The development and testing of a novel external-spraying injection dedusting device for the heading machine in a fully-mechanized excavation face

Process Safety and Environmental Protection 1 0 9 ( 2 0 1 7 ) 716–731

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The development and testing of a novel external-spraying injection dedusting device for the heading machine in a fully-mechanized excavation face Wen Nie a,b,∗ , Yanghao Liu a,b , Hao Wang a,b , Wenle Wei a,b , Huitian Peng a,b , Peng Cai a,b , Yun Hua a,b , Hu Jin a,b a

Key Laboratory of Ministry of Education for Mine Disaster Prevention and Control, Shandong University of Science and Technology, Qingdao 266590, China b College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao 266590, China

a r t i c l e

i n f o

a b s t r a c t

Article history:

This paper discusses the development of a novel external-spraying injection dedusting

Received 7 October 2016

device, which has the potential to reduce substantially the amount of dust produced by

Received in revised form 24 May

a heading machine during the cutting process in a fully-mechanized coal face. Firstly, the

2017

migration rules of the inner wind flow field are analyzed using numerical simulations, and

Accepted 6 June 2017

then the atomization function of the spraying nozzle is optimized. Consequently, the nozzle

Available online 15 June 2017

exhibited the highest airborne dust reduction rate at a spraying pressure of 4 MPa. The proposed device’s functionality was also tested by experiments, with the results indicating that

Keywords:

as the spraying pressure increased from 2 MPa to 8 MPa, the gas–liquid ratio increased at first

Coal dust

and then decreased, reaching a maximum of 1.269 at a spraying pressure of 4 MPa, which

Heading machine

was subsequently once again selected as the optimal spraying pressure. Finally, field exper-

Nozzle

iments were conducted on the developed dedusting device, with the results showing that,

External-spraying

compared with the original spraying dust reduction method, when the developed external-

Injection dedusting

spraying injection dedusting device was opened, using the dust reduction method (d), the

Dust reduction rate

average dust reduction rates of the total coal and respirable dust are enhanced by 17.0% and 18.3% respectively; moreover, using the dust reduction method (c), the average dust reduction rates of total coal and respirable dust are enhanced by 11.6% and 12.0% respectively. Therefore, it can be calculated that by using the injection dedusting method, the average reduction rates are enhanced by 5.4% and 6.3% respectively. In particular, the use of one of the proposed dust removal methods at the measuring point where the machine’s driver is located, led to the average concentrations of total dust and respirable dust decreasing from 305.1 mg/m3 and 129.5 mg/m3 to 88.2 mg/m3 and 38.3 mg/m3 respectively. © 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1.

Introduction

In an underground, fully-mechanized excavation face, most dust is produced by the heading machines’ cutting heads during the coal cutting process; this can account for 90% of the total dust produced in this

environment. This dust is blown by the wind and subsequently dispersed from the cutting heads to other areas of the coal mine, leading to a high level of dust concentration. The results of some surveys have shown that the dust concentration in certain areas of a coal mine can be as high as 3000 mg/m3 and more seriously, respirable dust accounts for 40% of the total amount. This coal dust is harmful and may cause

∗ Corresponding author at: College of Mining and Safety Engineering, Shandong University of Science and Technology, 579 Qianwangang Road Economic & Technical Development Zone, Qingdao 266510, Shandong Province, China. E-mail address: [email protected] (W. Nie). http://dx.doi.org/10.1016/j.psep.2017.06.002 0957-5820/© 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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Consequently, the external spaying method is better at controlling the dust produced by heading machines during the coal cutting process. When coal is being mined, the dust sources concentrate near the heading machine’s cutting head. Therefore, the creation of a spraying curtain near this cutting head is an effective method for controlling the diffusion of the dust. Such a curtain covers all of the dust and minimizes its dispersion, thereby enhancing the efficiency of the dust removal process. The core component of this dust reducing device is the nozzle, the atomization effect of which directly affects its performance in dealing with airborne dust. Generally, the nozzle’s atomization quality is characterized by normal atomization parameters, such as the atomizing angle and effective spraying range. However, few studies have systematically analyzed the atomization micro-parameters, such as the droplet diameter. Therefore, there is a significant lack of information and a randomness when selecting the type of nozzle and its spraying

Fig. 1 – The mechanism of the spraying-induced nozzle piston. explosions and other occupational hazards. In particular, the pneumoconiosis which can be caused by dust is very dangerous and is one of the most serious occupational diseases, with sufferers needing lifelong medical care (Ren et al., 2011; Colinet, 2010; Zhao, 2007; Seibel, 1976). According to findings based on incomplete statistics, more than 10% of those working in major state-owned coal enterprises have suffered from pneumoconiosis, and more than 50% of China’s pneumoconiosis patients are miners who were involved in excavating activities in a fully-mechanized face. Moreover out of these, 45% have subsequently been diagnosed with a malignant form of lung cancer. The number of patients that die of pneumoconiosis is approximately six times greater than those that die due to mine disasters and other industrial accidents. Currently, despite mechanization improving excavation processes, the number of pneumoconiosis cases for those working in this area continues to increase annually. Therefore, the question of how to improve dust prevention and control techniques during the coal cutting process is one that needs to be answered urgently. Currently, the most commonly used dust prevention and control techniques in fully-mechanized excavation faces include spraying, ventilation, the use of chemicals and injecting water into the coal seam. However, although the latter three methods can be effective with regard to controlling dust, they have been proven to have many shortcomings when applied in actual coal mines. When the ventilation method is used to remove dust, the press-in-type fan, dust removal fan and wind cylinder have to be used jointly; i.e., several devices are required simultaneously. Furthermore, it is very complex and costly to optimize and adjust the ventilation parameters. The problems regarding

pressure, which results in low levels of dust removal (Faschingleitner and Höflinger, 2011; Cousin and Nuglisch, 2001; Chakraborty, 2009; Cheng et al., 2010). Currently, the external nozzles on a heading machine are fixed in the ear region of the cutting head along the cutting arm, and this produces a horizontal spraying pattern; i.e., the spraying curtain generated barely covers the dust produced by the cutting head. Since dust is somewhat hydrophobic in nature, the spraying curtain is able cover the dust completely, but cannot deal with all of it, and thus some may still escape from the spraying curtain and be subsequently diffused outwards (Zhou et al., 2012; Ma et al., 2011; Liu, 2011). Several Chinese researchers, for example Kou and Zhao, have developed a circular external spraying device including 10–12 external nozzles, where the spraying curtain can cover most of the dust near the cutting heads. However, the nozzles performed poorly with regard to atomization; specifically, the droplet’s diameter generally exceeded 100 ␮m and even reached as high as 300 ␮m. In field applications when droplet coupling was used, the results with regard to dust settling were poor and the maximum dust reduction efficiency was only 47.8% (Ma and Kou, 2005; Zhao et al., 2014). However, when a piston and entrainment mechanism is utilized, the spraying nozzle can produce a negative pressure field near both the nozzle and spraying field. Consequently, the dust near the heading machine’s cutting head can be injected into the spraying field and the efficiency with regard to the dust reduction per unit of water consumption can be improved (Shao et al., 2001; Li, 2008; Xie and Jiang, 2003). Since the 1980s, some researchers have analyzed and tested the external spraying injection method with regard to its ability to remove dust from heading machines and have developed some external spraying injection devices; e.g., British Conflow-series products and Chinese SC-series products. These devices have been applied in China’s Nantun Coal Mine, Yanzhou Mining Group, and Gaozhuang Coal Mine, Zaozhuang Mining Group. However, the field observations and tests of these devices indicated that the dust was collected from

chemical dust depression methods include the general high cost of the

one direction only and the dusty airflow near the heading machine’s

chemical reagents and more seriously, such methods are not “green” or

cutting head could not be collected effectively. Indeed, when the spraying injection dust removal device was used, the dust removal process’s

environmentally friendly. Moreover, with regard to the coal seam injection method, some of its processes are in conflict with the cutting at, and supporting of, the site, thus limiting its application (Wang et al., 2013; Chen et al., 2012; Nie et al., 2012a; Zhou, 2009; Ren et al., 2013). Additionally, some other dust control techniques, such as ultrasonic atomization, magnetization and pre-charged water spraying, have not

efficiency was only enhanced by 3% (Wang, 2015; Nie, 2013; Wu, 2002). Therefore, this paper discusses the development of a novel external-spraying injection dedusting device for controlling dust in a fully-mechanized excavation face. This device forms a spraying cur-

been promoted sufficiently in the majority of China’s coal mines (Zhou,

tain that completely covers the sources of the dust and settles any dust escaping from the spraying curtain. Field tests on a fully-mechanized

2009; Zhou et al., 2016). In contrast, the spraying method only uses

excavation face are also conducted to prove the effectiveness of the

water and has a number of advantages; for example it is simple and reliable to operate, relatively inexpensive and pollution free. These advantages have led to spraying becoming the most widely applied

proposed device.

dedusting method in coal mines all over the world. There are two types of spraying method — internal and external. With the internal spraying

2. An analysis of the spraying injection dedusting theory

method, the nozzle is always installed at the heading machine’s cutting head near the tooth holder. Therefore, it is easily blocked during the coal cutting process and its water distribution system has a high failure rate. This makes the internal spraying method ineffective with

2.1. The mechanism of the spaying injection dedusting theory

regard to reliability and it has been rarely used in coal mines. With the external spraying method, the nozzle is always fixed on the cutting arm, at a certain distance away from the cutting head; therefore, blockages are less likely and the system has a relatively high reliability.

