Evaluation of Microstructural Changes and Performance Degradation

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Kinetics, Catalysis, and Reaction Engineering

Evaluation of Microstructural Changes and Performance Degradation in Iron-Based Oxygen Carriers during Redox Cycling for Chemical Looping Systems with Image Analysis Yuya Saito, Fumihiko Kosaka, Noriaki Kikuchi, Hiroyuki Hatano, and Junichiro Otomo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04966 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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Industrial & Engineering Chemistry Research

Evaluation of Microstructural Changes and Performance Degradation in Iron-Based Oxygen Carriers during Redox Cycling for Chemical Looping Systems with Image Analysis Yuya Saito1, Fumihiko Kosaka1, Noriaki Kikuchi1, Hiroyuki Hatano2, and Junichiro Otomo1＊

1

Department of Environment Systems, Graduate School of Frontier Sciences, The University of

Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan

2

Department of Integrated Science and Engineering for Sustainable Society, Faculty of Science

and Engineering, Chuo University 1-13-27, Kasuga, Bunkyo-Ku, Tokyo 112-8551, Japan

*Corresponding author

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT A coupled analysis of the reaction kinetics and microstructural changes of Fe2O3/Al2O3 oxygen carriers during redox reaction cycles for chemical looping systems was conducted. The microstructural changes in the oxygen carrier particles were investigated using an image analysis method with cross-sectional backscattered electron images, and microstructural information such as the particle size, porosity, and two-phase boundary between the iron oxide and pores was obtained. The microstructural changes and the degradation kinetics during redox cycles were investigated under various operating conditions (reaction temperatures, reduction times, oxygen partial pressures during the oxidation process, and weight ratios of the oxygen carriers). The degradation coefficient and the reaction enthalpy of the particles in the oxidation process are linearly related, implying that increasing the local temperature of the oxygen carriers causes the coarsening of iron oxide particles via sintering and microstructural changes. Our analysis contributes to the design of highly stable oxygen carrier particles and the improvement of the operating conditions for chemical looping systems.

KEYWORDS Chemical looping, degradation, image analysis, iron oxide, redox cycle

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INTRODUCTION Chemical looping systems such as chemical looping combustion and chemical looping reforming are attractive energy conversion systems to convert fuels such as coal, biomass, and hydrocarbons to thermal energy and to selectively generate gases such as CO2, H2, and N2.1–5 The thermal energy can be used for electric power generation using a steam turbine. In chemical looping systems, the reduction and oxidation reactions of metal oxides (MOs), such as NiO, CuO, and Fe2O3 are carried out in two reactors, an air reactor and a fuel reactor. Lattice oxygen in the metal oxides is used as an oxygen source for the oxidation of the fuels (eq 1), and the reduced metal oxide is re-oxidized in the air reactor (eq 2). (2n + m)MO + CnH2m → (2n + m)M + nCO2 + mH2O

(1)

M + 0.5O2 → MO

(2)

Therefore, the MO particles are referred to as oxygen carriers, and selective gas production and power generation can be achieved simultaneously in chemical looping systems. Furthermore, using steam iron reaction in an additional reactor (i.e., a steam reactor), hydrogen can be produced without a gas separation process (3Fe + 4H2O → Fe3O4 + 4H2).6–10 To design oxygen carriers with high reactivity and long lifetime, an understanding of the microstructural variations in the oxygen carriers during redox cycling is crucial, and this is also necessary for the development of chemical looping systems.1 Particle coarsening, which is the increase in the particle size of MOs in oxygen carriers, occurs because the redox reaction cycles are repeated at high temperatures, ranging from 700 to 1000 °C.1 The surface area of the oxygen 3



carrier material decreases and the reduction kinetics are degraded. Because the redox cycling of the oxygen carrier particles is repeated thousands of times in chemical looping systems,1 the design of oxygen carrier particles that can maintain high reactivity during redox cycles and have a long lifetime is very important. In previous studies, metal oxides such as Fe2O3, NiO, and CuO and natural ores such as ilmenite have been widely investigated as oxygen carriers in chemical looping systems, and the stability of oxygen carriers in redox cycles has also been examined.11–23 Concerning ilmenite, the surface area increases during the first several cycles and the redox kinetics improve.18 However, Fe is segregated to the surface of the ilmenite particles, and the porosity of the particles increases during redox cycles,18–20 resulting in a decrease in the mechanical strength of the particles. In addition, Knutsson et al. reported redox cycles of ilmenite particles in a 100-kW chemical-looping system, and cracks and pores were observed, as well as the formation of Fe rich islands, in the cross-sectional scanning electron microscopy (SEM)-energy dispersive X-ray spectrometry (EDX) images after system operation.19 In our previous studies, we also observed Fe segregation to the surface of ilmenite oxygen carrier particles during redox cycling.22,23 Concerning artificial oxygen carriers, these mostly consist of composites of MO particles such as Fe2O3, NiO, and CuO and support particles such as Al2O3 and TiO2, as summarized in ref 1. However, the coarsening of MO particles in the artificial oxygen carriers occurs, and the redox kinetics are degraded during redox cycles.21,24–39 Regarding the microstructural changes of the artificial particles, Sun et al. carried out 50 redox cycles of an Fe2O3/Al2O3 oxygen carrier in an

