Ionisation in premixed fuel-lean flames of H2, O2 and N2. Part 2.—Ions from alkali-metal additives

Abstract

Mass spectrometric measurements have shown that the charged species in an oxygen-rich flame are mainly H₃O⁺ and the free electron. These ions are most probably produced via H + OH⇌H₂O*(6), H₂O*+ H → H₃O⁺+ e^–(7) where H₂O* is excited, either vibrationally or electronically. The fact that the concentrations of the radicals H and OH early in such a flame exceed their values for final equilibrium, leads to the initial generation of ions being relatively rapid. When the alkali metals, Na or K, are added to the flame, Na⁺ and K⁺ appear much more quickly than can be explained by thermal ionisation, which dominates the production of ions from alkali metals in fuel-rich flames. The explanation appears to be the hitherto unsuspected reaction: H₂O*+ A → A⁺· H₂O + e^–, A⁺· H₂O + M = A⁺+ H₂O + M where A = Na or K. Kinetic parameters have been derived for many of these processes, and are presented. Alternative explanations, e.g. K + OH → K⁺+ OH^– or K + H₃O⁺→ K⁺+ H₂O + H, are not able to account for the rapid production of ions. It is also concluded that the H₂O* participating in reactions (6) and (7) is most probably in an antibonding electronic state.