When water is sprayed from a device towards the outside, if the spraying diameter is no less than the spraying pipe’s inner diameter, a spraying-induced piston is formed. Specifically,

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the air in front of the nozzle is pushed out continuously by the outpouring spray and a vacuum is formed behind. Consequently, a negative pressure field is formed at the nozzle, which can lead to the dusty airflow entering the pipe through its inhaling section. After repeated collisions with the spray, the inhaled dust is forced into the nozzle and coagulates with the droplets. Then, having been injected from the nozzle, the dust loses its suspension capability in the air and settles down. Meanwhile, the mixture composed of purified air and spray is ejected by the nozzle at high speed, and a negative pressure field is formed at the ejection end. Subsequently, the surrounding airflow containing the dust is entrained into the spraying field, wherein the dust in the airflow is further reduced (Cousin and Nuglisch, 2001; Shao et al., 2001; Li, 2008; Xie and Jiang, 2003). An illustration of the mechanism of the spraying-induced nozzle piston is provided in Fig. 1.

2.2. Numerical simulations of the two-phase flow field of suction wind and aerial fog To better illustrate the suction mechanism of the spraying injection, some numerical simulations were performed on the generated two-phase flow field (suction wind and aerial fog) using FLUENT software. During this process, dust enters the injection tube through the dust suction hoods due to the carrying action of the airflow. In this paper, the suction mechanism of the injection device and the migration rules of the inner wind flow field were ascertained through numerical simulations. Consequently, these numerical simulations can provide theoretical guidance for the research and development of the device.

2.2.1.

The mathematical model

d2 x dx = F − kx − d dt dt2

(1)

where x denotes the displacement of the fog drop from the equilibrium position. According to Taylor’s analogy: g u2 k d F   = CF = Ck 3 , = Cd l2 , m l r m l r m l r

and in combination with Eq. (2), Eq. (1) can be converted to the following dimensionless form: CF g u2 d2 y C  C  dy = − k 3 y − d 2l 2 dt Cb l r2 l r l r dt

(2)

where r denotes the fog drop diameter before deformation, using the unit m; u denotes the relative velocity between the air and fog drop, using the unit m/s; l denotes the density of the liquid, using the unit kg/m3 ; l denotes the viscosity of the liquid, using the unit Pa s; and  denotes the surface tension of the liquid, using the unit N/m2 . Assuming that y = x/Cb r,

(3)

where CF , Ck , Cd and Cb are the dimensionless constants. When y > 1, the fog drop is broken. For the undamped fog drops, if the relative velocity u is assumed to be unchanged, the following expression can be obtained by solving Eq. (3):

⎡

(y0 − Wec ) cos(ωt)+

y(t) = Wec + e−(t/td ) ⎣ 1 ω where Wec =

The Taylor analogy breakup (TAB) model is now widely applied to calculations relating to engineering water jets, and is also a classic method for the calculation of the fog drop breakup. In this paper, the two-phase flow field of suction wind and aerial fog were numerically simulated based on the TAB model. Moreover, the TAB model was established using the analogy of the fog drop’s vibration and deformation with the elastic mass system. Assuming that: F denotes the external force corresponding to the aerodynamics on the fog drop m; k denotes the tension on the wall of the fog drop obtained by the analogy with the elastic reaction; and d denotes the viscosity force applied on the fog drop obtained by the analogy with the damping force, the governing equation of the forced damped vibration can be written as (Gerke et al., 2009; Hou and Hou, 2007; Wang et al., 2017; Nie et al., 2012b):

m

Fig. 2 – The physical model of the spraying injection suction device.

CF Ck Cb We

 dy

=

0

dt

+

y0 − Wec td

2 CF g u r 1  , td Ck Cb

⎤ 

⎦

(4)

sin(ωt)



= Cd 2 lr2 , ω2 = Ck  r3 − l

l

1 t2 d

(Jiang et al., 2011; Bianchi and Pelloni, 2001).

2.2.2.

The physical model

The physical model of the spraying injection device was constructed using the GAMBIT model, and some mesh was also generated. The model is composed of a suction hood, an injection tube, a cone-shaped cap and a nozzle. The suction hood is a trapezoid, with a height of 0.12 m. The top of the suction hood is a large rectangle which is 0.4 × 0.1 m in size, whereas the bottom is a small rectangle which is 0.16 × 0.07 m in size. The bottom of the suction hood is connected to the injection tube and located at the top center of this tube. The injection tube is a cuboid with a dimension of 0.4 × 0.12 × 0.12 m. The top center of the injection tube is connected to the suction hood, whereas the lateral center is connected to the small circle of the cone-shaped cap. The cone-shaped cap has a height of 0.1 m, and the diameters of the small and large circles are 0.07 m and 0.1 m respectively. The nozzle is a cylinder with a length of 0.023 m and a diameter of 0.014 m. The bore diameter of the nozzle is 0.0022 m, and the nozzle is located 0.05 m away from the large circle of the cone-shaped cap. In the resulting physical model, the central point of the injection tube is taken as the origin. Moreover: the direction from the origin to the center of the large circle of the cone-shaped cap is taken as the positive direction of the X-axis; the direction from the origin to the center of the small rectangle on the lateral wall of the injection tube is taken as the positive direction of the Yaxis; and the direction from the origin to the center point of the rectangle on the top of the suction hood is taken as the positive direction of the Z-axis. The physical model of the spraying injection suction device can be seen in Fig. 2.

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Fig. 3 – A complete picture of the simulation results of the inner wind flow field migration.

2.2.3.

An analysis of the numerical simulation results

When the airflow enters the calculation region, the section boundary is set at the inlet and the outflow is set at the exit point. The injection sources were then constructed and set. The pressure/flat atomizing nozzle was selected for spraying along the positive direction of the X-axis. The inertia particles and water were both used in this process. The spraying lasted for 200 s, and the spraying pressure, water flow and atomizing angle were 4 MPa, 0.1 kg/s and 90◦ respectively. The outlet and inlet were set at the cone-shaped cap and the suction hood respectively, with an initial pressure of zero. The simulation results of the wind flow field inside the device after spraying, in which the numerical value column is measured in m/s, are displayed in Figs. 3 and 4.

According to Figs. 3 and 4, the following results can be obtained: (1) After spraying, the high-velocity fog drops have an impact on the static air near the nozzle and drive the air flow towards the outside. Subsequently, the air has a velocity as high as 95.3 m/s. Since the air inside the cone-shaped cap is pushed out by the spraying field, a negative-pressure field is formed at the nozzle. Consequently, the air at the top inlet of the suction hood is sucked into the negativepressure region near the nozzle, and then flows out from the cone-shaped cap. (2) After the nozzle sprays from the cone-shaped cap towards the positive direction of the X-axis, the wind flow at the top inlet of the suction hood moves towards the negative direc-

Fig. 4 – A cross-section diagram of the simulation results of the inner wind flow field migration; where: (a) Y = 0; (b) X = 0; (c) Z = 0; and (d) Z = 0.06 m, 0.01 m, 0.014 m, 0.18 m.

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Fig. 5 – The experimental platform for measuring the nozzles’ performance with regard to atomization and airborne dust settling and the layout of the devices; where: (a) Experimental platform; and (b) Layout of the devices. tion of the Z-axis (i.e., in the direction pointing towards the injection tube); however, the wind flow at the edges move towards the positive direction of the Z-axis (i.e., in the reverse direction pointing towards the injection tube). Moreover, due to the jet flow at the bottom of the injection tube, two vortex fields that are roughly symmetrical with each other are formed in the cylinder. (3) Since the suction hood is a trapezoid with a relatively large top and small bottom, the average velocity of the wind flow streaming from the suction hood to the injection tube increases from 2.64 m/s at the top of the hood to 9.39 m/s at the bottom, with a velocity range from 2.26–3.07 m/s to 9.26–9.58 m/s. Moreover, at the bottom and middle of the suction hood, the wind flow moves towards the negative direction of the Z-axis (i.e., in the direction pointing towards the injection tube).

3. Measuring the nozzles’ performance with regard to atomization and airborne dust settling In order to measure the performance of different nozzles with regard to atomization and airborne dust settling and select an optimal nozzle to apply on a fully-mechanized excavation face, a number of experiments were conducted in the Key Laboratory of Mine Disaster Prevention and Control, Shandong University of Science and Technology, China.

