Synthesis Gas Generation by Chemical-Looping Reforming in a

Article pubs.acs.org/IECR

Synthesis Gas Generation by Chemical-Looping Reforming in a Circulating Fluidized Bed Reactor Using Perovskite LaFeO3‑Based Oxygen Carriers Xiao Ping Dai, Jie Li, Jiang Tao Fan, Wei Sheng Wei,* and Jian Xu State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, People’s Republic of China S Supporting Information *

ABSTRACT: Perovskite-based LaFeO3/Al2O3−Kaolin oxygen carriers, prepared by a wet-mixing−kneading method, have been examined for chemical-looping reforming over a microfixed bed reactor and a circulating fluidized bed reactor. An oxygen carrier containing 60% LaFeO3 over 15Al2O3-25Kaolin exhibits higher reactivity, and can converted methane to syngas with high selectivity. The addition of Al2O3−Kaolin improves the oxygen migration rate from bulk to surface, and increases the amount of the very reactive oxygen species for CO2 formation. The CH4 conversion and CO selectivity rely heavily on reaction temperature and bed height. Increasing the fuel reactor temperature and bed height is beneficial for the reforming application with natural gas as fuel over a bubbling fluidized bed reactor. Preliminary results on the circulating fluidized bed reactor show that the CH4 conversion is about 25%, and CO selectivity is about 70% at an oxygen carrier-fuel molar ratio (1.32) and 900 °C. It is helpful to improve the redesign of the experimental facility and optimization of process parameters in further research. There is still a high potential for further improvement in mechanical strength and attrition resistance of perovskite-based oxygen carrier by optimization of binder and support.

1. INTRODUCTION Stream reforming of natural gas is a commercial process to produce syngas, which is the main source for the production of ammonia, methanol, hydrogen, and many other important products. However, because of its high endothermicity, this process is characterized by substantial capital costs and high energy consumption. On the other side, autothermal reforming uses a much more compact reactor, but the process involves the use of pure O2, commonly produced via cryogenic distillation and associated with nonzero explosion risks.1 For these reasons, an attractive alternative process for selective oxidation of CH4 to syngas would be desired. More recently, work has been directed toward partial oxidation based on the chemical-looping reforming concept (CLR), which is a novel and alternative technology that can be used for syngas production by partial oxidation and steam reforming of hydrocarbon fuels.2−4 Chemical-looping reforming, as examined in this paper, was originally proposed by Mattissson et al.5 Similar ideas have also been explored by Otsuka et al.,6 Stobbe et al.,7 Fathi et al.,8 Dai et al.,9 and Cimini et al.10 In this process, it is configured with two interconnected fluidized bed reactor: an air reactor and a fuel reactor. The oxygen carrier is circulated between the two reactors, which avoids the direct contact between fuel and air. Oxygen in the air is chemically fixed and converts to the lattice oxygen of oxygen carriers over an air reactor (reaction 1), and then methane is selectively oxidized to syngas by the oxygen species of oxygen carriers over the fuel reactor (reaction 2). The oxygen carrier goes through an oxidation/reduction cycle each time, and it circulates around the loop. The major advantage of this process is that the heat needed for converting CH4 is supplied by the flow of solid oxygen carrier from the exothermic air reactor to endothermic fuel reactor, without costly oxygen production, without diluting products © 2012 American Chemical Society

with N2, and without the mixing of air with fuel gases in the process.4,11 The net energy released in the reactor system is same as for ordinary partial oxidation of CH4 to syngas. air reactor: [ ]L + air → [O]L + N2

(1)

fuel reactor: CH4 + [O]L → CO + H 2 + CO2 + H 2O + [ ]L

(2)

where [O]L is framework oxygen and [ ]L is oxygen vacancy. The selection of oxygen carrier is a key issue for the CLR process. There are some works to explore different oxygen carriers for CLR, such as iron, nickel, copper, and manganese. NiO as oxygen carrier exhibited high reaction rate and good selectivity toward H2 and CO, while Fe2O3, CuO, and Mn2O3based oxygen carriers suffered from poor selectivity and thus produced CO2, H2O.12,13 The formation of deposit carbon was identified as a potential problem and was apparent for some of the experiments.2,13 For example, there was significant formation of deposit carbon in the fuel reactor with only natural gas as fuel for Ni18-αAl and Ni21-γAl, though the carbon formation can be reduced or eliminated by adding steam or CO2 to the fuel.4,14 Rydén et al.2,14 and de Diego at al.4 have demonstrated continuous chemical-looping reforming with Ni-based oxygen carriers in small laboratory units, but the formation of NiAl2O4 could decrease the reduction reactivity for NiO on Al2O3.4 These authors confirmed that the chemical-looping reforming concept Received: Revised: Accepted: Published: 11072

January 4, 2012 June 1, 2012 July 5, 2012 July 5, 2012 dx.doi.org/10.1021/ie300033e | Ind. Eng. Chem. Res. 2012, 51, 11072−11082

