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Fuel 87 (2008) 988–1001

Using continuous and pulse experiments to comparetwo promising nickel-based oxygen carriers for use

in chemical-looping technologies

Marcus Johansson a,*, Tobias Mattisson b, Anders Lyngfelt b, Alberto Abad c

a Department of Chemical and Biological Engineering, Environmental Inorganic Chemistry, Chalmers University of Technology, S-412 96 Goteborg, Swedenb Department of Energy and Environment, Energy Technology, Chalmers University of Technology, S-412 96 Goteborg, Sweden

c Department of Energy and Environment, Instituto de Carboquımica (CSIC), Miguel Luesma Castan 4, 500 18 Zaragoza, Spain

Received 16 April 2007; received in revised form 17 August 2007; accepted 17 August 2007Available online 11 September 2007

Abstract

Chemical-looping technologies have obtained widespread recognition as power or hydrogen production units with inherent carboncapture in a future scenario where CO2 capture and storage (CCS) is reality. In this paper three different techniques are described; chem-ical-looping combustion and two categories of chemical-looping reforming. The three techniques are all based on oxygen carriers that arecirculating between an air- and a fuel reactor, providing the fuel with undiluted oxygen. Two different oxygen carriers; NiO/NiAl2O4

(40/60 wt/wt) and NiO/MgAl2O4 (60/40 wt/wt) are compared. Both continuous and pulse experiments were performed in a batch lab-oratory fluidized bed working at 950 �C using methane as fuel. It was found that pulse experiments offer advantages in comparison tocontinuous experiments, particularly when evaluating suitable particles for autothermal chemical-looping reforming. Firstly, smallerconversion ranges can be investigated in more detail, and secondly, the onset and extent of carbon formation can be determined moreaccurately. Of the two oxygen carriers, NiO/MgAl2O4 offers several advantages at elevated temperatures, i.e. higher methane conversion,higher selectivity to reforming and lesser tendency for carbon formation.� 2007 Elsevier Ltd. All rights reserved.

Keywords: CO2 capture; Chemical-looping combustion; Chemical-looping reforming; Nickel oxides; Oxygen carriers

1. Introduction

1.1. Carbon capture and storage

It is widely accepted today that CO2 emissions escalatethe greenhouse effect and hence contribute to a higher aver-age temperature on earth. About a third of the global CO2

emissions come from the burning of fossil fuels in powerproduction and roughly a fifth arises from the transporta-tion sector [1]. Since a change to renewable fuels may bedifficult to achieve quickly enough, another option to pro-

0016-2361/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.fuel.2007.08.010

* Corresponding author. Tel.: +46 31 7722887; fax: +46 31 7722853.E-mail address: marcus.johansson@chem.chalmers.se (M. Johansson).

mote the reduction of emissions is to use CO2 capture andstorage (CCS) in fossil-fueled power plants. Most men-tioned suitable locations for CO2 storage are aquifers ordepleted gas and oil fields. If CO2 capture and storage isconnected to the use of fossil fuels in power productionthe result would be no net emission of CO2 to the atmo-sphere. For the transportation sector, CO2 sequestrationapplied on vehicles is not a realistic solution due to costand complexity. One option to reduce carbon dioxide emis-sions from vehicles is to use hydrogen as fuel. If there is tobe no net release of carbon dioxide, this requires that thehydrogen is produced from renewable resources or fromfossil fuels with CO2 capture. In this way CO2 could be sep-arated before the fuel reaches the vehicles and no net emis-sions of CO2 to the atmosphere would take place.

M. Johansson et al. / Fuel 87 (2008) 988–1001 989

Several technologies for CO2 capture at large fossilfueled plants have been proposed, including posttreatment,O2/CO2-firing and CO-shift [2]. A problem with theseoptions is the high costs and energy penalties associatedwith separation of gases [2]. A way to avoid the cost andenergy penalty of gas-separation is to use unmixed combus-tion. This could be achieved by oxidizing the fossil fuelwith oxygen available in metal oxides instead of air. Twoprocesses that use this technique are chemical-looping com-bustion (CLC) and chemical-looping reforming (CLR).CLC is a combustion method with no direct energy penaltyfor CO2 separation and similarly CLR is a method of pro-ducing hydrogen which could reduce the energy penalty forCO2 separation. These techniques are quite similar and areexplained in more detail below.

1.2. Chemical-looping combustion

Chemical-looping combustion (CLC) is a combustionprocess where the aim is to produce power with inherentCO2 capture. CLC consists of two reactors, an air and afuel reactor, as seen in Fig. 1a. Oxygen carriers, in the formof metal oxide particles, are circulating between the reac-tors and transferring oxygen from the combustion air tothe fuel. In this way direct contact between air and fuel isavoided and thus nitrogen and residual oxygen from the

Fig. 1. Schematic view of (a) chemical-looping combustion (CLC), (b) autothechemical-looping combustion (CLR(s)).

combustion air will not be mixed with the combustionproducts from the fuel reactor. The fuel is introduced tothe fuel reactor in a gaseous form where it reacts with anoxygen carrier to CO2 and H2O. The reduced oxygen car-rier is transported back to the air reactor where it is re-oxi-dized by air. The fuel could be syngas from coalgasification, natural gas or refinery gas. The overall reac-tions in the reactors are given below.

