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Investigation of Fe2O3 with MgAl2O4 for Chemical-Looping Combustion Marcus Johansson,*,† Tobias Mattisson,‡ and Anders Lyngfelt‡ Departments of Environmental Inorganic Chemistry and Energy Conversion, Chalmers University of Technology, S-412 96 Go¨ teborg, Sweden
Chemical-looping combustion (CLC) is a combustion technology with inherent separation of the greenhouse gas CO2. The technique involves the use of a metal oxide as an oxygen-carrier, which transfers oxygen from the combustion air to the fuel. The metal oxide Fe2O3 on MgAl2O4 support was examined, and particles of diameter 125-180 µm were prepared by freeze-granulation. The reactivity was evaluated in a fluidized-bed reactor, where the atmosphere was periodically changed. Reduction was performed in 50% CH4 and 50% H2O, while the oxidation was performed in 5% O2. The sintering temperature and the ratio of metal oxide/inert were varied in order to find the optimal particle properties. The particles were characterized with respect to crushing strength, surface structure, and chemical composition, and 60% Fe2O3 on 40% MgAl2O4 sintered at 1100 °C was found to be the most suitable oxygen carrier. The particle had a high reactivity and showed no tendency to agglomerate or break apart during the cyclic tests. The results indicate that the amount of material needed in the fuel reactor would be in the order of 150 kg/MW. Introduction Chemical-Looping Combustion. Chemical-looping combustion is a combustion technology where an oxygen carrier is used to transfer oxygen from the combustion air to the fuel, thus avoiding direct contact between air and fuel.1-3 The process is composed of two fluidized reactors, an air and a fuel reactor, as shown in Figure 1. The fuel is introduced to the fuel reactor in a gaseous form where it reacts with an oxygen carrier to CO2 and H2O. The reduced oxygen carrier is transported to the air reactor where it is oxidized back to its original state by air. The metal oxide in this work is hematite, or Fe2O3, and the reduced metal oxide is Fe3O4. There are other reduced oxides such as FeO, but formation of these is not compatible with full fuel conversion. The proposed fuel here is syngas from coal gasification, but other possible fuels are natural gas or refinery gas. The overall reactions in the reactors are given below Figure 1. Schematic view of chemical-looping combustion: (1) air reactor and riser, (2) cyclone, and (3) fuel reactor.
Reaction 1a is exothermic with ∆H950 ) -40.7/-8.2 kJ/mol, while reaction 1b is endothermic with ∆H950 ) 161.4 kJ/mol. Reaction 2 is exothermic with ∆H950 ) -481.9 kJ/mol. The total amount of heat evolved from reaction 1a or 1b plus reaction 2 is the same as for normal combustion where the oxygen is in direct contact with the fuel. However, the advantage with this system * To whom correspondence should be addressed. Tel: +46-31-7722887. Fax: +46-31-7722853. E-mail: kf95020@ chestud.chalmers.se. † Department of Environmental Inorganic Chemistry. ‡ Department of Energy Conversion.
compared to normal combustion is that the CO2 and H2O are inherently separated from the rest of the flue gases, and no major energy is expended for this separation. Thus, compared to other technologies for capture of CO2, CLC is potentially much cheaper since no costly gas-separation equipment is necessary.4 Although a real process could be either pressurized or atmospheric, a first step is to investigate CLC under atmospheric conditions. Expected temperature range could be 800-1200 °C for the air- and fuel-reactor, although the temperature would be higher in the air reactor due to its strong exothermic reaction. A CLC system of fluidized bed reactors adapts technology and components proven in applications such as circulating fluidized-bed combustion.3 However, the behavior of metal oxides in such a system is not known. Thus, the key issue is to find an oxygen carrier which is (i) reactive toward the fuel gas and air, (ii) resistant toward attrition, (iii) not apt to agglomerate, and (iv) possible to produce at reasonable cost.
