Energy 8z Fuels 1993, 7, 884-890
884
Structure of a Near-Laminar Coal Jet Flame B. Scott Brewster: and L. Douglas Smoot Advanced Combustion Engineering Research Center, Brigham Young University, 45 CTB, Provo, Utah 84602
Peter R. Solomon and James R. Markham Advanced Fuel Research, Inc., 87 Church St., East Hartford, Connecticut 06108 Received April 8, 1993. Revised Manuscript Received September 4, 199P
Flame data from a near-laminar coal jet have been compared with model predictions. Inclusion of gas turbulence with laminarization was necessary for adequately predicting the upper-flame and postflame regions and for predicting particle dispersion. Dispersion of gas and particles was insensitive to inlet turbulence intensity. Gas buoyancy induced radially inward flow that opposed particle dispersion. Gas temperature was predicted too high near the coal nozzle, perhaps due to neglecting finite-rate mixing of volatiles with the bulk gas and chemical kinetics effects. Single-particle burning effects were important in the flame zone, as evidenced by the sensitivity of particle temperature to direct enthalpy feedback from volatiles combustion. Particle burnout was insensitive to enthalpy feedback, heterogeneous C02 formation, and chemistry/turbulence interaction.
Introduction Improved understanding of coal combustion will promote the clean and efficient use of this abundant natural resource. Recently, detailed measurements of local flame properties obtained by intrusive and nonintrusive techniques were reported for three coal flames in a transparent wall reactor.112 The nonintrusive measurement technique consisted of tomographic reconstruction of line-of-sight values obtained by Fourier transform infrared (FT-IR) emission/transmission (E/T) spectroscopy. Properties thus measured include separate particle and gas temperatures. Measurements from grab sampling were also reported for particle burnout and particle residence time, together with temperature measured by a thermocouple. In this paper, model predictions of ignition distance, particle velocity, particle temperature, and gas temperature are compared with the data previously reported for one of the coal flames (Rosebud subbituminous). Effects of viscous and turbulent transport, gas buoyancy, particle Schmidt number (for particle dispersion), inlet gas turbulence intensity, gas turbulence/chemistry interactions, and direct enthalpy feedback from volatiles combustion to devolatilizing particles (i.e., single-particle burning mode) and from heterogeneous C02 formation to oxidizing particles are investigated. Experimental Section The transparent-wall reactor (TWR) and Montana Rosebud coal flame have been described previously.ls2 A diagram of the reactor with experimental conditions is shown in Figure 1. Coal is injected upwards on the center axis of a 10-cm-diameterstream
of preheated air flowing upward axisymmetrically in the 21.5cm-diameter, 70-cm high, octagonal, glass enclosure. The hot air flows through several layers of screen intended to flatten radial profiles as it enters the reactor. The hot air heats the coal, producing a flame that is stable in both shape and position, except at the ignition point, where rapid fluctuations in flame length of 1 5 mm are observed. The glass enclosure shields the flame from room drafts and allows the combustion products to be exhausted from the building. Room air inducted into the octagonalenclosure at the perimeter by an exhaust fan helps to cool the enclosure. The vertical fluctuation of the ignition point may be partially caused by inherent instabilities in the feeding system, which employs mechanical vibration to entrain coal particles into a carrier gas. Feed instabilitieswere minimizedby