Fuel reactor : ð2nþ mÞMyOx þ CnH2m

! ð2nþ mÞMyOx�1 þ mH2Oþ nCO2 ð1ÞAir reactor : ð2nþ mÞMyOx�1 þ ðnþ mÞO2

! ð2nþ mÞMyOx ð2Þ

____________________________________________________________________

Net reaction: CnH2mþ ðnþ mÞO2 ! mH2Oþ nCO2

The total amount of heat evolved from reaction (1) plus (2)is the same as for normal combustion where the oxygen isin direct contact with the fuel. However, the advantagewith this system compared to normal combustion is thatthe CO2 and H2O are inherently separated from the restof the flue gases, and no major energy is expended for thisseparation. Thus, compared to other technologies for

rmal chemical-looping reforming (CLR(a)) and (c) steam reforming using

990 M. Johansson et al. / Fuel 87 (2008) 988–1001

capture of CO2, CLC is potentially much cheaper since nocostly gas-separation equipment is necessary. In this studymethane is used as fuel, however, the fuel used can be syn-gas from gasification of coal, refinery gas or natural gas.Expected temperature range could be between 800 and1200 �C and the combustor could be either pressurized oratmospheric. For a detailed review on all experiments per-formed on oxygen carriers with different types of fuel, seepaper by Johansson et al. [3]. CLC has in the last yearsbeen successfully demonstrated in various prototype circu-lating fluidized bed systems [4–13].

1.3. Chemical-looping reforming

Chemical-looping reforming (CLR) is based on the sametechnology as CLC although the primary purpose here isproduction of H2 from natural gas with inherent CO2 cap-ture. Two proposed processes for the production of hydro-gen through chemical-looping technologies have beendescribed by Ryden et al. [14]. The first of these is calledCLR(a) where the ‘‘a’’ stands for autothermal. This isbased on partial oxidation of the hydrocarbon fuel, whichis shown in Fig. 1b. The gas mixture produced from thefuel reactor consists of mostly CO and H2 in addition tosmaller amounts of CO2 and H2O. To achieve a purehydrogen stream, this reformer-gas should be convertedin a water–gas shift reactor for optimization of producedhydrogen and finally CO2 and H2 could be separated withpressure swing adsorption or absorption with suitableamine solvent. CLR has recently been demonstrated in a300 W prototype reactor by Ryden et al. [15].

The second type of hydrogen production by chemical-looping is called CLR(s) where the ‘‘s’’ denotes steamreforming. The steam reforming part does not differ fromordinary steam reforming in the way that the reactions takeplace inside tubes using suitable catalysts and working atelevated pressure. However, the steam reforming tubesare here placed inside the fuel reactor or a fluidized bedheat exchanger in a CLC unit. Hence, the reformer tubesare not heated by direct firing but rather by the oxygen car-rier particles in the normal CLC process. The feed gas tothe fuel reactor is the offgas from the steam reformingwhich is a gas mixture of CH4, CO2, CO and H2. The pro-posed design of CLR(s) can be seen in Fig. 1c.

1.4. Oxygen carriers

Most of the work on CLC has been focused on thedevelopment and testing of oxygen carriers in particleform. Initial ideas for suitable oxygen carrier material tobe used in CLC and CLR(a,s) are mainly taken from het-erogeneous catalysis used for reforming of hydrocarbonfuel. However, it is important to point out that knowledgefrom research on catalysts for reforming is insufficient. Thereason for this is that both CLC and CLR(a,s) are based onprimary non-catalytic reactions and that the oxygen carri-ers act as a source of undiluted oxygen (i.e. without nitro-

gen). Even though the primary focus of CLC and CLR(a,s)differs, the exothermic oxidation of oxygen carriers with airin the air reactor is the driving force for the, most often,endothermic reactions in the fuel reactor. Because of theneed to transfer large amounts of oxygen between the airand fuel reactor, the oxygen carriers for chemical-loopingtechnologies have high ratios of active material to inertmaterial (typically 20–80%), as compared to heterogeneouscatalyst where the fraction of active material typically isless than 10%.

Almost all research on oxygen carriers have been direc-ted towards finding suitable materials for CLC. For CLR(a) only a limited number of papers exist [15–18]. ForCLR(s) the fuel feed mixture consists of reactive CH4,CO and H2 and unreactive CO2. Earlier studies of oxygencarriers clearly indicate that methane is much more difficultto convert than CO and H2 [19,20]. Therefore, the develop-ment of oxygen carriers for burning methane-rich fuels inCLC is highly relevant for CLR(s).

For the kind of fluidized bed systems outlined above, thecriteria for a good oxygen carrier are the following:

• Complete conversion of fuel.• High reactivity with fuel and air.• Low fragmentation and abrasion.• Low tendency for agglomeration.• Low production cost and preferably being environmen-

tally sound.

For CLC and CLR(s) you have the additionalrequirement:

• Able to convert the fuel to CO2 and H2O to the highestdegree possible (ideal 100%).

In a thermodynamic analysis of different oxygen carri-ers, Jerndal et al. concluded that the metal oxide pairs ofFe2O3/Fe3O4, Cu2O/Cu, Mn3O4/MnO and NiO/Ni werefeasible candidates to be used as oxygen carriers in CLCwhen using methane as a fuel [21]. For CLR(a), more oxidepairs could be used, as for example CoO/Co. These oxygencarriers would preferably be supported by an inert mate-rial. The inert material should add positive properties,among which the most important is to maintain the porestructure inside the particle. Of these suggested oxide sys-tems of transition metals, nickel oxides have been shownto have superior reactivity [22–24]. However, a drawbackwith nickel for use in CLC is the thermodynamic limitationto convert the fuel to 100% CO2 and H2O [21,25]. By this ismeant that small amounts of CO and H2 will always bepresent in the exit of the fuel reactor.

