Experimental investigation of the inerting effect of CO2 on explosion characteristics of micron-size Acrylate Copolymer dust

Journal Pre-proof Experimental investigation of the inerting effect of CO2 on explosion characteristics of micron-size Acrylate Copolymer dust Jie Yang, Yunhao Li, Yuan Yu, Qingwu Zhang, Liju Zheng, Yifan Suo, Juncheng Jiang PII:

S0950-4230(19)30509-1

DOI:

https://doi.org/10.1016/j.jlp.2019.103979

Reference:

JLPP 103979

To appear in:

Journal of Loss Prevention in the Process Industries

Received Date: 17 June 2019 Revised Date:

17 September 2019

Accepted Date: 6 October 2019

Please cite this article as: Yang, J., Li, Y., Yu, Y., Zhang, Q., Zheng, L., Suo, Y., Jiang, J., Experimental investigation of the inerting effect of CO2 on explosion characteristics of micron-size Acrylate Copolymer dust, Journal of Loss Prevention in the Process Industries (2019), doi: https://doi.org/10.1016/ j.jlp.2019.103979. 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. © 2019 Published by Elsevier Ltd.

Experimental investigation of the inerting effect of CO2 on explosion characteristics of micron-size Acrylate Copolymer dust Jie Yang a, Yunhao Li a, Yuan Yu1,a,b, Qingwu Zhang a, Liju Zheng a, Yifan Suo a

,Juncheng Jiang b

a

College of Safety Science and Engineering, Nanjing Tech University, Nanjing, 211800, China

b

Jiangsu Key Laboratory of Hazardous Chemicals Safety and Control, Nanjing Tech University,

Nanjing, 211800, China.

ABSTRACT To investigate the inerting effect of CO2 on micron-size Acrylate Copolymer (ACR) dust explosion, a standard 20-L spherical chamber, with a modified 1.2-L Hartmann tube and a Godbert-Greenwald furnace was adopted to determine the explosion severity and the ignition sensitivity of ACR dust under various CO2 concentrations (φ). The results indicated that the explosion severity and the ignition sensitivity of ACR dust decreased gradually as φ increased. When φ increased to 40 vol.%, decay rates of maximum explosion pressure (Pmax) and dust explosibility index (Kst) were 48.1% and 58.4%, respectively. Moreover, when φ increased to 30 and 40 vol.%, the minimum ignition energy (MIE) of ACR dust cloud increased respectively from 10 mJ to 207 and 267 mJ, where ACR dust had low sensitivity to ignition. Furthermore, the minimum ignition temperature (MIT) of ACR dust cloud was 460 °C. The MIT of ACR dust cloud increased as φ increased. The MIT increased by 50 °C when φ increased to 40 vol.%. The inerting mechanism of CO2 was that the chain reaction was interrupted or decelerated by CO2. Besides, the carbon layer formed by carbon residues prevented the heat from transferring to the internal of ACR particles.

Keywords: CO2；Acrylates Copolymer dust explosion；Explosion severity；Ignition sensitivity； Inerting mechanism 1

Correspondence to Y0uan Yu ([email protected]); Phone: 86-25- 8323 9965; Fax: 86-25-8323 9965 1

1.

Introduction

Dust explosion accidents occur frequently throughout the world, causing casualties, serious environmental pollution and huge property damage (Li et al., 2016). Table 1 lists several serious disasters caused by polymer dust explosions in the processing industry (Abbasi and Abbasi, 2007; Yuan et al., 2015; Combustible Dust Incident Database). Therefore, it is crucial to further study the explosion parameters of combustible polymer dust for preventing and controlling such industrial disasters. To reduce the risk of dust explosions, many investigations have been performed focusing on their prevention and mitigation, such as explosion venting, isolation and suppression (Amyotte 2006). Nie et al. (2011) found that foam ceramics can be developed into an isolation technology against multiple and continuous gas explosions in coal mines. Gao et al. (2016) studied the venting characteristics between microand nano-PMMA dust explosions. The results indicated that the venting effectiveness of micro-particles was superior to nano-particles. Chen et al. (2017) found that sodium bicarbonate with different granulometric distribution had a suppressing effect on the flame of aluminum dust explosion. Yu et al. (2018) experimentally demonstrated that the crystalline II type Ammonium Polyphosphate (APP-II) had an inerting effect on explosion characteristics of micron-size Acrylate Copolymer (ACR) dust. The results indicated that the explosion of ACR dust can be inerted completely by 80wt% APP-II. Jiang et al. (2018) found that the minimum inerting concentration significantly increased as the aluminum particle size decreased from 30 to 5 µm. CO2 has been identified as an effective inertant with the advantage of low-cost and environmental friendliness. Ma et al. (2010) found the explosion suppression effect of CO2 was greater than that of N2 under the same conditions. Luo et al. (2014) experimentally demonstrated that cooperative synergism exists between ABC powder and CO2. Pei et al. (2017) investigated the synergistic inhibition effects by CO2 with ultrafine water mist on methane/air explosion. Cui et al. (2018) found that the explosion suppression performance of the methane and air mixture increased gradually with the increase of CO2. 2