mixing entrained particles from three separate particle reservoirs prior to flowing into the hot gas of the TWR. From visual observation, it was included that the averaging of feed rate instabilities with the multireservoir system significantly reduced the vertical fluctuation of the ignition point when compared to a flame produced by a single reservoir. The fluctuation undoubtedly propagates through the flame but has minimal impact on the spectroscopic measurements since FT-IR signal averagingresults in an average measurement through the fluctuation distance. FT-IR E/T spectroscopy with tomographic reconstruction to obtain spatially resolved spectra provides a tool for obtaining local properties of the flame without intrusion. Movable KBr windows in the glass enclosure provide optical access. The method of obtaining line-of-sight spectra has been described previouslyz4 as well as the tomography technique for obtaining spatially resolved spectra.696 The spatial resolution was reported to be 1mm X 1 mm X 4 mm high. From the spatially resolved spectra, point values were obtained for several flame properties, including particle and COztemperatures. The spatially resolved values, particle burnout (measured by ash tracer), and residence
(3) Solomon, P. R.; Carangelo, R. M.; Best, P. E.; Markham, J. R.; Hamblen, D. G. Fuel 1987, 66, 897. (4) Solomon, P. R.; Best, P. E.; Carangelo, R. M.; Markham,J. R.; 0 Abstract published in Aduance ACS Abstracts, October 15, 1993. (1)Markham, J. R.;.Zhang, Y. P.! Carangelo, R. M.; Solomon, P. R. Chien, P. L.; Santoro, R. J.; Semerjian, H. G. Twenty-First Symposium Twenty-Third Symposium (Internotional)on Combustion, [Proceeding]; (International) on Combustion, [Proceeding];The Combustion InatiThe Combustion Institute: Pittsburgh, PA, 1991; pp 1869-1875. tute: Pittsburgh, PA, 1988; pp 1763-1771. (2) Solomon,P.R.;Chien,P.L.;Carangelo,R.M.;Best,P.E.;Markham, (5) Best,P.E.;Chien,P.L.;Carangelo,R.M.;Solomon,P.R.;Danchak, J. R. Twenty-Second Symposium (International) on Combustion, M.; Ilovici, I. Combust. Flame 1991, 85, 309-318. (6) Solomon, P. R.; Markham, J. R.; Zhang, Y. P.; Carangelo, R. M. [Proceeding];The Combustion Institute: Pittsburgh, PA, 1989; pp 211Prepr. Pap-Am. Chem. SOC.,Diu. Fuel Chem. 1990, 746. 221.
08S7-0624/93/2507-0884~04.00/0 0 1993 American Chemical Society
Energy & Fuels, Vol. 7, No. 6,1993 885
Structure of Near-Laminar Coal Jet Flame
I
t=:mm;l
focm
L, Figure 2. Coal injection nozzle.
Flowrates
Coal
0.015 ds 2.9 4.2 x 10-6 m3/s 4.1 x 10-5 m3/s
Preheated air Carrier air Room air
Temberatures 300 K
1123 K 300 K 300 K
Coal composition
c (daf)
H (da9 0 (W N (da9
0.724 0.049 0.203
S (daf, organic) Ash
Particle size
0.012 0.0 12 0.15 1 45-75 p
Figure 1. Transparent wall reactor (figureadapted from ref 3) and experimental conditions for Rosebud coal flame. time (calculatedfrom particlevelocitiesdeterminedfrom streaks on a video recording) were reported.' Radial thermocouple measurementa were also reported.2 Photographs of the flame appear in ref 1.
Modeling
General Description. A comprehensive model7p8for pulverized coal combustion was modified and applied to ~~~~
~
~~
(7) Smoot, L. D.; Smith, P. J. Coal combustion and gasification; Plenum: New York, 1985. (8) Brewster,B.S.;Boardman,R.D.;Huque,Z.;Berrondo,S.K.;Eaton, A. M.; Smoot, L. D., et al. 'User's Manual. 93-PCGC-2 Pulverized coal gasification and combustion model (Zdimensional) with a generalized coal reactions submodel (FG-DVC)";Solomon, P. R.; Serio, M. A.; Hamblen,D. G.;Smoot,L. D.;Brewster, B.S.Measurement and modeling of advanced cool conversion processes; Final Report, Volume 2; work performed under Contract No. DE-AC21-86MC23075, for U.S.Department of Energy (1993).
