fine ash formation during pulverized coal combustion

Fine Ash Formation during Pulverized Coal Combustion

Tsuyoshi Teramae* and Takayuki Takarada

Coal and EnVironmental Research Laboratory, Coal Business Office, Petroleum & Coal MarketingDepartment, Idemitsu Kosan Company, Limited, 3-1 Nakasode, Sodegaura, Chiba, Japan and Department

of Chemical and EnVironmental Engineering, Faculty of Engineering, Gunma UniVersity,1-5-1 Tenjin-cho, Kiryu, Gunma, Japan

ReceiVed August 13, 2008. ReVised Manuscript ReceiVed February 10, 2009

Fine particulates that are emitted from commercial coal combustion sources can be inhaled into humanrespiratory systems and have been known to cause various harmful effects. Therefore, legislation hasbeen enacted to limit the emission of fine particulate matter in many countries. A fundamental understandingof the mechanisms of fine particle formation is an important step to mitigating the environmental impactsof coal combustion. In this study, 15 pulverized coal samples were burnt in a drop-tube furnace to investigatethe formation of fine particulates and the influence of coal ash properties on their emission. Coal combustionwas carried out at 1673 K in air. Fine particles were collected by a cyclone and a low-pressure impactor.The elemental compositions of the collected particles were analyzed by scanning electron microscopywith energy-dispersive X-ray spectroscopy. We examined the chemical compositions of the fine particlesas a function of particle diameter and examined the proportions of the elements in the parent coal samples.We determined that almost all particles less than 0.22 µm in diameter were formed by means ofvolatilization-condensation of SiO2 and Al2O3 in the coal. We also demonstrated that the amount ofSiO2 in particle size less than 0.22 µm in diameter was related to the amount of fine included quartz andclay minerals in the parent coal. The primary components of particles greater than 0.76 µm in diameterwere SiO2 and Al2O3, and as the diameter of the particles decrease, the mass fractions of iron, magnesium,calcium, and phosphorus increased. However, the particle diameter at which this tendency commenceddiffered depending on the element. Particles between 0.22 and 0.76 µm in diameter were thought to havebeen formed by the fragmentation and coalescence of particles in the coal and by the simultaneouscondensation of volatilized elements onto other particles.

Introduction

Possible links between fine particles and health effects wereidentified in the late 1980s. Since then, there has been increasingstatistical evidence relating the concentration of fine particulatematter in ambient air to heart and lung problems.1 Studies haveindicated that fine particle toxicology could be responsible forthese adverse effects. The emission of fine particles is closelyassociated with the emission of toxic trace elements from coalcombustion because the fine particles are often enriched withthese toxic elements.2

Governments worldwide have acknowledged the adversehealth effects of ambient fine particles. As a result, standardshave been introduced to assist in reducing ambient fine particleconcentrations. In the United States, a National Ambient AirQuality Standard (NAAQS) has been established for particulatematter with an aerodynamic diameter less than 10 µm (PM10)and less than 2.5 µm (PM2.5). In Japan, environmental regulationof suspended particulate matter with a diameter less than 10µm has been established by the Ministry of the Environment.About 100 million tons of steaming coal is used in Japan each

year, and the emission of fine particulate matter from coal-firedboilers must be reduced.

To elucidate the mechanisms of PM10 formation, manyresearchers have measured the size distribution of PM10 forvarious coal types. In addition, they also have conductedcombustion experiments in which temperature and oxygenconcentration are varied. Measurements of particle size distribu-tion are an important means of elucidating the mechanisms offine particle formation. In recent years, there have been severalreports of particle size distribution appearing as a trimodaldistribution.3-5 The fine mode with an average diameter of about0.1 µm is formed by volatilization-condensation, whereas thecoarse mode of 5 µm or more is formed by the coalescence ofash particles. However, little progress has been made indiscussing formation mechanisms for the central mode. Yu andco-workers6 identified and examined the origin of this central

* To whom correspondence should be addressed. Telephone: 81-438-62-9511. Fax: 81-438-60-1177. E-mail: [email protected].

(1) Sloss, L. L. The importance of PM10/2.5 emissions; IEA CCC/89;IEA Clean Coal Centre: London, U.K., 2004.

(2) Lighty, J. S.; Veranth, J. M.; Sarofim, A. F. Combustion aerosols:factors governing their size and composition and implications to humanhealth. J. Air Waste Manage. Assoc. 2000, 50, 1565–1622.

