Primary fragmentation behavior investigation in pulverized coal

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Primary fragmentation behavior investigation in pulverized coal combustion with high-speed digital inline holography Xuecheng Wu, Xiaodan Lin, Longchao Yao, Yingchun Wu, Chenyue Wu, Linghong Chen, and Kefa Cen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01521 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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Primary fragmentation behavior investigation in pulverized coal combustion with high-speed digital inline holography Xuecheng Wu, Xiaodan Lin, Longchao Yao, Yingchun Wu,∗ Chenyue Wu, Linghong Chen, and Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China E-mail: [email protected]

Abstract Comprehension on the primary fragmentation of coal particles is important in understanding and optimizing practical coal combustion and creating credible combustion models. High-speed digital inline holography (DIH) at 6000 Hz is employed to investigate the primary fragmentation of China Ximeng lignite (90−154 µm) during coal devolatilization. The particle morphologies, 3D trajectories, and size evolutions of the parent coals as well as the corresponding fragments are captured by time-resolved holographic visualization. Three fragmentation modes, fragmentation at the particle center, exfoliation/fragmentation at the outer zone, and a hybrid fragmentation mode of both, are observed in this study. The particle morphologies show distinct appearances under the three different fragmentation modes. It is found that the pressure resulted from the volatile products inside particle under the mode of fragmentation at the particle center will accelerate or decelerate the the fragments. Statistical result on particle sizes

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on the recorded 26 coal particles undergoing fragmentation indicates that small particles formed because of coal fragmentation, whcih occupy 17.1% in the range of 26−40 µm. It is also concluded that the flame temperature and particle residence time during coal devolatilization have an obvious influence on the possibility of coal fragmentation in the operating condition and target research region. Digital inline holography is demonstrated to be capable of in-situ three-dimensional (3D) measurement of particle fragmentation during coal devolatilization.

1. Introduction The coal fragmentation has a great influence on the physical characteristics (e.g., mechanical, wetting, adsorption, and permeation properties) and the chemical reactivity (e.g., reaction rate), and affects the coal conversion and utilization processes, 1 such as coal devolatilizaiton. The devolatilizaiton is a complex physical and chemical process during the initial stage of coal combustion. In this process, the raw fuel particle could break up into smaller particles, which is called primary fragmentation. Primary fragmentation takes place due to the building up of the internal overpressure associated with volatile matter release and possibly thermal shock. 2,3 Thermal shock can occur because of a temperature gradient inside the particle, leading to limited heat transfer inside the particle. 4 Volatile release results in a rapid pressure increase as well as tensile and compressive stresses inside the fuel particle. Small particles generated during primary fragmentation are crucial to ash formation 5,6 and conversion efficiency of combustion. 7,8 Therefore, it is of great importance to investigate the primary fragmentation process during coal devolatilization. The primary fragmentation of coal particles during pyrolysis has already been investigated by Chirone et al. 9 30 years ago. Recently, several experimental investigations and numerical modelings on primary fragmentation in coal combustion have been reported. 10–12 Because fragmentation has a great influence on particle size distribution which the fluidized-bed combustor is particularly sensitive to, the particle fragmentations were previously studied in the 2


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fluidized reactor. 10,11 Umberto et al. 10 investigated primary and secondary fragmentation of two Kentucky No.9 coals during fluidized bed combustion. Zhang et al. 11 presented the influences of a variety of factors, such as particle size, bed temperature and coal rank, according to the study on the fragmentation of 10 typical Chinese coal ranks in a fluidized bed. The particle size of coals used in the fluidized bed are usually in millimeters. Apart from the fluidized bed, Cui et al. 12 applied a high-frequency induction furnace to investigate the primary fragmental characteristics of particles during rapid pyrolysis, where the pyrolysis temperature was controlled in the range of 1173 −1573 K. The experimental results showed that the pyrolysis process increased with temperature and time. They compared the pyrolysis progresses of three coal ranks and concluded that the possibility of fragmentation ranks lignite ≈ bituminous coal > anthracite. Senneca et al. 13–16 also reported a series of impressive works on coal fragmentation during rapid pyrolysis. They observed that primary fragmentation at high heating rates and high temperature can result in the formation of relatively coarse fragments and sometimes in a multitude of fines. 14 Subsequently, a model of primary fragmentation of coal under different heating conditions was proposed, which accounts for thermal stress and volatile over pressure. 15 Recently, Senneca et al. conducted coal fragmentation experiments in a laminar drop tube at 1573 K. The fragmentation model they used to predict the fate of Colombian coal particles was in very good agreement with experimental results. 16 However, the fragmentation model is based on relatively large particles, which are larger than 600 micrometers. The smaller coal particles during fragmentation process need a further research. The aforementioned literatures mostly adopted off-line sampling to analyze the phenomenon and extent of coal primary fragmentation. However, the on-line measurement is essential to capture the real-time process of primary fragmentation. Kim et al. 17 used a high-speed camera to capture the coal ignition process at high heating rate conditions (>105 K/s) in a flat-flame burner. They observed the fragmentation for anthracite and a lignite coal prior to ignition. Friedemann et al. 18 presented coal particle fragmentation inside a 3


