Chemical Engineering Science 62 (2007) 5682–5687www.elsevier.com/locate/ces

Solution combustion synthesized oxygen carriers forchemical looping combustion

Peter Erri, Arvind Varma∗

School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, USA

Received 15 June 2006; received in revised form 21 December 2006; accepted 10 January 2007Available online 25 January 2007

Abstract

Chemical looping combustion is an attractive method for CO2 capture in power plants, while maintaining high energy efficiency. The cyclicnature of the process, however, places significant demands on the redox and mechanical characteristics of the oxygen carrier. In this work,NiO/NiAl2O4 and (NiO)1−y(MgO)y/Ni(1−x)MgxAl2O4 were synthesized by solution combustion and investigated in two forms: porousextruded and dense pressed pellets. The pressed oxygen carriers of both compositions exhibited excellent stability during thermogravimetricanalysis at temperatures 800–1200 ◦C with cycling of oxidizing/reducing gases while, due to sintering effects, the extruded pellets showeddeteriorating performance. The addition of magnesium to the structure decreased reduction of the spinel, thus stabilizing the support. Attritiontests revealed superior performance of the (NiO)0.21(MgO)0.79/Ni0.62Mg0.38Al2O4, exceeding that of commercially available sintered ironoxide reference.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Chemical looping combustion; Solution combustion synthesis; Energy; Environment; Attrition; Reaction engineering

1. Introduction

The increasing carbon dioxide content of the atmosphere iswidely believed to cause global warming. To avoid or mini-mize this climate change, CO2 emissions need to be decreased(US Department of State, 2002, US Climate Action Report). Inthis context, carbon sequestration is an attractive strategy as itenables the continued use of currently available fossil fuel re-sources. A near term deployment opportunity of this approachis the capture of CO2 from large-scale emission sources, suchas power plants, with subsequent storage options such as under-ground formations. Various CO2 separation technologies exist,e.g. absorption in amines and combustion in pure oxygen, buteach has disadvantages such as large capital and/or operatingcosts (Dijkstra and Jansen, 2004).

Chemical looping combustion (CLC), a novel approachfor power generation, potentially offers inherent CO2 cap-ture, while maintaining high energy efficiency. In this system

∗ Corresponding author. Tel.: +1 765 494 4075; fax: +1 765 494 0805.E-mail address: avarma@purdue.edu (A. Varma).

0009-2509/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.ces.2007.01.015

(Fig. 1), a metal oxide circulates between two interconnectedfluidized bed reactors, being reduced by coal gas or natural gasin one and oxidized by air in the other (Jin and Ishida, 2004;Mattisson et al., 2006). The main advantage of CLC is that, evenwith the use of air as an oxidant, CO2 is not diluted with N2, butinstead can be obtained in high purity by simple condensationof water in the effluent from the fuel reactor. Note that theoverall heat evolved in CLC is the same as for conventionalcombustion, where the air is in direct contact with the fuel.Based on this concept, and given that metal oxide reductionby methane is frequently endothermic, low grade heat can beadded to the fuel reactor and subsequently recovered in the airreactor at higher temperature, as high grade energy.

The cyclic nature of the process makes CLC implementa-tion difficult, as the oxygen carriers must withstand repeatedoxidation and reduction steps, as well as mechanical stresses.An added challenge is that the oxidation must take place atelevated temperature to ensure high efficiency in the down-stream gas turbine (Wolf et al., 2005). A number of transitionmetals have been investigated as potential oxygen carriers,such as Fe, Cu, Mn and Ni, where particularly the latter hasshown promise with excellent reactivity, but is hindered by low

P. Erri, A. Varma / Chemical Engineering Science 62 (2007) 5682–5687 5683

MeO

Me

Air Fuel

(CH4 or CO/H2)

CO2, H2O N2, O2

Reactor Reactor

Fig. 1. Schematic representation of chemical looping combustion.

mechanical strength and sintering resistance (Cho et al., 2004).To improve these characteristics, the addition of refractory com-pounds has been investigated, showing the beneficial effect ofnickel aluminate and magnesium nickel aluminate, which sig-nificantly improve thermal stability even under reducing con-ditions (Ishida et al., 2002; Villa et al., 2003; Lif et al., 2004).

