Quench-induced nucleation of ash constituents during combustion of pulverized coal in a laboratory furnace

Twenty-Second Symposium(International)on Combustion/The CombustionInstitute, 1988/pp. 239-247

QUENCH-INDUCED NUCLEATION OF ASH CONSTITUENTS DURING COMBUSTION OF PULVERIZED COAL IN A LABORATORY FURNACE MARK V. SCOTTO, ERIC A. BASSHAM. JOST O. L. WENDT AND THOMAS W. PETERSON

Department of Chemical Engineering University of Arizona Tucson, AZ 85721 The hypothesis that nucleation of ash constituents in practical coal combustors can be influenced by the temperature quench rate was tested in a laboratory 2-5 kg/h pulverized coal furnace. Two Beulah lignite coals, similar in heating value and overall ash content but differing in the sodium content, were tested. Ash samples were withdrawn at various residence times down the combustor, segregated by size, and analyzed for their bulk, surface and interior composition. Increasing temperature quench rate beyond 400 K/s significantly increased the fraction of the inlet sodium appearing in the small particle size range. This was because a sodium-rich "fimae'" was frozen in the particulate phase during temperature quench, although surface enrichment of the sodium on the larger particles was also realized under these conditions. Subtle differences in these otherwise similar coals appeared to exert a large influence on the primary mechanisms forming the small particles. Although the low sodium coal contained approximately half the sodium of the other, it yielded less than one tenth of the amount of sodium in the small particle size range. Furthermore, while results for one coal supported a silicon vaporization/nucleation mechanism governing formation of small particles, those for the other coal did not.

Introduction In high temperature, entrained flow combustors and gasifiers, tile vaporization temperature of many alkali and trace metals contained in the parent coal is usually surpassed within the first hundred milliseconds of the process. In most cases, however, the exit stream from the process is at temperatures well below the metal dewpoint, and the alkali and trace metals, volatilized previously, nucleate and condense to form tiny submicron particles which are difficult and expensive to capture, Because these submicron particles can be enriched in toxic metals, they provide a mechanism through which human inhalation of those metals can occur. While the vaporization temperatures for the most volatile species are almost certainly exceeded under combustion conditions, the fraction of "volatilizable'" species that do in fact vaporize depends on the type of coal (or, more specifically, the type of ash) being burned. One must conclude that the vaporization, nucleation and condensation phenomena referred to above, are strongly dependent on: (a) the manner in which the metals are originally bound in the parent particles, (b) the structure, surface area and composition of the solid residues present in the post flame, and most importantly (c) the time, temperature and concentration environment the coal par239

titles see during the entire gasification or combustion process. The inhibition of homogeneous nucleation by control of dT/dt, the temperature quench rate, has been hypothesized by Neville and Sarofim, 1 predicted qualitatively by Sherman et al. z and quantitatively by MeNallan et al.a The latter indicate that if dT/dt can be reduced to less than 400 K/S new particle formation via nucleation can be reduced. Other researchers 4 imply that control of peak temperature might prevent nuclei-producing fragmentation processes. There is general agreement that T~a~ and dT/dt are critical variables in determining the number of nuclei formed. The enrichment of trace elements and alkali metals on the surface of submicron aerosols has been well established. 5'6'7"s The actual mechanism for the formation of submicron "nuclei" particles on which trace metals condense appears to be a matter in dispute, at least as far as phenomena in actual coal fired power plants is concerned. Smith4 reports data in which the ratio of concentration of aluminum in the smaller (0.1-3 p~m) fly ash to that in the larger (22-74 p,m) fly ash is of order one. Holve9 suggests that small ash particles exist loosely in the parent coal. Quann and Sarofim, 1° on the other hand, provide data from their well-defined drop tube experiment which shows the exact opposite, namely that

