High Temperature Indirect Solar Thermal Hydrogen Processes

More recently, higher temperature processes have been considered (at T > 2000 K), such as two-step thermal chemical cycles using metal oxide reactions.2 The first step is solar: the endothermic dissociation of the metal oxide to the metal or the lower-valence metal oxide. The second step is non-solar, and is the exothermic hydrolysis of the metal to form H2 and the corresponding metal oxide. The net reaction is H2O = H2 + 0.5 O2, but since H2 and O2 are formed in different steps, the need for high-temperature gas separation is thereby eliminated:

Table 3. Summary of multi-step chemical cycles for indirect thermochemical hydrogen generation, from Ref. 4.

No._Elements in cycle_Maximum temperature, K_Total reaction steps in cycle

1 Hg,Ca,Br 1050 4

2 Hg,Ca,Br 1050 4

3 Cu,Ca,Br 1070 4

4 Hg,Sr,Br 1070 3

12 Fe,Cl 1070 5

13 Fe,Cl 1070 5

14 Fe,Cl 1120 5

15 Mn,Cl 1120 3

16 Fe,Cl 920 3

18 S (hybrid) 1120 2

21 Fe,Cl 920 5

22 Fe,Cl 920 4

where M is a metal and MxOy is the corresponding metal oxide. Such a two-step cycle was originally proposed50 using the redox pair Fe3O4/FeO. The solar step, i.e., the thermal dissociation of magnetite to wustite at above 2300 K, has been thermo-dynamically examined51 and experimentally studied in a solar furnace.52,53 It was found necessary to quench the products in order to avoid re-oxidation, but quenching introduces an energy penalty of up to 80% of the solar energy input. The redox pair TiO2/TiOx (with x < 2) has been considered.54,55 Solar experiments on the thermal reduction of TiO2, conducted in an Ar atmosphere up to 2700 K, experienced losses due to the chemical conversion limited by the interfacial rate at which O2 diffuses.

Other redox pairs, such as Mn3O4/MnO and Co3O4/CoO have also been considered, but the yield of H2 in the reaction has been too low to be of any practical in-terest.53 H2 may be produced instead by reacting MnO with NaOH at above 900 K in a 3-step cycle.56 Steinfeld further suggests2 that partial substitution of iron in Fe3O4 by other metals (e.g., Mn and Ni) forms mixed metal oxides of the type (Fe1-xMx)3O4 that may be reducible at lower temperatures than those required for the reduction of Fe3O4, while the reduced phase (Fe1-xMx)1-yO remains capable of splitting water.57-59

Cpc For Solar Furnace

Fig. 5. Schematic of a rotating-cavity solar reactor concept for the thermal dissociation of ZnO to Zn and O2 at 2300 K, modifed from Ref. 2. It consists of a rotating conical cavity-receiver (#1) that contains an aperture (#2) for access of concentrated solar radiation through a quartz window (#3). ZnO particles are continuously fed by means of a screw powder feeder located at the rear of the reactor (#4). The gaseous products Zn and O2 continuously exit via an outlet port (#5) and are quenched.

Fig. 5. Schematic of a rotating-cavity solar reactor concept for the thermal dissociation of ZnO to Zn and O2 at 2300 K, modifed from Ref. 2. It consists of a rotating conical cavity-receiver (#1) that contains an aperture (#2) for access of concentrated solar radiation through a quartz window (#3). ZnO particles are continuously fed by means of a screw powder feeder located at the rear of the reactor (#4). The gaseous products Zn and O2 continuously exit via an outlet port (#5) and are quenched.

One of the most actively studied candidate metal oxide redox pair for the 2-step cycle, is ZnO/Zn. As reviewed by Steinfeld,2 several chemical aspects of the thermal dissociation of ZnO have been investigated.55,60,61 The reaction rate law and Arrhe-nius parameters for directly irradiated ZnO pellets has been derived.62 The condensation of zinc vapor in the presence of O2 by fractional crystallization in a temperature-gradient tube furnace was studied.63 Alternatively, electro-thermal methods for in situ separation of Zn(g) and O2 at high temperatures have been experimentally demonstrated to work in small-scale solar furnace reactors.64-67 High-temperature separation further enables recovery of the latent heat of the products (e.g., 116 kJ/mol during Zn condensation). Figure 5 shows the schematic configuration of a solar chemical reactor concept that features a windowed rotating cavity-receiver lined with ZnO particles that are held by centrifugal force.68 In this arrangement, ZnO is directly exposed to high-flux solar irradiation and serves simultaneously the functions of radiant absorber, thermal insulator, and chemical reactant. Solar tests carried out with a 10 kW prototype subjected to a peak solar concentration of 4000 suns proved the low thermal inertia of the reactor system. The ZnO surface temperature reached 2000 K in 2 s, and was resistant to thermal shocks.2 Cycles incorporating ZnO continue to be of active research interest.69-73

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