Twenty-Third Symposium (International) on Combustion/The Combustion Institute, 1990/pp. 1169-1176 THE EFFECT OF CO CONVERSION IN THE BOUNDARY LAYERS SURROUNDING PULVERIZED-COAL CHAR PARTICLES REGINALD E. MITCHELL AND ROBERT J. KEE Combustion Research Facility Sandia National Laboratories Livermore, CA 94550 PETER GLARBORG Laboratory of Heating and Air Conditioning Technical University of Denmark 2800 Lyngby, Denmark AND MICHAEL E. COLTRIN Surface Processes Division Sandia National Laboratories Albuquerque, NM 87185 We examine the extent of conversion of CO to CO2 in the boundary layers of char particles burning in gaseous environments containing 6% and 12% oxygen, 16% H20, and the balance N2. Our computational model considers elementary finite-rate homogeneous and heteroge- neous chemical reactions. The gas-phase CO/O~/H20 reaction mechanism has been well- established in numerous previous flame models. We have taken an accepted adsorption/de- sorption surface mechanism and made it quantitative by assigning rate parameters that lead to char-particle temperatures in agreement with measurements. For several ambient gaseous environments, we have applied the model over a range of particle diameters. The results show that relatively little CO2 is formed in the boundary layers of small particles (less than 100 microns). Correspondingly, little thermal energy is transferred to the particle surface as a result of CO conversion in the boundary layer. Thus for small particles, any CO2 formation must occur on the particle surface, not in the sur- rounding gas. Therefore, the results support the applicability of the one-film model for par- ticles in the pulverized-coal size range. Introduction Most researchers concerned with the burning rates of carbon and coal-char particles in the pulverized- Field's one-film model of coal size range employ 1 a burning carbon sphere. The model allows for both CO and CO~ formation at the particle surface, but assumes that no reaction occurs in the boundary layer surrounding the particle. However, until re- 2-5 cently, it was generally assumed that CO is the sole heterogeneous product at temperatures ex- ceeding 1500 K. Recent experiments have measured the sizes and temperatures of burning char particles. 6~ At tem- poratures as high as 2000 K, it was necessary to account for COa formation to satisfy both mass and energy balances. Without accounting for the COz, it was impossible to simultaneously predict the measured particle temperatures and mass-loss rates. 7,s In these studies, CO2 formation was var- iously postulated to be a consequence of catalysis by mineral impurities,6 homogeneous reaction in the boundary layers surrounding particles, 7 and heter- ogeneous reaction at the surfaccs, s It is possible that catalysis can lead to the direct conversion of carbon to CO~ or to the conversion of CO to C02 on the particle surface, but one would expect catalytic ef- fects to be relatively insignificant at high temper- atures. Because of the difficulties of measuring boundary layers surrounding small particles, no de- finitive experimental data exist at high tempera- tures to establish the relative importance of CO~ formation at the surfaces of particles and C02 for- 1169