Chemical Engineering Science 64 (2009) 3236 -- 3245

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Chemical Engineering Science

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CO2 looping cycleswith CaO-based sorbent pretreated in CO2 at high temperature

Vasilije Manovica, Edward J. Anthonya,∗, Davor Loncarevicb

aCanmetENERGY, Natural Resources Canada, 1 Haanel Drive, Ottawa, Ontario, Canada K1A 1M1bDepartment of Catalysis and Chemical Engineering, Institute of Chemistry, Technology and Metallurgy, Njegoseva 12, 11000 Belgrade, Serbia

A R T I C L E I N F O A B S T R A C T

Article history:Received 1 September 2008Received in revised form 27 March 2009Accepted 29 March 2009Available online 6 April 2009

Keywords:Ca looping cyclesFBCCO2 captureThermal pretreatmentSinteringLime based sorbents

In this study, pretreatment of CaO-based sorbent in a CO2 atmosphere at high temperature is investigatedfor its effect on CO2 capture. Three limestones from three widely different geographical locations are usedfor the tests: Kelly Rock (Canada), La Blanca (Spain), and Katowice (Poland). The particle sizes used aretypically as employed in fluidized bed conversion systems. Pretreatment was done in a tube furnace atdifferent temperatures and for different durations. The pretreated samples are characterized by nitrogenphysisorption tests, scanning electron microscopy (SEM), and carbonation/calcination conversion mea-surements in a thermogravimetric analyzer (TGA). The results obtained showed significant decrease ofsorbent surface area after pretreatment and the presence of smooth CaO grains was typical of the sorbentparticle surface morphology. The pore surface area of pretreated sorbent samples increased after CO2

cycling, with a peak in pore volume distributions at 50nm, and SEM images showed the reappearance ofsmaller CaO grains. In the case of Kelly Rock and Katowice samples, this led to an increase in CO2 captureactivity, up to 45% after 20 cycles. After that, conversions decreased but still remained 5–10% abovethose for the original (no pretreatment) samples. This beneficial effect means that particles of larger size,typical of fluidized bed combustion (FBC) systems, can be suitably pretreated for use in longer series ofCO2 capture cycles. An additional expected advantage of pretreating sorbent in this manner is reducedelutriation at any given FBC condition. Attempts to pretreat La Blanca failed, as they did when using N2,and it is believed that this is explained by the high Na content of this limestone.

Crown Copyright � 2009 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Fossil fuel combustion power plants represent a major sourceof anthropogenic CO2. It is believed that climate change is beingexacerbated by increased concentrations of greenhouse gases. Apotentially important approach for reducing such emissions frompower plants is the capture of CO2 from dilute gas mixtures (fluegas) to produce a concentrated stream suitable for liquefaction andsequestration/storage in underground geological formations and/orpossibly in the deep oceans (Freund, 1997; Herzog, 2001; Metzet al., 2005; Bachu, 2008).

CO2 separation from flue gas is the first and most energy-intensive step and much effort is being devoted to developing newapproaches to CO2 capture and separation. In general, there arethree main approaches in CO2 capture systems: post-combustion,pre-combustion and oxy-fuel combustion (Metz et al., 2005). Inthe post-combustion process, CO2 may be separated from a fluegas in looping cycles by absorption with organic solvents such as

∗ Corresponding author. Tel.: +16139962868; fax: +16139929335.E-mail address: banthony@nrcan.gc.ca (E.J. Anthony).

0009-2509/$ - see front matter Crown Copyright � 2009 Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ces.2009.03.051

monoethanolamine or by solids such as lime (CaO) (Yang et al., 2008;Anthony, 2008).

The investigation of CaO-based looping cycles and their applica-tion in fluidized bed conversion systems has received increasing at-tention recently (Shimizu et al., 1999; Salvador et al., 2003; Abanadeset al., 2004a, 2005; Stanmore and Gilot, 2005). The process is basedon the reversible exothermic carbonation reaction:

CaO(s) + CO2(g) = CaCO3(s) (1)

This is a typical gas–solid reaction with formation of a solid product(Prasannan et al., 1985; Duo et al., 1994). The critical stage in de-termining the reaction rate and conversion is the formation of solidcarbonate product that prevents contact of the reacting gas (CO2)with the solid reactant (CaO). In consequence, the reaction involvestwo quite different stages: first a fast one, kinetically controlled, andtaking minutes, followed by a slow stage, controlled by diffusionthrough the layer formed, which continues for hours to days (Bhatiaand Perlmutter, 1983; Abanades and Alvarez, 2003). Unfortunately,only the fast stage is interesting from the practical point of view. Itsrate at 650–700 ◦C is satisfactory for CO2 capture in real systems.Any further increase of temperature at atmospheric pressure is

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unfavorable because of reversibility; e.g., the concentration of CO2in the scrubbed flue gases becomes > 5% (Baker, 1962).

CO2 capture in FBC systems occurs in a fluidized bed of CaO (car-bonator). The reacted sorbent is then transferred to another fluidizedbed unit (calciner) operating at higher temperatures (> 900 ◦C) toproduce a concentrated CO2 stream (> 95%) (Hughes et al., 2005;Lu et al., 2008). In the calciner, the reverse reaction (calcination)occurs and the sorbent is regenerated. Ideally, a given amount ofsorbent can be used indefinitely. In reality, however, a number ofchemical and physical processes limit sorbent utilization; the mostimportant ones are: sintering (Barker, 1973; Silaban and Harrison,1995; Abanades, 2002; Abanades and Alvarez, 2003; Sun et al., 2007);

Table 1Elemental analysis of limestone samples used.

