The particle size, the temperature, and their interre-lation are objects of interest wherever pulverizedhigh-temperature particles are utilized or studied.Such applications are, for example, pulverized coalcombustion and coating by thermal plasma jets. Si-multaneous nonintrusive in situ measurement ofthe temperature and the size of individual movingparticles is a difficult task. Several groups pre-sented solutions for particle temperature and sizemeasurement in entrained flow and drop-tube com-bustion reactors.1–8 The solutions are similar inthermal plasma jets.9,10 In combustion reactors andthermal plasmas, particle temperatures are mostlymeasured by using optical two-color pyrometry. Forparticle sizing, however, a variety of methods areemployed: the coded aperture method,1–3 particleimaging,4–6 pyrometric particle sizing,7–9 and scat-tering of light.10 Modern laser-based sizing meth-ods, recently reviewed by Black et al.,11 includeseveral methods that are also applicable to particlesizing in combustion reactors and thermal plasmajets.
A two-color pyrometric method capable of simulta-
The authors are with the Department of Physics, Plasma Tech-nology Laboratory, Tampere University of Technology, P.O. Box692, FIN-33101 Tampere, Finland.
Received 22 December 1997; revised manuscript received 20February 1998.
neous in situ measurement of the temperature andthe size of individual fuel particles was recently de-veloped7 for particle measurements in a pressurizedentrained flow reactor. The particle sizing is basedon the proportionality between the particle cross-sectional area and the radiative flux emitted by aparticle at a known temperature. Since the methodis based on the magnitude of the flux, it is importantfor correct sizing that the full image of the particle bevisible to the detecting optics at the moment of sizeevaluation. For this purpose a particle discrimina-tion procedure based on two-focus optics was devel-oped. However, this procedure is applicable only toparticles moving in regular trajectories parallel to theoverall flow direction. Furthermore not all fully vis-ible particles can be included in the sizing. In thispaper a new particle discrimination procedure is pre-sented. The new procedure allows for irregular par-ticle motion, and the fraction of observed particlesthat are eligible for sizing is maximized. Results arepresented of experimental verification of the methodusing graphite particles in an inert gas flow.
The ambiguity in the measurement due to the par-ticle’s trajectory through the measurement volume iscrucial for all amplitude-based sizing methods, and anumber of solutions for this have been presented~e.g., two-color laser techniques, trigger beams, andcoded aperture methods!. The novel discriminationtechnique introduced in this paper has some advan-tages ~it is simple and compact; there are no lasers, noalignment of crossing optics, and no flow directionrequirements! compared with others, and it can also
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be applied to other techniques that require full visi-bility of particles.
An application to pulverized coal combustion ispresented to demonstrate a solution to a real prob-lem. In laboratory scale studies fuel particles aretypically in the range from 50 to 200 mm, the gastemperature is typically 1000–1700 K, and the par-ticle velocity is typically 1–3 mys. Measured tem-peratures have varied from the ambient gastemperature to as high as 2900 K.
2. Theory
The optical setup of the pyrometric measurements ina flow reactor is shown in Fig. 1. All observationsare made in one direction, and therefore only oneoptical port is necessary. In measurements with oneport only the thermal radiation from the reactor tubewall forms a background to the pyrometric signals.If two collinear optical ports are present, however,this background radiation is absent. This also al-lows measurement of the particles having tempera-tures close to or even lower than the reactor walltemperature.
The radiation emitted by a fuel particle is collectedthrough a lens system and focused to the end of anoptic fiber bundle. In the middle of the bundle is onelarge fiber, called the primary fiber. This fiber isused for pyrometry. The primary fiber is sur-rounded by smaller fibers, called reference fibers.They are used for particle discrimination. The fiberbundle is bifurcated so that the primary fiber formsone leg and all reference fibers form the other one.The radiation entering the primary fiber is measuredin a radiometric unit over two separate wavelengthbands, and the radiation entering the reference fibersis measured collectively in a separate unit over asingle wavelength band.
A. Particle Temperature and Size Determination
When a fuel particle hotter than the backgroundpasses through the field of view ~FOV! of the primaryfiber, a corresponding increase is detected in the py-rometric signal levels. Figure 2 shows the trajectoryof the particle’s image in the plane of the endface of
Fig. 1. Optical setup of the pyrometric particle measurements atan entrained flow reactor.
