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

Chemical reactions and physical processes responsible for the formation of polycyclic aromatic hydrocarbons and soot in hydrocarbon flames are reviewed. The discussion is focused on major elements in the present understanding of the phenomena, clarification of concepts central to the present state of the art, and a summary of new results.

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Reaction mechanism of soot formation in flames

A detailed kinetic modeling study of aromatics formation in laminar premixed acetylene and ethylene flames

Over the last two decades, our understanding of soot formation has evolved from an empirical, phenomenological description to an age of quantitative modeling for at least small fuel compounds. In this paper, we review the current state of knowledge of the fundamental sooting processes, including the chemistry of soot precursors, particle nucleation and mass/size growth. The discussion shows that though much progress has been made, critical gaps remain in many areas of our knowledge. We propose the roles of certain aromatic radicals resulting from localized π electron structures in particle nucleation and subsequent mass growth. The existence of these free radicals provides a rational explanation for the strong binding forces needed for forming initial clusters of polycyclic aromatic hydrocarbons. They may also explain a range of currently unexplained sooting phenomena, including the large amount of aliphatics observed in nascent soot formed in laminar premixed flames and the mass growth of soot in the absence of gas-phase H atoms. While the above suggestions are inspired, to an extent, by recent theoretical findings from the materials research community, this paper also demonstrates that the knowledge garnered through our longstanding interest in soot formation may well be carried over to flame synthesis of functional nanomaterials for clean and renewable energy applications. In particular, work on flame-synthesized thin films of nanocrystalline titania illustrates how our combustion knowledge might be useful for developing advanced yet inexpensive thin-film solar cells and chemical sensors for detecting gaseous air pollutants.

Formation of nascent soot and other condensed-phase materials in flames

Plasma assisted combustion: Dynamics and chemistry

Mechanisms of soot nucleation in flames—A critical review

Effect of molecular structure on incipient soot formation

Mechanisms for the formation of ions in flames

Summary Experimental results for hydrocarbon-air or oxygen flames are presented at pressures down to 3 mm Hg. These are interpreted to yield ion recombination coefficients which are independent of pressure and equal to 2×10−7 cc/sec, i.e., the rate of ion recombination is second order with respect to the ion concentration. The rate of ion formation is proportional to the pressure squared so the formation process is second order. The importance of ambipolar diffusion at pressures below 66 mm Hg is deduced, so it is necessary to consider diffusion in interpreting ion decay processes behind the flame front. These results with other available information are discussed in terms of ion production by chemi-ionization, e.g. by CH+O→CHO++e−, At 66 mm Hg, if the reactants are present in equal concentration, the reactant concentration would have to be 1011 molecules/cc. The initially produced ion rapidly reacts with H2O by an ionmolecule charge exchange reaction to produce H2O+. The half-life of the initially produced ion is very short, approximately 10−7 sec, so it is present in only small concentrations. In summary, a picture has been formulated of the mechanism of ion formation and removal in flames which is consistent with the known facts. Some details are still fuzzy, and the specific elementary steps to be chosen are still open to question. In such a complex system as a flame, with many reactions occurring, it is surpring that such an apparently simple picture seems to fit the experimental observations. With more accurate and detailed data, additional facets of the problem can be expected to appear.

Ion production and recombination in flames

Combustion is used as a source of heat to remove polluting volatiles from contaminated groundwater and subsurface soil. A burner assembly is placed in a well, either in or out of an aquifer, to produce steam and/or hot air and/or hot water which drives off the volatile contaminants which are collected through separate wells or vents depending upon the site. The combustion product gases are vented to atmosphere. Key elements are the addition of heat as close to the depth of the contaminated region as possible, transfer of heat through the aquifer and/or ground through the flow of steam and/or hot air and/or hot water to regions affected by the contaminants, and by drawing the flow to other well sites by which the contaminants are removed by pumping and subsequent treatment. Some of the contaminants are thermally destroyed. The design of the well casing can be tailored to proportion the quantities of water and steam and hot air delivered, with steam and air pressure driving the vapors at various levels below ground. The collected vapors from the pumped wells or vents are removed by a variety of techniques.

H.F. Calcote

Papers

Mechanisms of soot nucleation in flames—A critical review

Effect of molecular structure on incipient soot formation

Mechanisms for the formation of ions in flames

Ion production and recombination in flames

Submerged combustion process and apparatus for removing volatile contaminants from groundwater or subsurface soil