Experimental and numerical study of the effect of CO2 on the ignition

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Experimental and numerical study of the effect of CO2 on the ignition delay times of methane under different pressures and temperatures Yang Liu, Chun Zou, Jia Cheng, Huiqiao Jia, and Chuguang Zheng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02443 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 18, 2018

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Experimental and numerical study of the effect of CO2 on the ignition delay times of methane under different pressures and temperatures Yang Liu†, Chun Zou*†‡, Jia Cheng†, Huiqiao Jia†, Chuguang Zheng† †

State Key Laboratory of Coal Combustion, Huazhong University of Science and

Technology, Wuhan, 430074, P. R. China ‡ Shenzhen Huazhong University of Science and Technology Research Institute, Shenzhen, 518057, P. R. China *Corresponding author. Tel: +86 2787542417-8314; fax: +86 2787545526. E-mail address: [email protected] (C. Zou) Abstract: Pressurized oxy-fuel combustion is regarded as a new generation of oxy-fuel technology. The ignition delay times of methane in O2/N2 atmosphere (0.21O2 + 0.79N2) and O2/CO2 atmosphere (0.21O2 + 0.79CO2) were measured in a shock tube at pressure of 0.8 atm and an equivalence of 0.5 and within a temperature range of 1501 to 1847 K. The present and Hargis’s experiment data (Hargis and Peterson, 2015) at 1.75 and 10 atm were adopted to evaluate five representative chemical kinetic models. This paper studied the chemical effects (chaperon effects of CO2 and the effects of reactions containing CO2) and physical effects of CO2 on ignition of methane at different pressures and temperatures in detail using a modified model. Artificial materials X and Y were employed to analyze the chemical and physical effects. The analysis showed that the physical effects of CO2 inhibit the ignition of methane, and are not sensitive to temperature. The chemical effects of CO2 vary greatly with pressure and temperature. At 0.8 and 1.75 atm, the chemical effects of CO2 promote the ignition of methane at high temperature, while suppress the ignition of methane at low temperature. The chaperon effects of CO2 promote the 1

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ignition of methane in O2/CO2 atmospheres at high temperature mainly because of HCO + M CO + H + M. The chaperon effects of CO2 suppress the ignition of methane at low temperature because of the 2CH3(+M) C2H6(+M). The chemical effects of CO2 offset the half of the physical effects of CO2 at high temperature and those two effects are great at low temperature, which is the reason for the fact that the effect of CO2 is subtle at high temperature and evident at low temperature. At 10 atm, the chemical effects of CO2 suppress the ignition of methane at 1350 – 1700 K. The chaperon effects of CO2 suppress the ignition of methane mainly due to 2CH3(+M) C2H6(+M) and are strengthened with the decrease of the temperature. The inhibition of reactions involving CO2 mainly attributes to CO + OH CO2 + H, and weaken as the decrease of the temperature, thus, the chemical effects of CO2 on the ignition are almost not sensitive to the temperature. The effects of CO2 have almost not change with temperature at 10 atm. 1. Introduction Oxy-fuel combustion has increasingly caused research concern, because it is a promising technology for carbon capture and storage as well as lower the operational risk.1 In oxy-fuel combustion, air is replaced by oxygen and therefore the combustion occurs in the atmosphere of O2/CO2. Because the physical and chemical properties of CO2 are very different from that of N2, the effect of CO2 on the characteristics of combustion, such as flame temperature,2 flame speed,3 extinction limits,4 were successively conducted. Recently, pressurized oxy-fuel combustion has become a new research hotspot because of its high efficiency and low emission.5,6 The ignition of fuel in O2/CO2 atmospheres is crucial for pressurized oxy-fuel combustion.7 There are many studies of ignition delay time in O2/CO2 atmospheres. 2


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Ignition delay time measured using shock tubes has been used to investigate the ignition and chemical kinetics of oxy-fuel combustion.8-13 Barak et al.8 studied the ignition delay times in oxy-syngas mixtures, they found that the GIR 3.0 and Aramco 2.0 did not accurately predict the experimental data, but only predicted the trends. Pryor et al.9 experimentally and numerically investigated the ignition delay time of oxy-methane combustion using a high pressure shock tube. They mentioned that the GRI 3.0 under predicted the experimental data at 30 atm while the Aramco 1.3 showed reasonable results. CO2 is not chemically inert, but takes part in some reactions. Sabia et al.14 investigated the effect of CO2 on the ignition delay of methane and ethane blends. They mentioned that CO2 affected the ignition process because of the strong competition between the reactions involving CO2 and chain branching reactions for H atoms at high temperatures. Giménez-López et al.15 studied the oxy-fuel combustion of natural gas in a flow reactor at atmospheric pressure, and thought that CO2 can react with CH2 through the following reaction: CO2 + CH2 CH2O + CO. Vasu et al.11 studied the ignition delay of syngas diluted with 24.4% CO2 using shock tubes in the range of the pressure from 1.1 to 2.6 atm, and reported that reaction H + O2 + CO2 HO2 + CO2 exerted a significantly negative impact on the ignition of syngas in the whole experiment. Koroglu et al.12 measured the ignition delay time of CH4 with CO2 mole fractions of 0 to 0.6 at temperatures of 1577 to 2144 K, pressures of 0.53 to 4.4 atm, and found that CO + OH CO2 + H had the most suppressive effect on the ignition of methane among the reactions containing CO2. 3


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Moreover, Holton et al.16 performed the ignition delay times of methane and ethane mixtures with 10% CO2 addition at equivalence ratio of 0.5 and temperature of 1137 K. They found that CO2 inhibited the ignition due to the high third-body collision efficiencies of CO2. Hargis and Petersen13 investigated the ignition delay of methane in shock tubes with high levels of CO2 at 1.75 and 10 atm, temperature of 1450 to 1900 K, and showed that the third-body reaction HCO + M H + CO + M and CH3 + CH3(+M) C2H6(+M) were very important third-body reaction for methane ignition with 75% CO2. As for the effects of pressure on the ignition delay of methane, they noted that the effect of CO2 was minimal at 1.75 atm, while it was evident at (or near) 10.0 atm. The chemical effects of CO2 on ignition delay included (1) the effects of the reactions with CO2, such as those mentioned above, CO2 + CH2 CH2O + CO and CO + OH CO2 + H,

