Novel New Oxygen Carriers for Chemical Looping Combustion of Solid Fuels

Novel New Oxygen Carriers for Chemical Looping Combustion ofSolid FuelsYueying Fan†,‡ and Ranjani Siriwardane*,†

†National Energy Technology Laboratory, United States Department of Energy, 3610 Collins Ferry Road, Post Office Box 880,Morgantown, West Virginia 26507-0880, United States‡URS Corporation, 3610 Collins Ferry Road, Post Office Box 880, Morgantown, West Virginia 26507-0880, United States

ABSTRACT: Several bimetallic oxygen carriers, MFe2O4 (M = Co, Ni, Cu, Mg, Ca, Sr, and Ba) and MnFeO3, prepared by theprecipitation method in a microwave and the direct decomposition method, were tested for potential use in the application ofchemical looping combustion (CLC) of solid fuels. Thermogravimetric analysis (TGA) was used to study their reduction rate,oxidation rate, and cyclic reduction/oxidation properties. Comparative experimental data of novel bimetallic ferrites and pureFe2O3 and CuO showed that all bimetallic ferrites had better reduction rates than pure Fe2O3. The Group 2 metal ferrites hadbetter reduction and oxidation rates than transition-metal ferrites. BaFe2O4 was the highest performing among all bimetallicferrites during both reduction and oxidation reactions. The reduction rate of BaFe2O4 is comparable to that of CuO at higherreaction temperatures (>900 °C). A 10 wt % loading of an inert support on the surface of the bimetallic oxygen carrierssignificantly decreased the particle agglomeration during the cyclic tests, which contributed to a better cyclic reactionperformance.

1. INTRODUCTION

Concern about the global climate change prompted research onlowering CO2 emissions during fossil fuel combustion.Technologies or processes that prevent CO2 from reachingthe atmosphere have the disadvantage of contributing to largeenergy penalties, because of the high costs associated withseparating gases. The separation task can be simplified byreplacing conventional air oxidant with pure oxygen, so that theproducts from this “oxy-fuel” combustion are just carbondioxide and water, which are easily separated by condensation.However, the current commercial process for oxygen separationrequires cryogenic oxygen production from air, whichconsumes an appreciable amount of energy, making the oxy-fuel combustion process very energy-intense. Chemical loopingcombustion (CLC), which uses oxygen carriers, such as metaloxide, to supply oxygen instead of air for fuel combustion, is apromising technology that produces heat and energy, with thesignificant advantage of producing concentrated CO2 withoutrequiring any major energy for its separation.1 There aresignificant advantages2 to using the CLC process. Incomparison to normal combustion, CLC produces a sequestra-tion-ready stream of CO2 that is not diluted with N2 or flue gasand also reduces NOx emissions.Large-scale application of CLC is dependent upon the

availability of a suitable oxygen carrier. An ideal oxygen carriershould meet a number of requirements, including highreactivity, low fragmentation and attrition, low tendency foragglomeration, low cost, and stability under repeatedreduction/oxidation cycles at high temperatures. The carriershould also be environmentally benign. The development ofoxygen carriers possessing these desirable properties is criticalfor CLC.The traditional oxygen carriers,3 such as CuO, Fe2O3, NiO,

MnO, and CoO, have been tested extensively in the past for

CLC of coal and CH4. However, disadvantages, such as lowreactivity with Fe2O3, CoO, and MnO, low melting points, highagglomeration with CuO, and health concerns with NiO, mustbe addressed. None of the traditional metal oxides investigatedin previous studies3 appears to possess all of the desirablecharacteristics for CLC applications. Recently, researchers havebeen investigating mixed metal oxides for CLC to potentiallyresolve many of the shortcomings associated with thoseconventional single metal oxides. The synergetic effects ofhaving multiple oxides may enhance the performance of theoxides to obtain the desirable properties for CLC. For example,CuO possesses higher reactivity than Fe2O3 but has issues withparticle agglomeration, which limits its application in the CLCprocess. Iron oxide has the advantage of low agglomeration buthas slower reactivity and lower oxygen capacity. Thecomponents of bimetallic Cu−Fe oxygen carriers wereoptimized to achieve a better reactivity than Fe2O3 and betterstability than CuO in cyclic methane CLC and coal/carbonCLC reactions.4,5 Superior performance of CuFe2O4, ascompared to single metal oxides of either CuO or Fe2O3, wasalso reported for H2/air CLC application.6 However, when coalwas used, iron silicates were formed from the interaction ofreduced CuFe2O4 with ash and resulted in the insufficientreoxidation of reduced CuFe2O4.Jin et al.7,8 investigated NiO/YSZ, CoO/YSZ, and NiO−

