Chemical Looping Combustion of Isopropanol in Aqueous Solution with Fabricated Fe2O3/Al2O3 and Fe2O3/Al2O3/TiO2 Oxygen Carriers

Received: July 28, 2020
Revised: October 12, 2020
Accepted: October 16, 2020

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.

Cite this article:

Wu, H.C., Ku, Y. (2021). Chemical Looping Combustion of Isopropanol in Aqueous Solution with Fabricated Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃/TiO₂ Oxygen Carriers. Aerosol Air Qual. Res. 21, 200455. https://doi.org/10.4209/aaqr.2020.07.0455

HIGHLIGHTS

• Fe₂O₃/Al₂O₃/TiO₂ and Fe₂O₃/Al₂O₃ oxygen carriers provided with applicable property.
• Nearly 95% of IPA conversions were reached for IPA combustion in MBR.
• Carbon deposition was inhibited for IPA combustion with Fe₂O₃/Al₂O₃/TiO₂.
• Higher reaction rate was observed for Fe₂O₃/Al₂O₃/TiO₂ reduction with hydrogen.

Keywords: Chemical looping, Isopropanol, Reaction kinetics, Hydrogen, Fe2O3, Al2O3, Fabricated, Moving bed reactor

1 INTRODUCTION

Liquid waste combustion by chemical looping technology is aimed to eliminate liquid waste and simultaneously to generate hydrogen and/or heat. For chemical looping combustion (CLC) operation, the metal oxides provided the lattice oxygen to react with the fuel. Subsequently, the reduced particles are oxidized by air for cyclic applications. However, liquid injection is a critical concern for CLC operation, and is greatly influenced by the characteristics of liquid feedstocks. The mode of liquid fuel injection for CLC operation includes direct injection into the combustor, reforming before injection, and vaporization before injection. Because the temperature for the thermal pyrolysis of fuel is usually lower than that for fuel vaporization, fuel reforming to generate combustible gases for combustion with oxygen carriers would be preferred. Satisfactory hydrogen production for chemical looping reforming (CLR) of waste lubricating oil, waste cooking oil, scrap tyre pyrolysis oil (STPO) and other liquid fuels with NiO/Al₂O₃ oxygen carriers were reported by various researchers (Lea-Langton et al., 2010; Pimenidou et al., 2010; Giannakeas et al., 2012). However, the deterioration of hydrogen production was observed after multi-cycle operations, possibly due to the fouling of oxygen carriers by the carbon deposition or poisoning by the trace additives in the fuels. However, Serrano et al. (2017) investigated the application of Fe₂O₃/Al₂O₃ for chemical looping of combustion diesel and lubricant oil, and reported that the reactivity of Fe₂O₃/Al₂O₃ oxygen carriers was not affected by sulphur or impurities present in the fuels. This is because the formation of iron sulfide is thermodynamically feasible only under sub-stoichiometric conditions (fuel-rich), so sulphur does not react with the components existing in the Fe-based oxygen carrier during the combustion process.

Isopropanol (IPA) is a widely used solvent in semiconductor and liquid crystal display (LCD) industries for cleansing wafers and panels in the fabrication process (Ku et al., 2007). Spent solvents of high IPA concentrations, usually 30 wt.% or higher, may be considered to recover IPA. However, further treatment of spent solvents of lower IPA concentrations may be a serious concern for these industries. Chiu et al. (2014a) studied the IPA combustion with Fe₂O₃/Al₂O₃ in a moving-bed reactor, indicating that the IPA conversion and CO₂ yield of IPA combustion reached nearly 100% for experiments conducted at 900°C. The result also indicated that the processing efficiency was declined dramatically for lower IPA content, and when the IPA content is lower than 10%, the processing efficiency would be negative. The heat is possibly insufficient as process heat loss is included in a realistic CLC system for a very dilute solution as fuel in CLC. Hence, the IPA solution containing 10 vol.% IPA was selected to be the target liquid fuel. In this study, alumina- and alumina/titania-supported Fe₂O₃ (Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃/TiO₂) were fabricated to employ as oxygen carriers for the chemical looping combustion of aqueous solution containing 10 vol.% IPA. The reduction kinetics of the fabricated oxygen carriers with hydrogen was examined and described by a shrinking core model (SCM).