(3) Linak, W. P.; Miller, C. A.; Wendt, J. O. L. Comparison of particlesize distribution and elemental partitioning from the combustion ofpulverized coal and residual fuel oil. J. Air Waste Manage. Assoc. 2000,50, 1532–1544.

(4) Wendt, J. O. L. Pollutant formation in furnaces: NOx and fineparticulates. IFRF Combust. J. 2003, Article No. 200301 (http://www.journal.ifrf.net/library/may2003/200301Wendt.pdf, The IFRF ElectronicCombustion Journal).

(5) Seames, W. S. An initial study of the fine fragmentation fly ashparticle mode generated during pulverized coal combustion. Feel Process.Technol. 2003, 81, 109–125.

(6) Yu, D.; Xu, M.; Yao, H.; Sui, J.; Liu, X.; Yu, Y.; Cao, Q. Use ofelemental size distributions in identifying particle formation modes. Proc.Combust. Inst. 2007, 31, 1921–1928.

Energy & Fuels 2009, 23, 2018–20242018

10.1021/ef800658w CCC: $40.75 2009 American Chemical SocietyPublished on Web 03/23/2009

mode based on the basis of mass-fraction size distributions ofelements. They suggested that a heterogeneous condensation-adsorption of vaporized species on fine residual ash particlesappropriately accounts for the formation of the central mode.

The primary objectives of the present study were to utilizenumerous coal samples, to sample fine particles from each ofthese samples under the same conditions, to measure particlesize distribution and elemental mass-fraction size distribution,to compare the size boundary for each mode with those obtainedin previous research, and to investigate fine particle formationmechanisms. Previous studies used only a few types of coalsamples, whereas in the current research, we used 15 distinctcoal samples.

We also focused on silicon and aluminum, which are themain constituents of fine particles in ash, and elucidated theseelements’ effects on fine particle formation as well as thepaths they took in becoming fine particles. Quann and co-workers7 measured SiO2 contents of 1.6-44% in the sub-micrometer fumes from combustion of coal in 20% O2. Buhreand co-workers8 characterized small ash particles generatedfrom the combustion of bituminous coals in 21% and 50%O2. They showed that the main constituents of these samplesare sulfur, silicon, phosphorus, and sodium and that theelemental composition and yield of submicrometer ash aregreatly affected by increasing the O2 partial pressure. Theyalso indicated that the condensation of vaporized species wasresponsible for the formation of ash particles smaller than0.3 µm. In a further study by the same group, the correlationbetween the amounts of vaporized silica and the character-istics of five well-characterized Australian black coals wasexamined by combustion in 21% and 50% O2 in a drop-tubefurnace (DTF). They concluded that finely dispersed silicon-bearing minerals of a size less than 2 µm could substantiallycontribute to the amount of silica vaporization observed.

However, they did not quantify the amount of particles lessthan 2 µm in diameter nor did they directly express theseparticles’ contribution to silica vaporization.9 Zhang and co-workers10 indicated that mullite (3Al2O3 · 2SiO2) generatedfrom the decomposition of kaolinite (Al2O3 · 2SiO2 · 2H2O)contributed to the formation of fine particles with a size of0.1-1 µm. They noted that SiO2 is also produced stoichio-metrically by the decomposition of kaolinite, but the extentto which this generated SiO2 becomes incorporated intosubmicrometer particles is unknown. Numerous researchershave shown that the volatilization of SiO from the quartzcontained in coal forms fine particles. However, the effectsof clay minerals containing SiO2 and Al2O3 as sources ofsilicon in fine particles have hardly been discussed. In thisresearch, we examined the role of clay minerals as a sourceof silicon in fine particles and the differences in themechanisms of fine particle formation based on particlediameter.

Fifteen bituminous coal samples were characterized and thenburned in a DTF at 1673 K. Fine ash particles were collectedby means of a cyclone and a low-pressure impactor (LPI), andthe chemical composition of the particles was analyzed.

Experimental Section

Thirteen bituminous coals with a wide range of ash chemistrieswere selected for analysis. Table 1 lists the properties for thesecoals. Two of these coal samples (samples L and M in Table 1)were gravimetrically fractionated to obtain additional coal sampleswith a low ash content (samples N and O, respectively), bringingthe total sample count to 15. The float fraction, which was thefraction separated by means of a liquid with a specific density of1.3, was tested for the combustion experiments. The ash contentsand chemical compositions were dissimilar among the 15 coalsamples. For example, the SiO2 contents ranged from 44.5% to74.6%, and the Al2O3 contents ranged from 17.1% to 37.3%, withSiO2/Al2O3 ratios of 1.29-4.06.

