Fuel 124 (2014) 85–90

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Fuel

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The intrinsic mechanism of methane oxidation under explosioncondition: A combined ReaxFF and DFT study

http://dx.doi.org/10.1016/j.fuel.2014.01.0700016-2361/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors. Tel.: +86 01082687086.E-mail addresses: limin.liu@csrc.ac.cn (L.-M. Liu), wjzhu@caep.ac.cn (W. Zhu).

Zhenghua He a,b, Xi-Bo Li b,c, Li-Min Liu b,⇑, Wenjun Zhu a,⇑a National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, Mianyang 621900, Sichuan, Chinab Beijing Computational Science Research Center, Beijing 100084, Chinac Chengdu Green Energy and Green Manufacturing Technology R&D Center, Chengdu 610207, Sichuan, China

h i g h l i g h t s

� A complete chain reaction from CH4

to CO/CO2 under the explosioncondition is proposed at first time.� �OH and �HO2 are the main free radical

carriers for the whole reaction chain,and HCHO is the key intermediate forCH4 oxidation.� The reaction activity of gas explosion

system closely depends on theconcentration of the free radicals,such as �CH3, �OH and��HO2.� The methane explosion process will

be suppressed efficiently bycontrolling the concentration of freeradicals.

g r a p h i c a l a b s t r a c t

The concentration changes of main species involved in the methane–oxygen explosion process arerevealed along with MD simulation time. The CH4 is oxidized by forming the main intermediates, CH3OHand HCHO.

a r t i c l e i n f o

Article history:Received 11 September 2013Received in revised form 21 January 2014Accepted 22 January 2014Available online 5 February 2014

Keywords:Gas explosionOxidation mechanismChain reactionReaxFFDFT

a b s t r a c t

In order to develop efficient suppression techniques and isolation materials for gas explosion, it is greatlyvital to understand the intrinsic mechanism of gas explosion at atomic level. The methane explosion pro-cess was systematically explored by reactive force field molecular dynamics (MD) simulation and densityfunction theory (DFT) calculations. The results show that the initiation step of the gas explosion process ismainly induced by �OH free radical, and the chain transfer process is primarily carried by the conversionfrom �HO2 to �OH radical. The number of free radicals, such as �CH3, �HO2 and �OH, greatly affect the reac-tion velocity during the gas explosion process. These results not only identify the critical reaction inter-mediates and main radical carriers of methane explosion process, but also they will provide significanttheoretical guide for development of the novel and efficient gas explosion suppression methods and iso-lation materials.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, the production safety of petrochemical and coalmine industry have attracted numerous attentions from the indus-try community, and gas explosion is one of the most dangerous

86 Z. He et al. / Fuel 124 (2014) 85–90

fields [1]. It is well-known that gas explosion is a rapid and drasticchemical reaction process, involving a huge amount of energytransfer. The shock wave generated by explosion may cause fatalharm for either buildings or even human. The explosion processcan be automatically accelerated through the accumulation ofthe free radicals, and the concentrations of reactants and free rad-icals are widely believed to affect the methane branch explosion[2,3]. There is still no an effective technique to suppress this pro-cess in order to reduce the damage of gas explosion.

Because of the importance of reaction kinetics, the kinetic prop-erties of gas explosion become the focus of gas explosion research[4]. The basic mechanism of methane oxidation is widely studied,which is typically represented by the following equation [5,6]:

