5276r 2009 American Chemical Society pubs.acs.org/EF

Energy Fuels 2009, 23, 5276–5283 : DOI:10.1021/ef900444dPublished on Web 08/27/2009

Use of CaMn0.875Ti0.125O3 as Oxygen Carrier in Chemical-Looping with Oxygen

Uncoupling

Henrik Leion*

Department of Environmental Inorganic Chemistry, Chalmers University of Technology, S-412 96 Goteborg, Sweden

Yngve Larring, Egil Bakken, and Rune Bredesen

Department of Energy Conversion and Materials, SINTEF Materials and Chemistry, Oslo, Norway

Tobias Mattisson and Anders Lyngfelt

Department of Energy and Environment, Chalmers University of Technology, S-412 96 Goteborg, Sweden

Received May 12, 2009. Revised Manuscript Received July 23, 2009

Chemical-looping with oxygen uncoupling (CLOU) is a novel method to burn fuels in gas-phase oxygenwithout the need for an energy-intensive air separation unit. The carbon dioxide from the combustion isobtained separated from the nitrogen in the combustion air. The technique is based on chemical-loopingcombustion (CLC) but does not involve any direct reaction between the fuel and oxygen carrier. Instead,the CLOU process uses three steps in two reactors, one air reactor where a metal oxide captures oxygenfrom the combustion air (step 1), and a fuel reactor where the metal oxide releases oxygen (step 2) andwhere this oxygen reacts with a fuel (step 3). Thismeans that the fuel burns directly with gaseousO2. In thiswork CaMn0.875Ti0.125O3 will be used as oxygen carrier. Experiments were first performed with athermogravimetric analyzer (TGA). Here the sintering temperature, and thereby the porosity, for theproduced granulates was varied and optimized. The substitution of Ti onMn sites in CaMnO3 was chosensince this material showed no coke formation even in dry CH4 at high temperatures. This was followed byfluidized bed experiments with bothmethane and petroleum coke as fuel. TheCaMn0.875Ti0.125O3 particlesshowed promising results both for the tests performed in TGA and in fluidized bed experiments.CaMn0.875Ti0.125O3 released O2 both in inert and reducing atmosphere, making it a possible candidateas oxygen carrier in CLOU.

Introduction

It is today believed that the increase in greenhouse gasconcentrations has caused a rise in global temperature that inturn has caused changes in today’s climate.1 Therefore areduction in emissions of greenhouse gases, and in particularCO2, is necessary. It is today possible to store CO2 in depletedoil- and natural gas fields as well as in deep coal beds.2 Alsostorage in aquifers, which are geologically sealed formationsoften filled with saline water, provides a large potential forstorage of CO2. This is today practiced in the North Seaoutside Norway at the Utsira Formation.2

In order to store CO2 produced by combustion, it first needsto be separated from the combustion gases. Costs and effi-ciency losses due to storing and transport ofCO2 to the storagesite can be assumed to be equal for all combustion processes.However, the way of capturing CO2 differs between thetechnologies. The loss in efficiency and increased production

costs of electricity with respect to a conventional power plantdiffers between the CO2 capture techniques.3,4 In chemical-looping combustion (CLC) CO2 is inherently separated fromthe other flue gas components, that is, N2 and unused O2.Thus, no energy input is expended for the gas separation,whereas otherCO2 capture techniques, such as oxyfuel, pre- orpostcombustion involves a energy consuming, and therebycostly, gas separation step.5

The idea of CLC originates from a patent for CO2 produc-tion by Lewis and Gilliland6 in 1954. An oxygen carrier iscirculated between two fluidized bed reactors, an air and a fuelreactor. The fuel is introduced to the fuel reactor where itreacts with an oxygen carrier (MexOy) to CO2 and H2O,reaction 1. The reduced oxygen carrier is transported to

*Towhom correspondence should be addressed. Telephone:þ46-31-7722886. Fax: þ46-31-7722853. E-mail: Leion@chalmers.se.(1) Rosenzweig, C.; Karoly, D.; Vicarelli, M.; Neofotis, P.; Wu, Q.;

Casassa, G.; Menzel, A.; Root, T. L.; Estrella, N.; Seguin, B.;Tryjanowski, P.; Liu, C.; Rawlins, S.; Imeson, A. Nature 2008, 453(15), 353–357.(2) IPPC, I. P. o. C. C. Carbon Dioxide Capture and Storage; 2005.

(3) Davidson, J.; Thambimuthu, K. In Technologies for Capture ofCarbon Dioxide; 7th International Conference on Greenhouse Gas ControlTechnologies, Vancover, Canada, 2004.

(4) Herzog, H. J. In CO2 Capture and Storage: Cost and MarketPotential; 7th International Conference on Greenhouse Gas Control Tech-nologies, Vancover, Canada 2004.

(5) Metz, B.;Davidson,O.; deConinck,H.C.; Loos,M.;Meyer, L.A.IPCC Special Report on Carbon Dioxide Capture and Storage; Preparedby Working Group III of the Intergovernmental Panel on Climate Change. InIPCC, Cambridge University Press: Cambridge, United Kingdom and NewYork, USA, 2005.

(6) Lewis, W. K.; Gilliland, E. R. Production of Pure Carbon Dioxide.U.S. Patent 2665972, 1954.

