Hydrogen production from a chemical cycle of H2S splitting

The sulphur–iodine thermochemical water-splitting cycle (S–I cycle) developed for hydrogen production from water is fundamentally based on the following three chemical reactions: H2SO4→H2O+SO2+0.5O2,2H2O+SO2+I2→H2SO4+2HI,2HI→H2+I2.H2SO4→H2O+SO2+0.5O2,2H2O+SO2+I2→H2SO4+2HI,2HI→H2+I2.This paper proposes to replace the H2SO4 decomposition with a reaction between H2S and H2SO4 and the replacement gives rise to a H2S-splitting cycle that produces H2 and elemental S from H2S, shown as follows: H2S+H2SO4→S+SO2+2H2O,2H2O+I2+SO2→H2SO4+2HI,2HI→H2+I2.H2S+H2SO4→S+SO2+2H2O,2H2O+I2+SO2→H2SO4+2HI,2HI→H2+I2.Combined with the reactions such as O2+S→SO2,SO2+0.5O2→SO3,SO3+H2O→H2SO4,O2+S→SO2,SO2+0.5O2→SO3,SO3+H2O→H2SO4,this new cycle cannot only produce more H2 and extra H2SO4 but also facilitate flexible H2 to H2SO4 production ratio. Thermodynamic analysis shows that the new cycle is more energy-efficient than the S–I cycle of water-splitting because a series of endothermic reactions in H2SO4 decomposition have been replaced with an exothermic reaction between H2S and H2SO4. Technologies to be developed from this chemistry will be able to convert H2S from sour or acid gas into H2 and elemental S or H2SO4, which are more valuable than elemental S only, the product of the Claus process. In upgrading and refinery, the H2 produced can be returned to use in hydrotreating; and in gas plants, H2 from H2S splitting is an alternative clean fuel. Environmentally, H2 production based on this H2S-splitting cycle is carbon free.