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TiO2 oxygen carriers
Applied Energy 115 (2014) 374–383
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Studies on the redox reaction kinetics of Fe2O3–CuO/Al2O3 and Fe2O3/TiO2 oxygen carriers Ewelina Ksepko ⇑, Marek Sciazko, Piotr Babinski Institute for Chemical Processing of Coal, 1 Zamkowa, 41-803 Zabrze, Poland
h i g h l i g h t s Kinetics study for redox reactions of Fe2O3–CuO/Al2O3 and Fe2O3/TiO2 was performed. F1, R3 suitable for Fe2O3/TiO2; F1, D3 for Fe2O3–CuO/Al2O3 reduction reaction. CLOU effect of Fe2O3–CuO/Al2O3 can be described by the F1 or D3 models. R3 model is suitable for both oxidation reactions; F1 also for Fe2O3/TiO2. Activation energy for the oxidation reactions is close to 0 kJ/mole.
a r t i c l e
i n f o
Article history: Received 22 July 2013 Received in revised form 29 October 2013 Accepted 31 October 2013 Available online 2 December 2013 Keywords: Reaction model Kinetics Bi-metallic oxygen carriers for chemical looping combustion Hydrogen-fueled CLC
a b s t r a c t This paper contains the results of research work on chemical looping combustion (CLC). CLC is one of the most promising combustion technologies and has the main advantage of the production of a concentrated CO2 stream, which is obtained after water condensation without any energy penalty for CO2 separation. The objective of this work was to study the kinetics of both the reduction and oxidation reactions for the selected bi-metallic Fe2O3–CuO/Al2O3 and mono-metallic Fe2O3/TiO2 oxygen carriers. Based on our previous CLC research results, the most promising oxygen carriers were selected for the analysis. Tests were performed at isothermal conditions (600–950 °C) in multiple redox cycles using a thermogravimetric analyzer (Netzsch STA 409 PG Luxx). For the reduction, 3% H2 in Ar was used, and for the oxidation cycle, air was used. The activation energy and the pre-exponential factor were determined, and the reaction model was selected. The F1 (volumetric model) and R3 (shrinking core model) were suitable models for Fe2O3/TiO2, with Ea equal to 33.8 kJ/mole where F1 and D3 (3-dimensional diffusion model), were suitable for Fe2O3–CuO/Al2O3 reduction reaction kinetics decryption with Ea = 42.6 kJ/mole (F1 model). The best fits for oxidation reaction was obtained for R3 model, and F1 was also good for Fe2O3/TiO2 oxygen carrier. The chemical looping oxygen uncoupling (CLOU) effect of Fe2O3–CuO/Al2O3 material is the best described by the F1 or D3 models. The CLOU effect activation energy is equal to 22.2 kJ/mole. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The combustion of fossil fuels is one of the major sources of carbon dioxide emissions as a greenhouse gas component. Therefore, it is essential to develop new combustion technologies that allow utilization of fossil fuels while significantly reducing CO2 emissions. Existing carbon dioxide (CO2) capture technologies are costly and energy demanding. Chemical looping combustion (CLC) has been suggested as an energy-efficient fuel combustion method that produces high-purity CO2 [1,2]. This new combustion technology involves applying an oxygen carrier, typically a metal oxide, which transports oxygen from the air to the fuel. In this manner, direct ⇑ Corresponding author. Tel.: +48 32 271 00 41x228; fax: +48 32 271 08 09. E-mail address: [email protected] (E. Ksepko). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.10.064
contact between the fuel and air is avoided. The method is similar to the oxy-fuel combustion method; however, no direct supply of oxygen is needed, which avoids the costs and energy penalty of providing oxygen to the power plant. The CLC system contains two reactors: an air reactor and a fuel reactor. In the fuel reactor, the fuel reacts with the oxygen that is released from the metal oxide. The reduced metal oxide and/or its metallic form are oxidized (regenerated) in the air reactor back to the metal oxide. The metal oxide can then be used in another redox cycle. After water condensation, a pure stream of CO2, not diluted by N2, can be obtained from the fuel reactor. Therefore, CLC is one of the most promising combustion technologies and has the advantage of the production of a concentrated CO2 stream ready for sequestration with no additional energy penalty for its separation [2].
E. Ksepko et al. / Applied Energy 115 (2014) 374–383
There are special requirements for the practical utilization of oxygen carriers ina chemical looping combustion power plant, i.e., high redox reactivity; high selectivity towards complete oxidation products; resistance against carbon and sulfur, which could lead to oxygen carrier deactivation; sufficient durability under repeated reduction/oxidation cycles at high temperature; high mechanical strength associated with high circulation of the particles; non-toxicity, and low preparation cost [3]. The rates of the redox reactions of the oxygen carrier with the fuel, the metal oxide oxygen transfer capacities, and the redox reaction kinetics parameters must be determined to design a suitable CLC reactor system. Iron and copper oxides have been extensively studied worldwide due to the low cost of the former and the high reactivity of the latter [4–10]. However, the kinetic data for some oxygen carriers are still not fully available due to different possible combinations of chemical compositions of oxygen carriers. Son et al. [11] reported that for 60 wt.% Fe2O3/bentonite in TGA (thermogravimetric analyzer), within the temperature range of 700–1000 °C, and 10 vol% CH4, the reduction reaction follows the modified volumetric model with Ea (activation energy) equal to 29 kJ/mole, whereas for the oxidation reaction, it follows the shrinking core model with Ea = 6.0 kJ/mole. Abad et al. [12] studied 60 wt.% Fe2O3/Al2O3 with applied TGA and a 600–950 °C reaction range for 5–70 vol% H2 used as reducer and 5 vol% O2 used as oxidizer. For the reduction reaction Ea was 24 kJ/mole and for oxidation reaction, the calculated Ea was 14 kJ/mole. For both reactions, the shrinking core model was accurate. There are more kinetic data on copper oxygen carriers in the published literature [12–15]. For example, for 14 wt.% CuO/Al2O3, within 600–800 °C, applying 5–70 vol% H2 in TGA, Abad et al. [13] calculated the reduction reaction Ea to be equal to 20 kJ/mole, where shrinking core model was the most suitable. The 10 wt.% CuO/Al2O3 oxygen carrier was also studied extensively. TGA data collected at 600 – 800 °C, showed that the shrinking core model could be proposed with Ea of 33 kJ/mole and 15 kJ/mole for the reduction and oxidation reactions, respectively [14]. Extensive study of the reactions kinetics was performed by Chuang et al. [15] on significantly higher concentrations of copper oxide; 82 wt.% CuO/Al2O3. The kinetics of four reactions were studied in a fluid bed within 250–900 °C. While using 2–10 vol% H2 as the reducing agent, the CuO ? Cu2O and CuO ? Cu reduction kinetics were studied. The determined Ea was equal to 58 kJ/mole and 44 kJ/mole, respectively. For those reactions, the diffusion reaction model was appropriate. For Cu ? Cu2O and Cu2O ? CuO reaction an Ea of 40 kJ/mole 60 kJ/mole was calculated, respectively. Single metal oxides have been used extensively in the past as oxygen carriers [5,9,16], and recently, promising results have been obtained for bimetallic oxygen carriers due to synergistic effects between the two active metal oxides [6,10,17]. Moreover, the kinetic parameters of the redox reactions of 60 wt.% Fe2O3–20 wt.% CuO supported on 20 wt.% Al2O3 (bi-metallic oxygen carrier) and 80 wt.% Fe2O3 supported on 20 wt.% TiO2 with H2 have not been reported in the literature. The selected oxygen carriers for the kinetics study were chosen based on our previous research results [6]. We concluded, from ten-cycle reduction–oxidation data with simulated synthesis gas derived from steam gasification of Janina coal, that for the five carriers containing 60% Fe2O3–20% CuO, the supports had a great influence on the performance stability of these oxygen carriers. For Fe–Cu carriers supported on silica and sepiolite, there was a decrease in the oxygen capacity with an increasing number of cycles. The Fe–Cu on TiO2 showed the best stability, and the oxygen capacities were the highest for the Fe–Cu on bentonite and alumina. The stabilities during the 10-cycle test with a monometallic such as Fe/supports were similar to the stabilities with Fe–Cu carriers. The stable reduction rates that were observed with most of the supported Fe–Cu remained fairly constant, which
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indicated that incorporating Cu improves the rate and stability during cyclic tests. Fe/TiO2 had the best performance with stable oxygen capacities at both 800 and 900 °C. These results indicate that TiO2 and Al2O3 are appropriate supports for Fe and Fe–Cu carriers, respectively. The objective of the present work was to study the apparent kinetics of the reduction and oxidation reactions for selected biand mono-metallic oxygen carriers. The activation energy, preexponential factor, and reaction model for two promising oxygen carriers, Fe2O3–CuO supported on Al2O3 and Fe2O3 supported on TiO2, were investigated. To simulate the hydrogen/oxygen carrier reactions, the reactivity of the oxygen carriers was evaluated by conducting multi-cycle CLC tests in atmospheric TGA with oxygen carriers utilizing hydrogen as a fuel. In the work the research was conducted at specific for a CLC process temperatures and particle size which supposed to be used in large scale units. The kinetics investigated in this situation might reflect the effects of pure chemical kinetics affected by diffusion resistance or internal ion transfer rate. At this stage of mechanism understanding it is difficult to judge what is the extent of interference of physical phenomena into chemical reaction. This can be presented by the final data obtained, namely activation energy and its relation to temperature changes. Under these conditions the results obtained reflects apparent kinetics, which is has a very practical meaning. The models applied for data analysis were volumetric model (F1), shrinking core model (R3) and 3 – dimensional diffusion model (D3). All these models take account all of these specific resistances described above. The volumetric model assumes that the reaction occurs throughout the particles of the oxygen carrier and that the mass of the oxygen carrier grain changes linearly during the reaction. The shrinking core model assumes that the reaction occurs on the external surface of the grain, which changes during the reaction. The 3-dimensional diffusion model describes a reaction in which the diffusion of oxygen from the unreacted metal oxide through the metal layer to the surface of the particle is the reaction-limiting step. 2. Experimental section 2.1. Oxygen carrier preparation The oxygen carriers with compositions of 60 wt.