Micromechanism of the Initiation of a Multiple Flammable Gas Explosion

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Micromechanism of the Initiation of a Multiple Flammable Gas Explosion Zhenmin Luo, Bin Su, Qing Li, Tao Wang, Xiaofeng Kang, Fangming Cheng, Shuaishuai Gao, and Litao Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00480 • Publication Date (Web): 16 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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Micromechanism of the Initiation of a Multiple Flammable Gas Explosion Zhenmin Luoa,b,c*1, Bin Sua,c*, Qing Lia, Tao Wanga,c, Xiaofeng Kanga,c, Fangming Chenga,b,c, Shuaishuai Gaoa,c, Litao Liua,c (a School of Safety Science & Engineering, Xi’an University of Science and Technology, 58, Yanta Mid. Rd., Xi’an 710054, Shaanxi, PR China; b Shaanxi Key Laboratory of Prevention and Control of Coal Fire, 58, Yanta Mid. Rd, Xi'an, 710054, Shaanxi, PR China; c Shaanxi Engineering Research Center for Industrial Process Safety & Emergency Rescue, 58, Yanta Mid. Rd., Xi'an, 710054, Shaanxi, PR China)

Abstract To investigate the micromechanism of a multiple flammable gas explosion, CH4, CO, C2H6, C2H4 and H2 were selected to determine the thermodynamic and dynamic characteristics of the gas mixture explosion using Gaussian software. The initiation mechanism and primary initiation pathway of the joint explosion of the five combustible gases were analysed. A spherical experimental device for gas/dust explosions combined with a system for spectral measurements was implemented to obtain emission spectra of the ·H radical and CH2O during the gas mixture explosion process. The oxidation reaction of CH4 initiated the chain reaction for the entire explosive reaction rather than the CO/C2H6/C2H4/H2 mixture at the ignition moment of the explosion. Nevertheless, addition of the CO/C2H6/C2H4/H2 mixture had obvious effects on the chain-branching reactions of the CH4 explosion. The ·CH3 radicals produced during methane oxidation induced a hydrogen reaction, and ·H radicals generated in the reaction of ·CH3 and H2 triggered the involvement of other combustible gases in the explosion reaction, which considerably decreased the activation energy demand by multiple combustible gases to participate in the explosion reaction. The sequence of flammable gases involved in the explosion reaction was CH4, H2, CO, C2H4 and C2H6. Furthermore, the experimental results indicated that the ·H radical appeared before CH2O 1*

Corresponding author. Tel.:+86 13186055293; E-mail address: [email protected] (Zhenmin Luo) Tel.: +86 18392397316; E-mail address: [email protected] (Bin Su) 1 / 31

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during the explosion reaction process, which was consistent with the numerical simulation results. Moreover, the numerical simulation results provided a theoretical basis for the prevention of gas explosions and process safety in the petrochemical and mine industries.

Keywords: process safety; gas explosion; initiation mechanism; main reaction path; emission spectrum characteristics

1 Introduction Currently, gaseous fuels, such as methane (CH4), have become a good alternative to fossil fuels and are widely used because of their high caloric value, clean combustion process and abundant reserves. Moreover, large amounts of CH4 are used as raw materials to produce gaseous fuel and chemical products such as methanol and ammonia ， where combustible gases such as alkanes, alkenes, carbon monoxide and hydrogen may be produced during processing [1]. Additionally, the spontaneous combustion of coal produces a series of gases such as CO, CO2, H2O, H2, CH4, C2H4, and C2H6 [2]. Accordingly, gas mixture explosions are common accidents that pose a great threat to the safety of state property and people in industrial processes. Thus, it is essential and urgent to clarify micromechanism of the initiation of multiple flammable gas explosions on a molecular level. In recent years, much research has been performed on determining flammability and explosion parameters, such as flammable limits [3-11], explosion pressure and maximum rate of pressure rise [12-17], minimum ignition energy [18-25] and limiting oxygen concentration in gas mixtures [26-28]. Additionally, researchers have investigated the inerting effect of the addition of water mist and inert gases, such as nitrogen, carbon dioxide, helium and argon, on flammable gas mixture explosions [11, 29]. The abovementioned studies have provided a reliable reference for the prevention of gas explosions at the macroscopic level. Similarly, it is necessary to study the 2 / 31