In this work, the reactivity and mechanical characteris-tics of solution combustion synthesized nickel oxides areinvestigated as potential oxygen carriers for CLC. NiO wassynthesized with weight ratio 40:60 on two supporting spinelstructures: NiAl2O4 and Ni1−x MgxAl2O4 (Mg–Ni reagentmole ratio = 0.4). The resulting complex oxides were charac-terized, determining phase structure, morphology and attritionrate, and their redox properties were determined using thermo-gravimetric cycling under dilute methane and oxygen streams.

2. Experimental

2.1. Oxygen carrier preparation

The investigated metal oxides were prepared through solu-tion combustion synthesis, which has been described in detailelsewhere (Deshpande et al., 2004). In short, metal (Ni, Mg andAl) nitrates in the required stoichiometric amounts were mixedwith glycine in water to achieve mixing at the molecular level.The aqueous solution was then heated on a hotplate, causing wa-ter evaporation and, when a critical temperature was achieved,self-ignition and combustion, yielding fine oxide powder.

An example of the overall reaction between the metal ni-trates and the glycine fuel is represented below for the synthesisof NiO:

Ni(NO3)2 + 1.11�H2N(CH2)CO2H + 2.4975(� − 1)O2

→ NiO + �[2.22CO2 + 2.78H2O] + [1 + 0.555�]N2.

Note that a fuel to oxidizer ratio (�) of unity signifies thatno ambient oxygen is needed for the combustion reaction,while � > 1(< 1) implies net reducing (oxidizing) conditionsin the solution. A key aspect of this synthesis method is the

dual role of glycine: it acts as complexing agent for theprecursors and, through combustion, provides energy for theone-step synthesis of oxides. Further, at large � values, owingto incomplete combustion, glycine alters the reaction, makingit less vigorous. This property was used with the nickel oxidesynthesis where � = 1.5 yielded a slower moving reactionfront, thus allowing for the use of an open flask as reactionvessel.

The produced metal oxides were ball-milled for 12 h, andwere then pelletized in two ways: pressed at 5000 lbs or mixedwith 3:8 mass ratio of a 40 wt% polyacrylamide solution toform a smooth paste which was extruded and allowed to dry.The pressed and extruded pellets were then calcined for 3 h at1300 ◦C, followed by crushing and sieving to obtain the desiredparticle diameter (0.21–0.43 mm).

2.2. Structural characterization

The oxides carriers were characterized at various stages(as-synthesized, calcined, reduced, and cycled) using XRDanalysis (X1 Advanced diffraction system, Scintag Inc.) toascertain crystallinity and phase composition while changes inmorphology were determined by scanning electron microscopy(FEI Nano SEM).

The attrition rate of the calcined particles was investigated us-ing a modified version of the ASTM Standard Method (ASTMInternational, 2006). The pellets were dried under vacuum at300 ◦C, weighed and placed in a glass cylinder (L: 4 cm, d:2.0 cm) with at 4 mm baffle. The container was placed on a ballmill roller and rotated for 30 min at rate 60 rpm. Next, the pel-lets were sieved (+0.21 mm), dried as before and weighed tocalculate attrition as follows:

Loss on attrition(%) = A − B

A× 100,

where A is the Mass before test; B the Mass after test.Iron oxide (Sintered lump, 99.5%, Alfa Aesar), crushed and

sieved to the particle size of the oxygen carriers, served as anattrition standard to enable comparison by other investigators.

