Fabrication of Fe 2 O 3 / TiO 2 Oxygen Carriers for Chemical Looping Combustion and Hydrogen Generation

The characteristics and application of TiO2-supported Fe2O3 oxygen carriers were investigated for chemical looping combustion (CLC) and hydrogen generation (CLHG) in this study. Prepared Fe2O3/TiO2 oxygen carriers with 20–30 wt% of TiO2 were demonstrated to achieve satisfactory results even after 30-cycle operation by thermogravimetric analyzer (TGA). Oxygen carriers contain more than 33 wt% TiO2 were observed to be more porous and fragile after continuous high-temperature operation. Conversely, the reactivity of oxygen carriers contain less 10 wt% TiO2 was rapidly decayed after operated for merely 3 redox cycles because of their dense structure. The CO2 and H2 conversions through syngas combustion and steam oxidation, respectively, for experiments were demonstrated to be feasible using sieved oxygen carriers at higher operation temperatures from 400 to 1000°C in fixed bed reactor (FBR) system. Keyword: Fe2O3; Oxygen carrier; TiO2; Chemical looping combustion; Chemical looping hydrogen generation.


INTRODUCTION
The impact of global warming induced by the carbon dioxide emission has been an issue of international concern.Numerous technology options are proposed to reduce carbon dioxide (CO 2 ) emissions.Among these technologies, chemical looping combustion (CLC) has been proposed to generate energy with zero to negative carbon emission (Adanez et al., 2012;Reich et al., 2014;Zhou et al., 2014).For CLC process, Fig. 1 demonstrates that carbonaceous fuels are combusted with metal oxides (called oxygen carriers) instead of oxygen molecules in the air.Virtually pure CO 2 may be achieved from the combustion gas after H 2 O is condensed.The reduced oxygen carriers are then regenerated for repeated applications.Moreover, the research efforts of chemical looping have been extended to chemical looping hydrogen generation (CLHG) process.For CLHG, the reduced oxygen carriers are regenerated by steam to generate hydrogen.The operating concept behind CLHG resembles the well-known steam-iron process (Hacker et al., 2000).
Numerous research groups have focused on improving the activity and stability of various oxygen carrying materials for chemical looping operations (Cao et al., 2014;Kuo et al., 2015;Wang et al., 2016).However, most of the oxygen carriers suitable for CLC applications, such as CuO, Mn 2 O 3 , CoO and NiO, may not be suitable for CLHG operations due to the thermodynamic limitations; only iron oxides were reported to be appropriate to serve as oxygen carriers for CLHG (Abad et al., 2007;Kang et al., 2010).Additionally, oxygen carriers must be resistant to mechanical and chemical deterioration by attrition, agglomeration, fragmentation after long-term operations.Thus, oxygen carriers might be supported on various inert materials, such as Al 2 O 3 , CeO 2 , MgO, TiO 2 , ZrO 2 , MgAl 2 O 4 or calcium aluminate cements to retard the various negative effects on oxygen carriers during operation (Hossain and de Lasa, 2008;Manovic and Anthony, 2011;Sun et al., 2015).To date, numerous oxygen carriers composed of various metal oxides and support materials have been synthesized by different techniques and evaluated for their applications (Adánez et al., 2004), mostly following a trial-and-error approach (Ku et al., 2014).TiO 2 -supported iron oxides have been reported to be one of the most promising oxygen carriers because of their high mechanical resistance, proper chemical and physical stability, and considerable high melting point (Cho et al., 2004;Li et al., 2011).
This study is aimed to investigate the reactivity and stability of oxygen carriers prepared with various Fe 2 O 3 and TiO 2 fractions through consecutive redox-cycle operations by thermogravimetric analyzer (TGA), the crystalline phases and surface morphology of the oxygen carriers were identified by X-ray diffraction (XRD) and field-emission scanning electron microscope (FE-SEM), and the specific surface area and pore volume were calculated using the BET method, respectively.Experiments were also conducted in a fixed bed reactor system to discuss the reaction behavior for both CO 2 yield by syngas combustion and H 2 generation by steam oxidation with prepared Fe 2 O 3 /TiO 2 oxygen carriers.