% Fe2O3, 20 wt.% CuO/20 wt.% Al2O3 (bi-metallic) and 80 wt.% Fe2O3/ 20 wt.% TiO2 (mono-metallic) were prepared by the solid-state mixing method. Fe2O3 and CuO were mixed thoroughly with Al2O3. Then, deionized water was added to obtain a paste. The paste was dried and calcined at 850 °C in air for 20 h. After cooling, the sample was crushed and was mixed thoroughly. Then, it was calcined again at 950 °C. Each time, 10 wt.% of graphite (burn off material at 850 °C) was added to the mixture to improve the porosity/surface area of the samples. The calcined sample was sieved, and the fraction <250 lm was used for measurements. 2.2. Kinetics study using thermogravimetric analyzer Thermogravimetric analysis experiments were conducted in a Netzsch STA 409 PG Luxx thermogravimetric analyzer. In TGA, the weight change of the selected oxygen carriers was measured isothermally as a function of the time during the reduction–oxidation cycles. Three reduction–oxidation cycles at each temperature were conducted at atmospheric pressure for stable performance determination. A sample of approximately 10 mg was heated in argon in a plate crucible made of Al2O3 to the reaction temperature. Three percent H2/Ar was used for the reduction reaction, whereas
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air was used for the oxidation reaction. Three percent H2/Ar was used as a fuel due to the limitations of the TGA construction, where such a concentration is the upper limit for combustion/explosive gases. All reaction gas flow rates were 150 ml/min. In the experiments, the reduction time was 60–180 min, and the oxidation reaction time was 30–40 min, depending on temperature conditions. To avoid the mixing of reduction gases and air, the system was flushed with argon for 15–60 min before and after each reduction reaction. To understand the effect of temperature, the TGA experiments were performed at a range of temperatures from 600 °C and 950 °C. 2.3. Kinetic analysis Fractional conversions, i.e., fractional reduction and fractional oxidation, were calculated utilizing the TGA data of the second redox cycle. The fractional conversion (X) is defined as follows [6]:
M oxd M M oxd Mred
ð1Þ
M M red Fractional oxidation : X ¼ Moxd M red
ð2Þ
Fractional reduction : X ¼
where M is the instantaneous weight, Moxd is the weight of a completely oxidized sample in TGA (maximum weight after oxidation with air), and Mred is the weight of a completely reduced sample in TGA (minimum weight after reduction with hydrogen). Because the calculations described in this paper do account for the concentration of the gaseous substrate, a kinetic expression for the solid–gas reaction rate can be described by the following equation:
dX ¼ f ðXÞkðTÞ dt
ð3Þ
where f(X) is a structural factor or a model of the reaction that describes the physical or chemical properties during the reaction, and k(T) is a reaction rate constant that can be described by the Arrhenius Eq. (4): Ea
kðTÞ ¼ A0 e RT
ð4Þ
where A0 is a pre-exponential factor, Ea is the activation energy, and R is the gas constant, which is equal to 8.314 J/mol ⁄ K. In this paper, three models were chosen to find the model that best fit the experimental data (see Table 1) and that could describe the reaction in a proper manner. Among them were the volumetric model, the shrinking core model, and the 3-dimensional diffusion model (Jander’s type). Table 1 presents the models of these reaction as f(X) and g(X) functions. These three models were applied to all reactions occurring in the TGA system during the cycling experiments, i.e., reduction by H2, oxidation by O2 from synthetic air, and chemical looping oxygen uncoupling in the case of the Fe2O3–CuO/Al2O3 oxygen carrier. The first step of calculations was the model fitting of the raw TGA data. For this purpose, Eq. (4) was transformed to Eq. (5):
After integration, Eq. (5) can be described by the expression of the function g(X) for the following equation:
gðXÞ ¼
Z
dX f ðXÞ
ð6Þ
One method of experimental data model fitting is testing the linearity of the g(X) function versus time. The slope of this function results in the reaction rate constant k(T). For the calculation of the other kinetic parameters, such as Ea and A0, the reaction rate constant must be calculated for different temperatures. Then, the activation energy and the pre-exponential factors are calculated from the Arrhenius equation as the slope and intercept of ln (k(T)) versus 1/T, respectively, based on the following equation:
lnðkðTÞÞ ¼
Ea 1 þ lnðA0 Þ R T
ð7Þ
2.4. X-ray diffraction analysis X-ray diffraction (XRD) analyses were conducted using a Panalytical PW 3040 X-Pert Pro XRD system equipped with a Cu anode in the h/2 h configuration. The X-ray wavelength utilized was Cu Ka-1 at 1.5406 Å. Sample data were acquired at 40 kV and 30 mA, and the X-ray diffraction patterns were collected at room temperature in the 10–100 degrees (2h) range. 2.5. Porosimetry The automatic PoreMaster mercury intrusion porosimeter was used for the pore size analysis. The porosimeter allowed for the calculation of the porosity rate and a specific pore volume of the sample, in addition to the absolute, envelope and bulk densities. 3. Results and discussion 3.1. Oxygen carrier characterization The surface areas and XRD data of the 60 wt.% Fe2O3–20 wt.% CuO, 20 wt.% Al2O3 (bi-metallic) and 80 wt.% Fe2O3, 20 wt.% TiO2 (mono-metallic) oxygen carriers are listed in Table 2. They showed small surface areas of 0.59 and 1.09 m2/g, respectively as shown in Table 2. They were mostly composed of Fe2O3 and an inert phase of TiO2 or Al2O3. The mono-metallic oxygen carrier was also composed of Fe2TiO5. The mercury intrusion curves, into and out of the pores, showed the spherical shape of the pores. The calculated medium volume of the pores was 0.3633 and 0.4461 cm3/g, with a smaller size of the macropores of 2399 nm for the bimetallic and 4935 nm for the monometallic oxygen carriers. As we concluded in our previous paper [6], in Fe–Cu mixtures, the support had a significant effect on the stability and reactivity; however, the performance cannot be attributed to the surface area of the oxygen carriers because it is significantly low. 3.2. Kinetics study
dX ¼ kðTÞdt f ðXÞ
ð5Þ
Table 1 Tested reaction models [18]. Model
f(X)=
g(X)=
Volumetric model (F1) Shrinking core model (R3) 3-Dimensional diffusion model (D3)
1X 3(1 X)2/3
ln(1 X) 1(1 X)1/3
3 4 ð1
XÞ2=3 ð1 ð1 XÞ1=3 Þ
2
ð1 ð1 XÞ1=3 Þ
The cycling reduction/oxidation TGA data for the Fe2O3–CuO/ Al2O3 oxygen carrier over the range of 600 °C to 950 °C with 3% H2/Ar is shown in Fig. 1. Constant chemical looping combustion performance was observed. The stable second redox TGA data were used for the calculations and are marked with an asterisk. Mathcad v. 14.0 software was used for the kinetic parameters calculations. The entire redox cycle was composed of the reduction reaction, a flush with argon, and a regeneration reaction of the oxygen carrier. Due to the reduction reaction of the oxygen carrier, a decrease of the mass was observed because reducers, such as H2, were
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E. Ksepko et al. / Applied Energy 115 (2014) 374–383 Table 2 X-ray diffraction, BET surface area, pore analysis and density data for the oxygen carriers. Oxygen carrier
Phase composition
Surface area (m2/g)
VT (cm3/g)
Lmedium (nm)
Envelope density (g/cm3)
80 wt.% Fe2O3, 20 wt.% TiO2 60 wt.% Fe2O3, 20 wt.% CuO, 20 wt.% Al2O3
Mostly Fe2O3, Fe2TiO5, TiO2 Fe2O3, Fe3O4, Al2O3
1.089 0.589
0.3633 0.4461
4935 2399
3.13 2.47
Fig. 1. Cycling reduction/oxidation TGA data for the Fe2O3–CuO/Al2O3 oxygen carrier at 600 °C and 950 °C with 3%H2/Ar.
introduced into the TGA chamber. The system was flushed with inert argon to remove the combustion gases from the TGA. After the flush, the oxidation reaction occurs because the reduced oxygen carrier is regenerated; therefore, an increase of the mass on the TGA chart was observed. The entire TGA data redox cycle is shown in Fig. 2. The oxygen transport capacities were calculated for both 80 wt.% Fe2O3, 20 wt.% TiO2 and 60 wt.% Fe2O3–20 wt.% CuO/ 20 wt.% Al2O3 materials because it is an important practical parameter for oxygen carrier selection for a CLC power plant. The theoretical oxygen transfer capacity (wt.%) depends on the oxygen carrier
material and the final reduction state. The oxygen transfer capacities of the oxygen carriers under different reduction states are listed in Table 3 For the mono-metallic Fe-based carrier, it may vary from 2.67 to 24.05 wt.%, whereas for the Fe–Cu-based bimetallic carrier, it may vary from 4.01 to 22.06 wt.%. In our TGA experiments, maximal oxygen transport capacities were achieved, which means the final reduction state was obtained. Therefore, the kinetics data describe the final reduction of Fe2O3 to Fe within the sub-stage reactions Fe2O3)Fe3O4)FeO)Fe. For the Fe–Cu oxides, an additional three redox combinations can be observed. Additionally, for the bi-metallic oxygen carrier, the experimental value of the capacity was 22%, which indicated that
Table 3 Oxygen transfer capacity of the oxygen carriers under different reduction states.
Fig. 2. One full cycle of the reduction/oxidation TGA data for the Fe2O3–CuO/Al2O3 oxygen carrier with 3% H2/Ar.
Oxygen carriers
Reactions
Theoretical oxygen transfer capacity (wt.%)
80% Fe2O3/support
3Fe2O3 = 2Fe3O4 + 1/2O2 Fe2O3 = 2FeO + 1/2O2 Fe2O3 = 2Fe + 3/2O2
2.67 8.02 24.05
60% Fe2O3, 20% CuO/20% support
2CuO = Cu2O + 1/2O2 3Fe2O3 = 2Fe3O4 + 1/2O2 CuO = Cu + 1/2O2 3Fe2O3 = 2Fe3O4 + 1/2O2 2CuO = Cu2O + 1/2O2 Fe2O3 = 2FeO + 1/2O2 CuO = Cu + 1/2O2 Fe2O3 = 2FeO + 1/2O2 2CuO = Cu2O + 1/2O2 Fe2O3 = 2Fe + 3/2O2 CuO = Cu + 1/2O2 Fe2O3 = 2Fe + 3/2O2
4.01 6.03 8.02 10.03 20.05 22.06
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Fig. 3. Fractional reduction versus time for Fe2O3/TiO2.
