Physical and Morphological Studies of Size-Classified Coal Fly Ash Gerald L. Fisher”, Bruce A. Prentice, David Sllberman, John M. Ondovl, Arthur H. Biermannl, Richard C. Ragaini’, and Andrew R. McFarland2 Radiobiology Laboratory, University of California, Davis, Calif. 95616
A study of the physical and morphological properties of four fractions of size-classified coal fly ash is reported. Volume median diameters of the four size fractions are 2.2,3.2,6.3, and 20 pm, respectively. The size distributions of the four fractions are compared to isokinetically collected samples. Density variations and results of three standard particle sizing techniques are discussed in terms of particle size and morphological properties. Eleven morphological particle types are quantified by light microscopy. Relative abundances of the 11 morphological particle types within each size cut appear to be particle size dependent. The finest fraction is composed of 87% nonopaque solid spheres and 7.9% cenospheres in contrast to the coarsest fraction composed of 26% nonopaque solid spheres and 41% cenospheres. The density variation with particle size is explained in terms of the relative abundances of predominant particle types. On the basis of morphological appearance, a coal fly ash morphogenesis scheme is developed.
the physical properties and morphology. An understanding of the morphogenesis and structural heterogeneity of fly ash is required to ultimately evaluate the potential health hazards of coal combustion effluents. Since the physical and chemical properties of atmospheric fly ash depend on a variety of factors including coal mining sites, variability within coal seams, coal processing and combustion technologies, and particulate abatement technologies, health evaluation studies using fly ash from different sources may not be directly comparable. Therefore, interlaboratory comparisons of health effect studies require detailed physical and chemical characterizations of the materials under study. Furthermore, it is conceivable that a particular morphology of fly ash will be found to possess a greater toxicity than other morphologies; therefore, environmental release of this morphology should be more carefully controlled.
T o meet the increasing national demand for electricity, increased coal combustion is predicted throughout the remainder of this century. Associated with the combustion of coal is the release to the atmosphere of potentially hazardous by-products, including polycyclic aromatic hydrocarbons, oxides of sulfur and nitrogen, and fly ash. Based on an average in-plant collection efficiency of 9596, it has been estimated that 2.4 million metric tons of fly ash were emitted to the atmosphere from U.S. coal-fired electric plants in 1974 (I).Because the principal particulate emission control technologies, electrostatic precipitators (ESP) or wet scrubbers, have low collection efficiency for smaller particles (2), much of the released fly ash is in the “respirable” size range (aerodynamic diameters < l o pm) (3). This fine particle fraction presents the greatest potential health hazard since fine particles: have the longest atmospheric residence times and thus the greatest potential for ultimate human inhalation ( 4 ) ; are generally most efficiently deposited in deep lung and least efficiently removed by mucociliary transport (5);and have the highest concentrations of some potentially toxic elements, many of which appear to be surface deposited (6-8). As part of our efforts to determine the potential health hazards of coal combustion by-products, we have performed a detailed characterization of the physical and chemical properties of size-classified coal fly ash collected downstream of the ESP in the stack breeching of one power plant burning low-sulfur, high ash coal (9). This report describes in detail the physical and morphological properties of the four fly ash size fractions. Chemical analyses of the four size fractions by atomic absorption spectroscopy and instrumental neutron activation analysis have been presented (IO). Although many workers have recently described the bulk chemistry of coal fly ash (e.g., 6, 7), few have studied in detail
Lawrence Livermore Laboratory, Livermore, Calif. 94550.
* Civil Engineering Department, Texas A & M University, College
Station, Tex. 77843.