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H2/O2 atmosphere and observed the microstructural changes during the redox cycles using SEM-EDX analysis.37 They found that, over the 50 redox cycles, Fe diffuses to the surface of the Fe2O3/Al2O3 oxygen carrier particles and the amount of Fe inside the particles decreases. Qin et al. also reported the morphological changes of Fe2O3-TiO2 composite particles and the formation of Fe nanobelts on the surface of the composite particles. Moreover, a porous structure inside the particles after the redox cycles was observed.21 Because the redox reactions of oxygen carrier particles in chemical looping systems are conducted in fluidized bed reactors, mechanical degradation, that is, the attrition of oxygen carrier particles, is one of the main reasons for the performance degradation.40–46 Therefore, to obtain oxygen carrier particles with a long lifetime, chemically and mechanically stable particles should be designed. In this study, we focused on the microstructural changes, such as MO particle coarsening inside the oxygen carrier particles during redox cycles (chemical degradation), rather than the mechanical degradation (attrition). So far, microstructural analysis has only been reported based on SEM observation of the appearance and the cross-sectional images of the oxygen carrier particles, and there have been no reports of the quantitative measurement of the microstructural changes of oxygen carrier particles such as the particle size and the boundary area of the MO particles. In addition, there have been no reports that systematically summarize the influence of the redox reaction conditions on the deterioration of the particle performance. Furthermore, the correlation between the redox kinetics of Fe2O3 oxygen carriers and the variations in the microstructural changes of particles has not been investigated sufficiently, even though an

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understanding of this correlation can provide strategies to design oxygen carrier particles with improved stability and longer lifetime. Therefore, the detailed observation of the microstructural variations of oxygen carriers is crucial. In our preliminary experiments, we observed the microstructural changes of iron-based oxygen carrier particles during redox cycles with image analysis method.23 In this study, we reported detailed results of the microstructural changes of the oxygen carriers and focused on the relationship between the microstructural changes and the performance degradation during redox cycles of Fe2O3/Al2O3 oxygen carriers prepared by the spray granulation method. We obtained quantitative microstructural information such as the porosity and the particle sizes of the MO (iron oxide) and the support (Al2O3), as well as the two-phase boundary areas between the oxygen carrier and the gas phase, at which surface reaction for the redox of iron oxide occurs. This information was obtained using image analysis of the cross-sectional SEM images after redox reactions. In addition, important factors concerning the reaction conditions affecting the microstructural changes and the reduction kinetics of the oxygen carriers were investigated, such as the effect of reaction temperature, reduction time (i.e., the extent of the reduction reaction from Fe2O3 to Fe3O4, FeO, and Fe), oxidation conditions (i.e., oxygen partial pressure in the oxidation process), and the ratio of MO (Fe2O3) to support (Al2O3). Using the coupled analysis of the microstructural changes and reduction kinetics, the dominant factors affecting the degradation of the oxygen carrier particles, such as the reaction conditions and particle structures, are discussed. Although chemical looping technologies based on fluidized bed reactors have much progresses

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in recent years as summarized in previous reports,47,48 we believe that the detailed analysis of microstructural changes of oxygen carrier particles using image analysis in this study can also provide strategies to design particles with improved stability and longer lifetime. Furthermore, the analysis method based on the experiments with image analysis and TG in this study, i.e., coupled analysis between detailed microstructural evaluation and kinetic investigation, can be applied to the investigation of oxygen carrier particles in fluidized bed reactor.

2. Experimental Material Synthesis and Characterization Samples of 10, 25, and 50 wt.% Fe2O3/Al2O3 composite oxygen carrier particles were prepared by the spray granulation method (manufactured by Ohtsuka Ceramics, Inc., Japan). The samples were finally calcined at 1100, 1200, and 1300 °C for 1 h. The obtained Fe2O3/Al2O3 oxygen carriers were characterized by X-ray diffraction (XRD, SmartLab, RIGAKU Co., Japan), SEM (JSM-5600, JEOL Ltd., Japan) and EDX (Link ISIS, Oxford Instruments PLC, UK). The specific surface areas of the Fe2O3/Al2O3 oxygen carrier particles were also examined by the Brunauer-Emmett-Teller (BET) method with a NOVA2200e (Quantachrome Instruments, USA).

