SWELLING PROPERTIES OF COAL CHARS DURING RAPID PYROLYSIS AND COMBUSTION

Coal devolatilization experiments are commonly conducted at moderate temperatures (800 to 1300 K) and heating rates (10(3) to 10(4) K s-1) in inert environments in order to measure evolved species before secondary reaction in the gas phase. However, chars from these experiments exhibit different physical characteristics than chars obtained under typical combustion conditions (1500 to 2000 K, 10(5) K s-1, and 3 to 10 mol% oxygen). Experiments were conducted in two laminar, entrained-flow reactors to determine characteristics of coal chars in inert and oxygen-rich environments. One flow reactor was heated electrically, with gas temperatures of 1250 K, and the mol% oxygen was varied from 0 to 10%. The other flow reactor used a flat flame burner as the heat source, with gas temperatures of 1600 K, and the post-flame oxygen content was varied from 0 to 12 mol%. In both reactors, sampling was limited to regions during and immediately following devolatilization. Five coals of different rank were examined; for a given coal, similar total volatile yields were obtained in both flow reactors, and similarities in chemical compositions of the resulting chars are discussed. For softening coals, the apparent densities of chars obtained in the electrically heated reactor are much lower than that of chars from the flat-flame reactor, regardless of the gas phase oxygen content. This implies that changes in particle swelling behaviour between typical devolatilization experiments and char combustion experiments are not due to the presence of oxygen, but due to heating rate or post-flame gas species other than oxygen.