MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

taking into account convection, radiation and heat generation from reaction (Mitchell et
al., 1992).
Kinetic parameters (A 1p, E 1p, A p and E p) and pore structure parameters (r p1, r p2 and
M) were adjusted to minimize the difference between the predictions and the data. Three
observations were made:
1) The best fit used an intrinsic Langmuir rate equation that reduced to a zero-th order
reaction;
2) The diffusivity contributed from micropores can be neglected compared to the
diffusivity contributed from macropores.
3) Macropores are large enough so that Knudsen diffusivity can be neglected
compared to molecular diffusivity.
The first finding means that K pP os is much greater than 1 so that the Langmuir rate
equation reduced to a zero-th order rate equation:
r in ′ = k1 pPos 1+ K pPos ≈ k 1 p P os
K p P os
= k 1 p
K p
= k 0 = A 0 e −( E 0
Therefore the absolute values of k 1p and K p could not be determined but their ratio k 1p/K p
101
/ RT )
(7.9)
= k 0 was determined. The intrinsic zero-th order kinetics implied an apparent order of 0.5
in Zone II, which is repeatedly observed (Smith, 1982) or assumed (Mitchell et al., 1992)
for high temperature char oxidation:
r obs ′ = k0 = 1
MT dp Mc qobs = r obs ′ ′ 6
k 0 = 6
d p
= 12 2D e k 0
2D e P os
O k 0 RT k 0
O RT P os
1/2
= 6
d p
2D e k 0
O RT P os
1/2
(7.10)
(7.11)