Biomedical Engineering Reference
In-Depth Information
TABLE 5.4
Kinetic Rate Constants for One-Step Single-Reaction
Global Model
A
(s
2
1
)
Fuel
Temperature (K)
E
(kJ/mol)
References
10
11
Cellulose
520
1270
166.4
3.9
Lewellen et al. (1977)
3
10
9
Min (1977)
Hemicellulose 520
1270
123.7
1.45
3
10
8
Lignin
520
1270
141.3
1.2
Min (1977)
3
10
8
Wood
321
720
125.4
1.0
Nolan et al. (1973)
3
10
6
Almond shell
730
880
95
121
1.8
3
Font et al. (1990)
10
4
Beech
sawdust
450
700
84 (T
.
600K) 2.3
3
Barooah and Long
(1976)
Solving this equation we get:
ð
2
Þ
X
5
1
A exp
kt
(5.5)
where A is the preexponential coefficient, k
E/RT E is the activation energy
(J/mol), R is the gas constant (J/mol K), and T is the temperature (K).
Owing to the difficulties in extracting data from dynamic thermogravimetric
analysis, reliable data on the preexponential factor, A, and the activation
energy, E, are not easily available for fast pyrolysis (Reed, 2002, p. II-103).
However, for slow heating, we can obtain some reasonable values. If the effect
of secondary cracking and the heat-transfer limitation can be restricted, the
weight-loss rate of pure cellulose during pyrolysis can be represented by an
irreversible, one-stage global first-order equation.
For the one-step global reaction model,
Table 5.4
lists values of the acti-
vation energy E and the preexponential factor A, for the pyrolysis of various
biomass types at a relatively slow heating rate.
Other models are not discussed here, but details are available in several
publications, including Blasi (1993).
5
5.5 HEAT TRANSFER IN A PYROLYZER
The preceding discussions assume that the heat or mass transport rate is too
high to offer any resistance to the overall rate of pyrolysis. This is true at a
temperature of 300
400
C (Thurner and Mann, 1981), but at higher tem-
peratures heat and mass transport influence the overall rate and so cannot be
neglected. This section deals with heat transport during pyrolysis.
During pyrolysis, heat is transported to the particle's outer surface by
radiation and convection. Thereafter, it is transferred to the interior of the
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