240

COAL COMBUSTION: LABORATORY-SCALE COMBUSTION

the ratio of aluminum on the small ash to that in the large was only a tiny fraction, less than 0.1. Smith concludes that ash fragmentation must account for the presence of most small particles in the size range over which trace element concentrations do not vary with size (less than 1 txm). Sarofim and co-workers1'6"7"&a° conclude that vaporization of reduced forms of Si and Mg within the char particle, followed by diffusion to the outside, subsequent oxidation in the boundary layer and nucleation of the MgO and possibly SiOz in the oxidizing regions accounts for the presence of the small particles. Previous work at this laboratory has used a 2-5 kg/h downfired combustor that simulates, in a controlled manner, combustion events as they can be made to occur in full scale units. These studies H'lz examined the effects of staged combustion on the formation of submicron aerosols during pulverized coal combustion, and showed that for bituminous coals the effect of staged combustion on particle formation might be slight, while for lignites, staged combustion tended to increase the fraction of submicron aerosols, although significant differences between two superficially similar lignites were observed. These data are based on a change of first stage stoichiometry only, without control of the temperature profile. When temperatures in the combustion of a high Na lignite were increased through oxygen enrichment, a significant increase in the submicron mass was measured, indicating the practical importance of that variable. Such behavior poses the following questions: 1) For similarly ranked coals of different ash content, is it possible that complete vaporization of Na can occur in one coal and not the other? 2) If certain volatile species are vaporized regardless of combustion temperature (within certain ranges), can those volatile species be prematurely "forced out" of the vapor phase by rapid temperature quenching? In order to address these questions we now focus on one aspect of the environment that the coal particles see during combustion, namely the time-temperature history. Realizing that gas to particle conversion is a rate process dependent on the driving force of species saturation, the prospect of controlling the relative magnitudes of nucleation and condensation is studied. To the extent possible, coal composition is kept constant, although the effect of changes in coal composition are considered.

Experimental

System

Downfired Combustor: A laboratory combustor shown on Fig. 1 was designed to have the following attributes:

Pilot Gas

Cool r - j -Exhaust

SamplePorts ~ 4 upperlevel tOlowerlevel 14 total

~

1 1, Y;'~ I

[.:"

I

1

Middle Floor

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FIG. 1. Laboratory combustor.

1) It allowed self sustaining combustion to occur, with no external sources of heat required to maintain flame stability. This facet ensured that particle densities in the combustion zone are similar to those in industrial practice. 2) It allowed combustion of a wide variety of coals at temperatures and times representative of full scale units. 3) It was sufficiently large to allow for particle/particle and particle/gas interactions. 4) It was sufficiently small to allow for construction at reasonable cost. 5) It was versatile and enabled control of combustion parameters. 6) It was aerodynamically well defined to allow the extraction of rates and mechanisms. 7) It minimized particle deposition from the flue gas over long residence time spans. The final design consisted of an insulated vertical downflow firetube, (0.15 m I.D., 6 m long), with a thermal rating of 17 kW, corresponding to a feed rate of approximately 2.2 kg/h. Gas phase and particle residence times were of the order of six to twelve seconds, in order to simulate the passage of fuel and air components all the way from the burner to the air preheater in a practical furnace, and in

QUENCH-INDUCED NUCLEATION order to obtain adequate time resolved measurements describing the evolution of the ash aerosol over that period. Heat losses were minimized since the firetube wall consisted of overlapping cylindrical tubes of vacuum formed alumina, which were further insulated by a wrapped Kaowool blanket. This firetube, which extended over two floors, . contained 14 sample/access ports from which gas temperatures were measured and gas and particle samples were extracted. The "burner" at the top allowed fuel and air to be premixed prior to ignition, and allowed subsequent events to occur in an approximately one dimensional streamline mode.

