Evolution of char chemistry, crystallinity, and ultrafine structure during pulverized-coal combustion,

The carbonaceous structure of partially reacted char samples, generated by direct injection of pulverized coal into a laboratory 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. Typical combustion times and peak temperatures are insufficient to bring about true (three dimensional) graphitization for most coals, but rather, lead to the growth of regions of turbostratic order. This ordering is seen to occur over a time scale comparable to the 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 of burnout, which is critically important to the explanation and prediction of unburned carbon in flyash, is significantly impacted by the evolution of the carbonaceous matrix. Although significant evolution of internal surface area and hydrogen content (indicative of aromatic ring coalescence) 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 (H/C ratio, carbon dioxide surface area, crystallite dimensions by x-ray diffraction, and HRTEM images), the volume fraction of ordered material as determined by HRTEM fringe-imaging correlates best with the observed reactivity loss for Illinois #6 coal chars.