Effects of CaO, high-temperature treatment, carbon structure, and coal rank on intrinsic char oxidation rates

The low-temperature kinetics of oxidation of Dietz sub-bituminous coal char prepared in a methane flat-name burner (4% post-name oxygen) was studied by TGA both in the presence and in the absence of calcium minerals. The reactivities of untreated and calcium-reloaded chars at 600 K are 5 and 2 times higher than acid-washed char, indicating a significant catalytic effect for CaO. The intrinsic reactivities of these chars after oxidation in a drop-tube reactor at a particle temperature of about 1900 K and 5% O-2 also show a similar trend, although the reactivity of each char is lowered by about a factor of 10 due to this high-temperature oxidation. The physical properties of these chars are also significantly altered due to high-temperature oxidation treatment. Comparison of intrinsic oxidation rates of unloaded Spherocarb and demineralized (acid-washed) chars of Zap and Dietz coals based on available carbon mass shows a trend of increasing intrinsic rate with decreasing skeletal density suggesting that the intrinsic rate is a function of carbon structure. However, in the presence of CaO, the intrinsic oxidation rate based on CaO surface area is found to increase with decreasing coal rank. Dispersion of CaO is significantly higher for the original Dietz char prepared in a flat-flame burner (34%) than for the Ca-loaded char (12%), indicating that the Ca-reloading procedure could be improved.