A TPD study of coal chars in relation to the catalysis of mineral matter

The steam gasification mechanism of brown coal was studied by a temperature-programmed desorption (TPD)technique. A Morwell coal was devolatilized in N₂ and then gasified in steam at 1100 K. During the TPD of a partially gasified char, H₂O, CO₂ and CO evolved approximately at 640, 870 and 1020 K, respectively. The presence of mineral matter was found to be responsible for these gas evolutions, since essentially no gas evolution was observed during the TPD of the demineralized coal char. The comparison of the above TPD pattern with those determined for the cation-exchanged samples revealed which inorganic species is responsible for each TPD peak: H₂O evolution was due to Ca; CO₂ evolution to Ca and Mg; CO evolution to Na and/or Fe. The exchanged metal species like Ca and Na significantly catalysed the gasification reaction. The relation between the catalytic activity and TPD pattern was discussed in terms of surface oxygen complexes.