Ar Mixtures for

Sep 8, 2016 - Using shock tubes, Takahashi et al.7 and Shin et al.8 measured ... time (τ) is defined as the time interval between the sharp rise at t...
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Ignition Delay Time Measurements on CH4/CH3Cl/O2/Ar Mixtures for Kinetic Analysis J. C. Shi,†,‡ W. Ye,†,¶ B. X. Bie,†,¶ X. J. Long,†,§ R. T. Zhang,† X. J. Wu,† 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 ¶ School of Science, Wuhan University of Technology, Wuhan, Hubei 430070, People’s Republic of China § College of Physical Science and Technology, Sichuan University, Chengdu, Sichuan 610064, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Ignition delay times of CH4/CH3Cl/O2/Ar mixtures are measured using a shock tube at 1350−1950 K and 4−18 atm. Equivalence ratios of 0.5 and 1 and CH3Cl blending ratios ranging from 0 to 1 are explored. Correlations for the measured delay times are obtained through multiple linear regression. Increasing the blending ratio facilitates ignition, but this effect becomes saturated at a blending ratio of ∼0.2. Two existing chemical kinetic models for CH4/CH3Cl mixtures are examined against the measurements, and a modified chloromethane−Polimi−kin model incorporating the Aramco 2.0 model is proposed and validated through comparison with these new data. Based on the proposed model, sensitivity analysis, peak concentration analysis, and reaction pathway analysis are carried out to provide further insight into the ignition process of CH4/CH3Cl/O2/Ar mixtures.

1. INTRODUCTION The expected crude oil shortage and environmental issues associated with fuel processing and consumption demand cleaner fuels. As a clean energy source with large reserves, natural gas is usually considered as an alternative to crude oil.1 One promising pathway of converting CH4 to heavy hydrocarbons involves a two-stage process using CH3Cl as intermediate.2−4 Moreover, incineration is one of the most effective technologies for hazardous waste disposal, and the efficiency of incineration is significantly influenced by the presence of CH3Cl,5 resulting in increased emission of polyaromatic hydrocarbons (PAHs) and soots.6 Thus, it is desirable to understand in detail CH3Cl oxidation and its effects on oxidation of hydrocarbons, such as CH4. As an important combustion property, ignition delay time is widely used for establishing or validating chemical kinetics models. Using shock tubes, Takahashi et al.7 and Shin et al.8 measured ignition delay times of CH4/CH3Cl/O2/Ar mixtures. Their experimental results indicate that a small amount of CH3Cl promotes CH4 ignition. However, the effect of CH3Cl fraction on CH4 ignition is essentially untouched. Current chemical kinetics models9−12 for ignition of CH4/CH3Cl/O2/ Ar mixtures lack sufficient experimental constraints. Therefore, a more systematic investigation of CH4/CH3Cl/O2/Ar mixtures with varying CH3Cl fractions, equivalence ratios, and pressure−temperature conditions is necessary. In this study, ignition delay times of CH4/CH3Cl/O2/Ar mixtures are measured under a wide range of temperatures (T = 1350−1950 K), pressures (P = 4, 8, and 18 atm), equivalence ratios (ϕ = 0.5 and 1.0), and CH3Cl blending ratios (χCH3Cl = 0−1). The equivalence ratio, ϕ, depends on both CH4 and CH3Cl, and the definitions of equivalence ratio and CH3Cl blending ratio will © XXXX American Chemical Society

be presented in the next section. A modified chemical kinetics model based on the chloromethane−Polimi−kin (CPK) model10,12 is proposed and demonstrates successful comparison with these experimental data. This modified model is further used to provide some kinetic insights into ignition of CH4/ CH3Cl/O2/Ar mixtures.

