Chemistry of the Bunsen section of the Sulfur-Iodine Thermochemical Cycle: in situ Raman characterization of the HIx and H2SO4 phases

  • Mr Victor Ramos-Sanchez, High Temperature Science Laboratories, Department of Chemistry, University of Sheffield, United Kingdom
  • Miss Tina Conroy, High Temperature Science Laboratories, Department of Chemistry, University of Sheffield, United Kingdom
  • Robin Devonshire, High Temperature Science Laboratories, Department of Chemistry, University of Sheffield, United Kingdom

    • The first stage of the sulfur-iodine thermochemical water splitting cycle is the Bunsen reaction, an exothermic reaction carried out usually at 120oC. When operated with excess H2O and I2 present the reaction gives rise spontaneously to two separate product phases: a heavy "HIx" phase containing the majority of the total iodine; and a light "H2SO4" phase containing the majority of the total sulfur present.

    In the present study spontaneous Raman scattering measurements, made using a backscattering configuration, have been used to identify the chemical species present in the two process feeds at temperatures between 25oC and 200oC and for a range of different overall compositions.

    Figure 1 shows the classical Raman spectrum of an H2SO4 phase sample at 25oC. The spectral features apparent in the range from 500 cm-1 to 2000 cm-1 are from sulfur-containing species; the spectral features below 500 cm-1, shown in the inset, are from iodine-containing species found predominantly in the HIx phase but which are present as low level contaminants in the H2SO4 phase.

    A detailed analysis of the results shows that the major iodine-containing species in the HIx phase are: I3-, I-(I2), and I-(I2)2; and that the major sulfur-containing species in the H2SO4 phase are: HSO4-, SO4=, with the small amount of iodine present in this phase being in the form of I3-, and I-(I2) species.

    The study has provided a detailed chemical description of the two phases and demonstrates the potential of spontaneous Raman scattering as a non-intrusive, in situ, SI-TC process diagnostic.