Applied Energy 108 (2013) 373–382

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Applied Energy

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A comparative characterization study of Ca-looping natural sorbents

0306-2619/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.apenergy.2013.03.009

⇑ Corresponding author. Tel.: +30 210 6501507; fax: +30 210 6501598.E-mail address: itskos@certh.gr (G. Itskos).

Grigorios Itskos a,⇑, Panagiotis Grammelis a, Fabrizio Scala b, Halina Pawlak-Kruczek c, Antonio Coppola b,Piero Salatino b, Emmanuel Kakaras a

a Centre for Research and Technology Hellas, Chemical Process and Energy Resources Institute, 357-359 Mesogeion Avenue, Halandri GR-15231, Athens, Greeceb Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche, Piazzale Vincenzo Tecchio 80, 80125 Napoli, Italyc Wroclaw University of Technology, Institute of Heat Engineering and Fluid Mechanics, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland

h i g h l i g h t s

� Selected CO2 sorbents were tested in a batch FB reactor and a TGA apparatus.� We assessed the effect of flue gas sulfur on selected sorbent properties.� The sulfated FB carbonates were found much less reactive than the TGA ones.� A porous Ca–S layer on sorbent particles may mitigate the drawbacks of sulfation.

a r t i c l e i n f o

Article history:Received 16 July 2012Received in revised form 20 February 2013Accepted 4 March 2013Available online 12 April 2013

Keywords:CCS sorbentsCa-loopingScanning Electron Microscopy (SEM)SulfationN2-porosimetry

a b s t r a c t

In this study, six high-Ca limestones and one dolomite from Germany, Greece, Italy, and Poland weretested for their CO2-uptake capacity during carbonation–calcination experiments in a TGA apparatus,as well as in a lab-scale atmospheric bubbling FB reactor. The calcium looping experiments were carriedout both in the presence and absence of sulfur in gas phase, to study its likely inhibitory effect on thepenetration of CaO particles by CO2. The mineralogy and microstructure of fresh, sulfated/carbonated,and non-sulfated/carbonated sorbents have been comparatively evaluated by means of X-ray diffraction(XRD) and Energy Dispersive X-ray Spectroscopy–Scanning Electron Microscopy (EDS–SEM), respectively.Their specific surface area and pore size distribution have been determined by means of N2-porosimetry.All samples were examined after five cycles of carbonation–calcination. In most sulfated samples, a shellof anhydrite (CaSO4) has been identified peripherally to the CaO particles, preventing part of their corefrom further carbonating. The macro-porosity (%) of sulfated samples was increased, compared to thenon-sulfated ones, suggesting less sintering in the former, a fact also supported by the BET area measure-ments. On the other hand, micro-porosity showed no clear tendency with sulfation. The loss in micropo-rosity, observed in particular cases of sulfated samples, was attributed to a drop in the associatedconversion during carbonation. Overall, this work contains an integrated, comparative characterizationstudy of the tested sorbents, accompanied by suggestions on their utilization in Ca-looping processes.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon capture and storage (CCS) technologies hold the poten-tial to trap up to 90% of carbon dioxide (CO2) emissions from powerstations and industrial sites. They involve capturing, transportingand then storing CO2 in geological formations, so that it couldnot escape into the atmosphere and contribute to climate change[1]. Apart from storage, other options for captured CO2 are to bere-used for enhancing oil recovery or as a carbon resource convert-ible into other useful compounds [2]. Carbon capture is best ap-plied to large stationary sources such as power stations and

industrial plants, where CO2 can be separated from the flue gasesat some stage of the process. Currently, there are three generaltypes of CO2 capture systems, i.e. post-combustion capture, pre-combustion capture, and oxy-combustion [3–5]. The present studyfocuses on Ca-looping, a post-combustion process where CO2 cap-ture is performed from large point sources in a fluidized bed carb-onator at temperatures of 600–700 �C [6,7] using calcium oxide(CaO) sorbents. This process is based on the reversible gas–solidreaction between CaO and CO2, to form calcium carbonate (CaCO3)[8,9].

