Shock-Tube Experiments and Chemical Kinetic Modeling Study of

Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Shock-Tube Experiments and Chemical Kinetic Modeling Study of CH4 Sensitized by CH3NHCH3 J. C. Shi,†,‡ Y. L. Shang,‡ W. Ye,† R. T. Zhang,*,§ and S. N. Luo*,†,‡ †

The Peac Institute of Multiscale Sciences, Chengdu, Sichuan 610031, People’s Republic of China Key Laboratory of Advanced Technologies of Materials, Ministry of Education, Southwest Jiaotong University, Chengdu, Sichuan 610031, People’s Republic of China § Department of Chemistry, Sourthern University of Science and Technology, Shenzhen, Guangdong 518055, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Ignition delay times of stoichiometric CH4/CH3NHCH3/O2/Ar mixtures are measured with a shock tube in the temperature range of 1100−2000 K. Different pressures (4, 8, and 18 atm) and CH3NHCH3 blending ratios (0, 0.05, 0.1, 0.2, 0.5, and 1) are explored. CH3NHCH3 promotes CH4 ignition. Correlations for the measured ignition delay times are inferred through multiple linear regression. The measured ignition delay times are compared to the predictions of a recently developed CH3NHCH3 kinetic model with C0−C2 reaction subsets, and a new model for CH4/CH3NHCH3/O2/Ar mixtures is assembled and validated against the present measurements. To elucidate the effect of CH3NHCH3 on CH4 ignition, kinetic analyses including sensitivity and rate of production are performed. Perturbation of the radical pool (H, CH3, OH, and HO2) leading to the observed effects is discussed.

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INTRODUCTION Nitrogen-containing compounds are abundant in biomass as well as agricultural and municipal waste and result in the emission of NOx in combustion or incineration.1−7 Consequently, the chemistry of nitrogen fuel conversion is of great interest, and many studies have been performed along this line.8−38 Most of these studies involved the combustion of hydrocarbons with NOx additions9,10,12,15−38 and found that even small amounts of NOx have a significant effect on the oxidation and ignition characteristics of the hydrocarbon fuels. In the process of nitrogen fuel conversion, amines are widely identified and demonstrated to play an important role2,3,8,39,40 and have been investigated recently.41−46 In addition, the NH2 functional group is abundant in many propellants, and the solid propellants are usually dissolved in liquid hydrocarbons.47 However, few studies have addressed the interactions between amine and hydrocarbon. It has long been recognized that amine can impact the oxidation and ignition of hydrocarbon fuels.48−52 For instance, Cullis et al.48 investigated the ability of a variety of amines to retard cool flame propagation and spontaneous ignition of nheptane/air mixtures. Moore et al.49 studied the effect of amine additions on the low-temperature ignition of cyclohexane and n-heptane in a flow system and also found that amines inhibit ignition. In addition, the inhibiting effect of aliphatic amines on the oxidation of acetaldehyde or diethyl ether50−52 was attributed to the abstraction of hydrogen atoms from nitrogen-containing compounds by chain carriers derived from the fuels. Nonetheless, the detailed chemical kinetics of amine/hydrocarbon interactions and underlying mechanisms are still quite uncertain. In this work, ignition delay times of stoichiometric CH4/ CH3NHCH3/O2/Ar mixtures with different CH3NHCH3 © XXXX American Chemical Society

blending ratios (0, 0.05, 0.1, 0.2, 0.5, and 1) are measured at different temperatures (1100−2000 K) and pressures (4, 8, and 18 atm). A new model for the oxidation of CH4/CH3NHCH3/ O2/Ar mixtures is assembled and validated against the measured ignition delay times. On the basis of the assembled model, sensitivity, rate of production (ROP), and reaction pathway analyses have been conducted to elucidate the mechanisms with regard to the effect of CH3NHCH3 on CH4 ignition.

