Submicrometer Particle Formation and Control in a Bench-Scale

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Energy & Fuels 2001, 15, 510-516

Submicrometer Particle Formation and Control in a Bench-Scale Pulverized Coal Combustor Ye Zhuang and Pratim Biswas*,† Aerosol and Air Quality Research Laboratory, Environmental Engineering and Science Division, University of Cincinnati, Cincinnati, Ohio 45221-0071 Received April 17, 2000. Revised Manuscript Received January 26, 2001

Experimental studies were performed to investigate the formation mechanisms of submicrometer particles during combustion of a pulverized Ohio bituminous coal. Elemental compositions of total and submicrometer ash and particle size distributions were measured at different operating conditions. The submicrometer ash particles were formed by means of a vaporization-nucleationcondensation mechanism. A decreasing collection efficiency in a cylindrical electrostatic precipitator was observed for ash particles smaller than 100 nm due to insufficient charging. A vaporphase sorbent injection methodology was used in the coal combustion system, and was shown to suppress the nucleation mode of the ash particles.

Introduction Coal combustion is considered to be a potential major source of particulate matter emission to the atmosphere. While most of the large ash particles in the flue gas are effectively captured in air pollution control devices, the submicrometer-sized particles typically have a lower collection efficiency1 and comprise a larger fraction of aerosols emitted into the atmosphere. These submicrometer-sized particles are often enriched with toxic metallic species as reported by Smith et al.,2 Biswas and Wu,3 and Davison et al.4 The submicrometer aerosols are in high number concentrations, have a high specific surface area, and are often enriched with toxic species. Once emitted into the atmosphere they have long residence times, thus increasing the potential of exposure to humans. Once inhaled they are in size ranges that can penetrate deep into the alveolar regions of the lung. Therefore, there is a renewed interest in understanding the formation and capture characteristics of combustion aerosols. Extensive field-scale measurements have been made on toxic metallic species emissions from coal combustors. Davison et al.4 and Smith et al.2 demonstrated the enrichment of certain toxic elements, such as Pb, Hg, As, and Se, with decreasing particle size. Kaakinen et al.5 found As, Ni, Sb, Cd, and Hg were either condensed * Author to whom correspondence should be addressed. Tel: 314935-5482. E-mail: [email protected]. † Current address: Environmental Engineering Science, Washington University in St. Louis, One Brookings Drive, Campus Box 1180, St. Louis, Missouri 63130. (1) Markowski, G. R.; Ensor, D. S.; Hooper, R. G. Environ. Sci. Technol. 1980, 14, 1400-1402. (2) Smith, D. R.; Campbel, J. A.; Nielson, K. K. Atmos. Environ. 1979, 13, 607-617. (3) Biswas, P.; Wu, C. Y. Air Waste Manage. Assoc. 1998, 48, 113127. (4) Davison, R. L.; Natusch, D. F. S.; Wallace, J. R.; Evans, C. A. Environ. Sci. Technol. 1974, 8, 1107-1113. (5) Kaakinen, J. W.; Jorden, R. M.; Lawasani, M. H.; West, R. E. Environ. Sci. Technol. 1975, 9, 862-869.