3.1.

Experimental setups

The measurements were conducted on an independentlydeveloped experimental platform created to measure the

performance of the nozzle with regard to atomization and airborne dust settling. The platform consisted of a laser spraying particle size analyzer, an enclosed experimental box, a fan and a high-pressure pump. A picture of the experimental platform and the layout of the devices can be seen in Fig. 5. The experimental principles and procedures are now described. Firstly, in an enclosed experimental box that was 3.0 × 3.0 × 2.5 m in size, the nozzle was moved and fixed at a certain position using an automatic controller of the spray pipe’s movements. A laser spraying particle size analyzer (Winner 32, Jinan Winner Particle Technology Co., Ltd., China) and a computer were then switched on and the high-pressure pump was opened so that the water in the tank was set to a pre-set pressure. Then the water was delivered to the nozzle inside the model to be sprayed through the high-pressure rubber pipe (in the water delivery process, the dual-function high-pressure water meter was installed for measuring water pressure and flow). In the second step, the laser emitter installed outside the model sent out a green laser beam that passed through the spraying field, which consisted of a highpressure spray. The laser beam was then received by the laser receiver and transmitted to the computer; after the computer had processed this data, the particle size distribution was obtained. In the third step, an axial-flow fan was opened and adjusted to a certain rotation speed by the stepless frequency converter, so that a wind flow of a stable velocity was formed in the cuboid and enclosed experimental box. Then using the laser particle size analyzer, the particle size distribution of the droplets emanating from the nozzles at different pressures were measured at a certain wind speed in order to simulate the particle distribution of the droplets on the underground,

721

Solid cone

Fan-shaped

Solid and square cone

Lateral diversion core, centrifugal atomization Cross opening, including X-shaped diversion core, hybrid atomization Slope reflection, cone-shaped diversion groove; direct-injection type Including X-shaped diversion core, hybrid atomization Lateral diversion core, centrifugal atomization

Hollow cone

f

e Solid cone Including X-shaped diversion core, hybrid atomization

d

12# 13# 14# 15# 16# 17# 18# 19# 20# 21# 22# c Fan-shaped Cone-shaped diversion groove, direct-injection type

c

b

1# 2# 3# 4# 5# 6# 7# 8# 9# 10# 11# a

1.1 1.4 1.6 2.0 2.8 3.6 1.6 2.0 2.4 1.2 1.6

Atomization type Pore diameter/mm Serial number of nozzle

Table 1 – The classification table of the nozzles.

(1) The nozzle’s performance parameters, such as atomizing angle, effective spraying range and water flow, are in direct relation to the nozzle’s spraying pressure, pore diameter and structure. For any nozzle used in the experiments, as the spraying pressure increases from 2 to 8 MPa, its atomizing angle decreases, whereas the effective spraying range and water flow rate increase. The nozzle’s atomizing angle has little relationship with its pore diameter, however its effective spraying range and water flow increase with the pore diameter. At any spraying pressure used in the experiments, the atomizing angles, effective spraying ranges and water flows of different nozzles vary greatly, and the overall pattern of the spraying field is only related to the nozzle’s structure. As shown in Table 1: the nozzles of Type a and Type e are fan-shaped; those of Type b, Type d and Type f are solid cone-shaped; and those of Type c are hollow cone-shaped. (2) Nine nozzles (1#–6# of Type a and 17#–19# of Type e) are fan-shaped. Since the fan-shaped nozzles are injected

Spraying field pattern

An effective spraying range, atomizing angle and water flow are usually used to characterize a nozzle’s performance with regard to atomization; therefore these elements were measured on 22 different nozzles at various spraying pressures. After the spraying field reached a stable state, a picture was taken using a digital camera and then imported to the computer and processed using Photoshop to calculate the atomizing angle. In order to obtain a more accurate assessment, multiple measurements were conducted to acquire an average value. The nozzle’s effective spraying range was directly measured by a meter ruler in the spraying field. The water flow was measured using a high-pressure water meter and a second chronograph. Specifically, the water flow was calculated by dividing the difference between two adjacent reading values by the corresponding time. In the experiments, the most commonly used spraying pressures used in a fullymechanized excavation face, i.e., 2 MPa, 4 MPa, 6 MPa and 8 MPa, were selected. The measurement results at different spray pressures are listed in Table 2, from which the following conclusions can be drawn:

Serial number of nozzle

3.3. Measuring the nozzles’ atomization performance parameters

Type of nozzle

Pore diameter/mm

In order to select nozzles that were applicable to reducing dust in a fully-mechanized excavation face, and on the basis of a comprehensive investigation of both the coal mines of the major Chinese Mining Groups and nozzle manufacturers, 22 nozzles of 6 types (the nozzles are labeled as 1#–22#, with the types labeled as a–f) were selected for the experiments. The specifications of the different nozzles are listed in Table 1.

2.0 2.4 1.6 1.9 2.4 1.0 1.4 1.7 2.2 2.8 3.2

Atomization type

The selection of nozzles in the experiments

Type of nozzle

3.2.

Spraying field pattern

fully-mechanized excavation face. Finally, a blower was used to raise the dust, which would be dispersed steadily due to the wind flow in the experimental box. After the generated dust and spraying devices were in a stable state, a dust measurement device (AKFC-92A, Changshu Mine Mechanical and Electrical Equipment Co., Ltd., China), which was installed at the exit point of the model, was turned on. The dust was then collected by the sampling head so that the performance of the nozzle with regard to dust settling could be measured and analyzed.

Hollow cone

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Table 2 – The general performance parameters of different nozzles at different spray pressures. Serials number of nozzle

Spray pressure/MPa

Atomizing angle/◦

Effective spraying range/m

Water flow/(L/min)

Serials number of nozzle

Spray pressure/MPa

Atomizing angle/◦

Effective spraying range/m

Water flow/(L/min)

1#

2

11 1.7 111.2 110.8 110.5 125.1 123.6 122.0 121.9 99.5 99.1 99.0 98.9 27.7 27.4 27.3 26.9 28.9 24.9 – – 29.7 29.5 – – 67.7 65.2 62.6 59.1 69.8 59.3 56.2 55.5 72.1 61.6 59.9 59.3 94.5 84.5 82.9 75.8 90.0 88.3 83.6 81.5

1.0

3.45

12#

2

97.8

2.1

3.41

1.6 2.4 3.6 1.8 3.2 3.7 4.3 2.0 2.9 3.5 5.2 3.6 4.1 6.2 6.3 4.8 6.6 – – 5.2 8.3 – – 3.2 3.8 5.0 5.1 3.3 4.3 4.8 5.2 3.5 4.5 5.2 5.7 1.2 1.7 2.3 2.7 1.2 1.9 2.8 3.0

4.12 5.12 5.87 3.92 5.53 7.13 7.67 4.37 5.99 7.16 8.51 7.01 9.59 11.98 13.58 12.77 17.18 – – 17.69 25.50 – – 4.07 5.75 6.80 7.65 4.63 6.13 7.72 9.13 6.86 9.00 10.76 12.54 1.29 1.57 2.14 2.63 2.29 2.85 3.43 3.94

4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8

90.3 83.1 75.7 96.5 91.9 85.3 76.0 80.1 71.6 71.0 69.3 75.4 73.1 68.7 68.4 71.7 66.6 65.5 61.8 121.2 120.0 118.3 115.7 121.4 114.3 112.3 109.7 115.3 111.4 108.3 105.7 91.4 87.6 76.4 73.5 80.6 67.6 65.2 62.5 81.4 75.5 67.0 65.9

3.3 3.7 4.2 2.5 3.5 4.2 4.4 2.3 3.3 4.1 4.8 3.3 3.8 4.7 5.2 3.3 4.3 5.1 6.1 1.6 1.9 2.4 2.8 1.6 2.1 2.9 3.4 2.2 3.1 3.9 4.5 2.5 3.4 4.5 5.3 3.8 4.6 4.9 6.5 4.0 5.2 7.2 7.8

4.43 5.48 6.67 4.63 6.13 7.30 7.83 3.67 4.83 5.87 6.83 4.42 5.60 7.28 8.73 6.20 8.19 10.28 11.71 3.11 4.23 4.30 5.40 4.56 4.96 6.18 7.05 5.03 6.58 8.85 10.08 3.77 5.14 6.28 7.05 7.39 10.23 12.38 14.76 10.32 13.73 16.96 19.89

