Article pubs.acs.org/EF
Kinetics of Hematite to Wüstite by Hydrogen for Chemical Looping Combustion Esmail R. Monazam,† Ronald W. Breault,*,‡ and Ranjani Siriwardane‡ †
REM Engineering Services, PLLC, 3537 Collins Ferry Road, Morgantown, West Virginia 26505, United States National Energy Technology Laboratory, United States Department of Energy, 3610 Collins Ferry Road, Morgantown, West Virginia 26507-0880, United States
‡
ABSTRACT: Kinetics analysis of hematite (Fe2O3) reduction by hydrogen was evaluated by the thermogravimetric analyser (TGA) in the temperature range of 700−950 °C, using continuous streams of 5, 10, and 20% H2 concentrations in N2. A number of kinetic models have been considered, including the single and multi-step models to describe the experimental reduction data. The details of nucleation and growth during the isothermal reduction are described in terms of the local Johnson−Mehl−Avrami (JMA) exponent and local activation energy. The variations of n values (JMA exponent) and activation energies for the reduction conversion indicate the presence of the multi-step reaction process. The reduction was shown to be one-dimensional (1D) growth with a decrease in the nucleation rate.
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INTRODUCTION
Although there are several studies of hematite reduction with H2, CO, and mixtures of H2 and CO, kinetic studies of the hematite reduction with H2 at different concentrations are rather limited and lack certain details of the entire reduction process. When developing mathematical modeling of hematite reduction for designing a large-scale reactor system, it is very important to understand the kinetics of the reduction process to define the operating parameters and better reactor control of the system.11 Moon et al.12 investigated the reduction mechanism of hematite with a H2−CO gas mixture in the temperature range of 800−950 °C. They described the reduction behavior in terms of a single rate-determining step; the reduction process was initially controlled by the chemical reaction at the oxide− iron interface, while toward the end of the reaction, it was controlled by intraparticle diffusion. They also found that the rate of reduction with H2 was 2−3 times higher than that with CO. Towhidi and Szekely13 studied the reduction kinetics of commercial-grade hematite with a CO−H2 mixture over the temperature range of 600−1234 °C. They found that reduction with mixed gases was affected by the gas compositions; a higher H2 content contributes to a higher rate of reduction. They also observed significant carbon deposition below 780 °C, particularly at a higher CO concentration. Pang et al.14 studied the influence of the particle size of hematite on its reduction kinetics by H2 at a low temperature (450−600 °C). Their work showed that, when the particle size was decreased from 107.5 to 2.0 μm, the apparent activation energy decreased from 78.3 to 36.9 kJ/mol during reduction at a given temperature. Pineau et al.15 characterized the reduction of hematite to magnetite with H2 by an apparent activation energy of 76 kJ/mol and magnetite to metallic iron by apparent activation energies of 88
The United States Department of Energy (U.S. DOE) has set a goal to modify the existing pulverized coal (PC)-fired power plants to remove over 90% of the total carbon in the coal as CO2 for use or sequestration.1 Chemical looping combustion (CLC) is a promising combustion technology that produces a sequestration-ready CO2 stream that can be retrofit to existing PC and circulating fluidized-bed (CFB) boilers as well as is applicable to new installation.1 The CLC is a relatively new combustion process that consists of a fuel and an air reactor. In the fuel reactor, the solid oxygen carrier is reduced by the fuel that is converted to mainly CO2 and H2O, and in the air reactor, the reduced solid oxygen carrier is oxidized by O2 in the air.2 The solid oxygen carrier is a cornerstone in the CLC process. The desirable properties for solid oxygen carriers by CLC systems are high reactivity in both reduction by fuel gas and oxidation by oxygen in the air, high resistance to attrition, low agglomeration, and low fragmentation.3 Additionally, it is also an advantage if the metal oxide is low-cost and environmentally friendly.3 The reactivity of the four most