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micromechanism of combustible gas mixtures to better understand gas explosions and to fundamentally prevent explosions. Based on experimental data, Kee [30] proposed a model of methane combustion in stirred reactors, and methane oxidation was modelled using a detailed kinetic reaction mechanism by Dagaut et al. [31]. Currently, the kinetic mechanism provided by GRI-Mesh 3.0, which contains 325 elementary reactions involving 53 species [32,33], is widely used in studies on combustion and explosions. Liang and Zeng [34] suggested that the presence of water induced a prolonged explosion time and decreased the ·H, ·O and ·OH mole fractions during the gas explosion process, according to their numerical evaluation. Nie et al. [35] used CHEMKIN-III to obtain the chemical kinetic behaviours of methane explosions and factors that affect it. They found that the ignition delay time is the longest under a fuel-rich condition, and free radicals are obviously sensitive to the intermediate reactions (R156 and R158). Moreover, some simplified combustion mechanisms have been proposed for specific reaction conditions [36-38]. For instance, a reduced reaction mechanism was developed to evaluate the methane and ethane combustion processes. Additionally, ReaxFF and ab initio MD calculations were combined to study the mechanism of the intrinsic effect of water addition on gas explosions. The results showed that water can effectively suppress the methane oxidation process at the initial reaction stage and promotes gas explosion during the later reaction process [39]. He et al. [40] also investigated the intrinsic mechanism of methane oxidation under explosion conditions, and they discovered that the initiation step of the gas explosion process is mainly induced by ·OH free radicals. Ashraf and van Duin’s [41] work significantly improved the C1 chemistry predicted by ReaxFF and solved the low-temperature oxidation initiation problem. Although most of the previous studies examined the macroscopic properties and the reaction kinetic properties of the gas explosion process, the initiation mechanism for a multiple flammable 3 / 31


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gas mixture explosion is still unknown. Because they focused on the reaction mechanism of gas explosion for single or two components. Using experimental methods, it is difficult to obtain microparameters due to the complexity and fatal nature of the explosion process. Research achievements using numerical calculations have mainly reported the variations in the intermediate species and free radical fractions present during the process of gas explosions. We do not understand and have not clarified the initiation mechanism or primary initiation pathway for multiple flammable gas mixture explosions. This work aimed to understand the initiation mechanism and primary initiation pathway of a CH4/CO/C2H6/C2H4/H2 gas mixture explosion during the initial reaction stage. The purpose of this article is to present a detailed reaction initiation mechanism for a gas mixture explosion and provide a reference for process safety in the petrochemical and mine industries. In this study, density functional theory (DFT) was used to obtain the thermodynamic and dynamic parameters of the gas mixture explosion process via numerical calculations. Additionally, the optimized geometries of the reactants and products of the reactions were found, and the primary reaction pathway for the mixture was determined using an intrinsic reaction coordinate (IRC). Additionally, a series of experiments were performed to investigate the emission spectra of the intermediate products, which validated the reliability of the numerical calculation.

2 Computational method DFT (CAMB3LYP/6-31G) was employed to calculate the thermodynamic and dynamic parameters of the elementary reactions, where the positive reaction activation energy was taken as the activation energy, Ea, of the elementary reaction, and the activation energy of the reverse reaction was the activation energy, Eb, of the reverse reaction. The elementary reaction rate constant 4 / 31


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was calculated using the entropy change. The reaction rate constant for classical thermodynamic transition state theory is defined as:

k 

S  H TS k BT exp( TS ) exp( ) h R RT

(1)

  where H TS and STS are the neutralization and the activation entropy, respectively; h and kB

denote Planck’s constant and Boltzmann’s constant, respectively. R represents the ideal gas constant. T represents the temperature, and the unit of the rate constant is S-1.

3 Experimental 3.1 Experimental system The experimental system shown in Fig. 1 consists of an explosion reaction system, a monochromator (Omni-λ series, Zolix Instruments, Beijing, China), a digital oscilloscope (RTO1004, Rohde & Schwarz, Munich, Germany), an adjustable voltage regulator, and a data collection system. The explosion reaction system is composed of a 20-L spherical vessel, a gas distribution system, an electric ignition system, a data acquisition system, and a controlling computer. The fiber located at the center of the viewport, perpendicular to the ignitor, was connected to the monochromator and oscilloscope to obtain the flame spectral data. The experimental gases were prepared according to the partial-pressure method in a gas distribution system with an accuracy of 0.1%. After the gas was prepared, the circulation pump was turned on for 300 s to ensure the uniformity of the mixed gas. A pair of probes mounted at the center of the explosion vessel and a high-voltage pulse generator composed the electric ignition system. The ignition energy was alterable by changing the capacitor of the high-voltage pulse generator. In this study, the ignition energy was set as 1 J. After the ignition, the optical signal of an explosive flame in the 20-L spherical vessel is transmitted to the monochromator by fibre optics. After its 5 / 31


