Observation of nonuniform shrinkage and activation of highly porous chars during combustion in an improved electrodynamic chamber

Observation of Nonuniform Shrinkage and Activation of Highly Porous Chars during Combustion in an Improved Electrodynamic Chamber YONY WEISS and EZRA BAR-ZIV Department of Mechanical Engineering, Ben-Gurion Universityof the Negev (Y.W,;E.B-Z.), Nuclear Research Center--Negev, P.O. Box 9001 (E.B-Z.), Beer-Sheva, Israel Combustion of single particles of highly porous synthetic char has been investigated in an electrodynamic chamber (EDC). The main reasons for using the EDC for studying high temperature kinetics of single particles are to (1) sustain the particle without moving at all times at a known point, (2) eliminate heat and mass transfer limitations, (3) observe particle-to-particle differences, (4) fully characterize the particle prior to combustion, and (5) monitor the important properties of the single particle through its entire combustion history. In this device the particle is heated radiatively by a focused laser (CO 2) beam to the desired temperature. During the heating the particle should not move by more than 1% of its diameter since the waist of the beam is comparable to the particle diameter. A strong proportional-integral-derivative (PID) position controller was developed to maintain the particle at the center of the EDC, with position stability better than 0.6% of its diameter. Further development of the EDC included (1) real time measurement of the particle shape and diameter with a temporal resolution of 0.1 ms and (2) infrared optical pyrometry with wide spectral bands to determine the particle temperature to within + 10 K. Oxidation of synthetic char particles (Spherocarb) was studied in the EDC at temperatures around 900 K. Transients of the particle weight, size, shape, temperature, and position of the particle were measured in real-time. Using the present EDC two new phenomena were observed when highly porous chars were heated (in the range 800-900 K): (1) Prior to conversion there was a stage in which mass loss or size change did not occur. This is attributed to activation of the char and was found to depend on the particle temperature. (2) Nonuniform shrinkage during combustion--the initially spherical particles were consumed nonuniformly in all the numerous experiments. Eventually the spherical particle became a disk. Quantitative results are presented for both phenomena.

INTRODUCTION Further development of the electrodynamic chamber (EDC) for high-temperature studies of single particles enabled two phenomena to be observed during the burning of highly porous char particles: (1) a period with no change in the particle mass, prior to conversion, referred to as activation, and (2) nonuniform shrinkage of these particles. These phenomena were observed because of the ability of the EDC to maintain a single particle at high temperatures and monitor its properties in real time during its entire combustion history. Quantitative results of these phenomena are presented. The EDC has been primarily developed for kinetic studies of the combustion of single particles [1-10] and has been applied to a variety of other high-temperature applications. The main advantages of the EDC are the following

[1]: (1) The ability to study combustion kinetics of a single particle in well controlled conditions. (2) The ability to characterize the particle prior to reaction and monitor the important quantities needed for kinetic understanding in real time. (3) The elimination of heat and the mass transfer limitations of other methods that restrict kinetic measurements to slow burning rates. (4) The ability to study particle to particle variations. The following quantities can be measured in the EDC: (1) weight, (2) size and shape, (3) density, (4) temperature, (5) heat capacity, and (6) surface area. Central to the EDC is a position control system that can maintain the particle stationary at the center of the chamber [10-12]. This is of crucial importance since the particle is heated by a focused laser beam with a waist diameter equal to or smaller than the diameter of the particle. A robust position controller COMBUSTIONAND FLAME 101:452-460 (1995)

0010-2180/95/$9.50 SSDI 0010-2180(94)00221-D

Copyright © 1995 by The Combustion Institute Published by Elsevier Science Inc.

CHAR COMBUSTION IN ELECTRODYNAMIC CHAMBER enabled observation of new char oxidation phenomena. The temperature measuring system has been improved to measure to within 10 K. Measurements of size and shape, in real time, also have been developed and are presented.

453

imaging system to minimize the particle oscillations. An Ophir power meter model 30A-HP measured the laser power and extinction of the beam by the particle. High particle stability is vital during heating, since a small movement of the particle would upset the heat balance to change its temperature significantly.