studied supported oxygen carriers is in the descending order of NiO > CuO > Mn2O3 > Fe2O3.3 Among the metal oxides, ironbased (i.e., Fe2O3) solid oxygen carriers are believed to be the most promising for commercial CLC application because they are relatively inexpensive, readily available, and also environmentally safe compared to other metal oxides, such as NiO and CuO.4 Therefore, on the basis of the above advantages, the ironbased (natural hematite) metal oxide was selected as a solid oxygen carrier for CLC in this study. Hematite reduction is usually described as a three-step mechanism, i.e., Fe2O3 → Fe3O4 → FeO → Fe,5−7 rather than a two-step mechanism, Fe2O3 → Fe3O4 → Fe,7−9 when reacting with a gases, such as CO and H2. However, Slagtern et al.10 proposed a two-step reduction mechanism, Fe2O3 → FeO → Fe, when reacting with H2. © 2014 American Chemical Society
Received: May 14, 2014 Revised: June 18, 2014 Published: July 1, 2014 5406
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and 39 kJ/mol for temperatures lower and higher than 420 °C, respectively. Although there are few reported mechanistic studies and limited literature data on the reduction kinetic of hematite with H2, a complete understanding of the factors that control the reduction process for reactor modeling is necessary. The current study focuses on the effect of the temperature (700− 950 °C) and H2 concentration on the detailed kinetics of hematite reduction with H2. In our work on the reduction of hematite with methane, CO and H2 were both product gases. Therefore, it was necessary to investigate the kinetic rates of individual gases (CO and H2) for computational fluid dynamics (CFD) modeling efforts at the National Energy Technology Laboratory (NETL). We have previously reported the kinetic analysis for reduction of hematite with CO and oxidation of the reduced hematite using air. In this study, we present the kinetic rates for H2 (5, 10, and 20%) reduction of hematite. This range of H2 concentration was chosen on the basis of expected values in the reduction of hematite with methane.
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Figure 1. Typical mass and temperature measurement for the hematite particle of 200 μm using 20% H2 for reduction and air for oxidation reactions.
EXPERIMENTAL SECTION
A thermogravimetric analysis (TGA, TA Model 2050) method equipped with a mass spectrometer (MS, Pfeiffer Omnistar GSD301) was used for reduction of commercial hematite in different reductive atmospheres (5−20% H2 balance in N2). The hematite particles originating from the Wabush Mine, Canada. The hematite (94% Fe2O3 + 6% mineral) (the chemical analysis of the hematite particle is given by Monazam et al.16) was crushed in a lab to 100−300 μm with an average size of 200 μm. Hematite is currently being used as an oxygen carrier in our pilot-scale CLC unit (fluid bed) at the NETL. The particle size of hematite used in these tests is in the range of 100−300 μm. That is the reason why we used this particle size for our TGA tests. For a typical test, about 80 mg of hematite sample was heated in a platinum pan at a heating rate of 5 °C/min under N2 gas at a flow rate of 45 mL/min. The sample temperature ranging from 700 to 950 °C was maintained isothermally for 10 min prior to the reduction and oxidation cycles. The reduction−oxidation was conducted for 10 cycles with a reduction time of 20 min and an oxidation time of 30 min for all cycles. The system was flushed with ultrahigh-purity nitrogen for 10 min before and after each reaction segment. N2/O2 (air) used for the oxidation cycle was obtained from Butler Gas Products Co., Inc. The concentrations of H2, H2O, and O2 from the exit gas stream of the reactor were analyzed using a MS. The detailed analysis for selecting these conditions with no mass transfer or mixing issues has been described by Monazam et al.16 previously. Typical TGA experimental data on weight changes during reduction/oxidation cyclic tests at a given temperature are illustrated in Figure 1. In general, the performance becomes stable after about the fifth cycle, and we select the data after the fifth cycle for our analysis, averaging the data in cycles 5−10.
Figure 2. TGA experimental weight loss profiles obtained from the thermal reduction of hematite at different temperatures using 20% H2.
