Fuel Processing Technology 91 (2010) 1022–1027

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Attrition of limestones by impact loading in fluidized beds: The influenceof reaction conditions

Fabrizio Scala a,⁎, Piero Salatino a,b

a Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche, Napoli, Italyb Dipartimento di Ingegneria Chimica, Università degli Studi di Napoli Federico II, Napoli, Italy

⁎ Corresponding author. Tel.: +39 081 7682969; fax:E-mail address: scala@irc.cnr.it (F. Scala).

0378-3820/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.fuproc.2010.03.003

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 September 2009Received in revised form 10 March 2010Accepted 11 March 2010

Keywords:LimestoneAttritionFragmentationFluidized bedImpact

The extent of attrition associated with impact loading was studied for five different limestones pre-processedin fluidized bed under different reaction conditions. The experimental procedure was based on themeasurement of the amount and the particle size distribution of the debris generated upon impact of sorbentsamples against a target at velocities between 10 and 45 m/s. The effect of calcination, sulfation andcalcination/re-carbonation on impact damage was assessed. Fragmentation by impact loading of thelimestones was significant and increased with the impact velocity. Lime samples displayed the largestpropensity to undergo impact damage, followed by sulfated, re-carbonated and raw limestones.Fragmentation of the sulfated samples followed a pattern typical of the failure of brittle materials. On theother hand, the behaviour of lime samples better conformed to a disintegration failure mode, with extensivegeneration of very fine fragments. Raw limestone and re-carbonated lime samples followed either of the twopatterns depending on the sorbent nature. The extent of particle fragmentation increased after multipleimpacts, but the incremental amount of fragments generated upon one impact decreased with the number ofsuccessive impacts.

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1. Introduction

Attrition brings about substantial changes in the particle sizedistribution of sorbents in fluidized bed (FB) combustors [1–6]. Scalaet al. [3,5,7] classified sorbent attrition phenomena in fluidized bedson the basis of the relevant breakage mechanism and the size ofgenerated fragments. Primary fragmentation occurs immediately afterthe injection of the particles in the bed, as a consequence of thermalstresses and of internal overpressure due to CO2 emission. Primaryfragmentation occurs in the dense bed or in the splashing zone ofeither bubbling or circulating FB combustors, resulting in thegeneration of either coarse or fine fragments. Further breakage ofparticles occurs as a consequence of mechanical stresses experiencedby the particles during their lifetime in the reactor. Attrition byabrasion generates fine particles that are quickly elutriated. Thisbreakage process is related to the occurrence of surface wear as theemulsion phase of the fluidized bed is sheared by the passage ofbubbles. Secondary fragmentation generates coarser fragments thatare typically retained within the reactor for relatively long residencetimes. Secondary fragmentation may onset as a result of high-velocitycollisions against targets, which can be either bed material or reactorwalls and internals. High-velocity impact conditions are experienced

by the particles in the grid (jetting) region of FB combustors and areclosely related to the design of the gas distributor. The exit region ofthe riser and the cyclone are other potential locations of impactdamage of sorbent particles.

The influence of the progress of calcination and sulfation onattrition of limestone in FB combustors has long been recognized [1,3].These reactions cause significant modifications of the mechanical andmorphological properties of the sorbent particles, influencing theextent and mechanisms of particle breakage. In particular, theprogress of sulfation significantly decreases the attrition rate ofporous CaO, due to the formation of a tougher sulfate shell at theperiphery of the particle. Neglecting particle strengthening assulfation progresses in the particle outer shell leads to significantoverprediction of attrited fines generation. Most of the publishedinvestigations on sorbent attrition during sulfur capture in fluidizedbeds refer to moderate bubbling test conditions, which emphasize thecontribution due to surface wear. Only recently did attrition by impactdamage receive consideration [5,7–9]. Calcination and sulfation wereshown to significantly affect the extent and pattern of sorbentfragmentation due to impact.

The present study was focused on the characterization of thepropensity of Ca-based sorbents to undergo impact damage. The testprocedure consisted of entraining limestone particles in a gas stream atcontrolled flow rate and impacting them against a target in a purposelydesigned impactor. The particle size distribution of the debris remainingafter the impact was worked out to define a fragmentation index. The

Table 1Chemical analysis of the limestones tested.