Compound Kelly Rock La Blanca Katowice

Al2O3 (wt%) 1.0 < 0.10 0.24CaO (wt%) 51.61 54.39 54.10Fe2O3 (wt%) 0.37 < 0.01 0.09K2O (wt%) 0.27 < 0.02 0.06MgO (wt%) 0.52 0.11 0.89Na2O (wt%) < 0.10 1.07 < 0.20SiO2 (wt%) 3.70 < 0.10 0.85TiO2 (wt%) 0.06 < 0.03 < 0.03Loss on fusion (wt%) 41.69 44.20 43.64Sum 99.32 99.77 99.96

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Fig. 1. Carbonation–calcination cycles of pretreated KR sample (0.300–0.425mm). Pretreatment conditions: TGA, 24h at 1000 ◦C in 100% CO2; cycle conditions: TGA,isothermally at 800 ◦C, carbonation in 100% CO2 for 10min, calcination in 100% N2 for 10min.

Table 2Results of nitrogen adsorption–desorption tests: BET pore surface area (SBET) and total pore volume (Vp).

Sample Description SBET (m2/g) Vp (mm3/g)

KR Original calcined 4.7 35KR-30 cyc Original+30 CO2 cycles 2.5 33KR-24h 24h in 100% CO2 at 1000 ◦C 0.8 14KR-24h-30 cyc 24h in 100% CO2 at 1000 ◦C+30 CO2 cycles 8.4 101LB Original calcined 23.5 286LB-30 cyc Original+30 CO2 cycles 10.1 106LB-24h 24h in 100% CO2 at 1000 ◦C 1.8 24LB-24h-30 cyc 24h in 100% CO2 at 1000 ◦C+30 CO2 cycles 3.9 57KT Original calcined 13.5 164KT-30 cyc Original+30 CO2 cycles 6.3 94KT-24h 24h in 100% CO2 at 1000 ◦C 1.0 12KT-24h-30 cyc 24h in 100% CO2 at 1000 ◦C+30 CO2 cycles 7.5 108

sulphation (Ryu et al., 2006; Sun et al., 2006); and attrition/elutriation(Jia et al., 2007; Lu et al., 2008).

All of these diminish the sorbent's carrying capacity, and reducethe commercial advantages of any CaO-based fluidized bed CO2capture technology (Abanades et al., 2004b, 2007; MacKenzie et al.,2007). The loss of sorbent activity and capacity over increasing num-bers of cycles caused by sintering is probably the most investigatedof these phenomena. In the case of natural CaO-based sorbent, thefall of activity is monotonic and can be described by simple models(Abanades, 2002; Abanades and Alvarez, 2003; Wang and Anthony,2005; Grasa and Abanades, 2006). It is believed that sorbent activityis mainly determined by pore surface area, and moreover, that theshift from the fast to slow carbonation stage is determined by prod-uct layer thickness (Alvarez and Abanades, 2005a,b). Published para-metric studies (Grasa and Abanades, 2006; Sun et al., 2007; Lysikov etal., 2007; Manovic and Anthony, 2008a) showed that important pa-rameters are sorbent chemical compositions (including impurities/additives), the calcination temperature and the duration of boththe calcination and carbonation, reactor type (that determines thetemperature stresses experienced by the sorbent particles) and theeffective CO2 concentration during the calcination process. After lessthan 100 cycles the activity drops to the level where the sorbentis practically useless and must be replaced with fresh or make-upsorbent. This situation is significantly worsened under realistic con-ditions involving high-temperature calcination under high-CO2 con-centration conditions (Lu et al., 2009; Manovic et al., 2009a). Apartfrom the economic disadvantages, the use of extra sorbent leads to

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negative environmental impacts, because of large amounts of spentsorbent that must be disposed of.

There are different approaches to reducing problems relatedto sintering. These include spent sorbent reactivation by steam(Manovic and Anthony, 2007a; Manovic et al., 2008a) or moist air(Fennell et al., 2007a), which results in recovery of carbonationcapability. However, such reactivated sorbents appear to be me-chanically fragile and predisposed to elutriation from fluidized bedsduring additional cycling and, thus, the use of at least part of thissorbent stream for SO2 retention upstream of the carbonator is alsorecommended (Manovic and Anthony, 2007b, 2008b). Doping ofsorbent with Al2O3 appears to produce favorable results (Li et al.,2005; Manovic et al., 2008b), but doping with Na2CO3 has failed toshow any significant benefits (Salvador et al., 2003). The influenceof higher concentrations of Na2O in particular is shown to be un-favorable (Manovic et al., 2008b) but there is limited evidence thatsmall concentrations may generate higher carbonation conversions(Fennell et al., 2007b).

Fig. 3. SEM images of KR samples: (a) pretreated at 1000 ◦C for 24h, (b) pretreatedat 1000 ◦C for 24h and 30 CO2 cycles, and (c) 30 CO2 cycles.

