Flame suppression of 100 nm PMMA dust explosion by KHCO3 with different particle size

Flame suppression of 100 nm PMMA dust explosion by KHCO3 with different particle size

Process Safety and Environmental Protection 132 (2019) 303–312 Contents lists available at ScienceDirect Process Safety and Environmental Protection...

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Process Safety and Environmental Protection 132 (2019) 303–312

Contents lists available at ScienceDirect

Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep

Flame suppression of 100 nm PMMA dust explosion by KHCO3 with different particle size Jianhua Zhou, Haipeng Jiang, Yonghao Zhou, Wei Gao ∗ School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China

a r t i c l e

i n f o

Article history: Received 15 August 2019 Received in revised form 20 October 2019 Accepted 21 October 2019 Keywords: Nano-PMMA dust explosion Explosion suppression Flame propagations Suppression mechanisms

a b s t r a c t Flame suppression mechanism of KHCO3 for 100 nm PMMA dust explosions was investigated experimentally and computationally. The effect of KHCO3 particle size on the suppression efficiency was examined. The study revealed that the larger the proportion of added KHCO3 , the more significant the suppression effect on flame, for KHCO3 particles with a fixed particle size distribution. The suppression effects were significantly increased with the decreasing of KHCO3 particle size distributions. For endothermic mechanism, when the KHCO3 particles with three particle sizes were mixed with 100 nm PMMA, the heat absorption peaks exhibited at the initial stage, in which the heat absorption values were 28 J/g, 26 J/g and 21 J/g, respectively. A kinetics model was established to reveal the chemical suppression mechanism. It was presented that the mole fractions of key flame radicals (H and OH) were obviously reduced during the chain reaction processes with the addition of KHCO3 . For KHCO3 particles, the formation of catalytic cycle K ⇔ KOH converted H and OH radicals into a stable combustion product H2 O, resulting in a lower flame speed and flame temperature. © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Nanoscale particles are widely used in various applications, such as fireworks, cosmetics, pharmaceuticals, electronics and plastics (Azhagurajan and Selualzumar, 2014; Eckhoff, 2012). However, the risk of an explosion increases obviously as the particle size decreases (Azam and Mishra, 2019; Vignes et al., 2019). As a threat to the process industries, dust explosions have caused huge losses and casualties (Yuan et al., 2015). Therefore, “nano-safety” issue has caused widespread attention (Chunmiao et al., 2014). Dust explosions continue to occur, reflecting the lack of knowledge of explosion protection technology (Amyotte, 2014). Numerous experimental studies have been conducted on assessing ignition sensitivity and explosion severity of nano materials (Li et al., 2011; Wu et al., 2014; Yuan et al., 2017). However, these studies are not comprehensively enough to appropriately take measure against nanoscale particles explosion. Explosion pressure and even the consequences of explosion accidents are impacted by the flame propagation (Jiang et al., 2019). Flame temperatures, heats of combustion and fast flame speed of nanoscale particles flame can result in the seriously thermal damages and pressure built-up. For exam-

∗ Corresponding author. E-mail address: [email protected] (W. Gao).

ple, the flame propagation velocity of 100 nm PMMA dust can reach 0.99 m/s, and flame temperature up to 1691◦ , and the maximum explosion pressure can even reach 0.82 MPa (Gao et al., 2015; Zhang et al., 2016; Zhou et al., 2019). Hence, study on the suppression mechanisms of flame propagation is also one of the main key part to prevent the dust explosion. The explosion pentagon consisting of fuel, oxidant, ignition source, dispersion and the confinement. Removal of a component from one of these five elements can result in a mitigative or protective manner (Amyotte et al., 2009). Explosion suppression is an effective technology to mitigate and prevent dust explosions. The main techniques of explosion suppression include the use of solid suppressant, water mist, gas suppressant and reduction of oxygen concentration. Spraying water mist into the reactive system of dust explosion can reduce flame speed and flame temperature, but can also impact the flow field and distribution uniformity of dust cloud (Huang et al., 2019). Gas suppressants, such as halo replacements, always promote explosions at a low additive concentration (Xu et al., 2017; Yang et al., 2012). Inert gas can reduce the oxygen concentration of reactive system, but has a less effectiveness (Li et al., 2009; Lu et al., 2019). By comparison, solid powder suppressants appear to be the more effective suppressants (Amyotte, 2006; Eckhoff, 2009). As a chemical suppressant, potassium bicarbonate exhibits excellent efficiency of flame extinguishment (Williams, 1999; Tapscott, 2001). Hoorelbeke (2009)

https://doi.org/10.1016/j.psep.2019.10.027 0957-5820/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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Fig. 1. Dust explosion suppression experimental platform in open space.