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the air reactor where it is oxidized back to its original state byair, reaction 2.

CnH2m þ ð2nþmÞMexOy T nCO2 þmH2O

þ ð2nþmÞMexOy-1 ð1Þ

O2þ2MexOy-1 T 2MexOy ð2ÞThis gives one stream of oxygen-depleted air leaving the air

reactor and one stream of combustion gases, which mainlyconsists ofCO2 andH2O, leaving the fuel reactor. Thewater iseasily condensed and the CO2 can, after compression, betransported to an underground storage location. Becausethe fuel never meets the air, CO2 is inherently separatedwithout any direct loss in efficiency.

The CLC process has been successfully demonstrated usinggaseous fuel with different oxygen carriers in several proto-type units based on interconnected fluidized beds.7-17 Anoverview of literature concerning CLC is given by Lyngfeltet al.18 or Hossain and de Lasa.19 The major part of the workconcerning CLC has so far been with gaseous fuel, such asnatural gas or methane, but recent research is aiming to adaptthe technology to solid fuels.20-22 One way of doing this is tointroduce the coal directly to the fuel reactor where thegasification of the coal and subsequent reactions with the

metal oxide particles will occur simultaneously.23-29 Anotherway is to use an oxygen carrier that releases O2 in the fuelreactor andactually burns the fuelwith gas-phase oxygen.30-33

The last alternative is referred to as chemical-looping withoxygen uncoupling (CLOU)

CLOU involves three steps in two reactors, as shown inFigure 1. In the air reactor, an oxygen carrier captures oxygenfrom the combustion air (step 1), according to reaction 2. Theoxygen carrier is transported to the fuel reactor, where itreleases oxygen (step 2) according to the reverse reaction:

2MyOxT2MyOx-1 þO2ðgÞ ð3ÞThe released oxygen reacts with a fuel (step 3), in this case

assumed tobeonly carbonC, according tonormal combustion:

CþO2ðgÞ f CO2 ð4ÞThe reduced oxygen carrier is then recirculated to the air

reactor to be regenerated, that is, step 1. Since reactions 2 and3 cancel each other, the net reaction over the CLOU system issimply reaction4, that is, normal combustion.Thismeans thatthe total heat release over the fuel and air reactor is the sameasfor conventional combustion or regular CLC. The addedadvantage compared to regular CLC is that the slow gasifi-cation step when employing solid fuels is eliminated. This hasthe implication that much less oxygen carrier material isneeded in the system, which will also reduce the reactor sizeand thereby associated costs.

Figure 1. Schematic picture of the CLOU-process. Two intercon-nected fluidized bed reactors, one air and one fuel reactor, withcirculating oxygen carrying particles. The fuel is assumed to be purecarbon, C.

(7) Lyngfelt, A.; Thunman, H. Construction and 100 h of operationalexperience of a 10-kW chemical-looping combustor. Carbon DioxideCapture for Storage in Deep Geologic Formations;Results from the CO2

Capture Project 2005, 1, 625–645.(8) Lyngfelt, A.; Kronberger, B.; Ad�anez, J.; Morin, J.-X.; Hurst, P.

In The GRACE project. Development of oxygen carrier particles forchemical-looping combustion. Design and operation of a 10 kW chemical-looping combustor; The 7th International Conference on Greenhouse GasControl Technologies Vancouver, Canada, 2004.(9) Ryu,H.-J.; Jin, G.-T.; Bae,D.-H.; Yi, C.-K.ContinuousOperation

of a 50 kWth Chemical-Looping Combustor: Long-Term Operation withNi- and Co-Based Oxygen Carrier Particles; Presented at the 5th China-Korea Joint Workshop on Clean Energy Technology, October 25-28,Qingdao University, China, 2004; pp 221-230.(10) Abad, A.; Mattisson, T.; Lyngfelt, A.; Ryd�en, M. Fuel 2006, 85

(9), 1174–1185.(11) Johansson, E.; Mattisson, T.; Lyngfelt, A.; Thunman, H. Fuel

2006, 85 (10-11), 1428–1438.(12) de Diego, L. F.; Garcia-Labiano, F.; Gayan, P.; Celaya, J.;

Palacios, J. M.; Adanez, J. Fuel 2007, 86 (7-8), 1036–1045.(13) Adanez, J.; Gayan, P.; Celaya, J.; de Diego, L. F.; Garcia-

Labiano, F.; Abad, A. Ind. Eng. Chem. Res. 2006, 45 (17), 6075–6080.(14) Johansson, E.; Mattisson, T.; Lyngfelt, A.; Thunman, H. Chem.

Eng. Res. Des. 2006, 84 (A9), 819–827.(15) Abad, A.; Mattisson, T.; Lyngfelt, A.; Johansson, M. Fuel 2007,

86 (7-8), 1021–1035.(16) Linderholm, C.; Abad, A.; Mattisson, T.; Lyngfelt, A. Int. J.

Greenhouse Gas Control 2008, 2 (4), 520–530.(17) Proll, T.; Kolbitsch, P.; Bolh�ar-Nordenkampf, J.; Hofbauer, H.