Li et al. (2009) were the first to test the inerting effect of CO2 on dust explosions. The results of their research showed that CO2 had a certain inerting effect on magnesium dust explosion. However, so far, the inerting effect of CO2 on polymer dust explosion has not been investigated. As a result, CO2 was selected as the inert agent in this study. According to Yu et al. (2018), an ACR dust explosion can trigger great explosion power. Due to the explosion risk of ACR dust, in designing the plants for producing and processing ACR products, preventive and mitigative measures for ACR dust explosion must be seriously considered. In the present study, the inerting effect of CO2 on the maximum explosion pressure (Pmax), the explosibility index (Kst), the minimum explosion concentration (MEC), the minimum ignition energy (MIE) and the minimum ignition temperature (MIT) of ACR dust explosion were tested respectively. Moreover, the inerting mechanism was analyzed on the basis of the chain reaction and thermogravimetry (TG) tests.

3

2.

Methodology

2.1. Experimental apparatus for explosion severity parameters Experiments were performed in a standard 20-L spherical chamber (Fig. 1) according to International Standard ISO 6184/1. (International Organization for Standardization, 1985). The test system consisted of a 20-L stainless steel spherical chamber, a dust dispersion system, an ignition system, a data acquisition system unit, and a time controller. The ACR dust was stored in a 0.6-L container at a pressure of 20 bar. The container was connected to a fast-acting valve, which was mounted under the bottom of the 20-L chamber. After the injection of ACR powder through the nozzle, the ACR powder suspension was ignited by a centrally mounted chemical igniter, after a 60 ms delay to create a quasi-uniform dust cloud and allow the initial turbulence to decay. Fig. 2 shows a typical plot of pressure versus time during ACR dust explosion. The Most MEC measurements were also made in a standard 20-L spherical chamber (Fig. 1) according to BS EN 14034-3:2006 (British Standards Institution, 2006). Before the explosion test, a pre-weighed amount of dust was placed in the dust container which was pressurized up to 20 bar (gage) by compressed air, and the safely sealed explosion vessel was vacuumed in part to 0.4 bar (absolute). Then, the dust was discharged into the vessel via the nozzle to form dust cloud. Finally, after a time delay of 60 ms, the dust cloud was ignited by the centrally mounted ignitor. An ignition of the dust (dust explosion) will be considered to have taken place, when the measured overpressure (influence of chemical igniters included) relative to the initial pressure pi is ≥ 0.5 bar [pex ≥ (pi + 0.5 bar)]. Moreover, the igniter was composed of zirconium, barium peroxide and barium nitrate, with a mass ratio of 4:3:3. The ignition energy of the 0.48 g chemical igniter was 2 kJ (Yu et al., 2018). Each test was replicated three times, and the explosion parameter values were averaged.

4

According to (dP/dt)max and the volume of the spherical chamber (V), Kst, an international common value can be calculated by Eq.(1): K st = (dP / dt ) max × V −3

(1)

where, P – Pressure, bar t – Time, s V – Vessel volume, m3 Kst – Explosibility dust constant, bar m/s Kst values rounded to the nearest integer are used.