(7) Quann, R. J.; Nevill, M.; Janghorbani, M.; Mims, C. A.; Sarofim,A. F. Mineral matter and trace-element vaporization in a laboratory-pulverized coal combustion system. EnViron. Sci. Technol. 1982, 16, 776–781.

(8) Buhre, B. H. P.; Hinkley, J. T.; Gupta, R. P.; Wall, T. F.; Nelson,P. F. Submicron ash formation from coal combustion. Fuel 2005, 84, 1206–1214.

(9) Buhre, B. H. P.; Hinkley, J. T.; Gupta, R. P.; Nelson, P. F.; Wall,T. F. Factors affecting the vaporization of silica during coal combustion.Fuel Process. Technol. 2007, 88, 157–164.

(10) Zhang, L.; Ninomiya, Y.; Yamashita, T. Formation of submicronparticles matter (PM1) during coal combustion and influence of reactiontemperature. Fuel 2006, 85, 1446–1457.

Table 1. Properties of Coal Samples Used in This Study

coal sample A B C D E F G H I J K L M N O

proximate analysis (wt %)moisture 6.0 11.6 7.8 2.6 4.1 3.3 9.1 4.9 7.4 5.8 3.7 2.8 3.3 3.1 5.2ash 12.6 5.0 11.0 8.9 10.1 9.7 10.6 12.7 6.6 14.4 7.2 15.0 10.0 1.9 3.2volatile matter 30.2 38.6 33.9 38.7 34.0 34.4 41.4 35.6 27.4 31.8 35.6 26.3 27.7 32.7 33.3fixed carbon 51.2 44.8 47.3 49.8 51.8 52.6 38.9 46.8 58.6 48.0 53.5 55.9 59.0 62.3 58.3

ultimate analysis (wt %, daf)carbon 82.27 76.04 79.05 82.94 82.77 81.88 78.78 79.47 83.79 82.68 81.95 83.30 81.12 84.20 81.55hydrogen 5.17 5.61 5.42 6.13 5.78 5.44 6.65 5.65 5.15 5.62 5.24 4.97 4.64 5.27 5.04nitrogen 1.27 1.67 1.07 2.85 1.89 1.60 1.14 2.50 0.96 1.79 1.76 1.86 0.91 1.85 1.00sulfur 0.26 0.63 0.14 0.33 0.52 0.45 0.39 0.07 0.65 0.40 0.34 0.36 0.81 0.33 0.35oxygen (oxygen: by difference) 11.03 16.05 14.32 7.75 9.04 10.63 13.04 12.31 9.45 9.51 10.71 9.51 12.52 8.35 12.06

elemental composition of ash (wt %)SiO2 52.71 44.50 61.11 53.59 57.78 44.57 62.06 57.88 51.63 72.71 74.57 43.90 57.94 49.76 59.75Al2O3 29.70 21.88 17.12 19.98 23.41 32.58 24.29 17.79 18.63 19.03 18.36 34.02 19.01 37.30 15.06TiO2 1.12 0.88 0.64 0.81 1.29 1.61 2.10 0.72 0.73 0.90 1.16 1.60 0.65 2.97 0.47Fe2O3 3.52 12.45 3.99 7.44 6.11 5.29 2.60 6.93 14.11 2.10 1.61 5.50 12.89 1.43 16.71CaO 7.26 6.41 9.64 6.32 4.06 6.69 3.14 6.04 6.14 0.80 0.82 6.74 3.15 1.09 3.53MgO 0.80 1.53 1.43 2.72 1.40 2.38 1.09 2.40 0.89 0.57 0.53 1.31 0.78 0.72 0.82Na2O 0.31 0.53 0.22 1.24 0.39 0.42 0.85 1.59 0.51 0.30 0.33 0.48 0.28 1.33 0.53K2O 1.26 1.56 0.90 3.00 1.01 0.53 0.33 2.82 0.49 2.25 1.98 0.80 1.50 0.51 1.41P2O5 0.49 0.29 0.26 0.26 0.38 0.39 0.03 0.38 1.21 0.04 0.05 1.16 0.30 2.01 0.35MnO 0.05 0.12 0.04 0.05 0.04 0.08 0.07 0.07 0.07 0.00 0.00 0.04 0.24 0.08 0.11V2O5 0.02 0.05 0.02 0.03 0.06 0.05 0.06 0.02 0.02 0.02 0.03 0.03 0.02 0.40 0.00SO3 1.95 5.27 3.81 3.03 2.73 2.90 1.73 1.89 3.26 0.49 0.46 3.50 1.44 1.99 3.80

