Aerosol and Air Quality Research, 14: 981–990, 2014 Copyright © Taiwan Association for Aerosol Research ISSN: 1680-8584 print / 2071-1409 online doi: 10.4209/aaqr.2013.04.0135

Characterization and Evaluation of Prepared Fe2O3/Al2O3 Oxygen Carriers for Chemical Looping Process Ping-Chin Chiu1, Young Ku1*, You-Lin Wu1, Hsuan-Chih Wu1, Yu-Lin Kuo2, Yao-Hsuan Tseng1 1 Department of Chemical Engineering, National Taiwan University of Science and Technology, No. 43, Sec. 4, Keelung Rd., Da’an Dist., Taipei 106, Taiwan 2 Department of Mechanical Engineering, National Taiwan University of Science and Technology, No. 43, Sec. 4, Keelung Rd., Da’an Dist., Taipei 106, Taiwan ABSTRACT

Alumina supported Fe2O3 was prepared to evaluate its feasibility as an oxygen carrier for chemical looping using a thermogravimetric analyzer (TGA) and fixed bed reactor. Fe2O3/Al2O3 particles containing 60 wt% Fe2O3 and sintered at 1300°C demonstrated reasonable oxygen conversion. The reduction mechanism of Fe2O3/Al2O3 oxygen carriers was proposed based on the thermogravimetric analysis and XRD characterization. FeAl2O4 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 Fe2O3/Al2O3 oxygen carriers demonstrated high CO2 yields in relation to syngas and methane combustion in the fixed bed reactor operated at temperatures higher than 500 and 850°C, respectively. Moreover, hydrogen generation was demonstrated to be feasible by steam oxidation with reduced Fe2O3/Al2O3 oxygen carriers for experiments conducted at temperatures above 800°C. The crush strength and TGA of Fe2O3/Al2O3 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; Fe2O3/Al2O3 Oxygen carrier; TGA; Fixed bed reactor. INTRODUCTION

Carbon dioxide (CO2) as one of the anthropogenic GHGs has been rapidly grown by 39% above pre-industrial levels at the end of 2010 (IPCC, 2011). For satisfying the energy demand, fossil fuel supported around 80% of primary energy through combustion, and became the major CO2 emission point sources from industry and energy sectors, presently (IEA, 2010). For reduction of CO2 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 CO2 emissions in the near future, CO2 capture for utilization and storage (CCUS) is currently developed for mitigating CO2 emitted into atmosphere (Olajire, 2010; MacDowell et al., 2010). The CO2 capture technologies are typically including pre-combustion, post-combustion and oxyfuel-combustion. However, the efficiency loss was estimated from 6 to 14% for the above CO2 capture technologies due to the operation * Corresponding author. Tel.: +886-2-2378-5535; Fax: +886-2-2737-6644 E-mail address: ku508@mail.ntust.edu.tw

of additional units to obtain high purity CO2; in addition, for economical point of view, the cost for CO2 capture, transportation and sequestration were estimated as 30 to 60 €/tCO2 (Notz et al., 2011), which is much higher than the price of CO2 emission rights in European Energy Exchange (EEX) approximately 2.5 to 5.0 €/tCO2 in 2013. Hence, the CO2 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 CO2 capture (Figureoa et al., 2008).

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 CO2 and H2O 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, 2012). CnH2m + (2n + m)MexOy → nCO2 + mH2O + (2n + M) MexOy–1 (1) O2 + 2MexOy–1 → 2MexOy (2)

Chiu et al., Aerosol and Air Quality Research, 14: 981–990, 2014 982

Fig. 1. Schematic diagram of chemical looping process.

The CO2 yields were estimated to be higher than 90% for fuel combustion by chemical looping process with various metal oxides, such as Fe2O3, NiO, CuO, Mn2O3 and Co3O4, based on thermodynamic analysis (Cao and Pan, 2006; Gupta et al., 2007; Fan and Li, 2010). Chemical looping is considered a promising technology with high energy efficiency as well as inherent CO2 concentration for possible carbon reuse or storage (Figueroa et al., 2008). The efficiency of electricity generation for syngas (CO/H2) combustion by chemical looping process was estimated to be 43.2% with 99% CO2 yield (Xiang and Wang, 2008), much higher than those for pulverized coal power plants with amine absorption for CO2 capture (Li and Fan, 2008).