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the fiber bundle and the corresponding signals as afunction of time. The particle temperature Tp issolved using the ratio of the pulse heights measuredat the two wavelength bands. Tp can be solvedfrom7
where Ri 2 R0i is the pulse height at wavelengthband li~i 5 1, 2!, R0i is the system response when noparticles are in the FOV ~i.e., a signal value betweenthe peaks!, and Fi~T! is the system response cali-brated against a blackbody radiator at temperatureT. In two-color pyrometry it has been assumed thatthe particles are gray. The possible errors caused bynongrayness have been analyzed in Ref. 7.
The pyrometric response signal is proportional tothe particle cross-sectional area in the FOV. Whenthe particle temperature is known from measure-ment, this proportionality can be used for particlesizing. The cross-sectional area of particle Ap can besolved from7
εp ApyA0 5 ~Ri 2 R0i!y@Fi~Tp! 2 R0i#, (2)
where εp is the emissivity of the particle and A0 is thecross-sectional area of the FOV in the focal plane.The diameter Dp of an equivalent spherical particle iscalculated from Ap. If the emissivity of the particleis unknown, it leads to a corresponding uncertaintyin the value of Ap. In practice, coal particle emis-sivities are high, and the error is no greater than 10%when the emissivity is set to 0.9.12,13 The corre-sponding uncertainty in Dp is 66%.
B. Particle Discrimination
It follows from Eq. ~2! that to be correct, the particle-size evaluation must be performed at a moment whenthe whole particle is visible to the detecting device.It is clear from Fig. 2 that this condition is met onlywhen no radiation from the particle enters any of thereference fibers. If a particle does not meet this con-dition at any moment during its passage through theFOV, it is not eligible for sizing and must be rejected.This is done with the particle discrimination proce-
Fig. 2. Time dependence of primary and reference signals when afuel particle passes through the FOV of the optical probe.
dure ~PDP!. Figure 2 explains the temporal behav-ior of the light fluxes entering the fibers. Theeligibility criterion is met only if the light flux fromthe particle to the reference fibers goes to zero whilethe particle is in the FOV of the primary fiber. Thisis the basis for the PDP.
If the trajectory of the particle through the FOV isnot in the focal plane, the size of the particle image inthe fiber end plane is increased compared with theimage resulting from passage in focus and the imagebecomes blurred. As a consequence of the enlarge-ment of the image, the extent to which the particlemay deviate from the center line of the FOV without
Fig. 3. Region of full visibility for particles of different size. Themaximum deviation from the center line of FOV dr as a function ofdeviation from focal plane dx. The walls of the 70-mm reactortube are indicated by dashed lines ~effective focal length, 190 mm;fiber diameter, 1 mm; distance from optics to focal plane, 710 mm;lens aperture, 25 mm; magnification, 0.366!.
violating the discrimination criterion becomes pro-gressively smaller the more out of focus the particleis. Figure 3 shows an example of the region of ac-ceptance for sizing as a function of the deviation fromthe focal plane in the object space ~i.e., inside thereactor! dx and the deviation from the center line ofthe FOV dr. The center line is the x axis. Theactual region of acceptance for sizing is a rotationallysymmetric region in space generated by the rotationof the contours around the center line. The widthand the length of this region depend on the opticalarrangement. The example in Fig. 3 represents theactual geometry of the probe that was used in thepresent measurements, and the reactor walls are in-dicated by the dashed vertical lines.
The sizing procedure is based entirely on the mea-surement of light fluxes. Image quality is not im-portant. The method works also if the particletrajectory crosses the FOV outside the focal plane.In this case a small error occurs due to the variationin the solid angle of light collection. A comprehen-sive error analysis was performed in Ref. 7 and needsnot be reiterated here. In the present measuringgeometry the maximum size uncertainty due to out-of-focus crossing is 66%.
In Fig. 4 are shown six examples of typical signalsgenerated by a fuel particle passing through the FOVof the fiber bundle. These particles have been mea-sured at the Pressurized Entrained Flow Reactor~PEFR! of the Institute of Process Engineering andPower Plant Technology ~IVD! at the University ofStuttgart. The description of the reactor and the
Fig. 4. Typical signal pulses measured at a PEFR. Pulses ~a!, ~b!, and ~c! are accepted for particle sizing, whereas pulses ~d!, ~e!, and~f ! are rejected. The particle temperature can be determined in all six cases. Primary signals are shifted upward for clarity ~Garzweilerbrown coal, Tg 5 1370 K; p 5 0.3 MPa; c@O2# 5 12 vol.%, tr 5 300 ms!.