At these temperatures (700−950 °C), the weight reduction may proceed according to the following reactions: 3Fe2O3 + H 2 → 2Fe3O4 + H 2O
(1)
2Fe3O4 + 2H 2 → 6FeO + 2H 2O
(2)
According to reactions 1 and 2, hematite (Fe2O3) is reduced successively to magnetite (Fe3O4) and then to wüstite (FeO). Theoretically, weight changes in accordance with reaction stoichiometry for H2 reduction of iron oxide (Fe2O3) to FeO are a combination of reactions 1 and 2. Fe2O3 + H 2 → 2FeO + H 2O
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[ΔH800 °C = 28.89 kJ/mol,
RESULTS AND DISCUSSION Figure 2 illustrates the experimental weight loss curves obtained in TGA experiments at different temperatures (700−950 °C) using about 80 mg of Canadian hematite in 20% H2 with the balance N2. It can be seen in Figure 2 that the weight loss at the beginning of the reduction is fast, and then the reduction slows at the end of the cycle. It is also seen from Figure 2 that the weight loss increases significantly with the temperature. While weight loss of 10% was achieved in 20 min at 700 °C (Figure 2), the same weight loss was achieved after only 8 min at 950 °C. The weight change during reduction with H2 was due to oxygen removal of Fe2O3 to form H2O.
ΔG800 °C = −36.0 kJ/mol]
(3)
It was determined that the theoretical weight change for the transformation of Fe2O3 into Fe3O4, according to reaction 1 stoichiometry, corresponds to 3.3 wt % of the total sample. The transformation of Fe2O3 into FeO (reaction 3) and complete transformation to metallic iron correspond to theoretical weight decreases of 10 and 30 wt %, respectively. In this study, the weight of Fe2O3 was considered to be 94% of the hematite weight with the addition of 6% inerts. The extent of reduction was calculated using the following equation: 5407
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mo − m(t ) mo − mf
Article
(4)
where m(t) is the instantaneous weight of the solid during the exposure to H2. Parameters mo and mf are initial and final weights of the sample tested, respectively. In this study, the initial weight was considered as the weight of hematite and the final weight was considered as the weight of FeO (corresponding to a weight decrease of 10 wt %). Note that the resulting model, as will be shown, does not assume that the reduction occurs from one stage to the next only after completion of each stage but occurs simultaneously, as shown in eqs 1 and 2 above, to proceed in parallel, with some regions of the particle more reduced than others. Figure 3 clearly indicates that, with 20% H2, there is reduction of hematite (Fe2O3) to FeO−Fe mixtures (weight Figure 4. Mass spectral (ion current) data of H2O concentrations at the outlet and reduction conversion of Fe2O3 to FeO with 20% H2 and temperature range of 700−950 °C.
Figure 3. Effect of the reaction temperature on the reduction of hematite using 20, 10, and 5% H2 indication of the conversion limits corresponding to Fe3O4 and FeO.
Figure 5. Mass spectral (ion current) data of H2 concentrations at the outlet and reduction conversion of Fe2O3 to FeO with 20% H2 and temperature range of 700−800 °C.
loss of >10%) at all temperatures. However, Figure 3 also shows that, at lower H2 concentrations (10 and 5%), the weight loss is limited to 1 corresponds to a nucleation rate increasing with time.20 Using the repeat logarithm, eq 11 can be expanded to obtain eq 12.