2.2. Experimental apparatus for MIE According to the measurement procedures defined in EN ISO/IEC 80079-20-2 (British Standards Institution, 2016), a modified 1.2-L Hartmann tube was used in ignition energy tests. Fig. 3 shows a schematic diagram of the modified Hartmann apparatus (Yu et al., 2018). The dust diffusion and ignition were implemented in the Hartmann tube. All the tests were performed at atmospheric pressure and room temperature. The test coverage of the discharge energy values ranges from 1 to 2000 mJ. Dust dispersion is triggered by a compressed air blast at 7 bars. According to the standard, the electrode gap is set at 6 mm. In this study, the tests were performed with an inductance of the discharge circuit which was 1 mH. When the electronic pneumatic valve was opened, the ACR dust at the bottom of the tube was dispersed into the tube. Then the ACR dust was ignited by electrical spark triggered by the discharge of the high voltage unit. In addition, the time from dispersion to ignition was 60 ms. If the dust was ignited and the flame propagation appeared, a dust explosion was successfully induced. If the dust was non-ignitable in ten continual tests, this ignition energy could not induce a dust cloud explosion. The MIE is usually stated as a range of values rather than a single value. But in order to compare MIE of the different

5

combustible powders more clearly, only a single value, the lowest with ignition, was used here.

2.3. Experimental apparatus for MIT According to the measurement procedures defined in EN ISO/IEC 80079-20-2 (British Standards Institution, 2016), Godbert-Greenwald Furnace apparatus was used in minimum ignition temperature tests. A schematic diagram of Godbert-Greenwald Furnace apparatus is illustrated in Fig. 4. The apparatus is composed of nine parts: a heating furnace, a dust collector, an electromagnetic valve, a pressure reducing valve, a pressure gauge, a gasholder, an intake valve, a mirror and a heating controller. The measurement range of this equipment is 25–1000 °C. The injection pressure of the dust can be varied from 0.1 to 0.5 bar (typically 0.1 bar, 0.2 bar, 0.3 bar and 0.5 bar), and the volume of the heating furnace is 500 mL. After the furnace temperature and dust injection pressure were set to 500 °C and 0.5 bar, respectively, a weighed amount of ACR dust was placed in the dust container, and the dust sample was blown into the heating furnace with high-pressure gas after the electromagnetic valve was opened. Once the MIT was obtained, further tests were performed at a furnace temperature 5 °C below the MIT by varying the dust concentration to confirm the non-ignition state. The criterion for dust ignition was the observation of a flame at the bottom opening of the furnace. If there is flame eruption or flame lagging at the lower end of the furnace tube, it is judged to be on fire; if there are only sparks and there is no flame, it is judged as not burning. The minimum ignition temperature shall be recorded as the lowest temperature of the furnace that is ignited via the specified procedure. Concrete method is shown as follows:

Ttest > 300 °C , Tmin = Ttest − 20 °C; Ttest ≤ 300 °C , Tmin = Ttest − 10 °C.

6

2.4. Experimental materials ACR dust samples were provided by Suzhou Soken Chemical Co., Ltd., China. The air/CO2 mixtures with various concentrations of CO2 (φ) were provided by gas distribution company. The detailed conditions of the gas mixture were shown in Table 2. Table 3 summarizes the material properties of ACR powders. Fig. 5 shows the particle size distributions of the tested ACR powders. Fig. 6 shows the shapes of ACR powders. The shape of ACR powders was regular and spherical. The D50 of ACR powders was 7.55 µm.

7

3.

Results and discussion

3.1. Inerting effect of CO2 on the explosion severity of ACR powders To explore the inerting effect of CO2 on the explosion severity of ACR dust, the air/CO2 mixtures with various concentrations of CO2 (φ) were adopted in the tests. The concentrations of ACR powders for the tests were 100―1200 g/m3. Figs. 7 and 8 show the inerting effect of φ on Pex and K of ACR dust. It can be seen that Pex and K of ACR dust increased first and then decreased with the increase of dust concentration. It also can be seen that the explosion severity reduced when φ increased. The decay rates of Pmax and Kst in different φ are presented in Table 4. It can be seen that when φ increased to 10 and 40 vol.%, the decay rates of Pmax and Kst were 32.7% and 43.2%, 48.1% and 58.4%, respectively. Compared with the previous study (Yu et al., 2018), it can be seen that CO2 had a great inerting effect on ACR dust explosion. The reason is that when the activated molecules generated by ACR combustible vapor collide with the inerting molecule of CO2, the activated molecules lose the activation energy, and the chain reaction is interrupted or slowed down. Moreover, KSt is found at a concentration of 600 g/m³ for 10 vol.% CO2, 400 g/m³ for 20 and 30 vol.% CO2 and 200 g/m³ for 40 vol.% CO2.