Redox Cycle Thermogravimetric Measurements The Fe2O3/Al2O3 samples underwent redox cycling in a thermogravimetric analyzer (TG 8120, Rigaku, Japan). To avoid the influence of difference of Fe2O3 weight on reduction kinetics, the

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same weight of Fe2O3 (2 mg) was used for each TG measurement, i.e., 20, 8 and 4 mg of sample were used for 10, 25 and 50 wt.% of Fe2O3/Al2O3, respectively. The Fe2O3/Al2O3 powders were placed in an alumina pan, and a gaseous mixture of H2 and Ar (1:99 ratio) for reduction and that of air and Ar (x: 100 − x ratio, x = 33, 67, and 100) for oxidation were introduced into the TG. The redox reaction was carried out from 600 to 900 °C. The total flow rate of the gaseous mixtures was 300 mL min-1.

SEM Observation and Image Analysis Method The microstructural changes in the Fe2O3/Al2O3 oxygen carrier particles during the redox cycles were investigated using image analysis of cross-sectional SEM images. Microstructural information such as the porosity, particle sizes of the oxygen carriers (iron oxide) and support (Al2O3), and the two-phase boundary area between the iron oxide and the pore, at which surface reaction of redox of iron oxide occurs, was obtained. In this paper, the term “particle” was used for two types of particles “iron oxide particle” and “oxygen carrier particle”, i.e., “particle size” and ”oxygen carrier particle” were defined as “iron oxide particle” and “Fe2O3/Al2O3 composite particle”, respectively. To prepare samples for observation by SEM, the Fe2O3/Al2O3 powders were embedded in epoxy resin on a glass slide and were evacuated to infiltrate the resin into the powders. Then, the obtained samples were heated to 120 °C on a hot plate to fix the resin, which was then polished. Backscattered electron images (BEIs) were used for the image analysis because high contrast

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between the MO particles (Fe2O3), support particles (Al2O3), and pores in a Fe2O3/Al2O3 oxygen carrier particle can be obtained. The BEIs were taken of the fully reduced oxygen carrier particles to enhance the contrast between the reduced MO and the support (Fe and Al2O3). Fe, Al2O3, and pores correspond to the white, gray, and black areas in the BEIs. The microstructural information was obtained from the image analysis using the Image Processing Toolbox of MATLAB.49 The image analysis method has been used previously to observe the microstructure of a Ni-yttria-stabilized zirconia (YSZ) anode.50,51 Microstructural information, such as the particle size, pore size and volume, interfacial area, and triple phase boundary between Ni, YSZ, and the pores, was obtained from the image analysis, and the correlation between the fuel cell performance and the anode microstructure has been discussed.50,51 In this study, the obtained BEIs were recorded as three-valued data (white, gray, and black) to satisfy the initial theoretical ratio of the areas corresponding to the volume ratio of Fe, Al2O3, and pores. The particle size was calculated using equivalent diameters as the following equation, =

,

(3)

where, ri and Si are particle size and area, respectively, of each Fe particle i. About 10 particles were analyzed for each sample to average the data.

3. Results and Discussion 3.1 Characterizations of Fe2O3/Al2O3 Oxygen Carriers As-Prepared and After Redox Cycles

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Fe2O3/Al2O3 particles prepared by the spray granulation method were characterized by XRD. Figure 1 shows XRD patterns of Fe2O3/Al2O3 oxygen carrier particles calcined in air at 1100, 1200, and 1300 °C. Peaks corresponding to Fe2O3 and Al2O3 were observed, but no impurity phases were observed. The crystallite size of Fe2O3 in as-prepared Fe2O3/Al2O3 oxygen carrier particles, which was estimated from the XRD patterns using the Scherrer equation, was ca. 50 nm. The peak intensities of Fe2O3 decreased with an increase in the calcination temperature. The result may suggest that a part of Fe2O3 reacts with Al2O3 to form iron aluminates such as FeAlO3 and FeAl2O4 at relatively high calcination temperatures as shown in previous literatures.52,53 In fact, formation of the spinel phase, FeAl2O4, was observed after redox cycles of Fe2O3/Al2O3 oxygen carriers (Figure S1 in Supporting Information). In this paper, Fe2O3/Al2O3 calcined at 1100 °C was used to investigate the degradation behavior during redox cycles and the relationship between the microstructural changes and the reduction kinetics. ◇ Fe2O3

○

○

○ Al O

2

3

○ ○

○

1300℃

○(c) ○ ○ ○ (b) 1200℃

◇

◇

◇

◇

◇

◇

◇◇

(a) 1100℃

Figure 1. XRD patterns of Fe2O3/Al2O3 prepared by the spray granulation method and calcination at different temperatures. (a) 1100, (b) 1200, and (c) 1300 °C.

