Polycyclic Aromatic Hydrocarbons In Flames Research Articles

PAH Growth in Flames and Space: Formation of the Phenalenyl Radical.

Polycyclic aromatic hydrocarbons (PAHs) are intermediates in the formation of soot particles and interstellar grains. However, their formation mechanisms in combustion and interstellar environments are not fully understood. The production of tricyclic PAHs and, in particular, the conversion of a PAH containing a five-membered ring to one with a six-membered ring are of interest to explain PAH abundances in combustion processes. In the present work, resonant ionization mass spectrometry in conjunction with isotopic labeling is used to investigate the formation of the phenalenyl radical from acenaphthylene and methane in an electrical discharge. We show that in this environment the CH cycloaddition mechanism converts a five-membered ring to a six-membered ring. This mechanism can occur in tandem with other PAH formation mechanisms such as hydrogen abstraction/acetylene addition (HACA) to produce larger PAHs in flames and the interstellar medium.

Fluorescence Spectra of Polycyclic Aromatic Hydrocarbons and Soot Concentration in Partially Premixed Flames of Diesel Surrogate Containing Oxygenated Additives

Partially premixed laminar flames were formed using our purpose-built burner. The soot reduction mechanism of blends of diesel and oxygenated fuel was explored. The mixture of toluene and n-heptane(volume ratio, 20:80)(T20) was used as a diesel surrogate. Methanol, ethanol, n-butanol, methyl butyrate, and2,5-dimethylfuran(DMF) were blended with T20, whilst retaining a 4% oxygen content. Laser-induced fluorescence(LIF) was used to obtain spatial fluorescence spectra of polycyclic aromatic hydrocarbons(PAHs)in partially premixed co-flow flames. Laser-induced incandescence(LII) was used to measure soot concentration(volume fraction). The formation and growth of PAHs in flames varied with the fuel blend. Four-ring aromatics(A4) exhibited similar formation and oxidation to soot, so A4 was suitable for estimating soot formation and oxidation. With oxygenated additives, the content of toluene is reduced in T20 fuel, which is the major reason for the reduction of PAH fluorescence spectral intensity and soot concentration. The contribution of different oxygenated additives to PAH formation also affected soot reduction. The PAH-LIF spectral intensity and soot concentration of n-butanol/T20 blends were lower than those of fuels containing methanol, ethanol, methyl butyrate, and DMF. Therefore, n- butanol more effectively reduced PAHs and soot emission during the

Influence of substituted furans on the formation of Polycyclic Aromatic Hydrocarbons in flames

The structure of low-pressure (40kPa) rich laminar premixed flames of two potential new biofuels, 2,5-dimethylfuran (DMF) and 2-methylfuran (MF), has been investigated using gas chromatography after sampling by a sonic probe. This work follows a study of the same flames made using electron-ionization molecular-beam mass spectrometry (EI-MBMS). Temperature profiles have been measured using a thermocouple with and without the probe and compared with those obtained during EI-MBMS experiments, showing the important influence of the probe geometry. The mole fraction profiles above the burner of numerous products and intermediates (31 for MF, 40 for DMF) have been quantified, including all the isomers which were globalized during EI-MBMS experiments, as well as some heavier aromatic products such as ethylbenzene and styrene. The profiles of these aromatics confirmed their significantly enhanced formation in the case of DMF compared to MF, which was already shown in the previous study. The model of the oxidation of these two substituted furans have been up-dated to include new reactions for the formation of C8 aromatics, as well as reactions proposed in literature for large Polycyclic Aromatic Hydrocarbons (PAH) formation. Simulations show a significant enhancement effect on the formation of PAH, such as pyrene or phenanthrene: their maximum mole fractions are increased by a factor of more than 400 in the DMF flame compared to that of MF, while it is only a factor 3 in the case of benzene. Simulations show that PAH mole fractions are also slightly larger in the DMF flame than in a flame of a gasoline surrogate containing toluene as an octane improver under the same conditions. However the PAH mole fractions in the DMF flame are significantly lower than in a flame of pure toluene, showing that this biofuel could be an interesting octane improver in replacement of aromatics.