Table 4 Reduction reaction kinetics study for Fe2O3/TiO2. T (°C)
Model F1
Model R3 R2
k (s1) 600 700 800 850 900 950
4
6.862 10 9.484 104 1.468 103 2.075 103 2.596 103 2.082 103
0.858 0.947 0.977 0.989 0.974 0.996
Model D3 R2
k (s1) 4
4.471 10 6.323 104 1.022 103 1.437 103 1.810 103 1.420 103
0.714 0.826 0.986 0.971 0.986 0.922
R2
k (s1) 5
8.404 10 1.147 104 1.690 104 2.404 104 2.995 104 2.454 104
0.922 0.985 0.823 0.858 0.824 0.941
CuO was fully reduced to metallic Cu and that Fe2O3 was fully reduced to metallic Fe. The fractional conversions for the reduction and oxidation reactions were calculated as in Eqs. (1) and (2). Fig. 3 shows the fractional reduction for the mono-metallic 80 wt.% Fe2O3, 20 wt.% TiO2 oxygen carrier, which was calculated at a temperature range of 600–950 °C. The strong temperature effect is also observed because with the temperature increase, the reaction rate increased. In the literature, the shrinking core model and diffusion model are most frequently indicated [11–15]; therefore, for the Fe2O3/ TiO2 oxygen carrier in our calculations, we used both models. Additionally, other models, such as the volumetric model, were also tested. The results for the volumetric model (marked by F1), the shrinking core model (marked by R3) and the 3-dimensional diffusion model (marked by D3) are shown in Table 4. For all of the models, the k – kinetic constant and the correlation coefficient were calculated. Table 4 presents the fitting of these three models
to the reaction. The F1 and R3 models fit very well and can describe the behavior of the oxygen carrier during reaction, but in the lowest range of temperature (600 and 700 °C), the 3-dimensional diffusion model shows better correlation. Above 800 °C, the 3-dimensional diffusion model does not fit this reaction because the correlation coefficients are much lower than those obtained for the F1 and R3 models. Plots of the function g(X) versus time for the reduction of Fe2O3/TiO2 for these models are presented in Fig. 4. The calculated reduction reaction parameters for the F1 model are Ea = 33.808 kJ/mole, R2 = 0.927 and A0 = 0.069. For the D3 model, a similar activation energy was achieved, 32.357 kJ/ mole, which is also shown in Fig. 5, with a much poorer fit at the higher temperature range as shown previously in Fig. 4. The Ea value of reduction reaction of Fe2O3/TiO2, 33.8 kJ/mole is similar to that reported by Son et al. [11] for Fe2O3/bentonite that is equal to 29 kJ/mole. Different behavior was observed for the oxidation reaction. For all temperatures from 600 to 900 °C, no essential temperature effect on the oxidation reaction rate was observed (Fig. 6). In Fig. 3 the reaction rate at 950 °C drops. That behavior might be due to the melting of the mono-metallic Fe based carrier since the oxygen carrier is composed of large amount of Fe oxide, that is 80 wt.% of Fe2O3 and 20 wt.% of TiO2. That is known from literature that iron oxide III has a tendency to agglomerate. Perhaps such as amount of Fe2O3, and high temperature as high as 950 °C and hydrogen as reducing agent causes the melting of the particles, that effected in Fe agglomerates. Since they once appear, the 20 wt.% of inert material in such as high temperature does not prevent the agglomeration. The addition of TiO2 facilitated sintering prevention, which inhibited the contact between metallic iron particles in temperatures range of 600–900 °C. Metallic Fe that agglomerates can block the pathway for O2 to contact Fe in the interior of particles. Thus, local pores, created during reduction reaction, may be unavailable for oxidation reactions. The sintering effect causes particle agglomeration. Since, the metallic Fe agglomerates tend to be hard to oxidize, therefore that is perhaps the reason for the drop in oxidation rates observed at 950 °C. To support our conclusion, the additional TGA measurements were performed for Fe/TiO2 carrier. The visual observation of used sample, showed that the differences in reactivity’s may come from the melting of the sample at 950 °C. For the model fitting calculation, the range of the data that was taken into account was narrowed to approximately 15 s. As indicated by the data collected in Table 5, the F1 and R3 models are the most suitable for describing the sample behavior when oxidizing by air at 900 °C. Model D3 shows a poor fit to the data, so it is not suitable for describing the oxidation reaction behavior, which was also shown in Fig. 7. Fig. 8 shows the plot of the logarithm of the reaction rate constant versus inverse temperature – Arrhenius plot (without the
Fig. 4. Model fitting for the reduction reaction of Fe2O3/TiO2 (T = 800 °C).
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E. Ksepko et al. / Applied Energy 115 (2014) 374–383 Table 5 Oxidation reaction kinetics study for Fe2O3/TiO2. T (°C)
Model F1 k (s
600 700 800 850 900 950
Fig. 5. Arrhenius plot of the reduction reaction for Fe2O3/TiO2; (a) Model F1, (b) Model R3.
data collected at 950 °C). The calculated activation energy for the oxidation reaction is less than 0 kJ/mole, which indicates that there is no temperature dependence on the oxidation rate of the Fe2O3/TiO2 oxygen carrier in the range of temperatures from 600 to 900 °C. This lack of dependence may be due to the diffusion limitations of Fe2O3 layer formed rapidly on the surface of oxygen carrier
1
)
0.021 0.018 0.017 0.022 0.019 4.630 103
Model R3 2
R
0.904 0.972 0.965 0.960 0.986 0.831
k (s
Model D3
1
)
R 3
5.791 10 4.871 103 4.703 103 5.958 103 5.230 103 1.231 103
2
0.959 0.975 0.976 0.992 0.993 0.769
R2
k (s1) 3
1.858 10 1.610 103 1.547 103 1.937 103 1.726 103 4.539 104
0.592 0.765 0.736 0.696 0.778 0.964
which covers the unreacted core of the metallic Fe [19]. In this case, the reaction can be limited by Fe ion transport in solid product layer. The relative diffusivities of iron ions and oxygen ions are not expected to change at higher temperatures because iron ions are much smaller than oxygen ions. At higher temperatures, the iron will diffuse and sinter, leading to decreased reactivity over redox cycles [20]. The data presented by Son et al. [11] showed that the activation energy of the oxidation reaction could be 6 kJ/mole, but this result was obtained for another Fe-based oxide, 60 wt.% Fe2O3/bentonite. The second analyzed sample was the bi-metallic oxygen carrier Fe2O3–CuO/Al2O3, which presents both activities, i.e., chemical looping combustion and chemical looping oxygen uncoupling reaction (CLOU effect). In the CLOU effect for the examined material, approximately 4% of the total 20% oxygen capacity is involved. On the basis of this oxygen carrier behavior, the data from the reduction reaction were divided into two parts. One part represents the whole temperature range applied in this study, and the second part represents the temperature range from 850 to 950 °C, which covers the CLOU effect. The fractional reduction of the Fe2O3–CuO/Al2O3 oxygen carrier is presented in Fig. 9. The temperature effect is essential and is similar to the Fe2O3/TiO2 reduction reaction. In the case of the Fe2O3–CuO/Al2O3 oxygen carrier, the model fitting calculation gives different results from Fe2O3/TiO2, which are presented in Table 5, and the plots of the function g(X) versus time for the 800 °C data are presented in Fig. 10. As seen in Table 5 and Fig. 10, the most suitable model for the reduction
Fig. 6. Oxidation degree versus time for Fe2O3/TiO2. (Inlet-range taken into calculations.)
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Fig. 7. Model fitting for the oxidation reaction of Fe2O3/TiO2 (T = 800 °C).
Fig. 8. Arrhenius plot of the oxidation reaction for Fe2O3/TiO2; Model R3.
Fig. 9. Fractional reduction versus time for Fe2O3–CuO/Al2O3.
reaction is the D3 model, but model F1 is also appropriate for describing the behavior of Fe2O3–CuO/Al2O3 during the reduction reaction. F1 and R3 models are concluded to be suitable for Fe2O3/TiO2, and F1 and D3 models are suitable for Fe2O3–CuO/Al2O3 reduction reaction. The reason for the diverse models fitting for two oxygen carriers is different reduction reactions that are taking place since different material phase compositions was observed. In the Fe2O3/ TiO2 oxygen carrier the pathway for reduction is expected to be as follows Fe2O3)Fe3O4)FeO)Fe and meanwhile Fe2TiO5)Fe2TiO4, that is also supported by XRD data. For other Fe2O3–CuO/Al2O3 oxygen carrier sample, the fitted model describes the following reduction reactions: Fe2O3)Fe3O4)FeO)Fe, and Cu2O)Cu. The diffusion limitations of oxygen transport through the solid layer of the products, which are the reduced metal oxides, may play an important role in the reduction kinetics of the reduction of the Fe2O3–CuO/Al2O3 oxygen carrier. During the reduction step, the porosity of particles can rapidly increases because the volume of the solid products such as Cu2O and Fe3O4 and then FeO, Fe is lower than those for the solid reactants as CuO and Fe2O3. Solid product layer is formed on the surface of grain which limited the accessibility of the hydrogen to unreacted core. Based on that, it was assumed that the reduction can be controlled by the diffusion of ions in the solid. The calculations showed that the D3 model is more suitable than R3 model for Fe2O3–CuO/Al2O3 reduction reaction, because of the better correlation factor. The similar finding was indicated by authors of the papers [19–21]. The calculated activation energies are 42.684 kJ/mole for the D3 model and 41.318 kJ/mole for the F1 model and are shown in Fig. 11. The obtained values are higher than for the Fe2O3/TiO2 oxygen carrier. Abad et al. [13] and García-Labiano et al. [14] reported activation energies of reduction reactions equal to 20 kJ/mole and 33 kJ/mole. However, their data were obtained for 14 wt.% CuO/ Al2O3 and 10 wt.% CuO/Al2O3, respectively, and the shrinking core
Fig. 10. Model fitting for the reduction reaction of Fe2O3–CuO/Al2O3 (T = 800 °C) .
