Energy & Fuels 1994,8, 925-931
926
Improved Diameter, Velocity, and Temperature Measurements for Char Particles in Drop-Tube Reactors Richard I?. Cope,? Charles R. Monson,l Geoffrey J. Germane,$ and William C. Hecker*lt Departments of Chemical Engineering, 350 CB, and Mechanical Engineering, 242 CB, and the Advanced Combustion Engineering Research Center, Brigham Young University, Provo, Utah 84602 Received November 12,1993. Revised Manuscript Received March 21, 1994'
Coal combustion researchers have typically used the average temperature and residence time of a burning particle cloud to determine the high-temperature reactivity of coals and chars. These average values, however, cannot account for particle-to-particle variations or their possible causes. Researchers at Sandia National Laboratories developed a pyrometry technique to simultaneously measure the temperature, velocity, and diameter of individual char particles bumingin a transparentwall flat-flame facility. This work reports two significant advances relative to the optical pyrometry technique. First, pyrometer modifications together with a new analysis technique now permit the particle properties to be measured for smaller/cooler particles. Second, the modified pyrometer has been implemented in two heated-wall drop-tube reactors, rather than transparent-wall, flat-flame burners. This is significant because drop-tube reactors allow greater flexibility/control of gas environments and operating pressures during char oxidation. Glowingreactor walls, however, present some unique challenges for these optical measurements. Means of overcoming these challenges are discussed, and reliable in situ measurement of particle temperatures, velocities, and diameters is verified. The results of measurements made in these drop-tube reactors, both for calibration tests and actual oxidation tests with Spherocarb and a Utah bituminous coal char, are also presented.
Introduction Numerous groups throughout the world are studying the high-temperature oxidation of coal chars. Findings from early studies are reviewed by Laurendeau,' Smith? and M ~ r r i s o n .More ~ current work is reported by Harris and Smith: Hurt and M i t ~ h e l l Leslie , ~ ? ~ et al.,7McCollor et al.? Mitchell,s-12 Sahu et al.,13J4Saito et al.,16 Young and Niksa,le Young et al.,17and others. These studies are + Department of Chemical Engineering.
Department of Mechanical Engineering. Abstract published in Advance ACS Abstracts, May 1, 1994. (1) Leurendeau, N. M. Prog. Energy Combust. Sci. 1978,4,221. (2) Smith, I. W. Nineteenth Symposium (Intemtional)on Combustion; The Combustion Institute: Pittsburgh, PA, 1982; pp 1045-1065. (3) Morrison, G. F. IEA Coal Res. 1986, publication ICTS/TR34. (4) Harris, D. J.; Smith, I. W. Proceedings of the International Conference on Coal Science; Butterworth-Heineman: Newcastle, UK, 1991; pp 259-262. (5) Hurt, R. H.; Mitchell, R. E. Twenty-Fourth Symposium (Internatronal) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; pp 1233-1243. (6) Hurt, R. H.; Mitchell, R. E. Twenty-Fourth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; pp 1243-1251. (7) Leslie.. I. H.:. Jost.. M.:. and Kruaer. C. H. Combust. Flame 1989,78, 195. (8) McCollor, D. P.; Young,B. C.;Jones, M. L.; Benson, S. A. TwentySecond Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1988, pp 59-68. (9) Mitchell, R. E. Combust. Sci. Technol. 1987,53, 165. (10) Mitchell, R. E. Twenty-Second Symposium (International) on Combustion;The Combustion Institute: Pittsburgh,PA, 1988, pp 69-78. (11) Mitchell,R.E. Sirth Annu. Int.Pittsburgh Coal Conf.,Pittsburgh, PA l989,32-41. (12) Mitchell,R. E.;McKee, R. J.; Glarborg,P.;Coltrin,M.E. TwentyThird Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990, pp 1169-1176. (13) Sahu,R.; Northrup, P. S.; Flagan,R. C.;Gavalas, G. R. Combust. Sci. Technol. 1988, 60, 215. (14) Sahu, R.; Levendis,Y.A.;Flagan, R.C.;Gavalas, G. R. Fuel 1988, 67, 275. t
0aal-0624l94/25oa-0925$04.50l0
leading to a better fundamental understanding of coal char oxidation and aiding in the development and evaluation of comprehensive combustion models. The usefulness of char kinetics obtained from droptube experiments depends largely on the reliable measurement of particle temperature, velocity, and size. Previous studies conducted in drop-tube reactors have not measured these values for individual particles, relying instead on average values from particle ensembles. Such average values, however,do not account for particle-to-particle variations or their possible causes. Researchers at Sandia National Laboratories developed an optical pyrometry technique to simultaneously measure these parameters for individual burning char particles in a transparent-wall flat-flame burner.ls This work reports two significant advances relative to the optical pyrometry technique. First, pyrometer modifications together with a new analysis technique now permit the particle properties to be measured for smaller/cooler particles. This is significant since commercial pulverized fuel boilers typically burn particles which are smaller than 100 pm (e.g., 70% of particles less than 74 pm is common). Also, the ability to see cooler particles permits measurement over a broader range of particle temperatures and thus a broader range of reactor conditions. Second, the modified pyrometer (15) Saito,M.; Sadakata,M.;Sato, M.;Sakai, T. Int. Chem. Eng. 1989,
29, 494.