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., 2001; Johansson et al., 2004; Berguerand and Lyngfelt, 2009; Li et al., 2009; Kuo et al., 2013; Tseng et al., 2013). However, sintering and attrition of oxygen carriers during continuous, high-temperature chemical looping operation would decrease the reactivity of oxygen carriers. Therefore, support materials, such as Al2O3, MgAl2O4, ZrO2, TiO2 and bentonite, are combined with hematite (Fe2O3) to retard the sintering of oxygen carriers and to enhance their mechanical strength for chemical looping operation (Johansson et al., 2004; Mattisson et al., 2004; Son et al., 2006; Abad et al., 2007; Mattisson et al., 2007; Azis et al., 2010). Among the support materials, Al2O3 is frequently used for iron-based oxygen carriers to improve their reactivity and recyclability (Ishida et al., 2005). The reactors proposed for chemical looping process including circulating fluidized bed, bubbling fluidized bed, spouted bed, rotating bed and moving bed reactors (Lyngfelt, 2011; Sridhar et al., 2012). The design of moving bed fuel reactor with counter-flow pattern was proposed to increase the available oxygen for combustion with iron-based oxygen carriers (Gupta et al., 2007; Ku et al., 2014).

Hydrogen generation is one of the applications of chemical looping process using iron-based oxygen carriers (Gupta et al., 2007). The Fe2O3 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., 2010): 3Fe + 4H2O → Fe3O4 + 4H2 (3) 3FeO + 3H2O → Fe3O4 + 3H2 (4)

For practical H2 generation by chemical looping process, a 25 kW moving bed system demonstrated continuous generating 94.4 to 98.4% of H2 (Sridhar et al., 2012). The high purity of H2 can be used as raw material for chemicals production, such as methanol, ammonia and dimethyl ether (DME).

Preparation of Fe2O3/Al2O3 oxygen carriers was investigated by thermogravimetric analyzer (TGA) in this study. Experiments on the reduction of Fe2O3/Al2O3 oxygen carriers was conducted by TGA and characterized by X-ray diffraction (XRD). The performance of prepared Fe2O3/Al2O3 oxygen carriers on CO2 yield with syngas and methane combustions, and on H2 generation by H2O oxidation were determined in a fixed bed reactor. The mechanical strength of Fe2O3/Al2O3 pellets was evaluated by texture analyzer; and the reactivity and recyclability of Fe2O3/Al2O3 pellets were evaluated by TGA with continuous 50 redox cycles. EXPERIMENTAL Preparation of Fe2O3/Al2O3 Oxygen Carriers

Alumina-supported Fe2O3 oxygen carriers (Fe2O3/Al2O3) with various Fe2O3 fractions (20, 40, 50, 60 or 80wt%) were prepared by mixing Fe2O3 and Al2O3 powders (approximately 1 µm in particle size) with a ball miller operated at 380 rpm for 5 minutes. The Fe2O3/Al2O3 powders were then sintered in a muffle furnace at 900, 1000, 1100, 1200 or 1300°C for 2 hours. The prepared oxygen carriers were examined for reactivity by a thermogravimetric analyzer (TGA).

For preparation of Fe2O3/Al2O3 pellets for fixed bed experiments, Fe2O3 and Al2O3 powders were mixed in the de-ionized water in a stirring tank equipped for 30 minutes at room temperature. The well-mixed Fe2O3/Al2O3 slurry solution was dried at 80°C for 6 hours, and then pulverized and screened their particle sizes from 1.2 to 1.4 mm. Fe2O3/Al2O3 particles were sintered in a muffle furnace at 1300°C for 2 hours.