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EXPERIMENTAL SECTION

A 50 mm bore diameter shock tube is employed for all of the ignition experiments. Details of the shock tube were described previously53 and are briefly presented here. The shock tube is separated into a driver section (3.26 m in length) and a driven section (4.52 m in length) by a diaphragm, which is ruptured by an ohmic heating resistance wire upon firing. For each shot, the shock tube is evacuated to about 10 Pa and then He gas and a test gas mixture are injected into the driver section and the driven section, respectively. The incident shock wave speed at the endwall is calculated by linearly extrapolating the shock wave speeds at three upstream locations to the endwall. These three shock speeds are measured by four piezoelectric pressure transducers (113B24, PCB Piezotronics), located near the shock tube endwall. The temperature (T5) and pressure (P5) behind the shock wave reflected from the endwall are calculated with the Gaseq package.54 Different T5 and P5 values are achieved by simultaneously tuning the initial pressures of the driver and driven sections. For convenience, we use P and T to denote P5 and T5, respectively. Gas mixtures are prepared in a 15 L tank, and the fractions of constituent gases are determined by Dalton’s law of partial pressures. Constituent gases in a gas mixture are allowed to sufficiently blend with each other for over 12 h. The purity for CH4 and CH3NHCH3 is higher than 99.9%, and the purity of O2 Received: March 13, 2018 Published: March 20, 2018 A

DOI: 10.1021/acs.energyfuels.8b00860 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels and Ar is higher than 99.99%. All of the gases are provided by Chengdu Xiyuan Chemical Co., Ltd. A photomultiplier (CR131, Hamamatsu) with a narrowband filter centered at 307 ± 10 nm (307FS10-25, Andover) is used to capture OH* emission. Ignition delay time τ is determined as the time interval between the onset of the reflecting shock wave and the intersection between a background baseline and the downward extrapolation of the steepest rising portion of an OH* emission profile, as shown in Figure 1. The standard root-sum-square method55 is employed to evaluate the

To evaluate the performances of the present shock tube, the ignition delay times measured on pure CH4 and pure CH3NHCH3 are compared to other similar measurements from the literature. Our measurements of M0 at 4 atm (Figure 2a) and M100 at 3 atm (Figure 2b) agree with literature values within uncertainty.42,56

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RESULTS Ignition delay times measured for stoichiometric mixtures of CH4/CH3NHCH3/O2/Ar with varying blending ratios (χDMA = 0, 0.05, 0.1, 0.2, 0.5, and 1) are presented in Figure 3. The ignition delay time increases exponentially with a decreasing temperature, and the addition of CH3NHCH3 promotes CH4 ignition. As χDMA increases, the ignition delay time for CH4/ CH3NHCH3/O2/Ar mixtures decreases. For neat CH4, the ignition delay times are correlated with the pressure and temperature via regression analysis as ⎛ 24013 ⎞ ⎟ τ = 7.57 × 10−4P−0.75 exp⎜ ⎝ T ⎠

with the coefficient of determination R2 = 0.99. The ignition delay times of CH4/CH3NHCH3 mixtures are correlated with P and T as

Figure 1. Representative pressure and OH* emission histories. The definition of the ignition delay time (τ) is also indicated.

⎛ 18584 ⎞ −1.01 ⎟ exp⎜ τ = 4.7 × 10−4P−0.72χDMA ⎝ T ⎠

uncertainty in the ignition delay time measured with the present shock tube, and the uncertainty of τ is estimated to be below 20%. A detailed uncertainty analysis is provided in the Supporting Information. The blending ratio of CH3NHCH3 (dimethyl amine or DMA) is defined as nDMA χDMA = nDMA + nCH4 (1)

with R = 0.97. Here, τ is in μs, P is in atm, and T is in K. A comparison between the measured and correlated ignition delay times is provide in Figure 1s of the Supporting Information. Figure 4 shows an example of the pressure effect on the ignition delay time of CH4/CH3NHCH3/O2/Ar mixtures with χDMA = 0.2. The ignition delay time decreases with increasing pressure, while the promoting effect by pressure is more pronounced at lower temperatures.