(16) Young,B. C.; N i b 4 S. Proceedings of the International Conference on Coal Science; Elsevier Science Publishers B.V.: Amsterdam, The Netherlands, 1987; pp 819-821. (17) Young,B. C.; McCollor, D. P.; Weber, B. J.; Jones, M. L. Fuel 1988, 67, 40. (18) Tichenor, D. A.; Mitchell, K. R.; Hencken, K. R.; Niksa, S. TwentiethSymposium (International)on Combustion;The Combustion Institute: Pittsburgh, PA, 1984; pp 1213-1221.
0 1994 American Chemical Society
Cope et al.
926 Energy & Fuels, Vol. 8, No. 4, 1994 Secondary Gas Inlet Collection Probe
-a -Window
section
Vertical Reactor Section
T
Primary Gas Inlet 1
Feeder Preheater Injection Probe
1 .O meter
2.6 inc‘ters
C
ConnectorSection
Reaction Tube
Vertical Reactor Section Injection Probe
Window Section
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Secondary Gas Inlet
LJ
1
Primary Gas Inlet Collection Probe
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Figure 1. (a, left) Atmospheric-pressure drop-tube reactor cross section. (b, right) Elevated-pressure drop-tube reactor cross section.
has been implemented in two heated-wall drop-tube reactors, rather than transparent-wall, flat-flame burners. High-temperature char oxidationexperimentsare typically conducted in either drop-tubereactors or flat-flameburner facilities. Although the flat-flame burner closely approximates pulverized fuel combustor conditions, experiments are limited by the following: difficulty in obtaining isothermal gas temperature profiles, gas compositions comprising mainly combustion products (including significant concentrations of water), and decreased control of gas temperatures and operating pressures. Heatedwall drop-tube reactors, which can closely approximate typical combustor conditions,have a number of advantages for these experiments including relatively constant and well-characterized particle temperatures, unlimited gas compositions,direct sampling and in-situmeasurements of single particles, and operation at elevated pressures.
Experimental Section Drop-Tube Reactors. Two drop-tube reactors (DTRs), one operating at atmospheric pressure and the other at elevated pressures, have recently been constructed at Brigham Young University (BYU) to measure the high-temperature kinetics of coal char oxidation.19-21 Cross-sectional views of the two DTRa are shown in Figure 1. Each reactor’s 5.08 cm i.d. reaction tube is heated by MoSizresistanceheaters. Both reactors use nitrogen to transport the particles through uncooled (4.76mm i.d.) ceramic injection probes. As indicated in Figure 1,the particle inlet in the atmospheric-pressurereactor is located at the bottom whereas that of the elevated-pressure reactor is at the top. Collection systems allow sampling of the partially combusted char residue. The reactors have the following demonstrated capabilities: temperatures from 1000to 1800K, particle residence times from 30 to 2000 ms, variable gas compositions of inert and oxidizing (19) Monson, C. R. Ph.D. Dissertation, Department of Mechanical Engineering, Brigham Young University, Provo, UT, Dec. 1992. (20) Cope, R. F. Ph.D. Dissertation, Department of Chemical Engineering, Brigham Young University, Provo, UT, 1994. (21) Monson, C. R.; Germane, G. J. Energy Fuels 1993, 7,928.
