An official website of the United States government
We perform spatially resolved measurements of light scattering of soot in atmospheric pressure counterflow diffusion flames to complement previously reported data on soot pyrometry, temperature and gaseous species up to three-ring polycyclic aromatic hydrocarbons (PAHs). We compare two flames: a baseline ethylene flame and a toluene-seeded flame in which an aliquot of ethylene in the feed stream is replaced with 3500 ppm of pre-vaporized toluene. The goal is twofold: directly adding an aromatic fuel to bypass the formation of the first aromatic ring, widely regarded as the main bottleneck to soot formation from aliphatic fuels, and assessing the impact of a common component of surrogates of transportation fuels on soot formation. The composition of the fuel and oxidizer streams are adjusted to ensure invariance of the temperature-time history, thereby decoupling the chemical effects of the fuel substitution from other factors. The doping approach enables the comparison of very similar flames with respect to combustion products, radicals and critical precursors to aromatic formation (C2–C5 species), in addition to the temperature-time history. Doping with toluene boosts the aromatic content and soot volume fraction relative to the baseline ethylene flame, but, surprisingly, the soot number density and nucleation rate are affected modestly. As a result, the observed difference in volume fraction in the toluene-doped flame is reflective of larger initial particles at the onset of soot nucleation. The nucleation rate when soot first appears near the flame is of the same order as the dimerization rate of single-ring aromatics, in contrast with the expectation that the dimerization of larger PAHs initiates the process. Even though in and of itself nucleation contributes modestly to the overall soot loading, nucleation conditions the overall soot loading by affecting the size of the initial particle, which ultimately affects subsequent growth.
more »
« less
Detection of weakly bound clusters in incipiently sooting flames via ion seeded dilution and collision charging for (APi-TOF) mass spectrometry analysis
This study introduces an atmospheric pressure chemical ionization method that relies on low-energy thermal collisions (i.e., <0.05 eV) of aerosolized analytes with bipolar ions pre-seeded in a sample dilution flow and allows for the detection of weakly bound molecular clusters. Herein, the potential of the method is explored in the context of soot inception by performing mass spectrometric analysis of a laminar premixed flame of ethylene and air whose products are sampled through a tiny orifice and quickly diluted in nitrogen pre-flowed through a Kr85 based neutralizer to generate the bipolar ions. Analyses were performed with an Atmospheric Pressure Interface Time-of-Flight (APi-TOF, Tofwerk AG) Mass Spectrometer whose high sensitivity, mass accuracy, and resolution (over 4000) allowed for the discrimination of the flame products from the pre-seeded ions. Since ionization of neutrals occurs by either ion attachment or charge exchange following ion collision, the identification of the origin of each peak in the measured mass spectra is not-trivial. Nevertheless, the results provide valuable information on the overall elemental composition of the neutral flame products ionized in either polarity. Results show that the clustering of hydrocarbons lighter than 400 Da and having a C/H ratio between 2 and 3 leads to soot inception in the flame. The dehydrogenation of the flame products, expected to occur as they are convected in the flame, is observed only for measurements in positive polarity because of a higher probability of soot nuclei and precursors to get a positive rather than a negative charge.
more »
« less
- Award ID(s):
- 2013382
- PAR ID:
- 10225575
- Editor(s):
- Suuberg, Eric
- Date Published:
- Journal Name:
- Fuel
- Volume:
- 289
- ISSN:
- 2612-6958
- Page Range / eLocation ID:
- 119820
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
We perform spatially resolved measurements of temperature, gaseous species up to three-ring Polycyclic Aromatic Hydrocarbons (PAHs), and soot in atmospheric pressure counterflow diffusion flames. First, we characterize fully a baseline ethylene flame and then a toluene-seeded flame in which an aliquot of ethylene in the feed stream is replaced with 3500 ppm of prevaporized toluene. The goal is twofold: to investigate the impact of a common reference fuel component of surrogates of transportation fuels and bypass the main bottleneck to soot formation from aliphatic fuels, that is, the formation of the first aromatic ring. The composition of the fuel and oxidizer streams are adjusted to maintain a constant stoichiometric mixture fraction and global strain rate, thereby ensuring invariance of the temperature–time history in the comparison between the two flames and decoupling the chemical effects of the fuel substitution from other factors. Major combustion products and critical radicals are fixed by the baseline flame, and profiles of critical C2–C5 species precursors to aromatic formation are invariant in both flames. On the other hand, doping with toluene boosts the aromatic content and soot volume fraction, increasing the mole fraction of benzenoid structures and soot volume fraction by a factor of 2 or 3, relative to the baseline ethylene flame. This finding is consistent with the expectation that the formation of the first aromatic ring is no longer a bottleneck to soot formation in the doped flame. In addition, toluene bypasses completely benzene formation, opening a radical recombination pathway to soot precursors through the production of C14H14 (via dimerization of benzyl radical) and pyrene (through dimerization of indenyl radical).more » « less
