Fan_Advancements in CLC

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Hindawi Publishing CorporationInternational Journal of Chemical EngineeringVolume 2009, Article ID 710515, 16 pagesdoi:10.1155/2009/710515

Review Article Advancements in Development of Chemical-LoopingCombustion: A Review

He Fang, Li Haibin, and Zhao Zengli

The Renewable Energy and Gas Hydrate Key Laboratory of Chinese Academy of Sciences, Guangzhou Institute of Energy Conversion,Chinese Academy of Sciences, Guangzhou 510640, China

Correspondence should be addressed to He Fang, [email protected]

Received 9 June 2009; Accepted 18 August 2009

Recommended by B. Kulkarni

Chemical-looping combustion (CLC) is a novel combustion technology with inherent separation of greenhouse CO2. Extensiveresearch has been performed on CLC in the last decade with respect to oxygen carrier development, reaction kinetics, reactordesign, system efficiencies, and prototype testing. Transition metal oxides, such as Ni, Fe, Cu, and Mn oxides, were reported asreactive species in the oxygen carrier particles. Ni-based oxygen carriers exhibitedthe best reactivityand stability during multiredoxcycles. The performance of the oxygen carriers can be improved by changing preparation method or by making mixedoxides.The CLC has been demonstrated successfully in continuously operated prototype reactors based on interconnected fluidized-bed system in the size range of 0.350 kW. High fuel conversion rates and almost 100% CO2 capture efficiencies were obtained.The CLC system with two interconnected fluidized-bed reactors was considered the most suitable reactor design. Developmentof oxygen carriers with excellent reactivity and stability is still one of the challenges for CLC in the near future. Experiences ofbuilding and operating the large-scale CLC systems are needed before this technology is used commercially. Chemical-looping

reforming (CLR) and chemical-looping hydrogen (CLH) are novel chemical-looping techniques to produce synthesis gas andhydrogen deserving more attention and research.

Copyright 2009 He Fang et al. This is an open access article distributed under the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

On February 2, 2007, the United Nations scientific panelstudying climate change declared that the evidence of awarming trend is unequivocal, and that human activityhas very likely been the driving force in that change overthe last 50 years [1]. According to the Intergovernmental

Panel on Climate Change (IPCC) of the United Nations,the observed increase in globally averaged temperaturessince the mid-twentieth century is very likely to haveoccurred due to the increase in anthropogenic greenhouse-gas concentrations that leads to the warming of the earthssurface and lower atmosphere. The greenhouse effect is thephenomenon where water vapor, carbon dioxide, methane,and other atmospheric gases absorb outgoing infraredradiation resulting in the raising of the temperature. In itsturn, CO2 is essentially blamed to be the main factor causingthe greenhouse effect because it is the most importantanthropogenic greenhouse gas [2]. The concentration of CO2in the atmosphere has risen to a value of370 ppm today,

from the preindustrial value of 280 ppm [3]. Combustionof fossil fuels releases a huge amount of carbon as carbondioxide into the atmosphere. It was reported that fossilfuels fired power production contributes with one thirdof the total carbon dioxide release from fuels combustionworldwide [4]. It has been well known that increasingconcentration of CO2 to the atmosphere may affect theclimate of the earth. It is generally accepted that a reductionin emissions of greenhouse gases is necessary. At present,there are a number of CO2 capture processes as follows [5]:

(i) precombustion, in which the hydrocarbon fuels aredecarbonized prior to combustion;

(ii) oxyfuel combustion, which uses pure oxygen sepa-rated from air;

(iii) postcombustion separation, which separates CO2from the flues gases using different methods.

Most of these techniques have large energy penalty andhigh costs for separation of CO2 from the rest of the flue


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2 International Journal of Chemical Engineering

N2O2

CO2H2O

MexOy

MexOy1

Air Fuel

Airreactor

Fuelreactor

Figure 1: Chemical-looping combustion (CLC).

gas components, resulting in a significant decrease of theoverall combustion efficiency and as a result in a priceincrease of the energy because of the cost for CO2 capture.Chemical-looping combustion (CLC) offers a solution for

CO2 separation without energy penalty.

1.1. Chemical-Looping Combustion (CLC). The CLC uses asolid oxygen carrier to transfer the oxygen from the air to thefuel. The advantage with the technique compared to normalcombustion is that CO2 and H2O are inherently separatedfrom the other components of the flue gas, namely, N2 andunreacted O2, and thus no extra energy is needed for CO2separation. The CLC system is composed of two reactors, anair and a fuel reactor, as shown in Figure 1.

In CLC, the solid oxygen carrier is circulated between theair and fuel reactors. The fuel is fed into the fuel reactorwhere it is oxidized by the lattice oxygen of the oxygen

carriers according to

(2n + m) MexOy + CnH2m

(2n + m) MexOy1 + mH2O + nCO2,(1)

where MyOx is the fully oxidized oxygen carrier and MyOx1is the oxygen carrier in the reduced form which could be ametal or a metal oxide with lower oxygen content. The exitstream from the fuel reactor contains only CO2 and watervapor. The pure CO2 can be readily recovered by condensingwater vapor, eliminating the need of an additional energy forCO2 separation. The water-free CO2 can be sequestrated orused for other purpose.

Once fuel oxidation completed the reduced metal oxideMyOx1 is transported to the air reactor where it is reoxidizedaccording the reaction

MexOy1 +1

2O2 MexOy. (2)

The flue gas stream from the air reactor will have a hightemperature and contain N2 and some unreacted O2. Thisstream could be expanded through a gas turbine to produceelectricity. After energy recovered, these gases can be releasedto the atmosphere with minimum negative environmentalimpact. The reaction between the fuel and oxygen carrier inthe fuel reactor may be endothermic as well as exothermic

depending on the metal oxide used, while the reaction inthe air reactor is always exothermic. The CLC does not bringany enthalpy gains, thus, the total heat evolved in these tworeactions is the same as that of normal combustion in air.Its main advantage, however, is in the inherent separation ofboth CO2 and H2O from the flue gases. In addition, since air

and fuel go through two separated reactors and combustiontakes place without a flame, NOx formation should beavoided [6]. From the point of view of environmental-friendly characterizations, CLC has attracted wide attentionand extensive investigation in the past a few years. Thiswork will present an overview of the work which has beenconducted about the development and investigation on CLC.

2. Oxygen Carrier Development

When the CLC was firstly proposed by Richter and Knoche[7], the selection of the oxygen carrier was considered asone of the most important components of the CLC process.

The oxygen carrier particles are a cornerstone in the CLCtechnique. Important properties for oxygen carriers are highreactivity in both reduction by fuel gas and oxidation byoxygen in the air, as well as high resistance to attrition,fragmentation, and agglomeration. Additionally, it is also anadvantage if the metal oxide is cheap and environmentallyfriendly. Briefly, important criteria for a good oxygen carrierare the following:

(i) high reactivity with fuel and air;

(ii) low fragmentation and attrition, as well as lowtendency for agglomeration;

(iii) low production cost and environmentally benign;

(iv) be fluidizable and stable under repeated reduc-tion/oxidation cycles at high temperature.

A number of different transition-state metals and theircorresponding oxides have been investigated in literature aspossible candidates: Cu, Cd, Ni, Mn, Fe, and Co. Figure 2presents the mass ratio of active oxygen for different systemsof metal oxides.

Generally, these metal oxides are combined with an inertwhich acts as a porous support providing a higher surfacearea for reaction, as a binder for increasing the mechanicalstrength and attrition resistance, and additionally, as anion conductor enhancing the ion permeability in the solidparticles [8]. However, Al2O3, SiO2, TiO2, ZrO2, NiAl2O4,

and MgAl2O4 are usually used as the inert binder which wasproven to have the ability to increase the reactivity, durability,and fluidizability of the oxygen carrier particles. The inertmaterials are believed to enhance positive properties amongwhich the most important are to maintain the pore structureinside the particle and inhibit migration of the metals,which could lead to sintering of oxygen carrier particles. Inthe last decade, a number of researches on oxygen carriersfor chemical-looping combustion have been performed.The major affiliations have been Chalmers University ofTechnology in Goteborg, Sweden, CSIC in Zaragoza, Spain,Tokyo Institute of Technology in Japan, and Korea Instituteof Energy Research.


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International Journal of Chemical Engineering 3

Mn3O4-MnO

Fe2O3-

Fe3O4

Fe2

O3-Fe

Cu

O-Cu

N

iO-N

i0

0.1

0.2

0.3

Ratiooffreeoxygen

Figure 2: Mass ratio of active oxygen for different metal oxides.