The thermal pretreatment of sorbent is an interesting approach,based on the fact that exposure of sorbent to high temperature priorto CO2 cycles causes structural stabilization (Alvarez and Abanades,2005a,b; Lysikov et al., 2007;Manovic and Anthony, 2008c), and evenan increase of activity with increasing numbers of reaction cycles(self-reactivation). However, it should be mentioned that thermalpretreatment does not work for all limestones, and this, we believe,depends on the type of impurities present (Manovic et al., 2008b).Another interesting finding is that significant self-reactivation

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Fig. 4. SEM images of LB samples: (a) pretreated at 1000 ◦C for 24h, (b) pretreatedat 1000 ◦C for 24h and 30 CO2 cycles, and (c) 30 CO2 cycles.

behavior following preheating in a nitrogen atmosphere can only beproduced with ground limestones (Manovic and Anthony, 2008c),which is not desirable if the pretreated sorbent is to be used inFBC systems, unless pelletization is to be employed. In this studythermal pretreatment is investigated in more detail with differentlimestones and under different conditions with the aim of generat-ing self-reactivation effects for large-particle samples typically usedin FBC systems. Here, the pretreatment occurs in CO2 atmosphere,rather than the N2 atmosphere previously investigated.

Fig. 5. SEM images of KT samples: (a) pretreated at 1000 ◦C for 24h, (b) pretreatedat 1000 ◦C for 24h and 30 CO2 cycles, and (c) 30 CO2 cycles.

2. Experimental

Limestone samples from three widely different geological lo-cations were used in this study: Kelly Rock (KR) from Canada,particle size 0.300–0.425mm; La Blanca (LB) from Spain, particlesize 0.400–0.600mm; and Katowice (KT) from Poland, particle size0.400–0.800mm. The X-ray fluorescence (XRF) elemental analy-ses of samples investigated are given in Table 1. It can be seenthat LB and KT are very pure limestones, while KR contains 6%

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Fig. 6. Carbonation–calcination cycles (isothermally 800 ◦C, carbonation in 50% CO2

and calcination in 100% N2, each for 10min) of limestone samples pretreated fordifferent durations at 1000 ◦C: (a) KR, (b) LB, and (c) KT.

impurities with the highest concentration of impurities being SiO2and Al2O3, indicating the presence of silicates and aluminosilicatesin the sorbent. On the other hand, the content of Na2O in LB (1.07%)is unusually high for limestones.

Limestone samples for CO2 looping cycles were pretreated ina tube furnace (TF) at high temperatures (900, 950, 1000, and1100 ◦C) in an atmosphere of 100% CO2 for different durations: 6,24, and 64h. The flow rate of CO2 was 200 cm3/min and samplesused were 3.0 g. At the end of the experiments, during cooling, theTF atmosphere was replaced by 100% N2 to prevent reaction of hotsamples with atmospheric CO2 and moisture. The calcines obtainedwere characterized by nitrogen adsorption/desorption tests to ob-tain the data on Brunauer–Emmett–Teller (BET) pore surface areaand Barrett–Joyner–Halenda (BJH) pore volume distribution prior totheir treatment in a thermogravimetric analyzer (TGA).

The CO2 cycles were also carried out in the TF using the orig-inal limestone samples and those pretreated in CO2 at 1000 ◦C for

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Fig. 7. Carbonation rates in selected CO2 cycles of limestone pretreated in CO2 at1000 ◦C for 24h: (a) KR, (b) LB, and (c) KT.

24h. Calcination was performed in 100% N2 and carbonation in 100%CO2, both for 15min, isothermally at 800 ◦C. The aim was to ob-tain enough spent sample material during CO2 cycles (from originaland pretreated limestone) for both chemical and physical character-ization. The 30 calcination/carbonation cycles used 3.0-g limestonesamples under gas flow rates of 500 cm3/min.

A Mettler Toledo TGA/SDTA851e/LF/1100 ◦C instrument was usedfor TGA CO2 cycles. The reactive gas flow rates were controlled byflow meters, at a flow rate of 40 cm3/min. Calcination/carbonationcycles in the TGA were done under isothermal conditions at 800 ◦C,calcination in 100% N2 and carbonation in 50% CO2 (N2 balance). Thesample masses in the TGA experiments were equivalent to ∼15mgCaO and typically 45 cycles were carried out. Data on sample massduring the experiments were collected, and the degree of carbona-tion was calculated on the basis of the mass change, assuming thatmass change occurs only due to formation/decomposition of CaCO3.

Nitrogen adsorption–desorption isotherms were obtained on aSorptomatic 1990 Thermo Finnigan automatic system using nitrogen

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Fig. 8. Influence of pretreatment temperature on sorbent performance in CO2 loopingcycles of samples pretreated for 24h in 100% CO2: (a) KR, (b) LB, and (c) KT.

physisorption at 77K. Samples were degassed at 250 ◦C in vacuumfor 12h. The specific surface area of samples, SBET, was calculatedaccording to the BET method (Rouquerol et al., 1999) and pore vol-ume and distribution were calculated according to the BJH method(Barrett et al., 1953) from the nitrogen adsorption isotherms. Themorphologies of the samples were examined by scanning electronmicroscopy (SEM) using a Hitachi S3400 microscope with 20kV ofaccelerating voltage. The samples were coated with a 3-nm thicklayer of gold-palladium.

3. Results and discussion

A recent study (Manovic and Anthony, 2008c) on thermal pre-treatment of CaO-based sorbent showed that better performancewas achieved with sorbent grinding prior to pretreatment. More-over, the pretreated samples (original particle size) did not show anyincreased capture capacity along cycles. This difference was inter-esting and suggested further investigation because it is desirable toobtain activated sorbent with the original, “larger” particles, for use

in FBC systems. It can also be expected that unground sorbent oughtto be less susceptible to elutriation in a FBC, based on pilot-scaleresults for untreated sorbents (Marquard-Moellenstedt et al., 2008).