0013-936X/78/0912-0447$01 .OO/O
0 1978 American Chemical Society
Methods and Materials As previously described (9),kilogram quantities of fly ash were aerodynamically fractionated in situ from the stack breeching after the ESP of a 750-MW coal-fired electric power plant, burning low-sulfur, high ash, high moisture coal. The specially designed fractionator consisted of a series of two cyclone separators with cut sizes of 10 and 6.6 pm in aerodynamic equivalent diameter, respectively, and a centripeter (5.2 pm cut size). The resulting four fractions were collected directly in aluminum hoppers (cyclone fractions) or on fiberglass fabric filters and thence to aluminum containers (centripeter fractions). The fractionator system was operated at 30 cfm and approximately 100 OC to avoid condensation of stack gases. Volume size distributions of the fractions were determined with a Coulter Counter Model T system as previously described (9). Count size distributions were determined using a Zeiss particle analyzer from scanning electron micrographs (SEM)after HzO dispersal and deposition onto a 0.1-pm pore diameter filter. Agglomerates and particles of projected area diameter less than 1pm in the two cyclone fractions (fractions 1and 2) and less than 0.2 pm in the two centripeter fractions (fractions 3 and 4) were not counted. Mass size distributions were determined from the Stokes’ diameter after centrifugal sedimentation in an aqueous solution of 0.1% Daxad (Dewey and Almy Chemical Division, W. R. Grace and Co., Cambridge, Mass.) (I1). Density determinations were performed using a gravimetric liquid (l-propanol) displacement technique (9). Morphologic class counting and photomicroscopy were performed with a Zeiss Photomicroscope I11 with brightfield illumination. Photomicrographs utilizing a green VG9 filter were taken with Kodak Panatomic X film. Samples were dispersed by sonication in 0.2-pm filtered acetone for 20 min and then quickly dried on a glass coverslip prior to mounting in Carmount 165 high refractive index medium. Fields were chosen randomly, and approximately 1000 particles were counted on a slide; three slides of each size cut were counted (approximately 3000 total particles) and classified into the 11morphologic classes described. Scanning electron micrographs (SEM) were prepared with an Etec Autoscan electron microscope. Samples were mounted on aluminum SEM stubs and coated with Au-Pd (60:40) in a high vacuum evaporator. Volume 12. Number 4, April 1978
447
Table 1. Comparison of Particle Size Distribution Data of Four Fly Ash Fractions Analyzed by Three Sizing Technlques MMDb
CMDB
VMDC
rraalon
wm
a#
IM
%
pm
00
1 2 3 4
2.73 2.58 1.14 0.92
2.20 1.86 1.73 1.52
18.5 6.0 3.7 2.4
2.3 2.0 1.7 1.8
20.0 6.3 3.2 2.2
1.8 1.8 1.8 1.9
0.( p m P
csicv~ated
A Ie CMD
MMD
(SI'
0"
VMD
C V(%)9
34 6.94 6.82 11.9 8.55 (2.9) 22 4.60 2.92 3.75 3.76 (0.84) 17 1.79 2.43 1.91 2.04 (0.34) 1.20 1.43 1.19 1.27 (0.14) 11 * cDum median diameter determined from scanning electron micrographs. Mass median diameter d e t m i n e d from Stakes' law oi SBniing in aqu~ousdispmion. EVolume median diameter determined by Coulter analy$r. "Calculated diameter of average Mlume assuming log normal dislribution. e Average calculated 0,. 'Standard deviation. 0 Coefficient of variation = Slav. 0, X 100%. 1 2 3 4
40 0
4
.-.-. b
4 2
'b,
,
3 5 7 10 20 30 AERODYNAMIC DIAMETER I p m i
50 70 100
Flgure 2. Fractional mass distribution of four size-classified fly ash fractions ncfmalized to size dismbution data obtained by isokinetic stack sampling during normal plant operating conditions
Flgure 1. Scanning electron micrographs of four fly ash fractions (A) franion 1, VMD = 20 *In: (B) hadlOn 2. VMD = 6 3 !Am: (C)fraction3,VMD = 3 2 !Am; (0)fraction 4, VMD = 2 2 !Am
Results and Discussion Particle Size Distribution. Particle size distribution data are presented for each of the four size fractions (fractions 1 4 ) collected from the stack breeching of the power plant (Table I). Scanning electron micrographs of representative samples from the four size fractions are presented in Figure 1.Diameters of average volume (D,,) were calculated for comparison of the three sizing techniques from the count median diameters (CMDs), volume median diameters (VMD's), and mass median diameters (MMD's) and the respective geometric standard deviations (u,'s) using the Raahe modification of the Hatch-Choate equations (12). Good agreement was ohserved for Du'scalculated from the sizing data for fractions 2-4. The coarsestfraction, fraction 1, showed the greatest disparity between Do's and the highest associated coefficient of variation. The particle size distributions of the four sized-fractions may he compared to those collected isokinetically from the 448
Environmental Science & Technology
power plant stack during normal plant operating conditions (13).This approach allows for direct comparison of the sizefractionated material to fly ash representative of normal stack emissions. Figure 2 presents log-log plots of mass distribution normalized to the isokinetic stack sampling data as a function of aerodynamic equivalent diameter. The data used for the four size fractions are the individual volume distributions adjusted to aerodynamic diameter for direct comparison to the impactor (isokinetic) data. The aerodynamic equivalent diameters were calculated using the appropriate Cunningham's slip correction and density as a function of particle size. In this manner, the percentage mass in each size interval with respect to the totalmass of the individual fraction is compared to the percentage mass in each impactor interval. The individual plots clearly indicate the enhancement of fine particles and the depletion of coarse particles in fractions 3 and 4; conversely, the enhancement of coarse particles and depletion of fines is observed in fraction 1. Fraction 2 approximates the isokinetic sample fairly well, from diameters of 1.4 to 30
w.
