Manganese Minerals as Oxygen Carriers for Chemical LoopingCombustion of CoalDaofeng Mei,‡ Teresa Mendiara,*,† Alberto Abad,† Luis F. de Diego,† Francisco García-Labiano,†

Pilar Gayan,† Juan Adanez,† and Haibo Zhao‡

†Department of Energy and Environment, Instituto de Carboquímica-ICB-CSIC, Miguel Luesma Castan 4, 50018 Zaragoza, Spain‡State Key Laboratory of Coal Combustion, Huazhong Univeristy of Science and Technology, Wuhan 430074, People’s Republic ofChina

ABSTRACT: In this paper, four different manganese minerals were tested in a batch fluidized bed reactor to evaluate theirperformance in the chemical looping combustion (CLC) of solid fuels. The char gasification rate in the experiments with themanganese minerals tested was higher than the corresponding values for ilmenite and iron ore, which were taken as benchmarkmaterials. This phenomenon was attributed to the deposition of K, Na, and Ca from the manganese minerals on char particles sothat they catalyzed char gasification. The char gasification rate observed using manganese minerals decreased with the number ofcycles until a stable value was reached. Under these conditions, one of the materials presented higher char gasification rates thanthose observed for benchmark materials even when most of the K was already released. An estimation of the CO2 captureefficiency in a continuous unit using one of the selected manganese minerals showed that around 99% CO2 capture efficiencycould be reached at 1000 °C with a solid inventory in the fuel reactor as low as 300 kg/MWth if an efficient carbon stripper waspresent.

1. INTRODUCTION

Chemical looping combustion (CLC) technology is one of thecarbon dioxide capture technologies with lower cost and energypenalties associated.1 Thus, research on the different aspects ofthis technology has received significant attention during the lastyears. In CLC, combustion takes place without any contactbetween air and fuel. The oxygen required for combustion issupplied by a solid oxygen carrier which takes oxygen from airin the air reactor and then transfers it to the fuel in the fuelreactor, according to what is shown in Figure 1. This allows fora concentrated CO2 stream at the outlet of the fuel reactor,which facilitates CO2 capture. Therefore, the search for anoxygen carrier reactive enough and with good mechanicalproperties, with low attrition and absence of agglomeration to

withstand the successive redox cycles in the chemical loopingsystem, has turned into one of the main priorities for the scaleup of this technology.Materials based on oxides from different transition metals

such as nickel, copper, cobalt, iron, or manganese have beeninvestigated and tested at different scales during last years.1 Inthe case of the combustion of a solid fuel, it is especiallyvaluable that the oxygen carrier used in CLC was a low-costmaterial since part of the oxygen carrier can be lost duringoperation. As it can be seen in Figure 1, when a solid fuel(mainly coal) is combusted in a CLC system the ash generatedshould be periodically drained to avoid its accumulation. In thedrainage process, part of the oxygen carrier can be lost togetherwith the ashes. Thus, it would be desirable that the oxygencarrier was as cheap as possible.Many works have been published in the last years following

the objective of finding low-cost oxygen carriers. Most of themhave been dedicated to iron-based materials from differentorigins. Among the minerals, ilmenite, an iron titanate, has beenintensively used in pilot units from 0.5 kWth to 1 MWth burning

Received: January 19, 2016Revised: April 15, 2016Accepted: May 11, 2016Published: May 11, 2016

Figure 1. Scheme of the CLC process.

Article

pubs.acs.org/IECR

© 2016 American Chemical Society 6539 DOI: 10.1021/acs.iecr.6b00263Ind. Eng. Chem. Res. 2016, 55, 6539−6546

coal.2−8 Other authors evaluated different iron ores9−11 andalso industrial residues.12

According to recent literature studies, some manganeseminerals have shown higher reactivity than ilmenite. A Brazilianmanganese mineral was used as oxygen carrier in a 10 kWth unitwith petcoke as fuel and the results showed that the chargasification rate was four times that obtained under similarconditions with ilmenite.4 The increase in the char gasificationrate resulted in an increase of the CO2 capture efficiency fromaround 70% with ilmenite to 98% with the Brazilian manganesemineral. The value of oxygen demand also decreased to a valueof 10% with this manganese mineral. Nevertheless, a high fine-production rate (1.6%/h) compared to ilmenite was observedfor the Brazilian manganese mineral. Considering these results,different manganese minerals were selected and evaluated asoxygen carriers in a laboratory-scale fluidized-bed reactor in thetemperature range 900−970 °C.13 Six manganese mineralsfrom different origins (including the previously mentionedBrazilian mineral) were tested, and all of them exhibited higherrates of char gasification than ilmenite. Several possibleexplanations were argued. The first explanation would pointto a certain oxygen uncoupling effect in the material, i.e., arelease of oxygen from the manganese mineral due to thetransformation Mn2O3 to Mn3O4, which would favor charcombustion. The second explanation would correspond to thedecreased importance of H2 inhibition in char gasification dueto a higher reactivity of the manganese minerals to the gasesproduced in char gasification (H2 and CO) when compared toilmenite. However, the experimental evidence presented byArjmand et al.13 led to the conclusion that neither of theprevious explanations was the determining mechanism behindthe higher rate of char gasification observed with themanganese minerals compared to ilmenite. A third explanationwas then presented and discussed. These authors propose thatmanganese minerals have a catalytic effect on the chargasification.14 Carbonates, oxides, and hydroxides of potassiumand sodium present in different quantities in all the manganeseminerals analyzed have melting and boiling temperatures closeto the temperature in the fluidized bed. A possible reactionmechanism would assume that these K and/or Na containingcompounds were released in the gas phase and then bound tothe surface of the char. The authors supported this mechanismbased on the uniform distribution of K and Na elements foundon the surface of partially gasified petcoke particles extractedfrom the fluidized bed when the manganese minerals with thehighest amount of potassium and sodium were used. Moreover,these two samples had shown the highest rate of chargasification.Considering all this, manganese minerals exhibit a great