-
On the heels of a recent study in an atmospheric pressure ethylene diffusion flame in which the transition from parent fuel molecule to Polycyclic Aromatic Hydrocarbons (PAHs) and, eventually, soot was studied by spatially resolved measurements of PAH concentrations and soot quantities, we extended the focus to diffusion flames with self- similar structure in the 0.101–0.811 MPa pressure range. To that end, we complemented pyrometry based measurements of soot volume fraction with light scattering measurements that, once corrected for beam steering, yielded soot particle size and number concentration profiles. A chemistry model, that had been validated for all species up to 3 ring aromatics in one of the flames investigated at each pressure and up to 4-rings for the flame at atmospheric pressure, was used to compare profiles of chemical species to those of soot quantities. Further analysis yielded the assessment of number nucleation rates of soot and their comparison to the dimerization rates of PAHs. Soot nucleation rate is consistent only with the dimerization of one- and two-ring PAHs, an observation that confirms findings in the atmospheric pressure flame. Changes in pressure and temperature have a progressively larger impact on the concentration of aromatics of increasingly larger molecular weight and, even more so, on soot volume fraction and nucleation rate. A four-fold increase in pressure from 0.101 MPa to 0.405 MPa increases the soot nucleation rate and PAH dimerization rate in flames with constant peak temperature, which is primarily a concentration effect on bimolecular collision rates; a similar but less pronounced effect is observed in the higher (0.405–0.811 MPa) pressure range. Changes in pressure and temperature tend to be progressively more consequential on aromatics of increasing molecular weight and soot.more » « less
-
The growing use of fluorochemicals has elevated the need for non-targeted detection of unknown fluorinated compounds and transformation products. Elemental mass spectrometry coupled to chromatography offers a facile approach for such analyses by using fluorine as an elemental tag. However, efficient ionization of fluorine has been an ongoing challenge. Here, we demonstrate a novel atmospheric-pressure elemental ionization method where fluorinated compounds separated by GC are converted to Na2F+ for non-targeted detection. The compounds are first introduced into a helium dielectric barrier discharge (DBD) for breakdown. The plasma products are subsequently ionized by interaction with a nano-ESI plume of sodium-containing aqueous electrolytes. Our studies point to HF as the main plasma product contributing to Na2F+ formation. Moreover, the results reveal that Na2F+ is largely formed by the ion-neutral reaction between HF and Na2A(NaA)n+, gas-phase reagent ions produced by nano-ESI where A represents the anion of the electrolyte. Near-uniform fluorine response factors are obtained for a wide range of compounds, highlighting good efficiency of HF formation by DBD regardless of chemical structure of the compounds. Detection limits of 3.5 to 19.4 pg fluorine on-column are obtained using the reported GC-DBD-nano-ESI-MS. As an example of non-targeted screening, extractions from oil-and-water-repellent fabrics are analyzed via monitoring Na2F+, resulting in detection of a fluorinated compound on a clothing item. Notably, facile switching of the ion source to atmospheric-pressure chemical ionization with the exact same chromatographic method allows identification of the detected compound at the flagged retention time.more » « less
-
The study of fuel chemistry and soot inception in non-premixed combustion can be advanced by characterizing flame configurations in which the advection and diffusion transport can be finely controlled, with the ability to decouple pyrolysis from oxidation. Also, the ideal flames to be investigated should be perturbed minimally by probes and thick enough for sampling techniques to yield spatially resolved measurements of their structure. The Planar Mixing Layer Flame (PMLF) configuration introduced herein is established between a fuel and an oxidizer slot jet adjacent to each other and shielded from the ambient air by annularly co-flowing inert nitrogen. The PMLF flow is kept laminar and steady by an impinging flat plate equipped with a rectangular exhaust slit opening which anchors the position of the hot combustion products via buoyancy. The PMLF is accessible to sampling and its flow stability is preserved when using any tested probe. The experiments are complemented with 2DComputational Fluid Dynamics (CFD) modeling with detailed chemical kinetics. The results demonstrate that the PMLF has a self-similar boundary layer structure whose horizontal cross-sections are equivalent to properly selected and equally thick 1D- Counterflow Flames (CFs). The equivalence allows for excellent predictions of the PMLF thermochemical structure characterized experimentally but at a small fraction of the 2D-CFD computational cost. The 1D-CF equivalence affects even aromatics less than twofold despite their kinetics being known to be very sensitive to the temperature field. Importantly, the PMLF thickness is several millimeters and grows at increasing HABs so that the equivalent 1D-CFs have strain rates as small as 7.0 /s which cannot be studied in CF experiments. As a result, the PMLF emerges as a promising canonical non-premixed flame configuration for studying flame chemistry and soot inception on time scales of tens of milliseconds typical of many combustion applications.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/https://doi.org/10.1016/j.fuel.2020.119820
