Characterization and Evaluation of Prepared Fe 2 O 3 /Al 2 O 3 Oxygen Carriers for Chemical Looping Process

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1 Aerosol and Air Quality Research, 14: , 214 Copyright Taiwan Association for Aerosol Research ISSN: print / online doi: 1.429/aaqr Characterization and Evaluation of Prepared Fe 2 /Al 2 Oxygen Carriers for Chemical Looping Process Ping-Chin Chiu 1, Young Ku 1*, You-Lin Wu 1, Hsuan-Chih Wu 1, Yu-Lin Kuo 2, Yao-Hsuan Tseng 1 1 Department of Chemical Engineering, National Taiwan University of Science and Technology, No. 43, Sec. 4, Keelung Rd., Da an Dist., Taipei 16, Taiwan 2 Department of Mechanical Engineering, National Taiwan University of Science and Technology, No. 43, Sec. 4, Keelung Rd., Da an Dist., Taipei 16, Taiwan ABSTRACT Alumina supported Fe 2 was prepared to evaluate its feasibility as an oxygen carrier for chemical looping using a thermogravimetric analyzer (TGA) and fixed bed reactor. Fe 2 /Al 2 particles containing 6 wt% Fe 2 and sintered at 13 C demonstrated reasonable oxygen conversion. The reduction mechanism of Fe 2 /Al 2 oxygen carriers was proposed based on the thermogravimetric analysis and XRD characterization. FeAl 2 O 4 is formed and can serve as a supporting material as well as oxygen carrier in practical operations by using a moving bed reactor. The prepared Fe 2 /Al 2 oxygen carriers demonstrated high CO 2 yields in relation to syngas and methane combustion in the fixed bed reactor operated at temperatures higher than 5 and 85 C, respectively. Moreover, hydrogen generation was demonstrated to be feasible by steam oxidation with reduced Fe 2 /Al 2 oxygen carriers for experiments conducted at temperatures above 8 C. The crush strength and TGA of Fe 2 /Al 2 were examined, and the results showed that they had an appropriate crush strength and reasonable reactivity as oxygen carriers for use in a chemical looping process. Keywords: Chemical looping process; Fe 2 /Al 2 Oxygen carrier; TGA; Fixed bed reactor. INTRODUCTION Carbon dioxide (CO 2 ) as one of the anthropogenic GHGs has been rapidly grown by 39% above pre-industrial levels at the end of 21 (IPCC, 211). For satisfying the energy demand, fossil fuel supported around 8% of primary energy through combustion, and became the major CO 2 emission point sources from industry and energy sectors, presently (IEA, 21). For reduction of CO 2 emissions, use of renewable energies such as solar, wind, biomass is considered a long-term destination in place of fossil fuel to generate energy. In order to reduce CO 2 emissions in the near future, CO 2 capture for utilization and storage (CCUS) is currently developed for mitigating CO 2 emitted into atmosphere (Olajire, 21; MacDowell et al., 21). The CO 2 capture technologies are typically including precombustion, post-combustion and oxyfuel-combustion. However, the efficiency loss was estimated from 6 to 14% for the above CO 2 capture technologies due to the operation * Corresponding author. Tel.: ; Fax: address: ku58@mail.ntust.edu.tw of additional units to obtain high purity CO 2 ; in addition, for economical point of view, the cost for CO 2 capture, transportation and sequestration were estimated as 3 to 6 /t CO2 (Notz et al., 211), which is much higher than the price of CO 2 emission rights in European Energy Exchange (EEX) approximately 2.5 to 5. /t CO2 in 213. Hence, the CO 2 capture technologies should be substantially increased their energy efficiency to meet the economical requirement. Chemical looping technology is proposed and considered as a combustion technology with high potential to substantially cut down the cost of CO 2 capture (Figureoa et al., 28). Chemical looping process is typically consisted of a fuel reactor and an air reactor, as illustrated in Fig. 1, and utilizes the oxygen present in metal oxides as oxygen carriers circulating between the reactors for fuel combustion (Ishida et al., 1987). In the fuel reactor, combustion between oxygen carriers and carbonaceous fuels yields CO 2 and H 2 O as described by Eq. (1). In the air reactor, reduced oxygen carriers are oxidized as described by Eq. (2) to generate heat for applications (Chiu and Ku, 212). C n H 2m + (2n + m)me x O y nco 2 + mh 2 O + (2n + M) Me x O y 1 (1) O 2 + 2Me x O y 1 2Me x O y (2)

