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Experimental study on dust suppression at transhipment point based on the theory of induced airflow dust production
Building and Environment 160 (2019) 106200
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Experimental study on dust suppression at transhipment point based on the theory of induced airflow dust production
T
Xinghua Zhanga,b,c, Haifeng Wanga,b,c,*, Xi Chend, Chaonan Fand, Kun Tiana,b,c, Xiao Zhanga,b,c a
State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China National Professional Laboratory for Fundamental Research of Mine Gas and Dust Control Technology, School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China c School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China d College of Safety and Emergency Management Engineering, China Taiyuan University of Technology, Taiyuan, Shanxi, 030000, China b
ARTICLE INFO
ABSTRACT
Keywords: Induced airflow Dust concentration Dust suppression technology Chute Energy-saving equipment
Induced airflow is the main cause of dust problems at the transhipment point. However, the current measures for dust control are mainly from the perspective of dust control after production, focusing on blocking, lowering, discharging, removing and flushing. It is rare to manage dust from the source of occurrence, so dust pollution cannot be solved at the source. Based on the kinematics principles and the energy dissipation theory, this paper systematically summarizes the induced airflow dust production theory, improves the transfer chute, and proposes the compound straight-arc chute dust suppression technology. According to the similarity criteria, the experiment was carried out in the laboratory with the transportation of coal as an example. The experimental results show that with the decrease of particle size, the induced airflow velocity and dust concentration at the chute outlet show a linear growth trend and remains unchanged even when the feed volume changes. Compared with the upper-bend shape, when using a down-bend shape for the dust suppression chute in the process of transferring coal, the induced airflow velocity and dust concentration at the chute outlet can be reduced by 31% and 60%. Compared with a linear chute, the dust concentration is reduced by up to 77%. It indicates that the down-bend chute is a more energy-saving dust removal equipment that can be used to reduce operational costs while conveying coal.
1. Introduction Because of its simple, compact structure and high efficiency, belt conveyor is widely used in the transportation of bulk material. However, due to the limitation of single-period conveying distance, multi-periods are often needed to complete long-distance transportation [1,2]. Generally, bulk material is mostly transported vertically between belts, which not only has a large impact on the equipment, but also carries a large amount of induced airflow thus causes serious dust pollution [3–6]. In China, the annual incidence of pneumoconiosis accounts for 89.67% of the total number of occupational diseases, and the mortality rate is as high as 22.4%. The number of deaths is three times higher than that of other accidents, which caused great loss to the national economy [7]. However, currently, all kinds of dust removal technologies and equipment is active dust removal. Although the purification efficiency is high, and the dust removal speed is fast, they generally have complex structures, high operational costs, large
*
maintenance workloads, secondary pollution etc. [8–14,39]. In contrast, passive dust removal has the advantages of long life and low energy consumption [15,16], however, to solve the pollution problem, researchers need to understand the characteristics of dust sources thoroughly. Due to various factors, the mechanism of airflow induced by transhipment point are complex and fluctuates greatly. During the free fall, particles carry ambient air into the airflow, forming a particle plume, which is in a typical disordered state [17]. Therefore, in order to effectively solve the problem of dust pollution at the transhipment point, it is necessary to have a thorough understanding of the induced airflow. Aiming at the calculation of entrained air volume during bulk material transportation, Hemeon [18] established a physical model for the free fall of particles in an air-free flow environment under gravity. The study reported that in the process of falling, the force of particle flow on air is equal to the sum of the force of a single particle on air. Therefore, a single particle model of entrainment air volume is
Corresponding author. School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China. E-mail address: [email protected] (H. Wang).
https://doi.org/10.1016/j.buildenv.2019.106200 Received 15 April 2019; Received in revised form 6 June 2019; Accepted 8 June 2019 Available online 08 June 2019 0360-1323/ © 2019 Published by Elsevier Ltd.
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proposed, and the mathematical models of induced air volume and particle flow, thus cross section area, particle size, density, and falling information are deduced. Based on the mathematical model established by Hemeon, from the perspective of turbulent flow, Tooker [19,20] proposed a mathematical model of the total amount of dusty gas discharged by a closed ventilation system. On the basis of previous studies, Cooper [21] et al. studied the influencing factors of air volume in the falling process of particle flow, and considered that with the increase of the falling height of particle flow, the cross-sectional area of the particle plume fluid core gradually decreases, while the boundary layer crosssectional area of dusty gas increases. Therefore, the variation of entrainment air in the falling process of particle flow is obtained. Amos Ullmann [22] et al. conducted an experimental study and comparative analysis of previous studies, and proposed a new model for calculating induced airflow. Based on the conservation of energy, the mathematical model was improved by supplementing the equations of motion and fluid resistance in the material falling process. The calculation of material in turbulent zones was simplified, and the expression of airflow velocity in chute was obtained. In the process of falling Ogata [23] et al. studied the jet, dispersion and entrapment airflow of bulk materials, and obtained the law of porosity change and entrainment air volume, which was combined with the LVD test, PIV test and numerical simulation, thus the relationship of the final velocity of materials and the final velocity of airflow was obtained. To analyse the entrainment air volume during the free landing process Tomomi Uchiyama [24–28] and others used the vortex simulation method, and reported the relation between bulk materials’ diameter, density and induced airflow. Based on the two-phase flow model, Renaud Ansart [17,29,30] et al. described an experimental set up. The effect of the drop height of a free falling jet on segregation of particle size, particle velocity, changes the particle concentration and entrained air in the dust plume were investigated, and a model of predicting the particle and air velocities, and especially the volumetric flow of induced air in the column without dependence on any empirical constant was proposed. Xiaochuan Li [31,32] studied the effects of the conveyor chute on air entrainment and the quantitative factors of influence. The above researchers deduced the induced airflow from a view of mathematics, but they did not give out its governing method. In this paper, we decomposed the working process of the transhipment point into three parts: reserve zone, transport zone, impact zone, so that we can targeted solve the pollution. Generally, bulk material is compacted in the reserve zone without dust pollution. However, in the transport zone and the impact zone, the interaction between the particle and environment causes a large number of dust particles to be peeled off, resulting in floating dust, which is directly related to the formation of induced airflow, but the formation mechanisms in these two zones are substantially different [17,29,30]. In the transport zone, the movement of bulk material causes air to undergo plastic deformation around itself. During this process, with the increase of falling speed, due to viscous resistance, a negative pressure layer will be formed around the bulk material, which will absorb the surrounding air and produce a flow field with a velocity gradient [18,33,34]. As shown in Fig. 1, under the action of the airflow field, particle A on the left side directly contacts with the air, where the airflow is basically in full development and a turbulent wake appears that reduces the air pressure [34]. However, due to the influence of particle B, the air on the right side cannot fully expend in a short time, thus the resistance on the right side of A is smaller than the left side. Due to uneven force on the two sides, particle A will rotate clockwise, and its size depends on the difference in the air resistance between both sides, which will increase with the change of falling height and velocity. When it reaches a certain level, particle A rotates and increases the distance between the particles, which will increase the voidage of the coal stream [35–37]. In other words, more gas will be “wrapped” or “entrained” by the transporting coal stream thus entering into the lower equipment, as shown in Fig. 1a and b. At the same time, this differential
Fig. 1. Schematic diagram of coal flow and induced airflow in the chute.
