Influence of particle diameter on the wettability of coal dust and the dust suppression efficiency via spraying

Journal Pre-proof Influence of Particle Diameter on the Wettability of Coal Dust and the Dust Suppression Efficiency via Spraying Pengfei Wang, Xuanhao Tan, Lianyang zhang, Yongjun Li, Ronghua Liu

PII:

S0957-5820(19)31062-6

DOI:

https://doi.org/10.1016/j.psep.2019.09.031

Reference:

PSEP 1939

To appear in:

Process Safety and Environmental Protection

Received Date:

10 June 2019

Revised Date:

1 September 2019

Accepted Date:

27 September 2019

Please cite this article as: Wang P, Tan X, zhang L, Li Y, Liu R, Influence of Particle Diameter on the Wettability of Coal Dust and the Dust Suppression Efficiency via Spraying, Process Safety and Environmental Protection (2019), doi: https://doi.org/10.1016/j.psep.2019.09.031

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Influence of Particle Diameter on the Wettability of Coal Dust and the Dust Suppression Efficiency via Spraying

PengfeiWanga, c*, XuanhaoTanb, Lianyangzhang c, Yongjun Lib, RonghuaLiua

a

School of Resource, Environment & Safety Engineering, Hunan University of

b

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Science & Technology, Xiangtan411201, China

Work Safety Key Lab on Prevention and Control of Gas and Roof Disasters for

Southern Coal Mines, Hunan University of Science & Technology, Xiangtan 411201,

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China c

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Arizona, Tucson, AZ 8572, USA

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Department of Civil and Architectural Engineering and Mechanic, University of

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Corresponding author at: College of Resource, Environment & Safety Engineering, Hunan University of Science & Technology, Xiangtan 411201, China. Tel.: +8613789303851. E-mail address: [email protected] (P. Wang).

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Graphical abstract

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Highlights:

The micro-mechanisms of particle size effects on wettability were derived.

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Wettability dropped with the decrease of particle diameter.

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Spraying dust suppression efficiency was co-determined by wettability and △D50.

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

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Abstract: Spraying is a main technique means for the prevention and control of coal

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dust in coal mines. The dust suppression efficiency by spraying is highly correlated with the wettability of coal dust. There are many influencing factors for the

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wettability of coal dust, among which the particle diameter of dust is one of the most significant factors. In order to analyze the influence of particle diameter on the

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wettability of coal dusts and the dust suppression efficiency via spraying, 18 different

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samples with 3 different types of coal samples and 6 different particle diameters were selected in this study. A series of experiments were designed and performed to evaluate the micro-properties, the wettability, and the dust suppression performance via spraying of coal dust. According to experimental results on the micro-properties, the amount of hydrophilic oxygen-containing functional groups gradually dropped with the decrease of the particle diameter. As the particle diameter decreased, the

specific surface area of coal dust gradually increased while the average diameter of the internal pores decreased. Based on the experimental results on the wettability of coal dust, the wettability of the dust with the same property dropped with the decrease of particle diameter. Finally, based on the experimental results on the dust suppression efficiency via spraying, the dust suppression efficiency via spraying was determined by both the wettability of coal dusts and the value of △D50 (the absolute

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value of the difference between the droplet diameter and the dust particle diameter). As the particle diameter of coal dust increased, the dust suppression efficiency via spraying first increased and then decreased.

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Key words: coal dust; wettability; micro-properties; particle diameter; spraying dust

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suppression 1 Introduction

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Coal is a type of fossil energy resource and occupies approximately 30% of the

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total energy consumption in the world (Wang et al., 2019a; Liu et al, 2018a; Qiu et al., 2019). Plenty of environmental problems and natural disasters may occur during

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mining, i.e., roof fall, gas, fire and dust pollution（Ren et al., 2019; Wu et al., 2019; Tao et al., 2019, 2018）. A huge amount of dust is produced in the coal mining process

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and the workers in mine are exposed to a high-concentration dust environment, thus both physical health and life safety of the workers are under severe threat (Wang et al., 2019b, 2019c; Ma et al., 2018; Kumia et al., 2014). According to National Occupational Disease Report released by National Health Commission of the People’s Republic of China, 26756 cases of occupational disease were reported in

2017, among which 22701 cases were caused by pneumoconiosis. In other words, in 2017, the pneumoconiosis patients occupied 84.84% of total cases of occupational diseases in China. In addition, the newly-discovered cases of pneumoconiosis were mainly distributed in coal mining and nonferrous metal mining & dressing industries. Particularly, coal mining industry contributed 40% of the pneumoconiosis cases (Wang et al., 2019d; Yang et al, 2019). Therefore, it is extremely urgent to adopt

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efficient dust prevention and control measures to reduce dust concentration in coal production sites (Zhou et al., 2019a, 2019b; Wang et al., 2019e; Liu et al., 2019a). Currently, the main dust prevention methods in coal mine include pre-injection

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of water into coal, ventilation, exhausting of dust, purification of air by dust remover,

