Energy & Fuels 1993,7, 520-531
520
Effect of Fuel Particle and Droplet Size Distribution on Particle Size Distribution of Char and Ash during Pilot-Scale Combustion of Pulverized Coal and Coal-Water Slurry Fuels Sharon Falcone Miller and Harold H. Schobert' Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received August 10, 1992. Revised Manuscript Received April 22, 1993
The objective of this study was to investigate the effect of fuel form-pulverized coal and coalwater slurry fuel-on the particle size distribution (PSD) of char and ash formed during combustion. Two coals, Elk Creek (West Virginia) high volatile A bituminous coal and Beulah (North Dakota) lignite, were fired as a standard utility grind pulverized fuel and a coal-water slurry (CWSF) in a down-fired combustor at 316 MJ/h in 20% excess oxygen. The relationships of the PSDs of particles in the pulverized coal and CWSF and of the size distribution of the CWSF droplets to the char and ash samples vary between the Beulah and Elk Creek fuels. For Beulah pulverized coal and CWSF, fuel particle and droplet size alone does not determine the PSDs of the ashes. The CWSF ash PSD is coarser than the pulverized coal ash PSD even though both fuels have similar PSDs. The CWSF ash shows extensive agglomeration and coalescence while the pulverized coal ash exhibited fragmentation. The dm value of the inorganic phases in the Beulah pulverized coal is approximately 4 times larger than the d60 identified in the CWSF. However, the coarser PSD of the inorganics in the pulverized coal did not result in a coarser ash PSD, while the finer PSD of the CWSF inorganic phases resulted in a coarser ash PSD. The difference in ash PSDs between the two fuels is attributed to differences in the PSDs and behavior of the mineral matter in the fuels during combustion. For Elk Creek pulverized coal and CWSF, the ash PSDs are related to the original pulverized coal and CWSF droplet size distributions. The Elk Creek CWSF droplet size distribution is coarser than that of the pulverized coal. In turn, the Elk Creek CWSF ash is coarser than the pulverized coal ash. For the Elk Creek CWSF, the ash PSD is related to the coarser droplet size distribution rather than to the original CWSF particle size. The coarser PSD of the CWSF ash is due to the increased number of mineral matter particles, to other mineral particles, and to the carbonaceous portion of the fuel. This would increase the extent to which the mineral matter interactsand increasesthe local temperature experienced by the inherent mineral matter particles.
Introduction Coal-water mixtures were introduced in the 1950s as a potential way of supplementing fuel oil with coal as a liquid fuel.' The short supply and higher price of fuel oil in the 1970s sparked interest in the development of coal-water slurry fuels (CWSFs). The envisioned applications of CWSFs are the same as those of fuel oil: industrial and utility steam boilers, process heat, and heat engines.2 In these cases the CWSF would be substituted for oil with minimal retrofitting of the existing oil-firedsystem. CWSF technology may also be a way to utilize a fine wet coal stream produced in a coal preparation plant.3 The combustion of coal in any form inevitably leads to the production of ash. The ash particle size distribution (PSD) and ash composition are determined by the association of the inorganic components of coal with one another and with the burning carbon, the thermal behavior of the inorganics, and the processes by which they transform into ash.4 In turn, the ash PSD and composition determine the fouling and slaggingbehavior. The ash PSD (1) McHale, E. T. Energy h o g . 1988,5, 15. (2) Papachrietodoulou, G.; Trass, 0.Can. J. Chem. Eng. 1987,65,177.
( 3 ) Stoessner, R. D.; Zawadzki, E. A. h o c . Znt. Conj. Coal Slurry Technol. 1991,16, 699. (4) Bryers, R. W. Symp. Slagging Fouling Steam Generators 1987,63.
and composition can be affected by changes in any of the followingcharacteristics: the mineral matter composition, size, and occurrence in the coal; the coal particle composition and size; the morphology of the resulting char; the local atmosphere around the mineral particles; and the phase transformations of the mineral matter during combustion. Inorganic species are incorporated in coals in several ways: as ion-exchangeable cations, as coordination complexes, and as discrete minerals. The mineral matter can be associated within the coal particle, i.e., inherent, and as free particles not associated with the carbonaceous portion of the fuel, Le., extraneous. In the case of CWSFs, an agglomerate is formed during atomization and consists of coal particles containing inherent mineral matter and extraneous mineral matter. The process of preparing and burning a CWSF may result in changes in the inorganic composition, size, and occurrence in the coal; the coal particle and char size; and the phase transformations of the inorganics during combustion as compared to combustion of a pulverized coal. The discussion in this paper will focus on the effect of coal particle size on ash particle size. A companion paper will focus on the effect of the inorganic composition and PSD on ash composition and PSD.5 0 1993 American Chemical Society
Effect of Fuel Particle and Droplet Size
The formation of an agglomerate during atomization of a CWSF is of a particular interest in that the CWSF char may be larger than char produced from the same coal fired as a pulverized coal fuel, depending on the efficiency of atomization of the CWSF. Withless efficient atomization, the increased size of the CWSF char increases the time required for char burnout, and the presence of additional water associated with the droplet results in a delay of ignition, due to the time required for evaporation of the water. The increased burnout time for a CWSF char changes the temperature history of the minerals and their exposure to a localized gas atmosphere, relative to a pulverized coal char particle, thereby altering the characteristics of the resulting ash. During combustion, the inorganic portion of coal can undergo coalescence, agglomeration, shedding, fragmentation, vaporization, and condensati~n.~?