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Analysis of explosion pressure and residual gas characteristics of micro-nano coal dust in confined space
Journal Pre-proof Analysis of explosion pressure and residual gas characteristics of micro-nano coal dust in confined space Bo Tan, Zhuangzhuang Shao, Bin Xu, Hongyi Wei, Tian Wang PII:
S0950-4230(19)30541-8
DOI:
https://doi.org/10.1016/j.jlp.2020.104056
Reference:
JLPP 104056
To appear in:
Journal of Loss Prevention in the Process Industries
Received Date: 30 June 2019 Revised Date:
31 December 2019
Accepted Date: 20 January 2020
Please cite this article as: Tan, B., Shao, Z., Xu, B., Wei, H., Wang, T., Analysis of explosion pressure and residual gas characteristics of micro-nano coal dust in confined space, Journal of Loss Prevention in the Process Industries (2020), doi: https://doi.org/10.1016/j.jlp.2020.104056. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Analysis of explosion pressure and residual gas characteristics of micro-nano coal dust in confined space1 Bo Tan, Zhuangzhuang Shao, Bin Xu*, Hongyi Wei, Tian Wang College of Emergency Management and Safety Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China Abstracts: In order to study the explosion characteristics of micro and nano-sized coal dust, a 20L spherical explosion test device was used to compare and analyze the explosion pressure characteristics and residual gas characteristics of five kinds of micro-nano particle dusts of 800nm, 1200nm, 45µm, 60µm and 75µm under five concentrations (100g/m3、250g/m3、500g/m3、750g/m3、1000g/m3) .The results showed that the explosion pressure characteristics were significantly related to the particle size and concentration. The time required for the nano-sized coal dust to reach the maximum explosion pressure was less than the micron. Among the five different coal dust concentrations, the best explosion concentration was 500g/m3, the maximum explosion pressure of 800 nm coal dust was 0.73MPa, the maximum explosion pressure rate was 87.37 MPa·s-1, and the explosion index Kst was 22.35 MPa·m·s-1. The explosion hazard level of nano-sized coal dust was a strong level, while micro-sized coal dust was a weak level. Under the same coal dust concentration, the concentrations of O2, CO2 and CO after the nano-sized coal dust explosion were lower than the micro-sized.When the coal dust concentration is 500g/m3,the O2 and CO concentrations after 800nm coal dust explosion are 9.3% and 503ppm, respectively. However, the concentrations of O2 and CO after the 45um coal dust explosion were 10.6% and 2008ppm. The CO/CO2 ratio after the nano-sized coal dust explosion was less than the micron. Key words: Confined space; nanoscale; coal dust explosion; explosion pressure characteristics; residual gas 1.Introduction Mine dust can not only cause occupational diseases, but dust explosions will produce shock waves and a lot of harmful gases, which seriously threaten people's life and health and the safety production (Bai et al., 2011; Liu et al., 2010). For example, in the "1·12" major coal dust explosion accident at Lijiagou Coal Mine in Shanxi Province, China in 2019, 21 people were killed and the losses were heavy, and the impact was extremely bad. According to the CSB Investigation Report (Chemical SB et al., 2006; Di Benedetto et al., 2013), 281 major combustible dust accidents occurred between 1980 and 2005, resulting in 718 people injured, 119 people killed,and significant damage to industrial facilities. In the period of 2001–2015, the coal industry had significant problems resulting from dust explosions (Cao et al., 2015; Ye et al., 2014; Xin et al., 2014).The coal dust explosion law is studied by studying the characteristics of coal dust explosion and residual gas characteristics (Cao et al., 2017), which has important guiding significance for preventing coal dust explosion and ensuring coal mine safety. In the process of dust explosion (Bi et al., 2012; Ye et al., 2017), dust can be divided into two types: one is combustible dust, and the other is non-combustible dust.The combustible dust will explode when it meets the heat source, and the flame will spread quickly and spread to the whole space where the dust exists.The reaction rate is extremely fast, and a large amount of heat is released at the same time, resulting in high temperature and pressure, and has a strong destructive force. Mittal (2014) used 20 L spherical explosion vessel to study the explosion characteristics of micro and nano-sized magnesium powder. It was found that the maximum explosion pressure 1
∗ Corresponding author. E-mail address: [email protected] (B. Xu).
and detonation index of magnesium powder in the range of 30 nm-125 µm were roughly symmetrically distributed at 400 nm. Turkevich et al. (2015) studied the characteristics of minimum ignition energy and explosion pressure of various carbon nanopowders through the explosion experiments of nanomaterials such as graphite and graphene. Li et al. (2011) concluded that the effect of aluminum powder particle size on the maximum explosion pressure and explosion pressure rate was relatively small in the nanometer scale by the explosion experiment of aluminum powder with different particle sizes in a 20L sphere. Dufaud et al. (2010) studied the effect of dust particle size on the explosion of aluminum powder through a 20L spherical explosion experiment. The results showed that the maximum explosion pressure of the micro-sized aluminum powder, especially the maximum pressure rise rate, depended largely on the dust particle size. Li et al. (2010) used a 20L spherical explosion test device to experimentally study the explosion characteristics of three kinds of nanometer aluminum powders. It was found that the maximum explosion pressure and maximum pressure rise rate of nano-sized aluminum powder were affected by dust concentration. Cao et al. (2017) used 20L spherical explosion experiment to study the explosion parameters of coal dust. The results showed that the explosion pressure and explosion pressure rise rate of 10-100 µm coal dust increaseed with the increase of coal dust concentration. Li et al. (2016) studied the effect of particle size and dispersion on the severity of coal dust explosion. The results showed that the maximum explosion pressure increased with the decrease of dispersion and particle size. Gao et al. (2010) used a 20L spherical explosion test device to carry out an explosion test on four micro-sized pulverized coal powders. The results showed that the smaller the particle size, the smaller the lower explosion limit, and the explosion was more intense at the specified concentration. In summary, the research object of nano-sized dust explosion experiment was mainly metal dust, but the research on nano-sized coal dust has rarely been reported, mainly because nano-sized coal dust was relatively difficult to make. However, in the actual production process of underground coal mines, ultra-fine nano-sized coal dust often can be found. Therefore, five different particle size anthracites were studied in this paper, which were 800nm, 1200nm, 45µm, 60µm and 75µm, respectively. The standard 20L spherical explosion test device was used to analyze the dust explosion pressure and residual gas characteristics, and provide experimental theoretical support for the prevention and control of coal dust explosion prevention and accidents. 2 Experimental 2.1 Sample analysis 2.1.1 Proximate analysis The experimental samples are anthracites of the same degree of metamorphism from the same mining area, and the proximate analysis parameters are shown in Table 1. Before the test, the coal sample was placed in a constant temperature drying oven at 80 ℃ for drying and dehydration for 24 hours. Table 1 Proximate analysis of coal samples Characteristic index
Mad/%
Aad/%
Vad/%
FC/%
Test value
3.9
8.58
9.37
78.11
2.1.2 SEM characteristics analysis of samples The surface structure characteristics of coal dust with different particle sizes were obtained by scanning electron microscopy (SEM). The differences in surface characteristics of micro and nano-sized coal samples were analyzed. The test was performed using a Hitachi SU-8010 scanning electron microscope with a high-voltage limit (30 kV) resolution of 1.0 nm and low acceleration voltage imaging capability. The coal sample scanning test results are shown in Fig. 1.
