Environmental Engineering Reference
In-Depth Information
minerals) along their diffusion path (intraparticle) out of the biomass particle, with
other particles (interparticle), or in the vapor phase (homogeneous vapor-phase reac-
tions). Kinetic relations have also been derived for the conversion of vapors to (per-
manent) gases. An example is the relation derived by Wagenaar
(1994) for
homogeneous cracking, which uses a first-order decay reaction with
8×10
3
R
u
T
exp
−
87
:
6×10
5
k=1
:
ð
Eq
:
11
:
1
Þ
Obviously, the reaction rates of the secondary reactions will depend strongly on whether
these are homogeneous or heterogeneous reactions. Generally, once the vapors are
cooled down below 450
C, homogeneous cracking reactions become very slow.
11.4.2 Hydrothermal Liquefaction and Solvolysis
The simplest way of depicting hydrothermal liquefaction and solvolysis is by assuming
that it starts with the primary pyrolysis reactions. The main difference is then that the
reaction products are not being exposed to a vapor/gas phase, like in pyrolysis, but are
dissolved and diluted in a solvent both inside and outside the biomass particle. Whether
or not the solvent has part in the primary decomposition reactions is not unequivocally
settled. Possible roles of water in the primary reactions could the supply of OH
-
and H
+
ions for catalysis and the participation in hydrolysis reactions. The solvent can be water
or an organic solvent such as an alcohol, acid, or the liquefaction product itself. Typical
reactor temperatures are in the range of 250
400
C with pressures of 100
250 bar
(vapor pressure of water/solvent). An advantage of organic solvents over water is that
they can have lower vapor pressures leading to a lower reactor pressure.
Figure 11.7 depicts the hydrothermal liquefaction process of wood shred in an iso-
choric capillary reactor. It can be clearly seen that the reaction products dissolve in
water. After cooling the reactor effluent, two phases are present: an oil phase and an
aqueous phase. At the same temperature, less char is found for hydrothermal liquefac-
tion compared to pyrolysis. This might be explained by the rapid dilution (into the sol-
vent) of char precursors. On the other hand, under hydrothermal liquefaction conditions,
clearly, more decarboxylation takes place leading to an oil with a low(er) oxygen
content but with a higher molecular weight (see Table 11.1).
Further readings concerning the kinetics and chemistry involved in fast pyrolysis
are Antal (1982, 1985), Bradbury and Allan (1979), Chan et al. (1985), Cheng et al.
(2012), Dauenhauer et al. (2009), Di Blasi (2008), Haas et al. (2009), Lin et al. (2009),
Piskorz et al. (1989), Thurner and Mann (1981), and Wagenaar (1994).
-
-
11.5 PROCESSES AT THE PARTICLE LEVEL
Processes at the particle level are only discussed for fast pyrolysis of a wood particle.
At particle level, wood pyrolysis is a multilevel (length and time scale) process (see
Figure 11.8). Wood consists of cells (cavities) oriented in longitudinal direction; in a
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