the near-laminar coal flame. The model is two dimensional and steady state. Calculated gas properties are Eulerian while the particle properties are Lagrangian except for particle number density. Turbulence is modeled with the standard two-equation (k-e) approach. Particle dispersion is calculated with an empirical diffusion velocity which uses a Schmidt number to relate turbulent particle diffusivity to turbulent particle kinematic viscosity. Gasphase kinetics are assumed to be infinitely fast, and the probability density function (pdf) method is used for including the effects of turbulence on homogeneous chemistry. Devolatilization is treated with a comprehensive, coal-general modelgthat predicts the volatile amount and variability of volatiles composition and heating value with extent of devolatilization. Elemental composition of coal offgas (volatiles plus gasified char) is assumed to be constant in order to incorporate turbulence effects on the homogeneous chemistry. Char reaction occurs by diffusion (with correction for mass transfer) and surface reaction of the oxidizers. Gaseous and particulate radiation is modeled by the discrete ordinates method with non-isotropic scattering. Several modifications were made to the model for the near-laminar coal flame. First, gas buoyancy effects were included by adding a gravity term to the axial gas momentum equation. The gravity term was lumped with the source term. Second, the upward flow configuration was included by giving the appropriate sign (negative for upflow) to the gravity terms. And third, the turbulence model was extended to be applicable to low Reynolds number flow after the manner of ref 10. Their incorporated recommendations include (1)adding viscous transport, (2) allowing the model constants to depend upon a local Reynolds number of turbulence (which they define), and (3) adding terms to account for nonisotropic dissipation. The extended model was shown by ref 11 to give the expected predictions in the limits of low and high Reynolds numbers and reasonable predictions at intermediate Reynolds numbers (i.e., transitional flow). Inlet Conditions. Flow rates and temperatures of the carrier gas and preheated air are given in refs 6 and 1and summarized in Figure 1. The room air flow rate was deduced from hot-wire anemometer measurements of velocity near the wall. The inlet radial profiles were assumed to be flat. Particle concentration was assumed to be uniform in the primary stream. Model predictions (9) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A,; Deshpande, G. V. General Model of Coal Devolatilization.Energy Fuels 1988,2,405-422. (10)Jones, W. P.; Launder, B. E. Int. J. Heat Mass Transfer1973,16, 1119-1130. (11) Gillis, P. A. Ph.D. dissertation,BrighamYoung University,Provo, UT, 1989.
Brewster and Smoot
886 Energy &Fuels, Vol. 7, No. 6,1993
were sensitive to the primary stream diameter and to the inlet particle velocity. The particles were injected through a l-mm-diameter tube into a &"-diameter nozzle, which protruded 5 mm above the inlet screen foithe preheated air. A diagram of the nozzle is shown in Figure 2. Simulations which attempted to predict the flow in the nozzle did not adequately predict the particle spreading and ignition point. Detailed simulation of the nozzle revealed the presence of recirculation patterns which were not being adequately resolved in the overall flame simulations. Attempts to couple a detailed simulation of the nozzle with the overall simulations were unsuccessful. Therefore, an inlet coal stream diameter of 2 mm was selected, consistent with visual observation, and the inlet particle velocity was taken to be 95 % of the gas velocity, consistent with inlet particle velocity measurements. An inlet turbulence intensity of 1%was assumed for the preheated air and inducted room air. A nonzero value of turbulence intensity was assumed because these two streams flow through a screen at the reactor inlet. The sensitivity of the simulation results to inlet turbulence intensity was investigated by performing one simulation with an inlet turbulence intensity of 10%. The effect was negligible, since inlet turbulence quickly decays in the laminar flow near the bottom of the reactor. In the upper regions of the reactor, turbulence becomes more important, apparently due to fluctuations generated by combustion. The turbulence model with laminarization seems to predict the decay and generation of turbulence reasonably well. The inlet particle size distribution was also assumed to be uniform. Five particle sizes and 10 starting locations were used. Particle sizes varied from 45 to 75 pm, in accordance with the sample characteristics given in ref 1. Wall Boundary Conditions. Boundary conditions were needed for wall emissivity and temperature. Since the transparent enclosure is cold (cooled by room air), the sidewall temperature was taken as 300 K. Its emissivity was assumed to be 1.0, to simulate a cold (nonemitting), non-reflecting