Calibration system
Dmp-Tube Reactor
Particle Imaging System
: : : : : Secondary: : : : : Primary : . . . . . Lenses . . . . . Lenses .
Figure 2. Schematic of the optical pyrometer as implemented on the drop-tube reactors. gases, and optical access ports for in-situ diagnostics. The atmospheric-pressure DTR has a relatively constant gas temperature profile over the first 16 in. of the reaction zone. In the last 6 in. (the optical port region) the gas temperature decreases approximately 100 K/in. The elevated-pressure DTR is capable of maintaining an isothermal gas temperature profile over the entire reaction tube length because it contains individually controlled heating sections both before and after the optical port region. Other capabilities unique to the elevated-pressure DTR are: pressure from 1to 15 atm and a collection system that allows the separation and collection of tar, char, and gas. Optical Pyrometer. The opticaltechnique measures particle temperature by conventional two-color pyrometry, and in situ particle velocity and size by means of a coded aperture. As shown in Figure 2, the pyrometer consists of a particle imaging system which incorporates the coded aperture, a trigger system, and a calibration system. The role of each system in the pyrometer’s operation is discussed below, while technical specifications of their principal components are listed in Table 1. Incandescent particles moving along the reactor’s centerline are imaged at actual size onto the coded aperture by a pair of lenses. This coded aperture consists of a number of windows and blackouts (discussed later) which pass or block light emitted from the particle. The light transmitted through the aperture is split into two wavelength bands which are detected by a pair of photomultiplier tubes (PMTs) and recorded by a high-speed
Energy & Fuels, VoZ. 8, No. 4,1994 927
Char Particles in Drop-Tube Reactors Table 1. Pyrometer Components component primary lenses (cemented achromat)
technical specifications atmospheric-pressurereactor: 172 mm F.L. [f/3.81 elevated-pressurereactor: 356 mm F.L. [f/5.61 atmospheric-pressurereactor: 100 mm F.L. [f/3.31 elevated-pressurereactor: 127 mm F.L. [f/S.OI atmospheric-pressurereactor: 40 mm X 40 mm elevated-pressurereactor: 35 mm X 35 mm designed at BYU and manufactured by Align-Rite Corp. center wavelength (nm): 500; 700 bandwidth (nm):40; 40 Hamamatsu R928, thermoelectricallycooled to 250 K quantum efficiency: 16% at 500 nm; 4% at 700 nm 5 mW HeNe laser, 800 pm beam waist Thorn EM1 DA-603 laser detector Nicolet 2090-3C 2-MHz,12-bit digital oscilloscope Macintosh SE/30 microcomputer
secondary lenses (cemented achromat) beam splitter coded aperture narrow band-pass filters photomultiplier tube trigger system data acquisition data analysis 0
The large blackout between the first two windows completely occluded the 100-pmparticle and thus caused the signal voltage to return to the baseline. The fraction of the particle that was 10 pm occluded by the 70-, 25-,and 10-pm blackouts decreased with 25 pm decreasing blackout size, as evidenced by the decreasing depth 70 pm of the corresponding valleys. Particle Temperature, Velocity, and Diameter Determinations. Particle temperature and velocity are determinedfrom the traces in a manner similar to that previously used by Tichenor et al.18 Particle temperature is obtained by conventional two1 color pyrometry using unoccluded signal intensities (the broadest Signal Level (volts) peak in Figure 3) in both the 500- and 700-nm traces, together with Wien’s law and the gray body assumption. Particle velocity is determined from the broad peak’s width (i.e., time for the Figure 3. The BYU coded aperture and representative 100 pm particle to traverse the aperture’s 1000-pm window). diameter particle traces. Particle size is extracted from the traces by means of a unique fitting technique developed during this work. Because the 700digitizing oscilloscope. The result is a pair of traces that contains nm trace is typically the stronger of the two signals, it is used to information describing the particle temperature, velocity, and make the size determination. An initial estimate of the particle size. diameter is made in like manner to the Sandia technique: the The trigger system ensures that sampled particles are in focus trace is low-pass filtered (at 25 kHz) and the ratio of the occluded and centered on the coded aperture. A laser beam is positioned and unoccluded signal intensities (peak heights and valley depths) normal to the optical path and immediately upstream of the are determined and related to particle size. This estimated sample volume. While all particles passing through the laser diameter is, however, subsequently refined by a geometric/ beam scatter laser light, only those traveling along the reactor statistical analysis routine.23 The routine, derived from the centerline scatter this laser light through the aperture and into geometrical relations of a partially occluded circle, uses the the laser detector. The laser detector, upon receiving scattered particle’s velocity and estimated diameter to generate a noiseless laser light, generates a trigger pulse that initiates sampling by ideal particle trace. The ideal trace is compared to the measured a digital oscilloscope. (unfiltered)700-nmtrace. The measured diameteris determined The calibration system