potential to be considered as oxygen carriers in CLC processesusing solid fuels. Nevertheless, the extent of the effect of theirpresence on the char gasification rate should be analyzed as afunction of the redox cycle number in order to consider theiruse in a continuous unit. In line with this, the objective of thepresent work is to further investigate the enhancement of thechar gasification rate when manganese minerals are used asoxygen carriers and compare this behavior with the perform-ance of selected iron-based oxygen carriers taken as reference.The manganese minerals were selected considering theyshowed similar, or even higher, reactivity and even lowerattrition rate values than those showed for a Brazilian Mn-oretaken as a reference Mn-based material15

2. EXPERIMENTAL SECTION2.1. Manganese Minerals and Solid Fuel Used. Four

different manganese minerals were tested in this work. Thereactivity of these materials to the main gases involved incombustion processes (H2, CO and CH4) was already studiedand presented in a previous publication from the authors.15 Thesame identification of the manganese samples used in thisprevious work will be used in the present work. The manganeseminerals were named according to their origin as MnSA fromSouth Africa (SA); MnGBHNE and MnGBMPB from Gabon(GB) together with the abbreviated name of the supplier, i.e.,Hidro Nitro Espanola S.A. (HNE) and Mario Pilato Blatt S.A.(MPB); and finally, MnBR from Brazil (BR). Details about thetreatment of the manganese minerals before being used in CLCexperiments as well as the characterization of these materialscan be found elsewhere.15 Here, Table 1 is included

summarizing the main chemical and physical properties of allthe manganese minerals used. In addition to the fourmanganese minerals, activated ilmenite and iron ore previouslyused in other works from the authors were also used here asbenchmark materials for comparison purposes.6,9 The particlesize of the oxygen carrier materials was in the range 100−300μm in all the cases.Char from South African bituminous coal was used as solid

fuel in all the experiments. The use of char as fuel facilitated theevaluation of the influence of the presence of the differentmanganese minerals on the char gasification rate. Char wasobtained from coal particles in the size range 200−300 μm. Theprocedure followed for char preparation from coal and itscharacterization was described elsewhere.16

2.2. Batch Fluidized-Bed Reactor. The manganesematerials and reference iron-based materials were tested in abatch fluidized bed reactor adapted for the feeding of solid fuelbatches. Figure 2 shows the scheme of the experimental setupused. Details about this setup and the methodology used in theexperiments can be found elsewhere.16,17 The bubblingfluidized bed consisted in all the cases of 300 g of thecorresponding oxygen carrier. Steam was used as gasifyingagent and fluidization medium during the reduction period and

Table 1. Composition and Physical Properties of theManganese Minerals

MnSA MnGBHNE MnGBMPB MnBR

element distribution (wt %)Mn 39.8 46.6 53.0 44.6Fe 14.6 5.1 3.6 5.2Si 3.3 4.2 1.6 4.1Al 0.2 3.8 2.8 4.1Ca 5.3 0.9 0.7 1.0Mg 0.5 0.1 0.1 0.4Ti 0.1 0.1 0.3Na 0.1K 0.8 0.6 1.5Oa 36.2 38.4 37.5 38.9Bulk density (kg/m3) 3510 2570 2900 2230BET surface area (m2/g) 0.6 10.1 7.4 12.2porosity (vol %) 12.3 35.7 30.2 47.3crushing strength (N) 4.6 1.8 2.4 1.0oxygen transport capacity(wt %)

4.7 5.0 5.8 5.2

aOxygen was determined by difference.

