Hydrocarbon ions in fuel-rich, CH₄–C₂H₂–O₂ flames as a probe for the initiation of soot: interpretation of the ion chemistry

Abstract

. Can. J. Chem. 60,2766Chem. 60, (1982.The ion chemistry is discussed for fuel-rich, nearly sooting, methane-oxygen flames at atmospheric pressure with added acetylene. Different types of ion-molecule reactions, both positive and negative, which can contribute through chemical ionization (CI) processes are summarized including their dependence on temperature, pressure, and equivalence ratio 4. Extensive data were presented previously involving ion concentration profiles measured with a mass spectrometer as afunction of distance along the axis of conical flames. An understanding of the dominant CI processes provides insight into the early chemical stage of soot formation associated with the flame reaction zone. The negative ion profiles show moderately unsaturated hydrocarbon ions upstream formed by proton transfer followed by progressive dehydrogenation; the highly unsaturated, carbonaceous ions observed downstream appear to arise by two-and three-body electron attachment, charge transfer, and H-atom stripping. The negative hydrocarbon ions can all be explained in terms of polyacetylene derivatives. The same build-up of carbonaceous species downstream is evident from the positive ion profiles. A major role is ascribed to proton transfer reactions with lesser contributions from charge transfer and ion-molecule condensation; three-body association is probably insignificant. Experiments with added acetylene indicate extensive fuel pyrolysis early in the reaction zone. There is no evidence that an ionic mechanism is dominant in forming soot precursors compared with neutral condensation reactions. Because of complexities in the positive ion chemistry, the negative ions appear t o provide the more straightforward probe of the underlying neutral chemistry. probablement pas importante. Les experiences conduites avec de I'acCtylkne ajoute indiquent une abondante pyrolyse du carburant se produisant trts tBt dans la zone de reaction. 11 n'y a pas de preuve d'un mecanisme ionique dominant lors de la formation des precurseurs de suie par comparaison avec les reactions neutres de condensation. A cause de la complexite de la chimie de I'ion positif, les ions ntgatifs apparaissent comme les meilleures guides pour Ctudier la chimie neutre sous jacente.[Traduit par le journal] Introduction reaction chemistry. It is hoped that an understandIn a recent article (I), here designated as paper I, ing of the kinetics and energetics of the ionwe reported experimental data on the axial distri-molecule reactions responsible for the observed bution of both positive and negative ions monitored flame-ion distribution will lead t o further insights directly through the reaction zones of fuel-rich into the initial chemistry leading to Soot. CH4-C2H2-O2 flames at atmospheric pressure.The types of reactions commonly thought to The natural hydrocarbon ions c,H,? were studied dominate the ion chemistry of flames have been as a probe of the early chemical stages of soot conveniently classified. Those processes which formation. In that paper, the ph...

Ionic Mechanisms of Soot Formation in Flames.

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Reactions of Acetonitrile with Trapped, Translationally Cold Acetylene Cations

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