Article pubs.acs.org/EF
Study of Dimensional Changes during Redox Cycling of Oxygen Carrier Materials for Chemical Looping Combustion Anita Fossdal,† Ove Darell,† Arnold Lambert,‡ Erin Schols,§ Elodie Comte,‡ Rebecca Leenman,§ and Richard Blom*,∥ †
SINTEF Materials and Chemistry, Post Office Box 4760, NO-7465 Trondheim, Norway IFP Energies nouvelles, Rond-point de l’échangeur de Solaize, BP 3, 69360 Solaize, France § TNO Gas Treatment, NL-2600 JA Delft, Netherlands ∥ SINTEF Materials and Chemistry, Post Office Box 124 Blindern, NO-0314 Oslo, Norway ‡
ABSTRACT: Dimensional and phase changes of four candidate oxygen carrier materials for chemical looping combustion are investigated by dilatometry and high-temperature X-ray diffraction during four redox cycles. NiO/Ni2AlO4 does not exhibit significant dimensional changes during cycling, and it is shown that the support material also contributes to the oxygen carrying capacity. CaMn0.875Ti0.125O3 exhibited good chemical stability and small dimensional changes upon redox cycling. Cu0.95Fe1.05AlO4 showed a one-dimensional expansion of 9% after the experiments, and significant phase changes were seen. The complex set of reactions occurring during redox cycling of ilmenite (FeTiO3) was shown to be accompanied by dimensional changes, giving non-steady dimensional changes during the oxidation and reduction steps.
1. INTRODUCTION Chemical looping combustion (CLC) is a cyclic process with inherent CO2 separation, in which a metal oxide first is used to combust a carbon-containing fuel and then the reduced metal oxide is reoxidized in air before a new cycle can be carried out (see Figure 1).1 The interest in CLC has been boosted during the past decade because of its relatively high net energy efficiency2 and potentially low cost of CO2 capture.3
Fixed bed reactors can also be used for continuous CLC operation. Initial CLC experiments were carried out in single fixed bed reactors,5 and single fixed bed reactors are often used for basic characterization of oxygen carrier materials with respect to reaction kinetics and oxygen capacity. For continuous operation, such setups may involve two or more fixed bed reactors, where complex valving sequences ensure cyclic gas feeding to the reactors and optimal gas separation.6 Alternatively, a rotating bed reactor type, where the oxygen carrier material is rotated between the reducing and oxidizing gas streams, can be used.7 It has recently been shown that CFB and fixed bed CLC have similar process efficiencies; therefore, reactor choice will be dependent upon rector cost issues.8 In general, good oxygen carriers for CLC should have significant cyclic oxygen capacity; in addition, the kinetics of both the reduction and oxidation reactions should be fast. Also, because most oxygen carrier materials are bulk oxides, bulk diffusion of oxide ions to the particle surface, where the reactions take place, should be faster than the reaction kinetics to avoid surface oxygen deficiency. The stability of the oxygen carrier particles should also be high, both chemically and physically. Oxygen carrier particles are in most cases agglomerates of smaller grains. Changes in particle volume are induced by changes in the grain volume when reactions take place. The changing grain size model (CGSM) has been used to describe this phenomenon,9 and the model has later been used to estimate temperature variations within oxygen carrier materials.10 In the modeling, grains of the active phases were mixed with inert grains in the particle matrix and only a change in oxygen carrier grain volume was assumed; no change in the total particle volume was
Figure 1. Schematic drawing of the CLC process.