（a）800 nm （×20 000）
（b）1200 nm （×2 0000）
（d）60 µm （×2 000）
（c）45 µm （×2 000）
（e）75 µm （×2 000）
Fig.1. SEM pictures of coal samples
It can be seen from Fig. 1 that by magnifying the nano-sized coal particles to 20,000 times, it can be clearly seen that nano dusts tend to agglomerate, and the angular edges of the surface particles were relatively unclear. When the micro-sized coal sample particles were enlarged to 2 000 times, the coal sample particles were relatively dispersed, and most of them were in the form of flakes, and the particle edges were relatively distinct. Overall, the surface of the nano-sized coal particles was relatively flat and smooth relative to the micro-sized one. Bu et al. (2020) also found that smaller particles exhibited stronger intermolecular forces that affect coagulation. 2.1.3 Analysis of particle size characteristics of samples The BT-9300LD laser particle size analyzer dry method was used to test the particle size distribution of coal samples. The median particle size D50, volume average diameter, area average diameter and specific surface area (SSA) of the five coal samples were obtained. The average particle size of the coal sample can be reflected by the median diameter D50. The test results of coal sample particle size are shown in Table 2. Table 2 Coal sample size test results No.
D50 /µm
D[4,3]
D[3,2]
SSA
volume average diameter
area average diameter
specific surface area
/µm
/µm
（m2/kg）
1
0.825
0.916
0.856
1467
2
1.207
1.312
1.268
1059
3
45.36
46.78
46.27
213
4
61.28
62.17
61.59
189
5
74.82
75.72
75.14
174
2.2 Experimental explosion test device The experiment was carried out on a 20L spherical explosion test system device, which mainly consisted of four parts: an explosive spherical container, a data acquisition and control system, a gas distribution system and an ignition system. The inner and outer walls of the spherical container were made of stainless steel double-layer structure, and the spherical container was provided with vacuuming, exhaust gas introduction and compressed air cleaning interface. The gas-powder two-phase valve was installed at the bottom of the container, and the experimenter can pneumatically control the gas-powder two-phase valve to realize the opening and closing of the
powder spraying valve. The technical parameter of the test device was that the working pressure of the container was 2 MPa, the dust dispersion pressure was 2.0 MPa, the available range of the piezoelectric sensor was 2.758 MPa, and the chemical ignition energy was 10 KJ. A schematic diagram of a 20 L spherical explosion vessel is shown in Fig. 2.
Ignition source
Water inlet
Pressure sensor Vacuumize Spray point Venting
Dispersion nozzle Water outlet
2MPa Pneumatic valve air source Electromagnetic valve
Fuel gas
Gas-powder two-phase valve
Air Spray the power
The powder storage tank
Charge Amplifier
Data Acquisition
2MPa Experimental gas source
Fig.2. Experimental setup
3 Analysis of coal dust explosion characteristics 3.1 Coal dust explosion pressure characteristics The mass concentrations of five different particle sizes (800 nm, 1200 nm, 45 µm, 60 µm, 75 µm) were 100 g/m , 250 g/m3, 500 g/m3, 750 g/m3, and 1000 g/m3, respectively. Using a precision balance (accuracy 1 mg), weigh 2 g, 5 g, 10 g, 15 g, 20 g of coal dust at the corresponding concentration. The explosion pressure-time curve for each coal dust concentration is shown in Fig. 3. 3
（a）c =100 g/m3
（d）c =750 g/m3
（b）c =250 g/m3
（c）c =500 g/m3
（e）c =1000 g/m3
Fig.3. Explosion pressure-time curve of coal dust with different concentrations in the 20L sphere
Fig. 3 (a) shows the pressure-time curve of five different particle sizes of coal dust when the concentration of coal dust in the 20L explosion vessel was 100 g/m3. It can be seen that the pressure-time curve basically increased first, and then reached the peak, then slowly decreased, and finally tended to a stable value. The maximum explosion pressure decreased with the increase of the particle size, and the maximum explosion pressures corresponding to the five particle sizes of 800 nm, 1200 nm, 45 µm, 60 µm, and 75 µm were 0.53 MPa, 0.50 MPa, 0.40 MPa, 0.29 MPa, and 0.28 MPa, respectively. The particle size of coal dust can significantly affect the explosion pressure of coal dust. The explosion pressure of nano-sized coal dust was larger than that of micron, and the time required to reach the maximum explosion pressure was also shorter than that of micron. It can be seen from Fig. 3(b)~(e) that the trend of the explosion pressure-time curve was basically the same as (a), and the maximum explosion pressure decreased first and then decreased with the concentration of coal dust. The maximum explosion pressures of 800 nm coal dust at 250 g/m3, 500 g/m3, 750 g/m3, and 1000 g/m3 were 0.68 MPa, 0.73 MPa, 0.66 MPa, and 0.64 MPa, respectively. 3.2 Maximum rate of pressure rise The pressure-time curve of coal dust explosion at different concentrations was obtained through experimental measurements. And the maximum rate of pressure rise was calculated according to the formula (1) from the pressure rise section curve after the explosion dt dp
m
(myers, 2008)：
dt = ti+1 -t i dp
P
m
-P
i+1 i
(1)
max
The maximum rate of pressure rise of coal dust explosion is shown in Fig. 4. The maximum rate of pressure rise of 800nm and 1200nm coal dust explosion were 87.37 MPa·s-1 and 75.69 MPa·s-1, respectively, while the maximum rate of pressure rise of 45µm coal dust explosion was 45.77 MPa·s-1. It can be seen that the maximum rate of pressure rise of nano-sized coal dust explosion was extremely fast, almost twice that of micron, and the maximum rate of pressure rise of nano-sized coal dust explosion was always greater than that of micron at the same concentration.
