Devolatilization and cracking characteristics of Australian lumpy coals

An experimental study was conducted to investigate the devolatilization characteristics of five Australian coals in a thermogravimetric analysis (TGA) reactor by varying the coal lump size and the temperature. The swelling ratio was measured after thermal treatment of coal lumps in a horizontal tube furnace at 1273 K while the cracks generated in the lumpy char samples were examined using scanning electron microscopy (SEM). Physical and chemical properties of coal and char samples were measured using CO2 gas adsorption, Hg porosimetry, X-ray fluorescence (XRF) and X-ray diffraction (XRD). Under all the tested conditions, the total volatile-yield of lumpy coals was found to be not influenced by either the temperature or particle size and was similar to that indicated in proximate coal analysis. However, as expected, the devolatilization rates were found to increase with increasing temperature as well as the increasing amount of volatiles present in the coal. The study further demonstrated that the effect of coal properties on the devolatilization rates of lumpy coals may not be significant as the rates decrease with increasing lump size, such that coal lumps with sizes more than 10 mm indicated similar orders of reaction rates. The apparent activation energy of coal lumps indicated a linear correlation with the stack height of the carbon crystallite of coals. The study demonstrated that the cracking and swelling behavior of coals was influenced by physical as well as chemical properties, particularly their modification during devolatilization conditions. The study showed that coals with low volatiles indicated high cracking which would increase further with increasing lump size in accordance with the size effect. The cracking tendency of coals appeared to have a reciprocal association with swelling tendency such that less swelling coals are more vulnerable to cracking.