Page 242 - Biosystems Engineering

220 Cha pte r Se v e n

Because of their amorphous structure, hemicelluloses have lower
thermal stability than cellulose. Their thermal degradation starts at
temperatures as low as 200°C. The yields of furfural increase when
the substrate is impregnated with acid catalysts (Radlein et al. 1991a,
1991b).

Lignin
Thermal degradation of lignin occurs over a wide range of tempera-
tures (200 to 600°C), resulting in the formation of monomeric phenols,
guaiacols and syringols, formic acid, formaldehyde, methanol, carbon
dioxide, and water. Demethoxylation of lignin is the source of meth-
anol. In general, the thermal degradation of lignin can be described
by a competitive mechanism involving depolymerization and
condensation/carbonization reactions as suggested by Kawamoto
et al. (2007b). During fast pyrolysis, most of the lignin is converted
into monomers, dimers, and trimers. The dimers and trimers are
known as pyrolytic lignin. Many of these dimers and trimers are
released in the form of aerosols (Garcia-Pérez et al. 2008).

7.4 Single-Particle Models
Understanding how the thermal degradation of individual biomass
particles proceeds is critical to design and to operate more efficient
pyrolysis reactors. The conversion of a single biomass particle is con-
trolled by the physicochemical properties and dimensions of biomass
particles as well as by the reaction conditions (reactor temperature,
reacting environment, external heat transfer coefficient). The residence
time of biomass particles inside a pyrolysis reactor needed to achieve
complete conversion will depend on the devolatilization and heating
rates. Total conversion time can be calculated by single-particle mod-
els. The values of total devolatilization time for small biomass particles
(diameter less than 2 mm) in fluidized-bed reactors reported in the
literature are between 10 and 70 s (Kumar et al. 2006).
Many models have been developed over the last 60 years to
describe the thermal degradation of single biomass particles. In general,
it is accepted that for particles with length-to-diameter ratio smaller
than 3 should be described by two-dimensional models. One-
dimensional models should be used when describing biomass parti-
cles with length-to-diameter ratios larger than 3. Excellent reviews on
the single-particle models used to calculate biomass devolatilization
time can be found elsewhere (Kersten et al. 2005, Di Blasi 2008).
When designing a pyrolysis reactor it is very important to distin-
guish between the reactor temperature and the temperature at which
primary thermochemical reactions occur. The reactor temperature is
usually higher than the particle temperature, which controls the reac-
tion rate of primary thermochemical reactions (Bridgwater et al. 1999).