2.1. NiO-Based Oxygen Carriers. Ni-based oxygen carrierscan be used at high temperatures of 9001100C in a CLCprocess with full CH4 conversion although thermodynamiclimitations result in a small amount of CO and H2 inthe outlet gas of the fuel reactor [9]. Pure NiO withoutdoping inert binder used as looping material in CLC hasbeen studied and compared with NiO/YSZ (yttria-stabilizedzirconia) by using hydrogen or natural gas as the fuel [6].As a result, the NiO/YSZ particle permitted extremely fastoxidation compared to the pure NiO sample. The additionof YSZ to NiO gives a high solid diffusivity for the oxide ionespecially at high temperature and high porosity turns outto be a crucial feature that YSZ played an important role inincreasing the oxidation rate. The performance of NiO/YSZmixture used as oxygen carrier in CLC was investigatedby other researchers [1014]. It was shown that NiO/YSZdisplayed excellent reactivity and regenerability. However,the price of YSZ is higher in comparison to other inerts.

Another inert binder and support material which wasused in NiO-based oxygen carriers is Al2O3. It was shown

that the solid particles of NiO/Al2O3 had good reactivity andhigh mechanical strength [13]. Another advantage of usingAl2O3 as inert binder is that it is much cheaper than YSZpowder. Therefore, Al2O3 has attracted wide attention dueto its favorable fluidization properties thermal stability, andlow cost [1518]. It is noted that a part of NiO is convertedto form metal aluminum spinel compounds, NiAl2O4, viasolid-state reaction with Al2O3 in the course of the sinteringprocess [19]. The NiAl2O4 is inert or reacts very slowly withfuels or oxygen, therefore, the active metal oxide is addedin excess to obtain an NiO/NiAl2O4 of desired mass ratioto compensate for the loss of nickel as nickel aluminate[20]. NiAl2O4 is kept even in the cyclic reactions. It plays

the role in keeping the mechanical strength of the particle.To inhibit the interaction between NiO and Al2O3, Gayanet al. prepared NiO-based oxygen carriers with modifiedAl2O3 via thermal treatment or chemical deactivation withMg or Ca oxides [21]. Thermal treatment of -Al2O3 at1150C produced the phase transformation to -Al2O3.

The chemical pretreatment consisted in precoating the -Al2O3 support with MgO or CaO. The precoating wascarried out by dry impregnation on -Al2O3 with Mg or Canitrate solutions and forming the corresponding aluminatesby sintering at high temperature. Ni-based oxygen carriersprepared on -Al2O3, MgAl2O4, or CaAl2O4 as supportshowed very high reactivity and high methane combustionselectivity to CO2 and H2O because the interaction betweenthe NiO and the support was minimized.

As NiAl2O4 is formed during the sintering when Al2O3is used as support, a few of researchers have proposed usingNiAl2O4 as a support and binder instead of Al2O3 [2225]. As the results, the NiO/NiAl2O4 particles displayedgood reactivity and selectivity of oxidation of methaneto CO2 and H2O. The particles showed an increase inmechanical strength, no sign of deactivation, and no changein chemical composition after multicyclic chemical-loopingcombustion. Linderholm et al. [26] studied the performanceof NiO/NiAl2O4 oxygen carrier in a 10 kW reactor for 160hours. They found that a conversion of natural gas toCO2 and H2O of approximately 99% was accomplished. Nodecrease in reactivity was seen during the test period. Accord-ing to the previous investigations, NiO/NiAl2O4 oxygencarrier was considered as one of the most promising oxygencarriers for CLC due to its good reactivity, cyclic reactionability, and high bearing temperature of above 900C.

It was found that the presence of Mg in Ni-Al-O mixedoxides minimizes the sintering of the NiO and stabilizes theNi2+ in the oxide phase [23]. Zafar et al. [27] found thatthere was no or a limited interaction between the supportand the active phase in the course of cycling experimentsusing a NiO/MgAl2O4 as oxygen carrier. Therefore, MgAl2O4has been considered as a support for NiO to avoid aluminateformation. Johansson et al. [28] tested a chemical-loopingcombustor of 300 W using an oxygen carrier composed of60 wt% NiO and 40 wt% MgAl2O4. It was found that theNiO/MgAl2O4 particle gave a high conversion of the naturalgas. No methane was detected out in the exit gas out of thefuel reactor, and the fraction of CO varied between 0.5 and3%. Zafar et al. [29] determined the reactivity of the oxygen

carrier composed of 60 wt% NiO with 40 wt % MgAl2O4.The reactivity was investigated in a TGA at 8001000C using520% CH4 as a fuel gas for reduction and 315% O2 asan oxidizing gas for oxidation. The oxygen carrier showedvery high reactivity during reduction and oxidation. Thereaction rate was a function of the reacting gas concentrationand temperature both in reduction and oxidation reactions.However, it was found that the conversion of particles forthe reduction reaction was very low at 800850C, whichsuggested that it may not be feasible to use this oxygencarrier at lower temperature in a CLC system. Johansson et al.found that NiO/MgAl2O4 demonstrated several advantagesat elevated temperatures, that is, higher methane conversion,


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higher selectivity to reforming, and less tendency for carbonformation compared with NiO/NiAl2O4 [30]. In addition,several other researchers prepared NiO/MgAl2O4 oxygencarriers and investigated their performances for CLC [3133].

A few of researchers have tried to use SiO2, TiO2,

bentonite, or ZrO2 as support in the Ni-based oxygen carrierparticles for CLC [34, 35]. It was found that the oxygencarriers supported on Al2O3 or bentonite produced higherreactivity than those on TiO2. Moreover, the reactivity of themetal oxide particles increases with increasing temperatureand the amount of NiO. Natural titania contained mineral,rutile, was reported to be used as support and binder innickel-based oxygen carrier [36]. The results revealed that thereactions are fast, as CO2 is the only compound detected inthe outlet gas of the reduction stage. As the reaction proceeds,however, the thermal decomposition of methane appears asa side reaction which competes for methane consumptionwith the main reaction of the CLC. Zafar et al. [27] observedthat the reactivity of NiO/SiO

2decreased as a function of the

cycle number at 950C but was avoided below 850C. Thesame research group reported that NiO/SiO2 displayed highselectivity toward H2 during the later stages of reduction inchemical-looping reforming [37].

Generally speaking, the reactivity of the four moststudied supported oxygen carriers is in the descending orderof NiO > C u O > Mn2O3 > Fe2O3 [38] although thisorder depends on how the oxide is supported. Therefore,Ni-based oxygen carriers are believed the most promisingoxygen carrier candidate for CLC.

2.2. CuO-Based Oxygen Carriers. Among the possible metaloxides, CuO has the highest oxygen transport capacity [39].In the CLC system, the reaction between fuel and metaloxide in the fuel reactor may be endothermic as well asexothermic depending on the oxygen carrier used, whilethe reaction in the air reactor is always exothermic. WhenCuO is used as oxygen carrier, however, both the reactionsin fuel and air reactors are exothermic. Briefly, Cu-basedoxygen carriers have several advantages: (1) CuO has a highoxygen transport capacity, allowing system operating withlower solid flow rates circulating between the fuel reactorand air reactor [40]; (2) the reactions both reduction andoxidation are exothermic avoiding the need of heat supplyin the reduction reactor [41]; (3) CuO reduction is favored

thermodynamically to reach complete conversion of gaseoushydrocarbon fuels into CO2 and H2O [39]; (4) CuO is oneof the cheapest materials that can be used for CLC [42]; (5)Cu-based carriers are highly reactive in both reduction andoxidation cycles, which reduces the solids inventory in thesystem.

de Diego et al. [43] reported that the reaction rate ofpure CuO decreased quickly with the increasing numberof cycles and after three cycles of reaction the reactivity ofthe pure CuO was extremely low, reaching conversion ofonly 10% in more than 20 minutes. Therefore, to obtainbetter Cu-based oxygen carrier, an inert binder needs to beadded into the CuO. They examined the effects of the carrier

composition and preparation method on the property ofCu-based oxygen carriers. It was concluded that, to obtainCu-based oxygen carriers with high reduction and oxidationreaction rates while maintaining their mechanical propertiesfor a high number of successive reduction-oxidation cycles,the only effective preparation method was impregnation on

a support. The presence of a binder plays the role as anoxygen-permeable material and as a material to enhance themechanical strength of the particle for cyclic use and againstabrasion. Mattisson et al. [44] prepared CuO/Al2O3 oxygencarriers by means of dry impregnation and investigated itsreactivity. It was observed that the reduction rate is fast at alltemperature in the range 750950C for CuO/Al2O3. Minoramounts of CuO were decomposed to Cu2O during the inertperiod following the oxidation period at 950C. de Diegoet al. [45] found that CuO/Al2O3 oxygen carriers with aCuO content lower than 10 wt% never agglomerated in thefluidized bed and that the one with a CuO content greaterthan 20 wt% always agglomerated. In addition, the reactivityof the CuO/Al