on the small particles or emitted in the vapor phase. Norton6 reported the increased emissions of Cd, Cu, Hg, Pb, and Zn on the basis of a review on nine coal combustion sites. Meij7 measured the trace element distributions in raw coal, bottom ash, ash collected in the ESP (electro-static precipitator), and fly ash, and indicated the enrichment of some elements on the smaller-sized fly ash particles. Germani and Zoller8 and Bool III and Helble9 have reported higher emissions of As, Hg, Br, and Na with decreasing particle size. Such observations have motivated several studies aimed at understanding the mechanisms of submicrometer particle formation during coal combustion. The results of field- and laboratory-scale studies on emissions from pulverized coal combustors indicate that the particle size distributions are typically bimodal.1,10-12 The coarse particle (or residual ash) is formed by the coalescence of mineral inclusions in parent coal particles during the combustion process as described by Flagan,13 Kang et al.,14 Kang et al.,15 and Barta et al.16 The fine particle mode around 0.1 µm is formed via nucleation of vaporized ash components and growth by coagulation and heterogeneous condensation.12,17 Flagan13 proposed that certain species might vaporize in the form of (6) Norton, G. A. Environ. Prog. 1992, 11, 140-144. (7) Meij, R. Fuel Process. Technol. 1994, 39, 199-217. (8) Germani, M. S.; Zoller, W. H. Environ. Sci. Technol. 1988, 22, 1079-1085. (9) Bool, L. E., III; Helble, J. J. Energy Fuel 1995, 9, 880-887. (10) Taylor, D. D.; Flagan, R. C. Aerosol Sci. Technol. 1982, 103117. (11) Linak, W. P.; Peterson, T. W. Aerosol Sci. Technol. 1984, 7796. (12) Quann, R. J.; Neville, M.; Janghorbani, M.; Mims, C. A.; Sarofim, A. F. Environ. Sci. Technol. 1982, 16, 776-781. (13) Flagan, R. C. Symp. (Int.) Combust. 1979, 97-104. (14) Kang, S.-G.; Helble, J. H.; Sarofim, A. F.; Beer, J. M. 22nd Symp. (Int.) Combust. 1988, 231-238. (15) Kang, S. G.; Sarofim, A. F.; Beer, J. M. 24th Symp. (Int.) Combust. 1992, 1153-1159. (16) Barta, L. E.; Toqan, M. A.; Beer, J. M.; Sarofim, A. F. 24th Symp. (Int.) Combust. 1992, 1135-1144. (17) Senior, C. L.; Flagan, R. C. Aerosol Sci. Technol. 1982, 1, 371383.

10.1021/ef000080s CCC: $20.00 © 2001 American Chemical Society Published on Web 03/27/2001

Submicrometer Particle Formation in a Coal Combustor

suboxides, which are more volatile than the corresponding refractory oxides. Linak and Peterson11 studied the effects of coal type and residence time on particle size distributions, and concluded that the accumulation of particles in the submicrometer size ranges was the result of vaporization, nucleation, condensation, and coagulation. They suggested that the final particle size distributions in the convection section were primarily dictated by the coagulation rate. Scotto et al.18 reported that increasing the quench rate resulted in the enrichment of sodium in the small particles. Gallagher et al.19 examined alkali metal partitioning in ash obtained from pulverized coal combustors and found that increasing temperatures resulted in more vaporization of sodium. Most studies have reported that coal combustion and submicrometer particle formation mechanisms are very complex: coal type, temperature, fuel-air ratio, and residence time appear to be some of the important controlling factors. Sorbent injection into a combustion chamber is effective in reactively scavenging the metallic species vapors, thereby suppressing nucleation at the combustor exit. The trace metal compounds are thus associated with larger-sized particles which are more effectively removed in conventional particulate matter control devices. Moreover, these sorbents may also help immobilize the metallic species by forming glass complexes. Researchers have proposed the use of several bulk sorbent materials (such as silica, alumina, aluminosilicates, bauxite, limestone, hydrated lime, titania) for metals control using reactors of different geometries such as packed bed, fluidized bed, and direct injections a detailed review is presented in another paper.3 Although injection of bulk solid sorbents is potentially effective in removing certain trace metals from combustor gas streams, the use of such bulk materials is hindered by several physicochemical considerations such as mass transfer limitations due to outer surface reactions producing a metal sorbent complex which blocks the inner pore volume. The pathways and a mechanistic description of the various transformations have been summarized by Biswas and Wu.3 To overcome some of these limitations, Biswas and co-workers3,26 have developed a vapor precursor injection methodology wherein a high surface area/mass agglomerate sorbent oxide is produced in situ in the combustor. This sorbent oxide is stable at the high temperatures encountered in the combustor, and provides a surface for condensation and reaction of metal species vapors prior to their nucleation. (18) Scotto, M. V.; Bassham, E. A.; Wendt, J. O. L.; Peterson, T. W. 22th Symp. (Int.) Combust. 1988, 239-247. (19) Gallagher, N. B.; Bool, L. E.; Wendt, J. O. L.; Peterson, T. W. Combust. Sci. Technol. 1990, 74, 211-221. (20) Zhuang, Y.; Kim, Y. J.; Lee, T. G.; Biswas, P. J. Electrostat. 2000, 48, 245-260. (21) Biswas, P.; Zachariah, M. R. Environ. Sci. Technol. 1997, 31, 2455-2463. (22) Linak, W. P.; Peterson, T. W. 21st Symp. (Int.) Combust. 1986, 399-410. (23) Bool, L. E., III; Peterson, T. W.; Wendt, J. O. L. Combust. Flame 1995, 100, 262-270. (24) Finkelman, R. B. Fuel Process. Technol. 1994, 39, 21-34. (25) Handbook of Chemistry and Physics; David, R. L., Ed.; CRC Press: Boca Raton, FL, 1995-1996. (26) Owens, T. M.; Biswas, P. Ind. Eng. Chem. Res. 1996, 35, 792798.