2#

3#

4#

5#

6#

7#

8#

9#

10#

11#

4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8

directly, they have a large water consumption compared with the hybrid atomization nozzle with the same pore diameter. For example, at a spraying pressure of 2 MPa: nozzle 3# of Type a and nozzle 19# of Type e, with pore diameters of 1.6 and 1.7 mm respectively, have water consumptions of 4.37 and 5.03 L/min respectively; nozzle 7# of Type b and nozzles 11# and 14# of Type c, with a pore diameter of 1.6 mm but a cone-shaped spraying field, have water consumptions of 4.07, 2.29 and 3.67 L/min respectively; and nozzle 20# of Type f, with a pore diameter of 2.2 m, consumed 3.77 L water per minute. For the aforementioned nine nozzles of Type a and Type e, the atomizing angle decreases by 0.6◦ –11.7◦ when the spraying pressure increases from 2 to 8 MPa, with a small decreasing pattern. This indicates that the variation of the atomizing angle is affected slightly by the spraying pressure. For nozzle 2# (the direct injection type fan-shaped nozzle with a cone-shaped diversion groove), whose pore diameter is

13#

14#

15#

16#

17#

18#

19#

20#

21#

22#

1.4 mm, the atomizing angle ranges from 125.1◦ to 121.9◦ as the spraying pressure increases from 2 to 8 MPa, which is the highest out of these 22 nozzles. Additionally, nozzle 2# has a low water consumption (3.92–7.67 L/min) and when the spraying pressure increases to 4 MPa, the effective spraying range increases to 3.2 m. Accordingly, nozzle 2# has the best atomization performance out of the nine fan-shaped nozzles. (3) Nine nozzles (7#–9# of Type b, 14#–16# of Type d and 20#–22# of Type f) are all solid cone-shaped. At the same atomizing angle and effective spraying range, these solid cone-shaped nozzles are superior to the fan-shaped and hollow cone-shaped nozzles in terms of atomization range. Out of these nine solid cone-shaped nozzles, nozzle 20# (the hybrid type solid cone-shaped nozzle with an Xshaped diversion core), with a pore diameter of 2.2 mm, has the largest atomizing angle, ranging from 91.4◦ to 73.5◦ , as the spraying pressure increases from 2 to 8 MPa.

Process Safety and Environmental Protection 1 0 9 ( 2 0 1 7 ) 716–731

Moreover, nozzle 20# has a low water consumption, ranging from 3.77 to 7.05 L/min, which is slightly larger than that of nozzle 14#. As the spraying pressure increases to 4 MPa, the effective spraying range increases to 3. 4 m, which means that nozzle 20# has the best atomization performance out of these nine solid cone-shaped nozzles. (4) Four nozzles (10#–13# of Type c) are all hollow coneshaped. At the same atomizing angle and effective spraying range, these hollow cone-shaped nozzles are superior to the fan-shaped nozzles but inferior to the solid cone-shaped nozzles in terms of atomization range. Out of these four nozzles, nozzle 10# and nozzle 11# have short effective spraying ranges, which barely satisfy the requirements of long-distance dust collection in a fullymechanized excavation face. Compared with nozzle 12#, nozzle 13# has a larger atomizing angle and more effective spraying range; however its water consumption is 1.16–1.82 L/min, which is larger than that of nozzle 12#. Therefore, nozzle 12# has the best atomization performance out of these four hollow cone-shaped nozzles. Based on the above conclusions, three different types of nozzle (namely, nozzles 2#, 12# and 20#) with optimal overall atomization performances were selected from the nine fan-shaped nozzles, nine solid cone-shaped nozzles and four hollow cone-shaped nozzles. In terms of the atomizing angle, nozzle 2# is the best, followed by nozzle 12#, with nozzle 20# being the worst; in terms of the effective spraying range, nozzle 20# is the best, followed by nozzle 12#, with nozzle 2# being the worst; and in terms of water consumption, nozzle 12 # is the best, followed by nozzle 20#, with nozzle 2# being the worst.

3.4.

Fig. 6 – The layout of the measuring points.

excavation face. Clearly wind speed is another important factor affecting the nozzle’s performance with regard to atomization and airborne dust settling. According to China’s Coal Mine Safety Regulations, the wind speed on a coal tunnel’s fully-mechanized face should be within the range of 0.25–4 m/s and currently, the common wind speed is 0.3 m/s. Therefore, in the experiments conducted for this paper the wind speed in the enclosed experimental box was set as 0.3 m/s along the reverse direction of the X-axis.

3.4.2. An analysis of the measurements of the droplet diameters of different spraying fields The droplet diameters in the spraying fields of three nozzles, with spraying pressures measured at different measuring points and at a wind speed of 0.3 m/s, are listed in Tables 3–5 , from which the following conclusions can be drawn:

Measuring the spraying field’s droplet diameter

Having selected three nozzles with an optimal atomization performance, the droplet diameters of their spraying fields were measured and analyzed. The measured data include the surface mean diameter D32 (also known as the Sauter mean diameter), the volume mean diameter D43 (also known as the Herdan mean diameter) and the characteristic diameters, D0.1 , D0.5 and D0.9 , which indicate that the volume factions of the particles with a diameter less than the characteristic diameters occupy 10%, 50% and 90% of the total volume of all particles respectively.

3.4.1.

723

The design of the experimental scheme

(1) The layout of the measuring points To gain an accurate understanding of the distribution of the droplet diameters across the entire spraying field, seven measuring points in total were set in this field. The position of each measuring point can be described using the following coordinate expressions: the abscissa value denotes the distance between the laser beam and the nozzle jet’s horizontal direction; while the ordinate value denotes the distance between the laser beam and the nozzle jet’s vertical direction. Using this coordinate system, the seven measuring points were described as (0, 50), (250, 650), (0, 650), (−250, 650), (500, 1250), (0, 1250), and (−500, 1250), measured in mm, and labeled as A, B, C, D, E, F and G respectively. The layout of these measuring points can be seen in Fig. 6. (2) The settings of spraying pressure and wind speed The spraying pressures were also set as 2 MPa, 4 MPa, 6 MPa and 8 MPa, as these are commonly used in a fully-mechanized

(1) At a wind speed of 0.3 m/s, in any one spraying field of three nozzles, (D0.1 , D0.5 , D0.9 , D32 , D43 )A < (D0.1 , D0.5 , D0.9 , D32 , D43 )D , (D0.1 , D0.5 , D0.9 , D32 , D43 )B < (D0.1 , D0.5 , D0.9 , D32 , D43 )C < (D0.1 , D0.5 , D0.9 , D32 , D43 )G , and (D0.1 , D0.5 , D0.9 , D32 , D43 )E < (D0.1 , D0.5 , D0.9 , D32 , D43 )F ; (for example, (D0.1 , D0.5 , D0.9 , D32 , D43 )A denotes the droplet diameter at measuring point A). The distribution of droplet diameters exhibits the following variation rules: along the line from the nozzle’s pore center to any point within the spraying field’s effective range, the further away the point is from the nozzle, the better the atomization performance; and along the line perpendicular to the spray’s central line, the further away the point is from the perpendicular line, the better the atomization performance. Under the disturbance caused by the wind at a speed of 0.3 m/s, the droplet diameters in the spraying field on the windward side are smaller than those on the leeward side; and the measurement results of (D0.1 , D0.5 , D0.9 , D32 , D43 ) at point D are smaller than those at point B, whereas the measurement results at point G are smaller than those at point E. This is mainly due to the fact that the wind disturbance on the windward side is relatively strong, which is conducive to the formation of droplets with smaller diameters. Furthermore, on the windward side, the droplets are diffused towards the exit point due to the wind blowing, leading to the decline in droplet density and the probability of forming large droplets through droplet collisions. However, on the leeward side or at the center of the spraying field, the droplet density is increased, and therefore small droplets are more likely to collide with each other to form large droplets.

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Table 3 – The droplet diameters of the spraying fields of Nozzle 2# at different spray pressures. Position of the nozzle

A B C D E F G Average value

At a spray pressure of 2 MPa D0.1 /␮m

D0.5 /␮m

D0.9 /␮m

D32 /␮m

D43 /␮m

D0.1 /␮m

D0.5 /␮m

D0.9 /␮m

D32 /␮m

D43 /␮m

23.671 25.409 26.115 25.781 24.544 31.128 29.268 26.559

55.336 55.739 57.033 58.153 59.911 73.271 66.160 60.800

89.277 91.955 95.801 94.919 117.172 132.289 116.940 105.479

45.180 45.961 47.478 46.884 48.414 58.482 53.895 49.470

56.520 57.984 59.865 59.725 65.945 78.226 70.447 64.102

18.186 20.866 22.283 21.884 21.637 26.114 23.703 22.096

41.630 48.005 50.427 48.366 49.078 59.064 55.166 50.248

70.193 81.293 84.940 82.996 81.520 97.891 94.409 84.749

33.702 38.958 41.068 39.853 39.820 48.008 44.431 40.834

43.108 49.972 53.001 50.932 52.570 58.315 56.697 52.085

Position of the nozzle

A B C D E F G Average value

At a spray pressure of 4 MPa

At a spray pressure of 6 MPa

At a spray pressure of 8 MPa

D0.1 /␮m

D0.5 /␮m

D0.9 /␮m

D32 /␮m

D43 /␮m

D0.1 /␮m

D0.5 /␮m

D0.9 /␮m

D32 /␮m

D43 /␮m

18.416 18.208 18.516 18.400 18.824 22.903 21.578 19.549

39.155 39.724 41.054 41.456 41.631 51.788 48.937 43.392

62.504 64.704 66.423 64.426 70.567 86.314 76.882 70.260

32.231 32.398 33.196 32.893 33.884 42.126 39.383 35.159

39.833 40.665 41.767 41.176 43.474 54.158 50.672 44.535

16.677 16.639 17.687 18.217 18.874 20.337 20.138 18.367

33.099 34.005 37.296 36.948 42.087 42.643 45.745 38.832

63.182 59.352 64.274 58.867 73.480 80.140 79.520 68.402

28.826 28.702 30.938 31.141 34.591 38.060 37.551 32.830

37.064 36.170 39.058 37.809 44.182 49.239 49.055 41.797

Table 4 – The droplet diameters of the spraying fields of Nozzle 12# at different spray pressures. Position of the nozzle