(2) the chaperon (third-body) effects of CO2, such as those

mentioned above, HCO + M H + CO + M and CH3 + CH3(+M) C2H6(+M). However, few study has been conducted on the chemical and physical effects of CO2 at different pressures and temperatures in detail. Furthermore, the ignition delay time has been used to evaluate the performance of the combustion mechanism by many researchers. Zhang et al.17 evaluated the performance of UCS 2.0, GRI 3.0, UBC 2.018 and NUI Galway19 using the ignition delay times of methane/hydrogen blends under engine relevant pressure in 2012, they concluded that USC 2.0 shows the best match with experimental data. Zhang et al.20 evaluated the performance of NUI Galway, GRI 3.0, UCS 2.0, and Leeds 1.521 using 4


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the ignition delay times of CH4/H2/O2/Ar mixture at elevated pressures, the results show that the performance of four models is strongly dependent on pressure and hydrogen mole fraction. Hu et al.22 evaluated the performance of GRI 3.0, UCS 2.0 and Aramco 1.3 from the data of laminar flame speeds of methane-air mixtures in 2015, they found that the three models overpredict the experiment data at lean mixtures, and underpredict the experiment data at elevated temperatures and pressures. Most recently, Cai et al.23 evaluated GRI 3.0, UCS 2.0, “ITV”24, San Diego25 and Aramco 2.026 for oxy-methane combustion using extinction strain rates, ignition delay times, and laminar burning velocities. They found that Aramco 2.0 shows well agreement with experiment data of extinction strain rates with oxygen mass fractions of the oxidizer stream of 0.21, and all the five models overpredict the extinction strain rates with a fuel mass fraction of fuel steam of 0.2. Obviously, it is worth evaluating the performance of different reaction models at pressurized oxy-fuel combustion because pressurized oxy-fuel combustion is a new oxy-fuel technology. In this study, the ignition delay time of methane in O2/CO2 and O2/N2 atmospheres was recorded in a shock tube at pressure of 0.8, the equivalence ratio of 0.5, and within a temperature range of 1501–1847 K. The experiment data for 1.75 and 10 atm were adopted from Hargis and Petersen.13 Five models (“Ranzi”,27 USC 2.0,28 GRI 3.0,29 Aramco 1.3,30 FFCM31) were evaluated in the range of the pressure from 0.8 to 10bar using the present and previous experiment data.13 A detailed chemical kinetic model covering from 0.8 to 10 atm was updated to analyze the ignition delay time of methane. Artificial materials, chemical properties of which can 5


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be set according to needs, were employed to study the effects of the reactions CO2 was involved in and the chaperon effects of CO2 on the ignition delay of methane on the ignition. 2. Experimental and modeling 2.1. Shock-tube facility. The ignition delay time of the present work was measured in a shock tube with an inner diameter of 100 mm. The shock tube had a 4-m driver section and an 8-m driven section, and the two sections were separated by a diaphragm made of polyester terephthalate (PET). The shock wave was generated by a sudden burst of the diaphragm through a built-in spring needle. Diaphragms of 50 µm thickness was selected to produce 0.8 atm of pressure. Before each experiment, the shock tube was cleaned using high pressure air, and then the tube and mixing tank were evacuated to less than 1×10-5 bar using an oil-sealed, sliding, vane rotary vacuum pump (Oerlikon Leybold TRIVAC D40T) together with two root pumps (Oerlikon Leybold Ruvac WAU501). Table 1 listed Mix-1 and Mix-2 were prepared in the mixing tank using Dalton’s law of pressure and settled for 12 h. The purities of methane, oxygen, carbon dioxide, and nitrogen were 99.99%, 99.999%, 99.999%, and 99.999%, respectively. Table 1. Four different mixtures in the present work Mixture CH4

O2

CO2

N2

X

Y

0

0.75

0

0

Mix-1

0.05 0.20

Mix-2

0.05 0.20 0.75

0

0

0

Mix-X

0.05 0.20

0

0

0.75

0

Mix-Y

0.05 0.20

0

0

0

0.75

6


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Five piezoelectric pressure transducers (PCB 111A24) were arranged alongside of the shock tube to obtain the incident shock velocity. One piezoelectric pressure transducer (Kistler 603B1) was placed 0.02 m from the end-wall to monitor the pressure. The OH* radical emission was captured using a photomultiplier with a band pass filter of 307 ± 10 nm 0.02 m from the end-wall. The temperature ( T5 ) and pressure (

P5 ) behind the reflected shock were calculated using GasEq.32 The

uncertainty was mainly from the uncertainty of temperature which is determined by the incident shock velocity. According to Pterson et al.33 and Zhang et al.34, the standard root-sum-squares (RSS) method35 was used for the experiment uncertainty analysis. The typical error in this study was about ±17 K for the lowest temperature and ±26 K for the highest temperature. As a result, the uncertainty in the ignition delay time measurement was around 16-20% depending on the test conditions. The detailed analysis of the uncertainty is shown in Supporting Information. 2.2 Ignition delay times measurement. As shown in Fig. 1, the bifurcation happened when highly CO2 was in the mixture. When the bifurcation happens, it was hard to determine the arrival of the main reflected shock wave. The highly addition of CO2 result in the slight increase in the pressure during combustion. Therefore, in this study, the ignition delay time was determined by the time interval between the arrival of the reflected wave and the onset of ignition. The onset of ignition was defined by positioning the time of the steepest increase in normalized OH* radical emission and linearly extrapolating to the zero baseline.36 And the time of arrival of the reflected wave was determined through an empirical expression proposed by Petersen and 7


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Hanson.37 ∆t AO ( µ s ) = 4.6 M s 0.66γ 2 −7.1 M

where

0.57

(1)

M s was the incident shock Mach number, γ 2 was the specific heat ratio

upstream of the bifurcation, M was molecular weight of mixtures. The arrival of the reflected shock wave time

t A = ∆t AO + where

t A can be obtained as follows:

D + ti 2VR

(2)

ti was the time of initial pressure rise, D was diameter of the pressure

transducer (Kistler 603B1),

VR was the reflected-shock velocity. 2.5

Normalized Emission

1.0

Presure

2.0

0.8 1.5

tA

0.6

1.0 0.4

0.2

0.5

ignition delay time ti

0.0 -50

0

50

100

Presure (atm)

Normalized Emission

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.0

150

200

250

300

350

400

Time (µs)

Fig. 1. Example ignition delay definition for Mix-2: T = 1625 K, P = 1.87 atm. The time of arrival of the reflected wave was determined through an empirical expression.37 2.3 Modeling and analytic approach. Computations were made using CHEMKIN-PRO38 for ignition delay times and Rate Of Production (ROP) analysis, using Senkin39 from the CHEMKIN II software package40 for sensitivity analysis. As shown in Fig. 2, the experimental test time is more than 2000 µs. Koroglu et al. thought that the boundary layer influences can be minimized by large diameter of shock tube.12 In present tests, the ignition delay times were shorter than 2000 µs. 8


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Hence constant volume, zero-dimensional chemistry model (U, V assumption) was assumed without modification in the current study. A modified model was adopted to simulate the ignition delay time and conduct kinetic analysis. 1.50

Presure 1.25

Presure (atm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Horizontal line for average pressure

1.00

0.75

Driver gas:He Driven gas:Ar P=0.78atm T=1611K

0.50

0.25

0.00 0

500

1000

1500

2000

Time (µs)

Fig. 2. The experimental test time measurement. The chemical effects of CO2 on methane ignition delay in O2/CO2 included (1) the chaperon effects of CO2 (termed EFC1) and (2) the effects of the reactions with CO2 (termed EFC2). Artificial species X had the same thermochemical properties as CO2 and was chemically inert, while species Y had the same thermochemical properties and third-body collision efficiencies as CO2 but did not participate in any chemical reactions. Consequently, the differences in ignition delay time between the O2/X and O2/Y atmospheres represented EFC1, and the differences in ignition delay time between the O2/CO2 and O2/Y atmospheres represented EFC2. As a result, the differences in ignition delay time between O2/CO2 and O2/X represented the sum of EFC1 and EFC2, i.e., the overall chemical effects of CO2. The N2 was essentially chemically inert and the chaperon effects of N2 were minimal in the current study conditions. Thus, the differences in ignition delay time between the O2/X and O2/N2 atmospheres represented the physical effects of CO2 when CO2 was substituted for 9


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N2 . 3. Results and discussion 3.1 Comparison with previous data. In order to validate the present facilities and methods, the ignition delay time has been performed in the conditions of 5% CH4, 20% O2, 75% CO2 and 1.75 atm, which are the same as those in the experiment of Hargis and Peterson.13 Diaphragms of 75 µm thickness was selected to produce 1.75 atm of pressure. Fig. 3 shows the comparison between present measurements and previous data of methane from Hargis and Peterson with 75% of CO2 dilution at 1.75 atm.13 It can be seen from Fig. 3 that the present data conform fairly well to those from Hargis and Peterson.13 Therefore, the present facilities and methods are applicable to study the ignition delay of methane with 75% of CO2 dilution. The experimental data in present study are present in Supporting Information Table S1. Hargis and Peterson (2015)

Current Study Ignition delay time (µs)

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1000

P = 1.75atm Mix-2 CH4 0.05

100

O2 0.2 CO2 0.75 10 0.50

0.55

0.60

0.65

0.70

0.75

1000/T(K-1)

Fig. 3. Comparison between the ignition delay time measurements of present study and previous data for Mix-2. 3.2 Measured ignition delay times. Fig. 4 shows the ignition delay time of methane at equivalence ratio of 0.5 under O2/N2 and O2/CO2 atmosphere at 0.8 atm. As shown in Fig. 4, the ignition delay time of two atmospheres are very close, which 10


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indicates that CO2 has a minimal effect on ignition delay when CO2 is replaced by N2 in methane-oxidizer mixtures at 0.8 atm. This phenomenon was observed by Hargis and Peterson at 1.75 atm.13

Ignition delay time (µs)

Mix 2 Mix 1 1000

0.8 atm Mix 1 Mix-2 CH4 0.05 CH4 0.05

100

0.52

0.54

0.56

0.58

0.60

0.62

O2 0.2

O2 0.2

N2 0.75

CO2 0.75

0.64

0.66

0.68

0.70

-1

1000/T (K )

Fig. 4. Measured ignition delay times of Mix-1 and Mix-2 at 0.8 atm. 3.3 Model evaluation and modification. Fig. 5 displays the calculation results of the ignition delay time for Mix-1 (O2/N2 atmosphere) and Mix-2 (O2/CO2 atmosphere) at three pressures (0.8, 1.75, 10 atm) using five models, namely, “Ranzi”,27 USC 2.0,28 GRI 3.0,29 Aramco 1.3,30 and FFCM-131 which is a recently foundational fuel chemistry model proposed by Wang’s group and Simth. As shown in Fig. 5, as expected, the modeling results of the five models are different under different atmospheres and pressures. 10000

1000

10000

GRI 3.0 Aramco 1.3 "Ranzi" FFCM-1 USC 2.0

(a)

P=0.8atm Mix-2



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100

1000


P=0.8atm Mix-1

(b)

100

Date: Present Date: Present 10

10

0.56

0.60

0.64

0.56

1000/T (K-1)

0.60

1000/T (K-1)

11


0.64

0.68

Energy & Fuels

10000

P=1.75atm Mix-2


1000

(c)



10000

100


1000

(d)

P=1.75atm Mix-1

100

Date: Hargis & Peterson (2015)


10

10 0.56

0.60

0.64

0.68

0.52

0.56

0.60

1000/T (K-1)

0.64

0.68

0.72

1000/T (K-1)

10000

10000


1000

P=10atm Mix-2

(e)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100

1000


0.65

(f)

100


Date: Hargis & Peterson (2015) 10 0.60

P=10atm Mix-1

10 0.60

0.70

0.65

1000/T (K-1)