2.3. Redox experiments

The material redox properties were investigated using ther-mogravimetric measurements in the set-up shown schemati-cally in Fig. 2. An automated multiposition valve directed theselected gas stream from the UNIX mass flow controllers tothe TGA-instrument (TGA-DSC Q600, TA Instruments), whilethe non-selected gases were vented. In this manner, 22 mg ofoxygen carrier, placed in an alumina crucible, could be alter-natively exposed to a reductant (3% CH4, 3% H2O, balanceAr; water added to prevent coking) and an oxidant (10% O2,balance Ar), separated by an argon stream to flush the systemwith all flow rates set at 100 sccm. To prevent water conden-sation, the appropriate gas lines were heated to 50 ◦C. Priorto the material testing, several blank runs were conducted toquantify the buoyancy effects of the individual gases. Falsifica-tion of the activation energy due to mass transfer limitation was

5684 P. Erri, A. Varma / Chemical Engineering Science 62 (2007) 5682–5687

Vent

Vent

2

4

1 - Mass Flow

Controller

2 - Water Saturator

3 - Multiposition

4 - TGA

3

Vent

Ar

O2 + Ar

CH4

Ar

1

1

1

1

Valve

Fig. 2. Schematic diagram of the experimental apparatus.

investigated by performing the redox experiments at a seriesof fixed temperatures. The stability of the materials was deter-mined by repeating the oxidation–reduction cycle 10 times at950 ◦C, while the reaction steps during reduction were studiedby thermogravimetric analysis under the reducing gas streamwith heating to 1200 ◦C at rate 10 ◦C/ min.

3. Results and discussion

3.1. Structural characterization

Powder X-ray diffraction patterns of pressed (NiO)1−y

(MgO)y/Ni(1−x) MgxAl2O4 are displayed in Fig. 3. It canbe observed that the Ni phase in the as-synthesized powderis a mixture of Ni/NiO, where the presence of metal can beascribed to the fuel-rich conditions during synthesis. This isin agreement with the work of Jung et al. (2005), describingthe production of nickel/nickel-oxide mixtures by solutioncombustion under reducing conditions. In the present work,excess fuel was added to ensure a less vigorous reaction, en-abling the use of open flasks as reaction vessels. Note thatseparate phases of Mg and Al2O3 were not observed, whilethe XRD patterns of NiO and MgO are too similar to beresolved.

The XRD analysis of NiO–NiAl2O4produced identical pat-terns as (NiO)1−y(MgO)y/Ni(1−x)MgxAl2O4, except that thepeaks were located at higher 2� values as shown in Fig. 4, com-paring only the largest peaks of the calcined materials alongwith MgAl2O4, synthesized in identical manner. This shift isdue to the larger radii of the magnesium ions, indicating theirpartial substitution in the spinel in accordance with Vegards’Law. Next, attrition of the calcined particles was studied, us-ing commercially available sintered iron oxide for compari-son (with standard deviation 0.95%). Table 1 displays the ob-tained results for the two compositions, extruded and pressed.Note that the pressed magnesium-containing oxygen carrierperformed well, exhibiting higher resistance than the iron ox-ide. Pressed NiO–NiAl2O4 displayed a slightly larger mass lossduring testing which can be attributed to the larger content of

20 30 40 50 60 70 800

400

800

1200

1600

2000

2400

x

*

x

o - NiO/MgO

- Ni1-xMgxAl2O4

- Ni

o

*

*

*

xox

x

o

xo

x

x

o

xx

10 cycles

1/2 cycle

Calcined

As-synthesized

Inte

nsity [

a.u

.]

2θ, degrees

Fig. 3. XRD pattern of (NiO)1−y(MgO)y/Ni(1−x)MgxAl2O4 at differenttesting steps.

36 38 40 42 44 460

500

1000

1500

2000

NiO-NiAl2O4

(NiO)1-y(MgOy)-Ni1-xMgxAl2O4

MgAl2O4

Inte

nsity [a

.u.]

2θ, degrees

Fig. 4. XRD pattern of dominant peaks for NiO/NiAl2O4, (NiO)1−y(MgO)y/

Ni(1−x)MgxAl2O4 and MgAl2O4.

Table 1

Composition Preparationmethod

Attrition (%)

NiO/NiAl2O4 Pressed 5.6NiO/NiAl2O4 Extruded 15.2(NiO)1−y(MgO)y/Ni(1−x)MgxAl2O4 Pressed 1.7(NiO)1−y(MgO)y/Ni(1−x)MgxAl2O4 Extruded 10.3Fe2O3 – 3.9

free NiO (see Section 3.2). The extruded samples, on the otherhand, experienced significant attrition, likely due to their moreporous structure whichwill be further discussed below.