A B C D E F G Average value

At a spray pressure of 2 MPa D0.1 /␮m

D0.5 /␮m

D0.9 /␮m

D32 /␮m

D43 /␮m

D0.1 /␮m

D0.5 /␮m

D0.9 /␮m

D32 /␮m

D43 /␮m

20.550 26.905 29.745 22.859 30.382 34.447 33.488 28.339

45.979 59.720 60.528 50.465 66.489 77.215 78.122 62.646

88.956 94.504 110.706 81.919 118.405 134.782 136.703 109.425

38.115 48.884 52.228 41.097 55.085 60.722 59.425 50.794

50.528 60.757 65.960 51.627 70.985 81.440 82.277 66.225

16.561 21.461 25.147 20.246 27.602 28.146 26.065 23.604

31.317 51.857 53.694 39.575 64.623 59.565 55.786 50.917

56.822 85.683 90.198 69.562 100.647 105.238 90.775 85.561

27.511 41.100 45.273 34.290 49.791 49.639 46.150 41.965

34.311 53.020 56.189 42.282 71.526 61.746 57.429 53.786

Position of the nozzle

A B C D E F G Average value

At a spray pressure of 4 MPa

At a spray pressure of 6 MPa

At a spray pressure of 8 MPa

D0.1 /␮m

D0.5 /␮m

D0.9 /␮m

D32 /␮m

D43 /␮m

D0.1 /␮m

D0.5 /␮m

D0.9 /␮m

D32 /␮m

D43 /␮m

15.569 18.908 23.812 20.880 23.613 26.862 23.350 21.856

22.026 43.575 49.715 41.141 53.476 56.145 49.976 45.151

43.047 71.712 89.545 66.474 98.397 90.697 84.043 77.702

22.625 34.521 41.694 35.139 43.796 47.204 41.911 38.127

26.183 44.066 53.351 42.465 57.580 58.014 52.281 47.706

15.106 20.686 23.443 18.540 23.966 25.857 22.793 21.484

19.354 40.309 48.206 40.849 49.492 54.554 49.525 43.184

28.010 70.868 87.219 68.766 84.914 90.230 73.987 71.999

19.351 35.007 41.079 32.889 43.698 45.703 41.306 37.005

20.530 43.075 52.020 41.944 54.609 56.223 52.331 45.819

(2) At a wind speed of 0.3 m/s, among the spraying fields of three nozzles, the droplet diameters at seven measuring points decrease with the spraying pressure. As the spraying pressure increases from 2 to 4 MPa, the average values of (D0.1 , D0.5 , D0.9 , D32 , D43 ) of nozzles 2#, 12# and 20# decrease from (26.559 ␮m, 60.800 ␮m, 105.479 ␮m, 49.470 ␮m, 64.102 ␮m), (28.339 ␮m, 62.646 ␮m, 109.425 ␮m, 50.794 ␮m, 66.225 ␮m), and (22.072 ␮m, 49.369 ␮m, 78.777 ␮m, 40.703 ␮m, 50.734 ␮m) to (22.096 ␮m, 50.248 ␮m, 84.749 ␮m, 40.834 ␮m, 52.085 ␮m), (23.604 ␮m, 50.917 ␮m, 85.561 ␮m, 41.965 ␮m, 53.786 ␮m), and (20.467 ␮m, 45.529 ␮m, 76.058 ␮m, 38.263 ␮m, 47.526 ␮m). Then, when the spraying pressure increases to 8 MPa, the average values of (D0.1 , D0.5 , D0.9 , D32 , D43 ) of nozzles 2#, 12# and 20# are reduced to (18.367 ␮m, 38.832 ␮m, 68.402 ␮m, 32.830 ␮m, 41.797 ␮m), (21.484 ␮m, 43.184 ␮m, 71.999 ␮m,

37.005 ␮m, 45.819 ␮m), and (17.867 ␮m, 34.119 ␮m, 53.471 ␮m, 30.228 ␮m, 35.831 ␮m). From the perspective of the high-pressure atomization mechanism, as the spraying pressure increases, the turbulence of the high-speed liquid jet is enhanced, and the velocity difference of the air near the nozzle jet increases, leading to a strong disturbance of the airflow fluctuation. Therefore, the surface wave increases exponentially, and the wavelength and wave amplitude of the liquid jet’s surface wave increase dramatically. Meanwhile, more pressureinduced potential energy can be transformed into the kinetic energy of the liquid medium, and the liquid’s ability of overcoming air resistance and continuing to work is enhanced, giving rise to a rapidly enhanced disturbance of the liquid. Then the disturbance further intensifies with the increased liquid pressure, and the radial velocity of

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Table 5 – The droplet diameters of the spraying fields of Nozzle 20# at different spray pressures. Position of the nozzle

A B C D E F G Average value

At a spray pressure of 2 MPa D0.1 /␮m

D0.5 /␮m

D0.9 /␮m

D32 /␮m

D43 /␮m

D0.1 /␮m

D0.5 /␮m

D0.9 /␮m

D32 /␮m

12.727 20.024 21.656 15.400 27.776 30.417 26.503 22.072

27.515 40.126 51.698 40.406 53.705 71.018 61.113 49.369

46.215 68.461 78.250 68.007 85.872 109.741 94.889 78.777

23.543 34.800 41.921 32.019 46.527 55.986 50.126 40.703

28.566 42.104 53.474 41.153 55.320 71.262 63.262 50.734

10.485 16.375 21.689 13.163 27.088 28.589 25.884 20.467

19.851 29.971 50.840 33.911 55.172 69.614 59.341 45.529

37.262 43.474 86.783 61.699 93.579 110.075 99.529 76.058

19.742 28.239 41.771 29.168 44.107 56.057 48.761 38.263

Position of the nozzle

A B C D E F G Average value

At a spray pressure of 4 MPa

At a spray pressure of 6 MPa

D43 /␮m 20.432 29.314 51.883 35.230 64.677 70.522 60.624 47.526

At a spray pressure of 8 MPa

D0.1 /␮m

D0.5 /␮m

D0.9 /␮m

D32 /␮m

D43 /␮m

D0.1 /␮m

D0.5 /␮m

D0.9 /␮m

D32 /␮m

D43 /␮m

6.081 16.511 20.071 16.059 22.396 26.165 23.409 18.670

18.242 31.826 47.618 31.066 52.120 55.501 54.892 41.609

30.690 54.482 66.486 41.577 92.587 90.189 88.529 66.363

14.713 28.452 38.817 26.531 42.824 46.260 43.826 34.489

20.113 32.708 46.641 30.587 55.698 56.838 56.159 42.678

12.824 15.571 16.619 14.661 15.320 28.497 21.578 17.867

24.734 23.017 31.546 31.200 46.766 49.616 31.957 34.119

46.635 35.665 54.922 56.398 66.501 73.491 40.685 53.471

20.050 25.265 28.391 23.194 37.299 44.824 32.572 30.228

24.159 33.798 35.461 30.049 39.075 51.476 36.801 35.831

the local fluid (wave crest) on the surface of the liquid jet increases. Accordingly, a boundary separation can appear in the liquid due to the gas, and small liquid drops and silks are formed. These groups of liquid drops or silks have a higher energy and can suffer from stronger aerodynamic effects. Consequently, under the continuous action of aerodynamics and surface tension, the liquid drops or silks may be broken and form a finer droplet group. As the spraying pressure increases from 2 to 8 MPa, the droplet diameter of nozzle 20# is the smallest, followed by nozzle 2#, and finally by nozzle 12#.

3.5. Measuring the nozzles’ performance with regard to airborne dust settling As stated above, nozzle 20# (the hybrid-type solid cone-shaped nozzle including an X-shaped diversion core, with the pore diameter of 2.2 mm) has the best overall atomization performance out of the 22 nozzles; therefore it was selected for conducting the measurements regarding the airborne dust settling performance on the designed dust reduction measurement platform.

