Formation of polycyclic aromatic hydrocarbons and their growth to soot—a review of chemical reaction pathways

This review of the role of reaction kinetics in combustion chemistry traces the historical evolution and present state of qualitative and quantitative understanding of a number of reaction systems. Starting from the H2–O2 system, in particular from the reaction between H and O2, mechanisms and key reactions for soot formation, for the appearance of NOx, and for processes of peroxy radicals in hydrocarbon oxidation are illustrated. The struggle for precise rate constants on the experimental and theoretical side is demonstrated for the example of the reaction H + O2 → OH + O. The intrinsic complexity of complex-forming bimolecular reactions, such as observed even in this reaction, also dominates most other key reactions of the systems considered and can be unravelled only with the help of quantum-chemical methods. The multi-channel character of these reactions often also requires the combination with master equation codes. Although kinetics provides an already impressive database for quantitative modelling of simple combustion systems, considerable effort is still required to quantitatively account for the complexities of more complicated fuel oxidation processes.

Unravelling combustion mechanisms through a quantitative understanding of elementary reactions

https://depositonce.tu-berlin.de/bitstream/11303/1384/1/Dokument_44.pdf

Incoherent broad-band cavity-enhanced absorption spectroscopy

The equivalence ratio φ and temperature T are well known to have a significant effect on the quality of particulate formation, such as the soot volume fraction, particle diameter and number density. The purpose of this work is to clarify the φ-T dependence of soot formation for various kinds of fuels, including paraffinic hydrocarbon, aromatic hydrocarbon and oxygenated hydrocarbon, and to discuss a possibility for smokeless diesel combustion considering particulate size and number density. The sooting φ-T map, showing the tendency to generate soot particles as a function of φ-T and T, was made using a detailed soot kinetic model. The computational results show that oxygenated fuel reactions lead to a lower soot yield, smaller particle diameter, lower number density and narrower sooting φ-T region than those of aliphatic and aromatic fuels, due to the notable reduction in production of both acetylene and polycyclic aromatic hydrocarbons (PAHs). Furthermore, this lower sooting tendency is emphasize...

Mechanism of smokeless diesel combustion with oxygenated fuels based on the dependence of the equivalence ration and temperature on soot particle formation

Experimental and numerical studies indicate that particle coagulation plays an important role in sootformation. However, to date, neither method has been able to conclusively determine the stage at which chemical precursors begin to coalesce. In the present study, molecular dynamics (MD) with on-the-fly quantum forces was used to investigate binary collisions between pyrene molecules. The molecular structure and internal motion were treated explicitly. Most of the runs were performed at 1600 K, within the temperature window of soot nucleation in flames. The MD simulations were successful in producing dimers with substantial collisional frequency and lifetimes far exceeding the equilibrium-based predictions, demonstrating that pyrene dimerization is physically realisically realistic in flame environments. The principal finding of the present study is the development of internal rotors by colliding pyrene molecules, the phenomenon responsible for stabilization of the forming dimer. Analysis of the MD results shows that the mechanism of stabilization is rooted in the pattern of energy transfer. The translational energy of the individual colliding molecules is trapped in internal rotors that emerge upon collision and in the vibrational modes of the dimer, including the van der Waals bond established between the pyrene molecules. This model extends the view of stabilization of aromatic species dimerization, with the implication that aromatic dimers of species as small as pyrene can survive long enough to evolve into soot nuclei.

H.Gg. Wagner

Papers

Soot formation in combustion