Al2O3 Using the Solution

Article pubs.acs.org/IECR

Preparation and Characterization of Fe2O3/Al2O3 Using the Solution Combustion Approach for Chemical Looping Combustion Jianshe Zhang, Qingjie Guo,* Yongzhuo Liu, and Yu Cheng College of Chemical Engineering, Qingdao University of Science and Technology, Key Laboratory of Clean Chemical Processing Engineering of Shandong Province, Qingdao 266042, China ABSTRACT: Chemical looping combustion (CLC) is an attractive technology for CO2 capture with high energy efficiency. In this article, an Fe2O3/Al2O3 (Fe:Al = 3:1) oxygen carrier was first prepared by the solution combustion approach for the CLC process. The prepared oxygen carrier was characterized by different means. XRD identification has substantiated the necessity of calcinations to synthesize Fe2O3/Al2O3 oxygen carrier. SEM and TEM images showed the regular spherical and cubical shape and abundant porous structure in Fe2O3/Al2O3 oxygen carrier, respectively. Structural characteristics displayed that the pore shape of Fe2O3/Al2O3 particles was heterogeneous. The average pore size and surface area were 64.76 nm and 4.01 m2/g, respectively. Further, H2 temperature programmed reduction (TPR) of Fe2O3/Al2O3 oxygen carrier indicated that the reduction reaction had only one distinct DTG peak with the weight loss rate reaching 4.75 wt %/min. Finally, five cycles of red−ox reaction by alternating with CH4 and air demonstrated that Fe2O3/Al2O3 oxygen carrier had excellent reactivity and sintering resistance and consequently was capable of the CLC process.

1. INTRODUCTION The increasing carbon dioxide concentration of the atmosphere is widely believed to cause the global warming. It has increased significantly over the past decades as a result of the dependency on fossil fuels for energy production, from a preindustrial value of 280 to 390 ppm in 2010. To ensure the increase of average temperature was lower than 2 °C, which is considered as the limit to prevent the most catastrophic changes in the earth, the CO2 concentrations must not exceed 450 ppm.1 In recent years, a large number of techniques for CO2 capture have been proposed, such as, precombustion, oxy-fuel combustion, postcombustion separation, etc. However, a high energy penalty and costs are required for separating CO2 from the flue gas components, leading to a considerable decrease of the overall combustion efficiency. Apparently, a CO2 separation technology without the energy penalty would obviously be of great interest.2 Chemical-looping combustion (CLC) offers a new approach for CO2 separation without extra energy consumption. CLC uses a solid oxygen carrier to transfer the oxygen from the air to the fuel. Compared with the normal combustion, the technique possesses an advantage of CO2 inherently separation from the other components of the flue gas (mainly N2 and H2O) without the extra energy. The CLC system is composed of two reactors, an air and a fuel reactor, as depicted in Figure 1. The fuel is fed into the fuel reactor where it is oxidized by the lattice oxygen of the oxygen carriers according to reaction R1. The exit gases from the fuel reactor contain only CO2 and H2O. Thus, the pure CO 2 can be readily sequestrated by condensation.

Figure 1. Schematic illustration of a chemical-looping combustion.

The reduced metal oxide MyOx−1 is transported to the air reactor where it is reoxidized based on the reaction R2. MexOy − 1 + 1/2O2 → MexOy

For a given fuel/oxygen carrier combination, the reaction heats can be calculated from the thermodynamic data. Consequently, the fuel flow rate and the oxygen carrying capacity of the oxygen carrier can be established. These parameters determinate the solid circulation rate between fuel and air reactors.3 The reaction between the fuel and oxygen carrier in the fuel reactor is endothermic or exothermic depending on the metal oxide used, while the reaction in the air reactor is always exothermic. The CLC does not bring any enthalpy gains. Therefore, the total heat generated in these two reactions is the same as that of normal combustion in air.4 The performance of oxygen carrier is the key to CLC process. A good oxygen carrier for CLC should provide the Received: Revised: Accepted: Published:

(2n + m)MexOy + CnH 2m → (2n + m)MexOy − 1 + mH 2O + nCO2 © 2012 American Chemical Society

(R2)

(R1) 12773

July 8, 2012 August 26, 2012 September 6, 2012 September 6, 2012 dx.doi.org/10.1021/ie301804c | Ind. Eng. Chem. Res. 2012, 51, 12773−12781

Industrial & Engineering Chemistry Research

Article

The particles obtained from this method showed good reactivity for multiple cycles and high crushing strength. Table 2 presents a summary of oxygen carriers prepared by solution combustion approach in previous work.