The CLC redox process (the central unit in Figure 1) can be realized using various reactor setups. One obvious way is to use a circulating fluidized bed (CFB) configuration, where the metal oxide powder circulates between a fuel reactor, in which the combustion takes place, and an air reactor, where reoxidation takes place.4 In CFB reactors, the powder flows freely with the gas streams and gas mixing is avoided using gas locks between the two reactors. Quite high gas velocities are needed for powder transportation, giving rise to severe attrition when particles collide and hit the reactor walls. CFB reactors have until now gained by far the most attention within the CLC community because this reactor type is already commercial for combustion processes (boilers) and within refinery processes, such as fluidized catalytic cracking (FCC). © 2014 American Chemical Society
Received: September 7, 2014 Revised: November 4, 2014 Published: December 5, 2014 314
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parameters of the phases were calculated along with a quantitative estimation of the content of each phase by Rietveld analysis using the Topas software from Bruker. 2.4. SEM. Polished cross-sections of selected dilatometer samples were investigated using a Hitatchi S-3400N scanning electron microscope equipped with an Oxford Instruments energy-dispersive spectrometry (EDS) system and the Aztec software suite. Their methods of preparation are described in Table 1.
considered. Because both pure and supported oxygen carrier materials are candidates for used in CLC, it is of relevance to investigate how some of the more common oxygen carrier particles change volume and morphology during redox cycling. Volume changes of particles in a CFB reactor will lead to changes in fluidization properties; in addition, increased porosity might lead to an increased attrition rate. In a fixed bed column, some volume changes of the bed can be taken care of when the oxygen carrier pellets remain free-flowing (no interpellet sintering). However, if some sintering occurs, a reduction in particle volume might lead to shrinkage of the oxygen carrier bed and, thus, aggravated gas/solid contact because of gas bypass. Alternatively, an increased particle volume may lead to severe particle fragmentation, formation of fines, and increased pressure drop over the column. In a pre-study, we have shown that the dimensional changes during a redox cycle and upon further cycling were significantly different for two selected oxygen carrier materials.11 In the present contribution, we have studied four different oxygen carrier materials in more detail: NiO/NiAl2O4 (NNA), FeTiO3 (FTO, ilmenite), CaMn 0 . 8 7 5 Ti 0 . 1 2 5 O 3 (CMT), and Cu0.95Fe1.05AlO4 (CFA). Both NNA and FTO have been extensively studied for CLC applications, while CMT and CFA have been studied to a lesser extent. While NNA might be classified as a supported oxygen carrier material, the latter three can be classified as pure or unsupported systems. The dimensional changes of all four materials under redox conditions have been studied by dilatometry. Four redox cycles were run for each sample, which should both give information about major structural changes during the initial cycle and the trend during the succeeding cycles. The dilatometry experiments were supported by in situ powder X-ray diffraction (XRD) to investigate changes in crystallographic phases during operation and scanning electron microscopy (SEM) to obtain morphological information.
Table 1. Investigated Materials and Their Origin name
nominal composition
CFA CMT
NiO/NiAl2O4 (60% NiO) Cu0.95Fe1.05AlO4 CaMn0.875Ti0.125O3
FTO
FeTiO3
NNA
preparation method wet granulation (Marion Technologies, France) co-precipitation of metal nitrates10 spray pyrolysis (CerPoTech AS, Norway)12 crushed natural ore (from Norsk Titania AS)
3. RESULTS AND DISCUSSION Figure 2 shows the results of the dilatometry cycling experiments for the four investigated materials. The dimensional changes during cycling follow a different course for the four materials. NNA (Figure 2a) shows a steady but small contraction upon reduction, with no signs of leveling off during the 2 h segment. The largest contraction occurred in the first cycle, decreasing gradually in the three following cycles. During oxidation, a small expansion occurred during the first 30−40 min, followed by a minor contraction before the sample length stabilized. The peak in ΔL/L in the beginning of the segment is thought to be associated with a temporary local temperature increase caused by the exothermic oxidation of Ni. Within the given time frame, the expansion during oxidation only partially compensates for the contraction during the reduction step. A slow expansion was apparent during the overnight hold in air during the third cycle, as evidenced by the small jump in the curve in cycle three (marked by an asterisk). Although the material steadily contracts with increasing cycle number, the contraction in one dimension was less than 0.3% after four cycles. SEM analysis of a polished cross-section of the sample cylinder showed no apparent differences in morphology or porosity across the sample. HTXRD analysis showed that the NNA material cycled between the NiO/“NiAl2O4” spinel