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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.

Models for chemical mechanisms of hydrocarbon oxidation rely on spectrometric identification of molecular structures in flames. Carbonyl (keto) compounds are well-established combustion intermediates. However, their less-stable enol tautomers, bearing OH groups adjacent to carbon-carbon double bonds, are not included in standard models. We observed substantial quantities of two-, three-, and four-carbon enols by photoionization mass spectrometry of flames burning representative compounds from modern fuel blends. Concentration profiles demonstrate that enol flame chemistry cannot be accounted for purely by keto-enol tautomerization. Currently accepted hydrocarbon oxidation mechanisms will likely require revision to explain the formation and reactivity of these unexpected compounds.

a b s t r a c tFlame-sampling molecular-beam mass spectrometry of premixed, laminar, low-pressure flat flames has been demonstrated to be an efficient tool to study combustion chemistry. In this technique, flame gases are sampled through a small opening in a quartz probe, and after formation of a molecular beam, all flame species are separated using mass spectrometry. The present review focuses on critical aspects of the experimental approach including probe sampling effects, different ionization processes, and mass separation procedures. The capability for isomer-resolved flame species measurements, achievable by employing tunable vacuum-ultraviolet radiation for single-photon ionization, has greatly benefited flame-sampling molecular-beam mass spectrometry. This review also offers an overview of recent combustion chemistry studies of flames fueled by hydrocarbons and oxygenates. The identity of a variety of intermediates in hydrocarbon flames, including resonantly stabilized radicals and closed-shell intermediates, is described, thus establishing a more detailed understanding of the fundamentals of molecular-weight growth processes. Finally, molecular-beam mass-spectrometric studies of reaction paths in flames of alcohols, ethers, and esters, which have been performed to support the development and validation of kinetic models for bio-derived alternative fuels, are reviewed.

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Quantitative identification of isomers of hydrocarbon radicals in flames is critical to understanding soot formation. Isomers of C4H3 and C4H5 in flames fueled by allene, propyne, cyclopentene, or benzene are identified by comparison of the observed photoionization efficiencies with theoretical simulations based on calculated ionization energies and Franck-Condon factors. The experiments combine molecular-beam mass spectrometry (MBMS) with photoionization by tunable vacuum-ultraviolet synchrotron radiation. The theoretical simulations employ the rovibrational properties obtained with B3LYP/6-311++G(d,p) density functional theory and electronic energies obtained from QCISD(T) ab initio calculations extrapolated to the complete basis set limit. For C4H3, the comparisons reveal the presence of the resonantly stabilized CH2CCCH isomer (i-C4H3). For C4H5, contributions from the CH2CHCCH2 (i-C4H5) and some combination of the CH3CCCH2 and CH3CHCCH isomers are evident. Quantitative concentration estimates for these species are made for allene, cyclopentene, and benzene flames. Because of low Franck-Condon factors, sensitivity to n-isomers of both C4H3 and C4H5 is limited. Adiabatic ionization energies, as obtained from fits of the theoretical predictions to the experimental photoionization efficiency curves, are within the error bars of the QCISD(T) calculations. For i-C4H3 and i-C4H5, these fitted adiabatic ionization energies are (8.06 +/- 0.05) eV and (7.60 +/- 0.05) eV, respectively. The good agreement between the fitted and theoretical ionization thresholds suggests that the corresponding theoretically predicted radical heats of formation (119.1, 76.3, 78.7, and 79.1 kcal/mol at 0 K for i-C4H3, i-C4H5, CH3CCCH2, and CH3CHCCH, respectively) are also quite accurate.

A detailed chemical kinetic reaction mechanism has been developed for a group of four small alkyl ester fuels, consisting of methyl formate, methyl acetate, ethyl formate and ethyl acetate. This mechanism is validated by comparisons between computed results and recently measured intermediate species mole fractions in fuel-rich, low pressure, premixed laminar flames. The model development employs a principle of similarity of functional groups in constraining the H atom abstraction and unimolecular decomposition reactions in each of these fuels. As a result, the reaction mechanism and formalism for mechanism development are suitable for extension to larger oxygenated hydrocarbon fuels, together with an improved kinetic understanding of the structure and chemical kinetics of alkyl ester fuels that can be extended to biodiesel fuels. Variations in concentrations of intermediate species levels in these flames are traced to differences in the molecular structure of the fuel molecules.

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