particle size-density relation and

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Fuel Vol 74 No. 4, pp. 522-529, 1995Copyright kc 1995 Elsevier Science Ltd

Printed in Great Britain. All rights reserved0016-2361/95/ 10.00+0.00

Particle size density relation andcenosphere content of coal fly ashSarbajit Ghosal and Sidney A. SelfHigh Temperature Gasdyn amics Laboratory, Department of Mechanical Engineering,Stanford University, Stanford, CA 94305, USA(Received 14 February 1994; revised 4 May 1994)

The results are reported of detailed physica l characterization of six ashes from coals representative of thoseburned in US power plants. Centrifugal separation was used to classify the ashes into six density categoriesin the range < 1.6 to > 3.2 g cm- 3. The size distributions of all density classes were determined in the rangel-200 pm. For most of the density classes, log-normal functions, trun cated outside th e measuremen t limits,described the size distributions quite well. For all six ashes, the median diameter initially decrea sed an dthen increased with increasing particle d ensity. The influence of particle structure on this large variation(up to sixfold) in size is discussed. Centrifugal separation using a liquid of density 2.2 g cm- 3 was used toestimate the mass fraction of cenosp heres (i.e. particles with trapped interior bubbles) in the ashes. Thisfraction varied from < 5 to > 95 wt%. The cenosphere content was apparently uncorrelated with coal rankbut was positively correlated with the total mineral content of the coal. The median diameters of thecenosph eric fractions were found to be two to three times those of the non-cenospheric (solid) fractions.The density-size data were used to determine the Fe,O, distribution in the ashes.Keywords: fly ash; siz.e-density relation; cenospheres)

In pulverized coal boilers, fly ash is produced during charburnout from the melting of inorganic mineral matter inthe coal by a complex series of processes involving initialmineral fragmentation followed often by coalescence onthe char surface,. The molten ash particles, en trainedin the combustion gases after char fragmentation,are rapidly quen ched to primarily spherical, glassyparticles as they are swept away from the flame region.Microanalysis of ash collected in flue gas cleaning plantshow s that it consists prim arily of spherical particles ofimpure alum inosilicate glass3* 4. The particle size variesfrom submicrometre to > 100 pm.Ash particle densities vary significantly, o wing tovariation in compo sition from particle to particle, andbecause gas bubbles are trapped within many p articles.The latter class of particles, referred to as cenosph eres,is the more impo rtant cause of density va riation and isthe subject of this study. The bubbles may occur eitherin multiple form within a particle or in single, concentricform with a diameter that may be nearly as great as thatof the particle. Their occu rrence has been widely noted3-*.Quan titative data on size-density relations for fly ashare not available in the literature but would be helpful,for example, in understanding slagging and foulingmechanisms in combustors, and for improved models offly ash formation. This study however is part of an effortto provide data for reliable prediction of the effects of flyash on radiative heat transfer in combustors. Both thechemic al composition and physical structure of flyash particles influence their radiative propertiesg. Sixrepresentative ash samp les were therefore obtain ed for

* Current address: Department of Mechanical Engineering, Universityof Kentucky, Lexington, KY 40506, USA

study. Their physical characterization included opticaland scanning electron microscopy, size measurementusing the Coulter Multisizer, and density classificationby centrifugal separation .Fly ash has a submicrometre component resulting fromcondensation of mineral vapours and from extraneous(submicrom etre) fines in the pulverized coal. Howev er,in radiative tran sfer, the submicro metre compo nentmakes a negligible contribution to scattering andabsorption, so that particles of diameter 2 1 pm areof interest. Subm icrometre ash particles can be expectedto be solid (non-cenospheric), since the excess pressureneeded to sustain bubbles within such a small particle ismuch greater than that typical of compressed gasestrapped in the porous char or in the mineral particleitself. In studies of ash sections in the literature thesmallest detected cenosphere was found to be - 4 pm4.Hence, from the viewpoint of cenosph eres, data onsize-density relation are relevant for ash particles ofdiameter 2 1 pm.Insufficient accurate information is available on thesize distribution of fly ash. Size distribution, determine dusing cascade im pactors in terms of the aerodynamicdiamete r, is generally resolved into a few overlappingranges. Since the aerodynamic diameter, D,, includesdensity (p) information (D,ocD ), it is not useful fordetermin ing the sizeedensity relation. Current microsco pictechnique s can size only relatively sm all samp les (oneto two thousand particles). In contrast, the CoulterMultisizer reports the geometric diameter and is suitablefor sizing agglomerated powders such as fly ash, usingstatistically significant sample sizes12.This paper p resents the results of density classificationof six ashes using centrifugal separation in liquids, and

