Improved Temperature Measurements of Burning Char and Coal

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Energy & Fuels 1996, 10, 1133-1141

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Improved Temperature Measurements of Burning Char and Coal Particles Using an FT-IR Spectrometer Sønnik Clausen* Optics and Fluid Dynamics Department, Risø National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark

Lasse Holst Sørensen ReaTech, c/o Centre for Advanced Technology, P.O. Box 30, DK-4000 Roskilde, Denmark Received March 18, 1996. Revised Manuscript Received June 10, 1996X

A novel method for temperature measurements on individual burning char and coal particles with an FT-IR spectrometer has been developed. The technique is demonstrated for monitoring emission spectra of individual moving particles that require a few milliseconds to pass the field of view of a conventional scanning FT-IR spectrometer. The accurate particle surface temperature is calculated from a best match of the measured emission spectrum to a detailed physical radiance model spectrum. The technique is applied to measure the surface temperature of 90-125 µm particles with temperatures from 1000 to 2200 K in an entrained flow reactor. A one-temperature calibration of the FT-IR spectrometer is sufficient for accurate measurements throughout a broad temperature range. Background radiation and a fluctuating particle feeding rate are handled by subtraction of two successive measurements. The single-particle emission spectra are useful for testing the assumptions about particle emissivity as a function of wavelength. The findings in the present work justify the graybody assumption for the burning char particles as well as the burning coal particles. Under sooting conditions particle temperature errors of about 300 K were observed. The burn-off for four coal samples is analyzed and compared with particle temperatures at 1, 3, 6, 12, and 21 vol % oxygen. In addition to giving important information on the modeling of the combustion process, the particle temperature measurements and the burnoff give information that can be used for ranking of coal samples with respect to reactivity.

Introduction For combustion of solid particles, mass conversion and the derived heat production are main topics. Many groups have spent great efforts to study the hightemperature devolatilization and oxidation of coal chars. Typical high-temperature devolatilization and oxidation measurement are conducted in experimental facilities such as the flat-flame burner, the drop-tube reactor, and the entrained flow reactor. Good data are valuable since they lead to better direct understanding of particle combustion and aid in the development and evaluation of comprehensive combustion models. In many burner and flow reactor experiments, only particle mass loss is measured as a function of location and, thus, residence time if the gas velocity profile in the reactor is known. The conversion temperaturetime history is then modeled from the measured burnoff values with assumptions about the chemical kinetics as well as the evolution of the physical form and internal structure of the solid particle, i.e. a structural profile. Though leading to valuable and meaningful results, such results are still philosophical in nature. Contradictory results obtained by different modelers using the same data can only be satisfactorily settled if a number of parameters are properly measured; examples are the distributions of particle temperature, size, and velocity. Chemical parameters of interest are related to the high* Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, August 1, 1996.

temperature kinetics and to the heterogeneity of the sample and cannot be encompassed in detail. In some flat-flame burner and flow reactor studies, average temperatures from particle ensembles have been reported.1-3 However, due to the fact that particle-toparticle temperature variations may be large, average ensemble temperatures only roughly describe the important properties of the fuel sample. Consequently, kinetics determination may be deteriorating and meaningless. To get better data by the use of two-color pyrometry, in situ individual particle temperature, size, and velocity measurements in transparent-wall flat-flame burner systems have been carried out by Mitchell,4 Tichenor et al.,5 and Fletcher.6 Cope et al.7 recently used twocolor optical pyrometry in two heated-wall drop-tube reactors. The two- and three-color pyrometers are popular devices for rapid measurements of surface temperature (1) Peck, R. E.; Altenkirch, R. A.; Midkiff, K. C. Combust. Flame 1984, 55, 331-340. (2) Sørensen, L. H.; Biede, O.; Peck, R. E. Twenty-fifth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1994; pp 475-483. (3) Saastamoinen, J. J.; Aho, M. J.; Linna, V. Fuel 1993, 72, 599609. (4) Mitchell, R. E. Twenty-second Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1988; p 69. (5) Tichenor, D. A.; Mitchell, R. E.; Hencken, K. R.; Niksa, S. Twentieth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1984; pp 1213-1221. (6) Fletcher, T. H. Combust. Sci. Technol. 1989, 63, 89-105. (7) Cope, R. F.; Monson, C. R.; Germane, G. J.; Hecker, W. C. Energy Fuels 1994, 8, 925-931.