Sampling System: Particle sampling is performed using an aspirated isokinetic probe, which can be used to extract samples from any of the sampling ports throughout the furnace. The probe consists of the following components: 1) a water jacketed "delivery" system for the dilution air; 2) an aspiration-dilution system, whereby the dilution air is injected into the inner return tube, creating a low-pressure region which draws particle-laden sample from the furnace into the probe; 3) a water jacketed "return" system for the diluted sample. This probe was designed to provide a representative sample over all particle size ranges. The sample is rapidly quenched by an adjustable free turbulent jet which provides a high mixing rate, (approximately 10 to 1, depending on the inlet flow of the dilution air), fairly rapid dilution and a temperature quench of about 106 K/s. Plugging is avoided by free stream mixing. Nitric oxide measurements before and after dilution, together with the known flow of dilution air, were used to ensure that the sampling rate through the probe mouth was as close to isokinetic as possible. The samples drawn from the furnace through the particle probe are collected for chemical analysis in a particle sampling system. The purpose of this system is to provide representative samples in a range of particle sizes, so that chemical analyses can be performed on samples in each size range. The particle segregation and collection system is shown schematically in Fig. 2. Approximately 21 SLPM total flow (including both the aerosol sample extracted from the furnace and the dilution air) exists the probe. The diluted samples extracted from the furnace are distributed to various collection media as follows: 1) Approximately 7.5 SLPM is pulled through a 47 mm diameter 0.2 txm pore size Nucleopore polycarbonate filter. These collected particles can then be chemically analyzed, by means outlined below. 2) Approximately 13 SLPM is pulled through a cyclone built in this laboratory following John and

24

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,

,

-~

c ......

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If;

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FIG. 2. Particle segregation and collection sys tem. Reischl. la At this flow rate, the 50% cut-off di ameter is approximately 5 Ixm. Those particle larger than 5 txm are deposited in the botton cup of the cyclone (the cyclone "underflow," UFI while the smaller particles are carried throug'~ the primary outlet ("overflow") of the cyclone The cyclone overflow is further divided as fo] lows:

- Approximately 1.5 SLPM is pulled through 47 mm filter holder supporting a Nucleopor filter. This sample is referred to as the cyclon, overflow (OF). - Approximately 2 SLPM is pulled through single-jet cascade impactor which is capable c providing up to 11 discrete cuts in the siz range 0.05 to 10 txm. Usually, this impacto is used only for the purpose of collecting sam ples on the impactor after-filter (IAF) whicl includes all particles less than about 1 ~m. - Approximately 10 SLPM is shunted around th entire collection system, and is vented thougl exhaust lines. This bypass flow provides means for damping any flow fluctuations tha may occur in the overall sampling system, an, insures that relatively constant gas flows ar sent to the actual sample collection device (such as the cyclone, the total filter and th cascade impactor).

Chemical Analysis: Two analytical tools are used to determine th chemical composition of the ash particles collected First, atomic absorption spectroscopy is used to de termine the elemental chemical composition of a~ "'ensemble" of particles collected on filters. Second Scanning Auger Microscopy is used to determin detailed elemental analysis of selected individu~ particles.

Bulk Analysis: Bulk chemical analysis by atomic absorption (AA is performed on the following types of samples: te tal filter samples (TF), collected on 47 mm 0.2 Ixl~

242

COAL COMBUSTION: LABORATORY-SCALE COMBUSTION

pore size Nucleopore polycarbonate filters; cyclone undertow (CU) samples, collected from the bottom cup of the cyclone; cyclone overflow (CO) samples, collected on Nucleopore filters from the overflow stream of the cyclone; and impactor after-filter (1AF) samples, collected on Nucleopore filters. The major species examined by the atomic absorption technique for this work were AI, Si, Na, K, Ca, Mg and Fe.

Surface Analysis: IAF samples comprised primarily of submicron fly ash aerosol, were taken from two ports along the furnace. They have been scrutinized using several of the analytical techniques provided by a PHI 600 Auger Electron Microprobe. These include Auger Electron Spectroscopy (AES) surveys, profiles, and maps which provide elemental and some chemical information on the sample. Special procedures have been developed to treat filters on which fly ash aerosol samples are collected and situated for Auger spectrometry. Auger analysis of fly ash particles is difficult because of excessive charging. A carbon coating on silver membrane paper improved the visualization of fly ash particles and fume at high magnification. It also alleviated interferences caused by chlorine present as an impurity in the silver paper, and by the secondary silver peak at 302 ev, which can interfere with the Ca peak at 292 ev. In all cases, the aerosol density on the filter was kept low.

Coal Compositions: Two coals were examined in this work: a Beulah High Sodium Lignite and a Beulah Low Sodium Lignite. Parent coal and ash compositions are shown on Table I. Note that the high sodium coal contained slightly less than twice the sodium of the other, although in other respects these coals were quite similar.

