Chakravarthy_Thermodynamic Analysis of Alternatives to CLC

8/3/2019 Chakravarthy_Thermodynamic Analysis of Alternatives to CLC

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Energy Fuels 2011, 25, 656669 : DOI:10.1021/ef101336m

Published on Web 01/19/2011

Thermodynamic Analysis of Alternative Approaches to Chemical Looping Combustion

V. Kalyana Chakravarthy, C. Stuart Daw,* and Josh A. Pihl

Energy & Transportation Science Division, Oak Ridge National Laboratory, 2360 Cherahala Boulevard, Knoxville, Tennesee 37932

Received September 30, 2010. Revised Manuscript Received December 21, 2010

In this article, we review and clarify some of the points made by previous authors1,2 regarding chemicallooping combustion (CLC).Althoughmuch of therecent interest in chemical looping combustion hasbeenassociated with carbon sequestration, our primary interest here is its potential to increase the thermo-dynamic efficiency of converting fuel chemical energyinto useful work. We expand on several pointsaboutthe details of CLC that we feel have not previously been sufficiently explored and suggest alternative (andpossibly more practical) approaches that exploit some of the same thermodynamic concepts. We illustrateour key points with first and second law analyses of ideal conceptual processes, which, in addition to CLC,also include isothermal, nonequilibrium, preheated combustion and combustion with thermochemicalrecuperation. Our results suggest that a significant portion of the potential efficiency benefit of CLC mightbe achieved without the need to handle and transport large quantities of solid oxygen-storage material.Exploitation of this fact may lead to approaches for power generation from hydrocarbon fuel combustion

that can achieve second law efficiencies 10-

15% higher than those that are currently possible.

Introduction

Chemical looping combustion (CLC) is a two-stage processof burning fuel.1 The first stage involves oxidation of a solidmaterial using oxygen in the air, and in the second stage, it isreduced back to its original state by a fuel. To complete theprocess, the reduced oxygen carrier is recycled back to theoxidation stage where it can be reoxidized. The combustiongases leaving the second stage are not diluted with atmo-spheric nitrogen, and so, they are more concentrated in CO2than conventional flue gas.

In the current day, much of the interest in CLC is driven by itsvalue as a key step in carbon sequestration rather than for itsdirect benefit to fuel exergy preservation. Analyses of the exergybudget in CLC-based power generation plants have been per-formed in a fewpapers.

3-9Most such studies have typically beendirected atvery specific andcomplex applications,andthus,theyhave not been so concerned with clarifying the fundamentalthermodynamic impact of CLC relative to conventionalcombustion. One recent exception to this is the study byMcGlashan,

2which provides the kind of basic thermodynamic

analysis of CLC that we have found useful as a starting point. Inour analyses, we expand on points briefly noted by McGlashanand consider alternative ideal processes that capture some keyCLC features. We also point out practical problems that could

arise in implementation of these alternative processes.

Previous studies have emphasized that the expected effi-ciency advantage of CLC (relative to conventional unrest-rained combustion) is a consequence of constraining theoxidation and reduction reactions to separate gas-solidreaction stages that operate near chemical equilibrium, result-ing in lower exergy destruction and higher efficiency.1,2,6,8

Other approaches have been proposed for constraining gas-phase reactions to near equilibrium conditions also, but thedirect gas-phase reactions require very high temperatures toachieve equilibrium (e.g., typically above 3500 K2,10,11).Because the gas-solid reactions in CLC have much lower

equilibrium reaction temperatures, it offers the potential toavoid hightemperatures while still reducingexergy destruction.

One important aspect that has not always been stronglyemphasized in the CLC literature is that thermal equilibriumis also a critical factor.That is, all heat-transfer processes needto occur over very small temperature gradients to avoidgeneration of entropy and destruction of exergy.10-12 Inpractice, this means that all gas-gas, solid-solid, or gas-solid heat transfer between flowing streams should be counter-flow and must be thermally balanced so that temperaturepinch points or large gradients do not develop. This placessignificant constraints on the design of CLC systems and is anextremely important practical consideration. In the followingsections, we discuss this issue at length and its relationship to

the idea of isothermal combustion where chemical equilibri-um may or may not be present.

Two other importantfeatures of CLC are (1) the generationof two separate exhaust streams of different composition andtemperature and (2) the presence of an internal heat flow thatcan be used to generate additional work. The first featureis exploited in carbon sequestration, but, as far as we areaware, the precise role of this exhaust partitioning on entropy

*To whom correspondence should be addressed. E-mail: [email protected].

(1) Richter, H. J.; Knoche, K. F. ACS Symp. Ser. 1983, 235, 7185.(2) McGlashan,N. R. Proc. Inst. Mech. Eng., Part C2008, 222, 1005

1019.(3) Ishida, M.; Zheng, D.; Akehata, T. Energy 1987, 12, 147154.(4) Jin, H.; Ishida, M. Energy 1993, 18, 615625.(5) Ishida, M.; Jin, H. Energy 1994, 19, 415422.(6) Anheden, M.; Svedberg, G. Energy Convers. Manage. 1998, 39,

19671980.(7) Jin, H.; Ishida, M. Int. J. Hydrogen Energy 2000, 25, 12091215.(8) Brandvoll, O.; Bolland, O. J. Eng. Gas Turbines Power 2004, 126,

316321.(9) Wolf, J.; M. Anheden, J. Y. Fuel2005, 84, 9931006.

(10) Lutz, A.; Larson, R. S.; Keller, J. O. Int. J. Hydrogen Energy2002, 27, 11031111.

(11) Daw, C. S.; Chakravarthy, K.; Conklin, J. C.; Graves, R. L. Int.J. Hydrogen Energy 2006, 31, 728736.

(12) Dunbar, W. R.;Lior,N. Combust. Sci. Technol. 1994, 103, 4161.


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Energy Fuels 2011, 25, 656669 : DOI:10.1021/ef101336m Chakravarthy et al.

generation and exergy destruction has not been fully quanti-fied. We include an explicit discussion of this issue in theresults presented here. We also point out how the internalheat-flow feature of CLC can be mimicked by other processes,such as thermochemical recuperation (TCR), in which hydro-carbon fuels are first reformed to syngas using exhaust heatprior to being combusted. Taken together, we feel that theabove issues should be important considerations in futurediscussions about CLC and related concepts.

Thermodynamic Analysis

Work Extraction Calculations. Two kinds of work extrac-tion with heat engines are considered here. The first is workthat can be extracted using the heat available from anisothermal reactor, which is limited by the Carnot efficiency.The second is work that is continuously extracted from hotflue gas (produced by combustion) as it isobarically cools toambient conditions, which is limited by thermodynamicexergy. The Carnot limit and the concept of exergy (forisobaric systems) can easily be shown to be consistent witheach other as follows.

Assume infinitesimal heat dQ is drawn fromhot flue gas at

temperature T, which results in its cooling to temperature(T- dT). In the limit of dT being small, heat dQ can beassumed to be drawn at a constant temperature T. Themaximum amount of work, dW, that can be drawn fromdQ can be determined using the Carnot efficiency.

dW dQ 1-T0

T

Cp 1-

T0

T

dT CpdT-T0

Cp

TdT dH-T0dS

1

dH and dS in the above equation are the differentialchanges in enthalpy (H) and entropy (S) associated withtemperature change dT, which, respectively, equal CpdTandCp(dT/T). Integrating the above equation from T to T0(ambient temperature) results in the maximum amount of

work that can be extracted from hot flue gas using a series ofCarnot engines. Integration of (dH- T0dS) from tempera-ture T to ambient temperature T0 equals the exergy of themixture, [(H- H0) - T0(S- S0)], at atmospheric pressure,where H0 and S0 are the enthalpy and entropy of the mixtureat the reference dead state.