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Physical characterization of ashes: S. Ghosal and S. A. Self

size distribution mea sureme nts of each of the densityclasses with the Coulter Mu ltisizer. The parameters forthe best-fit log-normal functions to the size data a retabulated for all density c lasses. An estimate is made ofthe total c enosphere content of the ash by densityclassification, and the size distribution s of the twocompo nents are reported for each ash.EXPERIMENTALFly ash samples

The ashes originated from coals typical of those burnedin US power plants. Their properties, including mineraltransformation during combustion, have been studiedextensively13. The coals were Illinois N o. 6, KentuckyNo. 9, Upper Freeport, PA (all three bituminous), Beulah,ND, and San Miguel, TX (both lignites) and Eagle Butte,WY (subbituminous). The first four samples came fromthe bagho use of Foster-Wh eelers pilot-scale com bustor.The remaining two composite ashes came from full-scalepower plants in Colorado (Eagle Butte) and San Miguel(Texas lignite). The comb ustor conditions, howeve r, werenot available.

Table 1 Analyses (wt% as-received) of parent coals of fly ashes

For the Foster-Whe eler ashes, the fractions collectedin the cyclone w ere also supplied. However, the ashescould not be reconstituted, because the ma ss ratios ofbagho use to cyclone ashe s were not available. Except fora slightly different size distribution, the cyclone ash wa sfound to be quite similar to the baghouse ash for eachcoa19, and hence the measurements described here weremade only on the baghouse ashes.The ashes showed no detectable solubility in water.Chemical analyses of the coals and the ashes are givenin Tables I and 2 respectively. The average ashcomp ositions were obtained by electron microprobemeasurements on slags produced by melting 40-50g ofeach ash . It is seen that Beulah and Illinois No. 6 asheshad the highest iron oxide content (16-19 w t%). The silicacontents of the three bituminous coal ashes were - 50 wt%,and the SiO,/Al,O, ratio was -2:l. The San Miguellignite ash wa s rich in silica (65 wt%). The SiOJAl,O,ratio for the Beulah lignite slag was close to unity. TheEagle Butte subbituminous coal ash had an unusuallyhigh calcia content (CaO:SiO,:A1,0,=33:29:17). Thiscoal from the Powder River Basin is known to containmuch organically associated calcium.

KentuckyNo. 9

Proximate analysisVolatile matter 29.9Ash 18.4Moisture 12.1

Ultimate analysisCarbon 58.2Hydrogen 3.8Oxygen 1.6Nitrogen 1.0Sulfur 4.3

Forms of sulfurPyritic 2.3Organic 2.0

R. W. Bryers, personal communication, 1987

Illinois Upper EagleNo. 6 Freeport Butte

36.9 20.6 32.89.4 23.6 4.9

12.2 3.2 31.2

61.3 62.5 41.34.3 4.2 3.67.8 2.8 12.01.2 0.7 0.63.8 3.0 0.41.2 2.0 _2.6 1.0 _

Beulah

29.39.6

30.2

41.32.9

13.80.81.31.10.2

SanMiguel

25.340.420.3

23.12.8

11.80.41.30.31.0

Table 2 Compositions (wt%) of slags by electron microprobe analysisU. Freeport Illinois Kentucky E. Butte S. Miguel Beulah

