MnâFe Oxides with Support of MgAl - American Chemical Society

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Mn−Fe Oxides with Support of MgAl2O4, CeO2, ZrO2 and Y2O3−ZrO2 for Chemical-Looping Combustion and Chemical-Looping with Oxygen Uncoupling Golnar Azimi,*,† Henrik Leion,† Tobias Mattisson,‡ Magnus Rydén,‡ Frans Snijkers,§ and Anders Lyngfelt‡ †

Department of Environmental Inorganic Chemistry, Chalmers University of Technology, S-412 96 Göteborg, Sweden Department of Energy and Environment, Chalmers University of Technology, S-412 96 Göteborg, Sweden § VITO-Flemish Institute for Technological Research, B-2400 Mol, Belgium ‡

ABSTRACT: The feasibility of utilizing a combined oxide (Mn0.75Fe0.25)2O3 as an oxygen carrier for chemical-looping with oxygen uncoupling (CLOU) has been investigated. To increase the strength and attrition resistance of such particles, the oxygen carrier was prepared together with MgAl2O4, CeO2, ZrO2 and Y2O3−ZrO2 as supports. The oxygen-carrier particles were prepared using spray-drying. Each material was calcined for 4 h at 950, 1100 or 1200 °C. The materials were studied in a batch fluidized bed reactor to investigate their oxygen release and uptake potential and also their reactivity with CH4 and syngas. To gauge the mechanical stability of the different materials, the attrition resistance was measured in a jet-cup apparatus. With the exception of the material with MgAl2O4, the oxygen uncoupling property of the active combined oxides was largely kept intact using the added support materials. On the basis of the results from the reactivity tests and the measured attrition rates for all the particles, the material utilizing ZrO2 support seems to be the most promising candidate as an oxygen carrier for gaseous and solid fuels. However, due to phase transformations of the ZrO2 at higher temperatures, the calcination and operational temperature should likely not exceed 950 °C.

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INTRODUCTION

normal CLC oxygen carriers, e.g., high stability and good fluidization properties. Metal oxides of manganese, copper and cobalt have appropriate equilibrium pressures of gaseous oxygen within the range of 800 to 1200 °C for CLOU.1 Applying pure manganese oxide in CLOU is troublesome because the relevant equilibrium concentrations applicable for CLOU mean operation at relatively low temperatures, and it has been found that the oxidation of Mn3O4 to Mn2O3 is slow at the lower temperatures.16 However, the operational temperature can be raised by combining manganese oxide with other materials. Iron, nickel, silicon, magnesium and calcium are examples of materials that can react with manganese oxides to change the equilibrium and kinetic properties in a positive way.17−22 Combined manganese oxides have been examined as oxygen carriers in CLC and CLOU. Shulman et al.17,18 examined several combinations, for example, Mn/Mg, Mn/Ni, Mn/Si as well as the Fe/Mn oxide system. There are also some studies that have focused only on the Fe/Mn system.23−28 Because of the low price and favorable environmental properties of manganese and iron oxides, this system is of great interest for the development of CLOU. There are also a number of ores and minerals that contain a combination of Fe and Mn, and which could have CLOU properties.27

Chemical-looping with oxygen uncoupling (CLOU) is a variant of chemical-looping combustion (CLC).1 The CLOU system is composed of two fluidized bed reactors. One of them is an air reactor where an oxygen carrier, usually a reduced metal oxide, is oxidized by air. The oxygen carrier is transported to the second reactor, the fuel reactor. Here the oxygen-carrier material releases gas phase O2. The fuel then reacts with gasphase oxygen, like in normal combustion, and produces CO2 and H2O. In conventional CLC, no gaseous O2 is released and the fuel reacts directly with the oxygen carrier.1−4 The total amount of heat released from the fuel reactor and the air reactor is equal to the heat released from ordinary combustion. The CLC process has been demonstrated in different units5 of sizes from 0.3 kW to 1 MW using solid fuel,6−11 gaseous fuel12−14 and liquid fuel.15 A feasible CLOU oxygen carrier should be able to oxidize in the air reactor and also release gaseous O2 in the fuel reactor at relevant temperatures and oxygen partial pressures.1 This is possible if the oxide system has an oxygen equilibrium partial pressure that is lower than that of the outgoing gas stream in the air reactor, but high enough to enable sufficient release in the fuel reactor. Because of the presence of fuel, the oxygen concentration is very low in the fuel reactor, as the fuel will react with the oxygen released. This will remove any thermodynamic restriction and increase the driving force for oxygen release. The oxygen carrier should release oxygen at a sufficient rate to convert a significant part of the fuel. Further, the particles should have many of the same properties of © 2014 American Chemical Society