Preparation of Oxygen Carriers
Five oxygen carriers were prepared by mixing Fe 2 O 3 (0.1-0.5 mm, China Steel Corp., 99%) and TiO 2 (< 100 nm, Sigma-Aldrich, 99.9%) powder in a planetary ball mill (PM100, Retsch) at 380 rpm for 2 hours.The mass fractions of and theoretical oxygen contents of these oxygen carriers (named FT-01, FT-02, FT-03, FT-04 and FT-05) were listed in Table 1.All prepared oxygen carriers were pulverized and double sieved to get a size range distribution of 0.3-0.4mm.The sieved powder was then processed into cylindrical pellets of 3 mm in both diameter and height using an automatic rotary tablet press, then subsequently sintered at 1000°C, 1100°C, 1200°C and 1300°C in a muffle oven (Linderg BLUE/M UP550) for 2 hours.

Characterization and Analysis of Prepared Oxygen Carriers
Crystal phase of oxygen carriers were determined by x-ray diffraction (XRD, Bruker D2 PHASER).Incident radiation was generated using a Cu Kα line (λ = 1.5406Å) at 30 kV and 10 mA.The samples were examined at range of 2θ from 10 to 80° with a sampling interval of 0.05° and scan rate of 3° min -1 .The cross-section of oxygen carrier pellets was observed using a field-emission scanning electron microscope (FE-SEM, JEOL JSM-6500F).The crush strength of oxygen carriers was determined by a texture machine (TA.XT plus) following ASTM method D4179.The specific surface area and pore volume of prepared oxygen carriers were determined by a BET analyzer (BELSORP-max, BEL Japan, Inc.).

Thermogravimetric Analysis (TGA) of Prepared Oxygen Carriers
The reactivity and recyclability of prepared oxygen carriers were determined by a thermogravimetric analyzer (TGA, STA 449 F3, NETZSCH).Approximately 30 mg of prepared oxygen carriers was loaded in the alumina crucible of TGA; the temperature of TGA chamber was then increased with ramping rate of 30 °C min -1 in N 2 atmosphere, and finally kept isothermal at 900°C.200 mL min -1 reducing gas containing 20.4% H 2 (balance in N 2 ) was introduced into the TGA chamber for 5 minutes to reduce the oxygen carriers; and then 200 mL min -1 N 2 gas was passed through for 3 minutes to purge reducing gas from the TGA chamber.The reduced oxygen carriers were subsequently regenerated by oxygen with 200 mL min -1 air for 5 minutes.A 30redox cycle operation was conducted into the TGA chamber for each prepared oxygen carriers.

Chemical Looping Operation in a Fixed Bed Reactor
Fig. 2 demonstrates the fixed bed reactor (FBR) system used in this study, which was consisted of a reactor (a SS310 tubular reactor with 25.4 mm in ID, 30.48 in OD and 200 mm in high), a temperature control unit, gas flow control unit, and a gas analysis unit, with referred to our previous study (Ku et al., 2014).For chemical looping combustion (CLC) operation, 50 g of prepared oxygen carrier pellets were packed on a 200 mesh of porous disk located in the reactor.500 mL min -1 syngas (9.8% H 2 and 9.9% CO balanced in N 2 ) was introduced into the reactor and was oxidized by the prepared oxygen carrier; and the FBR was then purged by N 2 to sweep out the residual gas prior to subsequent oxidization of reduced oxygen carriers.For chemical looping hydrogen generation (CLHG) operation, DI water was pre-heated and injected into the reactor at flow rate of 4.0 mmol min -1 with 100 mL min -1 N 2 as carrier gas.The completely reduced oxygen carriers were then oxidized by steam in the reactor to generate hydrogen.All CLC and CLHG experiments were operated at constant temperature ranging from 400 to 1000°C.The exhaust gas from the FBR was passed through a cold trap to condense steam, and spent gas was subsequently analyzed by a non-dispersive infrared analyzer (NDIR, Molecular Analysis 6000i) to detect CO and CO 2 .A gas chromatography equipped with thermal conductivity detector (GC-TCD, Agilent 7890) was employed to measure H 2 and O 2 .