From screening of several types of metal oxides with dif-ferent inert material, different ratios of oxide and inert aswell as using various sintering temperature in manufactur-ing, two promising nickel oxides have been developed atChalmers University of Technology and tested in vari-ous fluidized bed reactor-systems. One is NiO/NiAl2O4

M. Johansson et al. / Fuel 87 (2008) 988–1001 991

(40/60 wt/wt) prepared by freeze granulation and sinteredat 1600 �C, denoted N4AN1600. This particle was selectedas one of the most promising from a screening of 58 differ-ent oxygen carriers tested in a laboratory fluidized bed withmethane as fuel [23]. It was further manufactured in largerscale and tested successfully in a 10 kW chemical-loopingcombustor prototype run for 100 h with natural gas as fuel[4,5]. A subsequent study of the used particles from the10 kW reactor indicated that no deactivation of particleshad taken place. Instead the particles were slightly harderand were as reactive as fresh particles [26]. The conclusionthat these nickel-based particles survived in a continuousCLC system is of great importance since many papers haveconfirmed the fast reactivity of oxygen carriers based onNiO/NiAl2O4 but expressed concern about its lack of phys-ical strength [12,22].

The other very promising nickel-based oxygen carrier isNiO/MgAl2O4 (60/40 wt/wt), also prepared by freeze gran-ulation and sintered at 1400 �C (N6AM1400). This carrierwas selected from another screening study of 50 oxygencarriers in laboratory fluidized bed using methane as fuel[24]. It was further successfully tested with natural gas asfuel in a 300 W reactor for chemical-looping combustion[8] and also in the first testing of CLR(a) in circulatingmode [15]. N6AM1400 have also been investigated andcompared with an iron- and manganese-based oxygen car-rier using syngas as fuel [19]. Recently this oxygen carrierwas also used in a mixed oxide system – where a small addi-tion of nickel oxides to a bed of iron oxides was shown togreatly increase the reactivity of the iron oxide bed [27].

The two promising nickel-oxides have also been com-pared by Johansson et al. in a study using both naturalgas and syngas as fuel in a 300 W reactor [7]. It was foundthat while N6AM1400 could convert all CH4 (when usingnatural gas as fuel) there was always methane out fromthe tests with N4AN1600. Further on CO was present inthe flue gas according to equilibrium for N4AN1600whereas the amount of CO out from experiments withN6AM1400 exceeded equilibrium. In another study byMattisson et al., similar nickel oxides but of lower sinteringtemperature were compared in a laboratory fluidized bed[25]. Also here it was found that the nickel oxide withMgAl2O4 as inert was better at converting methane. Villaet al. compared nickel oxides on the inert systems Al–Oand Mg–Al–O material in a thermogravimetric analyzerusing methane as fuel [28]. They concluded that the oxygencarrier containing Mg in the inert material was more stablein continuous redox reactions. This was explained by thefact that formation of NiO–MgO solid solution stabilizesNi2+ against reduction and sintering. Further on they con-

Table 1Oxygen carriers

Metal oxide Metal oxide (wt%) Inert Inert (wt%) Nom

NiO 60 MgAl2O4 40 N6ANiO 40 NiAl2O4 60 N4A

cluded that the nickel oxide with Mg was more apt to formCO and H2 and that it was better suited to avoid carbondeposition. It is well known that MgO can be used as anadditive to catalysts in steam reforming to promote gasifi-cation of carbon by aiding H2O adsorption [29,30]. Thekinetics of the two oxygen carrier has also been investi-gated in a thermogravimetric analyzer. With methane asfuel, the order of reaction and the activation energy werefound to be 0.8 and 78 kJ/mol for N4AN1600 [31] and0.4 and 114 kJ/mol for N6AM1400 [32].

The promising results obtained with both NiO/NiAl2O4

and NiO/MgAl2O4 both with respect to CLC and CLR(a,s) warrants a deeper investigation of these two particles.The objective of this paper is therefore:

• To compare the reactivity of N6AM1400 and N4AN1600and determine their feasibility for CLC and CLR (a,s).

• To compare two different experimental methods, i.e.continuous and pulse experiments, particularly in orderto find a suitable way of investigating oxygen carriers forCLR (a).

2. Experimental

2.1. Preparation of oxygen carriers

Spherical particles in the size range 125–180 lm com-posed of 60% NiO/40% MgAl2O4 (N6AM1400) and 40%NiO/60% NiAl2O4 (N4AN1600) were prepared by freezegranulation. The details of this procedure have beendescribed elsewhere [23,24]. The properties of the particlesare presented in Table 1. For N4AN1600, the inertNiAl2O4 was formed during heat treatment. Therefore,NiO was used in excess initially so that the amount ofactive material after heat treatment is 40%. For both parti-cles the sintering temperature used has been obtained froman optimization of strength (normally increases with sinter-ing temperature) and reactivity (normally decreases withsintering temperature).

The fact that the particles contain different amount ofactive material also means that they have a theoretical dif-ference in the amount of oxygen that can subsequently bereduced/oxidized within the looping system.

2.2. Reactivity investigation

The experiments were conducted in a fluidized bed reac-tor of quartz. The reactor had a length of 820 mm with aporous quartz plate of 30 mm in diameter placed 370 mmfrom the bottom. The inner diameters of the bottom and

enclature Porosity BET (m2/g) Sintering temperature (�C)

