Maxence Lailliau

Guillaume Dayma

Philippe Dagaut

PhD Zeynep Serinyel
email: zeynep.serinyel@cnrs-orleans.fr

A high pressure oxidation study of di-n-propyl ether

Introduction

Given the strict emission regulations for automotive sector and environmental concerns, there has recently been a growing need to find alternative feedstocks for the next generation biofuels. These include di-n-butyl ether (DBE, C4H9-O-C4H9), diethyl ether (DEE, C2H5-O-C2H5), and dimethyl ether (DME, CH3-O-CH3) among many other families of oxygenated molecules. DBE can be produced from lignocellulosic source, while DEE can be produced from bioethanol by dehydration. Very recently, DBE and DEE received a lot of interest. They were studied in different laboratory set-ups, in terms of oxidation, pyrolysis, ignition delays, laminar flame speeds, and laminar flame structure [START_REF] Yasunaga | A Multiple Shock Tube and Chemical Kinetic Modeling Study of Diethyl Ether Pyrolysis and Oxidation[END_REF][START_REF] Gillespie | Measurements of flat-flame velocities of diethyl ether in air[END_REF][START_REF] Werler | Ignition delay times of diethyl ether measured in a high-pressure shock tube and a rapid compression machine[END_REF][START_REF] Vin | Diethyl ether pyrolysis study in a jet-stirred reactor[END_REF][START_REF] Tran | Experimental and kinetic modeling study of diethyl ether flames[END_REF][START_REF] Serinyel | An experimental chemical kinetic study of the oxidation of diethyl ether in a jet-stirred reactor and comprehensive modeling[END_REF][START_REF] Uygun | Ignition studies of undiluted diethyl ether in a high-pressure shock tube[END_REF][START_REF] Tran | Low-temperature gas-phase oxidation of diethyl ether: Fuel reactivity and fuel-specific products[END_REF].

Our team has recently studied the oxidation of DBE and DEE in jet-stirred reactor [START_REF] Serinyel | An experimental chemical kinetic study of the oxidation of diethyl ether in a jet-stirred reactor and comprehensive modeling[END_REF][START_REF] Thion | A chemical kinetic study of the oxidation of dibutyl-ether in a jet-stirred reactor[END_REF]. An unusual oxidation behavior showing double-NTC region was observed with DBE, which was not the case with DEE under the same conditions, the latter having shown conventional low-temperature reactivity. Di-n-propyl ether (DPE, C3H7-O-C3H7), on the other hand, is not considered a potential biofuel and has not been studied in combustion. However, in terms of carbon number, this symmetric ether is between DEE and DBE. It is therefore of fundamental interest to study its oxidation behavior. The structures of these ethers are shown in figure 1. Oxidation of DPE, is therefore studied in the same experimental conditions as DEE and DBE, 10 atm, between 470 to 1160 K. A kinetic sub-model is developed for this fuel and compared to the present data only, given that this study is the first investigating di-n-propyl ether oxidation.

Experimental approach

Experiments were carried out in a fused silica jet-stirred reactor settled inside a stainless-steel pressure resistant jacket. An electrical oven enabled to perform experiments up to c.a. 1280K. The temperature within the reactor was continuously monitored by a Pt/Pt-Rh thermocouple located inside a thin wall fused silica tube to prevent catalytic reactions on the metallic wires. Initial fuel mole fraction was 1000 ppm for all experiments, pressure and residence time () were held constant 10 atm and 0.7s. The reactive mixtures were highly diluted by nitrogen to avoid high heat release inside the reactor and experiments were performed at temperatures ranging from 450 to 1160 K similar to our previous studies [START_REF] Serinyel | An experimental chemical kinetic study of the oxidation of diethyl ether in a jet-stirred reactor and comprehensive modeling[END_REF][START_REF] Thion | A chemical kinetic study of the oxidation of dibutyl-ether in a jet-stirred reactor[END_REF]. The liquid fuel was atomized by a nitrogen flow and vaporized through a heated chamber. Reactants were brought separately to the reactor to avoid premature reactions and then injected by 4 injectors providing stirring. Flow rates of the diluent and reactants were controlled by mass flowmeters. A low-pressure sonic probe was used to freeze the reactions and take samples of the reacting mixtures.

Online analyses were performed after sending the samples via a heated line to a Fourier transform infrared (FTIR) spectrometer for the quantification of H2O, CO, CO2, and CH2O. Samples were also stored at ca. 40 mbar in Pyrex bulbs for further analyses using gas chromatography (GC). Two gas chromatographs with a flame ionization detector (FID) were used: one equipped with a DB624 column to quantify oxygenated compounds and the other one with a CP-Al2O3/KCl column to quantify hydrocarbons. Identification of the products was done by GC/MS on a Shimadzu GC2010 Plus, with electron impact (70 eV) as the ionization mode. Hydrogen profiles were measured using a GC-TCD (thermal conductivity detector) equipped with a CP-CarboPLOT P7 column. The species quantified in this study include di-n-propyl-ether (DPE), H2, H2O, CO, CO2, C2H4, CH4, C2H6, C3H6, formaldehyde, propanal, acetaldehyde, and propanoic acid. Some other minor oxygenated species were also identified, such as 2-ethyl-4-methyl-1,3-dioxolane, 2-(propoxymethyl)oxirane, and traces of propyl formate and ethyl formate. The cyclic ethers cited are proper to DPE low-temperature chemistry, and they are formed in trace amounts. A quantification is therefore not done, also including the fact that these species were not directly calibrated and that we did not prefer to use an effective carbon method for molar fractions of few ppm. The carbon balance was checked for each sample and was found to be typically within ±10-15%.

Kinetic modeling

As the study of DPE is in line with our previous studies on diethyl ether (DEE) and di-n-butyl ether (DBE), a sub-mechanism was developed in a similar way to those of DEE and DBE and integrated into the mechanism as provided in [START_REF] Serinyel | An experimental chemical kinetic study of the oxidation of diethyl ether in a jet-stirred reactor and comprehensive modeling[END_REF]. In the present DPE sub-mechanism, rate constants of the main reactions were adopted from the literature, as follows:

-Reactions of hydrogen abstraction from fuel by OH radicals are taken from Zhou et al. [START_REF] Zhou | An ab initio/Rice-Ramsperger-Kassel-Marcus study of the hydrogen-abstraction reactions of methyl ethers, H3COCH3-x(CH3)x, x = 0-2, by [radical dot]OH; mechanism and kinetics[END_REF] for the alpha C-H site. For the beta C-H site, the rate constant is evaluated by fitting to the theoretical calculations performed by [START_REF] Zhou | Rate constants for hydrogen-abstraction by OH from n-butanol[END_REF][START_REF] Zádor | Hydrogen Abstraction From N-, I-Propanol And Nbutanol: A Systematic Theoretical Approach[END_REF][START_REF] Seal | Kinetics of the Hydrogen Atom Abstraction Reactions from 1-Butanol by Hydroxyl Radical: Theory Matches Experiment and More[END_REF] for the beta C-H in n-butanol. For the gamma C-H site, the rate constant is also assigned by fitting to the calculations of [START_REF] Zhou | Rate constants for hydrogen-abstraction by OH from n-butanol[END_REF][START_REF] Zádor | Hydrogen Abstraction From N-, I-Propanol And Nbutanol: A Systematic Theoretical Approach[END_REF][START_REF] Seal | Kinetics of the Hydrogen Atom Abstraction Reactions from 1-Butanol by Hydroxyl Radical: Theory Matches Experiment and More[END_REF] and the measurements of Droege and Tully [START_REF] Droege | Hydrogen atom abstraction from alkanes by hydroxyl. 5. n-Butane[END_REF] for the delta C-H bond in n-butanol, as this one is the further away from the alcoholic carbon.

-Reactions of hydrogen abstraction from fuel by H atoms are taken from the theoretical study of Ogura et al. [START_REF] Ogura | Rate coefficients of H-atom abstraction from ethers and isomerization of alkoxyalkylperoxy radicals[END_REF] for the alpha site. For the beta and gamma C-H bonds, rate constants are adopted from Tsang [START_REF] Tsang | Chemical Kinetic Data Base for Combustion Chemistry. Part 3: Propane[END_REF].

-H-abstraction rate constants by HO2 and CH3 radicals are adopted from the theoretical studies of Mendes et al. [START_REF] Mendes | Rate Constant Calculations of H-Atom Abstraction Reactions from Ethers by HȮ2 Radicals[END_REF] and Xu et al. [START_REF] Xu | Thermal decomposition of ethanol. III. A computational study of the kinetics and mechanism for the CH3+C2H5OH reaction[END_REF], respectively.

-Rate constants for R+O2 ⇌ RO2 reactions are adopted from Goldsmith et al. [START_REF] Goldsmith | Role of O2 + QOOH in Low-Temperature Ignition of Propane. 1. Temperature and Pressure Dependent Rate Coefficients[END_REF], both for 1 st and 2 nd addition.

-Rate constants for RO2 ⇌ QOOH, QOOH ⇌ cyclic ether + OH are adopted from Villano et al. [START_REF] Villano | High-Pressure Rate Rules for Alkyl + O2 Reactions. 1. The Dissociation, Concerted Elimination, and Isomerization Channels of the Alkyl Peroxy Radical[END_REF][START_REF] Villano | High-Pressure Rate Rules for Alkyl + O2 Reactions. 2. The Isomerization, Cyclic Ether Formation, and β-Scission Reactions of Hydroperoxy Alkyl Radicals[END_REF].

-Beta-scission reactions of fuel radicals and those of QOOH radicals are adopted from the theoretical calculations of Villano et al. [START_REF] Villano | High-Pressure Rate Rules for Alkyl + O2 Reactions. 2. The Isomerization, Cyclic Ether Formation, and β-Scission Reactions of Hydroperoxy Alkyl Radicals[END_REF], Sakai et al. [START_REF] Sakai | A computational study on the kinetics of unimolecular reactions of ethoxyethylperoxy radicals employing CTST and VTST[END_REF], and from our previous calculations on DBE [START_REF] Thion | A chemical kinetic study of the oxidation of dibutyl-ether in a jet-stirred reactor[END_REF].

-Other reactions related to low-temperature chemistry are taken analogous to our previous DBE model [START_REF] Thion | A chemical kinetic study of the oxidation of dibutyl-ether in a jet-stirred reactor[END_REF].

-Unimolecular decomposition reactions of DPE were taken from the study of Yasunaga et al. [START_REF] Yasunaga | A Multiple Shock Tube and Chemical Kinetic Modeling Study of Diethyl Ether Pyrolysis and Oxidation[END_REF] in analogy with DEE. These reactions have no importance under present experimental conditions.