Received: Revised: Accepted: Published: 10358

March 7, 2014 May 31, 2014 June 3, 2014 June 3, 2014 dx.doi.org/10.1021/ie500994m | Ind. Eng. Chem. Res. 2014, 53, 10358−10365

Industrial & Engineering Chemistry Research

Article

Table 1. Composition, Crushing Strength and Bulk Density of Iron−Manganese Oxide Materials denotation

molar composition

M45F_MgAl40

(Fe0.25Mn0.75)2O3/MgAl2O4

M45F_Z40

(Fe0.25Mn0.75)2O3/ZrO2

M45F_Ce40

(Fe0.25Mn0.75)2O3/CeO2

M45F_YZ40

(Fe0.25Mn0.75)2O3/Y2O3−ZrO2

M75F950

(Fe0.25Mn0.75)2O3

synthesis composition (wt %)

calcination temperature (°C)

crushing strength (N)

bulk density (kg/m3)

15.5% Fe2O3 44.5% Mn3O4 40% MgAl2O4 15.5% Fe2O3 44.5% Mn3O4 40% ZrO2 15.5% Fe2O3 44.5% Mn3O4 40% CeO2 15.5% Fe2O3 44.5% Mn3O4 40% Y2O3−ZrO2 25.9% Fe2O3 74.1% Mn3O4

950 1100 1200 950 1100 1200 950 1100 1200 950 1100 1200 950

0.6 0.8 1 0.7 0.6 0.5 0.4 0.4 0.5 0.4 0.4 0.6 0.4

918 1176 1676 1214 1323 1375 1258 1279 1486 1180 1261 1355 980

but because of problems with respect to mechanical stability, it would be of high interest to pursue the use of support materials. The purpose of this work is thus to investigate the reactivity and attrition resistance of a series of supported Mn−Fe oxygen carriers. Oxygen-carrier materials with Mn:Fe molar ratio of 75:25 with addition of MgAl2O4, CeO2, ZrO2 and Y2O3−ZrO2 as supports were tested. The ratio of Mn:Fe chosen was based on the results of the earlier work, where this combination proved especially interesting.28 These supports are thought to be relatively inert toward reaction with manganese and iron oxides and should not react with the active phases to a large extent, meaning that the active oxygen carrier should have a constant ratio of Mn to Fe. The materials were studied in a batch fluidized bed reactor to investigate their reactivity with CH4 and syngas and also their oxygen release in N2.

The phase diagram of the Fe−Mn−O system has been presented previously.28 The stable phases of Fe/Mn oxides at low temperature and oxidizing conditions are the fully oxidized states, i.e., hematite and bixbyite, both with the general formula Fe2−xMnxO3 or (Fe,Mn)2O3, whereas the reduced cubic spinel phases (Fe,Mn)3O4 and the tetragonal spinel are stable at high temperatures. There are also two-phase areas in which both forms, i.e., bixbyite/hematite and spinel, coexist at intermediate temperatures. Moving from low to high temperatures will result in a phase change from (Fe,Mn)2O3 to (Fe,Mn)3O4, which is accompanied by oxygen release (reaction 1) equivalent to a 3.3−3.4% change of mass. 6(Mn, Fe)2 O3 ↔ 4(Mn, Fe)3 O4 + O2 (g)

(1)

