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and lignin) appear (Koufopanos et al. 1991; Caballero et al. 1997;
Varhegyi et al. 1997; Orfao et al. 1999). The products of these primary
reactions could undergo additional depolymerization, fragmenta-
tion, or polycondensation in the solid phase or in a melted state to
form molecules of lower molecular weight and charcoal (Graham et
al. 1984; Agblevor et al. 1994). The resulting volatile species will con-
tinue reacting as they diffuse inside the pores of the particles (intra-
particle secondary reactions) either homogeneously in the gas phase or
heterogeneously with the partially converted solid biomass or with
the char (Hastaoglu and Berruti 1989). The intensity of these second-
ary reactions depends on the time-temperature history to which the
products of primary reactions will be subjected before collection in
the condensers (Bridgwater et al. 1999).

7.3.1 Mechanism of Primary Reactions

Cellulose
The mechanism of cellulose thermal degradation has been the source
of passionate discussions for many years (Piskorz et al. 2000). Several
models have been proposed to explain the complex cellulose thermal
decomposition pathways. The pioneering model of Broido and
Shafizadeh, shown in Fig. 7.6, is an important reference because it
introduced for the first time the concept of active cellulose to explain
the behavior of cellulose pyrolysis. This concept was developed as a
consequence of experimental evidences showing the formation of
cellulose at a lower degree of polymerization at temperatures as low
as 220°C (Golova 1975). Broido et al. (1973) found that crystalline cel-
lulose undergoes a large change in degree of polymerization before
weight loss occurs. Bradbury et al. (1979) noted that at low heating
temperatures (259 to 312°C), an initiation period occurs. Thus, the
model of Broido and Shafizadeh (Fig. 7.6; Broido and Nelson 1975;
Wooten et al. 2004) postulates that the cellulose is converted into a
more active form and that this is a rate-limiting step followed by the
formation of either char, gases, or tar.
An excellent overview of several of the semiglobal mechanisms
for primary pyrolysis of lignocellulosic materials can be found
elsewhere (Di Blasi 1998). Figure 7.7 is an overview of a more detailed
model describing more cellulose thermal-degradation reactions.
Much experimental evidence suggest that cellulose thermal degrada-
tion proceeds by the following seven major reactions: (1) hydrolysis
reactions to produce active cellulose (Broido et al. 1973; Golova 1975;

Volatile tar
Cellulose Active cellulose
Char + gases
FIGURE 7.6 The Broido–Shafi zadeh model.