in the oxidized state and Ni/ “γ-Al2O3” in the reduced state (the quotation marks indicate nominal composition). Both “NiAl2O4” and “γ-Al2O3” are indexed in space group Fd3̅m, and the unit cell parameters at 900 °C are shown as a function of the cycle number in Figure 3. The diffraction peaks (not shown) of both “NiAl2O4” and “γAl2O3” are quite broad, which indicates the presence of chemical inhomogeneities in both phases. The reported cell parameters are therefore to be seen as an average value. “NiAl2O4” is wellknown to be a solid solution phase, in which the Ni content (and, thus, the cell parameter) can vary.13 The solubility limit of NiO in γ-Al2O3 is not known, but it is expected to be low under reducing conditions. The unit cell parameter of “NiAl2O4” drops by ca. 0.03 Å between cycles one and two, pointing to a decrease in the Ni content after the initial cycle. Further cycling shows a further, minor, decrease in a. The unit cell parameter of “γ-Al2O3” shows the opposite behavior, increasing slightly upon cycling, which is thought to be related to increased retention (on average) of Ni in the alumina phase. The changes are, however, very small and not expected to have any particular impact on the CLC performance
2. EXPERIMENTAL SECTION 2.1. Material Preparation. The four investigated materials and all materials were sieved to a particle size fraction of 125−180 μm. 2.2. Dilatometry Experiments. Prior to dilatometric testing, the materials were subjected to a grinding step that reduced the particle size to below 40 μm, before cylindrical samples with a diameter of 10 mm and length of 15−20 mm were prepared by double-action uniaxial pressing. The pellets were then heat-treated at 1100−1200 °C to densify the pellets through sintering and ensure that any dimensional changes seen during the dilatometry test would be related to the changes in the surrounding atmosphere. Dilatometry experiments were performed using a Netzsch 402 E dilatometer. One face of the sample was placed against an immovable plate, where a spring-loaded pushrod measured the linear expansion/contraction behavior of the sample during thermal treatment. Each porous cylinder was heated to 900 °C in air and cycled 4 times according to the following isothermal steps: (1) 2 h in air (except cycle three, where the sample was kept overnight in air), (2) 10 min in N2, (3) 2 h in 5% H2 in N2, and (4) 10 min in N2. A total flow rate of 125 mL/min was used in all steps. 2.3. In Situ Powder XRD Experiments. High-temperature X-ray diffraction (HTXRD) experiments were performed using a Panalytical Empyreon diffractometer (Cu Kα radiation) equipped with an Anton Paar XRK 900 high-temperature camera and an automated gas switching system. The powders were heated to 900 °C in air, and redox cycling was performed following the procedure used for dilatometry, with the exception that there was no overnight dwell during the air step in cycle three. Scans (3 min) were recorded at 10 min intervals during the dwell periods. At the end of each dwell in air or reducing atmosphere, an 18 min scan was recorded to improve the signal-to-noise ratio. Phase evolution during cycling was evaluated. For NNA and FTO, unit cell 315
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Figure 2. Dimensional changes along the long axis of the sample cylinders as a function of time and atmosphere. The dotted lines indicate a change of atmosphere. (a) NNA, (b) CFA, (c) CMT, and (d) FTO. The asterisk in cycle three for all materials denotes a break in the curve because of the samples being kept in air overnight. All curves have been normalized to start at zero in the first cycle oxidation step. Note the differing scales on the y axis for the four graphs.
cycles and the material should be suitable for operation in both fixed and fluidized beds from a volume change standpoint. The CFA material (Figure 2b) behaved opposite to NNA during reduction, showing a steady, large, and close to linear expansion as a function of time. Again, a slight expansion followed by a contraction was seen at the start of the oxidation stage. This is again thought to be caused by the heat generated during the oxidation. Apart from the temperature-induced dimensional change seen in the start of the segment, the CFA sample length remained constant during oxidation. The CFA pellet was severely cracked and deformed after the dilatometry experiment, and the total one-directional expansion was as high as 9%. The bulk density of the pellet was 3.3 g/cm3 after the experiment, a decrease from 4.2 g/cm3 prior to the experiment. Correspondingly, the open porosity increased from 15.5 to 31.6% during the course of the experiment. Figure 4 shows representative SEM images (secondary electrons) taken of a polished cross-section of the CFA dilatometer sample after the redox cycling experiment. In the image taken near the sample edge, the amount of intragranular pores is much higher than near the sample center. The concentration of the reaction gases will vary throughout the dilatometry monolith because of mass-transfer limitations toward the center of the sample. The edge of the sample can
Figure 3. Unit cell parameters for “NiAl2O4” spinel (oxidation step) and “γ-Al2O3” (reduction step). Note the break in the y axis.