Thermochemistry of the fuel, fuel radical as well as all related low-temperature species were calculated using Thergas [START_REF] Muller | Thergas -A computer program for the evaluation of thermochemical data of molecules and free radicals in the gas phase[END_REF] which uses group additivity methods as proposed by Benson [START_REF] Benson | Thermochemical Kinetics[END_REF]. The JSR simulations were carried out with the Perfectly Stirred Reactor (PSR) code of Chemkin II package [START_REF] Glarborg | PSR: A Fortran Program for Modeling Well-Stirred Reactors[END_REF]. Results are shown in the following figures.

Results and discussion

Fuel conversion and fuel mole fractions for all experiments are presented in Figure 1. DPE shows a strongly pronounced low-temperature reactivity, followed by an NTC region (Figure 1a). For example, the NTC region for the  = 1 and 2 mixtures, begin around 530 K and arrive to a plateau around 600 K. Then within a temperature window of ~ 60 K, no further fuel conversion is observed, and past 660 K reactivity decreases again. This behavior resembles what was previously observed in DBE oxidation as double-NTC [START_REF] Thion | A chemical kinetic study of the oxidation of dibutyl-ether in a jet-stirred reactor[END_REF], better demonstrated in Figure 1b. For the  = 0.5 mixture, it would not be adequate to reach this conclusion as the single experimental point (600 K) that could prove this behavior is within experimental uncertainty. On the other hand, the  = 4 mixture does not show this behavior, also the temperature zone between the end of NTC and start of high-temperature reactivity is wider for this mixture, and approaches zero between 660-780 K.

The kinetic model can predict the NTC behavior except for the "2 nd NTC" region, although a small difference in the slope can be observed for the lean mixture. These kinetic uncertainties are due to the rate constants adopted in the low-temperature oxidation mechanism. Often, these rate constants are more adequate for alkane oxidation and analogies have to be made in developing mechanisms for oxygenated compounds. The effect of some of these rate constants will be illustrated in the coming sections.

In the following figures [START_REF] Gillespie | Measurements of flat-flame velocities of diethyl ether in air[END_REF][START_REF] Werler | Ignition delay times of diethyl ether measured in a high-pressure shock tube and a rapid compression machine[END_REF][START_REF] Vin | Diethyl ether pyrolysis study in a jet-stirred reactor[END_REF][START_REF] Tran | Experimental and kinetic modeling study of diethyl ether flames[END_REF], experimental results are presented along with simulations, a representative 15% uncertainty bar is added to the fuel profile. General tendency is well predicted by the model, some discrepancies exist due to the chosen rate constants in low-temperature oxidation sub-mechanism. From Figure 1, one can speculate that the model slightly under-predicts the experimental reactivity of the  = 0.5 and 1 mixtures in the NTC region. 0.0E+00 The most important low-temperature products observed were formaldehyde, propanal, acetaldehyde and propanoic acid. Formaldehyde is a typical marker of low-temperature reactivity of many fuels and is formed in large amounts in DPE oxidation as well. The typical oxygenated intermediate in the case of DPE is propanal. Similarly, butanal and acetaldehyde were observed in large quantities in DBE and DEE oxidation, respectively.

An interesting feature of ether oxidation turns out to be the formation of carboxylic acids. As an example, formic acid was identified and quantified in earlier studies by Curran et al. [START_REF] Curran | The reaction kinetics of dimethyl ether. II: Lowtemperature oxidation in flow reactors[END_REF] in a flow reactor and by Dagaut et al. [START_REF] Dagaut | The Low Temperature Oxidation of DME and Mutual Sensitization of the Oxidation of DME and Nitric Oxide: Experimental and Detailed Kinetic Modeling[END_REF] in a jet-stirred reactor and more recently by Moshammer et al. [START_REF] Moshammer | Detection and Identification of the Keto-Hydroperoxide (HOOCH2OCHO) and Other Intermediates during Low-Temperature Oxidation of Dimethyl Ether[END_REF] in a jet-stirred reactor and by Wang et al. [START_REF] Wang | Experimental and kinetic modeling study of the low-and intermediate-temperature oxidation of dimethyl ether[END_REF] in a flow reactor study. In contrast to these studies, in their jet-stirred reactor study on low-temperature DME oxidation by Rodriguez et al. [START_REF] Rodriguez | Experimental and Modeling Investigation of the Low-Temperature Oxidation of Dimethyl Ether[END_REF], no formation of formic acid was reported. In our DEE study, acetic acid was quantified at low temperatures in considerable quantities and in DBE oxidation butanoic acid was identified for some experiments but not quantified. The formation routes of these acids are unclear and probably due to an unconventional pathway followed by the radicals formed by ketohydroperoxide decomposition. Formation of propanoic acid cannot be explained by analogy to the formic acid formation pathway first proposed by Curran [START_REF] Curran | The reaction kinetics of dimethyl ether. II: Lowtemperature oxidation in flow reactors[END_REF] and later calculated by Wang and coworkers [START_REF] Wang | Experimental and kinetic modeling study of the low-and intermediate-temperature oxidation of dimethyl ether[END_REF]. This formic acid pathway involves an internal hydrogen transfer from the acyl site of the • OCH2OCHO radical (formed by C-O scission of the aldohydroperoxide) followed by its -scission to formic acid and CO. However, in DPE oxidation the most abundant carbonylhydroperoxide is a ketohydroperoxide (shown in Fig 7 as c3oc31ohhket1) hence such a pathway is not possible. Therefore, although propanoic acid is quantified with a 125 ppm peak for  = 0.5 mixture and 16 ppm peak for  = 4 mixture, its profile is not simulated. Generally speaking, for all conditions, kinetic model agrees reasonably well with the data. A factor of 1.5-2 discrepancy is observed with ethylene peak value and CO is slightly over-predicted in rich mixtures. C-H bond dissociation energies for DPE are calculated on G3B3 and CBS-QB3 levels using Gaussian09 [START_REF] Frisch | Revision D.01[END_REF] and are presented below. According to this, the alpha C-H bond is the weakest and therefore the dpe1 radical is favored, which was naturally the case with DBE and DEE. Also, note that the gamma C-H bond energy is similar to a primary C-H bond in alkane, confirming the choice of analogy. Main reaction pathways are presented in Figure 7 for the low-temperature oxidation of DPE, which is mainly consumed by H-abstraction reactions by OH radicals and this is the case at any temperature. The percentages are evaluated at 500 K for the  = 1 mixture, as an example. At low temperatures, addition of fuel radicals to molecular oxygen is dominant forming the RO2 radicals. The RO2 radicals can go through isomerization (internal hydrogen transfer) to form QOOH radicals. At these conditions the QOOH produced via 6-membered transition states are favored and lead to the species and pathways as presented below. Propanal formation at low temperature is via ketohydroperoxide decomposition, which is probably the most important in low-temperature chemistry. In fact, this reaction is often written in one step with chain branching fragments and an OH radical as products. The rate constant used for this reaction in various mechanisms comes from an experimental study by Sahetchian et al. [START_REF] Sahetchian | The pyrolysis of organic hydroperoxides (ROOH)[END_REF] focusing on the decomposition of organic hydroperoxides, and it can be approximately written as k = 1x10 16 exp (-43 kcal/RT). A slightly different pre-exponential factor or activation energy can be found in various literature mechanisms.

O

The Figure 8 shows the effect of this rate constant on the predictions of fuel mole fraction. Activation energy of the KHP decomposition was modified by up to 6 kcal/mol. This very low activation energy was proposed in the kinetic model of Cai et al. [START_REF] Cai | Chemical kinetic study of a novel lignocellulosic biofuel: Di-n-butyl ether oxidation in a laminar flow reactor and flames[END_REF] on DBE oxidation for all KHPs possible. The considerable effect of this rate constant on mole fraction profiles can be clearly observed. We have also included the theoretical study of Goldsmith et al. [START_REF] Goldsmith | Effect of non-thermal product energy distributions on ketohydroperoxide decomposition kinetics[END_REF] on the decomposition of HOOCH2CH2CHO, via various channels. This rate constant appears to be very close to the one obtained by reducing the original activation energy by 3 kcal, i.e. when using 1x10 16 exp (-40 kcal/RT) as the rate constant, which is therefore adopted here. Furthermore, in order to highlight the effect of R + O2 ⇌ RO2 rate constant, we have compared predictions with two theoretically calculated rate constants by Goldsmith [START_REF] Goldsmith | Role of O2 + QOOH in Low-Temperature Ignition of Propane. 1. Temperature and Pressure Dependent Rate Coefficients[END_REF] and Miyoshi [START_REF] Miyoshi | Systematic Computational Study on the Unimolecular Reactions of Alkylperoxy (RO2), Hydroperoxyalkyl (QOOH), and Hydroperoxyalkylperoxy (O2QOOH) Radicals[END_REF], both are widely used in kinetic mechanisms. Actually, this reversible reaction is very sensitive to thermochemistry and note that in this study, group additivity is used. Also, as is the case with the rate constants adopted from Villano et al. [START_REF] Villano | High-Pressure Rate Rules for Alkyl + O2 Reactions. 1. The Dissociation, Concerted Elimination, and Isomerization Channels of the Alkyl Peroxy Radical[END_REF][START_REF] Villano | High-Pressure Rate Rules for Alkyl + O2 Reactions. 2. The Isomerization, Cyclic Ether Formation, and β-Scission Reactions of Hydroperoxy Alkyl Radicals[END_REF] for this reaction class, very often these calculations are done for alkanes. The similarities assumed are therefore done compared to alkanes although the fuel is an ether. This probably adds an additional uncertainty to the predictions. Figure 9 illustrates the experimental fuel conversion of DEE, DPE and DBE, in order to compare their relative reactivity. On this plot one can clearly see the differences and similarities. All three ethers are very reactive, conversion begins right above 450 K, and they all exhibit a strong low-temperature reactivity and NTC behavior. On the other hand, DBE shows a clear double-NTC behavior, which is less pronounced in DPE oxidation and appears to be absent in the case of DEE. Among the three, DBE stands out with its very high conversion even in both NTC regions. For example, including the NTC region, DBE conversion is always higher than 60% for  = 0.5 mixture. The particularity in DBE oxidation stems from the fact that even at these low temperatures, owing to its long chain, formation of small radicals is possible. Its oxidation can produce radicals such as n-propyl, mostly from the ketohydroperoxide C3H7C(=O)OCH(OOH)C3H7 decomposition. Decomposition of this ketohydroperoxide also produces butanal, which gives n-propyl radicals by alpha-scission of its acyl radical at the low-intermediate temperature range, and n-butyl radicals at intermediate temperatures. Therefore the low-intermediate temperature regime in DBE oxidation is also controlled by the pathways followed by smaller radicals that have their own low-temperature reactivity. On the other hand in DEE oxidation, the chain is two carbon atoms smaller and these interactions do not take place. DPE, however, is in midway between DEE and DBE and produces ethyl radicals in abundance during its oxidation. Ethyl radicals add to molecular oxygen at lower temperatures and follow rather dismutation pathways (producing ethanol, for example). They do not trigger an NTC type of chemistry of their own like in DBE however they are more reactive than the methyl radicals in DEE oxidation. Note that these observations hold for the investigated conditions of 1000 ppm initial fuel mole fraction, 10 atm and for a residence time of 700 ms and that at other conditions reactivity profiles may be different. More details on DBE and DEE oxidation can be found in the corresponding papers [START_REF] Serinyel | An experimental chemical kinetic study of the oxidation of diethyl ether in a jet-stirred reactor and comprehensive modeling[END_REF][START_REF] Thion | A chemical kinetic study of the oxidation of dibutyl-ether in a jet-stirred reactor[END_REF]. At these temperatures thermochemistry is of crucial importance, hence the thermochemical values (especially RO2, QOOH, OOQOOH) have to be as accurate as possible in addition to the kinetics. Group additivity method may have its limits with such complex radicals. More kinetic data is certainly useful for our understanding of the detailed chemistry, however we should not neglect the effect of the uncertainties in thermochemistry, which are high with such large and complex radicals.