3.2. Inerting effect of CO2 on MEC of ACR powders MEC is the concentration boundary above which a dust–oxidant mixture will propagate a flame in the presence of adequate ignition source (Yuan et al., 2012). MEC is a crucial parameter for dust explosion evaluation and prevention. MEC of the ACR dust cloud from the previous study was 20―30 g/m3 (Yu et al., 2018). Therefore, it is very vital to raise MEC of ACR powders. To investigate the inerting effect of CO2 on the MEC of ACR dust, tests were conducted at φ of 10―40 vol.%. The experimental results of MEC for ACR powders with different φ are summarized in Table 5. It can be seen that CO2 had no inerting effect on MEC of ACR powders for 8

the φ of 10 vol.%. When φ increased to 20, 30 and 40 vol.%, the MEC of ACR dust increased to 30―35, 35―40 and 40―50 g/m3, respectively. This indicates that CO2 had an inerting effect on MEC of ACR powders, for oxygen concentration decreased due to the introduction of CO2.

3.3. Inerting effect of CO2 on MIE of ACR powders Ignition sensitivity can be characterized scientifically by the MIE of a dust cloud. MIE of the ACR dust cloud was low enough to be ignited by static electricity according to the previous study (Yu et al., 2018). Therefore, it is very necessary to reduce the ignition sensitivity of ACR powders. To investigate the inerting effect of CO2 on the MIE of ACR powders, the MIEs of ACR dust in the range of 240―1440 mg at φ of 10―40 vol.% were tested, respectively. The results of the lowest ignition energy (IE) of ACR powders with different φ are summarized in Table 6. Fig. 9 shows the inerting effect of φ on the lowest IE of ACR dust cloud. It can be seen that the lowest IE of ACR powders decreased first, then increased, and finally decreased with the increase of sample mass in the range of 240―1440 mg. Compared with the MIE results of ACR dust measured from the previous study (Yu et al., 2018), the result indicates that the MIE of ACR dust cloud increased respectively from 10 mJ to 60 and 100 mJ. This suggests that CO2 has an inerting effect on MIEs of ACR powders for the φ of 10 and 20 vol.%. When φ increased to 30 and 40 vol.%, MIEs of ACR dust increased greatly. Moreover, MIEs of several mass samples were already larger than 500 mJ. According to Table 7, the result shows that those mass samples of ACR dust in which MIEs exceed 500 mJ had low sensitivity to ignition. Therefore, CO2 can effectively mitigate the ignition sensitivity of ACR dust for the φ of 40 vol.%. To compare the inerting effects among different φ, the MIEs of ACR powders were calculated as Eq. (2) (Yuan et al., 2014).

log MIE =

log IE − NI × ( log IE − log NIE ) N +1 9

(2)

Here, N and NI denote the test number and combustion number, respectively, at energy level IE. Fig. 10 shows the MIEs of ACR dust for different φ. According to Table 7, ACR powders were difficult to be ignited by electrostatic discharges when 10 and 20 vol.% of CO2 was introduced into air. However, earthing personnel should be considered. When φ exceeded 30 vol.%, the plant should be earthed to prevent ACR dust explosion hazards. The reason could be explained that the presence of CO2 reduces the chance of effective collision between oxygen and flammable vapor. Moreover, the activated molecules produced by the flame front collide with CO2 and lose their activation energy, leading to interruption of the chain reaction.

3.4. Inerting effect of CO2 on MIT of ACR powders The MIT of dust cloud can be used to guide the selection of explosion-proof electrical equipment and the risk assessments of combustible dusts (Zhang et al., 2014). To understand MIT of ACR powders, the MITs of ACR dust in the range of 200―1200 g/m3 were experimentally investigated. The experimental results of the lowest ignition temperature (IT) of ACR powders with different φ were summarized in Table 8. Fig.11 shows the effect of the dust concentration on the lowest IT of ACR dust cloud. It can be seen that the dust concentration for MIT at 460 °C was 600 g/m3. It can be also seen that the lowest IT of ACR dust cloud decreased first and then increased with the increase of dust cloud concentration. The reason is that when the dust concentration is lower than 600 g/m3, the distance among ACR particles in the dust cloud is larger at lower dust, which results in the inefficient heat transfer among ACR particles. Of course, the dust reacts with oxygen even if the concentration is low, but higher temperatures are needed to observe an ignition. Therefore, more heat is required to be transferred to the dust cloud for ignition and explosion propagation. As the dust concentration increases, the number of particles per unit volume of ACR dust increases, which results in a decrease of distance among ACR particles. Thereby, the heat transfer and the explosion propagation between the particles are promoted with the increase of dust 10

