Attrition of sorbents during fluidized bed calcination and sulphation

Ž .Powder Technology 107 2000 153–167www.elsevier.comrlocaterpowtec

Attrition of sorbents during fluidized bed calcination and sulphation

Fabrizio Scala a, Piero Salatino a,), Renee Boerefijn b,1, Mojtaba Ghadiri b

a Dipartimento di Ingegneria Chimica, UniÕersita di Napoli Federico II, P.le Tecchio 80, 80125 Naples, Italy`b Department of Chemical and Process Engineering, UniÕersity of Surrey, Guildford, Surrey GU2 5XH, UK

Received 12 March 1999; accepted 22 June 1999

Abstract

The attrition behavior of two different limestones during calcination and sulphation in fluidized beds has been investigated by acombination of experimental techniques. The aim of the study is to shed light on the interactions between sorbent attrition and the changeof particle mechanical and morphological properties associated with the progress of chemical reactions. A number of differentexperimental techniques have been used to characterize breakage mechanisms relevant to particle attrition in different sections ofindustrial fluidized bed reactors operated at atmospheric pressure. Primary fragmentation and abrasive attrition were characterized in situby means of experiments carried out in a bench-scale fluidized bed reactor operated batchwise. Fragmentation under high velocity impactconditions was studied ex situ by means of single particle impact tests on pre-conditioned samples at room temperature. Scanning electronand optical microscopy analyses of the particles and EDX mapping of polished particle cross-sections were used to relate topography andinternal composition of sorbent particles to the attrition mechanism. q 2000 Elsevier Science S.A. All rights reserved.

Keywords: Attrition; Fluidized bed; Limestone; Calcination; Sulphation

1. Introduction

In situ reduction of sulphur oxides emissions in flu-idized bed combustion is usually accomplished by injec-tion of sorbents like limestone or dolomite. In the fluidizedbed, at typical atmospheric combustion conditions, sorbentparticles undergo simultaneous chemical reactionsŽ .calcination and sulphation and attrition. Sorbent attritionaffects the performance of the combustor, by influencingthe size distribution of the sorbent particles and the elutria-tion of fines in the flue gases, and possibly by enhancingthe extent of limestone conversion by exposing unreactedcalcium oxide.

A relatively large number of studies have addressedw xattrition of limestone in fluidized beds 1–7 . Most of the

effort, however, was directed towards the characterizationof attrition of lime in an inert atmosphere. The relevanceof the simultaneous occurrence of calcination and sulpha-tion reactions to limestone attrition was first recognized by

w x w xChandran and Duqum 8 and Couturier et al. 9 . Recently,

) Corresponding author. Tel.: q39-081-7682258; fax: q39-081-5936936; E-mail: [email protected]

1 Presently at Unilever Research Laboratory, P.O. Box 114, 3130 ACVlaardingen, The Netherlands.

w x w xScala et al. 10 and Di Benedetto and Salatino 11 haveinvestigated the interactions between attrition and chemicalreactions, through the changes of particle properties alongwith conversion. In particular, it was demonstrated thatneglecting the strengthening effect of the particle surfaceupon sulphation may lead to a consistent overprediction offines generation by attrition.

In the light of the different breakage mechanisms and ofw xthe typical size of generated fragments, Scala et al. 10

provided a classification of sorbent attrition phenomenaduring fluidized bed calcination and sulphation. Primaryfragmentation occurs immediately after the injection of theparticles in the bed, as a consequence of thermal stressesdue to rapid heating of the particles and of internal over-pressure due to carbon dioxide emission. This may resultin the generation of coarse as well as fine fragments.During their processing in the fluidized bed, sorbent parti-cles are also subject to attrition by abrasion and sec-ondary fragmentation as a consequence of mechanicalstresses due to collisions with other particles or with the

w xinternals of the reactor. According to Blinichev et al. 12these phenomena can be classified on the basis of thetypical size of the generated fragments: attrition by abra-sion generates fines which quickly elutriate; secondaryfragmentation generates coarser fragments, which cannot

0032-5910r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.Ž .PII: S0032-5910 99 00185-0

