Environmental Engineering Reference
In-Depth Information
k g
Gas
k l
Biomass
Vapors (oil)
k c
Char
FIGURE 11.5 Global lumped component scheme for pyrolysis (of wood).
TABLE 11.2 Kinetic rate coefficients for wood pyrolysis
Unspecified
wood species
(Chan et al., 1985)
Beech
(Di Blasi and
Branca, 2001)
Oak (Thurner and
Mann, 1981)
Pine (Wagenaar
et al., 1993)
k c [s -1 ]
7.4 × 10 5
exp(
1.1 × 10 7
exp(
3.1 × 10 7
exp(
3.3 × 10 6
exp(
107 ×
10 3 /R u T)
121 ×
10 3 /R u T)
-
125 ×
10 3 /R u T)
-
112 ×
10 3 /R u T)
-
k l [s -1 ]
4.1 × 10 6
exp(
2×10 8
exp(
9.3 × 10 9
exp(
1.1 × 10 10
exp(
113 ×
10 3 /R u T)
133 ×
10 3 /R u T)
-
149 ×
10 3 /R u T)
-
148 ×
10 3 /R u T)
-
k g [s -1 ]
1.4 × 10 4
exp(
1.3 × 10 8
exp(
1.1 × 10 11
exp(
4.4 × 10 9
exp(
88.6 ×
10 3 /R u T)
-
140 ×
10 3 /R u T)
-
177 ×
10 3 /R u T)
-
153 ×
10 3 /R u T)
-
For practical purposes, pyrolysis kinetics are often described based on so-called
lumped component schemes. The most used one includes three parallel
first-order reactions to
global
gas
,
vapors (which form oil after condensation)
, and
(see Figure 11.5).
Measured kinetic rate coefficients for different wood types are given in Table 11.2,
and Figure 11.6 (top) shows the liquid (oil) yield as a function of temperature for
pyrolysis in the absence of internal and external heat transfer limitations predicted
using these sets of kinetic data (see Section 11.5). It can be clearly seen that the pre-
dictions of the oil yield deviate considerably. This may be caused by errors in the
measurement techniques, the interpretation model employed (probably too simple),
and the effects of the biomass type used. In view of the actual complex reaction net-
work, including catalytic effects of minerals, it is indeed unrealistic to expect that
pyrolysis of woody biomass can be described accurately with such a simple kinetic
model. The existing kinetic schemes may have value in describing trends and as math-
ematical description of rates and yields for the purpose of reactor design for a partic-
ular biomass stream. For the latter, it has to be realized that the kinetic coefficients
have to be determined for every feedstock considered. Unfortunately, the existing
kinetic schemes cannot predict the composition (and thus the quality) of the produced
liquid. This would require models that provide a (much more) detailed description of
the chemistry and, at the particle level, mass and heat transfer effects (see Section 11.5).
The volatile chemical species (vapors and aerosols) that are formed by depolym-
erization may undergo secondary reactions with the nascent char (containing
char
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