Kinetics of Hematite to Wustite by Hydrogen for Chemical LoopingCombustionEsmail 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, WestVirginia 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 numberof 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 reductionconversion 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.

■ INTRODUCTION

The United States Department of Energy (U.S. DOE) has set agoal to modify the existing pulverized coal (PC)-fired powerplants to remove over 90% of the total carbon in the coal asCO2 for use or sequestration.1 Chemical looping combustion(CLC) is a promising combustion technology that produces asequestration-ready CO2 stream that can be retrofit to existingPC and circulating fluidized-bed (CFB) boilers as well as isapplicable to new installation.1 The CLC is a relatively newcombustion process that consists of a fuel and an air reactor. Inthe fuel reactor, the solid oxygen carrier is reduced by the fuelthat is converted to mainly CO2 and H2O, and in the airreactor, the reduced solid oxygen carrier is oxidized by O2 inthe air.2 The solid oxygen carrier is a cornerstone in the CLCprocess. The desirable properties for solid oxygen carriers byCLC systems are high reactivity in both reduction by fuel gasand oxidation by oxygen in the air, high resistance to attrition,low agglomeration, and low fragmentation.3 Additionally, it isalso an advantage if the metal oxide is low-cost andenvironmentally friendly.3 The reactivity of the four moststudied supported oxygen carriers is in the descending order ofNiO > CuO > Mn2O3 > Fe2O3.

3 Among the metal oxides, iron-based (i.e., Fe2O3) solid oxygen carriers are believed to be themost promising for commercial CLC application because theyare relatively inexpensive, readily available, and also environ-mentally safe compared to other metal oxides, such as NiO andCuO.4

Therefore, on the basis of the above advantages, the iron-based (natural hematite) metal oxide was selected as a solidoxygen 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 thana two-step mechanism, Fe2O3 → Fe3O4 → Fe,7−9 whenreacting with a gases, such as CO and H2. However, Slagtern etal.10 proposed a two-step reduction mechanism, Fe2O3 → FeO→ Fe, when reacting with H2.

Although there are several studies of hematite reduction withH2, CO, and mixtures of H2 and CO, kinetic studies of thehematite reduction with H2 at different concentrations arerather limited and lack certain details of the entire reductionprocess. When developing mathematical modeling of hematitereduction for designing a large-scale reactor system, it is veryimportant to understand the kinetics of the reduction processto define the operating parameters and better reactor control ofthe system.11

Moon et al.12 investigated the reduction mechanism ofhematite with a H2−CO gas mixture in the temperature rangeof 800−950 °C. They described the reduction behavior interms of a single rate-determining step; the reduction processwas initially controlled by the chemical reaction at the oxide−iron interface, while toward the end of the reaction, it wascontrolled by intraparticle diffusion. They also found that therate of reduction with H2 was 2−3 times higher than that withCO. Towhidi and Szekely13 studied the reduction kinetics ofcommercial-grade hematite with a CO−H2 mixture over thetemperature range of 600−1234 °C. They found that reductionwith mixed gases was affected by the gas compositions; a higherH2 content contributes to a higher rate of reduction. They alsoobserved significant carbon deposition below 780 °C,particularly at a higher CO concentration. Pang et al.14 studiedthe influence of the particle size of hematite on its reductionkinetics by H2 at a low temperature (450−600 °C). Their workshowed that, when the particle size was decreased from 107.5 to2.0 μm, the apparent activation energy decreased from 78.3 to36.9 kJ/mol during reduction at a given temperature. Pineau etal.15 characterized the reduction of hematite to magnetite withH2 by an apparent activation energy of 76 kJ/mol andmagnetite to metallic iron by apparent activation energies of 88

Received: May 14, 2014Revised: June 18, 2014Published: July 1, 2014

Article

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© 2014 American Chemical Society 5406 dx.doi.org/10.1021/ef501100b | Energy Fuels 2014, 28, 5406−5414

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 withH2, a complete understanding of the factors that control thereduction process for reactor modeling is necessary. Thecurrent study focuses on the effect of the temperature (700−950 °C) and H2 concentration on the detailed kinetics ofhematite reduction with H2.In our work on the reduction of hematite with methane, CO

and H2 were both product gases. Therefore, it was necessary toinvestigate the kinetic rates of individual gases (CO and H2) forcomputational fluid dynamics (CFD) modeling efforts at theNational Energy Technology Laboratory (NETL). We havepreviously reported the kinetic analysis for reduction ofhematite with CO and oxidation of the reduced hematiteusing air. In this study, we present the kinetic rates for H2 (5,10, and 20%) reduction of hematite. This range of H2concentration was chosen on the basis of expected values inthe reduction of hematite with methane.

