MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

7. Model Evaluation and Discussion
The HP-CBK model, using the Langmuir rate equation, the effectiveness factor
and the random pore model, was evaluated by comparison with five sets of pressure-
dependent reactivity data: 1) graphite flake data (Ranish and Walker, 1993); 2) rough
sphere combustion data (Banin et al., 1997a); 3) large particle oxidation data (Mathias,
1996); 4) pulverized char drop-tube data (Monson, 1992); and 5) TGA and FFB data
from this study.
Proper values have to be assigned to the kinetic and pore structure parameters in
order for the HP-CBK model to predict the char oxidation rates in agreement with
experimental data. An optimization model, StepIt (Chandler, 1999) was used to adjust the
kinetic parameters (A 1p, E 1p, A p, and E p) and pore structure parameters (r p1, r p2 and M)
within pre-set ranges to best fit the experimental data. The optimum values of the
parameters were used in the HP-CBK model. The HP-CBK model was then evaluated by
comparing the model calculations to experimental data. In this sense the HP-CBK model
unifies and explains experimental data with one different set of parameters for each set
of data, rather than predicts the reaction rates. This is still a substantial contribution,
since little is known about high-pressure char oxidation. The correlations between kinetic
parameters (activation energies and pre-exponential factors) and measurable char
properties are not yet possible, since experimental data at high pressures and
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