Coal-water fuel combustion

Twenty-first Symposium (International) on Combustion/The Combustion Institute, 1986/pp. 159-171

COAL-WATER

FUEL COMBUSTION

EDWARD T. MCHALE Atlantic Research Corporation Alexandria, Virginia 22312

Research related to the combustion of coal-water fuel (CWF) is being conducted throughout the world in government, industrial and university facilities. In this paper studies are reviewed of several of many aspects of the combustion process, with emphasis on: atomization and carbon burnout and the properties of CWF that influence these, mainly rheology and particle size distribution; combustion assist measures; ignition; emissions; and single droplet studies and combustion modeling. One combustion research problem receiving major attention is the relationship between the atomization process and CWF viscosity. A qualitative discussion of this subject is presented, as well as that of the droplet formation mechanism of CWF fuels. Lastly, single droplet burning and combustion modeling have been helpful in providing understanding of the combustion processes of vaporization, agglomeration, devolatilization and char formation and burnout.

Introduction

tally. A catalog of relevant combustion parameters would include the following:

This review represents an update and revision o f a p a p e r s covering coal-water fuel combustion technology that was p r e p a r e d about two years ago. T h e fuel itself is intended as a substitute for heavy oil to be used in large oil-designed boilers and direct-fire furnaces. T h e name coal-water fuel (CWF) has evolved as the more accepted designation to distinguish this product from older "mixtures" or "slurries" that were intended primarily for pipeline application and which have much different physical properties. T h e m o d e r n coal-water fuel contains typically 6 5 - 7 5 % by weight coal with the balance water and a small a m o u n t ( < 1%) of additives. It is intended to be b u r n e d without dewatering, and in fact can now be produced at such low viscosity that it may be transportable by pipeline. Besides the high coal concentration and low viscosity (generally less than 1000 centipoise), another distinguishing physical p r o p e r t y of CWF is its long-term stability. It can be stored for weeks to months with only minimal particle settling. Lastly, the particle size distribution (PSD), which tends to be much b r o a d e r than that o f a normal utility grind for pulverized coal burning, is critical. As will be evident in this review, an u n d e r s t a n d i n g of the combustion behavior o f C W F cannot be separated from a knowledge o f these fuel properties: solids content, rheology, PSD and additive package. Virtually every aspect of CWF combustion technology is being investigated experimen-

Atomization (air/steam) Carbon burnout (derating) Slagging tendency Tube fouling Fly ash size Ignition Flame stability

Burner turndown Effect of assist fuel Heat flux distribution Furnace radiation Secondary air preheat Slurry preheat NOn formation

Studies of single d r o p l e t burning and CWF use in fluidized beds are also being conducted. T h e r e has been some analytical modeling, but that which is directed specifically at the overall CWF combustion process, as distinct from the coal b u r n i n g alone, has been limited. Studies have concentrated heavily on high volatile bituminous coals, although some atention has been given to coals o f all ranks from anthracite to lignite. Selected topics a m o n g this listing are reviewed below. Studies of CWF Atomization and Carbon Burnout Historically the first r e p o r t e d combustion study was p e r f o r m e d u n d e r US Department of Energy (DOE) sponsorship e, followed shortly thereafter by an Electric Power Research Institute (EPRI) sponsored project. 3 These showed that (early generation) CWF could be atomized and burned, that secondary air swirl greatly improved combustion efficiency, and that high volatile coal p r o d u c e d greater carbon burnout than m e d i u m volatile.

159

160

INVITED LECTURE-COAL COMBUSTION

Quality of atomization has emerged as the leading aspect of C W F combustion requiring research, and considerable effort is being devoted to designing and evaluating atomizers, measuring d r o p l e t sizes in cold-flow tests, measuring carbon b u r n o u t in furnace tests, and studying how CWF properties and additives affect all of these. In Ref. 1, some 92 such studies were listed; and from Refs 4 and 5 another 10 or more could easily be a d d e d to the list. The studies involved furnaces of firing capacities ranging from 0.4 to 80 x 106 BTU/ hour (MMBTU/H), with carbon burnouts from 85% to over 99%. W h e n droplet sizes were measured in associated cold-flow tests using laser diffraction or interferometry (scattering), the mass median diameters (MMD) were reported to vary from 35 to 150 microns. This technique for d e t e r m i n i n g CWF droplet size is widely used and along with photographic measurements has been subjected to critical study by Meyer and Chigier. 6 Some observations can be made on the atomization process itself. T h e physical mechanism whereby droplets are formed using certain twin-fluid atomizers involves formation of sheets of the liquid in and/or immediately as it discharges from the nozzle, which d e f o r m into ligaments, which in turn disintegrate into droplets. ~ With liquids such as water and oil, viscosity alone opposes the shear forces of the atomization process. T h e same mechanism o f drol~let formation applies with CWF atomization~; however, more than just viscosity forces oppose the shear to tend to hold the material intact. As water of the CWF evaporates d u r i n g the sheet/ligamentJdroplet process, coal particles tend to a d h e r e strongly to each other, and the slurry masses congeal. CWFs are formulated with a particle size distribution that is designed to produce m a x i m u m packing density and mininmm void volume, and consequently a small loss of water has a large effect on the consistency o f slurry. Thus by their very nature CWFs resist efficient atomization by the normal droplet formation mechanism. It would seem that moderate dilution of a CWF would greatly improve atomization behavior, and in fact as will be discussed later this is the case. The atomizers fall into several classes: internal, external, Y-jet, T-jet, rotary cup, and others. Schematic diagrams illustrating several types are shown in Fig 1. Two-fluid atomization is used almost exclusively; there are no reports of pressure atomizers, and very little mention of the use of sonic or ultrasonic energy. No one type has e m e r g e d as clearly superior for CWF firing. Other listings and some test results may be found in references 8 and 9.