lattice oxygen for the fuel and regenerate by the oxidation of oxygen from air. Up to now, the studies on oxygen carriers have concentrated on metal oxides such as copper-, iron-, nickel-, and mixed-oxide based oxygen carriers5,6 and calcium sulfate.7 Although the Fe-based oxygen carrier particles have relatively slow reduction kinetic rate, they possesses favorable thermodynamic properties with less cost, physical strength, high melting temperature, and fewer environmental concerns.8 For the Febased oxygen carrier, different oxidation states can be found when Fe2O3 is reduced (Fe3O4, FeO, or Fe). Due to thermodynamic limitations, only the transformation from Fe2O3 to Fe3O4 may be applicable for industrial CLC systems based on interconnected fluidized beds. Further reduction to FeO or Fe would produce a high decrease in the CO2 purity obtained in the fuel reactor because of the increase in the equilibrium concentrations of CO and H2.9 However, reduction of Fe2O3 to FeO or Fe systems could be exploited with full combustion if special configurations of the fuel reactor was used, e.g. moving bed. A variety of materials has been employed as supports for this kind of oxygen carrier (Al2O3, MgAl2O4, SiO2, TiO2, etc.), with Al2O3 being the most usual. As mentioned earlier, the use of Al2O3 as support has a positive effect on the oxygen transport capacity of the oxygen carrier if FeAl2O4 is formed.10 Table 1 listed a summary of characteristics for Fe2O3 oxygen carrier supported on Al2O3 tested in the CLC process.

Table 2. Summary of Oxygen Carriers Prepared by the Solution Combustion Approach metal oxide NiO NiO NiO CuO Fe2O3 (NiO)1−y(MgO)y/ Ni(1−x)MgxAl2O4

facility

mechanical mixing mechanical mixing + pelletizing by extrusion impregnation impregnation impregnation spray drying dissolution freeze granulation

TGA TGA, batch fluidized bed

CH4 CH4

11 12

CLC (500 W) batch fluidized bed TGA, batch fluidized bed fixed bed TGA fixed bed, batch fluidized bed TGA TGA, pressurized TGA, batch fluidized bed, CLC (300 W)

PSA-off gas CH4 CH4 H2 H2, CO CH4

13 14 15 16 17 18

H2, CH4, CO nature gas, H2, CH4, CO, syngas

19 20−22

freeze granulation freeze granulation

fuel

facility

fuel

ref

MgAl2O4 NiAl2O4

TGA TGA TGA TGA

H2 CH4, syngas CH4, H2, syngas syngas, H2S

TGA TGA

H2 CH4

25 26 25, 26 10, 27 28 29

bariumhexaaluminate Al2O3

In this work, Fe2O3/Al2O3 oxygen carrier with molar ratio 3:1, which adopted cheap hydrated metal nitrates and urea as precursors, was first prepared by solution combustion approach for CLC process. By the comprehensive characteristics of staged products using FTIR and DTG, the formation mechanism of Fe2O3/Al2O3 oxygen carrier was obtained. In addition, the investigation into the characteristics of the prepared oxygen carrier including phase structure, morphology, and pore structure was carried out. Finally, the reaction and regeneration characteristic of the prepared oxygen carrier was investigated.

Table 1. Summary of Fe2O3/Al2O3 Tested in the CLC Progress preparation method

support material

ref

2. EXPERIMENTAL PROCEDURES 2.1. Materials. Previous investigations26,29 used glycine, citric acid, etc. as fuel in solution combustion synthesis. In particular, urea was suggested as the most convenient fuel for its ready availability and high exothermicity30,31 as well as effective complexion of amine group (i.e., −NH2) from urea with transition metals.32 Hydrated metal nitrates were chosen as the suitable oxidizer for its good solubility and lower decomposition temperature. In summary, the materials used in the present preparation were Al(NO3)3·9H2O (AR, 99.0%, BASF), Fe(NO3)3·9H2O (AR, 98.5%, BASF), and urea (AR, 99.0%, Alfa Aesar). 2.2. Preparation Procedure. The Fe2O3/Al2O3 oxygen carrier was prepared by solution combustion approach through the following procedure. First, stoichiometric compositions of hydrated metal nitrates, including Fe(NO3)3·9H2O, Al(NO3)3·9H2O, and urea were calculated at the desired molar ratio to yield 4 g Fe2O3/Al2O3 oxygen carrier particles. In terms of propellant chemistry theory, the molar ratio of Fe(NO3)3·9H2O, urea, and deionized water to Al(NO3)3·9H2O was correspondingly determined as 3, 3.5, and 31, ensuring that the equivalent ratio of the reducing valence of urea to the total oxidizing valence of metal nitrates was unity while the released heat reaches its maximum.33 Calculated amounts of Fe(NO3)3·9H2O, Al(NO3)3·9H2O, and urea were accurately weighed and dissolved in a beaker using deionized water as shown in Figure 2a, to ensure complete mixing. The mixture was then stirred on a hot plate exposed to air and aged at 60 °C until viscous colloidal suspension (sol) appeared in yellowish-red, as illustrated in Figure 2b. Then, the viscous sol was dried at 110 °C sequentially calcinated in an oven, as illuminated in Figure 2c. As Figure 2d depicts, the prepared dried gel was transferred

