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Detection of Trace Hydrocarbons in Flames Using Direct Sampling Mass Spectrometry Coupled with Multilinear Regression Analysis

Maria A. Puccio, and * J. Houston Miller

The George Washington University, 725 21st Street Washington DC, 20052

mpuccio@gwu.edu, houston@gwu.edu

A technique for the determination of species concentrations from the molecular growth regions of flames is presented. Samples are obtained by microprobe extraction from a nitrogen-diluted methane/air, non-premixed laminar flame supported on a co-annular burner. Quantification of measurements was accomplished by doping the flames fuel flow with argon at a level to match that in the laboratorys air. A library of 70 eV fragmentation patterns for several flame species was used in conjunction with a simplex algorithm to analyze mass spectra obtained at each flame location. Each fragmentation pattern was normalized for its integrated intensity and its ionization cross-section relative to argon. This technique provided sub part-per-million sensitivity of a large range of major and minor carbon-containing species ranging in size from C2 to C12 hydrocarbons. This flame could be forced to oscillate at a frequency emulating natural flame flickering behavior. Time-resolved measurements were obtained using a modified quartz microprobe synchronized to open and close with the flame oscillations. The near real-time sampling and analysis time and the relatively high sensitivity make this technique preferable to other extractive approaches of comparable flame measurements.

INTRODUCTION

The reduction of particulate carbon emissions is a key focus of current research in combustion science. Airborne particulate matter (PM) has been found to produce severe health effects, including increased mortality rates1, 2 increased risk of cardiorespiratory disease,3 a reduced lifespan for residents of highly polluted urban areas,4 and has been linked with cancer.5 In addition, recent reports have indicated that soot in the atmosphere, also known as black carbon, is a key climate forcer and thus plays a significant role in global warming.6-10

The primary source of soot particulate is the inefficient combustion that occurs for rich stochiometries or when mixing is not optimized. The goal of combustion research ultimately is an improvement of combustion processes through a better understanding of both chemistry and fluid mechanics, including soot inception and growth. A common strategy to meet this goal is the development of codes for the direct numerical simulation of combustion processes validated (and informed) by laboratory measurements in well defined systems. These studies should include not only flame structure measurements such as temperature and velocity fields, major species measurements, and total soot yield, but also an analysis of trace species concentrations.

Combustion Diagnostics for Trace Hydrocarbons. Polynuclear aromatic hydrocarbons (PAH) are often proposed to be the chemical intermediates between fuel and soot. Particularly at flame temperatures, PAH have broad absorption and fluorescence optical bands. Thus, although broadband, laser-induced fluorescence attributed to PAH has been observed in the sooting regions of flames,11-13 assignment of these features to specific PAH cannot be made. Non-optical methods of PAH analysis include microprobe extraction followed by gas chromatography mass spectrometry (GC/MS), resonance enhanced multiphoton ionization mass spectrometry (REMPI-TOFMS), single photon ionization mass spectrometry (SPI-TOFMS), molecular beam mass spectrometry (MBMS), and electron impact mass spectrometry. While this paper will focus on PAH analysis by electron impact mass spectrometry, a brief review of the advantages and disadvantages of similar techniques is valuable and is included below.

In reports over the past several decades, microprobe extraction followed by gas chromatography/mass spectrometry (GC/MS) has been used to examine PAH in flames.14-16 The main advantage of this technique is that GC/MS is able to differentiate individual isomers/congeners of PAH compounds. However, GC/MS analysis requires large amounts of sample (2 to 60 L of sample gas16), long analysis times (5 to 60 min14, 16), and may require extensive sample preparation such as filtering, drying, and chemical or thermal treatment of the extracted gas sample. Online gas chromatography that does not require sample preparation has been used to obtain PAH concentrations; however, heavier hydrocarbons with long elution times have not been fully resolved.15

Gittins and co-workers used resonance-enhanced multiphoton ionization time-of-flight mass spectrometry (REMPI-TOFMS) combined with standard addition methods to provide real time PAH concentrations in a methane + oxygen/argon flame.17 In REMPI, one or more photons with energy less than that required to ionize a molecule, raises the molecule to an excited electronic state. Additional photons then ionize the excited molecules. Because these are nonlinear techniques, signal levels are highly dependent on laser fluence and thus are sensitive to pulse-to-pulse intensity variation, particularly for species with similar ground and excited state cross-sections.18 Furthermore, the strongest signals are obtained when electronically excited states are accessed that have high Franck Condon overlap with both the ground state and the ion states (such as Rydberg levels). For these reasons, detection limits vary widely by species and the use of PAH standards is required for quantification. In addition, laser pulse energies that are too high may lead to absorption of a third photon and consequently to fragmentation.18 However, when combined with mass analysis of photo-ions (i.e., REMPI-TOFMS) REMPI is a very sensitive technique and detection limits as low as 10 ppbV have been observed.17