Industrial & Engineering Chemistry Research

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was then heated at 250 °C for 30 min with a fast exothermic reaction, and yielded brown powder precursor. Preparation of LaFeO3/Al2O3−Kaolin by wet-mixingkneading method. Conventional aluminum sol and kaolin were used as binder and support for preparation of LaFeO3/ Al2O3−Kaolin, respectively. The precursor was mixed uniformity with different amount of alumina sol binders (21 wt % Al2O3.nH2O, pH = 4, From Beijing Chemical Company) and kaolin (containing 37 wt % Al2O3 and 48 wt % SiO2, From Lanzhou PetroChemical Company, PetroChina Company, Ltd.), then extruded, and dried for 24 h at 80 °C. The mixture was calcined at 800 °C for 1 h, then increased to 900 °C and kept for 6 h. The samples were crushed, and sieved to a size range of 75−150 μm. 2.2. Materials Characterization. X-ray Diffraction Characterization. The crystal structures of catalysts were determined by a powder X-ray diffractometer (Shimadzu XRD 6000), using Cu Kα (λ = 1.54184 Å) radiation combined with a nickel filter operating at 40 kV and 10 mA. The diffractometer data were recorded for 2θ values between 15° and 80° with a scanning rate of 4°/min. Surface Area and Pore Volume. Nitrogen adsorption/ desorption was measured by using a Micromeritics, ASAP 2405N analyzer at −196 °C in liquid nitrogen. The Brunauer− Emmett−Teller (BET) surface area was calculated using experimental points at a relative pressure of P/P0 = 0.05−0.30. Prior to N2 sorption measurements, the samples were degassed at 250 °C, under vacuum, for at least 16 h. Attrition Resistance. The attrition resistance of the calcined oxygen carrier was measured with a modified three-hole air-jet attrition tester. The attrition resistance was determined at 10 L/ min (slpm) over 5 h. The attrition index (AI) is the percent of fines generated over 5 h. The fines are particles collected at the thimble, which was attached to the gas outlet. Lower AI values indicate better attrition resistance of the bulk particles.

is feasible, but the carbon formation could be an obstacle unless the fuel was mixed with some steam, and should be further investigated. Perovskite oxides, with a general formula ABO3, have been used effectively for catalytic oxidation reactions including hydrogenation and hydrogenolysis of hydrocarbons, CO oxidation, ammonia oxidation, and catalytic methane combustion. Because of their variable structure and chemical composition, perovskite oxides usually exhibit excellent reduction−oxidation properties, high reversible oxygen mobility, and chemical and thermal stability, and have potential application for chemical-looping process.9,15−27 LaFeO3 perovskite exhibits high activity and selectivity to syngas over a microfixed bed reactor, and make it attractive as an oxygen carrier by the chemical looping process.9,15−21 The surface adsorbed oxygen over LaFeO3 oxygen carrier is highly active to complete combustion, while the lattice oxygen is very selective to syngas. It is well-known that substitution of La3+ with cations of a lower oxidation state, such as Sr, leads a fraction of the B cations in a higher valence state and nonstoichiometric microstructural defects, which results in a better catalytic activity for perovskite oxides.22 LaxSr1‑xFeO3‑d perovskites provided very high selectivity toward CO/H2 and should be well suited for chemicallooping reforming. Substituting La for Sr was found to increase the oxygen capacity of these materials, and reduced slightly the selectivity toward CO/H2.16,23,24 Partial substitution of Fe with Co in La1‑xSrxCoyFe1‑yO3 perovskites, even very small amount (such as y = 0.1), resulted decrease in the structural stability and continuous oxygen supply in sequential reduction−oxidation cycles.16,19 Furthermore, the CO selectivity decreased dramatically as y increased, and it is also not suitable for chemicallooping reforming applications.23 Kharton et al. reported a correlation between the degree of reduction and the activity and selectivity for syngas formation over La0.3Sr0.7Fe0.8M0.2O3‑d (M = Ga, Al) and SrFe0.7Al0.3O3‑d, and concluded that the catalytic activity toward syngas formation correlates with the level of ionic conductivity.26 The physical mixture for a small amount of NiO and La0.7Sr0.3Cr0.05Fe0.95O3 exhibited higher H2 yield and good stability in repetitive reduction−oxidation cycles due to its being more reactive in methane decomposition.22 The conclusions drawn in these studies have generally been encouraging. Hence there are good reasons to further examine this potential application of LaFeO3-based perovskite as oxygen carrier. In the present work, LaFeO3/Al2O3−Kaolin composites, prepared by a wet-mixing−kneading method, are explored in regards to their physical−chemical properties and reaction performance for CLR over a microfixed bed reactor by CH4TPSR, pulse reaction, and continuous flow reaction. A preliminary run has been carried out over a circulating fluidized-bed reactor using natural gas as fuel, and the results will be helpful for the redesign of the experimental facility and the optimization of process parameters in further research.

AI (%) =

total fine collected for 5 h 100 amount of initial sample (50 g)

Mechanical Strength. Crushing strength was investigated using a QCY-602 crushing strength apparatus. Each sample is represented by 20 different particles collected from the sample, and the representative value for the crushing strength is obtained by calculating the mean value of the 20 collected values. 2.3. Reactivity Test over Microfixed-Bed Reactor. 2.3.1. Temperature Programmed Surface Reaction with CH4 (CH4-TPSR/MS). CH4-TPSR/MS was performed on a fixed-bed quartz microreactor (4.5 mm i.d.) packed with 250 mg samples. The sample was dehydrated for 1 h at 150 °C in pure Ar with 26 mL/min, then the mixture of 11 vol % CH4/Ar was used at a gas flow rate of 26 mL/min, and linear temperature was increased at a rate of 15 °C/min. The analysis of the reactants and products was carried out using an online mass spectrometer (AMTEK dycor System 1000). The quadrupole mass spectrometer can detect eight mass channels simultaneously with a minimum dwell time of 3 ms. Details about the experimental setup and procedures can be found in our previous work.26 2.3.2. Pulse Reactions. The pulse reactions were performed by injecting in a carrier gas (Ar) flowing continuously with a carrier flow rate of 26 mL/min through the catalyst bed loading 200 mg sample. The sample was pretreated with 11% O2/Ar at 26 mL/min at 900 °C for 30 min, and then switched to Ar for 15 min. A pulse of CH4 gas was introduced for time interval of 60 s

2. EXPERIMENTAL SECTION 2.1. Material Preparation. Synthesis of LaFeO3 perovskite precursor. The perovskite LaFeO3 precursor were prepared by the sol−gel method. Glycine (Industrial chemicals, +98%, From Hebei Xindonghua, Co., Ltd.) was added to an aqueous solution of La(NO3)3·6H2O (Industrial chemicals, From Shandong Zibo zhaoyi, Co., Ltd., 99.9%) and Fe(NO3)3·9H2O (Industrial chemicals, Shanxi Taiyuan xinjida, Co., Ltd. 98%) to have a ratio NH3/NO3−=1.05. Water was evaporated slowly from the mixed solution at 80 °C until a viscous gel was obtained. The gel 11073

