Effects of CO2 Atmosphere and K2CO3 Addition on the Reduction

Article pubs.acs.org/EF

Effects of CO2 Atmosphere and K2CO3 Addition on the Reduction Reactivity, Oxygen Transport Capacity, and Sintering of CuO and Fe2O3 Oxygen Carriers in Coal Direct Chemical Looping Combustion Zhongliang Yu,†,‡ Chunyu Li,† Xuliang Jing,†,‡ Qian Zhang,†,‡ Yitian Fang,*,† Jiantao Zhao,† and Jiejie Huang† †

Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China University of Chinese Academy of Sciences, Beijing 100049, P. R. China

‡

ABSTRACT: Chemical looping combustion (CLC) of coal has received increasing interest in recent years. However, few attempts have been made to examine the effects of CO2 atmosphere and K2CO3 addition on the reduction rate, the oxygen transport capacity (OTC), and the sintering of the oxygen carrier when coal is used directly in CLC. In this work, these issues for Fe2O3 and the CuO oxygen carriers were investigated by thermogravimetric analysis (TGA), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and inductively coupled plasma−atomic emission spectrometry (ICP-AES). The TGA results indicate that the reduction rates can be increased by either the CO2 atmosphere or the K2CO3 additive due to the enhanced CO2 gasification of coal char. Detailed analyses demonstrate that the CO2 atmosphere affects the OTC and the sintering of the oxygen carrier by thermodynamic restrictions. The CO2 atmosphere has no effect on the OTC of the CuO oxygen carrier, and there are no significant differences in sintering between the residues obtained under CO2 and N2 atmospheres. However, the CO2 atmosphere limits the OTC of the Fe2O3 oxygen carrier within the transformation Fe2O3−Fe3O4, and the sintering could be moderated because of the higher sintering resistance of Fe3O4. The K2CO3 addition does not affect the OTC because the catalyst has no impact on the equilibrium but promotes the sintering of the oxygen carrier due to its low Tammann temperature. Although severe sintering could be caused by the K2CO3 addition, the catalytic effect can be observed during several redox cycles.

1. INTRODUCTION CO2 capture and storage (CCS) from fossil fuel-based power plants is becoming increasingly important due to growing public concern about global warming. With the existing commercial CO2 capture technologies, the overall efficiency of a power plant will be significantly reduced because of the large energy cost for CO2 separation.1 Chemical looping combustion (CLC) is a novel combustion technology where the oxygen required for fuel combustion is transferred from air via an oxygen carrier.2 The fuel reactor and the air reactor are essential in a CLC system. In the fuel reactor, the fuel reacts with the oxygen carrier to produce a flue gas of CO2 and H2O. After condensation, almost pure CO2 can be obtained. In the air reactor, the reduced oxygen carrier from the fuel reactor is oxidized by air to its original form. The oxidized oxygen carrier is then returned to the fuel reactor to start a new cycle of combustion again. The net reaction of CLC is fuel combustion by air. However, the produced CO2 is not mixed with the atmospheric N2 because the fuel and air are introduced into two separate reactors, respectively. With CLC, the cost of CO2 capture can be remarkably reduced. In the past 2 decades, gaseous fuels such as CH4, coke oven gas, syngas, CO, and H2 havde been intensively selected for the chemical looping process, while fewer studies are currently available on CLC with solid fuels.3−7 CLC of solid fuels, such as coal and biomass, is very attractive because of their rich deposits, low cost, and wide distribution.8,9 The oxygen carrier is one of the most important components for a successful CLC process. An ideal oxygen carrier should be © 2013 American Chemical Society

inexpensive and environmentally benign and have sufficient reactivity, high oxygen transport capacity (OTC), low tendency for agglomeration, and high attrition resistance.1,10 Compared to gaseous fuels, the low reactivity between the solid fuel and the oxygen carrier is one of the major obstacles for CLC of solid fuels.11 For the reaction of oxygen carrier with solid fuel, there is a mixed mechanism of direct reduction by carbon and indirect reduction by gaseous intermediates.12 As reactions between solids are typically very slow due to the ineffective contacts, the reduction of oxygen carrier is believed to be mainly achieved with carbon monoxide.13 Figure 1 shows the

Figure 1. Schematic diagram of the mechanism of oxygen carrier reduction by solid fuel. Received: September 28, 2012 Revised: April 9, 2013 Published: April 11, 2013 2703

dx.doi.org/10.1021/ef400205t | Energy Fuels 2013, 27, 2703−2711

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Table 1. Proximate and Ultimate Analyses of Shenmu (SM) Char proximate analysis (wt %, air-dried basis)

a

ultimate analysis (wt %, air-dried basis)

sample

M

V

A

FC

C

H

Oa

N

S

SM char

0.86

1.00

17.35

80.79

76.74

0.93

2.99

0.60

0.53

By difference.