10.1021/ie049813c CCC: $27.50 © 2004 American Chemical Society Published on Web 09/25/2004
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Oxygen Carriers. In a thermodynamic analysis of different oxygen carriers, Mattisson and Lyngfelt found that some metal oxides of the transition state metals Fe, Cu, Mn, and Ni were feasible candidates to be used as oxygen carriers in CLC.5 Of these metals, iron is the least expensive in addition to being nontoxic. The use of iron oxides for CLC has been widely investigated. Ishida and co-workers have tested iron oxides on four different support materials in a thermogravimetric analyzer (TGA).6-9 Ada´nez et al. studied iron oxides on five different support materials in a TGA.10 In a recent study, Mattisson et al. developed iron oxides on six different inert materials and of different mass/inert ratios.11 Of these, 60% Fe2O3 on 40% MgAl2O4 showed the best reactivity in a fluidized bed reactor, as well as reasonable crushing strength and resistance toward agglomeration and fragmentation. Only a few articles have been written about the Mg-Fe-Al-O system and none regarding its feasibility as an oxygen carrier for chemical-looping combustion. The spinel MgAl2O4 offers a desirable combination of properties such as high melting point, high resistance to chemical attack, and high strength at elevated temperatures.12 When Fe2O3 and MgAl2O4 are mixed, complex phases may be formed where all four elements may interact.13-17 Thus, because of the promising results found with Fe2O3/MgAl2O4,11 the purpose of this work is to examine and optimize this oxygen carrier for chemical-looping combustion. Particles were prepared with different ratios of Fe2O3 to MgAl2O4 as well as different sintering temperatures. Multicycle redox tests were conducted with the effect of temperature and particle size determined. The testing conditions were chosen to give as much information and understanding as possible for further tests in a prototype of chemical-looping combustion. Experimental Preparation of Oxygen Carriers. The particles were produced in the following manner: A water-based slurry of the composite material, in the form of chemical powders with a size less than 99.99%), and the gas-analyzer is calibrated with high-pureness gases prior to each experiment. The mass flow regulators used has an accuracy of 99.0%, and fluctuations in the flow measurements are in the order of 1-2%. In total the errors should not match up to more than a couple of percent. Further, the main goal of this paper is to compare the reactivity of different oxygen carriers. Any errors in equipment should therefore have the same effect on all experiments. Figure 2 shows the outlet concentration of gaseous components for one reduction and one oxidation period conducted with F6AM950. During the reduction, Figure 2a, the incoming methane initially reacts to form only CO2 and H2O. As the reaction proceeds, unconverted CH4 is released, and there is formation of some minor amounts of CO and H2. The gas flow is measured in connection to the gas analysis, and flow variations upstream of the analyzers will rapidly affect the mea-
Xi ) Xi-1 -
∫tt
1
o
1 n˘ (4p + MoPtot out CO2,out 3pCO,out - pH2,out)dt (4)
and for the oxidizing period from
Xi ) Xi-1 +
∫tt
1
0
1 (n˘ p - n˘ outpO2,out)dt (5) MoPtot in O2,in
where Xi is the conversion as a function of time for period i, Xi-1 is the conversion after the preceding period, t0 and t1 are the times for the start and finish of the period respectively, Mo is the moles of active oxygen in the unreacted oxygen carrier, n˘ in and n˘ out are the molar flows of the gas going into and leaving the reactor after the water has been removed, Ptot is the total pressure, and pO2,in, pO2,out, pCO2out, pH2,out, and pCO,out are the partial pressures of incoming and outlet O2 and outlet partial pressures of CO2, H2, and CO after the removal of H2O. pH2,out was not measured online but assumed to be related to the outlet partial pressure of CO and CO2 through an empirical relation based on the equilibrium of the gas-shift reaction. The calculated concentration of H2 agreed reasonably well with the concentration as calculated from the difference 1-pCO2out-pCO,out-pCH4,out in the part of the reduction period where there is little or no back-mixing. The rate of reduction of the metal with methane was based on the conversion of the incoming methane and calculated from
1 dX ) n˘ (4p + 3pCO - pH2,out) dt MoPtot in,CH4 CO2
(6)
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where n˘ in,CH4 is the incoming molar flow rate of methane to the reactor. To facilitate a comparison between different oxygen carriers that contain varying amounts of oxygen depending upon the fraction of inert, a massbased conversion was defined as