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1134 Energy & Fuels, Vol. 10, No. 5, 1996

of single objects and short-lived events, when the temperature is determined from intensity measurements at two or three wavelengths. The two-wavelength pyrometers have typically been used for measuring the surface temperature of single moving 50250 µm diameter particles in the 600-2000 K range. The average error from calibration experiments is typically about 30 K in the temperature interval investigated depending on simulated particle size and temperature. The drawbacks are that preliminary assumptions have to be made, i.e. that the particles behave as graybodies following Wien’s law, and that the calibration of the system at many temperatures is elaborative. For a wide range of applications an instrument that makes measurements at two or three wavelengths is adequate, but little spectral information is obtained. In contrast to the poor spectral resolution of the pyrometer, infrared emission spectra can be obtained using a conventional FT-IR spectrometer with tens to thousands of spectral points. The emission spectra contain valuable information about the process, e.g. about the gas and the solid phase temperature in some specific spectral bands. Solid surfaces radiate a continuous spectrum, while infrared active gases radiate and absorb at characteristic spectral lines of the molecule; e.g. CO2 has strong bands at 2350 and 3700 cm-1 (see Figure 2). FT-IR spectroscopy has so far been applied on a group of particles or a flame for the analysis of particle emittance, composition, size, and temperature.3,8 In such experiments the experimental conditions must be stationary while the data are collected; i.e., a large constant number of particles have to be present in the field of view and the background must be constant. This is difficult to control in practice. For individually reacting particlessdifferent in size, conversion, and temperaturesthe mean temperature of the monitored sample represents some average value that may be misleading. The main contribution of this paper is the presentation of a new FT-IR technique for measurement of temperatures of individual burning particles moving ∼1.2 m/s. Measurements on moving pinholes, i.e. ideal particles with unit emissivity, have been presented elsewhere.9 In the present work the method has been extended to include the spectral emissivity of the particle and the reflected incoming radiation. One major advantage is that during in situ particle temperature measurements, emission spectra are monitored and validated. In other techniques spectral information is fixed a priori, which may eventually lead to false interpretation of data. Advantages of using the single-particle FT-IR technique are outlined, and results as well as detailed particle temperature data are presented for a char and a coal sample injected into an entrained flow reactor (EFR) under well-controlled conditions. Finally, it is shown how particle temperature and burn-off measurements can be used directly to evaluate and rank coal samples with respect to their reactivity. Experimental Equipment FT-IR Spectrometer. A commercially available FT-IR spectrometer, Bomem Model MB155, was used in all measure(8) Solomon, P. R.; Carangelo, R. M.; Best, P. E.; Markham, J. R.; Hamblen, D. G. Fuel 1987, 66, 897-908. (9) Clausen, S.; Sørensen, L. H. Measurement of Single Moving Particles Temperatures with an FT-IR Spectrometer. Appl. Spectrosc. 1996, in press.

Clausen and Sørensen

Figure 1. Optical setup for measurements of surface temperatures of single particles. E1, emission port used; M1, a focusing mirror (f ) 152.4 mm); A1, a field of view aperture; M2, a focusing mirror (f ) 101.6 mm); A2, aperture; FP, focal point. The distances between the components are |E1-M1| ]) 165 mm, |M1-A1| ) 180 mm, |A1-M2| ) 147.5 mm, and |M2FP| ) 320 mm.

Figure 2. Response function of spectrometer, optics, atmosphere, and ZnSe window. CO2 has strong bands at 2350 and 3700 cm-1. The lower curve is the response function measured by using a 200 µm pinhole in front of a blackbody at 1273 K. The upper curve is the actual response function in the experiments on the flow reactor corrected for the reduced path length in air (Figure 5). The spectral resolution is 32 cm-1. ments with a selected spectral resolution of 16 cm-1. The scan rate is 7.2 double-sided interferograms per second at 16 cm-1 resolution. The optical scan velocity is vopt ) 2.06 cm/s. An indium antimony (InSb) detector was used, giving a spectral range from ∼1750 to 7900 cm-1. The optical components are arranged on a small platform mounted on the spectrometer in front of the emission port (E1), as illustrated in Figure 1. Radiation is collimated with two off-axis paraboloidal concave mirrors. The first mirror (M1) collimates radiation into the spectrometer. The spectral response was uniform except at the border of the field of view (FOV). The ∼22 mrad half-angle FOV of the spectrometer is limited to 12 mrad by an iris (A1) with a 3.4 mm diameter opening placed in focus of the first mirror to improve the uniformity of the spectral response. The image of the particle is focused by the second mirror (M2) on the iris with a magnification of 0.46. The FOV of the spectrometer is defined by the size and shape of the iris, equivalent to a diameter of the measuring volume of 7.4 mm. A 30 mm aperture (A2) is placed in front of the second mirror to reduce aberrations from shape errors of the mirror. The response function of the instrumental setup must be found by calibrating it against a blackbody (Figure 2). The path length in air is 812.5 mm during calibration, but it was 607.5 mm in the experiments. Therefore, the measured response function is corrected in spectral intervals with strong gas absorption bands to reduce spectral distortions. A one-

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Temperature Measurements of Burning Char and Coal

