Page 190 - Advances in bioenergy (2016)

The worldwide production of biomass from terrestrial plants is 170–200 billion tons, with an
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estimated 70% made of plant cell walls. From an average estimate of 146 billion metric tons
of world biomass production per annum, about 20 metric tons/acre are produced from farm
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crops and trees. Lignocellulosic materials are usually categorized into agricultural and
forage residues (sugarcane bagasse, corn stover, wheat straw, rice straw, rice husk etc.),
dedicated energy crops (e.g., switch grass, timothy grass, hybrid poplar), forest wood residues
(e.g., saw mill residues, paper mill discards), and municipal paper waste. Typically,
lignocellulosic biomass contains 35–55% cellulose, 20–40% hemicellulose, and 10–25%
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lignin, while the rest consists of extractives and ash. With ash representing the mineral
components in biomass, extractives include structural biopolymers such as terpenoids,
steroids, resin acids, fats, lipids, waxes, and phenolic components.

Thermochemical conversion is the most widely known biogenic waste conversion pathway that
involves pyrolysis, hydropyrolysis, reforming, gasification, and liquefaction. 8-10 Through
gasification, the biomass is converted to hydrocarbons and synthesis gas (or syngas), while

pyrolysis and liquefaction directly convert biomass at high temperatures to bio-oils, gases, and
char. Biomass has tremendous potential to accelerate the realization and utilization of H as a
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major fuel of the future, as biomass is the fourth largest source of energy in the global context.
It accounts for 15% of world's primary energy consumption and about 38% of the primary
energy consumption in developing countries such as India, China, and Brazil.

LIGNO(HEMI)CELLULOSIC BIOMASS CHEMISTRY

Cellulose is a glucose polymer consisting of β (1, 4) linked D-glucose subunits, whereas
hemicellulose is a mixture of polysaccharides composed of C (e.g., xylose and arabinose) and
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C (e.g., galactose, glucose, and mannose) sugars as well as sugar acids such as
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methylglucuronic and galacturonic acids. In the polymeric form, cellulose consists of linear
chains of 1, 4-D-glucopyranose units with an average molecular weight of around
100,000 Da. 10-12 In the monomeric form, hemicellulose consists of D-glucose, D-mannose, D-
galactose, D-xylose, L-arabinose, L-rhamnose, D-glucuronic acid and D-galacturonic acid,
with an average molecular weight less than 30,000 Da. On the other hand, lignin is a phenyl
propane polymer linked with ester bonds that hold cellulose and hemicellulose together as a
matrix. The lignin in biomass is highly branched, substituted mononuclear aromatic polymer,
and consists of macromolecules containing phenolic groups. The average molecular weight of
lignin is around 20,000 Da.

During the biomass pretreatment, lignin can form furan compounds such as furfural and
hydroxymethyl furfural that could inhibit fermentation. 12-14 In contrast, thermochemical
conversion of biomass yields various products after degradation from lignin that have high
industrial values in the production of aromatic compounds, flavoring agents, food preserving
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agents, and other chemicals. The major structural components of lignin are the monolignols
(hydroxyl cinnamyl alcohols), such as coniferyl alcohols (guaiacyl propanol), coumaryl
alcohols (p-hydroxyphenyl propanol), and sinapyl alcohols (syringyl propanol).