0.70

0.75

1000/T (K-1)

Fig. 5. Comparison of experimental and simulated ignition delay times of Mix-1 and Mix-2 at three pressures with five models. Ignition delay times for experiments at 1.75 and 10 atm were obtained from Hargis and Peterson.13 In order to evaluate the performances of five models, the average absolute relative error (AARE) value was used and defined as follows:

1 N Ysim,i − Yexp,i AARE = ∑ ×100% N i=1 Yexp,i

(3)

Where N denotes the number of data points in the data set. The Yexp,i and Ysim,i represents the measured and calculated result for ith data point, respectively. Table 2 displays the AAREs of five models for Mix-1 and Mix-2 at 0.8, 1.75 and 10 atm. As shown in Table 2, at 0.8 atm, the AAREs of GRI 3.0, “Ranzi”, USC 2.0 and FFCM-1 are in the range from 16% to 30%, which indicates that the four models yield acceptable prediction of experimental data. And, the AAREs of Aramco 1.3 are higher 12


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than 47%, which indicates that it shows poor prediction of experimental data. Table 2. The average absolute relative error values for different mechanisms, mixtures and pressures. Pressure

Average absolute relative error (%)

Mixture [atm]

“Ranzi”

USC 2.0

GRI 3.0

Aramco 1.3

FFMC-1

Mix-1

0.8

19.38

25.10

16.58

49.53

20.56

Mix-2

0.8

18.59

29.69

20.58

60.84

28.59

Mix-1

1.75

15.61

21.43

14.64

48.44

13.48

Mix-2

1.75

19.07

9.10

9.40

42.99

9.76

Mix-1

10

9.92

23.49

39.00

9.99

19.60

Mix-2

10

16.48

22.32

39.31

11.27

18.43

At 1.75 atm, under O2/N2 atmospheres, as shown in table 2, the AAREs of GRI 3.0, “Ranzi” and FFCM-1 are about 15%, which means that GRI 3.0 and “Ranzi” and FFCM-1 show well agreement with experimental data. The AARE of Aramco 1.3 (48.44%) is more than thrice as large as the “Ranzi”, which indicates that it shows poor prediction of experimental data. Moreover, the AARE of USC 2.0 is 21.43%, which means that USC 2.0 yields acceptable prediction of experimental data. In the case of O2/CO2 atmospheres, the AAREs of “Ranzi”, USC 2.0, GRI 3.0, Aramco 1.3 and FFCM-1 are 19.07%, 9.10%, 9.40%, 42.99% and 9.76%, respectively. This means that USC 2.0, GRI 3.0 and FFCM-1 display fairly well agreement with the experimental data, and “Ranzi” yields acceptable prediction of experimental data, while Aramco 1.3 displays a poor prediction of experimental data. At 10 atm, under O2/N2 atmospheres shown in Table 2. The AAREs of Aramco 1.3 and “Ranzi” are the lowest value of approximately 10%, which indicates that the 13


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two models show fairly well agreement with experimental data. The AAREs of USC 2.0 and FFCM-1 are 23.49% and 19.60%, respectively. This means that USC 2.0 and FFCM-1 yield acceptable prediction of experimental data. And, the AARE of GRI 3.0 (39%) is about four times as large as the Aramco 1.3, which indicates that GRI 3.0 shows a poor prediction of experimental data. In the case of O2/CO2 atmospheres, the AAREs of “Ranzi”, USC 2.0, GRI 3.0, Aramco 1.3, FFCM-1 are 16.48%, 22.32%, 39.31%, 11.27%, 18.43%, respectively. These indicate that Aramco 1.3 shows a well agreement with experimental data, and “Ranzi”, USC 2.0 and FFCM-1 yield acceptable prediction of experimental data, and GRI 3.0 still displays a poor prediction of experimental data. It is worth noting from Fig. 4 that “Ranzi” shows poor prediction of ignition activation energy at 1.75 atm and 10 atm, especially in high temperature ranges, even though it displays a relatively small AARE. The model evaluation indicates that none of the five models can be suitable for the ignition of CH4 in O2/CO2 atmospheres in the pressure and temperature ranges from low to high. Therefore, a modified model has been proposed based on Aramco 1.3 in present study. Ranzi et al.27 and Westbrook et al.41 pointed out that the pressure-dependent reaction CH3 + CH3 (+M) C2H6 (+M) had great influence on methane overall ignition rate. In this study, the parameters of CH3 + CH3 (+M) C2H6 (+M) were updated by those expressed in “Ranzi”.27 Moreover, the channels of CH4→CH3 had significant influence on CH4 oxidation,42 the parameters of reaction CH4 + OH 14


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CH3 +H2O were updated by a newly fitted rate constant form Lee et al.43 and the parameters of reaction CH4 + H CH3 + H2 were updated according to Baulch et al.44 These reactions were listed detailedly in Table 3. Table 3. Reactions updated in the present studya Reaction

A

n

EA

Reference

CH4 + OH CH3 +H2O

2.30E+08

1.40

2850.0

43

CH4 + H CH3 + H2

4.78E+05

2.50

9588.0

44

CH3 + CH3 (+M) C2H6 (+M)

2.50E+13

0.00

0.00

27

Low-pressure limit

2.33E+34

-5.03

-1200.0

a = 0.38, T3 = 73.0, T1 = 1180.0 a

Rate constants were expressed as k = ATβ exp(-Ea/RT) with units of calories, cm3, mole, and second. To evaluate the simulation results of present model, the AAREs for the modified

model were carried out, as shown in Table 3. At 10 atm, the AAREs of Mix-1 and Mix-2 are 9.7% and 14%, respectively. At 1.75 atm, the AAREs of Mix-1 and Mix-2 are 10.38% and 9.12%, respectively. This means that the modified model shows fairly well agreement with the experiment data at 1.75 and 10 atm. Moreover, at 0.8 atm, the AAREs of Mix-1 and Mix-2 are 22.93% and 29.95%, respectively. This means that the modified model shows acceptable match with the experiment data. Table 4. The average absolute relative error value for the modified model Mixture