3.5.1. According to the measurement results of the nozzles’ atomizing performance parameters and the spraying fields’ droplet diameters, the three nozzles can be ranked (in a descending order based on a single parameter): in terms of the atomizing angle, nozzle 2# > nozzle 12# > nozzle 20#; in terms of the effective spraying range, nozzle 20# > nozzle 12# > nozzle 2#; in terms of the water consumption, nozzle 12# < nozzle 20# < nozzle 2#; and in terms of the droplet diameter, nozzle 20# < nozzle 2# < nozzle 12#. Since nozzle 20# has a solid cone-shaped spraying field with the largest coverage area, it is most suitable for large-area dust suppression; i.e., nozzle 20# is the best with regard to the overall atomization performance. As the spraying pressure further increases, the atomizing angles and droplet diameters of the three nozzles all decrease, whereas the effective spraying range and water consumption increase gradually. Therefore, the optimal spraying pressure still needs to be determined.

The design of the experimental scheme

Nozzle 20# was moved to the position of (0, 50) in the enclosed experimental box, and the wind speed was adjusted to 0.3 m/s. Then the high-pressure atomizing pump was opened and the pressure was adjusted to a certain value. After the spray from the nozzle was stable, the dust generator’s blower was opened to generate dust. The pulverized coal powder collected from a fully-mechanized excavation face in Jiangzhuang Coal Mine was emitted, and the dust capacity was controlled within 200 g/min. Using the dust measurement device (AKFC-92) at the exit point of the model, the dust concentrations at different spraying pressures were measured. It should be noted that the flow was controlled within 20 L/min and the dust collection lasted 2 min.

3.5.2.

An analysis of the results

The data of nozzle 20# at different spraying pressures, measured at a wind speed of 0.3 m/s, are displayed in Table 6, from which the following can be concluded:

Table 6 – The dust data of Nozzle 20# at different spray pressures. Spray concentration/(mg/m3 )

Spray pressure/MPa

0 2 4 6 8

Dust reduction rate/%

Total coal dust

Respirable dust

Total coal dust

Respirable dust

212.1 124.1 113.9 117.2 116.4

82.9 50.8 46.0 46.8 46.3

– 41.5 46.3 44.7 45.1

– 38.7 44.5 43.5 44.2

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Fig. 7 – The structure of the novel dedusting device.

(1) As the spraying pressure increases from 2 to 8 MPa, due to the combined action of several factors such as the formed atomizing angle of the spraying field and the droplet size, the reduction rates of coal and rock dust using nozzle 20# are highest at the spraying pressure of 4 MPa. Moreover, the reduction rates of the total coal and respirable dust are 46.3% and 44.5% respectively, suggesting that 4 MPa is the optimal spraying pressure for managing airborne dust. (2) As the spraying pressure increases, the difference between the total coal and respirable dust reduction rates decrease gradually. This indicates that a decline in droplet diameter can enhance the spraying field’s dust settling efficiency, especially for respirable dust.

4. The development of a novel dedusting device and performance measurement 4.1.

The development of a novel dedusting device

A novel spraying injection dedusting device was developed that was based on the dust production mechanism during the heading machine’s cutting process and the spray injection dedusting principle. In this device the primary spraying and negative-pressure secondary dedusting technique can be combined to achieve better control over dust production, and nozzle 20#, with a pore diameter of 2.2 mm, was selected due to its advantages outlined above. The structure of this novel

dedusting device is illustrated in Fig. 7 and its field installation is illustrated in Fig. 8. As shown in Figs. 7 and 8, the overall outline of the proposed spraying injection dedusting device for the heading machine used in coal mines looks like an arch. The device is installed on the heading machine’s cutting arm, specifically, on the left, top and right of this arm. It consists of a rigid water pipe, nozzles, cone-shaped caps, an injection tube and suction hoods. The main structural parameters of the injection tube are as follows: the outer and inner lengths are 1.14 and 0.9 m respectively; the width and height are 0.12 and 0.973 m respectively; and the width of the square tube is 0.12 m. The cone-shaped cap and suction hood are installed at the external wall of the injection tube, and the water pipe is installed inside this tube. All eight nozzles are fixed for spraying towards the cutting direction and are tilted 20◦ outwards. The nozzles on the bottom are tilted 30◦ downwards, whereas those on the top are tilted 20◦ upwards. Accordingly, the acquired spraying curtain can completely cover the dust produced by the cutting head. The cone-shaped cap and suction hood are each equipped with one nozzle. The cap is tilted at the same angle as the nozzle and has a total length of 0.1 m, with the diameters of the large and small circles being 0.07 and 0.1 m respectively. The distance between the nozzle jet and the outer boundary of the cap is 0.05 m. The suction hood at the top corner is shaped as an outer-wide and inner-narrow segment. The external arc is comparatively large, with a length and width of 0.41 and 0.1 m respectively; and the internal arc is comparatively small, with a length and width of 0.155 and 0.07 m respectively. The suction hood has a height of 0.12 m and is basically an outerwide and inner-narrow trapezoid. The external rectangle is comparatively large, being 0.4 × 0.1 m in size, and the internal rectangle is small, being 0.16 × 0.07 m in size, and is connected to the injection tube. As the novel spraying injection dedusting device is installed on the heading machine’s cutting arm a spraying curtain is always formed, which completely covers the dust produced in the cutting process, thereby collecting and settling the dust. Meanwhile, a spray-induced negative pressure is produced at the nozzle and near the spraying field, so that the dusty airflow can be sucked into the injection tube via the suction hood and then flow outwards from the cone-shaped cap. In this process,

Fig. 8 – An illustration of the device’s field installation.

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Fig. 9 – The atomization and suction performance of the proposed spraying injection dedusting device at the spray pressure of 4 MPa; where: (a) Atomization performance; and (b) Suction performance. Table 7 – The wind speeds at the suction ports and the amounts of air being sucked in at different spray pressures. Suction parameters

Spray pressure/MPa

Serial number of the suction hood

Average value (total)

1#

2#

3#

4#

5#

6#

7#

8#

Wind speed/(m/s)

2 4 6 8

1.91 2.67 3.14 3.32

1.93 2.69 3.15 3.35

1.92 2.65 3.12 3.33

1.96 2.71 3.18 3.39

1.97 2.72 3.21 3.41

1.95 2.66 3.14 3.34

2 2.74 3.23 3.45

2.01 2.77 3.26 3.47

1.956 2.701 3.178 3.382

Amount of air suction/(m3 /min)

2 4 6 8

4.584 6.408 7.536 7.968

4.632 6.456 7.56 8.04

4.7232 6.519 7.6752 8.1918

4.704 6.504 7.632 8.136

4.728 6.528 7.704 8.184

4.797 6.5436 7.7244 8.2164

4.8 6.576 7.752 8.28

4.824 6.648 7.824 8.328

37.792 52.183 61.408 65.344

the dust is mixed thoroughly with the spraying field to reduce the dust in the airflow. Since the novel dedusting devices are always kept a distance of 1.2–1.3 m away from the dust production sources and are installed along the cutting arm, the dusty wind flow near the cutting arm can be sucked away effectively, meaning the dust can be collected and settled completely.

4.2. Measuring the performance of the proposed dedusting device The performance of the proposed spraying injection dedusting device with regard to atomization and suction were subsequently analyzed. The water consumption and amount of air suction at different spraying pressures were measured, and thereby the gas–liquid ratio was calculated to determine the optimal spraying pressure. The gas–liquid ratio is defined as the ratio of the amount of air suction to water consumption. An air velocity meter (8347-VELOCICALC, TSI, USA) was used to measure the wind speed at different measuring points and the wind direction was measured using a red ribbon. The same four spraying pressures that were used to measure the nozzles’ performance with regards to atomization and airborne dust settling, i.e., 2 MPa, 4 MPa, 6 MPa and 8 MPa, were utilized again. The wind speeds at the suction ports of eight suction hoods were measured and thereby the total amount of air suc-

Table 8 – The gas–liquid ratios of the proposed device at different spray pressures. Spray pressure/MPa

Water consumption (L/min)

Amount of air suction/(m3 /min)

Gas–liquid ratio

2 4 6 8

30.16 41.12 50.24 56.40

37.792 52.183 61.408 65.344

1.253 1.269 1.222 1.159

tion could be calculated. The area of the external arc-shaped suction ports of suction hoods 3# and 6# is 0.041 m2 , and the area of the external rectangular suction ports of the other suction hoods is 0.04 m2 . The atomization and suction performances of the proposed dedusting device at the spraying pressure of 4 MPa are displayed in Fig. 9, the wind speeds at the suction ports and the amount of air suction at different spraying pressures are listed in Table 7, and the gas–liquid ratios at different spraying pressures are listed in Table 8. According to Fig. 9, Tables 7 and 8, the following conclusions can be drawn: (1) As the spraying pressure increases, the wind speed at the suction port and the amount of air suction increase grad-

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Fig. 10 – A comparison chart of the original external spraying device and proposed spraying injection dedusting device. ually. Specifically, as the spraying pressure increases from 2 MPa to 8 MPa, the average wind speed at the eight suction ports of the suction hood increases from 1.956 m/s to 3.382 m/s, and the average amount of air suction increases from 37.792 m3 /min to 65.344 m3 /min. (2) As the spraying pressure increases from 2 MPa to 8 MPa, the gas–liquid ratio increases from 1.253 at 2 MPa to 1.269 at 4 MPa, and then decreases to 1.159 at 8 MPa. The results demonstrate that the proposed device can attain an optimal gas–liquid ratio at a spraying pressure of 4 MPa. Moreover, the spraying curtain produced at a spraying pressure of 4 MPa can completely cover the dust that the heading machine produces during the cutting process, and can easily suck the red ribbon into the spraying field. Therefore, based on the measurement results, 4 MPa was selected as the optimal spraying pressure.