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Fig. 14. Arrhenius plot of the CLOU reaction for Fe2O3–CuO/Al2O3; model F1.
Fig. 11. Arrhenius plot of the reduction reaction for Fe2O3–CuO/Al2O3: (a) Model F1, (b) Model D3.
Fig. 12. Fractional reduction of the CLOU effect versus time for Fe2O3–CuO/Al2O3.
model was applied for both oxygen carriers. The data presented in this paper showed that the D3 model could describe the bi-metallic oxygen carrier, and the activation energy was higher than that presented in the literature for similar mono-metallic carriers. Fig. 12 presents the fractional reduction of the Fe2O3–CuO/Al2O3 oxygen carrier during the CLOU reaction, and Fig. 13 shows the model fitting of the experimental data. The range of data that was taken into account was between a 1 and 95% conversion degree. In this case, the F1 and D3 models have the best correlation, similar to the correlation that was observed for the reaction reduction by H2. The calculated activation energy for the F1 model, shown in Fig. 14, was equal to 22.239 kJ/mole, but the correlation coefficient was much lower (0.858) than calculated for the activation energy from the reduction reaction by H2 (0.968). This is most likely because the CLOU effect is quite small compared to the reduction of the oxygen carrier by H2 in chemical looping combustion and by a much narrower temperature range than was used in calculations. The oxidation reaction of the Fe2O3–CuO/Al2O3 oxygen carrier by air is similar to the oxidation of Fe2O3/TiO2. The plot of the conversion degree versus time is presented in Fig. 15. In this case, a narrower range of data was taken into account (up to 30 s), which corresponds to a conversion degree of 0.8–0.9. The rate of the oxidation reaction is independent on temperature. The same phenomenon is observed for the oxidation of the Fe2O3/TiO2 oxygen carrier. In Fig. 16, the plots of the function g(X) versus time at 800 °C are presented, and in Table 6, the reaction rate constants and correlation coefficients for the three models are shown. For the oxidation reaction of Fe2O3–CuO/Al2O3, the best correlations were obtained for the F1 and R3 models, but the fits are poorer than the fits obtained for the oxidation of Fe2O3/TiO2. Similar behavior was observed for the Janina coal synthesis gas CLC. Temperature had a positive effect on the rate for all of the carriers except Fe–Cu/ Al2O3 [6].
Fig. 13. Model fitting for the CLOU reaction of Fe2O3–CuO/Al2O3 (T = 800 °C).
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Fig. 15. Oxidation degree versus time for Fe2O3–CuO/Al2O3. (Inlet-range taken into calculations.)
Fig. 16. Model fitting for the oxidation reaction of Fe2O3–CuO/Al2O3 (T = 800 °C).
Table 6 Reduction reaction kinetics study for Fe2O3–CuO/Al2O3. T (°C)
Model F1
Model R3
Model D3
k (s1)
R2
k (s1)
R2
k (s1)
R2
Reduction with H2 600 700 800 850 900 950
8.435 104 1.087 103 1.924 103 2.649 103 3.369 103 4.295 103
0.861 0.797 0.916 0.951 0.947 0.981
1.824 104 2.314 104 4.233 104 5.890 104 7.506 104 9.727 104
0.713 0.643 0.782 0.828 0.826 0.893
1.041 104 1.361 104 2.346 104 3.194 104 4.032 104 5.055 104
0.943 0.899 0.975 0.979 0.951 0.921
CLOU effect 850 900 950
1.642 103 1.945 103 1.991 103
0.989 0.977 0.941
4.076 104 4.804 104 4.882 104
0.939 0.913 0.860
1.809 104 2.159 104 2.237 104
0.949 0.969 0.976
The calculated activation energy of the oxidation reaction of Fe2O3–CuO/Al2O3 was also less than 0 kJ/mole, as shown in Fig. 17. To further understand the possible reasons behind this phenomenon, as observed in the paper for both oxygen carriers i.e., Fe2O3–CuO/Al2O3 and Fe2O3/TiO2, the observed
behavior is possibly due to the another oxygen transport mechanism, due to defects of crystal structure in doped Fe oxygen carriers [21,22,23]. As a consequences that mechanism in some temperatures is more visible, in other less (see Table 7).
E. Ksepko et al. / Applied Energy 115 (2014) 374–383
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might be useful for CLC system modeling and facilitate facility design. Since the iron and iron–copper oxides are materials potentially suitable as oxygen carriers for a CLC and the degree of reduction of the iron–copper or iron species is highly important, therefore the performance must be assessed in multi-cycle tests. Acknowledgements This research was financed by the Polish Ministry of Higher Education and Science within the frame of State Subsidy Project No. 11.13.003.005. References Fig. 17. Arrhenius plot of the oxidation reaction for Fe2O3–CuO/Al2O3; model R3.
Table 7 Oxidation reaction kinetics study for Fe2O3–CuO/Al2O3. T (°C)
Model F1 k (s1)
600 700 800 850 900 950
0.062 0.058 0.055 0.056 0.055 0.056
Model R3 R2 0.821 0.840 0.867 0.922 0.890 0.833
k (s1) 0.017 0.016 0.016 0.016 0.015 0.016
Model D3 R2 0.892 0.889 0.912 0.948 0.925 0.890
R2
k (s1) 3
5.417 10 4.707 103 4.303 103 4.524 103 4.479 103 4.677 103
0.494 0.522 0.548 0.642 0.589 0.509
4. Conclusions In this study, the apparent kinetics of both the reduction and oxidation reactions, specifically for the 60 wt.% Fe2O3–20 wt.% CuO/20 wt.% Al2O3 (bi-metallic) and 80 wt.% Fe2O3/20 wt.% TiO2 (mono-metallic) oxygen carriers, were examined. Tests were performed at isothermal conditions (600–950 °C) in multiple redox cycles using a Netzsch STA 409 PG Luxx thermo-gravimetric analyzer. The activation energy and the pre-exponential factor were determined, and a reaction model was selected. The kinetic study results showed that: The F1 and R3 models are suitable models for the Fe2O3/ TiO2 oxygen carrier reduction reaction. The F1 and D3 models can be suitable for the Fe2O3–CuO/ Al2O3 oxygen carrier reduction reaction. The R3 model can be suitable for both oxygen carrier oxidation reactions, and F1 is also proper for the Fe2O3/TiO2 oxidation reaction. The activation energy for the oxidation reaction is close to 0 kJ/mole. This means that the reaction is not temperature dependent . The activation energies of the reduction reactions are as follows: 33.8 kJ/mole for Fe2O3/TiO2 and 42.6 kJ/mole (F1 model) for Fe2O3–CuO/Al2O3. The CLOU effect of Fe2O3–CuO/Al2O3 can be described by the F1 or D3 models, and its activation energy is equal to 22.2 kJ/mole. The model of the reduction and oxidation reactions is specific for a given oxygen carrier and should be designated for each specific oxygen carrier independently. The understanding of kinetic parameters on the performance of Fe- or Fe–Cu oxygen carriers is highly practically important for the real applications. The kinetic parameters obtained as a result of study of reduction and oxidation reactions of two potentially suitable oxygen carriers such as Fe2O3–CuO/Al2O3 and Fe2O3/TiO2
[1] Richter HJ, Knoche KF. Reversibility of combustion process. In: Gaggioli RA, editor. Efficiency and costing, second law analysis of process ACS symposium series 235. Washington: American Chemical Society; 1983. p. 71–85. [2] Brandvoll Ø, Bolland O. Chemical looping combustion: fuel conversion with inherent CO2 capture. J Eng Gas Turb Power 2004;126:316–21. [3] Krongberger B, Lyngfelt A, Löffler G, Hofbauer H. Design and fluid dynamic analysis of a bench-scale combustion system with CO2 separation-chemicallooping combustion. Ind Eng Chem Res 2005;44(3):546–56. [4] Mayer F, Bidwe RA, Schopf A, Taheri K, Zieba M, Scheffknecht G. Comparison of a new micaceous iron oxide and ilmenite as oxygen carrier for Chemical looping combustion with respect to syngas conversion. Appl Energy 2013;113:1863–8. [5] Bohn CD, Cleeton JP, Mueller CR, Chuang SY, Scott SA, Dennis JS. Stabilizing iron oxide used in cycles of reduction and oxidation for hydrogen production. Energy Fuels 2010;24(7):4025–33. [6] Siriwardane RV, Ksepko E, Tian H, Poston J, Simonyi T, Sciazko M. Interaction of Iron–Copper mixed metal oxide oxygen carriers with simulated synthesis gas derived from steam gasification of coal. Appl Energy 2013;107:111–23. [7] Fossdal A, Bakken E, Oeye BA, Schoening C, Kaus I, Mokkelbost T, et al. Study of inexpensive oxygen carriers for chemical looping combustion. Int J Greenh Gas Con 2011;5:483–8. [8] Imtiaz Q, Kierzkowska AM, Broda M, Müller CR. The synthesis of Cu-rich, Al2O3 stabilized oxygen carriers using a co-precipitation technique: Redox and carbon formation characteristics. Environ Sci Technol 2012;46:3561–6. [9] Siriwardane R, Tian H, Richards G, Simonyi T, Poston J. Chemical-looping combustion of coal with metal oxide oxygen carriers. Energy Fuels 2009;23(8):3885–92. [10] Ksepko E, Siriwardane RV, Tian H, Simonyi T, Sciazko M. Effect of H2S on chemical looping combustion of coal-derived synthesis gas over Fe–Mn oxides supported on sepiolite, ZrO2, and Al2O3. Energy Fuels 2012;26(4):2461–72. [11] Son SR, Kim SD. Chemical-looping combustion with NiO and Fe2O3 in a thermobalance and circulating fluidized bed reactor with double loops. Ind Eng Chem Res 2006;45:2689–96. [12] Abad A, García-Labiano F, de Diego LF, Gayán P, Adánez J. Reduction kinetics of Cu-, Ni- and Fe-based oxygen carriers using syngas (CO + H2) for chemical looping combustion. Energy Fuels 2007;21:1843–53. [13] Abad A, Adánez J, García-Labiano F, de Diego LF, Gayán P. Modeling of the chemical – looping combustion of methane using a Cu-based oxygen carrier. Combust Flame 2010;157:602–15. [14] García-Labiano F, de Diego LF, Adánez J, Abad A, Gayán P. Reduction and oxidation kinetics of a copper-based oxygen carrier prepared by impregnation for chemical-looping combustion. Ind Eng Chem Res 2004;43:8168–77. [15] Chuang SY, Dennis JS, Hayhurst AN, Scott SA. Kinetics of the chemical looping oxidation of CO by a co-precipitated mixture of CuO and Al2O3. Proc Combust Inst 2009;32:2633–40. [16] Wang J. Clean combustion of solid fuels. Appl Energy 2008;85:73–9. [17] Rydén M, Leion H, Mattisson T, Lyngfelt A. Combined oxides as oxygen-carrier material for chemical-looping with oxygen uncoupling. Appl Energy 2013;113:1924–32. [18] Galwey AK, Brown ME. Kinetic background to thermal analysis and calorimetry. In: Brown ME, editor. Handbook of thermal analysis and calorimetry, vol. 1. Principles and practice. Elsevier, Science B.V.: Amsterdam; 1998. p. 147–216. [19] Abad A, Adanez J, Cuadrat A, Garcia-Labiano F, Gayan P, de Diego LF. Kinetics of redox reactions of ilmenite for chemical-looping combustion. Chem Eng Sci 2011;66:689–702. [20] Sakata K, Nakamura T, Misono M, Yoneda Y. Reduction-oxidation mechanism of oxide catalysts-oxygen diffusion during redox cycles. Chem Lett 1979;8:273–6. [21] Ryu HJ, Bae DH, Han KH, Lee SY, Jin GT, Choi JH. Oxidation and reduction characteristics of oxygen carrier particles and reaction kinetics by unreacted core model. Korean J Chem Eng 2001;18:831–7. [22] Ishida M, Jin H, Okamoto T. A fundamental study of a new kind of medium material for chemical-looping combustion. Energy Fuel 1996;10:958–63. [23] Wang XH, Li JG, Kamiyama H, Katada M, Ohashi N, Moriyoshi Y, et al. Pyrogenic iron(III)-doped TiO2 nanopowders synthesized in RF thermal plasma: phase formation, defect structure, band gap, and magnetic properties. J Am Chem Soc 2005;127:10982–90.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Studies on the redox reaction kinetics of Fe2O3–CuO/Al2O3 and Fe2O3/TiO2 oxygen carriers Ewelina Ksepko ⇑, Marek Sciazko, Piotr Babinski Institute for Chemical Processing of Coal, 1 Zamkowa, 41-803 Zabrze, Poland
