Study of temperature, apparent spectral emissivity, and soot loading of a single burning coal particle using hyper-spectral imaging technique

Combustion and Flame 209 (2019) 267–277

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Study of temperature, apparent spectral emissivity, and soot loading of a single burning coal particle using hyper-spectral imaging technique Mengting Si, Qiang Cheng∗, Qi Zhang, Dongxu Wang, Zixue Luo∗, Chun Lou State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

a r t i c l e

i n f o

Article history: Received 28 September 2018 Revised 2 August 2019 Accepted 2 August 2019

Keywords: Hyper-spectral imaging device (technique) Single coal particle combustion Apparent spectral emissivity Soot volume fraction distribution

a b s t r a c t A hyper-spectral imaging device that can simultaneously capture three-dimensional data information of spectra, space, and time is employed to experimentally investigate the combustion behavior of a single coal particle. Three types of single pulverized coal particles are injected and burned on a Hencken flatflame burner, and their flames are captured by a high-speed camera and the hyper-spectral imaging device. The camera observations demonstrate that the three single coal particles undergo particle heat-up, volatile release and combustion, and volatile and char oxidation during the combustion process. Furthermore, the image captured by the hyper-spectral imaging device records the spectral and spatial history of the single burning coal particles throughout the combustion process. Then, the time–space temperature and apparent spectral emissivity of the single burning char particles are calculated from the time–space spectra radiative intensity obtained by the hyper-spectral imaging device. The gray characteristic exhibited by the spectral emissivity of the three single burning char particles in the longer wavelength range demonstrates the feasibility of calculating the temperature using the two-color method. Finally, the soot temperature and volume fraction distribution in the volatile flames are obtained to investigate the formation of soot during the combustion of a single coal particle. The results show that an elongated tail-like soot cloud is formed around the two single bituminous coal particles during the combustion process, but a nearly spherical soot cloud is formed around the single lignite coal particle. Moreover, the range of soot volume fractions in the three envelope volatile flames is comparable to the order of magnitude of the values measured by other researchers. © 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction Experimental investigations of combustion of single pulverized coal particles in a high-temperature gas environment provide an insight into the fundamental understanding of coal combustion and the mathematical description of combustion models. Nevertheless, the investigation of single coal particle combustion is not simple because of its heterogeneous characteristics, diminutive size, and fast movement. It mainly includes the chemical reactions of coal de-volatilization, homogeneous combustion of volatiles, and heterogeneous oxidation of char; the heat transfer between the coal particle and surrounding high-temperature products (soot, CO2 , H2 O); the release and evolution of soot, NOx, SOx, and alkali/alkaline-earth metals from the coal particle; and the size and motion of coal particle during the combustion process. Accurate information about the temperature of single burning coal particles contributes to analyzing the combustion process, ∗

Corresponding authors. E-mail addresses: [email protected] (Q. Cheng), [email protected] (Z. Luo).

improving thermal efficiency, and controlling pollutants. This is because it directly affects the chemical reaction and the nature of the gaseous and particulate burning products during the combustion process [1–4]. In recent years, the two-color and multicolor pyrometer-based methods for the measurement of temperature have been well developed by extensive research [4–13] and widely served as supplemental tools to investigate the combustion of single coal particles. For instance, temperature–time profiles throughout the luminous combustion history of a single particle were obtained using the two-color or multi-color pyrometer-based methods to study the combustion behavior of single coal particles [14–16], their ignition and combustion [17–19], the size of the single coal particle envelope flames and the formation of soot in these envelope flames [20–22], and the temporal release history of volatile alkali species during combustion of single coal particles [23–25]. It has been theoretically demonstrated that the measured temperature error of the two-color or multi-color pyrometer-based methods is minimized when the selection wavelengths are short and well apart [4,5,26]. However, for the two-color and multicolor pyrometer-based methods, the detection wavelength depends on the detector (pyrometer or CCD camera), which may lead to