flow can be considered as an airflow disturbance that will force the coal dust to passively enter the surrounding working space [17,21,29,30]. In the impact zone, the moving airflow is hindered by the subordinate equipment thus changes its movement direction, and the air dispersed in the void is rapidly “squeezed” and “discharged” into the environment. The dust adsorbed on the coal will then escape from the inside to the outside, as shown in Fig. 1c. These processes not only produce dust pollution but also disturb the stable airflow inside the equipment, resulting in the secondary dust pollution. With the change of transport operating parameters, such as feed volume, discrepancy in altitude etc., this phenomenon will lead to more serious dust pollution problem [38]. In view of the above problems, most of the current on-site production adopts the active method of applying dust removal equipment. This method often requires manpower, material and financial resources [39,40]. Although active method can alleviate the dust pollution in the workshop, it cannot fundamentally solve the dust pollution problem. Taking a belt conveyor transhipment point as the research object, through the change of the chute structure to limits the coal motion, this paper proposes a passive dust removal method named compound straight-curve chute dust suppression technology. The effectiveness and feasibility of the technology are verified by means of theoretical derivation and experimental comparative analysis. 2. Physical model of a chute According to the induced airflow dust production theory, the fundamental cause of dust pollution at transhipment point is the highspeed flow of coal [41]. However, in the design of traditional transfer chutes, various factors are not considered carefully, making it difficult to control the flow of coal in operation. This phenomenon not only hinders normal production but also increases unnecessary costs [38]. Therefore, it is necessary to analyse and improve the chute design and control the coal movement. 2.1. Motion analysis The motion of bulk material in chute can be accelerated or decelerated. When accelerated, the section area decreases. When decelerated, the section area increases, and the material is likely to be deposited even being blocked in the chute. To study the movement of bulk material in chute more conveniently, the movement process is simplified, and the following assumptions are made. (1) The bulk material is regarded as being composed of numerous micro elements; (2) The flow process is continuous; (3) Only the influence of friction on the flow between bulk material and the chute wall is considered, and the influence of its internal factors is ignored; (4) The influence of air resistance and temperature is ignored. 2
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Fig. 3. Sketch of the material movement at the exit of the chute.
v = cos v
Fig. 2. Diagram of coal force analysis in the process of transport.
+ ma + µE ( mg sin
(1)
+ man) = 0
To simplify:
a + µE an + g (µE sin
(2)
+ cos ) = 0
min
where Δm is the mass of coal micro elements, kg; aτ is the tangential acceleration of coal, m/s2; an is the normal acceleration of coal, m/s2; g is the gravitational acceleration, m2/s;μE is the equivalent friction coefficient of coal and chute;θ is the angle between gravity and the coordinate axis.
where v is the velocity of coal in the chute, m2/s; s is the initial velocity of coal entering the chute. In the arc part of the chute, the curvature radius is a fixed value, the velocity of the bulk material can be expressed as v = rθ, thus the flow velocity is as follows:
v=
H L
+ 5°
(6)
Generally, the pressure distribution of bulk material in a rectangular chute is shown in Fig. 4. It is assumed that the pressure exerted on the chute floor is uniformly distributed, and in the vertical direction, with the increase in bulk material height, the pressure increases linearly. According to the principle of mechanics, the expression of μE can be obtained. The total friction force on the chute wall is:
(3)
µE sin )
2gr [sin (1 + 2µE2 ) + 3µE cos ] + e 4µE2 + 1
= arctan µE 1 + K
2.4. Equivalent friction coefficient
According to the equilibrium equation of mechanics, formula (2) is further simplified to an equation of velocity. In the straight part of the chute, its curvature radius can be regarded as infinite, so the flow velocity of bulk material is:
v02 + 2gs (cos
(5)
where μE is the actual friction coefficient when bulk solid contacts with the chute surface; H is the height of the bulk material in the chute, m; L is the width of the chute, m; and K is the ratio of the vertical pressure and transverse pressure of the bulk material to the chute floor.
2.2. Kinematic equations
v=
tan
where θ is the angle between the and horizontal and the moving direction when the bulk material enters the chute, rad, and ϕ is the dynamic friction angle between the bulk material and the chute, rad. When θ+ϕ = 90°, v’ = 0. The smoother the chute surface is, the smaller the value of ϕ. To make the coal flow smoothly, it is necessary to make v’ > 0. Under the premise of coal material flow, the chute inclination angle is as small as possible. The minimum chute inclination angle is determined by the friction characteristics between the bulk material and chute. This angle can be derived from the following formula:
As shown in Fig. 2, the micro-element dm of bulk material in chute is selected as the research object. Take its moving position as the original coordinates, tangent movement direction as the transverse axis, and the normal direction as the longitudinal axis. Assuming that the force of coal is balanced during the course of movement, According to mechanical principles, the following formula is obtained
mg cos
sin
2µE
v0
(7)
F = µPn + KPn Hµ Making. F = µE Pn B The equivalent friction coefficient is:
µE = µ 1 + K
6µE rg
H B
(8)
It can be seen from formula (8) that the equivalent friction coefficient is proportional to the ratio of the height to the width of the bulk material in the chute, and the larger the ratio, the greater the equivalent
4µE2 + 1 (4)
2.3. Chute inclination The inclination of the chute directly determines whether the bulk material can flow smooth, which is a key factor in chute design. Therefore, the chute should have a reasonable inclination and proper roughness to ensure the smooth passage of the transported bulk material. As shown in Fig. 3, the ratio of the velocity v0 of coal on the belt to the velocity v when it leaves the chute coincides with the following function:
Fig. 4. Force analysis of the chute wall. 3
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friction coefficient, and the more easily to be blocked. 2.5. Chute structure Induced airflow is the main cause of dust pollution during bulk transportation, and the induced airflow velocity is directly related to the bulk material's flow velocity. Therefore, according to the energy dissipation theory, the author continuously reduces the angle of bulk material relative to the horizontal plane by means of a circular arc design to slow down the flow. Considering the large space occupied by a circular arc chute, part of the linear chute idea is introduced in the design, thus a composite straight-arc chute dust suppression technology is proposed. The upper part is the arc section and the lower part is the linear section, or, the upper part is the linear section and the lower part is the arc section. To reduce the impact of bulk material on the chute, to alleviate the wear of equipment, and to avoid the fragmentation, the chute is designed with a down-straight part and an upper-bend part (hereafter referred as upper-bend), which can not only make the coal enter the chute smoothly but also realize the purposes of slowing down the coal and decrease the dust suppression. To ensure that there is no blocking, the chute outlet inclination is set to be greater than the sum of the coal sliding friction angle and a certain angle. As shown in Fig. 5, when the dumping height and chute outlet angle were determined, we chose any number from 90°-α as the value of θ, then the radius R of the arc, central angle θ′ and straight line length S were derived. The geometric dimensions of each part of the chute are determined by the following formulas:
S=
y tan y
S=
2
R tan
A=
+ cos )
2
(sin
(10)
R=
(11)
(sin
+ cos )
(14)
Q 3600 v
(15)
3.1. Apparatus According to the on-site production situation, a transhipment point induced airflow experimental platform is designed and constructed, as shown in Fig. 7. Bulk material outflows from bunker, then passes through the electro-vibration feeder (GZV), chute, belt conveyor, and finally collected by the recycle bin. Here the bunker and GZV are considered as a whole. The bunker's upper opening is 900 mm × 900 mm, lower opening is 250 mm × 250 mm, and the height is 600 mm. The GZV chute size is 700 mm × 200 mm × 130 mm,
(12)
2
+ cos )
3. Experimental details
y S cos tan
(sin
sin
where A is the cross-section area of the chute, m ; Q is the feed volume, t/h; ξ is the filling factor; η is the bulk weight ratio, t/h3. V is the average flow velocity of the bulk material on the chute bottom plate, m/s.