ω)
m ) 1 + Ro(X - 1) mox
(7)
where Ro is the oxygen ratio, defined as
Ro ) (mox - mred)/mox
(8)
The oxygen ratio is the maximum mass fraction of the oxygen-carrier that can be used in the oxygen transfer and is dependent on the metal oxide used as oxygen carrier as well as the amount of inert in the particles. The mass-based reduction rate was calculated as
dX dω ) Ro dt dt
(9)
If the mass transfer resistance between the bubble and emulsion phases in the fluidized bed reactor is small, and assuming that the reaction between the methane and solid is first order with respect to methane, the exposure of particles to methane can be represented by a log-mean partial pressure of methane, pm, which can be defined in terms of the inlet and outlet partial pressure:
pm )
pin - pout pin ln pout
( )
(10)
An effective first-order reaction rate constant, keff, was defined as
keff )
1 dω pm dt
(11)
A reaction rate normalized to a reference partial pressure of methane was calculated as
(dω dt )
norm
) keff‚pref
(12)
where pref is a reference partial pressure of methane. Assuming an inlet partial pressure of methane of 1 and an outlet partial pressure of 0.001 the log-mean partial pressure would be 0.145, cf. eq 10. Below, pref is chosen to be 0.15, which accordingly corresponds to a high methane conversion. The fact that all of the oxygen was consumed in the initial part of the oxidation period suggests that there was good contact between all reacting gas and the particles in the reactor. Nevertheless, it should be noted that keff would include any masstransfer between the bubble phase and emulsion phase. A difficulty in applying a standardized test procedure is that the conversion of the gas will vary a lot both as a function of the conversion and the reactivity of the particles. By utilizing the effective rate constant, a better comparison of reactivity data obtained under different methane concentrations in the reactor can be made. Although the effective reaction rate constant does not measure detailed kinetics, it facilitates an assess-
Figure 3. Freeze granulated particles of 40% FeeO3/60% MgAl2O4 sintered at 1100 °C: (a) light microscope image (black bar ) 1 mm), (b) SEM image of particle (horizontal bar at bottom ) 150 µm), and (c) SEM image of surface (horizontal bar at bottom ) 45 µm).
ment of the reactivity of the oxygen carriers tested and provides a basis for comparison. Results and Discussion Analysis of Fresh Oxygen Carriers. Figure 3a shows a light microscope image of some particles of 40% Fe2O3/60% MgAl2O4 sintered at 1100 °C. Further, Figure 3b,c shows SEM images of a particle and its surface. It can be seen that the particles are spherical and display a smooth surface structure at this magnification. Furthermore it is seen in Figure 3a that many of the larger particles have smaller particles attached to them, which is believed to be an effect of the sintering procedure. The surface of F6AM particles sintered at
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Figure 4. Surface images of (a) F6AM1100, (b) F6AM1200, and (c) F6AM1300. Horizontal bar at bottom of images represents 45 µm.
1100, 1200, and 1300 °C is shown in Figure 4. It is clear that the sintering temperature strongly affects the surface structure, from smooth structure at 1100 °C to large grains at 1300 °C. The same tendency was seen for F4AM and F8AM. In Figure 5a-c the particles shown are sintered at 1100 °C with different amounts of Fe2O3, in the order 40%, 60%, and 80%. All three have a rather similar smooth structure. A conclusion from Figures 4 and 5 is that the sintering temperature seems to affect the structures of the particles to a larger degree than the differences in the ratio metal oxide/inert. All the fresh oxygen carriers were analyzed with X-ray powder diffraction (XRD) prior to experiments. A problem with interpretation of these data is that several different compounds are possible and many have intensity peaks at the same position in the XRD spectrum. It has been suggested that iron can take the place of either magnesium or alumina and produce changes in the lattice.12-16 Ortiz et al.16 showed that a mixture of hematite and the MgAl2O4 spinel may form MgO, Al2O3, MgAl2O4, Mg(Al,Fe)2O4, MgFe2O4, AlFeO3, Fe3O4, Al2FeO4, and Fe2O3 depending on the starting ratios and sintering temperature. With the exception of MgO and Al2O3, all of these phases may be possible in the investigated particles from interpretation of the XRD data. Ortiz et al. further noticed that an increased disorder of the formed iron-spinels is proportional to the fraction of added hematite and increased temperature. Without going deeper into the XRD investigations the
Figure 5. Surface images of (a) F4AM1100. Horizontal bar at bottom of images represents 45 µm.
