Comparison of a new micaceous iron oxide and ilmenite as oxygen carrier for Chemical looping...

Applied Energy 113 (2014) 1863–1868

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Applied Energy

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Comparison of a new micaceous iron oxide and ilmenite as oxygencarrier for Chemical looping combustion with respect to syngasconversion

0306-2619/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.apenergy.2013.04.056

⇑ Corresponding author. Tel.: +49 711 685 63790; fax: +49 711 685 63491.E-mail address: florian.mayer@ifk.uni-stuttgart.de (F. Mayer).

Florian Mayer a,⇑, Ajay R. Bidwe a, Alexander Schopf b, Kamran Taheri a, Mariusz Zieba a,Günter Scheffknecht a

a Institute of Combustion and Power Plant Technology (IFK), University of Stuttgart, Pfaffenwaldring 23, 70569 Stuttgart, Germanyb Institut für Mineralogie und Kristallchemie (IMK), University of Stuttgart, Azenbergstr. 18, 70174 Stuttgart, Germany

h i g h l i g h t s

�MIOX ME 400 is able to convert more syngas compared to ilmenite.� Activation of both natural oxygen carriers was shown.� Conversion of methane can be raised to 85–60% at 950 �C with MIOX ME 400.� Whereas syngas is almost totally converted already at 900 �C with MIOX ME 400.

a r t i c l e i n f o

Article history:Received 27 December 2012Received in revised form 15 April 2013Accepted 17 April 2013Available online 22 May 2013

Keywords:Chemical looping combustionCLCOxygen carrierIlmeniteMicaceous iron ore ‘‘MIOX’’

a b s t r a c t

Chemical looping combustion (CLC) is a promising carbon capture and storage (CCS) technology. One ofthe challenges is to find the most suitable oxygen carrier (OC). Using solid fuels makes it important to usecheap and natural oxygen carriers, since there will probably be some loss of bed material while discharg-ing ash from the system. Therefore ilmenite and a new micaceous iron oxide (MIOX ME 400) are com-pared with respect to syngas conversion in a 10 kWth bubbling fluidized bed (BFB) reactor. The OC wasalternatively reduced with either CO + H2 or CH4 + H2 and oxidized with air at 900 �C. The conversionof syngas with MIOX ME 400 is always higher (XCO, XH2 > 98%) than that with ilmenite. Conversion ofCH4 is also better for MIOX ME 400, even though it is still low. It can be raised by increasing fuel reactortemperature from 900 �C to 950 �C which results in a CH4 conversion of 85–60% instead of 60–40%.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Research on carbon capture and storage (CCS) technologies isgetting more and more important, since the climate prognosesare revealing increasing temperature on the earth. Scientists agreethat the temperature rise is linked with the rising greenhouse gasconcentration in the atmosphere. The main contributor to theworldwide anthropogenic greenhouse gas emissions is CO2 fromfossil fuels. By 2004, 27% of the total amount of emitted anthropo-genic CO2 had resulted from power generation [1]. Therefore it isimportant to use more renewable energies as well as CCS technol-ogies for clean power production from fossil fuels. CCS technologiesare classified into pre-combustion, oxyfuel and post-combustiontechnologies. Chemical looping combustion (CLC) is allocated to

the oxyfuel technologies and provides less electrical efficiency losscompared to other technologies [2]. Ekström et al. [2] calculated thenet electrical efficiency of a 445 MW bituminous coal CLC plantwith CO2 compression to 110 bar and without transport and storageto 42%. The reference case without CCS achieves an efficiency of44%, whereas the oxyfuel and pre-combustion processes achieve37% and 36% respectively. Adánez et al. [3] and Lyngfelt [4] alsoconcluded that CLC should be an economical alternative comparedto other CCS technologies.