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by a six-way gas-sampling valve equipped with measuring tubes (0.205 mL). The composition of the effluent gas from the reactor was monitored by a quadrupole mass spectrometer. 2.3.3. Continuous flow Reactions. Continuous flow reactions were carried out in a quartz microreactor loading with 420 mg sample. The sample was pretreated with 11% O2/Ar at 26 mL/ min at 900 °C for 30 min, and then flow of the reactant gas (11% methane in argon) at 26 mL/min was initiated and maintained for 4 min. The reactant and products were detected by mass spectrometer at m/e = 2 (H2), m/e = 15 (CH4), m/e = 18 (H2O), m/e = 28 (CO and CO2), m/e = 32 (O2) and m/e = 44 (CO2). For quantitative analysis, the mass spectrometer was calibrated by reference experiments using the same experimental procedure over an inert quartz bed. Cracking coefficients of methane, CO and CO2 were determined and used to calculate their concentrations. The conversion of methane and the selectivity of CO were calculated by the formulas as follows: CH4 conversion (%) =

SCO(%) =

A CH4,blank − A CH4,post reaction A CH4,blank

× 100

CO amount × 100 total amount of CO and CO2

where ACH4,postreaction is the integral area of remnant methane after reaction, ACH4,blank is the integral area of methane in the blank experiment, and SCO is the CO selectivity. 2.4. Preliminary Test over Circulating Fluidized-Bed Reactor. 2.4.1. Composition of Natural Gas. The pipeline natural gas was used for the reactivity test, after it was desulfurized deeply by combining hydrodesulfurization with Co−Mo catalyst and adsorption desulfurization with commercial ZnO catalyst over a fixed bed reactor. The sulfur content is less than 5 ppmv after sulphide removal. The composition of natural gas includes 95.8 vol % CH4, 1.8 vol % C2H6, 0.34 vol % C3H8, and 2.0 vol % CO2. 2.4.2. 7500 Wth Experimental Facility. Figure 1 shows the schematic diagram of 7500 Wth continuous atmospheric CLR apparatus, whose power was calculated according to the feeding gas flow in the fuel reactor and the air reactor, respectively, and heat loss (see Supporting Information). The process consists of a pipeline natural gas purifier (1), a heated reactant system, a 15.0 cm inner diameter (i.d.) and 100 cm height fuel reactor (2, FR) with a disengagement zone (6) and preheating zone just under the distributor, a 3.8 cm i.d. and 300 cm height air reactor (4 and 5, AR) with a disengagement zone (6), reactor effluent and sampling lines, and various process controllers and analytical instruments. A solid valve controls the flow rate of oxygen carriers fed to the AR. This allows the variation and control of oxygen carrier circulation flow rate between both reactors. Two hot filters located downstream from the fuel reactor recovered the solids elutriated from the bed during the successive reduction−oxidation cycles. The entire system was inside an electrically heated furnace. Thermocouples and pressure drop transducers located at different points show the current operating conditions at any time. Specific mass flow controllers give accurate flow rates of feeding gases. The gas outlet streams of the FR and AR are drawn to respective online gas analyzers to get continuous data of the gas composition. Typical operating conditions for chemical-looping experiments include approximately 0.39 m3/h natural gas, 6 m3/h air, particle size of 75−150

Figure 1. Schematic diagram for chemical-looping reforming: (1) desulfuration; (2) fuel reactor; (3) gas preheater; (4) air reactor; (5) riser; (6) cyclone; (7) high temperature valve; (8) powder collector.

μm, and 900 °C and atmospheric pressure. The operating conditions are shown in Table 1. Table 1. Operating Parameters of Circulating Fluidized Bed (CFB) Reactor for CLR operating item

operating parameters

system power materials for CFB temperature of fuel reactor temperature of air reactor outlet temperature of preheater for air outlet temperature of preheater for natural gas inner diameter of air reactor height of air reactor inner diameter of fuel reactor air flow in air reactor natural gas flow in fuel reactor solid−gas molar ratio

7500 Wth 0Cr25Ni20 900 °C 900 °C 800 °C 800 °C 38 mm 3500 mm 150 mm 6.0 m3/h 0.39 m3/h 1.32

The solid−gas ratio in this case was defined as molar ratio between the circulating molar flow rate of pure LaFeO3 loaded in the sample and molar flow rate of methane into the fuel reactor. R= 11074

NLaFeO3 NCH4

=

x% × moxygen carrier × ρoxygen carrier /MLaFeO3 mCH4 × ρCH /MCH4 4



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Figure 2. The schematic diagram of sequential column for gas analysis: (1) six-way valve; (2) Porapak N; (3) vacant column; (4) 13X column; (5) TCD; (6) signal switcher; (7) computer; (8) vent.

where, NLaFeO3 is the circulating molar flow rate of pure LaFeO3, mol; NCH4 is the molar flow rate of methane into the fuel reactor, mol; moxygen carrier and mCH4 are the circulating mass flow rate of oxygen carrier and methane in fuel reactor, m3/h; x% is mass percent of LaFeO3 in oxygen carrier sample; MLaFeO3, MCH4 are the molar mass of pure LaFeO3 (242.74 g/mol) and methane (16 g/mol); ρoxygen carrier, ρCH4 are the oxygen carrier density (1365 kg/m3) and methane density (0.75 kg/m3) at room temperature (298.15K), respectively. 2.4.3. Analysis of Reactant and Products. The reactant and products were analyzed on the thermal conductivity detector (TCD) in sequential connected Porapak N column, vacant column, and 13X molecule sieve column with online gas chromatograph (Agilent 1790T), as shown in Figure 2. All the components of products were detected in one TCD cell simultaneously. The conversion and selectivity were calculated by calibration area normalization over gas chromatograph as follows: XCH4 =

∑ CH4,in − ∑ CH4,out) ∑ CH4,in

3. RESULTS AND DISCUSSION 3.1. Physicochemical Characterization. The calcined samples are examined by X-ray diffraction patterns in order to identify the crystalline phases formed, as shown in Figure 3.