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Figure 1. Schematic diagram of the experimental system for coal combustion studies.

Though there have been several studies at understanding the formation of the submicrometer mode during coal combustion, there is a need to better understand the fate of the trace species in a coal combustor. Furthermore, the capture characteristics of the submicrometer aerosol mode have not been explored in great detail. In this paper, the formation of the submicrometer mode during combustion of an Ohio bituminous coal in a controlled drop tube furnace reactor is examined. The capture characteristics of the submicrometer mode of the resultant aerosol in a cylindrical electrostatic precipitator is established. The suppression of the formation of the submicrometer mode by injection of a vapor-phase sorbent precursor is demonstrated. Experimental Apparatus and Methods The schematic diagram of the experimental system, including a coal feeding system, a vapor-phase sorbent injection system, a combustion system, and a particle collection and measurement system, is shown in Figure 1. A pulverized Ohio, bituminous coal, with a mean particle size of 50 µm, was fed into the reactor by a fluidized bed coal feeder. The coal particles, carried by particle-free compressed air, were first introduced into a preheated section that was maintained at 673-773 K. The heated coal-gas mixture entered an electrically heated and insulated alumina tubular reactor, which was 91 cm long, 2.54 cm inside diameter. Thermocouples were installed at the preheated section and the reactor to monitor the temperatures. Particle-free nitrogen was added at the end of the reactor to quench the aerosol dynamic processes and the chemical reactions. A Mark III six-stage cascade impactor, with a final stage 50% cut-off particle size of 0.5 µm, was used downstream of the furnace to remove the large fly ash particles. After leaving the impactor, the flue gas was diluted again by a secondary dilution nitrogen flow to reduce the particle number concentrations to acceptable levels for particle

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Zhuang and Biswas to measure the corona current and the applied voltage. Particle size distributions were measured at the inlet and the outlet of the ESP by the SMPS to obtain the collection efficiencies as a function of particle size. The operating conditions are listed in Table 1. The vapor-phase sorbent injection methodology has been demonstrated to be effective in suppressing the formation of trace metallic submicrometer particles.3,21 Herein, we examined its effectiveness in the coal combustion system. Titania precursor (titanium, isopropoxide, 97% Ti[OCH(CH3)2]4, Aldrich), placed in a water bath at a temperature of 371 K, was introduced into the combustion system by bubbling air through the precursor solution. In the combustion environment, the titania precursor was converted into agglomerated TiO2 particles, providing a very high surface area for heterogeneous condensation of trace species vapor. The particle size distributions were measured by the SMPS with and without the presence of vapor-phase sorbent to establish its effect in a coal combustion environment.