EXPERIMENTAL

The chamber is of hyperboloidal configuration with a characteristic length of 6 mm. A schematic description of the EDC with its various operational and diagnostic systems is presented in Fig. 1. The study presents (1) a new position controller and (2) further development of fast methods for measuring particle temperature, size and shape. Measurements of other particle properties are described elsewhere in greater detail [1, 10]. Only synthetic char particles (Spherocarb) were studied. Particle Heating

The particle was heated vertically by a TEM~1 laser beam (doughnut shape profile), which was found to increase the stability of the heating [10]. An Optical Education cw CO 2 laser of 50 W with a 6-ram-diameter beam was used. A ZnSe converging lens (12 mm diameter, 1.1 m focal length) focused the beam to approximately the particle position (the center of the EDC). Fine adjustment of the lens was achieved by (1) the optical pyrometry system to maximize the particle temperature and (2) the To Power Meter Particle White

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Position Control and Size Measuring Systems

The positional control system is a further development of the system developed by Bar-Ziv and de Botton [11] and Baum et al. [12]. The present EDC includes also three independent means of measuring the particle size in real time. Figure 2 is a schematic diagram of the system and includes the optical, analog, and digital subsystems. A weak laser beam (Ar + or He-Ne) traversed horizontally through the center of the ring electrode and illuminated the particle. It passed through an objective lens of 50 mm focal length 50 mm from the particle. The beam was then split into two by a cube beam splitter. One was projected onto a 128 photodiode array (PDA) ( E G & G Reticon PDA model RLO128E) and the other passed through a piano-convex lens of 160 mm focal length, located 1250 mm from the objective lens. This lens magnified the image of the particle such that it captured about 60% of a 4096 PDA (EG & G Reticon photodiode array model RL4096NAG-011). The position was determined by the 128 PDA and the size by the two PDAs. The 128 PDA was placed vertically to measure the vertical position and the vertical dimension of the particle. The 4096 was placed horizontally to measure the horizontal dimension of the particle. Nonuniformity of particle shape could be detected by the two PDAs and be determined in real time. The 128 PDA was sampled at a rate of 7800 Hz and the 4096 PDA sampled the size at 500 Hz. To maximize the contrast, the photodiodes were operated at saturation conditions, enabling sharp transitions between the shadow of the particle and the light around it. A typical 128 PDA trace is shown in Fig. 2 in box A. Analog comparators were constructed to obtain square signals that showed clearly the border of the particle, see the "light" and "shadow" areas in

454

Y. WEISS AND E. BAR-ZIV

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h Fig. 2. Schematicdiagram of position control and size measuring systems.Figure shows a photodiode array trace of a synthetic char particle (Spherocarb) levitated in the EDC (A) and the traces after applying hard limiters to the signal (B and C). The "shadow" and "light" zones determine the position and diameter of the particle. boxes B and C in the figure. The widths of light and shadow were determined by counters which are then fed to the control computer (33 MHz 486 microprocessor) via the I n p u t / O u t p u t channel. These widths determined directly the position and size of the particle, with a resolution of about half a diode. The control computer operated as a proportional-integral-derivative (PID) controller and was used to minimize the error of the particle position by feedbacking voltage to the endcaps to maintain the particle at the center of the chamber. The particle size measured from the 128 PDA was fed to another computer that operated as a data acquisition system. The 4096 PDA was also used to determine the size of the particle with a much higher spatial resolution. Its signal also was fed into the data acquisition computer. The same computer collected temperature and mass data, in real time. Shape measurements are of importance especially, for quantifying the nonuniform

shrinkage phenomenon (see also [10]). These measurements were based on photography. A CCD camera, shown schematically in Fig. 1, was placed such that it viewed the particle when illuminated from the rear by white light and photographed the particle at a rate of 24 fps.

Controller Performance Details of the controller are presented in Ref. 12. Here, only the controller performance connected with experiments of single particle combustion is presented. To verify the application of the position controller in combustion conditions its ability to maintain the particle at the center of the EDC was tested when (1) the particle was irradiated abruptly with a focused laser beam and (2) the particle was exposed to gas flow. Simulations [12] evaluated controller parameters for the expected changes in the particle force balance. (The working parame-

C H A R C O M B U S T I O N IN E L E C T R O D Y N A M I C C H A M B E R ters are: VAC = 6400 V, v = 49 Hz, K~ = 12.5, K~ = 1, Kg = 7.5; see Ref. 12 for definitions.) Irradiation levitated the particle due to flee-convection of the surrounding gas and photophoresis. The sum of the initial free-convection and photophoretic forces can be as much as 40% of the particle weight and depends strongly on the particle and gas properties [1, 8]. The characteristic time of this phenomenon depends mainly on the dimensions of the particle [8]. Figure 3 shows the controller performance when a particle of 175/xm diameter was irradiated by a 22-W focused CO 2 laser beam. The ratio of the voltage to the initial voltage (equal also to the sum of freeconvection and photophoretic forces) is shown in the top figure for the periods when the laser was turned on and off. In the middle figure the correction is shown around t = 0. The particle location is shown in the bottom figure. Prior to irradiation the particle was oscillating a mere 0.5 micron (0.2% of the particle diameter). When irradiated, the particle was heated to about 900 K. Without the controller the parti-

