Article pubs.acs.org/EF
Novel New Oxygen Carriers for Chemical Looping Combustion of Solid Fuels Yueying Fan†,‡ and Ranjani Siriwardane*,† †
National Energy Technology Laboratory, United States Department of Energy, 3610 Collins Ferry Road, Post Office Box 880, Morgantown, West Virginia 26507-0880, United States ‡ URS Corporation, 3610 Collins Ferry Road, Post Office Box 880, Morgantown, West Virginia 26507-0880, United States ABSTRACT: Several bimetallic oxygen carriers, MFe2O4 (M = Co, Ni, Cu, Mg, Ca, Sr, and Ba) and MnFeO3, prepared by the precipitation method in a microwave and the direct decomposition method, were tested for potential use in the application of chemical looping combustion (CLC) of solid fuels. Thermogravimetric analysis (TGA) was used to study their reduction rate, oxidation rate, and cyclic reduction/oxidation properties. Comparative experimental data of novel bimetallic ferrites and pure Fe2O3 and CuO showed that all bimetallic ferrites had better reduction rates than pure Fe2O3. The Group 2 metal ferrites had better reduction and oxidation rates than transition-metal ferrites. BaFe2O4 was the highest performing among all bimetallic ferrites during both reduction and oxidation reactions. The reduction rate of BaFe2O4 is comparable to that of CuO at higher reaction temperatures (>900 °C). A 10 wt % loading of an inert support on the surface of the bimetallic oxygen carriers significantly decreased the particle agglomeration during the cyclic tests, which contributed to a better cyclic reaction performance.
1. INTRODUCTION Concern about the global climate change prompted research on lowering CO2 emissions during fossil fuel combustion. Technologies or processes that prevent CO2 from reaching the atmosphere have the disadvantage of contributing to large energy penalties, because of the high costs associated with separating gases. The separation task can be simplified by replacing conventional air oxidant with pure oxygen, so that the products from this “oxy-fuel” combustion are just carbon dioxide and water, which are easily separated by condensation. However, the current commercial process for oxygen separation requires cryogenic oxygen production from air, which consumes an appreciable amount of energy, making the oxyfuel combustion process very energy-intense. Chemical looping combustion (CLC), which uses oxygen carriers, such as metal oxide, to supply oxygen instead of air for fuel combustion, is a promising technology that produces heat and energy, with the significant advantage of producing concentrated CO2 without requiring any major energy for its separation.1 There are significant advantages 2 to using the CLC process. In comparison to normal combustion, CLC produces a sequestration-ready stream of CO2 that is not diluted with N2 or flue gas and also reduces NOx emissions. Large-scale application of CLC is dependent upon the availability of a suitable oxygen carrier. An ideal oxygen carrier should meet a number of requirements, including high reactivity, low fragmentation and attrition, low tendency for agglomeration, low cost, and stability under repeated reduction/oxidation cycles at high temperatures. The carrier should also be environmentally benign. The development of oxygen carriers possessing these desirable properties is critical for CLC. The traditional oxygen carriers,3 such as CuO, Fe2O3, NiO, MnO, and CoO, have been tested extensively in the past for © 2014 American Chemical Society
CLC of coal and CH4. However, disadvantages, such as low reactivity with Fe2O3, CoO, and MnO, low melting points, high agglomeration with CuO, and health concerns with NiO, must be addressed. None of the traditional metal oxides investigated in previous studies3 appears to possess all of the desirable characteristics for CLC applications. Recently, researchers have been investigating mixed metal oxides for CLC to potentially resolve many of the shortcomings associated with those conventional single metal oxides. The synergetic effects of having multiple oxides may enhance the performance of the oxides to obtain the desirable properties for CLC. For example, CuO possesses higher reactivity than Fe2O3 but has issues with particle agglomeration, which limits its application in the CLC process. Iron oxide has the advantage of low agglomeration but has slower reactivity and lower oxygen capacity. The components of bimetallic Cu−Fe oxygen carriers were optimized to achieve a better reactivity than Fe2O3 and better stability than CuO in cyclic methane CLC and coal/carbon CLC reactions.4,5 Superior performance of CuFe2O4, as compared to single metal oxides of either CuO or Fe2O3, was also reported for H2/air CLC application.6 However, when coal was used, iron silicates were formed from the interaction of reduced CuFe2O4 with ash and resulted in the insufficient reoxidation of reduced CuFe2O4. Jin et al.7,8 investigated NiO/YSZ, CoO/YSZ, and NiO− CoO/YSZ for CLC. They found that NiO−CoO/YSZ showed excellent overall performance with good reactivity, complete avoidance of carbon deposition, and significant regenerability for repeated cycles of reduction and oxidation. However, NiO/ YSZ and CoO/YSZ have drawbacks of either higher carbon Received: December 24, 2013 Revised: February 20, 2014 Published: February 21, 2014 2248
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Figure 1. XRD of fresh Group 2 metal ferrites before and after reduction with coal: (a) MgFe2O4, (b) CaFe2O4, (c) SrFe2O4, and (d) BaFe2O4.
