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Dust removal efficiency of high pressure atomization in underground coal mine
International Journal of Mining Science and Technology xxx (2018) xxx–xxx
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International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Dust removal efficiency of high pressure atomization in underground coal mine Wang Pengfei a,b,⇑, Tan Xuanhao b, Cheng Weimin b, Zhou Gang b, Liu Ronghua a a b
School of Mining and Safety Engineering, Shandong University of Science & Technology, Qingdao 266590, China School of Resource, Environment & Safety Engineering, Hunan University of Science & Technology, Xiangtan 411201, China
a r t i c l e
i n f o
Article history: Received 12 September 2017 Received in revised form 18 October 2017 Accepted 23 January 2018 Available online xxxx Keywords: Underground coal mine High pressure atomization Atomization characteristics Dust removal efficiency
a b s t r a c t To master theoretical calculation for dust removal efficiency of high pressure atomization in an underground coal mine, the corresponding atomization characteristics and dust removal efficiency were both comprehensively studied in theory by virtue of related theories of hydromechanics and aerosol. According to actual measurements of flow coefficients and atomization angles of X-type swirl nozzle, computational formula was derived for atomized particle sizes of such a nozzle in conjunction with relevant empirical equation. Moreover, a mathematical model for applying high pressure atomization to dust removal in underground coal mine was also established to deduce theoretical computation formula of fractional efficiency. Then, Matlab was adopted to portray the relation curve between fractional efficiency and influence factors. In addition, a theoretical formula was also set up for removal efficiency of respirable dust and total coal dust based on dust size and frequency distribution equations. In the end, impacts of dust characteristic parameters on various dust removal efficiencies were analyzed. Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Various links of production in underground mine can generate dusts in different extents. Dust suspended in air not only jeopardizes health of workers, but gives rise to bad influences on underground safety production. At present, measures such as dust removal by ventilation, water pre-infusion for coal mass and atomization have been taken both at home and abroad to reduce dust concentrations in the air of underground coal mines. In view of the advantages like cost-effectiveness, convenient and strong practicality, atomization has been extensively applied to remove dust in underground coal mines. However, its field practice in underground coal mine is lacking in theoretical guidance. As a consequence, the dust removal efficiency of atomization is not ideal. In particular, the efficiency of respirable dust removal is even below 40% [1–4]. In recent years, scholars begin to investigate atomization characteristics and dust removal efficiency of high pressure atomization. Cheng et al. used an experiment platform specially designed for high pressure atomization to carry out an experiment for analyzing the atomization characteristic of several commonly in the use of nozzles in coal face, and investigated dust removal ⇑ Corresponding author at: School of Mining and Safety Engineering, Shandong University of Science & Technology, Qingdao 266590, China. E-mail address: [email protected] (P. Wang).
efficiency on site to determine the relation of water supply pressure to atomization particle size and effect of dust removal [5–8]. Ma et al. took advantage of related theories such as fluid mechanics and aerosol to set up a mathematical calculation model for fractional dust removal efficiency of high pressure atomization in underground coal mines. In addition, they also analyzed the relation of dust removal efficiency with various operating parameters [9–11]. In the use of Fluent, Chen al. investigated impacts of water supply pressure on droplet size, droplet velocity and mist flow field [12,13]. Over the recent years, our research group also carried out a large number of experimental studies on atomization characteristics and dust removal effects of high pressure atomization, and found the relation between dust removal efficiency and various influencing factors [14,15]. Raj Mohan et al. also performed experimental studies regarding characteristics and dust removal efficiency for gas-water atomization, and determined how various operating parameters influence dust removal efficiency [16–18]. Evaluation on dust removal efficiency of high pressure atomization in underground coal mine is usually based on the removal efficiency of total coal dust and respirable dust. The calculation model for dust removal efficiency of atomization established by Chinese scholars can be only used for computing fractional efficiency. In other words, it can only calculate the efficiency of atomization in removing the dust of single particle size. Additionally, the findings of experiments about atomization-based dust removal all focus on
https://doi.org/10.1016/j.ijmst.2018.01.006 2095-2686/Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Wang P et al. Dust removal efficiency of high pressure atomization in underground coal mine. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.01.006
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the qualitative relationship between different influencing factors and dust removal efficiency. Their conclusions cannot be preferably applied to guide on-site engineering design and calculation of atomization-based dust removal efficiency. Therefore, the author set up a theoretical calculation model for removal efficiency of total coal dust and respirable dust based on the calculation model of existing fractional efficiency and the distribution equations of dust size and frequency. On this basis, the relations of different operating parameters and dust characteristic parameters to dust removal efficiency were analyzed. Our findings will provide a theoretical basis for applying high pressure atomization in dust removal to prevent and control dusts on underground coal mining face. 2. Characteristics of high pressure atomization 2.1. Flow coefficient and atomization angle of nozzle Factors that affect nozzle flow mainly include nozzle configuration, nozzle diameter and water supply pressure. For ordinary single-jet pressure nozzles, their water supply flow can be calculated according to the Eq. (1) [19]:
Q¼
p 4
2
Cqd
sffiffiffiffiffiffi 2p
q
103
ð1Þ
where Q is the volume flow rate of water supply, m3/s; and Cq the flow coefficient related to nozzle configuration; d the nozzle diameter, mm; p the water supply pressure, MPa; and q the water density, kg/m3. At room temperature, flow characteristics and atomization angle of the X-type swirl pressure nozzle commonly used in underground coal mine were measured based on the self-designed experiment platform for high pressure atomization. The diameter of the selected nozzle was 1.5 mm and the corresponding mist flow was in a shape of solid cone. Electromagnetic flowmeter and high speed camera were used to measure water flow rate and atomization angle under 5 water supply pressures (2, 4, 6, 8 or 10 MPa). According to Eq. (1), flow data measured during the experiment were regressively analyzed to acquire the flow coefficient of the nozzle, as shown in Table 1. Besides, atomization photos taken by the high speed camera were imported into Image-Pro Plus 6.0 to calculate atomization angle a. The related calculation results are given in Table 1. The measurements in Eq. (1) and Table 1 show that the water supply flow of nozzle goes up with the rise of atomization pressure and is in direct proportion to square root of water supply pressure. In addition, the results also indicate that the flow coefficient of nozzle independent of water supply pressure is only associated with nozzle configuration. Regarding a nozzle with fixed structure, there must be a flow coefficient corresponding to it. In the present study, the nozzle’s flow coefficient was 0.61. Table 1 also indicates that the atomization angle of nozzle drops slightly with the increase of water supply pressure. For example, when the water supply pressure increases to 10 from 2 MPa, the atomization angle only changes by 2.38°. For the convenience of calculation, the
Table 1 Flow rate and spray angle of nozzle under different water supply pressures. p (MPa)
Q (L/min)
Α (°)
Cq
Average a (°)
2 4 6 8 10
4.01 5.67 7.08 8.08 9.12
43.59 42.56 42.01 41.54 41.21
0.61
42.18
mean value within the interval of water supply pressure (42.18°) is used as the atomization angle of nozzle. 2.2. Atomized particle size of nozzle With regard to a single-jet atomizing pressure nozzle, atomized particle size can be calculated according to the empirical formula summarized by scholars of the former Soviet Union.