of NNA. The transformation from high-Ni spinel to low-Ni γAl2O3 requires cation diffusion to occur, and this cation diffusion is thought to contribute to the slight densification of the sample seen upon redox cycling. As mentioned earlier, the observed onedimensional contraction for NNA was less than 0.3% after four 316
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Figure 4. SEM images of a polished cross-section of the CFA dilatometry sample post-experiment: (a) center of the sample and (b) near the edge of the sample. Figure 5. X-ray diffractograms of CFA at 900 °C after the (a) oxidation in air and (b) reduction in 5% H2 steps of redox cycling.
be expected to cycle deeper (i.e., be more reduced and oxidized) than the center of the sample; hence, intragranular porosity is expected to increase upon cycling. The intragranular pores will mainly add to the closed porosity, but as the amount of pores increases, the pores will become gradually more interconnected and increase the open porosity as well. Because of the large expansion, crack formation in the sample will also increase the open porosity. HTXRD experiments (Figure 5) showed that CFA was singlephase (cubic Fd3m ̅ ) prior to the first reduction step. Upon reduction, decomposition of the cubic CFA phase was seen, yielding formation of a metallic phase and a cubic “CFA” phase, both of which appeared to be chemically inhomogeneous. Oxidation of the reduced sample gave a two-phase mixture of “CFA” phases. For each cycle, the diffraction peaks, in both the oxidation and reduction steps, became broader and decreased in intensity, indicating a decrease in domain size, rendering the material mainly X-ray amorphous. After six cycles, the intensity of the main oxidized “CFA” peak was reduced to 1.5% compared to the oxidized material in cycle one. Similarly, the main peak of the metallic phase decreased to 4.5% in the sixth cycle compared to cycle one. The high expansion and observed increase of porosity would most certainly be a challenge for using CFA in CLC fixed beds, because the pressure drop would change greatly with cycling and
time. Particle fragmentation would be likely to result from the combination of a limited volume in the reactor and the large volume increase during cycling, inducing stress on the particles in the bed. In a fluidized bed, the lifetime of this material would be severely limited because of reduction in mechanical strength upon cycling and subsequent production of fines, as caused by the increased porosity and phase separation. In addition to altering fluidization properties, the produced fines fraction would decrease the lifetime of downstream process components and increase the need for maintenance. Whereas the transient dimensional increase because of oxidation was small in NNA and CFA, it is more prominent in the CMT material, as evidenced in Figure 2c. The first cycle is very different from the following cycles, indicating different processes taking place. In the first cycle, an already fully oxidized material is the starting point; therefore, no dimensional changes are observed during the first oxidation step. During the flushing with inert and reduction steps, the dimensional increase is significant during the first cycle, around 0.5%. The three following cycles follow the same trend; after the temperatureinduced dimensional increase, the CMT cylinder contracts during oxidation. Switching to a nitrogen atmosphere initiates a dimensional increase, which continues during the reduction step 317
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in a 5% hydrogen atmosphere. The dimensional change is by far the largest during the first cycle, leveling off at approximately 0.7% higher than the initial length after four cycles. In situ XRD (Figure 6) shows a slight demixing of the CMT perovskite phase
Figure 7. SEM images of a polished cross-section of a CMT dilatometry sample after four redox cycles: (a) center of the sample and (b) edge of the sample.
