Effect of O₂ enrichment on acid gas oxidation and formation of COS and CS₂ in a rich diffusion flame

Highlights: •Oxidation of acid gas in a non-premixed flame was studied under oxygen enriched condition. •Reactivity of CO₂ was significantly promoted in an O₂ enhanced flame. •COS could be initially formed in the flame inner core by reaction involving CO₂ and sulfur species. •Formation of COS and CS₂ is promoted under O₂ enriched combustion. -- Abstract: O₂ enriched combustion is applied to a non-premixed acid gas (AG) flame, in which AG and air are injected separately into a vertical reactor via a coaxial burner at atmosphere pressure. Equilibrium predictions of AG oxidation were performed using Aspen Plus and lab-scale experiments with particular focus on formation of COS and CS₂ were undertaken under different combustion conditions. The effects of equivalence ratio (Φ = 0.8, 1.0 and 3.0) and initial O₂ concentration (OC) in air (21, 30 and 50 vol%) have been studied and the flame is interpreted by analyzing the axial temperature and species concentration distributions along the reactor. AG diffusion flame could be divided into three zones, namely AG decomposition, oxidation and complex reaction zones, among which decomposition zone is tightly associated with formation of COS and CS₂. It is shown that Φ generally determines the flame temperature and controls the H₂S oxidation degree and production rate of H₂ and CO. Reactivity of CO₂ primarily expressed via CO₂ + H = CO + OH during the fuel rich flame. COS is primarily produced in the flame inner core via reactions jointing CO₂ and sulfur species, consequently its formation shows a low sensitivity to Φ in the air-supplied flame. O₂ enrichment basically contributes to higher flame temperature, accelerated H₂S oxidation and advanced CO production. Also an increasing tendency for H₂S to decompose into H₂ and S₂ can be observed. And this factor directly triggers the formation of COS and CS₂ via increasing the presence of CO and sulfur species. COS is formed within extended channels involving the primary production by reaction of SH + CO₂ and the secondary by reactions between CO and sulfur species. The CS₂ formation is more complex, mainly comprises of reactions involving CS intermediate and evidently it is enhanced under O₂ enriched combustion.