OCR Text |
Show 10 such as high temperature combustion, the ultrafine carbon structure can also affect reactivity through its effect on microporous diffusion coefficients [Valix et aI., 1992; Mitchell et aI., 1993]. Although the relationship between reactivity and crystalline order is more complex than this simple scheme suggests, reactivity does, in general, decrease with increasing degree of order across the spectrum of carbon materials. The carbonaceous structure of partially reacted char samples, generated by direct injection of pulverized coal into our entrained flow reactor, was characterized by four techniques: elemental analysis, carbon dioxide vapor adsorption, X-ray diffraction, and fringe-imaging using high resolution transmission electron microscopy. It is observed that the early stages of heterogeneous oxidation proceed in parallel with the latter stages of carbonization, leading to preferential loss of hydrogen, a reduction in surface area, and the development of crystalline order. Although significant evolution of internal surface area and hydrogen content occurs during early char combustion, these two phenomena do not play a major role in the char deactivation noted in previous investigations. Among the four indicators of carbon structure evaluated herein (HIe ratio, carbon dioxide surface area, crystallite dimensions by X-ray diffraction, and HRTEM images), the fraction of carbon in an ordered phase, as determined by HRTEM fringe-imaging, correlates best with the observed reactivity loss for illinois #6 coal chars. Fringe images t were obtained for a series of partially reacted illinois #6 char samples and several residual carbon samples from commercial-scale boilers. The young char sampled at the end of the primary devolatilization zone was observed to be highly amorphous, with individual, curved carbon fringes predominating. At 72 msec, corresponding to 24% char carbon conversion, most of the fringe image fields examined are still amorphous, with a minority showing some recognizable crystallinity. The carbon crystalline structure, however, develops significantly from 72 to 117 msec (76% carbon conversion), suggesting a relationship between reactivity and a thermal annealing process that may be aided by preferential oxidation. Fringe imaging was also applied to biomass chars. Very little development of crystalline order under combustion conditions is noted for these chars. The lack of crystalline order developed in biomass chars, which were reacted under conditions identical to those of the coal chars, can be attributed to the high oxygencontent and structural rigidity in the nonfusible parent material [W om at et al., 1994]. A digital image analysis algorithm was developed to quantify the increase in crystalline order observed by HRTEM. One result of this analysis, the total length of fringes produced by the t This technique uses a transmission electron microscope to image individual graphitic layers of carbon. As crystallinity develops, these layers grow and align in parallel stacking arrangements. |