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single particle disintegrator (SPaltor) using a high-speed camera. Three coal ranks were applied to a detailed investigation of primary fragmentation. The results showed that the increments in some parameters, such as temperature, particle size, reactor height and calculated residence time, would lead to an increase of fragmentation of the anthracite and the high-volatile bituminous coal. Though high-speed photography could on-line visualize coal fragments, it only works when particles are well focused. In other words, the application of high-speed photography is limited by its small depth of field. The coal fragments distribute and move in a three-dimensional (3D) space so that the particles could be out of focus with high-speed photography. Furthermore, high-speed photography can only record the two dimensional (2D) information of coal particle. Compared with high-speed photography, digital inline holography (DIH) is able to measure dispersive particle fragments in 3D space. 3D position, size, concentration, velocity and even 3D morphology of the particle can be extracted simultaneously. 19–21 DIH has demonstrated its usefulness and robustness in 3D characterization of particles in particle (coal and aluminum particle) combustion 22–24 , droplets fragmentation 25,26 , and micro flow, 27 et al. In this study, the primary fragmentation of coal particles is investigated in a flat-flame burner using high-speed DIH. Three typical fragmentation modes are exhibited and the fragmentation processes are recorded at 6000 Hz. The sizes, morphologies and 3D trajectories of the parent coals and the corresponding fragments are analyzed and discussed.

2. Experimental setup The experimental configuration of high-speed DIH system to observe coal fragmentation is sketched in Figure 1, which has been applied in our previous work. 24 A continuous wave laser (Oxxius LCX-532S-300, wavelength of 532 nm) was used as the light source. The laser beam was attenuated using a neutral density filter and passed through a spatial filter consisting of an objective (10×) and a pinhole (30 µm). Then it was expanded and collimated to a

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plane wave with a diameter of about 5 cm to illuminate coal particles and form holograms. A band-pass filter (532 ± 10 nm) was mounted before the camera to reduce the influence of flame radiation on the holographic fringes. Holograms were recorded by a high-speed camera (Nac image technology, Memrecam HX−4, 1280×960 pixels, 22×22 µm2 /pixel) operated at 6000 Hz with an exposure time of 2 µs. A camera lens (Nikon AF 50 mm, 1:1.8D) with an extension tube was mounted in reverse to the camera to magnify the hologram, leading to an equivalent pixel size of 9×9 µm2 /pixel. The hologram recorded with the lens system can be regarded as the magnified image of the virtual hologram at the front focal plane of the imaging system. 28 Note that zr is the distance between particle and the recording plane. In the application of the high-speed photography, the aforementioned high-speed camera with the same camera lens was was set at the focal plane of burning coal particles field. The camera was operated at 2000 Hz with the exposure time of 10 µs. A water-cooled sintered bronze McKenna (Holthuis & Associates) burner was used to produce a high-temperature flue gas. The burner was composed of a bronze porous plug with a diameter of flame area of 62.6 mm and a central tube having an inner diameter of 7.7 mm. The bronze porous plug was supplied with premixed gas which was a mixture of methane of 2 standard (298K, 101325 Pa) liters per minute (slpm), O2 (1.96 slpm), and N2 (7.84 slpm), to produce a flat flame to ignite the coal particles. Three K-type thermocouples were placed at 120° of the circumference of the center tube above the burner. Then the flame temperature was obtained from the average of the three measured temperatures. The measurement time lasted 2 minutes in every height. The excessive CH4 in the fuel-oxygen mixture was provided to maintain the gas temperature. The flame temperature maintained stably over 1000 K from 3 cm to 5 cm above the burner to provide a high-temperature environment for coal particle fragmentation, as shown in Figure 1. In this work, the coal fragmentation processes were not investigated at over 5 cm above the burner. The pulverized coal particles were supplied by a screw feeder and carried by a gas mixture of O2 (0.44 slpm) and N2 (1.76 slpm) from the center tube. The mass flow rate of methane, N2 , O2 , and the 5


