Page 245 - Synthetic Fuels Handbook

FUELS FROM BIOMASS 231

Lignin makes up about one-quarter to one-third of the dry mass of wood and is generally
considered to be a large, cross-linked hydrophobis, aromatic macromolecule with molecular
mass that is estimated to be in excess of 10,000. Degradation studies indicate that the mol-
ecule consists of various types of substructures which appear to repeat in random manner.
The biosynthesis of lignin begins with the synthesis of monolignols (e.g., coniferyl
alcohol, sinapyl alcohol, and paracoumaryl alcohol) starting from an amino acid (phenyl-
alanine). There are a number of other monolignols present in plants but different plants use
different monolignols. The monolignols are synthesized as the respective glucosides which
are water soluble and allows transportation through the cell membrane to the apoplast
where the glucose moiety is removed after which the monolignols form lignin.
Lignin fills the spaces in the cell wall between cellulose, hemicellulose, and pectin com-
ponents and is covalently linked to hemicellulose. Lignin also forms covalent bonds to poly-
saccharides and thereby crosslinks different plant polysaccharides. It confers mechanical
strength to the cell wall (stabilizing the mature cell wall) and therefore to the entire plant.

8.3 PROCESSES

Biomass can be converted into commercial fuels, suitable substitute for fossil fuels
(Narayan, 2007). These can be used for transportation, heating, electricity generation, or
anything else fossil fuels are used for. The conversion is accomplished through the use of
several distinct processes which include both biochemical conversion and thermal conver-
sion to produce gaseous, liquid, and solid fuels which have high energy contents, are easily
transportable, and are therefore suitable for use as commercial fuels.

8.3.1 Process Types
Biochemical conversion of biomass is completed through alcoholic fermentation to produce
liquid fuels and anaerobic digestion or fermentation, resulting in biogas. Alcoholic fermen-
tation of crops such as sugarcane and maize (corn) to produce ethanol for use in internal
combustion engines has been practiced for years with the greatest production occurring in
Brazil and the United States, where ethanol has been blended with gasoline for use in auto-
mobiles. With slight engine modifications, automobiles can operate on ethanol alone.
Anaerobic digestion of biomass has been practiced for almost a century, and is very pop-
ular in many developing countries such as China and India. The organic fraction of almost
any form of biomass, including sewage sludge, animal wastes, and industrial effluents, can
be broken down through anaerobic digestion into methane and carbon dioxide. This biogas
is a reasonably clean burning fuel which can be captured and put to many different end uses
such as cooking, heating, or electricity generation.
Thermal conversion offers a more effective means for the recovery or conversion of the
energy content of wood and other lignocellulosic biomass. Wood and many other similar types
of biomass which contain lignin and cellulose, (agricultural wastes, cotton gin waste, wood
wastes, peanut hulls, etc.) can be converted through thermochemical processes into solid,
liquid, or gaseous fuels. Pyrolysis, used to produce charcoal since the dawn of civilization,
is still the most common thermochemical conversion of biomass to commercial fuel.
Fermentation. Traditional fermentation plants producing biogas are in routine use, rang-
ing from farms to large municipal plants. As feedstock they use manure, agricultural resi-
dues, urban sewage, and waste from households, and the output gas is typically 64 percent
methane. The biomass conversion process is accomplished by a large number of different