Limestone A B C D E

CaCO3 83.9 89.0 87.0 85.8 93.7MgCO3 0.9 1.3 3.2 0.9 0.7SiO2 14.4 9.0 8.9 12.4 5.2Others 0.8 0.7 0.9 0.9 0.4

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impact testing techniquewas applied tofive different limestones. Eitherraw or pre-processed sorbent samples were characterized from thestandpoint of impact damage. Pre-processed samples included:a) calcined limestone; b) limestone calcined and sulfated to exhaustion;c) limestone calcined and then re-carbonated. The pronouncedinfluence of particle pre-processing on attrition extent and patternwas highlighted. Multiple impact cycles were also tested on selectedlimestone samples.

2. Experimental

2.1. Apparatus

Calcination, sulfation and re-carbonation of limestone werecarried out in a stainless steel atmospheric bubbling FB reactor. Thereactor, 40 mm ID and 1 m high, was electrically heated. The gasdistributor was a perforated platewith 55 holes of 0.5 mmdiameter ina triangular pitch. Flue gases were continuously sampled for CO2 andSO2 concentration measurement by means of two on-line NDIRanalyzers, in order to monitor the progress of reactions. The bedmaterial consisted of mixtures of 20 g of limestone and 150 g of silicasand. Silica sand was sieved in the particle size range 0.85–1.0 mm.Further details on the FB apparatus can be found elsewhere [10].

Impact testing of either raw or pre-processed sorbent samples wascarried out in an apparatus based on the well established concept ofentraining particles in a gas stream at controlled velocity andimpacting them against a target [5,8,11–13]. The test rig consistedof a vertical stainless steel eductor tube equipped with a particlefeeding device. The eductor tube was 10 mm ID and 1 m high. Theparticle feeding device was fitted at the top of the eductor tube andconsisted of a stainless steel hopper with a 10 mm ID at the topsection and 4 mm ID at the bottom section. The bottom end of thehopper was connected through a valve to a steel tube (4 mm ID)running coaxially for 0.2 m inside the eductor tube. The air alsoentered the top section of the eductor tube and flowed downwardbetween the inner and outer tubes. When the valve was opened, thelimestone particles contained in the hopper flowed through the innertube driven by gravity and by the entrainment effect caused by the airflowing in the eductor tube. The hopper could be isolated from theenvironment by means of a top valve, to avoid air bypass when thebottom valve was open. After feeding, the particles were acceleratedby the air flow in the eductor tube. The particle velocity wascontrolled by regulating the air flow in the eductor tube, by meansof a flowmeter. When the particles exited the eductor tube, theyimpacted on a rigid target plate placed in a collection chamber 50 mmbelow the bottom end of the tube. The target was made of stainlesssteel and was inclined by 30° with respect to the horizontal. Thisinclination was chosen as a trade-off between the need of avoidinginterference between the impacting and reflected particles and theneed of minimizing the departure of results from those obtained witha target perpendicular to the particle trajectory [12,13]. The collectionchamber was made of glass, 80 mm ID and 0.55 m high. The air flowleft the collection chamber from the top section where it passedthrough a porous cellulose filter (for the capture of finer particulate).The impacted limestone particles settled at the bottom, where theywere collected for further analysis. Further details on the impact testapparatus can be found in ref. [7].

2.2. Procedures

Five different high-calcium limestones were used for the experi-ments. Chemical analysis of the limestones is reported in Table 1.Fresh limestone particles were first sieved in the particle size range0.4–0.6 mm, falling well within the range of particle sizes that aretypically employed in practical circulating FB combustion. Limestoneparticles were then either calcined (in air) or simultaneously calcined

and sulfated (1800 ppmv SO2) for 120 min in the FB reactor operatedbatchwise at 850 °C with a gas superficial velocity of 0.8 m/s,following the procedure reported by Scala et al. [10]. Some calcinedlimestone samples were also re-carbonated (10%v/v CO2) for 15 minin the same FB reactor operated at 700 °C at a gas superficial velocityof 0.7 m/s. At the end of each test the sorbent was discharged from thebed and easily sieved out of the bed material (sand) because of itssmaller particle size. Calcined, sulfated or re-carbonated limestoneparticles were again sieved in the particle size range of 0.4–0.6 mmand stored in a desiccator to prevent hydration and/or re-carbonationof the material.

