Effects of burnout on char oxidation kinetics

Abstract

The effects of both extent and type of burnout on char oxidation rates and rate parameters (apparent activation energy and oxygen reaction order) have been investigated for chars prepared from Dietz (subbituminuous B) coal. Intrinsic rates were determined using isothermal thermogravimetric analysis (TGA). N2 BET and CO2 DP surface areas were measured, as was hydrogen to carbon ratio (H/C). CaO surface area was measured for selected samples. Three types of burnouts were studied and compared. Devolatilization mass loss (DML) was studied by devolatilizing the Dietz coal to various extents in a flat-flame methane burner (FFB) and then comparing the oxidation rates and other properties of the resulting chars. Low-temperature oxidation burnout was studied by oxidizing a FFB char to a continuum of burnout oxidation rates. High-temperature oxidation burnout was studied by taking the same FFB char and oxidizing it to various conversion levels in a drop-tube reactor (DTR) at 1400 K (particle temperature). The oxidation rates and kinetics of these partially burned out char samples were then determined using TGA. The rate of oxidation was found to decrease with increasing devolatilization residence time, even after mass loss (DML) and H/C had become essentially constant. This decrease in reactivity was shown to correlate with a decrease in CaO surface area, consistent with the importance of CaO catalysis in low-temperature char oxidation. H/C shows an inverse linear correlation with DML. N2 and CO2 surface area data indicate dramatic increases in the mesopore surface are during devolatilization but not in the micropore surface are. Intrinsic rates of chars partially burned out at high temperatures were found to decrease with burnout level, while those of chars burned out at low temperatures were essentially constant with burnout level. Values of apparent activation energy increased with burnout level (or DML) for all three types of burnout by 20 to 30%. Oxygen reation orders ranged from 0.74 to 0.53 and generally showed a decrease with burnout (or DML).