Theoretical and kinetic modeling study of H₂S pyrolysis

Hydrogen sulfide pyrolysis was investigated theoretically and through chemical kinetic modeling. Reactions on the SHH potential energy surface, primarily S + H₂ (+Ar) ⇄ H₂S (+ Ar) (R1) and S + H₂ ⇄ SH + H (R6b) were characterized by ab initio calculations. Results for k₁ were in good agreement with experiment, but deviated strongly below 2000 K from values previously used in modeling. Collider efficiencies for H₂S, S₂, and N₂ compared to Ar were calculated for R1. Hydrogen sulfide decomposition experiments reported in literature were re-examined in terms of an updated detailed chemical kinetic model. Concentration profiles for the atomic S at high temperature in shock tubes supported the present value of k₁ and served to constrain the rate constants for reaction of S with SH and H₂S. To explain results from batch and flow reactors, conducted at high H₂S concentrations in the 900–1400 K range, a very fast rate constant was required for HSS + H ⇄ SH + SH. Under dilute conditions, the gas-phase chemistry was too slow to compete and the decomposition of H₂S was controlled by loss on the reactor surface.