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gas mixture were controlled by the mass flow controller (MFC, Alicat/MCV, ±1%). The mass flow rate of the pulverized coal was accurately controlled by adjusting the rotation speed of the step motor, with a rate of 0.15 g/min. In order to measure the pulverized coal particles in the flame from 3 cm to 5 cm above the burner, the burner was installed on a 3D motorized linear stage for accurate movement.

Figure 1: Experimental configuration of high-speed DIH measurement of the burning coal particles 24 Lignite coal from Xilingole League, Ximeng lignite, was used in this study. It was ground and sieved to the size range of 90−160 µm. The proximate analysis, ultimate analysis, and heating value of the coal sample are listed in Table 1.

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Table 1: Properties of Ximeng lignite Proximate analysis Air-dried, %

Moisture 0.86

Ash 18.68

Volatile matter 36.91

Fixed carbon 43.55

Ultimate analysis Air-dried, %

Carbon 58.26

Hydrogen 4.22

Nitrogen 0.74

Sulphur 0.74

Heating value Air-dried

Qb (J/g) 22765

Qb (cal/g) 5444

Oxygen 0.96

3. Hologram formation and post-processing Digital holography consists of two steps, that is digital recording and numerical reconstruction of a particle hologram. When a plane wave travels through the particle field, the light scattered by the particles is called object wave (EO ) while the undisturbed part serves as reference wave (ER ). The object wave interferes with the reference wave to form the hologram recorded by a digital camera.

IH = |EO + ER |2 = |EO |2 + |ER |2 + EO ER ∗ + EO ∗ ER ,

(1)

where the symbol ∗ denotes conjugation. Equation 1 indicates that the recorded hologram contains the original complex amplitude of the object wave which can then be reconstructed numerically by using the Fresnel-Kirchhoff integral.

E(x0 , y 0 , z 0 ) =

1 jλ

Z Z

q exp[j 2π (ξ − x0 )2 + (η − y0 )2 + (z0 )2 ] λ q IH (ξ, η) dξdη, 2 2 2 ∞ 0 0 0 (ξ − x ) + (η − y ) + (z )

(2)

where (ξ, η) and (x0 , y 0 ) are the coordinates of the hologram plane and reconstruction plane respectively, z 0 is the reconstruction distance, λ is the wavelength, and j is imaginary unit. A series of plane images are reconstructed to focalize particles in image slices at different depth position. To retrieve the information of the target particles, including particle size, velocity, etc., it is crucial to detect the particles and determine their positions. The best focal plane of the particle along the optical axis (z axis) is difficult to find, 29 although the 7


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transverse position(x−y position) is easy to determine with high precision. To extend the depth-of-field and focalize all the particles in a single synthesis image, reconstructed image slice is decomposed into four subimages with the discrete wavelet transform: HH, HL, LH, LL. Then the image gradient is detected: Gx,h = HL ⊗ Sx + HH ⊗ Sx , 0

Gy,h = LH ⊗ Sx + HH ⊗ Sx ,

(3)

Gx,l = LL ⊗ Sx , 0

Gy,l = LL ⊗ Sx . where 



 −1 0 1    , Sx =  −2 0 2     −1 0 1

(4)

0

is the Sobel operator, and Sx is the transpose matrix of Sx . The gradient magnitudes Gh and Gl in the high- and low-frequency bands are computed as q

G(x,h) 2 + G(y,h) 2 , q Gl = G(x,l) 2 + G(y,l) 2 .

