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Energy & Fuels 2009, 23, 2095–2102

2095

Synthesis Gas Generation by Chemical-Looping Reforming Using Ce-Based Oxygen Carriers Modified with Fe, Cu, and Mn Oxides Fang He,*,† Yonggang Wei,‡ Haibin Li,† and Hua Wang‡ Guangzhou Institute of Energy ConVersion, Chinese Academy of Sciences, Guangzhou 510640, China, and Faculty of Materials and Metallurgical Engineering, Kunming UniVersity of Science and Technology, Kunming 650093, China ReceiVed October 23, 2008. ReVised Manuscript ReceiVed January 12, 2009

Chemical-looping reforming (CLR) is a technology that can be used for partial oxidation and steam reforming of hydrocarbon fuels. It involves the use of a metal oxide as an oxygen carrier, which transfers oxygen from combustion air to the fuel. Composite oxygen carriers of cerium oxide added with Fe, Cu, and Mn oxides were prepared by co-precipitation and investigated in a thermogravimetric analyzer and a fixed-bed reactor using methane as fuel and air as oxidizing gas. It was revealed that the addition of transition-metal oxides into cerium oxide can improve the reactivity of the Ce-based oxygen carrier. The three kinds of mixed oxides showed high CO and H2 selectivity at above 800 °C. As for the Ce-Fe-O oxygen carrier, methane was converted to synthesis gas at a H2/CO molar ratio close to 2:1 at a temperature of 800-900 °C; however, the methane thermolysis reaction was found on Ce-Cu-O and Ce-Mn-O oxygen carriers at 850-900 °C. Among the three kinds of oxygen carriers, Ce-Fe-O presented the best performance for methane CLR. On Ce-Fe-O oxygen carriers, the CO and H2 selectivity decreased as the Fe content increased in the carrier particles. An optimal range of the Ce/Fe molar ratio is Ce/Fe > 1 for Ce-Fe-O oxygen carriers. Scanning electron microscopy (SEM) analysis revealed that the microstructure of the Ce-Fe-O oxides was not dramatically changed before and after 20 cyclic reactions. A small amount of Fe3C was found in the reacted Ce-Fe-O oxides by X-ray diffraction (XRD) analysis.

1. Introduction Synthesis gas, which is mainly composed of H2 and CO, is the most important intermediate for the production of ammonia, methanol, hydrogen, and many other chemical products. At present, steam or CO2 reforming of natural gas, where the reforming takes place in reactor tubes packed with catalyst, are the most widely used methods for synthesis gas production.1 The chemical reactions of the above-mentioned routes are shown in the following: Steam reforming: CH4 + H2O f CO + 3H2

(1)

CO2 reforming: CH4 + CO2 f 2CO + 2H2

(2)

These two reactions, which are highly endothermic reactions, consume a large amount of thermal energy, and the H2/CO ratio is unsuitable for the Fischer-Tropsch syntheses, in which the desired H2/CO ratio is 2.2 In comparison to the former two * To whom correspondence should be addressed. Telephone: +86-208705-7725. Fax: +86-20-8705-7737. E-mail: [email protected]. † Chinese Academy of Sciences. ‡ Kunming University of Science and Technology. (1) de Diego, L. F.; Ortiz, M.; Adanez, J.; Garcı´a-Labiano, F.; Abad, A.; Gaya´n, P. Synthesis gas generation by chemical-looping reforming in a batch fluidized bed reactor using Ni-based oxygen carriers. Chem. Eng. J. 2008, doi: 10.1016/j.cej.2008.06.004. (2) Wei, Y. G.; Wang, H.; He, F.; Ao, X. Q.; Zhang, C. Y. Ceria-based oxygen carrier partial oxidation of methane to syngas in molten salts: Thermodynamic analysis and experimental investigation. J. Nat. Gas Chem. 2007, 16 (1), 6–11.

methods, partial oxidation of methane to synthesis gas is a mild exothermic reaction and gives a H2/CO ratio of 2. In the view of the advantages of partial oxidation, it has attracted wide attention in recent years. Reaction of partial oxidation of methane: CH4 + 1/2O2 f CO + 2H2

(3)

Traditionally, in the partial oxidation of methane to synthesis gas, the oxygen source comes from pure oxygen. Therefore, an oxygen plant is needed for this method. It was reported that synthesis gas production accounted for about half of the capital investment in the gas-to-liquid process, because of the significant capital cost of the oxygen plant.3 If lattice oxygen of solid oxygen carriers instead of pure oxygen is used as the oxygen source of methane oxidation, the significant capital cost of building and running the oxygen plant will be avoided. Chemical-looping reforming of methane to synthesis gas is a novel methane partial oxidation process without pure oxygen consumption. Chemical-looping combustion (CLC) is a novel combustion technique with inherent separation of the greenhouse gas CO2 that involves the use of an oxygen carrier, which transfers oxygen from air to the fuel, avoiding the direct contact between them.4-6 The CLC system is made of two interconnected (3) Dai, X. P.; Yu, C. C.; Li, R. J.; Wu, Q.; Hao, Z. P. Synthesis gas production using oxygen storage materials as oxygen carrier over circulating fluidized bed. J. Rare Earths 2008, 26 (1), 76–80. (4) Mattisson, T.; Johansson, M.; Lyngfelt, A. The use of NiO as an oxygen carrier in chemical-looping combustion. Fuel 2006, 85 (5-6), 736– 747.

