Page 77 - Biofuels Refining and Performance

60 Chapter Two

with respect to the climatic conditions. Depending on the habitat, plants
differ in their characteristic makeup. Their cell walls have varying
amounts of cellulose, hemicellulose, lignin, and other minor components.
The relative proportion of cellulose and lignin is one of the selection cri-
teria in identifying the suitability of a given plant species as an energy
crop. Herbaceous plants are usually perennial, having a lower proportion
of lignin that binds together with cellulose fibers. Woody plants charac-
terized by slow growth are composed of tightly bound fibers resulting in
their hard external surface. Generally, cellulose is the largest component,
representing about 40–50% of the biomass by weight; the hemicellulose
portion represents 20–40% of the material by weight. Cellulose is a
straight-chain polysaccharide composed of D-glucose units. These units are
joined by -glycosidic linkage between C-1 of one glucose unit and C-4 of
the next glucose unit. The number of D-glucose units in cellulose ranges
from 300–2500. Hemicellulose is a mixture of polysaccharides, composed
almost entirely of sugars—such as glucose, mannose, xylose, and arabi-
nose—and methylglucuronic and galacturonic acids, with an average
molecular weight of <30,000 g. Cellulose is crystalline, strong, and resist-
ant to hydrolysis, whereas hemicellulose has a random, amorphous struc-
ture with little strength. It is easily hydrolyzed by dilute acid or base.
A complete structure of lignin is not well defined because the lignin
structure itself differs between plant species. Generally, lignin consists
of a group of amorphous, high-molecular-weight, chemically related
compounds. Phenylpropanes, three carbon chains attached to rings of
six carbon atoms, are the building blocks of lignin. These might have one
or two methoxyl groups attached to the rings. Sugar/starch feedstocks,
such as cereals, have been traditionally used in biochemical conversion
of biomass to liquids such as ethanol. High-cellulose content of biomass
is generally more efficient and therefore preferred over the lignin-rich
biomass for conversion of glucose to ethanol. Depending on the end use
and type of bioconversion preferred, the choice of the plant species
varies. In northern Europe, the C woody species especially grown on
3
short rotation coppice, such as willow and poplar, and forestry residues,
are used [14]. In Europe, there is wide interest in the use of oilseed rape
for producing biofuel [15]. Brazil was one of the first countries to begin
large-scale fuel alcohol production from sugarcane.

2.5 Harvesting Plants for Bioenergy
Biomass can be converted into different types of products, including:

1. Electrical/heat energy
2. Transport fuel
3. Chemical feedstock