 
2 MATERIAL AND METHODS

 
2.1 Preparation of Various Iron-based Oxygen Carriers

In this study, Fe₂O₃/Al₂O₃ oxygen carriers were formulated with 60 wt.% hematite (99.9% Fe₂O₃, China Steel) and 40 wt.% alumina (99% Al₂O₃, Chin Jung). Fe₂O₃/Al₂O₃/TiO₂ were formulated with 70 wt.% hematite, 20 wt.% alumina and 10 wt.% titania. Predetermined amounts of hematite, alumina and titania particles of roughly 1 µm were mixed thoroughly in deionized water at room temperature. The well-mixed slurry was desiccated at 130°C for 10 hours, and was subsequently crushed and partitioned for particles of size between 1.2 and 1.4 mm. The Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃/TiO₂ particles were later sintered in a muffle furnace for 2 hours.

The crush strength of fabricated Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃/TiO₂ was respectively determined to be 30.53 and 10.25 N by a texture machine (TA.XT plus). The attrition of fabricated Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃/TiO₂ was correspondingly determined to be 4.01 and 16.83% by an attrition analyzer following ASTM methods D4058-96. The particle density and porosity of oxygen carriers were measured by the Archimedes method in water. The particle densities of fabricated Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃/TiO₂ were determined to be 2,377 kg m^–3 and 1,937 kg m^–3, while the porosity of 49.13% and 59.60% were measured for fabricated Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃/TiO₂, respectively. The physical properties of these fabricated iron-based oxygen carriers are presented in Table 1.

 
2.2 Establishment of the Fixed-bed Reactor System

The fixed-bed reactor system employed in this study is composed of a stainless-steel tubular reactor and a PID-controlled heating element, as shown in Fig. 1. A plate with sixteen apertures of 0.25 mm in diameter was located in the lower segment of the reactor for supporting fabricated oxygen carriers. The temperature of the loaded reactor was then raised and eventually maintained at designated operating temperature. Hydrogen/nitrogen gas mixture was introduced into the reactor to reduce fabricated oxygen carriers. The outlet gas from the reactor was passed through a cold trap to condense steam, and was consequently analyzed by a non-dispersive infrared sensor (NDIR, Molecular Analysis 6000i) and a gas chromatography equipped with a thermal conductivity detector (GC-TCD, China Chromatography 2000) to detect the concentrations of carbon dioxide, carbon monoxide, methane, hydrogen, and oxygen. After reduction, nitrogen was introduced for sweeping residual gas contained in the reactor. Air at flowrate of 1 L min^–1 was subsequently introduced for 30 minutes to oxidize the reduced oxygen carriers for further replicate operations.

 
Fig. 1. Schematic diagram of the fixed-bed reactor system for CLP operation.

2.3 Establishment of the Moving-bed Reactor System

Schematic diagram of the annular dual-tube moving-bed reactor system (ADMBR) employed in this study is shown in Fig. 2. The reactor system was composed of a stainless-steel dual-tubular reactor and with a PID-controlled heating element, and two screw conveyors. For empty-bed operations of the ADMBR, the temperature for experiments was maintained at 850, 875, or 900°C. Aqueous solution containing 10% IPA was introduced into the inner tube with nitrogen. The gaseous products generated by thermal decomposition of IPA were then flown through the inner tube into the spacing between inner and outer tubes.

Fig. 2. Schematic diagram of the annular dual-tube moving-bed reactor (ADMBR) employed in this study.

For moving-bed operations, the fabricated oxygen carriers were initially packed in the spacing between inner and outer tubes of the ADMBR before operation. Supplementary oxygen carriers were then continuously fed into the packed reactor by a screw conveyer after the reactor was heated up to predetermined temperatures. For iron-based oxygen carrier, the fuel reactor should be operated above 750°C to avoid carbon deposition from methane decomposition (Zeng et al., 2015). However, the iron-based oxygen carrier at an operating temperature above 1200°C may form fusion and sintering, which can cause solid flow and particle reactivity problems. Hence, the operating temperature of the chemical looping system is typically in the range from 750 to 1200°C, according to previous studies (Fan et al., 2015). In this study, the temperature of the ADMBR was operated to about 900°C is due to the limitation of the heating element. IPA solution carried by nitrogen was then introduced into the inner tube for consequent combustion with fabricated oxygen carriers. The reduced oxygen carriers were collectively removed out of the reactor by another screw conveyor. The outlet gas from the reactor was cooled by a cold trap to condense water vapor and was analyzed by a GC-TCD and by a NDIR to detect H₂, CO₂, CO, CH₄, and O₂. The phase characteristics of fresh, reduced and regenerated oxygen carriers were detected by X-ray diffraction (XRD).