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Table 1. Compositions of Fuel Mixturesa

a

ϕ

CH4 (%)

CH3NHCH3 (%)

O2 (%)

Ar (%)

M0 M5 M10 M20 M50 M100

1 1 1 1 1 1

2.000 1.824 1.656 1.360 0.696 0.000

0.000 0.096 0.184 0.340 0.696 1.067

4.000 4.000 4.000 4.000 4.000 4.000

94.000 94.080 94.160 94.300 94.608 94.933

(3)

2

where n denotes the molar fraction. The gas mixtures explored in this study are listed in Table 1.

mixture

(2)

KINETIC MODELS AND ANALYSES The Senkin code57 in the Chemkin II package58 is used in chemical kinetics modeling under a constant-volume adiabatic condition. To consider the effect of boundary layer growth, the VTIM method (i.e., volume as a function of time t)59 is applied to the cases where the ignition delay time is longer than 1 ms with a pressure rise rate of 8%/ms (Figure 1). Consistently with the experiments, τ is defined on the basis of a simulated OH* emission history. A recently developed chemical kinetics model for CH3NHCH3 oxidation with C0−C2 reaction subsets (model

The percentages refer to molar ratios. ϕ = equivalence ratio.

Figure 2. Ignition delay time as a function of the inverse temperature obtained from this work and literature for (a) neat CH4 and (b) neat CH3NHCH3. B


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Figure 3. Ignition delay time as a function of the inverse temperature for different CH3NHCH3 blending ratios at (a) 4 atm, (b) 8 atm, and (c) 18 atm. Solid and dashed lines denote the predictions of the current model and that by Li et al.,42 respectively.

CH 3NHCH 3 decomposition and oxidation41 is further modified according to the works of Li et al.42 The rate constant for the unimolecular decomposition of CH3NHCH3, CH3NHCH3 (+M) = CH3NH + CH3 (+M), is updated.42 Moreover, the rate constants of reaction R989, CH3NCH3 + O2 = CH3NO + CH3O, and reaction R976, CH3NHCH2 = CH3 + CH2NH, are re-evaluated in this work. The rate constant of reaction R989 is increased by a factor of 2, as shown in Table 2. The rate constant of reaction R976 is estimated from that of CH3OCH2 = CH3 + CH2O,60 and at the high-pressure limit, kR976,∞ = 8.03 × 1012T0.44 exp(−26491/RT) s−1. Pressuredependent rate constants for reaction R976 also come from the same source (Table 2). The rate constants in Table 2 are provided in the form of k = ATn exp(−Ea/RT). The thermodynamic data are obtained from the same sources as the corresponding reaction subsets. The assembled model well predicts the measurements (Figures 3 and 4). A comparison of the predictions of the assembled model to the experimental values from the work of Li et al.42 is provided in Figures 2s and 3s of the Supporting Information. To find out the most important elementary reactions in the ignition process, sensitivity analysis is first performed. The sensitivity coefficient is defined as67

Figure 4. Ignition delay time as a function of the inverse temperature for mixture M20 at 4, 8, and 18 atm. Solid and dashed lines denote predictions of the current model and that by Li et al.42

by Li et al.) is first evaluated.42 As illustrated in Figure 3, this model can well predict the ignition delay time of neat CH3NHCH3 for the experimental conditions explored but underpredicts ignition delay times for CH4/CH3NHCH3 mixtures. Therefore, the reaction subset for the oxidation of CH4 needs improvement. In addition, the interactions between nitrogen-containing compounds and small hydrocarbons are not well characterized. To better predict ignition delay times of CH4/CH3NHCH3 mixtures, a new model is assembled. This model is based on the detailed chemical kinetics for C0−C2 species in Burke et al.,60 and the chemical kinetics for NH3,61 CH4/NH3,62 and CH3NHCH3 by Lucassen et al.41 are added. The work of Burke et al.60 includes the H2/CO sub-mechanism by Kéromnès et al.,63 the C1−C2 sub-mechanism by Metcalfe et al.,64 and the propene mechanism by Burke et al.65 The reaction subset for NH3 is mainly from Klippenstein et al.,66 and some key reactions are updated by Song et al.61 The CH4/ NH3 reaction subset62 is mainly taken from Tian et al.,8 Glarborg et al.,10 and Rasmussen et al.30 The reaction set for