Industrial & Engineering Chemistry Research Article

DOI: 10.1021/acs.iecr.6b00263Ind. Eng. Chem. Res. 2016, 55, 6539−6546

6540

it was fed at the bottom of the fluidized bed. A total flowcorresponding to a gas velocity of 0.1 m/s at 900 °C wasintroduced in the reactor. During the reduction period, 1 g ofchar was fed inside the bed and gasified. The gasificationproducts reacted with the oxygen carrier particles to produceCO2 and H2O. After a short purge period with nitrogen, theoxygen carrier in the bed was regenerated using a streamconsisting of 10% oxygen in nitrogen. During both reductionand oxidation, the gases leaving the reactor were analyzed aftercondensing the steam. Dry concentrations of the gases weredetermined in a nondispersive infrared analyzer (CO, CO2, andCH4), a thermal gas conductivity analyzer (H2), and aparamagnetic analyzer (O2).2.3. Data Evaluation. To evaluate the performance of the

different materials, several parameters were calculated from theexperimental results. A carbon balance to the gaseous species inthe reactor allowed the calculation of the rate of charconversion, rC(t). Methane was not considered as it wasnever detected.

= +r t y y F( ) ( )C CO CO out2 (1)

where the total dry basis outlet flow, Fout, was estimated from anitrogen flow introduced in the outlet stream of the reactorprior to analysis, FN2

:

=− ∑

FF

y(1 )i iout

N2

(2)

In eq 2, yi is the molar fraction of the component i (CO2,CO, or H2) at the outlet stream of the reactor.Using the rate of char conversion, the value of char

conversion at different reaction times, Xchar, can be calculated:

∫=X t

r t t

N( )

( ) dt

char0 C

C,char (3)

where NC,char is the moles of carbon fed into the reactor.

The instantaneous rate of char conversion, rC,inst, is defined asthe rate of gasification referred to the amount of nongasifiedcarbon in the reactor.

=−

r tr t

N X( )

( )(1 )C,inst

C

C,char char (4)

The oxygen balance to the gaseous species allowed tocalculate the rate of oxygen transferred from the oxygen carrierto the gasification products, rO(t):

= −

= · + + −

r t F F

F y y F F

( )

(2 )

O O,out O,in

out CO CO H O,out H O,in2 2 2 (5)

To calculate the flow of water at the reactor exit (FH2O,out) itwas assumed that the flow of hydrogen either in H2 or H2Ocame only from the introduced steam:

= −F F F yH O,out H O,in out H2 2 2 (6)

Including eq 6 in eq 5, the rate of oxygen transferred is

= + −r t F y y y( ) (2 )O out CO CO H2 2 (7)

Then the oxygen carrier conversion can be calculated fromthe integration of eq 7 with time as

∫= −X tN

r t t( ) 11

( ) dt

OO,OC 0

O(8)

where NO,OC are the moles of oxygen in the oxygen carrier:

=Nm R

MO,OCOC OC

O (9)

where mOC is the mass of oxygen carrier and MO is the oxygenatomic mass.

3. RESULTS AND DISCUSSION3.1. Char Combustion Experiments. Several cycles of

char combustion under CLC conditions using the fourmanganese minerals tested in this work were carried out at950 °C. Figure 3 shows the results using MnGBHNE as bed

material as representative of a typical cycle. The char fed wasgasified to CO and H2 after contacting with steam in thereactor. Afterward, these gasification products reacted with themanganese mineral to produce CO2 and H2O. In a previousstudy from the authors,15 the four manganese minerals used inthe present work showed high reactivity to H2 and CO, thus noor small amount of CO and/or H2 were expected at the outlet

Figure 2. Batch fluidized bed reactor setup.

Figure 3. Product distribution, as well as char and oxygen carrierconversion with time in experiments with MnGBHNE at 950 °C.

Industrial & Engineering Chemistry Research Article

DOI: 10.1021/acs.iecr.6b00263Ind. Eng. Chem. Res. 2016, 55, 6539−6546

6541

of the reactor using char as fuel. As it can be seen in Figure 3, allH2 and most of CO were oxidized to H2O and CO2,respectively, by MnGBHNE reaching a maximum dryconcentration of CO2 of 13%. Figure 3 also shows both thechar and oxygen carrier conversion evolution with time duringthe reduction period. The combustion of gasification productsconsumed the oxygen in the manganese mineral, thus adecrease in the oxygen carrier conversion (XO) was observed.After the purge with 100% N2, the manganese mineral wasrapidly regenerated by oxidation with 10% O2 and increase inXO with time was observed. It should be noted that no CO2and/or CO were registered during the oxidation process, whichsuggested the complete char gasification during the reductionperiod, in accordance with the value of Xchar = 1 reached at theend of reduction in Figure 3. For the other three manganeseminerals, i.e., MnSA, MnGBMPB, and MnBR, similar behaviorwas observed.3.2. Evaluation of the Char Gasification Rate with