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spraying, and dust isolation via enclosing (Wang et al., 2018a, 2019f; Zhou et al., 2019c; Liu et al., 2019b). Spraying has been extensively applied in underground

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mines for dedusting due to the low cost, convenient operation and practicability

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(Charinpanitkul et al., 2011; Wang et al., 2019g; Xu et al., 2019). However, according to the measured results in the underground mines, the dust suppression performance

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via spraying was far from ideal. Specifically, the dust suppression efficiency was generally lower than 50% for total dust and even below 30% for respiratory dust

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(Wang et al., 2019h; Cai et al., 2019; Mohan et al., 2008). In order to further enhance the dust suppression efficiency via spraying, researchers have made optimizations on structural and working condition of the pressure nozzles used in the spraying system and obtained some improvements in the spraying performance (Prostanski, 2013; Yu, et al., 2018; Liu et al., 2019c; Peng, et al., 2019a). Additionally, some scholars also

investigated the water consumption of spraying in underground mines and proposed to reduce the surface tension of the sprayed water by adding surfactant, thus enhancing the dust suppression efficiency of spraying. However, the practical applications of adding surfactant were restricted by the complication of the addition technology (Zhang et al., 2018; Xu et al., 2018; Han et al., 2019). Previous studies demonstrated that the dedusting efficiency of spraying was highly correlated with the

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wettability of coals. Generally, the dedusting efficiency via spraying is higher for coal with higher wettability (Wang et al., 2019i; Peng, et al., 2019b; Liu et al, 2018b). Since hydrophobic coal dust can hardly be combined with droplets in the air, a greater

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movement velocity and higher concentration of droplets are required for dust

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collection in hydrophobic environment. Consequently, the understanding of the correlation between coal dust wettability and the dedusting efficiency via spraying

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can provide significant guidance for the design of spraying scheme for dedusting and

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the prediction of dust suppression efficiency of the spraying system in underground coal mines.

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Researchers have performed a large amount of studies on the wetting characteristics of coal dusts. Dong et al. (2004) measured the contact angle of 5

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different types of ultra-fine coal powders with different metamorphism degrees using surface interface tension meter and found that the surface of ultrafine smashed coal was extremely hydrophobic. Zhou et al. (2015a, 2015b) examed the carbon skeleton structures of coal dust with 6 different metamorphism degrees using magnetic resonance imaging (MRI) and obtained the variation rules of the aromatic carbon and

the aliphatic carbon structures with the change of metamorphism degree. On that basis, they also measured the contact angles of different coal dust particles using an optical droplet morphology analysis system. In combination with MRI experimental results, the mechanisms of the differences in the wettability among coal dust particles with different metamorphism degrees were obtained and the traditional wetting theory for coal dust was improved. Using the software of MDI Jade 6.5, Wen et al.

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(2015) analyzed the inorganic mineral characteristics of coal dust, investigated the effect of inorganic minerals in long flame coal on the wettability, and found that quartz in coal dust ash can be used as an indicator of the hydrophily of coal dust.

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Cheng et al. (2014) and Wang et al. (2017) analyzed the functional groups on the

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surface of coal dust using Fourier transform infrared (FT-IR) spectrometer and pointed out that the stretching vibration transmittance of aromatic ring C-H occurred

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at 3050 cm-1, the anti-symmetric stretching vibration transmittance of S-O-Si in

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quartz occurred at 1020-1100 cm-1, and the content of carbon had significant effects on the wettability of coal dust. Yang et al. (2014, 2010) and Zhou et al. (2018)

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evaluated the surface characteristics and wettability of coal dust based on fractal dimension and revealed that the wettability of coal dust significantly increased with

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the increase of fractal dimension. Ulusoy et al. (2004) investigated the relationship between mineral surface roughness and the contact angle using the roughmeter (Surtronic3+, Taylor Hobson Co., Ltd., England) and found that mineral’s wettability increased with the increase of the surface roughness. Some scholars analyzed the physical properties and wettability of coal dust systematically and compared the

effects of some different types of surfactants on the wettability of coal dust. According to their results, the addition of surfactant improved the wettability of coal dust remarkably. Furthermore, the improvement of the wettability was correlated with both the type and the mass concentration of the added surfactant (Li et al., 2013; Chen et al, 2019, 2018; Tessum et al., 2017; Wang et al., 2019j). Currently, researchers from various countries have made great progress in the

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study on the wettability of coal dust, especially the influence of coal properties on the wettability of coal dust. In previous studies, the coal dust particles with a wide range of diameter were mainly investigated; however, the effect of the particle size on the

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wettability of coal dust was rarely analyzed. Meanwhile, only a single method was

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used for the measurement of wettability of coal dust, in which the contact angle was used as the evaluation index and the accuracy of the measurement was questionable.