demonstrated higher redox cycling behavior and better stability than pure NiO and Fe2O3. Perovskite-type materials, La0.8Sr0.2Co0.2Fe0.8O3−δ, had been investigated for a potential oxygen carrier by in situ X-ray diffraction (XRD) chemical-looping experiments. The results showed that La0.8Sr0.2Co0.2Fe0.8O3−δ does have the redox properties required for chemical looping. Reduction and oxidation of perovskite occur quickly under the conditions used. However, the low oxygen capacity observed would require a fast solid circulation or high solid inventory if this perovskite material were to be used in industrial CLC processes.15 La0.5Sr0.5Fe0.5Co0.5O3−δ was also found to be suitable for CLC applications.16 However, in comparison to other metal oxide materials, La0.5Sr0.5Fe0.5Co0.5O3−δ does not seem to offer any obvious advantages. The majority of the work performed to date on CLC has been performed using gaseous fuels. Few studies have been conducted using oxygen carriers for combustion of solid fuels, such as coal. The objectives of this research were to explore an oxygen carrier with high activity and great stability for coal CLC. In the current research, metal ferrites (MFe2O4) with M selected from Group 2 elements (M = Mg, Ca, Sr, and Ba) and transition metals (M = Cu, Ni, and Co) and MnFeO3 for coal CLC are reported.
deposition or lower ability to regenerate, accompanied with an increase in both the grain and pore sizes. Hossain et al.9 found that the activation energies for Co−Ni/Al2O3 reduction are significantly lower than those for the single metal oxide Ni/ Al2O3 reduction, which confirmed the favorable effect of Co on the reducibility of the bimetallic oxygen carrier. The bimetallic Fe−Mn oxides10 supported on ZrO2, sepiolite and Al2O3 prepared by solid-state mixing were found to be promising oxygen carriers for CLC. Using simulated synthesis gas and MnO also had a positive effect on the stability. These mixed oxides exhibited a lower oxygen transfer capacity than Ni-based materials. The capacity was also affected by the synthesis method.11 Mixtures of manganese and iron oxides have also been examined as oxygen carriers for CLC with natural gas as fuel in a circulating fluidized-bed reactor.12 It was concluded that the combined oxides of Mn and Fe have very interesting thermodynamic properties and could potentially be suitable for chemical-looping applications, but the physical strength of the materials would have to be improved. In addition, combined Fe−Mn oxides with molar ratios of Fe/Mn of 2:1 showed the best oxygen release ability, fluidizability, and methane conversion.13 They concluded that the mixed Fe−Mn oxides could contribute to faster fuel conversion, even though their oxygen release was less than copper-based chemicallooping with oxygen uncoupling (CLOU) materials during tests with solid fuel. Nickel ferrite (NiFe2O4) was also investigated as an oxygen carrier for CLC.14 Redox cycling of NiFe2O4 oxygen carriers was performed by thermogravimetric analysis (TGA) under pure CH4 gas and O2/air atmospheres. After five successive cycles, NiFe2O4 powder with a single phase of spinel structure
2. EXPERIMENTAL SECTION 2.1. Synthesis of Oxygen Carries. Both the precipitation method in a microwave (Anton Parr) and the direct decomposition method were used for the synthesis of bimetallic ferrite oxygen carriers. In the precipitation (microwave) method, metal nitrates or metal acetates were used as precursors to synthesize oxygen carriers. Metal nitrates or metal acetates were dissolved in the diethylene glycol, and the solution 2249