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experimental arrangement are given below. The de-termined particle temperature Tp and size Dp ~diam-eter of equivalent spherical particle! are given in theleft top corner of each figure. In Fig. 4~a! the particlepasses through the FOV close to the center line sothat it is fully visible to the primary fiber for ;3 ms,during which the primary signal is constant and thereference signal is at the background level. In Fig.4~b! the situation is similar, but the particle is signif-icantly colder. This is illustrated by the lower signallevels and signal-to-noise ratio. In Fig. 4~c! the par-ticle’s trajectory deviated from the center line of theFOV, and thus the primary signal reaches its maxi-mum and the reference signal its background levelonly momentarily. The particles in Fig. 4~a!–4~c!fulfill the discrimination criterion, and they are ac-cepted for sizing. In Figs. 4~d!–4~f ! the deviations ofthe particles’ trajectories from the center line of theFOV are too large and the reference signal does notrevert to the background level at the moment of theprimary signal maximum. Therefore sizing cannotbe performed correctly for these particles, and theyare rejected from the sizing process by the PDP. TheDp in the brackets is the apparent size, i.e., the frac-tion of the particle that was visible to the primaryfiber at the pulse maximum.
3. Experimental
A. Particle Pyrometer
The setup of the particle pyrometric device is shownin Fig. 1. The apparatus contains an optical probe,a bifurcated fiber bundle, a two-color pyrometer, areference signal detector, and a standard PCequipped with an analog-to-digital converter for dataprocessing.
The optical probe has a focusing two-lens systemthat has been optimized so that the effective focallength is the same at the center wavelengths of bothbands of the two-color pyrometer. In the probe usedin the measurements at the PEFR of the IVD theeffective focal length of the lens combination is 190mm, the lens aperture is 25 mm, the magnification is0.366, and the distance from the optics to the focalplane is 710 mm.
The receiving end of the fiber bundle contains aprimary fiber ~B1 mm! that is surrounded by 19smaller reference fibers ~B0.2 mm!. On the otherend the bundle is bifurcated between the primaryfiber and the reference fibers. The jackets and thebuffers of the fibers are stripped at the receiving endto minimize the gap between the active areas of thereference and the primary fibers. The size and thenumber of reference fibers were selected so that themaximum packing density can be achieved in theclose neighborhood of the primary fiber.
The radiation entering the primary fiber is trans-mitted to the two-color pyrometer, which is describedin Ref. 7. The wavelength bands were centered at650 and 1064 nm having bandwidths ~FWHM! of 60and 10 nm, respectively. In selecting the wave-lengths, we took care to avoid any interference with
3490 APPLIED OPTICS y Vol. 37, No. 16 y 1 June 1998
emission and absorption bands in the gas phase.7The radiation entering the reference fibers is trans-mitted to a separate radiometric unit where it is col-lectively measured over a wavelength band centeredat 1064 nm with a bandwidth of 10 nm.
The pyrometer was calibrated against a blackbodyradiator in the temperature range of 1173–1473 K.The accuracy of this calibration was better than61.5 K. Planck’s radiation law was used to extrap-olate the system response to temperatures of morethan 1473 K. Thus the uncertainty of calibrationgrows with increasing temperature. On the otherhand, the uncertainty in particle-temperature deter-mination due to the electric noise grows with decreas-ing particle temperature and size. The sources ofuncertainty in the particle sizing are the uncertaintyof the temperature determination @cf. Eq. ~2!#, of thecalibration, of particle emissivity, of the solid angle,and of the electric noise. For individual particleswith Tp . 1500 K and Dp . 50 mm the total uncer-tainty of Tp is better than 630 K, and the total un-certainty in the sizing of gray particles is at most615%. A detailed error analysis is given in Ref. 7.
B. Pressurized Entrained Flow Reactor
The PEFR of the IVD has a ceramic reactor tube of70-mm inner diameter and 2.0-m length. The reac-tor is equipped with observation ports at three levels.The pyrometric measurements were made at the firstlevel, which is located 560 mm below the top of thereactor tube. There are four ports at each level.Two of the ports form a collinear pair, and one ofthese was used for pyrometry. The reaction gas isheated by an electrical preheater. Pulverized fuel iscarried by nitrogen through a water-cooled injector toa mixing nozzle located at the top of the reaction tube.The preheated reaction gas and the fuel are mixed inthis nozzle and led to the reaction tube. The heatingzones that surround the reaction tube keep the tem-perature within narrow limits. The fuel particlesburn separately as a diluted suspension in the gasflow. The number of particles per unit gas volumewas less than 3 particlesycm3. Owing to the lowparticle density, the experimental conditions ~gastemperature, oxygen concentration, and flow pattern!remained nearly constant during the combustion.The residence time was calculated on the assumptionof a plug flow pattern.