Different Manganese Ores. Figure 4 presents the instanta-

neous rate of char gasification (rC,inst) in the first reductionperiod as a function of char conversion Xchar for the fourmanganese minerals at 950 °C. The values in Figure 4 arecompared with those obtained under similar conditions withactivated ilmenite and iron ore previously used by theauthors.6,9 These two iron-based oxygen carriers showedgood performance in continuous CLC operation with coaland this is the reason why they are here considered as referencefor comparison. In order to facilitate the comparison, a stablestage corresponding to Xchar = 0.4−0.8 was considered.According to the results in Figure 4, the rate of char gasificationin the presence of the manganese minerals MnSA, MnGBHNE,MnGBMPB, and MnBR was much faster than in the presenceof ilmenite and iron ore. Actually, the values of rC,inst usingilmenite and iron ore are around 20%/min, approximately halfof the average value obtained using manganese minerals.The higher rate of char gasification in the presence of

manganese minerals could be ascribed to the following factors:(i) oxygen uncoupling effect; (ii) reactivity of the manganesemineral to H2; (iii) solid−solid reaction between oxygen carrierand fuel; and/or (iv) catalytic effect of the manganese mineralon the char gasification. In the following sections each of thesefactors will be analyzed and discussed.3.2.1. Oxygen Uncoupling Effect. In the present work, the

char feeding to the bubbling fluidized bed was carried out after

the bed was purged in nitrogen, so that most of the gaseousoxygen released by the oxygen carrier was already lost duringthat period. Therefore, it seems that the release of oxygen fromthe manganese materials cannot be considered as anexplanation for the high char gasification rate observed. Thisfact confirms the behavior observed in a previous work,15 wherethe ability of these four manganese materials to release gaseousoxygen under successive redox cycles was evaluated in detail. Inthis previous study, the possible oxygen uncoupling effect wasstudied in a thermogravimetric analyzer and a fluidized bedreactor. The oxygen release only occurred in the first exposureto N2 and then disappeared as Mn3O4 was not oxidized back toMn2O3. Note that oxygen uncoupling capability was onlyavailable when Mn2O3 is reduced to Mn3O4.

18

3.2.2. Reactivity of the Manganese Minerals to H2.According to our previous tests with gaseous fuels (H2, COand CH4), the reactivity of these manganese minerals to H2 washigher than that observed for ilmenite under the sameconditions in a batch fluidized bed.15 The rate index in thereaction with H2 at 950 °C calculated for the manganeseminerals was in the range 1.5−2.0%/min while the rate indexcalculated for ilmenite was around 1.5%/min. Nevertheless, theH2 concentration observed in all the present experiments wasclose to zero due to the high oxygen carrier to char ratio used.Therefore, high gas conversion was observed in all cases, andlow inhibitory effect of H2 on the gasification rate is expectedregardless the oxygen carrier used. However, the chargasification rate observed in Figure 4 for any of the manganeseminerals was at least 1.5 times that for ilmenite and iron ore,which was unexpected considering the H2 inhibition effect.According to our results, the decrease in the H2 inhibition inthe presence of manganese materials cannot be considered themain mechanism to justify the rapid gasification observed.These results are in line to those reported in the investigationsof Arjmand et al.13

3.2.3. Solid−Solid Reaction between Manganese Mineralsand Fuel. Another possible explanation to the higher chargasification rate in the presence of manganese minerals wouldcome from a contribution to the total char consumption rate ofthe solid−solid reaction between the oxygen carrier and thechar. However, clear indications of the slow rate of this solid−solid reaction in a fluidized bed reactor were already discussedin the literature.19 More indeed, the present work also testedthe solid−solid reaction between manganese minerals and charin nitrogen at 950 °C. No combustion products were observedat the outlet of the reactor. According to the carbon balancesperformed, all the char introduced was already burned when10% oxygen was introduced during the oxidation period.Therefore, solid−solid reaction between manganese mineralparticles and fuel cannot illustrate the fast char conversionobserved.

3.2.4. Catalytic Effect of the Manganese Mineral on theChar Gasification. It is a known fact already reported inliterature that alkali metal elements K and Na have positiveeffects on the gasification of graphite and char.20,21 In additionto K and Na, Ca could also be considered as a catalyst forgraphite gasification, although lower catalytic effects than K andNa were observed.22 Regarding the CLC process, theenhancement on char gasification by the presence of K andNa in Mn-ores was demonstrated by Arjmand et al.13 using sixmanganese minerals. They observed that the instantaneous rateof char gasification increased with higher contents of K and/orNa in the manganese minerals and proposed that these

Figure 4. Instantaneous rate of char gasification (rC,inst) in the firstreduction period as a function of char conversion (Xchar) for the fourmanganese minerals at 950 °C.