concentration. In other words, in the case of lower dust concentrations, the MIT will be higher than that of the higher concentrations. When the concentration exceeds 600 g/m3, there is insufficient oxygen to support the complete combustion of ACR powders. In addition, excess dust particles are released to absorb heat without combustion. Therefore, the temperature of the flame drops and more heat will be absorbed from the heat source to reach the ignition temperature (Mishra and Azam, 2018). To understand the inerting effect of CO2 on MIT of ACR powders, MITs of ACR dust at φ of 10―40 vol.% were experimentally tested. Fig.11 also shows the effect of φ on MITs of ACR dust cloud. It can be seen that MITs of ACR dust also increased first and then decreased when CO2 was introduced into air. It also can be seen that the values of MIT increased by 20, 35, 45, and 50 °C, respectively. Therefore, MIT of ACR powders gradually increased when φ increased. This phenomenon may be because CO2 could absorb more heat which results in the inefficient heat transfer among ACR particles.

3.5. Inerting Mechanism The results indicated that the explosion severity and the ignition sensitivity of ACR dust decreased gradually as φ increased. The reason is that the suspended ACR powders are decomposed into flammable vapor due to pyrolysis under the heat source. The specific heat capacities of CO2 and air are 37.27 and 29.15 J K mol−1, respectively. Therefore, when introduced into air, CO2 will absorb more heat, and this results in the inefficient heat transfer between ACR particles. Then the flammable vapor mixed with CO2-air reaches the flammable limit and ignition is triggered by heat source. However, the presence of CO2 reduces the chance of effective collision between oxygen and flammable vapor on the flame front. Meanwhile, the activated molecules generated by the flame front collide with CO2 and lose the activation energy. Furthermore, the oxygen concentration decreases due to the introduction of CO2. Consequently, the chain reaction between oxygen and flammable vapor is 11

interrupted or decelerated. Next, the heat released from dust combustion transmits to the nearby suspended or blown powders through heat conduction and flame radiation, and thus the neighboring powders are heated and vaporized. The CO2 in the flame front and unburned areas absorbs a large amount of heat, and as a result the temperature in front of the flame drops. Besides, the burning rate, the heat release rate, and the flame propagation rate of ACR dust decrease in the process. At last, the ACR dust explosion is inerted by CO2. TG curves of ACR dust at different gas atmospheres are depicted in Fig. 12. It can be seen that the ACR powders started to lose weight at a temperature of approximately 257 °C. It decomposed completely at around 500 °C. It also can be seen that the char residues for ACR powders increased gradually as φ increased. The char residues of ACR powders at 800 °C were 0.64 wt % at air atmosphere. Under φ of 10―40 vol.%, the char residues of ACR powders at 800 °C were 2.84 wt %, 3.89 wt %, 7.62 wt % and 9.35 wt %, respectively. The results indicate that carbon residues increased with φ increasing, and carbon layers then formed on the surface of ACR particles. Moreover, the carbon layers act as an insulating barrier and prevent the heat from transferring to the internal of ACR particles. This results in incomplete pyrolysis and combustion of ACR particles. Therefore, the explosion severity and the ignition sensitivity of ACR dust were weakened by CO2.

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4.

Conclusions

The inerting effect of CO2 on ACR dust explosion was systematically investigated. The conclusions can be summarized as follows: (1) The addition of CO2 had a certain inerting effect on the explosion severity of ACR dust. Pmax and Kst of ACR powders gradually decreased with the increase of φ. When

φ increased to 40 vol.%, the decay rates of Pmax and dust Kst were 48.1% and 58.4%, respectively. (2) When φ increased to 10 vol.%, the value of MIE was larger than 25 mJ. So, ACR dust was not sensitive to static electricity. When φ increased to 30 and 40 vol.%, the MIE of ACR dust increased from 10 mJ to 207 and 267 mJ, respectively. In this case, ACR dust had low sensitivity to ignition. However, earthing personnel should still be considered. (3) The lowest IT of ACR dust cloud first decreased, and then increased with the increase of dust concentration and the MIT was about 460 °C. MIT of ACR dust cloud increased when φ increased. The MIT increased by 50 °C when φ increased to 40 vol.%. (4) The inerting mechanism is that the chain reaction was interrupted or decelerated owing to the endothermic heat of CO2, the reduction of the possibility of effective collisions between oxygen and flammable vapor and the decline of oxygen concentration. Moreover, CO2 could absorb more heat owing to its higher specific heat capacities. Furthermore, the carbon layer formed by carbon residues prevented the heat from transferring to the internal of ACR particles which resulted in the incomplete pyrolysis and combustion of ACR particles.

