MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

and 2850 K). For consistency with the rate data reduction, this assumption (that CO was
the sole primary product) was adopted in this study.
Banin et al. (1997a) calculated the reaction orders from the char oxidation rate data
and gave a 3-dimensinal representation of the reaction order as a function of the particle
temperature and oxygen partial pressure. From this plot, the reaction order decreases as
temperature increases, again suggesting that the activation energy of K is positive
(equivalently, E 1p is greater than E 0).
Another important observation from that plot is that at high temperature and high
oxygen partial pressure, the measured reaction order is less than 0.5. There are two
possible explanations: 1) the combustion occurs in Zone I or in the transition zone from
Zone I to Zone II, but not in Zone II, if the external combustion rate is assumed to be
negligible compared to the internal combustion rate; 2) combustion occurs in Zone II and
the external combustion rate cannot be neglected compared to the internal combustion
rate. The latter is the so-called rough sphere combustion hypothesis. On the external
surface area the combustion rate is not affected by internal diffusion and therefore allows
true kinetics and a reaction order below 0.5. Numerical calculations in this study showed
that the first explanation was inadequate to simultaneously explain the observed rates and
the reaction orders, and the rough sphere combustion hypothesis was examined
quantitatively.
Banin et al. used a simple Langmuir rate equation to describe the intrinsic reaction
rates and a microscopic pore structure model to account for the pore growth. It was
shown that the reaction rate was virtually constant over the whole range of burnout
except in the initial stage, which lasted only a short period of time. Since microscopic
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