Guillaume Dayma

Experimental study and detailed kinetic modeling of the mutual sensitization of the oxidation of nitric oxide, ethylene, and ethane

ABSTRACT New experimental results were obtained for the mutual sensitization of the oxidation of NO and ethane and NO and ethylene in fuel-lean conditions. An atmospheric fused-silica jet-stirred reactor operating over the temperature range 700–1150 K was used. The initial carbon mole fraction was 2500 ppm whereas that of NO varied from 0 to 1200 ppm. Sonic quartz probe sampling followed by on-line Fourier transform infrared analyses and off-line gas chromatography-thermal conductivity detection flame ionization detection analyses were used to measure the concentration profiles of the reactants, stable intermediates, and the final products. A detailed chemical kinetic modeling of the present experiments was performed (147 species, 1085 reversible reactions). An overall good agreement between the present data and modeling was obtained. Furthermore, the proposed model was able to simulate, better than in previous modeling efforts, plug-flow reactor experimental results available in the literature. According to the proposed model, the mutual sensitization of the oxidation of ethane or ethylene and NO proceeds mostly through the conversion of NO to NO2 by HO2 radicals. The NO-to-NO2 conversion is enhanced by the production of HO2 radicals from the oxidation of the fuel. The production of OH resulting from the oxidation of NO by the hydroperoxy radical promotes the oxidation of the fuel: NO + HO2 ⇒ OH + NO2 is followed by OH + C2H4 ⇒ C2H3 + H2O and OH + C2H6 ⇒ C2H5 + H2O. In the case of ethane, at low temperature, the reaction further proceeds via CH3 + O2 ⇒ CH3O2; CH3O2 + NO ⇒ CH3O + NO2; C2H5O2 + NO ⇒ C2H5O + NO2; C2H5 + O2 ⇒ C2H4 + HO2. At higher temperature, the sequence is followed by CH3O ⇒ CH2O + H; C2H5O ⇒ CH3CHO + H; C2H5O ⇒ CH3 + CH2O; CH2O + OH ⇒ HCO + H2O; HCO + O2 ⇒ HO2 + CO; and H + O2 ⇒ HO2. In the case of ethylene, the reaction further proceeds via C2H3 + O2 ⇒ CH2O + HCO; CH2O + OH ⇒ HCO + H2O; HCO + O2 ⇒ HO2 + CO; and H + O2 + M ⇒ HO2 + M. The main chemical kinetic differences between the two fuels in presence of NO were analyzed.

EFFECTS OF AIR CONTAMINATION ON THE COMBUSTION OF HYDROGEN—EFFECT OF NO AND NO2 ADDITION ON HYDROGEN IGNITION AND OXIDATION KINETICS

New kinetic results for the oxidation of hydrogen in diluted conditions, perturbed by the addition of various concentrations of NO or NO2 were obtained in a jet-stirred reactor. The experiments were performed over the temperature range 700–1150 K, for pressures of 1 to 10 atm, and equivalence ratios from 0.1 to 2.5. The experiments were performed at fixed residence time, τ, and variable temperature. Sonic probe sampling at low pressure followed by on-line and off-line chemical analyses (gas chromatography and FTIR) were used to follow the reaction. A detailed kinetic modeling of the present experiments was performed using the proposed kinetic scheme. Further validations of this kinetic reaction mechanism were performed simulating the ignition of hydrogen-air and hydrogen-NOx-air mixtures, and the burning velocities of hydrogen-air mixtures. The present modeling results agreed well with the flame data and the ignition data of H2-air-NO mixtures but disagree with the mixtures containing NO2. The present results support the low literature value of the rate constant for the reaction of NO2 with H2 whereas a much higher rate constant is needed to fit the ignition delays of the H2-air-NO2 mixtures.

Chemical kinetic study of the oxidation of a biodiesel-bioethanol surrogate fuel: methyl octanoate-ethanol mixtures.

There is a growing interest for using bioethanol-biodiesel fuel blends in diesel engines but no kinetic data and model for their combustion were available. Therefore, the kinetics of oxidation of a biodiesel-bioethanol surrogate fuel (methyl octanoate-ethanol) was studied experimentally in a jet-stirred reactor at 10 atm and constant residence time, over the temperature range 560-1160 K, and for several equivalence ratios (0.5-2). Concentration profiles of reactants, stable intermediates, and final products were obtained by probe sampling followed by online FTIR, and off-line gas chromatography analyses. The oxidation of this fuel in these conditions was modeled using a detailed chemical kinetic reaction mechanism consisting of 4592 reversible reactions and 1087 species. The proposed kinetic reaction mechanism yielded a good representation of the kinetics of oxidation of this biodiesel-bioethanol surrogate under the JSR conditions. The modeling was used to delineate the reactions triggering the low-temperature oxidation of ethanol important for diesel engine applications.

Combustion and Emissions Characteristics of Valeric Biofuels in a Compression Ignition Engine

AbstractNew-generation biofuels are mainly produced from nonfood crops or waste. Although second-generation ethanol is one of the main options, valeric esters can also be produced from lignocellulose through levulinic acid. However, only few experimental results are available to characterize their combustion behavior. Using a traditional compression ignition (CI) engine converted to monocylinder operation, the engine performances and emissions of butyl and pentyl valerate (BV and PenV, respectively) were investigated. This paper analyses the experimental results for blends of 20%vol of esters in diesel fuel, taking diesel fuel as the reference fuel. The BV and PenV have a smaller cetane number and consequently the ignition delay of the blends is slightly longer. However, engine performances and emissions are not significantly modified by adding 20%vol of esters to diesel fuel. The BV and PenV then represent very good alternative biofuels for CI engines.