3 RESULTS AND DISCUSSION

 
3.1 Decomposition of IPA in the Empty-bed Reactor

Effect of operating temperature on the fuel gas composition on IPA solution decomposition was investigated in an empty-bed reactor. As illustrated in Fig. 3, experimental results suggest that nearly complete IPA decomposition could be accomplished, and the main components of cooled outlet streams were determined to be H₂, CO₂, CH₄ and CO. IPA is assumed to be decomposed to form CH₄ and CO, which are subsequently reacted with H₂O to carry out the methane reforming and water-gas shift reactions, respectively. The reactions involved are described as:

Fig. 3. Effect of operating temperature on the composition of cooled outlet stream for IPA combustion in the empty bed.

However, the exothermic water-gas shift reaction is not favorable for experiments conducted at higher operating temperatures, comparing to the endothermic methane reforming reaction. Thus, CH₄ and CO₂ concentrations were decreased, whereas H₂ and CO concentrations were slightly increased, with increasing operating temperature, comparable to the results reported by previous study (Chiu et al., 2014a).

 
3.2 IPA Combustion with Fabricated Fe₂O₃/Al₂O₃ Oxygen Carriers in the ADMBR

Chemical looping combustion of IPA with fabricated Fe₂O₃/Al₂O₃ was conducted in the ADMBR operated at 900°C. The outlet gas was cooled to condense water vapor before further gas analysis. The main components of outlet streams for experiments operated with different inlet IPA flow rates were determined to be CO₂, CH₄, H₂ and CO, as shown in Fig. 4. Outlet gas containing nearly 100% CO₂ was achieved for experiments conducted with inlet IPA flow rate of 4.1 mmol min^–1. However, CO₂ concentration of outlet gas was found to be decreased, while CH₄ concentration was increased, for experiments carried out with higher inlet IPA flow rates, indicating that methane generated by IPA decomposition was not completely combusted by fabricated Fe₂O₃/Al₂O₃. Moreover, CO concentration of the outlet gas was slightly enhanced for experiments conducted with IPA flow rate ranged from 7.2 to 10.1 mmol min^–1, while H₂ concentration was notably increased.

Fig. 4. Effect of IPA flow rate on the composition of cooled outlet stream for IPA combustion with Fe₂O₃/Al₂O₃ in the ADMBR.

The carbon deposition for experiments carried out with higher IPA flow rate were observed which may be ascribed to the cracking of methane generated by IPA decomposition was further decomposed to form carbon and hydrogen, as depicted by Reaction (4) (Cho et al., 2005; Ku et al., 2014).

Based on carbon and hydrogen balance calculation, the steam generation rate (F_Steam) and carbon deposition rate (F_C) for IPA combustion are determined by Eqs. (5) and (6):

where F_IPA,in and F_H2O,in are the inlet mole flow rate (mmole min^–1) of IPA and H₂O, respectively; F_i is the outlet molar flow rate (mmole min^–1) of species i, i is denoted as CO₂, CO, H₂O, H₂ and CH₄. As shown in Fig. 4, the carbon deposition rate and steam generation rate of IPA combustion by fabricated Fe₂O₃/Al₂O₃ in the ADMBR were found to be increased for experiments conducted with higher IPA flow rates.

The carbon conversion (X_C) is defined as the conversion of inlet IPA to carbonaceous gases (CO₂, CO and CH₄), as described as Eq. (7). IPA conversion (X_IPA) and oxygen carrier conversion (X_OC) for IPA combustion were determined by Eqs. (8) and (9) (Zeng et al., 2015; Wu and Ku, 2016). The detailed mass balance data of the IPA combustion experiment, such as oxygen, carbon, hydrogen, and iron, were listed in Table 2.

where ṁ_OC is the mass flow rate (g min^–1) of the oxygen carriers.

As illustrated in Fig. 5, X_C and X_IPA are significantly decreased for experiments carried out with increasing inlet IPA flow rate, while less than 20% of the oxygen carrier conversion was achieved for most experiments, demonstrating the fabricated Fe₂O₃/Al₂O₃ was reduced mostly to Fe₃O₄, similar to the results reported by previous study (Luo et al., 2014).