Figure 3. Measured gas mean velocities along the center as a function of height above injection at 1173 K (dashed line) and 1603 K (solid line). Velocities were measured by a laser Doppler anemometer. Small TiO3 particles (d ≈ 1 µm) were added to the gas flow for measuring the gas velocity. The velocity profiles reflect the development of the gas flow and confirm that the carrier gas temperature is close to the reactor temperature. However, the gas flow is cooled by the unheated window section (Figure 7) that spans from 250 to 400 mm. temperature calibration is sufficient for the calibration with pinholes or small particles compared with the FOV of the spectrometer, as shown in the next section. A blackbody source, Mikron Model M360, was used for calibrating the spectrometer. The accuracy is specified as (0.2% of the temperature. The blackbody has a 25 mm diameter cavity opening with an emissivity of 0.998. The temperature calibration is traceable to international standards (NIST). As described by Clausen et al.,9 the collected data at 16 cm-1 spectral resolution were validated during operation on successive subtracted interferograms and application of a 128 cm-1 spectral resolution at the Fourier transformation to enhance the data rate. The effective collection rate of the spectra is limited to approximately three spectra per second using a 486 33 MHz PC. The validated interferograms at 16 cm-1 were stored for future analysis with other settings of the truncated spectral regions, resolution, thresholds, etc. The spectra were then analyzed with a spectral resolution of 32 cm-1 to enhance the signal-to-noise ratio. Entrained Flow Reactor. An electrically heated 62 mm × 1 m long laminar entrained flow reactor (EFR) described elsewhere by Sørensen et al.10 was used. The reactor is operated at uniform temperature except for the 150 mm long unheated window section. Temperatures of the three 300 mm long heated sections and the preheated reaction gas before the ceramic flow straightener are measured with Pt/Rh thermocouples for control and regulation of the reactor temperature. The particles are injected into the reactor with a nitrogen carrier gas and move along the center upward through the reactor tube. The nitrogen carrier gas of 0.7 L/min is preheated to the reactor temperature. The gas flow develops from a plug flow at the exit of the flow straightener toward a laminar flow profile upward through the reactor.10 The carrier gas is injected with higher velocity than the main gas flow to carry the fuel particles into the reactor and to maintain the particles at center after injection. Optical access to the EFR is available through optical ports 324 mm upstream from the injection of the particles. A laser Doppler anemometer was used to measure gas and particle velocities through observation windows (see Figure 3). The velocity profile close to the flow straightener (L ≈ 0 mm) varies with reactor temperature (10) Sørensen, L. H.; Clausen, S.; Astrup, P.; Rathmann, O.; Biede, O.; Lund, J. S. Experimental High-Temperature Investigations of Coal Particles; Risø National Laboratory Report Risø-R-871(EN), 1996 (available from Information Service Department, Risø National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark; telephone +4546774677, ext. 4004/4005; telefax +4546755627).

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Energy & Fuels, Vol. 10, No. 5, 1996 1135

Figure 4. Disturbance of gas flow by collector probe as a function of gas flow in the probe. The three curves are measured with laser Doppler anemometry for distances of 2.5, 12.5, and 22.5 mm between the point studied and the probe tip. Particles were sampled through a 4 mm hole in the collector probe. The mean velocity is 1.23 m/s in the flow reactor with the collector probe removed.

Figure 5. Experimental setup for measuring particle temperatures in an EFR. The window section of the reactor is mounted with a 50 mm ZnSe window (W) to allow optical access to the center of the reactor where the particles are passing. A low-temperature blackbody (BB) at the opposite measuring port is used as background. as the carrier gas flow is kept constant. The velocity of the carrier gas at L ≈ 0 mm confirms that the carrier gas flow has the reactor temperature. Furthermore, gas temperatures measured with a miniature suction pyrometer agree with reactor temperatures measured with Pt/Rh thermocouples. The development of the gas flow profile along the reactor center agrees with a fluid mechanical calculation.10 The velocity profiles are used to calculate particle residence times in the reactor. Particles were sampled and quenched with nitrogen using a 25 mm diameter insulated water-cooled collector. The disturbance from the sampling probe of the gas flow is small except very close to the collector probe tip (see Figure 4). The particles were collected in a filter. Particle temperature is measured in the window section, at position L ) 324 mm upstream. Samples are collected at various distances. Experiments are made at reactor temperatures of 1273 and 1603 K, oxygen partial pressures of 0, 1, 3, and 6 vol %, and residence times of 50-191 ms. The experimental setup for measuring particle temperatures is illustrated in Figure 5. A ZnSe window was originally mounted opposite the measuring port, but reflection of radiation from the reactor by the window (∼30%) gave a strong background saturating the detector/AD converter. Replacing this window with a water-cooled blackbody made of a 50 mm brass disk with V-grooves and coated with dull black paint reduced the background radiation significantly. Thermogravimetric Analyzer. The solid samples were subject to proximate and ultimate analysis. An SDT 2960 Simultaneous TGA-DTA Analyzer from TA-Instrument was used for the TGA proximate analysis. As the sample is progressively heated and weighed in the TGA, the effluent products are carried away in an N2 flow during devolatilization and during oxidation in an N2-O2 gas flow. Burn-off, symbolized by X, is calculated using the ash tracer technique. X ) (M0 - M)/M0, where M is the organic fraction of the sample.

