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Biomass Pyr olysis and Bio-Oil Refineries 217

7.2.4 Extractives
Extractives are nonstructural compounds formed by secondary
metabolism. They are both lipophilic and hydrophilic organic com-
pounds that can be extracted by hot water or with the aid of organic
solvents. Extractives confer odor, color, and durability to the wood
and protect the plants against predators. Bark contains 4 to 5 times
more extractives than bark-free wood.

7.2.5 Ash
The mineral materials (ash) are composed mainly of carbonates,
sulfates and phosphates of silica and metals from the first and
second group of the Periodic Table (Ca, Mg, Na, and K). These com-
pounds act as catalysts for polycondensation reactions leading to
char formation.

7.3 Overview of Biomass Pyrolysis Reactions
As the temperature of the biomass particle rises, thermal decomposi-
tion reactions start to take place on a cylindrical reaction front, which
advances toward the core of the particle at a speed determined by the
particle size, biomass thermal conductivity, and the temperature of
the environment (see Fig. 7.5). The cleavage of cellulose, hemicellu-
lose, lignin, and extractives chemical bonds occurs in well-defined
temperature ranges (Boroson et al. 1989a, 1989b; Evans and Milne 1987a,
1987b). In the literature, these reactions are called primary thermal
decomposition (pyrolysis) reactions. These reactions are commonly
studied by thermogravimetry. Two or three peaks assigned to each of
the biomass macrocomponents (extractives, cellulose, hemicellulose,

Unreacted
fibers

Heat

Volatiles
resulting
Reacted
from
fibers
pyrolysis
FIGURE 7.5 Representation of a biomass particle made up of hollow
cylindrical cells.