https://doi.org/10.1016/j.combustflame.2019.08.003 0010-2180/© 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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significant differences between the true and measured temperature. The problem of wavelength selection can be overcome by installing spectrum splitters to direct light toward narrowband interference filters on the pyrometer [4]; unfortunately, such a detection system is complicated and expensive. The hyper-spectral imaging device employed in this study can provide 128-wavelength spectral images, between 400 and 1000 nm, with a spectral resolution of approximately 4.69 nm. Therefore, the choice of wavelengths used to calculate the temperature varies in a wide range. Furthermore, following the calculation of temperature by the twocolor method, the spectral emissivity can be obtained, which facilitates the calculation of temperature. A deeper understanding of soot formation in flames is motivated by its beneficial radiative heat transfer effects on combustion systems and critical role in pollutant formation [22,27–29]. Extensive studies have been devoted to examining the soot concentration and morphological characteristics [30–37], soot formation mechanism [38–40], and soot radiative properties in gas flames [41–44]. However, relatively few studies on soot properties in coal flames have been reported, and even rarer are studies on soot formation in the envelope flames of a single burning coal particle. Ma et al. [45] employed the thermophoretic sampling method to extract soot from a high-temperature coal de-volatilization experiment to describe soot size. Ma et al. [46] also collected the soot particles in the volatiles cloud by using a water-cooled, nitrogenquenched suction probe to measure the soot yields. Khatami et al. [20,21] calculated the spatially averaged volume fraction of soot in volatile matter envelope flames around single coal particles by an emission-based pyrometric method. The contact (sampling) soot detection techniques are appropriate to simultaneously characterizing the local concentration and morphology of soot; however, they have a lower temporal and spatial resolution, and the sampling device may disturb the flame [33,47]. The soot volume fraction of a single burning coal particle obtained by the emissionbased pyrometric method is a spatially averaged value. In a different way, the hyper-spectral imaging device captures the time– space spectral image of single burning coal particles, from which the distribution of the soot volume fraction in the envelope flame of single coal particles can be obtained. In the present study, a hyper-spectral imaging technique, which has the potential advantages of simple structure, ease of operation, and low cost, is employed to experimentally investigate the combustion behavior of a single coal particle. Three types of single coal particles are selected to burn on a Hencken flat-flame burner. The combustion process of the three single coal particles is recorded by a high-speed camera, and their spectral images are captured by the hyper-spectral imaging device. Thereafter, the temperature and apparent spectral emissivity of the single char particles at different burning moments are obtained using the two-color method. Furthermore, the temperature and volume fraction distribution of soot in the envelope flame of single burning coal particles is calculated from the spectral intensity detected by the hyper-spectral imaging device. 2. Experiment 2.1. Experimental setup and samples The single coal particle combustion experiment was performed on a stand Hencken flat-flame burner from Holthius and Associates [48]. As schematically illustrated in Fig. 1, coal particles were injected by a SANKI piezo bowl vibratory feeder into the burner through a 3.7-mm I.D. tube, and their burning flames were captured by the imaging system. Three types of coal, Datong bituminous (DT), Shenhua bituminous (SH), and Indonesia lignite (IL), were selected to burn on the burner. The coals were ground and

Imaging system Powder feeder 1 Burner

1. Motorized translation stage

N2

CH4

Air

Fig. 1. Schematic illustration of experimental setup. Table 1 Proximate and ultimate analyses of three coals in the experiment. Sample

DT SH IL a

Proximate analysis (d, wt%)

Ultimate analysis (d, wt%)

V.M.

A

F.C.