The Fe2O3/Al2O3 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 1300°C for 2 hours. The Fe2O3/Al2O3 pellets were employed to 50 redox cycles test by TGA and crush strength test by texture analyzer. Reactivity Tests by Thermogravimetric Analyzer (TGA)

The reactivity and recyclability of Fe2O3 and Fe2O3/Al2O3 oxygen carriers were analyzed by a Netzsch STA 449F3 TGA. 200 mg oxygen carriers were loaded in an alumina crucible, the temperature of TGA chamber was increased with a ramping rate of 20°C/min in N2 atmosphere, and eventually kept at 900°C. 200 mL/min reducing gas composed of 20% H2 and 80% N2 was introduced into TGA chamber for 15 minutes to reduce oxygen carriers. After the reduction process, 200 mL/min N2 was introduced for 5 minutes for sweeping reducing gas contained in the TGA chamber. 200 mL/min air was then introduced for 10 minutes to oxidize the reduced oxygen carriers. The procedure of redox cycling was replicated for 50 times with Fe2O3 and Fe2O3/Al2O3 pellets to evaluate the reactivity and recyclability with respect to practical moving bed operation of chemical

Chiu et al., Aerosol and Air Quality Research, 14: 981–990, 2014 983

looping process. The redox cycling for oxygen carriers was carried out by repeating the procedure described above, while the reducing time was 20 minutes. The oxygen conversion for reduction of oxygen carriers is described by Eq. (5):

( )oOC

o r

m m tX

m m

(5)

where mo is weight of fully oxidized oxygen carrier; mr 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 Fe2O3/Al2O3 during redox cycle was characterized by X-ray diffraction (XRD, Bruker D2 Phaser), the incident beam was Cu Kα characteristic X-ray at 30 kV and 10 mA scanning with rate of 0.05 deg/s in the 2θ range from 10° to 80°. Surface morphology and interfacial behaviors of Fe2O3/Al2O3 oxygen carrier were examined by a field emission scanning electron microscope (FESEM, JOEL JSM-6500F). The crush strength of prepared Fe2O3/Al2O3 oxygen carriers were analyzed by a texture machine (TA.XT plus). Fixed Bed Reactor

Fixed bed reactor system employed in this study is shown

in Fig. 2, and is composed of a 25.4 mm ID stainless steel (SS310) reactor and a PID-controlled heating element covering 200 mm of reactor height. A plate with sixteen 0.25 mm apertures was located in the lower part of the reactor for supporting Fe2O3/Al2O3 oxygen carriers. For reduction of Fe2O3/Al2O3 oxygen carriers with methane and syngas, 1.5 L/min reducing gas was introducing into the fixed bed reactor at various temperatures, ranging from 100 to 900°C. The reducing gases were composed of 20% of CH4 and 80% of N2 for methane combustion and 10% of CO, 10% of H2 and 80% of N2 for syngas combustion, respectively. For H2 generation by steam oxidation of reduced Fe2O3/Al2O3 oxygen carriers, 50 mmol/min of steam was introduced into the fixed bed reactor at various temperatures, ranging from 500 to 900°C. The outlet stream from the fixed bed reactor was passed through a cold trap to condense steam, and 200 mL/min of spent gas was consequently analyzed by a non-dispersive infrared analyzer (Molecular Analysis 6000i) to detect CO, CH4 and CO2 separately by specific light beams, respectively. A gas chromatography analyzer equipped with thermal conductivity detector (Agilent 7890) was employed for measuring H2 concentration. The CO2 yield (YCO2) is defined as:

2

2

2 4 2

100%CO

COCO CO CH H

FY

F F F F

(6)

Fig. 2. Schematic diagram of fixed bed reactor system.

