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Energies 2020, 13, 420 5 of 96 depends on temperature and pressure. The process can be summarized by the overall endothermic reaction [71]: H2 O ↔ x1H2 O+ x2OH+x3O+x4H+x5H2 +x6O2 (1) The methods used for the energy transfer to water molecules are: (i) bulk heating of water vapor in a furnace cavity with a fluid tubular wall; (ii) direct contact of cold vapor with a hot solid layer covering the walls of the cavity reactor and (iii) direct contact of liquid water with the hot solid plate [72]. The separation of the resulting gas mixture constitutes a critical issue due to the risk during slow cooling of recombination of hydrogen and oxygen that could lead to an explosive mixture. Indeed, the lower limit of flammability of a mixture of oxygen and hydrogen is approximately 5%. The separation is realized via two main processes: at the reaction temperature or after a rapid quenching. The product separation at the gas temperature is carried out by the continuous removal of one of the reaction products enhancing the thermolysis reaction and preventing the recombination. The separation is obtained via a variety of methods [71,72]: • Microporous ceramic membranes made of refractory materials in sub-atmospheric pressure reactors that allow selective separation of a species [73]. • Membranes semipermeable to oxygen made by a high temperature solid electrolyte material (ZrO2, CaO, CeO2, Y2O3, perovskite) placed in a chemical potential gradient. The mixed ionic and electronic conducting (MIEC) membranes are realized with refractory oxides with a mixed type of electrical conductivity with ionic and electronic components. In the presence of a gradient of oxygen pressure, a chemical potential induces migration of oxygen ions across the membrane, and an electron current circulates in the opposite direction due to electronic conductivity of the material [74]. • Metallic membranes made of palladium and its alloys. Hydrogen is dissociated on the nonporous metallic membrane surface, and its atoms diffuse through the membrane and are desorbed and recombined after the membrane [20,75]. • A rotating cylindrical vessel in which the gas mixture could pass through. The centrifugal field of force modifies the equilibrium pressure of species, separating them [76]. • Supersonic jets generated by expanding a gas mixture rich of dense gases near the jet axis and with lighter gases on the external part. This distribution leads to beam deflection and gas separation due to centrifuge forces in the curved flow [77]. The diffusion flow increases with high temperature and a high-pressure gradient. The product gas separation via rapid quenching is realized by cooling the product gas at a cooling rate of 105–106 K/s by the direct contact with a cold auxiliary fluid. The quenching is a highly irreversible process, and it permits to recover hydrogen up to 90%. The temperature of the cold, inert fluid is 450–500 K, since the temperature should decrease by 1500–2000 K in a few milliseconds [78]. The cooling rate must be faster than the rate of heat released by the recombination reaction. The quenching process is realized either in an external vessel or internally in the reactor (auto-quenching). The hot gas is cooled down by heat exchange cooling, auto-cooling by the jet of cold stream, auto-cooling by direct contact with cold water or by rapid turbulent cold gas jets [79]. 2.2. Thermochemical Cycles Thermochemical cycles are a suitable process for the production of both syngas and hydrogen. Water splitting via thermochemical cycles addresses problems related to the separation of product gases since the production of hydrogen and oxygen occurs in different steps. The operating temperature needed is relatively low (1100–1500 K) [80]. If only water constitutes the inlet fluid, the product gas is hydrogen, whereas if a mixture of water and carbon dioxide is introduced in the reactor, syngas is generated. Therefore, water, carbon dioxide and heat are the input and oxygen, carbon monoxide and hydrogen the only output. The other reagents are regenerated in a closed loop. The production ofPDF Image | Green Synthetic Fuels
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