McEnally and co-workers used a custom-built single-photon photoionization mass spectrometer to obtain species concentrations from a methane/air, non-premixed flame.19 In SPI-TOFMS, a laser is used whose energy is strong enough to ionize the molecules, yet not high enough in energy to fragment the parent ions.19 SPI-TOFMS can suffer from electron-impact ionization from high energy photoelectrons produced in the ion extraction region.20 These photoelectrons are not dependent on the kinetic energy of incident photoelectrons and can result in multiply-charged ions, fragmentation, and may contribute to background noise in the spectrum. In addition, premature ion leakage into the acceleration region can increase noise in the spectra. Huang and coworkers20 found that by applying a small bias voltage to the spectrometer they could reduce these effects, but this must be done with care. A bias voltage too high will reduce signal levels by reducing ion density in the extraction region. Nevertheless, the SPI-TOFMS is a highly sensitive technique that provides part-per-million sensitivity for most C3-C12 hydrocarbons.19

Recently, synchrotron radiation from an advanced light source (ALS) has been coupled with molecular beam mass spectrometry (MBMS) for the analysis of hydrocarbons ranging in mass from 2 to 78 Dalton.20-23 This technique offers all of the advantages of MBMS, including identification of most species by molecular weight and ionization energy and quantification of a wide range of stable and radical species. In previous MBMS studies, an isomer pair needed threshold ionization energies at least 2 eV apart to distinguish them. With the addition of the ALS, resolution is improved to 0.1 eV. This has allowed analysis of specific isomers never before resolved in combustion systems, such as an entire class of molecules, the enols,21 and ethyl-methyl-ether.23 Unfortunately, this technique has only been applied to the study of low pressure flames.

All of the above techniques involve soft ionization processes to minimize fragmentation. In the past, our laboratory has used electron energies close to the ionization threshold ( 30 > 40 > 50).36 Peak methane concentrations follow the same trend as the exit velocity. Methane concentrations are lowest at the phase of the flame cycle with the lowest exit velocity and increase as velocity increases. They are largest at 20 ms and decrease as velocity decreases. This can be thought of intuitively as the fuel being drawn into the burner during decreasing velocity phases of the flame cycle and being forced outward during increasing velocity phases.

Benzene and other PAH follow a distinctly different pattern than the fuel and exit velocity. Maximum benzene concentrations are seen at the 50 ms phase where velocity and methane concentrations are lowest. At this phase, benzene peaks at ~38 mm HAB, 12 mm further downstream and at 1.5 times a larger maximum concentration than in the steady flame. Benzene also has a larger spatial distribution at this phase. As exit velocity increases, benzene concentrations decrease. There is a sudden drop in the benzene level between the 50 and 10 ms phases. Benzene levels then begin to increase with decreasing exit velocities.

The time lag between velocity and scalar quantities has been observed in previous time-dependent measurements.27, 36 This can be understood by walking through what happens to a packet of fuel during the course of a flame cycle. During the oscillation, increased fuel flow produces a large packet of fuel. As this packet reaches the top of the flame, the flow rate begins to slow as fuel is drawn back into the fuel tube during the minimum fuel velocity phase of the flame cycle. This leads to conditions not normally seen in steady state combustion. For instance, at the 50 ms phase of the flame cycle there are large benzene concentrations well within the high temperature reaction zone of the flame. The increased residence times of benzene and other PAH in molecular growth regions may help to explain the increased soot yields observed in time-dependent flames.31

Finally, note that the benzene distribution pinches inward at certain phases of the flame cycle. When the flame is forced, shear development between the cooler, slower moving oxidizer, and hotter, faster moving flame gases causes the formation of vortical structures in the flame.33 If the forcing is great enough, parts of the flame will extinguish, resulting in a detached flame tip. This tip-clipping phenomenon is captured in the bi-modal distribution of the benzene centerline measurements in the 10, 20, and 30 ms phases of the flame cycle.

CONCLUSIONS

In this work, concentration isopleths were presented for major species and several trace hydrocarbons. These contours can give information not available from centerline measurements, such as the decrease of spatial extent in the flame of trace hydrocarbon concentrations with increasing hydrocarbon size.

In addition, measurements were present from a time-dependent or flickering flame. These flames can give insights into turbulent combustion by presenting conditions not seen in a steady flame, yet in a reproducible flame cycle that can be measured by extractive techniques. In particular, increased benzene concentrations further downstream and at hotter regions of the flame at certain phases of the flame cycle may have a role in the increased soot production observed in these flames.31

For studying the difficult range between small molecules such as fuels and large molecules such as soot, microprobe extraction techniques are the preferred method. Current extraction/mass spectrometric methods may be too time-consuming or complex to obtain the vast number of measurements that are needed to fully resolve a two dimensional oscillating flame in the vertical, horizontal, and time dimensions. The technique described in this paper can be used as an additional tool to aid in such endeavors. The advantages of this technique lie in its simplicity and speed of data acquisition. However, the use of this technique for studying flame systems requires an internal standard whose cross section, and more importantly whose fragmentation pattern, must be known for accurate measurements. In addition, there must be a thorough list of expected species that make up the sample to be analyzed; otherwise, model fit will be reduced.

In conclusion, the technique described in this paper gives rapid measurements of combustion systems with detection limits on the order of parts-per-thousand for major species and parts-per-million for minor species and may be used to complement current methods of PAH analysis for the study of complex combustion systems.

ACKNOWLEDGMENT (Word Style TD_Acknowledgments). Partial funding of this work was provided by the National Science Foundation (Grant 0330230).

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