Page 243 - Synthetic Fuels Handbook

FUELS FROM BIOMASS 229

The energy in biomass is the chemical energy associated with the carbon and hydro-
gen atoms contained in oxidizable organic compounds which are the source of the carbon and
hydrogen in carbon dioxide and water. The conversion by plants of carbon dioxide and water to a
combustible organic form occurs by the process of photosynthesis in which solar energy and
chlorophyll are the important players.
Chlorophyll, present in the cells of green plants, absorbs solar energy and makes it available
for the photosynthesis, which may be represented by the simplified chemical reaction:
CO + H O → (CH O) + O
2 2 2 x 2
The oxidizable organic materials that are produced by photosynthesis and which deter-
mine the properties of the plant matter of relevance to biomass energy utilization are car-
bohydrates and lignin.
All of the carbohydrates present are saccharides (i.e., sugars) or polymers of sugars (i.e.,
polysaccharides) that fall into three types: (a) starch, (b) cellulose, and (c) hemicellulose.
The simple sugars include glucose, fructose, and the like while the polymeric sugars
such as cellulose and starch (Fig. 8.1) can be readily broken down to their constituent
monomers by hydrolysis, preparatory to conversion to ethanol or other chemicals.

OH HOCH 2
HOCH 2 O
O O HO O
O HO O
HO O
HOCH OH
OH 2
FIGURE 8.1 Generalized structure of cellulose.

Starch is a granular polysaccharide which accumulates in the storage tissues of plants
such as seeds, tubers, roots, and stem pith. It is an important constituent of corn, potato,
rice, and tapioca. Starch consists of 10 to 20 percent amylose, which is water soluble, and
80 to 90 percent amylopectin, which is insoluble in water. Both the constituents of starch
are polymers of glucose, with amylose linked in chain structures, while amylopectin is a
highly branched structure. Starch is not as chemically resistant as cellulose, and can be
readily hydrolyzed by dilute acids and enzymes to fermentable sugars.
Hemicelluloses are polysaccharides that occur in association with cellulose. They are
chemically different from cellulose, are amorphous, and have much lower molecular weight.
While cellulose is built from the single sugar glucose, most hemicelluloses contain two to
four different sugars as building blocks. Glucose is a component of some hemicelluloses,
although xylose is a dominant sugar in hardwood hemicellulose, and mannose is important
in softwood hemicellulose. Unlike the other sugars described so far, xylose contains only
5 carbon atoms and is a pentose.
The fraction of the cellulose containing xylose polymers is often referred to as pen-
tosan. Hemicellulose is more soluble than cellulose, is dissolved by dilute alkaline
solutions, and can be relatively readily hydrolyzed to fermentable sugars.
In contrast, lignin is a complex structure containing aromatic groups (Fig. 8.2) and is
less readily degraded. Although lignocellulose is one of the cheapest and most abundant
forms of biomass, it is difficult to convert this relatively unreactive material into sugars.
Among other factors, the walls of lignocellulose are composed of lignin, which must be
broken down in order to render the cellulose and hemicellulose accessible to acid hydroly-
sis. For this reason, many programs focused on ethanol production from biomass are based
almost entirely on the fermentation of sugars derived from the starch in corn grain.