M1400 0.42 1.24 1400N1600 0.36 0.4 1600

992 M. Johansson et al. / Fuel 87 (2008) 988–1001

top sections were 19 and 30 mm. The temperature wasmeasured 5 mm under, and 38 mm above the porousquartz plate, using 10% Pt/Rh thermocouples enclosed inquartz shells. The pressure drop over the bed was measuredby means of Honeywell pressure transducers and measuredat a frequency of 20 Hz. From the pressure fluctuations itwas possible to establish if the particles were fluidized dur-ing the oxidizing period. In the reducing period, this type ofanalysis was not possible because of flow variations. Asample of 15 g of oxygen carrier particles, in the size rangeof 125–180 lm, was initially heated in an inert atmosphereto 950 �C. As reducing gas 50% CH4/50% H2O was used.Because of the heat produced during the oxidation period,a gas mixture with 5% O2 in N2 was used instead of air.Thus, large temperature increases were avoided. To avoidair and methane mixing during the shifts between reductionand oxidation, nitrogen gas was introduced during 180 safter each period. The gas from the reactor was led to anelectric cooler, where the water was removed, and then toa gas analyzer where the concentrations of CO2, CO,CH4, and O2 were measured in addition to the gas flow.Hydrogen is not measured on-line but instead assumed tobe correlated to the outlet partial pressures of CO andCO2 through the water–gas shift equilibrium. The experi-ments were conducted with an inlet gas flow 900 mLn/min for the reducing gas. This means that the solid inven-tory corresponds to 56:8 kg=MWCH4

in these experiments.For the inert and oxidizing gas, the gas flow was 900 and1000 mLn/min, respectively.

Two types of experiments were used to compare the oxy-gen carriers; continuous and pulses tests. In a continuousexperiment the particles are exposed alternately to 5% O2

and 50% CH4/50% H2O, thus simulating the cyclic condi-tions of a CLC system.

In a pulse experiment, the reduction period is dividedinto numerous pulses of either 2, 4 or 6 s. Using shorterpulses is naturally more interesting since smaller conver-sion intervals can be investigated. However, the gas ana-lyzer collects data every 2 s – and the error in the massbalances will therefore be higher for shorter pulses. Here,all three pulse times were investigated in order to achieveas reliable results as possible. Between the pulses, inertgas is introduced for one minute to be sure that the remain-ing gas from the last pulse has left the reactor system. Aftera series of reduction pulses, and a subsequent period whereinert gas has purged the reactor system, the particles areoxidized completely in 5% O2 before a new reduction per-iod is started again.

2.3. Data evaluation

The degree of conversion, or oxidation, X, is defined as:

X ¼ m� mred

mox � mred

ð3Þ

where m is the actual mass of sample, mox is the mass of thesample when fully oxidized, and mred is the mass of the

sample in the fully reduced form. For a detailed descriptionof how to calculate X from the concentration and flow, seeother publication [25].

In order to facilitate a comparison between the twonickel oxides, a mass based conversion was defined as;

x ¼ mmox

¼ 1þ R0ðX � 1Þ ð4Þ

where R0 is the oxygen ratio of the oxygen carrier, definedas:

R0 ¼ ðmox � mredÞ=mox ð5ÞThe oxygen ratio is the maximum mass fraction of the oxy-gen carrier that can be used in the oxygen transfer, andwould here correspond to 0.086 (8.6%) for N4AN1600and 0.129 (12.9%) for N6AM1400. From this, the theoret-ical mass based conversion, x, of the two nickel oxides attheir fully reduced state should be 0.914 and 0.871,respectively.

The outlet fraction of methane in the bed is:

cCH4¼

pCH4;out

pCH4;out þ pCO2;out þ pCO;out

ð6Þ

where pi, out is the partial pressure of outgoing gaseous spe-cies i in dry basis. In order to evaluate oxygen carriers suit-able for CLR (a) the stochiometric ratio of the gasesleaving the reactor during the fuel addition phase, kF, is de-fined as:

kF ¼ð2pCO2;out þ pCO;out þ 2ðpCO2;out þ pCO;outÞ � pH2;outÞ

4ðpCH4;out þ pCO2;out þ pCO;outÞð7Þ

Here, the denominator denotes how many atoms of oxygenthat are needed to fully convert the incoming methane toCO2 and H2O and the numerator denotes how many oxy-gen atoms that have been supplied by the oxide in the out-going gas. Hence a kF of one equals full conversion of thebed to CO2 and a kF of about 0.3–0.4 is suitable forCLR(a) [33].

3. Results

3.1. Concentration profiles during continuous experiments

Concentration profiles of reduction and oxidation forcontinuous experiments are found in Fig. 2. The x-axisstarts when corresponding reducing or oxidizing agent isswitched on. As seen there is delay of approximately 20 sbefore the product gases reach the analyzer which is thenfollowed by a period of backmixing until all nitrogen fromthe preceding period is fully replaced. A more detaileddescription of the behavior of flow and concentration pro-files can be found elsewhere [27]. From the reduction pro-files it is clear that the yield of CO2 leaving the reactor ishighest at the initial stage of the reduction and thendecreases as time proceeds. For both nickel oxides thereis an initial small CH4 peak (not noticeable in the figures),

0 400 800 1200 1600 20000

1

2

3

4

5

0

0.2

0.4

0.6

0.8

1

Flow

O2

CO2 & CO

0 100 200 300time (s) time (s)

time (s)

0

20

40

60

80

100

conc

entra

tion

(%)

conc

entra

tion

(%)

conc

entra

tion

(%)

0

0.2

0.4

0.6

0.8

1

Flow

CO2

CH4

CO

0 1000 20000

1

2

3

4

5

0

0.2

0.4

0.6

0.8

1

flow

(LN/m

in)

flow

(LN/m

in)

flow

(LN/m

in)

O2

Flow

CO

CO2

a b

dc

Fig. 2. Concentration profiles (dry basis) from continuous batch experiments of N6AM1400 (a) reduction (b) oxidation and N4AN1600 (c) reduction (d)oxidation. Hydrogen not included in the figures. Vertical dashed lines indicate switch to inert gas.

M. Johansson et al. / Fuel 87 (2008) 988–1001 993

which is followed by a long period with virtually no meth-ane until later stages of the reduction period.

As can be seen for N4AN1600 in Fig. 2c, there is a longtail of CO leaving the reactor in the inert period as well asCO and CO2 leaving the reactor initially in the oxidizingperiod. This is a result of carbon formation and will bedescribed further ahead.