SiO, 51.36 49.39 47.19 28.53 64.61 30.81Al@, 27.51 21.09 28.95 17.44 21.49 32.51Fe@, 13.05 18.96 12.59 6.88 2.75 16.40CaO 2.59 2.94 5.17 33.00 4.85 7.35MgO 0.23 0.90 0.15 7.47 0.10 3.27Na,O 0.53 1.44 2.27 1.76 3.34 6.49K,O 3.16 2.17 2.24 0.00 1.80 1.45TiO, 1.08 0.68 1.06 1.11 0.9 1 0.65P*O, 0.40 0.19 0.25 0.27 0.03 0.38BaO 0.08 1.58 0.12 2.50 0.12 0.60Clb 0.02 0.64 0.00 1.04 0.01 0.02SO,b 0.00 0.02 0.00 0.01 0.00 0.01 From ref. 9b Most of the S and Cl volatilized during slag preparation

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Densi t y c l a ss l j k a t i o nThe density classification followed the scheme ofFuruya et al.14. Separa tion liquids of density 1.6,2.0, 2.4, 2.8 and 3.2 g cmm3 were created by mixingcarbon tetrachloride, dibromomethane and di-iodomethane(density 1.594, 2.497 an d 3.325 g cme3 respectively)in appropriate proportionsg. A standard centrifugal

separation procedure l 5was used, with some modificationsto improve accuracy. Relatively dilute ash suspensions(-0.8 g in 40 ml of liquid) w ere initially deagglom eratedby ultrasonic ag itation a nd gravitationally separated for10min. The centrifuge speed was gradually increasedto - 2000 rev min- , which subjected the ash to anacceleration of w 10009. These measures minimized thecarryover of smaller particles into the larger-sized andlighter cenospheric fraction. Becau se the density of mostof the ash was expected to fall between 2 and 3 g cme3,the liquid of density 2.4g cmm3 was used for initialseparation. Centrifugal separation was next performedwith the liquids of density 2.0 and 2.8 g cm-3 on the floatsand sinks respectively. Separation was carried out in thismanner for the other density classes. The reproducibilityof the separation process w as within 3% .S ize ana l ys i s

Fly ash has an unusually broad size distribution, withdiameters spanning more than three orders of magnitude.Hence it is importan t to measu re the dispersion ofthe size distribution (i.e. the stand ard deviation). Ingeneral, size data available in the literature give only themedia n diame ters. Furthermore, for calculatio ns, asuitable mathematical function is needed to describe thesize distribution.The Coulter Multisize r is particularly suited foraccurate sizing of agglomerated powders, such as fly ash,that are spherical and insulating, and whose density variesfrom particle to particle. A detailed method for sizingashes is described elsewhere.For each density class, the smallest possible Multisizerorifice that was not clogged by the larger ash particlesin that class was used for sizing. Orifices of diamete r 30,50, 70, 100, 140 and 280pm were used, allowingmeasurement of particle diameters in the range 1.2-180 pm.For most classes, 100 000-200 000 particles were sampledby the M ultisizer in 334 min. For a few classes the m assfraction of ash was small and the sample sizes wererestricted to 70 OO& lOO000 particles.For effective deagglo meration, the ashes in the twohighest-density classes were demagnetized by placing acoil carrying alternating current around a test tube ofash and then slowly withdrawing the test tube from the

Table 3 Density classification (wt%) of fly ashes

coil. The ash particles were thus subjected to a rapidlyalternating magnetic field that gradually decreased tozero. As a result the magnetic domains within the particleswere randomly orientated.D ete rm i na t i on o f cenosphere con ten t

The presence of cenosphe res in fly ash was noted byRaask5, who assumed that only those ash particles lighterthan water were cenospheric. However, among the oxidestypically present in fly ash, the lowest density (in thepure state) is that of silica (2.2 and 2.28 gcme3 forcrystalline and fused forms respectively). Conseque ntly,the minimum density of a solid (non-cenospheric) ashparticle is in the range 2.2-2.28 g cm- 3. In particular,ash particles with average density 52.2 g cmm3 canbe expected to contain significant v oids. Hence thecenospheric fraction was defined here as that having adensity ~2.2 gcme3 . Howeve r, it is to be noted thatparticles with density >2.2 g cmm3 may also containbubbles.