× 100%

⎛ ′ /fCH ⎞ A CH4,out f CH 4 4 ⎟ × 100% = ⎜⎜1 − ⎟ ′ ∑ A f / f × i ⎝ i i ⎠ ′ / fCO A COfCO

SCO =

∑ A i × f I′ / fi

Figure 3. XRD patterns of prepared LaFeO3-based oxygen carriers.

Characteristic diffraction lines of perovskite phase with weak peaks at 2θ = 25.0, 37.4, 47.3, 53.6, and 65.2, are observed for LaFeO3 oxide (JCPDS 35-1480) (Figure 3a), prepared using industrial raw materials. The LaFeO 3 perovskite shows orthorhombic structure with unit cell dimensions a = 5.553 Å, b = 7.843 Å, and c = 5.540 Å, space group pnma 62. All the samples (Figure 3b−h), prepared by the wet-mixing−kneading method, exhibit a strong perovskite phase, while the characteristic diffraction peaks of Al2O3 and Kaolin are not observed in this case (Figure 3i,j). It is notable that the strongest peak at 2θ = 32.2

× 100%

XCH4

where Ai is chromatographic peak area of i, fi is correction factor, and fi’ is TCD correction factor between the two arms. In this case, fi’ is 0.925, 0.897, 1 for CH4, CO and CO2, while fi is 44.7, 50.1, 55.3 for CH4, CO and CO2, respectively. The system errors are kept blow ±3%. 11075



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Figure 4. Comparison of XRD patterns between LaFeO3 and 60LaFeO3/15Al2O3-25Kaolin.

Figure 5. Typical CH4-TPSR/MS profiles in 11% CH4/Ar over LaFeO3-based oxygen carriers.

formation starts at 560 °C and reach the maximum at 675 °C over LaFeO3 oxide, but the amount is very minor. The major products are CO and H2 with the reaction temperature increased. More CH4 conversion takes place in CH4-TPSR above 730 °C on LaFeO3 oxide. This result suggests that the oxygen surface consumed for the formation of little amount of CO2 at the early stage of reduction reaction, whereas the reaction becomes more selective toward partial oxidation products from 730 to 900 °C. The CO reaches its peak at 825 °C, then decrease gradually; however, the intensity of H2 does not drop as CO intensity. It suggested that carbon deposition occurs on the LaFeO3 surface due to the lower concentration of oxygen species around the dissociative C* on the active sites.26 The surface oxygen and bulk oxygen species are responsible for the CH4 oxidation because the LaFeO3 oxide with low BET surface area (ca. 5.6 m2/g oxide) cannot afford the large amount of oxygen at the first surface layer. The oxygen species are transported by the surface reaction and bulk ion conduction mechanism.24,28 A relatively appropriate diffusion rate of oxygen species for balancing the dissociation of

shift progressively toward to high degree, and widen, compared with LaFeO3 (Figure 3a), with a simultaneous intensity decrease of the structure peaks of the orthorhombic phase. It indicates that the particles of LaFeO3 perovskite disperse well and grow with more small particles over Al2O3 and Kaolin. However, weak characteristic peaks for Fe2O3 appear in 33.4° and 35.8° (JCPDS 86-0550, as shown in Figure 4). The surface area of synthesized powers is about 5.6 m2/g for LaFeO3 perovskite oxide, and about 12.7 m2/g for 60LaFeO3/ 15Al2O3-25Kaolin oxygen carrier, while pore volume is only 0.023 cm3/g and 0.078 cm3/g, respectively, as they are prepared by calcination at high temperature (900 °C) for 6 h, but the surface area increase about 100% 60LaFeO3/15Al2O3-25Kaolin by wet-mixing−kneading method than that of pure LaFeO3 perovskite oxide. 3.2. CH4-TPSR/MS over LaFeO3-Based Oxygen Carriers. CH4-TPSR/MS was conducted with 11 vol% CH4/Ar over LaFeO3-based oxygen carriers, and the typical curves are shown in Figure 5. As observed in the insert of Figure 5, the CO2 11076



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of LaFeO3 over Al2O3−Kaolin (1:1). Attrition resistance is an important parameter for the application of circulating fluidized bed, because oxygen carriers will be shuttled between oxidizing and reducing atmospheres at high reaction temperature over circulating fluidized-bed reactor. All samples, prepared by the wet-mixing−kneading method, exhibit higher mechanical strength and attrition resistance than that of the pure LaFeO3 oxygen carrier (as shown in Table 3). Because of higher CO

CH4 and oxidation of carbon species by self-decoking over oxide is essential to oxidize selectively methane to syngas.29 Compared with LaFeO3 oxide, the CO2 formation starts at 520 °C, and reaches the first peak at 715 °C over 60LaFeO3/ 15Al2O3-25Kaolin (the insert of Figure 5). As the reaction temperature increased, the amount of CO2 formation enhances continuously to the second peak at 824 °C, then declines. The fast consumption of CH4 is delayed to 770 °C. The trend of CO and CO2 formation above 770 °C is similar, and a large amount of H2O produces simultaneously. It showed that the addition of 15Al2O3-25Kaolin by the wet-mixing−kneading method promotes its lattice oxygen mobility and makes oxygen carrier reduction facile, resulting in the benefit of CH4 total oxidation and the decrease of CO selectivity (as shown in Table 2).

