a good catalyst for steam gasification reaction in the metallic state.29 However, in this work, the NiO was not reduced to Ni. Therefore, it can be deduced that the enhanced coal gasification rate with Ni-OC sample as illustrated above was depended on its enhanced reactivity, not the catalytic effect of Ni. The XRD patterns of the fresh and spent K-OC sample are shown in Figures 12a and b, which evidenced the phase transformations during the coal CLC process. The main crystallographic phase constituting the fresh sample is K2Fe22O34, together with Fe2O3 and some Ca2Al2SiO7. The ferrite of K2Fe22O34 was formed after 950 °C calcination. The same ferrite was also found by Gu et al.,25 who used K2CO3 to be impregnated on the iron ore and after calcination to be an oxygen carrier in the CLC process of coal. Generally, the ferrite of K2Fe22O34 as one of the active phases of the catalyst in the industrial process has been widely used in the process of dehydrogenation of ethylbenzene.30,31 The interaction of potassium with the iron oxide matrix is complex, and the active state of the catalysts has been assigned to the equilibrium between two ferrite phases, KFeO2 ↔ K2Fe22O34, as discussed by Muhler and Schlögl.32,33 KFeO2 is hardly observable and actually it was not observed in the present XRD analysis, because, besides being extremely diluted, it transforms into amorphous species when exposed to air.34 However, it can form from K2Fe22O34 upon heating.35,36 As observed in Figure 9c, during this first reduction process, some coal gasification products such as CO, CH4, and H2 released without being oxidized by the oxygen carrier sample of K-OC. It indicated that the coal gasification process was significantly accelerated under the catalytic effect of potassium. However, only part of the gasification products were oxidized by the oxgen carriers. Further, during the following cycles, the CO2 concentration increased substantially indicating that the reactivity of this K-OC oxygen carrier became activated. Similar results were obtained by Bao et al.18 Another possible mechanism could be ascribed to the potassium loss during the cycles. This potential potassium loss made the catalytic effect weakened. The potassium loss was one of the most serious irreversible deactivating processes, not only for the K-based catalyst but for oxygen carriers in the CLC process of coal.25,34,37 Rossetti et al.34 investigated the deactivation of K-based catalyst in the process of ethylbenzene dehydrogenation to styrene. The deactivation was ascribed to the potassium loss. The results indicated that the progressive reduction of Fe2O3 reduction to Fe3O4 can cause the potassium loss at the expense of active phase of K2Fe22O34. Indeed, the Fe2O3 reduction to Fe3O4 was the favored step in the coal CLC process. However, the role of the oxygen carrier in such a CLC process was quite different from the one of catalysts in the process of ethylbenzene dehydrogenation. Fe2O3 was the active phase of the oxygen carrier, and the K2Fe22O34 in the oxygen carrier can be treated

Figure 10. Mass loss with the cycles for the oxygen carrier samples of Fe70C30, K-OC, and Ni-OC during 20 reducing/oxidizing cycles. Reduction temperature: 900 °C. Oxidation temperature: 900 ± 5−9 °C. Steam concentration: 50%. Coal fed weight for each cycle: 0.5 g. Cycles: 20 reducing/oxidizing cycles.

Figure 11. Attrition rate with the cycles for the oxygen carrier samples of Fe70C30, K-OC, and Ni-OC. Reduction temperature: 900 °C. Reduction time: 40 min. Oxidation temperature: 900 ± 5−9 °C. Oxidation time: 30 min. Steam concentration: 50%. Coal fed weight for each cycle: 0.5 g. Cycles: 20 reducing/oxidizing cycles.

(a) and after the first reduction (b), fresh Ni-OC (c) and after the first reduction (d). As shown from the Figure 12, it can be observed that a new phase called gehlenite (Ca2Al2SiO7, melting point: 1857 K) was produced. Also, it became one of the main components in the oxygen carriers. This material was formed according the interactive reaction, R5, of CaO and Al2O3 as well as SiO2, which were presented as the main compositions in the calcium aluminate cement. 2CaO + Al 2O3 + SiO2 = Ca 2Al 2SiO7

(R5)

As shown in Figure 12, in comparison with the XRD analyzing results for the oxygen carriers before and after used, it could be confirmed that the Ca2Al2SiO7 in the oxygen carrier samples was a stable material. Further, as shown in Figure 12b and d, the unique product for the active phase of Fe2O3 reduced by the gasification products was Fe3O4. The gasification products were oxidized during the reaction step of oxygen transferred from Fe2O3 to Fe3O4. Furthermore, as shown in Figures 12c and d, for the Ni-OC before and after use, it can be seen that the NiO in the oxygen carrier sample of Ni-OC was not reduced by the coal gasification products to Ni. According to the thermodynamic analysis given by Cao et al.28 and Adánez et al.,3 the equilibrium 9582