(10)
is illustrated in Figure 8 at different temperatures for 20% inlet H2 concentration. The model and experimental data agreed
ln(− ln(1 − X )) = ln(k) + n ln(t )
(12)
The plot of ln(−ln(1 − X)) against ln(t) is illustrated in Figure 9 for different reaction temperatures (700−950 °C) using 20% H2 concentrations in the conversion range of 0.15 < X < 0.5. Figure 9 indicated that a nonlinear relationship exists, implying that the nucleation and growth (n values) are not constant as reduction proceeds. The n value can give detailed information on the nucleation and growth during the reduction process. The local n(X) can be calculated by differentiating eq 12. n(X ) =
∂ ln( −ln(1 − X )) ∂ ln(t )
(13)
Figure 10 shows the average n value at different temperatures as reduction conversion progressed. At the beginning of reduction, the value of n increases rapidly and tends to decrease after a maximum value, implying a decrease in the nucleation rate. As reduction increases beyond X = 0.33, the n value rises slowly, indicating that another kind of nucleation may be taking place (Fe3O4 to FeO). Therefore, it is obvious that local n values are not constant during the reduction of hematite with H2, usually exhibiting an increase at the beginning when X < 0.16, a decline when 0.16 < X < 0.33, and an increase when X > 0.33. Hence, the n value variations during the reduction process suggest possibility of a multi-step reaction mechanism (Fe2O3 to Fe3O4, X < 0.333; Fe3O4 to FeO, X > 0.333).
Figure 8. Comparison of predicted data (solid lines) on conversion as a function of time, using the power law kinetic model and experimental data (symbol) at different temperatures.
well, except for the initial period of about 2 min at all temperatures. Because the power law model did not describe the complete conversion specifically during the initial 2 min, other models that include nucleation and growth were evaluated as follows. The conversion−time plot usually illustrates a sigmoidal shape that can be separated to three distinct regions: induction/ incubation period (0 < X < 0.15), acceleration period (0.15 < X < 0.5), and decay period (0.5 < X < 1).20 However, as Figure 3 5410
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Figure 11. Activation energy values as a function of X.
With increasing X, the activation energy initially decreased from 23 to 8 kJ/mol (0.025 < X < 0.1), increased to 47 kJ/mol (0.1 < X < 0.33), and decreased to 30 kJ/mol (0.33 < X < 1). These variations suggest the changes of nucleation and growth during the reduction process. Therefore, the dependencies of activation energy with X also suggest that the process involves multi-step reactions with different activation energies. Because of the multi-step nature of reactions, as evidence by Figures 10 and 11, the introduction of a supplementary model (i.e., double reaction scheme) usually proves to be more advantageous than using a single-model analysis because it considerably improves the quality of fit.27 Because the singlereaction model (power law) did not completely describe the conversion data, a supplementary model was applied to describe the conversion data. In this study, it is assumed that, at all temperatures, there are two reaction fronts, each linked to one of the single reactions (Figure 12). The progress of XAC of the double reaction can be
Figure 9. Sharp−Hancock plots for two temperature ranges of (A) 700−800 °C and (B) 900 and 950 °C for 20% H2 concentration using 0.15 < X < 0.5.
Figure 12. Progression of the Fe2O3 reaction to FeO. Figure 10. Local n values with reduction conversion.