~ These processes are not necessarily sequential but may be occurring simultaneously. The processes that occur are determined by the spatial relationship of the mineral matter to the coal, the initial composition of the mineral matter, the temperature history of the coal and ash particle, and the combustion conditions. Coalescence begins with the formation of molten inorganics on the exterior and interior of char particles. During combustion, the char surface recedes and the molten material coalesces. Partially molten mineral matter may agglomerate as mineral particles come into contact with one another. The majority of coalescence and agglomeration occurs during the late stages of combustion, with mineral inclusions retaining much of their chemical character up to that point.7 The net result of coalescence and agglomeration is an increase in the final ash PSD. Coalescence and agglomeration generally result in the formation of ash in the 1-100-pm size range.* As char burnout proceeds, some mineral matter particles on the char surface are liberated or shed into the gas stream as ash particles. Once free in the gas stream, they rarely interact with other ash particles. Mineral matter particles may also be separated from the char particle as the char fragments during burnout. Fragmentation of the char may form smaller char particles containing mineral inclusions. The number of mineral inclusions proximate to one another within the new, smaller char fragment is reduced relative to the original char particle, resulting in limited coalescenceand agglomeration of mineral matter. Also, the burnout time of the smaller char particles is less, resulting in less time for mineral inclusions to coalesce. The net result of shedding and fragmentation is a decrease in the overall ash PSD.g Vaporization and condensation of various inorganic components contribute to the formation of the submicron portion of the ash or form coatings on some of the larger particles. Vaporization is greatly influenced by the oxygen concentration in the immediate environment.1° Only about 1%of the mineral matter volatizes.ll Sarofim and co-workersconcluded that, for each lignitic coal particle burned, three ash particles are formed, while (5) Miller, S. F.; Schobert, H. H. Energy Fuels, companion paper in this issue. (6) Sarofim, A. F.; Howard, J. B.; Padia, A. S. Combust. Sci. Technol. 1977, 16, 187. ( 7 ) Raask, E. Mineral Impurities in Coal Combustion; Hemisphere Publishing Corp.: Washington, DC, 1985. (8)b a s k , E.; Williams, D. M. J. Znst. Fuel 1965, 38, 255. (9) Helble, J. J.; Srinivasachar, S.; Katz, C. B.; Boni, A. A. R e p . Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991,34,383. (10) Flagan, R. C. Roc. Symp. (Znt.) Combust. 1979,17,97. (11) Helble,J. J.;Neville,M.;Sarofim,A.F.Proc.Symp. (Znt.)Combust. 1986, 21, 411.
Energy & Fuels, Vol. 7, No. 4, 1993 521
five ash particles are formed per combusted particle of bituminous coal? These three to five ash particles are residual ash particles in the 10-100-pm size range subjected to inertial impaction.6 In this model, coal particle size had almost no impact on the number of ash particles produced per coal particle, but the PSD of the ash was proportional to the PSD of the parent coal. Subsequent work confirmed the number of 10-100-pm particles formed per coal particle and reported that 200-500 ash particles in the 1-4-pm size range are formed due to shedding and fragmentation.'l There is still debate as to the actual numbers of ash particles formed per coal particle. In situations in which both the coal and mineral matter PSDs are polydisperse and there is some variation of mineral matter among coal particles, as is the case for virtually all coal combustion situations, both coal and mineral PSDs are important in determining the ash PSD.12 The parameters determining the ash PSD are the char fragmentation, the mineral matter PSD, the coal PSD, and the variation in composition and weight percent of mineral matter among coal particles.12 Formation of one ash particle per mineral matter particle is favored by extensive char fragmentation, high-melting-point mineral matter, or both.13 Char fragmentation is favored by high char porosity, low mineral matter content, and large particle size. The formation of one ash particle per coal particle is favored by lack of char fragmentation in the early stage of burnout, low-melting-point mineral matter, or both.l3 Ranges in the composition and variations in the combustion kinetics based on coal rank, coal particle size, and char morphology all affect the fragmentation process. In addition, the PSD of the mineral matter is extremely important and varies with each coal. In general, these concepts and models can be applied to ash formation in CWSFs as well as pulverized coals. However, CWSF combustion is somewhat more involved, due to the nature of the fuel.14 The sequence of events can be classified into five major phases: atomization, evaporation, particle heat-up and devolatilization, char burnout, and fragmentati~n.'~J~ Atomization is controlled primarily by physical properties of the fuel, characteristics of the combustion facility, atomizer design, and operating conditions. Atomization quality, in addition to the coal PSD, determines the size of the coal-water slurry droplet. Droplet size determines the size of coal and char agglomerate, its morphology, and the time required for char burnout. The size of the char and its combustion history affect ash size and, possibly, ash composition. Droplet size also increases the number of the mineral particles in proximity to one another within the char agglomerate. Extraneous mineral matter may also be incorporated into the droplet and retained during char burnout, changing the temperature history of the mineral particle and its proximity to other mineral particles. The process of combustion is an exothermic reaction and raises the char particle temperature above the gas temperature. The (12) Kang,S. G.; Charon, 0.;Sarofim, A. F.; Beer, J. h o c . Sixth Znt. Pittsburgh Coal Conf. 1989, 1, 74. (13) Wilemski, G.; Srinivaaachar,S.; Sarofim, A. F. In Znorg. Transformations and Ash Deposition During Combustion; Benson, S. A., Ed.; Engineering Foundation: New York, 1991. (14) Monroe, L. S.; Farmayan, W. F.; Srinivasachar, S.; Beer, J. M. In Slugging and Fouling Due to Impurities in Combustion Gases; Barrett, R. E., Ed.;United Engineering Trustees: New York, 1989, pp 683-712. (15) Kang, S. W.; Sarofim, A. F.; Beer, J. M. R o c . Eur. Conf. CoalLiquid Mixtures 1987,3,179. (16) Kang, S. W.; Sarofim, A. F.; Teare, J.D.; Beer, J. M. Fundamental Aspects of Coal-Water Fuel Droplet Combustion and Secondary Atomization of Coal-WaterMixtures. Massachusetts Institute of Technology Rep. MIT-EL-87-002; MIT Press: Cambridge, MA, 1987; Vol. 1.