2 982 Chiu et al., Aerosol and Air Quality Research, 14: , 214 Fig. 1. Schematic diagram of chemical looping process. The CO 2 yields were estimated to be higher than 9% for fuel combustion by chemical looping process with various metal oxides, such as Fe 2, NiO, CuO, Mn 2 and Co 3 O 4, based on thermodynamic analysis (Cao and Pan, 26; Gupta et al., 27; Fan and Li, 21). Chemical looping is considered a promising technology with high energy efficiency as well as inherent CO 2 concentration for possible carbon reuse or storage (Figueroa et al., 28). The efficiency of electricity generation for syngas (CO/H 2 ) combustion by chemical looping process was estimated to be 43.2% with 99% CO 2 yield (Xiang and Wang, 28), much higher than those for pulverized coal power plants with amine absorption for CO 2 capture (Li and Fan, 28). Iron-based oxygen carriers are commonly applied for chemical looping process due to a number of advantages, including higher melting point, better mechanical strength, less impact on environment and health, and lower cost than most other oxygen carriers (Mattisson et al., 21; Johansson et al., 24; Berguerand and Lyngfelt, 29; Li et al., 29; Kuo et al., 213; Tseng et al., 213). However, sintering and attrition of oxygen carriers during continuous, hightemperature chemical looping operation would decrease the reactivity of oxygen carriers. Therefore, support materials, such as Al 2, MgAl 2 O 4, ZrO 2, TiO 2 and bentonite, are combined with hematite (Fe 2 ) to retard the sintering of oxygen carriers and to enhance their mechanical strength for chemical looping operation (Johansson et al., 24; Mattisson et al., 24; Son et al., 26; Abad et al., 27; Mattisson et al., 27; Azis et al., 21). Among the support materials, Al 2 is frequently used for iron-based oxygen carriers to improve their reactivity and recyclability (Ishida et al., 25). The reactors proposed for chemical looping process including circulating fluidized bed, bubbling fluidized bed, spouted bed, rotating bed and moving bed reactors (Lyngfelt, 211; Sridhar et al., 212). The design of moving bed fuel reactor with counter-flow pattern was proposed to increase the available oxygen for combustion with ironbased oxygen carriers (Gupta et al., 27; Ku et al., 214). Hydrogen generation is one of the applications of chemical looping process using iron-based oxygen carriers (Gupta et al., 27). The Fe 2 is reduced to Fe and FeO in a counter-flow moving bed fuel reactor, and moving to an oxidizer for hydrogen generation through steam oxidation as described reactions (Li et al., 21): 3Fe + 4H 2 O Fe 3 O 4 + 4H 2 (3) 3FeO + 3H 2 O Fe 3 O 4 + 3H 2 (4) For practical H 2 generation by chemical looping process, a 25 kw moving bed system demonstrated continuous generating 94.4 to 98.4% of H 2 (Sridhar et al., 212). The high purity of H 2 can be used as raw material for chemicals production, such as methanol, ammonia and dimethyl ether (DME). Preparation of Fe 2 /Al 2 oxygen carriers was investigated by thermogravimetric analyzer (TGA) in this study. Experiments on the reduction of Fe 2 /Al 2 oxygen carriers was conducted by TGA and characterized by X-ray diffraction (XRD). The performance of prepared Fe 2 /Al 2 oxygen carriers on CO 2 yield with syngas and methane combustions, and on H 2 generation by H 2 O oxidation were determined in a fixed bed reactor. The mechanical strength of Fe 2 /Al 2 pellets was evaluated by texture analyzer; and the reactivity and recyclability of Fe 2 /Al 2 pellets were evaluated by TGA with continuous 5 redox cycles. EXPERIMENTAL Preparation of Fe 2 /Al 2 Oxygen Carriers Alumina-supported Fe 2 oxygen carriers (Fe 2 /Al 2 ) with various Fe 2 fractions (2,, 5, 6 or 8wt%) were prepared by mixing Fe 2 and Al 2 powders (approximately 1 µm in particle size) with a ball miller operated at 38 rpm for 5 minutes. The Fe 2 /Al 2 powders were then sintered in a muffle furnace at 9, 1, 11, 12 or 13 C for 2 