C

H

Oa

N

S

27.93 32.32 44.41

20.79 10.27 5.15

51.28 57.41 50.44

61.79 73.15 68.84

3.73 2.99 5.29

11.77 12.25 18.76

1.06 1.06 1.39

0.86 0.28 0.57

Calculated by difference.

sieved to a particle size cut of 20 0–30 0 μm, and their proximate and ultimate analyses are listed in Table 1. The particle feeding rate was kept low to guarantee that the particles burned as isolated particles. The fuel gas was a mixture of 15 slpm air and 1.1 slpm methane, the shroud gas was 18 slpm nitrogen, and the entraining gas was 3 slpm air. The imaging system consists of a high-speed CMOS camera (FASTCAM SA4, Photron Ltd.) and a hyper-spectral imaging device (SOC710V, Surface Optics Corp.). The high-speed CMOS camera, coupled with an AF Zoom-Nikkor 24–85 mm f/2.8–4D lens, can capture 1024 (h) × 1024 (v) images at a shutter speed of 3600 fps. The hyper-spectral imaging device provides 128 sheets of spectral radiative images between 400 and 10 0 0 nm in one shot, with a spectral resolution of approximately 4.69 nm and a maximum spatial resolution of 696 (h) × 520 (v) pixels. The spectral radiative intensity of the hyper-spectral imaging device was calibrated on a high-temperature blackbody furnace (M330, LumaSense Technologies Inc.) [49,50]. 2.2. Hyper-spectral device imaging Figure 2 illustrates the picture of the experimental setup for scanning a stationary object by the hyper-spectral imaging device. In order to facilitate the explanations, a video of the scanning process is also provided in the supplementary material. As illustrated in Fig. 2, the hyper-spectral imaging device was placed on a motorized translation stage to provide it with a moving velocity, which was opposite to the scanning velocity of the device. In this case, the hyper-spectral imaging device has four types of scanning states depending on the relationship between the moving and scanning velocities. As shown in Fig. 3, the hyper-spectral imaging device scans the object at the scanning speed if the moving velocity vm = 0 (State 1), which is the normal state of the device.

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Fig. 2. Picture of the experimental setup for scanning a stationary object (a ruler) by the hyper-spectral imaging device.

Fig. 3. Images captured by the hyper-spectral imaging device at different scanning states.

269

Fig. 5. Image and schematic diagram of envelope flame formed by a single burning coal particle: (a) image, (b) schematic diagram.

When the scanning velocity is faster than the moving velocity, the hyper-spectral imaging device scans the object at a slower scanning speed (State 2). However, the hyper-spectral imaging device scans the object at an opposite scanning speed (State 3) and forms a reversed image when the scanning velocity is slower than the moving velocity. Once the scanning velocity equals the moving velocity, the hyper-spectral imaging device scans the object at a zeroscanning speed (State 4), implying that it always scans the same position of the object. Therefore, for stationary objects, each column of the image captured by the hyper-spectral imaging device is the same, as the image illustrated on the right side of the second row in Fig. 3. In the following sections, the scanning velocity of the hyper-spectral imaging device is always kept equal to the moving velocity of the motorized translation stage, i.e., the hyper-spectral imaging device is always in State 4. Scanning a single burning and moving coal particle by the hyper-spectral imaging device is complicated than scanning a stationary object. Figure 4(a) illustrates the complete process of scanning a single burning coal particle in a moment by the hyperspectral imaging device. The scanning spot of the device moves

Fig. 4. Schematic diagram of the process whereby a single burning coal particle is scanned by the hyper-spectral imaging device, where the “Flame” is the image of a single burning coal particle captured by the high-speed camera, the “image” is the image formed on the hyper-spectral imaging device, and the “T” denotes burning moment: (a) scanning a single burning coal particle in a moment by the device; (b) scanning the combustion process of a single coal particle by the device; (c) typical image of the single coal particle combustion process captured by the device.

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Fig. 6. Snapshot images illustrating the combustion process of single coal particles captured by the high-speed camera; the abscissa axis denotes time (× 2 ms) and the ordinate axis denotes vertical pixel (height): (a) DT, (b) SH, (c) IL.

from bottom to top at a speed that is much faster than the particle moving speed at which the particle is considered to be stationary during the scanning process. Therefore, the scanning spot quickly passes through the burning coal particle while forming a spectral image in a column of pixels, which records the intensity information of the particle along the height direction at the moment of scanning. Subsequently, the scanning spot moves from bottom to top again, while the coal particle is burning and moving upwards. Thus, the scanning spot will meet the particle at a higher height in the next moment, and a new spectral image is formed in the next column of pixels. By repeating this process, a burning coal particle will be scanned several times by the scanning spot, while an image in several adjacent columns of pixels is formed, as illustrated in Fig. 4(b). The several adjacent columns of images record the spectral information of the same coal particle at different heights and different moments over the combustion process. Figure 4(c) presents a typical image of the single coal particle

combustion process captured by the hyper-spectral imaging device; the device records the spectral information of the single burning coal particle at five burning moments during the combustion process in five columns of images.