Natural CO2 sorbents, such as limestone, become less reactiveupon cyclic operation, mainly due to sintering which occurs pre-dominantly during calcination at 900 �C or higher temperatures[10,3]. This normally leads to a decrease in the specific surface area

374 G. Itskos et al. / Applied Energy 108 (2013) 373–382

of the sorbent particles and, therefore, to a decrease in the reactiv-ity falling to an asymptotic value of about 0.08 g CO2/g sorbent inthe case of limestone [11]. Such effect may be overcome byincreasing the Ca/CO2 ratio in the carbonator, by increasing thepurge flow of deactivated material or by modifying the sorbentproperties [12]. Sorbent deactivation can also be caused by mate-rial losses from attrition and fragmentation, competing sulfationreactions and ash fouling [13]. Moreover, the mechanical stabilityshould also be taken into consideration for the selection of CaOprecursors in large-scale industrial applications.

Synthetic sorbents may possess a set of desirable properties,however their usage remains limited due to their high productioncost [14]. Limestone sorbents, on the other hand, are consideredacceptable due to low price, availability, and re-use as feedstockfor the cement industry [15]. As shown by previous studies [16–20], sulfation of limestone (Eqs. (1) and (2) [21]) plays a critical rolein sorbent reactivity. It mainly occurs on the surface of particles,forming an anhydritic layer (CaSO4), which obstructs further sulfa-tion and/or carbonation of the sorbent particles. In the currentwork, the effect of sulfation on the microstructure, mineralogy,specific surface area, and porosity of six high-Ca limestones andone dolomite (after 5 cycles of carbonation–calcination) has beencomparatively assessed.

CaCO3 ! CaO þ CO2 DH ¼ 182:1 kJ=gmol ð1Þ

CaOþ SO2 þ 1=2 O2 ! CaSO4 DH ¼ �2481:4 kJ=gmol ð2Þ

2. Materials and methods

2.1. Materials

The six raw sorbents used in this study were all high-calciumlimestones (calcite >94% by weight) from different European coun-tries (Italy, Germany, Greece and Poland). Table 1 shows theirchemical composition, as obtained by means of X-ray Fluorescence(Instrument: SRS 3400, Bruker). Gases used in the tests were mix-tures of air, CO2 and SO2/N2. The dolomitic sample used in thisstudy consisted of 95.6 wt.% dolomite (MgCa(CO3)2), 3.6 wt.% cal-cite (CaCO3), and 0.5 wt.% quartz (SiO2).

2.2. Experimental set-up

2.2.1. TGA apparatus testsThe cyclic carbonation and calcination reactions were experi-

mentally studied in a Perkin Elmer Diamond ThermogravimetricAnalyzer (TGA), especially adapted for long multicycle carbon-ation–calcination tests in different composition of gases. The TGAfurnace is capable of working at temperatures up to 1000 �C, butin stable way only up to 900 �C, with heating rate from 5 to200 �C/min. The sample holder was a platinum basket withdiameter of 6 mm. Temperature and sample weight were continu-ously recorded in a computer. The reacting gas mixture (CO2: 15%,N2 – balance and CO2: 15%, SO2: 1500 ppm, N2 – balance) flow was

Table 1Chemical composition of limestones (% by weight).

Sample Origin SiO2 Al2O3 Fe2O3 CaO

Massicci Italy 1.11 0.37 0.14 54.5Schwabian Germany 3.51 0.50 0.18 53.6EnBW Germany 0.30 0.13 0.08 56.0Xirorema Greece 0.83 0.26 0.36 55.1Tarnow Poland 1.73 0.34 0.39 54.0Czatkowice Poland 3.91 0.39 0.31 52.8Dolomite Poland 0.91 0.22 0.25 31.8

set by mass flow controllers at 200 ml/min. For each TGA run,around 80 mg of sorbent of particles size between 400 and600 lm were introduced in the sample holder. Calcination and car-bonation reactions were carried out in the same atmosphere, infive subsequent cycles, at 900 �C and 650 �C, respectively. Each cy-cle, with the exception of the first calcination cycle of 10 min,lasted for 5 min.