Preheated, Isothermal Gas-Phase Combustion. We beginour discussion with preheated, isothermal gas-phase combus-tion, which is the simplest concept analyzed here. In conven-tional (unrestrained) combustion, reactants enter the combus-tion chamber at ambient temperature, whereas the productsleave at the adiabaticflametemperature. Given that most fuelshave ignition temperatures much higher than ambient, thecombustion can be sustained only if there is some heat transferfrom already formed hot products to reactants. This exchangeis driven by a large temperature difference that results in highentropy generation and loss of exergy.

12

The gas-phase reaction between fuel and air can theoreti-cally be maintained at isothermal conditions by transferringthe heat generated by the combustion reactions to a heatengine and using the combustion products to preheat thereactants to the operating temperature. For a simplifiedanalysis, it is assumed that reactants can be preheated tothe reaction temperature using hot reaction products incompliance with both first and second laws. This enablescomparison with CLC, which is analyzed along similar linesbelow.

The maximum work that can be extracted using heatdrawn from an isothermal reaction, at temperature T, iseasily computed using the Carnot efficiency.

Wiso -H 1-T0

T

2

H is the enthalpy change associated with the reaction.This expression implies that work increases with reactiontemperature and is limited by the enthalpy change of com-

bustion and not exergy. This is problematic from a secondlaw perspective if the latter is lower. In such cases, however,there is an upper limit on operating temperature. For T >H/S, the conversion of reactants to products cannotproceed spontaneously because the Gibbs free energychange is positive. At this limiting operating temperature,Wiso can be shown to equal the exergy of the mixture.

The benefits of isothermal combustion over conventionalunrestrained combustion are due to the different nature ofthe heat exchange between products and reactants in the twoprocesses. In the former, heat is exchanged between hotproducts and reactants in a (counterflow type) preheaterwhere the heat exchange(at anylocation) is driven by a smalltemperature gradient between the two streams. Because heatis constantly extracted to maintain the reactions at anisothermal state, there is no heat exchange between reactantsand products when they are physically in contact in thereactor. Nevertheless, there is some entropy generation dueto composition change, which can only be avoided at theequilibrium temperature of the overall fuel-air reaction (ifone exists).2,10,11

We also analyzed preheated, gas-phase isothermal com-bustion considering losses associated with preheating whileusing temperature-dependent thermodynamic properties inan effort to understand the extent of deviations from idealityrepresented by eq 2. Figure 1 shows the theoretical secondlaw efficiency of this process for H2 fuel.

Figure 1. Performance of an ideal preheated, isothermal, stoichio-metric gas-phase combustion of H2 at atmospheric pressure with noheat losses. (a) Exergy budget nondimensionalized by initial exergy(the fraction of exergy that remains after unrestrained stoichio-metriccombustion is indicatedusinga symbolfor comparison attheadiabatic flame temperature); second law efficiency associated with

adiabatic, isobaric combustion is indicated by the symbol at theadiabatic flame temperature. (b) Temperature of the combustionproducts at the exit of the preheater.
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The exergy budget for this process has four components:work extracted from the heat engine, combustion irreversi-bility (which also includes irreversibility due to mixing),preheating irreversibility, and exergy in the exhaust. Theresults for this plot are generated as follows. A reactiontemperature is chosen, and the equilibrium composition ofthe products is determined. The product stream is used topreheat reactants to the maximum temperature possible.When the products are not hot enough to preheat reactants

all theway to thereaction temperature, thereactants are firstassumed to react partially so that the temperature rises to thechosen reaction temperature before heat is drawn to a heatengine. There is minimal exergy loss due to preheating in thisscenario. If there is more heat in the hot products than whatis needed to preheat the reactants to the required reactiontemperature, then the products exit the heat exchanger athigher than ambient temperature. This starts to happen asthe chosen reaction temperature starts to exceed the adia-batic flame temperature of H2. Instead of neglecting theexhaust heat, the exergy of the exhaust is added to the workthat is extracted from the heat engine (deriving heat fromreaction between preheated reactants at chosen reactiontemperature). There is, however, exergy loss associated with

heat transfer at the colder end of the heat exchanger in suchcases. Its increase with temperature is rapid enough beyondthis point that useful exergy (work and exhaust exergycombined) decreases with temperature despite decreasingcombustion irreversibility, and the 100% second law effi-ciency at equilibrium temperature, as suggested by eq 2 andthe Gibbs free energy constraint earlier, is never reached.

The same trend of highest efficiency when the reactiontemperature is near the adiabatic flame temperature is ob-served for other fuels. Preheated, isothermal gas-phase com-bustion at best has about half the combustion irreversibilityassociated with unrestrained gas-phase combustion betweenreactants. This is a valid comparison given that the peakefficiency of theformer is roughly aroundthe adiabaticflame

temperature, and so, the peak operating temperatures inboth cases are comparable.

It should be noted that implementing isothermal gas-phase combustion could be extremely difficult. At tempera-tures below the adiabatic flame temperature, gas-phasecombustion reactions are highly irreversible and sponta-neous. It is difficult to match the rate of heat extraction tothe rate of heat generation from highly spontaneous gas-phase reactions.

Conventional Chemical Looping Combustion. To help clar-ify the reasons behind the efficiency benefits of CLC, wediscuss two specific features that set it apart from conven-tional combustion. First, we review the importance of gen-erating two separate exhaust streams rather than a singlemixed exhaust. We then consider the impact of how thechemical reactions and internal heat transfer are managed.

Adiabatic CLC. The most distinctive global feature ofCLC highlighted in recent studies is that it generates twoseparate gaseous exhaust streams, one containingN2 (and O2under lean conditions) and the other containing H2O andCO2, as depicted in Figure 2. The two streams emergeseparately from staged oxidation and reduction reactors. Asolid oxygen carrier (typically some type of metal) shuttlesbetween the two reactors in reduced (Me) and oxidized(MeO) forms.

When no heat flows between the overall system and theenvironment, the enthalpy of the fuel and air mixture is split

between the two product streams. The temperatures of theproduct streams can be different because the heats of reac-tion for oxidation and reduction can be different. The firstlawrequires only that thenet enthalpy of thereactants equalsthe net enthalpy of two exhaust streams at steady state. Werefer to this generic case as adiabatic CLC. As we discussbelow,the separation of theexhaust gasesreflects an increasein the final exergy that is destroyed in conventional combus-tion when the exhaust gases are allowed to mix.

Without initially defining the details of how each reactoroperates, it is informative to consider just the second lawimpact of exhaust gas separation for a range of possibleexhaust temperatures from thetwo stages. Forexample, withstoichiometric H2 combustion, the product stream from theoxidation reactor only has N2 at temperature TN2, whereasthe product stream from the reduction reactor contains onlyH2O at a temperature ofTH2O. For TH2O ranging from 500 to3000 K, we computed values of TN2 so that the sum ofenthalpies of products equals that of the reactant streams(H2 and air). The sum of the exergies of the two productsdivided by the initial fuel-air exergy equals the second lawefficiency of the process. In Figure 3, the second law effi-ciency and TN

2

are plotted for a range of values ofTH2O. The

second law efficiency of conventional (isobaric gas-phasecombustion resulting in a single mixed product stream at theadiabatic flame temperature) is also shown for comparison.

As seen in the above figures, the efficiency benefit fromexhaust separation is minimal when the two streams havethe same temperature (which equals the adiabatic flametemperature). Even when the exhaust streams are at thesame temperature, however, there is also a slight benefitremaining because their compositions are different. At leasttheoretically, it is possible to exploit this concentrationdifference to produce work with a concentration engine.

The second law efficiency increases as the temperatures ofthe two streams diverge, which is expected. This is because

Figure 2. Simplified schematic of CLC.

Figure 3. Second law combustion efficiency (ratio of exergy after

combustion to initial exergy of unmixed fuel and air at ambientconditions)and temperatureof outlet streams for the stoichiometricsimplified (flue-gas generating) version of chemical looping com-bustion of H2. The oxidation reactor exhausts only N2, whereas thereduction reactor exhausts H2O.
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some work can be generated by operating a heat enginebetween the two streams in addition to the work that can begenerated by operating a heat engine between the mixture oftwo streams (when they are combined) and the ambient.