As described in Table 1, there are several methods for synthesis of oxygen carriers, including mechanical mixing, freeze granulation, dry impregnation, incipient wet impregnation, sol−gel, coprecipitation and dissolution, and solution combustion.23 The solution combustion is a relatively new method adopted for the synthesis of the chemical looping medium (oxygen carrier).24 The process involves dissolving the metal and support nitrates in the desired ratio in deionized water along with urea (combustible agent), respectively. The solution is heated until most of the water is driven out, and then is ignited. After combustion, the metal-oxide-based oxygen carrier particles are obtained. The oxygen carrier is ground in a mill to the desired particle size, and then calcined. This process is characterized by many advantages, such as low energy requirements, molecular scale mixing of precursors, desired composition, and microstructure. Moreover, the time-consuming drying steps are also avoided compared with other methods. 12774

dx.doi.org/10.1021/ie301804c | Ind. Eng. Chem. Res. 2012, 51, 12773−12781


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Figure 2. Images of the products at the different preparation stages: (a) sol solution; (b) viscous gel; (c) dried gel; (d) as-burnt product; (e) calcined product.

To avoid the explosion between H2 and air directly, before the reducing reaction of Fe2O3/Al2O3 with H2, the reaction chamber of the reactor was at first pumped air into vacuum state, and then high pure nitrogen was introduced into the reaction chamber for 10 min. To evaluate the cycle performance for the oxygen carrier, five cycles of red−ox reactions were explored, CH4 (50 vol %) with balance N2 and air alternate to react with oxygen carrier particles. To prevent the carbon formation of CH4, the reaction temperature remained at 850 °C. 2.4. Complexation and Combustion Mechanism. The mechanism of oxygen carrier preparation by solution combustion approach mainly includes complexation and combustion mechanism. As expected, the complextion mechanism is significant for the process of gel formation, while the combustion mechanism plays a vital role on the synthesis of oxygen carrier. Urea decomposes initially to biuret and NH3 at 198 °C and, at higher temperatures, to cyanic acid (HCNO) trimmer, while Fe(NO3)3·9H2O and Al(NO3)3·9H2O decompose to Fe2O3 and Al2O3, respectively, followed by a series of temperature stages (evaporation, dehydration, and dehydroxylation reaction).31,35,36 The behaviors of urea and nitrates while heating differ from those of their heating separately. The complex ligand would be formed by complexation, increasing solubility of metal ions in aqueous solution and stopping the formation of crystal segregation of metal ions with water evaporated; therefore, the synthesis method via complexation of urea and nitrates is well suited for the preparation of metal oxide. By analyzing the FTIR spectra of metal (Al)−urea complexes, the formation of oxygen-to-metal coordinate bonds (CO → M) was determined by Qiu et al.37 To further confirm the structure of the metal (Al)−urea complex, the 13C CP/MAS NMR and 27 Al MAS NMR spectra were presented. The 13C sharp resonance peak is assigned for the carbon atoms in urea, while the 27Al peak corresponds to the octahedral AlO6 coordinate bond, which proves the existence of MO bonds. Meanwhile, the extremely narrow half-height width of the 27Al resonance peak shows a highly symmetrical structure.38 Thus, the structure of metal (Al)−urea complexes (Figure 3) can be expected according to the analysis above. Urea molecules locate in the second coordination sphere near the carbon atom, while the nitrate radical anions locate in the outer coordination sphere. This structure stabilizes the vivid reactive Al(NO3)3 in an ambient atmosphere. Similar structures can be expected for metal (Fe)−urea complexes. Nevertheless, no reports are

