MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

It is interesting to compare Figures A.4 and A.6; the TGA reactivities (the steady
part of the curve) of Koonfontain chars are proportional to their N 2 BET surface areas,
but poorly correlated to their CO 2 surface areas. From Figure A.5 and A.7, the variations
of TGA reactivities of Middleburg chars are also well correlated to their N 2 BET surface
areas but are weakly correlated to their CO 2 surface areas.
It is believed that the N 2 BET surface area represents the mesopore surface area
while CO 2 surface area represents the micropore surface area (Gale et al., 1995). Internal
surface area is not accessible to oxygen unless the feeder pore is big enough or the reaction
is slow enough to allow oxygen to penetrate the pores before it is consumed (Laurendeau,
1978). It is generally accepted that at typical TGA temperatures, the burning rate of char
particles is slow enough to allow complete penetration of oxygen into all the pore
structures and onto all the surface area in the particles. The data produced by this project
seem to suggest:
1. At the temperature of the TGA experiments, all the surface area contributed by the
mesopores is accessible to oxygen, but not all the surface area contributed by
micropores is accessible.
2. In Zone I, oxygen completely penetrates bigger pore structures so that from a
macroscopic perspective, a particle burns uniformly throughout the whole particle.
However, oxygen fails to penetrate smaller pore structures, so that from a
microscopic perspective the burning of the particle is not uniform. That is to say,
complete oxygen penetration is a relative term; it occurs only for pore structures
bigger than a certain size. The relative nature of complete oxygen penetration, if
further substantiated, would imply that the activation energies measured in TGA
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