More of this additional work can be generated if there aretwo streams at two different temperatures Even in extremecases with high temperature differences where peak tempera-tures exceed 3000 K, the second law efficiency only increasesby about 7%, so exhaust gas separation can only account fora relatively modest improvement by itself.

Exhaust gas temperature differences are also constrainedby materials considerations. As in conventional combustion,materials temperature limits favor the use of lean combus-tion (high excess oxygen) to keep peak temperatures low.One potential benefit of chemical looping in this regard isthat it enables stable combustion below the conventionallean limit for gas-phase combustion. To consider how theabove estimates of second law efficiency for adiabatic CLC

are affected by lean operation, we also computed them for aglobal equivalence ratio of 0.5. These results are depicted inFigure 4. Even in this case, the second law efficiency benefitfrom the exhaust gas separation is only a few percent as longas the peak temperature is restricted to 2000 K. Thus, it isclear that exhaust gas separation alone does not provide amajor efficiency benefit in and of itself. If CLC is to bepursued for efficiency reasons, there are other importantfactors to consider.

Classical CLC. Two additional constraints usually asso-ciated with CLC are the isothermal and near-equilibriumoperation of each reactor. Analysis of these factors requiresconsideration of individual heat balances for each reactionstage. To be consistent with earlier analysis,

2 we assume heretemperature-independent specific heats.H1 andS1 repre-sent the enthalpy and entropy changes associated with theoxidation reaction in CLC, whereas H2 and S2 representthose of the reduction reaction. Obviously, H1 H2equals H( T2/T1,does not seem possible to meet with any combination ofoxygen carrier and fuel among several analyzed in this study

for reasonable values of T2. This is because T2 needs to behigh enough for the reduction reaction to be kineticallyfeasible (there are several material-fuel combinations wherethe reduction reaction is thermodynamically feasible atsubambient temperatures). For reasonable values of T2(around 600 K), no reduction reaction we found is endo-thermic enough (i.e., has a high enough Q2 to absorb all ofthe heat necessary to eliminate the discharge of heat to theenvironment). Further, the work output from the engine(W = Q1 - Q2) equals -H, the enthalpy change of theoverall fuel-air reaction, implying 100% first law efficiency.This is thermodynamically possible only if the enthalpychange of the overall fuel-air combustion is lower than theexergy of the fuel-air mixture. The condition, Q2/Q1 > T2/T1, if possible, would have to be met with fuels with thisfeature. Given that there are more practical limitations thatsupersede this condition (a discussion of which is includedlater), it is not necessary here to rule out the possibility thatthis condition could be met.

When the above condition is not met, two heat engines,one operating between the two reactors and the otheroperating between the oxidation reactor and ambient, asshown in Figure 5, are needed to extract the maximumpossible amount of work. It is assumed that T1

0 and T20 in

the schematic, which are, respectively, the temperatures ofpreheated air and fuel streams, equal T1 and T2 in thissimplified analysis to be consistent with the analysis of

Figure 5. Schematic of CLC for exothermic oxidation and endo-thermic reduction reactions. An internal engine generates work byextracting heat from theoxidation reactor andrejecting some of it tothe reduction reactor. Some additional heat needs to be extractedfrom the oxidation reactor by an external heat engine in order tomaintain the isothermal oxidation reaction.

Figure 4. Second law combustion efficiency and temperature ofoutlet streams for lean(equivalence ratioof 0.5),simplified chemicallooping combustion of H2. Because thecombustion is lean, only halfthe oxygen is consumed and the rest remains in the exhaust comingout of the oxidation reactor.
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McGlashan.2 In reality, however, T10 is lower than T1 because

N2 at temperature T1 cannot be used to preheat air, whichcontains the same amount of N2 and additional O2 to thesame temperature.

TheCarnot efficiencies of thetwo engines are [1- (T2/T1)]and [1- (T0/T1)], respectively. The work extracted from thetwo engines and the total work can be written as follows.

Wint Q2T1

T2

1-T2

T1 3

Wext Q1-Q2T1

T2

1-

T0

T1

4

W Wint Wext

Q1-Q2-T0Q1

T1-

Q2

T2

-H T0H1

T1H2

T2

5

Given that H1 e 0 and H2 g 0, the efficiency increases

with increasing T1 and decreasing T2. Thermodynamic fea-sibility conditions (negative free energy changes for thereactions) in addition to the previously noted facts thatS1 e 0 and S2 g 0 imply that T1 e Teq,1 and T2 g Teq,2(where Teq,1 and Teq,2 are equilibrium temperatures de-fined by McGlashan et al.2) for sustained operation of thereactors.

Teq;i Hi

Si; i 1;2 6

The maximum efficiency is, therefore, attained when T1 =Teq,1 and T2 = Teq,2, the choices suggested by McGlashanet al.2 The maximum work equals -H T0S, the exergyof the fuel-air mixture, and so the second law is never

violated. Note that operation of the idealized CLC systemat the oxidation and reduction equilibrium temperatureswould result in 100% second law efficiency.

For the fuel-oxygen carrier combination with exothermicoxidation and reduction reactions, two heat engines areneeded to maximize work extraction. A Carnot engineoperates between each reactor andthe ambient, as illustratedin Figure 6. As before, T1

0 and T20 areassumed to equal T1 and

T2, respectively. The total work extracted is determined bythe following expression.

W -H1 1-T0

T1

-H2 1-

T0

T2

-H T0

H1

T1H

2T2

7

Alternately, a combination of an internal Carnot enginedrawing heat from theoxidation reactor andrejecting heat tothe reduction reactor and an external Carnot engine drawingheat from the reduction reactor and rejecting heat to theambient could be used. The above expression for total workcan easilybe shown tobe applicablein this instance aswell. Itis also worth noting that this expression is exactly thesame asthe one derived for the combination of exothermic oxidationand endothermic reduction. In this case, however, workoutput increases with an increase in either T1 or T2. T1,

obviously, is limited by Teq,1, and this condition used in theabove expression results in the following inequality.

We-H T0S1 T0H2

T2

e-H T0S-S2 T0H2

T2

e -H T0ST0

T2H2-T2S2 8

Once again, Teq,2 imposes a lower limit on T2 to ensurethermodynamic feasibility. However, because H2 < 0 andS2 > 0, the calculated Teq,2 would be negative, leading totwo conclusions: there is no attainable equilibrium tempera-ture for theexothermic reduction reaction, andthe reductionreaction should occur spontaneously at all temperatures.

There does not seem to be an obvious thermodynamic upperlimit on T2, and efficiency increases monotonically withincreasing T2. T2 could theoretically be higher than T1. It isworth noting that the overall efficiency is actually increasingas T2 is moving away from the equilibrium reductiontemperature. In reality, T2 is limited by the melting pointsof the two solid materials involved. Even without theselimitations, the total work is limited by fuel-air exergy(-H T0S) in compliance with the second law giventhat the Gibbs free energy change for the reduction reactionH2 - T2S2 e 0 for all values ofT2.

In summary, it seems thermodynamically possible toachieve 100% second law efficiency using CLC for combina-tions of fuel and oxygen-storage material that have exo-thermic oxidation reactions and endothermic reductionreactions. For combinations where both reactions are exo-thermic, the second law efficiency is limited by the maximumpossible temperature where reduction reactions can be sus-tained. The overall efficiency for such cases, surprisingly,increases as the reduction reaction becomes more irreversible(i.e., T2 moves away from Teq,2).

IfT1 and T2 in eqs 5 and 7 are assumed to be equal, thentheexpressionfor total work foreither endothermic or exothermicreduction reactions becomes identical to the expression forpreheated, isothermal, gas-phase combustion given by eq 2. Ofcourse, there are different constraints on operating tempera-tures for the two kinds of combustion (as discussed above).

