Environmental Engineering Reference
In-Depth Information
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Lignocellulosic biomass
is the non-grain portion of biomass (e.g., cobs,
stalks), often referred to as agricultural stover or residues. Energy crops
such as switchgrass also contain valuable components, but they are not as
readily accessible as starch. These lignocellulosic biomass resources (also
called
cellulosic
) are comprised of cellulose, hemicellulose, and lignin.
Generally, lignocellulosic material contains 30 to 50% cellulose, 20 to 30%
hemicellulose, and 20 to 30% lignin. Some exceptions to this are cotton
(98% cellulose) and flax (80% cellulose). Lignocellulosic biomass is per-
ceived as a valuable and largely untapped resource for the future bioindus-
try; however, recovering the components in a cost-effective way presents a
significant technical challenge.
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Cellulose
is one of nature's polymers and is composed of glucose, a six-
carbon sugar. The glucose molecules are joined by glycosidic linkages,
which allow the glucose chains to assume an extended ribbon conforma-
tion. Hydrogen bonding between chains leads to the formation of the flat
sheets that lie on top of one another in a staggered fashion, similar to the
way staggered bricks add strength and stability to a wall. As a result, cel-
lulose is very chemically stable and insoluble and serves as a structural
component in plant walls.
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Hemicellulose
is a polymer containing primarily 5-carbon sugars such as
xylose and arabinose, with some glucose and mannose dispersed through-
out. It forms a short-chain polymer that interacts with cellulose and lignin
to form a matrix in the plant wall, strengthening it. Hemicellulose is more
easily hydrolyzed than cellulose. Much of the hemicellulose in lignocel-
lulosic material is solubilized and hydrolyzed to pentose and hexose sugars.
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Lignin
helps bind the cellulosic/hemicellulose matrix while adding flexibil-
ity to the mix. The molecular structure of lignin polymers is very random
and disorganized and consists primarily of carbon ring structures (ben-
zene rings with methoxyl, hydroxyl, and propyl groups) interconnected by
polysaccharides (sugar polymers). The ring structures of lignin have great
potential as valuable chemical intermediates; however, separation and
recovery of the lignin are difficult.
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Oils
and
proteins
are obtained from the seeds of certain plants (e.g., soy-
beans, castor beans) and have great potential for bioproducts. These oils and
proteins can be extracted in a variety of ways. Plants raised for this purpose
include soy, corn, sunflower, safflower, rapeseed, and others. A large portion
of the oils and proteins recovered from oilseeds and corn is processed for
human or animal consumption, but they can also serve as raw materials for
lubricants, hydraulic fluids, polymers, and a host of other products.
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Vegetable oils
are composed primarily of triglycerides, also referred to as
triacylglycerols. Triglycerides contain a glycerol molecule as the backbone
with three fatty acids attached to glycerol's hydroxyl groups.
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Proteins
are natural polymers with amino acids as the monomer unit. They
are incredibly complex materials and their functional properties depend
on molecular structure. There are 20 amino acids, each differentiated by
their side chain or R-group, and they can be classified as nonpolar and
