Ca0.9Mn0.5Ti0.5O3−δ: A Suitable Oxygen Carrier Material for Fixed-BedChemical Looping Combustion under Syngas ConditionsMehdi Pishahang, Yngve Larring,* Michael McCann, and Rune Bredesen

SINTEF Materials and Chemistry, Sustainable Energy Technology Sector, P.O. Box 124, Blindern, NO-0314 Oslo, Norway

ABSTRACT: Power generation using chemical looping combustion (CLC) technology has emerged as a promising CO2-capture-based alternative to conventional technology. In this study, the performance of a Ca0.9Mn0.5Ti0.5O3−δ perovskite-typeoxygen carrier material for use in fixed-bed CLC reactors is investigated. The main focus of the study is on the material’s oxygen-carrying capacity and reactivity with and conversion of syngas in the fixed-bed reactor. Pressurized thermogravimetric analysis inmodel gases indicates neither Boudouard coking nor reactivity of the oxygen-carrier material toward CO2. A fuel gas conversionof 95% was achieved in the fixed-bed reactor using this oxygen-carrier material.

■ INTRODUCTIONAtmospheric concentrations of CO2 have risen drasticallyduring the past century, and it is widely acknowledged thatincreased human-related emissions of CO2 contribute toclimate change. Fossil-fuel power plants are globally responsiblefor the biggest fraction of CO2 emission,1 and future demandsfor sustainable energy production can only be met using carboncapture and storage (CCS) as part of the solution. Presently,CO2 capture represents the main cost of the CCS chain, andfurther research is required to lower the cost significantly.Technologies for large-scale power generation and inherent

CO2 capture are usually categorized in three classes, namely,postcombustion and precombustion CO2 capture processes andoxyfuel combustion.2 In postcombustion CO2 capture, air is usedas the oxidizing agent in the combustion of fossil fuel and CO2is captured from the flue gas, which contains a relatively highconcentration of N2. This CO2/N2 separation is coupled with asignificant economic penalty. For precombustion techniques,where the fossil fuel is converted to a mixture of CO2 and H2,significant cost is related to CO2 separation upstream of the gasturbine. An advantage of this process is the possibility ofcombined H2 gas and power production. The sorption-enhanced water−gas shift3,4 and membrane-assisted water−gas shift5,6 processes are among the most promising processesfor combined H2 production and power generation with CO2capture. In oxyfuel processes, oxygen is used as the oxidizingagent and resulting flue gas, with which a high concentration ofCO2 can be separated from H2O simply by condensation. O2production by conventional cryogenic air separation is stillcostly in addition to the fact that the infrastructure forcryogenic O2 separation is highly priced.7

Chemical looping combustion (CLC) is an oxyfuel processwith the potential for cost-effective power generation incombination with CO2 capture. The chemical looping conceptis based on the idea of a combustion reaction in two separatesubprocesses;8,9 the first is O2 capture from an air stream usingan oxygen chemisorbent (often named “oxygen carriermaterial”, OCM), and in the second subprocess, the OCMreleases oxygen in the fuel stream in order to achievecombustion. The (often endothermic or slightly exothermic)reduction of the OCM with fuel during combustion yields CO2

and steam as the products, which can simply be separated bycondensation. In the second step, the reduced OCM istransported to the air reactor, where it is reoxidized by air.Oxidation of the OCM is exothermic, resulting in a hot airstream lean on O2, which is used for power generation. CLChas the additional advantage of eliminating problems associatedwith the thermal formation of NOx as nitrogen is not exposedto extremely high temperature flames.Coal-employed CLC has been focused on lately and reported

as very competitive, with only a 2.5% ideal CO2 capturepenalty10 and a calculated net efficiency of 43−45% for acombined cycle with CLC with a turbine inlet temperature of1200−1350 °C.11 Reaching the lowest CO2 capture penaltyrequires an OCM, which can fully deliver the required amountof oxygen for complete combustion, thus avoiding the need fora secondary oxygen supplier (e.g., air separation unit). Higherefficiencies can also be achieved by pressurizing the process andtherefore adding the possibility of including a gas turbine in theprocess, as notably implemented in natural gas combined cycle12

and integrated gasif ication combined cycle13 processes. Althoughpressurization of the process can technically be achieved innearly all CLC technologies, it is less convenient for fluidizedcirculating beds. One approach to ease the challengesassociated with pressurization is the selection of a fixed-bedreactor in which the reactor volume is significantly reduced andthe packing density of the OCM is high. In the alternating f ixed-bed reactors,14 transport of the OCM between the reactors isavoided. Instead each reactor repeatedly becomes a “fuelreactor” and an “air reactor” when fuel and air are introduced tothe reactor periodically.15 The CLC alternating fixed-bedreactor system is schematically presented in Figure 1.