3. Methodology Recent observations of single burning coal particles indicate that an envelope flame is formed surrounding the coal particles due to the combustion of volatiles released by the particles. Figure 5 illustrates the image and schematic diagram of the envelope flame, from which it can be seen that the radiation at the location of char particle (radiation of char particle) is slightly different from that at other locations (radiation of soot cloud). In the following sections, the two different modes of radiation are introduced in detail.

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271

3.1. Radiation of char particle As shown in Fig. 5, the radiative intensity received by the hyper-spectral imaging device from the location of char particle is jointly emitted from the char and the soot on the side of char. If the soot only absorbed and emitted, the radiative intensity can be calculated from the equation of radiative transfer as [51,52]:

Iλ = εchar,λ Ibλ exp(−τλ ) +

 τλ 0





Ibλ exp −(τλ − τλ ) dτλ ,

(1)

where

Ibλ =

C1

λ5 [exp(C2 /λT ) − 1]

.

(2)

The first term on the right-hand side of Eq. (1) is the contribution to the intensity by the char, which decays exponentially due to the extinction of soot on the side of char; the second term on the right-hand side of Eq. (1) is contribution from the soot on the side of char; ɛchar,λ is the spectral emissivity of char; Ibλ is the monochromatic blackbody radiative intensity governed by Planck’s law; C1 and C2 are the first and second radiation constants, respecS tively; T is the temperature; τλ = 0 κλ dS is the optical thickness (for absorption) of soot, κ λ is the spectral absorption coefficient of soot, and S is the path length through the soot. If the soot is assumed to be uniform and isothermal, Eq. (1) becomes

εchar,λ Ibλ exp (−κλ Lsoot ) + Ibλ [1 − exp (−κλ Lsoot )]   εchar,λ exp (−κλ Lsoot ) + 1 − exp (−κλ Lsoot ) = Ibλ ελ ,

Iλ =

= Ibλ

(3)

where, Lsoot is the total distance through the soot on the side of char; ελ =εchar,λ exp(−κλ Lsoot ) + [1 − exp(−κλ Lsoot )] is defined as the apparent spectral emissivity of the single burning char particle. Based on the two-color method [53], the ratio of the radiative intensities at two wavelengths is

Iλi Ibλi ελi = , Iλi+1 Ibλi+1 ελi+1

(4)

where

     εchar,λi exp −κλi Lsoot + 1 − exp −κλi Lsoot ελi      . = ελi+1 εchar,λi+1 exp −κλi+1 Lsoot + 1 − exp −κλi+1 Lsoot

(5)

The hyper-spectral imaging device provides 128-wavelength spectral images of the single burning coal particle, with a narrow spectral resolution of approximately 4.69 nm. The spectral emissivity of char and the spectral absorption coefficient of soot can be assumed to be constant in such a narrow wavelength range (4.69 nm). Therefore, Eq. (5) is approximately equal to 1, and temperature T can be calculated from Eq. (4):



Ti = −C2

1

λi

−

1

λi+1





/ ln

Iλi Iλi+1

 λ5i , i = 1, · · · , 127, λ5i+1

(6)

where i represents the ith wavelength between 1 and 127 and Iλi is the detected spectral radiative intensity at ith wavelength. Subsequently, the temperature T of the single burning char particle is obtained by averaging Ti over a range of wavelengths. How and why the temperature can be obtained by this method will be clarified in Section 4.2. After obtaining the temperature of the single burning char particle, the spectral blackbody radiative intensity at ith wavelength Ibλi is calculated by Eq. (2). According to Eq. (3), the apparent spectral emissivity of the single burning char particle is calculated by

ελi = Iλi /Ib λi .