Mix-1 Mix-2

Mix-1

Mix-2 Mix-1

Mix-2

Pressure (atm)

0.8

0.8

1.75

1.75

10

10

Average absolute relative error (%)

22.93

29.95

10.38

9.12

9.77

14.00

In addition, the modified model can accurately capture the ignition activation 15


Energy & Fuels

energy as shown in Fig. 5. The modified model compared with previous studies9,12 are shown in Supporting Information Fig. S1, it can be seen that the results by the modified model display good fit to the experimental results. Thus, the modified model can be employed to discuss the effect of CO2 for the present study. 10000 10000

0.8atm

Mix-1 Modification model Mix-2 Modification model Mix-1 Experiment Mix-2 Experiment

(b)



(a)

1000

100


1.75atm

1000

100


Date: Present 10

10

0.52

0.54

0.56

0.58

0.60

0.62

0.64

0.66

0.68

0.52

-1

0.56

0.60

0.64

0.68

0.72

1000/T (K-1)

1000/T (K ) 10000


(c)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 31

10atm

1000

100

Date: Hargis & Peterson (2015) 10 0.58

0.60

0.62

0.64

0.66

0.68

0.70

0.72

0.74

0.76

1000/T (K-1)

Fig. 6. Comparison of experimental and simulated ignition delay times of Mix-1 and Mix-2 at different pressures with the modified model. Ignition delay times for experiment at 1.75 and 10 atm were obtained from Hargis and Peterson.13 3.4 The physical and chemical effects of CO2. As shown in Fig. 6, the ignition delay times of Mix-2 (O2/CO2 atmosphere) are longer than that of Mix-1 (O2/N2 atmosphere), which means that the CO2 inhibits the ignition of methane in the pressure of 0.8, 1.75 and 10 atm. The difference between the Mix-2 and Mix-1 not changes with the temperature at 10 atm, while the difference between the Mix-2 and 16


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Energy & Fuels

Mix-1 increases as the temperature decreases at 0.8 and 1.75 atm. This means that the effect of CO2 on the ignition of methane depends on the initial temperature and pressure. Since CO2 has chemical and physical effects, the percent variation (PV)45 of effects was introduced in order to investigate the chemical and physical effects of CO2 quantitatively, and defined as:

PVEFC1 =

τ Mix−Y − τ Mix− X τ Mix− X

(4)

PVEFC 2 =

τ Mix −2 − τ Mix −Y τ Mix− X

(5)

PVPE = where

τ Mix−x −τ Mix−1 τ Mix− X

(6)

PVEFC1 , PVEFC2 and PVPE are the percent variation of chaperon effects,

percent variation of effects of the reactions with CO2 and present variation of physical effects, respectively; τ Mix−1 ,

τMix−2 , τMix−X

and

τMix−Y

are ignition delay times for

Mix-1, Mix-2, Mix-X and Mix-Y, respectively. According to the method proposed in section 2.3. The present variation of chemical effects of CO2 ( PVCE ) are the sum of

PVEFC1 and PVEFC2 . Moreover, the positive value of PV implied a suppression of ignition, and the negative value of PV implied a promotion of ignition. Fig. 7 shows the percent variation of physical and chemical effects of CO2 on the ignition delay of methane at 0.8, 1.75 and 10 atm. As shown in Fig. 7, at 0.8 atm, the

PVCE increases from -2.5% to 20.5% and the PVPE increases from 5.5% to 12.1% as the temperature changes from 1900 to 1400 K, which means that the chemical effects of CO2 promote the ignition at high temperature and suppress the ignition at low temperature, and the physical effects of CO2 inhibit the ignition of methane. 17


Energy & Fuels

Because the chemical effects of CO2 offset the half of the physical effects of CO2, the effect of CO2 on the ignition of methane is subtle at high temperature. Both of the chemical and physical effects of CO2 are great, the ignition delay is evident at low temperature in O2/CO2 atmospheres comparing with O2/N2 atmospheres, as shown in Fig. 6. The similar situation is observed at 1.75 atm. 30

20

Present variation (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

10

PVPE(0.8atm)

0

PVCE(0.8atm) PVPE(1.75atm)

-10

PVCE(1.75atm) PVPE(10atm)

-20

PVCE(10atm) -30 0.50

0.55

0.60

0.65

0.70

0.75

1000/T (K-1)

Fig. 7. The percent variation of physical and chemical effects of CO2 on the ignition delay of methane at three pressures (0.8, 1.75, 10 atm) At 10 atm, we can see from Fig. 7 that the

PVCE and PVPE are about 13% and

18%, respectively. Moreover, they slightly change with the temperature, which means that under high pressure, the chemical and physical effects of CO2 significantly suppress the ignition of methane, and these effects are almost not sensitive to temperature. Consequently, the line of the ignition delay times in O2/CO2 atmospheres is approximately parallel to that in O2/N2 atmospheres, as shown in Fig. 6. Fig. 8 shows the percent variation of chaperon effects of CO2 (EFC1) and effects of the reactions with CO2 (EFC2) on the ignition delay of methane at 0.8, 1.75 and 10 atm. At 0.8 atm, as shown in Fig. 8, the the

PVEFC1 increases from -17.4% to 11.9% and

PVEFC2 decreases from 14.9% to 8.6% as the temperature changes from 1900 to 18


Page 19 of 31

1400 K, respectively. This indicates that the EFC1 on the ignition of methane changes from the promotion at high temperature to suppression at low temperature. In contrast to propone, Sabia et al.45 found that the chaperon effects of CO2 on the ignition of propane changed from suppression to promotion as temperature decreased from 1400 to 800 K. Moreover, the profile of

PVEFC2 indicates that the EFC2 inhibits the

ignition of methane and is strengthened as temperature increases. The phenomenon was found in propone.45 It is worth nothing from Fig. 8 that the increase rate of

PVEFC1 is much higher than the decrease rate of PVEFC2 at 0.8 and 1.75 atm. Therefore, the chemical effects of CO2 on the ignition of methane evidently increase as temperature decreases from 1900 to 1400 K, as shown in Fig. 7. 30

