Environmental Engineering Reference
In-Depth Information
TABLE 11.1 Elemental composition and some characteristics of pinewood, fast
pyrolysis oil (from pinewood), hydrothermal liquefaction oil, and heavy fuel oil
Hydrothermal
liquefaction oil Heavy fuel oil
Pinewood
Pyrolysis oil
C (wt% db)
46.6
50-64
65-82
85
H (wt% db)
6.3
5-7
6-9
11
O (wt% db)
47.0
35-40
6-20
1
Water content (wt% ar)
9
5-35
3-6
0.1
Lower heating value (LHV)
(MJ.kg
-1
)
17-19
16-19
25-35
40
Viscosity (cP at 20
C)
10
4
—
40-150
180
Density (kg.m
-3
)
570
a
1150-1250
1050-1150
900
Data taken from Kersten and Garcia-Perez (2013) and Knezevic (2009).
a
Particle density.
11.4 CHEMISTRY AND KINETICS OF PYROLYSIS
In this section, firstly, the chemistry of pyrolysis is introduced. Secondly, similarities
and possible differences with hydrothermal liquefaction and solvolysis are briefly
discussed.
11.4.1 Pyrolysis
Lignocellulosic biomass is a very complex material that consists mainly of cellulose,
hemicellulose, and lignin (see Chapter 2). These building blocks together form
the cell wall of lignocellulosic materials. When lignocellulosic materials are heated
to temperatures over 200
C, reactions occur in the cell walls, which are known as
primary reactions. These are depolymerization reactions resulting in liquids, vapors,
and a solid residue. Upon subjecting lignocellulosic biomass to a temperature ramp,
hemicelluloses will decompose first followed by cellulose, while lignin reacts over a
broad temperature range. This behavior is illustrated in Figure 11.3, which shows
thermogravimetric analysis/analyzer (TGA) (see Chapter 2 for details on TGA) curves
of wood and its building blocks.
Because of the complex nature of the building blocks themselves (e.g., the mono-
mers and structure of the hemicelluloses and lignin vary from biomass type to biomass
type) and their interaction, many research works regarding the chemistry are carried
out using model compounds. Model compounds can be the individual building blocks
(e.g., cellulose), their monomers (e.g., levoglucosan) or decay products of the mono-
mers (e.g., glycolaldehyde). Generally, depolymerization of cellulose is proposed to
proceed via either an unzipping or a random chain cleavage mechanism (see Antal,
1982 for more information about the proposed cellulose decomposition models).
In dedicated experiments at high heating rates and with a quick product quenching
oligomer anhydrosugars with a degree of polymerization (DP) = 2 (cellobiosan) to
DP = 9 (cellononasan) have been observed, pointing toward the dominance of the
random chain mechanism (Piskorz et al., 2000).
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