Fig.4. Relationship between maximum explosion pressure rate and concentration of coal dust
3.3 Coal dust explosion index characteristics Explosion index Kst was an important index parameter for studying the explosive strength of combustible and explosive materials, and was also the main basis for determining the hazard of coal dust explosion. The explosion index Kst can be calculated by formula (2) (Amyotte and Eckhoff, 2010; Abbasi and Abbasi, 2007): dp
Kst = dt
max
1
·V 3
(2)
dp
Where Kst was coal dust explosion index, MPa·m·s-1; dt
max
was coal dust maximum explosion pressure
rise rate, MPa·s-1; V was explosive container volume, m3,which was 0.02 m3. According to the relevant literature (Zhang, 1999), the dust explosion hazard grading standard corresponding to the Kst value is shown in Table 3. Table 3 Dust explosion hazard grading standard Kst
Levels of danger
Explosion hazard
0＜Kst＜20
St1
weak
20≤Kst＜30
St2
strong
Kst≥30
St3
serious
According to formula (2), the coal dust explosion hazard level and explosion characteristic parameters of a certain particle size can be obtained, as shown in Table 4. The coal dust explosion index of nanometer particle size was 22.35 MPa·m·s-1 and 20.55 MPa·m·s-1, respectively, between 20~30 MPa·m·s-1, and the explosion hazard level was St2, which was a strong level. The micron-sized coal dust explosion index was 12.42 MPa·m·s-1, and the explosion hazard level was St1, which was a weak level. Table 4 The characteristic parameters and hazard grade of coal dust explosion Particle
Pmax
(dP/dt)max
Kst
hazard
size
(MPa)
(MPa·s-1)
(MPa·m·s-1)
level
800 nm
0.73
87.37
22.35
St2
1200 nm
0.69
75.69
20.55
St2
40 µm
0.65
45.77
12.42
St1
60 µm
0.57
11.29
3.07
St1
75 µm
0.52
11.89
3.23
St1
3.4 Maximum coal dust explosion pressure characteristics As shown in Fig. 5, the maximum explosion pressure of coal dust varied significantly with particle size. At the same coal dust concentration, the maximum explosion pressure tended to decrease as the particle size increases. For example, the maximum pressure of an 800 nm coal dust explosion is almost 1.5 times that of an explosion with 75 µm particles. This is because within a certain range, a smaller particle size contributes to dispersion of the coal dust, allowing more coal dust to participate in the explosion (Eckhoff, 2009). Smaller particle sizes contribute more to volatilization (Li et al, 2016). Therefore, the coal dust particle size is inversely proportional to the explosion pressure.The maximum explosion pressure of coal dust with different particle sizes reached a maximum at a concentration of 500 g/m3 which indicating that the best explosive concentration of coal dust at five test concentrations was 500 g/m3.
Fig.5. The maximum pressure of coal dust explosion with different particle sizes varies with concentration
4 Analysis of residual gas characteristics after explosion 4.1 Coal dust combustion explosion reaction mechanism The uncontrollable intense combustion of coal dust in a confined space can form a detonation explosion(Dong et al., 2012). The combustion and explosion process was mainly divided into four stages: (1) Heating and water evaporation process, coal dust encounters high temperature heat source will transfer heat, and water is gradually evaporated; (2) Thermal decomposition reaction process, the reaction mainly produces non-combustible carbon dioxide and flammable hydrocarbons, hydrogen and other volatiles to form residual coke; (3) Burning of volatile matter and residual coke; (4) Formation of residue and gas after combustion is completed. The four stages of the coal dust explosion were not in sequence, and may occur in series and cross each other during the explosion reaction, or even synchronously (Li et al., 2018; Houim and Oran, 2015). 4.2 Analysis of residual O2 and CO concentration after coal dust explosion The gas after the 20 L spherical explosion test was collected in an aluminum foil gas sample bag, and the collected gas samples were chromatographed by a gas chromatograph. The gas components after coal dust explosion were mainly N2, O2, CO2, CO, CH4, etc. In addition, there were generally gases such as C2H6 and C2H4 that were not completely deflagrated (Liu et al., 2016; Deng et al., 2016). The gas composition and concentration after coal dust explosion coal dust concentration, type, degree of metamorphism, particle size and ignition energy were related (Liu et al., 2017; Man and Gibbins, 2011). The relationship with the particle size of coal dust was generally that the smaller the particle size, the lower the CO2 content after the explosion, and the higher the CO content. From the perspective of accidental evidence analysis after the explosion, the main causes of gas products causing casualties were: lack of oxygen, asphyxiation and poisoning. Therefore, the two gas concentrations of O2 and CO were analyzed separately. The concentration of residual gas after the explosion was analyzed, the O2 concentration in the gas was much lower than the O2 concentration in the air, which is about 8% to 14%. According to the measured data of O2 concentration, the O2 concentration in the gas after coal dust explosion is shown in Fig. 6.We found that under the same particle size, the higher the coal dust concentration, the more oxygen was consumed during the explosion reaction process, and the less residual oxygen, the lower the O2 concentration after the explosion. At the same concentration, the smaller the particle size, the larger the surface area of the particles in contact with oxygen per unit volume, the more complete the explosion reaction, and the lower the O2 concentration after the explosion. The concentration of O2 in the gas after the nano-sized coal dust explosion was generally lower than that of the micron.