to a ceramic dish and ignited in the preheated muffle furnace at 600 °C for 15 min. Finally, such burnt products continued to be calcined in the same furnace at 950 °C for 2 h, as depicted in Figure 2e. 2.3. Characterization. The chemical structure of the products at three preparation stages was determined by the Fourier transforms infrared (FTIR) spectroscopy (BRUKER TENSOR-27). The thermal decomposition behavior of the dried gel was examined via thermogravimetric analysis− differential thermal analysis (TGA−DTA) using Netzsch STA 409 PC (Germany). For thermal analysis, the heating rate and the flow rate of air were fixed as 20 °C/min and 60 mL/min, respectively. BET surface area, pore volume, and average pore diameter were examined by analyzing the N2 adsorption/desorption isotherms at 77 K, using a 3H-2000PS2 surface area and pore size analyzer (Beishide Instrument-ST Co., Ltd.). The specific surface area of the synthesized Fe2O3/Al2O3 oxygen carrier was calculated by the Brunauer−Emmett−Teller (BET) theory assuming a closely packed BET monolayer. Consequently, the pore size distribution of the oxygen carrier was derived by the Barrett−Joyner−Halenda (BJH) model using the desorption branch of the isotherm. Crystalline structures of the prepared Fe2O3/Al2O3 oxygen carrier were analyzed by using a Rigaku D/MAX-2500 diffractometer with Cu Kα radiation in the 2θ ranging from 20° to 80° with a step of 0.02°/s. The working voltage and current of the X-ray tube were fixed at 40 kV and 200 mA. According to Scherrer’s equation,34 the average crystalline size was calculated from the X-ray line broadening. All X-ray diffraction patterns were analyzed using Jade 7.5 of Material Data, Inc. (MDI), and peak profiles of individual reflections were determined by a nonlinear least-squares fit of the Cu Kα2 corrected data. The morphology and compositional analysis of the prepared oxygen carrier were examined by using JEOL JSM-6700F scanning electron microscope with potential of 8.0 kV, and working distance of 7.9 mm linked with OXFORD INCA energy dispersive X-ray (EDX, England). To further observe the surface morphology of oxygen carrier, JEOL JEM-2100 transmission electron microscope was also employed. Temperature-programmed reduction (TPR) was performed with the mixture gas of H2 with N2 (50% volume) as reductant using Netzsch STA 409 PC thermal analyzer. A 15 mg portion of synthesized Fe2O3/Al2O3 was heated at 105 °C for 2 h to be dehydrated. The heating rate and the flow rate for the mixture gas were selected at 20 °C/min and 60 mL/min, respectively. 12775



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Figure 3. Expected structure of Fe (Al)−urea−nitrate complex. Figure 4. IR spectra of the formed Fe2O3/Al2O3 oxygen carrier: (a) the dried gel; (b) the ignition product; (c) the calcined product.

published on the ligand structure of urea and two or more nitrates complexes, which need further investigations. The high temperature to produce oxygen carrier particles is achieved by the exothermicity of the redox reactions between metal nitrate (oxidizer) and urea (fuel) during combustion. To explore the combustion mechanism of Fe(NO3)3·9H2O and urea for an example, the specific reaction equation is as follows:

was corresponded to the Fe−Al−O group. Formation of this complex group prevented the preferential precipitation of both ions of Fe3+ and Al3+ possibly involved in the coprecipitation.39 Moreover, the complex was further formed through the hydrolysis of iron and aluminum ions and then generated the complex with OH group and CO32−40,41 characterized as the hydrated basic carbonates.42 According to the IR spectra in curves b and c for the generated Fe2O3/Al2O3 oxygen carrier particles, the disappearance of the characteristic bands for CO32−, NH4+, and NO3− ions indicated that those ions were removed after the ignition and calcinations processes. The newly generation bands at 580 and 492 cm−1 were ascribed to Fe−O+Al−O and Fe−O groups, respectively. 3.2. Thermal Decomposition of the Dried Gel. To understand the potential transformation of the dried gel in the muffle furnace, TGA−DTA was performed using Netzsch STA 409 PC at heating rate of 20 K/min from ambient temperature to 900 °C under air atmosphere. As Figures 5 and 6 depicts, the

Fe(NO3)3 ·9H 2O + (5/2)CO(NH 2)2 → (1/2)Fe2O3 + (5/2)CO2 + 14H 2O + 4N2

(R3)

Simultaneously, gas phase reactions in the foam of combustible gases like ammonia and oxides of nitrogen occur accompanied by decomposition of metal nitrate and urea. In particular, stoichiometric chemical reaction between NH3 and NOX, as follows: 4x NH3 + 10NOx → (5 + 2x)N2 + 6x H 2O + 2xO2 (R4)

This reaction steps up combustion rate and eliminates the contaminated gases in the combustion process. It is concluded that the solution combustion approach was an efficient, quick, and simple preparation method of the oxygen carrier.