4. Results and Discussion
To assess the reliability and the accuracy of the sizeand the temperature measurement, we injected par-ticles of pulverized graphite into the flow of nitrogenand measured their temperature and size using themethod described above. The experiment was madeat two gas temperatures, 1662 and 1504 K. Thepressure was 0.15 MPa and the measurement posi-tion corresponded to a particle residence time in thereactor of 150 ms. The graphite powder was firstmechanically sieved, and then the fine particles wereaerodynamically removed from the fraction. The
Fig. 5. Results of the parti-cle temperature and sizemeasurement of pulverizedgraphite in a pure nitrogenenvironment at two gas tem-peratures: ~a! TemperatureTp and size Dp of individualparticles. ~b! Temperaturedistributions measured atboth gas temperatures. ~c!,~d! Size distribution mea-sured at 1662 and 1504 K.~e! Initial size distributionmeasured by optical imaging.The average value and thestandard deviation are alsogiven for each distribution~p 5 0.15 MPa, tr 5 150 ms!.
size distribution of the powder was determined byoptical imaging.
The results of the measurements are shown in Fig.5. Figure 5~a! shows ~Tp, Dp! pairs for individualgraphite particles at the two gas temperatures. Thefigure shows stochastic distributions for particle tem-perature and size for the two gas temperatures.Temperature and size are entirely uncorrelated inboth cases. This bears evidence for the indepen-dence of the sizing procedure from the particle tem-perature.
The particle-temperature distributions measuredat both gas temperatures are shown in Fig. 5~b!.The average particle temperatures are 1507 and1649 K, which are compared with the measured gastemperatures 1504 and 1662 K, respectively. Thestandard deviations are 15 K in either case. The gastemperature was determined from the mass flow andthermocouple measurements of preheated gas andfuel carrier gas. The accuracy of gas-temperaturedetermination is better than 610 K in the case ofperfect mixing of the two gas flows. However, theactual mixing is not necessarily perfect. Thereforespatial and temporal gas-temperature fluctuationsmay have been present. The agreement betweenthe gas temperatures and the average particle tem-peratures is remarkably good. The fluctuation of
the measured particle temperatures is modest. Tosome extent it probably reflects a true particle-temperature fluctuation caused by temperature fluc-tuation, but part of it may be due to electric noise.
The size distributions measured by the pyrometerat 1662 and 1504 K are shown in Figs. 5~c! and 5~d!,respectively. For the emissivity of graphite the 0.88value14 was used. The mean sizes ^Dp& of these twodistributions are almost identical ~190 and 193 mm!,and the standard deviation sD is 30 mm in either case.This is further evidence for the temperature indepen-dence of the sizing procedure. The initial size dis-tribution measured by optical imaging is shown inFig. 5~e!. Only particles larger than 50 mm weretaken into account in the analysis. The mean diam-eter is 5% larger than in the pyrometrically measureddistributions and the standard deviation is 36 mm.Thus the distribution measured by optical imaging isin good agreement with those measured by pyrom-etry. The difference in mean size can be explainedby the uncertainty of the emissivity for graphite.Measured values of the emissivity of graphite varyfrom 0.78 to 0.92.14–17 If a value of 0.80 is used foremissivity instead of 0.88, the mean diameters ob-tained by pyrometry are both closer than 1 mm to thevalue obtained by optical imaging. On the otherhand, the optical imaging has a slight tendency to
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overestimate the particle diameters, because the ir-regularly shaped graphite particles tend to settle onthe analyzer plate preferentially with their largestdimension facing the direction of observation,whereas in the PEFR directional isotropy of the par-ticles must be assumed. No attempt has been madeto quantify the extent of this effect. The rising edgeof the distribution in Fig. 5~e! is found to be somewhatsteeper than the one in Fig. 5~c! and 5~d!. This dif-ference is understandable when one considers theuncertainty in the pyrometric sizing due to the vari-ation in solid angle and electric noise. In summary,it can be stated that the agreement between the sizedistributions is remarkably good and that the ob-served minor differences are well understood. Theresults clearly indicate that the uncertainty in parti-cle diameter caused by the factors identified above iswell within the limits obtained by error analysis.