As previously reported (9) the apparent density of the fractions varies inversely with particle size. The apparent densities of fractions 1 4 were 1.85,2.19,2.36,and 2.45 g/cm3, respectively. A significant 0, < 0.05) negative correlation ( r
.-
W
E
F v 9
. i
.
‘”.
s .J*. .)
I.
‘
20pm U
15pm I
L_
9
Flgure 3. Eleven major morphological classes of coal fly ash determined by light microscopy
= -0.978) of VMD vs. density for the four stack fractions was observed using linear regression techniques. Optical Morphology. Light microscopy wm used to define 11major classes of coal fly ash particles (Figure 3A-K): (A) amorphous, nonopaque; (B) amorphous, opaque; (C) amorphous, mixed opaque and nonopaque; (D) rounded, vesicular, nonopaque; (E) rounded, vesicular, mixed opaque and nonopaque; (F) angular, lacy, opaque; (GInonopaque, cenosphere (hollow sphere); (H) nonopaque, plerosphere (sphere packed with other spheres); (I)nonopaque, solid sphere; (5)opaque, sphere, and (K) nonopaque sphere with either surface or internal crystals. On the basis of these morphological classes and their probable matrix compositions, we have developed a particle genesis scheme (Figure 4). Opacity, shape, and type of inclusions were used as characteristics for classification and determination of class frequencies. Opaque amorphous particles (Figure 3B) predominantly arise from coal components that have not been combusted completely. Completely opaque spheres (Figure 35) are mostly magnetite or other iron oxides, alone or in combination with silicates. Iron oxide particulates are easily identified in microscopic studies by taking advantage of their magnetic properties. The magnetic particles in a liquid mounting medium will move when a magnet is passed near the snecimen slide (14). In fact. since manv onaaue particles contain magnetite, small clusters of these particles may be observed (Figure 35). Nonopaque particles are mostly silicates derived from clays and siltstones associated with the coal (15). Shape characteristics of the particles depend on their exposure conditions (time and temperature) in the comhustion chamber (16). Amorphous particles (Figure 3A-C) with little rounding had limited exposure to high temperatures. These I
.
.