2.2.2. Batch FB reactor testsThe experiments were carried out in a stainless steel bubbling

fluidized bed reactor, 40 mm ID operated at atmospheric pressure.The reactor consists of three sections: (a) the preheater/premixer ofthe fluidizing gas, 0.66 m high; (b) the fluidization column, 0.95 mhigh; (c) the brass two-exit head placed on top of the reactor with ahopper to feed the solids in the reactor and connected with the ex-haust line. The gas distributor is a perforated plate with 55 holes of0.5 mm diameter in a triangular pitch. The reactor is electricallyheated with two semi-cylindrical ovens placed around the upperpart of the preheater/premixer and the lower part of the fluidizationcolumn. A type-K thermocouple, placed at 40 mm above the gasdistributor, allows for measuring the reactor temperature. The ther-mocouple is connected with a PID temperature controller whichregulates the electrical power supply for the ovens. The two-exithead is used to convey flue gases through either of two cylindricalsintered brass filters, whose filtration efficiency is one for >10 lm-particles. Alternated use of the filters enables time-resolved captureof elutriated fines at the exhaust. Fluidizing gases are supplied tothe reactor by means of three mass flow meters/controllers.Downstream of the two-exit head, a fraction of the exhaust gas iscontinuously sampled to measure CO2 and SO2 concentrations witha NDIR analyzer in order to monitor the progress of reactions. Con-centration signals are logged on a PC at a sampling rate of 1 Hz.

2.2.3. Batch FB reactor tests proceduresFive calcination/carbonation cycles were carried out in all the

experiments using an initial amount of 20 g of fresh limestone,sieved in the range size 0.4–0.6 mm. Limestone was diluted in abed of sand to avoid excessive temperature variations during calci-nation and carbonation reactions. The bed consisted of 150 g of sil-ica sand in the size range 0.85–1.0 mm and the fluidizing velocitieswere 0.7 and 0.6 m/s in the calcination and in the carbonationstages, respectively. It should be underlined that the presence ofsand may somewhat enhance sorbent attrition as discussed by Sca-la et al. [22]. The operating conditions of the experiments are givenin Table 2.

Before each test the reactor was charged with the silica sandand heated up to the desired temperature (940 �C). The bed wasfluidized with a gas mixture containing 70% CO2. These experimen-tal conditions were selected to simulate calcination in an oxy-firingenvironment. In particular, it was found in preliminary tests that abed temperature as high as 940 �C was necessary to calcine thelimestone at reasonable rates in an environment containing 70%CO2. When the set temperature was reached, the limestone parti-cles were injected in the reactor by means of the hopper. After

MgO K2O Na2O SO3 TiO2 LOI

0.44 0.06 0.02 0.00 0.02 43.10.51 0.08 0.02 0.00 0.02 41.90.26 0.00 0.02 0.00 0.01 43.50.56 0.00 0.01 0.00 0.02 42.80.94 0.00 0.02 0.00 0.02 42.60.99 0.00 0.02 0.00 0.03 41.4

20.9 0.00 0.00 0.00 0.02 45.9

Table 3CO2 capture capacity of sorbents in FB and TGA.a

g CO2/g sorbent (Batch FB) g CO2/g sorbent (TGA)

Sorbent With SO2 w/o SO2 With SO2 w/o SO2

M 0.004 0.023 0.021 0.066T–O 0.005 0.027 0.015 0.050Cz 0.019 0.067 0.020 0.068Sw 0.012 0.024 0.023 0.066EnBW 0.005 0.026 0.025 0.076Xr 0.010 0.041 0.033 0.075Db – – 0.060 0.101

a M: Massicci; T–O: Tarnow–Opolski; Cz: Czatkowice; Sw: Schwabian; En:EnBW; Xr: Xirorema; D: Dolomite Redziny.

b Tests with dolomite have not been performed in the FB reactor, but only in TGA.

Table 2Operating conditions of the FB calcination/carbonation experiments.

Calcination/carbonation Without SO2 With SO2

Duration (min) 20/15 20/15Temperature (�C) 940/650 940/650Inlet CO2 (%v/v) 70/15 70/15Inlet SO2 (ppmv) 0/0 1500/1500

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injection, the sorbent underwent rapid heating from ambient tem-perature to the reactor temperature, resulting in a thermal shock.The progress of calcination was followed during the run by mea-suring the CO2 concentration at the exhaust. The run ended whencalcination was complete. At this point the bed was rapidly dis-charged and cooled down. The sand was separated from the lime-stone by sieving and re-injected in the fluidized bed reactor. Thetemperature of the bed was then set to the carbonation tempera-ture (650 �C). When the new temperature was reached the carbon-ation reaction was started by fluidizing the bed with a mixture ofair and CO2 (15%v/v), and injecting the calcined sorbent particlesthrough the hopper.