Study of the formation of soot and its precursors in flames using optical diagnostics

Soot particle emission from combustion processes is a significant environmental and health problem. Optical detection is clearly the only viable choice for in situ PAH and soot measurement in turbulent flames. This paper reviews the considerable progress made in optical diagnostics of soot and its precursors formed this last decade. Advances include the spectroscopy of polycyclic aromatic hydrocarbons (PAHs) and optical diagnostics for PAHs in flames, new approaches to identify the structure and composition of incipient soot, and the latest refinements for soot particle detection and quantification. Improvements of selective and quantitative PAH measurements have been made possible with supersonic expansion of flame extracted samples, coupled with laser induced fluorescence (jet-cooled LIF) and mass-spectroscopy combined with resonance enhanced or synchrotron selective photoionisation. More accurate description of all facets of soot particles properties (optical properties, morphology, and volume fraction) has guided recent research. In particular, optical exploration has led to new insight into soot inception, including the identification of particular soot precursors from large PAHs to nanoparticles passing through stacked PAHs, polymeric-like structures and aliphatic-linked PAHs. Despite the various improvements, the ultimate accuracy of the soot diagnostic methods such as laser induced incandescence, light extinction and elastic light scattering relies on our knowledge of the soot optical properties which may vary as a function of soot age in flames. Nonetheless, with well designed experiments, comparative measurement of sooting trends within or between flames can be acquired with accuracies needed for the validation of numerical simulations of soot formation. These optical methods have been successfully adopted to classify the sooting tendencies of a large number of hydrocarbons and to examine the effect of pressure on soot formation. The ultimate objective is certainly to use optical diagnostics to provide useful data for soot formation modelling in turbulent flames and under practical combustion.

Polycyclic aromatic hydrocarbons in flames, in diesel fuels, and in diesel emissions

Numerous chemical analyses of gaseous and particulate samples from laboratory flames provide a library of data on the polycyclic aromatic hydrocarbon (PAH) species found in diverse flame types burning fuels consisting of pure gaseous hydrocarbons. The diesel fuels utilized by the more complex combustion in compression ignition engines are composed of thousands of hydrocarbon species. Mass spectrometry by the laser microprobe and gas chromatography were used in a complementary manner to distinguish the PAHs originating in the fuel from those produced by engine combustion. The C x H y PAH products of premixed and diffusion flame processes, which also occur in the unsteady diesel combustion, range in mass from 128 u (two rings, x = 10 , y = 8 ) to beyond 350 u (eight rings, x = 28 , y = 14 ). Graphs of the number of hydrogen atoms y vs the number of carbon atoms x for the species found by many investigators of laboratory flames show these pyrogenic PAHs to lie on or near the staircase curve that describes the most stable, pericondensed, benzenoid PAHs. In contrast, samples of diesel fuels from the United Kingdom and the United States contain petrogenic alkyl-PAHs with high hydrogen contents. Samples of diesel particulate emissions typical of the 1990s from two different sources display the full mass range of PAHs from 128 to 350 u, including both the benzenoid PAHs and the alkyl-PAHs. Thus diesel emissions, in general, may contain petrogenic fuel components ranging up to 206 u and also the combustion-generated four- to seven-ring species in the 228 to 302 u mass range that have greater carcinogenic potency. The absence of petrogenic components larger than 206 u facilitates their detection and delineation from pyrogenic PAHs by methods of chemical analysis.

Trapping Aryl Radicals with Acetylene: Evidence for C2-Accretion as a Mechanism for Polycyclic Aromatic Hydrocarbon Growth1

The formation of polycyclic aromatic hydrocarbons (PAH), fullerenes, soot, and other carbonaceous materials during the combustion or pyrolysis of low-molecular weight hydrocarbons requires, at a minimum, that small molecules and/or reactive intermediates somehow become joined to make larger ones. Most likely, more than one type of intermolecular C-C bond-forming reaction plays a role. The accretion of C{sub 2}-units has long been considered a probable pathway for the stepwise growth of PAH in flames, but evidence also points to the operation of bimolecular processes in which both partners can be relatively large. The experiments reported here address the former paradigm and provide clear support for a specific C{sub 2}-accretion pathway in which the key C-C bond-forming step involves the simple trapping of aryl radicals by acetylene (C{sub 2}H{sub 2}), both of which are abundant species in flames.