OCR Text |
Show lower reactor temperatures. This would tend to make the apparent reaction rate (based on external surface area) greater than would otherwise be expected, and thus reduce the effective slope of the data on an Arrhenius plot. Resolution of this question will ultimately require rate measurements based on internal surface area. Combining all of the Phase II data, the pre-exponential factor was determined to be 6.459 x 10~ g C/(cm s atm ). The resulting chemical rate expression for 30x35 mesh Kentucky No. 9 is /-ll,50o\ R^ = -6.459 x 10"2 exp I RT J P0°*6 [g C/cm2s]. (12) Per the earlier discussion on porous particle combustion, this was modified to include the effect of particle diameter to give R = -6.459 x 10"2 ( °a \0.0708 2 [g C/cm s], where d is the effective spherical diameter (in centimeters) of the oa r original coal particle based on external surface area.* The importance of making the diameter correction is demonstrated in Table 4. Note that Eq. (12) gives reaction rates for the large particles which are significantly lower than those experimentally observed, clearly an impossible situation. On the other hand, it is likely that the particle size correction should not be used below some minimum particle size or above some maximum particle size. It is more reasonable that the combustion rate per unit area approaches some minimum value as particle size is reduced and some maximum value as the size is increased. These limits will have to be determined from additional experimentation. "The coal diameter was used rather than the char diameter for reasons of simplicity. The true relationship may be considerably more complicated because of swelling effects. 33 |