To reduce the speed of leaving the chute and the induced airflow into the receiving equipment, and to alleviate the problem of collision dust, the chute is designed with an upper-straight part and down-bend part (hereafter referred to as down-bend). The geometric structural diagram of the chute described above is shown in Fig. 6. The geometric dimensions for each part of the chute are determined by the following formulas:
= 90°
2
2
+ cos )
sin
R tan
The cross sectional size of a chute is directly related to the bulk material blockage in transport. Generally, the chute outlet width is 2/3 of the horizontal projection width of the lower conveyor belt, and the height of bulk material in the chute should be 20–30% of the chute height. According to the conveyor capacity, the chute cross-section can be calculated by the following formula:
S sin (sin
y
2.6. Cross-sectional area
(9)
= 90° R=
Fig. 6. Down-bend feature of compound straight-curve type of dust suppression chute.
(13)
Fig. 5. Upper-bend feature of compound straight-curve type of dust suppression chute.
Fig. 7. Transhipment point induced airflow experimental platform. 4
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Fig. 8. Experimental coal sample.
device and ventilation system is turned off.
Table 1 Experimental coal sample sliding angle. Particle size
6–8 mm
4–6 mm
2–4 mm
0.5–2 mm
Sliding angle
21.3°
22.9°
24.2°
26°
3.3. Experimental procedure According to the relevant theory of powder mechanics [45], we can put the coal on the plate used for making the chute and incline it slowly. When the coal slips, the angle between the plate and horizontal direction is the required sliding friction angle. To eliminate the influence of systematic errors, 20 groups of coal samples of each particle size were taken, and the mean value was taken as the sliding friction. The results were shown in Table 1. To ensure that the coal sample can pass through the chute smoothly without blocking, the maximum value of 26° in the test results is used to represent the sliding friction angle of all coal samples. By bringing the coal sample sliding friction angle into formulas (1) ~ (14), the minimum values of formulas (3) and (4) are solved by computer. Then, we bring the curve radius R and the line length S of the chute into the upper bend calculation formulas (8) ~ (10) and lower calculation formulas (11) ~ (13), respectively. Finally, the structural size of the dust-suppressing chute that can effectively control the coal flow velocity at a limited distance under laboratory conditions is obtained, as shown in Table 2. In addition, to explore the effect of dust suppression by the dust suppression chute structure and installation requirements, we designed a linear chute as a control group. To determine the best data measurement point, COMSOL multiphysics software is used to simulate the transport process using the k-ε [17,29,30,46] model before the experiment. The internal flow field distributions of the dust suppression and linear chutes are compared and analyzed under laboratory conditions. The following assumptions are made:
and its outlet is 1400 mm above the ground. Through the adjustment of the input current of the GZV, the fluctuating electromagnetic attraction between the core and armature of the feeder can be changed, so that the GZV can produce reciprocating vibrations and adjust feeding volume. To eliminate the influence of turbulence, based on the calculation, inference and experimental conditions, the belt conveyor size was determined as 2400 mm × 400 mm × 400 mm. 3.2. Experimental sample Coal is a common porous ore with well-developed pore structure and easy to be broken. Therefore, this paper chose coal as a bulk material for experimental study [42,43]. To ensure the accuracy and scientific integrity of the experimental data, as shown in Fig. 8, to simulate different feeding conditions on-site, according to similar criteria, particle sizes of 0.5–2 mm, 2–4 mm, 4–6 mm, and 6–8 mm are selected, and to eliminate the interference of coal sample moisture content with the test data, the humidity is kept constant by air-drying and humidity test. During the experiment, a thermal anemometer and a ccd-500 explosionproof dust concentration tester were used to monitor and record the induced airflow velocity and PM10 dust concentration in real time, respectively. The precision of the instruments was 0.001 m/s and 0.01 mg/m3, respectively. During the testing process, to eliminate the influence of environmental factors on the data collected by the instruments, the experimental platform is placed in an underground windowless laboratory with few doors, and the exhaust system is closed, which can minimize the degree of indoor air flow. Because of serious dust pollution in the laboratory, a spray dust suppression system is installed around the experimental platform, and all electrical equipment inside the room is sealed, dust-proof and waterproof thus to ensure the safety of the laboratory staff. After the data collection for each group of experiments is completed, the spray device and ventilation system are turned on, and the indoor dust concentration is tested with the dust meter until it is reduced to 0.15 mg/m3 [44]. Then, the spray
Table 2 Structural parameters of the chute. Chute form
Upper-bend Down-bend Linear
5
Outlet angle of chute (°)
Chute width (m)
Arc part
Straight part
Central angle (°)
radius (m)
length (m)
26 26 26
0.15 0.15 0.15
61 58 –
1.5 2.1 –
0.58 1.34 1.02
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Fig. 9. Airflow field in the chute during coal transfer.
(1) Coal is evenly spread over the bottom of the chute; (2) The roughness of the bottom of the chute is used to represent the coal surface; (3) The airflow field is vertically and evenly distributed on the coal surface; (4) Air is incompressible; (5) The influence of air gravity is negelected; (6) The influence of temperature on the airflow field is neglected. Fig. 9 shows the simulation results. It was concluded that the downbend can effectively reduce the coal and induced airflow velocity. According to the theory of induced airflow dust production, this method can effectively solve the dust pollution problem at the transhipment point and prolong the service life of the equipment [6]. Fig. 10 shows the induced airflow velocity along the wall height direction at the outlet. The induced airflow is stable at level positions 4 cm away from the surface meeting the experimental requirements. Therefore, during the experiment, the wind meter and dust concentration tester probes are installed for data monitoring and recording, respectively. In the experiment, we adjust GZV's input current to the required feeding amount, close the power supply, install the chute, add coal to the bunker, then monitor and record the required time and instrumentation data for complete transport of all coal. The specific testing process is as follows: Firstly, under different feeding volume
Fig. 10. Induced flow velocity curves along the chute outlet wall height.
between the upper-band and lower-bend, we compare the change of induced airflow velocity and dust concentration in the process of transport coal with different particle sizes, then, to explore the dust suppression effect, we choose the lowest one velocity to compare with the traditional linear chute. Although dust control measures were designed during the experiment, indoor dust concentration could not be completely reduced to 6
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Fig. 11. Induced airflow velocity.
0 mg/m3. Therefore, the indoor air velocity is measured before each set of experimental tests carried out. When the test data does not fluctuate substantially, it was considered that there is no air flow in the room, and the measured data in the experiment is the induced airflow velocity. Then, we monitor and record the indoor dust concentration with dust meter. To eliminate the influence of errors on the conclusions, after the experiment, this data should be deducted and analyzed.
entrained in the pores, which will greatly increase the induced airflow in the chute during transportation. According to the relationship between the induced air flow and the dust concentration mentioned above, this is not conducive to dust reduction. With the increase of the feed volume, the induced airflow velocity increases, which remains unchanged with the change of coal particle size. In the process of increasing the feed rate from 1.39 to 13.27 t/h, the maximum increase of induced airflow is 41.87%. Because under the same particle size, increment of the feed volume is equival to the increasing of the total amount of coal entering the chute in a unit time, which makes the coal transfer more energy to the air in a unit time and induces a greater velocity air flow according to the energy conservation and transformation theorem. When using the upper-bend, coal can smoothly enter the chute without severe collision occurring, however, when the coal leaves the arc part and enters the straight part, its motion is similar to an object sliding on an inclined plane, as shown in Fig. 13. According to the principles of mechanics, the coal has a downward acceleration along the slope, the adjacent air is “entrained” and speeds up. This is the reason why the induced airflow velocity and dust concentration increased. In addition, the high-speed moving coal will produce more dust during the process of impacting on the belt.
