Effect of H₂S in methane/air flames on sulfur chemistry and products speciation

Reaction behavior of H₂S/O₂ under different equivalence ratios in methane/air flames is examined. Three equivalence ratios extending from fuel-lean (Φ=0.5), stoichiometric (Φ=1.0), to fuel-rich (Claus condition, Φ=3.0) are examined. The results revealed that the presence of H₂S prevents hydrogen oxidation in the primary reaction zone, while in the secondary reaction zone oxidation competition occurs between H₂ and H₂S. In presence of oxygen, oxidation of hydrogen sulfide forms sulfur dioxide. However, under Claus conditions, the depletion of oxidant causes the direction of hydrogen sulfide reaction to shifts towards the formation of elemental sulfur. Higher hydrocarbons are formed in trace amounts under Claus conditions wherein sulfur dioxide acts as a coupling catalyst which enhances the dimerization of CH₃ radical to form higher series of hydrocarbons. Under Claus conditions, sulfur deposits are formed in low temperature regions of the reactor including the sampling line. The deposits are analyzed using X-ray powder diffractometer and were found to be cyclo-S₈ (α-sulfur) with orthorhombic crystal structure. The formation of α-sulfur is mainly due to the agglomeration of elemental sulfur (S₂) during its condensation at low temperatures.