Table 1. Combustor and ESP Operating Conditions Combustor Operating Conditions coal feeding rate 0.85 g/h flow temperature

1.9 lpm

2.8 lpm

3.9 lpm

973 K 1073 K 1123 K 1173 K

x x x x

x

x

ESP Operating Conditions length of ESP (L) radius of collection surface of ESP (R) radius of corona wire (r0) pressure applied voltage (V) flow rate (LPM) temperature (T)

0.15 m 0.015 m 5 × 10-4 m 1 atm 9.5 kv 4 320 K

size distribution measurements by the Scanning Mobility Particle Sizer (SMPS). A filter assembly was also installed after the impactor to capture all the fly ash particles smaller than 0.5 µm. The combustion experiments were first performed at different temperatures, ranging from 973 to 1173 K, to study the effect of temperature on the fate of the submicrometer aerosols from pulverized coal combustion in the existing system. Second, a preheated particle-free air stream was introduced into the combustor (without influencing the coal feed rate) to alter the total air flow rate in the furnace to vary the residence time of coal particles in the reactor. The total air-flow rate through the reactor used in the experiments was varied from 1.9 to 3.8 lpm, resulting in a variation of the residence time from 4 s to 2 s. The experimental parameters used in the experiments are listed in Table 1. A cylindrical electrostatic precipitator was installed downstream of the impactor to investigate capture characteristics of the submicrometer ash particles. DC voltage (0 to 10 kv) was applied to the central discharge electrode wire in the ESP, and the outside collection surface was grounded. Details of the ESP system have been discussed in an earlier paper.20 A microammeter and a high voltage probe were connected to the ESP

Results and Discussions Ash Analysis. The elemental concentrations of metallic species in raw coal were measured by atomic absorption spectroscopy (AA) and proton X-ray emission spectroscopy (PIXE), and the results are presented in Table 2. The data provided by the supplier of the coal samples (McDermott Inc.) are also included for comparison. The measured elemental concentrations of the metallic species in the total ash (collected at the bottom of the furnace) and the submicrometer ash (collected after the impactor) are also shown in Table 2 as a function of temperature. The samples collected for a furnace temperature of 1073 K were reddish in color, indicative of a high concentration of iron oxide in the form of Fe2O3, while samples collected at 973 K were gray, suggesting the presence of carbon in the ash. Most of the elements in the total ash had higher concentrations at the higher temperature of 1073 K due to

Table 2. Composition of Feed Coal, Total Ash, and Submicrometer Fly Ash (Dp < 0.5 µM) feed coal (ppm based on coal)

aluminum silicon sulfur potassium calcium titanium copper zinc iron bromine strontium magnesium sodium antimony arsenic barium beryllium cadmium chromium cobalt Manganese mercury nickel selenium lead

total ash (ppm based on total ash)

PIXE

AA

McDermott

(50 µm)

(50 µm)

(50 µm)

602.3 1300.5 7024 192 529 53 14 6.02 3365 60 ND ND ND

990 NA Na 160 790 ND 9.1 18 11000 ND ND 100 210

ND ND ND ND ND 36 ND ND ND ND ND ND

ND 4.9 11 1.8 0.5 6.5 2.7 14 ND 6.7 ND 1.7

NA NA 13800 NA NA NA NA NA NA NA NA NA NA 0.2 7.97 17.46 3.2 0.19 19.61 5.04 16.1 0.26 15.72 2.57 6.26

submicrometer fly ash (ppm based on submicron ash)

PIXE

PIXE

AA

973 K

1073 LK

973 K

1073 K

1073 K

Macro Elements 4888.4 6910.8 2004.4 547.4 742.2 269.5 15.7 14.9 6320.7 ND ND ND ND