■ EXPERIMENTAL SECTIONA thermogravimetric analysis (TGA, TA Model 2050) methodequipped with a mass spectrometer (MS, Pfeiffer Omnistar GSD-301) was used for reduction of commercial hematite in differentreductive atmospheres (5−20% H2 balance in N2). The hematiteparticles originating from the Wabush Mine, Canada. The hematite(94% Fe2O3 + 6% mineral) (the chemical analysis of the hematiteparticle is given by Monazam et al.16) was crushed in a lab to 100−300μm with an average size of 200 μm. Hematite is currently being usedas an oxygen carrier in our pilot-scale CLC unit (fluid bed) at theNETL. The particle size of hematite used in these tests is in the rangeof 100−300 μm. That is the reason why we used this particle size forour TGA tests. For a typical test, about 80 mg of hematite sample washeated in a platinum pan at a heating rate of 5 °C/min under N2 gas ata flow rate of 45 mL/min. The sample temperature ranging from 700to 950 °C was maintained isothermally for 10 min prior to thereduction and oxidation cycles. The reduction−oxidation wasconducted for 10 cycles with a reduction time of 20 min and anoxidation time of 30 min for all cycles. The system was flushed withultrahigh-purity nitrogen for 10 min before and after each reactionsegment. N2/O2 (air) used for the oxidation cycle was obtained fromButler Gas Products Co., Inc. The concentrations of H2, H2O, and O2from 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 duringreduction/oxidation cyclic tests at a given temperature are illustratedin Figure 1. In general, the performance becomes stable after about thefifth cycle, and we select the data after the fifth cycle for our analysis,averaging the data in cycles 5−10.

■ RESULTS AND DISCUSSION

Figure 2 illustrates the experimental weight loss curves obtainedin TGA experiments at different temperatures (700−950 °C)using about 80 mg of Canadian hematite in 20% H2 with thebalance N2. It can be seen in Figure 2 that the weight loss at thebeginning of the reduction is fast, and then the reduction slowsat the end of the cycle. It is also seen from Figure 2 that theweight loss increases significantly with the temperature. Whileweight loss of 10% was achieved in 20 min at 700 °C (Figure2), the same weight loss was achieved after only 8 min at 950°C. The weight change during reduction with H2 was due tooxygen removal of Fe2O3 to form H2O.

At these temperatures (700−950 °C), the weight reductionmay proceed according to the following reactions:

+ → +3Fe O H 2Fe O H O2 3 2 3 4 2 (1)

+ → +2Fe O 2H 6FeO 2H O3 4 2 2 (2)

According to reactions 1 and 2, hematite (Fe2O3) is reducedsuccessively to magnetite (Fe3O4) and then to wustite (FeO).Theoretically, weight changes in accordance with reaction

stoichiometry for H2 reduction of iron oxide (Fe2O3) to FeOare a combination of reactions 1 and 2.

+ → +

Δ =

Δ = −°

°

H

G

Fe O H 2FeO H O

[ 28.89 kJ/mol,

36.0 kJ/mol]

2 3 2 2

800 C

800 C (3)

It was determined that the theoretical weight change for thetransformation of Fe2O3 into Fe3O4, according to reaction 1stoichiometry, corresponds to 3.3 wt % of the total sample. Thetransformation of Fe2O3 into FeO (reaction 3) and completetransformation to metallic iron correspond to theoretical weightdecreases of 10 and 30 wt %, respectively. In this study, theweight of Fe2O3 was considered to be 94% of the hematiteweight with the addition of 6% inerts.The extent of reduction was calculated using the following

equation:

Figure 1. Typical mass and temperature measurement for the hematiteparticle of 200 μm using 20% H2 for reduction and air for oxidationreactions.

Figure 2. TGA experimental weight loss profiles obtained from thethermal reduction of hematite at different temperatures using 20% H2.

