MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

Farrauto and Batholomew (1997) proposed a three-step mechanism (adsorption
of reactant, surface reaction, and desorption of product) and derived two expressions that
can be simplified to the SNDL form based on two totally different assumptions: reaction
control and desorption control. Since all four mechanisms can be described by the simple
Langmuir rate equation, the physical meaning of k 1, k 0, and K depend on the mechanism
leading to the SNDL. Therefore, it is more appropriate to k 1 and k 0 rather than k a and k d.
Although the Langmuir rate equation is used in this study to model the carbon-oxygen
reaction without specifying a mechanism, its application implies that the mechanism of
this reaction necessarily involves adsorption and desorption of reactant(s) and product(s).
Constant Fractional Reaction Order Issue
Suuberg et al. (1988) carried out a detailed study on the reaction order and
activation energy of char oxidation using phenol-formaldehyde resin chars with low
impurity levels. It was observed that the reaction order varied in a narrow range
(0.68±0.08) with burnout, char heat treatment temperature and oxidation temperature
(573-673 K). The oxygen partial pressure examined was 0.5-101 kPa. The activation
energy was observed to be about 130-150 kJ/mol (31 to 36 kcal/mol). Reade et al. (1995)
and Reade (1996) determined the reaction orders and activation energies at atmospheric
pressure for 7 different types of chars using a TGA. The reaction orders were observed
to be about 0.7, and the activation energies were about 31-35 kcal/mol, consistent with the
data by Suuberg and coworkers.
These experiments raised a question for the theoretical validity of the Langmuir
rate equation. In both of the above studies, the reaction order seems to be independent of
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