24083 66090 12362 12483 15217 5878 472 286 200712 37 651 ND ND

60873 202593 360252 6940 11997 5111 275.4 ND 72026 ND ND ND ND

58119 73907 118952 8574 13504 4976 ND 514 52196 ND ND ND ND

63000 NA NA 6666 12592 3259 ND 481 61481 ND ND 2592 3592

Trace Elements ND ND ND ND ND 11.31 ND ND ND ND ND ND

ND ND ND ND ND 188 ND 120 ND 57 ND 408

ND ND ND ND ND 112.8 ND ND ND ND ND ND

ND ND ND ND ND 1048 ND ND ND ND ND ND

ND ND 333 ND ND 985 ND 1111 ND 148 ND 74



preventing further vaporization (iron and silicon), whereas the other elements (i.e., zinc, chromium, manganese, and nickel) vaporize at a faster rate at the higher temperature. Elements such as aluminum, calcium, potassium, and titanium were equally distributed in the submicrometer and the total fraction of ash at both furnace temperatures. Submicrometer Particle Formation. The submicrometer particles are formed by nucleation of species that are formed by chemical reactions, and they grow by condensation and coagulation. The relative importance of these mechanisms can be estimated by comparing the characteristic times:3

τN )

Figure 2. Enrichment factor of elemental species in submicrometer-sized ash particles at two temperatures.

completed combustion. However, these elements showed different fates with the increasing temperatures in the submicrometer ash: the concentrations of silicon and iron decreased from 20.2% and 7.2% at the temperature of 973 K to 7.39% and 5.72% at the temperature of 1073 K, respectively. The elements aluminum, calcium, and potassium were almost at the same levels at the two different temperatures. Chromium increased from 112.8 to 1048 ppm on increasing the temperature from 973 to 1073 K. The elements arsenic, beryllium, cadmium, and cobalt were not detected both in the total ash and the submicrometer ash, even though they were present in the raw coal. All the observations support the hypothesis that the submicrometer ash particles are generated by nucleation of trace species vapor, a different mechanism compared to the coalescence mechanism for large particles. To evaluate the enrichment of various elements in the submicrometer ash, the concentrations were normalized by the concentration of a selected reference element (Al in this work, similar to Linak and Peterson22). The normalized concentrations were then divided by the corresponding normalized values in total ash to yield an enrichment factor, EF:

EF ) (X/Al)submicrometer/(X/Al)total

(1)

where (X/Al)submicrometer is the ratio of the concentration of an element X to that of Al in the submicrometer ash, and (X/Al)total is the same ratio for the total ash. The calculated EF values for several elements are shown in Figure 2. Silicon and iron were enriched in the submicrometer ash at 973 K relative to that at 1073 K. This was due to a reduced vaporization rate of iron and silicon because of the formation of a lower volatility glassy-type complex at the higher temperature, as also reported by Bool III et al.23 An increase in temperature, however, resulted in significant enrichments of zinc, chromium, manganese, and nickel in the submicrometer ash. Most likely these species are associated with the organic matrix24 in contrast to that of silicon and iron which are present as mineral inclusions dispersed in raw coal. During the combustion process, the mineral inclusions coalesce to form a glassy-type complex,

τC )

(C*)t)0 It)0

1 6Naνν1Cφ x2kBT/m1 πN

(

τcoag ) τxn )

)

2/3

(2) Nxπ

2 βNtot [C]i Ri

where τN, τc, τcoag, and τxn are the characteristic times for nucleation, condensation, coagulation, and chemical reaction, respectively. C* (cm-3) is the critical number concentration and is dependent on physical properties of the species; It)0 (cm-3 s-1) is the initial nucleation rate of the species and is dependent on the initial saturation ratio of the species in the present system; kB is the Boltzmann constant; T is the temperature (K); m1 is the molecular mass of the condensing species (kg); v1 is the molecular volume of the condensing species (cm3); Cφ is an equivalent initial vapor concentration (cm-3); β is the coagulation coefficient between particles (cm3 s-1); and Ntot is the initial total particle number concentration (cm-3). Due to the high temperatures in the combustion environment, chemical reactions are typically fast and a comparison of the nucleation, condensation and coagulation characteristic times is presented. The characteristic times are therefore estimated for iron oxide, a major component in the submicrometer ash particles. The physical property values for iron oxide at a combustion temperature of 1100 K were obtained from the CRC Handbook of Chemistry and Physics.25 By assuming that iron oxide in the submicrometer ash particles is a result of nucleation of iron oxide vapor, the characteristic times are τN ) 0.01 s, τc ) 0.5 s, and τcoag ) 5.4 s for this experimental system. The calculations indicate that nucleation occurs initially resulting in the formation of ultra-fine particles which then rapidly grow to submicrometer sizes by heterogeneous condensation. The measured particle size distributions of the submicrometer ash at the three temperatures are plotted in Figure 3a. The geometric mean particle size ranges (from 63 nm to 84 nm) are consistent with a vaporization-nucleation-condensation formation and growth mechanism.13 As the temperature is increased from 1073 K to 1123 K, the geometric mean particle size