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A similar release of O2 is expected to occur when moving from a high to a low partial pressure of oxygen, as when an oxygen carrier is transported from the air to the fuel reactor of a CLOU system. Thus, decomposition of bixbyite to spinel, reaction 1, should happen spontaneously in the fuel reactor. The released O2 would then be instantly consumed by the fuel, facilitating further O2 release. In the air reactor, reaction 1 is reversed, i.e., bixbyite is recreated by oxidation with air. Azimi et al.23,28 conducted extensive experiments with a material of general composition (MnxFe1−x)2O3 using a small fluidized bed reactor, and the material was found to have oxygen uncoupling properties. The rate of oxygen release from these types of oxygen carriers was a function of the Mn:Fe ratio and the temperature. Combined oxygen-carrier materials with Mn:Fe molar ratios in the range of 67:33 up to 80:20 were found to be especially interesting, as they released considerable gas phase oxygen at 850 °C and were capable of oxidizing CH4 completely and rapidly converting wood char to CO2 during experiments in a batch fluidized bed reactor. Although this material has been shown to chemically work excellently as a CLOU material, its mechanical strength needed improvement in order to have sufficient durability for commercial application.27 Support materials are often used together with active oxygen carriers in order to improve reactivity or mechanical properties. For instance, Al2O3, ZrO2, TiO2 or SiO2 are examples of materials that have been applied as support materials.29 Considering the fact that the Mn−Fe combined system shows very interesting properties with respect to reactivity,

EXPERIMENTAL SECTION Materials. The oxygen carriers studied in this work are particles with a Mn:Fe molar ratio of 75:25 with addition of MgAl2O4, CeO2, ZrO2 and Y2O3−ZrO2 as supports. For comparison, the results from tests with an unsupported material with the same ratio of Mn:Fe have also been included.28,30 The particles were produced by spray-drying by VITO (Flemish Institute for Technological Research) in Belgium. Powder mixtures of raw materials were dispersed in deionized water containing organic additives. Poly(ethylene oxide) (PEO, type PEO-1 Sumitomo Seika, Japan) and/or poly(vinyl alcohol) (PVA 1500 Fluka, Switzerland) and/or poly(ethylene glycol) (PEG 6000, Merck-Schuchardt, Germany) were used as organic binders, Darvan (type C, RT Vanderbilt, USA) and/or Dolapix (types A88, PC75 and PC80, Zschimmer & Schwarz, Germany) and/or Targon 1128 (BK Giulini Chemie, Germany) were used as dispersants. The water-based suspension was ball milled before being pumped to the spraydrier. Then, the slurry was continuously stirred with a propeller blade mixer while being pumped to the two-fluid spray-dry nozzle, positioned in the lower cone part of the spray-drier. After the spray-drying treatment, the fraction in the desired particle size range was obtained by sieving. In order to obtain oxygen-carrier particles with sufficient mechanical strength, calcination was performed in air at 1200, 1100 or 950 °C, for a dwell time of 4 h. After calcination, the particles were sieved again to the size range of 125−180 μm. 10359

dx.doi.org/10.1021/ie500994m | Ind. Eng. Chem. Res. 2014, 53, 10358−10365


Article

carriers by Rydén et al.31 The apparatus consists of a 39 mm high conical cup with an inner diameter of 13 mm at the bottom, and 25 mm at the top. At the bottom of the cup, there is a nozzle with a diameter of 1.5 mm that injects air with a velocity of approximately 100 m/s. The cup is located at the bottom of a 634 mm high cone with a maximum diameter of 216 mm. A particle filter with a 0.01 μm filter element is at the top of the apparatus. At the start of the experiments, the filter was weighed. Approximately 5 g of sample was placed in the cup. Every 10 min, the filter was weighed and the test was performed for 1 h. Data Evaluation. To be able to compare oxygen-carrier materials that contain different amounts of oxygen, a massbased conversion, ω, is defined as follows: m ω= mox (2)

