Sulfur K-edge XANES study of sulfur transformation during pyrolysis of four coals with different ranks

• Sulfur forms in coal and its thermal transformation were well correlated by combined GC-FPD, XRF and XAS
• S-containing gases during coal pyrolysis are related to the sulfur species and their inter-conversion
• Sulfur species in raw coals and chars were validated by S and Fe K-edge XAS
• Sulfur transformation during coal pyrolysis is closely related to the sulfur form and mineral contents
• Sulfur transformation during coal pyrolysis is very different for coals with different ranks.

Sulfur transformations during pyrolysis of four high sulfur coals with different ranks are studied by measuring the release of S-containing gaseous products using online gas chromatography with a flame photometric detector and analyzing the sulfur forms in raw coals and chars using sulfur K-edge X-ray absorption spectroscopy. Results show that the sulfur forms in different coals are different and they behave differently during pyrolysis. More active disulfide and sulfide in lower rank coal can be decomposed and released as gas products below 500 °C; while a more complex thiophenic structure in the higher rank coal is difficult to decompose even at 1000 °C. Decomposition of pyrite in coal plays an important role in sulfur transformation during pyrolysis of coals rich in pyritic-sulfur. Pyrite begins to convert to FeS above 300 °C; while FeS will also react with nascent char or volatiles above 600 °C. The inter-conversions among sulfur species in solid play a dominant role in the transformation of sulfur forms above 600 °C. The minerals in coal are valuable in understanding conversions of sulfate and sulfide. The sulfur retention in solid form is correlated well with the releasing trend of H2S and COS during the coal pyrolysis.