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Zhuang and Biswas

Figure 4. SEM picture of the submicrometer particles collected at the exit of the coal combustor.

Figure 3. (a) Submicrometer particle size distributions at three different temperatures for a furnace residence time of 2 s. (b) Submicrometer particle size distributions at two different residence times for a furnace temperature of 1173 K. The geometric mean diameter increased from 76 nm (3 s residence time) to 100 nm (4 s residence time).

shifts from 63 nm to 84 nm due to enhanced vaporization rates of metallic species followed by enhanced condensation rates. However, on further increasing the temperature to 1173 K, the geometric mean particle size reduced to 72.2 nm along with a reduction in the total particle number concentration. This is attributed to the decreased vaporization rates due to the formation of glassy structures (iron silicates) in the burning coal particles, and a similar explanation has also been reported by Bool III et al.23 The submicrometer particle size distributions were measured for different flow rates of 1.9 lpm and 2.8 lpm, with corresponding residence times of 4 s and 3 s, respectively. The furnace wall temperature was maintained at 1173 K, high enough to ensure complete combustion. The experimental data are plotted in Figure 3b. The geometric mean particle size decreased from 110 nm at 1.9 lpm (4 s residence time) to 85 nm at 2.8 lpm (3 s residence time) due to the reduced time available for condensation and coagulation growth processes. The SEM picture shown in Figure 4 indicates that particles smaller than 1 µm are spherical, supporting a nucleation and condensation mechanism for the formation and growth of submicrometer particles. Capture of Submicrometer Ash Particles in an Electrostatic Precipitator. The concentrations of chromium, lead, nickel, manganese in the submicrometer ash were 1016.2 ppm, 74.0, 148.0, and 1111.0 ppm, respectively, higher than their concentrations in raw coal: 20.7 ppm, 4.0, 11.2, and 15.0 ppm, respectively.

Figure 5. Collection efficiencies of submicrometer ash particles as a function of diameter. The dashed line is a fit to the experimental data.

The particle control devices have a minima in the collection efficiency in the submicrometer size range.4,20 Markowski et al.1 demonstrated that the penetration of submicrometer particles was 30 times the overall penetration in a full scale ESP. In the present study, the collection efficiencies determined from the measured size distributions at the inlet and the outlet of the ESP are plotted in Figure 5 as a function of particle size. The collection efficiency decreases for particles smaller than 100 nm as shown in the Figure 5. This is in contrast to the predictions of existing theories that the decreasing charge on the fine-sized particles is offset by the increased mobility, resulting in increased capture efficiencies. The average charge on the particles exiting the electrostatic precipitator was measured by a combination of an electrometer and a condensation particle counter (Figure 6). The submicrometer-sized particles had an average charge less than 1, indicating that a large fraction of particles is not charged. This is due to a partial charging type phenomena and this concept has



Figure 6. Average charge on the submicrometer particles at the outlet of the ESP. The dashed line is a fit of the data points.

