Energy & Fuels 1995,9, 484-492
484
Laser Ignition of Levitated Char Particles? Brian A. Wong,* George R. Gavalas,e and Richard C. Flaganf Departments of Environmental Engineering Science and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 Received September 6, 1994@
The temperature history of a char particle that was heated by a carbon dioxide laser to the ignition point was studied in the electrodynamic balance. The ignition characteristics of char particles are an important part of the overall coal combustion process, and the aim of research in this area is to improve efficiency and minimize waste products and pollution in current combustors. The experiments were conducted using 158-210 pm diameter Spherocarb particles, and 200-300 pm diameter char particles from a bituminous coal. Individual charged char particles were suspended in the electrodynamicbalance and heated by a 420 ms pulse of radiation from a C02 laser at a heating rate of approximately 4.5 x lo3 Ws.The temperature of the particle was measured with an optical pyrometer. The atmosphere in the electrodynamic balance was initially pure nitrogen. The oxygen concentration was slowly increased as successive laser pulses were applied, until the particle ignited. Upon ignition, the particle lost charge and dropped from the center of the balance. The particle temperature profile preceding ignition shows the ignition delay predicted by the general ignition condition of Semenov's thermal explosion theory. The measured temperature traces could be described well by calculations treating the particle as a sphere and using rate parameters given in the literature.
Introduction In pulverized coal combustion, small particles, typically 50 pm in size are burned while entrained in the combustion gases. As a coal particle heats up, it devolatilizes, losing a substantial fraction of its mass. The char residue that remains after devolatilization is a porous, primarily carbonaceous, material. Ignition occurs when the heat produced by the exothermic oxidation of gases and char exceeds the heat loss from the system due to conduction, convection, and radiation. This concept of ignition applies t o an entire system, e.g., a cloud of coal particles in a furnace, as well as to an individual particle (Field et a1.l and Essenhigh et a1.2 1. Two mechanisms of ignition have been identified for coal particles, homogeneous and heterogeneous.2 Homogeneous ignition occurs when the devolatilizing gases react with oxygen to ignite first, producing a flame front that initially stands off from the particle but eventually reaches the char surface as the devolatilization ceases. Heterogeneous ignition occurs directly on the particle surface as oxygen reacts with the solid char particle. Experimental (Howard and Essenhigh3 ) and theoretical (Annamalai and Durbetaki4)results have demonstrated that the heterogeneous ignition mechanism dominates Author to whom correspondence should be addressed. Current address: Chemical Industry Institute of Toxicology, Research Triangle Park, NC 27709. 'Presented in poster form at the 10th annual meeting of the American Association for Aerosol Reseach, Traverse City, MI, October 7-11, 1991. Department of Chemical Engineering. @Abstractpublished in Advance ACS Abstracts, April 1, 1995. (1)Field, M. A.; Gill, D. W.; Morgan, B. B.; Hawksley, P. G. W. Combustion of Pulverized Coal; The British Coal Utilisation Research Association: Leatherhead, U.K., 1967. (2) Essenhigh, R. H.; Misra, M. K.; Shaw, D. W. Combust. Flume 1989, 77, 3-30. (3) Howard, J. B.; Essenhigh, R. H. Symposium (International) on Combustion, [Proceedings] 11th; The Combustion Institute: Pittsburgh, 1967, pp 399-408.
0887-0624/95/2509-0484$09.00/0
for smaller ( O Field ~ ~ et a1.l cal/(cm s K)
0.8
emissivity activation energy, E, reaction order, n heat release, Q e ~
Monazam et al.23 83680 Jim01 Waters et al.30 0.5 Waters et al.30 0.35&cozS0.65Qco Waters et al.30 (170800 J/mol)
Table 2. Spherocarb Experiments
series
particle radius Cum) 100 98 75 99 99 105 82 79 95
laser intensity (W/m2) 3.13 4.65 4.11 3.04 3.45 3.46 4.54 3.85 3.54
x lo6 x lo6 x lo6 x lo6 x 106
x lo6 x 106 x x 106
lo6
density (kg/m3)
A (kg/ (m2 s kPao.5)
1100 1200
1000
3.6 4.2 6.1 3.7 5.0 3.6 3.6 5.2
1100
4.0
1000 800 1100 1000 1000
Po2
(%I 50 13 25 40 35 35 25 30 25