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was heated to 200−250 °C in the microwave reactor for 30−45 min. The resulting solid precipitate was washed with deionized (DI) water and separated by centrifugation. The material was dried in an oven at 100 °C overnight and calcined in air at 600−1000 °C for 6 h. The direct decomposition method was also evaluated as a preparation method because it is more cost-effective than the microwave precipitation method but yields bigger particle sizes. In this method, metal nitrates were mixed with citric acid to enhance bonding and prevent aggregation at high temperatures. The mixture was heated in a box oven to 1000 °C at a ramp rate of 3 °C/min in air and kept at 1000 °C for 6 h. 2.2. Synthesis of 10% Al2O3/BaFe2O4 and 40% Al2O3/ BaFe2O4. To decrease the agglomeration of oxygen carriers, 10% Al2O3 was incorporated into bimetallic oxygen carriers by the urea deposition−precipitation method after the synthesis of the oxygen carriers. A total of 2 g of bimetallic ferrites was added to 200 mL of an aqueous solution of Al(NO3)3(H2O)6 and 0.4 M urea. The suspension was loaded into a thermolysis reactor (Syrris sodium system) and vigorously stirred at 90 °C for 4 h. The sample was then washed with DI water, centrifuged several times to remove traces of acid and urea, and dried at 100 °C overnight in air. Then, 10% Al2O3/MFe2O4 was calcined in air at 900 °C for 1 h. For the preparation of 40% Al2O3/ BaFe2O4, 1 g of BaFe2O4 was mixed with 0.67 g of Al2O3. The mixture was then heated in a box oven to 1000 °C at a ramp rate of 3 °C/min in air and kept at 1000 °C for 6 h. 2.3. TGA of the CLC Performance Test. CLC tests of bimetallic oxygen carriers with coal were performed in TGA. The oxygen carrier mixed with coal (Illinois #6) or carbon black using a motor and pestle, and the mixture was loaded in TGA. A weight ratio of oxygen carrier/ coal = 1:0.067 was used during the performance comparison experiments, and a weight ratio of BaFe2O4/carbon = 1:0.075 was used for the cyclic test. The TGA samples were heated to 900−1000 °C at a ramping rate of 5 °C/min under N2 at a flow rate of 200 cm3/ min and kept at 900 or 1000 °C until there was no weight loss. The zero-grade air at a flow rate of 200 cm3/min was introduced for oxidation. The reaction rate of the oxygen carrier with coal or carbon black was calculated using TGA data as follows:
reduction rate = dx /dt ,
X = (Mo − M )/(Mo − M f )
oxidation rate = dx /dt ,
X = (M − M f )/(Moxd − M f )
samples. After reduction with coal, all ferrites had peaks corresponding to FexOy, with the exception of BaFe2O4, which had peaks corresponding to metallic Fe0 (Table 1). This indicated that Ba promoted further reduction of FexOy to Fe. The XRD patterns of the fresh transition-metal ferrites before and after reduction with coal are shown in Figure 2. XRD peaks indicate that the fresh transition-metal ferrites exist as MFe2O4 (M = Co, Ni, and Cu), with the exception of Mn, which exists as FeMnO3. After reduction with coal, all transition-metal ferrites had peaks corresponding to reduced FexOy. 3.2. TGA Reaction Performance of Metal Ferrites with Coal. To reduce agglomeration of the metal ferrites during reduction/oxidation reactions at high temperatures, 10% Al2O3 was incorporated in the metal ferrites. Figure 3 illustrates the reaction rate comparison before and after incorporating 10% Al2O3 loading on Group 2 metal ferrites. As shown in Figure 3, the rates were not affected by adding 10% Al2O3. However, the agglomeration was reduced significantly after incorporation of 10% Al2O3, and reactivity was maintained. The 10% Al2O3 loading on transition-metal ferrites also reduced agglomeration while maintaining reactivity (data not shown). A comparison of coal CLC reaction properties between single oxides and metal ferrites synthesized by the microwave precipitation method is shown in Figure 4. CuO has a better