lg
Dc50 0:35 lg We ¼ 4:47L0:133 p d
ð2Þ
where Dc50 is the mass median aerodynamic diameter (MMAD), i.e., the mass of fog drops with particle size below this value occupies 50% in the total mass of all fog drops, mm; d the feature size, mm; Lp the Laplace series; and We the Weber number, a parameter describing stability of droplets. d in Eq. (2) can be computed by Eq. (3).
d¼d 1
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi!. 1 C q cos
a 2
2 cos
a 2
ð3Þ
where d is the nozzle diameter, mm; and a the atomization angle, °. The diameter of the nozzle adopted in this paper is 1.5 mm. Moreover, the flow coefficient of nozzle measured is 0.61 (Cq = 0.61) and the atomization angle is 42.18° (a = 42.18°). Then, after all related data are substituted into the above equation, Eq. (4) can be obtained as below:
d ¼ 1:5ð1
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 0:61 cos 21:090 Þ=ð2 cos 21:090 Þ
¼ 0:276mm
ð4Þ
The Laplace series Lp can be calculated based on Eq. (5):
Lp ¼
qrd 103 l2L
ð5Þ
where r is the liquid surface tension coefficient, N/m; and l the fluid dynamic viscosity, N/(m2s). Relevant parameters of water in normal state, including r = 0.072 N/m, lL = 1.005 103 Pas and qL = 1000 kg/m3, are selected. Then:
Lp ¼
1000 0:072 0:276 1:0052 106
103 ¼ 19675
ð6Þ
The computational formula of We is
We ¼
qt2 d 103 r
ð7Þ
where t is the axial jet velocity of droplets, m/s, and it can be calculated according to nozzle flow coefficient Cq and water supply pressure p. That is
t ¼ Cq
sffiffiffiffiffiffi 2p
q
103
ð8Þ
Substituting Eqs. (8) into (7) together with related parameters, the value of Weber number is
We ¼ 2852:7p
ð9Þ
Substituting Eqs. (4), (6) and (9) into (2), a equation of atomized particle size. Then, we can obtain:
Dc50 ¼ 102:430:35 lg p lm
ð10Þ
Likewise, for an X-type swirl nozzle with a diameter of 1.2 mm, the computation equation of atomized particle size is
Dc50 ¼ 102:390:35 lg p lm
ð11Þ
Similarly, in terms of an X-type swirl nozzle with a diameter of 2.0 mm, the equation of atomized particle size is
Please cite this article in press as: Wang P et al. Dust removal efficiency of high pressure atomization in underground coal mine. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.01.006
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P. Wang et al. / International Journal of Mining Science and Technology xxx (2018) xxx–xxx
Dc50 ¼ 102:490:35 lg p lm
ð12Þ
According to Eqs. (10)–(12), Matlab is used to portray relation curves between atomized particle size Dc50 and water supply pressure p of nozzles in three different diameters, as shown in Fig. 1. It indicates that when the diameter of nozzle is invariant, atomized particle size keeps falling with the rise of water supply pressure. In the case that the water supply pressure changes between 0 and 8 MPa, the atomized particle size rapidly goes down with the increase of pressure. After the pressure reaches 8 MPa, changes in atomized particle size are minor although the water supply pressure continues to rise. Obtaining small fog drops by increasing water supply pressure has certain limitations. According to comparisons among three relation curves in Fig. 1, the nozzle with small diameter can be used to obtain fog drops with small particle size under the circumstance that water supply pressure is constant. As the dust removal efficiency of atomization is closely related to particle size of fog drops, those with small particle size can help to collect respirable dust. Thus, the nozzle with small diameter should be selected to guarantee higher removal efficiency of respirable dust in the case of fixed water supply pressure. Nevertheless, given that water used by atomization underground generally contains impurities which may block the nozzle with excessively small diameter, the nozzle with a diameter of 1.5 mm is selected.
3.2. Establishment of mathematical model As shown in Fig. 2, sectional area of a fully mechanized excavation face is supposed to be A and representative elemental volumes (REV) with a length of dx are adopted for analysis. Then, dusts inside the REV Adx satisfy a mass conservation equation in unit time: original dust capacity = aircurrent dust capacity after settlement + dust capacity collected by fog drops. The corresponding expression is
cU g A ¼ U g Aðc þ dcÞ þ dMdx
ð13Þ
where c is the dust concentration, g/m3; Ug the airflow speed, m/s; dc the dust concentration variation, g/m3; and dM the dust settling volume in unit time and unit length, g/(ms). Thus, according to the study by Ma and Hou, Eq. (14) can be obtained [11].
2
3 2 1 pðDC 106 Þ 5 3 U dg qgE cA 4 4 cA ¼ dM ¼ U dg qgE 6 2 2 DC 106 1 pðDC 10 Þ 6
ð14Þ
where Udg is the relative speed between fog drops and wind current, expressed in U dg ¼ U d þ U g . To be specific, Ud is the speed of fog drops, m/s. In addition, q is the volume content of droplets, namely q ¼ UQ LA, among which, QL is the total volume flow rate of droplets, d
3. Mathematical model for atomization-based dust removal
m3/s; gE is the dust-collecting efficiency of individual droplets; and DC is qualitative dimension of droplets (for spherical droplets, it represents their diameter, lm). After Eq. (14) is substituted into Eq. (13), we can acquire:
3.1. Assumed conditions
cU g A ¼ U g Aðc þ dcÞ þ
External atomization set on heading machine for the fully mechanized excavation face is taken as an example to perform related analysis. Sectional area of the fully mechanized excavation face in large coal mine is usually above 10 m2. Several X-type swirl nozzles with a diameter of 1.5 mm are uniformly arranged on rockshaft of the heading machine. Droplet efflux is ejected horizontally from the nozzle to the heading end socket. Under joint actions of multiple nozzles, the spray is able to evenly cover the entire roadway cross section. The spray’s flow columns that move in high speed within the spraying range impinge upon dust particles and are then bound and settled. In order to simplify the actual process of atomization-based dust removal, the following hypotheses are proposed to help establish a mathematical model for atomization-based dust removal.
Relevant parameters are further substituted into Eq. (15), and the following equation is obtained.
(1) Fog drops have the same particle size and are uniformly distributed in action zone of atomization, (2) Fog drops independently collect dusts without any mutual influence, (3) Dusts completely move along with wind currents and are also uniformly distributed in the entire roadway cross section.
Fig. 1. Relationship cure between droplet size and water supply pressure.
3 U dg qgE cA dx 2 DC 106
ð15Þ
U dg Q L gE dc 3 ¼ dx c 2 U g U d ADC 106
ð16Þ
Integral is conducted for both sides of Eq. (16), then
ln c ¼
3U dg Q L 2U g U d ADC 106
gE x þ a
ð17Þ
It is assumed that the original dust concentration is c0. When x = 0, c = c0 and a = lnc0, dust concentration c can be written into an equation below.
c ¼ c0 exp
3U dg Q L gE x
!
ð18Þ
2U g U d ADC 106
Then, the dust removal efficiency g can be expressed as
g¼
3U dg Q L gE x c0 c c ¼ 1 ¼ 1 exp c0 c0 2U g U d ADC 106
!