Figure 2. Effects of the CO2 atmosphere and K2CO3 addition on the reduction rates of (a) CuO and (b) Fe2O3 oxygen carriers by SM char.

may be affected by the CO2 atmosphere.10 However, the detailed interpretation of the OTC affected by the CO2 atmosphere has not been developed. Also, the effect of CO2 atmosphere on the sintering of the oxygen carrier is unclear. It has been reported that significant enhancement of the reduction rate of Fe2O3 oxygen carrier by coal char can be achieved by K2CO3 addition, but little is known about this effect on OTC, as well as the sintering of oxygen carrier.16,19 In this study, the reduction of CuO and Fe2O3 oxygen carriers by coal char were conducted with a thermogravimetric analysis (TGA). X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) analysis were performed to study the chemical properties and morphology of the reaction residues. The effects of CO2 atmosphere and K2CO3 addition on the reduction kinetics, OTC, and sintering of these oxygen carriers were investigated.

schematic diagram of the reduction mechanism, which proceeds through a chain reaction of the following two steps: CO + MexOy → CO2 + MexOy − 1

(1)

CO2 + C → 2CO

(2)

where the Boudouard reaction is the rate-limiting step.10,13 One of the advantages of CLC is that sequestration-ready CO2 can be obtained. It is important to study the reaction of oxygen carrier with solid fuel under concentrated CO2 atmosphere. However, many of the CLC experiments with solid fuels were conducted under N2 or CO2/N2 mixed atmospheres.12−18 More investigations are needed to study the effect of CO2 atmosphere on the CLC performance, such as the reduction rate, the OTC, and the sintering of the oxygen carrier. It has been known that the OTC 2704


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Figure 3. Reduction curves of the Fe2O3/CuO oxygen carrier by SM char under CO2 atmosphere with material ratios based on different stoichiometric equations.

2. EXPERIMENTAL SECTION A Shenmu (SM) bituminous coal char pyrolyzed at 900 °C (Table 1) was used as the solid fuel to reduce the impacts of volatiles on the reduction of the oxygen carrier. The Fe2O3 powders (99.9%, Tianda Chemical Reagent Factory, China) and the CuO powders (99.0%, Tianda Chemical Reagent Factory, China) were directly selected as the oxygen carriers. K2CO3 (99.0%, Tianda Chemical Reagent Factory, China) was used as the catalyst, which was added to the oxygen carrier by impregnation. The detailed procedures of char preparation and the catalyst loading were described elsewhere.16 The loading amount of K2CO3 was fixed at 4% in terms of the mass ratio of received K2CO3/ oxygen carrier. All the SM char−oxygen carrier mixtures were thoroughly mixed and then stored in a desiccator for use. The material ratios of the SM char to oxygen carrier were prepared according to the following stoichiometric reactions, respectively:

C + 2CuO → CO2 + 2Cu

(3)

C + 4CuO → CO2 + 2Cu 2O

(4)

1.5C + Fe2O3 → 1.5CO2 + 2Fe

(5)

C + 2Fe2O3 → CO2 + 4FeO

(6)

C + 6Fe2O3 → CO2 + 4Fe3O4

(7)

Figure 4. XRD patterns of the Fe2O3/CuO oxygen carrier after reduction by SM char under CO2 atmosphere with material ratios according to different stoichiometric equations. rapidly to the cooler chamber where the crucible was quenched under a N2 (or CO2) stream. When the crucible was cooled, the residue was removed and mixed thoroughly with another dose of SM char with an agate mortar and pestle to start the next new redox cycle. Simultaneously, about 22 mg of the newly prepared mixture was transferred to the TGA instrument to evaluate the redox behaviors of the oxygen carrier. The XRD patterns of the reduction residue were analyzed by a Bruker AXS D8 Advance X-ray powder diffractometer, which operated with Cu Kα radiation and a step size of 0.02° at a scanning speed of 6° 2θ/min from 2θ = 10° to 80°. A JEOL FESEM (JSM-7001F) was applied to characterize the sintering of the oxygen carriers. The element contents (K, Cu, and Fe) for the CuO and Fe2O3 oxygen carriers were determined by inductively coupled plasma−atomic emission spectrometry (ICP−AES, iCAP 6300, Thermo Fisher).

