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

Iron-based oxygen carriers supported on alumina or alumina/titania were fabricated and evaluated for chemical looping combustion of isopropanol (IPA). Hydrogen is the major combustible gas generated by IPA decomposition prior to combustion with oxygen carriers at temperatures above 800°C. Nearly complete combustion (above 95%) of IPA was achieved for experiments conducted with fabricated Fe2O3/Al2O3 and Fe2O3/Al2O3/TiO2 operated at lower inlet IPA flow rates. Carbon deposition during the chemical looping combustion of IPA was minimized using Fe2O3/Al2O3/TiO2 as an oxygen carrier. The reduction of Fe2O3/Al2O3 and Fe2O3/Al2O3/TiO2 by hydrogen was markedly increased with increasing inlet hydrogen concentration (5–20%), and was not obviously influenced by operating temperature (875–925°C). According to the shrinking core model, the mass transfer coefficients (kg) of Fe2O3/Al2O3 and Fe2O3/Al2O3/TiO2 reduction with H2 were found to be 0.22 and 0.24 mm s–1, while the effective diffusion diffusivity (De) of Fe2O3/Al2O3 oxygen carriers was more easily depended on the oxygen carrier conversion. The higher reduction conversions obtained for experiments conducted with Fe2O3/Al2O3/TiO2 because it can be further reduced to FeO and Fe; comparing to those with Fe2O3/Al2O3, which is 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.


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 substoichiometric 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).

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.

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.

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.
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 . 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 . 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).

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: 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).

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.
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 (FSteam) and carbon deposition rate (FC) for IPA combustion are determined by Eqs. (5) and (6): where FIPA,in and FH 2 O,in are the inlet mole flow rate (mmole min -1 ) of IPA and H2O, respectively; Fi 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 (XC) is defined as the conversion of inlet IPA to carbonaceous gases (CO2, CO and CH4), as described as Eq. (7). IPA conversion (XIPA) and oxygen carrier conversion (XOC) for IPA combustion were determined by Eqs. (8) and (9) 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 ṁOC is the mass flow rate (g min -1 ) of the oxygen carriers. As illustrated in Fig. 5, XC and XIPA 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).
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.

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. 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. 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 orthorhombicstructured Fe2TiO5 were completely reduced to the cubic-structured Fe3O4 and Al2FeO4,   Oxygen carrier conversion Carbon conversion IPA conversion Fe 2 O 3 /Al 2 O 3 /TiO 2 oxygen carrier reduced by 10.11 mmole min -1 IPA 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.

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 FH 2 ,in is the inlet mole flow rate (in mmole min -1 ) of H2; FH 2 and FH 2 O are the outlet molar flow rates (in mmole min -1 ) of H2 and H2O, respectively; mOC is the weight (g) of the fabricated oxygen carriers packed in the reactor; xFe 2 O 3 is the fraction of Fe2O3 contained in the oxygen carriers; MFe 2 O 3 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   11. Effect of inlet hydrogen concentration on the reduction of fabricated iron-based oxygen carriers in the fixed-bed reactor.
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 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). 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; τgf and τpl 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. CAg 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); kg is the mass transfer coefficient (m s -1 ) between gaseous fuel and oxygen carriers; De is the effective diffusion diffusivity (m 2 s -1 ) of gaseous fuel in the product layer; Dpl,0 (m 2 s -1 ) and Eapl (kJ mol -1 ) are the pre-exponential factor and activation energy for the product-layer diffusion, respectively; kd, kd,0 and Ead (kJ mol -1 ) are the decay constant, pre-exponential factor and activation energy for the product-layer diffusivity, respectively; Rg 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 (kg) 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 productlayer 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)

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 Table 3. Kinetics parameters for the reduction of fabricated iron-based oxygen carriers by H2.

Parameter
Symbol Unit Fe2O3/Al2O3 Fe2O3/Al2O3/TiO2 Mass transfer coefficient kg mm s -1 0.22 0.24 Pre-exponential factor for the product layer diffusion Dpl,0 mm 2 s -1 4.79 × 10 23 3.20 Activation energy for the product layer diffusion Eapl kJ mol -1 450.28 37.32 Pre-exponential factor for the product layer diffusivity kd,0 -501.80 6.50 × 10 8 Activation energy for the product layer diffusivity Ead kJ mol -1 26.25 214.09 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 (kg) 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 (Epl) estimated were about 450 and 37 kJ/mole, respectively. It was noticed that the effective diffusion diffusivity (De) of Fe2O3/Al2O3 oxygen carriers was more easily depended on the oxygen carrier conversion, due to high decay constant (kd) 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.