( )F. Scala et al.rPowder Technology 107 2000 153–167154

easily elutriate. Attrition by abrasion depends on the resis-tance of the bed particles to surface wear. Secondaryfragmentation, instead, should be related to the resistanceof the particles to impacts against walls and internals of

w xthe bed or in the jetting region of fluidized beds 13 .The aim of this work is to investigate the influence of

the progress of calcination and sulphation on the attritionof two limestones of different origin. In particular, primaryfragmentation is characterized in situ by comparing parti-cle size distribution of the original limestone with that oflime particles generated by in-bed calcination. Abrasiveattrition is characterized in situ by collection of elutriatedfines as they are generated during fluidized bed calcinationandror sulphation. A bench-scale apparatus operatedbatchwise has been used to this end. Attrition under highvelocity impact conditions is studied ex situ by means ofsingle particle impact tests at room temperature. The twotechniques are complementary, since they provide a mea-sure of the propensity to attrition in different fluidized bed

Ž .regions, where either low-energy dense phase or high-en-Ž .ergy jetting region, lean phase impacts prevail. More-

over, the effect of process conditions on the attritionpropensity of the sorbents has been investigated further by

Ž .scanning electron microscopy SEM analysis and EDXmapping of limestone particles.

2. Experimental

2.1. Apparatus and techniques for in situ experiments

The in situ experiments were carried out in a bubblingŽ .fluidized bed reactor, 40 mm ID and 1 m high Fig. 1 ,

operated at atmospheric pressure and Ts8508C. The reac-tor, made of stainless steel, was electrically heated. Thegas distributor was a perforated plate with 55 holes of 0.5mm diameter in a triangular pitch. The bed material con-sisted of mixtures of sorbent and sand, both double sieved

Fig. 1. Experimental apparatus for in situ tests.

( )F. Scala et al.rPowder Technology 107 2000 153–167 155

in the size range of interest. Two different sorbents wereŽ .tested, namely an Italian limestone Massicci and a

Ž .Swedish porous limestone Ignaberga , with chemicalcompositions as shown in Table 1.

Primary fragmentation was characterized by comparingŽ .particle size distributions PSD of the original limestones

with those of the lime remaining in the bed after calcina-tion at 8508C in nitrogen. To this end, 20 g of limestone ofsize 0.6–0.85 mm were added to a coarser bed made ofsilica sand in the size range 1.0–1.18 mm. PSD of theoriginal limestone was determined by image analysis underoptical microscope. After calcination was completed, thelime particles could be retrieved from the bed by discharg-ing the entire bed inventory and separating the sorbentfrom the sand by gently sieving. Change of apparentparticle volume upon calcination was limited, and so wasthe attrition propensity of the sand. This ensured that thematerial retrieved from the bed in the size range below0.85 mm consisted entirely of lime. PSD of lime sampleswere determined by optical microscopyrimage analysis ofsamples submerged in carbon tetrachloride before ambientmoisture uptake might have occurred to any significantextent.

Attrition was quantified during three different reactionstages:Ž . Ž .i calcination C ;Ž . Ž .ii sulphation of pre-calcined samples S ;Ž . Ž .iii simultaneous calcination and sulphation CS .

The calcination experiments were carried out in air. The Sand CS experiments were carried out fluidizing the bed

Ž .with a mixture of nitrogen, oxygen 8.5 vol.% and SO2Ž .1800 ppm . The fluidizing gas superficial velocity was

y1 Ž .always kept at 0.8 m s . A limited amount 20 g of freshŽ .limestone in C and CS experiments or of precalcinedŽ .limestone in S experiments was charged into the bed of

Ž .sand 150 g operated at 8508C. Both fresh limestone andsand were sieved in the nominal size range 0.425–0.6 mm.The attrition rate of bed material was determined bycontinuously measuring the amount of fines elutriatedfrom the bed. Fines were collected alternately in one oftwo parallel filters for definite periods of time. The differ-ence in weight of the filter before and after operation,divided by the time interval of collection, was taken as theaverage elutriation rate relative to that interval. Furtherdetails of the experimental apparatus and procedure are

w xreported elsewhere 10 .

Table 1Chemical composition of the limestones tested

Ž . Ž .Chemical composition Massicci % Ignaberga %

CaCO 96.8 91.03

MgCO 2.4 1.13

Others 0.8 7.9Ž .Loss on ignition CO 43.9 40.62

Fig. 2. Experimental apparatus for ex situ tests.

