Chemistry Reference
In-Depth Information
Hemicellulose is a sugar polymer that typically constitutes 15-30 wt.%
of biomass.
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Unlike cellulose, which is a polymer of only glucose,
hemicellulose is a polymer of multiple sugars. This complex poly-
saccharide occurs in association with cellulose in the cell walls. It
contains five-carbon (usually xylose and arabinose) and six-carbon sugars
(galactose, glucose, and mannose). The most abundant building block of
hemicellulose is xylan (a xylose polymer linked at the 1 and 4 positions).
Hemicellulose is amorphous because of its highly branched nature and it
is relatively easy to hydrolyze to its monomer sugars compared to
cellulose.
Lignin is a highly branched, substituted aromatic polymer found in the
cell walls of certain biomass, particularly woody biomass. In plant cell
walls, lignin fills the spaces between cellulose and hemicellulose, and it
acts like a resin that holds the lignocellulose matrix together. Although
the exact structure of the untreated lignin found in plants (also known as
''native lignin''), is still unknown, the biosynthesis of lignin involves the
polymerization of three primary monomers: coumaryl, coniferyl, and
sinapyl alcohols, which can still be recognized in Fig. 2.
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Polymerization by random radical-radical coupling reactions of
monomers under chemical control leads to lignin vascular plants. The
composition, molecular weight, and amount of lignin differ from plant
to plant, with lignin abundance generally decreasing in the order of
softwoodsWhardwoodsWgrasses. A schematic representation of the
lignin structure showing common linkages is depicted in Fig. 2.
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The
components derived from coumaryl, coniferyl, and sinapyl alcohol are
indicated with several examples of linkages between the components.
The linkages include b-O-4, 5-5, b-5, 4-O-5, b-1, spirodienone, phenyl-
coumaran, and b-b linkages, of which the b-O-4 linkage is the fragment
most often appearing, consisting of more than half of the linkage
structures of lignin.
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3.3 Biomass utilization and bio-oil production
In direct analogy to a petroleum refinery, which produces fuels and
chemicals from crude oil, a biorefinery is a facility that produces multiple
products, including fuel, power, and bulk and fine chemicals from bio-
mass.
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Mainly, three main types of processes are proposed to convert
biomass into liquid fuels as shown in Fig. 3 for cellulosic biomass.
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These processes include gasification,
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liquefaction,
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and pyr-
olysis.
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Furthermore, it can be directly combusted to produce heat.
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Biomass can be gasified to syn-gas (CO and H
2
) and methane; the
gasification process requires volatilization of water, decreasing the
overall energy eciency. Syn-gas can be used to produce alkanes by
Fischer-Tropsch synthesis (FTS), methanol by methanol synthesis,
hydrogen from water-gas shift reaction, and ethanol by fermentation.
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Monomer (sugar) units can be produced by hydrolysis of cellulosic
biomass, after which aqueous sugar, which mainly consists of single
units, and lignin is obtained. The monomer sugar units can then be
selectively converted into targeted fuels using different catalytic
processes. Biomass conversion into its monomer units depends on the