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Samples. A Norwegian coke sample from Longyear, Swalbard, was used in the char experiments. Coke particle sizes were analyzed on a number basis by image processing to 139 ( 18 µm of a 90-125 µm sieve cut. A Colombian coal sample (Cocerr) and three other coals (Samidd, Idpina, and Aublat), 25-45 and 90-125 µm sieve cuts, were used for the investigations. All particle temperature measurements were made on 90-125 µm sieve cut samples.

Measurement of Single-Particle Temperatures with FT-IR In an earlier work9 it was found that accurate lowresolution radiance spectra can be obtained with an FTIR spectrometer from rapid moving pinholes undergoing a large temperature gradient during a single doublesided scan. To obtain an informative spectrum of the particle, it is essential that the particle is present in the FOV of the spectrometer at zero path difference (ZPD). With the low particle concentration used in the experiments, i.e. ∼2 particles/cm3, it is highly probable that there will be no particles (≈97%) or a single particle (≈3%) present in the FOV at the time ZPD is passed during scanning of a measurement (interferogram). If a significant signal appears from two subsequent subtracted measurements, a particle is most likely detected. The signal is validated if it is symmetric, i.e. the particle is centrally located, and if the backgrounds of the previous and subsequent measurements are identical. The method was tested with a rotating calibration disk using different pinhole trajectories through the FOV along the optical axis and sideways. The mean monitored temperature was within (4 K, and the standard deviation varied from 4 K at the optical axis to 12 K near the edge. Physical Model of the Radiance from a Particle. The signal from a single particle is weak, which can be illustrated by the fact that the emitted energy falling on the detector element from a 297 K background is equal to a 100 µm particle at 2500 K. The method may appear unnecessarily complicated at first sight, but applying it is easy, accurate, and informative. The physical model for the emitted radiation from a large opaque particle (dp . λ) with temperature T is expressed as the product between the Planck function and the spectral emittance (emissivity) of the particle plus ambient radiation reflected by the particle. Radiation from the background and the instrument itself must be added to the recording. Using the formalism of Revercomb et al.,11 the complex spectrum is given by

projected area of the particle, A is the area of FOV cross section, Ω is the the solid angle of acceptance of the instrument, ΩIAI is the product of solid angle and area of the instrument, ψ(ν˜ ) is the phase response for internal instrument radiation, B(ν˜ ) is the background radiation, and G(ν˜ ,TI) is the offset from instrument emission, referred to input. Other terms might be included in eq 1, e.g. background radiation from gas, but they do not change the final result. The offset term, G(ν˜ ,TI)eiψ(ν˜ ), can be eliminated from eq 1 by subtracting the interferograms prior to Fourier transformation. This procedure is identical with the two-temperature method used to eliminate “background effects” and phase correction problems.12 The same procedure is used here to eliminate the offset term and all kinds of background radiation. The subtraction method is often used in high-quality infrared systems for correction of instrument radiation, but can also be used to eliminate disturbances from stationary gas emission bands; e.g. thermal emission from H2O and CO2 in the reaction gas is eliminated by using the subtraction method. Wavelength bands of pyrometers are normally chosen to avoid interference with emission from gas bands;13 i.e., they can only be used in spectral regions without gas radiation. Only interferograms scanned in the same direction having the same phase characteristics must be subtracted. Subtraction of spectra will not work because the offset term is significant compared with the weak particle signals. Applying Kirchhoff’s law, the reflectance for an opaque source is

F(ν˜ ,T) ) 1 - (ν˜ ,T)

(2)

The area of the particle is assumed to be small compared with the area of the FOV, and the background radiance is much smaller than that from the particle. Ap scales to A with an approximate factor 1:5000 with a particle diameter of 100 µm and a diameter of the FOV of 7 mm; therefore

A - Ap ≈ A

(3)

The signal from the particle is found by subtracting a measurement with no particle (the background) from a measurement with one particle present in the field of view. When we have inserted eqs 2 and 3 into eq 1 and phase corrected the interferogram, the spectrum (real) from the particle is

fp(ν˜ ,T) ) R(ν˜ ){(ν˜ ,T)L(ν˜ ,T) + I(ν˜ )[1 - (ν˜ ,T)]}ΩAp

emission

{

{

f ′(ν˜ ,T) ) R(ν˜ )eiφ(ν˜ ){[(ν˜ ,T)L(ν˜ ,T) + I(ν˜ )F(ν˜ ,T)]ΩAp + reflected

{

(1)