4. Results and discussion By removing the extreme values and averaging the data, the experimental results are shown in Figs. 11 and 12. We can find that the induced airflow velocity at the outlet increases with the decrease in coal particle size, and this trend does not change with the feed quantity. The reasons for this phenomenon can be considered from the following two aspects: (i). As the particle size of coal samples decreases, the number of particles in a unit space increases, which indicates that the increase of the drag effect on the air, suggesting that more air is pulled into the chute. (ii). The decrease in coal particle size reduces the stacking density and increases the porosity. Under the same conditions, more air will be
Fig. 12. Dust concentration. 7
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improve the transhipment point dust pollution problem for a business. This finding means that in the actual management, the work can be achieved without other dust removal equipment or less energy consumption, reducing the difficulty for the coal industry environmental protection and energy conservation. 5. Conclusion Taking the transhipment point of coal system belt conveyor as the research object, and relying on the theory of induced airflow dust production mechanism and energy dissipation, this paper improves the transporting chute, summarizes the compound straight-arc chute dust suppression technology, and carries out simulation experiments in the laboratory. The main work and conclusions are as follows:
Fig. 13. Force analysis of sliding objects on an inclined surface.
(1) In the process of coal flowing in the chute, under the action of air viscous resistance, the coal surface will not only form a negative pressure layer to absorb part of the air but also increase the coal porosity and entrap more air with the coal in the lower equipment, resulting in dust pollution. (2) When coal is transferred by down-bend, it will collide with the chute when it enters the straight part, but it will not generate excessive dust due to the low flow rate. When the coal flows through the straight part into the arc part, the arc chute makes it to slow down. The low-speed coal in front of the position has the function of cushioning and decelerating the back position, at the same time this low-speed flowing coal has the influence of dragging and decelerating the induced airflow. Because of the synthetically effect of deceleration method, the coal has a lower velocity when leaving the chute and controls the dust concentration at a lower level. Therefore, the dust suppression effect of the down-bend is obviously better than that of the upper-bend. (3) Compared with the linear chute, when using the down-bend chute, induced airflow velocity and dust concentration were reduced by 26% and 77%, respectively. Therefore, it can be considered that the composite straight-curve dust suppression chute technology proposed in this paper has a high potential feasibility.
When using the down-bend to transport, the coal will enter the chute's arc part from the straight part. In this process, the coal has less kinetic and potential energy, and the crushing effect is alleviated when it enters the chute compared with the upper-bend. After entering the arc part, the coal's transfer rate is gradually reduced due to circular motion. According to the energy dissipation theory, the coal particles are a complex energy dissipation system. The energy caused by impaction can be attenuated through the friction and viscous effects, and the force chain structure between particles can also extend the space and time of instantaneous local impact to achieve a cushioning effect [47,48], so that the energy of the coal material is consumed in a large amount, finally, the coal velocity is reduced sharply. In the meantime, through the drag effect, the coal can also reduce the induced airflow to alleviate dust pollution [49]. In summary, when transporting coal, the down-bend can suppress dust production more effectively than the upper-bend. According to Figs. 14 and 15, we can find that the induced airflow velocity and dust concentration of down-bend are significantly lower than that of the linear chute. When the feed volume is 1.92 t/h, the induced airflow velocity and dust concentration are reduced by approximately 25% and 77%, respectively. When the feed volume is 7.61 t/h, the data is 26% and 60% respectively. The dust concentration in the experiment is shown in Figs. 16 and 17. We can also find that in the process of increasing the feed volume from 1.92 t/h to 7.61 t/h, the dust concentration of the down-bend chute at the feed volume of 7.61 t/ h is far less than that of the linear chute at the feed volume of 1.92 t/h. Therefore, we considered that under the same operating conditions at the transhipment point, the use of a down-bend chute can effectively
Conflicts of interest All authors in this paper declare that there are no conflicts of interest regarding the publication of this article.
Fig. 14. Induced airflow velocity and dust concentration when the feed volume is 1.92 t/h. 8
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Fig. 15. Induced airflow velocity and dust concentration when the feed volume is 7.61 t/h.
Fig. 16. Dust pollution situation of transhipment point when the feed volume is 1.92 t/h.
Fig. 17. Dust pollution situation of transhipment point when the feed volume is 7.61 t/h.
Acknowledgements
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This research was financially supported by the National Science Foundation of China (No. 51874297 No. 51704145) and State Key Laboratory of Coal Resources and Safe Mining, CUMT (SKLCRSM16X03). References [1] D. He, Y. Pang, G. Lodewijks, X. Liu, Healthy speed control of belt conveyors on conveying bulk materials, Powder Technol. 327 (2018) 408–419. [2] M.S. Masaki, L. Zhang, X. Xia, A design approach for multiple drive belt conveyors minimizing life cycle costs, J. Clean. Prod. 201 (2018) 526–541. [3] A. Grincova, M. Andrejiova, D. Marasova, S. Khouri, Measurement and determination of the absorbed impact energy for conveyor belts of various structures under impact loading, Measurement 131 (2019) 362–371. [4] L. Xie, W. Zhong, H. Zhang, A. Yu, Y. Qian, Y. Situ, Wear process during granular flow transportation in conveyor transfer, Powder Technol. 288 (2016) 65–75. [5] X. Li, Q. Li, D. Zhang, B. Jia, H. Luo, Y. Hu, Model for induced airflow velocity of falling materials in semi-closed transfer station based on similitude theory, Adv. Powder Technol. 26 (1) (2015) 236–243.
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X. Zhang, et al. [15] Terrett, J. P. U.S. Patent No. 2,890,081. Washington, DC: U.S. Patent and Trademark Office. (1959). [16] H. Yan, Y. Kang, G. Fu, The application analysis of the buffer air lock in the bulk material handling system, Mech. Eng. 9 (2010) 153–154. [17] R. Ansart, J.J. Letourneau, A. De Ryck, J.A. Dodds, Dust emission by powder handling: influence of the hopper outlet on the dust plume, Powder Technol. 212 (3) (2011) 418–424. [18] W.C.L. Hemeon, Plant And Process Ventilation (No. TH7684. F2 H4 1963), Industrial Press, New York, 1963. [19] G.E. Tooker, Establishing design criteria for fugitive dust collection, Bulk. Solids Handl. 5 (4) (1985) 865–869. [20] G.E. Tooker, Controlling fugitive dust emissions in material handling operations, Bulk. Solids Handl. 12 (2) (1992) 227–232. [21] P. Cooper, P. Arnold, Air entrainment and dust generation from a falling stream of bulk material, KONA Powder Part. J. 13 (1995) 125–134. [22] A. Ullmann, A. Dayan, Exhaust volume model for dust emission control of belt conveyor transfer points, Powder Technol. 96 (2) (1998) 139–147. [23] K. Ogata, K. Funatsu, Y. Tomita, Experimental investigation of a free falling powder jet and the air entrainment, Powder Technol. 115 (1) (2001) 90–95. [24] T. Uchiyama, M. Naruse, Numerical simulation of gas-particle two-phase mixing layer by vortex method, Powder Technol. 125 (2–3) (2002) 111–121. [25] T. Uchiyama, T. Degawa, Numerical simulation for gas-liquid two-phase free turbulent flow based on vortex in cell method, JSME Int. J. Ser. B Fluids Therm. Eng. 49 (4) (2006) 1008–1015. [26] T. Uchiyama, Numerical analysis of particulate jet generated by free falling particles, Powder Technol. 145 (2) (2004) 123–130. [27] T. Uchiyama, M. Naruse, Three-dimensional vortex simulation for particulate jet generated by free falling particles, Chem. Eng. Sci. 61 (6) (2006) 1913–1921. [28] T. Uchiyama, A. Fukase, Vortex simulation of gas-particle two-phase compound round jet, Powder Technol. 165 (2) (2006) 83–91. [29] R. Ansart, A. De Ryck, J.A. Dodds, M. Roudet, D. Fabre, F. Charru, Dust emission by powder handling: comparison between numerical analysis and experimental results, Powder Technol. 190 (1–2) (2009) 274–281. [30] R. Ansart, A. De Ryck, J.A. Dodds, Dust emission in powder handling: free falling particle plume characterisation, Chem. Eng. J. 152 (2–3) (2009) 415–420. [31] X. Li, Q. Li, D. Zhang, B. Jia, H. Luo, Y. Hu, Model for induced airflow velocity of falling materials in semi-closed transfer station based on similitude theory, Adv. Powder Technol. 26 (1) (2015) 236–243. [32] X. Li, Q. Wang, X. Liu, Q. Li, H. luo, Y. Hu, H. Yang, Influence factors for induced airflow of bulk materials in chute transfer station, Mater. Sci. Eng. Powder Metallargy 20 (5) (2015) 683–689. [33] J. Niu, Y. Wang, D. Zhou, Effect of the outer windshield schemes on aerodynamic characteristics around the car-connecting parts and train aerodynamic
