OCR Text |
Show the less volatile condensed oxide within the reducing zone inside a coal particle. The reduction is believed to be a consequence of attack by CO according to the stoichiometric reaction. MOn(c) + CO = MOn-1(v) + C02 (9) In the absence of other sources of C0? within the particle, the partial pressure of MO and C0« are equal, since they are produced in equal amounts and given by PM0n . " pCO = Ke1/2 PCO (1°) assuming that the activity in the condensed state of MO is unity. This model, with appropriate allowances for internal and external diffusion limitations, has been found to adequately describe the vaporization of the Si, Ca, and Mg (10). For such species the slope in Eq. 8 will represent not the heat of vaporization but half the heat of the reaction given by Eq. 9. As evident in Figure 11, a linear relationship is obtained between the log of the fractional vaporizational rate versus particle combustion temperature for Mg, Fe, Ca, Al, Na, As, and Zn where the burning time and average particle combustion temperature were obtained from two-color pyrometry studies. At conventional combustion temperatures (~2000 K) the rate of vaporization increases in the sequence Al, Ca, Fe, Mg, Zn, As and Na, where the relative rates span over two orders of magnitude. CONCLUDING COMMENTS The results in the above section shows that ash vaporization is strongly temperature dependent. It is therefore important that the temperature history of a particle be determined for the proper interpretation of data on ash vaporization. In the present study the temperature of a particle is seen to be partially suppressed during devolatilization and to exhibit a maximum during char combustion. Time resolve measurements of ash vaporization show that fractional vaporization of elements is consistent with the temperature history of particles. Elements such as sodium which show a 3-22 |