In order to have an intensive contact of the oxygen carrier (OC)and the fuel CLC is normally carried out in a dual fluidized bed(DFB) system. The oxygen is transported from the air reactor(AR) to the fuel reactor (FR) by a solid OC. The OC is reduced bythe fuel in the FR and is afterwards recycled to the AR to be reox-idized with air again. In CLC all the three gaseous, liquid or solidfuels can be used. In the beginning of CLC Richter and Knoche [5]introduced CLC with gaseous fuels, followed by some important

1864 F. Mayer et al. / Applied Energy 113 (2014) 1863–1868

research performed on test facilities from 10–120 kWth [6–8].Since coal is more abundant and emits more CO2 per fuel powerthan natural gas, scientists have also started to investigate themore complex CLC process with solid fuels. The CLC of solid fuelsmainly faces two challenges. Since there is no solid–solid reaction,the solid fuel needs to be gasified either directly in the FR or in aseparate reactor [9]. At the moment, the main research is focusedon the direct gasification and the synthesis gas burning in the FRto keep the facility simple and cheap instead of using an additionalreactor for indirect gasification. Another advantage of direct gasifi-cation is the faster gasification of solid fuels while the gasificationproducts are burned immediately with the oxygen from the OC. Byburning these gasification products the inhibiting gases for gasifi-cation are removed immediately and the gasification is faster[10]. Consequently the first challenge is to totally burn the gasifiedcoal and the second is to reduce the transport of remaining un-burned coal or coke to the AR which would result in carbon captureefficiency loss. At Chalmers University in Sweden [11] and South-east University in China [12] investigations were performed onthese challenges using DFB systems of 10 kWth each. Cuadratet al. [9] also performed similar experiments on a DFB system with255 Wth. Liquid fuels are rather seldom investigated, even thoughthey also show good results in a batch fluidized bed reactor [13].

Finding suitable OCs for each CLC process with different fuelsand different facility configurations (for example direct and indi-rect gasification) is still one of the biggest challenges. In generalOC should be highly reactive with the fuel, environmentally harm-less, should have high oxygen capacity and good mechanicalstrength. For solid fuels with direct gasification it is also advanta-geous to use low cost OC, because the system will probably losesome OC during separation of ash out of the system. This is whycheap and natural occurring OC are favorable instead of using spe-cially prepared synthetic OC. Schwebel et al. [14] and Leion et al.[15] performed some remarkable material tests on natural OCand waste products from industry as OC.

The aim of this work is to compare two different natural OC inregard to fuel conversion ability. The system uses syngas as fueland mirrors in a simplified way direct coal combustion since therewould not be a reaction between the OC and coal but a reaction be-tween the syngas and the OC. Therefore a batch fluidized bed reac-tor was alternately fluidized with a certain syngas mixture toreduce the sample and air to oxidize it again. In between therewas always a period of purging with N2. The two tested materialsconsist of ilmenite as a kind of reference material [16] and a newmicaceous iron ore.

2. Experimental section

2.1. Oxygen carrier materials

Ilmenite is an often investigated OC for CLC process [8,14,15].Depending on the origin of the material it shows slightly differentbehavior. Schwebel et al. [14] concluded that the performance ofthe OC depends on the chemical composition and therefore onthe availability of active oxides. The OC used in this work is ilmen-

Table 1Composition of ilmenite from Capel, Australia and MIOX ME 400.

Compound Ilmenite (wt.%) MIOX ME 400 (wt.%)

Titan oxide 51.7 –Iron oxide 45.9 98.1Other mineral oxides 1.4 1.9Unknown impurities 1.0 –

