Two-step thermochemical H 2 S splitting process is a promising method to recover both H 2 and S from a toxic waste H 2 S gas via low/high metal sulfide conversion. Several studies have examined the potential of the process experimentally. Still, there is a lack of comprehensive research into such as to provide a guideline for selecting the appropriate metal for the process. The most common metals in the periodic Table (28 metals) have been examined to find stable low/high metal sulfide couples. A list consisting of 17 metals that can form thermodynamically stable couples has been shortlisted, in which six metals: K, Zr, Hf, Nb, Fe, and Ni, were found to be promising candidates with a high H 2 S splitting efficiency, high H 2 yield and good reversible transition between low and high sulfides. The transition between sulfide crystals has been experimentally demonstrated with the Ni 3 S 2 /Ni 3 S 2+x couple. This couple can decompose up to 90 mg H 2 S/g-Ni 3 S 2 at 500 °C to produce H 2 and high sulfides Ni 3 S 2+x that was successfully regenerated at 700 °C for multiple cycles. The thermodynamic calculations revealed that the sulfides of Zr, Hf, and Nb are favorable for the two-step H 2 S splitting process, but have not been explored experimentally. The sulfide of Zr was examined in this work, but they got oxidized by residual oxygen in the test gas before the crystalline transition was detected and remained as a challenge in the future works.
Associated with the rise in energy demand is the increase in the amount of H2S evolved to the environment. H2S is toxic and dangerous to life and the environment, thus, the need to develop efficient and costeffective ways of disposing of the H2S gas has become all-important. To this end, a two-step thermochemical H2S splitting cycle is proposed in this work which does more than just getting rid of the toxic gas but has the potential to produce valuable H2 gas as well as store the solar heat energy. Studies have proved that the type of material used, such as metal sulfides, is critical to the efficiency of this thermochemical splitting process. As follows, this study focuses on establishing a criterion to aid in selecting favorable metal sulfides for application and further development in the H2S thermochemical decomposition sphere. Using a computational approach, via the HSC Chemistry 8®, evaluations such as the equilibrium yield from the sulfurization and decomposition reaction steps, the temperature required for reaction spontaneity, and the Reversibility Index were determined. Investigations proved that sulfides of Zirconium, Niobium, and Nickel were auspicious candidates for the thermochemical decomposition.
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