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Energies 2020, 13, 420 8 of 96 metal oxide (MOred) reacts with an alkali metal hydroxide (2M’OH) generating a mixed oxide (M’2O•MOox) and hydrogen. In the third reaction, the mixed oxide is hydrolyzed by the steam regenerating the metal oxide and the alkali metal hydroxide [118]. Sulfur–iodine (S–I), copper–chlorine (Cu–Cl) and magnesium–chlorine (Mg–Cl) are the most promising and investigated three-step thermochemical cycles. In Figure 3 a schematic diagram of the sulfur–iodine thermochemical cycle is proposed. The maximum temperature required is 1100–1200 K in the first endothermic chemical reaction in which the sulfuric acid (H2SO4) is decomposed into steam, sulfur dioxide (SO2) and oxygen. In the second step, the Bunsen reaction occurs at 300–400 K. Iodine and water enter the reactor and promote the formation of sulfuric acid and hydroiodic acid (HI) reacting with sulfur dioxide. Finally, in the third step, the hydronic acid is split in iodine and hydrogen at 600–800 K [119]. Some drawbacks are the corrosive nature of the species and the poisoning of the aqueous acid phases (HI and H2SO4) that requires additional energy-expensive treatments to separate these compounds. Caple et al. [120] propose an experimental feasibility analysis of a novel sulfur-sulfur cycle that abolish the need for the processing of hydroiodic acid. Yilmaz et al. [121] evaluate the exergy and energy efficiency of the S–I thermochemical cycles by varying several parameters, including operational conditions, state Energies 2020, 13, x FOR PEER REVIEW 8 of 95 properties and reference environment. The computed thermochemical cycle energy efficiency is 43.85%, and the efficiency of the whole system is 32.76%. The exergy efficiency is 62.39% and 34.56% for 43.85%, and the efficiency of the whole system is 32.76%. The exergy efficiency is 62.39% and 34.56% the cycle and the whole system, respectively. for the cycle and the whole system, respectively. Figure 3. Scheme of a sulfur–iodine three-step thermochemical water splitting. In the first endothermic Figure 3. Scheme of a sulfur–iodine three-step thermochemical water splitting. In the first step, H SO is decomposed into steam, SO and oxygen that is released. Steam and SO react in 2422 endothermic step, H2SO4 is decomposed into steam, SO2 and oxygen that is released. Steam and SO2 the second reactor with I , resulting in the formation of H SO that is recirculated to the first-step 224 react in the second reactor with I2, resulting in the formation of H2SO4 that is recirculated to the first- reactor and HI that enters the third-step reactor. In the third reactor, HI is split into I2 and H2. I2 is step reactor and HI that enters the third-step reactor. In the third reactor, HI is split into I2 and H2. I2 recirculated to the second-step reactor and H2 is released. is recirculated to the second-step reactor and H2 is released. The copper–chlorine thermochemical water splitting requires a maximum temperature slightly The copper–chlorine thermochemical water splitting requires a maximum temperature slightly lower than S–I cycles. Low temperature allows the coupling with various heat sources (mainly lower than S–I cycles. Low temperature allows the coupling with various heat sources (mainly nuclear reactors) [122]. Moreover, in Cu–Cl cycles, fewer challenges of equipment material and nuclear reactors) [122]. Moreover, in Cu–Cl cycles, fewer challenges of equipment material and product separation occur, but the higher price of electricity than heat reduces these advantages product separation occur, but the higher price of electricity than heat reduces these advantages from from an economic viewpoint [123]. The oxygen production occurs at approximately 800 K, and it an economic viewpoint [123]. The oxygen production occurs at approximately 800 K, and it represents the maximum temperature needed. However, Cu–Cl cycles also involve electrical energy represents the maximum temperature needed. However, Cu–Cl cycles also involve electrical energy for the electrolysis step in which hydrogen is produced [124]. In the electrolysis step copper(I) chloride for the electrolysis step in which hydrogen is produced [124]. In the electrolysis step copper(I) (CuCl) reacts in an aqueous solution with hydrochloric acid (HCl) giving rise to copper(II) chloride chloride (CuCl) reacts in an aqueous solution with hydrochloric acid (HCl) giving rise to copper(II) (CuCl2) and H2. After drying, CuCl2•H2O is hydrolyzed and forms the solid-phase melanothallite chloride (CuCl2) and H2. After drying, CuCl2•H2O is hydrolyzed and forms the solid-phase (CuOCuCl2) and HCl. Finally, CuOCuCl2 generates the molten salt CuCl and oxygen through an melanothallite (CuOCuCl2) and HCl. Finally, CuOCuCl2 generates the molten salt CuCl and oxygen endothermic reaction [124,125]. Balta et al. [126] propose an energetic and exergetic analysis of a Cu–Cl through an endothermic reaction [124,125]. Balta et al. [126] propose an energetic and exergetic thermochemical cycle coupled with a geothermal source for hydrogen production via a parametric analysis of a Cu–Cl thermochemical cycle coupled with a geothermal source for hydrogen production study. The overall energy efficiency obtained is 21.67%, and the exergy efficiency is 19.35%. via a parametric study. The overall energy efficiency obtained is 21.67%, and the exergy efficiency is Magnesium–chlorine thermochemical cycles have been developed as an alternative to 19.35%. the single-step reverse Deacon reaction in a two-step chemical loop by Simpson et al. [127]. Three-step Magnesium–chlorine thermochemical cycles have been developed as an alternative to the single- step reverse Deacon reaction in a two-step chemical loop by Simpson et al. [127]. Three-step hydrogen production is a hybrid thermochemical-electrolytic process, such as Cu–Cl cycles. It involves two thermochemical reactions that require a maximum temperature of 700–800 K and one electrolytic process. In the first step, the magnesium dichloride (MgCl2) reacts with steam and is split in magnesium oxide (MgO) and HCl. The second step involves the reaction between MgO and chlorinePDF Image | Green Synthetic Fuels
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