Acknowledgments The authors are grateful for the financial support given by the National Key Research and Development Plan under grant no.2018YFC0800102 and the State Key Program of National Natural Science of China under grant no. 2143 6006 and the Natural Science Foundation of the Jiangsu Higher Education Institutions of China under grant

13

no.18KJB620002.The authors also thank Associate Prof. Yang Hongqi with Nanjing Tech University for improving this manuscript.

References Abbasi T., Abbasi S.A., 2007. Dust explosions—cases, causes, consequences, and control. J. Hazard Mater., 140, 7-44. Amyotte., 2006 Amyotte P.R., 2006. Solid inertants and their use in the dust explosion prevention and mitigation. J. Loss Prev. Process Ind., 19. 161–173. British Standards Institution, 1991. Code of Practice for Control of Undesirable Static Electricity. General Considerations. BSI, London BS 5958-1. British Standards Institution, 2006. Determination of explosion characteristics of dust clouds-Part 3: Determination of the lower explosion limit LEL of dust clouds. BSI, London BS EN 14034-3. British Standards Institution, 2016. Explosive Atmospheres-Part 20-2: Material Characteristics-Combustible Dusts Test Methods. BSI, London EN ISO/ IEC 80079-20-2 Combustible Dust Incident Database. https://dustsafetyscience.com/. Chen X., Zhang H., Chen X., Liu X., Niu Y., Zhang Y., Yuan B., 2017. Effect of dust explosion suppression by sodium bicarbonate with different granulometric distribution. J. Loss Prev. Process Ind., 49, 905–911. Cui C., Shao H., Jiang S., Zhang X., 2018. Experimental study on gas explosion suppression by coupling CO2 to a vacuum chamber. Powder Technol., 335, 42– 53. Gao W., Yu J., Li J., Zhang Q., Xie Q., Zhang X., Hu D., 2016. Experimental investigation on micro- and nano-PMMA dust explosion venting at elevated static activation overpressures. Powder Technol., 301, 713–722. International Organization for Standardization, 1985. Explosion Protection Systems−Part 1: Determination of Explosion Indices of Combustible Dusts in Air. ISO 6184/1.

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Jiang H., Bi M., Gao W., B. Gan, Zhang D., Zhang Q., 2018. Inhibition of aluminum dust explosion by NaHCO3 with different particle size distributions. J. Hazard. Mater. 344, 902–912. Li G., Yuan C.M., Y. Fu, Zhong Y.P., Chen B.Z., 2009. Inerting of magnesium dust cloud with Ar, N2 and CO2. J. Hazard. Mater. 170, 180–183. Li G.,. Yang H.X, Yuan C.M., Eckhoff R.K., 2016. A catastrophic aluminium-alloy dust explosion in China. J. Loss Prev. Process Ind. 39, 121–130. Luo Z., Wang T., Tian Z., Cheng F., Deng J., 2014. Experimental study on the suppression of gas explosion using the gas-solid suppressant of CO2/ABC powder. J. Loss Prev. Process Ind. 30, 17–23. Ma L., Y. Xiao, Deng J., Wang Q., 2010. Effect of CO2 on explosion limits of flammable gases in goafs. Mining Science and Technology. 20, 193–197. Mishra D.P., Azam S., 2018. Experimental investigation on effects of particle size, dust concentration and dust-dispersion-air pressure on minimum ignition temperature and combustion process of coal dust clouds in a G-G furnace. Fuel 227, 424–433. Nie B., He X., Zhang R., Chen W., Zhang J., 2011. The roles of foam ceramics in suppression of gas explosion overpressure and quenching of flame propagation. J. Hazard. Mater. 192, 741–747. Pei B., Yu M., Chen L., Wang F., Yang Y., Zhu X., 2017. Experimental study on the synergistic inhibition effect of gas-liquid two phase medium on gas explosion. J. Loss Prev. Process Ind. 49, 797–804. Yu Y., Li Y., Zhang Q., Ni W., Jiang J., 2018. Experimental investigation of the inerting effect of crystalline II type Ammonium Polyphosphate on explosion characteristics of micron-size Acrylates Copolymer dust. J. Hazard. Mater. 344, 558–565. Yuan J., Huang W., Ji H., Kuai N., Wu Y., 2012. Experimental investigation of dust MEC measurement. Powder Technol. 217, 245–251. 15