ð19Þ
Regarding the atomization–based dust removal where inertial impaction plays a dominant role, gE can be denoted by
Fig. 2. Micro-control volume of roadway.
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gE ¼ B0 gP
ð20Þ
where B0 is an experiment constant including retention and diffusion, which can be determined in experiments. gp isolated droplet collection efficiency of inertia impaction and can be expressed as
gp ¼
Kp K p þ 0:7
2 ð21Þ
where Kp is the non-dimensional inertial parameter of dust particle movement, which is also known as Stokes number. It can be expressed as
Kp ¼
BD2p qp U 0 106 9lg DC
ð22Þ
U dg ¼ U 0 ¼ U d
ð23Þ
By combining Eqs. (19)–(23), Eq. (24) can be obtained:
g ¼ 1 exp 4
3Q L x 2U g ADC 106
B0
BD2p qp U d 106 BD2p qp U d 106 þ 6:3lg DC
!2 3 5
ð24Þ
The dust-collecting mechanism of atomization-based dust removal mainly comprises inertia impaction, retention and diffusion. With regard to coal dust with particle size larger than 1 lm, inertia impaction plays a leading role. For the purpose of simplifying study, only inertia impaction effects between dusts and fog drops are taken into account when analyzing the mechanism of atomization-based dust removal. Thus, B0 = B = 1. The true density of coal dust qp = 1.05 103 kg/m3 is taken as the true density of coal dust particles. Moreover, the sectional area A of roadway is 12 m2; the average air velocity on fully mechanized excavation face is denoted by Ug = 1.5 m/s and the dynamic coefficient of viscosity for the air is expressed as lg = 1.8 105 Pas. Subsequently, the above parameters are substituted into the equation above. It can be further simplified into:
2
g ¼ 1 exp 6 4
0 12 3 pffiffiffi 4:02n px 1 A 7 g ¼ 1 exp 6 4 2:430:35 lg p @ 5 1 þ 100:352:13 10 lg p 2 pffiffi D p
ð28Þ
p
4. Analysis of factors influencing efficiency of atomizationbased dust removal The width of the fully mechanized excavation face selected is 4 m and n nozzle(s) is/are uniformly arranged on the rockshaft of heading machine. By means of on-spot observation, the effective action zone of atomization is approximately 4.0 m. After the above data are substituted into Eq. (28), it can be simplified into:
2
where B is Cunningham slip correction coefficient; Dp the diameter, lm; and qp the density of dust particles, kg/m3; U0 the relative velocity between dust particles and fog drops in upstream gases undisturbed, m/s (as the dust particle moves completely in pace with wind current, their velocity can be considered identical); and, lg the viscosity coefficient of the gas, Pas. Airflow velocity on the fully mechanized excavation face in underground coal mine is normally very low. Within the effective range of spray, the movement speed of fog drops is far higher than that of wind current or dust. Therefore, the speed of fog drops relative to wind current and dust can be approximately considered as the movement speed of fog drops. That is
2
2
0
Q L x 12DC 106
@
12 3 A 7 5 0:108DC 1
1þ
0 12 3 pffiffiffi 16:08n p 1 A 7 g ¼ 1 exp 6 4 2:430:35 lg p @ 5 1 þ 100:352:13 10 lg p 2 pffiffi D p
ð29Þ
p
where Dp is the dust with certain particle size. The measured efficiency of atomization-based dust removal is also targeted at the dust of such particle size, and is called fractional efficiency. For specific working face and atomizing nozzles in an underground coal mine, the factors influencing fractional efficiency of atomizationbased dust removal mainly include the number of nozzles n, the water supply pressure p and the dust particle size Dp. According to Eq. (9), Matlab is employed to portray the relation curves between various influencing factors and fractional efficiency, as shown in Figs. 3–5. Fig. 3 indicates the relation curves between fractional efficiency and the number of nozzles. During analysis, the dust particle size is set at 5 lm. Fig. 3 shows that the increase in number of nozzles is accompanied with constant rise of fractional efficiency when the water supply pressure is fixed. Moreover, when both the water supply pressure and the particle size of fog drops are constant, the water content and fog drop concentration in unit space go up as the number of nozzles rises. As a result, inertia impaction probability of dust and fog drops is improved. In addition, it can
ð25Þ Fig. 3. Relationship cure between fractional efficiency and number of nozzle.
D2p U d
In Eq. (25), the total volume flow rate QL of droplets can be calculated according to the number of nozzles n and the flow of single nozzle Q
np 2 Q L ¼ nQ ¼ Cq d 4
sffiffiffiffiffiffi 2p
q
pffiffiffi 103 ¼ 4:82n 105 p
ð26Þ
In Eq. (25), Ud, the velocity of fog drops, can be approximate to the half of initial velocity of the fog drops at the nozzle outlet, that is
Cq ud ¼ 2
sffiffiffiffiffiffi 2p
q
pffiffiffi 103 ¼ 13:64 p
ð27Þ
If the characteristic particle size of fog drops Dc in Eq. (25) is replaced with Dc50, both Eqs. (26) and (27) are substituted into Eq. (25), and a simplified formula can be obtained as follows.
Fig. 4. Relationship cure between fractional efficiency and water supply pressure.
Please cite this article in press as: Wang P et al. Dust removal efficiency of high pressure atomization in underground coal mine. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.01.006
P. Wang et al. / International Journal of Mining Science and Technology xxx (2018) xxx–xxx s s1 f ðDp Þ ¼ s ln 2Ds p50 Dp exp½ ln 2ðDp =D50 Þ
5
ð31Þ
where f(Dp) is the frequency of dusts with diverse particle sizes. Eq. (29) can be adopted to solve fractional efficiency of atomization– based dust removal only. In addition, the removal efficiency of total coal dust g0 can be written into the expression below according to Eqs. (29) and (31).
g0 ¼
Z
þ1
gf ðDp ÞdDp
ð32Þ
0
Likewise, the removal efficiency of respirable dust g00 can be expressed as Fig. 5. Relationship cure between fractional efficiency and dust size.
be also found in Fig. 3 that the fractional efficiency does not improve significantly with increasing the number of nozzles after it rises to a certain limit. Furthermore, excessive nozzles may increase water consumption of working face, and further affect the working environment and coal property. Therefore, the number of nozzles should be reasonably set when high pressure atomization is used for dust removal on the working face. In this way, both dust removal efficiency and proper water consumption can be guaranteed. Particle size of dusts remains unchanged (Dp = 5 lm). On this basis, the relation between fractional efficiency and water supply pressure is investigated. The corresponding results are presented in Fig. 4. Fig. 4 shows that in the case of a fixed number of nozzles, fractional efficiency rises with the increase in water supply pressure. The primary reason is that water content in unit space and droplet velocity keep going up due to continuous rise of pressure; in addition, as the water supply pressure goes up, atomized particle size decreases continuously. Consequently, the impaction probability between fog drops and dusts (especially respirable dusts) is improved, and the dust removal efficiency also goes up accordingly. It can be seen from Fig. 4 that the increase of fractional efficiency slightly slows down with the constant increase in water supply pressure. For example, in the case of n = 8, the water supply pressure rises from 8.0 to 10.0 MPa, while the fractional efficiency only goes up by 2.9%. The reason is that the fog drop’s particle size does not decrease significantly although the water supply pressure continues to rise after it has reached 8.0 MPa. The relation between fractional efficiency and dust particle size is shown in Fig. 5. During analysis, the number of nozzles is set at 6. According to Fig. 5, when both the number of nozzles and the water supply pressure are unchanged, the fractional efficiency rises in pace with the increase of dust particle size. However, once such particle size reaches a limit, the fractional efficiency does not increase significantly. Besides, after the water supply pressure reaches 4.0 MPa, the fractional efficiency for dust particles with a size greater than 2.0 lm is higher than 60%. By contrast, the fractional efficiency for dust particles with a size less than 2.0 lm drops sharply. Dust particle size for working face in underground coal mine conforms to Rosin-Rammler distribution in most cases. Its expression is given below.