S=

τ(2ki) − τ(0.5ki) 1.5τ(ki)

(4)

where ki is the pre-exponential factor for the ith elementary reaction. τ(2ki), τ(0.5ki), and τ(ki) are the corresponding ignition delay times when the rate constants of the ith elementary reaction are multiplied by 2, 0.5, and 1, respectively. A positive sensitivity coefficient means that the corresponding elementary reaction inhibits the ignition process, and it is the opposite for a negative value. We also compare the results of the sensitivity analyses between the model by Li et al.42 and the new assembled model in Figure 4s of the Supporting Information. In the new model, the ignition process is more

Table 2. Paramenter of Rate Coefficients for Updated Reactions reaction CH3NCH3 + O2 = CH3NO + CH3O CH3NHCH2 = CH3 + CH2NH

P (atm) 0.01 0.10 1.00 10.00 100.00

A (cm3 mol−1 s−1) 1.2 8.0 7.0 4.0 7.0 3.0 C

× × × × × ×

1013 1023 1028 1029 1027 1029

n

Ea (cal/mol)

reference

−4.52 −5.73 −5.61 −4.71 −4.94

4000.0 25236.0 27495.0 28898.0 29735.0 31785.0

estimated 60 60 60 60 60


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O2. Fuel-related reactions, reaction R966, CH3NHCH3 + H = CH3NHCH2 + H2, and reaction R970, CH3NHCH3 + OH = CH3NHCH2 + H2O, inhibit ignition through consuming reactive H and OH radicals, and the inhibition may also arise from the common product, CH3NHCH2. Our previous study68 demonstrated that CH3NCH3 can react with O2, yielding CH3 and NO, while one consuming pathway for CH3NHCH2 produces HCN by consuming reactive radicals (such as OH, O, and CH3) and H atoms. NO can promote ignition through reaction R1104.9,13,23,27,35,38,69 Reactions R129, R175, and R128, which are also among the most inhibiting reactions for neat CH4, inhibit the ignition process of CH4/CH3NHCH3 mixtures by consuming OH radicals, CH3 radicals, and H atoms, respectively. Figure 6 shows the sensitivity coefficients for mixture M20 at the temperature of 1300 K but different pressures (4, 8, and 18

sensitive to the interactions between nitrogen-containing compounds and small hydrocarbons. The sensitivity coefficients for the stoichiometric CH4/ CH3NHCH3/O2/Ar mixtures with χDMA varying from 0 to 1 at 8 atm and 1300 K are shown in Figure 5. For neat CH4, the

Figure 5. Sensitivity coefficients of elementary reactions for different CH3NHCH3 blending ratios (M0, M5, M20, and M100) at 8 atm and 1300 K.

