Isomer-Specific Influences on the Composition of Reaction Intermediates in Dimethyl Ether/Propene and Ethanol/Propene Flame

Effect of ethanol addition on soot precursors emissions during benzene oxidation in a jet-stirred reactor

A constant volume reactor model (PSR) was used to investigate the effect of ethanol addition on the formation of some pollutants during benzene oxidation in a jet-stirred reactor. The blended fuels were formed by incrementally adding 4% wt of oxygen (ethanol) to the neat benzene fuel and by keeping the inert mole fraction (nitrogen) and the equivalence ratio constants. The main objective of this work was to obtain fundamental understanding of the mechanisms through which the oxygenate compound affects soot precursor amounts. The modeling results showed that C2H2, C5H5, and C3H3 mole fractions decreased upon increasing the ethanol percentage in the fuel mixture.

EXPERIMENTAL AND MODELING STUDY OF PREMIXED LAMINAR FLAMES OF ETHANOL AND METHANE

To better understand the chemistry of the combustion of ethanol, the structure of five low pressure laminar premixed flames has been investigated: a pure methane flame (φ=1), three pure ethanol flames (φ=0.7, 1.0, and 1.3), and an ethanol/methane mixture flames (φ=1). The flames have been stabilized on a burner at a pressure of 6.7 kPa using argon as dilutant, with a gas velocity at the burner of 64.3 cm/s at 333 K. The results consist of mole fraction profiles of 20 species measured as a function of the height above the burner by probe sampling followed by online gas chromatography analyses. A mechanism for the oxidation of ethanol was proposed. The reactions of ethanol and acetaldehyde were updated and include recent theoretical calculations while that of ethenol, dimethyl ether, acetone, and propanal were added in the mechanism. This mechanism was also tested against experimental results available in the literature for laminar burning velocities and laminar premixed flame where ethenol was detected. The main reaction pathways of consumption of ethanol are analyzed. The effect of the branching ratios of reaction C2H5OH+OH→Products+H2O is also discussed.

On the Chemical Kinetics of Ethanol Oxidation: Shock Tube, Rapid Compression Machine and Detailed Modeling Study

Auto-ignition characteristics of ethanol were experimentally investigated using two Shock Tube (ST) facilities and a Rapid Compression Machine (RCM). Ignition delay times for stoichiometric ethanol-air mixtures were measured for temperatures between 775–1300 K in a High Pressure Shock Tube (HPST) at pressures close to 80 bar by probing pressure time histories and CH* emission. In some experiments the HPST was additionally employed for schlieren imaging to visualize ignition behavior by probing density gradients during ignition for ethanol-air mixtures. The ignition delay experiments in HPST were complemented by RCM measurements for extending the temperature regime to the Low Temperature Combustion (LTC) regime, down to 705 K, providing kinetic model validation data over a very wide temperature and pressure range. The current results also extend the earlier shock tube measurements performed in the same laboratory for pressures around 40 bar for temperatures down to 800 K [Heufer et al., Shock Waves 20 (2010) 307]. Furthermore, a Rectangular Shock Tube (RST) was solely used for additional schlieren imaging experiments to acquire information on ignition modes in stoichiometric ethanol-air mixtures around 10 bar. An improved chemical kinetic model was developed based on the Li et al. mechanism [Li et al., “Ethanol Model v1.0”, Princeton University, 2009] which was updated with evaluated rate parameters from the literature and validated through results obtained from the aforementioned facilities. The model predictions were compared to previously published low-pressure, premixed flat flame molecular beam mass spectrometry speciation data [Kasper et al., Combust. Flame 150 (2007) 220; Wang et al., J. Phys. Chem. A 112 (2008) 9255] where reasonable agreement is obtained considering the uncertainties in experiments and model. However, the model provides excellent agreement for the auto-ignition results obtained in the RCM and the high temperature shock induced ignition delays. Significant disparities with the model predictions are obtained for the shock tube results at temperatures below 1000 K as it transitions from the intermediate to the low temperature regime. The reasons for these deviations are assigned to strong fuel specific “pre-ignition” effects observed in ethanol auto-ignition, in contrast to other investigated fuels, which was satisfactorily explained through schlieren experimental results. To our knowledge this work is first of its kind that combines results from complementary experimental methods from three different facilities providing a holistic description on the auto-ignition behavior of ethanol. Furthermore, this paper reports ignition delay measurements for ethanol in air, at the highest pressures applicable to practical combustors.

Combustion Chemistry of the Butane Isomers in Premixed Low-Pressure Flames

The combustion chemistry of the two butane isomers represents a subset in a comprehensive description of C1–C4 hydrocarbon and oxygenated fuels. A critical examination of combustion models and their capability to predict emissions from this class of fuels must rely on high-quality experimental data that address the respective chemical decomposition and oxidation pathways, including quantitative intermediate species mole fractions. Premixed flat low-pressure (40 mbar) flames of the two butane isomers were thus studied under identical, fuel-rich (φ=1.71) conditions. Two independent molecular-beam mass spectrometer (MBMS) set-ups were used to provide quantitative species profiles. Both data sets, one from electron ionization (EI)-MBMS with high mass resolution and one from photoionization (PI)-MBMS with high energy resolution, are in overall good agreement. Simulations with a flame model were used to analyze the respective reaction pathways, and differences in the combustion behavior of the two isomers are discussed.

Biofuel combustion chemistry: from ethanol to biodiesel

Biofuels, such as bio-ethanol, bio-butanol, and biodiesel, are of increasing interest as alternatives to petroleum-based transportation fuels because they offer the long-term promise of fuel-source regenerability and reduced climatic impact. Current discussions emphasize the processes to make such alternative fuels and fuel additives, the compatibility of these substances with current fuel-delivery infrastructure and engine performance, and the competition between biofuel and food production. However, the combustion chemistry of the compounds that constitute typical biofuels, including alcohols, ethers, and esters, has not received similar public attention. Herein we highlight some characteristic aspects of the chemical pathways in the combustion of prototypical representatives of potential biofuels. The discussion focuses on the decomposition and oxidation mechanisms and the formation of undesired, harmful, or toxic emissions, with an emphasis on transportation fuels. New insights into the vastly diverse and complex chemical reaction networks of biofuel combustion are enabled by recent experimental investigations and complementary combustion modeling. Understanding key elements of this chemistry is an important step towards the intelligent selection of next-generation alternative fuels.

Sampling Probe Influences on Temperature and Species Concentrations in Molecular Beam Mass Spectroscopic Investigations of Flat Premixed Low-pressure Flames

New operating regimes for engines and combustors and the advocated use of non-conventional transportation fuels demand investigation of the combustion chemistry of different classes of chemicals, especially under premixed conditions. Detailed species compositions during combustion are needed to estimate hazardous emissions, and models for their prediction must be validated for the intended combustion conditions.Molecular-beam mass spectrometry (MBMS) is a common technique to measure quantitative species concentrations in flames. It is widely employed to characterize the flame chemistry of laminar premixed combustion, and it has been complemented with optical measurements for the detection of a number of molecular species and radicals. Significant progress has been made in recent studies through the introduction of synchrotron-based MBMS instruments. They have improved the identification process by using tunable vacuum-ultraviolet radiation for photoionization of the species to be detected, and isomer-specific measurements are now almost routinely possible. Along with quantitative species measurements, the temperature profile is needed as input parameter for chemical kinetic modeling. It is usually determined either using thermocouples or laser spectroscopic techniques.It is an ongoing discussion how sampling probes affect these measurements, and how MBMS results can be compared to combustion modeling. The present article is intended to contribute to this discussion by providing optical and MBMS results obtained with several sampling configurations.