The effects of particle grinding on the burnout and surface chemistry of coals in a drop tube furnace

Grinding coals to a pulverised coal specification for blast furnace injection can be costly, which is why some iron manufacturers choose a larger granulated coal size specification. However, there is a concern that these coals may have lower burnout in the raceway region so there is a technical and economic balance with coal grinding. This paper investigates how the process of grinding alters the physical properties, plus the surface chemistry, of coals and their chars formed in a drop tube furnace; it was found that in many cases the larger particle size coals gave improved combustion burnout compared to smaller sizes. The physical properties of the chars, formed from grinding coals to different sizes, resulted in char swelling in the smaller particle sizes, compared to char fragmentation for the larger size classifications. Minerals phases associated with better coal reactivity were found to undergo higher conversion to other chemical forms with the larger size coals, suggesting a potential catalytic or synergistic contribution to their burnout. A closer look at the surface chemistry suggests that the action of grinding coals has an important effect on the surface chemistry. The XPS spectra of the chars, formed in a drop tube furnace, indicated that grinding the coals to a smaller particle size reduced the carbon–oxygen and carbon-–mineral interactions compared to the larger sizes and correlated with the higher burnouts. An increasing trend was identified for the carbon sp bonding with larger size and higher rank coals which correlated with their burnout at low carbon conversions; however, this did not hold at higher conversions, suggesting other factors were more dominant.