MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

urn with shrinking diameters, and the particle temperatures (1480 ~ 2850 K) were
beyond the temperature range where Zone I combustion typically occurs. Zone I
corresponds to the complete penetration of oxygen into the particles and a non-shrinking
combustion mode. In Zone II, the apparent reaction order can never be less than 0.5,
unless the true reaction order is negative (which is unlikely true for carbon-oxygen
reaction) assuming the external surface area is negligible compared to the internal surface
area.
A “rough sphere” combustion theory was proposed to explain the conflicting
observations. When the contribution to reaction rate from the external surface is
comparable to that from the internal surface, rough sphere combustion occurs. Rough
sphere combustion occurs when reaction rate is controlled by both kinetics and pore
diffusion (Zone II), but allows the apparent reaction order to be less than 0.5 due to the
contribution from external surface area. However, a microscopic model was used in the
original work to account for the spatial and temporal variation of pore structure, which
required extensive computational efforts.
Data of Croiset et al.
Croiset et al. (1996) performed combustion experiments in a fixed-bed reactor at 2,
6, and 10 atm at temperatures between 850 and 1200 K with Westerholt bituminous coal
char with diameters in the range of 90-106 μm. The reaction was claimed to be first order
in both Zone I and Zone II. The pre-exponential factor, A, decreased when the total
pressure increased from 2 to 6 atm. Above 6 atm, the effect of total pressure was very
weak. High pressure also favored the combustion regime controlled by pore diffusion.
Attempts were made to apply the Langmuir rate equation to these data. However, the
mole fraction of oxygen was used instead of the oxygen concentration. The theoretical
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