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dispersion, the monochromatic light of a particular wavelength is transmitted to the data collection system again, and the digital oscilloscope records its voltage waveform. More details about the experimental systems and procedures can be found in previous work [42]. 3.2 Experimental conditions The experiments were carried out at room temperature and atmospheric pressure, and the relative humidity was 58–66%. The equivalent ratio of methane is 1 when it accounts for 9.5% of the system volume, which implies that methane just reacts completely with oxygen. When methane is less than 9.5% of the system volume, the system is an oxygen-rich state. Conversely, the system is in an oxygen-poor state when methane is more than 9.5% of the system volume. In this work, 7.0% and 11.0% methane were selected to represent oxygen-rich and oxygen-poor states, respectively. In industrial operations, a small amount of flammable gas (C2H4, C2H6, CO, and H2) tends to be mixed with methane. Experiments were performed to determine the effect of different volume fractions (0.4%, 0.8%, 1.2%, 1.6%, and 2.0%) of the C2H4/C2H6/CO/H2 mixture with a ratio of 1:1:1:1 for the emission spectral intensities of ·H and CH2O during the methane explosion process. The volume fraction ratio of 1:1:1:1 of C2H6, C2H4, CO, and H2 was determined to ensure the elimination of interference factor such as amounts of different gases. In our work, gas addition ratio was calculated according to the following equation [43], without considering the equivalence ratio. The experimental test scenarios are shown in Table 1. In addition, the wavelengths of ·H and CH2O were set to 656.25 nm and 412.1 nm [44, 45], respectively. The order of appearance of ·H and CH2O was compared according to the arrival time of the spectral peaks. R gas  

Vgas  VCH 4   VCO   VH 2   VC 2 H 4   VC 2 H 6   Vair 

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(2)

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4 Results and Analysis 4.1 Thermodynamic and dynamic analysis The thermodynamic and dynamic data for the 27 elementary reactions involved in this study are shown in Table 2. Besides, the 27 elementary reactions according to DFT are completely consistent with GRI-Mech 3.0 [46]. For the initiating stage of the CH4/CO/C2H6/C2H4/H2 gas mixture explosion, the combustible gases may react with oxygen to trigger chain reactions and an ethylene cracking reaction, as shown in reactions 1 to 6, at the moment of ignition. In contrast, the lowest positive activation energy of 56.99919 kcal/mol is required for reaction 1 (CH4+O2→·CH3+·HO2), which indicates that reaction 1 of the first six elementary reactions occurs at the beginning. Thus, CH4 is involved in the reaction first in the multi-component gas mixture. ·CH3 and·HO2 radicals appear in the system for the first time, and they initiate a series of elementary reactions. After reaction 1 occurs, the reactions that follow are reaction 5 (H2+O2→·HO2+·H), reaction 3 (C2H4+O2→ C2H4O+·O), reaction 4 (CO+O2→CO2+·O), reaction 6 (C2H4→·CH3CH), and reaction 2 (C2H6+O2 →·C2H5 +·HO2). Among these reactions, reaction 5, with an activation energy that is 2.00740 kcal/mol higher than that of reaction 1, has the relative advantage of reacting with oxygen. Chain reactions (from reaction 7 to 15) are induced by ·CH3 and ·HO2 radicals. And reaction 15(·HO2+·CH3→·CH3O+·OH) calculated by DFT is in good agreement with main reaction route according to MD result [40]. The positive activation energy of reaction 8 (·CH3+H2→CH4 +·H), which is only 13.83218 kcal/mol, is the lowest among the first 15 reactions. In this group of reactions, reaction 8 occurs first and generates the critical free radical, ·H. The first appearance of ·H in the main reaction path of the gas mixture explosion also induces follow-up reactions. This sequence demonstrates that H2 enters into the multi-component gas mixture after CH4. According to the comparison, reaction 8 is followed by reaction 12 (·HO2+C2H4 7 / 31