CH4 þ 2O2 ! CO2 þ 2H2O ð1Þ

This equation is highly refined from a large number of freeradical chain reactions [7,8]. Many detailed chemical kinetic mech-anisms of methane combustion have been suggested, whichalways contain dozens of species and hundreds of reactions[3,5,9,10]. The most popular kinetic mechanism is provided byGRI-Mesh 3.0, [3] which contains 325 elementary reactions involv-ing 53 species, based on experimental data [3,5]. The associatedrate constant expressions can be calculated from GRI-Mesh. Butthe kinetic models are rather complex to analyze the real thermo-dynamic and kinetic properties. To understand the detailed reac-tion mechanism, some reduced combustion mechanisms areproposed for some specific reaction condition [11–14]. For exam-ple, Peters suggested that a mechanism only contained 18 complexreactions and 14 species, which was simplified from the originalmechanism by using the steady-state and partial equilibriumhypotheses [12]. The ignition process, flame propagation and theexplosion pressure change during the gas explosion were alsowildly investigated, [15–19] basing on the kinetic model men-tioned above. Especially, Combustion-induced Rapid Phase Transi-tion (CRPT) was a new flame propagation and pressure generationmode proposed by Di Benedetto et al. [19]. They attributed theover-pressure during the gas explosion to the triggering of physicalexplosion (rapid phase transition) by a chemical reaction (combus-tion). The basic reactions and main species involved in the meth-ane combustion process can be recommended from thesemechanisms. But the detailed and intrinsic reaction processes,such as the actual reaction routes and key reaction steps, are stillnot fully explored at atomic level because of the complexity ofthe reactions. Meanwhile, these kinetic mechanisms based on thefundamental combustion experiments may fail to describe explo-sion system.

In the recent decades, many groups examined the effect of thethird component on intrinsic origin of the methane combustion[16,17,20–26]. Kneer and his coworkers [21] investigated the effectof CO2 on burning rate during the methane combustion. They re-ported that the addition of CO2 would cause a poor combustionperformance, which greatly decreased the burning velocity. Zengand Liao [24] discussed the effect of water addition on the gasexplosion. And they found that the induced explosion time wasprolonged with increasing water addition. On the other side, thegas explosion flame temperature and max-overpressure can be sig-nificantly attenuated by adding porous material [27–31]. Althoughthe current suppression methods for methane-air explosion candecrease the burning rate to some extent, there is still no an effec-tive way to meet the requirement of the industry productionsafety, especially for the continuous and multiple explosions [1].Thus, it is urgent to understand the detailed mechanism of gasexplosion at atomic level in order to further develop the more effi-cient suppression method.

In this article, a systemic study of the methane explosionprocess was carried out by a combination of first-principles

calculations and molecular dynamics (MD) simulation. The de-tailed properties of explosion process were examined by analyzingthe MD simulation trajectory. The results show that the initiationstep of the gas explosion process is mainly induced by �OH free rad-ical, and the chain transfer process is primarily carried by the con-version from �HO2 to �OH radical. The reaction activity of explosionsystem had a great dependence on the concentration of main spe-cies, such as �CH3, �HO2 and �OH, involved in the explosion process.Such results will not only help to promote the development of newgas explosion suppression method and isolation material, but alsoshed light on progress of the gas phase detonation theory.

2. Computational methods

In order to accurately understand the explosion mechanism ofmethane–oxygen system, both reactive force field moleculardynamics (MD) simulation and DFT calculations, were used tostudy the detailed reaction process and key reaction steps. TheMD was firstly run to examine the whole explosion process andmain reaction routes. After this, the intrinsic kinetics propertiesof key reaction steps were further checked by DFT calculations.

2.1. Molecular dynamics (MD) simulation

A 3-dimensions periodic system, containing 50 CH4 and 100 O2

molecules, was used in the calculations, with the density of0.2184 g/cm�3 (see Fig. S1 in the supplementary materials). TheLammps package was employed for whole MD calculation withthe reactive force field, as implanted in ReaxFF package [32,33].The parameters for C, H and O have been identified accuratelyfor such kind of oxidation process, as shown in the previous study[32].

The initial system was firstly equilibrated at 298 K for 50 ps toget the reasonable initial configuration. After the equilibrium, thesystem temperature was further raised to 2000 K from 298 K with-in 50 ps at uniform rate. The NVT canonical ensemble was em-ployed for these two stages to control the volume andtemperature of the system. During the first two stages, no reactionbetween molecules was observed. After these two stages, micro-canonical ensemble (NVE) was used to explore the detailedreaction process between CH4 and O2. While once upon the reac-tion occurs, the system will release a huge amount of energy.And the simulated system cannot transfer this energy to the envi-ronment or the other zone. Thus the temperature of the simulatedsystem may be too high to mimic the real situation at the finalstage. To solve this problem, we used the T-rescaling method tocontrol the temperature within 3000 K at the final stage of the sim-ulation to avoid the unrealistic results. The time step of 0.1 fs wasused for all calculations. The whole reaction process was run for2.5 ns.