In A Dual Circulating Fluidized Bed (DCFB) System for ChemicalLooping Processes; AIChE Annual Meeting, Philadelphia, USA, November16-21, 2008.(18) Lyngfelt, A.; Johansson, M.; Mattisson, T. In Chemical-looping

combustion - Status of development; 9th International Conference onCirculating Fluidized Bed (CFB-9), Hamburg, Germany,May 13-16, 2008.(19) Hossain, M. M.; de Lasa, H. I. Chem. Eng. Sci. 2008, 63, 4433–

4451.(20) Berguerand, N.; Lyngfelt, A. Fuel 2008, 87, 2713–2726.(21) Berguerand,N.; Lyngfelt, A. Int. J. GreenhouseGasControl 2008,

2 (2), 169–179.(22) Andrus,H. E., Jr.; Chiu, J. H.; Liljedahl,G.N.; Stromberg, P. T.;

Thibeault, P. R.; Jain, S. C. ALSTOM’s hybrid combustion-gasificationchemical looping technology development; Proceedings - 22nd AnnualInternational Pittsburgh Coal Conference, 2005; pp 122/1-122/20.(23) Scott, S. A.; Dennis, J. S.; Hayhurst, A. N.; Brown, T. AIChE J.

2006, 52 (9), 3325–3328.(24) Lyon, R. K.; Cole, J. A. Combust. Flame 2000, 121, 249–261.(25) Dennis, J. S.; Scott, S. A.; Hayhurst, A. N. J. Energy Inst. 2006,

79 (3), 187–190.(26) Rubel, A.; Liu, K.; Neatherya, J.; Taulbee, D. Fuel 2009, 88, 5.

(27) Gao, Z.; Shen, L.; Xiao, J. J. Chem. Ind. Eng. 2008, 59 (4), 1242–1250.

(28) Leion, H.; Mattisson, T.; Lyngfelt, A. CO2 Capture from DirectCombustion of Solid Fuels with Chemical-Looping Combustion; In 33rdInternational Technical Conference on Coal Utilization & Fuel Systems,Clearwater Florida, USA, 2008.

(29) Leion,H.;Mattisson, T.; Lyngfelt, A.Effects of steam andCO2 inthe fluidizing gas when using bituminous coal in Chemical-LoopingCombustion; In The 20th International Conference on Fluidized Bed Com-bustion, Xi�an, China, 2009.

(30) Mattisson, T.; Lyngfelt, A.; Leion, H. Int. J. Greenhouse GasControl 2009, 3 (1), 11–19.

(31) Mattisson, T.; Leion, H.; Lyngfelt, A. Fuel 2009, 88 (4), 683–690.(32) Leion, H.; Mattisson, T.; Lyngfelt, A. Combustion of a German

Lignite Using Chemical-Looping with Oxygen Uncoupling (CLOU); In33rd International Technical Conference on Coal Utilization& Fuel Systems,Clearwater Florida, USA, 2008.

(33) Leion, H.; Mattisson, T.; Lyngfelt, A. Using Chemical-Loopingwith Oxygen Uncoupling (CLOU) for Combustion Six Different of SolidFuels; In 9th International Conference on Greenhouse Gas Control Technol-ogies, Washington D.C, USA, 2008.

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In previously published work the oxygen carrier used inCLOU has been Cu-based.30-32 In this work CaMn0.875Ti0.125O3 will be used as oxygen carrier. Bakken et al.34 haveshowed that CaMnO3 material has a continuous loss ofoxygen when reducing the partial pressure of O2 stepwisefrom 100 kPa to approximately 1 kPa O2(g) by reduction ofMn4þ ions toMn3þ. Previous tests of this material shows thatit has a tendency to decompose at high temperature intoCa2MnO4 and CaMn2O4,

34 and this might restrict the fullreoxidation of this material when cycling between reducingand oxidizing conditions. The material has therefore beenstabilized by Ti in order to restrict this type of decomposition.Experiments were first performed with a thermogravimetricanalyzer (TGA) in order to verify the improved stability.Further, the reactivity at different sintering temperatures wereinvestigated and optimized. Thiswas followed by fluidized bedexperiments with both petroleum coke and methane as fuel.

Experimental Section

Production of Particles. The initial activities were to optimizethe material composition, that is, the Ti concentration inCaMn1-xTixO3, and optimising the sintering temperature ofthe freeze granulated materials. Small batches of Ca0.97Mn1-x-TixO3 (x = 0.1, 0.125, 0.175, and 0.25) were synthesized byspray pyrolysis and by freeze granulation before the capacitiesand kinetics were measured in a TGA.

Powders of CaMn1-xTixO3 were synthesized by spray pyr-olysis using standardized aqueous solutions of 2 M Ca(NO3)2,2 M Mn(NO3)2, and 0.85 M Ti-isopropoxide citric acid com-plex. The solutions weremixed to the desired stoichiometry. Thesolutions were sprayedwith a two-phase nozzle at a rate of 8 L/hdirectly into a rotating furnace at 875 �C.The outlet temperaturewas 560 �C.The as-prepared powderswere calcined at 800 �C for6 h. The calcined powders were ball milled (YSZ balls) in waterfor 18 h, dried, ground, and sieved to a fine size.

Porous granulates of proper size of CaMn1-xTixO3 weremade by freeze granulation and freeze-drying. The powder fromspray pyrolysis was mixed with water, dispersant, binder, andantifoam additive and then ball milled. The obtained slurry waspumped through a nozzle and sprayed into liquid N2. This gavefrozen granulates that contained water. The water was removedby freeze-drying. Granulates were finally sintered and sieved toobtain the desired properties and size fractions.