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At the present time, in the larger size combustors and employing practical operating conditions with the atomizers, it appears that droplet MMDs o f 5 0 - 8 0 microns and carbon burnout of about 99% are achievable. These are respectable results and indicate that combustion p e r f o r m a n c e o f CWF is comparable to that of pulverized coal. On the other hand, it is desirable to exceed the performance o f p.c. firing, and this should be achievable with further research. Since the MMD of coal particles in CWF is often 20 microns or less, this will not be a limiting factor, and there is definite room for i m p r o v e m e n t in atomization quality. Another i m p o r t a n t aspect of atomization quality is the fraction of large droplets. Depending on atomizer type and operating conditions, and on C W F quality, this can be held to about one percent greater than 300 microns at a ratio of about 0.15 by weight atomizing air to CWF. It is taken as axiomatic that carbon b u r n o u t and atomization d r o p l e t size are inversely related. Test data discussed later in connection with Fig. 3 and 4 s u p p o r t this, but as will be seen the situation can be complicated. Combustion Assist Measures

In o r d e r to achieve efficient combustion o f CWF the secondary air ordinarily is preheated,

COMBUSTION OF COAL-WATER MIXTURES data on which may be f o u n d in references 10, 11 and 12. Bortz et alJ 2 conducted a thorough study involving several coal types, burner configurations and furnace heat removal rates. This study and others show that air preheat to 250-400~ is usually necessary. An alternative may be to a d d enthalpy in another form, and for this p u r p o s e a small a m o u n t (3-4% on an energy basis) of gas or oil fuel co-firing is sometimes used. Essenhigh et al. 13 in fact have been using natural gas as an atomizing fluid, although in larger amounts. S o m m e r et al. 14 have r e p o r t e d a dual fuel rotary cup atomizer, with a small inner cup providing oil for assist. Tests at the International Flame Research Foundation (IFRF) 12 in Holland have shown that gas assist is beneficial. Tests at NEI International Combustion in England have shown a dramatic effect on the visual appearance of a CWF flame (becoming much more compact) with gas assist (private communication, J.W. Allen). Knell et al. 15 have b u r n e d a CWF o f m e d i u m volatile coal and have found increased carbon burnout, from 86% with no gas assist to 91% with 1 - 2 % gas assist. This represents evidence that higher rank coals (medium or low volatile bituminous) may yield higher combustion efficiency if b u r n e d with an assist fuel. Combustion of CWF using oxygen enrichment has not received as much attention as has that with fuel-assistance measures. In a recent p a p e r by McIlvried et al. 16, air with an extra two percent oxygen a d d e d was used in a small (0.16 MMBTU/H) combustor with methane assist. Results are fairly preliminary; a principal finding seems to be that N O . formation is increased. Hansel 17 has reviewed some experimental data which he feels indicate that oxygen enrichment would reduce boiler derating. T h e effect of preheating C W F itself and/or the atomizing air has also been studied 1] by measuring the mass median diameter of atomized droplets. CWF was heated to 150~ and atomizing air to 250~ and each c o m p a r e d to 95~ conditions at full and partial loads. These data are not precise, but a general trend toward smaller droplet size is discernible, in the most favorable cases a 10-15% reduction when atomizing air and CWF are both heated together. Because the a m o u n t o f sensible preheat is small relative to the heat o f vaporization of the water in a CWF, any effect might be attributable partially to a change in some physical property, possibly viscosity reduction. A very thorough study of the effect of fuel treatments on CWF combustion has been perf o r m e d recently at M I T Is. Comparison tests involving measurements o f spray droplet size,

161

carbon b u r n o u t and fly ash size were made for fuels that, i) were p r e h e a t e d u p to 150~ 2) contained CO2 a d d e d by pressurization, and 3) contained picric acid a d m i x e d with the CWF. T h e a d d e d substances were expected to improve combustion by p r o d u c i n g secondary atomization. Carbon dioxide p r o d u c e d better results than picric acid, but both were of marginal value. However, the thermal treatment yielded substantial improvement, both in spray droplet size and in carbon conversion. T h e atomization data are shown in Fig. 2, where a steady decrease in droplet MMD was found as a function of t e m p e r a t u r e at all atomizer air/fuel ratios tested. T h e authors attribute the decrease in d r o p l e t size to a reduction in CWF viscosity with heating to 100~ and to secondary atomization above this t e m p e r a t u r e .

Relationship of Combustion Performance to CWF Properties Essentially all of the combustion parameters catalogued earlier, with the possible exception of gaseous pollutants, are d e p e n d e n t on the physical properties o f CWF, and no serious experimental study can ignore this fact if the results are to be reproducible. However, for a given coal and t e m p e r a t u r e condition, one can r e g a r d atomization quality as the principal i n d e p e n d e n t variable a n d virtually all the other variables as dependent. In this section the CWF properties o f interest are primarily viscosity and particle size distribution and secondarily coal concentration a n d surface tension. A R C CWF ( 6 8 / 3 2 , standard] OR-KVB NOZZLE Fuel Flow Rate 2 7 2 k g / m i n

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162

INVITED LECTURE-COAL COMBUSTION

Research aimed at establishing the influence of CWF properties on combustion has been u n d e r t a k e n by n u m e r o u s organizations. A program at IFRF ~z involved seven CWFs, made from six different coals, and supplied by four producers. An on o i n g p gro ram supported by the US DOE 19, ~g. 21 is using 11 CWFs from four coals and five producers. A n o t h e r program u n d e r EPRI sponsorship 1~ involved six CWFs, five suppliers and six coals. Many variables necessarily are not controlled in these tests (including the coal and additives used by each producer) which, as will be seen, makes it difficult to draw conclusions applicable beyond each individual study. In addition each testing organization uses different atomizers, b u r n e r s and combustors of different firing capacity. Methods of measuring slurry properties are also not standardized.

content is d e p e n d e n t on the ease with which particles can slip past one another u n d e r applied stress. This "viscosity" measured u n d e r flow conditions could very well not be strongly related to the rate of breakage ofintermolecular or interparticle forces of CWF, and therefore even viscosity measured at high shear rates may not correlate well with quality of atomization. With the foregoing considerations in m i n d some test results can be considered. Data of Daley et al. 1~ in Fig. 4 (and Table 1) show no relationship between droplet MMD and low shear viscosity (Haake rotational viscometer) for six different CWFs. Furthermore, A not only was the most viscous but also was very dilatant at low shear rates (viscosity increasing with increasing shear rate). Yet in combustion tests the carbon conversion (97%) was highest for A of the six fuels. Fuel C of approximately equal viscosity to A at 100 s e c - ' w a s slightly pseudoplastic (viscosity decreasing with increas-