RðDp Þ ¼ exp½ ln 2ðDp =Dp50 Þs
g00 ¼
Z
7:07 0
gf ðDp Þ dDp 1 Rð7:07Þ
ð33Þ
The fractional dust removal efficiency g, dust frequency distribution formula f(Dp) and residual rate of materials retained on sieve R(Dp) in Eqs. (32) and (33), can be substituted by using Eqs. (29), (31) and (30) respectively. As Eqs. (32) and (33) become too redundant after substitution, they remain in their original forms. In conjunction with Eqs. (29)–(33), relational expressions between total coal dust/respirable dust removal efficiency and operating parameters (n and p)/dust particle size parameters (Dp50 and s) can be separately obtained. The analysis shows that the relation between total efficiency of coal dust/respirable dust removal and various operating parameters has a change rule similar to that of the fractional efficiency. Thus, it is unnecessary to go into details. The number of nozzles and the water supply pressure are respectively set at 6 and 4 MPa. Besides, the dust particle size distribution index s is defined as 1.5. Subsequently, Matlab is used to solve Eqs. (32) and (33). Thus, the relation curves for two dust removal efficiencies (total coal dust/respirable dust) and Dp50 are acquired, as shown in Fig. 6. Fig. 6 clearly shows that when the number of nozzles, the water supply pressure and the dust particle size distribution index are all constant, the removal efficiency of total coal dust and respirable dust goes up with the increase in median diameter of the dust. Based on the relation between fractional efficiency and dust particle size, they are directly proportional. In detail, when the dust particle size distribution index is fixed, the average particle size of dust keeps rising as its median diameter goes up. Consequently, the removal efficiency of total coal dust and respirable dust is improved. Fig. 6 indicates that the removal efficiency of total coal dust is higher than that of respirable dust in the case of the same operating parameters and dust characteristic parameters. Moreover, the gap between them is improved as the median diameter of dust increases. The reason is that the mass proportion of respirable dust in total coal dust continues to drop as the median diameter of dust increases, thus giving rise to increasingly larger differences between total coal dust removal efficiency and respirable dust removal efficiency.
ð30Þ
where R(Dp) is the residual rate of materials retained on sieve; Dp50 the median diameter of dusts, lm; and s the distribution index signifying uniformity of dust particle size. The larger the value of s is, the smaller the distribution range of particle sizes will be. Otherwise, a lower s implies a broader distribution range of particle size. For dusts of working face, s generally ranges from 0.8 to 1.8. Derivation is performed for Eq. (28), and the expression for dust frequency distribution can be obtained as follows [20].
Fig. 6. Relationship cure between dust removal efficiency and medium diameter.
Please cite this article in press as: Wang P et al. Dust removal efficiency of high pressure atomization in underground coal mine. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.01.006
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P. Wang et al. / International Journal of Mining Science and Technology xxx (2018) xxx–xxx
Fig. 7. Relationship cure between dust removal efficiency and distribution index.
In the case that both the number of nozzles and the water supply pressure remain unchanged and the median diameter of dust is set at 10 lm, the relations between two dust removal efficiencies and dust particle size distribution index are investigated. The related computing results are given in Fig. 7. According to the curves, the total coal dust/respirable dust removal efficiency rises in pace with the increase in dust particle size distribution index, provided that the number of nozzles, the water supply pressure and the median diameter of dust are all fixed. However, when the distribution index becomes larger, the corresponding dust particle size distribution range becomes narrower and narrower. Moreover, regarding particles with the size around the median diameter (10 lm), their volume frequency keeps rising. As for fine particles (Dp < 2 lm), their percentage in total coal dust/respirable dust falls. As a result, the removal efficiency of total coal dust and respirable dust can be improved. Meanwhile, as the distribution index goes up, the dust particle size distribution range becomes narrower and approaches to the median diameter. Hence, the average particle size of respirable dust also becomes increasingly closer to the median diameter with the increase in distribution index, leading to constant decrease of the difference between total coal dust removal efficiency and respirable dust removal efficiency. 5. Conclusions (1) By measuring flow coefficient and atomization angle of Xtype pressure swirl nozzle that is commonly used in underground coal mine, the equation of atomized particle size for the nozzle with a diameter of 1.5 mm is derived, i.e., Dc50 ¼ 102:430:35 lg p , in conjunction with relevant empirical equation. Similarly, the computational equation of atomized particle size for nozzles with a diameter of 1.2 or 2.0 mm are Dc50 ¼ 102:390:35 lg p and Dc50 ¼ 102:490:35 lg p respectively. (2) According to relevant theories of fluid mechanics and aerosol, a mathematical model for applying high pressure atomization to dust removal in underground coal mine is established. Based on this model, the expression of fractional efficiency g is derived. Then, by combining dust particle size distribution equation R(Dp) and dust frequency distribution equation f(Dp), the removal efficiency of total coal dust g0 R þ1 can be expressed as g0 ¼ 0 gf ðDp ÞdDp , while the removal efficiency of respirable dust g00 can be expressed as R g00 ¼ 07:07 gf ðDp Þ=½1 Rð7:07ÞdDp . (3) Under the circumstance that water supply pressure is fixed, fractional efficiency rises with the increase in the number of nozzles. Moreover, when the number of nozzles remains the same, the fractional efficiency constantly rises as the water supply pressure goes up. In addition, the fractional efficiency also rises with the increase of dust particle size. However, after the dust particle size reaches a certain limit, the fractional efficiency does not increase significantly.
(4) When the number of nozzles, the water supply pressure and the dust particle size distribution index are all constant, the removal efficiency of total coal dust and respirable dust goes up along with the increase in median diameter of dust. The removal efficiency of total coal dust is higher than that of respirable dust in the case of identical operating parameters and dust characteristic parameters. Moreover, the gap between them can be enlarged by increasing the median diameter of dust. By contrast, when the number of nozzles, the water supply pressure and the median diameter of dust are all fixed, the removal efficiency of total coal dust and respirable dust increases as the dust particle size distribution index rises. Furthermore, the difference between them decreases.
Acknowledgments Financial support for this work, provided by the National Natural Science Foundation of China (Nos. 51574123 and U1361118), and the China Postdoctoral Science Foundation (No. 2015M 582118), are gratefully acknowledged.