{

B(ν˜ )Ω(A - Ap) + G(ν˜ ,TI)ΩIAIeiψ(ν˜ )} background

instrument emission

∞ F(x)e-i2πν˜ x dx (f ′ is complex), ν˜ is where f ′(ν˜ ,T) ) 1/2∫-∞ the wavenumber (1/λ), x is the retardation of the optical path length, F(x) is the interferogram, R(ν˜ ) is the spectral response function of the spectrometer (real), φ(ν˜ ) is the phase response of the instrument, (ν˜ ,T) is the spectral emittance (emissivity) of the particle, L(ν˜ ,T) is the radiance from a blackbody at temperature T, I(ν˜ ) is the incoming radiation on the particle, F(ν˜ ,T) is the reflectance of the particle, Ap ) d2pπ/4, which is the

(4) Equation 4 is the basic equation for measuring the particle radiance and the surface temperature by applying subtraction of the interferograms. The FT-IR spectrometer must be calibrated against a blackbody ( ≈ 1) at a given temperature to find the response function R(ν˜ ) of the instrument. The spectral emittance (ν˜ ,T), the projected area Ap, and the particle temperature are the fundamental unknowns when the incoming radiation on the particle is given. The magnitude of R(ν˜ ) varies as a function of the position in FOV and, (11) Revercomb, H. E.; Buijs, H.; Howell, H. B.; LaPorte, D. D.; Smith, W. L.; Sromovsky, L. A. Appl. Opt. 1988, 27 (15), 3210-3218. (12) Griffiths, P. R.; A. de Haseth, J. Fourier Transform Infrared Spectroscopy; Wiley: New York, 1986; Vol. 83, pp 15-19 and 109. (13) Saastamoinen, J. J.; Martii, J. A.; Ha¨ma¨la¨inen, J. P.; Hernberg, R.; Joutsenoja, T. Energy Fuels 1996, 10, 121-133.

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Figure 6. Introduced error on the measured particle surface temperature due to reflected blackbody radiation from surrounding walls at 973 K (dashed lines) and 1273 K (solid lines). The particle emissivity is chosen as (ν˜ ) ) 0.7, 0.8, and 0.9. The spectral regions 2550-3500 and 4100-5000 cm-1 were applied in the simulations as well as in the experiments. The true particle surface temperature is given on the abscissa. The apparent (measurement) particle temperature is given by the true particle temperature plus the error.

therefore, it is not possible to estimate the particle temperature from the signal magnitude as the particle trajectory through the FOV is not known in the present study. Instead, the particle temperature is estimated from the shape of the emission spectrum. The singleparticle FT-IR method might be improved in future work using a laser as pointer or by recording a shadow image by backlighting the particle at times of zero path difference during scanning, which is a better procedure. If FT-IR is combined with simultaneous measurements of particle size and position at ZPD, the particle emissivity can be estimated. The spectrometer has a 220 ns TTL trigger signal output at ZPD during scanning for synchronization with external devices to, for example, trigger the grabbing of shadow images with a CCD camera to reveal the particle position and area. It is well-known among laboratories measuring temperatures of reflective surfaces that are below the ambient temperature with a pyrometer that reflected ambient radiation must be handled to get an accurate temperature measurement of the object. Similarly, the introduced error on the measured particle temperature due to reflected radiation by the particle must be handled. This is illustrated by an example. The incoming radiation from the wall on the particle is well approximated with blackbody radiation for an isothermal EFR reactor. The particle temperatures in Figure 6 are calculated from a least-squares fit to the shape of the particle emission spectrum calculated from eq 4 in the regions 2550-3500 and 4100-5000 cm-1 assuming constant emissivity and that results are not corrected for reflected wall radiation. In other words, the particle is assumed to emit as a graybody and the emissivity of the particle is not taken into account in calculation of the curves in Figure 6. When the reactor is colder than the particles, Tp is underestimated. The correction is important when Tp < Treactor. When Tp > Treactor, Figure 6 indicates that particle temperatures are overestimated. A correction is straightforward when the emissivity and incoming radiation on the particle are known. Although particle temperature measurements are feasible when Tp < Treactor, accurate values of Treactor and are essential.

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Figure 7. Gas temperature profile along the EFR axis. Furthermore, 324 mm upstream, monitored particle temperatures of Longyear coke particles in nitrogen are indicated. Mean particle temperature is symbolized with a box and one standard deviation marked with error bars. The unheated window section spans from 250 to 400 mm.