found that potassium-containing compounds performed very effectively at reducing burning velocity. Taveau (2015) demonstrated that potassium bicarbonate can even suppress aluminum dust explosion. Roosendans (2017) experimentally presented that the addition of potassium metal salt suppressants significantly suppressed methane/air explosions. Babushok (2017) studied the suppression effects of burning velocity and temperature of premixed CH4 /air flames with added KHCO3 . The decreasing in the burning velocity was the result of reductions in the radical concentrations due to the thermal effects of agent addition. It can be seen that the effectiveness of KHCO3 has been demonstrated in early studies. However, only few researchers have systematically studied the effect of KHCO3 on 100 nm PMMA dust explosion. Study on the suppression mechanisms of flame propagation is the main key part to prevent the dust explosion. The purpose of this paper is to understand the effects of KHCO3 particle size distributions on the flame propagation of 100 nm PMMA dust explosions, such as detailed suppression mechanisms of flame propagation, the effects of KHCO3 on flame propagation and temperature, and the particle size effect on suppression efficiency.

2. Experimental As shown in Fig. 1, the open-space dust explosion experimental system was the same as our previous study (Zhou et al., 2019), which was composed of a combustion system, an ignition system, a dust supply system, a high-speed video camera, a thermocouple, a data acquisition system unit and a time controller unit. The combustion system consisted of three cylindrical tubes. The upper tube was 60 mm high with a diameter of 95 mm. The movable tube was 115 mm high with a diameter of 95 mm. The bottom tube was 125 mm high with a diameter of 80 mm. The powder was placed in a hemispherical cup of the dispersion unit. After the dispersion of dust, the movable tube falls to creat an open space. The ignition system consisted of a pair of 0.4 mm tungsten wire electrodes and a 15 KV high-voltage transformer. The dust supply system included a pressure buffer tank, pipelines and solenoid valves. The flame propagation process was captured by a high-speed camera (Photron SA4) with a normal lens (Nikkor 50 mm f/1.2, Nikon), and the frame rate of the high-speed camera was 1000 frame/s. The thermocouple was composed of 13% Pt/Pt-Rh wires with a 25 ␮m diameter, whose measure range was 0–1900 ◦ . The experimental processes were controlled by a programmable logic controller (OMRON CPM1A). The time to trigger the programmable logic controller was set to 0 s. From 0 s to 0.5 s, the solenoid valve was opened. The particles were dispersed at the pressure of 0.5 MPa to form the homogeneous dust cloud. 0.2 s later, the electrical stoppers were triggered and the movable tube was moved down. From 0.5 s, the high speed cam-

Fig. 2. SEM and particle size distributions of 100 nm PMMA particles.

eras were triggered. When the dust cloud was relatively stable and uniform at about 1.0 s, the high voltage transformer was discharged for 0.01 s. Then the suspended particles were ignited. 100 nm PMMA particles consisting of MMA monomers were used in our experimental study. Due to the interaction of PMMA particles, hygroscopicity was strong and the particles were easily agglomerated. Therefore, PMMA particles were dried for more than 24 h at 35 ◦ before the experiment. KHCO3 particles were sifted into three sizes (32–75 ␮m, 75–100 ␮m and 100–212 ␮m). Particle size distributions of 100 nm PMMA particles and KHCO3 were shown in Table 1. D [4,3] was used to characterize the particle size of KHCO3 particles. D [4,3] values of KHCO3 with three particle sizes are 59 ␮m, 83 ␮m and 204 ␮m, respectively. Scanning electron microscope and particle size distributions of 100 nm PMMA particles and KHCO3 particles are exhibited in Figs. 2 and 3, respectively. The concentration of 100 nm PMMA used in this paper was 450 g/m3 . To achieve reliable results, 5–6 replicate experiments were conducted. 3. Kinetic model The suppression mechanisms of chemical reaction kinetics were studied using CHEMKIN 18.1. The 0-dimensional homogeneous reactor was selected. The combustion process of dust cloud is significantly complicated. Di Benedetto and Russo (2007) developed a model to simulate the dust explosion. To simulate the combustion reaction by means of a detailed reaction mechanism, the model assumed that the pyrolysis/devolatilization step of dust particles was very fast and then gas combustion was controlling dust explosion. They pointed out that the heat diffusion of intra-particle and inter-particle is very critical in affecting dust conversion. However,