is built into the pyrometer to provide by adjusting the estimated diameter until the best agreement is temperature, velocity, and diameter standards for calibrating obtained between the ideal and measured traces. In this manner the instrument. During calibration, a particle image is projected a large portion of the particle trace is used to determine the into the sample volume of the reactor. The image is formed by particle size, thereby increasing the measurement signal-to-noise focusing a tungsten strip lamp of known temperature onto a ratio above that possible with the Sandia technique. A comseries of pinholes mounted on a rotating disk. The pinholes chop mercially-available, nonlinear, general optimization package, the lamp light and form a particle image, which is projected into OPTDES.BYU,N determinesthe particle diameterthat produces the pyrometer’s sample volume by a pair of lenses. the best agreement between the ideal trace and the measured The key element in the particle imaging system is the coded trace. Agreement is quantified by the sum of the squared aperture. As shown in Figure 3, it contains a series of carefully differences between the two traces. The fitting technique is sized blackouts and windows. The narrow window is part of the applied only to the occlusion portion of a particle trace (between trigger system. The largest window, through which a particle is the aperture’s second and fifth windows, inclusive) where most fully visible, allows determination of the particle temperature of the size information is contained. Figure 4 shows the good and speed. The three remaining windows are separated by three agreement obtained between a known measured trace and its carefully sized blackouts which facilitate particle size measurecorresponding ideal (fitted) trace. Traces generated when a 100-pmparticle traveled past Pyrometer Implementation on the Heated-Wall Dropthe aperture are also displayed in Figure 3. Although the traces Tube Reactors. Significant development of the pyrometerwas appear somewhat ragged because of optical and electronic noise, required in order to adapt it to the two heated-wall drop-tube several features are readily apparent. Five major peaks and four reactors. A single burning particle emits significantly (ordersvalleys are prominent in the Figure 3 traces. All five peaks are of-magnitude) less energy than the glowing reactor walls, so the relatively flat, indicating that the particle was completely visible reactor light had to be eliminated from the pyrometer optical in each of the aperture’s five windows. The four valleys, which correspond to blackouts between the windows, indicate that (23) Cope,R. F.; Hecker,W. C.; Mown, C. R.; Germane,G.J. Preaented different amounts of the particle were occluded by each blackout. at the Western States Section;The Combustion Institute. LOBAngeles, CA, Oct. 1991. (24)Parkinson, A. R.; Balling, R. J.; Free, J. C. h o c . ASME Int. (22) Wells, W. F., personalcommunication;BrighamYoungUniversity, Comput. Eng. Conf., Las Vegas, NV 1984. Provo, UT, 1988. f
500nm
3 700nm
Cope et al.
928 Energy & Fuels, Vol. 8,No.4, 1994 Particle Parameters Temp: 1776K
-0.1
~
"
500
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"
1700
"
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"
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Time (psec) Figure 4. Comparison of a measured trace to ita ideal fitted trace.
I
I 900 1100 1300 1500 1700 1900 2100 Wall Temperature (K) Figure 6. Apparent particle temperature as a function of actual particle and wall temperatures, assuming full wall reflectance. path. This was accomplished, in part, by the diametricallyopposed view porta. The porta provided a clear view through the reactor and a cold background behind the glowing particles. Modifications to both the pyrometer and the reactor view porta minimized the amount of indirect wall emissions that entered the optical path. The view porta were enlarged and the near port was fitted with a water-cooled aperture to mask reactor radiation. An iris aperture was also added to the pyrometer (at the primary lens) and the alignment between the reactor and pyrometer was carefully optimized to further reduce background light. Collectively, these steps have adequately reduced signal noise from the glowing reactor walls. The possibilityof reactor wall radiation entering the pyrometer indirectly, as a result of reflection (or refraction) off of char particles passing through the sample volume, was also investigated. The effect of such reflections on particle temperature measurements was quantifiedusing Wien's law and the standard assumption of gray body particles. Char particle emissivity was conservatively taken as 0.8, while the reactor wall emissivity was set at 0.4.B "Worst-case" calculations determined the apparent particle temperature produced by summing radiation from both the particle and an equivalent area of the reactor wall (i.e., full wall reflectance). Calculated apparent particle temperatures are shown in Figure 5, as a function of actual particle temperature and reactor wall temperature. Actual particle temperatures were varied between 1400and 2000 K, the temperature range observed in the B W drop-tube reactors. Figure 5 shows that apparent (25) Touloukian, Y. S . Thermophysical Properties of Matter, Plenum: New York, NY,1970; Vol. 8, p 837.