P. Erri, A. Varma / Chemical Engineering Science 62 (2007) 5682–5687 5685

800 900 1000 1100 1200

86

88

90

92

94

96

98

100

102

Temperature (°C)

800 850 900 950 1000 1050 11000.0

0.5

1.0

1.5

2.0

2.5

3.0

Appare

nt R

eaction R

ate

[a.u

.]

Temperature (ºC)

(NiO)1-y(MgO)y/Ni1-xMgxAl2O4

(NiO)1-y(MgO)y/Ni1-xMgxAl2O4

NiO/NiAl2O4

NiO/NiAl2O4

We

igh

t, %

Fig. 5. TGA/TPR of NiO/NiAl2O4 and (NiO)1−y(MgO)y/Ni(1−x)MgxAl2O4 under 3% CH4, 3% H2O, balance Ar, at heating rate 10 ◦C/ min.

3.2. Redox experiments

Thermogravimetric analysis (TGA) was performed on press-ed NiO/NiAl2O4 and (NiO)1−y(MgO)y/Ni(1−x) MgxAl2O4,using 3% CH4, 3% H2O, balance Ar with heating rate10 ◦C/ min. This choice of reductant gas as opposed to hydro-gen, used traditionally, was made to simulate CLC conditions.Reduction of both oxygen carriers is a two-step process, asshown in Fig. 5, with initial fast reaction of the nickel oxidefollowed by the slower mass loss involving the spinel phase.When plotting the obtained data in terms of apparent reactionrate (inset in Fig. 5), this sequential progression is observedwith the largest peak at approximately 860 and 925 ◦C forNiO/NiAl2O4 and (NiO)1−y(MgO)y/Ni(1−x)MgxAl2O4, re-spectively. This temperature increase can be attributed to thepresence of Mg which stabilizes nickel, inhibiting sintering andreduction in both the cubic oxide and the spinel phase (Villaet al., 2003). Note also that mass of the Mg containing oxy-gen carrier stabilizes at ∼ 90 wt%, while the magnesium-freematerial continues to reduce.

The obtained data were analyzed to estimate oxygen carriercompositions. The initial rapid mass loss for the magnesium-free carrier is due to NiO reduction to Ni (Villa et al., 2003),yielding the amount of free NiO in the structure (40 wt%).The subsequent slower mass loss owes to reduction of thespinel. The smaller initial mass decrease of (NiO)1−y(MgO)y/

Ni(1−x)MgxAl2O4, on the other hand, suggests that not allreagent magnesium formed the spinel structure, but insteadwas present as MgO. Now, assuming the calcined sample to befully oxidized, these findings imply an approximat composition

400 600 800 1000 1200

86

88

90

92

94

96

98

100

102

850 °C900 °C

950

10001050

1100

Time (min.)

Weig

ht,%

Fig. 6. TGA of NiO/NiAl2O4 (dashed line) and (NiO)0.79(MgO)0.21/

Ni0.62Mg0.38Al2O4 (solid line) cycled under alternating dilute methane andoxygen at 950 ◦C.

of (NiO)0.79(MgO)0.21/Ni0.62Mg0.38Al2O4, yielding NiO tosupport ratio of (32: 68).

The temperature dependence of the redox properties wasfurther investigated with TGA, cycling the oxidizing (7 min)and reducing gases (15 min) at increasing temperatures. Fig. 6displays the thermogravimetric data at temperature inter-vals starting from 850 ◦C (negligible reduction occurredbelow this temperature), comparing pressed NiO/NiAl2O4

5686 P. Erri, A. Varma / Chemical Engineering Science 62 (2007) 5682–5687

Fig. 7. SEM images of (NiO)0.79(MgO)0.21/Ni0.62Mg0.38Al2O4: a, pressed/calcined; b, extruded/calcined; c, pressed/cycled; d, extruded/cycled.

with (NiO)0.79(MgO)0.21/Ni0.62Mg0.38Al2O4. The stabiliz-ing effect of Mg is again observed with the magnesium-free oxide exhibiting greater reduction at all temperatures.Also note that no increase in mass loss was seen for the(NiO)0.79(MgO)0.21/Ni0.62Mg0.38Al2O4 between the final twotemperature intervals, indicating that the maximum possible re-duction had been achieved at approximately 90 wt% (as notedpreviously, Fig. 5). Comparing with Fig. 5, it is clear that atelevated temperatures for both compositions, partial reductionof the spinel phase takes place. Since spinel support reduc-tion is not desirable for structural stability, these results setan upper limit for the fuel reactor temperature in actual CLCoperation.