Samples (approximately 2.0 g) of raw (R), calcined (C), sulfated(S) or re-carbonated (RC) limestones were weighed and used forfragmentation tests in the impact testing apparatus. The tests werecarried out in air with the following particle impact velocities v: 10,17, 24, 31, 38, and 45 m/s. These velocities were selected so as toreproduce impact conditions that are likely to establish near the gasdistributor of industrial-scale FB combustors. The tests were carriedout at room temperature. This should only moderately affect thepossibility to extend the present results to the actual operating FBtemperatures: the influence of temperature on attrition propensityhas been shown to be moderate as far as no chemical modifications ofthe sorbent are brought about by temperature changes [14,15]. Aftereach test the sample was retrieved from the collection chamber andweighed. Closure of themass balancewas checked to estimate the lossof material during testing. The closure was always within 3% of theinitial sample weight. The collected particles were then sieve-analyzed to obtain their particle size distribution. Few tests wererepeated to check the reproducibility of the particle size distributions.Multiple impact cycles were also tested on selected limestonesamples, by repeating several times the impact testing at the samevelocity on already impacted samples. Sieve-analysis was performedafter each impact.

3. Results

3.1. Impact fragmentation of raw limestones (R)

Fig. 1A reports the fractional mass f of fragments versus impactvelocity for R samples. By “fragments” we mean all the particlescollected after the impact tests whose size falls below the lower limitof the feed size interval (b0.4 mm). The log–log plot was chosen so asto better highlight the establishment of power-law f versus vrelationships (f ∝ vk) that are often used to correlate these data. Forthe raw limestones, results in Fig. 1A show that the extent of impactfragmentation is variable in a relatively broad interval, limestones Aand B being more prone to breakage, while limestone E appears to bethe most resistant. For all the limestones f increases moderately as vincreases from 10 to 45 m/s. Some of the R samples (limestones C, Dand E) display impact attrition patterns typical of brittle or semi-brittle materials [7]. The plots display a transition at impact velocitiesaround 25 m/s, witnessed by a change of the slope. For the other twolimestones (A and B) no change of slope is evident in the plots.

Fig. 1B reports the probability density function (PDF) of the sizes offragments collected after impact of R samples at 45 m/s. The PDF wasobtained by dividing the fractional mass of particles in a given size binby the width (in mm) of the size bin. Fig. 1B better highlights how the

Fig. 1. Raw limestone impact tests: A) weight fraction of fragmented material (f) as a function of the impact velocity; B) probability density function of particle size of fragments(db0.4 mm) collected after impact at v=45 m s−1.

1024 F. Scala, P. Salatino / Fuel Processing Technology 91 (2010) 1022–1027

size of fragments is distributed. At any impact velocity and for all thelimestones a dominance of relatively coarse fragments, of size justslightly smaller than 0.4 mm, is observed. R samples mostly undergoparticle splitting (breakage of a particle into a relatively small numberof fragments of size comparable with the parent particle size),possibly combined with moderate chipping (generation of a limitednumber of fragments of size much smaller than the parent particlesize), with prevailing generation of relatively coarse fragments. This issuggested by the non-uniform, monotonically increasing PDF ofparticle sizes with pronounced maxima close to the lower limit of thefeed particle size (0.4 mm).

3.2. Impact fragmentation of calcined limestones (C)

Fig. 2A reports the fractional mass f of fragments versus impactvelocity for C samples. The extent of impact fragmentation in this caseis generally larger than that observed for the raw limestones (Fig. 1A).Moreover, the effect of increasing v on the extent of particlefragmentation is more pronounced. The plots in Fig. 2A do not exhibitsingularities. Large values of f obeying a power-law relationship (f ∝vk) establish throughout the range of impact velocities investigatedfor all the limestones. Noteworthy, in this case limestone E is the mostsusceptible to breakage, contrary to what is obtained with the rawsample (Fig. 1A). It could be speculated that this result is related to the

Fig. 2. Calcined limestone impact tests: A) weight fraction of fragmented material (f) as a fun(db0.4 mm) collected after impact at v=45 m s−1.

chemical composition of the samples. Limestone E has the highercalcium content (Table 1), so it is likely that the raw sample ischaracterized by a compact structure made almost entirely of hardcarbonate, while the calcined sample is composed almost entirelyof soft and porous calcium oxide. The other samples have highercontents of impurities (mostly SiO2) which on one hand mightweaken the structure of the carbonate, on the other might increasethe toughness of the oxide.

