Hydrogen production by water splitting via modified sulphur-iodine thermochemical cycles

Splitting water molecule in order to obtain hydrogen and oxygen is on of the most ambitious and innovative target in the field of alternative energy research. Two realistic methods have been studied to achieve this result: water electrolysis (at ambient or high temperature) and thermochemical redox cycles based on inorganic metal compounds. Among thermochemical cycles both two-step and multi-step processes are extensively investigated. Two-step cycles feature theoretically higher overall efficiencies due to the reduced number of chemical reactions involved but, on the other side, multi-step cycles can work at quite lower process temperatures. In the latter case, the so-called “sulfur-iodine” (S-I) cycle is by far the most studied process. Particularly, ENEA is involved in national program on high-temperature hydrogen production by thermochemical water-splitting cycles powered by solar energy, and the solar powered S-I cycle extensively studied. Several process options are surveyed and analyzed in order to improve process management, efficiency, and economics. Here, a modified S-I route involving the use of metal (M) compounds (Ni, Fe, Cu ecc) recycling intermediates (M, MxIy, Mx(SO)y) is presented. The central reaction step is the Bunsen reaction (1), where an excess (x) of I2 is required to segregate the two formed acids (HI and H2SO4) into two corresponding immiscible liquid phases (H2SO4 phase and HIx phase). Differently from conventional S-I cycle, the two acids are neutralized to form the corresponding metal salts, which are dehydrated and subsequently decomposed. Hydrogen is quantitatively produced (instead, a conversion of about 20% is obtained by using thermal catalytic decomposition of HI) at “low” temperature by reaction of metal with the produced H2SO4 phase (2). The Mx(SO)y solution is dehydrated and the solid sulfate decomposed in a solar reactor (3), producing MxOy and a gas mixture SO3/SO2/O2 with ca. 80% SO2 yield. The obtained MxOy is added to the HIx phase to form the iodide MxIy (4) that is separated from I2 and H2O (recycled to the Bunsen reactor) and finally thermally decomposed (5) to regenerate I2 and M at 600-700°C. The whole process can be carried out by using one Bunsen reactor, and four reactors which, alternatively, perform the steps 2, 3, 4, and 5 respectively. The decomposition reactions (3) and (5) could be carried out in a directly irradiated fluidized bed system, which could be employed simultaneously as solar particle receiver and chemical reactor. Fluidized bed can provide high heat transfer coefficients, which could both enhance the absorption of solar energy and promote the metal sulphate/iodide decomposition. With respect to the “traditional” S-I cycle comprises two additional reactions and involves solid management. On the other hand, this route does not involve catalysts and special materials suitable for hydrated acids at high temperatures (> 200°C). Moreover, quantitative production of pure hydrogen is also obtained without complex or advanced separation and purification processes (e.g. membranes).