Summary

In line with our previous studies on the oxidation of ethers, in the present work high-pressure oxidation of di-n-propyl ether was studied in a jet-stirred reactor at various equivalence ratios ( = 0.5-4), for the first time. DPE exhibited an important low-temperature reactivity and a double-NTC behavior, although to a lesser extent that that of DBE. A kinetic model was developed in order to understand the oxidation patterns based on our previous efforts and literature. This model shows a good agreement in general, however some discrepancies arise from uncertainties in the rate parameters used. In order to extend this study to atmospheric pressure, more experiments will be performed. Also, theoretical calculations could be useful in interpreting the low-temperature oxidation behavior of the ether-related species.

Figure 1 .

 1 Figure 1. Structures of diethyl, di-n-propyl and di-n-butyl ethers

Figure 1 .

 1 Figure 1. DPE mole fraction profiles (a) and conversion (b) for all mixtures

Figure 2 .

 2 Figure 2. Mole fraction profiles as a function of reactor temperature ( = 0.5)

Figure 3 .

 3 Figure 3. Mole fraction profiles as a function of reactor temperature ( = 1)

Figure 4 .

 4 Figure 4. Mole fraction profiles as a function of reactor temperature ( = 2)

Figure 5 .

 5 Figure 5. Mole fraction profiles as a function of reactor temperature ( = 4)

Figure 6 .

 6 Figure 6. C-H bond dissociation energies of DPE calculated with G3B3 (bold) and CBS-QB3 (italic)

Figure 7 .

 7 Figure 7. Main reaction pathways at low temperatures (% are given at 500 K,  = 1)

DPE,  = 1 Figure 8 .Figure 9 .

 189 Figure 8. Effect of KHP decomposition (left) and R + O2 ⇌ RO2 (right) reactions on model predictions. For the figure on the left : (a) assuming Ea = 43 kcal/mol for all KHP, (b)assuming Ea = 37 kcal/mol for all KHP, (c) assuming Ea = 40 kcal/mol for all KHP, (d) assuming Ea = 37 kcal/mol except for c3oc31oohket1, (e) assuming Ea = 40 kcal/mol except for c3oc31oohket1, (f) using the rate constant calculated by Goldsmith et al.[START_REF] Goldsmith | Effect of non-thermal product energy distributions on ketohydroperoxide decomposition kinetics[END_REF] for the reaction HOOCH2CH2CHO → • OCH2CH2CHO + • OH.

Acknowledgements

Authors gratefully acknowledge funding received from Labex Caprysses (convention ANR-11-LABX-0006-01).


G Dayma

S Thion

Z Serinyel

P Dagaut

Experimental and kinetic modelling study of the oxidation of cyclopentane and methylcyclopentane at atmospheric pressure

Cyclopentane and methylcyclopentane oxidation was investigated in a jet-stirred reactor at atmospheric pressure, over temperatures ranging from 900 K to 1250 K, for fuel-lean, stoichiometric, and fuel-rich mixtures at a constant residence time of 70 ms. The initial mole fraction of both fuels was kept constant at 1000 ppm. The reactants were highly diluted by a flow of nitrogen to ensure thermal homogeneity. Samples of the reacting mixture were analyzed on-line and off-line by Fourier transform infrared spectroscopy and gas chromatography. A detailed kinetic mechanism consisting of 590 species involved in 3469 reactions was developed and simulation results were compared to these new experimental data and previously reported ignition delays. Reaction pathways analysis as well as sensitivity analyses were performed to get insights into the differences observed during the oxidation process of cyclopentane and methylcyclopentane.

Introduction

As compared to linear alkanes, the oxidation of cyclo-alkanes and substituted cycloalkanes, such as cyclo-pentane and methylcyclopentane, has received very little attention. However, with the growing interest on biomass-derived fuels, and fuels derived from oil sands and shale, the understanding of the combustion chemistry of these cyclic hydrocarbons becomes of major scientific and practical importance. Among cycloalkanes, cyclohexane is the most studied one. In their work on the low-and high-temperature oxidation of cyclohexane, Serinyel et al. 1 reported more than 20 previous studies on this compound. They also validated a detailed kinetic mechanism for the oxidation of cyclohexane under jet-stirred reactor conditions as well as laminar burning velocities and ignition delays in shock-tube and rapid compression machine. Furthermore, the combustion community has also shown interest to alkylated cyclohexanes, from methyl to propyl, under various experimental conditions. Despite recent attention, studies on cyclopentane or methylcyclopentane oxidation are still scarse.

Tsang 2 investigated the thermal decomposition of cyclopentane using a single-pulse shock tube. Several distinct pathways were considered and the isomerization to 1-pentene was found to be a major route of consumption under these conditions, while the formation of cyclopropane was also detected. High-temperature autoignition of cyclopentane was studied by Sirjean et al. [START_REF] Sirjean | Proceedings of the Combustion Institute[END_REF] in a shock tube. A detailed chemical kinetic mechanism was developed and used to explain the longer ignition delays observed as compared to cyclohexane. The ring opening of cyclopentyl radicals was demonstrated to be responsible for this lower reactivity. Cyclopentane ignition was also investigated at elevated pressures by Daley et al. [START_REF] Daley | [END_REF] and compared to that of cyclohexane. Mechanisms from the literature were tested and, although the general trends were captured, they all predicted ignition times generally longer than those measured. Recently, Al-Rashidi et al. [5][6][7] elucidated the different reactivity regimes in cyclopentane oxidation by means of jet-stirred reactor experiments, computational chemistry, and kinetic modelling. Their simulations reproduced the unique reactivity trend of cyclopentane at high-pressure and they showed that this peculiar behavior may be attributed to the C-C/C-H scission branching ratio of the cyclopentyl radical.

The oxidation of methylcyclopentane was first studied by Burgoyne and Silk 8 in a quartz bulb. They determined the pressure-temperature limits for spontaneous cool-and hot-flame ignitions of methylcyclopentane-oxygen mixtures. Brown and King 9 investigated the very-lowpressure pyrolysis of methylcyclopentane. The data were interpreted in terms of ring-opening bond fission pathways and bond fission to methyl and cyclo-alkyl radicals. They proposed highpressure rate expressions for the overall decomposition of methylcyclopentane and showed their data supported the assumption of a biradical mechanism for ring opening. More recently, Sivaramakrishnan and Michael 10 measured the high-temperature rate constants of methylcyclopentane H-abstractions by OH radicals in a shock tube and showed that these reactions are favored on ortho secondary carbons (carbon s1 in Fig. 1). Sirjean et al. 11 theoretically studied the thermal unimolecular decomposition of methylcyclopentyl radicals. For ring-opening reactions, they showed an increase of the activation energy when the double bond is formed in the ring (endo) in contrast to the cases in which the double bond is formed on the side chain (exo). Boehman and coworkers [12][13][14] focused on the low-temperature oxidation of cycloalkanes in engines. They observed methylcyclopentane showed little low-temperature reactivity prior to autoignition as compared to methylcyclohexane or decalin. Moreover, methylcyclopentenes were found to play an important role under their conditions, and methyl substitution on the ring was found to significantly promote the formation of propene relative to ethylene. Tian et al. 15 measured ignition delay times of cyclopentane/O2 and methylcyclopentane/O2 mixtures diluted in argon and developed a detailed kinetic mechanism validated against their experimental results. They observed methylcyclopentane had shorter ignition delays than cyclopentane and attributed this to the presence of the methyl group and the propensity of methylcyclopentane to produce H atoms. Finally, Fridlyand et al. 16 explored the low-temperature reactivity of cyclopentane, methylcyclopentane, and ethylcyclopentane under stoichiometric conditions at 20 and 50 bar in a rapid-compression machine. They observed ethylcyclopentane was the most reactive whereas cyclopentane exhibited a limited reactivity under these conditions, methylcyclopentane being in between.

In addition to the results available in the literature, the purpose of this work is to obtain new experimental data on cyclopentane and methylcyclopentane oxidation in order to further validate a detailed kinetic mechanism. Once validated, this mechanism will be used to delineate the different reactivity of cyclopentane and methylcyclopentane.