Fig.6. O2 concentration in gas after coal dust explosion
The CO concentration in the residual gas after the explosion is shown in Fig. 7. The CO concentration was at least 376.27 ppm and the highest was 3184.73 ppm. At the same particle size, the CO concentration in the residual gas after the explosion increased as the concentration of coal dust increased. The reason was that the coal dust concentration was larger, the coal dust particles in the unit explosion space volume increased, the oxygen content was limited, and the combination with oxygen was insufficient, resulting in incomplete combustion of coal dust.
Fig.7. CO concentration in gas after coal dust explosion
4.3 Analysis of CO/CO2 ratio after coal dust explosion Scholars have pointed out that the degree of explosive reaction can be reflected by the ratio of CO and CO2 in the residual gas. The higher the ratio, the less complete the explosion reaction (Liu et al., 2015). According to the chromatographic analysis, the ratio of CO and CO2 concentration can be plotted as Fig. 8. It can be seen from Fig. 8 that the CO content in the residual gas after explosion was generally greater than the CO2 content. When the coal dust concentration was constant, the smaller the coal dust particle size, the more severe the explosion reaction. , the lower the ratio. When the particle size of coal dust was constant, the higher the coal dust concentration, the less the reaction, and the ratio increased.
Fig.8. CO/CO2 of residual gas after explosion
5 Conclusions (1) The explosive characteristics of coal dust in 20 L spheres were significantly related to particle size and concentration. The maximum explosion pressure of the nano-sized coal dust was greater than the micron, and the time required for the nano-sized coal dust to reach the maximum explosion pressure was less than the micron. The explosion pressure of nano-sized coal dust at the same concentration, the maximum rate of pressure rise, and the
explosion index were all larger than those of micro-sized coal dust, indicating that nano-sized coal dust was more explosive. (2) Among the five different coal dust concentrations in this experiment, the optimal explosion concentration of nano-sized coal dust was 500g/m3, the maximum explosion pressure was 0.73MPa, the maximum explosion pressure rate was 87.37 MPa· s-1, and the explosion index Kst was 26.72 MPa· m· s-1. The level of nano-sized coal dust explosion was a strong level ,while micro-sized coal dust was a weake level. (3) Comparing the concentration of residual gas O2 and CO after explosion, it can be seen that under the same coal dust concentration, the O2 and CO concentrations after the nano-sized dust explosion were lower than the micron. And the ratio of CO/CO2 content after nano-sized coal dust explosion was also less than the micron. The reason for this analysis was that the nano-sized coal dust has a small particle size, a large specific surface area, more oxygen consumption, and a more thorough explosion reaction degree. Acknowledgements This research was supported by the National Key Research and Development Program of China (2016YFC0801800) and the National Nature Science Foundation of China (51774291、51864045). References Bai, C.H., Gong, G.D., Liu, Q.M., Chen, Y.H., Niu, G.T., 2011. The explosion overpressure field and flame propagation of methane/air and methane/coal dust/air mixtures. Safety Science. 49,1349-1354. https://doi.org/10.1016/j.ssci.2011.05.005. Liu, Q.M., Bai, C.H., Li, X.D., Jiang, L., Dai, W.X., 2010. Coal dust/air explosions in a large-scale tube. Fuel. 89,329-335. https://doi.org/10.1016/j.fuel.2009.07.010. Combustible Dust Hazard Study, 2006, Investigation Report-US. Chemical Safety and Hazard Investigation Board. Di Benedetto, A., Russo, P., Sanchirico, R., Di Sarli, V., 2013. CFD simulations of turbulent fluid flow and dust dispersion in the 20 liter explosion vessel. AIChE Journal. 59,2485-2496. https://doi.org/10.1002/aic.14029. Cao,W.G., Cao,W., Peng, Y.H., Qiu, S.S., Miao, N., Pan, F., 2015. Experimental study on the combustion sensitivity parameters and pre-combusted changes in functional groupsof lignite coal dust. Powder Technol. 283,512-518. https://doi.org/10.1016/j.powtec.2015.06.025 Ye, Q., Jia, Z.Z., 2014. Effect of the bifurcating duct on the gas explosion propagation characteristics. Combustion, Explosion, and Shock Waves. 50,424-428. https://doi.org/10.1134/s0010508214040108. Xin, H.H., Wang, D.M., Dou,G.L., Qi, X.Y., Xu,T., Qi,G.S., 2014. The infrared characterization and mechanism of oxygen adsorption in coal. Spectroscopy Letters. 47 ,664-675. https://doi.org/10.1080/00387010.2013.833940. Cao, W.G., Qin, Q.F., Cao, W., Lan, Y.H., Chen, T., Xu, S., Cao, X., 2017. Experimental and numerical studies on the explosion severities
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Highlights 1. The time required for the nano-sized coal dust to reach the maximum explosion pressure was less than the micron. 2. The best explosive concentration of coal dust at five test concentrations was 500 g/m3. 3. The explosion hazard level of nano-sized coal dust was a strong level, while micro-sized coal dust was a weak level. 4. The concentration of residual gas after nano-coal explosion was less than the micron.