Gh =

(5)

The variance of the gradient magnitude is obtained as follows: ε(H,z) =

XX

[Gh (n, m) − Gh (n, m)]2 ,

i=n i=m

ε(L,z) =

XX

(6) 2

[Gl (n, m) − Gl (n, m)] .

i=n i=m

where G(H,L) (n, m) is the average of G(H,L) over the local region with a block size of n × m. ε(H,z) is used as the focus metrics to determine the z location of particles, which reaches the 8


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peak at the focal plane and decreases rapidly with the degree of out-of-focus. Finally, the depth-of-field extended image with all the reconstructed particles focalized is obtained by applying the inverse discrete wavelet transform to the fused wavelet coefficients. Figure 2 displays the criterion curves for a parent coal and its corresponding two fragments after 2.5 ms. Particles have sharp boundaries and high contrast between the uniform particle inner region and outer background at the optimal focus. Therefore, both the parent coal and the other two fragments have obvious peaks at their best focal planes, zr = 92.55 mm for the parent coal, zr = 91.55 mm and zr = 91.45 mm for two fragments, respectively. This means the fragments move approximately 1.00−1.10 mm in the z direction, as displayed in the inset image II and IV in Figure 2. To detect the target particles, the synthesized image is transformed to a binary image. Three threshold criteria are applied to obtain the optimal threshold by applying a local adaptive threshold value in this work. I1 is set manually as a basic threshold. The second I2 used to remove the shadow caused by uneven illumination and the third one applied to judge whether the droplets detected have high contrast with the background are obtained as I2 = 1.2 × I(n, m), Imax (n, m) + I(n, m) . I3 = 2

(7)

where I(n, m) and Imax (n, m) are the mean and maximum gray values over the local region. Then the morphology, size, 3D position of coal particle fragments were obtained from the reconstructed image. The high measurement accuracy of z location and particle size have been validated in previous research 30,31 by measuring a fixed tungsten wire with the diameter of 300 µm in flame. The results showed that the fluctuation of measured depth(z) position was up to 500 µm, with a standard deviation of 300 µm. The mean diameter measured in flame was with deviations smaller than 10 µm.

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Figure 2: Autofocusing for a parent coal and its two corresponding fragments. (I) and (II) are the raw hologram and the corresponding reconstructed image of the parent coal, (III) and (IV) are the raw hologram and the corresponding reconstructed image of two fragments.

4. Results and discussions 4.1 Observation of fragmentation with high-speed photography High-speed photography, as a common visualization tool, is firstly applied to observe coal primary fragmentation in this study. Figure 3 shows a representative fragmentation process of a coal particle. The moment when the coal particle appears in the field of view is defined as the start time, 0 ms. The whole recording time lasts 13 ms. When the target particle is heated in the combustion environment, volatile products are released and ignited immediately. The volatile flame around the particle is so bright that it saturates the camera and thus is difficult to record the particle morphology clearly. The target particles are easily to be out of focus. After 1 ms, a small particle generates from the exfoliation at the particle surface as a consequence of the thermal shock, as marked in the red rectangle. Then a second small fragment comes from the exfoliation in succession at 4 ms. Fragments at the outer zone are separated from the parent coal while the main coal body remains. The main coal body continues burning and disintegrates into three fragments at 9 ms. The luminance of two fine fragments are getting increasingly weaker which may be accounted for two reasons: the burnout of surrounding volatiles and the out-of-focus movement of particles. It is observed

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that high-speed photography has limitation in clearly capturing particles in motion or even nonluminous particles. Moreover, it is difficult to distinguish the type of luminous particle, such as parent coal particle, fragment and micron-sized soot aggregation.

Figure 3: High-speed photograph sequences of coal fragmentation.