2O

3oxygen carriers, during the reduction and

oxidation reactions, was high and not affected by the numberof cycles carried out in the fluidized bed. Complete CH 4conversion to CO2 and H2O during most of the reductionperiod was obtained. Chuang et al. [46] developed Cu-basedoxygen carrier and studied the performance for burningsolid fuels using CLC. It was found that carriers madeby mechanical mixing and wet-impregnation were rejected,because they agglomerated and exhibited low reactivity.Co-precipitated carriers, however, did not agglomerate andshowed a high reactivity after 18 cycles of operation. Theeffect of the operating conditions, such as oxygen carrier-to-fuel ratio, fuel gas velocity, oxygen carrier particle size, andfuel reactor temperature, on fuel conversion was analyzedworking with a CuO/Al2O3 oxygen carrier prepared by dryimpregnation in a 10 kWth pilot fluidized-bed reactor [47].It was found that the most important parameter affectingthe CH4 conversion was the oxygen carrier-to-fuel ratio.Complete methane conversion, without CO or H2 emissions,was obtained with this oxygen carrier working at 800C andoxygen carrier-to-fuel ratios of>1.4.

SiO2 was studied as well as an oxygen carrier supportmaterial for copper-based oxygen carriers. Corbella et al. [48]studied the performance of a copper oxide silica-supportedoxygen carrier in a 20-cycle test of chemical-looping ofmethane in a fixed-bed reactor at 800C and atmosphericpressure. It was revealed that the reduction reaction rate is

fast and highly selective to CO2 formation, and CO emissionsare very low, only yielded at the end of the reduction stage. Inthe 20-cycle test neither performance decay nor mechanicaldegradation of the oxygen carrier has been observed. Son etal. [49] found that the oxygen particles supported on SiO2exhibit worse reactivity than those on Al2O3. In addition,ZrO2, TiO2, and bentonite were also studied as the inertsupport for copper-based oxygen carrier [5052].

2.3. Fe2O3-Based Oxygen Carriers. High reactivities in bothreduction by fuel and oxidation by oxygen in the air, as wellas high resistance to attrition, fragmentation, and agglom-eration are the important properties for oxygen carriers.


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In addition, it is also an advantage if the oxygen carrieris inexpensive and environmentally friendly. Advantageswith iron-based oxygen carriers are the environmentalcompatibility of its oxides magnetite (Fe3O4), hematite(Fe2O3), and wustite (FeO) and the lower price of ironoxides than other oxides such as nickel oxide (NiO) and

copper oxide (CuO) [53]. In view of the availability, lowprice, as well as environmental safe of iron oxides, hasattracted wide attention as oxygen carriers for using in CLC.It was confirmed that the reduction kinetics from hematiteto magnetite (Fe2O3 Fe3O4) is the fastest step with thesubsequent steps, the steps of magnetite to ferrous oxide(Fe3O4 FeO) and ferrous oxide to iron (FeO Fe) beingmuch slower. The subsequent steps were considered to befeasible for the chemical-looping reforming (CLR).

Pure Fe2O3 was studied as oxygen carrier at 720800C

and demonstrated excellent chemical stability and no loss ofactivity with cyclic redox. However, the same particles beganto agglomerate at 900C though the agglomeration rate wasslow [54]. Previous researchers found that the agglomerationand breakage of the particles could be avoided by addingAl2O3 into the particles [55]. Ishida et al. [56] preparedFe2O3/Al2O3 composite particles and evaluated their appli-cability as solid looping materials of the chemical-loopingcombustor. They found that two solid solutions, hematite(ss)and corundum(ss), were formed in Fe2O3/Al2O3 compositeparticles at temperatures above 1000C. The mechanicalstrength of the Fe2O3/Al2O3 particles was improved byincreasing the content of corundum(ss). Cho et al. [57]compared the iron-, nickel-, copper-, and manganese-basedoxygen carriers for chemical-looping combustion. It wasobserved that oxygen carriers based on nickel, copper,and iron showed high reactivity enough to be feasible forCLC system. However, samples of the Fe2O3/Al2O3 showedsigns of agglomeration. The same research group developedFe2O3-based oxygen carriers together with various inertssuch as Al2O3, ZrO2, TiO2, and MgAl2O4 finding thatFe2O3/Al2O3 exhibited good reactivity [58]. In our previouswork, Fe-based oxygen carrier composed of 80 wt% Fe2O3with 20wt% Al2O3 has been prepared by impregnationmethods [59]. The Fe2O3/Al2O3 oxygen carrier showed goodreactivity in 20-cycle redox tests in a TGA reactor. However,85% of the CH4 was converted to CO2 and H2O during mostof the reduction periods with minor formation of CO andH2. Abad et al. [60] investigated the performance of iron-based oxygen carrier in a continuously operating laboratory

CLC unit, consisting of two interconnected fluidized bedsusing natural gas or syngas as fuel. The combustion of fuelgas was stable during the operation of the reactor. Thecombustion efficiencies of syngas and natural gas reached99% and up to 94%, respectively. The reactivity and thecrushing strength of the oxygen carrier particles were notaffected significantly during operation. Agglomeration andcarbon deposition were not observed and no mass loss ofthe solids inventory was detected. It is clear that attritionand agglomeration of Fe2O3/Al2O3 oxygen carrier can becontrolled at very low level if it is operated under properconditions. Therefore, Fe2O3/Al2O3 oxygen carrier is one ofthe promising candidates for CLC.

Similar to Ni- and Cu-based oxygen carriers, othermaterials, such as YSZ, MgAl2O4, TiO2, and SiO2, werereported being used as binder and support in the oxy-gen carrier particles. Johansson et al. [61] suggested thatthe Fe2O3/MgAl2O4 oxygen carrier showed best reactivityamong the iron oxides supported on six different inert mate-

rials. As for this kind of oxygen carrier, the one containing60wt% Fe2O3 and 40wt% MgAl2O4 sintered at 1100C

exhibited reasonable crushing strength and resistance towardagglomeration and fragmentation. Leion et al. [62] studiedthe chemical-looping combustion of petroleum coke usingFe2O3/MgAl2O4 as oxygen carrier. It was revealed that theparticles Fe2O3/MgAl2O4 reacted rapidly with intermediategasification products such as CO and H2. Therefore, thepresence of an oxygen carrier enhanced the gasification ofpetroleum coke. TiO2 was also proposed being used as inertand support of the iron-base oxygen carriers. Corbella andPalacios [63] found that the available oxygen of Fe2O3/TiO2particles for methane combustion in the reduction stage waslower than expected due to the active phase interacts withthe support forming FeTiO3 ilmenite. Similar results wereobserved for Fe2O3/SiO2 oxygen carrier, the formation ofFe2SiO4 reduced the reactivity of the Fe2O3/TiO2 particles[37].

Natural iron ores, such as hematite and ilmenite, havealso considered as oxygen carriers for CLC, especially forsolid fuels CLC. The work in [64] reported the feasibilityof using ilmenite as oxygen carrier in CLC. It was foundthat ilmenite is an attractive and inexpensive oxygen carrierfor CLC. The ilmenite particles showed no decrease inreactivity after 37 cycles of redox in a laboratory fluidized-bed reactor system. Ilmenite gave high conversion of COand moderate conversion of CH4. Berguerand and Lyngfelt[65] investigated the CLC process of petroleum coke ina 10 kWth chemical-looping combustor. The petroleumconversion reached higher figures of 66 to 78%. Low lossof noncombustible fines from the system indicated very lowattrition of the ilmenite particles. The CO2 capture rangedfrom 60 to 75%. Natural hematite was also studied beingused as oxygen carrier in CLC with CH4 and air at 950

C[66]. It was found that the majority of CH4 was converted toCO2 with some small formation of CO during the reductionperiod. For the first reduction period the degree of methaneconversion was about 62%. Clear breakage on the surfaceof the reacted particles was observed due to the chemicalreactions occurring.