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=−−

Xm m t

m m( )o

o f (4)

where m(t) is the instantaneous weight of the solid during theexposure to H2. Parameters mo and mf are initial and finalweights of the sample tested, respectively. In this study, theinitial weight was considered as the weight of hematite and thefinal weight was considered as the weight of FeO (correspond-ing to a weight decrease of 10 wt %). Note that the resultingmodel, as will be shown, does not assume that the reductionoccurs from one stage to the next only after completion of eachstage but occurs simultaneously, as shown in eqs 1 and 2 above,to proceed in parallel, with some regions of the particle morereduced than others.Figure 3 clearly indicates that, with 20% H2, there is

reduction of hematite (Fe2O3) to FeO−Fe mixtures (weight

loss of >10%) at all temperatures. However, Figure 3 alsoshows that, at lower H2 concentrations (10 and 5%), the weightloss is limited to <10% at all of the temperatures, except at 950°C, which showed the formation of FeO from Fe2O3 with 10%H2. Therefore, to simplify the analysis in this study, thereduction conversion data were normalized to a value of 1 withrespect to the weight decrease of 10%. It should be noted thatthis does not assume that the particle is uniformly reduced. Theweight loss represents an average concentration. The particle islikely more reduced on the surface and less in the core.The reaction rate of hematite to magnetite (reaction 1) is

several orders of magnitude faster than that of reaction 2.17

This is also confirmed in this study, as shown in Figure 4, whichshows the variations of the product gas (H2O) concentrations(as measured by the MS) and the reduction conversion as afunction of time at different temperatures with inlet H2concentrations of 20%. The data in Figure 4 also show that,initially, the H2O concentration at the outlet increases rapidly,reaches a temporal maximum, and then increases slowly beforedecreasing to the final value. Figure 5 shows the H2concentration data at the TGA outlet and variation of reductionconversion at different temperatures (700−800 °C) with theinlet H2 concentration of 20%. The data in Figure 5 also showthat the H2 concentration at the outlet increases rapidly,reaches a temporal maximum, decreases slightly, and then

increases slowly to the final value. For clarity of Figure 5, thedata presented are limited to temperatures of 700−800 °C.Figures 4 and 5 also show that, when the conversion X

reaches 33.3%, there are initial peaks of both H2 and H2O,which means possibly the end of the process from hematite tomagnetite. There is a significant change of the slope of H2 andH2O concentrations when conversion X is above 33.3%,indicating the possible reaction of magnetite to wustite.

Kinetic Models. The reduction behavior of hematite inhydrogen has been investigated extensively, typically using athermogravimetric (TG) method. It was suggested that thereduction of hematite by hydrogen may proceed in one, two, oreven three steps, which mainly depends upon the reductionconditions employed and the type of sample used. Many kineticmodels, deduced from the spherical shrinking core model orthe formation and growth of the nuclei model, were employedas listed in Table 1, using the following rate equation:

=Xt

kf Xdd

( )(5)

where dX/dt represents the kinetic rate, t is the time, T is thetemperature, X is the extent of conversion, and f(X) is amathematical function that depends upon the kinetic modelused (see Table 1).

Figure 3. Effect of the reaction temperature on the reduction ofhematite using 20, 10, and 5% H2 indication of the conversion limitscorresponding to Fe3O4 and FeO.

Figure 4. Mass spectral (ion current) data of H2O concentrations atthe outlet and reduction conversion of Fe2O3 to FeO with 20% H2 andtemperature range of 700−950 °C.

Figure 5. Mass spectral (ion current) data of H2 concentrations at theoutlet and reduction conversion of Fe2O3 to FeO with 20% H2 andtemperature range of 700−800 °C.

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In eq 5, k is the rate constant, which is given as

= −⎜ ⎟⎛⎝

⎞⎠k A

ERT

exp(6)

For reaction kinetics under isothermal conditions, eq 5 can beanalytically integrated to yield

∫= =g XX

f Xkt( )

d( )

X

0 (7)

where g(X) is an integral mathematical expression related tomechanisms of solid-phase reactions.Therefore, the values of the kinetic rate constant k can be

determined at different temperatures from the slope of thestraight line obtained by plotting g(X) against time. Thesevalues can be subsequently inserted in the Arrhenius equationtogether with the corresponding temperature values to yieldactivation energy and pre-exponential factor values from theslope and intercept of the regression straight line.The proposed models in Table 1 are for a single-step

reaction process; therefore, if a straight line is not obtained, itimplies that the reaction proceeds via a multi-step process orthe reaction mechanism is not included in the proposed modelsidentified in Table 1.The TGA data were evaluated with all of the models listed in