Yuan C., Amyotte P.R., Hossain M.N., Li C., 2014. Minimum ignition energy of nano and micro Ti powder in the presence of

inert nano TiO2 powder. J. Hazard.

Mater. 274, 322–330. Yuan Z., Khakzad N., Khan F., Amyotte P., 2015. Dust explosions: a threat to the process industries. Process Saf. Environ. Prot. 98, 57–71. Zhang J., Liu X., Wang Y., Li Y., Li Q., 2014. Experimental research on minimum ignition temperature of 7-ACA dust cloud. Procedia Eng. 84, 467–471.

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Table 1 Accidents caused by polymer dust explosion (Abbasi and Abbasi, 2007; Yuan et al., 2015; Combustible Dust Incident Database). Date

Country

Dead (D)/Injured (I)

Material

1993 1994 1996 1998 1999 2002 2002 2003 2003 2003 2003 2004 2017 2017 2018 2018

USA Japan USA USA USA USA USA USA USA USA Canada UK Japan USA Germany Dominican Republic

2 d/2 i 5d/22 i 2d 16 d 2d 13 d 5d 38 d/ 6 i 37 d/7 i 7d 6 d/38 i 9 d/33 i 1 d/11 i 1i 3i 5 d/66 i

Plastic Rubber Rubber Plastic Plastic Rubber Rubber Rubber Plastic Resin Polyethylene Plastic Resin Fiber Plastic Plastic

Table 2 Experimental gas atmospheres for ACR dust explosion. φ (vol.%)

Air (vol.%)

Oxygen concentration (vol.%)

Gas composition

0 10 20 30 40

100 90 80 70 60

21 18.9 16.8 14.7 12.6

Air Air/CO2 Air/CO2 Air/CO2 Air/CO2

Table 3 Material properties of ACR powders. Supplier

Particle size distribution (µm)

Specific surface area (m2/g)

Suzhou Soken Chemical Co., Ltd

d(10)= 1.14 d(50)= 7.55 d(90)= 25.1

0.587

Table 4 Decay rates of Pmax and Kst in different φ (2kJ). φ (vol.%)

Pmax (bar)

Kst (bar m/s)

Decay rate of Pmax

Decay rate of Kst

Reference

0 10 20 30 40

10.4 7 6.5 5.9 5.4

416 226 211 193 173

― 32.7 37.5 43.3 48.1

― 43.2 49.3 53.6 58.4

（Yu et al., 2018） Present study Present study Present study Present study

Table 5 Experimental results of MEC for ACR powders with different φ (2kJ). φ (vol.%)

MEC (g/m3)

10 20 30 40

20―30 30―35 35―40 40―50

Table 6 Experimental results of the lowest IE for ACR powders with different φ (60ms). The lowest IE (mJ) for different sample mass

φ (vol.%)

240 mg

480 mg

720 mg

960 mg

1200 mg

1440 mg

0

190

90

60

30

10

50

（Yu et al., 2018）

10 20 30 40

200 240 330 540

100 135 225 270

70 120 210 385

80 100 245 445

85 110 400 510

90 135 475 610

Present study Present study Present study Present study

Reference

Table 7 Ignition sensitivity for flammable dust (BS EN 5958-1-1991, 1991). MIE (mJ)

Ignition sensitivity

500

Low sensitivity to ignition: Earth plant when ignition energy is at or below this level.

100

Consider earthing personnel when ignition energy is at or below this level.

25

The majority of ignition incidents occur when ignition energy is below this level. The hazard from electrostatic discharges from dust clouds should be considered High sensitivity to ignition. Take the above precautions and consider restrictions on the use of high resistivity materials (plastics). Electrostatic hazard from bulk powders of high resistivity should be considered

10

1

Extremely sensitive to ignition. Precautions should be as for flammable liquids and gases when ignition energy is at or below this level.