ite sand from Capel, Australia [16]. The composition is shown inTable 1. Due to its limited reactivity with fuel, especially CH4, itis important to find a new natural OC which achieves higher fuelconversion than ilmenite. MIOX ME 400 is a new micaceous ironore in the field of CLC and is supposed to have this potential. MIOXME 400 could be allocated to natural OC, although it is a commer-cially available product on the market which is used as a pigmentin paint industry. The particles are simply mined, crushed and clas-sified. Its composition mainly consists of iron and is shown in Ta-ble 1. Both ilmenite and MIOX ME 400 belong to the Geldart Bgroup and are easy to fluidize. The fluidization behavior of bothOC is tested in cold model and pressure drop studies in the BFBreactor. The pressure drop of the MIOX ME 400 bed during exper-iments was 46 mbar in average with a standard deviation of circa5 mbar. The shape of both OC is totally different and can be com-pared in Figs. 1 and 2. Ilmenite sand particles are rounded contraryto MIOX ME 400 which is rather flaky and possesses a sphericityanalogical to mica flakes of 0.28 [17]. The oxygen capacity isdependent on the degree of reduction. According to Adánez et al.[18] ilmenite consists of Fe2TiO5 + TiO2 after the first oxidationwhich can be reduced to FeTiO3 corresponding to reaction (I). Aftersome redox cycles the amount of Fe2O3 is rising and Fe2TiO5 + TiO2

is decreasing. Fe2O3 can be reduced stepwise to Fe3O4 (reaction(II)), FeO (reaction (III)) and Fe, although for CLC only the firstreduction is favorable, since highest fuel conversion is possible.Reaction (III) and even further reduction to elementary iron arenot favorable due to thermodynamic limitation in fuel conversion[19,20]. Each reduction is exemplarily shown for CO. Since MIOXME 400 mainly consists of Fe2O3 only reactions (II) and (III) aresupposed to occur. The theoretical oxygen transport capacitiesfor reactions (I), (II), (III) are 5 wt.%, 3.3 wt.% and 6.9 wt.% respec-tively, independently from the type of reducing gas. Since reaction(III) is afflicted with thermodynamic limitations the oxygen trans-port capacity of MIOX ME 400 is supposed to be 3.3 wt.%.

Fe2TiO5 þ TiO2 þ CO! 2FeTiO3 þ CO2 ðIÞ

3Fe2O3 þ CO! 2Fe3O4 þ CO2 ðIIÞ

Fe3O4 þ CO! 3FeOþ CO2 ðIIIÞ

2.2. Batch fluidized bed reactor

The experiments presented here are carried out in a single bub-bling fluidized bed (BFB) reactor which is alternatively under oxi-dizing and reducing atmosphere. The inner diameter is 0.15 m

Fig. 1. Flaky shape of fresh MIOX ME 400.

Fig. 2. Rounded shape of fresh ilmenite sand.

0.950.960.970.980.9910

102030405060708090

100

ω [−]

X H2[%

] ilmenite, increasing cycle number MIOX ME 400,

increasing cycle number

Fig. 3. Conversion of H2 with MIOX ME 400 and ilmenite as a function of x for

F. Mayer et al. / Applied Energy 113 (2014) 1863–1868 1865

and the height 3.5 m. All experiments were performed using thesame bubbling bed height equal to three times the reactor diame-ter which was tested before in a cold model. Therefore differentbed masses of 11 kg and 17 kg for MIOX ME 400 and ilmenite cor-respondingly were used. The tests of each material start with freshparticles which are first fully oxidized for more than 1 h at 900 �C.Air is used to oxidize the bed material, while for reduction, a fuelmixture of either CO, H2, N2 or CH4, H2, N2 (12.5 vol.%, 12.5 vol.%,75 vol.% respectively) is used. The thermal input of experimentswith CO and H2 is together 2.3 kW or 5 kW for experiments withCH4 and H2. The fluidization velocity is set to 0.19 m/s which cor-responds to two times the minimum fluidization velocity. Temper-ature is maintained constant at 900 �C except for one experimentwith methane which is at 950 �C. Between each reduction and oxi-dation, the bed is fluidized with only N2 to avoid fuel gases mixingwith air. Temperature in the bed is measured by thermocouplesand controlled by external electrical heaters. The inputs of gasesare controlled by mass flow controllers (MFCs). The outlet gas com-position is analyzed by an ABB advanced Optima 2020 continuousgas analyzer which measures the following gas components: CO2,CO, CH4, H2 and O2. Recording of temperatures, pressure drops, in-let gas flows and outlet gas concentrations from the gas analyzer isdone by LabVIEW�.