Viscosity It was natural at the outset of CWF research to assume that viscosity would be an important property controlling atomization because it does so with fuel oil. Many empirical expressions have been developed relating oil viscosity to droplet size for certain atomizer types. These are valid for two reasons: first, oil is a Newtonian fluid whose viscosity is i n d e p e n d e n t of shear rate, and therefore measurements at shear rates of, say, 100 sec -1 made in a laboratory viscometer are representative of viscosity at, say, 10 4 sec -r, which shear rates are typically e n c o u n t e r e d in atomizer passages. Secondly, viscosity of oil governs the rate of breakage of the forces b o n d i n g molecules together a n d as such provides a measure of the tendency of the substance to be deformed and disintegrated by an atomizer air or steam blast. With CWFs the first of these conditions does not hold because the fluids are not Newtonian; and it is not a p p a r e n t that the second condition is valid either. CWFs exhibit very complicated rheological behavior, and at low shear rates (a few h u n d r e d sec -1) may display dilatency or pseudoplasticity or even change from one to the other. Extrapolation to shear rate regimes of thousands or tens of thousands sec -~ is not reliable. Rheograms of two CWFs are presented in Fig. 3 that illustrate the changes in fluid behavior that can occur over a wide range of shear rates 22. Note the Newtonian behavior that develops u p o n dilution. In the case of CWF it is not obvious how viscosity relates to the tendency of the material to be broken into droplets by a fluid blast. Viscosity is a measure of resistance to flow, which for a two-phase fluid of high solids

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TABLE I CWF Data From Daley Et al. ~~

CWF

Percent Solids

A A B C D E F

75.3 70.5 69.3 69.4 69.9 74.9 70.0

Percent Carbon Conversion @ 3 MMBTU/H

Haake Droplet Viscosity (CP @ 100s -1) MMD (p.) 2000 400 1550 1650 500 500 950

87 -102 109 100 90 90

Coal Particle MMD (p.)

97 -94 87 92 96 93

35 35 20 50 25 35 25

(CP = 1.0 mpa. sec) ing shear), yet produced the lowest carbon conversion (87%), contrary to what one might expect based on analogy to oil. However, it can be seen from Fig. 4 that for a given CWF (C) a reduction in viscosity does produce a more easily atomized fuel. The same effect has been shown by Gillberg et al. z3 for a series of CWFs, In their work a master batch was produced at about 80% coal content and samples progressively diluted to about 68%. Viscosities (to 450 sec -~ with Haake) were measured for each, as were atomized droplet MMD. A monotonic decrease in droplet size from about 150 to 50 microns (at atomizing air/fuel = 0.2) was f o u n d as viscosity fell from about 950 cp to 40 cp. Others have found the same type of behavior. Hence, for a given CWF, if viscosity is reduced by dilution, as opposed to reduction by additive or particle size control, significantly improved atomization results. Levasseur et al. 21 have attempted to find a correlation between viscosity and atomization quality from data of 11 different CWFs (mentioned earlier) supplied by five producers. They used three types of viscometers and examined the data in three shear rate regimes to 1500 sec -1. In all cases, over a wide enough range of viscosity, there was a "trend" showing increasing droplet size with increasing viscosity. An analysis of their most extensive set of data at a shear rate of 100 sec -1 would yield a relationship D = 20.7

(,1])0.21

where D is droplet MMD in microns and ~l is viscosity in centipoise. However, the correlation coefficient is only 0.57. They concluded that the goal of their study of determining a reliable method for predicting CWF atomization quality using viscosity as a criterion was not realized. They also concluded that using a power law

exponent (from a viscosity-shear rate relationship) was not a dependable predictor either. Pohl et al. 24 attempted to find a power law exponent to relate viscosity to droplet MMD in a study in which a CWF was diluted sufficiently to make it a Newtonian fluid. The value of the exponent was 0,05. They also cite other studies, one z~ in which a value of 0.3 for diluted CWF was reported, a n d another 15 in which they determined the e x p o n e n t to be negative for several different CWFs, all at a shear rate of 5000 sec 1. At present there is no useful empirical relationship for CWF as there is for oil. The relationship between carbon conversion d u r i n g combustion and atomization droplet size follows a similar pattern to that just discussed in the sense that fuels from different sources show erratic results. For the six CWFs discussed, an inverse correlation does exist as seen in Fig. 5, but the relation is certainly weak when the fuels are all from different sources. However, for a single CWF whose droplet size is progressively decreased, there is a smooth 96

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INVITED LECTURE-COAL COMBUSTION

164

corresponding increase in carbon conversion (Fig. 7). Lest it be thought the lack of a general viscosity-combustion/atomization correlation is attributable to viscosity values being obtained at low shear rates, data at h i g h shear (Burrell viscometer) are presented in Table 2 from a study by Knell et al. 15 The CWFs are all different, and again there is no discernible trend a m o n g the parameters of interest. T h e "micronized" CWF exhibited the highest viscosity at high shear, yet produced the finest droplet size and very respectable combustion (carbon conversions are close for all the high volatile coals (Chinese Coal "X" is m e d i u m volatile), and therefore too much m e a n i n g cannot be attached to any ranking on this basis). The "unimodal," Chinese "O," and almost certainly the "bimodal" slurry, had lower viscosities but larger droplet sizes; possibly the Chinese "O" can be regarded as yielding poorer combustion. I n a n o t h e r study 25 involving two of these same type CWFs, atomization droplet size was measured as a function of dilution (70% and 66%) and related to measured viscosity at high shear rates (to 104 sec-1). T h e results are presented in Fig. 6 at two air/fuel ratios. T h e data showed much scatter, but a definite trend exists between atomization and high shear viscosity as was described for low shear viscosity for diluted CWFs. At this time it must be concluded that no

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consistent relationship has been shown to exist between viscosity a n d atomization quality when CWFs from different sources using the same or different coals are compared. This can be true even for CWFs from the same source using the same coal. A major exception is that reduction in viscosity by dilution does produce a substantial improvement in atomization, and correlations have been found.

Particle Size Distribution (PSD) The central question here is how the size distribution of the coal particles in CWF relate

TABLE II CWF Data from Knell et al. ~5 Elk Creek Coal

Percent Solids

Chinese Coal

Bimodal FCM30

Unimodal TT2&3

Micronized

68.92

70.57

68.42

"X"

"O"

68.49

68.47

Shear Rate Viscosity (CP)

(sec -1)

10 110 2,500 5,000 10,000 Atomized Droplet MMD (ix) % Carbon Burnout @ 4.5 MMBTU/H Coal Particle MMD 0x)

3,570 1,352 NM* NM NM

6,308 2,995 719 648 584

3,062 2,320 2,270 1,860 1,530

3,900 2,365 1,120 930 775

7,360 3,040 724 665 610

>>35

45

35

45

45

97.6

98.6

98.2

86

97

22

32

18

42

44

Chinese "X" is a medium volatile coal. * Not Measured.