Analysis of explosion pressure and residual gas characteristics of micro-nano coal dust in confined space
Bo Tan: Conceptualization, Methodology, Funding acquisition. Zhuangzhuang Shao: Investigation, Writing- Reviewing and Editing Bin Xu: Data curation, Writing- Original draft preparation. Hongyi Wei: Software, Visualization. Tian Wang: Supervision
Declaration of interests √ The authors declare that they have no known competing financial interests or personal relationships □ that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0950-4230(19)30541-8
DOI:
https://doi.org/10.1016/j.jlp.2020.104056
Reference:
JLPP 104056
To appear in:
Journal of Loss Prevention in the Process Industries
Received Date: 30 June 2019 Revised Date:
31 December 2019
Accepted Date: 20 January 2020
Please cite this article as: Tan, B., Shao, Z., Xu, B., Wei, H., Wang, T., Analysis of explosion pressure and residual gas characteristics of micro-nano coal dust in confined space, Journal of Loss Prevention in the Process Industries (2020), doi: https://doi.org/10.1016/j.jlp.2020.104056. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Analysis of explosion pressure and residual gas characteristics of micro-nano coal dust in confined space1 Bo Tan, Zhuangzhuang Shao, Bin Xu*, Hongyi Wei, Tian Wang College of Emergency Management and Safety Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China Abstracts: In order to study the explosion characteristics of micro and nano-sized coal dust, a 20L spherical explosion test device was used to compare and analyze the explosion pressure characteristics and residual gas characteristics of five kinds of micro-nano particle dusts of 800nm, 1200nm, 45µm, 60µm and 75µm under five concentrations (100g/m3、250g/m3、500g/m3、750g/m3、1000g/m3) .The results showed that the explosion pressure characteristics were significantly related to the particle size and concentration. The time required for the nano-sized coal dust to reach the maximum explosion pressure was less than the micron. Among the five different coal dust concentrations, the best explosion concentration was 500g/m3, the maximum explosion pressure of 800 nm coal dust was 0.73MPa, the maximum explosion pressure rate was 87.37 MPa·s-1, and the explosion index Kst was 22.35 MPa·m·s-1. The explosion hazard level of nano-sized coal dust was a strong level, while micro-sized coal dust was a weak level. Under the same coal dust concentration, the concentrations of O2, CO2 and CO after the nano-sized coal dust explosion were lower than the micro-sized.When the coal dust concentration is 500g/m3,the O2 and CO concentrations after 800nm coal dust explosion are 9.3% and 503ppm, respectively. However, the concentrations of O2 and CO after the 45um coal dust explosion were 10.6% and 2008ppm. The CO/CO2 ratio after the nano-sized coal dust explosion was less than the micron. Key words: Confined space; nanoscale; coal dust explosion; explosion pressure characteristics; residual gas 1.Introduction Mine dust can not only cause occupational diseases, but dust explosions will produce shock waves and a lot of harmful gases, which seriously threaten people's life and health and the safety production (Bai et al., 2011; Liu et al., 2010). For example, in the "1·12" major coal dust explosion accident at Lijiagou Coal Mine in Shanxi Province, China in 2019, 21 people were killed and the losses were heavy, and the impact was extremely bad. According to the CSB Investigation Report (Chemical SB et al., 2006; Di Benedetto et al., 2013), 281 major combustible dust accidents occurred between 1980 and 2005, resulting in 718 people injured, 119 people killed,and significant damage to industrial facilities. In the period of 2001–2015, the coal industry had significant problems resulting from dust explosions (Cao et al., 2015; Ye et al., 2014; Xin et al., 2014).The coal dust explosion law is studied by studying the characteristics of coal dust explosion and residual gas characteristics (Cao et al., 2017), which has important guiding significance for preventing coal dust explosion and ensuring coal mine safety. In the process of dust explosion (Bi et al., 2012; Ye et al., 2017), dust can be divided into two types: one is combustible dust, and the other is non-combustible dust.The combustible dust will explode when it meets the heat source, and the flame will spread quickly and spread to the whole space where the dust exists.The reaction rate is extremely fast, and a large amount of heat is released at the same time, resulting in high temperature and pressure, and has a strong destructive force. Mittal (2014) used 20 L spherical explosion vessel to study the explosion characteristics of micro and nano-sized magnesium powder. It was found that the maximum explosion pressure 1
∗ Corresponding author. E-mail address: [email protected] (B. Xu).
and detonation index of magnesium powder in the range of 30 nm-125 µm were roughly symmetrically distributed at 400 nm. Turkevich et al. (2015) studied the characteristics of minimum ignition energy and explosion pressure of various carbon nanopowders through the explosion experiments of nanomaterials such as graphite and graphene. Li et al. (2011) concluded that the effect of aluminum powder particle size on the maximum explosion pressure and explosion pressure rate was relatively small in the nanometer scale by the explosion experiment of aluminum powder with different particle sizes in a 20L sphere. Dufaud et al. (2010) studied the effect of dust particle size on the explosion of aluminum powder through a 20L spherical explosion experiment. The results showed that the maximum explosion pressure of the micro-sized aluminum powder, especially the maximum pressure rise rate, depended largely on the dust particle size. Li et al. (2010) used a 20L spherical explosion test device to experimentally study the explosion characteristics of three kinds of nanometer aluminum powders. It was found that the maximum explosion pressure and maximum pressure rise rate of nano-sized aluminum powder were affected by dust concentration. Cao et al. (2017) used 20L spherical explosion experiment to study the explosion parameters of coal dust. The results showed that the explosion pressure and explosion pressure rise rate of 10-100 µm coal dust increaseed with the increase of coal dust concentration. Li et al. (2016) studied the effect of particle size and dispersion on the severity of coal dust explosion. The results showed that the maximum explosion pressure increased with the decrease of dispersion and particle size. Gao et al. (2010) used a 20L spherical explosion test device to carry out an explosion test on four micro-sized pulverized coal powders. The results showed that the smaller the particle size, the smaller the lower explosion limit, and the explosion was more intense at the specified concentration. In summary, the research object of nano-sized dust explosion experiment was mainly metal dust, but the research on nano-sized coal dust has rarely been reported, mainly because nano-sized coal dust was relatively difficult to make. However, in the actual production process of underground coal mines, ultra-fine nano-sized coal dust often can be found. Therefore, five different particle size anthracites were studied in this paper, which were 800nm, 1200nm, 45µm, 60µm and 75µm, respectively. The standard 20L spherical explosion test device was used to analyze the dust explosion pressure and residual gas characteristics, and provide experimental theoretical support for the prevention and control of coal dust explosion prevention and accidents. 2 Experimental 2.1 Sample analysis 2.1.1 Proximate analysis The experimental samples are anthracites of the same degree of metamorphism from the same mining area, and the proximate analysis parameters are shown in Table 1. Before the test, the coal sample was placed in a constant temperature drying oven at 80 ℃ for drying and dehydration for 24 hours. Table 1 Proximate analysis of coal samples Characteristic index
Mad/%
Aad/%
Vad/%
FC/%
Test value
3.9
8.58
9.37
78.11
2.1.2 SEM characteristics analysis of samples The surface structure characteristics of coal dust with different particle sizes were obtained by scanning electron microscopy (SEM). The differences in surface characteristics of micro and nano-sized coal samples were analyzed. The test was performed using a Hitachi SU-8010 scanning electron microscope with a high-voltage limit (30 kV) resolution of 1.0 nm and low acceleration voltage imaging capability. The coal sample scanning test results are shown in Fig. 1.