In order to check the reliability of our model, another two MDsimulations with different initial configurations were also carriedout. The results show that the change trends of main species andthe product distribution in the oxidation process are quite closeto each other (see Fig. S2 in the supplementary materials).

2.2. Density functional theory (DFT) calculations

The complex reaction process of gas explosion was firstly ex-plored with MD simulation, and then the key reaction processesobserved in MD were examined by DFT calculations in order to fur-ther understand the reaction mechanism at the atomic level. Thekinetic properties of reaction system was calculated with theGaussian03 program package [34]. The geometrical structure opti-mization of the reactants, products, intermediates and transition

Z. He et al. / Fuel 124 (2014) 85–90 87

states were performed using B3LYP functional, [35,36] along withthe 6-31G(d) basis sets for all atoms. A spin-unrestricted set wasemployed for all open-shell species. Reaction potential energy sur-face (PES) scans were performed for all reactions to help for search-ing transition state (TS) [37]. Harmonic vibrational frequencycalculations were performed at the same level in order to confirmthe various stationary points to be either a minimum or a transi-tion state [38]. Intrinsic reaction coordinate (IRC) [39,40] calcula-tions were carried out to make sure that each TS connected itsreactions and products. The energies discussed below were relativeGibbs free energies (DG298K). The relative enthalpies (DH298K) andZPE corrected electronic energies (DE0K) were also provided for rel-evant discussion (see Figs. S4–S9 in the supplementary materials).

Fig. 2. Changes of the main free radical numbers as a function of simulation time(t).

3. Results and discussion

3.1. MD simulation of the methane oxidation

As mentioned above, the system was firstly equilibrated at298 K for 50 ps, and then it was raised to 2000 K gradually. Noreaction between CH4 and O2 was observed in such stages. The ini-tial reaction of the system occurs at about 200 ps during the MD-NVE simulation, and the main reactions completed after �2 ns.The molecular number of reactants (CH4 and O2) and main prod-ucts (CO2, H2O and CO) almost maintained constant. As for the finalstage, the MD simulation was continued for further 500 ps, onlyseveral reactions were observed owing to the lack of the reactants(only 4 of 50 methane molecules left) and free radicals [2,41] (�OHand �HO2) (see Figs. 1 and 2).

Fig. 1(a) displayed the changes of the main molecular numbersin the reaction system during the MD-NVE simulation process. Atthe first 300 ps, two methane molecules react with O2 to formCH3O2

� and CH3OOH, respectively. The oxidation of methane startswith the abstraction of the H atom from the CH4 by O2. Duringsuch process, the initial two free radicals, �CH3 and �HO2, are firstlyproduced (see Fig. 2(a) and (d)). This is a strong endothermic pro-cess with an energy of 60.60 kcal mol�1 (DH298K). After this, themethyl combines with the oxygen to form the methyl peroxide(CH3OO�) radical. This reaction is exothermic and has no energybarrier [42]. And the reverse reaction is also observed in later sim-ulation process. The first CH3OO� radical is consumed by reactingwith another methane molecule to form �CH3 radical and CH3OOHmolecule. And then the CH3OOH decomposes into CH3O� and hy-droxyl (�OH) radical, which are the key species for methaneoxidation.

Fig. 1. Changes of the main molecular numbers as a function of simulation time (t):(a) reactants and products and (b) main intermediates.

As shown in Fig. 1, the reaction speed becomes obviously fastafter 500 ps. During the 500–750 ps, 10 CH4 molecules are con-sumed, along with 7 of 10 CH4 molecules converting into HCHOmolecules. Within the later 150 ps (from 750 to 900 ps), a fasterreaction process is observed. The number of CH4 molecules de-creases rapidly from 36 to 15, and 11 CH3OH molecules are formedduring this period. But the number of HCHO molecules does not in-crease obviously, because some of them are consumed into CO andCO2 molecules, [43] and the intermediate of H2 is also producedduring this process (see Fig. 1(b)). The following stage is a steadystate for HCHO, and the total molecular number of HCHO is almostconstant. When the reaction proceeds to 2000 ps, the numbers ofmain species involved in the explosion process almost keep con-stant, as shown in Fig. 1.