4.2 Fragmentation modes visualized with high-speed DIH As addressed in the previous works, Senneca et al. and Parrika et al 15,32 classified the fragmentation modes of burning coal particles into three categories: exfoliation, fragmentation at the particle center, and fragmentation at the outer zone. These different modes cause diversity in the particle morphologies and fragmental characteristics. In this work, the 3D visualization of three fragmentation modes with high-speed DIH are presented. Figure 4 shows four image sequences of coal particle fragmentations reconstructed from holograms. The time of the first image in every holographic image sequence is set as t0 . Note that the particle circled in red is not the fragment of the parent coal, which is called the other coal hereinafter. Figure 4(a) and Figure 4(b) are two typical situations of fragmentation at the particle center for two different sizes of parent coals, 177 µm and 120 µm, respectively. This 11


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mode is characterized by several large fragments, which are generated from the fracture of the inner core. Senneca et al. 13,15 have also described this mode of fragmentation, and attributed it to the release of volatile matter. Because a large and abrupt release of volatiles during pyrolysis should result in large internal overpressure, unless a very open porous structure is present that facilitates volatiles escape. Particles during the fragmentation process also bear thermal stress simultaneously. In Figure 4(a), the parent coal breaks up into two large fragments at t0 +1.5 ms. The parent coal in Figure4(b) is taken apart into two fragments at t0 +1 ms. Then the corresponding fragments generate from the two fragments at t0 +1.5 ms. The mode of exfoliation/fragmentation at the outer zone displayed in Figure 4(c) is due to thermal shock associated with the fast heating rate of the particles, where the particle size of parent coal is 232 µm. Thermal stress along the tangential coordinate is always negtive in sign and is largest in absolute value close to the particle surface in the early instances. This mode is more likely to occur when the absolute value is large enough to determine the fragmentation on the out shell of particles. 13,15 Small particles from the outer shell gradually generate as a consequence of coal fragmentation. Actually, the fragmentation mode is mainly determined by particle size, heating rate, fuel type, and experimental temperature. The aforementioned two fragmentation modes sometimes can not occur independently in a certain condition. It is observed that the two modes take place simultaneously during coal fragmentation, as demonstrated by the fragmentation pattern in Figure 4(d). In this fragmentation mode, particle exfoliation firstly occurs at t0 +1 ms, then the main coal body starts to occur with obvious fissure at t0 +1.5 ms and breaks up into a few small fragments at inner radial position at t0 +2 ms, which is consistent with the fragmentation process shown in Figure 3. The mode of exfoliation/fragmentation at the outer zone is combined with fragmentation at the particle center in the third fragmentation mode. The third mode is named as hybrid fragmentation mode in this study. For convenience, the three modes, fragmentation at the particle center, exfoliation/fragmentation at the outer zone, hybrid fragmentation mode, are simply described as fragmentation mode 1, fragmentation mode 2 and fragmentation mode 12


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3, respectively. The morphologies of burning and moving parent coals and fragments are clearly visualized with high-speed DIH, which has demonstrated that high-speed DIH can be applied as a powerful tool to investigate coal fragmentation process.

4.3 Particle and fragment velocity measurement The velocity of coal particle has an influence on combustion efficiency and particulate formation. A pair of holograms with a time interval of 1/6000 µs are processed to identify the parent coals and the corresponding fragments. Then the particles are matched in a 2D way with the Hungarian algorithm, 33 which minimizes the total distance to yield the globally most possible pairing of particles in sequential frames. The particle velocities are calculated the displacement and the time interval(vx = δx/δt, vy = δy/δt, vz = δz/δt). Figure 5 exhibits the corresponding 3D positions, velocities and sizes of the parent coals and fragments under fragmentation mode 1 at three different stages, t0 +0.5 ms, t0 +1.5 ms, and t0 +2 ms in Figure 4(b). The colors of circles stand for particle size of the parent coal and the fragments. The parent coal located at (2.10 mm, 1.29 mm, 98.95 mm) is moving along y direction under the action of the carrying gas flow at t0 +0.5 ms. The corresponding four fragments of the parent coal generated at t0 +1.5 ms for the reason that the volatiles are released from the parent coal inside. It is observed that the parent coal and fragments follow the gas flow. In the meantime, the dispersed fragments diffuse slightly and radically along the flow due to thermal, jet expansion, 30 and momentum generated during coal fragmentation. So the four fragments at t0 +2 ms are distributed in a broader space with time. To further study the velocity differences between the parent coal and fragments during fragmentation process, the vertical velocities (y direction) of the parent coal and the corresponding four fragments at three different stages, t0 +0.5 ms, t0 +1.5 ms, and t0 +2 ms in Figure 5 are evaluated, as shown in Figure 6. The vertical velocity (Vy ) of the other coal circled in red in Figure 4(b) is compared with those of the parent coal and the four fragments as a reference. Because all particles mainly follow the gas flow along y direction, the changes 13


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Figure 4: Three possible fragmentation modes: (a) and (b) exfoliation/fragmentation at the particle center; (c) fragmentation at the outer zone; (d) hybrid fragmentation mode.

