The reactivity of
chemically isolated lignocellulosic blocks, namely,
α-cellulose, holocellulose, and lignin, has been rationalized
on the basis of the dependence of the effective activation energy
(E
α) upon conversion (α) determined
via the popular isoconversional kinetic analysis, Friedman’s
method. First of all, a detailed procedure for the thermogravimetric
data preparation, kinetic calculation, and uncertainty estimation
was implemented. Resulting E
α dependencies
obtained for the slow pyrolysis of the extractive-free Eucalyptus grandis isolated α-cellulose and
holocellulose remained constant for 0.05 < α < 0.80 and
equal to 173 ± 10, 208 ± 11, and 197 ± 118 kJ/mol,
thus confirming the single-step nature of pyrolysis. On the other
hand, large and significant variations in E
α with α from 174 ± 10 to 322 ± 11 kJ/mol in the region
of 0.05 and 0.79 were obtained for the Klason lignin and reported
for the first time. The non-monotonic nature of weight loss at low
and high conversions had a direct consequence on the confidence levels
of E
α. The new experimental and
calculation guidelines applied led to more accurate estimates of E
α values than those reported earlier.
The increasing E
α dependency trend
confirms that lignin is converted into a thermally more stable carbonaceous
material.
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