The preliminary pretreatment and cycling tests with originalparticle-size samples were done in the TGA. Different methods toproduce self-reactivation for these samples were examined, includ-ing variation of the temperature and duration of pretreatment andcycles, and the gas compositions. The most promising results wereobtained with pretreatment in CO2, and a typical result is shown inFig. 1 for a KR sample. It can be seen that, after the initial 24-h pre-treatment in pure CO2, conversions increased with the number ofcycles, reaching a maximum conversion of ∼45% in cycles 8–14. Sub-sequent pretreatments were carried out in the TF to obtain enoughsample material for characterization and for CO2 cycles in the TGA.

For the original sorbents, the reduction in activity with increas-ing numbers of CO2 cycles is usually accounted for by sintering andloss of pore surface area. However, the increase in activity of pre-treated sorbent appears to be best explained by a pore-skeletonmodel (Lysikov et al., 2007; Manovic and Anthony, 2008c), whichalso implies a transformation in the pore skeleton. The resulting ef-fect on pore surface area and total pore volume obtained by nitro-gen adsorption–desorption analyses is presented in Table 2. It canbe seen that cycling of the original sorbent led to decreased poresurface area, which is a typical result. It was also observed that sam-ple pretreating at high temperature for longer times led to very lowsurface area (0.8–1.8m2/g). However, cycling of these samples re-sulted in increased surface areas, which in the case of KR and KTwere actually higher than those after 30 cycles with the original,non-pretreated sample (7.5m2/g) vs. 6.3m2/g for KT; 8.4m2/g vs.2.5m2/g for KR). Interestingly, in the case of KR, the pretreated sam-ple after 30 cycles has a higher surface area than does the original,uncycled sample (8.4m2/g vs. 4.7m2/g). The pretreated LB samplealso showed an increase in surface area after cycling (3.9m2/g), butthis is significantly smaller than that for the original sample aftercalcination (23.5m2/g) and cycling (10.1m2/g).

Analysis of nitrogen physisorption isotherms for all samplestested shows that they correspond to Type II. Following IUPAC rec-ommendations for classifying of adsorption isotherms (Sing et al.,1985), that is characteristic of macroporous or non-porous ad-sorbent. Hysteresis loops, which appeared in the physisorptionisotherms, can be classified as Type H1. This is characteristic forsolids consisting of particles traversed by nearly cylindrical channelsor formed from aggregates or agglomerates of spheroidal particles.In this case pores can be of uniform size and shape. The texture andmorphology analyses of the sorbent also indicate that both meso-pores and macropores are present in the samples. However, whencarbonation is considered, pores of smaller diameter are more sig-nificant because they are the major contributor to the surface areaof the sorbent, which in turn determines the degree of conversion(Alvarez and Abanades, 2005a,b). Thus, nitrogen desorption datawere analyzed by the BJH model to obtain further information onthe pore size distribution of these pores.

The results of BJH pore volume distributions are presented inFig. 2. It can be seen that the contribution of pores ∼50nm decreasesafter cycling of the original samples, which effect is more pronouncedfor LB and KT. The loss of small pores is typical for samples spent inCO2 cycles and is a result of sintering. Pretreatment in CO2 at 1000 ◦Cgave samples with practically no pores below 200nm; i.e., a deeplysintered material was produced. However, after cycling of the sin-tered samples, a significant amount of pores with a peak maximumat about 50nm appeared. This indicates that pretreated (sintered)samples recovered their pore surface area during CO2 cycles, whichis very important for their subsequent good conversions.

The BJH results presented are in close agreement with resultsfrom the investigation of sample morphologies by SEM that are

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presented in Figs. 3–5. Similar images are seen for KR and KT samples(Figs. 3 and 5). After pretreatment the structure of sorbent is repre-sented by large grains and pores, on the 1�m scale. They are veryuniform in size and have very smooth surfaces. This means smallpores/grains were eliminated during pretreatment and transformedinto larger pores/grains. However, after CO2 cycles with pretreatedsamples, the grains with smooth surfaces are less pronounced; i.e., anumber of areas of roughness in the form of small grains with irreg-ular surfaces are apparent. That can be correlated with an increasein the degree of carbonation along subsequent cycles. The pretreatedsorbents after cycling have similar structures to the original sorbentsafter cycling (Figs. 3c and 5c).

LB limestone shows a quite different transformation during pre-treatment than seen for KR and KT. Thus, Fig. 4a shows that LBafter pretreatment is a highly sintered, low-porous material withcoalesced grains. Individual, non-melted grains are only seen irreg-ularly, and those are very large, typically >1�m. This sintered sam-ple after cycling did not noticeably change its morphology (Fig. 4b),which means that the sample can not significantly recover structureduring CO2 cycles, i.e., the self-reactivation effect is not produced.This is in close agreement with our recent study (Manovic et al.,2008b) on LB limestone pretreatment in nitrogen at high tempera-ture. The only difference seen after CO2 cycles was cracking at theparticle surface. In the case of LB limestone, the original sample afterCO2 cycles (Fig. 4c) is obviously different from that of the pretreatedone. The sample morphology here is represented by smaller grainsof very uniform size and shape.

The activities of samples pretreated in CO2 were tested in theTGA over 45 cycles and the results are presented in Fig. 6. Clearly,the performance of LB (Fig. 6b) is quite different from that of KR andKT (Figs. 6a and c, respectively). Carbonation conversions after pre-treatment are lower by more than a factor of two in comparison withthose for the original, non-pretreated LB sample. Except for the firstfew cycles for the sample pretreated for 64h, with increasing num-ber of carbonation/calcination cycles the low levels of carbonationfell further. For the 64-h sample, in the last cycles, the conversionwas only ∼10%. These results are fully consistent with the fact thatthe pretreated LB sample was heavily sintered, Fig. 4a.