COMBUSTION OF COAL-WATER MIXTURES FULLLOAD 4(]% EXCESS A I R

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97

96

105

115

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D R O P L E T MMD (p)

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to atomization and to carbon burnout. In fact particle size distribution (PSD) affects many combustion parameters, a n d there is particular interest in the relation to slagging, fouling and fly ash size. PSD considerations concern socalled "micronized," or fine particle CWF, and "standard" CWF. In the former, the fraction of particles smaller than 45 microns (325 mesh) is about 95%, and that less than 75 microns (200 mesh) is about 99%. This compares with a standard CWF where the a m o u n t less than 200 mesh is typically 85%, a n d that less than 325 mesh is about 70%. MMD for these fuels might be seven microns and 20 microns, respectively. (All these values are nominal and can vary considerably; MMD o f 40-60 microns is not uncommon.) A reason that particle size distribution is of such great interest a n d so much studied is that it is h o p e d that by r e d u c i n g the size of coal particles in CWF m o r e r a p i d b u r n o u t will be achieved, which will allow combustors to either be reduced in size or to operate at higher capacity (less derating). T o date this has not been realized because the effective particle size of coal d u r i n g CWF combustion is set, not by the particles in the slurry fuel, but by the atomized droplet size. As the water in a heated d r o p l e t evaporates, the coal particles devolatilize and they agglomerate. T h e y may also swell,

165

and substantial porosity may develop. But in any case the effective particle size during combustion is d e t e r m i n e d by the droplet size and the agglomeration process, not by the size of individual coal particles. This subject is discussed again in connection with CWF combustion modeling. In considering the PSD question, the data o f Tables 1, 2 and 3 can be examined. In Table 1 there is no correlation between coal particle MMD and either d r o p l e t MMD or carbon conversion (if the data are plotted, this becomes more evident). Also the data of Table 2, while they show a spread o f coal particle size, do not show much difference in d r o p l e t size or carbon burnout. Likewise in Table 3 no correlation exists between MMD o f coal particles and droplets (carbon conversion is high in all cases). All these results indicate that over the range of coal particle sizes (about 1 5 - 5 0 microns MMD) covered in these studies there is no trend indicating improved atomization or combustion performance with finer o r coarser coal grinds. As mentioned, fine particle CWFs have particle MMD below 15 microns, but there are no definitive studies with these fuels. A combustion study was p e r f o r m e d by Farmayan et al. 26, using the same CWFs of Elk Creek coal as listed in Table 2, and the same type o f atomizer. T h e micronized slurry was 95% < 325 mesh and 98% < 200 mesh; the unimodal was 72% < 200 mesh. These represent about the e x t r e m e ends o f the particle size range of CWFs now being p r o d u c e d or considered for boilers. T h e carbon conversions at a point in the combustion c o r r e s p o n d i n g to two seconds residence time in the furnace were 95.8% for the micronized CWF and 98.6% for the unimodal (standard CWF). T h e authors of the study discuss the problems of accurately determining carbon conversion by the ash tracer method when bottom and fly ash are involved, but nevertheless it is fairly certain that the carbon conversion values are reliable and that the coarser u n i m o d a l fuel did produce

TABLE III CWF Data From Hargrove Et al. z9

CWF/ Supplier/ Coal 1 2 3 4

(C) LK (D) SD (E) SD (C) SD

Percent Solids

Haake Viscosity (CP @ 100s -1)

Atomized MMD (ix)

Percent Carbon Conver.

Power Law Exponent

Coal Particle MMD (Ix)

66 69 66 70

1500 840 860 640

88 76 69 72

99.6 99.6 99.7 99.3

0.96 1.5 1.2 1.4

15 50 25 15

166

INVITED LECTURE-COAL COMBUSTION

significantly higher combustion rates than the finer CWF. T h e authors raise the point, often made by others, that fine particle CWFs will yield little benefit unless improved atomization can be developed to produce smaller droplets to take advantage o f the smaller coal particles. As of the present, droplet diameters, typically averaging 80 microns or somewhat lower for good atomizers a n d reasonable conditions, are roughly four times greater than coal particle diameters o f s t a n d a r d CWFs, about 20 microns. Hence it is difficult to make the case for finer grinds. In a study by Kikkawa et al. 27, findings were reported for CWFs of MMD - 2 0 microns (equivalent to the Farmayan 26 standard unimodal) and a n o t h e r of MMD - 4 5 microns, significantly coarser. Combustion efficiencies of approximately 98 and 95 percent, respectively, were obtained. T h e top size o f the coal particles was the same in both fuels, about 150 microns. This study then represents evidence that CWF with particles coarser than the "standard" grind may produce p o o r e r combustion performance. The evidence at this time indicates, but only weakly, that "standard" CWF, o f coal MMD approximately 2 0 - 3 0 microns with about 7 0 80% < 200 mesh, yields higher carbon conversion than either finer or coarser CWFs. O t h e r factors such as slagging and fouling and fly ash size may eventually become more i m p o r t a n t considerations in deciding fineness o f grind for CWF.

Surface Tension Quality o f atomization of fuel oil d e p e n d s on surface tension, although to a lesser extent than it does on viscosity. For CWF there has been one investigation o f this p r o p e r t y 1~ and the data are collected in Table 4. I f a plot o f these surface tension data against either d r o p l e t size or carbon conversion is made, r a n d o m scatter is TABLE IV CWF Data From Daley Et al. 1~

CWF

Surface Tension (dynes/cm)

A B C D E F

71 45 50 69 57 72

Atomized Percent Droplet Carbon MMD (Ix) Conversion 87 102 109 100 90 90

97 94 87 92 96 92

evident, indicating for this p r o p e r t y also that no relationship with combustion performance exists, at least a m o n g different CWFs. No studies have been r e p o r t e d where the surface tension of a single CWF was varied.

Ignition T h e heat r e q u i r e d to vaporize the water in a CWF is 3 - 4 % o f the heat of combustion and as such should have no substantial impact on the overall combustion process. However, this water evaporates at the outset o f the process and may affect ignition. T h e heat o f vaporization of water in a 70% CWF is roughly equal to the sensible heat required to raise the coal to its ignition t e m p e r a t u r e , of the o r d e r of 1000~ In a partly analytical study of CWF ignition Walsh et al. 28 have calculated, based on their model, that the time to ignition ( - 5 ms for an 80 micron droplet) is approximately double that required for a coal particle fired dry. T h e distance from the b u r n e r at which ignition occurs was estimated to be about double for CWF c o m p a r e d to pulverized coal. T h e y also report that the majority of the heat of evaporation/ignition is supplied by hot recirculated combustion gases, with only about 10% by radiation.