The progress of carbonation was followed during the run bymeasuring the CO2 concentration at the exhaust. Again, the runended when carbonation was complete. The bed was rapidly dis-charged and cooled down (in 100% CO2 to avoid possible calcina-tion). The procedure described before was then repeated in allthe cycles. The same duration of the first calcination stage wasused for the successive calcination stages, and the same criterionwas applied for the duration of the carbonation stages (Table 2).It must be noted that because of the above procedure the limestoneparticles underwent a severer thermal shock than that occurring ina real calcium looping process, where the particles are not cooleddown to ambient temperature.

Tests carried out to study the effect of SO2 on the calcium loop-ing process, were performed under the same conditions as thosewithout SO2, with the only difference being the presence of1500 ppm of sulfur dioxide in the inlet gas flow, both under calci-nation and carbonation stages. This concentration was selected tosimulate typical values of SO2 in uncontrolled flue gas.

Table 4Conversion (%) of CaO to CaSO4 in FB and TGA.

2.3. Characterization of sorbents

Raw, carbonated, and sulfated–carbonated sorbents were sub-jected to X-ray diffraction (XRD), using a Bruker D8 Advanceinstrument, to comparatively examine their mineralogy. Micro-structure of sorbents was examined by means of Scanning ElectronMicroscopy (SEM, 6300 JEOL). Chemical composition of particu-larly selected areas of sorbents was determined by means of En-ergy-Dispersive X-ray Spectroscopy (EDS) using an automatedanalysis system, installed on the SEM instrument. Some sorbentsamples were embedded in epoxy resin and then cut and polishedin order to analyze by SEM-EDS the internal structure of the parti-cle cross-section. Nitrogen adsorption measurements were per-formed using an Autosorb-1, Micropore version, static volumetricsystem (Quantachrome instruments), at 77 K. Prior to each mea-surement test, the samples were outgassed at 623 K under highvacuum.

Sorbent % Conversion to CaSO4 (Batch FB) % Conversion to CaSO4 (TGA)

M 25.8 4.66T–O 24.6 4.56Cz 21.1 3.64Sw 25.3 4.22EnBW 19.3 3.45Xr 20.9 3.94Da – 3.8

a Tests with dolomite have not been performed in the FB reactor, but only in TGA.

3. Results and discussion

3.1. CO2 capture capacity of sorbents

Table 3 shows the CO2 capture capacity of the various sorbentsafter five cycles (g CO2/g sorbent), both in the presence and in theabsence of SO2 in the gas phase, in FB and TGA tests. Table 4 reports

the final Ca conversion to CaSO4 during tests in the presence of SO2.It is clear that sulfur acts as an inhibitor of carbonation: it leads to adrop in CO2 capture capacity of limestone sorbents, as high as81.4% in the case of batch FB tests, and 75.5% in the case of TGA.In the case of dolomitic sorbent, CO2-capture capacity is decreasedby 40.5% in the presence of sulfur. The effect of sulfur dioxide onsorbent performance is harsh, especially in batch FB tests, wheretemperature is 40 �C higher than in TGA tests. Simultaneous sulfa-tion and carbonation reactions reduce the CO2 sorption capacity byblocking access to free CaO. It was inevitable to select a highertemperature, as CO2 concentration in gas phase was much higherin batch FB experiments and calcination had to be enhanced. Con-cerning conversion to CaSO4, in TGA tests sulfation rates were sig-nificantly lower than in FB tests, due to a shorter cycle durationtime and a lower temperature of calcination, as well as to possiblediffusional resistances in the sorbent layer. It must be noted thatduring the FB experiments, both attrition and fragmentation werevery low for all six limestones, as has been previously reported[23].