Ca1−yMn1−xTixO3−δ (CMT) as the OCM. Perovskite-typeoxides (with the general formula ABO3−δ) are known to be avery promising family of materials to be used as the OCM forCLC.16,17 The oxygen nonstoichiometry, δ, varies depending

Received: March 4, 2014Revised: May 16, 2014Accepted: June 6, 2014Published: June 6, 2014

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on the partial pressure of oxygen in the surroundings andtemperature.18−22 In isothermal conditions, as the surroundingoxygen partial pressure decreases, the defect concentrationincreases by releasing oxygen to the gas. CaMnO3−δ shows verypromising results because of its fast reduction at relatively highpartial pressures of oxygen.23 It is, however, previously shownby Bakken et al.24 that at 1000 °C the CaMnO3 materialcontinuously loses oxygen when the oxygen partial pressure isreduced from 1 to 10−5 atm, and the material reduction(oxygen release) is compensated for by the reduction of Mn4+

to Mn3+. They also show that this material tends to decomposeto Ca2MnO4 and CaMn2O4 at higher temperatures.24 Partialdecomposition might restrict the full recovery of material byquick oxidation. In fact, CaMnO3−δ releases its oxygen capacitytoo easily for fixed-bed applications (at relatively high oxygenpartial pressures). This leads to the release of oxygen to theinert gas when the OCM bed is flushed during thepostoxidation purge step. It is therefore crucial to adjust theoxidation enthalpy, reflecting the oxygen partial pressure rangewhere the material releases oxygen. One way to tailor thisthermodynamically fundamental property of the material is byadjusting its chemical composition.16,25−27 The addition oftitanium to the B-site sublattice in CaMnO3−δ stabilizes thestructure because titanium is more redox-stable (higherenthalpy of oxidation) and helps to keep the perovskitestructure at lower partial pressures of oxygen.28 Partialsubstitution of manganese with titanium at the B-site sublatticestabilizes the structure chemically but at the cost of a reducedoxygen capacity. This disadvantage, however, is offset by thefact that the reduced oxygen capacity helps to prevent thedetrimental decomposition during the reduction reaction.Selection of the OCM is the most important aspect in the

development of CLC processes. The OCM should possessadequate reduction−oxidation (redox) reactivity, structuralstability, and a reasonable capacity of oxygen. In previouspublications, we have developed and investigated the redoxthermodynamic properties of several perovskite-related OCMssuitable for CLC applications.20,21,28−30 The CaMn0.8Ti0.2O3−δ

composition shows CO2 uptake in a gas stream containing apartial pressure of CO2 equal to 4 atm due to the marginalformation of secondary phases. The stability of CMT towardCO2 may be improved by lowering the calcium content andincreasing the titanium content. Cation nonstoichiometry is aknown phenomenon for CaTiO3, where the ratio of Ti/Caexceeds unity, i.e., Ca1−xTiO3−δ.

31 An intentional decrease inthe calcium content inhibits the formation of secondary phasesbecause of this phenomenon. CaTiO3 and CaMnO3 are verysimilar in both terms of the formation enthalpy and theGoldschmidt tolerance factor.32 Still, they show different redoxbehaviors due to the difference in the redox characteristics oftitanium and manganese. Unlike CaMnO3, CaTiO3 does notshow oxygen nonstoichiometry and, therefore, does notparticipate in the redox reaction, thus keeping the structuretogether during the deep reductions that the solid solutionCMT undergoes. By acting as the skeleton in the solid solution,replacement of manganese by titanium in the solid solutionincreases the structural stability and restricts the reactivity ofsecondary phases with CO2.In this paper, we report the results of the performance of

stabilized Ca0.9Mn0.5Ti0.5O3−δ as OCM for fixed-bed CLC. Theexperiments are conducted in an in-house fixed-bed reactorsuitable for alternating the inlet gases in cycles of air−inert−fuel−inert−air. The experiments are performed under syngasconditions. Because the syngas consists of more than one activereducing agent, several chemical reactions may occur during thereduction and oxidation steps. The main reactions (1)−(3) arelisted below:

During the reduction:

+

⇌ +δ−

Ca Mn Ti O (s) CO(g)

Ca Mn Ti O (s) CO (g)0.9 0.5 0.5 2.9

0.9 0.5 0.5 2.9 2 (1)

+

⇌ +δ−

Ca Mn Ti O (s) H (g)

Ca Mn Ti O (s) H O(g)0.9 0.5 0.5 2.9 2

0.9 0.5 0.5 2.9 2 (2)

Figure 1. Schematic flow sheets for the two steps in the alternating fixed-bed CLC reactors.

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During the oxidation:

+

⇌δ−Ca Mn Ti O (s) O (g)

Ca Mn Ti O (s)0.9 0.5 0.5 2.9 2

0.9 0.5 0.5 2.9 (3)

Equation 3 is an oxidation reaction, involving a decrease in theentropy but the creation of some new bonds, which makes thisreaction exothermic. Reactions (1) and (2) are the sums of twosubreactions, the endothermic reduction of the OCM and theexothermic oxidation of CO to CO2 or H2 to H2O, andtherefore the net reaction can be either exothermic orendothermic depending on the oxidation enthalpy of the OCM.

■ EXPERIMENTAL PROCEDURE

The CLC experiments were conducted in a vertical fixed-bedreactor, schematically presented in Figure 2. The fixed-bedreactor consists of a boiler-grade stainless steel (MA253) tube(o.d. = 34 mm, i.d. = 18 mm, and l = 600 mm), which isresistant to temperatures as high as 1100 °C. A set of axialthermocouples, consisting of 6 K-type thermocouples (Skot-selv, Norway), was inserted into the reactor, which allowedmeasurement of the temperature at six positions along thereactor (40 mm between each). The reactor was positioned in avertical tubular furnace (i.d. = 40 mm and l = 500 mm), whichwas controlled separately by an external controller. A separateK-type thermocouple was used to measure the furnacetemperature, and the thermocouple was placed well belowthe hot zone of the furnace. Thus, the thermocouple’stemperature was not affected by the reactions occurring inthe reactor. As a result, the power output of the furnace

controller was kept constant during the experiment, thereforesetting an invariable temperature profile across the uppersection of the furnace where the OCM was placed.The bottom of the reactor was filled with a penetrable bed

consisting of clay ash (packed in the bottom 300 mm of thereactor). The OCM bed, 200 mm in height, was placed on topof the inert bed in the hot zone of the furnace with an averagebed porosity of ∼60%. A rather dense OCM used was in theform of cylindrical granulates with only 5% porosity (fabricatedby CTI, Salindres, France) and had an average diameter andlength of 2.7 and 15 mm, respectively. The length distributionwas such that 70% of the granulates had a diameter between 10and 20 mm). A high-temperature filter was located above theOCM in order to avoid damaging sensitive downstreamequipment by the potential fines abraded from the granulatesand carried in the gas stream.A gas mixing system consisting of six mass flow controllers

(MFCs) and two back-pressure controllers (BPCs) was used tosupply the reactive gas mixtures in the reactor and control thereactor’s pressure. This gas mixing system allowed mixtures ofargon (Ar) and O2 with a total flow of 4500 mL/min and Ar,CO, and H2 with a total flow of 900 mL/min for oxidizing andreducing gas mixtures, respectively. An automatic alternatingfour-way valve (VICI, Norway) was used to switch betweenoxidizing and reducing conditions. For safety reasons, a manualpressure relief valve was installed on the gas feed line with anopening pressure of 10 atm, in order to vent the entire feed gasto the exhaust in case the reactor or any of the outlet gas linesbecame clogged. In order to avoid mixing of the oxidizing andreducing gases, an inert purge with 1000 mL/min of Ar was

Figure 2. Schematic view of the fixed-bed reactor apparatus and the auxiliary components.

