Environmental Engineering Reference
In-Depth Information
Alternatively; biomass gasification, a potentially carbon neutral method; can be
used to break down biomass into H
2
, CH
4
, and CO, which can in turn be used for
steam reformation and the gas shift reaction. Although net CO
2
production is ob-
served during these processes, the overall process of biomass gasification can be
considered carbon neutral since new biomass generated during the photosynthesis
fixes CO
2
. With this process, municipal and agricultural wastes could also be turned
into valuable commodities. The electrolysis of water, which is one of the most com-
mon methods for H
2
production, is carried out by an electric current passing through
water, splitting it up into H
2
and O
2
. This is a very energy intensive process and
could be potentially emission free if only clean, renewable sources such as wind,
solar, hydro, or geothermal energy are used for the generation of electricity.
Hydrogen production via biotechnology (Bio-H
2
), on the other hand, employs
the use of either dark fermentative or light dependent hydrogen producing organ-
isms. This method of hydrogen production is an attractive alternative to conven-
tional thermo-chemical and electro-chemical methods since it is a potentially car-
bon neutral process which is carried out at lower temperatures and pressures, and is
therefore less energy-intensive than thermo-chemical and electro-chemical process-
es. Furthermore, unlike thermo-chemical methods, which involve the conversion of
nonrenewable fossil fuels into hydrogen, fermentative hydrogen production can uti-
lize renewable carbohydrate-rich substrates such as waste biomass from municipal,
agricultural, and forestry sectors, while light driven biological hydrogen production
processes utilize light energy, water, and/or CO
2
.
2
Biohydrogen Production
The term “biohydrogen” refers to hydrogen production by living organisms. Hydro-
gen production via biotechnology can be classified as follows:
2.1
Direct Biophotolysis of Water
(
)
2H O
+→+
light
2H
O
by cyanobacteria, microalgae
2
2
2
2.2
Indirect Biophotolysis of Water
( )
(
)
6H O
+
6CO
+ → +
light
C H
O
6O
a
by microalgae
2
2
6
12
6
2
(
)
C H
O
+→+ +
2H O
4H
2CH COOH
6CO
(b)
by cyanobacteria
6
12
6
2
2
3
2
