Andrea D’Anna

Modeling of particulate formation in combustion and pyrolysis

We use a detailed chemical kinetic mechanism to explore the effects of C/O ratio, temperature and pressure on the formation of high molecular weight aromatic species in premixed flames and shock tubes in a wide range of operating conditions. Key sequences of reactions in the formation of aromatics are the addition of acetylene to smaller aromatics, activated by H-atom abstraction and the combination of resonantly stabilized radicals, including cyclopentadienyl radical combination, propargyl addition to benzyl radicals and the sequential addition of propargyl radicals to aromatic rings. The modeling results are compared with those obtained by considering only the H-abstraction acetylene addition (HACA) route for aromatic formation to identify the controlling steps in different combustion regimes and to assess the viability of the HACA and resonantly stabilized radical (RSR) pathways to model aromatic growth. The full model is able to predict the concentration and formation rate of total organic carbon collected in slightly sooting rich flames at different temperatures and pressures. On the contrary, the HACA model predicts concentrations and formation rates more than one order of magnitude lower than those measured in experiments. This result indicates that a combination of resonantly stabilized radicals are the controlling steps in the formation of aromatics in flames. The oxidative environment is pivotal in the formation of aromatics to activate the pathways involving the resonantly stabilized radicals in the aromatic growth process. Since the model simulates only the formation of two- and three-ring aromatics, it can be hypothesized that total organic material collected in flames, i.e. a mass quantity much larger than the chromatographable polycyclic aromatic hydrocarbons, is the result of a fast reactive coagulation of small aromatics, forming structures of high molecular mass. Results indicate that reactions leading to the formation of two- and three-ring aromatics are rate-limiting, and combination reactions involving these aromatics control soot formation in oxidative or slightly sooting regimes. In very fuel-rich and pure pyrolysis conditions, the two models predict the same amount and formation rate of aromatics; this indicates that the HACA route controls formation in these conditions since the RSR pathways are activated only in oxidative environments.

Spectroscopic and Chemical Characterization of Soot Inception Processes in Premixed Laminar Flames at Atmospheric Pressure

Absorption, laser induced fluorescence and laser light scattering measurements have been performed in a slightly-sooting methane/oxygen flame at atmospheric pressure. Sampling and analysis has been also carried out with the specific aim of measuring and characterizing all of the condensable material formed during the combustion process. The combined use of scattering and extinction measurements in the ultra-violet (266 nm) and in the visible (532 nm) allowed to determine simultaneously the volume fraction, the average size and the number density of high molecular weight soot precursors and soot particles. The results of the optical measurements were in good agreement with the concentration measured by the sampling procedure. The formation of “particles” early in the flame with typical size of about 3–4 nm with no absorbtion and fluorescence in the visible could be shown. These “transparent” particles were considered to be precursors of soot particles on the basis of their decreasing concentration during soot inception. The results presented suggest that there is an initial fast polymerization process the building bricks of which are aromatic compounds with few condensed rings (not more than 2–3 rings) connected by aliphatic and oxygen bonds. Particle inception is primarily controlled by the internal arrangement of these polymers leading to structures with more condensed aromatic rings and more compact three dimensional shape thus forming the first soot nuclei.

Photoionization Study of Soot Precursor Nanoparticles in Laminar Premixed Ethylene/Ethanol Flames

The effect of ethanol addition in ethylene laminar premixed flames on the characteristics of soot precursor nanoparticles was investigated by aerosol photoionization technique. The fifth harmonic of a Nd:YAG laser, 213 nm (5.82 eV), was used as ionization source, while a differential mobility analyzer (DMA) coupled to a Faraday cup electrometer was used for ionized particle classification and detection. Carbonaceous nanoparticles in the nucleation mode, i.e., with sizes ranging from 1 to 10 nm, show a photoionization charging efficiency clearly dependent on the percentage of ethanol used as dopant in the flame. In particular, we observed that the more ethanol was added as fuel, the lower the particle photo-charging efficiency was. This result indicates a modification in the nanoparticle chemical structure as the amount of ethanol is increased. These experimental evidences may be explained by a decrease of aromaticity of the particles and/or by the presence of oxygen bonds within the nanoparticles.