522 Energy & Fuels, Vol. 7, No. 4, 1993
difference between gas temperature and particle temperature may be several hundred degrees, depending upon the rate of heat generation and the rate at which the particle dissipates the heat.” Therefore, mineral particles associated with carbon Ksee”higher temperatures during char combustionthan extraneous mineral particles. Extraneous mineral particles temperatures approximately equal to that of the gas stream. In the case of a mineral particle which undergoes oxidation in the gas stream, the particle temperature may be different than the gas temperature. In the case of pyrite, the particle temperature is actually higher than the surrounding gas temperature since the oxidation of pyrite is exothermic. During evaporation, a dry agglomerate of coal particles is formed having a larger diameter than that of the original coal particle. The dry agglomerate size distribution-not the original coal PSD-determines the pyrolysis and combustion character of the fuel.18 The processes of particle heat-up and devolatilization of a pulverized coal particle and a CWSF agglomerate are very similar. The obvious difference is the presence of the water by which the CWSF is introduced into the combustor. Atomization, formation of coal-water droplets, and evaporation have a profound effect on the ignition and char size and morphology. The ignition time of a CWSF agglomerate is twice as long as that of a pulverized coal particle of identical size, due to the delay by evaporation of water.14 Atomization and droplet formation change the size of the char to be considered when determining the time required for burnout. The peak temperature that the agglomerate will experience is inversely proportional to droplet and agglomerate size once combustion occurs under partial or full diffusion c o n t r ~ l . ~Once ~ - ~the ~ char is formed and char burnout is initiated, the ash formation processes that follow are similar for pulverized coal and CWSF combustion. Caking coals, fired as pulverized coal or CWSF, tend to swell, forming carbonaceous cenospheres.21 Noncaking coals do not form carbonaceous cenospheres during heating. The chars produced are weakly bonded, highly porous, and subject to fragmentation. During devolatilization, evolved volatiles impart a rotational motion to the char. In the case of a CWSF agglomerate,the rotational motion promotes separation of coal, ash, and char particles from the agglomerate.lsJ6 The separation of ash and char particles contributes to a finer ash PSD. Char and ash fragmentation are very important in determining the ash particle size and char burnout time. Char fragmentation that occurs before the inorganic portion of the coal begins to soften decreases the ash PSD by limiting the extent of mineral particle coalescenceand agglomerationduring char burnout. The size of the CWSF char may also set an upper limit of ash size, so that increased char size leads to increased ash size.14122 Although the ash formation processes are the same, the (17)Field, M. A.; Gill,D. W.; Morgan, B. B.; Hawksley, P. G. W. Combustion of Pulueriued Coal; The British Coal Utilisation Research Association: Leatherhead, England, 1967. (18) Zhou, Z. Q.;Hamdullahpur, F. R o c . Eur. Conf. Coal-Liquid
Mixtures 1987,3, 151.
(19) Boni, A. A.; Garman, A. R.; Johnson, S. A.; Thames, J. M. R o c . Int. Symp. Coal Water Slurry Combust. Technol. 1984,6, 667. (20) Holve, D. J.; Gomi, K.; Fletcher, T. H. Comparative Combustion Studies of Ultrafiie Coal-Water Slurries and Pulverized Coal. Sandia National Laboratory Rep. SAND 85-8706; Sandia National Laboratories: Albuquerque, NM, 1985. (21) Scaroni,A. W.; Khan, M. R.; Eser, R.; Radovic,L. R. In Ullmann’s Encyclopedia of Industrial Chemistry; Wolfgang Gerhartz, Ed.; VCH Verlag: Weinheim, Germany, 1986; pp 245-280. (22) Beer, J. M. Roc. Eur. Conf. Coal-Liquid Mixtures 1986,2,377.
Miller and Schobert
resulting products may differ on the basis of char morphology, size, and strength. The spatial relationships of the mineral matter in pulverized coal particles may be different from those in a CWSF agglomerate, possibly affecting the way minerals interact to form new species. Formation of an agglomerate increases the number of mineral grains and the amount of inorganics in proximity to one another within the CWSF agglomerate. In addition, extraneous mineral particles may be incorporated within the agglomerate. The differences in the resulting inorganic transformations and ash sizes may or may not be significant. In addition, the preparation procedures can greatly affect the physical and chemical makeup of a CWSF, altering its atomization characteristics and the resulting ash composition. The present paper reports results of a study to understand the effect that the form in which a coal is fired-pulverized or CWSF-and the particle or droplet size distribution has on the mechanisms responsible for the size of the char and ash produced during combustion and the final ash PSD. In a companion paper, we will report on the effect of mineral matter particle size on ash PSD.S In a series of subsequent papers we will discuss the effect of the composition and the occurrence of inorganics in the fuel on ash composition and size.
Experimental Section Fuel Preparation and Characteristics. Beulah lignite was obtained from the Beulah-Zap seam, Mercer County, North Dakota. The lignite was pulverized to 80% 1200 mesh (74Mm) at the University of North Dakota Energy and Environmental Research Center (UNDEERC) using a hammer mill. A portion of this material was then used at UNDEERC to prepare the CWSF by hydrothermal treatment of a 1:l mixture of deionized water and pulverized lignite for 5 min at 603 K and 15.2 MPa steam pressure.2g Ammonium lignosulfate, -3% by weight, was added as a dispersant, xanthan gum, -1% by weight, was added as a stabilizer, and formaldehyde =2 % ,by weight, was added as a biocide. Deionizedwater was used to dilute the CWSF to reduce the viscositysufficientlyto allow proper feeding. Elk Creek hvAb coal was obtained from the Island Creek Coal Company mine number 110 in Logan County, West Virginia. The coal was pulverized to 88% 1200 mesh by the OXCE Fuel Company in a C-E Raymond bowl mill. The Elk Creek CWSF was produced by OXCE using their proprietary process and additive package. The characteristics of the four fuels are summarized in Table I. Atomization Test Facility. This facility is used to determine the size distribution of CWSF droplets during atomization. A detailed description has been published elsewhere.u The same nozzle and pump used in the actual combustion tests are used in the atomization test facility. A Malvern model 2600C laser diffraction particle size analyzer is used to determine droplet size distribution. Down-FiredCombustor Facility. The configuration of the down-fired combustor used for firing pulverized coal and CWSF is shown in Figure 1. An extensive description of the unit and its operating procedures has been published elsewhere.”% Pulverized coals and CWSFs were fired at 316 MJ/h. All fuels were fired at 20% excess air. Pulverized coal is fed using an Accurate Model 302 dry material feeder adjusted to feed 19.1 kg/h of Beulah lignite and 10.0 kg/h Elk Creek bituminous coal. CWSF is fed using an atomizing gun assembly using a modified Delavan Model 33769-99 Variflo nozzle. (The nozzle was (23) Potas, T. A.; Baker, G.G.; Maas, D. J. J. Coal Qual. 1987,6,53. (24) Ramachaadran, P. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, 1990. (25) Miller, Sharon Falcone. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, 1992.