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Table 1 Characteristic diameters of 100 nm PMMA and three kinds of KHCO3 particles. Characteristic diameters

100 nm PMMA

32-75 ␮m KHCO3

75-100 ␮m KHCO3

100-212␮m KHCO3

D[4,3] (␮m) D[3,2] (␮m) d(0.1) (␮m) d(0.5) (␮m) d(0.9) (␮m)

24.90 22.90 15.06 25.86 31.99

58.63 24.79 18.94 53.55 106.51

82.51 44.46 41.77 77.74 133.30

203.96 159.01 108.23 190.14 323.73

Table 2 The summary of simplifying hypotheses. No.

Simplifying hypotheses

Reference

1. 2. 3. 4.

Dust particles completely decomposed in the reaction zone Intraparticle and interparticle heat diffusion were ignored Initial turbulence was ignored Ignition process was not considered.

Babushok et al. (2017) Di Benedetto and Russo (2007) Di Benedetto and Russo (2007) Glorian et al. (2016)

4. Results and discussions 4.1. Suppression effect of KHCO3 particle size on flame propagation behavior of 100 nm PMMA dust explosion

Fig. 3. SEM and particle size distributions of KHCO3 particles.

the purpose of this paper is to reveal a detailed gas–phase suppression mechanism of PMMA/air flame by K compounds in the reaction zone that located at flame front near the preheat zone. To understand how K–containing species and P–containing species react with flame radicals. Hence, it was assumed that all the particles (both PMMA and KHCO3 ) were completely vaporised into the gas phase in the reaction zone. Ignition process was also not considered in this paper. Additionally, the initial level of turbulence was considerably weak based on the results of our previous study (Jiang et al., 2019). The effect of initial turbulence could be ignored. The summary of simplifying hypotheses was shown in Table 2. The kinetic model is composed of sets of reactions:

(1) The first kinetic model was the reactions of PMMA combustion. For simplify, PMMA particles were assumed to be completely decomposed into gaseous monomer MMA and some small molecules, such as CH4 , CH3 OH, HCHO, CH3 COCH3 and so on (Rosser et al., 1963; Zeng et al., 2016a, 2016b). The kinetic data of these small molecule reactions were adopted from the National Institute of Standards and Technology (NIST) chemical kinetics database (Chemical Kinetics Database, 2001). Thermodynamic data were based on the JANAF table (JANAF tables, 1998). (2) The second kinetic model was the reactions of K-containing species. KHCO3 particles were assumed to be completely vaporized in the reaction zone. The kinetics and thermodynamic data of KHCO3 were adopted from Babushok et al. (Babushok et al., 2017). The reactions of K-containing species consisted of 85 elementary reactions and 12 species.

