Co-oxidation of methane (CH4) and carbon disulfide (CS2)

Abstract

Sulfur-containing species persist as important impurities in fossil fuels, affecting the combustion process of constituent hydrocarbons. This contribution reports the promotion effects of carbon disulfide (CS2) on oxidation of methane (CH4), with experiments conducted in a jet-stirred reactor (JSR), and explains the experimental findings from comprehensive kinetic modelling of CH4/CS2/O2 systems. This includes neat oxidation of CH4/O2 and CS2/O2 under stoichiometric conditions, as compared with co-oxidation of CH4/CS2/O2, containing different measures of CS2. Addition of small amounts of CS2 (50 ppm and 100 ppm) enhances the oxidation of CH4 (500 ppm) as characterised by a lower onset temperature (1300 K versus 1200 K and 1060 K). In contrast, the presence of CH4 delays the process of CS2 oxidation. Considering the similarity in the core charge of O and S, we propose the reactivity of S2, SO and in particular S towards CH4 to be responsible for the observed behaviours, in addition to an important effect of O radicals generated in oxidation of CS2 on engendering the oxidation of CH4. As reported in the literature, in analogy to O2 and O which oxidise CH4 into CO2, the S/S2/SO could also ‘sulfurdise’ CH4 into CS2 or COS under oxygen-lean conditions. Quantum chemistry calculations of the CH4 + O, CH4 + S, CH4 + S2, CH4 + SO and CH4 + O2 reactions further reveal the reactivity of O, O2, S, S2 and SO towards H abstraction from CH4. The sensitivity analysis of the kinetic modelling of the proposed co-oxidation reactions indicates that, the radicals formed during the CS2 conversion process promote the oxidation of CH4 at lower temperatures. However, the consumption of radicals in the CH4 oxidation also inhibits the decomposition of CS2 as observed in the experiments.