MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

adsorption control, respectively) as temperature increased. Third, the three-zone theory
assumes that the combustion rate contributed from the external surface area is negligible
compared to the rate contributed from the internal surface area. This assumption is true
for most cases, since the internal surface area is typically much larger than the external
surface. However, the external surface area can become important under some conditions,
these being favored by low internal surface area (typically in highly ordered carbons) or
severe pore diffusion limitations, which lead to an extremely low effectiveness factor
(Hurt, 1998). When the external combustion rate cannot be neglected compared to the
internal combustion rate, the so-called “rough sphere combustion” occurs (Banin et al.,
1997a, b).
Char Oxidation Model Classifications
Char oxidation models can be classified into two main categories: global models
and intrinsic models (Smith et al., 1994). Global models consider char particles
impervious to pore diffusion effects or else lump intraparticle diffusion effects into the
chemical reaction rate constants. These models are highly empirical, basing the reaction
rate on the particle’s external surface area and on the oxidizer concentration at the external
surface. In contrast, intrinsic models relate char oxidation rate to the active surface area
involved in the reaction and consider the non-uniform oxidizer concentration profile
within the particle. Intrinsic models rely on pore structure models to describe gaseous
diffusion through complex pore structures and to model the local oxidizer concentration at
the active surface area. Thus the intrinsic model approach has the potential of providing
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