2.2. Apparatus and technique for ex situ experiments

Ex situ experiments consisted of single particle impacttests, using the air eductor device shown in Fig. 2. Samplesof approximately 5 g were prepared by gently sieving a cutof 0.6–0.85 mm using BS 410 sieves. The materials used

Ž .were fresh F samples of Massicci and Ignaberga, as wellas samples resulting from the three in situ process stages,i.e., calcined, calcined and subsequently sulphated andsimultaneously calcined and sulphated. Particles were fedin a single array into the funnel-shaped inlet section at thetop of the eductor and subsequently accelerated on the airflow provided by a compressor. Upon exiting the eductortube the particles impacted on a rigid target plate, made ofsapphire. The particle velocity was controlled by regulat-ing the air flow and measured by means of a timer whichwas triggered by two diodes at the end of the eductor tube.The entraining air left the collection chamber through abrass porous plate, covered by a Whatman class 1 paperfilter, retaining the impact products down to 4 mm. The

Ž .collection chamber was kept at a low vacuum 2 mbar .After impact, the material was collected and the debrisseparated from the surviving mother particles using a sievewith a size of two BS 410 sieve sizes below the lower sizelimit of the original sieve cut, in this case 425 mm.Attrition was then quantified as the fractional mass loss,i.e., the ratio of the mass of debris to the mass of the feedmaterial. The latter was approximated by the sum of massof debris, M , and mother particles, M , collected afterd m

impact:

MdRs . 1Ž .

M qMm d


This expression was found to be insensitive to the effectsof moisture take-up. The procedure of collection of impactproducts and sieving will inevitably cause material losses,which may arise from the loss of the surviving mother

Ž .particles as well as from fine debris. Eq. 1 does not takeaccount of losses, but it can be shown that it is between thelower and upper limit of fractional mass loss, as defined by

w xPapadopoulos and Ghadiri 14 .

2.3. MicroscopyrEDX characterization of samples

The morphology of selected sorbent samples was char-acterized by the use of three different techniques. Motherparticles and debris from impact breakage tests were ob-served under a Meji Stereomicroscope at magnificationsup to 4.5 times. Polished cross-sections of preconvertedlimestone particles embedded in epoxy resin were ob-

Žserved under a scanning electron microscope Philips XL30.with LaB filament at magnifications up to 200 times.6

SEM observations were complemented by mapping ofsulphur throughout the particle cross-section by means of

Ž .an EDX probe EDAX DX-4 .

3. Experimental results

3.1. In situ experiments: assessment of primary fragmenta-tion

Cumulative particle undersize distributions of in-bedsorbents before and after calcination in the fluidized bed of

Massicci and Ignaberga are shown in Figs. 3 and 4,respectively.

The particle size distribution of calcined Massicci lime-Žstone is slightly shifted towards the smaller sizes by about

.40 mm throughout the size range when compared withthat of the fresh material. No significant population ofparticles finer than 500 mm is observed. It is likely thatparticle shrinkage associated to calcination and roundingoff are responsible for the moderate shift in the particlesize distribution. Altogether, it is concluded that no appre-ciable primary fragmentation takes place for this lime-stone.

Calcination of Ignaberga results in a similar shift inparticle size distribution towards smaller sizes when com-pared with the fresh material. Moreover a bimodal distribu-tion is observed in the calcined sample with a significantpopulation of fine particles: about 15% of the sorbent, onmass basis, consists of particles smaller than 500 mm,whereas no fresh material belonged to this size range. It isconcluded that primary fragmentation takes place to asignificant extent for this limestone.

It must be recalled here that elutriated fines were notcollected during experiments for primary fragmentationassessment, which relied on the characterization of the bedinventory only. The role of primary fragmentation as adirect or indirect source of elutriable fragments will befurther addressed in Section 3.2.

w xIn the light of experimental results of Scala et al. 10 ,secondary fragmentation was not investigated in this studyby means of in situ experiments. The authors, in fact,showed that in the present experimental conditions sec-

Fig. 3. Cumulative particle undersize distribution of fresh and calcined limestone: Massicci. Fresh sorbent particles size range: 0.6–0.85 mm. Sorbentsample mass: 20 g. Sand particles size range: 1.0–1.18 mm. Sand bed mass: 150 g. Bed temperature: 8508C.


Fig. 4. Cumulative particle undersize distribution of fresh and calcined limestone: Ignaberga. Fresh sorbent particles size range: 0.6–0.85 mm. Sorbentsample mass: 20 g. Sand particles size range: 1.0–1.18 mm. Sand bed mass: 150 g. Bed temperature: 8508C.

ondary fragmentation is negligible, as a consequence of thelow energy of particle collisions in the fluidized bed.

