L. Ma, Stanford University, Ph.D. Student, Bldg 520, Duena St., Stanford, CA 94305 and R. E. Mitchell, Mechanical Engineering, Professor, Bldg 520, Duena St., Stanford, CA 94305.
Despite the numerous studies concerned with char oxidation, the database on pressurized char reactivity to oxygen is rather limited. The data are insufficient to answer some of the most rudimentary questions concerning the impact of pressure on char reactivity. Some studies show a relatively small dependence of the rates of char oxidation on total pressure whereas other tests show a significant impact. Our efforts are directed at resolving this controversy. To this end, we have undertaken a project to increase the data available on the conversion rates of coal chars at elevated pressures and to use the data to characterize the separate effects of total pressure and oxygen mole fraction on char reactivity. In the initial phases of this project, we have measured the intrinsic chemical reactivities of synthetic chars at 0.1, 0.4 and 0.9 MPa and at oxygen mole fractions of 0.06 and 0.12 for temperatures between 673 K and 873 K in a thermogravimetric analyzer. In all combustion tests, the overall mass loss rates of the chars were governed by chemical kinetics effects. The measured char conversion rates during oxidation tests were combined with surface areas determined from gas adsorption measurements to yield intrinsic reactivities as functions of char conversion under zone I burning conditions. Experimental results show that under zone I burning conditions, char reactivities increase as the total pressure increases at constant oxygen mole fraction. At constant total pressures, char reactivities also increase with increasing oxygen mole fractions. The data were used to determine the kinetic parameters in a three-step heterogeneous reaction mechanism that has been used to describe accurately char reactivities at atmospheric pressures. Using the mechanism, it is possible to predict adequately the measured reactivities at the various oxygen mole fractions and total pressures used in the high-pressure experiments.