Pretreated KR and KT samples showed significantly increasingconversions for the initial cycles as presented in Figs. 6a and c. Itis also interesting that the maximum conversions seen are less pro-nounced and shift to higher cycle number with longer pretreatment.The fact that self-reactivation (Manovic and Anthony, 2008c) is oc-curring for larger particle sizes typically used in FBC systems, offersthe hope that no additional treatment (pelletization) will be required.It is also hoped that such particles may show less elutriation fromfluidized bed operation than that seen from pilot-plant unit behav-ior (Lu et al., 2008), although this has yet to be proved and thereare some preliminary new results which suggest that these particlesmay be more fragile than untreated sorbents (Chen et al., 2009).

For sorbent pretreated in CO2, performance in looping cyclescan be explained in terms of the pore-skeleton model, with theskeleton formed during high-temperature treatment. Recrystalliza-tion in CaO crystal lattices occurs at high temperature and leads toformation of a more stable skeleton structure. This subsequentlyprevents bulk mass transfer and sintering during formation anddecomposition of CaCO3 that has been identified as the criticalstep for sintering during CO2 cycles (Manovic and Anthony, 2008a).During pretreatment at high temperature, it is important to separatethe process of ion diffusion in CaO lattices that leads to stabilizationof the structure, from bulk mass transfer and melting that can takeplace. The role of CO2 during pretreatment is beneficial becauseit appears that the CO2 in contact with CaO helps in its recrys-tallization, but does not significantly enhance bulk diffusion andsintering.

Pretreatment leads to formation of recrystallized CaO that is avery stable structure and prevents sintering during cycles. It is ex-pected that such a stabilized solid structure would be less reactive,and this can be seen to be sconfirmed by Fig. 7, which shows thecarbonation history for the first and subsequent carbonation stepsfor the pretreated samples. There are clear differences between thecarbonation rate during the first carbonation step and those for sub-sequent cycles. It is evident that the first carbonation step differsfrom those in other cycles, since in all other cycles, a clear transitionfrom the fast initial, kinetically controlled, reaction step to the slowreaction step is apparent.

This suggests that a different controlling mechanism determinesthe reaction rate during the first carbonation step for a pretreatedsample. The absence of the typical reaction rate pattern (fast–slowshift) can also be explained by changes in the pore skeleton. Thus, ahard and less-reactive skeleton, formed during pretreatment, has amore stable crystal structure and is less reactive with CO2. During thefirst cycle, carbonation takes place at that less-reactive surface andis limited by the processes in the hard skeleton. During this step andfollowing calcination, a less-stable structure (soft skeleton) of CaO,which is more reactive, is formed. After that, calcination occurs at

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V. Manovic et al. / Chemical Engineering Science 64 (2009) 3236 -- 3245 3243

the surface of the soft skeleton with typical carbonation-controllingmechanisms. Apparently agreeing with this view, it should be notedthat a higher conversion rate is evident during the first cycle in thecase of the LB sample (Fig. 7b) in comparison to those seen for KRand KT. This can be explained by a higher Na content in LB thatrenders the structure of CaO less stable (defects in crystal latticestructure) and, thereby more reactive, as well as more accessibleto mass transfer and transformation to a soft skeleton. We believethese observations clearly support the validity of the pore-skeletonmodel.

The influence of pretreatment temperature can be seen in Fig. 8.It is obvious that changes in the temperature do not help in gener-ating a self-reactivation effect in LB. Fig. 8b shows that sorbent per-formance worsens with increased pretreatment temperature, whichis especially drastic at 1100 ◦C, where conversions are only 2.5% af-ter 45 cycles and appear to be further decreasing. This finding showsthat the final carbonation value may be different (in this case lower)than is usually reported for long series of reaction cycles for LB lime-stone, 7–8% (Grasa and Abanades, 2006).

It should also be noted that there are remarkable differences be-tween the performance of samples pretreated at 900 ◦C and that for

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[%]

Original limestone24h, 900 oC

24h, 1100 oC64h, 1000 oC

24h, 1000 oC

6h, 1000 oC 24h, 950 oC

0

10

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50

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70

80

90

Time [s]

Car

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tion

[%]

Original limestone

24h, 950 oC

6h, 1000 oC

24h, 1000 oC ≈ 64h, 1000 oC ≈≈ 24h, 1100 oC ≈ 24h, 950 oC

600500400300200100

0 600500400300200100

0 600500400300200100

Fig. A2. Comparison of carbonation rates in the first CO2 cycle of limestones treatedfor different times at different temperatures: (a) KR, (b) LB, and (c) KT.

samples pretreated at the higher temperatures investigated here.Conversions of the samples pretreated at 900 ◦C are very similar tothose of the original, non-pretreated samples. This implies that sor-bent in the carbonated form (CaCO3) is present at 900 ◦C in 100%CO2 (Baker, 1962), and its structure can not be stabilized at hightemperature. This has also been confirmed here by TGA. Even ifsome recrystallization in the crystal structure of CaCO3 occurs anda more stable skeleton structure is formed, it is eliminated duringthe first calcination cycle and a soft, reactive CaO skeleton is formed,which is predisposed to sintering in further cycles. Somewhat lowercarbonation conversions along cycles, in comparison with those forthe original samples, are most likely a result of sintering of CaCO3at 900 ◦C. This finding also supports the hypothesis that stabiliza-tion of the CaO skeleton structure is critical for stabilizing carbon-ation conversions along CO2 cycles. It is shown here, for the twoinvestigated samples (KR and KT) that are suitable for pretreatment,that the optimal pretreatment temperature is 1000 ◦C, but this mayvary with pretreatment duration, as can also be concluded fromFig. 6.