Data points expressing the fractional mass of fragments curves inFig. 2A for the different calcined limestones are far less scattered thandata points referring to the raw limestones (Fig. 1A). It can bespeculated that calcination plays an equalizing role on the apparentmechanical properties of the limestones.

Fig. 2B reports the probability density function of the fragmentssize collected after impact of C samples at 45 m/s. Compared withresults obtained with the R samples, the sizes of fragments are moreevenly distributed over the 0–0.4 mm size range, especially forlimestones A and E. The breakage of C particles takes place mostlikely according to the disintegration failure pattern typical of softmaterials (extensive loss of particle connectivity which results intothe generation of a large number of small fragments). The largeporosity of calcined samples is most likely the key for this behaviour.

In some samples (limestones A, B and C), calcination improves theattrition resistance at low impact velocities (Figs. 1A and 2A). This

ction of the impact velocity; B) probability density function of particle size of fragments

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result can be explained by the change in the nature of the particlestructure upon calcination. In fact, the carbonate, characterized bybrittle failure, changes into a softer oxide which is more prone to anelastic/plastic failure pattern. At low impact velocities the oxideparticles are able to better accommodate the stresses at the impactpoint without forming fractures, as compared with brittle materials.On the other hand, at higher impact velocities the particles are subjectto extensive disintegration, not observed with brittle materials.

3.3. Impact fragmentation of sulfated limestones (S)

S particles were obtained by sulfation of R particles to exhaustion,as revealed by inspection of SO2 profiles at the reactor outlet duringsulfation. The final degrees of calcium conversion of the limestoneswere: 30, 19, 14, 23 and 28% for limestones A, B, C, D and E,respectively. Fig. 3A reports the fractional mass f of fragments versusimpact velocity for S samples. When comparing these results withthose obtainedwith R and C samples, it can be observed that S samplesdisplay an intermediate propensity to undergo impact damage. Thecomposite nature of S particles is responsible for a more complexbreakage phenomenology. The fractional mass f for the S samples isvery low at low impact velocity, whereas it approaches that of thecalcined (C) samples at large impact velocity. It is likely that the hardsulfated shell acts as a shelter for the particle at small impact velocities,though the boundary between the sulfated shell and the unconvertedcore might be the location where stress concentration and crackgeneration/propagation might occur. At high impact velocity, theregion of the particle exposed to failure extends to the more porousunreacted core which undergoes fragmentation according to thedisintegrationmode. The transition between these regimes is reflectedby the change of the slope of the f versus the v plots. Interestingly, inthis case limestone E is the least susceptible to breakage. Again, the fcurves lie much closer one to the other than for the raw limestones.

Fig. 3B shows the PDF of the fragments size for S samples subjectedto impact fragmentation tests at 45 m/s. There is some resemblance ofthe fragments size PDF for the S and R samples: fragments are mostlycoarse (just below 0.4 mm) with a limited contribution of fragmentsof size smaller than 300 μm.

3.4. Impact fragmentation of re-carbonated limestones (RC)

RC particles were obtained from re-carbonation of C particles toexhaustion, monitored by inspection of CO2 profiles at the reactoroutlet during carbonation. The final degrees of calcium conversion ofthe limestones were: 61, 63, 52, 70 and 69% for limestones A, B, C, D

Fig. 3. Sulfated limestone impact tests: A) weight fraction of fragmented material (f) as a fun(db0.4 mm) collected after impact at v=45 m s−1.

and E, respectively. Fig. 4A reports the fractional mass f of fragmentsversus impact velocity for RC samples. Comparison of these resultswith those obtained with the R, C and S samples, shows that re-carbonation is effective in reducing the impact damage propensity ofthe sorbent, to the point of restoring or even exceeding themechanical resistance of the raw (R) samples. A composite natureof the particles might be expected for the RC sample as well. However,because of themuch larger conversion of CaO (nearly 70%), the degreeof heterogeneity of the RC particles is far less pronounced than that ofthe S sample. A change of slope is evident for the plots in Fig. 4A, for allthe limestones. Also in this case, pre-treatment of the limestonesclearly generates samples with more uniform mechanical properties,as witnessed by the closeness of the f curves reported in Fig. 4A. Fig. 4Bshows the PDF of the fragments size for RC samples subjected toimpact fragmentation tests at 45 m/s. The results are similar to thoseobtained with the R and S samples.

