Biofuels from Biomass - Novel Combustion Concepts for Sustainable Energy Development - page 28

Among these, dark fermentations and photo fermentations are most widely studied
for biohydrogen production because they accomplish waste reduction and energy
generation and support sustainability and economic viability of the process. In dark
fermentation, complex organic compounds are converted into biohydrogen, volatile
fatty acids (VFAs), and CO 2 by fermentative bacteria at ambient temperature.
However, for this, competent microbial cultures (Clostridium, Ruminococcus,
Fibrobacter, Enterobacter, Rhodobacter sp.) are required to use substrate effi- -

ciently, which are usually complex in nature.
Development of technologies for biohydrogen production is still in early stages;
however, some fermentation strategies have demonstrated with great ef ciencies

which are reasonable for implementation. To develop an ef cient system to attain

future hydrogen demands sustainably, it is mandatory to have a directed investment
in a strategic biohydrogen research agenda. In such context, biohydrogen produc-
tion via dark fermentation could be a promising method due to its higher hydrogen
evolution rate in the absence of light and conversion of waste into environmentally
benign material. Photo fermentation can be linked with dark fermentation with two
stages of fermentation to increase process viability as the organic acid produced
during dark fermentation can be converted into biohydrogen by second stage of
photo fermentative bacteria. Apart from general approach of process optimization
and metabolic engineering of biohydrogen pathway, thermophiles could be an
option to get increased yield of biohydrogen. Main advantage of employing ther-
mophiles is high temperature favors conversion toward hydrogen production due to
favorable thermodynamic conditions, and many industrial organic wastewaters are
discharged at elevated temperatures that can be directly used as feedstocks.
Continuous fermentation would be another promising approach to increase
hydrogen yield. To add economic viability of the process, it is desirable to minimize
organic acid production and biomass generation for which signi

cant fraction of
energy was utilized from substrate. In continuous process, cell recycling and pro-
cess optimization for maximum biohydrogen production would positively improve
substrate to main product conversion with higher biohydrogen yield.
Some of the major bottlenecks for the development of biohydrogen production at
commercial scale include low rate of H 2 production, low ef ciency, incomplete

substrate conversion, and its partial conversion into organic acids, viz. acetic,
butyric, propionic acids, etc. Major challenges for biohydrogen production include
developing bioreactor design, optimization of bioreactor conditions, and metabolic
engineering of microbes for desired properties for higher production. Biohydrogen
can be produced from a wide variety of primary energy sources. Abundant agro-
residual biomass and waste ef uents from various industries could be a source for

biohydrogen production, where combination of waste treatment and energy pro-
duction would be an advantage.
Hydrogen produces only water when combusted, generating a 2.75 times higher
energy yield than hydrocarbon fuels and offers a crosscut from electricity to trans-
portation fuels and energy storage, which is of special importance for a future dom-
inated by renewable energies. It can be used for ef

fl

cient power generation in fuel cells.
In fuel cells, hydrogen offers high performance and can achieve 30 90 % ef ciency.

-