10.1021/ef800922m CCC: $40.75  2009 American Chemical Society Published on Web 02/18/2009

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He et al.

Figure 1. Chemical-looping reforming.

reactors, designated as air and fuel reactors.7 In the fuel reactor, the fuel gas (CnH2m) is oxidized to CO2 and H2O by a metal oxide (MexOy), so-called oxygen carrier, which is reduced to a metal (Me) or a reduced form of MexOy-1 with less oxygen content. As shown in Figure 1, chemical-looping reforming (CLR) has the same basic principles as CLC, with the main difference that the target product in CLR is not heat but synthesis gas (H2 and CO). Therefore, in the CLR process, it is important to select an oxygen carrier that tends to partially oxide methane to synthesis gas rather than CO2 and H2O. In addition, the oxygen/fuel ratio is kept low to prevent the complete oxidation of the fuel. The oxygen carrier circulates between the reactors. In the fuel reactor, it is reduced by the fuel, which in turn is oxidized to H2 and CO according to reaction 4. In the air reactor, it is oxidized to its initial state with O2 from the combustion air according to reaction 5. Partial oxidation of methane with an oxygen carrier: CH4 + MexOy f CO + 2H2 + MexOy-1

(4)

Oxygen carrier regeneration: MexOy-1 + 1/2O2 f MexOy

(5)

Ryden et al.8 studied synthesis gas generation by CLR in a continuously operating laboratory reactor consisting of two interconnected fluidized beds. They prepared 60 wt % NiO and 40 wt % MgAl2O4 composed particles, which are used as bed material, oxygen carrier, and reformer catalyst. They observed that complete conversion of natural gas was achieved and the selectivity toward H2 and CO was high. The formation of solid carbon was identified as a potential problem and was apparent for some of the experiments with dry natural gas. For most experiments with natural gas and 25 vol % steam, there was no (5) Mattisson, T.; Jardnas, A.; Lyngfelt, A. Reactivity of some metal oxides supported on alumina with alternating methane and oxygenapplication for chemical-looping combustion. Energy Fuels 2003, 17 (3), 643–651. (6) Mattisson, T.; Lyngfelt, A.; Cho, P. The use of iron oxide as an oxygen carrier in chemical-looping combustion of methane with inherent separation of CO2. Fuel 2001, 80 (13), 1953–1962. (7) Lyngfelt, A.; Leckner, B.; Mattisson, T. A fluidized-bed combustion process with inherent CO2 separation; application of chemical-looping combustion. Chem. Eng. Sci. 2001, 56 (10), 3101–3113. (8) Ryden, M.; Lyngflelt, A.; Mattisson, T. Synthesis gas generation by chemical-looping reforming in a continuously operating laboratory reactor. Fuel 2006, 85 (12-13), 1631–1641.

accumulation of carbon in the reactors and the composition of the reformer gas was close to thermodynamical equilibrium, which indicates that the carbon deposition was very low. Zafar et al.9 prepared oxides of Ni, Cu, Fe, and Mn supported on SiO2 and MgAl2O4 by dry impregnation and investigated under alternating reducing and oxidizing conditions in a thermogravimetric analyzer at 800-1000 °C using fuel (10% CH4, 10% H2O, and 5% CO2) and oxidizing gas (5% O2). They observed that NiO and CuO supported on both SiO2 and MgAl2O4 showed very high reactivity. The reactivity of NiO/ SiO2 decreased as a function of the cycle number at 950 °C but was avoided below 850 °C. Mn and Fe oxides supported on SiO2 may not be feasible oxygen carriers to be used in the CLC or CLR because of the formation of irreversible silicates at high temperatures. However, Fe and Mn oxides supported on MgAl2O4 showed a rather high reactivity during reduction and oxidation and can possibly be used in the process. In the previous studies, there was only a limited amount of work that had been performed with rare earth oxides as the oxygen carriers for CLC or CLR. The purpose of this paper is to investigate the feasibility of using CeO2 as the active constituent in the oxygen carriers in CLR using methane as the fuel. The rates of both reduction and oxidation of oxygen carriers as well as the selectivity toward different gaseous products were determined. To improve the performance of the CeO2-based oxygen carriers, modification by doping impurity oxides into the CeO2 was examined. 2. Experimental Section 2.1. Preparation of Oxygen Carriers. Oxygen carriers were prepared by co-precipitation. Ce(NO3)3 · 6H2O, Fe(NO3)3 · 9H2O, Mn(NO3)2 · 6H2O, and Cu(NO3)2 · 3H2O were weighed accroding to the desired Ce/Me (Me ) Fe, Mn, and Cu) ratio. The salts were made in their solution, respectively, and then mixed slowly in one beaker. The mixed solution was sufficiently stirred and heated at 70 °C. A solution of 10% ammonia was gradually dropped to the mixture with stirring until the pH value increased to 10-11. Then, the resulting solution was maintained at 70 °C with continuous stirring for 1 h and was aged for 2 h. Subsequently, the precipitate was filtered and washed with distilled water and ethanol. The paste mixture was dried at 110 °C for 24 h after natural drying in the air overnight, and some dried gel-like substances with different colors were obtained. These dried gel-like substances were heated at 300 °C for 2 h and were ground into powder. These powders were calcined in air at 800 °C for 6 h. The main physical properties and composition of the oxygen carriers are listed in Table 1. 2.1. Thermogravimetric Analysis (TGA). TGA experiments were conducted on a thermogravimetric analyzer, in which the amount weight change of the oxygen carrier was measured as a function of the temperature or reacting time. The setup and operational procedure of the TGA tests were described in detail in our previous studies.10 Briefly, the reaction tube consisted of concentric double quartz tubes. In the bottom of the inner tube, the oxygen carrier particle was placed on a quartz pan suspended by a thin quartz chain hung from an electric microbalance. When the temperature reached the desired value, CH4 and air were alternately introduced into the reaction tube from the top of the outer tube. The flow rate of the reactant gas was set at 10 mL min-1 for all experiments. The weight of the oxygen carriers, temperature, and the reaction time were recorded continuously by an online computer. (9) Zafar, Q.; Mattisson, T.; Gevert, B. Redox investigation of some oxides of transition-state metals Ni, Cu, Fe, and Mn supported on SiO2 and MgAl2O4. Energy Fuels 2006, 20 (1), 34–44. (10) He, F.; Wang, H.; Dai, Y. N. Application of Fe2O3/Al2O3 composite particles as oxygen carrier of chemical looping combustion. J. Nat. Gas Chem. 2007, 16 (2), 155–161.

