MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

Table 7.5. (continued)
RL P XO2 Tg(0) Vp(0) B T3 T2 T1
6.5 15.4 5 987 0.24 23.4 987 714 635
8.5 15.5 5 987 0.25 43 987 712 642
6.5 15.3 10 987 0.25 70.2 1002 730 635
6.5 15.4 21 987 0.29 73 980 710 631
8.5 15.3 21 987 0.34 92.8 988 710 641
8.5 15.1 10 987 0.3 80.7 1009 730 660
* RL: reaction length L in cm; P: total pressure in atm; XO2: oxygen mole fraction in bulk gas; Tg(0): gas
temperature (K) at the collection point; Vg(0): the final velocity (m/sec) measured immediately before the
particles were collected; B: daf burnout; T1, T2 and T3: wall temperatures (K) at x = 1.3, 6.4 and 16.5 cm,
respectively.
In these experiments, the temperatures and velocities of the particles were
measured using a two-color pyrometer (Monson, 1992; Cope, 1994; Tichenor et al. 1992)
immediately before the particles entered the collection probe. The reported particle
temperatures seem unreasonably high, and do not permit balancing of the particle energy
equation. It is likely that many of the actual particle temperatures were below the
temperature measurement threshold, and only a few non-representative particles were
measured. Such errors were shown for other char oxidation experiments with low gas
temperatures by Fletcher and Hardesty (1992). Therefore, the measured particle
temperatures were not used. Instead, a CO/CO 2 product ratio was assumed and the
particle temperatures were calculated by the model. The carbon burnout (daf) of the
collected particles was determined following the contents of three tracers (ash, Ti and Al).
For each test, burnout values were calculated using each of the tracers. In almost every
case the three values agreed within a few percent, promoting confidence in the
determination of burnout.
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