3.5. Multiple impact fragmentation tests

A preliminary assessment of the effect of multiple impacts wasperformed by carrying out tests in which selected limestone sampleswere repeatedly impacted 10 times against the target at the samevelocity. Sieve-analysis was performed after each impact. Fig. 5Aand B reports the cumulative particle size distribution and thecumulative fractional mass of fragments collected after multipleimpacts. Data refer to raw (R) limestone E impacted at the velocity of31 m/s. As expected, the cumulative fractional mass of fragmentsincreases after multiple impacts. The incremental attrition is maxi-mum after the first two/three impacts, eventually approaching anearly linear attrition rate of about 0.8% incremental fractional mass offragments per impact for this sample, as represented by the brokenline in Fig. 5B. This value was the average of the incremental mass offragments experimentally obtained over the last 6 impacts.

4. Discussion

An assessment of the significance and likelihood of sorbent particlesundergoing multiple impacts at the gas distributor of a full-scale CFBcombustor is based on the following order of magnitude estimate.

The mass flow rate of sorbent collectively entrained in jets at thedistributor – per unit cross-sectional area of the distributor – is givenby:

Es = es⋅lj⋅N ð1Þ

ction of the impact velocity; B) probability density function of particle size of fragments

Fig. 4. Re-carbonated limestone impact tests: A) weight fraction of fragmented material (f) as a function of the impact velocity; B) probability density function of particle size offragments (db0.4 mm) collected after impact at v=45 m s−1.

1026 F. Scala, P. Salatino / Fuel Processing Technology 91 (2010) 1022–1027

where es is the sorbent entrainment rate per unit depth of the jet andN is the number of orifices per unit cross-sectional area of thedistributor. The penetration depth of jets lj within the bed at eachorifice may be expressed as lj= f(d0,U0), for given densities of the gasat the orifice and in the bed, where d0 and U0 are the orifice diameterand the gas velocity at the nozzle, respectively. The air distributor of aCFBC is typically designed so as to give rise to a pressure drop of ∼10%of the bed pressure drop, i.e. of the order of 1000 Pa. Such a pressuredrop would correspond to gas velocities at the orifices of the order ofU0≈50 m/s.

Eq. (1) can be worked out to yield:

Es = es⋅lj d0;U0ð Þ⋅ UU0

⋅ 1πd20

�4

ð2Þ

under the assumption that the densities of the fluidizing gas at thenozzle and in the bed are nearly the same. Values of the sorbententrainment rate es are reported in the literature [16]. A value of2 kg/s m has been assumed for the purpose of the present estimate.Several equations have been suggested in the literature to expresslj= f(d0,U0) [16]. The present computation has been based on theexpression due to Merry [17]. According to this equation, thedependence of the penetration depth on the orifice size under typicaloperating conditions departs only slightly from linearity, hence Eq. (2)

Fig. 5.Multiple impact tests on raw limestone E at v=31 m s−1: A) cumulative size distributimpacts as a function of the number of impacts.

expresses a nearly reciprocal dependence of Es on the orifice size.Taking d0=0.02 m and U0=50 m/s, a jet penetration depth of aboutlj≈0.2 m is computed. Accordingly, Eq. (2) yields an overall limestoneentrainment rate in the grid jets per unit cross sectional area ofthe distributor of about Es≈120 kg/s m2, with the further assumptionthat the fluidizing gas superficial velocity at the distributor (i.e. thatrelated to the primary air stream) is U=5 m/s.

On the other hand one should consider the typical value oflimestone inventory per unit cross sectional area of the combustor, inthe order of Ws=500 kg/m2, and the corresponding limestonefeeding rate per unit cross sectional area of the combustor, in theorder of Fs=0.03 kg/s m2 for typical fuel sulfur content and Ca/S ratio.Accordingly, the average residence time of the limestone particlesin the combustor is of the order of Ws

Fs≈16000s. The values of the

limestone inventory and entrainment rate in the jets at the distributorcan be worked out to give the average time interval elapsed betweenconsecutive impacts: Ws

Es≈4s. Accordingly, the average number of

impacts that one limestone particle is likely to experience over itsresidence time in the combustor would be: Es

Fs≈4000.

Despite the approximate character of this estimate, it supports theconclusion that impact attrition is extensive over the residence timeof limestone particles in industrial-scale combustors.