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455

cle would have been displaced upward from the center by several particle diameters. With the controller, it moved less than 2/zm (about 1% of the particle diameter) from the center and moved back after about 100 ms. This is approximately the characteristic time for the development of free-convection [8]. The controller correction was 16% of the particle weight (247 V was needed to levitate the particle). The response of the corrected voltage corresponds directly to the behavior of the particle location. The drag force on the particle from the imposed flow and its location are displayed in Fig. 4. The force rose gradually and relatively slowly until it reached a steady state value after about 1 s, of 60% of the particle weight. The location of the particle was not affected by the flow, since the phenomenon is slow and the controller responded readily.

Size Calibration Figure 5 displays calibration results of the particle diameter by the two PDA systems and the CCD camera. The calibration used spherical polystyrene particles with known diameters with a standard deviation of 2-3%. Each point in the figure represents 6 different particles. The results display a linear behavior for all three methods. 0.7

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Size [microns] Fig. 5. Calibration of size measurements: signal vs. calibrated diameter of polystyrene particles for two P D A systems and CCD camera.

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Temperature Measurements The temperature of the particle was determined from optical pyrometry measurements at two wide-wavelength bands. Figure 6 shows the optical pyrometry system. A CaF2 lens, of 90 mm focal length, for collecting the light emitted from the hot particle was placed such that it captured a maximum solid angle of the particle. A chopper was placed at the exit of the chamber for light modulation. A germanium beam splitter was placed after the collecting lens and the two images were projected, with a magnification power of unity, on to two IR detectors, at their centers. The two infrared, InSb, detectors of Infrared Associates, were cooled to 77 K, and had spectral responses in the ranges 2.0-5.0 /xm and 4.5-7 /zm. The active area of the two detectors was about 2.5 x 2.5 mm 2. The spatial response was verified to be homogeneous to within at least ten particle diameters. Ithaco lock-in amplifiers model 3962A determined the emission amplitude. The output amplitudes of the lockin amplifiers were transferred to the data acquisition computer. The temperature determination from emission measurements has been discussed in great detail in Ref. 1 and the references cited therein. Calibration of the system used an Infrared Industries black body source model IR-463-2 with a pin hole of 250 /xm, approximately the size of a single particle used in the experiments. The emission from the black body, which was placed exactly at the location of the

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particle, was measured by the two detectors. The natural logarithm of the ratio of the signals of the two detectors was plotted as a function of 1/T. This ratio was also calculated, assuming constant emissivity, by integrating over the spectral response of the two detectors. The results, presented in Fig. 6, show thattemperature can be determined at 450 K with an error of 15 K, and smaller at higher temperatures. RESULTS AND DISCUSSION The voltage between the two endcap electrodes measures the force balance on the particle, or

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C H A R COMBUSTION IN E L E C T R O D Y N A M I C C H A M B E R where q is particle charge, c is a dimensionless characteristic constant of the chamber (around 0.8), z 0 is a characteristic length of the chamber (8 mm), V is the voltage necessary to levitate the particle at the center of the EDC, F,. is the drag force on the particle due to the gas free-convection, Fp is the photophoretic force, g is gravitational acceleration, m is particle mass, p and d are the particle density and diameter, respectively, t is time, and T is temperature. The two forces F~ and Fp in Eq. 1 (free-convection and photophoresis) complicate the determination of the particle mass from the voltage measurements in the EDC. These forces depend on the particle temperature, density, diameter, and shape. By modulating the laser beam the sum of free-convection and photophoresis can be uncoupled. The force balance on the particle varies with the modulation. When the laser power is zero, then Am rn o

