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Energies 2020, 13, 420 9 of 96 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 chlorine (Cl2), generating O2 and MgCl2. The last step concerns the electrolysis of anhydrous HCl into H2 and Cl2 [128,129]. Balta et al. [130] perform the parametric study of energy and exergy efficiency also for this process. The overall energy efficiency obtained is 63.63%, and the exergy efficiency is 34.86%. 2.2.1. Reactors The solar reactor is the item that receives and transforms the solar radiation into thermal energy. The reactor has to endure high temperature and high heating rate, promote the heat transfer and reduce energy losses. The requirement of high temperatures is a critical issue of thermochemical water splitting due to the risk of the thermal shock of reactor walls. Further, the reactivity and corrosivity of some reactants (e.g., sulfur and iodine) and the strong oxidizing environment necessitate materials resistant to corrosion. Possible solutions are the use of compatible materials (e.g., ZrO2, graphite or silicon carbide) and the design of reactors to prevent direct contact between chemicals and wall materials [115]. In 1995, Tamaura et al. [131] proposed a first two-step water-splitting cycle, demonstrated experimentally in the solar furnace of the Paul Scherrer Institute, with a two-stage concentrator system and a tubular packed-bed reactor of quartz. Gokon et al. [132] propose and demonstrate on a laboratory-scale prototype reactor a thermochemical two-step water-splitting cycle using an internally circulating fluidized bed reactor with a redox system of iron-based oxides and ferrite. The cylindrical reactor was made of stainless steel with a beam-down transparent optics. The beam-down optics is a quartz window installed in the ceiling of the reactor to prevent contact between particles and the transparent window. Particles are transported upwards in a draft tube where are directly heated by concentrated solar radiation and move downward in the annulus region. The particles circulation guarantees solar energy transfer from the top to the bottom. The circulating fluidized bed reactor provides a more uniform temperature distribution compared to a packed-bed reactor. The German Aerospace Center (DLR) develops in the past years fixed coated ceramics reactors as a promising alternative to packed-bed reactors within the project HYDROSOL (HYDROgen production in a SOLar monolithic reactor) [133]. Reactors are realized by a ceramic multi-channeled monolithic honeycomb structure; within are incorporated active iron-oxide-based redox pairs. Walls are coated with silicon carbide (SiC) that ensures high thermal shock resistance, high mechanical strength and high thermal conductivity allowing the operational temperature of approximately 1073–1473 K. The whole plant was tested at the laboratory scale in the solar furnace at DLR, in Cologne. Fluid-wall reactor configuration appears as a promising technology to control high temperature and corrosivity of thermochemical cycles. The heating rate is very high (up to 105 K/s), and the graphite structure allows ultra-high operational temperatures. The graphite solar-powered fluid-wall aerosol flow reactor has been studied principally for the decarbonization of methane [134]. The reactor is realized with three concentric vertical tubes: the inner tube (“reaction tube”) is made of porous graphite; the central tube (“heating tube”) is composed of solid graphite; the outer tube (“protection tube”) is made of quartz. The concentrated solar power enters through the outer tube that is either totally or partially transparent and heats directly the center solid graphite tube that radiates to the inner porous graphite tube. A non-oxidizing atmosphere is guaranteed by feeding an inert or compatible gas such as argon, helium, neon or nitrogen that flows radially through the porous graphite tube to prevent contact and reactions between graphite and process chemicals (reactants and products). The realization of the protection tube with a quartz window instead of the totally transparent tube provides a reflective inner surface that increases the thermal efficiency [135].PDF Image | Green Synthetic Fuels
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