Table 8 Experimental results of the lowest IT for ACR powders with different φ (50KPa). The lowest IT for different ACR dust concentration (°C)

φ (vol.%)

200 g/m3

400 g/m3

600 g/m3

800 g/m3

1000 g/m3

1200 g/m3

0 10 20 30 40

490 500 505 510 515

480 490 495 500 510

460 480 500 505 525

470 500 505 515 540

490 515 520 540 550

500 530 535 555 560

Fig. 1. The 20-L spherical test apparatus. Notes: 1−Sealing cover, 2−Outer jacket, 3−Inner jacket, 4−Vacuum gauge, 5−Water Inlet, 6−Outlet Valve, 7−Base, 8−Peep hole, 9−Exhaust port, 10−Rebound Nozzle, 11−Dust Container, 12−Pressure gauge, 13−Pressure Sensor, 14−Water Outlet, 15−Safety limit switch, 16−Spark rod.

6 5

Pex

4

P, bar

3

(dP/dt)ex

2 1

Ingnition Injection

0 -1 0.0

0.1

0.2

0.3

0.4

0.5

0.6

Time, s

Fig. 2. Pressure–time curve recorded during the dust explosion test for the dust concentration of 1200 g/m3 (10 vol.% CO2).

Fig. 3. Schematic of the modified Hartmann tube (Yu et al., 2018).

Fig. 4. Godbert-Greenwald Furnace apparatus. Notes: 1−Heating Furnace, 2−Dust Collector, 3−Electromagnetic valve, 4−Pressure relief valve, 5−Pressure gauge, 6−gasholder, 7−Intake valve, 8−mirror, 9−Heating controller.

100

8

80

6

60

4

40

2

20

0

0 0.5

1

2

4

8

16

32

64

Paritcle size, µm

Fig. 5. Particle size distributions of ACR powders.

Fig. 6. Shapes of ACR powders under the microscope.

Cumulative volume distribution, %

Particle size distribution percentage, %

10

12

ACR 10 vol.% CO2

Yu et al.(2018)

10

20 vol.% CO2 30 vol.% CO2 40 vol.% CO2

Pex, bar

8 6 4 2 0 0

200

400

600

800

1000

1200

Concentration of ACR, g/m3

Fig. 7. Effect of φ on Pex of ACR dust (2kJ).

420

ACR 10 vol.% CO2

360

20 vol.% CO2

Yu et al.(2018)

30 vol.% CO2

K, bar.m/s

300

40 vol.% CO2

240 180 120 60 0 0

200

400

600

800

1000 3

Concentration of ACR, g/m

Fig. 8. Effect of φ on K of ACR dust (2kJ).

1200

700

ACR 10 vol.% CO2

600

20 vol.% CO2 30 vol.% CO2

Lowest IE, mJ

500

40 vol.% CO2

400 300 200 100 Yu et al.(2018)

0 200

400

600

800

1000

1200

1400

1600

ACR sample mass, mg

Fig. 9. Effect of φ on the lowest IE of ACR dust (60ms).

300 MIE Fitting curve

MIE, mJ

200

100

0 0

10

20

30

φ， vol.%

Fig. 10. Calculated MIEs vs. φ (60ms).

40

ACR 10 vol.% CO2 20 vol.% CO2 30 vol.% CO2

560 540

40 vol.% CO2 Lowest IT,

520 500 480 460 200

400

600

800

1000 3 Concentration of ACR, g/m

1200

Fig. 11. Effect of φ on the lowest IT of ACR dust (50KPa).

100 ACR 10 vol.% CO2

80

Mass loss, %

20 vol.% CO2 30 vol.% CO2

60

40 vol.% CO2

40

20

0 100

200

300

400

500

600

700

800

Temperature,

Fig. 12. TG curves of the ACR powders at different gas atmosphere.


When φ increased to 40 vol.%, decay rates of Pmax and dust Kst were 48.1% and 58.4%, respectively.
When φ increased to 30 and 40 vol.%, the MIE of ACR dust cloud increased from 10 mJ to 207 and 267 mJ, respectively.
MIT of ACR dust cloud was about 460 °C. The MIT increased when φ increased.
The inerting mechanism of CO2 was analyzed according to chain reaction and TG results.

Conflict of interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.