CWF Combustion Emissions T h e r e are three species of interest here: fly ash, SOx and NOx. In the case o f sulfur oxides emissions, practically all of the sulfur in the coal is emitted as SO2 (without control measures), and t h e r m o d y n a m i c calculations 29 show this to be the principal equilibrium product also. T h e small a m o u n t o f sulfur a p p e a r i n g as other oxides (SOs or SO42) is not always u n i m p o r t a n t but has been little studied. In the case of particulate emissions, a number of studies (see ref. 4 and the earlier Sixth Symposium) have included measurements of fly ash size from CWF burning. T h e data are too disparate to warrant review at this time, primarily it appears because size is a strong function of fly ash u n b u r n e d carbon content, which runs very high until combustion efficiency approaches or exceeds about 99%. Nitrogen oxides emissions are governed by chemical kinetic factors, d e p e n d strongly on the t i m e - t e m p e r a t u r e regime in a furnace, and hence cannot be reliably estimated by thermodynamics. T h e y have been the subject of several research efforts, which warrant review. In many cases the NOx formation from CWF

COMBUSTION OF COAL-WATER MIXTURES has been experimentally c o m p a r e d to that from pulverized coal firing; in some cases staged combustion testing has been conducted. Nitric oxide, the d o m i n a n t species, arises from both fuel-bound nitrogen and the fixed nitrogen in air, the f o r m e r usually being the d o m i n a n t source in unstaged combustion. A summary of the mechanism of formation can be found in reference 26. In this same p a p e r it is shown that NOx emission is a strong function of swirl number, being about 850 p p m at a swirl n u m b e r o f 0.7 and about 500 p p m at 1.5-2.0. It was also f o u n d that heat extraction in a furnace reduces NOx concentration. A discussion of "the role o f homogeneous and hetrogeneous reactions is to be found in reference 30, based on measurements of intermediate nitrogen species. For information on the effect of staged combustion o f CWF the r e a d e r is referred to 11, 26, 31, 32. T h e results o f these studies are consistent and show that if the first stage of the process is conducted at a stoichiometric ratio near about 0.8, NOx emissions can be reduced by about 50 percent for the entire process. T h e most meaningful studies compare NOx effluent concentrations for CWF and dry parent coal firing. For unstaged combustion Bortz et al. 12 r e p o r t NOx levels about the same for both fuels, while Smith et al. 11 r e p o r t them about the same at high firing rates but reduced for CWF relative to pulverized coal at low rates. T h r e e investigations, 15, 31, 32, cited above, which are very consistent with each other, have revealed substantial NO• reductions when CWF is b u r n e d and c o m p a r e d to the parent coal. T h e NOx emission levels varied considerably from 650 to 350 p p m for the dry coal firing because of the differences in combustion systems; however, there was a p p r o x i m a t e l y a 30% reduction in each study (at about 3% stack oxygen) with CWF. Data 31 are r e p r o d u c e d in Fig. 8 which are fairly representative of NO• emissions for parent coal and CWF, with and without staging. T h e quantity kBNR in the figure represents the stoichiometric ratio o f air to fuel in the first stage.

~00500[ o CWF

~

3OO

0 PARENT COAL

o

CWF

2OO

0 oo

I o 0

1

2

3

4

5

%6

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0'8

019

O2 A T FURNACE O U T L E 7 (%)

X BNR(

(A)

(B)

FIG. 8.

1 '0

1'1

1~

167

Fluidized Bed Combustion Coal-water fuel is completely suitable for use in fluidized bed combustors (FBC), and the particle size distribution can probably be more coarse than the standard grind. When CWF is injected there is a tendency for particle agglomeration, which substantially reduces the elutriation problem o f fines f o u n d with dry coal injection, particularly in bubbling beds. Another operational p r o b l e m with FBC is the solid feed system, which subject is discussed at length in reference 33. It is worse for pressurized than for atmospheric fluidized beds, and for both systems CWF seems to be ideally adaptable. In addition it offers the possibility of incorporating limestone into the C W F for sulfur capture. In a study by Roberts et al. 34 a coarse particle CWF was fired at a rate o f 4.4 MMBTU/H in a pressurized fluidized bed combustor at 16 atm. Combustion efficiencies o f > 99% were obtained, with sulfur retention as good as with crushed coal. T h e authors r e p o r t that with the liquid fuel, control, feeding and distribution are considerably simplified and less expensive. Other tests were also conducted by A r e n a et al. 35 and Rowley et al. 36 for atmospheric operation of a FBC using CWF, by Guoquan et al. 37 for three fluidized beds where sucessful operation was reported, and in a small sized u n i t y

Single Droplet Studies and Combustion Modeling Several studies o f single droplet burning have been r e p o r t e d ~9-46. These involved susp e n d i n g or injecting individual droplets in a combustor while making measurements and observations of the particle behavior. Matthews and Street 43 have taken high speed motion pictures of 4 5 0 - 6 0 0 micron droplets which reveal water vaporization followed by volatile release and ignition, often as a localized stream. T h e coal particles agglomerated within a droplet, and swelling began immediately with volatile release (for high swelling coals). T h e swelling led to a larger but porous particle, with some fragments breaking off d u r i n g char burnout. Low swelling coals f o r m e d particles the same size as the droplet. T h e surface area available for oxidation is therefore significantly different for coals of different free swelling indexes, and this p r o p e r t y should be quantitatively investigated for its effect on CWF combustion rates. Yao and Liu ~9 in their somewhat earlier r e p o r t observed similar behavior with slurries o f a high swelling bituminous and a low

