The Sulfur-Iodine and hybrid (Westinghouse) water-splitting cycles have suffered from the use of water as reaction media in their low temperature reactions. Separation of the various species from an aqueous medium results in significant energy expenditures and materials challenges in practical implementation. For the past few years we have examined an alternative route where the oxidation of sulfur step takes place in an ionic liquid medium. The application of ionic liquid reaction media for the low temperature reactions presents possible solutions to the problems of excess water removal, I2 solidification, and SO2 solubility.
The observation that carrying out the Bunsen reaction in ionic liquid media (Eq. 1 below) can be tailored to produce significant amounts of H2S gas from the reaction mixture has led to the development of a novel Sulfur-Sulfur cycle, shown below.
4SO2 + 4I2 + 8H2O → 4H2SO4 + 8HI Eq. 1
H2SO4 + 8HI → H2S + 4I2 + 4H2O Eq. 2
Net Low Temperature Reaction: 4SO2 + 4H2O → H2S + 3H2SO4 Eq. 3
H2S + 2H2O → SO2 + 3H2 Eq. 4
3H2SO4 → 3SO2 + 3H2O + 3/2O2 Eq. 5
The production of H2S (Eq. 2) following the Bunsen reaction, a normally undesired side reaction, can be intentionally enhanced under the appropriate conditions (concentration of I2 and water, temperature and reaction time). The I2 is regenerated in this reaction without having to transport HI to a decomposition section, and is therefore required only in catalytic amounts. The H2S spontaneously desorbs from the reaction medium and can then be steam reformed to produce H2 and SO2, (Eq. 4) the latter of which is recycled. The thermal decomposition of H2SO4 to produce O2 and SO2 (Eq. 5) is common to other Sulfur based thermochemical cycles. The H2S generation and steam reformation steps (Eq. 3 and 4) have been demonstrated in our lab. An overall thermal efficiency assuming a steam reformation temperature of 1100 K is estimated to be 55%.
This presentation will focus on the parametric dependence of the H2S production and steam reforming reactions on reactant concentrations, temperature, and residence time, and details of the estimated thermal efficiency calculations will be presented.
 A. Yokochi, N. AuYeung, “A Proposed New Sulfur-Sulfur Thermochemical Cycle” 2009 Meeting of the AIChE.