Table 1 shows the different particles used in this work. Here, M denotes Mn3O4 and the digits after M represents the mass fraction of Mn3O4 in the sample. Further, F denotes Fe2O3, the material after F denotes the applied support, the following digits show the mass fraction of support in the sample and the last digits indicate the calcination temperature of the sample. Experimental Procedure. The experiments were performed in a fluidized bed quartz reactor that has a length of 820 mm and a porous quartz plate of 22 mm in diameter placed 370 mm from the bottom. This system is not a circulating fluidized bed system, but instead emulates circulation by exposing the oxygen carriers alternatingly to oxidation with O2/N2 and a reduction by fuel. The system is flushed between cycles by an inert gas flow (nitrogen). All cycles were repeated at least two times. The gas from the reactor was led to an electric cooler for removing water and then to a Rosemount NGA 2000 multicomponent gas analyzer, which measured the concentrations of CO, CO2, CH4, H2 and O2 in the flue gas as well as the flow rate. From high frequency measurements of the pressure drop over the reactor, it was possible to see if the bed was fluidized. The CLOU property of the material was examined by monitoring the O2 release when the sample was fluidized with N2. Moreover, the reaction with both methane and synthesis gas (50/50% CO/H2) was examined. A sample of 15 g of oxygen-carrier particles with a diameter of 125−180 μm was placed on the porous plate. Then the reactor was heated to the temperature of interest in 900 mLn/min of a gas mixture consisting of 5% O2 in N2. This was done in order to prevent uncontrolled release of oxygen and to ensure that the oxygen carriers are adequately oxidized prior to the experiments. As the desired conditions were reached, the particles were fluidized by 600 mLn/min of pure N2 at 850 °C, and the outlet oxygen concentration was measured during the inert period. The particles were exposed to consecutive cycles of oxidizing and inert periods. For reactivity evaluation, the particles were exposed to 450 mLn/min CH4 or 450 mLn/min synthesis gas (syngas, 50/50% CO/H2) at 850 °C. The oxidation and the reduction periods were separated by an inert period during which the reactor was purged from reactive gases and gaseous products by introduction of N2. For all the materials, the temperature during the inert period and reducing period was 850 °C, whereas the oxidation temperature for all materials was 800 °C. The reason for this choice of temperatures was that for materials with support, oxidation was difficult and slow at 850 °C and on the other hand, no uncoupling properties were observed at 800 °C. Therefore, the temperature was decreased to 800 °C at the beginning of the oxidation period and increased to 850 °C at the beginning of the following inert period. In the previous study of M75F950 with solid fuels,30 difficulties with oxidizing the particles at 850 °C were seen already after three cycles using 5% O2. Therefore, also in that study30 the oxidation temperature was lowered to 800 °C. Although it may be possible to oxidize the particles utilizing a higher fraction of oxygen, this was not tested, and it may not be of high relevance for a CLC system, as the oxygen fraction needs to be kept relatively low at the outlet of the air reactor in order to limit energy losses with the flue gases. Still, it is believed that temperatures around 800 °C should be able to be used in combination with a normal steam power cycle. Attrition resistance of the particles was investigated in a jetcup apparatus previously used for study of attrition of oxygen

Here m is the actual mass of the sample and mox is the mass of the fully oxidized sample, i.e., bixbyite. The degree of conversion of oxygen carrier as a function of time during reduction with methane and syngas is calculated from the outlet gas volume fractions using eqs 3 and 4, respectively. ωi = ωi − 1 −

MO mox

∫t

t1

nout ̇ (4yCO ,out + 3yCO,out + 2yO ,out 2

0

2

− yH ,out )dt

(3)

2

ωi = ωi − 1 −

MO mox

∫t

t1

nout ̇ (2yCO ,out + yCO,out + 2yO ,out 2

0

− yH ,out )dt

2

(4)

2

Correspondingly, the degree of conversion is determined using the relationship in eq 5 for the inert period. ωi = ωi − 1 −

2MO mox

∫t

t1 0

nout ̇ yO ,out dt 2

(5)

In the equations presented above, ωi is the mass-based conversion as a function of time for a period i, ωi−1 is the massbased conversion after the foregoing period, t0 and t1 are the times for the start and the end of the period, MO is the molar mass of oxygen, and ṅout is the molar flow of dry gas entering the analyzer. yi,out is the outlet volume fraction of gas component i after removal of water vapor. For analysis of gas yields, for the two different gaseous fuels, the fraction of CO2 in the outlet gas flow was calculated as follows: yCO 2 γCH = 4 yCH + yCO + yCO (6) 4

γCO =

2

yCO

2

yCO + yCO 2

(7)

Characterization of Oxygen Carriers. The crushing strength, i.e., the force needed to fracture the particles, was examined using a Shimpo FGN-5 crushing strength apparatus. For each sample, 30 different particles in the size range of 180− 250 μm were tested and the mean value gives the crushing strength. The bulk density was measured simply by weighing 5 mL of particles in the size range of 125−180 μm in a graduated cylinder. The analysis of the phase compositions of the oxygen10360



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carrier particles was performed on a Siemens D5000 powder Xray diffractometer (Cu Kα, k = 1.540 56 Å). The shape and morphology of fresh and tested oxygen carriers were observed using environmental scanning electron microscopy (SEM, Quanta FEG 200, FEI).