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Figure 12. XRD analyzing results for the fresh sample of K-OC (a), after reduction (b), fresh Ni-OC (c), after reduction (d). Reaction temperature: 900 °C. Steam concentration: 50%. Coal fed weight: 0.5 g.

tigated. The attrition behavior and catalytic mechanism were further discussed. Some conclusions can be obtained: The mass ratio of iron ore to calcium aluminate cement was optimized as 70:30. When the oxygen carrier produced under this ratio was used in the CLC process of coal, the carbon gasification rate was higher than that for other oxygen carriers. Also, a high reducing temperature was beneficial for improving the carbon conversion and gasification rates. Further, at a reducing temperature of 900 °C, this oxygen carrier demonstrated a stable reactivity in the 20 reducing/oxidizing cycles, where the CO2 concentration in the reducing process was about 90%. By the addition of Ni in the oxygen carrier preparation process, the reactivity of the oxygen carrier was improved. Meanwhile, the coal conversion and gasification was enhanced in the coal CLC process. However, the reactivity of this oxygen carrier by the presence of Ni showed a slight decrease with the cycles. In the coal CLC process, the enhanced coal conversion was ascribed to the improved reactivity of this oxygen carrier, not the catalytic effect of Ni. When K as a catalyst was added, the newly developed oxygen carrier showed high reactivity in the coal CLC process. The time needed to reach a balanced coal conversion was significantly shortened. Meanwhile, the coal conversion rate and gasification rate were significantly improved. Also, even under a low reducing temperature of 800 °C, this oxygen carrier also showed a high reactivity. An optimized reducing temperature was 900 °C. Twenty reducing/oxidizing cycles experimental results suggested this oxygen carrier showed a stable reactivity after the initial five cycles. The CO 2 concentration in the reducing process was stabilized at nearly 97%. The enhanced coal conversion was due to the presence of catalytic active phases of K2Fe22O34 or KFeO2 in the oxygen carrier.

as a support with catalytic effect. Therefore, there was no information supported the potassium loss at the expense of K2Fe22O34. In this work, the potassium loss seems much less, as inferred from a stable coal gasification rate (Figure 8c) and gas yields (Figure 9c). However, for K-OC used, the material with the real catalytic effect on enhancing coal conversion in coal CLC process cannot be confirmed as K2Fe22O34. The equilibrium between the two ferrite phases of KFeO2 and K2Fe22O34 maybe exists in the coal CLC process with K-OC used. Additionally, Figure 13 shows the SEM photographs for the fresh K-OC oxygen carrier sample, after the first reduction, tenth reduction, and twentieth reduction, respectively. With the increase of cycles, the larger pores were gradually replaced by some smaller pores, indicating the growth of grains. The porous surface of the particles facilitated the diffusion of reactant gases into the core of oxygen carrier particles, enhancing the reactions between gas and oxygen carrier particles. For the KOC used, there was no obvious sintering on the surface of the particle. Further, for the fresh sample, the irregular grains were covered on the surface of the particle. The grains progressively became regular and were uniformly covered on the surface. It indicated that the product of Ca2Al2SiO7 was a good candidate as a support for the preparation of oxygen carrier samples, and also, it can prevent the sintering effect during the multiple cycles.

4. CONCLUSION Calcium aluminate cement as a very cheap material was used as a support to bind the iron ore powder to produce some novel oxygen carrier samples. Effect of additives, reducing temperature, and multiple reducing/oxidizing cycles on the reaction performance for these samples were experimentally inves9583



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Figure 13. SEM photographs for the fresh oxygen carrier samples of K-OC, after the first reduction, tenth reduction, and twentieth reduction. Reaction temperature: 900 °C. Oxidation: 900 ± 5−9 °C. Steam concentration: 50%. Coal fed weight for each cycle: 0.5 g.

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AUTHOR INFORMATION

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (51276037), the Key Technology Research and Development Program of Jiangsu Province of China (BE2012166), the Foundation of Graduate Creative Program of Jiangsu Province (CXZZ12-0098), and the

Corresponding Author

*Tel.: +86-25-8379 5598. Fax: +86-25-5771 4489. E-mail address: [email protected]. Notes

The authors declare no competing financial interest. 9584



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Scientific Research Foundation of Graduate School of Southeast University (YBJJ1214).

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