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Particle diameter is a key factor that affects the wettability of coal dust. However, as

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stated above, only few studies were performed regarding the effect of particle diameter on wettability. The mechanism and clear knowledge of the influence of the

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particle diameter on the wettability were lacking from the perspective of coal dust’s micro-structure. Moreover, in these studies regarding the effect of particle diameter

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on the wettability of coal dust, the dedusting efficiency via spraying was not taken into account and the effect of the particle diameter of coal dust on the dedusting efficiency by spraying still needed to be determined. In this study, the coal samples with different metamorphism degrees from some coal production sites in China were investigated and each coal sample was divided into six parts with 6 different particle

sizes. The micro-structure of coals was evaluated via infrared spectroscopy (IR), BET measurement, and scanning electron microscope (SEM). The influence mechanisms of dust’s particle diameter on the wettability at micro-scale were investigated based on the combination of the micro-structure of coals, the contact angle measurement, and the reverse osmosis experiment. Then a dedusting experiment platform via spraying was designed and used to test the dedusting efficiency via spraying for the

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coal samples with different properties and different sizes. Based on the test result, the effects of dust’s particle diameter were analyzed on the dedusting efficiency via spraying. This study can provide guidance for the design of spraying scheme for

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dedusting and theoretical prediction on the dedusting efficiency via spraying in

2.1 Experimental samples

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2 Experimental samples and scheme

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underground mines.

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In this study, three different coal samples with different metamorphism degrees from the main coal-producing areas in China were selected for the measurements of

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wettability and dedusting efficiency via spraying. According to the metamorphism degree (high to low), the selected coals were anthracite from Motian Coal Mine,

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Hunan, coking coal from Wanfeng Coal Mine, Shanxi, and lignite from Donghuai Coal Mine, Guangxi. The industrial analysis indexes of the three coal samples are listed in Table 1. The six coal samples were crushed by the crusher for 1 minute and sifted out using the standard industrial sieves with different specifications to obtain test pieces

with different particle diameters. These sifted-out coal samples were numbered consecutively in accordance with both the metamorphism degree and the particle size. Then the samples were dried in the vacuum drying oven at 80 ℃ for 480 minutes and collected into the sealing bags for later use. In this study, LS13320 laser particle analyzer was used for the analysis on the particle size. The serial numbers and the characteristic particle sizes of the coal samples are listed in Table 2. Fig. 1 shows the

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particle size distribution of six coal dust samples corresponding to anthracite. Due to space limitation, the particle size distribution of the other two types of coal samples is not included in this paper.

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2.2 Measurement of the micro-properties of coal dust

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According to previous results, the main intrinsic micro-factors that affect the wettability of coal dust include surface functional groups, specific surface area,

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roughness, and mean pore diameter. Both the coal samples and KBr were placed in

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the vacuum drying oven for 8 hours and took out. Then, the coal dust was uniformly mixed with KBr at a proportion of 1:150 and the mixture was placed into the

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compression mould to form the moulded sample. The surface functional groups of the moulded sample were analyzed using FT-IR spectrometer (Nicolet 6700). Then coal

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samples were cut into small cylinders by the cutting machine and the micro-properties were evaluated using a scanning electron microscope (SEM). Before the SEM test, the sample surface was cleaned by the compressed nitrogen and cleared by no-dust cloth and absolute ethyl alcohol (Jiang et al., 2018). Finally, the sample was placed inside the drying oven for 1 hour and the surface roughness was analyzed

by the new-generation ultra-high-solution SEM (SU3500, Hitachi, Japan). Meanwhile, the specific surface area and the mean pore diameter of the coal dust were measured by a surface area analyzer (ASAP2010, Micromeritics, America). The sample was put in a tube and connected to the degassing station for degasification and activation. After activation, the sample in the tube was connected into the analysis station for the

2.3 Measurement of the wettability of coal dust

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measurements of specific surface area and pore size.

The wettability of coal dust can be evaluated by both contact angle and reverse osmosis water absorption. In order to make the sample, 400 mg coal powder was

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added to the mould, then the mould was put into the desk-type powder compressing

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machine. The moulding pressure of 50 MPa was applied for 1 minute to obtain the cylindrical test pieces with smooth surface and 2 mm thickness. Three test samples

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were made out of each coal dust sample. The contact angle was measured by the

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CA100B contact angle measurement meter. For each coal dust sample, the measurement was performed on all the three test pieces to obtain the average value

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(Wang et al. 2014; Yang et al. 2009). The water absorption performances of coal samples were evaluated by the custom-designed reserve osmosis device. First, 3000

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mg coal powder was placed into the glass tube with a diameter of 10 mm and a sealing with the filter paper, and then the coal powder together with the glass tube were weighed. The glass tube was placed upside down into a water tank and water was poured into the tank to immerge the glass tube. The glass tube was kept inside the water tank for 10 minutes and then reweighed. The water absorption by the test

piece can be calculated from both weights. 2.4 Measurement of the dedusting efficiency via spraying Fig. 2 displays the dedusting experimental system via spraying, which can simulate the dust production, spraying, and ventilation conditions in the excavating and mining faces of mines. As shown in Fig. 2, the experimental system mainly consists of a tunnel model, a high-pressure water pump, a water storage tank, a

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control box, an aerosol generator, and the related pipes, valves and measurement instruments. Furthermore, the tunnel model can be divided into the entrance section, the measurement section, the spraying section, the axial flow fan, and the exit section.