2. EXPERIMENTAL SECTION Ignition experiments are conducted with a 50 mm bore diameter shock tube (Figure 1). The driver section (3.26 m in length) and driven section (4.52 m in length) are separated by a mylar diaphragm. The diaphragm is ruptured by a cross-shaped resistance wire, which is heated by capacitor discharge upon firing. The shock tube is evacuated to below 10 Pa by a vacuum pump (VP2200, Value Mechanical & Electrical Products Co.) prior to firing. Three incident shock wave speeds are measured with four piezoelectric pressure transducers (113B24, PCB Piezotronics), which are located at 971, 571, 271, and 11 mm from the shock tube endwall. Each incident shock wave speed is calculated through dividing the distance between adjacent pressure transducers by the corresponding time interval between incident shock wave arrivals. The endwall incident shock wave speed (Vendwall) is obtained by extrapolating the three measured incident shock wave speeds to the endwall. The temperature (T5) and pressure (P5) behind the shock wave reflected from the end wall (region 5) are calculated using Gaseq,13 with Vendwall, the initial driven section pressure (P1), and thermodynamic data of initial species in the driven section as input. A gas mixture is prepared in a 15 L stainless storage cylinder. The stainless storage cylinder is first evacuated to less than 10 Pa. Then, the concentration of each species in the gas mixture is determined according to Dalton’s law of partial pressures, and each constituent gas Received: June 16, 2016 Revised: September 2, 2016

A

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Energy & Fuels

Figure 1. Schematic of the shock tube experiments (PMT: photomultiplier).

Figure 2. (a) Representative pressure and OH* emission histories (τ: ignition delay time). (b) Comparison of the delay times obtained from this work and the literature for CH4. (c) Comparison of the delay times obtained from present and literature measurements on CH4/CH3Cl/O2/Ar mixtures. is injected into the stainless storage cylinder. The injected partial pressure of each constituent gas is monitored by a pressure transmitter (CYYZ15, Beijing Star Sensor Technology Co., Ltd.). The resultant gas mixture is allowed to sufficiently diffuse and mix for more than 10 h, and the concentration of each species in the as-prepared gas mixture is further confirmed by a gas chromatograph (7890B GC System, Agilent). The OH* emission at 307 nm is acquired by a narrow bandpass filter with a full width at half-maximum of 10 nm (307FS10-25, Andover) and a photomultiplier (CR131, Hamamatsu Photonics). Both pressure and OH* emission signals are recorded by digital oscilloscopes (HDO6104, Teledyne LeCroy). Representative pressure and emission histories are shown in Figure 2a. The sharp rise in intensity in the OH* emission signals the onset of ignition, and the ignition onset is determined as the intersection between the baseline and the downward extrapolation of the rising edge (t1). Ignition delay time (τ) is defined as the time interval between the sharp rise at t0 in the end wall pressure (P5) upon shock wave reflection by the end wall and the ignition onset (t1). The slow rise in the pressure history during the period of t0−t1 is due to the effect of boundary layers14 and is considered during chemical kinetics modeling. Figure 2b compares the ignition delay times obtained from this work and from the literature7 for 2% CH4/4% O2/94% Ar at P5 = 4 atm, and the agreement is excellent. Figure 2c shows comparison of the ignition delay times obtained from present shock tube and literature7 measurements on 2% CH4/0.1% CH3Cl/4% O2/93.9% Ar at P5 = 4 atm, and these two sets of data agree well with each other. The standard root-sum-squares method is employed to calculate the uncertainty in measured ignition delay time.15,16 The uncertainty in the arrival time difference between two neighboring pressure transducers is estimated to be 2 μs, and that in ϕ is ∼6%. The uncertainty in ignition delay time is approximately 20%. A detailed description of uncertainty analysis is provided in the Supporting Information. The CH3Cl blending ratio χCH3Cl is defined as

χCH Cl = 3

nCH3Cl nCH3Cl + nCH4

(1)

where nCH3Cl and nCH4 are the molar fractions of CH3Cl and CH4, respectively. The stoichiometry for CH4/CH3Cl/O2/Ar mixtures is determined by the following reaction