20

percent variation (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

10

0

PVEFC1(0.8atm) PVEFC2(0.8atm)

-10

PVEFC1(1.75atm) PVEFC2(1.75atm) -20

PVEFC1(10.0atm) PVEFC2(10.0atm)

-30 0.50

0.55

0.60

0.65

0.70

0.75

-1

1000/T (K )

Fig. 8. The percent variation of chaperon effects of CO2 and effects of the reactions with CO2 on the ignition delay times of methane at three pressures (0.8, 1.75, 10 atm) At 10 atm, the

PVEFC1 increases from 2.2% to 16.6% and the PVEFC2

decreases from 12.4% to 3.7% as the temperature changes from 1700 to 1350 K, which means that the suppression of the ignition due to the EFC1 is strengthened, and the suppression of the ignition due to the EFC2 is weaken as temperature decreases from 1700 to 1350 K. Consequently, the chemical effects of CO2 on the ignition are 19


Energy & Fuels

almost not sensitive to the temperature, as show in Fig. 7. In order to study the effect of CO2 on the ignition of methane at elevated pressures, the initial pressure is extended up to 60 atm. At 1500 K, as shown in Fig. 9, the PVPE increases significantly from 9.1% to 19.2% as the pressure increases from 0.8 to 60 atm, which means that the physical effects are strengthened. The PVCE sharply increases and reaches a peak at 11 atm, and then, it slowly decreases to 16% as the pressure increases to 60 atm. Therefore, the chemical effects of CO2 are larger than the physical effects of CO2 in the pressure range from 0.8 to 38 atm and smaller than that in the pressure range from 38 to 60 atm. 20

Present variation (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

15

PVPE PVCE PVEFC1 PVEFC2

10

T=1500K 5

0

10

20

30

40

50

60

Pressure (atm)

Fig. 9. The percent of effect of CO2 on the ignition of methane at different initial pressures. For the chemical effects, the PVEFC1 increases sharply from 3.9% to about 13% as the pressure increases from 0.8 to 20 atm, and it remains stable at pressure range from 20 to 60 atm, which indicates that the chaperon effects of CO2 are strongly correlated to the pressure in the pressure range from 0.8 to 20 atm, and nearly independent of the pressure in the pressure range from 20 to 60 atm. The PVEFC2 decreases significantly and monotonously from 10.6% to 3.3% as pressure increases 20


Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

from 0.8 to 60 atm. It is worth northing that the PVEFC1 is much higher that PVEFC1 in the pressure range from 20 to 60 atm, thus, the chaperon effects of CO2 are dominant in the chemical effects of CO2. Moreover, the reason for the fact that the chemical effects slowly decrease as pressure increase from 11 to 60 is that the PVEFC2 decreases significantly and monotonously with the increase in the pressure. 3.5 Chemical kinetic analysis of the chaperon effects of CO2. To further study the chemical mechanism of the chaperon effects of CO2 (EFC1) on the ignition of methane in an O2/CO2 atmosphere, the brute force sensitivity analysis were conducted at T = 1800 K, P = 0.8 atm; T = 1500 K, P = 0.8 atm; T = 1650 K, P = 10 atm; T = 1350 K, P = 10 atm for Mix-2. The sensitivity coefficient ( σ ) is calculated from the flowing formula:

σi =

τ (1.5ki ) − τ (0.5ki ) τ ( ki )

(7)

where ki and τ denote the rate coefficient of ith reaction and ignition delay times, respectively. The positive value of σ represents suppression of ignition, and the negative value of σ represents promotion of ignition. As shown in Fig. 10a, at 0.8 atm, there is only one third-body reaction, R30 (HCO + M CO + H + M), in Fig. 10a, and the sensitivity coefficient of R30 is negative, which indicates that R30 is beneficial to the ignition. At low temperature, there are two third-body reactions, R30 and R189 (2CH3(+M) C2H6(+M)) in Fig. 10b. The sensitivity coefficient of R189 is positive and absolute value is the larger than R30, which means that R189 is adverse to ignition and its effect on the ignition is greater than R30. The timing of 20% methane consumption was selected to analysis 21


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

the rate of production (ROP) of CH3 and H radicals. As shown in Fig. 11a, at 0.8 atm, the percent of CH3 consumption from R189 decreases sharply from 38.8% to 7.5% as the temperature changes from 1400 K to 1900 K. And, the percent of H production from R30 increases slowly from 25.5% to 33.6% as the temperature changes from 1400 K to 1900 K. Consequently, the change of EFC1 with temperature depends on the competition between R30 and R189, the ignition promotion by EFC1 at high temperature is mainly attributed to R30, and the ignition suppression by EFC1 at low temperature is mainly due to R189. (b)

R128 CH4+HCH3+H2

(a)

R128 CH4+HCH3+H2

R27 CO+OHCO2+H

R189 2CH3(+M)C2H6(+M)