CoO/YSZ for CLC. They found that NiO−CoO/YSZ showedexcellent overall performance with good reactivity, completeavoidance of carbon deposition, and significant regenerabilityfor repeated cycles of reduction and oxidation. However, NiO/YSZ and CoO/YSZ have drawbacks of either higher carbon

Received: December 24, 2013Revised: February 20, 2014Published: February 21, 2014

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© 2014 American Chemical Society 2248 dx.doi.org/10.1021/ef402528g | Energy Fuels 2014, 28, 2248−2257

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deposition or lower ability to regenerate, accompanied with anincrease in both the grain and pore sizes. Hossain et al.9 foundthat the activation energies for Co−Ni/Al2O3 reduction aresignificantly lower than those for the single metal oxide Ni/Al2O3 reduction, which confirmed the favorable effect of Co onthe reducibility of the bimetallic oxygen carrier.The bimetallic Fe−Mn oxides10 supported on ZrO2, sepiolite

and Al2O3 prepared by solid-state mixing were found to bepromising oxygen carriers for CLC. Using simulated synthesisgas and MnO also had a positive effect on the stability. Thesemixed oxides exhibited a lower oxygen transfer capacity thanNi-based materials. The capacity was also affected by thesynthesis method.11

Mixtures of manganese and iron oxides have also beenexamined as oxygen carriers for CLC with natural gas as fuel ina circulating fluidized-bed reactor.12 It was concluded that thecombined oxides of Mn and Fe have very interestingthermodynamic properties and could potentially be suitablefor chemical-looping applications, but the physical strength ofthe materials would have to be improved. In addition,combined Fe−Mn oxides with molar ratios of Fe/Mn of 2:1showed the best oxygen release ability, fluidizability, andmethane conversion.13 They concluded that the mixed Fe−Mnoxides could contribute to faster fuel conversion, even thoughtheir oxygen release was less than copper-based chemical-looping with oxygen uncoupling (CLOU) materials during testswith solid fuel.Nickel ferrite (NiFe2O4) was also investigated as an oxygen

carrier for CLC.14 Redox cycling of NiFe2O4 oxygen carrierswas performed by thermogravimetric analysis (TGA) underpure CH4 gas and O2/air atmospheres. After five successivecycles, NiFe2O4 powder with a single phase of spinel structure

demonstrated higher redox cycling behavior and better stabilitythan pure NiO and Fe2O3.Perovskite-type materials, La0.8Sr0.2Co0.2Fe0.8O3−δ, had been

investigated for a potential oxygen carrier by in situ X-raydiffraction (XRD) chemical-looping experiments. The resultsshowed that La0.8Sr0.2Co0.2Fe0.8O3−δ does have the redoxproperties required for chemical looping. Reduction andoxidation of perovskite occur quickly under the conditionsused. However, the low oxygen capacity observed wouldrequire a fast solid circulation or high solid inventory if thisperovskite material were to be used in industrial CLCprocesses.15 La0.5Sr0.5Fe0.5Co0.5O3−δ was also found to besuitable for CLC applications.16 However, in comparison toother metal oxide materials, La0.5Sr0.5Fe0.5Co0.5O3−δ does notseem to offer any obvious advantages.The majority of the work performed to date on CLC has

been performed using gaseous fuels. Few studies have beenconducted using oxygen carriers for combustion of solid fuels,such as coal. The objectives of this research were to explore anoxygen carrier with high activity and great stability for coalCLC. In the current research, metal ferrites (MFe2O4) with Mselected from Group 2 elements (M = Mg, Ca, Sr, and Ba) andtransition metals (M = Cu, Ni, and Co) and MnFeO3 for coalCLC are reported.

2. EXPERIMENTAL SECTION2.1. Synthesis of Oxygen Carries. Both the precipitation method

in a microwave (Anton Parr) and the direct decomposition methodwere used for the synthesis of bimetallic ferrite oxygen carriers. In theprecipitation (microwave) method, metal nitrates or metal acetateswere used as precursors to synthesize oxygen carriers. Metal nitrates ormetal acetates were dissolved in the diethylene glycol, and the solution

Figure 1. XRD of fresh Group 2 metal ferrites before and after reduction with coal: (a) MgFe2O4, (b) CaFe2O4, (c) SrFe2O4, and (d) BaFe2O4.