3.2. Microstructure of sorbents

Figs. 1a and 1b show micrographs of Czatkowice sorbent, freshand after carbonation (without S), respectively. It is clear that afterfive cycles of carbonation–calcination, ‘‘saturated’’ zones can beobserved, where the carbonate cannot grow any further. Such earlysaturation is not attributed to the nature of the tested limestones,but to the TGA conditions instead. The micrograph of raw Czatkow-ice limestone clearly shows that it consists of small, individual CaOgrains, either surrounded by voids, or bonded to their neighboringgrains. By comparing the micrographs of Figs. 1a and 1b, it is obvi-ous that carbonates are indeed partially sintered, as less regions ofseparate particles can be found on their surface. Rather, this sam-ple contains a lot of post-sintering regions, as well as fractures andtiny craters, attributed to gas penetration (a mixture of carbondioxide, nitrogen, and oxygen). It remains undefined whether thecurrent level of sintering has been reached after 5 min of thermaltreatment, or sintering progresses until the end of the 5th carbon-

Fig. 1d. Fracturing within carbonated Czatkowice sorbent. Micrograph at the scaleof 20 lm.

Fig. 1b. SEM micrograph of carbonated Czatkowice sorbent, at the scale of 10 lm.

Fig. 1a. SEM micrograph of raw Czatkowice sorbent, at the scale of 10 lm.

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ation stage (a total of 30 min of thermal treatment). Individual,unsintered particles of carbonated Czatkowice sorbent (Fig. 1c)can still be found, indicating an unfinished sintering process dueto time limitations. Fig. 1d illustrates (with arrows) aspects of frac-turing within the carbonated Czatkowice sample. It seems that thegas stream finds a way of penetration through inter-particle re-gions, leaving back linear and non-linear rifts. Thus, it can be in-ferred that a semi-finished sintering process may act beneficiallyto the CO2 uptake capacity of Ca-based sorbents, as it providesthe gas stream with an easier ‘‘entry path’’ to its innerenvironment.

The morphology of such rifts and voids is different when sulfuris present in the gas stream. Indeed, by the reaction between freeCaO and SO2, a ‘‘shell’’ of anhydrite (CaSO4) is formed peripherallyto the CaO particle, obstructing its further carbonation. Fig. 2a

Fig. 1c. Non-sintered CaO particles within carbonated Czatkowice sorbent. Micro-graph at the scale of 5 lm.

shows a selected SEM micrograph of the carbonated–sulfatedXirorema sample. The identified bright-colored cylindrical struc-tures refer to anhydrite, as it is concluded by the strong signal ofS in the respective EDS spectrum (Fig. 2b) (Au signal is apparentlydue to sample coating for SEM analysis), and X-ray diffraction,which revealed anhydrite as the predominant sulfurous ingredientof the sulfated sorbents. By dot-mapping the distribution of sulfurin the cross-section of a sorbent particle (Fig. 2c), this assumptionis confirmed, as sulfur is primarily concentrated in the boundariesof sorbent particles and much less in their core.

The sulfurous superficial formations of Xirorema carbonatedsorbent are shown at a higher scale in Fig. 3. The EDS spectra ofthe marked bright regions showed that they are all dominated byS and Ca. As it is inferred by the comparative assessment of thetested sulfated & carbonated sorbents by the use of EDS, Xiroremalimestone showed a quite high utilization rate of Ca, for the uptakeof S, which should normally hinder its further carbonation. As it isshown later in this paper, the change in porosity between carbon-ated and sulfated–carbonated Xirorema samples is less significantthan in the case of Czatkowice and EnBW sorbents (more carbon-ation-efficient), suggesting less shrinkage (and thus less CO2 up-take) in the former. This higher utilization rate is probably due tosmaller-sized particles. The superficial formation of uniformly sul-fated regions suggests that part of the CaO particle-core remainsunreacted (uncarbonated). Apart from the uniformly-sulfated indi-vidual particles, those can be also found in a network-mode [24],enhancing the thickness of the sulfurous outer layer of CaO parti-cles, thus preventing CO2 from further penetrating the CaO parti-cle. According to Laursen et al. [24], the network particles arecharacterized ‘‘by only being highly sulfated around the peripheryand in the proximity of fractures’’. By observing the micrograph of

Fig. 2a. SEM micrograph of carbonated–sulfated Xirorema sample, at the scale of100 lm.

Fig. 2c. SEM micrograph of Xirorema sample cross-section (left). Sulfur dot-

Fig. 2b. EDS spectrum of carbonated–sulfated Xirorema sorbent.

Fig. 4. Fracturing start-up at sulfated–carbonated Xirorema sorbent. Micrograph atthe scale of 50 lm.

Fig. 3. Sulfurous particles of the Xirorema carbonated–sulfated sorbent sample, atthe scale of 10 lm.