h i g h l i g h t s Kinetics study for redox reactions of Fe2O3–CuO/Al2O3 and Fe2O3/TiO2 was performed. F1, R3 suitable for Fe2O3/TiO2; F1, D3 for Fe2O3–CuO/Al2O3 reduction reaction. CLOU effect of Fe2O3–CuO/Al2O3 can be described by the F1 or D3 models. R3 model is suitable for both oxidation reactions; F1 also for Fe2O3/TiO2. Activation energy for the oxidation reactions is close to 0 kJ/mole.
a r t i c l e
i n f o
Article history: Received 22 July 2013 Received in revised form 29 October 2013 Accepted 31 October 2013 Available online 2 December 2013 Keywords: Reaction model Kinetics Bi-metallic oxygen carriers for chemical looping combustion Hydrogen-fueled CLC
a b s t r a c t This paper contains the results of research work on chemical looping combustion (CLC). CLC is one of the most promising combustion technologies and has the main advantage of the production of a concentrated CO2 stream, which is obtained after water condensation without any energy penalty for CO2 separation. The objective of this work was to study the kinetics of both the reduction and oxidation reactions for the selected bi-metallic Fe2O3–CuO/Al2O3 and mono-metallic Fe2O3/TiO2 oxygen carriers. Based on our previous CLC research results, the most promising oxygen carriers were selected for the analysis. Tests were performed at isothermal conditions (600–950 °C) in multiple redox cycles using a thermogravimetric analyzer (Netzsch STA 409 PG Luxx). For the reduction, 3% H2 in Ar was used, and for the oxidation cycle, air was used. The activation energy and the pre-exponential factor were determined, and the reaction model was selected. The F1 (volumetric model) and R3 (shrinking core model) were suitable models for Fe2O3/TiO2, with Ea equal to 33.8 kJ/mole where F1 and D3 (3-dimensional diffusion model), were suitable for Fe2O3–CuO/Al2O3 reduction reaction kinetics decryption with Ea = 42.6 kJ/mole (F1 model). The best fits for oxidation reaction was obtained for R3 model, and F1 was also good for Fe2O3/TiO2 oxygen carrier. The chemical looping oxygen uncoupling (CLOU) effect of Fe2O3–CuO/Al2O3 material is the best described by the F1 or D3 models. The CLOU effect activation energy is equal to 22.2 kJ/mole. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The combustion of fossil fuels is one of the major sources of carbon dioxide emissions as a greenhouse gas component. Therefore, it is essential to develop new combustion technologies that allow utilization of fossil fuels while significantly reducing CO2 emissions. Existing carbon dioxide (CO2) capture technologies are costly and energy demanding. Chemical looping combustion (CLC) has been suggested as an energy-efficient fuel combustion method that produces high-purity CO2 [1,2]. This new combustion technology involves applying an oxygen carrier, typically a metal oxide, which transports oxygen from the air to the fuel. In this manner, direct ⇑ Corresponding author. Tel.: +48 32 271 00 41x228; fax: +48 32 271 08 09. E-mail address: [email protected] (E. Ksepko). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.10.064
contact between the fuel and air is avoided. The method is similar to the oxy-fuel combustion method; however, no direct supply of oxygen is needed, which avoids the costs and energy penalty of providing oxygen to the power plant. The CLC system contains two reactors: an air reactor and a fuel reactor. In the fuel reactor, the fuel reacts with the oxygen that is released from the metal oxide. The reduced metal oxide and/or its metallic form are oxidized (regenerated) in the air reactor back to the metal oxide. The metal oxide can then be used in another redox cycle. After water condensation, a pure stream of CO2, not diluted by N2, can be obtained from the fuel reactor. Therefore, CLC is one of the most promising combustion technologies and has the advantage of the production of a concentrated CO2 stream ready for sequestration with no additional energy penalty for its separation [2].
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There are special requirements for the practical utilization of oxygen carriers ina chemical looping combustion power plant, i.e., high redox reactivity; high selectivity towards complete oxidation products; resistance against carbon and sulfur, which could lead to oxygen carrier deactivation; sufficient durability under repeated reduction/oxidation cycles at high temperature; high mechanical strength associated with high circulation of the particles; non-toxicity, and low preparation cost [3]. The rates of the redox reactions of the oxygen carrier with the fuel, the metal oxide oxygen transfer capacities, and the redox reaction kinetics parameters must be determined to design a suitable CLC reactor system. Iron and copper oxides have been extensively studied worldwide due to the low cost of the former and the high reactivity of the latter [4–10]. However, the kinetic data for some oxygen carriers are still not fully available due to different possible combinations of chemical compositions of oxygen carriers. Son et al. [11] reported that for 60 wt.% Fe2O3/bentonite in TGA (thermogravimetric analyzer), within the temperature range of 700–1000 °C, and 10 vol% CH4, the reduction reaction follows the modified volumetric model with Ea (activation energy) equal to 29 kJ/mole, whereas for the oxidation reaction, it follows the shrinking core model with Ea = 6.0 kJ/mole. Abad et al. [12] studied 60 wt.% Fe2O3/Al2O3 with applied TGA and a 600–950 °C reaction range for 5–70 vol% H2 used as reducer and 5 vol% O2 used as oxidizer. For the reduction reaction Ea was 24 kJ/mole and for oxidation reaction, the calculated Ea was 14 kJ/mole. For both reactions, the shrinking core model was accurate. There are more kinetic data on copper oxygen carriers in the published literature [12–15]. For example, for 14 wt.% CuO/Al2O3, within 600–800 °C, applying 5–70 vol% H2 in TGA, Abad et al. [13] calculated the reduction reaction Ea to be equal to 20 kJ/mole, where shrinking core model was the most suitable. The 10 wt.% CuO/Al2O3 oxygen carrier was also studied extensively. TGA data collected at 600 – 800 °C, showed that the shrinking core model could be proposed with Ea of 33 kJ/mole and 15 kJ/mole for the reduction and oxidation reactions, respectively [14]. Extensive study of the reactions kinetics was performed by Chuang et al. [15] on significantly higher concentrations of copper oxide; 82 wt.% CuO/Al2O3. The kinetics of four reactions were studied in a fluid bed within 250–900 °C. While using 2–10 vol% H2 as the reducing agent, the CuO ? Cu2O and CuO ? Cu reduction kinetics were studied. The determined Ea was equal to 58 kJ/mole and 44 kJ/mole, respectively. For those reactions, the diffusion reaction model was appropriate. For Cu ? Cu2O and Cu2O ? CuO reaction an Ea of 40 kJ/mole 60 kJ/mole was calculated, respectively. Single metal oxides have been used extensively in the past as oxygen carriers [5,9,16], and recently, promising results have been obtained for bimetallic oxygen carriers due to synergistic effects between the two active metal oxides [6,10,17]. Moreover, the kinetic parameters of the redox reactions of 60 wt.% Fe2O3–20 wt.% CuO supported on 20 wt.% Al2O3 (bi-metallic oxygen carrier) and 80 wt.% Fe2O3 supported on 20 wt.% TiO2 with H2 have not been reported in the literature. The selected oxygen carriers for the kinetics study were chosen based on our previous research results [6]. We concluded, from ten-cycle reduction–oxidation data with simulated synthesis gas derived from steam gasification of Janina coal, that for the five carriers containing 60% Fe2O3–20% CuO, the supports had a great influence on the performance stability of these oxygen carriers. For Fe–Cu carriers supported on silica and sepiolite, there was a decrease in the oxygen capacity with an increasing number of cycles. The Fe–Cu on TiO2 showed the best stability, and the oxygen capacities were the highest for the Fe–Cu on bentonite and alumina. The stabilities during the 10-cycle test with a monometallic such as Fe/supports were similar to the stabilities with Fe–Cu carriers. The stable reduction rates that were observed with most of the supported Fe–Cu remained fairly constant, which
375