0

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60C

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]Original limestone

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6h, 1000 oC

24h,1100 oC

24h, 950 oC 24h, 1000 oC

64h, 1000 oC

b

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Car

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tion

[%]

Original limestone

24h, 900 oC

24h, 1100 oC

64h, 1000 oC

24h, 950 oC

24h, 1000 oC

6h, 1000 oC

c

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60

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tion

[%]

Original limestone24h, 900 oC

6h, 1000 oC

24h, 1100 oC

24h, 1000 oC 64h, 1000 oC 24h, 950 oC

0Time [s]

600500400300200100

0Time [s]

600500400300200100

0Time [s]

600500400300200100

Fig. A3. Comparison of carbonation rates in the fifth CO2 cycle of limestones treatedfor different times at different temperatures: (a) KR, (b) LB, and (c) KT.

3244 V. Manovic et al. / Chemical Engineering Science 64 (2009) 3236 -- 3245

4. Conclusions

A modified method of high-temperature pretreatment of CaO-based sorbent has been examined. It is shown that treatment of thesorbent at high temperature in an atmosphere of CO2 is more ef-fective than that in N2. This treatment can produce a sorbent withpromising properties for CO2 capture, with suitable particle sizes forFBC systems. After pretreatment, the sorbent pore surface area wasreduced as a result of sintering, and there was a loss of small pores,producing low carbonation conversions in the initial CO2 cycles.However, these conversions increased, reaching a maximum, withshapes and development of the maximum as a function of reactioncycle depending on the temperature and duration of pretreatment.Three limestones were tested here and two of them (Kelly Rock andKatowice) were successfully self-reactivated. The best results wereobtained with pretreatment in CO2 at 1000 ◦C for 24h, with the sor-bent after 20 CO2 cycles achieving a carbonation conversion of ∼45%.These values decrease slightly with increasing numbers of reactioncycles, but they are still 5–10% above those for the original, untreatedsorbent samples. An additional benefit from sorbent pretreated inthis manner may be reduced elutriation from a FBC system.

The results obtained are explained by recrystallization of CaOat high temperatures and formation of a more stable skeleton thatproduces a pore texture more resistant during CO2 cycles. This isalso confirmed by pretreatment at 900 ◦C (presence of CaCO3 atthis temperature) in 100% CO2, which showed that pretreatmentof CaCO3 is not useful, producing instead a sorbent with poorer(and decreasing) activity than that of the original sorbent. Finally,it has been shown that performance of La Blanca limestone isnot improved, even after sintering in CO2 atmospheres, due to itshigh Na content, which causes extreme sintering at high tempera-tures, leading to low carbonation conversions that decrease alongcycles.

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KR

KTLB

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LB

Original limestones24h, 1000 oC

b

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tion

[%]

KRKT

LB

LBKT

KR

Original limestones24h, 1000 oC

0Time [s]

600500400300200100

0Time [s]

600500400300200100

Fig. A4. Comparison of carbonation rates in the first (a) and fifth (b) CO2 cycles ofdifferent limestones, original and samples treated at 1000 ◦C in CO2 for 24h.

Appendix A

During this study further comparisons between carbonation ki-netics of the original samples (Fig. A1) and samples pretreated fordifferent times, at different temperatures (Figs. A2–A4), as well ascomparison of calcination rates (Fig. A5), were made and they alsosupport the pore-skeleton approach. It has been seen that carbona-tion rates during the initial fast stage are almost the same for all sam-ples, with the exception of the first carbonation rates for pretreatedsamples. The only differences are conversion levels when carbona-tion rate shifts to the slow stage and reaction rate during the slowstage. The calcination rate patterns are also similar for different sam-ples and cycle number, including those after the first carbonation,which means that the reactive surface of the skeleton has alreadybeen formed after the first carbonation of pretreated samples.

The differences in calcination rates between the first and othercycles are not significant as is the case for carbonation. This supportsthe hypothesis that skeletons of similar reactivities are formed afterthe first carbonation in different samples. LB may be considered asan exception, where the calcination rate in the first cycle of

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]Original limestone24h, 1000 oC

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Cycle 1

Cycle 5

Cycle 5

b

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Cycle 1Cycle 5

Cycle 5

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tion

[%]

Original limestone24h, 1000 oC

Cycle 1

Cycle 1

Cycle 5

Cycle 5

0Time [s]

600500400300200100

0Time [s]

600500400300200100

0Time [s]

600500400300200100

Fig. A5. Comparison of calcination rates for the first and fifth CO2 cycles of theoriginal and limestone samples treated at 1000 ◦C in CO2 for 24h: (a) KR, (b) LB,and (c) KT.

V. Manovic et al. / Chemical Engineering Science 64 (2009) 3236 -- 3245 3245

pretreated sample is significantly lower than that for the originalsample (Fig. A5b). However, calcination rates in further cycles (thefifth is presented here) of the pretreated sample is the same as that inthe first cycle, which means that skeleton reactivity does not change.The lower rate can be attributed to the change in the particles' mor-phology rather than to any irreversible change in the CaO crystalstructure caused by thermal treatment. In particular, the significantand irreversible decrease of porosity after the pretreatment of LB hasbeen noted (Manovic et al., 2009b), which influences gas diffusion inporous sorbent particles and reduces calcination rate. The same ex-planation can be used for the lower carbonation rates of pretreatedsamples in comparison with those for the original sample (Fig. A3b).