The reaction process can be further understood by analyzing theconcentration changes of free radicals. As shown in Fig. 2, the con-centrations of the �CH3, �OH and �HO2 free radicals have a significantincrease after 500 ps. The reaction is apparently promoted by theaccumulation of the free radicals. During 790–890 ps, the numberof free radicals reaches the maximum successively. The concentra-tion of main free radicals also achieves the maximum at about810 ps (see Fig. 2(f)), which indicates that the system reaches thegreatest reaction activity. It should be noted that the T-rescalingbecomes active after �900 ps during the MD-NVE simulation (seeFig. S3 in the supplementary materials). When reaction proceedsto �900 ps, most of reactants (35 of 50 methane molecules) areconsumed to mainly form the key intermediates. And the free rad-ical accumulation process also completes at�810 ps. The followingreactions are only an extension of the methane oxidation. After2000 ps, the system was calmed down, and no �CH3 free radicalwas observed. And the total number of main radicals maintainedat a low level of 2.

Fig. 3. Main reaction routes from step 1 to 3 for the methane oxidation. (The greennumbers denote the number of the molecules, corresponding to the reactionroutes). (For interpretation of the references to color in this figure legend, the readeris referred to the web version of this article.)

Fig. 4. Main reaction routes from step 4 to 6 for the methane oxidation. (The greennumbers denote the number of the molecules, corresponding to the reactionroutes). (For interpretation of the references to color in this figure legend, the readeris referred to the web version of this article.)

88 Z. He et al. / Fuel 124 (2014) 85–90

During the MD simulations, there are many different reactionroutes for the same kind of reaction step. In order to distinguishthe main routes, the whole reactions for the different reactionsteps are analyzed. Figs. 3 and 4 show all the main reactions duringthe gas explosion process. In the simulation process, 46 methanemolecules are consumed into methyl free radicals at first step, witha conversion ratio of 92%. This conversion process could proceedthrough seven different paths. The main three routes are the reac-tions between CH4 with �OH, O2 and �H. The atomization of meth-ane into methyl and hydrogen atom was also observed (seeFig. 3(a)). After that, 31 �CH3 free radicals (67.39%) are consumedwith five different paths to generate the CH3O� free radicals. Themain three paths for this process are the reactions between �CH3

radical with �HO2, CH3OO� and �OH free radicals. Another 13 �CH3

radicals are consumed into formaldehyde (HCHO) through severalreaction steps, as listed in Fig. 3(b). The CH3OH molecule and�CH2OH radical are two important intermediates during theseprocesses. The decomposition of the CH3OO� radical is alsoobserved, resulting in generating HCHO and �OH. And then, 29CH3O� free radicals further convert into HCHO molecules. Most ofthe CH3O� free radicals (20 radicals with the fraction of 64.52%)are abstracted hydrogen atoms directly to form the HCHO. Andthe other 7 CH3O� radicals firstly convert into �CH2OH, which alsocan dehydrogenate to produce HCHO molecules finally.

As shown in Fig. 4, 42 �CH3 (91.30%) radicals convert into HCHOin all. Most of the HCHO (61.90%) molecules are further consumedinto �CHO radical and HCOOH. Such processes include several reac-tion paths, and the main paths are the reactions between HCHOwith �OH and �H radicals. 9 �CHO free radicals (45.00%) are com-bined with �OH radicals into HCOOH. The other �CHO radicals,and the whole HCOOH molecules convert into �CO2H free radicalsfinally, [44] which are the important species for the gas explosionsystem to produce the CO and CO2. During these processes, theintermediate, HOCO (see Fig. 4(c)), [32,45] is observed in manyreactions, and �CO2H radical can also be abstracted an H atom di-rectly to form CO2.