G. Itskos et al. / Applied Energy 108 (2013) 373–382 377

Fig. 4, which shows the initialization of fracturing due to gas pen-etration through carbonated particles, this statement is partiallyconfirmed, if we attribute the networking particles close to frac-tures, solely to anhydrite.

In dolomite, as it is shown in Figs. 5 and 6, fracturing progressesin a different mode, which results in longer and deeper rifts, prob-ably due to its locally softer surface. The micrograph of Fig. 5 showsa region presenting a high degree of sulfation. It seems that thisparticular crater depicted here, serves as the base upon whichthe whole sulfation process can get started. The fact that the mostsulfurous regions of dolomite are found proximate to craters andbig fractures, suggests that most of CaSO4 are formed in a networkmode, thus probably leaving some space for CO2 capture within thevarious reactive regions of this particular sorbent. It is noted thatsuch sorbents present a medium capacity for reactivation, as CaSO4

mapping distribution (middle) and Ca dot-mapping distribution (right).

is more likely to get decomposed by means of H2O in this case,compared to the uniformly-sulfated ones. Fig. 7 shows certain sul-fates upon the surface of dolomite.

Figs. 8a–8f show selected SEM micrographs of the tested sul-fated carbonates (EnBW, Xirorema, Schwabian, Tarnow, Massicci,and Czatkowice, respectively), after having underwent five cyclesof carbonation–calcination at the batch FB reactor. In Fig. 8a, a cer-tain fracture upon the surface of EnBW sample is shown, with itsneighboring, networking sulfated CaO particles. As in the case ofTGA testing, also in the FB reactor, the main sulfation mode isthe ‘‘unreacted core’’ one, whilst networking sulfated particlesare found mainly next to superficial fractures. Fig. 8b shows a sim-ilar region of fracturing surrounded by sulfated CaO particles inXirorema sample. By comparatively investigating the morphologyof sulfated TGA and FB carbonates, it can be inferred that sinteringtook place to a much lesser excess in the first case (Figs. 8c and 8d:indicative post-sintering phases). It is also quite clear that the pres-ence of sulfur in flue gases leads to higher porosity of sorbents(Fig. 8e), which in turn allows for an enhanced penetration of gas

Fig. 5. Sulfated–carbonated dolomite. Micrograph at the scale of 5 lm.

Fig. 8b. SEM micrograph of Xirorema sulfated sorbent, at the scale of 20 lm.

Fig. 8c. SEM micrograph of Schwabian sulfated sorbent, at the scale of 20 lm.

Fig. 8f. SEM micrograph of Czatkowice sulfated sorbent, at the scale of 20 lm.

Fig. 7. Sulfates on dolomite. SEM micrograph at the scale of 10 lm.

Fig. 6. Fracturing at dolomite. SEM micrograph at the scale of 10 lm.

Fig. 8d. SEM micrograph of Tarnow–Opolski sulfated sorbent, at the scale of 5 lm.

Fig. 8a. SEM micrograph of EnBW sulfated sorbent, at the scale of 20 lm.

Fig. 8e. SEM micrograph of Massicci sulfated sorbent, at the scale of 20 lm.

378 G. Itskos et al. / Applied Energy 108 (2013) 373–382

Fig. 9a. SEM micrograph of EnBW sample cross-section (left). Sulfur dot-mapping distribution (middle) and Ca dot-mapping distribution (right).

Fig. 9b. SEM micrograph of Schwabian sample cross-section (left). Sulfur dot-mapping distribution (middle) and Ca dot-mapping distribution (right).

G. Itskos et al. / Applied Energy 108 (2013) 373–382 379

particles (diffusion and adsorption of SO2 and CO2 into the sorbent)but the film of anhydrite that is consequently formed (gas–solidreaction) obstructs the process of carbonation over the next Ca-looping cycles. An indicative SEM picture of a less porous sulfatedsorbent is given in Fig. 8f. Figs. 9a and 9b show selected SEM pic-tures of the cross-sections of sulfated EnBW and Massicci carbon-ate particles, respectively, along with the respective dot-mappingdepictions of sulfur and calcium distribution. As in TGA, sulfur ismostly concentrated in the boundaries of particles, confirmingour interpretations, which have been based upon the ‘‘unreactedcore’’ approach.

