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Some microscopic features of fly-ash particles and their significance in relation to electrostatic precipitation
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Environmunt Pergamon Press 1970. Vol. 4, pp, 175-185. Printed in Great Britain.
SOME MICROSCOPIC FEATURES OF FXY-ASH PARTICLES AND THEIR SIGNIFICANCE IN REiLATION TO ELECTROSTATIC PRECIPITATION C. A. J. PMJLSON and A. R. ~~
CSIRQDivision of Miner& Chemistry, P.0. Box 175, -&wood, New South Wales, Australia
Abstract-E xamination of fly-ash samples from 5 technical-scale tests and one ful&scale test of &ectrostatic p~~~~on e&iency showed th& collection e&iency decreased as the propor6on of pa&&is sma&r than 5 m w in the fly-ash enter@ the precipitator. Where this proportion exceeded 45 per cent by weight the colleaion &ciency was uaaooeptably low (< 98*S”~).Super&e ily-ash consisting predominantly of submicron particles was encoontered in one test, apmently because of the high fusite content of the bituminous CoaI be& burned. Such ily-asb was especially di@cult to precipitate. In other tests, particle shape exerted an iniiuence, those sampIes consisting predominantly of spherical partick being collected more easily than the remainder. Evidence was obtaiued that, so far as bituminous coals 81~:concerned, the conchtsions dmwn from the present study are likely to be ~pp&-&k to full-scale pIantS 1. INTRODUCTION
precipitation is used extensively for collecting the fly-ash produced in modern coal&red power stations. In general, it is a very e5cient method, precipitation efficiencies of up to 99 per cent by weight being not uncommon in many parts of the world. In New South Wales, however, experience has shown that such high efficiencies are di5cult to achieve in a number of the existing power stations (KIRKWODD, 1962; WATSGN and BLECHER, 1965; WAFT, FLANAGAN and BLBXER,1965). The present investigation was carried out with special reference to those fly-ashes likely to be encountered at a 2000-MW pulverized-coal-tired power station being constructed at Liddell, New South Wales. Attention was directed to the microscopic features of the fly-ash particles and their relation to pr~pi~~~~ efficiency, as this aspect of the subject has previously received little attention. Earlier investigations of the chemical composition of these ashes (DURIE,1968), and the electrical resistivity (TASSICKBR, )IERcEGand MCLEAN1966a,b) ftiled to reveal any clear cut indicators of pr~pi~tion behaviour, The samples studied were obtained from 5 of the technicalscale tests aheady mentioned, and also from a full-scale precipitator test carried out at Vales Point power station, New South Wales (CSIRO, 1967).In each case, comparisons were sought between the fly-ash that entered the precipitator (inlet sample) and the fly-ash that escaped precipitation (outlet sample). EL~WTA~C
2, PREVfUUS
WORK
Despite theoretical and experimental investigations, electrostatic precipitators are still designed mainly on an empirical basis. The general subject of electrostatic precipitation has been treated by ROSEand WGGD(1956) and Wrzrrfz(1963), and specific 175
176
C.
A. J. PAULSON and A. R. RAMSDEN
problems
involved in the collection of fly-ash have been reviewed by WHITE (1955) and discussed briefly by MULLEN(1966). DEU-BCH (1922) developed a theoretical treatment which has long been used as a basis for comparing and analysing precipitator performance. As is well known, however, this theory does not accurately describe the precipitation process because it ignores the effects of relevant and interrelated factors such as combustion phenomena, properties of the gas stream, properties of the fly-ash particles, rate of electrostatic charging of the particles, formation of back corona, sparking, and re-entrainment of particles. The importance of the electrical properties of fly-ash has long been recognised (WHITE,1953, 1956) although the difficulties in obtaining unambiguous measurements of these properties are great. TASSICK~, HERCEGand MCLEAN(1966 a, b) have measured the electrical resistivities of three of the fly-ashes examined in the present work (from Bayswater, Great Northern and Wongawilli seam coals) and report that in each case the value is above that usually considered to be critical (2 x 10” &cm) for successful precipitation. The influence of gas conditioning has been investigated and in some cases improved precipitation efficiencies have been obtained. Thus, for example, DARBYand HJZINIUCH (1966) showed that the effective migration velocity (an indirect measure of the rate of particle precipitation) was increased by up to 85 per cent when a small quantity of sulphur trioxide was injected into the flue gas. TASICKER, HERCEGand MCLEAN (1966c) found that both sulphur trioxide and ammonia markedly reduced the resistivity of Bayswater and Great Northern fly-ashes. DUIUE(1968) has reported the chemical composition of numerous fly-ash samples from technical-scale precipitation tests (including some of those used in the present investigation). The ashes have high silica plus alumina contents (>80%), and ashes in which calcium was the predominant water-soluble cation showed good precipitation performance. Ashes having a silica plus ahunina content greater than 90 per cent usually behaved poorly. However, these results were valid only in a general sense; there was no direct correlation between chemical composition and precipitation efficiency for any given fly-ash. On the basis of large-scale precipitator tests and semi-theoretical calculations, HEINRICH(1961) showed that precipitation efficiency decreases when the dust contains a high proportion of fine particles and that particles in the O-5 pm size range exert a strong influence on precipitator performance. In general, however, the microscopic properties of fly-ash, such as the size and morphology of the particles, have received little attention in relation to electrostatic precipitation. Typical fly-ash particles from British bituminous coals have been described by HAMILTON and JAR~IS(1963) and classified by WATT and THORNE(1965). RAMSDEN (1969) used transmission electron microscopy to study fly-ash particles formed during combustion of Great Northern seam coal and PFEFFERKORN and BLAKHKE (1967) have drawn attention to the value of the scanning electron microscope for examining such particles. 3. METHODS OF INVESTIGATION Inlet and outlet samples from 5 technical-scale tests and 1 full scale test (see Table 1) were collected by inserting a suction probe into the duct and drawing out an
177
Microscopic Features of Fly-A& Particles
isokinetic sample of flue gas and fly-ash. This was passed through a filter-paper thimble which retained the fly-ash, and through a gas meter which measured the fluegas vohune. The fly-ash samples were studied microscopically and the particle-size distributions measured. Of the technical-scale tests, one (Great Northern) was used to check that the fly-ash obtained was comparable to that produced in a power station (full-scale test), and another (FVongawilli) was included because experience at the TABLE1. PRECiPITATXON EFFICIENCY AND
Coal seam
Average precipitation efficiency
PARTICLE-SJZE* DATA OF INLET FLY-ASH SAMPLES
Range
(%I
Precipitation efficiency for test sampled in the present study (%)
!I0 percentile
% less then 5pm
arm)
Reference samples
Great Northern seam (Vales Point power station) Great Northern seem (test rig) wollgawiUi LaWI area coals Liddell seem Bahnoral seam Bayswater seem
992
-
99.2
-40
32
98.7 90.6
96*8-99.7 85*3-95.5
99.7 85.3
16 12
40 55
99.2 93.8 92.5
98.2-99.9 92.3-959 794-98-J
999 95.9 85.2
17 23 15
43 47 52
+ On weight basis.