Fluidized Bed Experiments, Setup. The solid fuel experimentswere conducted in a fluidized-bed reactor of quartz presented inFigure 2. In order to achieve good solids mixing between fueland oxygen carrier, the reactor was conically shaped just abovethe distributor plate to enhance mixing in the bed. The reactorhad a total length of 870 mm with a porous quartz plate placed370 mm from the bottom of the reactor. The same reactorhas previously been used in CLC and CLOU experiments atChalmers.33,35

The fluidized bed experimentswithmethaneweremadewith astraight reactor with inner diameter of 22 mm. The reactor hadthe same length as the conical reactor, and the porous quartzplate was located at the same position as in the conical reactor,that is, 370 mm from the bottom.

A total of 15 g of oxygen carrier of size 0.09-0.125 mm wasplaced on the porous plate and tested under alternating oxidiz-ing and reducing atmospheres. During the oxidation, a flow of1000 mLn/min (0 �C, 100 kPa) with 5.5% of O2 in N2 wasadded from the bottom of the reactor. During reduction withpetroleum coke, 0.05 g of fuel, equivalent to a mass reduction of

the bed by 0.7%, was added from the top of the reactor, fallinginto the bed. During the reduction with petroleum coke, the bedwas fluidized with 50% steam in N2 at a flow of 600 mLn/min.The petroleum coke had a size of 0.125-0.180 mm, that is,somewhat larger than the oxygen carrier particles. An elemen-tary analysis of the petroleum coke is presented in Table 1. In theexperimentwheremethanewasused as fuel, a flowof 450mLn/minconsisting of pure CH4 was added from the bottom of the reactor.Between each oxidizing and reducing period, inert nitrogen witha flow of 450 mL/min gas was introduced for 60 s for both themethane and petroleum coke experiments.

The height of the bed was 30 mm, in both reactors, when thebed was not fluidized. The temperature was measured 5 mmunder and 10 mm above the porous quartz plate, using Pen-tronic CrAl/NiAl thermocouples enclosed inconel-600 in quartzshells. All temperature references for the fluidized bed experi-ments in this paper refer to the upper thermocouple in the actualbed. The steam was delivered by a steam generator (Cellkraft,Precision Evaporator E-1000).

The gas from the reactor was led to an electric cooler, wherethe steam was removed, and then to a gas analyzer (RosemountNGA-2000) where the concentrations of CO2, CO, CH4, andO2

were measured in addition to the volumetric gas flow. For theexperiments with petroleum coke, a small flow of N2 was addedin the top of the reactor, partly to sweep the fuel down into thereactor and partly to serve as carrier gas after the steamhad beenremoved in the cooler. A schematic layout of the laboratorysetup is presented in Figure 3.

Data Evaluation. The gas yield (γ) is used to quantify theconversion of gas in the gaseous fuel fluidized bed experiments.γ is the fraction of CO2 in the outgoing gas divided with the sumof the fractions of carbon-containing gases in the outgoing gas.Hence, a γ of 1 corresponds to total conversion of the fuel toCO2. With CH4 as reducing gas this gives:

γ ¼ xCO2

xCO2þ xCO þ xCH4

ð5Þ

where xi is the fraction of component i in the outgoing gas flow.All gas measurements were made on dry gases downstream ofthe cooler. However, eq 5 is independent of whether the gas iswet or dry.

The degree of mass-based conversion, ω, is used to describethe oxygen uptake of the oxygen carrier.ω is defined as themassof the oxygen carrier divided with the mass of the oxygen carrierin its most oxidized state:

ω ¼ m

moxð6Þ

wherem is the mass of the oxygen carrier at any given time, andmox is the mass of the oxygen carrier at the end of the oxidation.

Figure 2. Conical fluidized-bed reactor of quartz.

(34) Bakken, E.; Norby, T.; Stoelen, S. Solid State Ionics 2004, 176(1-2), 217–223.(35) Leion, H.; Mattisson, T.; Lyngfelt, A. Fuel 2007, 86 (12-13),

1947–1958.

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Note that due to the different oxygen uptake at differenttemperatures of the oxygen carrier used in this work, mox isdependent on the temperature of the oxygen carrying particles.In TGA m is directly measured, but since this is not possible inthe fluidized bed experiments ω was calculated as the timeintegral of exhaust gas concentrations through eq 7,

ωi ¼ ωi-1 -R t1t0

_nMO

moxð4xCO2þ 3xCO - xH2

Þ dt ð7Þ

where _n is the total molar flow rate, andMO is themolar mass ofoxygen.

In CLC-literature, the degree of conversion of the oxygencarrier,X, is often used to quantify the conversion of the oxygencarrier. X can easily be converted to ω by:

ω ¼ 1þROðX -1Þ ð8Þwhere RO is the oxygen transfer capacity, that is, the fraction ofavailable oxygen in the oxygen carrier. Again note that also RO

will be dependent on temperature for the oxygen carrier used inthis work.