The values for the parameters in this model are summarized in Table 1. The specific heat, C,, for Spherocarb was taken from Monazam et al.23 An apparent activation energy, E,, of 83 680 J/mol(20 kcal/ mol) and a reaction order with respect to oxygen, n, of one-half was assumed, as determined by Waters et a L 3 0 in their study of Spherocarb oxidation in the temperature range of 1400-2200 K. The diffusion of oxygen through the porous char matrix becomes the limiting factor in the carbon oxidation rate at these temperatures. The particle temperatures being measured were approaching this range, and hence these apparent rate parameters were selected. Also in the experiment by Waters et al.,30it was determined that the oxidation products were CO and C02 in a 65% to 35% ratio, and Q e R was calculated accordingly. Table 1gives the values used for each parameter and the literature source. The use of the TEM-01* mode greatly complicated the determination of the amount of energy absorbed by the particle because a flat intensity profile could not be assumed and based only on the overall intensity of the laser beam. Therefore, the value of HI was determined from the particle temperature trace when heated in nitrogen. In the absence of oxygen, Hgis zero, and at steady state, Hl = H , H,. After Hl was determined, the time-dependent eq 9 was solved using a fourth-order Runge-Kutta routine. The preexponential factor was varied until a reasonable fit of the curve to the experimental data from the ignition event was obtained. The calculated temperature traces are shown by the dashed lines in Figures 4a-c and 8a-c. The good agreement between calculated traces and measured traces for the nitrogen atmosphere (Figures 4a and 8a) simply reflects the choice of the parameter, Hl, which was adjusted to match these two traces. The calculated laser intensity is listed in Table 2. Either density or heat capacity from eq 9 could be adjusted to match the initial slope of the calculated trace to the experimental data. As the density of the Spherocarb particles was not measured in this set of experiments, and only Monazam's value23for heat capacity was available, we chose to adjust density. The estimated density values are larger than the density of 600 kg/m3 found for
+
Spherocarb particles by D'Amore et al.,34 but they cluster around the value of 1050 kg/m3 used by Monazam et al.23 when he measured the heat capacity of Spherocarb. Table 2 shows the fitted parameters for several particles. The fitted preexponential factors ranged from 3.6 t o 6.1. The preexponential factor that was determined by Waters et al.30was 10.6 kg m-2 s-l (mole 0 2 m-3)-n. Although the preexponential factor is the nominal adjustable parameter, Qeff and Pozn can also be lumped in with the preexponential factor. Therefore, the uncertainty in the fitted preexponential value is also indicative of uncertainties in Qeff and Pozn. D'Amore et aL19 determined an activation energy of 36 kcal/mol and a preexponential factor of 2.6 x lo6 s-l (at 21% oxygen) for Spherocarb particles oxidized at temperatures from 500 to 1200 K. A value of 42.9 kcaV mol from Smith (1978) was used in initial modeling efforts, which were not successful in matching the experimental data. With the previously mentioned values from Waters et al.,30the data could be modeled. According to DAmore et al.,19the conditions of combustion in the entrained flow reactor of Waters et al. is zone I1 (diffusion limited), while at their lower temperatures, oxidation is occurring in zone I (chemical kinetics limited). This suggests that under the conditions of these experiments, the ignition of the Spherocarb particles was occurring in zone 11. The calculated temperature trace in Figure 4b was determined using the fitted parameters from Figure 4, a and c. It appears to overpredict the measured temperature. This discrepancy may be due to experimental error such as fluctuations in the laser power, o r it may be due to the uncertainties in the Qeff and Pozn as mentioned previously. A general trend of increasing steady-state temperature with increasing oxygen concentration could be detected but the scatter in the data was substantial and the relationship could not be quantified. Figure 8, a, b, and c, shows that the char made from PSOC 1451 bituminous coal behave similarly to the Spherocarb particles. The experimental data was adequately modeled using an activation energy of 83 680 J/mol (20 kcaymol) and a preexponential factor of 8.0 kg/(m2s kPa0,5). Sahu et al.35determined an apparent activation energy of 17 kcaymol for this char made from PSOC 1451 bituminous coal with a preexponential factor of 107.1 g cm-2 s-l atm-l or 10.6 kg/(m2s kPa). No further attempt was made to reconcile the activation energies or the preexponential factors for the char or the Spherocarb particles. The isothermal sphere model is admittedly simple, and used solely to demonstrate that the ignition behavior of the particles, as presented in the temperature traces, is as expected. For better quantitation of the kinetic parameters, some problems must be addressed in future experiments. Variability in the experimental data and differences between the experimental and literature kinetic data may be due to experimental uncertainty (as discussed below), or to a changing reactivity or annealing of the particle. It has been demonstrated that the reactivity (34) DAmore, M.; Dudek, D. R.; Sarofim, A. F.; Longwell, J. P. Powder Technol. 1988,56,129-134. (35)Sahu, R.; Northrop, P. S.; Flagan, R. C.; Gavalas, G. R. Combust. Sci. Technol. 1988,60,215-230.
Wong et al.