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For the convenient measurement of droplet size using Malvern particle size analyzer,

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the tunnel model was mainly made up of transparent organic glass with a plate thickness of 1 cm.

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Lignite samples with favorable wettability were used to explore the effect of coal

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dust particle size on spraying suppression efficiency. Spraying suppression efficiencies on 6 different particle sizes under 4 different water supply pressures (0.5

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Mpa, 1.0 MPa, 1.5 Mpa and 2.0 Mpa, respectively) were measured. The commonly-used spiral-apertured pressure nozzle with an outlet diameter of 0.8 mm

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was used in this study. The structure of the nozzle is shown in Fig. 3. The Malvern particle size analyzer was used to measure the sizes of the droplet under 4 different water supply pressures. The measurements were performed at the central position of the downstream 50 cm away from the nozzle. Dust was generated by the dry powder aerosol generator (AG420, Germany), in

which dust-producing rate was set as 13 g/min. Two anti-explosion dust samplers were installed in the measurement sections in front of and at back of the spraying system, respectively. Therefore, dust particles in the two sections under different working conditions were collected for further analysis. Dust particles were collected at two measurement points at same time. Under each working condition, dust particles were sampled 3 times continuously to obtain the average. The sampling

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duration and flow rate of the sampler were set to be 2 minutes and 15 L/min, respectively. Dust mass concentration was then calculated by the analytical balance. During the measurement of dedusting efficiency via spraying, the airflow velocity in

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the tunnel was set as 1.0 m/s.

3.1Micro-properties of coal dust

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3 Experimental results and analysis

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Through measurements, a series of microscopic characteristic parameters of the

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coal dust particles with different sizes were obtained, including surface functional groups, surface roughness, specific surface area, and pore diameter. These

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microscopic characteristic parameters can be used to determine the internal mechanism of the wettability of coal dust.

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3.1.1 Surface functional groups Fig. 4 shows the measured FT-IR spectra of 3 different types of coal dust

samples with different metamorphism degrees. Apparently, the absorption peaks were observed

at

3431.568-3424.884

cm-1,

2924.200-2920.203

cm-1,

1618.380-1610.387-cm-1 and 921.0-920.6 cm-1 for all the 3 types of coal dust samples.

These 4 absorption peaks were corresponded to the stretching groups of aromatic hydroxyls, the stretching vibration of aliphatic series (CH), the stretching vibration of aromatic ring (C=C), and the deformation vibration of hydroxyls, respectively. In spite of the difference in metamorphism degree, the FTIR spectra of different coal dust samples had similar shapes and characteristic peaks, which indicated the similarity in the coal structure among different types of coal samples. Meanwhile, due

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to the difference in the metamorphism degree and coal-forming conditions, the characteristic absorption at the corresponding wave numbers of the three types of coal samples also varied.

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By comparing FT-IR spectra of the six coal dust samples with the same property

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but different particle sizes, it can be found that the transmittance of the characteristic peak significantly changed with the particle diameter of coal dust. Overall, as the

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particle diameter decreased, the transmittance increased. Previous studies

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demonstrated that the wettability of coal dust was highly correlated with some oxygen-containing functional groups on the surface of coal dust, including aromatic

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hydroxyls and hydroxyls (Zhu et al., 1999; Li et al., 2016). In general, coal dust with higher content of oxygen-containing functional groups showed higher hydrophilicity

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and better wettability. Therefore, the oxygen-containing functional groups corresponding to aromatic hydroxyls at 3431.568-3424.884 cm-1and hydroxyls at 921.0-2920.6 cm-1 were selected for further analysis. The transmittance of FT-IR spectra was transformed into absorbance based on Lambert-Beer Law and the areas of the absorption peaks corresponding to the oxygen-containing functional groups

were quantitatively analyzed using OMNIC 8.0 software, as the results shown in Fig. 5. From Fig. 5, the peak areas of the two oxygen-containing functional groups (aromatic hydroxyls and hydroxyls) both dropped with the decrease of the particle diameter. Accordingly, the content of hydrophilic oxygen-containing functional groups dropped after the sample was crushed and graded. This phenomenon was

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caused by the increase of incomplete molecules or unsaturated radicals on the surface of coal dust in the crushing and grading processes of coal, which led to the increase in the contents of both exposed groups and carbon as well as the decrease in the content

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of oxygen. By comparing the areas of the absorption peaks of the three different coal

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dust samples with the same particle size but different metamorphism degrees, it can be observed that the hydrophilic oxygen-containing functional groups on the surface

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of coal dust dropped gradually with the increase of coal rank. Among the different

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kinds of coal samples, the content of surface oxygen-containing functional groups was highest in lignite and lowest in anthracite. Moreover, for the anthracite, the

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content of surface oxygen-containing functional groups did not show obvious change with the particle size.