As discussed above, the methane oxidation process

under explosion condition contains six major steps: CH4 !step1

�CH3

!step2CH3O� !step3

HCHO !step4�CHO=HCOOH !step5

�CO2H !step6CO=CO2.

Although some of the species in this reaction route were discussedin previous work, [32] only one of the possible reaction route wasexamined. In our works, the main reaction route is examined bycomparing all the possible reaction paths involved in the gas explo-sion process. More importantly, the radical accumulation process isalso studied by MD-NVE method, which is vital to understand theexplosion properties in the real condition. In this article, the firstreaction step is the most important one because it is the initiationstep of the chain reaction, which is mainly induced by �OH free rad-ical (see Fig. 3(a)). The reaction from �CH3 to CH3O� is critical for thereaction chain transfer, which is primarily carried by the conver-sion from �HO2 to �OH radical. It is the key reaction for the forma-tion of �OH radical. Almost all CH4 molecules consumed areconverted into HCHO molecules, which are the most key interme-diate during the gas explosion process. At the initial reaction per-iod, the CH4 molecules prefer to convert into HCHO by formingCH3O� free radicals, and no CO or CO2 molecule is produced. After750 ps of MD-NVE simulation, some of the CH4 molecules preferto convert into CH3OH. Some of HCHO molecules are consumedinto the CO and CO2 molecules, [43] with several H2 moleculesforming. The reaction activity of explosion system closely dependson the concentrations of free radicals. The accumulations of �CH3,�OH and �HO2 free radicals apparently improve the reaction activityof gas explosion system.

3.2. DFT calculations of the main reaction routes

In this section, the chain reaction process of gas explosion wasdiscussed for the detailed steps. The kinetic properties of mainreactions observed during the MD simulation were analyzed tounderstand the explosion process. In the following, the R, MPX

and PX denoted reactant, intermediate and product, respectively.The main reaction route according to MD results was showed in

Fig. 5(a), and the entire reaction processes could refer to Figs. S4–S9 in the supplementary materials. The first step reaction(CH4 ! �CH3) is primarily induced by the �OH radicals, which startsby �OH free radical attacking the H atom of the CH4 to form the TS1.The bond of C1–H5 is apparently activated (lengthen from 1.09 to1.27 Å) by interaction with hydroxyl O, resulting in a �CH3 radicaland H2O molecule. This process needs to overcome a free energybarrier of 7.78 kcal mol�1 and 7.01 kcal mol�1 free energy is re-leased during the reaction process. The second reaction step isthe conversion from �CH3 to CH3O�, which begins with the adsorp-tion between C1 of �CH3 and O5 of �HO2 to produce CH3OOH. Theexergonic free energy of this process is 51.58 kcal mol�1. CH3OOHis decomposed into CH3O� and �OH radical finally [5]. The process isendothermic with a free energy of 30.82 kcal mol�1. It is one of theimportant reactions for the formation of �OH radical. The next reac-tion is a direct dehydrogenation process of CH3O� to form HCHO.This reaction is endothermic, which requires an activation free

Fig. 5. Potential energy profiles for the main reaction route (a) and kinetic favorite route (b) of methane oxidation.

Z. He et al. / Fuel 124 (2014) 85–90 89

energy of 27.12 kcal mol�1. It is the main reaction to form �H radi-cal, which is important for continuous reaction of system [5].

As shown in Fig. 4(a), the HCHO molecules mainly convert into�CHO by interaction with �OH radicals. The hydroxyl O initially at-tacks the H2 of HCHO to form the transition state TS3 (seeFig. 5(a)). This process requires an activation free energy of4.10 kcal mol�1, with an exergonic free energy of 23.11 kcal mol�1.The �CHO radical can easily combine with another �OH radical toproduce HCOOH, and this reaction is exothermic, with a free en-ergy of 97.10 kcal mol�1. The hydroxyl H of HCOOH can be ab-stracted directly by interaction with O2 to form the �CO2H radical.The process needs to absorb a free energy of 56.98 kcal mol�1 tosustain the reaction. The last step for methane oxidation is the con-version from �CO2H radical to CO2 [44,46,47]. The �CO2H radical candehydrogenate straightly to form CO2 and H atom through the TS4

