Page 43 - Biofuels Refining and Performance

26 Chapter One

One partial modification will be to collect oxygen during the day and
hydrogen at night, at the expense of accumulated reduced coenzymes,
made operative by anaerobically adapted microalgae or nonheterocystous
nitrogen- fixing blue-green algae. For product separation, the enzyme
technology or immobilization is inapplicable for biophotolysis. However,
there are potential practical applications of immobilized hydrogenase in
biochemical hydrogen–oxygen fuel cells. If such enzymes can be immobi-
lized on an electrode surface, an inexpensive fuel cell might be developed,
which would increase the energy recoverable for hydrogen to save fuels.
Awareness of the limitations due to efficiency, engineering, and the
economy of these principles will save disappointment and encourage con-
tinued research. Geographical location and frequency of weather change
limits the insolation. The best photosynthetic efficiency is only 6% of the
2
total incident solar radiation, i.e., 5 kg/(m
yr) of H by biophotolysis.
2
Half of this could be a very satisfactory achievement.
1.10.1 Heterocystous blue-green algae
(example, Anabaena cylindrica)
The heterocyst, regularly spread among more numerous vegetative cells
(ratio 1:15), receives carbon compounds fixed by the neighboring vege-
tative cells in exchange of the nitrogenous compounds fixed by them.
Nitrogenase, like hydrogenase, needs an anaerobic environment to func-
tion and can produce hydrogen only under certain conditions (absence of
molecular nitrogen). The ratio of evolution of hydrogen and oxygen roughly
corresponds to the ratio of the heterocysts and vegetative cells and also
with the ratio of nitrogen and carbon for nutritive requirements.
If the algal culture is exposed to argon for about 24 hours, due to nitro-
gen starvation, differentiation of the heterocysts increases from 6% up to
20%. In addition, a yellowish color appears due to the loss of the light-
trapping pigment phytocyanin, resulting in less carbon dioxide fixation,
i.e., oxygen evolution and an increase in light conversion efficiency by
almost 0.5%. Induction of reversible hydrogenase in the heterocysts, as its
theoretically higher turnover principle, is less affected by N 2 and O 2 , and
independent of ATP, it becomes more desirable and needs heterocysts to
be genetically improved.

1.10.2 Photofermentation by photosynthetic
bacteria (example, Rhodospirillium rubrum)
Hydrogen production by photoheterotrophic bacteria is principally sim-
ilar to that of blue-green algae, capable of fixing nitrogen and produc-
ing hydrogen. The microbes are capable of converting large varieties of
2
organic compounds to carbon dioxide and hydrogen up to 50 kg/(m
yr).
Practical applications of these bacteria are more of an engineering
problem than one of scientific “know-how.” The scope of newer research