Tallawarra power station, New South Wales, had shown that the fly-ash formed from this coal was particularly difficult to precipitate. The two tests on the Bayswater and Bahnoral seam coals were of special interest since these coals constitute about twothirds of the fuel reserves for the new Liddell power station, and their fly-ashes are difl?cult to precipitate {CSIRO, 1967). A test with Liddell seam coal was included since this is one of the coals mined in the Liddell area that are likely to be used at the new power station, and the fly-ash has good precipitation behaviour. 3.1 Parlic~e-sizeanalyGs In the 6 tests under consideration analyses of particle-size distribution were carried out on inlet and outlet samples to determine the minimum particle size capable of independent collection (i.e. without being transported on the surface, or entrained in the wake, of larger particles). A Bahco centrifugal classifier (WOLF, 1967) was used to analyse the inlet samples but the quantity of outlet sample was generally too small to be analysed in this way. Therefore, the outlet sampIes were analysed microscopically using a Zeiss TGZ3 particle-size analyser. The Bahco classifier uses an inertial method of size analysis and is a standard instrument for sizing fly-ashes. The sample is introduced into a spiral-shaped air current, where the smaller particles are accelerated by centrifugal force towards the periphery of the spiral, the rest of the dust being carried to the centre of the whirl by friction between the particles and the air. By varying the values of the tangential and radial
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C. A. J. PAULSONand A. R. RAM&DEN
components of the air velocity it is possible to change the grain size collected and thus to divide the material into a number of fractions each with a limited grain-size range. To increase the accuracy of the results in the present study, the method of size analysis was modified as suggested by WOLF(1967). The Zeiss particle-size analyser was developed from the work of ENDTER and GEBAUER (1956). It operates semi-automatically, the individual particles being measured on enlarged photomicrographs. A sharply defined circular spot of light is made equal in size to the particle on the photograph and then, by depressing a foot switch, this measurement is recorded automatically on 1 of 48 counters that are correlated to the spot size. At the same time, a punch marks the particle image so that it will not be counted again. It is generally a simple matter to evaluate the particle images on the photographs and to differentiate agglomerates into their constituent particles. In this study, the outlet samples (weighing a few milligrams) were first dispersed ultrasonically in a solution of O-1per cent collodionin amyl acetate, and the dilute suspenion spread on a clean glass slide. The amyl acetate evaporated to leave a thin membrane of collodion with fly-ash particles embedded in it. To obtain a representative number of particles, up to seven photographs were taken of each preparation, and these were printed at a total magnification of 760. From 3Of10to 5OOflparticles were counted for each sample, during which the agglomerates were resolved into their individual particles, since there was no way of distinguishing between original airborne aggregates and agglomerates formed during and after collection of the samples. Thus, the size-distributions obtained were of the individually resolved particles. It proved impossible to analyse an inlet sample microscopically because of the large size-range of particles present, and since the data obtained with the Bahco classifier are on a weight basis, whereas the microscopic method provides data on a number basis, no direct comparison of the inlet and outlet size distributions is possible. However, despite this limitation the data suggest a possible practical application of size analysis in predicting relative precipitation performance (see below). 3.2 Microscopy Fly-ash samples were examined by light microscopy and scanning electron microscopy. Samples for examination with the light microscope were first dispersed in collodion, as described above, in order to (1) permit examination of individual particles and (2) prevent the particles floating when oil-immersion objectives were used. The preparations were examined in transmitted and reflected light at magnifications up to 1575, the oil-immersion objectives being used for magnifications of 900 and greater. Samples for examination in the scanning electron microscope were prepared in two different ways: (1) a dry dispersion, prepared by dusting the particles lightly onto -5 mm2 of adhesive double-sided tape attached to a sample holder stub; and (2) a wet dispersion, prepared by dispersing the particles ultrasonically in redistilled ethyl alcohol and then pipetting onto a clean glass surface already cemented to a stub with silver dag. Two alternative methods were used because there was a tendency for the largest particles to be lost during pipetting whereas there was a strong tendency for clumping in the dry dispersions; the two preparations were thus complementary. Both types of preparation were coated with a conducting layer of vacuum-evaporated gold to prevent the samples becoming charged in the electron beam.
Microscopic Features of Fly-Ash Particles
179
4. RESULTS 4.1 Precipitation eficiency
Precipitation efficiency data for the 6 tests under consideration are given in Table 1. Samples were obtained by the isokinetic method described previously. The ash loading of the flue gas entering and leaving the precipitator was calculated from the volume of the flue gas and the weight of the associated fly-ash samples obtained by the above method, and the precipitator collection efficiency was then determined by simple proportion. Average values based on a more extensive series of tests (PAULSON, unpublished data) are given, as well as the results for the specific tests considered here, because the former provide a better indication of the overall precipitation behaviour of the ashes. For example, it is evident from the Table that for tests yielding poor precipitation efficiencies the result of a specific test can vary over quite a wide range. Great Northern fly-ash gave high precipitation efficiencies in both the full-scale and the technical-scale tests (essentially the same value being obtained for each), but, as at Tallawarra power station, the Wongawilli fly-ash gave low precipitation efficiency. Of the three Liddell area coals, the Liddell fly-ash gave high precipitation efficiency, the Bayswater low, and the Balmoral an intermediate value. 4.2 Particle-size distribution Cumulative particle-size distributions, plotted in FIG. 1, indicated that 98-99 per cent (by number) of all particles larger than 5 pm were collected in both the full-scale and the technical-scale precipitators. The distributions in the outlet samples were all very similar and the outlet samples of Great Northern fly-ash from the power station and from the test rig were identical, even though the inlet sample from the power station was much coarser than that from the rig. 4.3 Microscopy Under the light microscope, it was seen that the outlet samples were obviously finer than the inlet samples, and the observations suggested that the inlet samples from the tests of high precipitation efficiency contained a greater proportion of spherical and globular particles than the samples from tests of low precipitation efficiency. Although the distinction was not sharp it was nevertheless evident, that the Great Northern fly-ash (from both the power station and the test rig) and the Liddell fly-ash consisted predominantly of colourless spherical or globular glassy particles of the kind characterized as Type 1 by WATT and THORNE(1965), whereas particles in the Wongawilli, Balmoral and Bayswater fly-ashes were predominantly irregular in shape, glassy or partly crystalline, and of the kind characterized as Type 5. Other kinds of particle, present in small amounts in all samples, included black, irregularly-shaped particles (mostly char fragments), angular transparent or opaque mineral fragments, and coloured glassy spheres and globules. The spherical and globular coloured particles were black or various shades of translucent red, brown or yellow. They were generally smaller than 5 pm (many being less than 2 pm) and were noticeably more common in the outlet samples. Examination of the fly-ash samples with a “Stereoscan” scanning electron microscope revealed considerably more information than was obtainable from light microscopy at the same magnifications because the greater depth of focus and the wide field
180
C.
A.
J.
PAULAON
and k R
RAMSDEN
60
10 (01
20
GREAT 1 VALES
NORTHERN
I bl
GREAT
NORTHERN
POINT)
I2
w
0
10
20
30
100
u a
w
60
a 60 W >
LO
Ia
(cl
20
WONGAWILLI (TEST
(d)
LIOOELL
RIG)
_I
60
70
20
30
PARTICLE Fro.