Fig. 4 showed flame propagation behaviors of 100 nm PMMA dust with different proportion of 59 ␮m KHCO3 . As the proportion of KHCO3 increased, the flame propagation behaviors changed. When the proportion of KHCO3 was 10%, the flame propagation velocity became noticeably slow. The flame changed from a bright yellow flame to a dim orange-red flame at the initial stage of the development and the continuous flame front became more discrete. The suppression effects were more significant as the proportion of KHCO3 increased to 20% and the flame size at t =40 ms was almost equal to the flame size at t = 30 ms without suppressant. Flame propagation velocity was further slowed down when the proportion increased to 30 %. In summary, the proportion of 59 ␮m KHCO3 increased from 10 % to 30 % had an excellent suppression effect on flame development. Fig. 5(a) presented the dust cloud flame formed by mixing 100 nm PMMA particles with 83 ␮m KHCO3 particles as 1:1 ratio. It was apparent that the effect of KHCO3 on flame propagation were slight. As shown in Fig. 5(b), when the proportion of 83 ␮m KHCO3 was 150 %, the effect on the flame propagation behaviors were highlighted. The flame size at t =40 ms was close to the flame size at t = 30 ms without adding suppressant and the brightness at the edge of the flame was weakened. At t =60 ms, the flame front was dim and the flame center was bright, indicating that flame propagation was further limited. From what has been discussed above, it was found that the suppression effect was various when the particle size of KHCO3 was increased from 59 ␮m to 83 ␮m. It was likely that as the particle size increased, the specific surface area and surface atomic energy ratio was reduced. Eventually, the endothermic and combustion chemical reaction process were slowed down. When the particle size of KHCO3 increased to 204 ␮m continuously, the suppression effects of 204 ␮m KHCO3 particles with proportion 100 % and 200 % on flame propagation behaviors were not prominent. Only when the proportion of 204 ␮m KHCO3 particles was 300%, the flame propagation was slowed down (Fig. 6). In brief, for the same KHCO3 particle size, the slower the flame propagation velocity, the better the suppression effects of KHCO3 . But the suppression effects were significantly reduced with the increase of KHCO3 particle size. Based on the above results, it could be found that 100 nm PMMA (450 g/m3 ) flame could be suppressed when the proportion of 59 ␮m KHCO3 > 30 %. However, 100 nm PMMA (450 g/m3 ) flame could not be suppressed by 83 ␮m and 204 ␮m KaHCO3 with the proportion in the range studied.

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Fig. 4. Flame propagation of 100 nm PMMA dust cloud doped with 59 ␮m KHCO3 .

Fig. 5. Flame propagation of 100 nm PMMA dust cloud doped with 83 ␮m KHCO3 .

4.2. Suppression effect of KHCO3 particle size on flame propagation velocity of 100 nm PMMA dust explosion

the suppression ratio ı was defined as: ıi =

MATLAB program was used to recognize the flame front. The instantaneous flame velocity was the ratio of the distance in the vertical direction to the time (Zhang et al., 2016). To evaluate the suppression efficiency of KHCO3 particle sizes on flame velocity,

v0 − vi × 100% (i = 1, 2, 3) v0

(1)

v0 was the average flame velocity of 100 nm PMMA dust cloud; vi were the average flame velocities of PMMA dust doped with different proportions of KHCO3 particles. Flame propagation velocities

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Fig. 6. Flame propagation of 100 nm PMMA dust cloud doped with 204 ␮m KHCO3 .

Fig. 7. Flame propagation velocities of 100 nm PMMA dust cloud doped with 59 ␮m (a), 83 ␮m (b), 204 ␮m (c) KHCO3 particles.

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added, the average flame velocities were 1 m/s, 0.95 m/s, 0.78 m/s, and 0.70 m/s, and the suppression ratios were 0 %, 5 %, 22 %, and 30 %, respectively. The suppression ratio of adding 83 ␮m KHCO3 with the proportion of 100 % to 100 nm PMMA dust cloud was almost equal to that of adding 59 ␮m KHCO3 with the proportion of 10 %, which fully indicated that the particle size of suppressant affected its suppression effect. If the particle size of KHCO3 was increased sequentially, the effect of particle size on the suppression effect was more obvious. The average flame propagation velocities were 1 m/s, 0.98 m/s, 0.89 m/s, and 0.83 m/s, respectively, when 0 %, 100 %, 200 % and 300 % proportion of 204 ␮m KHCO3 particles were added to the 100 nm PMMA dust cloud. The suppression ratios were 0 %, 2 %, 11 %, and 17 %, respectively. Obviously, as the KHCO3 particles increased from 59 ␮m to 83 ␮m and 204 ␮m, the suppression effect on the flame velocities gradually weakened and this phenomenon was remarkable.

Fig. 8. Average flame propagation velocity of 100 nm PMMA/KHCO3 mixture.