(7)

Fig. 7. Images of the three coal-particle burning flames captured by the hyperspectral imaging device, 100 (h) × 520 (v), and corresponding grayscale of the images marked by the red line at the left, 20 (h) × 520 (v): (a) DT, (b) SH, (c) IL.

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Fig. 8. Spectral radiative intensity at 621 nm of the burning coal particles at different burning moments, where the height is converted from the vertical pixels, and the height at which the scanning spot starts to scan the single coal particles is treated as zero: (a) DT, (b) SH, (c) IL.

3.2. Radiation of soot cloud As illustrated in Fig. 5, the radiative intensity received by the hyper-spectral imaging device from other locations is only emitted by soot cloud. Based on Eq. (3), it can be calculated as:

Iλ = Ibλ [1 − exp (−κλ L )],

(8)

where L is the total distance through the soot flame of the single coal particle, which is the actual geometric length corresponding to the pixels occupied by the flame. The single-particle flame occupies a column of pixels in the image, and the actual geometric length of one horizontal pixel is measured to be 150 μm through the geometric calibration of the hyper-spectral imaging device prior to the experiment, thus, L = 150 μm. Temperature T can be calculated according to the two-color method as described in Section 3.1; thus, the absorption coefficient κ λ is obtained from Eq. (8). Then, the volume fraction of soot can be estimated based on the Rayleigh limit of Mie theory:

κλ =

6π E ( m ) f v

λ

,

(9)

where E (m ) = Im[(m2 − 1 )/(m2 + 2 )] is the refractive index absorption function of soot, m = 1.57−0.56i is the complex refractive index of soot [54], and fv is the soot volume fraction. 4. Results and discussion 4.1. Image and radiative intensity of single burning coal particles Figure 6 illustrates the sequential combustion images of the three single coal particles captured by the high-speed camera with a shutter speed of 500 fps. The images show that the three coal particles mainly experience three combustion sub-processes: 1) particle heat-up; 2) volatile release and combustion; and 3)

volatile and char oxidation. It should be noted that no clearly distinguishable boundary exists between two different sub-processes. As illustrated in Fig. 6, the simultaneous oxidation of volatile and char dominates the entire combustion process for the three coal particles. During the simultaneous oxidation process, the char particle near the center burns to form a bright flame whereas the volatiles released around the char particle burn to form a less luminous flame, finally forming an elongated tail-like flame because of velocity slip. Owing to the formation of a tail-like flame, the image of the single burning coal particles captured by the hyper-spectral imaging device is elongated and bright near the middle but it becomes gradually darkened on both sides. The bright pixels are the images of the burning char particle (approximately 2–3 pixels), and the dark pixels on both sides are the images of volatile flames. Figure 7 illustrates the image of the three burning coal particles captured by the hyper-spectral imaging device with a spatial resolution of 100 (h) × 520 (v) pixels and the corresponding spectral grayscale at 10 representative wavelengths with a spatial resolution of 20 (h) × 520 (v) pixels. As illustrated in Fig. 7, the hyperspectral imaging device captures the burning DT coal particle at six burning moments during the combustion process, which have been marked by characters A1–A6, and it captures the burning SH and IL coal particles at five burning moments, respectively (marked by characters B1–B5 and C1–C5, respectively). The hyper-spectral image of the three burning coal particles illustrated in Fig. 7 is converted to spectral radiative intensity through calibration on a high-temperature blackbody furnace. The results are presented in Fig. 8; it can be observed that the hyperspectral image simultaneously captures three-dimensional data information of spectra, space, and time. The radiative intensity of the three coal particles is weak at the first and last burning moments, which correspond to particle heat-up and near burnout processes, respectively. Following heat-up, the DT and SH single bituminous

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273

Fig. 9. Temperature of three char particles calculated from different spectral radiative intensities, where temperature at λi is calculated from the spectral radiative intensities captured by the hyper-spectral imaging device at wavelengths of λi and λi+ 1 : (a) DT, (b) SH, (c) IL.