The following results are not corrected for reflected wall radiation as the effect plays a minor and systematic role in this work for two main reasons. First, the particles are hotter than the reactor wall at the window section. The wall of the window section makes a total of 91% of the total solid angle (4π). The effective wall temperature is estimated to be ∼1273 K at the unheated window section for a reactor temperature of 1603 K. It is likely that ≈ 0.9 at the conditions used in the experiments. From this, the error ranges from around 0 K for Tp ≈ Treactor to a maximum of -25 K at Tp ≈ 1900 K. The impressive match of the measured emission spectrum and prediction shown in the next section is a strong indication that reflected radiation is not a problem. A poor match of measurement and prediction would be observed when the particle temperature error is large. Second, the minor error is systematic in nature and results are primarily used for comparison of different reacting coal samples obtained under identical operation conditions of the reactor. Therefore, in the present paper we prefer to present results without any corrections. However, when required, measurements can be reanalyzed or Figure 6 can be used for corrections of the presented results. Results and Discussion Char Particle Measurements. EFR experiments are made at a reactor temperature of 1273 K and for oxygen partial pressures of 0, 3, 6, 12, and 21 vol %. The gas temperature in the center of the reactor was measured by a miniature suction pyrometer; the gas temperature, Tgas, is shown as a function of axial position in Figure 7. The window section is not heated, which explains the decrease in gas temperature. The wall temperature also drops at the window section. For a 140 µm coke particle in nitrogen, the monitored particle temperatures were Tp - Tg < 10 K. From these results we estimate the radiation temperature, Trad ≈ Tg. This allows us to estimate the gas temperature at the window section from particle temperature measurements on coke particles in nitrogen. All data were obtained at approximately 0.5 g/h particle feeding rate. The feeding rate was manually adjusted from observation of the particle trajectories (see Figure 8), and the FT-IR signals were displayed on-line on a PC monitor. Increasing the particle concentration by a factor of 5 did not result in false particle temperature measurements, but the likelihood that

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Figure 8. Particles being collected with the suction probe. The particle feeding rate and the collection can be observed from particle trajectories. The hot ceramic tip of the collector probe appears bright in the picture.

Figure 9. Examples of interferograms of burning Longyear char particles at 0, 6, 12, and 20.9 vol % oxygen. The particle velocity is estimated at 1.1 m/s from the dip in the interferograms close to the gas velocity of 1.23 m/s found from laser Doppler velocimetry. The corresponding particle emission spectra are shown in Figure 10.

more than one particle turns up during measurement of the central part of the interferogram is increased. The capture of a particle in the FOV can be observed in the interferogram as shown in Figure 9. The mean velocity of the particles can be estimated from the dips in the interferograms. The cloud of gas released from the particle is elongated in the direction of the gas flow that is visible in interferograms where the thermal emission from CO2 is strong, e.g. the fine signal oscillations seen in Figure 9 at 20.9% oxygen caused by CO2 emission observed before the particle enters the FOV (-12 to -3 ms). An excellent graybody curve fit to the measured spectra is seen in Figure 10. The statistical uncertainty is about 1 K at low particle temperatures, increasing with temperature to ∼3 K. Four examples of emission spectra and best fit curves from individual char particles at 0, 6, 12, and 21% O2

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Figure 10. Examples of single-particle emission spectra and the corresponding least-squares fit curves. The particle temperature is estimated at 1095 K in nitrogen and the statistical uncertainty at 1 K. The spectral regions 2550-3500 and 41005000 cm-1 were applied to avoid disturbances from gas bands during combustion except in the experiment in nitrogen. The gas bands of CO2 are visible in the spectra with oxygen in the gas. The spectral resolution is 32 cm-1.

Figure 11. Particle temperature histograms for Longyear coke particles with 0, 6, 12, and 20.9 vol % oxygen. The histograms are based on measurements from approximately 70 particles at each setting.

are shown in Figure 10. The fit of the data with the physical model is excellent for results obtained on char particles. Notice the gas band from CO2 at 2350 cm-1 when combustion is taking place (6-21% O2). The gas band rises from the cloud of evolved gases from the burning particle, i.e. CO2 and perhaps CO. The width and height of the emission band reflect the mean gas temperature around the particle and the gas concentration. The interpretation of the gas emission spectra is beyond the scope of this paper. In Figure 11 four examples of particle temperature histograms at 0, 6, 12, and 21% oxygen are given for approximately 70 particles presented in each of the histograms. The particle temperatures increase with oxygen partial pressure. At 12 and 21% O2 the particle temperature distribution becomes bimodal. It is likely

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Figure 12. Coal particles in nitrogen at Treactor ) 1603 K. Particles and soot are sampled with the suction probe. The hot ceramic tip of the probe appears bright in the picture. The laminar flow and the good sampling efficiency of gas and particles are illustrated.

