MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

5. Theoretical Developments
The intrinsic reaction order of char oxidation has been observed to vary within
limits of zero and unity (Suuberg, 1988; Chan et al., 1987) although the intrinsic reaction
order is often observed to be about 0.7 in TGA experiments at atmospheric pressure
(Suuberg, 1988; Reade, 1995). No theory has been available to explain or predict how the
reaction order changes with experimental conditions. The global n-th order rate equation
has been under criticism (Essenhigh, 1996) and was shown to be inadequate in modeling
high pressure char oxidation. It is generally accepted that the carbon-oxygen reaction
involves adsorption of reactant(s), surface reactions, and desorption of products, although
the exact reaction mechanism of the carbon-oxygen reaction is still unknown. There is
little doubt that an appropriate Langmuir-Hinshelwood expression, with its sound
theoretical basis and more adjustable parameters, holds more potential to model char
oxidation rates over wide ranges of experimental conditions than the simplistic n-th order
rate equation. However, a complex Langmuir-Hinshelwood expression with many
parameters, which are difficult to determine both experimentally and theoretically, is not a
desirable engineering option. Recently, Essenhigh showed that the simple Langmuir rate
equation was satisfactory in modeling the carbon-oxygen reaction rate (Essenhigh, 1988;
Essenhigh, 1991; Essenhigh, 1994; Essenhigh and Mescher, 1996), although there is still
an unresolved question as to a suitable explanation of the empirical n-th order rate
equation (Essenhigh and Mescher, 1996). It was also recognized (Essehigh and Mescher,
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