0
10
SIZE,
1. Particle-size distribution curves.
20 /Um
30
LO
FIG. 2. Particles of Great Northern
FIG. 3. Great Northern
(Fuckzg p. 180)
fly-ash from inlet sample, Vales Point power station (wet dispersion).
fly-ash particles from Vales Point power station (dry dispersions) (a) Precipitator inlet; (b) Precipitator outlet.
FIG. 4. Comparison of fly-ash particles in inlet samples (wet dispersions). (a) Great Northern (Vales Point); (b) Great Northern (test rig); (c) Liddell (test rig); (d) Balmoral (test rig); (e) Wongawilli (test rig); (f) Bayswater (test rig).
FIG. 5. Partly reacted coal cenosphere in fly-ash from Great Northern coal (test rig).
FIG. 6. Agglomerates
of superfine particles in Bayswater fly-ash (wet dispersion).
Microscopic Features of Fly-Ash Particles
181
of view permitted many particles to be seen at the same time. Also, the scanning microscope was used at magnifications (up to 5,ooO) unattainable with a light microscope. The observations (FIGS. 2, 3 and 4) showed that Great Northern and Liddell flyashes consisted principally of smooth spherical particles and that particles in the Wongawilli and Bahnoral fly-ashes were generally less spherically symmetrical, consisting of globules that tended to be agglomerated (or fused) into irregularly-shaped particles. Particles in the Bayswater fly-ash were quite unlike those in the other samples. Most were irregular agglomerates formed through association of minute (mostly submicron) spherical particles. Although the agglomerates were of similar size to those in the Wongawilli and Bahnoral fly-ashes, the constituent particles were an order of magnitude smaller. Some partly reacted coal cenospheres (FIG. 5) up to 50 ,UII in diameter were observed in the inlet samples from the test rig but not in the inlet samples from the power station.
5. DISCUSSION
5.1 Comparison offull-scale and technical-scale tests The precipitation efficiencies measured from Vales Point power station and from the test rig were essentially the same (PAULSON, unpublished data) and the Great Northern fly-ashes from the full-scale and technical-scale tests had similar microscopic features. Differences observed could no doubt be attributed in part to problems involved in scaling-down from the power station boiler to the test rig, particularly with respect to residence time. Thus, for example, partly reacted coal cenospheres were present in the fly-ash from the rig (FIG. 5) but absent from the fly-ash from the power station boiler, where longer residence times would favour more efficient combustion of the coal particles (although, as shown in FIG. 4(a), a few small char fragments still occurred in this ash). Also, inlet fly-ash from the power station was coarser than that from the rig [FIG. l(a), (b)], a difference that was no doubt due to the design of the rig, in which, from aerodynamic considerations, the larger particles were expected to be preferentially deposited at the bottom of the combustion chamber. Possibly longer residence times in the full-scale boiler would favour production of larger particles through continued coalescence and growth of liquid ash drops in the furnace, following the mechanism suggested by RAMSDEN(1969). From these comparisons it is concluded that the test rig adequately reproduced the conditions prevailing in a full-scale boiler. From the fact that the outlet samples from the rig and the power station gave identical particle-size distributions, it is further concluded that the technical-scale data on precipitation performance are likely to be valid for full-scale installations in which bituminous coal is the source of the fly-ash but not necessarily for full-scale installations where the fly-ash is of radically different chemical composition, as, for example, that produced from some brown coals. 5.2 Significance of the microscopic features precipitation
of JEy-ash in relation to electrostatic
A comparison of the particle-size distributions (on a number basis) in the outlet samples from the 6 tests under consideration (FIG. 1) showed that-virtually all particles
182
C. A. J. P.WLSON and A. R. RAMSDEN
fagt~ than 5 m were colkcted by electrostatic pr~pi~tion. This accords with the conclusions of HEINRICH (1961). Using the empirically derived figure to compare the inlet particle-size distributions it was apparent that the precipitation efficiency fell as the proportion of particles smaller than 5 pm in the inlet samples increased (see TUBLE1). For bituminous coals, this suggests that difficulties in electrostatic precipitation may be expected where the proportion exceeds about 45 per cent (weight basis). Even though arrived at from a comparison between distribution curves based on numbers of particles and those based on weight percentages, this conclusion is valid and suggests that it may be possible to predict the precipitation behaviour of a given fly-ash from a rapid analysis on the Bahco classifier. The particle-size distribution above 5 m does not appear to affect the collection efficiency of the fly-ash entering the precipitator (compare the two Great Northern tests). However, for certain fly-ashes, the nature of the particles in this oversize range may be of considerable importance. This is well illustrated by the Bayswater fly-ash. Although the poor precipitation performance of this ash may, in part, be attributed to the high proportion (52 per cent) of particles smaller than 5 pm (TABLEl), it is clear from microscopic studies that a high proportion of the particles in the ash are actually sub-micron “spheres” (FIG. 4(f)), and that the larger particles indicated by the size analyses are in fact agglomerates of these (FIG.6). In the absence of direct evidence as to the original state of the fly-ash in the furnace gas stream, it is not possible to exclude the possibility that such agglomerates are primary particles in the sense that they represent stages in a process of coalescence and growth of liquid droplets within burning coal particles (RAMSDEN, 1969) that have been quenched as a result of rapid burn-out. On the other hand, the view that many of the superfine particles were initially independent of one another in the gas stream is suggested by certain indirect evidence (see Section 5.3), and it seems more probable that many of the agglomerates are secondary associations. MITCXELL and LEE(1962),discussing possible causes of agglomeration of superfine particles, concluded that the process starts by random collision due to aerodynamic forces. Bernoulli’s force of attraction, and Brownian motion, bring the particles together in the gas stream and van der Waals and electrostatic forces act to hold the agglomerate together. Cementation may subsequen~y take place. Thus, although the agglomerates in the Bayswater fly-ash tend to be large enough to collect, their formation is likely to be random and a statistically high proportion of supe&e particles either remain in the gas stream and escape collection, or become re-entrained during the rapping cycle. Moreover HIGIQYIT (1965) has shown that the motion of very small particles is determined by turbulence in the gas stream rather than by electrostatic force, and that the particles may, in fact, migrate towards the discharge rather than the collecting electrode. According to LOWE,DALMON and Hremrsr (1965) the deposition of such particles on the discharge electrode may cause abnormal corona discharge. The superfine nature of the fly-ash may explain the extremely variable results of many previous technical-scale tests with Bayswater seam coal (CSIRO, 1967), since the process of agglomeration and the stability of the agglomerates once formed are probably sensitive to con~tions in the gas stream. Indeed, it was reported previously (CSIRO, 1967) that increased atmospheric humidity resulted in improved collection efficiency of the fly-ash. The highest precipitation efficiency (98.5 per cent) was ob-
Microscopic Features of Fly-Ash Particles
183
