Sulphurous negative ions in a fuel-rich, CH4–O2 flame with OCS additive

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Abstract
Sulphurous negative ions S • SH • SO • SO2/S2• SO3• HSO3• SO4• and HSO4 were observed when 0.2% of carbonyl sulphide (OCS) was added to a conical, laminar, premixed. fuel-rich (equivalence ratio [Formula: see text]) CH4–O2 flame burning at atmospheric pressure. Profiles were obtained of ion concentration vs. distance along the flame axis by sampling the flame through a pinhole into a quadrupole mass spectrometer. Some of the ion signals observed in the flame reaction zone are very large, particularly that for HSO4. None of the sulphurous ions detected contain carbon. Of those listed above, only S−,•SH, • SO • and SO2 persist downstream through the burnt gas. The sulphurous ions are formed by chemical ionization processes of neutral sulphurous intermediates reacting with the naturally-occurring ions present in any hydrocarbon flame. The ion chemistry is discussed, as is the underlying neutral chemistry of sulphur relevant to the flame environment. The ion profiles show the rapidity with which OCS is oxidized through SH and SO to SO2 even within the reaction zone of this fuel-rich flame. No evidence was obtained for the presence of sulphuric or sulphurous acids, and the presence of S2: was not confirmed.

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