3.2. In situ experiments: assessment of abrasiÕe attrition

Fig. 5 shows the rate of sorbent fines elutriation Emeasured during the calcination and the subsequent sul-

phation of fresh samples of the two limestones. Duringcalcination, E decreases with time until a steady state

Ž .value E is reached. Two mechanisms may be invoked`,cŽ .to explain the observed trend: i rounding off of the

Ž .particle surface, and ii fines generation by primary frag-mentation due to thermal shock and calcination. The mech-anism through which primary fragmentation enhances attri-

Fig. 5. In situ experiments: sorbent elutriation rate during subsequent limestone calcination and sulphation. Fresh sorbent particles size range: 0.425–0.6mm. Sorbent sample mass: 20 g. Sand particles size range: 0.425–0.6 mm. Sand bed mass: 150 g. Fluidizing gas superficial velocity: 0.8 m sy1. Bedtemperature: 8508C.


tion in the early stage of sorbent processing is twofold: onthe one hand it may directly lead to the generation ofelutriable fragments; on the other it gives rise to relativelycoarse fragments with a highly angular shape, which aretherefore more prone to surface wear. The elutriation ofdirectly generated fines from the bed should take placeover a short time scale, comparable with the relaxationtime of both thermal stresses and mechanical stressesinduced by the release of gaseous CO upon calcination.2

This time is of the order of a few minutes or less depend-w xing on the operating conditions 10 . The generation of

fines by attrition of coarse fragments generated by primaryfragmentation, instead, should take place over a longertime scale. Fig. 6 reports the particle size distributionŽ .obtained by sieving of elutriated material cumulativelycollected during calcination experiments with both lime-stones. It appears that much coarser elutriable fines aregenerated during calcination of Ignaberga limestone thanduring calcination of Massicci limestone. Altogether, com-bined analysis of results from primary fragmentation ex-

Ž .periments Figs. 3 and 4 and from surface abrasion exper-Ž .iments Figs. 5 and 6 suggests that rounding off is the

dominant breaking mechanism active during calcination ofŽ w x.Massicci as reported earlier by Scala et al. 10 . For

Ignaberga, instead, primary fragmentation and roundingoff are both active at the same time.

At the steady state of the calcination stage, E is`,c

slightly larger for Massicci than for Ignaberga. This can bepartly attributed to the larger mass of Massicci limestone

Ž .remaining in the bed and exposed to abrasive attrition

after calcination. It is important to note here that theprocess of particle attrition in the bed is dominated byabrasion of the lime particles by the sand particles, whichare much harder than both types of lime.

Upon sulphation of precalcined lime the attrition rate ofboth types of limestones decreases dramatically until a

Ž . Ž .new steady-state value E is reached Fig. 5 , about one`,s

order of magnitude lower than that observed under calcina-w xtion conditions. As pointed out by Scala et al. 10 , this

trend can be related to surface strengthening of the particleas calcium oxide reacts to form a calcium sulphate surfaceshell. In contrast to calcination, where conversion takesplace far more rapidly than the decay of the fines elutria-tion rate, during sulphation the fines elutriation rate wasobserved to decay over a time scale comparable to thatassociated with the progress of conversion.

Figs. 7 and 8 compare the attrition rates of Massicci andIgnaberga limestone samples, respectively during calcina-tion of fresh limestone, during sulphation of precalcinedlime and during simultaneous calcination and sulphation offresh limestone. The feed particles of the S stage havebeen preconditioned in the calcination stage, hence thedifference in the initial rate of elutriation of C and S. Asexpected, for both limestones the profile corresponding tosimultaneous calcination and sulphation conditions lies inbetween those relative to separate calcination and sulpha-tion. Initially, the CS profile is close to that of C, asintuitively expected since little conversion has occurred.The long term attrition rate E is comparable under`,s

sulphation and simultaneous calcination and sulphation

Fig. 6. In situ experiments: cumulative particle undersize distribution of fines elutriated throughout the calcination stage. Fresh sorbent particles size range:0.425–0.6 mm. Sorbent sample mass: 20 g. Sand particles size range: 0.425–0.6 mm. Sand bed mass: 150 g. Fluidizing gas superficial velocity: 0.8 m sy1.Bed temperature: 8508C.