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→·C2H5+O2), which requires 15.58482 kcal/mol and is just 1.75264 kcal/mol higher than that of reaction 8. Reaction 12 thus has a relatively high reaction energy advantage. Similarly, the presence of ·H triggers the chain reactions (reactions 16 to 22). According to the numerical calculation, the positive activation energy of reaction 22 (·H+CO→·HCO) at 3.01581 kcal/mol is the lowest energy among the first 22 elementary reactions. Reaction 22 occurs at the beginning and produces another key free radical, ·HCO, which induces a range of reactions. The Ea of reaction 21 is only 0.22528 kcal/mol higher than that of reaction 22, which means that reactions 21 and 22 occur almost simultaneously. According to the calculation result, reaction 22 is followed by reactions 21 (·H+C2H4→·C2H5) and 20 (·H+C2H6→·C2H5+H2). Therefore, it can be considered that ·H, the key free radical with a high activity, induces CO, C2H4, and C2H6 to successively participate in the reaction. Moreover, chain reactions 23 to 27 are triggered by ·HCO. The Ea of reaction 24 (·HCO+·HO2→CH2O+O2) of only 0.39659 kcal/mol is the lowest of all reactions shown in Table 2. This finding means that reaction 24 occurs first, and the key intermediate product, CH2O, is generated for the first time in the main reaction pathway of the gas mixture explosion, indicating that ·H precedes CH2O. Additionally, reaction 1 (CH4+O2→·CH3+·HO2) has the largest reaction rate constant (k1=6.07087E+12 S-1) among the first six reactions. Reaction 8 (·CH3+H2→ CH4+·H) has the largest one (k8=6.17754E+12 S-1) among the chain reactions induced by ·CH3 and ·HO2 radicals. Among the chain reactions triggered by·H, reaction 22 (·H+CO→·HCO) has the largest reaction rate constant (k22=6.07087E+12 S-1), and those of reaction 20 (·H+C2H6→·C2H5+H2) and reaction 21 (·H+C2H4 →·C2H5) are very close to that of reaction 22. Reaction 27(·HCO+·HO2→CH2O+O2) has the largest one (k27=6.21137E+12 S-1) among the chain reactions initiated by·HCO. Generally, the reaction rate constants of the chain reactions triggered by·H are larger overall, especially ·H 8 / 31


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reacting with CO, C2H4 and C2H6. Therefore, ·H is the most critical free radical at the initial stage of the mixture explosion. 4.2 Analysis of the reaction process and the IRC reaction channel for the explosion of multiple flammable gases According to the thermodynamic and dynamic data given above, reactions 1, 8, 20, 21, 22, and 24 have a relatively high reaction advantage among the 27-step elementary reactions involved in the explosion of the CH4/C2H6/C2H4/CO/H2 gas mixture. In addition, the relative parameters, such as the reactants, transition state and configuration, bond length, bond angle, and dihedral angle of the products, for each reaction were obtained, as shown in Tables 3-8. The IRC reaction channels for each reaction were acquired, as shown in Figs. 2-7. Moreover, all bond distances are consistent with previous results using Perdew–Burke–Ernzerhof functional. RC-H (CH4) =1.09 Å [47], RO-O (O2) = 1.23 Å [48], RH-H (H2) =0.74 Å, RO-H (H2O) =0.97 Å [47], RC-O (CO2) =1.17 Å [47], RC-O (CO) =1.14 Å [47], RC-H (C2H4) =1. 09 Å, RC-H (C2H6) =1. 09 Å. Table 3 and Fig. 2 illustrate that in the chain initiation reaction of CH4+O2→ ∙CH3+∙HO2, R(C1,H3) gradually increases from the initial 1.092263 Å to 1.94555 Å, and the stretching vibration of the C-H bonds in CH4 becomes an inelastic stretching vibration due to the emergence of the oxygen atom. Eventually, H is released from CH4 and bonds with the O6 atom. The original tetrahedral structure of CH4 becomes planar as the structure of ∙CH3 due to the cleavage of C-H bonds. Moreover, the distance of R(O6,O7) gradually increases from 1.20565 Å to 1.31555 Å due to the bonding of H, and R(O6,O7) changes from a single-double bond to a single bond. Furthermore, 57.00 kcal/mol of energy is released, and ∙CH3 and ∙HO2 are generated in this process. Table 4 and Fig. 3 show that in the chain transfer reaction of·CH3+H2→CH4+·H, H2 gradually approaches ∙CH3, and R(C1, H5) gradually decreases from 3.88799 Å to 1.41938 Å. C1 appears in the direction of the elastic stretching vibration of H5 and H6, of which the elastic expansion is 9 / 31


