Phillip R. Westmoreland

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.

IUPAC Critical Evaluation of Thermochemical Properties of Selected Radicals. Part I

This is the first part of a series of articles reporting critically evaluated thermochemical properties of selected free radicals. The present article contains datasheets for 11 radicals: CH, CH2(triplet), CH2(singlet), CH3, CH2OH, CH3O, CH3CO, C2H5O, C6H5CH2, OH, and NH2. The thermochemical properties discussed are the enthalpy of formation, as well as the heat capacity, integrated heat capacity, and entropy of the radicals. One distinguishing feature of the present evaluation is the systematic utilization of available kinetic, spectroscopic and ion thermochemical data as well as high-level theoretical results.

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.

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.

Thermochemical and chemical kinetic data for fluorinated hydrocarbons

A comprehensive, detailed chemical kinetic mechanism was developed and is presented for C1 and C2 fluorinated hydrocarbon destruction and flame suppression. Existing fluorinated hydrocarbon thermochemistry and kinetics were compiled from the literature and evaluated. For species where no or incomplete thermochemistry was available, these data were calculated through application of ab initio molecular orbital theory. Group additivity values were determined consistent with experimental and ab initio data. For reactions where no or limited kinetics were available, these data were estimated by analogy to hydrocarbon reactions, by using empirical relationships from other fluorinated hydrocarbon reactions, by ab initio transition state calculations, and by application of RRKM and QRRK methods. The chemistry was modeled considering different transport conditions (plug flow, premixed flame, opposed flow diffusion flame) and using different fuels (methane, ethylene), equivalence ratios, agents (fluoromethanes, fluoroethanes) and agent concentrations. This report provides a compilation and analysis of the thermochemical and chemical kinetic data used in this work.

“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.

Measured flame structure and kinetics in a fuel-rich ethylene flame

Abstract Molecular-beam mass spectrometry (MBMS) has been used to map the structure of a fuel-rich C 2 H 4 /O 2 flame and infer new C 2 H 4 and C 2 H 3 kinetics. Axial profiles of concentration, area expansion ratio, and temperature were measured for a C 2 H 4 /O 2 /50% Ar (φ = 1.90) flame at 2.667 ± 0.001 kPa (20 Torr) and 62.5 cm/s burner velocity (300 K). Full concentration profiles were mapped for 42 radical and stable species. Elemental flux balances were within 12%, supporting the data’s accuracy and validity. Species flux-balance calculations were used to obtain net rates of reaction for the species. Rate constants were determined for H-abstraction from C 2 H 4 by H at 1850–2150 K, and their agreement with theory and previous lower-temperature data leads to recommendation of the ab initio /BAC-MP4 result: C 2 H 4 + H = C 2 H 3 + H 2 k = 4.49 × 10 7 × T 2.12 exp (−13,366/ RT ) in cm, mol, s, cal, K units. Data for abstraction by OH, combined with literature data, give: C 2 H 4 + OH = C 2 H 3 + H 2 O k = (5.53 ± 0.14) × 10 5 × T (2.310 ± 0.004) exp [−(2900 ± 60)/ RT ] for temperatures between 1400 and 1800 K. Rate constants for vinyl decomposition reaction C 2 H 3 = C 2 H 2 + H were analyzed, supporting the recent recommendations of Knyazev and Slagle [41] .

A reactive molecular dynamics model of thermal decomposition in polymers: I. Poly(methyl methacrylate)

The theory and implementation of reactive molecular dynamics (RMD) are presented. The capabilities of RMD and its potential use as a tool for investigating the mechanisms of thermal transformations in materials are demonstrated by presenting results from simulations of the thermal degradation of poly(methyl methacrylate) (PMMA). While it is known that depolymerization must be the major decomposition channel for PMMA, there are unanswered questions about the nature of the initiation reaction and the relative reactivities of the tertiary and primary radicals formed in the degradation process. The results of our RMD simulations, performed directly in the condensed phase, are consistent with available experimental information. They also provide new insights into the mechanism of the thermally induced conversion of this polymer into its constituent monomers.

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.

Concerted Reactions and Mechanism of Glucose Pyrolysis and Implications for Cellulose Kinetics

Concerted reactions are proposed to be keys to understanding thermal decomposition of glucose in the absence of ionic chemistry, including molecular catalysis by ROH molecules such as H(2)O, other glucose molecules, and most of the intermediates and products. Concerted transition states, elementary-reaction pathways, and rate coefficients are computed for pyrolysis of β-D-glucose (β-D-glucopyranose), the monomer of cellulose, and for related molecules, giving an improved and elementary-reaction interpretation of the reaction network proposed by Sanders et al. (J. Anal. Appl. Pyrolysis, 2003, 66, 29-50). Reactions for ring-opening and formation, ring contraction, retro-aldol condensation, keto-enol tautomerization, and dehydration are included. The dehydration reactions are focused on bicyclic ring formations that lead to levoglucosan and 1,6-β-D-anhydrousglucofuranose. The bimolecular ROH-assisted reactions are found to have lower activation energy compared to the unimolecular reactions. The same dehydration reaction to levoglucosan should occur for cellulose going to cellosan (e.g., cellotriosan) plus a shortened cellulose chain, a hypothesis supported by the very similar activation energies computed when alternate groups were substituted at the C1 glycosidic oxygen. The principles of Sanders et al. that distinguish D-glucose, D-fructose, sucrose, and cellulose pyrolysis prove useful in providing qualitative insights into cellulose pyrolysis.