Fig. 7. In situ experiments: sorbent elutriation rate during calcination, sulphation and simultaneous calcination and sulphation of Massicci limestone. Freshsorbent particles size range: 0.425–0.6 mm. Sorbent sample mass: 20 g. Sand particles size range: 0.425–0.6 mm. Sand bed mass: 150 g. Fluidizing gassuperficial velocity: 0.8 m sy1. Bed temperature: 8508C.

conditions, and much lower than that measured in the earlystage of sorbent sulphation. This is presumably because theexternal surface properties are the same for both samples.

3.3. Ex situ experiments: assessment of impact breakage

The analysis is carried out by comparing the attritionpropensity of the following samples: as-received ‘‘fresh’’

limestone, samples calcined for 20 min, samples sulphatedfor about 1 h after pre-calcination for 20 min and samplessimultaneously calcined and sulphated for about 1 h influidized bed. Results are reported as fractional mass losseson impact vs. impact velocity in Figs. 9 and 10 forMassicci and Ignaberga, respectively. For the series ofMassicci samples, it appears that the fresh sorbent is the

Fig. 8. In situ experiments: sorbent elutriation rate during calcination, sulphation and simultaneous calcination and sulphation of Ignaberga limestone. Freshsorbent particles size range: 0.425–0.6 mm. Sorbent sample mass: 20 g. Sand particles size range: 0.425–0.6 mm. Sand bed mass: 150 g. Fluidizing gassuperficial velocity: 0.8 m sy1. Bed temperature: 8508C.


Fig. 9. Ex situ experiments: fractional loss on impact of Massicci samples. Sorbent mother particles size range: 0.6–0.85 mm. Sorbent sample mass: 5 g.

one with the lowest propensity to impact damage. The C, Sand CS samples have comparable fractional losses uponimpact, showing a transition velocity at about 12–17 msy1, where the slopes of the curves change. This velocitymay be related to a change in impact breakage mechanism

w xfrom so-called chipping to fragmentation 14 as it will bediscussed further on. Ignaberga F, C and S samples, on the

Ž .other hand Fig. 10 , show comparable strengths, while theCS sample gives rise to a far larger fractional loss than the

others. Within the tested velocity range, Ignaberga C and Sappear to have similar transition velocities as the processedMassicci samples. However, the Ignaberga CS does notexhibit a clear transition velocity within the tested velocityrange. With the exception of Ignaberga CS, the extent ofimpact breakage in the chipping regime is not greatlydifferent between the various samples from different pro-cesses. Massicci limestone is, in general, less prone tofragmentation than Ignaberga.

Fig. 10. Ex situ experiments: fractional loss on impact of Ignaberga samples. Sorbent mother particles size range: 0.6–0.85 mm. Sorbent sample mass: 5 g.


3.4. Characterization by opticalrscanning electron mi-croscopy and EDX of samples

Figs. 11 and 12 show micrographs, taken from the MejiStereomicroscope at magnifications up to 4.5 times, of

sorbent samples subjected to different pretreatment. In thesame figures micrographs of the debris collected after

Ž . y1impact breakage ex situ experiments at about 15 m simpact velocity are shown. The following features can benoted.

Fig. 11. Ex situ experiments: micrographs of mother particles and debris after impact of fresh and processed Massicci samples. Sorbent mother particlessize range: 0.6–0.85 mm.


Fig. 12. Ex situ experiments: micrographs of mother particles and debris after impact of fresh and processed Ignaberga samples. Sorbent mother particlessize range: 0.6–0.85 mm.

3.4.1. Fresh samplesFresh Massicci has a pitted surface, here and there

covered with a thin layer of fines. The debris consists of

some very small fines and larger fragments. FreshIgnaberga has a glassy look under a fine layer of fines onthe surface. The debris of this material contains a large

Fig. 13. SEM micrograph and EDX sulphur map of polished cross-section of a sulphated Massicci particle. Fresh sorbent particles size range: 0.6–0.85mm.




amount of fines below 100 mm, indicative of brittle disin-tegration of the agglomerated mother particles.

3.4.2. Calcined samplesCalcined Massicci has a more rugged look, and the

debris contains less fines, than the fresh material. CalcinedIgnaberga exhibits the same surface roughness and textureas the fresh material. The amount of fines in the debris isless, and the debris consists generally of larger fragments.