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destroyed. R(H5, H6) increases from 0.74299 Å to 0.89117 Å, and the H5 and H6 bonds break. A(C1, H5, H6) enlarges from 165.113615° to 179.99831°, and D(H3, C1, H2, H4) narrows from 179.222° to 137.36°, where 8.81 kcal/mol is absorbed to form a transition state. R(C1, H5) continues to decrease to 1.09423 Å, and then C1 and H5 bond. R(H5, H6) increases to 2.663 Å, while D(H3, C1, H2, H4) decreases to 120.032°. The CH4 structure develops into a regular tetrahedron in which CH4 and ∙H are produced, releasing 12.37 kcal/mol. As determined from Table 5 and Fig. 4, H8 gradually approaches H9 in the chain transfer reaction (∙H+C2H6→·C2H5+H2). R(H8, H9) decreases from 2.61795 Å to 0.92412 Å, and R(C2, H9) increases from 1.9693 Å to 1.37111 Å. A transition state is developed with 9.43 kcal/mol produced in this process. R(H8, H9) decreases to 0.74348 Å once again, causing H8 and H9 to bond. R(C2, H9) continues to increase until the bond breaks.·C2H5 and H2 are finally generated, releasing 10.24 kcal/mol. Table 6 and Fig. 5 show that as H3 approaches C1, R(C1, H3) decreases from 3.59945 Å to 2.277792 Å, whereas R(C1, C2) increases from 1.3348 Å to 1.34133 Å in the chain transfer reaction (·H+C2H4→·C2H5). Meanwhile, A(H3, C1, C2) narrows from 179.771331° to 106.72812°, and A(H3, C1, H5) increases from 58.18046° to 82.2837°. A transition state is formed that absorbs 0.55 kcal/mol in this process. R(C1, H3) decreases to 1.10536 Å again, which causes the bonding of H3 and C1. R(C1, C2) continues to increase to 1.49105 Å, and A(H3, C1, H5) increases to 106.42611°. During this process, ·C2H5 is generated, releasing 40.90 kcal/mol. As shown in Table 7 and Fig. 6, R(C3, H1) decreases from 3.31394 Å to 2.12229 Å, and R(O2, C3) increases from 1.13737 Å to 1.14015 Å with H1 gradually approaching C3 in the chain transfer reaction of ·H+CO→·HCO. Additionally, A(O2, C3, H1) increases from 113.49927° to 116.44746°, and a transition state is formed that absorbs 1.13 kcal/mol. R(C3,H1) continues to 10 / 31


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decrease to 1.1250 Å, leading to the bonding of H1 and C3. R(O2,C3) continues to increase to 1.12823 Å as one of the triple bonds between O2 and C3 breaks, forming a carbon-oxygen double bond. A(O2, C3, H1) continues to increase to 123.764°, and ·HCO is finally produced, releasing 25.93 kcal/mol in the process. As shown in Table 8 and Fig. 7, R(C3,H6) decreases from 1.99994 Å to 1.50276 Å, whereas R(O4, H6) increases from 0.99939 Å to 1.11350 Å with H6 positioned near the C3 of the formaldehyde group in the chain reaction of ·HCO+HO2→·CH2O+O2. A(C3, H6, O4) increases from 149.35765° to 158.61782°, and A(H1, C3, H6) increases from 97.57241° to 104.13897°. The elastic stretching of H6 and O4 is destroyed, and the bonds of H6 and O4 break as a result of the approach of C3 in which a transition state is formed, absorbing 2.04 kcal/mol. R(C3, H6) continues to decrease to 1.10607 Å, leading to the bonding of H6 with C3. With an increase in the distance between H6 and O4, the distance between O4 and O5 decreases, forming a π bond. In the process, ·CH2O and O2 are finally generated, releasing 39.68 kcal/mol. On the basis of quantitative analysis of the reaction process for the explosion of multiple flammable gases, the reaction system of self-promotion and microcirculation regarding multiple flammable gases explosion was constructed as shown in Fig. 8. The chain reaction of the entire explosion reaction was initiated by ∙CH3 which created in the oxidation reaction of CH4. The presence of H2, CO, C2H4 and C2H6 promoted the chain reaction as a result of the increase of key radicals such as ·H, ·OH, ·O, ·HO2 and ∙CH3, which is in agreement with previous results[49]. Additionally, many research findings [34, 35] revealed that key radicals play a critical role in the process of gas explosion. From the perspective of statistical distribution, C2H6 and C2H4 are much less frequently involved in the collision than CO and H2 [50], and the activation energy of C2H6 and C2H4 is higher than that of CO and H2. Therefore, the sequence for multiple flammable gases 11 / 31