reduction rate than Fe2O3, consistent with what was reported previously.5 Data indicate that all metal ferrites have better reduction rates than Fe2O3. It is worth noting that the Group 2 metal ferrites have better reduction and oxidation rates than transition-metal ferrites. BaFe2O4 had the best performance among all metal ferrites during both reduction and oxidation phases. The reduction rate of BaFe2O4 at 1000 °C is comparable to that of CuO at 900 °C. The reaction temperature of CuO was restricted to 900 °C because of agglomeration and its low melting point. However, BaFe2O4 can be operated up to 1000 °C without agglomeration. BaFe2O4 was chosen for the cycling test because it had the best performance. It was also observed that metal ferrites had less agglomeration than single oxides Fe2O3 and CuO even without incorporation of 10% Al2O3. Commercial Fe2O3 and CuO used for comparison had particle sizes less than 5 μm. Fresh metal ferrites and Fe2O3 synthesized by the microwave precipitation method had particle sizes less than 1 μm. For proper comparison, 1 μm Fe2O3 synthesized by the microwave precipitation method was also tested. It showed significantly more agglomeration than metal ferrites after reaction, which contributed to a lower reaction rate than that with commercial Fe2O3, even though Fe2O3 from the microwave precipitation method had a smaller particle size. The theoretical carbon consumption for the reaction of carbon with metal ferrites and single metal oxides is shown in Table 2. The final reduction states of the oxides determined by XRD were used for these calculations. BaFe2O4 has the highest theoretical carbon consumption because the final reduction states are Ba and Fe0, and it is higher than that of Fe2O3 and comparable to that of CuO. The reason for the higher reactivity of metal ferrites compared to the reactivates of single metal oxides is not very clear. Metal ferrites with the general molecular formula M2+Fe23+O4 have a spinel-type structure similar to that of the mineral MgAl2O4, otherwise known as spinel. The lattice consists of 32 divalent oxygen ions, which are in direct contact with one another, forming a closed-pack, face-centered cubic arrangement with 64 tetrahedral interstitial sites (A sites) and
where M is the instantaneous weight of the metal oxide−coal mixture, Mo is the initial weight of the metal oxide−coal mixture, Mf is the weight of the reduced metal and ash after the reduction, and Moxd is the weight of the completed oxidized sample after introducing air. The reaction rate dx/dt was calculated by differentiating the fifth-order polynomial equation.
3. RESULTS AND DISCUSSION 3.1. Physical Characterization. The XRD patterns of fresh Group 2 metal ferrites before and after reduction with coal are shown in Figure 1, and phase identifications are listed in Table 1. XRD patterns corresponding to metal ferrites, MFe2O4 (M = Mg, Ca, Sr, and Ba), were observed with fresh Table 1. XRD-Identified Phases before and after Reduction with Coal metal ferrites, phase before reduction
phases after reduction
CuFe2O4 NiFe2O4 CoFe2O4 MnFeO3 MgFe2O4 CaFe2O4 SrFe2O4 BaFe2O4
Cu and Fe3O4 Fe, Ni3Fe, and Ni1.25Fe1.85O4 Co and Fe3O4 Fe, Fe0.95O, and MnO FeO and Mg0.7Fe0.23Al1.97O4 Fe0.902O and Ca2Fe2O5 FeO and SrAl2O4 Fe and BaO 2250
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Figure 2. XRD of transition-metal ferrites before and after reduction: (a) CoFe2O4, (b) CuFe2O4, (c) NiFe2O4, and (d) FeMnO3..
Figure 3. Reaction rate comparison of Group 2 metal ferrites before and after incorporation of 10% Al2O3 reduction with coal.
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Figure 4. Reaction rates and reaction temperatures of single metal oxides and metal ferrites with coal.
