Direct sampling mass spectrometry for the study of polynuclear aromatic hydrocarbons and other combustion intermediates and products.

摘要

The goal of this work is the analysis of combustion intermediates and products from non-premixed flames, in particular polynuclear aromatic hydrocarbons (PAH). These molecules have often been proposed as key intermediates in the pathways that lead from fuel to soot formation. Due to their large size, these species have broad absorption and fluorescence bands and individual PAH have not been optically quantified in flames. Generally, PAH and small hydrocarbons are examined through microprobe extraction from the flame followed by mass spectrometry. In the past, our laboratory has used microprobe extraction followed by electron impact mass spectrometry using 25 eV ionization energy. 1 Unfortunately, most atoms and molecules have low ionization efficiencies at this ionization energy. This reduces signal levels and key hydrocarbons and PAH, which often have concentrations at the ppm level, could not be quantified.;In this work, electron impact mass spectrometry (EI/MS) with an ionization energy of 70 eV was used. Most atoms and molecules have peak ionization efficiencies at 70 eV. This dramatically increases signal levels and even trace species such as naphthalene, which has peak concentrations on the order of 1 ppm in the systems studied in this work, could be quantified. In addition, 70 eV ionization energy is commonly used in mass spectral analysis and standard 70 eV ionization cross sections and fragmentation patterns are readily available in the NIST Standard Reference Database for most species.2 However, extensive fragmentation is associated with spectra collected from species ionized at this high of an ionization energy.;In order to deconvolute the spectra collected from the flame samples, we have developed a multilinear regression procedure that fits a simulated spectrum to the experimental spectrum to obtain species concentrations. The procedure uses a simplex algorithm for parameter optimization. The simulated spectrum is created using the ionization cross sections and fragmentation patterns of a select list of species expected to be found in the combustion systems studied here. The species concentrations are quantified using their signal relative to argon which is added to the fuel stream in the same concentration present in the laboratory air.;The microprobe extraction 70 eV EI/MS technique was employed to examine a host of combustion systems. These studies were mainly exploratory in nature and many represent the first measurements of trace hydrocarbons and PAH in the combustion systems examined.;The goal of these studies is two-fold. The first is to characterize base-case (unperturbed) flames through the measurement of major and minor species concentrations. The base-case systems studied in this work, a methane flame formed on a Wolfhard-Parker burner, a methane flame formed on a Santoro burner, and a 65% methane flame formed on a burner developed in collaboration with Yale University, were all chosen based on the availability of previously acquired major species concentrations and temperature data in the combustion literature. This provided an opportunity to validate the EI/MS technique and to add to the species concentration data available for these systems. For instance, Chapter 5 shows that the EI/MS measurements from the 65% methane flame are comparable to Raman measurements and the computational results of our collaborators from Yale University from the same flame system.;The second goal of these studies is to analyze the effects of perturbations to the fuel of the base-case flames. Chapter 6 explores time-dependent, flickering flames. These flames have combustion conditions not found in their steady counterparts. For instance, EI/MS measurements presented in Chapter 6 show regions of increased acetylene and PAH concentrations in the high temperature reaction zones of the flame at certain times in the course of the time-dependent flame cycle.;The last two chapters analyze the combustion of two different fuels which exhibit very different combustion behaviors. In a methane non-premixed flame, decomposition of the fuel is initiated through radical attack. In Chapter 7, combustion analysis of pyridine, a nitrogen-containing fuel found in coal, suggests that unlike methane, pyridine is most likely consumed through a combination of unimolecular and bimolecular processes under non-premixed conditions. Chapter 8 presents results from the combustion of methyl butanoate (MB), a molecule that has received much recent attention for its role as a surrogate in the analysis of biodiesel combustion. EI/MS analysis of an MB-doped flame reveals that decomposition of MB mainly occurs through unimolecular decomposition and not through radical attack. The molecularity of the mechanism through which a species decomposes can affect the types and the conditions under which combustion intermediates are formed. For instance, results presented in Chapter 8 show that the unimolecular decomposition of MB results in the formation of large amounts of radicals in relatively cool regions of the flame, promoting the formation of acetylene and PAH.