Chiu et al., Aerosol and Air Quality Research, 14: 981–990, 2014 984

where FCO2, FCO, FH2 and FCH4 are the molar flow rates of CO2, CO, H2 and CH4 in the outlet stream, respectively. RESULTS AND DISCUSSION Thermogravimetric Analysis of Fe2O3/Al2O3 Oxygen Carriers

The conversion of oxygen present in oxygen carriers for prepared Fe2O3/Al2O3 with various Fe2O3 fractions was conducted at 900°C by TGA, and is shown in Fig. 3. The conversion of oxygen was increased with the fraction of Fe2O3 for Fe2O3/Al2O3 oxygen carriers containing 20 to 60wt% Fe2O3, whereas decreased for Fe2O3/Al2O3 oxygen carriers containing 80wt% Fe2O3 possibly because of the

sintering of oxygen carriers during reduction and oxidation reactions.

The effect of sintering temperature for preparation of Fe2O3/Al2O3 on the conversion of oxygen present in oxygen carriers is shown in Fig. 4. For experiments conducted with Fe2O3/Al2O3 oxygen carriers sintered at temperature lower than 1000°C, the conversions of oxygen were independent on sintering temperature. For experiments conducted with Fe2O3/Al2O3 oxygen carriers sintered at temperature higher than 1100°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

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XO

C (

%)

Fe2O

3 Fraction (wt%)

Effect of Fe2O

3 Fraction in Fe

2O

3/Al

2O

3 Oxygen Carrier

Reaction temperature: 900°CReducing gas: 20% H

2

Gas flow rate: 200 mL/min

Fig. 3. Oxygen conversion of Fe2O3/Al2O3 oxygen carriers for 20, 40, 60 and 80wt% of Fe2O3 fractions by using H2 as reducing gas conducted by TGA at 900°C.

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Effect of Sintering TemperatureReaction temperature: 900°CReducing gas: 20% H

2

Gas flow rate: 200 mL/min

XO

C (

%)

Sintering Temperature (°C) Fig. 4. Oxygen conversion of Fe2O3/Al2O3 oxygen carriers for 900, 1000, 1100, 1200 and 1300°C of sintering temperature by using H2 as reducing gas conducted by TGA at 900°C.

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materials (Rahaman, 2008). Therefore, solid state sintering was suggested beginning at 1100°C which was exactly higher than 2/3 of melting point of Fe2O3 (1560°C), for well dispersion of Fe2O3 in Fe2O3/Al2O3 oxygen carriers. Thus, the reaction between Fe2O3 and reducing gas was increased as Fe2O3 was more uniformly dispersed in Fe2O3/Al2O3 oxygen carriers by increasing sintering temperature. It was reported that 60wt% of Fe2O3 supported by 40wt% of Al2O3 sintered at 1300°C demonstrated better reactivity (Mattisson et al., 2004), which is corresponded to the results in this study.

Experimental results for Fe2O3/Al2O3 oxygen carrier reduced by syngas for 60 minutes and oxidized by air for 10 minutes in TGA are depicted in Fig. 5. Approximately 70% of oxygen was reduced by syngas combustion within about 21 minutes of reaction time; furthermore, roughly 20wt% of oxygen was reduced from 21 to 60 minutes. The oxygen carrier was sampled at the reduction time at 0, 4, 21 and 60 minutes, and characterized by XRD for phase identification as shown in Fig. 6. Prior to starting reduction process, Fe2O3 and Al2O3 were the major crystalline phase of oxygen carriers. After 4 minutes of reduction, Fe3O4 was found to be the major crystalline phase. The conversion of oxygen for Fe2O3/Al2O3 oxygen carriers was around 10% for 4 minutes of reduction that corresponded to the amount of oxygen utilization from Fe2O3 to Fe3O4. FeO and FeAl2O4 were formed at the end of first stage, indicating that Fe3O4 was further reduced to FeO, and part of FeO reacted with Al2O3 to form FeAl2O4. The formation of FeAl2O4 was corresponded to the previous research reported by Ishida et al. (2005) using Al2O3 supported Fe2O3 in the reduction process. Therefore, reduction of Fe2O3/Al2O3 oxygen carrier in the first stage less than 4 minutes of reaction time, as illustrated in Fig. 5 is suggested by following reactions:

6Fe2O3 + 2CO/H2 → 4Fe3O4 + 2CO2/H2O (7) Fe3O4 + CO/H2 → 3FeO + CO2/H2O (8) FeO + Al2O3 → FeAl2O4 (9)