1 CH4 + m ⎛ 1⎞ → ⎜1 + ⎟CO2 ⎝ m⎠

CH3Cl +

⎛2 3⎞ ⎜ + ⎟O2 ⎝m 2⎠ ⎛ 2⎞ + HCl + ⎜1 + ⎟H 2O ⎝ m⎠

(2)

where m is the molar fraction ratio of CH3Cl to CH4. The equivalence ratio of CH4/CH3Cl/O2/Ar mixtures is defined as

ϕ=

2 m

+

3 2

actual O2 used

(3)

The gas mixtures used in this study are listed in Table 1. Two equivalence ratios, ϕ = 0.5 and 0.1, and five different blending ratios for each equivalence ratio are examined. In total, we investigate ignition of 10 mixtures under different pressure and temperature conditions. The purity for CH3Cl and CH4 is >99.9%, and that of O2 and Ar is higher than 99.99%. All above reactants are provided by Chengdu Xiyuan Chemical Co., Ltd.

3. CHEMICAL KINETICS MODELING Chemical kinetic simulations are performed using the Senkin subroutine17 in the Chemkin II package18 assuming a constant volume adiabatic model. Due to the ubiquitous presence of boundary layers, nonuniform pressure and temperature rising from incident shock attenuation and boundary layer growth are considered in the simulation by employing the option of B

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Energy & Fuels Table 1. Compositions of Fuel Mixturesa

combines the CPK model and the Aramco 2.0 model.25−28 Details will be presented in the next section.

mixture

χCH3Cl

ϕ

CH4(%)

CH3Cl (%)

O2 (%)

Ar (%)

M100 M90 M80 M50 M0

0.0 0.1 0.2 0.5 1.0

0.5 0.5 0.5 0.5 0.5

1.000 0.923 0.842 0.571 0.000

0.000 0.103 0.210 0.571 1.333

4.000 4.000 4.000 3.997 3.999

95.000 94.974 94.948 94.861 94.668

M100 M90 M80 M50 M0

0.0 0.1 0.2 0.5 1.0

1.0 1.0 1.0 1.0 1.0

2.000 1.846 1.684 1.143 0.000

0.000 0.205 0.421 1.143 2.667

4.000 4.000 4.000 4.000 4.000

94.000 93.949 93.895 93.714 93.333

a

4. RESULTS AND DISCUSSION 4.1. Measurements and Correlation. Ignition delay times of fuel-lean and stoichiometric CH4/CH3Cl/O2/Ar mixtures with CH3Cl blending ratios χCH3Cl = 0, 0.1, 0.2, 0.5, and 1 are measured at P5 = 4, 8, and 18 atm. In the discussion below, P and T denote P5 and T5, respectively, for convenience. Ignition delay times exhibit a strong Arrhenius dependence on temperature. Therefore, through multiple linear regression, ignition delay times of CH4/CH3Cl/O2/Ar mixtures are correlated with P, T, and ϕ at a given blending ratio as τ = AP aϕb exp(Ea /RT )

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

(4)

where A and a are correlated parameters; ignition time (τ) is in μs; pressure (P) is in atm; temperature (T) is in K; ϕ is the equivalence ratio; Ea is the overall activation energy in kcal mol−1; and R is the universal gas constant (1.986 cal mol−1 K−1). Figure 3 shows examples of the measurements and corresponding correlations, with ϕ = 0.5 and 1 and P = 4, 8, and 18 atm, for χCH3Cl = 0.5 (Figure 3a) and χCH3Cl = 0 (neat CH4, Figure 3b). The residual-square parameters, R2, are all above 0.973, indicating that the correlations can adequately describe the measured ignition delay times. The correlated parameters for the representative CH4/CH3Cl/O2/Ar mixtures explored in this study are summarized in Table 2. 4.2. Evaluation and Modification of Chemical Kinetics Models. A prerequisite for a useful chemical kinetics model of a binary-fuel mixture is its capability to predict ignition delay times of individual fuels in the mixture. We thus evaluate two existing models, the Leylegian and CPK models, by comparing their predictions against our measurements for neat CH4 and CH3Cl. The calculated ignition delay time is determined as the time interval between time zero and the intercept of the extrapolated OH curve with the baseline. While the calculated ignition delay times from the Leylegian model agree well with the measurements on neat CH4 at P = 4, 8, and 18 atm (M100, ϕ = 0.5; Figure 4a), the CPK model underestimates considerably these ignition delay times (Figure 4b). Because of the fluctuations in experiments, the experimental data in Figure 4a,b at 4 atm are more complex than that of the higherpressure cases. For the ignition of neat CH3Cl (M0, ϕ = 0.5) at the same pressures, the Leylegian model overestimates the