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Figure 2 shows the surface and cross-sectional SEM images of an as-prepared 10 wt.% Fe2O3/Al2O3 oxygen carrier particle. The particle is clearly spherical and has a diameter of ca. 100 µm (Figure 2a). Figure 2b shows a cross-sectional BEI, in which white, gray, and black areas correspond to Fe, Al2O3, and pores, respectively. In the as-prepared particles, the Fe and Al2O3 particles were 0.2–0.6 µm in diameter and were uniformly dispersed in the particles. The specific surface area of as-prepared 10 wt.% Fe2O3/Al2O3 oxygen carrier particles calcined at 1100 °C was 3.3 m2 g-1. The value agreed with the particle size of Fe and Al2O3 assuming that there were no significant meso and micro pores inside each Fe and Al2O3 particle.

(a)

20 µm

(b)

20 µm

Figure 2. Typical SEM images of (a) the appearance and (b) the cross-sectional image of an as-prepared 10 wt.% Fe2O3/Al2O3

The reduction and oxidation reactions of the Fe2O3/Al2O3 oxygen carrier particles were repeated 50 times in 1% H2/Ar and x% air/Ar (x = 33, 67, and 100) using TG. Figures 3a and 3b show cross-sectional SEM images of 25 and 50 wt.% Fe2O3/Al2O3 oxygen carrier particles after 50 redox cycles. The coarsening of the Fe particles inside the particle was observed for the 25 wt.% Fe2O3/Al2O3 sample, whereas Fe segregation at the surface of the particle was observed for

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the 50 wt.% Fe2O3/Al2O3 sample, as well as the coarsening of the Fe particles inside the particles. These results suggest that Fe diffused toward the surface of the oxygen carrier particles during redox cycles under some redox reaction conditions. Fe segregation has been reported by several research groups, and the results of this study agrees with those of previous studies.18–21,37 During the oxidation process, oxygen potential is relatively low inside oxygen carrier particles while that is relatively high at surfaces of the particles. Such large oxygen potential distribution causes the diffusion of cation. Therefore, Fe cations diffuse to the surface during oxidation reaction with driving force of a gradient of oxygen chemical potential.22,54,55 From the line analysis of Fe fraction inside the particles, the amount of Fe inside the particles was almost the same and Fe segregation was observed only at the surface of the particles.

(a)

Fe Al2O3

(b) Fe Al2O3

Figure 3 Fe segregation in Fe2O3/Al2O3 particles after 50 redox cycles at 900 °C in 1% H2/Ar for reduction and 33% air/Ar for oxidation: (a) 25 wt.% Fe2O3/Al2O3 and (b) 50 wt.% Fe2O3/Al2O3 particles after 50 redox cycles.

Fe segregation at the surface of the particles was observed in some redox reaction conditions, which are summarized in Table 1. Fe segregation was observed in the case of the 50 wt.% Fe2O3/Al2O3 reduced for relatively long time (10 min) during the redox cycles. Fe segregation

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may result in enhancing attrition of oxygen carrier particles in a practical chemical looping process (i.e., in a circulating fluidized bed reactor),19 which would result in a shorter lifetime of oxygen carriers because of attrition of the particles in a circulating fluidized bed reactor. Therefore, the design of particles and the improvement of the redox reaction operating conditions to prevent Fe segregation is important. As shown in Figure 3a, under some redox reaction conditions, although Fe segregation was not observed, internal sintering of the particles was observed, which resulted in a decrease in the redox reaction kinetics. For the design of particles with a long lifetime, a consideration of both attrition and sintering is important. In this study, we especially focused on the microstructural changes inside the oxygen carrier particles, and the relationship between the microstructural change inside the particles (i.e., microstructural changes caused by sintering) and the degradation of the reduction kinetics was investigated quantitatively using the image analysis method.

Table 1 Redox reaction conditions under which Fe segregation was observed in Fe2O3/Al2O3 particles after 50 redox cycles at 900 °C in 1%H2/Ar for reduction. The reduction was performed from Fe2O3 to Fe3O4 and FeO. ”n.d.” means that Fe segregation was not observed. Oxygen concentration for oxidation process

7%O2

14%O2

21%O2

Reduction time

0.5 min

10 min

0.5 min

10 min

0.5 min

10 min

10 wt.% Fe2O3/Al2O3

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

25 wt.% Fe2O3/Al2O3

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

50 wt.% Fe2O3/Al2O3

n.d.

Fe segregation

n.d.

Fe segregation

n.d.