served when the moisture content of the gases entering the precipitator was highest (N 10 per cent), and the lowest efficiency (< 80 per cent) when the content was lowest (H 7 per cent). Possibly the increased humidity favoured the formation of agglomerates, the stability of.which was increased by the surface tension of adsorbed moisture and/or by solution and cementation taking place at particle contacts. The difficulties with Bayswater fly-ash are therefore thought to be of two-fold origin: (1) the majority of particles are mostly submicron in size and therefore, in the absence of larger particles to transport them, are difficult to collect; and (2) although these small particles tend to agglomerate, the result is still a fly-ash in which more than 45 per cent (by weight) of the “particles” are smaller than 5 pm, an unacceptably high proportion, That in this case these factors are more relevant to precipitation behaviour than the electrical properties of the fly-ash is indicated by the fact that Bayswater and Great Northern fly-ash, whose electrical resistivities are of the same order (TASSICKER, HERCEG and MCLEAN,1966c) show considerable difference in precipitation behaviour. The complication introduced by the presence of superfine particles is less important for the other fly-ashes considered here, since most particles recorded as being larger than 5 pm are genuinely larger, and not agglomerates of superfine particles. For these ashes, a major factor influencing precipitation efficiency appears to be the proportion of particles smaller than 5 pm entering the precipitator. The particles in Balmoral and Wongawilli fly-ashes (low precipitation efficiency) are less spherically symmetrical (FIG. 4) than those in the Great Northern and Liddell fly-ashes (high precipitation efficiency), which suggests that the shape of the particle may also be a factor influencing precipitation efficiency. As particle size and shape are both readily determined, it may be possible to predict precipitation behaviour of fly-ash by the evaluation of these factors. Where, as with the Bayswater, Balmoral and Wongawilli fly-ashes in the present study, the predictions are unfavourable, possibly increasing the gas humidity might help promote agglomeration of the superfine particles of the Bayswater type, or, with Balmoral-type ash, adding fluxing agents to the fuel during combustion to promote formation of spherical particles. Lengthening particle residence times by increasing turbulence in the furnace might favour formation of larger ash particles. 5.3 Signi$cance of the petrographic composition of coal in relation to electrostatic precipitation The results of many technical-scale tests indicated that, apart from the Bayswater and Balmoral seam coals, the only other Liddell area coal likely to prove troublesome in regard to fly-ash precipitation was the Farrells Creek seam (CSIRO, 1967). Moreover, SMYTH(1968) has shown that the Bayswater and Farrells Creek seam coals differ in microlithotype composition from all the other Liddell area coals of the Tomago Coal Measures. In both these coals the proportion of vitrite is low (whereas it is generally high for the other coals) and the proportion of fusite is high. This would suggest, therefore, that it is coals having an abnormally high fusite content that yield superfine fly-ash. Vitrite is the most reactive component of bituminous coal and also has high swelling properties and good plasticity. It is this component which yields cenospheres such as the one shown in FIG. 5, during combustion of the pulverized coal. Fusite, on the
184
C. A. J. PMJISONand A. R. RNWEN
other hand, is relatively inert and does not swell or become plastic. It consists predominantly of two microscopic constituents (fusinite and semifusinite) that are also the principal constituents of anthracites. Investigations in progress with anthracite provide some support for the above statement that the fusite in the bituminous coals is the source of the superfine fly-ash. Partly burnt particles of anthracite are collected directly from a furnace gas stream onto 3 mm diameter disks of 200 mesh/in. stainless steel gauze, and then examined in an electron microscope. The technique is a modification of one described previously (RAMSDEN, 1968). The investigations have shown that discrete droplets of superfine fly-ash form at the surface of the anthracite but do not coalesce. Since, in a bituminous coal, fusite is the constituent whose properties most closely resemble those of anthracite, it is probably safe to infer that a similar ash-forming process occurs during its combustion. Thus, where fusite is the dominant microlithotype, the formation of a high proportion of superfine fly-ash particles may be expected, which may be difficult to precipitate. 6. CONCLUSIONS Examination of the microscopic features of fly-ash particles from full-scale and technical-scale precipitator tests with bituminous coal indicates that particle size and, possibly, particle shape influence precipitation behaviour. If the fly-ash entering the precipitator contains more than about 45 per cent by weight of particles smaller than 5 pm then difficulties may be expected in achieving efficient collection. Fly-ash particles which tend to be spherical precipitate more efficiently than those of irregular shape. Superfine fly-ash (i.e. that containing mostly submicron particles) may be especially difficult to collect on a predictable basis since, to a large extent, the precipitation behaviour appears to depend upon a random association of particles forming agglomerates larger than the critical size of 5 pm. Superfine fly-ash appears to be formed from coals of high fusite content. work forms part of an investigationinto problems of electrostaticprecipitation of fly-ash conducted in collaboration with the Elecb%AyCommissionof New South Wales. It is a pleasure to acknowledgethe assistanceof R. V. DELANDRO,T. DOEL and L. A. O’L~U~HLIN with the technical-scaletests, and of S. R. SILVA and Mrs. V. M. SILVA,of the Defence Standards Laboratory, Department of Supply, Melbourne, with the scanning electron microscopy. The authors Acknowle&ements-This
have benefited greatly from discussions with Dr. J. Boow and other colleagues. REFERENCES CSIRO (1967) Investigation of the Electrostatic Prec$itation of Fly-Ashes from Coals to be Supplied to the Liddell Power Station. Part 1. Tests on Bulk Samples from Trial Shafts and Existing OpenCuts, using a Combined Technical-Scale Combustion and Electrostatic-Precipitation Rig. Invest Rept. 68, CSIRO Div Min. Chem. DARBY K. and GINRICHD. 0. (1966) Konditionierung der Rauchgas von Kesselanlagen zur Verbesserung des Abscheidegrades von Elektroliltem. Sfaub-Reinhalt Llrfr 26 (1 l), 464-468. DEUTSCH W. (1922) Bewegung und Ladung der Elektrizittistrager im Zylinderkondensator. Annln Phys. 68,335-344. DURIER. A. (1968) Znvestigation of the Electrostatic Precipitation of ZYy-Ashes from Coals to be Supplied to the Liddell Power Station. Part 2. Characteristics of the my-Ashes Produced During Technical-Scale Tests on Special Bulk Coal Samples. Invest. Rept. 72, CSIRO Div. Min. Chem.
MicroscopicFeatures of Fly-Ash Farticles
185
ENDTERF. and GBBAUIXR H. (1956) Em einfaches Gergt zur statistischen Auswertung van mikroskopischen b.z.w, elektronmikroskopischechen Aufnahmen. Opfik 13,97-101. HAWLMN E. M. and JAR~I~H. D. (1963) Z&rI&ntt$cattkm of Atmospheric Dust by Use of the Microscope. Mono~ap4 Central Electricity Generating Board, London. Hznmrcrr D. 0. (1961) Study of electro-precipitator performance in relation to particle size distribution, level of collection etEciency and power input. 7Yan.r.Inst. them. Ekgrs 39,145-l 54. Hrom E. T. (1965) Particle charging in electrostatic precipitation. Colloquium on Electrostatic Precipitators, lnstn. Elect. Engrs, London (Feb.). &RKWOOD J. B. (1962) Electrostatic Precipitators for the CoRection of Fly-Ash from iarge P&eked Fuel-Fired Boilers. Paper 14 to Clean Air Conference, University of New South Wales. LowE H. J., DALMON J. and HIGNJIITE. T. (1965) The precipitation of ditBcuit dusts. Coiioquium on Ektrostatic Precipitators. Insltn. Elect. Engrs. London (Feb.). MITCHELL E. R. and Lzz G. K. (1962) Agglomeration of superlIne fly-ash in high-velocity gas streams. Forum on dust wllection. Zrm. Can. Inst. Min. Uet. 65,380-384. MULLENH. (1966) Solid emission problems. J. Am. Sot. Heat Refi. Air-Cot&it. Engrs l&35-37. R. (1967) Staubuntemuchungen mit Hiife des Raster-ElektronenF~~FERKORNG. and Bm mikroskops Stereoscan. Staub 27 (7), 324-326. RAMSDENA. R. (1969) A microscopic investigation into the formation of fly-ash during the combustion of a pulverized bituminous coal. Fuel, Lond. 48,121-137. RAMSDENA. R. (1968) Application of electron microscopy to the study of pulverized&oat combustion and &ash formation. J. Inst. Fuel 41,451-4X Rosa H. E. and WOOD A. J. (1956) An Introduction to Electrostatic Precipitation in Theory and Practice. Constable London. Sm, M. (1968) The petrography of some New South Wales Permian coals from the Tomago Coal Measures. Ptoc. Aust. Inst. Min. Met. No. 225,1-P. Tmcrcza 0. J., HERCEO2. and MCLEANK. (1966a) The ekctrical resistivity of precipitator fly-ash. Wollongong Univ. COB. Div. Engng Bull. No. 7.