Fine Ash Formation Energy & Fuels, Vol. 23, 2009 2019

The coal samples were milled and sieved to a particle size ofless than 74 µm. The coal samples were subjected to proximateand ultimate analyses (by JIS M 8812 and JIS M 8819, respectively)and analyzed by computer-controlled scanning electron microscopy(CCSEM). The chemical composition of ash generated by high-temperature ashing (1088 K, 2 h) of each sample was determinedby X-ray fluorescence analysis. Ashes were also generated fromthe samples by low-temperature ashing (LTA), after which theparticle size distribution of the ash was measured by a laserscattering particle size distribution analyzer.

After characterization, the coal samples were combusted in aDTF located at Idemitsu Coal & Environmental Research Labora-tory, Japan. The furnace, which features a heated tube 1200 mm inlength, is shown schematically in Figure 1. The coal particles werefed by a vibrating feeder into the primary air stream. The primaryair stream in turn fed the coal particles into the heated zone of thecombustor at a rate of 7 g/h via a water-cooled injector probe. Thesecondary air was heated to 973 K and fed into the heated zone ofthe DTF. Combustion of the coal particles was carried out in air at1673 K (gas temperature).

Generated ash larger than 2 µm was collected by a cycloneinstalled at the opposite end of the DTF, and the weight of the ashwas measured. Particles smaller than 2 µm were introduced into aLPI. The LPI consisted of 12 stages containing Teflon filter withaerodynamic cutoff diameters (µm) of 8.5, 5.7, 3.9, 2.5, 1.25, 0.76,0.52, 0.33, 0.22, 0.13, 0.06, and <0.06. The weights of the Teflonfilters on each stage were measured before and after combustionto calculate the weights of the fine particles. The chemicalcompositions of the ash samples were determined by scanningelectron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX). We considered this semiquantitative analysis to be sufficientto observe the variations in ash composition among the stages ofthe LPI. The chemical species measured by SEM/EDX were Na2O,MgO, Al2O3, SiO2, P2O5, K2O, CaO, TiO2, MnO, Fe2O3, and SO3.

To obtain the exact chemical composition data, a SEM/EDXanalysis procedure was established with ash particles of knowncomposition.

Results and Discussion

Figure 2 shows the size distribution of fine particles formedfrom combustion of 15 pulverized coal samples at 1673 K. Theamount of fine particles is reported for each sample as a weightpercent of the total coal ash (for convenience, the valuesobtained for particles <0.06 µm are plotted at 0.03 µm on thex axis). For most samples, a V-shaped distribution curve wasobtained, with the amount of 0.33 µm fine particles beingsmallest. The quantity of particles collected differed by morethan an order of magnitude among the 15 samples. Figure 3shows the particle diameter distribution of LTA ash as measuredby the laser scattering particle size distribution analyzer. Massfractions for the diameter ranges 0.510-0.584, 0.584-0.669,0.669-0.766, 0.766-0.877, 0.877-1.005, 1.005-1.151, 1.151-1.318,1.318-1.510,1.510-1.729,1.729-1.981,and1.981-2.269µm were obtained from the measurement apparatus, and thecumulative distribution is shown for particle diameters less than1.2 µm, the portion related to fine particle formation. Each pointin Figure 3 shows the mass for particles of less than themaximum diameter for each range. The smallest particlescontained in the LTA ash of each of the 15 coal types wereabout 0.5-0.9 µm in diameter. The proportion of fine particleswith a diameter greater than 0.33 µm declined with decreasing

Figure 1. Schematic diagram of drop tube furnace and ash collectiontrain.

Figure 2. Size distribution of fine particles formed from combustionof 15 samples at 1673 K in air.

Figure 3. Particle size distribution of LTA ash.