The redox kinetics of the oxygen carriers were evaluated with a Setaram Setsys TGA Instrument. In each test, about 20−40 mg of mixture was heated in a platinum crucible under high purity N2 (or CO2) atmosphere (100 mL/min) from 30 °C to 800 °C (for CuO) or 900 °C (for Fe2O3) with a heating rate of 10 °C/min. Subsequently, the sample was kept isothermally at this temperature for 120 min (for CuO) or 60 min (for Fe2O3). Finally, air was introduced for a 20 min oxidation. The multicycle experiments were conducted in a fixed bed. Details of the fixed bed used here have been reported by Ren et al.20 The experimental procedures can be briefly described as follows. First, about 1.2 g of sample was loaded into a cylindrical alumina crucible (35 mm i.d. × 10 mm deep). Then, the crucible was suspended into the cooler chamber of the reactor. After the reactor was flushed with a continuous stream of nitrogen for 60 min, the crucible was lowered quickly into the uniform temperature zone for a 120 min reduction under CO2 atmosphere for the CuO oxygen carrier (or a 60 min reduction under N2 atmosphere for the Fe2O3 oxygen carrier) and a 20 min air oxidation successively. The reaction temperatures for CuO and Fe2O3 were kept at 800 °C and 900 °C, respectively. All the flow rates of the purge gases were kept to 200 mL/min. Finally, the crucible was lifted

3. RESULTS AND DISCUSSION 3.1. Effects of CO2 Atmosphere and K2CO3 on the Reduction Reactivity of the Oxygen Carrier. In comparison to gaseous fuels, low reduction reactivity is one of the most important problems of CLC with solid fuels. To investigate the 2705


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Figure 5. CO2 gasification of SM char under the same experimental conditions as those used in the reduction experiments.

Figure 6. Comparison of the calculated pCO2/pCO of SM char gasification under CO2 atmosphere with the equilibrium constants of oxygen carrier reduction.

The enhanced reduction by K2CO3 addition can be ascribed to the catalyzed Boudouard reaction.16 In the TGA experiments, the char particles are in intimate contact with the oxygen carrier particles. The char particles are prone to be wetted by K2CO3 (low melting point) impregnated on the oxygen carrier particles during heating, which will then catalyze the Boudouard reaction. It should be especially noted that the final reduction values of the Fe2O3 oxygen carrier under CO2 atmosphere were remarkably lower than that of the K2CO3impregnated Fe2O3 oxygen carrier under N2 atmosphere, as discussed in section 3.2. 3.2. Effects of CO2 Atmosphere and K2CO3 on the Oxygen Transport Capacity (OTC) of the Oxygen Carrier. The OTC is another essential component for a selected oxygen carrier, which represents the amount of lattice oxygen that can be used in a redox cycle. Higher OTC means lower amount of solid loading needed by the CLC system. The OTC of the oxygen carrier highly depends on the reduction degree considered. For example, there are three OTCs for the Fe2O3 oxygen carrier

effects of CO2 atmosphere and K2CO3 addition on the reduction rates of the oxygen carrier by coal char, the material ratios of CuO/SM char and Fe2O3/SM char were fixed according to eqs 3 and 5, respectively. The results of redox tests conducted by TGA are shown in Figure 2. The reduction rates increase in the order of N2 atmosphere < CO2 atmosphere < N2 atmosphere with 4% K2CO3 addition < CO2 atmosphere with 4% K2CO3 addition. The observation of reduction rate enhancement by the CO2 atmosphere in this work differs from the findings of Siriwardane et al.18 The reduction of metal oxide oxygen carrier by coal char proceeds mainly through a mechanism of chain reactions of gas intermediates, as presented in eqs 1 and 2. Under N2 atmosphere, CO2 is only produced via the eq 1 of this mechanism and its concentration is low, especially at the initial stage. The low concentration of CO2 limits the occurrence of the Boudouard reaction (the rate-governing step), while under CO2 atmosphere, the CO2 concentration is much higher, which is beneficial to the CO2 gasification rate and thus enhances the overall reduction. 2706