Industrial & Engineering Chemistry Research Article

DOI: 10.1021/acs.iecr.6b00263Ind. Eng. Chem. Res. 2016, 55, 6539−6546

6542

elements were transferred to the gas phase under the reactionconditions used and then deposited on the char surface.Table 1 shows that the manganese minerals used in the

present work also possess certain amounts of Na (MnSA), K(MnGBHNE, MnGBMPB, and MnBR), and Ca in theircomposition. Therefore, the previously observed catalytic effectof K and/or Na from the manganese minerals on the chargasification rate can also be present in the results obtained inthe current work. This possibility is further analyzed next.In order to evaluate the possible catalytic effect of the Na, K,

and Ca in the manganese minerals on char gasification severalcycles of char combustion with the four manganese mineralswere performed in the fluidized bed reactor. It was found thatthe rate of char gasification decreased with the number of cyclesfor all these minerals. However, only results corresponding toMnSA and MnGBHNE will be shown in this analysis, as theywere the materials with the lowest attrition rates and highercrushing strength after the reactivity experiments with gases inthe batch fluidized bed.15 This makes these materials morepromising for further use than the MnGBMPB and MnBRminerals.Figure 5 plots the values of the instantaneous char

gasification rate (rC,inst) as a function of Xchar for MnGBHNE

and MnSA during successive redox cycles and compares thesevalues with those corresponding to the activated ilmenite andiron ore taken as reference. The values of char gasification ratedecreased with the number of cycles and then became stable,both for MnGBHNE and MnSA. These stable values of thechar gasification rate were achieved after ninth and 11th cyclesfor MnGBHNE and MnSA, respectively. Once the stable ratevalues were reached, MnGBHNE exhibited much higher chargasification rate than the iron ore and ilmenite, whereas MnSAshowed similar values to those obtained with the iron-basedreference materials. In order to clarify the different behaviorobserved for the MnSA and MnGBHNE materials, sampleswere extracted for different redox cycles to examine theevolution of K, Na, and also Ca contents in the oxygen carrierfor different reduction times using the EDX technique. Thereason to include Ca in the analysis is that this metal can alsoact as a catalyst for char gasification22 and according to Table 1,the content of Ca in the MnSA sample is high. Figure 6 showsthe results obtained in the SEM-EDX analysis to theMnGBHNE mineral. This material showed relevant K andCa content. In this case, potassium was only detected in thebeginning of tests and the intensity of the K pattern decreasedwith the number of redox cycles to become negligible at the

end of the tests. In Figure 6A, it can be observed that thedistribution of K is homogeneous inside the calcined particle.However, after the use in the batch fluidized bed, potassium hasalmost completely disappeared from the manganese mineral;see Figure 6B. In the case of MnSA, Na was initially detected inparticles, but no Na could be detected after the redox cycles. Itis assumed that all Na was released from the manganesemineral after the redox cycles were performed. In this case, onlyCa was detected in the MnSA particles, although the intensityof the Ca pattern in the MnSA material also decreased with thenumber of redox cycles. The decrease of K, Na, and Caintensity indicates the lower contents of these elements in theused MnGBHNE and MnSA, respectively, which cancontribute to explain the decrease in the char gasificationrates with the number of cycles. Nevertheless, it should bepointed out that the stable rates of char gasification withMnGBHNE are higher than those observed for MnSA aftersimilar number of cycles in Figure 5. This observation might beattributable to the fact that K has higher mobility than Caduring coal gasification which resulted in an easier and fasterdistribution of K on the basal plane of char than Ca.22

Therefore, although the amount of Ca in the MnSA mineral islarger than the amount of K in MnGBHNE, K results in a moreeffective catalyst for char gasification once it reaches the charparticles. This mechanism can contribute to explain the factthat MnGBHNE exhibited higher char gasification rates thanMnSA.Another fact that is interesting for future investigations is that

after the loss of most of K in the MnGBHNE mineral, itpresented higher char gasification rates than the referencematerials (ilmenite and iron ore). An additional test wasperformed in order to corroborate the catalytic effect of theMnGBHNE material on char gasification even when the majorpart of the K present is lost. After being used in 18 successiveredox cycles, the MnGBHNE sample was completely reducedat 950 °C to MnO using a gaseous stream composed of 30% H2

Figure 5. Instantaneous rate of char gasification (rC,inst) in thereduction period as a function of char conversion (Xchar) for MnSAand MnGBHNE manganese minerals at 950 °C after successive redoxcycles.

Figure 6. EDX mapping of the MnGBHNE showing K distributioninside the solid particle in calcined particles and used particles (after18th redox cycles).

Industrial & Engineering Chemistry Research Article

DOI: 10.1021/acs.iecr.6b00263Ind. Eng. Chem. Res. 2016, 55, 6539−6546

6543

in nitrogen. With the bed completely reduced, no oxygenuncoupling effect, or contribution of the oxygen carrier todecrease the H2 inhibition on char gasification, was possible.Moreover, this reduced sample has a negligible content ofpotassium, as it was shown in Figure 6. Therefore, the rate ofchar gasification using the reduced MnGBHNE as bed materialshould be similar to that obtained using an inert bed, e.g., sand.Thus, the same experiment was repeated using silica sand asbed material. Both experiments were performed at 950 °C.Char was fed to a bed of sand or reduced MnGBHNE andgasified using steam. Figure 7 compares the evolution of char

conversion rate with time in both experiments. As it can beobserved in the figure, the char conversion rate is clearly higherin the presence of the reduced MnGBHNE when compared toa sand bed. Moreover, the char gasification rate is closer to thatobserved in the last cycle (18th cycle) performed with theMnGBHNE. Further research to clarify this behavior of theMnGBHNE mineral needs to be undertaken.3.3. Influence of Temperature on the Char Gas-