2020 Energy & Fuels, Vol. 23, 2009 Teramae and Takayuki

average overall particle diameters in a given sample (Figure 2)mainly because, over this range of particle diameters, the amountof coal ash produced also declined as particle diameter declined.However, the smallest of the coal ash particles (0.5-0.9 µm,Figure 3) were large in diameter than the lowest proportion offine particles formed during combustion (0.33 µm). We inferredthat this discrepancy in diameters occurred because fine particlessmaller than the smallest of the coal ash particles are formedby fragmentation or decomposition of ash particles in parentcoal and that the factors leading to this fragmentation ordecomposition were more influential than factors leading to thecoalescence of ash particles during combustion. For fine particlediameters less than 0.33 µm, the proportion of fine particlesincreased as the particle diameter became smaller. For this rangeof particle diameters, we concluded that the amount of particlesformed via fragmentation of ash particles became extremelysmall and that the amount of particles formed by volatilization-condensation increased as particle diameter decreased. Sincethis distribution of particle sizes (Figure 2) has a boundary at0.33 µm, these observed proportions indicated that for theparticle diameter range larger than 0.33 µm, fine particles thatform from the fragmentation or coalescence of the coal ashparticles are predominant, whereas for the particle diameterrange smaller than 0.33 µm, fine particles that form viavolatilization are predominant. The above observations arerelated to the mechanisms of fine particle formation from theperspective of particle size distribution. Next, we consider thesemechanisms from the perspective of the chemical compositionof the fine particles.

Figures 4 and 5 show changes in the mean chemicalcomposition (as oxides) of the ash from the 15 samples versusparticle diameter. From these figures, we determined thatelements in particles of each size exhibited one the followingthree behaviors: (1) as particle diameter decreased, so did massfraction (SiO2, Al2O3); (2) as particle diameter decreased, massfraction increased (Na2O, K2O, MgO, P2O5, SO3); or (3) themaximum mass fraction value was observed at a particlediameter of about 0.5 µm (Fe2O3, CaO). Therefore, trends inmass-fraction variation differed depending on the element.However, we think that the fraction of fine particles formedvia volatilization increased with decreasing particle diameterfor all elements. Because the rate of elemental volatilizationvaries depending on each element, the mass fraction in particlesis determined by the product of the quantity of the element inthe coal and the volatilization rate. Most of the sulfur is thought

to become volatilized during combustion. At a particle diameterof 0.76 µm, the fraction of sulfur in fine particles started to riseand increased as the particle diameter became smaller. A certainproportion of the volatilized elements condense on the surfaceof particles in the combustion gas. Sulfur, in particular,condenses mostly in the form of sulfuric acid. We thought thatthe amount of condensation increased because the relativesurface area becomes larger, and consequently, the mass fractionof condensed elements in the fine particles increases, as theparticle diameter decreases. In such a case, the mass fractionof sulfur is expected to continuously increase with decreasingparticle diameter. In other words, even for particles greater than0.76 µm in diameter, condensation of volatilized sulfur on otherparticle surfaces should occur. However, Figure 4 shows thatthe sulfur content was roughly constant in particles greater than0.76 µm in diameter, but the sulfur content increased rapidlyfor particles less than 0.76 µm. The mass fractions of eachelement in Figures 4 and 5 are strictly relative values. Forparticles less than 0.76 µm, we thought that the amounts ofSiO2 and Al2O3 that became fine particles without beingvolatilized declined abruptly, concurrent with increasing sulfurmass fraction. For particles greater than 0.76 µm, we presumedthat the quantity of fine particles formed via the fragmentationor coalescence of coal ash particles was much higher than theamount of sulfur condensation, and thus, a variation in the sulfurmass fraction could not be detected.

Yu and co-workers11 carried out combustion experimentsusing a DTF and showed variations in sulfur mass fraction basedon particle diameter. They found that the particle diameter atwhich the mass fraction started to increase with decreasingparticle diameter was about 2 µm, whereas the increase in massfraction started to plateau when the particle diameter was about0.2 µm. This particle diameter at which the increase started toplateau is about the same as the diameter we found in this study,but the particle diameter at which the increase started differsbetween our study and Yu’s. One of the presumed causes forthis discrepancy is that the sulfur concentration of the coal thatYu and co-workers11 used was much higher than that of thecoal samples used in this study.

Sodium and potassium exhibited similar tendencies to sulfurregarding the particle diameter at which the mass fraction in

(11) Yu, D.; Xu, M.; Yao, H.; Liu, X.; Zhou, K. A new method foridentifying the modes of particulate matter from pulverized coal combustion.Powder Technol. 2008, 183, 105–114.