Fe segregation

13



3.2 Image Analysis for the Evaluation of the Microstructural Changes of Fe2O3/Al2O3 Oxygen Carriers During Redox Cycles The degradation of reduction kinetics of Fe2O3/Al2O3 was caused by the change in the microstructure of the oxygen carrier particles. Therefore, the investigation of the microstructural changes during the redox reaction and an understanding of the dominant factors affecting the microstructural changes are important. Image analysis of the Fe2O3/Al2O3 oxygen carrier particles before and after several redox cycles (0–50 cycles) was performed. The analysis yielded microstructural information about the particles such as the porosity, particle sizes of the oxygen carrier (iron oxide) and support (Al2O3), and the two-phase boundary area between the oxygen carrier and the gas phase, at which surface reactions for the redox of iron oxide occur. Decrease in porosity causes decrease in gas diffusion rate. Coarsening of iron oxide particles causes decrease in the two-phase boundary between iron oxides and gas phase, resulting in slow kinetics for surface reaction and solid-state diffusion. Figure 4 shows cross-sectional SEM images before and after the redox cycles. Figures 4a and 4b show the BEIs before and after the 50 redox cycles, respectively. White, gray, and black areas can be seen, which correspond to Fe, Al2O3, and pores, respectively. Figures 4c and 4d show images after the preprocessing of Figures 4a and 4b with the image analysis tool to analyze the microstructural information. The Fe particle size distributions estimated from Figures 4c and 4d by image analysis are shown in Figures 4e and 4f. In the figures, the y-axis shows area fraction, , , which was defined as follows, 14


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, = ∑

=

,

∑

(4)

where, nj is number of particles having the particle size of rj. The particle size distributions indicate that particle size of Fe increased after 50 redox cycles. Other microstructural information such as the porosity and the two-phase boundary area between the oxygen carrier and the pore were also calculated from the ternarized images. A change in the internal porosity of the Fe2O3/Al2O3 oxygen carrier particles was not observed over 50 redox cycles, and the two-phase boundary increased with increasing particle size.

Figure 4. Typical cross-sectional SEM images of 10 wt.% Fe2O3/Al2O3: (a) as-prepared and (b) after 50 redox cycles and (c) ternarized images of as-prepared and (d) after 50 redox cycles. Particle size distribution of Fe estimated by image analysis of the SEM images of the 10 wt.% Fe2O3/Al2O3 (e) before and (f) after 50 redox cycles. The cross-sectional SEM images of 25 wt.% and 50 wt.% Fe2O3/Al2O3 are shown in Figures S2 and S3 in Supporting Information, respectively.

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Figure 5 shows the change in the Fe particle size in the Fe2O3/Al2O3 oxygen carrier particles during the redox cycles under various redox reaction conditions. Particle sizes, r, before and after redox cycles in this study were defined as follows. = ∑ ,

(5)

In this study, to investigate the dominant reaction conditions affecting the microstructural changes and the reduction kinetics, we varied the redox cycling conditions used for the Fe2O3/Al2O3 oxygen carrier particles, as summarized in Table 2, such as the reaction temperature, reduction time (i.e., the reduction degree for the reduction to Fe, FeO, and Fe3O4), oxidation conditions (i.e., the oxygen concentration in the oxidation process), and the ratio of Fe2O3 to support (Al2O3). As shown in Figure 5, the Fe particle size increased with increasing number of redox cycles under all reaction conditions. In this study, typically, the particle size of Fe increased from 0.5 to 1–3 µm after 50 redox cycles under most redox reaction conditions, and the particle size continuously increased, reaching 8 µm after 50 cycles, for 50 wt.% Fe2O3/Al2O3. Because the Fe2O3 particles easily connect with others in 50 wt.% Fe2O3/Al2O3, the particle size continuously increases, and, thus, degradation easily occurs. The relationship between the microstructural changes and the degradation of the reduction kinetics is discussed in the following sections.

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Figure 5 Changes in iron oxide particle size over 50 redox cycles with different reduction and oxidation conditions for the Fe2O3/Al2O3 oxygen carriers: (a) temperature (900, 800, 700, and 600 °C), (b) oxygen concentration in oxidation reaction (7% O2, 14% O2, and 21% O2), (c) reduction time (i.e., the extent of reduction reaction) (0.5 min, 10 min, and 50 min), and (d) weight ratio of Fe2O3 (10, 25, and 50 wt.%). Standard redox reaction conditions (900 °C, 7% O2, 10 min reduction, and 10 wt.% Fe2O3) shown in Table 2 were employed and the other parameters were varied as shown in each Figure 5(a), (b), (c), and (d). Table 2 Redox reaction conditions conducted in this study. Parameters underlined are standard reaction conditions. Weight percentage of Temperature O2 concentration Reduction time Fe2O3 900 °C 800 °C 700 °C 600 °C

7%O2 14%O2 21%O2

0.5 min 10 min 50 min

17


10 wt.% Fe2O3 25 wt.% Fe2O3 50 wt.% Fe2O3


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3.3 Degradation in the Reduction Kinetics of Fe2O3/Al2O3 Oxygen Carriers During Redox Cycling Figure 6 shows typical time profiles for the reduction of the 10 wt.% Fe2O3/Al2O3 oxygen carrier in 1% H2/Ar during the TG measurements. The reduction of Fe2O3 proceeds via the three-step reaction shown in eq 6. "

#

: ! $ #

: ! $

: !$

Fe O %&&&&&&' Fe O %&&&&&&' 2FeO %&&&&&&' 2Fe

(6)

The conversion, X, of oxygen carrier particles in Figure 6 is defined by eq 7: ) =1−,

