Page 42 - Biofuels Refining and Performance

Energy and Its Biological Resources 25

1.9.1 Photosynthetic bacteria
Small vesicles, called chromatophores, can be isolated from the membranes
of photosynthetic bacteria, which exhibit two types of electron transfer
chains resembling mitochondria and chloroplasts. Chroma-tophores
supported on artificial membranes permit the generation of 200 mV on
illumination. The salt-bacteria (Halobacterium halobium) contain a
simple protein–vitamin A aldehyde, known as bacteriorhodopsin, when
supported on artificial membranes that generate 250 mV on illumina-
tion. This system is simpler than its counterpart. There is a probabil-
ity that the entire system may be successfully synthesized or assembled.
Solar photocells made of bacteriorhodopsin show great promise.

1.10 Biofuels
Prospects of ethanol and biodiesel as substitutes for conventional fuels
will not be discussed here; these two aspects are presented in sufficient
detail in Chaps. 3, 4, 5, and 6. One of the promising approaches for
future fuel is, perhaps, hydrogen and methane, both of which could be
obtained from living, particularly microbial resources.
Photosynthesis is the main route through which oxidized carbon is
reduced and again oxidized back to carbon dioxide for the generation
of energy. Based on this principle, we can utilize a few steps from
this life chain. This topic could be called biophotolysis—alternatively,
photobiolysis.
In the system, direct electron transport from water to hydrogen has
not been demonstrated as a technically feasible reaction. For this, con-
tinued research is required to elucidate the basic nature of FeS (PEA,
ferredoxin, and hydrogenase). This may lead ultimately to the practical
feasibility of production of hydrogen (ideally 20
L/h). Section 1.16 dis-
cusses hydrogen in detail. One inherent problem is the stability of the
hydrogenase system because of its sensitivity to molecular oxygen pro-
duced during photosynthesis.
However, one may design a two-step or two-compartment system.
Reduced Co II is the oxygen-stable electron carrier between photo-
synthesis and hydrogenase. A higher ratio of reduced Co II or Co II
helps the evolution of hydrogen, in spite of the unfavorable redox
potential of the coenzyme. Only Co II (reduced) can be pumped or
transported from one stage (compartment) to the other. Photosynthesis
and hydrogenase systems have to be encapsulated or immobilized sep-
arately in order to retain their respective activity; the two stages or
compartments may be connected through fiber filters. An example
could be to use appropriate algae to produce reduced organic com-
pounds which can be pumped into bath of photosynthetic bacteria of
hydrogen fermentation.