Figure 4. Distribution of mean mass fraction of SiO2, Al2O3, and SO3

in fine particles formed from combustion of 15 samples at 1673 K inair, reported as weight percent of ash content collected from each LPIstage.

Figure 5. Distribution of mean contents of Fe2O3, CaO, MgO, Na2O,K2O, and P2O5 in fine particles formed from combustion of 15 samplesat 1673 K in air, reported as weight percent of ash content collectedfrom each LPI stage.


the particles started to increase as particle diameters becameincreasingly small. However, compared to these elements, themass fractions of iron, calcium, magnesium, and phosphorusstarted to increase in particles that were of larger diameter, andboth the fragmentation and the coalescence of these particlesare believed to be responsible for the observed shift to largerparticle diameters. We plan to investigate the behaviors of theseelements more extensively in a future study.

Figure 4 shows that the mass fractions of SiO2 and Al2O3

decreased with decreasing particle diameter. The mean massfraction of SiO2 for particles less than 0.22 µm in all 15 sampleswas steady at about 33 wt %. In contrast, the mass fraction ofAl2O3 for particles less than 0.22 µm was only 3 wt %. SiO2,as quartz, is present with Al2O3 in clay minerals found in coal.In the next section of this discussion, variations in SiO2 massfraction and Al2O3 mass fraction based on particle diameter areshown for all 15 samples, and the formation mechanisms andthe origins of the SiO2 fine particles formed via volatilization-condensation are discussed together with the behavior of Al2O3.

Figure 6 shows the percentage by weight of SiO2 in particlesformed during combustion of the 15 coal samples and collectedfrom each filter in the LPI. The SiO2 mass fractions variedwidely among the coal samples; however, for most of thesamples, the patterns of variation in the SiO2 mass fractionswith increasing particle diameter were similar. In particular, asthe particle diameter decreased below about 1.2 µm, the SiO2

mass fractions also decreased for most of the samples, and massfractions for each sample became almost constant in particleswith diameters less than 0.22 µm. The variation in Al2O3 massfractions as a function of particle diameter followed the samegeneral trends as those observed for SiO2 for most of the samples(Figure 7). However, the mass fractions of Al2O3 in all sampleswere smaller than the mass fractions of SiO2, particularly forparticles with diameters less than 0.22 µm. The mass fractionsof SiO2 observed in particles with diameters less than 0.22 µmranged from approximately 10 to 60 wt %, whereas the massfractions of Al2O3 were less than 5 wt % in most samples.

Yu and co-workers6 examined variations in the proportionsof fine particles according to the particle diameters of siliconand aluminum. They classified fine particle formation into threemodes: the 0.0281-0.258 µm particle diameter range (Ultrafinemode), the 2.36-9.80 µm particle diameter range (coarse mode),and the particle diameter range lying between these (centralmode). When applying the results of this research to theclassifications of Yu and co-workers, the Ultrafine mode applies

to particles less than 0.22 µm in diameter, the coarse mode toparticles greater than 0.76 µm in diameter, and the central modeto the intermediate diameter range.

Figure 8 shows variations in particle diameter as a functionof Al2O3/SiO2 ratio. Although the mass fractions of both Al2O3

and SiO2 declined in the 0.52-0.76 µm range (Figures 6 and7), the Al2O3/SiO2 ratio for many samples did not change forparticles greater than 0.76 µm in diameter. For particles lessthan 0.52 µm, both the mass fractions and the ratio declined.We thought that for particles of 0.53-0.76 µm, the amounts ofparticles derived from quartz and clay minerals decreased inthe same ratio, and for particles less than 0.52 µm, the rate ofdecrease in the amount of particles derived from clay mineralswas higher than that for those derived from quartz. If many ofthe particles in this range of diameters are assumed to haveformed from fragmentation of minerals in the coal, then thefrequency of fragmentation of quartz must have been higherthan that of clay minerals. Further study is required to determinehow broken up particles contribute to the distribution ofdiameters of fine particles.