-!-./0

-./ 0# !-./0

1,

(7)

where m, 23$ , and 23 $# are the mass of the sample during reduction and the theoretical masses of the FeO/Al2O3 and Fe2O3/Al2O3, respectively. Kinetic analysis was performed to evaluate the change in reduction rate constants during redox cycles by applying the following Avrami–Erofe’ev equation, as shown in Figure 6.56–59 ) = 1 − exp4−56 7

(8)

Here, k, t, and n are the reaction rate constant, time, and Avrami exponent associated with the reaction mechanism and nuclei growth, respectively. The curves fit the experimental results well, as shown in Figure 6. The fitting was applied to the experimental reduction curves under various redox reaction conditions from 0 to 50 redox cycles.

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Figure 6. A typical time profile of 10 wt.% Fe2O3/Al2O3 reduction by 1% H2/Ar at 900 °C. Solid line: experimental value; broken line: fitting curve using the Avrami-Erofe'ev model (eq 8).

To investigate the influence of the redox reaction conditions such as temperature, reduction time, redox cycles, oxygen concentration in the oxidation reaction process, and weight ratio of Fe2O3 and Al2O3, the microstructural changes and degradation of the Fe2O3/Al2O3 oxygen carriers during redox cycling were examined under various redox conditions. Figure 7 shows the changes in the reduction rate constants of various weight percentages of Fe2O3/Al2O3 oxygen carriers under various redox conditions over 50 redox cycles. The redox cycles of 10 wt.% Fe2O3/Al2O3 in 1% H2/99% Ar for 10 min and 33% air/Ar for 2 min at 900 °C were selected as standard conditions, and the temperature, oxygen concentration, reduction time, and weight ratio of Fe2O3 were changed to evaluate the dominant factors affecting the reduction kinetics of the oxygen carriers, as summarized in Table 2. Avrami exponents, n, which was used for the fitting and the estimation of rate constants, k, are shown in Supporting Information. As shown in Figure 7, the reduction rate constants decreased rapidly over the first five cycles, after which the reduction rate gradually decreased. The reduction rate constant after 50 redox cycles was the highest for the redox cycling 19



carried out at 700 °C. This result suggests that a relatively low temperature is suitable for maintaining high reduction kinetics. However, to obtain high-grade heat, higher temperatures are favorable. Thus, the optimization of operating temperature should be considered in practical chemical looping systems. As for the variation in Avrami exponents (Figure S4), the value was around 1 at the first reduction. Then, it rapidly decreased during first 5 redox cycles and decreased slowly for further redox cycles. Corresponding cross-sectional SEM images of 10 wt.% Fe2O3/Al2O3 during 1-50 redox cycles are also shown in Figure S5 of Supporting Information. The results suggest that different degradation mechanisms occurred during the first 5 redox cycles and further redox cycles. The low Avrami exponent of ca. 0.3 after 5 redox cycles suggests that the rate-determining step changed during first 5 cycles and that the rate-determining step after a few cycles was solid-state diffusion, which agrees with the change of shapes of dX/dt curves for the reduction process.

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(a)

)

(c)

(b)

reduction rate constant, k (

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(d)

-3

5x10

-3

4x10

-3

3x10

-3

2x10

-3

1x10

0 0

10

20

30

40

50

60

Figure 7. Variations in the reduction rate constants over 50 redox cycles with different reduction and oxidation conditions: (a) temperature (900, 800, 700, and 600 °C), (b) oxygen concentration in oxidation reaction (7% O2, 14% O2, and 21% O2), (c) reduction time (reduction degree) (0.5 min, 10 min, and 50 min), and (d) weight ratio of Fe2O3 (10, 25, and 50 wt.%).

3.4 Factors Affecting the Redox Reaction Kinetics of the Fe2O3/Al2O3 Oxygen Carriers To evaluate the influence of the redox reaction conditions on the degradation behavior quantitatively and to predict the change in the reduction rate constant during redox cycles, the following degradation model was applied to fit the experimental results:60 58 = 59 − 5: exp4−= > − ?7 + 5: ,

(9)

where β, N, M, 58 , 59 , and 5: , are the degradation coefficient, cycle number, cycle number from which the degradation model was applied, rate constants after N and M redox cycles, and rate constant after full long-term redox cycles, respectively. The value of M = 5 was used in this study. A rapid decrease in the reduction rate constants during the first 1 to 5 redox cycles was observed,

21



and variations in the rate constants after the 5th cycles were relatively slow, which means that the degradation mechanism in the first five redox cycles was different from that in the later cycles. However, for the prediction of reaction rate after a large number of redox cycles, the degradation behavior after the initial rapid degradation observed in the initial cycles is of relevance. Thus, the degradation coefficient was determined at N > 5, that is, M = 5. Figure 8 shows an example of the fitting to the experimental data using the above degradation model, and the curves fit the experimental results well.