2.4. Mn3O4-Based Oxygen Carriers. Nickel-, copper-, andiron-oxides have been widely investigated in previous liter-atures as possible candidates for CLC. Manganese oxides,however, have been reported limitedly being used as oxygencarriers of CLC. Adanez et al. [8] developed manganeseoxides on five different inert materials, such as Al2O3,Sepiolite, SiO2, TiO2, and ZrO2. In their study, Al2O3,Sepiolite, SiO2, and TiO2 were found being unsuitable assupport for manganese-based oxygen carriers. ZrO2 wasfound to be the best inert material for manganese oxidesin the view of reactivity and strength. Mattisson et al. [44]prepared Mn3O4/Al2O3 oxygen carrier and investigated its


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reactivity. It was found that Mn3O4/Al2O3 appeared poorreactivity mostly due to the formation of MnAl2O4 duringsintering, which does not react with the fuels and oxygen.Cho et al. [57] also studied the performance of Mn2O4supported on Al2O3. Again the reactivity was found to bepoor because of the same reason. Johansson et al. [67]

synthesized manganese-based oxygen carriers together withpure ZrO2 and ZrO2 stabilized with CaO, MgO, and CeO2.They found that almost all investigated particles exhibitedhigh reactivity and limited physical changes during the cyclicreactions. The effect of the number of cyclic redox reactionson the reactivity of the all four samples was not clear. The onestabilized with MgO showed the highest reactivity amongthe four kinds of oxygen carriers. In a later study, the sameresearch group investigated the feasibility of the use of anMn-based oxygen carrier supported on zirconia stabilizedwith magnesium in a 300 W continuously operating reactorsystem [68]. As a result, Mn3O4/Mg-ZrO2 particles sinteredat 1150C showed a good reactivity for CLC. Very highefficiencies of>0.999 were obtained for syngas combustion atall temperatures in the range of 800950C. Agglomerationand attrition rate were not obviously observed. The sameauthors investigated the reduction and oxidation kineticsof Mn3O4/Mg-ZrO2 oxygen carrier particles for CLC in asubsequent work [69]. The order of reaction was 1 withrespect to CH4 and 0.65 with respect to O2. The activationenergy for the reduction and oxidation reactions were 119and 19 kJ/mol, respectively. The oxygen carrier particlesshowed high reactivity during both reduction and oxidationat all investigated temperatures of 800950C. The oxygencarrier was reduced to MnO in the course of reduction andoxidized to Mn3O4 during oxidation at all conditions. It isclear that ZrO2 stabilized by MgO is a good support and inertbinder for Mn-based oxygen carriers.

2.5. Other Oxygen Carriers. Beside the aforementioned oxy-gen carriers, other kinds of materials have been reportedbeing used as oxygen carriers for CLC, such as perovskite,CaSO4, as well as mixed-metal oxides. Actually, perovskitesare mixed oxides with the general formula ABO(3+), whereA is usually a ion of a rare earth metal, an alkali metal, oran alkaline earth metal B is a transition metal ion. Both Aand B can be partially substituted, leading to a wide varietyof compositions of general formula A1xA

xB1yB

yO(3+),characterized by structural and electronic defects owing totheir nonstoichiometry [70]. The -factor in perovskites can

be reduced or increased by changing factors in the conditionssuch as temperature or O2 partial pressure. This character-ization makes perovskites potentially to be used as oxygencarriers for CLC. Readman et al. [71] studied the feasibilityof La0.8Sr0.2Co0.2Fe0.8O3 as a potential oxygen carrierin a chemical-looping reactor. The results suggested thatLa0.8Sr0.2Co0.2Fe0.8O3 has the redox properties requiredfor chemical looping. Reduction and reoxidation of theperovskite takes place quickly enough for CLC. However, itdisplayed a low oxygen carrying capacity. Ryden et al. [72]reported the use of LaxSr1xFeyCo1yO3 as oxygen carriersfor CLC or chemical-looping reforming (CLR). They foundthat La0.5Sr0.5Fe0.5Co0.5O3 has properties that make high

conversion of CH4 into H2O and CO2 in the beginning ofeach reduction period. Substituting La for Sr could increasethe oxygen carrying capacity of the perovskite, but reducedthe reactivity with CH4.

The oxygen carriers of individual metal oxides havetheir own advantages and drawbacks. For example, Ni-

based oxygen carriers allow working at high temperature inCLC with high CH4 conversion, but the drawbacks are thehigher carbon formation rate as well as partial oxidationof CH4 to CO and H2. On the other hand, Cu-basedoxygen carriers give complete fuel combustion to CO2 andH2O, but the operating temperature is lower because ofthe low melting point of the Cu. Mixed-metal oxides mayintegrate the advantages of each sole oxides as well as partiallyavoid their disadvantages. Jin et al. in [14] reported thatdouble metal oxides of CoO-NiO/YSZ provided a betterperformance with good reactivity, complete avoidance ofcarbon deposition, and significant regenerability for repeatedcycles of reduction and oxidation than those of individualmetal oxides of NiO/YSZ and CoO/YSZ. Adanez et al.[73] prepared mixed Ni-Cu oxides and examined theirperformance for CLC of methane. It was revealed that thepresence of CuO in the Ni-Cu oxygen carriers gives thefull conversion of CH4 to CO2 and H2O with zero COand H2 emissions. Additionally, the presence of NiO inthe Ni-Cu oxygen carriers allows the particles operatingat high temperature (950C). Hossain and de Lasa [74]developed a bimetallic Co-Ni/Al2O3 oxygen carrier for afluidized bed chemical-looping combustion process. The Co-Ni/Al2O3 particles displayed excellent reactivity and stability.They confirmed that the addition of Co in the doublemetallic Co-Ni/Al2O3 particles influences the state of thesurface minimizing the formation of nickel aluminate, whichwas believed to be contributing to the inferior reactivityof Ni-based oxygen carriers. Besides, the addition of Coinhibits metal particle agglomeration during the cyclic redoxprocesses. The activation energy for Co-Ni/Al2O3 reductionwas found to be less than that of unpromoted Ni/Al2O3samples. This suggested that doping with Co decreases themetal-support interaction and the binding energies betweenthe metals and the fuel molecules [53]. Johansson et al.[75] investigated the synergy effect by using mixed oxides ofiron and nickel in combustion of CH4 in a CLC reactor. Itwas shown that the mixed-oxide system of 3% nickel oxidesin 97% iron oxides gives almost two times as much CO2per time unit in comparison to the sum of CO2 when the

oxides were tested separately. They believed that the presenceof metallic Ni could firstly catalyze the methane into COand H2, which then reacts with iron oxide at a considerablyhigher rate than methane.

Recently, CaSO4 was considered as oxygen carrier inCLC. Song et al. [76] studied the CLC of methane withCaSO4 oxygen carrier in a fixed bed reactor. The resultsshowed that the CaSO4 oxygen carrier has a good reductionreactivity and stability in a long-term redox test at 950C.The CH4 conversion rate was found as a function oftemperature, gas flow rate, mass of sample loaded, andparticle size of oxygen carrier. High temperatures lead toa high CH4 conversion into CO2, however, a significant


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SO2 formation was observed at higher temperatures. Ina later work, the same researchers investigated the cyclicperformance of a CaSO4-based oxygen carrier in alternatingreducing simulated coal gas and oxidizing conditions [77].It was found that a high concentration of CO2 could beobtained in the reduction. The coal gas conversions and

CO2 yield initially increased and finally decreased during thereduction in the multicycle tests. Similar to their previouswork, the formation of SO2 and H2S during the cyclic testswas found to be responsible for the decay of the reactivity ofthe CaSO4 oxygen carrier. Therefore, decomposition at high-temperature condition is a major problem for CaSO4 when itis used as oxygen carrier for CLC. Decomposition behaviorsof CaSO4 in chemical-looping combustion were studied byTian et al. [78]. They claimed that the most likely mechanismfunction in the decomposition of CaSO4 is characterized bythe Avrami-Erofeev equation. The decomposition reactionis dominated by the nucleation rate. The work in [76]represented that the release of SO2 and H2S can be avoidedby optimization of the operating conditions. Addition of aminor amount of fresh limestone into the system of CLC cancapture the SO2, and the products of CaSO4 and CaS can beused as oxygen carriers later [79].