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Figure 5: The 3D positions, velocities and particle sizes of the parent coal and the corresponding fragments under fragmentation mode 2 in Figure 4(b).

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for the vertical velocities of particles before and after coal fragmentation are analyzed here. The vertical velocities of the parent coal and the other coal at t0 +0.5 ms are 1.15 m/s and 1.22 m/s, respectively. The parent coal firstly breaks up into two fragments at t0 +1 ms, as shown in Figure 4(b). In the meantime, some cracks appear in the two fragments. Then the fragment on the left continues splintering and generating fragment 1 with 1.32 m/s and fragment 2 with 1.08 m/s at t0 +1.5 ms And the other fragment on the right smashes into fragment 3 with 1.12 m/s and fragment 4 with 1.24 m/s. Compared with the other coal whose vertical velocity increases with time, 1.23 m/s at t0 +1.5 ms and 1.38 m/s at t0 +2 ms, the vertical velocity of final four fragments show some differences. A force generates due to the pressure resulted from the volatile products inside particle under the fragmentation mode 2, and it spreads in all directions. It is obvious that an upward force will accelerate fragment 1 and a downward force will slow down the motion of fragment 2. Therefore, fragment 1 moves faster than the parent coal, while fragment 2 becomes slower than the parent coal. In the meanwhile, the parent coal and the corresponding fragments are all affected by gravity and the carrying gas flow. Fragment 3 and fragment 4 show the same behavior as fragment 2 and fragment 1, respectively. The four fragments also speed up as the consequence of the carrying gas flow due to thermal and jet expansion at t0 +2 ms.

4.4 Analysis on 26 particles undergoing fragmentation The flame temperature and particle residence time during coal devolatilization are two critical factors that influence coal fragmentation. To analyze the effects of flame temperature and particle residence time on coal fragmentation, the fragmentation frequency of coal particles from 0 cm to 5 cm above the burner is counted. The 3500 reconstructed images are analyzed at every height to investigate the fragmentation probability of coal particle. The statistical results are presented in Figure 7, where we do not distinguish the three different fragmentation modes. The corresponding flame temperature is stated in Figure 1. When coal particles are injected into the flame where the temperature is 579.8 K, no fragmentation 16


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Figure 6: Measured vertical velocity (along y direction) of the parent coals and the corresponding fragments related with three fragmentation modes. The four particles marked in green rectangle are the four target parent coal particles and other particles are the fragments in the corresponding fragmentation modes. is observed because of the low heating temperature. After about ten milliseconds, particle fragmentation happens because of the higher flame temperature. However, there are only two and three parent coals undergoing fragmentation process at 1 cm and 2 cm above the burner where the flame temperature is still below 1000 K. When particles move to about 3 cm, the number of parent coals with observations of fragmentation obviously increase to more than 8. Two reasons can account for the difference of particle fragmentation at different heights with different flame temperatures. Firstly, the higher flame temperature can contribute to a higher thermal stress and a higher pressure, 9 which makes the coal fragmentation occur easily. Secondly, the coal particles need enough heating time to produce the thermal stress for fragmentation. So the possibility of coal fragmentation increases with the height above the burner. In this part, the morphologies of the parent coals and the corresponding fragments are extracted from 3 cm to 5 cm above the burner to investigate the differences in the shape of the parent coals and fragments under different fragmentation modes. 26 coal particles undergoing fragmentation process and the corresponding fragments are observed and collected in Figure 17