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Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Dust removal efficiency of high pressure atomization in underground coal mine Wang Pengfei a,b,⇑, Tan Xuanhao b, Cheng Weimin b, Zhou Gang b, Liu Ronghua a a b
School of Mining and Safety Engineering, Shandong University of Science & Technology, Qingdao 266590, China School of Resource, Environment & Safety Engineering, Hunan University of Science & Technology, Xiangtan 411201, China
a r t i c l e
i n f o
Article history: Received 12 September 2017 Received in revised form 18 October 2017 Accepted 23 January 2018 Available online xxxx Keywords: Underground coal mine High pressure atomization Atomization characteristics Dust removal efficiency
a b s t r a c t To master theoretical calculation for dust removal efficiency of high pressure atomization in an underground coal mine, the corresponding atomization characteristics and dust removal efficiency were both comprehensively studied in theory by virtue of related theories of hydromechanics and aerosol. According to actual measurements of flow coefficients and atomization angles of X-type swirl nozzle, computational formula was derived for atomized particle sizes of such a nozzle in conjunction with relevant empirical equation. Moreover, a mathematical model for applying high pressure atomization to dust removal in underground coal mine was also established to deduce theoretical computation formula of fractional efficiency. Then, Matlab was adopted to portray the relation curve between fractional efficiency and influence factors. In addition, a theoretical formula was also set up for removal efficiency of respirable dust and total coal dust based on dust size and frequency distribution equations. In the end, impacts of dust characteristic parameters on various dust removal efficiencies were analyzed. Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Various links of production in underground mine can generate dusts in different extents. Dust suspended in air not only jeopardizes health of workers, but gives rise to bad influences on underground safety production. At present, measures such as dust removal by ventilation, water pre-infusion for coal mass and atomization have been taken both at home and abroad to reduce dust concentrations in the air of underground coal mines. In view of the advantages like cost-effectiveness, convenient and strong practicality, atomization has been extensively applied to remove dust in underground coal mines. However, its field practice in underground coal mine is lacking in theoretical guidance. As a consequence, the dust removal efficiency of atomization is not ideal. In particular, the efficiency of respirable dust removal is even below 40% [1–4]. In recent years, scholars begin to investigate atomization characteristics and dust removal efficiency of high pressure atomization. Cheng et al. used an experiment platform specially designed for high pressure atomization to carry out an experiment for analyzing the atomization characteristic of several commonly in the use of nozzles in coal face, and investigated dust removal ⇑ Corresponding author at: School of Mining and Safety Engineering, Shandong University of Science & Technology, Qingdao 266590, China. E-mail address: [email protected] (P. Wang).
efficiency on site to determine the relation of water supply pressure to atomization particle size and effect of dust removal [5–8]. Ma et al. took advantage of related theories such as fluid mechanics and aerosol to set up a mathematical calculation model for fractional dust removal efficiency of high pressure atomization in underground coal mines. In addition, they also analyzed the relation of dust removal efficiency with various operating parameters [9–11]. In the use of Fluent, Chen al. investigated impacts of water supply pressure on droplet size, droplet velocity and mist flow field [12,13]. Over the recent years, our research group also carried out a large number of experimental studies on atomization characteristics and dust removal effects of high pressure atomization, and found the relation between dust removal efficiency and various influencing factors [14,15]. Raj Mohan et al. also performed experimental studies regarding characteristics and dust removal efficiency for gas-water atomization, and determined how various operating parameters influence dust removal efficiency [16–18]. Evaluation on dust removal efficiency of high pressure atomization in underground coal mine is usually based on the removal efficiency of total coal dust and respirable dust. The calculation model for dust removal efficiency of atomization established by Chinese scholars can be only used for computing fractional efficiency. In other words, it can only calculate the efficiency of atomization in removing the dust of single particle size. Additionally, the findings of experiments about atomization-based dust removal all focus on
https://doi.org/10.1016/j.ijmst.2018.01.006 2095-2686/Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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P. Wang et al. / International Journal of Mining Science and Technology xxx (2018) xxx–xxx
the qualitative relationship between different influencing factors and dust removal efficiency. Their conclusions cannot be preferably applied to guide on-site engineering design and calculation of atomization-based dust removal efficiency. Therefore, the author set up a theoretical calculation model for removal efficiency of total coal dust and respirable dust based on the calculation model of existing fractional efficiency and the distribution equations of dust size and frequency. On this basis, the relations of different operating parameters and dust characteristic parameters to dust removal efficiency were analyzed. Our findings will provide a theoretical basis for applying high pressure atomization in dust removal to prevent and control dusts on underground coal mining face. 2. Characteristics of high pressure atomization 2.1. Flow coefficient and atomization angle of nozzle Factors that affect nozzle flow mainly include nozzle configuration, nozzle diameter and water supply pressure. For ordinary single-jet pressure nozzles, their water supply flow can be calculated according to the Eq. (1) [19]:
Q¼
p 4
2
Cqd
sffiffiffiffiffiffi 2p
q
103
ð1Þ
where Q is the volume flow rate of water supply, m3/s; and Cq the flow coefficient related to nozzle configuration; d the nozzle diameter, mm; p the water supply pressure, MPa; and q the water density, kg/m3. At room temperature, flow characteristics and atomization angle of the X-type swirl pressure nozzle commonly used in underground coal mine were measured based on the self-designed experiment platform for high pressure atomization. The diameter of the selected nozzle was 1.5 mm and the corresponding mist flow was in a shape of solid cone. Electromagnetic flowmeter and high speed camera were used to measure water flow rate and atomization angle under 5 water supply pressures (2, 4, 6, 8 or 10 MPa). According to Eq. (1), flow data measured during the experiment were regressively analyzed to acquire the flow coefficient of the nozzle, as shown in Table 1. Besides, atomization photos taken by the high speed camera were imported into Image-Pro Plus 6.0 to calculate atomization angle a. The related calculation results are given in Table 1. The measurements in Eq. (1) and Table 1 show that the water supply flow of nozzle goes up with the rise of atomization pressure and is in direct proportion to square root of water supply pressure. In addition, the results also indicate that the flow coefficient of nozzle independent of water supply pressure is only associated with nozzle configuration. Regarding a nozzle with fixed structure, there must be a flow coefficient corresponding to it. In the present study, the nozzle’s flow coefficient was 0.61. Table 1 also indicates that the atomization angle of nozzle drops slightly with the increase of water supply pressure. For example, when the water supply pressure increases to 10 from 2 MPa, the atomization angle only changes by 2.38°. For the convenience of calculation, the
Table 1 Flow rate and spray angle of nozzle under different water supply pressures. p (MPa)
Q (L/min)
Α (°)
Cq
Average a (°)
2 4 6 8 10
4.01 5.67 7.08 8.08 9.12
43.59 42.56 42.01 41.54 41.21
0.61
42.18
mean value within the interval of water supply pressure (42.18°) is used as the atomization angle of nozzle. 2.2. Atomized particle size of nozzle With regard to a single-jet atomizing pressure nozzle, atomized particle size can be calculated according to the empirical formula summarized by scholars of the former Soviet Union.
