Environmental Engineering Reference
In-Depth Information
Figure 9.2.3
Direct water splitting extent at different temperatures.
practice in the near future, although currently a high temperature up to 3,500
◦
C can
be obtained with solar concentrating furnaces of laboratory and pilot scales (Haueter
et al., 1999; Riveros-Rosas et al., 2010). At this high temperature, it is challenging
for refractory materials and construction of equipment. Moreover, the direct ther-
molysis product is a gas mixture of hydrogen and oxygen, which has a considerable
explosion risk.
To lower the water themolysis temperature, some auxiliary chemicals can be intro-
duced to form at least one intermediate in an association step and then the intermediate
releases hydrogen and/or oxygen separately in other dissociation steps. This will form
an “indirect'' water splitting cycle. The integration of the association and dissociation
steps forms a closed cycle wherein the net effect of the integration is that only water
is decomposed and the auxiliary chemicals are recycled inside the cycle. This type of
water decomposition is often termed as a “thermochemical'' cycle. If a small portion of
energy is supplied to the cycle as electricity, it is then a type of hybrid thermochemical
cycle rather than a purely thermal cycle. About three hundred thermochemical cycles,
either purely thermal or hybrid, have been reported previously (Abanades et al., 2006).
For fully thermal hydrogen production, two-step water splitting cycles based on
metal redox reactions are leading examples. These cycles usually consist of an endother-
mic reduction reaction where oxygen is produced from a metal oxide, and a hydrolysis
reaction where hydrogen is produced (Xiao et al., 2012; Le Gal et al., 2010):
M
x
O
y
→
M
x
O
y
−
1
+
½O
2
(9.2.7)
M
x
O
y
−
1
+
H
2
O
→
M
x
O
y
+
H
2
(9.2.8)
where M denotes a metal, and the subscripts x and y mean the numbers of the metal
and oxygen atoms in a metal oxide molecule. Zinc is a metal example currently under
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