primary interest here is to determine whether created phases are stable or not and to what degree an ironspinel can be reduced in comparison to free hematite. For example, the possible spinel, MgAlFeO4, is not stable in a reducing atmosphere but possible to reduce in two steps to pure iron with Mg1-xFexO as an intermediate. However, it reduces more slowly than hematite15 and is therefore not desirable from a reactivity point of view. For the samples containing 40% Fe2O3, the F4AM1100 consists of Fe2O3 and at least one more phase. The F4AM1200 and F4AM1300, however, do not contain any free Fe2O3, indicating that iron has interacted with alumina and magnesia to form new phases. It could be speculated that the surface changes observed for the particles sintered at the higher temperatures could be associated with these chemical reactions. A similar effect of the temperature can be seen with the 60% Fe2O3 particles. F6AM950 and F6AM1100 are identical and have a similar composition as F4AM1100, although with a higher Fe2O3 intensity. In the F6AM1200 sample it is possible to see that a lot of hematite has disappeared in favor of other phases, and in F6AM1300 almost no hematite can be identified. Again, this can partly explain the change in structures as seen in Figure 4a-c. All the three samples with 80% of Fe2O3 are very similar, with both free hematite and other phases. In general the starting spinel, MgAl2O4, is replaced in all the particles by Mg(Al,Fe)2O4, indicating cation interactions between the MgAl2O4 spinel and iron. This could be seen as a small displacement of the XRD intensity
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Figure 6. Crushing strength as a function of sintering temperature: F4AM (+), F6AM (]), and F8AM ([).
Reactivity of Oxygen Carriers. The reduction rate was either constant or increased slightly as a function of the cycle number, although all oxides showed a slower rate during the first reduction period. This effect is wellknown for similar experiments and particles,10,11,19 and it can be speculated that the particles need one cycle to attain a more favorable structure. Figure 8a-c shows the mass-based rate of reduction, dω/dt, as a function of the mass-based degree of conversion, ω, for the three different sets of particles. Because of the desire to avoid extensive formation of FeO, most of the reduction periods were terminated before there was any formation of FeO. Thus, the three metal oxides with 40%, 60%, and 80% Fe2O3 have a maximum ∆ω of 0.013, 0.02, and 0.027. From the figures it is clear that in general the reactivity decreases with increasing sintering temperature, which also means that the reactivity is a function of the porosity. Because more porous particles will probably have a higher active area available for reaction with the reactant gas, this it is not surprising. Further can be seen that despite the higher amount of iron oxide in the F8AM particles, they have a lower reactivity per mass unit than F6AM and even F4AM particles. This is probably a mixed effect of porosity and amount of iron oxide, where the properties of the F6AM particles seem to be optimal. The reason the graphs do not reach their maximum values immediately is due to the effect of back-mixing in the reactor. To facilitate a comparison of reaction rates between different oxygen carriers a rate index was defined as the normalized rate, expressed in %/min
rate index ) 60*100*(dω/dt)norm Figure 7. Crushing strength as a function of porosity: F4AM (+), F6AM (]), and F8AM ([).
peaks compared to fresh material investigated before heat treatment. The crushing strength of the particles was calculated as a mean value of tests on 30 particles and varied in wide range, as can be seen in Figure 6 where it is represented as a function of the sintering temperature. The general effect is that the crushing strength increases with higher sintering temperatures and higher Fe2O3 content in the particles. Sing et al.12 showed for the MgAl2O4 spinel that the sintering temperature and the time held at that temperature had a strong impact on the porosity of the particles, with decreasing porosity as a result of increasing temperature and time. The noticeable exception in this study is F8AM1300, where a significant decrease was seen in strength compared to the sample sintered at 1200 °C. However, all particles increase in strength between 1100 °C and 1200 °C. The correlation between crushing strength and porosity is shown in Figure 7. As can be seen the porosity varies between 0.28 and 0.71, while the crushing strength takes values from 0.7 N for the softest up to 12.7 N for the hardest. As mentioned, F8AM1300 has higher porosity and lower crushing strength than F8AM1200, which indicates that cracks may have developed during sintering. This could also be seen from the SEM image of the surface. For comparison, the crushing strength was also tested on a commercial catalyst particle, γ-Al2O3 (NWA-155, Sasol Germany Gmbh). The particles, in the larger size range of 0.355-0.500 mm, has a strength of 3.8N.18
(13)