4.3. Suppression effect of KHCO3 particle size on flame temperature of 100 nm PMMA dust explosion

of 100 nm PMMA doped with KHCO3 particles were illustrated in Fig. 7. Fig. 8. plotted the average flame propagation velocities of doped flames. It could be seen that 59 ␮m KHCO3 particles with proportion of 0 %, 10 %, 20 %, and 30 % were added and the average flame propagation velocities were 1 m/s, 0.76 m/s, 0.68 m/s and 0.52 m/s, respectively. The suppression ratios ı were 0 %, 24 %, 32 %, and 48 %, respectively. It was found that the average flame velocities significantly was reduced with the increase of the ratio of 59 ␮m KHCO3 . With the addition proportion of 59 ␮m KHCO3 was 30%, the average flame velocity was almost reduced by half. However, for 83 ␮m KHCO3 , the suppression effect was excellent only after a larger proportion of KHCO3 particles was added. In the same way, when 0 %, 100 %, 150 % and 200 % proportion of 83 ␮m KHCO3 particles were

A fine thermocouple (Pt-Pt/Rh 13 %) with 25 ␮m diameter wires was used in this study. The flame temperature histories of 100 nm PMMA/KHCO3 mixture were shown in Fig. 9. It could be seen that the maximum flame temperatures of 100 nm PMMA dust clouds decreased with the increase of KHCO3 particles proportion. After the addition of the suppressant, it took significantly longer time to rise to the maximum flame temperature. The maximum flame temperature of doped flame was shown in Fig. 10. It can be seen that after adding 0 %, 10 %, 20 % and 30 % proportion of 59 ␮m KHCO3 to the 100 nm PMMA dust cloud, the maximum flame temperatures were 1691 ◦ C, 1140 ◦ C, 991 ◦ C, and 791 ◦ C, respectively. With the increasing of the proportion of 59 ␮m KHCO3 , the suppression of flame temperature gradually increased. However, as

Fig. 9. Flame temperatures of 100 nm PMMA dust cloud doped with 59 ␮m (a), 83 ␮m (b), 204 ␮m (c) KHCO3 particles.

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Fig. 10. The maximum flame temperature of 100 nm PMMA/KHCO3 mixture.

the particle size of KHCO3 increased to 83 ␮m, the effects on flame temperatures were weakened. After adding 0 %, 100 %, 150 %, and 200 % proportion of 83 ␮m KHCO3 , the maximum flame temperatures were 1691 ◦ C, 1207 ◦ C, 885 ◦ C, and 763 ◦ C, respectively. As the particle size of KHCO3 was continue to increase to 204 ␮m, the suppression effect was less significant. After adding 0 %, 100 %, 200 %, and 300 % proportion of 204 ␮m KHCO3 , the maximum flame temperatures were 1691 ◦ C, 1274 ◦ C, 1103 ◦ C, and 917 ◦ C, respectively. 4.4. Suppression mechanisms 4.4.1. Physical suppression mechanism The physical suppression mechanisms consisted of the thermal mechanism and the barrier mechanism. The endothermic mechanisms of KHCO3 were analyzed in depth by thermal characteristics analysis. The results of thermal characteristics analysis of three particle sizes of KHCO3 at 59 ␮m, 83 ␮m and 204 ␮m were presented in Fig. 11. It was found that there was only one weight loss stage and only one endothermic stage of KHCO3 particles with three sizes from the DTG and DSC curves. From the TG curve, the temperatures at which KHCO3 particles with 59 ␮m, 83 ␮m and 204 ␮m began to lose weight were 120 ◦ C, 127 ◦ C and 131 ◦ C, respectively. It was indicated that the smaller of KHCO3 particle sizes, the lower of the temperatures at which weight loss began. Except that the temperature at which the weight losses ended were 194 ◦ C, 202 ◦ C and 215 ◦ C was clearly observed. It was further explained that higher temperature was required for the end of decomposition as the particle size increased. Multiple reactions proceeded simultaneously during the decomposition of KHCO3 . These reactions were shown as follows: 2KHCO3 → K2 CO3 + H2 O + CO2

(2)

K2 CO3 → K2 O + CO2

(3)

2K2 O → 4K + O2

(4)

As shown in Fig. 12, it obviously presented the thermal characteristics of 100 nm PMMA mixed with 59 ␮m, 83 ␮m and 204 ␮m KHCO3 . As shown in Fig. 12(a), there was significant weight loss stage and the endothermic peak in the DSC curve under the temperature of 148 ◦ C to 208 ◦ C. These phenomena were produced by the endothermic decomposition of 59 ␮m KHCO3 with the heat absorption of 28 J/g. It was also indicated the 83 ␮m KHCO3 endothermic weight loss stage under the temperature from 157 ◦ C to 218 ◦ C and the heat absorption was 26 J/g, as shown in Fig. 12(b). For DSC curve of 204 ␮m KHCO3 illustrated in Fig. 12(c), the endothermic stage was 175 ◦ C to 214 ◦ C and the heat absorption was 21 J/g.