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3.1.2 Surface properties and internal pore diameter of coal dust Previous studies revealed that the surface roughness affected the wettability of

coal dust to certain degree. Generally, for the non-hydrophobic material with a contact angle below 90°, greater roughness was indicative of higher wettability (Wenzel, 1949). Fig. 6 shows the SEM images of three different coal samples.

Because of the variations in the metamorphism degree of coal samples and the coal-forming condition, the three coal samples had different surface morphologies. The anthracite sample exhibited smooth surface without significant cracks or salient points. The coking coal sample had rugged surface with both gullies and fractures developed. The lignite sample was full of slight bumping on the surface. After the coal sample was crushed, the surface of the formed coal dust was

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irregular and rugged and pores were well developed inside the dust. Both specific surface area and internal pores affected the wetting process of the coal dust. In this study, the solid specific surface area and pore diameter of different coal samples were

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measured using nitrogen adsorption method. The measurement results are shown in

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Fig. 7. Apparently, with the decrease of the particle diameter, the specific surface area increased gradually while the mean pore diameter of coal dust dropped. It can also be

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observed that both specific surface area and internal pore diameter varied dramatically as the diameter changed in the small range of the particle size. As the

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particle size of anthracite dust dropped from 29.83 μm to 5.389 μm, the specific

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surface area increased from 4.99 mm2/g to 14.80 mm2/g while the pore diameter dropped from 240.93 nm to 15.21 nm. Therefore, the particle diameter of coal dust

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significantly affected specific surface area and internal pore diameter, especially in the small range of particle size. From Fig. 7, it can also be seen that coking coal dust has a larger specific surface area and internal pore size than the other two types of coal dust. 3.2 Measurement of the wettability of coal dust

3.2.1 Measurement of contact angle When a droplet is in contact with the solid surface, the interfaces are formed among the three phases, i.e., gas, solid, and liquid, and the angle between the solid-liquid interface and the gas-liquid interface is referred to as the contact angle. Contact angle is an important index for the characterization of the wettability of solid materials (Xie et al., 2004). After coal sample is crushed, both the physical and

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chemical properties of the surface changed. Moreover, different coal dust samples with different particle diameters had different contact capabilities with droplets and thus possessed different wettabilities. The contact angles between the measured coal

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samples and distilled water were measured by the CA100B contact angle

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measurement device and the measured results are shown in Fig. 8 and Fig. 9. As shown in Fig. 8 and Fig. 9, the contact angle of coking coal sample is much

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larger than those of other two coal samples, which suggest that coking sample had the

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poorest wettability among the three coal samples. The contact angle of lignite sample was slightly smaller than that of anthracite and had higher wettability. With the

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increase of coalification degree, the content of fixed carbon in the coal increased and the content of oxygen-containing functional groups dropped. Therefore, the contact

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angle of coking coal was larger than that of lignite and coking coal sample had poorer wettability than lignite coal. Despite of higher coalification degree, anthracite coal sample had fewer oxygen-containing functional groups and smoother surface. However, the ash content in anthracite coal sample was significantly higher than those in the other two coal samples, which enhanced the wettability. Accordingly, the

contact angle of anthracite coal sample was smaller than that of coking coal. Therefore, wettability of coal dust is affected by a lot of factors, including the content of fixed carbon, oxygen-containing functional groups, ash content, and surface structural properties. In another word, the wettability of coal dust is the result of the combined action of multiple factors. From Fig. 8 and Fig. 9, for the same type of coal sample, the contact angle

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increased with the decrease of the particle diameter of dust, which suggested that the decline in the particle size of coal dust caused the reduction of the wettability. Meanwhile, the contact angle of coking coal sample was decreased by 18.59%, while

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the contact angles of anthracite and lignite were decreased by 42.11% and 46.11%,

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respectively. Coking coal samples had stable wettability at different particle diameters. The measurement results on the micro-properties of coal dust showed that

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with the decrease of the particle size of coal dust, the specific surface area increased,

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while the surface oxygen-containing functional groups and the internal pores were both reduced, which affected the wettability. In general, the wetting ability of the coal

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dust decreased as the particle size decreased. 3.2.2 Reverse osmosis experiment

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Reverse osmosis is a conventional measurement method for the wettability of

powder material based on the principle of capillary (Yang et al., 2007). In this study, the water absorptions of above 18 coal dust samples were measured using the custom-designed reverse osmosis measurement instrument. The measurement results are shown in Fig. 10.