168

INVITED LECTURE-COAL COMBUSTION

swelling lignite. T h e fragmentation mentioned above was not observed, but a disruptive swelling occured d u r i n g gas release, p r o d u c i n g a "popcorn-like" char with the high swelling coal. T h e low swelling lignite stayed the same size t h r o u g h o u t ignition and devolatilization. A hard shell f o r m e d with the bituminous particles which the authors believe i m p e d e d oxygen intrusion a n d r e d u c e d oxidation rates in spite of the porosity. This shell did not develop with the lignite, which the authors feel accounted for faster b u r n rates. Generally it has been observed in the d r o p l e t studies that an agglomerate after vaporization and devolatilization does not burn as a shrinking sphere, but r a t h e r that the diameter stays fairly constant. A cenosphere type of particle usually develops. Most of the single d r o p l e t experimental studies are accompanied by analytical modeling o f the process or some aspect of it. Liu and Law 45 have modeled the char burnout phase, which can be p e r f o r m e d by modification of a coal particle model to allow for porosity. T h e b u r n i n g times followed a d2-rela tion, and as with all single d r o p l e t studies combustion was diffusion-limited because of the low Nusselt n u m b e r and relatively large droplet size. Fu et al. 46 have studied and modeled the evaporation and volatile b u r n i n g phases for three coals of different rank. Conditions were: 1 - 3 m m droplets of 3 0 - 4 0 % coal content; reactor temperatures of 600-2000~ The mechanism was found to involve sequential processes o f evaporation (two seconds) and devolatilization (considerably longer), and this was the basis for the modeling. It should be noted that the apparently clear-cut sequential mechanism may be unique to this study, which involved slurry o f high water content. T h e evaporation was treated as a heat transfer process with equilibrium t h r o u g h o u t the droplet and vaporization at the surface. T h e devolatilization was m o d e l e d as a two-parallel-reaction process as with pulverized coal 47, with a spherically symmetrical gas flame, which process e n d e d when the t e m p e r a t u r e gradient in the particle became zero (dT/dr = 0). Excellent agreement between calculated and measured evaporation a n d devolatilization times for droplets o f d i f f e r e n t diameters was shown. It is not quite clear how the terminal b o u n d a r y condition (dT/dr = 0) in the model was met, since theoretically it should take infinite time to reach this point. Murdoch et al. 4~ have r e p o r t e d two single droplet studies involving experimental measurments and m o d e l i n g with fuels o f several different ranks o f coal plus a coke which

contained 3 0 - 4 0 % water. Droplet sizes were 0.8-1.0 mm, and reactor temperatures were about 1100-1200~ T h e analytical treatment considered evaporation, devolatilization and char oxidation as simultaneous processes, although water evaporation d o m i n a t e d in the early stages. Energy balance and mass consumption involved the processes of conductive and radiant heat flux to the droplet, conduction through the d r o p l e t (or particle), and heat evolution by chemical reaction. T h e rate of this last process was incorporated as a combination of oxygen diffusion to the particle surface and chemical reaction:

kov -~ kz)kc/(kD + kc) where the subscripts on the reaction rates represent: overall, diffusion and chemical. A chemical reaction scheme was included, but evidently the overall reaction rate was controlled by diffusion since the particles were large, the Nussett n u m b e r 2, and a dE-law was found experimentally. T h e coal itself was assumed to pyrolyze in a single-step reaction to produce char and volatiles in some given ratio. T h e source o f heat was the char oxidation reaction (kc), which yielded CO. This p r o d u c t and the volatiles were assumed to b u r n in the gas phase and were not considered to be direct heat sources. T h e analytical treatment of Murdoch et al. produced curves o f t e m p e r a t u r e and mass loss as a function o f time for comparison with the experimental data. T h e later study which represented a m o r e detailed model showed quite good agreement; the earlier version o f the model, which was somewhat simplified, yielded only fair agreement. T h e authors concluded that their a p p r o a c h could be useful in making relative comparisons a m o n g different coal types. Two substantial efforts have been directed at modeling the overall CWF combustion process48'49.-In both cases detailed experimental measurements have been made in furnace tests for comparison. T h e first to be discussed is that of Baxter et al. 4s, who used a combustion chamber of a p p r o x i m a t e l y 0.4 M M B T U / H capacity, a commercially p r o d u c e d CWF of 70% high volatile A bituminous coal content, and an external atomizer with secondary air swirl. O f the many measurements that were made, the m a p p i n g of the CO2 a n d 02 profiles radially and axially through the furnace are of main interest here. T h e theoretical model that was used was an existing Brigham Young University 2-dimensional PCGC-2 c o m p u t e r code for pulverized coal combustion and gasification. It is a turbulent, reacting flow p r o g r a m applicable to parti-

COMBUSTION OF COAL-WATER MIXTURES cle-laden systems. A fuel d r o p l e t size distribution is an input p a r a m e t e r to the code. This may not necessarily be a serious limitation o f the model for application to C W F inasmuch as it should allow this p a r a m e t e r to be varied to examine its effect on the combustion process. T h e numerical techniques for solving the elaborate systems o f equations involving heat transfer processes, mixing and chemistry are described in the paper. T h e equations consider water evaporation, coal a n d char reaction as simultaneous processes; however, the authors state that the model predictions indicate that droplet/ particle consumption is mostly a sequential process of vaporization, devolatilization and oxidation. T h e combination o f chemistry/diffusion in the model is h a n d l e d by an assumption of local instantaneous equilibrium with statistical averaging to incorporate the local extent of turbulent mixing. Experimental data and predictions are shown in Fig. 9 from Ref. 48. T h e predictions indicate more r a p i d fuel consumption in the Radial 4

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early stages of combustion than was observed. T h e authors attribute this mainly to two factors: the limitations of t u r b u l e n t models in describing real flows, especially in recirculation zones; and the disturbance of the flow field near the b u r n e r end of the furnace by the intrusive sampling probe. It would seem that if these problems could be overcome and the accuracy o f the model improved, future analytical efforts might provide helpful guidance for CWF combustion technologists. T h e second theoretical and experimental study of CWF combustion is that of Srinivasachar et al. 49. This is an interesting study because both sequential and simultaneous ("two-zone") processes were modeled, and because sampling o f particles axially along the furnace allowed carbon b u r n o u t as a function o f particle size to be m e a s u r e d and c o m p a r e d with model predictions. T h e experimental studies were conducted in the furnace of the M I T Combustion Research Facility, which has a m a x i m u m firing capacity o f about 7 M M B T U / H but was only operated at one-third of that level. Pertinent data include: secondary air swirled and p r e h e a t e d to 503 ~ K; a commercial CWF o f 66% high volatile coal; and an internal mix atomizer of proprietary design. Gas t e m p e r a t u r e s a n d oxygen concentrations were m e a s u r e d and particles were collected for about one-third the length of the furnace to where the fraction o f b u r n o u t reached about 0.7. T h e collected particles were sized by sieving, and the fractions analyzed for ash and combustible content. T h e size ranges (in microns) r e p o r t e d were 53-75, 7 5 - 9 0 and 90-125. T h e assumption was made that a dried, agglomerated C W F particle does not change appreciably in d i a m e t e r up to about 70% b u r n o u t which allowed data of b u r n o u t vs. particle size to be obtained. T h e three processes of evaporation, devolatilization and char oxidation were modeled. In the (simpler) sequential version, evaporation and devolatilization were allowed to occur separately; in the two-zone or overlapping version of the model, only surface moisture evaporated initially, the r e m a i n i n g water vaporizing simultaneously with the o t h e r processes. Comparison o f theoretical and e x p e r i m e n t a l results was not precise enough to d e t e r m i n e which model version was truer. T h e rate o f devolatilization was modeled as two parallel, first-order reactions of differing activation energies 47'5~ A swelling factor (of the o r d e r o f 1.2-1.3) was included in the model and found to have a considerable effect. Both kinetics and convection are included in the char oxidation step. Particle diameter appears in all the rate equations.

