Environmental Engineering Reference
In-Depth Information
chemical reagents [
4
]. In these processes hydrogen is extracted from water thanks
to heat combined with closed-chemical cycles, necessary to reduce the very high
water decomposition temperatures ([2500C), difficult to be reached for heavy
limitations due to materials and heat source.
Electrolysis uses electricity to split water into hydrogen and oxygen by means
of an electrochemical approach (see
Sect. 2.1.2
). Hydrogen produced via elec-
trolytic processes can result in zero greenhouse gas emissions, depending on the
selected primary source of the electricity. In addition to renewable and nuclear
power, fossil fuels or biomass could be also used in stationary power plants to
produce electricity for water electrolysis (see Fig.
2.1
).
The analysis of photolytic methods (see
Sect. 2.1.3
) completes the discussion
about hydrogen production. They use sunlight energy to split water into hydrogen
and oxygen by photo-electrochemical and photo-biological approaches. These
direct sunlight-based processes are currently in the very early stages of research,
but could offer long-term potential for sustainable hydrogen production with low
environmental impact.
2.1.1 Thermal Processes
The thermal processes require the use of thermal energy to favour the advance of
chemical reactions providing hydrogen as direct product. Thermal approaches
involve, as reactants, various resources which contain hydrogen atoms as part of
their molecular structure, such as hydrocarbons or water, and the conversion
advance aimed at directly obtaining high hydrogen yield can be further improved
by catalyst addition (hydrocarbon reforming) or should require chemical com-
pound usage (water splitting by thermochemical cycles).
Natural gas SR, hydrocarbon partial oxidation or coal gasification are all
examples of 'thermal' methods and they are described in
Sects. 2.1.1.1
,
2.1.1.2
and
2.1.1.4
, respectively. The theoretical possibility to overcome the problem of
carbon dioxide emissions without using the CCS technology is based on other
possible 'thermal' methods, such as the hydrocarbon cracking (see
Sect. 2.1.1.3
),
or gasification of biomass-derived fuels (see
Sect. 2.1.1.5
). Also the thermal
production of hydrogen based on thermochemical cycles appears quite promising
(see
Sect. 2.1.1.6
), being its overall reaction based on the decomposition of water
aided by intervention of chemicals, anyway completely recycled.
2.1.1.1 Natural Gas SR
SR of hydrocarbons, in particular of Natural Gas (NG), is still today the major
industrial process for the manufacture of hydrogen [
6
-
8
].
This process was introduced in Germany at the beginning of the twentieth
century to produce hydrogen for ammonia synthesis, and it was further developed