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used between the oxidizing and reducing periods. In this study,the inert part of the syngas, N2, is replaced by Ar in order todifferentiate between the inert part and CO in the massspectrometer signal (N2 and CO have very similar molecularweights of 28 g/mol). A separate liquid MFC was used tocontrol the required flow of H2O into the reactor. The feed gasto the reactor was preheated to 200 °C by heating tapes inorder to make sure that the H2O supplied by the liquid MFC isevaporated completely before entering the reactor. Preheatingalso decreases the temperature change in the reactor because offorced convection caused by forcing the cool gas into the hotbed. The fixed-bed experimental procedure consists of periodicreduction and oxidation cycles; the experimental conditionsused are summarized in Table 1.

An in-house LabView program was used to control theexperiment and record the data from the MFCs and BPCs, aswell as log the temperatures, pressures, and mass spectrometerdata. A Spectra Mini-Lab mass spectrometer (MKS Instru-ments) was used to analyze the outlet gas of the reactor. Themass spectrometer was calibrated prior to experiments withcorresponding gas mixtures.A pressurized thermogravimetric analyzer with magnetic

suspension (Rubotherm, Bochum, Germany) was used to studythe reduction and oxidation capacities of the OCM underisothermal conditions at 700, 900, and 1100 °C. The totalpressure was constant at 10 atm during the whole experiment.

The total flow was kept constant at 1000 mL/min. Eachreduction was followed by an oxidation in 20% O2 for 30 min.Ca0.9Mn0.5Ti0.5O3−δ (fabricated by CTI, Salindres, France)

was used as the OCM. A paste corresponding to the finalcomposition was made from the raw minerals and directlyextruded to the form of cylindrical granulates. The granulateswere sintered at 1300 °C for 5 h. Ca0.9Mn0.5Ti0.5O3−δ has theperovskite structure, and the XRD shows asingle phase withonly traces of the minor secondary phase in the material.

■ RESULTS AND DISCUSSIONFigure 3 shows the thermogravimetric results giving thereduction and oxidation capacities of the OCM underisothermal conditions at 900, 1100, and 700 °C.The only cation taking part in the redox reaction is Mn ions,

which under oxidizing conditions mostly have a charge of 4+,giving an oxygen content of 2.9 (Ca0.9Mn0.5Ti0.5O2.9). Duringthe reduction, the calcium and titanium ions remain unreacted,and the Mn4+ cations are reduced to Mn3+. If all of the Mn4+

ions are reduced to Mn3+, the chemical composition becomesCa0.9Mn0.5Ti0.5O2.65 and the corresponding weight loss becomes3 wt %. From Figure 3, it can be estimated that the material’scapacity is slightly more than 3 wt %, indicating the furtherreduction of some Mn3+ to Mn2+. As is also seen from Figure 3,the material recovers its oxygen capacity fully during a fastoxidation.Another finding of this experiment is the resilience of this

OCM toward reactive coking. Because a significant portion ofthe reactive gas consists of CO, the formation of soot in thereactor according to the Boudouard, as shown by reaction (4),is also a possibility; however, no indication of soot formation isobserved even though the reducing gas is charged completelydry (Figure 3). The soot formation could have been revealed asa weight increase in TG and as CO2 release under reoxidationof the fixed-bed experiment.

⇌ +2CO(g) CO (g) C(s)2 (4)

Table 1. Experimental conditions in the fixed bedexperiments

parameter range

pressure P (atm) 1−7oxidation temperature (°C) 400−800reduction temperature (°C) 400−800inlet oxygen fraction (oxidation) (%) 20inlet CO and H2 fraction (reduction) (%) 60 and 20inlet H2O fraction (reduction) equal to molar CO + H2

Figure 3. Weight change imposed by cyclic reduction and oxidation in air of Ca0.9Mn0.5Ti05O3−δ at different temperatures and gas compositionsunder high pressure (10 atm). No stability issues toward CO2 were observed.

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Figure 4 shows the temperature increase evolution during theoxidation and reduction (inner box) reactions. During theexperiment, the initial bed temperature (T1−T6) was between700 and 750 °C. As observed in Figure 4, both the oxidationand reduction reactions are exothermic. The oxidation is,however, far more exothermic than the reduction, as expected.During the oxidation step, the temperature is increased by asmuch as 230 °C, resulting in outlet gas temperatures as high as950 °C. Figure 5 represents the temperature increase evolution