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involved in the explosion reaction is CH4, H2, CO, C2H4 and C2H6 according to quantitative analysis. 4.3 Emission spectral intensity of intermediate products during the gas explosion process Upon adding the C2H6/C2H4/CO/H2 mixture to methane (7.0%, 9.5%, or 11.0%), the emission spectra of ·H and CH2O were measured during the methane explosion process, as shown in Fig. 9. Wang et al. [42] and Luo et al. [51] also found that the effects of other flammable gases on the behavior of free radicals is in accordance with the explosion parameters such as explosion pressure.Because the spectral intensities of ·H and CH2O are very weak during the explosion process for 7% pure methane, they are not shown in the figure. As illustrated in Fig. 9, the emission spectral intensities of ·H and CH2O gradually increased during the explosion process for 7.0% methane as a mixture fraction from 0.4% to 2.0% was added. With regard to 9.5% methane, the emission spectral intensities of ·H and CH2O fluctuated approximately between 70 V and 80 V as the amount of the mixture increased from 0% to 2.0%. The intensity of the ·H emission peak significantly decreased only when 2.0% was added. After 2.0% mixture addition, the amount of O2 involved in the oxidation reaction declined sharply and the multiple flammable gas reacted in an oxygen-poor condition, which reduced the production of ·H significantly. For 11.0% methane, the intensities in the emission spectra of ·H and CH2O decreased upon adding the flammable gas mixture. However, the spectral intensity peak for ·H always appeared before that of CH2O during the combustible gas explosion process under the 17 operating conditions. Moreover, the order of the ·H and CH2O appearance was somewhat indicated by the arrival time of the spectral peaks, which demonstrates that the appearance of ·H precedes the appearance of CH2O during the CH4/C2H6/C2H4/CO/H2 explosion process. The numerical calculation shows that CH2O appeared in the subsequent reaction of ·H in the main chain reaction, which is consistent with the experimental 12 / 31


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results. This finding verifies that the numerical calculation was reliable to a certain extent. According to numerical and experimental results, the addition of C2H4/C2H6/ CO/H2 mixture make the chain reactions more complicated. It generated a tremendous amount of free radicals and increased the concentration of the activation center. Thus the CH4 explosion reaction was dramatically intensified.

5 Conclusions In present work, the primary initiation pathway for the CH4/CO/C2H6/C2H4/H2 gas mixture explosion at the initial reaction stage was obtained and as follows: ① CH4+O2 →·CH3+·HO2 ② ·CH3+H2→CH4+·H

③ ·H+CO→·HCO ④ ·H+C2H4→·C2H5 ⑤ ·H+C2H6→·C2H5+H2

⑥·HCO+·HO2→CH2O+O2. Therefore, the sequence for multiple flammable gases involved in the explosion reaction is CH4, H2, CO, C2H4 and C2H6. Clearly, the oxidation reaction of CH4 initiates the chain reaction of the entire explosion reaction rather than the involvement of the CO/C2H6/C2H4/H2 mixture at the moment that the explosion is ignited. The addition of the gas mixture has an obvious influence on the transition between different chain reactions. Moreover, ·CH3 produced during methane oxidation induced the involvement of H2 and its reaction product, ·H, triggering other flammable gases to enter into the reaction. This chain of events considerably decreased the activation energy of the gas mixture in the reaction. Furthermore, the experimental results showed that the intensity of the spectral peak of ·H always appeared before that of CH2O during this combustible gas explosion process, which verified the numerical calculation. Although informative, the experiments in this paper are relatively few because of imperfect experimental method. Future work should focus on experimental study on the initiation mechanism of the gas mixture. PLIF (planar laser induced fluorescence) technology will be used to investigate 13 / 31


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Page 14 of 31

spectral characteristics of the key radicals during the initial reaction stage, which could better reveal the micro mechanism of the multiple flammable gas explosion.

Acknowledgements This work is supported by the National Key Research and Development Program of China (Project No. 2017YFC0804702 & 2016YFC0800100), the National Natural Science Foundation of China (Grant No. 51674193 & 51504190) and the Fundamental Research Project for Natural Science of Shaanxi (No. 2017JM5068).

14 / 31


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Energy & Fuels

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[51] Luo Z, Wang T, Wen H, et al. Explosion and flame emission spectra characteristics of CH4-Air mixtures with CO addition. Journal of China Coal Society, 2019; DOI:10.13225/j.cnki. jccs.2018.1123

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Energy & Fuels

Appendix 1: Fig. 1 Schematic diagram of the apparatus Fig. 2 Schematics of the principle reaction between CH4 and O2 Fig. 3 Schematics of the principle reaction between ·CH3 and H2 Fig. 4 Schematics of the principle reaction between ·H and C2H6 Fig. 5 Schematics of the principle reaction between ·H and C2H4 Fig. 6 Schematics of the principle reaction between ·H and CO Fig. 7 Schematics of the principle reaction between ·HCO and·HO2 Fig. 8 Reaction mechanism of self-promotion and microcirculation for the explosion of multiple flammable gases Fig. 9 Changes in the emission spectral intensities of ·H and CH2O

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Fig. 1 Schematic diagram of the apparatus

Fig. 2 Schematics of the principle reaction between CH4 and O2

Fig. 3 Schematics of the principle reaction between ·CH3 and H2 22 / 31


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Energy & Fuels

Fig. 4 Schematics of the principle reaction between ·H and C2H6

Fig. 5 Schematics of the principle reaction between ·H and C2H4

Fig. 6 Schematics of the principle reaction between ·H and CO 23 / 31


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Fig. 7 Schematics of the principle reaction between ·HCO and·HO2