most promoting elementary reaction is reaction R148, CH3 + O2 = CH2O + OH, followed by reaction R1, H + O2 = O + OH. CH2O is an important intermediate species during hydrocarbon oxidation in the low-to-intermediate temperature range, and it can be converted to HCO, which can further generate a reactive H atom. Reaction R1 is an important chainbranching reaction, which converts H atom and O2 to reactive O and OH radicals. CH3O from reaction R144, CH3 + HO2 = CH3O + OH, is also a precursor to CH2O. The most inhibiting reactions are the consumption of the CH3 radical [reaction R175, CH3 + CH3 (+M) = C2H6 (+M)], H atom (reaction R128, CH4 + H = CH3 + H2), and OH radical (reaction R129, CH4 + OH = CH3 + H2O). For neat DMA, the most sensitive promoting reaction is reaction R1, followed by reaction R971, CH3NHCH3 + OH = CH3NCH3 + H2O, and reaction R973, CH3NHCH3 + CH3 = CH3NCH3 + CH4. Reaction R784, CH3 + NO2 = CH3O + NO, and reaction R144 also play an important role in promoting the ignition of DMA. The most sensitive inhibiting reaction is reaction R966, CH3NHCH3 + H = CH3NHCH2 + H2, followed by reaction R970, CH3NHCH3 + OH = CH3NHCH2 + H2O. In addition, by consuming CH3 radicals, reaction R175, reaction R127, CH3 + H (+M) = CH4 (+M), and reaction R145, CH3 + HO2 = CH4 + O2, inhibit the ignition of DMA. Different for the case of neat CH4, reaction R1 becomes dominant among the promoting reactions for M5 and M20. The sensitivity coefficient of reaction R1 is much higher that that of reaction R148, indicating that reaction R1 becomes much more reactive in CH4 oxidation with the addition of CH3NHCH3. The sensitivity coefficient of reaction R148 decreases gradually with increasing χDMA and is much smaller that that of reaction R1, in sharp contrast with the case of neat CH4. Among the remaining promoting reactions, reaction R144 and reaction R1104, NO + HO2 = NO2 + OH, are insensitive to χDMA, while it is the opposite for reaction R971, CH3NHCH3 + OH = CH3NCH3 + H2O, reaction R784, and reaction R973. The sensitivity coefficients of reactions R971 and R973 increase with increasing χDMA. Almost all of the most important inhibiting reactions are sensitive to χDMA, except reaction R145, CH3 + HO2 = CH4 +

Figure 6. Sensitivity coefficients of elementary reactions in the CH4/ CH3NHCH3/O2/Ar mixture (M20) at 1300 K and 4, 8, and 18 atm.

atm). Reaction R1 is dominant among the promoting reactions, and its sensitivity coefficient decreases with increasing pressure. The remaining promoting reactions are insensitive to pressure, except reaction R128. For mixture M20 at 8 atm but different temperatures (1300 and 1600 K), reaction R1 is predominant among the promoting reactions and its importance becomes more important at higher temperatures (Figure 7), consistent with previous studies.70−72 Ignition is more sensitive at 1300 K than at 1600 K to the reactions related to nitrogen-containing compounds, such as reactions R971, R784, R973, and R1104. The most inhibiting reaction is reaction R128, and its sensitivity coefficients are comparable at 1300 and 1600 K. Similar to the promoting

Figure 7. Sensitivity coefficients of elementary reactions in the CH4/ CH3NHCH3/O2/Ar mixture (M20) at 8 atm and 1300 and 1600 K. D


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concentration histories of H atoms and OH radicals are completely different from those of CH3 and HO2 radicals. OH radicals are almost absent during the induction period, while H atoms are gradually accumulated before the main ignition. We perform the next reaction pathway analysis on CH4 in stoichiometric CH4/CH3NHCH3/O2/Ar mixtures (M0, M5, and M20) at 1300 K, 8 atm, and timing of 20% CH4 consumption (Figure 9). In the following discussion, the

reactions, the inhibiting reactions are more readily influenced by nitrogen-containing species. As shown in Figures 5−7, CH3 radicals play an important role in the oxidation of CH4/CH3NHCH3 mixtures. For CH3NHCH3, the bond dissociation energies (BDEs) of C−H and N−H in CH3NHCH3 are 91.9 and 93.3 kcal/mol, respectively,73 and that of C−N bond is 84.9 kcal/mol.74 For CH4, the BDE of the C−H bond is 105 kcal/mol.75 Therefore, it is easier for CH3NHCH3 to decompose into the CH3 radical and H atom than for CH4. To further elucidate the effect of the concentration of CH3 radicals on the ignition of CH4/CH3NHCH3 mixtures, the concentrations of CH3 radicals during the induction periods of CH4/CH3NHCH3 mixtures with different χDMA are presented in Figure 8a. The time is normalized to the ignition delay time