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Figure 2. Experimental setup of the fixed-bed reactor. Table 1. Physical Properties and Composition of the Oxygen Carriers main crystalline phase specimen

particle size (µm)

specific surface area BET (m2/g)

CeO2 Ce-Fe-O-5/5 Ce-Cu-O-5/5 Ce-Mn-O-5/5 Ce-Fe-O-7/3

4-8 e5 5-10 4-6 e5

20.42 12.15 9.98 11.96 12.56

2.3. Fixed-Bed Reactor. Usually, two interconnected fluidized beds, a fuel reactor, and an air reactor are used in CLC and CLR. In the fuel reactor, the fuel is oxidized by a metal oxide, and in the air reactor, the reduced metal is oxidized back to the original state.11,12 However, the reactivity of the oxygen carriers and the gas product distribution during the reaction can also be investigated in a fixed-bed reactor by exposing the oxygen carriers to alternating fuel and air conditions, simulating a CLC system.13,14 In the present work, reduction-oxidation (redox) multicycles were conducted in a fixed-bed reactor to know the gas product distributions by exposing the oxygen carriers to an alternating methane and air atmosphere. Figure 2 shows the experimental setup of the fixedbed reactor for testing the oxygen carriers. A quartz tube with 1000 mm in length and 19 mm in inside diameter was put into a tubular furnace. For each test, 1.8 g of the oxygen carrier was placed in the middle part of the reactor between two flocks of quartz wool. The void part of the quartz tube was filled with quartz beads (380-830 µm). The temperature of the oxygen carrier was monitored by a thermocouple placed in the center of the oxygen carrier bed. The temperature of the furnace was maintained by an electronic controller. Prior to reactivity tests, the oxygen carrier was heated to 600 °C in flowing air at a rate of 10 °C min-1 and kept at this temperature for 1 h and then N2 was introduced to the reactor for half an hour, aiming to drive all of the air out of the reactor. The reactivity tests were performed by feeding methane (10 mL min-1) at a temperature range of 600-900 °C. The product gases were analyzed using an online gas chromatograph (GC) (11) Abad, A.; Mattisson, T.; Lyngfelt, A.; Ryde´n, M. Chemical-looping combustion in a 300 W continuously operating reactor system using a manganese-based oxygen carrier. Fuel 2006, 85 (9), 1174–1185. (12) Johansson, E.; Mattisson, T.; Lyngfelt, A.; Thunman, H. A 300 W laboratory reactor system for chemical-looping combustion with particle circulation. Fuel 2006, 85 (10-11), 1428–1438. (13) Ada´nez, J.; Garcı´a-Labiano, F.; de Diego, L. F.; Gaya´n, P.; Celaya, J.; Abad, A. Nickel-copper oxygen carriers to reach zero CO and H2 emissions in chemical-looping combustion. Ind. Eng. Chem. Res. 2006, 45 (8), 2617–2625. (14) Corbella, B. M.; de Diego, L. F.; Garcı´a-Labiano, F.; Ada´nez, J.; Palacios, J. M. Characterization and performance in a multicycle test in a fixed-bed reactor of silica-supported copper oxide as oxygen carrier for chemical-looping combustion of methane. Energy Fuels 2006, 20 (1), 148– 154.