lg
Dc50 0:35 lg We ¼ 4:47L0:133 p d
ð2Þ
where Dc50 is the mass median aerodynamic diameter (MMAD), i.e., the mass of fog drops with particle size below this value occupies 50% in the total mass of all fog drops, mm; d the feature size, mm; Lp the Laplace series; and We the Weber number, a parameter describing stability of droplets. d in Eq. (2) can be computed by Eq. (3).
d¼d 1
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi!. 1 C q cos
a 2
2 cos
a 2
ð3Þ
where d is the nozzle diameter, mm; and a the atomization angle, °. The diameter of the nozzle adopted in this paper is 1.5 mm. Moreover, the flow coefficient of nozzle measured is 0.61 (Cq = 0.61) and the atomization angle is 42.18° (a = 42.18°). Then, after all related data are substituted into the above equation, Eq. (4) can be obtained as below:
d ¼ 1:5ð1
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 0:61 cos 21:090 Þ=ð2 cos 21:090 Þ
¼ 0:276mm
ð4Þ
The Laplace series Lp can be calculated based on Eq. (5):
Lp ¼
qrd 103 l2L
ð5Þ
where r is the liquid surface tension coefficient, N/m; and l the fluid dynamic viscosity, N/(m2s). Relevant parameters of water in normal state, including r = 0.072 N/m, lL = 1.005 103 Pas and qL = 1000 kg/m3, are selected. Then:
Lp ¼
1000 0:072 0:276 1:0052 106
103 ¼ 19675
ð6Þ
The computational formula of We is
We ¼
qt2 d 103 r
ð7Þ
where t is the axial jet velocity of droplets, m/s, and it can be calculated according to nozzle flow coefficient Cq and water supply pressure p. That is
t ¼ Cq
sffiffiffiffiffiffi 2p
q
103
ð8Þ
Substituting Eqs. (8) into (7) together with related parameters, the value of Weber number is
We ¼ 2852:7p
ð9Þ
Substituting Eqs. (4), (6) and (9) into (2), a equation of atomized particle size. Then, we can obtain:
Dc50 ¼ 102:430:35 lg p lm
ð10Þ
Likewise, for an X-type swirl nozzle with a diameter of 1.2 mm, the computation equation of atomized particle size is
Dc50 ¼ 102:390:35 lg p lm
ð11Þ
Similarly, in terms of an X-type swirl nozzle with a diameter of 2.0 mm, the equation of atomized particle size is
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P. Wang et al. / International Journal of Mining Science and Technology xxx (2018) xxx–xxx
Dc50 ¼ 102:490:35 lg p lm
ð12Þ
According to Eqs. (10)–(12), Matlab is used to portray relation curves between atomized particle size Dc50 and water supply pressure p of nozzles in three different diameters, as shown in Fig. 1. It indicates that when the diameter of nozzle is invariant, atomized particle size keeps falling with the rise of water supply pressure. In the case that the water supply pressure changes between 0 and 8 MPa, the atomized particle size rapidly goes down with the increase of pressure. After the pressure reaches 8 MPa, changes in atomized particle size are minor although the water supply pressure continues to rise. Obtaining small fog drops by increasing water supply pressure has certain limitations. According to comparisons among three relation curves in Fig. 1, the nozzle with small diameter can be used to obtain fog drops with small particle size under the circumstance that water supply pressure is constant. As the dust removal efficiency of atomization is closely related to particle size of fog drops, those with small particle size can help to collect respirable dust. Thus, the nozzle with small diameter should be selected to guarantee higher removal efficiency of respirable dust in the case of fixed water supply pressure. Nevertheless, given that water used by atomization underground generally contains impurities which may block the nozzle with excessively small diameter, the nozzle with a diameter of 1.5 mm is selected.
3.2. Establishment of mathematical model As shown in Fig. 2, sectional area of a fully mechanized excavation face is supposed to be A and representative elemental volumes (REV) with a length of dx are adopted for analysis. Then, dusts inside the REV Adx satisfy a mass conservation equation in unit time: original dust capacity = aircurrent dust capacity after settlement + dust capacity collected by fog drops. The corresponding expression is
cU g A ¼ U g Aðc þ dcÞ þ dMdx
ð13Þ
where c is the dust concentration, g/m3; Ug the airflow speed, m/s; dc the dust concentration variation, g/m3; and dM the dust settling volume in unit time and unit length, g/(ms). Thus, according to the study by Ma and Hou, Eq. (14) can be obtained [11].
2
3 2 1 pðDC 106 Þ 5 3 U dg qgE cA 4 4 cA ¼ dM ¼ U dg qgE 6 2 2 DC 106 1 pðDC 10 Þ 6
ð14Þ
where Udg is the relative speed between fog drops and wind current, expressed in U dg ¼ U d þ U g . To be specific, Ud is the speed of fog drops, m/s. In addition, q is the volume content of droplets, namely q ¼ UQ LA, among which, QL is the total volume flow rate of droplets, d
3. Mathematical model for atomization-based dust removal
m3/s; gE is the dust-collecting efficiency of individual droplets; and DC is qualitative dimension of droplets (for spherical droplets, it represents their diameter, lm). After Eq. (14) is substituted into Eq. (13), we can acquire:
3.1. Assumed conditions
cU g A ¼ U g Aðc þ dcÞ þ
External atomization set on heading machine for the fully mechanized excavation face is taken as an example to perform related analysis. Sectional area of the fully mechanized excavation face in large coal mine is usually above 10 m2. Several X-type swirl nozzles with a diameter of 1.5 mm are uniformly arranged on rockshaft of the heading machine. Droplet efflux is ejected horizontally from the nozzle to the heading end socket. Under joint actions of multiple nozzles, the spray is able to evenly cover the entire roadway cross section. The spray’s flow columns that move in high speed within the spraying range impinge upon dust particles and are then bound and settled. In order to simplify the actual process of atomization-based dust removal, the following hypotheses are proposed to help establish a mathematical model for atomization-based dust removal.
Relevant parameters are further substituted into Eq. (15), and the following equation is obtained.
(1) Fog drops have the same particle size and are uniformly distributed in action zone of atomization, (2) Fog drops independently collect dusts without any mutual influence, (3) Dusts completely move along with wind currents and are also uniformly distributed in the entire roadway cross section.
Fig. 1. Relationship cure between droplet size and water supply pressure.
3 U dg qgE cA dx 2 DC 106
ð15Þ
U dg Q L gE dc 3 ¼ dx c 2 U g U d ADC 106
ð16Þ
Integral is conducted for both sides of Eq. (16), then
ln c ¼
3U dg Q L 2U g U d ADC 106
gE x þ a
ð17Þ
It is assumed that the original dust concentration is c0. When x = 0, c = c0 and a = lnc0, dust concentration c can be written into an equation below.
c ¼ c0 exp
3U dg Q L gE x
!
ð18Þ
2U g U d ADC 106
Then, the dust removal efficiency g can be expressed as
g¼
3U dg Q L gE x c0 c c ¼ 1 ¼ 1 exp c0 c0 2U g U d ADC 106
!
ð19Þ
Regarding the atomization–based dust removal where inertial impaction plays a dominant role, gE can be denoted by
Fig. 2. Micro-control volume of roadway.