In solid fuel experiments the rate of conversion for thepetroleum coke is given as an average rate, calculated from:

rAve ¼ 1

mtot

mt

tð9Þ

where t is the time elapsed since the start of the cycle, mtot is thetotal mass of carbon converted during the entire reducingperiod, and mt is the mass of carbon converted up until time t.The total carbon in theCLC solid fuel experiments is determinedfrom the integration of the outgoing CO and CO2 concentra-tions. The rate used in this paper, is the average rate for a massconversion of the fuel of 95%, identical to the rate used inpreviously published work.35

Results

TGA Experiments. Optimizing Ti Content. Only smalldifferences in the capacity and the reactivity were observedfor oxygen carriers with different Ti fractions. Tables 2 and 3show the oxygen capacities and the maximum reaction rates,respectively, in reducing atmosphere for 10% CH4 and 25%CO2 in Ar and the milder reduction of 5%H2 and 25% CO2

in Ar. The material was in the form of a fine powder, and thetests were made at temperatures of 800, 900, and 1000 �C.Not surprisingly, the reaction rate increased with increasingtemperature. The reactivity increased with increasing Ti

content up to a Ti fraction of 0.125, after which it startedto decrease. The mechanism behind this behavior is notcompletely understood, but the same trend was seen for alloperating temperatures. Since a Ti content of 0.125 showedthe best performance, this composition was chosen forfurther development and testing.

Reactivity of Oxygen Carrier. The sintering profile ofCaMn0.875Ti0.125O3 was determined by dilatometry, andthe reactivity of samples annealed at different temperatureswas measured by TGA. Tables 4 and 5 show the reactivity ofgranulates (125-180 μm) sintered at 1050 and 1250 �C andthe reactivity of powder that was annealed at 1200 �C. Themaximum rate is given as -dω/dt. It is clear that the reactionrate decreases with increasing sintering temperature. In thiswork the appropriate sintering conditions for sufficientattrition strength of the particles were determined to be 3 hat 1200 �C.

Available Oxygen in Tested Particles. The fraction ofavailable oxygen, RO, in the oxygen carriers of CaMn0.875-Ti0.125O3was obtainedwhen samples were reduced in dilutedhydrogen, 5%H2, 25%CO2, in Ar for 30min, and in diluted

Table 1. Fuel Analysis of Petroleum Coke

proximate [wt %, as received] [wt% dry ash free] ultimate [wt %, dry ash free]

Hi [MJ/kg](as received) moisture ash combustibles volatiles C H N S O

30.9 8.0 0.5 91.5 10.9 88.8 3.1 1.0 6.6 0.5

Figure 3. Schematic layout of the laboratory setup.

Table 2. The Capacity of CaMn1-xTixO3 (wt % O2), for reductionwith 10% CH4 and 25% CO2 and Oxidation with Air at Three

Different Temperaturesa

CaMn1-xTixO3 x = 0.1 x = 0.125 x = 0.175 x = 0.25

800 �C 9.4 9.4 8.6 9.1900 �C 9.4 9.4 8.6 9.21000 �C 9.0 9.1 8.3 8.9

aValues for capacity of CaMn1-xTixO3 (wt % O2) using 5% H2 and25% CO2 gives nearly the same results (deviation less than 0.1 wt %).

Table 3. Maximum Reaction Rate in the CH4-Containing Gas

Mixture at Different Temperatures As a Function of Composition As

Obtained in the TGA (-dω/dt (g/kg s))

CaMn1-xTixO3 x = 0.1 x = 0.125 x = 0.175 x = 0.25

800 �C 0.47 0.51 0.70 0.67900 �C 1.00 1.04 1.19 1.121000 �C 1.49 1.47 1.57 1.47

Table 4. Maximum Reaction Rate in 10% CH4 and 25% CO2 at

Different Temperatures As a Function of Temperature Treatment for

Powder Prepared by Spray Pyrolysis (-dω/dt (g/kg s))

CaMn0.875Ti0.125O3

1050 �C (125-180μm)

1200 �C(powder)

1250 �C (125-180μm)

800 �C 2.2 1.4 1.4900 �C 3.1 2.3 2.01000 �C 3.9 2.9 2.4

Table 5. Maximum Reaction Rate in 5% H2 and 25% CO2 at

Different Temperatures As a Function of Temperature Treatment

(Powder Prepared by Spray Pyrolysis) (-dω/dt (g/kg s))

CaMn0.875Ti0.125O3

1050 �C (125-180μm)

1200 �C(powder)

1250 �C (125-180μm)

800 �C 1.0 0.4 0.3900 �C 1.0 0.7 0.61000 �C 1.0 0.7 0.6D

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methane, 10%CH4, 25%CO2, in Ar for 30 min, at 800, 900,and 1000 �C. The results are presented in Figures 4 and 5 andare in the range of 8-9 wt % of the sample mass with thelowest values at the highest temperature.

Mechanical Strength and Chemical Stability. Crushingstrength is the average force (N) it takes to crush one particle.As a very general rule, particles used in a fluidized bed shouldhave a crushing strength of at least 1 N. The crushingstrength of CaMn0.875Ti0.125O3 particles sintered at 1050,1150, and 1250 �Cweremeasured. From these results, as wellas the reactivity investigations in TGA, a larger batch ofparticles was sintered at 1200 �C since this was considered togive particles with sufficient strength for fluidized bed ex-periments. Table 6 presents the crushing strength of allsintered particles with a size range of 125-180 μm. Notethat crushing strength values found in the literature aregenerally made on particles in the size range 180-250μm,36 this must be considered when comparing the resultsin this work to other published results.

