Page 82 - Biofuels Refining and Performance

Photosynthetic Plants as Renewable Energy Sources 65

which will not only produce sugar but also different by-products and
electricity [24]. The process can produce its own power.
Fermentation is equally important. Anaerobic and restricted aerobic
digestion with selected algae species allow us to harvest hydrogen and
clean fuels, without much loss of biomass and with the least amount of
waste products. In an aerobic process, the process is carried out by oxi-
dizing the volatile matter into biodegradable organic fractions of solid
waste. Air acts as a source of oxygen, and aerobic bacteria act as a cata-
lyst. The change occurring during the process may be represented as

sAerobic bacteriad S CO 1 H O 1 Organic manure
Biomass 1 O 2 2 2
Anaerobic digestion is carried out by segregating the nonbiodegrad-
ables and the biodegradables at the same time. This may be done man-
ually or mechanically. The smaller pieces of inorganic materials like clay
and sand may be removed by washing the biomass with water. The
washed material is then shredded into a size that will not interfere
with mixing and may be more amenable to bacterial action. The shred-
ded biomass is then mixed with sufficient quantity of water, and slurry
is fed into a digester system. If necessary, nutrients like nitrogen, phos-
phorus, and potassium have to be added to the digester. The process
involves four groups of bacteria in the digested slurry as follows:

1. Hydrolytic bacteria catabolize carbohydrates, proteins, lipids, and
, and CO .
so forth contained in the biomass to fatty acids, H 2 2
2. Hydrogen-producing acetogenic bacteria catabolize certain fatty acids
and some neutral end products to acetate, CO , and H .
2
2
3. Homoacetogenic bacteria synthesize acetate, using H , CO , and
2
2
formate.
4. In the final phase, called the methanogenic phase, methanogenic
bacteria cleave acetate to methane and CO .
2
Water acts as a catalytic agent in methane formation. Thus water is
acted upon by enzymes, itself breaking down to hydrogen and oxygen.
Hydrogen is used by microorganisms to reduce CO to CH , while oxygen
2
4
oxidizes carbon dioxide, i.e., makes it acidic (H CO ). In simple terms,
3
2
acetate (in presence of CoI) is simultaneously oxidized to CO and
2
reduced to CH . For details, refer to Chap. 1, methanation, and Baker’s
4
and Ganzalus pathway. Thus, methane-forming bacteria play an impor-
tant role in the circulation of substances and energy turnover in nature.
They absorb CO, CO , and H to give hydrocarbon and methane and help
2
2
synthesis of their own cell substances. During anaerobic digestion, gas
containing mainly CH and CO is produced. The gas is known as biogas,
2
4
which is used for the generation of electricity or fuel. The residual biomass