Sulphur anion chemistry in hydrocarbon flames with H2S, OCS, and SO2 additives

Abstract

A premixed, fuel-rich, methane–oxygen flame at atmospheric pressure was doped separately with 0.2 mol% of H2S, OCS, and SO2 to probe the behaviour of fuel sulphur during combustion. These three additives represent compounds occurring early, intermediate, and late in the oxidation sequence of fuel sulphur. They are chemically ionized in the reaction zone of a hydrocarbon flame to give large signals of sulphurous negative ions. Those detected include S−, SH−, SO− (uncertain), SO2− (S2−), SO3−, HSO3−, CH3O−•SO2, SO4− (S2O2−, S3−), and HSO4−. Ion concentration profiles of these ions were measured along the conical flame axis by sampling the flame into a mass spectrometer. The shapes of the profiles are insensitive to the nature of the additive, but their relative magnitudes are indicative of the additive's position in the sulphur oxidation sequence. For each additive, the very large HSO4− signal has analytical implications as an indicator for total fuel sulphur. The sulphurous anion chemistry is discussed for each additive in terms of roughly twenty ion (electron)-molecule reactions of six basic types, whose rate constants were known previously, or were measured at room temperature using the York flowing afterglow apparatus.