Oil can be recovered from kerogen
in oil shale by pyrolysis. The
devolatilization kinetics of the pyrolysis of oil shale from the Irati
Formation in Brazil was studied. Kinetic parameters were determined
from dynamic thermogravimetric analysis over the temperature range
323–1173 K, using different model-free methods. Evaluation
and validation were performed by pyrolysis at 673 K for 3 hours. It
was found that the activation energy depended on the extent of conversion.
Activation energy increased over the range 215–255 kJ/mol for
conversion in the range 0.15 ≤ α ≤ 0.55, where
α = 1 for pyrolysis at 1173 K. When the reaction rate was high,
the conversion calculated using kinetic parameters derived by the
Friedman method was more accurate than those calculated from the Flynn–Wall–Ozawa
and the Kissinger–Akahira–Sunose methods. The latter
two methods performed better when the reaction rate was lower, i.e.,
at higher conversion. Isothermal kerogen pyrolysis approached an incomplete
conversion limit that could be increased only by increasing the temperature;
this type of behavior was predicted by the conversion dependence of
activation energy. The observed activation energy is an average of
the different activation energies of the individual compounds in kerogen.
As conversion progresses, the compounds with lower activation energies
are more readily converted, so that the average activation energy
of the compounds that remain increases with increasing conversion.
The work highlighted the importance of employing conversion-dependent
kinetic parameters when modeling oil shale pyrolysis for process design,
especially when the process is designed for high kerogen conversion.