R146 CH3+OCH2O+H

R129 CH4+OHCH3+H2O

R129 CH4+OHCH3+H2O

R31 HCO+O2CO+HO2

R31 HCO+O2CO+HO2

R27 CO+OHCO2+H

R73 CH2O+HHCO+H2

R73 CH2O+HHCO+H2 R30 HCO+MH+CO+M

R75 CH2O+CH3HCO+CH4

R145 CH3+HO2CH4+O2

R131 CH4+HO2CH3+H2O2

R130 CH4+OCH3+OH

P=0.8atm

R30 HCO+MH+CO+M

P=0.8atm T=1500K

R39 CH2O+O2HCO+HO2

T=1800K

R144 CH3+HO2CH3O+OH

R39 CH2O+O2HCO+HO2 R147 CH3+O2CH3O+O

R147 CH3+O2CH3O+O

-0.6

-0.4

-0.2

R144 CH3+HO2CH3O+OH

R148 CH3+O2CH2O+OH

R148 CH3+O2CH2O+OH

R1 H+O2O+OH

R1 H+O2O+OH

0.0

0.2

0.4

0.6

-0.6

-0.4

Sensitivity coefficient

-0.2

R128 CH4+HCH3+H2

R17 2HO2H2O2+O2

R129 CH4+OHCH3+H2O

R145 CH3+HO2CH4+O2

R27 CO+OHCO2+H

R129 CH4+OHCH3+H2O

R31 HCO+O2CO+HO2

R128 CH4+HCH3+H2

R73 CH2O+HHCO+H2

R15 HO2+OHH2O+O2 R30 HCO+MH+CO+M R131 CH4+HO2CH3+H2O2

-0.2

0.6

R76 CH2O+HO2HCO+H2O2

P=10atm T=1350K

R147 CH3+O2CH3O+O

R39 CH2O+O2HCO+HO2

R131 CH4+HO2CH3+H2O2

R147 CH3+O2CH3O+O

R39 CH2O+O2HCO+HO2

R144 CH3+HO2CH3O+OH

R1 H+O2O+OH

R148 CH3+O2CH2O+OH

R144 CH3+HO2CH3O+OH

R1 H+O2O+OH -0.4

0.4

R193 C2H6+OHC2H5+H2O

R15 HO2+OHH2O+O2

P=10atm T=1650K

0.2

(d) R189 2CH3(+M)C2H6(+M)

(c) R189 2CH3(+M)C2H6(+M)

-0.6

0.0


0.0

0.2

R148 CH3+O2CH2O+OH 0.4

0.6 -0.6


-0.4

-0.2

0.0

0.2

0.4

0.6


Fig. 10. Sensitivity analysis of Mix-2 at: (a) T = 1800 K, P = 0.8 atm, (b) T = 1500 K, P = 0.8 atm, (c) T = 1650 K, P = 10 atm, (d) T = 1350 K, P = 10 atm.

22


45

50

(a)

40

40

35 30

30

25 20

20

15

2CH3(+M)C2H6(+M)

10

10

HCO+MCO+H+M

50

Percent of H radical production(%) Percent of CH3 radical consumtion(%)

50

Percent of CH3 radical consumtion(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

5

45

50

(b)

40

40

35 30

30

25 20

20

15

2CH3(+M)C2H6(+M) 10

10

HCO+MCO+H+M

Percent of H radical production(%)

Page 23 of 31

5

0

0 1400

1500

1600

1700

1800

0 1300

1900

0 1400

Temperature(K)

1500

1600

1700

Temperature(K)

Fig. 11. Percent of CH3 radical consumption and H radical production for Mix-2 at the timing of 20% methane consumption at (a) 0.8 atm, (b) 10 atm. As shown in Fig. 10c and Fig. 10d, at 10 atm, the third-body reactions with the highest sensitivity coefficient are R189 and R30 at high temperature, and only R189 appear at low temperature. It is worth northing that the percent of CH3 consumption from R189 is higher than 37.5% and decreases slowly only as the temperature changes from 1500 K to 1700 K, as shown in Fig. 11b, which is significantly different with the case at 0.8 atm. As a result, R189 is dominate in EFC1 at 10 atm at both high and low temperature. The promotion of R30 to the ignition is small at high temperature, and evidently weakened as the temperature decreases.

3.6 Chemical kinetic analysis of the effects of the reactions with CO2. As shown in Fig. 10a and Fig. 10b, at 0.8 atm, there is only one reaction containing CO2, R27 (CO + OH CO2 + H). The sensitive coefficient of R27 is positive, which indicates that R27 is adverse to ignition of methane, as reported by Koroglu et al.12 Moreover, the ROP analysis shows that the percent of H consumption from R27 increases from 5.3% to 14.4% as the temperature changes from 1400 K to 1900 K, as shown in Fig. 12, which indicates that more H radical is consumed through the R27 as 23


Energy & Fuels

temperature increases. As a result, the suppression of EFC2 on the ignition increases as temperature increases, and is mainly through R27 at 0.8 atm. 16

Percent of H radical consumption (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

14

0.8atm 10atm

12 10 8 6 4 2 1300

1400

1500

1600

1700

1800

1900

Temperature(K)

Fig. 12. Percent of H radical consumption from R27 for Mix-2 at the timing of 20% methane consumption at 0.8 and 10 atm. As shown in Fig. 10c and Fig. 10d, at 10 atm, the reaction containing CO2, which is the most sensitive to the ignition, is R27 at high temperature. But none of reaction containing CO2 appears at low temperature. Consequently, the suppression of the ignition due to the EFC2 is weaken as temperature decreases, as shown in Fig. 7.

4. Conclusions The five models, “Ranzi”, USC 2.0, GRI 3.0, Aramco 1.3, and FFCM-1, is not suitable for the ignition of CH4 in O2/CO2 atmospheres in the pressure range from 0.8 to 10 atm and temperature range from low to high. A modified model in the present work satisfactorily captured the ignition delay of methane in O2/N2 and O2/CO2 atmospheres at 0.8, 1.75 and 10 atm. The effect of CO2 always suppresses the ignition of methane in the pressure of 0.8, 1.75 and 10 atm. The physical effects of CO2 always inhibit the ignition of methane, and are not sensitive to temperature. The chemical effects of CO2 promote 24


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Energy & Fuels

the ignition at high temperature and suppress the ignition at low temperature in the pressure of 0.8 and 1.75. The chemical effects of CO2 on the ignition are negative and almost not sensitive to the temperature in the pressure of 10 atm. At 0.8 and 1.75 atm, the chaperon effects of CO2 (EFC1) on the ignition of methane change from the promotion at high temperature to suppression at low temperature, which depends on the competition between R30 (HCO + M CO + H + M) and R189 (2CH3(+M) C2H6(+M)). The effects of the reactions with CO2 (EFC2) inhibit the ignition of methane and are weakened as temperature decreases. At 10 atm, the suppression of the ignition due to the EFC1 (2CH3(+M) C2H6(+M)) is strengthened , and the suppression of the ignition due to the EFC2 (CO + OH CO2 + H) is weakened as temperature decreases from 1700 to 1350 K.

Acknowledgements This work was supported by the general program (No.51776081) of the National Natural Science Foundation of China and the fundamental research program of Shenzhen (JCYJ20170818164507350).

Supporting information The experimental conditions and ignition delay times, the validation of modified model, and the details of uncertainty in ignition delay time.