hours. The prepared oxygen carriers were examined for reactivity by a thermogravimetric analyzer (TGA). For preparation of Fe 2 /Al 2 pellets for fixed bed experiments, Fe 2 and Al 2 powders were mixed in the de-ionized water in a stirring tank equipped for 3 minutes at room temperature. The well-mixed Fe 2 /Al 2 slurry solution was dried at 8 C for 6 hours, and then pulverized and screened their particle sizes from 1.2 to 1.4 mm. Fe 2 /Al 2 particles were sintered in a muffle furnace at 13 C for 2 hours. The Fe 2 /Al 2 particles were pelletized by a bead machine to be pellets of 3 mm both in diameter and height, and then sintered in a muffle furnace at 13 C for 2 hours. The Fe 2 /Al 2 pellets were employed to 5 redox cycles test by TGA and crush strength test by texture analyzer. Reactivity Tests by Thermogravimetric Analyzer (TGA) The reactivity and recyclability of Fe 2 and Fe 2 /Al 2 oxygen carriers were analyzed by a Netzsch STA 449F3 TGA. 2 mg oxygen carriers were loaded in an alumina crucible, the temperature of TGA chamber was increased with a ramping rate of 2 C/min in N 2 atmosphere, and eventually kept at 9 C. 2 ml/min reducing gas composed of 2% H 2 and 8% N 2 was introduced into TGA chamber for 15 minutes to reduce oxygen carriers. After the reduction process, 2 ml/min N 2 was introduced for 5 minutes for sweeping reducing gas contained in the TGA chamber. 2 ml/min air was then introduced for 1 minutes to oxidize the reduced oxygen carriers. The procedure of redox cycling was replicated for 5 times with Fe 2 and Fe 2 /Al 2 pellets to evaluate the reactivity and recyclability with respect to practical moving bed operation of chemical

3 Chiu et al., Aerosol and Air Quality Research, 14: , looping process. The redox cycling for oxygen carriers was carried out by repeating the procedure described above, while the reducing time was 2 minutes. The oxygen conversion for reduction of oxygen carriers is described by Eq. (5): X OC mo m() t m m o r where m o is weight of fully oxidized oxygen carrier; m r is weight of fully reduced oxygen carrier; m(t) is weight of oxygen carrier after reduction period of t. Characterization of Oxygen Carriers The phase transformation of prepared Fe 2 /Al 2 during redox cycle was characterized by X-ray diffraction (XRD, Bruker D2 Phaser), the incident beam was Cu K α characteristic X-ray at 3 kv and 1 ma scanning with rate of.5 deg/s in the 2θ range from 1 to 8. Surface morphology and interfacial behaviors of Fe 2 /Al 2 oxygen carrier were examined by a field emission scanning electron microscope (FESEM, JOEL JSM-65F). The crush strength of prepared Fe 2 /Al 2 oxygen carriers were analyzed by a texture machine (TA.XT plus). Fixed Bed Reactor Fixed bed reactor system employed in this study is shown (5) in Fig. 2, and is composed of a 25.4 mm ID stainless steel (SS31) reactor and a PID-controlled heating element covering 2 mm of reactor height. A plate with sixteen.25 mm apertures was located in the lower part of the reactor for supporting Fe 2 /Al 2 oxygen carriers. For reduction of Fe 2 /Al 2 oxygen carriers with methane and syngas, 1.5 L/min reducing gas was introducing into the fixed bed reactor at various temperatures, ranging from 1 to 9 C. The reducing gases were composed of 2% of CH 4 and 8% of N 2 for methane combustion and 1% of CO, 1% of H 2 and 8% of N 2 for syngas combustion, respectively. For H 2 generation by steam oxidation of reduced Fe 2 /Al 2 oxygen carriers, 5 mmol/min of steam was introduced into the fixed bed reactor at various temperatures, ranging from 5 to 9 C. The outlet stream from the fixed bed reactor was passed through a cold trap to condense steam, and 2 ml/min of spent gas was consequently analyzed by a non-dispersive infrared analyzer (Molecular Analysis 6i) to detect CO, CH 4 and CO 2 separately by specific light beams, respectively. A gas chromatography analyzer equipped with thermal conductivity detector (Agilent 789) was employed for measuring H 2 concentration. The CO 2 yield (Y CO2 ) is defined as: Y CO2 FCO 2 F F F F CO2 CO CH4 H2 1% (6) Fig. 2. Schematic diagram of fixed bed reactor system.

