AN INVESTIGATION INTO THE CHANGES OF STRUCTURE AND REACTIVITY DURING DESULFURIZATION OF A BITUMINOUS COAL

A bituminous coal has been treated with molten (non-aqueous) caustic using several reaction temperatures and times. The caustic treatment removed all the pyritic sulphur and up to 80% of the organic sulphur. The extent of organic sulphur removed, as determined directly by scanning electron microscopy and energy-dispersive X-ray analysis (SEM-EDX), was linearly dependent on temperature. Optimal desulphurization conditions were investigated and correlated with losses in calorific value. The fate of pyrite and the organic sulphur functional groups during the desulphurization procedure was followed by the recently developed technique of temperature programmed reduction (TPR). Pyrite and the thermally labile sulphur compounds were removed, and with increasing desulphurization temperatures and times the thiophenic compounds were also removed. It has also been possible to follow the desulphurization quantitatively with TPR, i.e. the amount of aliphatic sulphur removed as indicated by the elemental analysis was the same as the amount obtained from the integrated TPR signals. These results underline the value of the TPR method as a technique for following sulphur functional group distribution during coal processing. The effect of the caustic treatment on the microstructure was clearly noticeable on the SEM micrographs showing the formation of depressions and microfissures and fragmentation into smaller particles. At higher reaction temperatures solid state C-13 cross-polarization and magic angle spinning nuclear magnetic resonance (C-13 CP/MAS) indicated an increased carbon aromaticity (f(a)C) and an increase of the non-protonated aromatic fraction (f(a)NP).