Experimental Section

The JSR experimental setup has been described in previous studies [START_REF] Cong | Journal of Engineering for Gas Turbines and Power-Transactions of the Asme[END_REF][START_REF] Dagaut | [END_REF] . Briefly, it consists of a 4 cm diameter fused silica sphere with four 1 mm i.d. nozzles. High purity nitrogen (< 100 ppm H2O, < 50 ppm O2, < 1000 ppm Ar, < 5 ppm H2, Air Liquide) was used as a diluent and mixed with the reactants before reaching the mixing point at the entrance of the injectors. High dilution (0.1 % mol. of fuel) was used to reduce temperature gradients and heat release inside the reactor. High-purity reactants were used: oxygen 99.995 % pure was provided by Air Liquide, while cyclopentane (CAS 287-92-3) > 98 % pure and methylcyclopentane (CAS 96-37-7) > 97 % pure were both obtained from Aldrich. 1.2 % of n-pentane was measured within cyclopentane and 0.7 % of n-hexane within methylcyclopentane. These impurities were taken into account for the simulations. To reduce temperature gradients inside the reactor, the reactants were preheated before injection. The fuel was delivered to a vaporizer assembly maintained at 473 K by means of a HPLC pump and an online degassing unit (Shimadzu LC10 AD VP and DGU-20 A3). Small temperature gradients (~ 1 K/cm) were measured along the vertical axis of the reactor using a thermocouple located inside a thin-wall fused-silica tube (0.1 mm Pt-Pt/Rh-10 %) to prevent catalytic reactions on the metallic wires. A fused-silica lowpressure sonic probe was used to sample the reacting mixtures and send samples to the analyzers via a Teflon heated line (393 K). Gas samples were analyzed online by FTIR. A resolution of 0.5 cm -1 was used. The sample cell had a 10 m optical path length and samples were analyzed at 200 mbar. Off-line GC analyses following collection and storage at ~ 50 mbar in 1L Pyrex bulbs were also performed. Gas chromatographs operating with capillary columns (a 0.32 mm i.d. DB-624, a 0.32 mm i.d. CP-Al2O3-KCl, a 0.53 mm i.d. CP-Carboplot-P7, and a 0.32 mm i.d. CP-SIL 5CB coupled to a 0.53 mm i.d. DB1-ms), a TCD (thermal conductivity detector), and a FID (flame ionization detector). Two GC-MS (Varian CP3800-V1200 and Shimadzu GC-MS 2010 Plus) operating with electron impact ionization (70 eV) were used for products identification. Thanks to this analytical equipment, it was possible to measure several species with different techniques or columns, giving high confidence into our measurements. The measured species were: CO, CO2, water, H2, O2, methane, formaldehyde, ethane, ethylene, acetylene, propene, allene, propyne, acroleine, 1-butene, 1,3-butadiene, vinylacetylene, 1pentene, 1-hexene, benzene, cyclopentene, cyclopentadiene, methylcyclopentene, methylcyclopentadiene, 1,3-cyclohexadiene, cyclopentane and methylcyclopentane.

All the experiments were performed at a mean residence time, τ, of 70 ms and a pressure of 1 atm. The reactants were constantly flowing into the JSR, while the reactor temperature was increased stepwise from 900 to 1250 K. A good repeatability of the measurements and a reasonably good carbon balance (typically 100 ± 15%) were obtained.

Kinetic Modeling

The kinetic mechanism developed by Tian et al. 15 was tested against our new experimental results and was found to be too reactive (see Figure S1 in the Supplementary Material). Therefore, a detailed kinetic reaction mechanism was developed based on a previously published mechanism for the oxidation of a variety of fuels up to C6 19,20 . It was extended by including the chemistry of cyclopentane and methylcyclopentane oxidation. Cyclopentene, cyclopentadiene, hex-1-ene and hex-2-ene sub-mechanisms were also revisited to improve our predictions. For the new species, the thermochemical properties were computed using group and bond additivity methods 21 . The kinetic modelling of cyclopentane and methylcyclopentane oxidation in jet-stirred reactor was performed using the PSR computer code 22 while ignition delays were computed thanks to the SENKIN code 23 , both from the CHEMKIN-II package. The detailed kinetic reaction mechanism consisting of 590 species involved in 3469 reactions and the thermochemical properties are provided as Supplementary Material. Figure 1 shows the molecular structure of cyclopentane and methylcyclopentane with carbon atoms labeled as in the mechanism. The chemical structure, the name used in the mechanism as well as the heat of formation and entropy at 298 K of the most important species in C5 cyclic species submechanism are gathered in Table 1. The fuel (either cyclopentane or methylcyclopentane) can react via thermal decomposition involving C-C, and C-H bond breaking. Previous it was shown that ring opening for cycloalkanes proceeds through the formation of biradical intermediates [24][25][26] . In the case of methylcyclopentane, homolytic C-C bond scissions yield 1-hexene, 2-hexene, or three different methylpentene isomers, whereas for cyclopentane either 1-pentene or propene and ethylene can be produced.

Table 1: Chemical structures, names used in the kinetic mechanism, and heat of formation (kcal/mol) and entropy (cal/K/mol) at 298 K of the most important species in cyclopentane and methylcyclopentane sub-mechanisms.

In the literature 2,24 , this last pathway produces cyclopropane and ethylene, but taking into account that cyclopropane was not considered in our mechanism and that, at 1000 K, this pathway is more than 100 times slower than that yielding 1-pentene, propene replaced cyclopropane. Rate constants for both of these pathways were taken from 2 . For methylcyclopentane, hexenes or methylpentenes can be produced through five distinct pathways in addition to cyclopentyl and methyl radicals. The rate constants of the formation of alkenes was adapted from Tsang 2 while that of the formation of cyclopentyl and methyl radicals was taken from Zhang et al. 26 from methylcyclohexane decomposition. The rate constant of C-H bond breaking leading to cyclopentyl was adopted from Al-Rashidi et al. 5 while the formation of the four primary radicals of methylcyclopentane was taken from Tian et al. 15 . Cyclopentane may react through bimolecular initiation with O2, the rate constant of which was taken from Al-Rashidi et al. 5 . Methylcyclopentane may also react through bimolecular initiations with O2, yielding the four primary radicals, where rate constants used by Tian et al. 15 were adopted. Propagation reactions proceed via H-abstraction reactions by small radicals (H, O, OH, HO2, CH3).

For cyclopentane, rate constants of H-abstraction reactions were adopted as follows:

-by OH radicals from Sivaramakrishnan and Michael 10 -by H atoms and HO2 radicals from Handford-Styring and Walker 27,28 -by O and CH3 radicals, no direct measurements or calculations were available and rate constants used in Dayma et al. 29 were chosen. For methylcyclopentane, H-abstraction reactions were adopted as follows:

-by H atoms, from Sivaramakrishnan et al. 30 on the methyl group, from Buda et al. 31 for the tertiary site, and Handford-Styring and Walker 28 for the secondary sites -by OH radicals, adopted from Sivaramakrishnan and Michael 10 who measured and calculated these rates over a large temperature range -by HO2 radicals on the four secondary carbons taken equal to that of cyclopentane while in the case of the primary and the tertiary carbons (m and t), rate constants used in our previous studies 32,33 were chosen -by O and CH3 radicals, rate constants were taken from 33 .

Figure 2 depicts a comparison of the rate constants for H-abstraction reactions by H, O, OH, HO2, and CH3 as well as the total rate constant defined as the sum of the previous five. Each of these graphs present the total rate for the small radical considered. Although some slight differences appear between the rate constants, it can be noticed that, despite an additional carbon atom and two hydrogens for methylcyclopentane, the rate constants for the Habstraction reactions are very close to that of cyclopentane, and the sum of all these rate constants for cyclopentane and methylcyclopentane is thus very similar. The decomposition reactions by β-scission of the primary radicals of both fuels were also considered. β-scission reactions involved in the consumption of cyclopentyl radicals were studied by several authors 5,11,34,35 with significantly different results in terms of branching ratios. For most of these authors, two pathways are considered: (i) the C-H β-scission yielding cyclopentene from cyclopentyl (reaction R2) and (ii) the ring opening (C-C β-scission) of cyclopentyl yielding pent-1-en-5-yl (reaction R1). Sun et al. [START_REF] Sun | The Journal of Physical Chemistry A[END_REF] proposed a third pathway, accounting for the prompt dissociation of pent-1-en-5-yl yielding ethylene and allyl radicals (R3). Figure 3 illustrates a comparison of the different branching ratios; in this figure, R2 and R3, calculated at 1 atm, were gathered in order to compare with the C-H and the C-C β-scission routes. As can be seen from this figure, for Tsang 34 and Sirjean et al. 11 , the C-H β-scission pathway involving the formation of cyclopentene dominates over the temperature range of interest while for Manion and Awan 35 and for Sun et al. [START_REF] Sun | The Journal of Physical Chemistry A[END_REF] , the C-C β-scission channel (R2+R3) dominates. Intermediately, Al-Rashidi et al. 5 calculated a crossover temperature (ca. 800 K) above which R1 dominates. The global reactivity of cyclopentane under fuel-rich conditions and high-pressure was found to be highly sensitive to this competition between the ring opening and the C-H β-scission. The pressure-dependent rate constants calculated by Sun et al. [START_REF] Sun | The Journal of Physical Chemistry A[END_REF] allow to capture this behavior remarkably well (see Figure S2 in the Supplementary Material) and were thus preferred in this study. Seven different C-C β-scissions were considered for methylcyclopentyl radicals for which pressure-dependent rate expressions calculated by Tian et al. 15 were preferred to those proposed by Sirjean et al. 11 (k∞). Actually, the branching ratios for the three possible C-C β-scissions of radical MCPs1 are quite different between these two authors as can be seen in Figure 4. Over our temperature range, both of them (rate constants were evaluated at 1 atm for Tian et al. 15 ) predicts that the dominant channel is the formation of cyclopentene, but while for Sirjean et al. 11 this channel represents 86-94 % of the total consumption of MCPs1, for Tian et al. 15 it strongly decreases with the temperature increase from 82 % at low temperature to 36 % at high temperature. Cyclopentene is mostly produced from this radical, and the decrease of the branching ratio as the temperature increases leads to a better prediction of cyclopentene and 1,3-butadiene (produced from the other two channels), but an underprediction of cyclopentadiene, and a slight increase of the global reactivity (see Figure S3 in the Supplementary Material). Indeed, the choice is very difficult to make between these two sets considering our experimental results alone, but ignition delay times are significantly improved when rate constants from Tian et al. 15 are used (see Figure S4 in the Supplementary Material). For C-H β-scissions, rate constants used in our previous works 32,33 were adopted in this study as well. For the oxidation reactions by O2 yielding cyclopentene and methylcyclopentenes or methylcyclopentadienes, the rate constants proposed by Battin-Leclerc 37 were selected. Finally, the methylcyclopentadiene sub-mechanism was mostly taken from Herbinet et al. 38 with the decomposition yielding methyl and cyclopentadienyl radicals coming from Sharma and Green 39 at atmospheric pressure. Fulvene formation was also considered following the work by Jin et al. 40 and Yang et al. 41 . Sub-mechanisms for the oxidation and pyrolysis of cyclopentene and cyclopentadiene were taken from our previous works 20,42 and updated using the works by Zhong and Bozzelli 43,44 , Wang et al. 45 , Oleinikov et al. 46 , Ghildina et al. 47 and Vermeire et al. 48 on 1,5-hexadiene.