Figure 6. Schematic of CLC for both exothermic reactions. Eachheat engine extracts heat from a reactor and rejects some of it toambient.
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Preheating also is different, as there are two exhaust streamsand two inlet streams in CLC. Therefore, the differencesbetween the two concepts stem primarily from constraintson operating temperatures

2and implementation issues.

As mentioned above, isothermal reactions can be hard tocontrol in preheated, gas-phase combustion. This is not aproblem in CLC because the oxidation reaction stops oncethe reactor temperature exceeds Teq,1 and will resume onlyafter heat is extracted to lower the temperature, so there is a

natural barrier against thermal runaway in CLC. Reductionreactions in CLC are spontaneous, but the heat generated/consumed by these reactions is relatively low for mostfuel-oxygen-storage material combinations.

A simplified analysis with constant specific heats of CLCand preheated, isothermal, gas-phase combustion has re-sulted in eqs 5, 7, and 2 for work outputs. A theoreticalcomparison can be made of the two kinds of combustion fora given fuel if an oxygen carrier is chosen for CLC. Theseexpressions, however, do not reflect the performance of real-world implementations of these idealized combustion pro-cesses. The analysis, for example, does not take into accountpreheating of the reactants. Minimizing the exergy lossesassociated with preheating is a challenge for both CLC and

preheated, isothermal, gas-phase combustion.The transport of solids required in CLC introduces an

even more difficult challenge. To achieve the isothermaloperation assumed above, the circulating solids must enterthe reactor at its operating temperature. Unless the reactorsare operating at the same temperature, the solid streamscirculating between the reactors must achieve complete heatexchange with minimal losses. That is, the solids exiting thehigh-temperature reactor must transfer enough heat to theother solid streamto cool to the operatingtemperature of thelow-temperature reactor.

MeOT1 MeT2 w MeOT2 MeT1 9

Implementing such a system would require identical heatcapacities of the solid materials, counter-flow heat exchange,and possibly an intermediate fluid to transfer the heatbetween the streams. Realizing such a system would clearlyinvolve many practical hurdles. Possibly because of thesehurdles, many implementations of CLC discussed in theliterature do not appear to include low-gradient heat ex-change for the carrier solids. Instead, the solids enter eachreactor at a temperature far different from the equilibriumreaction temperature. In cases of highly endothermic reduc-tion reactions, the resulting intrareactor temperature swingscan be quite high (as the reduction reactions consume largequantities of heat), and so, a highly endothermic reductionreaction is considered undesirable.13,14 On the other hand,

minimization of the endothermic stage reaction heat wouldappear to be counterproductive for efficiency based on theabove analyses and the work of McGlashan.2

If the solid-solid heat-exchange process is notmanagedinan optimal fashion, there is considerable generation ofentropy associated with the high-gradient heat transfer. Inthe limit, the two solids streams simply equilibrate to theirthermal capacity weighted average temperature.

MeOT1 MeT2 w MeOT1 T2

2

Me

T1 T22

10

The lost work potential can be determined easily as[mMeCp(T1 - T2)(T0/T2 - T0/T1)/2] per mole of fuel, where

mMe is the moles of Me involved in CLC. The thermalcapacity of the carrier solids (mMeCp) is usually much higherthan the thermal capacity of the gaseous reactants (when allmaterials are in stoichiometric proportions). As a result,CLC without optimal solids heat management ends uphaving low efficiency when T1 and T2 are very different.The problem of solids heat exchange can be avoided ifreactor temperatures are nearly equal, which is possible incases where both oxidation and reduction are exothermic, as

discussed later.Alternative to CLC: Staged Combustion with Oxygen

Transfer (SCOT). In addition to the gas and solids preheat-ing issues described above, CLC requires a mechanism forsolid circulation between the reactors. This difficulty aloneprecludes its use in nonstationary applications. It also in-troduces substantial cost and complexity into the systemrelative to other combustion approaches.

A version of chemical looping combustion with stationarysolid reactants, which mitigates some of the problems asso-ciated with CLC, was proposed and analyzed by Noormanet al.15,16 A similar concept, referred to as staged combustionwith oxygen transfer (SCOT) to distinguish it from conven-tional CLC, is analyzed here. Unlike in CLC, the solid

reactants in SCOT are stationary, and there is no solid-to-solid heat exchange. SCOT utilizes two reactors operating inopposite phases (one is under oxidation while the other isunder reduction). Once the solid reactants have been fullyconverted to products, the inlet gas compositions areswitched between air and fuel to change the operating phaseof the reactors.

Unlike the analyses performed in the previous sections,temperature-dependent thermodynamic properties and pre-heating inefficiencies are included in the analysis of this newconcept. We also revisit preheated, isothermal, gas-phasecombustion using this more general approach to provide abasis for comparison with SCOT. Estimates of maximumwork that canbe extracted forseveral operatingscenarios are

provided below. The calculations of these estimates are doneas described below.

The oxidation and reduction reactions are assumed to

occur under (mostly) isothermal conditions at temperaturesT1 and T2, respectively. When reactants are in stoichiometric

proportions, N2 is the only gaseous product of the oxidation

reactor. With the N2 at temperature T1, it will not be possible

to completely preheat ambient air (which contains O2 in

addition to the same amount of N2 as in the product stream)

to the required oxidation reaction temperature (T1). The

enthalpy of the CO2 and H2O exhaust from the reductionreactor is generally enough to preheat fuel to the reduction

reaction temperature. However, excess heat in the fuel pre-

heater cannot easily be used to make up for the deficit in theair preheater because typically T2 < T1. Even if the two

temperatures are equal,designs to do this routing canbe very

complex. Here, we consider simple heat-exchange processes

with outgoing N2 from theoxidation reactor at T1 preheating

the air and CO2 and H2O from the reduction reactor at T2

(13) Abad, A.; Mattisson, T.; Lyngfelt, A.; Johansson, M. Fuel2007,86, 10211035.

(14) Zafar,Q.; Abad, A.;Mattisson, T.; Gevert, B.;Strand, M. Chem.Eng. Sci. 2007, 62, 65566567.

(15) Noorman, S.; van Sint Annaland, M.; Kuipers, J. Ind. Eng.Chem. Res. 2007, 46, 42124220.

(16) Noorman,S.; vanSint Annaland,M.; Kuipers,J. Chem. Eng.Sci.2010, 65, 9297.


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preheating the fuel. The reaction schemes in the two reactors

can be symbolically denoted as follows.

airT10 MeT2 w N2T1 MeOT1 Q1T1 11

fuelT20 MeOT1 w CO2T2 H2OT2 MeT2

Q2T2 12

T10 and T20 are the temperatures of the air and fuel as theyenter the oxidation and reduction reactors, respectively,and, as explained above, may not equal T1 and T2. Q1 andQ2 denote the heat extracted from or added to the oxidationand reduction reactors, respectively. The signs of Q1 and Q2do not necessarily indicate whether the oxidation andreduction reactions are exothermic or endothermic, as ex-plained below. It is also worth noting that Q1 and Q2 do notnecessarily add up to -Hbecause T1

0 and T20 do not equal

T1 and T2. At the end of each stage of combustion, the solidmaterial is at a temperature that is different from what itshould be for the next stage. Therefore, neither the gas-phase nor thesolid-phase reactants are typically at the targetoperating temperatures at the beginning of the combustionstage. This complication is handled using the followingapproach.

The oxidation reaction is assumed to proceed at tempera-

tures below T1 until enough heat is generated to raise the

temperature of the reactor to T1. When the reactor reaches

temperature T1, heat is drawn continuously from it to

maintain the reaction under isothermal conditions until the

reaction is complete. If the solids and gases in the oxidation

reactor begin with temperatures much lower than the chosen

T1, the heat of the oxidation reaction alone may not be

sufficient to raise the temperature of the reactor to T1 and

additional heat may be necessary; that is, Q1 is negative.