Figure 8. Fitting to the change in the reduction rate over 50 redox cycles using the exponential degradation model. The reduction of 10 wt.% Fe2O3/Al2O3 by 1% H2/Ar at 900 °C over 50 redox cycles. Solid lines: experimental values; dashed lines: fitting curves using degradation model (eq. 9).

Figure 9 shows the relationships between the redox reaction conditions and degradation coefficient, β. The result suggests that the long-term degradation behavior depends on the redox reaction conditions, as well as the as-prepared structures of the oxygen carriers. The degradation coefficient strongly depends on the reaction temperature, reduction time, and weight percentage of Fe2O3 to Al2O3. High temperatures cause the agglomeration of iron oxides and long reduction 22


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times lead to a significant reduction of Fe2O3 to FeO and Fe, which causes a large change in the volume of iron oxide during the redox cycles and high heat generation, causing agglomeration. Although the differences in the effect of weight ratios of 10 wt.% Fe2O3 and 25 wt.% Fe2O3 was small, the 50 wt.% Fe2O3/Al2O3 oxygen carriers showed the highest degradation coefficient, which suggests that iron oxides can easily agglomerate with an increased probability that Fe2O3 particles with come into contact with each other in the Fe2O3/Al2O3 oxygen carrier, and a threshold may exist at around 25–50 wt.% of Fe2O3. In addition, the oxygen concentration in an oxidation process also has an effect on the degradation behavior; that is, a higher oxygen concentration results in a higher degradation rate. A higher oxygen concentration leads to higher oxidation reaction rate of the iron oxides. During the oxidation process, much heat is generated, as shown by eq 10.

(-)

0.5

Degradation coefficient,

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.5, 10, 50 (min)

10, 25, 50 (wt% Fe2O3)

0.4

600, 800 700, 900 (℃ ℃)

7, 14, 21 (O2%)

0.3 0.2 0.1 0 Fe2O3 wt% Temperature Reduction O2 time concentration

Figure 9. Degradation coefficient, β, over 50 redox cycles under various reduction and oxidation conditions

23



Fe + 3/4O2 → 1/2Fe2O3 +∆H (∆H = 404 kJ mol-1 at 900 °C)

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(10)

The high oxidation reaction rate resulting from the relatively high oxygen concentration can lead to locally high temperatures, resulting in a high degradation coefficient. This result suggests that the increase in temperature inside an oxygen carrier particle is an important factor affecting the agglomeration of the iron oxide particles. To reduce the cost of the oxygen carrier particles and improve the efficiency in chemical looping systems, a high-loading amount of Fe2O3, a high temperature, and the significant reduction of Fe2O3 to FeO and Fe are favorable. However, the results of this study show that the extreme reaction conditions caused sintering; that is, the efficiency and cost of the system and the lifetime of the oxygen carrier particles are in a trade-off relationship. Therefore, operating conditions should be selected with careful consideration and prediction of the degradation behavior and the lifetime of the oxygen carrier particles.

3.5 Correlation Between Microstructural Changes and Reduction Kinetics and the Effect of the Temperature Increase Inside the Oxygen Carrier Particles As mentioned above, we investigated the effect of the redox reaction conditions and the microstructure of the as-prepared Fe2O3/Al2O3 particles on the degradation behavior. The results suggest that the redox reaction operating conditions such as temperature, reduction and oxidation atmospheres, and time have a significant influence on the degradation behavior, and the operating conditions should be carefully chosen to maintain high activity during thousands of redox cycles.

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However, the direct reasons (factors) in the background influencing the microstructural change should be considered in the design of highly stable oxygen carrier particles. To investigate the relationship between the degradation behavior and the microstructural changes in the oxygen carrier particles, the degradation coefficient, β, under various redox reaction conditions was plotted against the rate of the microstructural changes, ∆S/∆N, (where ∆S/∆N is the change in the two-phase boundary area between the oxygen carrier and the gas phase during the redox cycles from 10 to 50 under various redox reaction conditions), as shown in Figure 10a. Although the reduction kinetics depend on both the two-phase boundary, S, and particle size, r, the variations in boundary area, ∆S, was used to investigate the relationship between the reduction kinetics and the microstructural changes. Since the iron oxides particles were not spherical, surface area was more appropriate to discuss the degradation mechanism than particle size. As shown in Figure 10a, a linear relationship was observed for the two parameters, β and ∆S/∆N. This result suggests that the degradation (that is, the decrease in the reduction rate) was caused by the microstructural changes in Fe2O3/Al2O3.

25



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Figure 10 (a) Relationship between the degradation coefficient, β, and ∆S/∆N from 10 to 50 redox cycles and (b) relationship between the degradation coefficient, β, and ∆H, under various redox reaction conditions.