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RESULTS Oxygen Uncoupling Properties. The ability of the oxygen-carrier particles to release oxygen was investigated by

Figure 3. Gas yield, γCH4, vs mass-based conversion, ω, using methane for 20 s at 850 °C.

Figure 1. Oxygen volume fraction as a function of time (s) during the inert periods at 850 °C.

exposing them to N2 in the fluidized bed reactor. Figure 1 illustrates the oxygen volume fraction as a function of time (s) during one inert period for the different oxygen carriers. Here the temperature is raised from 800 to 850 °C at the same time as the gas is switched from oxidizing to inert. The time needed for the temperature increase is around 250 s. Figure 1 shows that the oxygen concentration for all materials except M45F_MgAl40_950 during the inert period is in the range of around 0.2% to 0.5% with the highest release for the unsupported material. This may not be so surprising, considering that the amount of bed material is the same for all cases, meaning that the amount of active material is considerably less for the supported carriers. Still, there is a relatively large spread in the oxygen release rates for the

Figure 4. Measured dry gas volume fraction during 80 s of reduction of 15 g of M45F_Z40_950 with 450 mLn/min syngas at 850 °C.

different supports, with the best behavior seen for the material produced with pure zirconia and calcined at 1200 °C, while the material with the MgAl2O4 support had the least propensity to

Figure 2. Measured dry gas volume fraction during inert period and 20 s of reduction of 15 g M45F_Z40_950 with 450 mLn/min CH4 at 850 °C, left diagram; close-up of the reduction, right diagram. 10361



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Figure 3 shows the gas yield,γCH4, from eq 6, as a function of mass-based oxygen-carrier conversion at 850 °C with methane. The value for ω does not start at 1 because it decreases slightly due to release of oxygen during the inert period before the reduction. Figure 3 shows that the methane conversion for M75F950 and M45F_Z40_950 is higher than that for the other investigated materials. Generally, all the particles except M45F_MgAl40_950 and M45F_Ce40_1200 have fairly high CH4 conversion. The particles with a lower calcination temperature show better methane conversion than the particles calcined at higher temperatures. Overall, the CH4 conversion for the materials can be ranked according to the support used as follows: ZrO2 > Y2O3−ZrO2 > CeO2 > MgAl2O4. M75F950 and M45F_Z40_950 convert methane to CO2 and H2O almost completely at 850 °C. These particles were able to transfer oxygen corresponding to almost 2.5% of their mass in 20 s. In Figure 3, the gas yield, γ, for M45F_Z40_950 does not remain at 1 at the end of the CH4 period, but decreases somewhat, and only the unsupported M75F950 showed full gas yield throughout the whole period. However, this is not surprising because the amount of bed material is the same for all cases, meaning that the amount of active material is considerably less for the supported carriers. From these experiments, it is not possible to safely conclude which is the main mechanism for the oxygen transfer; whether the fuel reacts directly with the oxygen carrier or if the fuel reacts with the uncoupled oxygen. In previous work30 with M75F950, solid fuel tests were performed that indicated that the main mechanism of the fuel conversion is via CLOU. Here, since M45F_Z40_950 shows similar behavior as M75F950, it is likely that most or all of the oxygen is transferred via the CLOU mechanism, at least during the part of reaction period where there is an excess of oxygen. The oxygen carrier M45F_MgAl40_950 showed small oxygen release and very low fuel conversion. This can be attributed to the fact that the particles could not be oxidized with 5% O2 and therefore, it was in its reduced form during the entire experiment. This material could not even be oxidized at 700 °C. Reaction with Syngas. Syngas contains CO and H2, and oxygen-carrier reactivity toward these components is of interest when considering conversion of solid fuels where syngas forms from steam gasification of char. Figure 4 shows the outlet dry gas volume fraction during 80 s reduction of 15 g M45F_Z40_950 with 450 mLn/min syngas at 850 °C. Figure 4 shows a behavior similar to that in Figure 2 using methane. At time 375 s, syngas was added for 80 s. Syngas was fully converted with the oxygen from the (Mn0.75,Fe0.25)2O3 producing CO2 and heat, which results in a temperature increase of ∼40 °C, promoting the spontaneous release of O2, as is seen from the increase in the oxygen concentration during the reduction period. As mentioned earlier, part of this increase in the oxygen concentration is due to the fact that steam was being condensed. An interesting difference to the results with methane is that after oxygen goes to 0, there is no sign of unconverted fuel. This suggests that the direct reaction of syngas with the oxygen carrier is rapid, even after the material has started to release less oxygen. Figure 5 shows the CO yield, γCO, from eq 7, as a function of mass-based oxygen-carrier conversion during 80 s of reduction with 450 mLn/min syngas at 850 °C. As can be seen, all the