Table 3. Characterization of Mechanical Strength and Attrition Resistance over LaFeO3-Based Oxygen Carriers

Table 2. CO Selectivity for CH4-TPSR over LaFeO3-Based Oxygen Carriers x% LaFeO3/(y%)Al2O3-(z %)Kaolin (x:y:z)

CO selectivity (%)

x% LaFeO3/(y%) Al2O3-(z%)Kaolin (x:y:z)

CO selectivity (%)

20:40:40 40:30:30 60:20:20 80:10:10

65.3 66.7 71.6 83.8

60:25:15 60:15:25 60:10:30 100:0:0

61.1 85.8 68.4 96.3

x% LaFeO3/(y%)Al2O3-(z%) Kaolin (x:y:z)

crushing strength (N/mm)

attrition index (AI) (%)

40:30:30 60:20:20 60:15:25 60:10:30 80:10:10 100:0:0

14.3 13.0 12.8 8.3 2.5

32.6 39.1 38.6 40.9 55.2 72.3

selectivity (85.8%) and BET surface (12.7 m2/g), high mobility of oxygen species for CO formation, and similar mechanical strength and attrition resistance, it is deduced that 60LaFeO3/ 15Al2O3-25Kaolin is the best oxygen carrier among the tested oxides, so it is as the candidate material to further study. 3.3. Reactivity on 60LaFeO3/15Al2O3-25Kaolin Oxide over a Microfixed Bed Reactor. 3.3.1. Pulse reaction. To look further into the catalytic behavior of 60LaFeO3/15Al2O325Kaolin and to quantify the major oxidation products, a pulse reaction at 900 °C was performed. The product distributions over 60LaFeO3/15Al2O3-25Kaolin at 900 °C isothermal conditions are shown in Figure 7, and the CH4 conversion and CO selectivity are illustrated in Figure 8.

Notably, the temperature of CO2 peaks at 520 and 715 °C is close to the reported value over Fe2O3 oxide by CH4-TPSR, and the overlap of the second reduction peak for small amount of Fe2O3 and LaFeO3 phase indicates that the reduction of oxides may occur simultaneously.30,31 Therefore, the CO2 formation by CH4-TPSR can partially attribute to the small amount of Fe2O3 over 60LaFeO3/15Al2O3-25Kaolin. Moreover, the migration rates of oxygen species for CO formation have an obvious difference according to the various gradients of the CO curves, which reveals the CO formation rates can be controlled by both the mobility and the concentration of the bulk lattice oxygen29 (as shown in Figure 6). The results from CH4-TPSR (as shown in Table 2) show that partial oxidation products CO are predominant. With the ratio of Al2O3: Kaolin decreased over 60 wt % LaFeO3, CO selectivity increases from 61.1% to 85.8%, and then declines to 68.4%, while CO selectivity increases continuously with the enhancive amount

Figure 7. Response of CH4, CO, CO2 and H2 with pulse number over 60LaFeO3/15Al2O3-25Kaolin oxide at 900 °C.

During the first CH4 pulse, a large amount of CH4 (∼94%) is converted to CO2, and only a small amount of CO and H2 are detected by the online MS. The CO selectivity is initially close to 16%, and increases monotonically with pulse number. The H2 evolution pattern is similar to that for CO. It has been suggested that there are two types of oxygen species in reducible oxides:

Figure 6. CO normalized curves by CH4-TPSR over LaFeO3-based oxygen carriers at 900 °C. 11077



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Figure 8. CH4 conversion and CO selectivity vs pulse number over 60LaFeO3/15Al2O3-25Kaolin oxide at 900 °C.

very reactive giving full oxidation and less reactive giving predominantly partial oxidation.9 The two oxygen species could be ascribed to framework (‘‘lattice”) and loosely bound surface oxygen.32 The accessible amount of oxygen species over the oxygen carrier for methane oxidation is directly related to its degree of reduction. With pulse number increased, the oxide surface becomes more and more reduced and the syngas formation rises. A low concentration of extractable lattice oxygen favors selective oxidation of methane to CO and H2.33 Mudu et al.34 considered that the framework oxygen appears to play a key role for selective oxidation of methane. The observed CO selectivity does hence correlate with the degree of reduction of the perovskite oxide. However, CH4 conversion decreases continuously over the first 20 pulses, and then increases. Similar behavior has been observed over LaFeO3 oxide and 0.1%Rh/LSFC, respectively.26,32 The higher CH4 conversions and CO selectivity are maintained at the level of ∼90% with the increase to the 38th pulse. Furthermore, the selectivity of CO2 decreases monotonically from the first pulse. The change trend of CH4 conversion can be explained as follows: Exposure of 60LaFeO3/15Al2O325Kaolin to a reduced atmosphere can generate oxygen vacancies over LaFeO3 oxide due to the loss of framework oxygen and reduction of the oxidative state of the Fe ion simultaneously, providing pathways of oxygen transport through the lattice, and framework oxygen diffuses toward surface which is drived by oxygen concentration difference.33,34 The relationship between the rate of oxygen migration and concentration of oxygen vacancies has been confirmed by Rossetti and Forni.35 3.3.2. Continuous Flow Reaction. Transient product concentrations for continuous flow reaction of 11% CH4/Ar at 900 °C on 60LaFeO3/15Al2O3-25Kaolin are shown in Figure 9. The reactions are initially very fast, as it is known from CH4TPSR (as shown in Figure 5). The reduction could be divided into three distinct periods. First there is a combustion period, where the major part of CH4 is oxidized to CO2 and H2O. With surface oxygen consumption, CO2 and H2O begin to decrease. This period is very short for the LaFeO3 oxygen carrier.9 Following this is a period of partial oxidation where CH4 is converted mostly to syngas. Finally, CH4 begins to decompose to solid carbon and H2. CH4 conversion and CO and H2 selectivity change from 51.2%, 32.4%, and 28.9%, respectively, at the first

Figure 9. Continuous flow reaction with CH4 in 11% CH4/Ar over 60LaFeO3/15Al2O3-25Kaolin oxide at 900 °C.

