NREL - Glossary of Biomass Terms

  HYPERLINK "http://www.nrel.gov/biomass/glossary.html" 
http://www.nrel.gov/biomass/glossary.html  

hemicellulose: Hemicellulose consists of short, highly branched chains
of sugars. In contrast to cellulose a polymer of only glucose, a
hemicellulose is a polymer of five different sugars. It contains
five-carbon sugars (usually D-xylose and L-arabinose) and six-carbon
sugars (D-galactose, D-glucose, and D-mannose), and uronic acid. The
sugars are highly substituted with acetic acid. The branched nature of
hemicellulose renders it amorphous and relatively easy to hydrolyze to
its constituent sugars compared to cellulose. When hydrolyzed, the
hemicellulose from hardwoods releases products high in xylose (a
five-carbon sugar). The hemicellulose contained in softwoods, by
contrast, yields more six-carbon sugars.

cellulose: The carbohydrate that is the principal constituent of wood
and other biomass and forms the structural framework of the wood cells.
It is a polymer of glucose with a repeating unit of C6H10O5 strung
together by ß-glycosidic linkages. The ß-linkages in cellulose form
linear chains that are highly stable and resistant to chemical attack
because of the high degree of hydrogen bonding that can occur between
chains of cellulose (see below). Hydrogen bonding between cellulose
chains makes the polymers more rigid, inhibiting the flexing of the
molecules that must occur in the hydrolytic breaking of the glycosidic
linkages. Hydrolysis can reduce cellulose to a cellobiose repeating
unit, C12H22O11, and ultimately to glucose, C6H12O6. Heating values for
cellulose may be slightly different based upon the feedstock. Example
values are shown below (higher heating value [HHV] at 30°C, dry basis).
(Source: Domalski, E.S.; Milne T.A., ed. Thermodynamic Data for Biomass
Materials and Waste Components. The ASME Research Committee on
Industrial and Municipal Wastes, New York: The American Society of
Mechanical Engineers, 1987)

lignin: a glue-like polymer in the cell wall of plants that surrounds
cellulose to provide strength to fibers and to resist microbial decay.
Lignin is second only to cellulose as the largest pool of non-fossil
organic carbon, constituting up to a quarter of the dry mass of wood. 
it covers the glucan and xylan polymer cell walls and cements them
together. It is a complex, cross-linked polymeric substance with a
highly aromatic structure of molecular weight about 10,000 derived
principally from coniferyl alcohol (C10H12O3) by extensive condensation
polymerization. Higher heating value (oven dry basis): HHV=9111 BTU/LB
(5062 CAL/G, 21178 J/G). (Source: Domalski, E.S.; Milne T.A., ed.
Thermodynamic Data for Biomass Materials and Waste Components. The ASME
Research Committee on Industrial and Municipal Wastes, New York: The
American Society of Mechanical Engineers, 1987)

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  HYPERLINK
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net.html#top" 
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et.html#top  

A condensation polymer is formed by monomer molecules condensing out
small molecules (such as water) as the polymer chain forms. 

 

For example, when two glucose monomer molecules react through two
hydroxy groups -OH, an H-OH molecule is condensed out, leaving an -O-
linking the two monomer molecules. The first two glucose molecules to
join condense out an H-OH, and every glucose molecule added to the
growing chain then condenses out another H-OH.

  Cellulose is a flat, straight and rigid molecule. 

 

  The bulky -CH2OH groups, represented above as LOH , are on
alternate sides of adjoining glucose units. 

  

  Many of the hydroxy groups form hydrogen bonds that hold the
cellulose chains together. 

 

U

V

W

a

V

W

  The hydrogen bonding results in long strong cellulose fibres. This
accounts for why wood is a strong building material. The reduced
availability of hydroxy groups in the cellulose structure, due to their
involvement in hydrogen bonding between the chains, makes it insoluble
in water and resistant to chemical attack. 

  

  Cellulose is a natural condensation polymer. Most dry plant
material consists of up to 50% cellulose.

