Applied Energy 190 (2017) 1258–1274

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Applied Energy

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The redox reaction kinetics of Sinai ore for chemical looping combustionapplications

http://dx.doi.org/10.1016/j.apenergy.2017.01.0260306-2619/� 2017 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: eksepko@ichpw.pl (E. Ksepko).

Ewelina Ksepko a,⇑, Piotr Babinski a, Lori Nalbandian b

a Institute for Chemical Processing of Coal, 1 Zamkowa, 41-803 Zabrze, PolandbCenter for Research and Technology Hellas/Chemical Process and Energy Resources Institute, 6th km Harilaou – Thermi Rd, P.O. Box 60361, 57001 Thermi, Thessaloniki, Greece

h i g h l i g h t s

� Redox reaction kinetics of Fe-Mn-rich Sinai ore was determined by TGA.� The most suitable model for reduction was D3, while R3 for oxidation.� Activation energies 35.3 and 16.70 kJ/mole were determined for reduction and oxidation.� Repetitive redox reactions favor the formation of spinel phases in Sinai ore.� Multiple redox cycles induce formation of extensive porosity of the particles.

a r t i c l e i n f o

Article history:Received 25 August 2016Received in revised form 12 January 2017Accepted 14 January 2017Available online 25 January 2017

Keywords:Chemical looping combustionKineticsSinai oreFe–Mn-based oreNatural OCsIlmenite

a b s t r a c t

The objective of this work was to study the use of Sinai ore, a Fe–Mn-based ore from Egypt, as a low-costoxygen carrier (OC) in Chemical Looping Combustion (CLC). The Sinai ore was selected because it pos-sesses relatively high amounts of iron and manganese oxides. Furthermore, those oxides have low cost,very favorable environmental and thermodynamic properties for the CLC process. The performance of theSinai ore as an OC in CLC was compared to that of ilmenite (Norway Tellnes mine), the most extensivelystudied naturally occurring Fe-based mineral.The kinetics of the reduction and oxidation reactions with the two minerals were studied using a ther-

mogravimetric analyzer (TGA). Experiments were conducted under isothermal conditions, with multipleredox cycles, at temperatures between 750 and 950 �C. For the reduction and oxidation reactions, differ-ent concentrations of CH4 (10–25 vol.%) and O2 (5–20 vol.%) were applied, respectively. The kineticparameters, such as the activation energy (Ea), pre-exponential factor (A0), and reaction order (n), weredetermined for the redox reactions. Furthermore, models of the redox reactions were selected by meansof a model-fitting method. For the Sinai ore, the D3 model (3-dimensional diffusion) was suitable formodeling reduction reaction kinetics. The calculated Ea was 35.3 kJ/mole, and the reaction order wasdetermined to be approximately 0.76. The best fit for the oxidation reaction was obtained for the R3model (shrinking core). The oxidation (regeneration) reaction Ea was equal to 16.7 kJ/mole, and the deter-mined reaction order was approximately 0.72.The crystalline phases present, as well as the morphology and inhomogeneities in elemental composi-

tion were studied for both materials, fresh as well as after multiple redox cycles, by X-ray Diffraction(XRD) and Scanning Electron Microscopy (SEM) combined with X-ray Microanalysis - EnergyDispersive Spectroscopy (EDS). Structural and morphological changes were detected and correlated tothe reaction temperature as well as the reactant compositions and thus the stability of the ores in repet-itive CLC cycles was determined.

� 2017 Elsevier Ltd. All rights reserved.

1. Introduction

Chemical looping combustion (CLC) has been shown to be oneof most promising technologies for effective CO2 capture. In theCLC process, solid oxides are used to transport oxygen to fuelinstead of directly mixing air and fuel. The significant advantage

E. Ksepko et al. / Applied Energy 190 (2017) 1258–1274 1259

of CLC is that a concentrated CO2 stream can be obtained from thecombustion gas stream after water condensation without requiringany energy penalty for the separation or purification [1–4]. Addi-tionally, NOx emissions are also substantially reduced. Those twoare the main benefits of the CLC technology compared to otherknown CCS technologies [5,6].

In CLC, the ‘‘oxygen carriers” (OCs), that supply the oxygenrequired for fuel combustion, are transition metal oxides or theirmixtures. Potential OCs include numerous compositions of Ni, Cu,Fe, or Co oxides that are used as active materials, and they containSi, Al, Ti, or Zr oxides or lower-cost natural inert materials, such assepiolite [3,7–10]. Inert materials are added to extend the OCs’ life-times by reducing their attrition or improving their thermal resis-tivity. A suitable OC for the CLC process must have high reactivity,high selectivity for CO2 and H2O, suitable oxygen-transport capac-ity, high mechanical strength and low agglomeration tendency. Aneffective OC material must meet these expectations during cyclingreduction and oxidation reactions at high temperatures in a CLC-based power plant. High attrition resistance and high meltingpoints would also be favorable [11]. Furthermore, many challengesare associated with direct coal/biomass CLC using OCs because ofpossible partial loss of the carrier with the coal/biomass ash whichcan have both, environmental and economic consequences [12,13].

Suitable synthetic OC materials have been thus far extensivelydeveloped [3,7–9,14–16].

Recently some of emerging examples show that bimetallic com-pounds such as Fe-Mn oxides demonstrate both high reactivity andinteresting thermodynamic properties [11,17–19]. There are alsomany other advantages of using both Fe and Mn oxides as OCs.Both oxides are available at low cost, and they create minimalhealth and environmental concerns [18]. Ryden et al. [17] con-cluded that combined oxides of iron and manganese have interest-ing thermodynamic properties and could potentially be suitable forCLC applications. However, they also found that the physical stabil-ity of material was low. To address this problem, Ryden et al. [17]have suggested that improved stability could be achieved by theaddition of inert material. Ksepko et al. [18] showed that thebimetallic Fe-Mn oxides supported on inerts such as ZrO2, Sepio-lite, and Al2O3 were found to be promising OCs for synthesis gascombustion process. Comparison of Fe-Mn data with that ofmonometallic OCs supported that the addition of Mn oxide hadencouraging effect on stability over multi-cycle redox reactions.Furthermore, the presence of contaminant H2S in syngas did notappear to affect the oxygen carriers stability during tests, sincethe successful regeneration was achieved after cycling. Källénet al. [19] proved that combined iron/manganese/silicon oxidescould continuously operate with syngas and natural gas in flu-idized bed.

Lately there is increasing interest in using naturally occurringOCs, especially because, wide availability, and substantially lowerenvironmental impact than conventional Ni- or even Cu-basedOCs, and lower potential impacts on human health [11,3]. Finallythey are attractive because of their low cost of production [13]. Alower OC material cost would also influence the fact that the attri-tion rate of OCs will become of less importance during the process[19]. Some of ores require only simple crushing and sieving whichis particularly beneficial for overall CLC cost. However, althoughthe use of naturally existing materials as OCs is very promising,much less research work has been performed thus far, comparedto the research concerning synthetic OCs [3]. One natural materialthat has been extensively examined is ilmenite (FeTiO3); non-activated ilmenite has been shown to react slowly with H2/H2O,CO/CO2 and CH4/H2O [20,21], however its reactivity increases withthe number of redox cycles. Complete activation can be achievedafter 6 redox cycles. Recently, some other low-cost, natural mate-rials have been investigated in the literature. Wastes of various ori-

gins [13,22], minerals [11,23–27] and concentrates from minerals[25,28] have been tested as cost-effective OCs for CLC. Tian et al.[23] examined limonite (Fe-based) and chrysocolla (Cu-based),which both demonstrated excellent reactivity and stability in 50-cycle CLC thermogravimetric analysis (TGA) tests with methaneas a fuel. Other studies [29,30] indicated the potential of Mn-based ores. Fossdal et al. [29] tested a manganese ore, whichshowed a maximum oxygen capacity of 4.9 wt% at 1000 �C. Theywere able to further enhance the kinetics, chemical and mechani-cal stability and methane-conversion rate by adding excess cal-cium to the natural ore. Low-grade Sinai ore, a Fe-Mn-basedmineral [31], was reported recently as a promising OC material[11]. During the study, a screening test was performed, involvingredox cycle testing, at TGA, of 3 cheap Fe- and Mn-based minerals.20 cycles with wet 5%H2 + 25%CO2 followed by 20 cycles with wet10% CH4 + 25% CO2 at 800–1000 �C was conducted to identify thebest OC material. The Sinai-A ore demonstrated better reactionrates and higher capacities than ilmenite. It released oxygen moreeasily because of its Mn content and achieved a higher degree ofgas conversion with a lower oxygen demand.