period, to 30.6%, 69.4%, and 64.2% at the second period, then to 62.2%, 73.3%, and 88.4% at the third period (as shown in Figure 10). The trends for CH4 and CO are similar to that of pulse reaction in Figure 8. The average CH4 conversion is about 49.8%. CO and H2 accompanied by CO2 and H2O are almost part of the reduction period. The products distribution is of a remarkable difference with that of our previous report over LaFeO3 oxide, which is suitable for chemical-looping reforming.26 Since the reduction involves several steps, it may be useful for chemicallooping reforming and chemical-looping combustion. After continuous flow reaction with CH4, the reaction is switched from CH4/Ar to pure Ar for sweeping about 300 s, and then switched to 11%O2/Ar. The oxidation process could be divided into four distinct periods (as in Figure 11). First, a large amount of CO and CO2 are detected simultaneously. These could be the oxidation of the carbonaceous deposits on the catalyst surface, which originated from the methane dissociation in the third period in Figure 9. After that, nothing is detected in the second period. The missing oxygen was apparently merged into the oxygen vacancy via reoxidation of the oxygen carrier. Moreover, the O2 begins to be detected about 60 s after CO and CO2 decrease to a very low level, which demonstrates that 60LaFeO3/15Al2O3-25Kaolin catalyst has a high reactivity with 11078



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concentration.11 However, in this case, the effect of the reaction temperature was only investigated under atmospheric pressure, due to the safety concerns at high pressure and high temperature. The effect of reaction temperature was investigated in the range of 800−900 °C on a bubbling bed reactor, loading with 15 kg of 60LaFeO3/15Al2O3-25Kaolin (bulk density, 1365 kg/m3) (shown in Figure 12). At all reaction temperatures, the inlet volume flow of natural gas was maintained constant at 0.39 m3/h.

Figure 10. CH4 conversion, CO and H2 selectivity by continuous flow reaction with CH4 in 11% CH4/Ar over 60LaFeO3/15Al2O3-25Kaolin oxide at 900 °C.

Figure 12. Effects of reaction temperature on CH4 conversion and CO selectivity over 60LaFeO3/15Al2O3-25Kaolin oxide. Catalyst weight, 15 kg (bed height 0.6 m); flow of natural gas, 0.39 m3/h.

It can be observed that CH4 conversion and CO selectivity increase with increasing reaction temperature. At 800 °C, lower CH4 conversion and CO selectivity are obtained. With reaction temperature increased to 900 °C and with the same oxygen carrier-to-fuel ratio, the CH4 conversion and CO selectivity reach about 55.1% and 90.4%, respectively. It is important to note that reaction temperature increases the reaction rate of partial oxidation and total oxidation of methane simultaneously, and the increase in CO selectivity can be due to the more rapid increase rate for partial oxidation than that of total oxidation.9 Indeed, the mobility of the lattice oxygen of the perovskite-based oxygen carriers plays an important role in the oxidation of methane under CLR conditions. This mobility as well as the migration rate of oxygen from the bulk to the surface during methane combustion over 60LaFeO3/15Al2O3-25Kaolin is strongly affected by the temperature. In other words, the oxygen vacancies can be formed easily when perovskite is heated at a higher temperature, and consequently, more oxygen species will be available for the CH4 reaction, owing to the increased oxygen migration.39 From the point of view of thermodynamics, high temperature favors the equilibrium in the reduction reaction; more importantly, the reduction rate was enhanced by kinetics at a higher temperature. Therefore, operating the CLR process at a high temperature is beneficial, improving the rate of the reaction and reactivity of the carrier as well as minimizing the CO2 formation. 3.4.2. Effects of Bed Height over Fuel Reactor. The residence times and space velocities for runs with a catalyst are calculated from the height of the catalyst bed and CH4 flow at STP. However, the actual contact time of CH4 with the catalyst is a factor of 3−4 smaller than that and the space velocity is a factor of 3−4 higher, because of the expansion of the gas in the reaction zone at high temperature. In this case, the bed height is changed from 0.3 m, to 0.4−0.6 m, and keeps the natural gas flow about constant (0.39 m3/h), which means that the gas hourly space velocity (GHSV) values are 71 h−1, 55.2 h−1, and 35.5 h−1 at STP,

Figure 11. Response of O2, CO, CO2, and H2O switched from CH4/Ar to Ar, then to O2 after continuous flow reaction with CH4 for 220 s at 900 °C over 60LaFeO3/15Al2O3-25Kaolin oxide (flow rate = 24 mL/ min, m (cat) = 0.42 g).

gaseous oxygen. Oxygen signals are observed in the third and forth periods, and no other products are detected. The oxygen signal evolves via fast growth and slow growth. Reoxidation includes the surface adsorption of oxygen and the migration of adsorbed oxygen to oxygen vacancies in the bulk. It suggests that the complete regeneration of the reduced oxygen carrier needs a long period, and it could be controlled by the combination of two processes: surface diffusion of oxygen atoms along domain boundaries and bulk diffusion of oxygen atoms within the bulk of domains.36 Rao et al.37 and Ryu et al.38 observed similar phenomenon over oxidation of reduced ilmenite. 3.4. Preliminary Test over Circulating Fluidized Bed Reactor with Natural Gas As Fuel. 3.4.1. Effects of Reaction Temperature over Fuel Reactor. It is important to analyze the effect of the reaction temperature on CH4 conversion and CO selectivity at elevated pressures, because bed temperature is one of the most important operating parameters affecting the performance of methane oxidation, and the reaction rate is greatly accelerated by increasing temperature. Thermodynamic equilibrium indicated that an increase in operating pressure produced a decrease in CH4 conversion, an increase in the CO2 and H2O concentration, and a decrease in the H2 and CO 11079