3. CLC Reactor Design

Prior to the year 2001, most of the research work on CLCfocused on the development of oxygen carriers and onsystem studies, with limited information about how the CLCreactors could be designed [80]. In CLC process it is requireda good contact between fuels and oxygen carriers as well asa flow of solid material between the two reactors. Althoughthere are several options for CLC system designs, it is likelythat interconnected fluidized bed reactors are believed themost suitable reactor design. Lyngfelt et al. [4] proposed acirculating system composed of two interconnected fluidizedbeds, that is, a high-velocity riser and a low-velocity bubblingfluidized bed, see Figure 3. The bed material (oxygen carrierparticles) was circulated between the two fluidized beds. Inthe air reactor (the riser), oxygen is transferred from thecombustion air to the oxygen carrier. In the fuel reactor (thelow-velocity fluidized bed), oxygen is transferred from theoxygen carrier to the fuel. The authors presented the criticaldesign parameters, such as the solids inventory, recirculationrate of the oxygen carriers, the dimensions of the reactors,as well as the pressure drops. The same authors constructed

a 10 kW chemical looping combustor and operated for 100hours using NiO-based oxygen carriers and natural gas asfuel in 2005 [81]. However, 99.5% of the added methanewas converted into CO2 and H2O, with minor of CO, H2,and unreacted methane in the exit stream. There was nodetectable leakage between the two reactors. And CO2 escapefrom the system via the air reactor was not found. Therefore,almost 100% of the CO2 is captured in the process.

Kronberger et al. [82] designed a 10 kW CLC prototypeof a dual-fluidized bed reactor system. Gas velocities anddesigns were varied, while solids circulation rate and gasleakage between the reactor as well as static pressure balanceand residence time distribution of gas and particles were

H2O

CO2

Flue gas

Air

Fuel

1

2

3

Noncondensiblegas

Bleed

Figure 3: Layout of chemical-looping combustion process, withtwo interconnected fluidized beds. (1) air reactor, (2) cyclone, (3)fuel reactor.

measured. It was observed that the solid circulation rateswere sufficient and the gas leakage could be controlled at verylow level. In one of their previous work, the same authorsdeveloped a 300 W fluidized-bed reactor for CLC [83]. Thereactor was tested in various gas velocities and slot design.CLC in the same reactor was continuously operated usingMn-based oxygen carrier [68].

Linderholm et al. [26] examined the CLC process for160 hours in a 10 kW reactor system with an NiO/NiAl2O4oxygen carrier and natural gas as fuel. The prototypeconsisted of two interconnected fluidized-bed reactors, thefuel and the air reactors, a cyclone to separate solid from gasflow out of the air reactor, and two loop seals, see Figure 4.High fuel conversion to CO2 and H2O was achieved. Theoutlet gas stream out of the fuel reactor mainly containedCO2 with approximately 0.7% CO, 0.3% CH4, and 1.3%H2. The estimated particle life time was 4500 hours. Aconcentrated stream of CO2 was obtained when steamwas used as fluidization gas in particle locks. Adanez etal. [47] has designed and built a 10 kW pilot plant thatis composed of two interconnected bubbling fluidized-bed

reactors to demonstrate the CLC technology, see Figure 5.The system was operated for 200 hours, 120 hours of whichinvolved the burning of methane using CuO/Al2O3 oxygencarrier. Complete methane conversion was achieved and nodeactivation of the oxygen carrier was noticed at 800C.Similar results were obtained by the same authors [41].

Son and Kim [34] built an annular shape circulatingfluidized-bed (CFB) reactor with double loops for investi-gation into CLC. They tested the CLC of CH4 using mixedNiO-Fe2O3 particles supported on bentonite at 850

C. Fullconversion of CH4 to CO2 and H2O was achieved with verysmall amount of CO and no H2 emission was detected.Recently, a number of researchers started to investigate the


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6

15

14

13

12

1212

11

5

5

4

3

2

1

98710

Figure 4: Schematic diagram of the prototype reactor system. (1) air ractor, (2) riser, (3) cyclone, (4) fuel reactor, (5) upper and lowerparticle locks, (6) water trap, (7) nitrogen (8) natural gas, (9) argon, (10) air, (11) preheater, (12) heating coils (not available for test withnickel-based particles), (13) finned tubes for cooling of gas streams, (14) filters, and (15) connection to chimney.

feasibility of CLC for solid fuel. Berguerand and Lyngfelt[84] designed a 10 kW chemical-looping combustor for solidfuels, and tested with South African coal. The CLC systemfor solid fuels is very similar to the ones for gas fuels,which was reported in literature [22, 26] (see Figure 4). Toadapt the solid fuels important modifications in the fuelreactor chamber and the inclusion of an additional solidsrecirculation loop were made. Additionally, steam was usedas the fluidizing agent instead of the fuel gas flow. Thechemical-looping combustor has been operated with SouthAfrican coal for 22 hours, with approximately 12 hours ofstable CLC conditions. Ilmenite was proved to be a suitable

oxygen carrier for the tested solid fuel with good mechanicalproperties, low fragmentation/attrition, and good reactivity.The CO2 capture varied between 82.5% and 96.0% for thecoal tests.

Recently, a couple of CLC pilot plants at fuel powerof 10140 kW were built and tested with different fuels.Kolbitsch et al. [85, 86] built a 120kW CLC pilot unitwith dual circulating fluidized bed (DCFB) reactor system,and successfully put it into operation. Two different oxygencarriers, that is, ilmenite and a designed Ni-based particle,were tested in the CLC unit. The experimental results withilmenite and H2 rich gases as fuel showed very promisingresults. High gas conversion was observed. The Ni-based

oxygen carrier achieved thermodynamic maximum H2 andCO conversion as well as very high CH4 conversion when thesolids inventory is sufficiently high. The same research groupcompared the performance of two Ni-based oxygen carriers,that is, NiO/NiAl2O4 and NiO/NiMgAl2O4, for chemicallooping combustion of natural gas in the same CLC pilotrig on a scale of 140kW fuel power [87]. Both oxygencarriers showed high reactivity and no carbon formation wasobserved under any conditions. Linderholm et al. [88] inves-tigated the reactivity and physical characteristics of Ni-basedparticles in a 10 kW chemical looping combustor composedof two interconnected fluidized-bed reactors with natural

gas as fuel. Long-term of fuel operation (>1000 hours) wereachieved. The combustion efficiency was around 98% andthe methane fraction was typically 0.41% in the flue gasof fuel reactor. The particle lifetime was estimated up to33 000 hours based on calculation from the loss of fines. Shenet al. [89] carried out the experiments for CLC of biomasswith iron oxide as an oxygen carrier in a 10 kW continuousreactor of interconnected fluidized beds. The results showedthat an increase in the fuel reactor temperature produceda higher increase for the CO production from biomassgasification than for the consumption of CO oxidation toCO2. The transformation of Fe2O3 to Fe3O4 is the favoredstep in the process of iron oxide reduction with biomass


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4

Stack

95

68

7Stack

Air

Air N2 CH4 Air N2

Gas analysis

CH4, CO2,CO,H2

Gas analysis

O2,CO,CO2

Cu/CuO

CuO/Cu 3a

3b

10

11

25

1

5

12

9

Figure 5: Schematic diagram of the CLC facility of instituto de carboquimica (C.S.I.C.), Spain. (1) fuel reactor, (2) air reactor, (3) loop seals,(4) riser, (5) cyclone, (6) solid reservoir, (7) solids valve, (8) diverting solid valve, (9) filters, (10) oven, (11) air preheater, and (12) watercondenser.

syngas. Ryu and Jin [90] proposed a conceptual design a50 kW thermal chemical-looping combustor composed oftwo interconnected pressurized circulating fluidized beds.

They calculated the important parameters such as bed mass,solid circulation flux, and reactor dimension.

4. Chemical-Looping Reforming (CLR)and Chemical-Looping Hydrogen (CLH)

The chemical-looping technique can also be employed formethane reforming and production of hydrogen with inher-ent CO2 capture. Chemical-looping reforming (CLR) usesthe same basic principles as CLC, with the main differencethat the target products in CLR are H2 and CO instead ofheat. The basic principles of chemical-looping reforming areillustrated in Figure 6 [91].