Figure 13. Examples of single Cocerr coal char particle emission spectra and the corresponding least-squares fit curves. The spectral resolution is 32 cm-1.

that the low-temperature fraction is burning kinetically and the high-temperature fraction is burning mainly diffusion controlled. The result obtained here confirms modeled results made by Sørensen et al.14 The models were very sensitive to small variations of model parameters under present conditions. Coal Particle Measurements. EFR experiments are made at an aimed reactor temperature of 1603 K and for oxygen partial pressures of 1, 3, 6, 12, and 21 vol %. For Cocerr coal samples soot formation contaminates the sampled particles at 0 vol % oxygen (Figure 12) and the quantitative determination of volatiles easily fails; furthermore, the particle temperature measurements fail. Therefore, in the present work, experiments were carried out at 1 vol % oxygen or higher, at which volatiles are burned. The curve fits to the measured coal char particle spectra are presented in Figure 13. The standard deviations on the curve fits are on average ∼1.5 times higher than those found for the char particles. Particle temperature histograms are shown in Figure 14. An experiment with coal particles was made at 0 vol % oxygen to study the effects from soot radiation on measurements. The expected true particle temperature was estimated at 1459 K from measurements on Longyear coke particles in pure nitrogen as illustrated in Figure 7. For the coal particles the temperatures split into two groups with mean temperatures of approximately 1462 and 1783 K. Due to the inert condition, the highest temperatures are obviously false. Figure 15 shows the spectrum for a coal particle with an estimated temperature of 1793 K. The curve fit to the measured spectrum appears visually poor and must be rejected. The emissivity of soot soot is roughly proportional to the wavenumber,15 i.e. soot ∝ ν˜ , when ν˜ < 12 500 cm-1. Therefore, the measured particle temperature is expected to be too high when the particle is surrounded by a soot cloud, as actually observed in the experiment described. Notice that emission rather than

Figure 14. Particle temperature histograms for Cocerr coal at four oxygen partial pressures.

(14) Sørensen, L. H.; Saastamoinen, J.; Hustad, J. E. Fuel 1996, in press. (15) Siegel, R.; Howell, J. R. Thermal Radiation Heat Transfer; Hemisphere: New York, 1981; pp 658-668.

Figure 15. Example of a coal particle in nitrogen atmosphere with disturbance from soot radiation.

absorption by soot is important as Ap , A in measurements of single-particle temperatures (eq 1) and that pyrometers using the near-infrared and visible spectral regions are more sensitive to soot emission (∼1 order of magnitude) than those using the mid-infrared region. Soot can be detected from deviations in shape or slope of curves in two distinct spectral regions for particles

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Figure 16. Estimated burn-off vs residence time for two Cocerr coal samples at Treactor ) 1603 K. Particle size, oxygen vol %: b, 25-45 µm, 6%; 9, 25-45 µm, 3%; O, 90-125 µm, 6%; 0, 90-125 µm, 3%; 4, 90-125 µm, 1%.

Figure 17. Particle burn-off, X, in EFR as a function of oxygen particle pressure. Tset ) 1603 K and residence time is t ) 191 ms. 9, Idpina; 0, Aublat; 2, Samidd; ], Cocerr.

behaving like graybodies. This statement is general for any two distinct spectral regions in principle, but the best results are obtained when the regions are chosen to fit the specific application and temperature levels. Oxygen Partial Pressure Dependence. In Figure 16 burn-off X for the Cocerr sample is shown as a function of the residence time and 1, 3, and 6% oxygen for the two particle size fractions. The temperature distribution monitored for the 90-125 µm size fraction with residence time of 191 ms is shown in Figure 14. For these specific chars it should be noted that at 6% oxygen chars with X ≈ 0.9 maintain very high temperatures. A number of other coal-derived chars (Sørensen et al.10) show much less deviation from the gas temperature at the window section under the same conditions (see Figures 17 and 18). Particle temperatures were monitored 324 mm and 191 ms downstream of four 90-125 µm coal samples at three oxygen partial pressures in the EFR. At this location the sample burn-off (Figure 17) and the particle temperatures (Figure 18) are shown as a function of oxygen partial pressure, i.e. at 1, 3, 6, and up to 12 and 21% oxygen. When we compare the samples, Idpina has a higher temperature than the gas temperature at 1% oxygen, at which significant oxidation is already taking place. The temperature increases slightly until 3% oxygen, and then at 6% the particles get close to the ambient temperatures and, simultaneously, the burn-

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Figure 18. Particle (90-125 µm) mean temperature as a function of oxygen particle pressure. Reactor temperature Tset ) 1603 K and residence time is t ) 191 ms. 9, Idpina; 0, Aublat; 2, Samidd; ], Cocerr.