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Figure 7: Coal fragmentation frequency at different heights above the burner. 8, and Figure 8(a), Figure 8(b) and Figure 8(c) are the collections of fragmentation mode 1, 2 and 3, respectively. By comparing Figure 8(a) with Figure 8(b), particle shapes of parent coals differ in those two fragmentation modes to some extent. The parent coals with a convex structure on the surface are more likely to be frayed with exfoliation/fragmentation at the outer zone that is attributed to thermal stress. The edges of parent coals with fragmentation mode 1 are relatively more ragged than those of parent coals with fragmentation mode 2, and this can be explained by that the volatile products are ejected from the surface of particles in coal fragmentation process and the ragged edges roughly reflect the side view of blow holes. 34 A large proportion of parent coals undergoing fragmentation mode 3 are relatively larger and more ragged. If the parent coal is too small, it is difficult to continue breaking up into smaller secondary fragments after the generation of primary fragments . Coal fragmentation contributes to particle size evolution, which plays a significant role in coal combustion efficiency and fine particulate emission in power plants. The particle sizes of parent coals and fragments of the 26 particles undergoing fragmentaion are obtained with holographic data processing. The statistical results are shown in Figure 9. Note that the

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Figure 8: Collections of 26 fragmentations from 3 cm ≤ y ≤ 5 cm above the burner in three fragmentation modes: (a) fragmentation mode 1; (b) fragmentation mode 2; (c) fragmentation mode 3. 19


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particle size in this work is the equivalent diameter (d) of particle, defined as equation 8: r d=2

S , π

(8)

where S is the sum of the pixel areas occupied by the particle. Figure 9 shows the particle size distribution before and after coal primary fragmentation. The size of observed parent coals in fragmentation process is in the range of 95−230 µm. Though there are a few parent coals larger than 154 µm, they are mostly in the range of 90−160 µm, which is consistent with the sieve results of parent coals. The sizes of fragments approximately range from 14 µm to 200 µm. The number of fragments with the diameters of 26−40 µm accounts for 17.71%, which occupies the largest proportion. The large fragments are possibly resulted from slight exfoliation of the parent coals under fragmentation mode 2 with large main coal bodies left. The number size distribution of parent coals and fragments displays the relatively large parent coals undergo primary fragmentation leading to the formation of small particles and a few large main coal bodies.

Figure 9: The number fraction of parent coals and fragments.

5. Conclusions The primary particle fragmentation of Ximeng lignite during coal devolatilization in a flatflame burner is investigated with high-speed DIH. For a comprehension on the coal fragmen20


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tation, 3500 holographic reconstructed images at one height above the burner are processed and analyzed. 26 particles undergoing fragmentation are observed and analyzed from 3 cm to 5 cm above the burner. Four conclusions can be made as the following points: 1. Compared with high-speed photography, high-speed DIH has the distinct advantage of clearly capturing the particle sizes, morphologies, 3D trajectories of the parent coals and the corresponding fragments with micron-sized scale under primary fragmentation during coal combustion. 2. Three fragmentation modes are observed by holographic visualization, including fragmentation at the particle center, exfoliation/fragmentation at the outer zone, as well as hybrid fragmentation mode. By a close examination of the 26 particles undergoing fragmentation, it is found that there exist certain differences in the shape of parent coals and fragments under three different fragmentation modes. 3. The velocities of the parent coal and the corresponding fragments under fragmentation mode 1 are measured. The 3D trajectories display that the parent coal and fragments mostly move following the gas flow. A force generating due to the pressure resulted from the volatile products inside particle under fragmentation at the particle center will accelerate or decelerate the corresponding fragments, depending on the direction of the force. 4. The statistical results show that flame temperature and residence time have an influence on the possibility of coal fragmentation. The fragmentation rate increases with flame temperature and residence time in the operating condition and the target research region. Statistics on particle size indicate that small particles formed because of coal fragmentation and occupy 17.1% in the range of 26−40 µm.

Acknowledgement The authors express heartfelt thanks for the financial support from the National Natural Science Foundation of China (51576177), the National Natural Science Foundation of China

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(91741129), the Innovative Research Groups of the National Natural Science Foundation of China (51390491), Zhejiang Provincial Natural Science Foundation of China under Grant No. LQ19E060010, and the Fundamental Research Funds for the Central Universities.

Supporting Information Available The high-speed evolutions of coal primary fragmentation under the three fragmentation modes in Figure4(a), (c) and (d) are supplied as Supporting Information. The following files are available free of charge. • Filename: Fargmentation mode 1 in Figure4(a) • Filename: Fargmentation mode 2 in Figure4(c) • Filename: Fargmentation mode 3 in Figure4(d)

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Primary fragmentation behavior investigation in pulverized coal

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