Results and Discussion

For the sake of comparison, experiments were performed at a constant mean residence time of 70 ms and constant atmospheric pressure for both fuels. The equivalence ratio, φ, was varied from fuel-lean to fuel-rich conditions (0.5, 1, and 2); the fuel initial mole fraction was 1000 ppm for all experiments. The temperature ranged from 900 to 1250 K. A good repeatability was observed in the experiments and a reasonably good carbon balance of 100 ± 15% was achieved.

Figure 5 presents the evolution of the mole fraction of cyclopentane (CP) and methylcyclopentane (MCP) at three different equivalence ratios (0.5, 1, and 2) together with the simulations obtained from our mechanism. It can be noticed from this figure that the conversion of cyclopentane is more affected by the equivalence ratio than that of methylcyclopentane. Half of the initial cyclopentane is consumed at T = 1150 K, T = 1165 K, and T = 1180 K for the fuel-lean, the stoichiometric and the fuel-rich mixtures respectively, while half of the initial methylcyclopentane is consumed at T = 1135 K, T = 1140 K, and T = 1145 K for the fuel-lean, the stoichiometric and the fuel-rich mixtures respectively. In addition, at a given equivalence ratio, methylcyclopentane conversion occurs at lower temperatures than that of cyclopentane Thus, methylcyclopentane is slightly more reactive than cyclopentane despite a very similar H-abstraction total rate constant (Figure 2), indicating the radical pool is more abundant in the case of methylcyclopentane oxidation than in the case of cyclopentane. Moreover, the simulations performed with our detailed kinetic mechanism are in good agreement with our experimental data for both fuels at the three equivalence ratios.

Figure 5: Experimental (symbols) and computed (lines) mole fraction profiles obtained from the oxidation of cyclopentane (CP) and methylcyclopentane (MCP) in a JSR at p = 1 atm, τ = 0.07s, and  = 0.5 (red),  = 1 (green), and  = 2 (blue).

Figures 678show the results obtained for a fuel-lean, stoichiometric, and fuel-rich mixture of cyclopentane (empty triangles-dashed line) and methylcyclopentane (filled circles-full line). Under these conditions, methylcyclopentane consumption starts at a slightly lower temperature than the conversion of cyclopentane. These experiments were indeed performed starting at 710 K, but no evidence of reactivity below 1000 K was observed for both fuels under these conditions. The observed intermediates are mainly unsaturated compounds (C2H2, C2H4, C3H6, 1,3-C4H6, cyclopentene, cyclopentadiene, and benzene) regardless of the equivalence ratio. Except for water, CO, and CO2, formaldehyde and acrolein were the only oxygenated species identified. It should be noticed that 12 ppm of n-pentane and 7 ppm of n-hexane were identified as impurities in cyclopentane and methylcyclopentane, respectively. This small amount of impurity was taken into account in our simulations but no impact was observed on the overall reactivity or the species mole fractions. Also, less than 2 ppm of pent-1-ene and 1 ppm of hex-1-ene were measured either under fuel-lean, fuel-rich or stoichiometric conditions with cyclopentane and methylcyclopentane, respectively. The kinetic mechanism reproduces reasonably well the experimental data. Fuel consumption is well reproduced under all the conditions investigated. Nevertheless, some discrepancies between the simulations and the experimental data can be observed for vinylacetylene (C4H4), cyclopentene, or benzene, although these discrepancies are different depending on the fuel and the experimental conditions. For instance, vinylacetylene is well 0.0E+00 reproduced for both fuels under stoichiometric and fuel-rich conditions while it appears overestimated under fuel-lean conditions. Furthermore, although benzene is well reproduced for cyclopentane oxidation, it is patently underestimated regardless of the equivalence ratio for methylcyclopentane oxidation: Our mechanism predicts a similar amount of benzene produced by the oxidation of the two fuels whereas, experimentally, benzene accumulates twice as much for methylcyclopentane as for cyclopentane. It has to be mentioned that the mechanism developed by Tian et al. 15 also strongly under estimates benzene accumulation (see Figure S5 in the Supplementary Material) indicating that pathways are probably missing or some rate constants need to be reevaluated. Figure 9a depicts the reaction pathway analysis performed under stoichiometric conditions, at atmospheric pressure and 1150 K (the temperature at which ca. 45 % of the fuel is consumed) of the oxidation of cyclopentane. Under these conditions, the oxidation of cyclopentane is quite simple: 87.3 % of cyclopentane yields cyclopentyl radicals (H: 57.3 %, OH: 18.5 %, O: 6.5 %, CH3: 3.1 %, a-C3H5: 1.9 %) which in turn undergo a C-H β-scission (32.5 %) yielding cyclopentene, or two C-C β-scissions producing either pent-1-en-5-yl (30.7 %) or directly ethylene and allyl (36.4 %). According to our mechanism, 99.8 % of pent-1-en-5-yl gives ethylene and allyl radicals while 85.2 % of cyclopentene loses H2 and yields cyclopentadiene. Under these conditions, pent-1-ene unimolecularly decomposes into ethyl and allyl radicals (81 %) or ethylene and propene (11.3 %). Hydrogen is the most important radical as far as cyclopentane consumption is concerned. These atoms are mostly produced by C-H βscissions from allyl (37.8 %), cyclopentyl (29.3 %), and ethyl radicals (11.3 %). Allyl is also a key radical in the oxidation of cyclopentane: 54.5 % of the allyl produced gives allene after a hydrogen departure and 25 % of allyl also produces propene by recombination with hydrogen atoms (17.4 %) or H-abstraction reactions from a-C3H4 (4.8 %) or cyclopentane (2.8 %). Finally, 5.6 % of allyl recombines with methyl to give but-1-ene. As above-mentioned, benzene accumulation is well reproduced when cyclopentane is oxidized but under-estimated during methylcyclopentane oxidation. The reaction pathway analysis showed that ca. two third of the benzene proceeds from a reaction between a cyclopentadienyl radical and allene (38.3 %) or propyne (29.0 %) proposed by Wang et al. 45 while the other third comes from the reaction of propargyl with itself either directly (14.5 %) or through the formation of linear C6H6 intermediates as proposed by Leung and Lindstedt 49 .

Methylcyclopentane oxidation pattern is much more complicated. Figure 9b illustrates the reaction pathways analysis of methylcyclopentane under stoichiometric conditions, at atmospheric pressure and 1150 K, the temperature at which ca. 60 % of the fuel is consumed. Under these conditions, 27.6 % of the fuel is consumed through the decomposition yielding methyl and cyclopentyl radicals. The other unimolecular decompositions producing 1-and 2hexene only account for few percentages (4.5 % and 1.1 % respectively) of the total flow rate. Other important pathways for fuel consumption are H-abstraction reactions by H (36.7 %), OH (14.8 %), and CH3 (6.8 %) yielding the four methylcyclopentyl radicals. Among these methylcyclopentyl radicals mcps1 is the most abundant (24.7 %), followed by mcps2 (22.5 %) slightly less produced because of the difference in the rate constants of the H-abstraction by OH proposed by Sivaramakrishnan and Michael 10 , then mcpm (7.3 %) and mcpt (7.2 %) are produced in smaller quantities. Cyclopentyl radicals are consumed by ring opening (67.2 %) yielding pent-1-en-5-yl or allyl and ethylene. Cyclopentyl can also undergo C-H β-scissions (32.2 %) yielding cyclopentene + H. Cyclopentene then produces cyclopentadiene by dehydrogenation (82.5 %), cyclopentenyl (9.7 %) or 1,3-pentadiene (6.7 %). Cyclopentene is mainly produced by C-C β-scission from mcps1 (57.1 %) for which ring opening is a less minor channel (33.6 %). On the contrary, the other secondary methylcyclopentyl radical (mcps2) undergoes ring opening, mostly to form hex-1-en-5-yl (74.6 %). Finally, mcpm and mcpt are much less produced and participate to the production of 1,3-C4H6, allene and fulvene. Almost three quarter of the benzene comes from the reaction of cyclopentadienyl and allene (45.1 %) or propyne (27.6 %) while ca. 16 % comes from propargyl and less than 2 % from fulvene. These pathways would probably need to be reconsidered on the light of theoretical calculations to better reproduce our experimental results.

Conclusions

The oxidation of cyclopentane and methylcyclopentane was studied in a jet-stirred reactor at atmospheric pressure, a residence time of 70 ms and for various equivalence ratios (φ = 0.5, 1, and 2). A detailed kinetic mechanism was developed in this work based on our previous studies which is well able to reproduce our experimental results in JSR although the formation of benzene needs to be revisited in the case of methylcyclopentane. Thanks to this mechanism, the main consumption pathways for the oxidation of cyclopentane and methylcyclopentane under our experimental conditions were identified. It was found that methylcyclopentane was more reactive, under these conditions, than cyclopentane mostly because of the initiation step producing cyclopentyl and methyl radicals. In both systems, cyclopentyl and allyl are key intermediates as far as radicals are concerned while cyclopentene and cyclopentadiene are key intermediates among the stable species. However, experimental data under higher pressures and in different set-ups would be of importance to fully validate our detailed kinetic mechanism.

Figure 1 :

 1 Figure 1: Cyclopentane (left) and methylcyclopentane (right) with carbon labels.

  cyc5h10  f H° = -18.4 S° = 70.0 cyc5h9  f H° = 26.6 S° = 71.4 cyc5h8  f H° = 8.9 S° = 68.1 cyc5h7  f H° = 41.3 S° = 70.8 cpd  f H° = 24.2 S° = 65.5 mcp  f H° = -25.6 S° = 81.1 mcpm  f H° = 22.5 S° = 83.3 mcpt  f H° = 17.9 S° = 82.9 mcps1  f H° = 20.4 S° = 83.8 mcps2  f H° = 20.4 S° = 83.8 c6h12-1  f H° = -10.1 S° = 91.6 c6h1116  f H° = 38.7 S° = 93.6 c6h1113  f H° = 21.4 S° = 90.2 c6h1115  f H° = 36.5 S° = 93.8 c6h1126  f H° = 38.8 S° = 92.8 c6h12-2  f H° = -12.8 S° = 90.7 pent2m1d5  f H° = 35.4 S° = 92.5 pent3m1d5  f H° = 37.6 S° = 92.7 pent4m1d5  f H° = 36.4 S° = 91.7 c5h5  f H° = 63.1 S° = 64.0

Figure 3 :

 3 Figure3: Comparison of the branching ratio between ring opening cyC5H9⇌C5H915 (R1) and hydrogen atom elimination (R2). R3 is the prompt dissociation of pent-1-en-5-yl.

