Environmental Engineering Reference
In-Depth Information
All the existing types of biomass feedstocks can be divided in five groups: sugar crops,
starch crops, oil based materials, grasses, lignocellulosic materials and organic residues and
others. In the next sections, each of this group is depicted.
3.1.1. Sugar crops
Concerning sugar crops, they provide sugars in a simple form, i.e. mono- or
disaccharides, which can be readily used in conversion processes. Six-carbon, single-
molecule “monosaccharide” sugars (C
6
H
12
O
6
) include glucose, galactose and mannose, while
the most common 5-carbon sugars (C
5
H
10
O
5
) are xylose and arabinose. The two most
important sugar crops are sugar cane and sugar beet which, together with corn (a starch crop),
supply almost all the bioethanol that is produced today (Rajagopal and Zilberman, 2007). The
drawback of this type of feedstock is that requires dedicated hectares for the production and
can compete with the food industry. Chemical structures of glucose and xylose are reported in
Figure 3.
Figure 3. Chemical structures of glucose and xylose.
3.1.2. Starch crops
Starch crops provide grains containing starch. Starch (C
6
H
10
O
5
)
n
is a very large polymer
molecule composed of many hundreds or thousands of glucose molecules (polysaccharides),
which must be broken down into one or two molecule pieces prior to be fermented to
bioethanol or biochemicals. The most widespread starch crops are wheat and corn (Tuck et
al., 2006; Wright, 2006). The major drawback of the use of these crops in biorefinery is that
they are also used in the food and feed industry and their provision as biorefinery feedstock
can have adverse impacts on food supply, soil carbon content (if straws and corn stovers are
removed) and land pressure. The chemical structure of starch is illustrated in Figure 4.
