MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

This observation is somewhat surprising, since Gale et al. (1995) performed experiments
to show that the presence of steam in high heating rate experiments (drop tubes and
FFB's) significantly increased the N 2 BET surface areas of chars prepared in such
reactors. Reasons for this seeming discrepancy are not clear at this time.
TGA Reactivities: Effects of Reactor Conditions
The reactivities of Koonfontain chars and Middleburg chars prepared in these four
conditions were measured using a thermogravimetric analyzer (TGA) at 550 º C in 10%
oxygen. The results are shown in Figure A.6 and Figure A.7.
Figure A.6 indicates that the Koonfontain char #2 and char #4 collected at 1 inch
have similar TGA reactivities; char #1 and char #3 collected at 1 inch also have similar
TGA reactivities. However, chars #1 and #3 have higher TGA reactivities than chars #2
and #4. This means that the presence of oxygen in the preparation environment reduces
the TGA reactivity of the char. The effect of reduced steam concentration in the char
preparation environment is not significant for the Koonfontain chars. Similar trends with
post-flame O 2 environment are observed for the Middleburg chars, except that char #3 has
notably higher reactivity than that of char #1, indicating an effect of post-flame steam
concentration for this coal. This is consistent with the fact that char #3 has higher N 2
BET surface area than char #1. The reason why the reduced steam concentration affects
the N 2 BET surface area and TGA reactivity of the Middleburg char, but not the same
properties of the Koonfontain char, is not clear at this point.
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