(26) Hurley, J. P. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, 1990.
Effect of Fuel Particle and Droplet Size
Energy & Fuels, Vol. 7, No. 4, 1993 523
COAL HOPPER
WATER COOLED
SCREW FEEDER
1 0 0 . 1 RPM ~ VARIABLE SPEED
150 ATOMIZING AIR
STRAINERS DU
SECONDARY AIR 4
AlNER
I
1 - 2 -
VIEWING PORTS
1'
PROGRESSIVE CAVITY PUMP
WATER FLUSH
Figure 1. Down-fired combustor configuration for firing pulverized coal and coal-water slurry fuel. Table I. Selected ProDerties of Test Fuels Beulah PCa CWSEb proximate analysis, wt 7% (dry basis) moisture (30.0) (50.0) volatile matter 44.3 42.2 fixed carbon 47.1 49.5 ash 8.6 8.3 ultimate analysis, wt % ! (dry basis) carbon 65.3 66.6 hydrogen 4.3 4.2 nitrogen 1.0 1.1 sulfur 0.9 1.0 oxygen 19.9 18.8 ash 8.6 8.3 higher heating value, 25.2 25.0 MJ/kg (dry basis) free swelling index 0 a Pulverized coal. b Coal-water slurry fuel.
Elk Creek PC CWSF
(0.9) 30.5 63.4 6.1
(32.4) 32.2 61.8 6.0
81.0 4.9 1.6 0.7 5.8
6.0
80.4 5.0 1.5 0.7 6.4 6.0
33.6
33.5
6
originally designed for oil atomization, but was modified to accommodate the larger particle size, increased viscosity, and erosive nature of the CWSF.%) The overall length of the combustor is 3.1 m. A series of sampling ports extends the length of the combustor. Sample ports are numbered 1 through 10 starting a t the top of the combustor. During pulverized coal combustion tests,particulate and gas sampling were conducted in ports 1,2, and 10. Particulate and gas sampling during the CWSF tests were conducted in ports 3 , 4 , 8 , and 10. Samples were taken a t different locations for the CWSF than for the pulverized coal for two reasons: location of the atomization gun a t port 1 prevented sampling a t that port, and samples collected a t port 2 contained significant amounts of water which interfered with proper operation of the particle
sampling probe. Wall temperatures are monitored using type S thermocouples a t eight locations along the length of the combustor. During combustion, particles were sampled isokineticallyand classified using a water-cooled sampling probe and cyclones. A three-stage Anderson multicyclone and filter assembly was used to classify the particles collected with the probes. The cyclone aerodynamic diameter 50% ! cutpoints (Le., 50 !% of the particles are less than the indicated size) were 15 pm (cyclone l),2.54 pm (cyclone 2), and 0.42 pm (cyclone 3) for the pulverized coal tests and 15 pm (cyclone l ) , 3.22 pm (cyclone 2), and 0.57 pm (cyclone 3) for the coal-water fuel tests. Analytical Techniques. Ultimate analyses of fuels were conducted using a Leco CHN-600 elemental and analyzer and a Leco SC-132 Sulfur Determinator. Proximate analysis of coals and CWSF additives was conducted using a Leco MAC-400 proximate analyzer. A modified proximate analysis of cyclone samples was conducted thermogravimetrically using a Perkin Elmer Series 10 thermal analysis system. The use of thermogravimetry for proximate analysis has been discussed by other investigatomnVB Calorific values were measured using a Parr adiabatic calorimeter. The free swelling index was determined by ASTM Technique D720-67. Bulk inorganic compositional analysis for laboratory-prepared ash of each fuel was determined using a Spectrometrics Spectrospan 3 direct plasma spectrometer (DCP). Oxide composition of each ash is given in Table 11. Particle size analysis of the coals and particles collected a t each port was conducted with a Malvern 2600 particle size analyzer. Customarily, deionized and distilled water is used as a transporting medium, along with an aqueous solution of a wetting agent. We, however, used ethanol as the transporting and dispersing medium, because coal particles tended to agglomerate in the water and wetting agent, whereas ethanol easily wets the coal, char, and ash particles, inhibiting agglomeration (27)Ottaway, M.Fuel 1982,61,713. (28) Elder, J. P. Fuel 1983,62,580.