ification Rate. Figure 8 presents the instantaneous rate of char

gasification once stable reactivity was reached as a function oftemperature for MnGBHNE, MnSA, and the benchmarkmaterials (ilmenite and iron ore). As expected, the chargasification rate increased with temperature in all the cases.Little differences in the char gasification rate were observed forMnSA and the iron-based materials at the different temper-atures used. This fact agrees with the fact that the highgasification rate initially observed for the MnSA mineral

decreased with the number of redox cycles. Nevertheless, therate of char gasification with MnGBHNE was approximately 1.5times that observed for MnSA and the iron-based materials atthe different temperatures tested.

3.4. Analysis of the Particles after Use in the BatchFluidized Bed. The four manganese minerals were subjectedto several redox cycles in the batch fluidized bed. In total,MnSA was tested during 10 h, MnGBHNE during 20 h,MnGBMPB during 3 h, and MnBR during 3.5 h. No problemsof fluidization were observed during the whole experimentalcampaign with any of the materials. Nevertheless, after theexperiments performed at the highest temperature tested (1000°C) with both MnSA and MnGBHNE it was observed thatparticles tended to get stacked forming small agglomerates onlyvisible using a microscope. Figure 9 shows SEM pictures of the

particles extracted from the fluidized bed after experiments at1000 °C. According to these images, the number of aggregatesformed under similar conditions seems to be larger in theMnSA sample than in the MnGBHNE and the size of theaggregates formed is less than 500 μm. It is necessary to notethat these aggregates observed from SEM images did not leadto defluidization problems during the whole experimentalcampaign.

3.5. Estimation of the CO2 Capture Efficiency for aContinuous CLC Unit. In order to analyze the potentialperformance of the MnGBHNE mineral in a continuous CLCsystem an estimation of the carbon capture efficiency fordifferent solids inventories in the fuel reactor was performed.The methodology to estimate the CO2 capture efficiency wasalready detailed in a previous publication from the authors.23 It

Figure 7. Char conversion (Xchar) with time at 950 °C and using steamas gasifying agent in the presence of (i) sand, (ii) reduced MnGBHNE,and (iii) MnGBHNE after 18 redox cycles as bed materials.

Figure 8. Influence of temperature on the instantaneous rate of chargasification (rC,inst) for the MnSA and MnGBHNE minerals.

Figure 9. SEM pictures of the (A) MnSA and (B) MnGBHNEsamples after experiments at 1000 °C.

Industrial & Engineering Chemistry Research Article

DOI: 10.1021/acs.iecr.6b00263Ind. Eng. Chem. Res. 2016, 55, 6539−6546

6544

is based on the char gasification rate determined in this work,and it takes into account the properties of the oxygen carrierand solid fuel considered.Figure 10A presents the calculated CO2 capture efficiency,

ηCC, as a function of oxygen carrier inventory in the fuel reactor,

mFR. In this case, a carbon stripper is not considered in theCLC unit. Char particles not converted in the fuel reactor reachthe air reactor, and are burned there producing CO2 which willbe therefore not captured. As a consequence, full CO2 capturewould not be reached in the CLC process. According to Figure10(A), around 8000 kg/MWth of MnGBHNE would be neededto reach 90% CO2 capture efficiency at 900 °C. However, if thefuel reactor temperature is increased up to 1000 °C, around1600 kg/MWth would be enough to reach 95% CO2 captureefficiency.The use of a carbon stripper can effectively decrease the

oxygen carrier inventory required to obtain the same carboncapture efficiency, as it is shown in Figure 10B. As it was shownin Figure 1, a carbon stripper is a unit placed between fuel andair reactors. The function of the carbon stripper is to recoverunconverted char particles that might be leaving the fuelreactor. Thus, the char flow entering into the air reactor wouldbe decreased, and hence, the CO2 capture would be increased.In Figure 10B, a carbon separation efficiency of the carbonstripper equal to ηCS = 0.98 was assumed. This is a realisticvalue considering the carbon separation efficiencies reported inliterature for currently operating carbon stripper systems.24

Under these conditions, an oxygen carrier inventory as low as300 kg/MWth allows to reach a CO2 capture efficiency of 99%at 1000 °C using the MnGBHNE mineral. Considering all theprevious results, the MnGBHNE material has demonstrated tobe a potential material to be used as oxygen carrier in CLC withcoal.