Fig. 8 Reaction mechanism of self-promotion and microcirculation for the explosion of multiple flammable gases

(a) 0%

(b) 0.4%

24 / 31


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Energy & Fuels

(c) 0.8%

(d) 1.2%

(e) 1.6%

(f) 2.0%

Fig. 9 Changes in the emission spectral intensities for ·H and CH2O

25 / 31


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Appendix 2: Table 1 Experimental test scenarios Table 2 Activation energies and reaction rate constants of the elementary reactions Table 3 Optimized structures and parameters for the species in the reaction of CH4+ O2→·CH3+·HO2 Table 4 Optimized structures and parameters for the species in the reaction of ·CH3+ H2→CH4+·H Table 5 Optimized structures and parameters for the species in the reaction of ·H+ C2H6→·C2H5+H2 Table 6 Optimized structures and parameters for the species in the reaction of ·H+ C2H4→·C2H5 Table 7 Optimized structures and parameters for the species in the reaction of ·H+ CO→·HCO Table 8 Optimized structures and parameters for the species in the reaction of ·HCO +·HO2→CH2O+O2

26 / 31


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Energy & Fuels

Table 1 Experimental test scenarios Gases

Oxygen-rich

Stoichiometric

Fuel-rich

VCH4

7.0%

9.5%

11.0%

0%-2.0%

0%-2.0%

0%-2.0%

1- VCH4- Vmixture

1- VCH4- Vmixture

1- VCH4- Vmixture

Vmixture Mixture(CO/H2/C2H6/C2H4) Vair

27 / 31


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Table 2 Activation energies and reaction rate constants of the elementary reactions Free radical initiation

Reaction

reaction

number

Oxidation or cracking

Chain reaction

Activation energy of

Activation energy of

the positive reaction

the reverse reaction

(kcal/mol)

(kcal/mol)

Reaction rate constant(S-1)

1

CH4+O2→·CH3+·HO2

56.99919

-

6.07087E+12

2

C2H6+O2→·C2H5+·O2

119.09626

-

5.92052E+12

3

C2H4+O2→C2H4O+·O

66.19471

29.66237

6.04837E+12

4

CO+O2→CO2+·O

67.63735

70.47870

6.04476E+12

5

H2+O2→·HO2+·H

59.00659

-

6.05965E+12

6

C2H4→·CH3CH

75.46177

2.79053

6.02611E+12

7

·CH3+O2→CH2O+·OH

58.51211

27.82753

6.06701E+12

8

·CH3+H2→CH4+·H

13.83218

13.79328

6.17754E+12

9

·CH3+C2H4→CH4+·C2H3

21.06925

14.79353

6.15948E+12

10

·CH3+C2H6→CH4+·C2H5

18.77947

23.64769

6.16514E+12

11

·HO2+CH4→·CH3+H2O2

27.99821

5.36458

6.1425E+12

12

·HO2+C2H4→·C2H5+O2

15.58482

8.29442

6.17323E+12

13

·HO2+H2→·H+H2O2

26.07678

3.81714

6.14727E+12

14

·HO2+CO→CO2+·OH

20.13552

11.42004

6.16201E+12

15

·HO2+·CH3→·CH3O+·OH

26.44073

52.25710

6.14639E+12

16

·H+O2→·O+·OH

43.36403

23.27621

6.10446E+12

17

·H+·HO2→·O+H2O

21.26880

64.32536

6.15894E+12

18

·H+·CH3→·CH2+H2

15.32942

9.22313

6.17396E+12

19

·H+·C2H5→C2H4+H2

11.10189

15.88728

6.18448E+12

20

·H+C2H6→·C2H5+H2

9.11834

14.74208

6.18952E+12

21

·H+C2H4→·C2H5

3.24109

40.58982

6.20415E+12

22

·H+CO→·HCO

3.01581

20.53964

6.20475E+12

23

·HCO+O2→·HO2+CO

64.71567

90.16181

6.05204E+12

24

·HCO+·HO2→CH2O+O2

0.39659

40.59170

6.21137E+12

25

·HCO+H2→CH2O+·H

21.89882

4.52874

6.15746E+12

26

·HCO+·H→CH2O

5.35642

88.41858

6.19887E+12

27

·HCO+CH4→CH2O+·CH3

26.47524

8.95644

6.14622E+12

of multiple flammable gases

·CH3 and·HO2

·H

·HCO

28 / 31


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Energy & Fuels

Table 3 Optimized structures and parameters for the species in the reaction of CH4+ O2→·CH3+·HO2 Name

