OCR Text |
Show 0.6-0.7 cm in diameter. The average observed was about 53 K/s (96°F/s). Pillai et al. (16) reported heating rates of 53 K/s (96°F/s) in fluidized bed experiments with Pittsburgh No. 8 coal of the same size. Still another indication of particle heat-up rate is the time at which the volatiles flame extinguishes. For pure air (21 mole % 0£) and 1089 K (1500°F) nominal reactor temperature, we found that the volatiles flame usually extinguished after 30 to 35 s. This seems to be remarkably close to observations made during fluidized bed experiments at Massachusetts Institute of Technology according to personal communications from Byrd (17). In conclusion, it appears that the heating rates in the tubular reactor experiments were probably not much different from those which occur in fluidized-bed combustion. Thus the initially devolatilized tubular reactor char is probably similar to the initially devolatilized char produced in fluidized beds. Phase I - Devolatilization and swelling Initially, the particles underwent rapid heating and weight loss due to both drying (primarily inherent moisture) and some devolatilization. As heating continued, the particles became plastic and swelled considerably. Volatile bubbles could be seen erupting from the surface, causing a frothy appearance. At 21 mole % 0„, a volatiles flame was visible. The liquid-like behavior of the coal in region II was quite pronounced. Its appearance was similar to hot tar. This implies that coal particles devolatilizing in a fluidized bed will almost certainly come apart and possibly adhere to or be coated by pieces of bed material. The greater the agitation, the more likely it is that this will happen. This aspect of coal behavior, as well as the attrition of the resulting char, is probably the most important difference between the tubular reactor and an actual fluidized bed combustor. Since the tubular reactor data do not include fragmentation and attrition effects, it will be necessary to account for these separately in modeling fluidized bed combustors. 23 |