Centrifugal separation was carried out with liquid ofdensity 2.2 g cm - 3 on the six ashes to determine theircenosphere contents.RESULTS AND DISCUSSIOND ens i t y c l a ss i f i c at i o n

The results of the density c lassification are show n inTable 3. All but two ashe s have mass fractions of 2 80%in the density range 2 .0-2.8 g cme3, and average densitiesin the range 2.1-2.4 gcmP3. Of the two prominentexceptions, the Texas lignite fly ash from San Miguel has90% of its mass in the first two classes and a negligiblefraction in the highest-den sity class. The low densityindicates that the ash is overwhelmingly cenospheric. Theother exception is the Eagle Butte ash, more thantwo-thirds of which falls in the class 2.8-3.2 g cmm3. Asseen in Table 2, the reason for its relatively high densityis its high c alcia con tent. (The density of lime is 3.4 g cm- 3,but the effective density of Ca O in alumin osilicate glassiis N 3.9 g cm- 3.)Samples of Beulah and Eagle Butte ashes fromeach density c lass, embe dded in acrylic, were groundand polished to expose sections of the ash particles.Examination by scanning electron microscopy showedthat the first two cla sses (density < 2.0 g cm- 3, consistedalmost en tirely of cenosph eres. Sma ll ash particles weresometim es observed inside the voids of the cenosphe res(such particles are discussed later). Small amounts of charwere present in class 1; the total char content of each ashwas < 1 wt% as determined with a high-temperature

Density ofClass 1 2 3 4 5 6 whole ashDensity (g cmm3) 3.2 (gcme3)Kentucky No. 9 1.0 7.6 26.1 59.6 1.4 3.1 2.15Illinios No. 6 1.2 7.2 42.8 42.0 2.8 4.0 2.12Upper Freeport 1.3 6.1 27.0 57.0 2.4 5.6 2.29Eagle Butte 0.5 0.2 3.5 21.4 68.5 5.9 2.16Beulah 0.2 2.1 10.2 16.3 8.9 2.3 2.31San Miguel 35.6 52.9 7.0 3.5 1.0 0.0 1.73



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elemen tal analyser. In class 3, all the particles of diamete r> 15 pm are cenospheric. Few cenospheres are seen inclasses 4, 5 and 6.The ash particles in classes 5 and 6 are darker thanthe corresponding unseparated ashes. All the ash inclass 6, and a large fraction in class 5, is magnetic.Energy-dispersive X-ray spectroscopy (SEM -EDS ) of20-30 p articles of the two classes showed that theycontained high proportions of iron. A few particlescomposed of >90 wt% iron oxide were noted. Theselarge, non-glassy particles consist primarily of mag netite(Fe,O,) or haem atite (Fe,O ,) produced from comb ustionof excluded pyrite gra ins .7 Howeve r, most of the ironin the ash (originating from included pyrites a nd otheriron containing minerals embedded in the coal particle)is present in a glassy state in the aluminosilicate matrix,as found in several studies7*8,18.The occurrence of a widerange of iron fraction in the glassy ash may be predictedfrom studies on slags formed under oxidizing andreducing conditionslg.

For comp arison with the density-classified ash, theaverage density of the whole (unseparated) ash wasmeasu red using a specific gravity bottle a nd distilledwater. Bubble formation was minimized by graduallyadding water to ash in the bottle with an eye-dropper.The suspension was ultrasonically agitated to allow thebubbles to rise to the surface and escape. Multiplemeasu remen ts yielded a precision of 1.5% . The resultsare shown in the last column of Table 3. The averagedensity ranges from 1.73 g crnm3 (the highly cenosphericSan M iguel ash) to 2.76 gcmm3 (the high-calcia EagleButte ash).S ize -dens i t y re la t i onThe two-para meter log-norm al function is often usedto represent broa d particle size distribution s. Amodified form of the function, truncated outside themeasurement limits a and b, is used here, since no sizeinformation is available outside (a, b) l . The differentialvolume distribution is the fractional volume contributedby particles with diameters within d(ln D), and is given by

dF:,b( )d(ln D) = Inb

_2: l na , r .P - l z r ]

s1

lna ./L In IT,exp[ -kr$y]d(lnx) D a6Ddb (1)