References

Abanades, J.C., 2002. The maximum capture efficiency of CO2 using acalcination/carbonation cycle of CaO/CaCO3. Chemical Engineering Journal 90,303–306.

Abanades, J.C., Alvarez, D., 2003. Conversion limits in the reaction of CO2 with lime.Energy and Fuels 17, 308–315.

Abanades, J.C., Anthony, E.J., Alvarez, D., Lu, D., Salvador, C., 2004a. Capture of CO2

from combustion gases in a fluidized bed of CaO. A.I.Ch.E. Journal 50, 1614–1622.

Abanades, J.C., Rubin, E.S., Anthony, E.J., 2004b. Sorbent cost and performance in CO2

capture systems. Industrial and Engineering Chemistry Research 43, 3462–3466.Abanades, J.C., Anthony, E.J., Wang, J., Oakey, A., 2005. Fluidized bed combustion

systems integrating CO2 capture with CaO. Environmental Science andTechnology 39, 2861–2866.

Abanades, J.C., Grasa, G., Alonso, M., Rodriguez, N., Anthony, E.J., Romeo, L.M., 2007.Cost structure of a postcombustion CO2 capture system using CaO. EnvironmentalScience and Technology 41, 5523–5527.

Alvarez, D., Abanades, J.C., 2005a. Determination of the critical product layerthickness in the reaction of CaO with CO2. Industrial and Engineering ChemistryResearch 44, 5608–5615.

Alvarez, D., Abanades, J.C., 2005b. Pore-size and shape effects on the recarbonationperformance of calcium oxide submitted to repeated calcination/recarbonationcycles. Energy and Fuels 19, 270–278.

Anthony, E.J., 2008. Solid looping cycles: a new technology for coal conversion.Industrial and Engineering Chemistry Research 47, 1747–1754.

Bachu, S., 2008. CO2 storage in geological media: role, means, status and barriersto deployment. Progress in Energy and Combustion Science 34, 254–273.

Baker, E.H., 1962. The calcium oxide–carbon dioxide system in the pressure range1–300 atmospheres. Journal of the Chemical Society 70, 464–470.

Barker, R., 1973. The reversibility of the reaction CaCO3 = CaO + CO2. Journal ofApplied Chemical Biotechnology 23, 733–742.

Barrett, E.P., Joyner, L.G., Halenda, P.P., 1953. The determination of pore volumeand area distributions in porous substances. I. Computations from nitrogenisotherms. Journal of the American Chemical Society 73, 373–380.

Bhatia, S.K., Perlmutter, D.D., 1983. Effect of the product layer on the kinetics of theCO2–lime reaction. A.I.Ch.E. Journal 29, 79–86.

Chen, Z., Song, H.S., Portillo, M., Lim, C.J., Grace, J.R., Anthony, E.J., 2009. Long-termcalcination/carbonation cycling and thermal pre-treatment for CO2 capture bylimestone and dolomite. Energy and Fuels 23, 1437–1444.

Duo, W., Sevill, J.P.sK., Kirkby, N.F., Clift, R., 1994. Formation of product layers insolid–gas reactions for removal of acid gases. Chemical Engineering Science 49,4429–4442.

Fennell, P.S., Davidson, J.F., Dennis, J.S., Hayhurst, A.N., 2007a. Regeneration ofsintered limestone sorbents for the sequestration of CO2 from combustion andother systems. Journal of the Energy Institute 80, 116–119.

Fennell, P.S., Pacciani, R., Dennis, J.S., Davidson, J.F., Hayhurst, A.N., 2007b. Theeffects of repeated cycles of calcination and carbonation on a variety of differentlimestones, as measured in a hot fluidized bed of sand. Energy and Fuels 21,2072–2081.

Freund, P., 1997. International collaboration on capture, storage and utilisation ofgreenhouse gases. Waste Management 17, 281–287.

Grasa, G.S., Abanades, J.C., 2006. CO2 capture capacity of CaO in long series ofcarbonation/calcination cycles. Industrial and Engineering Chemistry Research45, 8846–8851.

Herzog, H., 2001. What future for carbon capture and sequestration? EnvironmentalScience and Technology 35, 148A–153A.

Hughes, R.W., Lu, D.Y., Anthony, E.J., Macchi, A., 2005. Design, process simulationand construction of an atmospheric dual fluidized bed combustion system for insitu CO2 capture using high-temperature sorbents. Fuel Processing Technology14–15, 1523–1531.

Jia, L., Hughes, R., Lu, D., Anthony, E.J., Lau, I., 2007. Attrition of calcining limestones incirculating fluidized-bed systems. Industrial and Engineering Chemistry Research46, 5199–5209.

Li, Z.S., Cai, N.S., Huang, Y.Y., Han, H.J., 2005. Synthesis, experimental studies, andanalysis of a new calcium-based carbon dioxide absorbent. Energy and Fuels 19,1447–1452.

Lu, D., Hughes, R., Anthony, E.J., Manovic, V., 2009. Sintering and reactivity of CaCO3-based sorbents for in-situ CO2 capture in fluidized beds under realistic calcinationconditions. Journal of Environmental Engineering, accepted for publication.

Lu, D.Y., Hughes, R.W., Anthony, E.J., 2008. Ca-based sorbent looping combustion forCO2 capture in pilot-scale dual fluidized beds. Fuel Processing Technology 89,1386–1395.