Because T1 > T2, this heat cannot be provided from within

thesystem, so an external heat pump is assumed to supplytheheat required to achieve temperature T1. The external heat

pump uses work generated from the reduction reactor (Q2 is

necessarily positive in such cases).Alternately, Q2 is sometimes positive even if the reduction

reaction is endothermic (it is always positive for an exother-

mic reduction reaction) because the solid-phase reactant

starts out at a much higher temperature than T2. Therefore,

heat has to be drawn to lower the temperature to T2 (and

maintainit there). Fora conservative estimation of work that

can be extracted, we assume that all of the heat is extracted

at T2.If the reduction reaction is sufficiently endothermic to

drop the reactor temperature to T2 at some point during the

reduction phase, heat is subsequently supplied at T2 using aninternal engine operating between the two reactors to main-

tain isothermal conditions. In cases with endothermic reduc-tion, the decision to add or remove heat has to be made

(based on the sign of Q2) prior to the beginning of the

combustion phase. This precludes the possibility of drawing

heat out while the reaction temperature is above T2 and

adding heat when it drops below.Once Q1 and Q2 are calculated, the following method is

used to compute the net amount of work that can be

generated. If both Q1 and Q2 are positive, then we assume

that each reactor is supplying heat at its nominal operating

temperature to a heat engine that is rejecting heat to the

ambient. The net work extracted can simply be estimated

as follows.

W1 Q1 1-T0

T1

13

W2 Q2 1-T0

T2

14

W

W1

W2

Q1 1-T0

T1

Q2 1-

T0

T2

15

The above expression suggests that highervalues ofT1 andT2 lead to higherefficiency. However, it is to be noted that Q1and Q2 in SCOT are notindependent ofT1 and T2, as evidentfrom eqs 11 and 12. When T1 and T2 are very different, theoxidation reaction starts with Me at T2, and no heat isextracted until the reactor temperature reaches T1. Theheating of the solids during this process leads to irreversi-bility that reduces the second law efficiency of the overallprocess. Efficiency is, therefore, expected to increase as T2 istaken closer to T1 when both Q1 and Q2 are positive.

IfQ1 is positive and Q2 is negative, an internal heat engineis used to supply heat to the reduction reactor just as in thecase of traditional CLC. Work generated by this engine isestimated assuming Carnot efficiency.

Wint -Q2T1

T21-

T2

T1

16

The heat drawn by theinternalheat engineis limited by theability of the reduction reactor to accept the heat by thisengine while maintaining isothermal conditions. Any heatnot drawn from the oxidation reactor by the internal heatengine is used to generate work using an external heat enginethat rejects heat to ambient. The heat left over to power this

external engine equals [Q1 Q2(T1/T2)], and the work itgenerates is estimated using its Carnot efficiency [1 - (T0/T1)].

Wext Q1 Q2T1

T2

1-

T0

T1

17

Wint Wext Q1 1-T0

T1

Q2 1-

T0

T2

18

For this combination (Q1g 0 and Q2e 0), the dependenceof efficiency on T1 and T1 is complicated by two opposingeffects. First, it can be argued that it is beneficial to generatemost of the work using the internal heat engine because thatminimizes the heat rejected to the ambient by the externalheat engine. This requires T1 and T2 to be far apart. How-ever, theheat-transfer processes required to reach thereactoroperating temperature at the beginning of the two combus-tion phases generate entropy and reduce efficiency. Theminimization of this entropy generation requires small dif-ferences between T1 and T2. Therefore, the optimal combi-nation of temperatures is determined by the interactionbetween these opposing effects.

This work expression also suggests that the optimal workextraction could be achieved by generating work using all theheat from the oxidation reactor in an external heat engineand using part of it to power a heat pump to supply heat tothe reduction reactor.


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As explained earlier, we also encounter cases where Q1 isnegative while Q2 is positive. In this instance, work isgenerated using heat drawn from the reduction reactor

(Q2) using an external heat engine and heat is supplied tothe oxidation reactor using a heat pump. The above equationfor net work output is found to be applicable for this case aswell.

Just asin the CLC case, ifall four temperatures are the same,then Q1 Q2 = -H (i.e., the heat in the combustion pro-ducts from two reactions is enough to preheat the air and fuel)and the expression for work extracted from SCOT (for allcombinations ofQ1 and Q2) is reduced to the following form.

W -H 1-T0

T1

19

This is essentially the same expression as was derived forpreheated, gas-phase isothermal combustion under similar

assumptions. Therefore, when T1 equals T2, the efficiency ofSCOT should be comparable to that of preheated, isother-mal, gas-phase combustion. Differences, if any, would resultfrom the way preheating is conducted in the two cases. Notethat there are two product streams in SCOT as in CLC,whereas gas-phase combustion has only one.

The SCOT concept is analyzed for combinations of threefuels, H2, CH4, and C8H18 (iso-octane) and three oxygen-storage materials, Ni/NiO, Mn3O4/Mn2O3, and Ce2O3/CeO2. These materials were chosen to provide high meltingpoints, readily available thermodynamic data, and a widerange of enthalpies for the reduction reactions (discussed inmore detail below). Ni/NiO and Mn3O4/Mn2O3 have beenstudied extensively for chemical looping (e.g., refs 14 and17-

22) with the former receiving most attention. Ceria hasbeen used extensively as an oxygen-storage material inautomotive catalysis. There is a large volume of data onredox kinetics, thermal aging, and durability of this materialthat could prove useful for SCOT application development.Typical temperatures of chemical looping, which are higherthan in automotive catalysts, will not be a problem becauseCe2O3 and CeO2 have very high melting points. Othermaterials that have multiple oxidation states, such as thosebased on iron, have been considered in CLC research (e.g.,refs 9, 13, 17, and23). Thesimultaneous existence of multipleoxides at equilibrium complicates the analysis, and there-fore, these materials are not included here. Mn can form

multiple oxides in different temperature ranges,

14

butthe Mn3O4/Mn2O3 couple is considered here for reasonsdiscussed below. More comprehensive discussions on thethermodynamics, kinetics, and stabilities of potential mate-rials for CLC can be found elsewhere.22,24-27

The oxygen-storage materials considered here span a

range of possibilities regarding exothermicity of the oxida-tion reaction relative to the overall exothermicity of fuel-aircombustion. The heats of oxidation of the materials areshown in Table 1, whereas the heats of reduction withvarious fuels are shown in Table 2. For easy reading, therelative exothermicities of oxidation reactions (H1/(H)for all combinations are shown in Table 3.

With Ce2O3/CeO2, both phases are exothermic, thoughmost of the heat is released during oxidation. For Ni/NiO,the reduction reaction is endothermic, making this a goodcase for testing the advantage of having an internal heatengine. For Mn3O4/Mn2O3, the heat release is split roughlyevenly between the oxidation and reduction reactions, mak-ing it a good candidate for an isothermal reactor with notemperature swings between combustion phases.

Tables 4 and 5 list the equilibrium temperatures for theoxidation and reduction reactions, respectively, computedusing eq 6.

The fractions of the more highly oxidized state in theoxygen storage material at equilibrium are plotted for thethree materials considered in Figure 7. It turns out that theequilibrium oxidation temperatures as determined usingheats of reaction and entropies of oxidation reactions understandard conditions and listed in the above table are indeedgood estimates for temperatures where conversion becomesequilibrium-limited. Note that both Ni and NiO melt belowthe estimated equilibrium temperature, Teq,1. Liquid-phase

thermodynamic properties are used to compute equilibriumcompositions above melting points.Equilibrium compositions were also computed when higher

oxides are exposed to different reductants in amounts thatwould allow for complete reduction to the desired lower

Table 1. Enthalpy Change (H1 in J) for Oxidation Reactions

Me H1 (J)

Ce2O3 -70808Ni -101055Mn3O4 -42587

Table 2. Enthalpy Change (H2 in J) for Reduction Reactions withVarious Fuels

fuel

MeO H2 CH4 iso-octane

CeO2 -30760 -13454 -14873NiO -512 16 794 15 374Mn2O3 -58980 -41674 -43094

Table 3. Percentage of Heat Released during Oxidation Stage

fuel


CeO2 70 84 83NiO 100 120 118Mn2O3 42 51 50

Table 4. Equilibrium Temperatures (in K) of Oxidation Reactions

Me Teq,1 (K)

Ce2O3 1609Ni 2300Mn3O4 1096

(17) Cho, P.; Mattisson, T.; Lyngfelt, A. Fuel2004, 83, 12151225.(18) Mattisson, T.; Johansson, M.; Lyngfelt, A. Fuel 2006, 85, 736

747.(19) Johansson, M.; Mattisson,T.; Lyngfelt, A. Chem. Eng. Res. Des.

