Measured and predicted char reactivity of three U.S. coals

The overall surface reactivity of three pulverized U.S. coals has been determined. The experiments were carried out in an atmospheric flow-tube reactor with mean gas temperatures between 1180 and 1940 K. Oxygen partial pressures were approximately 0.075 atm. The mean char surface reactivity of the three directly fired coals varied from 0.045 kg/m 2·s at a particle temperature of 1330 K, to 0.125 kg/m 2·s at 2075 K. These reactivity values are less than the diffusion limit and indicate that heterogeneous kinetics limited the overall reactivity. These results agree to within a factor of 2 of previously reported reactivity for prepared chars from coals of similar rank. Reactivity calculations based on the pore-tree model of Simons and Finson and a single-pore-size model predict mean experimental reactivity to within a factor of 2. Uncertainty in the data precludes choosing one model over the other; however, the temperature dependence of the reactivity suggests a non-Arrhenius form of intrinsic reactivity.