The objective of this work is to continue the evaluation of thelow-grade Sinai ore, as a promising OC material, by analyzing thekinetics of both reduction - oxidation reactions, as well as its reac-tivity and structural and morphological stability. Furthermore, theresults of Sinai ore are compared to the corresponding results ofpre-oxidized and activated ilmenite ore, obtained both during thisstudy, under identical experimental conditions and from literature[20,21].

Multicycle CLC tests were conducted in Thermogravimetric ana-lyzer to determine the reaction performance of the natural OCs andthe kinetic parameters of both reduction with CH4 and oxidationwith O2-mixtures. Methane was chosen as a fuel for thelaboratory-scale experiments because it allows the study of CLCwith both gaseous and solid fuels. During the chemical loopingcombustion of coal a sequence of different processes and reactionsare occurring as it can be seen in Fig. 1.

The first step is the coal pyrolysis when volatile matter isreleased and in-situ char is obtained. Methane is the major compo-nent in the gas obtained during coal pyrolysis [3,32]. The next stepis the gasification of in-situ char by CO2 and H2O when CO and H2

are produced. All the combustible gas compounds react with thesolid OC and are oxidized by it. Furthermore, methane has the low-est reactivity among all combustible gases of coal pyrolysis [3].

The obtained kinetic parameters, including the activationenergy (Ea), pre-exponential factor (A0) and reaction order (n),can be valuable for the design of CLC systems using Sinai ore asan OC. All the solid samples before and after multiple redox cycleswere physicochemically characterized by Inductively coupledplasma optical emission spectrometry (ICP–OES), X-ray Diffraction(XRD), N2 adsorption isotherms and SEM combined with X-rayEDS, in order to determine the retention of their structural andmorphological features.

2. Materials and methods

2.1. Oxygen carrier (OC) preparation

Sinai Manganese ore grade A (low grade), is a Fe-Mn-basedmaterial from the Um Bogma mine, Sinai, Egypt (one of the twomajor localities where economical deposits of manganese are pre-sent in Egypt as sandstone that is usually ferruginous and mangan-iferous) [31]. Ilmenite, was obtained from Norway’s Tellnes mine(one of the largest titanium mines in Norway).

For the reactivity tests and kinetics study of both ores, the 125–180-lm fraction was used. Prior to the TGA study, samples of both

Pyrolysis

CO2, H2O

Vola�le ma�er: H2, CH4, CO, CO2,

CnHm, H2O

CO, H2

Gasifica�on

Oxidized oxygen carrier

Char

Coal

CO2, H2O

Reduced oxygen carrier

Fig. 1. Graphical representation of coal CLC process.

1260 E. Ksepko et al. / Applied Energy 190 (2017) 1258–1274

ores were subjected to thermal treatment; the Sinai samples werecalcined at 950 �C, for 2 h, in air while the ilmenite samples at950 �C, for 8 h, in air. The calcination prior to testing is very impor-tant especially for ilmenite, because its reactivity, which is initiallyvery low, increases after air-calcination [21].

2.2. Physicochemical characterization

A Micromeritics 3Flex instrument was used to determine thepore volume and surface area via N2 adsorption isotherms at77 K. Prior to the measurements, the samples were degassed undervacuum at 350 �C for 4 h. The surface area and pore sizes were cal-culated using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively.

The elemental composition of the Sinai and ilmenite ores wasdetermined by ICP–OES, using a Thermo-Scientific iCAP 6500instrument.

The crystalline phases present in the prepared samples wereidentified by X–ray diffraction measurements. Powder XRD pat-terns were recorded with a Siemens D500 X–ray diffractometer,with auto divergent slit and graphite monochromator using CuKaradiation, having a scanning speed of 2�/min. The characteristicreflection peaks (d–values) were matched with JCPDS data filesand the crystalline phases were identified.

A Scanning Electron Microscope (JEOL 6300) equipped with anX-ray EDS analyzer (Oxford Isis 2000) was used for the morpholog-ical observation of the samples. In order to be able to examine themorphology and composition of the interior of the small particlesof the two ores (average size 125–185 lm), the powdered sampleswere embedded in epoxy resin. The epoxy blocks were ground andpolished to expose the interiors of the particles (approximatecross-sections), using silicon carbide paper to 1000 grid and dia-mond paste to 1 lm, respectively. Prior to data collection, a thingold film was deposited on all the samples by sputtering, in orderto make them conductive. Spot analysis and elemental mappingwere the techniques used for the study.

2.3. Kinetic study via TGA

TGA experiments were conducted using a HP 150 s TA pressur-ized TGA with a Rubotherm magnetic suspension balance element.In the TGA experiments, the weight change of the OC samples wasmeasured isothermally as a function of time during the reduction–oxidation cycles. Five reduction–oxidation cycles at each tempera-ture and atmospheric pressure were conducted. A sample ofapproximately 100 mg was heated in helium in a quartz crucibleto the reaction temperature.

The TGA reaction parameters, mass of the analyzed sample andgas flow rates, were experimentally determined prior to final test-

ing to ensure a kinetic region. In all experiments, the total gas flowrate was 1000 ml/min with a reduction time of 60–120 min andthe oxidation reaction time was 30–40 min, depending on the tem-perature. To avoid mixing the reduction and oxidation gases, thesystem was flushed with He for 30 min before and after eachreduction reaction.

For the reduction reaction, X% CH4/60% CO2/100-(60 + X)% Hegas mixtures were used. CO2 was added to the gas mixtures toallow the reduction of hematite only up to the magnetite phaseand prevent further reduction to wüstite or metallic Fe [3,21].For the oxidation reactions, Y% O2/100-Y% He gas mixtures wereused.

A matrix of experiments was created in order to evaluate the OCredox kinetics, i.e. reaction order with respect to gaseous reactants,activation energy and pre-exponential factor. Four different CH4

concentrations (X: 10, 15, 20 and 25%) were investigated duringthe reduction step at T = 900 �C, while four different O2 concentra-tions (Y: 5, 10, 15, and 20%) were investigated during the oxidationstep at T = 900 �C. The effect of temperature was studied by TGAexperiments performed at four different temperatures (750 �C,800 �C, 850 �C, 900 �C and 950 �C) with one selected concentrationof CH4 (20%) at the reduction stage and O2 (20%) at the oxidationstage.

2.4. Kinetic analysis

Fractional conversions—i.e., the fractional reduction and frac-tional oxidation conversions—were calculated using the TGA datafrom the 5th redox cycle. The fractional conversions for the reduc-tion and oxidation reactions are defined based on Eqs. (1) and (2)[21]:

Xred ¼ Moxd �MMoxd �Mred

ð1Þ

Xoxd ¼ M �Mred

Moxd �Mredð2Þ

where M is the instantaneous weight, Moxd is the weight of an oxi-dized sample obtained via TGA (maximum weight after oxidationwith O2/He mixture), and Mred is the weight of a reduced sampleobtained via TGA (minimum weight after reduction with CH4/He).In this study, Mred is considered to be the mass of Fe3O4, and 100%conversion means that the Fe2O3 (hematite) is completely con-verted to Fe3O4 (magnetite).

Because the calculations described in this paper account for thepartial pressure of the gaseous substrate, a kinetic expression forthe solid-gas reaction rate is described by Eq. (3):

dXdt

¼ k0Pnf ðXÞ ð3Þ

E. Ksepko et al. / Applied Energy 190 (2017) 1258–1274 1261

where f(X) is a structural factor or a model of the reaction thatdescribes the physical or chemical properties during the reaction;P is the partial pressure of methane or oxygen for the reductionand oxidation reactions, respectively; n is the reaction order withrespect to the gaseous reactant; and k0 is the reaction rate constantdescribed by the Arrhenius equation:

k0 ¼ A0e�EaRT ð4Þ

where A0 is the pre-exponential factor, Ea is the activation energy,and R is the gas constant (8.314 J/mole K).