bond lengths (energies) in the ferrite structure are different from those of single metal oxides, which may also contribute to the differences in reactivity. In our research reported here, Group 2 ferrites and transition-metal ferrites did show better reactivity with coal than single metal oxygen carriers. The Cu2+, Mg2+, Ni2+, and Co2+ ferrites form inverse spinel structure with the formula Fe3+[Me2+Fe3+]O4, in which Me2+ (Mg2+, Cu2+, Ni2+, and Co2+) is octahedral-coordinated with oxygen, while Fe3+ is distributed in both the tetrahedral- and octahedralcoordinated sites, which may have contributed better reactivity with coal than single metal oxygen carriers. The degree of inversion depended upon the metal cation in the ferrite structure.18 However, it was noted that barium ferrite (BaFe2O4) does not crystallize in a spinel structure but in an complicated orthorhombic structure19 because the size of Ba2+ is too large to be accommodated in the octahedral sites. In our present work, BaFe2O4 was also identified to be in the orthorhombic structure. Similar stuffed-framework structures are reported with Ca and Sr ferrites,20 but Ca ferrite has shown a completely different atomic arrangement from both Ba and Sr ferrites. Better reactivity of alkaline earth ferrites with coal or carbon, as compared to that of transition-metal ferrites, could be due to these structural differences. Molecular modeling work may be necessary to understand these differences.
Table 2. Theoretical Carbon Consumption Based on 1 g of Oxygen Carriersa oxygen carriers Fe2O3 CuO CuFe2O4 MnFeO3 NiFe2O4 CoFe2O4 CaFe2O4 MgFe2O4 SrFe2O4 BaFe2O4 a
reaction
theoretical carbon consumption (mg)
2Fe2O3 + C = 4FeO + CO2 2CuO + C = 2Cu + CO2 CuFe2O4 + C = Cu + 2FeO + CO2 MnFeO3 + C = MnO + Fe + CO2 NiFe2O4 + C = Ni + 2FeO + CO2 CoFe2O4 + C = Co + 2FeO + CO2 CaFe2O4 + C = CaO + FeO + CO2 MgFe2O4 + C = MgO + FeO + CO2 SrFe2O4 + C = SrO + FeO + CO2 BaFe2O4 + 4C = Ba + 2Fe + 4CO BaFe2O4 + 2C = Ba + 2Fe + 2CO2
37.5 75 50.16 75.576 51.19 51.15 27.81 60 45.57 153 76.67
The reduction phases identified by XRD were used.
32 octahedral interstitial sites (B sites). Of these, 8 tetrahedral (A sites) and 16 octahedral (B sites) sites are occupied by the divalent and trivalent cations.17 Thus, the large fraction of empty interstitial sites makes its crystal structure a very open structure that is conducive to cation migration. Therefore, a whole range of distributions of cations is possible in spinel ferrites, which can contribute to their remarkable magnetic, catalytic, optical, and electrical properties. The metal−oxygen
Figure 5. Cyclic CLC test data of carbon and 10% Al2O3/BaFe2O4 synthesized by the microwave precipitation method at 210 °C for 45 min. 2252
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Figure 6. Cyclic CLC tests of carbon with 10% Al2O3/BaFe2O4 synthesized by the microwave precipitation method at 240 °C for 30 min.
Figure 7. (a) Reaction rates, (b) reaction temperature corresponding to the maximum reaction rate during cyclic tests of 10% Al2O3/BaFe2O4 (direct decomposition method) and (c) XRD of fresh BaFe2O4.
3.3. TGA Cyclic Reduction/Oxidation Reaction of BaFe2O4 with Carbon. Figure 5 illustrates the TGA test data during cyclic tests of carbon CLC with 10% Al2O3/ BaFe2O4 synthesized by the microwave method. BaFe2O4 showed stable reduction rates during cyclic tests, while oxidation rates improved. The maximum reaction temperature slightly increased with increasing cycles because of the aggregation of the particle. Figure 6 illustrates that the performance of BaFe2O4, synthesized by the microwave method at 240 °C for 30 min, was similar to that synthesized at 210 °C for 45 min (Figure 5). Figure 7 illustrates the XRD patterns of fresh BaFe2O4 synthesized by the direct decomposition method, reaction rates, and temperatures corresponding to maximum reaction
rates during cyclic tests of 10% Al2O3/BaFe2O4 with carbon. Even though uniform small particle sizes can be obtained by the precipitation method in microwave, direct decomposition is a more convenient and cost-effective preparation method. XRD data of the sample prepared by the direct decomposition method (shown in Figure 7) indicate BaFe2O4 as the major phase with a trace amount of BaFe12O19. Reduction rates are similar for both materials synthesized by the precipitation (microwave) method and the direct decomposition method. However, the oxidation rate is higher for materials synthesized by the precipitation (microwave) method because of the smaller particle size. BaFe2O4 synthesized by the direct decomposition method also showed stable reduction and oxidation rates, while its reaction temperature increased slightly 2253
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Figure 8. Outlet gas analysis during the reaction of carbon black with 10% Al2O3/BaFe2O4 at various weight ratios: (a) ratio of carbon to 10% Al2O3/BaFe2O4 is 135 mg/1.1 g, and (b) ratio of carbon to 10% Al2O3/BaFe2O4 is 65 mg/1.1 g.
