Once all the moisture is evaporated and particle temperature has reached the pyrolysis threshold (about 280°C, as above mentioned) the devolatilization/pyrolysis process begins. The pyrolysis reaction can be generally represented by the following equation:

Biomass + heat ^ H2O + CO2 + H2 + CO + CH4 + C2H6 + CH2O +••• + tar + char

(5.16)

During the devolatilization phase a wide range of gaseous products are released through the decomposition of fuel (Fig. 5.6). The gaseous products most commonly produced by hemicellulose (composed by pentoses — such as xylan — and hexoses — d-glucose, d-galactose etc.) are: acetic acid, formaldehyde, carbon monoxide, hydrogen, but also furfural and furan. The first step in cellulose pyrolysis is the production of active cellulose (Browne, 1958; Sullivan and Ball, 2012; Liao and Ma, 2004), then if reaction temperature is low, activated cellulose will produce charcoal by dehydration.

As temperature increases, active cellulose will produce mainly levoglucosan (LG) and its isomeric anhydrosugar by cracking of glucosidic bonds at the same time the opening of acetal structural rings and cracking of internal carbon-carbon bond in pyroanoid rings will bring into the formation of hydroxyl-acetaldehyde (HAA), acetol, furfural, CO and other compounds. If secondary reactions happen then anhydrosugar will undergo a reaction similar to the opening and reforming of pyroanoid ring, producing small molecule gas and secondary char.

Lignin pyrolysis produces aromatic compounds and char. The production of char is higher if compared to cellulose. The initial breakdown during pyrolysis affects the straight chain links

Tempwelve [KJ

Figure 5.6. Pyrolysis and devolatilization mechanism.

which connect aromatic units such as: vanillyl, syringyl, guaiacols, cresols and catechols. The aromatic chains produce phenols, xylenols, guaiacols, cresols and catechols. The straight-chain links produce carbon dioxide, hydrocarbons, formic acid, acetic acid, higher fatty acids and methanol.

The development of biomass pyrolysis depends both on chemical and physical processes, so pyrolysis modeling is based on two approaches taking into account both kinetics rates and heat transfer rates (Di Blasi, 2008, Slopiecka etal., 2012). Some examples of pyrolysis kinetic values are proposed in Table 5.3.