3. RESULTS AND DISCUSSION 3.1. IR Analysis. The synthesis of Fe2O3/Al2O3 oxygen carrier includes drying process in the oven, igniting, and calcining in the muffle. For convenience, the generated products were designated as Fe3Al1gel, Fe3Al1ig, and Fe3Al1Si, respectively. The FTIR spectra of three-stage products are depicted in curves a−c in Figure 4. First, in curve a for the dried gel of Fe2O3/Al2O3 oxygen carrier, the evolution of urea was analyzed. The strong broad peak around 3460 cm−1 and the sharp band at 1385 cm−1 were ascribed to NH4+ group and CO32− ions (which resulted from the hydrolyzed reaction of Cyanate ion OCN−1), respectively. The presence of both NH4+ and CO32− ions clearly indicated the occurrence of the hydrolysis of urea during the preparation of Fe2O3/Al2O3 oxygen carrier. The medium band at 2370 cm−1 was ascribed to the groups CO2. In addition, the medium band at 1020 cm−1 was attributed correspondingly to the groups NH2 and CN, which implied the existence of free urea molecule in the dried gel without decomposition. Meanwhile, the evolution of iron and aluminum nitrates in the synthesis process was also analyzed. The band at 680 cm−1

Figure 5. TG curve for the thermal decomposition of the dried gel under air atmosphere.

TGA−DTA results indicated that during the heating of a sample at a given fraction of nitrates (Fe(NO3)3·9H2O and Al(NO3)3·9H2O) and urea, allowing the reaction R3 and the corresponding reaction of aluminum nitrate to occur. The TG curve, in Figure 5, delineated that the mass of the dried gel decreased from 100% to 20.91% as the temperature increasing 12776



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Figure 8. XRD patterns of calcined product. Figure 6. TGA−DTA curves for the thermal decomposition of the dried gel under air atmosphere.

according to matching with a XRD database. This observation supported the earlier conclusion that calcination was the necessary procedure to avoid the formation of spinel phase (FeAl2O4) between the active oxide and support, and a relatively low sintering temperature of 950 °C for 2 h was sufficient to sinter the as-synthesized Fe−Al oxygen carrier. Furthermore, the averaged crystalline sizes for α-Fe2O3 and αAl2O3 present in the Fe3Al1Si were correspondingly 47.9 and 70.8 nm evaluated by Scherrer’s equation, D = 0.9λ/(β cos θ),34 less than their separate counterparts in sizes of 107.3 and 79.1 nm.28 The possible reason was due to the aluminum substitution during the formation of sol and the ensuing partial reciprocal solubility between α-Fe2O3 and α-Al2O3.47 3.4. Structure Analysis. As the oxygen carrier, the Fe2O3/ Al2O3 particle served as the reactant in CLC process and the associated redox reactions were bulk reactions instead of a surface reaction like being used as a catalyst. Therefore, surface area, the pore structure, and the pore size distribution were also important for the reactivity of oxygen carriers which not only influence the reactivity of oxygen carriers, but also influence the mechanical strength and lifetime.4,5 N2 isothermal adsorption/ desorption was thus adopted to make a comprehensive investigation of the Fe2O3/Al2O3 oxygen carrier synthesized using solution combustion approach. Figure 9 indicates that the isotherms of nitrogen adsorption/ desorption was of type IV and had H1-type hysteresis loop,48 where P/P0 values were in the range of 0.92−0.98. If P/P0 is below 0.92, the adsorption branch increased slowly with the relative pressure, and then intersected without overlapping with the desorption branch. As P/P0 exceeding 0.98, the adsorption

from the ambient to around 400 °C, denoting the precursor decomposed completely below 400 °C to form various oxides. Similarly, the weight loss at 900 °C agrees well with the theoretical value corresponding to a complete decomposition to Fe2O3 and Al2O3. DTA traces show three main endothermic peaks. The first sharp endothermic peak (∼120 °C), indicating neither weight loss nor gas emission, which is due to sample melting of ferric nitrate, aluminum nitrate, and urea (placed at 47.2, 73, and 133 °C, respectively).43 The other two endothermic peaks (250 and 350 °C), which on the contrary involve both weight loss and formation of gaseous species, are caused by decomposition reactions occurring in various distinguishable steps. The sample underwent two decomposition steps in the temperature range of 140−350 °C, which appeared with strong sample weight loss, sharp endothermic effect and vigorous gas emission (NH3, HCNO, NOX, CO2 and H2O).44,45 The fourth exothermic reaction for the peak at 550 °C in the DTA curve was characterized by the typical combustion synthesis R4. This irregular shape of the exothermal peak seems to be structured, demonstrating that combustion occurs in a complex way which is related to formation of NO2, CO2, H2O, O2, and N2.46 3.3. Phase Identification. Phase identification of the asburnt and finished Fe2O3/Al2O3 oxygen carrier was performed using XRD, as shown in Figures 7 and 8, respectively. From Figure 7, after ignition, the peak intensity of Fe3Al1ig was considerably weak with 2θ ranging from 20° to 80°, not demonstrating good crystal structure. However, after 2 h of calcination at 950 °C, Figure 8, Fe3Al1 oxygen carrier was formed, composed of separate phases of α-Fe2O3 and α-Al2O3

Figure 9. Adsorption/desorption isotherms of Fe2O3/Al2O3 oxygen carrier.