2006, 84, 807818.(20) Sedora, K. E.; Hossain, M. M.; de Lasa, H. I. Chem. Eng. Sci.

2008, 63, 29943007.(21) Gayan, P.; Dueso, C.; Abad, A.; Adanez, J.; de Diego, L. F.;

Garcia-Labiano, F. Fuel2009, 88, 10161023.(22) Chandel,M. K.; Hoteit, A.; Delebarre,A. Fuel2009, 88, 898908.

(23) Mattisson, T.; Lyngfelt, A.; Cho, P. Fuel2001, 80, 19531962.(24) Abad, A.; Adanez, J.; Garcia-Labiano, F.; de Diego, L. F.;

Gayan, P.; Celaya, J. Chem. Eng. Sci. 2007, 62, 533549.(25) Hossain, M. M.; de Lasa, H. I. Chem. Eng. Sci. 2008, 63, 4433

4451.(26) Fang, H.; Haibin, L.; Zengli, Z. Int. J. Chem. Eng. 2009, 2009,

116.(27) Adanez, J. Oxygen Carrier Material for Chemical Looping

Processes - Fundamentals. 1st International Conference on ChemicalLooping, Lyon, France, 2010; keynote address.


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oxide. In the case of Mn2O3, excess reductant led to formationof MnO, but not elemental Mn. MnO is, in fact, the chosenreduced state in some CLC studies,19,28 but the choice ofMn3O4 was based on relative heats of oxidation and reduc-tion reactions, as explained earlier. As expected, reduction ofCeO2 and Mn2O3 is always thermodynamically possible atall temperatures above 300 K. The reduction of NiO to Ni is,however, not possible below the corresponding Teq,2 valuesin Table 5 when using CH4 or iso-octane. The fractions ofNiO at equilibrium under reducing conditions are plotted inFigure 8. Less noticeable in theplots is thefact that reductionseems limited due to the presence of CO at equilibrium athigher temperatures when using CH4 or iso-octane.

The entropies of reduction reactions, as mentioned pre-viously, are always positive, so, the sign of Teq,2 can be usedto determine if a reduction reaction is exothermic or endo-thermic. A positive Teq,2 indicates an endothermic reaction,whereas a negative Teq,2 indicates an exothermic reaction.Lower values ofTeq,2 indicate more exothermicity of reduc-tion. It is worth noting that no fuel-material combinationhas a Teq,2 that is high enough to thermodynamically limitthe reduction reaction.

At equilibrium, both reactants and products coexist.Complete oxidation is not possible unless the higher metaloxide is being removed continuously. Another option toachieve complete conversion is to run the oxidation reactorat a T

1slightly less than T

eq,1. To prevent melting, T

1also

needs to be lower than the melting points of both solid-phasecompounds. These two constraints provide an upper limitfor T1. Most of the oxygen-carrier materials used in theanalysis of McGlashan2 have Teq,1 and Teq,2 that are thermo-dynamically convenient for CLC but are higher than theirrespective melting points. As a result, those materials cannotbe considered for practical implementation of CLC orSCOT. Among the materials chosen here, the oxidationtemperature is limited by the melting point only for Ni/NiO (which melts around 1700 K).

In the results shown below, a series of T1 values areselected subject to these constraints and T2 is varied betweenT0 and T1. Potential kinetic limitations are not consideredhere. The exergy budget is computed with contributionsfrom work, irreversibility of reactions (which includes heat-ing/cooling thereactants in both phases to thetarget reactiontemperature as well as entropy generation due to the reac-tions themselves), preheater irreversibility, and residual ex-ergy in the exhaust streams. The last component is alwaysnonzero for SCOT because the two exhaust streams havedifferent compositions. Even if both are at ambient tempera-ture, the composition difference between these unmixedstreams has some exergy associated with it.

For comparison, the second law efficiencies associatedwith unrestrained (nonpreheated, noncompressed) gas-phase stoichiometric combustion of the three fuels underconsideration here are listed in Table 6. These efficiencies areassociated with corresponding adiabatic flame temperatures,which are all higher than 2200 K.

The efficiency plots for SCOT using Ce2O3/CeO2 with thethree fuels are shown in Figure 9. For comparison, thesecond law efficiencies associated with preheated, isother-mal, gas-phase combustion are also included. The peakefficiency always occurs when T2 is equal to T1 for reasonspointed out earlier for cases where both oxidation andreduction reactions are exothermic. Also, peak efficiencyincreases with increasing T1 (due to the nature of the Carnotefficiency) except when equilibrium starts to limit the extentof oxidation reaction. This is evident from the drop inefficiency as T1 is increased from 1400 to 1600 K. At 1600K, oxidation is equilibrium-limited and so about 14% of thefuel exits the reactor unutilized. Higher efficiency could beachieved by reducing the amount of fuel fed to the reactor towhat is actually consumed in reaching equilibrium.

As seen from the plots, slightly higher than conventionalefficiencies (shown in Table 6) are achieved using SCOT forall fuels when T1 is close to the equilibrium temperature. Theefficiency advantage would further be enhanced if heat lossesare included in the analysis given that the peak temperaturesfor the SCOT cases analyzed here are far lower than the

Figure 7. Equilibria of oxidation reactions. The equilibrium predic-tions aremade with thereduced state of oxygencarrier material and

just enough amount of air for its complete oxidation.

Table 5. Equilibrium Temperatures (in K) of Reduction Reactionswith Various Fuels

fuel


CeO2 -1461 -329 -307NiO -24 411 317Mn2O3 -3717 -1168 -995

Figure 8. Equilibria of NiO reduction reactions. The equilibriumpredictions are made with the fully oxidized state of oxygen-storagematerial and just enough reductant required for complete reductionto the desired reduced state.

Table 6. Second Law Efficiencies of Unrestrained Gas-Phase Stoi-chiometric Combustion of the Three Fuels

fuel 2

H2 0.77CH4 0.70iso-octane 0.69

(28) Ryden, M.; Lyngfelt, A.; Mattisson, T.; Chen, D.; Holmen, A.;Bjrgum, E. Int. J. Greenhouse Gas Control2008, 2, 2136.


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adiabatic flame temperatures (the peak temperatures duringconventional combustion) of the fuels.

For comparison, the efficiencies achieved through isother-mally constrained gas-phase combustion are also shown inthe plots. Excluding the differences in preheating, the effi-ciencies should be nearly the same as in SCOT when T1equals T2. The plots show a slightly higher efficiency for

isothermal gas-phase combustion. This is due to the fact thatthe enthalpy in the N2 exhaust from the oxidation reactor isnotsufficient to preheat theinletair to T1, sosome ofthe heatgenerated by the reactions is used to bring the oxidationreactor temperature to T1 before heat is drawn from it. Thegaseous products of the reduction reaction have more heatthan what is needed to preheat fuel to T2 (which equals T1),but the excess is not used to help in preheating air. The gas-phase combustion does not have this problem because asingle product stream is used to preheat both air and fuel.