Figure 4 :

 4 Figure 4: Comparison of the branching ratio between MCPs1⇌cC5H8+CH3, MCPs1⇌C6H1126, and MCPs1⇌pent3m1d5 (rate constants evaluated at 1 atm for Tian et al. 15 ).

Figure 6 :

 6 Figure 6: Experimental and computed mole fraction profiles obtained from the oxidation of cyclopentane (empty triangles-dashed line) and methylcyclopentane (filled circles-full line) in a JSR at φ = 0.5, p = 1 atm, τ = 0.07s.

Figure 7 :

 7 Figure 7: Experimental and computed mole fraction profiles obtained from the oxidation of cyclopentane (empty triangles and dashed line) and methylcyclopentane (filled circles and full line) in a JSR at φ = 1, p = 1 atm, τ = 0.07s.

Figure 8 :

 8 Figure 8: Experimental and computed mole fraction profiles obtained from the oxidation of cyclopentane (empty triangles and dashed line) and methylcyclopentane (filled circles and full line) in a JSR at φ = 2, p = 1 atm, τ = 0.07s.

Figure 9 :

 9 Figure 9: Reaction pathway analysis of the oxidation of a) cyclopentane and b) methylcyclopentane in a JSR at T = 1150 K, p = 1 atm, τ = 0.07s, and  = 1 (the size of the arrows is proportional to the flow rate).

  Evolution of the rate constants of H-abstraction reactions by H, O, OH, HO2, CH3, and total rate constant defined as the sum of the previous five for cyclopentane (full line, black) and methylcyclopentane (dashed line, red).

	k (cm 3 /mol/s) Figure 2:	1.0E+12 1.0E+13 1.0E+14	H-abstractions by H	MCP CP
		1.0E+11
	k (cm 3 /mol/s)	1.0E+12 1.0E+13 1.0E+14	H-abstractions by O	CP MCP
		1.0E+11
	k (cm 3 /mol/s)	1.0E+13	H-abstractions by OH	CP MCP
		1.0E+12
	k (cm 3 /mol/s)	1.0E+07 1.0E+08 1.0E+09 1.0E+10 1.0E+11	H-abstractions by HO 2	CP MCP
		1.0E+06
		1.0E+05
	k (cm 3 /mol/s)	1.0E+09 1.0E+10 1.0E+11	H-abstractions by CH 3	CP MCP
		1.0E+08
	k (cm 3 /mol/s)	1.0E+13 1.0E+14		Total	CP MCP
		1.0E+12
		0 . 5	1	1000/T (1/K)	1 . 5	2


Krishna Prasad Shrestha

Charles Lhuillier

Amanda Alves Barbosa

Pierre Brequigny

Francesco Contino

Christine Mounaïm-Rousselle

Lars Seidel
email: lars.seidel@logesoft.com

Fabian Mauss

An experimental and modeling study of ammonia with enriched oxygen content and ammonia/hydrogen laminar flame speed at elevated pressure and temperature

Keywords: ammonia, laminar flame speed, kinetic modeling, ammonia-hydrogen, NOx

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Figure 1 227 Figure 2 178 Figure 3 223 Figure 4 138 Figure 5 182 Figure 6 305 Figure 7 180 Figure 8 133 Figure 9 356 Table 1 145 Total(excl. abstract) 6075

Color figures in the electronic version only.

Supporting information is available

Introduction

Ammonia is a carbon free energy carrier. Its technical application is slightly hindered by the comparable low laminar flame speed. To consider ammonia (NH3) as single or blend fuel, it is essential to have a deep understanding of the detailed chemical processes of fuel oxidation to N2 and H2O and further oxidation to emission formation, mainly NO and NO2. Moreover, the knowledge on NH3 kinetics is also needed to improve the general NOx formation model. Ammonia is the smallest nitrogen containing fuel molecule. Several NH3-NOx mechanisms have been developed in recent years, each with a different conditions of interest [START_REF] Mathieu | Experimental and modeling study on the high-temperature oxidation of Ammonia and related NOx chemistry[END_REF][START_REF] Mei | Experimental and kinetic modeling investigation on the laminar flame propagation of ammonia under oxygen enrichment and elevated pressure conditions[END_REF][START_REF] Song | Ammonia oxidation at high pressure and intermediate temperatures[END_REF]. Shrestha et al. [START_REF] Shrestha | Detailed Kinetic Mechanism for the Oxidation of Ammonia Including the Formation and Reduction of Nitrogen Oxides[END_REF] published their NH3 model recently which covers a broad range of experimental conditions available from the literature, including NOx formation and reduction in dependence on the surrounding environment. Mathieu et al. [START_REF] Mathieu | Experimental and modeling study on the high-temperature oxidation of Ammonia and related NOx chemistry[END_REF] established an ammonia oxidation mechanism, based on experiments in a shock tube (ST). They compared their work against nine different literature models to underline the large discrepancies between the ammonia mechanisms. Hayakawa et al. [START_REF] Hayakawa | Laminar burning velocity and Markstein length of ammonia/air premixed flames at various pressures[END_REF] measured laminar flame speeds of ammoniaair in a cylindrical combustion chamber under elevated pressure conditions. They compared their data against the prediction of five detailed mechanisms from the literature. Most mechanisms failed to predict laminar flame experiments.

Xiao et al. [START_REF] Xiao | Chemical Kinetic Mechanism Study on Premixed Combustion of Ammonia/Hydrogen Fuels for Gas Turbine Use[END_REF] performed a comparative study on the performance of 12 different NH3-NOx kinetic mechanisms to identify the best model. They used NH3/H2 laminar flame speed data from [START_REF] Li | Study on using hydrogen and ammonia as fuels: Combustion characteristics and NOx formation[END_REF] and NH3 ignition delay time data from [START_REF] Mathieu | Experimental and modeling study on the high-temperature oxidation of Ammonia and related NOx chemistry[END_REF] to rate the model predictions. The majority of the kinetic models could not reproduce the experimental data for the investigated range of equivalence ratios.

Recently, Pochet et al. [START_REF] Pochet | Experimental and numerical study, under LTC conditions, of ammonia ignition delay with and without hydrogen addition[END_REF] and He et al. [START_REF] He | Auto-ignition kinetics of ammonia and ammonia/hydrogen mixtures at intermediate temperatures and high pressures[END_REF] studied autoignition of neat NH3 and NH3/H2 blends in a rapid compression machine (RCM). Both studies assessed the performance of the various NH3 kinetic models including our previous work [START_REF] Shrestha | Detailed Kinetic Mechanism for the Oxidation of Ammonia Including the Formation and Reduction of Nitrogen Oxides[END_REF] against their experimental data and concluded that none of the mechanisms was able to reproduce the experiments satisfactorily. Han et al. [START_REF] Han | Experimental and kinetic modeling study of laminar burning velocities of NH3/air, NH3/H2/air, NH3/CO/air and NH3/CH4/air premixed flames[END_REF] performed an experimental and kinetic modeling investigation on laminar flame speed for NH3/air and different NH3/H2/CO/CH4 air blends. The three investigated mechanisms from literature could not accurately predict the laminar burning velocity. Recently, Mei et al. [START_REF] Mei | Experimental and kinetic modeling investigation on the laminar flame propagation of ammonia under oxygen enrichment and elevated pressure conditions[END_REF] performed laminar flame speed experiments for oxygen enriched NH3/O2/N2 blends. They modified our previous work [START_REF] Shrestha | Detailed Kinetic Mechanism for the Oxidation of Ammonia Including the Formation and Reduction of Nitrogen Oxides[END_REF] to reach an agreement with their experiments. Their selection of the base model was founded on a previous comparative study, including work from different groups.

In general, the discrepancies between the kinetic model predictions are expected as the experimental database for ammonia oxidation is not yet broad enough. Uncertainties are found in the rate kinetic data as well as in the thermochemistry. Further experimental data are needed to allow further improvement of the kinetic models. The conclusion above includes that the further oxidation of intermediates to NOx is not yet fully understood. Several researchers have already put their effort into the development of an NH3-NOx kinetic model but still, a well-established and reliable model is not yet available. As discussed in [START_REF] Mei | Experimental and kinetic modeling investigation on the laminar flame propagation of ammonia under oxygen enrichment and elevated pressure conditions[END_REF] our recent model [START_REF] Shrestha | Detailed Kinetic Mechanism for the Oxidation of Ammonia Including the Formation and Reduction of Nitrogen Oxides[END_REF] was not validated for laminar flames but can be modified to include these experiments.

In this study, we present new experimental flame speed data together with a modification of our recent work to include these experiments in our validation database.

The aim of the present work is thus twofold:

1) to extend laminar flame speed data already published in [START_REF] Lhuillier | Experimental investigation on laminar burning velocities of ammonia/hydrogen/air mixtures at elevated temperatures[END_REF], in the case of NH3/H2 blend at higher pressure than atmospheric one, and also in the case of neat NH3 with enriched O2 conditions.Moreover, additional NH3 laminar flame speed measurement with He as an inert will be provided for the first time to investigate the influence of third body coefficients in pressure dependent reactions.

2) to improve our previous model [START_REF] Shrestha | Detailed Kinetic Mechanism for the Oxidation of Ammonia Including the Formation and Reduction of Nitrogen Oxides[END_REF], based on the improved and broadened experimental database.

This work is an ongoing effort to develop a reliable and comprehensive mechanism for NH3 and NH3/H2 blends. Although the focus of this study is on laminar flame speed, the derived model is critically tested against the previous validation database; i.e. ignition delay times (IDT), speciation in jet-stirred reactors (JSR), and burner stabilized flames (BSF).

Experimental set-up and processing

The experiments were performed in the 4.2 L (inner diameter 200 mm) spherical stainless steel combustion chamber, fully described in [START_REF] Brequigny | Laminar burning velocities of premixed nitromethane/air flames: An experimental and kinetic modeling study[END_REF][START_REF] Galmiche | Effects of high pressure, high temperature and dilution on laminar burning velocities and Markstein lengths of iso-octane/air mixtures[END_REF]. The fresh gases are heated up to a maximum initial temperature of 473K by means of a heating wire. Before filling the sphere, a vacuum pump allows the evacuation of residual gases until a pressure <0.009 bar. Then all reactant gases are injected by a thermal flow meter. Before being introduced in the sphere, the inlet valve heats the mixture up to the required temperature with a maximum of 2 K uncertainty. To guarantee a perfectly homogeneous mixture, a fan runs during few seconds, but is stopped 10 s before the ignition to avoid any perturbation. The maximum deviation between the effective initial pressure inside the combustion chamber and the required initial pressure was estimated about 0.5%. Two tungsten electrodes (1 mm diameter) with a conventional capacitive discharge ignition system are used, with an ignition coil time charge of 3 ms (i.e. discharge energy of less than 100 mJ). The electrodes gap is adjusted to ensure a sufficient energy deposit to sustain the flame.