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according to the three transformations Fe2O3−Fe3O4, Fe2O3− FeO, and Fe2O3−Fe, respectively. Without consideration of the reduction rate, the theoretical maximum reduction degree of the oxygen carrier by coal char under inert atmosphere can be approximately indicated by the stoichiometric equation. For a sample with a material ratio prepared according to eq 5 for example, the Fe2O3 oxygen carrier is expected to be reduced to Fe under N2 atmosphere. The reduction curves of CuO and Fe2O3 oxygen carriers with different OTCs under CO2 atmosphere and the corresponding XRD patterns of these reduction residues are shown in Figures 3 and 4, respectively. Because all these reductions had been completed under the experimental conditions (as shown in Figure 3), the OTCs of these two oxygen carriers can be indicated by the XRD patterns of the corresponding reduction residues (Figure 4). The reduction of the Fe2O3 oxygen carrier was limited to Fe3O4 under CO2 atmosphere, while the reduction degrees of the CuO oxygen carrier were not affected (Figure 4). This phenomenon needs further investigation. According to the reduction mechanism, the reduction degree of the oxygen carrier is determined by the thermodynamic restriction of eq 1. Provided CO and CO2 are perfect gases, the equilibrium constant, K, can be defined as K = (pCO /pCO )equilibrium 2

dnCO = −2 × dnC

(10)

dnCO2 = dnC

(11)

pCO /pCO = (p0 V0/RT0 + dnCO2)/dnCO

(12)

2

where dTG is the mass loss rate of coal char, MC is the molar mass of carbon, p0V0/RT0 represents the input CO2 of the TGA system (p0 = 101.325 kPa, V0 = 100 mL/min, T0 = 298.15 K), and dni and pi are the molar loss rate and the partial pressure of species i, respectively. The calculated pCO2/pCO quotient and

(8)

The spontaneity of eq 1 is controlled by the pCO2/pCO quotient of the reduction system: When pCO2/pCO < K, the forward reaction is spontaneous, and when pCO2/pCO > K, the reverse reaction is spontaneous. Because CO is derived solely from the Boudouard reaction, the minimum pCO2/pCO quotients during oxygen carrier reduction by SM char under CO2 atmosphere can be approximately simulated by the separate gasification of SM char. Figure 5 displays the CO2 gasification curve of SM char under the same experimental conditions with the reduction of the oxygen carrier. Assuming an ideal-gas behavior of all gases involved, the minimum pCO2/pCO quotients of oxygen carrier reduction can be calculated from the TGA data for char gasification according to the following steps:

dnC = dTG/MC

(9)

Figure 8. SEM images of CuO−SM char mixture (a) unreacted, and after reduction at 900 °C under (b-1) N2 atmosphere, (c-1) CO2 atmosphere, (d-1) N2 atmosphere with 4% K2CO3 addition, (e-1) CO2 atmosphere with 4% K2CO3 addition, and (f) after a five-cycle redox under CO2 and air atmospheres. Images (−2) are the corresponding images of (−1) after air oxidation.

Figure 7. XRD patterns of reduction residues of 4% K2CO3-catalyzed Fe2O3/CuO oxygen carrier by SM char under N2 atmosphere. 2707


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CO + 2CuO → CO2 + Cu 2O

the equilibrium constants of the oxygen carrier reductions by CO are depicted in Figure 6. The equilibrium lines of CO + Fe3O4 → CO2 + 3FeO

and

(13)

CO + Cu 2O → CO2 + 2Cu

and CO + FeO → CO2 + Fe

(17)

are located above the calculated line of pCO2/pCO, indicating that these forward reactions are spontaneous. It is reasonable to conclude that the reduction of Fe2O3 by SM char under CO2 atmosphere was limited to Fe3O4 due to the equilibrium restrictions. For the sample with the material ratio based on eq 5, the SM char particles have the ability to reduce the Fe2O3 particles to Fe under N2 atmosphere. Under CO2 atmosphere, however, these Fe2O3 oxygen carriers were merely reduced to Fe3O4. The extra char particles were only gasified by CO2, and the resulting CO made no more contribution to the reduction of the oxygen carrier, which can be the reason why the final reduction value of Fe2O3 under CO2 atmosphere was remarkably lower than that value obtained under N2 atmosphere (as shown in Figure 2b). By increasing the char content or decreasing the flow rate of CO2, the position of the calculated pCO2/pCO line can be lowered below the equilibrium lines for eqs 13 and 14, which makes the transformations of Fe3O4−FeO and Fe2O3−Fe feasible. However, with in situ gasification CLC (iG-CLC)

(14)

lie below the calculated pCO2/pCO line, which suggests that these transformations are thermodynamically unfavorable under this condition, while the equilibrium lines of CO + 3Fe2O3 → CO2 + 2Fe3O4

(16)

(15)

Figure 9. SEM images of Fe2O3−SM char mixture (a) unreacted, and after reduction at 900 °C under (b-1) N2 atmosphere, (c-1) CO2 atmosphere, (d-1) N2 atmosphere with 4% K2CO3 addition, (e-1) CO2 atmosphere with 4% K2CO3 addition, and (f) after a five-cycle redox under N2 and air atmospheres. Images (−2) are the corresponding images of (−1) after air oxidation.