volume as a function of time (VTIM).14 In this study, a pressure rise of 8%/ms is considered if the measured ignition delay time is more than 1 ms (Figure 2a). Two chemical kinetics models [Leylegian model9 and chloromethane−Polimi−kin (CPK) model10−12] for CH4/ CH3Cl oxidation are first considered. The Leylegian model originates from a kinetic model used in the simulation of CH3Cl oxidation (the Wang model),19 which consists of reaction chemistry of the C/H/O system for C1−C2 species, reactions of chlorinated species, and reactions involving chlorinated C2 species. The reaction chemistry for C1−C2 species is a modified version of GRI-Mech 1.2.20 The reactions of chlorinated species are mainly from Bozzelli and coworkers,21,22 and the reactions of chlorinated C2 species are from Tsang.23 Compiled from the Wang model and expanded with some elementary reactions involving heavily chlorinated C1−C2 species, the Leylegian model consists of 82 species and 505 elementary reactions. CPK model is compiled from the C1C3HT1412 module of the POLIMI mechanism,24 H2/CO mechanism,24 Kinetics−HCl−Cl2 module,12 and vco9410 module.11 The C1C3HT1412 module contains the reactions involving oxidation and pyrolysis of C1−C3 species. The H2/ CO mechanism involves the reactions for syngas. The HCl−Cl2 module consists of reactions of Cl and HCl from Pelucchi et al.12 The vco9410 module contains the reactions for oxidation and pyrolysis of CH3Cl and chlorinated species. We also develop a modified CPK model, referred to as the chloromethane−Polimi−Aramco−kin (CPAK) model, which

Figure 3. Comparison of the measured and regressed ignition delay times at different pressures: (a) ϕ = 0.5, M50 and (b) ϕ = 1, M100. Symbols, measured; lines, regressed. C

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Energy & Fuels Table 2. Summary of Correlated Parameters A

mixture M100 M90 M80 M50 M0

9.819 2.827 4.277 1.574 0.607

× × × × ×

a −4

10 10−4 10−4 10−4 10−4

−0.746 −0.722 −0.681 −0.731 −0.667

± ± ± ± ±

b 0.034 0.065 0.064 0.050 0.051

0.367 0.325 0.373 0.172 0.127

± ± ± ± ±

0.056 0.110 0.105 0.089 0.089

Ea (kcal/mol)

R2

± ± ± ± ±

0.990 0.973 0.978 0.988 0.985

46.8 49.6 47.4 50.2 52.3

0.7 1.6 1.2 1.0 1.3

Figure 4. Comparison of the experimental data with the predictions by the Leylegian and CPK models for neat CH4 and neat CH3Cl at ϕ = 0.5: (a) Leylegian model, CH4; (b) CPK model, CH4; (c) Leylegian model, CH3Cl; and (d) CPK model, CH3Cl. Symbols, measurements; lines, predictions.

Figure 5. Comparison of the measurements with the predictions of CPAK model for CH4/CH3Cl/O2/Ar mixtures with χCH3Cl = 0, 0.1, 0.2, 0.5, and 1: (a) P = 4 atm, ϕ = 0.5; (b) P = 8 atm, ϕ = 0.5; (c) P = 18 atm, ϕ = 0.5; (d) P = 4 atm, ϕ = 1; (e) P = 8 atm, ϕ = 1.0; and (f) P = 18 atm, ϕ = 1. Symbols, measurements; lines, calculations using CPAK model.