boundary. The temperature of the bottom (inlet) wall was taken as 1123 K since the screen through which the preheated air was flowing was at that temperature and was glowing bright orange. Numerical Grid Resolution. Since the model is designed for internal (duct) flows, the entire cross section of the glass enclosure was modeled, although the region of primary interest for data comparisons (Le., the flame) was the core region of 2-3 cm diameter. Modeling the entire cross section also allowed consideration of the possible effects of the cold room air on fluid mechanics and heat transfer in the reactor. Fifty-three non-uniformly-spacedgrid points were used in the radial direction, with most of these concentrated in the inner core (flame region). The spatial resolution in the radial direction was 0.5 mm in the flame region, one-half the resolution of the tomographic data. Sixty-seven grid points were used in the axial direction, with most of those concentrated in the particle heat-up and early flame regions. The grid spacing increased uniformly from 0.2 cm at the inlet to 2 cm at a distance of 50 cm. Grid independence was verified by performing calculations on different grid densities. Chemistry/Turbulence Interaction. Because of the near-laminar conditions in the preflame and early flame regions of the reactor, the effect of turbulent fluctuations on the gas-phase chemistry was neglected for most of the simulations. This assumption was validated by solving
for the local variance in gas composition and integrating gas properties over the corresponding probability density function. The effect was insignificant. Volatile Flame Feedback and Heterogeneous COZ Formation. Individual particles surrounded by volatiles flames were observed in the flame photographs of the kind presented in ref 1. Due to the possible effects of singleparticle burning, direct enthalpy feedback from volatiles combustion to devolatilizing particles was included by giving half of the heat of combustion of the volatiles to the particle that produced the volatiles and the other half to the gas. It was beyond the scope of this study to solve the boundary layer equations surrounding each particle as a function of distance along the particle trajectory and thereby calculate the fractional feedback. The significance of this assumed feedback was investigated by performing one simulation without feedback, which effect is discussed subsequently. In addition to enthalpy feedback from volatiles flames, particle enthalpy is also influenced by heterogeneous COz formation from char oxidation. Mitchell12has indicated that "as much as 1576 of the carbon content of the particle can be converted to COZat temperatures in the range 1600 to 1700 K", and this value was used in this study for all temperatures. The effect was investigated by performing one simulation with CO as the sole heterogeneous oxidation product. Char burnout kinetics of ref 13 as recorrelated by Baxter14were used. Model and Data Comparisons With gas buoyancy and laminarization included, the model gave reasonable predictions of the flowfield in the entire reactor. The flow was found to be laminar in the lower (preflame and volatilesflame) region and transitional in the upper (char burnout) region. Pure laminar predictions gave good agreement with radial gas profiles in the lower region, but inclusion of turbulence effects (with viscous diffusion and laminarization) was necessary to provide good agreement in the upper region. Simply neglecting the turbulence effects and using molecular diffusion properties substantially underpredicted the extent of gas mixing in the post-flame region. Particle dispersion was also underpredicted if turbulence was neglected. On the other hand, including the standard turbulence calculations without extension to low Reynolds number flow substantially overpredicted the mixing and particle dispersion. Neglecting gas buoyancy underpredicted the velocity at the centerline in the post-flame region. Modeling and experimental comparisons are shown below. Thermocouple Data. Predicted radial temperature profiles are compared with thermocouple data at several axial locations in Figure 3. The ignition point of the particle stream occurs at the 10 cm distance, and photographic inspection indicates that the majority of the ignited particle stream is less than 1.5 cm in diameter throughout its travel.'96 The data were obtained with an unshielded thermocouple and subsequently corrected for heat loss to (12) Mitchell, R. E. Twenty-Second Sympoeium (International) on Combustion, [Proceeding];The Combustion Institute: Pitteburgh, PA, 1989 DD 69-78. -(13)Goetz, G. J.; Nsakala, N. Y.; Patel, K. L.; Lao, T. C. Second Annu. - - - - z r r
Conf. Coal Gasification 1982. (14) Hedman, P. 0.;Smoot, L. D.; Smith, P. J.; Blackham, A. U. 'Entrained-flow gasificationat elevated pressure";DOE/MC/22069-2570; Combustion Laboratory, Chem. Eng. Dept., Provo, UT, 1987.
Structure of Near-Laminar Coal Jet Flame 2000I
a)
1000
5.