In this paper, three models were examined in order to select themodel that best fits the experimental data and thus can describethe reaction in an appropriate manner. Table 1 presents the volu-metric model (F1), the shrinking core model (R3) and the 3-Dimensional diffusion model (D3) (Jander’s type) as f(X) and g(X)functions [33]. The volumetric model assumes that the reactionoccurs throughout the OC particles and that the mass of the OCparticle changes linearly during the reaction. The shrinking coremodel assumes that the reaction occurs on the external surfaceof the grain, which changes during the reaction. The D3 modeldescribes a reaction in which the diffusion of iron ions from theunreacted metal oxide through the product layer to the surfaceof the particle is the reaction-limiting step. The three proposedmodels are graphically presented in Fig. 2.

These 3 models were applied to the reactions occurring in theTGA system during the cycling experiments - i.e., the reductionof the solid OC by CH4 and its oxidation by O2.

The first step of the calculations was the fitting of the modelwith the TGA data. For this purpose, Eq. (3) was transformed toEq. (5):

dXf ðXÞ ¼ kdt ð5Þ

And

k ¼ k0 � Pn ð6Þwhere k is a kinetic constant that relates the partial pressure of thereactant to the reaction order. After integration, Eq. (5) can bedescribed in terms of g(X) (Eq. (7)):

gðXÞ ¼Z

dXf ðXÞ ¼ k � t ¼ k0 � Pn � t ð7Þ

In order to fit each of the models to the experimental data, thelinearity of the g(X) function versus time (Eq. (7)) was examined.The slope of this function, when the partial pressure of gaseousreactant (P) is kept constant, is the reaction rate constant k0. Thesuitable model was chosen by using Root Mean Square Error(RMSE) which is defined by (Eq. (8)):

RMSE ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPN

i¼1ðXpred;i � Xobs;iÞ2N

sð8Þ

where Xobs,i is the actual value of conversion degree for object i,Xpred,i is the conversion degree for object i predicted by the modelunder investigation and N is the number of objects for whichXpred,i is obtained by prediction. The model with the smallest RMSEis the most suitable model for the reaction description.

Table 1Tested reaction models [33].

Model f ðXÞ gðXÞVolumetric (F1) 1� X � lnð1� XÞShrinking core (R3) 3ð1� XÞ2=3 1� ð1� XÞ1=33-Dimensional diffusion (D3) 3

2 ð1� XÞ2=3ð1� ð1� XÞ1=3Þ ð1� ð1� XÞ1=3Þ2

The activation energy (Ea) and the pre-exponential factor (A0)are calculated from the reaction rate constants determined at thevarious temperatures tested, using the Arrhenius equation (Eq.(9)):

lnðkÞ ¼ � Ea

R� 1Tþ lnðA0 � PnÞ ð9Þ

The reaction orders with respect to CH4 (reducer) and O2 (oxi-dizer) were calculated based on Eq. (6) and its logarithmic form(10):

lnðkÞ ¼ lnðk0Þ þ n � lnP ð10ÞThus, the reaction order with respect to the gaseous reactant

can be determined from Eq. (10) by the slope and intercept of ln(k) vs. ln(P) at a fixed temperature, 900 �C in this work. MathcadPrime 2.0 software was used to determine the kinetic parameters.

3. Results and discussion

3.1. Fresh and calcined oxygen carrier physicochemicalcharacterization

The N2 adsorption isotherms of raw samples revealed that Sinaiore presents a surface area of 17.85 m2/g, higher than ilmenite(0.12 m2/g), as shown in Table 2.

After calcination, the Sinai ore samples have a surface area ofonly 1.17 m2/g, which indicates that they lose a great part of theirsurface area during the thermal pretreatment. However, asexpected [21], the effect of thermal treatment is quite differentin the case of ilmenite ore samples, inducing a limited increasein their surface area (Table 2). The total pore volume alsodecreased significantly in the Sinai ore samples but remained con-stant in ilmenite samples. Since the surface area of the calcinedSinai ore samples is quite small, their reactivity and Oxygen Trans-fer Capacity cannot be attributed only to surface reactions. It cananticipated that bulk diffusion contributes to the reaction kinetics.

The elemental composition of the Sinai ore and ilmenite sam-ples, as determined by ICP-OES, is presented in Table 3. Both oreshave Fe as the main constituent. In the case of Sinai ore, Mn and,to a lesser extent, Si, Al and Ca are the elements with the higherconcentrations, while Ti and low contents of Si and Mg are presentin ilmenite. Iron and manganese oxides are known to actively par-ticipate in CLC redox reactions. Inert materials, such as titania, sil-ica and alumina, improve mechanical strength over multiple redoxCLC cycles at high temperatures, and they act as inhibitors forcation diffusion of active phase, but also to protect OC againstthe agglomeration of Fe oxide particles.

The XRD patterns of the Sinai ore samples, fresh and calcined at950 �C are shown in Fig. 3. Identification of crystalline phases isalso included in the Figure. In the fresh, uncalcined sample, ironis trivalent, in the hematite (Fe2O3) phase and manganese istetravalent, in the pyrolousite (MnO2) phase. In the XRD patternof the calcined sample all peaks attributed to MnO2 completely dis-appear, indicating that MnO2 has decomposed. From the figure, itcan be revealed that the dominating phases in the calcined oreare, besides inert silicon oxide, hematite (Fe2O3) and bixbyite(Mn2O3). Small quantities of the corresponding simple-oxide spinelphases, magnetite (Fe3O4) and hausmannite (Mn3O4) are alsoformed in the calcined sample.

The XRD pattern of an ilmenite sample, calcined in air at 950 �C,is included in Fig. 15 together with identification of its crystallinephases. As can be observed, the dominating phase in the fully oxi-dized sample is pseudobrookite, Fe2TiO5, while small quantities ofhematite (Fe2O3) and rutile (TiO2) are also present.

Fig. 2. Graphical representation of the proposed reaction mechanisms for the F1, R3, and D3 models.

Table 2BET and pore analysis data of OCs.

Sample SBET (m2/g) VT (cm3/g) Vmesopores (cm3/g)

Sinai ore Raw 17.85 0.0571 0.0364Calcined 1.17 0.0033 0.0016

Ilmenite Raw 0.12 0.0005 0.0003Calcined 0.26 0.0005 0.0005

1262 E. Ksepko et al. / Applied Energy 190 (2017) 1258–1274

The Sinai ore is a natural mineral and shows, as expected, greatcomposition heterogeneity. Fig. 4 shows the elemental mapping ofseveral grains of the ore, calcined at 950 �C. A relatively low mag-

nification (X200) is used in order to include a sufficient number ofparticles. It is observed that there are grains composed almostentirely of iron oxides, while other grains contain almost onlyMn oxides. There are of course a few granules containing both ele-ments, but without formation of mixed Mn–Fe oxides. Fe enrich-ment can be noticed at the surface of many Mn-rich grains andcorrespondingly, a Mn ‘‘corona” is observed around many Fe-richgrains. The maps of the lower concentrations elements, e.g. Si, Al,Ca and Mg show that they, also, are not evenly distributed. In theimages of Fig. 4 either whole grains or large grain parts can be

Table 3Chemical composition comparison of Sinai and ilmenite samples.

(%) Sinai Ilmenite

Fe 40.71 32.50Mn 13.33 0.22Si 3.67 1.27Al 0.99 0.17Ti 0.07 28.24Ca 0.98 0.26Mg 0.40 2.36

Fig. 3. X-ray diffraction pattern and phase identification

Fig. 4. SEM image and maps of the major elements Fe, Mn, Si

E. Ksepko et al. / Applied Energy 190 (2017) 1258–1274 1263

observed, with concentrations of these elements much higher thanaverage.

3.2. Kinetics study

A complete redox cycle was composed of a reduction step,helium flush and a regeneration step. Five redox cycles were per-formed during each experiment, in order to ensure reproducibilityand stability. During the reduction step, a decrease in the mass of

of unreacted and calcined in air at 950 �C Sinai ore.

, Al, Ca and Mg for the Sinai ore calcined in air at 950 �C.

