Abstract

Chars derived from Beulah-Zap (lignite A) and Dietz (subbituminous B) coals were prepared by three different methods utilizing three different reactor systems. These included a high heating rate method achieved in a methane flat flame burner, a moderate heating rate method achieved in a drop tube reactor, and a slow heating rate method achieved in a muffle furnace. The flat flame char was produced in a flame environment, while the drop tube and muffle furnace chars were produced in inert environments. Low temperature oxidation rates and kinetic parameters were determined using isothermal thermogravimetric analysis at temperatures between 550 K and 950 K. Reactivities at different oxidation burn-out levels (10–75%) were compared on both an initial mass and an available mass basis. Using the available mass basis, rates in the intrinsic regime were found to be nearly identical for the different burn-out levels. It was also found that the lower burn-out levels are more highly influenced by diffusional effects. This was manifest by a decrease in the slope of the Arrhenius plot which began at a temperature of ~ 750 K for the char at 10% burn-out compared with a temperature of nearly 900 K for the char at 75% burn-out. In comparing the chars produced by the three different methods, reactivities in the reaction control regime showed that, for both coals, the drop tube char was more reactive than either the flat flame or muffle furnace char. Further tests indicated that the drop tube chars had a hydrogen to carbon ratio that was 2.5-5 times greater than the char from either of the other reactors and the devolatilization conversion was significantly less. The activation energies for all three Beulah-Zap chars, and for the Dietz muffle furnace and flat flame chars, were found to be 118 ± 3 kJ mol −1. A comparison of the reactivities for the flat flame burner chars of the lignite and the subbituminous showed that the lignite chars were more reactive by a factor of two. This was consistent over all burn-out levels.

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