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Water for Energy and Fuel Production
changing the temperature of the reaction in various cycles gives rise to the
requirements to change the pressure too, and so it would be necessary to
pump gases from one temperature and one pressure to another and this is
similar to the Carnot cycle.
2. With three to four cycles and need to change apparatus for each, plant
capital costs for unit hydrogen production are likely to be more than
those that occur in the electrolysis. Furthermore, at temperatures such as
800°C–900°C, the corrosion will cause the plant life to be short.
3. Generally, it is assumed [131,132] that the reaction would take place along the
free-energy pathway, but in reality it takes place down a reaction rate pathway [133,134] and not necessarily on thermodynamic pathway. Also, because
of possible side reactions, the final product may not be what was intended.
Due to these reasons, if cyclicity in thermochemical steps fails even by 1%,
a considerable amount of unwanted materials will build up and calculated
economics based on the cyclical nature of the process is no longer valid.
In spite of these arguments, a considerable investigation on thermochemical cycles
to produce hydrogen at the temperatures lower than one required for the thermal dissociation has been carried out. The moderate temperatures used in these cycles, in
general, also cause less material and separation problems. More than 300 different
types of chemical cycles have been proposed and tested. In this section, we evaluate
some of the important thermochemical cycles.
Previously, thermochemical cycles were characterized as those that use process heat
at temperatures <950°C. These are expected to be available from high-temperature
nuclear reactors. These cycles required three or more chemical reaction steps, and
they are challenging because of material problems and inherent inefficiency involved
with heat transfer and product separation in each step. One example is hybrid sulfuric
acid cycle that requires two steps incorporating one electrolysis step. The leading candidates for multistep thermochemical cycles include mainly three-step sulfur–iodine
(S–I) cycle based on thermal decomposition of sulfuric acid at 850°C and four-step
UT-3 cycle based on hydrolysis of calcium and iron bromide at 750°C and 600°C,
respectively [87–131] (Funk, 2011, pers. comm.; Bamberger, 2011, pers. comm.).
Recent advancement in the development of optical systems for large-scale solar
concentrations capable of achieving mean solar concentration ratio that exceeds 5000
suns allows high radiation fluxes capable of getting temperature >1200°C. Such high
temperatures allowed the development of efficient two-step thermochemical cycles
using metal oxide–redox reactions (see Figure 11.3). Some of the important cycles
are briefly described in Sections 11.4.1.1 through 11.4.1.10.
11.4.1.1 the Ut-3 Cycle
The UT-3 cycle is based on two pairs of chemical reactions [91,93,94] (Funk, 2011,
pers. comm.). The first pair is as followa:
(11.10)
CaO Br
CaBr
O
C
+
→
+
°
2
2
2
1
2
550
(
)