expressed as a function of the partial progresses of XAB and XBC of the single reaction and can be written as28 XAC = wABXAB + wBCXBC (15)
Knowledge of the dependence of activation energy on X can also assist in determining multi-step processes.26 If the activation energy does not vary significantly with X, the process can be adequately described as single-step kinetics. If the activation energy varies with X, the process has to be described as multi-step kinetics. The variation of the activation energy as a function of X can be obtained by taking the logarithm and rearranging eq 7 as ln t = ( −ln A + ln g (X )) + E /RT
where wAB and wBC are weight fractions corresponding to the oxygen loss of each single reaction, with wAB + wBC = 1. Therefore, the progress of the double reaction can be expressed as two equations, which can be a series and/or parallel linear combination of Avrami’s model. This can then simulate kinetics of a dual-reaction process with two physically differentiable kinetic structures. The equations involved in isothermal processes are the following:16,29 parallel process can be defined by eq 16 as nAB nBC XAC = wAB(1 − e−aABt ) + wBC(1 − e−aBCt ) X∞ (16)
(14)
By plotting ln t versus 1/T according to eq 14, the activation energies were found at any given X value from the slope of a regression line. Figure 11 provides the activation energy for the temperature range of 700−800 °C using 20% H2 concentration. 5411
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cases, the values of nAB and nBC were around 2 and 1, respectively. The reduction rate constant for reactant AB was obtained as
series process can be defined by eq 17 as ⎛ ⎞ ⎛ ⎞ XAC 1 1 = wAB⎜ nAB ⎟ + wBC⎜ nBC ⎟ ⎝ 1 − e−aBCt ⎠ ⎝ 1 − e−aABt ⎠ X∞
(17)
1/2 kAB (min−1) = aAB
The values of nAB and nBC define the type of reaction mechanism for the process. Figure 13 shows a typical fit using the reaction model for both parallel (eq 16) and series (eq 17) processes. The reaction
(18)
and the reduction rate constant for reactant BC was obtained as kBC (min−1) = aBC
(19)
A plot of ln k versus 1/T for reduction of hematite for both reaction fronts is illustrated in Figure 14 at different reaction
Figure 13. Typical curve fitting of experimental reduction data to series and parallel reaction models at 750 °C. Figure 14. Arrhenius plot for the two-step reaction front mechanism for the temperature range of 700−800 °C.
models in series consistently underpredict the early stage and overpredict the later stage of reduction, as illustrated in Figure 13, indicating that the reaction process is not a series process. However, the data from the reaction model with a parallel process were very compatible with the experimental data, as also shown in Figure 13. Therefore, in this study, the reaction model with a parallel process was used for analysis of reduction of hematite with H2. For a given temperature, values of X∞, wAB, aAB, aBC, nAB, and nBC were determined by curve fitting the rate data of Figure 8 with the parameters in eq 16 using TABLECURVE available from Statistical Package for the Social Sciences. The kinetic parameters obtained by fitting the TGA isothermal data are summarized in Table 2. It can be seen that the weight fraction of reactant 1 (wAB) decreased with increasing temperature for all H2 concentrations under study. At temperatures above 800 °C, the value of wAB becomes insignificant and was close to zero. Hence, at T > 800 °C, reactant 1 was eliminated from multi-step reactions, and therefore, conversion−time data were analyzed as a single reaction and will be described later. The values of exponent “n” for reactants AB and BC are best described by nAB = 2 and nBC = 1. On the other hand, with regard to the exponent n, no significant variation was found for these parameters as a function of temperatures or H2 concentrations. In all of the
temperatures and inlet H2 concentrations (20 and 10%). The apparent activation energies for both reaction fronts AB and BC were estimated to be 6.15 ± 0.2 and 56.9 ± 1.1 kJ/mol, respectively. The significantly lower activation energy of the reaction front AB indicates that it is a less temperaturedependent process in comparison to reaction front BC. Piotrowski et al.21 reported an activation energy of 58.13 kJ/ mol for reduction of hematite to wüstite in the temperature range of 700−900 °C. The reduction time data for the temperature range of 850− 950 °C was fitted with high accuracy with the following equation: n X = (1 − e−at ) X∞ (20) The kinetic parameters obtained from fitting the TGA isothermal data for the temperature range of 850−950 °C as defined by eq 20 are summarized in Table 3. The reduction rate constant k was obtained as k (min−1) = a1/ n = A e−E / RT