Results

Both coals were burned at a stoichiometric ratio of 1.2 under unquenched and quenched conditions. Quenching was achieved by the insertion of cooling coils into Port 3, that is, shortly beforer 1 second residence time. For the high sodium lignite the unquenched and quenched runs are denoted as Runs 42 and 43, while for the low sodium coal they are labelled Runs 46 and 46B respectively. Resulting temperature profiles are shown on Fig. 3.

High Sodium Beulah Lignite: For Run 42, samples were withdrawn at Ports 4 (l.2 s), 6 (3.18 s), 8 (4.11 s) and 14 (7.5 s). Repli-

TABLE I Chemical analysis of coals considered Beulah Lignite Coal Proximate (wt %) Fixed C Volatiles Ash Moisture Ultimate (wt %) C H

N S O Ash HHV (BTU/lb) Oxide ash analysis (wt %) Si AI Fe Ca Mg Na K

S

Low Na

High Na

37.2 37.1 8.0 17.8

33.9 36.4 7.3 22.5

57.8 4.0 0.9 2.1 25.9 9.4

59.8 3.9 1.1 1.0 24.8 9.4

9,330

28.4 5,7 12.9 17.7 6.4 4.4 0.2 24.0

9,620

17.6 14.5 7.6 19.2 4.8 8.5 0.2 26.6

cates were drawn from Port 4. For each sample, particles were segregated into the following size fractions: TF, CU, CO, and IAF, as described in a previous section. Each size cut was then analyzed for its bulk composition by Atomic Absorption. Additional filter samples were reserved for individual particle surface analysis by Auger Microsopy. For the bulk analyses, precision between replicates was not adequate to discern a statistically significant time evolution in ash composition as measured from port to port. Therefore, results for each size class are averaged over all four ports as shown on Fig. 4a. It should be emphasized, however, that differences between TF, CU, CO and IAF compositions were significant for all samples, and are properly represented by the averaged values shown on Fig. 4a. Fig. 4a clearly shows enrichment of sodium on the particles collected on the IAF. Furthermore, aluminum is almost completely depleted for these small particles, while silicon is greatly enriched. These results are significant, since they suggest that the small particles do not arise from simple fragmentation of homogeneous bulk ash. Whatever the mechanism, the fate of silicon has a very great influence on the number and mass of small particles around which sodium, which certainly vaporizes, can condense or coagulate. Therefore, in order to

QUENCH-INDUCED NUCLEATION A) HIGH SODIUM BEULAH LIGNITE

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w~¢ (SECONDS) FI(;. 3. Measured t e m p e r a t u r e profiles (without correction) for quenched and unquenched combustion, Beulah low-sodium and high-sodium lignite coals.

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FIG. 4. Fraction of major species, as oxide, in TF, CU, CO and IAF, frmn combustion of high-sodium Beulah lignite coal, Data averaged over ports 4, 6, 8 and 14, (a) Unquenehed and (b) quenched combustion conditions.

243

determine the effects of combustion and coal composition variables oll the fate of alkali metals, one must also address the fate of silicon and other species in the ash. For Run 43 (quenched), samples were withdrawn at 1.28 s, 3.94 s, 4.66 s, and 8.35 s residence times. Replicate samples were taken at Port 4 (1.28 s). Resuits from all ports are averaged and shown on Fig. 4b. Again, the IAF samples are greatly enriched in sodium, greatly depleted in ahnninum, and also greatly enriched in silicon. These results are consistent with those of Run 42 (tmquenehed). Foeusing on the IAF samples only, it is clear that temperature quenching significantly increases the fraction of sodium on the small particles. Furthermore, when concentrations in the flue gas are compared (Fig. 5) it is clear that for this coal, the effect of a rapid temperature queuch is to increase the sodium mass concentration in the flue gas appearing in the small particle size fraction. Indeed, the amount of sodium in this range has increased by a factor of 2-1/2 and the increase in the total mass of small particles due to quenching is entirely due to the increase in sodium in that range. A Scanning Electron Micrograph of ash aerosol on the IAF, collected at Port 4 in the quenched case, provided strong evidence that a "fume" was formed either in the combnstor or in the sampling probe. Subsequent passage of the sample through the probe system did not coagulate the fim~c so substantially with existing aerosol particles that all evidence of it had disappeared by the time it was trapped on the impactor after filter. This fume is estimated to have particle sizes of less than 0.2 micron, Also present are larger particles of approximately 1 micron diameter. Thus the IAF contains at least two groups of particles, and so each group was examined separately in more detail. Fig. 6a shows an Auger area scan of the fiune aerosol on carbon coated silver membrane filters. It is clear that on the surface at least, the fimle consists of primarily sodium, sulfi)r and oxygen, suggesting that the fume may be primarily sodium sulfate. Neither severe depth profiling using the ion gun, nor gentle depth profiling using the electron beam definitively revealed any silicon or other key aerosol element below the fume particle surface. Therefore, the large amounts of silicon observed from the AA bulk analysis must reside in the larger 1 micron particles, which were still small enough to pass through the impaetor. Auger analysis of the larger 1 micron particles revealed that these also could be subdivided into two classes, namely those that contained both silieon and aluminum and those that contained only silicon (among the major species). The presence of any aluminum in some of the particles is at variance with the AA bulk analysis, which showed aluminum to be present in very small quantities. The surface composition of a 1 micron particle is

