DETAILED ION CHEMISTRY IN METHANE-OXYGEN FLAMES .1. POSITIVE-IONS

The detailed positive ion chemistry is discussed for two premixed methaneoxygen flames of fuel-lean and fuel-rich composition burning at atmospheric pressure. For each flame, a complete family of concentration profiles for natural, positive flame ions has been measured mass spectrometrically below 55 amu (atomic mass units), sampled along the axis through the flame front. The ion chemistry is explained with reference to published data on neutral concentration profiles, rate constants for ion-molecule reactions, and thermochemical values when available. The fuel-lean profiles form groups in essentially three regions along the flame axis. Commencing upstream, the first involves initiation of CHO+ by chemi-ionisation. The main second group is dominated by proton transfer to a wide variety of neutral species present in the reaction zone, with minor evidence for charge-transfer processes. In the third region downstream, H3O+ persists through the burnt gas with detectable signals of O2+ and O+. For the fuel-rich flame, the first two regions are essentially similar. Just downtream of the reaction zone, however, a new group of CnHx+ ions (n ≥ 1, x ≥ 0) are formed in a series of condensation reactions in which acetylene plays a major role. The fourth region, persisting through the burnt gas, is again dominated by H3O+ but with measurable equilibrium CHO+, which together initiate another proton-transfer regime leading to the detection of additional ionic species. These mechanisms are summarised in reaction flow diagrams showing the ion-molecule reactions that dominate in different flame regions. This knowledge of the ion chemistry reveals many aspects of the underlying neutral chemistry of combustion in a fairly detailed and sequential manner. © 1979.