Dynamic Behavior of Surface Oxygen Complexes during 0₂ Chemisorption and Subsequent Temperature-Programmed Desorption of Calcium-Loaded Coal Chars

The gas evolution during the oxygen chemisorption on coal char was determined together with the monitoring of weight change. From these data, the detailed mass balance during chemisorption was established. The addition of Ca to char accelerates the CO₂ evolution, whereas there was no effect on CO evolution. Approximately 70% of the reacted oxygen was trapped on the char surface. The stabilities of these trapped oxygen complexes were determined by a subsequent temperature-programmed desorption (TPD) experiment. The effect of Ca on the CO₂ and CO evolutions during TPD was somewhat different. The CO₂ evolutions increased with Ca loading, but the CO evolution was almost unchanged. From the chemisorption and TPD studies, the function of Ca on the formation of oxygen complexes was proposed. The scheme includes (1) oxygen transfer from CaO(O) to carbon, (2) formation of C(O), and (3) further reaction between C(O) and active oxygen to form C(O.O). From these results, the relationship among chemisorption, TPD, and gasification was discussed.