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

Abstract

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 [Formula: see text]. Extensive data were presented previously involving ion concentration profiles measured with a mass spectrometer as a function 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 to provide the more straightforward probe of the underlying neutral chemistry.