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As for the suppression effect of KHCO3 with three particle size distributions, 100 nm PMMA experienced the stage of rapid weight loss firstly, and then turned into the stage of slow weight loss. Therefore, as the particle size of KHCO3 increased, the heat absorption decreased significantly which resulted in poor suppression. Rosser (Rosser et al., 1963) and Mitani (Mitani, 1984) proposed that the suppression process of suppressant consisted of the following four stages: (1) the heating stage of the suppressant, (2) evaporation and decomposition of the suppressant, (3) gasification of the suppressant and generation of gas phase free radicals, (4) the suppression process of the combustion reaction. The stages (1) and (2) had been analyzed with the thermal characteristics analysis. Large particle size suppressant agent took longer time to be gasified. If KHCO3 with three particle sizes was in the flame region for a certain time, the small size particles were more easily get gasified completely in the same flame temperature region. And the suppression effects were fully exerted. However, the gasification rates of the large particles were slow and the suppression effects were limited. For the barrier effect of KHCO3 , previous studies (Wanigarathne et al., 2000) had found that the thermal radiation effect of flame was a key factor in igniting unburned areas, and then it promoted flame propagation. In this study, KHCO3 particles were such barriers. During the flame propagation of 100 nm PMMA dust cloud, the suppressants were distributed in the dust cloud in the unburned area, blocking the heat transfer between the flame and the unburned PMMA particles.

4.4.2. Chemical reaction kinetics suppression mechanism It was well known that H and OH radicals were the main free radicals which maintained the flame chain reactions. The mole fraction curves of H and OH during the reaction were shown in Fig. 13. As shown in Fig. 13(a), it could be found that the mole fractions of H and OH reached maximum values firstly. It was the result that the amount of H and OH generated by the reaction was significantly larger than that of the combustion chain. Then, the mole fractions of H and OH decreased, and eventually tended to be balanced. As shown in Fig. 13(b), the mole fractions of H and OH were significantly decreased after adding KHCO3 . In the early stage of reactions, not only did the flame chain reaction consume H and OH, but the active objects produced by KHCO3 decomposition also consumed H and OH, which resulted in a significant reduction in the amount of free H and OH. The maximum value of the mole fractions was significantly reduced and the consumption of H by KHCO3 was more intense. The H and OH mole fractions also decreased until equilibrium was reached in the later stage. The main reaction of H and OH production and consumption and the maximum rate of production (MROP) of H and OH with no suppressant and with the addition of KHCO3 were shown in Fig. 14. As shown in Fig. 14(a), the main reactions for the production of H with suppressant were R1-R5. The MROP of these five reactions decreased after the addition of KHCO3 and the MROP of H in R1 and R2 almost arrived at termination. The main reaction of consuming H was dominant at this time, indicating that the H in the flame was captured by the active objects generated by the decomposition of KHCO3 . Fig. 14(b) showed the main reactions for generating and consuming OH without suppressant. After adding KHCO3 , the main reactions for consuming OH were R13-R16. And the R14 reaction: K + OH + M=KOH + M was the highest maximum rate of consuming OH. The production rate of K was shown in Fig. 15, where a positive rate indicated the production of K and a negative rate indicated the consumption of K. It can be found that K and H2 O were formed when KOH combined with H. Further, K combined with OH to regenerate KOH. Two key reactions of KOH+H=K+H2 O and K + OH + M=KOH + M formed K ⇔ KOH catalytic cycling in which H combined with OH to form a more stable compound H2 O.

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Fig. 11. Thermal analysis results of KHCO3 with three particle sizes.

Fig. 12. Thermal analysis results of KHCO3 with three particle sizes.

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Fig. 13. Variation of H, OH radicals mole fraction.

Fig. 14. Maximum rate of production for H (a) and OH (b).

Fig. 15. Rate of production of K due to various reactions.