COAL COMBUSTION: LABORATORY-SCALE COMBUSTION

244 7

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FIG. 6. Auger analysis of aerosol collected on IAF during combustion of high-sodium Beulah lignite coal. Quenched combustion conditions, (a) fume, (b) surface analysis of 1 Ixm particle and (c) Auger intensity normalized by the original surface intensity as a function of sputter time for the same 1 Ixm particle. Legend: © Na, ~ S, + Si, /k Ca. be due to coagulated sodium particles rather than condensed sodium vapor, although this assertion is not definitive. Similar results were also found for those 1 micron particles which contained aluminum. An IAF sample from Port 6 yielded very similar results. Again, there is a fume, containing primarily sodium, sulfur, and some chlorine, and some larger particles which were similar to those analyzed from Port 4. One would not expect sodium vapor to exist at Port 6 (some time after cooling), and so it appears that the small particle fume exists in the combustor itself, This is significant, because it allows the inference that quenching for the high sodium Beulah Lignite allows sodium to appear in copious amounts of very small fume particles. This is consistent with the hypothesis proposed in a previous section of this paper, and with the bulk AA analysis shown on Fig. 5.

Low Sodium Beulah Lignite:

~ 0.5 :

=
CeO

=

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'-

Si02

20C

"O

~

~C

0

30 60 90 120 150 Sputter Time (seconds)

180

FIG. 5. Submieron particle concentration in flue gas, as measured on IAF, under unquenched and quenched combustion conditions.

shown on Fig. 6b. Note the presence of sodium, silicon, and the absence of aluminum. The concentrations of Na, S and Si as functions of time of ion milling (which is related to depth from the original surface) is shown on Fig. 6c. It is clear that Na and S signal intensities rapidly decrease as the surface of the particle is sputtered away while Si decreases only slightly. Since Na and S were sputtered away relatively quickly, the surface enrichment layer may

Two runs are reported for this coal. Note from Fig. 2b that Runs 46 and 46B have essentially the same peak temperature, but quite different temperature quench rates, even though the exhaust conditions are similar. Samples were withdrawn at Port 4 (1.45 s and 1.55 s for Runs 46 and 46B respectively), Port 6 (3.81 s, 4.31 s), Port 8 (4.94 s, 4.31 s) and Port 14 (8.85 s, 9.81 s). Replicate samples were withdrawn from Port 4, and, as before, 'although signflcant differences in composition among various size classes always occurred, variations between replicates was greater than variation between ports. Therefore, values for each size class are averaged over all ports. The total quantity of aerosol collected on the IAF was very much less than the amount collected over a similar time span for the High Sodium Beulah. Therefore, the aluminum values are close to the detection limit of AA, and should be viewed ap-