In addition, the degradation coefficient, β, values under various redox reaction conditions were plotted against heat generation, ∆H, during the oxidation processes (eqs 10-12), as shown in Figure 10b. As discussed above, significant heat is generated from the oxidation of iron oxides in the oxygen carriers, as described in eqs 10–12. FeO + 1/4O2 → 1/2Fe2O3 + ∆H (∆H = 138 kJ mol-1 at 900 °C)

(11)

1/3Fe3O4 + 1/12O2 → 1/2Fe2O3 + ∆H (∆H = 41 kJ mol-1 at 900 °C)

(12)

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Therefore, the increase in the internal temperature of the Fe2O3/Al2O3 oxygen carrier particles during the oxidation reaction is a plausible reason for the microstructural changes. As shown in Figure 10b, a linear relationship was observed between the redox reaction conditions (reduction time, oxygen concentration, and the weight percent of Fe2O3) and the degradation coefficient. This result suggests that the increase in the temperature inside the Fe2O3/Al2O3 oxygen carrier particles during the oxidation process may be the dominant factor affecting the microstructural changes during the oxidation of the oxygen carriers. As shown in Figure 10b, the effect of an increase in heat generation, ∆H, on the degradation coefficient, β, was higher than that of the O2 concentration. Increase in the O2 concentration during the oxidation process leads to increase the oxidation rate (Figure S6). The result suggests that dominant factor for the increase in the temperature and degradation behavior is the amount of heat generation during the oxidation process rather than oxidation reaction rate. The increase in temperature, assuming adiabatic oxidation of the oxygen carriers, estimated using the specific heat capacities of Fe2O3 and Al2O3, was 150 °C at 900 °C for the standard redox cycle condition (7% O2 for oxidation, 10 min reduction, and 10 wt.% Fe2O3/Al2O3). For 50 wt.% Fe2O3/Al2O3 and a relatively long reduction time (50 min reduction), the increases in temperature calculated were 400–800 °C at 900 °C. Although the practical increase in temperature in fluidized bed reactors would be lower than these values because convective heat transfer in fluidized bed reactors is higher than that in TG, this increase in temperature inside the particles will be a dominant factor affecting the sintering and the microstructural change during redox cycles. The result suggests that requirements for oxygen

27



carrier particles are high thermal conductivity and large thermal capacity. Fe2O3-loading amount and size of the oxygen carrier particles also influence the increase in temperature. Oxygen carrier particles should be designed based on these factors of particles. In addition, the findings of the relationship between the heat generation and the degradation coefficient, β, can be applied to predict the lifetime of the oxygen carrier particles. The results and models in this study can contribute to the development of highly efficient chemical looping systems and the reduction in the cost of oxygen carrier particles by designing particles with long lifetime.

4. Conclusions The microstructural change of the Fe2O3/Al2O3 oxygen carrier particles before and after redox reaction cycles for chemical looping systems was investigated. The redox cycles were conducted under various operating conditions, such as different reaction temperatures, reduction times, oxygen partial pressures in the oxidation process, and weight ratios of the oxygen carrier (Fe2O3) to support (Al2O3). The migration of Fe towards the surface of the particles and the microstructural changes inside the oxygen carrier particles were observed. Microstructural information such as the particle size, porosity, and the two-phase boundary between the iron oxide and pores was obtained using image analysis of cross-sectional BEIs of the oxygen carrier particles, and the relationship between the microstructural changes and the performance degradation (i.e., the decrease in the reduction reaction rate) was studied. During the redox cycles, the growth of iron oxide particles in the oxygen carrier particles was observed, and their reduction

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rate constants decreased with increasing iron oxide particle size in the oxygen carriers. The relationship between the particle size change and the degradation coefficient is linear, which suggests that the degradation was caused by the microstructural changes of the Fe2O3/Al2O3 particles. Furthermore, a linear relationship between the degradation coefficient and heat generation, ∆H, during the oxidation processes was observed for all redox reaction conditions (reaction temperature, reduction time, and oxygen concentration), which means the increase in the local temperature causes the sintering of iron oxide and the microstructural changes in the Fe2O3/Al2O3 oxygen carrier particles. These results can contribute to the design of highly stable oxygen carrier particles and be used to reduce the cost of the oxygen carrier particles in chemical looping systems.

29



AUTHOR INFORMATION Corresponding Author Junichiro Otomo, PhD, Associate Professor

Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan

Tel: +81-4-7136-4714; Fax: +81-4-7136-4715; E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS This work was supported by the New Energy and Industrial Technology Development Organization (NEDO, Japan) for the project on technological development for zero-emission coal power generation and a Grant-in-Aid for Scientific Research (B) (25281061) and (A) (17H00801) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT, Japan). The authors thank the Materials Design and Characterization Laboratory, Institute for Solid State Physics, The University of Tokyo for use of SEM-EDX and XRD facilities.

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Industrial & Engineering Chemistry Page Research 40 of 50 1 2 3 4 5 6 7 8 9 10 11 12


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Evaluation of Microstructural Changes and Performance Degradation

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