TA~~ICXR0. J., HERCEG2. and MCLEANK. (1966b) Mechanism of current conduction through precipitated fly-ash. Wollongong Univ. COB. Div. Engng Bull No. 10. TA~X~ICK~R 0. J., HERCE~Z. and MCLEANK. (1966c) Electrical resistivity of fly-ash from Bayswater and Newvale coals. Woi~~o~ Utdv. COB.Div. Eking Bull No. 11. WISTSON K. S. and B~scrrzu K. J. (1965) Further Investigation of Electrostatic Precipitators for large Puiverized-FueLFired Boiler. Paper 10 to Clean Air Conference, University of New South Waies (Aug.). WATSONK. S., FLANAGAN B. P. and Brzcuza K. J. (1965) Pilot plant testing as an aid to evaluating Precipitator performance. CoBoquium on Electrostatic Precipitators, Instn Elect. Engrs, London (Feb.). WAN J. D. and THORNE D. J. (1965) Composition and pozzoianic properties of pulverized-fuel ashes. J. appi. Chem. E&585-604. WH~TBH. J. (1953) Electrical resistivity of fly-ash. Proc. Air Pollut. Control Ass. 1953 (published 1954), 79-87 (ref. in Chem. Abstr. 48,6626e). WHITEH. J. (1955) Effect of fly ash characteristics on wllector performance. J. Air PO&t. Control Ass. 537-50. WHITE H. J. (1956) Chemical and physical particle conductivity factors in electrical precipitation. C&m. Engng Progr. 52 (6), 224-248. WHITEH. J. (1963) Zndustrial Electrostatic Precipitation. Addison-Wesley, Reading, Mass.
WOLF A. (1967) Die Bahco-Mikroskop-Sichtung, eine verbesserte Arbeitsweise mit dem BahwSichter. Stat& 27 (4), 190.
F
A
Environmunt Pergamon Press 1970. Vol. 4, pp, 175-185. Printed in Great Britain.
SOME MICROSCOPIC FEATURES OF FXY-ASH PARTICLES AND THEIR SIGNIFICANCE IN REiLATION TO ELECTROSTATIC PRECIPITATION C. A. J. PMJLSON and A. R. ~~
CSIRQDivision of Miner& Chemistry, P.0. Box 175, -&wood, New South Wales, Australia
Abstract-E xamination of fly-ash samples from 5 technical-scale tests and one ful&scale test of &ectrostatic p~~~~on e&iency showed th& collection e&iency decreased as the propor6on of pa&&is sma&r than 5 m w in the fly-ash enter@ the precipitator. Where this proportion exceeded 45 per cent by weight the colleaion &ciency was uaaooeptably low (< 98*S”~).Super&e ily-ash consisting predominantly of submicron particles was encoontered in one test, apmently because of the high fusite content of the bituminous CoaI be& burned. Such ily-asb was especially di@cult to precipitate. In other tests, particle shape exerted an iniiuence, those sampIes consisting predominantly of spherical partick being collected more easily than the remainder. Evidence was obtaiued that, so far as bituminous coals 81~:concerned, the conchtsions dmwn from the present study are likely to be ~pp&-&k to full-scale pIantS 1. INTRODUCTION
precipitation is used extensively for collecting the fly-ash produced in modern coal&red power stations. In general, it is a very e5cient method, precipitation efficiencies of up to 99 per cent by weight being not uncommon in many parts of the world. In New South Wales, however, experience has shown that such high efficiencies are di5cult to achieve in a number of the existing power stations (KIRKWODD, 1962; WATSGN and BLECHER, 1965; WAFT, FLANAGAN and BLBXER,1965). The present investigation was carried out with special reference to those fly-ashes likely to be encountered at a 2000-MW pulverized-coal-tired power station being constructed at Liddell, New South Wales. Attention was directed to the microscopic features of the fly-ash particles and their relation to pr~pi~~~~ efficiency, as this aspect of the subject has previously received little attention. Earlier investigations of the chemical composition of these ashes (DURIE,1968), and the electrical resistivity (TASSICKBR, )IERcEGand MCLEAN1966a,b) ftiled to reveal any clear cut indicators of pr~pi~tion behaviour, The samples studied were obtained from 5 of the technicalscale tests aheady mentioned, and also from a full-scale precipitator test carried out at Vales Point power station, New South Wales (CSIRO, 1967).In each case, comparisons were sought between the fly-ash that entered the precipitator (inlet sample) and the fly-ash that escaped precipitation (outlet sample). EL~WTA~C
2, PREVfUUS
WORK
Despite theoretical and experimental investigations, electrostatic precipitators are still designed mainly on an empirical basis. The general subject of electrostatic precipitation has been treated by ROSEand WGGD(1956) and Wrzrrfz(1963), and specific 175
176
C.