where (dω/dt)norm is the normalized average rate expressed in s-1, and calculated from eq 12, using the average rate constant in the interval of ∆ω ) 0.01 where the reactivity is as fastest. Further a reference partial pressure of methane of 0.15 was used to calculate the normalized rate in eq 12. The rate index is plotted as a function of crushing strength in Figure 9, see numbers 1-10. As a reference, a rate index of 1 and 3 is shown as horizontal dashed lines in the figures (explained in Solids Inventory). From Figure 9 it is easily seen that the rate index decreases with increasing strength of the particles. No particles underwent sintering during the experiments; however, some of the samples showed tendencies to agglomerate. This was verified in the pressures-drop measurements in the oxidizing period as a drastic decrease in the pressure fluctuations in comparison with earlier cycles in the experiment. The particles that agglomerated are the numbers represented with a surrounding circle in Figure 9, and this indicates that hard- and low-porous particles are more sensitive to agglomeration. It could be speculated that the agglomeration has to do with the fact that there was larger surface changes during redox reaction of these particles in comparison to the other particles sintered at lower temperatures. Another possibility could be that the agglomeration is caused by different fluidization conditions for denser particles. Higher density of particles will increase the minimum fluidization velocity, umf, and also give a more shallow bed. A summary of results for all the particles evaluated in this first batch can be seen in Table 1. The fact that the reactivity was constant as a function of cycle for most particles that did not agglomerate indicates that the experiments have good reproducibility. In this paper only one cycle from
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Figure 9. The rate index as a function of the crushing strength for all oxides investigated. Numbers with circle are those that showed tendencies of agglomeration. (1) F4AM1100, (2) F4AM1200, (3) F4AM1300, (4) F6AM950, (5) F6AM1100, (6) F6AM1200, (7) F6AM1300, (8) F8AM1100, (9) F8AM1200, and (10) F8AM1300. From the second batch: (A) F6AM1100P, (B) F6AM1125P, (C) F6AM1150P, (D) F6AM1175P, and (E) F6AM1200P. Table 1. Summary over Physical Properties and Reactivity of the Particles Investigated in the First Batch
particle F4AM1100 F4AM1200 F4AM1300 F6AM950 F6AM1100 F6AM1200 F6AM1300 F8AM1100 F8AM1200 F8AM1300
Figure 8. The mass based rate of reduction as a function of the mass conversion for (a) F4AM, (b) F6AM, and (c) F8AM. Particles sintered at 950 °C (0), 1100 °C (9), 1200 °C (O), and 1300 °C (b).
each experiment is represented; therefore, there is no real representative way to describe the reactivity of the poorer particles, however, since they are poor they are of less importance. Analysis of Reacted Oxygen Carriers. Images of the surface morphology of samples after reactivity investigations are shown in Figure 10. These experiments were terminated following a reducing period. For F6AM1100 almost no changes in comparison to fresh particles can be seen, while F6AM1200 and F6AM1300
fraction sintering apparent crushing rate of Fe2O3 temp density porosity strength index (%) (°C) (kg/m3) (-) (N) (%/min) 40 40 40 60 60 60 60 80 80 80
1100 1200 1300 950 1100 1200 1300 1100 1200 1300
1458 2593 2865 1318 2225 3321 3207 3357 3542 3286
0.66 0.40 0.34 0.71 0.52 0.28 0.30 0.31 0.28 0.33
0.7 4.2 7.5 0.7 1.8 11.8 12.5 7.5 12.7 7.3
3.90 1.37 0.83 4.05 3.01 1.12 1.20 0.71 0.75 0.75
have a rather irregular structure of the surface. For some of the particles sintered at high temperature, the existence of cracks can be detected. Interestingly, the particles that either agglomerated and/or exhibited cracks tended to increase in reactivity with the number of cycles. Thus, increase in reactivity may be explained by the particles developing larger “entrances” for the reducing gas compared to the original pore structure. Therefore those particles that had a reactivity which was independent of the cycle numbers were also those that will not agglomerate and/or develop cracks and hence would be more suitable for chemical-looping combustion. These were in particular: F4AM1100, F4AM1200, F6AM950, and F6AM1100. In a chemical-looping combustor a full conversion of the fuel is desired, and under those conditions formation of FeO and Fe is not expected. Therefore the experiments were carried out in such a way as to avoid the formation of FeO and Fe. Although there were differences between the XRD patterns for fresh samples sintered at different temperatures, as mentioned above, all of the particles with the same amount of metal oxide had similar XRD patterns after reduction. This indicates that the complex phases formed during sintering at the higher temperatures are not irreversible but rather are possible to reduce. Interestingly, Fe3O4 did not seem to be present in the reduced samples, instead iron formed other phases as it reduced. Small peaks of FeO, however, were discovered for a couple of the samples. Furthermore one experiment was conducted to reduce
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Figure 10. Surface images of reduced particles (a) F6AM1100, (b) F6AM1200, and (c) F6AM1300. Horizontal bar at bottom of images represents 45 µm.