IFI/
and actual particle temperatures differ by less than 2% when the particle is hotter than the reactor wall. Under such conditions, the maximum error (525 K)occurs when the wall is approximately 100 K cooler than the particle. If the wall temperature exceeds that of the particle, however, apparent particle temperatures approach the wall temperature and large errors are introduced. Since the preceding "worst-case" analysis assumed full wall reflectance, further calculations were performed to determine actual wall reflectance. The scattering behavior of a 25-pm char particle was determined%by solvingthe Maxwell electromagnetic equations which describe a particle's radiation field. It was assumed that the particle was spherical and had a complex refractive index of 2.0 - il.O.n In Figure 6a, incident radiation strikes the particle from a singleincident angle. The figureshows that nearly all scattered radiation (represented by the shaded region) leaves the particle in the forward direction (180" from the incidentradiation). TheahadedregioninFigure6brepresents the field of radiant energy scattered off the same particle as it passes through the optical plane of the elevated-pressure reactor. This radiation field was obtained by considering wall emissions that strike the particle from all possible incident angles (Le., integrating Figure 6a around the entire reaction tube), while allowing no radiation to be emitted from the view porta. Figure 6b indicates that, within the optical pyrometer's field of view, the relative intensity of scattered wall emissions is at least 2 orders of magnitude lower than it is at ita maximum (45" off the optical axis). The pyrometer actually collecta less than 1% of the radiation from the reactor walls, 80 particle temperature errors computed under the assumption of full wall reflectance (shown in Figure 5) are overestimated by more than 2 orders of magnitude. Consequently, reflected wall emissions have little effect on the particle temperature measurements, even if the reactor wall is hotter than the particle. Pyrometer Performance. The performance characteristics of the pyrometer were determined using the calibration system shown in Figure 2. The magnitude of the unoccluded 700-nm signal (the broad peak in Figure 3) was examined over a range of calibration temperatures and particle sizes and found to increase as either parameter increased. This magnitude has an exponential relationship with particle temperature, TpJand a squared dependence on particle diameter, d,,, as indicated by the following form of Wien's law. Ai is a system-dependent attenu-
ation constant [V/mal composed of the first radiation constant, the particle emissivity and the signal wavelength, Xi 1700 nml. C,, is the second radiation constant [1.439 X 10.'pm.Kl. Signal noise is critical to pyrometer performance in that it obscures trace features and thus dictates minimum values of particle temperature and size that are measurable. The strength of the geometric/statistical sizing technique is that it successfully interpreta previously unreadable signals from colder/smaller particles, in spite of excessivesignal noise. In ita fully automated mode, this s i z i i technique interpreta signals as low as 250 mV (e.g., Figure 4). If the technique is applied with user interaction, the detection limit is further extended to approximately 100mV. Successfully interpreted particle temperature/size pairs, which correspond to a 100 mV detection limit, are shown in Table 2. Note that particles as small as 25 pm can be detected if the corresponding particle temperature is at least 1800 K. Figure 7a-c contain the resulta from a number of calibration rum, illustrating the precision and accuracy associated with each of the pyrometer measurementa.19 Each of the pointa represents an average of 30 analyzed particle traces with the exception of the lowest temperature points at each size,which were generated usingabout tentraces each. Error bars show 1 standard deviation. In each case the standard deviation increases with decreasing ~
(26) Bohren,C.F.;Huffman,D.R.AbsorptionandScotteringojLight by Small Particles; Wiley: New York, 1983. (27) Menguc, M. P.;Viekenta, R.Combust. Sci. Technol. 1987,51,51.