Energy & Fuels Article

dx.doi.org/10.1021/ef402528g | Energy Fuels 2014, 28, 2248−22572249

was heated to 200−250 °C in the microwave reactor for 30−45 min.The resulting solid precipitate was washed with deionized (DI) waterand separated by centrifugation. The material was dried in an oven at100 °C overnight and calcined in air at 600−1000 °C for 6 h.The direct decomposition method was also evaluated as a

preparation method because it is more cost-effective than themicrowave precipitation method but yields bigger particle sizes. Inthis method, metal nitrates were mixed with citric acid to enhancebonding and prevent aggregation at high temperatures. The mixturewas heated in a box oven to 1000 °C at a ramp rate of 3 °C/min in airand kept at 1000 °C for 6 h.2.2. Synthesis of 10% Al2O3/BaFe2O4 and 40% Al2O3/

BaFe2O4. To decrease the agglomeration of oxygen carriers, 10%Al2O3 was incorporated into bimetallic oxygen carriers by the ureadeposition−precipitation method after the synthesis of the oxygencarriers. A total of 2 g of bimetallic ferrites was added to 200 mL of anaqueous solution of Al(NO3)3(H2O)6 and 0.4 M urea. The suspensionwas loaded into a thermolysis reactor (Syrris sodium system) andvigorously stirred at 90 °C for 4 h. The sample was then washed withDI water, centrifuged several times to remove traces of acid and urea,and dried at 100 °C overnight in air. Then, 10% Al2O3/MFe2O4 wascalcined in air at 900 °C for 1 h. For the preparation of 40% Al2O3/BaFe2O4, 1 g of BaFe2O4 was mixed with 0.67 g of Al2O3. The mixturewas then heated in a box oven to 1000 °C at a ramp rate of 3 °C/minin air and kept at 1000 °C for 6 h.2.3. TGA of the CLC Performance Test. CLC tests of bimetallic

oxygen carriers with coal were performed in TGA. The oxygen carriermixed with coal (Illinois #6) or carbon black using a motor and pestle,and the mixture was loaded in TGA. A weight ratio of oxygen carrier/coal = 1:0.067 was used during the performance comparisonexperiments, and a weight ratio of BaFe2O4/carbon = 1:0.075 wasused for the cyclic test. The TGA samples were heated to 900−1000°C at a ramping rate of 5 °C/min under N2 at a flow rate of 200 cm3/min and kept at 900 or 1000 °C until there was no weight loss. Thezero-grade air at a flow rate of 200 cm3/min was introduced foroxidation. The reaction rate of the oxygen carrier with coal or carbonblack was calculated using TGA data as follows:

= = − −x t X M M M Mreduction rate d /d , ( )/( )o o f

= = − −x t X M M M Moxidation rate d /d , ( )/( )f oxd f

where M is the instantaneous weight of the metal oxide−coal mixture,Mo is the initial weight of the metal oxide−coal mixture, Mf is theweight of the reduced metal and ash after the reduction, and Moxd isthe weight of the completed oxidized sample after introducing air. Thereaction rate dx/dt was calculated by differentiating the fifth-orderpolynomial equation.

3. RESULTS AND DISCUSSION3.1. Physical Characterization. The XRD patterns of

fresh Group 2 metal ferrites before and after reduction withcoal are shown in Figure 1, and phase identifications are listedin Table 1. XRD patterns corresponding to metal ferrites,MFe2O4 (M = Mg, Ca, Sr, and Ba), were observed with fresh

samples. After reduction with coal, all ferrites had peakscorresponding to FexOy, with the exception of BaFe2O4, whichhad peaks corresponding to metallic Fe0 (Table 1). Thisindicated that Ba promoted further reduction of FexOy to Fe.The XRD patterns of the fresh transition-metal ferrites beforeand after reduction with coal are shown in Figure 2. XRD peaksindicate that the fresh transition-metal ferrites exist as MFe2O4(M = Co, Ni, and Cu), with the exception of Mn, which existsas FeMnO3. After reduction with coal, all transition-metalferrites had peaks corresponding to reduced FexOy.