Figure 7. XRD patterns of ignition product. 12777



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particles, which will help increase the chemical reactivity of the oxygen carriers. EDX analysis was employed to determine the chemical composition of the synthesized Fe3Al1 oxygen carrier, Figure 12. The EDX spectra of the Fe3Al1 oxygen carrier indicate the

branch sharply ascended and reached the stable states. Because the Kelvin radius of adsorption process and desorption process is unequal, the two processes do not fully overlap at high pressure, there is a typical H1 hysteresis loop, the shape of which reflects definite pore structure (not lubricous column). The pore size analysis of the Fe2O3/Al2O3 was presented by the adsorption isotherm and calculated by Kelvin equation, as highlighted in Figure 10. It was observed that the Fe2O3/Al2O3

Figure 12. EDX profile of Fe2O3/Al2O3 oxygen carrier.

presence of Fe, Al, and O, and the weight percent of which were 51.34, 9.52, and 39.15, respectively. Similarly, the weight percent is nearly in accordance to the theoretical value i.e., the preliminary ratio of ferric nitrate and aluminum nitrate. 3.6. Reaction Characteristics. Compared with the oxidation reaction of the reduced oxygen carrier in air reactor, reduction reaction of the oxygen carrier with fuel in fuel reactor is generally considered as the limiting (control) step. Gaseous fuels such as CH4 or CO easily generated carbon deposition with complicated reduction reaction.49−51 To get a preliminary understanding of the reaction characteristics of the synthesized Fe3Al1 oxygen carrier, the reduction reaction with H2 was performed in a TGA, which is depicted in Figure 13. The overall temperature range consists of the rising and the constant temperature segments. In the rising temperature segment, when the temperature reaches 900 °C, a mixture of N2 and H2 is added in the reaction chamber simultaneously. And afterward, the temperature keeps at 900 °C for 20 min. It can be concluded from Figure 13 that the complete reaction occurs between the Fe2O3/Al2O3 oxygen carrier and H2 (reduction from α-Fe2O3 into Fe), the net mass loss was calculated as 25.74%. In this TPR experiment, however, the net mass loss was 20.5%. This is because the α-Fe2O3 present in the Fe2O3/Al2O3 oxygen carrier was not reduced to Fe completely, and the residual iron-based substances might consist of Fe3O4, FeO, and Fe.

Figure 10. Pore size, volume distribution, and BET surface area.

has pores of various sizes starting from 4 nm to the macropore region, greater than 100 nm. The average pore size approached 64.76 nm by the BJH method, and the total pore volume (P/P0 = 0.9958) was 0.0326 mL/g. The surface area of the Fe2O3/ Al2O3 oxygen carrier calculated by BET single point method, P/P0 = 0.20972 was 4.01 m2/g, larger than the surface area, 1.66 m2/g of the Fe1Al1 oxygen carrier prepared by the same method.26 3.5. Morphology and EDX Analysis. The SEM and TEM micrographs, Figure 11, exhibit the surface and the macrostructure of the prepared Fe2O3/Al2O3 oxygen carrier. One can see from Figure 11a, the SEM pattern of the Fe2O3/Al2O3 oxygen carrier particles were featured by mostly regular spherical and cubical shape (the diameter of the particles in the surface is approximately 100 nm), and no evident thermal agglomeration is observed in the calcining process (950 °C for 2 h). It can be observed from Figure 11b that the Fe2O3/Al2O3 oxygen carrier is apt to absorb the reducing gas (H2 and CH4 in this paper) due to its abundant porous structure. Such structures result in the large specific area of the oxygen carrier

Figure 11. SEM (a) and TEM (b) images of Fe2O3/Al2O3 oxygen carrier. 12778



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Figure 13. Isothermal reduction of Fe2O3/Al2O3 oxygen carrier with H2 at 900 °C. Figure 14. Five red−ox cyclings between the Fe2O3/Al2O3 oxygen carrier and alternately CH4 or air at 850 °C, respectively.