3.2. Concentration profiles during pulses

An example of a full cycle (reduction with pulses-inert-oxidation) of a pulse experiment with N6AM1400 can befound in Fig. 3. Included in the figure is the degree of con-

version. A summary of the different pulse experimentsinvestigated in this paper can be found in Table 2.

Results from two pulse experiments with 10 pulses of 6 sfor the two different nickel oxides can be seen in Fig. 4.Note that the concentrations displayed are diluted withnitrogen from the inert periods. Clearly, there are some sig-nificant differences between the two oxygen carriers. As canbe seen both oxygen carriers have a portion of methaneleaving the reactor in the first pulse that disappears(N6AM1400) or diminishes (N4AN1600) in the secondpulse. For N6AM1400 in Fig 4a the concentrations ofCO and CO2 are almost constant throughout the first sevenpulses, with a high conversion to CO2. Thus, the reactionproducts seem to be insensitive to the diminishing amount

Fig. 3. An example of a full cycle for a pulse experiment for N6AM1400, including 10 pulses of 6 s (dry basis), inert period and an oxidation period. Notethat nitrogen is introduced for one minute between each pulse. CO2 (s) and O2 (4), X (- - -). In the reduction only CO2 is shown for simplicity. For a moredetailed scenario of the reduction, see Fig. 4.

Table 2Pulse reductions investigated

Pulse time (s) N6AM1400 N4AN1600# pulses # pulses

2 20 104 10 and 20 206 10 and 20 6 and 10

994 M. Johansson et al. / Fuel 87 (2008) 988–1001

of oxygen in the bed. For N4AN1600 in Fig. 4b the con-centration of CO2 is decreasing and CO is increasing stea-dily with each pulse as the amount of available oxygen inthe carriers is diminishing.

3.3. Discreet reduction profile

By using pulse experiments a normalized concentrationcan be calculated for each pulse. Hence, an advantage ofusing the pulse experiments as compared to continuousexperiments is that the former can be displayed withoutdisturbances from backmixing of gases. These normalizedconcentrations are solely based on the volumetric fractionsof each gas out during a pulse where the nitrogen content isexcluded. The result of the concentration profiles as a func-tion of X and x for the two nickel oxides can be seen inFig. 5, where all the concentration profiles from the differ-ent cycles of the experiments are superimposed on eachother. As noted from the figures the results from the differ-ent pulse experiments have a high reproducibility, i.e. thegas composition as a function of the degree of conversionis similar for all experiments performed.

The profiles of the oxygen carriers in Fig. 5 are quite dif-ferent with N6AM1400 which have relatively stable con-centrations in the conversion interval 1–0.5 whereasN4AN1600 continuously releases less CO2 as the degreeof conversion gets smaller. Also note that for N6AM1400

a fully reduced sample is not achieved. This has beennoted before and is most likely related to reaction of activeand inert material during heat treatment with formationof compounds such as MgO, MgNiO2 and NiAl2O4

[18,25,32]. This means that the real amount of active mate-rial as NiO is less than the 60% stated. According to Zafaret al., experiments in a thermogravimetric analyzer indi-cated that the active material was 50% [32]. It is not easyto determine the exact composition of the sample after heattreatment because many of the possible compositionsformed have peaks that interact in the X-ray diffractioninvestigation. The formation of inactive compounds duringheat treatment of N6AM1400 means that the two oxygencarriers in the end have rather similar amount of availableoxygen, as can be seen when comparing the end-values ofthe mass based conversion, x, for the two profiles in Fig. 5.

3.4. Conversion of methane

From the different pulse experiments on the nickel oxi-des, unconverted methane always passed the reactor inthe first pulse, see Fig. 4. This is likely due to the lack ofNi0 sites on the oxidized particles. The reason for this isthat pure nickel is known to catalyze both steam reforming:

CH4 þH2O! COþ 3H2 ð8Þ

and methane pyrolysis:

CH4 ! Cþ 2H2 ð9Þ

In order to investigate to what extent the bed needs to bereduced in order to fully convert methane, smaller pulses of2 s were investigated. The outcome of this can be seen inFig. 6. For N6AM1400 methane was fully converted inthe second pulse. For N4AN1600 unconverted methanestill passed the reactor in the second pulse, but was

0 200 400 600 0 200 400 600time (s) time (s)

0

1020

30

4050co

ncen

tratio

n (%

)

4

8

120

0.40.8

1.2

1.6

200.2

0.4

0.60.8

1

0

1020

30

4050co

ncen

tratio

n (%

)

4

8

120

0.40.8

1.2

1.6

200.2

0.4

0.60.8

1

X (-)

0.88

0.92

0.96

1

ω (−

)

CH4

CO

CO2

X (-)

0.92

0.94

0.96

0.98

1

ω (−

)

CH4

CO

CO2

a b

Fig. 4. Two pulse experiments with 10 cycles of 6 s (dry basis) of (a) N6AM1400 and (b) N4AN1600. Note that the sums do not add up to 100% due todilution with nitrogen from inert periods.

1 0.8 0.6 0.4 0.2 0 1 0.8 0.6 0.4 0.2 0X

0

20

40

60

80

100

Nom

aliz

ed C

onc.

CO

2, CO

, H2 (%

)

Nom

aliz

ed C

onc.

CO

2, CO

, H2 (%

)

0

4

8

12

Nom

aliz

ed C

onc.

CH

4 (%)

Nom

aliz

ed C

onc.