far beyond Fe3O4, and the XRD peaks clearly indicated that free Fe was created. Two experiments with F6AM1100 and F6AM1200 were stopped after oxidation. Interestingly they both showed similar composition as fresh F6AM1100 sample, which indicates that all the additional phases created at sintering at 1200 °C were reduced at 950 °C and not oxidized back at this lower temperature. Fresh samples of F6AM1100 and F6AM1200 had crushing strengths of 1.8 and 11.8 N, respectively. When tested after experiments and stopped after oxidation the crushing strengths were 3.5 and 5.2 N, respectively. First it agrees with the observation that the particles achieve a similar state after reaction and second, in the case of F6AM1100, it shows that the
particles have increased in strength during experiments. This might be an effect of restructuring to a more favorable structure and has also been noticed for oxygen-carriers based on nickel oxide by Mattisson et al.18 It can be speculated that the sintering temperature of the particles may not determine the particles properties in a long perspective but only the starting point of porosity and strength of the particles. However, in the present experimental data, with a limited number of cycles, the rate index is strongly dependent on the sintering temperature, see Figure 9. Second Batch of Particles. It is assumed that particles with crushing strength below 1 N are too soft to be suitable for chemical-looping combustion. Thus, the best particle in the first batch would be F6AM1100, number 5 in Figure 9. A second batch with smaller intervals of sintering temperature was made in order to optimize porosity and crushing strength and to find the sintering temperature at which major chemical and structural changes occur. Figure 9 shows the large difference between F6AM1100 and F6AM1200 (number 5 and 6) regarding reactivity, crushing strength, and agglomeration. For this reason the second batch of F6AM was manufactured and sintered at 1100, 1125, 1150, 1175, and 1200 °C. The only difference in preparation was that the smallest fractions (355 µm) were separated prior to sintering in order to avoid the smaller particles being attached to the larger, as seen in Figure 3a. The same nomenclature follows for these particles but with an additional P afterward, as for example F6AM1100P. The particles follow the same tendencies as the previous regarding porosity and crushing strength. SEM images indicate similar differences in surface structures between 1100 and 1200 °C as in the first batch. The sample sintered at 1125 °C showed minor enlargements of the surface grains compared to the sample sintered at 1100 °C, and this effect became more prominent for particles sintered at the higher temperatures (not shown here). The XRD analysis of the fresh particles F6AM1100P and F6AM1125P looks identical to that of F6AM1100 from the first batch. For F6AM1150P the hematite intensity peaks are somewhat lower compared to the particles sintered at lower temperatures. For F6AM1175P and F6AM1200P the intensity of the hematite peaks are further reduced, while it is clear that there are intensity peaks associated with at least two other different phases. From the reactivity investigations in the fluidized bed, F6AM1100P and F6AM1125P had a reactivity more or less independent of the number of cycles, while the three others showed increased reactivity over the cycles. None of the particles agglomerated or showed any formation of cracks. Worth noticing is that F6AM1200P had much lower reactivity than F6AM1200 and that the former did not agglomerate, the reason for this difference is not clear. F6AM1125P has a bit lower reactivity but is harder than F6AM1100P. The rate indexes of the particles from the second batch are included in Figure 9 as A-E. Thus, the second batch strengthened the conclusions from the first batch that a sintering temperature of approximately 1100 °C is suitable, considering reactivity, strength, and tendency of agglomeration. Similar to batch 1 particles, the XRD investigations of the reduced particles from batch 2 show that they all have equal chemical composition. A summary of the evaluation of the particles from the second batch is seen in Table 2.