Table 4Theoretical oxygen-transport capacity for the Sinai ore and ilmenite samples.

No Initial state Reduced state Maximum weightchange (%)

Sinai ore1 Fe2O3, Mn2O3 FeO, MnO 7.252 Fe2O3, Mn2O3 Fe3O4, Mn3O4 2.593 Fe2O3, Mn2O3 Fe3O4, MnO 3.88

Ilmenite4 Fe2TiO5 (pseudobrookite) FeTiO3 (ilmenite) 6.665 Fe2O3 Fe3O4 2.22

1264 E. Ksepko et al. / Applied Energy 190 (2017) 1258–1274

the solid OC was observed while a corresponding increase in masswas observed during the oxidation step.

Based on the XRD study, the major phases in the calcined sam-ple are Fe2O3 and Mn2O3. The theoretical oxygen capacity was cal-culated, based on the results of the chemical analysis, and wascompared to the values observed in the reactivity tests. For theSinai ore, the theoretical oxygen-transport capacities for bimetallicmixtures (40.71% Fe and 13.33% Mn corresponding to 58.20% ofFe2O3 and 19.15% of Mn2O3) was calculated for various reductionstates of Mn and Fe and are shown in Table 4. During the reductionstep of the TGA experiments, CH4/CO2 mixtures were used and theexperimental conditions were adjusted such as to obtain a totalweight reduction close to 3% wt., in order to assure that reductionof Fe would not proceed further than Fe3O4. As is shown in Table 4,the weight change of �3 wt% lies between the reduction of bothMn3+ and Fe3+ into the corresponding spinel phases Fe3O4, Mn3O4

(2.59 wt.%, Table 4, No. 2) and the reduction of Fe3+ into Fe3O4

and of Mn3+ into MnO (3.88 wt.%, Table 4, No. 3).In the case of ilmenite, the reducible compounds in the pre-

oxidized, activated sample, in its fully oxidized state, are Fe2TiO5

and Fe2O3. They can be reduced to FeTiO3 and Fe3O4 respectively,with the theoretical weight changes shown in Table 4.

In the present work a CO2/CH4 mixture was used as a fuel-reducing agent, with CO2-to-CH4 ratio higher than 2:1, in orderto prevent the reduction of iron beyond Fe3O4, to FeO [21]. Com-parison of the experimentally measured mass change, equal to3.69 wt.%, with the data of Table 4, verifies that no FeO is formedduring the TGA study.

3.2.1. Reduction stepFig. 5a shows the fractional reduction for the Sinai ore, at four

different temperatures in the range 750–950 �C. The temperaturehas a positive effect on the reaction rate: as the reaction tempera-ture increases, the reaction rate also increases.

Fig. 5. Fractional reduction conversion versus time for Sinai ore at (

To determine the effect of the reductive gas (CH4) concentrationon the kinetics of the reaction, experiments at constant tempera-ture (900 �C) and four different methane concentrations (10, 15,20 and 25%) were performed. The obtained results are shown inFig. 5b. An increase in the reduction rate is observed when thereductive gas concentration is increased.

Calcined ilmenite, which has been repeatedly studied as apromising, naturally occurring OC, was also tested during thiswork, for comparison, using exactly the same measurement proce-dure described above. Experiments with ilmenite were performedonly at four different temperatures in the range 800–950 �C, sinceilmenite had practically zero reactivity at temperatures below800 �C.

The fractional reduction curves of ilmenite is presented inFig. 6a and b. The temperature effect is positive also in this case.By comparing the data of Figs. 5 and 6, it can be observed that Sinaiore exhibits higher reaction rates, achieving the same conversionlevels in reaction time less than one third of the correspondingreaction time with ilmenite.

In order to fit the experimental data to a kinetic model, plots ofg(X) vs. time were prepared using each of the models described inSection 2.4. One step reaction was assumed because the reductionreaction was stopped at Fe3O4 and Mn3O4 oxidation level by usingthe given concentration of CO2 in the gas mixture. The plots for thereduction of Sinai ore at various temperatures and CH4 concentra-tions are shown in Fig. 7. In order to examine the appropriatemodel, linear regression of the g(X) function was used. The kineticconstants values were calculated based on the slope of the fittedlines, and the RMSE was calculated for each of the 3 tested models.The obtained results for all three models are shown in Table 5.Based on the RMSE values, over the entire temperature range(750–950 �C) and partial pressure of CH4, the 3-Dimensional Diffu-sion model shows the lowest RMSE value for Sinai ore reduction.The D3 model was chosen for Sinai ore reduction reaction, becauseit has the lowest sum of the RMSE values for the whole experimen-tal matrix.

Similarly, for ilmenite ore, the model-fitting results are shownin Table 6. Based on the RMSE values of g(X) vs. time (Table 6),the most suitable model for the reduction reaction is the D3 model,although the Shrinking-Core (R3) model is also appropriate fordescribing the behavior of ilmenite during the reduction reactionin the lower temperature.

The reduction reaction parameters for the Sinai-ore, accordingto the 3-Dimensional Diffusion (D3) model, are calculated fromthe Arrhenius plot of the reduction reaction (Fig. 8) based on Eq.(8). The obtained values are: Ea = 35.3 kJ/mole andA0 = 8.15 � 10�3 s�1�Pan with R2 = 0.930. The calculated Ea is in

a) different temperatures and (b) different CH4 concentrations.

Fig. 6. Fractional reduction versus time for ilmenite at (a) different temperatures and (b) different CH4 concentrations.

Fig. 7. Model fitting for the reduction reaction of Sinai ore under different temperatures

Table 5Reduction reaction kinetics of Sinai ore.

T (�C) Model F1 Model R3 Model D3

k (s�1) RMSE k (s�1) RMSE k (s�1) RMSE

750 1.20 � 10�3 3.21 � 10�2 2.94 � 10�4 5.42 � 10�3 1.35 � 10�4 8.79 � 10�5

800 1.36 � 10�3 3.11 � 10�2 3.35 � 10�4 5.47 � 10�3 1.53 � 10�4 9.57 � 10�5

850 1.63 � 10�3 1.54 � 10�2 4.01 � 10�4 3.76 � 10�3 1.81 � 10�4 1.81 � 10�4

900 1.79 � 10�3 9.68 � 10�3 4.43 � 10�4 3.01 � 10�3 1.98 � 10�4 2.92 � 10�4

950 2.52 � 10�3 6.55 � 10�3 6.26 � 10�4 2.57 � 10�3 2.79 � 10�4 3.32 � 10�4

PCH4 (Pa) CCH4 (%)

10,130 10 1.18 � 10�3 1.32 � 10�3 2.95 � 10�4 1.24 � 10�3 1.29 � 10�4 6.16 � 10�4

15,195 15 1.48 � 10�3 7.20 � 10�3 3.66 � 10�4 2.57 � 10�3 1.63 � 10�4 3.73 � 10�4

20,260 20 1.79 � 10�3 9.68 � 10�3 4.43 � 10�4 3.01 � 10�3 1.98 � 10�4 2.92 � 10�4

25,325 25 2.46 � 10�3 1.82 � 10�3 6.13 � 10�4 9.48 � 10�4 2.67 � 10�4 7.17 � 10�4

Table 6Reduction reaction kinetics of Ilmenite ore.

T (�C) Model F1 Model R3 Model D3

k (s�1) RMSE k (s�1) RMSE k (s�1) RMSE

800 3.44 � 10�4 1.30 � 10�1 8.77 � 10�5 1.14 � 10�3 3.59 � 10�5 5.03 � 10�3

850 4.24 � 10�4 8.40 � 10�2 1.08 � 10�4 4.96 � 10�4 4.45 � 10�5 3.99 � 10�3

900 6.53 � 10�4 1.01 � 10�2 1.62 � 10�4 3.84 � 10�3 7.19 � 10�5 6.91 � 10�4

950 7.73 � 10�4 2.67 � 10�2 1.90 � 10�4 7.04 � 10�3 8.62 � 10�5 2.51 � 10�4

PCH4 (Pa) CCH4 (%)

10,130 4.97 � 10�4 3.61 � 10�2 1.25 � 10�4 2.58 � 10�4 5.33 � 10�5 2.52 � 10�3

15,195 15 7.26 � 10�4 1.84 � 10�2 1.79 � 10�4 5.99 � 10�3 8.06 � 10�5 3.68 � 10�4

20,260 20 6.53 � 10�4 1.01 � 10�2 1.62 � 10�4 3.84 � 10�3 7.19 � 10�5 6.91 � 10�4

25,325 25 8.63 � 10�4 4.35 � 10�2 2.12 � 10�4 9.36 � 10�3 9.65 � 10�5 2.03 � 10�4

E. Ksepko et al. / Applied Energy 190 (2017) 1258–1274 1265

Fig. 8. Arrhenius plots of the reduction reactions of Sinai ore and ilmenite for the applied models (a), and reduction reaction order determination (b).