In general, the reactivity falls with rising sintering tem-perature since the surface and porosity of the particlesdecreases, Tables 4 and 5. However, the mechanical proper-ties become more favorable at higher sintering temperature.Hence the optimal sintering temperature is a compromisebetween reactivity and mechanical strength.37

Further TGA experiments with the batch sintered at1200 �C included 45 cycles at 950 �C using 10% CH4 in Arduring a 5 min reduction and 20% O2 during a 5 minoxidation, see Figure 6. In these tests CO2 was omitted,and the conditions were thus rather severe during the redu-cing gas steps. The oxygen loss and uptake in CaMn0.875-Ti0.125O3 was reversible through the whole test, and no cokeon the particles was found during any of the reductions.

Fluidized Bed Experiments with Gas. A CLOU oxygencarrier releases O2 as long as the partial pressure of oxygenaround the particles is lower than the thermodynamic partialpressure for that particular material. To evaluate the magni-tude of O2 release, a few cycles were made at differenttemperatures but with only inert N2 instead of fuel. Figure 7gives theO2 concentration as a function of time. The inlet gasis switch from5.5%O2 in nitrogen to pure nitrogen after 30 s.After 390 s, the flow is switched back to 5.5%O2. Thismeansthat any oxygen in the outgoing flow between 30 and 390 s is

Figure 4. Oxygen release as a function of time for CaMn0.875-Ti0.125O3 under reducing conditions. Capacity as a function of timeusing 5 min Ar(g) and then 30 min of Ar(g) with 5%H2(g) and 25%CO2(g).

Figure 5. Oxygen released a function of time CaMn0.875Ti0.125O3

under reducing conditions. Capacity as a function of time using 5min of Ar(g) and then 30 min of Ar(g) with 10% CH4(g) and 25%CO2(g).

Table 6.Crushing Strength (inN) for aParticle of 0.125-0.180mmat

Different Sintering Temperatures

sintering temperature (�C) 1050 1150 1200 1250crushing strength (N) 0.6 1.09 1.25 2.48

Figure 6. Experiments with CaMn0.875Ti0.125O3 in the TGA usingparticles of size 1.4-4.0 mm at 950 �C for 45 cycles with: 5 min of10% CH4, 1 min of Ar(g), and 5 min of 20% O2.

Figure 7. Concentration profile of oxygen for the inert phasebetween two oxidation phases for a number of different tempera-tures as measured in the fluidized bed reactor using the CaMn0.875-Ti0.125O3 oxygen carrier.

(36) Johansson, M.; Mattisson, T.; Lyngfelt, A. Therm. Sci. 2006, 10(3), 93–107.

(37) Mattisson, T.; Johansson, M.; Lyngfelt, A. Fuel 2006, 85 (5-6),736–747.

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released by the particles. As also seen in Figure 7, higheroxygen release at higher temperatures also gives a higheroxygen uptake in the following oxidation. This is promisingsince it indicates that the material can be fully recovered, interms of oxygen content, at all temperatures. However, thereis no apparent difference between 900 and 950 �C. Also notethe very low O2 level at 630 �C.

Apart from the O2 release phenomena presented inFigure 7, which is dependent on the surrounding partialpressure of oxygen, the amount of oxygen that theCaMn0.875Ti0.125O3 particles can hold during the fluidizedbed experiments also seemed to be dependent on the tem-perature of the particles. Fully oxidized particles were heatedand cooled down while being exposed to a fluidizing gascontaining 5.5% O2. Figures 8 and 9 give the O2 concentra-tion in the outgoing flow during heating and cooling ofCaMn0.875Ti0.125O3. As seen, the particles release oxygenwhen the temperature rises and take up oxygen when thetemperature falls. This means that the available amount ofoxygen in the particles is dependent on the temperature of thematerial. This is in line with the lower RO value obtained athigher temperatures in Figures 4 and 5.

Figure 10 shows the outlet dry gas concentrations as afunction of time for the third reducing period with 15 g ofCaMn0.875Ti0.125O3 sintered at 1200 �C. The CH4 is turnedon at time 0, but the residence time in the system delays theresponse with around 20 s before the CO2 rapidly increases.After roughly 20 s the CO2 concentration reaches a maxi-mum. The CH4 is then turned off and replaced by inert N2.O2 is released during the inert phase as well as during the

beginning of the reduction phase. This gives complete con-version of the incoming CH4 to CO2 up to a point where theparticles are unable to release sufficient amount of O2,resulting in a decreased O2 concentration and increasingamounts of CH4. Note that no CO at all is detected through-out the whole cycle.

Figure 11 presents the gas yield, γ, as a function of ω forseveral particles using methane as fuel. The particles sinteredat 1050 �Chave higher reactivity than the particles sintered at1200 �C. A higher temperature in the bed also gives a slightlyhigher reactivity. However, even at the lower bed tempera-ture, particles sintered at 1200 �C have a conversion of CH4

that is higher than any previously investigated manganese-or iron-based particle found in the literature.18,19

Figure 12 shows the concentration of O2 at 900 �C fromFigure 7, whichwasmade on fresh particles, together with anidentical cycle made on the particle that bad been usedduring the methane experiments. These two concentrationprofiles are so similar that they are actually hard to distin-guish. Both show a gradually decreasing release of O2 during6 min in a flow of inert nitrogen. If there would have been norelease of O2 from the particles, then the O2 concentrationwould be expected to decrease to zero within 20 s, such as inthe case of 630 �C in Figure 7. Over 30 reducing cycles with

Figure 8. The O2 concentration (solid curve) variation with tem-perature (dashed curve) during a fluidized bed experiment. Inlet O2

concentration is 5.5%.

