Environmental Engineering Reference
In-Depth Information
1.1.1.1 Production of Biomass by Photosynthesis
Photosynthesis is a biochemical reaction that converts light energy into chemi-
cal energy. Starting materials for photosynthesis are H
2
O and CO
2
. In typical
aerobic photosynthesis, electrons from water combine with CO
2
so that the
oxidation state of carbon decreases from +4 to the average oxidation state of
biomass of around 0 [7]. This reduction is illustrated by the formation of
carbohydrate, CH
2
O, whose C has an oxidation state of 0. The O in H
2
Ois
oxidized to form O
2
:
Half reaction for water: 2H
2
OO
2
þ
4H
þ
þ
4e
Half reaction for carbohydratesynthesis : CO
2
þ
4H
þ
þ
4e
CH
2
O
þ
H
2
O
Whole reaction :
H
2
O
þ
CO
2
CH
2
O
þ
O
2
While this reaction is thermodynamically unfavorable, photosynthetic organ-
isms capture energy from light to energize the electron removal from H
2
O and
drive the reduction of CO
2
(approximately G
0
0
= 480 kJ/mol CH
2
O). Organ
isms that fix carbon for biomass synthesis using light energy are called photo-
autotrophs (photo = light; auto = self; troph = nourishment).
The goal of all biomass-based systems for generating useful energy is to
efficiently drive the photosynthetic reaction in reverse. A glucose molecule
(C
6
H
12
O
6
) is an example of carbohydrate that can form from photosynthesis:
6H
2
O
þ
6CO
2
C
6
H
12
O
6
þ
6O
2
The glucose reaction as written requires light energy of at least 2870 kJ to form
every mole of glucose. It is the absorption of solar energy that fuels the food
chain on Earth and also gives the Earth's atmosphere its characteristic 21%O
2
.
A successful biomass-based energy technology captures this energy efficiently,
usefully, and without creating serious environmental harm.
The method for energy capture depends on the type of biomass. Petroleum
and dry materials like coal and wood can be combusted. Coal and petroleum
are non-renewable, while the production rate of new wood is slow. Other types
of biomass, such as algae and cyanobacteria, are rich in lipids that can be
efficiently extracted as biodiesel and combusted. Animal manures, wastewater,
and many agricultural residues are wet and cannot be combusted efficiently;
they require microbiological processing (next sections) to convert the biomass
to convenient biofuels. Figure 1.2 provides an overview of the steps catalyzed by
microorganisms.
1.1.1.2 Hydrolysis and Fermentation
Hydrolysis is the first step in converting biomass, such as agricultural residue,
animal manure, and wastewater sludge, to biofuel (Fig. 1.2). Hydrolysis con-
verts organic solids and polymeric macromolecules to soluble molecules that