Figure 9. (a) XRD of 10% Al2O3/BaFe2O4 after reduction with carbon, (b) XRD of Fe2O3 after reduction with carbon, and (c) XRD of 10% Al2O3/ BaFe2O4 after cyclic tests.
Thus, the oxidation rates of BaFe2O4 were affected by the particle size. Outlet gas composition during the reaction of carbon with two different amounts of 10% Al2O3/BaFe2O4 is shown in Figure 8. When the ratio of carbon to 10% Al2O3/BaFe2O4 (135 mg/1.1 g) was higher, more CO [CO2/(CO + CO2) = 0.48] was produced, according to the reaction: BaFe2O4 + 4C = Ba + 2Fe + 4CO. When the ratio of carbon to 10% Al2O3/ BaFe2O4 (65 mg/1.1 g) was lower, more CO2 [CO2/(CO + CO2) = 0.80] was produced, according to the reaction: BaFe2O4 + 2C = Ba + 2Fe+ 2CO2. Therefore, pure CO2 or CO can be obtained using the appropriate ratio of carbon to BaFe2O4.
as the number of cycles increased. TEM showed that the particle size of BaFe2O4, synthesized by the microwave precipitation method, was about 500−600 nm, which is smaller than that synthesized by the solid reaction method (particle size is in the range of 1−2 μm). The particle size affected the oxidation rates but not the reduction rates. During the reduction cycle, oxygen from the surface of the metal oxide is released first, facilitating the diffusion of gases from the interior to the exterior of metal oxide. Therefore, reduction rates of BaFe2O4 were not affected because of different particle sizes. During the oxidation cycle, oxygen in air oxidizes the surface of the metal, restricting the diffusion of oxygen inside metal oxide. 2254
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3.4. Effect of Reactor Bed Dilution. Al2O3, bentonite and BaAl2O4 were tested for the suitability as diluting materials for BaFe2O4 in the application of CLC. BaAl2O4 was chosen to be tested as a diluting material because of a possible reaction between Ba and Al2O3 during reduction/oxidation cycles. BaAl2O4 was synthesized by the direct decomposition method. Barium nitrate and aluminum nitrate were mixed with citric acid and heated to 1000 °C for 6 h at a ramping rate of 3 °C/ min in air. Al2O3 (40%), BaAl2O4, or bentonite was mixed with BaFe2O4 and heated to 1000 °C for 6 h at a ramping rate of 3 °C/min in air before the TGA performance test with carbon. Figure 12 shows carbon CLC performance of 60% BaFe2O4 diluted with 40% Al2O3, BaAl2O4, or bentonite. A reduction rate of 40% Al2O3/BaFe2O4 with carbon decreased with an increasing number of cycles, similar to that of 10% Al2O3/ BaFe2O4, as shown in Figure 7. The reduction rate with 40% Al2O3 is lower than that with 10% Al2O3. The contacts between carbon and BaFe2O4 will be lower with increasing Al2O3 content, which may contribute to a lower reduction rate. However, the oxidation rate increased with more Al2O3 dilution compared to 10% Al2O3/BaFe2O4 synthesized by both the microwave precipitation method (Figures 5 and 6) and the direct decomposition method (Figure 7). Al2O3 and BaAl2O4 showed better performance as support dilute materials for CLC with carbon, while bentonite, as a support material for BaFe2O4, showed significant decreases in both the reduction and oxidation rates during the second cycle. Cyclic performance tests were not continued with 40% bentonite/BaFe2O4. It has been reported2 that Fe oxide/bentonite and Fe/SiO2 showed the worst stability at 800 °C while improving at 900 °C. It was believed that the interaction between the metal and supports, as well as agglomeration, may have contributed to the decrease in stability. In the present work, it is possible that the main component SiO2 in bentonite may have reacted with Ba or Fe to form compounds that are inactive for CLC. Costs of the ferrites should be comparable to the other metal-oxide-based oxygen carriers that have been reported for the CLC process. Barium ferrite is often used in the manufacture of permanent magnets, magnetic storage media, magnetic materials, and pigments. The heightened interest in ferrites is mainly due to the abundance of starting materials and a low production cost.21