performance, Mech. Syst. Signal Process. 130 (2019) 1–16. [34] X. Yang, F. Sotiropoulos, On the dispersion of contaminants released far upwind of a cubical building for different turbulent inflows, Build. Environ. 154 (2019) 324–335. [35] B. Kim, K.T. Tse, A. Yoshida, Z. Chen, P. Van Phuc, H.S. Park, Investigation of flow visualization around linked tall buildings with circular sections, Build. Environ. 153 (2019) 60–76. [36] S. Wang, M. Vanella, E. Balaras, A hydrodynamic stress model for simulating turbulence/particle interactions with immersed boundary methods, J. Comput. Phys. 382 (2019) 240–263. [37] S. Wang, S. Ge, X. Chen, Experimental study on influencing factors of induced airflow in transshipment point of coal preparation plant, J. Saf. Sci. Technol. 13 (9) (2017) 53–57 23. [38] Y. Huang, Y. Wang, L. Liu, P.V. Nielsen, R.L. Jensen, F. Yan, Reduced-scale experimental investigation on ventilation performance of a local exhaust hood in an industrial plant, Build. Environ. 85 (2015) 94–103. [39] A.B. Cecala, A.D. O’brien, J. Schall, J.F. Colinet, W.R. Fox, R.J. Franta, M.J. Schultz, Dust Control Handbook for Industrial Minerals Mining and Processing, (2012). [40] Z. Wang, Y. Cheng, K. Zhang, C. Hao, L. Wang, W. Li, B. Hu, Characteristics of microscopic pore structure and fractal dimension of bituminous coal by cyclic gas adsorption/desorption: an experimental study, Fuel 232 (2018) 495–505. [41] Z. Wang, Y. Cheng, Y. Qi, R. Wang, L. Wang, J. Jiang, Experimental study of pore structure and fractal characteristics of pulverized intact coal and tectonic coal by low temperature nitrogen adsorption, Powder Technol. 350 (2019) 15–25. [42] Ministry Of Health, PRC, Hygienic Standard for Inhalable Partculate Matter in Indoor Air, (1998) GBT17095-1997. [43] R.L. Brown, J.C. Richards, Principles of Powder Mechanics: Essays on the Packing and Flow of Powders and Bulk Solids, Elsevier, 2016. [44] K. OGATA, K. FUNATSU, Y. TOMITA, Entrained air flow characteristics due to the powder jet, J. Soc. Powder Technol., Japan 37 (3) (2000) 160–167. [45] C.Y. Wu, L.Y. Li, C. Thornton, Energy dissipation during normal impact of elastic and elastic–plastic spheres, Int. J. Impact Eng. 32 (1–4) (2005) 593–604. [46] I. Etsion, Y. Kligerman, Y. Kadin, Unloading of an elastic–plastic loaded spherical contact, Int. J. Solids Struct. 42 (13) (2005) 3716–3729. [47] P.W. Cleary, J.E. Hilton, M.D. Sinnott, Modelling of industrial particle and multiphase flows, Powder Technol. 314 (2017) 232–252. [48] X. Chen, C. Wheeler, Computational Fluid Dynamics (CFD) modelling of transfer chutes: a study of the influence of model parameters, Chem. Eng. Sci. 95 (2013) 194–202. [49] X.L. Chen, C.A. Wheeler, T.J. Donohue, R. McLean, A.W. Roberts, Evaluation of dust emissions from conveyor transfer chutes using experimental and CFD simulation, Int. J. Miner. Process. 110 (2012) 101–108.
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Experimental study on dust suppression at transhipment point based on the theory of induced airflow dust production
T
Xinghua Zhanga,b,c, Haifeng Wanga,b,c,*, Xi Chend, Chaonan Fand, Kun Tiana,b,c, Xiao Zhanga,b,c a
State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China National Professional Laboratory for Fundamental Research of Mine Gas and Dust Control Technology, School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China c School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China d College of Safety and Emergency Management Engineering, China Taiyuan University of Technology, Taiyuan, Shanxi, 030000, China b
ARTICLE INFO
ABSTRACT
Keywords: Induced airflow Dust concentration Dust suppression technology Chute Energy-saving equipment
Induced airflow is the main cause of dust problems at the transhipment point. However, the current measures for dust control are mainly from the perspective of dust control after production, focusing on blocking, lowering, discharging, removing and flushing. It is rare to manage dust from the source of occurrence, so dust pollution cannot be solved at the source. Based on the kinematics principles and the energy dissipation theory, this paper systematically summarizes the induced airflow dust production theory, improves the transfer chute, and proposes the compound straight-arc chute dust suppression technology. According to the similarity criteria, the experiment was carried out in the laboratory with the transportation of coal as an example. The experimental results show that with the decrease of particle size, the induced airflow velocity and dust concentration at the chute outlet show a linear growth trend and remains unchanged even when the feed volume changes. Compared with the upper-bend shape, when using a down-bend shape for the dust suppression chute in the process of transferring coal, the induced airflow velocity and dust concentration at the chute outlet can be reduced by 31% and 60%. Compared with a linear chute, the dust concentration is reduced by up to 77%. It indicates that the down-bend chute is a more energy-saving dust removal equipment that can be used to reduce operational costs while conveying coal.
1. Introduction Because of its simple, compact structure and high efficiency, belt conveyor is widely used in the transportation of bulk material. However, due to the limitation of single-period conveying distance, multi-periods are often needed to complete long-distance transportation [1,2]. Generally, bulk material is mostly transported vertically between belts, which not only has a large impact on the equipment, but also carries a large amount of induced airflow thus causes serious dust pollution [3–6]. In China, the annual incidence of pneumoconiosis accounts for 89.67% of the total number of occupational diseases, and the mortality rate is as high as 22.4%. The number of deaths is three times higher than that of other accidents, which caused great loss to the national economy [7]. However, currently, all kinds of dust removal technologies and equipment is active dust removal. Although the purification efficiency is high, and the dust removal speed is fast, they generally have complex structures, high operational costs, large
*
maintenance workloads, secondary pollution etc. [8–14,39]. In contrast, passive dust removal has the advantages of long life and low energy consumption [15,16], however, to solve the pollution problem, researchers need to understand the characteristics of dust sources thoroughly. Due to various factors, the mechanism of airflow induced by transhipment point are complex and fluctuates greatly. During the free fall, particles carry ambient air into the airflow, forming a particle plume, which is in a typical disordered state [17]. Therefore, in order to effectively solve the problem of dust pollution at the transhipment point, it is necessary to have a thorough understanding of the induced airflow. Aiming at the calculation of entrained air volume during bulk material transportation, Hemeon [18] established a physical model for the free fall of particles in an air-free flow environment under gravity. The study reported that in the process of falling, the force of particle flow on air is equal to the sum of the force of a single particle on air. Therefore, a single particle model of entrainment air volume is
Corresponding author. School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China. E-mail address: [email protected] (H. Wang).
https://doi.org/10.1016/j.buildenv.2019.106200 Received 15 April 2019; Received in revised form 6 June 2019; Accepted 8 June 2019 Available online 08 June 2019 0360-1323/ © 2019 Published by Elsevier Ltd.