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oxygen species for CH4 oxidation and CO selectivity were related to reoxidation time.21 The static surface of the oxygen carrier is favorable to providing oxygen species from the oxygen carrier for the oxidation of methane.23 Another possible explanation is the presence of gas bubbles within the bed gas flow flowing through the fluidized bed and uneven solid contact, which affects product uniformity and results in decrease of CH4 conversion. Multiphase CFD simulations by Deng et al40 discovered that the upward flowing gas bubbles provide the energy to keep the oxygen carrier and fuel gas highly mixed, but the formation of fast and big size bubbles will lead to a lower reactant conversion rate. It is noteworthy that no carbon formation could be observed on the oxygen carrier and fuel reactor wall at the lower oxygen carrier-fuel molar ratio without the addition of steam. The postreaction of 60LaFeO3/15Al2O3-25Kaolin oxide by XRD characterization (as shown in Figure 14) indicates that this

respectively. That is, the oxygen carrier (LaFeO3)-to-fuel ratio (expressed as ratio between the moles of LaFeO3 loaded in the sample and methane moles fed ratio fed into the fuel reactor) is 0.66, 0.88, and 1.32, respectively. The effects of bed height are shown in Figure 13. The CH4 conversion and CO selectivity

Figure 13. Effects of bed weight on CH4 conversion and CO selectivity over 60LaFeO3/15Al2O3-25Kaolin oxide. Reaction temperature, 900 °C; flow of natural gas, 0.39 m3/h.

remains varying in a wide range. Increasing the bed height by 100% (from 0.3 to 0.6 m) with the same temperature and flow of natural gas increased the CH4 conversion by ∼53% (from 36.1% to 55.1%), while CO selectivity enhances remarkably by ∼55% (from 58.4% to 90.4%). The high bed height means an increase in residence time, which is favorable to the oxidation of surface carbide from the dissociation of methane. So, it can be concluded that the rate-limiting step for methane oxidation is the dissociation of methane, and it shows that the high oxygen carrier-fuel ratio is favorable for CH4 conversion and CO selectivity over the bubbling bed reactor among a series of bed height studied for this reaction. 3.4.3. Preliminary Test over Circulating Fluidized Bed Reactor. The total solids inventory in the system is about 15 kg of 60LaFeO3/15Al2O3-25Kaolin oxygen carriers. The temperature in the fuel reactor and air reactor are always kept constant at about 900 ± 5 °C. The gas feed to the fuel reactor is natural gas with a flow of 0.39 m3/h, and the inlet air flow in the air reactor is 6.0 m3/h. The solid circulation flow rate is controlled by means of the solids valve. The solid−gas ratio (oxygen carrier (LaFeO3)-fuel (natural gas) molar ratio) is 1.32, and the “static” bed weight is 0.4 m in fuel reactor. Preliminary results for 120 min after steady-state operation suggest that CO selectivity is maintained about 70%; the H2/CO ratio is kept about 1.99. However, the CH4 conversion is only 25%. Compared with similar “static” bed height over bubbling bed reactor (as shown in Table 3), the CO selectivity remains almost the same as that in the single bubbling bed reactor, but CH4 conversion decreases from 42% to 25%. There are some differences between the bubbling bed reactor (or fixed-bed reactor) and the circulating fluidized bed reactor. There are many possible reasons for the decrease in CH4 conversion. Oxygen carriers are shaken slightly by natural gas in a bubbling bed reactor or fixed bed reactor, while particles vigorously move in a circulating fluidized bed reactor. The behavior of particles in a bubbling bed reactor and fixed bed reactor is similar in some degree. According to our previous works in a fixed bed reactor,16 the different results were obtained by a continuous flow reaction and sequential redox cycle, which indicated that the amount of

Figure 14. Comparison of XRD profile between pre- and postreaction for 60 min over 60LaFeO3/15Al2O3-25Kaolin oxide.

consists almost entirely of perovskites structure, and that there is no sign of decomposition into metals or metal oxides. There are also trace peaks appearing at 2θ = 26.1, 30.03, and 35.0, which cannot be attributed to the oxides's ownership. LaFeO3-based perovskites as oxygen carriers are potential candidates for chemical-looping reforming (CLR) process. If some effort is put into the manufacturing process for this kind of perovskites, it may be possible to improve the properties of such materials (mechanical strength and attrition resistance) significantly by further optimization of binder and support in the preparation process. The preliminary run is a helpful experience for the redesign of the experimental facility, optimization of process parameters, mechanical strength, and attrition resistance of oxygen carrier in further research.

4. CONCLUSIONS The addition of Al2O3−Kaolin over LaFeO3 oxide improves oxygen migration for CH4 conversion, and can oxidize CH4 selectively to syngas, but CO selectivity decreases. Reaction temperature and bed height over fuel reactor are great influential in CH4 conversion and CO selectivity. High reactor temperature and bed height is beneficial for reforming application on bubbling fluidized fuel reactor. Preliminary results on circulating fluidized bed reactor with 15 kg oxygen carrier of total solids inventory show that 25% CH4 conversion and 70% CO selectivity without carbon formation are achieved at oxygen carrier-fuel molar ratio (1.32) and 900 °C. LaFeO3-based perovskites as oxygen carriers are potential candidates for chemical-looping reforming (CLR) process, but if more effort is put into the manufacturing process, 11080