Therefore, to better predict ignition delay times of CH4/ CH3Cl mixtures, the CPK model is modified as follows: (i) Its H2/O2 submodel is completely replaced by the H2/O2 reaction set of Kéromnès and co-workers.29 (ii) For the elementary reactions of C1−C3 submodel common to the CPK model and the Aramco 2.0 model,25−28 the reaction rates in the CPK

ignition delay times (Figure 4c), but the CPK model predictions are in accord with the measurements (Figure 4d). These comparisons suggest the deficiency of the CPK model in describing the elementary reactions of nonchlorinated species for CH4/CH3Cl mixtures. D

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Figure 6. Measured and predicted ignition times as a function of χCH3Cl for CH4/CH3Cl/O2/Ar mixtures at different ϕ, P, and T conditions. Symbols, measurements; lines, calculations using CPAK model.

model are replaced by those in the Aramco 2.0 model. (iii) The remaining elementary reactions for hydrocarbon species in the Aramco 2.0 model but not included in the CPK model are added to the CPK model. The modified CPK model is referred to as the chloromethane−Polimi−Aramco−kin (CPAK) model. The CPAK model consists of 557 species and 4592 elementary reactions. Figure 5 shows the comparison between the measured ignition delay times and the predicted values by the CPAK model of CH4/CH3Cl/O2/Ar mixtures for T = 1350−1950 K; P = 4, 8, and 18 atm; equivalence ratio ϕ = 0.5 and 1.0; and CH3Cl blending ratio χCH3Cl = 0, 0.1, 0.2, 0.5, and 1. The excellent agreement attests to the accuracy of the CPAK model. The experimental results demonstrate that the presence of CH3Cl promotes CH4 ignition by reducing the ignition delay time, consistent with previous studies.7,8 The influence of CH3Cl fraction or blending ratio is illustrated in Figure 6. With increasing blending ratio, ignition delay time decreases. The boosting effect of CH3Cl on ignition begins to saturate for χCH3Cl > 0.2. 4.3. Chemical Kinetics Analysis. To gain insight into ignition of CH4/CH3Cl mixtures, sensitivity, peak concentration, and reaction pathway analyses are conducted. Sensitivity analysis of ignition of CH4/CH3Cl/O2/Ar mixtures is conducted first to single out important elementary reactions in ignition. Sensitivity of a particular reaction is defined as16 S=

τ(2ki) − τ(0.5ki) 1.5τ(ki)

Figure 7. Sensitivity analysis for CH4/CH3Cl/O2/Ar mixtures with different χCH3Cl at T = 1700 K, P = 8 atm, and ϕ = 0.5.

O + OH is the most important elementary reaction for expediting ignition of CH4/CH3Cl mixtures. O2 + H = O + OH and O2 + H (+M) = HO2 (+M) play a unique role in different combustion conditions.30 At high temperatures, O2 + H = O + OH is usually the most important elementary reaction in the oxidation of all types of hydrocarbon (including hydrogen). At lower temperatures, O2 + H = O + OH becomes less important, while O2 + H (+M) = HO2 (+M) becomes more important. Because the temperature in this study is above 1350 K, O2 + H = O + OH is more important than O2 + H (+M) = HO2 (+M). With increasing χCH3Cl, sensitivity (S) of R5 increases dramatically to its peak at χCH3Cl = 0.2 then deceases sharply. The second most important elementary reaction is R2781: CH3Cl = CH3 + Cl; its sensitivity also peaks at χCH3Cl = 0.2. The third most important elementary reaction is R91: CH3 + O2 = CH2O + OH, with its sensitivity peak located at χCH3Cl = 0.2.