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proposed, and the mathematical models of induced air volume and particle flow, thus cross section area, particle size, density, and falling information are deduced. Based on the mathematical model established by Hemeon, from the perspective of turbulent flow, Tooker [19,20] proposed a mathematical model of the total amount of dusty gas discharged by a closed ventilation system. On the basis of previous studies, Cooper [21] et al. studied the influencing factors of air volume in the falling process of particle flow, and considered that with the increase of the falling height of particle flow, the cross-sectional area of the particle plume fluid core gradually decreases, while the boundary layer crosssectional area of dusty gas increases. Therefore, the variation of entrainment air in the falling process of particle flow is obtained. Amos Ullmann [22] et al. conducted an experimental study and comparative analysis of previous studies, and proposed a new model for calculating induced airflow. Based on the conservation of energy, the mathematical model was improved by supplementing the equations of motion and fluid resistance in the material falling process. The calculation of material in turbulent zones was simplified, and the expression of airflow velocity in chute was obtained. In the process of falling Ogata [23] et al. studied the jet, dispersion and entrapment airflow of bulk materials, and obtained the law of porosity change and entrainment air volume, which was combined with the LVD test, PIV test and numerical simulation, thus the relationship of the final velocity of materials and the final velocity of airflow was obtained. To analyse the entrainment air volume during the free landing process Tomomi Uchiyama [24–28] and others used the vortex simulation method, and reported the relation between bulk materials’ diameter, density and induced airflow. Based on the two-phase flow model, Renaud Ansart [17,29,30] et al. described an experimental set up. The effect of the drop height of a free falling jet on segregation of particle size, particle velocity, changes the particle concentration and entrained air in the dust plume were investigated, and a model of predicting the particle and air velocities, and especially the volumetric flow of induced air in the column without dependence on any empirical constant was proposed. Xiaochuan Li [31,32] studied the effects of the conveyor chute on air entrainment and the quantitative factors of influence. The above researchers deduced the induced airflow from a view of mathematics, but they did not give out its governing method. In this paper, we decomposed the working process of the transhipment point into three parts: reserve zone, transport zone, impact zone, so that we can targeted solve the pollution. Generally, bulk material is compacted in the reserve zone without dust pollution. However, in the transport zone and the impact zone, the interaction between the particle and environment causes a large number of dust particles to be peeled off, resulting in floating dust, which is directly related to the formation of induced airflow, but the formation mechanisms in these two zones are substantially different [17,29,30]. In the transport zone, the movement of bulk material causes air to undergo plastic deformation around itself. During this process, with the increase of falling speed, due to viscous resistance, a negative pressure layer will be formed around the bulk material, which will absorb the surrounding air and produce a flow field with a velocity gradient [18,33,34]. As shown in Fig. 1, under the action of the airflow field, particle A on the left side directly contacts with the air, where the airflow is basically in full development and a turbulent wake appears that reduces the air pressure [34]. However, due to the influence of particle B, the air on the right side cannot fully expend in a short time, thus the resistance on the right side of A is smaller than the left side. Due to uneven force on the two sides, particle A will rotate clockwise, and its size depends on the difference in the air resistance between both sides, which will increase with the change of falling height and velocity. When it reaches a certain level, particle A rotates and increases the distance between the particles, which will increase the voidage of the coal stream [35–37]. In other words, more gas will be “wrapped” or “entrained” by the transporting coal stream thus entering into the lower equipment, as shown in Fig. 1a and b. At the same time, this differential
Fig. 1. Schematic diagram of coal flow and induced airflow in the chute.
flow can be considered as an airflow disturbance that will force the coal dust to passively enter the surrounding working space [17,21,29,30]. In the impact zone, the moving airflow is hindered by the subordinate equipment thus changes its movement direction, and the air dispersed in the void is rapidly “squeezed” and “discharged” into the environment. The dust adsorbed on the coal will then escape from the inside to the outside, as shown in Fig. 1c. These processes not only produce dust pollution but also disturb the stable airflow inside the equipment, resulting in the secondary dust pollution. With the change of transport operating parameters, such as feed volume, discrepancy in altitude etc., this phenomenon will lead to more serious dust pollution problem [38]. In view of the above problems, most of the current on-site production adopts the active method of applying dust removal equipment. This method often requires manpower, material and financial resources [39,40]. Although active method can alleviate the dust pollution in the workshop, it cannot fundamentally solve the dust pollution problem. Taking a belt conveyor transhipment point as the research object, through the change of the chute structure to limits the coal motion, this paper proposes a passive dust removal method named compound straight-curve chute dust suppression technology. The effectiveness and feasibility of the technology are verified by means of theoretical derivation and experimental comparative analysis. 2. Physical model of a chute According to the induced airflow dust production theory, the fundamental cause of dust pollution at transhipment point is the highspeed flow of coal [41]. However, in the design of traditional transfer chutes, various factors are not considered carefully, making it difficult to control the flow of coal in operation. This phenomenon not only hinders normal production but also increases unnecessary costs [38]. Therefore, it is necessary to analyse and improve the chute design and control the coal movement. 2.1. Motion analysis The motion of bulk material in chute can be accelerated or decelerated. When accelerated, the section area decreases. When decelerated, the section area increases, and the material is likely to be deposited even being blocked in the chute. To study the movement of bulk material in chute more conveniently, the movement process is simplified, and the following assumptions are made. (1) The bulk material is regarded as being composed of numerous micro elements; (2) The flow process is continuous; (3) Only the influence of friction on the flow between bulk material and the chute wall is considered, and the influence of its internal factors is ignored; (4) The influence of air resistance and temperature is ignored. 2
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Fig. 3. Sketch of the material movement at the exit of the chute.
v = cos v
Fig. 2. Diagram of coal force analysis in the process of transport.
+ ma + µE ( mg sin
(1)
+ man) = 0
To simplify:
a + µE an + g (µE sin
(2)
+ cos ) = 0
min
where Δm is the mass of coal micro elements, kg; aτ is the tangential acceleration of coal, m/s2; an is the normal acceleration of coal, m/s2; g is the gravitational acceleration, m2/s;μE is the equivalent friction coefficient of coal and chute;θ is the angle between gravity and the coordinate axis.
where v is the velocity of coal in the chute, m2/s; s is the initial velocity of coal entering the chute. In the arc part of the chute, the curvature radius is a fixed value, the velocity of the bulk material can be expressed as v = rθ, thus the flow velocity is as follows:
v=
H L
+ 5°
(6)
Generally, the pressure distribution of bulk material in a rectangular chute is shown in Fig. 4. It is assumed that the pressure exerted on the chute floor is uniformly distributed, and in the vertical direction, with the increase in bulk material height, the pressure increases linearly. According to the principle of mechanics, the expression of μE can be obtained. The total friction force on the chute wall is:
(3)
µE sin )
2gr [sin (1 + 2µE2 ) + 3µE cos ] + e 4µE2 + 1
= arctan µE 1 + K
2.4. Equivalent friction coefficient
According to the equilibrium equation of mechanics, formula (2) is further simplified to an equation of velocity. In the straight part of the chute, its curvature radius can be regarded as infinite, so the flow velocity of bulk material is:
v02 + 2gs (cos
(5)
where μE is the actual friction coefficient when bulk solid contacts with the chute surface; H is the height of the bulk material in the chute, m; L is the width of the chute, m; and K is the ratio of the vertical pressure and transverse pressure of the bulk material to the chute floor.