170

INVITED LECTURE-COAL COMBUSTION OISTAHCE

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A set o f results s h o w i n g very g o o d a g r e e m e n t between the theoretical and e x p e r i m e n t a l studies is r e p r o d u c e d in Fig. 10. (In the figure, "interstitial water" refers to the fraction within a particle, the r e m a i n d e r b e i n g surface water.) T h e r e was o n e a n o m a l y in the study: the m e a s u r e d o x y g e n c o n c e n t r a t i o n a l o n g the j e t axis r e a c h e d a v e r y low value at a p o i n t w h e n " b u r n o u t " was still in an early stage. T h e t e m p o r a r y o x y g e n d e p l e t i o n along the axis resulted f r o m volatile oxidation. T h e " b u r n o u t fraction" a l o n g the o r d i n a t e in Fig. 10 m o r e p r o p e r l y refers to coal devolatilization, which o c c u r r e d to the e x t e n t o f 60% or m o r e u n d e r the t e m p e r a t u r e a n d h e a t i n g rates within the furnace. T h e u p p e r , flatter segments o f the curves in Fig. 10 r e p r e s e n t the char oxidation, which was p r o c e e d i n g at a slow rate a l o n g the c e n t e r line at the o n e - m e t e r p o i n t in the f u r n a c e but which accelerated later as o x y g e n a l o n g the center line increased. Based on results such as those o f Fig. 10, this e x p e r i m e n t a l a n d m o d e l i n g a p p r o a c h can be highly successful, and f u r t h e r refinement could provide understanding of details of the C W F c o m b u s t i o n process.

Abbreviations and Conversion Factors cp centipoise CWF coal-water fuel MMBTU/H million B T U p e r h o u r MMD mass m e d i a n d i a m e t e r PSD particle size distribution 1.0 M M B T U = 0.252 kcal = 1055 J o u l e 1000 cp = 1.0 Pascal 9 second

1. MCHALE, E.T.: Energy Progress 5, 15 (1985). 2. MCHALE, E.T., SCHEVFEE,R.S., AND ROSSMEISSE, N.P.: Combustion and Flame 45, 121 (1982). 3. FARTHING, G.A., JR., JOHNSON, S.A., AND VECCI, S.J.: Combustion Tests of Coal-Water Slurry, Final Report Research Project 1895-2, EPRI, Palo Alto, CA 94303, 1982. 4. Seventh International Symposium on Coal Slurry Fuels, US DOE PETC, Pittsburgh, PA 15236, held in New Orleans, LA, May 21-24, 1985. 5. Second European Conference on Coal Liquid Mixtures, Pergamon Press, Inc., New York, N.Y. 10523, held in London, England, Sept. 16-18, 1985. 6. CHIGIER, N. AND MEYER, P.L.: reference 4 above, p. 402. 7. CHIGXER,N.A.: Prog. Ener. Comb. Sci. 2, 97 (1976). 8. GERMANE, G.J., SMOOT, L.D., RAWLINS, D.C., JONES, R.G. and EATOE'GH, C.N., reference 4 above, p. 568. 9. LAFLESH,R.C., RINI, M.J., LACHOWICZ,Y.V. AND LEVASSEUR,A.A.: reference 4 above, p. 596. 10. DALLY, R.D., FARTHING, G.A., JR., AND VECCI, S.J.: Coal-Water-Slurry Evaluation, 2, Final Report CS-3413, Research Project 1895-3, EPRI, Palo Alto, CA 94303, 1984 11. SMITH, D.A., RINI, M.J., LAFLESH, R.C. MARION, J.L., AND BORIO, R.W.: Coal-Water-Slurry Technology Development, 1, Final Report CS-3374, Research Project 1895-4, EPRI, Palo Alto, CA 94303, 1984. 12. BORTZ, S., ENGELBERTS, E.D., AND SCHREIER, W.: Sixth Symposium on Coal Slurry, p. 710. 13. ESSENHIGH, R.H., ZONGMENLi, AND KYU-ILHAL': Performance Characteristics of a Hot-Wall Furnace Fired with Coal Water Slurry Using Gas/Air Atomization, American Flame Research Committee (IFRF), Fall Meeting, October 1983. 14. SOMMER, H.T., MARNICIO, R.J., THYLANDER, L.G., AND LANBAEUS,K.: Sixth Symposium on Coal Slurry, p. 1052, DOE, Orlando, FI., June 1984. 15. KNELL, E.W., MuzIo, L.J., AND AROUD, J.K.: Combustion Characteristics of Occidental CoalWater Mixtures, American Flame Research Committee (IFRF), Fall Meeting, October 1983. 16. MCILVRIED, T.S., SCARONI, A.W. AND JENKINS, R.G.: Oxygen-Enriched Combustion of a CoalWater Fuel, American Chemical Society Meeting Spring 1986, New York, N.Y., Vol. 31, No. 2, Fuel Chemistry Division. 17. HANSEL,J.G.: Energy Progress 6, 44 (19"36). 18. Yu, T.U., KANG, S.W., TOQAN, M.A., WALSH, P.M., TEARE,J.D., BEER,J.M. ANDSAROVIM,A.F. : this symposium. 19. HARGROVE, M.J., LEVASSEVR, A.A. AND CHOW, O.K., Sixth Symposium on Coal Slurry, p. 127. 20. ALBAUGH, E.W., DAVIS, B.E., LEVASSEUR,A.A.: Sixth Symposium on Coal Slurry, p. 282.