indicated that incorporating Cu improves the rate and stability during cyclic tests. Fe/TiO2 had the best performance with stable oxygen capacities at both 800 and 900 °C. These results indicate that TiO2 and Al2O3 are appropriate supports for Fe and Fe–Cu carriers, respectively. The objective of the present work was to study the apparent kinetics of the reduction and oxidation reactions for selected biand mono-metallic oxygen carriers. The activation energy, preexponential factor, and reaction model for two promising oxygen carriers, Fe2O3–CuO supported on Al2O3 and Fe2O3 supported on TiO2, were investigated. To simulate the hydrogen/oxygen carrier reactions, the reactivity of the oxygen carriers was evaluated by conducting multi-cycle CLC tests in atmospheric TGA with oxygen carriers utilizing hydrogen as a fuel. In the work the research was conducted at specific for a CLC process temperatures and particle size which supposed to be used in large scale units. The kinetics investigated in this situation might reflect the effects of pure chemical kinetics affected by diffusion resistance or internal ion transfer rate. At this stage of mechanism understanding it is difficult to judge what is the extent of interference of physical phenomena into chemical reaction. This can be presented by the final data obtained, namely activation energy and its relation to temperature changes. Under these conditions the results obtained reflects apparent kinetics, which is has a very practical meaning. The models applied for data analysis were volumetric model (F1), shrinking core model (R3) and 3 – dimensional diffusion model (D3). All these models take account all of these specific resistances described above. The volumetric model assumes that the reaction occurs throughout the particles of the oxygen carrier and that the mass of the oxygen carrier grain changes linearly during the reaction. The shrinking core model assumes that the reaction occurs on the external surface of the grain, which changes during the reaction. The 3-dimensional diffusion model describes a reaction in which the diffusion of oxygen from the unreacted metal oxide through the metal layer to the surface of the particle is the reaction-limiting step. 2. Experimental section 2.1. Oxygen carrier preparation The oxygen carriers with compositions of 60 wt.% Fe2O3, 20 wt.% CuO/20 wt.% Al2O3 (bi-metallic) and 80 wt.% Fe2O3/ 20 wt.% TiO2 (mono-metallic) were prepared by the solid-state mixing method. Fe2O3 and CuO were mixed thoroughly with Al2O3. Then, deionized water was added to obtain a paste. The paste was dried and calcined at 850 °C in air for 20 h. After cooling, the sample was crushed and was mixed thoroughly. Then, it was calcined again at 950 °C. Each time, 10 wt.% of graphite (burn off material at 850 °C) was added to the mixture to improve the porosity/surface area of the samples. The calcined sample was sieved, and the fraction <250 lm was used for measurements. 2.2. Kinetics study using thermogravimetric analyzer Thermogravimetric analysis experiments were conducted in a Netzsch STA 409 PG Luxx thermogravimetric analyzer. In TGA, the weight change of the selected oxygen carriers was measured isothermally as a function of the time during the reduction–oxidation cycles. Three reduction–oxidation cycles at each temperature were conducted at atmospheric pressure for stable performance determination. A sample of approximately 10 mg was heated in argon in a plate crucible made of Al2O3 to the reaction temperature. Three percent H2/Ar was used for the reduction reaction, whereas
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air was used for the oxidation reaction. Three percent H2/Ar was used as a fuel due to the limitations of the TGA construction, where such a concentration is the upper limit for combustion/explosive gases. All reaction gas flow rates were 150 ml/min. In the experiments, the reduction time was 60–180 min, and the oxidation reaction time was 30–40 min, depending on temperature conditions. To avoid the mixing of reduction gases and air, the system was flushed with argon for 15–60 min before and after each reduction reaction. To understand the effect of temperature, the TGA experiments were performed at a range of temperatures from 600 °C and 950 °C. 2.3. Kinetic analysis Fractional conversions, i.e., fractional reduction and fractional oxidation, were calculated utilizing the TGA data of the second redox cycle. The fractional conversion (X) is defined as follows [6]:
M oxd M M oxd Mred
ð1Þ
M M red Fractional oxidation : X ¼ Moxd M red
ð2Þ
Fractional reduction : X ¼
where M is the instantaneous weight, Moxd is the weight of a completely oxidized sample in TGA (maximum weight after oxidation with air), and Mred is the weight of a completely reduced sample in TGA (minimum weight after reduction with hydrogen). Because the calculations described in this paper do account for the concentration of the gaseous substrate, a kinetic expression for the solid–gas reaction rate can be described by the following equation:
dX ¼ f ðXÞkðTÞ dt
ð3Þ
where f(X) is a structural factor or a model of the reaction that describes the physical or chemical properties during the reaction, and k(T) is a reaction rate constant that can be described by the Arrhenius Eq. (4): Ea
kðTÞ ¼ A0 e RT
ð4Þ
where A0 is a pre-exponential factor, Ea is the activation energy, and R is the gas constant, which is equal to 8.314 J/mol ⁄ K. In this paper, three models were chosen to find the model that best fit the experimental data (see Table 1) and that could describe the reaction in a proper manner. Among them were the volumetric model, the shrinking core model, and the 3-dimensional diffusion model (Jander’s type). Table 1 presents the models of these reaction as f(X) and g(X) functions. These three models were applied to all reactions occurring in the TGA system during the cycling experiments, i.e., reduction by H2, oxidation by O2 from synthetic air, and chemical looping oxygen uncoupling in the case of the Fe2O3–CuO/Al2O3 oxygen carrier. The first step of calculations was the model fitting of the raw TGA data. For this purpose, Eq. (4) was transformed to Eq. (5):
After integration, Eq. (5) can be described by the expression of the function g(X) for the following equation:
gðXÞ ¼
Z
dX f ðXÞ
ð6Þ
One method of experimental data model fitting is testing the linearity of the g(X) function versus time. The slope of this function results in the reaction rate constant k(T). For the calculation of the other kinetic parameters, such as Ea and A0, the reaction rate constant must be calculated for different temperatures. Then, the activation energy and the pre-exponential factors are calculated from the Arrhenius equation as the slope and intercept of ln (k(T)) versus 1/T, respectively, based on the following equation:
lnðkðTÞÞ ¼
Ea 1 þ lnðA0 Þ R T
ð7Þ
2.4. X-ray diffraction analysis X-ray diffraction (XRD) analyses were conducted using a Panalytical PW 3040 X-Pert Pro XRD system equipped with a Cu anode in the h/2 h configuration. The X-ray wavelength utilized was Cu Ka-1 at 1.5406 Å. Sample data were acquired at 40 kV and 30 mA, and the X-ray diffraction patterns were collected at room temperature in the 10–100 degrees (2h) range. 2.5. Porosimetry The automatic PoreMaster mercury intrusion porosimeter was used for the pore size analysis. The porosimeter allowed for the calculation of the porosity rate and a specific pore volume of the sample, in addition to the absolute, envelope and bulk densities. 3. Results and discussion 3.1. Oxygen carrier characterization The surface areas and XRD data of the 60 wt.% Fe2O3–20 wt.% CuO, 20 wt.% Al2O3 (bi-metallic) and 80 wt.% Fe2O3, 20 wt.% TiO2 (mono-metallic) oxygen carriers are listed in Table 2. They showed small surface areas of 0.59 and 1.09 m2/g, respectively as shown in Table 2. They were mostly composed of Fe2O3 and an inert phase of TiO2 or Al2O3. The mono-metallic oxygen carrier was also composed of Fe2TiO5. The mercury intrusion curves, into and out of the pores, showed the spherical shape of the pores. The calculated medium volume of the pores was 0.3633 and 0.4461 cm3/g, with a smaller size of the macropores of 2399 nm for the bimetallic and 4935 nm for the monometallic oxygen carriers. As we concluded in our previous paper [6], in Fe–Cu mixtures, the support had a significant effect on the stability and reactivity; however, the performance cannot be attributed to the surface area of the oxygen carriers because it is significantly low. 3.2. Kinetics study
dX ¼ kðTÞdt f ðXÞ
ð5Þ
Table 1 Tested reaction models [18]. Model
f(X)=
g(X)=
Volumetric model (F1) Shrinking core model (R3) 3-Dimensional diffusion model (D3)
1X 3(1 X)2/3
ln(1 X) 1(1 X)1/3
3 4 ð1
XÞ2=3 ð1 ð1 XÞ1=3 Þ
2
ð1 ð1 XÞ1=3 Þ
The cycling reduction/oxidation TGA data for the Fe2O3–CuO/ Al2O3 oxygen carrier over the range of 600 °C to 950 °C with 3% H2/Ar is shown in Fig. 1. Constant chemical looping combustion performance was observed. The stable second redox TGA data were used for the calculations and are marked with an asterisk. Mathcad v. 14.0 software was used for the kinetic parameters calculations. The entire redox cycle was composed of the reduction reaction, a flush with argon, and a regeneration reaction of the oxygen carrier. Due to the reduction reaction of the oxygen carrier, a decrease of the mass was observed because reducers, such as H2, were
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E. Ksepko et al. / Applied Energy 115 (2014) 374–383 Table 2 X-ray diffraction, BET surface area, pore analysis and density data for the oxygen carriers. Oxygen carrier
Phase composition
Surface area (m2/g)
VT (cm3/g)
Lmedium (nm)
Envelope density (g/cm3)
80 wt.% Fe2O3, 20 wt.% TiO2 60 wt.% Fe2O3, 20 wt.% CuO, 20 wt.% Al2O3
Mostly Fe2O3, Fe2TiO5, TiO2 Fe2O3, Fe3O4, Al2O3
1.089 0.589
0.3633 0.4461
4935 2399
3.13 2.47
Fig. 1. Cycling reduction/oxidation TGA data for the Fe2O3–CuO/Al2O3 oxygen carrier at 600 °C and 950 °C with 3%H2/Ar.