In the fuel reactor of CLR process, the fuels are partiallyoxidized using a solid oxygen carrier to produce synthesisgas, a mix of H2 and CO, instead of being oxidized into CO2and H2O. Therefore, the ratio of oxygen to fuel is kept low toprevent the fuel from becoming fully oxidized. Pure oxygenproduction plant which is needed in normal natural gasreforming is avoided. Ryden et al. [91] investigated the CLRof natural gas in a circulating fluidized-bed reactor using Ni-based oxygen carriers. It was found that CLR is a feasibleconcept for production of synthesis gas and hydrogen. Theconversion of natural gas into synthesis gas was 96%100%depending on oxygen carrier and experimental conditions.The same authors [31] evaluated the feasibility of synthesis

Nitrogen

N2

Reformer gas

H2, CO

(H2O,CO2)

MeO

Me

AirO2, N2

FuelCnHm (H2O,CO2)

Airreactor

Fuelreactor

Figure 6: Schematic description of chemical-looping reforming.

gas generation by CLR of natural gas in a continuouslyoperating laboratory reactor using a Ni-based oxygen carrier.As a result, complete conversion of natural gas was achievedand the selectivity towards H2 and CO was high. Carbondeposition was found for some cases, which was eliminatedby adding 25 vol% steam to the natural gas. de Diego etal. [92] studied the synthesis gas generation by chemical-looping reforming of methane using a Ni-based oxygencarrier in a 900 Wth CLR pilot plant. The effect of operatingconditions, such as fuel reactor temperature, H2O/CH4molar ratio, and solid circulation, on CH4 conversion and gasproduct distribution was analyzed. The CH4 conversion rate


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reached >98% in all operating conditions. When NiO/CH4molar ratio was 1.25, a dry gas product composition of65 vol% H2, 25 vol% CO, 9 vol% CO2, and 11.5vol% CH4was obtained in the CLR process. Also, de Diego et al. [ 93]studied the CLR of CH4 in a thermogravimetric analyzer(TGA) and in a batch fluidized bed reactor using Ni-based

oxygen carriers. The support used to prepare the oxygencarriers exhibited an important effect on the reactivity ofthe oxygen carriers, on the gas product distribution, and onthe carbon deposition. The Ni-based oxygen carrier impreg-nated on -Al2O3 showed the highest reactivity during thereduction reaction. For all of the tested oxygen carriers,the reduction time without carbon deposition increasedwith increasing the reduction reaction temperature and theH2O/CH4 molar ratio in the feed. The H2/CO molar ratioin the gas product generated during CH4 reforming wasbetween 2 and 3 in the reduction period. In a later article,the same researchers presented the experimental resultsof autothermal CLR of methane in a 900 Wth circulatingfluidized bed reactor under continuous operation using Ni-based oxygen carriers [94]. It was observed that the CH4conversion was very high (>98%) in all operating conditionswith both oxygen carriers of 21 wt% NiO supported on -Al2O3 and 18wt% NiO on -Al2O3. The oxygen carriercirculation rate, that is, the NiO/CH4 molar ratio was themost important factor affecting the gas product distribution.Synthesis gas of H2/CO ratio near 2.6/1 with minor amountof CO2 and unreacted CH4 was obtained. Significant changesin the reactivity, surface texture, and solid structure of theoxygen carrier particles were not observed after 50 hoursof operation. From these previous literatures, it can be seenclearly that reforming of methane or natural gas by chemical-looping technique is feasible.

Recently, chemical-looping process was proposed beingused to produce hydrogen. Chemical-looping hydrogen(CLH) generation, which originates from CLC, actually isa type of water splitting process with a redox of a metaloxide. Similarly to CLC, the CLH system is composed oftwo reactors: a fuel reactor for burning fuels, and a steamreactor for water decomposition, which is different fromthe air reactor in the CLC. Fuel is introduced into the fuelreactor to reduce the metal oxide particles, meanwhile thefuel is oxidized into CO2 and H2O. The reduced metal oxideis transported to the steam reactor, where it decomposeswater to generate H2. The outlet gas stream from the fuelreactor contains only CO2 and H2O at complete conversion

of the fuel, while the exit gas from the steam reactor isjust H2 with excess H2O. Therefore, pure H2 and CO2 canbe obtained with H2O condensation without any furtherseparation process [95]. The work in [95] reported that 3.7 Lof H2 was generated through reaction between per kilogramfully reduced copper-based oxide and steam. Go et al. [96]investigated hydrogen production by chemical-looping ofmethane in a fluidized-bed reactor using iron-based oxygencarrier. It was found that pure hydrogen with free-CO2 canbe obtained from the reaction of FeO Fe3O4 in steamreactor at 900C. Additionally, the authors proposed thedesign basis for the continuous two step steam-methane-reforming (SMR) with double-loop solids circulation system

Oxygen

depleted air

H2O,CO2

Fe2O3

FeO

FR

Natural gas

H2, H2O

H2O

Fe3O4

SRAR

Air

Figure 7: Conceptual scheme of the three-reactor CLH system. AR:air reactor; FR: fuel reactor, SR: steam reactor.

for fuel and steam. Solid fuels, such as coal char, werereported being used as reducing materials in CLR. Yang etal. [97] investigated H2 production from steam-iron processwith reduction of iron oxide by CLC of coal char. Iron oxidewas used to oxidize the coal char into CO2 and H2O leadingto the reduction of Fe2O3 to FeO and metallic Fe in the fuelreactor. The reduced iron oxide, FeO and Fe, was oxidized bysteam in the steam reactor, while H2 was generated by waterdecomposing. Usually, FeO and Fe are oxidized to Fe3O4 inthe steam reactor. Therefore, an air reactor is needed, wherethe Fe3O4 is oxidized to the original Fe2O3 by air. The authorsproved that FeO and Fe produced from the Fe2O3 reductionof char were feasible for H2 production through the steam-iron process. The total H2 produced was 1000 mL of H2 pergram of tested coal char, where the energy efficiency was50.2% with respect to the energy ratio of H2/char.

Similarly, Chiesa et al. [98] proposed a CLH productionsystem with three reactors. The conceptual scheme of thethree reactor of CLH is shown in Figure 7. Compared tocommercially available technologies, this CLH process showssimilar efficiency but much better environmental benignbecause of the inherent CO2 separation. They concluded thatthe CLH system is a promising hydrogen production processdeserving substantial research and development activitiesin the near future. In the steam-iron process to producehydrogen, deactivation of the iron oxide is one of theproblems during the operation. Bleeker et al. [99] consideredthat the deactivation of the iron oxide is caused by a decreasein the surface area of the oxygen carrier particles. Further, a


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N2, O2 CO2, H2O

Airreactor

Fuelreactor

MexOy

MexOy2

MexOy2 + O2(g) MexOy MexOy MexOy2

+ O2(g) C + O2(g) CO2

Air Fuel (+H2O/CO2)

Figure 8: Principal layout of chemical-looping oxygen uncoupling.The oxygen carrier is denoted by MexOy and MexOy2, whereMexOy is a metal oxide and MexOy2 is a metal or metal oxide withlower oxygen content. Here, the fuel is carbon. The fuel reactorcould be fluidized using recirculated CO

2or steam when burning

solid fuel [101].

higher conversion degree of the oxygen carriers in the redoxcycles will give a stronger deactivation.

5. Chemical-Looping with OxygenUncoupling (CLOU)

When CLC process is used to burn solid fuel, there aretwo approaches operating the combustor to adapt the solidfuels. One option is to introduce the solid fuels directlyto the fuel reactor where the gasification of the solid fuelsand subsequent reactions with the oxygen carriers will occursimultaneously. Another strategy is to use an oxygen carrierwhich releases O2 in the fuel reactor firstly and therebyallowing the fuel to burn with gas-phase oxygen [100].As for the first method, there is usually a need for anintermediate gasification step of the solid fuels with steam orcarbon dioxide to form reactive gaseous compounds whichthen react with the oxygen carrier particles. Generally, thegasification of solid fuels with H2O and CO2 is inherentlyslow leading to slow overall reaction rates. The second optionis referred to as chemical-looping with oxygen uncoupling(CLOU). In the CLOU combustion, the slow gasification of

solid fuels with H2OandCO2 is avoided, since the fuels reactsdirectly with gas-phase oxygen. The combustion technique ofCLOU involves three steps in two reactors, see Figure 8.