3.2. TGA Reaction Performance of Metal Ferrites withCoal. To reduce agglomeration of the metal ferrites duringreduction/oxidation reactions at high temperatures, 10% Al2O3was incorporated in the metal ferrites. Figure 3 illustrates thereaction rate comparison before and after incorporating 10%Al2O3 loading on Group 2 metal ferrites. As shown in Figure 3,the rates were not affected by adding 10% Al2O3. However, theagglomeration was reduced significantly after incorporation of10% Al2O3, and reactivity was maintained. The 10% Al2O3loading on transition-metal ferrites also reduced agglomerationwhile maintaining reactivity (data not shown).A comparison of coal CLC reaction properties between

single oxides and metal ferrites synthesized by the microwaveprecipitation method is shown in Figure 4. CuO has a betterreduction rate than Fe2O3, consistent with what was reportedpreviously.5 Data indicate that all metal ferrites have betterreduction rates than Fe2O3. It is worth noting that the Group 2metal ferrites have better reduction and oxidation rates thantransition-metal ferrites. BaFe2O4 had the best performanceamong all metal ferrites during both reduction and oxidationphases. The reduction rate of BaFe2O4 at 1000 °C iscomparable to that of CuO at 900 °C. The reactiontemperature of CuO was restricted to 900 °C because ofagglomeration and its low melting point. However, BaFe2O4can be operated up to 1000 °C without agglomeration.BaFe2O4 was chosen for the cycling test because it had the bestperformance. It was also observed that metal ferrites had lessagglomeration than single oxides Fe2O3 and CuO even withoutincorporation of 10% Al2O3. Commercial Fe2O3 and CuO usedfor comparison had particle sizes less than 5 μm. Fresh metalferrites and Fe2O3 synthesized by the microwave precipitationmethod had particle sizes less than 1 μm. For propercomparison, 1 μm Fe2O3 synthesized by the microwaveprecipitation method was also tested. It showed significantlymore agglomeration than metal ferrites after reaction, whichcontributed to a lower reaction rate than that with commercialFe2O3, even though Fe2O3 from the microwave precipitationmethod had a smaller particle size.The theoretical carbon consumption for the reaction of

carbon with metal ferrites and single metal oxides is shown inTable 2. The final reduction states of the oxides determined byXRD were used for these calculations. BaFe2O4 has the highesttheoretical carbon consumption because the final reductionstates are Ba and Fe0, and it is higher than that of Fe2O3 andcomparable to that of CuO.The reason for the higher reactivity of metal ferrites

compared to the reactivates of single metal oxides is not veryclear. Metal ferrites with the general molecular formulaM2+Fe2

3+O4 have a spinel-type structure similar to that of themineral MgAl2O4, otherwise known as spinel. The latticeconsists of 32 divalent oxygen ions, which are in direct contactwith one another, forming a closed-pack, face-centered cubicarrangement with 64 tetrahedral interstitial sites (A sites) and

Table 1. XRD-Identified Phases before and after Reductionwith Coal

metal ferrites, phase before reduction phases after reduction

CuFe2O4 Cu and Fe3O4

NiFe2O4 Fe, Ni3Fe, and Ni1.25Fe1.85O4

CoFe2O4 Co and Fe3O4

MnFeO3 Fe, Fe0.95O, and MnOMgFe2O4 FeO and Mg0.7Fe0.23Al1.97O4

CaFe2O4 Fe0.902O and Ca2Fe2O5

SrFe2O4 FeO and SrAl2O4

BaFe2O4 Fe and BaO



Figure 2. XRD of transition-metal ferrites before and after reduction: (a) CoFe2O4, (b) CuFe2O4, (c) NiFe2O4, and (d) FeMnO3..

Figure 3. Reaction rate comparison of Group 2 metal ferrites before and after incorporation of 10% Al2O3 reduction with coal.



32 octahedral interstitial sites (B sites). Of these, 8 tetrahedral(A sites) and 16 octahedral (B sites) sites are occupied by thedivalent and trivalent cations.17 Thus, the large fraction ofempty interstitial sites makes its crystal structure a very openstructure that is conducive to cation migration. Therefore, awhole range of distributions of cations is possible in spinelferrites, which can contribute to their remarkable magnetic,catalytic, optical, and electrical properties. The metal−oxygen