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P. Wang et al. / International Journal of Mining Science and Technology xxx (2018) xxx–xxx
gE ¼ B0 gP
ð20Þ
where B0 is an experiment constant including retention and diffusion, which can be determined in experiments. gp isolated droplet collection efficiency of inertia impaction and can be expressed as
gp ¼
Kp K p þ 0:7
2 ð21Þ
where Kp is the non-dimensional inertial parameter of dust particle movement, which is also known as Stokes number. It can be expressed as
Kp ¼
BD2p qp U 0 106 9lg DC
ð22Þ
U dg ¼ U 0 ¼ U d
ð23Þ
By combining Eqs. (19)–(23), Eq. (24) can be obtained:
g ¼ 1 exp 4
3Q L x 2U g ADC 106
B0
BD2p qp U d 106 BD2p qp U d 106 þ 6:3lg DC
!2 3 5
ð24Þ
The dust-collecting mechanism of atomization-based dust removal mainly comprises inertia impaction, retention and diffusion. With regard to coal dust with particle size larger than 1 lm, inertia impaction plays a leading role. For the purpose of simplifying study, only inertia impaction effects between dusts and fog drops are taken into account when analyzing the mechanism of atomization-based dust removal. Thus, B0 = B = 1. The true density of coal dust qp = 1.05 103 kg/m3 is taken as the true density of coal dust particles. Moreover, the sectional area A of roadway is 12 m2; the average air velocity on fully mechanized excavation face is denoted by Ug = 1.5 m/s and the dynamic coefficient of viscosity for the air is expressed as lg = 1.8 105 Pas. Subsequently, the above parameters are substituted into the equation above. It can be further simplified into:
2
g ¼ 1 exp 6 4
0 12 3 pffiffiffi 4:02n px 1 A 7 g ¼ 1 exp 6 4 2:430:35 lg p @ 5 1 þ 100:352:13 10 lg p 2 pffiffi D p
ð28Þ
p
4. Analysis of factors influencing efficiency of atomizationbased dust removal The width of the fully mechanized excavation face selected is 4 m and n nozzle(s) is/are uniformly arranged on the rockshaft of heading machine. By means of on-spot observation, the effective action zone of atomization is approximately 4.0 m. After the above data are substituted into Eq. (28), it can be simplified into:
2
where B is Cunningham slip correction coefficient; Dp the diameter, lm; and qp the density of dust particles, kg/m3; U0 the relative velocity between dust particles and fog drops in upstream gases undisturbed, m/s (as the dust particle moves completely in pace with wind current, their velocity can be considered identical); and, lg the viscosity coefficient of the gas, Pas. Airflow velocity on the fully mechanized excavation face in underground coal mine is normally very low. Within the effective range of spray, the movement speed of fog drops is far higher than that of wind current or dust. Therefore, the speed of fog drops relative to wind current and dust can be approximately considered as the movement speed of fog drops. That is
2
2
0
Q L x 12DC 106
@
12 3 A 7 5 0:108DC 1
1þ
0 12 3 pffiffiffi 16:08n p 1 A 7 g ¼ 1 exp 6 4 2:430:35 lg p @ 5 1 þ 100:352:13 10 lg p 2 pffiffi D p
ð29Þ
p
where Dp is the dust with certain particle size. The measured efficiency of atomization-based dust removal is also targeted at the dust of such particle size, and is called fractional efficiency. For specific working face and atomizing nozzles in an underground coal mine, the factors influencing fractional efficiency of atomizationbased dust removal mainly include the number of nozzles n, the water supply pressure p and the dust particle size Dp. According to Eq. (9), Matlab is employed to portray the relation curves between various influencing factors and fractional efficiency, as shown in Figs. 3–5. Fig. 3 indicates the relation curves between fractional efficiency and the number of nozzles. During analysis, the dust particle size is set at 5 lm. Fig. 3 shows that the increase in number of nozzles is accompanied with constant rise of fractional efficiency when the water supply pressure is fixed. Moreover, when both the water supply pressure and the particle size of fog drops are constant, the water content and fog drop concentration in unit space go up as the number of nozzles rises. As a result, inertia impaction probability of dust and fog drops is improved. In addition, it can
ð25Þ Fig. 3. Relationship cure between fractional efficiency and number of nozzle.
D2p U d
In Eq. (25), the total volume flow rate QL of droplets can be calculated according to the number of nozzles n and the flow of single nozzle Q
np 2 Q L ¼ nQ ¼ Cq d 4
sffiffiffiffiffiffi 2p
q
pffiffiffi 103 ¼ 4:82n 105 p
ð26Þ
In Eq. (25), Ud, the velocity of fog drops, can be approximate to the half of initial velocity of the fog drops at the nozzle outlet, that is
Cq ud ¼ 2
sffiffiffiffiffiffi 2p
q
pffiffiffi 103 ¼ 13:64 p
ð27Þ
If the characteristic particle size of fog drops Dc in Eq. (25) is replaced with Dc50, both Eqs. (26) and (27) are substituted into Eq. (25), and a simplified formula can be obtained as follows.
Fig. 4. Relationship cure between fractional efficiency and water supply pressure.
Please cite this article in press as: Wang P et al. Dust removal efficiency of high pressure atomization in underground coal mine. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.01.006
P. Wang et al. / International Journal of Mining Science and Technology xxx (2018) xxx–xxx s s1 f ðDp Þ ¼ s ln 2Ds p50 Dp exp½ ln 2ðDp =D50 Þ
5
ð31Þ
where f(Dp) is the frequency of dusts with diverse particle sizes. Eq. (29) can be adopted to solve fractional efficiency of atomization– based dust removal only. In addition, the removal efficiency of total coal dust g0 can be written into the expression below according to Eqs. (29) and (31).
g0 ¼
Z
þ1
gf ðDp ÞdDp
ð32Þ
0
Likewise, the removal efficiency of respirable dust g00 can be expressed as Fig. 5. Relationship cure between fractional efficiency and dust size.
be also found in Fig. 3 that the fractional efficiency does not improve significantly with increasing the number of nozzles after it rises to a certain limit. Furthermore, excessive nozzles may increase water consumption of working face, and further affect the working environment and coal property. Therefore, the number of nozzles should be reasonably set when high pressure atomization is used for dust removal on the working face. In this way, both dust removal efficiency and proper water consumption can be guaranteed. Particle size of dusts remains unchanged (Dp = 5 lm). On this basis, the relation between fractional efficiency and water supply pressure is investigated. The corresponding results are presented in Fig. 4. Fig. 4 shows that in the case of a fixed number of nozzles, fractional efficiency rises with the increase in water supply pressure. The primary reason is that water content in unit space and droplet velocity keep going up due to continuous rise of pressure; in addition, as the water supply pressure goes up, atomized particle size decreases continuously. Consequently, the impaction probability between fog drops and dusts (especially respirable dusts) is improved, and the dust removal efficiency also goes up accordingly. It can be seen from Fig. 4 that the increase of fractional efficiency slightly slows down with the constant increase in water supply pressure. For example, in the case of n = 8, the water supply pressure rises from 8.0 to 10.0 MPa, while the fractional efficiency only goes up by 2.9%. The reason is that the fog drop’s particle size does not decrease significantly although the water supply pressure continues to rise after it has reached 8.0 MPa. The relation between fractional efficiency and dust particle size is shown in Fig. 5. During analysis, the number of nozzles is set at 6. According to Fig. 5, when both the number of nozzles and the water supply pressure are unchanged, the fractional efficiency rises in pace with the increase of dust particle size. However, once such particle size reaches a limit, the fractional efficiency does not increase significantly. Besides, after the water supply pressure reaches 4.0 MPa, the fractional efficiency for dust particles with a size greater than 2.0 lm is higher than 60%. By contrast, the fractional efficiency for dust particles with a size less than 2.0 lm drops sharply. Dust particle size for working face in underground coal mine conforms to Rosin-Rammler distribution in most cases. Its expression is given below.