3. Data evaluation

Data evaluation is according to the previous publication of Bid-we et al. [16]. This paper focuses on fuel conversion in the FR, sinceit is one of the biggest challenges in CLC. The gas conversions of CO(XCO), CH4 (XCH4) and H2 (XH2) in the syngas experiments of equi-molar CO:H2 or CH4:H2 mixtures are calculated using followingEqs. (1)–(3).

XCO ¼ yCO2ð Þ= yCO þ yCO2ð Þ ð1Þ

XCH4 ¼ 1� yCH4= yCH4 þ yCO2 þ yCOð Þð Þ ð2Þ

XH2 ¼ n�

H2 in � n�

tot outyH2

� �= n�

H2 in

� �ð3Þ

n�

iin is the inlet flow rate of compound i in kmol/h and yi is the vol-umetric fraction of gas component i in the dry flue gas measured bygas analyzer. ntot is the total exit flow rate of gas in kmol/h. It is cal-culated using the analogy that n

�CO in ¼ n

�CO out þ n

�CO2 out and

n�

CH4 in ¼ n�

CH4 out þ n�

CO2 out þ n�

CO out respectively, assuming that COand CH4 respectively converts only to CO2. Therefore n

�tot is calcu-

lated as defined in Eqs. (4) and (5) respectively:

n�

tot ¼ ðn�

CO inÞ=ðyCO þ yCO2Þ ð4Þ

n�

tot ¼ ðn�

CH4 inÞ=ðyCH4 þ yCO2 þ yCOÞ ð5Þ

A mass based conversion of OC (x) is normally used to comparethe reactivity of different OC [16]. Thereby it is easy to calculate theneeded circulation rate of OC between the AR and the FR for a cer-tain oxygen demand and mass based conversion of the OC in theFR. It is simply the ratio of mass of OC (m) to the mass of OC inits most oxidized state (mox) as shown in Eq. (6). In this experimen-tal setup, x is calculated using Eqs. (7), (8) with MO as the molec-ular weight of oxygen. x is set to 1 at the beginning of everyreduction cycle.

x ¼ m=mox ð6Þ

x ¼ 1� ðM0=moxÞZ t1

t0ðXCOn

�CO in þ XH2n

�H2 inÞdt ð7Þ

x ¼ 1� ðM0=moxÞZ t1

t0ðXCH4n

�CH4 in þ XH2n

�H2 inÞdt ð8Þ

4. Results and discussion

4.1. Activation of ilmenite versus MIOX ME 400

Figs. 3 and 4 show the conversion of CO and H2 with ilmenite andMIOX ME 400 respectively. Each line represents one reduction pro-cess, whereas for ilmenite reduction number 2–5 and for MIOX ME400 reduction number 2, 4 and 5 are shown. The conversion is risingwith rising cycle number. The reason for this activation is theincreasing porosity of the particles which can be seen in Figs. 5and 6. Fig. 5 shows the used MIOX ME 400 after 31 h and sevenredox cycles whereas Fig. 6 shows the used ilmenite after 31 hand six redox cycles in the BFB system. Additionally to the shownactivation process, a well defined step of gas conversion for MIOXME 400 can be seen at x = 0.97 which represents the lower fuelconversion of reaction (III) in comparison to reaction (II). MIOXME 400 always shows higher conversion compared to ilmenite.

4.2. Temperature variation for MIOX ME 400 with methane

Conversion of CH4 with MIOX ME 400 is relatively low, whichcan be seen in Fig. 7. The lower graph corresponds to reductionnumber 6 with CH4 and H2 at 900 �C and the upper one to reduc-tion number 7 at 950 �C. Thus increasing the temperature is alsoincreasing the fuel conversion, although there could be a minor ef-

cycles 2–5 at 900 �C.