Figure 9. The O2 concentration (solid curve) variation with tem-perature (dashed curve) during a fluidized bed experiment. Inlet O2

concentration is 5.5%.

Figure 10.Concentration profile for a reducing cycle forCaMn0.875-Ti0.125O3 sintered at 1200 �C. The inlet gas is pure CH4, and thetemperature is 900 �C.

Figure 11. Gas yield, γ, as a function of ω for experimentswith CaMn0.875Ti0.125O3 and CH4 as fuel in the fluidized bedreactor.

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CH4 were made at different temperatures between the cyclespresented in Figure 12. Still, the particles did not show anysign of decreased reactivity, that is, change in the rate andability to release and take up O2.

Fluidized Bed Experiments with PetroleumCoke.Figure 13shows the outlet gas concentrations after condensation ofwater as a function of time for a reducing period withpetroleum coke as fuel and at a temperature of 950 �C with50% steam in the fluidizing gas. The inert period with 100%N2 in the fluidizing gas starts at time equal to zero, and theactual reduction starts 1min laterwhen the petroleum coke isadded and the fluidizing gas is switched to 50% steam in N2.The initial transient decrease in O2 is due to backmixing, butafter half a minute more-or-less all O2 originates from theCaMn0.875Ti0.125O3 particles.

When the fuel is added the CO2 concentration increases asthe petroleum coke reacts with the O2 released from theparticles. Small peaks of CO and CH4 are initially detectedand are most likely due to volatiles released from thepetroleum coke that do not have sufficient time to react.The CO2 gradually decreases during the whole cycle. Thepulse-like behavior of the CO2 concentration is due to theirregular feeding ofwater in the pump in the steamgenerator.Small amounts of O2 are visible for a large part of the period,indicating that the CaMn0.875Ti0.125O3 continues to releaseO2. Almost no CO is detected apart from the initial volatiles,indicating that CaMn0.875Ti0.125O3 is able to completelyconvert the fuel to CO2.

Table 7 presents the conversion times and the averageconversion rates for 95 and 80% conversion of the fuel. It is

clear that a higher temperature increases the conversion rateand decreases the conversion time.

Fluidization Properties. In the fluidized bed experiments,pressure drop measurements over the bed were used todetermine if the bed was fluidized or not. The CaMn0.875-Ti0.125O3 particles sintered at 1050 �C generally showed verygood fluidization properties, and the sample appeared un-affected when removed from the reactor. However, only afew cycles were performed with these particles, and as themechanical strength of the particles was rather low, use in acontinuous system for longer periods of time may not befeasible.

The CaMn0.875Ti0.125O3 particles sintered at 1200 �Cshowed very good fluidization properties for temperaturesup to 900 �C as long as the particles were not reduced too far.When the reduction of the particles was moderate, theparticles showed good fluidization properties also at highertemperatures. Further, in the cases where defluidizationoccurred due to a high degree of reduction at higher tem-peratures, the particles could easily be fluidized again bylowering the temperature to 850 �C or lower. Also, as thesample was removed from the reactor the particles lookedunaffected by the experiment and no sign of defluidizationcould be seen.

X-ray Diffraction Investigations. An XRD measurementshows that the sample consisted ofCa0.97(Mn,Ti)O3 and verysmall amounts of CaMn2O4 and Ca2MnO4 before the TGAexperiment. The XRD pattern of the sample after the TGAexperiments was of rather low quality, which limits thedegree of information obtained. However, it could be seenthat the amount of CaMn2O4 and Ca2MnO4 had increasedcompared to Ca0.97(Mn,Ti)O3, indicating that the oxidationis not 100% complete.

In-situ XRD measurements of Ca097Mn0.875Ti0.125O3

have been performed at 800 �C in diluted hydrogen and airto check structure stability and recoverability. The samplewas evenly distributed on a Pt strip that was heated to obtainthe desired temperature. The sample was placed in an in situcell allowing for various gas mixtures. The gas compositionand gas flows were regulated with mass flow controllers.

In air, before the cycle test started, the main crystal phase isCa097Mn0.875Ti0.125O3with unit cell dimensions a (A)=5.351,b (A) = 7.554, c (A) = 5.371, V (A3) = 217.10 (Vc = 54.28).After reduction, themain constituent isCa0.97Mn0.875Ti0.125O2

with unit cell dimensions; a (A) = 4.679. This shows a fullreduction of the perovskite to an oxide with cubic structure.The proposed reduction mechanism is:

Ca0:97Mn0:875Ti0:125O3 f Ca097Mn0:875Ti0:125O3-δ

f Ca097Mn0:875Ti0:125O2 ð10ÞThe reoxidation of Ca097Mn0.875Ti0.125O2 in the XRD

setup was not 100% complete. The main oxidized consti-tuent is most probably the partially oxidized Ca3Mn3O8.These results indicate that the structures can be slow toreoxidize, after prolonged reduction, in this case more than24 h. The perovskite phase has been shown to be stable down

Figure 12. Concentration profile for the first and last inert phasebetween two oxidation phases for CaMn0.875Ti0.125O3 sintered at1200 �C. The temperature is 900 �C andO2 concentration is 5.5% inthe oxidation phases and 0% in the inert phase inbetween.