Miller and Schobert
624 Energy &Fuels, Vol. 7, No. 4, 1993
Table 11. Laboratory Prepared Ash Composition and Particle Size of Inorganics Identified by CCSEM in Each Fuel Beulah Elk Creek PC CWSF PC CWSF 23.87 13.21 0.64 13.21 8.02 27.71 0.09 12.67 0.57 21.45
26.73 12.51 0.63 15.26 8.03 27.61 8.52 0.63 17.57
55.35 30.92 1.68 7.16 0.93 1.27 0.02 0.69 1.99 0.93
55.40 30.56 1.66 7.46 0.88 1.21 0.02 0.74 2.07 0.51
K2.2 17.0 80.0
K2.2 4.0 20.0
K2.2 4.6 20.0
K2.2 3.0 17.0
0.10
inorganic phases PSDb(wm) dio dw dso
SO3is reported as normalized to 100%. All remainingoxides are reported on a SO3 free basis. b The dlo, dw, and d~ values are the particle sizes where,respectively,lo%,50%,and 90% of the particles, by volume, are less than the indicated particle size. Values obtained from culumative frequency curves. and ensuring dispersion. Particle size distribution of the inorganics in the fuels was determined by computer controlled scanningelectron microscopy (CCSEM). CCSEM allows for the in situ identification and sizing of the inorganic phases in coal matter. Selected size parameters for the inorganicsidentified in each fuel are given in Table 11. The number of particlesclassified by CCSEM in the fuel samples ranged from 1054 to 2278, with an average of 1675particles. A more detailed description of the CCSEM sample preparation and procedure has been published by Zygarlicke and Steadman.B Photomicrographs were taken with an IS1 model ABT SX40A scanningelectronmicroscopeto study morphology of various char and ash samples. Samples were mounted on double-sided tape and gold coated. A classificationscheme to categorize char morphology was adapted from Kleesatte1.m The ash particles were classifiedby shape and associationwith other ash particles as sphericaltype A, completely sphericalwith no surfacefeatures; spherical type B, similar to type A but covered with condensed alkaline sulfates giving the particle surface a rough appearance; and sintered,which have gone through a partial melt phase and are attachedto other particlesvia a neck of once-moltenmaterial. Type A particlesmay occur as agglomeratedmasses or as isolated particles. Type B particles generallyoccur as isolated particles.
Results and Discussion Combustion Characteristics of Fuels. Burnout or carbon conversion and residence time for the pulverized coal char ash samples collected at porta 1,2, and 10 and for the CWSF char ash collected at porta 3,4,8, and 10 are listed in Table 111. The carbon burnout is calculated using the ash tracer technique explained in detail by Scaroni31 and Rama~handran.~3 The ash percent values used to determine the percent burnout, or carbon conversion, were those based on complete oxidation of the inorganics originally present in the char. Fuel burnout was calculated using ash as a tracer in composite cyclone samples. The percent carbon conversion or burnout was calculated using the following formula: '
(29)Zygarlicke, C.J.; Steadman, E. N. Scanning Microsc. Int. 1990, 4, 579.
(30)Kleeeattel,D.R. TerminologyUsed in UNDERC Char Morphology Analyses. University of North Dakota Energy Research Center Rep., 1986. (31)Scaroni, A. W. M.S. Dissertation, The Pennsylvania State University, University Park, PA, 1979.
Table 111. Residence Time, Burnout, and Particle Size of Test Fuels and Char Ash at Different Locations within the Furance
volume
pulverized coal port 1 port 2 port 10
coal-water slurry fuel droplet port 3 port 4 port 8 port 10
pulverized coal port 1 port 2 port 10
coal-water slurry fuel droplet port 3 port 4
port 8 port 10
Beulah Fuels 0.16 0.41 2.09
10.8 83.1 99.6
0.25 70.9 0.41 76.6 1.03 97.6 1.68 97.8 Elk Creek Fuels 0.18 0.46 2.37
63.7 90.2 99.4
0.25 0.38 0.89 1.39
71.8 75.9 98.7 99.2
8.7 16.5 5.9 2.4 5.0 9.3 7.2 10.6 3.1 2.6 5.3 5.9 5.7 2.3 5.2 9.1 7.4 9.1 4.4 4.1
46.4 129.4 64.4 125.1 45.6 133.2 8.2 22.5 41.7 111.6 42.7 114.2 48.8 92.8 67.4 123.0 12.0 128.9 13.1 132.2 19.4 28.1 21.1
8.6 19.3 31.3 32.6 38.2 13.6 12.2
58.9 99.3 56.8 27.8 55.5 80.3 76.8 72.4 48.1 31.2
Burnout calculationsbasedon ash tracer technique. Ash percents on an oxidized basis obtained from the thermogravimetric analyzer.
where: Ac = percent ash yield of the coal (dry basis) Ar = percent ash yield of the residue obtained by thermogravimetric analysis (dry basis) Carbon burnout levels of 99.4% and 99.2 % are observed at port 10 for the Elk Creek pulverized coal and CWSF, respectively. Comparable values for the Beulah fuels are 99.6% burnout for pulverized lignite and 96.8% for the CWSF. Port 10 samples are taken to represent the final burnout level of each fuel, since port 10 is the last access port into the furnace; some continued burnout may occur beyond port 10. Residence times for the pulverized coal were based on calculated combustion gas and terminal particle velocities. For CWSFs, initial droplet velocities were estimated following the method of Walsh et Droplet velocities of 180 m/s and 260 m/s were used for the Beulah and Elk Creek CWSF, respectively. The time necessary to travel to each port was calculated based on the initial droplet velocities for the penetration distance (0.38 m for Beulah and 0.31 m for Elk Creek CWSFs) and the combustion gas velocities for the remaining distance to each port. Equilibrium wall and gas temperature profiles observed during combustion of the four fuels are shown in Figures 2 and 3. The maximum gas temperature for the Beulah pulverized coal and CWSF is 1800and 1720K, respectively. The maximum gas temperature for the Elk Creek pulverized coal and CWSF is 1872 and 1784 K, respectively. The maximum temperature observed during combustion (32)Waleh, P. M.; Zhang, M.; Farmayan, W. F.; Beer, J. M. h o c . Symp. (Znt.)Combustion 1984, 1401. (33)Essenhigh, R. H.;Yorke, G. C. Fuel 1965,44, 177. (34)Littlejohn, R.F. J.Znst. Fuel 1966, 40, 128.