Figure 9. Reaction pathways of CH4 for stoichiometric mixtures of CH4/CH3NHCH3/O2/Ar at 1300 K and 8 atm, with the blending ratio of CH3NHCH3 ranging from 0 to 0.2. Black fonts, M0; red fonts, M5; and violet fonts, M20.

numbers in the parentheses indicate the molar fractions of corresponding species. The reaction pathways for neat CH4 and neat CH3NHCH3 were detailed previously.15,53,68 Therefore, we will focus on the effect of CH3NHCH3 addition on the reaction pathway of CH4. With CH3NHCH3 addition, hydrogen abstraction by H atom becomes more important (15.8% for M0, 24.5% for M5, and 31.9% for M20) with increasing χDMA, while hydrogen abstractions through OH radicals (66.8% for M0, 59.3% for M5, and 53.1% for M20) and HO2 radicals (4.4% for M0, 1.1% for M5, and 0.6% for M20) become less important. In the three main channels for consuming CH 3 radicals, the selfcombination reaction to form C2H6 become more dominant (49.4% for M0, 64.7% for M5, and 73.2% for M20) with increasing χDMA, transferring an oxygen atom from HO2 to form CH3O becomes less important (10.9% for M0, 7.0% for M5, and 7.8% for M20), the formation of CH2O through the interactions with the O radical (0.5% for M0, 1.3% for M5, and 2.8% for M20) increases modestly, and the formation of CH2O through the interactions with O2 (22.1% for M0, 10.9% for M5, and 5.2% for M20) becomes less important. The percentage of CH3O decomposing to CH2O remains nearly constant in CH4/ CH3NHCH3 mixtures. For hydrogen abstractions (converting CH2O to HCO radicals), the ratios of abstractions through OH radicals and O radicals remain constant, the channel through H atoms becomes more important (20.1% for M0, 38.2% for M5, and 53.4% for M20), and the channel via CH3 radicals becomes

Figure 8. (a) CH3 concentration histories during induction time for CH4/CH3NHCH3 mixtures performed at 1300 K and 8 atm and (b) ROP for the formation of CH3 radicals during oxidation of the CH4/ CH3NHCH3 mixture (M50) at 1300 K and 8 atm.

in each case. CH3 radicals are present before the main ignition as CH3NHCH3 is added, and its concentration increases with increasing χDMA. To identify the sources of the CH3 radicals, ROP analysis for the formation of CH3 radicals during the induction period of CH4/CH3NHCH3 mixtures (M50) is performed at 1300 K and 8 atm (Figure 8b). During the induction period, reaction R976, CH3NHCH2 = CH3 + CH2NH, and reaction R783, CH3 + NO (+M) = CH3NO (+M), contribute to the formation of CH3 radicals. Therefore, CH3NHCH3 addition increases the reactivity of CH4 mixtures by introducing CH3 radicals before main ignition of CH4. Moreover, we also illustrate the concentrations of H atoms, HO2 radicals, and OH radicals, as shown in Figure 5s of the Supporting Information. The concentration histories of HO2 radicals are similar to those of CH3 radicals, but their values are much smaller than those of CH3 radicals. However, the E


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−1.01 mixtures, the correlation is τ = 4.7 × 10 −4 P−0.72 χ DMA exp(18584/T), where τ is in μs, P is in atm, and T is in K. A kinetic model is proposed, assembled from the reaction sub-mechanism of C0−C2 species, reaction subset for NH3 oxidation, chemical kinetics of CH4/NH3 interactions, and CH3NHCH3 reaction sets. Predictions of this model are in agreement with the present measurements. Kinetic analyses including sensitivity analysis, reaction pathway analysis, and ROP analysis are performed with this model. For neat CH4, the sensitivity coefficients of reaction R148, CH3 + O2 = CH2O + OH, and reaction R1, H + O2 = O + OH, are comparable, while reaction R1 becomes predominant as CH4 is blended with CH3NHCH3. The concentration histories of CH3 radicals show that CH3 radicals are present during induction time with CH3NHCH3 addition, and these radicals are mainly produced by the reaction R976, CH3NHCH2 = CH3 + CH2NH, and reaction R783, CH3 + NO (+M) = CH3NO (+M). Reaction pathway analysis illustrates that the H-atom-involved reactions become increasingly important with the addition of CH3NHCH3 compared to the case of neat CH4. ROP analysis for the H atom shows that certain reactions related to nitrogencontaining compounds (reaction R702, HCN + O = NCO + H, and reaction R1037, NH + O = NO + H) contribute to the production of H atoms. The reactive CH3 radicals, generated before the main ignition, and the H atoms, partially produced by the reactions related to nitrogen-containing compounds, interrupt the initial chemical reaction process and, thus, promote the ignition of CH4.

