Terrill A. Cool

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.

Selective detection of Isomers with Photoionization mass spectrometry for studies of hydrocarbon flame chemistry

We report the first use of synchrotron radiation, continuously tunable from 8 to 15 eV, for flame-sampling photoionization mass spectrometry (PIMS). Synchrotron radiation offers important advantages over the use of pulsed vacuum ultraviolet lasers for PIMS; these include superior signal-to-noise, soft ionization, and access to photon energies outside the limited tuning ranges of current VUV laser sources. Near-threshold photoionization efficiency measurements were used to determine the absolute concentrations of the allene and propyne isomers of C3H4 in low-pressure laminar ethylene–oxygen and benzene–oxygen flames. Similar measurements of the isomeric composition of C2H4O species in a fuel-rich ethylene–oxygen flame revealed the presence of substantial concentrations of ethenol (vinyl alcohol) and acetaldehyde. Ethenol has not been previously detected in hydrocarbon flames. Absolute photoionization cross sections were measured for ethylene, allene, propyne, and acetaldehyde, using propene as a calibration standard. PIE curves are presented for several additional reaction intermediates prominent in hydrocarbon flames.

Overtone emission spectroscopy of HF and DF: Vibrational matrix elements and dipole moment function

Measurements of the emission intensities for overtone bands of HF and DF from a chemical laser source have been used to determine the dipole moment function for the ground electronic states of HF and DF for internuclear separations from 1.25–3.1 bohr. Vibrational matrix elements and Einstein coefficients have been determined for the fundamental through fifth overtone bands for all transitions from upper vibrational levels below v=10 for HF and v=13 for DF. The results are compared with recent ab initio and semiempirical dipole moment calculations.

Photoionization mass spectrometer for studies of flame chemistry with a synchrotron light source

A flame-sampling molecular-beam photoionization mass spectrometer, recently designed and constructed for use with a synchrotron-radiation light source, provides significant improvements over previous molecular-beam mass spectrometers that have employed either electron-impact ionization or vacuum ultraviolet laser photoionization. These include superior signal-to-noise ratio, soft ionization, and photon energies easily and precisely tunable [E∕ΔE(FWHM)≈250–400] over the 7.8–17-eV range required for quantitative measurements of the concentrations and isomeric compositions of flame species. Mass resolution of the time-of-flight mass spectrometer is m∕Δm=400 and sensitivity reaches ppm levels. The design of the instrument and its advantages for studies of flame chemistry are discussed.

“Imaging” combustion chemistry via multiplexed synchrotron-photoionization mass spectrometry

The combination of multiplexed mass spectrometry with photoionization by tunable-synchrotron radiation has proved to be a powerful tool to investigate elementary reaction kinetics and the chemistry of low-pressure flames. In both of these applications, multiple-mass detection and the ease of tunability of synchrotron radiation make it possible to acquire full sets of data as a function of mass, photon energy, and of the physical dimension of the system, e.g. distance from the burner or time after reaction initiation. The data are in essence an indirect image of the chemistry. The data can be quantitatively correlated and integrated along any of several dimensions to compare to traditional measurements such as time or distance profiles of individual chemical species, but it can also be directly interpreted in image form. This perspective offers an overview of flame chemistry and chemical kinetics measurements that combine tunable photoionization with multiple-mass detection, emphasizing the overall insight that can be gained from multidimensional data on these systems. The low-pressure flame apparatus is capable of providing isomer-resolved mass spectra of stable and radical species as a function of position in the flame. The overall chemical structure of the flames can be readily seen from images of the evolving mass spectrum as distance from the burner increases, with isomer-specific information given in images of the photoionization efficiency. Several flames are compared in this manner, with a focus on identification of global differences in fuel-decomposition and soot-formation pathways. Differences in the chemistry of flames of isomeric fuels can be discerned. The application of multiplexed synchrotron photoionization to elementary reaction kinetics permits identification of time-resolved isomeric composition in reacting systems. The power of this technique is illustrated by the separation of direct and dissociative ionization signals in the reaction of C(2)H(5) with O(2); by the resolution of isomeric products in reactions of the ethynyl (C(2)H) radical; and by preliminary observation of branching to methyl + propargyl products in the self-reaction of vinyl radicals. Finally, prospects for future research using multiplexed photoionization mass spectrometry are explored.

Synchrotron photoionization measurements of combustion intermediates: Photoionization efficiency and identification of C3H2 isomers

Photoionization mass spectrometry using tunable vacuum-ultraviolet synchrotron radiation is applied to the study of C3H2 Sampled from a rich cyclopentene flame. The photoionization efficiency has been measured between 8.5 eV and 11.0 eV. Franck-Condon factors for photoionization are calculated from B3LYP/ 6-311++-G(d,p) characterizations of the neutral and cation of the two lowest-energy C3H2 isomers, triplet propargylene (HCCCH, prop-2-ynylidene) and singlet cyclopropenylidene (cyclo-HCCCH). Comparison of the calculated Franck-Condon envelopes with the experimental photoionization efficiency spectrum determines the adiabatic ionization energy of triplet propargylene to be (8.96 +/- 0.04) eV. Ionization energies for cyclopropenylidene, propargylene and propadienylidene (H2CCC) calculated using QCISD(T) with triple-zeta and quadruple-zeta basis sets extrapolated to the infinite basis set limit are in excellent agreement with the present determination of the ionization energy for propargylene and with literature values for cyclopropenylidene and propadienylidene. The results suggest the presence of both propargylene and cyclopropenylidene in the cyclopentene flame and allow reanalysis of electron ionization measurements of C3H2 in other flames. Possible chemical pathways for C3H2 formation in these flames are briefly discussed.