fresh CeO2 CeO2, CeO2, CeO2, CeO2,

after reduction Ce2O3 CeO2, Ce2O3, CeO2, Ce2O3, CeO2, Ce2O3, CeO2, Ce2O3,

Fe2O3 CuO Mn2O3 Fe2O3

FeO, Fe3C Cu2O, Cu Mn3O4, MnO FeO, Fe3C

equipped with a thermal conductivity detector (TCD) and an active carbon column (2 m), employing argon as a carrier gas. The GC data were processed with a computer workstation. The qualitative analysis of product gase composition was made by residence time and quantitative analysis by the area unitary method. On the basis of the analysis data of GC, the selectivity of H2, SH2, was calculated as

SH2 )

nH2,out nH2,out + nH2O,out

× 100%

(6)

The selectivity of CO, SCO, was calculated as below

SCO )

nCO,out × 100% nCO,out + nCO2,out + nC

(7)

The selectivity of CO2, SCO2, was calculated as

SCO2 )

nCO2,out nCO,out + nCO2,out + nC

× 100%

(8)

where nH2,out, nH2O,out, nCO,out, and nCO2,out are the molar fractions of H2, H2O, CO, and CO2 in the outlet gas from the reactor, respectively. nC is the molar fraction of carbon deposition in the reactor during the reduction reactions. It can be calculated from the inlet of CH4 and O2 and outlet of CH4, H2, CO, CO2, H2O, and O2. The amount of unreacted CH4 out of the reactor was not taken into account during the calculation of the selectivity of H2, CO, and CO2. 2.4. Characterization of Oxygen Carriers. The X-ray diffraction (XRD) experiments were performed for all oxygen carrier samples in a Japan Science D/max-R diffractometer with Cu KR radiation (λ ) 0.154 06 nm), operating voltage of 40 kV, and current of 40 mA, and the diffraction angle (2θ) was scanned from 10° to 80°. Morphology and microstructure of the fresh and reacted oxygen carriers were determined by means of a Philips XL30 scanning

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Figure 3. TG curves of CeO2 in methane and air atmospheres.

Figure 4. TG curves of Ce-Fe-O, Ce-Cu-O, and Ce-Mn-O oxygen carriers in a methane atmosphere.

electron microscope (SEM) attached with energy dispersive spectroscopy (EDS).

3. Results and Discussion 3.1. Thermogravimetric Analysis of Oxygen Carriers in TGA. Weight loss curves as a function of the temperature of pure CeO2 in air (as a blank test) and methane are shown in Figure 3. It can be seen that the pure CeO2 is stable in air at the temperature range from 0 to 1000 °C. This means that it is difficult of pure CeO2 to loss its lattice oxygen in the absence of reducing gas reactants. In the presence of CH4, however, CeO2 starts to loss its lattice oxygen at a temperature of 590 °C. Clearly, the TG curve of CeO2 in methane flow is divided into two stages: the first is starting from 590 to 770 °C, and the latter is from 770 to 930 °C. During the whole process, the weight loss ratio is 3.04% for the pure CeO2. Theoretically, if the CeO2 is totally reduced to Ce2O3, the weight loss ratio will be 4.65%. Therefore, it is difficult to reduce all of the CeO2 to Ce2O3 when cerium oxide is used as the oxygen carrier and CH4 is used as the fuel. A part of CeO2 may be reduced to nonstoichiometric oxides, such as CeO1.83 and CeO1.72. Figure 4 shows the TG curves of Ce-Fe-O, Ce-Cu-O, and Ce-Mn-O oxygen carriers in methane flow, where the molar ratio of Ce/Me (Me ) Fe, Cu, and Mn) in these oxygen carriers is 1:1. In comparison to pure CeO2, the composite oxygen carriers have a higher ability of oxygen donation. The

He et al.

