Ionization in a methanol-oxygen flame

Abstract

The combustion of methanol was studied in a conical methanol—oxygen flame of fuel-lean composition (equivalence ratio φ = 0.25) burning at atmospheric pressure surrounded by a flowing argon shield. Oxygen bubbled through liquid methanol contained in two gas saturators provided a premixed flame of excellent stability and reproducibility. The flame gas was sampled into a mass spectrometer which was used to detect a variety of naturally occurring ionic species, both positive and negative. Although the maximum level of ionization in this flame had the rather low value of 5 × 10 8 ions ml −1, nevertheless concentration profiles were drawn as a function of distance along the flame axis at 17 individual mass numbers ( m/z) for positive ions and 11 for negative ions; that is, whenever an appreciable signal could be detected below 100 amu. The positive profiles in the reaction zone are dominated by proton transfer processes which indicate that CHO + is the primary ion. A variety of protonated combustion intermediates and products were detected including both stable molecules (H 2O, HCHO, the CH 3OH fuel, CH 2CO, CH 3CHO, HCOOH and possibly CH 3OCH 3/C 2H 5OH) and radicals (C, CH, CH 2, HCO, CH 2OH or CH 3O, CH 3CO or CH 2CHO, and possibly CH 3OO). Chemical ionization by charge transfer is certainly operative for two species (O 2 and NO as an impurity) and perhaps others (CH 3, O). As far as the negative ions are concerned, a number of species can be ionized in the reaction zone by three-body electron attachment and subsequent charge transfer processes (O 2 initially, and then O, OH, HO 2, HCOO and possibly HCOOO). Some of these negative ions are sufficiently strong bases to abstract protons from other species in proton transfer reactions (CH 3OH, possibly H 2O 2, HCOOH, and perhaps the peroxidic intermediates HCOOO and HCOOOH). The remaining negative ions detected can be interpreted as cluster ions (OH − · H 2O, HO 2 − · O 2 or HO 2 − · CH 3OH, CHO 3 − · H 2O). All of this ion chemistry was compared with that observed previously for an analogous fuel-lean methane—oxygen flame. Many similarities were noted, particularly downstream, for both positive and negative ions. However, the combustion path leading to formic acid is more prominent in the methanol flame.