A. J. PAULSON and A. R. RAMSDEN
problems
involved in the collection of fly-ash have been reviewed by WHITE (1955) and discussed briefly by MULLEN(1966). DEU-BCH (1922) developed a theoretical treatment which has long been used as a basis for comparing and analysing precipitator performance. As is well known, however, this theory does not accurately describe the precipitation process because it ignores the effects of relevant and interrelated factors such as combustion phenomena, properties of the gas stream, properties of the fly-ash particles, rate of electrostatic charging of the particles, formation of back corona, sparking, and re-entrainment of particles. The importance of the electrical properties of fly-ash has long been recognised (WHITE,1953, 1956) although the difficulties in obtaining unambiguous measurements of these properties are great. TASSICK~, HERCEGand MCLEAN(1966 a, b) have measured the electrical resistivities of three of the fly-ashes examined in the present work (from Bayswater, Great Northern and Wongawilli seam coals) and report that in each case the value is above that usually considered to be critical (2 x 10” &cm) for successful precipitation. The influence of gas conditioning has been investigated and in some cases improved precipitation efficiencies have been obtained. Thus, for example, DARBYand HJZINIUCH (1966) showed that the effective migration velocity (an indirect measure of the rate of particle precipitation) was increased by up to 85 per cent when a small quantity of sulphur trioxide was injected into the flue gas. TASICKER, HERCEGand MCLEAN (1966c) found that both sulphur trioxide and ammonia markedly reduced the resistivity of Bayswater and Great Northern fly-ashes. DUIUE(1968) has reported the chemical composition of numerous fly-ash samples from technical-scale precipitation tests (including some of those used in the present investigation). The ashes have high silica plus alumina contents (>80%), and ashes in which calcium was the predominant water-soluble cation showed good precipitation performance. Ashes having a silica plus ahunina content greater than 90 per cent usually behaved poorly. However, these results were valid only in a general sense; there was no direct correlation between chemical composition and precipitation efficiency for any given fly-ash. On the basis of large-scale precipitator tests and semi-theoretical calculations, HEINRICH(1961) showed that precipitation efficiency decreases when the dust contains a high proportion of fine particles and that particles in the O-5 pm size range exert a strong influence on precipitator performance. In general, however, the microscopic properties of fly-ash, such as the size and morphology of the particles, have received little attention in relation to electrostatic precipitation. Typical fly-ash particles from British bituminous coals have been described by HAMILTON and JAR~IS(1963) and classified by WATT and THORNE(1965). RAMSDEN (1969) used transmission electron microscopy to study fly-ash particles formed during combustion of Great Northern seam coal and PFEFFERKORN and BLAKHKE (1967) have drawn attention to the value of the scanning electron microscope for examining such particles. 3. METHODS OF INVESTIGATION Inlet and outlet samples from 5 technical-scale tests and 1 full scale test (see Table 1) were collected by inserting a suction probe into the duct and drawing out an
177
Microscopic Features of Fly-A& Particles
isokinetic sample of flue gas and fly-ash. This was passed through a filter-paper thimble which retained the fly-ash, and through a gas meter which measured the fluegas vohune. The fly-ash samples were studied microscopically and the particle-size distributions measured. Of the technical-scale tests, one (Great Northern) was used to check that the fly-ash obtained was comparable to that produced in a power station (full-scale test), and another (FVongawilli) was included because experience at the TABLE1. PRECiPITATXON EFFICIENCY AND
Coal seam
Average precipitation efficiency
PARTICLE-SJZE* DATA OF INLET FLY-ASH SAMPLES
Range
(%I
Precipitation efficiency for test sampled in the present study (%)
!I0 percentile
% less then 5pm
arm)
Reference samples
Great Northern seam (Vales Point power station) Great Northern seem (test rig) wollgawiUi LaWI area coals Liddell seem Bahnoral seam Bayswater seem
992
-
99.2
-40
32
98.7 90.6
96*8-99.7 85*3-95.5
99.7 85.3
16 12
40 55
99.2 93.8 92.5
98.2-99.9 92.3-959 794-98-J
999 95.9 85.2
17 23 15
43 47 52
+ On weight basis.
Tallawarra power station, New South Wales, had shown that the fly-ash formed from this coal was particularly difficult to precipitate. The two tests on the Bayswater and Bahnoral seam coals were of special interest since these coals constitute about twothirds of the fuel reserves for the new Liddell power station, and their fly-ashes are difl?cult to precipitate {CSIRO, 1967). A test with Liddell seam coal was included since this is one of the coals mined in the Liddell area that are likely to be used at the new power station, and the fly-ash has good precipitation behaviour. 3.1 Parlic~e-sizeanalyGs In the 6 tests under consideration analyses of particle-size distribution were carried out on inlet and outlet samples to determine the minimum particle size capable of independent collection (i.e. without being transported on the surface, or entrained in the wake, of larger particles). A Bahco centrifugal classifier (WOLF, 1967) was used to analyse the inlet samples but the quantity of outlet sample was generally too small to be analysed in this way. Therefore, the outlet sampIes were analysed microscopically using a Zeiss TGZ3 particle-size analyser. The Bahco classifier uses an inertial method of size analysis and is a standard instrument for sizing fly-ashes. The sample is introduced into a spiral-shaped air current, where the smaller particles are accelerated by centrifugal force towards the periphery of the spiral, the rest of the dust being carried to the centre of the whirl by friction between the particles and the air. By varying the values of the tangential and radial
178
C. A. J. PAULSONand A. R. RAM&DEN
components of the air velocity it is possible to change the grain size collected and thus to divide the material into a number of fractions each with a limited grain-size range. To increase the accuracy of the results in the present study, the method of size analysis was modified as suggested by WOLF(1967). The Zeiss particle-size analyser was developed from the work of ENDTER and GEBAUER (1956). It operates semi-automatically, the individual particles being measured on enlarged photomicrographs. A sharply defined circular spot of light is made equal in size to the particle on the photograph and then, by depressing a foot switch, this measurement is recorded automatically on 1 of 48 counters that are correlated to the spot size. At the same time, a punch marks the particle image so that it will not be counted again. It is generally a simple matter to evaluate the particle images on the photographs and to differentiate agglomerates into their constituent particles. In this study, the outlet samples (weighing a few milligrams) were first dispersed ultrasonically in a solution of O-1per cent collodionin amyl acetate, and the dilute suspenion spread on a clean glass slide. The amyl acetate evaporated to leave a thin membrane of collodion with fly-ash particles embedded in it. To obtain a representative number of particles, up to seven photographs were taken of each preparation, and these were printed at a total magnification of 760. From 3Of10to 5OOflparticles were counted for each sample, during which the agglomerates were resolved into their individual particles, since there was no way of distinguishing between original airborne aggregates and agglomerates formed during and after collection of the samples. Thus, the size-distributions obtained were of the individually resolved particles. It proved impossible to analyse an inlet sample microscopically because of the large size-range of particles present, and since the data obtained with the Bahco classifier are on a weight basis, whereas the microscopic method provides data on a number basis, no direct comparison of the inlet and outlet size distributions is possible. However, despite this limitation the data suggest a possible practical application of size analysis in predicting relative precipitation performance (see below). 3.2 Microscopy Fly-ash samples were examined by light microscopy and scanning electron microscopy. Samples for examination with the light microscope were first dispersed in collodion, as described above, in order to (1) permit examination of individual particles and (2) prevent the particles floating when oil-immersion objectives were used. The preparations were examined in transmitted and reflected light at magnifications up to 1575, the oil-immersion objectives being used for magnifications of 900 and greater. Samples for examination in the scanning electron microscope were prepared in two different ways: (1) a dry dispersion, prepared by dusting the particles lightly onto -5 mm2 of adhesive double-sided tape attached to a sample holder stub; and (2) a wet dispersion, prepared by dispersing the particles ultrasonically in redistilled ethyl alcohol and then pipetting onto a clean glass surface already cemented to a stub with silver dag. Two alternative methods were used because there was a tendency for the largest particles to be lost during pipetting whereas there was a strong tendency for clumping in the dry dispersions; the two preparations were thus complementary. Both types of preparation were coated with a conducting layer of vacuum-evaporated gold to prevent the samples becoming charged in the electron beam.