CH

4 (%)

1 0.98 0.96 0.94 0.921 0.96 0.92 0.88ω

X

0

20

40

60

80

100

0

4

8

12

ωa b

Fig. 5. Discreet normalized concentration profiles (dry basis and without nitrogen) calculated from pulse experiments as a function of X and x for (a)N6AM1400 and (b) N4AN1600. CH4 (+), CO2 (s), CO (h), H2 (4). Note that CH4 concentrations are on the right hand side y-scale.

M. Johansson et al. / Fuel 87 (2008) 988–1001 995

suppressed in the third cycle. This indicates that very smallamounts of pure nickel, Ni0, are needed to totally convertthe incoming methane. The degree of conversion needed

for the nickel oxides to totally convert the incoming meth-ane can then be calculated. For N4AN1600 it was found tobe at X = 0.94 (i.e. x = 0.995) or higher and for

0 40 80 120 160time (s)

0

0.4

0.8

1.2

1.6

2

conc

entra

tion

(%)

0

0.4

0.8

1.2

1.6 a

b

Fig. 6. CH4 concentration (dry basis) for first 3 pulses of 2 s. (a)N6AM1400 and (b) N4AN1600. Note that there is dilution with nitrogenfrom inert periods.

996 M. Johansson et al. / Fuel 87 (2008) 988–1001

N6AM1400 X = 0.985 (i.e. x = 0.998) or higher. In a realapplication where particles continuously flow into the fuelreactor, it will contain a mix of particles with varyingdegrees of conversion.

Another interesting effect of methane conversion isshown in Fig. 7a and b where the fraction of methaneout from continuous and pulse experiments are compared.The pulse experiments are shown as discreet fractions cal-culated from the different pulse cycles superimposed oneach other. For N6AM1400 the methane fraction is basi-cally zero for both continuous and pulse experiments afterthe initial peak at high degrees of conversion in the bed.However, for N4AN1600 the methane fraction is lowerfor the continuous than the pulse experiments for most partof the reduction period. This could be explained by the twodifferent ways the experiments are conducted. Since meth-ane pyrolysis (9) probably takes place at an early stage[34,35] it could be speculated that carbon formed on the

1 0.8 0.6 0.4 0.2 0X

0

4

8

12

γC

H4 (%

)

a b

Fig. 7. Methane fractions for (a) N6AM1400 and (b) N4AN1600

particle interior and exterior surfaces catalyzes methaneconversion. For the pulse experiments, any deposited car-bon in each pulse has sufficient time in the inert periodsbetween the pulses to react by solid–solid phase reactionwith oxygen present in the oxygen carrier before the nextpulse is started (see next section). In this way an autocata-lytic effect of carbon deposition is not seen to such largeextent. On the other hand, for the continuous experimentsmethane again starts passing through the reactor uncon-verted at a higher degree of conversion. An explanationto this could be that the accumulated carbon for the con-tinuous experiments at a certain stage represents a com-plete fouling of the bed material with blocking of activesites as a result. For example, steam reforming catalystsmay remain active through a long period, although theyhave accumulated carbon several times their own weight,because deposited carbon dissolves into the particles leav-ing the surface of the particles active for catalysis [30,36].Hence, the situation may occur that nickel continues to cat-alyze and accumulate carbon for a long period until itreaches the point where the catalytic effect dropsdrastically.

The fact that N6AM1400 is better than N4AN1600 atconverting methane is in accordance with previous studiesof the two oxygen carriers [7,25,28]. However, two of thesestudies also indicated that the carriers with the addition ofmagnesium were less good at oxidizing CO and H2. Fromthe results here the opposite seems valid. This could beexplained by the high temperature used in this study,950 �C. The reactivity of N6AM1400 has been shown tobe highly temperature dependant and displayed a largedecrease in reactivity with methane between 1000 and800 �C [32]. Furthermore, a large decrease in reactivitywas also found in experiments with syngas between 950and 650 �C [19]. Since production of syngas most likely isan intermediate when using methane as fuel, this couldexplain the high concentrations of CO and H2 found in pre-vious studies where the temperature was lower [7,28]. On

1 0.8 0.6 0.4 0.2 0X

0

4

8

12

γC

H4 (%

)

. Results from continual experiments (-) and from pulses (d).

M. Johansson et al. / Fuel 87 (2008) 988–1001 997

the other hand, earlier experiments of N4AN1600 withmethane at different temperatures showed no significantdecrease in reactivity between 950 and 750 �C [35].

3.5. Carbon deposition

What is clear from the continuous experiments displayedin Figs. 2c–d is that there is carbon formation forN4AN1600. This is both seen as the large tail of CO leavingthe reactor in the inert period and the CO and CO2 leavingthe reactor in the subsequent oxidation period. Using 50%of steam with the methane flow should be enough to sup-press carbon formation [21,34] but this is obviously notthe case here. Carbon formation could occur through eithermethane pyrolysis (9) or the Boudouard reaction:

2CO! CO2 þ C ð10Þ

In Fig. 2c there is a lot of CO leaving the reactor into theinert period. Since there is no other oxygen source than theoxygen on the particles present in the bed at this stagethe reaction must be solid–solid according to:

0 20 40 60

0 20 40 60

0

10

20

30

40

50

time (s)

time (s)

0

10

20

30

40

50

conc

entra

tion

CO

2 (%

)co

ncen

tratio

n C

O2

(%)

a

c

Fig. 8. Concentration profiles from selected pulses of 6 s for N6AM1400 (20 pPulse # 1 (+), # 4 (e), #6 (�),#7 (·), #8 (j), #9 (h), #10 (s), #13 (d), #15 (4The x-axes show time after start of pulse.