0.950.960.970.980.9910

102030405060708090

100

ω [−]

X CO

[%] ilmenite,

increasing cycle number MIOX ME 400, increasing cycle number

Fig. 4. Conversion of CO with MIOX ME 400 and ilmenite as a function of x forcycles 2–5 at 900 �C.

Fig. 5. Porous structure of used MIOX ME 400 after 31 h of experiments and sevenredox cycles.

Fig. 6. Porous structure of used ilmenite after 31 h of experiment and six redoxcycles.

0.950.960.970.980.9910

102030405060708090

100

ω [−]

X CH

4[%]

950°C

900°C

Fig. 7. Conversion of CH4 with MIOX ME 400 as a function of x for cycles 6 + 7 at900 �C and 950 �C respectively.

0.950.960.970.980.9910

102030405060708090

100

ω [−]

X CH

4, CO

, H2[%

]

CH4, ilmenite CH4, MIOX ME 400

CO and H2, ilmenite

CO and H2, MIOX ME 400

Fig. 8. Summary of gas conversions with MIOX ME 400 and ilmenite at 900 �C.

1866 F. Mayer et al. / Applied Energy 113 (2014) 1863–1868

fect because of activation. But there is an upper temperature limitwhere the particles start to melt and agglomerate. This phenome-non could be observed at the successive oxidation at 950 �C whichis probably boosted due to the low fluidizing velocity used in theseexperiments. Another reason for melting of ilmenite and iron basedOC is given by Cuadrat et al. [21] and Cho et al. [22]. According to

them agglomeration and defluidisation can happen during oxida-tion of strongly reduced ilmenite and iron based OC at high tem-peratures. Testing the raw MIOX ME 400 in an oven at 1000 �Cfor more than 1 h did not cause any agglomeration and therebyproofed its usability related to melting for a real CLC plant.

4.3. Comparison of gas conversions with both oxygen carriers

Fig. 8 shows the summary of all mentioned gas conversions forthe 5th (CO and H2) and 6th (CH4) reduction cycle each at 900 �C.Reduction cycle number 6 for ilmenite with CH4 at 900 �C is addedto complete the comparison. It can be concluded that MIOX ME400 possesses the ability to convert a higher amount of any men-tioned gas compared to ilmenite. The favorable reduction degreeof MIOX ME 400 is limited to x = 0.97. This is where all Fe2O3 is re-duced to Fe3O4 and the next reduction to FeO starts. The CO con-version with ilmenite achieved within this study is lower thanthat presented by Leion et al. [15]. The authors achieve a conver-sion of CO of 100–95% with Norwegian ilmenite at 950 �C duringthe 15th reduction cycle. The reduction was performed down tox = 0.95. The ilmenite used here converts CO to 99–80% at 900 �Cduring the 5th reduction down to x = 0.97. The higher CO conver-sion presented by Leion et al. [15] results doubtless from the higherprocess temperature and the higher cycle number. Such high con-version can also be linked to the different chemical composition ofthe ilmenite and thus to its different reactivity properties. Thehigher reactivity of the Norwegian ilmenite compared to the testedilmenite sand is emphasized by mentioning the solid inventory ofOC in relation to the fuel power input. Normally the conversion in-creases by using more inventory. But even using 7391 kg/MW re-

0 100 200 300 400 500 600 700 8000

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100

particle diameter [µm]

cum

ulat

ive

volu

met

ric c

onte

nt [%

] raw ilmenite

ilmenite after 31 h

Fig. 9. Unchanged particle size distribution of raw and used ilmenite after 31 h ofexperiment and six redox cycles.

0 100 200 300 400 500 600 700 8000

102030405060708090

100

particle diameter [µm]

cum

ulat

ive

volu

met

ric c

onte

nt [%

]

raw MIOX ME 400

MIOX ME 400 after 18 h MIOX ME 400 after 31 h

MIOX ME 400 from 1.cyclon after 18 h MIOX ME 400 from 2.cyclon after 18 h

Fig. 10. Changing particle size distribution of raw and used MIOX ME 400.