QUENCH-INDUCED NUCLEATION propriately. Fig. 7 shows the fraction oxide values for particles gathered on the TF, CU, CO and IAF, each averaged over all ports. Comparison of unquenched and quenched cases show bulk ash compositions that are very similar. In each case some aluminum and appreciable calcium appear in the IAF fraction. That fraction is significantly enriched in sodium, but not so in silicon, which distinguishes these results from those obtained for the high sodium coal. It would appear it is the calcium that is behaving very differently. For the High Sodium Beulah, calcium appeared in the CO fraction but not significantly in the IAF fraction while in the Low Sodium Beulah it was equally concentrated in both. Furthermore, appreciable amounts of iron were found among the small particles for this coal, but not for the high sodium lignite. Comparison of unquenched and quenched results also show greater enrichment of the IAF particles in sodium for the quenched case. This is consistent with the hypothesis presented previously and with the data for the high sodium coal. Taken as a whole, these data suggest that fragmentation plays a dominant role in the creation of IAF captured particles for this coal, and this is a very different mechanism from that suggested for the High Sodium coal data. It would appear, therefore, that A) Run 46 Low No Beuloh Lignite Unquenched 12o"I.. ~00~'

No

t BOY,,

/W Co I

40'7,,

I

Si g.'l//////~

2OR,

Fe :::::::::::::: 'IF

03

CO

7~F

Size Froction

B) Run 46b Low N(~ Beuloh Lignite Quenched 120g 100~.

I

6ON.

I

Co I

40g

Og

•

Si

"iF

OJ

CO

~

ii:::::::::::: K

Size Fmetk~n

FIG. 7. Fraction of major species, as oxide, in TF, CU, CO and IAF, from combustion of low-sodium Beulah lignite coal. Data averaged over ports 4, 6, 8 and 14. (a) Unquenched and (b) quenched combustion conditions.

245

mechanisms governing the amount of small particles do depend on both temperature quench rate and coal constitution. This is important frmn a practical point of view, since the fate of sodimn is coupled with the fate of all the uther elements constituting the small particles. Because the actual amount of IAF particles collected was small (less than 10% of that collected tot the high sodium coal) precision in the data was insufficient to quantify the effect uf temperature quench on the total m a s s of small particles for this coal. Discussion and Conclusions Data clearly show that for both coals, tinder similar sampling conditions, the enrichment of sodium in the small particles increases as the combustur temperature quench rate is increased, even though the peak temperature may not vary. For the High Sodium Beulah lignite, a change o[~ (gross) temperature quench rate from 170 K/s to 420 K/s increased, by a factor of 2.5, the total amount of sodium present as a "fume" or attached to small particles in the flue gas. This agrees with both the qualitative and quantitative predictions of Sherman et al. 2 and MeNallan et al. 3 respectively, and demonstrates the applicability of their conclusions to practical coal combustion systems. Coal composition effects are exceedingly complex. Although the low sodium lignite contained approximately half the sodium of the high sodium lignite, it produced less than one tenth of the amount of sodium in the small particle size range. Given the superficial similarity between the two coals, differences in the ash composition were great, especially as regards the depletion (or lack of depletion) of aluminum in the small particles. Ash compositions from the high sodium lignite would support the silicon vaporization and condensation mechanism of Sarofim and eoworkers. Analogous data from the low sodium coal would not. It is unlikely that the 100 K peak temperature difference between the high and low sodium coal could account for all this. Clearly, a detailed knowledge of the parent coal ash structure is required before these large eft'cots of coal composition on ash formation mechanisms can be predicted. Surface analyses of the ash from the high sodium coal suggested that the effect of temperature quench was to freeze in a "fume" consisting of sodium salts. This fume self-coagulated or coagulated and condensed around existing small particles, most of which contained primarily silicon and negligible aluminum. Quenching did not have a large effect on the composition in the interior of these particles. Partieles within the same size range and with similar morphologies may have very different composi-

246

COAL COMBUSTION: LABORATORY-SCALE COMBUSTION

tions, even for a single ash sample. Therefore, single particle analysis should be conducted in conjunction with an average bulk analysis in order to determine mechanisms governing average behavior. Acknowledgments

The authors would like to acknowledge the Department of Energy for their continued support of t h i s work, u n d e r M E T C c o n t r a c t D E - A C 2 1 84MC21386 and PETC contract DE-AC2286PC90751. REFERENCES 1. NEVILLE, M. AND SAROFIM, A. F., Nineteenth Symposium (International) on Combustion, p. 1441, The Combustion Institute, 1983. 2. SHERMAN, P. M., GLASS, D. R., POSTMAN, D. AND WASHBAUGH, P., Department of Energy Report No. PC30305-TS, Final Report on Grant DE-FG-22-PC-30305, PETC-DOE. (1982). 3. MCNALLAN, n . J., YUREK, G. J. AND ELLIOTI', J. F., Comb. Flame 42, 45-60, (1981). 4. SMITH, R. D., Prog. Energy Comb. Sci., 6, 53119 (1980).