Table 1. As illustrated by Figure 6, only one of the expressions(4) listed in Table 1, which corresponds to the power lawkinetic model, provided straight lines, except for the initial 2min. This may indicate that the hematite reduction rate with H2follows power law kinetics for the whole time period after 2min as

=Xt

kX

dd

23 1/2 (8)

Then, the pre-exponential and activation energy can beobtained by plotting ln k (k values were obtained from Figure6) versus T−1, as illustrated by Figure 7. Therefore, thetemperature dependence of the rate constants by combining allof the values of k for different H2 concentrations (Figure 7) canbe expressed by the following equation:

= −−k y T(min ) 33.3 exp( 4727.3/ )1H2 (9)

where yH2is the mole fraction of H2. The activation energy was

found to be 39.3 ± 3.08 kJ/mol. Moon et al.12 obtained anactivation energy of 42.12 kJ/mol for reduction of hematitewith 100% H2 in the temperature range of 800−950 °C, whichis comparable to this study. Gray and Henderson18 studied theeffect of temperatures (400−1000 °C) on H2 reduction ofdense hematite using a particle size of 0.07−10 mm. They

Table 1. Basic Kinetic Models and Properties of f(X) and g(X) Functions

number kinetic model f(X) g(X)

1 power law 4X3/4 X1/4

2 3X2/3 X1/3

3 2X1/2 X1/2

4 2/3X−1/2 X3/2

contraction model5 zero order 1 X6 2D 2(1 − X)1/2 (1 − (1 − X)1/2)7 3D 3(1 − X)2/3 (1 − (1 − X)1/3)

kinetics order models8 first order (1 − X) −ln(1 − X)9 3/2 order (1 − X)3/2 2((1 − X)−1/2 − 1)10 second order (1 − X)2 (1 − X)−1 − 111 third order (1 − X)3 1/2((1 − X)−2 − 1)

nucleation model12 n = 1.5 3/2(1 − X)(−ln(1 − X))1/3 (−ln(1 − X))2/3

13 n = 2 2(1 − X)(−ln(1 − X))1/2 (−ln(1 − X))1/2

14 n = 3 3(1 − X)(−ln(1 − X))2/3 (−ln(1 − X))1/3

15 n = 4 4(1 − X)(−ln(1 − X))3/4 (−ln(1 − X))1/4

diffusion model16 1D 1/(2X) X2

17 2D 1/(−ln(1 − X)) (1 − X)ln(1 − X) + X18 3D (Jander) (3/2)(1 − X)2/3(1 − (1 − X)1/3) (1 − (1 − X)1/3)2

19 3D (Grinstling) (3/2)((1 − X)−1/3 − 1) (1 − 2X/3) − (1 − X)2/3

aThis table includes kinetic-controlled (contraction model 7) and diffusion-controlled (diffusion model 19) shrinking core models (SCMs)

Figure 6. Examination of the linear relationship based on the powerlaw reaction model.

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reported an activation energy of 38.04 kJ/mol for temperaturesof greater than 570 °C. Piotrowski et al.19 reported anactivation energy of 28.08 kJ/mol for reduction of Fe2O3 + 10%H2 + 90% N2 → FeO in the temperature range of 700−900 °C.The value of the pre-exponential increased with increasing

the H2 inlet concentration. The apparent activation energy, E,was fairly constant for different H2 concentrations, as evidencedby the parallel lines of ln k versus 1/T in Figure 7.Equation 9 also shows that the order of reaction with respect

to the gaseous reactant (H2) was obtained to be 1. This issimilar to the stoichiometric ratios of H2/Fe2O3 in eq 3.The comparison of the experimental hematite reduction, X,

data to the power law reaction model as presented by thefollowing equation:

=X kt( )2/3(10)

is illustrated in Figure 8 at different temperatures for 20% inletH2 concentration. The model and experimental data agreed

well, except for the initial period of about 2 min at alltemperatures. Because the power law model did not describethe complete conversion specifically during the initial 2 min,other models that include nucleation and growth wereevaluated 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

illustrates, the induction/incubation period occurs over therange (0 < X < 0.1), the acceleration period occurs over therange (0.1 < X < 0.33), and the decay period occurs over therange (0.33 < X < 1). It is observed that, with an increase of thereaction temperature, the conversion−time plot exhibits thedisappearances of the induction period, especially at temper-atures above 800 °C (Figure 5). This was also observed byPiotrowski et al.21 on their kinetics study of reduction ofhematite to wustite using CO.To test the multi-step reaction rate of conversion−time data,

the approach by Hancock and Sharp [plot of ln(−ln(1 − X))versus ln t] was used for deviation from a straight line(especially in the acceleration period, 0.15 < X < 0.5). Hancockand Sharp19 used a generalized Johnson−Mehl−Avrami(JMA)22−25 model as the classical method for analyzing alltypes of kinetic data for gas−solid reactions. The conversion−time as described in the generalized JMA model is

= − −X 1 e kt n(11)

where X is the reduction conversion at time t, k is the overallrate constant, and n is the kinetic exponent, which dependsupon the mechanism of growth and the dimensionality of thenuclei. The values of n define the type of reaction mechanismfor the process and can be interpreted as the sum of α + β,where α is the dimensionality of growth [an integer value ofone-dimensional (1D), two-dimensional (2D), or three-dimen-sional (3D)] and β is the contribution of the nucleation processto the overall kinetics, where β = 0 corresponds toinstantaneous nucleation (nucleation rate is zero, i.e., constantdensity of the nucleation site), 0 < β < 1 corresponds to asporadic nucleation rate decreasing with time, β = 1corresponds to a constant nucleation rate, and β > 1corresponds to a nucleation rate increasing with time.20

Using the repeat logarithm, eq 11 can be expanded to obtaineq 12.

− − = +X k n tln( ln(1 )) ln( ) ln( ) (12)

The plot of ln(−ln(1 − X)) against ln(t) is illustrated in Figure9 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, implyingthat the nucleation and growth (n values) are not constant asreduction proceeds. The n value can give detailed informationon the nucleation and growth during the reduction process.The local n(X) can be calculated by differentiating eq 12.

= ∂ − −∂

n XX

t( )

ln( ln(1 ))ln( ) (13)

Figure 10 shows the average n value at different temperaturesas reduction conversion progressed. At the beginning ofreduction, the value of n increases rapidly and tends todecrease after a maximum value, implying a decrease in thenucleation rate. As reduction increases beyond X = 0.33, the nvalue rises slowly, indicating that another kind of nucleationmay be taking place (Fe3O4 to FeO). Therefore, it is obviousthat local n values are not constant during the reduction ofhematite with H2, usually exhibiting an increase at thebeginning when X < 0.16, a decline when 0.16 < X < 0.33,and an increase when X > 0.33. Hence, the n value variationsduring the reduction process suggest possibility of a multi-stepreaction mechanism (Fe2O3 to Fe3O4, X < 0.333; Fe3O4 toFeO, X > 0.333).

Figure 7. Arrhenius plot for reduction of hematite to wustite based onthe power law kinetic reaction model.

Figure 8. Comparison of predicted data (solid lines) on conversion asa function of time, using the power law kinetic model andexperimental data (symbol) at different temperatures.

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Knowledge of the dependence of activation energy on X canalso assist in determining multi-step processes.26 If theactivation energy does not vary significantly with X, the processcan be adequately described as single-step kinetics. If theactivation energy varies with X, the process has to be describedas multi-step kinetics. The variation of the activation energy as afunction of X can be obtained by taking the logarithm andrearranging eq 7 as

= − + +t A g X E RTln ( ln ln ( )) / (14)

By plotting ln t versus 1/T according to eq 14, the activationenergies were found at any given X value from the slope of aregression line. Figure 11 provides the activation energy for thetemperature range of 700−800 °C using 20% H2 concentration.