3.3. Specific surface area and porosity of sorbents

Figs. 10a–10d show diagrams comparing the specific surfacearea (BET, m2/g), porosity (%), total pore volume (ml/g), and micro-porosity (%) of the tested fresh, carbonated, and sulfated-carbon-ated sorbents, after five cycles of carbonation–calcination in TGAApparatus, respectively. It seems that BET area of the sulfated–car-bonated sorbents is higher than the non-S-carbonated ones(Fig. 10a), revealing less sintering in the former, due to the forma-tion of the non-porous shell of anhydrite. It is known that sinteringof CaO during calcination gives rise to a sharp reduction in the sur-face area, which can in turn affect the reaction rates of the calcinewith SO2 and CO2 [25]. From the tested limestone sorbents, theonly two cases in which BET area of the sulfated–carbonated sam-ple is smaller than the carbonated one, are Tarnow and Schwabianlimestone, an indication that these sorbents are either sulfated to amuch lesser degree, or sulfation takes place in a network-particlemode (and is not uniform). In this case, it is also likely that theanhydrite shell is more porous, a possible indication of direct sul-fation (CaCO3 + SO2 + O2 = CaSO4 + CO2) [26]. BET area of the sul-fated–carbonated dolomite is also slightly lower than of thecarbonated one, a fact attributed to the network-mode sulfatedparticles of the sorbent, identified by means of SEM. Althoughthe higher specific surface area of sorbents is an indicator for ahigher CO2-uptake potential, it is still likely that advantage of thisproperty cannot be taken up to a full extent, due to the anhydriteshell occurring around the core of the CaO particles (this hypothe-

sis takes for granted that in most of the tested cases the sulfationmode is the ‘‘unreacted particle core’’).

Carbonated samples with sulfur demonstrate higher porositythan the non-sulfurous ones (Fig. 10b), which is also an indicationof less sintering in the former. This is again attributed to the crys-tallization of anhydrite that takes place peripherally to the CaOparticle and obstructs the deeper penetration of gas particles,and thus the further carbonation of CaO. The most drastic increaseis demonstrated by EnBW sample, a result of the significantly high-er BET area of the sulfated EnBW sorbent and, as a result, the moreexcessive reaction between Ca and S. Thus, it seems that the pres-ence of sulfur in flue gases leads to less sintering between CaO par-ticles. Micro-porosity (<2 nm) reduces during carbonation–calcination cycles, most likely due to CaO particles sintering.Simultaneously, (meso- and macro-) porosity increases, due toCaCO3 recrystallization. Fast recrystallization of the newly formedcarbonate, produces randomly oriented crystals of a much smallersize than the original crystalline grains, thus leaving networks oflarger pores in-between. Such large pores allow for an easy pene-tration of CO2, even in highly converted particles. Micro-porosity,which generally reduces with carbonation, showed no clear ten-dency between sulfated and non-sulfated sorbents (Fig. 10d). Thiscan be probably because the sintering/shrinking processes in theinner part of a CaO particle are not directly affected by the CaSO4

‘‘shell’’ that is formed in its outer part, and the two processesmay occur simultaneously. On the other hand, the loss in micropo-rosity is translated into a drop in the associated conversion duringcarbonation, while it is also confirmed that this loss is proportionalto the increase in macro-porosity. It is assumed that the increase inmacro-porosity is proportional to the loss of microporosity, aftereach one of the five cycles of carbonation–calcination the samplesunderwent.

Figs. 11a–11d show diagrams comparing the specific surfacearea (BET, m2/g), porosity (%), total pore volume (ml/g), andmicroporosity (%), respectively, of the sulfated TGA and FB car-bonates. In these Figures, the respective values of fresh sorbentsare also cited as a reference. It is quite obvious that in the caseof batch FB samples, sintering is almost complete (porosity rangesfrom 0.5% to 1%, compared to 4.3–8.1% of TGA samples), attrib-

Fig. 10a. BET area (m2/g) of fresh, carbonated, and carbonated–sulfated sorbents,tested at TGA.

Fig. 10d. Micro-porosity (%) of fresh, carbonated, and carbonated–sulfated sor-bents, tested at TGA.

Fig. 11b. Porosity (%) comparison of fresh and carbonated–sulfated sorbents, testedat TGA and FB.

Fig. 11a. BET area (m2/g) comparison of fresh and carbonated–sulfated sorbents,tested at TGA and FB.