Ce-Fe-O oxygen carrier lost its weight of 2.1% from 300 to 540 °C and 10.3% from 640 to 880 °C, respectively. The total weight loss rate reached 12.4%, which is much higher than that of pure CeO2 (3.04%). It is clear that doping Fe2O3 into CeO2 dramatically increased the oxygen donation performance of the Ce-Fe-O oxygen carrier. Generally, it is believed that both CeO2 and Fe2O3 in the Ce-Fe-O composite can provide oxygen during CH4 partial oxidation. For the Ce-Fe-O composite with a Ce/Fe molar ratio of 1:1, if all of the CeO2 was reduced to Ce2O3 and Fe2O3 reduced to Fe, the theoretical weight loss rate is 16.86%. Apparently, not all of Fe2O3 was reduced to metallic Fe. As for the Ce-Mn-O oxygen carrier, there are also two weight loss stages in its TG curve (see Figure 4). From 480 to 675 °C, the weight of the sample lost 1.8%, and from 675 to 900 °C, the weight of the sample lost 2.6%. The total weight loss rate of the Ce-Mn-O oxygen carrier during the reaction was 4.4%, which is a little higher than that of pure CeO2. XRD analysis revealed that the Mn element in the Ce-Mn-O composite was mainly found in the form of Mn2O3, which was transformed into Mn3O4 first and MnO subsequently when exposed in CH4 flow. If all of CeO2 and Mn2O3 were reduced into Ce2O3 and MnO, respectively, the theoretical weight loss rate of Ce-Mn-O was 6.06%, while the actual value was 4.4%. It is indicated that not all of Mn2O3 was reduced to MnO, and this point was verified by the XRD analysis by which Mn3O4 was found in the reduced oxygen carrier. Another TG curve in Figure 4 was assigned to the Ce-Cu-O oxygen carrier. Similarly, there are two weight loss stages at a temperature of 300-510 and 690-875 °C, respectively. A level off at a temperature from 510 to 690 °C was found in the TG curve of Ce-Cu-O. The weight loss rates of the two stages were 5.7 and 2.1%, respectively. XRD analysis showed that CuO was reduced to both Cu and Cu2O during the reaction. On the basis of the above analysis, it can be seen that the addition of Fe, Mn, and Cu oxides into CeO2 can substantially improve the ability of oxygen donation of the oxygen carriers. Caputo and co-workers15 found similar results. They claimed that the enhanced activity of these composite oxides was contributed to the interaction between copper (or iron/manganese) clusters and cerium oxide and address to ceria the role of the oxygen source. It is worth noting that the composite oxygen carriers of Ce-Fe-O, Ce-Mn-O, and Ce-Cu-O started to react with CH4 at 300, 480, and 300 °C, respectively, while the pure CeO2 did not react with CH4 until the temperature reached 590 °C. To some extent, the reaction taking place at lower temperature indicates that the oxygen carrier has better reactivity. Therefore, the composite oxygen carriers have better reactivity than the pure CeO2. To test the cyclic reactivity of these oxygen carriers, multicycles of redox of the oxides exposed to alternating methane and air conditions were carried out in the TGA reactor at 850 °C. Here, we used the degree of oxidation or conversion to characterize the conversion of oxygen carriers in redox cycles. The degree of oxidation or conversion was defined as X)

m - mred mox - mred

(9)

where m is the instantaneous mass of the sample, mox is the mass of the sample when fully oxidized, and mred is the mass of the sample in the reduced form. The reduced form is either (15) Caputo, T.; Lisi, L.; Pirone, R.; Russo, G. On the role of redox properties of CuO/CeO2 catalysts in the preferential oxidation of CO in H2-rich gases. Appl. Catal., A 2008, 348 (1), 42–53.

Synthesis Gas Generation by CLR

Figure 5. Conversion, X, as a function of time for the first five redox cycles with Ce-Fe-O at 850 °C.

the metal or a metal oxide with lower oxygen content. Therefore, the difference between mox and mred in eq 9 is the amount of active oxygen in the oxygen carrier. In this work, the difference of mox and mred is calculated on the basis of the following transformations between metal oxide and a reduced metal oxide or metal: (i) CeO2 - Ce2O3, (ii) Fe2O3 - FeO, (iii) Mn2O3 MnO, and (iv) CuO - Cu. As a result, stable reactivity was observed during the 20cycle test for all three kinds of oxygen carriers at 850 °C. In reduction, the reaction rate was fast when the degree of oxidation changed from 1.0 to 0.6. If the oxygen carrier was further reduced from the degree of oxidation of 0.6 to less, the reaction rate became slower. In an actual CLC or CLR process, to obtain a desired reaction rate, a suitable degree of oxidation change in the course of redox cycles should be selected. Fully reducing the oxygen carrier to the theoretical reduced form, i.e., X ) 0, is usually not necessary. However, decreasing the degree of oxidation change means more oxygen carrier is needed during the reactions. At the end stage of reduction, the X value will increase if carbon deposition takes place rapidly. However, X increasing was not obvious at the later stage of reduction for the three composite oxygen carriers. This indicated that carbon deposition was not significant in reduction with the oxygen carriers. Additionally, carbon formation can be significantly inhibited by humidifying the methane before being introduced into the fuel reactor. The oxidation rates of the reduced oxygen carriers in air flow for the three kinds of oxides were quite faster compared to their reduction stages. This suggests that these oxygen carriers have good regenerability. The degree of oxidation as a function of time for the first five redox cycles with Ce-Fe-O at 850 °C is shown in Figure 5. Similar profiles were observed for the other two oxygen carriers. 3.2. Reactivity of Oxygen Carriers in a Fixed-Bed Reactor. In the previous study, we investigated the methane conversion reacting with pure ceria and composite Ce-Me-O oxygen carriers as a function of the temperature. In addition, the gas product distribution as a function of reacting time for the redox cycles was investigated.16 As a result, the methane conversion rate of composite oxygen carriers was clearly higher than that of pure CeO2 in a wide temperature range, and the conversion was above 90% after 850 °C. In this work, we were (16) Li, K. Z.; Wang, H.; Wei, Y. G.; Liu, M. C. Preparation and characterization of Ce1-xFexO2 complex oxides and its catalytic activity for methane selective oxidation. J. Rare Earths 2008, 26 (2), 245–249.

