Coal char reactivity and structural evolution during combustion—Factors influencing blast furnace pulverized coal injection operation

A fresh char was prepared and reacted with oxygen under conditions similar to those prevailing in the raceway region of the blast furnace (BF) during pulverized coal injection (PCI), using a well-characterized drop-tube furnace (DTF). Char combustion under the present conditions was found to be controlled by the combination of pore diffusion and chemical reaction. Both the char density and size gradually decrease with burnoff, while the char surface area increases up to a burnoff of 40 to 50 pct due to the formation of a large amount of meso- and micropores, which were observed by high-resolution field-emission scanning electron microscopy (FESEM) and gas adsorption measurements. Despite the obvious increase in surface area, the char combustion reactivity decreases with burnoff. This is due to the loss of the intrinsic reactivity of char during combustion, as confirmed by fixed-bed (FB) measurements of fresh char and chars partly burnt in a DTF. The structural characterization by quantitative X-ray diffraction analysis (QXRDA) shows that the amorphous concentration (fam) of the char decreases during combustion, while the aromaticity (far) and the average crystallite size (L002) of the char increase. The char becomes more ordered during combustion, which is in accordance with the observations made using high-resolution transmission electron microscopy (HRTEM). The char structural ordering observed was found to be responsible for the loss of char intrinsic reactivity during combustion. Based on the QXRDA, a char structure model has also been suggested to explain the char structural evolution observed during combustion. The implications of char structural evolution for char combustion during a PCI operation are also discussed.