Figure 10. XRD patterns of reduction residues of the (a) CuO and (b) Fe2O3 oxygen carriers by SM char. 2708


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Figure 11. Five-cycle experiment of catalytic chemical looping combustion of SM char with the (a) CuO and (b) Fe2O3 oxygen carriers.

fluidized by CO2, it is almost impossible to get a pCO2/pCO suitable for transformations Fe3O4−FeO and Fe2O3−Fe because the slow gasification and the fluidization need to be satisfied simultaneously. The amount of solid fuel loaded into the fluidized bed should be within the transformation Fe2O3− Fe3O4, because the redundant solid fuel will produce the unwanted CO. The effect of K2CO3 addition on the OTC was also studied. Figure 7 shows the XRD patterns of reduction residues of K2CO3-catalyzed samples under N2 atmosphere. The target products, Fe and Cu, were obtained as shown in Figure 7, which means that the OTCs of the Fe2O3 and CuO oxygen carriers were not affected by the K2CO3 additive. This can be attributed to the catalyst just increasing the reduction kinetics but not affecting the equilibrium. 3.3. Effects of CO2 Atmosphere and K2CO3 on the Sintering of the Oxygen Carrier. Figures 8 and 9 display the SEM images of the CuO and Fe2O3 oxygen carrier obtained from the fixed bed tests, respectively. It can be seen from Figure 8 that the reduced CuO particles were highly agglomerated as compared to the fresh particles. However, there were no significant differences in the size of grain among the used CuO

particles, suggesting that the CO2 and N2 atmospheres have a similar effect on the sintering of the CuO oxygen carrier. Comparing Figures 9b,c with Figure 9a, we observed no remarkable increase in the grain size of the Fe2O3 oxygen carrier. Under N2 atmosphere, the K2CO3 addition significantly aggravated the sintering of the Fe2O3 oxygen carrier (Figure 9d-1). However, less sintering was caused by K2CO3 under CO2 atmosphere (Figure 9e-1). Comparing the images of reduction residues with that of oxidation residues in Figures 8 and 9, we conclude that the reduction period has an important effect on the sintering of the oxygen carrier. The sintering of the oxygen carriers can be expected because the reactions occurred at temperatures in excess of their Tammann temperatures (which are about one-half the melting point in Kelvin). However, differences in sintering of the oxygen carrier are shown in Figures 8 and 9. To further study these observed differences in sintering, the XRD patterns of these reduction residues were studied, as shown in Figure 10. From Figure 10a, the Cu phase can be found in all the reduction residues of the CuO oxygen carriers, which may be the reason why no significant differences in sintering occurred among the used CuO oxygen carriers (as shown in Figures 8b−e). From Figure 10b, the Fe2O3 oxygen carriers were only reduced to 2709


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Fe3O4 under CO2 atmosphere. The Tammann temperature of Fe3O4 (799 °C) is slightly higher than that of Fe2O3 (783 °C), which could lead to no severe increase in the grain size as shown in Figure 9c-1. K2CO3 can promote the sintering of the iron oxide oxygen carrier due to its low Tammann temperature (451 °C). Note the differences in sintering between Figure 9d-1 and Figure 9e-1, which suggest that sintering of FeO/Fe is promoted by K2CO3 much more easily than that of Fe3O4. The oxygen carriers should possess stable reactivity during multicycle CLC. The sintering has been considered as a main factor to deactivate the reactivity of the oxygen carrier. To evaluate this effect, another four successive redox cycle tests were conducted with the most sintered CuO oxygen carrier (Figure 8e) and Fe2O3 oxygen carrier (Figure 9d), respectively, as shown in Figure 11. It can be observed that even though severe sintering was caused (Figures 8e-2 and 9d-2), the K2CO3-added oxygen carriers still show higher reduction rates than that of the non-K2CO3-added oxygen carrier (Figure 11). The reactivities of the CuO and Fe2O3 oxygen carriers decreased with the increase in redox cycles. There are no remarkable differences in sintering between residues after the first cycle (Figures 8e-2 and 9d-2) and the fifth cycle (Figures 8f and 9f), which indicates the sintering may not be the main reason for these decreases. Table 2 shows the K/Cu and K/Fe mass ratios

sintering of the oxygen carrier due to its low Tammann temperature. (4) Although significant sintering was caused, the K2CO3catalyzed oxygen carrier still possesses a higher reduction rate during several redox cycles.