The qualitative trends of SCOT with Mn3O4/Mn2O3,shown in Figure 10, are similar to those observed withCe2O3/CeO2. This is because both oxidation and reductionreactions are exothermic when using either of these materi-als. The peak efficiencies for each value of T1 are alsocomparable to those of preheated, isothermal, gas-phasecombustion at the same temperature. There is, however, adifference that is not fully evident in the efficiency plots. TheCe2O2 oxidation reaction has an enthalpy change that isabove 80% of the overall enthalpy of fuel-air combustion,whereas for Mn3O4, it is near 50%. Therefore, the oxidationreaction is much less exothermic in the case of Mn3O4. Theoxidation reaction starts out with the higher oxide being atT2, and, as explained earlier, ifT2 is much lower than T1, theoxidationreaction involvingsome solid materials may notbeexothermic enough for the oxidation reactor to reach thetarget temperature T1. Therefore, heat needs to be added to

the oxidation reactor using a heat pump in such cases; that is,Q1 is negative. The work in such cases is generated using heatdrawn from the reduction reactor. When T2 is low (and closeto T0), the efficiency of this process is very low and much ofthe heat is rejected to the ambient. As a result, for very lowvalues of T2, the efficiency could even drop to negativevalues, as seen in the plot. When T2 is close to T1 (where

efficiencies are higher), however, both Q1 and Q2 are posi-tive. Peak efficiency increases with T1 until equilibrium limitsthe extent of the oxidation reaction, and fuel utilizationbegins to drop just as for the Ce2O3/CeO2 system.

The efficiency plots for Ni/NiO in Figure 11 show verydifferent trends than those for the other two materials. This isdue to the fact that the NiO reduction reactions are neutral orendothermic for all three fuels. In the case of H2, the reductionis only slightly exothermic andQ2 is positive (this is because thereduction reaction starts out with NiO at T1, which is higherthan the target reaction temperature ofT2). For the other twofuels, Q2 is actually negative and SCOT with these fuels (usingNi/NiO) is close to the chemical looping combustion casewhere an internal engine takes heat from the oxidation reactorand rejects heat to the reduction reactor. For such cases, theoptimal efficiency using CLC is achieved by operating thereactors at corresponding equilibrium temperatures. Thisseems to be true for SCOT even when, unlike CLC, thesolid-phase reactants in both reactors start out at temperaturesdifferent from the intended target temperatures.

The peak efficiency for all values of T1 occurs when T2 iscloser to the equilibrium reduction temperature than in thecase of the other two oxygen carrier materials considered. AsT1 approaches Teq,2, the extent of the reduction reaction andfuel consumption become equilibrium-limited, as evidentfrom Figure 8. The efficiency drops as a result of unusedfuel exiting the reactor.

Figure 9. Comparison of secondlaw efficiencies of SCOT using Ce2O3/CeO2 and preheated,isothermal, gas-phase combustion (symbols). T1 isthe temperature at which the oxidation reaction is isothermally constrained.


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One noteworthyfactin Figure11 is that the efficiency ofSCOTnever actually exceeds that of preheated, isothermal, gas-phasecombustion forcomparablepeak temperatures. It is important torecognize that the abscissa is the reduction temperature and not

thepeak temperature forSCOT, whereasin thecaseof preheated,isothermal, gas-phase combustion, it is the peak temperature.

In summary, SCOT provides a way to harness some of theefficiencyadvantages (over unrestrainedgas-phasecombustion)

Figure 10. Comparison of second law efficiencies of SCOT using Mn3O4/Mn2O3 and preheated, isothermal, gas-phase combustion (symbols).T1 is the temperature at which the oxidation reaction is isothermally constrained.

Figure 11. Comparison of second law efficiencies of SCOT using Ni/NiO and preheated, isothermal, gas-phase combustion(symbols). T1 isthetemperature at which the oxidation reaction is isothermally constrained.


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of CLC without the problems associated with moving solidsand heat exchange between them. Although the efficiency ofSCOT is lower than that of isothermal, preheated, gas-phasecombustion, the former does offer some practical advan-tages. SCOT has a natural barrier against thermal runawaybecause the oxidation reaction, during which much of theheat release occurs, becomes thermodynamically limited asthe temperature increases beyond its equilibrium tempera-ture, which, in the case of most material-fuel combinations,is well below the adiabatic flame temperature. There is alsono lean flammability limit associated with SCOT. There are,however, some practical problems with SCOT. It is not asteady-state process. Thermal stresses and fatigue of solid-

phase materials resulting from temperature swings can posea challenge for long-duration use. In the case of SCOT withan internal engine, heat needs to be removed and added atdifferent times to the same location. The phasing of suchalternating heat transfer is hard to achieve. These problemsare not so acute if temperature swings are minimized andperfectly isothermal SCOT can be achieved. Two of theoxygen-carrier materials seem to be good candidates forsuch a process.

Alternative to CLC: Thermochemical Recuperation (TCR).As discussed above, some of the efficiency benefits of CLCand SCOT are derived from the use of an internal heatengine between the two reactors, thereby avoiding or mini-mizing heat rejection to the ambient. There may be otherprocesses that can be used to achieve this effect. Consider,for example, the schematic of thermochemical recuperationillustrated in Figure 12. The process is similar to preheated,isothermal combustion except that the fuel is reformed tosyngas during preheating. The fuel stream needs to includewater for steam reformation when using fuels other thanmethanol. Given that hydrocarbon steam reformation isendothermic, the preheater requires more heat than what isavailable from the combustor exhaust gases. This heat isprovided by an internal heat engine that is drawing heatfrom the isothermal combustor at temperature (TC) andrejects heat (QR) at temperature TR to the fuel preheater/reformer.

We first provide a simplified analysis of this novel process;a more detailed exploration with temperature-dependentthermodynamic properties and iso-octane fuel follows.

Just as in our simplified analysis of preheated, isothermalcombustion andCLC, we assume that there is enoughheat inthe combustion products to preheat both reactant streams tothe target combustion temperature (TC). The additionalheat, QR, coming from the internal heat engine is assumedto equal the heat of the reformation reaction. Thus, the

combination of the heat in the combustion products and QRis sufficient to facilitate entry of air and syngas into thecombustion chamber at temperature TC. The heat of thesyngas combustion reaction would, therefore, equal -HQR. This amount of heat needs to be removed from thecombustion chamber on a continual basis for maintainingisothermal conditions. Assuming that the internal engineoperates at Carnot efficiency, the following relationships areeasily derived.

Wint TC

TR-1

QR 20

Wext -H 1-TC

TR QR" # 1- T0

TC 21

If TR is chosen as per the following equation, there is noneed for an external engine.

-H 1-TC

TR

QR 0 22

All the work is generated by the internal engine, and itequals-H, which means a 100% first law efficiency. This,as stated previously, is possible if the exergy of the fuel ishigher than its heating value, which is the case with most ofthe hydrocarbon fuels.

The assumptions made in the above simplified analysismay, however, be invalid in practice.The temperatures of the

two product streams as they exit the two preheaters ( T00 andT0

00 ) in Figure 12 may be different from ambient temperaturefor the preheating and reforming to be compliant with bothlaws of thermodynamics. To explore this possibility, weperform a simple analysis of the process with iso-octane fuelfor a range of combustion-chamber temperatures. For eachcombustion-chamber temperature, a range of TR values ischosen and overall second law efficiencies are computed. Itturns out that, for each value ofTC, there is a minimum valueofTR below which the preheating and reforming of the fuelstream to syngas is impossible for any value of QR. Thisminimum value of TR varies little with TC and is roughlyaround 640-650 K. This, coincidentally, is roughly thetemperature at which the degree of steam reformation of

iso-octane at equilibrium reaches 50%.29

Figure 13 shows the dependence of overall second lawefficiency on TR for a range of TC values. The efficiency ishighest when TR is close to the minimum possible tempera-ture (of 650 K) for all values of TC. This is because thefraction of combustion heat (which is fixed for a given TC)extracted by internal engine is highest in this limit. Thoughthe efficiency of the internal engine is lower, the heat rejectedis utilized internally within the system for reformation. Onthe other hand, heat rejected by the external heat engine is

Figure 12. Simplifiedschematicof proposed isothermal combustionwith reforming. This process differs from preheated, gas-phase,isothermal combustion in that fuel in it is reformed in the process ofgetting preheated and also the heat rejected from the heat engine isutilized (in addition to heat from combustion products) for pre-heating.