5. DAVISON, R. L., NATUSCH, D. F. S., WALLACE, J. R. AND EVENS, C. A., Environ. Sci. Techn., 8, 1107-1113 (1974). 6. MIMS, C. A., NEVILLE, M., QUANN, R. J., HOUSE, K. & SAROFIM, A. F., AIChE Symposium Series No. 201, (76), 188-194, (1980). 7. NEVILLE, M., QUANN, R. J. HAYNES, G. S. AND SAROEIM, A. F., Eighteenth Symposium (International) on Combustion, p, 1267, The Combustion Institute, 1981. 8. QUANN, R. J., NEVILLE, M., JANGHORBANI, M., MIMS, C. A. AND SAROFIM, A. F., Environ. Sci. and Tech., 16, 776-781 (1982). 9. HOLVE D. J., In Situ Measurements of Flyash Formation from Pulverized Coal, Report No. SAND85-8683, Saudia Nat'l. Lab., Livermore CA, (1985). 10. QUANN, R. J. AND SAROFIM, A. F., Nineteenth Symposium (International) on Combustion, p. 1429, The Combustion Institute, 1983. 11. LINAK, W. P. AND PETERSON, T. W., Aerosol Sci. Tech., 3, 77-96, (1984). 12. LINAK, W. P. AND PETEI~SON, T. W., TwentyFirst Symposium (International) on Combustion, p. 399, The Combustion Institute, 1988. 13. JOHN, W. AND REISCnL, G., J. Air Poll. Cont. Assoc., 30(8), 872-876, (1980).

COMMENTS D. E. Rosner, Yale Univ., USA. Using seeded laminar flat flame techniques we have b e e n experimentally studying the nucleation of "'mixed" alkali sulfate vapors (e.g., Na2SO4 + K2804) in combustion gas thermal boundary layers (BLs) with equivalent quenching rates AT(Sm/2)/D) i between 103 and 104 K/s. Our principal objective is to detect and understand the effects of ruicrodroplet formation on alkali sulfate mass transfer rates across such BLs. Our results for initially undersaturated mainstreams ~ indicate that nucleation to form solution droplets is much more likely to occur than in single-salt systems. Also, even in the absence of s u s p e n d e d i n o r g a n i c p a r t i c u l a t e m a t t e r (e.g., MgO(s)), condensation was inferred to occur at much lower supersaturations (with respect to binary solution equilibrium vapor pressures) than those expected based on "'classical" binary homogeneous nucleation theory. It is also interesting to note that fine mist formation (Na2SO4, K2804, ...) in the BL is observed to reduce the rate of deposition of these salts (cf. vapor deposition) z on exposed tubes/surfaces.l This reduction will normally not be an order of magnitude effect (because of the compensatory mechanism of droplet thermophoresis, which over-

whelms microdroplet Bunncour (?) diffusion in the presence of wall heat transfer). Yet, because of its influence on sticking probabilities, 3 even a small reduction in alkali sulfate deposition rate can cause disproportionate reductions in the capture rates of impacting supermicron ash particles. The case of simultaneous vapor and microdroplet deposition from a mainstream which is locally saturated, also relevant to boiler generation, has recently been treated theoretically;4 total alkali sulfate deposition rates are again predicted to be less than those expected in the absence of the (mainstream) microdroplets. Are the alkali sulfate condensation data obtained/ analyzed using your downflow coal combustor facility consistent with (some of) these potentially important trends?

REFERENCES 1. LIANG, B., GOMEZ, A., CASTILLO, J., AND ROSNER, D. E., "Experimental Studies of Nucleation Phenomena Within Thermal Boundary Layers--Influence on Chemical Vapor Deposition Rate Processes," C h E Comun. (submitted 1988).