4 984 Chiu et al., Aerosol and Air Quality Research, 14: , 214 where F CO2, F CO, F H2 and F CH4 are the molar flow rates of CO 2, CO, H 2 and CH 4 in the outlet stream, respectively. RESULTS AND DISCUSSION Thermogravimetric Analysis of Fe 2 /Al 2 Oxygen Carriers The conversion of oxygen present in oxygen carriers for prepared Fe 2 /Al 2 with various Fe 2 fractions was conducted at 9 C by TGA, and is shown in Fig. 3. The conversion of oxygen was increased with the fraction of Fe 2 for Fe 2 /Al 2 oxygen carriers containing 2 to 6wt% Fe 2, whereas decreased for Fe 2 /Al 2 oxygen carriers containing 8wt% Fe 2 possibly because of the sintering of oxygen carriers during reduction and oxidation reactions. The effect of sintering temperature for preparation of Fe 2 /Al 2 on the conversion of oxygen present in oxygen carriers is shown in Fig. 4. For experiments conducted with Fe 2 /Al 2 oxygen carriers sintered at temperature lower than 1 C, the conversions of oxygen were independent on sintering temperature. For experiments conducted with Fe 2 /Al 2 oxygen carriers sintered at temperature higher than 11 C, the conversion was increased with increasing sintering temperature. Based on the experience of ceramic sintering, solid state sintering between ceramic materials would be started as sintering temperature higher than 2/3 of its melting point for uniformly dispersion of multiple ceramic Effect of Fe 2 Fraction in Fe 2 /Al 2 Oxygen Carrier Reaction temperature: 9 C Reducing gas: 2% H 2 Gas flow rate: 2 ml/min X OC (%) Fe 2 Fraction (wt%) Fig. 3. Oxygen conversion of Fe 2 /Al 2 oxygen carriers for 2,, 6 and 8wt% of Fe 2 fractions by using H 2 as reducing gas conducted by TGA at 9 C Effect of Sintering Temperature Reaction temperature: 9 C Reducing gas: 2% H 2 Gas flow rate: 2 ml/min X OC (%) Sintering Temperature ( C) Fig. 4. Oxygen conversion of Fe 2 /Al 2 oxygen carriers for 9, 1, 11, 12 and 13 C of sintering temperature by using H 2 as reducing gas conducted by TGA at 9 C.

5 Chiu et al., Aerosol and Air Quality Research, 14: , materials (Rahaman, 28). Therefore, solid state sintering was suggested beginning at 11 C which was exactly higher than 2/3 of melting point of Fe 2 (156 C), for well dispersion of Fe 2 in Fe 2 /Al 2 oxygen carriers. Thus, the reaction between Fe 2 and reducing gas was increased as Fe 2 was more uniformly dispersed in Fe 2 /Al 2 oxygen carriers by increasing sintering temperature. It was reported that 6wt% of Fe 2 supported by wt% of Al 2 sintered at 13 C demonstrated better reactivity (Mattisson et al., 24), which is corresponded to the results in this study. Experimental results for Fe 2 /Al 2 oxygen carrier reduced by syngas for 6 minutes and oxidized by air for 1 minutes in TGA are depicted in Fig. 5. Approximately 7% of oxygen was reduced by syngas combustion within about 21 minutes of reaction time; furthermore, roughly 2wt% of oxygen was reduced from 21 to 6 