For oxygen carriers sampled at 21 minutes of reduction, Fe and FeAl2O4 were observed to be the major crystalline phases, while FeO and Fe3O4 were absence in the XRD pattern. The reduction of Fe2O3/Al2O3 oxygen carrier in the second stage (4 to 21 minutes of reaction time) is suggested by the following reactions: FeO + CO/H2 → Fe + CO2/H2O (10) FeAl2O4 + CO/H2 → Fe + Al2O3 + CO2/H2O (11)

The XRD intensity of Fe was enhanced at 60 minutes of reduction indicating that FeAl2O4 was continuously reduced to Fe. Hence, reduction of FeAl2O4 was much slower than Fe2O3 and Fe3O4 resulted in the slow reduction rate in the third stage (21 to 60 minutes of reaction time). The counter-flow moving bed reactor might promote the utilization of oxygen in iron-based oxygen carriers up to 50%, which was much higher than that by the fluidized bed reactors (Li and Fan, 2008). Accordingly, FeAl2O4 and Fe are suggested the major crystalline phases as using Fe2O3/Al2O3 oxygen carriers for a counter-flow moving bed reactor as illustrated in Fig. 6. Hence, the reduction mechanism of Fe2O3/Al2O3 oxygen carriers was proposed in accordance with thermogravimetric analysis and XRD characterization. In addition, the intermediate crystalline phase, FeAl2O4, would serve as the supporting material as well as oxygen carrier in practical operation by moving bed reactor.

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Second stage

Fe/FeAl2O

4

Fe/FeAl2O

4

Fe3O

4/FeO/FeAl

2O

4

Wei

ght (

%)

Time (min)

TG Analysis of Reduction of Fe2O

3/Al

2O

3

Weight of Fe2O

3/Al

2O

3: 200 mg

Reaction temperature: 900CReducing gas: 10% CO+10% H

2

Oxidizing gas: airGas flow rate: 200 mL/min

Fe2O

3+Al

2O

3

First stage

Fig. 5. Oxygen conversion of Fe2O3/Al2O3 oxygen carriers for 900, 1000, 1100, 1200 and 1300°C of sintering temperature by using H2 as reducing gas conducted by TGA at 900°C.

Chiu et al., Aerosol and Air Quality Research, 14: 981–990, 2014 986

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XRD Analysis of Fe2O

3/Al

2O

3 Oxygen Carrier Reduction

Weight of Fe2O

3/Al

2O

3: 200 mg

Reaction temperature: 900 °CReducing Gas: syngas

Fe2O

3/Al

2O

3 reduction for 60 min

Fe2O

3/Al

2O

3 reduction for 21 min

Fresh Fe2O

3/Al

2O

3

Fe2O

3/Al

2O

3 reduction for 4 min

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nsit

y (a

.u.)

2

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●▼

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XRD Analysis of Fe2O

3/Al

2O

3 Oxygen Carrier Reduction

Weight of Fe2O

3/Al

2O

3: 200 mg

Reaction temperature: 900 °CReducing Gas: syngas

Fe2O

3/Al

2O

3 reduction for 60 min

Fe2O

3/Al

2O

3 reduction for 21 min

Fresh Fe2O

3/Al

2O

3

Fe2O

3/Al

2O

3 reduction for 4 min

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nsit

y (a

.u.)

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■

■

■

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Fig. 6. XRD patterns of Fe2O3/Al2O3 oxygen carriers sampled at each time level of reduction at 0, 4, 21 and 60 minutes conducted by TGA at 900°C (■: Fe2O3, □: Al2O3, ◆: Fe, ▲: FeAl2O4, ▼: Fe3O4, ●: FeO).

