Environmental Engineering Reference
In-Depth Information
development of a more complex concept of water decomposition, based on multi-
step thermochemical processes. This approach is founded on the characteristics of
several chemical reagents, capable to lower the temperatures of water decompo-
sition down to a commercially viable value, inferior to 1200C.
A schematic overall process involves at least two steps:
H
2
O
þ
2X2XH
þ
1
=
2
O
2
ð
2
:
20
Þ
2XH2X
þ
H
2
ð
2
:
21
Þ
where X represents the generic chemical agent. Obviously the net balance reaction
is the reverse of Eq.
2.13
:
H
2
OH
2
þ
1
=
2
O
2
DH
¼þ
288 kJ/mol
ð
2
:
22
Þ
where DH is calculated considering the water in liquid form. The nature and the
role of intermediate compounds (XH) are the key point for a successful process,
strictly related to the reaching of the following targets [
4
]:
1. Gibbs free energy variation of all individual reaction steps must approach zero.
2. The different steps should be minimal.
3. Direct and reverse reaction rates of the different steps need to be very fast.
A lot of thermochemical cycles have been proposed in literature [
4
,
67
,
68
],
potentially able to exploit the high-temperature energy coming from nuclear or
concentrating solar plants (CSP).
In particular iodine-sulphur reaction results quite attractive [
68
]. It consists of
three steps at different operation temperatures, which involve the H
2
SO
4
and HI
dissociation and the re-production of both acids starting from I, SO
2
and H
2
O.
Particular interest is also focused on CeO
2
/Ce
2
O
3
cycle, cerium-chlorine cycle
(Ce-Cl), Zinc-zinc-oxide cycle (Zn/ZnO), but also on a Cu-Cl cycle, which is a
cycle with an electrochemical step [
69
].
This technology appears really promising for a massive efficient hydrogen
production but it is still far to be practically realized in few years, basically
because of engineering and material constraints associated with high operation
temperature (not inferior to 900-1000C).
2.1.2 Electrolytic Processes
The possibility to store the surplus of electric energy produced by the power plants
into a hydrogen carrier represents an attractive potential solution to optimize the
overall efficiency of energy production and utilization. This idea requires a
technology able to transform the excess of produced electric energy into the
chemical energy of hydrogen molecule.