XRD data of BaFe2O4 and Fe2O3 after reduction with carbon are shown in Figure 9. BaFe2O4 was reduced to Fe and Ba, while Fe2O3 was reduced to Fe3O4 and FeO. Ba acted as a promoter to enhance deeper reduction of Fe3O4/FeO to Fe, which contributed to a better performance in coal CLC. XRD data in Figure 9c showed that barium ferrite still existed as BaFe2O4 after multiple reduction/oxidation cycles synthesized by both the direct decomposition method and the microwave precipitation method. This indicated that, even though BaFe2O4 separated into Ba and Fe during the reduction cycle, they combined during the oxidation cycle to form BaFe2O4. Transmission electron microscopy (TEM) of 10% Al2O3/ BaFe2O4 before and after the cyclic reactions with carbon is shown in Figure 10. TEM data indicated that the particle size of
4. CONCLUSION Bimetallic oxygen carriers, selected from Group 2 metal ferrites (MgFe2O4, CaFe2O4, SrFe2O4, and BaFe2O4) and transitionmetal ferrites (NiFe2O4, CuFe2O4, MnFeO3, and CoFe2O4) synthesized by both the microwave precipitation method and the direct decomposition method, were evaluated for the coal CLC reaction. Group 2 metal ferrite oxygen carriers showed better reduction and oxidation rates than Fe2O3 for coal CLC. Among all of the novel metal ferrites, BaFe2O4 showed the highest reduction and oxidation rates for coal CLC; its reduction rate is comparable to CuO but can be operated at a higher reaction temperature, up to 1000 °C, without agglomeration. The CuO performance is generally restricted to 900 °C because of agglomeration and its low melting point. Cyclic tests on BaFe2O4 showed very stable performance for coal CLC without agglomeration, even at a high temperature (1000 °C). BaFe2O4 showed stable reduction rates, and its oxidation rates improved during cycling tests. Even though BaFe2O4 showed the best performance, each of the other metal ferrites tested can be used for coal CLC, because they all had
Figure 10. TEM of 10% Al2O3/BaFe2O4: (a) before and (b) after carbon CLC cyclic tests.
BaFe2O4 did not change significantly even after cyclic tests, which may have also contributed to the stable performance during cyclic tests. XRD showed that barium oxide in BaFe2O4 had participated in the CLC reaction with coal or carbon. BaO has never been reported as a CLC oxygen carrier. To investigate this, CLC of carbon with pure BaO was also performed. The results are shown in Figure 11, and the CLC performance of BaO with carbon is better than that with Fe2O3. The reduction rate of BaO is lower than that with BaFe2O4 but higher than that with Fe2O3, and the reaction temperature of BaO is higher than that with BaFe2O4. 2255
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Figure 11. Performance for the CLC reaction of carbon with BaO synthesized by the direct decomposition method.
Figure 12. Performance test of carbon black with 60% BaFe2O4 on supports: 40% BaAl2O4, Al2O3, and bentonite.
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ACKNOWLEDGMENTS This work was performed in support of the National Energy Technology Laboratory’s ongoing research under the Research and Engineering Services (RES) Contract DE-FE0004000. The authors also greatly appreciate Dr. Yun Chen from West Virginia University (WVU) and James A. Poston from the National Energy Technology Laboratory, U.S. Department of Energy (DOE), for help with scanning electron microscopy (SEM) measurements.
better performance than commercial Fe2O3. Metal ferrites have lower agglomeration than CuO. The cost of metal ferrites is comparable to that of Fe2O3 and CuO and can be prepared using readily available materials.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
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Notes
Disclaimer: This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. The authors declare no competing financial interest.
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dx.doi.org/10.1021/ef402528g | Energy Fuels 2014, 28, 2248−2257
