MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

combustion is rough sphere combustion (since external combustion cannot be neglected in
Zone III), but does not allow the rough sphere phenomenon to be observed.
Factors that reduce the value of S int would favor the occurrence of rough sphere
combustion. These factors include:
1) Small specific surface area (typically in highly ordered carbon).
2) Factors that reduce the effective diffusivity D e in the generalized Thiele
modulus (pore constriction, blind pore and low porosity) and hence reduce the
effectiveness factor.
3) Very fast kinetics, which increases the value of the general Thiele modulus and
hence reduces the effectiveness factor.
Small pore size in char particles seems to fall into the second category (factors
that reduce the effective diffusivity). However, small pore size is typically associated
with large specific internal surface area, since for cylindrical pores the pore radius is
inversely proportional to the internal surface (r p = 2 V p/S i nt; Smith, 1981). These two
effects cancel out and therefore small pore size is not a factor that favors rough sphere
combustion.
It is important to distinguish the external surface area ( d 2 ) from the geometric
external surface area, which is simply d 2 for spherical particles. The roughness factor ,
accounts not only for the apparent geometric distortions from a smooth and spherical
shape but also penetration of oxygen molecules within a pore before the first collision
with the pore wall, which determines the Knudsen diffusion coefficient (Banin et al.,
1997).
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