(5)

where ki is the pre-exponential factor of the ith reaction. That is, ignition delay times are obtained from the CPAK model by varying the prefactor values (0.5k, 1.5k, and 2k) for calculating S. Figure 7 shows sensitivity (S) values of key reactions in ignition of CH4/CH3Cl/O2/Ar mixtures for different χCH3Cl at T = 1700 K, P = 8 atm, and ϕ = 0.5. The reaction R5: O2+H = E

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Energy & Fuels Because the C−Cl bond dissociation energy (83 kcal/mol)31 of CH3Cl is lower than that of C−H bond (105 kcal/mol)32 of CH4, reaction R2781 yields reactive CH3 and Cl radicals before unimolecular decomposition of CH4. The CH3 radicals abstract O from O2 and HO2, producing more reactive O radicals as well as OH radicals. The most inhibiting elementary reaction for ignition of CH4/CH3Cl mixtures is reaction R44: CH4 + H = CH3 + H2, which involves elimination of H radicals. The second most inhibiting elementary reaction is reaction R194: 2CH3(+M) = C2H6(+M). This elementary reaction reduces CH3 radicals and produces nonreactive C2H6. The reactions R46: CH4 + OH = CH3 + H2O and R2791: OH + CH3Cl = CH2Cl + H2O reduce active OH radicals and produce final product H2O. Sensitivity values for these four elementary reactions reach their maximums at χCH3Cl = 0.2, except in neat CH4. Sensitivity analysis of CH4/CH3Cl/O2/Ar mixtures with different ϕ at P = 8 atm, T = 1700 K, and χCH3Cl = 0.5 is shown in Figure 8. The sensitivity values for ϕ = 0.5 and 1.0 are

Figure 9. Sensitivity analysis for CH4/CH3Cl/O2/Ar mixtures at T = 1700 K; ϕ = 0.5; χCH3Cl = 0.5; and P = 4, 8, and 18 atm.

CH2O, and chlorinated species (Cl, HCl, and CH2Cl) are analyzed (Figure 10). Figure 10a illustrates peak concentration as a function of blending ratio χCH3Cl for O, OH, H, CH3, and CH2O in fuel-lean CH4/CH3Cl/O2/Ar mixtures at T = 1700 K and P = 8 atm. The peak concentrations of O, OH, H, CH3, and CH2O decrease moderately with increasing χCH3Cl. However, the peak concentrations of Cl, HCl, and CH2Cl increase nonlinearly with increasing χCH3Cl (Figure 10b); such an increase is dramatic for χCH3Cl ≤ 0.2 and slows at larger blending ratios. For stoichiometric CH4/CH3Cl/O2/Ar mixtures at T = 1700 K and P = 8 atm, the peak concentrations follow a similar trend for fuel-lean mixtures (Figure 10c,d). Reaction pathway analyses of CH4/CH3Cl/O2/Ar mixtures are presented in Figure 11, conducted at 20% fuel consumption.33 Figure 11a illustrates the reaction pathways of fuel-lean (ϕ = 0.5) CH4/CH3Cl/O2/Ar mixtures at T = 1700 K and P = 8 atm, for CH3Cl blending ratios of 0, 0.5, and 1. Neat CH4 is mainly consumed by H-abstraction reactions from O (17.3%), H (17.7%), and OH radicals (62.5%), producing CH3 radicals. The main products of CH3 radicals are CH2O (31.0%), HCO (11.8%), and C2H6 (39.5%). Mixed with CH3Cl at χCH3Cl = 0.5, the consuming pathway of CH4 changes markedly. CH4 is mainly consumed by O (7.2%), H (9.1%), OH (27.6%), and Cl radicals (55.9%). Cl radicals from CH3Cl decomposition facilitate the H abstraction of CH4 and thus its ignition. The resultant CH3 radicals are further converted to C2H4 (40.8%), CH2O (6.0%), and C2H6 (44.4%). Thus, the existence of chlorine in CH4/CH3Cl/O2/Ar mixtures stimulates the formation of C2H4, increasing the possibility of forming PAHs and soots.34,35 The production of CH2O (31.0%) and CHO radicals from CH3 radicals substantially decreases. Neat CH3Cl is mainly decomposed into CH3 (38.7%) and CH2Cl (60.7%) radicals. CH3 radicals are further consumed, yielding C2H6 (25.1%) and C2H4 (62.4%); 34.5% CH2Cl radicals are converted to C2H4. Mixed with CH4, CH3Cl is also mainly decomposed into CH3 radicals (54.4%) and CH2Cl radicals (45.5%). CH3 radicals are mainly converted into C2H6 (56.0%) and C2H4 (36.9%), and 58.2% of CH2Cl is converted to C2H4. Figure 11b shows reaction pathways of stoichiometric CH4/ CH3Cl/O2/Ar mixtures at T = 1700 K and P = 8 atm. Neat