CþMyOx ! CO=CO2 þMyOx�1=2 ð11Þ

This reaction has also been reported earlier [25,34].Earlierstudies have also proposed that carbon formation through(9) followed by

CþH2O! COþH2 ð12Þ

may be a possible reaction path for NiO on NiAl2O4 par-ticles where CO and H2 subsequently act as reagents[34,35]. Thus, one cannot exclude the possibility that car-bon could be an intermediate also when no tailing of COis evident in the inert period and no carbonous speciesare leaving the reactor in the oxidizing period. It mightbe that carbon formation occurs but at a rate where it israpidly gasified, thus preventing major amounts of net car-bon to form.

By examining the CO2 and CO profiles out from eachpulse, it can be seen when carbon formation starts tobecome a problem. This can be seen in Fig. 8 for experi-ments with 6 s pulses where selected CO2 and CO profilesare shown. As seen, CO2 out from the first cycles is high

0 20 40 60

0 20 40 60time (s)

time (s)

0

5

10

15

20

25

conc

entra

tion

CO

(%)

0

4

8

12

conc

entra

tion

CO

(%)

b

d

ulses) of (a) CO2, (b) CO and N4AN1600 (10 pulses) of (c). CO2, (d) CO), #17 (m), #19 ($) and #20 (.). Note that the scales on the y-axis differs.

998 M. Johansson et al. / Fuel 87 (2008) 988–1001

and the concentration peak of CO2 decreases as the pulsesproceed, whereas the opposite is valid for the CO pulses.Furthermore, the time of the CO2 and CO pulses, indepen-dent of their concentration peak, is also more or less simi-lar for the first pulses. However, as can be seen in the lastpulses there is significant ‘‘tailing’’ of either CO2 and/orCO leaving the reactor. This tailing can be explained bysolid–solid state reaction between carbon and oxygen inthe particles, reaction (11). An interesting differencebetween the two nickel carriers is the selectivity towardsCO2 or CO. For N6AM1400 there is no tailing at all ofCO2 – the selectivity of Eq. (11) is totally towards COfor the pulses investigated. For N4AN1600, tailing of bothCO2 and CO is initiated after a couple of pulses, but for thesubsequent pulses it seems as if the selectivity is turningtowards formation of CO. An explanation for the selectiv-ity could be the amount of available oxygen present in thebed. For N6AM1400, there is very little easy available oxy-gen left when net formation starts and hence CO is pro-duced. For N4AN1600 there is still much oxygen left inthe bed when carbon formation is initiated and thereforesome CO2 can be formed, but less CO2 is formed as theoxygen in the bed is consumed.

In order to quantify the amount of carbon formed theconcentration of the total carbon, i.e. CH4, CO2 and COversus time, was derived for each pulse. Furthermore, anintegration of these pulses verified that the total carbonin each pulse had high reproducibility. From these carbonbased concentration plots, a standardized distributioncould be identified for pulses without carbon formationsince they were almost identical. If carbon formationoccurred, this was identified as a deviation from the stan-dardized curve in the transient period at the end, i.e. wherethe tailing occurs. By integrating this deviation, the carbonformed could be quantified as percentage of total carbonspecies in one pulse. The result from this is displayed inFig. 9. For N6AM1400, carbon formation is first noticedafter the 13th pulse and for N4AN1600 already in the third

0 4 8 1 2 16 20Pulse #

0

0.2

0.4

0.6

0.8

1

Cde

posi

ted/C

H4,

in

a

Fig. 9. The ratio of deposited carbon for each

pulse. The fact that N6AM1400 is less sensitive to carbonformation is in accordance with previous studies [7,28].Hence, between the two nickel carriers there is a significantdifference in the degree of conversion for which tailing isinitiated; for N6AM1400 at 0.33 < X < 0.36 (0.913 < x <0.917) and for N4AN1600 at 0.73 < X < 0.82 (0.977 <x < 0.984). Normally this low conversion degree shouldbe avoided in CLC but since lower degrees of conversionare used in CLR(a) care should be taken to avoid theselower degrees of conversion where net formation of solidcarbon could be a problem.

To investigate the oxygen carriers’ suitability for CLR(a), the stochiometric ratio and the outlet methane fractionwas calculated. The outcome of this can be seen in Fig. 10,where results from continuous experiments are alsoincluded. For CLR(a) optimization of the process isachieved if kF equals approximately 0.35 and the methaneconversion at the same time is complete [33]. In the figures,the two vertical dashes lines indicate in-betweenwhich pulses that carbon formation was initiated. ForN6AM1400 carbon formation was initiated before suchlow stochiometric ratio was reached. For N4AN1600 car-bon formation is initiated already at very high values ofthe stochiometric ratio, almost when there still is completecombustion. This finding is very much in line with previousstudies where carbon formation was suggested as a reactionintermediate for particles consisting of NiO on NiAl2O4

[34,35].Since hydrogen concentrations are calculated from

water–gas shift equilibrium, a small error in the hydrogenconcentration is introduced as soon as carbon formationis initiated. The more carbon that is deposited the largerwill be the error and it will naturally also affect the massbalances and normalized concentrations calculated here.Hence, in Fig. 10, the error will start on the right-hand sideof the vertical dashed lines and increase as we go further tothe right. This is also true for the profiles in Fig. 5 using thesame limits for carbon initiation.

0 2 4 6 8 10Pulse #

0

0.2

0.4

0.6

0.8

1

Cde

posi

ted/ C

H4,

in

b

pulse. (a) N6AM1400 and (b) N4AN1600.

1 0.8 0.6 0.4 0.2 0X

0.2

0.4

0.6

0.8

1λ F

0

0.02

0.04

0.06

0.08

0.1

γ CH

4

1 0.96 0.92 0.88ω

1 0.8 0.6 0.4 0.2 0X

0.2

0.4

0.6

0.8

1

λ F

0

0.02

0.04

0.06

0.08

0.1

γ CH

4

1 0.98 0.96 0.94 0.92ωa b

Fig. 10. The stochiometric ratio (pulse �, continuous d) of the gases leaving the reactor and the fraction of methane out (pulse e, continuous s) as afunction of the degree of conversion for (a) N6AM1400 and (b) N4AN1600. In-between dashed vertical lines indicates region where carbon deposition isinitiated for the pulse experiments.

