MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

effectiveness factor for the Langmuir rate equation with a correction function to improve
its accuracy; 3) a pore structure model for calculation of the effective diffusivity, taking
into account both Knudsen diffusion and molecular diffusion; and 4) general correlations
for Nusselt and Sherwood numbers, which allowed the HP-CBK model to be used for
both entrained-flow pulverized char oxidation and large-particle combustion in fixed beds.
The HP-CBK model was evaluated by comparison with five sets of high-pressure
experimental data: 1) graphite flake oxidation 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.
Results showed that the HP-CBK model was able to quantitatively explain: 1)
the effects of temperature, total gas pressure, oxygen mole fraction, particle size and gas
velocity on reaction rates; 2) the change of reaction order with temperature and oxygen
partial pressure observed by Ranish and Walker (1993) and by Banin et al. (1997); 3) the
reaction order (typically about 0.7) and activation energy (typically 34 kcal/mol)
observed in TGA experiments (Reade, 1996); and 4) the apparent reaction order of 0.5
typically observed at high temperatures, implying a true order of zero (Smith, 1982).
Therefore, the Langmuir rate equation, when used with the appropriate effectiveness
factor, seems to be satisfactory for modeling char oxidation over wide ranges of
experimental conditions.
In modeling the data by Mathias (1996) and those by Monson (1992), it was
found that the Langmuir rate equation reduced to an intrinsic zero-th order equation for
both cases. The intrinsic zero-th order equation implies an apparent order of 0.5 in Zone
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