Page 189 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

166 Biomass Gasification, Pyrolysis and Torrefaction

TABLE 5.4 Kinetic Rate Constants for One-Step Single-Reaction
Global Model
21
Fuel Temperature (K) E (kJ/mol) A (s ) References
Cellulose 520 1270 166.4 3.9 3 10 11 Lewellen et al. (1977)
Hemicellulose 520 1270 123.7 1.45 3 10 9 Min (1977)
Lignin 520 1270 141.3 1.2 3 10 8 Min (1977)
Wood 321 720 125.4 1.0 3 10 8 Nolan et al. (1973)
Almond shell 730 880 95 121 1.8 3 10 6 Font et al. (1990)
Beech 450 700 84 (T . 600K) 2.3 3 10 4 Barooah and Long
sawdust (1976)

Solving this equation we get:
X 5 1 A expð2ktÞ (5.5)
where A is the preexponential coefficient, k 5 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 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