Energy & Fuels, Vol. 7, No. 4, 1993 525
Effect of Fuel Particle and Droplet Size Port
"
1
2
-
g
0.23
0.23
0.60
0.60
0.98
0.98
1.21
1.21
1.66
1.66
2.12
2.12
2.60
2.60
b,
E
3.07
11
D
3.07
1zM)
1300
Temperature (K) Beulah Pulverized Cod
1400
IS00
IMX)
1700
1800
Temperature (K) Beulah CWSF
Figure 2. Stable temperature profile when firing the Beulah pulverized coal and coal-water slurry fuel. Port
0
0
0.23
0.23
0.60
0.60
3
0.98
0.98
4
1.21
1.21
1.66
1.66
1 2
P 4 E
Y
6
P
f 8
2.12
10
2.60
2.60
11
3.07
3.07 1300
2.12
1400
1Mo
1600
1700
1800
Temperature (K) Temperature (K) Elk Creek Pulverized Coal Elk Creek CWSF Figure 3. Stable temperature profile when firing the Elk Creek pulverized coal and coal-water slurry fuel.
of the two pulverized coals was measured at port 2 versus port 4 for the CWSFs. The shift of the maximum temperature down the combustor during CWSF combustion is due to the shift in the location of the flame reflecting the delay in ignition caused by the evaporation of water from the CWSF. Beulah Pulverized Coal and Coal-Water S l u r r y Fuel. A summary of dlo, dm, and dgg volumetric diameters for the particle sizes of the four fuels, CWSF droplet sizes, and chars are given in Table 111. (The dlo, d ~and , d~ volumetric diameters are the volume diameters below which l o % , 50 % ,and 90 % of the volume of the particles lie.) The size distribution of the pulverized coal and CWSF particles and the CWSF droplets is shown in Figure 4. The PSDs of the coal particles in the CWSF and the pulverized coal are very similar for particles 120 pm. However, agreater percentage of the total CWSF particle volume comprises fine particles. The finer PSD of the CWSF particles may be due to the slurry preparation process. The reduction in the volume percent of the
smaller size fraction of the CWSF particles, relative to the CWSF droplets, is due to the incorporation of smaller particles within the droplet during atomization, which results in a larger droplet size. In contrast to the model of Monroe et al.14 that indicated droplets consisting of numerous coal particles of similar size, our results suggest that the CWSF droplet consists of a larger coal or mineral particle surrounded by significantlysmaller coal or mineral particles and a thin film of water. The PSDs of the chars and ash collected at the various ports during combustion, and the original coal PSD are shown in Figure 5. The shift to a finer PSD from port 1 to port 10 is consistent with increasing burnout. The overall PSD of the char collected at port 1is coarser than the original coal PSD, due to the removal of the smaller particles by combustion. The first particles to undergo complete carbon burnout are those making up the smaller size fraction of the total particle size population. As the burnout increases from 10.8% to 83.1% from port 1 to port 2 the char surface recedes, taking on a jagged
Miller and Schobert
526 Energy & Fuels, Vol. 7, No. 4, 1993
10
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Figure 4. Size distribution of the Beulah pulverized coal and coal-water slurry fuel particles and coal-water slurry fuel droplets. 100
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Figure 5. Size distribution of the Beulah pulverized coal particles and char/ash particles collected at various locations within the
combustor. appearance (Figure 6A). The char is claasifiedasunfused.m The visible ash spheres, classified as spherical type A, are =5 pm diameter and may be shed into the gas stream or coalesce with one another as the char surface recedes. At port 10, burnout is 99.6%. The majority of the material collected is ash, showing numerous spherical type B and sintered particles (Figure 6B). The PSDs of the CWSF particles, char, and ash, and the droplet size distribution are shown in Figure 7. The shift to a finer size distribution of chars and ash collected from porta 3 to 10 is consistent with increasing burnout. There is little difference between the CWSF droplet size distribution and the PSD of the CWSF char collected at port 3 (Table 111). This suggests that the droplet size closely approximates the agglomerate size after evaporation and that no significant breakup of the agglomerate occurs during evaporation. The appearance of the CWSF char collected at port 3 is similar to that of the pulverized coal char collected at port 2 (Figure 6A). The PSD of the char collected at port 4 is somewhat coarser than that of
the port 3 char, possibly as a result of removal of fines with increased burnout. The PSD of the char ash collected at port 8 and port 10 is similar, because of the similar extent of carbon burnout a t these porta. Many of the ash particles collected at port 8 exhibit bridging with other ash particles. At port 10, the ash particles are highly agglomerated, showing extensive bridging (Figure 8). Although the size distributions of the pulverized coal, CWSF particles, and CWSF droplets are similar (Figure 41, the final PSDs of the port 10 char/ash samples are quite different than might have been anticipated from the similarity of the fuels. The CWSF ash is much coarser than the pulverized coal ash (Table 111). There is little difference between the PSDs of the pulverized coal and CWSF port 10 ashes in the size fractions 1 3 pm. There is a significantly greater volume percent of coarser particles in the CWSF ash ( d =~132.2 pm) than in the pulverized coal ash (& = 22.5 pm). If the coal particle size alone determines the final ash PSD, then the PSDs of the port 10 samples for the two fuels should be nearly identical.