coal particles undergo initial de-volatilization, which can be identified by the first peak of radiative intensity at the A2 and B2 burning moments illustrated in Fig. 8(a) and (b) [55]. A low-intensity stage resulting from further char heat-up follows; then, the particles experience the simultaneous oxidation of char and volatiles yielded by secondary de-volatilization [55]. In contrast, as illustrated in Fig. 8(c), the radiative intensity of the IL single lignite coal particle at different burning moments does not reveal clearly distinguishable combustion stages [14]. Furthermore, the spectral radiative intensity of the three single coal particles at different burning moments increases with increasing height, reaches a peak, and then decreases as height increases. Since the size of the single coal particles is about 20 0–30 0 μm and one vertical pixel of the hyper-spectral image is measured to be 168 μm, the radiative intensity emitted from the single burning char particles will actually occupy 2–3 pixels along the vertical (height) direction. In addition, the temperature and physicochemical properties inside a char particle are unlikely to be abrupt, and therefore, the radiative intensity of these 2–3 pixels emitted by the burning char particles is basically the same. As illustrated in the enlarged view in Fig. 8(a), the peak of the radiative intensity at different burning moments always occupies 2–3 consecutive heights (pixels), which coincide with the number of pixels occupied by the char particles. Besides, there is no similar situation that the radiative intensity of 2–3 consecutive pixels remains unchanged in the spectral radiative intensity plots. It demonstrates that the char particles are located at the height where the radiative intensity peaks. Therefore, this study considers the maximum radiative intensity to be emitted from the char particle, and the rest from soot in the volatile flames. 4.2. Temperature and apparent spectral emissivity of single burning char particles Figure 9 illustrates the temperature of the three single char particles calculated according to Eq. (6); it can be observed that the temperature is stable and reliable over a wide wavelength range,

Fig. 10. Apparent spectral emissivity of three burning char particles at different burning moments, the error bars represent standard deviations of the data: (a) DT, (b) SH, (c) IL.

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Fig. 11. Radiative intensity ratio of DT single coal particle at different burning moments.

Fig. 12. Soot volume fraction calculated from different spectral radiative intensities at six random points (D1–D6) of SH coal.

which benefits from the quite narrow spectral resolution (4.69 nm) of the hyper-spectral imaging device. Additionally, in Fig. 9, the temperature is almost constant in the range of 751–906 nm. It is expected that the two-color method is robust in this range owing to the feasibility of the gray assumption, which will be demonstrated by the apparent spectral emissivity calculated in the next section. Therefore, this study obtains the temperature by averaging the values between 751 and 906 nm, and the accuracy of the temperature is characterized by the standard deviation of these values. The temperature and temperature error of the three char particles at different burning moments is listed in Fig. 9. Combined with Fig. 8, it is found that the temperature of the three char particles during secondary volatile/char combustion is in the range of 170 0–220 0 K. From the time-temperature profiles of the two bituminous char particles it is also found that the temperature reaches minima at the A4 and B3 burning moments, resulting from further char heat-up. However, the char particles recover to high temperature during the volatile/char combustion process. In comparison, the temperature of the IL lignite char particle changes gently during the combustion process; however, it is a little higher than that of the other two bituminous char particles at the corresponding burning moments [14]. Following the calculation of temperature, the apparent spectral emissivity of the three char particles at different burning moments is obtained according to Eq. (7), and the apparent spectral emissivity error caused by temperature measurement error is also

Fig. 13. Temperature and soot volume fraction in the volatile flame of three burning coal particles at different burning moments, the error bars represent standard deviations of the data: (a) DT, (b) SH, (c) IL.