3.4.3. Calcined and then sulphated samplesThe sulphated Massicci has a shiny surface, and the

debris contains almost no fines whatsoever. The shine ofthe surface indicates that the surface porosity is very low.Sulphated Ignaberga has a shiny surface, with a higherroughness scale than fresh and calcined material. Theprotuberances are much more rounded and much largerthan in the other two materials. The debris of this materialconsists of highly angular fragments with only very littlefine debris.

3.4.4. Calcined and simultaneously sulphated samplesCS Massicci has a similar appearance as the sulphated

material: the surface is smoother and has a slight shine.The debris contains some clear surface cracks. The outersurface has a different structure from the open crack faces,which have a far higher roughness. There is hardly anyfine debris. CS Ignaberga has the same shiny surface andrough texture as the sulphated material. Some particlesexhibit clear crack patterns on the surface, indicative ofcore-shell behaviour: the shell has started to crack, but thecrack did not propagate throughout the core.

The establishment of a core-shell particle structure uponw xsulphation has been demonstrated 15,16 and is directly

confirmed with reference to Massicci and Ignaberga parti-cles in Figs. 13 and 14. These report SEM micrographs ofpolished cross-sections of particles that had been calcinedand sulphated for about 2 h at 8508C in an atmospherecontaining 1800 ppm SO and 10% O , the balance being2 2

nitrogen. Particles were embedded in epoxy resin, cut andpolished prior to observation.

Inspection of the SEM micrographs reveals the exis-tence, for both limestones, of a surface layer, about 100mm thick, whose appearance is brighter than the core. Thechemical nature of the shell is disclosed by the parallelexamination of EDX maps of the same particles. Sulphurmostly concentrates in the brighter cortical region, rich inthe sulphate, whereas it is practically absent in the core,consisting almost exclusively of lime, apart from some

Ž .sulphur penetration into existing cracks see Fig. 14 .Other cracks in the sorbent particles shown in the micro-graphs might have been generated by the particle cuttingand polishing procedure.

4. Discussion

Three phenomena are clearly the keys to the mechanicalbehavior of the sorbent particles along conversion:Ž .i the onset of internal stresses due to thermal shockandror internal overpressures associated to particle cal-cination;Ž .ii rounding off of the particle roughness, either originalor left behind by primary fragmentation;Ž .iii the establishment of a core-shell structure uponparticle sulphation.The first phenomenon turns out to be strongly sorbent-

dependent, as confirmed by the large discrepancies ob-served between the two materials investigated. The othersgive rise to attrition patterns which are to a large extentsimilar for the two limestones.

Results of in situ experiments suggest that surfaceabrasion of the fully sulphated particle is usually negligibleand is not affected by the properties of the unreacted core.Accordingly, loss of sorbent fines by surface abrasion overthe particle lifetime is controlled by the time scale overwhich the harder sulphated surface layer is formed, whichin turn depends on the kinetics of lime sulphation. The rateof lime attrition prior to the formation of the sulphate layeris largely related to particle rounding off, and is thereforedependent on the roughness of the particle, either original,or formed by previous primary fragmentation of the parti-cle. This mechanism has been studied in depth by Scala et

w x w xal. 10 and modelled by Di Benedetto and Salatino 11who introduced rounding off and sulphation time constantsto follow the surface abrasion patterns along particle life-time. Major differences are observed in the attritionpropensity during the early calcination of the two lime-stones, probably a remnant of the previous primary frag-mentation. Differences between the two materials vanishwith the progress of sulphation.

Analysis of the results of impact tests is done in theframework of theories of semi-brittle failure of materials,as the particles are angular and porous, and may thereforeexperience some local plastic deformation under compres-sive loading. Impact testing was carried out at ambientconditions. Therefore, it may not entirely reflect the parti-cle breakage behavior at high temperature, as the particlesbecome softer. It is believed, however, that it shouldprovide a meaningful indication of structural differencesbetween samples which underwent different chemical his-tories. Upon impact of a particle against a target, a plastic

w xdeformation zone is created around the impact area 17,18 .At the edge of the contact area between particle andimpact target tensile stresses, arising from the elastic re-covery around the plastic deformation zone upon rebound,may cause fracture. A newly formed crack may propagate

Fig. 14. SEM micrograph and EDX sulphur map of polished cross-section of a sulphated Ignaberga particle. Fresh sorbent particles size range: 0.6–0.85mm.


subsurface from the base of the plastic zone into theparticle, eventually curving to the surface of the particle.This leads to chipping of the particle. When the impactenergy is increased, i.e., at higher impact velocities, chipsbecome larger and ultimately the crack does no longercurve out towards the particle surface but may traverse theparticle, causing particle splitting into fragments. Thisscenario is further complicated by the heterogeneous na-ture of the particle: the tough shell that establishes itselfupon sulphation can be relatively thinner than the particleradius. Crack propagation might be thus retarded by thecore-shell structure, and the boundary between the tworegions might provide preferential direction of crack prop-agation, as suggested by visual inspection of the debrisŽ .Figs. 11 and 12 and of the particle cross-sections in Figs.13 and 14.