Configuration parameters

Configuration

Bond length

Bond angle

Dihedral angle

R(C1,H3)

A(H5,C1,H3)

D(H5,C1,H3,O6)

1.09263

109.44287

72.24478

R(H3,O6)

A(C1,H3,O6)

3.58780

85.96156

R(C1,H3)

A(H5,C1,H3)

D(H5,C1,H3,O6)

1.94555

86.57053

-0.00026

R(H3,O6)

A(C1,H3,O6)

1.00223

170.20063

CH4+O2 -

CH3+ HO2 -

Table 4 Optimized structures and parameters for the species in the reaction of ·CH3+ H2→CH4+·H Name


Configuration Bond length

Bond angle

Dihedral angle

R(C1,H5)

A(C1,H5,H6)

D(H3,C1,H2,H4)

3.88799

165.13615

-179.22259

R(H5,H6)

A(H3,C1, H4)

0.74299

120.03048

R(C1,H5)

A(C1,H5,H6)

D(H3,C1,H2,H4)

1.41938

179.99831

-137.35948

R(H5,H6)

A(H3,C1, H4)

0.89117

115.01699

R(C1,H5)

A(C1,H5,H6)

D(H3,C1,H2,H4)

1.09423

179.87989

-120.03284

R(H5,H6)

A(H3,C1, H4)

2.66348

109.47718

·CH3+H2

Transition state

-

-

CH4+·H

29 / 31


-

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Table 5 Optimized structures and parameters for the species in the reaction of ·H+ C2H6→·C2H5+H2 Configuration parameters Name


Bond angle

Dihedral angle

R(C2,H9)

A(C2,H9,C8)

D(H6,C2,C1,H9)

1.09693

175.85505

-120.00638

R(H8,H9)

A(C1,C2,H9)

2.61795

111.32811

R(C2,H9)

A(C2,H9,C8)

D(H6,C2,C1,H9)

1.37171

178.38686

-112.23301

R(H8,H9)

A(C1,C2,H9)

0.92412

106.93128

R(C2,H9)

A(C2,H9,C8)

D(H6,C2,C1,H9)

3.37313

177.04807

-94.18482

R(H8,H9)

A(C1,C2,H9)

0.74348

101.64077

·H+C2H6 -

Transition state -

·C2H5+H2 -

Table 6 Optimized structures and parameters for the species in the reaction of ·H+ C2H4→·C2H5 Configuration parameters Name


Bond angle

Dihedral angle

R(C1,H3)

A(H3,C1,C2)

D(H4,C1,C2,H5)

3.59945

179.77731

-180.00000

R(C1,C2)

A(H3,C1,H5)

1.33480

58.18046

R(C1,H3)

A(H3,C1,C2)

D(H4,C1,C2,H5)

2.27792

106.72821

173.73786

R(C1,C2)

A(H3,C1,H5)

1.34133

83.28376

R(C1,H3)

A(H3,C1,C2)

D(H4,C1,C2,H5)

1.10536

111.95627

121.34829

R(C1,C2)

A(H3,C1,H5)

1.49105

106.42611

·H+C2H4

Transition state

·C2H5

30 / 31


-

-

-

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Energy & Fuels

Table 7 Optimized structures and parameters for the species in the reaction of ·H+ CO→·HCO Configuration parameters Name


Bond angle

R(C3,H1)

A(O2,C3,H1)

3.31394

113.49927

Dihedral angle

-

·H+CO R(O2,C3) 1.13737

Transition state

R(C3,H1)

A(O2,C3,H1)

2.12229

116.44746

R(O2,C3) 1.14015

·HCO

-

-

R(C3,H1)

A(O2,C3,H1)

1.12500

123.76400

R(O2,C3) 1.18283

-

-

-

-

-

-

Table 8 Optimized structures and parameters for the species in the reaction of ·HCO +·HO2→CH2O+O2 Name



Bond angle

Dihedral angle

R(C3,H6)

A(C3,H6,O4)

D(H1,C3,O2,H6)

1.99994

149.35765

-180.00000

R(O4,H6)

A(H1,C3,H6)

0.99939

97.57241

R(C3,H6)

A(C3,H6,O4)

D(H1,C3,O2,H6)

1.50276

158.61782

-179.98969

R(O4,H6)

A(H1,C3,H6)

1.11350

104.13897

R(C3,H6)

A(C3,H6,O4)

D(H1,C3,O2,H6)

1.10607

102.87170

-179.99954

R(O4,H6)

A(H1,C3,H6)

2.90125

116.43889

·HCO+·HO2

Transition state

CH2O+O2

31 / 31


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Micromechanism of the Initiation of a Multiple Flammable Gas Explosion

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