in a more elaborate way. The 3D plot in Figure 5 shows howthe temperature increase varies with time and reactor length.The horizontal lines in this figure represent the thermocouplepositions, and the temperature between the thermocouplepoints is extrapolated linearly. The gas flow used is in theregime of plug flow, which is mainly due to the fairly longtubular reactor with a significant ratio of length/diameter. Aplug-flow regime is crucial to ensure that the gas is pushed outof the reactor as it is periodically switched between theoxidizing and reducing gases. It ensures that these gases will notmix within the reactor, both for safety reasons and to preventthe fuel from being diluted with nitrogen from air (which is themain idea behind CLC).Another interesting observation from Figure 5 is the time

frame in which the energy is released during the oxidation andreduction steps. It is evident that a significant portion of theenergy during the oxidation is released in only 200 s. The gasvelocity is 0.137 m/s (corresponding to a flow of 4500 mL/min), and thus the oxidation kinetics of the reaction are fastenough such as not to be the controlling factor in the reactionrate; rather, the physical availability of oxygen is the controllingparameter in the oxidation kinetics. This is more pronounced inFigure 6, which represents the maximum temperature achievedat three different gas speeds. As illustrated in the inset of Figure6, the maximum achieved temperature increase during theoxidation is almost linearly proportional to the oxidizing gasspeed.Unlike many other OCM systems (such as ilmenite), the

reduction reaction is also exothermic (as shown in Figures 4and 5). Although the maximum temperature increase observedduring the reduction is only 30 °C, this is still very beneficialbecause it can at least help maintain the reactor temperaturethroughout the reduction step. The slightly exothermic natureof the reduction step avoids several challenges related to strongendothermic reactions such as reduced conversion related tocooling of the bed and unreacted fuel in the effluent gas stream.The reaction rate of the reduction is not as fast as that of theoxidation, which may be attributed to two factors. First, a lowergas speed of 0.0274 m/s was used during this experiment forthe reduction (corresponding to a flow of 900 mL/min).Second, the syngas (i.e., H2 and CO) reactivity is reducedbecause of dilution of the fuel with H2O vapor. This yields ahigher oxygen partial pressure of the fuel and therefore lowerchemical reactivity. In practice, for stationary applications, morethan two reactors could be coupled in parallel with cyclicswitching between the oxidation and reduction modes to adjustfor the difference in reactivity between fuel and air conditions.Another finding of this investigation is the effect of the

starting bed temperature on the maximum temperature increaseand the maximum temperature reached during the oxidation.The results are summarized in Figure 7. Here the results forCa0.9Mn0.5Ti0.5O3−δ are compared with our previous results foranother material of the same family, CaMn0.8Ti0.2O3−δ in similarand comparable conditions. It should be noted that the resultspresented in this figure are obtained from breakthroughreduction experiments at the same bed temperature followedby an oxidation. As this figure shows, the maximumtemperature increase during the oxidation increases withincreased starting bed temperature. The temperature increaseis correlated to the chemical energy release as heat, due to theoxidation enthalpy of the material. The portion of manganeseions (the only cations participating in the redox reaction) issignificantly higher in CaMn0.8Ti0.2O3−δ compared to

Figure 4. Temperature increase evolution during the oxidation andreduction (inner box) reactions. The initial bed temperature is 700−750 °C.

Figure 5. Bed temperature versus time and the reactor length: (a)during the reduction; (b) during the oxidation.

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Ca0.9Mn0.5Ti0.5O3−δ, indicating that the energy capacity ofCaMn0 . 8T i 0 . 2O3− δ i s much higher than that o fCa0.9Mn0.5Ti0.5O3−δ. Therefore, much more energy is expectedto be released during the oxidation of CaMn0.8Ti0.2O3−δ, whichleads to higher values for the maximum temperature increase. Ahigher oxidation enthalpy of Ca0.9Mn0.5Ti0.5O3−δ reduces theexpected impact of having close to twice the oxygen capacity inCaMn0.8Ti0.2O3−δ (the maximum temperature increase is lessthan a factor of 2, with the inlet temperature above 600 °C). Atlower temperatures, the reaction rate of the reduction is nothigh enough, and some of the fuel can leave the reactorunreacted. Post gas analysis by the mass spectrometer duringthe reduction process is plotted in Figure 8 and confirms this

hypothesis. As the temperature increases, the reaction rateincreases, and therefore more of the chemical energy of the fuelis stored in the material, in the form of Mn2+ and Mn3+.Because the oxidation reaction is exothermic, increasing thetemperature leads to less thermodynamic motivation for thereaction to move forward, leading to stabilization of themaximum temperature increase at a maximum value.In order to investigate the chemical reactions in the reactor

during the oxidation and reduction, the gas outlet of the reactoris analyzed by in-line mass spectrometry at different startingbed temperatures. The results are presented in Figure 9. Itshould be noted that H2O in the outlet gas is removed via acondenser before mass spectrometry in order to avoiddamaging the mass spectrometer. As a result, H2O is notvisible in the gas analysis curves in this figure. It is evident that