Q U E N C H - I N D U C E D NUCLEATION 2. L1ANG, B. AND BOSNER, D. E., "Laboratory Studies of Binary Salt CVD in Combustion Gas Environments, AIChE 33, No. 12, 1937-1948 (1987). 3. ROSNER, D. E. AND NAGARAJAN, R., "Toward a Mechanistic Theory of Net Deposit Growth from Ash-Laden Flowing Combustion Gases: SelfRegulated Sticking of Impacting Particles and Deposit Erosion in the Presence of Vapor 'Glue'," M C h E Symposium Series, 83, No. 257, pp. 289296 (1987). 4. CAST~LLO, J. AND ROSNER, D. E,, ChE Science (in press, 1988).

Author's Reply. The results you present on the heteromoleeular homogeneous nucleation of potassium and sodium sulfates are indeed important to the determination of salt deposition in combustion systems. Unfortunately, on the scale in which we perform our experiments, it is impossible to obtain the finely resolved data (spatial, temporal, particle size and composition) required to observe such mechanisms. What we can say from onr data is that, particularly for the high sodium lignite, our results are consistent with at least a portion of the vaporized sodium species homogeneously nucleating, even in the presence of other ash particles. These partides can then coagulate with themselves and with other ash particles. Further, it is also probable in our system that some of the sodium (presumably after the saturation ratio has been diminished due to nucleation) heterogeneously condenses on existing particles. Both mechanisms (nucleation followed by coagulation, and heterogeneous condensation) can lead to the experimental results we observe namely, the surface enrichment of Na species.

M. Jones, Univ. of N. Dakota, USA. The sodium is vaporized in the flame. Vapor phase alkali can react with the surface of the entrained ash in the combustor. Have you looked at the total surface area of the ash in the two coal samples to see if the lower Na in fine particulate may be due to a "'gathering" action by the entrained ash? Author's Reply. We have not made surfaee area measurements of the resultant ash from either of the lignite coals studied here. However, we would expect that if the Na in the low-Na lignite were somehow inhibited from homogeneously nucleating due to the "'gathering" by large surface area particles, we would see greater Na surface enrichment in these samples than those found in the high-Na

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case. In the limited Auger analyses pertbrmed, that did not appear to be the ease.

j. Itelble, PSI Technology, USA. The differing sodium concentrations in the fume ohserved dm'ing combustion of the two beulah coals are intriguing. Do you suspect a difference in tbe original form of sodium between the two coals? Also, have you been able to close a mass balance for these two cases? If so, were differences in the distribution of (1) total species and (2) sodium noted? Author's Reply. As we point out in our paper, there are two important differences seen in the aerosol behavior of high and low sodium Beulah lignites. First, the ratio of small particle sodium enrichment in the high sodium lignite to the low sodium lignite is much greater than the ratio of the relative sodium concentrations in the original asia. Second, small particle silicon enrichment is seen for the high sodimn lignite but not for the low sodium lignite. We have only the chemical analyses of the coal ash materials, not their mineralogical analyses. We suspect, however, that the sodium behavior may be attributed to the relative amounts of sodium silicates in the two coals. With regard to the silicon behavior, it could be explained by either a silicon vaporization mechanism, or possibly by the pres ence, in the original coal, of silicon-rich fines. These questions will hopefully he answered by current experinaents on more completely characterized coals. Finally, we were able, within experimental error, to close a mass balance on the major species measured by atomic absorption. In our system, we extract only a portion of the ash for analysis. Since we collect various size-segregated samples, measure sample flow rates, and measure the duration of sample extraction, we actually have an overdefincd system. This allows us to perform least squares adjustment of all data (flow rates, element concentrations and sample times) to achieve a mass balance. This method also provides a means |br highlighting "suspect'" data, since a high degree of data adjustment (from actual measured values) would be apparent. Results from this procedure indicated that in Run 42 (unquenched), 15% of sodium and 8% of the total nmss ended up in the small particle class, while in Iqun 43 (quenched) 22% of the inlet sodium and 8% of the total mass were found there. The low sodium coal had much lower values tbr fraction of inlet sodinm and total mass fmmd in the small particle size range.