It might be expected that such a high number of collisions wouldbring about complete particle disintegration if incremental rates of

ion and, B) cumulative fractional mass of fragments (db0.4 mm) collected after multiple

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impact attrition similar to those reported in Fig. 5B would applythroughout the particle residence time in the reactor. However, itshould be considered that limestones, like many other granular solids,may be assumed to be composed of assemblies of sub-grainscharacterized by a well-defined natural grain size, as suggested byRay et al. [18]. As the native particles approach, due to attrition, thenatural grain size, it is expected that attrition is progressivelyhindered, due to combined effects of reduced particle inertia andincreased toughness. Accordingly, the incremental attrition rate isexpected to vanish over prolonged exposure to attrition. Additionalexperiments are required to fully assess the attrition behaviour overprolonged attrition periods, in particular as particle size reduction issuch that the natural grain size is approached.

The previous estimate was only referred to impacts suffered byparticles in the jetting region of the distributor. It might be interestingto compare this contribution with that associated with particlesimpacts possibly suffered in the exit region of the riser and in thecyclone. This is an aspect that is currently very poorly characterized. Inparticular it is difficult at present to assess the actual mechanics ofparticle–particle and particle–wall interaction in the exit region of theriser and in the cyclone, with specific reference to the relativeimportance of the abrasive versus impact components. A roughestimate of the relative importance of contributions to impactattrition in the jetting region and in the exit+cyclone region mightbe based on the comparison between the entrainment rate of particlesin the jets, Es≈120 kg/s m2, and the axial solids mass flux along theriser which is usually in the order of Gs≈10–20 kg/s m2. Thiscomparison suggests that, for given bed inventory Ws, in-bedcollisions would take place at frequencies which are about oneorder of magnitude larger that the collisions in the upper riser and inthe cyclone. Full assessment of the relative extent of impact attritionin the jetting and in exit+cyclone regions would require theknowledge of impact patterns and velocities that are possiblyexperienced by particles in the different sections. Additional researchis required to cover these relatively unexplored aspects.

Attrition/fragmentation phenomena give rise to conflicting pro-cesses as regards the extent of sorbent exploitation. On one side,attrition decreases the particle size and generates elutriable fines thatquickly report to fly ash. On the other side, attrition may inducebreakage of the hard sulfate (or carbonate) shell leading to improvedsorbent conversion. Predicting which of these effects dominates in apractical process is not straightforward, and this paper moves just onestep further in clarifying this point. A recent study, focused on thecombined effects of attrition/fragmentation of a selected limestone ondesulfurization in a CFB combustor, suggests that the detrimentaleffect of enhanced loss of sorbent overcomes the second, positive one[19]. On the other hand, there is evidence [9] that for a particularlimestone impact attrition can enhance the sulfation performance bydisclosing unreacted calcium oxide. At present, it is not possible todraw general conclusions on the influence of attrition on sorbentexploitation, but sorbent-specific characterization of the attrition/sulfation behaviour is needed.

5. Conclusions

Five limestones were characterized as regards their propensity toundergo fragmentation by impact loading. The test protocol wasapplied to both raw and pre-processed (calcined, sulfated, re-carbonated) limestone samples. The following conclusions could bedrawn:

• Calcination, sulfation and re-carbonation significantly affect thefragmentation extent and pattern of the sorbent. The raw and re-carbonated materials are the most resistant to impact loading.

• The fully sulfated limestones exhibit a fragmentation pattern whichis affected by the composite nature of the particles. A transitionbetween chipping and splitting is observed at impact velocitiesbeyond 25 m/s.

• Lime samples undergo impact fragmentation according to a particledisintegration pattern into a population of polydisperse fragments.No change of regime is observed as impact velocity is increased.

• Raw and re-carbonated limestones exhibit either of the twopatterns depending on the sorbent nature.

• The cumulative fractional mass of attrited fragments increases aftermultiple impacts, to a larger extent after the first two/three impacts,to a smaller and nearly constant extent after the successive impacts.The significance of multiple impacts in practical fluidized bedcombustion has been illustrated by an order of magnitude estimateof the number of impacts of sorbent particles over their lifetime inthe reactor.

Acknowledgements

The experimental support of Mr. Antonio Lombardo is gratefullyacknowledged.

Financial support from the program MIUR-FISR: “Sistemi integratidi produzione di idrogeno e sua utilizzazione nella generazionedistribuita” is acknowledged.

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