RðDp Þ ¼ exp½ ln 2ðDp =Dp50 Þs
g00 ¼
Z
7:07 0
gf ðDp Þ dDp 1 Rð7:07Þ
ð33Þ
The fractional dust removal efficiency g, dust frequency distribution formula f(Dp) and residual rate of materials retained on sieve R(Dp) in Eqs. (32) and (33), can be substituted by using Eqs. (29), (31) and (30) respectively. As Eqs. (32) and (33) become too redundant after substitution, they remain in their original forms. In conjunction with Eqs. (29)–(33), relational expressions between total coal dust/respirable dust removal efficiency and operating parameters (n and p)/dust particle size parameters (Dp50 and s) can be separately obtained. The analysis shows that the relation between total efficiency of coal dust/respirable dust removal and various operating parameters has a change rule similar to that of the fractional efficiency. Thus, it is unnecessary to go into details. The number of nozzles and the water supply pressure are respectively set at 6 and 4 MPa. Besides, the dust particle size distribution index s is defined as 1.5. Subsequently, Matlab is used to solve Eqs. (32) and (33). Thus, the relation curves for two dust removal efficiencies (total coal dust/respirable dust) and Dp50 are acquired, as shown in Fig. 6. Fig. 6 clearly shows that when the number of nozzles, the water supply pressure and the dust particle size distribution index are all constant, the removal efficiency of total coal dust and respirable dust goes up with the increase in median diameter of the dust. Based on the relation between fractional efficiency and dust particle size, they are directly proportional. In detail, when the dust particle size distribution index is fixed, the average particle size of dust keeps rising as its median diameter goes up. Consequently, the removal efficiency of total coal dust and respirable dust is improved. Fig. 6 indicates that the removal efficiency of total coal dust is higher than that of respirable dust in the case of the same operating parameters and dust characteristic parameters. Moreover, the gap between them is improved as the median diameter of dust increases. The reason is that the mass proportion of respirable dust in total coal dust continues to drop as the median diameter of dust increases, thus giving rise to increasingly larger differences between total coal dust removal efficiency and respirable dust removal efficiency.
ð30Þ
where R(Dp) is the residual rate of materials retained on sieve; Dp50 the median diameter of dusts, lm; and s the distribution index signifying uniformity of dust particle size. The larger the value of s is, the smaller the distribution range of particle sizes will be. Otherwise, a lower s implies a broader distribution range of particle size. For dusts of working face, s generally ranges from 0.8 to 1.8. Derivation is performed for Eq. (28), and the expression for dust frequency distribution can be obtained as follows [20].
Fig. 6. Relationship cure between dust removal efficiency and medium diameter.
Please cite this article in press as: Wang P et al. Dust removal efficiency of high pressure atomization in underground coal mine. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.01.006
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P. Wang et al. / International Journal of Mining Science and Technology xxx (2018) xxx–xxx
Fig. 7. Relationship cure between dust removal efficiency and distribution index.
In the case that both the number of nozzles and the water supply pressure remain unchanged and the median diameter of dust is set at 10 lm, the relations between two dust removal efficiencies and dust particle size distribution index are investigated. The related computing results are given in Fig. 7. According to the curves, the total coal dust/respirable dust removal efficiency rises in pace with the increase in dust particle size distribution index, provided that the number of nozzles, the water supply pressure and the median diameter of dust are all fixed. However, when the distribution index becomes larger, the corresponding dust particle size distribution range becomes narrower and narrower. Moreover, regarding particles with the size around the median diameter (10 lm), their volume frequency keeps rising. As for fine particles (Dp < 2 lm), their percentage in total coal dust/respirable dust falls. As a result, the removal efficiency of total coal dust and respirable dust can be improved. Meanwhile, as the distribution index goes up, the dust particle size distribution range becomes narrower and approaches to the median diameter. Hence, the average particle size of respirable dust also becomes increasingly closer to the median diameter with the increase in distribution index, leading to constant decrease of the difference between total coal dust removal efficiency and respirable dust removal efficiency. 5. Conclusions (1) By measuring flow coefficient and atomization angle of Xtype pressure swirl nozzle that is commonly used in underground coal mine, the equation of atomized particle size for the nozzle with a diameter of 1.5 mm is derived, i.e., Dc50 ¼ 102:430:35 lg p , in conjunction with relevant empirical equation. Similarly, the computational equation of atomized particle size for nozzles with a diameter of 1.2 or 2.0 mm are Dc50 ¼ 102:390:35 lg p and Dc50 ¼ 102:490:35 lg p respectively. (2) According to relevant theories of fluid mechanics and aerosol, a mathematical model for applying high pressure atomization to dust removal in underground coal mine is established. Based on this model, the expression of fractional efficiency g is derived. Then, by combining dust particle size distribution equation R(Dp) and dust frequency distribution equation f(Dp), the removal efficiency of total coal dust g0 R þ1 can be expressed as g0 ¼ 0 gf ðDp ÞdDp , while the removal efficiency of respirable dust g00 can be expressed as R g00 ¼ 07:07 gf ðDp Þ=½1 Rð7:07ÞdDp . (3) Under the circumstance that water supply pressure is fixed, fractional efficiency rises with the increase in the number of nozzles. Moreover, when the number of nozzles remains the same, the fractional efficiency constantly rises as the water supply pressure goes up. In addition, the fractional efficiency also rises with the increase of dust particle size. However, after the dust particle size reaches a certain limit, the fractional efficiency does not increase significantly.
(4) When the number of nozzles, the water supply pressure and the dust particle size distribution index are all constant, the removal efficiency of total coal dust and respirable dust goes up along with the increase in median diameter of dust. The removal efficiency of total coal dust is higher than that of respirable dust in the case of identical operating parameters and dust characteristic parameters. Moreover, the gap between them can be enlarged by increasing the median diameter of dust. By contrast, when the number of nozzles, the water supply pressure and the median diameter of dust are all fixed, the removal efficiency of total coal dust and respirable dust increases as the dust particle size distribution index rises. Furthermore, the difference between them decreases.
Acknowledgments Financial support for this work, provided by the National Natural Science Foundation of China (Nos. 51574123 and U1361118), and the China Postdoctoral Science Foundation (No. 2015M 582118), are gratefully acknowledged.
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Please cite this article in press as: Wang P et al. Dust removal efficiency of high pressure atomization in underground coal mine. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.01.006