been studied experimentally and theoretically,20 and explains the observed trends in Figure 5. Effectiveness of Vapor-Phase Sorbent Injection on Suppression of Submicrometer Mode. Previous controlled studies3,26 have shown that in-situ generated sorbent particles can effectively suppress formation of submicrometer toxic particles such as lead oxide. The effect of an in-situ generated titania sorbent agglomerate on coal fly ash distribution was established (Figure 7). For coal combustion alone, a clear submicrometer mode with a geometric mean diameter of 49.9 nm, a geometric standard deviation, σg ) 1.49, and a total number concentration of 1.94 × 106 #/cm3 was observed. Two modes of injection of the vapor phase sorbent precursor were used: one, wherein the titanium isopropoxide vapors were premixed with the coal particles before it entered the high-temperature furnace, and second, wherein the titanium isopropoxide was first oxidized and the resultant titanium dioxide agglomerates mixed with the coal particles. The TiO2 particles had a larger particle size: geometric mean diameter of 100.9 nm with a broader size distribution, σg ) 1.72, and a total number concentration of 2.87 × 106 #/cm3 (Figure 7a). On injection of the titania precursor and the raw coal into the reactor together, the particle size distribution was significantly different from that of the coal combustion alone. The geometric mean diameter increased to 96.9 nm (compared to the 49.9 nm for coal combustion alone). The geometric standard deviation was higher: σg ) 1.84, and the total particle number concentration was slightly lower: 1.48 × 106 #/cm3. The concentrations of particles smaller than 100 nm were reduced as the ash vapors were scavenged by the agglomerated titania particles prior to their undergoing homogeneous nucleation. This is also supported by the shift in the size distribution of the resultant coal ash particles to larger sizes (shift to the right, Figure 7a). It should be noted that there is a complex interaction between the titania sorbent precursor and the combusting coal particles, resulting in an alteration of the resultant size distributions. A possible explanation could also be the coagulation of the titania particles with the large mode of the coal fly ash particles, moving them out of the measurement range of the SMPS. The second experiment was conducted by first oxidizing the vapor phase precursor to form agglomerated

Figure 7. (a) Resultant size distributions of particles at the exit of the combustor for a furnace temperature of 1173 K, and residence time of 4 s for injection of the vapor-phase titanium isopropoxide precursor in conjunction with the coal particles. (a) Resultant size distributions of particles at the exit of the combustor for a furnace temperature of 1173 K, and residence time of 4 s. The vapor-phase titanium isopropoxide precursor was injected into a furnace resulting in the formation of agglomerated titanium dioxide particles which were then mixed with the coal particles and introduced into the coal combustion furnace.

titanium dioxide particles (done in a separate furnace). These agglomerated particles consisting of primary particles in the nanometer size ranges was mixed with the coal feed and introduced into the combustor. The results are shown in Figure 7b, and the geometric mean diameter increased from 53 to 214 nm. This injection method was more effective at suppression of the formation of the nucleation mode of the coal combustion aerosol. Clearly, more tests have to be performed in pilot scale systems to establish the optimal injection location for the vapor-phase sorbent precursor. Such studies would help optimize the conditions and parameters for effectively reducing the resultant submicrometer mode of the combustion aerosol. Conclusions The size distributions and the elemental compositions of fly ash particles from pulverized coal combustion have been measured at different operating conditions. A submicrometer mode was observed, indicating a vapor-

516


ization-nucleation-condensation mechanism for formation of the submicrometer ash during coal combustion. The modes of occurrence of the elements in raw coal affect the formation of the submicrometer particles. This resulted in a decreasing enrichment of iron and silicon with increasing temperature, whereas the trace species associated with the volatile fraction of the coal matrix were significantly enriched in the submicrometer sizes. Capture efficiencies of the submicrometer particles below 100 nm decreased with decreasing particle size. This was readily explained by a partial charging type phenomena for submicrometer-sized coal fly ash particles. An in-situ vapor phase sorbent precursor injection

Zhuang and Biswas

methodology was demonstrated to decrease the nucleation rates and increase the resultant mean size of the coal combustion aerosol by promoting condensation on agglomerated sorbent particles. The injection conditions of the sorbent precursor are important, and need to be optimized for more effective capture of coal fly ash particles in conventional particulate control devices. Acknowledgment. This work was partially supported by a grant from the Ohio Coal Development Office, Grant OCRC-99-B4.7. EF000080S

Submicrometer Particle Formation and Control in a Bench-Scale

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