EVOLUTION OF CHAR CHEMISTRY, CRYSTALLINITY, AND ULTRAFINE STRUCTURE DURING PULVERIZED-COAL COMBUSTION

The carbonaceous structure of partially reacted char samples, generate d by direct injection of pulverized coal into a laboratory entrained f low reactor, was characterized by four techniques, elemental analysis, carbon dioxide vapor adsorption, x-ray diffraction, and fringe-imagin g using high-resolution transmission electron microscopy. It is observ ed that the early stages of heterogeneous oxidation proceed in paralle l with the latter stages of carbonization, leading to preferential los s of hydrogen, a reduction in surface area, and the development of cry stalline order. Typical combustion times and peak temperatures are ins ufficient to bring about true (three dimensional) graphitization for m ost coals, but rather, lead to the growth of regions of turbostratic o rder. This ordering is seen to occur over a time scale comparable to t he combustion process itself-here, on the order of 100 ms at particle temperatures of 1800 K and oxygen concentrations of 12 mol%. This work presents evidence that the reactivity of chars in the latter stages o f burnout, which is critically important to the explanation and predic tion of unburned carbon in flyash, is significantly impacted by the ev olution of the carbonaceous matrix. Although significant evolution of internal surface area and hydrogen content (indicative of aromatic rin g coalescence) occurs during early char combustion, these two phenomen a do not play a major role in the char deactivation noted in previous investigations. Among the four indicators of carbon structure evaluate d herein (H/C ratio, carbon dioxide surface area, crystallite dimensio ns by x-ray diffraction, and HRTEM images), the volume fraction of ord ered material as determined by HRTEM fringe-imaging correlates best wi th the observed reactivity loss for Illinois #6 coal chars.