2.2. Kinematic equations
v=
tan
where θ is the angle between the and horizontal and the moving direction when the bulk material enters the chute, rad, and ϕ is the dynamic friction angle between the bulk material and the chute, rad. When θ+ϕ = 90°, v’ = 0. The smoother the chute surface is, the smaller the value of ϕ. To make the coal flow smoothly, it is necessary to make v’ > 0. Under the premise of coal material flow, the chute inclination angle is as small as possible. The minimum chute inclination angle is determined by the friction characteristics between the bulk material and chute. This angle can be derived from the following formula:
As shown in Fig. 2, the micro-element dm of bulk material in chute is selected as the research object. Take its moving position as the original coordinates, tangent movement direction as the transverse axis, and the normal direction as the longitudinal axis. Assuming that the force of coal is balanced during the course of movement, According to mechanical principles, the following formula is obtained
mg cos
sin
2µE
v0
(7)
F = µPn + KPn Hµ Making. F = µE Pn B The equivalent friction coefficient is:
µE = µ 1 + K
6µE rg
H B
(8)
It can be seen from formula (8) that the equivalent friction coefficient is proportional to the ratio of the height to the width of the bulk material in the chute, and the larger the ratio, the greater the equivalent
4µE2 + 1 (4)
2.3. Chute inclination The inclination of the chute directly determines whether the bulk material can flow smooth, which is a key factor in chute design. Therefore, the chute should have a reasonable inclination and proper roughness to ensure the smooth passage of the transported bulk material. As shown in Fig. 3, the ratio of the velocity v0 of coal on the belt to the velocity v when it leaves the chute coincides with the following function:
Fig. 4. Force analysis of the chute wall. 3
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friction coefficient, and the more easily to be blocked. 2.5. Chute structure Induced airflow is the main cause of dust pollution during bulk transportation, and the induced airflow velocity is directly related to the bulk material's flow velocity. Therefore, according to the energy dissipation theory, the author continuously reduces the angle of bulk material relative to the horizontal plane by means of a circular arc design to slow down the flow. Considering the large space occupied by a circular arc chute, part of the linear chute idea is introduced in the design, thus a composite straight-arc chute dust suppression technology is proposed. The upper part is the arc section and the lower part is the linear section, or, the upper part is the linear section and the lower part is the arc section. To reduce the impact of bulk material on the chute, to alleviate the wear of equipment, and to avoid the fragmentation, the chute is designed with a down-straight part and an upper-bend part (hereafter referred as upper-bend), which can not only make the coal enter the chute smoothly but also realize the purposes of slowing down the coal and decrease the dust suppression. To ensure that there is no blocking, the chute outlet inclination is set to be greater than the sum of the coal sliding friction angle and a certain angle. As shown in Fig. 5, when the dumping height and chute outlet angle were determined, we chose any number from 90°-α as the value of θ, then the radius R of the arc, central angle θ′ and straight line length S were derived. The geometric dimensions of each part of the chute are determined by the following formulas:
S=
y tan y
S=
2
R tan
A=
+ cos )
2
(sin
(10)
R=
(11)
(sin
+ cos )
(14)
Q 3600 v
(15)
3.1. Apparatus According to the on-site production situation, a transhipment point induced airflow experimental platform is designed and constructed, as shown in Fig. 7. Bulk material outflows from bunker, then passes through the electro-vibration feeder (GZV), chute, belt conveyor, and finally collected by the recycle bin. Here the bunker and GZV are considered as a whole. The bunker's upper opening is 900 mm × 900 mm, lower opening is 250 mm × 250 mm, and the height is 600 mm. The GZV chute size is 700 mm × 200 mm × 130 mm,
(12)
2
+ cos )
3. Experimental details
y S cos tan
(sin
sin
where A is the cross-section area of the chute, m ; Q is the feed volume, t/h; ξ is the filling factor; η is the bulk weight ratio, t/h3. V is the average flow velocity of the bulk material on the chute bottom plate, m/s.
To reduce the speed of leaving the chute and the induced airflow into the receiving equipment, and to alleviate the problem of collision dust, the chute is designed with an upper-straight part and down-bend part (hereafter referred to as down-bend). The geometric structural diagram of the chute described above is shown in Fig. 6. The geometric dimensions for each part of the chute are determined by the following formulas:
= 90°
2
2
+ cos )
sin
R tan
The cross sectional size of a chute is directly related to the bulk material blockage in transport. Generally, the chute outlet width is 2/3 of the horizontal projection width of the lower conveyor belt, and the height of bulk material in the chute should be 20–30% of the chute height. According to the conveyor capacity, the chute cross-section can be calculated by the following formula:
S sin (sin
y
2.6. Cross-sectional area
(9)
= 90° R=
Fig. 6. Down-bend feature of compound straight-curve type of dust suppression chute.
(13)
Fig. 5. Upper-bend feature of compound straight-curve type of dust suppression chute.
Fig. 7. Transhipment point induced airflow experimental platform. 4
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Fig. 8. Experimental coal sample.
device and ventilation system is turned off.
Table 1 Experimental coal sample sliding angle. Particle size
6–8 mm
4–6 mm
2–4 mm
0.5–2 mm
Sliding angle
21.3°
22.9°
24.2°
26°
3.3. Experimental procedure According to the relevant theory of powder mechanics [45], we can put the coal on the plate used for making the chute and incline it slowly. When the coal slips, the angle between the plate and horizontal direction is the required sliding friction angle. To eliminate the influence of systematic errors, 20 groups of coal samples of each particle size were taken, and the mean value was taken as the sliding friction. The results were shown in Table 1. To ensure that the coal sample can pass through the chute smoothly without blocking, the maximum value of 26° in the test results is used to represent the sliding friction angle of all coal samples. By bringing the coal sample sliding friction angle into formulas (1) ~ (14), the minimum values of formulas (3) and (4) are solved by computer. Then, we bring the curve radius R and the line length S of the chute into the upper bend calculation formulas (8) ~ (10) and lower calculation formulas (11) ~ (13), respectively. Finally, the structural size of the dust-suppressing chute that can effectively control the coal flow velocity at a limited distance under laboratory conditions is obtained, as shown in Table 2. In addition, to explore the effect of dust suppression by the dust suppression chute structure and installation requirements, we designed a linear chute as a control group. To determine the best data measurement point, COMSOL multiphysics software is used to simulate the transport process using the k-ε [17,29,30,46] model before the experiment. The internal flow field distributions of the dust suppression and linear chutes are compared and analyzed under laboratory conditions. The following assumptions are made:
and its outlet is 1400 mm above the ground. Through the adjustment of the input current of the GZV, the fluctuating electromagnetic attraction between the core and armature of the feeder can be changed, so that the GZV can produce reciprocating vibrations and adjust feeding volume. To eliminate the influence of turbulence, based on the calculation, inference and experimental conditions, the belt conveyor size was determined as 2400 mm × 400 mm × 400 mm. 3.2. Experimental sample Coal is a common porous ore with well-developed pore structure and easy to be broken. Therefore, this paper chose coal as a bulk material for experimental study [42,43]. To ensure the accuracy and scientific integrity of the experimental data, as shown in Fig. 8, to simulate different feeding conditions on-site, according to similar criteria, particle sizes of 0.5–2 mm, 2–4 mm, 4–6 mm, and 6–8 mm are selected, and to eliminate the interference of coal sample moisture content with the test data, the humidity is kept constant by air-drying and humidity test. During the experiment, a thermal anemometer and a ccd-500 explosionproof dust concentration tester were used to monitor and record the induced airflow velocity and PM10 dust concentration in real time, respectively. The precision of the instruments was 0.001 m/s and 0.01 mg/m3, respectively. During the testing process, to eliminate the influence of environmental factors on the data collected by the instruments, the experimental platform is placed in an underground windowless laboratory with few doors, and the exhaust system is closed, which can minimize the degree of indoor air flow. Because of serious dust pollution in the laboratory, a spray dust suppression system is installed around the experimental platform, and all electrical equipment inside the room is sealed, dust-proof and waterproof thus to ensure the safety of the laboratory staff. After the data collection for each group of experiments is completed, the spray device and ventilation system are turned on, and the indoor dust concentration is tested with the dust meter until it is reduced to 0.15 mg/m3 [44]. Then, the spray
Table 2 Structural parameters of the chute. Chute form
Upper-bend Down-bend Linear
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Outlet angle of chute (°)
Chute width (m)
Arc part
Straight part
Central angle (°)
radius (m)
length (m)
26 26 26
0.15 0.15 0.15
61 58 –
1.5 2.1 –
0.58 1.34 1.02
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Fig. 9. Airflow field in the chute during coal transfer.
