Biomedical Engineering Reference
In-Depth Information
4
Emerging Technologies for Biohydrogen Production......................................................
153
4.1
Limitation of Dark Fermentation .............................................................................
153
4.2
Combination of Dark Fermentation and Photofermentation...................................
154
4.3
Coupling of Dark Fermentation with an MEC........................................................
155
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Perspectives .......................................................................................................................
158
References ...............................................................................................................................
159
1 Fundamentals of Anaerobic Hydrogen Fermentations
Biological hydrogen production under anaerobic conditions can be classified into
two categories according to the types of microorganisms employed: photofer-
mentative hydrogen production and dark-fermentative hydrogen production.
1.1 Photofermentation
Photofermentative hydrogen production in purple non-sulfur photosynthetic
bacteria is catalyzed by nitrogenase under nitrogen-deficient conditions using
light energy and reduced compounds [ 1 ]. The overall biochemical pathway of the
photofermentation process can be expressed as follows:
Hydrogen production by photofermentative bacteria has been widely studied for a
long time. Purple non-sulfur photosynthetic bacteria have photosystem I, but not
photosystem II, which makes these anoxygenic bacteria suitable for hydrogen pro-
duction since photosystem II generates oxygen, which inactivates nitrogenases
irreversibly. Batch and continuous processes have been developed [ 2 - 4 ], and cells
immobilized by supporting matrices such as poly(vinyl alcohol), agar gel, and porous
glass have also been explored to enhance hydrogen production [ 5 - 8 ]. However, there
are significant disadvantages in the photofermentative hydrogen production, such as
high energy demand by the nitrogenases and low light energy harvest efficiency and
hydrogen productivity. Therefore, photofermentation alone is not economically
competitive for hydrogen production, but it can be integrated with dark fermentation
to improve substrate conversion as well as hydrogen production [ 9 ].
1.2 Dark Fermentation
Dark-fermentative microorganisms are mainly anaerobic bacteria, which can
generate molecular hydrogen during the decomposition of carbohydrate substrates.
The main soluble products are organic acids, including acetic, propionic, and
butyric acids, and ethanol [ 10 ]. The dark fermentation is exergonic. However,
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