Effect of Fuel Particle and Droplet Size
Energy & Fuels, Vol. 7,No. 4,1993 527
The data suggest that different mechanisms are responsible for ash formation in the two fuels and that fuel PSD is not solely responsible for determining the final ash PSD. The ash contents of the Beulah fuels are very similar (Table I). Detailed mineralogical and chemical analysis verifies that the size and occurrence of inorganics in the Beulah fuels are not identical (Table 11). The dm and dw values of the inorganic phases in the Beulah pulverized coal are approximately4times larger than those identified in the CWSF. However, the coarser PSD of the inorganics in the pulverized coal did not result in a coarser ash PSD, and the finer PSD of the CWSF inorganic phases did not result in a finer ash PSD. The pulverized coal ash collected at port 10 is significantly finer than the original inorganics in the coal (Tables I1 and 111). The dm of the inorganics as measured by CCSEM in the coal is 17 pm versus 6.5 pm in the ash. This suggests that one mineral particle results in the formation of several ash particles, Le., fragmentation. The CWSF ash collected at port 10 is coarser than the inorganics in the fuel (Tables I1 and 111). The d~ of the inorganics in the coal is 4.0 pm, versus 13.1 pm in the ash. One other compositional difference between the two fuels is the reduction in the amount of sodium present in the CWSF as compared with that in the pulverized coal. Sodium plays an important role in effecting the sintering behavior of inorganics in low-rank coals. The differences in the inorganic size and compositional character between the two fuels are attributed to the CWSF preparation process. The data suggest that the size distribution and occurrence of the inorganics in the Beulah fuel are more important than the coal particle size in determining the mechanism for ash formation and final ash PSD. Detailed discussion of the effect of mineral matter size and inorganic composition of these fuels on ash PSD will be presented in a companion paper.5 Elk Creek Pulverized Coal and Coal-Water Slurry Fuel. The PSDs of the pulverized coal and the CWSF, and the CWSFdroplet size distribution are shown in Figure 9. The PSDs of the pulverized coal and coal particles which make up the CWSF are very similar (Table 111). The size distribution of the CWSF droplets formed is coarser than that of the individual coal particles comprising the CWSF. The dlo, d ~and , dw values of the CWSF
Figure 6. (A, top) Scanning electron micrograph of the Beulah pulverized coal char collected at port 2. Numerous ash spheres are exposed as the jagged char surface recedes during char burnout. (B,bottom) Scanningelectron micrograph of the Beulah pulverized coal ash collected at port 10. Ash particles are highly sintered.
SEM micrographs show extensive agglomeration and coalescence of ash particles in the CWSF ash but single ash particles in the pulverized coal ash (Figures 6 and 8). 100
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Size (Microns) Figure 7. Size distribution of the Beulah coal-water slurry fuel particles, droplets, and char/ash collected a t various locations within the combustor.
528 Energy & Fuels, Vol. 7, No. 4, 1993 P
Figure 8. Scanning electron micrograph of the highly agglomerated Beulah coal-water slurry fuel ash collected at port 10.
droplets are d.5-1.8 times greater than the corresponding volume diameters of the CWSF particles (Table 111).This suggests that the increase in particle size due to atomization occurs throughout the entire size range. The PSDs of chars collected during combustion of the pulverized coal are shown in Figure 10. The char particles collected at port 1 are slightly coarser than the original coal particles. The greatest change is in the volume fraction of the larger size fractions. For this coal, which has an FSI of 6, swelling of the larger char particles offsets the reduction in char mass due to combustion. The importance of swelling depends on particle size.lg Swelling is due, in part, to pressure generated by volatiles production. Increased volatiles evolution will result in a greater pressure differential between the interior and exterior of the particle.35 Since the small and larger particles have the same percentage of volatile matter, the absolute amount of volatiles evolved from a larger particle is greater than that from a smaller particle. The pressure differential between the particle interior and exterior is greater in larger particles than in smaller particles, resulting in greater swelling of the larger particles. Also, gas evolved in a larger particle must travel a greater distance to reach the surface. Since the distance acts as a resistance to diffusion of gas through the pores, the greater distance results in the gas being contained in the particle f m a greater period of time before it escapes, thereby contributing to swelling. Swelling also depends on heating rate.36 Smaller particles have higher heating rates relative to larger ones. At higher heating rates, maximum fluidity increases. The low-viscosity plastic phase allows volatiles to escape, resulting in a solid char cenospherethat has experienced little or no swelling. The result is an increased swelling of larger particles during heating. The particles collected a t port 1are predominantly type A carbonaceous ceno~pheres,~~ as shown in Figure 11A. Increased char size due to swelling can account for a 50 % increase in the diameter of single particles in air.% Swelling coals in the size range 15-40 pm can yield chars in the range 15-105 pm (ref 34). The smaller particles burn out rapidly leaving larger expanded particles. The char collected at port 2 is characterized as honeycomb or lacy. The char is much more fragile as burnout (35) Solomon, P. R.; Hamblen, D. G. In Chemistry of Coal Conuersion; Schlosberg, R. H., Ed.; Plenum Press: New York, 1985; pp 121-252. (36) van Krevelen, D.W. Coal; Elsevier: Amsterdam, 1961.
Miller and Schobert
increases; therefore, the number of char fragments also increases moving down the combustor. The ash at port 10 is spherical type A and highly agglomerated (Figure 11B). Some char fragments are still present. The difference in the surface appearance of the Elk Creek and Beulah samples may be related to the inorganic compositions of the coals and the inorganic reactions occurring during combustion. The PSDs of the Elk Creek CWSF, the CWSF droplet sizes, and chars collected during combustion of Elk Creek CWSF are shown in Figure 12. The PSD of the char collected at port 3 is very similar to the droplet size distribution, suggesting that droplet size closely approximates agglomerate size after evaporation. The port 3 chars are type B cenospheres. The PSD of the char collected at port 4 reflects a combination of increased size due to swelling and loss of smaller particles due to burnout. The coarser PSD is due to the dominant influence of swelling of larger coal particle agglomeratesforminglarger char particles relative to the swelling of individual pulverized coal particles. The char collected at port 4 is also classified as type B cenospheres, with some of the char surfaces showing exposed individual molten mineral particles. The port 8 char is honeycomb or lacy in appearance; numerous type A spherical ash particles are present with this char. The majority of the sample collected at port 10 is composed of highly agglomerated and coalesced type A spherical ash particles similar to those observed in the Elk Creek pulverized coal port 10 ash (Figure 11B). There is little difference in the inorganic composition and PSD between the two fuels. The ash percent as determined by proximate analysis is the same for both Elk Creek fuels (Table I). The inorganic composition and the PSDs of the inorganic portion of the Elk Creek pulverized coal and CWSF are very similar (Table 11). The d~ of the inorganic phases in the two fuels is approximately 4 pm (Table 11). In addition, the dm of the particles in the two fuels are almost identical (Table 111). However, the PSD of the CWSF port 10 ash is slightly coarser than the PSD of the pulverized coal port 10 ash even though the extents of carbon burnout are comparable (99.4 and 99.2 % ,respectively). The d~ of the CWSF ash is =1.4 times larger than that of the pulverized coal ash (Table 111). Since the d~ of the CWSF droplets is 1.6 times larger than the dm of the pulverized coal particles, the difference in PSDs of the ashes is likely due to the difference in the initial size of the pulverized coal particle versus the CWSF droplet and not the size of the inorganic phases in the fuel. The similarity between the inorganic composition and PSD of the two fuels highlights the importance of char size in determining ash PSD. The larger droplet size results in a larger agglomeratethat burns as an individual particle. The larger agglomerate size enhances the extent to which mineral particles and other inorganic phases are agglomerated or coalesced, due to the increased number of inorganic particles in proximity to one another within the CWSF char particle. In the case of the Elk Creek CWSF, the port 10 ash PSD is affected by the coarser droplet size distribution. As noted, the d~ of the droplets is 1.6 times larger than that of the individual CWSF and pulverized coal particles. For the Elk Creek CWSF atomization is not efficient, in respect to the droplet size approximating the individual coal particle size. The droplet size is approximately the actual
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Figure 10. Particle size distributionof the Elk Creek pulverized coal and char/ash collected at various locations within the combustor. size of the agglomerate after evaporation has been completed since no significant breakup of the agglomerate occurs prior to or during e~aporati0n.l~ The droplet size is important in determining the final char size, since formation of an agglomerate upon atomization increases the number of mineral matter particles to other mineral and coal particles.