Fixed Bed Experiments with Fe2O3/Al2O3 Oxygen Carriers The combustions of methane and syngas with Fe2O3/Al2O3

oxygen carriers were employed in a fixed bed reactor at temperatures ranging from 100 to 900°C. As shown in Fig. 7, the YCO2 of syngas combustion was noticeable for experiment conducted at 300°C, and reached 100% for experiments conducted at temperatures higher than 500°C. Based on the thermodynamic analysis by previous researchers (Li et al., 2010), CO and H2 can be completely converted to CO2 and H2O through the reduction from Fe2O3 to Fe3O4 at temperatures ranging from 200 to 1200°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 400°C. The CO2 yields of methane combustion were observed for experiment conducted at 600°C and reached 100% for experiments conducted at temperatures higher than 850°C, as shown in Fig. 7. Therefore, methane combustion with Fe2O3/Al2O3 oxygen carriers was suggested to carry out at temperatures higher than 850°C for chemical looping process, which corresponded to the previous study that natural gas (over 90% of CH4) combustion with iron-based oxygen carriers to achieve complete conversion at 950°C (Abad et al., 2007). Hence, the prepared Fe2O3/Al2O3 oxygen carriers demonstrated high CO2 yields regarding to syngas and methane combustion in the fixed bed reactor.

Experiments with regards to steam oxidation with completely reduced oxygen carriers for H2 generation in a fixed-bed reactor were conducted at temperature varied from 500 to 900°C. As shown in Fig. 8, the flow rate of H2 in the outlet stream was significantly increased with reaction temperature and reached a steady value for experiments conducted at temperature above 800°C. Based on the mole balance calculation, the conversion of H2O to H2 reached

roughly 75% for experiments conducted at temperature above 800°C. The results of conversions were corresponded to the thermodynamic analysis of oxidation of Fe by H2O reported by previous study (Li et al., 2010) that 60 to 70% of H2O would be reduced to form H2 using reduced Fe2O3 oxygen carriers at temperature ranging from 600 to 1000°C. Hence, the prepared Fe2O3/Al2O3 oxygen carriers could be feasible for H2 generation by chemical looping process.

Performance Evaluation of Fe2O3/Al2O3 Oxygen Carriers for Chemical Looping Process

For applications of Fe2O3/Al2O3 oxygen carriers in moving bed reactors, the Fe2O3/Al2O3 oxygen carriers are usually pelletized to provide stronger mechanical strength to sustain the bumping in the reactor system. The crush strength of Fe2O3 is examined to be 33 and 60 N for 1.2 to 1.4 mm and 1.4 to 2.0 mm of particles size, respectively, as listed in Table 1. The crush strength of Fe2O3/Al2O3 particles were lower than Fe2O3 examined to be 22 and 28 N for 1.2 to 1.4 mm and 1.4 to 2.0 mm of particles size, respectively. The interspace between Fe2O3 and Al2O3 particles is possibly greater than Fe2O3 particles because the crystallite size of Fe2O3 is different with Al2O3 particles, which generated more space in the prepared Fe2O3/Al2O3 particles. The comparatively loose packed Fe2O3/Al2O3 particles resulted in lower crush strength than Fe2O3 particles. Pelletizing of Fe2O3 and Fe2O3/Al2O3 particles effectively enhanced their crush strength to be 251 and 230 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 Fe2O3 and Fe2O3/Al2O3 oxygen carriers were used for experiments in TGA to evaluate their reactivity and recyclability.

Chiu et al., Aerosol and Air Quality Research, 14: 981–990, 2014 987

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Y CO

2 (%

)

Reaction Temperature (°C)

1.5 L/min of syngas (10 % CO+10 % H2)

1.5 L/min of CH4 (10 % CH

4)

Oxygen carrier: 100 gFixed Bed Experiments

Fig. 7. CO2 yields for syngas and methane combustions varied with reaction temperatures in the fixed bed reactor packed with 100 g of Fe2O3/Al2O3 particles.

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50H

2 Generation in Fixed Bed Reactor

Fe2O

3/Al

2O

3 oxygen carriers: 10 g

Steam flow rate: 50 mmol/min

Reaction Temperature (°C)

H2 (

mm

ol/m

in)

0

20

40

60

80

100

H2 O

Conversion (%

)

Fig. 8. H2 generation by steam oxidation of reduced Fe2O3/Al2O3 particles in the fixed bed reactor at 500, 600, 700, 800 and 900°C.