minutes. The oxygen carrier was sampled at the reduction time at, 4, 21 and 6 minutes, and characterized by XRD for phase identification as shown in Fig. 6. Prior to starting reduction process, Fe 2 and Al 2 were the major crystalline phase of oxygen carriers. After 4 minutes of reduction, Fe 3 O 4 was found to be the major crystalline phase. The conversion of oxygen for Fe 2 /Al 2 oxygen carriers was around 1% for 4 minutes of reduction that corresponded to the amount of oxygen utilization from Fe 2 to Fe 3 O 4. FeO and FeAl 2 O 4 were formed at the end of first stage, indicating that Fe 3 O 4 was further reduced to FeO, and part of FeO reacted with Al 2 to form FeAl 2 O 4. The formation of FeAl 2 O 4 was corresponded to the previous research reported by Ishida et al. (25) using Al 2 supported Fe 2 in the reduction process. Therefore, reduction of Fe 2 /Al 2 oxygen carrier in the first stage less than 4 minutes of reaction time, as illustrated in Fig. 5 is suggested by following reactions: 6Fe 2 + 2CO/H 2 4Fe 3 O 4 + 2CO 2 /H 2 O (7) Fe 3 O 4 + CO/H 2 3FeO + CO 2 /H 2 O (8) FeO + Al 2 FeAl 2 O 4 (9) For oxygen carriers sampled at 21 minutes of reduction, Fe and FeAl 2 O 4 were observed to be the major crystalline phases, while FeO and Fe 3 O 4 were absence in the XRD pattern. The reduction of Fe 2 /Al 2 oxygen carrier in the second stage (4 to 21 minutes of reaction time) is suggested by the following reactions: FeO + CO/H 2 Fe + CO 2 /H 2 O (1) FeAl 2 O 4 + CO/H 2 Fe + Al 2 + CO 2 /H 2 O (11) The XRD intensity of Fe was enhanced at 6 minutes of reduction indicating that FeAl 2 O 4 was continuously reduced to Fe. Hence, reduction of FeAl 2 O 4 was much slower than Fe 2 and Fe 3 O 4 resulted in the slow reduction rate in the third stage (21 to 6 minutes of reaction time). The counterflow moving bed reactor might promote the utilization of oxygen in iron-based oxygen carriers up to 5%, which was much higher than that by the fluidized bed reactors (Li and Fan, 28). Accordingly, FeAl 2 O 4 and Fe are suggested the major crystalline phases as using Fe 2 /Al 2 oxygen carriers for a counter-flow moving bed reactor as illustrated in Fig. 6. Hence, the reduction mechanism of Fe 2 /Al 2 oxygen carriers was proposed in accordance with thermogravimetric analysis and XRD characterization. In addition, the intermediate crystalline phase, FeAl 2 O 4, would serve as the supporting material as well as oxygen carrier in practical operation by moving bed reactor. 1 Fe 2 +Al 2 First stage 8 Fe 3 O 4 /FeO/FeAl 2 O 4 Weight (%) 6 Second stage TG Analysis of Reduction of Fe 2 /Al 2 Weight of Fe 2 /Al 2 : 2 mg Reaction temperature: 9 C Reducing gas: 1% CO+1% H 2 Oxidizing gas: air Gas flow rate: 2 ml/min Fe/FeAl 2 O 4 2 Third stage Fe/FeAl 2 O Time (min) Fig. 5. Oxygen conversion of Fe 2 /Al 2 oxygen carriers for 9, 1, 11, 12 and 13 C of sintering temperature by using H 2 as reducing gas conducted by TGA at 9 C.