Fig. 9. Oxidation degree versus time of Sinai ore for different temperature (a) and reagent (O2) concentration (b) conditions.

1266 E. Ksepko et al. / Applied Energy 190 (2017) 1258–1274

good agreement with the results reported in the literature withsimilar Fe-based OCs: for Fe2O3/TiO2, this value was reported tobe 33.8 kJ/mole [9], while 29 kJ/mole was reported for Fe2O3/ben-tonite [34].

In Fig. 8a the Arrhenius plot of the reduction reaction of ilme-nite is also shown, determined using the 3-dimensional (D3)model. The calculated activation energy is 62.4 kJ/mole. Τhe activa-tion energies for ilmenite’s reduction reaction with CH4 reportedelsewhere [21] are 165.3 kJ/mole and 135.3 kJ/mole for the pre-oxidized and activated ilmenite samples, respectively. The param-eters obtained herein lie between the activation energies reportedfor ilmenite and Fe-based ores supported on inert substrates [9,28].This could be attributed to the large quantities of hematite that arepresent in the tested ilmenite samples after 5 redox cycles, asshown in Fig. 18 and discussed in Section 3.3.2.

The reduction-reaction order for the Sinai ore and ilmenite aredetermined from the graph shown in Fig. 8b, based on Eq. (9).The calculated reduction-reaction orders are n = 0.76 and 0.52 forthe Sinai ore and ilmenite, respectively. Good R2 values (0.961;0.970) for n determinations are obtained.

A reaction order of 0.76 suggests that a more complex mecha-nism is involved in the reduction of the Sinai ore. According tothe calculations, the reduction reaction proceeds uniformlythroughout the whole grain, and there is no diffusion limitation.This reaction order can also be explained by a complex reactionmechanism involving dry reforming of CH4. Both CO2 and CH4

are feed in the reaction mixture, therefore reaction (11) occurs:

CH4 þ CO2 ! 2 COþ 2 H2 ð11Þ

The yield of reaction (11) increases with increasing tempera-ture, leading to increased CO/H2 concentrations. Furthermore, theratio of substrates may strongly influence the concentrations ofthe products (and, thus, alter the reaction kinetics). In other words,changing the stoichiometry of substrates from Eq. (11), may lead toproduce for example more H2 or CO. Meanwhile products of thereaction may react with an oxygen carrier effecting (increasing)the kinetics of redox reactions.

Concluding, models F1 and R3 are found to be suitable for ilme-nite, whereas models D3 and F1 are suitable for Sinai ore reduction.The reason underlying this observation is that the two OCs differ interms of both their chemical and phase compositions. Therefore,different reduction reactions are expected to occur. In the ilmenite(Fe2O3- and TiO2-based) OC, the reduction pathway is expected tobe Fe2TiO5 ? FeTiO3 with parallel Fe2O3 ? Fe3O4; this expectationis supported by the mass changes observed during TGA testing. Forthe Sinai ore (Fe2O3, Mn2O3, SiO2, Al2O3) OC sample, the fittedmodel describes the following reduction reactions: Fe2O3 ?Fe3O4, Mn2O3 ?Mn3O4 and Fe2O3 + Mn2O3 ?MnFe2O4.

3.2.2. Oxidation stepFig. 9 shows the oxidation conversion versus time for the Sinai

ore under different reaction conditions. The presented curves showthe fractional oxidation conversions at five different reaction tem-peratures (Fig. 9a) and four different O2 concentrations (Fig. 9b).Since the oxidation reaction is significantly faster than the reduc-tion reaction, the range of data considered for the model-fittingcalculations was narrowed to approximately 60 s, which corre-sponds to a conversion degree ca. 0.8.

Fig. 10. Oxidation degree versus time of ilmenite for different temperature (a) and different reagent (O2) concentration (b) conditions.

E. Ksepko et al. / Applied Energy 190 (2017) 1258–1274 1267

Fig. 9a shows that the temperature also has a positive effect onthe oxidation reaction rate, similarly as with the reduction reac-tion. An increase in the O2 concentration leads to a correspondingincrease in the oxidation reaction rate, as shown in Fig. 9b. Thus,both temperature and O2 concentration have a positive effect onthe oxidation rates of the Sinai ore. The increase is more pro-nounced when increasing from 5 to 10% O2.

The oxidation of the ilmenite ore by O2 is shown inFig. 10a and b, by plotting the fractional oxidation conversion vs.time for four different temperature and four different reagent(O2) concentrations, respectively. These figures show that the O2

concentration barely affects the reaction rate, which supports theresults reported in the literature [21]. This is because the reactionis controlled by diffusion inside the particle, which will beexplained further, while discussing obtained oxidation reactionorder.

Table 7 lists the reaction rate constants and RMSE for the 3 stud-ied models. The most suitable model for describing the sample

Table 7Oxidation kinetics study of Sinai ore.

T (�C) Model F1

k (s�1) RMSE

750 6.16 � 10�2 7.21 � 10�3

800 6.63 � 10�2 1.49 � 10�2

850 7.47 � 10�2 1.06 � 10�2

900 8.35 � 10�2 1.86 � 10�2

950 8.21 � 10�2 1.39 � 10�2

PO2 (Pa) CO2 (%)

5,065 4.35 � 10�2 6.18 � 10�3

10,130 10 6.61 � 10�2 1.08 � 10�2

15,195 15 7.52 � 10�2 2.86 � 10�3

20,260 20 8.35 � 10�2 7.36 � 10�3

Table 8Oxidation kinetics study of Ilmenite ore.

T (�C) Model F1

k (s�1) RMSE

800 2.25 � 10�3 7.67 � 10�2

850 2.39 � 10�3 9.92 � 10�2

900 6.12 � 10�3 1.93 � 10�1

950 1.08 � 10�2 3.18 � 10�1

PO2 (Pa) CO2 (%)

5,065 6.00 � 10�3 9.87 � 10�2

10,130 10 6.21 � 10�3 1.68 � 10�1

15,195 15 6.96 � 10�3 2.15 � 10�1

20,260 20 6.12 � 10�3 1.96 � 10�1

behavior during O2 oxidation in the given temperature range wasthe Shrinking Core (R3) model. As indicated by the data collectedin Table 7, the D3 model also provided good results.

For ilmenite, the oxidation reaction rate significantly dependson the regeneration reaction temperature, as shown in Fig. 10.For the oxidation reaction of ilmenite, the best correlations wereobtained for the D3 model as shown in Table 8.

Fig. 11a shows the Arrhenius plots of the oxidation reactions ofSinai ore and ilmenite obtained by applying R3 and D3 modelsrespectively. The oxidation-reaction order values for the Sinai oreand ilmenite are determined from the graph shown in Fig. 11b.The calculated Ea and n were 16.70 kJ/mole and 0.72, respectively,for Sinai ore, and 125.7 kJ/mole and 0.07, respectively, for ilmenite.