Performance of Prepared Oxygen Carriers
For most CLP applications, the oxygen carriers are usually pelletized to enhance their mechanical strength for withstanding the collision during the operation.As depicted in Table 2, crush strength of prepared oxygen carrier pellets was noticed to be moderately increased from 56.6 to 79.0 N mm -1 with the increase in Fe 2 O 3 content and sintering temperature.Previous researchers has identified that TiO 2 is not only a good inert support for oxygen carriers, but also a good nucleating agent to improve the crystallization property of Fe 2 O 3 (He, 1999).Fe 2 O 3 and TiO 2 are believed to form a solid solution at the high temperature, resulting in the production of a large amount of small nucleus.The small nucleus are subsequently grew to microstructure during sintering process.The mechanism strength of oxygen carriers along with microstructure can be enhanced by increasing the sintering temperature.Besides, Table 2 also shows that Fe 2 TiO 5 is the major crystalline phase for all oxygen carriers prepared at temperature higher than 1000°C, confirming the formation of mixed solid solution of Fe 2 O 3 and TiO 2 (Neri et al., 2004).
Experimental results of the conversion for prepared oxygen carriers reacted with 20.4% H 2 as fuel in the TGA operated for 30 redox cycles were illustrated in Fig. 3. Gas conversions were relatively constant for experiments carried out with FT-01, FT-02, FT-03 and FT-04 oxygen carriers,   (Chiu et al., 2014).This could be ascribed to the strong metal-support interaction (SMSI) effect, which is characterized by significant changes in the physical and chemical properties of metallic particles (e.g., Pt, Pd, Au, Fe, Co and Ni) dispersed over reducible oxides (e.g.TiO 2 ) at temperatures above 500°C via H 2 or CO reduction (Tauster et al., 1978;Nobile Jr and Davis Jr, 1989;Nobile Jr et al., 1991).SMSI refers only to the bond that exists at the interface between the metallic particles and the support.As analyzed by our previous study, the metallic Fe would be formed from Fe 2 TiO 4 or FeTiO 3 phases during the H 2 reduction (Ku et al., 2014).The H 2 might be dissociated by metallic Fe to atom form and distribute over the TiO 2 surface.Thus, the TiO 2 surface might be partially reduced to TiO 2-x , to form Ti 3+ defects and oxygen vacancies (Liu et al., 2003).Belton et al. (1984) has reported that the TiO 2-x was found to migrate considerable distances in short terms and to appear on initially pristine surface of metal under reducing atmosphere (Belton et al., 1984a, b).The migration appears to occur via grain boundaries, since it is far too rapid to be accounted for by diffusion of titanium and oxygen through the metal (Raupp and Dumesic, 1985).In our case, similarly, the reduced TiO 2-x and titania species (e.g., Fe 2 TiO 4-x and FeTiO 3-x ) were able to decorate, partially cover, or even completely mask the metal particles (e.g., metallic Fe) during CLC operation (Ponec and Bond, 1995;Vishwanathan and Narayanan, 1988;Hayek et al., 1997), resulting in the porous structure of titania-contained oxygen carriers.The possible mechanism of this phenomenon is the reductant gas, H 2 , is significantly adsorbed on the metallic Fe.The adsorbed hydrogen atom will react with TiO 2 and iron oxide to produce the Fe 2 TiO 4-x and FeTiO 3-x .
The porosity of titania-contained oxygen carriers is thus increased by the active migration behaviour of titania.Thus understanding of the origin of SMSI and the consequent oxygen carriers activity deserved a new look in CLC area.
Additionally, the effect of SMSI on activity for structuresensitive reaction is frequently disastrous.When TiO 2 was excessive, the SMSI would have an evident effect for Fe 2 O 3 /TiO 2 oxygen carriers, and caused the disintegration of FT-01 and FT-02 structures.Figs.5(a) and 5(b) demonstrates the specific surface area and pore volume for prepared oxygen carriers during the cycling redox operation.The specific surface area and pore volume for FT-01 and FT-02 were relatively higher than those for other oxygen carriers, probably due to the presence of excessive TiO 2 .The porosities of these two samples were increased with cycling time, but the reactivity was not enhanced with the increase of specific surface area.On the contrary, FT-05 with less TiO 2 content, will be aggregated gradually in the CLC process.
In the other hand, the reactivity of FT-05 oxygen carriers with few TiO 2 content was rapidly decayed after operated for only 3 cycles, in agreement with previous experimental observations (Gao et al., 2009;Kang et al., 2010).As shown in Fig. 4(e), the texture of reacted FT-05 pellets seems to be denser than the fresh pellets.This result may be expected because at high operating temperature, excessive sintering may occur and lead to the collapse of the pore structure (Mattisson et al., 2004).FT-03 and FT-04 oxygen carriers consisted of 20-30 wt% TiO 2 could only exhibited great reactivity and maintained complete structure during 30-cycle operation (see Figs. 4(c) and 4(d)).It indicates the CLC activity of iron-based oxygen carrier can be kept at a constant value as its TiO 2 content is larger than 20 wt%.
Based on the TGA experimental results discussed above, both FT-03 and FT-04 oxygen carriers were suitable for subsequent experiments because of their great reactivity and durability.However, FT-04 contains more oxygen that would provide longer reaction time and require lower circulation rate; thus, FT-04 were used for FBR experiments for discussion in the following sections.