Energy & Fuels, Vol. 23, 2009 2099

Figure 6. Selectivity of CO and CO2 and the H2/CO molar ratio versus temperature using Ce-Fe-O as the oxygen carrier.

focusing on the selectivity of H2, CO, and CO2 when the Ce-Me-O composite oxides were used as oxygen carriers. Figure 6 shows an example of the selectivity of CO and CO2 and the H2/CO molar ratio at different temperatures using Ce-Fe-O as the oxygen carrier. Before 750 °C, the selectivity of CO2 was higher than that of CO. CO2 and H2O were the overwhelming products, although H2 and CO were observed at the early stage of the reaction before around 750 °C. This indicates that CH4 was mainly oxidized to CO2 and H2O in total oxidation at the beginning of the reaction at lower temperatures. After 750 °C, the selectivity of CO increased rapidly and reached close to 100% after 800 °C. Generally speaking, the oxygen species in oxygen carriers is classified into two types: surface oxygen (also called adsorption molecular oxygen) and bulk lattice oxygen.17 Surface oxygen contributed to the complete oxidation of methane to CO2 and H2O because it has a higher reactivity. Also, the lattice oxygen released, corresponding to the transformation Fe2O3 f Fe3O4, is prone to CH4 total oxidation.18 This is easy to explain the reason why methane was oxidized to CO2 and H2O at the initial stage of the reaction and then selectively oxidized into CO and H2 at higher temperatures. In addition, as shown in the Figure 6, the H2/CO molar ratio reduced from approximately 3 to 2 from 700 to 800 °C and then the ratio was almost kept a constant at around 2. In the view of the ratio of H2/CO at 2 being ideal for Fischer-Tropsch synthesis or other continuous use, the Ce-Fe-O composite oxygen carrier displays its obvious advantages. The selectivity of CO and CO2 and the H2/CO molar ratio along with the temperature using Ce-Mn-O as the oxygen carrier are shown in Figure 7. The reaction initially proceeded to a high concentration of CO2 with little outgoing CO and H2 in the earlier stage. As the reaction proceeds, the CO concentration increased continuously and reached a level off close to 100% after 850 °C. The H2/CO molar ratio increased rapidly when the temperature was higher than 750 °C, reaching 29.8 at 900 °C. It seems that a part of methane was decomposed to H2 and C. This can be verified by the fact that a small amount of CO2 was released at the start of the oxidation periods, which indicates that little carbon formation occurred in the course of (17) Otsuka, K.; Wang, Y.; Sunada, E.; Yamanaka, I. Direct partial oxidation of methane to synthesis gas by cerium oxide. J. Catal. 1998, 175 (2), 152–160. (18) Zafar, Q.; Mattisson, T.; Gevert, B. Integrated hydrogen and power production with CO2 capture using chemical-looping reforming-redox reactivity of particles of CuO, Mn2O3, NiO, and Fe2O3 using SiO2 as a support. Ind. Eng. Chem. Res. 2005, 44 (10), 3485–3496.

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Figure 9. CH4 conversion versus the reaction temperature using Ce-Fe-O-X as oxygen carriers. Figure 7. Selectivity of CO and CO2 and the H2/CO molar ratio versus temperature using Ce-Mn-O as the oxygen carrier.

Figure 10. CO selectivity versus the reaction temperature using Ce-Fe-O-X as oxygen carriers.

Figure 8. Selectivity of CO and CO2 and the H2/CO molar ratio versus temperature using Ce-Cu-O as the oxygen carrier.

the reduction periods.18 Far from 2 of the H2/CO molar ratio in the produced synthesis gas is not convenient for subsequent use. The selectivity of CO and CO2 and the H2/CO molar ratio as a function of the temperature of the Ce-Cu-O oxygen carrier are shown in Figure 8. The incoming methane reacted with the oxygen carrier to form CO2 and H2O, with minor formation of H2 and CO before 800 °C. As the reaction proceeds, there was an increase in the concentrations of both CO and H2 and the concentration of CO was higher than that of CO2 after the temperature reached 825 °C. The selectivity of CO reached 100% at 850 °C, which means that almost all of methane reacting with the oxygen carrier converts to H2 and CO. It can be observed that the H2/CO molar ratio was about 2 at 850 °C; however, it increased dramatically after this temperature and attained 8.3 at 900 °C. This indicates that methane starts to decompose obviously when the temperature is higher than 850 °C. Therefore, 850 °C is an ideal temperature of CLR if Ce-Cu-O used as the oxygen carrier. Methane decomposition on both Ce-Mn-O and Ce-Cu-O oxygen carriers is not favorable for their use in CLR. From the point of view of the oxygen-donation ability, reactivity, selectivity, as well as the cost, Ce-Fe-O is a promising oxygen carrier candidate for methane CLR. Therefore, the effect of the Ce/Fe molar ratio on the reactivity of the Ce-Fe-O oxygen carrier was investigated in the following section of this work. 3.3. Effect of the Ce/Fe Ratio on the Reactivity of the Ce-Fe-O Oxygen Carrier. It is well-know that complex metal