5. Conclusions Flame suppression mechanism of 100 nm PMMA dust explosion by different particle size KHCO3 had been investigated experimentally and computationally. The physical suppression mechanism was revealed by analyzing the endothermic mechanism and barrier mechanism. A detailed kinetic model of effect of potassiumcontaining compounds on PMMA/air flames was first developed. The kinetic model based on some simplifying hypotheses was then used to explain the mechanism of flame suppression by Kcontaining species in the reaction zone. The conclusions could be summarized as follows:

(1) Suppression effects were significantly reduced with the increase of KHCO3 particle size. As the KHCO3 particles increased from 59 ␮m to 83 ␮m and 204 ␮m, the suppression effect on the flame propagation velocities significantly weakened. Besides, the maximum temperature of flame for adding 59 ␮m KHCO3 with the proportion of 10 % to 100 nm PMMA dust cloud was almost equal to that of adding 83 ␮m KHCO3 with the proportion of 100 %. (2) The physical suppression mechanismconsisted of the endothermic mechanism and the barrier mechanism. When the three particle size KHCO3 (59 ␮m, 83 ␮m and 204 ␮m) were mixed with 100 nm PMMA, the heat absorption peaks exhibited at the initial stage in which the heat absorption values were 28 J/g, 26 J/g and 21 J/g, respectively. For barrier mechanism, KHCO3 was distributed in the dust cloud in the unburned area, blocking the heat transfer between the flame and the unburned PMMA particles and absorbing a large amount of heat radiation generated by the combustion. (3) Numerical results indicated that the maximum mole fractions and the mole fractions at equilibrium of H and OH radicals were reduced in the chain reaction processes with the addition of KHCO3 . For KHCO3 particles, KOH + H = K + H2 O and K + OH + M = KOH + M formed a catalytic cycling to promote the combination of H and OH radicals, and then produced stable H2 O. (4) Initial turbulence was ignored in the model. Intra-particle and inter-particle heat diffusion was also not considered. Future work should be devoted to include initial turbulence and the heat diffusion of intra-particle and inter-particle in the model.

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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, “Flame suppression of 100 nm PMMA dust explosion by KHCO3 with different particle size” (PSEP 2019 1513). Declaration of Competing 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, “Flame suppression of 100 nm PMMA dust explosion by KHCO3 with different particle size” (PSEP 2019 1513). Acknowledgments The authors appreciate the financial supported by the National Natural Science Foundation of China (No. 51874066and No. 51674059), Key Laboratory of Building Fire Protection Engineering and Technology of MPS (KFKT2016ZD01), the Fundamental Research Funds for the Central Universities (DUT16RC(4)04), Liaoning Provincial Natural Science Foundation of China (20170540160) and the Fundamental Research Funds for the Central Universities (DUT18RC (3)038). The authors would like to thank Dr. Babushok Valeri I. for his help in the gas-phase mechanism of inhibition. References Amyotte, P.R., 2006. Solid inertants and their use in dust explosion prevention and mitigation. J. Loss Prev. Process Ind. 19, 161–173. Amyotte, P.R., 2014. Some myths and realities about dust explosions. Process Saf. Environ. 92, 292–299. Amyotte, P.R., Pegg, M.J., Khan, F.I., 2009. Application of inherent safety principles to dust explosion prevention and mitigation. Process Saf. Environ. 87, 35–39. Azam, S., Mishra, D.P., 2019. Effects of particle size, dust concentration and dust-dispersion-air pressure on rock dust inertant requirement for coal dust explosion suppression in underground coal mines. Process Saf. Environ. 126, 35–43. Azhagurajan, A., Selualzumar, N., 2014. Impact of nano particles on safety and environment for fireworks chemicals. Process Saf. Environ. 92, 732–738. Babushok, V.I., Linteris, G.T., Hoorelbeke, P., Roosendans, D., Wingerden van, K., 2017. Flame inhibition by potassium-containing compounds. Combust. Sci. Technol. 189, 2039–2055. Chemical Kinetics Database, 2001. NIST Standard Reference Database 17. http:// kinetics.nist.gov/. Chunmiao, Y., 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. Di Benedetto, A., Russo, P., 2007. Thermo-kinetic modelling of dust explosions. J. Loss Prev. Process Ind. 20, 303–309.

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