Detailed kinetic modeling of homogeneous H2S-CH4 oxidation under ultra-rich condition for H2 production

This study presents a detailed kinetic investigation into ultra-rich oxidation of H2S-CH4 under high temperature (900-1250 °C) and ambient pressure. Effects of temperature, initial H2S/CH4 ratio and equivalence ratio (Φ) on reactants conversions and products distributions were experimentally studied in a tubular flow reactor and kinetically analyzed by CHEMKIN software. A detailed kinetic mechanism involving 85 species and 515 reactions has been developed and validated using reference data for H2S-CH4 decomposition and results from extended experimental conditions involving the O2 addition. For H2S-CH4 system, conversion of H2S increased steady with the rising temperature while reactivity of CH4 was weak at temperature below 1000 °C. At temperature higher than 1000 °C, conversion of CH4 increased rapidly and devoted further formation of H2 and CS2 mainly via reacting with H2S decomposition products. The H2 production efficiency was negatively associated with initial H2S fraction as H2S decomposition was dominant H2 source within 1150 °C. The stoichiometric ratio for H2S/CH4 merely showed its advantage in H2 production at higher temperature under which CH4 reached its equilibrium conversion swiftly. Introduction of little amount of O2 (Φ = 6 or higher) accelerated the whole reaction process and triggered H2S partial oxidation and H2 formation at lower temperature. CH4 explicitly showed inferior position in oxidation competition with H2S and maintained poor conversion at temperature below 950 °C. The results of rate of production (ROP) analysis at condition without O2 showed that CH4 reactivity showed dependence on free S radical via S + CH4 = SH + CH3, and the formed CH3 was mainly converted via reacting with SH and H radicals. CH3 could be concurrently reverted to CH4 via reactions with H2S and H2. O2 activated the whole system by forming chain branching radicals O and OH. These radicals promoted H2S and CH4 conversions to form richer S, H and CH3 radicals. SH + CS = CS2 + H was important for CS2 formation and with presence of O2, CS2 was likely to be consumed via oxidation reactions. Finally reaction pathways for H2S, CH4 conversion and H2, CS2 formation were presented.