oxides may sometimes provide better properties than those of individual metal oxides.19 In comparison to the properties of the above three type of composite oxygen carriers, Ce-Fe-O demonstrated a better performance on the whole. Therefore, the performance of the Ce-Fe-O composite with various Ce/Fe molar ratios was emphatically studied and compared in this section. We employed Ce-Fe-O-X to stand for the composite oxygen carriers, where, X is the Ce/Fe molar ratio. For example, Ce-Fe-O-9/1 means the Ce/Fe molar ratio is 9:1 in the oxygen carrier particles. Johansson et al.20 found that mixed metal oxides had higher methane conversion rates than those of sole oxides because of synergy effects between the mixed oxides. Additionally, the molar ratio of the two different oxides in the mixture could affect the performance of the oxygen carriers.21 Figure 9 presents the CH4 conversion as a function of the temperature for Ce-Fe-O-X oxygen carriers. It can be seen clearly that composite oxygen carriers of ceria with added Fe2O3 give higher CH4 conversion than those of individual CeO2 and Fe2O3 at a wide temperature range of 600-900 °C. It is clear from Figure 9 that the addition of Fe2O3 into ceria has beneficial effects with respect to CH4 conversion. The CH4 conversions were lower than 20% for all nine types of oxygen carriers before 750 °C. After that temperature, the CH4 conversions increased (19) Jin, H. G.; Okamoto, T.; Ishida, M. Development of a novel chemical-looping combustion: Synthesis of a looping material with a double metal oxide of CoO-NiO. Energy Fuels 1998, 12 (6), 1272–1277. (20) Johansson, M.; Mattisson, T.; Lyngfelt, A. Creating a synergy effect by using mixed oxides of iron and nickel oxides in the combustion of methane in a chemical-looping combustion reactor. Energy Fuels 2006, 20 (6), 2399–2407. (21) Ryden, M.; Lyngfelt, A.; Mattisson, T.; Chen, D.; Holmen, A.; Bjørgum, E. Novel oxygen-carrier materials for chemical-looping combustion and chemical-looping reforming; Lax Sr1-xFeyCo1-yO3-δ perovskites and mixed-metal oxides of NiO, Fe2O3 and Mn3O4. Int. J. Greenhouse Gas Control 2008, 2 (1), 21–36.

Synthesis Gas Generation by CLR

Figure 11. H2 selectivity versus the reaction temperature using Ce-Fe-O-X as oxygen carriers.

dramatically for the composite oxygen carriers, with the exception of individual CeO2 and Fe2O3. However, the effect of the Ce/Fe molar ratio on the reactivity of the oxygen carriers is not simply monotonic increasing or decreasing. It can also be seen that the Ce-Fe-O-7/3 gave the best reactivity among all of the oxygen carriers, and the conversion rate reached more than 95% at 900 °C. Jin and co-workers19 reported that the reactivity of mixed metal oxides could be reduced if a solid solution was formed as redox reactions proceeded. XRD patterns show that the formation of the solid solution between CeO2 and Fe2O3 was not found during the reactions. Figure 10 represents the CO selectivity of Ce-Fe-O-X oxygen carriers with respect to the reaction temperature. Broadly speaking, CeO2 and the seven complex oxygen carriers provided higher CO selectivity than Fe2O3. This indicates that individual Fe2O3 oxygen carriers display high methane total oxidation ability rather than partial oxidation. Similar results have been obtained by other authors.22 Cho et al.23 observed that the

Energy & Fuels, Vol. 23, 2009 2101

reduction of Fe2O3 to Fe3O4 is able to convert methane completely to carbon dioxide and water. For reduction to a lower iron oxide, such as FeO, it is possible to convert methane partially to carbon monoxide and hydrogen. The sample of the Ce/Fe ratio of 2:8 had property somewhat resembling that of sole Fe2O3 oxygen carriers. In most parts of the temperature range from 600 to 800 °C, pure CeO2 had higher CO selectivity than the mixed oxides, except for Ce-Fe-O-9/1 and Ce-Fe-O8/2, which gave higher CO selectivity than individual CeO2 at 680-800 and 710-765 °C, respectively. After 800 °C, however, Ce-Fe-O-7/3 demonstrated the highest CO selectivity. The H2 selectivity along with reaction temperatures for various oxygen carriers is illustrated in Figure 11. Also, a strong temperature dependence is seen; the H2 selectivity for all oxygen carriers increased with the temperature. It can be seen that the individual Fe2O3 oxygen carrier had the lowest H2 selectivity and, on the contrary, CeO2 gave the highest H2 selectivity in the whole temperature range. The H2 selectivity of the mixed oxides was just between those of the two individual oxides (Fe2O3 and CeO2). Besides, the more percentage of CeO2 contained in the mixture, the higher the H2 selectivity at the same temperature. At 800 °C and above, however, the complex oxygen carriers with a Ce/Fe molar ratio of 7:3, 8:2, and 9:1 had H2 selectivity very close to the ones with only CeO2. As shown in Figures 9-11, it is clear that there exists a range of Ce/Fe molar ratios (Ce-Fe-O-X, X > 5/5) where the conversion of CH4 and selectivity of H2 and CO were the highest at temperatures above 850 °C. In addition, H2 and CO selectivity of the oxygen carriers reduced as the Fe content increased in the carriers. Therefore, from the viewpoint of CH4 conversion and CO/H2 selectivity, the optimal Ce/Fe molar ratio in the composite oxygen carriers is Ce/Fe > 1 for methane CLR.