Char Particles in Drop-Tube Reactors
Energy & Fuels, Vol. 8, No. 4,1994 929 90’
(b)
(a)
Figure 6. (a) Radiation scatter plot and (b) scattered-radiation field (700nm wavelength) for a 25-pm particle in the optical plane of the elevated-pressure reactor. Table 2. Successfully Interpreted Temperature/Diameter Pairs from Particle Signals of Approximately 100 mV temp (K) 1375 1475 1600 1800 diameter (pm) 100 75 50 25 ~~
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temperature and size. Errors in the measured temperature and size for the 50 pm and larger particles are seen to be fairly small, e.g. f12 K, &5 pm, and 11.2 cm/s at ZOO0 K. Errors in the same measurements for the 25-pm particles are significantly larger. This is a result of a decreased signal-to-noise ratio (while the noise component in the 25-pm traces is of the same magnitude as those in the larger particle traces, the signal is much smaller). The particle diameter measurements all have a positive offset that increases with decreasing size. This offset is accounted for in actual measurements with a calibration equation. In all cases, errors in the velocity measurements are less than 3 cm/s when measured velocities range from 1to 2 m/s. The preceding measurements were made using ideal particle images (spherical, uniform temperature, and known emissivity) generated by the calibration system at typical reactor wall temperatures. Background noise from the hot reactor wall is included in the calibration traces but additionalnoise from actual char particles is not present. As a result, errors in actual char measurements may be somewhat larger than those discussed above. The larger noise component in char measurements is a result of nonspherical particles, varying particle emissivity (and possibly some nongray behavior), nonuniform surface temperatures, and collected light from other particles in the vicinity of a sampled particle. Although char particles are not necessarily spherical, scanning electron micrographs indicate that most (particularly cenospheres)are rounded. While temperature and velocitymeasurements are not affected by a nonsphericalparticle, the determined size would effectivelybe the height of the particle (the dimension of the particle image in a direction perpendicular to the aperture occlusions). Measurements in the elevated-pressure reactor showed a pressure limitation for the pyrometer. At pressures above 10 atm, density gradients in the gas distorted particle images and prevented size measurements below 100 pm. Accurate temperature and velocity measurements were obtained over the complete operating range of the elevated-pressurefacility (1-15 atm pressure).
Results The pyrometer described above has been used in both the atmospheric- and the elevated-pressure drop-tube reactors to measure the particle properties of Spherocarb (a commercially available synthetic char) and several coal
~
chars. Particle properties (temperature, velocity, and diameter) determined from these measurements and the associated analyses, hereafter referred to as “predicted” particle properties,will now be discussed. Char reactivities determined from these particle properties have been reported else~here.~9,~0@ The 500- and 700-nm traces obtained from a burning 125/150 pm Spherocarb particle are shown in Figure 8. Both traces have the same general appearance, but significantly different signal magnitudes. “Predicted” particle properties determined from these two traces are inserted in the figure. No independent means of checking particle temperature was available, but the “predicted” value correlates well with that obtained from an energy balance calculation. The “predicted” particle velocity closely matches the calculated gas velocity, as expected, since these particles experiencelittle slip. The “predicted” particle diameter of 127 pm is within the diameter range of the starting particles. Figures 9,10, and 11show the results of measurements made while oxidizing 63-74 pm (-200/+230 mesh) Utah bituminous coal char particles in the elevated-pressure drop-tube reactor.lg Particles in this size range would not be detectable by the Sandia pyrometer as described by Tichenor et al.18 Reactor conditionsfor these experiments were as follows: atmospheric pressure, 21% oxygen, 1130 K wall temperature, 1200 K gas temperature, and 42-ms particle residence time. Subsequentanalyses indicated a burnout of 47 % on a dry ash free (daf) basis, an apparent activation energy of 15 kcal/mol, a decrease of 7% in particle diameter, and a decrease of 30% in apparent density. These results indicate that the particles were burning on both internal and external surfaces. Figure 9 shows the 700-nm traces obtained from four representative Utah coal char particles, along with the idealtrace that the analysisroutine fit to each. “Predicted” values of temperature, velocity, and diameter and the calculated sum-of-squared error, determined during the fitting process, are also shown for each trace. The agreement between the measured and ideal traces is excellent, and taken with the calibration discussed previously, imparts confidence in these “predicted” values. Figure 9, a and b, illustrates the effect of particle (28) Monson, C. R.; Germane, G. J.; Blackham, A. U.;Smoot, L. D. Combust. Flame, in press.