(see Fig. 5(a)), with a free energy barrier of 3.99 kcal mol�1.In order to compare with the main reaction process, the kinetic

favorite route was also discussed, which was showed in Fig. 5(b).The first step of the favorite route is quite similar to the one inthe main reaction route. After the first step, the methyl C can inter-act with the hydroxyl O to form CH3OH molecule directly, with anexergonic free energy of 78.61 kcal mol�1. The CH3OH can furtherreact with another �OH by forming a hydrogen bond between H3and O7 (TS2 in Fig. 5(b)). In TS2, the bond of C1–H3 is expandedfrom 1.10 to 1.17 Å, resulting in a �CH2OH radical and H2O mole-cule. This reaction has a small free energy barrier of3.59 kcal mol�1. The C1 and hydroxyl H6 of �CH2OH interactswith O2 molecule to form a 5-members ring complex, MP2 (seeFig. 5(b)). The complex can decompose into HCHO and �HO2

through the transition state TS3. In TS3, the bond of O5–H6 is acti-vated from 0.99 to 1.27 Å by the interaction of O5–H6–O7. Thisprocess requires an activation free energy of 8.90 kcal mol�1. Thereaction process from HCHO to HCOOH is also same with that inthe main reaction route, as discussed above. The following reactionstarts by attacking of the hydroxyl O to the hydroxyl H of HCOOH,and the formed intermediate, MP3, can further convert into �CO2Hthrough the TS5. In TS5, the interaction between H7 and O8 isstrengthened (the length of the bond is shortened from 1.70 to1.25 Å), leading to the bond between O6 and H7 broken to formthe �CO2H radical (see Fig. 5(b)). This reaction only requires a smallactivation free energy of 1.52 kcal mol�1. The last reaction formethane oxidation is still same with that in the main route.Although some of the �CO2H free radicals can convert into HOCOby intramolecular rearrangement, [32] this process needs to over-come an energy barrier of 26.54 kcal mol�1 (see Fig. S9 in the sup-plementary materials).

From the discussion above, the reaction of forming the HCHOmolecule, is the rate-limiting step for the methane oxidationboth in the two reaction routes, which has the highest energybarrier of 27.12 kcal mol�1 for CH3O� ! HCHO and 8.90 kcal mol�1

for �CH2OH! HCHO, respectively. In these two routes shown inFig. 5, almost all the key reactions involve �OH free radical, whichis the critical species for the methane oxidation, especially for theinitial radical formation (step 1). If the concentration of the �OH freeradical can be effectively controlled, the methane oxidation reac-tion will be quenched because the other routes need higher energybarriers to overcome. The reaction velocity will be decreased, andthe gas explosion process should be inhibited efficiently.

4. Conclusions

The intrinsic reaction mechanism of gas explosion processwas thoroughly examined based on both molecular dynamicsand density functional theory calculations. A detailed chain reac-tion process for methane oxidation under explosion condition isproposed. The results show that the reaction activity of gasexplosion system is closely associated with the concentrationof free radicals (�CH3, �HO2, and �OH). The �OH free radical isthe most important free radical carrier for the methane explo-sion process. If the concentrations of free radicals, such as the�OH and �HO2, could be effectively controlled, the reaction activ-ity of explosion system would be decreased. Such results notonly clarify the complex reaction process of gas explosion, butalso provide a great theoretical guide for the further develop-ment of the efficient gas explosion suppression methods and iso-lation materials.

Acknowledgments

This work was supported by the National Natural Science Foun-dation of China (Nos. 51222212 and 11102194), the CAEP founda-tion (Grant No. 2012B0302052), the MOST of China (973 Project,Grant No. 2011CB922200). The computations supports from Infor-malization Construction Project of Chinese Academy of Sciencesduring the 11th Five-Year Plan Period (No. INFO-115-B01) are alsohighly acknowledged.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.fuel.2014.01.070.

90 Z. He et al. / Fuel 124 (2014) 85–90

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