Table 1. Crush strength of Fe2O3 and Fe2O3/Al2O3 particles and pellets examined by texture analyzer.

Oxygen Carriers Oxygen Content (wt%) Particle Size (mm) Crush Strength (N)

Fe2O3 30 1.2–1.4 33 1.4–2.0 60

3.0 (pellet) 251

Fe2O3/Al2O3 18 1.2–1.4 22 1.4–2.0 28

3.0 (pellet) 230

The reactivity and recyclability of Fe2O3 and Fe2O3/Al2O3 pellets using syngas as reducing gas were evaluated in TGA for 50 redox cycles. As depicted in Fig. 9, the conversion

of Fe2O3 oxygen carriers was decayed rapidly after the second redox cycle. However, the conversion of Fe2O3/Al2O3 pellet was maintained at above 70% during continuous

Chiu et al., Aerosol and Air Quality Research, 14: 981–990, 2014 988

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. 10. The grain size of fresh Fe2O3/Al2O3 was observed to be larger than fresh Fe2O3 because Fe2O3/Al2O3 pellets were sintered at 1300°C for 2 hours during preparation. The agglomeration of Fe2O3/Al2O3 sintered at 1300°C was also observed in previous literature (Mattisson et al., 2004). As demonstrated in Fig. 10(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 50 redox cycles, no obvious agglomeration was

observed comparing with fresh Fe2O3/Al2O3 pellets, as shown in Fig. 10(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 1780°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.

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Redox Cycling by TGA

XO

C (

%)

Redox Cycle

Reaction Temperature: 900°CReducing gas: 10% CO+10% H

2

Gas flow rate: 200 mL/minReducing time: 20 min

3 mm Fe2O

3 pellet

3 mm Fe2O

3/Al

2O

3 pellet

Fig. 9. Oxygen conversion of Fe2O3 and Fe2O3/Al2O3 pellets using syngas as reducing gas conducted by TGA at 900°C.

(a) Fresh Fe2O3 (c) Fe2O3 after 5 redox cycles

(b) Fresh Fe2O3/Al2O3 (d) Fe2O3/Al2O3 after 50 redox

Fig. 10. SEM images of (a) fresh Fe2O3, (b) fresh Fe2O3/Al2O3, (c) Fe2O3 after 5 redox cycles and (d) Fe2O3/Al2O3 after 50 redox cycles conducted by TGA at 900°C.

Chiu et al., Aerosol and Air Quality Research, 14: 981–990, 2014 989

CONCLUSIONS

Fe2O3/Al2O3 particles containing 60wt% Fe2O3 and sintered at 1300°C is a suitable preparation parameter to demonstrate reasonable oxygen conversion. The reduction mechanism of Fe2O3/Al2O3 oxygen carriers was proposed in accordance with thermogravimetric analysis and XRD characterization. In addition, the intermediate crystalline phase, FeAl2O4, would serve as the supporting material as well as oxygen carrier in practical operation by moving bed reactor. The prepared Fe2O3/Al2O3 oxygen carriers demonstrated high CO2 yields regarding to syngas and methane combustion in the fixed bed reactor, operated at temperatures higher than 500 and 850°C, respectively. Moreover, hydrogen generation was demonstrated to be feasible by steam oxidation with reduced Fe2O3/Al2O3 oxygen carriers for experiments conducted at temperature above 800°C. 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. ACKNOWNLEDGEMENT

This research was supported by grant NSC-98-3114-E-007-013, NSC- 100-3113-E-007-005 and NSC-101-3113-E-007-005 from the National Science and Technology Program-Energy in Taiwan. The authors appreciate China Steel Corp. to provide hematite powders for preparation of oxygen carriers. REFERENCES Abad, A., Mattisson, T., Lyngfelt, A. and Johansson, M.

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Received for review, April 24, 2013 Accepted, August 27, 2013