COMBUSTION OF COAL-WATER MIXTURES 21. LEVASSEU, A.A. AND LAFLESH, R.C.: reference 4 above, p. 507. 22. HEATON, H.L. AND MCHALE, E.T.: reference 5 above, p. 73. 23. GILLBERG, L., LARSSON, N., MATHIESEN, M., NGSTROM, 0., AND PERSSON,J.: Fifth International Symposium on Coal Slu~wy Combustion and Technology, US DOE PETC, Pittsburgh, PA 15236, 1983. 24. POHL, J.H., SEPULVEDA,J. AND ROTHFELD, L.B.: reference 4 above, p. 357. 25. TSAI, S.C. AND KNELL, E.W.: First Pittsburgh Coal Conference, US DOE PETC, Pittsburgh, PA 15236, September 17-21, 1984. 26. FARMAYAN, W.F., SRINIVASACHAR, S., MONROE, L., DITARANTO, F., TEARE, J.D., AND BEER,J.M.: Sixth Symposium on Coal Slurry, p, 165. 27. KIKKAWA,H., OKIURA, K., AND ARIKAWA,Y.: Sixth Symposium on Coal Slurry, p. 205. 28. WALSH, P.M., ZHANG, M., FARMAYAN,W.F., AND BEER, J.M.: Twentieth Symposium (International) on Combustion, T h e Combustion Institute, Pittsburgh, PA 15236, 1984. 29. HENDERSON, C.B., SCHEFFEE, R.S., AND MCHALE, E.T.: Energy Progress 3, 69 (1983). 30. GERMANE, G.J., RICHARDSON, K.H., RaWLINGS, D.C., HEDMAN, P.O., AND SMOOT, L.D.: Sixth Symposium on Coal Slurry, p. 143. 31. KURODA, H., MASAI, T., TAKABASHI, Y., AND WATANABE, S., Fifth International Symposium on Coal Slurry Combustion and Technology, US DOE PETC, Pittsburgh, PA 15236, 1983. 32. SAKaI, M., TOKUDA, K., KANEBO, S., TAgATSUKA, H., AND IMAMOTO, T.: Sixth Symposium on Coal Slurry, p. 960. 33. SHANG, J.Y.: Fluidized Bed Boilers: Design and Application, Basu, P., Editor, Pergammon Press, New York, N.Y. 10523, 19849 34. ROBERTS, A.G., PILLAI, K., BARKER, S.N., AND BYARN, J.W.: First European Conference on Coal Liquid Mixtures, Pergammon Press, Inc., New York, N.Y. 10523, 1983. 35. ARENA, U., DEMICHELE, G., MARESCA, A., MISSIMILLA, L., AND MICGIO, M.: Sixth Symposium on Coal Slurry, p. 29.

171

36. ROWLEY, D.R., LAu, I.T. AND FRIEDRICH, F.D., reference 4 above, p. 612. 37. GUOQLTAN,H., MINGJIANG, N., XINYU, C., ZHEYU, H., ZHONGYANG L., DESHOU, L., Minghu X. and Kefa, C.: Sixth Symposium on Coal Slurry, p. 756. 38. TRIVETT, G.S., MACKAY, G.D.M. AND TAWEEL, A.: reference 4 above, p. 630. 39. YAO, S. ANn LILT, L., Combustion and Flame 51, 335 (1983). 40. MURDOCH, P.L., POURKASHANIAN, M. AND WIL LIAMS, A.: Twentieth Symposium (International) on Combustion, p. 1409, T h e Combustion Institute, Pittsburgh, PA 15236, 1984. 41. MURDOCH,P., POURKASHANIAN,M. ANDWILLIAMS, A.: reference 5 above, p. 177. 42. MALONEY,D.J., MCCARTHY, L.A., LAWSON, W.F., FASCHING, G.E. AND CASLETON, K.H.: Sixth Symposium on Coal Slurry, p. 1051. 43. MATTHEWS, K.J. AND STREET, P.J.: *Sixth Symposium on Coal Slurry, p. 109. 44. DEsnou, L., MINGHU, X., XINGYU, C., BAIXUN, G., GUOQUAN, H., ZHONGYANG, L., MINGJIANG, N., FENGYING, C. AND KEVA, C.: Sixth Symposium on Coal Slurry, p. 731. 45. Ltv, G.E. AND LAW, C.K.: Fuel 65, 171 (1986). 46. Fu, W-B., WEI, J-B., HAU, H-Q. ANn ZHANG, Y-P.: Combust. Sci. and Tech. 43, 67 (1985). 47. UBHAYAKAR, S.K., STICKLER, D.B., VON ROSEN BERG, C.W. ANDGANNON, R.E. : Sixteenth Symposium (International) on Combustion, p. 427, The Combustion Institute, Pittsburgh, PA 15219, 1979. 48. BAXTER, L.L., SMITH, P.J. ANn SMOOT, L.D,: Western States Section/The Combustion Institute 1985 Spring Meeting, San Antonio, Texas, April 22-24, 1985. 49. SRINIVASACHAR, S., FARMAYAN, W.F. AND BEER, J.M.: reference 5 above, p. 193. 50. KOBAYASHI, H., HOWARD, J.B. AND SAROF IM, A.F.: Sixteenth Symposium (International) on Combustion, p. 411, the Combustion Institute, Pittsburgh, PA 15219, 19779 Sixth Symposium on Coal Slurry citations refer to Sixth International Symposium on Coal Slurry Combustion and Technology, US DOE, Pittsburgh, PA 15236, held in Orlando, FL, J u n e 1984.

COMMENTS D. R. Hardesty, Sandia National Laboratory, USA. (1) The lack of correlation between coal/water slurry dropsize and viscosity at high solids loading may, in part, be due to the failure to determine viscoscity at the high shear rates typical of fuel atomizers. Such dynamic shear measurements are now being made for coal liquifaction products and may be of use for coal/water mixtures. (2) Your review did not mention the extensive

results of Holve, Dunn-Rankin et al at Sandia Livermore on the evolution of the particle size-number distribution (0.3~m < d < 100~m)during fundamental bench scale studies of coal-water slurry combustion. These results are among the very few quantitative ones which clearly show the effects of agglomeration, swelling and burnout of slurry mixtures relative to the initial particle size distribution.