With increasing X, the activation energy initially decreased from23 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). Thesevariations suggest the changes of nucleation and growth duringthe reduction process. Therefore, the dependencies ofactivation energy with X also suggest that the process involvesmulti-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 moreadvantageous than using a single-model analysis because itconsiderably improves the quality of fit.27 Because the single-reaction model (power law) did not completely describe theconversion data, a supplementary model was applied todescribe 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

expressed as a function of the partial progresses of XAB and XBCof the single reaction and can be written as28

= +X w X w XAC AB AB BC BC (15)

where wAB and wBC are weight fractions corresponding to theoxygen loss of each single reaction, with wAB + wBC = 1.Therefore, the progress of the double reaction can be expressedas two equations, which can be a series and/or parallel linearcombination of Avrami’s model. This can then simulate kineticsof a dual-reaction process with two physically differentiablekinetic structures. The equations involved in isothermalprocesses are the following:16,29

parallel process can be defined by eq 16 as

= − + −∞

− −XX

w e w e(1 ) (1 )a t a tACAB BC

n nAB

ABBC

BC

(16)

Figure 9. Sharp−Hancock plots for two temperature ranges of (A)700−800 °C and (B) 900 and 950 °C for 20% H2 concentration using0.15 < X < 0.5.

Figure 10. Local n values with reduction conversion.

Figure 11. Activation energy values as a function of X.

Figure 12. Progression of the Fe2O3 reaction to FeO.

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series process can be defined by eq 17 as

=−

+−∞

− −

⎛⎝⎜

⎞⎠⎟

⎛⎝⎜

⎞⎠⎟

XX

we

we

11

11a t a t

ACAB BCn n

ABAB

BCBC

(17)

The values of nAB and nBC define the type of reactionmechanism 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

models in series consistently underpredict the early stage andoverpredict the later stage of reduction, as illustrated in Figure13, indicating that the reaction process is not a series process.However, the data from the reaction model with a parallelprocess were very compatible with the experimental data, asalso shown in Figure 13. Therefore, in this study, the reactionmodel with a parallel process was used for analysis of reductionof 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 8with the parameters in eq 16 using TABLECURVE availablefrom Statistical Package for the Social Sciences.The kinetic parameters obtained by fitting the TGA

isothermal data are summarized in Table 2. It can be seenthat the weight fraction of reactant 1 (wAB) decreased withincreasing temperature for all H2 concentrations under study.At temperatures above 800 °C, the value of wAB becomesinsignificant and was close to zero. Hence, at T > 800 °C,reactant 1 was eliminated from multi-step reactions, andtherefore, conversion−time data were analyzed as a singlereaction 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, nosignificant variation was found for these parameters as afunction of temperatures or H2 concentrations. In all of the

cases, the values of nAB and nBC were around 2 and 1,respectively.The reduction rate constant for reactant AB was obtained as

=−k a(min )AB1

AB1/2

(18)

and the reduction rate constant for reactant BC was obtained as

=−k a(min )BC1

BC (19)

A plot of ln k versus 1/T for reduction of hematite for bothreaction fronts is illustrated in Figure 14 at different reaction

temperatures and inlet H2 concentrations (20 and 10%). Theapparent activation energies for both reaction fronts AB and BCwere estimated to be 6.15 ± 0.2 and 56.9 ± 1.1 kJ/mol,respectively. The significantly lower activation energy of thereaction front AB indicates that it is a less temperature-dependent 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 wustite in the temperaturerange of 700−900 °C.The reduction time data for the temperature range of 850−

950 °C was fitted with high accuracy with the followingequation:

= −∞

−XX

(1 e )at n

(20)

The kinetic parameters obtained from fitting the TGAisothermal data for the temperature range of 850−950 °C asdefined by eq 20 are summarized in Table 3.The reduction rate constant k was obtained as

= =− −k a A(min ) en E RT1 1/ /(21)

Figure 13. Typical curve fitting of experimental reduction data toseries and parallel reaction models at 750 °C.

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∞ 1.71 1.52 1.54 1.62 1.58 1.51 1.172 1.9333 3.6WAB 0.068 0.060 0.040 0.0674 0.0632 0.0610 0.110 51 0.079 94 0.0339aAB 0.519 0.562 0.600 0.131 0.141 0.150 0.0205 0.0261 0.0329nAB 2 2 2 2 2 2 2 2 2aBC 0.0405 0.0645 0.0769 0.0190 0.0289 0.0368 0.011 88 0.0086 0.0057nBC 1 1 1 1 1 1 1 1 1R2 0.9998 0.9998 0.9998 0.9998 0.9997 0.9998 0.9996 0.9993 0.9996

Figure 14. Arrhenius plot for the two-step reaction front mechanismfor the temperature range of 700−800 °C.