Figure 13. Concentration profiles during the reduction with petro-leum coke in the fluidized bed reactor. The inlet H2O content is 50%and the temperature is 950 �C.

Table 7. Conversion Times and Conversion Rates with Petroleum

Coke at 900 and 950 �C

conversion time [min] rate [%/min]

95% conversion 900 �C 7.5 12.880% conversion 900 �C 4.2 19.395% conversion 950 �C 4.5 21.480% conversion 950 �C 2.7 29.6

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toABO2.3-2.5 for some perovskites, so the phase transfer to acubic structure is expected when this material is reduced tothe final state. Under conditions in a real CLC-system thematerial will not be reduced so extensively, and this phasechange should be avoided. It can still be noticed that thematerial can be reoxidizing back to the original perovskitestructure.

Thermochemical Analysis. The enthalpy of oxidation ofCaMn0.875Ti0.125O3-δ was measured by combined TG-DSCat 1000 �C when the material is oxidized according to:

CaMn0:875Ti0:125O3-δ þ δ0 -δ2

!O2ðgÞ

¼ CaMn0:875Ti0:125O3-δ0 ð11ÞThe obtained enthalpy is similar to the enthalpy of oxida-

tion of CaMnO3-δ,34 and when taking the experimental

uncertainties of the TG-DSC into consideration, the experi-mental values for these two materials were not possible todistinguish. The measured enthalpy of oxidation isΔoxH

o=-272( 40 kJ/(molO2(g)). CaMn0.875Ti0.125O3will thus havean exothermic reaction both in the fuel and air reactor aslong as the reduction of the material is moderate.

Discussion

The concept of CLOU has been demonstrated usingCaMn0.875Ti0.125O3 as oxygen carrier. The rate of conversionin the solid fuel experimentswith petroleumcoke is lower thanpreviously published experiments with a Cu-based oxygencarrier.31,33 The reason for this lower conversion rate is thatthe release of oxygen from the particles is not as fast as in thecase of a Cu-based oxygen carrier. However, compared toregular CLC at the same temperature, the conversion of thefuel is faster.35 Also, all fuel in these experiments was fullyconverted toCO2, except for some small amounts likely due tovolatile release in the beginning of the cycle. This is in line withprevious CLOU results. And just as with Cu-based CLOU,the reactions inboth the fuel and air reactor are exothermic forCaMn0.875Ti0.125O3.This reduces the needed particle circu-lation in an actual system, as compared to a previouslypublished CLC fuel reactor system.35

One of the main differences for CaMn0.875Ti0.125O3 com-pared to Cu is the oxidation of particles. Oxidation of Cu isstrongly governed by thermodynamics, and thereby tempera-ture, which limits the possible operation temperatures, whereas

the temperature during oxidation of CaMn0.875Ti0.125O3

seems to be of less importance. Figure 7 show that the particlesrelease roughly the same amount of oxygen at 900 and 950 �C.However, Figures 4 and 5 indicate that the amount of availableoxygen is less at higher temperatures.

It is possible that further improvements of the particles canlead to better fuel conversion, for example the particlessintered at 1050 �C showed a very high reactivity, but maybe too soft for long-term operation. Doping with othermaterials may improve the properties or decrease the cost.

Thus, CaMn0.875Ti0.125O3 showed good fluidization prop-erties and was chemically stable over a large number of cyclesmaking it an interestingoxygen carrier forCLOUapplication.Even if the fuel conversion for CaMn0.875Ti0.125O3 is lowerthan for Cu-based particles, the price of CaMn0.875Ti0.125O3 isexpected to be lower than for Cu particles.

Conclusions

The use of CaMn1-xTixO3 has been investigated as apossible oxygen carrier for CLOU. The reactivity has beeninvestigated in both a TGA and fluidized bed reactor usingboth gaseous and solid fuel. Results are very promising, withvery high reaction rates, superior to all earlier-investigatedMn-based oxygen carriers. The most important results foundin the current study are:

• The substitution of Ti on someMn sites in CaMnO3waschosen for investigation since this material showed nocoke formation even in dry CH4. The amount of Ti wasoptimized and the composition CaMn0.875Ti0.125O3 wasfound most interesting for CLOU applications, releas-ing oxygen in the gas phase during inert and reducingconditions.

• CaMn0.875Ti0.125O3 shows promising result both for thetests performed in TGA and in fluidized bed experi-ments with very high rates of reaction found for bothpetroleum coke and methane.

• NoCOwas detected during the solid fuel experiments inthe fluidized bed reactor.

• The particles sintered at 1200 �C showed good fluidiza-tion properties and were chemically stable over a largenumber of cycles.

• The enthalpy of oxidation ofCaMn0.875Ti0.125O3-δwhenthe material is oxidized is ΔoxH

o = -272 ( 40 kJ/(molO2(g)) at 1000 �C. Thus, the material will have anexothermic reaction both in the fuel and air reactor.

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