（a）800 nm （×20 000）
（b）1200 nm （×2 0000）
（d）60 µm （×2 000）
（c）45 µm （×2 000）
（e）75 µm （×2 000）
Fig.1. SEM pictures of coal samples
It can be seen from Fig. 1 that by magnifying the nano-sized coal particles to 20,000 times, it can be clearly seen that nano dusts tend to agglomerate, and the angular edges of the surface particles were relatively unclear. When the micro-sized coal sample particles were enlarged to 2 000 times, the coal sample particles were relatively dispersed, and most of them were in the form of flakes, and the particle edges were relatively distinct. Overall, the surface of the nano-sized coal particles was relatively flat and smooth relative to the micro-sized one. Bu et al. (2020) also found that smaller particles exhibited stronger intermolecular forces that affect coagulation. 2.1.3 Analysis of particle size characteristics of samples The BT-9300LD laser particle size analyzer dry method was used to test the particle size distribution of coal samples. The median particle size D50, volume average diameter, area average diameter and specific surface area (SSA) of the five coal samples were obtained. The average particle size of the coal sample can be reflected by the median diameter D50. The test results of coal sample particle size are shown in Table 2. Table 2 Coal sample size test results No.
D50 /µm
D[4,3]
D[3,2]
SSA
volume average diameter
area average diameter
specific surface area
/µm
/µm
（m2/kg）
1
0.825
0.916
0.856
1467
2
1.207
1.312
1.268
1059
3
45.36
46.78
46.27
213
4
61.28
62.17
61.59
189
5
74.82
75.72
75.14
174
2.2 Experimental explosion test device The experiment was carried out on a 20L spherical explosion test system device, which mainly consisted of four parts: an explosive spherical container, a data acquisition and control system, a gas distribution system and an ignition system. The inner and outer walls of the spherical container were made of stainless steel double-layer structure, and the spherical container was provided with vacuuming, exhaust gas introduction and compressed air cleaning interface. The gas-powder two-phase valve was installed at the bottom of the container, and the experimenter can pneumatically control the gas-powder two-phase valve to realize the opening and closing of the
powder spraying valve. The technical parameter of the test device was that the working pressure of the container was 2 MPa, the dust dispersion pressure was 2.0 MPa, the available range of the piezoelectric sensor was 2.758 MPa, and the chemical ignition energy was 10 KJ. A schematic diagram of a 20 L spherical explosion vessel is shown in Fig. 2.
Ignition source
Water inlet
Pressure sensor Vacuumize Spray point Venting
Dispersion nozzle Water outlet
2MPa Pneumatic valve air source Electromagnetic valve
Fuel gas
Gas-powder two-phase valve
Air Spray the power
The powder storage tank
Charge Amplifier
Data Acquisition
2MPa Experimental gas source
Fig.2. Experimental setup
3 Analysis of coal dust explosion characteristics 3.1 Coal dust explosion pressure characteristics The mass concentrations of five different particle sizes (800 nm, 1200 nm, 45 µm, 60 µm, 75 µm) were 100 g/m , 250 g/m3, 500 g/m3, 750 g/m3, and 1000 g/m3, respectively. Using a precision balance (accuracy 1 mg), weigh 2 g, 5 g, 10 g, 15 g, 20 g of coal dust at the corresponding concentration. The explosion pressure-time curve for each coal dust concentration is shown in Fig. 3. 3
（a）c =100 g/m3
（d）c =750 g/m3
（b）c =250 g/m3
（c）c =500 g/m3
（e）c =1000 g/m3
Fig.3. Explosion pressure-time curve of coal dust with different concentrations in the 20L sphere
Fig. 3 (a) shows the pressure-time curve of five different particle sizes of coal dust when the concentration of coal dust in the 20L explosion vessel was 100 g/m3. It can be seen that the pressure-time curve basically increased first, and then reached the peak, then slowly decreased, and finally tended to a stable value. The maximum explosion pressure decreased with the increase of the particle size, and the maximum explosion pressures corresponding to the five particle sizes of 800 nm, 1200 nm, 45 µm, 60 µm, and 75 µm were 0.53 MPa, 0.50 MPa, 0.40 MPa, 0.29 MPa, and 0.28 MPa, respectively. The particle size of coal dust can significantly affect the explosion pressure of coal dust. The explosion pressure of nano-sized coal dust was larger than that of micron, and the time required to reach the maximum explosion pressure was also shorter than that of micron. It can be seen from Fig. 3(b)~(e) that the trend of the explosion pressure-time curve was basically the same as (a), and the maximum explosion pressure decreased first and then decreased with the concentration of coal dust. The maximum explosion pressures of 800 nm coal dust at 250 g/m3, 500 g/m3, 750 g/m3, and 1000 g/m3 were 0.68 MPa, 0.73 MPa, 0.66 MPa, and 0.64 MPa, respectively. 3.2 Maximum rate of pressure rise The pressure-time curve of coal dust explosion at different concentrations was obtained through experimental measurements. And the maximum rate of pressure rise was calculated according to the formula (1) from the pressure rise section curve after the explosion dt dp
m
(myers, 2008)：
dt = ti+1 -t i dp
P
m
-P
i+1 i
(1)
max
The maximum rate of pressure rise of coal dust explosion is shown in Fig. 4. The maximum rate of pressure rise of 800nm and 1200nm coal dust explosion were 87.37 MPa·s-1 and 75.69 MPa·s-1, respectively, while the maximum rate of pressure rise of 45µm coal dust explosion was 45.77 MPa·s-1. It can be seen that the maximum rate of pressure rise of nano-sized coal dust explosion was extremely fast, almost twice that of micron, and the maximum rate of pressure rise of nano-sized coal dust explosion was always greater than that of micron at the same concentration.