evaluated, the results are illustrated in Fig. 10. The apparent spectra-emissivity plots of the three char particles at different burning moments generally exhibit a sharp rise in a shorter wavelength range (419–565 nm), a tendency that becomes constant in a longer wavelength range (751–906 nm), and a transitional state from sharp rise to constant in the range of 565–751 nm. Furthermore, the apparent spectral emissivity is well fitted by a secondorder polynomial in the shorter wavelength range, and by a constant in the longer wavelength range. This demonstrates that the apparent spectral emissivity strongly depends on wavelength in the shorter wavelength range. But it is almost independent of wavelength in the longer wavelength range, which confirms the

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275

Fig. 14. Temperature and soot volume fraction in the volatile flame of three burning coal particles plot along the motion direction of particles inside the soot cloud, the height at which the char particles are located is treated as zero: (a) DT, (b) SH, (c) IL.

aforementioned expectation that the gray assumption is feasible in this range. This further demonstrates that the temperature calculated in this range by the two-color method is robust. The apparent spectral emissivity of the three char particles increases first and then decreases with the combustion of single coal particles, and then peaks at the 4th burning moment, which corresponds to the char combustion process. And they are considerably small at the first and last burning moments, which correspond to the particle heat-up and burnout processes. Moreover, from Fig. 10 it also can be seen that the apparent spectral emissivity of the DT char particle is the largest, followed by the SH char particle and by the IL char particle. As illustrated in Fig. 10, the apparent spectral emissivity of the three char particles at different burning moments are very low

compared with other published results of pulverized coal powder. According to the definition of the apparent spectral emissivity of single burning char particle: ελ =εchar,λ exp(−κλ Lsoot ) + [1 − exp(−κλ Lsoot )], it is attributed to the char and the soot on the side of char (marked in Fig. 5). On the one hand, the attenuation of char emission due to the absorption of soot on its side reduces the apparent spectral emissivity, on the other hand, the augmentation of radiative intensity due to emission of soot on the side of char increases the apparent spectral emissivity. In order to give a deeper insight, the radiative intensity ratio of the char (char ) and the soot on the char’s side (soot ), at the location of char particle (marked in Fig. 5), is estimated. As illustrated in Fig. 11, P2 denotes the location of char particle, P1 and P3 denote the locations near the edges of the char particle. Thus, the radiative intensity

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of the soot on the side of char (at P2) is approximately equal to the average of the radiative intensity at P1 and P3. Then, soot , which is equal to the ratio of the radiative intensity of the soot at P2 to the total radiative intensity at P2, is obtained to quantify the contribution of the soot on the char’s side to the radiative intensity. Figure 11 shows the calculated results of DT char particle at six different burning moments, from which it can be seen that the radiative intensity contributed by the soot on the side of char exceeds 90%. This demonstrates that the radiative intensity received by the hyper-spectral imaging device from the location of char particle mainly comes from the contribution of the soot on the side of char. And the radiation of char particles is significantly reduced by the absorption of soot on the char’s side, which finally results in a small apparent spectral emissivity of char particles.

4.3. Temperature and volume fraction of soot in the envelope flames The temperature of soot in the volatile flame is calculated using the aforementioned two-color method from the spectral radiative intensity captured by the hyper-spectral imaging device. Thereafter, the volume fraction of soot can be obtained from different spectral radiative intensities, and the results at six random points of SH coal are shown in Fig. 12. It is found that the soot volume fraction calculated from different spectral radiative intensities remains almost constant over the wavelength range, demonstrating the reliability of calculating the soot volume fraction using the Rayleigh limit of Mie theory. It should be noted that the radiative intensity at smaller wavelengths is quite weak and susceptible to environmental factors, thus, only the results of the volume fraction calculated from the spectral radiative intensity in the longer wavelength range are given here. Figure 13 illustrates the temperature and volume fraction of soot in the volatile flame around single burning coal particles calculated from the spectral radiative intensity captured by the hyperspectral imaging device. In comparison, more soot is produced by the DT single burning char particle than those by SH and IL char particles. The soot volume fraction in the envelope volatile flame of the three single burning particles is in the range of 0–60 ppm. This is in good agreement with the order of magnitude of the values measured by Timothy et al. [22] based on a two-color optical pyrometry and Khatami et al. [20] using an emission-based method, and numerically computed by Lau and Niksa [56]. Negligible soot is yielded around the single coal particles during the particle heat-up process, slight soot is yielded during the near burnout process, but abundant soot is yielded during the volatile release/combustion and volatile/char combustion processes. The soot yield of single coal particles is much larger at the 4th burning moment (A4) for DT coal, and at the 3rd burning moment (B3 and C3) for SH and IL coal, which corresponds to the moment when the char particle temperature reaches minima. To facilitate the discussion of soot formation around single burning coal particles, the distribution of the temperature and volume fraction along the motion direction of particles inside the soot cloud is plotted in Fig. 14. The location of char particle is marked by a red dotted line. The length of the soot cloud ahead of the char particle is denoted by la , and behind the char particle is denoted by lb , which can be obtained from the distribution of volume fraction along the height. As illustrated by the green line in Fig. 14, la of the two bituminous coals at different burning moments is much longer than lb , which means that an elongated tail-like soot cloud (shown in Fig. 14(b)) is formed around the single coal particles. However, la is slightly longer than lb for the IL coal, indicating the formation of a nearly spherical soot cloud. The temperature and volume fraction of soot in the envelope volatile flames at

