MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

The second and third findings mean that the effective diffusivity is only determined by
the macro-porosity (ε M):
2
De = MDO2,
N2 =1.523 × 10 −5 1.67
Tp 102
2
M / P (7.12)
These three findings greatly simplified the model and reduced the number of adjustable
parameters to three: A 0, E 0, and M. The best-fit kinetic and pore structure parameters are
listed in Table 7.4. By using this set of parameters, the HP-CBK model was able to
quantitatively explain the effects of all six experimental variables: total pressure, oxygen
partial pressure, oxygen mole fraction, gas velocity, gas temperature, and particle size
with a standard deviation of 14% and a maximum error of 22% (see Appendix B). The
resulting comparison of the HP-CBK model to the experimental data is shown in Figure 4.
Table 7.4. Parameters Used in Modeling the Data by Mathias.
A 0 = 0.75 mol/cm 3 /sec E 0 = 18.2 kcal/mol
Porosity = 0.65 (Calculated and Pre-set)
Macroporosity ε M = 0.28
The activation energy E 0 seems low compared the values observed for pulverized
char oxidation. However, these char particles were prepared from large coal particles and
might differ from the char prepared from small particles in reactivity and pore structure.