Fig. 10c. Total pore volume (TPV, ml/g) of fresh, carbonated, and carbonated–sulfated sorbents, tested at TGA.

Fig. 10b. Porosity (%) of fresh, carbonated, and carbonated–sulfated sorbents,tested at TGA.

380 G. Itskos et al. / Applied Energy 108 (2013) 373–382

uted to the higher calcination temperature of the latter (940 �Cagainst 900 �C). Higher temperature is selected to better facilitatecalcination, which could not be fully performed due to the highbulk CO2 concentration in the gas phase (70%, against 15% inthe respective TGA ones). Reasonably, the TGA carbonates aremore reactive (higher BET) and porous than the FB ones. By com-paring reactivity and porosity of fresh and batch FB sulfated car-bonates, no major differentiation is observed, suggesting that ahigh calcination temperature along with parallel sulfation canonly lead to the severe deterioration of sorbent properties (mainlythrough rapid pore closure), even after only five cycles of carbon-ation–calcination. Given that the number of cycles is the most

important parameter in sorbents CO2-uptake performance [27],such a result is far from encouraging, at least when ultra-highlysulfurous calcination conditions are concerned. It is noted thatporosity in FB sulfated carbonates occurs basically in the formof fractures and meso- and macro-pores, as microporosity is neg-ligible (ranging from 0.06% to 0.1%). Overall, Schwabian, EnBW,Czatkowice, and Xirorema samples are best in terms of reactivity,however mild calcination conditions (temperature around 900 �Cand lowest possible sulfur concentration, depending on the prop-erties of the available feedcoal) are apparently required to main-

Fig. 11d. Micro-porosity (%) comparison of fresh and carbonated–sulfated sorbents,tested at TGA and FB.

Fig. 11c. Total pore volume (TPV, ml/g) comparison of fresh and carbonated–sulfated sorbents, tested at TGA and FB.

G. Itskos et al. / Applied Energy 108 (2013) 373–382 381

tain their reversibility at a high level, as it has been also reportedin literature [28].

4. Conclusions

� In most cases, the core of sorbent particles remained unreacted(uncarbonated), as a result of sulfur presence in the gas phase.Networking sulfated particles were also observed, mainly closeto fractured regions of the sorbent surfaces.� The BET area of sulfated TGA carbonates was higher than the

non-sulfated ones, revealing less sintering in the former, dueto the formation of a non-porous anhydritic shell, peripherallyto the sorbent particle.� From the TGA-tested limestone sorbents, the only two cases in

which BET area of the sulfated–carbonated sample was smallerthan the carbonated one, are Tarnow–Opolski and Schwabianlimestone, an indication that sulfation took place in a net-work-particle mode (and was not uniform).� Sulfated carbonates presented higher porosity than the non-sul-

fated ones. This may be attributed to the crystallization of anhy-drite peripherally to the CaO particles that hindered a deeperpenetration of gas in the particles, and thus the further carbon-ation of CaO. The most drastic increase was presented by EnBWsample, a result of the significantly higher BET area of the sul-fated EnBW sorbent, and subsequently the enhanced reactionbetween Ca and S.� Microporosity, which generally reduces with carbonation,

showed no clear tendency between the sulfated and non-sul-fated TGA sorbents. The loss in microporosity was translated

into a drop in the associated conversion during carbonation,while it is also confirmed that this loss was proportional tothe macro-porosity increase. Microporosity of the sulfated FBcarbonates was negligible due to the excessive sintering theyunderwent.� Concerning carbonation, it is possible that the formation of a

porous anhydrite shell peripherally to CaO particle could miti-gate the undesirable consequences of sulfation. Thus, the partialpressure of CO2 may be deliberately increased, so to lead to thedirect sulfation of calcines and thus get as much porous anhy-drite as it can be formed.� As a result of the stronger sintering conditions in the FB reactor

experiments, the sulfated FB carbonates presented much lowerreactivity and porosity than the TGA ones. The properties of theFB sulfated carbonates after five cycles were comparable tothose of the respective raw sorbents.

Acknowledgements

This work was funded by EU RFCS project CaL-Mod, ContractNumber: RFCR-CT-2010-00013. TITAN S.A. is acknowledged forthe XRF analyses of sorbents.

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