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Am is particle mass loss due to conversion, m is mass, V is voltage, superscripts off and on are laser off and on, respectively, and subscript o is initial. Figure 7 presents a voltage recorded from a typical laser modulated experiment. Tempera~ure was constant to within 25 K. Since no reaction occurs when the particle is not hot, it is required to subtract the time with no laser radiation. Into Fig. 7 a close look-up of the voltage raw data is inserted. The time which should be subtracted is also indicated in the figure. During the laser-off period no reaction takes place and hence this time should be subtracted. The final, uncoupled, results are presented in Fig. 8, showing transients of particle weight ("laser-off") and the sum of weight, free convection, and photophoretic force (assigned as "laser-on"). Fitted polynomials are presented by solid lines in the figure. The diameter tran-

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sient of the particle is presented in the inset for comparison, showing the correspondence between the mass and diameter behavior. After the uncoupling, the time scale in Fig. 8 is half of that of Fig. 7. The particle weight changes little up to 800 s and then rapidly decreases. This behavior indicates some sort of activation prior to conversion. The particle temperature (900 K) was constant, to within 25 K, until the rapid decrease in weight when it rose by about 50-70 K. In Fig. 9 transients of conversion (top) and the ratio of the sum of free-convection and photophoretic forces to particle weight (bottom) are shown. (Conversion is defined as the fraction of particle mass loss due to reaction.)

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This ratio was determined from Fig. 8 by Eq. 3, Conversion stays constant up to about 800 s, then increases rapidly. The sum of flee-convection and photophoretic forces follows approximately the conversion behavior; however, it begins to rise a little earlier than conversion. Surprisingly, it grows quite rapidly to values equivalent to the particle weight. Of interest is the nonuniform shrinkage. The initially spherical particle shrank nonuniformly until it eventually became a disk [10, 13]. Over a hundred experiments showed a reproducible pattern of behavior. Plausible explanations are presented below. Free convection for spherical particles can be estimated from the study of Dudek et al. [8], who solved the Navier-Stokes equation and found a perfect fit between simulations and measurements. An empirical expression to correlate flee-convection force to temperature, density, and diameter was found to be

Fc = mg

PD(a o + aid + biT + b2 T2 + coTd p +clTd 2 + c2T2d),

-0.04416, a x = 0.0001 /xm -1, b 1 = 0.00027 K - l , b 2 = -6.3966 x 10-8 K-2, Co = -2.3198 X 10 - 7 K - 1 / x m - 1 , c 1 = -1.6806 X 10 - 9 K - 1 ~ m - 2 , c 2 = 2 . 9 5 3 9 X 10 - l ° K - 2 /zm-1. The expression is valid for the ranges of diameters 50-250 /~m and temperatures 500-1500 K. It holds only for spherical particles. Estimates of the contribution of the asphericity to free convection show no more than 5% difference [14]. Within this uncertainty, Eq. 4 can be used to estimate the free convection force and hence determine the photophoretic force from the measured value of p(t), T(t), and d(t). From Fig. 9 the sum of free-conversion force and photophoretic force were calculated as a function of conversion and these are shown in Fig. 10. Figure 9 also shows the free convection force as calculated by Eq. 4 and the photophoretic force (by subtraction). Surprisingly, while the free-convection force increases very slightly with conversion, photophoresis increases quite dramatically. This dramatic increase might be connected with the change in thermal conductivity of the char during oxidation. In a recent study by Weiss et al. [13] thermal conductivity was determined from photophoresis. Clearly, the change in thermal conductivity is caused by the evolution of the pore structure. In a study now in progress [15] the thermal conductivity of highly porous chars is being modeled. Non-uniform shrinkage is attributed mainly to uneven irradiation of the particle (the particle is heated from the bottom in these experi•

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C H A R C O M B U S T I O N IN E L E C T R O D Y N A M I C C H A M B E R ments). Experiments in a thermogravimetric analyzer [13] revealed uniform shrinkage under uniform heating. As shown from a modeling study of nonuniform shrinkage [16], this phenomenon sheds light into the evolution of the pore structure. As oxidation has been shown to proceed throughout the internal microstructure [17-19], the observed change in the shape cannot be ascribed to the preferential consumption on the overheated external surface exposed to radiation. The change in the shape is accompanied by a decrease in particle dimensions. The features of nonuniform consumption are rather unusual, since it reveals the threshold nature. Nonuniform shrinkage is clearly observed at a conversion of 40-50%. This shows preferential consumption beginning at the top of the particle, although the particle is heated from the bottom. As oxidation continues the disk configuration becomes more clear. At very high conversions the particle becomes a disk that is preferentially consumed from its center, to form a centered hole. The phenomenon of nonuniform shrinkage can be explained by the uneven transformation of the micropore structure [16] due to oxidation of unevenly heated particles. The microstructure is represented as randomly distributed interconnected microcrystals with a continuous break and restoration of the internal joints of the pore subskeleton [18]. It can be shown [16] that though the char particle is heated from the bottom, shrinkage occurs over the entire volume. As the contraction of certain zones is nonuniform, the outer surface becomes distorted. Nonuniform shrinkage, probably a unique feature of uneven heating, reveals the importance of the pore structure to the reactivity of char. The evolution of this structure can be further elucidated from the measurement of thermal conductivity of single particles, which is a unique feature of the EDC. The mutual effect of the pore structure and reactivity has received little attention. SUMMARY AND CONCLUSIONS The further development of a new position "ontrol system in the EDC enables the particle