The experimental results in Fig. 10 show that when the particle sizes of the three types of coal dust samples are in the same range, the water absorption of coking coal is obviously smaller than that of lignite and anthracite. From the previous analysis, the wettability of lignite was slightly better than that of anthracite. Fig. 10 also shows that in general, the water absorption of lignite is slightly higher than that of anthracite; however, this law is not always met. From the figure, by comparing the results of the

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two coal dust samples labeled with size grades of 5 and 6, the reverse osmosis of anthracite is slightly higher than that of lignite. The above phenomena may be due to the fact that the three types of coal dust samples in the same size range don’t have the

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identical size in the experiment. From the characteristic particle size data of coal dust

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in Table 1, the particle size of anthracite dust samples labeled with the size grades of 5 and 6 is larger than that of lignite with the same grade. As the particle size was

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larger, the wettability of coal dust was better, which may lead to the above results in

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the experiment. There certainly could be other reasons to cause the difference in experimental results between the two types of coal dust. For instance, the difference

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in surface structure of coal dust could be another possible cause. The difference in the experimental results might also be caused by a comprehensive effect of multiple

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factors.

Fig.10 also shows that the water absorption of coal dust dropped with the

decrease of particle diameter for the same type of coal. Specifically, the water absorptions of lignite changed from 5706 mg to 882 mg with the reduction ratio of 84.54%, the water absorption of anthracite dropped by 3702 mg with the reduction

ratio of 78.79%, and the water absorption of coking coal dropped by only 183 mg but still with the reduction ratio of higher than 78.20%. Accordingly, for different kinds of coal samples, wettability reduced with the decrease of the particle diameter of coal dust with a reduction ratio of higher than 75%. The results of the reverse osmosis experiment basically agreed with the measurement results of contact angle. Therefore, the evaluations of coal dust wettability using contact angle and reserve osmosis water

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absorption are feasible and the present conclusions regarding coal dust wettability are accurate and reliable.

3.3.1 Atomization characteristics of nozzle

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3.3 Dedusting experiment via spraying

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The dust suppression efficiency via spraying is highly correlated with the atomization characteristic parameters of the nozzle (Cheng et al., 2011; Yu et al.,

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2018). In order to analyze the correlation between the wettability of coal dust and the

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dust suppression efficiency via spraying, the nozzle’s atomization characteristic parameters were measured under 4 different water supply pressures. The results are

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summarized in Table 3.

From Table 3, the flow rate of the nozzle and the volume concentration of

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droplets increased with the increase of water supply pressure, which resulted in the increase in the volume in unit space of the atomized water and the enhancement of the dust particle collection. Furthermore, all the parameters of the droplet diameter, including D10, D50, D90, D[3, 2] and D[4, 3], dropped with the increase of water supply pressure. At the water supply pressure of 0.50 MPa, D50 equaled to 181.5 μm; as

water supply pressure increased to 1.0 MPa, 1.5 MPa and 2.0 MPa, D50 was reduced to 150.7 μm, 136.0 μm and 127.9μm, respectively. Fig. 11 shows the distribution patterns of the droplet diameter under 4 different water supply pressures. With the increase of water supply pressure, the frequency peak gradually moved toward the left, i.e., the frequency peak moved toward smaller droplet diameter. It can also be observed from the curves of cumulative volume fraction that D90, D50 and D10 all

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decreased gradually with the increase of water supply pressure. 3.3.2 Dust suppression efficiency via spraying

Fig. 12 displays the experimentally measured spraying dust suppression

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efficiencies of 6 different samples under different working conditions. As shown in

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Fig. 12, the dust suppression efficiency via spraying first increased and then dropped with the increase of the particle diameter of coal dust. The results shown in Fig. 12

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were not completely coincident with the experimental results on the wettability of

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coal dust. For the same type of coal dust sample, the dust suppression efficiency via spraying was determined both the wettability and particle diameter of coal dust. In

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general, when the wettability of coal dust was higher, the dust suppression efficiency via spraying was greater. According to previous studies, when the droplet diameter

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was closer to the dust particle diameter, i.e., the absolute value of the difference between the droplet diameter and the dust particle diameter (△D50) was small, the dust suppression efficiency via spraying would increase (Wang et al., 2018b, 2015). In order to provide contrastive analysis, the dust suppression efficiency via spraying, contact angle, and △D50 of different coal samples under varying water supply

pressures were plotted, as shown in Fig. 13. In Fig. 13, the contact angle was used for the characterization of the wettability of coal dust. As shown in Fig. 13, with the increase of coal dust particle diameter, the dust suppression efficiency via spraying under the four different water supply pressures all first rose and then dropped. When the coal property and spraying condition were constant, the dust suppression efficiency via spraying was determined by both the

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wettability of coal dust and the value of △D50. With the increase of the particle diameter of coal dust, the contact angle decreased gradually and the wettability of coal dust was enhanced. Meanwhile, as the particle diameter of coal dust increased

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constantly, △D50 first increased and then dropped, resulting in an inflection point in

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the variation curve of △D50. Based on above analyses, for the coal dust sample with the smallest particle diameter (No. 6 sample), as the particle diameter of coal dust

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increased from the smallest value to the inflection point, both the contact angle and

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the corresponding △ D50 dropped gradually. As a result, the dust suppression efficiency via spraying increased with the increase of the particle diameter of coal