930 Energy & Fuels, Vol. 8, No. 4, 1994
Cope et al.
407
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temperature on signal magnitude. The hotter particle's signal is nearly two times that of the cooler particle (3.75 versus 2.0 V),yet the traces indicate that the two particles are of comparablesize (the firstvalley in both signalsbriefly touches the dashed baseline). Noise has a similar magnitude in each of these traces, but a stronger signal lessens its significancein the hotter particle's trace. Consequently, fitting an ideal trace to the stronger signal produced a smaller sum-of-squareddifferences (7V2)than fitting one to the weaker signal (27V2). "Predicted" diameters of 58 and 52 pm closely approximate a 7% decrease in the 63pm starting particles. Figure 9, c and d, illustrate the dependence of signal magnitude on relative particle size. These figures were generated by two different particles with similar tem-
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peratures and velocities, but different diameters (49 and 33 pm). The larger particle's signal is 50% greater than that of the smaller particle (1.5versus 1.0 V). The 33-pm particle produced deeper valleys (relative to the dashed baseline) than the 49-pm particle, because a larger fraction of its projected area was occluded by the aperture's three blackouts. As might be expected, the sum-of-squared
Energy & Fuels, Vol. 8, No. 4,1994 931
Char Particles in Drop-Tube Reactors 30
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Figure 10. Particle diameter histograms for Utah bituminous coal char (a) as measured by a Coulter LSlOO before oxidation, and (b) as measured in situ by the pyrometer after partial oxidation (ca. 47% burnout daf).
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'
temperature 1250 K, 0 2 partial pressure 0.21 atm, and particle residence time 46 ms) with Utah bituminous coal char. No relationship between particle temperature and diameter is apparent from the individual values (open squares). A fairly large spread in particle temperature is observed at fixed values of particle diameter. These variations are believed to be the result of physical and chemical differences between the comparably-sized particles. It is interesting to note that, under these reaction conditions, particle temperatures are 450-800 K hotter than the gas. The closed circles in Figure 11are average temperatures for all particles contained within each of seven particle size bins (31-38,38-45,45-54,54-65,6578, 78-94, and 94-112 pm). The closed circles indicate that, excepting the 33-pm point which is the average of only two particles, particle temperatures decreased by about 70 K as particle diameter increased from 40 to 105 pm. Similar cross plots of particle size and temperature have been shown for larger particles reacting in a flatflame reactor,9sm but Figure 10 is the first such representationfor either (1)particles below 40pm or (2)particles reacting in a heated wall drop-tube reactor. The particle temperature/diameter relationship (decreased temperature with increased size)shown in Figure 11is not universal, since the opposite trend has also been observed in this study and in the preceding citations. Rather, this relationship is dependent on burnout level, char type and reaction conditions (oxygen concentration and reactor temperature).
Conclusions An optical pyrometer has been implemented on two heated-walldrop-tubereactors. Associatedproblems were successfully overcome to reduce noise in in situ measurements of particletemperature, velocity, and size. A unique geometric/statisticalfittingtechnique has been developed to improveparticle size measurementsand to significantly reduce the lower limit on particle temperature/size measurements such that a 25-pm diameter can be measured for particles at 11800 K. Results from oxidation tests with Spherocarb and Utah bituminous coal chars have verified that particle temperatures, velocities, and diameters are reliably determined in a size range significantly smaller than that obtained by the Sandia system.l* This measurementtechnique will enable char reactivities to be accurately determined for a variety of reacting char particles, under a variety of conditions, in both atmospheric- and elevated-pressuredrop-tube reactors.
Acknowledgment. The authors appreciate the financial support of this work by the Advanced Combustion Engineering Research Center (ACERC)/BYU and Advanced Fuels Research (AFR)/Department of Energy (METC). The assistance of C. B. Arrington, L. Swindlehurst, A. R. Parkinson, and B. G. Porter is also greatly appreciated. (29) Hurt, R. H.; Hardesty, D. R. US DOE QuarterlyProgress Report for contract DE-AC0476DP00789;Sandia National Laboratories, Livermore, CA, May, 1991; pp 2-1-2-28.