The coal-combustion experiments were made withtwo pulverized coals of different type. The effects ofprocess conditions on the fuel particle temperatureand size distributions were studied. A large num-ber, typically 3000–5000, of individual fuel particleswere measured at each fixed process condition. Thefuel particle temperature was determined for all thedetected particles. Typically between 45% and 60%of the detected particles passed the PDP, and forthese particles the size was determined too. Theresults of these experimental series will be reportedand analyzed elsewhere.18
Two examples of the results of pyrometric particlemeasurement at fixed conditions are shown in Fig. 6to demonstrate the method. The particle tempera-ture Tp and size Dp of individual particles of pulver-ized Gottelborn high volatile bituminous coal isshown in Fig. 6~a! and Garzweiler brown coal in Fig.6~b!. The process conditions were fixed during mea-surement of the population. The gas temperaturewas ;1450 K, the pressure was 0.3 MPa, and oxygenconcentration was 12 vol.%. This comparison dem-onstrates that the Tp versus Dp dependence may varyconsiderably between different coal types. Since themeasurements were made against a cold background,the detection of particles at the gas temperature waspossible. The particles with temperatures aroundthe gas temperature are ash particles that haveburned out completely. The agreement between theburnout particles and the estimated gas temperatureis good in Fig. 6~b!, but in Fig. 6~a! the average of themeasured temperatures of the burnout particles is;15 K below the estimated gas temperature. Thisdeviation can be caused, at least partially by non-grayness of the ash particles.
The detection limit of the pyrometric device is alsoindicated in Fig. 6. The detection limit, which isdefined by the minimum signal-to-noise ratio atwhich a particle can be detected, is a result of anall-inclusive optimization of the device ~optics, wave-length bands, detectors, signal amplification! for theobject of the measurement ~particle size, tempera-ture, velocity and number density, spectral featuresof object and media!.
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The extent of the region where full visibility of theparticle can be achieved depends on the particle size.We can assume that the particles have linear trajec-tories in the measurement volume, and the compo-nent of the velocity vector in the direction of the lineof sight is small compared with the diameter of thereactor. Thus the probability of the detection of afully visible particle is proportional to the area withinthe contours in Fig. 3. In a 70-mm reactor tube thisarea is 20% larger for 50-mm particles than for400-mm particles, and this difference should be takeninto account in analyzing the results of measure-ments of the type shown in Fig. 6~a!. However, thedependence is relatively small ~7%! within such a
Fig. 6. Particle temperature Tp and size Dp of individual ~a! Got-telborn high volatile bituminous coal and ~b! Garzweiler browncoal particles. The process conditions were fixed during measure-ment of the population. The detection limit of the pyrometricdevice ~solid curves! and the estimated gas temperature ~dashedlines! are also indicated.
narrow size fraction ~150–275 mm! that was mea-sured in the graphite experiments ~Fig. 5! and it canbe neglected ~D^Dp& , 1 mm!.
5. Conclusions
A method for simultaneous in situ measurement ofthe temperature and the size of individual particlesin flow reactors was presented. The method is basedon two-color pyrometry, and both parameters are de-termined from the same pyrometric signals mea-sured at two wavelength bands. A novel particlediscrimination procedure based on coaxial referenceoptics was developed for particle sizing. The proce-dure makes use of reference signals obtained from abundle of thin optical fibers surrounding the primaryfiber. A major advantage of the method is that oneoptical viewport is sufficient for the measurements,and it can be used for measurement of the particlesmoving irregularly. Another advantage is that ab-errations of the image of the particle do not affect thesize measurement. No more than one particle at atime may be present in the FOV. This sets an upperlimit for the particle loading at which the method canbe applied. The pyrometric-sizing procedure de-pends on the emissivity of the particle, but the un-certainty arising from this is not great when coal orchar particles are measured. The measurement ap-paratus is simple and compact, it can be built atrelatively low cost, and it is very flexible for modifi-cation and installation into reactors of different typeand dimensions. The discrimination method basedon coaxial reference optics can also be useful for otherapplications where full visibility of detected particleis required, for example, particle-sizing methodsbased on the amplitude of scattered radiation.
Testing the method and the measurements wasdone at the pressurized entrained flow facilities ofVTT Energy, Finland, and Institute of Process Engi-neering and Power Plant Technology ~IVD!, Univer-sity of Stuttgart, Germany. The authors thankMartti Aho from VTT Energy and Hartmut Spliethoffand Tobias Reichelt from IVD for pleasant coopera-tion.
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