introduced into the TGA chamber. The system was flushed with inert argon to remove the combustion gases from the TGA. After the flush, the oxidation reaction occurs because the reduced oxygen carrier is regenerated; therefore, an increase of the mass on the TGA chart was observed. The entire TGA data redox cycle is shown in Fig. 2. The oxygen transport capacities were calculated for both 80 wt.% Fe2O3, 20 wt.% TiO2 and 60 wt.% Fe2O3–20 wt.% CuO/ 20 wt.% Al2O3 materials because it is an important practical parameter for oxygen carrier selection for a CLC power plant. The theoretical oxygen transfer capacity (wt.%) depends on the oxygen carrier
material and the final reduction state. The oxygen transfer capacities of the oxygen carriers under different reduction states are listed in Table 3 For the mono-metallic Fe-based carrier, it may vary from 2.67 to 24.05 wt.%, whereas for the Fe–Cu-based bimetallic carrier, it may vary from 4.01 to 22.06 wt.%. In our TGA experiments, maximal oxygen transport capacities were achieved, which means the final reduction state was obtained. Therefore, the kinetics data describe the final reduction of Fe2O3 to Fe within the sub-stage reactions Fe2O3)Fe3O4)FeO)Fe. For the Fe–Cu oxides, an additional three redox combinations can be observed. Additionally, for the bi-metallic oxygen carrier, the experimental value of the capacity was 22%, which indicated that
Table 3 Oxygen transfer capacity of the oxygen carriers under different reduction states.
Fig. 2. One full cycle of the reduction/oxidation TGA data for the Fe2O3–CuO/Al2O3 oxygen carrier with 3% H2/Ar.
Oxygen carriers
Reactions
Theoretical oxygen transfer capacity (wt.%)
80% Fe2O3/support
3Fe2O3 = 2Fe3O4 + 1/2O2 Fe2O3 = 2FeO + 1/2O2 Fe2O3 = 2Fe + 3/2O2
2.67 8.02 24.05
60% Fe2O3, 20% CuO/20% support
2CuO = Cu2O + 1/2O2 3Fe2O3 = 2Fe3O4 + 1/2O2 CuO = Cu + 1/2O2 3Fe2O3 = 2Fe3O4 + 1/2O2 2CuO = Cu2O + 1/2O2 Fe2O3 = 2FeO + 1/2O2 CuO = Cu + 1/2O2 Fe2O3 = 2FeO + 1/2O2 2CuO = Cu2O + 1/2O2 Fe2O3 = 2Fe + 3/2O2 CuO = Cu + 1/2O2 Fe2O3 = 2Fe + 3/2O2
4.01 6.03 8.02 10.03 20.05 22.06
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Fig. 3. Fractional reduction versus time for Fe2O3/TiO2.
Table 4 Reduction reaction kinetics study for Fe2O3/TiO2. T (°C)
Model F1
Model R3 R2
k (s1) 600 700 800 850 900 950
4
6.862 10 9.484 104 1.468 103 2.075 103 2.596 103 2.082 103
0.858 0.947 0.977 0.989 0.974 0.996
Model D3 R2
k (s1) 4
4.471 10 6.323 104 1.022 103 1.437 103 1.810 103 1.420 103
0.714 0.826 0.986 0.971 0.986 0.922
R2
k (s1) 5
8.404 10 1.147 104 1.690 104 2.404 104 2.995 104 2.454 104
0.922 0.985 0.823 0.858 0.824 0.941
CuO was fully reduced to metallic Cu and that Fe2O3 was fully reduced to metallic Fe. The fractional conversions for the reduction and oxidation reactions were calculated as in Eqs. (1) and (2). Fig. 3 shows the fractional reduction for the mono-metallic 80 wt.% Fe2O3, 20 wt.% TiO2 oxygen carrier, which was calculated at a temperature range of 600–950 °C. The strong temperature effect is also observed because with the temperature increase, the reaction rate increased. In the literature, the shrinking core model and diffusion model are most frequently indicated [11–15]; therefore, for the Fe2O3/ TiO2 oxygen carrier in our calculations, we used both models. Additionally, other models, such as the volumetric model, were also tested. The results for the volumetric model (marked by F1), the shrinking core model (marked by R3) and the 3-dimensional diffusion model (marked by D3) are shown in Table 4. For all of the models, the k – kinetic constant and the correlation coefficient were calculated. Table 4 presents the fitting of these three models
to the reaction. The F1 and R3 models fit very well and can describe the behavior of the oxygen carrier during reaction, but in the lowest range of temperature (600 and 700 °C), the 3-dimensional diffusion model shows better correlation. Above 800 °C, the 3-dimensional diffusion model does not fit this reaction because the correlation coefficients are much lower than those obtained for the F1 and R3 models. Plots of the function g(X) versus time for the reduction of Fe2O3/TiO2 for these models are presented in Fig. 4. The calculated reduction reaction parameters for the F1 model are Ea = 33.808 kJ/mole, R2 = 0.927 and A0 = 0.069. For the D3 model, a similar activation energy was achieved, 32.357 kJ/ mole, which is also shown in Fig. 5, with a much poorer fit at the higher temperature range as shown previously in Fig. 4. The Ea value of reduction reaction of Fe2O3/TiO2, 33.8 kJ/mole is similar to that reported by Son et al. [11] for Fe2O3/bentonite that is equal to 29 kJ/mole. Different behavior was observed for the oxidation reaction. For all temperatures from 600 to 900 °C, no essential temperature effect on the oxidation reaction rate was observed (Fig. 6). In Fig. 3 the reaction rate at 950 °C drops. That behavior might be due to the melting of the mono-metallic Fe based carrier since the oxygen carrier is composed of large amount of Fe oxide, that is 80 wt.% of Fe2O3 and 20 wt.% of TiO2. That is known from literature that iron oxide III has a tendency to agglomerate. Perhaps such as amount of Fe2O3, and high temperature as high as 950 °C and hydrogen as reducing agent causes the melting of the particles, that effected in Fe agglomerates. Since they once appear, the 20 wt.% of inert material in such as high temperature does not prevent the agglomeration. The addition of TiO2 facilitated sintering prevention, which inhibited the contact between metallic iron particles in temperatures range of 600–900 °C. Metallic Fe that agglomerates can block the pathway for O2 to contact Fe in the interior of particles. Thus, local pores, created during reduction reaction, may be unavailable for oxidation reactions. The sintering effect causes particle agglomeration. Since, the metallic Fe agglomerates tend to be hard to oxidize, therefore that is perhaps the reason for the drop in oxidation rates observed at 950 °C. To support our conclusion, the additional TGA measurements were performed for Fe/TiO2 carrier. The visual observation of used sample, showed that the differences in reactivity’s may come from the melting of the sample at 950 °C. For the model fitting calculation, the range of the data that was taken into account was narrowed to approximately 15 s. As indicated by the data collected in Table 5, the F1 and R3 models are the most suitable for describing the sample behavior when oxidizing by air at 900 °C. Model D3 shows a poor fit to the data, so it is not suitable for describing the oxidation reaction behavior, which was also shown in Fig. 7. Fig. 8 shows the plot of the logarithm of the reaction rate constant versus inverse temperature – Arrhenius plot (without the
Fig. 4. Model fitting for the reduction reaction of Fe2O3/TiO2 (T = 800 °C).
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E. Ksepko et al. / Applied Energy 115 (2014) 374–383 Table 5 Oxidation reaction kinetics study for Fe2O3/TiO2. T (°C)
Model F1 k (s
600 700 800 850 900 950
Fig. 5. Arrhenius plot of the reduction reaction for Fe2O3/TiO2; (a) Model F1, (b) Model R3.
data collected at 950 °C). The calculated activation energy for the oxidation reaction is less than 0 kJ/mole, which indicates that there is no temperature dependence on the oxidation rate of the Fe2O3/TiO2 oxygen carrier in the range of temperatures from 600 to 900 °C. This lack of dependence may be due to the diffusion limitations of Fe2O3 layer formed rapidly on the surface of oxygen carrier
1
)
0.021 0.018 0.017 0.022 0.019 4.630 103
Model R3 2
R
0.904 0.972 0.965 0.960 0.986 0.831
k (s
Model D3
1
)
R 3
5.791 10 4.871 103 4.703 103 5.958 103 5.230 103 1.231 103
2
0.959 0.975 0.976 0.992 0.993 0.769
R2
k (s1) 3
1.858 10 1.610 103 1.547 103 1.937 103 1.726 103 4.539 104
0.592 0.765 0.736 0.696 0.778 0.964
which covers the unreacted core of the metallic Fe [19]. In this case, the reaction can be limited by Fe ion transport in solid product layer. The relative diffusivities of iron ions and oxygen ions are not expected to change at higher temperatures because iron ions are much smaller than oxygen ions. At higher temperatures, the iron will diffuse and sinter, leading to decreased reactivity over redox cycles [20]. The data presented by Son et al. [11] showed that the activation energy of the oxidation reaction could be 6 kJ/mole, but this result was obtained for another Fe-based oxide, 60 wt.% Fe2O3/bentonite. The second analyzed sample was the bi-metallic oxygen carrier Fe2O3–CuO/Al2O3, which presents both activities, i.e., chemical looping combustion and chemical looping oxygen uncoupling reaction (CLOU effect). In the CLOU effect for the examined material, approximately 4% of the total 20% oxygen capacity is involved. On the basis of this oxygen carrier behavior, the data from the reduction reaction were divided into two parts. One part represents the whole temperature range applied in this study, and the second part represents the temperature range from 850 to 950 °C, which covers the CLOU effect. The fractional reduction of the Fe2O3–CuO/Al2O3 oxygen carrier is presented in Fig. 9. The temperature effect is essential and is similar to the Fe2O3/TiO2 reduction reaction. In the case of the Fe2O3–CuO/Al2O3 oxygen carrier, the model fitting calculation gives different results from Fe2O3/TiO2, which are presented in Table 5, and the plots of the function g(X) versus time for the 800 °C data are presented in Fig. 10. As seen in Table 5 and Fig. 10, the most suitable model for the reduction
Fig. 6. Oxidation degree versus time for Fe2O3/TiO2. (Inlet-range taken into calculations.)