5.

Field experiments

5.1.

The experimental layout

The field experiments with regard to the proposed spraying injection dedusting device were conducted on the 3below 614 roadway’s fully-mechanized excavation face, Jiangzhuang Coal Mine, Zaozhuang Mining Group, China. A heading machine (EBZ-160A, Shandong China Coal Mining Group, China) was used for coal production on the working face. The original external-spraying nozzles were installed at the two ears of the cutting head, and in total 8 nozzles were used. The type and number of the original nozzles are identical to the nozzles used in the proposed external-spraying injection dedusting device. The water flows are also identical at the same spraying pressure, and the booster pump was used for constant-pressure spraying at 4 MPa. The original external spraying device can only form a horizontal surface spray, and cannot form a stereo dedusting spray field which can completely envelop the dust source in the cutting head. Additionally, there is no airflow suction system in the original external spraying device, and thus the dust cannot be further reduced. The overall outline of the proposed spraying injection dedusting device was a 3/4 semi enclosed structure, and it was installed at the heading machine’s cutting arm, located at the

left, top and right. The spray nozzles can produce a water mist curtain without a dead-end covering the cutting head, and the dust produced by the cutting head can be captured effectively. Meanwhile, the water mist curtain can generate a negative pressure around the nozzle and the spray field, thus forming a more obvious negative pressure field in the cone-shaped cap. The dusty airflow can be sucked into the injection tube by the suction hood, and then flow out through the cone-shaped cap. If in the above process the dust and spray field can be fully mixed, then the dusty airflow can be cleaned. A comparison chart of the original external spraying device and proposed spraying injection dedusting device is shown in Fig. 10. In order to understand the dust emanating from a working face, a certain number of points for measuring it should be arranged in the field to determine the levels of dust concentration at a fully-mechanized excavation face. Specifically, the selection of sampling points should be based upon the principle areas representing the harm caused to a human being’s health. Taking into consideration the diffusion law of dust sources in space and time, the 5 sampling points are: I — the measuring point where the cutting head is located; II — the measuring point where the machine’s driver works; III — the measuring point where the reversed loader is located; IV — the measuring point 50 m away from the end of the cutting head; and V — the measuring point 100 m away from the end of the cutting head. The dust sampling work was carried out after the tunneling machine had been in operation for 30 min. The AKFC-92A type of mine dust sampler and an organic membrane with a diameter of 40 mm was used in the dust sampling process. The sampling flow was controlled at 20 L/min, and the dust collection time was controlled at 10 min. During the experiments on the working face, the dust concentrations at each measuring point using different dust reduction methods were compared. These different dust reduction methods are: (a) without dust reduction measures; (b) the original externalspraying dust reduction device was opened; (c) the original external-spraying dust reduction device was closed and the proposed external-spraying injection dedusting device was opened, with the suction ports of the suction hoods blocked; and (d) the proposed external-spraying injection dedusting device was opened, with the suction ports of the suction hoods opened. The average value of each measuring point was sam-

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Table 9 – The dust concentrations of each measuring point using different dust reduction methods. Measuring points Method (a)

I

Average II

Average III

Average IV

Average V

Average

Method (b)

Method (d)

Total dust/(mg/m3 )

Respirable dust/(mg/m3 )

Total dust/(mg/m3 )

Respirable dust/(mg/m3 )

Total dust/(mg/m3 )

Respirable dust/(mg/m3 )

Total dust/(mg/m3 )

Respirable dust/(mg/m3 )

508.7 527.1 535.6 523.8 306.9 308.6 299.8 305.1 250.6 230.1 257.9 246.2 187.6 204.5 200.4 197.5 161.8 170.4 158.6 163.6

217.5 201.2 195.1 204.6 117.6 133.4 137.5 129.5 108.7 89.5 100.9 99.7 84.1 89.5 82.3 85.3 68.4 79.3 72.5 73.4

241.3 260.9 270.3 257.5 142.3 137.5 143.8 141.2 105.1 122.6 125.1 117.6 100.3 96.2 92.7 96.4 80.4 80.7 76.5 79.2

99.6 111.4 107.6 106.2 64.1 65.4 59.8 63.1 52.8 52.3 48.5 51.2 43.9 45.5 40.8 43.4 34.1 38.63 40.1 37.5

201.2 194.5 196.2 197.3 110.2 115.6 99.7 108.5 85.8 84.1 99.5 89.8 69.6 68.5 75.2 71.1 54.1 60.9 66.2 60.4

80.1 84.7 79.7 81.5 46.2 48.4 51.2 48.6 38.6 42.7 35.4 38.9 35.1 34.1 28.9 32.7 30.2 30.8 25.4 28.8

170.8 174.5 169.5 171.6 90.5 82.4 91.7 88.2 78.4 75.3 75.8 76.5 59.8 60.4 66.4 62.2 55.7 49.9 48.6 51.4

71.6 69.3 67.3 69.4 43.5 41.5 29.9 38.3 32.2 28.7 35.4 32.1 27.8 30.1 26.7 28.2 28.6 20.8 25.3 24.9

Fig. 11 – The measured dust concentrations at different measuring points using different dust reduction methods. pled three times, and the mean value of the final levels of dust and dust concentration were measured. The experimental data are shown in Table 9 and the mean dust concentrations at different measuring points using different dust reduction methods are listed in Fig. 11.

5.2.

Method (c)

An analysis of the results

(1) For measuring point II where the machine’s driver works, using the dust reduction methods (b)–(d), the average total coal dust reduction rates are 53.7%, 64.6% and 71.1% respectively; and the average respirable dust reduction rates are 51.3%, 62.5% and 70.4% respectively. Compared with the original external-spraying method, by using the novel external-spraying injection dedusting device for the heading machine, the average reduction rates of the total coal and respirable dust at the measuring point where the machine’s driver works are enhanced by 17.4% and 19.1% respectively. When no measures are taken, the average concentrations of total coal and respirable dust

are 305.1 mg/m3 and 129.5 mg/m3 respectively, and these figures decrease to 88.2 mg/m3 and 38.3 mg/m3 respectively once the dust reduction method (d) is used. The results show that the proposed spraying injection dedusting device can greatly improve the dust reduction rate at the driver’s location; in particular, the concentration of respirable dust is reduced. The driver’s working environment is therefore clearly improved, and the physical and mental health of mine workers in general is enhanced. (2) Using the dust reduction methods (b)–(d), the average total coal dust reduction rates are 51.9%, 63.5% and 68.9% respectively; and the average respirable dust reduction rates are 49.2%, 61.2% and 67.5% respectively. Compared with the original external-spraying method, using the dust reduction method (d), that is when the proposed external-spraying injection dedusting device was opened, with the suction ports of the suction hoods also open, the average dust reduction rates of the total coal and respirable dust are enhanced by 17.0% and 18.3% respectively; moreover, using the dust reduction method (c), that is when the proposed external-spraying injection dedusting device was opened, with the suction ports of the suction hoods blocked, the average dust reduction rates of total coal and respirable dust are enhanced by 11.6% and 12.0% respectively. Therefore, it can be calculated that by using the injection dedusting method, the average reduction rates are enhanced by 5.4% (17.0–11.6 = 5.4) and 6.3% (18.3–12.0 = 6.3) respectively. The results indicate that by using the novel injection dedusting device, the spraying curtain can envelop the dust produced in the cutting process completely, which is better with regard to dust settling on a working face. Moreover, the dust escaping from the spraying curtain can be sucked back due to the negative pressure; i.e., the developed device performs very favorably when settling the respirable dust that is composed of very small particles.

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6.