Reduction and Oxidation Behavior of Chemical Looping Operation in the FBR System
Investigation on the exhaust gas distribution from chemical looping combustion of syngas (9.8% H 2 and 9.9% CO balanced in N 2 ) were carried out in a FBR filled with FT-04 oxygen carriers operated at different temperatures.The reactions occurring during the syngas combustion are summarized by the Eqs.( 1)-( 5) according to previous studies (Ku et al., 2014;Gao et al., 2009).
Fig. 6 exhibits the composition of exhaust gas from the FBR filled with FT-04 operated at 900°C using syngas as fuel and air as oxidizing gas.CO 2 was formed immediately to achieve virtually full conversion of syngas.The application of FT-04 oxygen carriers can maintain full conversion stage for 50 minutes.The previous study has identified that Fe 2 O 3 and Fe 2 TiO 5 would be converted to Fe 3 O 4 and TiO 2 mixture in this stage (Ku et al., 2014).After about 50 min of reaction time, CO 2 concentration was observed to be decreased rapidly because the oxygen contained of Fe 2 O 3 and Fe 2 TiO 5 phases was increasingly exhausted.This corresponds closely to the level where all Fe 2 O 3 and Fe 2 TiO 5 have reacted to Fe 2 TiO 4 , FeTiO 3 and Fe mixture.Besides, carbon deposition was reported to be formed because of the incomplete combustion of CO during the reduction period via the Bourdouard mechanism (Leion et al., 2008;Sridhar et al., 2012).The carbon deposited oxygen carrier particles can contribute to producing CO as described Eq. ( 6) in the H 2 generation process (Ishida et al., 1998;Jerndal et al., 2006;Wang et al., 2008), lowering the purity of H 2 .
CO and CO 2 were instantly observed in exhaust gas after air was introduced for the regeneration of reduced oxygen carriers, possibly because of the combustion of carbon generated during the reduction period.Oxygen in exhaust gas was untraceable for more than 10 minutes because the oxygen in the air was consumed for the oxidation of reduced oxygen carriers.For the case of using reduced FT-04 at 900°C under syngas, generated carbonaceous gas was only 2.4% of total carbon balance.Therefore, the effect of carbon deposition on FT-04 could be ignored in subsequently evaluation and analysis.