(29) Chakravarthy, V. K.; Daw, C. S.; Pihl, J. A.; Conklin, J. C.Energy Fuels 2010, 24, 15291537.


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lost to the surroundings. The exergy loss in the fuel pre-heater/reformer also decreases as TR decreases. It ap-proaches zero as TR approaches the 650 K. Therefore, the

preheating losses are also lower in this limit. The onlysignificant exergy loss then is due to combustion of thereformate.

The optimal efficiency (with TR fixed at 650 K) is plottedas a function ofTC in Figure 14. Also shown in this figure forcomparison are the efficiencies associated with preheated,isothermal, gas-phase combustion of iso-octane (withoutreformation) and H2. The efficiency with thermochemicalrecuperation is higher because of lower combustion reversi-bility of syngas when compared to that of iso-octane. Com-bustion irreversibility and preheater losses are the onlyexergy losses in preheated, isothermal combustion, with theformer being the most significant when preheating is doneoptimally. Efficiency increases with temperature becausecombustion irreversibility decreases with temperature. Be-cause H2 and syngas have similar combustion irreversibil-ities, when reforming is done optimally (with TR close to650K), iso-octane canbe combustedwith thesame efficiencyas that of preheated, isothermal H2 combustion. This isevident from Figure 14. At lower combustion temperatures,however, differences between combustion efficiencies ofreformed iso-octane and H2 are evident. This is because thereformation is thermodynamically limited at lower tempera-tures and is complete only around 1000 K.

29Note that TR is

the temperature at which heat is supplied to the reformer/preheater but is not the temperature at which reformingoccurs; that is, reforming is not an isothermal process.

We have previously analyzed and reported thermochemi-cal recuperation for reducing irreversibility in an earlierstudy.29 There are several differences between the previouslyanalyzed concept and the one we propose here. The combus-tion in theearlier concept was adiabaticinstead of isothermalas in the present instance. The reformation in the previousstudy was analyzed under far from ideal conditions. Here, wehave attempted to study reformation in the limit of minimalexergyloss (which occurs when TR approaches 650 K). There

was also an intercooler in the previous study thatwas used tocool the reformate (to keep the peak temperatures low), andthis cooling introduces an exergy loss into the system. As aconsequences of all these differences, the efficiencies re-ported here are much higher. The practical problems asso-ciated with reforming pointed out in our earlier study alongwith the difficulty in maintaining isothermal combustion aresignificant challenges to be overcome to realize the presentconcept.

Summary and Conclusions

The main thermodynamic efficiency benefits of CLC de-pend on maintaining the isothermal reaction (and associated

heat release and work generation) and low-temperature-gra-dient heat exchange between the reactants and products ofboth the oxidation and the reduction stages. In the absence ofthese features, CLC only provides a way of generating twoexhaust-gas streams with different temperatures and compo-sitions. Although beneficial for carbon sequestration, ex-haust-gas partitioning alone does not offer a significantexergy savings over conventional unconstrained combustion.

For CLC, oxygen carriers that support endothermic reduc-tion reactions produce higher efficiencies because of theinternal heat sink that is made available as an alternative torejecting heat to the environment. For oxygen carriers thatinvolve exothermic reduction reactions, it is not generallypossible to achieve a feasible equilibrium temperature in the

reduction reactor. Nevertheless, it appears that the efficiencycan still increase as the reduction reactor temperature in-creases, and the reaction becomes more irreversible. Thus, itappears that requiring the reactions to occur closer to chemicalequilibrium is not, by itself, sufficient to improve efficiency.

The need for transporting large masses of oxygen-carriersolids and small-temperature gradient heat exchange betweencounterflowing oxygen-carrier solid streams and betweenvolumetrically unbalanced gas streams seems to be one ofthe biggest practical drawbacks in implementation of CLC. Amodified form of CLC that utilizes a fixed bed of solids(referred to here as SCOT) overcomes the need for transport-ing solids. However, SCOT suffers from a significant exergypenalty associated with heating and cooling the stationaryoxygen-carrier solids during each gas modulation cycle. It isnot clear how this drawback can be overcome when endo-thermic and exothermic reactions at different temperaturesare required.

Isothermal combustion with simultaneous work generationand carefully constrained heat exchange between productsand reactants are the keys to achieving most of the efficiencybenefits expected from CLC. It appears likely that these effectscould be achieved without the need of circulating oxygen-carrier solids or utilizing impractical oxygen-storage materialsthat have the endothermic properties required by CLC.Analysis of the simple isothermal process proposed hereindicates that overall efficiencies similar to those projected

Figure 13. Dependence of isothermal combustion with preheated,reformed reactants on operating temperatures.

Figure 14. Variations of efficiencies of isothermal combustion ofiso-octane with and without reforming during preheating withcombustion temperature. Efficiency variation of preheated, iso-thermal combustion of H2 is also shown for comparison.


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for CLC can be achieved with relatively modest peak combus-tion temperatures.

One of the most challenging aspects of isothermal combus-tion is likely to be simultaneous matching of heat generationand work output required to maintain constant temperature.It is to be noted here that this idea was analyzed computa-tionally in past studies30-32 that reported no efficiency bene-fits. That was due to the fact that there was no preheating ofreactants in any of thesestudies and therewas no exhaustheat

recovery by any other means. Constant-temperature combus-tion might be achieved practically with an intelligent combus-tion and engine control system, but this may be extremelydifficult in a load-following context. On the other hand, it maybe possible to usea modified SCOT reactor that operates withan oxygen carrier having both a highly exothermic oxidationreaction and a mildly exothermic reduction reaction. Materi-als such as cerium oxide might be feasible oxygen-storagecandidates in this case. Duringthe oxidation phase, the chemi-cal equilibrium of the oxidation reaction would effectivelylimit heat release and prevent thermal overshoot when workextraction is diminished.

Combining TCR with isothermal reaction appears to offeran additional way to obtain some of the theoretical exergy

benefit of CLC. Like the endothermic reduction reaction inCLC, hydrocarbon reforming can provide an internal heatsink for generating work that does not result in external heattransfer to the environment. The TCR concept itself has beenof interest also for a number of years, butas far as we know, itssimilarity to this aspect of CLC has not been recognized.When combined with an isothermal reaction and heat-trans-fer optimization, TCRappears to offer importantadvantages.Together, these advanced combustion approaches may be

able to achieve second law efficiencies 10-

15% higher thanthose that are currently possible.

Acknowledgment. Thisresearch was initially sponsored by theLaboratory Directed Research and Development Program ofOak RidgeNational Laboratory,managedby UT-Battelle,LLC,for the U.S. Department of Energy and continued later withsupport from the U.S. Department of Energy (DOE) underContract No. DE-AC05-00OR22725 with the Oak Ridge Na-tional Laboratory, managed by UT-Battelle, LLC. The authorsspecifically thank Gurpreet Singh of the DOEs Office of VehicleTechnologies for sponsoring this work. The authors also thankDrs. K. Dean Edwards and Charles Finney of Oak RidgeNational Laboratory for enlightening discussions and insights.This research was sponsored by the US Department of Energy

under contract number DE-AC05-00OR22725 with the OakRidge National Laboratory, managed by UT-Battelle, LLC.The authors specifically thank Gurpreet Singh of DOEs Officeof Vehicle Technologies for sponsoring this work. The authorsalso thank Dr. K. Dean Edwards and Dr. Charles Finney of OakRidgeNationalLaboratory for enlighteningdiscussions and insights.

(30) Rice,M. J. M.Sc.Thesis, Virginia Polytechnic Institute and StateUniversity, Blacksburg, VA, 2004.

(31) Druecke, B. C. M.Sc. Thesis, University of Wisconsin, Madison,WI, 2006.

(32) Teh, K.-Y.;Edwards,C. F. J. Dyn. Syst.,Meas., Control2008, 130,130-139.

Chakravarthy_Thermodynamic Analysis of Alternatives to CLC

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