In the limit a-+0 and b -+oo , the denominator +l andthe distribution shown in Equation (1) approaches thestanda rd form of the untrunca ted distribution.Half the volume is due to particles with diameterssmaller than the median diameter, D ,. The geometricstandard deviation, (TV,s a measure of the breadth of thedistribution. About 68% of the particles have diametersbetween D,fo , and ogDv . A value of a,>2 indicates avery broad distribution, where as a closely mono dispersepowder has o,+l. Other quantities of interest such asthe number and area median diameters can be obtainedfrom D, and ggg.The log-norm al function (Equation (1)) wa s fitted to thesize data of all density classes for the six ashes. The sizeparameters are listed in Table 4, but graphs of sizedistributions are given here only for the Uppe r Freeport



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0 8

0 68I ?

0 4

0 2

1 2 4 6 810 20 40 60 80 100D W I

Figure 1 Cumu lative volume distribution, F,(D), for all six densityclasses of Upper Freeport ash. For clarity, only every twelfth datumpoint is plotted. The lines show the best-fit log-norm al functions

ash. The data for cumu lative distribution by volume,F,(D), are plotted in Figure 1 for all six size classes, withthe correspondin g best-fit log-norm al functions. The sizedistributions of unseparated fly ashes are known to beclosely log-normal ,* but it is interesting to find that thesize distribution s of the density classified fractions tooare very well represented by log-norma l functions. Clas ses1 and 2 have smaller values of og than do the otherclasses, indicating that cenospheres have narrower sizedistribution s than do non-ceno spheric particles. Theexception is the San M iguel ash, which is almost entirelycenosphe ric. The values of gB for most classes lie in therange 1.2 c eBc 2.0. The unseparated ash encompasses alldiameter classes and h as the highest eB (bottom row ofTable 4).In the following discussion, a simplified model for acenosphere is considered: one consisting of a singleconcentric bubble w ithin the particle, and characterizedby the quantity R,, the ratio of the inner to the outerdiameter, which is assumed constant for all particles. Inreality, of course, the situation is far more complex,with multiple (isolated or interconnected) voids thatmay be non-conce ntric, with variable shell thickne ss.Nevertheless, this simple model (after Raask5) can beuseful in characterizing the general nature of the

cenospheric fraction.From Figure 2, it is seen that for all ashes, D, has aV-shaped distribution when plotted as a function ofdensity (class). The lightest class (containin g thin-wa lledcenospheres with R , closest to unity) also has thelargest median diameter. The median diameter decreasesprogressively to classes 2 and 3. Class 4, having thesmallest D, and containing no cenospheres, representsthe largest m ass fraction for four of the ashes (althoughfor the Illinois No. 6 ash, the mass fractions in classes 3and 4 are almost equal). The two exceptions are EagleButte and San Miguel. The Eagle Butte ash, with its highcalcia content, has the smallest size and largest m assfraction in class 5. The San M iguel ash, containing95 wt% cenospheres, has the smallest D , in class 3, themaximum mass fraction in class 2, and the average densityin class 1.It is reasonab le to expect the average density of eachash (last column of Table 3) to belong to the class

representing the highest mass fraction. Howeve r, it isobserved that the average density of each ash falls in theclass containing the second-highest mass fraction, whichis the density class just below that containing the highestmass fraction. This skewness probably arises because themedian diameters in classes 1 and 2 are larger than thosein classes 5 and 6.A possible mechanism of cenosphere formation is aprocess similar to glass blowing, either when a moltenash droplet on a char particle surface blocks a poreemitting gas under pressure, or when gas is generatedwithin the ash particle during melting. If two molten ashparticles of comparable diameter are subjected unequallyto the blowing action, the particle that is blown morewill produce a cenosphere with a larger diameter, athinner wall (i.e. R,+l) and a lower density. Thisobservation is borne out by the first three class es, wherethe median diameter increases as the class numberdecreases from 3 to 1.A possible explanation for the relatively large sizes of