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Fig. 7. Model fitting for the oxidation reaction of Fe2O3/TiO2 (T = 800 °C).
Fig. 8. Arrhenius plot of the oxidation reaction for Fe2O3/TiO2; Model R3.
Fig. 9. Fractional reduction versus time for Fe2O3–CuO/Al2O3.
reaction is the D3 model, but model F1 is also appropriate for describing the behavior of Fe2O3–CuO/Al2O3 during the reduction reaction. F1 and R3 models are concluded to be suitable for Fe2O3/TiO2, and F1 and D3 models are suitable for Fe2O3–CuO/Al2O3 reduction reaction. The reason for the diverse models fitting for two oxygen carriers is different reduction reactions that are taking place since different material phase compositions was observed. In the Fe2O3/ TiO2 oxygen carrier the pathway for reduction is expected to be as follows Fe2O3)Fe3O4)FeO)Fe and meanwhile Fe2TiO5)Fe2TiO4, that is also supported by XRD data. For other Fe2O3–CuO/Al2O3 oxygen carrier sample, the fitted model describes the following reduction reactions: Fe2O3)Fe3O4)FeO)Fe, and Cu2O)Cu. The diffusion limitations of oxygen transport through the solid layer of the products, which are the reduced metal oxides, may play an important role in the reduction kinetics of the reduction of the Fe2O3–CuO/Al2O3 oxygen carrier. During the reduction step, the porosity of particles can rapidly increases because the volume of the solid products such as Cu2O and Fe3O4 and then FeO, Fe is lower than those for the solid reactants as CuO and Fe2O3. Solid product layer is formed on the surface of grain which limited the accessibility of the hydrogen to unreacted core. Based on that, it was assumed that the reduction can be controlled by the diffusion of ions in the solid. The calculations showed that the D3 model is more suitable than R3 model for Fe2O3–CuO/Al2O3 reduction reaction, because of the better correlation factor. The similar finding was indicated by authors of the papers [19–21]. The calculated activation energies are 42.684 kJ/mole for the D3 model and 41.318 kJ/mole for the F1 model and are shown in Fig. 11. The obtained values are higher than for the Fe2O3/TiO2 oxygen carrier. Abad et al. [13] and García-Labiano et al. [14] reported activation energies of reduction reactions equal to 20 kJ/mole and 33 kJ/mole. However, their data were obtained for 14 wt.% CuO/ Al2O3 and 10 wt.% CuO/Al2O3, respectively, and the shrinking core
Fig. 10. Model fitting for the reduction reaction of Fe2O3–CuO/Al2O3 (T = 800 °C) .
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Fig. 14. Arrhenius plot of the CLOU reaction for Fe2O3–CuO/Al2O3; model F1.
Fig. 11. Arrhenius plot of the reduction reaction for Fe2O3–CuO/Al2O3: (a) Model F1, (b) Model D3.
Fig. 12. Fractional reduction of the CLOU effect versus time for Fe2O3–CuO/Al2O3.
model was applied for both oxygen carriers. The data presented in this paper showed that the D3 model could describe the bi-metallic oxygen carrier, and the activation energy was higher than that presented in the literature for similar mono-metallic carriers. Fig. 12 presents the fractional reduction of the Fe2O3–CuO/Al2O3 oxygen carrier during the CLOU reaction, and Fig. 13 shows the model fitting of the experimental data. The range of data that was taken into account was between a 1 and 95% conversion degree. In this case, the F1 and D3 models have the best correlation, similar to the correlation that was observed for the reaction reduction by H2. The calculated activation energy for the F1 model, shown in Fig. 14, was equal to 22.239 kJ/mole, but the correlation coefficient was much lower (0.858) than calculated for the activation energy from the reduction reaction by H2 (0.968). This is most likely because the CLOU effect is quite small compared to the reduction of the oxygen carrier by H2 in chemical looping combustion and by a much narrower temperature range than was used in calculations. The oxidation reaction of the Fe2O3–CuO/Al2O3 oxygen carrier by air is similar to the oxidation of Fe2O3/TiO2. The plot of the conversion degree versus time is presented in Fig. 15. In this case, a narrower range of data was taken into account (up to 30 s), which corresponds to a conversion degree of 0.8–0.9. The rate of the oxidation reaction is independent on temperature. The same phenomenon is observed for the oxidation of the Fe2O3/TiO2 oxygen carrier. In Fig. 16, the plots of the function g(X) versus time at 800 °C are presented, and in Table 6, the reaction rate constants and correlation coefficients for the three models are shown. For the oxidation reaction of Fe2O3–CuO/Al2O3, the best correlations were obtained for the F1 and R3 models, but the fits are poorer than the fits obtained for the oxidation of Fe2O3/TiO2. Similar behavior was observed for the Janina coal synthesis gas CLC. Temperature had a positive effect on the rate for all of the carriers except Fe–Cu/ Al2O3 [6].
Fig. 13. Model fitting for the CLOU reaction of Fe2O3–CuO/Al2O3 (T = 800 °C).
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Fig. 15. Oxidation degree versus time for Fe2O3–CuO/Al2O3. (Inlet-range taken into calculations.)
Fig. 16. Model fitting for the oxidation reaction of Fe2O3–CuO/Al2O3 (T = 800 °C).
Table 6 Reduction reaction kinetics study for Fe2O3–CuO/Al2O3. T (°C)
Model F1
Model R3
Model D3
k (s1)
R2
k (s1)
R2
k (s1)
R2
Reduction with H2 600 700 800 850 900 950
8.435 104 1.087 103 1.924 103 2.649 103 3.369 103 4.295 103
0.861 0.797 0.916 0.951 0.947 0.981
1.824 104 2.314 104 4.233 104 5.890 104 7.506 104 9.727 104
0.713 0.643 0.782 0.828 0.826 0.893
1.041 104 1.361 104 2.346 104 3.194 104 4.032 104 5.055 104
0.943 0.899 0.975 0.979 0.951 0.921
CLOU effect 850 900 950
1.642 103 1.945 103 1.991 103
0.989 0.977 0.941
4.076 104 4.804 104 4.882 104
0.939 0.913 0.860
1.809 104 2.159 104 2.237 104
0.949 0.969 0.976
The calculated activation energy of the oxidation reaction of Fe2O3–CuO/Al2O3 was also less than 0 kJ/mole, as shown in Fig. 17. To further understand the possible reasons behind this phenomenon, as observed in the paper for both oxygen carriers i.e., Fe2O3–CuO/Al2O3 and Fe2O3/TiO2, the observed
behavior is possibly due to the another oxygen transport mechanism, due to defects of crystal structure in doped Fe oxygen carriers [21,22,23]. As a consequences that mechanism in some temperatures is more visible, in other less (see Table 7).
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might be useful for CLC system modeling and facilitate facility design. Since the iron and iron–copper oxides are materials potentially suitable as oxygen carriers for a CLC and the degree of reduction of the iron–copper or iron species is highly important, therefore the performance must be assessed in multi-cycle tests. Acknowledgements This research was financed by the Polish Ministry of Higher Education and Science within the frame of State Subsidy Project No. 11.13.003.005. References Fig. 17. Arrhenius plot of the oxidation reaction for Fe2O3–CuO/Al2O3; model R3.
Table 7 Oxidation reaction kinetics study for Fe2O3–CuO/Al2O3. T (°C)
Model F1 k (s1)
600 700 800 850 900 950
0.062 0.058 0.055 0.056 0.055 0.056
Model R3 R2 0.821 0.840 0.867 0.922 0.890 0.833
k (s1) 0.017 0.016 0.016 0.016 0.015 0.016
Model D3 R2 0.892 0.889 0.912 0.948 0.925 0.890
R2
k (s1) 3
5.417 10 4.707 103 4.303 103 4.524 103 4.479 103 4.677 103
0.494 0.522 0.548 0.642 0.589 0.509
4. Conclusions In this study, the apparent kinetics of both the reduction and oxidation reactions, specifically for the 60 wt.% Fe2O3–20 wt.% CuO/20 wt.% Al2O3 (bi-metallic) and 80 wt.% Fe2O3/20 wt.% TiO2 (mono-metallic) oxygen carriers, were examined. Tests were performed at isothermal conditions (600–950 °C) in multiple redox cycles using a Netzsch STA 409 PG Luxx thermo-gravimetric analyzer. The activation energy and the pre-exponential factor were determined, and a reaction model was selected. The kinetic study results showed that: The F1 and R3 models are suitable models for the Fe2O3/ TiO2 oxygen carrier reduction reaction. The F1 and D3 models can be suitable for the Fe2O3–CuO/ Al2O3 oxygen carrier reduction reaction. The R3 model can be suitable for both oxygen carrier oxidation reactions, and F1 is also proper for the Fe2O3/TiO2 oxidation reaction. The activation energy for the oxidation reaction is close to 0 kJ/mole. This means that the reaction is not temperature dependent . The activation energies of the reduction reactions are as follows: 33.8 kJ/mole for Fe2O3/TiO2 and 42.6 kJ/mole (F1 model) for Fe2O3–CuO/Al2O3. The CLOU effect of Fe2O3–CuO/Al2O3 can be described by the F1 or D3 models, and its activation energy is equal to 22.2 kJ/mole. The model of the reduction and oxidation reactions is specific for a given oxygen carrier and should be designated for each specific oxygen carrier independently. The understanding of kinetic parameters on the performance of Fe- or Fe–Cu oxygen carriers is highly practically important for the real applications. The kinetic parameters obtained as a result of study of reduction and oxidation reactions of two potentially suitable oxygen carriers such as Fe2O3–CuO/Al2O3 and Fe2O3/TiO2
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