particles are probably similar to precombustion particles and may he derived from coal, silicates, or both. Some nonopaque, amorphous particles contain small inclusions of dark to opaque crystals resembling rutile (Ti02) in quartz (17). Amorphous particles with rounded edges and vesicles (Figure 3D-E) are probably coal and silicates exposed to higher temperatures or longer residence times in the comhustion zone. Spherical particles make up most of the fly ash, especially in the finer fractions. These spheres are glassy and mostly transparent (Figure 3G-I, K), indicating complete melting of silicate minerals. At least some of the spheres are hollow (cenospheres), and some are filled with other spheres (plerospheres), amorphous particles, and crystals (Figure 3G, H, and K). A SEM-micrograph of a broken plerosphere is presented in Figure 5A. The few opaque SphereslFigure 35) are mostly magnetite or other iron oxide particles. Spheres may range in color from water-white through yellow and orange to deep red or brown to opaque, probably because of varying iron oxide contents within the glass or on the surface of the particles. The crystals observed within spheres by light microscopy (LM) appear to be acicular and water-white (Figure 3K). Eloneate C N S ~ ~are ~ SoccasionaIlv observed within solid elass spheres. They generally number from 1to 10 per sphere and radiate from one or two points on the sphere surface through the sphere or around, hut within, the surface. These “quench” crystals (18) probably are formed from heterogeneous nucleation at the surface of the molten silicate droplets during rapid quenching of the droplet (19).Their composition will differ somewhat from that of the matrix because growth from the melt will alter the matrix composition. Crystals observed I
I
Volume 12, Number 4. April 1978 449
Flgure 4. Fly ash morphogenesis scheme illustrating probable relationship of opacity to particle composition, and relationship of particle shape to exposure in combustion chamber
on the surface of spheres by LM and SEM were acicular, elongate blades, or cuboid (Figure 5B-D). Electron microprobe analysis has tentatively demonstrated some of the larger acicular crystals to be calcium sulfate (gypsum or anhydrite) (15).Under various conditions, Cas04 can be formed in any of the three crystal habits observed with SEM. The formation of surface crystals is probably the result of sulfuric acid formed on the particle surface reacting with metal oxides in the fly ash (15)to form metal sulfates, or reacting with naturally occurring ambient ammonia to form ammonium sulfates. Similarly, ammonium sulfate crystals have been observed on the downstream side of high-volume filter samples collected in Chicago (20). The few opaque, lacy particles (Figure 3F) seen predominantly in the coarsest fly ash fractions appear to be soot of incompletely combusted carbonaceous material that has accumulated on internal surfaces of the boiler (21). Relative abundances of the 11classes within each size cut (Table 11) were determined by LM counting to assess the relative contribution of each morphologic class to the respective size fraction. As a practical measure, only particles of diameters greater than the SEM-determined CMD of the appropriate size cut were counted. Due to the inability to differentiate between opaque particles and angular, lacy opaque particles in fine fractions 3 and 4, the results are reported for the combined classes (Class L) for these two frac-
- 60 5
5
50
>
=,
40
30
%
20 10 'IBCDPFGHIJIL
Flgure 5. Scanning electron micrographs of fly ash collected from electrostatic precipitator hopper indicating (A) typical plerosphere and ( E D ) variety of observed crystal habits on fine fly ash particles
MORPHOLOGIC CLASSES FRACTION I
LBCOfFGHIJXL
IICOEF6HIJIL
ABCDEFGHIJKL
MORPHOLOGIC
MORPHOLOGiC
MORPHOLOGIC CLASSES FRACTION 4
CLASSES
CLASSES
FRIICTION 2
FRACTION 3
Flgure 6. Frequency polygon illustrating relative abundance of 11 morphological classes in four fly ash fractions
Table II. Relative Abundance ( % ) of Morphologic Particle Classes in Four Fly Ash Fractions Fracllo"
Ptrlicie class ~
A. Amorphous. nonopaque B. Amorphous, opaque C. Amorphous,mixedopaqueandnonopaque
Rounded, vesicular, nonopaque E. Rounded, vesicular, mixed opaque and nonopaque F. Angular, lacy. opaque G. Nonopaque, cenosphere H. Nonopaque, plerosphere I. Nonopaque, solid sphere J. Opaque sphere K. Nonopaque sphere with crystals L. Combined particle classes B and F b D.
a
1 (Ra"se)a
2(Rangs)
7.25 (5.62)
2.13 (0.95)
0.42 (0.13)
0.18 (0.20)
-3 range)^^-. 0.79 (0.50)
...
Environmental Science 8. Technology
...
0.77 (0.71)
0.09 (0.20)
...
...
12.39 (2.61)
6.67 (3.00)
2.91 (0.10)
2.99 (1.58)
2.27 (0.40)
0.24 (0.09)
...
0.03 (0.10)
1.34 (0.33)
0.57 (0.51)
0.27 (0.14)
0.33 (0.10)
41.41 (6.36)
26.22 (3.50)
13.20 (5.44)
7.91 (2.02)
0.51 (0.27)
0.21 (0.34)
...
...
25.58 (0.34)
56.01 (0.67)
79.16 (6.02)
86.99 (4.76)
1.56 (0.83)
0.90 (0.96)
0.33 (0.12)
0.24 (0.21)
6.80 (3.64)
6.79 (1.25) .