Mattisson et al. [102] presented the chemical-loopingwith oxygen uncoupling for combustion of petroleum coke.Thermodynamic analysis showed that Mn2O3/Mn3O4 andCuO/Cu2O are the most two promising candidates ofoxygen carrier for CLOU combustion of solid fuels. TheCLOU combustion of petroleum coke was tested in a batchlaboratory fluidized-bed reactor and compared to the resultsof normal CLC. It was found that the reaction rate ofpetroleum coke was 50 times higher using CLOU than thereaction rate of the same fuel with a Fe2O3-based oxygen

carrier in normal CLC. In a later work, the same authors[101] studied the CLOU using CuO/ZrO2 as oxygen carrierand petroleum coke as fuel in a laboratory fluidized bedreactor of quartz. The temperature gave an obvious effect onthe reaction rate of petroleum coke. Conversion rates variedbetween 0.5%/s and 5%/s at a set-point temperature from

895

C to 985

C. These reaction rates are significantly higherthan the rates with the same fuel in regular CLC. Therefore,the CLOU is a promising alternative for combustion of solidfuels inherently CO2 capture.

6. Carbon Formation

Carbon formation may occur during reduction period ifcarbon-containing fuels are used in CLC. The carbon forma-tion is believed taking place as two mechanisms: pyrolysis ofmethane and Boudouard reaction (see (3)):

CH4 C + 2H2

pyrolysis

,

2CO C + CO2 (Boudouard reaction).

(3)

The pyolysis of methane is thermodynamically favoredat high temperature as an endothermic reaction. TheBoudouard reaction is exothermic more likely to occur atlower temperatures. Chandel et al. [103] suggested that bothof the pyrolysis and Boudouard reactions are slow in CLCprocess without a catalyst. However, transition metals suchas Ni and Fe can act as a catalyst for methane decomposition.Especially, metallic nickel is well known a good catalyst forthermal decomposition of hydrocarbons. This side reactionis undesirable in a CLC process since it increases methaneconsumption in clear competition with the main reaction inthe reduction period. The formed carbon will be burned intoCO2 in oxidation stage resulting in lower efficiency of CO2capture of the whole CLC process. Carbon formation usingnickel-based oxygen carriers can be decreased by using mixedoxides as oxygen carriers or adding steam into the fuel gas[48]. Other conditions such as oxygen availability of oxygencarriers, fuel conversion, temperature, and pressure couldaffect the carbon formation. Generally, carbon is prone tobe formed at low temperature and small amounts of addedoxygen. Here, the oxygen added ratio, , is defined as theactual amount of O, added with the oxide and/or with steam,over the stoichiometric amount needed for full conversion ofthe fuel [104]:

=n

O,addednO,stoich . (4)

Carbon deposition is thermodynamically favorable ata lower oxygen added ratio. Figure 9 shows the carbonformation as a function of temperature and oxygen-addedratio at various pressure independent on the oxygen carrierused when CH4 is used as fuel in CLC. It is clear that anincreased pressure will enhance carbon formation at lowtemperature, while an increased pressure will reduce carbonformation at high temperature. This is due to the fact thatless carbon is formed at higher pressures through methanepyrolysis while more carbon is formed at higher pressures bythe Boudouard reaction.


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120011001000900800700600

T(C)

p = 1bar

p = 15bar

p = 30bar

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Figure 9: The oxygen added ratio, , needed to avoid carbon

formation when CH4 is used as fuel [104].

Addition of water vapor into the fuels before beingintroduced to the fuel reactor would obviously inhibitthe carbon deposition for NiO/NiAl2O4 particle [13]. Thedecrease in carbon formation with the addition of steam tomethane may be explained with steam reforming and theshift reaction as follows:

CH4 + H2O CO + 3H2

Steam reforming

,

CO + H2O CO2 + H2 (Shift reaction).(5)

Apart from above two reactions, carbon formation maybe reduced by another reaction as (6) in the presence ofsteam:

C + H2O CO + H2

Carbon gasification

. (6)

The products H2 and CO from the aforementionedreactions can be further oxidized by the oxygen carriers toform CO2 and H2O. Therefore, addition of steam to the fuelscan significantly reduce the carbon formation in CLC. Theaddition of water vapor resulted in the formation of COand H2 as intermediate products. Sometimes, some of theintermediate gases will not be completely oxidized by theoxygen carriers leading to an increase of their concentration

in the outlet gas from fuel reactor.Cho et al. [20] investigated the carbon formation con-

ditions on oxygen carriers based on nickel oxide and ironoxide. It was observed that the carbon formation of nickeloxide was considerably affected by the availability of oxygen.Carbon formation was not obvious when sufficient oxygenin the nickel-base oxygen carrier was still available regardlesswith or without steam addition. For CLR, carbon depositioncould be more of a problem since the fuel reactor would beoperated under more reducing conditions. For the particlesof iron oxide, there was no or very little carbon was formed.Jin et al. [14] reported that carbon deposition of NiO/YSZoxygen carrier can be completely avoided by doping CoO

into the NiO/YSZ. This suggests that carbon formation isalso affected by the chemical nature of the oxygen carriersin addition to the operating conditions such as temperature,pressure, gas composition, and particle size.

Basically, carbon formation should not be a problemunder the conditions used in a CLC system where a high

conversion rate of the fuel is achieved. For a temperature of950C, no carbon formation is expected as long as more thanone-fourth of the oxygen needed for complete oxidation ofCH4 is supplied [17]. If the CLC process is operated at desiredconditions, and a percentage of steam or CO2 is added intothe fuels, carbon formation is considerably inhibited.

7. Conclusions

CLC is a novel promising technology for fossil fuels con-version with inherent CO2 separation. Extensive researchhas been performed on CLC in the last decade with respectto oxygen carrier development, reaction kinetics, reactordesign, system efficiencies, and prototype testing. Ni, Fe,Cu, Mn, and Co oxides are potential candidates for reactivespecies in the oxygen carrier particles. Ni-based oxygencarriers exhibited the best reactivity and stability duringmultiredox cycles among these oxides. However, carbonformation is easily generated on Ni-based oxygen carrierswhen carbon-containing fuels are used in CLC. Doping animpurity such as CoO into the NiO, and adding a proportionof steam into the fuels can considerably inhibit the carbonformation in the reduction period. The performance ofother oxygen carriers above mentioned can be improved bychanging preparation method or by making mixed oxides.

The CLC process has been demonstrated successfullyboth in bench scale fixed-bed reactors and in continu-ously operated prototype reactors based on interconnectedfluidized-bed system in the size range of 0.3140 kW usingvarious types of oxygen carriers and different fuels. In theseprevious investigations, very high fuel conversion rates up to100% were achieved in most cases. Therefore, CO2 captureefficiencies were close to 100%. Although there are severalapproaches to design the CLC reactor system, it is likely thatthe interconnected fluidized-bed reactors are considered tobe the most suitable reactor design.

In recent years, CLR using the same basic principles asCLC has been proposed to generate synthesis gas by partiallyoxidizing the natural gas or methane with oxygen carriersin fuel reactor. CLH has been presented to produce pure H2

with CO2 capture by means of water splitting process with aredox of an oxygen carrier. Both CLR and CLH have beendemonstrated to be feasible in laboratory scale fluidized-bed reactors. In order to adapt to the characterizations ofsolid fuels, CLOU, which originates from normal CLC, wasproposed to burn solid fuels such as coal, petroleum coke,and biomass.

Despite CLC has attracted extensive research in recentyears, there are still a number of issues that require furtherinvestigation. For example, development of oxygen carrierswith excellent reactivity and stability is still one of thechallenges for CLC. Construction and operation of large-scale CLC systems is needed before this technology is used


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commercially. As novel techniques of producing synthesisgas and hydrogen using chemical-looping process, CLR andCLH deserve further research in the near future.

Acknowledgment

The financial support of National Natural Science Foun-dation of China (50574046 and 50774038) is gratefullyacknowledged. This work was also supported by the DirectorFoundation of Guangzhou Institute of Energy Conversion(o807z2 and o807rf), Chinese Academy of Sciences.

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[19] P. Cho, T. Mattisson, and A. Lyngfelt, Defluidization con-ditions for a fluidized bed of iron oxide-, nickel oxide-, andmanganese oxide-containing oxygen carriers for chemical-looping combustion, Industrial and Engineering ChemistryResearch, vol. 45, no. 3, pp. 968977, 2006.

[20] P. Cho, T. Mattisson, and A. Lyngfelt, Carbon formationon nickel and iron oxide-containing oxygen carriers forchemical-looping combustion, Industrial and EngineeringChemistry Research, vol. 44, no. 4, pp. 668676, 2005.