Microscopic Features of Fly-Ash Particles
179
4. RESULTS 4.1 Precipitation eficiency
Precipitation efficiency data for the 6 tests under consideration are given in Table 1. Samples were obtained by the isokinetic method described previously. The ash loading of the flue gas entering and leaving the precipitator was calculated from the volume of the flue gas and the weight of the associated fly-ash samples obtained by the above method, and the precipitator collection efficiency was then determined by simple proportion. Average values based on a more extensive series of tests (PAULSON, unpublished data) are given, as well as the results for the specific tests considered here, because the former provide a better indication of the overall precipitation behaviour of the ashes. For example, it is evident from the Table that for tests yielding poor precipitation efficiencies the result of a specific test can vary over quite a wide range. Great Northern fly-ash gave high precipitation efficiencies in both the full-scale and the technical-scale tests (essentially the same value being obtained for each), but, as at Tallawarra power station, the Wongawilli fly-ash gave low precipitation efficiency. Of the three Liddell area coals, the Liddell fly-ash gave high precipitation efficiency, the Bayswater low, and the Balmoral an intermediate value. 4.2 Particle-size distribution Cumulative particle-size distributions, plotted in FIG. 1, indicated that 98-99 per cent (by number) of all particles larger than 5 pm were collected in both the full-scale and the technical-scale precipitators. The distributions in the outlet samples were all very similar and the outlet samples of Great Northern fly-ash from the power station and from the test rig were identical, even though the inlet sample from the power station was much coarser than that from the rig. 4.3 Microscopy Under the light microscope, it was seen that the outlet samples were obviously finer than the inlet samples, and the observations suggested that the inlet samples from the tests of high precipitation efficiency contained a greater proportion of spherical and globular particles than the samples from tests of low precipitation efficiency. Although the distinction was not sharp it was nevertheless evident, that the Great Northern fly-ash (from both the power station and the test rig) and the Liddell fly-ash consisted predominantly of colourless spherical or globular glassy particles of the kind characterized as Type 1 by WATT and THORNE(1965), whereas particles in the Wongawilli, Balmoral and Bayswater fly-ashes were predominantly irregular in shape, glassy or partly crystalline, and of the kind characterized as Type 5. Other kinds of particle, present in small amounts in all samples, included black, irregularly-shaped particles (mostly char fragments), angular transparent or opaque mineral fragments, and coloured glassy spheres and globules. The spherical and globular coloured particles were black or various shades of translucent red, brown or yellow. They were generally smaller than 5 pm (many being less than 2 pm) and were noticeably more common in the outlet samples. Examination of the fly-ash samples with a “Stereoscan” scanning electron microscope revealed considerably more information than was obtainable from light microscopy at the same magnifications because the greater depth of focus and the wide field
180
C.
A.
J.
PAULAON
and k R
RAMSDEN
60
10 (01
20
GREAT 1 VALES
NORTHERN
I bl
GREAT
NORTHERN
POINT)
I2
w
0
10
20
30
100
u a
w
60
a 60 W >
LO
Ia
(cl
20
WONGAWILLI (TEST
(d)
LIOOELL
RIG)
_I
60
70
20
30
PARTICLE Fro.
0
10
SIZE,
1. Particle-size distribution curves.
20 /Um
30
LO
FIG. 2. Particles of Great Northern
FIG. 3. Great Northern
(Fuckzg p. 180)
fly-ash from inlet sample, Vales Point power station (wet dispersion).
fly-ash particles from Vales Point power station (dry dispersions) (a) Precipitator inlet; (b) Precipitator outlet.
FIG. 4. Comparison of fly-ash particles in inlet samples (wet dispersions). (a) Great Northern (Vales Point); (b) Great Northern (test rig); (c) Liddell (test rig); (d) Balmoral (test rig); (e) Wongawilli (test rig); (f) Bayswater (test rig).
FIG. 5. Partly reacted coal cenosphere in fly-ash from Great Northern coal (test rig).
FIG. 6. Agglomerates
of superfine particles in Bayswater fly-ash (wet dispersion).
Microscopic Features of Fly-Ash Particles
181
of view permitted many particles to be seen at the same time. Also, the scanning microscope was used at magnifications (up to 5,ooO) unattainable with a light microscope. The observations (FIGS. 2, 3 and 4) showed that Great Northern and Liddell flyashes consisted principally of smooth spherical particles and that particles in the Wongawilli and Bahnoral fly-ashes were generally less spherically symmetrical, consisting of globules that tended to be agglomerated (or fused) into irregularly-shaped particles. Particles in the Bayswater fly-ash were quite unlike those in the other samples. Most were irregular agglomerates formed through association of minute (mostly submicron) spherical particles. Although the agglomerates were of similar size to those in the Wongawilli and Bahnoral fly-ashes, the constituent particles were an order of magnitude smaller. Some partly reacted coal cenospheres (FIG. 5) up to 50 ,UII in diameter were observed in the inlet samples from the test rig but not in the inlet samples from the power station.
5. DISCUSSION
5.1 Comparison offull-scale and technical-scale tests The precipitation efficiencies measured from Vales Point power station and from the test rig were essentially the same (PAULSON, unpublished data) and the Great Northern fly-ashes from the full-scale and technical-scale tests had similar microscopic features. Differences observed could no doubt be attributed in part to problems involved in scaling-down from the power station boiler to the test rig, particularly with respect to residence time. Thus, for example, partly reacted coal cenospheres were present in the fly-ash from the rig (FIG. 5) but absent from the fly-ash from the power station boiler, where longer residence times would favour more efficient combustion of the coal particles (although, as shown in FIG. 4(a), a few small char fragments still occurred in this ash). Also, inlet fly-ash from the power station was coarser than that from the rig [FIG. l(a), (b)], a difference that was no doubt due to the design of the rig, in which, from aerodynamic considerations, the larger particles were expected to be preferentially deposited at the bottom of the combustion chamber. Possibly longer residence times in the full-scale boiler would favour production of larger particles through continued coalescence and growth of liquid ash drops in the furnace, following the mechanism suggested by RAMSDEN(1969). From these comparisons it is concluded that the test rig adequately reproduced the conditions prevailing in a full-scale boiler. From the fact that the outlet samples from the rig and the power station gave identical particle-size distributions, it is further concluded that the technical-scale data on precipitation performance are likely to be valid for full-scale installations in which bituminous coal is the source of the fly-ash but not necessarily for full-scale installations where the fly-ash is of radically different chemical composition, as, for example, that produced from some brown coals. 5.2 Significance of the microscopic features precipitation
of JEy-ash in relation to electrostatic
A comparison of the particle-size distributions (on a number basis) in the outlet samples from the 6 tests under consideration (FIG. 1) showed that-virtually all particles
182
C. A. J. P.WLSON and A. R. RAMSDEN