Figure 8. Sensitivity analysis for CH4/CH3Cl/O2/Ar mixtures at T = 1700 K, P = 8 atm, χCH3Cl = 0.5, and ϕ = 0.5 and 1.

similar. Therefore, ϕ has negligible effects on the sensitivity of the elementary reactions. Sensitivity analysis of CH4/CH3Cl/ O2/Ar mixtures under different pressures at T = 1700 K, ϕ = 0.5, and χCH3Cl = 0.5 is shown in Figure 9. The sensitivity coefficients for elementary reactions are comparable except for R2781: CH3 = CH3 + Cl and R43: CH3+H(+M) = CH4(+M), for which the rate constants are pressure-dependent. When the pressure increases from 4 to 18 atm, the absolute sensitivity coefficient of R2781: CH3 = CH3+Cl decreases by 15% and that of R43: CH3 + H(+M) = CH4(+M) increases by 106%. As shown in Figures 7−9, chlorine-related elementary reactions play important roles in the oxidation of CH4/ CH3Cl/O2/Ar mixtures. For different blending ratios, the most important chlorine-related elementary reaction for promoting ignition, R2781: CH3 = CH3 + Cl, has its sensitivity coefficient peaked at χCH3Cl = 0.2. However, for different equivalence ratios and pressures, the sensitivity coefficients of chlorine-related elementary reactions are nearly unchanged. To further understand the effects of small radicals and species on ignition of CH4/CH3Cl mixtures, the peak concentration of small radicals such as O, OH, H, CH3, F

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Figure 10. Concentrations of key species in fuel-lean and stoichiometric CH4/CH3Cl/O2/Ar mixtures with varying χCH3Cl at T = 1700 K and P = 8 atm: (a) H, O, OH, CH3, and CH2O radicals for ϕ = 0.5; (b) Cl, HCl, and CH2Cl species for ϕ = 0.5; (c) H, O, OH, CH3, and CH2O radicals for ϕ = 1; and (d) Cl, HCl, and CH2Cl species for ϕ = 1.

Figure 11. Reaction pathways of fuel-lean (ϕ = 0.5) and stoichiometric (ϕ = 1) CH4/CH3Cl/O2/Ar mixtures at T = 1700 K and P = 8 atm with χCH3Cl = 0, 0.5, and 1: (a) fuel-lean CH4/CH3Cl/O2/Ar mixtures with χCH3Cl = 0, 0.5, and 1 and (b) stoichiometric CH4/CH3Cl/O2/Ar mixtures with χCH3Cl = 0, 0.5, and 1. Black fonts, CH4, M100; green fonts, CH4, M50; red fonts, CH3Cl, M0; blue fonts, CH3Cl, M50.

CH3Cl decomposes into CH3 radicals (53.5%) and CH2Cl radicals (46.1%). CH3 radicals are further converted to C2H6 (48.4%) and C2H4 (35.5%), and 58.7% of CH2Cl radicals are converted to C2H4. As shown in Figure 11, the reaction pathways of CH4/ CH3Cl/O2/Ar mixtures at ϕ = 0.5 and ϕ = 1 are similar. CH4 is mainly consumed via H abstraction reactions. Before chlorine is introduced, CH4 is consumed by small nonchlorinated radicals (O, H, OH). When CH3Cl is involved in the ignition of CH4, the H abstraction reaction by Cl radical becomes a dominant channel for CH4 consumption. In addition, upon introducing CH3Cl into the ignition of CH4, the production of nonsaturated species C2H4, yielded from CH3, is prompted. The main products from CH3Cl reactions are CH3 and CH2Cl.