The transition between chipping and fragmentation isassociated with an increase in the fractional mass loss, anda change of slope in the fractional mass loss vs. impactvelocity curves is observed at that point. This is more orless what is observed for all the present processed materi-als around 12 m sy1. Within the velocity range tested here,a transition velocity for Ignaberga CS was not observed. Itis likely that in this case the transition takes place aroundor below the free fall velocity, and that the materialfragments already at the lowest velocities.

Comparison of results for the various samples suggeststhat fractional loss upon impact decreases in the sequenceCS ) C ) S 4 F for Massicci, CS 4 C ) S f F forIgnaberga. Calcium oxide appears to be less resistant thanboth the fresh calcite and the sulphate. The unexpected

Žresult is that the CS sample is more prone far more in the.case of Ignaberga to impact damage than the others. In

particular it is noteworthy that, from the standpoint ofimpact resistance, simultaneous calcination and sulphationof limestone is not at all equivalent to the path consistingof calcination of limestone followed by sulphation of lime.One tentative explanation of this observation might be thattwo different structures evolve: in the CS experiments, thesulphation occurs at the same time as rounding off of theparticles, whereas in the S experiments it acts on materialwhich has survived the process of calcination. It is possiblethat sulphation strengthens the particle surface in the CSexperiment, limiting further rounding off of the calcinedparticles. In the light of this argument, the enhanced

Žtendency of CS samples to undergo impact fracture but.not surface wear might be related to the presence of

micro-flaws at the surface which could facilitate crackpropagation, leading to progressive fragmentation. More-over, the simultaneous occurrence of calcination and ofsurface sulphation in CS experiments might leave behindresidual stresses after cooling to ambient conditions whichenhance structural failure upon impact loading. In Fig. 12,one CS particle clearly exhibits fissures and cracks on thesurface. These cracks were observed in a number of parti-cles in the CS sample, and none were found in the S

sample. It is likely that a combination of increased surfacehardness and of residual stresses comes into play in deter-mining the much larger propensity of CS samples toimpact damage at ambient conditions. On the other hand,the sulphated materials would have a much lower hardnessand larger toughness at high temperature because of theproximity to the glass transition temperature of CaSO .4

In summary, the comparison of results of ex situ singleparticle impact tests with those of in situ experimentssuggests that different mechanisms of particle breakageprevail in the two cases. Length scales of zones within theparticle affected by mechanical stressesrcrack propagation

Žare different. A relatively thin surface layer thinner thanthe sulphated shell, as postulated by Di Benedetto and

w x.Salatino 11 is likely to be the location where failure isconfined when surface wear is the dominant attritionmechanism, as in the dense phase of a fluidized bed.Surface properties should therefore control the extent ofattrition in this case. Under impact loading, and dependingon the velocity, mechanical stressesrcrack propagationmay extend deeper within the particle, possibly extendingbeyond the sulphated surface shell. A combination of

Ž .surface hardness and roughness and bulk propertiesshould come into play in this case.

5. Conclusions

Primary fragmentation and attrition by either surfacewear or impact breakage of two limestones during calcina-tion and sulphation in a fluidized bed have been character-ized by a combination of in situ and ex situ experimentaltechniques.

Primary fragmentation is strongly dependent on theŽnature of the sorbent: it is negligible for the Italian Mas-

. Ž .sicci limestone, extensive for the Swedish Ignabergaone.

Sorbent elutriation is significant in the early stage ofcalcination, especially for the Ignaberga limestone. Directfines generation by primary fragmentation and roundingoff of surface asperities, either originally present or leftbehind by primary fragmentation, are the keys to theobserved attrition patterns. Differences in surface abrasionpropensities of the two materials vanish after prolongedcalcination in the fluidized bed.

Sulphation brings about a dramatic decrease of the rateof fines generation by surface abrasion. The formation of ahard sulphate shell is responsible for the negligible propen-sity to surface abrasion of fully sulphated sorbent particles.

