Influence of Inherent Oxygen Species on the Fluidity of Coal during Carbonization

The evolution of gaseous oxygen-containing species (CO, CO2, and H2O) during carbonization of 10 types of caking coals has been investigated mainly using a fixed-bed quartz reactor to reveal the influence of inherent oxygen species on the Gieseler fluidity of the coal particles. The heating rate and temperature were 3 degrees C/min and 1000 degrees C, respectively. CO evolution apparently started after 350 degrees C, and the rate profile for CO evolved showed the main or shoulder peak at about 650 degrees C in many cases. On the other hand, CO2 started to evolve at low temperatures of 200-250 degrees C for almost all of the coals, and the profile for the rate of CO2 evolution exhibited a main peak at 400-450 degrees C and a shoulder or small peak at about 600 degrees C in all cases. H2O formation occurred significantly between 400 and 800 degrees C, irrespective of coal type. The Gieseler fluidity analyses also revealed that the initial softening, maximum fluidity (MF), and resolidification temperatures of the 10 coals were in the ranges of 375-435, 435-480, and 450-505 degrees C, respectively, and the MF values were 0.78-4.1 log(ddpm). Interestingly, the MF values tended to decrease with increasing total amount of CO, CO2, or (HO)-O-2 up to the initial softening temperature mentioned above. Further, the addition of oxygen -containing compounds [phthalide (C8H6O2), 2-naphthoic acid (C11H8O2), and fluorescein (C20H12O5)] to an Australian caking coal decreased the MF value considerably; specifically, the value decreased from 2.2 log(ddpm) originally to 0.28-1.5 log(ddpm), and the degree of decrease was greatest with 2-naphthoic acid containing COOH. In contrast, benzofuran (C8H60) mixed with the coal did not affect the MF value significantly. These observations indicate that some of the oxygen-containing functional groups naturally present in coal have a negative effect on coal fluidity and suggest that this effect is particularly strong for carboxyl and/or acid anhydride groups, which can be readily converted to gaseous oxygen -containing species during heating to the initial softening temperature.