Conclusions The fuel particle size distribution and the droplet size distribution of the pulverized coal and CWSF are important in determining the PSD of the respective chars and ash formed during combustion when the PSD of the mineral matter and the composition and occurrence of inorganics in the two fuels are similar, as in the Elk Creek fuel. However, when the PSD of the mineral matter and the occurrence of inorganics differ between the two fuel forms, fuel PSD and droplet size may not reflect the final ash PSD, as in the case of the Beulah fuels. Fuel particle size and droplet size affect two variables: the number and proximity of mineral particles to one
another and the time required for particle burnout. In most cases, the PSD of mineral matter and the inorganic composition determines the mechanisms responsible for ash formation. The relationship of the PSDs of particles in the pulverized coal and CWSF and of the size distribution of the CWSF droplets to the char and ash samples varies between the Beulah and Elk Creek fuels. For the Beulah pulverized coal and CWSF, fuel particle size alone does not determine the PSDs of the ashes collected at port 10, even though both fuels had similar PSDs. If the coal particle size alone determines the final ash PSD, then the PSDs of the port 10samples for the two Beulah fuels should be nearly identical. The initial morphologiesof the Beulah pulverized coal and CWSF do not differ significantly; therefore, the difference in ash PSDs between the two fuels is probably due to differences in the PSDs of the mineral matter and the occurrence of inorganics in the fuels and their behavior during combustion. In the Elk Creek fuels, the original PSDs of the mineral matter are similar and the dominant mechanism of ash
530 Energy & Fuels, Vol. 7, No. 4,1993
Miller and Schobert
Figure 11. (A, top) Scanning electron micrograph of the Elk Creek pulverized coal char collected at port 1. Carbonaceous cenospheres having rupture holes due to devolalitization are predominant. (B, bottom) Scanningelectron mcirograph of the highly agglomerated Elk Creek pulverized coal ash and char fragment collected at port 10.
formation is coalescence and agglomeration of the inorganic portion of the fuels during combustion (Le., several mineral
particles contribute to the formation of an individual ash particle). Since the same mechanism is responsible for ash formation in both Elk Creek fuels, the size of the individual fuel particle or agglomerate in the combustor determines the final ash PSDs. In CWSFs, less-thanoptimum atomization results in formation of agglomerates having a coarser PSD than the original pulverized coal used to prepare the CWSF. The Elk Creek CWSF agglomerate consists of individual coal particles as well as mineral matter particles. With the formation of a larger CWSF agglomerate, the number of mineral particles in proximity within the agglomerate is greater than that in an individual coal particle. The coal-mineral particle agglomerate remains intact during evaporation and forms the CWSF char. The mineral particles in the char experience higher temperatures during combustion than mineral particles extraneous to the char. The higher temperatures enhance the formation of melt phases, forming new inorganic phases and enhancing coalescence and agglomeration of molten particles at the char surface and within the char. If completecoalescenceof the mineral particles occurs during combustion, the resulting CWSF ash particles would be larger than ash particles formed during combustion of the pulverized coal particles. The result is a slightly coarser PSD of the CWSF ash, compared to the pulverized coal ash, even though the PSDs of the mineral matter in the two fuels are similar. This was observed in the Elk Creek CWSF; the PSD of the CWSF ash was slightly coarser than the PSD of the pulverized coal ash. In both the Beulah and Elk Creek coals, firing as pulverized coal or CWSF had little effect on char morphology. Char size played a greater role in the time required for char burnout and the point at which char fragmentation begins. For example, fragmentation of the larger Elk Creek CWSF char was delayed, allowing for the coalescenceof mineral matter to occur over a greater period of time, hence resulting in a slightly coarser ash PSD. In a companion paper, we will report on the effect of the particle size of the mineral matter and the occurrence of inorganics on the resulting ash particle size di~tribution.~
100 90 80
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Figure 12. Size distribution of the Elk Creek coal-water slurry fuel particles, droplets, and char/ash collected at various locations within the combustor.
Effect of Fuel Particle and Droplet Size
Acknowledgment. Financialsupport for thisworkwas provided by The Pennsylvania State University Cooperative Program for Coal Research and the Coal Water Fuel Project sponsored by the Commonwealth of Pennsylvania. OXCE Fuel Company and Energy International provided
Energy & Fuels, Vol. 7, No. 4, 1993 531
the Elk Creek fuels at no cost. We thank Carl Martin for construction of particle sampling probes; Prakash Ramachandran and Naiyi Hu for their help during the combustion tests; and John Hurley for his suggestions regarding sample collection.