During the oxidation reaction, which is faster than the reduc-tion reaction, a Fe2O3 product layer can form around the unreactedcore of Fe3O4, and thus, the resistance to the diffusion of O2� or Fe3+

ions may play an important role. A comparison to data reported inthe literature [9] for Fe2O3/bentonite revealed an Ea that was lower

Model R3 Model D3

k (s�1) RMSE k (s�1) RMSE

1.79 � 10�2 2.48 � 10�4 4.36 � 10�3 3.56 � 10�4

1.93 � 10�2 3.31 � 10�4 4.54 � 10�3 8.71 � 10�4

2.19 � 10�2 2.90 � 10�4 4.96 � 10�3 5.68 � 10�4

2.42 � 10�2 3.68 � 10�4 5.85 � 10�3 1.12 � 10�3

2.38 � 10�2 3.31 � 10�4 5.81 � 10�3 7.63 � 10�4

1.25 � 10�2 2.43 � 10�4 3.25 � 10�3 2.65 � 10�3

1.86 � 10�2 2.97 � 10�4 5.37 � 10�3 5.60 � 10�4

2.16 � 10�2 1.92 � 10�4 5.61 � 10�3 8.46 � 10�5

2.42 � 10�2 2.56 � 10�4 5.85 � 10�3 3.42 � 10�4

Model R3 Model D3

k (s�1) RMSE k (s�1) RMSE

5.78 � 10�4 8.80 � 10�3 2.39 � 10�4 2.27 � 10�4

6.10 � 10�4 1.13 � 10�2 2.55 � 10�4 1.28 � 10�4

1.55 � 10�3 1.92 � 10�2 6.64 � 10�4 6.83 � 10�4

2.72 � 10�3 2.79 � 10�2 1.21 � 10�3 1.76 � 10�4

1.54 � 10�3 1.15 � 10�2 6.37 � 10�4 1.24 � 10�4

1.58�10�3 1.68�10�2 6.67 � 10�4 5.77 � 10�4

1.76�10�3 2.05�10�2 7.61 � 10�4 8.36 � 10�4

1.55�10�3 1.92�10�2 6.64 � 10�4 6.83 � 10�4

Fig. 11. Arrhenius plots of the oxidation reactions of Sinai ore and ilmenite using the applied models (a), and oxidation reaction order determination (b).

Table 9Summary of kinetic data for both the reduction and oxidation reactions.

Sinai ore Ilmenite

Reduction Oxidation Reduction Oxidation

Ea (kJ/mole) 35.3 16.70 62.4 125.7A0 (s�1) 2.40 � 10�2 1.02 � 10�4 2.21 � 10�3 129n 0.76 0.72 0.52 0.07Reaction model D3 R3 D3 D3

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(6 kJ/mole) than that obtained in this paper. However, the twosamples differ in terms of composition—i.e., Fe-Mn for Sinai oreand Fe for Fe2O3/bentonite, which might be a reason for the dis-crepancy in the Ea values.

For the D3 model of the ilmenite sample, the calculated activa-tion energy of the oxidation reaction of was 125.7 kJ/mole, and thereaction order n was 0.07. This low reaction order, which is close to0, means that the concentration of O2 in the gas phase does notinfluence the oxidation reaction rate of ilmenite. This may be aresult of O2� diffusion through the product layer in the grain,which does not depend on the O2 concentration in the oxidizinggases. This supports previous observations, which suggested thatthe reaction mechanism probably changed from chemical reaction

Fig. 12. X-ray diffraction pattern and phase identification of Sinai ore after 5 redox cy

control to diffusional control in the particle because of the loss ofporosity inside the particle as oxidation proceeds [21].

The obtained kinetic data were summarized and are shown forboth OC samples in Table 9. Based on these results, each OC in thekinetics study should be treated specifically, even if it contains thesame active metal oxide—here, Fe oxide.

3.3. Physicochemical characterization after 5 redox cycles

3.3.1. Sinai oreFig. 12 presents the X-ray diffraction (XRD) patterns of Sinai ore

samples which have ‘‘reacted” during five redox cycles, at four dif-ferent temperatures, 750 �C, 850 �C, 900 �C and 950 �C. The ‘‘fuelmixture” as well as the ‘‘oxidation mixture” compositions werekept constant during all the above cycles, with 20% CH4/60%CO2/20% He and 20% O2/80% He, respectively. The major compo-nents of all four oxidized samples are Fe2O3 and Mn2O3. However,careful comparison of the XRD patterns indicates that the compo-sition of the oxidized samples is gradually modified, as the temper-ature of the redox reactions is increased. More specifically, therelative content of hematite and bixbyite, decreases while theformation of the mixed valence spinel phases, magnetite and haus-mannite, continuously increases with increasing reactiontemperature.

cles, with 20% CH4/60% CO2/20% He and 20% O2/80% He at different temperatures.

Fig. 13. (a) X-ray diffraction pattern of the Sinai ore after 5 redox cycles at 900 �C, analyzed into the 2 peaks attributed to hematite [214] and mixed Mn-Fe spinel [440], (b)area of the two peaks as a function of reaction temperature.

E. Ksepko et al. / Applied Energy 190 (2017) 1258–1274 1269

The above observation is more clearly shown in Fig. 13. InFig. 13a the 60.5� < 2h < 63.5� region of the XRD pattern of the‘‘reacted” Sinai ore sample, has been analyzed into 2 peaks, oneof which is the (214) band of hematite (JPDCS No. [1-89-0597])and the other is the (440) band of the mixed Mn-Fe spinel MnFe2-O4 (JPDCS No. [1-73-1964]). The ratio of the area of the two abovepeaks, which is directly connected to the relative content of thetwo corresponding phases, has been plotted in Fig. 13b as a func-tion of the reaction Temperature. As can be observed in Fig. 13b,the relative content of the spinel phase increases with increasingreaction temperature, at the expense of the hematite phase.

In Table 10, SEM images are compared, at the same magnifica-tion level, of the Sinai ore, after 5 redox cycles at four differenttemperatures. The corresponding images from a sample calcinedat 950 �C are also included for comparison. The particles in the leftcolumn are composed almost 100% of Fe-oxides while those in theright column are rich in Mn. Different effects of the subsequentredox reaction cycles, in the morphology of the particles, can beobserved, depending on the chemical composition. The Mn-richparticles undergo a great change in their morphology with the for-mation of a high level of porosity, even at low reaction tempera-ture. It can be observed that the extent of disintegration is afunction of the reaction temperature. The Fe-rich particles aremuch more stable. They seem to remain almost unaffected at750 �C, a small change starts at 850 �C, which becomes graduallymore pronounced as the reaction temperature increases.

Fig. 14 presents the XRD patterns of Sinai ore samples whichhave ‘‘reacted” during five redox cycles, at 900 �C, with variablemethane and oxygen contents in the ‘‘fuel mixture” and the ‘‘oxi-dation mixture”, respectively. Comparison of the three XRD pat-terns indicates that the composition of the samples is, withinexperimental error, not modified. This shows that the effect ofthe reactant concertation on the structure of the OC samples, dur-ing both the reduction and the oxidation steps, is much less pro-nounced than the effect of the reaction temperature.

In Table 11, SEM images are compared, at the same magnifica-tion level, of the Sinai ore, after redox reaction during five cycles, at900 �C, with variable methane and oxygen contents in the ‘‘fuelmixture” and the ‘‘oxidation mixture”, respectively. The particlesin the left column are composed almost 100% by Fe-oxides, thosein the middle column are rich in Mn, while the images of the rightcolumn have been obtained from particles that are rich in both Feand Mn. The effect of the multiple redox cycles on the morphologyof all three types of granules does not seem to be strongly affectedby the different reactions conditions, as also shown in the XRDstudy. The disintegration of the Mn-rich particles is more pro-nounced, the Fe-rich particles maintain more dense texture, in all

three samples while the mixed Fe-Mn rich particles lie somewherein-between.

The intermixing between Mn and Fe oxides, described by Lar-ring et al. [11] for the same mineral, is not observed in this work,possibly due to the different severity that the samples haveencountered during redox cycles, prior to characterization. Theimages of Tables 10 and 11 are obtained from samples after only5 redox cycles at temperatures between 750-950 �C, while thesamples in Ref. [11] have been subjected to 200 cycles at the high-est temperature, 1000 �C. Thus, the effect of the redox cycles in themorphology of the granules are, in the case of this work, much lesspronounced.

3.3.2. Ilmenite oreFig. 15 presents XRD patterns of oxidized ilmenite ore samples

which have ‘‘reacted” during five redox cycles, at three differenttemperatures, 850 �C, 900 �C and 950 �C. As in Fig. 12, the ‘‘fuelmixture” and the ‘‘oxidation mixture” compositions were kept con-stant during all the above cycles, with 20% CH4/60% CO2/20% Heand 20% O2/80% He, respectively. The corresponding XRD patternfrom a sample calcined, in air, at 950 �C, for comparison and iden-tification of crystalline phases are also included in the figure.

