Aromatics Oxidation and Soot Formation in Flames [report]

J. B. Howard, H. Richter
2005 unpublished
Scope This project is concerned with the kinetics and mechanisms of aromatics oxidation and the growth process to polycyclic aromatic hydrocarbons (PAH) of increasing size, soot and fullerenes formation in flames. The overall objective of the experimental aromatics oxidation work is to extend the set of available data by measuring concentration profiles for decomposition intermediates such as phenyl, cyclopentadienyl, phenoxy or indenyl radicals which could not be measured with molecular-beam
more » ... ss spectrometry to permit further refinement and testing of benzene oxidation mechanisms. The focus includes PAH radicals which are thought to play a major role in the soot formation process while their concentrations are in many cases too low to permit measurement with conventional mass spectrometry. The radical species measurements are used in critical testing and improvement of a kinetic model describing benzene oxidation and PAH growth. Thermodynamic property data of selected species are determined computationally, for instance using density functional theory (DFT). Potential energy surfaces are explored in order to identify additional reaction pathways. The ultimate goal is to understand the conversion of high molecular weight compounds to nascent soot particles, to assess the roles of planar and curved PAH and relationships between soot and fullerenes formation. The specific aims are to characterize both the high molecular weight compounds involved in the nucleation of soot particles and the structure of soot including internal nanoscale features indicative of contributions of planar and/or curved PAH to particle inception. Recent Progress A) Experimental Experimental work assessing PAH reaction pathways has been finished. Mole fraction profiles of PAH radicals and the corresponding parent molecules were determined using nozzle beam sampling followed by radical scavenging reactions with dimethyl disulfide. Scavening products, i.e., methylthio compounds (R-SCH 3 ) and stable PAH were analysed by gas chromatography coupled with mass spectrometry (GC-MS). PAH isomers including specific radical sites were unambiguously identified using authentic standards, commercially available or synthesized in the present work. Two benzene/oxygen/argon flames (30% argon, cold gas velocity v= 50 cm s -1 , 20 torr) which were nearly sooting or slightly sooting with equivalence ratios of φ= 1.8 and 2.0, respectively, were investigated. All PAH profiles in the φ= 2.0 flame were shifted by ≈ 2 mm towards the burned gases compared to the φ= 1.8 flame. No increase of peak mole fractions was observed for one-and two-ring aromatics while an at least 2-fold increase was found for larger PAH. Experimental uncertainties were assessed by means of control experiments using equipment developed and built by Homann and coworkers, including combustion chamber, sampling and scavenging equipment. Shape and location of all measured compounds were very similar in both experiments while peak mole fractions were mostly lower in the control experiments. The results of the comparison ranged from a nearly perfect match such as for indene to ≈ 2 -5-fold discrepancies. Larger discrepancies, observed in a few cases such as for 5ethynylacenaphthylene, were attributed to possible experimental errors, e.g., incomplete peak separation in the GC analysis. As a general trend, partial equilibrium of (8) PAH + H PAH• + H 2 is more and more closely approached with increasing size of the molecule. The experimental 2-naphthyl : phenyl mole fraction ratio was found to exceed the partial equilibrium value. Therefore, decomposition of PAH radicals forming butadiyne, e.g., the reverse of reaction (10) phenyl + C 4 H 2 2-naphthyl is suggested as contributing to the consumption of some PAH radicals. Thermodynamic considerations were found to be important but not sufficient for a complete understanding of PAH formation and consumption. Background, data, their analysis and conclusion are reported below.
doi:10.2172/838109 fatcat:rg46n2b7nrc4ne3wkof2f2xr6m