0 2000
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f 2000
1
looo 0
2000
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A X I A L L O C A T I O N (CM) 0 2000
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Figure 3. Predicted and measured radial gas temperature profiles in a near-laminar coal jet flame for combustion of Montana Rosebud subbituminous coal: (-) turbulence with laminarization; (- - -) no turbulence; (- - -) no laminarization; (0) measured; ( 0 )corrected for heat loss. the cold surroundings. Both the uncorrected and corrected values are shown. Thermocouple measurements within the volume of the particle stream (i.e., the centerline measurements) are influenced by the accumulation of particles, soot, and ash on the wire junction. The recording of the maximum indicated temperature was attempted before surface agglomeration significantly insulated the junction. Multiple thermocouple measurements were performed at each height in the centerline, each with initially a “clean” junction. The maximum indicated temperature was consistent for these multiple measurements, but the influence of coating coupled to the response time of the thermocouple probably resulted in a slightly low temperature indication at the centerline. Spectroscopic measurements of centerline gas and particle temperatures presented later in this article support this conclusion. Interestingly, the uncorrected thermocouple measurements near the inlet (0.2 and 5 cm) agree better with the predictions than the corrected values. Also, the measurements at a radius greater than 5 cm are too high. This discrepancy is probably due to radiation from the hot screen to the thermocouple which was not accounted for in the temperature corrections. Also, heat-loss corrections in coal flames are quite uncertain owing to the effects of gas and soot radiation in the flame and coating of the thermocouple with soot.
Figure 4. Predicted particle trajectories for (a) the base case (Sc = 0.7,turbulence with relaminarization,gas buoyancy), (b) no gas buoyancy, (c) no turbulence, (d) particle Sc = 0.35, and (e) no laminarization. Pure laminar, pure turbulent, and turbulent (with relaminarization) predictions are shown. Near the inlet, the flow is laminar and no effect of turbulence can be seen. From the ignition point (10 cm) upward, turbulence effects become increasingly significant. This turbulence is probably generated by combustion and gas buoyancy effects. The unusual predicted centerline peakedness at 50 cm is due to radially inward flow induced by gas buoyancy. The overprediction of centerline temperature in the coal flame at 50 cm may be due to neglecting heat loss by soot radiation. Particle Trajectories. Computed particle trajectories are shown in Figure 4. Only the portion of each trajectory where the particle is burning is shown. For purposes of this plot, burning is assumed to begin when the particle temperature reaches lo00 K and end when the temperature decreases below 1000K or complete burnout occurs. The particles are predicted to ignite at from 5 to 9 cm above the exit of the nozzle. The particles near the outer edge of the stream ignite first, as expected, since they are the first to mix with the preheated air. They also react the most quickly and burn out first, causing the width of the ignited particle stream to reach a maximum and then taper in toward the center of the reactor as particles are consumed. The width of the particle stream also undergoes a rapid expansion at the ignition point as hot gases generated by combustion suddenly force the particle stream outward. The predicted flame shape in Figure 4a which includes the effects of gas buoyancy, turbulent fluctuations, and relaminarization, is in good agreement with the shape observed from photographs.lI8 The effects of neglecting gas buoyancy, neglecting gas turbulence, decreasing particle Schmidt number (Sc),and neglecting laminarization are also shown in Figure 4. There is no significant effect on onset of ignition. Particle dispersion is most sensitive to gas turbulence and lami-
Brewster and Snoot
888 Energy & Fuels, Vol. 7, No. 6, 1993 I
Ignition point (observed)
I
I
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100
-8 a
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I
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l
I
I 1, 0 10 20 30 40 50 Axial distance (cm) Figure 5. Predicted and measured (0) particle velocity for the base case: (-) 45-gm particle; (- - -) 75-pm particle. 1
i