CH4 is mainly consumed by H-abstraction reactions, yielding CH3 radicals. The proportions of the consuming pathway of CH4 abstracted by O, H, and OH radicals are 14.1%, 26.5%, and 56.1%, respectively. CH3 radicals are further consumed, yielding CH2O (24.6%), HCO radicals (15.1%), and C2H6 (43.4%). Mixed with CH3Cl, the proportions of CH4 abstracted by O, H, OH, and Cl radicals are 2.6%, 4.7%, 13.0%, and 79.2%, respectively. The main consuming products of CH3 radicals are C2H4 (34.7%), CH2O (11.1%), and C2H6 (49.3%). Neat CH3Cl decomposes into CH3 radicals (37.6%) and CH2Cl radicals (62.3%). CH3 radicals are further converted to C2H6 (22.0%) and C2H4 (64.1%), and the conversion ratio of CH2Cl radicals to C2H4 is 31.3%. Blended with CH4 at χCH3Cl = 0.5, G

DOI: 10.1021/acs.energyfuels.6b01466 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

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When CH4 is involved in the ignition of CH3Cl, the main products of CH3Cl are still CH3 and CH2Cl as in neat CH3Cl ignition. The branching ratio of CH3 from CH3Cl increases by ∼15% for ϕ = 0.5 and ∼16% for ϕ = 1, and that of CH2Cl decreases by ∼15% for ϕ = 0.5 and ∼16% for ϕ = 1. Although the total production of C2H6 from CH3Cl increases by ∼20% for ϕ = 0.5 and ∼18% for ϕ = 1, that of C2H4 from CH3Cl remains nearly constant (∼45% for ϕ = 0.5 and ∼44% for ϕ = 1).

5. CONCLUSIONS Ignition delay times of CH4/CH3Cl/O2/Ar mixtures are measured at temperatures of 1350−1950 K; pressures of 4, 8, and 18 atm; equivalence ratios of 0.5 and 1.0; and CH3Cl blending ratios of 0−1. Increasing χCH3Cl leads to reduced ignition delay time, but this effect becomes saturated approximately at χCH3Cl > 0.2. The Leylegian and chloromethane−Polimi−kin models under- or overestimates ignition delay times; a modified model incorporating the Aramco 2.0 model, i.e., the chloromethane−Polimi−Aramco−kin or CPAK model, is proposed and validated through comparison with the measurements. Based on the CPAK model, sensitivity, peak concentration, and reaction pathway analyses are conducted. R2781: CH3Cl = CH3 + Cl is a most important reaction for promoting ignition of CH4/CH3Cl mixtures, and its sensitivity peaks at χCH3Cl = 0.2. The peak concentrations of small radicals such as O, OH, H, CH3, and CH2O decrease moderately with increasing χCH3Cl, but the corresponding values of Cl, HCl, and CH2Cl increase nonlinearly with increasing χCH3Cl. Reaction pathway analysis shows that CH3Cl stimulates the formation of C2H4 in CH4/CH3Cl mixtures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01466. Measured ignition delay times of CH4/CH3Cl/O2/Ar mixtures and the detailed description of the uncertainty analysis (ZIP)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the 973 Project of China (2014CB845904) and the Scientific Challenges Project of China. We thank Dr. M. Pelucchi at Politecnico di Milano for sharing the CPK model before publication. L. Lu, D. Fan, X. M. Zhou, and J. Wang at PIMS are thanked for their kind help with constructing the shock tube facility, and X. Zhang and L. Pu for their help with experiments.

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DOI: 10.1021/acs.energyfuels.6b01466 Energy Fuels XXXX, XXX, XXX−XXX