4. CONCLUSIONSFour manganese minerals (MnSA, MnGBHNE, MnGBMPB,and MnBR) were tested in a batch fluidized bed reactor atdifferent temperatures in the range 900−1000 °C using charfrom bituminous coal as fuel in order to evaluate theirperformance as oxygen carriers for CLC of coal.Initially, all of the manganese minerals showed values of

instantaneous char gasification rate (rC,inst) higher than thoseobtained under the same experimental conditions with iron-based reference materials already used in continuous CLC units(i.e., ilmenite and iron ore). This result was attributed to the

presence of K, Na, and Ca in the manganese minerals that werereleased during gasification to the gaseous atmosphere and thenbound to the char particles, where they acted as catalysts forchar gasification.The values of char gasification rate obtained with the

manganese minerals decreased with the number of redox cyclesuntil they reached a stable value as catalytic elements such asNa and K were lost. In the case of the MnGBHNE material thestable value of char gasification rate was still higher than that ofilmenite or the iron ore, although in this case most of the K inthe manganese mineral had already been released and itscatalytic effect on the gasification of char particles would nolonger be of relevance.The effect of temperature on the char gasification rate

obtained using MnGBHNE and MnSA minerals was relevant,especially in the case of MnGBHNE. At the differenttemperatures used, the rate of char gasification withMnGBHNE was approximately 1.5 times that observed forMnSA and the iron-based materials.Finally, an estimation of the CO2 capture efficiency that

could be reached in a continuous CLC unit using MnGBHNEas oxygen carrier was done. According to the calculationspresented, an oxygen carrier inventory of MnGBHNE as low as300 kg/MWth would allow us to reach a CO2 capture efficiencyof 99% at 1000 °C.

■ AUTHOR INFORMATION

Corresponding Author*Tel.: +34 976 733 977. Fax: +34 976 733 318. E-mail:tmendiara@icb.csic.es.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was supported by the Spanish Ministry of Economyand Competitiveness (Project ENE 2013-45454-R) and by theEuropean Regional Development Fund (ERDF), by the CSIC(Project 2014-80E101). T.M. thanks the ‘“Ramon y Cajal”’postdoctoral contract awarded by the Spanish Ministry ofEconomy and Competitiveness. D.M. is grateful for the supportp r o v i d e d b y t h e Ch i n a S c ho l a r s h i p Coun c i l(CSC201306160054). The authors also thank Hidro NitroEspanola S.A. for providing MnSA and MnGBHNE materialsand Leopoldo Alcazar (CT-GAS) for providing the MnBRmaterial.

■ NOMENCLATURE

FH2O,in = steam molar flow fed to the reactor (mol/s)FH2O,out = steam molar flow exiting the reactor (mol/s)FN2

= nitrogen molar flow introduced before analysis (mol/s)Fout = total dry flow exiting reactor (mol/s)FO,in = oxygen molar flow entering the reactor (mol/s)FO,out = oxygen molar flow exiting the reactor (mol/s)MO = oxygen molar weight (kg/mol)mOC = mass of oxygen carrier (kg)Nc,char = moles of carbon fed into the reactorNO,OC = moles of oxygen in the oxygen carrier active forCLC processrc(t) = rate of char conversion (mol/s)rc,inst(t) = instantaneous rate of char conversion (s−1)

Figure 10. Estimation of the CO2 capture efficiency for different solidinventories in the fuel reactor at different temperatures in the range900−1000 °C using MnGBHNE (A) without and (B) with a carbonstripper system. Fuel: South African bituminous coal.

Industrial & Engineering Chemistry Research Article

DOI: 10.1021/acs.iecr.6b00263Ind. Eng. Chem. Res. 2016, 55, 6539−6546

6545

ro(t) = rate of oxygen transferred from the oxygen carrier tothe fuel gas (mol/s)ROC = oxygen transport capacityXchar(t) = char conversionXo(t) = oxygen carrier conversionyi = molar fraction of the component i at the outlet of thereactor