In the XRD pattern of the calcined sample the dominating phaseis pseudobrookite, Fe2TiO5, in which iron is trivalent. Small quanti-ties of hematite (Fe2O3) and rutile (TiO2) are also present. Compar-ison of the three XRD patterns of the reacted samples indicates thatafter 5 reduction – oxidation cycles, the Pseudobrookite phase isonly partially formed in the oxidized samples, while the hematite(Fe2O3) and rutile (TiO2) phases are dominating. The relative con-tent of the pseudobrookite is gradually increasing, as the tempera-ture of the redox reactions is increased and becomes significantonly in the sample reacted at 950 �C. This is in agreement withthe results of previous works [21,35,36] where it was reported thatpseudobrookite is the only end product when the oxidation is car-ried out at high temperature (>900 �C) and long oxidation time,while a mixture of hematite (Fe2O3) and rutile (TiO2) phases aredominating at lower temperatures or short oxidation time.

In Table 12, SEM images together with the corresponding ele-mental maps of Ti and Fe, at the same magnification level, are com-pared, obtained from Ilmenite ore samples, after 5 redox cycles atthree different temperatures. In the 1rst row, the sample reacted atthe lowest temperature, shows a slightly increased Fe content nearthe particle edge. Moreover, big pores can be observed in which therelative Ti content is very much increased. In the images presentedin the middle row, obtained from samples after 5 cycles at theintermediate temperature, 900 �C, the increased Fe content nearthe particle edge is more pronounced, while the high Ti-content

Table 10SEM images of the Sinai ore calcined at 950 �C and after 5 redox cycles at different temperatures with 20% CH4/60% CO2/20% Heand 20% O2/80% He.

1270 E. Ksepko et al. / Applied Energy 190 (2017) 1258–1274

Fig. 14. X-ray diffraction pattern and phase identification of Sinai ore after 5 redox cycles, at 900 �C with variable methane and oxygen contents.

Table 11SEM images of Fe-rich, Mn-rich and Fe-Mn rich particles from a Sinai ore after 5 redox cycles at 900 �C with variable methane and oxygen contents.

E. Ksepko et al. / Applied Energy 190 (2017) 1258–1274 1271

pores seem to decrease in area and in depth. Finally, after 5 redoxcycles at the highest temperature tested, 950 �C, shown in the lastrow of Table 12, the pores almost disappear and the interior of theparticle becomes more smooth and with uniform elemental com-position. At the same time, the Fe-content at the particle edgeincreases further.

This observation is in agreement with the interpretation of theXRD patterns, shown in Fig. 15. As already written above, themixed Fe-Ti pseudobrookite phase is only partially formed in the

‘‘reacted” samples after the oxidation step. In the contrary, the sin-gle element hematite (Fe2O3) and rutile (TiO2) phases are dominat-ing. In the elemental maps of Table 12 it is shown that, in thesamples reacted at low temperatures, the two single elementphases are spatially separated, and due to this reason they do notrecombine to form the mixed phase Fe2TiO5. As the temperatureincreases, the mobility of the two single phases increases and thus,they recombine easier and more mixed phase, pseudobrookite, isformed.

Table 12SEM images of the Ilmenite ore after 5 redox cycles at different temperatures with 20% CH4/60% CO2/20% He and 20% O2/80% He.

Fig. 15. X-ray diffraction pattern and phase identification of Ilmenite ore and after 5 redox cycles at different temperatures.

1272 E. Ksepko et al. / Applied Energy 190 (2017) 1258–1274

4. Conclusions

Low-grade Sinai ore, a naturally occurring Fe-based material, isa potentially suitable oxygen carrier for Chemical Looping Com-bustion. Ilmenite is well known as a natural OC material and has

been extensively investigated in the past; however, Sinai oreshows superior kinetics in both its reduction and oxidation reac-tions, which makes it a more favorable OC material in terms ofpractical utilization in CLC power plants. However, the structureand morphology of the Sinai ore appears to be less stable, therefore

E. Ksepko et al. / Applied Energy 190 (2017) 1258–1274 1273

further study is necessary in order to optimize the reaction condi-tions in order to maximize its performance and stability.

The paper data is an excellent prove that synthetic Fe-Mn OCsmight be substituted by naturally occurring iron manganese oressuch as Sinai ore from the Um Bogma mine, Egypt. The only con-cern might be that if it is a deposit in one place meaning the lowavailability for future applications. The geological analysis fromUnited States Geological Survey shows that large quantities ofmanganiferous iron ores are present in many countries such as:South Africa, USA, Norway, Australia, Republic of Korea, Georgia,Gabon etc. In other words there are huge quantities of Fe-Mn oresavailable to be used as OCs in the future CLC based power plants.

The kinetic parameters of its reduction and oxidation reactionswere determined and can be useful in designing CLC reactors andmodeling such systems. The best reaction models for both thereduction and oxidation reactions were identified and the activa-tion energy, the pre-exponential factor and the reaction order weredetermined. The 3-Dimensional Diffusion (D3) model was the mostappropriate for the reduction reaction of Sinai ore, the activationenergy (Ea) is equal to 35.3 kJ/mole and reaction order (n) is 0.76.For the oxidation reaction, the Shrinking Core (R3) model providedthe best correlation with the data, with Ea and n of 16.70 kJ/moleand 0.72, respectively.

The kinetic study for ilmenite ore samples, which was per-formed for comparison, showed that the 3-Dimensional Diffusion(D3) model was the most suitable for describing the sample behav-ior during the reduction reaction with Ea and n of 62.4 kJ/mole and0.52, respectively. For the ilmenite oxidation reaction, the 3-Dimensional Diffusion (D3) provided the best correlation withthe data, with Ea and n equal to 125.7 kJ/mole and 0.07,respectively.

Hematite and bixbyite are the dominating compounds in theoxidized Sinai ore samples after 5 redox cycles at all tested temper-atures. As the temperature of the redox reactions increases, theirrelative content decreases, while the formation of the mixedvalence spinel phases, magnetite and hausmannite continuouslyincreases. The effect of the reactant concentrations, during boththe reduction and the oxidation steps, is much less pronounced.The formation of a high level of porosity was observed, duringthe SEM-EDS study, at the Mn-rich particles, even at low reactiontemperature. The extent of disintegration is a function of the reac-tion temperature. The Fe-rich particles are much more stable. Thedifferent reactant concentrations do not have a significant effect onthe formed porosity.

The dominating phase in the calcined ilmenite ore sample ispseudobrookite, Fe2TiO5. In the oxidized samples, after 5 redoxcycles, the Pseudobrookite phase is only partially formed whilethe hematite (Fe2O3) and rutile (TiO2) phases are dominating.The relative content of the pseudobrookite is gradually increas-ing, as the temperature of the redox reactions is increased andbecomes dominant at the highest temperature used, 950 �C. Atthe lowest tested temperature, 850 �C, big pores are formed inthe ilmenite ore particles, in which the relative Ti content isvery much increased, while Fe content increases near theparticle edge. With increasing temperature the pores tend todisappear while the increased Fe-content at the particle edgesis enhanced.

The results for the Sinai ore confirm that it is among the miner-als that can be utilized in CLC without the need for pre-treatmentprior to utilization, thus with great cost savings. The extent of theparticles disintegration observed is only minor and is not expectedto cause severe problems during fluidization in CLC reactors. Itshould, furthermore, be keep in mind that TGA cycling gives deepreduction, which causes more severe damage to the particles microstructure, compared to faster and shorter cycling under CLC reactortest condition.

Acknowledgments

This study was financially supported by The NationalCenter of Research and Development Poland, project No.NCBiR/FENCO–NET2/2013 and the General Secretariat for Researchand Technology Hellas, project No. 13FENCO–13–478A ‘Mineralsfor Sustainable COst and energy efficient Chemical LoopingCombustion Technology’ – MINERAL SCOUT.

Authors would like to acknowledge Dr. Yngve Larring and Dr.Mehdi Pishahang from SINTEF Materials and Chemistry (Norway)for providing the Sinai and ilmenite minerals used in this study.