different burning moments generally exhibit a unimodal distribution with height. The volume fraction peaks ahead of the char, but the temperature peaks behind the char for the DT and SH single bituminous coal particles. For the IL lignite coal, the volume fraction generally peaks behind the char and the temperature peaks at the location of char particle at C1–C4 burning moments but behind the char at the last burning moment.

5. Conclusions The temperature, apparent spectral emissivity, and soot formation of three types of single coal particles burned on a Hencken flat-flame burner were experimentally investigated using a hyperspectral imaging technique. The envelope flame of the single burning coal particles was observed by a high-speed camera, and the space–time spectral image of the single burning coal particles was captured by a hyper-spectral imaging device. Thereafter, the space– time spectra radiative intensity was converted from the hyperspectral image, from which the temperature and apparent spectral emissivity of the single burning char particles at different burning moments were calculated. Thanks to the narrow spectral resolution (4.69 nm) of the hyper-spectral imaging device, the temperatures of the three single char particles calculated from different radiative intensities by means of the two-color method are stable in a wide wavelength range. The apparent spectral emissivity of the three char particles is almost independent of wavelength in the range of 751– 906 nm, which enables the temperature calculated using the twocolor method to be nearly constant within this range. The emission of char particles is significantly attenuated by the absorption of the soot on the char’s side, resulting in a very low apparent spectral emissivity of the three char particles. The soot temperature and volume fraction distribution in the envelope flame of the three single burning coal particles were also yielded from the space–time spectra radiative intensity. The volume fraction of soot in the envelope volatile flame of the three single coal particles is in the range of 0–60 ppm, which is in line with the order of magnitude of the values measured by other researchers. The DT and SH single bituminous coal particles burn to form an elongated tail-like soot cloud around the particles, and the IL single lignite coal particle burns to form a nearly spherical soot cloud. The DT single bituminous coal particle produces the largest amount of soot, and the smallest amount of soot is produced around the IL lignite coal. The hyper-spectral imaging technique simultaneously carries three-dimensional data information of spectra, space, and time, which supplies abundant radiative intensity information on single coal particle combustion, making it extremely valuable for investigating the combustion process of single coal particles. It is recommended in future experiments to achieve the movement of the hyper-spectral imaging device in the vertical direction to capture the single burning coal particles more times during the combustion process. Simultaneous measurements of the alkali species (sodium, potassium) released by a single burning coal particle using the proposed hyper-spectral imaging technique are also planned.

Acknowledgments This research was supported by the National Natural Science Foundation of China (Nos. 51776078, 51676077, 51676078, and 51827808); Foundation of State Key Laboratory of Coal Combustion (No. FSKLCCB1901); Fundamental Research Funds for the Central Universities (No. 2019kfyXKJC033).

M. Si, Q. Cheng and Q. Zhang et al. / Combustion and Flame 209 (2019) 267–277

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