492 Energy & Fuels, Vol. 9,No. 3, 1995
of a char decreases as it o ~ i d i z e s . ~This ~ - ~may ~ be due to a burnout of the more reactive components, leaving the less reactive material, or a structural reordering of Conceivthe carbon atoms into less reactive ably, in this experiment, the char particles could be undergoing an annealing or deactivation during the pulses of heating a t different oxygen concentrations. This change would not be detectable with this experimental procedure. The effects of annealing could possibly be detected by igniting particles in a single pulse, after they had been oxidized to a certain level of conversion. Presumably, with increasing conversion, the deactivated or annealed particle would require a greater laser intensity t o ignite, at a given oxygen atmosphere. There are several problems with this experiment which are not apparent from the results presented. The first is related to the laser stability. The laser power, as measured on a power meter, fluctuated and occasionally drifted over a long period of time. To minimize the effect of drift, the experiments were performed only when it appeared that the laser was operating stably and the power was not drifting. Also, the laser energy absorbed by the particle could not be precisely determine because of the use of the TEM-01* mode, as discussed above. Another difficulty was that the laser heated the particle from the top only. Video recordings of a Spherocarb particle during heating clearly show a thermal gradient across the particle. Thus, the model used to describe the experiments is an oversimplification. Bar-Ziv et al.15 and Monazam et alaz3addressed this problem for a particle heated on two opposite sides and concluded that the maximum temperature difference between the surface and center of the particle was about 8 K for a particle 1000 "C above ambient. They concluded that this difference may be disregarded for most combustion studies. For one-sided heating, however, the temperature difference would be greater. A particle heated on one side might lead to ignition at a point, or a very small area, as discussed by Levendis et al.39 These authors used two-color pyrometry to measure the temperature of a char particle burning in a drop-tube furnace. They observed signal intensities which increased in both detectors while the ratio of the signal intensities remained relatively constant, indicating a relatively uniform temperature. They attributed the increase in signal intensity t o localized ignition on the particle surface which gradually spread over the surface of the particle. In the present experiments we did not measure the individual signal intensities but only their ratios and, therefore, it is not known whether the detector signal levels were changing while their ratio remained constant. Observations from the video recording of the particle ignition did not show localized ignition on a Spherocarb particle. The particle appeared to glow uniformly as the temperature neared the ignition point (although saturation of the video camera (36)Davis, K. A.; Hurt, R. H.; Ottesen, D. K. Chem. Phys. Processes Combust. 1993,251-254. (37) Beeley, T. J.;Gibbins, J. R.; Hurt, R.; Man, C. K.; Pendlebury, K. J.; Williamson, J. Prepr. Pap.-Am. Chem. Soc., Diu.Fuel Chem. 1994. 39. _ ___ ~~, , 564-568. ~ . . (38)Hurt, R. H. Energy Fuels 1993,7 , 721-733. (39) Levendis, Y. A,; Sahu, R.; Flagan, R. C.; Gavalas, G. R. Fuel 1989,68,849-855. ~~~
precluded quantitative interpretation of these images). As the particle ignited, it appeared to expand to about twice its original diameter. This expansion may have been due to an oscillation of the particle that was too rapid to be resolved by the video camera, or it may have resulted from camera saturation. Shortly after ignition, the particle lost a significant amount of charge, and dropped from the center of the chamber. No hot spots were visible, no jetting was observed, and the Spherocarb particles always moved in the downward direction. Hence, it is believed that the Spherocarb particles were igniting uniformly across the surface of the particle. In contrast, there was clear evidence of localized ignition on char particles. Video recordings showed that, when a coal char particle was heated, small areas glowed considerably more intensely than other areas. Since the char was irregularly shaped, this was not unexpected. Even after a char particle ignited, hot spots often were visible.
Conclusions This experiment has shown that char ignition can be studied in the electrodynamic balance. With the particle suspended without physical contact, heat transfer other than by conduction to the surrounding gas and radiation is avoided. The use of the optical pyrometer to measure the particle temperature eliminates the need to calculate it from an energy balance. There are some difficulties which need to be addressed in order to increase the utility of this experiment. The first is the temperature gradient along the particle due to heating from the top side only. When we attempted to heat from both sides, the particle bounced around excessively. This difficulty might be avoided by using a more powerful laser, such as used by Monazam et al.,23in the Gaussian mode. The problem of charge loss at high temperatures also limits the usefulness of the electrodynamic balance in that the particle is lost once it reaches ignition temperatures. There are other techniques that can levitate an uncharged particle which are as yet untested for char oxidation studies. The temperature traces of particles that ignited clearly show an induction time, or ignition time lag, as predicted by the solution of the time-dependent equation for a heated particle. The effect of increasing the oxygen concentration is also seen in the increasing equilibrium temperature at subignition conditions, though it was not quantifiable in this study. A simple isothermal sphere model was used with parameters obtained from the literature which adequately predicted the experimentally obtained time-temperature histories of igniting Spherocarb particles. These experiments provide the first example in which the oxidation of char during ignition is modeled and compared with existing literature data. The electrodynamic balance can be used in ignition experiments t o measure the apparent char oxidation rate parameters, and probe particle-to-particle variations in the apparent kinetics.
Acknowledgment. This research was supported by the US. Department of Energy University Coal Research Program under grant DE-FG22-89PC89765. EF940167B