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The linear regression analysis of the experimental data of ln kversus 1/T was used to determine E/R. Plots of ln k versus 1/Tare shown in Figure 15 at different reaction temperatures

(850−950 °C) and H2 concentrations. When all of the values ofk for different H2 concentrations are combined (Figure 16), thefollowing equation is obtained:

= −−k y T(min ) 517.6 exp( 7715.9/ )1H2 (22)

where yH2is the mole fraction of H2. The apparent activation

energy was estimated to be 64.15 ± 0.5 kJ/mol. Equation 22also shows that the order of reaction with respect to thegaseous reactant (H2) was obtained to be 1 (Figure 16). This issimilar 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 eq16 for the temperature range of 700−800 °C and eq 20 for the

temperature range of 850−950 °C are illustrated in Figure 17.The model data and experimental data agree over the entire

conversion time, with overall variance (R2) greater than 99.9%.It should be noted that only a few chosen data points are shownat each temperature for clear illustration of the trend of thecurves in Figure 17. The solid lines represent the model fit toexperimental data (symbols).The present study indicates that the reduction of hematite

with H2 cannot be described by a single model for all of thetemperatures (700−950 °C) as reported in the literature. Thesignificance of the present study was identifying the temper-ature regions and corresponding kinetic rate models specific tothe temperature range. After the experimental data werecompared to used models in detail, it was determined that amulti-step model is most suitable for the 700−800 °C range,while a single-step model is more suitable for the 850−950 °Crange.

■ SUMMARYThe reduction kinetics of hematite with H2 was investigated byTGA and MS. In the temperature range of 700−800 °C, amulti-step kinetic rate model best defined the reductionprocess, which included an induction period and anucleation/growth period. At higher temperatures (850−950°C), the induction period becomes shorter and the nucleation/growth period becomes longer, which resulted in single-stepkinetic rate. The isothermal experimental data indicated thatthe nucleation is 1D with a decreasing nucleation rate. TheJMA “n” value was found to vary within the range of 0.8−2depending upon the temperature and H2 concentration. Anempirical rate is developed to predict the course of reduction ofhematite with H2 in CLC.

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∞ 1.41 1.28 1.20 1.34 1.23 1.13 0.89 1.05 1.06a 0.095 0.084 0.098 0.060 0.049 0.044 0.0220 0.0188 0.0147n 1.05 1.27 1.39 1.00 1.21 1.39 1.09 1.27 1.43R2 1 0.9997 0.9999 0.9992 0.9997 0.9998 0.999 0.9996 0.9998

Figure 15. Arrhenius plot for the temperature range of 850−950 °C.

Figure 16. Effect of the H2 concentration on the reaction rate for thetemperature 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) andsingle nucleation/growth (850−950 °C) model using 20% H2.

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■ AUTHOR INFORMATIONCorresponding Author*Telephone: 304-285-4486. Fax: 304-285-4403. E-mail: ronald.breault@netl.doe.gov.NotesDisclaimer: The U.S. DOE, NETL, and REM contributions tothis paper were prepared as an account of work sponsored byan agency of the United States Government. Neither theUnited States Government nor any agency thereof, nor any oftheir employees, makes any warranty, express or implied, orassumes any legal liability or responsibility for the accuracy,completeness, or usefulness of any information, apparatus,product, or process disclosed, or represents that its use wouldnot infringe privately owned rights. Reference herein to anyspecific commercial product, process, or service by trade name,trademark, manufacturer, or otherwise does not necessarilyconstitute or imply its endorsement, recommendation, orfavoring by the United States Government or any agencythereof. The views and opinions of authors expressed herein donot necessarily state or reflect those of the United StatesGovernment or any agency thereof.The authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors acknowledge the U.S. DOE for funding theresearch through the office of Fossil Energy’s GasificationTechnology and Advanced Research funding programs. Specialthanks go to Duane D. Miller, Hanjing Tian, and ThomasSimonyi of URS Energy and Construction, Inc. for theirassistance with experimental work and data.