narization. Neglecting either one produces results that are inconsistent with visual observation of the flame and with the data. Close-up photographs of the coal stream as it exits the nozzle show that the coal stream spreads to approximately the diameter of the nozzle (0.5 cm) within the first 3-4 cm of axial distance. The effect on particle dispersion of the radially inward flow induced by gas buoyancy can be seen by comparing parts a and b of Figure 4. The effect of particle Schmidt number can be seen in Figure 4d. A value of 0.7 was used in the base case and seems to give the best overall results in this near-laminar flame. This value is consistent with the recommendation of ref 15 for turbulent jets. A value of 0.35 was recommended by ref 16 for pulverized coal combustion. Particle Velocity. Particle velocity data and predictions are presented in Figure 5. Calculations for two particle sizes and two starting locations are shown. The outer particles initially decelerate, probably due to the sudden expansion of the gas as it enters the flow domain. As buoyancy forces increase the velocity at the center, the particles accelerate. The outer-edge particles slow down as they move away from the centerline where the velocity is lower. Outer-edge particles are not seen in the video recordings past 30 cm because they are burned out. Centerline particles continue to accelerate and match the trend of the data. The predicted particle velocities match the experimental data at the inlet. A t approximately 6 cm, the predictions show a sudden acceleration which does not match the experimental data. This sudden acceleration corresponds with a sudden increase in predicted gas temperature which also does not match the experimental data (seethe tomographic datacomparisons shown below). Possible reasons for this sudden increase in predicted gas temperature are discussed below. Without the sudden increase, the predicted particle velocities would show good agreement with the measured values. Particle Burnout. Total (radially integrated) particle burnout is shown in Figurs 6. The shape of the initial (devolatilization) portion of the profile is predicted very well. The exact location of ignition is sensitive to the inlet particle velocity. Use of the observed particle stream inlet diameter of 2 mm gives a reasonable prediction. Char burnout rate is underpredicted, probably due to overpredicting particle velocity and underpredicting residence (15) Melville, E. K.; Bray, N. C. Int. J.Heat Mass Transfer 1979,22, 647-656. (16) Fletcher, T. H. Ph.D. dissertation, Brigham Young University, Provo, UT, 1983.
LL
U D
20 0 0
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20 30 40 Axial distance (cm)
0
50
Experimental Base prediction
-
No volatile flame feedback No gas buoyancy Pure laminar
. . . . . No laminarization
Figure 6. Predicted and measured particle burnout. time. When gas buoyancy is neglected, particle residence time and burnout are increased. Interestingly, neglecting gas buoyancy gives the best agreement with the data, but this discrepancy is thought to be due to overpredicting the particle drag coefficient rather than to overpredicting the gas velocity. Gas velocity data are not available to test this hypothesis. Neglectinglaminarization overpredicts the early portion of the curve due to overpredicting particle dispersion and mixing with the preheated air. Burnout never reaches completion because larger particles at the outer edge of the stream are extinguished by mixing with cold room air. Neglecting gas turbulence greatly underpredicts the burnout curve because of inadequate particle dispersion and mixing. The predicted effect of volatile flame enthalpy feedback on burnout was only moderate. This was tested by giving all of the volatiles enthalpy to the gas. In the base case, half of the standard heat of combustion of the volatiles was given to the particles. As shown below, volatile flame feedback increases the particle temperature during devolatilization, which leads to accelerated weight loss. The effect of neglecting heterogeneous COZformation (157%) was insignificant. Tomographic Data. Comparison between FT-IR tomographic data and predictions is shown for the base case in Figure 7a for gas and particle temperatures. A different symbol is shown for the predicted temperature of each particle size at the indicated axial distance. A t 6 cm, the particles are being heated by the gas and, therefore, lag the gas temperature. Measurements indicate the presence of both ignited (at 1700 K)and unignited (at 700 K) particles; however, the prediction shows a continuous distribution of particle temperatures a t 6 cm. This discrepancy may be due to underpredicting the extent of particle dispersion at this point, or it may be that ignition is occurring with even smaller particles than are included
Energy & Fuels, Vol. 7 , No. 6, 1993 889
Structure of Near-Laminar Coal Jet Flame
0
0.2 0.4 0.6 0.8 Radial distance (cm)
1
0
0.4 0.6 0.8 Radial distance (cm)
0.2
1
Figure 7. Predicted and measured (by FT-IRE/Tspectroscopy and tomography) particle and gas temperature (a, left) with and (b, -) unignited particles, (-) right) without direct enthalpy feedback from volatiles combustion. Measured (- 0 -) COz, (ignited particles, Predicted gas: (- -). Predicted particles: (0) 45 pm, (0)52.5 M, (0)60 pm, (X) 67.5 pm, (+) 75 pm.