(1) Coal is evenly spread over the bottom of the chute; (2) The roughness of the bottom of the chute is used to represent the coal surface; (3) The airflow field is vertically and evenly distributed on the coal surface; (4) Air is incompressible; (5) The influence of air gravity is negelected; (6) The influence of temperature on the airflow field is neglected. Fig. 9 shows the simulation results. It was concluded that the downbend can effectively reduce the coal and induced airflow velocity. According to the theory of induced airflow dust production, this method can effectively solve the dust pollution problem at the transhipment point and prolong the service life of the equipment [6]. Fig. 10 shows the induced airflow velocity along the wall height direction at the outlet. The induced airflow is stable at level positions 4 cm away from the surface meeting the experimental requirements. Therefore, during the experiment, the wind meter and dust concentration tester probes are installed for data monitoring and recording, respectively. In the experiment, we adjust GZV's input current to the required feeding amount, close the power supply, install the chute, add coal to the bunker, then monitor and record the required time and instrumentation data for complete transport of all coal. The specific testing process is as follows: Firstly, under different feeding volume
Fig. 10. Induced flow velocity curves along the chute outlet wall height.
between the upper-band and lower-bend, we compare the change of induced airflow velocity and dust concentration in the process of transport coal with different particle sizes, then, to explore the dust suppression effect, we choose the lowest one velocity to compare with the traditional linear chute. Although dust control measures were designed during the experiment, indoor dust concentration could not be completely reduced to 6
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Fig. 11. Induced airflow velocity.
0 mg/m3. Therefore, the indoor air velocity is measured before each set of experimental tests carried out. When the test data does not fluctuate substantially, it was considered that there is no air flow in the room, and the measured data in the experiment is the induced airflow velocity. Then, we monitor and record the indoor dust concentration with dust meter. To eliminate the influence of errors on the conclusions, after the experiment, this data should be deducted and analyzed.
entrained in the pores, which will greatly increase the induced airflow in the chute during transportation. According to the relationship between the induced air flow and the dust concentration mentioned above, this is not conducive to dust reduction. With the increase of the feed volume, the induced airflow velocity increases, which remains unchanged with the change of coal particle size. In the process of increasing the feed rate from 1.39 to 13.27 t/h, the maximum increase of induced airflow is 41.87%. Because under the same particle size, increment of the feed volume is equival to the increasing of the total amount of coal entering the chute in a unit time, which makes the coal transfer more energy to the air in a unit time and induces a greater velocity air flow according to the energy conservation and transformation theorem. When using the upper-bend, coal can smoothly enter the chute without severe collision occurring, however, when the coal leaves the arc part and enters the straight part, its motion is similar to an object sliding on an inclined plane, as shown in Fig. 13. According to the principles of mechanics, the coal has a downward acceleration along the slope, the adjacent air is “entrained” and speeds up. This is the reason why the induced airflow velocity and dust concentration increased. In addition, the high-speed moving coal will produce more dust during the process of impacting on the belt.
4. Results and discussion By removing the extreme values and averaging the data, the experimental results are shown in Figs. 11 and 12. We can find that the induced airflow velocity at the outlet increases with the decrease in coal particle size, and this trend does not change with the feed quantity. The reasons for this phenomenon can be considered from the following two aspects: (i). As the particle size of coal samples decreases, the number of particles in a unit space increases, which indicates that the increase of the drag effect on the air, suggesting that more air is pulled into the chute. (ii). The decrease in coal particle size reduces the stacking density and increases the porosity. Under the same conditions, more air will be
Fig. 12. Dust concentration. 7
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improve the transhipment point dust pollution problem for a business. This finding means that in the actual management, the work can be achieved without other dust removal equipment or less energy consumption, reducing the difficulty for the coal industry environmental protection and energy conservation. 5. Conclusion Taking the transhipment point of coal system belt conveyor as the research object, and relying on the theory of induced airflow dust production mechanism and energy dissipation, this paper improves the transporting chute, summarizes the compound straight-arc chute dust suppression technology, and carries out simulation experiments in the laboratory. The main work and conclusions are as follows:
Fig. 13. Force analysis of sliding objects on an inclined surface.
(1) In the process of coal flowing in the chute, under the action of air viscous resistance, the coal surface will not only form a negative pressure layer to absorb part of the air but also increase the coal porosity and entrap more air with the coal in the lower equipment, resulting in dust pollution. (2) When coal is transferred by down-bend, it will collide with the chute when it enters the straight part, but it will not generate excessive dust due to the low flow rate. When the coal flows through the straight part into the arc part, the arc chute makes it to slow down. The low-speed coal in front of the position has the function of cushioning and decelerating the back position, at the same time this low-speed flowing coal has the influence of dragging and decelerating the induced airflow. Because of the synthetically effect of deceleration method, the coal has a lower velocity when leaving the chute and controls the dust concentration at a lower level. Therefore, the dust suppression effect of the down-bend is obviously better than that of the upper-bend. (3) Compared with the linear chute, when using the down-bend chute, induced airflow velocity and dust concentration were reduced by 26% and 77%, respectively. Therefore, it can be considered that the composite straight-curve dust suppression chute technology proposed in this paper has a high potential feasibility.
When using the down-bend to transport, the coal will enter the chute's arc part from the straight part. In this process, the coal has less kinetic and potential energy, and the crushing effect is alleviated when it enters the chute compared with the upper-bend. After entering the arc part, the coal's transfer rate is gradually reduced due to circular motion. According to the energy dissipation theory, the coal particles are a complex energy dissipation system. The energy caused by impaction can be attenuated through the friction and viscous effects, and the force chain structure between particles can also extend the space and time of instantaneous local impact to achieve a cushioning effect [47,48], so that the energy of the coal material is consumed in a large amount, finally, the coal velocity is reduced sharply. In the meantime, through the drag effect, the coal can also reduce the induced airflow to alleviate dust pollution [49]. In summary, when transporting coal, the down-bend can suppress dust production more effectively than the upper-bend. According to Figs. 14 and 15, we can find that the induced airflow velocity and dust concentration of down-bend are significantly lower than that of the linear chute. When the feed volume is 1.92 t/h, the induced airflow velocity and dust concentration are reduced by approximately 25% and 77%, respectively. When the feed volume is 7.61 t/h, the data is 26% and 60% respectively. The dust concentration in the experiment is shown in Figs. 16 and 17. We can also find that in the process of increasing the feed volume from 1.92 t/h to 7.61 t/h, the dust concentration of the down-bend chute at the feed volume of 7.61 t/ h is far less than that of the linear chute at the feed volume of 1.92 t/h. Therefore, we considered that under the same operating conditions at the transhipment point, the use of a down-bend chute can effectively
Conflicts of interest All authors in this paper declare that there are no conflicts of interest regarding the publication of this article.
Fig. 14. Induced airflow velocity and dust concentration when the feed volume is 1.92 t/h. 8
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Fig. 15. Induced airflow velocity and dust concentration when the feed volume is 7.61 t/h.
Fig. 16. Dust pollution situation of transhipment point when the feed volume is 1.92 t/h.
Fig. 17. Dust pollution situation of transhipment point when the feed volume is 7.61 t/h.
Acknowledgements
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