MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

m obs = 0 at high temperatures) and activation energy, although the values of reaction order
and activation energy were assumed rather than measured. In the future, the measured
values of reaction order and activation energy of these chars may become available, and
more extensive comparisons will become possible.
A global n-th order rate equation is inadequate to unify reaction rates in both Zone
I (TGA data) and Zone II (FFB data). Although an intrinsic m-th order rate equation has
the possibility to unify the rates in both Zone I and Zone II, it cannot explain the change
of reaction order with temperature, such as those observed by Ranish and Walker (1993).
Assuming a constant reaction order of 0.7 would yield an apparent reaction order of 0.85
in Zone II, which is not commonly observed at high temperatures.
Summary
The HP-CBK model theoretically could use up to 7 adjustable parameters (E 1p,
A 1p, E p, A p, r p1, r p2 and M). Compared to the n-th order rate equation, which has three
parameters (A, E obs, and n), the HP-CBK model has more adjustable parameters.
However, in many cases the number of adjustable parameters were reduced. For example,
for non-porous graphite flakes, no pore structure parameters (r p1, r p2 and M) were
needed. In some cases, the Langmuir rate equation reduced to a zero-th order rate
equation, and E 0 (which is equal to E 1p-E p) and A 0 (which is equal to A 1p/A p) were required
as input parameters instead of E 1p, A 1p, E p, and A p. The curve-fitted parameters and
assumptions used for each set of data are listed in Table 7.9.
The global n-th order rate equation has obvious difficulties: it cannot be
extrapolated between Zone I and Zone II; it insufficiently describes the effects of total
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