off is high. The other samples have low particle temperatures at 1% oxygen. This probably reflects that these chars have pyrolyzed during rapid heating and following burns at a moderate rate. The Aublat and Cocerr samples approach the gas temperature at the window section in 12 and 21% oxygen, respectively. The very high particle temperatures of Cocerr at 6 and 12% suggest that this sample ignites late and that it has just recently ignited before being collected. Experiments were only made on Samidd up to 6% oxygen. This sample shows a moderately increasing temperature with partial pressure and a moderate burn-off. We do expect that for this sample temperatures will be increasing even up to 21% oxygen. Ranking. In addition to being an indispensable parameter for combustion modeling, particle temperature measurements give important rapid information that can be used for ranking coal samples. The basic idea is to measure the particle temperatures as a function of oxygen partial pressure, i.e. from ≈0 vol % oxygen and upward with the other parameters fixed in the experiments. A very reactive particle will reach a maximum temperature at a lower oxygen partial pressure compared with a less reactive particle at a certain residence time. Both particles will approach the gas temperature at low oxygen partial pressure or when they are reacted completely (nearly 100% burn-off). The monitored particle temperature curves give important information that may be used for the ranking of coal samples. (a) Ranking from EFR Burn-off. From Figure 17 alone, Idpina is the most reactive and Samidd the least reactive of the samples. At low oxygen partial pressures Aublat is more reactive than Cocerr, but at high oxygen partial pressures the comparison gets more difficult. Cocerr seems to be fairly reactive but eventually ignites late. From the EFR experimental data we suggest the ranking

Idpina . Aublatt ≈ Cocerr . Samidd where “>” means more reactive than. For Idpina and Samidd this is in good agreement with experience gained with these coal types use in boilers. It is also in agreement with expectations that the reactivity of Aublatt and Cocerr is located between the reactivities of these two.

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(b) Ranking from Particle Temperature Measurements. From Figure 18 one may suggest that if we assume that the return to low temperature reflects burn-out, the ranking is

Idpina > Aublatt > Cocerr >? Samidd where “>?” means that we expect the necessity of high oxygen partial pressures to burn-out Samidd. The temperature measurements as a reliable reactivity index would gain from increasing the number of oxygen partial pressures investigated. Also, it will be beneficial to monitor temperatures at one or two locations more downstream together with particle sizes. Oppositely, a number of sample collections (but not all) and the following elaborative and time-consuming TGA measurements may be omitted. Conclusion Particle temperature measurements in an EFR on individual burning and moving char and coal particles have for the first time been demonstrated using an FTIR spectrometer. The FT-IR technique offers many advantages over the pyrometer, all leading to more trustful and accurate results. The single-particle FT-IR technique has five major advantages. First, a low-resolution emission spectrum of the particle can be obtained from which the particle surface temperature can be found. The actual spectral resolution in the present measurements is about 80 cm-1 in the range 1750-7900 cm-1 (5.7-1.27 µm). Second, the technique uses a conventional FT-IR spectrometer with few extra components. The alignment of the instrumentation is simple and easy. The same spectrometer is also used for solids-laden stream and gas temperature measurements in large-scale combustors.16 Third, for measuring the surface temperature from small objects, a one-temperature calibration of the FT-IR spectrometer is sufficient to cover a large temperature range. The temperature errors are reduced


by a factor of 2-10 compared with the accuracy of a typical two-color pyrometer.17 Fourth, a varying background and a fluctuating particle feeding rate are handled. Subtraction of two successive measurements eliminates the background for single-particle measurements, e.g. background radiation from gas, reflections, windows, room, etc. Fifth, the particle emission spectra are useful for testing if assumptions about particle emissivity, possible disturbances from soot radiation, spectral range applied to estimate temperature, etc. are fulfilled. Analyzing measurements with other settings afterward might be advantageous, e.g. selection of other spectral regions than the ones first selected. If the FT-IR technique is combined with the simultaneous measurements of particle size and position at ZPD during scanning, the spectral emissivity of single particles may be measured simultaneously. Another aspect is that the statistics of the particle temperature histograms can be improved as particles that are not validated can be taken into account and particle size effects might be studied. For coal samples soot formation contaminates the sampled particles at 0 vol % oxygen and the quantitative determination of volatiles easily fails; furthermore, the particle temperature measurements fail. When these problems appear, experiments must be carried out instead with a low oxygen content at which volatiles are burned. Besides giving important information about the modeling of the combustion process, the monitored particle temperatures can be used in conjunction with measured burn-off to give important information that may be used for ranking coal samples. Acknowledgment. We acknowledge financial support from The Danish Research Academy. We thank Peter Mørk for manufacturing noncommercial mechanical parts and Peter A. Jensen for good critical comments. EF960044M (16) Clausen, S. Infrared Combustion Diagnostics in Fluctuating Flames. Presented at SPIE 2506, Air Pollution and Visibility Measurements, Munich, June 19-23, 1995. (17) Maswadeh, W.; Tripathi, A.; Arnold, N. S.; DuBow, J.; Meuzelaar, H. L. C. J. Anal. Appl. Pyrolysis 1994, 28, 55-70.

Improved Temperature Measurements of Burning Char and Coal

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