Experiments were carried out at various initial pressure, temperature, global equivalence ratio from 1-10 bar, 298-473 K and 0.8-1 respectively. The volume content of H2, xH2, was varied from 0 to 30% (in fuel). Synthetic air (20.9% O2 / 79.1% N2) was used as ambient gas, but experiments were also performed for the neat ammonia cases with a variation of oxygen content, xO2 (23, 25, 27 and 30%vol, in oxidizer). Last, a set of experiments was also conducted by using He as inert gases for 27 and 30% vol. O2 cases. For each condition, at least 3 tests were carried out in order to obtain average and standard deviation values, represented by error bars in the following figures.

To measure laminar flame speeds, the Schlieren technique was implemented thanks to two opposite and transparent windows (diameter 70 mm). The Schlieren pathway is made by using a continuous LED, coupled with a 0.8 mm pinhole and 2 convex mirrors (864 mm focal length). At the focus point of the second mirror, a 0.5 mm dot is placed with two lenses (250 mm and 400 mm) to focus the Schlieren images on the Phantom v1210 high-speed CMOS ship. Images of 640

x 800 pixels² were recorded with a magnification ratio of 0.11 mm/pixel. The frame rate was adapted from 5 to 8kHz. The temporal evolution of the expanding spherical flame was then processed to get the flame front radius, rf. The laminar flame propagation speed, Vs was calculated from the time derivative of rf. The non-linear methodology as suggested by Kelley and Law [START_REF] Kelley | Nonlinear effects in the extraction of laminar flame speeds from expanding spherical flames[END_REF] and Halter et al. [START_REF] Halter | Nonlinear effects of stretch on the flame front propagation[END_REF] was used to estimate the unstretched laminar flame velocity Vs 0 , as This methodology was applied for flame diameter greater than 6.5 mm to avoid any ignition effect and lower than 25 mm corresponding to a burned gas volume less than 1% of sphere volume, to consider constant pressure chamber and avoid confinement effect.

(
Then the fundamental unstretched laminar burning velocity SL 0 is obtained with the expansion factor, ρb/ρu as 𝑆 𝐿 0 = 𝜌 𝑏 𝜌 𝑢 𝑉 𝑠 0 , with 𝜌 𝑏 and 𝜌 𝑢 the burned and the unburned gases densities respectively.

For low unstretched laminar burning velocity (<20 cm.s -1 ), radiative heat loss induced error can be of importance. Those effects were already addressed in [START_REF] Lhuillier | Experimental investigation on laminar burning velocities of ammonia/hydrogen/air mixtures at elevated temperatures[END_REF]. In the present work, using the correlation of Yu et al. [START_REF] Yu | Radiation-induced uncertainty in laminar flame speed measured from propagating spherical flames[END_REF], the worst case is obtained at 473 K, 3 bar where an underprediction of about 12% is calculated for neat ammonia corresponding to an absolute error of 1 cm/s. The error decreases down to 1% for the fastest flames.

Kinetic modeling

The kinetic model used in this work follows our previous publication [START_REF] Shrestha | Detailed Kinetic Mechanism for the Oxidation of Ammonia Including the Formation and Reduction of Nitrogen Oxides[END_REF] and was revised to predict the new experimental data determined in this work and in our recent work [START_REF] Lhuillier | Experimental investigation on laminar burning velocities of ammonia/hydrogen/air mixtures at elevated temperatures[END_REF] for the wide range of conditions. In this work, the reaction rate parameter of several reactions were updated based on the literature study. The changes were made in the NH3, NH2, NH, N2H2, and H2NO submechanisms and modified reactions are listed in Table 1. The model developed in this work is validated against 104 sets (approximately 500 data points) of laminar flame speed data (covering ϕ = 0.7-1.7, T = 298-473 K, P = 1-10 bar). The complete kinetic scheme is provided as supporting information (SI). For lean and stoichiometric conditions, the model agrees with most measurements. For fuel-rich conditions, the best agreement is found with the older measurements from Ronney 1988 [START_REF] Ronney | Effect of Chemistry and Transport Properties on Near-Limit Flames at Microgravity[END_REF] and with the very recent experiments from Mei et al. 2019 [START_REF] Mei | Experimental and kinetic modeling investigation on the laminar flame propagation of ammonia under oxygen enrichment and elevated pressure conditions[END_REF]. Furthermore, particularly for fuel-rich conditions the model in this work shows an improved agreement against the measurements, compared to the model from our previous work [START_REF] Shrestha | Detailed Kinetic Mechanism for the Oxidation of Ammonia Including the Formation and Reduction of Nitrogen Oxides[END_REF]. This improvement is mainly due to the update in the rate parameters for the reactions involving N2H2 (listed in Table 1). Reaction flow analysis (see Fig. 2, only major paths are shown here) reveals that reaction paths involving N2H2 are favored more at rich conditions. As can be seen in Fig. 2 at rich environment (ϕ=1.4red numbers) most of the NH2 reacts to N2H2 reacting with NH or self-recombining. This route increases its flux by 60% when compared to stoichiometric conditions. This increased importance of N2H2 formation via the NH2 route is compensated by the shrink in the NH formation route. Figure 3 shows the new measured laminar flames speeds (symbols) compared against the model predictions using our previous work [START_REF] Shrestha | Detailed Kinetic Mechanism for the Oxidation of Ammonia Including the Formation and Reduction of Nitrogen Oxides[END_REF] and the model improvements from this work for NH3/H2/air at 473 K and 1 [START_REF] Lhuillier | Experimental investigation on laminar burning velocities of ammonia/hydrogen/air mixtures at elevated temperatures[END_REF] and 3 bar. Simulations were performed using a detailed multicomponent diffusion model to consider the strong thermo diffusion of the H and H2. It can be seen that as the hydrogen content in the mixture is increased laminar flame speed is also increased. The experiments show peak laminar flame speed at ϕ = 1.1, which is well predicted by the models. It can be observed that below 10% H2 the laminar flame speed does not increase significantly, but does for H2 contents in the blend above 10% (see Fig. 1b). For 1 bar (Fig. 3a) at ϕ = 1.1 with 10% and 30% H2, laminar flame speed is increased by factor 1.4 and 2.8 respectively and for 3 bar (Fig. 3b) at same condition laminar flame speed is increased by factor 1.3 and 2.0 respectively compared to neat ammonia flame. This leads to the conclusion that the impact of H2 on the laminar flame speed of NH3/air is reduced with increasing pressure. Figure 3 also demonstrates that the model predictions from this work agree well with the most of the measurements while slightly underpredicting for 30% H2 (Fig. 3a) at lean to stoichiometric conditions.

Figure 4 shows new data the laminar flame speeds of NH3/H2/air at ϕ = 1.1, 473 K, and pressure of 5, 7 and 10 bar respectively as a function of H2 content in the fuel blend. It can be observed that the model predicts slightly higher flame speed compared to measurements but remain within the error margins of experiments. There are limited number of experimental data in literature at higher pressure (we believe this study is first to report data at 7 and 10 bar). To outline important reactions with regard to NH3/H2/air laminar flame speeds a reaction sensitivity analysis is conducted which is presented in Fig. 5 for ϕ = 1.0 at 1 bar and 473 K and different H2 contents (0-30%) in the fuel blend. In Fig. 5 positive sensitivity means reaction promotes the reactivity (increased flame speed) and negative sensitivity means reaction retards the reactivity. The chain branching reaction from the hydrogen chemistry (O2+H=OH+O) is also for Ammonia the most sensitive reaction under all conditions. Its sensitivity increases as the H2 content is increased and explains the accelerating laminar flame speed with increasing H2 content.

Other most sensitive reactions involve the NNH radical, which is a key radical in the ammonia oxidation chemistry. The thermal dissociation NNH=N2+H competes with the oxidation NNH+O2=N2+HO2. The former reaction promotes the reactivity by producing more reactive Hatoms while the latter retards the system reactivity by producing less reactive HO2 radicals.

Sensitivity increases with increasing H2 content in the blend for both reactions. The reactions, NH2+NO=NNH+OH and N2H2+M=NNH+H+M are chain branching reactions promoting the reactivity of the system. With respect to the increasing H2 content, the sensitivity of the former reaction increases while sensitivity of later reaction decreases. A further important reaction is NH2+NH=N2H2+H, with the highest contribution to the formation of N2H2 and consuming the important key radicals (NH2 and NH). The sensitivity of this reaction also increases with increase in H2 content. Similar observations can be drawn for other reactions as well. Additional model validation of NH3/air and NH3/H2/air laminar flame speeds for wide range of experimental condition is shown in SI (see Fig. S1 -S8).

Furthermore, for comparison study simulation has also been performed using three different mechanisms from literature (Glarborg et al. [START_REF] Glarborg | Modeling nitrogen chemistry in combustion[END_REF], Otomo et al. [START_REF] Otomo | Chemical kinetic modeling of ammonia oxidation with improved reaction mechanism for ammonia/air and ammonia/hydrogen/air combustion[END_REF] and Stagni et al. [START_REF] Stagni | An experimental, theoretical and kinetic-modeling study of the gas-phase oxidation of ammonia[END_REF]). The performance of these mechanism against the experimental data (shown in Fig. 1, Fig. 3 and Fig. 4) are provided in SI (Fig. S12-S15). Among these, model from Glarborg [START_REF] Glarborg | Modeling nitrogen chemistry in combustion[END_REF] overpredicts the laminar flame speed at all condition, Otomo model [START_REF] Otomo | Chemical kinetic modeling of ammonia oxidation with improved reaction mechanism for ammonia/air and ammonia/hydrogen/air combustion[END_REF] underpredicts mainly for NH3/H2 blends, and Stagni model [START_REF] Stagni | An experimental, theoretical and kinetic-modeling study of the gas-phase oxidation of ammonia[END_REF] performance is similar as our mechanism from this work. well for all the conditions. For completeness it is also stated that the agreement is better for 298 and 373 K than at 323 K. The laminar flame speed depends almost linearly on the O2 content in the oxidizer (see also Fig. S17 in SI). The influence of O2 is mainly explained by the increased adiabatic flame temperature. The influence of the chemistry is discussed in Fig. 7. It should be noted that with increasing H2 in fuel blend the growth is exponential while this does not hold true for O2 (see Fig. 2b and Fig. S17).