Effect of Reaction Temperature on CO 2 Yield and H 2 Generation
The CO 2 concentration (XCO 2 ) in exhaust gas from the FBR operated at various temperatures (from 400°C to 1000°C) for syngas combustion with FT-04 oxygen carriers was demonstrated in Fig. 7.The combustion of syngas was noticeable for experiment conducted at 400°C, and reached 100% for experiments conducted at temperatures higher than 500°C.Based on the thermodynamic analysis by previous researchers (Wang et al., 2008;Li et al., 2010), CO and H 2 can be completely converted to CO 2 and H 2 O through the exothermic reaction with Fe 2 O 3 (or Fe 2 TiO 5 ) at temperatures ranging from 200 to 1200°C.However, the reaction rates for experiments conducted at 400°C (or lower) might be too slow to reach equilibrium conversion.
The reactivity with H 2 generation was also investigated for different temperatures at the range of 400-1000°C.After reduction and purge stages for 90 and 30 min, respectively, the reduced FT-04 oxygen carriers could be subsequently employed for chemical looping hydrogen generation (CLHG) through steam oxidation, as described by reactions from ( 7) to (9), as analyzed by the previous study (Ku et al., 2014).
Fig. 8 exhibits the H 2 concentration (XH 2 ) of exhaust gas from the FBR for the regeneration of the reduced FT-04 oxygen carriers by stream operated at various temperatures.
The hydrogen conversion is defined as: where FH 2 O and FH 2 are the molar flow rates of H 2 O (l) and H 2 , respectively.In order to produce H 2 , oxygen carriers must be reduced to Fe 2 TiO 4 , FeTiO 3 , FeO or even metallic Fe in the reduction period.Besides, the steam oxidation using Fe 2 O 3 based oxygen carriers is exothermic, so that the H 2 equilibrium constant is at lower temperatures (Fan, 2010).Thus, the temperature adopted in the steam oxidation phase had a limited impact on the total amounts of H 2 gas.For instance, the steady H 2 generation from steam using FT-04 only could be achieved from 16.4 to 69.7% with increasing reaction temperature from 400 to 900°C.Generally, high temperature favors reaction equilibrium to the side product in the reduction; and more importantly, the reduction rate enhanced with increasing temperature.Moreover, steam oxidation only can regenerate oxygen carriers to Fe 3 O 4 , complete oxidation of fuels to CO 2 and H 2 O cannot be achieved in these processes.

CONCLUSIONS
TiO 2 supported Fe 2 O 3 oxygen carriers were prepared and further pelletized.Crush strength of prepared oxygen carrier pellets was observed to be increased with increasing both Fe 2 O 3 content and sintering temperature.Fe 2 O 3 /TiO 2 oxygen carriers contain more than 33 wt% TiO 2 were observed to be fragile and porous after 30 redox cycle  operation of TGA.These could be attributed to influence of the SMSI effect.The disintegration of oxygen carrier structure implies that the interaction between metallic Fe and reduced TiO 2-x or titania species (e.g., Fe 2 TiO 4 and FeTiO 3 ) occurred during the redox cycle operation of TGA.Besides, the effect of both specific surface area and pore volume for Fe 2 O 3 /TiO 2 oxygen carriers were not very significant.Prepared Fe 2 O 3 /TiO 2 oxygen carriers contain 20-30 wt% of TiO 2 and sintered at 1100°C were exhibited to accomplish satisfactory gas conversion and maintain complete structure.CO 2 conversions by syngas combustion achieved almost 100% for experiment conducted in the FBR reactor through temperature operated at 500°C.Besides, H 2 conversions by steam oxidation were achieved almost 70% at 900°C.

Fig. 5 .
Fig. 5. (a) Specific surface area and (b) pore volume for various oxygen carriers during 30 redox-cycle operation of TGA.

Fig. 6 .
Fig. 6.Temporal variation of CO 2 , CO and H 2 concentrations of exhaust gas from syngas combustion with FT-04 oxygen carriers at 900°C.

Fig. 7 .
Fig. 7. Temporal variation of CO 2 concentration of exhaust gas from syngas combustion with FT-04 oxygen carriers at various temperatures.

Fig. 8 .
Fig. 8. Temporal variation of H 2 concentration and conversion from steam oxidation with FT-04 oxygen carriers at various temperatures.
Fig. 2. Schematic layout of laboratory fixed bed reactor system.

Table 2 .
Crystalline phase and crushing strength of prepared Fe 2 O 3 /TiO 2 oxygen carriers.