Figure 5. CO yield, γ, vs mass-based conversion, ω, using syngas for 80 s at 850 °C.

release oxygen. Hence, it seems as if the interesting CLOU property which is now well-known for the pure combined oxide, is, to a large extent, retained using Ce- and Zr-based supports. It should be mentioned that because of the poor results with M45F_MgAl40_950, the two remaining materials with MgAl2O4 calcined at the higher temperatures were not tested. Reaction of the Oxygen Carriers with CH4. Figure 2 shows the outlet dry gas volume fraction of CO2, CO and CH4 during 20 s of reduction of 15 g of M45F_Z40_950 with 450 mLn/min CH4 at 850 °C. In Figure 2 the O2/N2 was shifted to nitrogen at the time 20 s. Here, the temperature was also increased from 800 to 850 °C. The figure shows that M45F_Z40_950 does not release much oxygen at 800 °C but by increasing the temperature to 850 °C, it spontaneously decomposes, giving ≈0.2% of oxygen in the exiting gas. At the time 375 s, methane was added for 20 s. Methane reacts directly with the oxygen released from the (Mn0.75,Fe0.25)2O3 producing CO2 and heat, which results in a temperature increase of around 20 °C, promoting the spontaneous release of O2, as is seen from the increase in the oxygen concentration during the reduction period. It needs to be mentioned that part of this increase in the oxygen concentration is due to the fact that the produced steam is being condensed to liquid water before analysis. This greatly influences measured O2 concentration compared to experiments with N2, because a certain O2 concentration of wet gas in the reactor will appear much higher when measured on a dry basis. There is some dispersion of the gas before it reaches the analyzer. As seen in Figure 2, the initial transient in the O2 concentration when nitrogen is turned on is approximately 10 s long. Similar transients of approximately 10 s due to dispersion are expected when oxygen release from the particles is slowing down and methane starts to appear in the outlet gas. This would explain the overlapping period in Figure 2 when O2 and CH4 are measured simultaneously for 5 s. The actual concentration of O2 in the reactor likely goes to zero as methane starts to rise. 10362



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Figure 6. SEM images of (left) M75F950 and (right) M45F_Z40_950. The upper ones are the used particles before attrition test and the lower ones are the used particles after attrition test. The scale bars at the bottom right of the images are 500 μm.

The crystalline phase composition of the materials was examined with X-ray powder diffraction. The active phases identified of materials with CeO2, ZrO2 and Y2O3−ZrO2 as supports were similar to those of the material without support, M75F950, again confirming the intact and operational combined oxide materials. The oxidized active phase in these samples is a cubic bixbyite structure of (Mn,Fe)2O3 and the reduced phase is a tetragonal spinel structure of (Mn,Fe)3O4. The additional inert phase in the material with ZrO2 as a support is the monoclinic structure of ZrO2 both in the reduced and oxidized states. For ZrO2-supported material sintered at 1200 °C, some tetragonal and cubic structures of ZrO2 were also seen. With Y2O3−ZrO2 as the support, the cubic and tetragonal structures of Y2O3−ZrO2 were identified, and with CeO2 as the support, the cubic structure of CeO2 was seen. In the case of CeO2, a small fraction of Fe or Mn could possibly be mixed with CeO2. Figure 7 shows the diffractograms for o xi d i z e d a n d r e d u c e d p h a s e s o f M 7 5 F 9 5 0 an d M45F_Z40_950.

