THERMOCHEMICAL CONVERSION: AN INTRODUCTION TO FAST PYROLYSIS - Biomass as a Sustainable Energy Source for the Future - page 371

λ

Py = ð pyrolysis number Þ ð Eq : 11 : 3 Þ

c p R 2

k ρ

h

Py 0 = pyrolysis 0 number ð Þ ð Eq : 11 : 4 Þ

k ρ c p R

The symbols used in these equations are explained in the general symbols list at the
beginning of this chapter.
Besides heat transfer, also, mass transfer can play an important role at the particle
level. As mentioned before, the actual pyrolysis reactions occur in the cell walls pro-
ducing liquids, vapors, and a residual carbonaceous solid called char. The liquid
intermediates formed during pyrolysis (see publication and movies of Haas et al.,
2009) can remain in the cell at reaction conditions and are important because they
accelerate dehydration and cross-linking reactions, which result in the formation
of more char. Aerosols form thin liquid layers of reacting cellulose (see Teixeira
et al., 2011). It is argued that their subsequent entrainment is an important mechanism
for transporting heavy, nonvolatile products out of the biomass particle. Aerosols and
vapors formed may undergo secondary reactions with the nascent char, containing
(alkali) minerals, on their way out of the biomass particle. These secondary reactions
could be cracking/depolymerization reactions leading to the formation of gases/light
vapors or polymerization reactions leading to the formation of char, CO 2 and water.
Inside the particles, secondary polymerization reactions leading to the formation of
more char are dominant. It may be argued that if the transport distance of vapors/aero-
sols or the residence time of liquids in the particles decreases, this will lead to the
production of more oil and less char. This indeed has been observed by making
the transport distances very small by pyrolyzing only cell wall fragments and very
rapid removal (e.g., under vacuum) of the (volatile) products (see the smallest
particle sizes in Figure 11.9).
The heat and mass transfer effects described can be clearly observed by comparing
oil and char yields of particles of different size (heating rate) as shown in Figure 11.9.
Obviously, heat and mass transfer effects may interfere with attempts to unravel the
reaction pathways and measure the intrinsic kinetics of the primary pyrolysis reactions.
More on the subject of heat and mass transfer and particle models can be found in
the works of Di Blasi (1997, 2008), Di Blasi and Branca (2001), Hoekstra et al.
(2012), Kersten et al. (2005), Pyle and Zaror (1984), Teixeira et al. (2011), Wang
et al. (2005), and Westerhof et al. (2012).

11.6 A CLOSER LOOK AT PYROLYSIS OIL

Pyrolysis oil, produced by fast pyrolysis, is a complex mixture of water and a large
variety of different oxygenated organic compounds, originating from mainly depo-
lymerization and fragmentation reactions of the cell wall compounds. The composi-
tion of the pyrolysis oil largely depends on the biomass feedstock, process conditions,
and pyrolysis oil handling. The compounds in the pyrolysis oil cover a large range of