Environmental Engineering Reference
In-Depth Information
1.1.1 A General Cycle for Biomass-Based Renewable Energy
Figure 1.1 shows the general cycle for biomass-based renewable energy, in
which microorganisms convert biomass to fuel. The cycle has three parts: (1)
production of biomass by photosynthesis, (2) generation of a convenient bio-
fuel, and (3) generation of useful energy from the biofuel. Photosynthesis
captures solar energy to reduce carbon dioxide and generate biomass in the
form of plants, algae, and cyanobacteria. The biomass can be used directly as
the biofuel source, or it may be used as input to the food-supply system. In the
latter case, processed biomass outputs of the food-supply system, such as wastes
and residues, become the biofuel source.
In most cases, the biomass used as a biofuel source is made of polymers,
especially proteins, lipids, and polysaccharides. However, these complex poly-
mers are not usually convenient for direct use in energy generation. Hence,
microbiological reactions are needed to convert biomass into useful biofuels,
such as methane, hydrogen, and ethanol. Biofuels can be collected and reacted
with oxygen to generate useful energy. The carbon dioxide produced during
microbial metabolism in parts 2 and 3 is recycled in photosynthesis and, hence,
the biomass technology is carbon neutral.
Carbon changes its oxidation state during the general cycle. A carbon atom
can have an oxidation state between +4 and -4, and the carbon in CO
2
is at its
most oxidized form, +4. The next sections show that following the oxidation
state of carbons helps understand the general cycle.
BIOMASS POLYMERS
(1)
Production of
biomass by
Photosynthesis
Proteins, polysaccharides, lipids
(2) Generation of
convenient biofuels
CO
2
Hydrogen, methane,
ethanol,
acetate
(3)
Generation of
useful energy
Fig. 1.1 Schematic of how microbial processes fit into the overall scheme for biomass-based
renewable energy technologies. Three major biofuels (ethanol, hydrogen, and methane)
appear in bold. Another useful biofuel is acetate. Microbial systems can participate in all
three steps. (1) Photosynthesis to produce algae or cyanobacteria. (2) Fermentative conver-
sion to produce hydrogen, methane, or ethanol, and acetate. (3) Generation of useful energy
from biofuels (e.g., electricity generation from acetate via an MFC)