manganites, in particular, the effect is likely to be related to the large difference in ionic radius of the Mn3+ and Mn4+ ions.16 Cracking of the sample will increase the sample volume. It is expected that cracking will mainly occur during the first cycle, because most of the strain will have been released. Subsequent cycling would therefore have a minor effect on the sample volume. This behavior is indeed seen in the dilatometry experiment. For CMT particles intended for use in a CLC process, cracks induced by chemical expansion will have a detrimental effect on the mechanical properties of the material, resulting in production of fines. Unsupported CMT materials can therefore be expected to have shorter lifetimes in a CLC process than oxygen carriers that do not exhibit chemical expansion upon oxygen release. CMT can be expected to be more suitable for fixed bed applications rather than fluidized beds, because of lower requirements for mechanical properties in fixed beds. The FTO dilatometry curves (Figure 2d) show a more complex signature than the other three materials. Rather than exhibiting the expected transient dimensional increase at the start of the oxidation step because of the exothermic oxidation reaction, FTO contracts noticeably, followed by an increase in dimensions. The expansion continues during the overnight dwell in air, as seen by the step in the curve after the oxidation step for cycle three. The behavior during reduction differs between the cycles. In cycles one and three, where the material can be expected to be close to fully oxidized at the start of reduction, the
Figure 6. X-ray diffractograms of CMT at 900 °C after the (a) oxidation in air and (b) reduction in 5% H2 steps of redox cycling.
material into two perovskite phases during heating in air (Figure 6a), demixing further after cycle two. The relative amounts of the two oxidized phases stabilize from the second cycle onward. The initial demixing is a likely origin of the differing behavior observed in the first dilatometry cycle compared to the following three. The XRD pattern after the reduction step did not change with cycling (Figure 6b); hence, XRD gave no indication that changes in phase relations are the cause of the observed dimensional changes between cycles. Investigation of a polished cross-section of the CMT dilatometry sample (Figure 7) revealed cracking in the outer regions of the sample cylinder, presumably because of the relatively low mechanical strength of the porous sample combined with the stress of chemical expansion/contraction caused by the cyclic reduction and oxidation. Chemical expansion is a well-known phenomenon upon reduction of perovskites, e.g., seen for ferrites14 and manganites.15 For 318
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initial reduction phase gives a decreasing sample length. In cycle one, this decrease continues all of the way through the reduction step. In cycle three, however, the sample length reaches a minimum after ca. 230 min of the cycle (ca. 90 min into the reduction step), after which a slight expansion is seen. Cycles two and three display an initial dimensional increase upon onset of reduction, after which the length of the sample decreases, goes through a minimum, and then expands slowly again. The total one-dimensional change for FTO spans 4% during the four cycles in this experiment. The relevant phases in the Fe−Ti−O system and the abbreviations used in this work are described in Table 2. The Table 2. Composition of and Abbreviations for the Observed Fe−Ti−O Phases during Cycling abbreviation
name
composition/solid solutiona
FTO US PSB RUT HEM
ilmenite ulvospinel pseudobrookite rutile hematite
(Fe,Ti)2O3, high (≈1) Ti (Fe,Ti)3O4 (Fe,Ti)3O5 TiO2 (Fe,Ti)2O3, low (≈0) Ti
Figure 9. Phase composition during redox cycling (second cycle) for FTO.
study, untreated FTO was used; hence, activation will occur during cycling. Figure 9 illustrates that HEM is first formed alongside RUT when the atmosphere is changed from reducing to oxidizing. HEM and RUT then react, forming increasing amounts of the PSB solid solution during the dwell under oxidizing conditions. Contrary to the expected enhanced kinetics resulting from activation, however, the rate of the PSB formation reaction appears to decrease with the cycle number, because the amount of PSB at the end of the 2 h oxidation step decreases with increasing the cycle number (Figure 8). The observation is due to a change in the amounts of each phase and not in chemical composition, because the unit cell parameters for the HEM phase remain constant during the cycle (Figure 10). Investigations have not been made to determine the cause of the reduced reaction rate for PSB formation, because this would go beyond the scope of the current study. The reactions occurring under reducing conditions appear less affected by cycling, because the relative amount of the product phases is constant after cycle two. An increase in the Fe content (and, thus, a lowering of the average oxidation state of Fe) is seen after the second reduction compared to the first, stabilizing upon
a
Solid solutions have varying composition on one or more crystallographic sites. The notation (X,Y)z means that the molar content of X + Y = z but that the relative content of X and Y is unknown or can vary.