Figure 6. Highest bed temperature increase during the oxidation at different gas speeds; 4500, 3000, and 1500 mL/min. Inset: Maximumtemperature increase achieved during the oxidation versus gas speed.

Figure 7. Effect of the starting bed temperature on the maximumtemperature increase and the maximum temperature reached duringthe oxidation for Ca0.9Mn0 .5Ti0 . 5O3−δ (CMT0955) andCaMn0.8Ti0.2O3−δ (CMT1082).

Figure 8. Syngas conversion versus starting bed temperature duringthe reduction, using a 200 mm bed height of the granulates.

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the reactivity of the feed fuel increases as the starting bedtemperature increases, and ultimately at temperatures above700−750 °C, the conversion reaches its highest value. In thistemperature range, the hydrogen in the fuel is completelyconsumed, and only a small minimal amount of CO remains inthe flue gas (Figure 9c). The amount of CO in the flue gas was5%, and no hydrogen was detected in the flue gas. Given thevery short length of the OCM bed (200 mm) and the relativelyhigh speeds at which the gases are fed into the reactor, this is asignificant achievement. As expected, the reactivity of H2 ismuch faster than that of CO. The concentration of CO in thefeed is 3 times higher than the concentration of H2, but theaddition of H2O eases the consumption of CO via the water−gas-shift reaction, as illustrated by equation (5). The higherreactivity of H2 compared to CO is due to the higher gasdiffusion rate for H2, as well as the higher thermodynamicdriving force of H2 reacting with O2 compared to CO.

+ ⇌ +CO(g) H O(g) CO (g) H (g)2 2 2 (5)

Because a significant portion of the syngas consists of CO,the formation of soot in the reactor, according to theBoudouard reaction in eq 4, is also a possibility. One problemwith the solid carbon produced during the coking is the damageit will cause to the downstream processing equipment (i.e.,filters, valves, turbine) as well as the steel reactor itself. Thesolid carbon formed during the reduction step will be oxidizedin the oxidation step and lead to unwanted CO2 emission.Mixing the syngas with steam shifts the CO content of thesyngas to CO2 and H2 according to eq 5 and therefore reducesthe coking driving force in the reactor by decreasing the COpartial pressure. In our case, all of the experiments in the fixed-bed reactor were performed with a syngas/steam ratio of 1:1,and it is evident that the Boudouard coking is inhibited in thereactor.

■ CONCLUSIONThe thermogravimetric measurements confirm thatCa0.9Mn0.5Ti0.5O3−δ shows neither any coking under 5 atm ofCO nor any CO2 uptake under 4 atm partial pressure of CO2.Both lowering the calcium content and increasing the titaniumcontent contributed to stabilizing the material. The temperatureincrease during the oxidation for Ca0.9Mn0.5Ti0.5O3−δ is lowerthan that of CaMn0.8Ti0.2O3−δ. This is explained by the loweramount of manganese, which is the cation controlling theamount of oxygen uptake. Because of the high oxidationenthalpy, Ca0.9Mn0.5Ti0.5O3−δ does not give full combustion ofthe syngas; still 95% conversion is achieved.

■ AUTHOR INFORMATIONCorresponding Author*Tel: +47 9828 3956. Fax: +47 2206 7350. E-mail: Yngve.Larring@sintef.no.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe work presented in this article is conducted under theEuropean Union Seventh Framework Programme (FP7/2007-2013) under Grant 268112 (Project acronym DEMOCLOCK).The authors thank CTI for supplying the materials and, inparticular, Eric Louradour and Didier Tournigant for providingthe material used in this investigation.

Figure 9. Flue gas analysis by mass spectrometry at different startingbed temperatures.

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dx.doi.org/10.1021/ie500928m | Ind. Eng. Chem. Res. 2014, 53, 10549−1055610556