Fig.4. Relationship between maximum explosion pressure rate and concentration of coal dust
3.3 Coal dust explosion index characteristics Explosion index Kst was an important index parameter for studying the explosive strength of combustible and explosive materials, and was also the main basis for determining the hazard of coal dust explosion. The explosion index Kst can be calculated by formula (2) (Amyotte and Eckhoff, 2010; Abbasi and Abbasi, 2007): dp
Kst = dt
max
1
·V 3
(2)
dp
Where Kst was coal dust explosion index, MPa·m·s-1; dt
max
was coal dust maximum explosion pressure
rise rate, MPa·s-1; V was explosive container volume, m3,which was 0.02 m3. According to the relevant literature (Zhang, 1999), the dust explosion hazard grading standard corresponding to the Kst value is shown in Table 3. Table 3 Dust explosion hazard grading standard Kst
Levels of danger
Explosion hazard
0＜Kst＜20
St1
weak
20≤Kst＜30
St2
strong
Kst≥30
St3
serious
According to formula (2), the coal dust explosion hazard level and explosion characteristic parameters of a certain particle size can be obtained, as shown in Table 4. The coal dust explosion index of nanometer particle size was 22.35 MPa·m·s-1 and 20.55 MPa·m·s-1, respectively, between 20~30 MPa·m·s-1, and the explosion hazard level was St2, which was a strong level. The micron-sized coal dust explosion index was 12.42 MPa·m·s-1, and the explosion hazard level was St1, which was a weak level. Table 4 The characteristic parameters and hazard grade of coal dust explosion Particle
Pmax
(dP/dt)max
Kst
hazard
size
(MPa)
(MPa·s-1)
(MPa·m·s-1)
level
800 nm
0.73
87.37
22.35
St2
1200 nm
0.69
75.69
20.55
St2
40 µm
0.65
45.77
12.42
St1
60 µm
0.57
11.29
3.07
St1
75 µm
0.52
11.89
3.23
St1
3.4 Maximum coal dust explosion pressure characteristics As shown in Fig. 5, the maximum explosion pressure of coal dust varied significantly with particle size. At the same coal dust concentration, the maximum explosion pressure tended to decrease as the particle size increases. For example, the maximum pressure of an 800 nm coal dust explosion is almost 1.5 times that of an explosion with 75 µm particles. This is because within a certain range, a smaller particle size contributes to dispersion of the coal dust, allowing more coal dust to participate in the explosion (Eckhoff, 2009). Smaller particle sizes contribute more to volatilization (Li et al, 2016). Therefore, the coal dust particle size is inversely proportional to the explosion pressure.The maximum explosion pressure of coal dust with different particle sizes reached a maximum at a concentration of 500 g/m3 which indicating that the best explosive concentration of coal dust at five test concentrations was 500 g/m3.
Fig.5. The maximum pressure of coal dust explosion with different particle sizes varies with concentration
4 Analysis of residual gas characteristics after explosion 4.1 Coal dust combustion explosion reaction mechanism The uncontrollable intense combustion of coal dust in a confined space can form a detonation explosion(Dong et al., 2012). The combustion and explosion process was mainly divided into four stages: (1) Heating and water evaporation process, coal dust encounters high temperature heat source will transfer heat, and water is gradually evaporated; (2) Thermal decomposition reaction process, the reaction mainly produces non-combustible carbon dioxide and flammable hydrocarbons, hydrogen and other volatiles to form residual coke; (3) Burning of volatile matter and residual coke; (4) Formation of residue and gas after combustion is completed. The four stages of the coal dust explosion were not in sequence, and may occur in series and cross each other during the explosion reaction, or even synchronously (Li et al., 2018; Houim and Oran, 2015). 4.2 Analysis of residual O2 and CO concentration after coal dust explosion The gas after the 20 L spherical explosion test was collected in an aluminum foil gas sample bag, and the collected gas samples were chromatographed by a gas chromatograph. The gas components after coal dust explosion were mainly N2, O2, CO2, CO, CH4, etc. In addition, there were generally gases such as C2H6 and C2H4 that were not completely deflagrated (Liu et al., 2016; Deng et al., 2016). The gas composition and concentration after coal dust explosion coal dust concentration, type, degree of metamorphism, particle size and ignition energy were related (Liu et al., 2017; Man and Gibbins, 2011). The relationship with the particle size of coal dust was generally that the smaller the particle size, the lower the CO2 content after the explosion, and the higher the CO content. From the perspective of accidental evidence analysis after the explosion, the main causes of gas products causing casualties were: lack of oxygen, asphyxiation and poisoning. Therefore, the two gas concentrations of O2 and CO were analyzed separately. The concentration of residual gas after the explosion was analyzed, the O2 concentration in the gas was much lower than the O2 concentration in the air, which is about 8% to 14%. According to the measured data of O2 concentration, the O2 concentration in the gas after coal dust explosion is shown in Fig. 6.We found that under the same particle size, the higher the coal dust concentration, the more oxygen was consumed during the explosion reaction process, and the less residual oxygen, the lower the O2 concentration after the explosion. At the same concentration, the smaller the particle size, the larger the surface area of the particles in contact with oxygen per unit volume, the more complete the explosion reaction, and the lower the O2 concentration after the explosion. The concentration of O2 in the gas after the nano-sized coal dust explosion was generally lower than that of the micron.