phases and their estimated weight fraction (the uncertain chemical composition in the two solid solutions has not been taken into account; hence, some uncertainty in the numbers is introduced through the use of the nominal compositions in the Rietveld analysis) at the end of the oxidation and reduction cycles, as found by HTXRD, are shown in Figure 8.
Figure 8. Observed phases and their estimated weight fractions at the end of each of the four (left) oxidation and (right) reduction steps for FTO, as found from HTXRD.
The phase evolution observed during the second redox cycle is shown in Figure 9. All cycles showed similar trends. It is clear from Figure 8 that repeated cycling changes the kinetics of the phase evolution, particularly during the oxidation step. A so-called activation process, entailing enhanced FTO performance in CLC operation after activation, has been described previously by, e.g., Cuadrat et al.17 In the current
Figure 10. Unit cell parameters of the FTO and HEM phases during the second redox cycle. 319
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ACKNOWLEDGMENTS This work was carried out as a cooperation between TNO, IFP Energies nouvelles (IFPEN), and SINTEF within the strategic TRI4CCS constellation. Dr. Anna Lind (SINTEF) is gratefully acknowledged for carrying out the HTXRD measurements.
further cycling. The phase evolution route during reduction also consists of two steps. First, FTO and US are formed, and an increase in the US content and a decrease in the FTO content are seen. Second, precipitation of metallic Fe is seen, accompanied by a decrease in the US content and an increase in the FTO content. The chemical composition of FTO remains constant throughout the cycle (Figure 10). Note that, under CLC conditions, the material would not be as reduced as under the conditions employed here, avoiding metallic Fe formation. The complex phase relations in the FTO sample as a function of the oxygen partial pressure and time results in volume changes because of changes in the sum of molar volumes of the constituent phases. Estimations of volume changes based on molar volumes available in the Inorganic Crystal Structure Database (ICSD) and the calculated phase fractions from HTXRD correspond well with the non-steady expansion behavior observed by dilatometry. Further investigations are in progress. Although FTO shows non-steady expansion behavior, the numerical value of the dimensional change is small and should not by itself constitute a problem for use of FTO in fixed or fluidized beds for CLC. The extensive cation diffusion involved in the phase evolution during reduction and oxidation can however be expected to result in increased porosity and decreased strength with cycling. An increase in porosity and decrease in strength is confirmed in the study by Cuadrat et al.17
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REFERENCES
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4. CONCLUSION Changes in particle volume during redox cycling can lead to severe particle degradation in fixed bed systems in which the room for bed expansion is limited. Fluidized bed systems handle particle volume changes of this kind more easily, and only fluidization behavior of the powder will be affected. Mechanical strength and attrition resistance is however very important for this reactor type. In this work, it is shown that four different oxygen carrier materials show significantly different dimensional changes during redox cycling. NNA shows minimal dimensional changes, in line with its proven history of mechanical stability as an oxygen carrier in a CLC process. CFA decomposes chemically and mechanically during redox cycling and would have a very limited lifetime as an oxygen carrier. CMT exhibits a slight chemical demixing upon initial reduction and oxidation but stabilizes upon further cycling. Chemical expansion is, however, seen to induce stress in the CMT material, causing stress release by cracking and, thus, lowered mechanical strength. The amount of stress buildup in a particle will be strongly dependent upon its morphology, and it may be possible to tailor particles that can tolerate the dimensional changes through the use of a support material. FTO shows a complex phase evolution scheme during a redox cycle, giving rise to non-steady dimensional changes during both the reduction and oxidation steps. The dimensional changes are, however, quite small and are not expected to have a negative impact on FTO as a CLC oxygen carrier.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 320
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