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Figure 11. The rate index as a function of the reaction temperature for F6AM1100. Table 2. Summary over Physical Properties and Reactivity of the Particles Investigated in the Second Batch
particle F6AM1100P F6AM1125P F6AM1150P F6AM1175P F6AM1200P
fraction sintering apparent crushing rate of Fe2O3 temp density porosity strength index (%) (°C) (kg/m3) (-) (N) (%/min) 60 60 60 60 60
1100 1125 1150 1175 1200
1898 2371 2977 3436 3518
0.59 0.48 0.35 0.25 0.23
1.5 2.3 3.9 6.8 10.3
2.87 1.94 0.81 0.49 0.53
The Effect of Particle Size. To examine the effect of particle size further tests were carried out with F6AM1100. Three different size intervals of 90-125 µm, 125-180 µm, and 180-250 µm were tested, and no significant differences were found in the reactivity. The Effect of Reaction Temperature. For the effect of the reaction temperature the same particles, F6AM1100, were run at 650, 750, 850, and 950 °C. The results in form of rate index versus reaction temperature can be seen in Figure 11. It is clear that the reaction temperature has a strong effect on the reactivity, although even at the very low temperature of 650 °C some reactivity was seen. To see the conversion of methane to carbon dioxide we introduce the gas yield, defined as
γred ) CO2,out/(CO2,out + CO,out + CH4,out) The dependence of γred on the reactivity temperature is seen in Figure 12a. For 950 °C with F6AM1100 there is almost full conversion of the gas in the initial part of the reduction and then a gradual decline to the point where FeO starts to form (corresponding to ω ) 0.98), where the gas yield falls rapidly. In Figure 2a there is seen some formation of CO in the later part of the reduction. To investigate the amounts of CO created at different temperatures the ratio COout/(COout + CH4,out) was calculated and is displayed in Figure 12b. As can be seen this ratio is almost zero for 650 °C and increases with higher reaction temperatures, and it is rather constant over the mass based conversion range. A possible interpretation is that CO is a reaction intermediate, i.e., that carbon is sequentially oxidized as k1
k2
CH4 98 CO 98 CO2
(15)
Figure 12. (a) The gas yield as a function of the mass conversion for F6AM1100 at different reaction temperatures: 950 °C (9), 850 °C (b), 750 °C (]), and 650 °C (+). (b) The ratio of CO/(CO+CH4) as a function of the mass conversion for F6AM1100 at different reaction temperatures: 950 °C (9), 850 °C (b), 750 °C (]), and 650 °C (+).
The results obtained here may indicate that the first step in (15) is more temperature dependent than the second, and hence that the formation of CO is the rate determining step of the reduction. Solids Inventory. Possible amounts of bed material in a CLC system using iron ore were discussed by Lyngfelt et al., and an amount of 500 kg/MW was assumed to be realistic.3 The rate index is closely connected to needed bed mass, so that a rate index of 1 corresponds to 500 kg/MW and a rate index of 3 corresponds to approximately 150 kg/MW in the fuel reactor. Therefore we can conclude from Figure 9 that several out of the 10 initially tested particles would be suitable. Excluding those that agglomerated and those that were believed to be too soft, F6AM1100 and F4AM1200 were the two that were best suited. From the second batch we could also include F6AM1125P in these. From the reaction temperature comparison in Figure 11 it can also be concluded that the reactivity is sufficient for F6AM1100 at lower reaction temperatures. Conclusion The reactivity of the metal oxide Fe2O3 on MgAl2O4 support has been investigated. The effect of sintering temperature, the amount of metal oxide, particle size, and reaction temperature has been investigated in order to determine the best characteristics of the oxygencarriers for use in CLC. The sintering temperature had