Fig. 5. Effect of IPA flow rate on fuel and oxygen carrier conversions for IPA combustion with Fe₂O₃/Al₂O₃ in the ADMBR.

Based on the XRD patterns illustrated in Fig. 6, the main crystalline phases of reduced Fe₂O₃/Al₂O₃ for IPA combustion operated in the ADMBR, and the crystalline phases of reduced Fe₂O₃/Al₂O₃ were mostly Fe₃O₄ and Al₂FeO₄. Comparable observation was reported by previous researchers (Ishida et al., 2005; Ku et al., 2014). Zhu et al. (2016) analyzed the structural evolution during the reduction of α-Fe₂O₃ nanowires, and noticed that more oxygen vacancies were formed as the reduction continues. Thus, the rhombohedral-structured α-Fe₂O₃ was transformed to the cubic-structured Fe₃O₄. In this study, Fe₃O₄ was further reduced to form Al₂FeO₄. Al₂FeO₄ generated might serve as support materials as well as oxygen carriers in a moving-bed reactor for practical chemical looping operation, as stated by previous study (Chiu et al., 2014b). Subsequently, the reduced oxygen carriers were completely oxidized to Fe₂O₃ and Al₂O₃, which were observed in the XRD pattern.

Fig. 6. X-ray diffraction patterns of fresh, reduced and regenerated Fe₂O₃/Al₂O₃ for IPA combustion.

 
3.3 IPA Combustion with Fabricated Fe₂O₃/Al₂O₃/TiO₂ in the ADMBR

Chemical looping combustion of IPA by fabricated Fe₂O₃/Al₂O₃/TiO₂ in the ADMBR was examined in this study. The composition of cooled outlet streams for combustion experiments of IPA was illustrated in Fig. 7. Outlet gas containing more than 97% CO₂ were achieved for experiments conducted with inlet IPA flow rate lower than 7.2 mmol min^–1, while the CO₂ concentration of outlet gas was dropped for experiments carried out with higher IPA flow rates. More CH₄, H₂ and CO were observed in the outlet gas for experiment conducted with IPA flow rate greater than 8.7 mmol min^–1, possibly because part of the combustible gas generated via Reactions (1) to (3) was not consumed by Fe₂O₃/Al₂O₃/TiO₂. However, CO₂ and CH₄ concentration of the outlet gas were found to be increased for experiment carried out with inlet IPA flow rate of 10.1 mmol min^–1 than that with inlet IPA flow rate of 8.7 mmol min^–1, whereas H₂ and CO concentrations were decreased. Compared with the results by Fe₂O₃/Al₂O₃, more IPA was oxidized by Fe₂O₃/Al₂O₃/TiO₂ to generate more CO₂ and H₂O with less H₂ and CO. The calculated steam generation rates for IPA combustion with Fe₂O₃/Al₂O₃/TiO₂ were higher than those with Fe₂O₃/Al₂O₃, as demonstrated in Fig. 7.

Fig. 7. Effect of IPA flow rate on the composition of cooled outlet stream for combustion of IPA with Fe₂O₃/Al₂O₃/TiO₂ in the ADMBR

As shown in Figs. 4 and 7, less carbon deposition were observed for IPA combustion with Fe₂O₃/Al₂O₃/TiO₂ than that with Fe₂O₃/Al₂O₃. CH₄ concentration of outlet gas was maintained at around 10% for IPA combustion with Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃/TiO₂ with higher IPA flow rate. Fig. 8 demonstrates that carbon conversion, IPA conversion and oxygen carrier conversion for experiments conducted with Fe₂O₃/Al₂O₃/TiO₂ in the ADMBR were higher than those with Fe₂O₃/Al₂O₃.

Fig. 8. Effect of IPA flow rate on fuel and oxygen carrier conversions for IPA combustion with Fe₂O₃/Al₂O₃/TiO₂ in the ADMBR.

