EFFECTS OF PREOXIDATION ON RAPID PYROLYSIS BEHAVIOR AND RESULTANT CHAR STRUCTURE OF CAKING COALS

An experimental investigation was conducted to determine the effects of mild preoxidation on subsequent rapid pyrolysis behavior of bituminous coals. Resulting char properties (surface area, morphology, C/H ratios, reactivity in air) were then analyzed to ascertain the effects of preoxidation on overall coal gasification potential. Two highly caking bituminous coals, and preoxidized samples of each coal (up to 2.5% oxygen added by wt.), were pyrolyzed in an entrained flow tube furnace (> 10('4) K/s heating rate) at temperatures between 1073 and 1273 K with heat treatment times up to 0.270 s. Complementary slow pyrolysis (12 K/min) experiments were conducted to determine the effects of pyrolysis conditions on subsequent char properties.Chars produced by rapid heating of the unoxidized coals have classical cenosphere structures with particle diameters 2 times larger than the starting coals and apparent densities less than 0.1 g/cc. Corresponding micropore surface areas are found to be significantly (2 to 3 times) higher than those observed for the parent coals. At the levels of oxidation employed, neither coal forms the cenosphere structures produced by the unoxidized precursors however, some thermoplastic behavior is evident. The amount of preoxidation required to eliminate thermoplastic properties appears to be a function of the subsequent heating rate. During the early stages of pyrolysis (times < 0.3 s), preoxidation has little influence on resulting char surface area and reactivity. Following longer heat treatment times (1 h), however, preoxidized coals yield chars of higher surface area and reactivity than do the unoxidized parent coals. Heating rate has little effect on subsequent char reactivity.Volatile yields during rapid pyrolysis are found to increase significantly with increasing temperature and approach asymptotic levels within 0.1 to 0.2 s. The apparent first order rate constants for pyrolysis are dependent on the extent of reaction as well as on the temperature. Devolatilization kinetics appear to be limited by thermal transport processes. As the level of preoxidation is increased, volatile yields are progressively reduced. Lower pyrolysis yields are paralleled by dramatic reductions in the quantities of tar liberated.