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[29] Q. Zafar, A. Abad, T. Mattisson, and B. Gevert, Reactionkinetics of freeze-granulated NiO/MgAl2O4 oxygen carrierparticles for chemical-looping combustion, Energy andFuels, vol. 21, no. 2, pp. 610618, 2007.

[30] M. Johansson, T. Mattisson, A. Lyngfelt, and A. Abad,Using continuous and pulse experiments to compare twopromising nickel-based oxygen carriers for use in chemical-

looping technologies, Fuel, vol. 87, no. 6, pp. 9881001,2008.

[31] M. Ryden, A. Lyngfelt, and T. Mattisson, Synthesis gasgeneration by chemical-looping reforming in a continuouslyoperating laboratory reactor, Fuel, vol. 85, no. 12-13, pp.16311641, 2006.

[32] E. Johansson, T. Mattisson, A. Lyngfelt, and H. Thunman,Combustion of syngas and natural gas in a 300 W chemical-looping combustor, Chemical Engineering Research andDesign, vol. 84, no. A9, pp. 819827, 2006.

[33] A. Lyngfelt, M. Johansson, and T. Mattisson, Chemical-looping combustion: status of development, in Proceedingsof the 9th International Conference on Circulating FluidizedBeds (CFB 08), Hamburg, Germany, May 2008.

[34] S. R. Son and S. D. Kim, Chemical-looping combustion withNiO and Fe2O3 in a thermobalance and circulating fluidizedbed reactor with double loops, Industrial and EngineeringChemistry Research, vol. 45, no. 8, pp. 26892696, 2006.

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[45] L. F. de Diego, P. Gayan, F. Garca-Labiano, J. Celaya, A. Abad,and J. Adanez, Impregnated CuO/Al2O3 oxygen carriersfor chemical-looping combustion: avoiding fluidized bedagglomeration, Energy and Fuels, vol. 19, no. 5, pp. 18501856, 2005.

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Labiano, and A. Abad, Chemical looping combustion in a10kWth prototype using a CuO/Al2O3 oxygen carrier: effectof operating conditions on methane combustion, Industrialand Engineering Chemistry Research, vol. 45, no. 17, pp. 60756080, 2006.

[48] B. M. Corbella, L. de Diego,F.Garca-Labiano, J. Adanez, andJ. M. Palacios, Characterization and performance in a mul-ticycle test in a fixed-bed reactor of silica-supported copperoxide as oxygen carrier for chemical-looping combustion ofmethane, Energy and Fuels, vol. 20, no. 1, pp. 148154, 2006.

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[55] P. Cho, T. Mattisson, and A. Lyngfelt, Reactivity of ironoxide with methane in a laboratory fluidized bed-applicationof chemical looping combustion, in Proceedings of the 7thInternational Conference on Fluidized Bed Combustion, p. 599,Niagara Falls, Canada, 2000.

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cle reduction and oxidation of different types of ironoxide particles-application to chemical-looping combus-tion, Energy and Fuels, vol. 18, no. 3, pp. 628637, 2004.

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[60] A. Abad, T. Mattisson, A. Lyngfelt, and M. Johansson, Theuse of iron oxide as oxygen carrier in a chemical-loopingreactor, Fuel, vol. 86, no. 7-8, pp. 10211035, 2007.

[61] M. Johansson, T. Mattisson, and A. Lyngfelt, Investigationof Fe2O3 with MgAl2O4 for chemical-looping combustion,Industrial and Engineering Chemistry Research, vol. 43, no. 22,pp. 69786987, 2004.

[62] H. Leion, T. Mattisson, and A. Lyngfelt, The use ofpetroleum coke as fuel in chemical-looping combustion,Fuel, vol. 86, no. 12-13, pp. 19471958, 2007.

[63] B. M. Corbella and J. M. Palacios, Titania-supported ironoxide as oxygen carrier for chemical-looping combustion ofmethane, Fuel, vol. 86, no. 1-2, pp. 113122, 2007.

[64] H. Leion, A. Lyngfelt, M. Johansson, E. Jerndal, and T. Mat-tisson, The use of ilmenite as an oxygen carrier in chemical-looping combustion, Chemical Engineering Research andDesign, vol. 86, no. 9, pp. 10171026, 2008.

[65] N. Berguerand and A. Lyngfelt, The use of petroleumcoke as fuel in a 10kWth chemical-looping combustor,International Journal of Greenhouse Gas Control, vol. 2, no.2, pp. 169179, 2008.

[66] T. Mattisson, A. Lyngfelt, and P. Cho, The use of ironoxide as an oxygen carrier in chemical-looping combustionof methane with inherent separation of CO2, Fuel, vol. 80,no. 13, pp. 19531962, 2001.

[67] M. Johansson, T. Mattisson, and A. Lyngfelt, Investigationof Mn3O4 with stabilized ZrO2 for chemical-looping com-bustion, Chemical Engineering Research and Design, vol. 84,no. A9, pp. 807818, 2006.

[68] A. Abad, T. Mattisson, A. Lyngfelt,and M. Ryden, Chemical-looping combustion in a 300W continuously operatingreactor system using a manganese-based oxygen carrier,Fuel, vol. 85, no. 9, pp. 11741185, 2006.

[69] Q. Zafar, A. Abad, T. Mattisson, B. Gevert, and M. Strand,Reduction and oxidation kinetics of Mn3O4/Mg-ZrO2oxygen carrier particles for chemical-looping combustion,

Chemical Engineering Science, vol. 62, no. 23, pp. 65566567,2007.

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[71] J. E. Readman, A. Olafsen, Y. Larring, and R. Blom,La0.8Sr0.2Co0.2Fe0.8O3 as a potential oxygen carrier in achemical looping type reactor, an in-situ powder X-raydiffraction study, Journal of Materials Chemistry, vol. 15, no.19, pp. 19311937, 2005.

[72] M. Ryden, A. Lyngfelt, T. Mattisson, D. Chen, A. Hol-men, and E. Bjrgum, Novel oxygen-carrier materials forchemical-looping combustion and chemical-looping reform-

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[73] J. Adanez, F. Garca-Labiano, L. F. de Diego, P. Gayan, J.Celaya, and A. Abad, Nickel-copper oxygen carriers to reachzero CO and H2 emissions in chemical-looping combustion,Industrial and Engineering Chemistry Research, vol. 45, no. 8,

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of Co-Ni/Al2O3 oxygen carrier in multicycle CLC, AIChEJournal, vol. 53, no. 7, pp. 18171829, 2007.

[75] M. Johansson, T. Mattisson, and A. Lyngfelt, Creating asynergy effect by using mixed oxides of iron- and nickeloxides in the combustion of methane in a chemical-loopingcombustion reactor, Energy and Fuels, vol. 20, no. 6, pp.23992407, 2006.

[76] Q. Song, R. Xiao, Z. Deng, et al., Chemical-loopingcombustion of methane with CaSO4 oxygen carrier in a fixedbed reactor, Energy Conversion and Management, vol. 49, no.11, pp. 31783187, 2008.

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[80] T. Mattisson, Q. Zafar, M. Johansson, and A. Lyngfelt,Chemical-looping combustion as a new CO2 managementtechnology, in Proceedings of the 1st Reginal Symposium onCarbon Management, Dhahran, Saudi-Arabia, May 2006.

[81] A. Lyngfelt and H. Thunman, Construction and 100hof operational experience of a 10-kW chemical-loopingcombustor, in Carbon Dioxide Capture for Storage in DeepGeologic Formations-Results from the CO2 Capture Project,vol. 1, pp. 625645, 2005.

[82] B. Kronberger, A. Lyngfelt, G. Loffler, and H. Hofbauer,Design and fluid dynamic analysis of a bench-scale com-bustion system with 2 separation-chemical-looping combus-tion, Industrial and Engineering Chemistry Research, vol. 44,no. 3, pp. 546556, 2005.

[83] B. Kronberger, E. Johansson, G. Loffer, T. Mattisson, A.Lyngfelt, and H. Hofbauer, A two-compartment fluidizedbed reactor for 2 capture by chemical-looping combustion,Chemical Engineering and Technology, vol. 27, no. 12, pp.

13181326, 2004.[84] N. Berguerand and A. Lyngfelt, Design and operation of a

10kWth chemical-looping combustor for solid fuelstestingwith South African coal, Fuel, vol. 87, no. 12, pp. 27132726,2008.

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chemical-looping combustor using natural gas as fuel, Fuel,vol. 88, no. 11, pp. 20832096, 2009.

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