Jet-Stirred Reactors

The jet-stirred reactor is a type of ideal continuously stirred-tank reactor which is well suited for gas-phase kinetic studies. It is mainly used to study the oxidation and the pyrolysis of hydrocarbon and oxygenated fuels. These studies consist in recording the evolution of the conversion of the reactants and of the mole fractions of reaction products as a function of different parameters such as reaction temperature, residence time, pressure, and composition of the inlet gas. Gas chromatography is classically used for the analysis of the species in the gas phase, but recent studies aimed at coupling new types of analytical devices to a jet-stirred reactor to observe new types of species and to gain accuracy in the identification and the quantification of species.

Quantitative Measurements of HO2 and Other Products of n-Butane Oxidation (H2O2, H2O, CH2O, and C2H4) at Elevated Temperatures by Direct Coupling of a Jet-Stirred Reactor with Sampling Nozzle and Cavity Ring-Down Spectroscopy (cw-CRDS)

For the first time quantitative measurements of the hydroperoxyl radical (HO2) in a jet-stirred reactor were performed thanks to a new experimental setup involving fast sampling and near-infrared cavity ring-down spectroscopy at low pressure. The experiments were performed at atmospheric pressure and over a range of temperatures (550-900 K) with n-butane, the simplest hydrocarbon fuel exhibiting cool flame oxidation chemistry which represents a key process for the auto-ignition in internal combustion engines. The same technique was also used to measure H2O2, H2O, CH2O, and C2H4 under the same conditions. This new setup brings new scientific horizons for characterizing complex reactive systems at elevated temperatures. Measuring HO2 formation from hydrocarbon oxidation is extremely important in determining the propensity of a fuel to follow chain-termination pathways from R + O2 compared to chain branching (leading to OH), helping to constrain and better validate detailed chemical kinetics models.

An Experimental and Kinetic Modeling Study of Premixed Laminar Flames of Methyl Pentanoate and Methyl Hexanoate

Abstract Detailed chemical structures of stoichiometric and rich premixed laminar flames of methyl pentanoate and methyl hexanoate were investigated over a flat burner at 20 Torr and for methyl pentanoate at 1 atm. Molecular beam mass spectrometry was used with tunable synchrotron vacuum ultraviolet (VUV) photoionization for low pressure flames of both methyl pentanoate and methyl hexanoate, and soft electron-impact ionization was used for atmospheric pressure flames of methyl pentanoate. Mole fraction profiles of stable and intermediate species, as well as temperature profiles, were measured in the flames. A detailed chemical kinetic high temperature reaction mechanism for small alkyl ester oxidation was extended to include combustion of methyl pentanoate and methyl hexanoate, and the resulting model was used to compare computed values with experimentally measured values. Reaction pathways for both fuels were identified, with good agreement between measured and computed species profiles. Implications of these results for future studies of larger alkyl ester fuels are discussed.

Engine Performances and Emissions of Second-Generation Biofuels in Spark Ignition Engines: The Case of Methyl and Ethyl Valerates

As an alternative to second generation ethanol, valeric esters can be produced from lignocellulose through levulinic acid. While some data on these fuels are available, only few experiments have been performed to analyze their combustion characteristics under engine conditions. Using a traditional spark ignition engine converted to mono-cylinder operation, we have investigated the engine performances and emissions of methyl and ethyl valerates. This paper compares the experimental results for pure valeric esters and for blends of 20% of esters in PRF95, with PRF95 as the reference fuel. The esters propagate faster than PRF95 which requires a slight change of ignition timing to optimise the work output. However, both the performances and the emissions are not significantly changed compared to the reference. Accordingly, methyl and ethyl valerate represent very good alternatives as biofuels for SI engines. Future studies will focus on testing these esters in real application engines and performing endurance tests.

Towards HCCI Control by Ozone Seeding

Nowadays, the main objectives in the automobile engine field are to reduce fuel consumption and pollutant emissions. HCCI engines can be a good solution to meet pollutant emission requirements and to achieve high combustion efficiency. However, before an HCCI engine is used as a conventional engine, several problems must be overcome, in particular control of the progression of combustion. Many studies have been conducted into possible control methods. A new strategy consists in using oxidizing chemical species such as ozone to seed the intake of a HCCI engine. As increasingly smaller ozonizers are now being designed, this kind of device could be integrated on a vehicle and on a HCCI engine, in order to control combustion phasing and promote the future use of this engine as a conventional engine. In the present study, experiments on a HCCI engine fuelled with iso-octane were carried out with ozone seeding in the intake. Results showed that when assisted by the addition of ozone, combustion can be enhanced and moved forward. Consequently, the use of oxidizing chemical species can be a means to control HCCI combustion. Depending on the inlet temperature, the control of combustion phasing may be more or less easy due to sensitivity to the ozone concentration. The present results also show the existence of a cool flame in the case of iso-octane combustion, indicating that ozone seeding can also be used in order to study iso-octane cool flame in a HCCI engine.

Experimental Study of the Oxidation of N-Tetradecane in a Jet-Stirred Reactor (JSR) and Detailed Chemical Kinetic Modeling

The kinetics of oxidation of n-tetradecane was studied experimentally in a jet-stirred reactor (JSR) at high pressure (10 atm), at temperatures ranging from 560 to 1030 K, at a constant residence time (τ) of 1 s, and for three equivalence ratios (φ = 0.5, 1.0, and 2.0). Chemical analyses by Fourier transform infrared spectrometry and gas chromatography yielded mole fractions of reactants, stable intermediates, and final products as a function of temperature. A kinetic reaction mechanism based on previous studies from this laboratory was developed and validated by comparison with the present experimental results. The proposed reaction mechanism consisted of 7,885 reversible reactions involving 1,813 species. Experimental data and simulation results obtained in the current work were found in good agreement. Ignition delays of n-tetradecane/air mixtures were also simulated.