The fabricated Fe₂O₃/Al₂O₃/TiO₂ oxygen carriers for IPA combustion operated in the ADMBR were identified. As shown in Fig. 9, Fe₂O₃, Fe₂TiO₅ and Al₂O₃ were the major crystalline phases of fresh Fe₂O₃/Al₂O₃/TiO₂, and the crystalline phases of reduced oxygen carriers were mostly Fe₃O₄, Al₂FeO₄, Al₂O₃ and TiO₂. Hence, the rhombohedral-structured Fe₂O₃ and the orthorhombic-structured Fe₂TiO₅ were completely reduced to the cubic-structured Fe₃O₄ and Al₂FeO₄, comparable observations was previous reported by previous researchers (Abad et al., 2011; Zhu et al., 2016). For regenerated oxygen carriers, Fe₂O₃, Fe₂TiO₅ and Al₂O₃ were observed in the XRD pattern, demonstrating that the reduced oxygen carriers were completely oxidized.

 Fig. 9. X-ray diffraction patterns of fresh, reduced and regenerated Fe₂O₃/Al₂O₃/TiO₂ for IPA combustion.

3.4 Kinetic Parameter Determination for the Reduction of Fabricated Oxygen Carriers by Hydrogen

Because hydrogen is the major combustible gas generated by IPA decomposition at temperature above 800°C, the reduction of ferric-oxide oxygen carriers by hydrogen is described by the following simplified reaction (Fan, 2010):

Based on the mass balances of oxygen and hydrogen, the conversion of oxygen carriers is determined as:

where F_H2,in is the inlet mole flow rate (in mmole min^-1) of H₂; F_H2 and F_H2O are the outlet molar flow rates (in mmole min^–1) of H₂ and H₂O, respectively; m_OC is the weight (g) of the fabricated oxygen carriers packed in the reactor; x_Fe2O3 is the fraction of Fe₂O₃ contained in the oxygen carriers; M_Fe2O3 is 159.69 g mole^–1 as the molecular weight of Fe₂O₃.

The reduction of fabricated Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃/TiO₂ by feeding H₂/N₂ gas mixtures containing 5 to 20 vol.% H₂ was examined in the fixed-bed reactor operated at 875, 900 and 925°C. As shown in Fig. 10, the calculated reduction conversions for experiments conducted with Fe₂O₃/Al₂O₃/TiO₂ were evidently higher than those with Fe₂O₃/Al₂O₃ after 4,500 second of operation time. For experiments conducted after the Fe₂O₃/Al₂O₃ conversion was observed to be maintained at about 33.33% after 4,500 seconds. The reduction of fabricated oxygen carriers by hydrogen were barely influenced by operating temperature, similar to the observations reported by previous researchers (de Diego et al., 2014; Abad et al., 2015). Fig. 11 exhibits the effect of inlet hydrogen concentrations on the chemical looping combustion of H₂ by fabricated oxygen carriers. The conversions of Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃/TiO₂ reduction were increased for experiments conducted with higher inlet hydrogen concentrations, as described by Reaction (10).

Fig. 10. Effect of operating temperature on the reduction of fabricated iron-based oxygen carriers by hydrogen in the fixed-bed reactor.

Fig. 11. Effect of inlet hydrogen concentration on the reduction of fabricated iron-based oxygen carriers in the fixed-bed reactor.

The kinetics for the reduction Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃/TiO₂ by hydrogen were calculated with the shrinking core model (SCM), exercised by most researchers for the application of various oxygen carriers for chemical looping (Abad et al., 2007; Cabello et al., 2014; Abad et al., 2015; Wu and Ku, 2018; Wu and Ku, 2019), as described in Eqs. (13) to (18):

where R and r are the radius (m) of the fresh particle and the unreacted core; τ_gf and τ_pl are the time required for complete conversion of the oxygen carrier when the reaction is controlled by the gas-film diffusion and the product-layer diffusion, respectively. The product-layer (Fe₃O₄, Al₂FeO₄, FeTiO₃, FeO, or Fe) generated on the surface of the reduced Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃/TiO₂ is a barrier for the diffusion of fuel molecules and significantly decelerated the reaction rate, especially when high reduction conversions were achieved. C_Ag is the inlet molar concentration (mole m^–3) of the gaseous fuel; b is the stoichiometric coefficient of gaseous fuel combusted with Fe₂O₃, which is determined to be 1/3 by Reaction (10); k_g is the mass transfer coefficient (m s^–1) between gaseous fuel and oxygen carriers; D_e is the effective diffusion diffusivity (m² s^–1) of gaseous fuel in the product layer; D_pl,0 (m² s^–1) and Ea_pl (kJ mol^–1) are the pre-exponential factor and activation energy for the product-layer diffusion, respectively; k_d, k_d,0 and Ea_d (kJ mol^–1) are the decay constant, pre-exponential factor and activation energy for the product-layer diffusivity, respectively; R_g is the ideal gas constant; T (K) is the operating temperature.