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dust in this diameter range. In addition, the inflection points of △D50 were different at the four different water supply pressures due to the difference in the particle

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diameter of droplet. When the water supply pressure was equal to 0.5 MPa and 1.0 MPa, the inflection points of △D50 appeared in the second range of the particle diameter. When the water supply pressure exceeded 1.0 MPa, the inflection point of △D50 appeared in the third range of the particle diameter. After △D50 reached the inflection point, further increase of the particle diameter

of coal dust may either enhance or degrade the dust suppression efficiency via spraying. On one hand, the wettability of coal dust was improved with the increase of particle diameter, which was beneficial to the enhancement of dust suppression efficiency. On the other hand, the increase of △ D50 also inhibited the dust suppression efficiency via spraying. Therefore, when the particle diameter of coal dust exceeded the value at the inflection point of △D50, either enhancement or

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degradation of the dust suppression efficiency via spraying can be observed with the further increase of the particle diameter of coal dust, depending on whether the wettability or △D50 played the dominant role. As shown in Fig. 13, the peak of the

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dust suppression efficiencies was achieved when the particle diameter was in the

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second range under all four water supply pressures. In the second range of the particle diameter, the wettability of coal dust was the second most beneficial to the dust

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suppression while the value of △D50 was not high. In the first diameter range of the coal dust, the wettability of coal dust achieved the best but the corresponding value of

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△D50 was too high, which led to lower dust suppression efficiency compared to the

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case when the diameter was in the second range. Based on the above analyses, when the coal property and the spraying condition

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were constant, the dust suppression efficiency via spraying was determined by both the wettability of coal dust and the value of △D50. With the increase in the particle diameter of coal dust, the dust suppression efficiency via spraying first increased and then dropped. To achieve high dust suppression efficiency, the value of △D50 should be reduced while the wettability of coal dust should be improved. For the respiratory

dust with the particle diameter below 7.07 μm, water supply pressure should be appropriately enhanced to reduce the droplet diameter and improve dust suppression efficiency. 4 Conclusions In this study, the micro-properties and wettability of coal dust were investigated, and the influence mechanism of the particle diameter on the wettability of coal dust

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was analyzed. On that basis, a custom-developed dedusting experimental platform via spraying was used to investigate the effects of the wettability of coal dust and the value of △D50 on the dust suppression efficiency via spraying. According to the

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measurement results of coal dust micro-properties, hydrophilic oxygen-containing

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functional groups dropped gradually with the decrease of particle diameter. Coal dust particle size greatly affected specific surface area and pore diameter especially in

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small particle diameter range. With the decrease of particle diameter, coal dust’s

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specific surface area increased gradually and simultaneously mean pore diameter dropped gradually. The experimental results of coal dust wettability revealed that

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wettability dropped with the decrease of particle diameter for the coal dust from a same property of coal. The measured water absorptions via reserve osmosis test were

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in good consistency with the measured contact angles. It was feasible to evaluation coal dust wettability using the contact angle and reverse osmosis water absorptions. The measured dust suppression efficiencies revealed that the dust suppression efficiency via spraying was co-determined by both the wettability of coal dusts and the value of △D50; As the particle diameter of coal dust increased, the dust

suppression efficiency via spraying first increased and then decreased. Acknowledgements Financial support for this work, provided by the National Natural Science Foundation of China (No. 51574123), and the Scientific Research Project of Hunan

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Province Office of Education (No. 18A185), are gratefully acknowledged.

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Yang, J., Tan, Y.Z., Wang, Z.H., et al., 2007. Study on the coal dust surface characteristics and wetting mechanism. J. Journal of China Coal Society. 32(07), 737-740. Cheng, W.M., Nie, W., Zhou, G., et al., 2011. Study of dust suppression by atomized water from high-pressure sprays in mines. J. Journal of China University of Mining & Technology. 40(02), 185-190.

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Wang, P.F., Tan, X.H., Cheng, W.M., et al., 2018b. Dust removal efficiency of high pressure atomization in underground coal mine. J. International Journal of Mining Science and Technology. 28, 685-690.

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Wang, P.F., Liu, R.H., Tang, M., et al., 2015. Experimental study on atomization

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characteristics and dust suppression efficiency of high-pressure spray in

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underground coal mine. J. Journal of China Coal Society. 40(09), 2124-2130.

Fig. 1. Coal dust particle size distribution of anthracite sample : (a) NO. 1-1, (b)

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NO. 1-2, (c) NO. 1-3, (d) NO. 1-4, (e) NO. 1-5 and (f) NO. 1-6.

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Fig. 2. Illustration of the established spraying dedusting experimental system.

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Fig. 3. Illustration of the structure of the used spiral-apertured pressure nozzle.

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Fig. 4. FT-IR spectra of different coal dust samples: (a) anthracite samples, (b)

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coking coal samples and (c) lignite samples.