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(6) Otsuka, K.; Wang, Y.; Sunada, E.; Yamanaka, I. Direct Partial Oxidation of Methane to Synthesis Gas by Cerium Oxide. J. Catal. 1998, 172, 152. (7) Stobbe, E. R.; De Boer, B. A.; Geus, J. W. The reduction and oxidation behaviour of manganese oxides. Catal. Today 1999, 47, 161. (8) Fathi, M.; Bjorgum, E.; Viig, T.; Rokstad, O. A. Partial oxidation of methane to synthesis gas: Elimination of gas phase oxygen. Catal. Today 2000, 63, 489. (9) Dai, X. P.; Li, R. J.; Yu, C. C.; Hao, Z. P. Unsteady-state direct partial oxidation of methane to synthesis gas in a fixed-bed reactor using AFeO3 (A = La, Nd, Eu) perovskite-type oxides as oxygen storage. J. Phys. Chem. B 2006, 110, 22525. (10) Cimini, R. J.; Marler, D. O.; Shinnar, R; Teitman, G. J. Integration of steam reforming unit and cogeneration power plant. U.S. Patent 5624964, 1995. (11) Ortiz, M.; de Diego, L. F.; Abad, A.; García-Labiano, F.; Gayán, P.; Adánez, J. Hydrogen production by auto-thermal chemical-looping reforming in a pressurized fluidized bed reactor using Ni-based oxygen carriers. Int. J. Hydrogen Energy 2010, 35, 151. (12) Zafar, Q.; Mattisson, T.; Gevert, B. Integrated hydrogen and power production with CO2 capture using chemical-looping reformingredox reactivity of particles of CuO, Mn2O3, NiO, and Fe2O3 using SiO2 as a support. Ind. Eng. Chem. Res. 2005, 44, 3485. (13) Linderholm, C.; Abad, A.; Mattisson, T.; Lyngfelt, A. 160 h of chemical-looping combustion in a 10 kW reactor system with a NiObased oxygen carrier. Int. J. Greenhouse Gas Control 2008, 2, 520. (14) Rydén, M.; Lyngfelt, A.; Mattisson, T. Chemical-looping combustion and chemical-looping reforming in a circulating fluidizedbed reactor using Ni-based oxygen carriers. Energy Fuels 2008, 22, 2585. (15) García, V.; Caldes, M. T.; Joubert, O.; Gautron, E.; Mondragón, F.; Moreno, A. Methane oxidation by lattice oxygen of Ni/ BaTi1‑xInxO3‑d catalysts. Catal. Today 2010, 157, 177. (16) Dai, X. P.; Yu, C. C.; Wu, Q. Comparison of LaFeO3, La0.8Sr0.2FeO3, and La0.8Sr0.2Fe0.9Co0.1O3 perovskite oxides as oxygen carrier for partial oxidation of methane. J. Nat. Gas Chem. 2008, 17, 415. (17) 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 Earth 2008, 26, 76. (18) Dai, X. P.; Yu, C. C.; Li, R. J.; Wu, Q.; Shi, K. J.; Hao, Z. P. Effect of calcination temperature and reaction conditions on methane partial oxidation using lanthanum-based perovskite as oxygen donor. J. Rare Earth 2008, 26, 341. (19) Dai, X. P.; Yu, C. C.; Li, R. J.; Wu, Q. Direct methane oxidation in the absence of gaseous oxygen using La0.8Sr0.2Fe0.9Co0.1O3 perovskite oxide as the oxygen carrier. Chin. J. Catal. 2008, 29, 954. (20) Dai, X. P.; Yu, C. C.; Li, R. J.; Hao, Z. P. The optimization of preparation, reaction conditions and synthesis gas production by redox cycle using lattice oxygen. Stud. Surf. Sci. Catal. 2007, 167, 391. (21) Mihai, O.; Chen, D.; Holmen, A. Catalytic consequence of oxygen of lanthanum ferrite perovskite in chemical looping reforming of methane. Ind. Eng. Chem. Res. 2011, 50, 2613. (22) Nalbandian, L.; Evdou, A.; Zaspalis, V. La1‑xSrxMyFe1‑yO3 perovskites as oxygen-carrier materials for chemical-looping reforming. Int. J. Hydrogen Energy 2011, 36, 6657. (23) Rydén, M.; Lyngfelt, A.; Mattisson, T.; Chen, D.; Holmen, A.; Bjørgum, E. Novel oxygen-carrier materials for chemical-looping combustion and chemical-looping reforming: LaxSr1‑xFeyCo1‑yO3‑d perovskites and mixed-metal oxides of NiO, Fe2O3 and Mn3O4. Int. J. Greenhouse Gas Control 2008, 2, 21. (24) Zeng, Y.; Tamhankar, S.; Ramprasad, N.; Fitch, F.; Acharya, D.; Wolf, R. A novel cyclic process for synthesis gas production. Chem. Eng. Sci. 2003, 58, 577. (25) Nalbandian, L.; Evdou, A.; Zaspalis, V. La1‑xSrxMyFe1‑yO3 perovskites as oxygen-carrier materials for chemical-looping reforming. Int. J. Hydrogen Energy 2011, 36, 6657. (26) Dai, X. P.; Wu, Q.; Li, R. J.; Yu, C. C.; Hao, Z. P. Hydrogen Production from a combination of the water−gas shift and redox cycle process of methane partial oxidation via lattice oxygen over LaFeO3 perovskite catalyst. J. Phys. Chem. B 2006, 110, 25856.

Table 4. Comparison of CH4 Conversion and CO Selectivity over a Fixed Bed, Bubbling Bed, and Circulating Fluidized Bed Reactor microfixed bed

bubbling bed

thermodynamic equilibrium dataa

catalyst loading gas flow

0.42 g

reaction temperature solid−gas ratio GHSV (h−1) CCH4 (%)

900 °C

18.6 50.9

55.2 42.4

55.2 25.1

49.9

SCO (%) SCO2 (%)

88.2 11.8

68.9 31.1

67.8 32.2

99.9 0.1

26 mL/min 11 vol % CH4/Ar

9.64 kg

circulating fluidized bed (fuel reactor)

3

0.39 m / h natural gas 900 °C

9.64 kg 0.39 m3/h natural gas 900 °C

900 °C

1.32

a

Note: Thermodynamic equilibrium data were calculated according to the mass balance of circulating fluidized bed (fuel reactor) for 2 h. O/ CH4 = 0.5 (mol/mol), T = 900 °C (without carbon formation).

it may be possible to improve the properties of such materials significantly (such as mechanical strength, attrition resistance).

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Additional tables and figures. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*Tel.: 86-10-89734981. Fax: 86-10-89734979. E-mail: weiws@ cup.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by China National Petroleum Corp. is gratefully acknowledged. The authors also gratefully acknowledge Dr. Changchun Yu, Dr. Ranjia Li, Mr. Erzhong Chen, Prof. Xiaojun Bao, China University of Petroleum (Beijing, China), for helpful discussion and preparation of some samples.

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REFERENCES

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Synthesis Gas Generation by Chemical-Looping Reforming in a

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