6 986 Chiu et al., Aerosol and Air Quality Research, 14: , 214 XRD Analysis of Fe 2 /Al 2 Oxygen Carrier Reduction Weight of Fe 2 /Al 2 : 2 mg Reaction temperature: 9 C Reducing Gas: syngas Fe 2 /Al 2 reduction for 6 min Intensity (a.u.) Fe 2 /Al 2 reduction for 21 min Fe 2 /Al 2 reduction for 4 min Fresh Fe 2 /Al Fig. 6. XRD patterns of Fe 2 /Al 2 oxygen carriers sampled at each time level of reduction at, 4, 21 and 6 minutes conducted by TGA at 9 C ( : Fe 2, : Al 2, : Fe, : FeAl 2 O 4, : Fe 3 O 4, : FeO). Fixed Bed Experiments with Fe 2 /Al 2 Oxygen Carriers The combustions of methane and syngas with Fe 2 /Al 2 oxygen carriers were employed in a fixed bed reactor at temperatures ranging from 1 to 9 C. As shown in Fig. 7, the Y CO2 of syngas combustion was noticeable for experiment conducted at 3 C, and reached 1% for experiments conducted at temperatures higher than 5 C. Based on the thermodynamic analysis by previous researchers (Li et al., 21), CO and H 2 can be completely converted to CO 2 and H 2 O through the reduction from Fe 2 to Fe 3 O 4 at temperatures ranging from 2 to 12 C. However, as illustrated in Fig. 7, the equilibrium was not achieved because the reaction rates were too slow to reach complete conversion at temperatures ranging lower than C. The CO 2 yields of methane combustion were observed for experiment conducted at 6 C and reached 1% for experiments conducted at temperatures higher than 85 C, as shown in Fig. 7. Therefore, methane combustion with Fe 2 /Al 2 oxygen carriers was suggested to carry out at temperatures higher than 85 C for chemical looping process, which corresponded to the previous study that natural gas (over 9% of CH 4 ) combustion with iron-based oxygen carriers to achieve complete conversion at 95 C (Abad et al., 27). Hence, the prepared Fe 2 /Al 2 oxygen carriers demonstrated high CO 2 yields regarding to syngas and methane combustion in the fixed bed reactor. Experiments with regards to steam oxidation with completely reduced oxygen carriers for H 2 generation in a fixed-bed reactor were conducted at temperature varied from 5 to 9 C. As shown in Fig. 8, the flow rate of H 2 in the outlet stream was significantly increased with reaction temperature and reached a steady value for experiments conducted at temperature above 8 C. Based on the mole balance calculation, the conversion of H 2 O to H 2 reached roughly 75% for experiments conducted at temperature above 8 C. The results of conversions were corresponded to the thermodynamic analysis of oxidation of Fe by H 2 O reported by previous study (Li et al., 21) that 6 to 7% of H 2 O would be reduced to form H 2 using reduced Fe 2 oxygen carriers at temperature ranging from 6 to 1 C. Hence, the prepared Fe 2 /Al 2 oxygen carriers could be feasible for H 2 generation by chemical looping process. Performance Evaluation of Fe 2 /Al 2 Oxygen Carriers for Chemical Looping Process For applications of Fe 2 /Al 2 oxygen carriers in moving bed reactors, the Fe 2 /Al 2 oxygen carriers are usually pelletized to provide stronger mechanical strength to sustain the bumping in the reactor system. The crush strength of Fe 2 is examined to be 33 and 6 N for 1.2 to 1.4 mm and 1.4 to 2. mm of particles size, respectively, as listed in Table 1. The crush strength of Fe 2 /Al 2 particles were lower than Fe 2 examined to be 22 and 28 N for 1.2 to 1.4 mm and 1.4 to 2. mm of particles size, respectively. The interspace between Fe 2 and Al 2 particles is possibly greater than Fe 2 particles because the crystallite size of Fe 2 is different with Al 2 particles, which generated more space in the prepared Fe 2 /Al 2 particles. The comparatively loose packed Fe 2 /Al 2 particles resulted in lower crush strength than Fe 2 particles. Pelletizing of Fe 2 and Fe 2 /Al 2 particles effectively enhanced their crush strength to be 251 and 23 N, respectively. The interspace of the pelletized particles would be substantially reduced for more compact packing in the pellet. Therefore, the crush strength is greatly enhanced by pelletizing process. The pelletized Fe 2 and Fe 2 /Al 2 oxygen carriers were used for experiments in TGA to evaluate their reactivity and recyclability.