■

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by Shanxi Natural Science Foundation Committee (no. 2012021005-4) and Strategic Priority Research Program of the Chinese Academy of Sciences (no. XDA07050100). Prof. Yang Wang and Dr. Zhiqing Wang are thanked for helpful discussions.

■

CuO oxygen carrier,a K/Cu mass ratio

Fe2O3 oxygen carrier,b K/Fe mass ratio

unreacted after the first cycle after the fifth cycle

0.0288 0.0272 0.0268

0.0289 0.0287 0.0303

a

REFERENCES

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Table 2. K/Cu and K/Fe Mass Ratios of K2CO3Impregnated CuO and Fe2O3 Oxygen Carrier, Respectively sample

AUTHOR INFORMATION

Under CO2 atmosphere bUnder N2 atmosphere

determined by ICP. From Table 2, no significant change in the K amount can be observed during the multicycle tests, suggesting that these deactivations might not be primarily caused by the catalyst loss, too. Due to the ashes accumulated in the multicycle tests, it is conjectured that these decreasing reactivities may be mainly caused by contamination of the catalyst by ashes.

4. CONCLUSIONS The effects of CO2 atmosphere and K2CO3 addition on the reduction rates, OTC, and the sintering of CuO and Fe2O3 oxygen carriers were investigated. The main conclusions are summarized as follows: (1) Both the CO2 atmosphere and K2CO3 addition can improve the reduction reactivity of the CuO and Fe2O3 oxygen carrier by SM char. These enhancements can be ascribed to the increased CO2 gasification rate of SM char, which is the rate-controlling step of the overall reduction. (2) The CO2 atmosphere affects the OTC of the oxygen carrier by raising the pCO2/pCO quotients: the OTC of the CuO oxygen carrier is not affected, while that of the Fe2O3 oxygen carrier is restricted within the reduction of Fe2O3 to Fe3O4. The K2CO3 additive does not limit the OTC of oxygen carriers because the catalyst has no effect on the equilibrium. (3) The sintering of the CuO oxygen carrier is almost not affected by the CO2 atmosphere. However, the sintering of the Fe2O3 oxygen carrier could be reduced to some extent by the CO2 atmosphere because only Fe3O4 is produced. The K2CO3 added could promote the 2710


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(14) Wang, B.; Yan, R.; Zheng, Y.; Zhao, H.; Zheng, C. Mechanistic investigation of chemical looping combustion of coal with Fe2O3 oxygen carrier. Fuel 2011, 90 (7), 2359−2366. (15) Scott, S.; Dennis, J.; Hayhurst, A.; Brown, T. In situ gasification of a solid fuel and CO2 separation using chemical looping. AIChE J. 2006, 52 (9), 3325−3327. (16) Yu, Z.; Li, C.; Fang, Y.; Huang, J.; Wang, Z. Reduction Rate Enhancements for Coal Direct Chemical Looping Combustion with an Iron Oxide Oxygen Carrier. Energy Fuels 2012, 26 (4), 2505−2511. (17) Saha, C.; Roy, B.; Bhattacharya, S. Chemical looping combustion of Victorian brown coal using NiO oxygen carrier. Int. J. Hydrogen Energy 2011, 36 (4), 3253−3259. (18) Siriwardane, R.; Tian, H. J.; Richards, G.; Simonyi, T.; Poston, J. Chemical-Looping Combustion of Coal with Metal Oxide Oxygen Carriers. Energy Fuels 2009, 23 (8), 3885−3892. (19) Gu, H.; Shen, L.; Xiao, J.; Zhang, S.; Song, T.; Chen, D. Iron ore as oxygen carrier improved with potassium for chemical looping combustion of anthracite coal. Combust. Flame 2012, 159 (7), 2480− 2490. (20) Ren, H. J.; Zhang, Y. Q.; Fang, Y. T.; Wang, Y. Co-gasification behavior of meat and bone meal char and coal char. Fuel Process. Technol. 2011, 92 (3), 298−307.

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