■ REFERENCES(1) Adrus, H. E. J.; Chiu, J. H.; Thibeault, P. R.; Brutsch, A. Alstom’scalcium oxide chemical looping combustion coal power technologydevelopment. Proceedings of the 34th International Technical Conferenceon Clean Coal and Fuel Systems; Clearwater, FL, May 31−June 4, 2009.(2) Richter, H. J.; Knoche, K. F. Reversibility of combustionprocesses. In Efficiency and Costing: Second Law Analysis of Processes;Gaggioli, R. A., Ed.; American Chemical Society: Washington, D.C.,1983, ACS Symposium Series, Vol. 235, Chapter 3, pp 71−85.(3) Fang, H.; Haibin, L.; Zengli, Z. Int. J. Chem. Eng. 2009,No. 710515, DOI: 10.1155/2009/710515.(4) Hossain, M. M.; Sedor, K. E.; de Lasa, H. I. Chem. Eng. Sci. 2007,62, 5464−5472.(5) Jozwiak, W. K.; Kaczmarek, E. T.; Maniecki, P.; Ignaczak, W.;Maniukiewicz, W. Appl. Catal., A 2007, 326, 17−27.(6) Bukur, D. B.; Lang, X.; Rossin, J. A.; Zimmerman, W. H.;Rosynek, M. P.; Yeh, E. B.; Li, Ch. Ind. Eng. Chem. Res. 1989, 28,1130−1140.(7) Munteanu, G.; Ilieva, L.; Andreeva, D. Thermochim. Acta 1997,291, 171−177.(8) Lin, H. Y.; Chen, Y. W.; Li, Ch. Thermochim. Acta 2003, 400, 61−67.(9) Wimmers, O. J.; Arnoldy, P.; Moulijn, J. A. J. Phys. Chem. 1986,90, 1331−1337.(10) Slagtern, A.; Swaan, H. M.; Olsbye, U.; Dahl, I. M.; Mirodatos,C. Catal. Today 1998, 46, 107−115.(11) Bonalde, A.; Henriquez, A.; Manrique, M. ISIJ Int. 2005, 45 (9),1255−1260.(12) Moon, I. J.; Rhee, C. H.; Min, D. J. Steel Res. 1998, 69 (8), 302−306.(13) Towhidi, N.; Szekely, J. Ironmaking Steelmaking 1981, 6, 237−249.(14) Pang, J-m.; Guo, P-r.; Zhao, P.; Cao, C-z.; Zhang, D-w. J. IronSteel Res. Int. 2009, l6 (5), 7−11.

(15) Pineau, A.; Kanari, N.; Gaballah, I. Thermochim. Acta 2006, 447,89−100.(16) Monazam, E. R.; Breault, R. W.; Siriwardane, R.; Richards, G.;Carpender, S. Chem. Eng. J. 2013, 232, 478−487.(17) Hou, B.; Zhang, H.; Li, H.; Zhu, Q. Chin. J. Chem. Eng. 2012, 20(1), 10−17.(18) Gray, N. B.; Henderson, J. Trans. Metall. Soc. AIME 1966, 236,1213−1217.(19) Piotrowski, K.; Mondal, K.; Lorethova, H.; Stonawski, L.;Szyman ski, T.; Wiltowski, T. Int. J. Hydrogen Energy 2005, 30, 1543−1554.(20) Liu, H.; Sullivan, R. M.; Hanson, J. C.; Grey, C. P.; Martin, J. D.J. Am. Chem. Soc. 2001, 123, 7564−7573.(21) Piotrowski, K.; Mondal, K.; Wiltowski, T.; Dydo, P.; Rizeg, G.Chem. Eng. J. 2007, 131 (1−3), 73−82.(22) Johnson, W. A.; Mehl, R. F. Trans. Am. Inst. Min. Eng. 1939, 135,416.(23) Avrami, M. J. Chem. Phys. 1939, 7, 1103−1112.(24) Avrami, M. J. Chem. Phys. 1940, 8, 212−224.(25) Avrami, M. J. Chem. Phys. 1941, 9, 177−184.(26) Jankovic, B.; Adnađevic, B.; Jovanovic, J. Thermochim. Acta2007, 452, 106−115.(27) Roduit, B. Thermochim. Acta 2000, 355, 171−180.(28) Bessieres, J.; Bessieres, A.; Heizmann, J. J. Int. J. Hydrogen Energy1980, 5, 585−595.(29) Velisaris, C. N.; Sefeis, J. C. Polym. Eng. Sci. 1986, 26, 1574−1581.

Energy & Fuels Article

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