To explore the important reactions for predicting the laminar flame speeds at varying O2 content, reaction sensitivity analysis is performed. The prediction of different models from literature [START_REF] Glarborg | Modeling nitrogen chemistry in combustion[END_REF][START_REF] Otomo | Chemical kinetic modeling of ammonia oxidation with improved reaction mechanism for ammonia/air and ammonia/hydrogen/air combustion[END_REF][START_REF] Stagni | An experimental, theoretical and kinetic-modeling study of the gas-phase oxidation of ammonia[END_REF] was compared against the present experimental data (see SI Fig. S22 -Fig. S24). The model from Glarborg [START_REF] Glarborg | Modeling nitrogen chemistry in combustion[END_REF] overpredicts the laminar flame speed at all conditions, the Otomo model [START_REF] Otomo | Chemical kinetic modeling of ammonia oxidation with improved reaction mechanism for ammonia/air and ammonia/hydrogen/air combustion[END_REF] underpredicts mainly at rich side, and

Stagni model [START_REF] Stagni | An experimental, theoretical and kinetic-modeling study of the gas-phase oxidation of ammonia[END_REF] performance is similar as our model from this work.

Model validation against literature data for IDT, speciation in BSF and JSR

Finally, we demonstrate for completeness of this paper that the modified model still predicts the validation experiments from previous work. Figure 9a shows the comparison between the model predictions against selected experimental data for ignition delay times in shock tube [START_REF] Mathieu | Experimental and modeling study on the high-temperature oxidation of Ammonia and related NOx chemistry[END_REF] and RCM [START_REF] He | Auto-ignition kinetics of ammonia and ammonia/hydrogen mixtures at intermediate temperatures and high pressures[END_REF] for NH3/O2/Ar blend. The model predictions are in good agreement with the experimental data for a wide range of pressure over the whole temperature range investigated.

Conclusion

New data of the laminar flame speed of NH3 enriched with O2 and NH3/H2 blends have been studied experimentally at 1-10 bar, 298-473 K using a constant pressure combustion vessel. Based on the present measurements and published experimental data, a reaction mechanism for the oxidation of NH3 and NH3/H2 blends is developed. A number of published experiments have been selected to demonstrate that important features of the ammonia chemistry are well captured by the mechanism that was modified to include laminar flame speed prediction. Overall, there is a good agreement between model predictions and experimental measurements for the wide range of experimental conditions investigated. It is found that laminar flame speed of neat ammonia can be increased by increasing the oxygen content or using hydrogen as fuel blend. To rate the sensitivity of H2 addition to the fuel blend or O2 enrichment in the oxidizer we compare the data from Fig. 1b and Fig. 6a at 1 atm, 298K and ϕ = 1.0. For 30% O2 (in oxidizer stream) the laminar flame speed of NH3 is increased by factor 2.8 while 30% H2 (in fuel stream) increases flame speed by factor 2.9. Hence, 9% increase of O2 (compared to air) content has the same effect as 30% H2 in the fuel blend.

List of figure captions

  K the stretch rate impacting the flame, estimated by 𝐾 = Lb the Markstein length for burned gases.

4. 1

 1 Figure 1a displays the laminar flame speeds of NH3/air at 1 atm and 298 K as a function of

Figure 1b shows the

  Figure1bshows the laminar flame speed of NH3/H2/air blends at stoichiometric and standard

Figure 1 :

 1 Figure 1: Laminar flame speed at 1 atm and 298 K for NH3/air (a) and for stoichiometric

Figure 2 :

 2 Figure 2: Integrated N-atom mass flux analysis at 1 atm and 298 K for ϕ = 1.0 and ϕ = 1.4 NH3/air

Figure 3 :

 3 Figure 3: Laminar flame speeds of NH3/H2/air at 473 K and 1 bar (a) and 3 bar (b) at varying H2

Figure 4 :

 4 Figure 4: Laminar flame speed of NH3/H2/air at 473K and varying H2 content at ϕ = 1.1 and 5-

Figure 5 : 4 . 2 Figure 6

 5426 Figure 5: Reaction sensitivity analysis for laminar flame speed of NH3/H2/air at ϕ = 1.0, 1 bar and

Figure 7

 7 displays the 15 highest sensitive reactions at ϕ = 1.0, 1 bar and 298 K. The highest sensitivity of the branching reaction O2+H=OH+O is expected. Its sensitivity increases with increasing O2 content in the oxidizer. Most of the sensitive reactions are unchanged when compared to Fig.5(H2 addition to the fuel). Reaction NNH=N2+H and NNH+O2=N2+HO2 which showed an increased sensitivity trend with an increase in H2 content in the NH3/H2/air blend (Fig.5), shows a decreasing sensitivity trend with an increase in O2 content. Similarly, reaction N2H2+M=NNH+H+M and N2H2+H=NNH+H2 show an increasing trend in sensitivity contrary to Fig.5. However, other important reactions NH2+NO=NNH+OH and NH2+NH=N2H2+H show a similar trend decreasing and increasing. Hence the chemical effect of O2 enrichment is more complex. It is, however governed by the chain branching reaction O2+H=OH+O.

Figure 6 :

 6 Figure 6: Laminar flame speed of NH3/O2/N2 at 1 bar and different temperatures with varying O2

Figure 7 :

 7 Figure 7: Reaction sensitivity analysis for laminar flame speed of NH3/O2/N2 at ϕ = 1.0, 1 bar and

Figure 8

 8 Figure 8 shows the laminar flame speeds of NH3/O2/He blend compared to the model prediction

Figure 8 :

 8 Figure 8: Laminar flame speed of NH3/O2/He at 1 bar and 373K and different O2 content.

Figure 9 (

 9 b: speciation in NH3/NO/Ar, BSF (295K, 7.2kPa), (c, d): speciation during H2/O2/N2 oxidation in JSR doped with 220 ppm of NO) compares the model prediction against the experimental data from the literature and good agreement is found. Additional model validation for IDT, BSF, and JSR is shown in SI to offer a broad data base (Fig. S25 -S31).

Figure 9 :

 9 Figure 9: Comparison between model prediction and experimental data from literature for

Figure 1 :

 1 Figure 1: Laminar flame speed at 1 atm and 298 K for NH3/air (a) and for stoichiometric

Figure 2 :

 2 Figure 2: Integrated N-atom mass flux analysis at 1 atm and 298 K for ϕ = 1.0 and ϕ = 1.4 NH3/air

Figure 3 :

 3 Figure 3: Laminar flame speeds of NH3/H2/air at 473 K and 1 bar (a) and 3 bar (b) at varying H2

Figure 4 :

 4 Figure 4: Laminar flame speed of NH3/H2/air at 473K and varying H2 content at ϕ = 1.1 and 5-

Figure 5 :

 5 Figure 5: Reaction sensitivity analysis for laminar flame speed of NH3/H2/air at ϕ = 1.0, 1 bar and

Figure 6 :

 6 Figure 6: Laminar flame speed of NH3/O2/N2 at 1 bar and different temperatures with varying O2

Figure 7 :Figure 8 :

 78 Figure 7: Reaction sensitivity analysis for laminar flame speed of NH3/O2/N2 at ϕ = 1.0, 1 bar and

Figure 9 :

 9 Figure 9: Comparison between model prediction and experimental data from literature for

Table 1 :

 1 List of reactions with updated rate parameter in the detailed model, found in the SI. The symbols in figures are experimental data from this work or from the literature, lines are model prediction. All the simulations are performed with the LOGEresearch v1.10 [24] software suit. The model from this work is validated against 104 data sets of laminar flame speed measurements

	Reactions	Source of adopted rate
	NH2+H(+M)=NH3(+M)	Altinay and Macdonald [17]
	NH3+H=NH2+H2	Nguyen and Stanton [18]
	NH2+H=NH+H2	Samu et al. [19]
	NH2+O=HNO+H	Miller et al. [20] /2
	NH2+HO2=NH3+O2	Sumathi et al. [21]
	NH2+HO2=H2NO+OH	Sumathi et al. [21]
	NH2+HO2=HNO+H2O	Sumathi et al. [21]
	NH2+HO2=HON+H2O	Sumathi et al. [21]
	N2H2+M=NNH+H+M	Mei et al. [2] x 0.75
	N2H2+H=NNH+H2	Zheng et al. [22] x 2
	NH2+NH=N2H2+H	Klippenstein et al. [23] x 2
	NH2+NH2=N2H2+H2	Klippenstein et al. [23] x 2

from different sources (32 data are from this work and 40 data (for NH3/H2) are from our recent work

[START_REF] Lhuillier | Experimental investigation on laminar burning velocities of ammonia/hydrogen/air mixtures at elevated temperatures[END_REF]

and others are from published literature

[START_REF] Mei | Experimental and kinetic modeling investigation on the laminar flame propagation of ammonia under oxygen enrichment and elevated pressure conditions[END_REF][START_REF] Hayakawa | Laminar burning velocity and Markstein length of ammonia/air premixed flames at various pressures[END_REF][START_REF] Li | Study on using hydrogen and ammonia as fuels: Combustion characteristics and NOx formation[END_REF][START_REF] Han | Experimental and kinetic modeling study of laminar burning velocities of NH3/air, NH3/H2/air, NH3/CO/air and NH3/CH4/air premixed flames[END_REF][START_REF] Lee | Effects of ammonia substitution on hydrogen/air flame propagation and emissions[END_REF][START_REF] Ichikawa | Laminar burning velocity and Markstein length of ammonia/hydrogen/air premixed flames at elevated pressures[END_REF][START_REF] Jabbour | Burning velocity and refrigerant flammability classification[END_REF][START_REF] Zakaznov | Determination of normal flame velocity and critical diameter of flame extinction in ammonia-air mixture[END_REF][START_REF] Pfahl | Flammability limits, ignition energy, and flame speeds in H2-CH4-NH3-N2O-O2-N2 mixtures[END_REF][START_REF] Ronney | Effect of Chemistry and Transport Properties on Near-Limit Flames at Microgravity[END_REF][START_REF] Kumar | Experimental and modeling study of chemical-kinetics mechanisms for H 2-NH3-air mixtures in laminar premixed jet flames[END_REF][START_REF] Takizawa | Burning velocity measurements of nitrogen-containing compounds[END_REF]

).

Acknowledgements

The research leading to these results has received funding from the French Government's "Investissement d'Avenir" program: "Laboratoire d'Excellence CAPRYSSES" (Grant No ANR-11-LABX-0006-01).