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a large effect on the structure and the reactivity of the particles. Particles sintered at 1100 °C had a smooth surface structure, while particles sintered at 1300 °C formed large surface grains, believed to be associated with solid-state reactions within the particles. Particles of 60% Fe2O3 sintered at 1100-1125 °C were very reactive and sufficiently hard. The strength of the 1100 °C particles increased after reaction. The particle size did not seem to affect the reactivity. The effect of the reaction temperature on the reaction rate is strong. The particles of 60% Fe2O3 sintered at 1100 °C are believed to be well suited for a fluidized-bed reactor of chemicallooping combustion. Acknowledgment This work was financed by the ECSC project, Capture of CO2 in Coal Combustion (CCCC), 7220-PR-125. A special thanks to Christina Lager for conducting some of the experiments. Literature Cited (1) Ishida, M.; Zheng, D.; Akehata, T. Evaluation of a ChemicalLooping Combustion Power-Generation System by Graphic Exergy Analysis. Energy 1987, 12, 145-154. (2) Anheden, M.; Svedberg, G. Exergy analysis of chemicallooping combustion systems. Energy Convers. Manage. 1998, 39, (16-18), 1967-1980. (3) 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, 3101-3113. (4) Lyngfelt, A.; Leckner, B. Technologies for CO2 separation. Minisymposium on Carbon Dioxide Capture and Storage, Chalmers University of Technology and Go¨ teborg University, Go¨ teborg, 1999. (5) Mattisson, T.; Lyngfelt, A. Capture of CO2 using chemicallooping combustion. in Scandinavian-Nordic Section of Combustion Institute. 2001. Go¨ teborg, 2001. (6) Jin, H.; Okamoto, T.; Ishida, M. Development of a Novel Chemical-looping Combustion: Synthesis of a Solid Looping Material of NiO/NiAl2O4. Ind. Eng. Chem. 1999, 38, 126-132. (7) Jin, H.; 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, 1272-1277.
(8) Ishida, M.; Jin, H.; Okamoto, T. Kinetic Behaviour of Solid Particle in Chemical-looping Combustion: Supressing Carbon Deposition in Reduction. Energy Fuels 1998, 12, 223-229. (9) Ishida, M.; Jin, H. A Novel Combustor Based on Chemicallooping Reactions and its Reaction Kinetics. J. Chem. Eng. Jpn. 1994, 27, 296-301. (10) Ada´nez, J.; de Diego, L. F.; Garcı´a-Labiano, F.; Gaya´n, P.; Abad, A.; Palacios, J. M. Selection of Oxygen Carriers for Chemical-Looping Combustion. Energy Fuels 2004, 18(2), 371-377. (11) Mattisson, T.; Johansson, M.; Lyngfelt, A. Multi-Cycle Reduction and Oxidation of Different Types of Iron Oxide Particles - Application of Chemical-Looping Combustion. Energy Fuels 2004, 18(3), 628-637. (12) Singh, V. K.; Sinha, R. K. Low-Temperature Synthesis of spinel (MgAl2O4). Mater. Lett. 1996, 31, 281-285. (13) Wang, J.; Li, C. A study of surface and inner layer compositions of Mg-Fe-Al-O mixed spinel sulfur-transfer catalyst using Auger electron spectroscopy. Mater. Lett. 1997, 32, 223227. (14) Wang, J.-A.; Zhu, Z.-L.; Li, C.-L. Pathway of the cycle between the oxidative adsorption of SO2 and the reductive decomposition of sulfate on the MgAl2-xFexO4 catalyst. J. Mol. Catal. A: Chem. 1999, 139, 31-41. (15) Ge, X.; Mingshi, L.; Shen, J. The Reduction of Mg-Fe-O and Mg-Fe-Al-O Complex Oxides Studied by TemperatureProgrammed Reduction Combined with in Situ Mo¨ssbauer Spectroscopy. J. Solid State Chem. 2001, 161, 38-44. (16) Ortiz, U.; Aguilar, J.; Kharissova, O. Effect of iron over the magnesia-alumina spinel lattice. Adv. Technol. Mater. Mater. Process. J. 2001, 2, 107-116. (17) Harrison, R. J.; Dove, M. T.; Knight, K. S.; Putnis, A. In Situ Neutron Diffraction Study of Non-Convergent Cation Ordering in the (Fe3O4)1-x(MgAl2O4)x Spinel Solid Solution. Am. Miner. 1999, 84, 555-563. (18) Mattisson, T.; Ja¨rdna¨s, A.; Lyngfelt, A. Reactivity of some metal oxides supported on alumina with alternating methane and oxygen - application for chemical-looping combustion. Energy Fuels 2003, 17, 643-651. (19) 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, 1953-1962.
Received for review March 8, 2004 Revised manuscript received June 29, 2004 Accepted August 11, 2004 IE049813C