M. Johansson et al. / Fuel 87 (2008) 988–1001 999

4. Discussion

Although the theoretical oxygen ratio of the nickel-particles is as high as 8.6% (N4AN1600) and 12.9%(N6AM1400) all available oxygen would not be used in areal application when using natural gas (or methane) asfuel. Due to the endothermic reaction in the fuel reactor,the solid circulation of particles needs to be sufficiently highto transfer enough heat from the air reactor to the fuel rec-tor [21]. As an example, by using a realistic recirculationrate of particles of 8 kg/s, MWth, using a Dx = 0.01 wouldmean a temperature drop of approximately 50 �C. There-fore, the conversion difference should not exceed typicallyDx � 0.02 in a real application in order to avoid too lowtemperatures in the fuel reactor. With the fuel flow usedhere, 15 s is needed to convert a bed 2 wt-%, i.e. Dx� 0.02, assuming full conversion to CO2 and H2O. As anexample, 2–3 pulses of 6 s, 4 pulses of 4 s or 7–8 pulsesof 2 s is a good estimation of the desired working rangeof the oxygen carriers for use in CLC and CLR (s). ForCLR(a) neither full conversion to CO2 and H2O or as highdegrees of conversion as Dx � 0.02 are needed, howeverthe same amount of pulses could probably be assumed tocover the desired working range.

Almost no methane was found for the experimentswith N6AM1400 and small amounts were found forN4AN1600. In a real application, since there is no thermo-dynamic limitation to convert methane with nickel oxides[21], methane could be suppressed by using a higher solidsinventory per MW of fuel. However, less solids inventorywould be needed for N6AM1400 which is an advantage.

For CLC and CLR (s) a high yield of CO2 and full con-version of CH4 is desired and hence for both nickel oxidesit is an advantage if the conversion region of the particles

used is as high as possible. Investigating the reactivity ofoxygen carriers for CLC and CLR (s) is therefore astraightforward task and performing continuous experi-ments is sufficient to obtain satisfying knowledge.

For CLR(a), full conversion of methane is desired aswell as a high yield of reformer-gas consisting mainly ofCO and H2. However, in order to heat the incomingstreams of fuel, oxygen and circulating oxygen carriers,some produced amounts of CO2 and H2O are desired inorder to optimize the system. Hence a kF of approximately0.35 has been proposed as appropriate for CLR(a) [33].This means that the working window of an oxygen carrierfor CLR(a) would be at much lower degrees of conversionthan for CLC and CLR (s), as can be seen from the gascompositions in Fig. 8. Even though neither of the particlesmanaged to reach a kF of 0.35 without carbon formation,N6AM1400 is still seen as promising for CLR(a). Carbonformation could, for example, be further suppressed byraising the reaction temperature. Besides, although thispaper indicates that carbon formation could be a problemin a real application, it is important to bear in mindthat both oxygen carriers already have been tested inchemical-looping prototypes in successful experimentswithout carbon formation; N4AN1600 for CLC [4,5,7]and N6AM1400 for CLC [7,8] and CLR(a) [15].

For CLR (a), pulse experiments have two features thatmake them suitable for investigating oxygen carriers:

• As lower degrees of conversion are used in CLR (a), theparticles that enter the fuel reactor are not fully oxidizedas in CLC. Naturally, the particles are also totally freefrom any possibly deposited carbon. Since it has beenshown that autocatalytic carbon formation has an effecton the methane conversion, continuous experiments

1000 M. Johansson et al. / Fuel 87 (2008) 988–1001

from fully oxidized samples will not give a proper meth-ane conversion at lower degrees of conversion. Withpulse experiments, every pulse always starts withoutany deposited carbon on the particles, which is a morerealistic situation.

• With pulse experiments small snapshots of the conver-sion only based on reaction products can easily beobtained, avoiding the effect that carbon formationcould have on subsequent methane decomposition. Bycombining these snapshots with investigation of possibletailing of CO2 and CO, start and extent of carbon for-mation is obtained.

It could be argued that due to the high dispersion ofgases in the analyzer, see Fig 8, the beds of oxygen carriersare exposed to lower concentrations of methane and steamin the pulse experiments because of dilution with inertnitrogen. This would mean that the study of the pulseexperiments would not be so representative for a real appli-cation. However, almost all dispersion in the laboratoryequipment is expected to take place after the bed of parti-cles, to some extent above the bed but mainly in the con-denser after the reactor, due to its relatively large sizeand low temperature.

5. Conclusion

Of the two investigated oxygen carriers, N6AM1400 andN4AN1600, the former is both more suitable for chemical-looping combustion and the two types of chemical-loopingreforming at 950 �C. It has a better methane conversion,better reforming properties and is better at avoiding netcarbon formation.

For N4AN1600, carbon formation is initiated very earlyon in the reduction even though the selectivity to CO2 isvery high. On the contrary, for N6AM1400 carbon forma-tion takes place when the reaction products are mainly COand H2.

It has been demonstrated that pulse experiments haveseveral advantages compared to continuous experiments,and could be a useful method to investigate oxygen carrier,especially for CLR(a). The conversion of the gas as well asthe extent of carbon formation can be obtained as a func-tion of the solids conversion.

Acknowledgement

This work was made with financial support from theSwedish Energy Agency, under project 21670-1, and theEU project Cachet, Contract Nr. 019972 (SES6).

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