Ka. Davis et al., EVOLUTION OF CHAR CHEMISTRY, CRYSTALLINITY, AND ULTRAFINE STRUCTURE DURING PULVERIZED-COAL COMBUSTION, Combustion and flame, 100(1-2), 1995, pp. 31-40
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.