denser, iron-rich particles may lie in their formationprocesses. P articles that are high in pyrite content do notdecrease in size as much as coal particles of averagemineral content. A larger m ass fraction (i.e. the carbon)of the latter type of coal particles is converted to gaseousform during comb ustion, and the former type form moreash.For all the ashes except San Miguel, the class with thelargest ma ss fraction (class 5 for Eagle B utte, c lass 4 forthe rest) also has the smallest median diameter and isclosest in median diameter to the unseparated ash. Theash particles in this class contain few bubbles and arethe most representative of the unseparated ash. Class 5(2.8-3.2 g cm- 3, contains particles that are rich in ironand other relatively heavy oxides (especially calcia forthe Eagle Butte ash, and alumina for the others). Theseoxides are present in ash particles with a broad range ofdiameters. Hence this class has the highest value of (r8,with the exception of the Beulah as h.It was found that the Fe,O, content obtained bymicroanalysis of > 1000 individual ash particles was only20-50% of that obtained by electron m icroprobemeasurements on slag made by melting the ash. The

100

80

2 ISoa 40

0 0 1 2 3 4 5 6 IDensity Class

Figure 2 Distribution of volume m edian diam eter, D,, of density-classified ashes, as a function of density class



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reason for this discrepancy is that high-iron ashparticles and magnetite particles belong to densityclasses 5 and 6, with significantly higher mediandiameters, so they are fewer in number, and a samplesize of a few thousa nd particles is not statistically largeenough to detect sufficient num bers of such particles.The location of the iron is especially importan t for opticalcharacterization , because iron plays an importa nt rolein radiative transfer throug h its absorption in thewavelength range l-4 ~m21,22.Cenosphere content

The results of the determin ation of cenosphere contentsare shown in Table 5. The San Miguel ash is almostentirely cenospheric, whereas the Eagle Butte ash havevery few cenosphe res. The three bitumin ous coal asheshave cenosphere contents of 14-24 wt%.There a re two sources of uncertainty in the methodused for determin ing cenosphere content. First, thismethod does not encompass cenospheres with particledensity >2.2 g cme3. However, SEM exam ination ofsections of ash particles showed that only a smallfraction of the hea vier ash particles (p>2.2 g cme3)are cenospheric. Second, some of the smaller, non-cenospheric ash particles are carried over into the

Table 5 Characterization of cenosphere content (wt%) of fly ashesProportion (wt%) of ash with

density (g cme3)B/A< 2.2 (A) 2.O


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Physical characterization of ashes: S. Ghosal and S. A. Sel f

Table 6 Size parameters for best-fit log-normal functions forcenospheric and non-cenospheric (solid) ash fractions; D,, and in pmDensity (g cm-) 2.2

D cg (a, b) D, og (a, b)Illinois No. 6 32.5 1.63 4.9,88.0 13.7 3.40 1.7,33.0Kentucky No. 9 19.3 1.88 3.563.0 8.4 2.10 1.7,33.0Upper Freeport 24.7 2.01 3.5,63.0 7.7 2.41 1.7,33.0Eagle Butte 40.0 1.96 9.8J80.0 14.4 3.21 1.7,33.0Beulah 43.3 1.61 4.9, 88.0 13.4 2.17 3.563.0

log-norm al, and the param eters for the best-fit functionsfor all the ashes (except S an Migue l) are presented inTable 6. It is seen that D , for the cenospheric fraction ofan ash is two to three times larger than that for thenon-cenospheric fraction.A few large cenospheres with broken walls wereobserved u nder the microscope to contain smallerash particles. Some investigator& l4 have referred to suchparticles as plerospheres (Greek pleres, full). However,it is difficult to conceive of processes that couldresult in their direct formation. A simpler a nd moreplausible explanation is that the small particles filled thecenospheres after their thin walls had been rupturedduring collection, storage or samp le preparation . Theobservations of Carpen ter et a1.25 appear to support thishypothesis. They crushed several large, low-densityash particles in situ under an optical microscope. Allthe particles contained voids, but none contained smallerparticles.

