OCR Text |
Show Table 3. The effect of swelling on char burnout time Nominal reactor R. t Series temperature r.,n r,/n / r,/1 2 . /> \ r„ ,0 I ?M d60 d40 (g C/cm s) (s) /Jj. [K ( F)] ^!*AlL B 1096 (1512) 1.34 1.12 1.28 x 10_4 520 I 874 (1114) 1.75 1.55 7.34 x 10"5 275 K 982 (1308) 1.42 1.40 9.97 x 10~5 388 r,,_ = diameter at 60% weight remaining/initial coal diameter. r,,_ = diameter at 40% weight remaining/initial coal diameter. in the chemical rate expression. The coal was sized in a narrow sieve cut between 30 and 35 mesh (sieve openings 595 to 500 ym), giving it an effective spherical diameter of 708 ym (based on surface area). Thus, according to the results of Phase I, it should burn almost completely under chemical reaction control. The activation energy and reaction order were determined by measuring the combustion rate at three nominal reactor temperatures (867, 978, and 1089 K) and three oxygen concentrations (2, 5, and 10 mole % ) . Figures 11 and 12 illustrate the results of these experiments. From these plots, the activation energy was estimated to be V11.5 kcal/g-mole and the reaction order estimated to be 0.6. Based on the results reported in the literature [see for example Smith (18)], this activation energy appears unusually low. Most data for cokes and chars have indicated values of between 16 and 35 kcal/g-mole. The unusually low value could indicate significant catalysts from metallic impurities, or possibly a different reaction mechanism at the low temperatures of this experiment. Mulcahy and Smith (2) point out that catalysis effects should be enhanced at lower temperatures. The reaction order of 0.6 fits nicely in the expected range of 0.5 to 1.0. Another possible explanation for the low apparent activation energy may be that the internal surface area of the char is higher at 30 |