3.18 (0.05)
0.95 (1.44)
0.15 (0.09)
0.24 (0.19)
...
...
Range 01 average abundance from three samples. Combined due to inability to dislingui9h between these classes lor the finer pariicies.
450
4 (Rawe)-
0.33 (0.52)
tions. The size dependence of the class frequencies is graphically illustrated in Figure 6. Nearly all particle classes demonstrate a size frequency relationship. The greater morphological heterogeneity of the coarser fractions (1and 2) is clearly indicated. Most striking differences are observed for fraction 4 vs. fraction 1. Fraction 4 is composed of 87% nonopaque, solid spheres (Class I) and 7.9% cenospheres (Class G) compared to the 26%solid spheres (Class I) and 41%cenospheres (Class G) of fraction 1. Chi-square analyses of the frequency data (22) indicate that the relative fractional distributions of the morphological particle classes in each fraction are highly significantly different ( p < 0.001) for all six possible comparisons. Linear regression analyses of the fractional distribution data with the densities of the individual size fractions indicate significant ( p < 0.05) negative correlations for the following particle classes: (A) amorphous, nonopaque; (D) rounded, vesicular, nonopaque; (G) cenospheres, and (J) opaque spheres. Significant positive correlation (p < 0.01) was found only for (I) solid nonopaque spheres. Extrapolation of the linear regression equation ( r = 0.997, p < 0.01) of the fractional distribution data for the solid, nonopaque spheres with the apparent densities to 100% solid, transparent spheres results in an extrapolated density of 2.57 g/cm3. The extrapolated density is in good agreement with the density of 2.60-2.63 g/cm3 (23) reported for a major mineral component of coal, kaolinite (15). Conclusions Coal fly ash derived from a Western U S . power plant utilizing low-sulfur, high ash coal is composed of a variety of morphological particle types. The particle shape and degree of opacity are characteristic of the particle composition and the duration and extent of exposure to combustion temperatures within the boiler. A morphogenesis scheme has been developed on the basis of the light microscopic evaluation of particle shape and opacity. Not only is the morphogenesis scheme useful in understanding the relationship between particles, but it provides a logical basis for quantification of morphological heterogeneity. Small (24) has reported 11 morphologic particle types in coal fly ash on the basis of surface characteristics determined by SEM. However, no comparison of SEM with LM appearance was reported. We have found by three-color, x-ray mapping that fly ash particles with similar SEM morphologies may have dramatically different elemental compositions (25). A study is presently under way to evaluate the SEM morphology and elemental composition of the 11 particle types observed by LM in this study. The particle size analyses indicate greater discrepancies between data from SEM, Coulter, and centrifugal sedimentation techniques for the coarser fractions (1and 2) compared to the fine fractions (3 and 4). This is probably due, in part, to the greater morphologic heterogeneity, including nonsphericity and vesicularity of fractions 1 and 2. Therefore, aerodynamic sizes may be appropriately estimated (26) for fractions 3 and 4 from the data presented describing geometric size, physical density, and relative abundance of spherical particles. Apparent density was found to negatively correlate ( p < 0.05) with the VMD’s of the size fractions. This inverse correlation can be explained by the greater abundance of solid spheres and lesser abundance of vesicular particles in the finer fly ash fractions.