The maximum observed reaction rates, in an Arrhenius plot,generated activation energy values of approximately 0.09 and0.43 kJ/mol for oxidation and reduction steps, respectively. Asexpected, given the elevated temperatures and placement of theparticles in the TGA crucible, the reactions are transport-limitedwith severe falsification of the activation energies (Aris, 1975).

Finally, the effect of preparation method was examined, com-paring the pressed and extruded samples. As seen in SEM im-ages (Fig. 7), polyacrylamide addition yields a highly porousstructure, which can be ascribed to gas evolution from polymerdecomposition during calcination. The TGA results for cycled(NiO)0.79(MgO)0.21/Ni0.62Mg0.38Al2O4, are shown in Fig. 8.The extruded sample prepared with polyacrylamide exhibited

P. Erri, A. Varma / Chemical Engineering Science 62 (2007) 5682–5687 5687

100 150 200

90

92

94

96

98

100

102

We

igh

t %

Time (min.)

Extruded

250 300 350

Pressed

Fig. 8. TGA of (NiO)0.79(MgO)0.21/Ni0.62Mg0.38Al2O4, pressed and ex-truded, cycled under alternating dilute methane and oxygen at 950 ◦C.

deteriorating reoxidation in contrast to its pressed counterpart.This suggest that greater sintering of NiO, with a correspond-ing loss in oxidizability during cycling (Ishida et al., 2002),occurred for the extruded sample. BET measurements, usingkrypton owing to low surface areas, are currently in progressto confirm this hypothesis.

4. Concluding remarks

Solution combustion synthesis of nickel oxides for chemicallooping combustion (CLC) was investigated. To improve ther-mal stability and cyclic oxidation/reduction characteristics, theactive metal was supported on two spinel structures: NiAl2O4and Ni(1−x)MgxAl2O4, enabling study of the effect of magne-sium addition. The oxygen carriers were characterized by XRD,confirming formation of the spinel structure after calcinationat 1300 ◦C. Thermogravimetric analysis of the materials underreducing conditions suggested approximate compositions ofNiO/NiAl2O4 and (NiO)0.79(MgO)0.21/Ni0.62Mg0.38Al2O4for the two samples with NiO fractions 40 and 24.6 wt%,respectively. The reduction occurs in two steps: initial rapidreduction of nickel oxide, followed by the slower spinel re-action. The stabilizing effect of magnesium was evident as(NiO)0.79(MgO)0.21/Ni0.62Mg0.38Al2O4 exhibited peak reduc-tion at ∼ 925 ◦C as compared to ∼ 860 ◦C for NiO/NiAl2O4.The degree of reduction was also altered as the former exhib-ited limited reduction of the spinel, while the latter reducedcontinuously throughout the test. This stability improvement

was again observed during cycling of alternating dilute oxygenand methane, simulating CLC operation, where the magnesiumfree oxide was reduced more than its substituted counterpart.

The effect of preparation conditions was also examined,comparing pressed samples with those extruded with polyacry-lamide. The latter exhibited decreased reoxidation, indicatingsintering effects. Finally, structural stability of the oxygencarriers was investigated using a variation of the ASTM at-trition test for catalysts. In this, pressed (NiO)0.79(MgO)0.21/

Ni0.62Mg0.38Al2O4 displayed excellent mechanical strengthwith lower attrition than the reference iron oxide used for com-parison. The results presented in this work suggest that nickeloxide, supported on magnesium substituted nickel aluminate,is a promising oxygen carrier for CLC.

Acknowledgment

This work was supported by the National Science Foundation(Grant CTS-0446529).

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