7 Chiu et al., Aerosol and Air Quality Research, 14: , Fixed Bed Experiments Oxygen carrier: 1 g 1.5 L/min of syngas (1 % CO+1 % H 2 ) 1.5 L/min of CH 4 (1 % CH 4 ) Y CO2 (%) Reaction Temperature ( C) Fig. 7. CO 2 yields for syngas and methane combustions varied with reaction temperatures in the fixed bed reactor packed with 1 g of Fe 2 /Al 2 particles. 5 H 2 Generation in Fixed Bed Reactor Fe 2 /Al 2 oxygen carriers: 1 g Steam flow rate: 5 mmol/min 1 8 H 2 (mmol/min) H2 O Conversion (%) Reaction Temperature ( C) Fig. 8. H 2 generation by steam oxidation of reduced Fe 2 /Al 2 particles in the fixed bed reactor at 5, 6, 7, 8 and 9 C. Table 1. Crush strength of Fe 2 and Fe 2 /Al 2 particles and pellets examined by texture analyzer. Oxygen Carriers Oxygen Content (wt%) Particle Size (mm) Crush Strength (N) Fe (pellet) Fe 2 /Al (pellet) 23 The reactivity and recyclability of Fe 2 and Fe 2 /Al 2 pellets using syngas as reducing gas were evaluated in TGA for 5 redox cycles. As depicted in Fig. 9, the conversion of Fe 2 oxygen carriers was decayed rapidly after the second redox cycle. However, the conversion of Fe 2 /Al 2 pellet was maintained at above 7% during continuous

8 988 Chiu et al., Aerosol and Air Quality Research, 14: , 214 redox cycling indicating that the recyclability of Fe2O3 was considerably improved with Al2O3 support. The SEM images of Fe2O3 and Fe2O3/Al2O3 pellets are shown in Fig. 1. The grain size of fresh Fe2O3/Al2O3 was observed to be larger than fresh Fe2O3 because Fe2O3/Al2O3 pellets were sintered at 13 C for 2 hours during preparation. The agglomeration of Fe2O3/Al2O3 sintered at 13 C was also observed in previous literature (Mattisson et al., 24). As demonstrated in Fig. 1(c), the Fe2O3 oxygen carriers were obviously agglomerated after 5 redox cycles to decrease the surface area and reactivity of Fe2O3 pellets. For Fe2O3/Al2O3 pellets after 5 redox cycles, no obvious agglomeration was observed comparing with fresh Fe2O3/Al2O3 pellets, as shown in Fig. 1(d). Fe2O3/Al2O3 pellet was illustrated to provide better reactivity owing to the inhibition of agglomeration as observed by SEM image. Formation of FeAl2O4 by reduced FeO and Al2O3 was characterized by XRD in Fig. 6, and the melting point of FeAl2O4 is 178 C, which is higher than that of FeO, thus mitigated agglomeration of pellets. Hence, Fe2O3/Al2O3 pellets that examined by crush strength and TGA demonstrated proper crush strength and reasonable reactivity, indicating Fe2O3/Al2O3 pellets are preliminary validated as oxygen carrier for chemical looping process XOC (%) 7 6 Redox Cycling by TGA 5 Reaction Temperature: 9 C Reducing gas: 1% CO+1% H2 Gas flow rate: 2 ml/min Reducing time: 2 min 3 mm Fe2O3 pellet 3 3 mm Fe2O3/Al2O3 pellet Redox Cycle Fig. 9. Oxygen conversion of Fe2O3 and Fe2O3/Al2O3 pellets using syngas as reducing gas conducted by TGA at 9 C. (a) Fresh Fe2O3 (c) Fe2O3 after 5 redox cycles (b) Fresh Fe2O3/Al2O3 (d) Fe2O3/Al2O3 after 5 redox Fig. 1. SEM images of (a) fresh Fe2O3, (b) fresh Fe2O3/Al2O3, (c) Fe2O3 after 5 redox cycles and (d) Fe2O3/Al2O3 after 5 redox cycles conducted by TGA at 9 C.

9 Chiu et al., Aerosol and Air Quality Research, 14: , CONCLUSIONS Fe 2 /Al 2 particles containing 6wt% Fe 2 and sintered at 13 C is a suitable preparation parameter to demonstrate reasonable oxygen conversion. The reduction mechanism of Fe 2 /Al 2 oxygen carriers was proposed in accordance with thermogravimetric analysis and XRD characterization. In addition, the intermediate crystalline phase, FeAl 2 O 4, would serve as the supporting material as well as oxygen carrier in practical operation by moving bed reactor. The prepared Fe 2 /Al 2 oxygen carriers demonstrated high CO 2 yields regarding to syngas and methane combustion in the fixed bed reactor, operated at temperatures higher than 5 and 85 C, respectively. Moreover, hydrogen generation was demonstrated to be feasible by steam oxidation with reduced Fe 2 /Al 2 oxygen carriers for experiments conducted at temperature above 8 C. 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