bond lengths (energies) in the ferrite structure are differentfrom those of single metal oxides, which may also contribute tothe differences in reactivity. In our research reported here,Group 2 ferrites and transition-metal ferrites did show betterreactivity with coal than single metal oxygen carriers. The Cu2+,Mg2+, Ni2+, and Co2+ ferrites form inverse spinel structure withthe formula Fe3+[Me2+Fe3+]O4, in which Me2+ (Mg2+, Cu2+,Ni2+, and Co2+) is octahedral-coordinated with oxygen, whileFe3+ is distributed in both the tetrahedral- and octahedral-coordinated sites, which may have contributed better reactivitywith coal than single metal oxygen carriers. The degree ofinversion depended upon the metal cation in the ferritestructure.18 However, it was noted that barium ferrite(BaFe2O4) does not crystallize in a spinel structure but in ancomplicated orthorhombic structure19 because the size of Ba2+

is too large to be accommodated in the octahedral sites. In ourpresent work, BaFe2O4 was also identified to be in theorthorhombic structure. Similar stuffed-framework structuresare reported with Ca and Sr ferrites,20 but Ca ferrite has showna completely different atomic arrangement from both Ba and Srferrites. Better reactivity of alkaline earth ferrites with coal orcarbon, as compared to that of transition-metal ferrites, couldbe due to these structural differences. Molecular modeling workmay be necessary to understand these differences.

Figure 4. Reaction rates and reaction temperatures of single metal oxides and metal ferrites with coal.

Table 2. Theoretical Carbon Consumption Based on 1 g ofOxygen Carriersa

oxygencarriers reaction

theoretical carbonconsumption (mg)

Fe2O3 2Fe2O3 + C = 4FeO + CO2 37.5CuO 2CuO + C = 2Cu + CO2 75CuFe2O4 CuFe2O4 + C = Cu + 2FeO + CO2 50.16MnFeO3 MnFeO3 + C = MnO + Fe + CO2 75.576NiFe2O4 NiFe2O4 + C = Ni + 2FeO + CO2 51.19CoFe2O4 CoFe2O4 + C = Co + 2FeO + CO2 51.15CaFe2O4 CaFe2O4 + C = CaO + FeO + CO2 27.81MgFe2O4 MgFe2O4 + C = MgO + FeO + CO2 60SrFe2O4 SrFe2O4 + C = SrO + FeO + CO2 45.57BaFe2O4 BaFe2O4 + 4C = Ba + 2Fe + 4CO 153

BaFe2O4 + 2C = Ba + 2Fe + 2CO2 76.67aThe reduction phases identified by XRD were used.

Figure 5. Cyclic CLC test data of carbon and 10% Al2O3/BaFe2O4 synthesized by the microwave precipitation method at 210 °C for 45 min.



3.3. TGA Cyclic Reduction/Oxidation Reaction ofBaFe2O4 with Carbon. Figure 5 illustrates the TGA testdata during cyclic tests of carbon CLC with 10% Al2O3/BaFe2O4 synthesized by the microwave method. BaFe2O4showed stable reduction rates during cyclic tests, whileoxidation rates improved. The maximum reaction temperatureslightly increased with increasing cycles because of theaggregation of the particle. Figure 6 illustrates that theperformance of BaFe2O4, synthesized by the microwavemethod at 240 °C for 30 min, was similar to that synthesizedat 210 °C for 45 min (Figure 5).Figure 7 illustrates the XRD patterns of fresh BaFe2O4

synthesized by the direct decomposition method, reactionrates, and temperatures corresponding to maximum reaction

rates during cyclic tests of 10% Al2O3/BaFe2O4 with carbon.Even though uniform small particle sizes can be obtained by theprecipitation method in microwave, direct decomposition is amore convenient and cost-effective preparation method. XRDdata of the sample prepared by the direct decompositionmethod (shown in Figure 7) indicate BaFe2O4 as the majorphase with a trace amount of BaFe12O19. Reduction rates aresimilar for both materials synthesized by the precipitation(microwave) method and the direct decomposition method.However, the oxidation rate is higher for materials synthesizedby the precipitation (microwave) method because of thesmaller particle size. BaFe2O4 synthesized by the directdecomposition method also showed stable reduction andoxidation rates, while its reaction temperature increased slightly

Figure 6. Cyclic CLC tests of carbon with 10% Al2O3/BaFe2O4 synthesized by the microwave precipitation method at 240 °C for 30 min.