in the model calculations. In addition, the measured gas temperature is higher than the predicted temperature where ignition has occurred (at a radius greater than 5 mm). The predicted gas temperature goes through a maximum at approximately 2 mm radial distance from the centerline, indicating that particle combustion is occurring, whereas the measured gas temperature is flat except at the outer edge where ignition has occurred. Thus, while the experiment indicates local ignition for individual particles, the predictions show combustion averaged over many particles. The discrepancy in gas temperature at a radius of 2 mm is probably due to an ignition delay caused by mixing and homogeneous kinetics. The model assumes local, instantaneous mixing of volatiles with the bulk gas as they are released and infinite-rate chemical kinetics, neither of which may be a good assumption during the early stages of particle heat-up and devolatilization. At 10.5 cm, predictions show the outer-edge particles have ignited. The temperatures of ignited particles exceeds the measured values by 20&300 K. The measured particle temperature profile is flat, whereas the particles are predicted to be colder in the core region. Agreement between measured and predicted gas temperature is quite good here and at higher locations.
Similar comparisons are made in Figure 7b for the case where enthalpy feedback from volatiles flames is neglected. Predicted particle temperature is significantly lower at the observed ignition point (approximately 10 cm). Particles on the centerline are only partially ignited at a distance of 14 cm, whereas they are all ignited at 10.5 cm when volatile flame feedback is included. Particle ignition initiates in an annular region which begins at the outside edge of the coal stream and moves inward to the core. In Figure 7b, ignition is occurring at 0.3-0.8cm radial distance at an axial distance of 10.5 cm. At 14 cm, it is occurring at the core. I t is interesting to note that particle temperature depends more strongly on particle size in the ignition zone when volatile flame feedback is neglected. The opposite might have been expected, since particle size effects (e.g., differing thermal capacities) would be expected to be dampened with decreased rate of enthalpy transfer to the particles. Apparently, including volatiles enthalpy feedback raises the temperature of the larger particles more than that of the smaller particles, since the smaller particles quickly reach and exceed the gas temperature, and are limited in further temperature increase by heat loss to the gas. The dependence of particle temperature on particle size is therefore reduced when
890 Energy & Fuels, Vol. 7,No. 6,1993
volatiles enthalpy feedback is included. Improved agreement is obtained when direct enthalpy feedback effects due to single-particle burning are included. Conclusions
A comprehensive,pulverized coal combustion model was applied to predict a near-laminar coal flame. The consideration of gas buoyancy, turbulence with laminarization, and direct enthalpy feedback to particles from local volatiles flames gave reasonable agreement with experimental data. Instantaneous mixing of volatiles and infinite-rate kinetics may not be good assumptions in the early stages of particle heat-up and devolatilization, and this is an area in need of further work, Effects of inlet turbulence intensity, heterogeneous COz formation, and chemistry/turbulence interactions were negligible for this flame. Overprediction of gas temperature in the postflame region is probably due to neglecting soot radiation.
Brewster and Smoot
Future efforts in comprehensive modeling need to be directed at finding suitable ways to incorporate finiterate, homogeneous kinetics, especially in the near-burner region, without ignoring chemistry/turbulence interactions. In conjunction with accounting for the finite-rate of volatiles combustion, methods are also needed for accounting for the varying composition of the volatiles and the finite rate of mixing between the volatiles and the bulk gas. A flame-standoff model, incorporating effects of mass transport and kinetics, may be needed to accurately model enthalpy feedback from volatiles combustion and to accurately predict particle ignition. Acknowledgment. This work was supported by the
U.S. Department of Energy, Morgantown Energy Technology Center, Contract No. DE-AC21-86MC23075. Drs. Norman Holcomb and Richard Johnson were the project managers, and Advanced Fuel Research, Inc. was the prime contractor.