The mass loss in the TG curve for the Fe2O3/Al2O3 oxygen carrier reacting with H2 was actually resulting from the loss of lattice oxygen present in the oxide carrier. Accordingly, the DTG value implied the transfer rate of the lattice oxygen in the oxides to H2, a greater DTG value meant a larger transfer rate. Figure 13 displays that there was only one distinct DTG peak in the reduction reaction of as-synthesized Fe3Al1 oxygen carrier at 900 °C and the weight loss rate reached a maximum (4.75 wt %/min at the reaction time of 49.5 min, larger than the DTG value of Ni40Al-FG and Fe45Al-FG oxygen carriers investigated by Abad et al.51). Meanwhile, the left mass from the TG curve reaches 92.86%, which meant Fe3Al1Si had a greater oxygen transfer rate and was more suitable for CLC application. When an oxygen carrier was used in CLC system, the high oxygen transfer rate meant the decreased bed inventory in fuel reactor and less recirculation of oxygen carrier from air reactor to fuel reactor. 3.7. Red−Ox Reactivity of the Fe2O3/Al2O3 Oxygen Carrier with CH4 and Air. Excellent red−ox properties and sintering resistance are of the great concern to sustain the good reactivity of the oxygen carrier. To evaluate red−ox and the sintering-resistant characteristics for the Fe2O3/Al2O3 oxygen carrier, five cycles of red−ox reactions between oxygen carrier and alternating by CH4 in 50 vol % with balance N2 and then air were performed. The result of mass loss (i.e., TG curve) over the five cycles of the red−ox process is plotted in Figure 14. It was observed from Figure 14 that though the first red−ox reaction rates with CH4 and air corresponded to 7.7 and 11.3%/min, however, with the reaction further proceeding, the reaction rates related to the reduction by CH4 and oxidization by air were increased, stabilizing around 11.9 and 16.4%/min, respectively. The low reduction and oxidation rate for the first cycle mainly arose from the dense layer Fe2O3 initially formed in the outer surface for the fresh oxygen carrier during preparation. Both the dense layer formed outside the oxygen carrier particle and the uneven pore size distribution hamper CH4 in accessing the active reaction surface. After the first red− ox cycle, the abundant pores of different size were developed over the ensuing red−ox process by enlargement in the reduction stage and contraction in the oxidization stage, which favors increasing the reaction rates. The shape and morphology of the Fe2O3/Al2O3 oxygen carrier after five cyclic reactions is denoted in Figure 15. The surface of the fresh oxygen carrier, Figure 11, is composed of

individual grains, where many micropores exist. However, after the cyclic reactions the diameter of the individual grains increases obviously, implying that sintering behavior appears in the inside of the individual oxygen carrier particle, but the particle still exhibits a porous structure. It is worth noting that the sintering behavior among different particles is not observed after the multiple cyclic reactions. In conclusion, Fe2O3/Al2O3 oxygen carriers prepared by solution combustion approach have an excellent chemical reactivity and good recyclability, which is suitable to employ in the CLC process.

4. CONCLUSIONS In this work, the solution combustion approach was used to prepare a potential oxygen carrier for CLC process. The main conclusions were summarized as follows. (i) The mechanism of oxygen carrier preparation by the solution combustion approach mainly includes the complexation and combustion mechanism. The complex structure stabilizes the vivid reactive Al(NO3)3 or Fe(NO3)3 in an ambient atmosphere. In addition, the high temperature is achieved by the exothermicity of the redox reactions between nitrates and urea during combustion, especially the reaction between NH3 and NOX. (ii) Calcination was a necessary step to produce Fe2O3/ Al2O3 oxygen carrier with separate phases of α-Fe2O3 and α-Al2O3. (iii) DTA curve of the dried gel showed three main endothermic and one exothermic peaks. The three endothermic peaks referred to melting of nitrates, weight loss, and formation of gaseous species. The one irregular exothermic peak was attributed to complex combustion synthesis. (iv) XRD identification of the Fe3Al1ig and Fe3Al1Si verified that calcination at 900 °C for 2 h was necessary to obtain Fe2O3/Al2O3 oxygen carrier. The pore shape of Fe2O3/ Al2O3 particles was heterogeneous. The average pore size and surface area were 64.76 nm and 4.01 m2/g, respectively. SEM and TEM photos showed the regular spherical, cubical shape, and abundant porous structure of the prepared Fe2O3/Al2O3 oxygen carrier, respectively. 12779