Figure 7. (a) Reaction rates, (b) reaction temperature corresponding to the maximum reaction rate during cyclic tests of 10% Al2O3/BaFe2O4(direct decomposition method) and (c) XRD of fresh BaFe2O4.



as the number of cycles increased. TEM showed that theparticle size of BaFe2O4, synthesized by the microwaveprecipitation method, was about 500−600 nm, which is smallerthan that synthesized by the solid reaction method (particle sizeis in the range of 1−2 μm). The particle size affected theoxidation rates but not the reduction rates. During thereduction cycle, oxygen from the surface of the metal oxide isreleased first, facilitating the diffusion of gases from the interiorto the exterior of metal oxide. Therefore, reduction rates ofBaFe2O4 were not affected because of different particle sizes.During the oxidation cycle, oxygen in air oxidizes the surface ofthe metal, restricting the diffusion of oxygen inside metal oxide.

Thus, the oxidation rates of BaFe2O4 were affected by theparticle size.Outlet gas composition during the reaction of carbon with

two different amounts of 10% Al2O3/BaFe2O4 is shown inFigure 8. When the ratio of carbon to 10% Al2O3/BaFe2O4

(135 mg/1.1 g) was higher, more CO [CO2/(CO + CO2) =0.48] was produced, according to the reaction: BaFe2O4 + 4C =Ba + 2Fe + 4CO. When the ratio of carbon to 10% Al2O3/BaFe2O4 (65 mg/1.1 g) was lower, more CO2 [CO2/(CO +CO2) = 0.80] was produced, according to the reaction:BaFe2O4 + 2C = Ba + 2Fe+ 2CO2. Therefore, pure CO2 or COcan be obtained using the appropriate ratio of carbon toBaFe2O4.

Figure 8. Outlet gas analysis during the reaction of carbon black with 10% Al2O3/BaFe2O4 at various weight ratios: (a) ratio of carbon to 10%Al2O3/BaFe2O4 is 135 mg/1.1 g, and (b) ratio of carbon to 10% Al2O3/BaFe2O4 is 65 mg/1.1 g.

Figure 9. (a) XRD of 10% Al2O3/BaFe2O4 after reduction with carbon, (b) XRD of Fe2O3 after reduction with carbon, and (c) XRD of 10% Al2O3/BaFe2O4 after cyclic tests.



XRD data of BaFe2O4 and Fe2O3 after reduction with carbonare shown in Figure 9. BaFe2O4 was reduced to Fe and Ba,while Fe2O3 was reduced to Fe3O4 and FeO. Ba acted as apromoter to enhance deeper reduction of Fe3O4/FeO to Fe,which contributed to a better performance in coal CLC. XRDdata in Figure 9c showed that barium ferrite still existed asBaFe2O4 after multiple reduction/oxidation cycles synthesizedby both the direct decomposition method and the microwaveprecipitation method. This indicated that, even thoughBaFe2O4 separated into Ba and Fe during the reduction cycle,they combined during the oxidation cycle to form BaFe2O4.Transmission electron microscopy (TEM) of 10% Al2O3/BaFe2O4 before and after the cyclic reactions with carbon isshown in Figure 10. TEM data indicated that the particle size of

BaFe2O4 did not change significantly even after cyclic tests,which may have also contributed to the stable performanceduring cyclic tests. XRD showed that barium oxide in BaFe2O4had participated in the CLC reaction with coal or carbon. BaOhas never been reported as a CLC oxygen carrier. Toinvestigate this, CLC of carbon with pure BaO was alsoperformed. The results are shown in Figure 11, and the CLCperformance of BaO with carbon is better than that with Fe2O3.The reduction rate of BaO is lower than that with BaFe2O4 buthigher than that with Fe2O3, and the reaction temperature ofBaO is higher than that with BaFe2O4.