Acknowledgment The authors are indebted to 0. G. Raabe for reviewing the manuscript, L. S. Rosenblatt for aid in statistical analysis, and M. Brummer for scanning electron microscopy. Literature Cited (1) Estimate by authors based on USERDA Balanced Program Plan Vol. 111, Coal Extraction Processing and Combustion, ERDA Pub. #116, pp 3,16,1976, which indicated U S . power plants combusted a total of 600 X lo6 tons of coal of which 11%is ash. Eighty percent of total ash is fly ash of which approximately 5% is released to atmosphere. (2) Vandergrift, A. E., Shannon, L. J., Gorman, P. G., Chem. Eng., 80,107-14 (1973). (3) . , See. for examde. Task G r o w on Lune Dvnamics. Health Phvs.. 12,173-207 (1966); Hatch, T. @.,Gross, k.,;‘Pulmonary DeposiGon and Retention of Inhaled Aerosols”, Academic Press, New York, N.Y., 1964. (4) Mercer, T. T., “Aerosol Technology in Hazard Evaluation”, pp 21-62, Academic Press, New York, N.Y., 1973. (5) Yeh, H. C., Phalen, R. F., Raabe, 0. G., Enuiron. Health Perspect., 15, 147-56 (1976). (6) Davison, R.., Natusch, D. F. S., Wallace, J. R., Evans, C. A., Jr., Enuiron. Sci. Technol., 8,1107-13 (1974). (7) Fisher, G. L., Prentice, B. A., Silberman, D., Ondov, J. M., Ragaini, R. C., Biermann, A. H., McFarland, A. R., Pawley, J. B., “Size Dependence of the Physical and Chemical Properties of Coal Fly Ash,” American Chemical Society Meeting, Div. Fuel Chem. Symp., Properties of Coal Ash, Montreal, Canada, May 1977, in press. (8) Linton, R. W., Loh, A., Natusch, D. F. S., Evans, C. A., Jr., Williams, P., Science, 191,852-4 (1976). (9) McFarland, A. R., Bertch, R. W., Fisher, G. L., Prentice, B. A., Enuiron. Sci. Technol., 11,781-4 (1977). (10) Ondov, J. M., Ragaini, R. C., Heft, R. E., Fisher, G. L., Silberman, D., Prentice, B. A., “Interlaboratory Comparison of Neutron Activation and Atomic Absorption Analyses of Size-Classified Stack Fly Ash,” pp 565-72,8th Materials Res. Symp., NBS, Gaithersburg, Md., GPO, 1977. (11) Whitbv. K. T.. Trans. A m . SOC.Heat Refrip. , - Air Cond. Ene.. 61. ‘ 33-50,4i4-62 (1955). (12) Raabe. 0. G.. J.Aerosol Sci.. 2.289-303 (1971). (13) Ondov, J. M., Ragaini, R. C , Biermann, A. H., American Chemical Society Meeting, Div. Environ. Chem., S.F., pp 200-203, Aug. 29-Sept. 3,1976. (14) Draftz, R. G., Illinois Institute of Technology Research Institute, Chicago, Ill., private communication, 1976. (15) Fisher, G. L., Chang, D. P. Y., Brummer, M., Science, 192,553-5 (1976). (16) McCrone, W. C., Delly, J. G., “The Particle Atlas”, 2nd ed., pp 323-4, Ann Arbor Science. Ann Arbor. Mich., 1973. (17) Hurlbut, C. s., Jr., “Dan’a’sManual of Mineralogy”, 17th ed., pp 296-7, Wiley, New York, N.Y., 1963. (18) . , Green. H. W.. Universitv of California. Davis. Calif.. Drivate communication, 1977. (19) Hurt, J., Biechnicki, D. J., “Ultrafine-Grain Ceramics from Melt Phase”, in “Ultrafine-Grain Ceramics”, J. J. Burke, N. L. Reed, and V. Weiss, Eds., Proc. 15th Sagamore Army Materials Res. Conf., pp 286-7, Syracuse Univ. Press, Syracuse, N.Y., 1970. (20) Draftz, R. G., “Types and Sources of Suspended Particles in Chicago”, Illinois Institute of Technology Research Institute, Rep. No. C9914-CO1, Chicago, Ill. (21) McCrone, W. C., Delly, J. G., ibid., p 547. (22) Snedecor, G. W., Cochran, W. G., “Statistical Methods”, 6th ed., pp 228-52, Iowa State Univ. Press, Ames, Iowa, 1972. (23) Hurlbut. C. S.. Jr.. ibid.. D 461. (24) Small, J.’A,, PhD disserta’tion, University of Maryland, College Park, Md., 1976. (25) Pawley; J. B., Fisher, G. L., J . Micros., 110,87-101 (1977). (26) Raabe, 0. G., J. Air Pollut. Control Assoc., 26,856-60 (1976).
Received for review May 31,1977. Accepted October 17,1977. Study funded by the U.S. Energy Research and Deuelopment Administration through the Radiobiology Laboratory and Lawrence Liuermore Laboratory.
Volume 12, Number 4, April
1978
451