(21)
Table 2. Parameters of the Parallel Kinetics Model for Different Temperatures and H2 Concentrations 20% H2
10% H2
5% H2
T (°C)
700
750
800
700
750
800
700
750
800
X∞ WAB aAB nAB aBC nBC R2
1.71 0.068 0.519 2 0.0405 1 0.9998
1.52 0.060 0.562 2 0.0645 1 0.9998
1.54 0.040 0.600 2 0.0769 1 0.9998
1.62 0.0674 0.131 2 0.0190 1 0.9998
1.58 0.0632 0.141 2 0.0289 1 0.9997
1.51 0.0610 0.150 2 0.0368 1 0.9998
1.172 0.110 51 0.0205 2 0.011 88 1 0.9996
1.9333 0.079 94 0.0261 2 0.0086 1 0.9993
3.6 0.0339 0.0329 2 0.0057 1 0.9996
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Table 3. Parameters of the Single Kinetics Model for Different Temperatures and H2 Concentrations 20% H2
10% H2
5% H2
T (°C)
850
900
950
850
900
950
850
900
950
X∞ a n R2
1.41 0.095 1.05 1
1.28 0.084 1.27 0.9997
1.20 0.098 1.39 0.9999
1.34 0.060 1.00 0.9992
1.23 0.049 1.21 0.9997
1.13 0.044 1.39 0.9998
0.89 0.0220 1.09 0.999
1.05 0.0188 1.27 0.9996
1.06 0.0147 1.43 0.9998
temperature range of 850−950 °C are illustrated in Figure 17. The model data and experimental data agree over the entire
The linear regression analysis of the experimental data of ln k versus 1/T was used to determine E/R. Plots of ln k versus 1/T are shown in Figure 15 at different reaction temperatures
Figure 15. Arrhenius plot for the temperature range of 850−950 °C. Figure 17. Comparison of the experimental hematite reduction, X, data (symbols) to the multi-step reaction scheme (700−800 °C) and single nucleation/growth (850−950 °C) model using 20% H2.
conversion time, with overall variance (R2) greater than 99.9%. It should be noted that only a few chosen data points are shown at each temperature for clear illustration of the trend of the curves in Figure 17. The solid lines represent the model fit to experimental data (symbols). The present study indicates that the reduction of hematite with H2 cannot be described by a single model for all of the temperatures (700−950 °C) as reported in the literature. The significance of the present study was identifying the temperature regions and corresponding kinetic rate models specific to the temperature range. After the experimental data were compared to used models in detail, it was determined that a multi-step model is most suitable for the 700−800 °C range, while a single-step model is more suitable for the 850−950 °C range.
Figure 16. Effect of the H2 concentration on the reaction rate for the temperature range of 850−950 °C.
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SUMMARY The reduction kinetics of hematite with H2 was investigated by TGA and MS. In the temperature range of 700−800 °C, a multi-step kinetic rate model best defined the reduction process, which included an induction period and a nucleation/growth period. At higher temperatures (850−950 °C), the induction period becomes shorter and the nucleation/ growth period becomes longer, which resulted in single-step kinetic rate. The isothermal experimental data indicated that the nucleation is 1D with a decreasing nucleation rate. The JMA “n” value was found to vary within the range of 0.8−2 depending upon the temperature and H2 concentration. An empirical rate is developed to predict the course of reduction of hematite with H2 in CLC.
(850−950 °C) and H2 concentrations. When all of the values of k for different H2 concentrations are combined (Figure 16), the following equation is obtained: k (min−1) = 517.6yH exp( −7715.9/T ) 2
(22)
where yH2 is the mole fraction of H2. The apparent activation energy was estimated to be 64.15 ± 0.5 kJ/mol. Equation 22 also shows that the order of reaction with respect to the gaseous reactant (H2) was obtained to be 1 (Figure 16). This is similar to the stoichiometric ratios of H2 to Fe2O3 in eq 3. The comparative data for the experimental hematite reduction, X, data with 20% inlet H2 concentration using eq 16 for the temperature range of 700−800 °C and eq 20 for the 5413
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AUTHOR INFORMATION
Corresponding Author
*Telephone: 304-285-4486. Fax: 304-285-4403. E-mail: ronald.
[email protected]. Notes
Disclaimer: The U.S. DOE, NETL, and REM contributions to this paper were prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the U.S. DOE for funding the research through the office of Fossil Energy’s Gasification Technology and Advanced Research funding programs. Special thanks go to Duane D. Miller, Hanjing Tian, and Thomas Simonyi of URS Energy and Construction, Inc. for their assistance with experimental work and data.
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