Figure 12. SEM-EDS images of fresh and reacted Ce-Fe-O-7/3 oxygen carriers.

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3.4. Characterization of the Ce-Fe-O Oxygen Carrier. The surface of the Ce-Fe-O-7/3 oxygen carrier was studied with a SEM before and after 20 reduction and oxidation cyclic reactions (see Figure 12). The grain size of a fresh Ce-Fe-O-7/3 oxide was about 2-4 µm in diameter, and small pores existed. After 20 cyclic reactions, the particle size of the reacted oxygen carrier was under 5 µm. Sintering between the particles was not found. EDS analysis indicated that the fresh oxygen carrier are just composed of three kinds of elements: O, Ce, and Fe. It is worth noticing that the microstructures of the fresh and reacted oxides were not substantially changed. In other words, the microstructure of the Ce-Fe-O-7/3 was stable in the course of the cyclic reaction, and agglomeration was not observed after 20 cyclic redox reactions. The obvious difference of the EDS analysis of the fresh and reacted oxygen carriers can be found in Figure 12. Except for O, Ce, and Fe elements, C and a very small amount of Al and Si were detected on the surface of the reacted oxygen carrier. The traces of Al and Si elements were considered coming from the quartz tube and wool of the reactor or quartz beads packed in the reactor tube. The presence of C element on the surface of the oxygen carrier suggested that the carbon deposition reaction was taking place during the reduction periods. Carbon formation could occur through either methane pyrolysis or the Boudouard reaction. Carbon deposition reactions are side reactions, which should be inhibited of methane reforming. The addition of a percentage of steam into methane flow can efficiently suppress carbon formation.24 XRD analysis revealed that a part of carbon reacted with the iron oxides forming Fe3C, which was believed to be contributing to the inferior of the (22) Abad, A.; Mattisson, T.; Lyngfelt, A.; Johansson, M. The use of iron oxide as oxygen carrier in a chemical-looping reactor. Fuel 2007, 86 (7-8), 1021–1035. (23) Cho, P.; Mattisson, T.; Lyngfelt, A. Comparison of iron-, nickel-, copper- and manganese-based oxygen carriers for chemical-looping combustion. Fuel 2004, 83 (9), 1215–1225. (24) Johansson, M.; Mattisson, T.; Lygfelt, A.; Abad, A. Using continuous and pulse experiments to compare two promising nickel-based oxygen carriers for use in chemical-looping technologies. Fuel 2008, 87 (6), 988–1001. (25) Hossain, M. M.; de Lasa, H. I. Chemical-looping combustion (CLC) for inherent CO2 separationssA review. Chem. Eng. Sci. 2008, 63 (18), 4433–4451.

He et al.

reactivity of Fe-based oxygen carriers.25 Controlling the Fe2O3 not to be reduced to Fe can inhibit the formation of Fe3C in the course of reduction periods. 4. Conclusion Composite ceria-based oxygen carriers for methane CLR modified with iron, copper, and manganese oxides were prepared by means of co-precipitation. The performances of the composite oxygen carriers were investigated in TGA and fixed-bed reactors. The characterizations of the fresh and reacted oxygen carriers were studied using SEM-EDS and XRD techniques. It has been demonstrated that doping transition-metal oxides can improve the oxygen-donation ability and reactivity of a ceria-based oxygen carrier. The Ce-Fe-O, Ce-Mn-O, and Ce-Cu-O oxygen carriers gave high CO and H2 selectivity at above 800 °C. Methane was converted to synthesis gas with a H2/CO molar ratio closing to 2:1 by reacting with a Ce-Fe-O oxygen carrier at temperatures of 800-900 °C while converting to syngas with a H2/CO ratio of much more than 2:1 by reacting with Ce-Mn-O and Ce-Cu-O at above 850 °C. This indicated that the thermolysis reaction of methane was taking place on Ce-Mn-O and Ce-Cu-O when the temperature is higher than 850 °C. Of the three investigated composite oxygen carriers, Ce-Fe-O presented the best performance for methane CLR. The Ce/Fe molar ratio in the mixed oxides had an obvious effect on the performance of the oxygen carriers. The H2 and CO selectivity was decreased when the Fe content was increased in the carrier particles. From the viewpoint of reactivity and CO/H2 selectivity, an optimal range of the Ce/Fe molar ratio is Ce/Fe > 1 for Ce-Fe-O mixed oxides. SEM analysis revealed that the microstructures of the Ce-Fe-O-7/3 oxides were not dramatically changed before and after 20 cyclic reactions. Acknowledgment. The financial support of the National Natural Science Foundation of China (50574046 and 50774038) is gratefully acknowledged. This work was also supported by the Director Foundation of Guangzhou Institute of Energy Conversion (0807z2 and 0807rf), Chinese Academy of Sciences. EF800922M