459

to be maintained with oscillations of less than 0.5% of its diameter and severe changes in the force balance to be overcome. New improvements are also reported: (1) real-time size and shape measurements, based on shadowgraphy and photography; and (2) optical pyrometry with wide-band detectors in the infrared. Synthetic char particles, spherically shaped (Spherocarb) were oxidized in one atmosphere of oxygen under kinetically controlled conditions at around 900 K (heated by a focused CO 2 laser beam). Nonuniform shrinkage and activation of the char under oxidation were observed. The observation of the two phenomena of nonuniform shrinkage and activation of char during oxidation were possible due to the capability of the EDC to follow the entire transient conversion of a single particle. These phenomena are quantifiable. This study was partially supported by a United States-Israel Binational Science Foundation (grant number 88-157). The authors acknowledge the assistance of David Baum in operation of the position controller and in some combustion experiments.

REFERENCES 1. Bar-Ziv,E., and Sarofim,A. F., Prog.Energy Combust. Sci. 17:1-65 (1991). 2. Bar-Ziv, E., Jones, D. B., Spjut, R. E., Dudek, D. R., Sarofim, A. F., and Longwell, J. P., Combust. Flame 75:81-106 (1989). 3. Spjut, R. E., Bar-Ziv, E., Sarofim, A. F., and Longwell, J. P., Re~'. Sci. Instrum. 57:1604-1610 (1986). 4. Phuoc, T. X., and Maloney, D. J., Twenty-Second Symposium (International) on Combustion, Combustion Institute, Pittsburgh, 1988,pp. 125-134. 5. Maloney,D. J., and Spann, J. F., Twenty-SecondSymposium (International) on Combustion, Combustion Institute, Pittsburgh, 1988, pp. 1999-2008. 6. D'Amore, M., Dudek, R. D., Sarofim, A. F., and Longwell, J. P., Powder Technol. 56:129-134(1988). 7. Tognotti, L., Longwell, J. P., and Sarofim, A. F., Twenty-Third Symposium (International) on Combustion, Combustion Institute, 1990, pp. 1207-1213. 8. Dudek, D. R., Fletcher, T. H., Longwell, J. P., and Sarofim, A. F., Int. J. Heat Mass Trans. 31:863-873 (1988). 9. Weiss, Y., and Bar-Ziv, E., International Workshop on Heterogeneous Combustion, The Dead Sea, Israel, January 5-10, 1992.

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Y. WEISS AND E. BAR-ZIV

10. Weiss, Y., and Bar-Ziv, E., Combust. Flame 95:362-373 (1993). I1. Bar-Ziv, E., and deBotton, G., Aerosol Sci. Technol. 15:1-7 (1991). 12. Baum, D., Har-Nov, Y., Guterman, H., and Bar-Ziv, E., Rev. Sci. Instrum. 64:3627-3633 (1993). 13. Weiss, Y., Ben-Ari, Y., Bar-Ziv, E., Krammer, G., Modestino, A., and Sarofim, A. F., presented at Twenty-Fifth Symposium (International) on Combustion, 1994.

14. Krammer, Gernot, private communication.

15. Kantorovich, I., and Bar-Ziv, E., in preparation. 16. Kantorovich, I., and Bar-Ziv, E., submitted. 17. Kantorovich, I., and Bar-Ziv, E., International Workshop on Heterogeneous Combustion, January 1992, The Dead Sea, Israel. 18. Kantorovich, I., and Bar-Ziv, E., Combust. Flame 97:61-78 (1994). 19. Kantorovich, I., and Bar-Ziv, E., Combust. Flame 97:79-87 (1994). Received 22 September 1993; revised 16 September 1994