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Figure 15. SEM images of Fe2O3/Al2O3 oxygen carrier after five cycles of red−ox reactions (850 °C, 1 atm, reduction in 50 vol % CH4, balance N2, and oxidized in air). deposits and Sulfur evolution in Chemical-looping combustion system. Int. Rev. Chem. Eng. 2009, 1, 51−57. (8) Li, F. Chemical looping gasification process. Ph. D. thesis, The Ohio State University: Columbus, OH, 2009. (9) Abad, A.; García-Labiano, F.; de Diego, L. F.; Gayán, P.; Adánez, J. Reduction kinetics of Cu-, Ni- and Fe-based oxygen carriers using syngas (CO + H2) for chemical- looping combustion. Energy Fuels 2007, 21, 1843−1853. (10) Jerndal, E.; Mattisson, T.; Lyngfelt, A. Thermal analysis of chemical-looping combustion. Chem. Eng. Res. Des. 2006, 84, 795− 806. (11) Son, S. R.; Kim, S. D. Chemical-looping combustion with NiO and Fe2O3 in a thermobalance and circulating fluidized bed reactor with double loops. Ind. Eng. Chem. Res. 2006, 45, 2689−2696. (12) Adánez, J.; García-Labiano, F.; de Diego, L. F.; Gayán, P.; Celaya, J.; Abad, A. Characterization of oxygen carriers for chemical looping combustion. In Proceedings of the 7th International Conference on Greenhouse Gas Control Technology (GHGT-7), Vancouver, Canada, Sept 5−9, 2004. (13) Gayán, P.; Forero, C. R.; de Diego, L. F.; Abad, A.; GarcíaLabiano, F.; Adánez, J. Effect of gas composition in chemical-looping combustion with copper based oxygen carriers: Fate of light hydrocarbons. Int. J. Greenhouse. Gas Control 2010, 4, 13−22. (14) Mattisson, T.; Johansson, M.; Lyngfelt, A. Multicycle reduction and oxidation of different types of iron oxide particles- Application to chemical-looping combustion. Energy Fuels 2004, 18, 628−637. (15) He, F.; Wang, H.; Dai, Y. Application of Fe2O3/Al2O3 composite particles as oxygen carrier of chemical looping combustion. J. Nat. Gas Chem. 2007, 16, 155−161. (16) Ishida, M.; Takeshita, K.; Suzuki, K.; Ohba, T. Application of Fe2O3-Al2O3 composite particles as solid looping material of the chemical-loop combustor. Energy Fuels 2005, 19, 2514−2518. (17) Jin, H.; Okamoto, T.; Ishida, M. Development of a novel chemical-looping combustion: synthesis of a solid looping material of NiO/NiAl2O4. Ind. Eng. Chem. Res. 1999, 38, 126−132. (18) Cho, P.; Mattissson, T.; Lyngfelt, A. Reactivity of iron oxide with methane in a laboratory fluidized bed - application of chemical looping combustion. In Proceedings of the 7th International Conference on Circulating Fluidized Beds (CFB-7), Niagara Falls, Ontario, May 5− 8, 2002; pp 599−606. (19) Moghtaderi, B.; Song, H. Reduction properties of physically mixed metallic oxide oxygen carriers in chemical looping combustion. Energy Fuels 2010, 24, 5359−5368. (20) Mattisson, T.; García-Labiano, F.; Kronberger, B.; Lyngfelt, A.; Adánez, J.; Hofbauer, H. Chemical-looping combustion using syngas as fuel. Int. J. Greenhouse. Gas Control 2007, 1, 158−169. (21) Abad, A.; Mattisson, T.; Lyngfelt, A.; Johansson, M. The use of iron oxide as oxygen carrier in a chemical-looping reactor. Fuel 2007, 86, 1021−1035. (22) Cho, P.; Mattisson, T.; Lyngfelt, A. Carbon formation on nickel and iron oxide containing oxygen carriers for chemical-looping combustion. Ind. Eng. Chem. Res. 2005, 44, 668−676.

(v) H2-TPR of Fe2O3/Al2O3 oxygen carrier in TGA indicated that the reduction reaction of the oxygen carrier had only one distinct DTG peak and the weight loss rate reached a maximum (4.75 wt %/min). The αFe2O3 present in the Fe2O3/Al2O3 oxygen carrier was not reduced to Fe completely, and the residual iron-based substances may consist of Fe3O4, FeO, and Fe. In particular, five cycles of red-ox reaction by alternating with CH4 or air demonstrated the synthesized oxygen carriers had good reactivity and sintering resistance.

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support by the Natural Science Foundation of China (21276129), the Key Technologies R & D Program of Shandong province (2009GG10007001) and the Project supported by the National Science Foundation for Distinguished Young Scholars of Shandong province (JQ200904) is gratefully acknowledged.

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Al2O3 Using the Solution

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