Oxidation of high sulphur coal. Part 1. Desulphurisation and evidence of the formation of oxidised organic sulphur species

A systematic study has been made on the desulphurisation of organic sulphur in both feed and mercury-treated coal for their unoxidised samples as well as their oxidised samples prepared at 50, 100 and 150°C in air under normal atmospheric pressure. A continuous increase of organic sulphur loss has been observed; the magnitude of which is substantial in mercury-treated samples. Formation of oxidised organic sulphur functionalities (SO and –SO2) are identified by infrared spectroscopy. The band in the region 1080–1020 cm−1 assignable to SO stretch for all oxidised sulphur species (sulphoxide, sulphone, sulphonic acid, sulfenic acid and sulphinic acid) while the band in the region 1350–1320 cm−1 attributable to assymmetric and 1170–1120 cm−1 to symmetric for –SO2 stretch for sulphones, sulphonic acid and sulphinic acid, increase in intensity up to 100°C which decline at 150°C. The absorbance of these bands are qualitatively correlated with those of aliphatic and aromatic C–H content. The aliphatic C–H content follows similar pattern as observed for the absorbance for SO and –SO2 strech; however, the trend is exactly similar with the mercury-treated sample. The difference of absorbance between SO and –SO2 gives an indication of the extent of sulphoxide and sulfenic acid formation. Mercury has a catalytic effect in both accelerating the oxidised species formation and their desulphurisation. This study further reveals that up to 50°C desulphurisation is primarily due to the release of free radicals of sulphur species while SO species along with these radicals predominantly removed beyond 50°C. Knowledge gained from this study would be helpful for various coal conversion processes.