OCR Text |
Show As shown in Fig. 7, the post-devolatilization profiles were significantly different when oxygen was present. Just after the completion of devolatilization, char ignition was indicated by a rapid rise in the particle temperature. The peak temperature reached during this period depended very much on the surrounding oxygen concentration. For pure air (i.e., 21 mole % 0~), it was found to be as much as 260 K (470°F) above the nominal reactor temperature. From this point the particle temperature remained fairly constant until the fuel was nearly consumed. Usually there was a pronounced tail-off in the rates of temperature and weight loss indicating complete burnout (extinction). For convenience, the characteristic regions in Fig. 7 have been assigned Roman numerals as follows: initial heat-up - region I, devolatilization and swelling - region II, char ignition - region III, char combustion - region IV, and extinction -region V. Both weight loss and temperature profiles were developed for series A-Q. Only weight loss measurements were made for series R and S, since the particles were too small to attach to the thermocouple. Temperature measurements for the intermediate size particles clearly indicated that decreasing particle size resulted in a smaller difference between the particle temperature during combustion and the type K thermocouple temperature. Since thermocouple measurements were not made for series S (and subsequently for Phase II), the particle combustion temperature was assumed to be equal to the type K thermocouple temperature for analysis purposes. This assumption was also supported by optical pyrometer measurements. Phase I - Initial heating rate Recent sudies have clearly demonstrated that initial heating rate can have a significant effect on the properties of the devolatilized char [see for example Essenhigh (11)]. Thus it is important to compare the heating rates in these experiments to those that are likely to occur in fluidized beds. Reasonably good approximations of heating rate were made from the temperature vs time profiles discussed previously. Observed values varied between 28 and 96 K/s (50 and 173°F/s) for coal particles 22 |