Fractional fragment losses on impact of the fresh lime-stones are similar. Large differences are instead observedbetween impact breakage patterns of processed samplesfrom the two limestones. Residual stresses and flaws leftbehind by primary fragmentation to an extent which de-pends on the nature of the limestone should be responsible


for the observed discrepancies. Transition between chip-ping and fragmentation failure modes is generally ob-served for the processed sorbents at impact velocitiesslightly in excess of 10 m sy1.

The influence of the internal structure on the impactresistance is emphasized by the comparison of the attritionrates of subsequently and simultaneously processed materi-

Ž .als S, CS , especially Ignaberga. Simultaneously calcinedand sulphated Ignaberga exhibit a dramatic increase of theimpact attrition propensity when compared with fresh,calcined and subsequently calcined and sulphated lime-stones. Mechanical stresses associated with the simultane-ous occurrence of carbon dioxide release and of build-upof a sulphated surface shell should be responsible for thisfeature.

6. Notation

Ž y1 .E Fines elutriation rate kg sE Ultimate fines elutriation rate during calcination`,c

Ž y1 .kg sŽE Ultimate fines elutriation rate during sulphation kg`,s

y1 .sŽ .M Mass of debris kgd

Ž .M Mass of surviving mother particles kgmŽ .R Fractional mass loss on impact yry

Acknowledgements

ENEL, Polo Termico, Pisa, Italy and B. Leckner and A.Lyngfelt, Chalmers University of Technology, Sweden,supplied, respectively, Massicci and Ignaberga limestone

samples. The support of Mrs. C. Zucchini and Mr. S.Russo in SEMrEDX particle characterization and of Mr.A. Cammarota in setting up and performing in situ experi-ments is acknowledged.

References

w x Ž .1 D. Merrick, J. Highley, AIChE Symp. Ser. 137 1974 366.w x2 J. Franceschi, A. Kolar, G. Miller, V. Zakkay, C. Ho, W. Skelley, S.

Hakim, Proc. 6th Int. Conf. on FBC, 1980, p. 1028.w x Ž .3 W.G. Vaux, A.W. Fellers, AIChE Symp. Ser. 205 1981 107.w x Ž .4 W.G. Vaux, J.S. Shruben, AIChE Symp. Ser. 222 1983 97.w x Ž .5 Y. Ray, T.S. Jiang, C.J. Wen, Powder Technol. 49 1987 193.w x6 S. Lee, X. Jiang, T.C. Keener, S.J. Khang, Ind. Eng. Chem. Res. 32

Ž .1993 2758.w x7 J.L. Cook, S.J. Khang, S.K. Lee, T.C. Keener, Powder Technol. 89

Ž .1996 1.w x8 R.R. Chandran, J.N. Duqum, in: J.R. Grace, L.W. Shemilt, M.A.

Ž .Bergougnou Eds. , Fluidization VI, Engineering Foundation, NewYork, 1989, 571.

w x Ž .9 M.F. Couturier, I. Karidio, F.R. Steward, in: A.A. Avidan Ed. ,Circulating Fluidized Bed Technology IV, Am. Inst. Chem Eng.,New York, 1993, 672.

w x10 F. Scala, A. Cammarota, R. Chirone, P. Salatino, AIChE J. 43Ž .1997 363.

w x Ž .11 A. Di Benedetto, P. Salatino, Powder Technol. 95 1998 119.w x12 V.N. Blinichev, V.V. Strel’tsov, E.S. Lebedeva, Int. Chem. Eng. 8

Ž .1968 615.w x Ž .13 M. Ghadiri, R. Boerefijn, KONA Powder Part. 14 1996 5.w x Ž .14 D.G. Papadopoulos, M. Ghadiri, Adv. Powder Technol. 7 1996

183.w x Ž .15 K. Dam-Johansen, K. Østergaard, Chem. Eng. Sci. 46 1991 839.w x16 T. Mattisson, Sulfur capture during combustion of coal in circulating

fluidized bed boilers, PhD Thesis, Goteborg University, Sweden,¨1998.

w x Ž .17 N. Arbiter, C.C. Harris, G.A. Stamboltzis, Trans. AIME 244 1969118.

w x18 Z. Zhang, Impact attrition of particulate solids, PhD Thesis, Univer-sity of Surrey, UK, 1994.

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Attrition of sorbents during fluidized bed calcination and sulphation