Page 214 - Materials Chemistry, Second Edition

200 LIFE CYCLE ASSESSMENT HANDBOOK

Numerous researchers anticipate that hydrogen will replace petroleum prod-
ucts for fuelling transportation vehicles. Although hydrogen atoms exist
in abundance in nature in water, molecular hydrogen (which is the form of
hydrogen energy) does not. Hydrogen needs to be produced and there are
several production methods including steam reforming of natural gas, coal
gasification, water electrolysis and thermochemical cycles. In this chapter,
the application and benefits of exergetic life cycle assessment are illustrated
through a case study involving a comparative environmental assessment of
nuclear-based hydrogen production via thermochemical water splitting using
a copper-chlorine (Cu-Cl) cycle. In the assessment, exergy efficiencies and air
pollution emissions are evaluated for all process steps (e.g., uranium process-
ing, nuclear plant operation and hydrogen production), and the following
impact categories are considered: acidification potential, eutrophication poten-
tial, global warming potential and ozone depletion potential.
Thermochemical water splitting decomposes water into hydrogen and
oxygen, and has the potential to be a cleaner and more cost-effective hydro-
gen production method than other processes. A cyclic approach is required
since the temperature required to split water directly in one step is too high
to be practical. A series of selected chemical reactions can split water at much
lower temperatures (Serban et al., 2010). A variety of thermochemical water
decomposition cycles have been identified (Funk, 2001), but few have pro-
gressed beyond theoretical calculations to working experimental demonstra-
tions. Most of these cycles require process heat at temperatures exceeding
800°C. Due to its lower temperature requirements (around 530°C), the Cu-Cl
thermochemical water decomposition cycle has some advantages over other
cycles (Naterer et a\., 2008), including reduced material and maintenance costs.
Moreover, the Cu-Cl cycle has some advantages over other hydrogen produc-
tion methods, and can utilize low-grade or waste heat to improve its efficiency
(Naterer et al, 2009).
Fossil fuels, nuclear energy and renewable energies can be used as energy
sources for producing hydrogen. Fossil fuel use impacts the environment
significantly. Although renewables are usually considered the most environ-
mentally benign alternative, an important challenge is to obtain sustainable
large-scale hydrogen production. Using nuclear energy for hydrogen pro-
duction is advantageous for two main reasons: (1) nuclear plants do not emit
GHGs during operation, and (2) nuclear energy can contribute to large scale
hydrogen production (Orhan, 2008). For these reasons, thermochemical water
decomposition linked with nuclear plants is seen as a promising alternative for
hydrogen production. The Generation IV SCWR (super-critical water cooled
reactor) is viewed as a particularly suitable option for pairing with the Cu-Cl
thermochemical cycle.
Although hydrogen is a relatively clean energy carrier, since its oxidation
emits mainly water, negative environmental impacts can arise during its pro-
duction. But the environmental impact of hydrogen use is highly dependent
on the method employed for its production, so the environmental impact of
hydrogen production methods needs to be investigated. Thermochemical water