The kinetic parameters for the reduction of prepared iron-based oxygen carriers by hydrogen were summarized in Table 3. The mass transfer coefficients (k_g) for the reduction of Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃/TiO₂ were calculated to be 0.22 and 0.24 mm s^–1, respectively; demonstrating that hydrogen molecules can more easily diffuse through the product-layer on Fe₂O₃/Al₂O₃. The higher reduction conversions obtained for experiments conducted with Fe₂O₃/Al₂O₃/TiO₂ than those with Fe₂O₃/Al₂O₃ indicates that Fe₂O₃/Al₂O₃ is primarily reduced to FeO, while the Fe₂O₃/Al₂O₃/TiO₂ can be further reduced to FeO and Fe. The crystal structure of Fe is denser than that of FeO, and is probably more difficult for hydrogen molecules to pass through the product-layer generated on Fe₂O₃/Al₂O₃/TiO₂. Therefore, the mechanism for the overall reduction reaction of Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃/TiO₂ oxygen carriers with H₂ are proposed as Eqs. (19) to (22) based on the experimental results of this study.

For Fe₂O₃/Al₂O₃/TiO₂ reduction:

 
4 CONCLUSIONS

Hydrogen is the major combustible gases generated during IPA decomposition at above 800°C. Approximately, more than 95% IPA was combusted for most experiments conducted in the moving-bed reactor with fabricated Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃/TiO₂ operated at inlet IPA flow rate ranged from 4.1 to 5.8 mmol min^–1 and 4.1 to 7.2 mmol min^–1, respectively; in addition, the oxygen carrier conversions were respectively reached less than about 16.3% and 17.5%. Carbon deposition during IPA combustion using Fe₂O₃/Al₂O₃/TiO₂ as an oxygen carrier was noticeably avoided. According to the XRD characterization, the cubic structure of Fe₃O₄ and Al₂FeO₄ are the major crystalline phases generated during the chemical looping combustion of IPA with Fe₂O₃/Al₂O₃. For experiments conducted with Fe₂O₃/Al₂O₃/TiO₂, Fe₃O₄, Al₂FeO₄, Al₂O₃, and TiO₂ were identified by the XRD pattern, demonstrating Al₂FeO₄ generated might serve as support materials as well as oxygen carriers during chemical looping combustion. According to the shrinking core model, the mass transfer coefficients (k_g) of Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃/TiO₂ reduction with H₂ were found to be 0.22 and 0.24 mm s^–1, while the activation energy for the product layer diffusion reaction (E_pl) estimated were about 450 and 37 kJ/mole, respectively. It was noticed that the effective diffusion diffusivity (D_e) of Fe₂O₃/Al₂O₃ oxygen carriers was more easily depended on the oxygen carrier conversion, due to high decay constant (k_d) was obtained for experiments with Fe₂O₃/Al₂O₃ than those with Fe₂O₃/Al₂O₃/TiO₂. Therefore, the reduction of fabricated Fe₂O₃/Al₂O₃/TiO₂ by hydrogen was observed to further progress to FeO and Fe, and the conversion was obviously higher than that of Fe₂O₃/Al₂O₃, which was primarily reduced to FeO. Hydrogen molecules are found to diffuse more easily through the FeO product-layer on Fe₂O₃/Al₂O₃ than the FeO/Fe product-layer on Fe₂O₃/Al₂O₃/TiO₂. However, both Fe₂O₃/Al₂O₃ and Fe₂O₃/Al₂O₃/TiO₂ reductions were markedly enhanced with increasing inlet hydrogen concentration and were not obviously influenced by operating temperatures.

ACKNOWLEDGMENTS

This research was supported by Grant MOST 106-3113-E-007-002- from the National Science and Technology Program-Energy, Taiwan, and by Grant MOST 105-2622-E-011-019-CC2 and MOST 103-2221-E-011-002-MY3 from the Ministry of Science and Technology, Taiwan. The authors appreciated China Steel Corp. for providing hematite powders for the preparation of oxygen carriers.