■ REFERENCES(1) Adanez, J.; Abad, A.; Garcia-Labiano, F.; Gayan, P.; de Diego, L.F. Progress in chemical-looping combustion and reforming tech-nologies. Prog. Energy Combust. Sci. 2012, 38 (2), 215.(2) Cuadrat, A.; Abad, A.; García-Labiano, F.; Gayan, P.; de Diego, L.F.; Adanez, J. The use of ilmenite as oxygen-carrier in a 500 Wthchemical-looping coal combustion unit. Int. J. Greenhouse Gas Control2011, 5, 1630.(3) Berguerand, N.; Lyngfelt, A. Design and operation of a 10 kWthchemical-looping combustor for solid fuels - Testing with SouthAfrican coal. Fuel 2008, 87 (12), 2713.(4) Linderholm, C.; Lyngfelt, A.; Cuadrat, A.; Jerndal, E. Chemical-looping combustion of solid fuels - Operation in a 10 kW unit withtwo fuels, above-bed and in-bed fuel feed and two oxygen carriers,manganese ore and ilmenite. Fuel 2012, 102, 808.(5) Markstrom, P.; Linderholm, C.; Lyngfelt, A. Chemical-loopingcombustion of solid fuels - Design and operation of a 100kW unit withbituminous coal. Int. J. Greenhouse Gas Control 2013, 15, 150.(6) Abad, A.; Perez-Vega, R.; de Diego, L. F.; García-Labiano, F.;Gayan, P.; Adanez, J. Design and operation of a 50kWth ChemicalLooping Combustion (CLC) unit for solid fuels. Appl. Energy 2015,157, 295.(7) Thon, A.; Kramp, M.; Hartge, E. U.; Heinrich, S.; Werther, J.Operational experience with a system of coupled fluidized beds forchemical looping combustion of solid fuels using ilmenite as oxygencarrier. Appl. Energy 2014, 118, 309.(8) Strohle, J.; Orth, M.; Epple, B. Chemical looping combustion ofhard coal in a 1 MWth pilot plant using ilmenite as oxygen carrier.Appl. Energy 2015, 157, 288.(9) Mendiara, T.; de Diego, L. F.; García-Labiano, F.; Gayan, P.;Abad, A.; Adanez, J. On the use of a highly reactive iron ore inChemical Looping Combustion of different coals. Fuel 2014, 126, 239.(10) Ma, J.; Zhao, H.; Tian, X.; Wei, Y.; Rajendran, S.; Zhang, Y.;Bhattacharya, S.; Zheng, C. Chemical looping combustion of coal in a5kWth interconnected fluidized bed reactor using hematite as oxygencarrier. Appl. Energy 2015, 157, 304.(11) Ge, H.; Shen, L.; Gu, H.; Song, T.; Jiang, S. Combustionperformance and sodium transformation of high-sodium ZhunDongcoal during chemical looping combustion with hematite as oxygencarrier. Fuel 2015, 159, 107.(12) Mendiara, T.; de Diego, L. F.; García-Labiano, F.; Gayan, P.;Abad, A.; Adanez, J. Behaviour of a bauxite waste material as oxygencarrier in a 500Wth CLC unit with coal. Int. J. Greenhouse Gas Control2013, 17, 170.(13) Arjmand, M.; Leion, H.; Mattisson, T.; Lyngfelt, A. Investigationof different manganese ores as oxygen carriers in chemical-loopingcombustion (CLC) for solid fuels. Appl. Energy 2014, 113, 1883.(14) Keller, M.; Leion, H.; Mattisson, T. Mechanisms of Solid FuelConversion by Chemical-Looping Combustion (CLC) using Man-ganese Ore: Catalytic Gasification by Potassium Compounds. EnergyTechnology 2013, 1, 273.(15) Mei, D.; Mendiara, T.; Abad, A.; de Diego, L. F.; García-Labiano, F.; Gayan, P.; Adanez, J.; Zhao, H. Evaluation of ManganeseMinerals for Chemical Looping Combustion. Energy Fuels 2015, 29(10), 6605.(16) Mendiara, T.; Gayan, P.; Abad, A.; de Diego, L. F.; García-Labiano, F.; Adanez, J. Performance of a bauxite waste as oxygen-carrier for chemical-looping combustion using coal as fuel. Fuel Process.Technol. 2013, 109, 57.

(17) Cuadrat, A.; Abad, A.; de Diego, L. F.; García-Labiano, F.;Gayan, P.; Adanez, J. Prompt considerations on the design ofChemical-Looping Combustion of coal from experimental tests. Fuel2012, 97, 219.(18) Mattisson, T.; Lyngfelt, A.; Leion, H. Chemical-looping withoxygen uncoupling for combustion of solid fuels. Int. J. Greenhouse GasControl 2009, 3 (1), 11.(19) Mendiara, T.; García-Labiano, F.; Gayan, P.; Abad, A.; de Diego,L. F.; Adanez, J. Evaluation of the use of different coals in ChemicalLooping Combustion using a bauxite waste as oxygen carrier. Fuel2013, 106, 814.(20) McKee, D. W. Gasification of graphite in carbon dioxide andwater vapor-the catalytic effects of alkali metal salts. Carbon 1982, 20(1), 59.(21) Kwon, T. W.; Kim, J. R.; Kim, S. D.; Park, W. H. Catalytic steamgasification of lignite char. Fuel 1989, 68 (4), 416.(22) Lang, R. J.; Neavel, R. C. Behaviour of calcium as a steamgasification catalyst. Fuel 1982, 61 (7), 620.(23) Abad, A.; Cuadrat, A.; Mendiara, T.; García-Labiano, F.; Gayan,P.; de Diego, L. F.; Adanez, J. Low-cost Fe-based oxygen carriermaterials for the iG-CLC process with coal. 2. Ind. Eng. Chem. Res.2012, 51 (50), 16230.(24) Abad, A.; Adanez, J.; de Diego, L. F.; Gayan, P.; García-Labiano,F.; Lyngfelt, A. Fuel reactor model validation: Assessment of the keyparameters affecting the chemical-looping combustion of coal. Int. J.Greenhouse Gas Control 2013, 19, 541.

Industrial & Engineering Chemistry Research Article

DOI: 10.1021/acs.iecr.6b00263Ind. Eng. Chem. Res. 2016, 55, 6539−6546

6546