materials except M45F_MgAl40_950, showed full conversion of syngas. Analysis of the Oxygen Carrier Particles. Besides the reactivity of particles, mechanical strength, attrition resistance, bulk density and crystal structure of the particles are important. Attrition tests were performed on the used oxidized materials from the above reactivity tests using a customized jet-cup apparatus, which was briefly described above. All the materials except M45F_Z40_950 showed very poor attrition resistance and were fragmented or turned to dust. For M45F_Z40_950, an attrition index, Ai, of 8.5 wt %/h was found, cf. Rydén et al.31 The attrition index is simply the rate of attrition as determined by material caught on the filter during the last 30 min of experiments.31 Of the materials tested, M45F_Z40_950 also had the highest crushing strength, see Table 1. Figure 6 shows the SEM images of M45F_Z40_950 and M75F950 particles for fresh and the used particles removed from the jet cup after the attrition test. Clearly, the use of the ZrO2 improved the stability, as the unsupported particles clearly fragmented to some extent in the jet cup. 10363



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different support materials were tested together with the active material. Particles with a Mn:Fe molar ratio of 75:25 were prepared with addition of MgAl2O4, CeO2, ZrO2 and Y2O3− ZrO2 as support. For reference, also oxygen carriers without support was prepared and tested. All oxygen carriers were investigated in a batch fluidized bed reactor to investigate their oxygen release and uptake potential (CLOU), their reactivity with CH4 and syngas. The attrition resistance was measured using a jet-cup apparatus. All the materials except the material with MgAl2O4 as support showed full conversion of syngas and good CLOU properties, indicating that the interesting oxygen transfer properties of this combined oxide system were retained. Further, all the materials, except M45F_MgAl40_950 and M45F_Ce40_1200, had fairly high CH4 conversion using a bed mass corresponding to 57 kg/MW in the bed. All the materials, except the oxygen carrier with ZrO2 as support calcined at 950 °C, showed very poor attrition resistance and were fragmented or turned to dust, and thus it seems as if the addition of supports does not really have much of a beneficial effect on the material. For addition of ZrO2 as support, much more promising results were found with respect to attrition for the materials calcined at the lowest temperature, 950C. However, it is believed that it is necessary to utilize low calcination and operating temperatures with this material in order to avoid the phase transformation of ZrO2 at higher temperature, which will likely cause mechanical failure.

Figure 7. Diffractograms for oxidized and reduced phases of M75F950 and M45F_Z40_950.

The phase identified for the material with MgAl2O4 as the support, M45F_MgAl40_950, was MgxFeyMnzAl3−x−y−zO4. This indicates that the MgAl2O4 is not an inert support and reacts with the iron manganese oxide. Judging from the reactivity experiments, this phase is apparently rather inactive for reactions with methane and syngas. The phase analysis supports the reactivity results that CeO2, ZrO2 and Y2O3−ZrO2 could be good candidates as supports for the oxygen-carrier materials based on Fe−Mn whereas the MgAl2O4 is a poor candidate. However, based on the attrition data, only the ZrO2 showed substantial improvement compared to the unsupported oxygen carrier. Table 1 presents the measured crushing strength and bulk density of different materials. As can be seen from Table 1, the crushing strength varies between 0.4 and 1 N. Generally, the crushing strength and density increase as a function of calcination temperature. However, for the materials with ZrO2, the crushing strength is somewhat lower for materials sintered at higher temperature. This can be attributed to the ZrO2 crystal structure transformation at higher temperatures. ZrO2 has a monoclinic crystal structure at temperatures below 1170 °C and by increasing temperature, it changes to tetragonal and cubic structures, which have a smaller volume.32 Therefore, cooling of these materials causes cracking because of inducing stresse as a result of volume expansion of crystal structure transformation from the cubic to tetragonal to monoclinic forms.32 This explanation correlates well with the X-ray diffraction analysis of these materials, which showed that the inert phase in the material with ZrO2 as the support is the monoclinic structure of ZrO2 both in the reduced and oxidized states. For ZrO2-supported material sintered at 1200 °C, some tetragonal or cubic structures of ZrO2 were also seen. In summary, although ZrO2 does show some promise, the calcination temperature could be of substantial importance, and it is advised to use rather low calcination temperatures.

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AUTHOR INFORMATION

Corresponding Author

*G. Azimi. Tel.: +46 31 7722887. Fax: +46 31 7722853. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financed by the Swedish Energy Agency, project number 32368-1. REFERENCES

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CONCLUSIONS In this work, oxygen carriers based on the (Mnx,Fe1−x)2O3 combined oxide system have been investigated. In order to improve the mechanical stability of this interesting system, 10364



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