MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

temperature profile of the post-flame gases and the residence time of the particles are
kept relatively constant from condition to condition.
Mass Release
The mass release of char is commonly normalized by the original mass of the coal
on a dry, ash-free (daf) basis. The mass release is determined by assuming mass
conservation of one of the following elements or material: Al, Ti, Si, and ash. It can also
be determined by overall mass balance, i.e., by the amount of char collected and the
amount of coal fed in the experiment. In this project the mass release of a char is
determined using all five of these methods. In most cases, these five methods produced
relatively consistent values of mass release. In this analysis, the average values of the
mass releases based on Ti and Al conservation were used for the calculation of the high
temperature reactivity of Koonfontain chars made under conditions #2 and #4, since the
mass releases based on Si were sometimes subject to question.
The mass release data from Table A.4 are plotted in Figure A.2. It can be seen
that for both Koonfontain and Middleburg coals, the mass release of char #2 (CH 4, fuel-
lean) is higher than that of char #1 (CH 4, fuel-rich), and the mass release of char #4 (CO,
fuel-lean) is higher than that of char #3 (CO, fuel-lean). In other words, the presence of
oxygen in the preparation environments increases the mass release of the char, as expected
due to the initiation of char oxidation in the fuel-lean reactor conditions.
The decrease of steam concentration did not seem to affect mass release of chars in
fuel-lean conditions (#3 vs. #1), but seemed to decrease mass release slightly in fuel-rich
conditions (#4 vs. #2).
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