fagt~ than 5 m were colkcted by electrostatic pr~pi~tion. This accords with the conclusions of HEINRICH (1961). Using the empirically derived figure to compare the inlet particle-size distributions it was apparent that the precipitation efficiency fell as the proportion of particles smaller than 5 pm in the inlet samples increased (see TUBLE1). For bituminous coals, this suggests that difficulties in electrostatic precipitation may be expected where the proportion exceeds about 45 per cent (weight basis). Even though arrived at from a comparison between distribution curves based on numbers of particles and those based on weight percentages, this conclusion is valid and suggests that it may be possible to predict the precipitation behaviour of a given fly-ash from a rapid analysis on the Bahco classifier. The particle-size distribution above 5 m does not appear to affect the collection efficiency of the fly-ash entering the precipitator (compare the two Great Northern tests). However, for certain fly-ashes, the nature of the particles in this oversize range may be of considerable importance. This is well illustrated by the Bayswater fly-ash. Although the poor precipitation performance of this ash may, in part, be attributed to the high proportion (52 per cent) of particles smaller than 5 pm (TABLEl), it is clear from microscopic studies that a high proportion of the particles in the ash are actually sub-micron “spheres” (FIG. 4(f)), and that the larger particles indicated by the size analyses are in fact agglomerates of these (FIG.6). In the absence of direct evidence as to the original state of the fly-ash in the furnace gas stream, it is not possible to exclude the possibility that such agglomerates are primary particles in the sense that they represent stages in a process of coalescence and growth of liquid droplets within burning coal particles (RAMSDEN, 1969) that have been quenched as a result of rapid burn-out. On the other hand, the view that many of the superfine particles were initially independent of one another in the gas stream is suggested by certain indirect evidence (see Section 5.3), and it seems more probable that many of the agglomerates are secondary associations. MITCXELL and LEE(1962),discussing possible causes of agglomeration of superfine particles, concluded that the process starts by random collision due to aerodynamic forces. Bernoulli’s force of attraction, and Brownian motion, bring the particles together in the gas stream and van der Waals and electrostatic forces act to hold the agglomerate together. Cementation may subsequen~y take place. Thus, although the agglomerates in the Bayswater fly-ash tend to be large enough to collect, their formation is likely to be random and a statistically high proportion of supe&e particles either remain in the gas stream and escape collection, or become re-entrained during the rapping cycle. Moreover HIGIQYIT (1965) has shown that the motion of very small particles is determined by turbulence in the gas stream rather than by electrostatic force, and that the particles may, in fact, migrate towards the discharge rather than the collecting electrode. According to LOWE,DALMON and Hremrsr (1965) the deposition of such particles on the discharge electrode may cause abnormal corona discharge. The superfine nature of the fly-ash may explain the extremely variable results of many previous technical-scale tests with Bayswater seam coal (CSIRO, 1967), since the process of agglomeration and the stability of the agglomerates once formed are probably sensitive to con~tions in the gas stream. Indeed, it was reported previously (CSIRO, 1967) that increased atmospheric humidity resulted in improved collection efficiency of the fly-ash. The highest precipitation efficiency (98.5 per cent) was ob-
Microscopic Features of Fly-Ash Particles
183
served when the moisture content of the gases entering the precipitator was highest (N 10 per cent), and the lowest efficiency (< 80 per cent) when the content was lowest (H 7 per cent). Possibly the increased humidity favoured the formation of agglomerates, the stability of.which was increased by the surface tension of adsorbed moisture and/or by solution and cementation taking place at particle contacts. The difficulties with Bayswater fly-ash are therefore thought to be of two-fold origin: (1) the majority of particles are mostly submicron in size and therefore, in the absence of larger particles to transport them, are difficult to collect; and (2) although these small particles tend to agglomerate, the result is still a fly-ash in which more than 45 per cent (by weight) of the “particles” are smaller than 5 pm, an unacceptably high proportion, That in this case these factors are more relevant to precipitation behaviour than the electrical properties of the fly-ash is indicated by the fact that Bayswater and Great Northern fly-ash, whose electrical resistivities are of the same order (TASSICKER, HERCEG and MCLEAN,1966c) show considerable difference in precipitation behaviour. The complication introduced by the presence of superfine particles is less important for the other fly-ashes considered here, since most particles recorded as being larger than 5 pm are genuinely larger, and not agglomerates of superfine particles. For these ashes, a major factor influencing precipitation efficiency appears to be the proportion of particles smaller than 5 pm entering the precipitator. The particles in Balmoral and Wongawilli fly-ashes (low precipitation efficiency) are less spherically symmetrical (FIG. 4) than those in the Great Northern and Liddell fly-ashes (high precipitation efficiency), which suggests that the shape of the particle may also be a factor influencing precipitation efficiency. As particle size and shape are both readily determined, it may be possible to predict precipitation behaviour of fly-ash by the evaluation of these factors. Where, as with the Bayswater, Balmoral and Wongawilli fly-ashes in the present study, the predictions are unfavourable, possibly increasing the gas humidity might help promote agglomeration of the superfine particles of the Bayswater type, or, with Balmoral-type ash, adding fluxing agents to the fuel during combustion to promote formation of spherical particles. Lengthening particle residence times by increasing turbulence in the furnace might favour formation of larger ash particles. 5.3 Signi$cance of the petrographic composition of coal in relation to electrostatic precipitation The results of many technical-scale tests indicated that, apart from the Bayswater and Balmoral seam coals, the only other Liddell area coal likely to prove troublesome in regard to fly-ash precipitation was the Farrells Creek seam (CSIRO, 1967). Moreover, SMYTH(1968) has shown that the Bayswater and Farrells Creek seam coals differ in microlithotype composition from all the other Liddell area coals of the Tomago Coal Measures. In both these coals the proportion of vitrite is low (whereas it is generally high for the other coals) and the proportion of fusite is high. This would suggest, therefore, that it is coals having an abnormally high fusite content that yield superfine fly-ash. Vitrite is the most reactive component of bituminous coal and also has high swelling properties and good plasticity. It is this component which yields cenospheres such as the one shown in FIG. 5, during combustion of the pulverized coal. Fusite, on the
184
C. A. J. PMJISONand A. R. RNWEN
other hand, is relatively inert and does not swell or become plastic. It consists predominantly of two microscopic constituents (fusinite and semifusinite) that are also the principal constituents of anthracites. Investigations in progress with anthracite provide some support for the above statement that the fusite in the bituminous coals is the source of the superfine fly-ash. Partly burnt particles of anthracite are collected directly from a furnace gas stream onto 3 mm diameter disks of 200 mesh/in. stainless steel gauze, and then examined in an electron microscope. The technique is a modification of one described previously (RAMSDEN, 1968). The investigations have shown that discrete droplets of superfine fly-ash form at the surface of the anthracite but do not coalesce. Since, in a bituminous coal, fusite is the constituent whose properties most closely resemble those of anthracite, it is probably safe to infer that a similar ash-forming process occurs during its combustion. Thus, where fusite is the dominant microlithotype, the formation of a high proportion of superfine fly-ash particles may be expected, which may be difficult to precipitate. 6. CONCLUSIONS Examination of the microscopic features of fly-ash particles from full-scale and technical-scale precipitator tests with bituminous coal indicates that particle size and, possibly, particle shape influence precipitation behaviour. If the fly-ash entering the precipitator contains more than about 45 per cent by weight of particles smaller than 5 pm then difficulties may be expected in achieving efficient collection. Fly-ash particles which tend to be spherical precipitate more efficiently than those of irregular shape. Superfine fly-ash (i.e. that containing mostly submicron particles) may be especially difficult to collect on a predictable basis since, to a large extent, the precipitation behaviour appears to depend upon a random association of particles forming agglomerates larger than the critical size of 5 pm. Superfine fly-ash appears to be formed from coals of high fusite content. work forms part of an investigationinto problems of electrostaticprecipitation of fly-ash conducted in collaboration with the Elecb%AyCommissionof New South Wales. It is a pleasure to acknowledgethe assistanceof R. V. DELANDRO,T. DOEL and L. A. O’L~U~HLIN with the technical-scaletests, and of S. R. SILVA and Mrs. V. M. SILVA,of the Defence Standards Laboratory, Department of Supply, Melbourne, with the scanning electron microscopy. The authors Acknowle&ements-This
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