3.4. Effect of Reactor Bed Dilution. Al2O3, bentonite andBaAl2O4 were tested for the suitability as diluting materials forBaFe2O4 in the application of CLC. BaAl2O4 was chosen to betested as a diluting material because of a possible reactionbetween Ba and Al2O3 during reduction/oxidation cycles.BaAl2O4 was synthesized by the direct decomposition method.Barium nitrate and aluminum nitrate were mixed with citricacid and heated to 1000 °C for 6 h at a ramping rate of 3 °C/min in air. Al2O3 (40%), BaAl2O4, or bentonite was mixed withBaFe2O4 and heated to 1000 °C for 6 h at a ramping rate of 3°C/min in air before the TGA performance test with carbon.Figure 12 shows carbon CLC performance of 60% BaFe2O4diluted with 40% Al2O3, BaAl2O4, or bentonite. A reductionrate of 40% Al2O3/BaFe2O4 with carbon decreased with anincreasing number of cycles, similar to that of 10% Al2O3/BaFe2O4, as shown in Figure 7. The reduction rate with 40%Al2O3 is lower than that with 10% Al2O3. The contacts betweencarbon and BaFe2O4 will be lower with increasing Al2O3content, which may contribute to a lower reduction rate.However, the oxidation rate increased with more Al2O3 dilutioncompared to 10% Al2O3/BaFe2O4 synthesized by both themicrowave precipitation method (Figures 5 and 6) and thedirect decomposition method (Figure 7). Al2O3 and BaAl2O4showed better performance as support dilute materials for CLCwith carbon, while bentonite, as a support material for BaFe2O4,showed significant decreases in both the reduction andoxidation rates during the second cycle. Cyclic performancetests were not continued with 40% bentonite/BaFe2O4. It hasbeen reported2 that Fe oxide/bentonite and Fe/SiO2 showedthe worst stability at 800 °C while improving at 900 °C. It wasbelieved that the interaction between the metal and supports, aswell as agglomeration, may have contributed to the decrease instability. In the present work, it is possible that the maincomponent SiO2 in bentonite may have reacted with Ba or Feto form compounds that are inactive for CLC.Costs of the ferrites should be comparable to the other

metal-oxide-based oxygen carriers that have been reported forthe CLC process. Barium ferrite is often used in themanufacture of permanent magnets, magnetic storage media,magnetic materials, and pigments. The heightened interest inferrites is mainly due to the abundance of starting materials anda low production cost.21

4. CONCLUSIONBimetallic oxygen carriers, selected from Group 2 metal ferrites(MgFe2O4, CaFe2O4, SrFe2O4, and BaFe2O4) and transition-metal ferrites (NiFe2O4, CuFe2O4, MnFeO3, and CoFe2O4)synthesized by both the microwave precipitation method andthe direct decomposition method, were evaluated for the coalCLC reaction. Group 2 metal ferrite oxygen carriers showedbetter reduction and oxidation rates than Fe2O3 for coal CLC.Among all of the novel metal ferrites, BaFe2O4 showed thehighest reduction and oxidation rates for coal CLC; itsreduction rate is comparable to CuO but can be operated ata higher reaction temperature, up to 1000 °C, withoutagglomeration. The CuO performance is generally restrictedto 900 °C because of agglomeration and its low melting point.Cyclic tests on BaFe2O4 showed very stable performance forcoal CLC without agglomeration, even at a high temperature(1000 °C). BaFe2O4 showed stable reduction rates, and itsoxidation rates improved during cycling tests. Even thoughBaFe2O4 showed the best performance, each of the other metalferrites tested can be used for coal CLC, because they all had

Figure 10. TEM of 10% Al2O3/BaFe2O4: (a) before and (b) aftercarbon CLC cyclic tests.



better performance than commercial Fe2O3. Metal ferrites havelower agglomeration than CuO. The cost of metal ferrites iscomparable to that of Fe2O3 and CuO and can be preparedusing readily available materials.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

NotesDisclaimer: This report was prepared as an account of worksponsored by an agency of the United States Government.Neither the United States Government nor any agency thereof,nor any of their employees, makes any warranty, express orimplied, or assumes any legal liability or responsibility for theaccuracy, completeness, or usefulness of any information,apparatus, product, or process disclosed, or represents that itsuse would not infringe privately owned rights. Reference hereinto any specific commercial product, process, or service by tradename, trademark, manufacturer, or otherwise does notnecessarily constitute or imply its endorsement, recommenda-tion, or favoring by the United States Government or anyagency thereof. The views and opinions of authors expressedherein do not necessarily state or reflect those of the UnitedStates Government or any agency thereof.The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was performed in support of the National EnergyTechnology Laboratory’s ongoing research under the Researchand Engineering Services (RES) Contract DE-FE0004000. Theauthors also greatly appreciate Dr. Yun Chen from WestVirginia University (WVU) and James A. Poston from theNational Energy Technology Laboratory, U.S. Department ofEnergy (DOE), for help with scanning electron microscopy(SEM) measurements.

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Figure 11. Performance for the CLC reaction of carbon with BaO synthesized by the direct decomposition method.

Figure 12. Performance test of carbon black with 60% BaFe2O4 on supports: 40% BaAl2O4, Al2O3, and bentonite.



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Novel New Oxygen Carriers for Chemical Looping Combustion of Solid Fuels

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