less important (49.9% for M0, 33.2% for M5, and 17.7% for M20). In the HCO−CO conversion process, CH3NHCH3 has a negligible effect and the ratios of the two channels through decomposition and hydrogen abstraction are almost constant in all CH4/CH3NHCH3 mixtures. As illustrated in Figure 9, with the addition of CH3NHCH3, the slow reactions via O2 including the interactions between CH3 radical and O2, between CH2 and O2, and between HCO and O2 become less important, while the roles of the fast reactions via H atoms become more important. These fast reactions include the interactions between CH4 and H atom and between CH2O and H atom. As discussed above, H atom plays an important role in the consuming pathways of CH4. Therefore, ROP analysis for the formation of H atoms in oxidation of mixture M50 is performed at 1300 K and 8 atm (Figure 10). Reaction R3,

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ASSOCIATED CONTENT

S Supporting Information *

Figure 10. ROP for the formation of H atoms during oxidation of the CH4/CH3NHCH3/O2/Ar mixture (M50) at 1300 K and 8 atm.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.8b00860. Comparison of the measured and correlated ignition delay times, comparison of the simulated ignition delay times using the current model and the literature experimental values, comparison of the sensitivity coefficients of the new model and the model by Li et al., H, HO2, and OH concentration histories, uncertainty analysis, and measured ignition delay times in the present work (ZIP)

OH + H2 = H + H2O, and reaction R27 are the dominant pathways for the formation of H atoms, followed by reaction R2, O + H2 = H + OH, and reaction R146, CH3 + O = CH2O + H. Reaction R325, C2H2 + O = HCCO + H, is another important channel to generate H atoms. Furthermore, reactions related to nitrogen-containing compounds, reaction R702, HCN + O = NCO + H, and reaction R1037, NH + O = NO + H, contribute to the production of H atoms as well. Through the kinetic analysis, we find that the CH3 radical and H atom play important roles in the ignition and oxidation processes. With a symmetric and similar isoelectronic structure to CH3NHCH3,42 we compare the results of our kinetic analysis to the work of Chen et al.76 They investigated the effects of dimethyl ether (DME, CH3OCH3) on the hightemperature ignition and burning properties of CH4 and also found that the rapid buildup of CH3 radicals results in a significant ignition enhancement. For CH4/CH3OCH3 mixtures, the chain propagation reaction via CH3 replaces the slow reactions via CH3 in neat CH 4. However, for CH4 / CH3NHCH3 mixtures, the slow reactions via O2 are replaced by the reactions via H atoms.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

S. N. Luo: 0000-0002-7538-0541 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was support in part by the 973 Project of China (2014CB845904) and the National Natural Science Foundation of China (NSFC, 11627901).

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CONCLUSION Ignition delay times of stoichiometric mixtures of CH4/ CH3NHCH3 are measured at 1100−2000 K and 4−18 atm. Different CH3NHCH3 blending ratios ranging from 0 to 1 are explored. CH3NHCH3 promotes the ignition of CH4. The ignition delay times for neat CH4 can be correlated as τ = 7.57 × 10 −4 P −0.75 exp(24013/T), and for CH 4 /CH 3 NHCH 3

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REFERENCES

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