Fig. 5. Calculated areas of the absorption peaks of oxygen-containing function groups: (a) stretching of aromatic hydroxyls at 3431.568- 3424.884 cm-1 for

anthracite, (b) stretching of aromatic hydroxyls at 3431.568- 3424.884 cm-1 for coking coal, (c) stretching of aromatic hydroxyls at 3431.568- 3424.884 cm-1 for lignite, (d) deformation vibration of hydroxyls at 921.0-920.6 cm-1 for anthracite, (e) deformation vibration of hydroxyls at 921.0-920.6 cm-1 for coking coal and (f)

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deformation vibration of hydroxyls at 921.0-920.6 cm-1 for lignite.

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Fig. 6. SEM images of different coal samples: (a) anthracite image with

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magnification of 300, (b) coking coal image with magnification of 300, (c) lignite image with magnification of 300, (d) anthracite image with magnification of

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1500, (e) coking coal image with magnification of 1500 and (f) lignite image with

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magnification of 1500.

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Fig. 7. Trend of specific surface area and mean pore diameter with the change of

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particle diameter.

Fig. 8. Measurement of contact angle for different coal samples: (a) anthracite

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samples, (b) coking coal samples and (c) lignite samples.

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Fig. 9. Measured contact angles of different coal dust samples.

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Fig. 10. Water absorptions of different coal dust samples.

Fig. 11. Droplet particle size distribution under different water supply pressures:

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(a)p=0.5 MPa, (b) p=1.0 MPa, (c) p=1.5 MPa and (d) p=2.0 MPa.

Fig. 12. Measured spraying dust suppression efficiencies of different coal

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samples

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Fig. 13. Dust suppression efficiencies via spraying, contact angles, and the values of △D50 of different coal samples under different water supply pressures: (a)

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p=0.5 MPa, (b) p=1.0 MPa, (c) p=1.5 MPa and (d) p=2.0 MPa.

Table 1 Industrial analysis indexes of experimental coal samples. Coal Sampling site

Mad (%)

Aad (%)

Vad (%)

FCad(%)

3.25

23.50

5.10

68.15

2.56

12.72

14.94

69.78

1.59

10.75

property Motian Coal Mine,

Anthracit

Hunan

e

Wanfeng Coal Mine,

Coking coal

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Shanxi Donghuai Coal Mine,

Lignite Guangxi

39.87

49.79

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Note: ‘Mad’ refers to the moisture content of air-dried basis, ‘Aad’ refers to the ash

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content of air-dried basis, ‘Vad’ refers to the volatile content of air-dried basis, and

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‘FCad’ refers to the content of fix carbon.

Table 2 Serial numbers and characteristic particle sizes of coal samples. Anthracite Serial

D10

D50

Coking coal D90

D10

D50

Lignite D90

Serial

D10

D50

D90

Sampli numb (μm (μm (μm

(μm (μm (μm

numb (μm (μm (μm

ng site )

)

)

)

177. 259. 352.

)

er

196. 247. 307. 2-1

8

4

3

7

2

6

1

3

6 2-4

3

3

0

19.9 29.8 64.5

3

1

0

8

3

4

1-6 2

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1.42 5.38 19.9 9

8

87.5 103. 159.

2

0

0

3

3-4 9

4

8

13.3 19.6 49.3 3-5

3

7

3

0.93 5.07 16.7

2-6

2

2

29.0 41.9 83.1

3

2-5

5

9

21.5 39.3 63.0

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1-5

7

22.9 46.6 79.1

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1-4

7

3-3

9

23.2 69.3 106.

4

105. 192. 256.

33.4 97.7 158. 2-3

6

3

3-2

6

92.3 142. 195. 1-3

3

126. 181. 238. 2-2

8

)

133. 230. 326.

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1-2

)

3-1 6

141. 204. 275.

)

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1-1

)

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er

0

7

0.97 4.52 15.8 3-6

6

1

0

3

5

5

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Note: D10, D50 and D90are coal dust’s characteristic particle diameters, i.e., the volume of the particles with the diameters below D10, D50 and D90 occupy 10%, 50% and 90% of total particle volume, respectively.

Table 3 Nozzle’s atomization characteristic parameters under different water supply pressures. p

Q

Cv

D10

D50

D90

D[3, 2]

D[4, 3]

(MPa)

(L/min)

(10-6)

(μm)

(μm)

(μm)

(μm)

(μm)

0.5 1.0 1.5 2.0

0.50 1.17 1.50 2.00

138.0 257.2 333.9 319.8

103.8 91.23 79.47 67.76

181.5 150.7 136.0 127.9

312.6 243.3 225.1 234.7

145.8 132.6 117.6 106.6

196.8 159.9 145.0 142.1

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Note: D[3, 2]and D[4, 3]refer to Sauter mean diameter (SMD) and volume-weighed mean diameter, respectively, α refers to the atomizing angle, Cv refers to the volume concentration of droplets, p refers to water supply pressure, and Q refers to the

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nozzle’s flow rate.