References (1) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Oxy-fuel combustion technology for coal-fired power generation. Prog. Energy Combust. Sci.

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(2) Song, Y.; Zou, C.; He, Y.; Zheng, C. The chemical mechanism of the effect of CO2 on the temperature in methane oxy-fuel combustion. Int. J. Heat Mass Transfer 2015, 86 (1), 622-628. (3) Natarajan, J.; Lieuwen, T.; Seitzman, J. Laminar flame speeds of H2/CO mixtures: effect of CO2 dilution, preheat temperature, and pressure. Combust. Flame 2007, 151 (1), 104-119. (4) Li, X.; Jia, L.; Onishi, T.; Grajetzki, P.; Nakamura, H.; Tezuka, T.; Hasegawa, S.; Maruta, K. Study on stretch extinction limits of CH4/CO2 versus high temperature O2/CO2 counterflow non-premixed flames. Combust. Flame 2014, 161 (6), 1526-1536. (5) Xia, F.; Yang, Z.; Adeosun, A.; Gopan, A.; Kumfer, B. M.; Axelbaum, R. L. Pressurized oxy-combustion with low flue gas recycle: Computational fluid dynamic simulations of radiant boilers. Fuel 2016, 181, 1170-1178. (6) Zebian, H.; Mitsos, A. Pressurized oxy-coal combustion: Ideally flexible to uncertainties. Energy 2013, 57, 513-526. (7) Chen, L.; Yong, S. Z.; Ghoniem, A. F. Oxy-fuel combustion of pulverized coal: characterization, fundamentals, stabilization and CFD modeling. Prog. Energy Combust. Sci. 2012, 38 (2), 156-214. (8) Barak, S.; Pryor, O.; Lopez, J.; Ninnemann, E.; Vasu, S.; Koroglu, B. High-Speed Imaging and Measurements of Ignition Delay Times in Oxy-Syngas Mixtures With High CO2 Dilution in a Shock Tube. J. Eng. Gas Turbines Power 2017, 139 (12). (9) Pryor, O.; Barak, S.; Lopez, J.; Ninnemann, E.; Koroglu, B.; Nash, L.; Vasu, S. 26


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High Pressure Shock Tube Ignition Delay Time Measurements During Oxy-Methane Combustion With High Levels of CO2 Dilution. J. Energy Resour. Technol 2017, 139 042208-042208-6. (10) Pryor, O.; Barak, S.; Koroglu, B.; Ninnemann, E.; Vasu, S. S. Measurements and interpretation of shock tube ignition delay times in highly CO2 diluted mixtures using multiple diagnostics. Combust. Flame 2017, 180, 63-76. (11) Vasu, S. S.; Davidson, D. F.; Hanson, R. K. Shock tube study of syngas ignition in rich CO2 mixtures and determination of the rate of H+ O2+ CO2→ HO2+ CO2. Energy Fuels 2011, 25 (3), 990-997. (12) Koroglu, B.; Pryor, O. M.; Lopez, J.; Nash, L.; Vasu, S. S. Shock tube ignition delay times and methane time-histories measurements during excess CO2 diluted oxy-methane combustion. Combust. Flame 2016, 164, 152-163. (13) Hargis, J. W.; Petersen, E. L. Methane Ignition in a Shock Tube with High Levels of CO2 Dilution: Consideration of the Reflected-Shock Bifurcation. Energy Fuels

2015, 29 (11), 7712-7726. (14) Sabia, P.; Lubrano Lavadera, M.; Sorrentino, G.; Giudicianni, P.; Ragucci, R.; de Joannon, M. H2O and CO2 Dilution in MILD Combustion of Simple Hydrocarbons. Flow, Turbul. Combust. 2016, 96 (2), 433-448. (15) Giménez-López, J.; Millera, A.; Bilbao, R.; Alzueta, M. U. Experimental and kinetic modeling study of the oxy-fuel oxidation of natural gas, CH4 and C2H6. Fuel

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Autoignition Delay Time Measurements of Methane, Ethane, and Propane Pure Fuels and Methane-Based Fuel Blends. J. Eng. Gas Turbines Power 2010. 132 (9), 091502. (17) Zhang, Y.; Jiang, X.; Wei, L.; Zhang, J.; Tang, C.; Huang, Z. Experimental and modeling study on auto-ignition characteristics of methane/hydrogen blends under engine relevant pressure. Int. J. Hydrogen Energy 2012, 37 (24), 19168-19176. (18) Huang, J.; Bushe, W. K.; Hill, P. G.; Munshi, S. R. Experimental and kinetic study of shock initiated ignition in homogeneous methane–hydrogen–air mixtures at engine-relevant conditions. Int. J. Chem. Kinet 2006, 38 (4), 221-233. (19) Petersen, E. L.; Kalitan, D. M.; Simmons, S.; Bourque, G.; Curran, H. J.; Simmie, J. M. Methane/propane oxidation at high pressures: Experimental and detailed chemical kinetic modeling. Proc. Combust. Inst 2007, 31 (1), 447-454. (20) Zhang, Y.; Huang, Z.; Wei, L.; Zhang, J.; Law, C. K. Experimental and modeling study on ignition delays of lean mixtures of methane, hydrogen, oxygen, and argon at elevated pressures. Combust. Flame 2012, 159 (3), 918-931. (21) Hughes, K. J.; Turányi, T.; Clague, A. R.; Pilling, M. J. Development and testing of a comprehensive chemical mechanism for the oxidation of methane. Int. J. Chem. Kinet 2001, 33 (9), 513-538. (22) Hu, E.; Li, X.; Meng, X.; Chen, Y.; Cheng, Y.; Xie, Y.; Huang, Z. Laminar flame speeds and ignition delay times of methane–air mixtures at elevated temperatures and pressures. Fuel 2015, 158, 1-10. (23) Cai, L.; Kruse, S.; Felsmann, D.; Thies, C.; Yalamanchi, K. K.; Pitsch, H. Experimental Design for Discrimination of Chemical Kinetic Models for 28


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Experimental and numerical study of the effect of CO2 on the ignition

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