Further useful data (outside the scope of this work)may be obtained from chemic al a nalysis of the densityclasses, and will contribute to better u nderstan ding of flyash formation. To the authors knowledge, such studiesare currently available for only two ashes7 ,s (thoughwithout data on coal mineralogy). Some idea of thechemical composition may however be obtained fromphysical properties. For example, since the most denseash fractions are rich in oxides th at a re dense in theirpure states (CaO , Fe,O,, Al,O,), the lightest fractions(cenospheres) will be poor in these oxides and hencerelatively rich in SiO, 8.The fact that the cenospheric components containlower-than-a verage Fe,O, is appare nt from their lightercolour compa red with the solid fraction for all the ashes.This observation contradicts an earlier hypothesis thatthe cenospheric fraction of an ash is positively correlatedwith its iron oxide content. The present da ta show thatthe San Miguel ash, with the lowest Fe,O, content, h asthe highest cenosphere content. H owever, from Figure 4it is clear that the cenosphere content is positivelycorrelated with the total ash content of the coal. Thisrelation sugge sts a significant role of the mineral matter,as well as the mineral type (such as zeolites in theTexas lign ite coal, and illite2j in the Upper Freeportcoal), in the formation of cenosph eres. The connectionbetween cenosphere formation and the mineralogy andassociation of the minerals within the coal matrix needsfurther inve stigation. Surface tension an d viscosity of themelt, and the transformation of the minerals duringcombu stion (e.g. gas generation during pyrolysis), areexpected to be importa nt factors in cenosphere formation.


100 Keatucky 9r3 SO_ 0 Illinois 6B - 0 UpperFreeportg 60 - 0 EagleButte8u I + Beulah

v011o1 I I-I0 10 20 30 40 50Mineral Content (Mass %)

Figure 4 Dependence of cenosphere content of fly ash on total mineralcontent of coal. The line represents the best-fit quadratic polynomialfunction

CONCLUSIONSDensity classification of six US coal ashes by centrifugalseparation shows that >80 wt% of four of them lies inthe density range 2.c2.8 g cme3. More than 95 wt% ofthe San Miguel ash, which consists almost entirely ofcenospheres, is of density < 2.4 g cmm3, and more thantwo-thirds of the high-calcium Eagle Butte as h is ofdensity 2.8-3.2 g cm - 3.Truncated log-norm al func tions describe the sizedistribution s of the ashes very well. The median diame tersof the low-density (cenospheric) classes and the high-density (rich in CaO , Fe,O, and Al,O,) classes areconsiderably larger than that of the whole (unseparated)ash.

The cenospheric fraction of the ash, defined as that ofdensity < 2 g cmm3, varies from < 5 wt% for Eagle Butteto > 95 wt% for San Miguel. The median diameter ofthe cenospheric fraction is significantly larger (two- tothreefold) than that of the non-cenosp heric fraction.Howev er, for some of the ashes, a considerab le fractionof the cenosphe res contains only small bubbles (i.e. itsaverage density is 2.s2.2 g cm - 3).Density classification of an ash is a useful techniquein optical characterization because the optical properties(more specifically, the complex refractive index) of a flyash particle depend on its composition, and since densityand compo sition are correlated, this technique helps indetecting the distribution of infrared-active oxides suchas Fe,O,.The scattering and absorption characte ristics of a flyash particle also depend on its geometry; hence the needto quantify th e cenospheric fraction. H owever, fromthe standp oint of radiative properties, the effectivecenosphere content is somewhat lower than the measuredvalue bec ause for particles of low porosity ( < 10 vol.%)the radiation penetrating the particle w ill be absorbedbefore it detects the presence of the bubble (s).This study, together with complementary data, showsthat as much as half of the Fe,O, in the ash isconcentrated in large (D > 20 pm) spherical particles thatare relatively few in number and some of which arenon-glassy. Hence the influence of iron in radiativetransfer is less than if it is distributed more uniformly inthe ash.The cenosphere content of an ash is directly related tothe total ash conten t of the coal.


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ACKNOWLEDGEMENTSThis work was supported by US DOE contract numberDE-AC 22-87PC 79903. The authors would like to thankRod Leach for his help in centrifugal separation , andJon Ebert for several helpful discussions.

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