F. Mayer et al. / Applied Energy 113 (2014) 1863–1868 1867

sults in a lower conversion than the conversion of fuel with Norwe-gian ilmenite using 149 kg/MW. However MIOX ME 400 showsbetter conversions than all materials tested by Leion et al. [15].Such performance is achieved even by a low number of activationcycles. The experiments using MIOX ME 400 are performed with4783 kg/MW and need to be proofed with either more fuel inputor less inventory to get a direct comparison to the experimentsperformed by Leion et al. [15]. The experiments shown here areperformed with a relatively high amount of bed material to see ifit helps to get higher fuel conversion especially with ilmenite. Adá-nez et al. [18] calculated a relative bed mass for direct coal com-bustion with ilmenite of 350–1600 kg/MW which should betaken for further experiments with MIOX ME 400.

4.4. Comparison of attrition of both oxygen carriers

In order to compare attrition of both OC a particle size distribu-tion of the fresh and used particles is prepared. Each particle sizedistribution shows the diameter of round particles that would havethe same volume than the analyzed particles. The tests are per-formed with the ‘‘Malvern Mastersizer 3000’’. As can be seen fromFig. 9, ilmenite sand keeps its original particle size distributionafter 31 h of experiments in the bubbling fluidized bed and doesnot lose much material due to attrition. The lines are equal in therange of uncertainty. Fig. 10 shows an opposite behavior for MIOXME 400. The raw MIOX ME 400 clearly provides bigger particlesthan the used material after 18 or 31 h of experiments. This lostmaterial due to attrition could be recovered at the bottom of thefirst and second cyclone of flue gas cleaning. Noticeable to see isthat the graphs of the used material after 18 and 31 h are equalin the range of uncertainty. This indicates an initial loss of material

and clearly less attrition of particles after the first 18 h of experi-ment. This can also be confirmed by the collected fines from the cy-clones which give a number for the lost material per hour. MIOXME 400 loses 0.073% per hour in the first 18 h which decreasedto 0.031% per hour during the next 13 h. However Ilmenite loses0.013% per hour during the first 24 h of experiments followed by0.04% per hour during the next 7 h. So attrition of ilmenite is lesscompared to MIOX ME 400 but both attrition rates are low. Com-pared to Cuadrat et al. [21] who performed a similar experimentin a smaller scale with 0.076% fines per hour, these values are inthe same range.

5. Conclusions

The comparison of two different oxygen carriers with respect tosyngas conversion was performed in a 10 kWth bubbling fluidizedbed reactor. It can be concluded, that the new micaceous iron orecalled MIOX ME 400 provides higher fuel conversion than ilmenite.Since MIOX ME 400 mainly consists of Fe2O3, it provides high fuelconversion (XCO, XH2 > 98%) down to x = 0.97, which correspondsto the reduction to Fe3O4. The next reduction step to FeO providesless conversion. The conversion of CH4 is higher with MIOX ME 400than with ilmenite, but still relatively low. Even rising the fuelreactor temperature from 900 �C to 950 �C improves the conver-sion only to 85–60%. Characterizing a new oxygen carrier alsoneeds to analyze attrition. MIOX ME 400 loses some fine particlesout of the system but the amount should be manageable for a realCLC plant since it is in the same range than the Norwegian ilmenitetested by Cuadrat et al. [21]. All in all MIOX ME 400 could be apromising new oxygen carrier for direct coal combustion but it stillneeds to be tested in a more realistic fuel reactor with about1600 kg/MW or less [18].

Acknowledgements

This work is performed under a joint research project with theIFK, University of Stuttgart, the Institute of Energy Systems and theInstitute of Solids Process Engineering and Particle Technology ofthe TUHH. The financial support is provided by the German FederalMinistry of Economics and Technology (FKZ 0327844B/CLOCK)with additional funding from BASF, EnBW Kraftwerke AG, E.ONEnergie AG, Hitachi Power Europe GmbH, RWE power AG and Vat-tenfall Europe Generation AG.

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