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Conclusions

(1) Numerical simulations of the wind suction and inner wind flow of the spraying injection suction device were performed using FLUENT software. The simulation results demonstrate the spray injection suction mechanism visually, and provide a theoretical basis for the development of new dust removal device. (2) Using the self-developed experimental platform that measures performance with regard to spray atomization and dust settling, the related performances of 22 kinds of nozzles were analyzed. The results show that as the spraying pressure increases from 2 MPa to 8 MPa, nozzle 22# (a hybrid-type solid cone-shaped nozzle including an Xshaped diversion core, with a pore diameter of 2.2 mm) has the largest water consumption, largest droplet diameter and widest coverage range of the spraying field; i.e., nozzle 20# is optimal in terms of overall atomization performances. Moreover, nozzle 20# exhibits the highest airborne dust reduction rate at a spraying pressure of 4 MPa, and the gas–liquid ratio of the developed dedusting device firstly increases and then decreases, reaching a maximum of 1.269 at a spraying pressure of 4 MPa. Consequently, 4 MPa was set as the optimal spraying pressure. (3) In order to resolve the issue of dust production during a heading machine’s coal cutting process, a novel external-spraying injection dedusting device was developed. Compared with the original external-spraying dedusting device, the spray nozzles of the proposed external-spraying injection dedusting device can produce a water mist curtain which can cover the cutting head with a larger range, thus the dust produced by the cutting head can be captured by the water mist curtain. Meanwhile, the air in front of the nozzle is pushed out continuously by the outpouring spray and a vacuum is formed behind; thus, the nozzle produced a more obvious negative pressure field that was nearer the spraying field. The dusty airflow can subsequently be sucked into the injection tube by the suction hood, and then flow out through the cone-shaped cap. During this process, the dust is mixed thoroughly with the spraying field to reduce the dust in the airflow. (4) According to the field experimental results, compared with the original external-spraying method, using the dust reduction method (d), that is when the proposed external-spraying injection dedusting device was opened, with the suction ports of the suction hoods also open, the average dust reduction rates of the total coal and respirable dust are enhanced by 17.0% and 18.3% respectively; moreover, using the dust reduction method (c), that is when the proposed external-spraying injection dedusting device was opened, with the suction ports of the suction hoods blocked, the average dust reduction rates of total coal and respirable dust are enhanced by 11.6% and 12.0% respectively. Therefore, it can be calculated that by using the injection dedusting method, the average reduction rates are enhanced by 5.4% (17.0–11.6 = 5.4) and 6.3% (18.3–12.0 = 6.3) respectively. Additionally, by using the novel external-spraying injection dedusting device, the average dust reduction rates of the total coal and respirable dust at the measuring point where the heading machine’s driver works are enhanced by 17.4% and 19.1% respectively. Therefore, the spraying injection dedusting

method displays a remarkable dust settling ability, especially with regard to respirable dust.

Acknowledgements This work was funded by the Key Program of the Coal Joint Funds of the National Natural Science Foundation of China (NO.U1261205), the National Natural Science Foundation for Young Scientists of China (NO.51404147), the China Postdoctoral Science Foundation (NO.2015M570601), and the Graduate Science and Technology Innovation Project (NO.SDKDYC170202, NO. SDKDYC170101).

References Bianchi, G.M., Pelloni, P., 2001. Modeling atomization of high-pressure diesel sprays. ASME J. Eng. Gas Turbine Power 23, 419–427. Chakraborty, S.N., 2009. Combating coal mine fire-application of high pressure water jet technology adopted by Goma Engineering Pvt. Ltd. J. Mines Metals Fuels 57 (11), 398–418. Chen, G., Wang, D.M., Wang, H.T., et al., 2012. The technology of controlling dust with foam for fully mechanized excavation face of large cross-section rock tunnel. J. China Coal Soc. 37 (11), 1859–1864. Cheng, W.M., Zhou, G., Zuo, Q.M., et al., 2010. Experimental research on the relationship between nozzle spray pressure and atomization particle size. J. China Coal Soc. 35 (8), 1308–1313. Colinet, J.F., 2010. Best Practices for Dust Control in Coal Mining. US Department of Human Health Services, Pittsburgh, PA, pp. 1, 65. Cousin, J., Nuglisch, H.J., 2001. Modeling of internal flow in high pressure swirl injectors. SAE Trans. 110 (3), 806–814. Faschingleitner, J., Höflinger, W., 2011. Evaluation of primary and secondary fugitive dust suppression methods using enclosed water spraying systems at bulk solids handling. Adv. Powder Technol. 22, 236–244. Gerke, H.H., Badorreck, A., Einecke, M., 2009. Single-and dual-porosity modelling of flow in reclaimed mine soil cores with embedded lignitic fragments. J. Contam. Hydrol. 104 (1–4), 90–106. Hou, L.Y., Hou, X.C., 2007. Spray Technical Manuals, Second edition. Beijing China Petrochemical Press, China. Jiang, B., Wang, Z.Y., Fu, X.Z., et al., 2011. Numerical study on fan-shaped air-blast atomizer parameters. Chem. Ind. Eng. Progress 30 (2), 269–274. Li, Q., 2008. Study on the Negative-pressure Absorb Dust Device Used at the Coal Draw Point of the Hydraulic Support on the Fully-Mechanized Caving Face. Taiyuan University of Technology. College of Mechanical Engineering, Taiyuan, pp. 3–7. Liu, B.M., 2011. Research on road header rotating spray dustfall device, D. Taiyuan: Taiyuan University of Technology. Coll. Mech. Eng., 3–7. Ma, S.P., Kou, Z.M., 2005. Study on mechanism of reducing dust by spray. J. China Coal Soc. 30 (3), 297–300. Ma, Z.F., Yan, Z.B., Chen, J.X., et al., 2011. CFD numerial simulation and experiment of the water-air rotating jet flow dust-controlling system on the comprehensive mechanized mining faces. J. China Coal Soc. 36 (5), 818–822. Nie, W., Cheng, W.M., Yu, Y.B., et al., 2012a. The research and application on whole-rock mechanized excavation face of pressure ventilation air curtain closed dust removal system. J. China Coal Soc. 37 (7), 1165–1170. Nie, W., Cheng, W.M., Zhou, G., et al., 2012b. Experimental study on atomized particle size as affected by airflow disturbance at the heading face. J. China Univ. Min. Technol. 41 (3), 378–382. Nie, Wen, 2013. Research on the Airborne Dust Migration Rule and Inhibition Technology in Mechanized Excavation Face. Shandong University of Science and Technology. College of

Process Safety and Environmental Protection 1 0 9 ( 2 0 1 7 ) 716–731

Natural Resources and Environmental Engineering, Qingdao, pp. 3–9. Ren, T., Plush, B., Aziz, N., 2011. Dust controls and monitoring practices on Australian longwalls. Procedia Eng. 26, 1417–1429. Ren, T., Karekal, S., Cooper, G., et al., 2013. Design and field trials of water-mist based venturi systems for dust mitigation on longwall faces. 13th Coal Operators’ Conference, University of Wollongong, The Australasian Institute of Mining and Metallurgy & Mine Managers Association of Australia, 209–220. Seibel, R.J., 1976. Dust Control at a Transfer Point Using Foam and Water Sprays. US Department of the Interior, Bureau of Mines, Pittsburgh, PA, pp. 1–12. Shao, H.Z., Zhang, G.J., Li, Z., et al., 2001. Theoretical and experimental study of air ejector with high-pressure spray for dust suppression. J. China Coal Soc. 26 (3), 299–302. Wang, H.T., Wang, D.M., Ren, W.X., et al., 2013. Application of foam to suppress rock dust in a large cross-section rock roadway driven with roadheader. Adv. Powder Technol. 24, 257–262. Wang, G., Li, W.X., Wang, P.F., Yang, X.X., Zhang, S.T., 2017. Deformation and gas flow characteristics of coal-like materials under triaxial stress conditions. Int. J. Rock Mech. Min. Sci. 91, 72–80. Wang, D.M., 2015. Mine Dusts. Science Publishing, Beijing, pp. 167–173.

731

Wu, Tonghai, 2002. Research on Application of Efficient Spraying Dust-Settling Technology in The Digging Working Face. Taiyuan University of Technology. College of Mechanical Engineering, Taiyuan, pp. 2–5. Xie, Y.S., Jiang, X.Y., 2003. Dust removal technology with negative pressure and its application. J. China Univ. Min. Technol. 32 (5), 567–570. Zhao, L.J., Tian, Z., Wang, Y., 2014. Numerical simulation of shearer external spray system. J. China Coal Soc. 39 (6), 1172–1176. Zhao, T.C., 2007. Technology of Dust Control in Underground Mines. China Coal Industry Publishing House, Beijing, pp. 61–88. Zhou, G., Cheng, W.M., Nie, W., et al., 2012. Extended theoretical analysis of jet and atomization under high-pressure spraying and collecting dust mechanism of droplet. J. Chongqing Univ. 35 (3), 121–126. Zhou, D., Luo, Z.Y., Jiang, J.P., et al., 2016. Experimental study on improving the efficiency of dust removers by using acoustic agglomeration as pretreatment. Powder Technol. 289, 52–59. Zhou, G., 2009. Research of Theory about Dust Prevention by Water-Cloud and Relevant Techniques for Fully Mechanized Caving Coal Face. Shandong University of Science and Technology, College of Mining and Safety Engineering, Qingdao, pp. 1–2.