Figures 9 and 10 show the amounts of SiO2 and Al2O3 infine particles as a percentage by weight of the amounts of SiO2

and Al2O3, respectively, found in the parent coal samples(referred to hereafter as the conversion rate). The y-axis limitsare 1% in Figure 9 and 0.06% in Figure 10 to show clearly thedistributions of the oxide conversion rates for particles with

Figure 6. Distribution of SiO2 mass fraction in fine particles formedfrom combustion of 15 samples at 1673 K in air, reported as weightpercent of ash content collected from each LPI stage.

Figure 7. Distribution of Al2O3 mass fraction in fine particles formedfrom combustion of 15 samples at 1673 K in air, reported as weightpercent of ash content collected from each LPI stage.

Figure 8. Distribution of Al2O3/SiO2 ratios in fine particles.


diameters smaller than 0.22 µm. For particles greater than 0.33µm, the conversion rates of both SiO2 and Al2O3 decreased asparticle diameter decreased to 0.22 µm; the conversion rate wereobserved to increase for particles smaller than 0.22 µm. Animportant point to emphasize is that with particles smaller than0.22 µm, the conversion rates of SiO2 were substantially higherthan those of Al2O3. In particles smaller than 0.06 µm, the SiO2

conversion rates were between 0.82% and 0.04%, whereas theAl2O3 conversion rates were below 0.06%. These results clearlyshow that Al2O3 is more difficult to volatilize than SiO2 underthese experimental conditions of 1673 K in air. Previous reportshave also found that the volatility of SiO2 is higher than that ofAl2O3.12 According to the data of Buhre and co-workers,8 theamount of Al2O3 vaporized would increase if the char temper-ature under combustion was higher.

The conversion rates of SiO2 and Al2O3 for particles less than0.22 µm tended to decline as particle diameter increased (Figures9 and 10). Volatilized particles, after passing through the high-temperature region of the furnace, form nuclei and aggregate,and particles thereby grow. In the current study, these particlesgrew to at least about 0.2 µm in diameter. If particle growthdoes indeed proceed because of collisions between formed nuclei

and because of the condensation of volatilized components ontothe surfaces of larger particles, then it can be surmised that onefactor that determines particle diameter is the number ofcollisions between particles. The number of collisions must belarge for the particles to grow to a large size. For example, thenumber of collisions needed to grow a 0.2 µm particle mustbe greater than the number of collisions needed to grow aparticle smaller than 0.2 µm, and we believe that the probabilityof the former scenario is smaller than the probability of the latter.This reasoning explains the observed trend of declining conver-sion rates for SiO2 and Al2O3 with increasing particle diameter.Furthermore, from this reasoning we concluded that particlesof 0.22 µm or less were formed solely by means of thevolatilization-condensation mechanism.

Volatilization of SiO2 results from carbothermal reduction.A considerable number of studies have been conducted on thekinetics of reactions between carbon and SiO2 that produce SiOgas under high temperatures.13-15 Fine particles of SiO2 thatarise from the volatilization-condensation mechanism originatefrom quartz in the parent coal; however, the extent to whichclay minerals influence the amount of SiO generated iscontroversial. Buhre and co-workers9 reported that no apparentcorrelation between the amount of silica vaporized and theamount of clay minerals in the parent coal is observed.

Figure 11 displays the correlation between the mass percentof SiO2 particles smaller than 0.22 µm on the basis of the amountof SiO2 in coal and the ratio of included quartz and clay mineralsin coal particles with diameters of 1.0-2.2 µm to the total oneswith the same size () included/(included + excluded)) detectedby CCSEM analysis. The smallest size range that could beanalyzed by the CCSEM used in this study was 1.0-2.2 µm;we selected the data of this smallest range of coal particlesbecause these particles had the largest surface area and becausewe thought that the carbothermal reduction occurred more easilyon smaller particles. The relationship between the amounts ofvolatilized SiO2 and the amounts of included quartz and clayminerals (Figure 11) would be more apparent if we had beenable to determine the amounts of included quartz and clay

(12) Neville, M.; Quann, R. J.; Haynes, B. S.; Sarofim, A. F. Vaporiza-tion and condensation of mineral matter during pulverized coal combustion.18th Symposium on Combustion; The Combustion Institute: Pittsburgh, PA,1981; pp 1267-1274.

(13) Ozturk, B.; Fruehan, R. J. The rate of formation of SiO by thereaction of CO or H2 with silica and silicate slags. Metall. Mateer. Trans.B 1985, 16B, 801–806.