Lysikov, A.I., Salanov, A.N., Okunev, A.G., 2007. Change of CO2 carrying capacity of CaOin isothermal recarbonation-decomposition cycles. Industrial and EngineeringChemistry Research 46, 4633–4638.

MacKenzie, A., Granatstein, D.L., Anthony, E.J., Abanades, J.C., 2007. Economics ofCO2 capture using the calcium cycle with a pressurized fluidized bed combustor.Energy and Fuels 21, 920–926.

Manovic, V., Anthony, E.J., 2007a. Steam reactivation of spent CaO-based sorbentfor multiple CO2 capture cycles. Environmental Science and Technology 41,1420–1425.

Manovic, V., Anthony, E.J., 2007b. SO2 Retention by reactivated CaO-based sorbentfrom multiple CO2 capture cycles. Environmental Science and Technology 41,4435–4440.

Manovic, V., Anthony, E.J., 2008a. Parametric study on the CO2 capture capacity ofCaO-based sorbents in looping cycles. Energy and Fuels 22, 1851–1857.

Manovic, V., Anthony, E.J., 2008b. Sequential SO2/CO2 capture enhanced by steamreactivation of a CaO-based sorbent. Fuel 87, 1564–1573.

Manovic, V., Anthony, E.J., 2008c. Thermal activation of CaO-based sorbent andself-reactivation during CO2 capture looping cycles. Environmental Science andTechnology 42, 4170–4174.

Manovic, V., Lu, D.Y., Anthony, E.J., 2008a. Steam hydration of sorbents from a dualfluidized bed CO2 looping cycle reactor. Fuel 87, 3344–3352.

Manovic, V., Anthony, E.J., Grasa, G., Abanades, J.C., 2008b. CO2 looping cycleperformance of a high-purity limestone after thermal activation/doping. Energyand Fuels 22, 3258–3264.

Manovic, V., Charland, J.-P., Blamey, J., Fennell, P.S., Lu, D.Y., Anthony, E.J., 2009a.Influence of calcination conditions on carrying capacity of CaO-based sorbent inCO2 looping cycles. Fuel, submitted for publication.

Manovic, V., Anthony, E.J., Loncarevic, D., 2009b. SO2 retention by CaO-based sorbentspent in CO2 looping cycles. Industrial and Engineering Chemistry Research,submitted for publication.

Marquard-Moellenstedt, T., Specht, M., Zuberbuehler, U., Koppatz, S., Pfeifer, C.,Rauch, R., Soukup, G., Hofbauer, H., Koch, M., 2008. Transfer of the absorptionenhanced reforming (AER)-process from the pilot scale to an 8MWth gasificationplant in Guessing, Austria. In:16th European Biomass Conference and Exhibition,June 2–6, Valencia, Spain.

Metz, B., Davidson, O., de Coninck, H., Loos, M., Meyer, L. (Eds.), 2005. Special Reporton Carbon Dioxide Capture and Storage, Intergovernmental Panel on ClimateChange. Cambridge University Press, Cambridge.

Prasannan, P.C., Ramachandran, P.A., Doraiswamy, L.K., 1985. A model for gas–solidreactions with structural changes in the presence of inert solids. ChemicalEngineering Science 40, 1251–1261.

Rouquerol, F., Rouquerol, J., Sing, K., 1999. Adsorption by Powders and Porous Solids.Academic Press, London.

Ryu, H., Grace, J.R., Lim, C.J., 2006. Simultaneous CO2/SO2 capture characteristicsof three limestones in a fluidized-bed reactor. Energy and Fuels 20,1621–1628.

Salvador, C., Lu, D., Anthony, E.J., Abanades, J.C., 2003. Enhancement of CaOfor CO2 capture in an FBC environment. Chemical Engineering Journal 96,187–195.

Shimizu, T., Hirama, T., Hosoda, H., Kitano, K., Inagaki, M., Tejima, K., 1999. A twinfluid-bed reactor for removal of CO2 from combustion processes. Transactionsof IChemE 77 (part A), 62–68.

Silaban, A., Harrison, P., 1995. High temperature capture of carbon dioxide:characteristics of the reversible reaction between CaO(s) and CO2(g). ChemicalEngineering Communications 137, 177–190.

Sing, K.S.W., Everett, D.H., Haul, R.A.W., Moscou, L., Pierotti, R.A., Rouquerol, J.,Siemieniewska, T., 1985. Reporting physisorption data for gas/solid systems withspecial reference to the determination of surface area and porosity. Pure andApplied Chemistry 57, 603–619.

Stanmore, B.R., Gilot, P., 2005. Review—calcination and carbonation of limestoneduring thermal cycling for CO2 sequestration. Fuel Processing Technology 86,1707–1743.

Sun, P., Grace, J.R., Lim, C.J., Anthony, E.J., 2006. Removal of CO2 by calcium-basedsorbents in the presence of SO2. Energy and Fuels 21, 163–170.

Sun, P., Grace, J.R., Lim, C.J., Anthony, E.J., 2007. The effect of CaO sintering on cyclicCO2 capture in energy systems. A.I.Ch.E. Journal 53, 2432–2442.

Wang, J.S., Anthony, E.J., 2005. On the decay behavior of the CO2 absorption capacityof CaO-based sorbents. Industrial and Engineering Chemistry Research 44,627–629.

Yang, H., Xu, Z., Fan, M., Gupta, R., Slimane, R.B., Bland, A.E., Wright, I., 2008. Progressin carbon dioxide separation and capture: a review. Journal of EnvironmentalSciences 20, 14–27.