Environmental Engineering Reference
In-Depth Information
BIOMASS POLYMERS
Proteins, polysaccharides, lipids
Hydrolysis
MONOMERS AND OLIGOMERS
Fatty acids, amino acids, and sugars
Fermentation
SIMPLE ACIDS AND ALCOHOLS
Volatile fatty acids
Alcohols (e.g.,
ethanol
)
H
2
+
CO
2
Acetate
Acetogenesis
Methanogenesis
CH
4
and CO
2
Fig. 1.2 Different stages at which biofuels such as acetate, ethanol, hydrogen, and methane
are generated. Steps enclosed within dotted oval show generation of biofuels
are bioavailable for microbial metabolism. Many microorganisms produce
enzymes that can hydrolyze polymers to monomers and oligomers. However,
some lignocellulose materials, present in many plants, are difficult to degrade
microbiologically or enzymatically. Lignocellulosic structures provide a physi-
cal barrier to enzymes or can be chemically resistant to enzymatic reaction. In
these cases, mechanical, physicochemical, or chemical pretreatment can make
lignocellulosic materials susceptible to enzymatic attack [8].
In biofuels production, fermentation generates biofuels from monomers by
rearranging the molecular bonds within the compounds and breaking them
down into smaller oxidized and reduced products. Oxidized products are typi-
cally CO
2
, which carries no electrons from the original product; therefore
capturing of the rest of electrons by the reduced byproduct brings the average
carbon oxidation state of the fermentation products to a value more negative