(14) Weimer, A. W.; Nilsen, K. J.; Cochran, G. A.; Roach, R. P. Kineticsof Carbothermal reaction synthesis of beta silicon carbide. AIChE J. 1993,39, 493–503.

(15) Agarwal, A.; Pal, U. Influence of pellet composition and structureon carbothermic reduction of silica. Metall. Mater. Trans. B 1999, 30B,295–306.

Figure 9. Distribution of SiO2 conversion rates in collected fineparticles, reported as a weight percent of the amount of SiO2 in theparent coal samples.

Figure 10. Distribution of Al2O3 conversion rates in collected fineparticles, reported as a weight percent of the amount of Al2O3 in theparent coal samples.

Figure 11. Relationship between the conversion rate of SiO2 withdiameters less than 0.22 µm and the ratio of included quartz and clayminerals with 1-2.2 µm diameter to included + excluded minerals ofthe same diameter detected by CCSEM.


minerals less than 1 µm in diameter. When the amount ofvolatilized SiO2 was plotted against only the included quartz, aweak correlation was observed (Figure 12). Bechtold andCutler16 concluded that the carbothermal reduction of kaolinclay at temperatures of 1632-1778 K is a viable method ofrecovering Al2O3 from the clay by removing SiO2 via formationof an SiO gas intermediate. Furthermore, the only major oxidein kaolin clay that is reduced by carbon at 1573-1773 K isSiO2, as shown by the research of Wright and Wolff.17

Therefore, it seems reasonable to conclude that clay mineralsalso played a role in producing fine SiO2 particles via thevolatilization-condensation mechanism.

Conclusion

Fifteen bituminous coal samples, which showed a wide rangeof chemical compositions in their ash, were combusted in a DTFto investigate the mechanisms of fine particle formation. Theamounts and chemical compositions of the fine particles weredetermined for each particle size collected by the LPI.

On the basis of the distribution of particle diameters, thefollowing conclusions were reached for the mechanisms of fineparticle formation.

(1) <0.22 µm: Particle formation via volatilization-conden-sation predominates for particles in this diameter range. In thisexperimental system, the maximum particle diameter attainedvia volatilization-condensation is thought to be 0.22 µm. TheSiO2 and Al2O3 conversion rates decline as particle diametersincrease. A large number of collisions between volatilizedelements are required in order for particles to grow. The amountof SiO2 in particles less than 0.22 µm in diameter is related tothe amount of quartz and clay minerals included in fine particlesof coal. Volatilization of SiO by way of carbothermal reductionfrom minerals in the coal occurred not only with quartz butalso with clay minerals.

(2) 0.22-0.76 µm: SiO2 and Al2O3 particles with this rangeof diameters are formed by the fragmentation of minerals inthe coal, but further study is required to confirm this conclusion.The mass fraction of sulfur in these particles increases as particlediameter decreases. This same relationship between massfraction and particle diameter is also observed for other volatileelements. The volatilized elements scarcely form particles ofthis diameter range by nucleus formation and subsequent growth;the elements condense onto the surfaces of other particles. Asparticle diameter increases, the quantity of fine particles notformed via volatilization becomes greater than the quantity ofelemental particles formed via volatilization and the quantityof the condensed elements onto the surfaces of other particles.

(3) >0.76 µm: Volatilized elements condense onto the surfaceof particles of this size. The content of sulfur, potassium, andsodium was relatively stable in particles greater than 0.76 µmin diameter, but in particles below 0.76 µm the mass fractionof these elements increased. No relative change in mass fractionsis evident because the amount of condensation is small. Theparticle diameter at which the iron, calcium, magnesium, andphosphorus contents start to rise as particle diameter decreasesis larger than that for sulfur, potassium, and sodium. Both thefragmentation and the coalescence of these particles are believedto be involved in the formation mechanism of larger particles.

EF800658W

(16) Bechtold, B. C.; Cutler, I. B. Reaction of clay and carbon to formand separate Al2O3and SiC. J. Am. Ceram. Soc. 1980, 63, 271–275.

(17) Wright, R. E.; Wolff, H. I. Refractry problems in production ofhydrogen by pyrolysis of natural gas. J. Am. Ceram. Soc. 1948, 31, 31–38.

Figure 12. Relationship between the conversion rate of SiO2 withdiameters less than 0.22 µm and the ratio of included quartz with 1-2.2µm diameter to included + excluded quarts of the same diameterdetected by CCSEM.


fine ash formation during pulverized coal combustion

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