Kinetic model for the decomposition of DMMP in a hydrogen/oxygen flame

Abstract A kinetic model of the combustion chemistry of a hydrogen/oxygen base flame, doped with dimethyl methylphosphonate, a useful simulant for chemical warfare agents (CWAs), has been developed to assist in the controlled thermal destruction of CWA stockpiles. Laser-ionization mass spectrometry is employed to record concentration profiles of radical intermediates in a low-pressure premixed laminar flame. These measurements, combined with ab initio estimates of thermochemical properties of organophosphorus compounds, lead to a kinetic model incorporating several key reaction intermediates, which include methyl metaphosphate CH 3 OPO 2 , methyl dioxophosphorane CH 3 PO 2 , and monomethyl methylphosphonate PO(OH)(CH 3 )(OCH 3 ).

Isomer-specific influences on the composition of reaction intermediates in dimethyl ether/propene and ethanol/propene flame.

This work provides experimental evidence on how the molecular compositions of fuel-rich low-pressure premixed flames are influenced as the oxygenates dimethyl ether (DME) or ethanol are incrementally blended into the propene fuel. Ten different flames with a carbon-to-oxygen ratio of 0.5, ranging from 100% propene (phi = 1.5) to 100% oxygenated fuel (phi = 2.0), are analyzed with flame-sampling molecular-beam mass spectrometry employing electron- or photoionization. Absolute mole fraction profiles for flame species with masses ranging from m/z = 2 (H2) to m/z = 80 (C6H8) are analyzed with particular emphasis on the formation of harmful emissions. Fuel-specific destruction pathways, likely to be initiated by hydrogen abstraction, appear to lead to benzene from propene combustion and to formaldehyde and acetaldehyde through DME and ethanol combustion, respectively. While the concentration of acetaldehyde increases 10-fold as propene is substituted by ethanol, it decreases as propene is replaced with DME. In contrast, the formaldehyde concentration rises only slightly with ethanol replacement but increases markedly with addition of DME. Allyl and propargyl radicals, the dominant precursors for benzene formation, are likely to be produced directly from propene decomposition or via allene and propyne. Benzene formation through propargyl radicals formed via unsaturated C2 intermediates in the decomposition of DME and ethanol is negligibly small. As a consequence, DME and ethanol addition lead to similar reductions of the benzene concentration.

Vibrational Energy Transfer and De‐excitation in the HF, DF, HF–CO2, and DF–CO2 Systems

The laser excited fluorescence method has been employed to determine the key rate constants for energy transfer and deactivation processes in the HF, DF, HF–CO2, and DF–CO2 chemical laser systems at a temperature of 350°K. The self‐deactivation rates for HF (v=1) and DF(v=1) molecules by ground state molecules were found to be kHF–HF=5.25 ± 0.30 × 104 sec−1 · torr−1 and kDF–DF = 2.0 ± 0.2 × 104 sec−1 · torr−1, respectively. The measured rates of V → V transfer from HF(v=1) and DF(v=1) to the CO2(0001) state were kHF–CO2 = 3.7 ± 0.3 × 104 sec−1 · torr−1 and kDF–CO2 = 17.5 ± 2.5 × 104 sec−1 · torr−1. The respective deactivation rates of CO2(0001) by ground state HF and DF were determined to be kCO2–HF = 3.6 ± 0.3 × 104 sec−1 · torr−1 and kCO2–DF = 1.9 ± 0.4 × 104 sec−1 · torr−1. The large rates for these processes can be attributed to energy transfer to rotation under the influence of a sizable attractive (hydrogen bonded) intermolecular potential well and enhanced repulsion at close range.

Two‐photon spectroscopy of Rydberg states of jet‐cooled C2H4 and C2D4

Spectroscopic studies of two‐photon resonant vibronic bands of the (π,3s), (π,3p), and (π,nf ) (n=4–18) Rydberg states of jet‐cooled C2H4 and C2D4 are reported. New vibronic bands are resolved by these low temperature (<10 K) observations and previous vibronic assignments of two‐photon 3s and 3p transitions are confirmed. A new vibronic progression is observed in the 3s spectrum, tentatively assigned to two‐photon allowed 1B2g←1Ag transitions, vibronically induced via excitation of the b1u ν6 CH2 antisymmetric scissors mode. Polarization ratio measurements and identification of the previously unreported 3pσ origin band show that the energies of the 3p levels are in good agreement with theoretical predictions. Higher‐lying members of the s‐ and p‐Rydberg series are not observed, suggestive of the onset of strong predissociation. These first observations of the two‐photon allowed nf‐Rydberg series yield ionization potential estimates of 84 799±5 cm−1 for C2H4 and 84 918±5 cm−1 for C2D4 that compare favorabl...