References

[1] Gaggioli RA. Efficiency and costing. Second law analysis of process. Cary,NC: Oxford University Press; 1983. p. 71.

[2] Siriwardane RV, Ksepko E, Tian H, Poston J, Simonyi T, Sciazko M. Interaction ofiron–copper mixed metal oxide oxygen carriers with simulated synthesis gasderived from steam gasification of coal. Appl Energy 2013;107:111–23.

[3] Adanez J, Abad A, Garcia-Labiano F, Gayan P, de Diego LF. Progress in chemical-looping combustion and reforming technologies. Prog Energy Combust Sci2012;38:215–82.

[4] Albrecht KJ, Jackson GS, Braun RJ. Thermodynamically consistent modeling ofredox-stable perovskite oxides for thermochemical energy conversion andstorage. Appl Energy 2016;165:285–96.

[5] Stec M, Tatarczuk A, Wiecław-Solny L, Krótki A, Scia _zko M, Tokarski S. Pilotplant results for advanced CO2 capture process using amine scrubbing at theJaworzno II Power Plant in Poland. Fuel 2015;151:50–6.

[6] Wiecław-Solny L, Tatarczuk A, Stec M, Krótki A. 12th International conferenceon greenhouse gas control technologies, GHGT-12 advanced CO2 capture pilotplant at tauron’s coal-fired power plant: initial results and furtheropportunities. Energy Procedia 2014;63:6318–22.

[7] Pishahang M, Larring Y, McCann M, Bredesen R. Ca0.9Mn0.5Ti0.5O3�d: a suitableoxygen carrier material for fixed-bed chemical looping combustion undersyngas conditions. Ind Eng Chem Res 2014;53:10549–56.

[8] Zhang Y, Doroodchi E, Moghtaderi B. Chemical looping combustion of ultra lowconcentration of methane with Fe2O3/Al2O3 and CuO/SiO2. Appl Energy2014;113:1916–23.

[9] Ksepko E, Sciazko M, Babinski P. Studies on the redox reaction kinetics ofFe2O3–CuO/Al2O3 and Fe2O3/TiO2 oxygen carriers. Appl Energy2014;115:374–83.

[10] Huang L, Tang M, Fan M, Cheng H. Density functional theory study on thereaction between hematite and methane during chemical looping process.Appl Energy 2015;159:132–44.

[11] Larring Y, Pishahang M, Sunding MF, Tsakalakis K. Fe–Mn based minerals withremarkable redox characteristics for chemical looping combustion. Fuel2015;159:169–78.

[12] de Diego LF, Gayán P, García-Labiano F, Celaya J, Abad A, Adánez J.Impregnated CuO/Al2O3 oxygen carriers for chemical-looping combustion:avoiding fluidized bed agglomeration. Energy Fuels 2005;19:1850–6.

[13] Ksepko E. Feasible utility of inorganic remains from potable water purificationprocess in chemical looping combustion studied in TG. J Therm Anal Calorim2015;120:457–70.

[14] Chen L, Zhu Q, Hao Z, Zhang T, Xie Z. Development of a Co–Ni bimetallicaerogel catalyst for hydrogen production via methane oxidative CO2 reformingin a magnetic assisted fluidized bed. Int J Hydrogen Energy2010;35:8494–502.

[15] Gu Z, Li K, Wang H, Qing S, Zhu X, Wei Y, et al. Bulk monolithic Ce–Zr–Fe–O/Al2O3 oxygen carriers for a fixed bed scheme of the chemical loopingcombustion: reactivity of oxygen carrier. Appl Energy 2016;163:19–31.

[16] Lisi L, Mancino G, Cimino S. Chemical looping oxygen transfer properties of Cu-doped lanthanum oxysulphate. Int J Hydrogen Energy 2015;40:2047–54.

[17] Rydén M, Lyngfelt A, Mattisson T. Combined manganese/iron oxides as oxygencarrier for chemical looping combustion with oxygen uncoupling (CLOU) in acirculating fluidized bed reactor system. Energy Procedia 2011;4:341–8.

[18] Ksepko E, Siriwardane RV, Tian H, Simonyi T, Sciazko M. Effect of H2S onchemical looping combustion of coal-derived synthesis gas over Fe–Mn oxidessupported on sepiolite, ZrO2, and Al2O3. Energy Fuels 2012;26:2461–72.

[19] Källén M, Rydén M, Lyngfelt A, Mattisson T. Chemical-looping combustionusing combined iron/manganese/silicon oxygen carriers. Appl Energy2015;15:330–7.

[20] Azis MM, Jerndal E, Leion H, Mattisson T, Lyngfelt A. On the evaluation ofsynthetic and natural ilmenite using syngas as fuel in chemical-loopingcombustion (CLC). Chem Eng Res Des 2010;88:1505–14.

[21] Abad A, Adánez J, Cuadrat A, García-Labiano F, Gayán P, de Diego LF. Kinetics ofredox reactions of ilmenite for chemical-looping combustion. Chem Eng Sci2011;66:689–702.

[22] Leion H, Jerndal E, Steenari B-M, Hermansson S, Israelsson M, Jansson E, et al.Solid fuels in chemical-looping combustion using oxide scale and unprocessediron ore as oxygen carriers. Fuel 2009;88:1945–54.

[23] Tian H, Siriwardane R, Simonyi T, Poston J. Natural ores as oxygen carriers inchemical looping combustion. Energy Fuels 2013;27:4108–18.

1274 E. Ksepko et al. / Applied Energy 190 (2017) 1258–1274

[24] Wang K, Tian X, Zhao H. Sulfur behavior in chemical-looping combustion usinga copper ore oxygen carrier. Appl Energy 2016;166:84–95.

[25] Leion H, Mattisson T, Lyngfelt A. Use of ores and industrial products as oxygencarriers in chemical-looping combustion. Energy Fuels 2009;23:2307–15.

[26] Perreault P, Rifflart S, Nguyen E, Patience GS. Pyrolusite: an alternative oxygencarrier for chemical looping combustion. Fuel 2016;185:630–8.

[27] Wang X, Liu H, Jin B, Sun C, Snape CE. Experimental evaluation of a Chinesesulfur-containing lean iron ore as the oxygen carrier for chemical-loopingcombustion. Ind Eng Chem Res 2016;55:428–35.

[28] Ksepko E, Babinski P, Evdou A, Nalbandian L. Studies on the redox reactionkinetics of selected, naturally occurring oxygen carrier. J Therm Anal Calorim2015;124:137–50.

[29] Fossdal A, Bakken E, Øye BA, Schøning C, Kaus I, Mokkelbost T, et al. Study ofinexpensive oxygen carriers for chemical looping combustion. Int J Greenh GasControl 2011;5:483–8.

[30] Mohammad Pour N, Azimi G, Leion H, Rydén M, Mattisson T, Lyngfelt A.Investigation of manganese–iron oxide materials based on manganese ores as

oxygen carriers for Chemical Looping with Oxygen Uncoupling (CLOU). EnergyTech 2014;2:469–79.

[31] Fahim MS, El Faramawy H, Ahmed AM, Ghali SN, Kandil AEHT.Characterization of Egyptian manganese ores for production of high carbonferromanganese. J Miner Mater Charact Eng 2013;1:68–74.

[32] Wang B, Xiao G, Song X, Zhao H, Zheng C. Chemical looping combustion ofhigh-sulfur coal with NiFe2O4-combined oxygen carrier. J Therm Anal Calorim2014;118:1593–602.

[33] Szekely J, Evans JW, Sohn HY. Gas-solid reactions; 1976.[34] Son SR, Kim SD. Chemical-looping combustion with NiO and Fe2O3 in a

thermobalance and circulating fluidized bed reactor with double loops. IndEng Chem Res 2006;45:2689–96.

[35] Itoh S, Sato S, Ono J, Okada H, Nagasaka T. Feasibility study of the new rutileextraction process from natural ilmenite ore based on the oxidation reaction.Metall Mater Trans B 2006;37:979–85.

[36] Bhogeswara Rao D, Rigaud M. Kinetics of the oxidation of ilmenite. Oxid Met1975;9:99–116.