Fig.6. O2 concentration in gas after coal dust explosion
The CO concentration in the residual gas after the explosion is shown in Fig. 7. The CO concentration was at least 376.27 ppm and the highest was 3184.73 ppm. At the same particle size, the CO concentration in the residual gas after the explosion increased as the concentration of coal dust increased. The reason was that the coal dust concentration was larger, the coal dust particles in the unit explosion space volume increased, the oxygen content was limited, and the combination with oxygen was insufficient, resulting in incomplete combustion of coal dust.
Fig.7. CO concentration in gas after coal dust explosion
4.3 Analysis of CO/CO2 ratio after coal dust explosion Scholars have pointed out that the degree of explosive reaction can be reflected by the ratio of CO and CO2 in the residual gas. The higher the ratio, the less complete the explosion reaction (Liu et al., 2015). According to the chromatographic analysis, the ratio of CO and CO2 concentration can be plotted as Fig. 8. It can be seen from Fig. 8 that the CO content in the residual gas after explosion was generally greater than the CO2 content. When the coal dust concentration was constant, the smaller the coal dust particle size, the more severe the explosion reaction. , the lower the ratio. When the particle size of coal dust was constant, the higher the coal dust concentration, the less the reaction, and the ratio increased.
Fig.8. CO/CO2 of residual gas after explosion
5 Conclusions (1) The explosive characteristics of coal dust in 20 L spheres were significantly related to particle size and concentration. The maximum explosion pressure of the nano-sized coal dust was greater than the micron, and the time required for the nano-sized coal dust to reach the maximum explosion pressure was less than the micron. The explosion pressure of nano-sized coal dust at the same concentration, the maximum rate of pressure rise, and the
explosion index were all larger than those of micro-sized coal dust, indicating that nano-sized coal dust was more explosive. (2) Among the five different coal dust concentrations in this experiment, the optimal explosion concentration of nano-sized coal dust was 500g/m3, the maximum explosion pressure was 0.73MPa, the maximum explosion pressure rate was 87.37 MPa· s-1, and the explosion index Kst was 26.72 MPa· m· s-1. The level of nano-sized coal dust explosion was a strong level ,while micro-sized coal dust was a weake level. (3) Comparing the concentration of residual gas O2 and CO after explosion, it can be seen that under the same coal dust concentration, the O2 and CO concentrations after the nano-sized dust explosion were lower than the micron. And the ratio of CO/CO2 content after nano-sized coal dust explosion was also less than the micron. The reason for this analysis was that the nano-sized coal dust has a small particle size, a large specific surface area, more oxygen consumption, and a more thorough explosion reaction degree. Acknowledgements This research was supported by the National Key Research and Development Program of China (2016YFC0801800) and the National Nature Science Foundation of China (51774291、51864045). References Bai, C.H., Gong, G.D., Liu, Q.M., Chen, Y.H., Niu, G.T., 2011. The explosion overpressure field and flame propagation of methane/air and methane/coal dust/air mixtures. Safety Science. 49,1349-1354. https://doi.org/10.1016/j.ssci.2011.05.005. Liu, Q.M., Bai, C.H., Li, X.D., Jiang, L., Dai, W.X., 2010. Coal dust/air explosions in a large-scale tube. Fuel. 89,329-335. https://doi.org/10.1016/j.fuel.2009.07.010. Combustible Dust Hazard Study, 2006, Investigation Report-US. Chemical Safety and Hazard Investigation Board. Di Benedetto, A., Russo, P., Sanchirico, R., Di Sarli, V., 2013. CFD simulations of turbulent fluid flow and dust dispersion in the 20 liter explosion vessel. AIChE Journal. 59,2485-2496. https://doi.org/10.1002/aic.14029. Cao,W.G., Cao,W., Peng, Y.H., Qiu, S.S., Miao, N., Pan, F., 2015. Experimental study on the combustion sensitivity parameters and pre-combusted changes in functional groupsof lignite coal dust. Powder Technol. 283,512-518. https://doi.org/10.1016/j.powtec.2015.06.025 Ye, Q., Jia, Z.Z., 2014. Effect of the bifurcating duct on the gas explosion propagation characteristics. Combustion, Explosion, and Shock Waves. 50,424-428. https://doi.org/10.1134/s0010508214040108. Xin, H.H., Wang, D.M., Dou,G.L., Qi, X.Y., Xu,T., Qi,G.S., 2014. The infrared characterization and mechanism of oxygen adsorption in coal. Spectroscopy Letters. 47 ,664-675. https://doi.org/10.1080/00387010.2013.833940. Cao, W.G., Qin, Q.F., Cao, W., Lan, Y.H., Chen, T., Xu, S., Cao, X., 2017. Experimental and numerical studies on the explosion severities
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Highlights 1. The time required for the nano-sized coal dust to reach the maximum explosion pressure was less than the micron. 2. The best explosive concentration of coal dust at five test concentrations was 500 g/m3. 3. The explosion hazard level of nano-sized coal dust was a strong level, while micro-sized coal dust was a weak level. 4. The concentration of residual gas after nano-coal explosion was less than the micron.
Analysis of explosion pressure and residual gas characteristics of micro-nano coal dust in confined space
Bo Tan: Conceptualization, Methodology, Funding acquisition. Zhuangzhuang Shao: Investigation, Writing- Reviewing and Editing Bin Xu: Data curation, Writing- Original draft preparation. Hongyi Wei: Software, Visualization. Tian Wang: Supervision
Declaration of interests √ The authors declare that they have no known competing financial interests or personal relationships □ that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: