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BIOFUELS AND BIOFUEL CELLS FROM MICROORGANISMS
AN ALTERNATIVE SOURCE OF ENERGY
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Photosynthetic microorganisms,
such as micro-algae and cyanobacteria are able to
harness low-intensity solar energy and store it as
latent chemical energy in the biomass. This energy
can then be released via biochemical conversion. The
structural and storage carbohydrates in biomass have
low energy content and it is necessary to concentrate
the energy content further for fuel application. Anaerobic
microbial fermentation is an efficient and widely
used method for such conversion process. Useful renewable
fuels produced by microorganisms include hydrocarbon,
ethanol, methane and hydrogen. Biofuel cells which
can release energy in fuel chemicals to generate electrical
energy at ambient temperature have been developed. |
Fuel produced by microbes has
the potential for helping to meet world energy demands.
Living organisms assimilate and concentrate energy
in their biomass and products. Hence, biomass in its
various forms is an attractive alternative source
of energy (Lee, 2003). Through photosynthesis, biomass
collects and stores low-intensity solar energy. This
can then be harvested and released via biochemical
conversion. Biological process are involved in both
the harnessing of solar energy and upgrading of low
energy feed stocks to biomass fuels. |
Photo-biological hydrogen
production:
Chloroplast of some photosynthetic
microorganisms such as the green alga chlorella
in the presence of suitable electron acceptors is
capable of producing H2 and O2
through direct photolysis of water. In the system,
the substrate (electron donor) is water, sunlight
as the energy source is unlimited, and the product
(hydrogen) can be stored and is non-polluting. Moreover,
the process is renewable, because when the energy
is consumed, the substrate (water) is regenerated. |
| Conversion
of biomass energy:
Structural and storage carbohydrates in biomass having
low energy content cannot be used as fuel directly.
It is necessary to concentrate the energy content
further for fuel applications. The use of microorganisms
to produce commercially valuable fuels depends on
getting the right microorganisms which can produce
the desired fuel efficiently. The quantum of substrate
they require for fermentation should be low and inexpensive
(Tanaka et al., 1988). It is imperative that
the production of synthetic fuels does not consume
more of natural fuels than what they produce. Anaerobic
microbial fermentation is an efficient and widely
used route for such conversion processes. |
| a) Alcohol (ethanol) Production:
The microbial production of ethanol has become an
important source of a valuable fuel, particularly
in regions of the world that have abundant supplies
of plant residues. Fermentation production of fuel
alcohol can be through microbial conversion of low
cost agricultural substrates high in starch and sugar
content. Numerous microorganisms are capable of producing
ethanol, but not all are suitable for industrial processes.
Yeast cultures, particularly saccharomyces,
have been most extensively examined because they are
very efficient in converting sugars into ethanol,
i.e. cost competitive and are not as strongly inhibited
by high ethanol concentrations as are other microbes. |
The following
equation illustrates the basic biochemical mechanism
by which the ethanol is produced through the fermentation
process.
The yeasts commonly used in industrial
alcohol production include Saccharomyces
cerevisiae (ferment glucose, fructose,
maltose and maltoriose), S.uvarum, S.diataticus
etc. The ethanol productivity ranges between 1
and 2 g ethanol/h/g cells.
Selected bacterial cultures were
examined for use in ethanol production processes
because of their higher temperature tolerance.
However, their yield of ethanol was not as high
as in yeast fermentation (Lee, 2003). Recently,
the bacterium Zymomonas mobilis has been
selected to achieve a high productivity of 2.5-3.8
g ethanol/h/g cells. |
| b) Methane production:
Methane (CH4) is an energy-rich fuel that
can be used for the generation of mechanical, electrical
and heat energy. Large amounts of methane can be produced
by anaerobic decomposition of waste materials. Efficient
generation of methane can be achieved using algal
biomass. In microbial production of methane, naturally
occurring mixed anaerobic bacteria population is always
used and cells are retained within the digester. During
the fermentation process, a large amount of organic
matter is degraded, with a low yield of microbial
cells, while about 90% of the energy available in
the substrate is retained in the easily purified gaseous
products CH4. The end product is a mixture
of methane gas and CO2 (also called biogas).
Fermentative bacteria hydrolyze the degradable primary
substrate polymers such as proteins, lipids and polysaccharides
and decompose to smaller molecules with the production
to acetate and other saturated fatty acids, CO2
and H2 as major end products. The second
group is the obligate H2 producing acetogenic
bacteria, which metabolise low molecule organic acids
(end products of the first group) to H2
and acetate (and sometimes CO2). |
Electricity from biofuel
cells:
Biofuel cells could release energy in fuel chemicals
to generate electrical energy at ambient temperature.
Fuel cells covert energy more efficiently than conventional
power engines such as the internal combustion engine
and produce almost no pollution. The basic-set up
of the fuel cell is two electrodes placed in an electricity
conducting electrolyte, separated by an ion exchange
membrane. The arrangement allows the electrochemical
equivalent of combustion to occur.
a) Generation of electricity
from hydrogen gas:
Hydrogen is a very attractive fuel because of its
high energy content (18.7 kJ/g), which is about four-fold
greater than ethanol and over two-fold higher than
methane. A wide range of microorganisms produce hydrogen
as a part of mechanisms for disposing of electrons
that are generated during metabolic reactions: |
| The generation of hydrogen using microorganisms,
or cell-free systems based on microbial components,
is still very much in its infancy. However, there
are three possible routes of production (Waites et
al., 2001):
Biophotolysis of water:
It involves splitting water using light energy and
does not require an exogenous substrate. In vivo,
the energy generated is normally used to form
reduced nicotinamide adenine dinucleotide phosphate
(NADPH). In the presence of a bacterial hydrogenase
and an appropriate electron carrier, molecular hydrogen
can be generated.
Photoreduction: it is a
light dependent process of decomposition of organic
compounds performed by photosynthetic bacteria. This
is an anaerobic process requiring light and an exogenous
organic substrate, which is inhibited by oxygen, dinitrogen
and ammonium ions. Formation of hydrogen is attributed
to a nitrogenase that can reduce protons as well as
di-nitrogen. Members of the Chlorobioaceae, Chromatiaceae
and Rhodospirillaceae, carry out photoreduction. Bacteria
having higher potential are probably the purple non-sulphur
bacteria, such as Rhodospirillium spp., which
photometabolize organic acids.
Fermentation: fermentation
of organic compounds by many bacteria generates a
small amount of hydrogen. For example, some enterobacteria
produce hydrogen and CO2 by cleaving formate.
In clostridia it is produced from reduced ferrodoxin.
The role of microorganisms in electricity generation
may involve microbially produced gaseous and liquid
fuels, such as ethanol or methane, being used to drive
conventional mechanical generators. Possible routes
are via intact microorganisms or microbial enzymes
incorporated within fuel cells (Fig. 1).
Hydrogen gas as a fuel enters at one electrode and
oxidant, usually oxygen from the air, at the other.
The anode is coated with the enzyme hydrogenase, while
the cathode with the laccase. On the anode, hydrogen
molecules split into their constituent protons and
electrons. The enzymes catalyze a reduction-oxidation
reaction across the membrane, releasing energy, which
pushes electrons round an external circuit. At the
cathode, the protons and electrons combine with oxygen
to form water. The fuel does not actually burn; therefore
fuel cells do not produce pollutants associated with
combustion, such as carbon oxides and oxides of nitrogen.
Cells that use hydrogen generate only water compared
to fossil fuel, which produce water and carbon dioxide
as waste product. This type of fuel cells is called
proton exchange membrane cell (PEM).
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b) Generation
of electricity form methanol:
In this case, the fuel is methanol.
The direct methanol fuel cell (DMFC) runs on a dilute
mixture of about 2% methanol in water. The methanol
is converted into formate on the anode. The proton
then reacts with oxygen as in a PEM cell.
The metabolically active microorganisms,
such as Proteus vulgaris and Anabaena
variabillis immobilized in a biofuel cell could
convert energy in their substrate (glucose for the
former and light for the later) into electricity (Allen
and Bennetto, 1993). A biofuel cell in which bacteria
Proteus vulgaris and Escherischia coli
were used as sulfate reduction catalysts was in operation
for 5 years, demonstrating thus its long-term stability.
The disadvantage of biofuel
cell is that the power output is low (1 kW at 40 mA/cm2).
Thus, it is used for specific purposes, such as small
medical and military apparatuses used in the field
and in space missions. Biofuel cells are considered
to be ecofriendly and can be used as substitutes in
order to reduce green houses gas emission.
References:
Allen, R. M. and H. P. Bennetto,
1993. Microbial fuel cells: electricity production
from carbohydrates. Appl. Biochem. Biotechnol.,
39/40: 27-40.
Lee, Y.K., 2003. Microorganisms
and production of alternative energy. In: microbial
Biotechnology: Principles and Applications, (L.E.
Kun, Ed.), World Scientific Publishing Co. Pte. Ltd.,
655-670.
Tanaka, K., N. Kashiwagi and
T. Ogawa, 1988. Effects of light on the electrical
output of bioelectrochemical fuel-cells containing
Anabaena variabilis M-2: Mechanism of the
post- illumination burst. J. Chem. Technol. Biotechnol.,
42: 235-240.
Waites, M. J., N. L. Morgan,
J.S. Rockey and G. Higton, 2001. Industrial Microbiology:
An introduction. Blackwell Science Ltd., 288 pp.
Published by: Surajit Das,
P. S. Lyla and S. Ajmal Khan
CAS in Marine Biology
Annamalai University
Parangipettai - 608 502.
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